Calcined Copper Concentrate: Pyritic Concentrates Roasted to 8 % Sulphur in Allen-O'Harra Calciners (Butte, Peters 1905)

Pyritic copper concentrates carry about 40 % sulphur, far too much to smelt to a rich matte. At Butte they were dragged along two 90 ft hearths by chain-drawn ploughs while wood fires burned part of the sulphur away. Result: calcines with 8 to 9 % sulphur, 50 tons a day from each furnace; the concentrates lose 26 % of their dry weight. Source: Edward D. Peters, Modern Copper Smelting (1905), pp. 201-204; picture: his Plate II, the Allen-O'Harra calciner. Not given: temperature, time on the hearths. Roasting gives off sulphur dioxide, a poisonous gas.