Isn't it easier to leach PM out of a fine sand or silt size particles versus course sand to fine gravel, which is more 'typical' of a heap?
You don't grind any finer than the economic limit. At a certain min max point the marginal capex, solvent cost, comminution cost, meets the marginal recovery of the finer ground ore. This is the best extraction size. The low capex of crush heap ores and low thru put cost means much smaller and lower grade ores can be tackled. On a typical Az-Nv body you would run about 7 scenarios of crush-heap leach to full grind mills. Crush-leach might get 70% or maybe 60%. But it might only cost 5 million to get going. Full mill might get 90% but it could cost 20 million. Is the 30% worth it?
The release size porosity is gone into. Too fine a crush and the water just runs off. Too tight an ore and the cyanide takes too long to penetrate. Too much sulphide and the cyanide dies. The release size might be 1/2 inch or it might be 1/8 inch.
The Box Mine in Sk has a 60 mesh release size. That in CDN terms is huge. It is also all-gravity. It saves millions in mill size, cost and thruput cost. But it is hard to convince the investor that 1/15 of an ounce underground will make money.
Stripping ratio, depth to ore top, geometry, hardness of rock all play a part. Believe it or not a really hard tight rock is not that much of a disadvantage in a full milling open pit. The rock walls can be very steep. Saves money.
Mining is a deep subject. Seems neandrathal and easy at first rude glance, but it requires ponderation. To optimise fully, it can stretch the limit of any discipline of science, chemistry physics, math or computation.
The need to empty the pits of Cornwall led to the development of the steam engine in the 16th century and led to the industrial revolution. The need to transport coal led to the development of rail. The need for massive amounts of steel for railroads and bridges led to the development of the steel industry, hence larger mines, larger equipment and the need for more and better distributed energy, hence the need for more coal and faster mining methods there too. All this led to the science of calculative engineering, first practiced to high art in Edinburgh Scotland.
Mining needs to be at the cutting edge of industrial technique to make money. It can generate predictable amounts of cash, but it is a business of engineering margin with exquisite precision.
Why don't those Montanans switch technological horses? Lack of drive to risk on new technique. They lack the essential element of the modern industrial engineer, not an iron ring but a brass ball.
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