There is an ever-constant struggle to mill ores and recover minerals and metals more efficiently and cheaply as with other branches of the mining industry. Hydrometallurgical, or Leaching, processes have been developed for the recovery of uranium and other metals.
Sherritt Gordon Ltd. created a leaching process for producing nickel, copper and cobalt from concentrates for its Lynn Lake mine in northem Manitoba. The process employs an ammonia pressure leach; this is sometimes known as the Forward Process after the name of its inventor, F.A. Forward.
In the case of Sherritt, the copper and nickel were concentrated separately at the mine (these concentrates are graded about 29% copper and 12% nickel). Afterwards, the metal is leached from the concentrates by a mixture of ammonia-air. Both the nickel and the copper, together with a small quantity of cobalt, are dissolved and subsequently recovered and separated. The sulphur is converted to ammonium sulphate, which is recovered as a byproduct and used in the industry of fertilizer.
Employing a sulphuric acid leach, is used in some uranium mills somewhat different adaptation of the leaching process.
These processes are coming into wider use both for base metal and uranium ores. At the same time that they are innovative, they are not different from the cyanidation process which was described earlier, but these later leaching plants are more complex and expensive to construct.
In the last few years, solvent extraction and electrowinning (SX-EW) has become a quite popular hydrometallurgical method, at copper mines in particular. The principal feature of this method is the use of special synthetic organic liquids that are able to extract copper so that it can be deposited by electrolysis. This is used in combination with dump or heap leaching, this relatively cheap method can make it possible to economically process very low grade ores. Waste products and air emissions are most of the time minimal, making it doubly attractive to the miners of these days.
A method for processing lateritic nickel ores is the high pressure acid leaching which is coming to the front. The ores are leached with sulphuric acid; the nickel is after recovered by solvent extraction and electrolysis similar to the solvent extraction and electrowinning (SX-EW) method for copper recovery.
Both solvent extraction and electrowinning (SX-EW) for copper and acid leaching for nickel permit the producer to bypass the expensive refining and smelting stages in processing the ores.