The Rotowinner modular pilot plant now commissioned at the Prieska Copper Zinc Mine for water treatment and minerals recovery trials. Image supplied by Orion Minerals
Orion Minerals Limited has commenced a series of field trials for the treatment of mine water at the Prieska Copper-Zinc Mine (PCZM) in South Africa’s Northern Cape Province, using a proprietary electrolytic technology to produce saleable products.
The process has the potential to extract valuable products such as calcium and magnesium for agri-nutrient use from water pumped out of the underground mine at Prieska. In addition, other products with potential for by-product sales in the chemicals industry will also be extracted.
The trials have commenced as Orion prepares to start the dewatering of the underground mine and to implement other key elements of its Early Production Plan as it advances the PCZM towards production.
The background
The extensive underground workings at Prieska hold approximately 9 million cubic metres of water, which have accumulated since the mine was closed by Anglovaal in 1991. While the quality of the mine water is relatively benign, with pHs ranging between 6.6 to 7.1 (neutral pH is 7.4), and total dissolved solids measured in the shaft water range between 6 000 and 11 000mg/l, some water treatment is required if the water is to be considered for off-site discharge and subsequent agricultural or potable applications.
The process
The water treatment trials are being undertaken in collaboration with Free Radical Process Design (FRPD), a South Africa-based technology company which has developed an innovative and proprietary process and equipment for the continuous extraction of metals and other valuable minerals from water using electrowinning via a rotating cathode (Electrosoftner).
This will advance Orion’s vision of establishing a ‘green’ footprint for the Prieska Project with broad stakeholder benefits as part of its objective to have a world-class ESG framework to support its operations.
The commencement of the field trials follows the success of laboratory scale test work, where water samples from PCZM were analysed and were passed through an Electrosoftner demonstration unit, producing precipitates of iron hydroxide, calcium hydroxide/carbonate and magnesium hydroxide while concentrating sodium hydroxide that can be bled off as a liquid from PCZM mine water.
Figure 2: Tailings remediation application of the Rotowinner process. Image supplied by Orion Minerals
The dewatering setup, which is being implemented, consists of pump sets and reticulation that will extract mine water from underground workings at a pumping rate that will build up from 120m3/hr to 500m3/hr. Once on the surface, some of the water will be treated by reverse osmosis (RO) while the balance will be treated by electrolysis. Some brine water will be retained in evaporation dams. The treated water will be used for irrigation projects, planned to benefit local host communities.
Dewatering is planned to occur over a three-and-a-half-year period, during which time, mining is planned to commence in those parts of the Prieska deposit above the water level so that copper and zinc concentrate production can commence within less than three years, as scheduled in the original project development plan (BFS-20 Plan).
Rotowinner metal extraction process
FRPD has patented technology for the continuous electrowinning of selected minerals from mineralised leachate using a rotating cathode (Rotowinner). The Rotowinner consists of a rotating cathode, submerged in pregnant leachate with a scraper arrangement to remove the precipitation of minerals on the cathode. The scraped material is discharged into a collection launder and the spent leacheate is collected to be recycled in the leach circuit (Figure 1). A mobile demonstration scale plant has been constructed that has been successfully used to produce base metals on a continuous basis.
Figure 3: Cross-section through the Rotowinner illustrating how the continuous rotating cathode harvests minerals from pregnant leachate. Image supplied by Orion Minerals
The process offers the advantage over conventional electrowinning set-ups of:
- Being a continuous production process;
- Having a lower energy requirement than standard electrowinning;
- With built-in cathode stripping offering a simpler, safer and more cost-effective operation;
- Having no lifting equipment requirements during operation;
- Having smaller inter-electrode distances;
- Completely enclosed construction;
- Reduced acid mist generation;
- Modular set-up for versatility and easy scale-up or down;
- Robust design;
- Wider and more efficient operating ranges
- Significantly reduced capital cost for establishment.
Application
Following on from laboratory-scale test work and water quality assessments of the Prieska Mine water, it is anticipated that the Rotowinner will be able to produce agricultural use water while extracting calcium, magnesium and iron as products, as well as generating hydrogen, oxygen and chlorine gases and with a sodium hydroxide solution residue (Figure 4). This would result in valuable use of the whole water treatment stream.
Figure 4: Water treatment and minerals recovery flowsheet for the Prieska dewatering program. Image credit: supplied by Orion Minerals
Orion’s Managing Director and CEO, Errol Smart, commented, “As we prepare to start dewatering the underground mine at the Prieska Mine, we continually review engineering and technology solutions to find ways of reducing costs and maximising ESG outcomes at our operations. Any technology or new process that offers benefits to our key stakeholders and local host communities while supporting shareholder returns will always be prioritised in our operating plans.
“We are particularly pleased to have the opportunity to collaborate with South African technology developers, Free Radical Process Design, who have worked with The University of Pretoria to develop an innovative technology that could have a global impact in the treatment of contaminated mine water and which is directly applicable to our Prieska Mine dewatering challenge.
“We look forward to trialing the Rotowinner Technology to treat our Prieska Mine water while simultaneously recovering ‘clean & green’ agricultural nutrients and delivering treated water which would be available for community agriculture purposes. “In addition to agricultural nutrients, other products can potentially be extracted using the Rotowinner Technology which are used in the chemical industry, opening up interesting future by-product sales opportunities.”
Field trials are expected to continue for six months. Subject to the success of these trials, implementation of a production-scale Rotowinner assembly will be undertaken. The current dewatering infrastructure layout has made provision for incorporating the water treatment solution at the end of the trial period.