BY JOSE PALU-AY DACUDAO
THE MOST commonly used metals come from mineral metal oxides. The top four in their elemental forms (alone or admixed in alloys with other elemental metals) are iron, aluminum, magnesium and chromium. These are the most abundantly used metals because they are the main structural metals that human world civilization utilizes. They literally build up human-made structures.
In the previous articles, we talked about decreasing anthropomorphic CO2 emission from the iron and aluminum industries. By far in terms of quantity, the most important elemental metal is iron. Most of the iron goes into steel-making, and the world produces a whopping 1.2 billion tons or more of steel alone (not counting pure iron and its niche uses) each year. (China is the leading producer with more than 500 million tons of steel annually.) A far second to iron is aluminum, with around 60 million tons produced annually. (Fifty five percent comes from China.)
Next are magnesium and chromium, at nearly 30 million tons each per year. Almost all of the chromium goes into the production of ferrochromium (FeCr) (also called Ferrochrome), which is an alloy of chromium (50% to 70% chromium by mass) and iron. Ferrochrome is added to iron in smelters to produce steel, and especially stainless steel, which is 10% to 20% chromium. Thus, chromium usage mostly cannot be separated from iron usage, as both are needed to make stainless steel.
Commercial Ferrochrome comes from the mineral chromite (FeCr2O4). Note that chromite is an oxide and already contains iron. Ferrochrome (FeCr) is made by the carbothermic reduction of chromite, usually in electric arc furnaces, as it requires a higher temperature than that of iron ore smelting. (As previously discussed, elemental iron is also produced by carbothermic reduction, mostly in blast furnaces.)
Metal oxides (which form the mineral ores of iron, aluminum, magnesium, and chromium) consists of metallic elements chemically bonded to oxygen.
Carbothermic literally means carbon + heat (thermic). In carbothermic reduction, elemental carbon is used to ‘steal’ the oxygen from the metal oxide, leaving behind the elemental metal.
As explained before, the over-all chemical formula for the reduction of iron oxide to elemental iron by carbon and heat is: 2 Fe2O3 (iron oxide) + 3 C (elemental carbon) → 4 Fe (elemental iron) + 3 CO2 (carbon dioxide).
In the production of Ferrochrome FeCr, chromite FeCr2O4 is chemically reduced by carbon and heat. Roughly: Chromite + C (elemental carbon) → FeCr (Ferrochrome) + CO2 (carbon dioxide).
As explained previously, unlike iron oxide, it takes highly energy-consuming electrolysis, the Hall–Héroult process, in order to separate elemental aluminum from its oxide because of the strong affiliation of aluminum for oxygen. In practice, mere carbothermic reduction can’t do it. Thus, production of one kilogram of aluminum requires 7 kilograms of oil energy equivalent, as compared to 1.5 kilograms for that of iron. (To be continued)/PN
