Assay Sampling Protocols for Industrial Metal Scrap Valuation
Assay sampling protocols establish true dry weight and payable metal content, directly governing settlement values, working capital allocations, and lender borrowing base limits.
Multi Acid Digestion
Meaning
Wet chemical sample dissolution represents the preparation method used to dissolve a powder sample in a mixture of strong acids for multi-element analysis. Performing a multi acid digestion involves heating the rock powder with a combination of nitric, hydrochloric, hydrofluoric, and perchloric acids. This aggressive acid mix breaks down the silicate minerals to put the metals into solution.
Chemical Dissolution
The sample is heated in a Teflon beaker with the acid mixture on a hot plate. This process must be conducted in specialised fume cupboards designed to handle perchloric acid. Hydrofluoric acid is necessary to attack the silica matrix of the rock.
This releases the metals that were locked in the silicate minerals. The mixture is heated until it is nearly dry, and the residue is then redissolved in dilute hydrochloric acid to prepare the solution for the spectrometer.
Analytical Scope
This dissolution method is often referred to as a near-total digestion. It dissolves almost all the minerals present in the rock, making it suitable for base metal analysis. Some highly resistant minerals like zircon or chromite may not dissolve completely.
This limitation requires a fusion prep if those specific minerals are of interest.
Recovery Evaluation
Comparing the assay results of this digestion to a fusion assay reveals the recovery rate. This comparison is used to assess if any target metals were left in the residue. High recovery rates confirm that the acid mixture was sufficient.