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Effect of flocculant on settling and filtration rates (2 replies)

2 months ago
Charlie 2 months ago

Hello everyone, generally, by how much is:

  1.  The settling rate of a flotation concentrate increased when the correct flocculant is applied?
  2.  The filtration rate of that same flotation concentrate increased when the correct type and amount of floc is applied?

Am looking for a crude approximation.

Thank you!

Alan Carter
2 months ago
Alan Carter 2 months ago
1 like by David

Hello Charlie,

Re Thickening:

“Generally” more is the answer.  As you know, the purpose of floc is not just to increase settling rate, that’s the effect but the reason is so that everything can be smaller.  Smaller tank, smaller foundations, better clarity, better density, it probably helps filtration (as long as not overdosed), aids raking (slight lubrication) bound to be more.  So smaller, cheaper, more consistent operation.  Un-flocculated settling rate vs flocked settling rate is never tested as in modern world operations using no floc is never done. 

Optimal flocculation is what a dynamic test work will try to determine; the right dose and right kind of flocc to achieve the thickener underflow density target.

Re Filtration:

In filtration you’re looking into flocc, coagulants and surfactants to reduce cycle time improving capacity. However in reverse to the more the merrier, over dose can lead to cloth blinding reducing your filtration capability to nearly zero. Right flocc (or aids) can improve pressure filtration in the vicinity of 5-20% for specific application, even more on vacuum filtration.

Typically, whatever amount of flocc was introduced into the thickener, is all that the filter wants to see.

From non-floc to flocc, the size difference can be 5 to 10 times smaller, sometimes even more than that.

2 months ago
johnclark 2 months ago
1 like by David

With the right flocculant, the settling rate should increase significantly. The added flocculant will require the "optimum" (or higher) conditioning time prior to the concentrate being added to the thickener. Conditioning (mixing time is determined from tests) - can you add floc. to the pump (or may have to be the pump discharge pipe) and achieve the required conditioning time? Some flocs condition faster than others. Adding floc. to the thickener underflow storage tank may be a bit dicey, although I did this once and things worked out of on the leaf filters). Floc. addition method really important and depends on the type of floc. Alan's comment on overdosing the floc and then have a negative consequence should be tested to avoid surprises! The floc. supplier may assist - guess it's better to locate a floc that does not decrease settling/filtration when overdosed. It appears that some flocs do this and some do not. If the filtration system is not currently removing enough water, the floc. should make a big difference (testing required to select a floc. that is best for filtration rather than settling). The Kemess mine successfully used a floc. resulting in eliminating the dryer stage. 

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