Oxidation of sodium thiocyanate (NaSCN) in Stretford aqueous liquor using air and commercial hydrogen peroxide (H2O2)

Abstract

Stretford processes are most widely utilized for the absorption of H2S from gaseous streams from the coal gasification process and converting the absorbed H2S into elemental sulfur. Hydrogen cyanide in the gaseous streams is efficiently absorbed and quickly ionised in the Stretford liquor during the absorption of H2S. The free cyanide ions react with polysulphide and suspended elemental sulfur to form thiocyanate ions. Sodium thiocyanate (NaSCN), which is a very expensive reagent, is used for bacterial control; for suppressing formation of sodium thiosulphate and for reducing the consumption of sodium anthraquinone 2,7-disulphonate (Na2[ADA]) in Stretford liquors. The objective of this paper is to report on the degree of oxidation of NaSCN in Stretford liquors using air and hydrogen peroxide (H2O2). The X-ray fluorescence and X-ray diffraction results indicated that NaSCN in the Stretford liquors oxidized to form sticky sodium and ammonium based salts (Na2SO4, Na6(CO3)(SO4)2, NaHSO4, and (NH4)2SO4) when using air and H2O2. The NaSCN oxidation led to high concentrations of Na2SO4 that enhanced crystallization and precipitation that resulted in severe blockages of plant equipment and contamination of the element sulfur. The results also indicated clearly that the concentration of NaSCN in the Stretford liquors steadily decreased, while the concentration of Na2SO4 increased during the oxidation of Stretford liquors using air and H2O2. Therefore, it is critical to ensure that NaSCN does not oxidize to form sodium based salts under operating conditions as a result of increased concentrations of dissolved oxygen from the oxidizing plant equipment and from hydrogen peroxide formed from catalysts. The oxidation of NaSCN in the Stretford liquors with H2O2 and air to sulfate salts also revealed that hydrogen cyanide and carbon dioxide gases were evolved. The results obtained from the NaSCN oxidation experiments can be used in conjunction with process changes to optimize plant parameters and critical chemical concentrations