Abstract

The nature of alkali used for the preparation of chromic hydroxide has been found to have profound influence on its settling behaviour. The settled volume of chromic hydroxide produced using NaOH, Na,C0 3,Ca(OH), and MgO has been found to vary from 72 to 8%. Similar variations have been observed in their particle size distribution. The mode of addition of alkali has been found to influence the settled volume of chromic hydroxide, with slow addition of alkali producing the most compact chromic hydroxide sludge. A pH of 8.5 9.0 has been found to be ideal for effective settling of chromic hydroxide using MgO as the precipitating agent. Precipitation of chromic hydroxide using NaOH, Na,C0 3and MgO has been found to leave 3.8, 3.6 and 1.4 ppm respectively of Cr(III) in the supernatant. Calcium hydroxide on the other hand was found to leave only 0.08 ppm of Cr(JII) in the supernatant. Scanning electron micrographs of the dried chromic hydroxide show that the morphology of dry chromic hydroxide is influenced by the alkali used to produce the chrornic hydroxide.