 Cadmium is a common impurity in zinc ores , and it is most often isolated during the production of zinc . Some zinc ores concentrates from <unk> zinc ores contain up to 1 @.@ 4 % of cadmium . In the 1970s , the output of cadmium was 6 @.@ 5 pounds per ton of zinc . Zinc sulfide ores are roasted in the presence of oxygen , converting the zinc sulfide to the oxide . Zinc metal is produced either by <unk> the oxide with carbon or by electrolysis in sulfuric acid . Cadmium is isolated from the zinc metal by vacuum distillation if the zinc is <unk> , or cadmium sulfate is precipitated from the electrolysis solution . 
