Alumina production process and basic characteristics

   

  Data shows that China is the world's largest producer of alumina, with a production capacity of more than 43.5 million tons per year under construction and under construction, of which the production capacity for processing domestic bauxite is 32.5 million tons per year. In 2018, the national alumina output 48.96 million tons, making it the world's largest producer of alumina. The demand for alumina is also increasing year by year. So, what exactly is alumina? In fact, alumina is a high-hardness compound with a melting point of 2054 ° C and a boiling point of 2980 ° C. It is an ionic crystal that can be ionized at high temperatures and is often used in the manufacture of refractory materials. Industrial Al2O3 is prepared from bauxite (Al2O3 ▪ 3H2O) and hard boehmite. For Al2O3 with high purity requirements, it is generally prepared by chemical methods. Do you want to know more about alumina? The following editor will introduce the production process, basic characteristics, classification, use, main components, and factors affecting adsorption performance of alumina. Hope to help everyone!

　　Alumina production process

　　1. Bayer method

　　The principle is to use caustic soda (NaOH) solution to heat and dissolve alumina in bauxite to obtain sodium aluminate solution. After the solution is separated from the residue (red mud), the temperature is lowered, and aluminum hydroxide is added as the crystal seed. After long-term stirring, sodium aluminate is decomposed to precipitate aluminum hydroxide, washed, and calcined at a temperature of 950-1200 ° C to obtain the finished alumina product. The solution after precipitating aluminum hydroxide is called the mother liquor, which can be recycled after evaporation and concentration.

　　1. The brief chemical reaction of the Bayer method is as follows:

　　The main progress of modern Bayer method:

　　① Large-scale and continuous operation of equipment;

　　② Automation of the production process;

　　③ Save energy, such as high pressure enhanced dissolution and fluidized roasting;

　　④ To produce sandy alumina to meet the needs of aluminum electrolysis and flue gas dry purification. The process flow of the Bayer process is shown in the figure below.

　　2. The advantages of the Bayer method are mainly simple process, low investment and low energy consumption.

　　The economic effect of Bayer process production is determined by the quality of bauxite, mainly the SiO2 content in the ore, which is usually expressed in terms of the aluminum-silicon ratio of the ore, that is, the weight ratio of the Al2O3 and SiO2 content in the ore. Because during the dissolution process of the Bayer process, SiO2 is converted into sodium aluminosilicate hydrate (Na2O · Al2O3 · 1.7SiO2 · nH2O) of the albite type, which is discharged with red mud.

　　2. Soda lime sintering method

　　1. For the treatment of high-silicon bauxite, bauxite, sodium carbonate and lime are mixed and batched in a certain proportion, and sintered in a rotary kiln into a clinker composed of sodium aluminate (Na2O · Al2O3), sodium ferrite (Na2O · Fe2O3, orthosilicate (2CaO · SiO2) and sodium titanate (CaO · TiO2). Then use a dilute alkali solution to dissolve the sodium aluminate in the clinker. At this time, NaOH obtained by hydrolysis of sodium ferrite also enters the solution. If the dissolution conditions are properly controlled, calcium orthosilicate will not react with sodium aluminate solution in large quantities, but will be discharged with calcium titanate, Fe2O3 · H2O, etc. to form red mud. The sodium aluminate solution obtained by dissolving the clinker undergoes a special desilication process, and SiO2O forms hydrated sodium aluminosilicate (called sodium silica slag) or hydrated garnet 3CaO · Al2O3 · xSiO2 · (6-2x) H2O precipitation (where x 😉 0.1) to purify the solution. The CO2 gas is introduced into the refined sodium aluminate solution, and the seed crystal is added to stir to obtain the aluminum hydroxide precipitate and the mother liquor whose main component is sodium carbonate. Aluminum hydroxide is calcined to become a finished alumina. Al2O3 in hydrated garnet can be extracted and recovered with Na 2CO3-containing mother liquor.

　　2. The main chemical reactions of the soda lime sintering method are as follows:

　　3. The main technical achievements in the production of alumina by the alkali lime sintering method are: the formula with low alkali ratio is used in the clinker sintering, and the two-stage abrasive and low molecular ratio solution are used in the clinker dissolution process to inhibit the side reaction during dissolution. Loss, so that the dissolution rates of Na2O and Al2O3 in the clinker reach 94-96% and 92-94%, respectively. The total collection rate of Al2O3 is about 90%, and the consumption of Na2CO3 per ton of alumina is about 95 kg. The alkali-lime sintering method can deal with low-grade ores that cannot be economically utilized by the Bayer method, and its Al-Si ratio can be as low as 3.5. The comprehensive utilization of raw materials is better and has its own characteristics.

　　3. Bayer-sintering method

　　It can give full play to the advantages of the two methods, learn from each other's strengths and make up for their weaknesses, and use bauxite with a lower aluminum-silicon ratio to obtain better economic results. There are many forms of the joint method, all of which are dominated by the Bayer method and supplemented by the sintering method. According to the purpose of the joint method and the different connection methods of the process, it can be divided into three technological processes: series method, parallel method and hybrid method.

　　1. The series method is to recover Na2O and Al2O3 in the red mud of the Bayer process by sintering method, and to deal with the boehmite-type bauxite that cannot be used economically by the Bayer process. The raw material resources are expanded, the alkali consumption is reduced, and the cheaper soda is used instead of caustic soda, and the collection rate of Al2O3 is also high.

　　2. The parallel method is the parallel operation of the Bayer method and the sintering method, which deal with bauxite respectively, but the sintering method only accounts for 10-15% of the total capacity. The NaOH produced by the process transformation of the sintering method is used to supplement the consumption of NaOH in the Bayer process.

　　3. The hybrid method is a synthesis of the first two joint methods. The sintering method in this method not only deals with Bayer red mud, but also deals with some low-grade ores.

　　Basic properties of alumina

　　1. Properties: White solid that is difficult to dissolve in water, odorless, tasteless, extremely hard, easy to absorb moisture but not deliquence (burned non-hygroscopic). Amphoteric oxide, soluble in inorganic acids and alkaline solutions, almost insoluble in water and non-polar organic solvents; relative density (d204) 4.0; melting point 2050 ℃.

　　2. Storage: sealed and dry.

　　3. Uses: used as analytical reagents, dehydration of organic solvents, adsorbents, organic reaction catalysts, abrasives, polishing agents, raw materials for smelting aluminum, and refractory materials.

　　Classification of alumina

　　1. Gamma-type alumina

　　Gamma-type alumina is insoluble in water and can be dissolved in strong acid or alkali solutions. When it is heated to 1200 ° C, it is completely converted into α-type alumina. Gamma-type alumina is a porous material with an inner surface area of hundreds of square meters per gram and strong adsorption capacity. Industrial products are often colorless or slightly pink cylindrical particles with good pressure resistance. In petroleum refining and petrochemical industry, it is a commonly used adsorbent, catalyst and catalyst carrier; in industry, it is a deacidifier for transformer oil and turbine oil, and it is also used for color layer analysis; in the laboratory, it is a neutral strong desiccant, Its drying capacity is no less than that of phosphorus pentoxide, and it can be regenerated and reused after heating below 175 ° C. At present, the alumina produced by the Bayer process accounts for more than 90% of the total output in the world. Most of the alumina is used to make metal aluminum, and less than 10% is used for other purposes.

　　2. α-type alumina

　　In the lattice of α-type alumina, oxygen ions are tightly packed in hexagons, and Al3 + is symmetrically distributed in the coordination center of the octahedron surrounded by oxygen ions. The lattice energy is very large, so the melting point and boiling point are very high. Alpha-type alumina is insoluble in water and acid, and is also called alumina in industry. It is the basic raw material for making metal aluminum; it is also used to make various refractory bricks, refractory crucibles, refractory tubes, and high-temperature experimental instruments; it can also be used as abrasives, flame retardants, fillers, etc.; high-purity α-type alumina is also a raw material for the production of artificial corundum, artificial ruby and sapphire; it is also used to produce board bases for modern large-scale integrated circuits. Type alumina is obtained by dehydration of aluminum hydroxide at a low temperature of 140-150 ° C. It is also called activated alumina and aluminum glue in industry. Oxygen ions in its structure are approximately tightly packed in the center of the cubic face, and Al3 + is irregularly distributed in the octahedral and tetrahedral voids surrounded by oxygen ions.

　　Alumina use

　　The uses of alumina generally have the following categories:

　　1. For refractory materials, such as: refractory board, fire insulation brick, heat insulation;

　　The refractory material is built in the shell of the smelting furnace, which plays the role of heat insulation and heat preservation for the smelting furnace, and also protects the shell of the smelting furnace from being melted by the high temperature in the smelting furnace. Refractory bricks are mainly used in the metallurgical industry, such as refractory bricks for steel kiln masonry.

　　2. For abrasives, divided into ordinary abrasives and special abrasives

　　The crystal form of aluminum emery is suitable for grinding materials and cutting tools because of its high hardness; special abrasives should be used for stainless steel polishing and grinding.

　　3. Ceramic use: Due to the high mechanical strength of alumina ceramics, high insulation resistance, high hardness, wear resistance, corrosion resistance and high temperature resistance, it can be used as electronic ceramics, such as vacuum devices, circuit substrates, spark plugs Insulated porcelain, etc.

　　A. Use high strength and hardness as structural ceramics, such as abrasives, knives and scrapers for the paper industry, and textile porcelain parts;

　　B. With its good chemical stability, it can be used as chemical and biological ceramics, such as artificial joints, crucibles;

　　C. Fine ceramics: mechanical bearings;

　　D. Ceramic coating.

　　4. Glaze: sanitary ware.

　　5. Gem material, laser material.

　　The main component of alumina

　　Alumina is obtained by chemically treating bauxite raw materials to remove oxides such as silicon, iron, and titanium. It is an alumina raw material with high purity. The Al2O3 content is generally above 99%. The mineral phase is composed of 40% to 76% γ-Al2O3 and 24% to 60% α-Al2O3. γ-Al2O3 can be converted into α-Al2O3 (corundum) at 950-1200 ° C, and significant volume shrinkage occurs at the same time.

　　Factors Affecting the Adsorption Properties of Alumina

　　1. Chloride ion and sulfate ion;

　　2. The initial fluorine concentration of raw water: the higher the initial fluorine concentration, the larger the adsorption capacity, and the main factors affecting the adsorption function of activated alumina;

　　3. Particle size: The smaller the particle size, the higher the adsorption capacity, but the smaller the particle size, the lower the particle strength, which affects its service life;

　　4. Alkalinity of raw water: the concentration of bicarbonate in raw water is high, and the adsorption capacity will decrease;

　　5. PH value of raw water: When the pH value is greater than 5, the lower the pH value, the higher the adsorption capacity of activated alumina, and the main factors affecting the adsorption function of activated alumina;

　　6. The influence of arsenic: Alumina has an adsorption effect on arsenic in water, and the accumulation of arsenic on activated alumina reduces the adsorption capacity of fluoride ions, and makes it relatively difficult to elute arsenic ions during regeneration.