Occurrence and extraction of metals

The earth’s crust is the major source of metals. Seawater also contains some soluble salts such as sodium chloride, magnesium chloride, etc. The elements or compounds, which occur naturally in the earth’s crust, are known as minerals. At some places, minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. These minerals are called ores.

Some metals like gold, silver, platinum etc are found in the free state (nugget) in the earth’s crust because they are least reactive. Most metals are found as oxides, carbonates, sulphides, halides etc.

Minerals: are elements or compounds which occur naturally inside the earth’s crust.

Ore: is a mineral from which metals can be extracted profitably.

Gangue: is the impurities present in the ore like rock particles, sand particles, clay particles etc.

Extraction of metals from their ores:

Metals are extracted from their ores in three main steps. They are:

1) Concentration of the ore (Enrichment of the ore) is the removal of gangue (impurities) from the ore by different methods.

2) Reduction to the metal (by O₂, H₂, C, CO)

3) Refining (Purification of the metal by electrolysis).

Some metals are found in the earth’s crust in the free state. Some are found in the form of their compounds.

The metals at the bottom of the activity series are the least reactive. They are often found in a free state. For example, gold, silver, platinum and copper are found in the free state. Copper and silver are also found in the combined state as their sulfide or oxide ores.

The metals at the top of the activity series (K, Na, Ca, Mg and Al) are so reactive that they are never found in nature as free elements.

The metals in the middle of the activity series (Zn, Fe, Pb, etc.) are moderately reactive. They are found in the earth’s crust mainly as oxides, sulphides or carbonates. The ores of many metals are oxides. This is because oxygen is a very reactive element and is very abundant on the earth.

Thus on the basis of reactivity, we can group the metals into the following three categories:

1 Metals of low reactivity (after H atom);

2 Metals of medium reactivity (between Al and Pb);

3 Metals of high reactivity (above Mg metal atom).

Different techniques are to be used for obtaining the metals falling in each category. Metallurgy as Per Reactivity:

· Electrolysis of molten is used for highly active metals such as K, Na, Ca, Mg and Al.

· Reduction with carbon is used for medium active metals such as Zn, Fe, Pb and Cu.

· Least reactive metals such as Ag and Au are found in free state in nature.

Refining of Metals:The metals produced by various reduction processes described above are not very pure. They contain impurities, which must be removed to obtain pure metals. The most widely used method for refining impure metals is electrolytic refining.

Electrolytic Refining:Many metals, such as copper, zinc, tin, nickel, silver, gold, etc., are refined electrolytically. In this process, the impure metal is made the anode and a thin strip of pure metal is made the cathode. A solution of the metal salt is used as an electrolyte. On passing the current through the electrolyte, the pure metal from the anode dissolves into the electrolyte. An equivalent amount of pure metal from the electrolyte is deposited on the cathode. The soluble impurities go into the solution, whereas, the insoluble impurities settle down at the bottom of the anode and are known as anode mud.