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I. Overview
The primary aluminum produced in the aluminum electrolytic cell differs greatly in quality. In addition, it also contains some metal impurities, gas and non-metallic solid inclusions. The task of aluminum ingot casting is to increase the utilization of low-grade aluminum liquid and remove impurities as much as possible. The impurities in the primary aluminum can be classified into the following three categories: the first type is a metal element such as iron, silicon, copper, calcium, magnesium, titanium, vanadium, boron, nickel, zinc, gallium, tin, lead, phosphorus, etc. The main elements are iron and silicon; the second type is non-gold dust solid inclusions, Al 2 O 3 , AlN and Al 4 C 3 ; the third type is gas, H 2 , CO 2 , CO, CH 4 , N 2 , The main one is H2. At 660 C, approximately 0.2 cm 3 of hydrogen was dissolved in 100 g of aluminum liquid. The solubility of the gas in the aluminum solution increases with increasing temperature. The aluminum liquid sucked from the electrolytic cell is subjected to purification treatment to remove a part of impurities, and then cast into a commercial aluminum ingot (99.85% A1). Containing 99.996% Al pure aluminum (aluminum wire φ2mm, hard pull), the resistivity is 2.668×10-8 Ω·m. If there is an impurity element in pure aluminum, the electrical resistivity increases. . The most influential are chromium, vanadium, manganese, lithium, and titanium. The less influential are indium, lead, zinc, cadmium, tin, antimony, and iron.
1. Balance of impurity elements in aluminum
In the industrial alumina produced from the bauxite by the Bayer process, the content of impurities is greatly reduced relative to the raw material bauxite. In addition to the base brought from the lye, the total amount of the analytical value of the impurity element is usually less than 1%. The main impurities are SiO2 and Fe2O3. In addition to the impurities brought by the alumina to the electrolytic cell, the carbon anode and the flux cryolite also bring a lot of impurities. The impurities brought by the carbon anode are mainly iron and silicon, as are cryolite.
If all the impurity elements of the raw material are precipitated in the primary aluminum, the grade of the obtained aluminum is only 99.7% Al. However, the actual aluminum produced has a higher grade of 99.8% Al. This difference is mainly due to evaporation of impurity elements. Iron, titanium, phosphorus, zinc and gallium are the majority of alumina, while silicon and vanadium are the majority of carbon anodes. The impurity element from the flux is mostly phosphorus, accounting for about 20% of the total amount of phosphorus, and the remaining silicon, iron, titanium and vanadium are few.
The balance sheet expenditure, silicon and iron are more than the amount brought from raw materials, of which silicon is more than 60%, iron is more than 37%. The lining materials of the electrolytic cell, such as high ash bottomed carbon blocks and carbon paste and refractory materials, are another important source of these impurity elements. In addition, iron is also in equilibrium due to erosion of the operating tool and the cathode steel rod. The remaining elements are close to balance.
The amount of impurity elements in the distribution of primary aluminum and exhaust gas is different. The largest amount of evaporation is phosphorus, accounting for 72% of total revenue, vanadium accounting for 64.4%, iron accounting for 62.4%, titanium accounting for 57.7%, gallium accounting for 49.6%, and zinc accounting for 19.7%. The smallest is silicon, which accounts for only 13.3% of total revenue. The reason for this is that: 1 silicon and zinc are present in the electrolyte in the form of compounds that are more difficult to evaporate or even evaporate, such as SiO 2 , ZnO or ZnF 2 . Silicon and zinc are apparently accumulated in the aluminum liquid. The extent to which aluminum is contaminated with silicon and zinc is primarily determined by the total amount of silicon and zinc compounds supplied in the material balance. In this case, the collection efficiency of the tank cover does not matter. 2 Iron, gallium, titanium and nickel are present at least partially in the form of volatile compounds in the system. These compounds are probably produced after entering the electrolyte. Possible compounds are Fe(CO) 5 , Ni(CO) 4 , TiF 3 , TiF 4 and GaF3 and the like. If the collection efficiency of the trough is increased, the quality of the aluminum will be affected to some extent. 3 Vanadium and phosphorus are only present in the form of volatile compounds. Possible compounds are firstly fluorides (VF3 and PF3) and phosphorus pentoxide (P 2 O 5 ). Since the increase of phosphorus content in the electrolyte will affect the current efficiency, the increase of the amount of vanadium in the aluminum will reduce the conductivity of the aluminum, so it can be expected that the collection efficiency of the groove cover will bring about the quality of the original aluminum and the best production effect. damage.
2. Classification of aluminum ingots
Aluminum ingots are divided into aluminum ingots, high-purity aluminum ingots and aluminum alloy ingots according to different compositions. They can be divided into strips, round ingots, ingots, T-shaped ingots, etc. according to their shape and size. a common aluminum ingot;
Aluminum ingot for remelting - 15kg, 20kg (≤99.80% Al):
T-shaped aluminum ingot - 500kg, 1000kg (≤ 99.80% Al):
High-purity aluminum ingot - l0kg, 15kg (99.90% ~ 99.999% Al);
Aluminum alloy ingot -10kg, 15kg (Al--Si, Al--Cu, Al--Mg);
Ingots - 500 ~ 1000kg (for board);
Round ingot - -30 ~ 60kg (for wire drawing).
3. Aluminum ingot casting process
Aluminum - slag - check - ingredients - furnace - scouring - casting - remelting aluminum ingot - finished product inspection - finished product inspection - storage
Aluminum - slag - check - ingredients - furnace - scouring - casting - alloy ingots - casting alloy ingots - finished product inspection - finished product inspection - storage
Second, the original aluminum purification
The aluminum liquid sucked from the electrolytic cell contains various impurities, and therefore needs to be purified before casting. In the industry, purification methods such as clarification, flux, and gas are mainly used, and some methods are used for purification by directional solidification and filtration methods.
Flux purification
Flux purification is to use a flux added to the aluminum liquid to form a large number of fine droplets, so that the oxides in the aluminum liquid are wetted by the droplets to adsorb and dissolve, and a new droplet is formed to rise to the surface, and after cooling, the scum is removed.
The flux for purification is composed of a salt having a low melting point, a small density, a small surface tension, a large activity, and a strong adsorption capacity for the oxidized slag. When using, first put a small piece of flux into the iron cage, and then insert it into the bottom of the mixing furnace and stir it up. After the fluxing is finished, take out the iron cage and stand still for 5-10 minutes. The surface scum can be removed and cast. The flux can also be removed from the surface to cover as needed.
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