Assaying – analysis of ore samples

assay – noun
1. the analysis of an ore, etc. to find out the nature and proportion of the ingredients.
2. the report of such analysis. Sampling of the solution during processing.

How are ore and waste defined?

Samples of rock from within the pit or from underground are sent to the SGS laboratory in Waihi to determine gold content.

The data generated from assaying is used to map out which areas are suitable to process as ore and which areas are waste material to be used to build the waste rock embankment or to be used as backfill underground.

In process testing - double checking

Throughout the extraction process at the Waihi Gold mill, samples of solution, solids and activated carbon are removed for analysis to determine the efficiency of the process. The solids and the carbon samples are analysed by the fire assay technique for gold and aqua regia for silver. The solutions are aspirated (turned into a fine mist) into an AA Spectrophotometer.

Fire assay

Fire assay involves adding a ground portion of the solid (or carbon) to a crucible containing a flux. Flux is used to lower the melting point of the samples to ensure the entire sample becomes liquid during firing. The flux contains borax, soda ash, silica flour, litharge (lead oxide) and silver nitrate in various quantities. In the firing stage everything turns liquid and the lead oxide forms into small globules of metallic lead. The globules fall through the liquid and form an amalgam with any other metals it comes into contact with, including gold and silver. This all collects in the bottom of the crucible. After about an hour at 1,000 degrees Celsius the contents of the crucible are poured into a conical mould to cool. Again the lead, being more dense than other material, quickly settles to the tip of the conical mould and solidifies along with the glass-like rock remains.

When cooled, the lead is separated from the glass and then cupelled. Cupellation involves placing the lead ‘button’ into a pre-heated cupel at 1,000 degrees C. A cupel is a small cup made of magnesium sulphate which is a material that is capable of absorbing the lead. At this temperature the metals liquefy and lead is absorbed into the cupel. Gold and silver having a higher surface tension than lead, are not absorbed and remain as a small ‘prill’ in the bottom of the cupel. The cupel and prill are removed from the furnace and allowed to cool. The prill is then dissolved in aqua regia. The resulting solution is aspirated into an AA Spectrophotometer to determine the gold content.

Aqua regia

Silver in the solids and carbons is determined by dissolving the sample in aqua regia, a mixture of hydrochloric and nitric acids. The sample has specific volumes of each acid added, then is placed on a heating block to digest. When completed the resulting solution is diluted and then aspirated into an AA Spectrophotometer.

References and acknowledgements
The information on this page was kindly supplied by SGS NZ Ltd.