4.10.1.4 - Phytomining and bioleaching

Limited ores and low-grade resources (Higher)

A low-grade ore contains a relatively small proportion of the useful metal compound.

An ore is rock containing enough of a metal or its compounds for extraction to be worthwhile. “Enough” depends on the available technology, energy prices and value of the metal, not simply whether any copper is present. A low-grade ore contains a lower proportion of the desired material than a high-grade ore. To obtain the same amount of metal, a conventional route may have to dig, move and process much more rock.

The Earth's metal-ore resources are limited. Accessible copper ores can become scarce as the richest deposits are used. Even where low concentrations of copper remain, a route involving large amounts of waste rock and high processing costs may be unattractive. Chemistry and biological processes provide alternative ways of concentrating and recovering the metal.

Extracting from the higher-grade rock

Illustrative ore comparison: $1000\,\mathrm{kg}$ of rock P contains $20\,\mathrm{kg}$ of copper, while the same mass of Q contains $2\,\mathrm{kg}$. Their copper percentages are:

To obtain $20\,\mathrm{kg}$ of copper from P with complete recovery, divide by its copper mass fraction.

Extracting from the lower-grade rock

For the same $20\,\mathrm{kg}$ target from Q:

This is an ideal limit: real extraction loses some copper and would need more material. The lower grade requires ten times the rock mass to supply the same copper.

Phytomining uses plants and bioleaching uses bacteria. They can exploit low-grade resources while avoiding or reducing traditional methods of digging, moving and disposing of large amounts of rock. They do not make copper atoms or turn a finite metal resource into a renewable one; they recover copper that already exists in compounds.

Concentrating metal compounds by phytomining (Higher)

Phytomining concentrates metal compounds in plant material before the metal is recovered chemically.

Some plants absorb metal compounds through their roots while growing in soil containing those compounds. The metal becomes incorporated into the plant material. Suitable plants can therefore gather metal from a large area containing low concentrations, including material that would be uneconomic to treat by conventional mining.

The plants are harvested and burned. Burning removes much of the organic plant material as gases, leaving an ash containing metal compounds. Because much of the plant mass is removed, the ash has a greater concentration of the metal compounds than the starting plant material. The ash is not pure copper: burning is a concentration step, not the final conversion of a copper compound into copper metal.

A suitable chemical treatment can transfer copper compounds from the ash into a solution, from which copper can then be recovered. This separates two problems: first collect and concentrate the copper compounds, then obtain the metal itself. A complete account must include harvest and burning rather than saying only that plants “take up copper”.

The rate is limited by plant growth, uptake and the growing season. Land is needed and not every plant tolerates or absorbs useful amounts of the metal. Burning and processing the ash still have energy and environmental costs. The route can reduce the movement of bulk rock, but whether it is worthwhile depends on yield, timescale and the available site.

Bioleaching and recovering copper from solution (Higher)

Bioleaching uses bacteria to produce a liquid containing dissolved metal compounds, called a leachate.

Bacteria act on suitable metal-containing material and help produce soluble metal compounds. The resulting leachate solution is collected. Copper in the leachate is present as dissolved copper compounds, not chunks of copper metal. The rate depends on conditions suitable for the bacteria, such as temperature and access to the materials they need. A leachate must be controlled so that dissolved metal compounds do not contaminate surrounding water or soil.