Trust almost all the mineral beneficiation plants based on wet grinding and flotation uses this phenomenal practise of using the recycled water from thickeners , filters and tailing ponds which will have presence of slimes, pH variation, and of course a bit of flocculent polymer molecules also. This acts in opposite nature of flotation. The effect may be very meagre but however flotation test work comparison using fresh water and recycled water as base for your ore with same reagent dosing will quantify the difference and thereby can envisage the fall in recovery or con grade. I am trying to simulate the existing plant situation in lab.

Please permit me to me share an irrelevant but similar case study of coal washery using thickener overflow water as feed to plant.

In Coal washeries the deslimed slimes fed to thickener, where the particle's natural settling rates are low and flocculent dosing needs to be on higher side to enhance settling rates. At times the increase in flocculent doses will result in increase of recycled water's viscosity which will reduce the magnetite recovery (especially the fine magnetite) in LIMS.

Given the fragility of flocs and short activity times I cannot see any problem unless decant water from the thickener flowed by gravity to the flotation feed.

The only test work I know on flocculants in Mo flotation was by Boliden who used Magnafloc 351. As most of us their greater concern was any NaHS getting back to the copper circuit.

http://is.gd/GNgqf3

The residual floc in process water going back to the milling circuit will have negligible impact. The amount of floc is small in comparison with the tonnage of solids and the milling circuit will destroy whatever floc comes through.

Adding floc in the bulk concentrate thickener is usually avoided as that does impact the Moly recovery.

Thank you so much for sharing your experience. As mentioned, in order to ignore floc impact on Moly flotation, it is a common practice to design copper-moly concentrate thickener without adding floc.

Furthermore, the concern that was stated in paper was due to using NaHS in copper-moly which causes acid generation in thickener O/F is unlikely to happen in our case. Since we are working in molybdenum mine and it is not a Polymetal mine so we do not need to use any NaSH for copper depression. In conclusion, it seems that it hasn't any notable effect.

The flocculent if applied properly should reside in the under-flow not in the over-flow. There could be situations were residual flocculent in the over-flow that may cause issues. However, as most of the fine gentlemen have already state, the impact should be little at best. You would have to be overdosing in a big way for the flocculent to have a noticeable effect in flotation.

One example is where cooling tower water was feeding into a thickener in which the reagents had an effect on flocculation. Huge amounts of residual flocculent resided in the overflow. The thickener performance showed decreasing amount of performance in solids, settling, and clarity even though more flocculent was being used.

This water was being used in a flotation process. However, no attempt to qualify the performance was made.

Point is that there could be situations whereby residual flocculent could be in the overflow. Float tests with increasing amounts of residual flocculent could help with the quantification of the impact. A simple turbidic method for what I would suppose is an anionic flocculent could be done to determine if you have residual flocculent and quantity in the overflow. 

I agree with the comments re flocculent having minimal effect on MoS2 recovery using recycle water. Turbulence in mechanical cells, plus the fact that dispersants (inorganic as well as organic) are often also added to avoid MoS2 aggregating with silicates through divalent cation bridging. That said, often fine (<10 um say) MoS2 is often improved by some level of aggregation, that can then be addressed in cleaning.

I concur with the many comments indicating that there should be minimal impact of the flocculent used for thickening the concentrate as most of it would be with the solids in the underflow.

Having said that, this implies that there is no operational issue with the thickener itself.

In any event, a few flotation tests with addition of flocculent reflecting a 10 to 25% proportion of flocculent reporting to the overflow should indicate whether this would be an issue or not.

We used a low MW floc in the collective/bulk thickener and also in the smaller in-circuit moly thickener in the moly plant at a large South American Cu/Mo porphyry and didn't have any troubles. We had tested it in the lab and then ran a plant trial and didn't notice any impact on moly flotation recovery. Overall recovery went up as the floc helped control loss of fine moly to the o/f. Suggest you get some samples and try it in the lab.

Are you facing any such problems? If yes you change flow sheet design by adding cyclone before thickener and avoid flocculent addition.