Sphalerite flotation and separation from other sulphide minerals are strongly dependent on pulp solution conditions such as pH and copper concentration but also temperature. The surface of sphalerite needs to be activated with copper ions to enable collector adsorption and therefore sphalerite flotation. It was found that the collectorless flotation recovery and rate of sphalerite increased with increasing copper concentration up to a maximum value before decreasing with further copper addition independently of temperature. This increase in recovery is attributed to the copper-activation of sphalerite with formation of the hydrophobic species of polysulphide/elemental sulphur. Several complementary solution and surface analytical studies have shown that the maximum recovery corresponds to the maximum number of surface zinc ions that can be replaced with copper ions (copper-activation mechanism). The decrease in recovery is not the result of further adsorption of copper hydroxide in molecular form at the sphalerite surface but it is attributed to copper hydroxide precipitation covering the hydrophobic surface species. Addition of xanthate collector restores the flotation recovery and rate of copper-activated sphalerite. It was also found that the flotation recovery and rate of copper-activated sphalerite at pH 10.5 are reduced when temperature is below 12 degrees C in the absence or presence of collector, which is in good agreement with observations made at several flotation plants that zinc recovery is on average less during winter than in summer. Possible causes for this decrease in sphalerite flotation at lower temperatures are discussed. (C) 2015 Elsevier B.V. All rights reserved.