Metals

To convert a metal-containing compound to a metal requires an oxidation-reduction reaction. **Oxidation** is the loss of electrons. **Reduction** is the gain of electrons. In a metal-containing compound, the metal exists as a positively charged ion because it has lost one or more electrons to its bonding partner. To convert metal ions to neutral metal atoms requires that they gain electrons.

The tendency of a metal ion to be reduced depends on its location in the periodic table. Metals on the left of the table easily lose electrons. Any ionic compound tends to be stable. Metals on the lower left of the table require the most energy-intensive methods of recovery, which includes electrolysis. During electrolysis, an electric current supplies electrons to positively charged metal ions, thus reducing them. Electrolysis commonly uses metals of group one through three, and most frequently as halides, carbonates, and phosphates. Aluminum is also recovered by electrolysis and other metals are obtained using electrolysis when very high purity is needed.

Limestone reacts with ore impurities-predominantly silicon compounds- to form slag. High temperatures cause both metal and slag to be matter. They drain to the bottom of the blast furnace, where they collect in two layers. Slag with less density stays at the top ,while the metal layer is topped off through an opening at the bottom of the furnace. The metal from a blast furnace is known as a cast metal once it has cooled off. A **cast metal** is brittle and soft because it contains impurities such as phosphate, sulfur, and carbon. To remove the impurities, oxygen is blown through the molten cast metal in a basic oxygen furnace. The oxygen oxidizes the impurities to form additional slag, which floats to the surface and is later skimmed off. Most phosphorus and sulfur impurities are removed in a basic oxygen furnace but purified metals still contain about 3% sulfur.

Metal sulfides can be purified by flotation. **Flotation** is a technique that takes advantage of the fact that metal sulfides are relatively non-polar and therefore are attracted to oil. An ore that contains a metal sulfide is first ground into a fire power and then mixed vigorously with lightweight oil and water. Air is forced up through the mixture. As the air bubbles rise, they become coated with oil and metal sulfide particles. At the surface of the liquid, the coated bubbles form a floating froth that is very rich in the metal sulfide, which is then skimmed off. The metal sulfides recovered from the froth are then roasted in the presence of oxygen.



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