With a significant portion of large mineral deposits having been discovered worldwide, and, in many cases, reaching the end of viable economic recovery, phytomining is one option that could be chosen to extract the remaining value from this mined ground, Florida International University geosciences research professor Stephen Haggerty tells Mining Weekly.
Phytomining is the exploitation of ‘hyper- accumulator’ plants to obtain valuable elements from degraded or already mined land. These unusual plants can accumulate exceptionally high concentrations of certain elements in their living biomass.
Agromining refers to the full agronomic chain in using hyperaccumulator plants as ‘metal crops’ and is considered a variant of phytomining technology.
The process involves farming such crops on subeconomic deposits or industrial or mineral wastes to obtain valuable elements from their harvested biomass through the production of a ‘bio-ore’ (which is incinerated biomass rich in metals). This bio-ore can then be processed through hydrometallurgical or pyrometallurgical processes to obtain high-value products.
In agromining, the plants are grown, harvested for biomass, dried, incinerated, or ‘ashed’, and processed to recover the target metals.
Australian Research Council postdoctoral research fellow at the University of Queensland, in Australia, and the Université de Lorraine, in France, Dr Antony van der Ent agrees, reiterating that the increasing demand for critical elements challenges conventional methods of resource extraction.
Van der Ent’s work focuses on biogeo- chemistry, with a specialisation in hyperaccumulator plants. He is part of a team developing agromining in the Asia-Pacific region.
“Agromining has created new opportunities for the recovery of metals from natural ore deposits, or for the use of resources that are otherwise uneconomic, such as wastes,” he tells Mining Weekly. He emphasises the value of agromining as part of the circular economy for the effective recovery of valuable elements from mining or industrial wastes.
Haggerty agrees, and notes that, if this concept of farming for minerals is successful in Australia, it can be equally successful in South Africa: “Both countries are richly endowed in mineral deposits and have equally variable agroclimatic conditions.”
In agromining, hyperaccumulator plants can be grown as ‘metal crops’ on very low-grade mineral deposits, Haggerty explains, noting that the metal concentrations in such deposits, for example, most ultramafic soils, are uneconomic to process, or are beyond the extraction capabilities of currently available technology.
Van der Ent highlights that ultramafic soils typically contain 1 000 to 5 000 parts per million (ppm) nickel, while cutoff grades for conventional mining projects are generally 10 000 ppm, or 1%, nickel.
Haggerty however suggests that it depends on the deposit, as, for example, nickel in lateritic serpentinite, as in New Caledonia, has very high-Ni, as opposed to nickel in sulphide deposits, such as moderate to low nickel at Kilembe, in Uganda.
Hyperaccumulator plants can also be used in the rehabilitation of mine sites that have left minor quantities of metals in the ground, or can be used in the clean-up of (and to gain value from) tailings dams and other types of mineral wastes.
“Adapted plant species can also play an enormously useful role in the removal of toxic waste products and metals for agricultural purposes,” Haggerty underscores.
Agromining on low-productivity soils can target large areas that have low productivity for food production, Van der Ent further suggests, noting that agromining in these areas would be superior to conventional agricultural production, possibly generating better economic returns for farmers.
Plants can be central not only to this new form of mining for metals but can also be used as key exploration tools for prospecting to locate orebodies for metals, such as gold, silver, platinum, lead, zinc, copper and molybdenum, Haggerty highlights.
In Liberia, West Africa, the spiny, palm-like plant, Pandanus candelabrum, has been useful in finding kimberlite pipes that may contain diamonds, as the plant seems to have an affinity for growing on kimberlite- derived soils, Haggerty points out.
Meanwhile, the Swedes have used the small pink flowering Alpine plant, the Lychnis alpina, as far back as the Middle Ages to delineate the presence of copper, while the Haumaniastrum katangense, a purple- flowered herb in Central Africa, was used centuries later to outline Central Africa’s rich Copperbelt, according to Haggerty.
Agromining could have definitive economic advantages in the commercial production of metals, metal products or catalysts and provides access to resources that are not easily accessible using conventional mining techniques.
In addition to being an environment-friendly practice, compared with large-scale opencast mining, the socioeconomic benefits derived from agromining include job creation and income for local ‘metal farmers’.
The successful application of agromining depends on the target metal, Van der Ent tells Mining Weekly. For example, tropical regions that host extensive ultramafic soils are the most prospective for nickel, he says, adding that, as agromining renders abundant nonviable resources into accessible reserves, resources such as ultramafic soils with a nickel content as low as 0.1% can be agromined.
Additionally, the criteria for the selection of ‘metal crops’ include high biomass yield, combined with high nickel concentrations of 1% to 2% in the biomass, which typically translates into 10% to 25% in ash (bio-ore).
The performance of agromining is further closely related to available resources of the target elements and the ability of plants to grow and extract these elements from the resources.
“In particular, the extent of the bioavail- ability of the target elements in soils is a major factor that determines the potential of a site for agromining,” Van der Ent says.
However, agromining should always use local, or indigenous, hyperaccumulator species, which are also adapted to local climatic conditions as ‘metal crops’, he advises.
In previous conversations with Mining Weekly, Haggerty highlighted that, in Western Australia, the bark and leaves of eucalyptus trees were shown to be gold-bearing in the gold-rush area of Kalgoorlie. The botanical indicator is being tested throughout Australia to locate potential gold deposits.
Meanwhile, Van der Ent and his research team from the Sustainable Minerals Insti- tute’s Centre for Mined Land Rehabilitation at the University of Queensland, with its partners from the Université de Lorraine, are developing tropical nickel phytomining in Sabah, Malaysia. The French university is also developing field trials in Greece, Spain and Albania.
Research from the centre, to date, has led to the discovery of 120 hyperaccumulator plants new to science. Research into discover- ing more of these plants has been intensified in recent years using advanced X-ray fluorescence methods for the mass screening of herbarium collections. This has led to a significant increase in the global pool of known hyperaccumulator plants suited to a range of different climates and conditions for use as ‘metal crops’, Van der Ent says.
In Africa, the copper/cobalt belt of the Demo- cratic Republic of Congo and Zambia is one of the most important metallogenic regions, and has more than 30 copper and cobalt hyperaccumulator plants.
“This concomitant occurrence of copper/cobalt hyperaccumulator plants and large areas with metal-enriched and -contaminated soils makes a compelling case for developing phytotechnologies in Zambia,” Van der Ent states.
A team of researchers from Australia, which included Van der Ent, as well as researchers from South Africa and Belgium, in collaboration with researchers from the Copperbelt University, in Zambia, conducted fieldwork in October and November of 2014, visiting sites such as Canadian mining and metals company First Quantum Minerals’ Kansanshi mine site.
Van der Ent studied the potential of Zambian copper/cobalt hyperaccumulator plants for the phytoremediation of mining- and smelter-pollu- ted soils. The research was sponsored by the International Mining for Development Centre.
Locally, national mineral research organisation Mintek scientist Tshiamo Legoale has attracted interest in her research in phytomining, as she proposes that gold can be harvested from crops grown on mine waste tailings dumps. In South Africa alone, there is an estimated 17.7-million tons of gold waste.
Leogale, who holds a degree in geology from the University of the Free State and a degree in mineral resources management from the University of the Witwatersrand, presented her winning proposal on gold phytomining at the Famelab South Africa competition, in Johannesburg, in April, as well as the international leg of the competition in the UK in June.
According to Legoale, this is one technological innovation that is intended to be trans- fered to the communities that can use it.
“Hopefully, in future, this can help to employ a few people – it will be fields of gold to harvest,” Legoale stated in a press statement.
Plants, however, do not normally accumulate gold; the metal must be made soluble before uptake can occur. However, some plants exude natural lixiviants that can mobilise gold in soil. Laboratory and greenhouse research has shown that the uptake of gold can be induced by treating the soil or tailings with lixiviants such as cyanide and thiocyanate.
Van der Ent notes that gold phytomining necessitates the use of thiocyanate to liberate gold from the tailings so that plants can take it up. Therefore, this approach is only suitable for enclosed environments, where appropriate controls can be implemented to avoid pollution. Phytomining for gold has been successfully demonstrated, as reported in the article ‘Harvesting a crop of gold in plants’, which appeared in the magazine Nature in 1998.
To progress the concept of using plants as exploration tools and in mineral extraction, Haggerty suggests initiating cataloguing and surveys for hyperaccumulator plants.
“Cross-disciplinary educa- tional programmes in plant science and economic geology are necessary,” Haggerty points out, noting that an appreciation that minerals are nonrenewable resources is paramount.
Van der Ent also suggests that, ultimately, agromining should be subjected to the same rules as conventional agricultural systems for food crops, where production is driven by the market, which controls demand in the quantity and quality of the products (in this case, pure metals, catalytic materials or metal salts).
Despite the untapped potential in Africa for phytomining, there has been little progress to date, Van der Ent acknowledges.
He reiterates that, while the mining industry, locally and worldwide, is hesitant to embrace “radically new” technologies to extract metals, it is important to emphasise that agromining will not compete with conventional mining operations.
“Instead, agromining’s niche would be low-grade surface deposits found in ultramafic soils and as part of the rehabilitation strategy after strip mining has taken place.”
To further build the business case for the minerals industry, a large-scale demonstration is needed to work through operational challenges and provide “real-life” evidence of profitability, Van der Ent concludes.