Factors in gravitational settling of chromic hydroxide in aqueous media

The nature of alkali used for the preparation of chromic hydroxide has been found to have profound influence on its settling behaviour. The Settled vo lume of chromic hydroxide produced using NaOH, Na2CO3, Ca(OH)(2) and MgO ha s been found to vary from 72 to 80/0. Similar variations have been observed in their particle size distribution. The mode of addition of alkali has be en found to influence the settled volume of chromic hydroxide, with slow ad dition 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 hyd roxide using MgO as the precipitating agent. Precipitation of chromic hydro xide using NaOH, Na2CO3 and MgO has been found to leave 3.8, 3.6 and 1.4 pp m respectively of Cr(III) in the supernatant. Calcium hydroxide on the othe r band was found to leave only 0.08 ppm of Cr(III) in the supernatant. Scan ning electron micrographs of the dried chromic hydroxide show that the morp hology of dry chromic hydroxide is influenced by the alkali used to produce the chromic hydroxide.