Gold standard in water management

Heap leaching, as a technique for recovering low-grade deposits of gold, silver and copper, is a highly cost-effective method of extraction, offering recovery rates of between 60 and 70%. However, as the process can involve aggressive and dangerous chemicals , every aspect of heap leaching must be carefully managed to mitigate negative impact on the surrounding environment and water courses. The key to ensuring a safe yet efficient system lies in the intelligent specification of the constituent components --particularly those which come into contact with both corrosive and abrasive particles.

A low-cost method of extracting valuable minerals from lower grade ores, heap leaching relies on first crushing the aggregate into smaller pieces to allow an acidic solution to permeate the rock and dissolve the metals. While this is a time intensive process -- taking anywhere from weeks to years -- it can cope with large quantities of ore and minerals requiring little labour, and is therefore one of the more cost-effective extraction techniques. However, initial investment must be made in quality, robust system components to ensure the longevity and efficacy of such a system.

Arguably the most important components within a heap leaching system, the pipes which capture and collect the impregnated solution before carrying it to ponds or treatment facilities have to withstand the most extreme of conditions. Not only are they buried under heavy, abrasive support aggregate but they must also carry the weight of the crushed ore above. As the heap is piled over the pipes at a sloping angle, slippage can occur due to erosion or the vibration caused by tipper lorries delivering fresh loads of ore. This means the support aggregate surrounding the pipes can shift and so a completely rigid pipe would be at risk of cracking, meaning the specified solution must offer a rigid yet flexible construction to withstand abrasion and heavy loads yet cope with shifting rock around them.

Within the pipe, resistance to chemical corrosion is clearly vital to prevent harmful chemicals entering the surrounding soil, groundwater and water courses as well as the costly loss of the dissolved minerals themselves. While the perforated pipes are surrounded in a semi-permeable geotextile wrap to prevent larger particles of rock or sand entering the pipe, abrasion resistance is still a key characteristic in case of damage to the wrap or stormwater flooding the pipe from the other end.

Pipes and water management systems have, over the years, been fabricated from a variety of materials. Traditional concrete and cast or ductile iron pipes, while robust, suffer from a number of drawbacks where heap leaching processes are concerned. Due to their heavy weight, they are cumbersome to manoeuvre on site and into position. They are supplied in relatively short lengths meaning a large number of joints, thus increasing the likelihood of leaks. Also their construction does not lend itself well to being perforated.

High-density polyethylene (HDPE) is quickly becoming the material of choice for a variety of mining water management applications including tailings and heap leaching, and is proven to withstand the demands of extreme operating environments. A critical characteristic is its resistance to long-term exposure to an expansive pH range from 1.5 to 14.0 -- as both alkaline cyanide and sulphuric acids are used in differing heap leach applications, the same product can be used for a variety of mineral extraction processes. HDPE is also resistant to both galvanic corrosion and bacterium sediment build-up -- unlike steel or concrete pipes which can be badly damaged.

HDPE offer inherent strength and impact resistance, while still possessing the flexibility to cope with ground settlement or shifting after installation. With an extremely smooth bore, they are less likely to block and so are also more easily maintained., offering excellent abrasion resistance to slurries, salts and small particles. As an additional benefit where large volumes of liquid are being transported, there is very low friction, meaning less drag or turbulence at high flow rates and greater resistance to scaling than water absorbent materials.

Structured-wall HDPE drainage pipes are lighter and more robust than most rigid materials -- typically weighing 94% less than their concrete equivalents -- are also quick and easy to install, no heavy lifting equipment during fitting. As they are lighter and more portable, they can be supplied in far longer lengths than cast iron or concrete pipes which results in fewer joints and less possibility of leakage. If required HDPE pipes can be butt-welded or electro-fused depending upon the application and can be supplied in bespoke lengths or angles to allow curved pipelines to be constructed around ponds or obstructions.

As well as a requirement to protect the environment from chemical leakage, sustainability is a major consideration for materials used in the construction of permanent infrastructures and modern mining projects are required to have a minimal environmental impact at all stages. This includes the carbon footprint of materials used on site and, as HDPE pipes can be made from recycled material, as well as being fully recyclable at the end of their working life, plastics are a safe environmental choice.

For many mining applications across Africa, the Pressure and Non-Pressure HDPE Pipe solutions from Polypipe are in high demand. For example Ridgidrain, a twin-wall pipe designed specifically for non-pressurised surface and subsurface drainage. Its excellent structural integrity and high strength-to-weight ratio is balanced with the flexibility to withstand shifting of the surrounding substrate without cracking or leaking, making it ideal for heap leaching applications, alongside its resistance to chemical corrosion and abrasion.

One example is the successful Ridgidrain installation at the Bissa Gold/Nordgold mine in Burkina Faso.

Polypipe have recently successfully supplied our Ridgidrain pipe systems into Heap Leachate applications at the Nordgold Bouly Gold Project, Bissa Mine in Burkina Faso. Working closely with the mining company, Polypipe were able to successfully value-engineer the project and offer significant savings on product and installation costs.

Philip Wood, Polypipe's International Business Developmenet Manager said : "Through working closely with Nordgold's team at the mine, we were able to fully understand their requirements for the project from design, through to delivery and installation. Polypipe's strong experience in handling difficult and complex projects and their shipping and logistics requirements, ensured that the project ran smoothly and is now operational."

As water is so integral to many key mining applications, water management systems will always be central to the design and planning of a successful mine operation. Involving expert supply partners in the early stages of the commissioning process will ensure the best possible products and solutions are specified for demanding applications such as heap leaching, and guarantee protection of the environment as well as the greatest possible mineral yield.

For further information please visit www.polypipe.softwareimc.app/mining