This depends on the liberation sizes for the lead and zinc minerals. Have you done any liberation analysis? Are you doing a lead flotation followed by zinc flotation? If you look at the work from MIM (Aust), MRM (Aust) and Brunswick Mining & Smelting (Canada) you will have some idea of the required analysis and potential difficulty re establishing grind and regrind size.

This also depends on the target grade you want to achieve and the trade-off of cost vs. revenue for the product. As noted above, mineralogical analysis is the first step to determine grain size and the particle size required for liberation. Depending on your downstream processing there will be other factors to consider in the selection of regrind size.

It may vary from

Mineral to mineral of lead and zinc metal, Deposit to deposit with and without interlocking of non valuable mineral P 80 of lead and zinc may also vary. Detail liberation study is required on representable sample to establish optimum p80 .grind size.

I have seen different p80 grind sizes for different mines having similar minerals of lead (Galena) and zinc (sphalerite) varying from 65 to 110 micron associated with pyrite within 200 to 300 Kms span in India.

In general I would recommend only grinding fine enough to maximize the net revenue generated (at an acceptable product grade) by the subsequent flotation stage. Anything finer than that defined by your mineralogy and marketing requirements represents a waste of power and consumables. As stated by others, yours is not a straightforward question and there is a fair bit of work involved in grasping the liberation requirements for a particular ore type. At MIM we typically cut a final pre-cyclone overflow P80 of 7-10 micron for sphalerite liberation, but even for us, this requirement may vary considerably from ore-type to ore-type. First point of call might be your Geology team to see what data is already on-hand.

I would rather suggest conducting series of flotation tests with the size ranging from -65 mesh to -400 mesh under the same conditions and study each products mineralogy & liberation sizes. This may give clue for further tests varying different parameters. Do you have any liberation data?

I'd start with some mineralogical analysis. Most labs almost exclusively go with automated SEM (QEMSCAN, TESCAN, and MLA) mineralogy. If you do your mineralogy on a size by size basis (recommend at least 4 size fractions) then you can plot degree liberation (>90% of surface area being a "liberated" mineral) versus particle size. General rule of thumb we use is the primary grind size p80 should be at the size where ~40% of the target mineral is liberated (assuming there would be a regrind ahead of downstream cleaning circuits). This would be a starting point anyway. As previous posters have said, your next step would then be to conduct a primary grind size versus recovery batch flotation test work program. 3-4 grind p80s would typically be sufficient, making sure to test grind sizes above and below your target ~40% mineral liberation.

Good luck, where it gets interesting is if your galena and sphalerite minerals have different liberation characteristics!

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It is obvious thing that the mineral beneficiation process is to enrich the interested mineral quantity in the concentrate. It directly derives from the optimum liberation of the minerals from the host minerals; which in turn can be achieved over a particular mesh of grind /P80. But, the increment in liberation increases the grade & recovery to a particular theoretical grade recovery limit after which the increment in liberation may result in decrement in the grade & recovery as the minerals will be fine grounded and the irrelevant flotation starts as the significance of entrainment and entrapment phenomena gets increased. Due to this the plant consumables like power and reagent dosage will also be multiplied for the same concentrate yield.