Selective removal of dissolved silicon and aluminium ions from gas liquor by hydrometallurgical methods

Abstract

Sasol raw gas liquor emanating from the gasification plant is contaminated by trace amounts of silicon, iron and aluminium which can crystallize or precipitate from gas liquor to form synthetic clay during either: (1) gas liquor transportation from the gasification plant to the phenosolvan plant or (2) during the liquid–liquid extraction process. Phenol is a major high value component of gas liquor which can be selectively recovered by diisopropyl ether (DIPE) from gas liquor during a liquid–liquid extraction process. A colloidal alumina/silica precipitate readily occurs on the heat exchanger plates during the liquid–liquid extraction process. This gelatinous precipitate formation is an operational problem which results in a severe blockage of the heat exchanger plates and eventually needs to be unblocked with hydrofluoric acid, a toxic and corrosive chemical. The precipitate formation can be attributed to the presence of the aluminium, silicon and iron species in the gas liquor. Characterisation studies identified metal ions such as Al 3+, Ca 2+, Fe 3+, Mg 2+, Si 4+ and K + in all liquid samples analysed in this study. Major anions detected in the liquid samples are sulphate, fluoride, chloride, bicarbonate, carbonate and nitrate. Thermodynamic modelling of inorganic compounds in the gas liquor sample indicates that Al(OH) 3, CaCO 3 and Fe(OH) 3 are dominant precipitates between pH 8 and 10 and a temperature of 40 °C. In this study, various purification processes such as: (1) desilication, (2) flocculation, and (3) selective precipitation were evaluated to selectively remove silicon, aluminium and iron species from the phenol-containing gas liquor. In addition a complexing of Al (III) and Fe (III) ions with citric acid was investigated to retain both aluminium and silicon species in the phenol-containing gas liquor during the extraction of phenol. The concentration of phenol present in the gas liquor must not be affected during the removal of aluminium, silicon and iron ions from the gas liquor. The preliminary results obtained indicate that in terms of removing the aluminium and silicon species from the gas liquor, the addition of seed crystals such as alumina, gibbsite and silica to gas liquor coupled with flocculation proved superior to all hydrometallurgical processes evaluated in this study. The availability of this technology to eliminate or substantially reduce silica and alumina fouling would reduce expensive maintenance and down time requirements and also enhance the plant efficiency.