What happens when NaCN is added in ore containing PbS and ZnS during concentration by froth floatation method?

NaCN is used as a depressant which selectively allows PbS to come with froth, but prevents ZnS from coming to froth. This happens because NaCN reacts with ZnS to form Na2[Zn(CN)4]. 4NaCN + ZnS → Na2[Zn(CN)4] + Na2S