Biological Extraction Methods (OCR Gateway GCSE Chemistry: Combined Science)

• Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal, such as iron ores and aluminium ores
• For low grade ores (ores with lower quantities of metals) other techniques are being developed to meet global demand
• This is happening in particular with nickel and copper as their ores are becoming more and more scarce
• Phytoextraction and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes
• Both of these methods avoid the significant environmental damage caused by the more traditional methods of mining
• Traditional mining involves a great deal of digging, moving and disposing of large amounts of rock
• Biological methods are, however, very slow and also require either displacement or electrolysis to purify the extracted metal
• Both techniques are also used to extract metals from mining wastes, which may contain small quantities of metals or toxic metals that need to be removed from that environment

Phytomining

• This process takes advantage of how some plants absorb metals through their roots
• The plants are grown in areas known to contain metals of interest in the soil
• As the plants grow the metals are taken up through the plants vascular system and become concentrated in specific parts such as their shoots and leaves
• These parts of the plant are harvested, dried and burned
• The resulting ash contains metal compounds from which the useful metals can be extracted by displacement reactions or electrolysis

Copper in soil can be extracted by phytoextraction

Bioleaching

• Bioleaching is a technique that makes use of bacteria to extract metals from metal ores
• Some strains of bacteria are capable of breaking down ores to form acidic solutions containing metals ions such as copper(II)
• The solution is called a leachate which contains significant quantities of metal ions
• The ions can then be reduced to the solid metal form and extracted by displacement reactions or electrolysis
• This method is often used to extract metals from sulfides e.g. CuS or Fe₂S
• Although bioleaching does not require high temperatures, it does produce toxic substances which need to be treated so they don't contaminate the environment
• Bioleaching is not only used for the primary extraction of metals, but it is also used in mining waste clean up operations