Dissolution of aluminium from metakaolin with oxalic, citric and lactic acids

This study examines the leaching of aluminium from calcined kaolin (metakaolin) with citric, oxalic and lactic acid and binary combinations of these organic acids. The investigated parameters were the pulp density, acid concentration, pH, agitation speed, temperature and contact time. The kinetics of aluminium dissolution from metakaolin in binary organic acid mixtures were determined and scanning electron microscopy examination and energy-dispersive spectrometry mapping of Si and Al of untreated kaolin and residual solids were also carried out. Aluminium dissolution increased with temperature, time, acid concentration, pulp density and acidity. At maximum dissolution, citric-oxalic (1:4 w/w) and lactic-oxalic (1:4 w/w) mixtures dissolved 77% and 78% aluminium, respectively, from metakaolin in 11 h. The activation energy ranged from 57.8 to 74.6 kJ/mot. The most effective parameter in the dissolution of aluminium was the temperature, indicating that the reaction was not diffusion-controlled. It was concluded on the basis of the activation energy values and the sensitivity of the reaction to temperature that the dissolution was under chemical-reaction control.