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Assay Techniques

Assays are regularly used to determine the purity of precious metals. Assay Techniques - Doral Refining Corp.

A quantitative determination of the amount of a given substance in a particular sample. They can be performed by wet methods or dry methods. In the wet method, the sample is dissolved in a reagent, like an acid, until the purified metal is separated out. In the dry method, the sample is mixed with a flux (a substance such as borax or silica that helps lower the melting temperature) and then heated to the point where impurities in the metal fuse with the flux, leaving the purified metal as a residue.

Doral Refining regularly performs 3 types of assays: XRF Spectrometer, Fire Assay, and Aqua-Regia

XRF Spectrometer

Energy dispersive X-ray fluorescence technology (ED-XRF) provides one of the simplest, most accurate, and most economic analytical methods for the determination of the chemical composition of many types of materials. It is non-destructive and reliable, requires no, or very little, sample preparation and is suitable for solid, liquid and powdered samples. It can be used for a wide range of elements, from sodium (11) to uranium (92), and provides detection limits at the sub-ppm level; it can also measure concentrations of up to 100% easily and simultaneously.

Doral uses this form of assay for the majority of its testing purposes.

Fire Assay

Fire assaying is the quantitative determination in which a metal or metals are separated from impurities by fusion processes and weighed in order to determine the amount present in the original sample.

Fire assays take about 3-4 business days to perform at a cost of $40 per assay. This is used to determine gold and silver in the sample provided.

Aqua-Regia Process

The gold refining technique described here is the rather ancient wet chemical method whereby the gold-bearing scrap is dissolved. The gold solution is then filtered and the jewelers bench dirt, sandpaper grit, etc remains on the filter as a solid sludge, together with any silver present, which will be in the form of silver chloride. The filter and sludge are washed with repeated small amounts of water to wash all gold chloride solution down through the filter. Other metals that were in the alloy or in the scrap (nickel, zinc, copper, iron, etc.) are also in this solution, which is usually green in color. The nitric acid from the excess of aqua-regia used in the digestion is removed either by boiling or chemical reaction. To recover the gold as metal, a reducing chemical is added to selectively change the gold chloride into solid gold particles and leave the other metal chlorides unchanged and in solution. When tests show this to be complete, the solution is filtered and the gold in the filter thoroughly washed. The clean gold is then melted and poured into molds or made into shot. This process is also used to remove stones from finished jewelry.

Doral also offers this service for a fee of $375. It commonly takes about 3-4 business days and all precious and semi-precious stones are returned to the customer on completion of the job.
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