Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Blend in some coarse, review rake design and speed, address negative clays outside of the thickener.

I can unfortunately not agree with the blending in of coarse it will increase the underflow density and increase the volume to be pumped, with no benefit in retaining process water (dewatering) and often a water balance will show that more water will be lost.

An increase of (inert rock flour) fines will influence the bed permeability; I agree that the rake design and speed will help to dewater the bed.
If it is an increase in clay content, then the process water chemistry will be the determining factor.

V = D2 g(Ps–Pl)/18n - Stokes law looks at both the particle diameter and density.

Coarse particles with flocculent may provide a better environment for fine fractions and change the compression - one Q would be, what is Moh after wrt %S? How far into the compression will you be and is this a paste thickener? This has been done on plants where hydro cyclones are positioned ahead of thickeners - taking a bleed from the coarse underflow and adding it to the feed. Ratios are important to control /review the water balance.

Point taken.

Thanks a lot for your comments.

This case is the first experience of employing paste thickener in copper industries. If there was a hydro cyclone after thickener, addition of coarse particles is possible. If not, we have to look for another way. To my mind searching for the methods to decrease shear yield stress or producing flocs with lower strength may be a solution. Utilizing a Low Molecular weight polymer may be a remedy for this problem.

Yes agree - flocculent is a nuisance in compression

I did lots of work at Escondido, RadomiroTomic and other copper plants. The other issue we had was water them - get the pH right - work on the negative charged clays (we do this outside the thickener)

Conventional flocculants (Acrylic Acid / Acrylamide co-polymers) do form large flocs which can be difficult to compress due to entrained water. Rheomax DR Flocculants replace the Acrylic Acid with a sulphonate based acid. These flocculants produce flocs with a higher fractal density which leads to higher underflow density. The added bonus is that the higher density comes without an increase in Yield Stress.

A little known Perth Based company has made significant advances in tailings dewatering using a containerized modular screw press system specially configured for tailings and coal fines. In one Rare Earth tailings case comprised primarily of super fine clays, the units can replace the tailings thickener altogether. Pilot units are reading to go, Tangible results can be evaluated within weeks of engagement, and delivery time is short with almost zero civil and additional engineering time. Consider that a single container and grasshopper conveyor unit can be positioned at every spigot on a tailings dam, and convert it from paste or wet tailings to dry stacked tailings deposition for as little $1.3M for a 30T/h unit. Even a large Laterite mine would only need say 10-15 units. Sounds too good to be true right. Go test it. These costs are super low. So low in fact that this can be considered as potential disruptive technology in paste & tailings dewatering.

I agree the screw press might be a good option for small applications with compressible solids - that RE tailings is challenging stuff. However for large copper tailings application, say 1500 t/hr, you would need 50 off units of the screw press. Hardly a practical solution! I suspect it might take exception to being a "little known Perth Based company" :).

I agree that blending in coarse can help "ballast" the floc's for better settling, though I'd also review your dilution ratio to ensure you are at optimum solids.

Disruptive developments tend to challenge the conventional mindset. Watch that space.

Shear thinning is a good option for handling slurries with high yield stress or where the thickener designers have decided to put the underflow pumps miles away from the thickener itself. However, shear thinning will not increase the underflow density proper.

Increasing the floc dosage may improve the overflow clarity. In theory, a faster sedimentation rate gives you a longer residence time in the compaction zone - which is good - but the time frames are negligible if you're talking about sedimentation around 5-10 m/h. Higher floc dosage also increases underflow yield stress, costs more reagent and you don't want to go there.

Deep underflow beds are good in theory but tend to ignore that most of the compaction in a bed is done with the rake squeezing the slurry ahead of the blades. Plus, there is a need for the water to be released from the bed, which is hindered by an increased path length to the supernatant zone. I'm a big fan of dewatering rods, but no-one seems to build them properly. Very often they finish up in the bottom of the thickener or wrapped around something they shouldn't. I've got a few ideas and it seems that we've all suffered long enough. Remind me to have a chat to Outotec or similar to fix this up...

Two issues come to mind - maintenance and number of units, only worth considering for small tonnages.

How much opex costs do you want to have on tailings? Most plants want as little as practically possible. The rule of thumb for tailings is: Cheapest most suitable"