Issues Relevant to Recycling of Stainless-Steel Pickling Sludge

Stainless-steel pickling sludge is a hazardous solid waste. Harmless treatment and recycling of this sludge are major research topics. Based on the sludge formation process, two typical types of stainless-steel pickling sludge were chosen and their physiochemical properties determined by chemical analysis, differential thermal analysis/thermogravimetry, and x-ray diffraction analysis. Possible approaches to recycle the sludge were suggested, and relevant problems are discussed. Based on its high water and sulfur contents, as well as the valuable metals that could be recovered from sludge, drying, roasting, and reduction tests were carried out. It was found that Fe and Cr were mainly present in oxides, Ca in Ca(OH)₂ and CaF₂, and sulfur in CaSO₄. Natural drying was a very slow process, but forced drying at temperatures above 423 K was effective. Sulfur could be partly removed from sludge through roasting and reduction at high temperatures, with most of the sulfur being transferred to gas and slag in the reduction process; however, this treatment did not allow sludge with high sulfur content to be used as a suitable raw material in terms of sulfur level requirements. It is suggested that prepickling and pickling waste liquors should be treated to reduce output of high-sulfur sludge to promote sludge recycling. Optimized reduction conditions were identified in terms of additional carbon content (1.3 times as suggested by theoretical data) and temperature (above 1673 K) for high metal recovery.