Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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COF density at gold plant (5 replies)

5 months ago
vekian 5 months ago

Hi people. I've put this idea by 2 metallurgists at the gold plant I work at. 1 agrees with me that it is worth trying. The other disagrees.

They want a COF density target of 46 with a SG of 2.7. Extra water is needed to be added to the mill discharge hopper to achieve this which is pumped to the cyclone cluster. The problem with this is, some of the added water inevitably finds its way to the CUF which feeds to the mill feed end and affects the mill density/grind. Taking feed water off just means more is needed at the discharge end to achieve 46 COF density and so the circle goes.

We have a thickener: The overflow water feeds a nearby process water pond which has other water from various sources. I suggested putting a tank between the thickener and process water pond. When it is full the overflow would continue on to the process water pond via an overflow pipe. The water in the new tank could be pumped directly to the overflow cluster. This would have no effect on the cyclones and would flow straight from the overflow 'well'. A similar effect would be to add a hose. The water from the tank would already have cyanide and pH (lime) as per the discharge from thickener via the last CIL tank in the chain. Theoretically the cyanide and pH reading should be the same or close to that that of the last CIL tank. It was asked why not just pump from the process water pond, to which I replied that the cyanide and pH is diluted from other water sources which are added. I feel this would save money on reagents and also solve the problem of unwanted water going to the mill feed end.

What do you think? Is it viable?

Thanks for your advice, in advance.


Paul Morrow
5 months ago
Paul Morrow 5 months ago
1 like by David

Hi Kevin,

addition water to your cyclone feed box with increase the cyclone pressure and thicken the underflow. A roping cyclone underflow is a cyclone in which the apex percent solids is too high. That is alleviated by reducing the cyclone load/pressure/feed-density to reduce its efficiency. 

Your "disagreeable" metallurgist is correct.

You can add that water to the cyclone feed as long as your ball mill can handle the demand caused by the increased viscosity in the mill and circulating load.

If there is an increase in viscosity, internal friction in the load will be reduced and the load will slump. This will shift the center of gravity closer to the mill center line thereby reducing power consumption. Reduced friction and lift means that less grinding will be accomplished. Since viscosity rises sharply with % solids at some critical value, a viscous overload will occur around this value.  


If COF density target of 46 is what you want?  What % are you at now?

If you know you can save that much (how much) on cyanide and lime and pump back water from the pond; do your intermediate box idea, yes. Otherwise, I would jump hand a submersible pump in the center of a rubber tubing and have it float on top of your thickener.

It is all worth in capital investment?

5 months ago
vekian 5 months ago
1 like by David

Hi Paul. Thanks for the reply. The idea is not to actually have any interference at all with the cyclone. It will have no effect on how the cyclone operates as it would be added to the overflow discharge 'after' it is split between underflow and overflow. The cyclones work well. The overflow density runs naturally at about 48/49ish. The water addition is needed to target the desired 46. I'm thinking of adding it as if it was like a hose dangling in the 1st tank just to get 46. To add water to the circuit undermines the how the the mill performs. 


5 months ago
Colette 5 months ago
2 likes by Paul Morrow and David

Hi Kevin

If i understand your query correctly then you are simply trying to achieve density control to your CIL - in which case I agree wholeheartedly that running your cyclones at a lower density will potentially cause you a headache. You could fiddle with the cyclone vortex/apex ratios to maintain a constant underflow density, but this can be tricky and if it is all working well then best to leave it alone.

I agree that using thickener overflow should have a benefit in terms of recycled reagents, cyanide may well degrade in the pond and the other water sources could introduce reactions that negate some of the reagents - however, this needs to be offset against the capital cost of your tank, piping and pumps to achieve the configuration you are suggesting. You will also only be using a very small percentage of water from the thickener overflow in comparison to the bulk coming from the pond for density control in the milling circuit.

I think you might struggle to justify the capital, particularly if in order to utilise the pond as a source you only need to make piping modifications. Its a tricky one to justify, as you are unlikely to see the benefit until the money is spent - there are just too many unknowns.

The suggestion of a pump in the thickener supernatent has the virtue of pragmatism - I would however be wary of causing a zone where the settling is affected by the flow patterns set up as the pump extracts the solution, this could result in uneven settling on the thickener bed and potentially create a less stable operation that has a tendency to slime - much like one sees when a thickener overflow isn't completely level, and one sees the area that is "lower" having a tendency to draw solids.

I'd implement the lowest cost option - using the water directly from the process pond - first, see if there is any detectable change in reagent consumption from the time this is done, and then use that to justify your idea. If there is no detectable change on the plant (which can be difficult to see) maybe you need to run a lab trial - which could also be a bit painful, you'd need to run a control, one with just pond water and one with the same mix of pond and thickener as you'd have on the plant. Alternatively you could model it, after doing analyses on the water streams, and try to justify it that way? If you have a handle on the chemistry this would be easy enough to do in Excel with iterations switched on to get rid of any circular error.


Good luck!

5 months ago
vekian 5 months ago

Thanks for the input Colette. I think it would be a minimal capital expense of a tank, pump, valves, flow meter and pipework. The only thing we don't have would be a tank. Everything else is either already in our warehouse or can be used from seconds, for a test run. In theory the water is the overflow from the thickener so it should be clear and slimeless. I think there is a good chance of recovering reagents (whether it's enough is the question)  and achieving the 46 COF density without undermining the grinding side of things. 

4 months ago
Dielgarcia 4 months ago


Hi, can you tell me the thickener u/f?, if you have 46% in the COF, but, what density have in your CIL tanks?

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