bauxide residue issue 3
Bauxite residue issues:III.Alkalinity and associated chemistry
M.Gr?fe,G.Power,C.Klauber ?
CSIRO Process Science and Engineering (Parker CRC),Light Metals National Research Flagship,PO Box 7229,Karawara,WA 6152,Australia
a b s t r a c t
a r t i c l e i n f o Article history:
Received 20August 2010
Received in revised form 1February 2011Accepted 14February 2011
Available online 24February 2011Keywords:
Bauxite residue Red mud
Bauxite residue management Bauxite residue disposal Bauxite residue utilization Bauxite residue neutralization Buffering chemistry Alkaline solids Surface charge
A detailed understanding of the complex buffering and neutralization chemistry of bauxite residue remains the key to improved management,both in terms of reduced environmental impact for current storage practices,legacy costs and for the utilization of the material as an industrial by-product for other applications.In spite of 120years of continuous industrial production,the nature of residue and the chemistry of remediation is still poorly understood.This review brings together what is known of residue behavior and outlines the existing knowledge gaps in our understanding.It examines those aspects of the Bayer process that relate to the creation of the alkalinity in residue and discusses in detail the complex chemical reactions that govern the neutralization behavior.pH is the “master variable ”in the chemistry of residue and is strongly buffered by the presence of multiple alkaline solids.The pH in untreated residue liquor (washer over ?ow)ranges over 9.2–12.8with an average value of 11.3±1.0.This high alkalinity is the primary reason for residue classi ?cation as a hazardous material,and in conjunction with the sodic content the primary reason that residue will not support plant life.The pH is highly buffered by the presence of alkaline solids (various hydroxides,carbonates,aluminates and aluminosilicates)that are formed by the action of caustic soda on bauxite in the Bayer process re ?nery.The presence of such Bayer process characteristic solids causes the acid neutralization behavior of residues to be highly complex and makes impractical the removal of alkalinity by simply washing with water.This chemistry also impacts physical properties e.g.bulk density,sedimentation rates and compaction,hydraulic conductivity,drying rates and dusting behavior,and physical strength after drying.Understanding how surface charge develops,distributes and abates in the residue mineral assemblage as a function of acid input will be paramount to understanding neutralization reactions overall,to successfully model them and eventually to implement the most effective neutralization measures that create conditions at the surface conducive to reduced environmental impact,e.g.to enable plant growth.Once this is understood a model can be constructed for the neutralization behavior of bauxite residue based on the underlying mineralogy and its relationship to overall surface charge.This is the third in a series of four reviews examining bauxite residue issues in detail.
Crown Copyright ?2011Published by Elsevier B.V.All rights reserved.
1.Introduction
In the prior two reviews covering bauxite residue issues (Power et al.,2011-this issue;Klauber et al.,2011-this issue )the disposal management practices and options for residue utilization are consid-ered.The presence of well engineered land based impoundments for residue storage,which applies to most of the world's re ?nery operations,does not provide a ?nal solution for the dilemma of billions of tons of residue currently in storage.Ensuring that the impounded residues do not cause harm to surrounding ecosystems nor diminish the
amenity and aesthetics of the landscape is not a comprehensive long-term solution.Vertical and horizontal embankment integrity only ensures that hazardous components are con ?ned for a ?nite period.
Notwithstanding some of the innovative technical options that have been developed for the use of residue none have been integrated into existing industrial processes to any major extent (Klauber et al.,2011-this issue ).Unless particular industrial synergies happen to arise through local co-existence,the most likely path forward for high volume rehabilitation of residue is conversion into an effective soil component (Klauber et al.,2011-this issue;Gr?fe and Klauber,2011-this issue ),i.e.transforming it from a hazardous waste into a form that supports plant life via a bio-remediation approach.This is arguably the most sustainable approach for the future of bauxite residue management.Impor-tantly this should not be confused with impoundment closure practice.While it is feasible to overlay residues with layers of
Hydrometallurgy 108(2011)60–79
?Corresponding author.Tel.:+61893348060;fax:+61893348001.E-mail address:craig.klauber@csiro.au (C.
Klauber).
0304-386X/$–see front matter.Crown Copyright ?2011Published by Elsevier B.V.All rights reserved.doi:
10.1016/j.hydromet.2011.02.004
Contents lists available at ScienceDirect
Hydrometallurgy
j o u r n a l h o me p a g e :w w w.e l s ev i e r.c o m/l o c a t e /hyd ro m e t
fertile materials to help plants to establish,continuous manage-ment over an extended period is required to ensure the ongoing effectiveness.High volume remediation needs go much further.Investigations at the Gove re ?nery have shown that the success of the impoundment closure method is critically dependent on drainage and the ability to withstand resurging alkalinity from lower lying residues (Wehr et al.,2006).While not surprising,this emphasizes that the key to the problem is an understanding of the residue chemistry,in particular the complex buffering and neutral-ization behavior.This is equally true whether conventional options are chosen for improved management (reduced environmental impact for current storage practices and reduced legacy costs)or for the utilization of the material as an industrial by-product or for returning it to a medium capable of supporting life.For example,even something as pivotal as the reactivity and longevity of the naturally occurring and synthetic sealants that are used to maintain the security of residue impoundments in relation to leaching of alkaline waters are not well documented in the literature.
The high alkalinity is the primary reason for classi ?cation of residue as a hazardous material,and in conjunction with the sodic content,the primary reason that residue will not support
Table 1
Selected summary of chemical and physical characteristics (average and range)of unamended bauxite residues.Detailed data is presented in Tables 7and 8.See glossary for extended de ?nitions.
Average
Std a Max Min n b Units pH 11.3 1.012.89.744–
EC 7.4 6.028.4 1.446mS cm ?1[Na +]101.481.6225.88.99mmol +L ?1SAR 307.2233.167331.510–ESP
68.919.69132.110–
ANC,7.00.940.3 1.640.6813ANC,5.5c 4.56––
–1PZC d 6.9 1.08.25 5.111(pH)BD 2.50.7 3.5 1.613g cm ?3SSA
32.7
12.2
58.0
15.0
30
m 2g ?1
a std =standard deviation of the population (n).
b
n=population size from which the average and std were calculated.c
Only one study is quoted in this table.Additional studies are discussed in text where the ANC,5.5was estimated from graphs.d
Measurements include bauxite residues with and without ?occulant
addition.
Fig.1.Schematic of a general Bayer process “red side ”,highlighting those processes affecting the disposal and storage of bauxite residues.The right arrows for the counter-current wash train (1–5)indicate the ?ow of wash water opposite to the ?ow of residue.The ?nal residue preparation stage may be a deep cone thickener (5),superthickener (6)or a ?lter.Some re ?neries practice a variety of pre-disposal neutralization amendments.For more detail and explanation of the bauxite residue production process,please see Power et al.(2011).
61
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
plant life.The sodic-alkaline content also causes the formation of friable dust-prone surfaces(Klauber et al.,2008)and might contribute to embankment failure in the long term.However,a range of toxic trace metals and naturally occurring radioactive materials(NORMs)are also known to be present in residue,but little is known of their mineralogy,chemical speciation or leaching behavior,especially in relation to neutralization.Without an understanding of the chemistry,speciation information is discon-
nected from an understanding of the residue behavior as a whole.With an understanding of the latter it becomes feasible to systematically realize:
?Microbiologically assisted bio-remediation of residues.?Hydrological modeling of liquid?ow in residue impoundments.?Optimized amendments for the development of residue structure conducive to plant growth.
?Sustainable vegetative covers in bauxite residue disposal areas.?Best agronomic practices for managing vegetative covers overlying bauxite residue disposal areas.
For those unfamiliar with the Bayer process and residue related terminology please refer to the glossaries at the end of the Part II review(Klauber et al.,2011-this issue)and this review.
2.Overview of residue chemistry
2.1.Bauxite residue:generation and composition
Bauxite residue is the slurry by-product generated during the treatment of bauxite ores using the Bayer process to produce alumina.Bauxite residue is also commonly referred to in the literature as red mud,Bayer process tailings,or bauxite process tailings.In this review “bauxite residue”is the preferred term.
In2008,60.5million tonnes(Mt)of alumina were produced worldwide(International Aluminium Institute,2008).As a global average,the production of a tonne of alumina generates some1.5 tonnes of bauxite residue so approximately91Mt of bauxite residue was produced in2008based on IAI data and120Mt taking remaining re?neries into account.Presently an estimated2.7billion tonnes(Bt) is in storage(Power et al.,2011-this issue).
Bauxite residues are strongly alkaline,have a high salt content and electrical conductivity(EC)dominated by sodium(Na+)and the particles are compacted(high bulk density(ρ)).Trace metals can be of concern and may exceed regulatory levels in certain circumstances(Batley et al.,2003;Goldstein and Reimers,1999; Kutle et al.,2004).Some bauxite residues may emit ionizing radiation above natural background rates due to the presence of naturally occurring radioactive materials(NORMs):238U and/or 232Th and members of their decay chains(Bardossy and Aleva, 1990;Cooper et al.,1995;McPharlin et al.,1994;Pinnock,1991; Somlai et al.,2008;von Philipsborn and Kuhnast,1992;Wong and Ho,1993).Little is known about the speciation of trace
Table2
Bayer digest variables(Hudson,1987).
Main Al mineral Gibbsite Al(OH)3Boehmiteγ-AlOOH
Temperature(°C)104–145200–232
Pressure(atm) 1.0–3.0 6.0
NaOH(M)8.9–3.6 5.0–3.6Table3
Main elemental and mineralogical composition of bauxites(Bardossy and Aleva,1990). The minerals are listed in order of general abundance.
Element
(as oxides)
Content,wt.%Main mineral
phases
Unit cell
formula
Min Avg Max
Al2O32026–6070Gibbsite
Boehmite
Al(OH)3
γ-AlOOH
Fe2O30.510–3565Goethite
Hematite
α-FeOOH
α-Fe2O3
TiO20.12–425Anatase
Rutile
Ilmenite
TiO2
TiO2
TiFeO3
SiO20.14–815Kaolinite
Quartz
Si4Al4O10(OH)8
SiO2
Notes to Table4
1.Atun and Hisarli(2003).
2.Newson et al.(2006).
3.Liu et al.(2007b).
4.Zhang et al.(2008).
5.Alp and Goral(2003b).
6.Atasoy(2005).
7.Atasoy(2007).
8.Cablik(2007).
9.Hamdy and Williams(2001)).
10.Hirosue et al.(1980).
11.Kasliwal and Sai(1999).
12.Lopez et al.(1998).
13.Ochsenkuhn-Petropoulou et al.(1996).
14.Park and Jun(2005).
15.Polcaro et al.(2000).
16.Sayan and Bayramoglu(2001).
17.Sglavo et al.(2000).
18.Summers(1996).
19.Vachon et al.(1994).
20.Xenidis et al.(2005).
21.Zouboulis and Kydros(1993).
22.Fuller et al.(1982).
23.Li and Rutherford(1996)).
24.Li(1998).
25.Kainuma et al.(1979).
26.Ercag and Apak(1997).
27.Li et al.(2006).
28.Prasad and Subramanian,1997).
29.Xiang et al.,2001).
30.Zhang et al.(2001).
a LOI=loss on ignition measured between900and1100°C.
b DDI=double deionized water.
62M.Gr?fe et al./Hydrometallurgy108(2011)60–79
metals and NORMs in bauxite residues,particularly with regard to pH neutralization and the accompanying changes in the mineral and solution phases.Typical residue characteristics are listed in Table 1.
The physical and chemical characteristics of the residues make re-vegetation challenging,and established ?oras are dif ?cult to sustain without amendments (Meecham and Bell,1977a;Meecham and Bell,1977b;Wehr et al.,2006;Wong and Ho,1994a,b;Wong and Ho,
1991;Wong and Ho,1992;Wong and Ho,1993;Woodard et al.,2008).
2.2.Mineral,chemical and physical inputs from the Bayer process In order to understand the nature of bauxite residues,it is useful to brie ?y summarize the Bayer process including its mineral,chemical and physical inputs (Fig.1).The Bayer process is a high
Table 4
Elemental compositions of bauxite residues determined by XRF.Bauxite origin Re ?nery
Neutralization or treatment Al 2O 3Fe 2O 3SiO 2TiO 2CaO Na 2O LOI a Ref.Unknown Unknown Acid 17.5837.2616.94 5.55 4.388.317.17
1Ghana Burntisland Acid
17.0451.7521.028.360.190.672China Zhengzhou Bayer process and limestone 12.519.9 6.741.6 2.43China Chalco Hot water,acid,DDI b -H 2O 18.36 6.8114.4910.4525.22 5.534Turkey
Seydisehir None 20.2439.8415.27 4.15 1.89.438.795Guinea,South Africa Aughinish None 23.630.69.6517.85 6.4 5.310.1
6Turkey Seydisehir None 20.2439.8415.24 4.15 1.89.437Guinea
Aughinish None 23.630.49.6517.85 6.4 5.37Ex Jugoslavia Birac Alumina None 14.1448.511.53 5.42 3.967.57.258Unknown Point Comfort None 17.8409.598.487.57 2.6910.39Unknown Mobile None 19.426.410.29.4n/a 5.49Unknown Gramercy
None 1551.5 1.7 6.770.979.39Unknown Corpus Christi None 8.8952.5 4.48 6.6410.85 3.178.469Unknown Unknown None 19.247.37.58.3 3.8 4.38.710Unknown Unknown None 15308181041211Unknown San Ciprian None 20.131.8 6.122.6 4.8 4.78.7
12Ghana Burntisland
None 23.4336.3118.25 5.97 4.3812.362Greece Alumine de Grece None 15.642.59.2 5.919.7 2.413Unknown S.Korean re ?nery None 23.716.622.9 6.7 6.711.614Weipa Eurallumia None 1841.89.988.87 5.57 6.478.715Turkey Seydisehir None 17.2737.7217.1 4.81 4.547.1310.2216Weipa
Eurallumia None 2035.211.69.2 6.77.57.317Darling Range Pinjarra None 17.136.223.8 3.9 3.9 1.610.418Unknown Arkansas None 23.110.123.1 3.647.2 3.6 4.418Unknown Suriname None 24.333.416 3.6581418Unknown Arvida
None 20.631.68.9 6.2 1.710.321.1
19Greece Alumine de Grece None 15.8548 6.967.0614.84 3.2620Greece
None
None
16.9439.34 6.95 4.7913.221Southern Brazil Laboratory Nalco 7879,BM1 6.871.9 1.357.8 3.20.410.422Southern Brazil Laboratory Nalco 7879,BM27.965.30.610.95012.823Southern Brazil Laboratory Nalco 7879,BM310.864.60.88 2.90.110.523Southern Brazil Laboratory Nalco 7879,BM41262.2 2.39.1 3.20.79.523Southern Brazil Laboratory Nalco 7879,BM59.461.7 3.410.4 4.2 1.512.523Southern Brazil Laboratory Nalco 7879,BM613.560.40.89.3 3.20.48.123Southern Brazil Laboratory Nalco 7879,BM77.865.7 3.89.7 3.6 1.423Jamaica Laboratory Nalco 7879,JHM18.1653.50.810.914.80.311.623Jamaica Laboratory Nalco 7879,JHM2 2.3462.1 1.312.915.90.8 4.624Jamaica Laboratory Nalco 7879,JHM37.4657.80.611.911.30.310.724Jamaica Laboratory Nalco 7879,JHM4 2.1264.20.713.314.20.6524Jamaica Laboratory Nalco 7879,JHM5 6.1359.10.612.113.40.48.324Jamaica Laboratory Nalco 7879,JHM6 5.9859.40.612.913.20.47.6
24China
Pingguo
None
26.826.913.17.3
23.5
3Weipa,Bintan Tomakomai Works Red mud (none)17.945.312.4 6.925Weipa,Bintan Tomakomai Works Red sand (none)19.653.912.8 2.025Weipa Eurallumia Seawater 17.930.59.588.617.7712.112.415Turkey Seydisehir Unknown 19.137.615.7 4.9 2.49.57.826China Shandong Unknown 6.912.819.1 3.4346.0 2.37 5.7327India Renukoot Unknown 21.928.17.515.610.2 4.512.228Hungary
Ajka Unknown 14.842.113.5 5.2 6.18.98.228Darling Range Pinjarra Unknown 17.136.223.8 3.9 3.9 1.610.428India Muri Unknown 24.324.5 6.218.0 5.328India Korba
Unknown 19.427.97.316.411.8 3.312.628India Damanjodi Unknown 14.854.8 6.4 3.7 2.5 4.89.528India
Belgaum Unknown 19.244.57.013.50.8 4.010.028Unknown Gramercy
Unknown 15.051.5 1.7 6.77.0 1.09.328Unknown Pocos de Caldas Unknown 21.929.617.5 4.4 2.98.311.528Virgin Islands St.Croix Unknown 33.022.98.512.9 3.5 6.012.428Unknown Arvida Unknown 28.427.414.39.8 1.38.89.928Jamaica Kirkvine Unknown 13.249.4 3.07.39.4 4.012.528Unknown Gramercy
Unknown 14.648.9 2.7 6.99.1 1.511.629Unknown Corpus Christi Unknown 11.747.8 5.4 6.48.7 2.712.829Unknown Point Comfort Unknown 20.332.79.38.9 6.87.413.0
29China
Shanxi
Unknown
7.3
6.8
13.9
2.5
33.9
2.7
30
63
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
temperature,high-pressure dissolution process extracting gibbsite (Al(OH)3)and/or boehmite(γ-AlOOH)from bauxite by dissolving these constituents in concentrated NaOH solution.This process takes advantage of the solubility of Al3+as aluminate(Al(OH)4?) relative to other constituents in bauxite.The preferred reaction temperature,pressure and concentration of NaOH(Table2)depend on the mineralogical composition of the bauxite(Table3).It should be noted that diaspore can also be extracted at high temperatures (240–260°C)by the Bayer process(although other processes are generally used).Due to the solubility of phyllosilicate clays such as kaolinite,a pre-desilication step is often necessary to minimize the silica contamination in the green liquor(for control of product quality).In pre-desilication slaked lime(Ca(OH)2)may be added prior to the digestion process to form cancrinite instead of sodalite. The desilication products(DSPs)of sodalite and cancrinite are a sub-set of the more general Bayer process characteristic solids (BPCSs)that are disposed with bauxite residues and impart signi?cant acid neutralizing capacity(alkalinity)to the residues as a whole.
Following bauxite digestion,the NaAl(OH)4-rich solution (green or pregnant liquor)is separated(clari?cation)from the remaining solids,i.e.,the bauxite residues.The separation process usually occurs in settlers with the aid of?occulants (Amano et al.,1992;Ishikawa et al.,2001;Yang et al.,2007)and occasionally in pressure-decanters(Iida et al.,1994).Other additives such as iron oxides(Anon,1967),MgSO4(Pohland and Schepers,1985),apatite solids(Roach et al.,2007) and(very seldomly)slaked lime(Pohland and Schepers,1985) may be used for speci?c purposes according to the re?nery and bauxite;to minimize the contamination of the green liquor by soluble and colloidal iron,carbonate,organics including oxalate,phosphate and other unwanted impurities.The solids generated from these reactions,e.g.calcite(CaCO3),tri-calcium aluminate(Ca3Al2(OH)12),whewellite(CaC2O4·H2O)and/or apatite(Ca10[PO4]6(OH)2)become part of the bulk bauxite residue mixture.
After separation from the green liquor,bauxite residues are sequentially washed in counter-current decantation washer trains employing high volume settler tanks to recover entrained NaOH and Al(OH)4?,which are returned to the Bayer process.Flocculants such as polyacrylates and polyamides are used to optimize solid–liquid separation.In the?nal washing step,bauxite residues are thickened to a paste with a speci?c solids concentration(wt.%)before being sent to bauxite residue disposal areas(Power et al.,2011-this issue). Any Ca input as a?lter aid causes the formation of further Ca-bearing minerals that report to the residue,including:CaCO3,TCA, cancrinite and hydrocalumite.Additional residue calcite comes from the causticization step that replaces carbonate ions with hydroxyl
Table5
Mineralogical compositions of bauxite residues.
Bauxite origin Re?nery Neutralization
or treatment Hematite
α-Fe2O3
Goethite
α-FeOOH
Magnetite
Fe3O4
Diaspore
α-AlOOH
Boehmite
γ-AlOOH
Gibbsite
Al(OH)3
Quartz
SiO2
Rutile
TiO2
China Zhengzhou Aged10years7.6–0–––––China Zhengzhou Aged5years8.2–7.8–––––China Zhengzhou Bayer Process+limestone7–5–––––China Zhengzhou None7.4–8–––––China Pingguo Normal Bayer process19––––––3 Darling Range Kwinana None●●––●●●–Darling Range Kwinana None8.724.3–– 1.3– 4.9–Ghana Burntisland Acid16.123.8–––– 1.3 5.4 Ghana Burntisland None13.521.8–––– 1.2 4.6 Greece Alumine de Grece None●–●●–––●Guinea Aughinish None●●––●●●●Guinea,South Africa Aughinish None●●––●●●●India(Bihar)Renukoot Causticized13.87.3–0.59.61– 1.1 India(Bihar)Renukoot None22.210.9–0.613– 1.8 NaTiO3n/d8
Jamaica Arvida None●●––●●––Jamaica Kirkvine None●●––●–––Turkey Seydisehir None●●––●●●●Turkey Seydisehir None●––●●●––Weipa Eurallumia Acid27–––54––Weipa Eurallumia None●–––●●●●Weipa Eurallumia None27–––932
Weipa Eurallumia None29–––65––Weipa QAL Seawater●–––●●●–
Anatase TiO2Sodalite Na8
[Al6Si6O24]
[(OH)2]
Cancrinite Na6
[Al6Si6O24]?
2CaCO3
Calcite
CaCO3
Kaolinite
Al4Si4O10
(OH)8
Imogolite
Al2SiO3
(OH)4
Perovskite
CaTiO3
Ilmenite
FeTiO3
TCA
Ca3Al2
(OH)12
Hydrocalumite
Ca4Al2(OH)12·
CO3
Other Amorphous Reference
–not found.
●present.
n/d not determined.
1.Liu et al.(2007b).
2.Thornber and Hughes(1986).
3.Thornber and Hughes(1992).
4.Newson et al.(2006).
5.Ochsenkuhn-Petropoulou et al.(1996).
6.Atasoy(2007).
7.Atasoy(2005).
8.Venugopalan et al.(1986).
9.Yong and Ludwig(1986).
10.Alp and Goral(2003a).
11.Alp and Goral(2003b).
12.Castaldi et al.(2008).
13.Sglavo et al.(2000).
14.Garau et al.(2007).
15.Hanahan et al.(2004).
64M.Gr?fe et al./Hydrometallurgy108(2011)60–79
ions in solution by precipitating the former as calcium carbonate (calcite):
Na2CO3tCaeOHT2?2NaOHtCaCO3esT
The NaOH is then recycled into the Bayer circuit.Even after repeated washing,bauxite residues remain strongly alkaline,because much of the alkalinity is in form of these various slowly dissolving solid phases.
In2007,the majority(~65%)of alumina producers were dry-stacking bauxite residues in land-based disposal areas(Power et al.,2011-this issue).Most other re?neries used wet lagooning(~20–30wt.%solids)as their disposal method,but at least two(Gardanne and Alumine de Grece) disposed their residues to the offshore marine environment(Pagano et al.,2002;Varnavas et al.,1986).Several alumina re?ners apply a degree of amendment to their bauxite residues.For example,at Queensland Alumina Re?nery(QAL,Gladstone,Queensland),bauxite residues are slurried with seawater for pumping the slurry to the bauxite residue disposal area(BRDA)which has the bene?cial side-effect of lowering the pH(Graham and Fawkes,1992).Neutralization by seawater is also practiced at Eurallumina(Sardinia,Italy)and for supernatant liquor (washer over?ow)at Gove(Northern Territory,Australia)(Anderson et al.,2008;Leoni and Penco,2002).At Alcoa's Kwinana re?nery(Western Australia),bauxite residues are treated with CO2prior to disposal to reduce the pH(Cooling et al.,2002),but notably also to improve settling characteristics and?nal dried residue strength(Nikraz et al.,2007).In other cases bauxite residues are disposed as dry?lter cakes e.g.,Renukoot, at70%solids(Pagano et al.,2002;Shah and Gararia,1995;Varnavas et al., 1986).In the context of this review,the term residue liquor is used for lake water,washer over?ow or any liquid phase associated with the residue,including that which arises from rainfall on the BRDA.
In summary,bauxite residues are highly alkaline solid–liquid mixtures,whose properties are a result of the treatment of bauxite by the application of NaOH,heat and pressure,as well as lime and other chemical additives.In some cases the residue is partly neutralized,but in all cases its overall chemical and physical characteristics inhibit the establishment of vegetation and pose a barrier to many possibilities for utilization.
3.Characterization of bauxite residues
3.1.Mineralogy of bauxite residues
The identity and quantity of mineral phases in bauxite resi-dues are important to the overall behavior of residue alkalinity. Knowing which minerals and how much are present in residues provides information about the residues'buffering capacity as the minerals dissolve in acid.In addition,physical parameters such as particle size distribution,speci?c surface area(SSA)andρare relevant with respect to the reactivity of the solids.For
Table5(continued)
Anatase TiO2Sodalite Na8
[Al6Si6O24]
[(OH)2]
Cancrinite Na6
[Al6Si6O24]?
2CaCO3
Calcite
CaCO3
Kaolinite
Al4Si4O10
(OH)8
Imogolite
Al2SiO3
(OH)4
Perovskite
CaTiO3
Ilmenite
FeTiO3
TCA
Ca3Al2
(OH)12
Hydrocalumite
Ca4Al2(OH)12·
CO3
Other Amorphous Reference
–––●––11.5–––Illite20.71
–––●––10.9–––Illite24.61
–––●––11–––Illite221
–––●––10.2–––Illite20.81
–––––32–10––221
●●–●––––●–Muscovite n/d2
0.3 2.7–11.2––––––Muscovite(5.8)48.33
––––––––––n/d514
–17.5––––––––Muscovite38.24
–●–●––●–––Calcium silicate n/d5
–●●–––●–●–n/d n/d6
–●––––●–●–n/d n/d7
11 2.301––0–––Hydrogrossular n/d8
3.8 3.7
4.71–– 1.1–––Hydrogrossular
●●–●–––––––9
●––●––––––Bayerite n/d9
–●●–––––––Calcium silicate n/d6
––●●––––––n/d n/d10,11 52429–––––––n/d n/d12●–●–––––––Bayerite,chantalite n/d13 351––––––5n/d14 51633–––––––n/d n/d12–●●●––––●●Brucite,whewellite n/d1565
M.Gr?fe et al./Hydrometallurgy108(2011)60–79
example,SSA in?uences the rates of dissolution reactions andρrelates to the packing density and hence to hydraulic conductivity. Tables4and5summarize the elemental and mineralogical compositions respectively of a range of bauxite residues that have been examined.Bauxite residues are solid-solution mixtures ranging in initial solids content from20to80%by weight (depending on the disposal method of the re?nery)with a typical order of elemental abundance of Fe N Si~Ti N Al N Ca N Na(Table3, Table4).
Bauxite residues consist on average of approximately70%(by weight)crystalline phases and30%amorphous materials(Table5). The elemental abundance and mineralogical composition of bauxite residues in comparison to bauxite(Tables3and6)are very distinct. Hematite is present in all bauxite residues with a concentration range of7%to29%.Goethite is particularly prevalent in bauxite residues generated from Jamaican and Darling Range bauxites(Li, 1998;Li and Rutherford,1996)(Fig.2).Boehmite(γ-AlOOH), gibbsite(Al(OH)3),anatase,rutile(both TiO2),ilmenite(FeTiO3), perovskite(CaTiO3),and quartz(SiO2)are the other minerals commonly present in bauxite residues.Clearly there is the mineralization that carries over from the original bauxite and the mineralization that is a consequence of the Bayer process.The latter can be conveniently termed Bayer process characteristic solids (BPCSs);they primarily contain(in no particular order)Al3+,Ca2+, Na+,H3SiO4?/H2SiO42?,OH?and CO32?and are generally absent in the bauxites.As noted,within this group of BPCSs is a smaller generic subset of the DSPs(sodalites and cancrinites)that also play a signi?cant role in re?nery scaling issues as well as residue chemistry.
A convenient way to think of the BPCSs is in terms of the metal centres(Na,Al,Ca and Ti)and the terminal or bridging groups (such as O,SiO4)and1CO3)that constitute the mineral array; sodalite,cancrinite,hydrated Ca-silicates,tri-calcium aluminates, hydrocalumites,calcite/aragonite,sodium carbonates and perov-skite(Table6).Sodalite is the most common DSP forming during pre-desilication,while cancrinite may form in the presence of Ca in the elevated temperature regimes during the digestion of boehmi-tic bauxites.Sodalite concentrations of16–24%(Castaldi et al., 2008)and up to50%cancrinite(Garau et al.,2007)have been measured in bauxite residues from Eurallumina,which processes Weipa bauxites.Perovskite(CaTiO3)and calcite/aragonite(both CaCO3)are common in bauxite residues due to the addition of lime during the Bayer process,for example as reported for Chinese re?neries by Liu et al.(2007b).Hydrated Ca-silicates,tri-calcium aluminates and hydrocalumites form from adding slaked lime(Ca (OH)2)during digestion and causticization(Fig.1).The formation of sodium carbonates(e.g.,trona or nahcolite)at the surfaces of residues in the drying areas due to evaporation emphasizes that the solution contains a substantial concentration of Na+and CO3. Occasionally magnetite Fe3O4is a constituent and in one unusual case(Liu et al.,2007b)imogolite(Al2SiO3[OH]4).The presence of imogolite in residue is presumably an indication of incomplete dissolution during pre-desilication and digestion.
3.2.Physical characteristics of bauxite residues
The particle size of bauxite residues averages2to100μm with a typical range of100nm to200μm(Pradhan et al.,1996;Roach et al., 2001).It is therefore on average in the silt to?ne sand textural class(Gee and Bauder,1986).Newson et al.(2006)reported a particle size range for bauxite residues from the UK(likely Burntisland,Scotland)between 1and300μm with50%of the particles being larger than5μm.Fuller et al.(1982)have shown that the texture of the residues can be dependent on the location within the disposal area(Fig.3).Table7summarizes the main physical characteristics of a range of bauxite residues that have been examined.
In the residue area of the decommissioned Alcoa re?nery at Mobile (Alabama,USA),the residues become increasingly?ner with increas-ing distance from the embankment.Ten to20m from the embank-ment,the residues are sandy(sand,silt,and clay%=76–90,4–16,and 4–8,respectively);30–40m away,they are loamy(sand,silt,and clay %=19–43,30–43,and26–38,respectively);and at N50m,the residues are silty–clayey(sand,silt,and clay%=5–7,41–43,and 53–54,respectively).The size fractionation with distance from embankment shows that the residues began to separate immediately from the point of discharge near the embankment,consistent with residues disposed of as wet slurries in lagoons rather than as thickened pastes discharged for dry-stacking.
The averageρof bauxite residues is reported as2.5±0.7g cm?3 (Table1).Bulk densities exceeding1.5g cm?3impede root penetra-tion and therefore the establishment of plants,and atρvalues above 1.6g cm?3healthy plant growth is unlikely.Nikraz et al.(2007) worked with residues that had aρof1.85prior to treatment,and treatment with CO2resulted in aρof1.8g cm?3.By comparison bitterns loweredρto1.62g cm?3,at which level root penetration would be possible.
The average SSA of bauxite residues is32.7±12.2m2g?1and ranges between15and58m2g?1(Table1),which is consistent with the approximate size/distribution and textural class of the residue. This SSA is small in comparison with many soils,particularly with soils that have high amorphous mineral content and/or2:1type clays,which have high internal surface areas,i.e.substantial micropore structure would appear to be absent from bauxite residue. Both these types of minerals are absent in bauxite residues because they are unstable under the digestion conditions in the Bayer process
(i.e.they dissolve in digestion).
4.Residue alkalinity and associated chemicophysical characteristics 4.1.pH:the master variable
Many geochemists refer to pH as the master variable,because most reactions are at least a partial function of pH and therefore changing the pH can either drive a reaction forward or backward.For example,Al (OH)3(s)is quite stable at pH7.5,however at pH4.0and at pH13,it will readily dissolve.
Table8summarizes the most important chemical character-istics of a range of bauxite residues that have been reported. The pH in untreated residue liquor ranges over9.2–12.8with an average value of11.3±1.0.The alkaline anions in bauxite residue solution are OH?,CO32?/HCO3?,Al(OH)4?/Al(OH)3(aq)and H2SiO42?/ H3SiO4?.These anions are the dissolution products of most BPCSs. Thornber and Binet(1999)conducted an experiment in which they sequentially exchanged bauxite residues with H2O.The authors determined that the weight of the solids decreased with sequential washings,but neither the pH,Na+,Al(OH)4?,CO32?nor the OH?concentration changed in solution.This simple experi-ment demonstrated that the solution pH of the residues was buffered by alkaline solids,and that the pH did not change until these solids were completely dissolved and their reaction products removed.
4.2.Acid neutralizing capacity(ANC)
The experiment by Thornber and Binet(1999)and that of others (Fuller et al.,1982;Meecham and Bell,1977a)also demonstrated that
1When generally referring to polyprotic acids/bases we do not indicate the pH
dependent chemical species,but refer to them in more general terms,e.g.,carbonates=
CO3,silicate=SiO4,phosphate=PO4.Where speciation is relevant,we indicate the
appropriate chemical speciation,e.g.:at pH13,H2SiO42?dominates in solution.
66M.Gr?fe et al./Hydrometallurgy108(2011)60–79
bauxite residues have the ability to neutralize acid.How much acid bauxite residues can neutralize is expressed by their acid neutrali-zation capacity (ANC),which measures the amount of mineral acid required to reach a speci ?c pH endpoint (Table 1)(Carter et al.,2008;Lin et al.,2004;Liu et al.,2007b;Snars et al.,2004a ).Fig.4shows an acid neutralization capacity curve for bauxite residues from Pinjarra (Alcoa,Western Australia),desilication product (DSP)(probably sodalite)and calcite.The ANC to pH 7.0of this bauxite residue is about 1.2mol H +kg ?1of residue,while that of DSP is 2.6mol H +kg ?1.Calcite buffers the pH at 7.0up to 5moles of H +acid being added per kg of calcite.
In ?ection points on ANC curves indicate complete mineral dissolution and a new phase assuming the buffering role.The length of the plateau is related to the amount of buffer present.DSP exhibits its greatest buffering capacity near pH 7and an abrupt in ?ection point near pH 5as the last of the DSP is dissolved.This result is similar to the ?ndings of Wong (1988)who showed the disappearance of XRD peaks of natrodavyne (a cancrinite group member)at pH 6.3and its complete disappearance by pH 5.4.
Carter et al.(2008)described the ANC of bauxite residues from nine different Alcoa re ?neries.The buffering curves of untreated residues showed multiple in ?ection points suggesting that several solids were buffering the pH.The ANC of the residues to pH 7ranged from 1to 3.5mol H +kg ?1residue,which is greater than the ANC (pH 7)measured previously by Snars et al.(2004a)for bauxite residues of similar origin.The explanation could be related to the dependency of ANC on the equilibration period used after each acid addition.Thornber and Hughes (1986)showed that the change in pH increased signi ?cantly as the equilibration period was varied from 1min to 5days,and that the shape of the buffering curve was also highly dependent on equilibration time.Under well-mixed conditions,such that reactions were not diffusion limited,the results show that the neutralization reaction in solution was instantaneous;while the neutralization reactions related to the solids were limited by the rates of dissolution of the solid phases.The ANC to pH 7increased from about 0.2mol H +kg ?1solids at a 1-minute equilibration period to 1mol H +kg ?1solids at a 5day equilibration period.
These results are consistent with the observations made by Liu et al.(2007b)who found also that ANC values were higher at longer
Table 6
Elemental and mineralogical composition of bauxite residues.Points of zero charge were taken from (Hanawa et al.,1998;Hu et al.,2003;Sparks,2003;Stumm,1992).Bayer process characteristic solids (BPCSs)are identi ?ed by bold lettering.Element (n)a
Content Minerals Unit cell formula
PZC c
Min Avg ±std b Max Fe 2O 3(63)
6.8
40.9±15.6
71.9
Hematite α-Fe 2O 38.7–9.8Goethite α-FeOOH 7.5–8.5Magnetite Fe 3O 4
6.8Boehmite γ-AlOOH 8.2Al 2O 3(62) 2.1216.3±6.433.1
Gibbsite γ-Al(OH)3 5.0Diaspore α-AlOOH
6.4Sodalite Na 6[Al 6Si 6O 24]·[2NaOH,Na 2SO 4]d n/d Cancrinite
Na 6[Al 6Si 6O 24]·2[CaCO 3]·0[H 2O]e n/d f SiO 2(63)
0.6
9.6±6.7
23.8
Quartz SiO 2
b 2.0Other Illite,muscovite Rutile TiO 2 4.6Anatase TiO 2
5.9–
6.3TiO 2(61) 2.58.8±4.422.6
Perovskite CaTi IV O 38.1Ilmenite Ti IV Fe II O 3n/d Calcite CaCO 38.3Perovskite CaTi IV O 38.1Whewellite CaC 2O 4g
n/d CaO (76)0.68.6±9.447.2
TCA Ca 3Al 2(OH)12n/d Hydro-calumite Ca 4Al 2(OH)12·CO 3·6H 2O h
n/d Sodalite Na 6[Al 6Si 6O 24]·[2NaOH,Na 2SO 4]d n/d Na 2O (78)0.1 4.5±3.312.4
Cancrinite Na 6[Al 6Si 6O 24]·2[CaCO 3]·0[H 2O]e n/d Dawsonite
NaAl(OH)2·CO 3
n/d
LOI (46)
4.410.0±2.821.1
a n =number of samples.
b
Avg ±std =average ±standard deviation of the population.c
PZC =point of zero charge of the corresponding mineral.n/a =not applicable,n/d =not determined.d
Sodalite has between 0and 6waters of hydration (Whittington,1996;Whittington et al.,1998).e
Cancrinite has 0to 2waters of hydration depending on the ions in the cage (Whittington,1996):0for 2·CaCO 3,1for 2·NaOH,and 2for Na 2SO 4.f
The point of zero-charge for the desilication products sodalite and cancrinite has not been determined yet.As tectosilicates,their PZC may be similar to that of feldspar,~2–2.4.g
Whewellite is also known as calcium oxalate.h
After Tables 2and 3in (Vieillard and Rassineux,1992).
c o u n t s
80
706050403020102*theta (degrees)
Gb Gb
Qtz Gt
Hm
C
Qtz
Ca
Hm
Gt
C Hm
C Gt
Hm
C
Hm
C
Hm
C
Hm
Hm
C
Bo Gt
C - corundum (internal standard)Hm - hematite Gt - goethite Qtz - quartz Gb - gibbsite Ca - calcite Bo - boehmite A - anatase Ilm - ilmenite
A
Hm Ilm
Fig.2.XRD pattern of a Darling Range (Western Australia)bauxite residue.Corundum serves as an internal standard for calibration and determination of amorphous phase content.The unassigned peaks refer to sodalite.
67
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
equilibration https://www.wendangku.net/doc/c82514608.html,ing a residue that had not been pre-treated,the authors measured an ANC to pH 5.5and allowed each acid aliquot to equilibrate initially over a 24-hour period and later in the experiment over several days,before adding additional acid.The experiment was terminated after 780days and 367titration steps and recorded an ANC (to pH 5.5)of 10mol H +kg ?1.
These results suggest that the acid neutralization of bauxite residues is governed by a complex set of reactions that depend on the interplay between multiple solids and the solution phase.Consequently,as many researchers have shown,the ANC curves of bauxite residues do not resemble the acid neutralization behavior of pure minerals (Fig.4).Unfortunately,data relating dissolution behavior over time in a speci ?c buffering pH region is absent from the literature,but is critical if a well-founded acid neutralization theory is to be realized to model ANC based on
mineralogy.
Fig.3.Textural distribution of bauxite residues deposited at the BRDA of the Alcoa Mobile plant (Alabama,USA).The textural class in ?uences the chemical properties of the residues from pH to Na,Ca and Mg and therefore ESP.Figure based on data reported by Fuller et al.(1982).
Notes to Table 7
1.Snars et al.(2004a).
2.Snars et al.(2004b).
3.Castaldi et al.(2008).
4.Newson et al.(2006).
5.Snars et al.(2003).
6.Nikraz et al.(2007).
7.Zhang et al.(2008).
8.Atasoy (2005).
9.Cablik (2007).
10.Chevdov et al.(2001).
11.Hamdy and Williams (2001).12.Hirosue et al.(1980).13.Kasliwal and Sai (1999).14.Lopez et al.(1998).
15.Ochsenkuhn-Petropoulou et al.(1996).16.Park and Jun (2005).17.Sglavo et al.(2000).18.Summers (1996).19.Vachon et al.(1994).20.Wong and Ho (1993).21.Wong and Ho (1994a).22.Wong and Ho (1994b).23.Fuller et al.(1982).
24.Li and Rutherford,1996).25.Li (1998).
26.Liu et al.(2007b).
27.Hanahan et al.(2004).28.Kainuma et al.(1979).
29.Meecham and Bell (1977a).30.Meecham and Bell (1977b).31.Zhang et al.(2001).a
BD =bulk density.b
SSA =speci ?c surface area.c
PZC =point of zero charge.
68M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
4.3.Sodium (Na +)
In clay –water mixtures,Na +is associated with clay dispersal,poor aggregate structure,cementation and dust formation upon drying at the surface (Klauber et al.,2008;McBride,1994).Its presence in
signi ?cant quantities at alkaline pH is a signi ?cant inhibitor to the creation of aggregate structure and hydraulic conductivities conducive to plant growth.In addition,the presence of large concentrations of Na +elevates the electrical conductivity of the solution beyond tolerable limits for plants and denies plants the uptake of water.
Table 7
Physical characteristics of bauxite residues.Bauxite origin Re ?nery
Neutralization or treatment
BD a g/cm 3
Void ratio (e)
SSA b m 2/g PZC c pH,σo
Ref.Darling Range Kwinana 10wt.%gypsum
231,2Darling Range Kwinana 10wt.%gypsum,4years age 241,2Darling Range Pinjarra 5wt.%gypsum and leaching 261,2Weipa Eurallumia Acid 25.2
5.3
3Ghana
Burntisland Acid 1.75
4Darling Range Pinjarra Acid 29.95Darling Range Kwinana Bitterns
27
1,2Darling Range Kwinana Bitterns,winter 1.62 2.84
6Darling Range Kwinana CO 2
19
1,2Darling Range Kwinana CO 2,summer 1.8 1.976Darling Range Kwinana CO 2,winter 1.75
1.78
6Darling Range Pinjarra Dilute acid
301,2Darling Range Pinjarra Gypsum and leaching 26.25China Chalco Hot water,acid,DDI d
14.348.257China Chalco Hot water,acid,DDI,Na-polyacrylate 10.17.67China
Chalco Hot water,acid,DDI,polyacrylamide 11.88
8.24
7Guinea,South Africa Aughinish None 2.758Ex Jugoslavia Birac Alumina None 3.059Weipa Eurallumia None 5.13Weipa
Laboratory None 2.9 6.510Claremont,Jamaica Laboratory None 3.57.8
10Boke +Brazil Laboratory None 3
10Unknown Mobile
None 48.711Unknown Corpus Christi None 18.911Unknown Unknown None 3.4722.212Unknown Unknown None 2.913Unknown San Ciprian None 14.3 6.914Ghana Burntisland
None 1.75 3.05
1.784Greece Alumine de Grece None
2.93415Unknown S.Korean re ?nery None 0.7
16Weipa
Eurallumia None 5817Darling Range Pinjarra None 295Weipa Eurallumia None 271,2Weipa QAL
None 251,2Unknown Corpus Christi None 241,2Unknown San Ciprian None 251,2Unknown VAW Stade None 151,2Unknown Arkansas,USA None 2718Unknown Suriname None 2118Darling Range Pinjarra None 2318Unknown Arvida None 2919Darling Range Kwinana None 2520Darling Range
Kwinana None
25.521,22Claremont,Jamaica Laboratory 190ppm Nalco 9779and Alclar 665 3.226.8610Unknown Mobile 20m from dike
23Boke +Brazil Laboratory 200ppm Alclar 665and 663320610Weipa
Laboratory 50ppm Nalco 9779321.9
6
10Southern Brazil Laboratory Nalco 7879,BM1 3.9924Southern Brazil Laboratory Nalco 7879,BM2 3.8724Southern Brazil Laboratory Nalco 7879,BM3 3.7424Southern Brazil Laboratory Nalco 7879,BM4 3.6924Southern Brazil Laboratory Nalco 7879,BM5 3.6624Southern Brazil Laboratory Nalco 7879,BM6 3.724Southern Brazil Laboratory Nalco 7879,BM7 3.833324Jamaica Laboratory Nalco 7879,JHM1 3.6750.425Jamaica Laboratory Nalco 7879,JHM2 4.174025Jamaica Laboratory Nalco 7879,JHM3 3.7546.625Jamaica Laboratory Nalco 7879,JHM4 4.2247.425Jamaica Laboratory Nalco 7879,JHM5 2.9246.725Jamaica
Laboratory Nalco 7879,JHM6 3.9441.625Darling Range Kwinana None,summer 1.87 1.7251.76Darling Range Kwinana None,winter 1.82
1.57
19.26China Pingguo None
44.826Weipa
QAL
Partly-neutralized 2227Weipa,Bintan Tomakomai Works Red mud (none)44.528Weipa
QAL
Red mud (none)32.6529,30Weipa,Bintan Tomakomai Works Red sand (none)12.228China
Shanxi
Unknown
73
31
69
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
Studies by Fuller et al.(1982)on residues from the Alcoa-Mobile (AL,USA)re ?nery have shown that the concentrations of Na +in solution range between 17and 200mmol L ?1,exceeding Ca 2+and Mg 2+concentrations by two to four orders of magnitude.Other studies by Liu et al.(2007b)and Courtney and Timpson (2005)showed water soluble Na +values from residue of 89(fresh residue)mmol kg ?1and 49(mud)to 157(sand)mmol kg ?1,respectively,exceeding Ca 2+and Mg 2+concentrations in both studies by two and three orders of magnitude.These concentrations are the result of the low solubility of Ca and Mg carbonates above pH 10.
The buffering capacity of the residue solids for Na +has not been studied in detail although Thornber and Binet (1999)conducted sequential washings of bauxite residues in water and showed that the release of Na +was related to the total alkalinity (TA,according to a modi ?ed method of Watts and Utley (1953,1956))of the solution extracted after washing.The authors argued that the main source of Na +was DSP,where Na +was exchanged from the cages of DSP particles.The authors did not mention,however,which cation(s)(incl.H +)were substituting for Na +,whether DSP was dissolving or what general mechanism was responsible for enabling the “exchange ”to occur.
4.4.Electrical conductivity (EC)
Electrical conductivity is related to cation and anion concen-trations in solution and therefore to the ionic strength of a solution.In the absence of detailed solution data,it is the only feasible quantity to measure total cation and anion concentrations and to estimate ionic strength.The ionic strength is relevant as it determines the double-layer thickness of charged particles,a property that applies to bauxite residue particles (see next section on surface charge),and which in turn relates to physical behavior such as dispersion and coagulation.In bauxite residues,the high EC is due to high Na +concentrations in solution and in the solid phases.Calcium,magnesium and other cations do not contribute signi ?cantly to the EC as their concentrations are negligible in solution at pH above 10.Anions of relevance in solution are OH ?
and SO 42?
.
The electrical conductivity (EC)of bauxite residues in deionized water averages 7.4±6.0mS cm ?1and ranges from 1.4to 28.4mS cm ?1(Table 1).In amended residues,the EC varies from as low as 0.3to as high as 60mS cm ?1.Acid neutralized residues tend to have lower ECs,whereas seawater-neutralized residues have higher ECs (Castaldi et al.,2008;Meecham and Bell,1977a;Snars et al.,2004a ).Electrical conductivity is related to total cation and anion concentrations in solution,expressed as mmoles of charge per litre,by approximately a factor of 10(McBride,1994):
Total cations mmol etTL ?1 e
EC mS cm ?1
?
10e1TTotal anions mmol e?TL ?1 e EC mScm ?1
?
10:e2TMarion and Babcock (1976)established a similar relationship,
which measured the ionic concentration (IC,corrected for ion pairs)in solutions as a function of EC,which works well up to 15mS cm ?1:LogIC =0:955+1:039logEC :
e3T
Due to its relation to total cation and anion concentration,the EC can also be converted to an approximate ionic strength (IS).In natural aqueous solutions,Grif ?n and Jurinak (1973)determined that:
IS e 0:0127?EC :
e4T
In addition,the EC is related to the quantity of total dissolved solids (TDS,(McBride,1994)):
TDS mgL ?1 e
EC mScm ?1
?
640:e5T
How applicable these relationships are for bauxite residues is not known,but they have worked well for soil solutions,which are similarly complex but not nearly so alkaline.For example,in the absence of detailed solution analysis data,Eq.(4)and Table 1can be used to estimate the ionic strength in solution range to be between 0.02and 0.4mol L ?1and on average 0.1±0.08mol L ?1.Taking data from Fuller et al.(Fuller et al.,1982),EC values ranged from 1.4to 16.9mS cm ?1.
Assuming that a quarter of the Na +concentration is balanced by CO 32?
and the remainder by OH ?,the ionic strength indicated using Grif ?n and Jurinak's (1973)equation is in reasonable agreement (86%)in the absence of a more accurate solution description.4.5.Surface charge
Residue is a mixture of ?ne-sized heterogeneous solids.Inter-and intra-particle behavior in the presence of water (and the inherent dissolved salts)is critically dependent on the surface charge of the constituent particles.This affects not only gross macroscopic physical behavior such as rheology,aggregation and coagulation,but also the entire range of particle chemistry including surface hydration,ion-exchange,and redox behavior.Surface charge concepts such as point of zero charge (PZC),and double layer thickness are very complex both from an operational de ?nition point of view (i.e.how it is measured)and how it subsequently affects the aggregation and repulsion behavior of particles (Fig.5).More detail on this subject may be found elsewhere (McBride,1994;Sposito,1984;Stumm,1992;Zelazny et al.,1996)but it is worthwhile brie ?y considering the origins and natures of charge in minerals and mineral assemblies.
Charge development in pH-sensitive mineral –solution phases regulates ion exchange and adsorption/desorption reactions of ions at the mineral –water interface.The charged state of the mineral assemblage is therefore a signi ?cant regulator for ions participating in the acid neutralization reactions to either dissolve into or be removed from solution.The nature of the charge is strongly related to its origin;the mineral composition of bauxite residues reveals that pH-dependent,variably charged surfaces exist (Fe-,Al-and Ti-oxides+SiO 2)alongside permanently,negatively-charged surfaces stemming from DSPs.Thus,two origins of mineral surface charge are recognized:variable or pH-dependent charge and permanent or pH-independent charge.
The origin of charge is either a local (surface)charge imbalance caused by a lack or abundance of H +or a structural charge imbalance caused by the substitution of a host metal center with a metal of different charge to that of the host metal,e.g.Al 3+occupying a Si 4+site in sodalite (DSP).Permanently charged minerals are either negatively charged (e.g.,DSP)or positively charged (e.g.,hydro-calumite and hydrotalcite).In un-neutralized bauxite residues,both permanently charged (DSPs)and pH-dependent charge minerals (e.g.,metal oxides)are present.In seawater-neutralized bauxite residues,the precipitation of hydrotalcite adds a permanent,positively charged mineral to the residues (Palmer et al.,2009a ).
The charge stemming from a mineral surface or crystal structure is distributed throughout the oxygen-layer structure of the mineral and extends into solution,where it creates the electric or diffuse double layer.The thickness of this layer is dependent (among other things)on ionic strength and therefore on the concentration and charge of the cations and anions in solution.
The composition of the residue solution is thus not only important with respect to pH buffering,but also with respect to how surface charge and electric double layer thickness change (Stumm,1992).The
70M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
Table 8
Chemical characteristics of bauxite residues.Bauxite origin
Re ?nery
Neutralization or treatment
pH
EC a
Na
Ca
Mg
SAR b
ESP c %
CEC d
ANC e Ref.
mmol (+)/L
pH 7.0
pH 5.0
mol H +
/kg solid
Darling Range Kwinana 10wt.%gypsum
11.48.5 1.041Darling Range Kwinana 10wt.%gypsum,4years age 10.70.720.521Darling Range Pinjarra 5wt.%gypsum and leaching 9.2 2.40.88
1Weipa
Eurallumia Acid 70.398.2
2Darling Range Pinjarra
Acid
7.50.74
0.45
3Jamaica,Africa Corpus Christi Acidic gypsum
8.1 2.0 4.04Jamaica,Africa Corpus Christi
Acidic gypsum and wood waste 8 2.0 5.04China Zhengzhou Aluminium Ltd Aged 10years 9.615.4 2.80.390.06 5.98.1496.65China
Zhengzhou Aluminium Ltd Aged 5years 10.617.2 5.20.250.0313.9
17.3
514.6
5Darling Range Kwinana Bitterns 9.9 6.7 1.31Darling Range Kwinana CO 2
10.9 6.3
1.05
1Australia Unknown CO 2,5min 10.26Australia Unknown CO 2,10min 8.56Australia Unknown CO 2,20min 76Australia
Unknown CO 2,24min 7.36Point Comfort DDI f -H 2O 12.57Darling Range Pinjarra
Dilute acid 8.40.67
0.45
1Jamaica,Africa Corpus Christi Elemental S
7.810.0 4.04Jamaica,Africa Corpus Christi Elemental S and wood waste 8.711.0 5.04Guinea
Aughinish Gypsum 7.93
14.80.04
1.1 1.68Jamaica,Africa Corpus Christi Gypsum
8.674
4Darling Range Pinjarra
Gypsum and leaching 8.7 1.04
0.88
3Jamaica,Africa Corpus Christi Gypsum and wood waste 8.57.0 5.04Jamaica,Africa Corpus Christi H 2SO 4
8.6 3.0 6.04Jamaica,Africa
Corpus Christi H 2SO 4and wood waste 7.8 5.0
5.0
4Guinea,South Africa Aughinish None 12.79Ex Jugoslavia Birac Alumina None 12.2226
10Weipa Eurallumia None 11.5 2.1
106.52Guinea Aughinish None 9.79.7
0.04
0.02
56.0
45.7
11Weipa
Eurallumia None 11.5 2.1107
12Not speci ?ed Not speci ?ed None 12.71213Jamaica,Africa Corpus Christi None 10 3.77360
66.0
30.14Australia Unknown None 12.66India Damanjodi None 11.1 4.9
14China Pingguo
None 10.5 4.56
15China Zhengzhou Aluminium Ltd None 11.5828.4
8.9
0.15
0.01
31.5
32.1
883.65?
San Ciprian None 10.210.8
16Ghana Burntisland
None 11.617Greece Alumine de Grece None 1018Weipa
Eurallumia None 12.5819Darling Range Kwinana None 11.530.751Darling Range Pinjarra None 11.6 6.1 1.081Darling Range Wagerup None 12 2.70.681Darling Range Worsley None 12.6 6.30.881Gove Gove None 12.410.8 1.641Weipa QAL
None 10.28.20.921Brazil Alunorte None 12.2 3.3 1.251?
VAW Stade None 12.1 2.60.611Weipa Eurallumia None 9.818.20.941?San Ciprian None 12.6 2.30.861?
Corpus Christi None 11 3.60.771Darling Range Pinjarra None 11.75
1.08
3Darling Range Pinjarra None 10.520?
Suriname None 10.612.5
20Darling Range Kwinana None 10.5 3.9158.6
70.4
21Darling Range Kwinana
None
10.5
7.741.9
2223unknown Point Comfort None 60.87Greece Alumine de Grece None 12.817.66380.7
24Greece None
None
12.125?Mobile,AL —USA None,10m from dike 9.2 1.417.40.080.0276.453.417.5326?Mobile,AL —USA None,20m from dike 9.3 2.226.40.080.02117.063.726.5226?Mobile,AL —USA None,30m from dike 10.79.81290.110.02510.088.4129.8926?Mobile,AL —USA None,45m from dike 11.516.91980.160.01673.191.0198.5726?Mobile,AL —USA None,60m from dike 11.112.41580.140.01578.289.7158.1626?
Mobile,AL —USA None,85m from dike 11.211
139
0.13
0.01
511.2
88.5
139.88
26Jamaica Laboratory None,JHM111.627Jamaica Laboratory None,JHM210.927Jamaica Laboratory None,JHM311.827Jamaica Laboratory None,JHM411.227Jamaica Laboratory None,JHM5
11.727Weipa
QAL
Partly-neutralized
11
1.9 3.68
28
(continued on next page)
71
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
PZC of bauxite residues has been addressed by only a few studies (Atun and Hisarli,2000;Chevdov et al.,2001;Lopez et al.,1998;Zhang et al.,2008)and varies signi ?cantly among them due to the
different origin of the bauxites,variability in the Bayer process variables,the addition of different types of ?occulants and the composition of the background electrolyte.Results by Zhang et al.(2008)and Chevdov et al.(2001)suggest that polyacrylate is adsorbing speci ?cally to the surface of residue particles and thereby decreases the PZC from 8.3to 7.6and 7.8to 6.0,respectively.In contrast,polyacrylamide had little or no effect on the PZC (Zhang et al.,2008).Even lower PZC (6.0and 5.3,respectively)values for residue have been reported by Lopez et al.(1998)and Castaldi et al.(2008).
For re ?neries that are lagooning slurries,although knowledge over where the PZC lies could be exploited to help settle residue particles via coagulation,?occulation is the preferred option of thickening residues as approaching the PZC of the mineral assembly would require permanent neutralization of the alkaline constituents to maintain the PZC and a concomitant reduction in ionic strength.Flocculation operates on a different mechanism from coagulation;as a ?occulant requires the cation concentration in solution to bridge the negative charge stemming from the mineral surfaces and the negatively charged polymer (Fawell et al.,2002;Kirwan et al.,2004)it is more suited to strongly pH buffered and high Na +concentrations in solution.
Table 8(continued )Bauxite origin
Re ?nery
Neutralization or treatment pH
EC a
Na Ca Mg
SAR b
ESP c %
CEC d ANC e Ref.
mmol (+)/L
pH 7.0
pH 5.0
mol H +/kg solid
Weipa QAL Red mud (none)11.137.7546 5.9 1.6283811329,30Weipa QAL
Red sand (none)9.530.2428
0.60
0.70
531
89
8.529,30Weipa Eurallumia Seawater
91619Weipa QAL Seawater-neutralized 8.821.8 3.74
28Weipa
QAL
Seawater-neutralized 10.5 3.44
2.39
15Jamaica,Africa
Corpus Christi
Wood-waste
8.6
14
6
4
1.Snars et al.(2004a).
2.Castaldi et al.(2008).
3.Snars et al.(2003).
4.Ippolito et al.(2005).
5.Liu et al.(2007b).
6.Jones et al.(2006).
7.Woodard et al.(2008).
8.Courtney and Timpson (2004).9.Atasoy (2005).10.Cablik (2007).
11.Courtney and Timpson (2005).12.Garau et al.(2007).13.Hirosue et al.(1980).14.Krishna et al.(2005).15.Lin et al.(2004).16.Lopez et al.(1998).17.Newson et al.(2006).
18.Ochsenkuhn-Petropoulou et al.(1996).19.Polcaro et al.(2000).20.Summers (1996).
21.Wong and Ho (1993).22.Wong and Ho (1994a).23.Wong and Ho (1994b).24.Xenidis et al.(2005).
25.Zouboulis and Kydros (1993).26.Fuller et al.(1982).27.Li (1998).
28.Hanahan et al.(2004).
29.Meecham and Bell (1977a).30.Meecham and Bell (1977b).a
EC =electrical conductivity.b
SAR =sodium adsorption ratio.c
ESP =exchangeable sodium percentage.d
CEC =cation exchange capacity.e
ANC =acid neutralization capacity.f
DDI =double deionized water.
bauxite residue
DSP
CaCO 3
1412108
6420p H
5
432
1
moles H +
added per kg solid
Fig.4.Acid neutralization capacity curves of Pinjarra bauxite residues,calcite and DSP.Redrawn from Snars et al.(2004a).
72
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
Developing an understanding of how surface charge develops,distributes and abates in the residue mineral assemblage as a function of acid input is therefore critical to understanding neutralization reactions overall,to successfully model them,and ultimately to implement the most effective neutralization measures that create conditions at the surface conducive to plant growth.5.Discussion
5.1.Alkalinity and buffering
The main contribution to pH buffering is the ability of the solids to maintain the concentration of the alkaline anions in solution.This is known as the buffering capacity of the residue and entails alkaline anions that are present in both soluble and solid forms.In order to buffer,the solids need to be soluble to some degree,and some degree of H +-acceptance (Br?nsted base behavior with distinct acid dissociation constant,pKa)by the alkaline anion in solution (Table 9)is required.Above pH 10.2and in the absence of excess
Ca 2+,Na 2CO 3controls the concentration of HCO 3?/CO 3
2?
in solution,because the calcite (CaCO 3)alternative is virtually insoluble.Hence,Na 2CO 3and other alkaline solids which are more soluble at pH N 10buffer the solution pH.
In bauxite residues,the main alkaline anions buffering the solution
are HCO 3?/CO 32?,Al(OH)4?
and OH ?.Other,less concentrated anions,
which may help to buffer the solution pH as well,are H 2SiO 42?
/
H 3SiO 4?/H 4SiO 4,VO 43?/HVO 42?and PO 43?/HPO 42?/H 2PO 4?
.Their reac-tions in solution are summarized in Table 9.The pH region between two pKa values is the buffering region for the Br?nsted acid –base pair,e.g.,the buffering regime of HCO 3?is around pH 8.3(the average of
10.2and 6.35).Note that Al(OH)4?
precipitates rapidly below pH 10to form gibbsite.
To enhance the performance of the overall extraction of gibbsite and/or boehmite from bauxite,Ca in the form of slaked lime (Ca(OH)2)may
be introduced at various stages prior to,during and after digestion (Whittington,1996).This leads to the formation of a number of BPCSs,which impart signi ?cant buffering capacity to bauxite residue solutions.The abundance of these solids in bauxite residues is dependent on the exact conditions of the bauxite processing.In the case of boehmitic bauxites,Ca(OH)2increases the dissolution of boehmite at temperatures above 240°C if titanium is present.In the presence of kaolins (kaolinite and/or halloysite),which are present in bauxites,Ca(OH)2helps reduce the formation of (hydroxy)sodalite (which constitutes a signi ?cant loss of Na +from the digestion liquor)by instead transforming some to cancrinite (Table 10)which reduces the SiO 4content in solution,thereby producing a pregnant liquor with lower SiO 4(impurity)content (Whittington,1996).The addition of Ca(OH)2furthermore favors the formation of a hydrogrossular;tricalcium aluminate (TCA),which in the
presence of SiO 4transforms by the exchange of 4OH ?for one SiO 44?
.The extent of this substitution reaction appears limited however:n ≤0.6(Whittington,1996).The paucity of dissolution constants and or solubility products in the literature makes it impossible to assess the buffering capacity of most of the BPCSs listed in Table 10.Only from the
Fig.5.Approximate regions of zero surface charge for individual minerals commonly present in bauxite residues.Hydrotalcites and hydrocalumites,and DSPs have permanent-positive and permanent-negative charge,respectively.The metal oxides (Fe,Al,Ti and Si)have pH dependent charge.pH-dependent charge may also occur at edge sites of DSPs and other permanently charged minerals.A fundamental question is whether surface charge retains an individual mineral character or if a single value exists for the mineral assembly.
Table 9
Buffering reactions of common weak bases in aqueous solution of bauxite residues (Gustafsson,2006;Stumm and Morgan,1981).Reaction
Acidity constants OH ?+H 3O +?2H 2O
pK w =14.0
Al(OH)4?·2H 2O+H 3O +
?Al(OH)3·3H 2O (s)+2H 2O pK a 4~10.2CO 32?+H 2O ?HCO 3?
+OH ?
pK a 2=10.2HCO 3?+H 3O +?H 2CO 3+OH
?
pK a 1=6.35H 2SiO 42?+H 2O ?H 3SiO 4?
pK a 2=12.95H 3SiO 4?+H 2O ?H 4SiO 4
pK a 1=9.85PO 43?+H 2O ?HPO 42?+OH
?pK a 3=12.35HPO 42?
+H 2O ?H 2PO 4+OH ?
pK a 2=7.2H 2PO 4?+H 3O +?H 3PO 4+OH
?
pK a 1=2.25
73
M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
certain knowledge that the oxide minerals of Fe,Al,Ti and Si are not the solid buffering agents do we know that the BPCSs must be responsible for the buffering.The lack of dissolution constants and overall understand-ing of the dissolution mechanism of BPCSs is a signi?cant knowledge gap.
The solubility product of the BPCSs is particularly important in the context of dissolution in water and the contact that this alkaline solution makes with the vertical and horizontal sealants in the residue disposal area.Many of the earlier sealant approaches are clay based and therefore have the potential to leak in alkaline pH conditions as has been witnessed at the Kwinana(Western Australia)re?nery (Thomas et al.,2002).The safeguarding of the sealants is paramount to maintain the alkaline water within the con?nes of the disposal area embankments.
The mechanisms and factors controlling the rates of the dissolution reactions are critical,because they determine the rates of replenishment of ions into solution and therefore the timeframes over which neutralization reactions need to be evaluated.When comparing ANCs of bauxite residues from the literature,it becomes rapidly apparent that the dissolution reactions of the minerals vary considerably.Wong (1988)conducted dissolution experiments of residue from Kwinana (Alcoa,Western Australia)and showed that the XRD signature of natrodavyne(Na5Ca2[Al6Si6O24]Cl2(OH)),a desilication product similar to cancrinite,began disappearing for samples treated to a pH below6.3. This suggests that desilication products are sparingly soluble in water and need to be treated with mineral acids in order to completely dissolve,consistent with Thornber and Binet's(1999)theory of Na+ exchange,possibly from DSPs,as a source of Na+in solution.In contrast, recent work by Khaitan et al.(2009a)suggests that DSPs begin dissolving at pH just below9.The variability of these results con?rms the need to understand the mechanisms and factors responsible for the dissolution of alkaline solids present in bauxite residues if effective neutralization measures are to be devised.
In addition to the dissolution of minerals bearing alkaline anions, exchange reactions occurring at the mineral surface of the metal oxides need to be taken into account.Chevdov et al.(2001)argued that part of the buffering capacity of bauxite residues between pH9and6can be attributed to the titration of surface hydroxyl(―OH)groups and that their buffering capacity was signi?cantly enhanced by speci?cally sorbing polyacrylate?occulants.Hematite,the most abundant of all minerals,has the highest point of zero charge at approximately 8.7–9.8(Table6).Hence at pH N10,all mineral surfaces are negatively charged and therefore will repel anions and attract cations to their surfaces.In the absence of alternative cations in solution,hematite may exchange electrostatically bound Na+at its surface for a proton and thereby contribute to the buffering capacity of the residues:
S―O?ààNattH2O?S―OH0tNattOH?
where S―O?is a deprotonated,and hence negatively charged surface site on hematite,whose charge is compensated by an Na+ion.
Similar exchange reactions are possible at edge sites of other minerals.As the pH drops below9.8(PZC)during acid neutralization and declines further,the negative surface charge of the residues overall becomes increasingly smaller,and OH?and other anions in solution become attracted to the surface as the surface charge becomes increasingly positive.
The positions of the PZC reported above for bauxite residues are questionable,because PZC determinations made by point of zero salt effect titrations rely on the assumption that the charging surfaces dissolve only minimally(Zelazny et al.,1996).Bauxite residues have a number of distinctly soluble mineral species as evident from their ANC,which dissolve and re-precipitate over the course of a neutralization reaction and therefore cannot meet the conditions required for a point of zero salt effect.Charge development in pH-sensitive mineral–solution phases regulates ion exchange and adsorption/desorption reactions of ions at the mineral–water inter-face.The charged state of the mineral assemblage is therefore a signi?cant regulator for ions participating in the acid neutralization reactions to either dissolve into or be removed from solution.
5.2.Alternative means of neutralizing bauxite residues
5.2.1.Seawater
Even though Ca2+,Mg2+and HCO3?are not the most abundant ions in seawater(Table11),the addition of seawater to residue lowers the concentration of OH?and Al(OH)4?in solution due to the precipitation of brucite(Mg3(OH)6),calcite/aragonite(CaCO3), hydrotalcites(Mg6Al2(CO3)(OH)16·4H2O,aluminohydrocalcite (CaAl2(CO3)2(OH)4·3H2O),hydrocalumite(Ca4Al2(OH)12·CO3),pyroaur-ite(Mg6Fe2(CO3)(OH)16·4H2O)among others(Hanahan et al.,2004; Menzies et al.,2004).Depending on the solid-to-seawater ratio,the neutralization proceeds through two to three buffering stages before reaching a pH of8.2to9.0(Menzies et al.,2004).This pH regime is further stabilized by the increased solubility of carbonate phases(calcite and aragonite)in seawater(Stumm and Morgan,1981).
Table10
Dissolution reactions of common buffering solids present in bauxite residues
(Greenberg and Chang,1965;Greenberg et al.,1960;Stumm and Morgan,1981;
Vieillard and Rassineux,1992).
Dissolution reaction Solubility products a
Natron-decahydrate
Na2CO3·10H2O(s)+H2O?2Na++HCO3?+OH?+10H2O
pK sp=1.31
Calcite
CaCO3(s)?Ca2++CO3?
pK sp=8.42(6.2)b
Hydrocalumite
Ca4Al2(OH)12·CO3·6H2O+7H2O?4Ca2++
2Al(OH)3(aq)+HCO3?+7OH?+6H2O
pK sp=n/d
Tri-calcium aluminate(TCA or hydrogrossular,n=0)
Ca3Al2[(OH)12?4n](SiO4)n(s)+
H2O?3Ca2++2Al(OH)3+6OH?
pK sp=n/d
Hydrogrossular(0b n≤0.6in Bayer process)
Ca3Al2[(OH)12?4n](SiO4)n(s)+6H2O?3Ca2++
2Al(OH)3+nH4SiO4+6?4OH?
pK sp=n/d
Hydroxysodalite c
Na6[Al6Si6O24]?2Na OH+24H2O?8Na++8OH?+
6Al(OH)3+6H4SiO4
pK sp=n/d
Cancrinite d
Na6[Al6Si6O24]?2CaCO3+26H2O?6Na++
2Ca2++8OH?+2HCO3?+6Al(OH)3+6H4SiO4
pK sp=n/d
a pKsp=?log Ksp.
b Value in parentheses corresponds to dissolution constants in seawater.
c In sodalite2·OH?are replace
d by2·Cl?;in noselite,2·OH?ar
e replaced by1·SO42?.
d In vishnevite,2·CaCO
3
are replaced by2·Na2SO4and2·H2O;in hydroxycancrinite,
2·CaCO3are replaced by2·NaOH and1·H2O.
Table11
Composition of seawater for a total salinity of3.5wt.%(Stumm and Morgan,1981).
Ion Concentration(g kg?1)
Na+10.77
Mg2+ 1.29
Ca2+0.4121
K+0.399
Sr2+0.0079
Cl?19.354
SO42? 2.712
HCO3?0.1424
Br?0.0673
F?0.0013
B0.0045
Total35
74M.Gr?fe et al./Hydrometallurgy108(2011)60–79
pH neutralization by seawater operates differently from mineral acid neutralization in that Ca2+and Mg2+remove alkaline anions from solution as precipitates in place of the simple reactions of hydroxide and other alkaline anions that occur with acid,i.e.H+.The minerals that are formed exhibit their own buffering capacities according to their solubility products.Effectively,the buffering capacity of soluble alkali species is shifted to less soluble alkaline species.The buffering capacity and ANC to pH5.5is enhanced through the addition of seawater,in part due to the formation of calcite/aragonite.
Palmer et al.(2009b)and Palmer and Frost(2009)examined seawater neutralized residue with infrared,Raman and UV–visible spectroscopy to cover electronic as well as vibrational spectra.The major conclusions were that neutralization had no effect on structural anion substitutions such as Al3+,Fe2+,Fe3+or Ti3+;which is consistent with the known neutralization chemistry.The oxalate compound of whewellite was also noted to be observable with infrared.
5.2.2.CO2and SO2
Large scale neutralization of bauxite residues with CO2began in2000 at Alcoa's Kwinana re?nery(Western Australia)(Cooling et al.,2002; Guilfoyle et al.,2005)reaching full capacity to treat all Kwinana residue by2007with the construction of a CO2pipeline from a nearby ammonia plant.Re?neries in Japan(Sumitomo)and Italy(Eurallumina)have been using residue to scrub SO2from?ue gasses,thus neutralizing small quantities of residue,since the mid1970s and early2000respectively (Anon,1970;Cooling,2007;Cooling et al.,2002;Fois et al.,2007;Leoni and Penco,2002).The neutralization reactions by either gas phase are based on the diffusion of the gasses into solution.In the presence of O2, Na2SO3oxidizes further to Na2SO4(Anon,1970;Anon,1972).Prolonged treatment of bauxite residues with SO2(g)depletes(free)Na+in solution and increases the H+concentration and thereby contributes to the dissolution of Na-bearing minerals,including the desilication products (Leoni and Penco,2002):
Na8?Al6Si6O24 ?eOHT2 t4H2SO3t16H2O?4Na2SO3t6H4SiO4t6AleOHT3
4Na2SO3t2O2→4Na2SO4
H2CO3*is a much weaker acid than H2SO3*,with its?rst dissociation constant at pH6.35and the second at10.2.Khaitan et al.(2009b)have shown that the pH of bauxite residue slurries treated with CO2decreases within1day to an apparently stable value,but that it will subsequently increase over time.A30day exposure to1atm of CO2was required to stabilize the pH of the bauxite residue slurries at7.5.This is explained on the basis that the initial neutralization reaction occurs primarily in the liquid phase,and that the rebounding of the pH is caused by the continuous dissolution of buffering solids,primarily TCA(Cooling et al., 2002;Khaitan et al.,2009b).The kinetics of these dissolution reactions in the HCO3?and H2CO3*stability regions have not been studied in detail.Khaitan et al.(2009)and Cooling et al.(2002)have suggested that any TCA present in bauxite residue will dissolve during CO2 treatment and re-precipitate as CaCO3and gibbsite,based on results of pure TCA neutralization by CO2(Smith et al.,2003).A further observed reaction is the formation of dawsonite(NaAl(CO3)(OH)2),which is stable between pH4.1and7.8(Su and Suarez,1997).In addition,HCO3?may form inner-sphere surface complexes on amorphous gibbsite, which would contribute to an overall lowering of the point of zero charge of the mineral–solution assemblage.The extent of CO2 sequestration by a residue body is therefore limited by its soluble Ca-bearing solids,the ability to precipitate with Al and Na,and by additional external inputs,e.g.,gypsum,bitterns,to promote the formation of CaCO3.Note that in the absence of carbonation prior to disposal residue bodies will experience at least partial neutralization by atmospheric CO2 with time.This is evident from the average residue liquor pH of11.3±1.0(Table1)being lower than might be expected.Liu et al.(2007b) observe a residue pH decline from11.6to9.6over a10-year aging period.
5.2.3.Gypsum
Gypsum(CaSO4)lowers the pH of bauxite residues by precipitat-ing excess OH?,Al(OH)4?and CO32?as Ca(OH)2,TCA,hydrocalumite and CaCO3.The pH of a5and8wt.%gypsum amended bauxite residue is about8.6suggesting that the pH of the residue solution is buffered by CaCO3as either calcite or aragonite(Wong and Ho,1992;Wong and Ho,1993).The soluble product of the reaction is Na2SO4which increases the EC of the residue solution considerably,but which can be leached from the system under adequate drainage conditions with water having a lower salinity.
The ef?cacy of gypsum in lowering the pH is related to the ability of gypsum to dissolve and release Ca2+into the solution to react with OH?, Al(OH)4?and CO32?.Polcaro et al.(2000)and Xenidis et al.(2005) observed that gypsum solubility limited the extent of the pH reaction and only upon activation with H2SO4did the desired pH reductions take place.This limitation may have also occurred due to the precipitation of CaCO3on gypsum particles as has recently been observed by other researchers(Kopittke et al.,2004).In addition,common ion effects, particle-size and SSA control the dissolution rate of gypsum.
5.2.4.Microbial neutralization
Bauxite residue neutralization by microbial means has been investigated by only a small number of researchers despite early results showing signi?cant promise(Hamdy and Williams,2001; Krishna et al.,2005;Vachon et al.,1994;Williams and Hamdy,1982). Column studies conducted on bauxite residues from Alcoa's Mobile (AL,USA)plant demonstrated that the pH could be lowered from13to 6–7provided that the bacterial cultures received adequate nutrients, for example in the form of hay or sterile nutrient solutions(Williams and Hamdy,1982).Similar results were achieved several years later by Krishna et al.(2005)using Aspergillus tubingensis.While the exact mechanism of the neutralization reactions is not fully understood,it is assumed to be a combination of organic acids released by the microbes and the diffusion of the respiratory gasses into solution(Hamdy and Williams,2001).Bauxite residues incubated at24°C for34days with alfalfa hay showed the presence of lactic,acetic,propanoic and butyric acid ranging from0.6–1.0(butyric acid)to10–14(acetic acid)g L?1 (Hamdy and Williams,2001)originating from more than150cultures of bacteria including Bacillus,Lactobacillus,Leuconostoc,Micricoccus, Staphylococcus,Pseudomonas,Flavobacterium and Endobacter.
In contrast to the mineral acids(H2SO3*and H2CO3*)weaker organic acids are known to operate in a two-fold manner concerning the dissolution of minerals.Proton promoted dissolution can be enhanced by the chelation of the released metal with excess organic acids in solution thereby lowering the activity of the free metal in solution.Alternatively, speci?c adsorption of the organic acid to the surface of the solid can destabilize bonds between the surface metal and the bulk mineral by either ligand-to-metal or metal-to-ligand charge transfers that ulti-mately promotes the dissolution of metals at the surface(Stumm,1992).
The neutralization of bauxite residues using microorganisms is signi?cant in four aspects:
1.The neutralization is continuously controlled by a biological entity
rather than the(anthropogenic)application of an acid.
2.The pH is buffered by microbes as long as they are provided with
nutrients rather than buffering being controlled by the dissolution of alkaline bearing solids.
3.The bulk structure of the solids is improved due to the presence of
microbes,which improves drainage,nutrient exchange,and the chances for establishing a plant cover.
4.Microbes are an integral component of functioning rhizospheres of
plants covering mine and residue spoils.
75
M.Gr?fe et al./Hydrometallurgy108(2011)60–79
Bauxite residues treated with alfalfa hay showed survival of red wigglers and night crawlers for up to 300days in addition to various plants including poplar,monkey and pampas grass (Hamdy and Williams,2001).Based on these results,there is considerable potential to optimize bauxite residue neutralization by microorgan-isms and combine it with a revegetation program.5.3.Na +and its effect on structure
The high concentration of exchangeable sodium (Na +)relative to divalent cations,in particular Mg 2+and Ca 2+,is of major concern given the strong association to colloidal dispersion,which gives rise to poor structural characteristics including swelling,surface crusting/sealing,and erosion (McBride,1994).Because of the single positive charge on Na +and its high solubility in water,Na +ions do not dehydrate as readily as divalent cations,indicative of very stable hydration layers.In addition,sodium ions do not coordinate readily among themselves (e.g.,sharing water or OH ?molecules among Na +ions)or with negatively charged surfaces.The effect is that particles do not aggregate well and usually tend to be massive and crusted in the dry state as Na +eventually precipitates out as nahcolite or trona (Na 2CO 3species),which concurrently leads to the formation of dust at the surfaces of residue disposal areas (Klauber et al.,2008).Poor structural conditions at the surface are ultimately also detrimental to the revegetation of bauxite residues (Cablik,2007;Courtney and Timpson,2005;Fuller et al.,1982;Liu et al.,2007a ).The sodium adsorption ratio (SAR)is frequently used in agriculture to delineate whether a soil is sodic or non-sodic.SAR =Na th i =Ca 2++Mg
2+h i
?
0:5 0:5e6T
where the [concentration]values are in mmol of charge or mEq L ?1.A SAR of N 15is indicative of the soil to be sodic.The SAR is related to the exchangeable sodium percentage by:0:015SAR =ESP =100?ESP eT:
e7T
Soils with an ESP N 30are impermeable and would restrict plant growth and root penetration considerably.As noted,studies by Fuller et al.(1982)on residues from Mobile (AL,USA)have shown that the concentration of Na +in solution can exceed that of Ca 2+and Mg 2+by two to four orders of magnitude.An SAR of 76in the sandy layers near the embankment and SAR values of 673at 45m from the embankment in the silty clayey layers have been measured.The corresponding ESP values range from 53to 91.Other researchers
(Courtney and Timpson,2005;Liu et al.,2007a )have also shown SAR/
ESP values signi ?cantly exceeding 30.
Poor hydraulic conductivity is one of the main consequences of high ESP values.Hydraulic conductivity measurements of unamended bauxite residues range between 0.2and 0.3mm h ?1with an average void ratio (e)of 1.6–1.7(Nikraz et al.,2007;Woodard et al.,2008).Neutralization measures have mostly had limited success in improv-ing hydraulic conductivity,however bitterns treated residues seem to have higher hydraulic conductivities (0.4mm h ?1)than untreated residues and higher void ratios (e =2.3–2.9)(Nikraz et al.,2007).
In addition to lowering the pH,seawater and gypsum lower the SAR due to an increase in exchangeable Ca 2+and Mg 2+in solution and a decrease of exchangeable Na +(Courtney and Timpson,2005;Ippolito et al.,2005).The latter re ?ects on the one hand the greater preference for Mg 2+,Ca 2+and K +on the exchange complex than for Na +(Helferich's series (Helfferich,1956;Helfferich,1962a;Helfferich,1962b;Helfferich,1965)),on the other hand it is also possibly a coating effect of neo-precipitates blocking DSPs and other exchangeable Na +sources (Hanahan et al.,2004;Menzies et al.,2004).The net effect of such reactions should be a marked improvement in residue structure and the creation of (more)viable pore networks,which improve the hydraulic conductivity of the residues (Wong and Ho,1991;Wong and Ho,1993).
6.Conclusions
The chemistry of bauxite residues is dominated by the presence of multiple alkaline solids,which impart signi ?cant acid neutralizing capacity.The pH in untreated residue liquor ranges over 9.2–12.8with an average value of 11.3±1.0.Thus residues are highly alkaline,hazardous,and will not support plant life.The alkaline pH of residues is strongly buffered by the presence of alkaline solids (hydroxides,carbonates,aluminates and aluminosilicates)that are formed by the action of caustic soda on bauxite in the Bayer process re ?nery,leading (among others)to the formation of Bayer process characteristic solids (BPCSs).The buffering action of multiple BPCSs causes the acid neutralization behavior of residue to be highly complex.It is impractical to remove the alkalinity from residue by washing with water or mineral acids.Ultimately,however,the alkalinity of residue needs to be abated because it has profound implications for all aspects of residue,including:storage requirements,raw material usages and recoveries,neutralization,sedimentation rates,ρ,compaction,hydraulic conductivity,drying rates,dusting behavior,and physical strength after drying.
Table 12
Summary of the key knowledge gaps in relation to residue alkalinity and associated chemistry and areas of suggested research.Knowledge gap
Research project
Neutralization chemistry and the behavior of solid alkalinity is poorly understood.
Develop thermodynamic and kinetic models for the neutralization behavior of bauxite residue and their component minerals and their relationship to surface charge.
Dissolution behavior of Bayer process speci ?c solids is poorly understood.
Establish a comprehensive data set relating to the dissolution behavior of (in particular)Bayer process speci ?c solids.
Limited knowledge on the speciation and behavior of trace metals and radionuclides in residue.
Detailed investigations into the nature,concentrations,speciation and leaching behavior of trace metals and radionuclides in bauxite residues under a range of neutralization,storage,rehabilitation and reuse scenarios.
Limited knowledge of dissolved salts and liquor transport within residue pro ?les.
Develop a combined reaction transport and hydrological model of solution ?ow and reaction processes in bauxite residue pro ?les applicable to impoundments.
Limited knowledge on all aspects of residue bio-remediation.
Develop the science and practice of microbiologically assisted bio-remediation of bauxite residues.
Develop methods for the optimization of residue amendments for the development soil structure conducive to plant growth.
Develop selection criteria for vegetative covers in bauxite residue disposal areas.Develop a set of best agronomic practices for managing vegetative covers overlying bauxite residue disposal areas.
BRDA liner design could be methodically improved.
Review the materials used for lining residue storage areas and research their reactivities under accelerated test conditions.
76M.Gr?fe et al./Hydrometallurgy 108(2011)60–79
Future progress on improved storage strategies,remediation, rehabilitation and utilization will be dependent upon the development of a better understanding of the complex buffering and neutralization chemistry of residue.In particular,knowing how surface charge develops,distributes and abates in the residue mineral assemblage as a function of acid input is critical to understanding neutralization reactions overall,to successfully model them and ultimately to implement the most effective neutralization measures.The principal bene?ciaries of effective neutralization measures would be microbes and plants.The establishment of techniques for creating self-managing, sustainable ecosystems from bauxite residue impoundments is the most realistic solution to the large and increasing inventory(currently2.7Bt increasing to4Bt by2015)of bauxite residue globally.A summary of the key knowledge gaps in residue chemistry and the suggested areas for future research are outlined in Table12.
Acknowledgements
This project received funding from the Australian Government as part of the Asia-Paci?c Partnership on Clean Development and Climate.The views expressed herein are not necessarily the views of the Commonwealth,and the Commonwealth does not accept responsibility for any information or advice contained herein.The support of the CSIRO Light Metals National Research Flagship and the Parker CRC for Integrated Hydrometallurgy Solutions(established and supported under the Australian Government's Cooperative Research Centres Program)is gratefully acknowledged.
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Glossary
A:alumina in solution expressed as expressed as equivalent g/L of Al2O3. Amorphous content:wt.%content that is amorphous(as opposed to crystalline)as measured during a Rietveld re?nement of a sample with a known quantity of an internal standard with known crystallinity.ANC:acid neutralization capacity normalized to the weight of the residue to a given pH endpoint(e.g.to reach a pH value of5.5or7)using a strong mineral acid.
Bulk density:(ρ),generally this is the overall dry packed solids density as would be relevant in(for example)transport of the solids or de?nition of soil properties.
This?gure is dependent on factors such as extent of pre-washing,drying, constituent particle size distribution and the packing pressure.Due to the entrained porosity bulk densities are always lower the densities of the constituent particles.See speci?c gravity.In the soil science context it is the dry mass of a sample divided by its moist state volume(also known as dry density).
BRDA:bauxite residue disposal area,usually an engineered dam repository for bauxite residue or red mud.
CAC:calcium aluminum carbonate
CEC:cation exchange capacity(cmol+kg?1)
EC:electrical conductivity(mS cm?1)
ESP:exchangeable sodium percentage is the availability of sodium in residue expressed as a percentage of the overall exchangeable cations[ESP/(100?ESP)]=0.015?SAR IS:ionic strength
LOI:loss on ignition,mass loss measured during XRF analysis accounting for water, bicarbonates,carbonates and other temperature sensitive components.
PZC:point of zero charge for a given slurry and background electrolyte is the pH value at which particulates have no net surface charge.
SAR:adsorption ratio of sodium to that of calcium and magnesium combined SAR= [Na+]/{([Ca2+]+[Mg2+])/2}1/2note that[Na+,Ca2+,Mg2+]must be mmol+L?1 SNL:supernatant liquor
SSA:speci?c surface area(m2g?1)
Surface free energy:increase of free energy when the residue surface area is increased Swell index:free swelling index of dry residue
TCA:tricalcium aluminate Ca3Al2(OH)12
TDS:total dissolved salts
Total alkali or TA:for residue associated spent liquor caustic concentration plus the concentration of carbonate expressed as equivalent g/L of Na2CO3.Also referred to as S or TA(for total alkali).
Total caustic or TC:for residue associated spent liquor,combination of the aluminate and excess hydroxide expressed as equivalent g/L of Na2CO3.Also referred to as C or TC(for total caustic).
Void ratio:volume of voids in a material mixture divided by the volume of solids
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小学数学典型应用题归纳汇总30种题型
小学数学典型应用题归纳汇总30种题型 1 归一问题 【含义】在解题时，先求出一份是多少（即单一量），然后以单一量为标准，求出所要求的数量。这类应用题叫做归一问题。 【数量关系】总量÷份数＝1份数量 1份数量×所占份数＝所求几份的数量 另一总量÷（总量÷份数）＝所求份数 【解题思路和方法】先求出单一量，以单一量为标准，求出所要求的数量。 例1 买5支铅笔要0.6元钱，买同样的铅笔16支，需要多少钱？ 解（1）买1支铅笔多少钱？0.6÷5＝0.12（元） （2）买16支铅笔需要多少钱？0.12×16＝1.92（元） 列成综合算式0.6÷5×16＝0.12×16＝1.92（元） 答：需要1.92元。 2 归总问题 【含义】解题时，常常先找出“总数量”，然后再根据其它条件算出所求的问题，叫归总问题。所谓“总数量”是指货物的总价、几小时（几天）的总工作量、几公亩地上的总产量、几小时行的总路程等。 【数量关系】1份数量×份数＝总量 总量÷1份数量＝份数 总量÷另一份数＝另一每份数量 【解题思路和方法】先求出总数量，再根据题意得出所求的数量。 例1 服装厂原来做一套衣服用布3.2米，改进裁剪方法后，每套衣服用布2.8米。原来做791套衣服的布，现在可以做多少套？ 解（1）这批布总共有多少米？ 3.2×791＝2531.2（米） （2）现在可以做多少套？2531.2÷2.8＝904（套） 列成综合算式 3.2×791÷2.8＝904（套） 答：现在可以做904套。。 3 和差问题 【含义】已知两个数量的和与差，求这两个数量各是多少，这类应用题叫和差问题。 【数量关系】大数＝（和＋差）÷2 小数＝（和－差）÷2 【解题思路和方法】简单的题目可以直接套用公式；复杂的题目变通后再用公式。
TOEFL Speaking 模板范例
Question 1 1. Person=Personalities 1. Parents My mother is the person whom I admire most due to her so many good personali ties like talented, tolerant and most of all, her spirit of dedication. First of all, she is such a talented person who not only has great achievements in her work, but also can cook delicious food for my dad and me. Furthermore, since she is well-e ducated, she can tolerate different opinions from varied people. And the most bea utiful thing about her is that she has a beautiful mind. Thrifty as she is in the dai ly life, she saves the money and donates all of them to the people who may need them. The ways she does and thinks make me want to be the person like she is. That is why I think my mother is the person I admire most. 2. Teacher (万能人物；可替换的角色有：parent, leader, frie nd, ideal spouse, celebrity) A good teacher should have the following personalities. First of all, a good teacher makes herself available to all students and she knows which students need extra assistance. Furthermore, a good teacher is an effective communicator, who knows when she needs to change her communicating techniques to be sure students can grasp instructional concepts. What is more, she would show her great ability when her students are making mistakes, she would let them know why they are wrong and how they are going to do to correct them, rather than simply punish them. For most students, a good teacher is also a helper who can lift them to new heig hts. Just like an old saying goes, " GIVE ME A FISH AND I EAT FOR A DAY, T EACH ME TO FISH AND I EAT FOR A LIFE TIME". This must be a philosophy of every good teacher. 3. Friends A good friend should have the following personalities like trustworthy, helpful and positive. First of all, a trustworthy person is someone whom I can rely on especi ally when I am in difficulty; he/she will be just a phone-call away to get me out of trouble. Secondly, he/she must be someone who can give me some suggestions when I lose my heart. I clearly remember last time I had a bad experience on my job, I was so sad during those period of time and my friend Nana just sat besid es me and was such a good listener to support me and inspired me by saying tha t I deserved a second chance and never pushing myself too hard would be a bette
汇编语言调试DEBUG命令详解
汇编语言调试DEBUG命令详解 1、显示命令D ① D [地址] ② D [范围] 如不指定范围，一次显示8行×16个字节。 －D ；默认段寄存器为DS，当前偏移地址 －D DS:100 / －D CS:200 －D 200:100 －D 200；200为偏移地址，默认段寄存器DS －D DS:100 110/ －D 100 L 10 2.修改命令E ① E 地址；从指定地址开始，修改（或连续修改）存储单元内容。DEBUG首先显示指定单元内容，如要修改，可输入新数据；空格键显示下一个单元内容并可修改，减号键显示上一个单元内容并可修改；如不修改，可直接按空格键或减号键；回车键结束命令。 ② E 地址数据表；从指定的地址开始用数据表给定的数据修改存储单元。 －E DS:100 F3 ‘AB’ 8D。 3.添充命令F F 范围数据表； 将数据表写入指定范围的存储单元；数据个数多，忽略多出的数据，个数少，则重复使用数据表。 －F DS:0 L5 01,02,03,04,05 －F DS:0 L5 01 02 03 04 05（空格分隔） －F DS:0 L5 FF ；5个字节重复使用FF 4.显示修改寄存器命令R R；★显示所有寄存器和标志位状态； ★显示当前CS：IP指向的指令。 显示标志时使用的符号： 标志标志=1 标志=0 OF OV NV DF DN UP IF EI DI SF NG PL ZF ZR NZ AF AC NA PF PE PO CF CY NC
5.汇编命令A A [地址]；从指定的地址开始输入符号指令；如省略地址，则接着上一个A命令的最后一个单元开始；若第一次使用A命令省略地址，则从当前CS:IP 开始（通常是CS：100）。 注释:①在DEBUG下编写简单程序即使用A命令。 ②每条指令后要按回车。 ③不输入指令按回车，或按Ctrl+C结束汇编。 ④支持所有8086符号硬指令，伪指令只支持DB、DW，不支持各类符号名。 6.反汇编命令U ① U [地址]；从指定地址开始反汇编32个字节的机器指令；省略地址时,则接着上一个U命令的最后一个单元开始；若第一次使用U命令省略地址，则从当前CS:IP开始（通常是CS：100）。 ② U 范围；对指定范围的单元进行反汇编。 －U －U100 －U100L10 7.运行程序命令G ① G；从CS:IP指向的指令开始执行程序，直到程序结束或遇到INT 3。 ② G=地址；从指定地址开始执行程序，直到程序结束或遇到INT 3。 ③ G 断点1[，断点2，…断点10]；从CS:IP指向的指令开始执行程序，直到遇到断点。 ④G=地址断点1[，断点2，…断点10] －G ；从CS:IP指向的指令开始执行程序。 －G=100 ；从指定地址开始执行程序。 －G=100 105 110 120 8.跟踪命令（单步执行命令）T ① T；从当前IP开始执行一条指令。 ② T 数值；从当前IP开始执行多条指令。 ② T =地址； ③ T =地址数值； －T －T5 / －T=100 5 9.跟踪执行并跳过子程序命令P P [=地址] [数值]；类似T命令，但跳过子程序和中断服务程序。 10.退出DEBUG命令Q Q；返回DOS环境。 －Q 11.命名命令N N 文件标示符；指定文件，以便用W命令在磁盘上生成该文件，或者用L命令从磁盘装入该文件。 －N MY_https://www.wendangku.net/doc/c82514608.html,
六年级奥数题型分类
六年级奥数： 第一类：比和比例问题 一块合金内铜和锌的比是2∶3，现在再加入6克锌，共得新合金36克，求新合金内铜和锌的比。(试题选自华罗庚学校数学课本) 第二类：上坡问题 一条路全长60千米，分成上坡、平路、下坡三段，各段路程长的比依次是1：2：3，某人走各段路程所用时间之比依次是4∶5∶6，已知他上坡的速度是每小时3千米，问此人走完全程用了多少时间。(试题选自华罗庚学校数学课本) 第三类：长方形和正方形 如下图，一个边长为3a厘米的正方体，分别在它的前后、左右、上下各面的中心位置挖去一个截口是边长为a厘米的正方形的长方体（都和对面打通）．如果这个镂空的物体的表面积为2592平方厘米，试求正方形截口的边长。(试题选自华罗庚学校数学课本) 第四类：工程问题 蓄水池有一条进水管和一条排水管．要灌满一池水，单开进水管需5小时．排光一池水，单开排水管需3小时．现在池内有半池水，如果按进水，排水，进水，排水…的顺序轮流各开1小时．问：多长时间后水池的水刚好排完（精确到分钟）(试题选自华罗庚学校数学课本) 第五类：几何问题
如图所示，四边形ABCD为直角梯形，三角形APB的面积为2，且2AD=BC,EP:PB=1:2，求直角梯形ABCD的面积。 第六类：飞镖比赛 在新年联欢会上，某班组织了一场飞镖比赛.如右图，飞镖的靶子分为三块区域，分别对应17分、11分和4分.每人可以扔若干次飞镖，脱靶不得分，投中靶子就可以得到相应的分数.若恰好投在两块(或三块)区域的交界线上，则得两块(或三块)区域中分数最高区域的分数.如果比赛规定恰好投中120分才能获奖，要想获奖至少需要投中-------次飞镖. 第七类：发帽子 小明和8个好朋友去李老师家玩.李老师给每人发了一顶帽子，并在每个人的帽子上写了一个两位数，这9个两位数互不相同，且每个小朋友只能看见别人帽子上的数.老师在纸上又写了一个数A，问这9位同学：“你知不知道自己帽子上的数能否被A整除知道的请举手.”结果有4人举手.老师又问：“现在你知不知道自己帽子上的数能否被24整除知道的请举手.”结果有6人举手.已知小明两次都举手了，并且这9个小朋友都足够聪明且从不说谎，那么小明看到的别人帽子上的8个两位数的总和是----------. 第八类：计算综合 一个长方形能把平面分成2部分，那么三个长方形最多把平面分成多少部分
英语万能模板
英语万能作文(模板型） Along with the advance of the society more and more problems are brought to our attention, one of which is that.... 随着社会的不断发展，出现了越来越多的问题，其中之一便是 ____________。 As to whether it is a blessing or a curse, however, people take different attitudes. 然而，对于此类问题，人们持不同的看法。(Hold different attitudes 持不同的看;Come up with different attitudes 有不同的看法) As society develops, people are attaching much importance to.... 随着社会的发展，人们开始关注............ People are attaching more and more importance to the interview during job hunting 求职的过程中，人们慢慢意识到面试的重要性。 As to whether it is worthwhile ....., there is a long-running controversial debate. It is quite natural that people from different backgrounds may have divergent attitudes towards it. 关于是否值得___________的问题，一直以来争论不休。当然，不同的人对此可能持不同的观点。 In the process of modern urban development, we often find ourselves in a dilemma. 在都市的发展中，我们往往会陷入困境。 Recently the phenomenon has aroused wide concern, some people are in alarm that.... 最近，这种现象引起了人们的广泛关注，有人开始担心______________。 The human race has entered a completely new stage in its history, with the increasingly rapid economic globalization and urbanization, more problems are brought to our attention. 人类进入了一个历史的崭新的阶段，经济全球化、都市化的速度不断加快，随之给我们带来了很多问题。 ...... plays such an important role that it undeniably becomes the biggest concern of the present world, there comes a question, is it a blessing or a curse?" _______显得非常重要而成为当今世界所关注的最大的问题，这是无可厚非的。不过，问题是："我们该如何抉择?" Now we are entering a new era, full of opportunities and challenges, 现在我们正在进入一个充满机会和挑战的新时代。
常用时序分析SDC 命令参考
常用时序分析SDC 命令参考（一） 1. Define design environment 1.1. Set_operating_conditions 1.2. Set_wire_load_model 1.3. Set_driving_cell 1.4. Set_load 1.5. Set_fanout_load 1.6. Set_min_library 2. Set design constraints 2.1. Design rule constraints 2.1.1. Set_max_transition 2.1.2. Set_max_fanout 2.1. 3. Set_max_capacitance 2.2. Design optimization constraints 2.2.1. create_clock 2.2.2. create_generated_clock 2.2. 3. Set_clock_latency 2.2.4. Set_propagated_clock 2.2.5. Set_clock_uncertainty 2.2.6. Set_input_delay 2.2.7. Set_output_delay 2.2.8. Set_max_area 3. Other commands 3.1. set_clock_groups 3.2. set_false_path 3.3. set_case_analysis 3.4. set_max_delay 1. Do not exist in timing fix SDC file: 1.1. Set_max_area 1.2. set_operation_conditions 1.3. set_wire_load_model 1.4. set_ideal_* 2. Must be placed in timing fix SDC file: 2.1. Set_clock_uncertainty, 2.2. set_max_transition 2.3. set_propagated_clock
微机原理与接口技术汇编语言指令详解吐血版
第一讲 第三章 指令系统--寻址方式 回顾: 8086/8088的内部结构和寄存器，地址分段的概念，8086/8088的工作过 程。 重点和纲要：指令系统--寻址方式。有关寻址的概念；6种基本的寻址方式及 有效地址的计算。 教学方法、实施步骤 时间分配 教学手段 回 顾 5”×2 板书 计算机 投影仪 多媒体课件等 讲 授 40” ×2 提 问 3” ×2 小 结 2” ×2 讲授内容： 3.1 8086/8088寻址方式 首先，简单讲述一下指令的一般格式： 操作码 操作数 …… 操作数 计算机中的指令由操作码字段和操作数字段组成。 操作码：指计算机所要执行的操作，或称为指出操作类型，是一种助记符。 操作数：指在指令执行操作的过程中所需要的操作数。该字段除可以是操作数本身外，也可以是操作数地址或是地址的一部分，还可以是指向操作数地址的指针或其它有关操作数的信息。 寻址方式就是指令中用于说明操作数所在地址的方法，或者说是寻找操作数有效地址的方法。8086／8088的基本寻址方式有六种。 1．立即寻址 所提供的操作数直接包含在指令中。它紧跟在操作码的后面，与操作码一起放在代码段区域中。如图所示。 例如：MOV AX ，3000H
立即数可以是8位的，也可以是16位的。若是16位的，则存储时低位在前，高位在后。 立即寻址主要用来给寄存器或存储器赋初值。 2．直接寻址 操作数地址的16位偏移量直接包含在指令中。它与操作码—起存放在代码段区域，操作数一般在数据段区域中，它的地址为数据段寄存器DS加上这16位地址偏移量。如图2-2所示。 例如： MOV AX，DS：[2000H]； 图2－2 （对DS来讲可以省略成 MOV AX，[2000H]，系统默认为数据段）这种寻址方法是以数据段的地址为基础，可在多达64KB的范围内寻找操作数。 8086/8088中允许段超越，即还允许操作数在以代码段、堆栈段或附加段为基准的区域中。此时只要在指令中指明是段超越的，则16位地址偏移量可以与CS或SS或ES相加，作为操作数的地址。 MOV AX，[2000H] ；数据段 MOV BX，ES：[3000H] ；段超越，操作数在附加段 即绝对地址＝（ES）＊16＋3000H 3．寄存器寻址 操作数包含在CPU的内部寄存器中，如寄存器AX、BX、CX、DX等。 例如：MOV DS，AX MOV AL，BH 4．寄存器间接寻址 操作数是在存储器中，但是，操作数地址的16位偏移量包含在以下四个寄
系统指令分析
指令系统分析 一、指令系统概述： 指令系统是指某一种计算机所有指令的集合。对计算机而言，这是一组二进制数的输入，实际上是一组电平的输入。这些输入能在一个指令周期内产生人们预先规定的动作。显然这不是一组随机的二进制数据输入。指令系统是联系硬件和软件的桥梁。指令系统对计算机的作用相当于逻辑函数表对逻辑器件（门、触发器、加法器、移位寄存器）的作用。当然指令系统要比逻辑函数表复杂得多。一种计算机指令系统设计的好坏，往往标志着该计算机功能的强弱，计算机的设计往往先从设计它的指令系统开始。 二、指令格式： 指令由操作码和操作数两部分组成，操作数可以是要被操作的数据本身（立即数），也可以是数据所在单元的地址或寄存器。指令格式包括指令长度和指令内部信息的安排等。一条长指令通常可以分成几段存放和处理，每一段的长度与微处理机的字长相等。8位微处理机是以8位二进制数（字节）为基础，MCS-51单片机采用变长指令，有单字节、双字节和三字节三种指令格式。 一字节指令（49条） 这类指令的二进制代码既包含了操作码信息，又包含了操作数信息。有两种情况： 二字节指令（46条） 这类指令的第一字节为操作码，第二字节为操作数或操作数的地址。 三字节指令（16条） 这类指令的第一字节为操作码，后跟两个字节的操作数。 三、寻址方式： 每条指令存数或取数都需要知道如何获得该数据，如何产生操作数或其地址就称作寻址。指令的寻址方式是指如何获得操作数的方法，一个指令系统的寻址方式越多，计算机的功能越强，灵活性越大。所以寻址方式的多样性和灵活性是衡量计算机性能的一项重要指标。 MCS-51 单片机有七种寻址方式。 1．寄存器寻址（Register Addressing） 在指令中指出存放操作数的寄存器，因为寄存器在片内RAM中，所以其特点是速度快。如MOV A，R0，INC DPTR，MUL AB，DEC R7等。 寄存器包括32个工作寄存器（分4组）以及部分专用寄存器。指令只能使用当前寄存器组的8个通用寄存器R0～R7，对不同寄存器组可以通过PSW 中的RS0，RS1来选择。 2．直接寻址（Direct Addressing） 指令中给出操作数的8位直接地址，可用这种寻址方式的存储空间只限于内
《汇编语言》习题及解答
第1章基础知识 检测点(第9页) （1）1个CPU的寻址能力为8KB，那么它的地址总线的宽度为13位。 （2）1KB的存储器有1024个存储单元，存储单元的编号从0到1023。 （3）1KB的存储器可以存储8192（2^13）个bit，1024个Byte。 ~ （4）1GB是24（2^30）个Byte、1MB是1048576（2^20）个Byte、1KB是1024（2^10）个Byte。 （5）8080、8088、80296、80386的地址总线宽度分别为16根、20根、24根、32根，则它们的寻址能力分别为: 64（KB）、1（MB）、16（MB）、4（GB）。 （6）8080、8088、8086、80286、80386的数据总线宽度分别为8根、8根、16根、16根、32根。则它们一次可以传送的数据为: 1（B）、1（B）、2（B）、2（B）、4（B）。 （7）从内存中读取1024字节的数据，8086至少要读512次，80386至少要读256次。 （8）在存储器中，数据和程序以二进制形式存放。 解题过程： ' （1）1KB=1024B，8KB=1024B*8=2^N，N=13。 （2）存储器的容量是以字节为最小单位来计算的，1KB=1024B。 （3）8Bit=1Byte，1024Byte=1KB（1KB=1024B=1024B*8Bit）。 （4）1GB=24B（即2^30）1MB=1048576B（即2^20）1KB=1024B（即2^10）。 （5）一个CPU有N根地址线，则可以说这个CPU的地址总线的宽度为N。这样的CPU最多可以寻找2的N次方个内存单元。（一个内存单元=1Byte）。 （6）8根数据总线一次可以传送8位二进制数据（即一个字节）。 （7）8086的数据总线宽度为16根（即一次传送的数据为2B）1024B/2B=512，同理1024B/4B=256。 （8）在存储器中指令和数据没有任何区别，都是二进制信息。
奥数知识点分类汇总(包含公式)
奥数知识点分类汇总（包含公式） 1．和差倍问题 和差问题和倍问题差倍问题 已知条件几个数的和与差几个数的和与倍数几个数的差与倍数 公式适用范围已知两个数的和，差，倍数关系 公式①(和－差)÷2=较小数 较小数＋差=较大数小学奥数很简单，就这30个知识点 和－较小数=较大数 ②(和＋差)÷2=较大数 较大数－差=较小数 和－较大数=较小数 和÷(倍数＋1)=小数 小数×倍数=大数 和－小数=大数 差÷(倍数-1)=小数 小数×倍数=大数 小数＋差=大数 关键问题求出同一条件下的 和与差和与倍数差与倍数 2．年龄问题的三个基本特征： ①两个人的年龄差是不变的； ②两个人的年龄是同时增加或者同时减少的； ③两个人的年龄的倍数是发生变化的； 3．归一问题的基本特点：问题中有一个不变的量，一般是那个“单一量”，题目一般用“照这样的速度”……等词语来表示。 关键问题：根据题目中的条件确定并求出单一量； 4．植树问题 基本类型在直线或者不封闭的曲线上植树，两端都植树在直线或者不封闭的曲线上植树，两端都不植树在直线或者不封闭的曲线上植树，只有一端植树封闭曲线上植树 基本公式棵数=段数＋1 棵距×段数=总长棵数=段数－1 棵距×段数=总长棵数=段数 棵距×段数=总长 关键问题确定所属类型，从而确定棵数与段数的关系 5．鸡兔同笼问题 基本概念：鸡兔同笼问题又称为置换问题、假设问题，就是把假设错的那部分置换出来；基本思路： ①假设，即假设某种现象存在（甲和乙一样或者乙和甲一样）：
②假设后，发生了和题目条件不同的差，找出这个差是多少； ③每个事物造成的差是固定的，从而找出出现这个差的原因； ④再根据这两个差作适当的调整，消去出现的差。 基本公式： ①把所有鸡假设成兔子：鸡数＝（兔脚数×总头数－总脚数）÷（兔脚数－鸡脚数） ②把所有兔子假设成鸡：兔数＝（总脚数一鸡脚数×总头数）÷（兔脚数一鸡脚数） 关键问题：找出总量的差与单位量的差。 6．盈亏问题 基本概念：一定量的对象，按照某种标准分组，产生一种结果：按照另一种标准分组，又产生一种结果，由于分组的标准不同，造成结果的差异，由它们的关系求对象分组的组数或对象的总量． 基本思路：先将两种分配方案进行比较，分析由于标准的差异造成结果的变化，根据这个关系求出参加分配的总份数，然后根据题意求出对象的总量． 基本题型： ①一次有余数，另一次不足； 基本公式：总份数＝（余数＋不足数）÷两次每份数的差 ②当两次都有余数； 基本公式：总份数＝（较大余数一较小余数）÷两次每份数的差 ③当两次都不足； 基本公式：总份数＝（较大不足数一较小不足数）÷两次每份数的差 基本特点：对象总量和总的组数是不变的。 关键问题：确定对象总量和总的组数。 7．牛吃草问题 基本思路：假设每头牛吃草的速度为“1”份，根据两次不同的吃法，求出其中的总草量的差；再找出造成这种差异的原因，即可确定草的生长速度和总草量。 基本特点：原草量和新草生长速度是不变的； 关键问题：确定两个不变的量。 基本公式： 生长量=（较长时间×长时间牛头数-较短时间×短时间牛头数）÷（长时间-短时间）； 总草量=较长时间×长时间牛头数-较长时间×生长量； 8．周期循环与数表规律 周期现象：事物在运动变化的过程中，某些特征有规律循环出现。 周期：我们把连续两次出现所经过的时间叫周期。 关键问题：确定循环周期。 闰年：一年有366天； ①年份能被4整除；②如果年份能被100整除，则年份必须能被400整除； 平年：一年有365天。 ①年份不能被4整除；②如果年份能被100整除，但不能被400整除； 9．平均数 基本公式：①平均数=总数量÷总份数 总数量=平均数×总份数
汇编语言指令集合 吐血整理
8086/8088指令系统记忆表 数据寄存器分为: AH&AL＝AX(accumulator)：累加寄存器，常用于运算;在乘除等指令中指定用来存放操作数,另外,所有的I/O指令都使用这一寄存器与外界设备传送数据. BH&BL＝BX(base)：基址寄存器，常用于地址索引； CH&CL＝CX(count)：计数寄存器，常用于计数；常用于保存计算值,如在移位指令,循环(loop)和串处理指令中用作隐含的计数器. DH&DL＝DX(data)：数据寄存器，常用于数据传递。他们的特点是,这4个16位的寄存器可以分为高8位:AH,BH,CH,DH.以及低八位：AL,BL,CL,DL。这2组8位寄存器可以分别寻址，并单独使用。 另一组是指针寄存器和变址寄存器，包括： SP（Stack Pointer）：堆栈指针，与SS配合使用，可指向目前的堆栈位置； BP（Base Pointer）：基址指针寄存器，可用作SS的一个相对基址位置； SI（Source Index）：源变址寄存器可用来存放相对于DS段之源变址指针； DI（Destination Index）：目的变址寄存器，可用来存放相对于ES段之目的变址指针。 指令指针IP(Instruction Pointer) 标志寄存器FR(Flag Register) OF(overflow flag) DF(direction flag) CF(carrier flag) PF(parity flag) AF(auxiliary flag) ZF(zero flag) SF(sign flag) IF(interrupt flag) TF(trap flag) 段寄存器(Segment Register) 为了运用所有的内存空间，8086设定了四个段寄存器，专门用来保存段地址： CS（Code Segment）：代码段寄存器； DS（Data Segment）：数据段寄存器； SS（Stack Segment）：堆栈段寄存器；
小学五年级下册奥数题型分类讲义 (附答案)
小学五年级奥数分类讲义含答案 图形问题 专题1 长方形、正方形的周长 一、专题解析 同学们都知道，长方形的周长=（长＋宽）×2，正方形的周长=边长×4。长方形、正方形的周长公式只能用来计算标准的长方形和正方形的周长。 那么如何应用所学知识巧求表面上看起来不是长方形或正方形的图形的周长呢？还需同学们灵活应用已学知识，掌握转化的思考方法，把复杂的图形转化为标准的图形，以便计算它们的周长。 二、精讲精练 【例题1】 有5张同样大小的纸如下图（a）重叠着，每张纸都是边长6厘米的正方形，重叠的部分为边长的一半，求重叠后图形的周长。 【思路导航】根据题意，我们可以把每个正方形的边长的一半同时向左、右、上、下平移（如图b），转化成一个大正方形，这个大正方形的周长和原来5个小正方形重叠后的图形的周长相等。因此，所求周长是18×4=72厘米。 练习1 1、右图由8个边长都是2厘米的正方形组成，求这个图形的周长。 2、右图由1个正方形和2个长方形组成，下方长方形长为50cm，求这 个图形的周长。 3、有6块边长是1厘米的正方形，如例题中所说的这样重叠着，求重叠后图 形的周长。
【例题2】 一块长方形木板，沿着它的长度不同的两条边各截去4厘米，截掉的面积为192平方厘米。现在这块木板的周长是多少厘米？ 【思路导航】把截掉的192平方厘米分成A、B、C三块（如图），其中AB的面积是192－4×4=176（平方厘米）。把A和B移到一起拼成一个宽4厘米的长方形，而此长方形的长就是这块木板剩下部分的周长的一半。176÷4=44（厘米），现在这块木板的周长是44×2=88（厘米）。 练习2 1、有一个长方形，如果长减少4米，宽减少2米，面积就比原来减少44平方米，且剩下部分正好是一个正方形。求这个正方形的周长。 2、有两个相同的长方形，长是8厘米，宽是3厘米，如果按下图叠放在一起，这 个图形的周长是多少？ 3、有一块长方形广场，沿着它不同的两条边各划出2米做绿化带，剩下的部分仍是长方形，且周长为280米。求划去的绿化带的面积是多少平方米？ 【例题3】 已知下图中，甲是正方形，乙是长方形，整个图形的周长是多少？ 【思路导航】从图中可以看出，整个图形的周长由六条线段围成，其中三条横着，三条竖着。三条横着的线段和是（a＋b）×2，三条竖着的线段和是b×2。所以，整个图形的周长是（a＋b）×2＋b×2，即2a＋4b。 练习3
汇编语言期末考试试题及复习资料
汇编语言模拟试题及答案 一，单项选择题（在每小题的四个备选答案中，选出一个正确的答案，并将其号码填在题干后的括号内，每小题1分，共20分） 1．指令JMP FAR PTR DONE属于参考答案为:C A．段内转移直接寻址 B．段内转移间接寻址 C．段间转移直接寻址 D．段间转移间接寻址 [解析]略 2．下列叙述正确的是参考答案为:C A．对两个无符号数进行比较采用CMP指令，对两个有符号数比较用CMPS指令 B．对两个无符号数进行比较采用CMPS指令，对两个有符号数比较用CMP指令 C．对无符号数条件转移采用JAE/JNB指令，对有符号数条件转移用JGE/JNL 指令 D．对无符号数条件转移采用JGE/JNL指令，对有符号数条件转移用JAE/JNB 指令 [解析]对于无符号数和有符号数的比较都采用CMP指令； CMPS指令是串比较指令； 对两个无符号数的条件转移指令应是：JAE、JNB、JBE、JNA； 对两个有符号数的条件转移指令应是：JGE、JNL、JLE、JNG。 3．一个有128个字的数据区，它的起始地址为12ABH：00ABH，请给出这个数据区最末一个字单元的物理地址是参考答案为:C A．12CSBH B．12B6BH C．12C59H D．12BFEH [解析]末字与首字相隔（128－1＝）127个字，且每个字占用2个字节，因此末字单元的物理地址应为： 首字单元的物理地址＋（128－1）×2 即 12ABH×10H＋00ABH＋（128－1）×2＝12C59H。 4．在下列指令的表示中，不正确的是参考答案为:C A．MOV AL，[BX+SI] B．JMP SHORT DONI C．DEC [BX] D．MUL CL [解析]当只有一个存储器操作数时，这个操作数的类型不明确，例如选项C中的[BX]，没有明确的说明访问该存储单元的类型，此时存储器操作数就必须需用类型说明，如 DEC BYTE PTR [BX] 或 DEC WORD PTR [BX] 但是在题目的选项C中，没有指出存储器操作数类型，所以该指令是不正确的；而其它选项中的指令均是正确的。
新GRE写作Issue 模板 By Jason Lee
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小学数学知识点汇总以及题型归纳整理
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英语口语模板(完整版)
英语口语万能模板 一、图表作文 As is shown by the figure/percentage in the table/picture，____ has been on rise/ decrease，significantly/dramatically rising/decreasing from ____ in _____ to _____ in _____. From the sharp rise/decline in the chart，it goes without saying that _____. There are at least two good reasons accounting for _____. In one hand，____.In the other hand，_____ is due to the fact that ______.In addition，______ is responsible for _____. Maybe there are some other reasons to show ______.But itis generally believed that the above mentioned reasons are commonly convincing. As far as I am concerned，I hold the point of view that _______. I am sure my opinion is both sound and well-grounded. 二、书信作文模板 Your address Month，Date，year Receiver's address Dear …， I am extremely pleased to hear from you. And I would like to write a letter to tell you that_____. …… I will greatly appreciate a response from you at your earliest convenience/I am looking f0rward to your replies at your earliest convenience. Best regards for your health and success.
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