A method and system for producing metallurgical grade alumina from high alumina blended coal

Abstract

This invention discloses a method and system for producing metallurgical-grade alumina from high-alumina mixed coal. The system comprises a fluidized bed boiler, a pre-desiliconization tank, and a quicklime sintering device. During production, a high-alumina mixed coal (comprising high-alumina coal and coal gangue) is used as raw material and burned in the fluidized bed boiler to produce fly ash. This fly ash is then pre-calcined to obtain activated fly ash residue, which undergoes a pre-desiliconization reaction to obtain desiliconized liquid and fly ash concentrate. The fly ash concentrate is then sintered using the quicklime method to obtain metallurgical-grade alumina. This invention represents a novel process for producing metallurgical-grade alumina from high-alumina coal or coal gangue, solving the technical challenges of producing alumina from high-alumina fly ash both domestically and internationally. It offers high desiliconization efficiency and significant economic and social benefits.

Description

Technical Field

[0001] This invention relates to a method and system for producing metallurgical-grade alumina from high-alumina mixed coal, specifically an industrial production method and system for producing metallurgical-grade alumina in a fluidized bed using high-alumina mixed coal as raw material, belonging to the field of coal resource recycling. Background Technology

[0002] my country possesses abundant high-alumina coal resources. The high-alumina fly ash produced after the combustion of high-alumina coal can be used to extract mineral resources such as alumina, lithium, and gallium, achieving efficient and comprehensive utilization of these resources. Currently, methods for extracting alumina from high-alumina fly ash mainly involve limestone sintering, pre-desiliconization-sintering, acid processes, and other technical routes for producing alumina from high-alumina fly ash.

[0003] The limestone sintering method combines thorough carbon separation of crude alumina solution with low-temperature Bayer leaching to extract alumina from high-alumina fly ash. The waste product, calcium silicate slag, from the alumina extraction process can be used to produce silicate cement clinker. However, this method requires a high SiO2 content in the fly ash, resulting in high raw material consumption and low alumina yield, making it economically unprofitable. The pre-desiliconization-sintering method is an improvement on the limestone sintering method. Before the sintering process, an alkaline solution reacts chemically with the active SiO2 in the fly ash to generate a sodium silicate solution, which separates from the solid fly ash. This can improve the A / S ratio of the pre-desiliconized fly ash concentrate before sintering to produce qualified alumina. However, this method still only improves the A / S ratio of the fly ash concentrate to around 2.0, making it uneconomical. The acid process utilizes the principle that SiO2 in fly ash is insoluble in acid to effectively separate the two main components, Al2O3 and SiO2, in high-alumina fly ash, thereby achieving the purpose of extracting Al2O3. Although this method solves the adverse effects of SiO2 impurities on alumina production, it also introduces a batch of metal oxide impurities, such as iron oxide, alkali metals, and alkaline earth metals, which have adverse effects on alumina production. According to alumina process science and practice, the acid process also has many technical challenges, such as severe corrosion of equipment materials, difficulty in removing impurities from aluminum salt solutions, and the comprehensive impact of alkali metal and alkaline earth metal impurities on all processes, not just on a single product. The process also involves large material flow rates, high heat of aluminum salt formation, and large differences in the bulk density of alumina products. Therefore, it is difficult to scale up production to an industrial scale.

[0004] Other known technical routes for producing alumina from high-alumina fly ash include low-calcium sintering, calcium-free sintering, and high-alumina fly ash beneficiation. Among these, the low-calcium and calcium-free sintering processes are currently only in the experimental stage, and their industrial application prospects are unclear. The high-alumina fly ash beneficiation method aims to effectively separate the main components, Al2O3 and SiO2, through beneficiation to facilitate subsequent alumina extraction. However, due to the combustion temperature of pulverized coal boilers above 1350℃, the phase composition of fly ash shows that Al2O3 and SiO2 mainly form mullite (3Al2O3·2SiO2) or 2Al2O3·SiO2. Furthermore, the combustion temperature of fluidized bed boilers is in the range of 870–970℃, where Al2O3 and SiO2 in fly ash mainly exist as amorphous glassy minerals. Therefore, beneficiation methods cannot effectively separate Al2O3 and SiO2.

[0005] In the prior art, the invention patent with publication number CN101941725A describes a method for extracting alumina and co-producing active calcium silicate from coal gangue. The method involves crushing the coal gangue, calcining and activating it, and then subjecting the coal gangue clinker to an alkaline desilication reaction to obtain desilicationated coal gangue and sodium silicate solution. The sodium silicate solution can be used to prepare active calcium silicate micro powder after a causticization reaction. Limestone and ammonium carbonate solution can be added to the desilicationated coal gangue and ball-milled to form a raw meal slurry. The clinker after calcination of the raw meal slurry is then dissolved in water to obtain sodium aluminate solution and calcium silicate slag. The calcium silicate slag can be used as a cement raw material after treatment. The crude sodium aluminate solution is then subjected to deep desilication, CO2 carbon removal, and calcination to obtain the alumina product. The patent discloses the utilization of pure coal gangue. However, due to the fact that the activated roasting process does not take into account the rational utilization of carbon in the coal gangue, does not combine it with fluidized bed power plant boilers, lacks mining and processing, coal washing, aluminum solidification and silicon activation links, has large losses of pre-desiliconized alumina, and has poor technical and economic indicators of sintering method, the production cost is extremely high and it cannot be put into industrial application.

[0006] The invention patent with publication number CN101306826A describes a method for extracting metallurgical-grade alumina from fly ash or slag. This method is applicable to fly ash or slag from coal combustion in thermal power plants. After screening, flotation, pre-desiliconization, and silica production processes, metallurgical-grade alumina can be obtained, enabling co-production of cement and reducing the difficulty of waste residue treatment. This patent directly uses fly ash and slag as raw materials. The fly ash concentrate obtained from pre-desiliconization is fed into the alumina plant, with an A / S ratio of ≤2, which differs significantly from the requirements of existing sintering alumina processes. However, its production cost is twice that of traditional alumina plants, resulting in high processing costs. In addition, this patented method cannot produce qualified silica. Due to the strong adsorption properties of silica, during the aging process, silica will adsorb various impurities in the pre-desiliconization liquid. Therefore, the quality of the silica produced does not meet the requirements, resulting in poor economic benefits.

[0007] The invention patent with publication number CN108275695A describes a method for preparing 4A zeolite for detergent additives from high-alumina coal gangue. This method involves pulverizing high-alumina coal gangue, adding an activator (aluminum hydroxide or aluminum oxide-containing ore), and calcining it. Then, it undergoes alkaline pre-desiliconization to obtain sodium silicate solution and desiliconized fly ash. The desiliconized fly ash is processed using the Bayer process to obtain sodium aluminate solution, which can then be used to prepare 4A zeolite. This method can solve the resource waste and environmental pollution problems caused by long-term stockpiling of high-alumina coal gangue, realizing the resource utilization of industrial waste. It involves only one sintering process, resulting in low energy consumption, low product cost, high modulus and concentration of sodium silicate solution, easy process control, stable product quality, low waste residue yield, and thorough desiliconization. Although this patent involves adding alumina-containing ores (bauxite or kaolinite) to high-alumina coal gangue, bauxite or kaolinite does not contain carbon and cannot burn on its own; it can only burn with the help of coal. After bauxite or kaolinite is burned with coal, its water of crystallization is removed, and the resulting coal gangue and coal have a fly ash content and composition that are suitable for producing 4A zeolite (aluminosilicate), which is completely different from the structure and composition of metallurgical-grade alumina. Summary of the Invention

[0008] The purpose of this invention is to provide a method for producing metallurgical-grade alumina from high-alumina mixed coal. This method involves the combustion of high-alumina coal and coal gangue in a fluidized bed boiler, followed by alumina-fixing pre-baking, pre-desiliconization, and sintering. This is a novel process for producing metallurgical-grade alumina from high-alumina coal or coal gangue, solving the technical challenges of producing alumina from high-alumina fly ash both domestically and internationally. It boasts high desiliconization efficiency and significant economic and social benefits. Furthermore, this invention also provides a system for producing metallurgical-grade alumina from high-alumina mixed coal.

[0009] This invention is achieved through the following technical solution: a method for producing metallurgical-grade alumina from high-alumina mixed coal, comprising the following steps:

[0010] S1. High-alumina mixed coal, consisting of high-alumina coal and coal gangue, is used as raw material and burned in a circulating fluidized bed boiler to produce fly ash. The mineral composition of the fly ash satisfies the following conditions: Al2O3 ≥ 45%; SiO2 ≤ 40.9%; A / S ≥ 1.10; CaO ≤ 3.5%.

[0011] S2. The fly ash is processed in an alumina pre-baking furnace, with the baking temperature controlled at 840-970℃ and the holding time at 30-120 min.

[0012] S3. The activated fly ash residue after solid alumina pre-baking is subjected to a pre-desiliconization reaction with alkaline solution to obtain desiliconization solution and fly ash concentrate, wherein the mineral composition of the fly ash concentrate has A / S ≥ 3.0;

[0013] S4. The fly ash concentrate is sintered using the soda lime method to obtain metallurgical grade alumina.

[0014] The high-alumina coal and coal gangue are blended after mining to obtain high-alumina mixed coal, and the calorific value of the high-alumina mixed coal is controlled to be >2700kcal / kg.

[0015] The high-alumina mixed coal is successively crushed, medium crushed, and washed before being fed into a circulating fluidized bed boiler.

[0016] The combustion temperature of the circulating fluidized bed boiler is controlled at 840-950℃, and no desulfurizing agent is added to the circulating fluidized bed boiler during combustion.

[0017] During the pre-desiliconization reaction, sodium hydroxide with a mass concentration of 10-30% is used to react with activated fly ash, and the liquid-to-solid ratio L / S is controlled to be 4-15, the reaction time to be 30-240 min, and the reaction temperature to be 90-110℃.

[0018] The soda lime sintering method includes raw slurry preparation, clinker calcination, clinker leaching, red mud separation and washing, medium-pressure desilication, carbon separation, sintering seed separation, evaporation of carbon and seed separation mother liquor, aluminum hydroxide separation and washing, and aluminum hydroxide roasting.

[0019] During the preparation of the raw meal slurry, the formulation of the raw meal slurry is controlled to meet the following conditions: [CaO] / [SiO2] = 2.0~2.1, [Na2O] / [Al2O3+Fe2O3] = 0.92~0.96.

[0020] The desilication liquid is reacted with lime or lime milk to obtain calcium silicate powder. The filter cake after filtering and washing the calcium silicate powder is dried to obtain calcium silicate by-product. The filtrate and washing liquid are evaporated and then returned to the pre-desilication process.

[0021] Another technical solution of the present invention is: a system for producing metallurgical-grade alumina from high-alumina mixed coal, comprising a circulating fluidized bed boiler, an alumina pre-baking furnace, a pre-desiliconizing tank, and a quicklime sintering device. The circulating fluidized bed boiler is used to burn the high-alumina mixed coal to obtain fly ash; the alumina pre-baking furnace is used to perform alumina-fixing and silica-activating roasting on the fly ash to obtain active fly ash slag; the pre-desiliconizing tank is used for the pre-desiliconizing reaction of the active fly ash slag with alkaline solution to obtain desiliconizing liquid and fly ash concentrate; and the quicklime sintering device is used to prepare metallurgical-grade alumina from the fly ash concentrate.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] (1) The present invention combines high-alumina coal with coal gangue to form high-alumina mixed coal, which is then used for combustion in a fluidized bed boiler. This invention can solve the current problem of treating solid wastes such as coal gangue and fly ash, as well as ordinary industrial waste residues, avoiding the occupation of large amounts of land and serious environmental pollution. The method of the present invention can make comprehensive use of these wastes. The carbon in the coal gangue is used as fuel for power plants, and the high-alumina fly ash after combustion is used as bauxite, thus alleviating the problem of insufficient domestic bauxite supply.

[0024] (2) By controlling the mineral composition of the fly ash after combustion, this invention can improve the A / S ratio of coal blending and ensure that the A / S ratio of the fly ash obtained after coal combustion is ≥1.10. This satisfies the requirement that the A / S ratio of the fly ash concentrate after pre-desiliconization in the subsequent alumina plant can be increased to above 3.0, which is suitable for the sintering process of alumina production and ensures that the alumina plant is similar to the traditional bauxite sintering alumina plant. At the same time, the CaO content in the fly ash after combustion is ≤3.5%, which can also prevent CaO from combining with SiO2 to form various calcium silicates, which seriously affect the desiliconization rate of the fly ash.

[0025] (3) In this invention, after combustion in a fluidized bed boiler, the fly ash undergoes a solid alumina pre-baking process. Since the fly ash produced by fluidized bed boiler combustion is mainly composed of Al2O3, SiO2, and Fe2O3, primarily consisting of an amorphous glassy phase, it is chemically active and readily dissolves in caustic alkali during subsequent pre-desiliconization. This leads to a significant dissolution of Al2O3 from the fly ash, reducing the Al2O3 content in the concentrate fly ash and consequently affecting the Al2O3 recovery rate and red mud residue quantity during sintering production. Therefore, this invention utilizes a solid alumina pre-baking process to convert active Al2O3 to inert Al2O3 through specific temperature and holding time treatment, without affecting the activity of SiO2 in the fly ash. The process control is simple.

[0026] (4) In order to further improve the subsequent desiliconization efficiency of fly ash, the present invention changes the traditional desulfurization method of boilers, that is, the addition of lime or limestone to the furnace of fluidized bed boilers for desulfurization is changed to flue gas desulfurization after the furnace. That is, before the flue gas is emitted, the SO2 index of the flue gas emission is ensured to meet the emission standard, which can reduce the CaO content in fly ash and ensure the activity and pre-desiliconization efficiency of fly ash.

[0027] (5) This invention has good economic and social benefits. Practical experience has shown that approximately 2.5-3.0 tons of fly ash produce 1 ton of alumina and 236 kg of activated calcium silicate powder. The estimated production cost of alumina is 2400 yuan / ton, and the production cost of activated calcium silicate powder is 800 yuan / ton, while the selling price of alumina is 3000 yuan / ton and the selling price of activated calcium silicate powder is 1800 yuan / ton. It has good economic and social benefits. Considering an alumina plant with a scale of 2 million tons / year, it is estimated to consume 5-6 million tons / year of high-alumina fly ash, with an investment of approximately 9 billion yuan, sales revenue of approximately 6.85 billion yuan, and profit of approximately 1.9 billion yuan. The project's internal rate of return is approximately 17%, higher than the average internal rate of return of 12% for traditional bauxite alumina plants. It also completely solves the world's scientific and technological problem of extracting alumina from high-alumina fly ash. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0030] Example 1:

[0031] according to Figure 1 The process flow shown involves setting up specialized mining, blending, crushing, and washing processes and equipment for high-alumina coal and high-alumina coal gangue in the coal mine. High-alumina mixed coal is obtained through processes such as coal mining, blending, crushing, washing, and grinding. The coal crushing process can be carried out as follows:

[0032] Serial Number Process Equipment Name unit Incoming crushing particle size Output particle size 1 Coal crushing Jawstone crushing and mm 500-1500 125-400 2 Coal crushing cone crusher mm 125-400 50-100 3 Coal crushing cone crusher mm 50-100 5-25 4 coal pulverizer Hammerstone Coal Mill mm 5-25 1-8

[0033] In this embodiment, the calorific value of the high-alumina mixed coal is 3300 kcal / kg. Based on the coal analysis and ash composition, the fly ash composition is calculated as follows: Al2O3: 48%; SiO2: 38%; A / S: 1.26; CaO: 3.0%.

[0034] To meet the feed requirements of the circulating fluidized bed boiler, high-alumina mixed coal is ground to a particle size ≤8mm by a hammer crusher before being fed into the circulating fluidized bed boiler of the coal gangue power plant for combustion. The boiler combustion temperature is 930℃, and no desulfurizing agent is added to the furnace. The fresh steam generated by the boiler is sent to the generator unit for power generation. The fly ash after combustion accounts for 55% of the fly ash residue, and its fineness is ≤20μm. It is sent to an alumina-silica calcining furnace to perform alumina-silica calcination on the fly ash. The furnace is held at 940℃ for 60 minutes to reduce the activity of Al2O3 in the fly ash, changing it from active Al2O3 to inert Al2O3. At this time, the activity of SiO2 remains basically unchanged.

[0035] The fly ash discharged from the bottom slag outlet of the boiler accounts for 45% of the total fly ash slag, that is, the ash / slag ratio of fly ash is 55:45, the particle size of fly ash slag is ≤8mm, and after grinding, the particle size is ≤63μm. It is mixed with the fly ash obtained from the boiler flue gas dust collection equipment and enters the previous fly ash solid alumina and activated silica roasting process to carry out solid alumina and activated silica roasting. The roasting conditions are also 940℃ and held for 60 minutes.

[0036] The flue gas discharged from the boiler is desulfurized by adding a desulfurizing agent, and the flue gas is discharged in accordance with the power plant's environmental protection standards.

[0037] The pre-calcined activated fly ash residue is sent to a fly ash alumina plant for the production of metallurgical-grade alumina. Specifically, 1000 kg of activated fly ash residue is subjected to alkaline pre-desiliconization under the following conditions: liquid-to-solid ratio (L / S) = 10, NaOH concentration 15%, desiliconization time 120 min, and desiliconization temperature 105℃, using a flat-bottomed mechanically stirred decomposition tank. Under these conditions, the desiliconization rate is 68%, and the dealuminization rate is 17%. The high-alumina fly ash concentrate after pre-desiliconization has an A / S ratio of 3.28. This concentrate is then used in the existing soda lime sintering process, which mainly includes raw material slurry preparation, clinker calcination, clinker leaching, red mud separation and washing, medium-pressure desiliconization, carbon separation, sintering seed separation, carbon and seed mother liquor evaporation, aluminum hydroxide separation and washing, and aluminum hydroxide roasting, producing 345 kg of metallurgical-grade alumina. Therefore, the activated fly ash consumption index for metallurgical-grade alumina is 2.9 t / t-Al2O3.

[0038] The pre-desilicon liquid after pre-desiliconization contains 38% SiO2 from the active fly ash, of which 68% (258.4 kg) enters the pre-desilicon liquid; while the Al2O3 content is 48%, of which 17% (81.6 kg) enters the pre-desilicon liquid. This pre-desilicon liquid is used to produce activated calcium silicate powder.

[0039] The sintering process used in this embodiment is a mature soda lime sintering technology, which will not be described in detail here.

[0040] The metallurgical grade alumina prepared in this embodiment meets the first-grade standard in GB / T24487-2022.

[0041] Its chemical composition is shown in the table below:

[0042] <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[Fe2O3]]> <![CDATA[Na2O]]> LOI ≥98.6 ≤0.02 ≤0.02 ≤0.45 ≤1.0

[0043] Physical properties must meet the following requirements: α-Al₂O₃ content 5-20%, loss on ignition (LOI) ≤ 1.0%, particle size -45μm ≤ 20%, +125μm ≤ 15%, specific surface area ≥ 60m². 2 / g, with a bulk density of 0.95–1.05 tons / m³ 3 .

[0044] This metallurgical-grade alumina has a wide range of applications. Over 95% can be used in the electrolytic aluminum industry, specifically in the production of aluminum ingots via molten salt electrolysis. The remaining ≤5% of this metallurgical-grade alumina can be used in over 1000 varieties of alumina chemicals, widely used in ceramics, petroleum, chemicals, fire protection, water purification agents, medical applications, coatings, and more.

[0045] Example 2:

[0046] This embodiment uses the same production method as Embodiment 1, the only difference being:

[0047] The calorific value of the high-alumina mixed coal is 3100 kcal / kg. Based on the coal analysis and ash composition, the fly ash composition is calculated as follows: Al2O3: 50%; SiO2: 37%; A / S: 1.35; CaO: 3.3%.

[0048] The combustion temperature of the circulating fluidized bed boiler is 900℃. The fly ash after combustion accounts for 58% of the fly ash residue, and its fineness is ≤20μ. It is then sent to the solid alumina and activated silica calcination furnace to solidify alumina and activate silica, and is kept at 920℃ for 30 minutes.

[0049] The fly ash discharged from the bottom slag outlet of the boiler accounts for 42% of the total fly ash slag, that is, the fly ash ash / slag ratio is 58:42, and its particle size is ≤8mm. After grinding, its particle size is ≤63μm. It is mixed with the fly ash obtained from the boiler flue gas dust collection equipment and enters the previous fly ash solid alumina and activated silica roasting process for solid alumina and activated silica roasting. The roasting conditions are also 920℃ and held for 30 minutes.

[0050] 1000 kg of active fly ash residue was subjected to alkaline pre-desiliconization under the following conditions: liquid-to-solid ratio (L / S) = 12, NaOH concentration 13%, desiliconization time 180 min, and desiliconization temperature 100℃. A flat-bottomed mechanically stirred decomposition tank was used for the desiliconization. Under these conditions, the desiliconization rate was 70%, and the dealuminization rate was 15%. The high-alumina fly ash concentrate after pre-desiliconization had an A / S ratio of 3.50. This concentrate was then used in the existing soda lime sintering process to produce 368 kg of metallurgical-grade alumina (also meeting the first-grade standard in GB / T24487-2022). Therefore, the active fly ash consumption index for metallurgical-grade alumina is 2.72 t / t Al₂O₃.

[0051] The pre-desilicon liquid after pre-desiliconization contains 37% SiO2 from the active fly ash, of which 70% (259 kg) enters the pre-desilicon liquid; while the Al2O3 content is 50%, of which 15% (75 kg) enters the pre-desilicon liquid. This pre-desilicon liquid is used to produce activated calcium silicate powder.

[0052] In summary, this invention utilizes high-alumina coal gangue mixed with high-alumina coal, allowing the carbon to be burned in a fluidized bed boiler for power generation and extraction of metallurgical-grade alumina. This represents the highest level of resource utilization for coal gangue, treating it as coal or bauxite with significant economic value. The following example illustrates this: Washed gangue produced by a coal washing plant with 1 ton of gangue yielding 1700 cal / kg is used as an example.

[0053] (1) Coal gangue is the tailings produced in coal production. It has no value and is usually stored in the open or backfilled in coal mines. It occupies land and pollutes the environment. If it spontaneously combusts, it will also pollute the air and cause accidents such as the collapse of the stockpile.

[0054] (2) If resource utilization is adopted, the carbon content is utilized first. The calorific value of coal gangue is 1700cal / kg, which is equivalent to 1 / 3 of the calorific value of coal with 5000cal / kg. The price of coal is 800 yuan / ton, so this value is 800 / 3 = 266.7 yuan / ton.

[0055] (3) If resource utilization is adopted, the alumina content is utilized first. For example, if burning coal gangue produces 500 kg of high-alumina fly ash, and bauxite costs 400 yuan / ton, the value is 0.5 tons × 400 = 200 yuan. The alumina content in high-alumina fly ash is 45%, while the alumina content in imported bauxite is 40%. Therefore, the alumina content in high-alumina fly ash is greater than or equal to the 40% alumina content in imported bauxite.

[0056] (4) Due to the severe shortage of bauxite, 110 million tons of bauxite are imported annually, accounting for 60% of the bauxite used in the domestic alumina industry. The comprehensive utilization of high-alumina coal gangue and high-alumina fly ash has become a research focus, but existing research has not made any breakthroughs in this area.

[0057] (5) This invention utilizes coal gangue resources, which has a coal gangue content of 1700cal / kg. The coal value of each ton is 266.7 yuan / ton, and the aluminum ore value is 200 yuan, totaling 466.7 yuan. This can achieve a circular economy and turn waste into treasure.

[0058] (6) Although the method of the existing patent CN101941725A discloses the process of extracting alumina from coal gangue, it does not consider the need to use a power plant fluidized bed boiler during activation roasting. Instead, it uses ordinary low-calorific-value coal gangue. Low-calorific-value coal gangue has low alumina, high silica, and low A / S ratio, resulting in poor economic indicators and failing to meet the requirements of industrial production.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A process for producing metallurgical grade alumina from high alumina blended coal, characterized by: Includes the following steps: S1. Using high-alumina coal and high-alumina coal gangue as raw materials, a high-alumina mixed coal is obtained through blending. The calorific value of the high-alumina mixed coal is controlled at 3100-3300 kcal / kg. The high-alumina mixed coal is then subjected to coarse crushing, medium crushing, and coal washing, and then fed into a circulating fluidized bed boiler. Fly ash is produced by combustion at 840-950℃ in the circulating fluidized bed boiler without adding desulfurizing agent. The mineral composition of the resulting fly ash meets the following requirements: Al2O3: 48-50%, SiO2: 37-38%, A / S: 1.26~1.35 CaO: 3.0–3.3%; S2. The fly ash is roasted in an alumina pre-baking furnace, with the roasting temperature controlled at 920-940℃ and the holding time at 30-60 min. S3. A pre-desiliconization reaction is carried out with sodium hydroxide at a mass concentration of 13-15% and activated fly ash slag after solid alumina pre-baking. The liquid-to-solid ratio L / S is controlled at 10-12, the reaction time is 120-180 min, and the reaction temperature is 100-105℃. After the pre-desiliconization reaction, desiliconization liquid and fly ash concentrate are obtained. The mineral composition of the fly ash concentrate has A / S ≥ 3.

0. S4. The fly ash concentrate is sintered using the soda lime sintering method to obtain metallurgical grade alumina.

2. The method of claim 1, wherein: The soda lime sintering method includes raw slurry preparation, clinker calcination, clinker leaching, red mud separation and washing, medium-pressure desilication, carbon separation, sintering seed separation, evaporation of carbon and seed separation mother liquor, aluminum hydroxide separation and washing, and aluminum hydroxide roasting.

3. The method of claim 2, wherein: During the preparation of the raw meal slurry, the formulation of the raw meal slurry is controlled to meet the following conditions: [CaO] / [SiO2]=2.0~2.1, [Na2O] / [Al2O3+ Fe2O3]=0.92~0.

96.

4. The method of claim 1, wherein: The desilication liquid is reacted with lime or lime milk to obtain calcium silicate powder. The filter cake after filtering and washing the calcium silicate powder is dried to obtain calcium silicate by-product. The filtrate and washing liquid are evaporated and then returned to the pre-desilication process.

5. A system for producing metallurgical-grade alumina from high-alumina mixed coal, characterized in that: The apparatus includes a circulating fluidized bed boiler, a solid alumina prebaking furnace, a pre-desilicon tank, and a quicklime sintering device, for performing the method according to any one of claims 1 to 4, wherein: The circulating fluidized bed boiler is used to burn high-alumina mixed coal and obtain fly ash; Aluminum-fixing pre-baking furnace is used to calcine fly ash to fix aluminum and activate silica, and to obtain active fly ash slag. The pre-desilicon tank is used for the pre-desiliconization reaction of activated fly ash residue with alkaline solution to obtain desiliconized liquid and fly ash concentrate; The soda lime sintering unit is used to prepare metallurgical grade alumina from fly ash concentrate.

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