South Africa’s national mineral research organisation, Mintek, highlighted its commitment to protecting South Africa’s scarce water resources when it demonstrated its process solutions for the treatment of solid and liquid mine effluent during a technology showcase in Randfontein.
Mintek understands the importance of preventing and remediating pollution caused by mining activities and has therefore spent several years of research and development on process solutions for solid and liquid mine effluent treatment.
This article first appeared in Mining Review Africa Issue 6 2018
As a result, Mintek is now able to offer an integrated suite of mineral processing technologies for the sustainable treatment of highly polluted mine-impacted water and tailings.
The research organisation is using its technology demonstration site in Randfontein as a platform to showcase how different technologies can be used to treat highly polluted waste water sites with high uranium concentrates. Mintek chose the nearby Robinson Lake – a former community recreation site in Randfontein on the West Rand of Johannesburg – as the test site from which to extract polluted waste water.
Made possible through a partnership with gold mining company Sibanye-Stillwater, the Mintek technology demonstration site is located on Sibanye-Stillwater-owned land and is adjacent to the gold miner’s water treatment plant.
The demonstration site provides a platform for Mintek and other industry players to demonstrate and evaluate suitable technologies for the treatment of mine-impacted waste water, including acid mine drainage (AMD) and the removal of toxic and radioactive elements from the water such as uranium to minimise the environmental impact of mining and mining legacies.
The technologies showcased included SAVMIN, MetRIX, NIC Membrane, Biological Oxidation, and Biological Sulphate Reduction – some of which are trademarked Mintek processes.
Broadly, these technologies work to both neutralise AMD as well as extract toxic components from the mine effluent.
Mintek developed a flow-sheet that includes these technologies, integrated in a manner that treats the water and mine effluent and enables the generation of streams of various water qualities, including drinking water and potable water, treated solids that can be stored on a lined tailings dams as well as product streams containing high concentrations of uranium, gold and base metals.
Speaking at the technology showcase, Mintek acting CEO, David Msiza, explained that the technology integration platform provides applicable processes for the remediation and potential restoration of heavily polluted natural sources such as lakes and rivers.
“By providing technologies for the prevention and treatment of pollution caused by mining activities, the sustainability of the mining industry is ensured, while also protecting the inhabitants of the impacted environments, as well as the national heritage sites and other natural resources in the proximity of mining activities,” he said.
With a mining history that spans over 150 years, South Africa has experienced negative legacy issues in large mining areas such as in the Gauteng, North West, Free State and Mpumalanga provinces.
Mintek has seen an opportunity to develop technologies and implement initiatives to assist the mining sector and government to deal with legacy issues.
In light of on-going drought conditions and stringent water use requirements, there is an economic driver now prompting mining companies to consider mine effluent treatment owing to the growing limit of available water resources.
Mintek therefore plans to commercialise these technologies in order to offer a technically viable and economically competitive solution to the mining industry for the recovery of valuable products as well as the rehabilitation of mine-impacted sites.
Also speaking at the technology showcase, Mintek board chairperson, Dickson Masemola, said that through innovation and metallurgical responsibilities, Mintek contributes to mining growth and development in South Africa – which is one of the major pillars of the country’s economy.
“It is important for Mintek as an organisation working together with the mines to strengthen our relations going forward so that those areas which have been long impacted by mining activities get the appropriate responses,” he added.
Technologies at a glance
SAVMIN is Mintek’s trademarked cost-effective technology for the treatment of mine-impacted water, including AMD that contain high levels of metals and sulphates.
The SAVMIN process, which was developed by Mintek in the 1990’s, is a precipitation-based process, operating at atmospheric temperature and pressure, and removes heavy metals and sulphates from mine-impacted water.
The four-stage SAVMIN process can be applied to treat AMD, mine-impacted water in the coal, gold, platinum and base metals mining industries or process water that requires sulphate treatment or removal prior to reuse or discharge.
Since its development, several advancements have been made in the process resulting in noteworthy improvements in the capital and operating cost of the process.
In terms of capital costs, process simplification and reduction of unit operation resulted in around 80% reduction in the original capital cost estimate for the process from R945 million to R205 million for a 1 000 m3/h plant, which includes the total capital costs of the engineering, construction and commissioning phase but excludes the costs associated with prefeasibility and feasibility phases of the project.
In terms of the operating costs, the reagent cost for the SAVMIN process is not static, since the value changes as the sulphate concentration in the feed water changes.
At exceptionally high sulphate concentrations of around 5 000 mg/L, the reagent cost of SAVMIN is R14/m3, whereas this cost reduces to around R10/m3 for feed water containing reasonable concentrations of sulphate around 2 000 mg/L.
The SAVMIN pilot plant at the Mintek technology demonstration site, has a capacity to treat 4 m3/h of AMD.
Biological Sulphate Reduction
Biological Sulphate Reduction (BSR) is a process to remove sulphates from waste waters using sulphate-reducing, naturally-occurring bacteria that are immobilised on a suitable substrate, such as woodchips.
The process can be used for the treatment of AMD to remove the sulphates from solution and stabilise the toxic metal ions present as metal sulphides, while simultaneously producing alkalinity that raises the effluent pH.
The water emanating from the BSR treatment process can be used for applications including irrigation, flushing of toilets and various industrial processes.
Biological treatment of mining effluents using sulphate-reducing bacteria offers a cost-effective and sustainable alternative to conventional treatment of effluents, such as lime neutralisation, which does not reduce the sulphate concentrations in the treated effluent to limits that are safe for discharge.
Moreover, conventional lime neutralisation produces large quantities of sludge that carries disposal issues while biological sulphate reduction which produces considerably less solid waste with decreased toxicity and increased stability.
The BSR process is passive treatment process primarily targeted as a long-term, sustainable treatment technology for treating AMD that decants from derelict or owner-less mines and can become part of the mining industry’s long-term rehabilitation strategy post coal or gold mine closure.
The BSR process was developed by Mintek over the past five years and has been operated at pilot scale for the past two years and has been proven to remove more than 95% of sulphates and increase the pH of the effluent to 7.
Ongoing work is investigating more passive approaches to the treatment technology.
The NICMembrane is a Mintek-trademarked low-fouling ultrafiltration membrane that can be used for the treatment of AMD and mine-impacted water for the removal of particulates and macromolecules (turbidity and suspended solids) for reuse.
Additionally, NICMembrane can be used for the removal of particulates and pathogens from grey water to produce drinkable water.
The NICMembrane capillary ultrafiltration membrane is being demonstrated on pilot scale at the Mintek demonstration site on a single 4 m3 membrane module and produces 1.5 m3/day of treated water.
This testing is focused on the assessment of the productivity and efficiency of the membrane.
Metal Recovery through Ion-eXchange is a Mintek trademarked continuous resin-in-pulp ion exchange process for the removal of uranium or base metals from dense slurries.
The three-stage process can operate with slurries containing up to 50% solids and therefore does not require any upfront filtration.
Mintek has conducted research and development over the past 10 years on the recovery of uranium from low-grade uranium slurries and more recent development in the resin-in-pulp technology have focused on the recovery of base metals from slurries.
Evaluations have also been carried out on the recovery of nickel, zinc, cobalt and copper at laboratory-scale and zinc at pilot plant scale.
The unique benefit of MetRIX is that it eliminates solid/liquid separation steps, subsequently lowering capital and operating costs.
Biological oxidation is a process for extraction of precious and base metals from sulfide-containing ore in tailings dumps with the aid of naturally-occurring micro-organisms.
The micro-organisms act as a catalyst and oxidize the solid metal sulfides into soluble metals.
Once the metals have been liberated by the micro-organisms, the metals can be subsequently recovered from solution using hydro-metallurgical standard techniques.
Bioleaching of refractory gold and base metals concentrates is growing in popularity as oxidised, near surface resources become depleted and old waste deposits are increasingly being investigated as potential reserves of valuable base and precious metals.
Bioleaching of refractory gold in agitated tanks has become an established commercial technology over the past three decades and Mintek is one of the leading technology suppliers.
Mintek has also undertaken several major base metals bioleaching projects including a 500 kg/day demonstration plant in Mexico and a nickel bioleaching plant, designed to treat about 12 000 tpa of nickel concentrate, in Finland.
SAVMIN as a possible next step in treating South Africa’s AMD challenge
Head of the technology metals group within the hydro-metallurgy division at Mintek, Michelle van Rooyen, has played an instrumental role in the technology development and establishment of the Mintek demonstration site and the SAVMIN pilot plant.
She explained that in South Africa, the AMD treatment plants in operation on the Witwatersrand’s West, Central and East Basins are currently only equipped to correct the pH of the water and remove the heavy metals in the water – but the water remains high in sulphates.
To provide a complete solution for the treatment of AMD, these sulphates need to be removed from the water, Van Rooyen explains.
As a long-term, sustainable solution to the AMD challenge, the Department of Water and Sanitation is in the process of integrating the initial AMD treatment programme into a sustainable long-term strategy for South Africa’s AMD challenge through sulphate removal as the next step in producing fully-treated water.
Van Rooyen believes that the SAVMIN process is capable of providing the ‘next step’ needed in South Africa’s AMD treatment plan and should be considered a strong contender to reverse osmosis and desalination technology.