Phthalic anhydride, benzoic acid and maleic acid were selectively removed from solid petrochemical wastes (called molten phthalic anhydride) using supercritical carbon dioxide extraction technique. A central composite design was used to optimize the effective extraction variables for the semicontinuous extraction of phthalic anhydride, benzoic acid and maleic acid from their mixture with benzoic acid, maleic acid, phthalic anhydride, aldehydes, phthaldehyde, toluic acid, and some minor impurities. Variables such as temperature (35-95 degrees C), pressure (100-300 bar), static time (0-20 min), dynamic time (10-90 min), and flow rate (0.2-1 mL/min) were used. The chemical composition of extracted samples was analyzed by a gas chromatograph-flame ionization detector column. The results indicate that supercritical fluid extraction is a viable technique for extraction from solid waste. (C) 2012 Elsevier B.V. All rights reserved.