Scrap is one of the most valuable and important raw materials in the steel industry. Since the content of harmful elements such as S and P in this category is low, it is beneficial to shorten the smelting time and reduce iron consumption and auxiliary material consumption. At present, with the development of industry, there are more and more industrial scrap steel in society. In order to effectively utilize this part of resources, countries around the world are trying to increase the scrap ratio in the process of electric furnace smelting. The amount of scrap accounts for about 70%-80% of the total amount of steel. It can be seen that scrap has become the main charge for electric furnace steel production.
Source and classification of scrap
The sources of scrap are mainly returned scrap, dismantled scrap, scraps and waste scrap from the processing industry, etc.
- Returned scrap is a high-quality charge. It is produced in the process of steelmaking, slag steel and forging or finishing, such as short lengths, waste ingots, soup channels, injection residues and slag steel in the steelmaking workshop or cut heads, cut tails and other forms of waste in the forging workshop. The processing preparation work of returned scrap is small, and it is classified and grouped according to the amount of elements and their contents, so it can be returned to the furnace for use anytime and anywhere. For furnace steel producers, the use of returned scrap has great economic significance. This is because the use of returned scrap as furnace charge can be smelted by non-oxidation method or back-blowing method, which can not only recover a large amount of precious alloy elements, but also reduce costs, shorten smelting time, and thus improve the productivity of electric furnaces.
- Scrap obtained from dismantling various old machines, cars, ships, scrapped rails and building components, various old weapons and tools, etc. is called dismantling scrap. The more highly developed the industry is in a country or region, the greater the proportion of dismantling scrap in the total steel volume. However, its return cycle is longer, and it often takes a long time, or even decades, to be returned to the furnace for use.
- In the manufacturing process of steel products, various scraps, turnings and material heads are also one of the main sources of scrap.
- Garbage scrap is mainly recycled from cans and thin materials in urban garbage, which contain high Sn or Zn; they must be separated before use.
Scrap can be roughly divided into heavy scrap, medium scrap, small scrap, light scrap, slag steel and turnings according to its shape, size and requirements for its composition and density.
General requirements for scrap in electric furnace steelmaking
- Scrap used in electric furnace steelmaking cannot be severely corroded or contain too much rust, otherwise the weight of the molten metal cannot be accurately determined, and the inaccurate weight of the molten steel will affect the accurate control of the chemical composition during the smelting process; during pouring, due to insufficient weight of the molten steel, it is easy to cause short-length scrap; when smelting with non-oxidizing, it is easy to increase the burning loss of alloy elements.
- Scrap should try to avoid being stained with mud, slag and mixed with rubber, bone, enamel, refractory materials, concrete blocks, etc. The slag content of slag steel should not be greater than 15% by volume, because their presence will reduce the conductivity of the charge.
- Scrap should not be mixed with non-ferrous metals such as Cu, Pb, Zn, and Sn. Zn has a low melting point and is very volatile. Its oxidation product, zinc oxide, corrodes the refractory material of the furnace cover, especially the silica brick furnace cover. Pb has a lower melting point, high density, and is volatile. It is difficult to dissolve in steel. Therefore, if the charge is mixed with Pb, it can not only poison and pollute the air, but also easily destroy the furnace bottom.
- Cu and Sn make steel hot and brittle.
Processing of scrap
Except for returned scrap, scrap is generally not clean. Not only is the surface stained with mud, ice and snow, but it is also mixed with many harmful elements and non-metallic elements. Its shape, size and density are also very irregular, which brings many difficulties to transportation, storage, loading, melting, etc. In order to solve such problems, it is necessary to adopt different processing methods for pre-processing.
Pre-processing of scrap is also a very complex and important task. Many large special steel mills have special workshops and equipment for this operation. Heavy scrapis broken by cutting, cold shearing, hammering and blasting. Light and thin materials are packed and briquetted after sorting and removing impurities. Cuttings are compacted by barreling or cold or hot briquetting. In addition, various waste weapons and guns and shells should be handled safely, that is, fuses, fuses and unexploded ammunition must be removed. Some important special furnace materials must also be pickled to remove iron oxide and surface impurities.
