Non-domain steel production
It includes two principal processes: the production of metal pellets from the ore, bypassing the blast furnace, and steel from pellets.
This method is the most resource and energy saving in many senses and should gradually replace traditional ones. For example, it allows you to conduct the process without consuming metallurgical coke, replacing it with other types of fuel. In addition, it is possible to obtain pure metal due to the development of methods for deep ore dressing.
At present, there are many ways of direct iron production. Their diversity is explained by the characteristics of processed raw materials and fuel. The most widely used methods of recovery using various aggregates: shaft furnaces and retort, rotary kilns, moving grate, fluidized bed reactors.
Consider one of the ways to produce steel from metallized pellets. It consists of three main stages: the preparation of oxidized pellets from iron ore concentrate, their metallization in special installations for the direct reduction of iron (up to an iron content of 90-95%), and the smelting of steel in electric arc furnaces.
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Iron ore is being enriched at mining and processing plants. Then the moisture is removed from the concentrate and bentonite is added to it, which in the rotating pelletizing pellets glues the particles into small balls (diameter 1-2 cm) - gray pellets, which are burned to give them the required strength.
The metallization of the concentrate is carried out at special metallization plants (Figure 4.8).
Fig. 4.8. The scheme for obtaining metallized pellets:
1.8 - pipeline; 2 - the receiving device; 3 - the tower-furnace; 4 - the system for supplying purified gas; 5 - conversion installation; 6 - cooling air supply system; 7 - pellets
The main part of the installation is a 64-meter cylindrical tower-furnace 3 with an internal diameter of 5 m, in which the reduction of iron occurs. Piles are conveyed by the conveyor 1 into the receiving device 2 and are evenly loaded into the shaft tower-furnace 3, operating on the countercurrent principle. To reduce iron from the pellets, carbon monoxide and hydrogen are supplied through the pipeline 4 , which are obtained in the conversion plant 5 from the blend of top and natural gases. In the reduction zone of the mine, oxidized pellets at a temperature of 500-1100 ° C are reduced to sponge iron. As a result, the content of iron in them reaches 90-95%. Metallized pellets have a constant chemical composition. They are cooled in the lower part of the shaft by purging with cold air supplied through the pipeline 6.
The cooled pellets 7 are delivered to the conveyor 8 and fed to the hopper, where they are stored in an inert atmosphere before melting in the electric furnace.
Ways to improve the quality of steel
The technology of steel production in melting furnaces does not allow to produce a metal with a low content of gases, harmful impurities and nonmetallic inclusions. At the same time, the development of industry makes ever-increasing demands on the quality of metal: its strength, ductility, and gas content. Reducing the content of harmful impurities in melting furnaces is economically inexpedient, therefore the characteristics improve "in-line" methods; processing by synthetic slag, vacuum degassing, electroslag remelting (ESR), vacuum-arc remelting (VDP), remelting in electron beam (ELP) and plasma furnaces, etc.
Metal processing with synthetic slag. Synthetic slag, consisting of 55% CaO, 40% Al2O3, a small amount of SiO2, MgO and a minimum of FeO, is melted in electric furnaces and poured into a ladle 1 (Fig. 4.9, a) i> Then the steel 4 is poured into the same bucket (Figure 4.9, b).
Fig. 4.9. Scheme of metal treatment with synthetic slag:
1 - bucket; 2 - slag; 3 - a stream of slag; 4 - steel; a - the process of pouring slag; b is the end of the steel treatment
Intensive mixing of steel and slag sharply increases the surface of their contact. Gases and nonmetallic inclusions are adsorbed by the slag and diffuse into it, sulfur is bound by calcium oxide and also passes into the slag. Due to this, as well as the low content of iron oxide in the slag, steel treated in this way contains less sulfur, oxygen and non-metallic inclusions, its plasticity and strength are improved. These steels are used to make responsible machine parts.
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The same effect is achieved by blowing the steel with a powdered slag-forming mixture consisting of lime and fluorspar.
A simple way to improve the properties of steel is to blow it in a ladle with inert gases, most often argon, through the bottom of the bucket or tuyere. Passing through the metal gas bubbles adsorb to themselves non-metallic inclusions, they penetrate the dissolved in the steel hydrogen, nitrogen and oxygen.
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