Liquids called reagents in which one attracts metal and other makes bubbles are added to the slurry in a large container in the presence of air. As agitators mix the slurry, metal and reagents are attracted to each other. After this the heavy metals (copper & Gold) are settled down in the container and the lighter liquid which contains other impurities, reagents and water rests at the top of the container which is then drained away.
Extraction of Gold
Bacterial Leaching of mineral Arsenopyrite (FeAsS):-
The Gold metal can be extracted from minerals like Arsenopyrite, Pyrite,
Chalcopyrite. The bacterial leaching of Chalcopyrite involves two main bacteria Thiobacillus ferrooxidans and thiooxidans.
The Process
Stage 1:-
Bacteria catalyse the mineral arsenopyrite (FeAsS) by the oxidation of Sulphur and arsenic metal to higher oxidation states and reduction of oxygen by hydrogen and iron. This reaction occurs at the cell membrane of the bacteria.
Direct mechanism:-
FeAsS (s) + ½ O2 (g) + 2H+ (g) + Fe+3 (s) Fe+2 (aq) + As+3(aq) + S+6(aq) + H2O(aq)
FeAsS(s) Fe+3(s) 2H+ (g) CO2(g)
½ O2 (g)
H2O(aq)
Fe+2 (aq) As+3(aq) S+6(aq)
Stage 2:-
Oxidation of ferrous ion (Fe+2) to ferric ion (Fe+3).Through oxidation the bacteria convert the iron into the less soluble ferric form.
2Fe+2(aq) + ½ O2 (g) + 2H+ (g) 2Fe+3(aq) + H2O (aq)
2Fe+2(aq) ½ O2 (g) 2H+ (g) CO2 (g)
2Fe+3(aq) H2O (aq)
Oxidation of Sulphur (IV) ions to sulphuric acid (IV).
S+6(aq) + H2O (aq) +2 O2 (g) Thiooxidans S2O3-2(aq) + 2OH-(aq)
2S2O3-2(aq) not stable in alkaline solution S4O6-2(aq) +2e-
S4O6-2(aq) + 2H+ (g) Thiooxidans H2S4O6
(400 words)
Oxidation of Arsenic metal (III) to arsenic metal (V). Arsenic metal (V) is present in complex forms.
As+3(aq) Fe+3 / FeS2 As+5 (aq)
Thiobacillus bacteria
The overall reaction for Arsenopyrite is:-
2FeAsS (s) + 7O2 (g) + 4H+ (g) + H2O (aq) 2Fe+3 (aq) +2HSO-4(aq) +2H3AsO4 (aq)
Now, the Gold metal is left in the original solution which can be obtained by treating the solution with sodium cyanide (NaCN). Cynidation process increases the gold recovery from 10% to 100%.
Extraction from Mixture:-
Pure solid Gold metal can be extracted by Cynidation reaction in the presence of free oxygen.
Au (aq) + 2NaCN (aq) + 1/4O2 + ½ O2 (g) Na [Au (CN) 2] (aq) + NaOH- (s)
2Na [Au (CN) 2] (aq) +Zn(s) 2NaCN (aq) + Zn (CN) 2 + Au (s)
Extraction of copper
Copper can be extracted from their low grade ores like chalcopyrite (CuFeS2), Pyrite (FeS2), Covellite and others. It is similar to the extraction process of gold.
The process
Stage 1:-
The acid leaching solution (contains H2SO4, Thiobacillus thiooxidans and thioferrooxidans) is sprayed on mineral chalcopyrite (CuFeS2) which is then oxidised into copper (II) ions, ferrous ions (Fe+2), ferric ions (Fe+3), sulphate (II).
CuFeS2(s) +O2 (g) +H+ (g) +H2SO4 (aq Cu+2(aq) +H2O (aq) +Fe+2(aq) +S-2(aq) +Fe+3(aq)
Stage 2:-
Oxidation of ferrous ion (Fe+2) to ferric ion (Fe+3).Through oxidation the bacteria convert the iron into the less soluble ferric form.
2Fe+2(aq) + ½ O2 (g) + 2H+ (g) 2Fe+3(aq) + H2O(aq)
Oxidation of Sulphur (II) to Sulphate and then to Sulphuric acid (II).
S-2(aq) + 4H2O (aq SO4-2 (aq) + 8H+ (g) + 8e-
SO4-2 (aq) + H+ (g) H2SO4 (aq)
Overall reaction:-
4CuFeS2(s) +17O2 (g) +4H+ (g) 4Cu+2(l) +2H2O (l) + 8SO4-2 (aq) +4Fe+3(l)
Extraction from mixture:-
By ligand exchange solvent extraction, copper ions are removed from the solution leaving other ions in the solution. The ligand is a large molecule consists of lone pairs with each small group. The ligand is then dissolved in organic solvent Kerosene and then shaken with the solution.
Cu2+(aq) + 2LH(organic) CuL2(organic) + 2H+(aq)
(600 words)
L represents the ligand.
The electrons denoted by the ligand are taken up by the copper, which produces a complex compound- a central metal copper bonded to two molecules of ligand. The reaction is reversible and copper metal can be obtained by adding the concentrated acid which reverses the reaction and copper ions go back into aqueous solution.
It can also be separated by displacing the copper with Fe from Scrap iron.
O- OH-
:N
C
H
R
Cu2+(aq) + Fe(s) Cu(s) + Fe2+(aq)
The electrons denoted by Fe are taken by Cu as it is oxidising agent and Fe is reducing agent.
Q2:- Discuss the advantages and disadvantages of using bacterial leaching rather than traditional extraction methods and suggest why bacterial leaching is used only as a secondary extraction process for copper but as a primary extraction process for Gold?
Advantages and Disadvantages:-
Bacterial leaching process is simple to operate, mild operation conditions. It uses naturally occurring resources such as micro-organisms, Water and air. It is very friendly towards environment but in smelting and roasting process, emissions of SO2 are released. It requires low energy input, low pressure and temperature process rather than traditional extraction methods. Traditional method is expensive as bioleaching involves low capital costs. Bioleaching is a slow process as it takes decades rather than years to extract metals but as traditional method is expensive but it is faster. As facts had revealed that bacterial leaching is the cause of acid mine drainage.
Secondary extraction process for copper but as a primary extraction process for Gold:-
(800 words)
Bacterial leaching is considered as secondary process for copper for the fact that it is not able to perform on a large scale and copper is present in large amounts in their ores. It is also an extremely slow process and uneconomic. For gold however it proved to be cost effective as it recovered almost all the gold.
Q3:-Summarise the development stages that a new mining process (such as bacterial leaching) must pass through before it can operate commercially?
- It must be safe not damaging the environment.
- Able to meet the demand of the trade market.
- The process should be cost effective to produce the metal.
- Funding should be provided by government and political powers to regulate a new process.
- Tests should be carried out in labs to make sure the process works well.
- The location used for bacterial leaching should be suitable to the favourable conditions of the process,
- A large scale plant should be built for industrial use.
(950 words)
Summary:-
Basically microbiological research focuses on the biochemistry, physiology and genetics of the involved micro-organisms and on the complex interrelationship in the community of the leaching species.
(976 words)
References:-
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Ask search engine for bacterial leaching
- Chemical story line, M1, page 59, Rock-eating bacteria.
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Asked for information of bacterial leaching, bio hydrometallurgy, thiobacillus thiooxidans and thioferrooxidans.