Acid consumption and gangue mineralogy

The leach-chemistry topic drew a line between the stoichiometric reagent a dissolution requires and the larger amount a real circuit consumes. This topic is about the gap. For an acid leach, much of the acid the circuit pays for is consumed not by the metal it is recovering but by the gangue minerals it is not, and the size of that consumption is set by the ore’s mineralogy. It is one of the strongest levers on a leach circuit’s operating cost, and it is invisible in the reaction that dissolves the value.

The acid-consuming gangue problem

Many of the minerals that accompany a target metal react with acid even though they carry nothing the circuit wants. Carbonates are the worst offenders — calcite and dolomite consume acid readily, neutralising it as fast as it is added — and a range of other gangue minerals contribute their share. The acid spent on them is acid that did no useful work: it dissolved gangue, raised the consumption, and left in the residue. A feed with a few percent of reactive carbonate can demand several times the acid its metal alone would, and the difference is paid every tonne, every day.

Mineralogy sets the opex

This is why acid consumption is a mineralogy question rather than a stoichiometry one. The stoichiometric demand of the value reaction is fixed and small; the gangue’s demand depends on which minerals the ore happens to contain and how much, and it is measured on the actual feed — an acid-consumption number from testwork, not a figure read off the metal grade. Two ores of the same metal grade can have completely different reagent bills because one carries acid-hungry gangue and the other does not. Reading a leach economically means asking not only how much metal there is but what the gangue will cost in reagent to leach through.

The consumption calculator below turns the dose that mineralogy sets — supplied from testwork or plant data — into a reagent rate, a daily tonnage and a cost, and the worked thread shows how sensitive that cost is to the dose. The sensitivity is the gangue’s story: a change in the acid demand of a kilogram or two per tonne, well within the spread that carbonate content produces, moves the daily reagent bill by an amount that can dominate an operating-cost comparison. The calculator sizes the consumption of a stated dose; the dose itself comes from the mineralogy, which is the point of the topic.