197 Ligand takes up Cu2+ ions in the following reaction:
Cu2+(aq) + 2LH (organic) ↔ CuL2 (organic) + 2H+(aq)
(L represents Ligand)
and if like Kerosene, the Ligand is immiscible with water, the new solution can be easily removed. Alternatively, scrap iron can be used to reduce the Cu2+ ions. When the Cu2+ ions are alone in solution, electrolysis can be used to reduce them, the copper collects at the negative electrode in sheets. The leftover solution can then be pumped back up to the top of the ore pile and used again (providing any Fe2+ ions have been oxidised to Fe3+ ions first)
The whole process can be summarised in this diagram. (left)
Bacteria can also be used to extract gold from within refractory minerals like iron pyrites (FeS2), arsenopyrite (FeAsS) and chalcopyrite (FeCuS2).
If you take arsenopyrite as an example, the oxidation of the mineral occurs in two stages, firstly the oxidation of:
FeAsS → Fe(II) + As (III) + S(VI) (in the presence oxidising microbes)
and secondly the oxidation of Fe(II) and As(III):
Fe(II) → Fe(III) (in the presence of oxidising microbes)
and
AS(III) → AS(V) (in the presence of oxidising microbes)
This means that the main products of the reactions(S(VI), Fe(III) and AS(V)) are all water soluble. The remaining solution is then treated with ground limestone or slaked lime, this neutralises the acidic conditions and causes the iron(III) arsenates, iron oxohydroxdide and gypsum to form a gelatinous precipitate.
397 The gold is then extracted from this mixture using a process called cyanidation. In cyanidation, metallic gold is oxidized and dissolved. The oxidant employed is atmospheric oxygen, which causes the dissolution of gold and the formation of sodium cyanoaurite and sodium hydroxide.
The reaction involved with this process is as follows:
4Au + 8NaCN + 2H2O + O2 → 4NaAu(CN)2 + 4NaOH
When gold dissolution is complete, the gold-bearing solution is separated from the solids and the gold can be removed by electrolysis.
The main advantage of using bacteria to oxidise insoluble ions and release metal ions into solution is the benefit to the environment. During traditional copper extraction, the ore is smelted at very high temperatures, the sulphur in it is converted into sulphur dioxide. This waste gas is the main contributor to acid rain, which in the past before sulphur dioxide emissions were strictly regulated caused major damage to lakes, forests and buildings. Typically, to produce one tonne of copper, two tonnes of sulphur dioxide formed. During bacterial leaching, sulphur dioxide is not produced, therefore reducing the cost of sulphur dioxide extraction processes and the potential threat to the environment. The use of microbes as an alternative to copper ore smelting is cheaper; smelting ore can cost between $130 and $200 per kilogram whereas the cost of
603 bacterial oxidation can be as little as $70 per kilogram.
For both gold and copper extraction, the use of bacteria in production results in a higher percentage of metal being retrieved, for example it is worthwhile in copper extraction to use ore that has already been refined using previous methods. In the extraction of gold bacterial oxidation increased some companies retrieval rate by 90%, as illustrated in the graph below.
Another advantage of bacteria oxidation during gold extraction is that arsenic (III) oxide is not produced. During the roasting of gold containing ores, this compound is produced and is very toxic.
Another advantage of bacterial oxidation is that base metals can also be extracted, at the Youanmi plant in Perth, Australia, almost 90% of copper, nickel, cobalt and zinc in the ore is being retrieved.
A major disadvantage with the use of bacteria in mining processes is the time the microbes need to work on the ore. Conventional mining techniques may recover all of the copper or gold in an ore deposit in a matter of months whereas with the use of bacterial leaching, this process may take years or decades.
The release of the compound iron(III) arsenate(V) as one of the
807 products of the gold extraction process may pose a threat to the environment. It is possible that that acidic rain or river water may slowly dissolve arsenic(V) and release into the water. However, it is unlikely that the concentrations will reach hazardous levels.
One reason why microbes are used as a primary extraction process for gold and not copper is the speed at which the microbes work. Companies are hesitant adopt the technology as they will not see the benefits for a long time. Gold however, is much more valuable, so profit can be made although the process takes more time.
A new mining process like bacterial leaching must go through several stages before it can operate commercially. Firstly the technology has to be developed on a laboratory scale. Then plans to apply it to field locations have to be produced as well as larger scale prototypes. When the process is well refined (i.e. what works best at what temperatures, pH, pressure e.t.c) the technology has to be applied full scale. Then the necessary adjustments made to previous plants, or new plants constructed. Only when the new process is working effectively and making a profit will the adapted
999 or new plants be operating commercially.
