Acid Method for the Digestion of Gold Ore Samples

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Over the previous decades, applications of acid digestions for the determination of gold have reported low or variable gold recoveries from geological samples on a consistent basis.^1,2 Poor gold recoveries have been associated with the ‘nugget effect’, believed to be caused by an unequal distribution of coarse gold particles.

Acid Method for the Digestion of Gold Ore Samples

Solutions to this problem typically propose the use of larger sample sizes, typically 30 g or greater.³ Furthermore, when using acid digestions, in the absence of hydrofluoric acid, low recoveries have been associated with the encapsulation of gold in the siliceous rock.^1,2,4,5 Here, it is presumed that gold is secreted within a refractory matrix, making it inaccessible to solvent solutions.

Conducting their 1989 review of bias in the determination of gold in mineral phases, Hall et al. concluded that a general low bias, varying between 24 and 42%, was discovered over a variety of determination methods, where wet techniques (aqua regia (AR) with minor variations) were employed.² Hall et al., associated these low values to a potential lack of mechanical contact between the sample and the solvent and/or secretion of a portion of the gold in the intractable gangue.

They observed that the introduction of hydrofluoric acid to samples culminated in recoveries similar to those from instrumental neutron activation analysis (INAA).² In spite of the complications outlined above, due to cost-effectiveness and simple implementation, acid digestion has remained a desired but elusive method for the proper determination of gold. Although fire assay (FA) has been the preferred method for the determination of gold and other precious metals for some time, this method is time-consuming and costly.

Moreover, FA tends to have relatively high detection limits due to the effects of impurities connected with the required fluxes.^5,6 Recently, a paper by Wang et al., did indeed report that aqua regia delivered low recoveries for gold.⁷ However, when the ratio of HNO₃:HCl was altered from 1:3 to 3:1 (reverse aqua regia or Lefort aqua regia), a great improvement in the recovery of gold was observed. This development can be associated with the release of encapsulated gold, particularly from within sulphidic mineral phases. Celep et al., have demonstrated that gold can be encased within a variety of matrices⁸ and nitric acid has proven to break down more of these matrices than hydrochloric acid.^8,9 Therefore, a considerable presence of excess nitric acid ensures the deterioration of some of the gold-bearing minerals.

This also establishes the fact that the residual concentration of nitric acid is adequate enough to oxidize the gold that has been released. However, in some samples following acid digestion using the 3:1 ratio consisting of high levels of sulfur, beads of elemental sulfur can build-up in which gold may be secreted. Another feature that impedes the recovery of gold can be adsorption, either onto the material of the digestion vessel or onto the system that pumps the solution into the spectrometer (wash-in, wash-out effect).

This effect is also troublesome at the end of the sample introduction process when a gradual release of adsorbed gold from the introduction system into the flowing solution causes a considerable memory effect for various instrumental analyses. When investigating this issue, Mohammadnejad et al. proposed through ligand exchange reactions between gold chloride and hydroxyl groups on the silica surface AuCl₄^- can adsorb onto silica surfaces.¹⁰ Subsequently, this can produce decreased gold recoveries, as the gold adsorbs to the surfaces of the glass vessel.^10,11 Moreover, they proposed that Au(III) could be diminished by hydrogen or silicon radicals to produce gold(0) nanoparticles. Chen et al. reviewed these adsorption effects covering all the significant details and proposed a solution that used Lcysteine to eradicate them.¹²

More recently, a paper by Wang and Brindle described how bromide can act as an improved complexing agent when compared to chloride ions. This can be attributed to the use of HBr in place of HCl in modified aqua regia, which recovers gold from geological samples more effectively than conventional aqua regia.¹³ This observation inspired analysis of the use of alternative bromine-containing reagents for gold digestions rather than using bromine itself, which is difficult to handle in laboratory practice.

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References and Further Reading

1. N. Baharun and O. P. Ling, Characterization and gold assaying methods in the assessment of low grade gold ore from Malaysia, Adv. Mater. Res., 2014, 858, 243–247.
2. G. E. M. Hall, J. A. Vaive, J. A. Coope and E. F. Weiland, Bias in the analysis of geological materials for gold using current methods, J. Geochem. Explor., 1989, 34(2), 157–171.
3. S. C. Dominy, A. E. Annels, A. E. Johansen and B. W. Cuffley, General considerations of sampling and assaying in a coarse gold environment, Appl. Earth Sci., 2000, 109(3), 145–167.
4. E. L. Hoffman, J. R. Clark and J. R. Yeager, Gold analysis— Fire assaying and alternative methods, Explor. Min. Geol., 1998, 7(1), 2.
5. M. I. Leybourne and S. Rice, Determination of Gold in Soils and Sediments by Fire Assay or Aqua Regia Digestion: Choosing the Optimal Method, Explore, 2013, (135), 1–10.
6. T. T. Chao and R. F. Sanzolone, Decomposition techniques, J. Geochem. Explor., 1992, 33(1–3), 65–106.
7. Y. Wang, L. A. Baker and I. D. Brindle, Determination of gold and silver in geological samples by focused infrared digestion: A re-investigation of aqua regia digestion, Talanta, 2016, 148, 419–426.
8. O. Celep, I. Alp, H. Deveci and M. Vicil, Characterization of refractory behaviour of complex gold/silver ore by diagnostic leaching, Trans. Nonferrous Met. Soc. China, 2009, 19(3), 707–713.
9. L. Lorenzen and V. Deventer, The identification of refractoriness in gold ores by the selective destruction of minerals, Miner. Eng., 1993, 6, 1013–1023.
10. S. Mohammadnejad, J. L. Provis and J. S. Deventer, Reduction of gold(III) chloride to gold(0) on silicate surfaces, J. Colloid Interface Sci., 2013, 389(1), 252–259.
11. J. A. Pask and R. M. Fulrath, Fundamentals of Glass-to-Metal Bonding: VIII, Nature of Wetting and Adherence, J. Am. Ceram. Soc., 1962, 45(12), 592–596.
12. W. Chen, P. Wee and I. D. Brindle, Elimination of the memory effects of gold, mercury and silver in inductively coupled plasma atomic emission spectroscopy, J. Anal. At. Spectrom., 2000, 15(4), 409–413.
13. Y. Wang and I. D. Brindle, Rapid high-performance sample digestion for ICP determination by ColdBlock™ digestion: part 2: gold determination in geological samples with memory effect elimination, J. Anal. At. Spectrom., 2014, 29(10), 1904–1911.

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