Principles of Metal Extraction: Metals are found in the combined state in ores where they exist as positive ions. During the extraction of metals, the metallic ions must first be reduced to the corresponding metal atoms. This reduction process can be done electrolytically or by thermal or chemical methods. The particular method chosen for the extraction of metal depends on the stability of the ore, which also depends on the position of the metal in the activity series. Electrolytic reduction process is used to reduce the more reactive metals(from their ores) that have very negative electrode potential values and a great tendency to exist as positive ions. It is difficult to reduce these sorts of metals by using common reducing agents such as coke. Ores of the less reactive metals can be reduced by using reducing agent, chemical or thermal methods because these metals have a relatively lower tendency to exist as positive ions which is denoted by their less negative electrode potential values.
Over 80% of the known elements are metals and they are widely distributed in the earth’s crust either in a combined state or as free elements. The form in which a metal exists is dependent on its level of reactivity as the most reactive metals such as sodium and potassium are found as chlorides and as trioxocarbonates(IV), which are described as very stable compounds. Moderately reactive metals such as lead and zinc are found as oxides and sulphides, while the least reactive metals such as gold and silver are found in a free state in nature.
In the extraction of metals, some factors are taking into consideration such as the chemical composition of the compound containing the metal, level of impurities and the extent of chemical reactivity of the ores or mineral constituents of these metals. Generally, ores are first concentrated and then changed into oxides before extraction.
Procedures for Metal Extraction
- Concentration of Ores: Ores may be concentrated either by (i) Washing away the earthy materials in a stream of water in order to leave behind the heavier ores as is the case with tin ore or by (ii) Froth flotation where the ores are churned up with oil and water to form froth. This froth which contains the ore is skimmed off by blowing air. This is particularly used in the concentration of zinc ores and (iii) By passing a magnetic ore such as copper ore over a magnetic separator. It is noticed that ores with a high metal content are deflected into a pile, while the earthy impurities and alongside ores with a low metal content pass straight on through the separator.
- Roasting in Air: If the ore is not an oxide, then the ore concentrate is normally roasted in air so as to change it to the oxide because ores which are oxides are easier to deal with during the extraction of metals compared to ores that are sulphides or trioxocarbonates(IV).
- Chemical and Thermal Reduction: Metals that are less electropositive such as lead(Pb), tin(Sn), iron(Fe) and zinc(Zn) are usually obtained by reducing their oxides with coke or carbon(II) oxide. These reducing agents are used because they are cheap and are readily available. For instance, in the extraction of zinc from zinc blende(ZnS), the sulphide is first converted to the oxide by roasting and the resultant oxide is then heated with coke and releases the electrons neededfor the reduction of the zinc ions.
Some metals are obtained by heating their ores in air. Mercury is obtained by heating mercury(II) sulphide in air while in the case of platinum, the ore is first of all converted to ammonium hexachloroplatinate(IV), which is then subsequently reduced to platinum by heating.
- Electrolytic Reduction: The more electropositive metals such as potassium, sodium, calcium and magnesium are obtained from their ores by electrolysis which is a strong redox process. During electrolysis, the cathode acts as a reducing region by supplying the electrons to the metallic ions in the electrolyte, resulting in the deposition of the free metal. The electrolyte is usually a salt of the metal such as a chloride or an oxide, with a high melting point.
Electrolytic processes are expensive to install and maintain. They are used only when chemical or thermal reduction of the ores is not possible.