​Innovative Solutions For Solving Water Supply Problems

Access to clean water is now a declared fundamental human right as per the UN's 2010 declaration. It's also a fundamental ingredient in mining operations because no mine can operate without water. However, the two needs often clash. Indeed, cynics might argue that the two uses are mutually exclusive. They have plenty of evidence to back up their view too. Some of the worst mining related environmental disasters in history have revolved around water incidents and each time it happens, it hands another weapon to anti-mining groups.

Mining Processes And Water

Water is involved in many mining processes. From a clean potable supply for human consumption in administration buildings and accommodation facilities to dust control on waste dumps and haul roads, water is essential. Some mining operations transport ore via slurry pipes, which are more economical, use less energy, and make less impact environmentally than roads and railways. However, the trade off is that water is needed to keep the slurry liquid enough to flow through the pipes. All these operations and processes require significant amounts of water but for most mine sites it's the hydrometallurgical processes that are the major water users.
Industrial water use in many countries is now heavily regulated and requires a licence. To obtain a licence companies are increasingly required to provide detailed water and waste management plans before they will be granted one. Socially and environmentally responsible mining companies also seek to involve local communities in their water management plans as part of their social licence to operate. Some have come up with very innovative plans too that benefit not just the company but also the surrounding communities. We take a look at what some mining companies have come up with in South America.

​Reclaiming waste water for the Cerro Verde copper mine in Peru (Freeport-McMoRan Copper & Gold Sociedad Minera)

When the operators of the Cerro Verde mine in Peru looked into the feasibility of expanding their operations they ran into a major problem. Notably a water supply problem. Whilst the Rio Chili surface water was enough to meet their current requirements, it would not cover the proposed expansion. Another source was required.

The local communities in the region are already plagued by a scarcity of clean water and a shortage of wastewater treatment facilities compounds the problem. The Rio Chili is now heavily contaminated by untreated sewage from both domestic and industrial sources. Downstream, this also adversely affects the agricultural areas that supply food to the region. If this wastewater could be treated for use in mining operations, it would kill a number of birds with one stone. Or give them a better quality of life is probably more apt. It would clean up the river system, improve the quality of water being used to grow food downstream, and reduce the problem of water-born diseases. It would also comply with Peruvian government initiatives to get mining companies to use treated wastewater in their operations.

The mine's operators had already identified this wastewater problem and taken steps to rectify it as part of their 'licence to operate'. They'd entered into an agreement with local and national authorities to help fund a potable water treatment plant and a wastewater treatment plant. Construction of the potable water plant was started in 2011 and completed in 2012. None of the water treated by the plant is intended for use in mining operations. Planning for the wastewater plant started in 2011 with feasibility studies into the use of treated wastewater for mining operations. The mine's operators will fund it as part of the expansion of their existing mines but the plant itself will be operated by Peru's national water authority. The plant will supply 1 cubic metre of treated wastewater per second to the mine site.

Minera Esperanza in Chile – using untreated seawater (Antofagasta Minerals)

The Atacama Desert in Chile is a very dry place. It does however contain commercially significant quantities of copper and gold, which makes sourcing water from somewhere to run a floatation plant to extract it, worthwhile. That's exactly what Antofagasta Minerals set out to do with the Minera Esperanza copper and gold mine. The milling and flotation plant requires 20 million cubic metres of water annually, water that had to come from somewhere.

One of the advantages with mining and water is that so long as the water that leaves the processing plant at the end of the processing is clean, many mining processes can use poor quality water. Including salt water. Innovative folk at Antofagasta Minerals hit on the idea of piping in untreated seawater from their port facility 145 kilometres away in Mejillones. Before it begins its journey up to the plant, the seawater is filtered to remove particulate matter and treated with a corrosion-inhibiting reagent to reduce corrosion in the pipes. It then passes through 4 pumping stations and rises 2300 metres along its 145 km journey. The power to run the pumping stations, some 20 megawatt-hours hours, is supplied by the local electricity grid.

The majority of the seawater is used by the plant, which needs 600 litres per second. The remainder is treated in a desalination plant and used for drinking, sanitation, and other processes. The port in Mejillones also has a desalination plant and treats seawater for its own similar usage. To further reduce water usage and the potential for environmental contamination either through evaporation or leakage, the tailings are treated with thickeners that reduce the liquid waste to a product that is two-thirds solid. This semi solid mass is pumped to the tailings dam where it solidifies. Once it has, more tailings can be deposited on top of it.

Reusing waste water and monitoring cyanide levels at Cerro Vanguardia S.A. in Argentina (AngloGold Ashanti)

The Cerro Vanguardia SA is a gold and silver mine in the Patagonia area of Argentina. Processing of ore is via cyanidation and the company has installed a Cyanisorb plant that removes around 95% of the cyanide from the tailings slurry before it heads to the tailings storage facility (TSF). The slurry is allowed to separate in the TSF and the water is then pumped back into the system for reuse. 

Because the slurry and the residue contains traces of cyanide, the company has set up an extensive system of test boreholes and wells around the TSF to detect any cyanide leaching into the soil and groundwater beneath the tailings dam. The water in these holes and wells is continually sampled and tested by independent external labs, and the results monitored closely and reported to local and national authorities. Heavy metals are also tested. 

To date there has only been one isolated incident where cyanide readings from one borehole showed traces of cyanide. Tailings discharge to that area of the facility was stopped and extensive tests carried out to determine how the cyanide was leaking out. Eventually it was found that the cyanide was travelling along a quartz vein directly underneath the dam. The vein was exposed and covered with 2 layers of thick high-density polyethylene. Electric sensors were placed in between the two layers to pick up any leakage. So far, ongoing testing and monitoring has not found any further evidence of leakage. 

Identifying water wastage at the Lomas Bayas mine in Chile (Xstrata Copper)

The Lomas Bayas mine is a copper mine in the very dry Atacama Desert in Chile. It, and the surrounding communities, rely on the Loa River for water. As environmental focus on the use of natural water ways for these purposes increases, the mine's operators decided to take a good hard look at their water usage processes to determine where they were using the most and how much they were losing through things like evaporation.

Needless to say, the solution ponds and leach pads were quickly identified as major problem areas. Evaporation from these 2 operations alone accounted for over 40% of the mine's total water loss. The heap leaching pad was particularly bad because it used a sprinkler system to distribute the water over the ore, whilst other sections were irrigated. Evaporation loss from these distribution methods was found to be significant. The sprinklers were replaced with a more water efficient drip-feed system and plastic covers were put over the irrigated areas to reduce evaporation. The result was a 54% reduction in the evaporation rate, or a saving of 4 ½ litres of water a day (down from 9.8 litres). As a result of the improvements, the operators were able to expand leaching operations by 70% without requiring any additional water.
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Friday, 23 August 2019
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