Roasting (Heat) or Leaching (Liquid) Minerals: Which is More Environmentally Sound – by Marilyn Scales

Marilyn Scales is a field editor for the Canadian Mining Journal, Canada’s first mining publication. She is one of Canada’s most senior mining commentators.

One the news releases that landed in my inbox last week made me wonder about the advancement of mineral processing technology. Is it going forward … or back?

The item in question came from a Canadian company testing a South American property for its zinc potential. The company and project shall remain nameless. What caught my eye was that the favourable results were obtained in a kiln. That made me wonder why hydrometallurgical methods were not being tested.

Anytime an ore is subjected to heating it gives off gases. The worst case scenario is the burning of sulphide ores and resulting clouds of SO2 and NOx responsible for widespread environmental damage. Such was the case in the early days of smelting nickel and copper ores from the mines near Sudbury, ON.

Fortunately today we have gas containment units, acid plants and various means of controlling particulate and other emissions. These technologies help protect the environment, but they come with certain costs and limitations.

Hydrometallurgical means of metal recovery does not produce such deleterious emissions. Methods have been adapted in recent history to leach almost every kind of ore imaginable and to create the purest metals. Hydrometallurgy is considered far cleaner than roasting ores.

Leaching comes with its own set of startup and operating costs, but the byproducts are frequently solids that are more easily managed than gaseous emissions.

That is what made me wonder: Is this South American project going backwards on the technology continuum? Why not test and consider modern leaching methods?

From where I sit, choosing a mineral processing technology (roasting) that would harm even a small corner of South America is a non-starter. When a better environmental option is at hand (hydrometallurgy), should not it be tested, too? Somewhere along the line the operator would have to do his sums on both options and balance them against environmental risk.

It might be determined that roasting is the most cost-effective option. But if the company doesn’t test leaching, how can it make an informed decision?