Calcined Alumina

1. Calcined Alumina manufacturing method

Calcined alumina is processed from bauxite through alumina calcining kiln and other equipment. Calcined alumina mainly includes α-alumina and γ-alumina, which can be extracted from bauxite in industry.

Bauxite (Al2O3 · H2O and Al2O3 · 3H2O) is the main mineral of aluminum in nature. After crushing, it is impregnated with high-temperature calcined sodium oxide solution to obtain sodium aluminate solution. Filter and remove the residue, cool the filtrate and add calcined alumina crystal. After stirring for a long time, the sodium aluminate solution will decompose and precipitate aluminum hydroxide. The precipitate is separated and washed, and then calcined at 950-1200 ℃, will get α-alumina powder, mother liquor can be recycled.

This method was invented by Austrian scientist K · J · Bayer in 1888. Today, it is still the main method for industrial production of alumina, known as "Bayer method".

At present, alumina produced by Bayer process accounts for more than 90% of the total output in the world. Most of alumina is used to make metal aluminum and less than 10% is used for other purposes.

2. types of Calcined Alumina

2.1 α-alumina

In the lattice of α-alumina, the oxygen ions are hexagonal and closely packed, and Al3 + is symmetrically distributed in the octahedral coordination center surrounded by oxygen ions. The lattice energy is very large, so the melting point and boiling point are very high. α-alumina is insoluble in water and acid.

It is also known as aluminum oxide in industry. It is the basic raw material for making metal aluminum. It is also used for making all kinds of refractory bricks, refractory crucibles, refractory pipes and high temperature resistant experimental instruments. It can also be used as abrasives, flame retardants, fillers, etc.

High purity α-alumina is also the raw material for the production of brown fused alumina and white fused alumina, artificial ruby and sapphire. It is also used to produce the board base of modern large-scale integrated circuits.

2.2 γ-alumina

γ Type calcined alumina is prepared by dehydration of aluminum hydroxide at a low temperature of 140-150 ℃. It is also called activated alumina and aluminum glue in industry. In its structure, the oxygen ions are close packed at the center of the cube, and Al3 + is irregularly distributed in the octahedral and tetrahedral voids surrounded by oxygen ions.

γ-alumina is insoluble in water and can be dissolved in strong acid or alkali solution. When it is heated to 1200 ℃, it is completely converted into α-alumina. γ-alumina is a porous material with an internal surface area of hundreds of square meters per gram, high activity and strong adsorption capacity.

Industrial products are usually colorless or slightly pink cylindrical particles with good pressure resistance. It is commonly used as adsorbent, catalyst and catalyst carrier in petroleum refining and petrochemical industry. In industry, it is a deacidification agent for transformer oil and turbine oil. It is also used for chromatographic analysis. It is a neutral strong desiccant in the laboratory, and its drying capacity is no less than that of phosphorus pentoxide. It can be regenerated and reused after heating below 175 ℃ for 6-8h.

3. Why should alumina be calcined?

Because alumina has many crystal forms, of which α-alumina has high hardness and good chemical corrosion resistance, so it has a wide range of applications. α-alumina must be calcined at more than 1000 degrees in order to convert low-temperature alumina into α Crystalline alumina. So, to α-alumina is calcined at high temperature.

High temperature calcined alumina powder takes industrial aluminum hydroxide or industrial alumina as raw material and calcines into stable crystal form at appropriate temperature α-alumina products. To calcine α-alumina powder is made from alumina by ball milling.

These product has high melting point, excellent mechanical strength, hardness, high resistivity and thermal conductivity. It can be widely used in industries such as electronic equipment, structural ceramics, refractories, wear-resistant materials, polishing materials and so on.

However, not all alumina should be calcined at high temperature. Other crystalline forms of alumina also have their special, such as γ-alumina has high porosity and is often used as catalyst carrier or desiccant.

4. Classification of high temperature calcined alumina powder

The production technology of aluminum oxide powder is getting higher and higher, and aluminium oxide powder below 1 micron can be produced. Nano alumina liquid can be dissolved in any aqueous or oily solvent. At the same time, it can be used as all kinds of glass coating materials, gemstones, precision instrument materials and so on.

Nano alumina is a layer of deep processing screening of alumina, adding to all kinds of acrylic resin, polyurethane resin, epoxy resin, melamine resin, silicone acrylic emulsion and other resin in the aqueous liquid, adding 5% to 10%, can improve the hardness of the resin, hardness up to 6-8H Completely transparent.

Alumina micro powder products are divided into 325 mesh, 500 mesh, 600 mesh, 800 mesh micro powder and fine crystal micro powder. According to different use needs, five series of products have been formed, including dry pressing and isostatic pressing process series, hot die casting and grouting molding series, refractory series, casting shaping series, alumina structural ceramic granulation powder and so on.

4.1 Dry pressing and isostatic pressing process series.

This series of calcined alumina products are refined by advanced formula and strict process. The calcined alumina product has reasonable particle size distribution, good particle fluidity, loose combination between grains, good grindability and easy sintering. Its porcelain parts are compact, smooth and clean, high mechanical strength and good electrical insulation. It is an ideal raw material for manufacturing special ceramics.

4.2 Hot die casting and grouting molding series.

This series of calcined alumina products have large original grain size, stable performance, good molding performance, small product shrinkage and wide application range. It is especially suitable for hot die casting and grouting molding series. It can be used for various high-temperature resistant porcelain parts (spark plug, etc.), wear-resistant porcelain parts (mortar pump shaft plug, pump lining, impeller, grinding medium ball, etc.), electronic substrate, electronic vacuum tube, etc.

4.3 Refractory series.

The crystal form of the calcined alumina product is stable, which can improve the fire resistance and mechanical properties of the product. It is suitable for the production of amorphous refractories (various high aluminum castables), shaped refractories (corundum bricks, etc.), furnace building materials, etc., meet the requirements of customers for high temperature, excellent thermal shock stability and processing characteristics, and prolong the service life of refractories.

4.4 Casting shaping series.

This calcined alumina product has uniform grains, good filling performance and pouring activity, which can improve the sintering density, high-temperature flexural and compressive strength of castable, and enhance the wear resistance, corrosion resistance, scouring resistance and other properties of castable. It can be used for low cement, ultra-low cement or cementless castables, shaped refractory materials and precision polishing.

4.5 Alumina ceramic granulation powder.

It is composed of automatic grinding, pulping and spray drying. Uniform particle size distribution, good fluidity and moderate strength; The green body has high strength, excellent demoulding performance and low sintering temperature; The product has smooth surface and dense structure. It can meet the process requirements of rapid dry pressing of precision ceramics. It is an ideal raw material for the production of electronic ceramics and structural ceramic components.

I believe that we have a certain foundation for our understanding of alumina and calcined alumina, but we should also be able to see that the processing of alumina into calcined alumina powder is not a simple process that can meet the needs of high-value. We use professional equipment and advanced production experience to obtain qualified and high-quality products.

Our calcined alumina is made of high-quality industrial alumina as raw material and processed by new high-temperature kiln and scientific calcination process. The product has high conversion rate and low sodium content. After being discharged from the kiln, it is manually selected with less impurities and clear classification. It is an ideal raw material for making all kinds of special ceramics, abrasives and polishing materials.