Bioleaching

Microbial leaching is the process by which metals are dissolved from are bearing rocks using microorganism.

1. BIOLEACHING

2. INTRODUCTION :- Microbial leaching is the process by which metals are dissolved from are bearing rocks using microorganism . The world there are vast quantities of such low grade copper ores that cannot be profitably purified by conventional chemical method. but that could by microbial leaching. There are also significant quantities of nickel , lead and zinc ores which could be leached. Leaching was discovered as a process occurring in pumps and pipelines installed in mine pits containing acidic water.

3. I t was subsequently developed foe the recovery of metal from low grade ores. For many metals, there are now leaching methods which permit extraction from metal or other ores. The metals are converted to water – soluble metal sulfates with the aid of biochemical oxidation processes. commonly used microorganisms are: Mesophiles moderately extremophiles

4. MICRORGANISM USED IN BIOLEACHING:- TWO most commonly used organism in microbial leaching are thiobacillus thiooxidans and thiobacillus terrooxidans . A number of others may also be used including: Thiobacillus concretivorus , pseudomonas fluorescens , P. achromobacter , bacillus licheniformis , B. polymyxa and several thermophilic bacteria including thiobacillus thermophilica , . The heterotrophic organism listed have not as yet actually been used ,but it seems likely that processes will be developed by which metals are extracted from ores with microbially produced organic acid via chelate and salt formation .

5. CHEMISTRY OF BIOLEACHING :- The reaction mechanisms are of two types, 1 Direct bacterial leaching 2 indirect bacterial leaching 1. Direct bacterial leaching in this process, a physical contact exist between bactria and ores and oxidation of minerals takes place though enzymatically catalysed steps

6. ex; pyrite is oxidised to ferric sulphate 2FeS2+ 7O2+ 2H2O 2Feso4 + 2H2so4 2 indirect bacterial leaching IN THIS process the microbes are not in direct with minerals , but leaching agents are produced by these microbes which oxidize the ores.

7. The process involving thiobacillus ferroxidans is being used since 1960 s in canada for uranium recovery , and since 1970 s in south africa for recovery of gold . In USA and some other countrics it is being used mainly for the recovery for copper ; about 10-20 % of the world copper supply is derived by this process .

8. COMMERCIAL PROCESS OF BIOLEACHING :- Naturally occur bioleaching process is very slow . For commercial extraction of metal by bioleaching the process is optimized by controlling the Ph , temperature. Three method of the commercial process used in bioleaching : - Slope leaching Heap leaching In-situ leaching

9. 1 Slope leaching Finely ground ores ( up to 10,000 tons) are dumped in large piles down a mountainside and continuously sprinkled with water containing thiobacillus . The water is collected at the bottom and reused after metal extraction and possible regeneration of the bacteria in an oxidation pool. 2 HEAP LEACHING The ore is arranged in large heaps and treated as in slope leaching .

10. 3 IN SITU – LEACHING In this process the ore remains in its original position in earth . Surface blasting of earth is done to increase the permebility of water. Water containing is pumped through drilled passages to the ores. Acidic water seeps through the rocks and collect at bottom. Again water is pumped from bottom Mineral is extracted and water is reused after generation of bacteria.

11. Bioleaching of some metals:

12. MERITS of using microbes for ore leaching:- It does not require high energy imputs It can be applied both on small and large scales It is self – regenerating if soluble iron is present since fe+2 is oxidised to fe+3 by T. ferroxidance The process can be used to extract a variety of metals

13. LIMITATIONS OF MICROBIAL ORE LEACHING:- The desired metal is recovered as a dilute solution of its salf and not as elemental metal. This makes a recovery process from the solution essential. The microorganisms must be kept viable by providing approprite conditions