Evaluating biological metal extraction

The best way to obtain a metal depends on the ore source and the evidence available. Phytomining and bioleaching use living organisms and can be compared with traditional mining and extraction to decide which method is more suitable.

In phytomining, plants take up metal compounds from the ground. The plants are harvested and burned, leaving ash from which the metal can be extracted. In bioleaching, microorganisms help produce a solution containing metal compounds; the metal is then recovered from the solution. Both methods therefore need further processing before the metal is obtained.

Comparing extraction methods

Biological methods can be useful for low-grade ores, which contain relatively little metal. They can reduce the need to dig up, move and dispose of large amounts of rock to obtain that metal. This may reduce environmental damage and some costs, although the actual consequences depend on the method and site.

An evaluation should weigh that potential benefit against the evidence for the particular ore source. Relevant questions include:

• Amount obtained: How much metal can each method recover from the ore?
• Speed: How long does extraction take? A slower method may be less suitable when metal is needed quickly.
• Further processing: What is needed to recover the metal from the ash or solution, and what does that add to the cost or environmental impact?
• Economic and environmental consequences: What costs, energy use, waste or other effects are given for each method?

A strong judgement links these factors rather than simply naming a preferred method. For example, if a low-grade ore would require a large amount of rock to be moved by traditional mining, a biological method may be advantageous because it reduces that work. But if the evidence also shows that the biological method recovers much less metal or takes too long, those drawbacks may outweigh the benefit. The conclusion should state which method is more suitable for that ore source and explain why the evidence supports it.

Need for alternative copper extraction

Copper ores are a finite resource: there is a limited amount available in the Earth. High-grade copper ores, which contain a relatively large proportion of copper, are becoming harder to find.

A low-grade ore contains only a small amount of useful copper compared with the amount of unwanted rock. To obtain a given amount of copper from low-grade ore using traditional methods, much more rock must be dug up and moved, and more unwanted material must be disposed of. This makes traditional mining and extraction less attractive.

Alternative methods are used for some copper ores because they can extract copper from low-grade sources while reducing the need to remove rock on a large scale.

Phytomining

Phytomining uses plants to help extract metal from low-grade ore: material that contains only a small proportion of the metal. The plants take up metal compounds from the ground as they grow, collecting them in their tissues.

The plants are then harvested and burned. Burning leaves ash containing the metal compounds. Because the ash is a much smaller amount of material than the original ore, the metal compounds are more concentrated in it.

The ash does not necessarily contain the metal in its pure form. It is processed further to recover the metal.

Bioleaching

A low-grade ore contains only a small proportion of useful metal compounds. Bioleaching extracts metal from such ores using bacteria.

The bacteria act on the ore, helping transfer metal compounds from the rock into a solution called the leachate. For example, when copper-containing ore is bioleached, the leachate contains copper compounds. The copper metal can then be recovered from the solution in a later step.

Bioleaching transfers useful compounds into solution rather than relying on the removal of huge amounts of rock and mechanical concentration of the ore. The leachate contains metal compounds, not the recovered metal itself.

Recovering copper from compound solutions

Copper obtained as a compound is not yet copper metal. When the compound is dissolved, copper can be recovered from the solution by displacement with scrap iron or by electrolysis. These methods can be used after bioleaching, or after copper compounds obtained by phytomining have been processed into solution.

Displacement with scrap iron

Iron is more reactive than copper, so it can displace copper from a solution containing copper ions. Iron atoms enter the solution as iron ions, while copper ions form copper metal. The copper is deposited and can be collected.

For a solution containing $$\ce{Cu^2+}$$ ions, the displacement can be represented as:

Recovery by electrolysis

In electrolysis, an electric current supplies electrons to the negative electrode. Copper ions in the solution gain electrons at this electrode and form copper metal, which is deposited there.

For $$\ce{Cu^2+}$$ ions, the change is: