6.2. Brief technology of metals

Cast-iron and steel production. Technological process of ferrous metal production includes melting of cast-iron from iron ores with the next processing in steel.

The basic method of the cast-iron production is blast-furnace. Blast-furnace process consists of three stages: iron reduction from the oxides containing in ore, iron carbonization and slag-forming. The iron-stones and fluxes are the raw materials.

Iron ores are subjected to the previous processing and crushing, to concentration and pelletizing before the melting. The crushed iron-stones are often concentrated by the magnetic separation. For the exception of sandy and clay particles ore is cleansed by water. Fine and dusty iron ores are pelletized with the agglomeration - baking on the fire grates of the sintering machines or running in the granulator with the subsequent drying and burning-out. The basic fuel at melting of cast-iron is cokes which is the source of heat and directly participates in the iron carbonization. Fluxes (limestones, dolomites or sandstones) are applied to reduce the temperatures of melting of spoil and binding it with the ash of fuel in slag.

T he blast furnace is a vertical mine stack with walls made of fire-brick, placed in a steel jacket (Fig. 6.6). The prepared raw materials are layerwise stoked from above in a furnace. As a result of coke burning at presence of oxygen from the air which is heated in the underbody of furnace, the carbon oxide forms which reducts iron from ore and can chemically react with it. At the same time the silicon, phosphorus, manganese and other admixtures are reduced.

Molten at a temperature of 1380-1420°C the cast-iron and slag is passed through the notch. Cast-iron is teemed into forms, and a slag goes on the reprocessing. The cast-iron, smelted in the blast furnaces intended for a steel production, foundry-iron – for obtaining various cast-iron elements. Special types of cast-iron (ferrosilicon, ferromanganese) are used in a steel production as the deoxidizing agents or alloy additions.

Steel is smelted from the iron by the oxidation with the open-hearth, converter and electrosmelting methods. The open-hearth method is basic method, but oxygen-converter method which has substantial technical-economic advantages in the last years become widespread.

At open-hearth method steel is smelted in open-hearth furnace, where gas or fuel oil is burned, and in the special chambers- regenerators air and gas fuel are heated due to the accumulated warmth of rejected burning products. The charge contains iron and metal scrap - debris and iron ore. Steel is produced as a result of charge melting, at which appears large amount of protoxide iron; products of oxidization of carbon and other admixtures and deoxidation- reduction of iron from a protoxide by additions of ferrosilicon, ferromanganese or aluminum.

Converter method of steel production consists in blowing out air or oxygen through the iron, poured in a converter (Fig. 6.7). Upon the termination of process the converter is returned horizontally, the blowing is stopped, steel composition is checked up and outpoured in a scoop. At present oxygen-converter method at which blowing is carried out with the clean oxygen is widespread. Oxygen-converter method is characterized with a high productivity, does not need large capital investments and fuel. Coefficient of heat use at this method is about 70 %.

E lectro-smelting method of the steel production is the most modern one. In the electric furnaces the high-quality steel is smelted which has a high purity and good deoxidation. The steel scrap is a basic material for the electro-smelting of steel.

Electro-smelting is carried out in arc (Fig. 6.8) or induction furnaces. In arc furnaces metal is heated due to the heat of the voltaic arc, which appears between an electrode and molten metal, in induction one - due to the high frequency currents.

Steel production by duplex process is a progressive method, when liquid cast-iron is processed into steel in the converters, and then in electric furnaces steel is taken to the set chemical state.

The electroslag and plasma-beam remelting are the modern methods of steel production. High-quality alloyed steel is manufactured from the bars of ordinary steel at the electroslag remelting, which melts due to a heat which is generated at passing of the electric current through them. A molten metal passes through the layer of liquid slag and clears up from harmful admixtures and gases. At the plasma-beam melting plasma arc is a source of the heat, and at electron beam melting – electrons flow which is emanated by the cobalt unit with forming of deep vacuum in the melting space.

The steel production from ore by the direct reduction without blast-furnace is an effective process. The sponge iron which is semi-finished product at this method is obtained in rotary or blast furnaces and subjected to the crushing and separating from a barren rock. In the case of direct reduction of iron from ore there is no need in by-product-coking production - the basic one at a b last-furnace process that substantially promotes the productivity of steel production.

Steel, produced in the furnaces, is inundated in the special cast-iron patterns – casting molds or supply on the continuous casting. In the case of continuous casting the stream of molten metal goes at first to the crystallizer which is cooled down with water, and then to the area of aftercooling and automatically is cut on the ingots of required length.

The continuous casting, comparatively with an piece one, enables to promote the high labour productivity, quality of steel and to decrease the amount of the recrements of the casting industry.

Production of aluminum. The rocks with high content of Al₂O₃ - bauxites, nephelines, alumstone and others are the raw material for the production of aluminum. The process of aluminum production consists of three technological stages: manufacturing of pure alumina, its electrolysing and extraction of original aluminum and finishing (that is cleaning). For the production of the alumina aluminum ore is processed in alkalis or acids depending on the composition of admixtures. Water solution of sodium aluminate forms at the alkaline method of admixtures extraction. It is easily decayed with formation of the aluminum hydrate and then Al₂O₃ formed after burning. Aluminum salts, which also consistently transform in Al(OH)₃ and Аl₂О₃, appear at a acid method.

Metallic aluminum is manufactured by the method of electrolysis of alumina in a liquid electrolyte - molten cryolite Na₃AlF₆. Electrolysis is carried out in electrolysis baths which are consistently connected in series of 80-100 units. The liquid aluminum is cleared or refined in the closed scoop by the blowing out with the chlorine at a temperature 750-770°C.