The Removal Of Thiocyanate Ions And Phenols From Toxic Aqueous Wastes From Coal Gasification Processes

The operation of the British Gas Slagging Gasifier results in the production of aqueous waste streams, in which two of the least acceptable contaminants are thiocyanate ions and phenolic compounds. Ion pair and ion exchange extraction processes were investigated for the removal of these toxic components. In excess of 95% of thiocyanate ions extraction would be achieved if tetrapentylphosphonium chloride or hydrogensulphate were to be employed as the quaternary compounds and octan-2-ol or dichloromethane as the organic diluent in ion pair extraction systems. These quaternary compounds are relatively expensive and not available in tonnage quantities. An empirical relationship was found for the rate of mass transfer indicating second order kinetics within the individual run, but where the apparent second order rate constant depends on the reciprocal of the square root of the thiocyanate ions and quaternary concentrations for both extraction systems. Ion exchange systems were preferred because of their low water solubility and availability in large quantities. It was shown that, using Alamine 336/HCl in Escaid 100, less than three extraction stages are required to reduce thiocyanate ions to less than 5 mg dm from both simulated and actual effluent containing 1000 mg dm at one to one phase ratio. Extraction values in excess of 90% were also obtained with phenol when employing tetrabutylammonium hydrogensulphate and octan-2-o0l or Alamine 336/HC1l in iso-octyl alcohol. Thus, simultaneous extraction of thiocyanate ions and phenol from the Westfield plant effluent was attempted with Alamine 336/HC1l in iso-octyl alcohol. Equilibrium diagrams showed that less than four theoretical stages are needed to reduce these toxic compounds to below 5 mg dm at ome to one phase ratio when employing excess of Alamine 336/HCl. Upto 70% recovery was obtained when the loaded organic phase was stripped with concentrated chloride solutions. The dispesal of the stripped solution and the back extraction of phenol requires further work. Overall, an extraction system can be proposed to treat the toxic effluent produced in the Slagging Gasifier based on an ion-exchange route.