Hydrometallurgy

— The following is an excerpt from Mining Explained, published by The Northern Miner.

As with other branches of the mining industry, there is an ever-constant struggle to mill ores and recover minerals and metals more efficiently and cheaply. Leaching, or hydrometallurgical, processes have been developed for the recovery of uranium and other metals.

Sherritt Gordon developed a leaching process for producing nickel, copper and cobalt from concentrates for its Lynn Lake mine in northern Manitoba. The process employs an ammonia pressure leach, sometimes known as the Forward Process, after its inventor, F.A. Forward.

In Sherritt’s case, the nickel and copper were concentrated separately at the mine (these concentrates graded about 12% nickel and 29% copper). Under this process, the metal is then leached from the concentrates by an ammonia/air mixture. Both the nickel and the copper, together with a small amount of cobalt, are dissolved and subsequently separated and recovered. The sulphur is converted to ammonium sulphate, which is recovered as a byproduct and used in the fertilizer industry.

A somewhat different adaptation of the leaching process, employing a sulphuric acid leach, is used in some uranium mills.

These processes are coming into wider use both for base metal and uranium ores. While innovative, they are not unlike the cyanidation process described earlier, though these later leaching plants are more complex and costly to construct.

In recent years, solvent extraction-electrowinning (SX-EW) has become a popular hydrometallurgical method, particularly at copper mines. This method entails the use of synthetic organic liquids that are able to extract copper so that it can be deposited by electrolysis. Used in combination with heap or dump leaching, this relatively inexpensive method is used to process low-grade ores. Air emissions and waste products are generally minimal, making it doubly attractive to today’s miners.

High-pressure acid leaching is coming to the fore as a method for processing lateritic nickel ores. The ores are leached with sulphuric acid, then the nickel is recovered by solvent extraction and electrolysis similar to the SX-EW method for copper recovery.

Both SX-EW for copper and acid leaching for nickel enable the producer to bypass the expensive smelting and refining stages in ore processing.

Bacterial leaching

As early as the days of the Roman Empire, the action of water-entrained oxygen on metallic sulphides has been recognized and used, particularly on copper-iron minerals. As it turns out, a microscopic organism known as thiobacillus ferroxidans is responsible for this action. This one-celled bacterium breaks down the sulphide minerals and generates weak sulphuric and sulphurous acids, which take the metal into solution as sulphates and sulphites, often leaving a precipitate of iron.

This bacterial leaching process is now used deliberately, mainly to recover copper and uranium from mine drainage waters and from surface waters percolating through old mine dumps and waste heaps.

The phenomenon of bacterial leaching has given rise to an active field of research and is producing economic benefits in the heap leaching of low-grade copper ores in open pits, particularly in the southwestern U.S.

In the low-grade, underground uranium mines of Elliot Lake, Ont., blasted ore was left in the stopes and leached with bacteria-rich solutions in situ. This reduced production proved costly since only the uranium-bearing solution was pumped to surface, instead of tonnes of ore.

Another area where bacterial leaching is being evaluated is in the treatment of refractory gold ores, in which the gold occurs as sub-microscopic particles inside the sulphide minerals. Bacterial oxidation destroys the sulphide grains, freeing the gold for subsequent cyanidation.