[Abstract] This paper describes our secondary aluminum production process, a comprehensive analysis of the current situation and the lack of quality of the metallurgy of aluminum regeneration, it pointed out that the main obstacle to purify deterioration is restricting China's development of renewable production of aluminum that make our recycled aluminum When production is at a new level, it is necessary to increase technical innovation and increase investment in new processes and new equipment.
Keywords: Aluminum recycling metallurgical quality <br> I. Introduction <br> aluminum alloy with its excellent performance has been widely used in various sectors of the national economy, the production of aluminum-silicon alloy There are two main methods :( a) Primary aluminum smelting and alloying; (2) recycling and recycling of used aluminum alloy. Waste aluminum recycling has low energy consumption and consumes only 5% of the original aluminum production [1] . The recycling rate is high and can be recycled many times. Moreover, the investment in recycled aluminum production is small, the yield period is short, and it has great cost and energy efficiency advantages compared with the production of aluminum-silicon alloy by primary aluminum. In the past ten years, China's aluminum recycling industry has developed rapidly. Among them, only the recycling rate of old cans has reached more than 60%. In 2001, the net import volume of used aluminum alloys in China reached 360.019 million tons [2] . However, from the overall point of view, China's aluminum recycling technology is backward, sorting, compounding and smelting processes are simple. Many enterprise component testing and quality control methods are still not perfect. Some enterprises only judge whether the products are based on the surface quality and fracture morphology of aluminum ingots. Qualified, the production of metallurgical quality is unstable, and quite a few enterprises can only produce several low value-added cast aluminum alloys, such as the ZLA102. Based on the status quo of waste aluminum recycling in China, this paper systematically analyzes the metallurgical quality problems and current status in China's aluminum regeneration process, discusses the ways to improve the quality status, and puts forward some suggestions.
Second, aluminum recycling and environmental protection <br> With the strengthening of people's environmental awareness, the academic and industrial circles have put forward green production calls [3] , requiring materials to be exhausted, processed, manufactured, used, recycled and recycled. Liquid, solid waste is harmless to humans and the environment, or meets acceptable emission standards. The recycling of waste aluminum is more than the production of primary aluminum, which not only greatly reduces energy consumption, but also eliminates and reduces the emission of harmful gases and wastes during the electrolytic production of aluminum. Therefore, the recycling of waste aluminum has improved social and environmental benefits and is worth promoting.
The regeneration of aluminum alloy is conducive to protecting the environment and increasing the utilization of aluminum resources. In the process of aluminum regeneration, in order to reduce the pollution during the regeneration process and realize the green production principle of regenerative production, Shanghai University uses the rare earth composite fine agent treatment technology in production [4] to greatly reduce the toxic substances in the flue gas, and the flue gas passes. The two-stage spray adsorption tower is connected in series to remove dust, and the flue gas emission standard can be achieved without further treatment. At the same time, due to the deterioration and purification of rare earth on recycled aluminum, the metallurgical quality and stability of recycled aluminum are greatly improved, and the scrap rate is reduced.
Third, China's recycled aluminum production process
There are three central steps in the process of producing recycled aluminum: pretreatment process, batch melting process, purification and deterioration process, which are closely related to the metallurgical quality of recycled aluminum. In order to obtain high-quality recycled aluminum, it is necessary to grasp the basic elements that affect the quality of metallurgy around these three central links. At present, China's production enterprises are mainly small and medium-sized enterprises. With the enhancement of people's quality awareness, these three links are getting more and more people's attention. Of course, we should also see that quite a few enterprises have realized this. It is still shallow, for this reason, we will mainly analyze the metallurgical quality of recycled aluminum in China from these three central links.
Analysis of the status quo of China's recycled aluminum-silicon metallurgy quality
1. The purpose of the pretreatment pretreatment is to classify the waste aluminum according to the alloy composition, remove the water, remove the oil paint, and remove other metal impurities. The general process is: (1) large pieces and slicing, sorted by composition → remove other metals and inserts → baking to remove oil paint; (2) powder and waste materials, classification → dry dewatering → magnetic separation , wind selection, parabolic sorting [5] → roasting; (three) cans, chopped (→ roasting paint) → briquetting. Developed countries use special equipment when sorting waste materials, such as gravity sorting and magnetic separation. Most enterprises in China basically use manual sorting, firstly stacking large pieces and slices of easy-to-sort known components, and then manually sorting the scrap aluminum. It can be seen that some small enterprises did not consider the composition of the alloy of recycled aluminum when sorting, so that there is actually no first good quality of metallurgical quality, which not only reduces the grade of recycled aluminum, but also wastes Resources. The fine powdery aluminum is not well sorted, and the composition of the components is complicated. If it is not treated, it will affect the chemical composition of the finished product to some extent, and even cause the whole furnace to be scrapped. Therefore, in general, be cautious. Use, preferably after blocking and testing the ingredients.
The surface treatment of scrap aluminum raw materials, especially the decontamination of waste aluminum (such as cans) containing oil stains and paints, is one of the effective measures to improve the quality of recycled aluminum metallurgy. Large-scale enterprises in China often use preheating ovens to remove paint, degrease and remove water when dealing with oil stains and paints. Most of the small recycling enterprises are not subject to their own conditions, and there is no treatment process for removing oil stains on the surface of waste materials. Most of them are directly thrown into the furnace, which is not only polluting the environment, but also easily brings hydrogen and impurities into the molten pool to affect the metallurgical quality of the aluminum ingot.
The pretreatment process is the first step in the process of reclaimed aluminum production. The quality of this first entry is crucial to the grade and metallurgical quality of recycled aluminum. Strictly control moisture during pretreatment, prevent sand, oxidized inclusions, organic matter from being brought in, grasp the sorting and sorting of waste, increase investment, improve pretreatment level, and realize the standardization of pretreatment is the current production of recycled aluminum in China One of the problems that need to be solved. [next]
2. Ingredient melting 2.1 Ingredients Ingredients refer to the preparation of the charge according to the composition requirements of the produced product. The ingredients are directly related to the ability to produce qualified products. Ingredients should follow the following principles: Starting with the production of qualified products, close contact with the inventory of scrap aluminum and raw materials of the factory, and optimizing the proportion of the prepared materials on the premise of known stock materials and scrap components. For products with narrow alloy composition range, the common practice of Chinese enterprises is pure aluminum ingot + alloying raw material (intermediate alloy) + waste aluminum with known composition; for products with wide range of composition requirements, usually aluminum scrap with known composition is used. + Alloying raw materials (intermediate alloys), adding appropriate amounts of pure aluminum ingots and intermediate alloys when the composition is deviated. Since different alloying alloys are alloyed differently, crystalline silicon is directly added to the alloy when alloying silicon; when alloying iron and copper , in order to reduce the cost, iron, wire or copper wire is directly added, and their sources are widely used. Low cost, but they have a long melting time and a high melting temperature, which increases the energy consumption and the burning of the aluminum liquid. And because the absorption rate is related to the processing technology, it tends to cause fluctuations in the Fe and Cu contents. Therefore, the method of adding the A1-Fe and A1-Cu master alloys is often used in the production of alloy ingots with stricter composition control. In fact, whether it is the alloying of intermediate alloys or the direct use of metal raw materials, we should not only focus on the immediate interests, but also comprehensively evaluate the cost-benefit ratio to determine what materials to prepare. In short, when mixing ingredients, avoiding the wrong ingredients, holding the quality of the raw materials, and clarifying the waste aluminum components, it ensures the first level of chemical composition of recycled aluminum.
2.2 Melting and melting is the process of transforming waste aluminum and raw materials from solid to liquid. When melted, the melt is in contact with the furnace atmosphere, the furnace wall and other solids, which may cause some substances to melt or penetrate into the melt. The composition of the melt, which in turn affects the metallurgical quality of the recycled aluminum. Most of China's recycled aluminum enterprises use melting furnaces with low melting costs to smelt. A few large and medium-sized enterprises also have some electric furnaces and induction furnaces for special needs. Reverberatory furnace with fuel oil or coal gas as fuel, lower than the cost of an electric furnace, an electric furnace in the control but not as good metallurgical quality.
In the melting process, secondary pollution of the melt is the main reason that affects the metallurgical quality of the recycled aluminum alloy. Therefore, the following points should be noted during the production operation: First, dry the charge, refining agent, precision agent and all operating tools to avoid Bring water into the water. Secondly, the charge should remove sand, mud and other harmful materials as much as possible. The iron tools used for production should be coated to avoid direct contact with the aluminum liquid, resulting in iron seepage (iron is one of the elements that seriously affect the quality of recycled aluminum metallurgy). Finally, choose the best melting temperature, pouring temperature and pouring time, try to avoid the aluminum liquid heating too high, and the holding time is too long. If necessary, before the feeding (such as when changing metal varieties and grades), pure aluminum or the corresponding alloy washing furnace is also applied. Some enterprises in China have not noticed the importance of the influence of washing furnace on metallurgical quality.
After ensuring the premise of metallurgical quality, another problem that needs attention is the problem of smelting cost, the consumption of molten fuel and the burning of aluminum liquid directly affect the cost. In the actual production process, by preheating the charge, the energy consumption can be effectively reduced by 10%-15%, and the non-flux refining modifier (such as rare earth composite fine modifier) ​​can be used to reduce the burning loss [4] . Through the above measures, the reproduction cost can be reduced, and at the same time, the metallurgical quality can be better.
3. Purification and deterioration 3.1 Purification The gas content and impurity content of recycled aluminum ingots directly affect the mechanical properties and process properties of alloy aluminum. The main task of the purification process is to ensure that the gas content and impurity content of the recycled aluminum ingot meet user standards and ensure the performance of the recycled aluminum ingot. Refining and purification often use the well-known floating method [6] . At present, the slag removing agent and refining agent used by Chinese enterprises generally use chlorine salt and nitrate as the refining agent. Such refining agent reacts with aluminum during the treatment to generate harmful gases such as AlCl 3 and Cl 2 . Gas damages environmental equipment and personal health. In the current situation of the country's vigorous promotion of sustainable development with special emphasis on environmental protection, how to find green non-toxic and non-polluting alternatives is an urgent problem for China's aluminum alloy smelting enterprises. Although there is no environmental problem with N2 or Ar refining, the effect is not good. Some foreign companies adopt dynamic vacuum degassing method, SNIF method, Alpur method, MINT method [7], although they can all achieve results, but these methods are generally applicable to large continuous casting furnaces and not suitable for more than 90% of China's reverberatory furnaces. Recycling aluminum companies. The compound refinement method invented by Shanghai University in recent years is a pollution-free green refining method developed for the characteristics of furnaces used in China's aluminum production enterprises [4] . It not only solves the problem of quality of reclaimed aluminum metallurgy, but also ensures the quality of the surrounding environment. A technique worthy of promotion. In the pouring, the filter or ceramic filter is placed in front of the aluminum ingot pouring port to further improve the aluminum liquid to reduce the product quality and wrinkle caused by the slag and scale. This method is widely used in China's recycling enterprises. [next]
3.2 Modification of metamorphic Al-Si alloys is a process to improve the mechanical properties of aluminum alloys in order to improve the morphology of the silicon in the alloy. At present, the commonly used modifiers in China are as follows: (1) The sodium salt modifier is a short-acting modifier, and the deterioration effect is good and the failure is fast. Although the sodium salt can be used for deterioration, it destroys the dense oxide film on the surface of the melt, causing the aluminum liquid to re-inhale and oxidize, deteriorating the metallurgical quality, especially for ZL102. Therefore, the use of short-acting modifiers is not recommended for the production of such eutectic aluminum alloy ingots [8] . (B) Normal depot modifier, (1), strontium (Sr), and common A1-10% Sr A1-14% Si-10% Sr master alloy. The metamorphism effect can be maintained for 6-8 hours, it is long-lived, the incubation time is 40 minutes, and the relative price is high, the source is difficult, which limits its application in aluminum regeneration [6] . In addition, niobium will also loosen the oxide film on the surface of the molten metal, making the pinhole degree worse. In order to ensure that the pinhole degree of the metallurgical quality meets the standard requirements, the necessary degassing measures are usually adopted, and the type of deteriorating agent is not used alone. (2) barium, tellurium, antimony, barium, though having the effect of long-term degeneration eutectic Al-Si alloy, but many tests show that the effect of modification of these elements is greatly affected by the cooling rate, and refining agent, slag The agent will strongly reduce the deterioration of the elements such as bismuth [6] . Therefore, it is rarely used in the production of recycled aluminum. (3) Rare earth-based modifiers, rare earths can not only have metamorphism on the silicon phase, but also have obvious effects in refining a-A1. Practice also shows the hereditary nature of metamorphism after metamorphism of rare earths [4/9] . It can be seen that compared with the above three types of modifiers, the rare earth modifier has good deterioration effect and has certain purification effect, and is also beneficial to environmental protection, stable effect and cost-effective, and is a preferred modifier for recycled aluminum production.
3.3 Metallurgical quality control before furnace The metallurgical quality control before furnace is an extremely important part of ensuring the production of qualified aluminum ingots. Pre-furnace analysis can avoid major metallurgical quality accidents, recover some losses and reduce costs. Pre-furnace quality inspection and control usually use two methods: chemical analysis, spectral analysis. The former has high analysis accuracy, but its analysis time is long, often unable to keep up with the beat of the production line, but also affected by the level of the operator and the analysis environment. In the latter case, the analysis speed is fast and easy to use. Although the equipment is expensive, it is still widely used. In the detection of product components exceeding the standard and non-standard, the composition must be adjusted. Before pouring, in addition to ensuring chemical composition, there is also checking the pinhole. Usually, metallographic statistics are used after macrocorrosive corrosion. Some joint ventures have equipped a molten aluminum hydrogen analyzer for pre-furnace quality control. When the gas content exceeds the standard, N2 and Ar are degassed until they pass the test.
V. Conclusion
The recycling of waste aluminum is a cause of benefiting the country and the people, and it will also bring huge profits to the enterprise. However, it must be noted that the metallurgical quality problems existing in the production of recycled aluminum enterprises in China are directly affecting the development of China's recycled aluminum in the depth direction. . The author made a detailed investigation on the production process of recycled aluminum in China, and specifically analyzed the effects of three main production steps of pretreatment, batch melting and purification on the metallurgical quality of recycled aluminum alloy in aluminum regeneration process, and proposed to improve the metallurgical quality. Related recommendations and programs. It is worth mentioning that in the three main production links, the purification and deterioration of the link has largely restricted the production of high value-added products by Chinese enterprises. Therefore, we should increase investment in technological transformation and new technology, break through the technical bottle, eliminate backward processes and equipment, and promote the transformation of China's recycled aluminum industry from the current quantitative expansion to the quality and efficiency. It is believed that in the near future, China's recycled aluminum industry will have a win-win situation of environmental protection and efficiency.
references:
[1] Cheng Yue Some views on the production of recycled aluminum in China China Material Regeneration, 1999, (1): 12-13
[2] Wang Zhutang China's Recycling Aluminum Industry Annual Meeting of Nonferrous Metals Recycling and Utilization, 2002:38-53
[3] Qian Hancheng, Wu Qifeng, Zhao Jianhua, etc. Discussion on Greening of Casting Hypoeutectic Al-Si Alloys Special Casting and Non-Ferrous Alloys, 2002(6):1-4
[4] Mao Xiemin, Tang Duoguang, Zhang Jinlong et al. Environmental load assessment of green aluminum alloy rare earth compound refining and metamorphic treatment process Journal of Nonferrous Metals, 2002(3): 4347
[5] Fangyuan Waste aluminum retreatment technology status and discussion Chongguo resources comprehensive utilization, 2000, (5): 11-13
[6]Lu Shuzhen, Gu Kaidao, Zheng Laisu Colored casting alloy and smelting Xi'an: National Defense Industry Press, 1983
[7] Gao Yinheng Aluminum and Aluminum Alloy Melt Treatment Technology Light Alloy Processing Technology, 1989, (1): 1-11
[8] Pan Ye, Sun Guoxiong, Chen Jiansheng Proceedings of the 3rd China Youth Materials Science Symposium, 1991, (9)
[9] Tang Duoguang A good material for casting alloy refining and transformation - rare earth alloy special casting and non-ferrous metals, 1999, (5): 42-43
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