Treatment of Sludge containing Chromium and Heavy Metals from Surface Finishing Recesses for Making it Harmless

2004 Fiscal Year Final Research Report Summary

• Project/Area Number: 15560711
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Recycling engineering
• Research Institution: Musashi Institute of Technology
• Principal Investigator: SHIMPO Ryokichi Musashi Institute of Technology, Faculty of Engineering, Associate Professor, 工学部, 助教授 (40154398)
• Co-Investigator(Kenkyū-buntansha): HOSHINO Shigeo Musashi Institute of Technology, Faculty of Engineering, Professor, 工学部, 教授 (00061515)
• Project Period (FY): 2003 – 2004
• Keywords: Sludge / Hexavalent Chromium / Treatment of Waste / Recycling

Research Abstract

Sludge recovered from the waste water treatment of the surface finishing industries contains not only much amount of water but also heavy metals such as chromium. This study has been conducted in order to reducing the volume and weight of this industrial waste and to recycle the sludge as magnetic materials by the calcining operation. It has been pointed out so far that the chromium in the sludge can be converted to the toxic hexavalent state during the calcination according to the heating conditions.
In this study, first of all, it became clear that the conversion ratio of hexavalent chromium has a peak at around 400℃ under which the water in the calcined sludge is not sufficiently eliminated and the crystal structure is amorphous like, and above which the sludge becomes crystallized and the amount of the hexavalent chromium decreases. The similar result has been obtained in the calcination of pure chromium hydroxide reagent where the conversion ratio of hexavalent chromium gas the pea k at 300℃. The content of hexavalent chromium became sufficiently low comparing with the environmental standard in Japan when the sludge examined was calcined at 900℃.
In the next, in order to examine the effects of the co-existing elements in sludge on the conversion ratio of hexavalent chromium, artificial sludge was made by mixing hydroxide reagents of nickel, copper, zinc, and iron with chromium hydroxide, heated at 200℃ to 700℃ in atmosphere, and examined. It has turned out that when one of those hydroxides was mixed with chromium hydroxide and calcined, no significant effect of the co-existing elements on the conversion ratio of hexavalent chromium was found except for the copper hydroxide mixing test where hexavalent chromium content became lower to some extent. However, when the those hydroxides were mixed with chromium hydroxide in the similar molar ratio to the actual sludge examined in this study, the conversion ratio of hexavalent chromium became very low as same as the result of the actual sludge calcinations.