Automated environmental mineralogy; the use of liberation analysis in humidity cell testwork

Abstract The prediction of acid rock drainage and metal leaching (ARDML) from mine sites requires a detailed understanding of the site specific rates and mechanisms of weathering. This is typically obtained through geochemical and mineralogical characterisation testwork that is undertaken on representative materials. One of the key components of this characterisation work is kinetic testing which generates critical data on the rate at which minerals within waste materials weather in the presence of moisture and oxygen. One of the most commonly applied kinetic tests is the humidity cell test (HCT) in which granulated material is exposed to cyclical dry and humid air and periodic leaching. Knowing when to terminate a HCT is one of the key challenges encountered whilst undertaking a kinetic program. It is essential to balance the economics of continuing tests for extended durations with the danger of missing critical data if the test is terminated too early. This is particularly true where there are discrepancies in prediction between basic diagnostic (static) tests and kinetic tests. This may occur for example when corresponding static geochemical tests have predicted a significant potential for ARDML but the HCT has continued for well over a year without showing any acid-generation. In these cases, the initial period of HCT data can be used to estimate the time to onset of acidic conditions, which may be many years. However, this estimate does not explain why such a discrepancy exists. Likewise, static testing, bulk and basic mineralogical characterisation do not provide sufficient information to explain such discrepancies which are commonly related to such factors as sulfide and carbonate mineral liberation, mineral associations and grain size distributions. This paper explores a novel mineralogical response to the uncertainty by applying a metallurgical mineralogy technique of mineral liberation analysis on sulfide minerals from pre leach HCT material. The results show that quantitative sulfide mineral liberation analysis can be used to understand HCT leachate quality. This information could be used to increase confidence in the termination of cells which contain no significant sulfide encapsulation. Altogether, quantitative liberation analysis opens up the possibility of improving the overall efficiency and data quality of a geochemical kinetic program as opposed to the historic method of running uncertain HCT samples indefinitely to observe the long term behaviour.