DETERMINATION OF HUMIC-TYPE SUBSTANCES IN BAYER PROCESS LIQUORS BY A SPECTROPHOTOMETRIC METHOD
Tolentino, E.N. Boom, E.A.
The organics present in Bayer process liquors are diverse ranging from low molecular weight compounds such as acetate to humate with molecular weights above 100,000. Humate, with similar characteristics to flocculants, has several, mainly adverse, impacts on the process. It causes the liquor to foam, modifies oxalate crystallisation and lime behaviour through changes in crystal morphology, increases liquor viscosity and contributes to occluded soda in product hydrate. Control of the humate concentration is a necessity and must therefore be measured in refinery liquor to provide monitoring and removal steps.
A method based on UV-Vis Absorption Spectrophotometry has been developed to measure humic-type substances (H-TS) in refinery liquors. The quantitative method evolved from the results of gravimetric analysis and solid-phase extraction (SPE) techniques of various digest liquors. The components of the humic-type substances in refinery liquors were fractionated into two based on their solubility at varying pH’s. The humic acids (HA) are precipitated at pH < 2 while the fulvic acids (FA) remain in solution. The extinction coefficients for refinery HA and FA were obtained and used to choose the appropriate standards (from International Humic Substances Society) to create the respective calibration curves. Control bauxite was digested and the extraction coupled with UV-Vis method gave a SD of ± 0.002g/L HA (N=9) and ± 0.01g/L FA (N=5). Comparison of the UV-Vis method and Elementar TOC (total organic carbon) Analyser by linear regression gave a correlation coefficient r=0.9455 (N= 81), slope of 1.05 ± 0.08 and an intercept of 0.031 ± 0.048 at 95% CL.
The UV-Vis method has been successfully applied to generate carbon profiles of bauxite samples as well as carbon balance in refinery humate surveys. Analysis of organic content is important to identify mining areas which will significantly contribute to organic impurities in the refinery. Refinery humate surveys established that humates are removed from process liquor with several streams when a solid phase is formed.