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optimizing pre leach thickeners (2 replies)

martie
2 weeks ago
martie 2 weeks ago

Hello there,  am interning at a gold mine where there are problems with the clarity of the overflow of the pre-leach thickeners. Initially, only flocc was used at 0.1% strength.  Now we are testing a coagulant as well at different dilutions on the plant. The clarity has improved over the 2 weeks we have been testing. How do I make sure the coagulant and flocc compliment each other in the overall clarity and settling performance? 

Rheomet
2 weeks ago
Rheomet 2 weeks ago
1 like by David

Suggest coagulant screening first, because you need to coagulate the fine particles (i.e. neutralize the charges) in order to enable subsequent flocculation. Test a couple of generic coagulants at the same additions, say 20 g/t each, then cut to half or double until see effect, if any. For each test, once the coagulant added, add the same amount of flocculant you are currently using, and at the same dosage. Run a base-line witness test without any coagulant so you can compare the effect visually. Do these fast little tests in 100 mL cylinders. You can also test some competing flocs once you pinned down a good coagulant. Refer to one of my previous postings for next steps and related data reduction options. For scaling up, please note, stock concentration (1 g/L in your case) appears alright for mixing in the big unit , but still, I'd make sure that the floc is mixed alright prior to feedwell, and that the internal dilution is working proper. Remember to check the thickener feed solids content (%wt.) as this will determine the correct specific addition vs. dry feed, make sure the pulp density {Marcy scale or similar) returns the correct SG.  Finally, for best results, set-up a bench for impromptu off-line validation of your lab results, so you won't risk upsetting the unit. Hope this helps. Cheers,

Alex.

 

Sammy Rabie
2 weeks ago
Sammy Rabie 2 weeks ago
3 likes by Marshal Meru, Anupam and David

Hi Martie

It might also be a good idea to check if your material even needs a coagulant. A coagulant is generally only needed in cases where the material properties is such that floc does not work, normally material with problematic clay or lots of ultra fines. To test this simply take a sample of the thickener feed material before floc and lime addition, put it in a measuring cylinder and let it stand for a few hours. 

If after a few hours the water above the slurry is still dirty a coagulant is probably needed before floc addition. if the water is clear just adding floc to the thickener feed should produce clean overflow if the following is correct:

  • Correct floc is being used.
  • Correct floc dose is being added.
  • The floc dosing point is in the optimal place for the thickener.
  • Thickener is controlled well.

I was involved in a project end of last year where we worked on pre-leach thickeners for a South-African gold mine treating Witwatersrand material. There the biggest problems was the floc addition point, which was to far away from the thickener and caused all of the flocs that was formed to be broken down by the time the slurry reached the thickener. As well as poor thickener control, in this case the mud bed forming in the thickener actually overflowed in to the overflow launder (slimming) as there was no mud bed control present. 

Sammy 

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