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 analyzed 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 is this:
1 Silicon and zinc exist in the electrolyte in the form of compounds that are difficult to evaporate or even evaporate, such as SiO2, ZnO or ZnF2. 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, TiF3, TiF4, 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 (P2O5). 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. Common aluminum ingots; aluminum ingots 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); ingot--500~1000kg (for board making);
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 of aluminum - slag - check - ingredients - furnace - refining - casting - alloy ingots - casting alloy ingots - finished product inspection - finished product inspection - storage 2, primary 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 cleaning 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 and adsorbed by the droplets, 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, then insert it into the bottom of the mixing furnace and stir it up until the flux is finished, take out the iron cage and stand still for 5~10min. The surface scum can be removed and cast. The flux can also be removed from the surface to cover as needed.
2. Gas purification
Gas purification is a major primary aluminum purification process. The gas used is a mixture of chlorine, nitrogen or chlorine and nitrogen.
(1) Chlorine purification. Previously, reactive gas chlorine was used as a purifying agent (chlorination method). In the chlorination method, a large amount of abnormally fine AlCl3 is formed when chlorine gas is introduced into the aluminum liquid, and the bubbles are sufficiently mixed in the aluminum liquid. Hydrogen dissolved in the aluminum liquid, as well as some mechanical inclusions, are adsorbed on the AlCl3 bubbles and are discharged as the AlCl3 bubbles rise to the surface of the aluminum liquid. Chlorine gas can also chlorinate some elements that are more negatively charged than aluminum, such as calcium, sodium, magnesium, etc., which are separated by chlorine gas to form corresponding chloride. Therefore, the chlorination method is a very effective primary aluminum purification method. The amount of chlorine gas is 500-700g per ton of aluminum. However, because oxygen is toxic and relatively expensive, in order to avoid air pollution and reduce the cost of aluminum ingot production, the chlorination process has been gradually abolished into an inert gas-nitrogen purification method in the modern aluminum industry.
(2) Nitrogen purification method. Also known as the smokeless continuous purification method, alumina balls (418 mm) are used as the filter medium. N2 is directly introduced into the aluminum liquid. The aluminum liquid is continuously fed into the purification furnace, passed through the alumina ball filter layer, and is flushed by nitrogen, so that the non-metallic inclusions and dissolved hydrogen in the aluminum liquid are removed, and then continuously discharged, thereby uniformly distributing the fine nitrogen bubbles. It acts as a purifying agent in the treated aluminum liquid. Nitrogen has no pollution to the atmosphere, and the purification treatment is large. It can process 200~600kg of aluminum liquid per minute, and the amount of aluminum loss caused by the purification process is relatively reduced, so it is widely used now. But it does not remove calcium, sodium, and magnesium from aluminum in the same way as chlorine.
(3) Mixed gas purification method. A mixture of chlorine and nitrogen is used to purify the aluminum liquid, which acts to remove hydrogen and separate oxides on the one hand, and to remove certain metal impurities (such as magnesium) from aluminum. The usual composition is 90% nitrogen + 10% chlorine. . There are also 10% chlorine + 10% carbon dioxide + 80% nitrogen. This effect is better, carbon dioxide can make chlorine and nitrogen diffuse well, which can shorten the operation time.
Ingot casting process
Nowadays, the aluminum ingot casting process generally adopts a casting process, that is, the aluminum liquid is directly poured into the mold, and is taken out after being cooled.
The quality of the product is mainly at this step, and the entire casting process is also based on this process. The casting process is a physical process in which liquid aluminum is cooled and crystallized into solid aluminum ingots.
Continuous casting can be divided into two ways: mixing furnace casting and external casting. Both use a continuous casting machine. The mixing furnace is cast after the aluminum liquid is charged into the mixing furnace and cast by the mixing furnace, and is mainly used for producing aluminum ingots and casting alloys for remelting. The outer casting is cast directly from the lifting bag to the casting machine, mainly in the case where the casting equipment cannot meet the production, or the incoming material quality is too poor to be directly into the furnace. Since there is no external heating source, it is required to have a certain temperature for the lifting bag, generally 690~740°C in summer and 700~760°C in winter to ensure a good appearance of the aluminum ingot.
The mixing furnace is cast, firstly through the ingredients, then poured into the mixing furnace, stirred evenly, and then added flux to refine. The cast alloy ingot must be clarified for more than 30 minutes. After clarification, the slag can be cast. When casting, the furnace eye of the mixing furnace is aligned with the second and third casting molds of the casting machine, so that the flow can be changed and the maneuverability can be changed when the mold is changed. The furnace eye and the casting machine are connected by a launder, and the launder is shorter, which can reduce the oxidation of aluminum and avoid vortexing and splashing. When the casting machine is deactivated for more than 48 hours, the mold is preheated for 4 hours before restarting. The aluminum liquid flows into the mold through the launder, and the oxide film on the surface of the aluminum liquid is removed by a shovel, which is called slag. After the mold is full, the runner is moved to the next mold, and the casting machine is continuously advanced. The mold advances in sequence, and the aluminum liquid is gradually cooled. When it reaches the middle of the casting machine, the aluminum liquid has solidified into an aluminum ingot, and the printer has a melting number. When the aluminum ingot reaches the top of the casting machine, it has completely solidified into an aluminum ingot. At this time, the mold is turned over, the aluminum ingot is released from the mold, and falls on the automatic pick-up trolley, which is automatically stacked and bundled by the stacker to become the finished aluminum. ingot. The casting machine is cooled by water spray, but it must be fed after the casting machine has been turned full. Approximately 8-10 tons of water is consumed per ton of aluminum liquid, and a surface is required for cooling in the summer. The ingot is cast in a flat mold, the solidification direction of the aluminum liquid is from bottom to top, and the upper part is finally solidified, leaving a groove-shaped collapse. The solidification time and conditions of the various parts of the aluminum ingot are not the same, so the chemical composition will also vary, but it is in line with the standard as a whole.
Common defects in remelting aluminum ingots are: 1 vent. Mainly because the casting temperature is too high, the aluminum liquid contains more gas, the surface of the aluminum ingot has many pores (pinholes), the surface is dark, and hot cracks occur in severe cases. 2 slag. The main reason is that the first is that the slag is not clean, causing surface slag inclusion; the second is that the temperature of the aluminum liquid is too low, causing internal slag inclusion. 3 ripples and flash. Mainly because the operation is not fine, the aluminum ingot is too large, or the casting machine is not stable. 4 cracks. The cold crack is mainly because the casting temperature is too low, so that the crystal of the aluminum ingot is not dense, causing looseness and even cracking. Hot cracks are caused by high casting temperatures. 5 components segregation. Mainly caused by uneven mixing during casting of the alloy.
2. Vertical semi-continuous casting
Vertical semi-continuous casting is mainly used for the production of aluminum wire ingots, ingots and various deformed alloys for processing profiles. After the aluminum liquid is mixed, it is poured into the mixing furnace. Due to the special requirements of the electric wire, it is necessary to add titanium and vanadium (wire ingot) in the aluminum liquid before the casting, and the Al-Ti--B alloy is added to the ingot. (Ti5% B1%) was refined. Make the surface tissue fine. High-magnesium alloy plus 2# refining agent, the dosage is 5%, stir evenly. After standing for 30 minutes, the scum can be removed and cast. The casting machine chassis is raised before casting, and the moisture on the chassis is blown off with compressed air. Then raise the chassis into the crystallizer, apply a layer of lubricating oil to the inner wall of the crystallizer, put some cooling water into the water jacket, and put the dry preheated distribution plate, automatic adjusting plug and launder to make the distribution plate The port is located in the center of the crystallizer. At the beginning of casting, press the automatic adjusting plug by hand to block the nozzle, cut the furnace eye of the mixing furnace, and let the aluminum liquid flow into the distribution tray through the launder. When the aluminum liquid reaches 2/5 in the distribution tray, release the automatic The plug is adjusted so that the aluminum liquid flows into the crystallizer, and the aluminum liquid is cooled on the chassis. When the aluminum liquid reaches 30mm in the crystallizer, the chassis can be lowered and the cooling water can be sent. The automatic adjustment plug controls the aluminum liquid to flow into the crystallizer evenly, and keeps the aluminum liquid level in the crystallizer unchanged. The scum and oxide film on the surface of the aluminum liquid should be removed in time. When the length of the aluminum ingot is about 6m, the furnace eye is blocked, the distribution plate is removed, the aluminum liquid is completely solidified, the water supply is stopped, the water jacket is removed, and the cast aluminum ingot is taken out by the monorail crane, and the required size is on the sawing machine. Saw off and then prepare for the next casting.
During casting, the temperature of the aluminum liquid in the mixing furnace is maintained at 690~7l0 °C, the temperature of the aluminum liquid in the distribution tray is maintained at 685-690 ° C, the casting speed is 190~21Omm/min, and the cooling water pressure is 0.147~0.196 MPa. The casting speed is proportional to the wire ingot with a square cross section:
VD=K, where V is the casting speed, mm/min or m/h; D is the side length of the ingot section, mm or m; K is a constant value, m2/h, generally 1.2 to 1.5.
The vertical semi-continuous casting is a sequential crystallization method. After the aluminum liquid enters the casting hole, it starts to crystallize on the chassis and the inner wall of the crystallizer. Since the cooling conditions of the center and the side are different, the crystal forms a form with a low middle and a high periphery. The chassis drops at a constant speed. At the same time, the upper part is continuously injected with aluminum liquid, so that there is a semi-solidification zone between the solid aluminum and the liquid aluminum. Since the aluminum liquid shrinks during condensation, and a layer of lubricating oil is added to the inner wall of the crystallizer, the solidified aluminum decreases with the lowering of the chassis. The crystallizer is withdrawn, and there is a circle of cooling water in the lower part of the crystallizer. The cooling water can be sprayed onto the surface of the aluminum ingot which has been removed, for secondary cooling, until the entire wire is cast.
Sequential crystallization can establish relatively satisfactory solidification conditions, which are favorable for the crystal size, mechanical properties and electrical conductivity. There is no difference in mechanical properties in the height direction of the ingot, the segregation is smaller, the cooling rate is faster, and a fine crystal structure can be obtained.
The surface of the aluminum wire ingot should be smooth and smooth, without slag inclusions, cracks, pores, etc., the surface crack length is not more than 1.5mm, the surface slag and rib wrinkle crack depth must not exceed 2mm, the section should not have cracks, pores and slag, less than There are no more than 5 slag inclusions in lmm.
The defects of aluminum wire ingots mainly include: 1 crack. The reason is that the temperature of the aluminum liquid is too high, the speed is too fast, and the residual stress is increased; the silicon containing silicon is greater than 0.8%, and the aluminum silicon is formed into the same melt, and a certain amount of free silicon is formed, which increases the thermal cracking property of the metal: Or the amount of cooling water is insufficient. When the surface of the crystallizer is rough or no lubricating oil is used, cracks may also occur on the surface and corners of the ingot. 2 slag. The slag inclusion on the surface of the aluminum wire ingot is caused by the fluctuation of the aluminum liquid, the cracking of the oxide film on the surface of the aluminum liquid, and the scum of the surface entering the side of the ingot. Sometimes the lubricant can also carry some slag. The internal slag is caused by the low temperature of the aluminum liquid, the high viscosity, the slag cannot float in time or the aluminum liquid surface changes frequently during casting. 3 cold partitions. The formation of the cold partition is mainly due to the excessive fluctuation of the aluminum liquid level in the crystallizer, the casting temperature