制盐工艺流程

盐田饱和卤水直接进蒸发罐真空制盐工艺介绍

靳志玲

(中盐制盐工程技术研究院,天津300450)

内容摘要:利用滩田饱和卤水直接进蒸发罐蒸发结晶制取氯化钠,在制盐过程中采取三次洗涤方法,除去钙镁离子,以获得纯度超过99.1%的氯化钠和浓度较高的制盐母液。

关键词:滩田饱和卤水真空精制盐

1 简介

目前,国内大部分井矿盐企业采用以井矿盐卤为原料,北方部分生产企业采用原盐化成饱和盐水再精制真空盐的生产方法。随着国内“两碱”的迅速发展和人民生活水平的提高,原盐供需矛盾已越来越突出。特别是在海盐区,以原盐溶解成饱和卤水再生产精制盐,不仅增加了原盐的消耗,而且降低了盐田面积的使用效率,增加投资,生产成本高,不同程度上影响了食用盐生产企业的经济效益,造成许多食用盐定点生产企业减产、亏损,直接影响了百姓的生活。而采用滩田饱和卤水直接进蒸发罐制盐,即可以节约结晶面积,其母液还可以为生产氯化钾、氯化镁等盐化工产品提供高质量原料苦卤,达到充分利用卤水中的各种有效成分,实现零排放、无污染、循环经济的目的。

2. 工艺流程简述

卤水经滩田日晒蒸发,至饱和后引入一个卤水库中储存。卤水库的作用,一是储存一定量的饱和卤水,以备真空制盐生产的需要;二是卤水在此有一定的停留时间,可以使其中的一些杂质颗粒沉淀下来,达到净化卤水的目的。净化后的饱和卤水从卤水库引出后,进入蒸发制盐车间的精卤桶。采用预热后的卤水进罐,顺流转料、末效排盐浆,集中排母液的方式进行生产。I效蒸发罐盐箱中的盐排到II效下循环管中,II效盐箱中的盐排到III效下循环管中,III效盐箱中的盐排到IV效下循环管中,最后集中在IV效盐箱;IV效排出盐浆,同时由IV效分离罐排出母液。工艺流程简图见图1。

制盐工艺流程

3. 工艺的优缺点

运用该工艺已在中国北方建设了一个年产15万吨的真空制盐生产线,相比再制盐生产工艺,其实施效果如下:

本工艺采用饱和卤水制盐,不需要淡水化原盐,淡水消耗低,可节约淡水资源,根据生产情况,1t盐消耗淡水 1.4-0.2m3,而再制盐生产工艺需要淡水3.5-4.53.73 m3,吨盐节约了2.1-4.3 m3淡水。

2)节约日晒盐场土地,减少原盐消耗,提高了盐田利用率

对再制盐生产线来说,一般生产1t精制盐需要消耗1.2t日晒原盐,年产15万t的精制盐需要18万t原盐,需要结晶面积136.36公顷(按目前中国北方盐场有效结晶单产1650t/公顷,有效面积占80%),采用饱和卤水直接进蒸发罐工艺,不需要饱和卤水进结晶池继续蒸发制盐,只需要建设一个约2公顷的饱和卤库,因此,节约了土地面积134.36公顷,节省的结晶面积可用于增加日晒盐产量8547t,按日晒盐销售价160元/t计算,日晒盐增加产值136.8万元。如果土地按3万元/亩计算,节约土地价值为6000多万元。

3)采用三次洗涤方法洗涤盐浆以去除钙镁离子,使氯化钠含量超过99.4%,精制盐达到优级盐质量。蒸发结晶过程中盐浆会夹带大量的母液,母液中含有大量的钙、镁杂质,因此,采用三次不同的洗涤方式洗涤盐浆,提高精制盐质量。

4)用饱和卤水直接生产精制盐母液损失小,提高了母液的利用率,实现盐化联产

采用盐田饱和卤水直接生产优级真空精制盐工艺,每产1t精制盐副产母液约1.7m3,以年产15万t精制盐计算,全年可产生25.5万m3蒸发母液,且排出的苦卤氯化钾含量达到了24~30g/l,浓度为29~34°Be′,钠镁比值为1.4~0.6,排出的苦卤中氯化钾总含量为6589t,氯化镁总量为36540t,硫酸镁总量为18900t,氯化钠总量为37600t,溴总量510t。而年产18万t原盐,排出的苦卤量一般只有18万m3,经过复晒、渗透损失、沟道输送进入化工厂后只有原来的45%左右,也就是说还有8.1万m3,这时氯化钾总含量为2036t,氯化镁总量为15280 t,硫酸镁总量为7727.4 t,氯化钠总量为11128 t,溴总量162t。因此,该工艺可为盐化工生产提供更多的原料,提高了生产效率,降低了各种消耗,节约能源。该工艺排出的制盐蒸发母液温度在55.04℃以上,可通过管道直接进入下步生产氯化钾的工序,生产氯化钾产品,节约氯化钾生产过程中所需热量,折标煤1157吨/年

5)由于滩田饱和卤水较化盐卤水氯化钠含量低,因此单位产品的蒸发水量较高,吨盐产品电、蒸汽消耗量较大,能耗较高。

6)蒸发设备换热面积较化盐溶解工艺稍高,因此设备投资相对较高,但由于节省大量盐田结晶面积,总体投资低。

4. 与原盐溶解再制盐工艺技术经济指标比较

国内海盐区生产真空盐的工艺主要有滩田饱和卤水进罐蒸发制盐和原盐溶解制成饱和卤水蒸发制盐两种方法。原盐溶解制成饱和卤水蒸发制盐工艺优缺点有:(1)需建结晶池,工程总体投资较高;(2)单位产品蒸发水量小,设备投资稍低,(3)吨盐消耗电、蒸汽量较小;(4)水消耗较大,吨盐产品耗水 2.4m3,(5)蒸发母液损失大。比较两种制盐方法技术经济指标如表。

表主要经济技术指标比较

制盐工艺流程

制盐工艺流程

两种工艺在投资和单位产品生产成本中有较大的差别,饱和卤水法单位产品总投资低于原盐溶解法,单位产品成本低于原盐溶解法30%。

5. 结论

对于北方海盐区生产精制盐企业来说,利用滩田饱和卤水直接进蒸发罐制取精制盐是一个较好的生产工艺,节约投资,节约土地,特别是将宝贵的原盐溶解成饱和卤水再进行制盐是对土地资源的浪费,再有其生产成本低,企业有较高的利润空间;排出的制盐母液综合利用生产钾、溴、镁等产品,使卤水全部吃干榨净,没有废物排放,减少了环境污染,有利于可持续发展。

New technology of producing refined salt by adding salt field saturated brine into evaporator directly

Jin zhiling Ge Jinyang

(Salt Research Institute China Natural Salt Industry Corporation, TianJin 300450)

Abstract: After Making saline saturated brine flow in evaporating pot, the sodium chloride is obtained by evaporative crystallization. Calcium and magnesium ions can be removed by washing salt three times in order to get the sodium chloride that the purity is over 99.1 percent and the mother liquor that have high concentration.

Key words: saline saturated brine, vacuum, refine salt

1 BRIEF INTRODUCTION

At present, the materials of making vacuum salt are using the well and rock salt in most of the well and rock salt corporations and the saturated brine got by dissolving crude salt. Along with the rapid development of caustic alkali and sodium carbonate and the growth in the people’s living standard, the contradiction of crude salt in supply and require is more and more stood out.

Particularly in the sea salt areas, producting refine salt with saturated brine that is got by dissolving crude salt can increase the consumption of raw salt , reduce the availability factor of salt field, add investment and production cost. This will influence the economic efficiency of edible salt manufacturing enterprises , make them reduce output and affect people’s live.

However, using the method of making saline saturated brine flow in evaporating pot and then obtaining sodium chloride by evaporative crystallization can saving crystal area. Furthermore, the mother liquor can be used as material to produce chemically products such as potassium chloride, magnesium chloride etc. in order to make use of all kinds of effective constituents in the brine and achieve the objects of no discharge ,no pollution and circulating economy.

2. INDICATION OF THE PROCESS FLOW

After being solarized to saturation in beach field brine was inducted to brine tank and stored. On the one hand brine tank can store some saturated brine that is used to produce vacuum salt. On the other hand ,brine have some residence time here so some impurities can be deposited in order to make the brine be purified. After being drawouted from brine tank the cleaned saturated brine was pumped to fine brine tank in the manufacture salt plant. And production can be gone on by using the methods of preheating brine , transferring stuff in parallel flow, discharging salt slurry in the last effect and exporting mother liquor concentratedly. The salt in the first effect tank is discharged to the circulation pipe of the second effect, The salt in the second effect tank is discharged to the circulation pipe of the third effect, The

salt in the third effect tank is discharged to the circulation pipe of the fourth effect. At last the salt is collected in the fourth effect tank where the salt slurry and the mother liquor are discharged. Fig. 1 is the diagram of the process flow.

制盐工艺流程

Figure.1 Diagram of the process flow

3. RELATIVE MERITS OF THE PROCESS

In the north of China a production line has been built with this process, which can produce 1.5 million tons salt per year. Comparing with rework salt production process, the effect of this process is as follows:

1)The saturated brine is used in this process take the place of dissolving crude salt with fresh water. Such can reduce the consumption of fresh water and save the resource. According to the produce conditions, producing 1 ton salt will consume 1.4-0.2 m3 fresh water. And that if use the reworking salt process 3.5-4.53 m3fresh water will be used. That is to say 2.1-4.3 m3 fresh water can be saved corresponding to 1 ton salt . 2) Saving the solar salt field, reduce the consumption of crude salt, increase the availability of the salt field.

For the reworking salt production line, as a general rule producing 1 ton refine salt will consume 1.2 tons solar salt. Producing 1.5 million ton refine salt needs 1.8 million tons crude salt and 136.36 hectare crystal area(According to the experience that one hectare can produce 1650 ton crystal and the effective area is 80 percent). Using the process that the saturated brine is pumped to evaporating tank directly the brine need not evaporate in crystal pond but for a saturated brine storeroom of about 2 hectare. Such can save surface area of 134.36 hectare and this area can be used to increase 8547 ton output of solar salt. Hence the productive value of solar salt will increase 136.8 ten

thousand yuan according to the market price of solar salt which is 160yuan/t. And land value of 60 million yuan can be saved if the land is 30,000 yuan per mou.

3)Adopting the method of washing salt three times in order to get rid of calcium and magnesium ions , make the purity of sodium chloride exceed 99.4% and make the refine salt reach first quality. In the process of evaporative crystallization much mother liquor will exist in salt slurry. The mother liquor contains a lot of impurity of calcium and magnesium. So salt slurry is washed in three different ways in order to improve the quality of refine salt.

4)Using the saturated brine to produce refine salt can reduce the consume of mother liquors, enhance it’s availability and realize the production of salt and chemicals together.

The process that producing excellent vacuum salt with salina saturated brine directly can get 1.7 m3 mother liquor in relation to 1t refine salt. On the base of 150,000 ton refine salt, we can get 255,000 m3 evaporation mother liquor. In bitter brine whose concentration is 29~340Be′the content of potassium chloride is 24~30g/l and the ratio of sodium and magnesium is 1.4~0.6. And 6589t potassium chloride, 36540t magnesium chloride,18900t magnesium sulfate,37600t sodium chloride and 510t bromine are contained in the bitter brine.

The bitter brine is about 180,000 m3 in relation to 180,000t crude salt. After being solarized repeatedly and consuming for filter the brine is left 45% when it is pumped to chemical plant. That is to say there are 81,000 m3 brine. By now there are 2036t potassium chloride, 15280t magnesium chloride, 7727.4t magnesium sulfate, 11128t sodium chloride and 162t bromine. So the process can provide more material for salt and chemicals plants, heighten production efficiency, reduce all kinds of consumption and save energy. The temperature of the mother liquor got from this process is over 55.04℃. So it can be pumped to the potassium chloride process with pipe to produce potassium chloride. Such will save the heat quantity in the process of producing potassium chloride. And the heat quantity is equal to 11578t/y standard coal.

5) Because the content of sodium chloride in the beach saturated brine is lower than the content in the brine that is got by dissolving salt, the evaporated water of unit produce is higher, the electricity and steam consumption are more. That is to say the energy consumption is higher.

6)The heat exchange area is a littler bigger than the process of dissolving salt, so the equipment investment is higher. But because of saving a lot of salina crystal area the total investment is lower.

4. COMPARE THE ECONOMIC INDICATORS WITH THE PROCESS OF DISSOLVING CRUDE SALT AND THEN REMAKING SALT

In China There are two main methods to producing vacuum salt in sea salt area. One is pumping the beach saturated brine into evaporating pot. The other is using the saturated brine got by dissolving crude salt. Comparing the second method with the first method the

relative merits of the second method is as follows:(1)need to build crystal pond and the total investment is higher.(2)The evaporated water of unit product is smaller and the equipment investment is lower.(3)The consuming of electricity and steam is lower.(4)The consuming of water is more and 2.4 m3 water will be consumed in relation to 1 ton salt.(5)The loss of mother liquor is bigger. The economic indicators of the two methods are as follows:

Comparison of the main economic and technical norms

制盐工艺流程

Between the two processes there is bigger difference in the investment and production cost of the unit product. The total investment of the unit product is lower and the cost of unit product is lower 30%than the method of dissolving crude salt.

For the plant that producing refine salt in the sea salt area in north using the method of pumping beach saturated brine into evaporating pot and producing salt is a good production process. It can save investment and land. Particularly dissolving crude salt into saturated salt and then making salt is a kind of waste to land resource. Moreover, the production cost is lower. And corporations will get more profit. The mother liquor can be used to product potassium, bromine and magnesium etc. Brine will be utilized completely. No waste be discharged and it will reduce environment pollution. This is in favor of the sustainable development.

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