A process for recovering potassium sulfate from a potassium sulfate and sodium sulfate mixed salt solution

Abstract

The application discloses a process for recovering potassium sulfate from a potassium sulfate and sodium sulfate mixed salt solution, and is characterized by comprising the following steps: S1, separating NaK3(SO4)2 crystals from the potassium sulfate and sodium sulfate mixed salt solution, wherein the mass percentage of sodium sulfate in the potassium sulfate and sodium sulfate mixed salt solution is x0, and the mass percentage of potassium sulfate is y0; S2, dissolving the NaK3(SO4)2 crystals in pure water to obtain slurry a3, controlling the temperature of the slurry a3 to be t a3 , 10℃<t a3 <20℃, so that the mass percentage of sodium sulfate in the slurry a3 is x a3 , the mass percentage of potassium sulfate is y a3 , 2%<x a3 <10%, 15%<y a3 <20%, and K2SO4 crystals are precipitated. The application utilizes the heterogeneous composition of NaK3(SO4)2 (sodium potassium mirabilite), dissolves part of K2SO4 and all Na2SO4 in the sodium potassium mirabilite by adding water in an appropriate amount, forms a co-saturated solution, and leaves pure K2SO4 crystals at the same time, so as to realize the purpose of separating the pure K2SO4 from the sodium potassium mirabilite.

Description

Technical Field

[0001] This invention relates to a potassium sulfate recovery process, and more particularly to a process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate. Background Technology

[0002] Potassium salts, as an essential agricultural fertilizer for plant growth, strengthen plant stems and improve their resistance to diseases, pests, drought, cold, and lodging. In today's booming lithium battery industry, given the scarcity of spodumene in China, potassium-bearing hard rock ores such as lepidolite and feldspar have become the main raw materials for lithium extraction by domestic lithium mining companies. However, my country not only faces a significant lithium resource shortage but is also a severely potassium-deficient country, particularly lacking soluble potassium resources. While my country lacks soluble potassium ores, it possesses substantial reserves of insoluble potassium resources. Currently, most feasible hard rock potassium extraction technologies require high-temperature calcination to convert insoluble potassium rocks into soluble potassium salts for leaching. The selling price of these soluble salts is much lower than the extraction cost, thus limiting the widespread adoption of hard rock potassium extraction processes.

[0003] However, with the continuous development of lithium ore resources, utilizing the heat energy from calcining lithium ore to simultaneously dissociate potassium from its stable crystal structure into soluble potassium salts has become an effective shortcut for obtaining potassium resources while extracting lithium. However, since the leached lithium-containing solution contains a large amount of sodium ions, separating potassium and sodium from the lithium precipitation mother liquor after conversion and precipitation is a crucial step in producing usable potash fertilizer from the lithium extraction mother liquor.

[0004] Because there is currently no separation of potassium and sodium salts in lithium precipitation mother liquor, a large amount of potassium sulfate and sodium sulfate mixed salts are treated as waste. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a process for recovering potassium sulfate from a mixed salt solution of potassium sulfate and sodium sulfate, which does not introduce external anions and cations other than water, and recovers pure potassium sulfate and sodium sulfate.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a process for recovering potassium sulfate from a mixed salt solution of potassium sulfate and sodium sulfate, comprising the following steps:

[0007] S1. Separate NaK3(SO4)2 crystals from a mixed salt solution of potassium sulfate and sodium sulfate, wherein the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate is x0 and the mass percentage of potassium sulfate is y0.

[0008] S2 and NaK3(SO4)2 crystals are dissolved in pure water to obtain slurry a3. The temperature of slurry a3 is controlled at t. a3 10℃<t a3<20℃, so that the mass percentage of sodium sulfate in slurry a3 is x a3 The mass percentage of potassium sulfate is y. a3 2% < x a3 <10%, 15% <y a3 <20%, K2SO4 crystals precipitate.

[0009] This invention utilizes the heterogeneity of NaK3(SO4)2 (sodium potassium sulfate). By adding an appropriate amount of water, some K2SO4 and all Na2SO4 in sodium potassium sulfate are dissolved to form a co-saturated solution (5.7%, 9.7%), while leaving pure K2SO4 crystals, thereby achieving the purpose of separating pure K2SO4 from sodium potassium sulfate.

[0010] Based on the heterogeneous composition of NaK3(SO4)2, adding water at various temperatures will completely dissolve both Na2SO4 and some K2SO4, leaving solid K2SO4. The remaining task is to find the temperature at which the maximum amount of K2SO4 can be separated from a given amount of solid NaK3(SO4)2. According to the molecular formula of NaK3(SO4)2, its K2SO4 / Na2SO4 ratio is 7.8614 / 2.1386 = 3.676. Comparing this to experimental data on the K2SO4 / Na2SO4 ratio in solutions where K2SO4 can be separated, at temperatures below 10℃, the K2SO4 / Na2SO4 ratio is significantly higher. a3 This value is smallest at temperatures below 20℃, and differs most from 3.676. Therefore, the optimal temperature for separating K2SO4 from NaK3(SO4)2 can be determined as t. a3 10℃<t a3 At temperatures below 20°C, the maximum mass of K2SO4 can be separated from solid NaK3(SO4)2. Optimally, the maximum mass of K2SO4 can be obtained by separating it from solution or solid NaK3(SO4)2 at 15°C.

[0011] In a preferred embodiment of the present invention, when y0 / x0 > 1.702, S1 includes the following steps:

[0012] A mixed salt solution of potassium sulfate and sodium sulfate is obtained by adding water or evaporating water. 10 , to make the mixed salt solution a 10 The mass percentage of sodium sulfate in the medium is Mixed salt solution a 10 The temperature is t a1 10℃<t a1 <25℃, from mixed salt solution a 10 K2SO4 crystals precipitate out, and the remaining solution is a co-saturated solution. 11 ;

[0013] saturated liquid a 11 Continue evaporating the water to obtain a mixed salt solution a20 , to make the mixed salt solution a 20 The mass percentage of sodium sulfate in the medium is x a2 The mass percentage of potassium sulfate is y. a2 20% < x a2 <30%, 40% <y a2 <50%, adjust the mixed salt solution a 20 temperature t a2 25℃<t a2 At temperatures below 45℃, NaK3(SO4)2 crystals were separated, and the remaining solution was a co-saturated solution. 21 .

[0014] By adjusting the water volume through evaporation and other methods, NaK3(SO4)2 (sodium potassium sulfate) crystals will precipitate directly from the potassium-sodium solution at the corresponding temperature. When 25℃ < t a2 At temperatures below 45℃, the most NaK3(SO4)2 crystals are separated.

[0015] This invention obtains K2SO4 crystals from a saturated liquid by changing the temperature and adjusting the water content of the system.

[0016] The order of K2SO4 precipitation from highest to lowest temperature is 15℃ > 30℃ > 35℃ > 60℃ > 75℃ > 100℃ > 25℃. When 10℃ < t a3 The amount of K2SO4 crystals precipitated is the largest at temperatures below 20℃.

[0017] In a preferred embodiment of the present invention, when y0 / x0 < 0.623, S1 includes the following steps:

[0018] A mixed salt solution of potassium sulfate and sodium sulfate is obtained by adding water or evaporating water to obtain mixed salt solution b. 10 The mass percentage of sodium sulfate in mixed salt solution b1 is _____. Mixed salt solution b 10 The temperature is t b1 10℃<t b1 <25℃, from mixed salt solution b 10 Pure Na₂SO₄·10H₂O crystals were separated, and the remaining solution was the co-saturated solution b after the separation of Na₂SO₄·10H₂O. 11 ;

[0019] The co-saturated solution b after separating Na2SO4·10H2O 11 Evaporation of water yields mixed salt solution b 20 , so that the mixed salt solution b 20 The mass percentage of sodium sulfate in the medium is x b2 The mass percentage of potassium sulfate is y. b2 20% < x b2< 40%, 10% < y b2 < 30%, adjust the mixed salt solution b 20 of the temperature t b2 , 25°C < t b2 < 45°C, separate out the NaK3(SO4)2 crystal, and the remaining solution is the co-saturated solution b 21 .

[0020] By adding water or evaporating water to adjust the water volume of the system, the separate crystallization of Na2SO4 or Na2SO4·10H2O can be achieved (when the temperature < 32.24°C, the crystallization product is S10: Na2SO4·10H2O). Then, to increase the yield of Na2SO4, the lower the K2SO4 / Na2SO4 (mass ratio) of the feed liquid, the more sodium sulfate is precipitated. The order of sodium sulfate precipitation amount from large to small by temperature is: 15°C > 100°C > 25°C > 75°C > 60°C > 30°C > 35°C. As can be seen from the above, when the ratio of K2SO4 / Na2SO4 (mass ratio) of the feed is small, to obtain the Na2SO4·10H2O crystal, it is better to operate at 10 - 25°C.

[0021] Optimally, separating Na2SO4·10H2O from the solution at 15°C can obtain the maximum mass.

[0022] In a preferred embodiment of the present invention, when 0.623 ≤ y0 / x0 ≤ 1.702, S1 includes the following steps:

[0023] Add water or evaporate water from the mixed salt solution of potassium sulfate and sodium sulfate to obtain the mixed salt solution c 10 , make the mixed salt solution c 10 in which the mass percentage of sodium sulfate is t]] Adjust the temperature of the mixed salt solution c 10 to t c1 , 25°C < t c1 < 45°C, precipitate the NaK3(SO4)2 crystal from the mixed salt solution c 10 , and the remaining solution is the co-saturated solution c 11 .

[0024] In a preferred embodiment of the present invention, the method for adjusting the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate to x2 is:

[0025] When x0 < x2, evaporate the water in the solution so that the mass percentage of sodium sulfate in the mixed salt solution after evaporation of water is x2. ed solution

[0026] In a preferred embodiment of the present invention, the method for adjusting the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate to x2 is:

[0027] When x0 > x2, water is added so that the mass percentage of sodium sulfate in the mixed salt solution after adding water is x2.

[0028] More preferably, 10℃ < t a1 <20℃.

[0029] In a preferred embodiment of the present invention, the co-saturated liquid a 21 Adding water yields solution a 22 Make the mass percentage of sodium sulfate x a22 The mass percentage of potassium sulfate is y. a22 x a22 =26.194%, y a22 =4.266%, cooled to 15℃, NaK3(SO4)2 and Na2SO4·10H2O continued to precipitate.

[0030] In t a2 The following describes the separation of NaK3(SO4)2 from the solution (because Na2SO4·10H2O needs to be separated from the saturated solution after separating NaK3(SO4)2 at 15℃, and t is required). a2 If the temperature is varied between 15℃ and 15℃, then the separation of NaK3(SO4)2 needs to be carried out at t a2 conduct).

[0031] In a preferred embodiment of the present invention, for the co-saturated liquid b 21 Add water to obtain solution b 22 Make the mass percentage of sodium sulfate x b22 The mass percentage of potassium sulfate is y. b22 x b22 =5.7%, y b22 =9.7%, and cooled to 15℃ to dissolve the excess NaK3(SO4)2, and filtered to obtain pure Na2SO4·10H2O crystals.

[0032] In a preferred embodiment of the present invention, the co-saturated liquid c 11 Adding water yields solution c 20 , so that solution c 20 The mass percentage of sodium sulfate in the medium is x c2 The mass percentage of potassium sulfate is y. c2 20% < x c2 <30%, 2% <y c2 <10%, adjust co-saturated solution c 20 The temperature is t c2 10℃<t c2 At <20℃, filtration yielded Na2SO4·10H2O crystals and a co-saturated solution c3.

[0033] In a preferred embodiment of the present invention, 10℃ < tb1 <20℃.

[0034] In a preferred embodiment of the present invention, 30℃ < t a2 <40℃.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] On the one hand, the heterogeneity of NaK3(SO4)2 (sodium potassium sulfate) is utilized. By adding an appropriate amount of water, some K2SO4 and all Na2SO4 in sodium potassium sulfate are dissolved to form a co-saturated solution (sodium sulfate mass percentage 5.7% and potassium sulfate mass percentage 9.7%), while leaving pure K2SO4 crystals, thereby achieving the purpose of separating pure K2SO4 from sodium potassium sulfate.

[0037] On the other hand, the co-saturated solution (30.7%, 5%) after evaporating NaK3(SO4)2 (sodium potassium sulfate) from the lithium precipitation mother liquor will precipitate a mixed salt of Na2SO4·10H2O + NaK3(SO4)2 at low temperatures (below 30℃). This mixed salt slurry is diluted with an appropriate amount of water. When the system just enters the Na2SO4·10H2O crystallization zone, all the potassium sulfate in the NaK3(SO4)2 solid solution is dissolved, leaving only Na2SO4·10H2O solid, thus achieving the purpose of extracting Na2SO4·10H2O from the co-saturated solution (30.7%, 5%).

[0038] Combining these two crystallization processes allows for the complete separation of K2SO4 and Na2SO4 from lithium precipitation mother liquor. Attached Figure Description

[0039] Figure 1 This is a flowchart illustrating an embodiment of the present invention.

[0040] Figure 2 This is a flowchart illustrating an embodiment of the present invention.

[0041] Figure 3 This is a flowchart illustrating an embodiment of the present invention. Detailed Implementation

[0042] After conversion and roasting of lepidolite or lithium feldspar ore, Li + and K + After dissolving in water, the lithium sulfate will enter the leachate along with the precipitate. Generally, to reduce the entrainment loss of Li, sodium carbonate is added to precipitate lithium first. After precipitation, the lithium sulfate content in the solution is very low (approximately 2-8 g / L), so the influence of Li can be ignored. The solution is then simplified, and the lithium precipitation mother liquor is analyzed as a ternary aqueous salt system of sodium sulfate and potassium sulfate. Specifically, in this embodiment, the mixed salt solution of potassium sulfate and sodium sulfate uses the lithium precipitation mother liquor.

[0043] Discuss the process routes for separating potassium sulfate and sodium sulfate from the lithium precipitation mother liquor in the following three cases. Let the mass content of Na2SO4 in the solution be x and the mass content of K2SO4 be y.

[0044] For lithium precipitation mother liquor in Case 1, when K2SO4 / Na2SO4 > 1.702 in the mother liquor:

[0045] K2SO4 / Na2SO4 = k

[0046] Separate K2SO4 from the lithium precipitation mother liquor at 15°C. The boundary line for precipitating pure K2SO4 is x / 0.57 = (1 - y) / 9.03, and the boundary line for K2SO4 starting to precipitate from nothing is x / 0.57 = (y - 0.093) / 0.04. The intersection points of the evaporation line y = kx of the lithium precipitation mother liquor with the two boundary lines are (x1, y1) and (x2, y2). When x < x2, evaporate the water in the solution until x = x2 (when x < x1, it is also evaporated to x = x2) to maximize the amount of precipitated K2SO4; when x > x2, add water to make x = x2 in the solution, wash away the precipitated Na2SO4 in the solution, and leave pure K2SO4.

[0047] The co-saturated solution after separating K2SO4 is (5.7%, 9.7%). Further evaporate the solution until it intersects with the complete precipitation boundary line of NaK3(SO4)2, (y - 0.05) / 7.3614 = (0.307 - x) / 0.9314, at the point (25.781%, 43.873%). Stop evaporation, filter out pure NaK3(SO4)2 at 35°C, and the remaining saturated solution is (30.7%, 5%).

[0048] Cool this saturated solution to 15°C, and NaK3(SO4)2 and Na2SO4·10H2O will continue to precipitate. Add water until it intersects with the complete dissolution line of NaK3(SO4)2, y / 0.76 = (0.441 - x) / 3.19, at the point (26.194%, 4.266%). Dissolve all the precipitated NaK3(SO4)2 during cooling, and filter to obtain pure Na2SO4·10H2O product and a saturated solution (12.2%, 7.6%).

[0049] This saturated solution can be mixed with the previously precipitated K2SO4 saturated solution (5.7%, 9.7%) and enter the next evaporation cycle to extract NaK3(SO4)2 at 35°C.

[0050] For the obtained NaK3(SO4)2 crystals, add water to (5.123%, 18.834%) at 15°C, wash away the Na2SO4 in the crystals, and obtain pure K2SO4 solid and a co-saturated solution (5.7%, 9.7%). This co-saturated solution then enters the next evaporation cycle to precipitate NaK3(SO4)2 at 35°C.

[0051] Case 2 of the lithium precipitation mother liquor, when K2SO4 / Na2SO4 in the mother liquor < 0.623:

[0052] Separate S10 from the lithium precipitation mother liquor at 15 °C. The evaporation line of the lithium precipitation mother liquor y = kx intersects the boundary line of the precipitation of pure S10 y / 0.76 = (0.441 - x) / 3.19 at (x3, y3). When x < x3, evaporate the solution to x = x3 to maximize the precipitation of Na2SO4·10H2O; when x > x3, add water to the slurry to x = x3 to just completely dissolve NaK3(SO4)2 in the crystalline substance, leaving pure Na2SO4·10H2O.

[0053] The co-saturated solution after separating S10 is (12.2%, 7.6%). Evaporate this solution until it intersects the boundary line of the complete precipitation of NaK3(SO4)2 (y - 0.05) / 7.3614 = (0.307 - x) / 0.9314 at the point (29.043%, 18.093%). Stop evaporation and filter out pure NaK3(SO4)2 at 35 °C. The remaining saturated solution is (30.7%, 5%). Cool this saturated solution to 15 °C and add water until it reaches the intersection point (26.194%, 4.266%) with the complete dissolution line of NaK3(SO4)2 y / 0.76 = (0.441 - x) / 3.19. Filter to obtain pure S10 product and saturated solution (12.2%, 7.6%). The saturated solution (12.2%, 7.6%) enters the aforementioned evaporation cycle to produce pure NaK3(SO4)2.

[0054] Add water to the obtained pure NaK3(SO4)2 crystals at 15 °C to (5.123%, 18.834%) to wash away Na2SO4 in the crystals, obtaining pure K2SO4 solid and co-saturated solution (5.7%, 9.7%). This co-saturated solution can be mixed with the co-saturated solution (12.2%, 7.6%) after separating S10. Evaporate the solution line y = kx (0.623 < k < 1.702) until it intersects the boundary line of the complete precipitation of NaK3(SO4)2 (y - 0.05) / 7.3614 = (0.307 - x) / 0.9314 at the intersection point (x4, y4). Separate NaK3(SO4) solid from the co-saturated solution (30.7%, 5%) at 35 °C. The co-saturated solution (30.7%, 5%) enters the aforementioned cooling system to recycle and produce S10.

[0055] Case 3 of the lithium precipitation mother liquor, when 0.623 ≤ K2SO4 / Na2SO4 ≤ 1.702:

[0056] Separate NaK3(SO4)2 from the lithium precipitation mother liquor at 35°C. The evaporation line of the lithium precipitation mother liquor is y = kx, which intersects with the complete precipitation boundary line of NaK3(SO4)2, ((y - 0.05) / 7.3614 = (0.307 - x) / 0.9314), at the point (x4, y4). When x < x4, evaporate the solution to x = x4 to maximize the production of pure NaK3(SO4)2 crystals; when x > x4, add water to the slurry at 35°C to x = x4 to dissolve Na2SO4 in the mixed salts, leaving pure NaK3(SO4)2 solids. Separate the NaK3(SO4)2 solids at 35°C to obtain a co-saturated solution (30.7%, 5%). Cool this co-saturated solution to 15°C and add water to the intersection point (26.194%, 4.266%) with the complete dissolution line of NaK3(SO4)2, y / 0.76 = (0.441 - x) / 3.19, and filter to obtain pure S10 product and a co-saturated solution (12.2%, 7.6%).

[0057] Add water to the obtained pure NaK3(SO4)2 crystals at 15°C to (5.123%, 18.834%) to wash away Na2SO4 in the crystals and obtain pure K2SO4 solids and a co-saturated solution (5.7%, 9.7%).

[0058] The co-saturated solution (5.7%, 9.7%) can be mixed with the co-saturated solution (12.2%, 7.6%) after separating S10 for evaporation recycling, and produce NaK3(SO4)2 solids and a co-saturated solution (30.7%, 5%) again at 35°C. The co-saturated solution (30.7%, 5%) then enters the cooling crystallization cycle to reproduce S10.

[0059] The previous analysis proposed that the optimal temperature for extracting sodium potassium mirabilite (NaK3(SO4)2) after evaporation is 35°C. However, considering that the lower the temperature during the evaporation process, the higher the vacuum degree, and the larger the volume of secondary steam extracted during evaporation, the power increase of the MVR steam compressor is too large, which is very uneconomical in terms of energy consumption. Therefore, the evaporation process should be carried out under the most suitable condition of 85°C for MVR, and then the slurry is cooled to 35°C for crystallization to remove NaK3(SO4)2 crystals. When using multi-effect evaporation to treat the lithium precipitation mixed solution, the discharge end can be set at the last effect to make the discharge temperature as low as possible. If the temperature of the last effect can be exactly 35°C, then there is no need for further cooling crystallization, and the slurry of the last effect directly separates out NaK3(SO4)2 crystals and crystallization mother liquor. The crystallization mother liquor is then added with water and cooled to 15°C, and finally the Na2SO4·10H2O crystallization product is removed. To reduce the loss of Li + in the entrained liquid of Na2SO4·10H2O, mirabilite should be redissolved by adding water and heating, and solid-liquid separation should be carried out at a temperature above 33°C (the saturated crystallization temperature of Na2SO4·10H2O is 32.24°C) to obtain anhydrous sodium sulfate products, and then the separated filtrate is returned to the processes such as leaching before lithium precipitation for recycling.

[0060] Both dilution and redissolution water additions come from the condensate of the secondary steam during evaporation. Excess condensate is either leached back or used to prepare sodium carbonate alkali. Specific Implementation Example 1:

[0062] like Figure 1 As shown, when K2SO4 / Na2SO4 < 0.623, the following scheme is used to separate potassium and sodium from a 100 t / h lithium precipitation mother liquor (Na2SO4: 23.60%, K2SO4: 7.63%):

[0063] Adding 26.136 t / h of pure water to 100 t / h of lithium precipitation mother liquor and performing solid-liquid separation at 15℃ yields 25.741 t / h of pure Na2SO4·10H2O and 100.395 t / h of co-saturated liquor (12.2%, 7.6%). If the water addition is less than 26.136 t / h, the precipitated solid will contain Na2SO4·10H2O and NaK3(SO4)2, affecting product quality. If the water addition is greater than 26.136 t / h, the S10 yield will decrease, and the recovery rate will drop. When the water addition is greater than 97.415 t / h, no S10 will be produced, and the S10 recovery rate will be 0.

[0064] 100.395 t / h of cosaturated liquid (12.2%, 7.6%) was added to 141.26 t / h of cosaturated liquid (12.2%, 7.6%) after subsequent cooling and sodium precipitation. After mixing, 140.13 t / h of water was evaporated. Solid-liquid separation was carried out at 35°C to obtain 18.048 t / h of pure NaK3(SO4)2 solid and 83.471 t / h of cosaturated liquid (30.7%, 5%).

[0065] 18.048 t / h of pure NaK3(SO4)2 solid was added to 57.23 t / h of water, and the solid-liquid mixture was separated at 15 °C to obtain 7.63 t / h and 67.65 t / h of pure K2SO4 solid co-saturated liquid (5.7%, 9.7%). If the water addition rate is greater than 57.23 t / h, the amount of pure K2SO4 obtained will be less than 7.63 t / h, resulting in a decrease in yield; if the water addition rate is less than 57.23 t / h, some NaK3(SO4)2 will be mixed in with the recrystallized K2SO4 solid, affecting product quality.

[0066] A mixture of 67.65 t / h cosaturated liquor (5.7%, 9.7%) and 83.471 t / h cosaturated liquor (30.7%, 5%) was prepared, and 17.911 t / h of water was added to obtain a mixed liquor with a capacity of 169.032 t / h. Solid-liquid separation of this mixture at 15°C yielded 27.774 t / h of pure S10 solid and 141.26 t / h of cosaturated liquor (12.2%, 7.6%). If the water addition is less than 17.911 t / h, the S10 crystals will contain impurities such as NaK3(SO4)2, affecting the quality of S10; if the water addition is greater than 17.911 t / h, the yield of pure S10 will be less than 27.774 t / h.

[0067] The 141.26 t / h co-saturated liquid (12.2%, 7.6%) after cold precipitation of S10 is returned to the evaporation process for recycling to produce pure NaK3(SO4)2 solid.

[0068] For the detailed process flow of this example, please refer to the appendix. Figure 1 . Specific Implementation Example 2:

[0070] When K2SO4 / Na2SO4 > 1.702, the following scheme is used to separate potassium and sodium from lithium precipitation mother liquor (containing 3% Na2SO4 and 22% K2SO4) with a feed rate of 100 t / h:

[0071] The lithium precipitation mother liquor was first evaporated at a rate of 30.47 t / h of water, then cooled to 15°C to separate K2SO4 at a rate of 16.895 t / h, yielding a corresponding co-saturated solution of 52.632 t / h (5.7%, 9.7%). If the evaporated water rate is greater than 30.47 t / h, NaK3(SO4)2 will be mixed into the generated K2SO4 solid, reducing product quality; if the evaporated water rate is less than 30.47 t / h, the generated K2SO4 rate is less than 16.895 t / h, and the yield of K2SO4 per batch decreases.

[0072] 52.632 t / h of co-saturated solution (5.7%, 9.7%) was added to 45.27 t / h of co-saturated solution (5.7%, 9.7%) after potassium precipitation, followed by 8.7 t / h of co-saturated solution (12.2%, 7.6%) after cooling. After mixing, water was further evaporated at a rate of 81.29 t / h, and the temperature was lowered to 35°C. Pure NaK3(SO4)2 was separated at a rate of 12.08 t / h, and a co-saturated solution of 13.231 t / h (30.7%, 5%) was obtained.

[0073] The 13.231 t / h cosaturated liquor (30.7%, 5%) was mixed with 2.28 t / h of water and cooled to 15°C, separating 6.8 t / h of S10 (Na2SO4·10H2O) to obtain 8.7 t / h of cosaturated liquor (12.2%, 7.6%).

[0074] Adding 38.29 t / h of water to 12.08 t / h of pure NaK3(SO4)2 solid and separating the slurry at 15°C yields 5.11 t / h of K2SO4 product and 45.27 t / h of co-saturated liquor (5.7%, 9.7%). This co-saturated liquor is mixed with the aforementioned 8.7 t / h co-saturated liquor (12.2%, 7.6%) and the 52.632 t / h co-saturated liquor (5.7%, 9.7%) obtained from the first evaporation and cooling potassium precipitation, and then returned to the evaporation process for recycling and extraction of pure NaK3(SO4)2 solid.

[0075] For the detailed process flow of this example, please refer to the appendix. Figure 2 . Specific Implementation Example 3:

[0077] When 0.623≦K₂SO₄ / Na₂SO₄≦1.702, the following scheme is used to separate potassium and sodium from lithium precipitation mother liquor (containing 6% Na₂SO₄ and 6% K₂SO₄) with a feed rate of 100 t / h:

[0078] The lithium precipitation mother liquor of 100 t / h was mixed with the potassium extraction co-saturated liquor of 53.198 t / h (5.7%, 9.7%) and the sodium extraction co-saturated liquor of 17.409 t / h (12.2%, 7.6%), and 129.95 t / h of water was evaporated. Solid-liquid separation was carried out at 35 °C to obtain 14.193 t / h of pure NaK3(SO4)2 and 26.462 t / h of co-saturated liquor (30.7%, 5%). If the evaporation rate is less than 129.95 t / h, the precipitated NaK3(SO4)2 will not reach its maximum, which will reduce the single yield of K2SO4 in subsequent batches. Furthermore, the amount of S10 produced per batch by cooling the resulting saturated solution (x < 30.7%, y > 5%) will also be reduced. If the evaporation rate is greater than 129.95 t / h, Na2SO4 mixed salt will precipitate along with NaK3(SO4)2, which will make it difficult to obtain pure K2SO4 by adding water to NaK3(SO4)2, and may even prevent the obtaining of pure K2SO4 after adding water.

[0079] Adding 26.462 t / h of co-saturated solution (30.7%, 5%) to 4.55 t / h of fresh water and cooling to 15°C will precipitate 13.61 t / h of S10 (Na2SO4·10H2O), while simultaneously yielding 17.409 t / h of co-saturated solution (12.2%, 7.6%). If the added water volume is less than 4.55 t / h of fresh water, NaK3(SO4)2 will precipitate along with S10 at 15°C, resulting in no pure S10 product. If the added water volume is greater than 4.55 t / h, the amount of pure S10 precipitated will be less than 13.61 t / h.

[0080] Adding 45.01 t / h of water to 14.193 t / h of pure NaK3(SO4)2 and performing solid-liquid separation at 15℃ yields 6 t / h of pure K2SO4 and 53.198 t / h of co-saturated liquor (5.7%, 9.7%). If the water addition is less than 45.01 t / h, pure K2SO4 will not be obtained, affecting the quality of K2SO4; if the water addition is greater than 45.01 t / h, the yield of K2SO4 will be reduced (K2SO4 < 6 t / h).

[0081] The 53.198 t / h co-saturated liquor (5.7%, 9.7%) and 17.409 t / h co-saturated liquor (12.2%, 7.6%) were returned and mixed with the feed lithium precipitation mother liquor, and then recycled and evaporated to extract NaK3(SO4)2.

[0082] For the detailed process flow of this example, please refer to the appendix. Figure 3 .

[0083] The processes of wet material centrifugation and product drying in the product manufacturing process are not described in detail, but they are all integral parts of this invention and are within the scope of this invention.

[0084] Whether the evaporation process specifically employs MVR, multi-effect evaporation, or other evaporation methods is within the scope of this invention and is not limited to any particular form.

[0085] The specific cooling process, whether using a cooling unit (lithium bromide, calcium chloride, or ethanol-water, etc.), steam refrigeration, or other refrigeration methods, is within the scope of this invention and is not limited to any particular form.

[0086] The principle of separating potassium and sodium salts using crystallization in this invention is mainly based on the following set of experimental data:

[0087] Experimental Analysis 1: When the system is at 15℃, K2SO4 / Na2SO4 > 9.7 / 5.7 = 1.702; or at 25℃, K2SO4 / Na2SO4 > 10.85 / 3 = 3.617; or at 30℃, K2SO4 / Na2SO4 > 11.6 / 5.9 = 1.966; or at 35℃, K2SO4 / Na2SO4 > 12.2 / 5.95 = 2.05; or at 60℃, K2SO4 / Na2SO4 > 14 / 6 = 2.33; or at 75℃, K2SO4 / Na2SO4 > 16.6 / 5.9 = 2.814; or at 100℃, K2SO4 / Na2SO4 > 18.8 / 5.6 = 3.357. K₂SO₄ can be crystallized and precipitated individually by adjusting the water volume in the system through evaporation or by adding water. This also indicates that the greater the difference between the potassium-to-sodium ratio in the feed and the boundary value, the more K₂SO₄ can be precipitated. When the K₂SO₄ / Na₂SO₄ ratio is at its minimum at 15℃, the amount of precipitated K₂SO₄ is the maximum. The order of K₂SO₄ precipitation from largest to smallest, arranged by temperature, is: 15℃ > 30℃ > 35℃ > 60℃ > 75℃ > 100℃ > 25℃.

[0088] Experimental Analysis 2: When the K₂SO₄ / Na₂SO₄ ratio of the feed is less than the K₂SO₄ / Na₂SO₄ ratios at the temperatures mentioned above, and at 15℃, K₂SO₄ / Na₂SO₄ < 7.6 / 12.2 = 0.623; at 25℃, K₂SO₄ / Na₂SO₄ < 6.2 / 22.2 = 0.279; at 30℃, K₂SO₄ / Na₂SO₄ < 5.1 / 29.6 = 0.172; at 35℃, K₂SO₄ / Na₂SO₄ < 5 / 30.7 = 0.163; at 60℃, K₂SO₄ / Na₂SO₄ < 5.9. When K₂SO₄ / Na₂SO₄ = 0.199 at 75℃ and K₂SO₄ / Na₂SO₄ < 7.5 / 28 = 0.268 at 100℃ and K₂SO₄ / Na₂SO₄ < 9 / 27 = 0.333 at 100℃, the individual crystallization of Na₂SO₄ or Na₂SO₄·10H₂O can be achieved by adjusting the water volume in the system through adding water or evaporation (when the temperature is < 32.24℃, the crystal is S₁₀:Na₂SO₄·10H₂O). Therefore, to increase the yield of Na₂SO₄, the lower the K₂SO₄ / Na₂SO₄ ratio in the feed solution and the greater the difference from the boundary value, the more sodium sulfate will precipitate. The sodium sulfate precipitation amount, arranged from highest to lowest temperature, is: 15℃ > 100℃ > 25℃ > 75℃ > 60℃ > 30℃ > 35℃. As can be seen from the above, when the K2SO4 / Na2SO4 ratio of the feed is relatively small, it is better to operate at 15℃ to obtain Na2SO4·10H2O crystals; and it is better to operate at 100℃ to obtain Na2SO4.

[0089] Experimental Analysis 3: When the K₂SO₄ / Na₂SO₄ ratio of the feed is at various temperatures ranging from 15 to 100℃, at 15℃, 0.623≦K₂SO₄ / Na₂SO₄≦1.702; at 25℃, 0.279≦K₂SO₄ / Na₂SO₄≦3.617; at 30℃, 0.172≦K₂SO₄ / Na₂SO₄≦1.966; and at 35℃, 0.163≦K₂SO₄ / Na₂SO₄≦ When SO4≦2.05; at 60℃, 0.199≦K2SO4 / Na2SO4≦2.33; at 75℃, 0.268≦K2SO4 / Na2SO4≦2.814; and at 100℃, 0.333≦K2SO4 / Na2SO4≦3.357, by adjusting the water volume through evaporation, NaK3(SO4)2 (sodium potassium sulfate) crystals will precipitate directly from the potassium and sodium solutions at the corresponding temperatures. Further consideration is needed on how to separate Na2SO4 from this co-saturated solution and how to separate K2SO4 from NaK3(SO4)2.

[0090] It should be noted that it is impossible to further separate potassium and sodium salts from the saturated solution after separating NaK3(SO4)2 without changing the temperature. This is because the solid composition obtained by evaporating the solution to dryness has the same proportion of potassium and sodium salts as in the saturated solution. Therefore, Na2SO4 crystals can only be obtained by further crystallizing from the saturated solution by changing the temperature and adjusting the water content of the system.

[0091] Experimental analysis 2 has shown that the K2SO4 / Na2SO4 boundary value for separating Na2SO4 (or S10) from solution varies at different temperatures. The temperature at which the difference between the two boundary values ​​is greatest is the optimal operating temperature for separating Na2SO4 from the co-saturated solution. The maximum value (0.623) is found at 15℃, and the minimum value (0.163) is found at 35℃. Therefore, it can be determined that the optimal operating temperature for extracting Na2SO4 from the co-saturated solution after extracting NaK3(SO4)2 is the change from 35℃ to 15℃.

[0092] Based on the heterogeneous composition of NaK3(SO4)2, adding water at various temperatures will completely dissolve both Na2SO4 and some K2SO4, leaving solid K2SO4. The remaining task is to find the temperature at which the maximum amount of K2SO4 can be separated from a given amount of solid NaK3(SO4)2. According to the molecular formula of NaK3(SO4)2, its K2SO4 / Na2SO4 ratio is 7.8614 / 2.1386 = 3.676. Comparing experimental data on the K2SO4 / Na2SO4 ratio in solutions where K2SO4 can be separated, this value is smallest at 15℃, with K2SO4 / Na2SO4 = 1.703, the largest difference from 3.676. Therefore, the optimal temperature for separating K2SO4 from NaK3(SO4)2 is 15℃.

[0093] In summary, the optimal operating conditions for the physical separation of K2SO4 and Na2SO4 from any mixed solution are: separating K2SO4 from the solution or from solid NaK3(SO4)2 at 15℃; separating Na2SO4·10H2O from the solution at 15℃; and separating NaK3(SO4)2 from the solution at 35℃ (because separating Na2SO4·10H2O from the saturated solution after separating NaK3(SO4)2 at 15℃ requires a temperature change operation from 15℃ to 35℃, therefore the separation of NaK3(SO4)2 needs to be carried out at 35℃).

Claims

1. A process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate, characterized in that... It includes the following steps: S1. Separate NaK3(SO4)2 crystals from the mixed salt solution of potassium sulfate and sodium sulfate, where the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate is x0, and the mass percentage of potassium sulfate is y0; S2 and NaK3(SO4)2 crystals are dissolved in pure water to obtain slurry a3. The temperature of slurry a3 is controlled at t. a3 10℃<t a3 <20℃, so that the mass percentage of sodium sulfate in slurry a3 is x a3 The mass percentage of potassium sulfate is y. a3 2% < x a3 <10%, 15% <y a3 <20%, K2SO4 crystals precipitate; S3. Cool the filtrate in S2 to 15°C, dissolve the excess NaK3(SO4)2, and filter to obtain pure Na2SO4·10H2O crystals; When y0 / x0 > 1.702, S1 includes the following steps: A mixed salt solution of potassium sulfate and sodium sulfate is obtained by adding water or evaporating water. 10 , to make the mixed salt solution a 10 The mass percentage of sodium sulfate in the medium is Mixed salt solution a 10 The temperature is t a1 10℃<t a1 <25℃, from mixed salt solution a 10 K₂SO₄ crystals precipitate out, and the remaining solution is a co-saturated solution. 11 ; saturated liquid a 11 Continue evaporating the water to obtain a mixed salt solution a 20 , to make the mixed salt solution a 20 The mass percentage of sodium sulfate in the medium is x a2 The mass percentage of potassium sulfate is y. a2 20% < x a2 <30%, 40% <y a2 <50%, adjust the mixed salt solution a 20 temperature t a2 25℃<t a2 At temperatures below 45℃, NaK3(SO4)2 crystals were separated, and the remaining solution was a co-saturated solution. 21 ; When y0 / x0 < 0.623, S1 includes the following steps: A mixed salt solution of potassium sulfate and sodium sulfate is obtained by adding water or evaporating water to obtain mixed salt solution b. 10 , so that the mixed salt solution b 10 The mass percentage of sodium sulfate in the medium is Mixed salt solution b 10 The temperature is t b1 10℃<t b1 <25℃, from mixed salt solution b 10 Pure Na₂SO₄·10H₂O crystals were separated, and the remaining solution was the co-saturated solution b after the separation of Na₂SO₄·10H₂O. 11 ; The co-saturated solution b after separating Na2SO4·10H2O 11 Evaporation of water yields mixed salt solution b 20 , so that the mixed salt solution b 20 The mass percentage of sodium sulfate in the medium is x b2 The mass percentage of potassium sulfate is y. b2 20% < x b2 <40%, 10% <y b2 <30%, adjust mixed salt solution b 20 temperature t b2 25℃<t b2 At <45℃, NaK3(SO4)2 crystals were separated, and the remaining solution was the co-saturated solution b. 21 ; When 0.623 ≤ y0 / x0 ≤ 1.702, S1 includes the following steps: A mixed salt solution of potassium sulfate and sodium sulfate is obtained by adding water or evaporating water to obtain a mixed salt solution c. 10 , so that the mixed salt solution c 10 The mass percentage of sodium sulfate in the medium is Adjusting the c of the mixed salt solution 10 The temperature is t c1 25℃<t c1 <45℃, from mixed salt solution c 10 NaK3(SO4)2 crystals precipitate out, and the remaining solution is a co-saturated solution. 11 .

2. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, The method for adjusting the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate to x2 is as follows: When x0 < x2, evaporate the water in the solution so that the mass percentage of sodium sulfate in the mixed salt solution after evaporating the water is x2.

3. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, The method for adjusting the mass percentage of sodium sulfate in the mixed salt solution of potassium sulfate and sodium sulfate to x2 is as follows: When x0 > x2, add water so that the mass percentage of sodium sulfate in the mixed salt solution after adding water is x2.

4. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, 10℃＜t a1 ＜20℃。 5. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, co-saturated liquid a 21 Adding water yields solution a 22 Make the mass percentage of sodium sulfate x a22 The mass percentage of potassium sulfate is y. a22 x a22 =26.194%, y a22 =4.266%, cooled to 15℃, NaK3(SO4)2 and Na2SO4·10H2O continued to precipitate.

6. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, For saturated liquid b 21 Add water to obtain solution b 22， Make the mass percentage of sodium sulfate x b22 The mass percentage of potassium sulfate is y. b22 x b22 =5.7%, y b22 =9.7%, and cooled to 15℃ to dissolve the excess NaK3(SO4)2, and filtered to obtain pure Na2SO4·10H2O crystals.

7. The process for recovering potassium sulfate from a mixed solution of potassium sulfate and sodium sulfate according to claim 1, characterized in that, To the co-saturated liquid c 11 Adding water yields solution c 20 , so that solution c 20 The mass percentage of sodium sulfate in the medium is x c2 The mass percentage of potassium sulfate is y. c2 20% < x c2 <30%, 2% <y c2 <10%, adjust co-saturated solution c 20 The temperature is t c2 10℃<t c2 At <20℃, filtration yielded Na2SO4·10H2O crystals and a co-saturated solution c3.

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