November 2013Analytical Instrumentation & Environmental Monitoring
The fire assay method is an accepted standard for determining the amount of precious metals, such as gold, silver and platinum, in a sample of ore. In mine assay laboratories therefore, reliable instruments are essential to allow accurate production forecasts by management.
Mettler Toledo’s range of Excellence balances offers accuracy, robustness and long-term reliability in the harsh weighing environments of precious metal mining companies and refineries. Specially designed accessories, and LabX One Click weighing solutions, make weighing secure and sample handling easier whilst providing fast results.
Weighing plays a crucial role in the fire assay process since errors in the measurement of the ore sample can have huge financial implications. To ensure accuracy, many assay laboratories use a high resolution analytical balance to determine the exact weight of the original sample, and a microbalance to weigh the final gold bead. With several samples being analysed at a time, the process requires much care to ensure that samples are tracked correctly.
Samples of ore from the field are prepared for analysis by crushing or grinding to obtain a uniform particle size. This is split into samples of approximately 30 g to be representative of the original larger sample. The samples are placed in a crucible and mixed with lead oxide and flux. The crucibles are then placed in a furnace at around 1000°C for a predetermined time, after which the molten contents are poured into a mould and allowed to cool. The lead in the sample ‘collects’ the gold which sinks to the bottom with the remaining contents forming a glassy slag on top.
The resulting lead ‘button’ is detached from the glassy slag and placed in a cupel. This small crucible is made of bone ash or magnesium oxide and can absorb up to twice its weight in lead. The cupels are placed back in the furnace. As the button melts, the lead is absorbed into the cupel to leave a small bead of gold.
The final weighing of this tiny bead of gold is critical to determine the gold content of the original sample with results typically in the order of 0,1 mg. Once the final gold bead is weighed, the percentage of gold content is calculated automatically and this determines if the ore is economically viable.
The fire assay method is a differential weighing procedure which can be customised and stored in the LabX software of the balance according to individual process requirements. Users can access the method directly with the LabX One Click shortcut on the balance touch screen and are guided through the process step-by-step.
The process allows the user to enter the number of samples and crucible IDs for easy sample identification. Results are saved automatically and confirming each step on the balance ensures operating procedures are followed precisely.
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