Process method for purifying sodium sulfate and co-producing potassium-sodium mixed salt

Abstract

The invention discloses a process method for purifying sodium sulfate and co-producing potassium-sodium mixed salt. The process method comprises the following steps: step 1, pretreating byproduct sodium sulfate; 2, carrying out solid-liquid separation treatment; step 3, solid matter sodium sulfate treatment; step 4, concentration treatment; the mixed salt obtained in the step 4 is subjected to solid-liquid separation to obtain a potassium-sodium mixed crystal substance and a crystallization mother solution, the potassium-sodium mixed crystal substance is the co-produced potassium-sodium mixed salt, and the remaining crystallization mother solution is returned to the step 4 to be circularly evaporated and crystallized; 6, precious metal salt is recycled and prepared. The method has the advantages that the byproduct sodium sulfate is used as a raw material, potassium is extracted from the byproduct sodium sulfate, the sodium sulfate with higher purity is obtained, the purified sodium sulfate product sodium sulfate meets the quality requirements of GB / T6009-2014 sodium sulfate second-class qualified products or above, the extraction rate can reach 95% or above, the prepared potassium-sodium mixed salt can be used as a raw material for preparing potassium fertilizer, and the potassium-sodium mixed salt is suitable for industrial production. Further, an opportunity is provided for cyclic utilization of potassium.

Description

Technical Field

[0001] The invention relates to a process for purifying sodium sulfate and co-produced potassium-sodium mixed salt, and in particular to a process for purifying sodium sulfate and co-produced potassium-sodium mixed salt from lithium carbonate by-product salt, belonging to the technical field of resource recycling. Background Art

[0002] With the development of economy, the demand for lithium carbonate and lithium hydroxide is increasing day by day. The production of lithium carbonate and lithium hydroxide produces a large amount of sodium sulfate as a by-product. Yichun city alone produces millions of tons of sodium sulfate products as a by-product every year. However, the by-product sodium sulfate is currently sold directly and has a low added value.

[0003] The main uses of the product sodium sulfate are in the detergent, glass, printing and dyeing industries. The consumption at home and abroad has increased year by year. In particular, technological advances in recent years have given it new application areas, such as feed grade, pharmaceutical grade, ultrafine grade, color grade, etc. It is also used in the battery industry, energy storage industry, etc.

[0004] Potassium is one of the three elements of fertilizer. The potassium content in plants generally accounts for 0.2% to 4.1% of the dry matter weight, second only to nitrogen. During the growth and development of plants, potassium participates in the activation of more than 60 enzyme systems, photosynthesis, the transportation of assimilated products, the metabolism of carbohydrates and the synthesis of proteins. China is both a major agricultural country and a potassium-poor country. China's exploitable potassium reserves account for about 9% of the world's total, and its consumption accounts for about 26% of the world's total. In 2020, the annual demand for potassium salts exceeded 10 million tons, with a self-sufficiency rate of less than 60%, and a high degree of dependence on imports. Domestic potash fertilizers are mainly produced from potassium-containing minerals, salt lake resources, seawater bitterns and other minerals as raw materials to produce potassium sulfate and potassium-magnesium sulfate fertilizers, which consume non-renewable mineral resources and are bound to affect the ecological environment where the resources are located.

[0005] Therefore, how to utilize the by-product sodium sulfate as a resource and co-produce potassium-sodium mixed salt is a topic worthy of study. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a process for purifying sodium sulfate and co-producing a sodium-potassium mixed salt, extracting sodium and potassium elements from the by-product sodium sulfate and preparing a sodium sulfate and sodium-potassium mixture, and converting potassium from ore into potassium salt for use in industry or agriculture, thereby realizing the recycling of potassium resources.

[0007] The present invention is achieved by the following scheme: a process for purifying sodium sulfate and co-produced potassium-sodium mixed salt, using by-product sodium sulfate as raw material to purify sodium sulfate and co-produced potassium-sodium mixed salt, which comprises the following steps:

[0008] Step 1: Pretreatment of by-product sodium sulfate: After crushing and screening the by-product sodium sulfate, the by-product sodium sulfate is washed with a saturated sodium sulfate solution to obtain a saturated by-product sodium sulfate solution;

[0009] Step 2, solid-liquid separation treatment: the by-product sodium sulfate saturated solution obtained in step 1 is subjected to solid-liquid separation to obtain a wet sodium sulfate and saturated potassium sodium mixture solution;

[0010] Step 3: solid sodium sulfate treatment: drying, sieving and packaging the wet sodium sulfate obtained in step 2;

[0011] Step 4: Concentration treatment: After precipitating and removing impurities and adjusting the pH value of the saturated potassium-sodium mixture solution obtained in step 2, evaporation and crystallization are performed to obtain a mixed salt;

[0012] Step 5, preparation of potassium-sodium mixed salt: passing the mixed salt obtained in step 4 through a two-stage pusher centrifuge to obtain potassium-sodium mixed crystals and crystallization mother liquor after solid-liquid separation. The potassium-sodium mixed crystals are the co-produced potassium-sodium mixed salt, and the remaining crystallization mother liquor is returned to step 4 for cyclic evaporation and crystallization;

[0013] Step 6: Recovering and preparing precious metal salts: Recovering the precious metal salts from the crystallization mother liquor obtained in step 5.

[0014] The by-product sodium sulfate product in the step 1 is crushed and screened by a double-stage crushing and hammer crusher, and then screened by a 16-mesh rotary vibrating screen.

[0015] In the step 1, washing is performed by stirring, the stirring is performed by upturning and down pressing, and the stirring speed is 40-60r / min.

[0016] The washed sodium sulfate solution in step 2 is passed through a two-stage pusher centrifuge with a rotation speed of 1800-2000 r / min to separate the solid sodium sulfate and the saturated potassium-sodium mixture solution.

[0017] The moisture content of the sodium sulfate separated in step 2 is ≤4%, and the saturated potassium-sodium mixture solution is sent to the impurity removal process.

[0018] In the step 3, the wet sodium sulfate after centrifugal separation is heated by a drum dryer, an air flow dryer or a fluidized bed dryer.

[0019] In the step 3, the heating temperature in front of the furnace is controlled at 220-240° C., the temperature at the end of the furnace is controlled at 100-110° C., the furnace speed is controlled at 40-60 r / min, and the moisture content of the finished sodium sulfate is ≤0.05%.

[0020] In step 4, the saturated potassium-sodium mixture solution after precipitation and removal of heavy metals is evaporated and crystallized in an MVR evaporator.

[0021] The concentration treatment in step 4 adopts MVR evaporator for cyclic evaporation concentration and cyclic crystallization, and the yield of potassium and magnesium sulfate cyclic crystallization reaches more than 90%. The mother liquor returns to the system for cyclic evaporation and crystallization to produce mixed salt.

[0022] In step six, after the Li+, Rb+, and Cs+ ions in the crystallization mother liquor are enriched, the precious metal salt compound is recovered by precipitation reaction or resin adsorption.

[0023] The beneficial effects of the present invention are:

[0024] 1. The present invention uses by-product sodium sulfate as a raw material to extract potassium from it, thereby obtaining sodium sulfate with higher purity. The first step of deep processing of by-product sodium sulfate is to wash the by-product sodium sulfate, that is, the by-product sodium sulfate needs to be washed with a saturated sodium sulfate solution to separate solid sodium sulfate and soluble potassium-sodium mixed salt, and separate crystalline sodium sulfate and potassium-sodium mixed solution before subsequent processing. In the washing process, the soluble potassium element in the by-product sodium sulfate is dissolved into the washing solution together with sodium sulfate and other components, thereby providing an opportunity for the recycling of potassium;

[0025] 2. The present invention extracts potassium from the by-product sodium sulfate to prepare a potassium-sodium mixture, realizes the recycling of potassium, reduces the consumption of mineral resources and salt lake resources, achieves carbon neutrality, protects the ecological environment, and makes up for the dependence on the import of potassium raw materials to a certain extent, which is of great significance to China's agricultural development and environmental protection;

[0026] 3. The present invention provides a process route for extracting potassium from by-product sodium sulfate soaking liquid to obtain sodium sulfate and potassium-sodium mixed salt, that is, realizing the extraction of solid waste sodium sulfate of lithium carbonate → harvesting industrial by-product sodium sulfate and potassium-sodium mixed salt → recycling potassium element that can be used for potash fertilizer, reducing the consumption of mineral resources, achieving carbon neutrality, and to some extent making up for the dependence on import of potassium raw materials, which is of great significance to China's agricultural development and environmental protection;

[0027] 4. The process method of the present invention can purify sodium sulfate and extract potassium-sodium mixed salt from the by-product sodium sulfate. The purified sodium sulfate product meets the quality requirements of Class II qualified products of GB / T 6009-2014 sodium sulfate and the extraction rate can reach more than 95%. The prepared potassium-sodium mixed salt can be used as a raw material for preparing potash fertilizer;

[0028] 5. The present invention prepares sodium sulfate and potassium-sodium mixed salt by purifying the by-product sodium sulfate, thereby realizing the recycling of potassium element in crop planting, agricultural product processing, and potassium element extraction and recovery to manufacture potash fertilizer;

[0029] 6. The present invention includes the steps of pre-treating the sodium sulfate extract, separating the sodium sulfate and potassium-sodium mixed solution, evaporating, concentrating, crystallizing, and drying, so as to separate the potassium-containing sodium sulfate from high-value ions such as potassium and potassium-sodium mixed salt, and improve the purity of the sodium sulfate crystal from 90% to 95% to 98% to 99%;

[0030] 7. The potassium sulfate and sodium sulfate mixed solution of the present invention is separated and purified by a centrifuge, and then concentrated and crystallized to directly obtain a potassium-sodium mixture. Compared with the traditional salt lake brine process to generate sodium sulfate and then undergo a complex water-salt conversion process to obtain the finished sodium sulfate, the process flow is shortened, the single process yield is improved, and the cost is reduced;

[0031] 8. The present invention belongs to resource recycling technology and application, which is characterized by wide sources of raw materials, good quality and low secondary processing cost;

[0032] 9. The present invention removes potassium salt from the raw materials by washing; removes harmful impurities such as thallium, beryllium, and fluorine; prepares potassium and sodium mixed salts used in potassium sulfate production; recovers enriched lithium, rubidium, cesium and other precious metal compounds, and ensures that the product quality meets the national standards at the lowest cost, thereby achieving better economic benefits;

[0033] 10. The present invention is a resource utilization method, with no discharge of waste water, waste gas or waste residue, and no environmental pollution;

[0034] 11. The process of the present invention is short, the product quality is excellent, and the recovery rate is high. Compared with the traditional process, there are only three processes: washing, drying, and evaporation. Because of the small number of processes, the energy consumption is low, the control is simple, and the labor is also less;

[0035] 12. The investment of the present invention is only about 12% of the traditional process of the same scale, and the economic benefits are obvious. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The present invention is a schematic flow diagram of a process for purifying sodium sulfate and co-producing a potassium-sodium mixed salt. DETAILED DESCRIPTION

[0037] Combine the following Figure 1 The present invention is further described, but the protection scope of the present invention is not limited to the described contents.

[0038] For the sake of clarity, not all features of an actual embodiment are described. In the following description, well-known functions and structures are not described in detail because they would obscure the invention with unnecessary detail. It should be recognized that in the development of any actual embodiment, a large number of implementation details must be made to achieve the specific goals of the developer, such as changing from one embodiment to another according to relevant system or relevant business limitations. In addition, it should be recognized that such development work may be complex and time-consuming, but it is merely routine work for those skilled in the art.

[0039] Example 1: A process for purifying sodium sulfate and co-produced potassium-sodium mixed salt, using by-product sodium sulfate as raw material to purify sodium sulfate and co-produced potassium-sodium mixed salt, which comprises the following steps:

[0040] Step 1, pretreatment of by-product sodium sulfate: after crushing and screening the by-product sodium sulfate, wash the by-product sodium sulfate with a saturated sodium sulfate solution to obtain a saturated solution of by-product sodium sulfate, the insoluble sodium sulfate solid is used for sodium sulfate production and processing, and the saturated solution of the by-product sodium sulfate after washing is collected for extracting potassium-sodium mixed salt;

[0041] Step 2, solid-liquid separation treatment: the by-product sodium sulfate saturated solution obtained in step 1 is subjected to solid-liquid separation to obtain a wet sodium sulfate and saturated potassium sodium mixture solution;

[0042] Step 3: solid sodium sulfate treatment: drying, sieving and packaging the wet sodium sulfate obtained in step 2;

[0043] Step 4, concentration treatment: the saturated potassium-sodium mixture solution obtained in step 2 is subjected to precipitation and impurity removal and pH adjustment according to the content of impurity ions such as thallium and fluorine therein, and then evaporated and crystallized to obtain a mixed salt;

[0044] Step 5, preparation of potassium-sodium mixed salt: passing the mixed salt obtained in step 4 through a two-stage pusher centrifuge to obtain potassium-sodium mixed crystals and crystallization mother liquor after solid-liquid separation. The potassium-sodium mixed crystals are the co-produced potassium-sodium mixed salt, which is sold externally, and the remaining crystallization mother liquor is returned to step 4 for cyclic evaporation and crystallization;

[0045] Step 6: Recovering and preparing precious metal salts: Recovering the precious metal salts from the crystallization mother liquor obtained in step 5.

[0046] In step 1, the by-product sodium sulfate product is crushed and screened by a double-stage crusher plus a hammer crusher, and then screened by a 16-mesh rotary vibrating screen to meet the material requirements.

[0047] In step 1, washing is performed by stirring, the stirring is performed by upturning and down pressing, and the stirring speed is 40 r / min.

[0048] The washed sodium sulfate solution in step 2 is passed through a two-stage pusher centrifuge with a rotation speed of 1800 r / min to separate the solid sodium sulfate and the saturated potassium-sodium mixture solution.

[0049] The moisture content of the sodium sulfate after separation in step 2 is 3.5%, and the saturated potassium-sodium mixture solution is sent to the impurity removal process.

[0050] In step three, the wet sodium sulfate after centrifugal separation is heated by a drum dryer, an air flow dryer or a fluidized bed dryer.

[0051] In step 3, the heating temperature in front of the furnace is controlled at 220°C, the temperature at the tail of the furnace is controlled at 100°C, the furnace speed is controlled at 40r / min, and the moisture content of the finished sodium sulfate is 0.04%, which meets the index requirements of Class I, second-class products and above in GB / T 6009-2014.

[0052] Step 4: The saturated potassium-sodium mixture solution after precipitation and removal of heavy metals is evaporated and crystallized in an MVR evaporator. The working process is as follows: the secondary steam generated by the saturated solution in the MVR evaporator is compressed by the compressor to increase its temperature and pressure. The compressed high-temperature and high-pressure steam is sent to the shell-and-tube heat exchanger to heat the liquid to be evaporated. In this way, the secondary steam that was originally to be discarded is fully utilized, the latent heat is recovered, and the thermal efficiency is improved. The key components of the MVR evaporator include steam compressor, plate-type lifting film evaporator, gas-liquid separator and pipeline system. It can significantly reduce the demand for external energy and improve energy utilization efficiency.

[0053] The concentration treatment in step 4 adopts MVR evaporator for cyclic evaporation concentration and cyclic crystallization. The yield of potassium sulfate and magnesium sulfate cyclic crystallization reaches more than 90% based on potassium sulfate. The mother liquor returns to this system for cyclic evaporation and crystallization to produce mixed salt.

[0054] Step 6: After the Li+, Rb+, and Cs+ ions in the crystallization mother liquor are enriched, the precious metal salt compound is recovered by precipitation reaction or resin adsorption.

[0055] Example 2: A process for purifying sodium sulfate and co-produced potassium-sodium mixed salt, using by-product sodium sulfate as raw material to purify sodium sulfate and co-produced potassium-sodium mixed salt, which comprises the following steps:

[0056] Step 1, pretreatment of by-product sodium sulfate: after crushing and screening the by-product sodium sulfate, wash the by-product sodium sulfate with a saturated sodium sulfate solution to obtain a saturated solution of by-product sodium sulfate, the insoluble sodium sulfate solid is used for sodium sulfate production and processing, and the saturated solution of the by-product sodium sulfate after washing is collected for extracting potassium-sodium mixed salt;

[0057] Step 2, solid-liquid separation treatment: the by-product sodium sulfate saturated solution obtained in step 1 is subjected to solid-liquid separation to obtain a wet sodium sulfate and saturated potassium sodium mixture solution;

[0058] Step 3: solid sodium sulfate treatment: drying, sieving and packaging the wet sodium sulfate obtained in step 2;

[0059] Step 4, concentration treatment: the saturated potassium-sodium mixture solution obtained in step 2 is subjected to precipitation and impurity removal and pH adjustment according to the content of impurity ions such as thallium and fluorine therein, and then evaporated and crystallized to obtain a mixed salt;

[0060] Step 5, preparation of potassium-sodium mixed salt: passing the mixed salt obtained in step 4 through a two-stage pusher centrifuge to obtain potassium-sodium mixed crystals and crystallization mother liquor after solid-liquid separation. The potassium-sodium mixed crystals are the co-produced potassium-sodium mixed salt, which is sold externally, and the remaining crystallization mother liquor is returned to step 4 for cyclic evaporation and crystallization;

[0061] Step 6: Recovering and preparing precious metal salts: Recovering the precious metal salts from the crystallization mother liquor obtained in step 5.

[0062] In step 1, the by-product sodium sulfate product is crushed and screened by a double-stage crusher plus a hammer crusher, and then screened by a 16-mesh rotary vibrating screen to meet the material requirements.

[0063] In step 1, washing is performed by stirring, the stirring is performed by upturning and down pressing, and the stirring speed is 60r / min.

[0064] The washed sodium sulfate solution in step 2 is passed through a two-stage pusher centrifuge with a rotation speed of 2000 r / min to separate the solid sodium sulfate and the saturated potassium-sodium mixture solution.

[0065] The moisture content of the sodium sulfate after separation in step 2 is ≤4%, and the saturated potassium-sodium mixture solution is sent to the impurity removal process.

[0066] In step three, the wet sodium sulfate after centrifugal separation is heated by a drum dryer, an air flow dryer or a fluidized bed dryer.

[0067] In step 3, the heating temperature in front of the furnace is controlled at 240°C, the temperature at the tail of the furnace is controlled at 110°C, the furnace speed is controlled at 60r / min, and the moisture content of the finished sodium sulfate is ≤0.05%, which meets the index requirements of Class I, second-class products and above in GB / T 6009-2014.

[0068] Step 4: The saturated potassium-sodium mixture solution after precipitation and removal of heavy metals is evaporated and crystallized in an MVR evaporator. The working process is as follows: the secondary steam generated by the saturated solution in the MVR evaporator is compressed by the compressor to increase its temperature and pressure. The compressed high-temperature and high-pressure steam is sent to the shell-and-tube heat exchanger to heat the liquid to be evaporated. In this way, the secondary steam that was originally to be discarded is fully utilized, the latent heat is recovered, and the thermal efficiency is improved. The key components of the MVR evaporator include steam compressor, plate-type lifting film evaporator, gas-liquid separator and pipeline system. It can significantly reduce the demand for external energy and improve energy utilization efficiency.

[0069] The concentration treatment in step 4 adopts MVR evaporator for cyclic evaporation concentration and cyclic crystallization. The yield of potassium sulfate and magnesium sulfate cyclic crystallization reaches more than 90% based on potassium sulfate. The mother liquor returns to this system for cyclic evaporation and crystallization to produce mixed salt.

[0070] Step 6: After the Li+, Rb+, and Cs+ ions in the crystallization mother liquor are enriched, the precious metal salt compound is recovered by precipitation reaction or resin adsorption.

[0071] Example 3: A process for purifying sodium sulfate and co-produced potassium-sodium mixed salt, using by-product sodium sulfate as raw material to purify sodium sulfate and co-produced potassium-sodium mixed salt, which comprises the following steps:

[0072] Step 1, pretreatment of by-product sodium sulfate: after crushing and screening the by-product sodium sulfate, wash the by-product sodium sulfate with a saturated sodium sulfate solution to obtain a saturated solution of by-product sodium sulfate, the insoluble sodium sulfate solid is used for sodium sulfate production and processing, and the saturated solution of the by-product sodium sulfate after washing is collected for extracting potassium-sodium mixed salt;

[0073] Step 2, solid-liquid separation treatment: the by-product sodium sulfate saturated solution obtained in step 1 is subjected to solid-liquid separation to obtain a wet sodium sulfate and saturated potassium sodium mixture solution;

[0074] Step 3: solid sodium sulfate treatment: drying, sieving and packaging the wet sodium sulfate obtained in step 2;

[0075] Step 4, concentration treatment: the saturated potassium-sodium mixture solution obtained in step 2 is subjected to precipitation and impurity removal and pH adjustment according to the content of impurity ions such as thallium and fluorine therein, and then evaporated and crystallized to obtain a mixed salt;

[0076] Step 5, preparation of potassium-sodium mixed salt: passing the mixed salt obtained in step 4 through a two-stage pusher centrifuge to obtain potassium-sodium mixed crystals and crystallization mother liquor after solid-liquid separation. The potassium-sodium mixed crystals are the co-produced potassium-sodium mixed salt, which is sold externally, and the remaining crystallization mother liquor is returned to step 4 for cyclic evaporation and crystallization;

[0077] Step 6: Recovering and preparing precious metal salts: Recovering the precious metal salts from the crystallization mother liquor obtained in step 5.

[0078] In step 1, the by-product sodium sulfate product is crushed and screened by a double-stage crusher plus a hammer crusher, and then screened by a 16-mesh rotary vibrating screen to meet the material requirements.

[0079] In step 1, washing is performed by stirring, the stirring is performed by upturning and down pressing, and the stirring speed is 50 r / min.

[0080] The washed sodium sulfate solution in step 2 is passed through a two-stage pusher centrifuge with a rotation speed of 1900 r / min to separate the solid sodium sulfate and the saturated potassium-sodium mixture solution.

[0081] The moisture content of the separated sodium sulfate in step 2 is 4%, and the saturated potassium-sodium mixture solution is sent to the impurity removal process.

[0082] In step three, the wet sodium sulfate after centrifugal separation is heated by a drum dryer, an air flow dryer or a fluidized bed dryer.

[0083] In step 3, the heating temperature in front of the furnace is controlled at 230°C, the temperature at the tail of the furnace is controlled at 105°C, the furnace speed is controlled at 50r / min, and the moisture content of the finished sodium sulfate is 0.05%, which meets the index requirements of Class I, second-class products and above in GB / T 6009-2014.

[0084] Step 4: The saturated potassium-sodium mixture solution after precipitation and removal of heavy metals is evaporated and crystallized in an MVR evaporator. The working process is as follows: the secondary steam generated by the saturated solution in the MVR evaporator is compressed by the compressor to increase its temperature and pressure. The compressed high-temperature and high-pressure steam is sent to the shell-and-tube heat exchanger to heat the liquid to be evaporated. In this way, the secondary steam that was originally to be discarded is fully utilized, the latent heat is recovered, and the thermal efficiency is improved. The key components of the MVR evaporator include steam compressor, plate-type lifting film evaporator, gas-liquid separator and pipeline system. It can significantly reduce the demand for external energy and improve energy utilization efficiency.

[0085] The concentration treatment in step 4 adopts MVR evaporator for cyclic evaporation concentration and cyclic crystallization. The yield of potassium sulfate and magnesium sulfate cyclic crystallization reaches more than 90% based on potassium sulfate. The mother liquor returns to this system for cyclic evaporation and crystallization to produce mixed salt.

[0086] Step 6: After the Li+, Rb+, and Cs+ ions in the crystallization mother liquor are enriched, the precious metal salt compound is recovered by precipitation reaction or resin adsorption.

[0087] Although the technical solutions of the present invention have been described and listed in detail, it should be understood that it is obvious to those skilled in the art to make modifications to the above embodiments or adopt equivalent alternatives. These modifications or improvements made without departing from the spirit of the present invention are within the scope of protection required by the present invention.

Claims

1. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt, characterized in that: Using by-product sodium sulfate as raw material, purifying sodium sulfate and co-producing potassium-sodium mixed salt, which includes the following steps: Step 1: Pretreatment of by-product sodium sulfate: After crushing and screening the by-product sodium sulfate, the by-product sodium sulfate is washed with a saturated sodium sulfate solution to obtain a saturated by-product sodium sulfate solution; Step 2, solid-liquid separation treatment: the by-product sodium sulfate saturated solution obtained in step 1 is subjected to solid-liquid separation to obtain a wet sodium sulfate and saturated potassium sodium mixture solution; Step 3: solid sodium sulfate treatment: drying, sieving and packaging the wet sodium sulfate obtained in step 2; Step 4: Concentration treatment: After precipitating and removing impurities from the saturated potassium-sodium mixture solution obtained in step 2 and adjusting the pH value, evaporation and crystallization are performed to obtain a mixed salt; Step 5, preparation of potassium-sodium mixed salt: passing the mixed salt obtained in step 4 through a two-stage pusher centrifuge to obtain potassium-sodium mixed crystals and crystallization mother liquor after solid-liquid separation. The potassium-sodium mixed crystals are the co-produced potassium-sodium mixed salt, and the remaining crystallization mother liquor is returned to step 4 for cyclic evaporation and crystallization; Step 6: Recovering and preparing precious metal salts: Recovering the precious metal salts from the crystallization mother liquor obtained in step 5.

2. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: The by-product sodium sulfate product in the step 1 is crushed and screened by a double-stage crushing and hammer crusher, and then screened by a 16-mesh rotary vibrating screen.

3. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: In the step 1, washing is performed by stirring, the stirring is performed by upturning and down pressing, and the stirring speed is 40-60r / min.

4. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: The washed sodium sulfate solution in step 2 is passed through a two-stage pusher centrifuge with a rotation speed of 1800-2000 r / min to separate the solid sodium sulfate and the saturated potassium-sodium mixture solution.

5. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: The moisture content of the separated sodium sulfate in step 2 is ≤4%, and the saturated potassium-sodium mixture solution is sent to the impurity removal process.

6. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: In the step 3, the wet sodium sulfate after centrifugal separation is heated by a drum dryer, an air flow dryer or a fluidized bed dryer.

7. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: In the step 3, the heating temperature is controlled at 220-240° C. in front of the furnace, 100-110° C. in the rear of the furnace, the furnace speed is controlled at 40-60 r / min, and the moisture content of the finished sodium sulfate is ≤0.05%.

8. The process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: In step 4, the saturated potassium-sodium mixture solution after precipitation and removal of heavy metals is evaporated and crystallized in an MVR evaporator.

9. A process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: The concentration treatment in step 4 adopts MVR evaporator for cyclic evaporation concentration and cyclic crystallization, and the yield of potassium and magnesium sulfate cyclic crystallization reaches more than 90%. The mother liquor returns to the system for cyclic evaporation and crystallization to produce mixed salt.

10. The process for purifying sodium sulfate and co-producing potassium-sodium mixed salt according to claim 1, characterized in that: In step six, after the Li+, Rb+, and Cs+ ions in the crystallization mother liquor are enriched, the precious metal salt compound is recovered by precipitation reaction or resin adsorption.

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