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

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).