Thermodynamic modelling was used in conjunction with the results of a sampling campaign to investigate the liquidus temperature and the copper losses in slag from the Teniente converter (TC) and from the slag cleaning electrical furnace (EF) at the Paipote smelter in Chile. The present paper (Part 1) describes the smelter flow sheet and discusses the results of thermodynamic calculations. The impact of process parameters such as the Fe/SiO2 ratio, the minor oxide levels in slag (CaO, Al2O3, MgO and ZnO), the sulphur dioxide partial pressure p(SO2), temperature and matte composition were examined with respect to the slag liquidus and copper level in the slags. The operating window of the Paipote TC to produce good slag quality correspond to a vessel temperature between 1200 and 1250 degrees C, and an Fe/SiO2 ratio in slag between 1.5 and 1.8 when the iron content of the matte is in the range of 3-6 wt-% Fe (or 76.5-73.2 wt-% Cu). Under these conditions, the TC slag (average total copper in slag similar to 6 wt-% Cu) contains less than 1 wt-% Cu as soluble copper with approximately 90% of total copper in the slag is entrained matte; it is noted that 0-10 wt-% solid magnetite is in suspension in the slag. As regards the EF, over the nominal temperature range from 1200 and 1270 degrees C, the EF slag is fully molten over a wide range of Fe/SiO2 ratios for slag containing the present level of minor oxides. An operating temperature of about 1225 degrees C and a Fe/SiO2 ratio in the range 1.4-1.8 are considered good conditions for optimum EF performance. The EF slag typically averages 0.82 wt-% Cu and it was estimated that under present conditions, about 50% of this copper was as soluble copper with the balance as entrained matte.