Bioleaching

Bioleaching, or microbial ore leaching, is a process used to extract metals from their ores using bacterial micro-organisms.
The bacteria feed on nutrients in the minerals, causing the metal to separate from its ore.

1. BIOLEACHING  Prasanna R Kovath Assistant Professor Department of Biotechnology St. Mary's College Thrissur

2. Bioleaching, or microbial ore leaching, is a process used to extract metals from their ores using bacterial micro-organisms. The bacteria feed on nutrients in the minerals, causing the metal to separate from its ore. The metals commonly extracted using this process include gold, silver, zinc, copper, lead, arsenic, antimony, nickel, molybdenum cobalt, and uranium.

3. Using micro-organisms helps to reduce production costs, minimize environmental pollution, compared to conventional leaching processes that use cyanide, and to efficiently extract metals, even when their concentration in the ore is low. This process is growing in popularity, as the bacteria can grow naturally in mining environments, and can also be easily cultivated and recycled . Bioleaching is performed mostly by iron and sulfide oxidizing bacteria, or acid producing fungus. Some of the types of bacteria used in this process include Leptospirillum ferrooxidans , Thiobacillus ferrooxidans , and some species of sulfolobus , acidianus , and sulfobacillus .

4. PRINCIPLES OF MICROBIAL METAL LEACHING • Mineralytic effects of bacteria and fungi on minerals are mainly based on three principles, acidolysis , complexolysis , and redoxolysis . Microorganisms are able to mobilize metals through the following processes: Formation of organic and inorganic acids (proton formation) 2 . Excretion of complexing agents (ligand formation) 3 . Oxidation and reduction reactions

5. Heap leaching  is the most common method for bioleaching and is mainly used for secondary copper ores.  Stirred tank leaching  is used for refractory gold  concentrates where gold is locked into the pyrite/ arsenopyrite matrix . As the microbes do not necessarily need to contact the valuable metal-bearing material that is bioleached, they can be physically separated from it: Direct bioleaching . The microbes are kept together with the valuable metal-bearing material Indirect bioleaching . The microbes are kept in a pond external to the valuable metal-bearing material and provide the leaching chemicals at a distance. Bioleaching involves abiotic and  biotic reactions , often with different physicochemical requirements. Indirect bioleaching is a way of satisfying the requirements independently by separating the biotic and abiotic reactions. In direct bioleaching the challenge is to select microbes whose living conditions are as close to the optimal conditions of the abiotic leaching reactions as possible.

6. DIRECT BIOLEACHING In this bioleaching, bacteria directly oxidize minerals and solubilize metals. In direct leaching , a physical contact exist between bacteria and ores and oxidation of minerals takes place through enzymatically catalysed steps. Example; pyrite is oxidised to ferric suphate 2FeS2+7O2+ 7H2O 2feSO4+ 2H2SO4

7. INDIRECT BIOLEACHING In this type of bioleaching, bacteria produces the strong oxidizing agent such as ferric ion and sulfuric acid on oxidation of soluble iron or soluble sulfur respectively. Acidic environment is absolutely essential in order to keep ferric iron and other metals in solution. Acidic environment maintain by oxidation of iron, sulfur, metal sulfides or by dissolution of carbonate ions. Ex; bioleaching of uranium UO2+Fe(SO4)3 ----- UO2SO4+2FeSO4

8. SLOPE LEACHING In slope leaching the ore is finely ground and kept in large pile in a slope which is subjected to continuous sprinkling of aqueous solution of microorganisms. The liquor collected at the bottom of the ore is processed for metal recovery . HEAP LEACHING In heap leaching the ore is arranged in heap and goes through the same treatment such a in slope leaching. The aqueous solution containing microorganisms works on the heap of ore and produces the leach liquor. The leach liquor is used for metal recovery.

9. IN-SITU LEACHING • In situ leaching ore is subjected to bioleaching in it’s natural occurrence. • Aqueous solution of microorganisms is pumped through drilled passages within the ore. • The leach liquor collected at the bottom of the ore used for the metal extraction. • The permeability of rocks is increased by blasting of rock. • As the acidic water seeps through the rock, it collects at the bottom which is used for metal extraction. • This water can be recycled and reuse.

10. HEAP LEACHING COPPER

11. LIST OF BIOLEACHING OR BIOOXIDATION PLANTS METAL LEACHING METHOD PLANT Cu Heap Cerro Colorado , Chile Nickel heap BioNIC , South Africa Au Tank Sao Bento , Brazil Co (from a cobaltic pyrite) Tank Kasese , Uganda Polymetallic heap Penoles Mexico Zn (Sphalerite) heap Namibia Au Tank Youanmi , Australia

12. FACTORS INFLUENCING BIOLEACHING

13. Nutrients : Bacteria used for metal extraction are chemolithoautotrophs ( inorganic compounds ).Mineral nutrients obtained from environment and from material to be leached. O2 and CO2 : For good growth of organisms adequate amount of oxygen is require. In laboratory this can be achieved by aeration, stirring, shaking . PH : Adjustment of correct PH is required for the growth of leaching bacteria. It is necessary for the solubilization of metals . PH values in range of 2.0- 2.5 are optimum for bacterial oxidation of ferrus iron and sulphide .

14. Temperature : Optimum temperature for ferrous iron and sulfide oxidation by T. ferrooxidans is 28-30 degree c.At low temperature decrease in metal extraction occur. At high temperature thermophilic bacteria can be used for leaching purpose. Surfactants and organic compounds Surfactant used and organic compounds used in extraction have inhibitory effect on leaching bacteria due to decrease in surface tension.

15. ADVANTAGES Cheaper than chemical extraction Simpler than other conventional processes Fewer specialists are required Environmentally friendly, with no sulfur dioxide emissions as in smelters Lower capex and opex than in conventional smelting and refining processes High pressure or temperature is not required Leaching residues are less active than in physico -chemical processes Can partly replace extensive crushing and grinding processes used in a conventional process, which then helps to cut costs and energy consumption

16. DISADVANTAGES OF BIOLEACHING The heat create from the dissolving process can kill the bacteria. Time consuming( 6-24 months or longer). Low yield of minerals. Requires a large open area for treatment. High risk of contamination.