The gold mining industry in Tanzania is facing highly-sensitive environmental challenges related to the storage of treated gold plant residues in tailings ponds, mixer and peristaltic pumps manufacturer Afromix MD Eugene Els tells Mining Weekly.
Afromix has been active in Tanzania for the past six years and was called on, four years ago, by various gold mining companies to supply high-flow gas dispersion agitators in their cyanide destruction plants in Tanzania, explains Els.
Existing, former and future gold mines in Tanzania include, among others, Bulyanhulu, Buzwagi, Geita, Golden Pride, New Luika, Sekenke and Tulawako.
However, Els notes that, despite the increasing value of gold, which improved following Britian’s exit, or Brexit, from the European Union this year, mining companies in Tanzania and other countries are still wary of overcapitalising projects, as they are concerned that the gold price will decline and, therefore, negatively impact on the sustainability of their businesses.
The World Gold Council (WGC) said in August that investment in gold had been the largest component of gold demand for two consecutive quarters – the first time this has ever happened.
The WGC said in June that, as a result of Brexit, it expected to see strong and sustained inflows into the gold market, driven by the “staggering” level of protracted uncertainty that investors faced.
The council said, at the time, that gold’s surge to $1 313.85/oz provided investors with a much-needed safe haven. The price of gold was $1 326.91/oz as at September 21.
Dangers of Gold Tailings
Els explains that, treated gold plant residues contain cyanide that is harmful to the environment and, in most countries worldwide, it has become mandatory to destroy the cyanide in tailings before their deposition for storage.
Hazardous materials, including cyanide, affect not only humans but also ecological receptors. For mining environments, three groups of ecological or environmental receptors are of concern: mammals, reptiles and amphibians; birds (especially migratory wildfowl); and fish and other aquatic life.
Cyanide is a fast-acting poison that is capable of killing a person within minutes if he or she is exposed to a sufficiently high dose. Humans may be exposed to cyanide by inhalation, ingestion or absorption through the skin. Cyanide prevents oxygen from being used by bodily cells, causing tissue hypoxia and “cyanosis” (a bluish discolouration of the skin). The respiratory system fails to nourish the cells with oxygen, a condition, which, if untreated causes rapid, deep breathing followed by convulsion, loss of consciousness and suffocation.
There are many every day sources of exposure to cyanide (such as automobile exhaust, tobacco smoke and fires, for example); however, these sources of cyanide do not accumulate in tissues because the body transforms such small amounts into a less toxic compound called ‘thiocyanate’, which is then excreted. Cyanide is not known to cause cancer or birth defects or adversely affect reproduction.
The International Cyanide Management Code for the manufacture, transport and use of cyanide in the production of gold was developed by a multistakeholder steering committee under the guidance of the United Nations Environmental Programme and the then-International Council on Metals and the Environment.
The code is an industry voluntary programme for gold mining companies and focuses exclusively on the safe management of cyanide and cyanidation mill tailings and leach solutions. Companies that adopt the code must have their mining operations that use cyanide to recover gold audited by an independent third party to determine the status of code implementation. Those operations that meet the code requirements can be certified. A unique trademark symbol can then be utilised by the certified operation. Audit results are made public to inform stakeholders of the status of cyanide management practices at the certified operation.
The objective of the code is to improve the management of cyanide used in gold mining and assist in the protection of human health and the reduction of environmental impacts.
There are several processes available for the removal of cyanide from gold tailings, with the most common process being the sulphur dioxide method, which includes the destruction of the cyanide in the presence of oxygen and sulphur dioxide. The oxygen is provided through large volumes of air or pure oxygen with sulphur dioxide gas or a solid reagent sodium called meta besulphite.
“Afromix, a division of AFX Holdings, the holding company of Afromix in Africa, has many years of experience in solid suspension and gas dispersion applications. This includes the P4 impeller for gas dispersion in Biomin BIOX plants and, lately, for base metals bioleaching plants,” says Els.
To achieve maximum oxygen use during the treatment process, Afromix has developed specialised impellers that yield oxygen utilisation from compressed air of between 20% and 40%, depending on the percentage of solids in the slurry being treated. When pure oxygen gas is introduced, this number can increase to over 80%.
“Before this process can be initiated at the cyanide destruction plants, our specialised team conducts internal case studies, whereby our sales staff and representatives ensure that they understand the process in terms of figures, volumes of gas and the gas rates before supplying the detox plant with the equipment necessary for the specific procedure,” explains Els, concluding that
South Africa and Tanzania have a golden opportunity to collaborate in terms of proficiency and equipment, as the country presents a stable business environment.”