BROSS PROCESS - BIOTREATMENT OF ALKALINE SLAGS PRODUCED DURING LEAD RECYCLING

The production of lead by reprocessing of spent car batteries in rotar y furnaces produces a highly alkaline slag that is currently landfille d at registered sites. When exposed to moist air the solid slag slowly exfoliates; in the presence of excess water (e.g. during atmospheric precipitation) rapid disintegration occurs and some of the slag compon ents dissolve to produce a highly alkaline solution. Although the slag passes a standard heavy-metal leaching test (DIN 38414), disposal to landfill may result in contamination of the underlying groundwater. Si nce few locations in Europe have landfill sites where this problem can be contained, companies that produce these slags are investigating pr ocesses that have the potential of avoiding slag disposal. One such pr ocess is based on bio-remediation of the sulphidic slag (BROSS). The p rocess operates at 45 degrees C in a neutral (pH 7), aerobic aqueous e nvironment in which the sulphide moiety present in the slag is convert ed to soluble sulphate. A two-stage version has been developed that in volves quantitative chemical pre-oxidation of the total sulphide prese nt in the slag to thiosulphate followed by complete bio-oxidation of t hiosulphate to sulphate. The two-stage process, which allows ready sep aration of any metallic lead present in the slag, has a high sulphate production rate and low acid and nutrient consumptions. Microorganism growth is recognized as the rate-limiting step in the process, requiri ng a minimum liquid residence time in the bioreactor of about 7 h. Alt hough large quantities of oxygen are needed for sulphide oxidation, it has been demonstrated that sufficient gas mass transfer can be attain ed in an air-lift reactor. The process produces an aqueous stream that contains sodium sulphate and only trace levels of toxic heavy metals in concentrations that would meet current EC environmental criteria. A benign, toxic heavy metal-containing sludge that could be recycled ba ck to the smelter or disposed of by landfilling is also produced. Eval uation of the BROSS process indicates that its introduction should red uce environmental impact while increasing both lead recoveries and lea d smelting capacity.