SEQUESTRATION OF ACID-GENERATING MILL CONCENTRATES: CRIPPLE CREEK

Cripple Creek is an epithermal gold deposit in Colorado, USA. The processing flow at Cripple Creek & Victor Gold Mine (CC&V) features both milling and heap leaching circuits. Floatation tailings and spend mill concentrates are mixed and placed on the leach pad for additional gold extraction. The presence of finely ground sulfide concentrates raises questions about long-term chemistry of the leach pad circuit. The purpose of this project was to investigate possible routes to stabilize the concentrate and minimize acid generation from the oxidation of sulfide minerals.

The mill concentrates were mixed with Portland cement to create ~1” diameter experimental pellets at a range of cement: mill concentrate ratios from 20 to 200 lbs/ton. The pellets were leached at pH 10.5 in NaOH solution in a bench-scale heap leach system, in a matrix of either acrylic balls or crushed ore from Cripple Creek. Agglomerate degradation was examined over 8 week trials using Field Emission Scanning Electron Microscopy (FE-SEM), and an automated mineralogy system to observe the difference between the initial samples and leached samples. Major cations were analyzed in the leachate at 2 week intervals during the trials.

After 8 weeks of leach testing, no significant sulfide oxidation was observed in any of the trials, and most minerals from the mill concentrate did not undergo significant alteration. However, the pellets with less cement physically degraded where cement material had been leached away. In samples with low cement ratios, fracturing occurred within 2 days and the mineralogical analyses indicated that cement had mostly been leached away. At higher cement concentrations, even though the samples did not appear macroscopically damaged, a cement-reduced rim was visible using FE-SEM. At all cement concentrations, the leachate cation data indicated that Ca, Ba, and Sr concentrations decreased in the pellets over time, consistent with cement dissolution. Na and Mg concentrations increased over time indicating more exposure of the concentrate material as the cement degraded. Fe and Cu concentrations peaked at 4 weeks, consistent with exposure of mill concentrate. Although higher cement content produces a physically stronger agglomerate, degradation still occurs, which may lead to sulfide oxidation once the agglomerates break apart.