Production of colloidal sulphur from reduction of sulphur dioxide by contact deposition with 304 stainless steel

Abstract

This paper deals with the use of 304 stainless steel as a reducing agent for the transformation of sulphur dioxide from aqueous acid solutions into elemental sulphur. The kinetic behaviours of the sulphur dioxide reduction and the dissolution of stainless steel were analysed with the use of a rotating disc electrode demonstrating, by means of an Evans diagram, the feasibility of the process. Experiments carried out with rotating three-dimensional cylinders made from an ordered packing of woven-wire meshes of 304 stainless steel, mesh size 8, confirmed the spontaneous production of elemental sulphur and the exhaustion of sulphur dioxide in solution. Likewise, the metal phase can be fully dissolved in the electrolyte. The experiments were made in a solution of 5 g dm−3 SO2 in 0.5 mol dm−3 H2SO4 at temperatures ranging from 30 °C to 80 °C and rotation speeds of 500 rpm or 1000 rpm. The best results were achieved at 50 °C and 500 rpm giving 1.11 g of elemental sulphur with a space time yield of 5 kg m−3 h−1, in an experiment of 15 min duration. This means that 46% of sulphur dioxide was converted into elemental sulphur. The remaining sulphur dioxide was mainly reduced to polythionates, which stays in solution. The consumption of 304 stainless steel was 6.3 g per gram of sulphur produced. The sulphur particles are of high purity as well as porous with different irregular shapes with an average size of 8.4 μm.