At present, most of China's electrolytic aluminum plants pass the electrolytic aluminum liquid through refining, remove the slag and degas, and then cast it into a remelted aluminum ingot for sale; while the aluminum profile factory purchases the above aluminum ingot, remelting and casting, and producing aluminum alloy aluminum for extrusion. ingot.
If the alloy is added to the high-temperature electrolytic aluminum liquid, it is directly smelted into an aluminum alloy aluminum ingot, which shortens the process, saves the aluminum ingot remelting process, and saves equipment investment and labor wage. Direct production of aluminum ingots by electrolytic primary aluminum liquid, as long as the process measures are right, the product quality fully meets the requirements of the extrusion production process. The cost of smelting and casting is 300 yuan/t-A, of which about 0.8% is burned, fuel consumption is 4kg/t-A, power consumption is 14kw/t-A, and labor wage is 24 yuan/t-A.
The short-process production process makes full use of the high-temperature heat energy of electrolytic aluminum liquid, saves precious energy, reduces the pollution of smoke and dust to the environment, reduces the burning loss of aluminum liquid by about 1%, and saves oil per ton of product. More than 60kg, saving more than 30 degrees, each ton of products at least create economic benefits of 500-600 yuan.
However, some production units use equipment too simplistic, lack of understanding of the characteristics of electrolytic aluminum liquid, improper process measures, quality of round ingots, non-metallic inclusions, high gas content, coarse crystals, cracks and other quality defects. As long as appropriate process technology measures are taken and management is strengthened, high-quality aluminum ingots can be produced to meet the needs of users.
Second, the characteristics of electrolytic primary aluminum liquid
1. Electrolytic raw aluminum liquid has high temperature and high gas content
It is well known that the electrolytic aluminum production process is an electrochemical process under high temperature conditions. Nowadays, with the development of science and technology, the capacity of the electrolytic cell is getting larger and larger, the current intensity is as high as 350-500KA, and the current efficiency is 94-95%. The electrolytic cell uses a carbon block as an anode and a semi-graphitized carbon block as a cathode, and the temperature is generally between 950 and 960 °C. The composition of the electrolyte is very complicated. In addition to the main component cryolite, Al2O3, CaF2, MgF2, etc. are added, and their contents are 1.38-2.88%, 4.88-5.88% and 0.47-0.87%, respectively; the water content is AlF3 ≤7.5%. Others are less than or equal to 1.0%. The water in the above materials is attached water and crystal water respectively. The attached water is easily evaporated under high temperature conditions, and the crystal water is chemically reacted in the electrolytic bath 3H2O+2AL→Al2O3+6[H], [H] dissolved In electrolytic aluminum, the higher the temperature of the aluminum liquid, the greater the saturation concentration of atomic hydrogen.
In addition to hydrogen, electrolytic aluminum also contains CO2, CO, CH4 and N2. The composition of the electrolytic aluminum liquid gas content is also wide, H2: 53-96%; CO2: 2.5-30%; CO: 20%; CH4: 2.5%; N2: 2.5%. For electrolytic aluminum liquids with high gas content, effective degassing measures must be taken to eliminate their hazards.
2, the amount of impurities in the electrolytic raw aluminum liquid is large
The impurities in the liquid primary aluminum are mainly non-metallic impurities and also a small amount of metal impurities. The non-metallic impurities are at most alumina, followed by fluoride salts, aluminum carbide and aluminum nitride. During the electrolysis process, the alumina added to the electrolysis cell is a sand-like alumina. The sand-like alumina is composed of α-Al2O3 and γ-Al2O3, α-Al2O3 ≥ 25%; α-Al2O3 is spherical, large in specific gravity, compact in texture and small in surface area. In the electrolyte, the dissolution rate is slow, and it is often too late to dissolve α-Al2O3. Precipitate at the bottom of the tank and mix a small amount into the cathode aluminum solution. The γ-Al2O3 has a large activity and a fast dissolution rate, and generally does not deposit at the bottom of the electrolytic cell. Generally, the electrolytic bath extracts the aluminum liquid once every 24 hours, and the α-Al2O3 mixed in the aluminum liquid enters the aluminum liquid with the aluminum liquid and is poured into the melting furnace of the melting and casting production system.
The metal impurities in the electrolytic raw aluminum liquid are silicon, iron, zinc, titanium, sodium, etc., but the most important metal impurities are silicon and iron. There are two main sources of these impurities: one is from raw materials and materials. For example, alumina raw materials produced by alkali method contain impurities such as SiO2, Fe2O3, TiO2, ZnO, Na2O, etc., and Si, Fe, Ti are formed by electrolysis. Metal such as Zn or Na. The second is from the lining of the electrolytic cell, tools, equipment, dust and others.
By the way, in high-temperature liquid aluminum, carbon reacts with titanium to form TiC. TiC is a non-spontaneous nucleus of liquid aluminum alloy during solidification. TiC nucleation is better than TiB2 particles, which is beneficial to alloy aluminum liquid crystallization. When the grain is refined, the content is too low.
China is the country with the largest production of primary aluminum in the world. It adopts short process and directly produces flat ingots and round ingots for aluminum processing by using electrolytic aluminum liquid solar cells. This is the only way for the development of aluminum industry and also the height of relevant departments. Pay attention to it. Some large-scale electrolytic aluminum enterprises, such as Guizhou Aluminum Plant, Qinghai Aluminum Plant, Baotou Aluminum Plant, Qingtongxia Aluminum Plant, etc., have introduced advanced technology and advanced equipment from Europe and the United States in recent years, and have successively built a first-class melting and casting production line with high degree of automation. The main equipment includes large-scale melting furnace, large-scale hydraulic tilting static furnace, high-power electromagnetic stirrer, online argon degassing system, hydraulic casting machine and ingot homogenizing equipment. Through the production of test and production, the quality of the products is excellent, and good economic and environmental benefits have been achieved.
There are also some companies that invest millions of yuan to build three 15-ton melting furnaces, two wire rope casting machines, a single-rail crane, and a set of water circulation systems. The production capacity can reach 75,000 tons/year. However, the equipment is too rudimentary, the technical equipment is poor, and the discipline is not respected, the management is neglected, the production is shoddy, the quality of the products is poor, and the problems are piled up. For this type of enterprise, how to carry out equipment transformation, strengthen technical management, improve process technology measures, improve the process system, and produce qualified products. Here, talk about your own views for peer reference.
Third, the use of electrolytic raw aluminum liquid to produce ingots process technology measures
1. Improvement of smelting process technology
The aluminum alloy ingot is produced by electrolyzing the original aluminum liquid, and the ordinary 6063 aluminum alloy accounts for more than 90%, and occasionally produces a little aluminum alloy such as 3003, 6061, 6082, 6005, 5052. These aluminum alloys have a small solid-liquid phase temperature range, a small tendency for hot and cold cracking, and low technical difficulty. Unlike the 2XXX series of hard aluminum and 7XXX series of super-hard aluminum, the alloy has many components and a wide range. Due to the wide crystallization temperature range, the solid-liquid zone has low plasticity and has a great tendency to form hot cracks and looseness. Considering the influence of the ratio of impurity elements Si and Fe on the properties of the alloy, it is considered to use copper, manganese and chromium to improve the corrosion resistance of the alloy, and titanium and zirconium are added to improve the ingot and recrystallized structure.
The smelting furnace adds the dry head and tail without water to the bottom of the melting furnace before pouring the aluminum water. The temperature of the aluminum electrolytic bath is very high, up to 950-965 ° C under normal conditions, and the temperature of the aluminum liquid at the bottom of the tank is not lower than 940 ° C. After the aluminum liquid is poured into the melting furnace, there is still 820-860 °C. In order to prevent oxidation of the aluminum liquid at high temperatures, a layer of covering agent is immediately applied to cover the surface of the melt. Since the melting point of the covering agent is lower than the temperature of the aluminum liquid, the density is smaller than that of the melt, and the wettability is good, a continuous liquid protective film is formed on the surface of the melt to separate the melt in the furnace from the air.
In general, oxygen or water vapor cannot or rarely react with the aluminum liquid through the coating layer, and the hydrogen atoms dissolved in the electrolyte can penetrate the coating layer and escape due to the small radius.
Due to the high content of the raw aluminum liquid gas and impurities, it is necessary to strengthen the melt refining process in view of the above composition characteristics. Refining is usually carried out using a powdered refining agent and nitrogen blowing; the quality of the refining agent and nitrogen must first be ensured. Production of 6063 aluminum alloy ingots, generally using 1# refining agent; currently, there are many kinds of refining agents available in the domestic market, and most enterprises are based on confidentiality, do not explain the composition of the components to the users, and the price competition between suppliers is fierce. Individual defective refining agent manufacturers will cut corners from chemical raw materials selection, formula matching and processing methods, and refining agents must ensure quality and careful selection. The quality of the refining agent of Shenzhen Pai Rui Ke Zhi Jin Materials Co., Ltd. is good, you may wish to try it out. Nitrogen cannot be manufactured by itself. It is commercially available industrial nitrogen with a purity of 99%. Then nitrogen has 1% oxygen and water. The longer the blowing time, the more water and oxygen are brought into the aluminum liquid. The nitrogen used for refining must ensure that the purity of nitrogen is ≥99.995%. A nitrogen production line with a flow rate of 20M3 will be built. The equipment will be put into operation less than 200,000 yuan, and the equipment investment will be recovered for up to one year. It not only guarantees the purity of nitrogen, but also saves production costs.
The refining is carried out twice at a temperature of 730-750 ° C using a high quality refining agent and high purity nitrogen. The first refining refining agent dosage is 2.5kg/t-A, the refining time is 25-30min; after refining, the slag is slag, magnesium is added, and the fire is repaired. The second refining refining agent dosage is 1.5kg/t, and the refining time is 15-20min. After the second refining is completed, the slag, sampling analysis, and ingredients are passed, and then a layer of covering agent is sprayed, and the furnace is allowed to stand for 30 minutes.
With flux adsorption refining, the refining effect is closely related to the purity of the refining agent and the purity of the nitrogen, and is also closely related to the refining operation. Because the air refining is based on the partial pressure diffusion and degassing and flotation slag removal. Only the areas that come into close contact with the refined gas and powder have refining effects. The operator must follow the operating procedures and let the refiner move evenly along the bottom of the furnace at the four corners of the furnace, so that all the aluminum liquid is in full contact with the refining agent and nitrogen without leaving a dead angle. The effect of gas refining depends more on the gas dispersion and the size of the bubbles. The smaller the size of the bubble, the better the degassing effect. Since the smaller the bubble, the more the number of bubbles caused by the same volume of gas, the larger the surface area; and the slower the floating speed; the longer the contact with the melt. When the ordinary T-shaped refiner is used, the diameter of the bubble is about 10mm, and the effect is very good. If a 15mm diameter steel pipe is used as the refiner, the bubble diameter can reach 300mm or more, and more nitrogen is used, and the degassing effect is still poor.
The flux adsorbing the fine agent, the refining agent is directly in contact with the aluminum melt, and the adsorption and diffusion effect is achieved, thereby achieving the good effect of removing the slag. At the same time, the refining agent also has a degassing effect. The degassing effect of the flux is mainly manifested in three aspects: one is with the complex γ-Al2O3. The removal of XH removes a portion of the complexed hydrogen absorbed by the oxidized inclusions. Second, the flux is decomposed to form a gaseous product when it interacts with the melt, and diffusion is performed to remove hydrogen. Third, the oxide film on the surface of the melt is dissolved by the cryolite in the flux, so that the atomic hydrogen melted in the aluminum liquid is easily diffused into the atmosphere.
The purpose of standing: First, it is convenient to bring the gas and fine non-metallic inclusions in the aluminum liquid out of the liquid surface upstream of the refined oil carrier, and the second is to facilitate the sinking of large non-metallic inclusions to the bottom of the furnace.
2. Improvement of casting technology
Since the electrolytic aluminum liquid and the aluminum ingot remelted aluminum liquid are refined into an aluminum alloy, the properties are still different. The former has a slightly higher impurity content, a slightly higher viscosity, and a slightly less fluidity. Therefore, in casting a round ingot, in order to ensure fluidity, the casting temperature of the former should be about 10 ° C higher than the latter, and for 6063 aluminum alloy, the casting temperature should be controlled at 725-740 ° C. In continuous casting, the better the fluidity of the alloy liquid, the shrinkage cavities generated during the solidification of the ingot are more easily compensated, and the hot cracks generated by the ingot at the end of solidification are more easily obtained by timely welding, the surface of the ingot The tendency to form a cold barrier is also smaller, and it is also advantageous for the floating of gases and impurities. Therefore, only a slight increase in the casting temperature of the electrolytic aluminum alloy liquid and the improvement of the fluidity thereof can obtain an excellent round ingot.
According to the crystallization characteristics of the aluminum alloy, the formation of the nucleus is not derived from the fact that the aluminum alloy melt is supercooled and self-generating, and is actually always generated in the molten aluminum alloy. Although the impurity content in the electrolytic aluminum liquid alloy is higher than that in the aluminum alloy remelting alloy liquid, the impurities in the former are in the high temperature environment of 940-960 ° C for a long time, and the activity has been lost to become an inert impurity. Become the core of crystallization. Therefore, the use of electrolytic raw aluminum liquid to produce aluminum alloy ingots, only relying on external crystal nucleus, increase the amount of Al-Ti-B wire, in order to obtain a fine grained equiaxed crystal structure. Through production practice, the amount of Al-Ti-B wire should be increased to 3.5-4.0kg/t, and the specific gravity molten aluminum ingot alloy liquid should be increased by 50%. Otherwise, coarse equiaxed crystal or columnar crystal structure will be produced in round casting. .
The crystallization rate of the ingot is an important factor in determining the quality of the ingot. Generally, the higher the crystallization rate, the finer the crystal structure of the ingot, and the better the mechanical properties. Reducing the temperature of the helium ring water and increasing the pressure of the cooling water can reduce the surface temperature of the ingot, increase the temperature gradient in the ingot shell, increase the heat conduction strength, and improve the crystallization speed of the ingot. In the production practice, to ensure that the ingot does not produce hot cracks, try to increase the casting speed.
The 6063 aluminum alloy ingot produced by electrolytic primary aluminum liquid has great difference between the head and tail material and the remelted aluminum ingot. The head tends to produce coarse crystal structure and the tail is heavily gas-filled. At the beginning of casting, it is conscious to put some Al-Ti-B wire in the launder, or increase the speed of the wire feeder, or it is difficult to eliminate the coarse crystal structure. Therefore, when sawing, increase the length of the head to the diameter of the ingot 2 Double, the coarse crystal structure can be removed.
At the end of the ingot, the grain structure is still a small equiaxed crystal structure, which is mainly characterized by loose structure of the ingot and high hydrogen content. Since the casting process is coming to an end, the amount of molten metal in the furnace is small, the temperature is low, and the fluidity is poor. Appropriately reducing the casting speed, the alloy liquid is still difficult to completely replenish the fine and dispersed pores generated by the volume shrinkage of the alloy in the liquid and solidified state, thus leading to the formation of macroscopic and microscopic shrinkage. The length of the tail saw is controlled at 1.5 times the diameter of the ingot. Only the cut-off waste can ensure the quality of the product.
Fourth, the conclusion
1. Electrolytic raw aluminum liquid has the characteristics of high temperature, high gas content, many impurities, and few crystal nucleation active particles.
2. In the flux nitrogen blowing refining, select high-quality flux, high-purity nitrogen, increase the flux dosage to 4.0kg/t, refine the time to 40-50min, fully remove the slag exhaust, and improve the quality of the aluminum alloy liquid.
3, 6063 aluminum alloy casting temperature is 725-740 ° C, Al-Ti-B refiner added amount is 3.5-4.0kg / t, reduce water temperature, increase water pressure, increase crystallization speed, can produce fine grain High quality round ingots.
4. Keep the head and tailings, remove the quality defects in the head and tail sections, and ensure the quality of the products.