[China Aluminum Industry Network] The aluminum produced in aluminum electrolyzers has a large difference 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 liquids and remove as much impurities as possible. The impurities in the primary aluminum can be divided into the following three categories: the former are metal elements such as iron, silicon, copper, calcium, magnesium, titanium, vanadium, boron, nickel, zinc, antimony, tin, lead, phosphorus, etc. The main elements are iron and silicon; the second is non-gold solid inclusions, Al2O3, AlN and Al4C3; the third is gas, H2, CO2, CO, CH4, N2, of which the main one is H2. At 660C, approximately 0.2 cm3 of hydrogen was dissolved in 100 g of aluminum liquid. The solubility of gas in aluminum liquid increases with increasing temperature. The aluminum liquid sucked from the electrolytic cell is subjected to a purification process to remove some of the impurities and cast into a commercial aluminum ingot (99.85% A1). With 99.996% Al pure aluminum (aluminum wire φ2mm, hard puller), the resistivity is 2.668×10-8Ω·m. If there is an impurity element in pure aluminum, the resistivity increases. The larger ones are chromium, vanadium, manganese, lithium, and titanium. The less affected are indium, lead, zinc, cadmium, tin, antimony, and iron.
1. Balance of Impurity Elements in Aluminum The amount of impurities in industrial alumina produced from bauxite by the Bayer process is greatly reduced relative to the raw material bauxite. In addition to the bases brought from the lye, the total amount of analysis of the impurity elements is usually less than 1%. The main impurities are SiO2 and Fe2O3. In addition to impurities in the electrolytic cell caused by alumina, the carbon anode and the flux cryolite also cause a lot of impurities. Impurities caused by carbon anodes are mainly iron and silicon, as are cryolites.
If all of the impurity elements of the raw material are precipitated in the primary aluminum, the resulting aluminum grade is only 99.7% Al. However, the actual production of aluminum has a higher grade of 99.8% Al. This difference is mainly due to the evaporation of impurity elements. Iron, titanium, phosphorus, zinc, and bismuth are the majority of aluminum oxide, while silicon and vanadium are the majority of carbon anodes. The impurity element from the flux is phosphorus, which accounts for about 20% of the total phosphorous. The remaining silicon, iron, titanium and vanadium are few.
In the balance sheet, silicon and iron exceeded the amount brought by raw materials, of which silicon exceeded 60% and iron exceeded 37%. The lining materials of the electrolyzers, such as high ash bottomed carbon blocks and carbon paste and refractories, are another important source of these impurity elements. In addition, due to the erosion of the operating tool and the cathode steel bar, the iron is also balanced. The rest of the elements are close to balance.
Expenditure allocation is different in the amount of impurity elements in primary aluminum and exhaust gas. Phosphorus, which accounts for the largest amount of evaporation, accounted for 72% of the total income. Vanadium accounted for 64.4%, iron accounted for 62.4%, titanium accounted for 57.7%, thorium accounted for 49.6%, and zinc accounted for 19.7%. The smaller is silicon, which only accounts for 13.3% of total revenue. This is the reason why:
1 Silicon and zinc exist in the electrolyte in the form of compounds that are more difficult to evaporate or even evaporate, such as SiO2, ZnO, or ZnF2. Silicon and zinc clearly accumulate in the liquid aluminum. The extent to which aluminum liquid is contaminated with silicon and zinc is mainly determined by the total amount of silicon compounds and zinc compounds fed into the material balance. In this case, the collection efficiency of the slot hood does not matter.
2 Iron, ruthenium, titanium and nickel are present in the system at least partly in the form of volatile compounds. These compounds are probably produced after entering the electrolyte. Possible compounds are Fe(CO)5, Ni(CO)4, TiF3, TiF4 and GaF3. If the collection efficiency of the tank cover increases, the quality of aluminum will be affected to some extent.
3 Vanadium and phosphorus exist only in the form of volatile compounds. The possible compounds are first fluoride (VF3 and PF3) and phosphorus pentoxide (P2O5). Since the increase of the phosphorus content in the electrolyte will affect the current efficiency, the increase in the amount of vanadium in the aluminum will reduce the conductive properties of aluminum, so it can be expected to improve the collection efficiency of the tank cover will bring about the quality of primary aluminum and better production results damage.
2. Classification of Aluminum Ingots Aluminum ingots are divided into aluminum ingots, high-purity aluminum ingots, and aluminum alloy ingots according to their composition. They can be classified into strips, ingots, ingots, and T-shaped ingots according to their shapes and sizes. Several, following are several common aluminum ingots; aluminum ingots for remelting - 15kg, 20kg (≤ 99.80% Al):
T-shaped aluminum ingot - 500kg, 1000kg (≤ 99.80% Al):
High-purity aluminum ingots - l0kg, 15kg (99.90% to 99.999% Al); aluminum alloy ingots - 10kg, 15kg (Al--Si, Al--Cu, Al--Mg); ingots - 500 to 1000kg (Plate making);
Ingot - 30~60kg (for drawing).
3. Aluminum ingot casting process aluminum - slag dregs - test kiln - Ingredients - Furnace - scouring - casting - remelting aluminum ingots - finished product inspection - finished product inspection jin - storage out of aluminum - slag residue - test jin - ingredients - Furnace installation - refining - Casting - Alloy ingots - Casting alloy ingots - Finished product inspection - Finished product inspection - Storage II. Primary aluminum purification The aluminum liquid extracted from the electrolytic cell contains various impurities, so it needs to be purified prior to casting. In the industry, clarification, flux, gas, and other purification methods are mainly used, and some methods of directional solidification and filtration are used for purification.
1. flux purification flux purification is the use of molten 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, forming a new droplets rise to the surface, cooling after the formation of the float Slag removed.
The flux used for purification is composed of salt with low melting point, low density, small surface tension, high activity, and strong adsorption capacity for oxidized slag. When using, firstly put a small amount of flux into the iron cage, and then insert it into the bottom of the mixing furnace to stir it. After the fluxing is completed, remove the iron cage and stand still for 5 to 10 minutes. Remove surface scum and cast. The flux can also be withdrawn on the surface as needed to provide coverage.
2. Gas Purification Gas purification is a major primary aluminum purification method. The gas used is a mixture of chlorine, nitrogen, or chlorine and nitrogen.
(1) Chlorine gas purification. In the past, active gas chlorine was used as a scavenger (chlorination method). In the chlorination process, when chlorine gas is introduced into the aluminum liquid, many abnormally fine AlCl3 and bubbles are generated and sufficiently mixed in the aluminum liquid. Hydrogen dissolved in the molten aluminum, as well as some mechanical inclusions, are adsorbed on the AlCl 3 bubbles, and are discharged as the AlCl 3 bubbles rise to the surface of the aluminum liquid. Chlorine can also be used to chlorinate certain elements that are more negative than aluminum, such as calcium, sodium, magnesium, etc. Chlorine is generated by the introduction of chlorine and can be separated. 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 the oxygen is toxic and more expensive, in order to avoid air pollution and reduce the cost of aluminum ingot production, modern aluminum industry has gradually abolished the chlorination method into inert gas - nitrogen purification method.
(2) Nitrogen purification method. Also known as smokeless continuous purification method, alumina ball (418mm) is used as filter media. N2 directly into the liquid aluminum. The molten aluminum is continuously fed into the purifier, passes through the alumina ball filter, and is flushed with nitrogen. The non-metallic inclusions and dissolved hydrogen in the molten aluminum are then removed and continuously discharged, so that the fine nitrogen bubbles are evenly distributed. In the treatment of aluminum liquid play a role in purification. Nitrogen has no pollution to the atmosphere, and has a large amount of purification treatment. It can handle 200~600kg of aluminum liquid per minute, and the loss of aluminum caused by the purification process is relatively reduced, so it is now widely used. However, unlike chlorine, it does not remove calcium, sodium, and magnesium from aluminum.
(3) Mixed gas purification method. A mixture of chlorine and nitrogen is used to purify the aluminum liquid. Its role is to remove hydrogen and separate oxides on the one hand, and to remove certain metal impurities (such as magnesium) on aluminum. The commonly used composition is 90% nitrogen + 10% chlorine. . There are also 10% chlorine + 10% carbon dioxide + 80% nitrogen. This works better. Carbon dioxide can spread chlorine and nitrogen well, reducing operating time.
Third, the ingot casting process Now the aluminum ingot casting process generally uses a casting process, that is, the aluminum liquid is poured directly into the mold, until it is cooled and removed.
The quality of the product is mainly in this step, and the entire casting process is also based on this process. The casting process is a physical process in which liquid aluminum cools and crystallizes into a solid aluminum ingot.
1. Continuous Casting Continuous Casting can be divided into two ways: mixing furnace casting and external casting. Both use continuous casting machines. Mixing furnace casting is the casting of aluminum liquid into the mixing furnace and casting by the mixing furnace. It is mainly used to produce aluminum ingots for remelting and casting alloys. Outer casting is carried out directly from the bag to the casting machine. It is mainly used when the casting equipment cannot meet the requirements of the production, or the quality of the incoming material cannot be directly introduced into the furnace. Because there is no external heat source, the bag is required to have a certain temperature, generally 690 ~ 740 °C in summer and 700 ~ 760 °C in winter, to ensure that aluminum ingots get a better appearance.
Mixing furnace casting, first through the ingredients, and then poured into the mixing furnace, stir evenly, then add the flux refining. The cast alloy ingot must be clarified for more than 30 minutes, and the slag can be cast after clarification. During casting, the furnace furnace eye is aligned with the second and third casting molds of the casting machine. This ensures that the fluid flow changes and there is a certain degree of maneuverability during mold change. The furnace eye and the casting machine are connected by a runner, and the runner 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 to be preheated for 4 hours before being restarted. The molten aluminum flows into the mold through the flow cell, and the oxide film on the surface of the aluminum liquid is removed with a shovel, which is called slag. After a full mold is reached, the runner is moved to the next mold and the caster continues to advance. The mold advances in sequence, the aluminum liquid gradually cools, and the molten aluminum has solidified into an aluminum ingot when it reaches the middle of the casting machine, and the printer prints the melting number. When the aluminum ingot reaches the top of the casting machine, it has been completely solidified into aluminum ingots. At this time, the mold is turned over, the aluminum ingot is ejected from the mold, and it falls on the automatic splice car, and is automatically stacked and bundled by the stacker to become finished aluminum. ingot. The casting machine is cooled by the spray of water, but the water must be supplied only after the casting machine has been fully rotated. About 8 to 10 tons of water are consumed per ton of aluminum liquid, and it is necessary to add a blower for surface 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 middle is solidified later, leaving a groove-shaped depression. The solidification time and conditions of the various parts of the aluminum ingot are not the same, so the chemical composition will also be different, but it is in line with the standard as a whole.
The common defects of aluminum ingots for remelting are: 1 blowhole. Mainly due to the casting temperature is too high, aluminum liquid contains more gas, aluminum ingot surface pores (pinholes), the surface dark, severe thermal cracks. 2 slag inclusions. It is mainly due to the fact that the slag is not clean, resulting in slag on the surface; the second is that the temperature of the aluminum liquid is too low, resulting in internal slag inclusion. 3 Ripples and flashes. The main reason is that the operation is not precise, the aluminum ingot is too large, or the running of the casting machine is not stable. 4 cracks. The cold cracking is mainly due to the casting temperature is too low, resulting in the crystal of the aluminum ingot is not dense, causing loose or even cracks. Hot cracks are caused by high casting temperatures. 5 component segregation. It is mainly caused by uneven stirring when casting the alloy.
2. Vertical semi-continuous casting Vertical semi-continuous casting is mainly used for the production of aluminum ingots, slab ingots and various deformed alloys for processing profiles. The aluminum liquid is poured into the mixing furnace after compounding. Due to the special requirements of the wire, before the casting, it is necessary to add titanium and vanadium (line ingot) in the aluminum liquid after adding the intermediate combining disk Al-B; and the aluminum ingot needs to be added to the Al-Ti--B alloy. (Ti5%B1%) is refined. Make surface tissue fine. High magnesium alloy plus 2 # refining agent, the amount of 5%, stirring evenly, after standing for 30min to remove scum, you can cast. Raise the caster chassis before casting and blow off the moisture from the chassis with compressed air. Then raise the chassis into the crystallizer, apply a layer of lubricating oil to the inner wall of the mold, put some cooling water into the water jacket, place the dry preheated distribution plate, self-adjusting plug and flow cell, and make the distribution plate The mouth is in the center of the crystallizer. At the beginning of casting, press the self-adjusting plug with your hand to block the flow nozzle, cut the opening of the mixing furnace, allow the aluminum liquid to flow into the distribution plate through the flow cell, and wait until the molten aluminum reaches 2/5 in the distribution tray, and let it open automatically. The stopper is adjusted so that the aluminum liquid flows into the crystallizer and the aluminum liquid is cooled on the chassis. When the liquid aluminum reaches 30mm high in the crystallizer, the chassis can be lowered and the cooling water can be started. The self-adjusting plug controls the molten aluminum to flow evenly into the crystallizer and keeps the liquid height in the crystallizer constant. The scum and oxide film on the aluminum surface should be promptly removed. When the length of the aluminum ingot is about 6m, the furnace is blocked and the distribution plate is removed. After the aluminum solution is completely solidified, the water supply is stopped, the water jacket is removed, and the cast aluminum ingot is removed by a monorail crane. The required size is shown on the sawing bed. Saw and then prepare for the next casting.
During the casting, the temperature of the aluminum liquid in the mixing furnace is kept at 690-710°C, the aluminum liquid in the distribution pan is maintained at 685-690°C, the casting speed is 190-21Omm/min, and the cooling water pressure is 0.147-0.196MPa. The casting speed is proportional to the square wire ingot:
VD=K, where V is the casting speed, mm/min or m/h; D is the side length of the ingot, mm or m; K is the constant value, m2/h, generally 1.2 to 1.5.
The vertical semi-continuous casting is a sequential crystallization method. After the molten aluminum enters the casting hole, it begins to crystallize on the bottom plate and on the inner wall of the crystallizer. Because the cooling conditions of the center and the edge are different, the crystal forms a low middle and a high perimeter. 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 the condensation, plus a layer of lubricating oil on the inner wall of the crystallizer, the solidified aluminum decreases with the lowering of the chassis. Exit the crystallizer, there is a circle of cooling water eye in the lower part of the crystallizer, and the cooling water can be sprayed onto the surface of the aluminum ingot that has been pulled out, for the second cooling, until the entire wire ingot is cast.
Sequential crystallization can establish relatively satisfactory solidification conditions, which are favorable for crystallized grain 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 very fine crystal structure can be obtained.
The surface of 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 depth of surface slag and edge wrinkle cracks must not exceed 2mm. There should be no cracks, air holes, and slag inclusions on the surface. There are no more than 5 slag inclusions in lmm.
The defects of aluminum wire ingots mainly include: 1 cracks. 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 aluminum liquid contains more than 0.8% of silicon, generates aluminum silicon and the same melt, and then generate a certain amount of free silicon, increasing the thermal cracking of the metal: Or the amount of cooling water is insufficient. When the surface of the crystallizer is rough or no lubricant is used, the surface and corners of the ingot also crack. 2 slag inclusions. The inclusion of slag on the surface of the aluminum wire ingot is caused by the fluctuation of the aluminum liquid, the breakdown of the oxide film on the surface of the aluminum liquid, and the inflow of scum on the surface of the ingot. Sometimes lubricants can also carry some slag. The internal slag inclusions are caused by the low temperature of the aluminum liquid, the large viscosity, and the frequent fluctuation of the molten aluminum surface when the slag cannot be floated or cast. 3 cold separated. The formation of cold septa is mainly due to the excessive fluctuation of aluminum liquid in the crystallizer, the low casting temperature, the slow casting ingot speed, or the inconvenience caused by the inconvenience caused by casting machine vibration and uneven drop. The pores mentioned here refer to small pores with a diameter of less than 1 mm. The reason for this is that the casting temperature is too high and the condensation is too fast, so that the gas contained in the aluminum liquid can not be escaped in time. After solidification, small bubbles are accumulated in the ingot to form pores. 5 rough surface. Because the inner wall of the crystallizer is not smooth, the lubrication effect is not good, and when it is serious, the aluminum tumor on the crystal surface is formed. Or because the iron-silicon ratio is too large, uneven cooling caused by segregation. 6 leak aluminum and re-analysis. The main problem is that the operation is serious. It also causes tumors.
3. Quality assurance of ingots (1) Aluminum ingots for remelting. The more important technical condition in the ingot casting process is the casting temperature. In the casting process, the casting temperature must be strictly controlled, and is generally higher than the solidification temperature of the aluminum liquid by 30 to 50°C.
(2) Ingots. The casting of wire ingots is somewhat complicated, and the conditions to be controlled are the ingot speed. The ingot speed is related to the ingot diameter. The casting temperature is maintained at 680~690°C, the cooling water pressure is 0.147~0.196MPa, and the aluminum liquid level in the inner wall of the crystallizer is controlled at about 30mm. Control the above conditions, and strengthen the operation and management, you can get better quality.
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