Method for preparing refined sodium chloride from sodium-containing crude salt

Abstract

The invention discloses a method for preparing refined sodium chloride from sodium-containing crude salt, and belongs to the field of chemical engineering and environmental protection. The method comprises the following steps: dissolving sodium-containing crude salt in water to form a near-saturated salt solution or a saturated salt solution, heating and stirring, adding ammonium chloride, converting Na < + > ions into NaCl by using the same ion effect of Cl <-> ions when the ammonium chloride is dissolved to reach or close to saturation, selectively crystallizing and separating out, filtering while hot to obtain sodium chloride crystals and salting-out liquid, and drying to obtain the sodium chloride. Sodium chloride crystals are washed and dried to obtain refined sodium chloride, salting-out liquid is cooled and crystallized to separate out ammonium chloride, ammonium chloride crystals obtained through filtering return to the salting-out procedure to be recycled, ammonium chloride serves as a transformation agent and a salting-out agent in the technological process, and the method has the advantages of being simple in technology, high in production efficiency, high in raw material adaptability, economical, environmentally friendly and the like; and large-scale industrial production of refined sodium chloride from sodium-containing crude salt is facilitated.

Description

Technical Field

[0001] The present invention belongs to the fields of chemical engineering and environmental protection, and particularly relates to a method for preparing refined sodium chloride from crude sodium salt containing sodium. Background Art

[0002] Common sodium salts include sodium chloride, sodium sulfate, sodium nitrate, etc. In addition to these common sodium salts, crude sodium salt containing sodium may also contain impurities such as calcium chloride, magnesium sulfate, insoluble substances, and organic substances. Generally speaking, mineral salt, well salt, sea salt, lake salt, natural mirabilite, waste salt residue generated in the chemical production process, and crystals formed by evaporation and concentration of sodium-containing industrial wastewater are all crude sodium salt containing sodium.

[0003] Ammonium chloride is a common chemical product, and more than 90% of it comes from the by-products of the soda ash production process. Ammonium chloride is mainly used in the fields of chemical fertilizers, batteries, hydrometallurgy, food, medicine, etc., and more than 95% of it is used as chemical fertilizers for crops. However, although chlorine is a trace element required by crops, excessive use will be harmful, causing leaf burning of crops, soil compaction and salinization, etc. Therefore, there is a large backlog of ammonium chloride at home and abroad at present. If ammonium chloride can be split and chlorine and ammonia can be utilized separately, a new way out for the consumption of ammonium chloride will be found.

[0004] As is well known, the refined sodium chloride obtained by purifying crude salt can be used as a raw material for the production of soda ash or as a raw material for the production of chlor-alkali. The refined sodium chloride solution is carbonated to precipitate sodium bicarbonate, and sodium bicarbonate is calcined to obtain sodium carbonate. The refined sodium chloride solution is subjected to membrane electrolysis to obtain chlorine and caustic soda solution, and the caustic soda solution is further evaporated and concentrated to obtain liquid caustic soda (sodium hydroxide solution with a concentration of more than 30%) or solid sodium hydroxide products. The solution of refined sodium chloride is generally prepared by purifying and removing impurities after dissolving raw salt in water. The refined sodium chloride crystals need to first dissolve the crude salt in water, purify and remove impurities, and then evaporate and concentrate to crystallize out refined sodium chloride. To prepare 1 ton of refined sodium chloride, nearly 3 tons of water need to be evaporated in the process, with high energy consumption, and soluble impurities will also crystallize out with sodium chloride during the evaporation and concentration process. Soda ash, liquid caustic soda, and solid caustic soda are all essential basic raw materials in the modern chemical industry, and their use will produce sodium-containing wastewater. The sodium-containing wastewater is then purified, decontaminated, evaporated, and concentrated to form industrial waste salt. Industrial waste salt is divided into single salt and mixed salt. Single salt is a salt with a single component, while mixed salt refers to a salt with two or more components. According to statistics, only about 20% of industrial waste salt is single salt, and the rest is mixed salt. In addition to sodium salts, mixed salts may also contain potassium salts and ammonium salts. Among them, the mixture of sodium chloride and sodium sulfate is a typical representative of such mixed salts. Sodium sulfate and sodium chloride are both inexpensive sodium salts, and the economic benefit of purifying, separating, and recovering sodium sulfate and sodium chloride from sodium-containing waste salt is negative.

[0005] Therefore, many enterprises are currently facing a severe dilemma: the sodium-containing wastewater generated during the production process must be purified, but there is a lack of effective resource utilization channels for the sodium-containing waste salts obtained after purification. This has led to a backlog of a large amount of industrial waste salts, which has become a problem that enterprises urgently need to solve. Summary of the Invention

[0006] Aiming at the defects in the prior art of refining sodium chloride, such as high energy consumption, difficulty in treating sodium-containing wastewater and industrial waste salts, the object of the present invention is to provide a method for preparing refined sodium chloride from sodium-containing crude salt. This method uses ammonium chloride as both a transformation agent and a salting-out agent, and can obtain high-grade refined sodium chloride. Moreover, various products such as sodium nitrate crystals, ammonium nitrate crystals and ammonium sulfate crystals can be obtained during this process, solving the problem of the poor sales of by-product ammonium chloride in industry.

[0007] To achieve the above technical object, the present invention provides a method for preparing refined sodium chloride from sodium-containing crude salt. The method is to dissolve the sodium-containing crude salt in water to form a nearly saturated salt solution or a saturated salt solution. Ammonium chloride is added to the nearly saturated salt solution or saturated salt solution while heating and stirring. After ammonium chloride is dissolved to reach or approach saturation, the common ion effect of Cl - ions forces the Na + ions to selectively crystallize out as NaCl. After filtration and separation, sodium chloride crystals and salting-out mother liquor are obtained. The sodium chloride crystals are washed, filtered and dried to obtain refined sodium chloride; the salting-out mother liquor is cooled and crystallized to precipitate ammonium chloride crystals. After filtration and separation, ammonium chloride crystals and ammonium chloride crystallization mother liquor are obtained. The ammonium chloride crystals are returned to the salting-out process for recycling, and the ammonium chloride crystallization mother liquor is returned to the sodium-containing crude salt dissolution process for continuous use until it has an obvious hindrance effect on the dissolution of sodium-containing crude salt, and then it is discharged for comprehensive recovery.

[0008] The key to the technical solution of the present invention lies in using ammonium chloride as both a transformation agent and a salting-out agent to indirectly recrystallize sodium-containing crude salt, and the added ammonium chloride can be separated from other by-products by using solubility differences to achieve recycling.

[0009] Specifically, the present invention first adds sodium-containing crude salt to water, heats and stirs it to dissolve, and then cleverly uses the characteristic that the solubility of ammonium chloride increases significantly with the increase of temperature, and adds ammonium chloride to the solution while heating and stirring. Through the common ion effect of Cl - ions generated by the hot concentrated ammonium chloride solution, the Na +The ions are converted into NaCl and selectively crystallized and precipitated, so as to realize the separation of sodium sulfate, sodium nitrate and other non-sodium chloride salts from sodium chloride. After separation, the non-sodium chloride salts such as sodium sulfate and sodium nitrate in the solution are converted into ammonium sulfate, ammonium nitrate and other non-ammonium chloride salt solutions. Through hot filtration, sodium chloride crystals and the mother liquor after sodium chloride crystallization are obtained. The obtained mother liquor after sodium chloride crystallization is an ammonium chloride solution containing ammonium sulfate or ammonium nitrate or other non-chlorinated ammonium salts. Since the solubility of ammonium sulfate and ammonium nitrate is much higher than that of ammonium chloride at the same temperature, ammonium chloride preferentially crystallizes and precipitates during the cooling process. The ammonium chloride crystals obtained by liquid-solid separation can be recycled, thus completing the mission of ammonium chloride as both a transformation agent for converting other non-chlorinated sodium salts such as sodium sulfate and sodium nitrate into sodium chloride and a salting-out agent for sodium chloride crystallization in the process.

[0010] In the present invention, the main reactions for preparing refined sodium chloride by transforming sodium-containing crude salt with ammonium chloride are as follows:

[0011] 2NH 4 Cl (in excess) + Na 2 SO 4 =Δ= 2NaCl↓ + (NH 4 ) 2 SO 4 (1)

[0012] NH 4 Cl (in excess) + NaNO 3 =Δ= NaCl↓ + NH 4 NO 3 (2)

[0013] As a preferred scheme, the amount of ammonium chloride added to the nearly saturated salt solution or saturated salt solution is 1 to 3 times the amount required to reach saturation when dissolved in the obtained nearly saturated salt solution or saturated salt solution, including the amount required to convert non-sodium chloride salts into sodium chloride and the amount required to dissolve ammonium chloride to near saturation or saturation.

[0014] As a preferred scheme, the sodium-containing crude salt is a solid mixture of sodium salts containing at least one of sodium chloride, sodium sulfate and sodium nitrate, including mineral salt, well salt, sea salt, lake salt, natural mirabilite, waste salt residue obtained in the chemical production process and crystals obtained by evaporation and concentration of sodium-containing industrial wastewater. In addition to sodium salts, the sodium-containing crude salt may also contain calcium salts, magnesium salts, ammonium salts, potassium salts, insoluble impurities and organic substances. Before transformation, the organic substances in the sodium-containing crude salt can be removed by pyrolysis.

[0015] As a preferred scheme, when the sodium-containing crude salt contains organic impurities and / or other inorganic impurities, impurity removal pretreatment is required before adding ammonium chloride, and the impurity removal pretreatment includes at least one of the following methods:

[0016] When the sodium-containing crude salt contains organic impurities, pyrolysis treatment or oxidation treatment is carried out on the sodium-containing crude salt;

[0017] When the sodium-containing crude salt contains insoluble inorganic impurities, filtration treatment is carried out on the nearly saturated solution or saturated solution formed by dissolving the sodium-containing crude salt;

[0018] When the sodium-containing crude salt contains water-soluble inorganic impurities, anion impurity removal treatment and cation impurity removal treatment are carried out on the nearly saturated solution or saturated solution formed by dissolving the sodium-containing crude salt: The anion impurity removal treatment process is: adding iron salt and / or aluminum salt, and adjusting the pH of the solution system to 3.5-6.5, stirring for 0.5-1.5 h, to remove anion impurities including phosphate and silicate; The cation impurity removal treatment process is: using the lime-soda method to remove cations including Mg 2+ , Fe 3+ , Mn 2+ , Zn 2+ and Cu 2+ and other cation impurities. After adding iron salt and / or aluminum salt, phosphate in the solution precipitates out as iron phosphate or aluminum phosphate, while silicate precipitates out in the form of iron silicate or aluminum silicate. The addition amount of iron salt and / or aluminum salt is measured at 1-1.5 times of the stoichiometry. After using the lime-soda method to remove Mg 2+ , Fe 3+ , Mn 2+ , Zn 2+ and Cu 2 + and other cation impurities, the pH of the solution system is adjusted to 5-7 with hydrochloric acid. The iron salt and / or aluminum salt includes at least one of ferric sulfate, aluminum sulfate, ferric nitrate and aluminum nitrate.

[0019] As a preferred scheme, the sodium-containing crude salt is dissolved at a temperature of 25-75 °C to obtain a nearly saturated solution or saturated solution at this temperature.

[0020] As a preferred scheme, the temperature of the selective crystallization of NaCl is 65-115 °C. In the present invention, the selection of the temperature of the selective crystallization has a direct impact on the selective precipitation effect of sodium chloride. The temperature has little effect on the solubility of sodium chloride, while the solubility of ammonium chloride increases sharply with the increase of temperature. And when the temperature is relatively low, the solubility of sodium chloride is greater than that of ammonium chloride. As the temperature increases, the solubility of ammonium chloride increases greatly, much greater than that of sodium chloride, and the greater the temperature, the greater the difference in solubility between the two. Therefore, within the temperature range selected in the present invention, adding an excessive amount of ammonium chloride can utilize the common ion effect generated by Cl - ions, and can force the Na +The ions are converted into NaCl and selectively crystallize out, where the excessive ammonium chloride is used to convert non-sodium chloride salts into sodium chloride crystals.

[0021] As a preferred solution, in the washing process of the sodium chloride crystals, a saturated sodium chloride solution at 25 - 95 °C is used as the washing liquid, the solid-liquid ratio of washing is 1:(0.5 - 5) g / mL, the pH of the washing liquid is adjusted to 8.5 - 10.5, and the washing time is 0.25 - 2.5 h. In the present invention, sodium hydroxide or sodium carbonate salt is used to adjust the pH of the washing liquid.

[0022] As a preferred solution, when the crude sodium salt contains a solid mixture of at least one of sodium sulfate and sodium nitrate, the hindering effect comes from the enrichment of ammonium sulfate and / or ammonium nitrate.

[0023] As a preferred solution, for the ammonium chloride crystallization mother liquor whose hindering effect comes from the enrichment of ammonium nitrate, its comprehensive recovery means first heating and concentrating, then cooling and crystallizing, and filtering to obtain ammonium nitrate crystals and ammonium nitrate crystallization mother liquor;

[0024] Or;

[0025] First, HCl gas is introduced into the ammonium chloride crystallization mother liquor, then cooled or frozen and crystallized, and filtered to obtain ammonium chloride crystals and a regenerated industrial nitric acid solution;

[0026] Or;

[0027] First, concentrated hydrochloric acid is added to the ammonium chloride crystallization mother liquor, then cooled or frozen and crystallized, and filtered to obtain ammonium chloride crystals and a regenerated industrial nitric acid solution;

[0028] Or;

[0029] First, sodium chloride is added to the ammonium chloride crystallization mother liquor, then cooled or frozen and crystallized, and filtered to obtain ammonium chloride crystals and a sodium nitrate solution. The sodium nitrate solution is further evaporated and concentrated to crystallize out sodium nitrate crystals.

[0030] As a preferred solution, for the ammonium chloride crystallization mother liquor whose hindering effect comes from the enrichment of ammonium sulfate, its comprehensive recovery means first adding ammonium sulfate crystals according to the solubility of ammonium sulfate at this temperature, heating and stirring to dissolve it, cooling and crystallizing, filtering to obtain ammonium chloride crystals, and then evaporating and concentrating the filtrate, and cooling and crystallizing to obtain ammonium sulfate crystals;

[0031] Or;

[0032] Add a calcium chloride-containing solution with a calcium chloride concentration ≥ 10 wt.% to the ammonium chloride crystallization mother liquor, heat and stir to precipitate sulfate, filter to obtain precipitated calcium sulfate and the post-precipitation liquid, cool and crystallize the post-precipitation liquid, filter the ammonium chloride crystals and their crystallization mother liquor, and return the ammonium chloride crystallization mother liquor to the sodium-containing crude salt dissolution process for continued use. The sulfate transformation process is to convert calcium chloride and soluble sulfate into calcium sulfate precipitate and soluble chloride. The addition amount of calcium chloride is adjusted to make the Ca 2+ / SO 4 2- molar ratio ≥ 0.95.

[0033] As a preferred solution, the precipitated calcium sulfate is at least one of calcium sulfate dihydrate, calcium sulfate hemihydrate, and anhydrous calcium sulfate.

[0034] As a preferred solution, the utilization rate of Na in the sodium-containing crude salt > 98%, and the grade of the obtained refined sodium chloride ≥ 99.5%.

[0035] As a preferred solution, if the sodium-containing crude salt also contains potassium salts, refined potassium chloride can be prepared by ammonium chloride transformation because the crystallization behaviors of potassium chloride and sodium chloride in hot ammonium chloride solution are similar, or (NH 4 ) 2 SO 4 obtained from reaction (1) can be used as a transformation agent to prepare potassium sulfate because the solubility of potassium sulfate is much smaller than that of potassium chloride and potassium nitrate at the same temperature. The specific process is as follows: Add an excess of (NH 4 ) 2 SO 4 to the dissolution solution of sodium- and potassium-containing crude salt, and use the common ion effect generated by SO 4 2- to force the K + in the solution to selectively crystallize out in the form of K 2 SO 4 . Filter while it is hot to obtain K 2 SO 4 crystals and its post-crystallization liquid, and then cool and crystallize NH 4 Cl or evaporate and crystallize the mixed agricultural fertilizer of ammonium sulfate and ammonium nitrate. Among them, the principle of ammonium sulfate transformation to recover potassium sulfate is:

[0036] (NH 4 ) 2 SO 4 (in excess) + 2KCl = Δ = K 2 SO 4 ↓ + 2NH 4 Cl (4)

[0037] (NH 4 ) 2 SO4 (Excess) + 2KNO 3 = Δ = K 2 SO 4 ↓ + 2NH 4 NO 3 (5)

[0038] As a preferred solution, if the sodium - containing crude salt contains ammonium salts in addition to sodium salts, the sodium - containing crude salt containing ammonium salts can also be transformed with ammonium chloride to prepare refined sodium chloride, and the ammonium salts therein can be comprehensively recovered.

[0039] Compared with the prior art, the technical solution of the present invention has the following technical advantages and effects:

[0040] The present invention ingeniously uses ammonium chloride as a transformation agent and a salting - out agent, without evaporation and concentration, and realizes the indirect recrystallization and refining process of sodium chloride. The influence of temperature on the solubility of sodium chloride is small, while the solubility of ammonium chloride increases sharply with the increase of temperature. Below 16 °C, the solubility of sodium chloride is greater than that of ammonium chloride, and above 16 °C, the solubility of ammonium chloride increases significantly and is much greater than that of sodium chloride, and the greater the temperature, the greater the difference in solubility between the two. Therefore, as long as the sodium - containing crude salt is added to a solution with a temperature above 65 °C to make it dissolve close to saturation, and ammonium chloride is added thereto, by using the common - ion effect generated by Cl - ions, the Na + ions can be forced to selectively crystallize out as NaCl. It can be seen from this that the added ammonium chloride is both a transformation agent for non - sodium chloride salts in the sodium - containing crude salt and a salting - out agent for the crystallization of sodium chloride in the solution. After the sodium chloride crystals precipitate, filter while it is hot, cool the filtrate to below 16 °C, and add sodium chloride to the filtrate during the cooling process and stir to dissolve. At this time, sodium chloride in turn becomes a salting - out agent for the crystallization of ammonium chloride. Filter to obtain ammonium chloride crystals, and the obtained ammonium chloride crystals are returned to the salting - out process for continued use. In this way, the indirect recrystallization and refining process for preparing refined sodium chloride from sodium - containing crude salt is realized.

[0041] The present invention uses ammonium chloride as a transformation agent to convert crude salt containing sodium sulfate or sodium nitrate into sodium chloride, and comprehensively utilizes ammonium sulfate or ammonium nitrate in the transformed solution. This not only realizes the large - cycle of sodium source production - sales - use - regeneration, greatly simplifies the process route for comprehensive utilization of sodium - containing crude salt, but also drives the large - cycle of chlorine source (ammonium chloride) production - sales - use - regeneration, and solves the problem of poor sales of industrial by - product ammonium chloride.

[0042] The present invention has the advantages of simple process, high production efficiency, strong raw material adaptability, economy and environmental protection, and is suitable for large - scale industrial production of refined sodium chloride from sodium - containing crude salt. Specific embodiments

[0043] The present invention will be further described below in conjunction with embodiments. The following embodiments are intended to illustrate the present invention rather than further limit the present invention.

[0044] Example 1

[0045] First, industrial waste salt containing 95.73 wt% NaCl and 2.54 wt% Na 2 SO 4 is added with water, stirred, heated and dissolved to prepare a saturated NaCl solution at 65°C. A sulfuric acid-ferric sulfate solution is added in an amount 2.5 times the amount required for phosphate precipitation. Then, the pH of the solution is adjusted to 4.1 with an alkali, and the mixture is stirred at a constant temperature for 1 h. The anionic impurities such as phosphate and silicate are removed by filtration. Then, a mixture of lime milk and sodium carbonate with a Ca 2+ / CO 3 2- molar ratio of 1:1 is added, the pH is adjusted to 10.5, and the mixture is stirred for 0.5 h. The cationic impurities such as calcium, magnesium and iron are removed by filtration. Then, the purified liquid is heated and stirred to 65°C, and ammonium chloride crystals are slowly added thereto as a salting-out agent in an amount 1.2 times the solubility of ammonium chloride in the solution at 65°C. After all the ammonium chloride is added and completely dissolved, the sodium chloride crystals and the salting-out mother liquor are obtained by hot filtration while maintaining the temperature. The sodium chloride crystals are washed by stirring at atmospheric pressure with a saturated sodium chloride solution with a pH of 9.7 (the solid-liquid ratio of washing is 1:1 g / mL), the washing time is 0.5 h, and then filtered and dried to obtain refined sodium chloride with a purity of 99.9% (the contents of Ca and Mg are both <0.01%). While the obtained salting-out mother liquor is cooled to less than 16°C, industrial salt is added with stirring to increase the NaCl concentration therein to 250 g / L to improve the crystallization rate of ammonium chloride. Ammonium chloride crystals and a crystallization mother liquor are obtained by filtration. The crystallization mother liquor is returned to the industrial salt dissolution process for recycling until the ammonium sulfate therein is enriched to have an obvious hindering effect on the dissolution of industrial salt, and then it is discharged. And calcium chloride is added according to a Ca 2+ / SO 4 2- molar ratio of 0.95 (the concentration of calcium chloride is greater than 10 wt.%), and the mixture is heated and stirred to precipitate sulfate radicals at a temperature of 75-85°C. Gypsum dihydrate and a post-precipitation liquid are obtained by filtration. The filtrate is cooled and crystallized, and ammonium chloride crystals and an ammonium chloride crystallization mother liquor are obtained by filtration. The ammonium chloride crystallization mother liquor is returned to the industrial salt dissolution process for recycling.

[0046] Example 2

[0047] Take a sodium sulfate saturated solution prepared with industrial mirabilite at a temperature of 60°C. First, slowly add ammonium chloride crystals as a salting-out agent and a transformation agent at 1.6 times the amount required for ammonium chloride to dissolve to saturation in this solution at 60°C. Heat and stir. Wait until the ammonium chloride crystals are completely dissolved and the solution temperature rises to 90°C. Keep stirring at a constant temperature for 0.5 h. Filter while it is hot and keep warm to obtain sodium chloride crystals and the salting-out solution. The sodium chloride crystals are washed by negative-pressure stirring with a sodium chloride saturated solution at a temperature of 75°C with a pH of 8.9 for 0.5 h (the solid-liquid ratio for washing is 1:2 g / mL), then filtered and dried to obtain refined sodium chloride with a purity of 99.7%. The obtained salting-out solution is cooled and crystallized to precipitate ammonium chloride. Filter to obtain ammonium chloride crystals and ammonium chloride crystallization mother liquor. The ammonium chloride crystals are returned to continue to be used as a salting-out agent, and the obtained ammonium chloride crystallization mother liquor is returned to the mirabilite dissolution process for recycling until the ammonium sulfate in it is enriched to have an obvious obstructive effect on the dissolution of industrial mirabilite, and then it is discharged. First, add ammonium sulfate crystals according to the solubility of ammonium sulfate at this temperature, heat and stir to dissolve it, cool and crystallize, filter to obtain ammonium chloride crystals, and then evaporate and concentrate the filtrate, cool and crystallize to obtain ammonium sulfate agricultural fertilizer.

[0048] Example 3

[0049] Take the evaporated and concentrated solution after purification of sodium-containing wastewater with a pH of 8.6, which contains Na + 5.8 mol / L, Cl - 1.6 mol / L, SO 4 2- 2.1 mol / L. First, heat it to 75°C, then add ammonium chloride as a transformation agent by stirring according to the stoichiometry required for all the sodium sulfate in it to be converted into NaCl, and at the same time slowly add ammonium chloride crystals as a salting-out agent at 1.2 times the amount required for ammonium chloride to dissolve to saturation in it at this temperature. Stir and heat to raise the solution temperature to 95°C. Keep stirring at a constant temperature for 1 h. Filter while it is hot and keep warm to obtain sodium chloride crystals and its salting-out solution. The sodium chloride crystals are washed by stirring with a sodium chloride saturated solution at a temperature of 95°C with a pH of 10.2 (the solid-liquid ratio for washing is 1:2.5 g / mL), the washing time is 0.75 h, then filtered and dried to obtain refined sodium chloride with a purity of 99.6%. Then, according to the molar ratio of SO 4 2- to Ca 2+ 1:1, add anhydrous calcium chloride to the obtained salting-out solution, raise the temperature and stir to precipitate the sulfate radical at a temperature of 75 - 85°C. Filter while it is hot to obtain calcium sulfate filter cake and its post-precipitation solution. The calcium sulfate filter cake is washed and dried to obtain a calcium sulfate dihydrate product with a purity of 99.5%. The obtained post-precipitation solution is cooled and crystallized, filtered to obtain ammonium chloride crystals and ammonium chloride crystallization mother liquor. The ammonium chloride crystals are returned to the wastewater purification process for recycling, and the ammonium chloride crystallization mother liquor is sent to the sodium-containing wastewater evaporation and concentration process.

[0050] Example 4

[0051] First, the waste salt residue containing 78.64 wt% NaCl, 6.68 wt% K 2 SO 4 6, 5.97 wt% organic matter, 6.91 wt% moisture, and 1.73 wt% water-insoluble matter is thermally decomposed at 400 °C to remove the organic matter, and then it is added to water and stirred to dissolve, and the water-insoluble matter is removed by filtration to obtain a dissolved solution. The obtained dissolved solution is first heated to 60 °C, and then ammonium chloride crystals are slowly added as a salting-out agent and a transformation agent at 1.4 times the solubility of ammonium chloride therein, heated and stirred. After the ammonium chloride is completely dissolved and the solution temperature rises to 96 °C, it is stirred at a constant temperature for 0.5 h, and then filtered while hot and kept warm to obtain sodium chloride crystals and its post-salting-out liquid. The sodium chloride crystals are stirred and washed with a saturated sodium chloride solution with a pH of 9.3 and a temperature of 45 °C (the washing solid-liquid ratio is 1:1.5 g / mL), the washing time is 1 h, filtered, and dried to obtain refined sodium chloride with a purity of 99.8%. After the post-salting-out liquid obtained is cooled and crystallized to separate ammonium chloride, it is returned to the waste salt residue dissolution process for recycling until ammonium sulfate is enriched to have an obvious hindering effect on the dissolution of the waste salt residue, then it is opened, heated and stirred to add ammonium sulfate to dissolve it, and the common ion effect generated by SO 4 2- forces the K + in the solution to crystallize out in the form of K 2 SO 4 . It is filtered while hot to obtain K 2 SO 4 crystals and its post-crystallization liquid. The post-crystallization liquid is first cooled and crystallized for ammonium chloride, and then evaporated and crystallized for ammonium sulfate fertilizer.

[0052] Comparative Example 1

[0053] Industrial waste salt containing 42.81 wt% NaCl and 57.04 wt% Na 2 SO 4 is added with water, stirred, heated and dissolved to make a mixed salt solution saturated with NaCl at 40 °C. The fractional crystallization method is used to separate and recover sodium chloride and sodium sulfate therein: first, the temperature of the mixed salt solution is lowered to 20 - 25 °C to allow some of the sodium sulfate to crystallize out, and the mirabilite crystals and its crystallization mother liquor are obtained by filtration. Then, the obtained crystallization mother liquor is evaporated and concentrated to crystallize out a mixed crystal of sodium chloride and sodium sulfate. Then, the obtained mixed crystal is added to a newly prepared mixed salt solution and stirred, heated and dissolved to keep the NaCl concentration in the solution at 17 - 19 wt%. Then, the temperature of the solution is lowered to 20 - 25 °C to allow some of the sodium sulfate to crystallize out, and the mirabilite crystals and its crystallization mother liquor are filtered. Then, the obtained crystallization mother liquor is evaporated and concentrated to crystallize out a mixed crystal of sodium chloride and sodium sulfate. The above operations are repeated until sodium sulfate is nearly completely crystallized out, and then evaporated and concentrated to crystallize out industrial salt. The mirabilite obtained by crystallization is washed and then heated and dehydrated to obtain Na 2 SO 4Industrial anhydrous sodium sulfate product with 97.82% and NaCl 1.91%. The obtained industrial salt is washed and dried to obtain industrial salt containing 98.64% NaCl and Na 2 SO 4 1.02%. Due to the need for continuous dissolution and evaporation, the energy consumption in the separation and recovery process of sodium chloride and sodium sulfate is more than 10 times that of Example 1.

Claims

1. A method for preparing refined sodium chloride from crude sodium salt, characterized in that: Sodium-containing crude salt is dissolved in water to form a nearly saturated salt solution or a saturated salt solution, and ammonium chloride is added to the nearly saturated salt solution or the saturated salt solution under heating and stirring. When the ammonium chloride is dissolved until it reaches or is close to saturation, the ammonium chloride is used to - The common ion effect of ions forces Na + The ions are converted into NaCl, which is selectively crystallized and separated by filtration to obtain sodium chloride crystals and a salting-out liquid. The sodium chloride crystals are washed, filtered and dried to obtain refined sodium chloride. The salting-out liquid is cooled and crystallized to obtain ammonium chloride crystals, which are filtered and separated to obtain ammonium chloride crystals and an ammonium chloride crystallization mother liquor. The ammonium chloride crystals are returned to the salting-out step for recycling, and the ammonium chloride crystallization mother liquor is returned to the sodium-containing crude salt dissolution step for continued use until it has a significant inhibitory effect on the dissolution of the sodium-containing crude salt, and then the circuit is opened for comprehensive recovery.

2. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 1, wherein: The sodium-containing crude salt is a sodium salt solid mixture containing at least one of sodium chloride, sodium sulfate and sodium nitrate.

3. A method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 1, characterized in that: When the sodium-containing crude salt contains organic impurities and / or other inorganic impurities, it is necessary to perform impurity removal pretreatment before adding ammonium chloride, and the impurity removal pretreatment includes at least one of the following methods: When the sodium-containing crude salt contains organic impurities, the sodium-containing crude salt is subjected to pyrolysis or oxidation treatment; When the sodium-containing crude salt contains insoluble inorganic impurities, filtering the nearly saturated solution or saturated solution formed by dissolving the sodium-containing crude salt; When the sodium-containing crude salt contains water-soluble inorganic impurities, the nearly saturated solution or saturated solution formed by dissolving the sodium-containing crude salt is subjected to anion impurity removal treatment and cationic impurity removal treatment: the anion impurity removal treatment process is: adding iron salt and / or aluminum salt, and adjusting the pH of the solution system to 3.5-6.5, stirring for 0.5-1.5h, and removing anion impurities including phosphate and silicate; the cationic impurity removal treatment process is: using lime-soda method to remove Mg 2+ , Fe 3+ , Mn 2+ 、Zn 2+ and Cu 2+ The iron salt and / or aluminum salt comprises at least one of ferric sulfate, aluminum sulfate, ferric nitrate and aluminum nitrate.

4. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 3, wherein: The sodium-containing crude salt is dissolved at a temperature of 25 to 75° C. to obtain a nearly saturated solution or a saturated solution at the temperature; The temperature of the NaCl selective crystallization is 65-115°C.

5. A method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 4, characterized in that: The washing process of the sodium chloride crystals uses a saturated sodium chloride solution at 25 to 95° C. as a washing liquid, the washing solid-liquid ratio is 1:(0.5 to 5) g / mL, the pH of the washing liquid is adjusted to 8.5 to 10.5, and the washing time is 0.25 to 2.5 hours.

6. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 2, characterized in that: When the sodium-containing crude salt contains a solid mixture of at least one of sodium sulfate and sodium nitrate, the hindering effect comes from the enrichment of ammonium sulfate and / or ammonium nitrate.

7. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 6, wherein: For the ammonium chloride crystallization mother liquor after the enrichment of ammonium nitrate, the comprehensive recovery means first heating and concentrating, then cooling and crystallizing, filtering to obtain ammonium nitrate crystals and ammonium nitrate crystallization mother liquor; or; First, HCl gas is introduced into the ammonium chloride crystallization mother liquor, and then cooled or frozen for crystallization, and filtered to obtain ammonium chloride crystals and regenerated industrial nitric acid solution; or; First, add concentrated hydrochloric acid to the ammonium chloride crystallization mother liquor, then cool or freeze the crystallization, filter to obtain ammonium chloride crystals and regenerate industrial nitric acid solution; or; First, add sodium chloride to the ammonium chloride crystallization mother liquor, then cool or freeze to crystallize, filter to obtain ammonium chloride crystals and sodium nitrate solution, further evaporate and concentrate the sodium nitrate solution, and crystallize to precipitate sodium nitrate crystals.

8. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 6, characterized in that: The comprehensive recovery of the mother liquor of ammonium chloride crystallization after the hindering effect comes from the ammonium sulfate enrichment is to first add ammonium sulfate crystals according to the solubility of ammonium sulfate at this temperature, heat and stir to dissolve it, cool and crystallize it, filter to obtain ammonium chloride crystals, and then evaporate and concentrate the filtrate, cool and crystallize it to obtain ammonium sulfate crystals; or; A solution containing calcium chloride with a calcium chloride concentration of ≥10wt.% is added to the ammonium chloride crystallization mother liquor, the temperature is raised and stirred to precipitate sulfate, the precipitated calcium sulfate and the post-precipitation liquid are filtered, the post-precipitation liquid is cooled and crystallized, the ammonium chloride crystals and the crystallization mother liquor are filtered, and the ammonium chloride crystallization mother liquor is returned to the sodium-containing crude salt dissolution process for continued use.

9. The method for preparing refined sodium chloride from a sodium-containing crude salt according to claim 8, characterized in that: The precipitated calcium sulfate is at least one of calcium sulfate dihydrate, calcium sulfate hemihydrate and anhydrous calcium sulfate.

10. The method for preparing refined sodium chloride from crude sodium salt according to any one of claims 1 to 9, characterized in that: The utilization rate of Na in the sodium-containing crude salt is greater than 98%, and the grade of the obtained refined sodium chloride is greater than or equal to 99.5%.