Environment-friendly process for preparing alumina

Abstract

This invention discloses an environmentally friendly alumina preparation process, specifically relating to the field of alumina preparation technology, including the following steps: Step 1: Ore pretreatment, the raw ore is dry-crushed to 10–30 mm, and then rolled in a closed-circuit high-pressure roller mill to d80≈0.2–0.3 mm, followed by desliming and classification to reduce the entry of low-grade siliceous fine mud into the leaching stage; Step 2: Prepare sodium aluminate mother liquor with a Na2O concentration of 180–220 g / L and a Na2O / Al2O3 molar ratio R=1.5–1.7, and adopt a two-stage leaching process of low temperature and medium temperature, pre-dissolving at atmospheric pressure at 95–110℃, and then mainly dissolving at 140–145℃ for 2–4 h using residual heat from flash evaporation. This invention utilizes dry pre-crushing combined with high-pressure roller milling to replace part of wet grinding, saving 10-20% of electricity, reducing water consumption in grinding slurry, reducing steam consumption and carbon emissions through two-stage leaching, and featuring a double-layer composite seepage prevention system in the stockpile equipped with a drainage system to achieve zero direct discharge from filtration. It also enables the resource-based treatment of red mud generated from pressure filtration, improving resource utilization and significantly enhancing the environmental friendliness of the invention. This results in better practical application effects of the invention.

Description

Technical Field

[0001] This invention relates to the field of alumina preparation technology, and more specifically, to an environmentally friendly alumina preparation process. Background Technology

[0002] Alumina (Al2O3) is a white amorphous or crystalline powder with high hardness, wear and corrosion resistance, and excellent electrical insulation and thermal stability. It is mainly produced by the Bayer process: alkaline dissolution of bauxite, removal of red mud, decomposition to obtain aluminum hydroxide, and calcination to produce alumina. It is widely used in electrolytic aluminum, refractory materials, ceramics, polishing materials, catalyst carriers, etc. However, the current technology for preparing alumina is not environmentally friendly, resulting in serious environmental pollution and is not conducive to green environmental protection.

[0003] Therefore, there is an urgent need for an environmentally friendly alumina preparation process to solve the above problems. Summary of the Invention

[0004] To overcome the aforementioned deficiencies of the prior art, embodiments of the present invention provide an environmentally friendly alumina preparation process. This invention utilizes dry pre-crushing combined with high-pressure roller milling to replace part of the wet grinding process, saving 10-20% of electricity and reducing water consumption in the grinding slurry. Two-stage leaching reduces steam consumption and carbon emissions. The stockpile features a double-layer composite seepage prevention system equipped with a drainage system, achieving zero direct discharge from the filtration process. It also enables the resource-based treatment of red mud generated from pressure filtration, improving resource utilization and significantly enhancing the environmental friendliness of the invention. This results in better practical application effects and addresses the problems mentioned in the background section.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an environmentally friendly alumina preparation process, comprising the following steps: Step 1: Perform ore pretreatment by dry pre-crushing the raw ore to 10–30 mm, then close-circuit high-pressure roller milling to d80≈0.2–0.3 mm, followed by desliming and classification to reduce the entry of low-grade siliceous fine mud into the leaching section. Step 2: Prepare sodium aluminate mother liquor with Na2O concentration of 180–220 g / L and Na2O / Al2O3 molar ratio R = 1.5–1.7. Use a two-stage dissolution process with low and medium temperatures. Pre-dissolve at atmospheric pressure at 95–110℃, then use flash evaporation to raise the temperature to 140–145℃ for main dissolution for 2–4 hours. Use either a biomass steam boiler or a natural gas boiler for steam supply. Add either limestone or slaked lime for limeification. Control the precipitation of active Si into perovskite and hydrotalcite co-precipitation to inhibit desilication and reverse dissolution. Step 3: Solid-liquid separation and red mud treatment are carried out. High-efficiency flocculants are used in conjunction with deep cone thickeners to achieve a red mud concentration of ≥55% solid content, reducing the storage area and leachate. Overflow water is returned to the leaching section and dry-pile process is adopted. The water content is reduced to <25% by pressure filtration. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by pressure filtration is red mud. The red mud is treated for resource utilization. The pH is adjusted and neutralized to 8-9 to prepare admixtures for building materials. It is combined with clinker and slag powder. Heavy metal leaching meets GB 5085. Step 4: Precipitate aluminum hydroxide. Cool the sodium aluminate mother liquor to 50–65℃, add fine seed crystals for decomposition, and let it stand for 36–72 h to generate Al(OH)3. Multi-stage series decomposition is carried out, and the mother liquor is returned to the final stage for dissolution, realizing a closed-loop circulation. The mother liquor circulation rate is >98%, and the discharge of alkaline wastewater is close to zero. The finer seed crystal circulation reduces the entrainment of organic matter. Activated carbon and ozone oxidation decolorize the mother liquor, reduce the accumulation of organic carbon, improve circulation stability, and avoid organic matter being discharged with wastewater. Step 5: Graded washing and filtration can be used to obtain coarse and fine aluminum hydroxide. Countercurrent washing removes alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns the alkali to the mother liquor, reducing the consumption of new alkali and the alkalinity of the wastewater. Step Six: The obtained crude and fine aluminum hydroxide are calcined. Dehydration occurs in the preheating section at 350–500℃, and the calcination section at 900–1100℃ to generate the γ to α phases. The calcined aluminum hydroxide is then rapidly cooled to <200℃ to prevent over-burning. The calcination flue gas is purified using a high-temperature cyclone separator and bag filter to achieve dust emissions ≤10 mg / Nm³. 3 ; Step 7: Using NaAlO2 solution as the aluminum source, introduce compressed CO2 to carbonize to pH≈7–8 to generate Al(OH)3 precipitate, then filter, wash and dry at low temperature in sequence. The dried Al(OH)3 is then calcined at medium temperature of 600–900℃ to obtain γ-Al2O3, and then further calcined at 1100–1200℃ to obtain α phase. Step 8: The alumina is sieved, and then particle size is shaped and homogenized using an air jet mill to obtain the finished alumina with the required particle size.

[0006] In a preferred embodiment, the high-efficiency flocculant in step three is anionic PAM.

[0007] The technical effects and advantages of this invention are as follows: This invention first pre-treats the ore, dry-crushing the raw ore to 10–30 mm and then high-pressure roller milling it to d80≈0.2–0.3 mm. Next, desliming and classification are performed to reduce the entry of low-grade siliceous fine mud into the leaching stage. Sodium aluminate mother liquor is prepared with a Na2O concentration of 180–220 g / L and a Na2O / Al2O3 molar ratio R=1.5–1.7. A two-stage leaching process is employed, involving low-temperature and medium-temperature leaching. Pre-leaching occurs at atmospheric pressure (95–110℃), followed by flash evaporation to raise the temperature to 140–145℃ for primary leaching (2–4 saturations). h. Steam is supplied by either a biomass steam boiler or a natural gas boiler. Limestone or slaked lime is added for limeification. Active Si is controlled to precipitate as perovskite and hydrotalcite co-precipitation, inhibiting desilication and resolution. Solid-liquid separation and red mud treatment are carried out. A high-efficiency flocculant is used in conjunction with a deep cone thickener to ensure that the red mud concentration is ≥55% solid content, reducing the stockpile area and leachate. Overflow water is returned to the leaching section. A dry-pile process is used, and the water content is reduced to <25%. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by the filter press is red mud. The red mud is treated for resource utilization. The pH is adjusted and neutralized to 8-9 to prepare admixtures for building materials. It is combined with clinker and slag powder. Heavy metal leaching meets GB standards. 5085 involves solid-liquid separation and red mud treatment, employing a high-efficiency flocculant in conjunction with a deep cone thickener to achieve a red mud concentration of ≥55% solids content, reducing storage area and leachate. Overflow water is returned to the leaching section, where a dry-pile process is used, followed by pressure filtration to a moisture content of <25%. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by pressure filtration is red mud, which is then used for resource recovery. The red mud is acidified and neutralized to a pH of 8–9 to prepare admixtures for building materials, combined with clinker and slag powder. Heavy metal leaching meets GB5085 standards, aluminum hydroxide is precipitated, and the sodium aluminate mother liquor is cooled to 50–65℃, with the addition of fine seed crystals for decomposition, and a residence time of 36–72 seconds. h, Al(OH)3 is generated, and multi-stage series decomposition occurs. The mother liquor is returned to the dissolution stage in the final stage, realizing a closed-loop cycle with a mother liquor circulation rate >98%. The discharge of alkaline wastewater is close to zero. Seed crystal circulation refinement reduces organic matter entrainment. Activated carbon and ozone oxidation decolorize the mother liquor, reducing organic carbon accumulation and improving circulation stability. Organic matter is avoided from being discharged with wastewater. Staged washing and filtration can obtain coarse and fine aluminum hydroxide. Countercurrent washing removes alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns alkali to the mother liquor, reducing the consumption of new alkali and the alkalinity of wastewater. Staged washing and filtration... The process involves filtration to obtain coarse and fine aluminum hydroxide, countercurrent washing to remove alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns the alkali to the mother liquor, reducing fresh alkali consumption and wastewater alkalinity. Using NaAlO2 solution as the aluminum source, compressed CO2 is introduced to carbonize to pH ≈ 7–8, generating Al(OH)3 precipitate. This precipitate is then sequentially filtered, washed, and dried at low temperatures. The dried Al(OH)3 is subsequently calcined at a medium temperature of 600–900℃ to obtain γ-Al2O3, and further calcined at 1100–1200℃ to obtain the α-phase. The alumina is then sieved.Then, an air jet mill is used for particle size shaping and homogenization to obtain the finished alumina of the desired particle size. This invention utilizes dry pre-crushing combined with a high-pressure roller mill to replace part of the wet grinding, saving 10-20% of electricity and reducing water consumption in the grinding slurry. Two-stage leaching reduces steam consumption and carbon emissions. The stockpile features a double-layer composite seepage prevention system equipped with a drainage system, achieving zero direct discharge from filtration. The red mud generated from pressure filtration is treated resource-wise, improving resource utilization and significantly enhancing the environmental friendliness of this invention. This results in good practical application effects. Detailed Implementation

[0008] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0009] Example This invention provides an environmentally friendly alumina preparation process, comprising the following steps: Step 1: Perform ore pretreatment by dry pre-crushing the raw ore to 10–30 mm, then close-circuit high-pressure roller milling to d80≈0.2–0.3 mm, followed by desliming and classification to reduce the entry of low-grade siliceous fine mud into the leaching section. Step 2: Prepare sodium aluminate mother liquor with Na2O concentration of 180–220 g / L and Na2O / Al2O3 molar ratio R = 1.5–1.7. Use a two-stage dissolution process with low and medium temperatures. Pre-dissolve at atmospheric pressure at 95–110℃, then use flash evaporation to raise the temperature to 140–145℃ for main dissolution for 2–4 hours. Use either a biomass steam boiler or a natural gas boiler for steam supply. Add either limestone or slaked lime for limeification. Control the precipitation of active Si into perovskite and hydrotalcite co-precipitation to inhibit desilication and reverse dissolution. Step 3: Solid-liquid separation and red mud treatment are carried out. High-efficiency flocculants are used in conjunction with deep cone thickeners to achieve a red mud concentration of ≥55% solid content, reducing the storage area and leachate. Overflow water is returned to the leaching section and dry-pile process is adopted. The water content is reduced to <25% by pressure filtration. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by pressure filtration is red mud. The red mud is treated for resource utilization. The pH is adjusted and neutralized to 8-9 to prepare admixtures for building materials. It is combined with clinker and slag powder. Heavy metal leaching meets GB 5085. Step 4: Precipitate aluminum hydroxide. Cool the sodium aluminate mother liquor to 50–65℃, add fine seed crystals for decomposition, and let it stand for 36–72 h to generate Al(OH)3. Multi-stage series decomposition is carried out, and the mother liquor is returned to the final stage for dissolution, realizing a closed-loop circulation. The mother liquor circulation rate is >98%, and the discharge of alkaline wastewater is close to zero. The finer seed crystal circulation reduces the entrainment of organic matter. Activated carbon and ozone oxidation decolorize the mother liquor, reduce the accumulation of organic carbon, improve circulation stability, and avoid organic matter being discharged with wastewater. Step 5: Graded washing and filtration can be used to obtain coarse and fine aluminum hydroxide. Countercurrent washing removes alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns the alkali to the mother liquor, reducing the consumption of new alkali and the alkalinity of the wastewater. Step Six: The obtained crude and fine aluminum hydroxide are calcined. Dehydration occurs in the preheating section at 350–500℃, and the calcination section at 900–1100℃ to generate the γ to α phases. The calcined aluminum hydroxide is then rapidly cooled to <200℃ to prevent over-burning. The calcination flue gas is purified using a high-temperature cyclone separator and bag filter to achieve dust emissions ≤10 mg / Nm³. 3 ; Step 7: Using NaAlO2 solution as the aluminum source, introduce compressed CO2 to carbonize to pH≈7–8 to generate Al(OH)3 precipitate, then filter, wash and dry at low temperature in sequence. The dried Al(OH)3 is then calcined at medium temperature of 600–900℃ to obtain γ-Al2O3, and then further calcined at 1100–1200℃ to obtain α phase. Step 8: The alumina is sieved and then particle size is shaped and homogenized using an air jet mill.

[0010] In a preferred embodiment, the high-efficiency flocculant in step three is anionic PAM.

[0011] This invention first pre-treats the ore, dry-crushing the raw ore to 10–30 mm and then high-pressure roller milling it to d80≈0.2–0.3 mm. Next, desliming and classification are performed to reduce the entry of low-grade siliceous fine mud into the leaching stage. Sodium aluminate mother liquor is prepared with a Na2O concentration of 180–220 g / L and a Na2O / Al2O3 molar ratio R=1.5–1.7. A two-stage leaching process is employed, involving low-temperature and medium-temperature leaching. Pre-leaching occurs at atmospheric pressure (95–110℃), followed by flash evaporation to raise the temperature to 140–145℃ for primary leaching (2–4 saturations). h. Steam is supplied by either a biomass steam boiler or a natural gas boiler. Limestone or slaked lime is added for limeification. Active Si is controlled to precipitate as perovskite and hydrotalcite co-precipitation, inhibiting desilication and resolution. Solid-liquid separation and red mud treatment are carried out. A high-efficiency flocculant is used in conjunction with a deep cone thickener to ensure that the red mud concentration is ≥55% solid content, reducing the stockpile area and leachate. Overflow water is returned to the leaching section. A dry-pile process is used, and the water content is reduced to <25%. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by the filter press is red mud. The red mud is treated for resource utilization. The pH is adjusted and neutralized to 8-9 to prepare admixtures for building materials. It is combined with clinker and slag powder. Heavy metal leaching meets GB standards. 5085 involves solid-liquid separation and red mud treatment, employing a high-efficiency flocculant in conjunction with a deep cone thickener to achieve a red mud concentration of ≥55% solids content, reducing storage area and leachate. Overflow water is returned to the leaching section, where a dry-pile process is used, followed by pressure filtration to a moisture content of <25%. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by pressure filtration is red mud, which is then used for resource recovery. The red mud is acidified and neutralized to a pH of 8–9 to prepare admixtures for building materials, combined with clinker and slag powder. Heavy metal leaching meets GB5085 standards, aluminum hydroxide is precipitated, and the sodium aluminate mother liquor is cooled to 50–65℃, with the addition of fine seed crystals for decomposition, and a residence time of 36–72 seconds. h, Al(OH)3 is generated, and multi-stage series decomposition occurs. The mother liquor is returned to the dissolution stage in the final stage, realizing a closed-loop cycle with a mother liquor circulation rate >98%. The discharge of alkaline wastewater is close to zero. Seed crystal circulation refinement reduces organic matter entrainment. Activated carbon and ozone oxidation decolorize the mother liquor, reducing organic carbon accumulation and improving circulation stability. Organic matter is avoided from being discharged with wastewater. Staged washing and filtration can obtain coarse and fine aluminum hydroxide. Countercurrent washing removes alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns alkali to the mother liquor, reducing the consumption of new alkali and the alkalinity of wastewater. Staged washing and filtration... The process involves filtration to obtain coarse and fine aluminum hydroxide, countercurrent washing to remove alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns the alkali to the mother liquor, reducing fresh alkali consumption and wastewater alkalinity. Using NaAlO2 solution as the aluminum source, compressed CO2 is introduced to carbonize to pH ≈ 7–8, generating Al(OH)3 precipitate. This precipitate is then sequentially filtered, washed, and dried at low temperatures. The dried Al(OH)3 is subsequently calcined at a medium temperature of 600–900℃ to obtain γ-Al2O3, and further calcined at 1100–1200℃ to obtain the α-phase. The alumina is then sieved.Then, an air jet mill is used for particle size shaping and homogenization to obtain the finished alumina of the desired particle size. This invention utilizes dry pre-crushing combined with a high-pressure roller mill to replace part of the wet grinding, saving 10-20% of electricity and reducing water consumption in the grinding slurry. Two-stage leaching reduces steam consumption and carbon emissions. The stockpile features a double-layer composite seepage prevention system equipped with a drainage system, achieving zero direct discharge from filtration. The red mud generated from pressure filtration is treated resource-wise, improving resource utilization and significantly enhancing the environmental friendliness of this invention. This results in good practical application effects.

[0012] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An environmentally friendly alumina preparation process, characterized in that, Includes the following steps: Step 1: Perform ore pretreatment by dry pre-crushing the raw ore to 10–30 mm, then close-circuit high-pressure roller milling to d80≈0.2–0.3 mm, followed by desliming and classification to reduce the entry of low-grade siliceous fine mud into the leaching section. Step 2: Prepare sodium aluminate mother liquor with Na2O concentration of 180–220 g / L and Na2O / Al2O3 molar ratio R = 1.5–1.

7. Use a two-stage dissolution process with low and medium temperatures. Pre-dissolve at atmospheric pressure at 95–110℃, then use flash evaporation to raise the temperature to 140–145℃ for main dissolution for 2–4 hours. Use either a biomass steam boiler or a natural gas boiler for steam supply. Add either limestone or slaked lime for limeification. Control the precipitation of active Si into perovskite and hydrotalcite co-precipitation to inhibit desilication and reverse dissolution. Step 3: Solid-liquid separation and red mud treatment are carried out. High-efficiency flocculants are used in conjunction with deep cone thickeners to achieve a red mud concentration of ≥55% solid content, reducing the storage area and leachate. Overflow water is returned to the leaching section and dry-pile process is adopted. The water content is reduced to <25% by pressure filtration. The stockpile is equipped with a double-layer composite seepage prevention system and a drainage system to achieve zero direct discharge of leachate. The filter cake produced by pressure filtration is red mud. The red mud is treated for resource utilization. The pH is adjusted and neutralized to 8-9 to prepare admixtures for building materials. It is combined with clinker and slag powder. Heavy metal leaching meets GB 5085. Step 4: Precipitate aluminum hydroxide. Cool the sodium aluminate mother liquor to 50–65℃, add fine seed crystals for decomposition, and let it stand for 36–72 hours to generate Al(OH)3. Multi-stage series decomposition is carried out, and the mother liquor is returned to the final stage for dissolution, realizing a closed-loop circulation. The mother liquor circulation rate is >98%, and the discharge of alkaline wastewater is close to zero. The finer seed crystal circulation reduces the entrainment of organic matter. Activated carbon and ozone oxidation decolorize the mother liquor, reduce the accumulation of organic carbon, improve circulation stability, and avoid organic matter being discharged with wastewater. Step 5: Graded washing and filtration can be used to obtain coarse and fine aluminum hydroxide. Countercurrent washing removes alkali, followed by vacuum filtration and low-temperature drying. Countercurrent multi-stage washing returns the alkali to the mother liquor, reducing the consumption of new alkali and the alkalinity of the wastewater. Step Six: The obtained crude and fine aluminum hydroxide are calcined. Dehydration occurs in the preheating section at 350–500℃, and the calcination section at 900–1100℃ to generate the γ to α phases. The calcined aluminum hydroxide is then rapidly cooled to <200℃ to prevent over-burning. The calcination flue gas is purified using a high-temperature cyclone separator and bag filter to achieve dust emissions ≤10 mg / Nm³. 3 ; Step 7: Using NaAlO2 solution as the aluminum source, introduce compressed CO2 to carbonize to pH≈7–8 to generate Al(OH)3 precipitate, then filter, wash and dry at low temperature in sequence. The dried Al(OH)3 is then calcined at medium temperature of 600–900℃ to obtain γ-Al2O3, and then further calcined at 1100–1200℃ to obtain α phase. Step 8: The alumina is sieved, and then particle size is shaped and homogenized using an air jet mill to obtain the finished alumina with the required particle size.

2. The environmentally friendly alumina preparation process according to claim 1, characterized in that: The high-efficiency flocculant in step three is anionic PAM.