• Electrolytic refining: The process of electrolysis is used to obtain very highly purified metals. It is very widely used to obtain refined copper, zinc, tin, lead, chromium, nickel, silver, and gold metals. In this process, the anode is made as impure slab of metal and cathode as pure thin sheet of same metal and a salt solution of the metal is used as the electrolyte. On passing current, pure metal from the electrolyte is deposited on the cathode. The impure metal dissolves from the anode and goes into the electrolyte. The impurities collect as the anode mud below the anode (Fig. 14.13).

• Liquation process: In this process, the block of impure metal is kept on the sloping floor of a hearth and heated slowly. The pure metal liquefies (melts) and flows down the furnace. The non-volatile impurities are infusible and remain behind (Fig. 14.14).

• Distillation process: In this process, metals with low boiling point, such as zinc, calcium, and mercury are vaporized in a vessel. The pure vapor are condensed into pure metal in a different vessel. The non-volatile impurities are not vaporized and so are left behind.

• Oxidation process: In this process, the impurities are oxidized instead of the metal itself. Air is passed through the molten metal. The impurities like phos­phorus, sulfur, silicon, and manganese get oxidized and rise to the surface of the molten metal, which are then removed.

All these methods are effective but result in the generation of toxic chemical sludges and waste products.

Another approach which involves aqueous chemistry for the recovery of pure metals from ores is termed as hydrometallurgy. It is typically divided into three general areas:

• Leaching

• Solution concentration and purification

• Metal recovery

Fig. 14.13 Electrolytic refining

Fig. 14.14 Liquation process

Leaching

Leaching involves the use of aqueous solutions containing a lixiviant is brought into contact with a material containing a valuable metal. The lixiviant in solution may be acidic or basic in nature. In the leaching process, oxidation potential, temperature, and pH of the solution are important parameters, and are often manipulated to optimize dissolution of the desired metal component into the aqueous phase. The three basic leaching techniques are in situ leaching, heap leaching, and vat leaching.

After leaching, the leached solids and pregnant solution are usually separated prior to further processing.

Solution concentration and purification

After leaching, the leach liquor must normally undergo concentration of the metal ions that are to be recovered. Additionally, some undesirable metals may have also been taken into solution during the leach process. The solution is often purified to eliminate the undesirable components. The processes employed for solution concentration and purification include:

• Precipitation

• Cementation

• Solvent Extraction

• Ion Exchange

Metal Recovery

Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step.

Sometimes, however, further refining is required if ultra-high purity metals are to be produced. The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation.