GeoBiotics, LLC has developed a proprietary heap bioleaching technology for the processing of sulphide base metal and gold concentrates. In this process, known as GEOCOAT((R)), thickened flotation concentrate is contacted during heap stacking with gravel-sized support rock, forming a thin adherent concentrate coating on the support rock particles. The stacked heap has an open structure and is highly permeable to the flows of solution and air. Acid solution, which contains acidophilic bacteria, is circulated through the heap to biooxidise and leach the contained metals. The heat produced by the exothermic oxidation reactions causes the internal temperature of the heap to rise. Heat is transferred to the percolating solution and to the air blown up through the heap. Rates of solution application and aeration can be varied to control the heap temperature and maintain it within the optimum range for bacterial activity. GeoBiotics and Kumba Resources have investigated the feasibility of applying the GEOCOAT((R)) process to the leaching and recovery of zinc from a low-grade sphalerite concentrate produced from accumulated flotation tailings at Kumba's Rosh Pinah zinc mine in Namibia. The concentrate contained 18% Zn, 4.5% Fe, 12.8% S2-, 0.2% Cu, and the D-80 particle size was 53 mu m. To confirm that a GEOCOAT((R)) heap to bioleach this concentrate would operate autothermally, a large engineering column test was conducted. The insulated stainless steel column, 6.5 m high and 1.2 m in diameter, was equipped with sample ports, airflow measurement, oxygen analysis and thermocouples to monitor temperatures during the test. The large column was designed to simulate a unit cell from a commercial heap and similar operating parameters where applied to its operation. The column was filled to a 6 m height with concentrate coated support rock which was produced using a batch coating method. After acid stabilization to reduce the solution pH to 1.5-1.8, the column was inoculated with an adapted mixed mesophilic bacterial culture. Through control of the exothermic biooxidation reactions and the solution and aeration rates, the temperature in the column was increased from ambient to a maximum of 49 degrees C. The solution application rate and the aeration rate adjusted to control the temperature and prevent it from rising beyond the maximum tolerable by mesophiles. The column was operated for a total of 90 days. Final zinc dissolution after 90 days was 91% with a corresponding sulphide sulphur oxidation of 89%. Concentrations of in excess of 90 g/l Zn in the column effluent did not appear to inhibit the microbial oxidation. The comparison of the zinc dissolution kinetics between the large diameter autothermal column and that exhibited by previous heated small diameter columns (6.5 m high x 0.15 m diameter) showed excellent correlation. The small diameter column test produced a zinc dissolution of 92% in 63 days compared to 91% after 60 days in the large column. The operation of the large diameter column was successful in demonstrating that the GEOCOAT((R)) process can be operated autothermally at mesophilic temperatures treating a low-grade sphalerite concentrate. Zinc dissolution in excess of 90% can be achieved in a leach time of 60 days, while results of small diameter column testing indicate zinc dissolutions in excess of 95% in 60-100 days. (c) 2004 Elsevier Ltd. All rights reserved.