Phytomining

Phytomining trial crop in Brazil a few years ago.

Defined...

Croesus Projects Ltd. defines phytomining as the cultivation of a crop where the specific purpose is to process metal out of the harvested biomass for sale to the international market. Phytomining can therefore be considered as ‘farming for metals’.

Phytomining was first described for nickel in 1994, by two scientists from the United States Department of Agriculture (Larry Nicks and Michael Chambers). These scientists grew the nickel accumulating plant Streptanthus polygaloides in plots of nickel-rich soil in California, and predicted that a nickel yield of about 100 kg of metal per hectare could be achieved.

Throughout the 1990s and 2000s, scientists further investigated the feasibility of nickel phytomining in Brazil, South Africa, the USA and Mexico. Several patents were taken out on nickel phytomining between 1998 and 2004. However, nickel phytomining is still not in operation as a commercially viable activity anywhere in the world.

Nickel phytomining makes use of plants that naturally accumulate metals, and there are more than 400 species known to do this. But similar species are not known for the more valuable metals gold, platinum and palladium. This is because these metals are very insoluble in soil. Because uptake is proportional to solubility, very little uptake of precious metals occurs naturally. In order to phytomine these metals, chemicals must be added to the soil to promote solubility. Uptake can then occur naturally as a function of mass flow as a plant is absorbing water from the soil during evapotranspiration.

Metal Solubility

Metal solubility is what the mining industry is promoting when gold is recovered from rock using conventional mining technology. Chemicals like cyanide are used to leach gold from rock, and the now soluble gold can then be recovered as pure metal.

Precious metal phytomining is therefore strongly linked to the ideas and technology of conventional mining. The key difference is that instead of leaching gold and processing a gold-rich solution, plants are used to absorb this gold, and the precious metal is recovered from the plants.

In practical terms, a precious-metal phytomining operation may work something like this:

Grow plants with a large leaf area on an area of precious-metal rich soil;
Once the plants have maximum leaf development, irrigate a chemical onto the soil that will dissolve some of the gold in the soil. This irrigation occurs over about 30 minutes;
Over the next days plants will accumulate precious metal, but also more toxic metals made soluble;
Accumulation of these metals will kill the plants, so when this happens harvest the crop;
Next the crop is processed, the metal(s) recovered, and the bullion sold to the international market.

Unfortunately, the limited solubility of precious metals under normal environmental conditions dictates that chemicals must be used to recover them from rock. Phytomining uses a small amount of chemical for a short period of time. But even in phytomining these chemicals must be used properly to ensure that there is no environmental risk. Phytomining for precious metals should only be conducted under instruction from trained professionals. Croesus Projects Ltd. can work with clients at all stages of precious metal phytomining.

Mexico - plot 8 weeks after seeding

Mexico - plot 48 hours after treatment, plants are dead and ready to harvest.

Resource for Gold Phytomining

The decision on whether to conduct phytomining is made based upon economics. Where the metal recovered from harvested biomass generates sufficient revenue to cover all costs, an operation can proceed. The biggest determinant of revenue is the amount of metal in the plants, and this is proportional to the concentration of metal in the soil, waste rock or tailings (resource) being farmed. A rough cut of grade for metal in soil is about 1 mg/kg (the same as 1 g/t).

A cut-off-grade of 1 g/t limits the locations where a phytomining operation can be run. Modern mines in developed countries are efficient, and the concentration of metals in the waste is generally less than 1 g/t. Phytomining is not particular relevant to such resource. Mines in developing countries are often less efficient, so phytomining may be applicable. But the single greatest application may be to small deposits of historic tailings or waste rock with precious metal concentrations greater than 1 g/t. Such material can be found throughout the world, but is more common in developing countries.

However, availability of resource for phytomining has been limited in recent years. As the gold price crept closer to US$2,000/ounce, companies began to review options for re-processing historic tailings using existing facilities. In this scenario gold phytomining is actually competing for resource with conventional technology. But at a gold price closer to US$1,000/ounce, conventional technology is not economically viable, leaving phytomining as a realistic option to generate profit from waste material.

Phytomining, nanoparticles and catalysts...

In 2002, a research group led by Prof. Jorge Gardea-Torresdey at the University of Texas at El Paso (USA) reported their discovery of gold nanoparticles in alfalfa plants. This fascinating report precipitated a plethora of research studies that have analysed and imaged the properties of gold in plants. Mounting evidence shows that a range of precious metals are stored in plants as nanoparticles of metal, and there is real potential that these nanoparticles could be recovered and used in a range of useful catalytic applications.

Where is precious-metal phytomining being used today?

Of the precious metals, phytomining for gold is the most developed. There is significant interest in this idea from private groups, companies, and scientists around the world.

Croesus Projects Ltd. is involved with the development of gold phytomining projects in China, Mexico and Indonesia. In each of these cases there is environmental and social value associated with phytomining. Gold revenue at each location is helping promote management of mercury (Indonesia) and arsenic (China) in mine waste, while in Mexico, phytomining is being used as a mechanism to create employment in poor areas.

Platinum and palladium phytomining was first explored by Amplats (Anglo American) in South Africa from 2001-2003. Today the technology is being developed by the G8-funded PHYTOCAT consortium as a mechanism to create nanoparticles of precious metal that can catalyse industrially important chemical reactions. Croesus Projects Ltd. is supporting this work through association with the University of British Columbia in Vancouver, Canada.

For more information see the following publications:

In A. Hunt (Ed) Element Recovery and Sustainability (pp. 114-139) RSC Green Chemistry No. 22

Anderson C. W. N. (2013)

Hyperaccumulation by Plants

ACS Sustainable Chemistry & Engineering, 1(6), 640-648

Anderson, C. W. N., Bhatti, S. M., Gardea-Torresdey, J. and Parsons, J. (2013)

In vivo effect of copper and silver on synthesis of gold nanoparticles inside living plants

Journal of Environmental Management, 111, 249-257.

Wilson-Corral, V., Anderson, C. W. N. and Rodriguez-Lopez, M. (2012).

Gold phytomining. A review of the relevance of this technology to mineral extraction in the 21st Century.