Globally, ore grades are declining and it is becoming ever more difficult – and therefore costlier – to extract target elements. Dr van der Ent said the increasing demand for critical elements challenges conventional methods of resource extraction. For example, the massive uptake of lithium-ion batteries that use nickel and/or cobalt cathodes, such as those produced in Tesla’s Gigafactory, places further strains on the global demand chain. Agromining of so-called “metal crops” can provide a source of sustainably sourced nickel and cobalt with low environmental impact.
Metal crops or hyperaccumulator plants, explained Dr van der Ent, are unusual plants that have the capacity to naturally accumulate exceptionally high concentrations of certain elements in their biomass. Farming metal crops and harvesting their metal-rich biomass is called ‘agromining’. This innovative approach enables access to resources not accessible by conventional mining techniques and abundant low-grade sources of valuable elements.
Agromining could also benefit local communities, by providing new income opportunities for farmers in Indonesia and the Philippines where low-grade nickel sources are located. It can also be applied for cleaning of metal-polluted sites, including various industrial wastes. Hyperaccumulator plants are now known for elements that pose a major risk to human health, such as arsenic and cadmium, and for nutritional elements such as selenium and zinc. Applications of agromining therefore span from the remediation of pollution to the production of bio-sourced industrial metals and organic supplements for human consumption.
Hyperaccumulator plants are able to accumulate 4% nickel in their leaves, translating to over 300 kg of nickel per hectare per year in harvested biomass. A unique characteristic of the bio-sourced nickel is the high purity affected by the metal crop, with the ashed biomass containing 20–30% nickel and few of the impurities usually associated with nickel ores. This makes these bio-ores ideally suited for specific applications, particularly the electrochemical industry producing rechargeable batteries.
