Formation of Oxalic Acid by the Oxidation of Yallour Coal with Oxygen in Concentrated Sodium Hydroxide Solution

Abstract

Oxidation of Yallourn coal, consisting 66.8% carbon on dryash-free basis, was carried out using a conventional 300 cm³-autoclave under reaction conditions varied in the following ranges reaction temperature, 225-300°C (Fig.1); concentration of NaOH, 15-, 35 mol/kg-H₂O(Fig.2); O₂S pressure, 10-50 kg/cm2 (Figs.3 and 4); revolution of magnetically driven stirrer, 400-1200 rpm (Figs.5 and 6); and initial coal/NaOH weight ratio, 0.4 (Fig.7). The reaction afforded CO₂S, oxalic acid, benzenepolycarboxylic acids (BPCA), and water-insoluble acids. Maximal oxalic acid yield of 88.5 wt%, corresponding to 42.3% on the basis of carbon recovery, was attained when the oxidation was undertaken at 250°C for 2 h under the following set of reaction conditions: concentration of NaOH, 25 mol/kg-H2O; O₂S pressure, 50 kg/cm2, revolution of stirrer, 1200 rpm; and coal/NaOH ratio, 0.2 (Fig.3). It was revealed, throughout the course of the oxidation, that the fraction of coal converted to oxalic acid was nearly equal to that converted to CO₂S (Fig.10). It was also =revealed that relative abundance of BPCA's decreased in the order: tri->tetra->di- pentacarboxylic acids (Table 4). The oxidation took place only slowly at 225°C. Upon raising the temperature above 250°C, however, the oxidation to produce oxalic acid and CO₂S became significantly faster thereby emerging globular suspensions composed of coal, water-insoluble acids, Na₂C₂O₄ and Na₂CO₃. To explain this oxidation at high temperature a reaction model, in which the diffusion of dissolved O₂S in the globular body plays rate-determining role, is proposed. The model is compatible with the effects of O₂S pressure as well as mechanical agitation (Fig.8). Both products yield and composition of BPCA were compared with those previously reported on nitrate oxidation of Yallourn coal (Fig.11, Table 5).