Process for preparing potassium sulfate by using whole-mirabilite method

Abstract

The invention discloses a process for preparing potassium sulfate by a whole-mirabilite method. The process comprises the following steps: (1) dehydrating mirabilite; (2) first-stage conversion; (3) carrying out second-stage conversion; (4) evaporating; (5) primary flash evaporation; (6) secondary flash evaporation; and (7) washing and drying to obtain a potassium sulfate product. The sodium chloride slurry separated out by forced evaporation is elutriated by the first-conversion mother liquor, so that the temperature of the sodium chloride separation liquid is reduced, and the material requirement of equipment is reduced; meanwhile, the potassium content in sodium chloride is reduced, and the comprehensive yield of potassium is improved; mother liquor at all levels is fully utilized, and energy consumption and cost are low. Sodium-potassium separation is achieved through evaporation separation, the separation purity is high, the K2O content in the obtained potassium sulfate product can reach 52% or above, the NaCl content in the sodium chloride product can reach 94% or above, and joint production of potassium sulfate and sodium chloride wet products is achieved.

Description

Technical Field

[0001] The invention relates to a process for preparing potassium sulfate, in particular to a process for preparing potassium sulfate by a full-glauber's salt method. Background Art

[0002] Potassium sulfate is an important chlorine-free potassium fertilizer, mainly used for economic crops such as tobacco, sugarcane, and fruit. The sulfur element can not only increase the yield of agricultural products, but also improve their quality, and there is a large market demand for it.

[0003] 50% of the world's potassium sulfate fertilizers come from minerals, and about 40% of potassium sulfate is produced by converting potassium chloride with sulfuric acid or sulfate. With the continuous exploitation of mineral resources, resources are becoming increasingly scarce. The process of producing potassium sulfate by the mirabilite method is conducive to improving the industrial status of potassium sulfate production. In my country, the total proven reserves of mirabilite mineral resources are about 20 billion tons (in terms of anhydrous sodium sulfate), ranking first in the world, and are widely distributed, with good mineral quality and easy to mine. In addition, a large amount of mirabilite can be produced as a by-product in other industrial production processes, which has low market utilization value and has become an environmental burden on enterprises. The process of producing potassium sulfate by the full mirabilite method can turn waste into treasure, save energy and protect the environment, and has a very broad market prospect.

[0004] When producing potassium sulfate using the full-glauber's salt method, the crystal water brought in by sodium sulfate decahydrate will cause a large amount of water in the first-stage conversion reaction, and the potassium glauber's salt yield is low. Usually, the glauber's salt is first dehydrated to evaporate the water, and then dehydrated to anhydrous sodium sulfate slurry and then converted into a first-stage conversion. This not only requires a large investment, but the evaporation will also increase energy consumption.

[0005] CN1189456A discloses a production process of potassium sulfate, which is to separate sodium chloride from the glauberite mother liquor obtained by a first-stage conversion and separation through a high-temperature evaporation, and then to participate in a double decomposition reaction after being evaporated by negative pressure to form a supersaturated salt-making mother liquor. However, the first-stage addition of glauberite results in a large amount of water in the first stage. In order to ensure the yield of the glauberite in the first stage, a large amount of water must be evaporated from the negative pressure evaporation material returned, resulting in a large intensity of negative pressure evaporation and high energy consumption. In addition, a large amount of sodium chloride is precipitated during the negative pressure evaporation process. When the sodium chloride is returned to the first stage, it may not be able to completely dissolve, resulting in a high sodium chloride content in the glauberite precipitated in the first stage, which affects the quality of the glauberite. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a process for preparing potassium sulfate by a full-salt salt method with low production energy consumption, and the obtained potassium sulfate product has high purity.

[0007] The technical solution adopted by the present invention to solve the technical problem is a process for preparing potassium sulfate by a full-glauber's salt method. The sodium sulfate is prepared by a Glauber's salt method combined with a thermal dehydration and a flash evaporation cooling method. Sodium sulfate decahydrate does not need to be evaporated and dehydrated first. The heat of a first-stage flash evaporation liquid is used to thermally dehydrate and precipitate Glauber's salt. The slurry from which sodium sulfate is precipitated is subjected to a first conversion step. The mother liquor does not need to be further evaporated and concentrated. The first-stage flash evaporation and the second-stage flash evaporation are used to supplement the water loss of the flash evaporation. The first-stage conversion mother liquor is forced to evaporate and precipitate sodium chloride, and the precipitation of sodium chloride is controlled while the precipitation of other components is not allowed (ensuring the maximum precipitation of sodium chloride and the purity of sodium chloride, and reducing the system mother liquor circulation amount). Sodium chloride is separated to obtain an evaporated mother liquor for two-stage flash evaporation. In order to ensure that sodium chloride is not precipitated during the flash evaporation cooling process, a nitrate mother liquor is added during the flash evaporation process (the nitrate mother liquor is the mother liquor after the sodium sulfate slurry is separated).

[0008] The specific steps include:

[0009] (1) Dehydration of Glauber's Salt: Using a primary flash liquid (initially, there is no primary flash liquid, and steamed saltpeter is used) to heat Glauber's Salt (sodium sulfate decahydrate) to dehydrate the Glauber's Salt to precipitate sodium sulfate, and then separate to obtain a sodium sulfate slurry, i.e., anhydrous Glauber's Salt, and a separated mother liquor;

[0010] The sodium sulfate slurry is sent to a conversion step, and the separated mother liquor is sent to a primary flash distillation step and a secondary flash distillation step.

[0011] (2) One-stage conversion: the sodium sulfate slurry obtained in step (1) is mixed with potassium chloride, water and / or a mother liquor containing potassium sulfate (the second-stage conversion mother liquor of step 3 and / or the secondary flash liquid of step 6), wherein the mass concentration of potassium element in the system after mixing is 6.5 to 8.0 wt % (preferably 6.8 to 7.5 wt %), and the mass concentration of sulfate ion is 3.5 to 5.0 wt % (preferably 3.8 to 4.5 wt %), and the mixture is stirred. After glauberite is precipitated by conversion reaction, solid-liquid separation is performed to obtain glauberite and the first-stage conversion mother liquor;

[0012] (3) Second-stage conversion: the potassium sulfate obtained in step (2) is mixed with potassium chloride, water or a mother liquor containing potassium sulfate (the repulp washing water in step 7), wherein the mass concentration of potassium element in the system after mixing is 19.0 to 23.0 wt% (preferably 20.0 to 22.0 wt%), and the mass concentration of sulfate ion is 16.0 to 20.0 wt% (preferably 17.0 to 19.0 wt%), and the mixture is stirred. After potassium sulfate is precipitated by conversion reaction, solid-liquid separation is performed to obtain a crude potassium sulfate product and a second-stage conversion mother liquor;

[0013] The second-stage conversion is a double decomposition reaction, and the principle is the quaternary phase diagram. The principle of adding materials is to control the composition of the mother liquor of the conversion so that the amount of potassium sulfate precipitation is maximized and the second-stage potassium yield is the highest. Too high potassium elements and sulfate ions will affect the purity of the product, while too low will affect the second-stage yield.

[0014] (4) Evaporation: The first-stage conversion mother liquor obtained in step (2) is subjected to forced evaporation of water equivalent to 13-15 wt% (preferably 14 wt%) of the first-stage conversion mother liquor to obtain sodium chloride slurry and evaporated mother liquor; the sodium chloride slurry (the mass of sodium chloride is equivalent to about 20% of the slurry concentration) is elutriated with the first-stage conversion mother liquor (the first-stage conversion mother liquor used is 80%-100% of the slurry amount), and the elutriated slurry is thickened (preferably thickened by a thickener) and then solid-liquid separation is performed to obtain sodium chloride and elutriated mother liquor;

[0015] In step (4), the mother liquor is evaporated and subjected to two-stage flash cooling in the subsequent step.

[0016] The main purpose of washing is to lower the material temperature and thus reduce the temperature requirements of the equipment material. The second purpose is to reduce the potassium content of the mother liquor, reduce entrainment losses, and increase potassium yield.

[0017] (5) primary flash evaporation: the evaporation mother liquor obtained in step (4) is subjected to primary flash evaporation of water equivalent to 3.0 to 5.0 wt % of the evaporation mother liquor to obtain a primary flash liquid, 13% to 15% of the mass of the primary flash liquid is used for dehydration in step (1), and the rest is used for secondary flash evaporation in step (6);

[0018] The principle of controlling the amount of primary flash liquid used for dehydration in this step is to balance the amount of flash water loss with the amount of water replenished. If it is too little, the amount of water replenished is insufficient and sodium chloride is precipitated during the flash evaporation process; if it is too much, the concentration of the nitrate slurry is too high, which is not conducive to transportation and the thickener equipment is too large.

[0019] The primary flash evaporation is a process of evaporating water and cooling down. After the primary flash evaporation, the temperature is reduced to 75-85°C to obtain the primary flash liquid.

[0020] (6) Secondary flash evaporation: the primary flash liquid obtained in step (5), except for the portion used for dehydration in step (1), is subjected to flash evaporation of water equivalent to 2.0 to 3.0 wt % of the mass of the evaporation mother liquid to obtain a secondary flash liquid, which is returned to step (2) as a mother liquid ingredient containing potassium sulfate;

[0021] Secondary flash evaporation is the process of evaporating water and cooling down. After the secondary flash evaporation, the temperature is reduced to 45-55°C to obtain the secondary flash liquid.

[0022] (7) Washing and drying: The crude potassium sulfate product obtained in step (3) is re-slurried, washed, filtered, and the filter cake is dried to obtain a potassium sulfate product.

[0023] Furthermore, in step (1), the content of Na2SO4 in the sodium sulfate is ≥37.0wt%. If the content of Na2SO4 in the sodium sulfate is too low, there may be more impurities other than sodium chloride, and more tails, which will affect the comprehensive yield of potassium; if the sodium chloride in the sodium sulfate is too high, the first conversion potassium yield will be affected.

[0024] The purpose of the first-stage conversion reaction is mainly to produce glauberite; sodium chloride is precipitated from the obtained first-stage conversion mother liquor by forced evaporation, the evaporated mother liquor is flash-evaporated to cool down, and the mother liquor separated by dehydration of glauberite is added during the flash evaporation process to supplement the amount of water evaporated during the flash evaporation process, so as to avoid the precipitation of sodium chloride during the flash evaporation process affecting the quality of glauberite. At the same time, the mother liquor separated by dehydration of glauberite is omitted from further evaporation and concentration, thereby reducing the water consumption, energy consumption and equipment investment of the system; the glauberite obtained by the first-stage conversion is converted with potassium chloride in the second-stage conversion reaction to produce potassium sulfate product.

[0025] Furthermore, in step (1), the temperature of the dehydration of the mirabilite is 45-70°C. The dehydration time of the mirabilite is 0.5-2h. If the temperature is too high, the temperature of the first turn is affected; if the temperature is too low, the nitration speed is slow. If the time is too long, the equipment investment is large; if the time is too short, the nitration is incomplete, affecting the first turn.

[0026] In step (1), the slurry after dehydration of the mirabilite is subjected to a first-stage conversion, and the separated mother liquor is subjected to a first-stage flash evaporation and a second-stage flash evaporation.

[0027] Furthermore, in step (2), after mixing the ingredients, the mass concentration of potassium in the control system is controlled to be 6.5-8.0wt%, and the mass concentration of sulfate ions is controlled to be 3.5-5.0wt%. This is mainly to ensure that the ions in the raw materials can fully react with each other to obtain high-quality potassium sulfate, and at the same time, the phase diagram point of the first-stage conversion mother liquor can be better controlled at 50°C Na+, K+ / / Cl-, SO4 2- -H2O quaternary water-salt system equilibrium phase diagram of sodium chloride and potassium glauberite co-saturation line. (50℃ conversion speed is faster, too high temperature will cause serious corrosion to equipment, low temperature will cause slower conversion, and equipment and operation costs will be higher. At the same time, the amount of sodium chloride precipitated by evaporation of mother liquor at 50℃ is large, and the amount of chlorine precipitated during the evaporation process is large, which can reduce the circulation of materials and thus reduce the operation cost).

[0028] The purpose of the first-stage conversion reaction in step (2) is mainly to produce glauberite.

[0029] Furthermore, in step (2), the obtained first-stage conversion mother liquor is subjected to forced evaporation to precipitate sodium chloride.

[0030] The mother liquor is flash-evaporated to cool down, and the mother liquor separated by dehydration of the thenardite in step (1) is added to the flash process to supplement the evaporation of water in the flash process, thereby avoiding the precipitation of sodium chloride in the flash process that affects the quality of the potassium sulfate. At the same time, there is no need to further evaporate and concentrate the mother liquor separated by dehydration of the thenardite, thereby reducing the water consumption, energy consumption and equipment investment of the system; the potassium sulfate obtained by the first-stage conversion is converted with potassium chloride in the second-stage conversion reaction to prepare a potassium sulfate product.

[0031] Furthermore, in step (2), the temperature of the first stage conversion reaction is 40-60°C. The time is 0.5-2h (preferably 1.5-1.8h). When the temperature is 40-60°C, the conversion speed is relatively fast. If the temperature is too high, the equipment will be corroded greatly. If the temperature is low, the conversion is relatively slow, and the equipment and operation costs are relatively high. At the same time, when the temperature is 40-60°C, the amount of sodium chloride precipitated from the mother liquor by evaporation is relatively large, and the amount of chlorine precipitated during the evaporation process is relatively large, which can reduce the circulation amount of materials and thus reduce the operation cost. The time of the conversion reaction is to ensure that the conversion reaction is complete.

[0032] Furthermore, in step (3), the temperature of the second-stage conversion reaction is 30-40°C and the time is 0.5-2h (preferably 1.5-1.8h). If the temperature is too high, the yield is low, and if the time is too short, the conversion is incomplete, which affects the product quality.

[0033] Furthermore, the second-stage conversion mother liquor obtained in step (3) is a mother liquor containing potassium sulfate, which can be returned to step (2) for batching. The second-stage conversion mother liquor can be fully utilized by recycling the potassium and sulfate resources contained in the mother liquor, reducing the raw materials of potassium chloride and anhydrous sodium sulfate in step (2), reducing production costs, and improving potassium yield. When the system runs stably, the second-stage conversion mother liquor is repeatedly used in this system through circulation, and can also be used for initial batching of other potassium sulfate production systems.

[0034] Furthermore, in step (4), the temperature of evaporation of the first-stage conversion mother liquor is 105-115° C. Evaporation within this temperature range is a comprehensive consideration of the temperature range of the saturated mother liquor co-saturation point and energy saving, so as to evaporate and separate the sodium chloride byproduct to the greatest extent and effectively reduce the system material circulation amount.

[0035] In step (4), the amount of evaporated water is controlled during evaporation of the first-stage conversion mother liquor, mainly to ensure that only sodium chloride crystals are precipitated during the evaporation and crystallization process, while potassium sulfate is not precipitated.

[0036] Furthermore, in step (4), the byproduct sodium chloride is re-pulped and washed, and filtered to obtain a sodium chloride wet product. In the re-pulped washing, the amount of water used is 10% to 30% (preferably 15% to 25%) of the mass of the sodium chloride product, and the washing time is 10 to 30 minutes; the washing water can be returned to the evaporation step of step (4) for recovery (recovering sodium chloride and reducing emissions).

[0037] Furthermore, the secondary flash liquid obtained in step (6) is returned to step (2) as a mother liquor containing potassium sulfate for batching. The secondary flash liquid can be circulated to make full use of the potassium and sulfate resources contained in the mother liquor, reduce the raw materials of potassium chloride and sodium sulfate in step (2), reduce production costs, and improve potassium yield. After the system runs stably, the secondary flash liquid is circulated and reused in this system, and can also be used for initial batching of other potassium sulfate production systems.

[0038] Furthermore, in step (7), the slurry is washed again, the amount of water used is 10% to 30% of the mass of the crude potassium sulfate product, and the washing time is 10 to 30 minutes.

[0039] Furthermore, the washing water obtained after the re-pulping and washing in step (7) is returned to step (3) as a mother liquor containing potassium sulfate for batching. The washing water can replace part of the fresh water, effectively reducing the amount of fresh water. When the system runs stably, the washing water is reused in the system through circulation, and can also be used for initial batching of other potassium sulfate production systems.

[0040] The present invention combines the Glauber's salt method with thermal dehydration (mother liquor dehydration) and flash evaporation cooling to produce potassium sulfate, which solves the shortcomings of the existing traditional process such as incomplete evaporation of the first-stage conversion mother liquor and a large amount of returned mother liquor. The beneficial effects are specifically embodied in the following aspects:

[0041] (1) The present invention realizes sodium-potassium separation by evaporation separation, and the separation purity is high. The K2O content in the obtained potassium sulfate product can reach more than 52%, and the NaCl content in the sodium chloride product can reach more than 94% (the sodium chloride product is industrial wet salt, GBT5462-2015 national standard: the secondary index of industrial wet salt is that the sodium chloride content is greater than 93.3%), thereby realizing the joint production of potassium sulfate and sodium chloride wet products, thereby improving economic benefits;

[0042] (2) In the process of the present invention, the dehydration of the mirabilite utilizes part of the primary flash liquid to dehydrate the nitrate, making full use of the heat of the primary flash liquid and reducing the heat consumption of the system;

[0043] (3) In order to maintain the sodium and water balance of the system, the present invention performs forced evaporation on the first-stage conversion mother liquor, which can greatly reduce energy consumption and production costs compared with the traditional cycle evaporation-cooling process; the present invention uses the first-stage mother liquor to wash the sodium chloride slurry precipitated by forced evaporation, thereby reducing the temperature of the sodium chloride separation liquid, thereby reducing the material requirements of the equipment; at the same time, the potassium content in the sodium chloride is reduced, and the comprehensive potassium yield is improved;

[0044] (4) The process of the present invention is to thermally dehydrate the sodium sulfate. The dehydrated sodium sulfate is thickened and then removed. The mother liquor is no longer evaporated and concentrated further, but is directly used for the first and second flash evaporation to supplement the water loss of the flash evaporation, thereby reducing the investment in evaporation equipment, saving evaporation energy consumption, and reducing the water consumption required for the flash evaporation process.

[0045] (5) The present invention eliminates the need for an evaporation device for dehydrating and separating the mother liquor from the sodium sulfate, thereby reducing system energy consumption and water consumption.

[0046] The method uses all sodium sulfate decahydrate as raw materials, and the obtained potassium sulfate product has high purity, and no additional sodium sulfate evaporation and dehydration step is required. The mother liquors at all levels are fully utilized, and the energy consumption and cost are low. The K2O content in the obtained potassium sulfate product reaches more than 52%. DETAILED DESCRIPTION

[0047] The present invention will be further described below in conjunction with the embodiments.

[0048] Unless otherwise specified, the chemical reagents used in the embodiments of the present invention were obtained through conventional commercial channels.

[0049] Example 1

[0050] (1) Dehydration of Glauber's Salt: 14% of the total amount of the primary flash liquid was mixed with Glauber's Salt (Na2SO4 content: 39.16 wt%), and the Glauber's Salt was dehydrated by heat to precipitate sodium sulfate. The temperature for dehydration of the Glauber's Salt was 55°C, and the time was 1.5 h. The slurry was separated by a thickener to obtain sodium sulfate slurry (the mass concentration of sodium sulfate in the slurry was 21%). The separated mother liquor was subjected to primary and secondary flash evaporation.

[0051] (2) One-stage conversion: The sodium sulfate slurry in step (1) is mixed with potassium chloride and a second-stage conversion mother liquor (prepared by oneself when there is no second-stage conversion mother liquor in the first step) until the mass concentration of potassium in the system is 7.19 wt % and the mass concentration of sulfate ion is 4.15 wt %. The mixture is stirred and the conversion reaction is carried out at 53° C. for 1.5 h to precipitate glauberite, followed by solid-liquid separation to obtain glauberite and a first-stage conversion mother liquor.

[0052] (3) Second-stage conversion: The potassium sulfate obtained in step (2) is mixed with potassium chloride and water until the mass concentration of potassium in the system is 20.97 wt % and the mass concentration of sulfate is 17.99 wt %. The mixture is stirred and the reaction is carried out at 37.5° C. for 2 h to precipitate potassium sulfate, followed by solid-liquid separation to obtain a crude potassium sulfate product and a second-stage conversion mother liquor.

[0053] (4) Evaporation: The first-stage conversion mother liquor obtained in step (2) is subjected to forced evaporation of water equivalent to 13.94 wt% of the first-stage conversion mother liquor at 110°C to obtain sodium chloride slurry and evaporation mother liquor; (the evaporation mother liquor is subjected to two-stage flash evaporation cooling in the subsequent steps). The evaporated sodium chloride slurry is washed with the first-stage conversion mother liquor of step (2) (the mass of the first-stage conversion mother liquor used is 80% of the slurry amount), and the washed slurry is thickened (thickened by a thickener) and then solid-liquid separation is performed to obtain sodium chloride and the washed mother liquor, and the washed mother liquor is heated and returned to the evaporator;

[0054] Add water equivalent to 15% of the mass of the by-product sodium chloride obtained in step (4) to perform re-slurry washing for 20 minutes, and filter to obtain a sodium chloride wet product.

[0055] (5) primary flash evaporation: the evaporation mother liquor obtained in step (4) is subjected to primary flash evaporation of water equivalent to 4.0 wt% of the evaporation mother liquor to obtain a primary flash liquid at about 80° C. 14% of the primary flash liquid is used for nitration in step (1), and the rest is used for secondary flash evaporation in step (6);

[0056] (6) Secondary flash evaporation: 86% of the primary flash liquid obtained in step (5) is subjected to flash evaporation of water equivalent to 2.5 wt% of the mass of the evaporation mother liquid to obtain a secondary flash liquid at about 50° C., which is used in step (2) as a mother liquid ingredient containing potassium sulfate;

[0057] (7) Washing and drying: Add water equivalent to 15% of the mass of the crude potassium sulfate product obtained in step (3) to perform reslurry washing for 30 minutes, filter, and dry the filter cake at 105° C. for 3 hours to obtain a potassium sulfate product.

[0058] In this embodiment, the potassium sulfate product contains 52.0% K2O, which meets the requirements of the superior product of potassium sulfate for agricultural use (GB / T20406-2017). The sodium chloride wet product contains 94.0% NaCl, and the quality of the byproduct industrial salt meets the requirements of China's "Industrial Salt"

[0059] (GB / T5462-2015) Secondary quality index requirements for refined industrial wet salt.

[0060] Example 2

[0061] (1) Dehydration of Glauber's Salt: 14% of the total amount of the primary flash liquid is mixed with Glauber's Salt (Na2SO4 content is 38.5wt%), and the Glauber's Salt is heated to dehydrate and precipitate sodium sulfate. The temperature for dehydration of the Glauber's Salt is 60°C and the time is 1 hour. The slurry is separated by a thickener to obtain sodium sulfate slurry (the mass concentration of sodium sulfate in the slurry is 21%) for a first-stage conversion, and the separated mother liquor is subjected to primary and secondary flash evaporation.

[0062] (2) One-stage conversion: The sodium sulfate slurry (21%) from step (1) is mixed with potassium chloride and the second conversion mother liquor from step (3) of Example 1 until the mass concentration of potassium in the system is 7.15 wt % and the mass concentration of sulfate is 4.12 wt %. The mixture is stirred and the conversion reaction is carried out at 55° C. for 1.5 h to precipitate glauberite. The mixture is then solid-liquid separated to obtain glauberite and the first-stage conversion mother liquor.

[0063] (3) Second-stage conversion: The potassium sulfate obtained in step (2), potassium chloride, water, and the re-washing water of the crude potassium sulfate product obtained in step (7) of Example 1 are mixed and prepared until the mass concentration of potassium element in the system is 20.97 wt % and the mass concentration of sulfate ion is 17.99 wt %. The mixture is stirred and subjected to conversion reaction at 37.5° C. for 2 h to precipitate potassium sulfate, followed by solid-liquid separation to obtain a crude potassium sulfate product and a second-stage conversion mother liquor.

[0064] The second-stage conversion mother liquor obtained in step (3) is the mother liquor containing potassium sulfate, which can be returned to step (2) for batching.

[0065] (4) Evaporation: The first-stage conversion mother liquor obtained in step (2) is evaporated at 110°C to obtain water equivalent to 14.05 wt% of the first-stage conversion mother liquor, thereby obtaining sodium chloride slurry and evaporation mother liquor; the evaporation mother liquor is subjected to two-stage flash cooling in the subsequent steps. The evaporated sodium chloride slurry is washed with the first-stage conversion mother liquor of step (2) (the first-stage conversion mother liquor used is 80% of the slurry amount), and the washed slurry is thickened (thickened by a thickener) and then solid-liquid separation is performed to obtain sodium chloride and elution mother liquor, and the elution mother liquor is heated and returned to the evaporator;

[0066] Add water equivalent to 20% of the mass of the by-product sodium chloride obtained in step (4) to perform re-slurry washing for 20 minutes, and filter to obtain a sodium chloride wet product.

[0067] (5) primary flash evaporation: the evaporation mother liquor obtained in step (4) is subjected to primary flash evaporation of water equivalent to 4.0 wt% of the evaporation mother liquor to obtain a primary flash liquid at about 80° C. 14% of the primary flash liquid is used for nitration in step (1), and the rest is used for secondary flash evaporation;

[0068] (6) Secondary flash evaporation: 86% of the primary flash liquid obtained in step (5) is subjected to flash evaporation of water equivalent to 2.5 wt% of the mass of the evaporation mother liquid to obtain a secondary flash liquid at about 50° C., which is used in step (2) as a mother liquid ingredient containing potassium sulfate;

[0069] (7) Washing and drying: Add water equivalent to 20% of the mass of the crude potassium sulfate product obtained in step (3) to perform reslurry washing for 30 minutes, filter, and dry the filter cake at 105° C. for 2 hours to obtain a potassium sulfate product.

[0070] In this embodiment, the potassium sulfate product contains 52.4% K2O, which meets the requirements of the superior product of potassium sulfate for agricultural use (GB / T20406-2017). The sodium chloride wet product contains 94.5% NaCl, and the quality of the byproduct industrial salt meets the requirements of China's "Industrial Salt"

[0071] (GB / T5462-2015) Secondary quality index requirements for refined industrial wet salt.

[0072] Example 3

[0073] (1) Dehydration of Glauber's Salt: 14% of the total amount of the primary flash liquid was mixed with Glauber's Salt (Na2SO4 content: 37.6 wt%), and the Glauber's Salt was dehydrated by heat to precipitate sodium sulfate. The temperature for dehydration of the Glauber's Salt was 50°C and the time was 2 h. The slurry was separated by a thickener to obtain sodium sulfate slurry (the mass concentration of sodium sulfate in the slurry was 21%) for a first-stage conversion, and the separated mother liquor was subjected to primary and secondary flash evaporation.

[0074] (2) One-stage conversion: The sodium sulfate slurry (21%) obtained in step (1) is mixed with potassium chloride and the second conversion mother liquor of step (3) of Example 2 until the mass concentration of potassium in the system is 7.09 wt % and the mass concentration of sulfate is 4.09 wt %. The mixture is stirred and the conversion reaction is carried out at 52° C. for 1.5 h to precipitate glauberite, followed by solid-liquid separation to obtain glauberite and the first-stage conversion mother liquor.

[0075] (3) Second-stage conversion: The potassium sulfate obtained in step (2), potassium chloride, water, and the re-washing water of the crude potassium sulfate product obtained in step (7) of Example 2 are mixed and prepared until the mass concentration of potassium element in the system is 20.97 wt % and the mass concentration of sulfate ion is 17.99 wt %. The mixture is stirred and the conversion reaction is carried out at 37.5° C. for 1 h to precipitate potassium sulfate, and then solid-liquid separation is carried out to obtain a crude potassium sulfate product and a second-stage conversion mother liquor.

[0076] (4) Evaporation: The first-stage conversion mother liquor obtained in step (2) is subjected to forced evaporation at 110°C to remove water equivalent to 14.00 wt% of the first-stage conversion mother liquor, thereby obtaining a sodium chloride slurry and an evaporation mother liquor; the evaporation mother liquor is subjected to two-stage flash cooling in the subsequent steps. The evaporated sodium chloride slurry is washed with the first-stage conversion mother liquor of step (2) (the first-stage conversion mother liquor used is 80% of the slurry amount), and the washed slurry is thickened (thickened by a thickener) and then solid-liquid separation is performed to obtain sodium chloride and an elutriation mother liquor, which is heated and returned to the evaporator;

[0077] Add water equivalent to 18% of the mass of the by-product sodium chloride obtained in step (4) to perform re-slurry washing for 20 minutes, and filter to obtain a sodium chloride wet product.

[0078] (5) primary flash evaporation: the evaporation mother liquor obtained in step (4) is subjected to primary flash evaporation of water equivalent to 4.0 wt% of the evaporation mother liquor to obtain a primary flash liquid at about 80° C. 14% of the primary flash liquid is used for nitration in step (1), and the rest is used for secondary flash evaporation;

[0079] (6) Secondary flash evaporation: 86% of the primary flash liquid obtained in step (5) is subjected to flash evaporation of water equivalent to 2.5 wt% of the mass of the evaporation mother liquid to obtain a secondary flash liquid at about 50° C., which is returned to step (2) as a mother liquid ingredient containing potassium sulfate;

[0080] (7) Washing and drying: Add water equivalent to 18% of the mass of the crude potassium sulfate product obtained in step (3) to perform reslurry washing for 30 minutes, filter, and dry the filter cake at 105° C. for 4 hours to obtain a potassium sulfate product.

[0081] In this embodiment, the potassium sulfate product obtained contains 52.2% K2O, which meets the requirements of the superior product of potassium sulfate for agricultural use (GB / T20406-2017). The sodium chloride wet product obtained contains 94.2% NaCl, and the quality of the byproduct industrial salt meets the requirements of China's "Industrial Salt"

[0082] (GB / T5462-2015) Secondary quality index requirements for refined industrial wet salt.

Claims

1. A process for preparing potassium sulfate by the full-salt salt method, characterized in that: Potassium sulfate is prepared by combining the method of Glauber's salt method with thermal dehydration and flash cooling. Sodium sulfate decahydrate does not need to be evaporated and dehydrated first. The heat of the first-stage flash liquid is used to thermally dehydrate and precipitate Glauber's salt. The slurry of the precipitated sodium sulfate goes through a first conversion step. The mother liquor does not need to be further evaporated and concentrated. The first-stage flash evaporation and the second-stage flash evaporation are used to supplement the water loss of the flash evaporation. The first-stage conversion mother liquor is forced to evaporate and precipitate sodium chloride, and the precipitation of sodium chloride is controlled while the precipitation of other components is not precipitated. The sodium chloride is separated to obtain the evaporated mother liquor for two-stage flash evaporation.

2. The process for preparing potassium sulfate by the full-salt salt method according to claim 1, characterized in that: The following steps are involved: (1) Dehydration of Glauber's Salt: Using a primary flash liquid to heat Glauber's Salt, the Glauber's Salt is dehydrated to precipitate sodium sulfate, and sodium sulfate slurry, i.e., anhydrous Glauber's Salt, and separated mother liquor are obtained; (2) one-stage conversion: the sodium sulfate slurry obtained in step (1) is mixed with potassium chloride, water and / or a mother liquor containing potassium sulfate, wherein the mass concentration of potassium element in the system after the mixing is 6.5-8.0 wt %, and the mass concentration of sulfate ion is 3.5-5.0 wt %. The mixture is stirred, and after glauberite is precipitated by the conversion reaction, solid-liquid separation is performed to obtain glauberite and a one-stage conversion mother liquor; (3) Second-stage conversion: mixing the glauberite obtained in step (2) with potassium chloride, water or a mother liquor containing potassium sulfate, wherein the mass concentration of potassium element in the system after the mixing is 19.0 to 23.0 wt %, and the mass concentration of sulfate ion is 16.0 to 20.0 wt %. The mixture is stirred, and after potassium sulfate is precipitated by the conversion reaction, solid-liquid separation is performed to obtain a crude potassium sulfate product and a second-stage conversion mother liquor; (4) Evaporation: The first-stage conversion mother liquor obtained in step (2) is subjected to forced evaporation of water equivalent to 13-15 wt% of the first-stage conversion mother liquor to obtain sodium chloride slurry and evaporated mother liquor; the sodium chloride slurry is elutriated with the first-stage conversion mother liquor, and the elutriated slurry is thickened and then subjected to solid-liquid separation to obtain sodium chloride and elutriated mother liquor; (5) primary flash evaporation: the evaporation mother liquor obtained in step (4) is subjected to primary flash evaporation of water equivalent to 3.0 to 5.0 wt % of the evaporation mother liquor to obtain a primary flash liquid, 13% to 15% of the mass of the primary flash liquid is used for dehydration in step (1), and the rest is used for secondary flash evaporation in step (6); (6) Secondary flash evaporation: the primary flash liquid obtained in step (5), except for the portion used for dehydration in step (1), is subjected to flash evaporation of water equivalent to 2.0 to 3.0 wt % of the mass of the evaporation mother liquid to obtain a secondary flash liquid, which is returned to step (2) as a mother liquid ingredient containing potassium sulfate; (7) Washing and drying: The crude potassium sulfate product obtained in step (3) is re-slurried, washed, filtered, and the filter cake is dried to obtain a potassium sulfate product.

3. The process for preparing potassium sulfate by the full-salt salt method according to claim 2, characterized in that: In step (1), the content of Na2SO4 in the Glauber's salt is ≥37.0wt%; and / or, in step (1), the temperature for dehydrating the Glauber's salt is 45-70°C; and / or, the time for dehydrating the Glauber's salt is 0.5-2h.

4. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: In step (2), after mixing the ingredients, the mass concentration of potassium element in the control system is controlled to be 6.5-8.0wt%, and the mass concentration of sulfate ion is controlled to be 3.5-5.0wt%.

5. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: In step (2), the obtained first-stage conversion mother liquor is subjected to forced evaporation to precipitate sodium chloride; and / or, in step (2), the temperature of the first-stage conversion reaction is 40 to 60° C., the time is 0.5 to 2 hours, and the temperature is 40 to 60° C.

6. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: In step (3), the temperature of the second-stage conversion reaction is 30-40° C. and the time is 0.5-2 h.

7. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: The second-stage conversion mother liquor obtained in step (3) is the mother liquor containing potassium sulfate, which is all returned to step (2) for batching.

8. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: In step (4), the temperature of evaporation of the first-stage conversion mother liquor is 105-115° C.; and / or, in step (4), the obtained by-product sodium chloride is re-pulped, washed, and filtered to obtain a sodium chloride wet product.

9. The process for preparing potassium sulfate by the full-salt-salt method according to claim 2 or 3, characterized in that: The secondary flash liquid obtained in step (6) is returned to step (2) as mother liquor containing potassium sulfate for batching.

10. The process for preparing potassium sulfate by the full-salt salt method according to claim 2 or 3, characterized in that: In step (7), the repulp washing is performed, the amount of water used is 10% to 30% of the mass of the crude potassium sulfate product, and the washing time is 10 to 30 minutes; and / or, the washing water after the repulp washing obtained in step (7) is returned to step (3) as a mother liquor containing potassium sulfate for batching.