Oxidation of sodium thiocyanate (NaSCN) in high ionic-strength process aqueous liquor
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The use of liquid redox processes (Stretford Process) for the absorption of H2S from gaseous streams and converting the absorbed H2S into elemental sulphur is widely used. The chemistry of Stretford process aqueous liquors is very complicated and maintaining the critical chemical parameters is imperative. Sodium thiocyanate (NaSCN) is used for bacterial control and for reducing the consumption of sodium anthraquinone 2,7-disulphonate (Na2[ADA]) in Stretford process aqueous liquors. A series of experiments were conducted to investigate the degree of oxidation of NaSCN in aqueous solutions and Stretford process aqueous liquors using oxidants such as hydrogen peroxide (H2O2), air, intermediate H2O2 from sodium ammonium vanadate (SAV) and sodium anthraquinone 2,7-disulphonate (Na2[ADA]). A decrease in the total alkalinity and Na2[ADA] concentration was also observed under these oxidising conditions. Some of the techniques employed during the study were X-ray diffraction (XRD), X-ray fluorescence (XRF), Ion Chromatography (IC), Fourier Transform Infrared (FTIR), automated titration and Gas Chromatography-Mass Spectrometry (GC-MS). The products formed from the oxidations were identified. The FTIR and XRD results confirmed that a mixture of (NaSCN)x and amorphous polymeric (SCN)x was formed as an intermediate when NaSCN solutions were oxidised with H2O2. XRD and XRF results confirmed the presence of Na2SO4 as a major product from the H2O2 oxidation of NaSCN solutions. NaSCN in the Stretford process aqueous liquor is oxidised to form sodium sulphate, burkeite and mascagnite when using air and H2O2. When HCl and NaOH were used at pH values of 7.0 and 9.0 respectively, the concentration of NaSCN remained predominantly unaffected. The oxidation of NaSCN using increased concentrations of Na2[ADA] and SAV did not impact the NaSCN concentration. Small amounts of H2O2 are liberated from Na2[ADA] and SAV during the re-oxidation of Na2[ADA] using SAV. These liberated H2O2 concentrations are not sufficient to facilitate NaSCN oxidation using increased concentrations of these catalysts (i.e. Na2[ADA] and SAV). During oxidation of Stretford process aqueous liquors using air and H2O2, the concentration of NaSCN steadily decreased while the concentration of Na2SO4 increased. During the oxidation of Stretford process aqueous liquor using H2O2, HCN was found to be the main gas liberated. The formation of Na2S2O3 during the oxidation of Stretford process aqueous liquors using H2O2 was not found to be significant since the conditions of increased pH and temperature that facilitate its formation were not met.