Almost a no brainer and exactly for the reasons you mention. But- try and have the thickener in the plant at source of the tailings- the thickened underflow helps reduce pump and pipe size and the overflow is immediately usable via either a process pond or storage facility thereby reducing water losses.

Less water on the tailings is always a surplus, if you build day walls or spigot, a thick deposit is first price, if you use cyclones- the thicker density will affect the cyclone efficiency- review your water balance

The overflow would be used as part of the mill feed water at the usual flowrate, not sure how this would affect the cyclones? Also need some guidance on conducting a cost analysis to justify the cost of installing.

The cyclone argument is for the feed density into the cyclone...as a rule of thumb the lighter the feed density the better the classification- should not be a T piece.

The cost savings is really a LONG term thing

Assume you change the residue line size and install more efficient pumps for lower volumes but higher densities

Total Cost of thickener and ancillary equipment installed versus Plant to tailings residue Pump Power savings over LOM, maintenance cost reduction LOM , cost savings not pumping water back to the plant over long distance, cost of water handling on the tailings dam ( effluent treatment cost/ evaporation cost if mechanical/pump cost and power costs for the pumps/pontoon maintenance and upkeep/labour.)- (capital deferring - if you don't need to raise the dam walls too quick because you are pumping lower volumes, the rate of rise is inhibited and you can spread your capital out longer for that- it helps)

If it is a cyanide residue the cost savings of cyanide recovered back to the plant, the capital cost savings of a smaller cyanide neutralization plant if you go that way- ( also have e to look at the thickener OPEX as not to be bias.)

If you pay for fresh water make up, calculate the value of the water not lost due to evaporation and seepage due to the new thickener installed this should go far in doing the financial study.

Do not forget to account for soluble losses. Even at today's metal prices it does not take much to pay for a thickener.

very much so

One thing to consider if you do install tails thickener and overflow goes back to grinding is that you are now "grinding in cyanide". There can be quite a bit of dissolution in mills, so need to consider spills etc of solution that can result in gold loss. If, in addition to tails thickener, you have a preleach thickener it may be worth considering putting CIC columns on overflow to prevent gold solution buildup in the grinding area. If you have gravity recovery in grinding this may drop off a little with dissolution taking place.
If you do go down the tails thickener route take advantage of it and add your reclaim or makeup water to the tails thickener feed and "wash" cyanide solution from tails to grinding. Not sure whether you have CN destruct, but you can calculate water balance and cyanide washing that will occur. Main part of financial justification can be reduced cyanide as well as reduced reagents in destruct circuit.

As I understand it you are already operating a gold plant and therefore the CAPEX saving for smaller piping size etc is probably not relevant as an economic justification.Depending on what you are currently pumping to the TSF now,in terms of pulp density, you may have to consider settling of solids in the pipe due to the lower slurry transport velocity.
The retrofitting of a TSF thickener is often considered not so much due to cyanide cost savings but usually due to lack of sufficient water supply. If you have sufficient water supply then as Willem says it would be a long term payback.
The discussion regarding classification efficiency due to lower feed density is often held as a justification for the installation of a pre-leach thickener. In this case you can run the cyclones at a lower density and increase separation efficiency thus resulting in a finer overflow size distribution. The cyclone overflow then gravitates to the thickener and you can then run your leaching circuit at a higher density resulting in longer residence time and cyanide and possibly lime cost savings as well. This is important if your ore is grind dependent and then the economic justifications are often much more easily made as it results in higher gold production.

Hi all, thanks for your comments, will have to study them closely so I don't overlook any parameters that need considering. The idea initially seemed attractive as we did some quick pH buffer tests and noticed a big drop in lime consumption when using filtered water from the tailings compared to using the decant water from the TSF. Obviously if these savings are minimal compared to the cost of installation and maintenance, there would be no need for it as we have sufficient water supply. Haven't really looked at the pre-leach thickener.

I think more light must be thrown on the economic viability of this project. Let the expects be precise in their answers. A little cost savings could go a long way to improve a mine's financial regime. I think the decision will also be based on the mine life as well so that becomes a long term project. I believe installing a tailings thickener sounds pretty nice idea. Cost savings on cyanide detoxification, lime usage and short distance to pump process water from the thickener to mill. But we need the expects to tell us more about its benefits then we look at the economic implications. I know most mines have the tailings thickener. This is a nice discussion. Thanks to all who have contributed up until now.