CN107935003A - A kind of method that high-purity magnesium sulfate is prepared by magnesium sulfate discarded object - Google Patents

Abstract

The invention discloses a kind of method that high-purity magnesium sulfate is prepared by magnesium sulfate discarded object, which further includes the impurity such as water-insoluble, other inorganic salts, heavy metal, arsenide, organic matter, microorganism；This method includes：Water-insoluble is not contained in S1, regulation and control magnesium sulfate discarded object, and magnesium sulfate primary product are used as using the mass percent that anhydrous magnesium chloride is counted as 3%~20%；S2, roast magnesium sulfate primary product 0.5h~3.5h at 400 DEG C~650 DEG C, obtains product of roasting；S3, after product of roasting is mixed with water, aquation 2h~10h at 60 DEG C~90 DEG C, obtain hydrated product；Hydrated product, is carried out a separation of solid and liquid by S4, and the first liquid phase of gained is concentrated, crystallizes, secondary separation of solid and liquid, obtains magnesium sulfate crystals, magnesium sulfate crystals are washed, dry, obtain high-purity magnesium sulfate.The method according to the invention only needs to adjust the content of magnesium chloride in magnesium sulfate primary product, raw material and process costs are cheap, technique is simple.

Description

A method for preparing high-purity magnesium sulfate from magnesium sulfate waste

technical field

The invention belongs to the technical field of industrial waste recycling, and specifically relates to a method for preparing high-purity magnesium sulfate from magnesium sulfate waste.

Background technique

High-purity magnesium sulfate refers to substances with a magnesium sulfate content of more than 99.9%, which can be widely used in food, medicine and other fields. At present, there is no unified standard for high-purity magnesium sulfate; according to GB 29207-2012 records, food-grade magnesium sulfate It is required that the mass percentage of magnesium sulfate is not lower than 99.0%, therefore, high-purity magnesium sulfate is limited to a purity of not lower than 99.9%.

In industry, wastes rich in magnesium sulfate are generally produced by-products. These wastes mainly include potassium extraction tail liquid and by-products of magnesium sulfate subtype salt lakes, boron extraction tail liquid of boron-magnesium mines, seawater sun-dried salt brine, and serpentine decomposition. Mother liquor etc., if the magnesium sulfate in this part of magnesium sulfate waste can be reclaimed, used as the production of high-purity magnesium sulfate, not only has improved economic added value, and has reduced pollution and waste; But, except containing In addition to a large amount of magnesium sulfate, it also contains mud sand, chloride, other sulfates, heavy metal ions, iron ions, arsenic compounds, flotation agents and microorganisms. It greatly increases the technical difficulty and process cost.

At present, there are many processes for preparing high-purity magnesium sulfate. For example, Ma Jun and others have prepared high-purity magnesium sulfate through multi-step recrystallization, decolorization and washing by using multi-stage series equipment; Ding Hongxia and others have used industrial magnesium sulfate heptahydrate to produce high-grade magnesium sulfate Pure magnesium sulfate, which removes impurities and decolorizes by adding alkaline additives, activated carbon, etc., and then obtains high-purity magnesium sulfate; Zhang Baoyuan adopts multiple fine filtration and recrystallization of magnesium sulfate heptahydrate industrial products, and dehydrates at high temperature to obtain High-purity anhydrous magnesium sulfate; Wu Yaoming et al. used magnesite to carry out multi-step neutralization and removal of impurities on laterite nickel ore leaching solution, and obtained high-purity magnesium sulfate through concentrated crystallization; Hu Jianhua reacted with high-purity magnesium oxide and sulfuric acid. Concentrate and crystallize to prepare high-purity magnesium sulfate; Xiong Yijun neutralizes the serpentine acid solution with waste alkali to obtain magnesium salt precipitation, and after pyrolysis, the decomposition product is dissolved in sulfuric acid, and the serpentine powder adjusts the acidity and concentrates to obtain High-purity magnesium sulfate; Su Xuesong used hydrofluoric acid to precipitate impurity calcium ions in magnesium sulfate to obtain high-purity magnesium sulfate; Cheng Huaigang et al. used gypsum to remove impurities in brine, then used multi-step natural evaporation to concentrate brine, and finally forced evaporation to concentrate Brine, to obtain high-purity magnesium sulfate hexahydrate; and so on. Obviously, in the process method of preparing high-purity magnesium sulfate in prior art, adopt high-purity magnesium oxide raw material to produce high-purity magnesium sulfate, sulfuric acid used needs to use high-purity product equally, and the high input of raw material will pull up production cost greatly; Secondary recrystallization purification will produce a large amount of magnesium-rich waste liquid, and the entrainment and adsorption of chloride ions and organic matter are difficult to eliminate through recrystallization. Multiple recrystallizations need to consume a lot of heat energy for evaporation and concentration, and a large amount of money needs to be invested to purchase enough Advanced evaporation and crystallization equipment, and more importantly, a large amount of magnesium-rich waste liquid needs to be processed, resulting in great environmental pressure and waste of magnesium resources; natural evaporation of brine combined with forced evaporation and concentration to obtain high-purity magnesium sulfate by crystallization takes a long time. The entrainment and adsorption of organic matter and chloride ions in brine are difficult to solve; if industrial magnesium sulfate is used as raw material, heavy metal ions, iron ions and toxic compounds are precipitated by adding alkaline additives, and organic matter is removed by adding decolorizers such as activated carbon, and then Obtain high-purity magnesium sulfate, but this method is not suitable for complex systems.

Therefore, be necessary to search a kind of with above-mentioned magnesium sulfate waste as raw material, in order to prepare the technical method of high-purity magnesium sulfate.

Contents of the invention

In order to solve the problems in the above-mentioned prior art, the invention provides a method for preparing high-purity magnesium sulfate from magnesium sulfate waste, which is applicable to Complex systems with many impurities such as chemical compounds, organic matter, microorganisms, etc., have a wider scope of application, and the process is simple and the cost is low.

In order to achieve the above-mentioned purpose of the invention, the present invention has adopted following technical scheme:

A method for preparing high-purity magnesium sulfate from magnesium sulfate waste, wherein the magnesium sulfate waste also includes at least one of mud sand, chloride, sulfate, heavy metal ions, iron ions, arsenic, organic matter, and microorganisms; Said method comprises the following steps:

S1. Controlling that the magnesium sulfate waste does not contain water-insoluble matter, and wherein the mass percentage of anhydrous magnesium chloride is 3% to 20% as the primary product of magnesium sulfate; wherein, the dried solid phase of the magnesium sulfate waste The mass percentage calculated by anhydrous magnesium sulfate in the magnesium sulfate is not less than 10%, and the mass percentage calculated by anhydrous magnesium sulfate in the solid phase of the magnesium sulfate primary product after drying is not less than 20%;

S2. Roasting the magnesium sulfate primary product at 400°C to 650°C for 0.5h to 3.5h to obtain a roasted product;

S3, mixing the roasted product with water and then hydrating at 60°C to 90°C for 2h to 10h to obtain a hydration product;

S4. The hydration product is subjected to solid-liquid separation once, and the obtained first liquid phase is concentrated, crystallized, and solid-liquid separated twice to obtain magnesium sulfate crystals, and the magnesium sulfate crystals are washed and dried to obtain high-purity magnesium sulfate .

Further, the magnesium sulfate waste is the potassium extraction tail liquid and by-products of the magnesium sulfate subtype salt lake brine, the boron extraction tail liquid of boron magnesium ore, the old brine of seawater drying salt, and the mother liquor of serpentine acid decomposition.

Further, the high-purity magnesium sulfate is at least one of high-purity anhydrous magnesium sulfate, high-purity magnesium sulfate monohydrate, and high-purity magnesium sulfate heptahydrate.

Further, in the step S1, the magnesium sulfate primary product is obtained by filtering and/or flotation and/or concentration-crystallization on the magnesium sulfate waste.

Further, the method for controlling the mass percentage of anhydrous magnesium chloride in the primary product of magnesium sulfate to be 3% to 20% includes: adding magnesium chloride to magnesium sulfate waste free of water-insoluble matter, or adding magnesium sulfate free of water-insoluble matter The waste is subjected to flotation to remove part of the magnesium chloride.

Further, in the step S2, the HCl gas corresponding to the roasted product is also obtained through roasting; in the step S4, the second gas corresponding to the first liquid phase is also obtained through a solid-liquid separation. a solid phase; the first solid phase is used to absorb and neutralize the HCl gas to generate by-product magnesium chloride.

Further, the by-product magnesium chloride is added to the magnesium sulfate waste for removing water-insoluble matter, so that the mass percentage of anhydrous magnesium chloride in the magnesium sulfate primary product is 3% to 20%.

Further, in the step S4, the hydration product is subjected to a solid-liquid separation using a plate-and-frame filter press separation method.

Further, in the step S4, the hydrated product is subjected to a solid-liquid separation while it is hot.

Further, in the step S4, the method of concentrating the first liquid phase is forced heating, evaporation and concentration.

Further, in the step S4, the washing water used for washing the magnesium sulfate crystals is double distilled water.

Further, the step S2 further includes: recovering the thermal energy of the roasted product with water to obtain recovered hot water; the recovered hot water is used for the hydration treatment in the step S3.

Beneficial effect:

(1) The present invention does not need to use high-purity raw materials, but adopts the magnesium sulfate waste that contains various impurities therein as raw material, and promptly this process method is to containing silt, chloride, sulfate, heavy metal ion, iron ion, arsenide The magnesium sulfate system with various impurities such as microorganisms, organic matter, etc. has wide applicability and adjustability;

(2) according to the method of the present invention, only need to adjust the content of magnesium chloride in the primary product of magnesium sulfate, need not add compounds such as acid, alkali, decolorizing agent in addition, raw material and process cost are low;

(3) The roasting process of the present invention is to utilize the high temperature environment to make the magnesium chloride in the primary product of magnesium sulfate in-situ generate active magnesium oxide, so that the organic matter of microorganisms and flotation agents is carbonized, and the subsequent hydration process makes active magnesium oxide in-situ Be converted into magnesium hydroxide; That is, the roasting-hydration process in the present invention makes the in-situ conversion of impurities such as magnesium chloride, microorganisms and flotation agents in the primary product of magnesium sulfate into precipitant, adsorbent, and hydrogen required for the purification of magnesium sulfate products. Magnesium oxide and carbonized products have achieved complete removal of impurities such as heavy metal ions, iron ions, and toxic arsenic compounds through co-precipitation and adsorption, reducing the content of the above-mentioned impurities to the requirements of high-purity magnesium sulfate. Therefore, the roasting of the present invention- The hydration process cleverly realizes the in-situ resource utilization of impurities in the system;

(4) The process of the present invention is simple, easy to operate, and ingeniously realizes the in-situ resource utilization of impurities in the raw material system of magnesium sulfate waste, specifically converting magnesium chloride into magnesium oxide in situ through the roasting process, microorganisms and flotation Magnesium oxide is transformed into carbon compounds in situ, and magnesium oxide is converted into magnesium hydroxide in situ through the hydration process. The above processes do not separate magnesium chloride, microorganisms, and flotation agents, and they are directly converted into purified flotation agents in the waste mixing system. The required precipitant and adsorbent.

Detailed ways

Hereinafter, embodiments of the present invention will be described in detail. This invention may, however, be embodied in many different forms and should not be construed as limited to the specific embodiments set forth herein. Rather, the embodiments are provided to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to particular intended uses.

It will be understood that although the terms "first", "second", etc. may be used herein to describe various substances, these substances should not be limited by these terms, which are only used to distinguish one substance from another. At the same time, the terms "primary", "secondary", etc. are only used to describe various process operations, and these terms are only used to distinguish one process operation from another identical process operation.

The invention discloses a method for preparing high-purity magnesium sulfate from magnesium sulfate waste, which is a kind of magnesium sulfate containing mud sand and/or chloride and/or sulfate and/or heavy metal ions and/or iron ions and /or impurities such as arsenide and/or organic matter and/or microorganisms, which need to meet the following requirements: the mass percentage of anhydrous magnesium sulfate in the solid phase after drying is not less than 10%; and, the magnesium sulfate is discarded The substances mainly come from the potassium extraction tail liquid and by-products of magnesium sulfate subtype salt lake brine, the boron extraction tail liquid of boron magnesium ore, the old brine of seawater sun-dried salt, and the mother liquor of serpentine acid decomposition.

The above-mentioned degree of dryness means that the solid phase does not contain obvious liquid water and wet slag phase.

The method for preparing high-purity magnesium sulfate according to the magnesium sulfate waste of the present invention may further comprise the steps:

In step S1, the magnesium sulfate waste is adjusted to contain no water-insoluble matter, and the mass percentage calculated by anhydrous magnesium chloride is 3%-20% as the primary product of magnesium sulfate.

Specifically, it is necessary to control the mass percentage of anhydrous magnesium sulfate in the dried solid phase of the obtained magnesium sulfate primary product to not be less than 20%.

It is worth noting that the content of magnesium chloride in the waste based on the raw material magnesium sulfate is not fixed. Therefore, if the content in terms of anhydrous magnesium chloride has reached 3% to 20%, it is determined whether it contains water-insoluble substances such as silt or sand. Carry out filtration operation, to remove water-insoluble matter such as silt; Or when the content in anhydrous magnesium chloride is lower than 3%, then can add part of magnesium chloride to the system, so that the content in anhydrous magnesium chloride in the primary product of magnesium sulfate Reach the range of 3%～20%; Or when the content in anhydrous magnesium chloride is higher than 20%, then part of the magnesium chloride can be removed by flotation. In this way, water-insoluble substances such as mud sand and ore slime can be removed from the magnesium sulfate waste, and the initial enrichment of magnesium sulfate can be realized.

In addition, it should be noted that the above-mentioned anhydrous magnesium chloride and anhydrous magnesium sulfate are only the quantitative nominal compounds adopted for the convenience of expressing the mass percentage when counting the contents of magnesium sulfate waste and magnesium sulfate primary products. Does not represent the actual form of magnesium chloride and magnesium sulfate in magnesium sulfate waste and magnesium sulfate primary products.

In step S2, the magnesium sulfate primary product is calcined at 400° C. to 650° C. for 0.5 h to 3.5 h to obtain a calcined product.

In step S3, the calcined product is mixed with water and then hydrated at 60° C. to 90° C. for 2 h to 10 h to obtain a hydrated product.

In step S4, the hydration product is subjected to solid-liquid separation once, and the obtained first liquid phase is concentrated, crystallized, and solid-liquid separated twice to obtain magnesium sulfate crystals, which are washed and dried to obtain high-purity magnesium sulfate.

Specifically, the obtained high-purity magnesium sulfate refers to at least one of high-purity anhydrous magnesium sulfate, high-purity magnesium sulfate monohydrate, and high-purity magnesium sulfate heptahydrate; and, the high-purity mentioned here refers to the product The purity in terms of mass percentage is not less than 99.9%.

Preferably, the plate and frame filter press separation method can be used to conduct a solid-liquid separation of the hydrated product, and preferably the hydrated product before separation is directly separated while hot without cooling.

More preferably, the first liquid phase is concentrated using a forced heating evaporation concentration method.

Generally, the magnesium sulfate crystals are washed with double distilled water, and the number of washings and the corresponding drying methods are not limited, and the time for each washing can be controlled to be 20s-1min.

The above-mentioned method for preparing high-purity magnesium sulfate from magnesium sulfate waste of the present invention will be illustrated below by specific examples.

Example 1

The magnesium sulfate waste in the present embodiment is specifically the magnesium sulfate subtype salt lake brine extracting potassium by-product, and its content in the solid phase after drying is 25% in terms of anhydrous magnesium sulfate, and the content in terms of anhydrous magnesium chloride is 3%, and there are other inorganic salts such as sodium chloride and sodium sulfate, as well as impurities such as mud sand and organic matter in brine itself.

The magnesium sulfate waste is pretreated by flotation to obtain the primary product of magnesium sulfate.

In the primary product of magnesium sulfate, the content of magnesium sulfate is 35%, the content of magnesium chloride is 3%, and it also contains impurities such as other inorganic salts, flotation agents brought in by the flotation process, and organic matter in the brine itself.

The primary product of magnesium sulfate was calcined at 400° C. for 0.5 h to obtain a calcined product.

Add water to the roasted product to cool, and age the roasted product-water system at 60° C. for 10 h to obtain a hydrated product.

Centrifuge the hydration product, heat, evaporate and concentrate the obtained first liquid phase, and crystallize to form magnesium sulfate hydrate crystals; the magnesium sulfate hydrate crystals are separated from solid and liquid, washed twice with double distilled water, and then dried to obtain high-purity magnesium sulfate. The purity of the high-purity magnesium sulfate reaches 99.96%.

In this embodiment, the above percentages all represent mass percentages.

Example 2

The magnesium sulfate waste in the present embodiment is specifically the boron extraction tail liquid of the boron magnesium ore in the salt lake, and the content in the solid phase after drying is 30% in terms of anhydrous magnesium sulfate, and the content in terms of anhydrous magnesium chloride is 10% , and there are other impurities such as sodium chloride, sodium sulfate and other inorganic salts and organic substances.

The dried solid phase of the magnesium sulfate waste was used as the primary product of magnesium sulfate, and it was calcined at 650° C. for 2 hours to obtain a calcined product.

Add water to the roasted product to cool, and age the roasted product-water system at 90° C. for 2 hours to obtain a hydrated product.

Centrifuge the hydration product, heat, evaporate and concentrate the obtained first liquid phase, and crystallize to form anhydrous magnesium sulfate crystal; the anhydrous magnesium sulfate crystal is separated from solid and liquid, washed twice with double distilled water, and dried to obtain high-purity sulfuric acid Magnesium, the purity of this high-purity magnesium sulfate reaches 99.99%.

In this embodiment, the above percentages all represent mass percentages.

Example 3

The magnesium sulfate waste in the present embodiment is specifically the old brine of coastal seawater sun-dried salt, and its content in the solid phase after drying is 15% in terms of anhydrous magnesium sulfate, and the content in terms of anhydrous magnesium chloride is 25%, and There are also inorganic salts such as sodium chloride and calcium salts, and impurities such as mud and sand, organic matter in seawater itself.

The dried solid phase of the magnesium sulfate waste is pretreated by flotation to obtain the primary product of magnesium sulfate.

In the magnesium sulfate primary product, the magnesium sulfate content is 20%, the magnesium chloride content is 20%, and contains impurities such as inorganic salts, flotation agents and organic matter.

Calcined at 500°C for 3.5h to obtain a calcined product.

Water was added to the roasted product to cool, and the roasted product-water system was aged at 70° C. for 6 hours to obtain a hydrated product.

Centrifuge the hydration product, heat, evaporate and concentrate the obtained first liquid phase, and crystallize to form magnesium sulfate hydrate crystals; the magnesium sulfate hydrate crystals are separated from solid and liquid, washed twice with double distilled water, and dried to obtain high-purity magnesium sulfate. The purity of the high-purity magnesium sulfate reaches 99.98%.

In this embodiment, the above percentages all represent mass percentages.

Example 4

The magnesium sulfate waste in the present embodiment is specifically serpentine sulfuric acid decomposition liquid, and the content of anhydrous magnesium sulfate in its dried solid phase is 10%, and there are also inorganic salts such as sodium sulfate and calcium salt and mud sand and other impurities, but almost no magnesium chloride.

The dried solid phase of the magnesium sulfate waste is pretreated by flotation, and at the same time, part of magnesium chloride is added to the system to obtain a primary product of magnesium sulfate with a magnesium chloride content of 8%.

In the primary product of magnesium sulfate, the content of magnesium sulfate is 20%, and it also contains impurities such as inorganic salts, heavy metal ions, arsenic compounds and the like.

Calcined at 550°C for 2h to obtain a calcined product.

Water was added to the roasted product to cool, and the roasted product-water system was aged at 65° C. for 4 hours to obtain a hydrated product.

Centrifuge the hydration product, heat, evaporate and concentrate the obtained first liquid phase, and crystallize to form magnesium sulfate hydrate crystals; the magnesium sulfate hydrate crystals are separated from solid and liquid, washed twice with double distilled water, and dried to obtain high-purity magnesium sulfate. The purity of the high-purity magnesium sulfate reaches 99.95%.

In this embodiment, the above percentages all represent mass percentages.

Example 5

The magnesium sulfate waste in the present embodiment is specifically the solid by-product of oilfield water extraction of potassium, and its content in the solid phase after drying is 10% in terms of anhydrous magnesium sulfate, and the content in terms of anhydrous magnesium chloride is 5%. In addition, there are inorganic salts such as sodium chloride and sodium sulfate, as well as impurities such as mud sand and organic matter.

The magnesium sulfate waste is pretreated by flotation to obtain the primary product of magnesium sulfate.

In the magnesium sulfate primary product, the magnesium sulfate content is 25%, the magnesium chloride content is 8%, and contains impurities such as inorganic salts, flotation agents and organic matter.

The primary product of magnesium sulfate was calcined at 550° C. for 1.5 h to obtain a calcined product.

Add water to the roasted product to cool, and age the roasted product-water system at 80° C. for 6 hours to obtain a hydrated product.

Centrifuge the hydration product, heat, evaporate and concentrate the obtained first liquid phase, and crystallize to form magnesium sulfate hydrate crystals; the magnesium sulfate hydrate crystals are separated from solid and liquid, washed twice with double distilled water, and then dried to obtain high-purity magnesium sulfate. The purity of the high-purity magnesium sulfate reaches 99.98%.

In this embodiment, the above percentages all represent mass percentages.

While the invention has been shown and described with reference to particular embodiments, it will be understood by those skilled in the art that changes may be made in the form and scope thereof without departing from the spirit and scope of the invention as defined by the claims and their equivalents. Various changes in details.

Claims (12)

1. a method for preparing high-purity magnesium sulfate by magnesium sulfate waste, it is characterized in that, described magnesium sulfate waste also includes silt, chloride, sulfate, heavy metal ion, iron ion, arsenide, organic matter, microorganism at least one of; said method comprises the steps of: S1. Controlling that the magnesium sulfate waste does not contain water-insoluble matter, and wherein the mass percentage in terms of anhydrous magnesium chloride is 3% to 20% as the primary product of magnesium sulfate; wherein, the magnesium sulfate waste is dried The mass percentage calculated as anhydrous magnesium sulfate in the solid phase is not less than 10%, and the mass percentage calculated as anhydrous magnesium sulfate in the solid phase after the magnesium sulfate primary product is dried is not less than 20%; S2. Roasting the magnesium sulfate primary product at 400°C to 650°C for 0.5h to 3.5h to obtain a roasted product; S3, mixing the roasted product with water and then hydrating at 60°C to 90°C for 2h to 10h to obtain a hydration product; S4. The hydration product is subjected to solid-liquid separation once, and the obtained first liquid phase is concentrated, crystallized, and solid-liquid separated twice to obtain magnesium sulfate crystals, and the magnesium sulfate crystals are washed and dried to obtain high-purity magnesium sulfate ; Wherein, the purity of the high-purity magnesium sulfate is not less than 99.9%. 2. The method according to claim 1, characterized in that, the magnesium sulfate waste is the potassium tail liquid and the by-product of the magnesium sulfate subtype salt lake brine, the boron tail liquid of the boron magnesium ore, seawater drying salt Halogen, serpentine acid decomposition mother liquor. 3. according to the described method of claim 1 and 2, it is characterized in that, described high-purity magnesium sulfate is at least one in high-purity anhydrous magnesium sulfate, high-purity magnesium sulfate monohydrate, high-purity magnesium sulfate heptahydrate. 4. The method according to claim 1 or 2, characterized in that, in the step S1, the magnesium sulfate primary product is obtained by filtering and/or flotation the magnesium sulfate waste. 5. method according to claim 4, is characterized in that, the method that the mass percent of anhydrous magnesium chloride in the magnesium sulfate primary product of regulation and control is 3%～20% comprises: in the magnesium sulfate waste that does not contain water-insoluble Add magnesium chloride, or flotation the magnesium sulfate waste without water-insoluble matter to remove part of the magnesium chloride. 6. The method according to claim 5, characterized in that, in the step S2, HCl gas corresponding to the roasted product is also obtained through roasting; in the step S4, after a solid-liquid separation, the A first solid phase corresponding to the first liquid phase is obtained; the first solid phase is used to absorb and neutralize the HCl gas to generate magnesium chloride as a by-product. 7. method according to claim 6, is characterized in that, described by-product magnesium chloride is used to be added in the magnesium sulfate waste that removes water-insoluble matter, so that the mass percent of anhydrous magnesium chloride in the described magnesium sulfate primary product 3% to 20%. 8. The method according to claim 1 or 2, characterized in that, in the step S4, a solid-liquid separation of the hydration product is performed by using a plate-and-frame filter press separation method. 9. The method according to claim 8, characterized in that, in the step S4, the hydrated product is subjected to a solid-liquid separation while it is hot. 10. The method according to claim 1 or 2, characterized in that, in the step S4, the method of concentrating the first liquid phase is forced heating, evaporation and concentration. 11. The method according to claim 1 or 2, characterized in that, in the step S4, the washing water for washing the magnesium sulfate crystals is double distilled water. 12. The method according to claim 1 or 2, characterized in that the step S2 further comprises: recovering the thermal energy of the roasted product with water to obtain recovered hot water; the recovered hot water is used in the step S3 hydration treatment.