Question 6 of 6: Bonus — Canadian Mineral Commodities Not Concentrated by Froth Flotation

Question 6: Bonus — Canadian Mineral Commodities Not Concentrated by Froth Flotation (2 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Two clear examples are iron ore and diamonds.

Canadian iron ore, from the Labrador Trough operations at Labrador City, Wabush and Mont Wright, is concentrated entirely by physical means: spirals and other gravity devices on the specular hematite ores, and low-intensity magnetic separation where magnetite is present. The density and magnetic contrasts between the iron oxides and the quartz gangue are large enough that no surface chemistry is needed, although reverse flotation of silica is used at some plants elsewhere as a final polishing step.

Diamonds, from Ekati, Diavik, Gahcho Kue and Renard, are recovered by crushing the kimberlite gently to avoid breaking the stones, then dense-medium cyclone separation in a ferrosilicon medium, followed by X-ray luminescence sorting and grease-table finishing. Flotation plays no part at all; the recovery depends on density, on X-ray fluorescence and on the natural hydrophobicity of a diamond surface at the grease table.

Other equally acceptable answers include Saskatchewan potash produced by solution mining and crystallisation rather than by the flotation route used at conventional shaft mines, rock salt from Goderich and the Magdalen Islands, which is mined and simply sized, aggregate and industrial silica sand, gypsum, and uranium from the Athabasca Basin, which is concentrated hydrometallurgically by acid leaching rather than by any physical concentration step.

It is worth being explicit about why these commodities escape flotation, because the reasoning is the same in every case and it is what the two marks are really testing. A separation only needs surface chemistry when no bulk property gives a wide enough contrast at the liberation size the ore demands. Magnetite is strongly ferromagnetic and hematite is half again as dense as quartz, so a magnetic or gravity device does the job at a fraction of the reagent cost. A diamond is far denser than its kimberlite host and fluoresces under X-rays, so two independent bulk properties are available. Salt and potash dissolve, so a chemical route is simply cheaper than a physical one. Sulphide ores have none of these advantages: chalcopyrite, galena, sphalerite and pyrite have densities within about 25 per cent of one another and none is usefully magnetic, so their surfaces are the only handle left.

The Canadian context is worth a sentence as well. By value, Canada's largest mineral outputs are gold, potash, iron ore, coal, copper and diamonds, and of those only copper is principally a flotation product. The impression that flotation dominates mineral processing comes from the base-metal sulphide industry that the discipline grew up around, not from the national production statistics.