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I am looking for any suggestions/input on methods to more completely digest a nickel laterite sample. Currently, my lab is using an aqua regia digestion using 5 g of pulverized sample (-200 mesh) with 25 mL of aqua regia on a hotplate. We have been having difficulties in mass balancing the nickel amounts in our head assay size fractions relative to the nickel measured in our composite, 'bulk' sample. Specifically, we note that the calculated head value based on our size fraction analyses are approximately 70% of the value measured in our composite sample. After confirming the accuracy of our analytical numbers from flame AAS, we have begun to suspect that this discrepancy may be due to an incomplete digestion of our sample material by aqua regia. Performing a second aqua regia digest on the residual material from the first digestion extracts more nickel, but still not enough to bring the mass accounting into balance.
Any suggestions or insight would be most welcome! I will say that we are not currently set up to use HF or perchloric acid in our digestions, so that's why we have not opted for a more complete '4-acid' digestion.