The effect of caustic soda addition on the decoppering power of pyrite to decrease copper content in hard lead containing 0.56% weight percent copper has been studied. The process block diagram includes remelting of the hard lead samples, addition of pyrite and caustic soda. Experiments are conducted within the temperature range from 350 to 550 °C for a period of more than 1h. Factors such as stoichiometric ratio of pyrite, NaOH concentration, time, and temperature affecting the decoppering process, yield, and antimony content were studied. The results revealed that pyrite 2 stoichiometric ratio and 0.5 % caustic soda at 450°C decreases copper content in the hard lead alloy from about 0.56% to 0.03 %. Decoppering time about 30 minutes affects both the yield and the quality of the product. Under these experimental conditions, decoppering efficiency amounts to 92% with a yield of 95%. A little decrease in antimony content from 2.77 % to 2.4 % was achieved. A model explaining the findings was suggested which assumes that chalcopyrite is the main product of decoppering reactions involved in the early period of the process. The chalcopyrite so formed goes into slag more readily as the alkalinity increase due to decreasing slag viscosity.