Heap leaching is a process that extracts valuable metals from ores by dissolving the metals in a solvent. The valuable metals can then be recovered from the solution by a variety of methods.
Heap Leaching is an ancient process; leaching metals out of ore goes back nearly 500 years. Mines in Hungary recycled copper-bearing solutions through waste heaps in the 1550s, Spanish miners percolated acid solutions through large heaps of oxide copper ore on the banks of the Rio Tinto in 1752 and one of the classical methods for the manufacture of copperas (iron sulphate) was to heap up iron pyrite and collect the leachate from the heap which was then boiled with iron to produce iron sulphate.
Heap leaching also provides a low cost process for nickel laterites such as those found at the Piauí Nickel Project; providing huge capital cost savings and competitive advantages over other nickel laterite operators.
In the heap leaching process, very dilute sulphuric acid (similar in strength to some acid‐based household cleaning products) is slowly applied to crushed nickel ore on a lined pad to dissolve the nickel and cobalt contained in the rock in a several month process cycle. The resulting metal-rich solution is then treated in a simple ambient temperature and pressure precipitation process to firstly remove impurities and then to recover the valuable nickel and cobalt from solution. The final product is an intermediate mixed hydroxide product containing nickel and cobalt that can then be sold to a large customer base of existing nickel plants and possible new intermediate customers.
Sulphuric acid leaching of copper ores is common and safely used around the world, including in the US, Canada and Chile. This process has been successfully demonstrated for nickel laterites by the management team at Çaldağ in Turkey, Acoje in the Philippines and CMSA in Colombia over the past decade. Over 1 million tonnes of ore have been economically treated at Murrin Murrin in Australia as part of an integrated nickel laterite operation. Further Yuanjiang in China is a small standalone nickel heap leach which commenced production in 2007, and has until the end of 2014 produced approximately 10,000 tonnes of nickel contained in product.