Refining of Metals
Once a metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. is reduced, it is still not necessarily pure enough for all uses to which it might be put. An obvious example is the brittleness and low tensile strength of pig iron, characteristics which make it suitable for casting, but little else. These adverse properties are due to the presence of impurities, a typical analysis of blast-furnace iron showing about 4% C, 2% P, 2.5%, Si, 2.5%Mn, and 0.1% S by weightA measure of the gravitational force on an object; directly proportional to mass.. Further refining to remove these impurities (especially carbon) produces steel, a much stronger and consequently more useful material.
Steelmaking involves oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. of the impurities in basic oxygen, open hearth, or electric furnaces. Some oxidation products (CO, CO2, and SO2) are volatile and easily separated. The others end up, along with some iron oxides, in a slag which floats on the surface of the molten steel.
In the open-hearth furnace oxidation is due to air at the surface of the molten metal. This method of steel refining was developed in the mid 1800s, contemporaneous with the industrial revolution. This method requires up to 12 hours—and natural gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. or other fuel must be burned to keep the metal liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container. Thus the open hearth wastes large amounts of free energyGibbs energy: a thermodynamic function corresponding to the tendency for spontaneous change in a system; represented by the symbol G.. The use of fossil fuel does make it possible to recycle as much as 50 percent scrap iron, however, and the longer meltingThe process of a liquid forming from a solid. time allows somewhat greater control over the composition of a batch of steel.
Developed in the 1950s, the basic oxygen furnace has replaced the open-hearth furnace as the primary steelmaking method. In this process, pure oxygen is directed onto the surface of molten pig iron in a large crucible. Some of the iron is oxidized to Fe3O4 and Fe2O3, forming an oxidizing slag. The impurities, namely, C, P, Si, Mn, and S, are all oxidized at the same time. Since all these reactions are spontaneousCapable of proceeding without an outside source of energy; refers to a reaction in which the products are thermodynamically favored (product-favored reaction). and exothermicDescribes a process in which energy is transferred to the surroundings as a result of a temperature difference., they provide enough heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. so that up to 25 percent solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. scrap iron may be melted in the crucible without cooling it to the point where solid iron would remain. Oxidation of one batch of pig iron and scrap normally takes slightly more than half an hour. Thus, this method is far quicker than the open-hearth furnace, is three times more efficient.
Computers are now used to interpret spectroscopic analyses of steel in basic oxygen furnaces, indicating in a few minutes what metals must be added to obtain the desired composition. This has largely eliminated the last advantage of the open hearth and speeded up changeovers to basic oxygen. It has also decreased recycling of iron because the latter furnace cannot handle as much scrap. Much recycling of iron is now done in electric-arc furnaces which can melt a charge of 100 percent scrap.
In addition to the chemical oxidations used in steelmaking, electrolytic oxidation and reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. is quite important in refining metals. The electrolytic refining of copper, and aluminum production has already been described.