Abstract
The mechanism for formation of Cr6+ compounds which were produced by high temperature oxidation reaction of Cr-Ca and Cr(OH)3 systems was investigated. The following conclusions were drawn: (1) When Cr(OH)3-Ca(OH)2 system was burned, Cr6+ compounds were produced by two different mechanisms, which were distinguished by the boundary temperature of 300°C. At temperatures above 300°C, Cr6+ compounds were produced by the mechanism proposed by Nishino et al.; but at temperatures below 300°C, Cr6+ compounds were produced by the following reaction: 4Cr(OH)3+4Ca(OH)3+3O2→4CaCrO4+10H2O (2) CrO3 was prouced by burning of Cr(OH)3 including oxidable anions such as SO42+; and the amount of CrO3 compounds produced was maximum at 250°C. Therefore, it was impossible to prevent the production of Cr6+ compounds in the low temperature burning of real sludge. (3) When Cr-Ca system was burned, the amount of Cr6+ compounds produced became constant at a time at a temperature of 440-600°C; but, at temperatures above 700°C, the surface began to fuse and the reaction proceeded to the interior of particles so that the amount of Cr6+ compounds produced increased with the rise of temperature.
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