In-field carbon analysis to determine preg-robbing in gold

Gold ores containing organic carbon must be identified before the ore is processed in order to avoid significant loss in gold recovery and revenue due to the effects of preg-robbing.

The SciAps Z-300 handheld analyzer utilizes an optical technique called Laser Induced Breakdown Spectroscopy, or LIBS, that is very sensitive to the low atomic number elements such as C yet able to measure virtually the entire periodic table in a matter of seconds. Using the capability to measure C and to differentiate between C present as organic and inorganic (carbonate) offers exciting opportunities in better understanding gold ore and optimized recovery of gold through management of the processing of the ore. Optimizing the recovery of gold in mining operation is a mission-critical objective. The ore being delivered to processing plants vary in composition and characteristics throughout the mining process on both short and long timescales. Predictively understanding potential impacts on recovery requires quality information delivered in a timely fashion.

The extraction of Au from gold ores is commonly achieved by cyanide leaching followed by recovery of the Au from the leachate by fixing the Au onto activated carbon. Gold ore can be roughly divided into three classes: free-milling, refractory or double refractory. Free milling ores deliver good recoveries with minimal processing, while the refractory classes require additional treatment to optimize recoveries due to sulphides and carbonaceous matter impeding the recovery of Au from the ore. Carbonaceous matter in gold ores can both lock up a proportion of the Au in the ore by inhibiting leaching and by absorbing already leached Au from the pregnant leachate solution by a process known as preg-robbing. Preg-robbing may be defined as the phenomenon whereby the gold cyanide complex (Au(CN)2) is removed from solution by the constituents of the ore including organic carbon and other minerals. Characterizing the preg-robbing potential of ore is important, and the differentiation of organic carbon from carbon present in carbonate minerals is one of the key aspects requiring measurement to achieve characterization.

Conventional methods of measuring carbon content and total organic carbon require expensive laboratory equipment and take significant periods of time to prepare and analyze samples. LIBS has the capability to analyze and measure total C content of ore and also differentiate between C present as organic carbon and C incorporated into carbonate minerals. For best results the sample must be milled, homogenised and pressed into a pellet. But following that, the actual analysis time is a matter of seconds. An example of the data achievable is shown below. This plot shows the estimation of TOC using the industry standard LECO analysis technique vs the SciAps Z-300.