Modern technology can handle any arsenic issues in the few deposits with arsenic present
By Ron Hall
In his article The Cobalt Conundrum on July 3rd, 2018, geologist Alf Stewart highlighted the major issues facing the cobalt market: on the one hand demand is increasing rapidly due to its growing use in lithium batteries required to power electric vehicles but on the other hand supply is being challenged by two major factors:
- Most cobalt is produced as a by-product of copper and nickel mining which limits the flexibility of producers to respond to changes in market demand so as demand picks up for cobalt and those markets remain weak as they are now then cobalt supply is restricted.
- Some 60% of the world’s cobalt comes from the Democratic Republic of Congo, a country that ranks high on the ‘risk of doing business’ index because of its political instability; another 16% of primary cobalt production is from Russia, Cuba, and China combined.
With price of cobalt rising significantly (now trading at US $30/lb compare to US $10/lb as recently as 2015), the hunt for alternative sources is on. Although the largest known cobalt resources are found in the Fe-Mn nodules and crusts on the Pacific seabed, mining of these are not currently feasible due to technical and legal issues.
According to the U.S. Geological survey the deposit types currently in production can be summarised as follows:
- 60% of global production comes from Stratiform sediment hosted Cu-Co and only one such deposit contains economic cobalt resources; ie, the Central African Copperbelt. Cobalt is present as the mineral carrollite, a copper-nickel-cobalt sulphide.
- 23% comes from Magmatic Ni-Cu-Co-PGE sulphides found in Canada, China and Russia.
- 15% comes from Laterites in Australia, Brazil, Cuba and New Caledonia.
Aside from these major supplies, another 2% comes from other sources which includes Five-Element Vein Deposits recognized as distinctive ore types since the early 20th century. The deposits contain Ag-Ni-Co-As-Bi and some have U-REE. The historic silver mines of Europe all contained these deposits. In Canada, the major districts are Cobalt‐Gowganda, Thunder Bay, and Echo Bay. In the United States, similar deposits were mined at Wickenberg (Arizona) and Black Hawk (Silver City, New Mexico). All these deposits produced silver, some produced uranium, and a few produced cobalt.
The Cobalt-Gowanda area in Ontario is centred around the town of Cobalt, some 500 km north of Toronto but despite its name the town was actually built on silver. Over 100 years ago, contractors looking for lumber to expand the Canadian railroad discovered visible silver in the loose rock. Within two years, 600 prospecting licenses had been issued and by 1907, that had swelled to almost 10,000, according to records housed in the Cobalt Mining Museum. By 1908 Cobalt produced 9% of the world’s silver, and in 1911 produced 31,507,791 ounces of silver. Although mining continued until the 1930s, it then slowed to a trickle. Activity renewed in the 1950s then slowly dropped off again, and since the 1980s, there have been no operating mines in the area.
Although the silver deposits in the Cobalt area were associated with cobalt (which back then had little or no economic value and so was not extracted) they also contained significant amounts of arsenic that was also not recovered and ended up in the tailings and waste rock piles. In those days, waste rock was frequently disposed of in dumps that extended outwards from mine headframes and tailings were normally disposed of in the closest convenient depression in the land, sometimes even right beside the mill, or even into nearby lakes.
Today, arsenic continues to leach from these mining wastes, and most of the lakes and streams around Cobalt are laden with arsenic with some of the highest concentrations of arsenic in water anywhere in Canada. Despite recent remediation projects, Cobalt remains one the largest sources in Canada of releases of arsenic with estimates of the amount of arsenic discharged each year into Lake Temiskaming ranging from 10 to 18 tonnes – more than all operating mines in Canada, combined.
With cobalt explorers scouring the world for new sources of cobalt, there is obviously renewed interest in the Cobalt area with the potential for not only new discoveries but for reprocessing of dumps, tailings and other effluents to extract the metal.
Modern exploration and mining requires an integrated approach to insure arsenic control is present at each phase of the mine life, with a goal of maximizing arsenic rejection in its original minerals at the mine and mill through careful evaluation of the arsenic minerals and optimization of mineral processing. Currently, the primary approach for arsenic control is to stabilize arsenic as a stable product through metallurgical processing.
The technologies that have been practiced or studied for the treatment of arsenic containing effluents include:
- Neutralization with lime
- Co-precipitation with ferric ion or other chemicals
- Biotechnical approach
Lime neutralization to a high pH (~12) was widely practiced for the treatment of effluent to remove arsenic, due to the convenience in its operation. But it is no longer considered acceptable in terms of the high As solubility and the environmental instability of the produced calcium arsenite or arsenate sludge.
The co-precipitation of arsenic with ferric ion is considered an environmentally more acceptable method and the produced arsenic ferrihydrite sludge can be disposed in an environmentally safe way. Over the past decade, this method has gradually replaced the lime neutralization method.
Adsorption is a widely recognized technology for the removal of arsenic from water, e.g, the naturally occurring low arsenic-containing underground and surface water. The adsorbents available include alumina, natural or artificial minerals, and ion exchange resins. For the effluent from mine sites and metallurgical processing, the adsorption technology is less likely to be an efficient and cost-effective solution due to the relatively high level of arsenic and other species which may compete with arsenic for absorption sites or contaminate the surface of the adsorbent particles.
Biological treatment of arsenic-containing effluents is based on the biological formation of arsenic sulfide, i.e the reduction by bacteria and formation of insoluble arsenic sulfide complex under anaerobic conditions. It could be practiced in a bioreactor or anaerobic/wetlands cell.
A new solution may be found in Sweden where scientists have recently discovered a moss that purifies water contaminated with poisonous arsenic so successfully that it becomes safe to drink. Researchers at Stockholm University say the aquatic moss, warnstofia fluitans, which flourishes in northern Sweden, can rapidly absorb arsenic, removing as much as 82% of the toxins within one hour in some tests.
Although in many ways the area around Cobalt stands as a testament to just how much mining has changed in the last century – the prospecting and mining methods used in Cobalt back then would be completely unacceptable today – the current staking and exploration rush is naturally raising concerns about exacerbating what is already a major environmental issue; however, the technology exists to handle arsenic concerns.