Energy efficiency in the mining and mineral processing sector continues to be a hot topic, especially in South Africa, where increases in the cost of energy sources – namely electricity, diesel and coal – along with that of labour, exceed inflation.
“Mining companies should critically evaluate their flowsheets to identify opportunities to become more energy efficient and incorporate renewable-energy sources to replace fossil fuels. Mining companies should also look to the future and start planning how to manage disruptive changes to their value chain due to the need to reduce greenhouse-gas emissions to zero within the next 20 to 30 years,” says Lina Hockaday, a senior engineer in the pyrometallurgy division at minerals research organisation Mintek.
Although it will be a complex affair, transitioning to a green economy, thereby reducing energy costs, is feasible, as alternative technologies such as renewable- energy solutions do exist, she tells Mining Weekly.
“Barriers such as the entrenchment of fossil fuel technology, a lack of large demonstration-scale projects and high capital investments delayed the imple- mentation of these technologies in the past. Now, with increasing global funds available to support the greening of mining and mineral processing, the main remaining barrier is resistance to change.”
Hockaday avers that energy efficiency will become increasingly important as the local industry strives to remain competitive in global markets. She notes that the South African mining sector is aware of the importance of energy efficiency and the long-term benefits of energy efficiency improvements.
Combined, the mining and manu- facturing sectors account for about 60% of the country’s energy consumption, according to a February 2017 report prepared by professional services firm Deloitte for State-owned power utility Eskom, in which an overview of energy consumption and pricing in South Africa is provided. The percentage is this high despite the combined gross domestic product (GDP) of these sectors having declined from 31% in 1995 to about 22% in 2015. Today, the mining and manufacturing sectors contribute just over 20% of South Africa’s GDP, states the Energy Intensive Users Group (EIUG).
With Eskom again calling for substantial price increases, the energy-intensive mining industry is set to be placed under greater pressure. But the EIUG has called for an electricity price compact that confines tariff increases to as close as possible to consumer price inflation for the coming five years. This proposal is a precursor to the EIUG’s official response to Eskom’s latest revenue application, which will be adjudicated by the National Energy Regulator of South Africa in the coming months. If granted, it would equate to tariff hikes of 15%/y over the three years.
The EIUG states that, over the past 12 years, consumers have absorbed tariff hikes totalling 427%, or 15.7% each year, with its mining and industrial member companies collectively accounting for 40% of Eskom’s yearly sales.
There are several ways of improving energy efficiency in mining processes, states Hockaday. “These include power factor correction for electrical motors and process waste heat recovery for electricity generation and/or heating, but the energy efficiency measures will depend on the operations at each site and can differ for different processes.”
Energy technology innovations involving advanced materials, energy storage, smart grids, renewable energy conversion, superconductivity, nondeto- nating solutions and high-energy lasers should also be considered by mining companies, she adds.
One innovation that is set to drastically reduce the cost of reprocessing mining waste is a technology developed by Scotland-based Scoria Environmental. The company is edging closer to commercialising its patented pyrometal- lurgical technology, the Minex Process. Scoria Environmental MD Matthew Dorman tells Mining Weekly that the company is closing off a deal for the supply of material and the financing of a pilot plant as part of its final optimisation programme.
In June, the company announced that its board of directors had approved a proposed private placement offering of up to $7.5-million from Spain-headquartered corporate finance firm Cognitive Corporate Finance.
“We finished an initial optimisation of material and require a further $7.5- million to build the pilot plant, with the location of the plant [to depend] on the origin of the funding,” states Dorman. He notes that it is likely the plant will be built in Canada, where, historically, most of the test programmes for the Minex Process have taken place. Dorman adds that the optimisation of the Minex Process could also be undertaken outside Canada should the funders have a geographical preference. Scoria will provide its technical oversight team to assist the test facility staff in ensuring a smooth start-up and optimisation of the pilot plant.
Developed to extract metals contained in smelter and other metallurgical process wastes in an environmentally sound way, the Minex Process uses less energy than alternative traditional processing methods. It is estimated that recovering metals from these wastes may be 70% less expensive than discovery and recovery from primary operations.
It is the use of chloride gas in the patented pyrometallurgical process, instead of atmospheric air, that significantly lowers the boiling points of metals, thus reducing energy consumption materially. For example, while gold melts at 2 800 ºC in atmospheric air, it melts at 500 ºC when the Minex process is used.
“While the use of chloride gas is not new to metals extraction, its use has not really been investigated in a pyrometallurgical process and [it] has solved one of the biggest challenges with pyrometallurgical processes for the reprocessing of mining waste – the economic viability of the process, owing to high energy costs,” explains Dorman.
Improving the economics further is that chloride can also be manufactured on site, obviating the need to ship large quantities of the gas to site. The chloride is made by means of electrolysis, using salt and water, with the by-product being caustic soda – a neutraliser for chloride gas – should this be required.
The process can be fuelled by any hydrocarbon-based fuel – such as diesel, oil or gas – or electricity.
The energy consumption of the plant, at $60/t, is one-third of the total operating cost, while chloride gas, other processing and general and administrative costs account for $60/t each, bringing the total all-in sustaining cost to $180/t.
The process is able to target all metals in the waste stream for recovery in a single pass, with development work to date indicating that the process effectively removes 90% of the targeted metals.
The dry pyrometallurgical process, which also largely eliminates the use of water, another scarce resource in Africa, has the added downstream benefit of obviating the need for tailings storage dams or other wet storage facilities, thus eleminating the costs associated with the management and closure of these facilities. The process removes the need for tailings storage facilities – considered to be environmental and economic liabilities, as well as eyesores – while generating revenue.
Besides producing saleable recovered metals, the Minex Process also produces a stable, environmentally nonhazardous by-product – ferrocalcine – which can be sold for use in ferroconcrete for building or asphalt for roads.
“This is a very interesting development in mining and metals, particularly with regard to the cyclical nature of mining and the importance of secondary metals production and the reuse of discharge materials,” says Dorman, adding that there is currently no efficient technology available to recover all the metals found in mining waste in an environmentally sound manner.
Mining waste is a global issue, with an estimated two-billion tons scattered around the world and the World Bank noting that 3 000-plus sites globally have a general health and environmental impact. Scoria Environmental has in its sights potential target areas globally, but the immediate focus is on “safer jurisdictions”, and these do not include the Democratic Republic of Congo, which boasts about 100-million tons of waste across the Copperbelt.
Scoria Environmental is, however, considering several sites across sub-Saharan Africa, particularly in South Africa, where it has been granted patents; the greatest potential in South Africa is presented by the country’s gold smelting and other refineries. Namibia, which has about five-million tons of waste, Dorman notes, also holds promise, specifically the waste dumps owned by Canadian-headquartered gold mining company Dundee Precious Metals.
“We could build a regionally central plant to which waste material is transported or a global central plant, or even have plants based at mines should they have enough material – more than five-million tons. Royalty and licensing agreements could also be [entered into with the] owners of slag, enabling them to build their own plants and Scoria Environmental to grow organically and inorganically,” says Dorman
Locally, Hockaday is working on technology that will use concentrating mirrors to not only convert the sun’s radiation into heat that can be used in the energy-intensive ferroalloy and steel industries but also store it. She believes that, in future, diesel and coal combustion heating will be replaced by concentrating solar thermal heat for use in minerals processing.
This technology is four years away from being realised, with Mintek having joined a partnership that includes Norwegian science council SINTEF, German aerospace agency DLR, manganese-ore smelting operation Transalloys and Stellenbosch University to demonstrate its potential. The project is funded by the European Union’s Horison2020 programme.
The technology involves capturing and storing heat during daylight hours, using ceramic particles as the storage medium. These thermal storage units lose just 1 ºC of heat a day. The technology is 30% to 50% cheaper than diesel-generated electricity per megawatt hour, depending on the process temperature needed, which, Hockaday says, is a massive saving for industry.
“There is a good overlap between South Africa’s mineral and solar resources, [which creates] unique opportunities – opportunities not just in increasing the use of renewable energy in mining, but also in establishing new businesses for the beneficiation of mineral resources relying on sustainable renewable-energy technologies for electricity, process heating, cooling and transport.”
Given that solar thermal technologies are most cost effective in locations with high solar irradiation, solar thermal energy in South Africa is cost competitive globally and rapid uptake of the technology is expected once it has been proven. International companies could also be encouraged to establish operations in the country to take advantage of its lower energy costs.
Several other solar technologies are available to provide process heat at temperatures of up to 400 ºC, with thermal storage technology enabling constant operation for up to 80% of the year.
“If these renewable-energy options are used to replace heating by fossil fuels and electricity (generated by coal-fired power stations), the greenhouse-gas emissions reduction will be astounding. The technology boundaries are continuously being stretched and improved and new particle receiver technologies aim to increase the temperature limit of concentrating solar thermal process heat to well above 600 ºC,” Hockaday states.
Although high-temperature smelting processes can use electric heating to reach the required temperatures (above 1 200 ºC), carbon is required as a final reductant to produce steel and ferroalloys. Hockaday says this carbon cannot be replaced with other reductants, such as hydrogen, owing to thermodynamic constraints. For these energy-intensive processes, as well as processes relying on the decomposition of carbonate minerals, research into energy efficiency, waste heat recovery and preheating with renewable energy is urgent.
Hockaday says the uptake of renewable energy in the South African mining and mineral processing industry is variable. “This is [perhaps] to be expected, given the diversity of companies and the traditionally conservative [nature of the] industry.”
This is despite South Africa being blessed with excellent renewable solar and wind resources, with the cost of solar photovoltaic- and wind-generated electricity decreasing to below grid parity. Thus, over their lifetime, these technologies can provide electricity more cheaply than the national grid.
“However, with increased financing available in the form of grants and/or loans to lower the barrier of the high capital costs of renewable solutions for industry and a global imperative to change to greener processing, the race is on to adapt to the renewable-energy revolution,” Hockaday concludes.