Melting method of aluminum

Thermal reduction method can be used to smelt aluminum, but the cost is too high.

Electrolysis is used in industrial aluminum smelting. The main principle is Hall-Elu aluminum electrolysis: pure alumina is used as raw material to produce aluminum by electrolysis. Because pure alumina has a high melting point (about 2045℃), it is difficult to melt. Therefore, molten cryolite (Na3AlF6) is used as a flux in industry, so that alumina can be dissolved in liquid cryolite at about 1000℃ and become a melt of cryolite and alumina.

A comprehensive introduction is as follows:

Production and processing of aluminum

There are four links in the production process of aluminum to form a complete industrial chain: aluminum mining-alumina production-electrolytic aluminum smelting-aluminum processing production.

Generally speaking, two tons of aluminum ore produce one ton of alumina; Two tons of alumina produce one ton of electrolytic aluminum.

(A) the production method of alumina

So far, many methods have been proposed to extract alumina from aluminum ore or other aluminum-containing raw materials.

Due to technical and economic reasons, some methods have been eliminated, and some are still in the experimental research stage.

The production methods of alumina can be divided into four categories, namely, alkali method, acid method, acid-base combination method and thermal method.

At present, only alkali method is used in large-scale industrial production.

Bauxite is the most important aluminum resource in the world, followed by alunite, nepheline and clay.

At present, in the world alumina industry, except for some alumina produced by Russian nepheline, almost all alumina in the world is produced by bauxite.

Bauxite is an ore mainly composed of gibbsite, diaspore or diaspore.

Up to now, all bauxite resources available for alumina production in China are diaspore bauxite.

The content of alumina in bauxite varies greatly, from about 30% low to more than 70% high.

The main impurities in bauxite are silicon oxide, iron oxide and titanium oxide besides alumina.

In addition, it also contains a small amount or trace of calcium and magnesium carbonate, potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur and other compounds and organic matter.

Although the content of gallium in bauxite is small, it will gradually accumulate in circulating mother liquor during alumina production, so it can be effectively recovered and become the main source of gallium production.

One of the main indicators to measure the quality of bauxite is the ratio of alumina content to silica content in bauxite, commonly known as aluminum-silicon ratio.

When producing alumina by alkali method, aluminum ore is treated with alkali (NaOH or Na2CO3) to convert alumina in the ore into sodium aluminate solution.

Impurities such as iron, titanium and most of silicon in the ore become insoluble compounds.

Insoluble residue (red mud) is separated from the solution, washed and discarded or comprehensively treated to recover useful components.

Pure sodium aluminate solution can be decomposed to separate out aluminum hydroxide, and alumina products can be obtained after separation, washing and calcination.

The decomposed mother liquor is recycled to treat another batch of ore.

The processes of producing alumina by alkaline method include Bayer process, sintering process and Bayer-sintering combined process.

Bayer process was invented by Austrian chemist K·J· Bayer in 1889 ~ 1892 to extract alumina from bauxite.

Over the past 100 years, there have been many improvements in technology, but the basic principles have not changed.

In order to commemorate this great contribution of Bayer, this method has always used the name Bayer method.

Bayer process includes two main processes.

The first is to dissolve alumina from bauxite under certain conditions (the term used in alumina industry is leaching).

The following is the same) process, and then the process of dissolving aluminum hydroxide from supersaturated sodium aluminate solution, which are two patents put forward by Bayer.

The essence of Bayer process is to extract alumina from bauxite by hydrometallurgy.

In the process of alumina production by Bayer process, silicon-containing minerals will cause the loss of Al2O3 and Na2O.

In Bayer process, bauxite, after crushing, enters wet grinding with lime and circulating mother liquor to make qualified pulp.

After pre-desilication, the pulp is preheated to the dissolution temperature for dissolution.

After the dissolved pulp is cooled by self-evaporation, it enters the processes of dilution, sedimentation and red mud separation (dissolved solid residue).

The secondary steam generated in the process of self-evaporation is used for preheating pulp.

The settled and separated red mud is washed into the red mud yard, and the separated crude liquid (sodium aluminate solution containing solid suspended matter, the same below) is sent to the leaf filter.

The crude liquid after removing most of the suspended solids by leaf filtration is called * * * liquid.

* * * enters the decomposition process, and performs seed crystal decomposition to obtain aluminum hydroxide.

After the decomposed aluminum hydroxide is classified, separated and washed, a part of it is returned to the seed crystal decomposition process as a seed crystal, and the other part is roasted to obtain an alumina product.

The decomposed mother liquor separated after seed crystal decomposition is evaporated and returned to the dissolution process to form a closed cycle.

Alumina is obtained by roasting aluminum hydroxide.

Different types of bauxite require different dissolution conditions.

The gibbsite type bauxite can be leached well at 105℃, while the diaspore type bauxite is leached faster at 200℃, while the diaspore type bauxite must be leached at a temperature higher than 240℃, and its typical industrial leaching temperature is 260℃.

The dissolution time is not less than 60 minutes.

Bayer process for treating bauxite with high Al-Si ratio has the advantages of simple process, high product quality and far better economic effect than other methods.

When it is used to treat gibbsite bauxite which is easy to dissolve out, its advantages are more prominent.

At present, more than 90% of alumina and aluminum hydroxide produced in the world are produced by Bayer process.

Due to the particularity of bauxite resources in China, about 50% of alumina in China is produced by Bayer process at present.

The combined process of Bayer process and sintering process is called combined production process.

The combination mode can be divided into parallel combination mode, series combination mode and mixed combination mode.

What method is used to produce alumina mainly depends on the grade of bauxite (that is, the aluminum-silicon ratio of ore).

From the general technical and economic point of view, when the aluminum-silicon ratio of ore is about 3, sintering method is usually used; Bayer process can be used for ores with Al/Si ratio higher than 10. When the grade of bauxite is between the two, the combined method can give full play to the respective advantages of Bayer process and sintering process and obtain better technical and economic indicators.

At present, the annual output of alumina in the world is about 55 million tons, and that in China is about 6.8 million tons.

(2) Production methods of primary aluminum, aluminum alloy and aluminum material.

At present, the only method for industrial production of primary aluminum is Hall-Elu aluminum electrolysis.

It was invented in 1886 by Hall of America and Elou of France.

Hall-Elu aluminum electrolysis is an electrolyte composed of alumina as raw material and cryolite (Na3AlF6) as flux. At 950-970℃, alumina in electrolyte melt is decomposed into aluminum and oxygen by electrolysis. Aluminum precipitates in liquid phase at the carbon cathode, and oxygen escapes as carbon dioxide gas at the carbon anode.

Each ton of primary aluminum can produce 1.5 tons of carbon dioxide, and the comprehensive power consumption is about 15000kwh.

Industrial aluminum electrolytic cells can be generally divided into three types: side-inserted anode self-baking cell, up-inserted anode self-baking cell and prebaked anode cell.

Self-baking cell technology is gradually being eliminated because of its high power consumption in electrolysis process, which is not conducive to environmental protection

At present, the annual output of primary aluminum in the world is about 28 million tons, and that in China is about 7 million tons.

If necessary, the primary aluminum obtained by electrolysis can be refined to obtain high purity aluminum.

At present, the main production method of aluminum alloy is melt blending.

Because aluminum and its alloys have excellent machinability, plates, strips, foils, tubes, wires and other profiles are produced by forging, casting, rolling, stamping and pressing.