MHYSTI

Development of a Management system for HYdroxyde sludge produced by the Surface Treatment Industry

The surface treatment industry generates large quantities of metallic hydroxide sludge from the purification of waste water. In order to comply with environmental constraints, this sludge must be eliminated without causing adverse effects to human health or to the environment.

In 1991 the European Commission modified the 1976 general directive on waste, by adopting the principle of prohibiting, wherever possible, storage or dumping, and prioritising recycling, re-use and recovery.

The present research project now terminated was part of this initiative. It proposed a global approach to hydroxide sludge management that could be adapted to surface treatment workshops of all sizes, by developing technically viable and industrially cost-effective methods for drying, inerting and recycling.

The quantity of metallic hydroxides produced by the purification of waste water from surface treatment plants within the European Community is reckoned to be at least 1,000,000 metric tons.

Currently, these hydroxides are stored as sludge in landfills offering maximum guarantees for the protection of ground water. However, these landfills are too few or are already saturated, and any new request to open such sites is subject to extremely stringent conditions.

The main project goals were to reduce by at least 60% the volume of non-recyclable sludge dumped, and to make this sludge hydrophobic to prevent subsequent leaching.

Because a large proportion of sludge (some 20%) cannot be treated by the existing means and does not have enough intrinsic value to deserve recovering the metals, it can be inerted by incorporation into materials that are themselves inert. The last fraction (some 10%) contains metals such as nickel, chrome, copper or tin, with value superior to recovery cost, and will therefore be reprocessed.

The drying of sludge, especially when followed by chemical fixing to prevent the sludge from subsequent leaching, is a technique that is immediately available for industrial application. In fact, it is the only short term solution to address the environmental concerns expressed by the European Community nations.