Salt-containing solution purification method

Abstract

The invention provides a salt-containing solution purification method, and belongs to the technical field of wastewater treatment. Heavy metal ions in the salt-containing solution can enter an evaporative crystallization system after being subjected to the procedures of heavy metal removal, magnesium removal, calcium removal and the like, and sylvite and sodium salt with the purity of 99% are obtained. The method comprises the following specific process steps: 1, adjusting the pH value of a salt-containing solution to 7.5-8.5 by using an alkali solution, then adding a de-weighting agent, and carrying out solid-liquid separation after the reaction is completed to obtain a magnesium-containing solution; 2, adjusting the pH value of the magnesium-containing solution to 11-13 by using an alkali solution, and after the reaction is completed, carrying out solid-liquid separation to obtain a calcium-containing solution; 3, adding a precipitant into the calcium-containing solution, and obtaining an evaporation stock solution after precipitation is completed; 4, the evaporation stock solution enters an evaporation crystallization system, and potassium salt and sodium salt are obtained after two times of forced circulation. The method is simple in process and suitable for large-scale production, heavy metal ions in various salt-containing wastewater can be removed, potassium salt and sodium salt can be obtained, and deep purification and resource utilization of the salt-containing wastewater are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of wastewater treatment, and in particular to a method for purifying a saline solution. Background Art

[0002] Salty wastewater generally comes from domestic sewage and industrial wastewater. Wastewater with a salt content of more than 1% is called high-salt wastewater. These discharged wastewaters contain a large amount of Cl - 、SO4 2- , K + 、Na + , Ca 2+ Inorganic salt ions, especially in the chemical, pharmaceutical, petroleum and other industries, there are more saline wastewater, the components in the wastewater are more complex, and it is more difficult to treat. The direct discharge of these saline wastewaters will seriously affect the local ecosystem, so the treatment of saline wastewater is imperative. At the same time, if the potassium and sodium salts of saline wastewater can be extracted as industrial products, it will not only reduce resource waste and create economic benefits, but also solve the discharge of saline wastewater. It is the best way to achieve green and sustainable development in the chemical industry.

[0003] At present, there are three main methods for treating saline wastewater: thermal concentration, membrane treatment, and biological treatment. Thermal concentration mainly includes multiple-effect evaporation technology, mechanical steam regeneration technology, and wastewater incineration technology. Chinese patent CN114314978 A announced "a process for resource utilization and treatment of thermal cracking wastewater of organic solid hazardous waste in the steel industry". This method obtains saline wastewater by filtering and oil-water separation of thermal cracking wastewater of organic solid waste in the steel industry. The saline wastewater is evaporated by multiple effects to obtain crystal water and potassium and sodium salts. This method has a simple process and a large processing capacity, but it has high energy consumption, and the wastewater contains a large amount of heavy metal ions, which puts a large load on the equipment and is difficult to operate for a long time. In addition, the crystallized potassium and sodium salts contain heavy metals. Improper treatment will cause environmental pollution and affect the use of product salt. Chinese patent CN 117902597 B discloses "a method for preparing potassium chloride, a food additive, by MVR evaporation and crystallization". This method passes industrial potassium chloride solution through MVR evaporation and crystallization equipment after coarse filtration and fine filtration to obtain potassium chloride, a food additive. The purity of the prepared potassium chloride reaches 99.4-99.7%. However, the raw material used in this method is industrial-grade potassium chloride, which has poor treatment capacity for industrial wastewater with higher salt content. In addition, since the ions in the saline wastewater are more complex, it will cause scaling and blockage of the equipment, affecting the normal operation of the equipment. Chinese patent CN 114890604 B discloses "a salt-containing waste liquid treatment system and method based on liquid incineration technology". This system mixes high calorific value waste liquid with natural gas and sends it into the furnace for combustion to obtain crystalline miscellaneous salts. This method can burn and utilize organic matter in the wastewater, and can also utilize waste heat to achieve harmless treatment of wastewater. However, it is only suitable for saline wastewater with higher calorific value, and requires consumption of a large amount of fuels such as petroleum and natural gas, with large energy consumption and serious pollution. The crystalline salt components produced are numerous and mixed, and it is difficult to obtain efficient utilization. Compared with the thermal concentration method, the membrane treatment method has higher separation efficiency and lower energy consumption for high-salt wastewater. Different compositions of high-salt wastewater are suitable for different desalination processes, mainly including microfiltration (0.1-10 microns), ultrafiltration (0.01-0.1 microns), nanofiltration, reverse osmosis and electrodialysis. Chinese patent CN 113912226 A discloses a "treatment method and application of high-salt wastewater". This method uses ceramic membrane, ultrafiltration membrane and nanofiltration to filter high-salt wastewater, and then the filtered water is treated with electrodialysis membrane to obtain purified water. The hardness of the purified water is less than 2mg / L. However, this method has a small processing capacity, the membrane is easy to be contaminated, the anti-structural performance is poor, and the long-term operation and maintenance costs are high. It can only achieve wastewater reduction treatment, and the remaining wastewater still needs to be treated before it can be discharged.Chinese patent CN 105906147 A discloses "a biological purification method for industrial wastewater". This method uses physical and chemical methods to remove suspended solids and oil film in saline wastewater, and then dilutes the treated wastewater and introduces it into an artificial wetland planted with purification plants. The biologically purified wastewater can meet the discharge standards. However, this method takes a long time to process and is only applicable to wastewater with a low salt content. Wastewater with a high salt content needs to be diluted before it can be processed. The maintenance cost is high, and it usually requires a combination of physical and chemical methods for coordinated treatment. In addition, the inorganic salts cannot be recovered, resulting in a waste of resources.

[0004] In summary, the existing treatment technology has different treatment methods for saline wastewater with different compositions, which can treat saline wastewater to the discharge standard, but there are still certain limitations. It is mainly reflected in the following points: 1. The treatment depth is not enough, the composition of the saline solution is complex, and the treated saline wastewater has a high content of calcium and magnesium ions when entering the evaporation crystallization system, which will cause the equipment to scale and affect normal operation. At the same time, these impurity ions will also affect the purity of the salt output, making it difficult to fully utilize it; 2. The maintenance cost of the treatment equipment is high, especially the membrane treatment technology. The membrane is easy to clog, the treatment capacity is limited, and it cannot be completely treated. It can only be reduced, and the salt content of the remaining wastewater will be further enriched. 3. The scope of application is narrow, and it can only treat saline wastewater with a specific salt content, and the inorganic matter in the saline wastewater cannot be recycled, resulting in a waste of resources. Summary of the invention

[0005] The purpose of the present invention is to provide a method for purifying a salt solution, which is simple to operate, easy to control the process, has low equipment maintenance cost, is easy to mass produce, and the treated wastewater can be directly discharged or continued to be used as industrial water without secondary pollution.

[0006] The technical solution of the present invention is achieved in this way: The present invention provides a method for purifying saline wastewater, comprising saline wastewater → weight removal → magnesium removal → calcium removal → evaporation and crystallization to prepare potassium chloride and sodium chloride, and comprising the following key steps: Step 1: Use alkali solution to adjust the pH of the saline wastewater to 7.5-8.5, then add a deweighting agent, stir at room temperature for 0.5-1.5 hours, and then perform solid-liquid separation to obtain a magnesium-containing solution and a zinc-containing solid. The magnesium-containing solution enters the next step of magnesium removal, and the zinc-containing solid is rinsed 1-2 times at a liquid-solid ratio of 1-3:1. The rinsed solid is crude zinc hydroxide, which can be dissolved with acid and then extracted to recover zinc; Step 2: Use alkali solution to adjust the pH of the magnesium-containing solution to 11-13, stir at room temperature for 0.5-1.5 hours, and after the reaction is completed, perform solid-liquid separation to obtain a calcium-containing solution and a magnesium-containing solid. The calcium-containing solution enters the next calcium removal process, and the magnesium-containing solid is rinsed 1-2 times at a liquid-solid ratio of 1-3:1. The rinsed solid is crude magnesium hydroxide, which can be dissolved with acid and then extracted to recover magnesium; Step 3: adding a precipitant to the calcium-containing solution, stirring the reaction at room temperature for 0.5-1.5 hours, and then performing solid-liquid separation to obtain an evaporation stock solution and a calcium-containing solid. The evaporation stock solution and the calcium-containing solid are rinsed 1-2 times at a liquid-solid ratio of 1-3:1. The rinsed solid is the calcium carbonate product; Step 4: The evaporated stock solution is sent to the evaporation crystallization system. Potassium chloride is crystallized first during evaporation, and then sodium chloride is crystallized after 2-3 forced cycles.

[0007] As a further improvement of the present invention, the salt-containing wastewater used in step 1 is one or more of ore dressing wastewater, steel dust treatment water, and coal chemical wastewater; the alkali solution used is one or more of sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide solution and lime water; the concentration of the alkali solution used is 100-200g / L, and the addition method is dropwise addition under stirring; the deweighting agent used is one or more of sodium sulfide, organic sulfur, and polyaluminum chloride; the organic sulfur is trisodium thiocyanate solution, which is obtained by polymerization of cyanuric chloride, sodium thiocyanide, and sodium hydroxide as raw materials at a pH of 8-10; the amount of the deweighting agent added is 0.5-2.5mL / L.

[0008] As a further improvement of the present invention, the alkali solution used in step 2 is one or more of sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide solution and lime water; the concentration of the alkali solution used is 100-200g / L, and the addition method is dropwise addition under stirring.

[0009] As a further improvement of the present invention, the precipitant used in step 3 is one or more of sodium carbonate, sodium hydroxide, and carbon dioxide, and the amount of the precipitant added is 100-120% of the measured value.

[0010] As a further improvement of the present invention, the salt in the evaporation stock solution fed into the evaporation crystallization system in step 4 includes potassium chloride and sodium chloride, the chloride ion concentration in the solution is 2-6 mol / L, and the potassium-sodium ratio is 1.5-6.5:1.

[0011] As a further improvement of the present invention, the crystallizer used for evaporation in step 4 includes at least one of an MVR evaporator, a triple-effect evaporator, a TVR evaporator, and a low-temperature crystallization evaporator.

[0012] As a further improvement of the present invention, the deweighting agent used is modified trisodium thiocyanate, and the preparation method is as follows: S1. A trisodium thiocyanate solution is obtained by polymerization of cyanuric chloride, sodium thiocyanate and sodium hydroxide at a pH of 8-10; S2. Add silane coupling agent KH580 to the trisodium thiocyanate solution, heat and stir to react, remove the solvent under reduced pressure, wash, and dry to obtain modified trisodium thiocyanate.

[0013] As a further improvement of the present invention, the mass ratio of cyanuric chloride, sodium thiocyanate, sodium hydroxide and silane coupling agent KH580 is 10:3-5:1-2:2-3.

[0014] As a further improvement of the present invention, the temperature of the heating and stirring reaction is 35-45°C.

[0015] As a further improvement of the present invention, the heating and stirring reaction time is 1-2h.

[0016] The deweighting agent prepared by the present invention contains abundant active groups such as amino, thiol, hydroxyl and the like, and can remove a large amount of calcium, magnesium and heavy metal ions in saline wastewater by forming complex bonds, thereby greatly reducing the load of evaporation equipment. The purity of the obtained potassium and sodium salts can reach 99%. At the same time, there is a synergistic effect between the two raw materials of cyanuric chloride and sodium thiocyanide.

[0017] The present invention has the following beneficial effects: 1. It does not require coordinated treatment with other methods. A large amount of calcium, magnesium and heavy metal ions in saline wastewater can be removed by simply adding deweighting agents, precipitants and other methods, which greatly reduces the load of the evaporation equipment. The purity of the obtained potassium and sodium salts can reach 99%.

[0018] 2. Suitable for wastewater treatment with various salt contents. The higher the salt content, the better the treatment effect and the higher the economic value.

[0019] 3. The operation is simple, the process is easy to control, the equipment maintenance cost is low, and it is easy to produce on a large scale. The treated wastewater can be directly discharged or used as industrial water without secondary pollution. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] Preparation Example 1: Preparation of modified trisodium thiocyanate Here’s how: S1. 10g of cyanuric chloride, 3g of sodium thiocyanate and 1g of sodium hydroxide were used as raw materials and polymerized at pH 8 to obtain a trisodium thiocyanate solution; S2. Add 2 g of silane coupling agent KH580 to the trisodium thiocyanate solution, heat to 35°C, stir and react for 1 hour, remove the solvent under reduced pressure, wash and dry to obtain modified trisodium thiocyanate.

[0022] Preparation Example 2: Preparation of modified trisodium thiocyanate Here’s how: S1. 10g of cyanuric chloride, 5g of sodium thiocyanate and 2g of sodium hydroxide were used as raw materials and polymerized at pH 10 to obtain a trisodium thiocyanate solution; S2. Add 3 g of silane coupling agent KH580 to the trisodium thiocyanate solution, heat to 45°C, stir and react for 2 h, remove the solvent under reduced pressure, wash and dry to obtain modified trisodium thiocyanate.

[0023] Preparation Example 3: Preparation of modified trisodium thiocyanate Here’s how: S1. 10g of cyanuric chloride, 4g of sodium thiocyanate and 1.5g of sodium hydroxide were used as raw materials and polymerized at pH 9 to obtain a trisodium thiocyanate solution; S2. Add 2.5 g of silane coupling agent KH580 to the trisodium thiocyanate solution, heat to 40°C, stir and react for 1.5 hours, remove the solvent under reduced pressure, wash and dry to obtain modified trisodium thiocyanate.

[0024] Comparative Preparation Example 1 The difference between Preparation Example 1 and Preparation Example 3 is that no cyanuric chloride is added in step S1.

[0025] Here’s how: S1. 14g of sodium thiocyanate and 1.5g of sodium hydroxide were used as raw materials and polymerized at pH 9 to obtain a trisodium thiocyanate solution; S2. Add 2.5 g of silane coupling agent KH580 to the trisodium thiocyanate solution, heat to 40°C, stir and react for 1.5 hours, remove the solvent under reduced pressure, wash and dry to obtain a modified deweighting agent.

[0026] Comparative Preparation Example 2 Comparative Preparation Example 2 is different from Preparation Example 3 in that sodium thiocyanate is not added in step S1.

[0027] Here’s how: S1. 14g of cyanuric chloride and 1.5g of sodium hydroxide were used as raw materials and polymerized at pH 9 to obtain trisodium thiocyanate solution; S2. Add 2.5 g of silane coupling agent KH580 to the trisodium thiocyanate solution, heat to 40°C, stir and react for 1.5 hours, remove the solvent under reduced pressure, wash and dry to obtain a modified deweighting agent. Embodiment 1:

[0028] The composition of high-salt wastewater after treating sintering machine head ash in a steel plant is shown in Table 1: Table 1 Composition of high-salinity wastewater

[0029] As shown in Table 1, the potassium-sodium ratio in saline wastewater is 1.9:1, in which the content of metal ions such as calcium, magnesium, and zinc is relatively high. Prepare 100g / L lime water, then slowly add it into the saline wastewater, adjust the pH of the system to 7.5, and then add the modified trisodium thiocyanate prepared in Preparation Example 1 at a dosage of 0.5mL / L. Stir at room temperature for 0.5h and perform solid-liquid separation to obtain a magnesium-containing solution and a zinc-containing solid. The magnesium-containing liquid is subjected to the next step of magnesium removal, and the zinc-containing solid is rinsed once with distilled water at a liquid-solid ratio of 1:1, and then dissolved with a 1mol / L sulfuric acid solution to extract and recover zinc. The composition of the magnesium-containing solution is shown in Table 2 below: Table 2 Composition of magnesium-containing solution

[0030] As shown in Table 2, 100 g / L lime water was prepared and slowly added to the magnesium-containing solution, the pH of the system was adjusted to 11, and solid-liquid separation was performed after stirring at room temperature for 0.5 h to obtain a calcium-containing solution and a magnesium-containing solid. The calcium-containing solution was subjected to the next calcium removal process, and the magnesium-containing solid was rinsed once with distilled water at a liquid-solid ratio of 1:1, and magnesium was extracted and recovered after being dissolved with a 1 mol / L sulfuric acid solution. The composition of the magnesium-containing solution is shown in Table 3 below.

[0031] Table 3 Composition of calcium-containing solution

[0032] As shown in Table 3, sodium carbonate as a precipitant is added to the calcium-containing solution to remove calcium ions in the solution. The amount of sodium carbonate added is 100% of the measured value of calcium ions in the solution. After stirring for 0.5 hours, solid-liquid separation is performed to obtain an evaporation stock solution and a calcium-containing solid. The calcium-containing solid is rinsed once with distilled water at a liquid-solid ratio of 1:1 to obtain a calcium carbonate product. The composition of the evaporation stock solution is shown in Table 4 below.

[0033] Table 4 Composition of the evaporation stock solution

[0034] The evaporated stock solution is sent to the evaporation crystallization system, and the crystallizer used during evaporation is an MVR evaporator. During evaporation, potassium chloride is crystallized first, and then sodium chloride is crystallized after two forced cycles. Embodiment 2:

[0035] The composition of high-salt wastewater after treating sintering machine head ash in a steel plant is shown in Table 5: Table 5 Composition of high-salt wastewater

[0036] As shown in Table 5, the potassium-sodium ratio in the saline wastewater is 2.6:1, in which the content of metal ions such as calcium, magnesium, and zinc is relatively high. Prepare 150g / L lime water, then slowly add it to the saline wastewater, adjust the pH of the system to 8.0, and then add the modified trisodium thiocyanate prepared in Preparation Example 2 at a dosage of 1.5mL / L. After stirring at room temperature for 1h, perform solid-liquid separation to obtain a magnesium-containing solution and a zinc-containing solid. The magnesium-containing liquid undergoes the next step of magnesium removal, and the zinc-containing solid is rinsed twice with distilled water at a liquid-solid ratio of 2:1, and is dissolved with a 2mol / L sulfuric acid solution to extract and recover zinc. The composition of the magnesium-containing solution is shown in Table 6 below: Table 6 Composition of magnesium-containing solution

[0037] As shown in Table 6, 150 g / L lime water was prepared and slowly added to the magnesium-containing solution, the pH of the system was adjusted to 12, and solid-liquid separation was performed after stirring at room temperature for 1 hour to obtain a calcium-containing solution and a magnesium-containing solid. The calcium-containing solution was subjected to the next calcium removal process, and the magnesium-containing solid was rinsed twice with distilled water at a liquid-solid ratio of 2:1, and magnesium was extracted and recovered after being dissolved with a 2 mol / L sulfuric acid solution. The composition of the magnesium-containing solution is shown in Table 7 below.

[0038] Table 7 Composition of calcium-containing solution

[0039] As shown in Table 7, sodium carbonate as a precipitant is added to the calcium-containing solution to remove calcium ions in the solution. The amount of sodium carbonate added is 110% of the measured value of calcium ions in the solution. After stirring for 1 hour, solid-liquid separation is performed to obtain an evaporation stock solution and a calcium-containing solid. The calcium-containing solid is rinsed twice with distilled water at a liquid-solid ratio of 2:1 to obtain a calcium carbonate product. The composition of the evaporation stock solution is shown in Table 8 below.

[0040] Table 8 Composition of the evaporation stock solution

[0041] The evaporated stock solution is sent to the evaporation crystallization system, and the crystallizer used during evaporation is a triple-effect evaporator. During evaporation, potassium chloride is crystallized first, and sodium chloride is crystallized after three forced cycles. Embodiment 3:

[0042] The composition of high-salt wastewater after treating sintering machine head ash in a steel plant is shown in Table 9: Table 9 Composition of high-salt wastewater

[0043] As shown in Table 9, the potassium-sodium ratio in the saline wastewater is 2.9:1, in which the content of metal ions such as calcium, magnesium, and zinc is relatively high. Prepare 200g / L lime water, then slowly add it into the saline wastewater, adjust the pH of the system to 8.5, and then add the modified trisodium thiocyanate prepared in Preparation Example 3 at a dosage of 2.5mL / L. Stir at room temperature for 1.5h and perform solid-liquid separation to obtain a magnesium-containing solution and a zinc-containing solid. The magnesium-containing liquid is subjected to the next step of the magnesium removal process, and the zinc-containing solid is rinsed twice with distilled water at a liquid-solid ratio of 3:1, and then dissolved with a 3mol / L sulfuric acid solution to extract and recover zinc. The composition of the magnesium-containing solution is shown in Table 10 below: Table 10 Composition of magnesium-containing solution

[0044] As shown in Table 10, 200 g / L lime water was prepared and slowly added to the magnesium-containing solution, the pH of the system was adjusted to 13, and solid-liquid separation was performed after stirring at room temperature for 1.5 hours to obtain a calcium-containing solution and a magnesium-containing solid. The calcium-containing solution was subjected to the next calcium removal process, and the magnesium-containing solid was rinsed twice with distilled water at a liquid-solid ratio of 3:1, and then dissolved with a 3 mol / L sulfuric acid solution to extract and recover magnesium. The composition of the calcium-containing solution is shown in Table 11 below.

[0045] Table 11 Composition of calcium-containing solution

[0046] As shown in Table 11, sodium carbonate as a precipitant is added to the calcium-containing solution to remove calcium ions in the solution. The amount of sodium carbonate added is 120% of the measured value of calcium ions in the solution. After stirring for 1.5 hours, solid-liquid separation is performed to obtain an evaporation stock solution and a calcium-containing solid. The calcium-containing solid is rinsed twice with distilled water at a liquid-solid ratio of 3:1 to obtain a calcium carbonate product. The composition of the evaporation stock solution is shown in Table 12.

[0047] Table 12 Composition of the evaporation stock solution

[0048] The evaporated stock solution is sent to the evaporation crystallization system, and the crystallizer used during evaporation is a TVR evaporator. During evaporation, potassium chloride is crystallized first, and sodium chloride is crystallized after three forced cycles.

[0049] Comparative Example 1 The difference between Comparative Example 1 and Example 3 is that the weight-removing agent is prepared by Comparative Preparation Example 1.

[0050] The composition of the magnesium-containing solution is shown in Table 13 below.

[0051] Table 13 Composition of magnesium-containing solution

[0052] The composition of the calcium-containing solution is shown in Table 14 below.

[0053] Table 14 Composition of calcium-containing solution

[0054] The composition of the evaporated stock solution is shown in Table 15 below.

[0055] Table 15 Composition of the evaporation stock solution

[0056] The evaporated stock solution is sent to the evaporation crystallization system, and the crystallizer used during evaporation is a TVR evaporator. During evaporation, potassium chloride is crystallized first, and sodium chloride is crystallized after three forced cycles.

[0057] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is that the weight-removing agent is prepared by Comparative Preparation Example 2.

[0058] The composition of the magnesium-containing solution is shown in Table 16 below.

[0059] Table 16 Composition of magnesium-containing solution

[0060] The composition of the calcium-containing solution is shown in Table 17 below.

[0061] Table 17 Composition of calcium-containing solution

[0062] The composition of the evaporated stock solution is shown in Table 18 below.

[0063] Table 18 Composition of the evaporation stock solution

[0064] The evaporated stock solution is sent to the evaporation crystallization system, and the crystallizer used during evaporation is a TVR evaporator. During evaporation, potassium chloride is crystallized first, and sodium chloride is crystallized after three forced cycles.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for purifying saline wastewater, comprising: saline wastewater → weight removal → magnesium removal → calcium removal → evaporation and crystallization to prepare potassium chloride and sodium chloride, characterized in that: The key steps include: Step 1: Use alkali solution to adjust the pH of the saline wastewater to 7.5-8.5, then add a deweighting agent, stir at room temperature for 0.5-1.5 hours, and then perform solid-liquid separation to obtain a magnesium-containing solution and a zinc-containing solid. The magnesium-containing solution enters the next step of magnesium removal, and the zinc-containing solid is rinsed 1-2 times at a liquid-solid ratio of 1-3:

1. The rinsed solid is crude zinc hydroxide, which can be dissolved with acid and then extracted to recover zinc; Step 2: Use alkali solution to adjust the pH of the magnesium-containing solution to 11-13, stir at room temperature for 0.5-1.5 hours, and after the reaction is completed, perform solid-liquid separation to obtain a calcium-containing solution and a magnesium-containing solid. The calcium-containing solution enters the next calcium removal process, and the magnesium-containing solid is rinsed 1-2 times at a liquid-solid ratio of 1-3:

1. The rinsed solid is crude magnesium hydroxide, which can be dissolved with acid and then extracted to recover magnesium; Step 3: adding a precipitant to the calcium-containing solution, stirring the reaction at room temperature for 0.5-1.5 hours, and then performing solid-liquid separation to obtain an evaporation stock solution and a calcium-containing solid. The evaporation stock solution and the calcium-containing solid are rinsed 1-2 times at a liquid-solid ratio of 1-3:

1. The rinsed solid is the calcium carbonate product; Step 4: The evaporated stock solution is sent to the evaporation crystallization system. Potassium chloride is crystallized first during evaporation, and then sodium chloride is crystallized after 2-3 forced cycles.

2. The method according to claim 1, characterized in that: The salt-containing wastewater used in step 1 is one or more of ore dressing wastewater, steel dust treatment water, and coal chemical wastewater; the alkali solution used is one or more of sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide solution, and lime water; the concentration of the alkali solution used is 100-200g / L, and the addition method is dropwise addition under stirring; the deweighting agent used is one or more of sodium sulfide, organic sulfur, and polyaluminum chloride; the organic sulfur is trisodium thiocyanate solution, which is obtained by polymerization of cyanuric chloride, sodium thiocyanide, and sodium hydroxide as raw materials at a pH of 8-10; the amount of the deweighting agent added is 0.5-2.5mL / L.

3. The method according to claim 1, characterized in that: The alkali solution used in step 2 is one or more of sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide solution and lime water; the concentration of the alkali solution used is 100-200g / L, and the addition method is dropwise addition under stirring.

4. The method according to claim 1, characterized in that: The precipitant used in step 3 is one or more of sodium carbonate, sodium hydroxide, and carbon dioxide, and the amount of the precipitant added is 100-120% of the measured value.

5. The method according to claim 1, characterized in that: The salt in the evaporation stock solution fed into the evaporation crystallization system in step 4 includes potassium chloride and sodium chloride, the chloride ion concentration in the solution is 2-6 mol / L, and the potassium-sodium ratio is 1.5-6.5:

1.

6. The method according to claim 1, characterized in that: The crystallizer used for evaporation in step 4 includes at least one of an MVR evaporator, a triple-effect evaporator, a TVR evaporator, and a low-temperature crystallization evaporator.

7. The method according to claim 2, characterized in that: The deweighting agent used is modified trisodium thiocyanate, and the preparation method is as follows: S1. A trisodium thiocyanate solution is obtained by polymerization of cyanuric chloride, sodium thiocyanate and sodium hydroxide at a pH of 8-10; S2. Add silane coupling agent KH580 to the trisodium thiocyanate solution, heat and stir to react, remove the solvent under reduced pressure, wash, and dry to obtain modified trisodium thiocyanate.

8. The method according to claim 7, characterized in that: The mass ratio of cyanuric chloride, sodium thiocyanate, sodium hydroxide and silane coupling agent KH580 is 10:3-5:1-2:2-3.

9. The method according to claim 7, characterized in that: The temperature of the heating and stirring reaction is 35-45°C.

10. The method according to claim 7, characterized in that: The heating and stirring reaction time is 1-2h.

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