7th November 2018 – Josh Powell writes for MiningIR – Follow our ‘Mining 101’ series, where we introduce investors to all the key concepts that contribute to a project’s overall value, HERE including an in depth article on Acid Mine Drainage from earlier this year.
Financing, finding and freeing ore from its incarceration in the crust is only half the battle. Once a deposit has been blasted and crushed comes the long process of turning billions of tonnes of rock containing a few percent of metal into pure ingots. There are a variety of ways to achieve this.
Historically, smelting was the answer. It is an effective process and was used extensively in Cornwall (Illustration 1), however this resulted in high levels of toxins in the soil which is unacceptable under modern environmental laws as we now care about the environment, people’s health and the planet’s atmosphere.
This has forced geologists, metallurgists and sciences in general to come up with new ways of extracting what we want out of the earth with minimal destruction. Initially as smelting was phased out companies turned to hydrometallurgical processes as it has a much lower environmental impact. These involve pulverising the material and mixing it with a chemical concoction that draws the metal out of its minerals forms via a series of chemical reactions.
An example of this is the addition of cyanide to gold bearing ores. A mixture of cyanide, carbon ( in the from of coconut shells) and acids are added to a pulp containing gold. The gold and cyanide form a compound which is absorbed by the carbon and removed from solution. An adjustment to the pH and the gold is released where it can be collected and refined. Other metals and minerals work best with different acids at different pH, temperatures, density and many other factors, which can be adjusted to get better recovery.
The world’s increasing supply of low grade ore presents an economic challenge to larger scale hydrometallurgical process which necessitate large chemical plants and vast amounts of reactants which is rarely economical viable.
To remove the metal a cheaper option is needed. Bioleaching has provided the mining industry with an answer. First observed in 23-79 AD “leaving water running through a mine in winter leaves a copper decomposition” (4) and actively used in the the 16th century where miners left material to decompose naturally in the sun ( Illustration 2). In 1961 the method was first official identified and detailed by Rio Tinto. The process involves the application of certain species of bacteria that gain energy from the oxidation (removal of elections) of sulphur or iron rather then oxygen.
These hungry bacteria belong to the family Acidithiobacillus ferrooxidans or in simple terms iron oxidising bacteria which are the most commonly applied. Given the right conditions (species vary on temperature and local acidity but usual the lower the pH the better) the bacteria will oxidise the iron or sulphur in the mineral (Illustration 3) causing it to break down into its constituent components. Usually the copper can be collected from solution in the case of chalcopyrite ( a common copper ore) for example iron and sulphur remain in place.
Bioleaching operations require little initial expenditure on machinery. All you need is a method of fluid collection ( PVC Pipes ), bacteria ( which can be ordered from labs) and billions of tonnes of low grade ore (which unfortunately is becoming increasingly common). After the initial piles are built a weak solution is slowly dripped through them to ensure the microbes are healthy. Apart from that you sit back and let the bacteria feast while you collect the targeted metal.
A similar alternative is leaching which relies on acid being slowly dripped on the piles and a chemical not biological process leading the the break down of the mineral. The major disadvantage is that non metal minerals also react with the acid which consumes it. This means depending on the rock type the acid cant be extremely high.
As more funding is provided for research into Bioleaching new strains of bacterium will be discovered and better methods of implementation will be designed. This will see its popularity increase, especially when applied to low grade ore is available.
An area where Bioleaching is becoming increasingly popular is in the treatment of the arsenic rich copper ore: Enargite. Historically this material was ignored but as copper resources run low globally companies reviewing these preliminary rejected resources. Bioleaching have proven a effective method as the bacteria can become acclimatised to the high concentrations of arsenic (3), separating it from the copper while trapping the arsenic in place.
With very few large scale copper projects coming online and 200 projects finishing in 2035 there will be significant shortcomings in copper production while global demand for copper is expected to rise ( illustration 4). As such cleaner and more efficient methods of copper extraction that can economically process low grade ores must be found and Bioleaching is well positioned to take a substantial share of the market in the upcoming years.
REFERENCES:
- Agricola, G.E.O.R.G.I.U.S (1555). De Re Metallica. Italy: .
- Cecilia Jamasmie. 2018. Copper supply crunch earlier than predicted — experts. [ONLINE] Available at: http://www.mining.com/copper-supply-crunch-earlier-predicted-experts/. [Accessed 29 October 2018].
- Sadegh Safarzadeh , Michael S. Moats & Jan D. Miller (2014) Recent Trends
in the Processing of Enargite Concentrates, Mineral Processing and Extractive Metallurgy Review: An International Journal, 35:5, 283-367, DOI: 10.1080/08827508.2012.723651 - Plinius G. secundus. Naturalis historiae libri XXXVII [on-line access under http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Pliny_the_Elder/home.html]
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