Combined preparation method of water-soluble potassium sulfate and feed-grade potash magnesium sulfate fertilizer

Abstract

The invention belongs to the technical field of chemical fertilizer production and livestock feed, and particularly relates to a combined preparation method of water-soluble potassium sulfate and feed-grade potash magnesium sulphate fertilizer. According to the method, the picromerite concentrate wet material is taken as an initial raw material, the picromerite concentrate wet material with relatively low potassium and magnesium content is purified into high-grade potassium sulfate and the potash magnesium sulphate fertilizer by utilizing hot melt mother liquor at different mother liquor points through multi-stage hot melt and multi-stage vacuum evaporative crystallization, so that efficient distribution and conversion of potassium and magnesium elements in the raw material are realized, resource waste is avoided, and the production cost is reduced. The comprehensive utilization rate of raw materials is greatly improved. Two high-added-value products, namely the water-soluble potassium sulfate and the feed-grade potassium-magnesium fertilizer, are prepared at the same time, and the blank of combined preparation of raw materials required by agriculture and animal husbandry is filled.

Description

Technical Field

[0001] This invention belongs to the field of fertilizer production and livestock feed technology, specifically relating to a method for the combined preparation of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer. Background Technology

[0002] With the rapid development of agriculture and animal husbandry, the demand for high-quality water-soluble fertilizers and feed additives is increasing. Water-soluble potassium sulfate, as a high-quality potassium fertilizer, is widely used in drip irrigation and can be quickly absorbed and utilized by crops, improving crop yield and quality. Meanwhile, feed-grade potassium magnesium fertilizer plays a key role in the physiological processes of animal growth, development, and metabolism, providing strong support for animal husbandry.

[0003] This technology discloses a method for the combined preparation of fully water-soluble potassium sulfate and ordinary agricultural potassium sulfate using a water-salt system. The method utilizes cooling evaporation crystallization and conversion crystallization to jointly prepare water-soluble potassium sulfate and ordinary agricultural potassium sulfate. A drawback of this method is that a large amount of magnesium in the raw materials cannot be recovered, resulting in resource waste. Existing related technologies often focus on the preparation of single fertilizer products or have shortcomings in process conditions, raw material utilization, and product quality control. Some traditional potassium sulfate preparation methods only focus on improving product purity, neglecting the comprehensive utilization of resources during the preparation process, leading to the loss of beneficial elements such as magnesium, which not only wastes resources but also increases production costs. Furthermore, some potassium-magnesium fertilizer preparation processes cannot control the potassium and magnesium content in the product, making it difficult to meet the high standards required for feed-grade products and affecting their application in animal husbandry. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the combined preparation of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer. This invention achieves the combined preparation of high-grade water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer, realizing the efficient distribution and conversion of potassium and magnesium elements in the raw materials, avoiding resource waste, and greatly improving the comprehensive utilization rate of raw materials.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for the combined preparation of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer, comprising the following steps:

[0007] (1) Dilute the wet molten potassium magnesium sulfate concentrate with water to obtain a slurry;

[0008] (2) The slurry is heated and dissolved to obtain the first hot-dissolved mother liquor;

[0009] (3) The first hot-melt mother liquor and the first flocculant are mixed and subjected to first heat preservation sedimentation to obtain the first impurity-removed hot-melt mother liquor;

[0010] (4) The first impurity-removing hot-melt mother liquor is subjected to first vacuum evaporation crystallization, second vacuum evaporation crystallization, and third vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the first vacuum evaporation crystallization are greater than the absolute pressure and temperature of the second vacuum evaporation crystallization and the absolute pressure and temperature of the third vacuum evaporation crystallization to obtain water-soluble potassium sulfate wet material and first crystallization mother liquor. The water-soluble potassium sulfate wet material is subjected to first drying to obtain the water-soluble potassium sulfate.

[0011] (5) Heat the first crystallization mother liquor to obtain the second hot-melt mother liquor, and subject the second hot-melt mother liquor to a second heat preservation sedimentation to obtain the second impurity-removed hot-melt mother liquor.

[0012] (6) The second impurity-removing hot melt mother liquor is subjected to fourth vacuum evaporation crystallization, fifth vacuum evaporation crystallization, and sixth vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the fourth vacuum evaporation crystallization are greater than those of the fifth vacuum evaporation crystallization and the sixth vacuum evaporation crystallization, to obtain feed-grade potassium magnesium sulfate fertilizer wet material and second crystallization mother liquor. The feed-grade potassium magnesium sulfate fertilizer wet material is subjected to a second drying to obtain the feed-grade potassium magnesium sulfate fertilizer.

[0013] Preferably, the components of the wet molten potassium magnesium sulfate concentrate include K + 15-16 wt%, Mg 2+ 5.5–6 wt%, SO4 2- 37.5–40 wt%; In step (1): the ratio of the volume of the dilution water to the mass of the wet molten potassium magnesium sulfate concentrate is 1–2 m³. 3 1kg;

[0014] The slurry contains 40 ± 3 wt% solids.

[0015] Preferably, in step (2): before the heating and dissolving, water is further added to the slurry for dilution, and the ratio of the volume of water used for heating and dissolving to the mass of the wet soft potassium magnesium alum concentrate is 2-3 m³. 3 1 kg; the temperature for heating and dissolving is 80±2℃, and the heating and dissolving is carried out under stirring conditions;

[0016] The components of the first hot-melt mother liquor include: K + 7.5–8 wt%, Mg 2+ 2.5–3 wt%, SO4 2- 18-19 wt%, Cl - 9–9.5 wt%.

[0017] Preferably, the first flocculant is polyacrylamide, and the mass percentage of the first flocculant in the wet molten potassium magnesium alum concentrate is 0.003 to 0.01%; the overflow of the first heat-insulating sedimentation is the first impurity-removing hot-melt mother liquor, and the first heat-insulating sedimentation also yields a first sedimentation underflow. When the mass concentration of insoluble matter in the first sedimentation underflow is ≥3±0.5%, the first sedimentation underflow is discharged.

[0018] The components of the first impurity-removing hot-melt mother liquor include: K + 7.5–8 wt%, Mg 2+ 2.4–2.8 wt%, SO4 2- 18–18.5 wt%, Cl - 9–9.5 wt%.

[0019] Preferably, the absolute pressure of the first vacuum evaporation crystallization is 15-20 kPa and the temperature is 65±2℃; the absolute pressure of the second vacuum evaporation crystallization is 5-7 kPa and the temperature is 40±2℃; and the absolute pressure of the third vacuum evaporation crystallization is 1-2 kPa and the temperature is 21±2℃.

[0020] The components of the water-soluble potassium sulfate wet material include: K + 43.3–43.5 wt%, Mg 2+ 0.03–0.05 wt%, SO4 2- 52.3–52.6 wt%, Cl - 0.03wt%;

[0021] The temperature of the first drying process is 180±2℃, and the time is 20±1min.

[0022] Preferably, the components of the water-soluble potassium sulfate include: K + 44.5–45 wt%, Mg 2+ 0.015–0.02 wt%, SO4 2- 55.4–55.6 wt%, Cl - 0.06~0.07wt%, H2O≤0.03wt%, water-insoluble matter 0.03~0.07wt%.

[0023] Preferably, in step (5), the heating is carried out under stirring conditions, and the temperature of the second hot-melt mother liquor is 82±2℃;

[0024] The second hot-melt mother liquor consists of: K + 3.8–3.9 wt%, Mg 2+ 3.2–3.3 wt%, SO4 2- 16.2–16.5 wt%, Cl -9.2–9.3 wt%;

[0025] Before the second heat preservation sedimentation, the process further includes mixing the second hot-melt mother liquor and the second flocculant; the second flocculant is polyacrylamide, and the mass percentage of the second flocculant in the wet molten potassium magnesium alum concentrate is ≤0.0035%; the overflow of the second heat preservation sedimentation is the second impurity removal hot-melt mother liquor, and the second heat preservation sedimentation also yields a second sedimentation underflow. When the mass concentration of insoluble matter in the second sedimentation underflow is ≥3±0.5wt%, the second sedimentation underflow is discharged.

[0026] The components of the second impurity-removing hot-melt mother liquor include: K + 3.8–3.9 wt%, Mg 2+ 3.0–3.1 wt%, SO4 2- 16.1–16.3 wt%, Cl - 9.0–9.1 wt%.

[0027] Preferably, the absolute pressure of the fourth vacuum evaporation crystallization is 15-20 kPa and the temperature is 65±2℃; the absolute pressure of the fifth vacuum evaporation crystallization is 5-7 kPa and the temperature is 40±2℃; and the absolute pressure of the sixth vacuum evaporation crystallization is 1-2 kPa and the temperature is 21±2℃.

[0028] The components of the feed-grade potassium magnesium sulfate fertilizer wet feed include: K + 19.5–20.3 wt%, Mg 2+ 5.9–6.1 wt%, SO4 2- 47.3–47.9 wt%, Cl - 0.03–0.05 wt%;

[0029] The second drying temperature is 180±2℃ and the time is 20±1min.

[0030] Preferably, the feed-grade potassium magnesium sulfate fertilizer comprises: K + 22.3–22.5 wt%, Mg 2+ 8.1–8.5 wt%, SO4 2- 62.5–63.3 wt%, Cl - 0.05~0.08wt%, water-insoluble matter 0.01~0.02wt%.

[0031] Preferably, after obtaining the second crystallization mother liquor, the method further includes: combining the second crystallization mother liquor and the first crystallization mother liquor and heating them to prepare a second hot-melt mother liquor.

[0032] This invention provides a combined preparation method for water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer. The invention uses wet soft potassium magnesium sulfate concentrate (obtained by potassium mixed salt flotation process, also referred to as wet potassium magnesium sulfate fertilizer concentrate) as the initial raw material. It employs multi-stage hot dissolution and multi-stage vacuum evaporation crystallization. During multi-stage vacuum evaporation crystallization, different mother liquor points are utilized (during evaporation crystallization, solutes (such as potassium and magnesium sulfates) in the solution crystallize out as their concentration reaches saturation, and the remaining uncrystallized solution is the "mother liquor"). Because it is a "multi-stage crystallization," the conditions for each stage of crystallization are different. Different compositions of the precipitated crystals (target product) will result in variations in the composition of the residual mother liquor (potassium, magnesium content, proportion of other impurities, etc.). In this invention, these mother liquors with specific potassium-magnesium ratios, corresponding to a particular crystallization stage (referred to as "mother liquor points"), are used to purify wet soft potassium magnesium sulfate concentrate with low potassium and magnesium content into high-grade water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer. This achieves efficient allocation and conversion of potassium and magnesium elements in the wet soft potassium magnesium sulfate concentrate raw material, avoiding resource waste and greatly improving the comprehensive utilization rate of raw materials. This invention simultaneously prepares two high-value-added products: water-soluble potassium sulfate and feed-grade potassium magnesium fertilizer, filling the gap in the joint preparation of raw materials needed for agriculture and animal husbandry.

[0033] Compared with the prior art, the beneficial effects of the present invention include:

[0034] (1) The water-soluble potassium sulfate prepared by this invention has high purity, low impurity content, complete crystal structure, and good solubility. Compared with traditional potassium fertilizer, it can increase the absorption efficiency of potassium by crops by 20-30%. The feed-grade potassium-magnesium fertilizer prepared by this invention has high purity, stable and qualified potassium and magnesium content, and can meet the needs of animals at different growth stages for these nutrients.

[0035] (2) The process conditions of the hot melting, heat preservation sedimentation and vacuum evaporation crystallization steps adopted in this invention are mild, which reduces the investment cost and operating energy consumption of the equipment. No waste gas is generated in the entire preparation process, and the wastewater can be reused, which meets the national environmental protection standards.

[0036] (3) This invention can simultaneously prepare water-soluble potassium sulfate and feed-grade potassium magnesium fertilizer, which can meet the needs of the two major markets of agriculture and animal husbandry, broaden the sales channels of the products, and allow enterprises to flexibly adjust the production ratio of the two products according to market demand, thereby improving the market competitiveness of the enterprise. Attached Figure Description

[0037] Figure 1 This is a process flow diagram of the combined preparation method of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer. Detailed Implementation

[0038] This invention provides a method for the combined preparation of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer, comprising the following steps:

[0039] (1) Dilute the wet molten potassium magnesium sulfate concentrate with water to obtain a slurry;

[0040] (2) The slurry is heated and dissolved to obtain the first hot-dissolved mother liquor;

[0041] (3) The first hot-melt mother liquor and the first flocculant are mixed and subjected to first heat preservation sedimentation to obtain the first impurity-removed hot-melt mother liquor;

[0042] (4) The first impurity-removing hot-melt mother liquor is subjected to first vacuum evaporation crystallization, second vacuum evaporation crystallization, and third vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the first vacuum evaporation crystallization are greater than the absolute pressure and temperature of the second vacuum evaporation crystallization and the absolute pressure and temperature of the third vacuum evaporation crystallization to obtain water-soluble potassium sulfate wet material and first crystallization mother liquor. The water-soluble potassium sulfate wet material is subjected to first drying to obtain the water-soluble potassium sulfate.

[0043] (5) Heat the first crystallization mother liquor to obtain the second hot-melt mother liquor, and subject the second hot-melt mother liquor to a second heat preservation sedimentation to obtain the second impurity-removed hot-melt mother liquor.

[0044] (6) The second impurity-removing hot melt mother liquor is subjected to fourth vacuum evaporation crystallization, fifth vacuum evaporation crystallization, and sixth vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the fourth vacuum evaporation crystallization are greater than those of the fifth vacuum evaporation crystallization and the sixth vacuum evaporation crystallization, to obtain feed-grade potassium magnesium sulfate fertilizer wet material and second crystallization mother liquor. The feed-grade potassium magnesium sulfate fertilizer wet material is subjected to a second drying to obtain the feed-grade potassium magnesium sulfate fertilizer.

[0045] In this invention, unless otherwise specified, all raw materials / components used in the preparation are commercially available products well known to those skilled in the art.

[0046] This invention involves diluting wet molten potassium magnesium alum concentrate with water to obtain a slurry. In this invention, the wet molten potassium magnesium alum concentrate is preferably wet molten potassium magnesium alum concentrate produced through a potassium mixed salt flotation process (also known as potassium magnesium sulfate fertilizer wet concentrate). The preferred components of the wet molten potassium magnesium alum concentrate include: K... + 15-16 wt%, Mg 2+ 5.5–6 wt%, SO4 2- 37.5–40 wt%, in the examples, K can be: + 15.1 wt%, Mg 2+ 5.5 wt%, SO4 2- 38.2 wt%; or K + 15.3 wt%, Mg 2+5.6 wt%, SO4 2- 37.8 wt%; or K + 15.9 wt%, Mg 2+ 5.9 wt%, SO4 2- 39.8 wt%.

[0047] In this invention, the volume ratio of the dilution water to the mass of the wet molten potassium magnesium sulfate concentrate is preferably 1-2 m³. 3 1 kg. In this embodiment of the invention, the dilution water can be slightly brackish water. The mass content of solids in the slurry is preferably 40 ± 3 wt%, and in this embodiment it can be 43 wt%.

[0048] After obtaining the slurry, the present invention heats and dissolves the slurry to obtain a first hot-dissolved mother liquor. In the present invention, after obtaining the slurry, it is preferable to introduce the slurry into a hot-dissolved tank. In the present invention, before the heating and dissolving step, it is preferable to further dilute the slurry with water. The volume ratio of the water used for heating and dissolving to the mass of the wet soft potassium magnesium sulfate concentrate is preferably 2-3 m³. 3 1 kg. In this embodiment of the invention, the water used for heating and dissolving can be slightly saline water. The preferred temperature for heating and dissolving is 80±2℃, and in this embodiment it can be 80℃. The preferred holding time for heating and dissolving is 1-2 hours. The heating and dissolving is preferably carried out under stirring conditions. The components of the first hot-dissolving mother liquor preferably include: K + 7.5–8 wt%, Mg 2+ 2.5–3 wt%, SO4 2- 18-19 wt%, Cl - 9–9.5 wt%, which can be K in the examples. + 7.6 wt%, Mg 2+ 2.5wt%, SO4 2- 18.3 wt%, Cl - 9.3 wt%; or K + 7.7 wt%, Mg 2+ 2.6 wt%, SO4 2- 18.4 wt%, Cl - 9.2 wt%; or K + 7.9 wt%, Mg 2+ 2.7 wt%, SO4 2- 18.8 wt%, Cl - 9.1 wt%.

[0049] After obtaining the first hot-melt mother liquor, the present invention mixes the first hot-melt mother liquor and the first flocculant for a first heat-insulating sedimentation to obtain a first impurity-removed hot-melt mother liquor. In the present invention, after obtaining the first hot-melt mother liquor, it is preferably pumped into a heat-insulating sedimentation tank. The first flocculant is preferably polyacrylamide. The mass percentage of the first flocculant relative to the wet mass of the soft potassium magnesium sulfate concentrate is preferably 0.003–0.01%, and in the embodiments, it can be 0.01%, 0.006%, or 0.003%. In the present invention, the temperature of the first heat-insulating sedimentation is preferably 80±2℃, and in the embodiments, it can be 78℃. The time of the first heat-insulating sedimentation is preferably 30–50 min.

[0050] In this invention, the first heat-insulating sedimentation yields an overflow and a bottom flow. The overflow of the first heat-insulating sedimentation is a first impurity-removing hot-melt mother liquor. Preferably, the first heat-insulating sedimentation also yields a first sedimentation bottom flow, and when the mass concentration of insoluble matter in the first sedimentation bottom flow is preferably ≥3±0.5wt%, the first sedimentation bottom flow is discharged.

[0051] In this invention, the components of the first impurity-removing hot-melt mother liquor preferably include: K + 7.5–8 wt%, Mg 2+ 2.4–2.8 wt%, SO4 2- 18–18.5 wt%, Cl - 9–9.5 wt%; in the examples, K can be used. + 7.5 wt%, Mg 2+ 2.4 wt%, SO4 2- 18.2 wt%, Cl - 9.2 wt%; or K + 7.5 wt%, Mg 2+ 2.5wt%, SO4 2- 18.3 wt%, Cl - 9.1 wt%; or K + 7.7 wt%, Mg 2+ 2.6 wt%, SO4 2- 18.4 wt%, Cl - 9.0 wt%.

[0052] After obtaining the first impurity-removing hot-melt mother liquor, the present invention sequentially performs first vacuum evaporation crystallization, second vacuum evaporation crystallization, and third vacuum evaporation crystallization on the first vacuum evaporation crystallization, wherein the absolute pressure and temperature of the first vacuum evaporation crystallization are greater than those of the second vacuum evaporation crystallization and the third vacuum evaporation crystallization, to obtain water-soluble potassium sulfate wet material and the first crystallization mother liquor. The water-soluble potassium sulfate wet material is then subjected to a first drying process to obtain the water-soluble potassium sulfate. In the present invention, the absolute pressure of the first vacuum evaporation crystallization is preferably 15-20 kPa, and the temperature is preferably 65±2℃. The absolute pressure of the second vacuum evaporation crystallization is preferably 5-7 kPa, and the temperature is preferably 40±2℃. The absolute pressure of the third vacuum evaporation crystallization is preferably 1-2 kPa, and the temperature is preferably 21±2℃. The first, second, and third vacuum evaporation crystallizations are all preferably carried out in a vacuum crystallizer. The primary underflow and primary overflow obtained from the first vacuum evaporation crystallization are simultaneously used as the mother liquor for the second vacuum evaporation crystallization. The secondary underflow and secondary overflow obtained from the second vacuum evaporation crystallization are simultaneously used as the mother liquor for the third vacuum evaporation crystallization. The third vacuum evaporation crystallization yields a three-stage overflow and a three-stage underflow. The three-stage overflow and underflow obtained from the third vacuum evaporation crystallization are introduced into a crystallization thickener for first crystallization thickening. The first crystallization thickening yields a first thickened underflow and a first thickened overflow. In this invention, the first thickened underflow preferably undergoes a first solid-liquid separation to obtain the wet potassium sulfate material and the liquid phase component of the first thickened underflow. The liquid phase component of the first thickened underflow obtained from the first solid-liquid separation and the first thickened overflow form the first crystallization mother liquor. The first solid-liquid separation is preferably centrifugal separation.

[0053] In this invention, the components of the water-soluble potassium sulfate wet material preferably include: K + 43.3–43.5 wt%, Mg 2+ 0.03–0.05 wt%, SO4 2- 52.3–52.6 wt%, Cl - 0.03wt%; in the examples, K can be used. + 43.3 wt%, Mg 2+ 0.03wt%, SO4 2- 52.3 wt%, Cl - 0.03wt%; or K + 43.4 wt%, Mg 2+ 0.04wt%, SO4 2- 52.4 wt%, Cl - 0.03wt%; or K + 43.5 wt%, Mg 2+ 0.05wt%, SO4 2-52.6 wt%, Cl - 0.03 wt%.

[0054] The preferred temperature for the first drying step is 180±2℃, and the preferred time is 20±1 min. The water-soluble potassium sulfate preferably comprises: K + 44.5–45 wt%, Mg 2+ 0.015–0.02 wt%, SO4 2- 55.4–55.6 wt%, Cl - 0.06–0.07 wt%, H₂O ≤ 0.03 wt%, water-insoluble matter 0.03–0.07 wt%; in the examples, K can be used. + 44.5 wt%, Mg 2+ 0.02wt%, SO4 2- 55.4 wt%, Cl - 0.06 wt%, H₂O 0.03 wt%, water-insoluble matter 0.03 wt%; or K + 44.6 wt%, Mg 2+ 0.02wt%, SO4 2- 55.6 wt%, Cl - 0.07wt%, H2O 0.03wt%, water-insoluble matter 0.04wt%; or K + 45.0 wt%, Mg 2+ 0.02wt%, SO4 2- 55.4 wt%, Cl - 0.06wt%, H2O 0.03wt%, water-insoluble matter 0.07wt%.

[0055] After obtaining the first crystallization mother liquor, the present invention heats the first crystallization mother liquor to obtain a second hot-melt mother liquor, and then subjectes the second hot-melt mother liquor to a second heat preservation and sedimentation process to obtain a second impurity-removed hot-melt mother liquor. In the present invention, after obtaining the first crystallization mother liquor, the second crystallization mother liquor is preferably introduced into a hot-melt tank. The heating is preferably carried out under stirring conditions. The temperature of the second hot-melt mother liquor is preferably 82±2℃, and the second hot-melt mother liquor is subjected to heat preservation treatment after heating, the heat preservation treatment time is preferably 1-2 hours. The composition of the second hot-melt mother liquor preferably includes: K + 3.8–3.9 wt%, Mg 2+ 3.2–3.3 wt%, SO4 2- 16.2–16.5 wt%, Cl - 9.2–9.3 wt%; in the examples, K can be used. + 3.9 wt%, Mg 2 + 3.2 wt%, SO42- 16.2 wt%, Cl - 9.2 wt%; or K + 3.8 wt%, Mg 2+ 3.3 wt%, SO4 2- 16.4 wt%, Cl - 9.3 wt%; or K + 3.9 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.5 wt%, Cl - 9.2 wt%.

[0056] In this invention, prior to the second heat-insulating sedimentation, the invention preferably further includes mixing the second hot-melt mother liquor and the second flocculant. The second flocculant is preferably polyacrylamide. The mass percentage of the second flocculant relative to the wet mass of the soft potassium magnesium sulfate concentrate is preferably ≤0.0035%, and in the examples, it can be 0.0033% or 0.002%. The temperature of the second heat-insulating sedimentation is preferably 82±2℃, and the time of the second heat-insulating sedimentation is preferably 30–50 min.

[0057] The second heat-insulating sedimentation process yields an overflow and a bottom flow. The overflow from the second heat-insulating sedimentation is a second impurity-removing hot-melt mother liquor. The second heat-insulating sedimentation process also yields a second sedimentation bottom flow. When the mass concentration of insoluble matter in the second sedimentation bottom flow is ≥3±0.5wt%, the second sedimentation bottom flow is discharged. The preferred components of the second impurity-removing hot-melt mother liquor include: K + 3.8–3.9 wt%, Mg 2+ 3.0–3.1 wt%, SO4 2- 16.1–16.3 wt%, Cl - 9.0–9.1 wt%; in the examples, K may be used. + 3.8 wt%, Mg 2+ 3.0 wt%, SO4 2- 16.1 wt%, Cl - 9.1 wt%; or K + 3.9 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.2 wt%, Cl - 9.0 wt%; or K + 3.8 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.3 wt%, Cl - 9.1 wt%.

[0058] After obtaining the second impurity-removing hot-melt mother liquor, the present invention sequentially performs fourth, fifth, and sixth vacuum evaporation crystallization processes on the second impurity-removing hot-melt mother liquor. The absolute pressure and temperature of the fourth vacuum evaporation crystallization are greater than those of the fifth and sixth vacuum evaporation crystallization processes, resulting in feed-grade potassium magnesium sulfate fertilizer wet material and a second crystallization mother liquor. The feed-grade potassium magnesium sulfate fertilizer wet material is then subjected to a second drying process to obtain the feed-grade potassium magnesium sulfate fertilizer. In this invention, the absolute pressure of the fourth vacuum evaporation crystallization is preferably 15–20 kPa, and the temperature is preferably 65 ± 2 °C. The absolute pressure of the fifth vacuum evaporation crystallization is preferably 5–7 kPa, and the temperature is preferably 40 ± 2 °C. The absolute pressure of the sixth vacuum evaporation crystallization is preferably 1–2 kPa, and the temperature is preferably 21 ± 2 °C. The fourth, fifth, and sixth vacuum evaporation crystallization processes are preferably carried out in a vacuum crystallizer. The four-stage underflow and four-stage overflow obtained from the fourth vacuum evaporation crystallization are simultaneously used as the mother liquor for the fifth vacuum evaporation crystallization. The fifth-stage underflow and fifth-stage overflow obtained from the fifth vacuum evaporation crystallization are simultaneously used as the mother liquor for the sixth vacuum evaporation crystallization. The sixth vacuum evaporation crystallization yields a sixth-stage overflow and a sixth-stage underflow.

[0059] The sixth-stage overflow and sixth-stage underflow obtained from the sixth vacuum evaporation crystallization are introduced into a crystallization thickener for a second crystallization thickening. The second crystallization thickening yields a second thickened underflow and a second thickened overflow. In this invention, the second thickened underflow preferably undergoes a second solid-liquid separation to obtain the liquid phase component of the potassium sulfate wet material and the second thickened underflow. The liquid phase component of the second thickened underflow obtained from the second solid-liquid separation and the second thickened overflow form the second crystallization mother liquor. The second solid-liquid separation is preferably centrifugal separation.

[0060] In this invention, the feed-grade potassium magnesium sulfate wet fertilizer preferably comprises: K + 19.5–20.3 wt%, Mg 2 + 5.9–6.1 wt%, SO4 2- 47.3–47.9 wt%, Cl - 0.03–0.05 wt%; in the examples, K can be used. + 19.5 wt%, Mg 2+ 5.9 wt%, SO4 2- 47.9 wt%, Cl - 0.03wt%; or K + 20.1 wt%, Mg 2+ 5.9 wt%, SO4 2- 47.8 wt%, Cl - 0.04wt%; or K +20.3 wt%, Mg 2+ 6.1 wt%, SO4 2- 47.3 wt%, Cl - 0.05 wt%.

[0061] In this invention, the temperature of the second drying is preferably 180±2℃, and the time is preferably 20±1min.

[0062] In this invention, the feed-grade potassium magnesium sulfate fertilizer preferably comprises: K + The content was 22.3–23.5 wt%, Mg 2+ It is 8.1–8.5 wt%, SO4 2- 62.5–63.3 wt%, Cl - The content is 0.05–0.08 wt%, and the water-insoluble matter is 0.01–0.02 wt%; in the examples, it can be K. + It is 22.5 wt%, Mg 2+ It is 8.1 wt%, SO4 2-： 62.5 wt%, Cl - It is 0.08 wt%, and the water-insoluble matter is 0.02 wt%; or it is K. + 22.3 wt%, Mg 2+ 8.1 wt%, SO4 2- 62.9 wt%, Cl - 0.07 wt%, water-insoluble matter 0.02 wt%; or K + 23.3 wt%, Mg 2+ 8.2 wt%, SO4 2- 63.3 wt%, Cl - 0.06 wt%, water-insoluble matter 0.01 wt%; or K + 23.8 wt%, Mg 2+ 8.5 wt%, SO4 2- 62.9 wt%, Cl - 0.05wt%, water-insoluble matter 0.01wt%.

[0063] In this invention, after obtaining the second crystallization mother liquor, the invention preferably further includes: combining the second crystallization mother liquor and the first crystallization mother liquor and heating them to prepare a second hot-melt mother liquor.

[0064] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0065] The following embodiments are in accordance with Figure 1 The preparation process is carried out according to the flowchart shown.

[0066] Example 1:

[0067] 3000 kg of wet soft potassium magnesium sulfate concentrate was selected, and its components include: K + 15.1 wt%, Mg 2+ 5.5 wt%, SO4 2- 38.2 wt%. The ratio of water volume to the mass of wet molten potassium magnesium sulfate concentrate is 1 m³. 3 Add 1kg to 3000m 3 Water was used to prepare a slurry with a solid content of 43 wt%, which was continuously fed into the hot melt tank. Then, 6000 mg of the slurry was added. 3 Water was stirred at 80°C for 1 hour to obtain the first hot-melt mother liquor, the chemical composition of which included: K + 7.6 wt%, Mg 2+ 2.5wt%, SO4 2- 18.3 wt%, Cl - 9.3 wt%. Add 0.3 kg of polyacrylamide to the first hot-melt mother liquor, keep warm and allow to settle for 30 minutes, then reduce the overflow temperature to 78°C to obtain the first purified hot-melt mother liquor, whose chemical composition includes: K + 7.5 wt%, Mg 2+ 2.4 wt%, SO4 2- 18.2 wt%, Cl - 9.2 wt%. The purified hot-melt mother liquor is then subjected to three-stage evaporation and crystallization: the first purified hot-melt mother liquor is introduced into a stage I vacuum crystallizer, operating at an absolute pressure of 20.0 kPa and a secondary steam temperature of 65 ± 2℃; the underflow and overflow from the stage I vacuum crystallizer are introduced into a stage II vacuum crystallizer, operating at an absolute pressure of 7 kPa and a secondary steam temperature of 40 ± 2℃; the underflow and overflow from the stage II vacuum crystallizer are introduced into a stage III vacuum crystallizer, operating at an absolute pressure of 2 kPa and a secondary steam temperature of 21 ± 1℃. The underflow and overflow from the stage III crystallizer are introduced into a crystallization thickener. The overflow from the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow from the crystallization thickener is centrifuged to obtain water-soluble potassium sulfate wet material, the chemical composition of which includes: K + 43.3 wt%, Mg 2+ 0.03wt%, SO4 2- 52.3 wt%, Cl - 0.03 wt%. After drying at 180℃ for 20 min, a water-soluble potassium sulfate product was obtained, with the chemical composition including: K + 44.5 wt%, Mg 2+ 0.02wt%, SO4 2- 55.4 wt%, Cl - 0.06wt%, H2O 0.03wt%, water-insoluble matter 0.03wt%.

[0068] The overflow from the crystallizer thickener and the filtrate from the centrifuge were pumped into two sets of hot melt tanks and stirred at a constant temperature of 84°C for 1 hour to obtain the second hot melt mother liquor, the chemical composition of which includes K. + 3.9 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.2 wt%, Cl - 9.2 wt%. Add 0.1 kg of polyacrylamide to the second hot-melt mother liquor, keep warm and allow to settle for 30 minutes, then reduce the overflow temperature to 82°C to obtain the second purified hot-melt mother liquor, whose chemical composition includes: K + 3.8 wt%, Mg 2+ 3.0 wt%, SO4 2- 16.1 wt%, Cl - 9.1 wt%. The second hot-melt mother liquor is then subjected to three-stage crystallization: two sets of Stage I vacuum crystallizers, operating pressure (absolute pressure) 20.0 kPa, secondary steam temperature 65±2℃; the underflow and overflow of the two sets of Stage I vacuum crystallizers are introduced into two sets of Stage II vacuum crystallizers, operating pressure (absolute pressure) 7 kPa, secondary steam temperature 40±2℃; the underflow and overflow of the Stage II vacuum crystallizers are introduced into two sets of Stage III vacuum crystallizers, operating pressure (absolute pressure) 2 kPa, secondary steam temperature 21±1℃. The underflow and overflow of the two sets of Stage III crystallizers are introduced into a crystallization thickener. The overflow of the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow of the crystallization thickener is centrifuged to obtain feed-grade potassium magnesium sulfate fertilizer wet feed, the chemical composition of which includes: K + 19.5 wt%, Mg 2+ 5.9 wt%, SO4 2- 47.9 wt%, Cl - 0.03 wt%. After drying at 180℃, feed-grade potassium magnesium sulfate fertilizer was obtained. Its chemical composition includes: K + 22.3 wt%, Mg 2+ 8.1 wt%, SO4 2- 62.9 wt%, Cl - 0.07 wt%, water-insoluble matter 0.02 wt%. The centrifuge filtrate is returned to the second set of hot melt tanks.

[0069] Example 2:

[0070] 5000 kg of wet soft potassium magnesium sulfate concentrate was selected. Its chemical composition includes: K + 15.3 wt%, Mg 2+ 5.6 wt%, SO4 2- 37.8 wt%. The ratio of water volume to the mass of wet molten potassium magnesium sulfate concentrate is 1 m³. 3 Add 1kg to 5000m 3Water was used to prepare a slurry with a solid content of 42 wt%, which was continuously fed into the hot melt tank. Then, 10,000 ml of the slurry was added. 3 Water was stirred at a constant temperature of 80°C for 1 hour to obtain the first hot-melt mother liquor, the chemical composition of which includes: K + 7.7 wt%, Mg 2+ 2.6 wt%, SO4 2- 18.4 wt%, Cl - 9.2 wt%. Add 0.3 kg of polyacrylamide to the first hot-melt mother liquor, keep warm and allow to settle for 30 minutes, then reduce the overflow temperature to 78°C to obtain the first purified hot-melt mother liquor, whose chemical composition includes: K + 7.5 wt%, Mg 2+ 2.5wt%, SO4 2- 18.3%, Cl - 9.1 wt. The purified hot-melt mother liquor was then subjected to three-stage evaporation and crystallization: the first purified hot-melt mother liquor was introduced into a stage I vacuum crystallizer, operating at an absolute pressure of 20.0 kPa and a secondary steam temperature of 65 ± 2℃; the underflow and overflow from the stage I vacuum crystallizer were introduced into a stage II vacuum crystallizer, operating at an absolute pressure of 7 kPa and a secondary steam temperature of 40 ± 2℃; the underflow and overflow from the stage II vacuum crystallizer were introduced into a stage III vacuum crystallizer, operating at an absolute pressure of 2 kPa and a secondary steam temperature of 21 ± 1℃. The underflow and overflow from the stage III crystallizer were introduced into a crystallization thickener. The overflow from the crystallization thickener went directly into the first crystallization mother liquor tank. The underflow from the crystallization thickener was centrifuged to obtain water-soluble potassium sulfate wet material, the chemical composition of which includes: K + 43.4 wt%, Mg 2+ 0.04wt%, SO4 2- 52.4 wt%, Cl - 0.03wt%. After drying at 180℃, a water-soluble potassium sulfate product is obtained, with the chemical composition including: K + 44.6 wt%, Mg 2+ 0.02wt%, SO4 2- 55.6 wt%, Cl - 0.07 wt%, water-insoluble matter 0.04 wt%.

[0071] The overflow from the crystallizer thickener and the filtrate from the centrifuge were pumped into two sets of hot melt tanks and stirred at a constant temperature of 85°C for 1 hour to obtain the second hot melt mother liquor, the chemical composition of which includes K. + 3.8 wt%, Mg 2+ 3.3 wt%, SO4 2- 16.4 wt%, Cl - 9.3 wt%. Add 0.1 kg of polyacrylamide to the second hot-melt mother liquor, keep warm and allow to settle for 30 minutes, then reduce the overflow temperature to 83°C to obtain the second impurity-removed hot-melt mother liquor, whose chemical composition includes: K +3.9 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.2 wt%, Cl - 9.0 wt%. The second hot-melt mother liquor is then subjected to three-stage crystallization: two sets of Stage I vacuum crystallizers, operating pressure (absolute pressure) 20.0 kPa, secondary steam temperature 65±2℃; the underflow and overflow of the two sets of Stage I vacuum crystallizers are introduced into two sets of Stage II vacuum crystallizers, operating pressure (absolute pressure) 7 kPa, secondary steam temperature 40±2℃; the underflow and overflow of the Stage II vacuum crystallizers are introduced into two sets of Stage III vacuum crystallizers, operating pressure (absolute pressure) 2 kPa, secondary steam temperature 21±1℃. The underflow and overflow of the two sets of Stage III crystallizers are introduced into a crystallization thickener. The overflow of the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow of the crystallization thickener is centrifuged to obtain feed-grade potassium magnesium sulfate fertilizer wet feed, the chemical composition of which includes: K + 20.1 wt%, Mg 2+ 5.9 wt%, SO4 2- 47.8 wt%, Cl - 0.04 wt%. After drying at 180℃, feed-grade potassium magnesium sulfate fertilizer was obtained. Its chemical composition includes: K... + 23.3 wt%, Mg 2+ 8.2 wt%, SO4 2- 63.3 wt%, Cl - 0.06 wt%, water-insoluble matter 0.01 wt%. The centrifuge filtrate is returned to the second set of hot melt tanks.

[0072] Example 3:

[0073] 10,000 kg of wet soft potassium magnesium sulfate concentrate was selected. Its chemical composition includes: K + 15.9 wt%, Mg 2+ 5.9 wt%, SO4 2- 39.8 wt%. The ratio of water volume to the mass of wet molten potassium magnesium sulfate concentrate is 1 m³. 3 Add 1kg to 10000m 3 A slurry with a solid content of 41 wt% was prepared by continuous feeding with water and stirred at a constant temperature of 80°C for 1 hour to obtain the first hot-melt mother liquor, the chemical composition of which includes: K + 7.9 wt%, Mg 2+ 2.7 wt%, SO4 2- 18.8 wt%, Cl - 9.1 wt%. Add 0.3 kg of polyacrylamide to the first hot-melt mother liquor, keep warm and allow to settle for 30 minutes, then reduce the overflow temperature to 78°C to obtain the first purified hot-melt mother liquor, whose chemical composition includes: K + 7.7 wt%, Mg 2+2.6 wt%, SO4 2- 18.4 wt%, Cl - 9.0 wt%. The purified hot-melt mother liquor is then subjected to three-stage evaporation and crystallization: the first purified hot-melt mother liquor is introduced into a stage I vacuum crystallizer, operating at an absolute pressure of 20.0 kPa and a secondary steam temperature of 65 ± 2℃; the underflow and overflow from the stage I vacuum crystallizer are introduced into a stage II vacuum crystallizer, operating at an absolute pressure of 7 kPa and a secondary steam temperature of 40 ± 2℃; the underflow and overflow from the stage II vacuum crystallizer are introduced into a stage III vacuum crystallizer, operating at an absolute pressure of 2 kPa and a secondary steam temperature of 21 ± 1℃. The underflow and overflow from the stage III crystallizer are introduced into a crystallization thickener. The overflow from the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow from the crystallization thickener is centrifuged to obtain water-soluble potassium sulfate wet material, the chemical composition of which includes: K + 43.5 wt%, Mg 2+ 0.05wt%, SO4 2- 52.6 wt%, Cl - 0.03wt%. After drying at 180℃, a water-soluble potassium sulfate product is obtained, with the chemical composition including: K + 45.0 wt%, Mg 2+ 0.02wt%, SO4 2- 55.4 wt%, Cl - 0.06wt%, water-insoluble matter 0.07wt%.

[0074] The overflow from the crystallizer thickener and the filtrate from the centrifuge were pumped into two sets of hot melt tanks and stirred at a constant temperature of 85°C for 1 hour to obtain the second hot melt mother liquor, the chemical composition of which includes K. + 3.9 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.5 wt%, Cl - 9.2 wt%. After settling at a constant temperature for 30 minutes, the overflow temperature was reduced to 82°C, yielding the second impurity-removing hot-melt mother liquor. Its chemical composition includes: K... + 3.8 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.3 wt%, Cl -9.1 wt%. The second hot-melt mother liquor is then subjected to three-stage crystallization: two sets of Stage I vacuum crystallizers, operating pressure (absolute pressure) 20.0 kPa, secondary steam temperature 65±2℃; the underflow and overflow of the two sets of Stage I vacuum crystallizers are introduced into two sets of Stage II vacuum crystallizers, operating pressure (absolute pressure) 7 kPa, secondary steam temperature 40±2℃; the underflow and overflow of the Stage II vacuum crystallizers are introduced into two sets of Stage III vacuum crystallizers, operating pressure (absolute pressure) 2 kPa, secondary steam temperature 21±1℃. The underflow and overflow of the two sets of Stage III crystallizers are introduced into a crystallization thickener. The overflow of the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow of the crystallization thickener is centrifuged to obtain feed-grade potassium magnesium sulfate fertilizer wet feed, the chemical composition of which includes: K + 20.3 wt%, Mg 2+ 6.1 wt%, SO4 2- 47.3 wt%, Cl - 0.05 wt%. After drying at 180℃, feed-grade potassium magnesium sulfate fertilizer was obtained. Its chemical composition includes: K + 23.8 wt%, Mg 2+ 8.5 wt%, SO4 2- 62.9 wt%, Cl - 0.05wt%, water-insoluble matter 0.01wt%.

[0075] Comparative Example 1:

[0076] 10,000 kg of wet soft potassium magnesium sulfate concentrate was selected. Its chemical composition includes: K + 15.8 wt%, Mg 2+ 5.7 wt%, SO4 2- 39.7 wt%. The ratio of water volume to the mass of wet pyrophosphate concentrate is 1 m³. 3 Add 1kg to 10000m 3 A slurry with a solid content of 42 wt% was prepared by continuous feeding with water and stirred at a constant temperature of 80°C for 1 hour to obtain the first hot-melt mother liquor, the chemical composition of which includes: K + 7.9 wt%, Mg 2+ 2.8 wt%, SO4 2- 18.7 wt%, Cl - 9.2 wt%. The first hot-melted mother liquor was kept at a constant temperature for 30 minutes to allow it to settle, reducing the overflow temperature to 78°C. The overflow chemical composition included: K... + 7.8 wt%, Mg 2+ 2.8 wt%, SO4 2- 18.5 wt%, Cl -9.1 wt%. The overflow is then subjected to three-stage evaporation crystallization: the first impurity-removing hot-melt mother liquor is introduced into a stage I vacuum crystallizer, operating at an absolute pressure of 20.0 kPa and a secondary steam temperature of 65 ± 2℃; the underflow and overflow from the stage I vacuum crystallizer are introduced into a stage II vacuum crystallizer, operating at an absolute pressure of 7 kPa and a secondary steam temperature of 40 ± 2℃; the underflow and overflow from the stage II vacuum crystallizer are introduced into a stage III vacuum crystallizer, operating at an absolute pressure of 2 kPa and a secondary steam temperature of 21 ± 1℃. The underflow and overflow from the stage III crystallizer are introduced into a crystallization thickener. The overflow from the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow from the crystallization thickener is centrifuged to obtain water-soluble potassium sulfate wet material, the chemical composition of which includes: K + 43.4 wt%, Mg 2+ 0.05wt%, SO4 2- 52.5 wt%, Cl - 0.03wt%. After drying at 180℃, a water-soluble potassium sulfate product is obtained, with the chemical composition including: K + 44.7.0 wt%, Mg 2+ 0.02wt%, SO4 2- 55.4 wt%, Cl - 0.08 wt%, water-insoluble matter 0.18 wt%.

[0077] The overflow from the crystallizer thickener and the filtrate from the centrifuge were pumped into two sets of hot melt tanks and stirred at a constant temperature of 85°C for 1 hour to obtain the second hot melt mother liquor, the chemical composition of which includes K. + 3.9 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.4 wt%, Cl - 9.2 wt%. After 30 minutes of heat preservation and settling, the overflow temperature decreased to 82°C. The overflow chemical composition included: K + 3.8 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.3%, Cl - 9.1 wt. The overflow is then passed through a three-stage crystallization process: two sets of Stage I vacuum crystallizers, operating pressure (absolute pressure) 20.0 kPa, secondary steam temperature 65±2℃; the underflow and overflow from the two sets of Stage I vacuum crystallizers are introduced into two sets of Stage II vacuum crystallizers, operating pressure (absolute pressure) 7 kPa, secondary steam temperature 40±2℃; the underflow and overflow from the Stage II vacuum crystallizers are introduced into two sets of Stage III vacuum crystallizers, operating pressure (absolute pressure) 2 kPa, secondary steam temperature 21±1℃. The underflow and overflow from the two sets of Stage III crystallizers are introduced into a crystallization thickener. The overflow from the crystallization thickener directly enters the first crystallization mother liquor tank. The underflow from the crystallization thickener is centrifuged to obtain feed-grade potassium magnesium sulfate fertilizer wet feed, the chemical composition of which includes: K + 20.2 wt%, Mg2+ 6.1 wt%, SO4 2- 47.3 wt%, Cl - 0.05 wt%. After drying at 180℃, feed-grade potassium magnesium sulfate fertilizer was obtained. Its chemical composition includes: K + 22.4 wt%, Mg 2+ 8.4 wt%, SO4 2- 62.8 wt%, Cl - 0.09 wt%, water-insoluble matter 0.11 wt%. In this comparative example, because no flocculant was added during the heat preservation and sedimentation process of the first and second hot-melt mother liquors, the final product Cl... - The product is substandard due to excessive levels of water-insoluble matter.

[0078] In this comparative example, because no flocculant was added in either the first or second hot-melt mother liquor's heat preservation and sedimentation process, the final product Cl... - The product is substandard due to excessive levels of water-insoluble matter.

[0079] As can be seen from the above embodiments, this invention breaks the traditional single fertilizer preparation mode, and innovatively uses ordinary soft potassium magnesium sulfate concentrate wet material as the core raw material to simultaneously prepare two high-value-added products: water-soluble potassium sulfate and feed-grade potassium magnesium fertilizer, filling the gap in the joint preparation of raw materials required by agriculture and animal husbandry. In terms of process route, it innovatively uses multi-stage hot dissolution and evaporation crystallization, utilizing hot dissolution mother liquors at different points to purify soft potassium magnesium sulfate with low potassium and magnesium content into high-grade potassium sulfate and potassium magnesium sulfate fertilizer, achieving efficient allocation and conversion of potassium and magnesium elements in the raw materials, avoiding resource waste, and greatly improving the comprehensive utilization rate of raw materials.

[0080] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for the combined preparation of water-soluble potassium sulfate and feed-grade potassium magnesium sulfate fertilizer, characterized in that, Includes the following steps: (1) Dilute the wet molten potassium magnesium sulfate concentrate with water to obtain a slurry; (2) The slurry is heated and dissolved to obtain the first hot-dissolved mother liquor; (3) The first hot-melt mother liquor and the first flocculant are mixed and subjected to first heat preservation sedimentation to obtain the first impurity-removed hot-melt mother liquor; (4) The first impurity-removing hot-melt mother liquor is subjected to first vacuum evaporation crystallization, second vacuum evaporation crystallization, and third vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the first vacuum evaporation crystallization are greater than the absolute pressure and temperature of the second vacuum evaporation crystallization and the absolute pressure and temperature of the third vacuum evaporation crystallization to obtain water-soluble potassium sulfate wet material and first crystallization mother liquor. The water-soluble potassium sulfate wet material is subjected to first drying to obtain the water-soluble potassium sulfate. (5) Heat the first crystallization mother liquor to obtain the second hot-melt mother liquor, and subject the second hot-melt mother liquor to a second heat preservation sedimentation to obtain the second impurity-removed hot-melt mother liquor. (6) The second impurity-removing hot melt mother liquor is subjected to fourth vacuum evaporation crystallization, fifth vacuum evaporation crystallization, and sixth vacuum evaporation crystallization in sequence. The absolute pressure and temperature of the fourth vacuum evaporation crystallization are greater than those of the fifth vacuum evaporation crystallization and the sixth vacuum evaporation crystallization, to obtain feed-grade potassium magnesium sulfate fertilizer wet material and second crystallization mother liquor. The feed-grade potassium magnesium sulfate fertilizer wet material is subjected to a second drying to obtain the feed-grade potassium magnesium sulfate fertilizer.

2. The combined preparation method according to claim 1, characterized in that, The components of the wet molten potassium magnesium sulfate concentrate include: K + 15-16 wt%, Mg 2+ 5.5–6 wt%, SO4 2- 37.5–40 wt%; In step (1): the ratio of the volume of the dilution water to the mass of the wet molten potassium magnesium sulfate concentrate is 1–2 m³. 3 1kg; The slurry contains 40 ± 3 wt% solids.

3. The combined preparation method according to claim 1, characterized in that, In step (2): before the heating and dissolving process, water is further added to the slurry for dilution, and the ratio of the volume of water used for heating and dissolving to the mass of the wet soft potassium magnesium alum concentrate is 2-3 m³. 3 1kg; The temperature for the heating and dissolution is 80±2℃, and the heating and dissolution is carried out under stirring conditions; The components of the first hot-melt mother liquor include: K + 7.5–8 wt%, Mg 2+ 2.5–3 wt%, SO4 2- 18-19 wt%, Cl - 9–9.5 wt%.

4. The combined preparation method according to claim 1, characterized in that, The first flocculant is polyacrylamide, and the mass percentage of the first flocculant in the wet molten potassium magnesium alum concentrate is 0.003 to 0.01%. The overflow of the first heat-insulating sedimentation is the first impurity-removing hot-melt mother liquor. The first heat-insulating sedimentation also yields a first sedimentation underflow. When the mass concentration of insoluble matter in the first sedimentation underflow is ≥3±0.5%, the first sedimentation underflow is discharged. The components of the first impurity-removing hot-melt mother liquor include: K + 7.5–8 wt%, Mg 2+ 2.4–2.8 wt%, SO4 2- 18–18.5 wt%, Cl - 9–9.5 wt%.

5. The combined preparation method according to claim 1, characterized in that, The absolute pressure of the first vacuum evaporation crystallization is 15-20 kPa and the temperature is 65±2℃; the absolute pressure of the second vacuum evaporation crystallization is 5-7 kPa and the temperature is 40±2℃; the absolute pressure of the third vacuum evaporation crystallization is 1-2 kPa and the temperature is 21±2℃. The components of the water-soluble potassium sulfate wet material include: K + 43.3–43.5 wt%, Mg 2+ 0.03–0.05 wt%, SO4 2- 52.3–52.6 wt%, Cl - 0.03wt%; The temperature of the first drying process is 180±2℃, and the time is 20±1min.

6. The combined preparation method according to claim 1 or 5, characterized in that, The components of the water-soluble potassium sulfate include: K + 44.5–45 wt%, Mg 2+ 0.015–0.02 wt%, SO4 2- 55.4–55.6 wt%, Cl - 0.06~0.07wt%, H2O≤0.03wt%, water-insoluble matter 0.03~0.07wt%.

7. The combined preparation method according to claim 1, characterized in that, In step (5), the heating is carried out under stirring conditions, and the temperature of the second hot-melt mother liquor is 82±2℃; The second hot-melt mother liquor consists of: K + 3.8–3.9 wt%, Mg 2+ 3.2–3.3 wt%, SO4 2- 16.2–16.5 wt%, Cl - 9.2–9.3 wt%; Before the second heat preservation sedimentation, the process further includes mixing the second hot-melt mother liquor and the second flocculant; the second flocculant is polyacrylamide, and the mass percentage of the second flocculant in the wet soft potassium magnesium alum concentrate is ≤0.0035%; the overflow of the second heat preservation sedimentation is the second impurity removal hot-melt mother liquor, and the second heat preservation sedimentation also yields a second sedimentation underflow. When the mass concentration of insoluble matter in the second sedimentation underflow is ≥3±0.5%, the second sedimentation underflow is discharged. The components of the second impurity-removing hot-melt mother liquor include: K + 3.8–3.9 wt%, Mg 2+ 3.0–3.1 wt%, SO4 2- 16.1–16.3 wt%, Cl - 9.0–9.1 wt%.

8. The combined preparation method according to claim 1, characterized in that, The absolute pressure of the fourth vacuum evaporation crystallization is 15-20 kPa and the temperature is 65±2℃; the absolute pressure of the fifth vacuum evaporation crystallization is 5-7 kPa and the temperature is 40±2℃; the absolute pressure of the sixth vacuum evaporation crystallization is 1-2 kPa and the temperature is 21±2℃. The components of the feed-grade potassium magnesium sulfate fertilizer wet feed include: K + 19.5–20.3 wt%, Mg 2+ 5.9–6.1 wt%, SO4 2- 47.3–47.9 wt%, Cl - 0.03–0.05 wt%; The second drying temperature is 180±2℃ and the time is 20±1min.

9. The combined preparation method according to claim 1, characterized in that, The feed-grade potassium magnesium sulfate fertilizer comprises: K + 22.3–22.5 wt%, Mg 2+ 8.1–8.5 wt%, SO4 2- 62.5–63.3 wt%, Cl - 0.05~0.08wt%, water-insoluble matter 0.01~0.02wt%.

10. The combined preparation method according to claim 1, characterized in that, After obtaining the second crystallization mother liquor, the process further includes: combining the second crystallization mother liquor and the first crystallization mother liquor and heating them to prepare a second hot-melt mother liquor.

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