Basic knowledge of aluminum

Date:2017-11-25 Hits:
1. What is aluminum

Aluminum is a silver white metal, which ranks third in the earth's crust next to oxygen and silicon. The density of aluminum is small, only 34.61% of iron and 30.33% of copper, so it is also called light metal. Comparing the same volume of aluminum to steel and copper, the weight of the steel is 2.9 times that of the aluminum, and the weight of the copper is 3.3 times of the aluminum. Therefore, aluminum has become the preferred metal material for the light weight of various facilities.

Aluminum is the world's largest nonferrous metal after production and consumption. It is not only a common production and living material, but also an important strategic material. It has the advantages of portable, conductive, thermal, plastic (easy to stretch, Yi Yanzhan), corrosion resistance (not rust) and so on, so it has become an important raw material in the industries of Electromechanical, electric, aviation, aerospace, shipbuilding, automobile manufacturing, packaging, transportation, daily use and real estate. We have to deal with aluminum every day. At home, we have tableware, cookers, doors and windows, electrical appliances, computers in the office, telephone, stationery, tables and chairs, machine equipment in the factory, and transportation tools, all of which are related to aluminum. From daily necessities to submarines, fighters and even rockets and spaceships, aluminum is everywhere and everywhere. With the rapid development of modern science and technology and the continuous emergence of new industrial fields, the use of aluminum is becoming more and more extensive, increasing demand and playing a more and more important role in the national economy. Among the 124 industries in China, 113 sectors use aluminum products, accounting for 91%. Among them, 96 of the 101 production departments consumed aluminum smelting products or calendering products, accounting for 95%. Now, aluminum and aluminum alloys have been able to replace wood, steel, plastic and other materials, which are widely used to make the various aspects of modern life directly affected. Therefore, at present, all countries in the world are actively developing their own aluminum industry according to their respective conditions.

 Two. Properties of aluminum

The excellent features of aluminum and its alloys are good appearance, light quality, machinability, good physical and mechanical properties and corrosion resistance. It is considered the most economical and practical in many fields of application.

The density of aluminum is only 2.7 x 103 /m3, about 1/3 of the density of steel, copper or brass. Because of its light material, it is often used in the manufacture of vehicle, train, subway, ship, aircraft, rocket, spaceship and other land, sea and air vehicles to reduce the weight of self weight and increase the load. Aluminum is also widely used in military industry.

Under most environmental conditions, aluminum can display excellent corrosion resistance, including air, water (or brine), petrochemistry and many chemical systems.

The surface of aluminum is highly reflective. Radiant, visible, radiant and electric waves can be effectively reflected by aluminum. The surface of anodized and dark anodized surfaces can be reflective and absorbable. The polished aluminum is reflected in a wide wavelength range and thus has a variety of decorative purposes. Reflective functional use.

Aluminum usually shows excellent electrical conductivity, its electrical conductivity is about 2/3 of copper, but because its density is only 1/3 of copper, so compared to equal mass and equal length aluminum wire and copper wire, the electrical conductivity of aluminum is about two times that of copper, and the price is lower and the application cost is low, so it is often used by electric power industry and electricity. The selection of the sub industry. At present, some specific aluminum alloys with high electrical resistivity have also been successfully developed. These alloys can be used in motors such as high turning motors. The thermal conductivity of aluminum is very high, second only to copper. The thermal conductivity of aluminum is 3 times larger than that of iron, which is about 50% to 60% of copper. The thermal conductivity of copper is the best, but the weight of the same size workpiece is much larger than that of aluminum, and the price is much more expensive than aluminum. Therefore, the manufacture of radiator aluminum is still the first choice. The cost performance of aluminum is advantageous to the manufacture of heat exchangers, evaporators, heating appliances, cooking utensils, and cylinder heads and radiators of automobiles.

Aluminum is non-ferromagnetic, which is an extremely important characteristic for the electrical industry and the electronics industry.

Aluminum is not self ignition, which is very important for the industries involved in handling or contacting inflammable and explosive materials. Aluminum is very toxic and is usually used to make containers for food and beverages. In recent years, aluminum foil has been widely used in cigarette, medicine and food packaging, and has become an important material in the packaging industry.

The natural surface state of aluminum has a pleasant appearance. It is soft and glossy, and can also be coloured or dyed with texture patterns for aesthetic purposes. In modern life, aluminum has been widely used in construction industry and general merchandise. Aluminum also has good sound absorption properties. According to this characteristic, some of the broadcasting rooms, the ceiling of modern large buildings and so on have adopted aluminum.

The plasticity of aluminum is very good. The pure aluminum is soft, not strong, has good ductility, can be pulled into filaments and rolled into chaff. It has good machinability and is widely used in wire, cable manufacturing and radio industry and packaging industry. Adding a small amount of aluminum in some metals can greatly improve its properties. If a small amount of magnesium and copper is added to aluminum, a tough aluminum alloy can be made; also like bronze aluminum (aluminum 4% ~ 15%) alloy, it has high strength corrosion resistance, the hardness is close to low carbon steel, and there is a hard and dark metal luster, often used in Yu Zhubao's ornaments and construction industry, for machine parts and tools, for pickling. Equipment and other equipment that contact with dilute sulfuric acid, hydrochloric acid and hydrofluoric acid can make electric welding machine brush and handle, heavy gear and worm wheel, metal molding mould, machine tool guide, tool without spark, non magnetic chain, pressure vessel, heat exchanger, compressor blade, ship screw pulp and anchor. A magnesium alloy is made in aluminum, which is made of magnesium alloy. Its hardness is much larger than pure magnesium and aluminum, and its light quality is retained. It is often used to make aircraft's fuselage, rocket arrows, doors and windows, beautify the living environment and make ships. Adding other metal components in aluminum can become all kinds of aluminum alloys. Some aluminum alloys even exceed structural steels in strength.

The connection of aluminum is also easy, using a variety of methods, including welding, resistance welding, hard welding, soft welding, bonding, and mechanical methods such as riveting and plugs.

Compared with iron, aluminum is not easy to rust and prolong its service life. Therefore, aluminum has a very high recyclability, and the characteristics of recycled aluminum are almost the same as that of native aluminum, which makes aluminum the darling of environmentalists.

   Three, the main use of aluminum

Because of its excellent physical properties, aluminum has been widely used in various sectors of national economy and national defense industry. Aluminum is light structural material with light weight and strong strength. All kinds of vehicles such as sea, land and air, especially aircraft, missile, rocket, artificial earth satellite, are all used in a large amount of aluminum. The aluminum amount of a supersonic aircraft accounts for 70% of its own weight, and more than 10% of the total weight of a missile is aluminum. Aluminum and aluminum are used. All kinds of vehicles made by alloy can reduce energy consumption, which can save much more energy than the energy consumed by aluminum smelting; aluminum alloys are used in the construction industry to make doors and windows and structural materials; aluminum is used to make solar collectors, which can save energy; in power transmission, the amount of aluminum is the first and 90% high voltage wire. It is made of aluminum; in the food industry, from the storage tank to the canned box and to the beverage container, most of the aluminum can be made of aluminum; the aluminum powder can be used as a reductant for refractory metals (such as molybdenum) and deoxidizer in steelmaking. Most of the pan, disc and spoon used in daily life are made of aluminum.

Aluminum compounds such as aluminum hydroxide, such as aluminum hydroxide, can be used to prepare aluminum salts, adsorbents, mordant and ion exchange agents, as well as raw materials for porcelain glazes and refractory materials, as acids in medicine, neutralizing gastric acid and treating ulcers; sodium aluminate is used in printing and dyeing fabrics; anhydrous aluminum chloride is an oil industry and organic synthesis. A commonly used catalyst; six hydrated aluminum chloride can be used to prepare deodorants and safety disinfectants; six sodium aluminate (i. e. cryolite) used as an insecticide in agriculture; aluminum phosphide is used as a fumigant for insect killing in a barn, as a fumigant for insect killing in a barn; and aluminum sulfate is used as a paper filling. Aluminum nitrate can be used to tanning leather and making white thermoelectric filament, and also used as a mordant; aluminum silicate is often used for making glass, ceramics, paints and fillers of paint, rubber and plastics. It has a lasting effect and can form a protective membrane. It has a strong protective effect on the gastric mucosa and the action of making acid, helps to regenerate the mucous membrane, promotes the healing of the ulcer, and has low toxicity. It is a good therapeutic agent for gastrointestinal ulcer.

In recent years, some new aluminum compounds have been developed, such as three oxidation two aluminum, alkyl aluminum and nano alumina, which are used as the wear-resistant layer of composite wood floor. With the development of science, people will make more and better use of aluminum and compounds to benefit mankind.

Four, aluminum production and processing

Aluminum has four links in the production process to form a complete industrial chain: aluminum ore mining - alumina production - electrolytic aluminum smelting - aluminum processing production.

In general, two tons of aluminum ore produce one ton of alumina; two tons of alumina produce one ton of electrolytic aluminum.

(1) the production method of alumina

   So far, many methods have been proposed for the extraction of alumina from aluminum ores or other aluminum containing materials. Due to technical and economic reasons, some methods have been eliminated and some are still in the experimental stage. The methods of alumina production can be classified into four types: alkali, acid, acid-base and thermal methods. At present, only alkali method is used for large-scale industrial production.

Bauxite is the most important aluminum resource in the world, followed by alunite, nepheline, clay and so on. At present, the world alumina industry, in addition to the Russian use of nepheline production of some alumina, almost all of the world's alumina is made of bauxite as raw materials.

Bauxite is a kind of ore mainly composed of gibbsite, diaspore or diaspore. So far, all bauxite resources that can be used for alumina production in China are diaspore bauxite.

The content of alumina in bauxite varies greatly, with a low level of only about 30% and a high content of 70%. The chemical composition of bauxite includes alumina, oxide, iron oxide and titanium oxide. In addition, it 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 compounds. The content of gallium in bauxite is small, but it will accumulate in the circulating mother liquid gradually in the process of alumina production, so it can be effectively recovered and become the main source of gallium production.

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

When alumina is produced by alkali method, the aluminum ore is treated with alkali (NaOH or Na2CO3), so that the alumina in the ore is transformed into sodium aluminate solution. Iron, titanium and other impurities in the ore and most of the silicon are insoluble compounds. The insoluble residue (red mud) is separated from the solution and washed or discarded or processed to recover useful components. Pure sodium aluminate solution can decompose and separate aluminum hydroxide. After separation, washing and calcination, alumina products can be obtained. The mother liquor is recycled to treat another batch of ore. Alumina production by alkali process includes Bayer process, sintering process and Bayer sintering combined process. Bayer process is a method of extracting alumina from bauxite by Austria chemist Bayer (K. J. Bayer) in 1889~1892. In the past more than 100 years, many improvements have been made in process technology, but the basic principles have not changed. In order to commemorate the great contribution of Bayer, this method has always used the name of Bayer method.

Bayer process includes two main processes. First is the dissolution of 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 digestion of aluminum hydroxide from supersaturated sodium aluminate solution, which is the two patent put forward by Bayer. The essence of Bayer process is to extract alumina from bauxite by means of hydrometallurgy. In the process of alumina production in Bayer process, silicon containing minerals can cause losses of Al2O3 and Na2O.

In Bayer process, bauxite is broken and then enters into wet grinding with lime and circulating mother liquor to make qualified pulp. The pulp is preheated to the dissolution temperature after pre desilication. After dissolving, the pulp is then evaporated and evaporated to enter the settling and separating process of dilution and red mud (solid waste after dissolution). The two steam generated by the self evaporation process is used for pre heating of slurry. After separation and separation, the red mud is washed into the red mud storage yard, and the crude liquid separated (sodium aluminate solution containing solid floating matter) is sent to the leaf filter.  The coarse liquid is called semen after removing most of the planktonic matter through leaf filtration. The semen enters the decomposition process and is decomposed by crystal seeds to obtain aluminum hydroxide. After decomposition and washing, the decomposed aluminum hydroxide is used as a seed to return to the seed decomposition process, and the other part is calcined to obtain alumina products. The decomposed mother liquor separated from the crystal seed is evaporated back to the dissolution process to form a closed cycle. Alumina is obtained by roasting aluminum hydroxide.

   Different types of bauxite need different dissolution conditions. The diaspore bauxite can be dissolved well at 105 C. The dissolution rate of a diaspore bauxite can be faster at 200 degrees centigrade, while a diaspore bauxite must be dissolved at a temperature above 240 degrees C, and its typical industrial dissolution temperature is 260. The dissolution time is not less than 60 minutes.

Bayer process is used to treat bauxite with high alumina to silicon ratio. The process is simple, the product quality is high, and its economic effect is much better than other methods. The advantages of the bauxite bauxite are also outstanding. 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.

The combination of Bayer process and sintering process is called the combined process of two processes. The combined method can be divided into parallel connection method, series connection method and mixed combination method. The way to produce alumina is determined mainly by the grade of bauxite, i.e. the ratio of aluminum to silicon. From the general technical and economic point of view, the ore aluminum and silicon ratio is about 3 or so, and the Bayer method can be used for the ore with the ratio of aluminum and silicon to higher than 10. When the grade of the bauxite is between the two, the combined method can be used to give full play to the advantages of the Bayer process and the sintering method, so as to achieve better technical and economic indicators.

At present, the annual output of alumina in the world is about 55 million tons, and the output of alumina in China is about 6 million 800 thousand tons.

(two) production methods of raw aluminum, aluminum alloy and aluminum materials

At present, the only way to produce raw aluminum in industry is the Holzer aluminum electrolysis process. Invented by Holzer in the US and France in 1886. Holzer Alu aluminum electrolysis is an electrolyte composed of alumina as raw material and Na3AlF6 as flux. Under the condition of 950-970 C, the alumina in the electrolyte melt is decomposed into aluminum and oxygen by electrolysis. The aluminum is precipitated in the liquid form in the carbon cathode and oxygen is dissolved in the carbon anode in the form of carbon dioxide gas. Out. Each production of one ton of raw aluminum can produce 1.5 tons of carbon dioxide, and the total power consumption is about 15000kwh.

Industrial aluminum electrolysis cells can be generally divided into three types: side insert anode self baking tank, self inserted anode anode baking oven and prebaked anode slot. Since the power consumption of the self baking cell technology is high in the process of electrolysis and is not conducive to the protection of the environment, the technology of self baking is being phased out. At present, the annual output of the world's original aluminum is about 28 million tons, and the annual output of China's original aluminum is about 7 million tons.

When necessary, the raw aluminum obtained by electrolysis can be refined to obtain high purity aluminum. The current method of aluminum alloy production is mainly melt blending. As aluminum and its alloys have excellent machinability, forging, casting, rolling, punching, pressing and so on are used to produce plates, belts, foils, pipes and wires.