Analytical Methods

Here are some word or talk/terms of the trade we using in assaying: Annealing – the heating of gold after parting to change its colour from black to gold.
Bead – Particle of gold and silver that is obtained at the end of the cupellation process.
Borax – An acid flux.
Crucible – A bone ash vessel in which the sample and fluxes are melted.

Assaying the Platinum group Elements (PGE’s) is done via the same fire assy analysis technique explained earlier. The standard lead collection scheme used for gold gives good recovery for platinum and palladium as well. They are soluble in aqua regia and can be measured with gold in a multi element instrument such as ICP/OES or ICP/MS. The detection limits are low ppb.

One should first pick and flatten the beads and put them into 16 x 125mm disposable glass test tube. One should ensure water bath is a minimum of 75 0C. Another option is the use of fine sand. The sand gets far hotter than a water bath.

Proper Gravimetric  Technique starts with ensuring parting cups are clean before using. The assayer should put a flatten bend in hot 6:1 nitric to start, and watch the bead for its reactivity, add stronger acid if it is needed. This should be done on a hot plate and inside a fume hood. If stronger acid is needed it should be hot. One should carefully add and remove the acid with a disposable pipette. To make sure that floaters are fully parted, use glass rod or tap cup to sink to sink them. One can add ammonia rinse solution and to remove any AgCl that is present.

Parting is the process of dissolving the Ag (Silver) in the bead from the cupellation to leave the Au (Gold) behind for weighing or to be read on the AAS.
If Ag is to be determined gravimetrically as well, the bead is weighed before parting. The very small beads are weighed using sensitive micro-balances.

Cupellation separates the gold and silver from the lead. This consists of an oxidizing fusion in a porous vessel called a cupel.

Roasting is a technique used to handle difficult gold or precious metal samples. Minerals decomposed in this way include sulphides, arsenides, antimonides and tellurides. A successful roasting can render samples amenable to standard flux mixes. Weigh 50 grams of a sample into a shallow dish and place it into an open furnace at 100 0C.

One must try to produce an ideal slag because it will help with the recovery of the precious metals. The best slags are a balance between acidic and basic properties. Slags are classified according to the proportion of oxygen in the acid to the oxygen in the base. A mono-silicate slag has equal amounts of oxygen in each whereas bi-silicate slags have twice as much oxygen in the acid as in the base. The best slags are those that range between a subsilicate and a bisilicate. The different fluxing reagents all have a role in producing an ideal slag.

In the fire assay speiss is usually a combination of Fe & As. Speiss is formed when the iron method is used on ores containing arsenic. It is a hard, fairly tough, tin-white substance found on the top of the lead. If a small amount of arsenic is present, the speiss will form a little button on the top of the lead. If there is lot arsenic is present, the speiss may cover the entire top of the lead. Speiss contains Au & Ag, and may be cupelled along with the lead if there is less than a gram of speiss.

When the slag is cooled, the slag is separated from the Pb button by pounding it off with a hammer and then cubing the Pb button so it can be easily picked up. The lead button should be bright, it should separate from the slag easily and there should be no evidence of matte or speiss on the top surface of the button. The Pb buttons should be between 15 to 35 grams.