As seen in Mining Review Africa.
The global mining industry is going green – or at least is in the process as it looks for environmentally sustainable solutions that demonstrate a clear journey towards greater ESG compliance. A large-scale gold operation in West Africa has recently taken a positive step in this direction, having invested in a water treatment plant that purifies treated process water to a level suitable for discharge into the environment, writes LAURA CORNISH.
The design, construction, installation and commissioning of the water treatment plant was undertaken by metallurgical and process equipment firm Multotec.
Unathi Mkruqulwa, process engineer at Multotec shares the journey in providing a world-class plant that demonstrates the company’s water treatment capabilities and collaborative approach to delivering sustainable solutions for the mining industry.
“Our client was looking to discharge water from its operation into the environment but was unable to due to the high volumes of arsenic in the water,” Mkruqulwa starts. “With 10 ppm of contained arsenic, this is well above the International Finance Corporation’s required discharge limit.”
The starting point, Mkruqulwa continues, was test work, which Multotec conducted at its facilities in South Africa after securing the necessary water samples from site. Having determined a process that would work, in 2019, the company secured the full-scale project in February 2020.
With an initial small scope change to the project – which saw the plant relocate from its original intended position at the tailings storage facility to the return water dam – it was completed and commissioned over January and February 2021.
Now fully operational, the plant removes the arsenic using a two stage ferric sulphate treatment process. “Housed within the plant is an additional circuit which first converts ferrous sulphate to ferric sulphate – which is then able to precipitate the arsenic from the water,” Mkruqulwa explains. Iron flocculants are recognised as the most effective compound for removing contained arsenic and is also fast at doing so.
Designed to treat 150 m3/h of process water, the plant is successfully reducing the arsenic level from its original 10 ppm to <0.1ppm, a globally acceptable standard to release into the environment.
Impressively, the final plant design and construction required further adaptation when Multotec discovered an unexpected challenge – the presence of cyanide in the water. Quick to adapt, the company added a cyanide treatment circuit to the plant which neutralises the cyanide upfront before moving through the arsenic precipitation phase.
“This in some ways could be considered the easier challenge to overcome,” Mkruqulwa notes. “Delivering a plant to a remote region in Africa during ongoing COVID-19 related lockdown measures presented its own difficulties and required additional planning and preparation. Known for our flexibility and adaptability and absolute commitment to helping our customers succeed, this ultimately posed no additional problems as the Multotec team constructed the plant on site over the Christmas period ensuring it could be completed on time. This was followed by a three week commissioning period on site in February.”
Multotec brought to the project its holistic dewatering solutions by adding its filter press to dewater the sludge produced from the process to a 50% moisture level. The water abstracted during this process is recycled upstream of the arsenic treatment plant.
“The plant has been a phenomenal success and is treating the arsenic to the desired <0.01 ppm,” Mkruqulwa highlights. On the back of this, the company is however looking to conduct a process optimisation exercise which includes the addition of an online arsenic analyser.
Typically, arsenic levels are monitored in a laboratory and can take days to return results. “Through the incorporation of an online analyser, we will be able to monitor arsenic levels in the water in real time, allowing us to address any potential problems quickly and efficiently, a value-add offering which will enhance the overall performance of our plant significantly.” This value add service from Multotec is in line with the company’s move towards the future world of digital solutions providing real-time monitoring of clients’ processes. This enables quick reaction to negative changes in plant performance or condition and allows clients to maintain a continuously optimised operation.
While Multotec has and will continue to offer training to on site personnel, it remains committed to providing any necessary or required after sales support. The company has a branch in Ghana, which provides localised and speedy support in the West Africa region. “Ghana is the hub from which we service all of our West African customers,” Mkruqulwa indicates.
“Not only is this the largest arsenic precipitation plant we’ve successfully delivered to date, but the incorporation of an online arsenic analyser further demonstrates the commitment we have to providing the industry with digitally relevant products that enhance the performance of our equipment and solutions,” Mkruqulwa concludes.