The concentration of phosphate rock is strongly affected by the presence of ionic species dissolved in flotation pulp due to the interaction of ions with the surface of apatite particle. Process water containing large amounts of calcium, magnesium, fluoride and phosphate (phosphorus) directly influences the performance of apatite flotation (recovery and grade). This study quantified and analyzed the effect of these ions in bench scale flotation tests. The experiments were aimed at evaluating the effect of individual calcium, magnesium, fluoride and phosphate ions on the apatite recovery and P2O5 content. Interactions between the ionic species at low contaminant concentrations were also evaluated using a central composite design (CCD). The results were compared with the standard test (without ionic contamination). The results indicate that the presence of calcium and phosphate ions substantially reduced the apatite recovery, reaching values below 10%. Magnesium was also found to have a harmful effect not only on the recovery rate, but most notably on the P2O5 content, affecting the selectivity of the process. The effects of concentration of each ionic species were quantified. The interactions between variables also significantly affected the recovery and grade, probably due to the formation of insoluble compounds among the ions of this study, causing their removal from the system. (C) 2010 Elsevier B.V. All rights reserved.