Fire assay techniques assays

Gold often occurs in concentrations which are too small for direct instrumental analysis. Prior to the determination, it is necessary to carry out some pre-concentration and/or pre-separation of Au from many other substances in the complex matrices. The most common analytical method is the classical fire assay technique (Van Loon and Barefoot 1991, Qu 1996, Rao etal. 

Like the refining of the PGMs, the analysis is compHcated by the chemical similarity of the metals. The techniques used depend on the elements present and their concentration in the sample. For some low grade samples, analysis is preceded by a concentration stage using fire assay with collection into a lead or nickel sulfide button. The individual metals can then be determined. 

The fire assay, the antecedents of which date to ancient Egypt, remains the most rehable method for the accurate quantitative determination of precious metals ia any mixture for concentrations from 5 ppm to 100%. A sample is folded iato silver-free lead foil cones, which are placed ia bone-ash cupels (cups) and heated to between 1000 and 1200°C to oxidize the noimoble metals. The oxides are then absorbed iato a bone-ash cupel (ca 99%) and a shiny, uniformly metaUic-colored bead remains. The bead is bmshed clean, roUed fiat, and treated with CP grade nitric acid to dissolve the silver. The presence of trace metals ia that solution is then determined by iastmmental techniques and the purity of the silver determined by difference. 

Critical evaluation of the fire assay for separation of gold [14—16] and platinum group metals [17] have been published. The problem of gold losses in the cupellation technique has... 

Parry SJ (1992) Fire assay for the preconcentration of the platinum group elements and gold. In Alfassi ZB and Wai CM, eds. Preconcentration Techniques for Trace Elements, pp. 401-415. CRC Press, Boca Raton. 

Several methods have been proposed for digestion of samples of differing origin. With respect to the sample composition and the intended analytical technique, acidic dissolution, microwave assisted digestion, chlorination and fire assays have been proposed. Generally, these procedures allow removal of matrix as well as simultaneous enrichment of analytes. 

Some mention should be made of the fire-assay methods for silver, selenium, and tellurium 502-504). Although these methods are not often convenient, they can be used to isolate these elements from most of the matrix of an evaporated sample by a high-temperature treatment and lead dissolution. The concentrated material can then be analyzed by more sensitive techniques. 

The pyrognostk tests also called/ire assays are simple qualitative laboratory techniques used by mineralogists in the field or in a laboratory to indentify quickly without complex equipment the chemical elements present in an unknown mineral sample. Five major types of fire assays are used ... 

The distribution of ruthenium and rhodium traces in the ocean has been investigated by RIS -VETA. Both elements are rare and their distribution is of great geological and geochemical importance. The abundance of Ru and Rh in the Earth s crust is estimated to be 1-100 pg/g and can not be directly measured by current analytical techniques. The fire assay method and concentration by polymer sorbents were used prior to RIS analysis. Concentrations of Ru varied from 1.3 pg/mL in ocean water to 7.6 ng/g in Red Sea sediments. Concentrations of Rh in ocean water were 3- lOpg/mL. 

Fire assay and cupel methods are routine separation techniques for the assay of the noble metals. These have been extended by the use of the cupel techniques as a preseparative method for the n.a.a. of noble-metal traces in geological materialand crude platinum. ... 

---