Quick metallurgical lesson from guy who’s been there and done it. Wayne McCrae.

Ok, yes I have built from scratch a number of Gold processing plants. Gravity, Carbon in Pulp, (CIP) Heap
Leach and Vat Leach. However, lot of investors, stockbrokers, industry people sometimes only understand 50% of what they are being told whether at a Gold Seminar or a pitch by a CEO/MD for cap raise.

CARBON IN PULP (CIP) what it means: standard plant : say 200-500 tonnes of ore throughout per annum.

First after mining the ore (rock) needs to be crushed. The first process is the ore is fed into a crusher, in most cases a Jaw Crusher. It will reduce the ore after being mined to between 3” to 6” size, then along a conveyor to a secondary crusher, usually a cone style crusher that will bring the rock size down to around 12mm. Once crushed to this size it’s ready for the ball mill.

The ball mill is a grinding circuit that grinds the ore conveyed from the secondary crusher to around -100 micron that’s about the size of your talcum powder in your bathroom. For every tonne of ore put into the ball mill, we add a tonne of water to make a runny slurry. The ball mill is a cylindrical type of apparatus with a width usually half its length. 8×4, 18.9 etc. It is set on bearings and is approx. 1/3rd to 1/2 full of 3” round mild steel balls. (these can vary depending upon metallurgical test work) The ball mill is driven by an electric motor and revolves around at between 20 and 40 revolutions per minute. The balls grind the ore (rock) into a slurry or fine powdered rock and water.

The slurry with constant feed into one end of the ball mill overflows the opposite end of the ball mill into a slurry pump. (similar to water pump but with extra linings). The slurry is pumped into a big tank 300-500x m3 with a big stirrer (impeller) in the middle of the tank that mixes the slurry and keep slurry from settling.

This first tank is called a primary leach tank. Cyanide pellets, mixed with water and dissolved, is added into the tank with a little pump that delivers around 1x kg of cyanide for every tonne of ore into the primary tank. (Note: cyanide when mixed with water and put into a slurry that contains gold the cyanide will dissolve the gold into a liquid just like water does to sugar.) Once cyanide is added, the slurry overflows from the primary tank into a series of additional tanks called carbon in pulp or leach tanks. ( there are other names for them). Each one of those tanks also have large stirrers with large impellers on the bottom to stir the slurry now with cyanide in it. The slurry with the cyanide overflows from one tank to another in a series. The tank after the primary leach tank, being the first tank after the primary leach tank has carbon added into the slurry. Now we have a slurry with carbon in it. (Bit like a cake mix with chocolate chips in it.) The quantity of carbon again depends on metallurgical testwork)

What is Carbon:
Carbon added is actually “burnt coconuts shells and crushed with the fines (dust) screened out, the size can vary in size front 4mm to 8mm. (Called activated carbon)

Then What:
Once carbon added into the slurry it is important that the carbon stays in suspension evenly in the slurry (don’t sink and don’t float) the activated carbon absorbs all of the Gold in the slurry and becomes loaded with the gold. The carbon is then determined pregnant carbon. Each of the series of tanks has activated carbon with the first take after the primary leach tank absorbing most of the gold. After this process the carbon is screened out by running the slurry of a sieve screen which allows the carbon to be screen out which is loaded with the gold. The slurry is then pumped to a tailings dam as waste.

OK now we have the carbon loaded with Gold. That’s what a CIP processing Plant is in simple terms. Crush and grind the rock to a paste add water to a slurry, mix it in a tank, add cyanide, dissolve the gold the add carbon to absorb the gold. Take the carbon out.

Then comes how to get the Gold out of the carbon.

Tomorrow, I’ll tell you how. This post is already too long.


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