Method for preparing composite soil conditioner from aluminum extraction residue and comprehensive utilization of coal-based solid waste

Abstract

The present application relates to the field of coal-based solid waste comprehensive utilization, and discloses a method for preparing a composite soil conditioner from aluminum extraction residue and comprehensive utilization of coal-based solid waste. The method comprises: carrying out a first heating reaction on a mixture of aluminum extraction residue, calcium agent and optional magnesium agent, and water to obtain a first product slurry; wherein the concentration of sulfate in the aluminum extraction residue is 1wt%-20wt%; carrying out a first solid-liquid separation on the first product slurry, carrying out a second heating reaction on the obtained solid phase and alkali liquor to obtain a second product slurry; carrying out a second solid-liquid separation on the second product slurry, and carrying out washing and drying on the obtained solid phase to obtain a composite soil conditioner. The present application uses the sulfuric acid method to extract aluminum, so that the residual acid in the aluminum extraction residue can be effectively utilized, and the neutralization with alkali to produce salts that can cause soil salinization is avoided. At the same time, through the improvement of the production process, a low-alkali silicon fertilizer is also prepared, and the comprehensive utilization of coal-based solid waste is realized.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization of coal-based solid waste, specifically to a method for preparing composite soil conditioners from aluminum extraction residues and the comprehensive utilization of coal-based solid waste. Background Technology

[0002] China is a country with scarce bauxite resources, with its ore mainly consisting of low- to medium-grade monohydrate gibbsite. Its production process is complex, with high energy consumption and production costs. With China's rapid economic development, bauxite resources are becoming increasingly scarce, and the shortage is widening. Due to China's large bauxite imports, global bauxite prices have risen sharply, and ore grades fluctuate significantly. This shortage of ore resources severely restricts the development of China's alumina industry. On the other hand, China is the world's largest coal producer, with most of its coal used for power generation, generating large amounts of fly ash annually. Currently, the utilization of fly ash in China is concentrated in low-value-added applications in the construction and road engineering sectors, with limited applications in high-value-added fields. The main method of treating coal gangue is still landfill, which occupies large amounts of land, pollutes the environment, and imposes a significant economic burden on enterprises and society. Coal gangue produced during coal mining and washing, fly ash from coal-fired power generation, and gasification slag from coal gasification are mainly composed of alumina and silicon dioxide, representing a potential silicon-aluminum resource. Therefore, the treatment and comprehensive utilization of coal-related solid wastes, such as coal gangue, fly ash and gasification slag, are receiving increasing attention.

[0003] Currently, technologies for extracting alumina from coal-related solid wastes, such as coal gangue, fly ash, and gasification slag, have been developed. The main technologies for alumina extraction from coal gangue, fly ash, and gasification slag can be divided into alkaline and acidic methods. Alkaline fly ash alumina extraction, including limestone sintering, soda-lime sintering, and hydrochemical methods, has successfully extracted alumina. The main problem is that alkaline alumina extraction requires the addition of calcium oxide, resulting in more solid waste after extraction and a lack of utilization pathways, making long-term commercial operation difficult. Acidic methods, including sulfuric acid, hydrochloric acid, and nitric acid methods, produce less solid waste after alumina extraction, but the waste is acidic and requires neutralization with alkali to meet discharge standards. Alumina extraction generates a large amount of alumina extraction waste residue, causing secondary environmental damage.

[0004] In response to soil salinization and silicon deficiency in my country, efforts have been made to prepare soil conditioners using fly ash and coal gangue, with some progress. During the acid-based aluminum extraction process from fly ash, harmful metals can be effectively removed, making the residue an ideal raw material for silicon fertilizer production. However, the residue itself contains significant acid residue, which is difficult to completely remove through washing. Furthermore, the dilute acid produced during washing is difficult to utilize effectively, and neutralization with alkali increases costs and leads to secondary waste salt pollution. Simultaneously, according to existing patented technologies for roasting or wet silicon fertilizer production, if aluminum extraction residue with high residual acid content is used, the resulting salts from neutralization with added alkali will lead to the loss of valuable alkali and further soil salinization. Existing methods for preparing silicon fertilizer from fly ash aluminum extraction residue generally have high alkali content (based on Na2O), which can easily lead to soil salinization with long-term use. For example, the roasting process requires alkali as an additive, resulting in an alkali content in the silicon fertilizer exceeding 10-20%. The wet process requires the addition of acid-process aluminum extraction residue and fertilizer in the presence of alkaline solution, which can reduce the alkali content to some extent, but it can still be as high as 6%. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of difficult comprehensive utilization of coal-based solid waste and secondary pollution caused by existing technologies, and to provide a method for the comprehensive utilization of coal-based solid waste. This method achieves efficient aluminum extraction while reducing and effectively utilizing residual acid in the aluminum extraction residue, preparing a low-alkali composite soil conditioner, efficiently supplementing silicon to the soil, reducing alkali loss and preventing soil salinization.

[0006] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a composite soil conditioner from aluminum extraction residue, comprising:

[0007] (1) A mixture of aluminum extraction residue, calcium agent, water and optional magnesium agent is subjected to a first heating reaction to obtain a first product slurry; wherein, the concentration of sulfate in the aluminum extraction residue is 1wt%-20wt%;

[0008] (2) The first product slurry is subjected to a first solid-liquid separation, and the obtained solid phase is subjected to a second heating reaction with alkaline solution to obtain a second product slurry;

[0009] (3) The second product slurry is subjected to a second solid-liquid separation, and the obtained solid phase is washed and dried to obtain a composite soil conditioner.

[0010] A second aspect of this invention provides a method for the comprehensive utilization of coal-related solid waste, comprising:

[0011] S1. Aluminum extraction from coal-based solid waste is carried out using the sulfuric acid process. After solid-liquid separation, aluminum extraction liquid and aluminum extraction residue are obtained.

[0012] S2. The aluminum extraction solution is treated to obtain aluminum sulfate, which is used to prepare alumina products;

[0013] S3. Using any one of the methods described above, the aluminum extraction residue is made into a composite soil conditioner.

[0014] A third aspect of the present invention provides a composite soil conditioner, wherein the composite soil conditioner is prepared by the method described in any one of the preceding claims, or the composite soil conditioner is prepared by the comprehensive utilization method described in any one of the preceding claims, wherein the effective silicon content of the composite soil conditioner is greater than 20 wt% and the alkali content is less than 0.5 wt%.

[0015] The fourth aspect of this invention provides the application of the aforementioned composite soil conditioner in the efficient replenishment of silicon in soil and the improvement of salinized soil.

[0016] This invention first involves adding a calcium agent and, optionally, a magnesium agent to aluminum extraction residue for a first heating reaction, followed by adding an alkaline solution for a second heating reaction. After adding the calcium agent, the sulfate ions remaining in the residue react with the calcium in the calcium agent to form calcium sulfate. Ultimately, a composite soil conditioner is generated, primarily composed of calcium sulfate and calcium silicate compounds, with the silicon-to-calcium ratio in the calcium silicate compound being variable. This controlled method of adding the calcium agent first and then the alkaline solution prevents the formation of sodium aluminosilicate, avoids sodium consumption, and facilitates alkaline solution recovery. Simultaneously, it significantly reduces the alkali content of the final composite soil conditioner, ensuring an effective silicon content greater than 20 wt% while maintaining an alkali content less than 0.5 wt%. This composite soil conditioner can be used for soil silicon replenishment and the improvement of saline-alkali soils.

[0017] This invention addresses the challenges of utilizing coal-based solid waste. Using coal gangue, fly ash, and gasification slag as raw materials, it employs a sulfuric acid process for aluminum extraction. This method effectively utilizes the residual acid in the aluminum extraction residue, avoiding the formation of salts that could lead to soil salinization through neutralization with alkali. Furthermore, by leveraging the advantages of the acid-based aluminum extraction process and improving the production process, a low-alkali silicon fertilizer is also prepared, achieving comprehensive utilization of coal-based solid waste. Detailed Implementation

[0018] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0019] In this invention, the liquid-solid ratio refers to the ratio of the mass of liquid material to the mass of solid material in a mixture.

[0020] The first aspect of the present invention provides a method for preparing a composite soil conditioner from aluminum extraction residue, comprising:

[0021] (1) A mixture of aluminum extraction residue, calcium agent, water and optional magnesium agent is subjected to a first heating reaction to obtain a first product slurry; wherein, in the aluminum extraction residue, the concentration of sulfate is 1wt%-20wt%, preferably 3wt%-15wt%, more preferably 6wt%-12wt%.

[0022] (2) The first product slurry obtained in step (1) is subjected to a first solid-liquid separation, and the obtained solid phase is subjected to a second heating reaction with alkaline solution to obtain a second product slurry;

[0023] (3) The second product slurry obtained in step (2) is subjected to a second solid-liquid separation. The obtained solid phase is washed and dried to obtain a composite soil conditioner.

[0024] According to a preferred embodiment of the present invention, the aluminum extraction residue is aluminum extraction residue from the sulfuric acid process or aluminum extraction residue from coal-based solid waste from the ammonium sulfate process, wherein the coal-based solid waste is at least one of coal gangue, fly ash, and gasification slag. Specifically, the aluminum extraction residue from the sulfuric acid process is obtained by extracting aluminum from coal-based solid waste using the sulfuric acid process; the aluminum extraction residue from the ammonium sulfate process is obtained by extracting aluminum from coal-based solid waste using the method of generating ammonium alum.

[0025] The sulfuric acid method for aluminum extraction includes aluminum extraction by dilute sulfuric acid leaching, aluminum extraction by concentrated sulfuric acid leaching, or aluminum extraction by concentrated sulfuric acid roasting.

[0026] In some embodiments, the aluminum extraction by dilute sulfuric acid leaching includes: adding dilute sulfuric acid to coal-based solid waste to carry out an acid dissolution reaction; wherein the concentration of dilute sulfuric acid may be, for example, 10-25 wt%, the mass ratio of dilute sulfuric acid to coal-based solid waste is 10, the reaction temperature is 180°C, the pressure is 1 MPa, and the time is 0.5-3 h.

[0027] In some embodiments, the aluminum extraction by concentrated sulfuric acid leaching includes: adding concentrated sulfuric acid to coal-based solid waste to carry out an acid dissolution reaction; wherein the concentration of concentrated sulfuric acid is 98 wt%, the liquid-to-solid ratio of concentrated sulfuric acid to coal-based solid waste is 4, the reaction temperature is 200-280℃, the reaction pressure is atmospheric pressure, and the reaction time is 0.5-3h.

[0028] In some embodiments, the aluminum extraction method by concentrated sulfuric acid roasting includes: adding concentrated sulfuric acid to coal-based solid waste and roasting it at 280-320°C; wherein the concentration of concentrated sulfuric acid is 80-98 wt%, the liquid-to-solid ratio of concentrated sulfuric acid to coal-based solid waste is 1.5, and the roasting reaction time is 0.5-3 h.

[0029] In some embodiments, the ammonium sulfate method for aluminum extraction mainly involves roasting ammonium sulfate with fly ash to produce ammonium alum. Alternatively, ammonium bisulfate can be used instead of ammonium sulfate for roasting. Specifically, this includes: mixing fly ash and ammonium sulfate with water, heating to 200℃-700℃ for roasting, absorbing the ammonia gas produced by the reaction with water to obtain ammonia water; dissolving the obtained solid roasting product in water, and filtering to obtain an ammonium aluminum sulfate solution and aluminum extraction residue.

[0030] This invention preferably uses the sulfuric acid process for aluminum extraction because the sulfate ions remaining on the aluminum extraction residue can react with calcium agents to form insoluble calcium sulfate (CaSO4) precipitate, which is easily separated from the solution. Calcium sulfate is also a soil conditioner component used to improve saline-alkali soils. Through the processing of this invention, a composite conditioner containing calcium sulfate and calcium silicate compounds can be further generated, with the silicate-calcium ratio being variable. In contrast, if hydrochloric acid or nitric acid is used for aluminum extraction, the calcium salts formed by their reaction with calcium agents are soluble waste salts, unlike calcium sulfate, which can be separated from the solution and used together with calcium silicate compounds as a soil conditioner for effective utilization. Furthermore, after solid-liquid separation in step (2), calcium chloride and calcium nitrate remain in the solid. In step (3), when reacting with alkali, sodium chloride and sodium nitrate are generated, consuming valuable alkali and affecting the quality of the alkali solution, hindering its recycling and reuse.

[0031] In this invention, the composition of the aluminum extraction residue, based on oxides, mainly contains SiO2 and Al2O3, with SiO2 content of approximately 60-90 wt% and Al2O3 content of approximately 5-20 wt%. For example, a Shenhua Inner Mongolia Zhungeer fly ash acid extraction residue contains approximately 76.5 wt% SiO2 and approximately 11.8 wt% Al2O3. The silica mainly consists of undissolved silica activated by the acid extraction process, and the alumina mainly consists of undissolved aluminum activated by the acid extraction process and residual dissolved but unwashed aluminum.

[0032] In this invention, in step (1), the amount of calcium agent added to the mixture is calculated as CaO, and the amount of magnesium agent added is calculated as MgO. The calculation method is as follows:

[0033] The mass molar composition of sulfate, silica, and aluminum (calculated as alumina) in aluminum extraction residue was determined, with units of mol / kg.

[0034] Calculate the total amount of CaO and MgO added according to formula (1);

[0035] M1+M2=C1+C2+C3…………(1)

[0036] In equation (1), M1 ≥ C1;

[0037] M1 is the amount of CaO added, in mol / kg;

[0038] M2 represents the amount of MgO added, expressed in mol / kg.

[0039] C1 = (1-1.5)X; C2 = (0.6-4)Y; C3 = (0.2-4)Z; where X represents the molar composition of sulfate ions in mol / kg; Y represents the molar composition of silicon dioxide in mol / kg; and Z represents the molar composition of aluminum in alumina in mol / kg.

[0040] As can be seen from equation (1), the relationship between the amount of calcium oxide and magnesium oxide added and the amount of sulfate, silicon dioxide, and aluminum (calculated as alumina) in the aluminum extraction residue is as follows: Taking the addition of all calcium oxide as an example, equation (1) shows that the molar ratio of (calcium oxide) to (sulfate) is 1-1.5, the molar ratio of (calcium oxide) to (silicon dioxide) is 0.6-4, and the molar ratio of (calcium oxide) to (alumina) is 0.2-4. The sum of the three amounts of calcium oxide is the amount of calcium oxide added. Some calcium oxide can also be replaced by an equimolar amount of magnesium oxide, but it is necessary to ensure that the amount of calcium oxide added is not less than C1, and to ensure that the sulfate ions can react with the calcium oxide to form calcium sulfate.

[0041] In this embodiment of the invention, the calcium agent is selected from at least one of calcium oxide, calcium hydroxide, and lime milk; the magnesium agent is selected from at least one of magnesium oxide, magnesium hydroxide, and magnesium hydroxide emulsion.

[0042] It should be understood that magnesium hydroxide emulsion is a saturated aqueous solution of magnesium hydroxide. Since magnesium hydroxide is only slightly soluble in water, it forms an emulsion.

[0043] Furthermore, in the mixture, the mass ratio of water to the sum of aluminum extraction residue, calcium agent, and magnesium agent is 0.5-5.

[0044] In this invention, the temperature of the first heating reaction is 20-99℃, and the time is 0.5-6h;

[0045] Preferably, the temperature of the first heating reaction is 60-95°C, and the time is 1-3 hours.

[0046] In some embodiments, a calcium agent is added first without a magnesium agent to fix sulfate and aluminum. The calcium agent reacts with the sulfate to form calcium sulfate. At this time, according to formula (1), the molar ratio of aluminum (calculated as alumina) to calcium in the formed calcium aluminate compound is 1:0.2-4.

[0047] In this invention, calcium aluminate compounds include, but are not limited to, monocalcium disodium aluminate, monocalcium aluminate, and tricalcium aluminate.

[0048] In some embodiments, calcium and magnesium agents are added simultaneously to fix sulfate and aluminum. The calcium agent reacts with the sulfate to form calcium sulfate. In this case, according to formula (1), the molar ratio of aluminum (calculated as alumina) to calcium + magnesium in the formed calcium aluminate / magnesium compound is 1:0.2-4.

[0049] In this invention, the calcium / magnesium aluminate compound includes calcium aluminate compound, magnesium aluminate compound, and calcium magnesium aluminate compound.

[0050] It is important to note that no alkali is added in this step to prevent the silica and alumina from dissolving in the alkaline solution and forming sodium aluminosilicate precipitate, which would consume sodium.

[0051] In this invention, the alkaline solution is a 5-50 wt% sodium hydroxide solution, and the liquid-to-solid mass ratio after adding the alkaline solution is 0.5-5.

[0052] The second heating reaction is carried out at a temperature of 20-99℃ for a time of 0.5-6 hours.

[0053] Preferably, the temperature of the second heating reaction is 60-95°C, and the time is 1-3 hours.

[0054] According to the present invention, the NaOH solution added in step (2) exists as a catalyst, participates in the chemical reaction but is theoretically not consumed, and can be separated from the obtained reaction product to recover NaOH.

[0055] In the method of this invention, the alkali solution does not bind tightly to the obtained reaction products and is easily removed by a general water washing operation, thereby reducing the alkali content in the solid phase and minimizing the negative impact of soil conditioners on the soil. Preferably, the sodium oxide content in the solid phase after water washing is less than 0.5 wt%.

[0056] In this invention, sodium hydroxide acts as a catalyst and is not consumed itself, but it can cause the silica in the aluminum extraction residue to react with lime milk to form calcium silicate compound, and the silica-calcium ratio is variable.

[0057] Since the aluminum in the aluminum extraction residue has been fixed by forming aluminum-calcium / magnesium compounds, the generation of sodium aluminosilicate, which consumes alkali and increases the alkali content of the filter cake (calculated as Na2O), can be avoided. After the reaction is completed, the filter cake and filtrate are obtained by filtration. After washing the filter cake several times, it is dried to obtain a composite soil conditioner mainly composed of calcium sulfate and calcium / magnesium silicate compounds (the ratio of silicon, calcium, and magnesium is variable under the condition of satisfying formula (1)). It can be used for the improvement of saline-alkali soil and to supplement the available silicon in the soil. The available silicon content is greater than 20%, and the alkali content is less than 0.5%.

[0058] In some embodiments, only calcium is added without magnesium. In this case, according to formula (1), the molar ratio of silicon to calcium in the formed calcium silicate compound is 1:0.6-4.

[0059] In this invention, calcium silicate compounds include, but are not limited to, monocalcium silicate, dicalcium silicate, and tricalcium silicate.

[0060] In some other embodiments, calcium and magnesium agents are added simultaneously. In this case, according to formula (1), the molar ratio of silicon to calcium + magnesium in the formed calcium silicate / magnesium compound is 1:0.6-4.

[0061] In this invention, the calcium / magnesium silicate compound includes calcium silicate compound, magnesium silicate compound, and calcium magnesium silicate compound.

[0062] In this invention, step (2) further includes: returning the liquid phase obtained from the first solid-liquid separation to step (1) to replace part of the water. Returning the liquid phase for reuse can realize the recycling of water and save water resources.

[0063] In this invention, step (3) further includes: combining the liquid phase obtained from the second solid-liquid separation with the solid washing liquid, concentrating it to a set concentration, and returning it to step (2) to replace part of the alkali solution.

[0064] This step can make full use of raw materials and avoid the discharge of alkaline solution, which would cause secondary pollution. The NaOH recycling efficiency (η) can be calculated by the following method. The mass percentage of Na element used: [(absolute mass of Na element added in step (2) - absolute mass of Na element in the product) / absolute mass of Na element added in step (2)] × 100%. The recovered NaOH solution is used in the process of step (2) to satisfy the aforementioned feeding relationship.

[0065] This invention uses aluminum extraction residue containing 1wt%-20wt% sulfate as raw material. After adding calcium agent to the aluminum extraction residue, the sulfate remaining in the sulfur-containing aluminum extraction residue will react with the calcium in the calcium agent to form calcium sulfate. Finally, a composite soil conditioner with calcium sulfate and calcium silicate compound (the ratio of silicon and calcium is variable) as the main components will be generated. If magnesium agent is added at the same time as calcium agent, then a composite soil conditioner with calcium sulfate, calcium silicate compound (the ratio of silicon and calcium is variable), magnesium silicate compound (the ratio of silicon and magnesium is variable), and calcium magnesium silicate compound (the ratio of silicon, calcium, and magnesium is variable) as the main components will be generated. This invention introduces a controlled method of first adding calcium to the aluminum extraction residue and then heating it before adding alkali solution (NaOH). This method prevents the formation of sodium aluminosilicate, which would consume sodium and ultimately affect the recovery of the alkali solution. Simultaneously, it significantly reduces the alkali content of the final composite soil conditioner, ensuring an effective silicon content greater than 20 wt% while maintaining an alkali content less than 0.5 wt%. In contrast, existing technologies add calcium under alkaline conditions. Under alkaline conditions, silica and alumina dissolve, forming sodium aluminosilicate precipitate, which then enters the product, increasing the alkali content and resulting in the loss of a large amount of valuable alkali (alkali is about ten times the price of acid). If hydrochloric acid or nitric acid is used for aluminum extraction, it will remain in the extraction residue and can also react with alkali to form small amounts of sodium chloride or sodium nitrate, leading to soil salinization with long-term use. Adding calcium first and then alkali means first fixing the sulfate and aluminum in the soil by adding calcium agent to prevent the formation of sodium aluminosilicate and reduce sodium consumption. The alkali content of the composite soil conditioner of this invention is less than 0.5%, and the sodium recycling efficiency can reach more than 99%, such as 99.3%, 99.5%, 99.8%, etc.

[0066] A second aspect of this invention provides a method for the comprehensive utilization of coal-related solid waste, comprising:

[0067] S1. Aluminum extraction from coal-based solid waste is carried out using the sulfuric acid process. After solid-liquid separation, aluminum extraction liquid and aluminum extraction residue are obtained.

[0068] S2. The aluminum extraction solution is treated to obtain aluminum sulfate, which is used to prepare alumina products;

[0069] S3. Using the aforementioned method, the aluminum extraction residue is prepared into a composite soil conditioner; specifically as follows:

[0070] (1) The mixture of aluminum extraction residue, calcium agent, water and optional magnesium agent obtained in step S1 is subjected to a first heating reaction to obtain a first product slurry; wherein, the concentration of sulfate in the aluminum extraction residue is 1wt%-20wt%.

[0071] (2) The first product slurry is subjected to a first solid-liquid separation, and the obtained solid phase is subjected to a second heating reaction with alkaline solution to obtain a second product slurry;

[0072] (3) The second product slurry is subjected to a second solid-liquid separation, and the obtained solid phase is washed and dried to obtain a composite soil conditioner.

[0073] In step S1 of this invention, aluminum is extracted using the sulfuric acid method. The residual sulfate ions can react with the calcium agent to form a poorly soluble precipitate of calcium sulfate (CaSO4), which can be separated from the solution to improve the quality of alkali recovery. At the same time, calcium sulfate is also a component of a soil conditioner, which can simultaneously generate a composite conditioner containing calcium sulfate and calcium silicate compounds. The ratio of calcium silicate to calcium silicate is variable.

[0074] In step S2 of this invention, the aluminum extraction solution is mainly composed of aluminum sulfate, and also contains ferric sulfate and acid-soluble salts such as lead and mercury that are harmful to soil. It requires impurity removal and concentration before aluminum sulfate can be obtained. These processes include, but are not limited to, impurity removal and concentration steps.

[0075] In this invention, the alumina product is at least one of industrial activated alumina, metallurgical grade alumina, and high-purity alumina.

[0076] The following example illustrates the specific process of producing alumina from aluminum sulfate:

[0077] Preparation of industrial activated alumina: Crude aluminum sulfate solution is evaporated and concentrated, then cooled to obtain a concentrated aluminum sulfate solution; the concentrated aluminum sulfate solution is heated and then cooled and recrystallized, separating the precipitate and the clear liquid to obtain a refined crystalline aluminum sulfate slurry; the obtained refined crystalline aluminum sulfate slurry is subjected to solid-liquid separation to obtain low-iron crystalline aluminum sulfate filter media; the obtained low-iron crystalline aluminum sulfate filter media is dried to obtain high-purity industrial-grade aluminum sulfate with an Fe content of less than 0.02%; the obtained high-purity industrial aluminum sulfate is calcined to obtain industrial activated alumina with an Fe2O3 content of less than 0.5%. The Chinese patent application "Method for producing industrial activated alumina from fly ash" (CN 201010601489.X) describes a specific implementation method for preparing industrial activated alumina from crude aluminum sulfate solution, the entire contents of which are incorporated herein by reference.

[0078] Preparation of metallurgical-grade alumina: Crude aluminum sulfate solution is evaporated and concentrated, then cooled to obtain a concentrated aluminum sulfate solution. An organic alcohol is added to the concentrated aluminum sulfate solution, stirred, and filtered to obtain an aluminum sulfate filter cake. Water is added to dissolve the filter cake, an organic alcohol is added, stirred, and the ferric sulfate is dissolved, precipitating aluminum sulfate. The filter cake is then filtered to obtain another aluminum sulfate filter cake. The aluminum sulfate filter cake is dried to obtain high-purity industrial-grade aluminum sulfate with an Fe content of less than 20 ppm. The high-purity industrial-grade aluminum sulfate is then calcined at high temperature to obtain metallurgical-grade α-Al₂O₃ with an Fe₂O₃ content of less than 0.02%. The organic alcohol used above is any one or a mixture of more than one of methanol, ethanol, isopropanol, and n-propanol. The Chinese patent application "Method for producing metallurgical-grade alumina from fly ash" (CN201010601681.9) describes a specific method for preparing metallurgical-grade alumina from crude aluminum sulfate solution, and its entire contents are incorporated herein by reference.

[0079] Preparation of high-purity alumina: The obtained crude aluminum sulfate solution is passed through a cation exchange resin column to remove iron, resulting in a low-ferrous aluminum sulfate solution; the obtained low-ferrous aluminum sulfate solution is evaporated and concentrated to 1 / 10-1 / 20 of its original volume at 90℃-130℃, and after cooling, a concentrated aluminum sulfate solution is obtained; an organic alcohol is added to the concentrated aluminum sulfate solution at a volume ratio of 1:1-4:1, and the mixture is stirred thoroughly for 1-4 hours and filtered to obtain a low-ferrous aluminum sulfate filter cake; the obtained low-ferrous aluminum sulfate filter cake is dried at 70℃-100℃ to obtain ultra-high-purity aluminum sulfate with an Fe content of less than 0.2 ppm; the obtained ultra-high-purity aluminum sulfate is calcined at 800℃-1200℃ for 3-6 hours to obtain ultra-high-purity α-Al2O3 with an Fe2O3 content of less than 2 ppm. The Chinese patent application “A process for producing ultra-high purity alumina from fly ash and its comprehensive utilization” (CN 201010601190.4) describes a specific method for preparing high-purity alumina from crude aluminum sulfate solution, and the entire contents of which are incorporated herein by reference.

[0080] In this invention, step S3 further includes: adding water to the aluminum extraction residue for multiple washing and filtration until the sulfate content in the aluminum extraction residue is below 20 wt% and the aluminum content calculated as alumina is below 20 wt%.

[0081] Preferably, the washing filtrate of the aluminum extraction residue is combined with the aluminum extraction liquid in step S2.

[0082] A third aspect of the present invention provides a composite soil conditioner, wherein the composite soil conditioner is prepared by the aforementioned method, or the composite soil conditioner is prepared by the aforementioned comprehensive utilization method, wherein the effective silicon content of the composite soil conditioner is greater than 20 wt% and the alkali content is less than 0.5 wt%.

[0083] In this invention, if only calcium agent is added to the aluminum extraction residue, the composite soil conditioner is mainly composed of calcium sulfate and calcium silicate compounds, and the ratio of calcium silicate to calcium silicate is variable.

[0084] In this invention, if calcium and magnesium agents are added simultaneously to the aluminum extraction residue, the composite soil conditioner mainly consists of calcium sulfate, calcium silicate compound (the ratio of silicon to calcium is variable), magnesium silicate compound (the ratio of silicon to magnesium is variable), and calcium-magnesium silicate compound (the ratio of silicon to calcium to magnesium is variable).

[0085] The fourth aspect of this invention provides the application of the aforementioned composite soil conditioner in the improvement of saline-alkali soil.

[0086] The aforementioned method of this invention yields a composite soil conditioner primarily composed of calcium sulfate and calcium / magnesium silicate salts. This conditioner can be used to improve saline-alkali soils and supplement available silicon in the soil, with an available silicon content greater than 20% and an alkali content less than 0.5%. The calcium and magnesium agents are readily available, sourced from limestone, dolomite, and magnesite, among others. Calcium and magnesium are also essential nutrients for plants. The calcium sulfate generated during the reaction can improve saline-alkali soils, and the resulting aluminum-calcium / magnesium compounds can fix aluminum from aluminum extraction residues. Therefore, the composite soil conditioner of this invention can effectively improve saline-alkali soils.

[0087] This invention provides a scheme for the acid extraction of alumina from coal-related solid waste, especially coal gangue, fly ash and gasification slag, and the combined production of a composite soil conditioner, thereby realizing the comprehensive resource utilization of coal-related solid waste.

[0088] To further understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0089] Unless otherwise specified, all reagents involved in the embodiments of this invention are commercially available products and can be purchased through commercial channels. The invention will be described in detail below through embodiments.

[0090] The chemical composition and alkali content of the samples were determined by ICP method, and the effective silicon (calculated as SiO2) content was determined according to NY / T797-2004 "Silicon Fertilizer".

[0091] In the following embodiments, the coal-related solid waste used are fly ash, coal gangue, and gasification slag, wherein,

[0092] The average particle size of fly ash is 43 micrometers, and its composition is shown in Table 1.

[0093] Coal gangue (calcined at 850 degrees Celsius, with an ash content of 93%, composition shown in Table 2) has an average particle size of 39 micrometers after crushing.

[0094] The gasification slag (calcined at 850 degrees Celsius, with an ash content of 87%, and its composition is shown in Table 3) has an average particle size of 40 micrometers after crushing.

[0095] Table 1 Composition of fly ash

[0096] <![CDATA[Na2O]]> MgO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[P2O5]]> <![CDATA[SO3]]> <![CDATA[K2O]]> High <![CDATA[TiO2]]> <![CDATA[Fe2O3]]> 0.13 0.86 48.26 40.20 0.10 0.41 0.49 3.67 1.34 1.56

[0097] Table 2 Composition of Coal Gangue

[0098] <![CDATA[Na2O]]> MgO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[P2O5]]> <![CDATA[SO3]]> <![CDATA[K2O]]> High <![CDATA[TiO2]]> <![CDATA[Fe2O3 <!-- 7 -->]]> 0.27 0.96 26.19 62.55 0.07 0.46 2.45 0.42 1.13 5.29

[0099] Table 3 Composition of Gasification Slag

[0100] <![CDATA[Na2O]]> MgO <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[P2O5]]> <![CDATA[SO3]]> <![CDATA[K2O]]> High <![CDATA[TiO2]]> <![CDATA[Fe2O3]]> 1.85 1.09 23.87 54.40 0.19 1.23 1.73 6.72 1.28 7.36

[0101] Example 1

[0102] (1) Take 1000g of fly ash and add concentrated sulfuric acid with a concentration of 98wt% to make the liquid-solid mass ratio 4. Control the reaction temperature at 220℃, the reaction pressure at atmospheric pressure, and the reaction time at 1h. After filtering the concentrated sulfuric acid, add water to the aluminum extraction residue for multiple washings and filtration. Combine the filtrate and washing liquid to obtain a crude liquid with aluminum sulfate as the main component, which is used to extract alumina. The aluminum element of the aluminum extraction residue can be determined to be 93% of the alumina dissolution rate in the fly ash.

[0103] (2) The aluminum extraction residue washed in step (1) is dried. After analysis, the aluminum content (calculated as alumina) in the aluminum extraction residue is 6 wt%, the residual acid (sulfate) content is 12 wt%, and the silica content is 73 wt%. According to formula (1), the molar ratio of (calcium oxide) / (sulfate) is 1, the molar ratio of (calcium oxide) / (silica) is 1.2, and the molar ratio of (calcium oxide) / (alumina) is 3. Calcium oxide and water are added to the dried aluminum extraction residue to make the liquid-solid mass ratio 2. The mixture is heated to 95°C and reacted for 2 hours to obtain the first product slurry.

[0104] (3) The first product slurry obtained in step (2) is filtered and washed to obtain a first filter cake. A 20wt% sodium hydroxide solution is added to the first filter cake to make the liquid-solid mass ratio 3. The mixture is heated to 95°C and reacted for 1 hour to obtain a second product slurry. The second product slurry is filtered, washed, and dried to obtain a composite soil conditioner. The filtrate and washings are concentrated into a 20% concentration alkali solution, which can be used as an alkali solution. The composite soil conditioner of this embodiment has an effective silicon content of 36wt%, a calcium sulfate content of 8.6%, and an alkali content of 0.2wt%.

[0105] Calculations show that the recycling rate of NaOH during the preparation process is 99.8%.

[0106] Example 2

[0107] (1) Take 1000g of coal gangue, add 20wt% dilute sulfuric acid to make the liquid-solid mass ratio 10, control the reaction temperature at 180℃, and the reaction time at 3h. After filtering the dilute sulfuric acid, add water to the aluminum extraction residue for multiple washings and filtration. Combine the filtrate and washing liquid to obtain a crude liquid whose main component is aluminum sulfate, which is used to extract alumina. ICP analysis of the aluminum extraction residue showed that the alumina dissolution rate in the coal gangue was 86%.

[0108] (2) The aluminum extraction residue washed in step (1) is dried. After analysis, the aluminum content in the aluminum extraction residue is 5 wt% (calculated as alumina), 8 wt% (sulfate), and 75 wt% (silica). According to formula (1), the molar ratio of (calcium oxide) / (sulfate) is 1, the molar ratio of (calcium oxide) / (silica) is 3, and the molar ratio of (calcium oxide) / (alumina) is 3.5. Calcium hydroxide and water are added to the dried aluminum extraction residue to make the liquid-solid mass ratio 0.8. The mixture is heated to 60°C and reacted for 4 hours to obtain the first product slurry.

[0109] (3) The first product slurry obtained in step (2) is filtered and washed to obtain a first filter cake. A 10 wt% sodium hydroxide solution is added to the first filter cake at a liquid-to-solid mass ratio of 5. The mixture is heated to 90°C and reacted for 1 hour to obtain a second product slurry. The second product slurry is filtered, washed, and dried to obtain a composite soil conditioner. The filtrate and washings are concentrated into a 10% alkaline solution, which can be used as an alkaline solution. Testing showed that the effective silicon content of the composite soil conditioner in this embodiment is 21.2 wt%, the calcium sulfate content is 3.5%, and the alkali content is 0.4 wt%.

[0110] Calculations show that the recycling rate of NaOH during the preparation process is 99.6%.

[0111] Example 3

[0112] (1) Take 1000g of gasification slag, add 80wt% concentrated sulfuric acid to make the liquid-solid mass ratio 1.5, calcine at 300℃ for 1h, filter the concentrated sulfuric acid, add water to the aluminum extraction residue for multiple washings and filtration, combine the filtrate and washing liquid to obtain a crude liquid with aluminum sulfate as the main component, which is used to extract alumina. ICP analysis of the aluminum extraction slag showed that the alumina dissolution rate in the fly ash was 95%.

[0113] (2) The aluminum extraction residue washed in step (1) is dried. After analysis, the aluminum content (calculated as alumina) in the aluminum extraction residue is 4 wt%, the residual acid (sulfate) content is 7 wt%, and the silica content is 81 wt%. According to formula (1), the molar ratio of (calcium oxide) / (sulfate) is 1.2, the molar ratio of (calcium oxide) / (silica) is 2, and the molar ratio of (calcium oxide) / (alumina) is 4. Calcium oxide and water are added to the dried aluminum extraction residue to make the liquid-solid mass ratio 4. The mixture is heated to 40°C and reacted for 6 hours to obtain the first product slurry.

[0114] (3) The first product slurry obtained in step (2) is filtered and washed to obtain a first filter cake. A 30% sodium hydroxide solution is added to the first filter cake to make the liquid-to-solid mass ratio 4. The mixture is heated to 80°C and reacted for 6 hours to obtain a second product slurry. The second product slurry is filtered, washed, and dried to obtain a composite soil conditioner. The filtrate and washings are concentrated into a 30% alkaline solution, which can be used as an alkaline solution. Testing showed that the effective silicon content of the composite soil conditioner in this embodiment is 29 wt%, the calcium sulfate content is 3.6%, and the alkali content is 0.3 wt%.

[0115] Calculations show that the recycling rate of NaOH during the preparation process is 99.7%.

[0116] Example 4

[0117] (1) Take 1000g of fly ash, add 3200g of ammonium sulfate, and mix thoroughly with water. Roast at 500℃ for 3 hours. Absorb the generated ammonia gas with water. After cooling, add water for dissolution. After solid-liquid separation, obtain aluminum extraction slag and crude ammonium aluminum sulfate solution, which are used to extract alumina. ICP analysis of the aluminum extraction slag showed that the alumina dissolution rate in the coal gangue was 86%.

[0118] (2) The aluminum extraction residue washed in step (1) was dried. After analysis, the aluminum content (calculated as alumina) in the aluminum extraction residue was 7 wt%, the residual acid (sulfate) content was 15 wt%, and the silica content was 72 wt%. According to formula (1), the molar ratio of (calcium oxide) / (sulfate) is 1.5, the molar ratio of (calcium oxide) / (silica) is 1.2, and the molar ratio of (calcium oxide) / (alumina) is 3. Calcium hydroxide and water g were added to the dried aluminum extraction residue to make the liquid-solid mass ratio 0.8. The mixture was heated to 60°C and reacted for 4 h to obtain the first product slurry.

[0119] (3) The first product slurry obtained in step (2) is filtered and washed to obtain a first filter cake. A 10 wt% sodium hydroxide solution is added to the first filter cake at a liquid-to-solid mass ratio of 5. The mixture is heated to 90°C and reacted for 1 hour to obtain a second product slurry. The second product slurry is filtered, washed, and dried to obtain a composite soil conditioner. The filtrate and washings are concentrated into a 10% alkaline solution, which can be used as an alkaline solution. Testing showed that the effective silicon content of the composite soil conditioner in this embodiment is 34 wt%, the calcium sulfate content is 10.3%, and the alkali content is 0.4 wt%.

[0120] Calculations show that the recycling rate of NaOH during the preparation process is 99.6%.

[0121] Example 5

[0122] Aluminum extraction and preparation of a composite soil conditioner were carried out according to Example 1. Unlike Example 1, in (calcium oxide) / (silicon dioxide) and (calcium oxide) / (aluminum oxide) mixtures, calcium oxide was completely replaced by magnesium oxide, i.e., according to formula (1), the molar ratio of (calcium oxide) / (sulfate) was 1, the molar ratio of (magnesium oxide) / (silicon dioxide) was 1.2, and the molar ratio of (magnesium oxide) / (aluminum oxide) was 3. The composite soil conditioner of this example was found to have an effective silicon content of 39 wt%, a calcium sulfate content of 9.9%, and an alkali content of 0.4 wt%.

[0123] Calculations show that the recycling rate of NaOH during the preparation process is 99.7%.

[0124] Comparative Example 1

[0125] Unlike Example 1, alkali was added first, followed by calcium. The prepared soil conditioner contained 33 wt% available silicon, 0.2% calcium sulfate, and 6.3 wt% alkali.

[0126] Calculations show that the recycling rate of NaOH during the preparation process is 96.1%.

[0127] Comparative Example 2

[0128] Unlike Example 1, the (calcium oxide) / (silicon dioxide) ratio is 5, the (calcium oxide) / (sulfate) ratio is 2, and the (calcium oxide) / (alumina) ratio is 5. The prepared soil conditioner contains 14 wt% available silicon, 3.6% calcium sulfate, and 0.2 wt% alkali.

[0129] Calculations show that the recycling rate of NaOH during the preparation process is 99.7%.

[0130] Comparative Example 3

[0131] Unlike Example 1, the (calcium oxide) / (silicon dioxide) ratio is 0.5, the (calcium oxide) / (sulfate) ratio is 0.9, and the (calcium oxide) / (alumina) ratio is 0.1. The prepared soil conditioner contains 5 wt% available silicon, 11.8% calcium sulfate, and 8.7 wt% alkali.

[0132] Calculations show that the recycling rate of NaOH during the preparation process is 95.7%.

[0133] Comparative Example 4

[0134] Unlike Example 1, hydrochloric acid was used for aluminum extraction. The prepared soil conditioner contained 35 wt% effective silicon and 0.2 wt% alkali. Sodium chloride, converted from calcium chloride by the reaction with alkali, and sodium chloride generated from the reaction of residual hydrochloric acid with alkali in the aluminum extraction residue, entered the recycled alkali solution, making the alkali solution unusable.

[0135] As seen in Examples 1-5, aluminum extraction from fly ash, coal gangue, and gasification slag is achieved using concentrated sulfuric acid, dilute sulfuric acid, sulfuric acid roasting, and ammonium sulfate methods. Aluminum extraction residues are obtained, and the amounts of calcium oxide / magnesium added are calculated. Corresponding calcium / magnesium agents are then added, and the resulting reaction yields calcium sulfate, which can be used to improve saline-alkali soils and fix the aluminum in the extraction residues. Adding sodium hydroxide solution catalyzes the reaction between silica and the calcium / magnesium agents, yielding a composite soil conditioner. This soil conditioner contains more than 20% active silicon, less than 0.5% alkali, and abundant calcium sulfate.

[0136] Comparing Comparative Examples 1-4 with Example 1, it can be seen that only by ensuring that reasonable molar ratios of (calcium oxide) / (silicon dioxide), (calcium oxide) / (sulfate), and (calcium oxide) / (alumina) are met simultaneously, and by using a process of adding calcium and magnesium first and then adding alkali, can a composite soil conditioner with high-activity silicon, low alkali and abundant calcium sulfate be obtained, so as to maximize the recycling of alkali solution.

[0137] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a composite soil conditioner from aluminum extraction residue, characterized in that, include: (1) A mixture of aluminum extraction residue, calcium agent, optional magnesium agent, and water is subjected to a first heating reaction to obtain a first product slurry; wherein, the concentration of sulfate in the aluminum extraction residue is 1wt%-20wt%; (2) The first product slurry is subjected to a first solid-liquid separation, and the obtained solid phase is subjected to a second heating reaction with alkaline solution to obtain a second product slurry; (3) The second product slurry is subjected to a second solid-liquid separation, and the obtained solid phase is washed and dried to obtain a composite soil conditioner; The aluminum extraction residue is aluminum extraction residue from the sulfuric acid process or aluminum extraction residue from coal-based solid waste from the ammonium sulfate process; wherein, the coal-based solid waste is at least one of coal gangue, fly ash, and gasification slag. The calcium agent is selected from at least one of calcium oxide, calcium hydroxide, and lime milk; The magnesium agent is selected from at least one of magnesium oxide, magnesium hydroxide, and magnesium hydroxide emulsion; In the mixture, the amount of calcium agent added is calculated as CaO, and the amount of magnesium agent added is calculated as MgO, and the calculation method is as follows: The mass molar composition of silica, sulfate, and aluminum (calculated as alumina) in aluminum extraction residue was determined, with units of mol / kg. Calculate the total amount of CaO and MgO added according to formula (1); M1+M2=C1+C2+C3…………(1) In equation (1), M1 ≥ C1; M1 is the amount of CaO added, in mol / kg; M2 represents the amount of MgO added, expressed in mol / kg. C1 = (1-1.5)X; C2 = (0.6-4)Y; C3 = (0.2-4)Z; where X represents the molar composition of sulfate ions in mol / kg; Y represents the molar composition of silicon dioxide in mol / kg; and Z represents the molar composition of aluminum in alumina in mol / kg.

2. The method according to claim 1, wherein, The aluminum extraction process in the sulfuric acid extraction residue includes aluminum extraction by dilute sulfuric acid leaching, aluminum extraction by concentrated sulfuric acid leaching, or aluminum extraction by concentrated sulfuric acid roasting.

3. The method according to claim 1, wherein, In step (1), the mass ratio of water to the aluminum extraction residue, calcium agent and optional magnesium agent is 0.5-5.

4. The method according to claim 1, wherein, In step (1), the temperature of the first heating reaction is 20-99℃ and the time is 0.5-6h.

5. The method according to claim 4, wherein, The temperature of the first heating reaction is 60-95℃, and the time is 1-3h.

6. The method according to claim 1, wherein, In step (2), the alkaline solution is a 5-50 wt% sodium hydroxide solution, and the mass ratio of the alkaline solution to the solid phase is 0.5-5. The second heating reaction is carried out at a temperature of 20-99℃ for a time of 0.5-6 hours.

7. The method according to claim 6, wherein, The second heating reaction is carried out at a temperature of 60-95℃ for 1-3 hours.

8. The method according to claim 1, wherein, Step (2) further includes: returning the liquid phase obtained from the first solid-liquid separation to step (1) to replace part of the water.

9. The method according to claim 1, wherein, Step (3) further includes: combining the liquid phase obtained from the second solid-liquid separation with the solid phase washing liquid obtained from the washing, concentrating it to a set concentration, and then returning it to step (2) to replace part of the alkali solution.

10. A method for comprehensive utilization of coal-bearing solid waste, characterized in that, include: S1. Aluminum extraction from coal-based solid waste is carried out using the sulfuric acid process. After solid-liquid separation, aluminum extraction liquid and aluminum extraction residue are obtained. S2. The aluminum extraction solution is treated to obtain aluminum sulfate, which is used to prepare alumina products; S3. Using the method described in any one of claims 1-9, the aluminum extraction residue is made into a composite soil conditioner.

11. The comprehensive utilization method according to claim 10, wherein, Step S3 further includes: adding water to the aluminum extraction residue for multiple washing and filtration until the sulfate content in the aluminum extraction residue is below 20 wt% and the aluminum content calculated as alumina is below 20 wt%.

12. The comprehensive utilization method according to claim 11, wherein, The washing filtrate of the aluminum extraction residue is combined with the aluminum extraction liquid in step S2.

13. A composite soil conditioner, characterized in that, The composite soil conditioner is prepared by the method described in any one of claims 1-9, or by the comprehensive utilization method described in any one of claims 10-12, wherein the effective silicon content of the composite soil conditioner is greater than 20 wt% and the alkali content is less than 0.5 wt%.

14. The composite soil conditioner according to claim 13, wherein, The composite soil conditioner is mainly composed of calcium sulfate and calcium silicate compounds; And / or, the composite soil conditioner is mainly composed of calcium sulfate, calcium silicate compound, magnesium silicate compound, and calcium magnesium silicate compound.

15. The application of the composite soil conditioner according to claim 13 or 14 in soil silicon supplementation and salinization soil improvement.