Method of recovering base metals from low grade ores and residues

A method for the recovery of at least one metal from a low grade metal sulphide ore or residue, wherein the method includes the following steps:

• a) cyanide-leaching the low grade ore or residue, at an alkaline pH of between 10 and 11, to produce a pregnant leach solution which contains at least one metal cyanide;
• b) upgrading the pregnant leach solution by removing leached or barren solids from the pregnant leach solution;
• c) subjecting the upgraded pregnant leach solution to ultrafiltration to produce a clarified liquor;
• d) subjecting the clarified liquor to nano-filtration so as to separate metal cyanide complexes in a retentate from free cyanide in a permeate, thereby to produce a metal cyanide concentrate and a permeate containing free cyanide;
• e) subjecting the permeate to a reverse osmosis process to form a high-quality water stream and a free cyanide concentrate which is recycled to the cyanide leaching step a);
• f) subjecting the metal cyanide concentrate to acidification by the addition of sulphuric acid to lower the pH thereof to a value between 2 and 6 and then to sulphidisation to produce a solution which contains a metal-sulphide and hydrogen cyanide;
• g) subjecting the solution formed in step f) to a solid/liquid separation step, thereby to recover at least some of the metal sulphide and at least some of the hydrogen cyanide;
• and
• h) recycling the recovered hydrogen cyanide to the cyanide leaching (step a)).