Preparation method of feed-grade potash magnesium sulphate fertilizer

Abstract

The invention belongs to the technical field of livestock feeds, and particularly relates to a preparation method of a feed-grade potash magnesium sulphate fertilizer. According to the preparation method, picromerite concentrate is used as a preparation raw material, and meanwhile flotation tail liquid and / or crystallization mother liquid are / is used as a size mixing solution; the feed-grade potash magnesium sulphate fertilizer is obtained through size mixing, hot melting, heat preservation sedimentation, three-stage vacuum crystallization, solid-liquid separation and drying in sequence. The preparation method provided by the invention has the advantages of stable product quality and high purity, and meanwhile, the picromerite concentrate, the crystallization mother liquor and the flotation tail liquid are combined for use, so that the feed-grade potash magnesium sulphate fertilizer product is obtained, and the requirement of the market on the high-quality feed-grade potash magnesium sulphate fertilizer can be met.

Description

Technical Field

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

[0002] In recent years, the global livestock industry has experienced rapid development, with farming increasingly shifting towards large-scale and intensive operations. The importance of scientific feeding and precise nutritional formulation has become increasingly prominent, and feed additives have become key elements in ensuring healthy animal growth and improving farming efficiency. Among various feed additives, mineral additives containing potassium and magnesium play an irreplaceable role due to their crucial role in maintaining acid-base balance, nerve conduction, and muscle contraction in animals. For example, in dairy farming, appropriate supplementation of potassium and magnesium in feed can effectively prevent metabolic diseases such as hypomagnesemia and convulsions in dairy cows, significantly improving milk yield and quality. Therefore, ensuring an adequate supply of potassium and magnesium in feed has become a fundamental requirement of modern livestock farming.

[0003] Potassium magnesium sulfate, as an ideal source of potassium and magnesium, has been used in the global feed industry for over 50 years, and its safety and effectiveness have been fully verified. Current potassium magnesium sulfate production processes mainly include ore extraction, brine crystallization, and chemical synthesis. The content of various elements in products from these traditional processes is difficult to meet the stringent requirements for feed-grade potassium magnesium sulfate, which specifies potassium content ≥18%, magnesium content ≥8%, and heavy metals ≤10ppm. Furthermore, the chloride ion content is difficult to control. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing feed-grade potassium magnesium sulfate fertilizer. The preparation method provided by this invention has the advantages of stable product quality and high purity, which can meet the market demand for high-quality feed-grade (potassium content ≥18%, magnesium content ≥8%, heavy metals ≤10ppm) potassium magnesium sulfate fertilizer, while also achieving effective control of chloride ion content.

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

[0006] This invention provides a method for preparing feed-grade potassium magnesium sulfate fertilizer, comprising the following steps:

[0007] (1) The soft potassium magnesium alum concentrate is prepared by using a slurry preparation solution to obtain a slurry. The soft potassium magnesium alum concentrate is obtained by flotation of potassium mixed salt ore, and the grade of the soft potassium magnesium alum concentrate is K. + ≥12.0wt%, Mg 2+ ≥5.0wt%, SO4 2-≥34.0 wt%, the flotation also yields flotation tailings, the slurry preparation solution comprises at least two of water, flotation tailings and / or the crystallization mother liquor obtained in step (3), wherein the crystallization mother liquor contains Cl - The content is <14wt%;

[0008] (2) The slurry is heated to the hot dissolution temperature for hot dissolution to obtain hot dissolution mother liquor. The hot dissolution mother liquor is kept at a constant temperature and allowed to settle to obtain purified hot dissolution mother liquor.

[0009] (3) The purified hot-dissolved mother liquor is vacuum crystallized to obtain a crystallized mother liquor and a concentrated crystallized solution. The vacuum crystallization includes sequentially performing first-stage vacuum crystallization, second-stage vacuum crystallization and third-stage vacuum crystallization.

[0010] (4) Separate the solid and liquid phases of the crystallized concentrated solution to obtain a wet product and a separated concentrated liquid phase;

[0011] (5) The wet product is dried to obtain the feed-grade potassium magnesium sulfate fertilizer.

[0012] Preferably, the composition of the soft potassium magnesium sulfate concentrate includes: K + 12.0–16.0 wt%, Mg 2+ 5.0–6.0 wt%, SO4 2- 34.0–40.0 wt%;

[0013] The composition of the flotation tailings includes: K + 2.0–3.0 wt%, Mg 2+ 4.0–5.0 wt%, SO4 2- 6.0–8.0 wt%, Cl - 12–16 wt%;

[0014] The composition of the crystallization mother liquor includes: K + 2.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 10.0–12.0 wt%, Cl - 11-12 wt%.

[0015] Preferably, the mass content of the solid phase in the slurry is 40-45%.

[0016] Preferably, the hot melting temperature is 75-80°C, and the composition of the hot melting mother liquor includes: K + 3.0–5.0 wt%, Mg 2+ 2.9–3.3 wt%, SO4 2- 15.0–17.0 wt%, Cl - 9–10 wt%.

[0017] Preferably, the temperature for heat preservation and sedimentation is 75–80°C, and the composition of the purified hot-dissolved mother liquor includes: K + 3.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 14.0–17.0 wt%, Cl - 9–10 wt%.

[0018] Preferably, the operating pressure of the first-stage vacuum crystallization is 18-20 kPa and the temperature is 60-65°C; the operating pressure of the second-stage vacuum crystallization is 7-8 kPa and the temperature is 40-45°C; and the operating pressure of the third-stage vacuum crystallization is 2-5 kPa and the temperature is 21-30°C.

[0019] Preferably, the solid phase content in the concentrated crystallization solution is 45-50% by mass, the moisture content of the wet product is ≤6wt%, and the composition of the wet product includes: K + 18.0–20.0 wt%, Mg 2+ 5.7–6.1 wt%, SO4 2- 47.0–48.0 wt%, Cl - 0.05–0.1 wt%.

[0020] Preferably, the drying temperature is 170–180°C; and the free water content in the feed-grade potassium magnesium sulfate fertilizer is ≤1 wt%.

[0021] Preferably, the composition of the feed-grade potassium magnesium sulfate fertilizer includes: K + 25.2–27.0 wt%, Mg 2+ 8.0–8.2 wt%, SO4 2- 62.0–65.0 wt%, Cl - 0.05–0.5 wt%, water-insoluble matter 0.01–0.05 wt%.

[0022] Preferably, after obtaining the separated concentrated liquid phase, the method further includes reusing the separated concentrated liquid phase. The reuse includes returning the separated concentrated liquid phase to mix with the purified hot-soluble mother liquor to obtain a mixed mother liquor, and using the mixed mother liquor as the raw material for step (3).

[0023] This invention provides a method for preparing feed-grade potassium magnesium sulfate fertilizer. The method uses soft potassium magnesium sulfate concentrate as the raw material, and utilizes at least two of the following: water, flotation tailings, and crystallization mother liquor as a slurry conditioning solution. The process involves sequentially conditioning the slurry, hot dissolving, heat-insulating sedimentation, three-stage vacuum crystallization, solid-liquid separation, and drying to obtain feed-grade potassium magnesium sulfate fertilizer. Compared with existing technologies, this invention, using soft potassium magnesium sulfate concentrate as the raw material, offers the following superior advantages in preparing feed-grade potassium magnesium sulfate fertilizer:

[0024] This invention employs a raw material formulation system that combines soft potassium magnesia concentrate, crystallization mother liquor, and flotation tailings produced by flotation. In this system, the grade of the soft potassium magnesia concentrate only needs to reach K. + ≥12.0wt%, Mg 2+ ≥5.0wt%, SO4 2- A content of ≥34.0 wt% is sufficient to meet production requirements, reducing the raw material grade requirements and greatly expanding the range of raw material sources. This invention maximizes the utilization efficiency of raw materials by recycling the crystallization mother liquor as the main hot-melt slurry and using the flotation tail liquor as a supplementary slurry mother liquor, thereby realizing the resource utilization of production waste liquid and achieving significant economic and environmental benefits.

[0025] In terms of product performance, the finished feed-grade potassium magnesium sulfate fertilizer produced by this invention fully meets the requirements of feed-grade standards, solving the problem of difficult control of chloride ion content in feed-grade potassium magnesium sulfate products, keeping it below 0.5 wt%. The product also possesses excellent characteristics such as complete water solubility, large particle size, good slow-release properties, and strong stability. Furthermore, this invention can produce a series of potassium magnesium sulfate products with different water of crystallization by adjusting the mother liquor point (during vacuum crystallization, solutes in the solution (such as potassium and magnesium sulfates) will crystallize out when the concentration reaches saturation, and the remaining uncrystallized solution is the "mother liquor." Because it is a "multi-stage crystallization," the conditions of each stage of crystallization are different, resulting in different compositions of the precipitated crystals (target product), and the composition of the residual mother liquor (potassium and magnesium content, proportion of other impurities, etc.) will also vary. In this invention, these mother liquors with specific potassium-magnesium ratios corresponding to a certain crystallization stage are called "mother liquor points") and drying process parameters, demonstrating significant development potential.

[0026] This invention produces feed-grade potassium magnesium sulfate fertilizer from soft potassium magnesium sulfate concentrate. The product is characterized by complete water solubility, large particle size, and slow release. In particular, it achieves industry-leading levels in key indicators such as potassium content (25-27 wt%), magnesium content (8.0-8.2 wt%), and water-insoluble matter content (0.01-0.05 wt%), realizing a significant upgrade in the purity of potassium magnesium sulfate products from traditional industrial grade to feed grade, thus greatly increasing the product's added value. The product's advantages, such as complete water solubility and low chlorine content, ensure stable market demand not only in traditional livestock farming but also demonstrate promising application prospects in emerging high-end markets such as pet healthcare and organic agriculture. It possesses strong market premium capabilities, can effectively compete with imported products, and has broad market prospects.

[0027] This invention achieves a balance between economic and environmental benefits using soft potassium magnesium sulfate concentrate. In terms of comprehensive resource utilization, it significantly reduces system water consumption and realizes the resource utilization of waste liquid through efficient recovery of flotation tailings and recycling of crystallization mother liquor. The production process employs clean technology, requiring no additional chemical reagents, resulting in low energy consumption and minimal pollution, fully meeting clean production requirements. The preparation method provided by this invention not only improves the comprehensive utilization level of salt lake resources but also offers new ideas for the sustainable development of the industry.

[0028] Furthermore, this invention precisely controls the K content in the hot-melt mother liquor by applying hot-melt treatment and heat preservation sedimentation operating conditions. + The concentration can be flexibly adjusted to optimize the potassium sulfate ratio, allowing for formulation optimization based on specific needs, thereby ensuring the stable production of high-purity, high-quality feed-grade potassium magnesium sulfate fertilizer. This refined production process not only improves product uniformity and quality stability but also enhances fertilizer utilization efficiency, meeting the growing demand of modern agriculture for efficient and environmentally friendly fertilizers.

[0029] This invention employs multi-stage vacuum crystallization technology, enabling the entire production process to be carried out at lower temperatures and pressures, thereby reducing energy consumption. By precisely controlling production parameters, production efficiency is improved, the production cycle is shortened, and production costs are further reduced. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating the preparation of feed-grade high-magnesium potassium sulfate fertilizer using soft potassium magnesium sulfate concentrate as the raw material. Detailed Implementation

[0031] This invention provides a method for preparing feed-grade potassium magnesium sulfate fertilizer, comprising the following steps:

[0032] (1) The soft potassium magnesium alum concentrate is prepared by using a slurry preparation solution to obtain a slurry. The soft potassium magnesium alum concentrate is obtained by flotation of potassium mixed salt ore, and the grade of the soft potassium magnesium alum concentrate is K.+ ≥12.0wt%, Mg 2+ ≥5.0wt%, SO4 2- ≥34.0 wt%, the flotation also yields flotation tailings, the slurry preparation solution includes at least two of the following: water, flotation tailings, and crystallization mother liquor obtained in step (3), wherein the crystallization mother liquor contains Cl - The content is <14wt%;

[0033] (2) The slurry is heated to the hot dissolution temperature for hot dissolution to obtain hot dissolution mother liquor. The hot dissolution mother liquor is kept at a constant temperature and allowed to settle to obtain purified hot dissolution mother liquor.

[0034] (3) The purified hot-dissolved mother liquor is vacuum crystallized to obtain a crystallized mother liquor and a concentrated crystallized solution. The vacuum crystallization includes sequentially performing first-stage vacuum crystallization, second-stage vacuum crystallization and third-stage vacuum crystallization.

[0035] (4) Separate the solid and liquid phases of the crystallized concentrated solution to obtain a wet product and a separated concentrated liquid phase;

[0036] (5) The wet product is dried to obtain the feed-grade potassium magnesium sulfate fertilizer.

[0037] 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.

[0038] This invention uses a slurry conditioning solution to condition the soft potassium magnesia concentrate, resulting in a slurry. The soft potassium magnesia concentrate is obtained by flotation of potassium mixed salt ore, and the grade of the soft potassium magnesia concentrate is K. + ≥12.0wt%, Mg 2+ ≥5.0wt%, SO4 2- ≥34.0 wt%, the flotation also yields flotation tailings, the slurry preparation solution comprises at least two of water, flotation tailings, and crystallization mother liquor, wherein the crystallization mother liquor contains Cl - The content is <14wt%.

[0039] The present invention does not have any special requirements for the specific implementation of the flotation method.

[0040] In this invention, the composition of the soft potassium magnesium sulfate concentrate preferably includes: K + 12.0–16.0 wt%, Mg 2+ 5.0–6.0 wt%, SO4 2- 34.0–40.0 wt%. In embodiments of the present invention, the composition of the soft potassium magnesium sulfate concentrate includes: K + 13.2 wt%, Mg 2+ 5.4 wt%, SO4 2- 35.7 wt%. Or the composition of the soft potassium magnesium sulfate concentrate includes: K+ 13.0 wt%, Mg 2+ 5.5 wt%, SO4 2- 35.0 wt%. Or the composition of the soft potassium magnesium sulfate concentrate includes: K + 15.7 wt%, Mg 2+ 6.3 wt%, SO4 2- 38.8 wt%.

[0041] In this invention, the composition of the flotation tailings preferably includes: K + 2.0–3.0 wt%, Mg 2+ 4.0–5.0 wt%, SO4 2- 6.0–8.0 wt%, Cl - 12-16 wt%. In embodiments of the present invention, the composition of the flotation tailings includes: K + 2.3wt%, Mg 2+ 4.4 wt%, SO4 2- 7.2 wt%, Cl - 13.8 wt%. Or the composition of the flotation tailings includes: K + 2.6wt%, Mg 2+ 4.5 wt%, SO4 2- 6.8 wt%, Cl - 15.1 wt%. Or the composition of the flotation tailings includes: K + 2.8 wt%, Mg 2+ 4.8 wt%, SO4 2- 7.1 wt%, Cl - 15.6 wt%.

[0042] In this invention, the Cl in the crystallization mother liquor - The content of K is <14wt%, and the composition of the crystallization mother liquor preferably includes: K + 2.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 10.0–12.0 wt%, Cl - 11-12 wt%. In embodiments of the present invention, the composition of the crystallization mother liquor includes: K + 2.1 wt%, Mg 2+ 3.1 wt%, SO4 2- 11.7 wt%, Cl - 11.3 wt%. Alternatively, the composition of the crystallization mother liquor preferably includes: K + 2.6wt%, Mg 2+ 3.1 wt%, SO4 2- 11.2 wt%, Cl- 11.6 wt%. Alternatively, the composition of the crystallization mother liquor preferably includes: K + 2.8 wt%, Mg 2+ 3.2 wt%, SO4 2- 11.8 wt%, Cl - 12.0 wt%. In this invention, when the Cl in the crystallization mother liquor... - When the content is ≥14wt%, the crystallization mother liquor is discharged and is no longer used as the slurry preparation mother liquor.

[0043] In this invention, the raw material is soft potassium magnesium sulfate concentrate, and the slurry conditioning solution is preferably two or three of the following: the mother liquor obtained from vacuum crystallization, the flotation tailings, and water. During the initial startup, since the mother liquor has not yet been prepared, the slurry conditioning solution is preferably the flotation tailings and water.

[0044] In this invention, the slurry preparation is preferably carried out in a slurry preparation tank. The slurry preparation is preferably carried out under stirring conditions. The mass content of the solid phase in the slurry is preferably 40-45%, and in the embodiments, it can be 45%.

[0045] After obtaining the slurry, the present invention heats the slurry to a hot-dissolving temperature for hot dissolving to obtain a hot-dissolving mother liquor. The hot-dissolving mother liquor is then subjected to heat preservation and sedimentation to obtain a purified hot-dissolving mother liquor. Preferably, the present invention introduces the slurry from the slurry preparation tank into the hot-dissolving tank for hot dissolving. Before heating, the present invention preferably further includes diluting the slurry with a slurry preparation solution, and then heating it to the hot-dissolving temperature for hot dissolving treatment to ensure that all the soft potassium magnesium alum in the solid phase of the slurry dissolves. The present invention obtains a hot-dissolving mother liquor in the hot-dissolving tank. In the present invention, the hot-dissolving temperature is preferably 75–80°C.

[0046] In this invention, the composition of the hot-melt mother liquor preferably includes: K + 3.0–5.0 wt%, Mg 2+ 2.9–3.3 wt%, SO4 2- 15.0–17.0 wt%, Cl - 9-10 wt%. In embodiments of the present invention, the composition of the hot-melt mother liquor includes: K + 3.3 wt%, Mg 2+ 2.9wt%, SO4 2- 15.9 wt%, Cl - 9.1 wt%. Or the composition of the hot-melt mother liquor includes: K + 3.6 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.2 wt%, Cl -9.5 wt%. Or the composition of the hot-melt mother liquor includes: K + 3.7 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.2 wt%, Cl - 9.4 wt%.

[0047] In this invention, the hot-melt mother liquor in the hot-melt tank is preferably introduced into an insulated settling tank for insulated settling. The insulated settling preferably results in an overflow and a slurry. The overflow enters the hot-melt mother liquor tank, and the overflow is the purified hot-melt mother liquor. When the insoluble matter in the slurry from the insulated settling preferably exceeds 3%, the slurry is discharged.

[0048] In this invention, the preferred temperature for the heat preservation and sedimentation is 75–80°C. The composition of the purified hot-dissolved mother liquor preferably includes: K + 3.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 14.0–17.0 wt%, Cl - 9-10 wt%. In embodiments of the present invention, the composition of the purified hot-soluble mother liquor includes: K + 3.1 wt%, Mg 2+ 2.7 wt%, SO4 2- 14.8 wt%, Cl - 9.0 wt%. Or the composition of the purified hot-melt mother liquor includes: K + 3.4 wt%, Mg 2+ 3.2 wt%, SO4 2- 15.9 wt%, Cl - 9.4 wt%. Or the composition of the purified hot-melt mother liquor includes: K + 3.8 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.3 wt%, Cl - 9.2 wt%.

[0049] After obtaining the purified hot-melt mother liquor, the present invention performs vacuum crystallization of the purified hot-melt mother liquor to obtain a crystallized mother liquor and a concentrated crystallized solution. The vacuum crystallization includes sequentially performing first-stage vacuum crystallization, second-stage vacuum crystallization, and third-stage vacuum crystallization. In the present invention, the vacuum crystallization is preferably performed by vacuum evaporation crystallization. In the present invention, the first-stage vacuum crystallization is performed in a stage I vacuum crystallizer, and the first-stage vacuum crystallization yields a first-stage underflow and a first-stage overflow. The second-stage vacuum crystallization is performed in a stage II vacuum crystallizer, and the second-stage vacuum crystallization yields a second-stage underflow and a second-stage overflow. The third-stage vacuum crystallization is performed in a stage III vacuum crystallizer, and the third-stage vacuum crystallization yields a third-stage underflow and a third-stage overflow. The first-stage underflow and the first-stage overflow jointly enter the stage II vacuum crystallizer for second-stage vacuum crystallization. The second-stage underflow and the second-stage overflow jointly enter the stage III vacuum crystallizer for third-stage vacuum crystallization.

[0050] In this invention, the operating pressure (absolute pressure) of the first-stage vacuum crystallization is preferably 18-20 kPa, and in the embodiment it can be 20.0 kPa; the temperature is preferably 60-65°C, and in the embodiment it can be 65°C. The operating pressure (absolute pressure) of the second-stage vacuum crystallization is preferably 7-8 kPa, and in the embodiment it can be 7 kPa; the temperature is preferably 40-45°C, and in the embodiment it can be 40°C. The operating pressure (absolute pressure) of the third-stage vacuum crystallization is preferably 2-5 kPa, and in the embodiment it can be 2 kPa; the temperature is preferably 21-30°C, and in the embodiment it can be 21°C.

[0051] In this invention, the three-stage underflow enters the crystallization thickener through the III-stage vacuum crystallizer. A portion of the three-stage overflow enters the crystallization thickener, and the remainder enters the crystallization mother liquor tank. The three-stage underflow undergoes heat-concentration in the crystallization thickener, forming a thickener overflow and a thickener underflow. The thickener overflow enters the crystallization mother liquor tank, where crystallization mother liquor is obtained. The thickener underflow is the concentrated crystallization solution.

[0052] Alternatively, in this invention, the three-stage underflow and the three-stage overflow jointly enter the crystallizer thickener, where they are thickened under heat to obtain a thickener overflow and a thickener underflow. The thickener overflow enters the crystallization mother liquor tank, where crystallization mother liquor is obtained. The thickener underflow is the crystallization thickened solution.

[0053] In this invention, the temperature of the concentrated crystallization solution is preferably 20-22°C, and the mass content of the solid phase in the concentrated crystallization solution is preferably 45-50%.

[0054] In this invention, the composition of the thickener overflow preferably includes: K + 2.0–4.0 wt%, Mg2+ 2.7–3.1 wt%, SO4 2- 10.0–12.0 wt%, Cl - 11-12 wt%. In embodiments of the present invention, the thickener overflow comprises: K + 2.1 wt%, Mg 2+ 3.1 wt%, SO4 2- 11.7 wt%, Cl - 11.3 wt%. Or the thickener overflow may consist of: K + 2.6wt%, Mg 2+ 3.1 wt%, SO4 2- 11.2 wt%, Cl - 11.6 wt%. Or the thickener overflow may consist of: K + 2.8 wt%, Mg 2+ 3.2 wt%, SO4 2- 11.8 wt%, Cl - 12.0 wt%.

[0055] In this invention, the composition of the thickener overflow is basically the same as that of the crystallization mother liquor, and the composition of the thickener overflow is the same as that of the crystallization mother liquor.

[0056] After obtaining a concentrated crystalline solution, the present invention performs solid-liquid separation to obtain a wet product and a separated concentrated liquid phase. In this invention, the solid-liquid separation is preferably centrifugal separation. The centrifugal separation is preferably carried out in a centrifuge, and the present invention preferably introduces the concentrated crystalline solution into the high-level tank of the centrifuge.

[0057] In this invention, the moisture content of the wet product is preferably ≤6wt%, and the composition of the wet product preferably includes: K + 18.0–20.0 wt%, Mg 2+ 5.7–6.1 wt%, SO4 2- 47.0–48.0 wt%, Cl - 0.05–0.1 wt%. The composition of the wet product described in the examples includes: K + 18.6 wt%, Mg 2+ 5.7 wt%, SO4 2- 47.1 wt%, Cl - 0.1wt%. Or the composition of the wet product includes: K + 19.1 wt%, Mg 2+ 5.8 wt%, SO4 2- 47.5 wt%, Cl - 0.07wt%. Or the composition of the wet product includes: K+ 19.4 wt%, Mg 2+ 5.8 wt%, SO4 2- 47.5 wt%, Cl - 0.08 wt%.

[0058] In this invention, after obtaining the separated concentrated liquid phase, the invention preferably further includes reusing the separated concentrated liquid phase. The reuse preferably includes returning the separated concentrated liquid phase to mix with the purified hot-melt mother liquor to obtain a mixed mother liquor, and using the mixed mother liquor as a raw material for vacuum crystallization.

[0059] After obtaining the wet product, the present invention dries the wet product to obtain the feed-grade potassium magnesium sulfate fertilizer. In the present invention, the drying is preferably oven drying. The drying temperature is preferably 170–180°C. The free water content in the feed-grade potassium magnesium sulfate fertilizer is ≤1 wt%.

[0060] In this invention, the feed-grade potassium magnesium sulfate fertilizer preferably comprises: K + 25.2–27.0 wt%, Mg 2+ 8.0–8.2 wt%, SO4 2- 62.0–65.0 wt%, Cl - 0.05–0.5 wt%, water-insoluble matter 0.01–0.05 wt%. In the examples, the composition of the feed-grade potassium magnesium sulfate fertilizer includes: K + 25.8 wt%, Mg 2+ 8.02 wt%, SO4 2- 63.1 wt%, Cl - 0.2 wt%, water-insoluble matter 0.05%. Alternatively, the composition of the feed-grade potassium magnesium sulfate fertilizer includes: K + 26.3 wt%, Mg 2+ 8.1 wt%, SO4 2- 63.8 wt%, Cl - 0.2 wt%, water-insoluble matter 0.02%. Alternatively, the composition of the feed-grade potassium magnesium sulfate fertilizer includes: K + 26.8 wt%, Mg 2+ 8.2 wt%, SO4 2- 64.7 wt%, Cl - 0.07 wt%, water-insoluble matter 0.01%.

[0061] 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.

[0062] Examples 1-3 and Comparative Example 1 below use soft potassium magnesium sulfate concentrate as the raw material.

[0063] Example 1

[0064] (1) Raw material preparation: Soft potassium magnesia concentrate produced by flotation, crystallization mother liquor, and flotation tailings from the flotation process are selected as raw materials. The chemical composition of soft potassium magnesia concentrate includes: K + 13.2 wt%, Mg 2+ 5.4 wt%, SO4 2- 35.7 wt%. The chemical composition of the crystallization mother liquor includes: K + 2.1 wt%, Mg 2+ 3.1 wt%, SO4 2- 11.7 wt%, Cl - 11.3 wt%. The chemical composition of the flotation tailings includes: K + 2.3wt%, Mg 2+ 4.4 wt%, SO4 2- 7.2 wt%, Cl - 13.8 wt%.

[0065] (2) Hot dissolution sedimentation: Soft potassium magnesium alum concentrate and crystallization mother liquor are added to a slurry mixing tank to form a slurry with a solid content of 45%. This slurry is then introduced into a hot dissolution tank, with the remaining crystallization mother liquor added along with a suitable amount of flotation tailings. Steam is introduced to raise the hot dissolution temperature to 75–80°C, ensuring that all the soft potassium magnesium alum in the concentrate dissolves in the liquid phase to obtain the hot dissolution mother liquor. The chemical composition of the hot dissolution mother liquor in the hot dissolution tank includes: K + 3.3 wt%, Mg 2+ 2.9wt%, SO4 2- 15.9 wt%, Cl - 9.1 wt%. The hot-melt mother liquor is introduced into an insulated settling tank, maintaining a temperature of 75℃~80℃. The overflow (i.e., purified hot-melt mother liquor) enters the hot-melt mother liquor tank. When the insoluble matter in the bottom water of the insulated settling tank is ≥3%, the bottom water is discharged. The chemical composition of the purified hot-melt mother liquor in the hot-melt mother liquor tank includes: K + 3.1 wt%, Mg 2+ 2.7 wt%, SO4 2- 14.8 wt%, Cl - 9.0 wt%.

[0066] (3) Three-stage crystallization: The liquid phase in the hot melt mother liquor tank is introduced into the first-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 20.0 kPa and a secondary steam temperature of 65℃; the underflow from the first-stage vacuum crystallizer is introduced into the second-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 7 kPa and a secondary steam temperature of 40℃; the underflow from the second-stage vacuum crystallizer is introduced into the third-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 2 kPa and a secondary steam temperature of 21℃.

[0067] (4) Concentration and Filtration: The underflow and overflow of the Stage III vacuum crystallizer are jointly introduced into the crystallization thickener. The overflow of the crystallization thickener is returned to the slurry preparation tank and hot melt tank as crystallization mother liquor. The chemical composition of the overflow of the crystallization thickener includes: K + 2.1 wt%, Mg 2 + 3.1 wt%, SO4 2- 11.7 wt%, Cl - 11.3 wt%. The underflow from the crystallizer (solid content 45%) enters the high-level tank of the centrifuge, operating at 20°C. The moisture content of the centrifuge filter cake is ≤6 wt% (wet basis), and the centrifuged filtrate is returned to the hot-melt mother liquor tank.

[0068] (5) Drying: The main components of the centrifuged wet material are various potassium magnesium sulfates containing different amounts of water of crystallization. The drying temperature is 180℃, and the free water content of the product is ≤1wt%. The final product obtained is potassium magnesium sulfate powder. The chemical composition of the centrifuged wet material includes: K + 18.6 wt%, Mg 2+ 5.7 wt%, SO4 2- 47.1 wt%, Cl - 0.1wt%. The chemical composition of high-quality potassium magnesium sulfate fertilizer includes: K + 25.8 wt%, Mg 2+ 8.02 wt%, SO4 2- 63.1 wt%, Cl - 0.2 wt%, water-insoluble matter 0.05%.

[0069] Example 2

[0070] (1) Raw material preparation: Soft potassium magnesia concentrate produced by flotation, crystallization mother liquor, and flotation tailings from the flotation process are selected as raw materials. The chemical composition of soft potassium magnesia concentrate includes: K + 13.0 wt%, Mg 2+ 5.5 wt%, SO4 2- 35.0 wt%. The chemical composition of the crystallization mother liquor includes: K + 2.6wt%, Mg 2+ 3.1 wt%, SO4 2- 11.2 wt%, Cl - 11.6 wt%. The chemical composition of the flotation tailings includes: K + 2.6wt%, Mg 2+ 4.5 wt%, SO4 2- 6.8 wt%, Cl - 15.1 wt%.

[0071] (2) Hot dissolution sedimentation: Soft potassium magnesium alum concentrate and crystallization mother liquor are added to a slurry mixing tank to form a slurry with a solid content of 45%. This slurry is then introduced into a hot dissolution tank, with the remaining crystallization mother liquor added along with a suitable amount of flotation tailings. Steam is introduced to raise the hot dissolution temperature to 75–80°C, ensuring that all the soft potassium magnesium alum in the concentrate dissolves in the liquid phase to obtain the hot dissolution mother liquor. The chemical composition of the hot dissolution mother liquor in the hot dissolution tank includes: K + 3.6 wt%, Mg 2+ 3.2 wt%, SO4 2- 16.2 wt%, Cl - 9.5wt%. The hot-melt mother liquor is introduced into an insulated settling tank, maintaining a temperature of 75℃~80℃. The overflow (i.e., purified hot-melt mother liquor) enters the hot-melt mother liquor tank. When the insoluble matter in the bottom water of the insulated settling tank is ≥3%, the bottom water is discharged. The chemical composition of the purified hot-melt mother liquor in the hot-melt mother liquor tank includes: K + 3.4 wt%, Mg 2+ 3.2 wt%, SO4 2- 15.9 wt%, Cl - 9.4 wt%.

[0072] (3) Three-stage crystallization: The liquid phase in the hot melt mother liquor tank is introduced into the first-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 20.0 kPa and a secondary steam temperature of 65℃; the underflow from the first-stage vacuum crystallizer is introduced into the second-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 7 kPa and a secondary steam temperature of 40℃; the underflow from the second-stage vacuum crystallizer is introduced into the third-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 2 kPa and a secondary steam temperature of 21℃.

[0073] (4) Concentration and Filtration: The underflow and overflow of the Stage III vacuum crystallizer are jointly introduced into the crystallization thickener. The overflow of the crystallization thickener is returned to the slurry preparation tank and hot melt tank as crystallization mother liquor. The chemical composition of the overflow of the crystallization thickener includes: K + 2.6wt%, Mg 2 + 3.1 wt%, SO4 2- 11.2 wt%, Cl - 11.6 wt%. The underflow from the crystallizer (45% solids content) enters the high-level tank of the centrifuge, operating at 20°C. The moisture content of the centrifuge filter cake is ≤6 wt% (wet basis), and the centrifuged filtrate is returned to the hot-melt mother liquor tank.

[0074] (5) Drying: The main components of the centrifuged wet material are various potassium magnesium sulfates containing different amounts of water of crystallization. The drying temperature is 180℃, and the free water content of the product is ≤1wt%. The final product obtained is potassium magnesium sulfate powder. The chemical composition of the centrifuged wet material includes: K + 19.1 wt%, Mg 2+ 5.8 wt%, SO4 2-47.5 wt%, Cl - 0.07wt%. The chemical composition of superior potassium magnesium sulfate fertilizer includes: K + 26.3 wt%, Mg 2+ 8.1 wt%, SO4 2- 63.8 wt%, Cl - 0.2 wt%, water-insoluble matter 0.02%.

[0075] Example 3

[0076] (1) Raw material preparation: Soft potassium magnesia concentrate produced by flotation, crystallization mother liquor, and flotation tailings from the flotation process are selected as raw materials. The chemical composition of soft potassium magnesia concentrate includes: K + 15.7 wt%, Mg 2+ 6.3 wt%, SO4 2- 38.8 wt%. The chemical composition of the crystallization mother liquor includes: K + 2.8 wt%, Mg 2+ 3.2 wt%, SO4 2- 11.8 wt%, Cl - 12.0 wt%. The chemical composition of the flotation tailings includes: K + 2.8 wt%, Mg 2+ 4.8 wt%, SO4 2- 7.1 wt%, Cl - 15.6 wt%.

[0077] (2) Hot dissolution sedimentation: Soft potassium magnesium alum concentrate and crystallization mother liquor are added to a slurry mixing tank to form a slurry with a solid content of 45%. This slurry is then introduced into a hot dissolution tank, with the remaining crystallization mother liquor added along with a suitable amount of flotation tailings. Steam is introduced to raise the hot dissolution temperature to 75–80°C, ensuring that all the soft potassium magnesium alum in the concentrate dissolves in the liquid phase to obtain the hot dissolution mother liquor. The chemical composition of the hot dissolution mother liquor in the hot dissolution tank includes: K + 3.7 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.2 wt%, Cl - 9.4 wt%. The hot-melt mother liquor is introduced into an insulated settling tank, maintaining a temperature of 75℃~80℃. The overflow (i.e., purified hot-melt mother liquor) enters the hot-melt mother liquor tank. When the insoluble matter in the bottom water of the insulated settling tank is ≥3%, the bottom water is discharged. The chemical composition of the purified hot-melt mother liquor in the hot-melt mother liquor tank includes: K + 3.8 wt%, Mg 2+ 3.1 wt%, SO4 2- 16.3 wt%, Cl - 9.2 wt%.

[0078] (3) Three-stage crystallization: The liquid phase in the hot melt mother liquor tank is introduced into the first-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 20.0 kPa and a secondary steam temperature of 65℃; the underflow from the first-stage vacuum crystallizer is introduced into the second-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 7 kPa and a secondary steam temperature of 40℃; the underflow from the second-stage vacuum crystallizer is introduced into the third-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 2 kPa and a secondary steam temperature of 21℃.

[0079] (4) Concentration and Filtration: The underflow and overflow of the Stage III vacuum crystallizer are jointly introduced into the crystallization thickener. The overflow of the crystallization thickener is returned to the slurry preparation tank and hot melt tank as crystallization mother liquor. The chemical composition of the overflow of the crystallization thickener includes: K + 2.8 wt%, Mg 2 + 3.2 wt%, SO4 2- 11.8 wt%, Cl - 12.0 wt%. The underflow from the crystallizer (solid content 45%) enters the high-level tank of the centrifuge, operating at 20°C. The moisture content of the centrifuge filter cake is ≤6 wt% (wet basis), and the centrifuged filtrate is returned to the hot-melt mother liquor tank.

[0080] (5) Drying: The main components of the centrifuged wet material are various potassium magnesium sulfates containing different amounts of water of crystallization. The drying temperature is 180℃, and the free water content of the product is ≤1wt%. The final product obtained is potassium magnesium sulfate powder. The chemical composition of the centrifuged wet material includes: K + 19.4 wt%, Mg 2+ 5.8 wt%, SO4 2- 47.5 wt%, Cl - 0.08wt%. The chemical composition of superior potassium magnesium sulfate fertilizer includes: K + 26.8 wt%, Mg 2+ 8.2 wt%, SO4 2- 64.7 wt%, Cl - 0.07 wt%, water-insoluble matter 0.01%.

[0081] Comparative Example 1

[0082] (1) Raw material preparation: Soft potassium magnesia concentrate produced by flotation, crystallization mother liquor, and flotation tailings from the flotation process are selected as raw materials. The chemical composition of soft potassium magnesia concentrate includes: K + 15.6 wt%, Mg 2+ 6.1 wt%, SO4 2- 38.5 wt%. The chemical composition of the crystallization mother liquor includes: K + 2.8 wt%, Mg 2+ 2.7 wt%, SO4 2- 11.8 wt%, Cl -14.0 wt%. The chemical composition of the flotation tailings includes: K + 2.8 wt%, Mg 2+ 4.8 wt%, SO4 2- 7.1 wt%, Cl - 15.6 wt%.

[0083] (2) Hot dissolution sedimentation: Soft potassium magnesium alum concentrate and crystallization mother liquor (flotation tailings used at startup) are added to the slurry mixing tank and mixed to form a slurry with a solid content of 45%. This slurry is then introduced into the hot dissolution tank, and the remaining crystallization mother liquor is added while simultaneously supplementing with an appropriate amount of flotation tailings. Steam is introduced to raise the hot dissolution temperature to 75-80℃, ensuring that all the soft potassium magnesium alum in the concentrate dissolves in the liquid phase to obtain the hot dissolution mother liquor. The chemical composition of the hot dissolution mother liquor in the hot dissolution tank includes: K + 3.3 wt%, Mg 2+ 3.1 wt%, SO4 2- 11.2 wt%, Cl - 12.4 wt%. The hot-melt mother liquor is introduced into an insulated settling tank, maintaining a temperature of 75℃~80℃. The overflow (i.e., purified hot-melt mother liquor) enters the hot-melt mother liquor tank. When the insoluble matter in the bottom water of the insulated settling tank is ≥3%, the bottom water is discharged. The chemical composition of the purified hot-melt mother liquor in the hot-melt mother liquor tank includes: K + 3.2 wt%, Mg 2+ 3.1 wt%, SO4 2- 11.0 wt%, Cl - 12.1 wt%.

[0084] (3) Three-stage crystallization: The liquid phase in the hot melt mother liquor tank is introduced into the first-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 20.0 kPa and a secondary steam temperature of 65℃; the underflow from the first-stage vacuum crystallizer is introduced into the second-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 7 kPa and a secondary steam temperature of 40℃; the underflow from the second-stage vacuum crystallizer is introduced into the third-stage vacuum crystallizer, with an operating pressure (absolute pressure) of 2 kPa and a secondary steam temperature of 21℃.

[0085] (4) Concentration and Filtration: The underflow and overflow of the Stage III vacuum crystallizer are jointly introduced into the crystallization thickener. The overflow of the crystallization thickener is returned to the slurry preparation tank and hot melt tank as crystallization mother liquor. The chemical composition of the overflow of the crystallization thickener includes: K + 2.8 wt%, Mg 2 + 2.7 wt%, SO4 2- 10.8 wt%, Cl - 13.0 wt%. The underflow from the crystallizer (solid content 45%) enters the high-level tank of the centrifuge, operating at 20°C. The moisture content of the centrifuge filter cake is ≤6 wt% (wet basis), and the centrifuged filtrate is returned to the hot-melt mother liquor tank.

[0086] (5) Drying: The centrifuged wet material is dried at 180℃, and the free water content of the product is ≤1wt%, to obtain the final product. The chemical composition of the centrifuged wet material includes: K + 15.4 wt%, Mg 2+ 3.8 wt%, SO4 2- 47.5 wt%, Cl - 0.08wt%. The chemical composition of superior potassium magnesium sulfate fertilizer includes: K + 21.8 wt%, Mg 2+ 7.1 wt%, SO4 2- 64.7 wt%, Cl - 0.07 wt%, water-insoluble matter 0.01%. In this comparative example, due to the excessively long mother liquor circulation time, the Cl in the crystallization mother liquor... - Exceeding the standard, Na + Concentration saturation precipitation reduces the magnesium content in the final product, resulting in substandard product quality.

[0087] As can be seen from the above embodiments, the present invention provides a method for preparing feed-grade potassium magnesium sulfate fertilizer. The preparation method provided by the present invention has the advantages of stable product quality and high purity, and can meet the market demand for high-quality feed-grade potassium magnesium sulfate fertilizer.

[0088] 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 preparing feed-grade potassium magnesium sulfate fertilizer, characterized in that, Includes the following steps: (1) The soft potassium magnesium alum concentrate is prepared by using a slurry preparation solution to obtain a slurry. The soft potassium magnesium alum concentrate is obtained by flotation of potassium mixed salt ore, and the grade of the soft potassium magnesium alum concentrate is K. + ≥12.0wt%, Mg 2+ ≥5.0wt%, SO4 2- ≥34.0 wt%, the flotation also yields flotation tailings, the slurry preparation solution includes at least two of the following: water, flotation tailings, and crystallization mother liquor obtained in step (3), wherein the crystallization mother liquor contains Cl - The content is <14wt%; (2) The slurry is heated to the hot dissolution temperature for hot dissolution to obtain hot dissolution mother liquor. The hot dissolution mother liquor is kept at a constant temperature and allowed to settle to obtain purified hot dissolution mother liquor. (3) The purified hot-dissolved mother liquor is vacuum crystallized to obtain a crystallized mother liquor and a concentrated crystallized solution. The vacuum crystallization includes sequentially performing first-stage vacuum crystallization, second-stage vacuum crystallization and third-stage vacuum crystallization. (4) Separate the solid and liquid phases of the crystallized concentrated solution to obtain a wet product and a separated concentrated liquid phase; (5) The wet product is dried to obtain the feed-grade potassium magnesium sulfate fertilizer.

2. The preparation method according to claim 1, characterized in that, The composition of the soft potassium magnesium sulfate concentrate includes: K + 12.0–16.0 wt%, Mg 2+ 5.0–6.0 wt%, SO4 2- 34.0–40.0 wt%; The composition of the flotation tailings includes: K + 2.0–3.0 wt%, Mg 2+ 4.0–5.0 wt%, SO4 2- 6.0–8.0 wt%, Cl - 12–16 wt%; The composition of the crystallization mother liquor includes: K + 2.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 10.0–12.0 wt%, Cl - 11-12 wt%.

3. The preparation method according to claim 1, characterized in that, The solid content in the slurry is 40-45% by mass.

4. The preparation method according to claim 1, characterized in that, The hot-melting temperature is 75–80°C, and the composition of the hot-melting mother liquor includes: K + 3.0–5.0 wt%, Mg 2+ 2.9–3.3 wt%, SO4 2- 15.0–17.0 wt%, Cl - 9–10 wt%.

5. The preparation method according to claim 1, characterized in that, The temperature for heat preservation and sedimentation is 75–80°C, and the composition of the purified hot-dissolved mother liquor includes: K + 3.0–4.0 wt%, Mg 2+ 2.7–3.1 wt%, SO4 2- 14.0–17.0 wt%, Cl - 9–10 wt%.

6. The preparation method according to claim 1, characterized in that, The operating pressure of the first-stage vacuum crystallization is 18–20 kPa, and the temperature is 60–65 °C; the operating pressure of the second-stage vacuum crystallization is 7–8 kPa, and the temperature is 40–45 °C; the operating pressure of the third-stage vacuum crystallization is 2–5 kPa, and the temperature is 21–30 °C.

7. The preparation method according to claim 1, characterized in that, The solid phase content in the concentrated crystallization solution is 45-50% by mass, the moisture content of the wet product is ≤6wt%, and the composition of the wet product includes: K + 18.0–20.0 wt%, Mg 2+ 5.7–6.1 wt%, SO4 2- 47.0–48.0 wt%, Cl - 0.05–0.1 wt%.

8. The preparation method according to claim 1, characterized in that, The drying temperature is 170–180°C; the free water content in the feed-grade potassium magnesium sulfate fertilizer is ≤1 wt%.

9. The preparation method according to claim 1 or 8, characterized in that, The composition of the feed-grade potassium magnesium sulfate fertilizer includes: K + 25.2–27.0 wt%, Mg 2+ 8.0–8.2 wt%, SO4 2- 62.0–65.0 wt%, Cl - 0.05–0.5 wt%, water-insoluble matter 0.01–0.05 wt%.

10. The preparation method according to claim 1, characterized in that, After obtaining the separated concentrated liquid phase, the method further includes reusing the separated concentrated liquid phase. The reuse includes returning the separated concentrated liquid phase to mix with the purified hot-soluble mother liquor to obtain a mixed mother liquor, and using the mixed mother liquor as the raw material for step (3).

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