BACKGROUND: To explore the extraction properties of aromatic acids produced in the oxidation of coal, the extraction equilibrium of phthalic acid (as a typical product) was studied using three solvents, 1-octanol, 50% tributylphosphate (TBP)/kerosene, and 10% trialkylphosphine oxide (TRPO)/kerosene, and the feasibility of separating phthalic, [1,1'-biphenyl]-2,2'-dicarboxylic and trimellitic acids is discussed. RESULTS: Phthalic acid extraction followed the sequence: 10% TRPO > 50% TBP > 1-octanol, with recoveries from the TRPO and TBP systems being much larger than that for 1-octanol. The stoichiometry of formation of the complexes of TBP and TRPO with phthalic acid was 1 : 1. The apparent extraction equilibrium constant for TRPO is much larger than that for TBP. The distribution coefficient of [1,1'-biphenyl]-2,2'-dicarboxylic acid is much larger than that of the other two acids using 1-octanol as the extractant and this acid could be removed by 1-octanol from a mixture of the three acids. The extraction equilibrium correlations obtained for the individual acid component systems can be used to predict that of the acid mixture. CONCLUSION: The extent of phthalic acid extraction by the three solvents investigated is as follows: 10% TRPO > 50% TBP > 1-octanol. The stoichiometry of the complex formation of TBP or TRPO and phthalic acid is 1 : 1, and the apparent extraction equilibrium constant for TRPO is much larger than that for TBP. 1-octanol shows a good extractive selectivity for [1,1'-biphenyl]-2,2'-dicarboxylic acid as compared with phthalic and trimellitic acids. (C) 2011 Society of Chemical Industry