Q :What kind of gold resources are there?
A: Gold can occur in loose oxide near the surface or as the deposit gets deeper the host rock becomes harder. Sometimes as depth increases and sulphide minerals occur, the gold will be in the silicates as well as the sulphide minerals. The deep level ore is called hard rock, the surface ore is called oxides.
Alluvial deposits are associated with rivers and the gold occurs as nuggets or fine gold. There is no gold in the hard rock in an alluvial deposit.
An elluvial deposit is one that was also deposited by rivers or water a long time ago and it may now be far from any river. Again the gold is free and there is no gold in the rocks.
Tailings are what is left over after miners have treated the ore before. There is little or no free gold left.
Q: What process do I choose for each of the types of resource?
A: For alluvial or elluvial ore you first dig up the material, then put it through a scrubber to break up lumps and liberate all the gold particles. After this, the slurry is passed through a gravity concentrator.
For surface oxides the ore is crushed down to liberate the gold and then the resulting slurry goes through a gravity concentrator. Sometimes if the ore is ‘soft’ it helps to use a scrubber up front to break up and liberate the ‘soft’ part and then you only have to crush the hard component. The slurry from the scrubber and from the crushing go together through a gravity concentrator. *See the later section on ‘High Energy Impacting’ which APT uses for this type of ore. If there is still enough gold left in the gangue (or waste) particles after impacting, then they must be sent to cyanidation. *See ‘Types of Cyanidation’ further down.
Hard Rock needs to be crushed and milled to about 75 microns in a jaw crusher followed by a ball mill. The fine slurry is then sent to an agitated cyanidation system. If there is enough free gold present, say 30% or more, then a gravity circuit can be installed with the ball mill and the gravity tailings go to the cyanidation agitators. If the gold occurs with the sulphides then it may be possible to float out the sulphides after milling using a system like APT’s TriFloat. The flotation concentrate is then either ground further and agitated in cyanide, or simply sold as is into the market. The cyanidation of a flotation circuit cannot happen with carbon as in a CIL, but requires thickening, filtration and Merrill Crowe zinc precipitation. This is a specialized process that cannot be performed at small scale.
Q: What kinds of cyanidation plants are there?
A: The simplest form is vat leaching where you charge the dry material into the first set of tanks and then fill them with cyanide solution. The solution then percolates through the charge and dissolves the gold. Finally the solution with the gold in it passes through carbon or zinc to deposit the gold out of the solution. Usually only coarser material like sand is suitable for percolation leaching or the solution wont pass through easily. Vat leaching takes longer (4 to 8 days) and is normally not as efficient as stirred tank cyanidation. The simplest stirred tank agitated cyanidation system is CIL (Carbon In Leach). This is used when the gold dissolves rapidly so that dissolution is complete within 12 hours. In this case the carbon is added to the first tank and gold dissolution and gold adsorption onto the carbon happen together. Where the dissolution is slower requiring more than 18 hours, the usual system is CIP (Carbon In Pulp). Here the dissolution is carried out first without carbon and when the gold is dissolved, carbon is added in a second set of tanks called the adsorption train.
In this article we are referring to CIL or CIP, not vat leaching.
Q: Will a gravity system recover all of the gold?
A: No, it will only recover part of it. There will always be something left in the gravity tailings. That is why it is common to start with a gravity system and then add a cyanidation module on later when it is required and you have built up some capital.
Q: Which is cheaper: gravity or cyanidation?
A: Gravity is much cheaper in terms of both the initial capital outlay for the plant and then the operating cost. Cyanidation equipment is more expensive to buy and more expensive to operate as the power consumption is higher and you also need to buy cyanide and lime.
Q: Do you need to grind to the same size for both gravity and cyanidation recovery?
A: Cyanidation recovery requires the ore to be finely ground to about 75 microns and to achieve this you need a ball mill and classifier, as well as the pre-crushing steps to get the ore to about 20mm in size. Gravity recovery will also benefit from fine grinding in a ball mill, but you can get almost the same gravity recovery from using a High Energy Impactor. This is because the ore is shattered by the impact bars and breaks on the grain boundaries where the gold usually is. So you get gold liberation without grinding the rest of the grains. This is called ‘Early Liberation’.
Q: What size do I need to grind to with a High Energy Impactor to get early liberation?
A: The usual size is 2mm, as dictated by the internal screen fitted to the High Energy Impactor.
Q: How does a High Energy Impactor differ from a normal hammer mill?
A: The High Energy Impactor has two solid bars that are much bigger than in a hammer mill which has many little hammers like fingers. The bars are rotated much faster than a hammer mill and impart far higher breakage energy, without which you won’t get early liberation. The machine needs to be far stronger and built with proper bearings to withstand the forces. APT makes High Energy Impactors called the ‘RD15’ (up to 1.5tph) and the ‘RD50’ (up to 5tph).
Q: How much do the impactor bars wear?
A: The usual wear rate in APT RD’s is about 500 grams of bar per tonne of ore fed through the machine. This is about half of the wear rate in a ball mill for example, but because the bars only weigh about 9kg each in the RD15 and 18kg in the RD50, you need to replace the impactor bars every 8 hours os so.
Q: Is the cost of the bars very expensive?
A: The bars are manufactured so they cost more than the balls that go into a mill, but overall the cost of impacting including bar wear is about US$6 to US$10 per tonne, so it is just a small consumable cost to be taken into account.
Q: If I started off with an RD type gravity system, can I simply add a CIL cyanidation section?
A: No, you need to add a ball mill as well to reduce the size to 75 microns. This is not only necessary to maximise the gold dissolution, but also to prevent the sand from settling in the agitator tanks.
Q: What kind of gravity concentrators are there?
A: The simplest and least expensive is a sluice but the recovery is poor, especially for fine gold below about 150 microns. A sluice operates at 1 G force. Then comes the bowl concentrators. These are non-fluidised and operate at about 6G force. There are many models; APT makes the GoldKacha (up to 3tph) which has a deeper bowl and a reflux system to enhance fine gold recovery. Then there are the fluidized concentrators, which typically run at 60G force. The fluitized concentrators are more efficient but cost more too and also require clean water for fluidization. Examples are Knelson, Falcon and Icon. APT manufactures its own high G force fluidized concentrator called GK-X (10-15tph). We have designed the GK-X in such a way that it has many advanced features such as it is easy to use, clean and cheaper to buy and operate.
Q: How do I get the gold out of the gravity concentrate?
A: It the old days this was done with mercury amalgamation but this is no longer used as it is toxic to humans and the environment. APT uses a system whereby the concentrate from either the GoldKacha or the GK-X is first upgraded over a special concentrating table that has a bump-stop action. Typical models are the Gemeni and ST tables from South Africa. The table concentrate is now high grade and suitable for smelting with borax to get a gold button. If the concentrate is not high enough grade and if there is not enough gold, the smelt will not work. One solution is to accumulate table concentrates over several days and then put them over the table once more. But even then, you are not ready to smelt yet; the final stage is to dry the gold concentrate and then run a hand magnet over it to remove all the iron and magnetite. You then smelt with a mixture of concentrate and borax, 2:1 by volume, at 1200 degrees C. APT supplies the ST range of tables as well as electric muffle furnaces called P42 or P100 for large mines.
Q: How do I get the gold out of the carbon from the CIP or CIL system?
A: The pregnant carbon (loaded with gold) is placed into an elution column and hot cyanide and sodium hydroxide solution is pumped around through the bed of carbon. The solution dissolves the gold off the carbon and then flows through a series of electrowinning cells which plate the gold from the solution onto steel wool. The elution column and the electrowinning cells are in series so that the solution goes through both each time it is pumped around. The solution typically has 0.2% cyanide and 2% sodium hydroxide strength. The electrowinning section needs to work at about 4.2 Volts.
The elution process in an unpressurised (or open Zadra) type elution system that is used for small mines and needs a cycle of 2 days. If the elution system is pressurized (closed Zadra) this is reduced to 1 day, but these systems cost more and are more involved to operate and therefore are usually used by larger mines only.
Finally the steel wool with the gold on it is treated with nitric acid. This dissolves the steel wool and the remaining sludge is dried and smelted with borax to get a gold button.
CAUTION: All cyanidation processes (including elution) are potentially fatal if not performed properly. These processes should only be undertaken by qualified and experienced personnel and all safety procedures should be adhered to.
Q: How much gold do I need in the ore to be successful?
A: It depends upon many things, and the figures given below are just a general guideline. APT accepts no responsibility for the viability or otherwise of your project using these figures:
Small scale alluvial or elluvial up to 3tph Minimum recovery 3 g/t Au
Medium scale alluvial or elluvial 10-20tph Minimum recovery 2 g/t Au
Large scale alluvial or elluvial >50tph Minimum 0.6g/t Au
Small scale <10tph oxide ore gravity only Minimum 3.5g/tAu, add 1g/tAu for milling and cyanide
Medium scale <20tph oxide ore gravity only Minimum 2.5g/tAu, add 1 g/tAu for milling and cyanide
Large scale >50tph oxide ore gravity only Minimum 2g/t Au, add 1g/tAu for milling and cyanide
Small scale hard rock mill, CIL leach, <3tph Minimum 5g/t Au
Medium scale hard rock mill, CIL leach <20tph Minimum 4g/t Au
Large scale hard rock mill, CIL leach 20-50 tph Minimum 3g/t Au
Tailings grind and CIL leach, small scale <3tph Minimum 4g/t Au
Tailings grind and CIL leach, medium <20tph Minimum 3g/t Au
Tailings grind and leach, large scale 20-50 tph Minimum 1.5 g/t Au