Water management at mines – every drop counts

(This article, penned by Munesu Shoko, is reproduced from the August 2020 edition of Modern Mining.)

It is important that every operation prioritises the most efficient control and management of valuable water resources to maximise the efficient use and reuse of every drop of water that is involved with a mine site operation. This also minimises the long-term environmental liabilities that could result from the mismanagement of water resources.

He is of the view that water pollution from mine waste rock and tailings may need to be managed for decades, if not centuries, after closure, as the water sources continue to naturally produce sulphuric acid when sulphides in rocks are exposed to air and water. This results in oxidation and acidification processes, which continue to leach trace metals from the exposed rock face, and are discharged into the environment. 

“Furthermore, chemicals used in leaching or flotation process, such as cyanide or sulphuric acid, enter the process water that is being recirculated within an operation, and some of these solvents remain in that water and, as it migrates, the toxic solvents are carried into the agricultural soils and into the water source of downstream communities,” explains Ridgard. 

Significance of water in mining

Water, according to Ridgard, is arguably the second most valuable asset on a mine after the ore body itself. Strangely enough, he reasons, it is more often considered an “afterthought” for many design houses and mine owners. 

Mining uses water for mineral processing, including comminution practices, classification by screening and hydrocyclones, dust suppression, slurry transport and employees’ needs, among others. It is also used in some underground operations for hydro-powered equipment. 

“Mining operations commonly seek water from groundwater, rivers and lakes, or through municipal water service suppliers. It plays an essential role in most mining and extractive processes, and today, responsible water use is a critical business issue for the mining sector as a whole,” adds Ridgard.

“Maintaining a constant water balance on your site is critical for both mining and mineral processing. For underground mining, you constantly need to dewater your shaft to allow for mining practices to continue. This water could now be contaminated by naturally leached components such as arsenic, and first needs to be treated before it can be discharged to the environment to maintain a water balance,” he says. 

On the surface, the water needs of the processing plant must be balanced with what is now in the tailings facility. Any excess water needs to be treated and discharged from the tailings facility to maintain the water balance. Overcapacity can be catastrophic, with dam failures inevitable, explains Ridgard. 

 Maintaining a constant water balance on your site is critical for both mining and processing.

Effect of contaminants in process water

Water quality can have a detrimental effect on process efficiency and recovery, but it is often the last place that mine operators look when experiencing a drop in recovery. Water hardness (high concentration of CaCO₃ and MgCO₃) in process waters can cause scaling of pipelines, which results in reduced throughput. 

“Furthermore, it has been proven that contaminated waters can have a significant effect in recovery efficiency of hydrometallurgical processes. Compared to uncontaminated fresh water, lower recovery of target metals can be attributed to the presence of various metal ions in the process water,” explains Ridgard. 

For gold operations, for example, the gangue minerals are insoluble in cyanide solution. Some metallic minerals, however, are soluble and deprive the solution of its oxygen and cyanide. 
“Utilising completely clean water, however, does have its disadvantages in that residual reagents which carry over back into the process have been removed, and your operational cost increases to supplement this. Thus, it is critical to implement a fit-for-purpose water treatment system which removes target contaminants while allowing other elements to make up the necessary process water composition,” adds Ridgard.

High recovery water treatment systems

The cost of effluent treatment is significant, hence a high recovery system is essential. One of the common ways of treating effluent water is Reverse Osmosis (RO). RO was initially designed for sea water desalination to remove monovalent salt molecules (NaCI). Due to its success in this application, it has since been introduced to other sectors such as industrial and mining. 

“The problem is that sources of wastewaters also include a wide variety of other elements, such as divalent and trivalent elements which cause scaling of membranes. This means that when a standalone RO plant is utilised to treat these waters, it is operated at lower recoveries to enhance the lifespan of the membranes. It results in large volumes of highly concentrated brine streams, which are either recirculated within the system or require very expensive effluent treatment systems,” says Ridgard. 

The utilisation of resin-based chemistry for the removal of target species has long been understood and respected globally in the industry, he says. It offers the selective extraction of contaminants by the exchanging of ionic functional groups, engineered on the resin beads, for target elements in the surrounding solution of like charge. The problem, however, has always been that there has not been a suitable technology to effectively facilitate the enormous advantages provided by the resin chemistry. 

“The Continuous Counter Current Ion Exchange technology, engineered by Clean TeQ and supplied to the African market by Multotec Process Equipment, is a game-changing, moving bed technology,” he says. 

Contrary to the conventional fixed-bed systems, the use of resin transfer mechanisms allows the CIF (Continuous Ionic Filtration) to: 

• handle up to 150 ppm of solids (conventional systems need a 100% clean liquor), hence offering simultaneous removal of TSS (total suspended solids) and TDS (total dissolved solids); 
• offer optimised resin inventory (resin is the most expensive part of the plant and hence it is critical to ensure the longevity is maximised and the volume is minimised);
•  provide very high water recoveries; 
• handle in column precipitation; 
• offer low power consumption (given the limited power availability on isolated mine sites in Africa, this is another major advantage); and 
• produce valuable by-products and/or trace metal recovery.

Fit-for-purpose treatment systems

potentially comprise of a combined HDS (High Density Sludge) and Continuous Counter Current IX system. If environmentally compliant dischargeable water is required, then a simple HDS system is perhaps the ideal solution.  

“Even if the end goal is to change mine service water to potable drinking water, we design a high recovery system to meet these needs – this could potentially consist of an HDS, Continuous Counter Current IX and RO system,” explains Ridgard. 

Depending on the customer’s ultimate water quality requirements, Continuous Counter Current IX is combined and fully integrated with RO to produce a high-recovery or Zero-Liquid-Discharge (ZLD) solution. 

“Remember that RO was designed to remove monovalent ions, while IX is more selective to larger divalent and trivalent ions. Hence, by combining the two technologies and allowing the IX to firstly remove the elements which scale up the RO membranes, you allow the RO plant to do what is was designed to do, which is to remove monovalent salts at significantly higher recoveries. Furthermore, we can potentially provide a ZLD system by recirculating the concentrated sodium brine stream to regenerate the resin in the ion exchange plant.” 

Proving capabilities

In one of the flagship Minimum Liquid Discharge (MLD) systems, Multotec designed and supplied a complete system to a mining operation that is extremely sensitive to water usage and waste production in the desert of the Middle East.

One of the major advantages of utilising ion exchange for the treatment of effluent and/or tailings streams, reasons Ridgard, is that in addition to being environmentally compliant, potentially increasing recoveries and reducing reagent consumption by providing a fit-for-purpose process water, ion exchange offers the possibility of recovering residual trace metals, which would have otherwise been lost to the mine owner.

“Mining operations spend millions of dollars to liberate and recover their target elements, but despite their best efforts, 100% recovery of these elements is simply not possible and large percentages end up in the tailings dams or is lost to the environment,” says Ridgard.

“What the Clean-IX Continuous Counter Current Ion Exchange technology offers is the opportunity to recover what is lost from the processing plant and potentially provide an economic benefit which significantly offsets the cost of the water treatment plant. Depending on the concentration of the valuable metal and the total flowrate that is being treated, a complete payback within a matter of months could be possible,” concludes Ridgard. 

Key takeaways

• One of the greatest challenges facing mining operations is the development and management of water resources.
• Water is arguably the second most valuable asset on a mine after the ore body itself.
• Multotec offers fit-for-purpose, niche technologies specifically suited to the treatment of divalent and trivalent containing mining waters.
• Multotec has partnered with Australian based Clean TeQ Water to provide the African mining market with a continuous counter current ion exchange technology.
• Multotec provides the mining industry with a high recovery (>90%) system to provide fit-for-purpose process waters to be utilised within the water balance or discharged safely to the environment.