The performance of Denver sub-A flotation cells in rougher and 1st cleaner service in Inco＇s Matte Separation plant was studied using the technique of Variance Spectrum Analysis. The procedure involves fitting a Fourier Series to real-time plant data and filtering out the low frequency disturbances on the assumption that performance would be enhanced with improved equipment installation - in this case Denver DR flotation cells. The resulting series is then recalculated to estimate the response of the process under improved control. In the case under study, recovery of nickel to the high grade nickel sulfide product was projected to increase by 5% as a result of implementing the equipment upgrade in the rougher circuit. In addition, it was projected that the quality specification target of a secondary nickel sulfide product could be met simultaneously with that of the copper sulfide product, by upgrading the 1st cleaner cells. These targets had previously been difficult to achieve concurrently due to the strong inverse correlation between them. Based on this study an equipment upgrade was carried out. Subsequent operation of both circuits confirmed the projected results and the validity of the methods as basis for justifying capital investment based on improved metallurgical performance.