grade ore and tailings left from any previous mining is piled up (1). The ground below is
impermeable, allowing the solutions to drain away. The pile is then sprayed with the micro-
organisms, who thrive in the acidic environment. The bacteria convert the insoluble sulphide
materials into a solution containing Cu2+, Fe2+, Fe3+ and SO42- ions (2). The solution containing
copper ions is drained away, and then separated by the process of ligand exchange solvent
extraction. This process transfers the copper ions from the leaching solution (low concentration) to
the organic solvent (high concentration). This whole process depends on the pH. Figure 1 shows the
reaction that happens, where L is the Ligand. A good example of a Ligand that is used with copper
is kerosene. The copper is then extracted from the new solution by an electric current being passed
through it, and the copper collecting on the negative electrode.
The new process that revolutionised the gold industry is a cyanide leaching. It allows 1 part of gold
to 300,000 parts of worthless material can be worked at a profit (3). It is a multiple stage extraction
process. Firstly, the ore must be crushed dry in a gyratory crusher and then ground wet in a
semiautogenous grinding mill, while cyanide and lime are added. The ore leaves the tank in a slurry
of muddy water, where carbon - which picks up the gold - is introduced as it flows into a huge
stirring tank. The gold leaches out in about 20-30 hours where it is screened from the slurry and
pumped into an desorption tank where it is gold stripped. Typically a 20% ethanol solution is used at
80oC. The gold is now concentrated in the stripping solution and is pumped into electrowinning
tanks. The gold then plates onto the steel wool within the tanks, when a golden fleece is created
after hours. The golden fleece is the melted with the flux which removes the steel and impurities. The
molten gold is the made into shiny bars and usually sold for further refining. Nothing is wasted: the
carbon is heated with oxygen to reactivate it and restore it to the slurry tanks; the slurry eventually
ends up in a tailing pond. Some lower grade ores may not go through this process, as it - like
copper - is crushed and placed in heaps and slowly leached with cyanide solution. This process is
cheaper but gives lower gold recovery.
The new process of bacterial leaching has many advantages over previos methods. Firstly, it
is more environmentally safe. In previous method of extracting copper, excessive amounts of
Sulphur Dioxide - an intensely poisonous gas and major cause of acid rain - were produced,
typically (weight-wise) twice as much as the copper. In the production of gold, in previous methods,
not only created sulphur dioxide, but arsenic (III) oxide. As the whole procedure is more
environmentally friendly, new businesses will find it easier to get mining permits. Bacterial leaching is
quite considerably cheaper as well. Copper, using the smelting technique, cost between $130 and
$200 per kilo. The biohydrometallurgy cut the cost to less than $70 per kilo (4). For gold, the plant
for cyanide leaching costs only $30 per tonne of concentrate, including powder, chemicals,
maintenance, and labour (5). The lower costs of production means that operations to extract copper
or gold require little start-up capital. Also, as most of the work is done by the micro-organisms,
man-power is lowered (lowering the cost further) meaning that energy consumption is lower and
work related accidents are substantially decreased (6). However, despite all of the advantages of
bacterial leaching, there numerous limitations and disadvantages. Bacterial leaching takes
considerably longer than previous methods. This is because the leaching is fairly quick to extract the
first 90%, but then decreases substantially for the last 10%. Also, weather conditions may hinder
progress. If there is heavy rainfall, permafrost or freezing, then the leaching will not occur. Also if the
pH is not good, then it may slow progress. There must also be sufficient area for tailings disposals
and water must be readily available for a plan to be even considered. Leaching also requires extra
safe-guards as there continually hazardous or corrosive chemicals. Bacterial leaching also is
restrictive as leaching gold doesn’t recover the base metals such as copper and visa versa.
Bacterial Leaching is used a primary extraction method for gold due to the economic
factors. As it only costs $30 per tonne of gold to produced, businesses can make a considerable
profit extracting gold. For copper though, it is only secondary. This may be because it is “easier” to
use previous methods. The new methods still require research and conversions of old plants. This
requires considerable money, so businesses find it easier to continue as they are. Also, there are no
tight environmental regulations on the amount of Sulphur Dioxide produced, so businesses are able
to continue.
For a new mining process to operate commercially it must pass through development stages.
Firstly, there must be sufficient capital available to begin research. There must also be a quandary
with the current process, which could be improved. It must be tested and proved to be more
economically and environmentally viable than the original.
Bibliography
(1), (2), (4) - Article 1
(3) - "Gold," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights
reserved.
(5) - Article 3
(6) - McGraw-Hill encyclopaedia of science and technology 16 (SAB-SON)
McGraw-Hill encyclopaedia of science and technolgy 4 (CLI-CYT)
McGraw-Hill encyclopaedia of science and technolgy 8 (GEO-HYS)
McGraw-Hill encyclopaedia of science and technolgy 16 (SAB-SON)
www.ask.co.uk “process of bacterial leaching”
www.innovations.copper.org
www.ejb.org
Salters Advanced Chemistry: Chemical Storylines
