Aluminum and aluminum alloy waste pretreatment process

[China Aluminum Network] Most of the raw materials of the recycled aluminum production enterprises are scraps of aluminum and aluminum alloys recovered through circulation. These wastes come from complex sources and come in different shapes, often with some other metals and non-metals. The composition varies widely and the surface is unclean. If these substances are not removed before smelting, not only will the aluminum melt be seriously aspirated, but defects such as pores, porosity, etc. will be generated in the subsequent solidification process, and the composition and performance of the material will be unacceptable. In addition, a large amount of harmful gases will be generated in the production, which will seriously pollute the environment. Therefore, in order to ensure the product quality of recycled aluminum and improve the economic efficiency of the company, strict pretreatment of scrap aluminum must be performed before smelting.

The purpose of aluminum and aluminum alloy waste pretreatment is to: - remove other metals and non-metals from the waste; second, classify the aluminum scrap by alloy composition; third, remove the oil pollution and coating on the aluminum scrap surface; Aluminum is used more economically and rationally.

1 The status quo of China's aluminum waste pretreatment process China's renewable aluminum industry has made considerable progress after nearly 10 years of development, but it is still lagging behind in waste pretreatment. The pretreatment process used is generally as follows:

1 For aluminum- and aluminum-alloy scraps that are single-type or substantially free of other impurities, they are generally not subjected to complicated pretreatment, except that the bulk aluminum scrap is crushed (sheared or otherwise) into pieces of a specification that meets the requirements. The loose aluminum waste is pressed into pieces and classified according to the type and composition of the waste;

2 For high-grade scrap aluminum chips (including cast aluminum alloy, deformed aluminum alloy and pure aluminum, etc.), in the large-scale reclaimed aluminum companies - generally remove dirt and other inclusions directly into the melting furnace smelting; , - General use of artificial cast aluminum alloy, deformed aluminum alloy and pure aluminum, etc., and then use them separately;

3 For the low-grade slicing, due to its complex composition (including scrap steel, scrap copper, waste lead and other metals and non-metallic inclusions such as plastics, soil, rubber, sawdust, etc.), mainly rely on manual sorting. First screen out dirt and waste, then pick out non-metallic and metal scrap. The waste aluminum that is sorted out is mixed, and it is no longer subdivided. This method is used for pretreatment of aluminum scrap from Shanghai Xinge Non-Ferrous Metals Co., Ltd., that is, water washing followed by manual sorting.

In short, the waste pretreatment technology of China's reclaimed aluminum enterprises (including large-scale reclaimed aluminum companies) is still very backward, and simple manual sorting is still used. The tools used are magnets, steel files, hammers, pliers, and reeling tools. The experience is that the labor intensity, high cost, and poor quality are all factors that restrict the development of China's aluminum recycling enterprises.

2 Foreign aluminum waste pretreatment technology The United States, Japan, Germany, Australia and other developed countries have started earlier in the research and application of aluminum waste pretreatment technology, and have realized the mechanization and automation of aluminum waste pretreatment, which can be removed to a greater extent. Metal and non-metal impurities increase production efficiency and separation quality, reducing production costs. The pretreatment process used includes sorting, disassembling, packing, drying and degreasing, organic coating on the surface, and the like.

2.1 Sorting and sorting methods There are mainly morphological sorting, mechanical sorting, heavy medium sorting, fire sorting and electrostatic sorting.

2.1.1 Morphological Separation Form Separation refers to sorting of physical forms, that is, sorting according to appearance marks (color, section characteristics, hardness, mass density, magnetism, etc.) and physical signs (part names, winning prizes, etc.) . Visual methods can be used, but also by spot analysis, spectroscopic analysis and special instruments. For example, iron, copper, and aluminum alloys can be distinguished according to their colors; white metals can be distinguished from aluminum, magnesium, zinc, lead, cupronickel, tin, etc. according to the difference in surface color after cleaning; Materials of the same part, such as magnesium parts are lighter than aluminum parts; use of magnetic can determine the steel parts of aluminum or aluminum.

When the color, density and magnetic properties cannot be identified, spot analysis, spectral analysis, and special instruments can be used for sorting. In the spot analysis method, chemicals are dropped on the smooth surface of the metal or alloy, and the presence of a certain element can be judged according to the displayed color. The type of the alloy can be determined by sequentially dropping different reagents. Such as using CdSO. (5g) Aqueous solution (100mL) of NaCl (10g) and hydrochloric acid (20mL) was spotted onto the metal surface. If it appeared black after 10-20s, it was a magnesium alloy; if it was gray, it was an aluminum-zinc alloy. ; For aluminum and aluminum alloys, it is a transparent color. At the same time, it can also roughly estimate the content of an element based on the depth of the color. Droplet analysis is simple and easy to use. For some special parts, if you need to accurately determine the chemical composition, you must use spectral analysis.

2.1.2 Mechanical Sorting Mechanical sorting includes particle size sorting method, wind selection method, magnetic separation method, flotation method and vortex selection method.

2.1.2.1 Granular sorting method When it is necessary to sort fine or large components from aluminum scrap, the size sorting method may be adopted. Sorting process - generally using screening machines, including fixed sieves, trommel and vibrating screens.

There are grid type and horizontal type fixed sieves, suitable for sorting aluminum scrap with particle size J larger than 50mm. The screening machine shall not be less than 350 when sifting bulk materials, and not less than 50 when sorting flat-shaped materials or damp pieces. . The screening efficiency of the grid-type screening machine does not exceed 65%. The required sieve area F can be calculated as follows:

F=Q/2.4a

Where Q - sorting capacity by raw scrap /t·h-1

a-Screen strip gap width/mm

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