How does bauxite become alumina?

Bauxite actually refers to the generic name of ore that can be used industrially and consists mainly of gibbsite, boehmite, or dicalite. Bauxite is the most important ore resource in current alumina production. More than 99% of alumina in the world is produced from bauxite.

The method of producing alumina from bauxite can be roughly divided into four categories, namely alkali method, acid method, acid-base combination method and thermal method. However, almost all of them used in industrial production are alkali methods. This article will briefly introduce the production methods of these four aluminas. The Bayer process is the most widely used in the alkali process. Therefore, the Bayer process is described in detail in this paper.

First, the alkaline method

Alkali process flow text description:

Alkali production of alumina uses alkali (NaOH or Na2CO3) to treat the aluminum ore and converts the alumina in the aluminum ore to a sodium aluminate solution. Impurities such as iron and titanium in the ore and most of the silicon form insoluble compounds. The insoluble residue (usually containing a large amount of iron oxide, red, and customarily called red mud) is separated from the solution and washed. After discarded or comprehensively used to recover the useful components. The pure sodium aluminate solution is analyzed under the appropriate conditions to separate aluminum hydroxide, which is separated from the mother liquor, washed and then calcined to obtain an alumina product. The decomposing mother liquor can be recycled for processing another batch of ore.

Alkali production of alumina is divided into Bayer process, sintering process and Bayer process - sintering method and other processes.

1 Bayer process

The Bayer process is a chemical process widely used in the industry to produce alumina from bauxite. Submitted by KJ Bayer in 1889-1892, it has had many improvements over a hundred years. It is suitable for the treatment of low-silica-alumina, especially when dealing with trihydrate bauxite, it has the characteristics of simple process, convenient operation, high product quality and high economic benefit.

Basic principle of Bayer process: The alumina hydrate in bauxite is converted into sodium aluminate with concentrated sodium hydroxide solution, and aluminum hydroxide is re-precipitated by diluting and adding aluminum hydroxide seeds. The remaining sodium aluminate solution is also The mother liquor was re-used to process the next batch of bauxite to achieve continuous production. The following figure shows the basic flow chart for the production of alumina by the Bayer process. Each plant may have slightly different process flow due to different conditions, but in principle, there is no essential difference between them.

The basic process flow from Bayer process can be divided into the following major production processes: raw ore slurry preparation, high pressure dissolution, dilution of dissolved ore slurry, separation and washing of red mud, seed decomposition, aluminum hydroxide grading With the washing, aluminum hydroxide roasting, mother liquor evaporation and a water soda causticization, the specific process as shown below.

a, raw slurry preparation

Raw slurry preparation is the first step in alumina production. The so-called raw ore pulp preparation is to prepare raw materials used for the production of alumina by the Bayer process, such as bauxite, lime, sodium aluminate solution, etc., and prepare a raw ore pulp that is dispersible and dispersed in accordance with the dissolution requirements in a certain proportion.

b, high pressure dissolution

Dissolution is one of the two main processes for the production of alumina by the Bayer process. The purpose of the dissolution is to dissolve the alumina hydrate in the bauxite into a sodium aluminate solution. The dissolution effect has a direct impact on the technical and economic indicators of Bayer's production of alumina.

c. Dilution of dissolved pulp and separation and washing of red mud

In order to promote the decomposition of sodium aluminate solution, it is necessary to dilute the dissolved ore slurry, and at the same time reduce the viscosity of the sodium aluminate solution so as to facilitate the sedimentation separation of the red mud. Since the separated red mud is accompanied by a part of sodium aluminate solution, the red mud should be washed in order to reduce the loss of Al2O3 and Na2O and environmental protection.

d, seed decomposition

The seed crystal decomposition is to reduce the temperature of the sodium aluminate solution, increase its degree of supersaturation, add aluminum hydroxide as a seed crystal, and stir it to precipitate aluminum hydroxide. It is another key process for the production of alumina by the Bayer process. This process has a major impact on the product's output, quality, and the entire plant's technical and economic indicators. The seed crystals are released to obtain aluminum hydroxide, and at the same time, a mother liquor with a relatively high causticity is obtained as a circulating mother liquor for dissolving bauxite, thereby constituting a closed circuit for the production of alumina by the Bayer process.

e. Separation and washing of aluminum hydroxide

The aluminum hydroxide slurry obtained after the seed crystal is decomposed must be separated to obtain the desired aluminum hydroxide and seed mother liquor. The aluminum hydroxide obtained after the separation was returned to the process for seeding without washing, and the remaining part was washed to recover aluminum oxide and sodium oxide accompanying the aluminum hydroxide to become a product of aluminum hydroxide. The seed solution is returned to the process for reuse.

In order to achieve separation of aluminum hydroxide and mother liquor, different methods can be used, such as sedimentation or filtration. The sedimentation method with large slurry to solid ratio can be used, and the liquid-solid ratio can be filtered. Since the liquid to solid ratio of the slurry affects the filtration efficiency, the aluminum hydroxide slurry is first concentrated and then separated by filtration.

f, calcination of aluminum hydroxide

The calcining is to remove aluminum hydroxide and crystal water from the aluminum hydroxide at a high temperature, and transform the crystal form of the aluminum hydroxide to obtain a process of alumina that meets specific requirements. Therefore, many physical properties of alumina, especially the specific surface area, the α-Al 2 O 3 content, the angle of repose, the density, etc., are mainly determined by the calcination conditions. Granularity and strength are also strongly related to calcination conditions. The calcination process also affects the content of impurities (mainly SiO2) in the alumina product.

The quality indicators of calcined products include: chemical purity, burn reduction, α-Al2O3 content, particle size, and repose angle. The technical and economical indicators of the calcination process include calcining temperature, fuel consumption, and production.

g, the evaporation of the seed mother liquor

The main purpose of evaporation is to eliminate excess moisture in the process, maintain the balance of liquid in the circulating system, and evaporate and concentrate the mother liquor to meet the Bayer's method for preparing bauxite. Excluding water from the red mud and calcined aluminum hydroxide, excess water in the process is eliminated by the evaporation process.

h. Causticization of sodium carbonate monohydrate

Bauxite contains a small amount of carbonate (such as limestone, siderite, etc.), and the lime added when the bauxite is dissolved also contains a small amount of limestone due to incomplete calcination. Carbonate reacts with high concentrations of caustic solution to produce sodium carbonate. NaOH in sodium aluminate solution also absorbs CO2 in the air to produce sodium carbonate. This reaction is called anti-causticization.

2NaOH + CaCO3 (aqueous solution) → Na2CO3 + Ca (OH) 2 (aqueous solution)

The caustic alkali produced in the Bayer process, due to the back causticization produced during the leaching process and the absorption of CO2 in the air by the sodium aluminate solution, is converted into about 2% of the alkalinity of the carbonates. These carbonates are solid phase in the evaporation process. Sodium carbonate monohydrate precipitated. To reduce caustic consumption, sodium carbonate is treated to recover caustic. The following is a caustic reaction of a water soda.

Na2CO3·H2O+Ca(OH)2 →2NaOH+CaCO3+H2O

The production of 1t alumina by the Bayer process generally requires 1.7 to 3.4 tons of ore, 60 to 150 kg of alkali (NaOH), 200 to 350 kW·H of electricity, and a total energy consumption of 7.4 to 32.6 GJ.

2 sintering method

The basic principle of the soda lime sintering method is to convert the oxides in the burden to high temperature sintering to convert sodium aluminate (Na2O·Al2O3), sodium ferrite (Na2O·Fe2O3), calcium orthosilicate (2CaO·SiO2) and titanic acid. When calcium (CaO·TiO 2 ) is dissolved in water or dilute alkali solution, sodium aluminate is dissolved into the solution, sodium ferrite is hydrolyzed into NaOH and Fe 2 O 3 ·H 2 O precipitates, and calcium orthosilicate and calcium titanate are not dissolved into sludge, and mud is separated and removed. After the slag, sodium aluminate solution is obtained, and then CO2 is introduced for carbonation decomposition, Al(OH)3 is precipitated, and the carbon split mother liquor (the main component is Na2CO3) is concentrated by evaporation and can be returned to the batch sintering and recycled. Al(OH)3 is calcined as product Al2O3.

Soda lime sintering method features: a suitable for low A / S mine, A / S3 ~ 6; b complex process, high energy consumption, high cost; c product quality than Bayer process is low.

3 joint method

Bayer process and soda lime sintering process are currently the main methods for industrial production of alumina. They have their own advantages and disadvantages and scope of application. When the scale of production is large, the Bayer process and the sintering process can be used to combine the advantages of the two methods, eliminating their shortcomings, achieving better economic results than a single method, and making full use of aluminum. Mineral resources. The joint method can be divided into three basic processes: parallel connection, series connection and hybrid connection. It is mainly applicable to low-grade bauxite mines with A/S 7-9.

The following table compares the three processes of the alkaline process.

China's bauxite resources are characterized by low-grade, high-aluminum and high-silica diaspore. The production methods used are sintering method and joint method. Bayer method in combination method (high dissolution temperature and high concentration of caustic soda). It is also different from the Bayer process for the treatment of gibbsite-type bauxite in foreign countries (low dissolution temperature and low concentration of caustic soda). Therefore, compared with foreign countries, alumina production in China is energy-intensive and costly.

Second, acid method

That is, the aluminum-containing raw material is treated with a mineral acid such as nitric acid, sulfuric acid, or hydrochloric acid to obtain an acidic aqueous solution of the corresponding aluminum salt. These aluminum salts or hydrate crystals (by evaporative crystallization) or basic aluminum salts (hydrolysis crystallization) are then precipitated from the solution. It is also possible to neutralize these aqueous aluminum salt solutions with alkali to precipitate them as aluminum hydroxide. Aluminum oxide is obtained by calcining aluminum hydroxide, various aluminum salt hydrates, or basic aluminum salts.

Three, acid and alkali combined method

First, the acid method is used to prepare impure aluminum hydroxide containing titanium, iron, and other impurities from the high-silica-alumina, and then it is treated with the alkali method (Bayer method). The essence is to remove silicon by acid method and remove iron by alkaline method.

Fourth, thermal method

Suitable for processing high-silicon high-iron-iron-aluminum ore, the essence of which is the reduction and smelting of ore in an electric furnace or blast furnace. At the same time, silicon-iron alloy (or pig iron) and alumina-containing slag are obtained. The two are differentiated by density and then alkalinized. Oxidation is extracted from the slag.