In any copper production plant, more than two tons of slag is discarded with 0.7-2.3%Cu to produce each ton of copper. Therefore, minimizing copper losses is crucial during the copper matte smelting. In order to reduce the copper losses to slag in copper production, the addition of colemanite (a boron compound; 2CaO.3B(2)O(3).5H(2)O) to slag was investigated in the present study. In experiments, the effects of other oxides such as ZnO, PbO, CaO and Al2O3 which are generally known to be present in copper matte smelting slag were not taken into account by using a synthetic slag and a matte. Therefore, a slag consisting of FeO-Fe2O3-SiO2 and a matte with Cu-Fe-S were only used. After producing synthetically a slag without copper and a matte, they were melted together at 1250 degrees C under nitrogen atmosphere in silica crucibles. During these experiments, calcined colemanite addition in various amounts (0%, 2%, 4% and 6% of charge), and duration (0.5, 1, 2, 4 h) were chosen as variables. At the end of experiments, the results showed that when the calcined colemanite addition to synthetic slag-matte couple was increased from 0% to 6%, the copper content in resultant slag decreased substantially from about 1.50% to about 0.40%. It was also seen that the addition of calcined colemanite reduced the duration to reach equilibrium. By applying this method, the quality of production process could be improved as well as reducing the cost of processing of slag. (C) 2012 Elsevier Masson SAS. All rights reserved.