An investigation into the permeability of a PGM slag freeze lining to sulphur

Abstract

Sulphur driven off from the sulphides in the black top of PGM smelters has in past years caused failure of copper coolers. When the magnesia-chromite bricks in the smelter were replaced with graphite blocks, however, copper cooler corrosion drastically decreased and furnace campaign lives improved. This study explores the role of the slag freeze lining in the permeation of sulphur to the copper cooler. This was done by measuring the gas permeabilities of two freeze linings and comparing them against the permeabilities of typical graphite and magnesia-chromite bricks that are used in PGM smelters; measuring the gas permeabilities of the different layers (hot face, crystalline layers, and cold face) of a freeze lining; and testing the permeation of liquid sulphur through freeze lining samples at 120 and 180°C. It was found that the freeze lining samples had gas permeabilities of 1.5 and 0.8 cd at a differential pressure of 32 kPa, compared to the permeabilities of the magnesia-chromite (10.0 cd) and graphite bricks (4.85 cd). The cold face layer of the freeze lining had the lowest permeability (1.8 cd), and the hot face layer the highest (2.4 cd). At 120°C liquid sulphur permeated the freeze lining through cracks. The freeze lining can therefore participate in preventing sulphur permeation towards the copper coolers, as long as its cold face remains intact and crack free. An added benefit of using graphite blocks in the smelter is that it establishes a glassy cold face layer in the freeze lining, which has a very low gas permeability.