The process for copper is very similar. The mineral chalcopyrite CuFeS2 (pictured) follows the two stages of being dissolved and then further oxidised, with Cu2+ ions being left.
Copper can be extracted from low-grade ore by means of bacterial leaching. The low-grade ore and tailings are put onto an area of impermeable ground. This is then covered in an acidic leeching solution that contains the bacteria, T. ferro-oxidans and T. thio-oxidans. These bacteria are best suited to an acidic environment. All the bacteria need are Fe2+ ions or S2- ions, oxygen, carbon dioxide and bacterial nutrients containing phosphorus and nitrogen. The final result is that the bacteria convert the insoluble chalcopyrite into a solution containing Cu2+, Fe2+, Fe3+ and
SO2- ions. The solution carrying the copper ions can be straightforwardly drained off as it is on an impermeable base layer. T. ferro-oxidans catalase the oxidation of Fe2+ and Fe3+ ions.
Extraction from the Mixture using bacterial leaching
Copper (Cu2+) ions are removed from the solution by ligand exchange solvent extraction which leaves other ions in the solution.
Ligand - is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. A lot of such ligands around a centre are termed a complex.
The copper is removed by bonding to a ligand, which is a large molecule consisting of a number of smaller groups each possessing a lone pair. The ligand is dissolved in an organic solvent such as kerosene and shaken with the solution producing this reaction:
Cu2+(aq) + 2LH(organic) -> CuL2(organic) + 2H+(aq)
The ligand donates electrons to the copper, producing a complex - a central metal atom (copper) bonded to 2 molecules of the ligand. Because this complex has no charge, it is no longer attracted to polar water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial reaction is reversible, and therefore not a displacement reaction, it is determined by pH. Adding concentrated acid reverses the equation, and the copper ions go back into an aqueous solution.
Then the copper is passed through an electro-winning process to increase its purity: an electric current is passed through the resulting solution of copper ions. Because copper ions have a 2+ charge, they are attracted to the negative cathodes and collect there.
The copper can also be concentrated and separated by displacing the copper with Fe from scrap iron:
Cu2 + (aq) + Fe(s) -> Cu(s) + Fe2+ (aq)
the electrons lost by the iron are taken up by the copper. Copper is the oxidising agent, and iron is the reducing agent
(Reference: , 23rd February 2008)
(Reference: , 23rd February 2008)
The crushed ore of gold is treated with H2 SO4, The slurry of crushed ore and liquid ore is moved from tank to tank to ensure adequate time for arsenopyrite oxidation, Bacteria oxidise the iron or arsenic or sulphur in mineral; and release gold that is encapsulated. Also the Bacterial leaching solution contains Sulpholobus acidocalderius. It is then extracted by treating with sodium cyanide (cyanidation) gold forms a soluble complex with cyanide ion.
‘Bacteria can expose 85-100% of the latter to extraction by cyanide’
(Reference: Salters, ‘Advanced chemistry’, ‘activities booklet’, chapter M2.5, page 83)
advantages and disadvantages of bacterial leaching compared to traditional methods
Bacterial leaching is cheaper and quieter and less polluting than smelting process.
(Reference: Salters' Advanced Chemistry: Chemical Storylines, page 49, chapter M2, published 1994, by George Burton)
Environmental advantages also include that the bacteria used can be recycled for reuse, decreasing expenditure.
Some environmental disadvantages are associated with bacterial leaching including that heavy ions such as iron, zinc and arsenic, can leak out of the mine during draining, forming an ionic precipitate when diluted by fresh water.
Bacterial leaching requires little labour so it’s profitable compared to the Traditional methods.
Sulphur dioxide is not released in bacterial leaching and doesn’t need high
temperatures to run
Development stages of new mining process
Bacterial leaching is used a secondary for copper as it is extracted too slow and the companies can’t profit at all-because the metal recovery may take decades. Bacterial leaching is used as primary extraction for gold: the arsenic produces is non toxic and greater % of gold is extracted by bacterial leaching.
A lot of research has gone into to find to find best bacterial culture and conditions. A lot of funding has gone into developing the pilot stages.
‘Research at BacTech quickly progressed from 450 dm3 laboratory plant then to a 32m3 transportable pilot plant.’
(Reference: Chemistry Salters, 1st march 1999, Open book, article 3, golden opportunity, golden opportunism)
Bibliography
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(Reference: , 23rd February 2008)
X 2.
(Reference: Salters, ‘Advanced chemistry’, ‘activities booklet’, chapter M2.5, page 83)
(Reference: Salters' Advanced Chemistry: Chemical Storylines, page 49, chapter M2, published 1994, by George Burton)
(Reference: Chemistry Salters, 1st march 1999, Open book, article 3, golden opportunity, golden opportunism)
