Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 259

Bioleaching of Sulphide Ores

Only oxidized ores are amenable to atmospheric leaching using sulphuric acid. Other, less
oxidized, cuprite and sulphide ores, such as chalcocite, bornite, covelite, and chalcopyrite,
require the addition of ferric sulphate and oxygen (as oxidants) to accomplish leaching.
Leaching ores containing bornite and chalcopyrite with ferric sulphate is very slow, even
at elevated temperatures (Weiss 1985).
Bacterial leaching, otherwise also known as bioleaching, is the extraction of a metal
from these less oxidized or sulphide ores using materials found native to the environment:
water, air and microorganisms. In other words, bioleaching is the commercialization of
the ability of certain bacteria, found in nature, to catalyze the oxidation of sulphide miner-
als (Brierley 2000). Leaching sulphide materials, whether in ore or concentrate, requires a
chemical oxidizing agent - ferric ions (Fe 3 ). These can be generated by reactions with air
assisted by bacteria. The oxidation can also be assisted by pressure as in an autoclave.
The results of natural microbial leaching, not the cause, have been known since ancient
times. Iron-rich acidic waters draining from abandoned coal and metal mines as well as
from unmined mineralized areas provide evidence of microbial leaching. In fact, history
records that mine water problems began at the same time that mining activities began. At
Rio Tinto in Spain, seventeenth century records describe the occurrence of copper-bearing
waters. The UK based mining company, Rio Tinto, which was formed in 1873, owes its
name to these copper-bearing waters, although it is iron that imparts the red colouration.
Commercial application of bacterial leaching, however, only began in the 1950s at
Kennecott's Bingham mine near Salt Lake City, Utah. It was noticed that blue copper-
containing solutions were produced in waste piles that contained copper sulphide minerals -
a condition that should not have happened in the absence of powerful oxidizing agents
and acid. On investigation it was found that naturally occurring bacteria were oxidizing
iron sulphides and the resulting ferric sulphate was acting as an oxidizer and leachant for
copper sulphides. These bacteria, thriving at pH 1.5 to 3.0 (Blesing et al . 1975, Hawley
1977), were given the name ferro-oxidans for their action in oxidizing iron sulphides. A
second set of bacteria were also identified and given the name thio-oxidans for their action
in oxidizing sulphur to yield sulphuric acid.
Bacterial leaching offers a method of exploiting small ore bodies with a minimum of
capital investment. Today most commercial operations leaching copper from ore dumps
are located in the Southern Hemisphere: in Australia, Chile, Myanmar and Peru (Dresher
2001). The process consists of injecting the material to be leached with cultivated strains of
appropriate bacteria and maintaining conditions that are conducive to their effective oper-
ation and propagation. Air, for instance, is blown into the heap through air lines situated
under the leach pad.
Copper bacterial leaching is still confined to the leaching of ore, but pilot plant tests
are underway for the leaching of chalcopyrite concentrates that would normally be proc-
essed by flotation and smelting. Unlike heap leaching, industrial leaching processes are not
subject to climatic conditions at the mine site. Leaching of concentrate using vats allows
the metallurgist to apply various combinations of temperature and bacteria or temperature
and pressure.
Leaching with cyanide has been applied almost exclusively to gold and silver as discussed
later in this chapter, but cyanide has also been applied to copper for both oxidized and low-
grade sulphide ores. The effectiveness of cyanide in leaching depends on the ability of the
cyanide ion to form stable complexes with the majority of transition metals. These complexes
are strong enough to overcome the relative inertness of gold and silver and the insolubility of
copper minerals, such as chalcocite, to form copper-cyanide complexes (Weiss 1985).

Bacterial leaching is the
extraction of a metal from these
less oxidized or sulphide ores
using materials found native to
the environment: water, air and
micro-organisms.

Bacterial leaching offers a
method of exploiting small ore
bodies with a minimum of capital
investment.