It's important to know some additional information such as copper and zinc grades, copper minerals, flotation feed size (K80). Right now, based on the information you  provided, the addition of PAX is not appropriate in the copper flotation circuit the collector is very strong. If the addition of sodium sulphide is low or high, results will not be good. The zinc/copper ratio is other important factor to consider in the evaluation. You should perform kinetics flotation tests to determine the optimum flotation time in the copper rougher flotation circuit and in what moment the problem starts (i.e. first seconds). 

Thanks, Jorge

the feed grade Zn/Cu ratio is ~7:1, feed size being 80% passing 75mic. but I found 30%zinc recovered with 9% cu in first 1,2 min and in 3,6 min almost all Zn recovered.

I think you should evaluate the grinding environment that promotes the production of more copper or led ions in the slurry. In order to do this, it is possible to evaluate different sets of depressors to identify the less complicated grinding environment. Test 1, tap water (blank test); Test 2, NaCN, 50 g/t; Test 3, NaCN, 50 g/t and ZnSO4, 1200 g/t; Test 4, SMBS, 600 g/t; Test 5, ZnSO4, 1200 g/t and SMBS, 600 g/t; Test 6, NacN, 50 g/t, SMBS, 600 g/t. In all cases is important to follow the same grinding procedure (e.g. ore, 500 g; water, 1000 mL; grinding time). The solution should be assayed for copper and lead.

Did you evaluate the potential activation of sphalerite at coarser sizes than 75 um K80? Was the activation the same?

A possible suggestion is try adding some sodium cyanide to your float circuit in the conditioner.  The cyanide will remove copper coatings on sphalrrite. You don't need much, so a low concentration should be used.  You want it to go after the thin copper coatings on the spalerite, not the chalcopyrite or other copper minerals.  Be mindful of the pH of the process water, you want it to be alkaline, above pH 9 to 10.  A small amount of cyanide will complex quickly and there will be little free cyanide left as a concern.  If you are floating with an acid pH, you don't want to use cyanide as it will quickly disassociate to HCN, a poisonous gas.

Hi, you can try agregating 1,1.5 kg de CaO in the mill for to obtain ph 9.5-10 during this process, in the condittioner adjust the pH at 6.5 with sodium meta bisulfide or amonium bisulfide.

The objetive of CaO is to precipitate all ions of copper that maybe can to generate in  the mill, i made this in a plant in Zacatecas with good results