2 Metallurgy

2.1. Materials Used in Metallurgy

Metallurgy is a science and engineering which studies the methods used to obtain metals in free condition from compounds that occur in nature.

Only some metals are mined in the native state. Among these are gold, silver, platinum, mercury, tin and, partly, copper. Most of the metals, however, are found in the earth in the form of oxides, silicates, sulphides, carbonates, etc. Native metals and metal compounds are associated with considerable amount of foreign materials, such as rock, gravel, sand, clay and other impurities that require removal.

Metal ore is its chemical compounds plus foreign impurities. Ore may be rich or poor. In the last case it must be dressed (concentrated) to remove undesirable impurities. When ore may be mined and converted in metal with commercial profit it is called pay or able ore. Ores of different metals are put through various dressing processes to obtain them with small quantity of impurities, i.e. to obtain almost pure metal compound and then to obtain metal in the free condition.

To convert ore in metal the high temperature is usually required. To obtain high temperature we use fuel, which may be:

- gaseous (native gas, blast-furnace gas);

- liquid (black mineral oil, or mazut);

- solid (anthracite, coke, charcoal).

Electric energy is also used in metallurgy to receive heat.

Some impurities, in the main oxides, find their way into a furnace together with ores. As a rule, oxides have high melting point: A1₂O₃-2040°C, CaO-2570°C, MgO-2800°C, etc. Besides that, an ash is formed on account of fuel burning.

To remove these impurities and ashes from the furnace fluxes are used. Fluxes render the impurities fusible at operating temperatures, combine with them and carry them off into slag.

All metallurgical processes are accompanied by slag formation, representing oxides, suphides, nitrides, and other chemical compounds. Slags are formed on the account of added fluxes as well as damage of furnace lining. The slag importance in metallurgy is very high: such important reactions as oxidation and deoxidation are possible because of slags, sulphur and phosphorous are assimilated and carried away with slag. Slags protect metal against gas saturation, facilitate heat accumulation in metal. The principal components of ferrous metallurgy slags are: CaO, SiO₂, P₂O₅, Al₂O₃, FeO, MnO, CaS, MnS, etc. The main slag characteric is its basicity mostly determined as CaO/SiO₂.

Metallurgical furnaces are operated at high temperatures. To prevent damage their walls are covered with refractory or fireproof materials. Refractory are materials that can stand at high temperatures from 1580 to 2400°C and more without considerable mechanical damage and withstand chemical attack of molten metal and slag.

According to chemical composition refractories are divided into:

acid materials:

basic materials:

inert refractory:

Correspondingly, slags may be acid, neutral or basic. Only acid slag may be built up in a furnace with acid lining and, on the contrary, in the basic furnace basic slag must be formed, because of acid slag reaction with basic lining, or basic slag reaction with acid lining with formation of easy fusible compound: SiO₂+CaO=CaSiO₃ and with destruction of lining.