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

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-04-04T06:57:46+00:00
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Effect of Fine on Paste Thickener Performance (10 replies)

Maya Rothman
2 years ago
Maya Rothman 2 years ago

The effect of feed fine fractions on paste thickeners performance. The application of paste thickeners has been on rise in mineral industries. The efficiency of compression in the bed is a determinant factor which affect dewatering efficiency in the paste thickeners. However, a number of parameters such as increasing feed fine fraction or clays content may affect the compressibility of sediment and consequently dewatering efficiency of paste thickeners. Do you have a practical solution which could be utilized when fine fraction of feed increases? 

Note that, the objective is improving compression in the thickener bed when feed fine fraction increases.

Alan Carter
2 years ago
Alan Carter 2 years ago

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

Bob Mathias
2 years ago
Bob Mathias 2 years ago

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.

Alan Carter
2 years ago
Alan Carter 2 years ago

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

Coarse particles with flocculant 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 hydrocyclones 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. You are right with the water balance.

Maya Rothman
2 years ago
Maya Rothman 2 years ago

This case is the first experience of employing paste thickener in copper industries. If there was a hydrocyclone after thickener, addition of coarse particles is possible. if not, we have to looking 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. 

Alan Carter
2 years ago
Alan Carter 2 years ago

Yes agree - flocculant is a nuisance in compression. Escondida, Radomiro Tomic and other copper plants all do.  The other issue we had was water them - get the pH right - work on the negative charged clays (we do this outside the thickener).

Victor Bergman
2 years ago
Victor Bergman 2 years ago

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. Contact your local BASF representative and he will fill you in on the details.

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

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 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 civils 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 a potential disruptive technology in paste & tailings dewatering.

Helena Russell
2 years ago
Helena Russell 2 years ago

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 Ishigaki 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.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

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.

David Kano
2 years ago
David Kano 2 years ago

2 issues come to mind - maintenance and number of units. You are right - 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"

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