Sampling and assaying are key elements
of our process.
Silver contents are determined
for each consignment of material received.
The sampling can be witnessed. Assaying
is carried out according to classical fire
assay techniques and universally accepted
chemical methods.
Fire Assaying
The term "fire assay" describes
a quantitative procedure in which a fusion
is used to separate the precious metals
from a known amount of sample. The fusion--or
melt--is done in a furnace at high temperatures.
This method is the standard for determining
gold and silver in ores, concentrates, and
a variety of metal alloys.
Fire assay begins with weighing a sample
into a crucible and thoroughly blending
it with a mixture of dry chemicals. One
of the ingredients is lead oxide; another
is a reducing agent. The crucible is then
placed into a 1000°C furnace. The reducing
agent reduces the lead oxide to lead. This
forms thousands of small droplets of lead,
which collect all the precious metals from
the sample and sink to the bottom of the
crucible. Meanwhile, the non-precious metal
contaminants are carried off into the slag.
The crucible is removed from the furnace
and poured into a mold. When cool, the slag
is discarded and the lead "button" cleaned
of any residual slag. This entire process,
therefore, selectively collects all the
precious metals from the sample and alloys
them with lead.
The next step involves a "cupel". A heated
cupel has the unique capability of absorbing
metallic oxides. The cupel is preheated
to 950°C, the lead button placed into
it. The lead melts and the surface oxidizes,
the oxide absorbed, exposing more lead,
which oxidizes, etc. Since gold and silver
do not oxidize, they continue to concentrate
until the last of the lead is absorbed;
leaving only a bead in the bottom of the
cupel. This bead contains all the precious
metals from the original sample. It is called
the "dore" bead. Its weight is recorded
to get the total precious metal content
of the sample.
Once the dore is weighed, the silver is
dissolved from it with nitric acid--which
dissolves silver but does not affect gold.
This step is called "parting". After parting,
the weight of gold is recorded. The difference
between the dore and the gold is silver.
A fully equipped and versatile sampling
department prepares and samples all incoming
customer materials.
Incineration, calcination, drying, milling,
and smelting procedures are utilised to
obtain a fully representative quality sample
of the lot of material. A final quality
sample is assayed in a modern laboratory.
Platinum
The noble metals are analysed more frequently by nickel collection. This technique uses nickel oxide as a collector, and it is possible to determine a range of metals in one procedure, namely platinum, palladium, rhodium, iridium and ruthenium. This method does not use cupellation to separate the collected precious metals, but instead dissolves the pulverized nickel button in acid. The nickel is removed by filtration, the remaining residue of precious metals is treated and the final determination of the solution is made using instrumentation such as ICP.
ICP ORS: Inductively Coupled Plasma Optical Emission Spectrometry.
ICP spectrometry analyses solutions of samples of known weights into known volumes by the most appropriate preparation methods for this type of sample. The resulting solution from direct dissolution of the sample or from the parting of the precious metal beads from fire assay is then analysed directly on an ICP spectrometer. The method requires comparative standards and reference samples and has the advantage that it can measure a range of elements sequentially.
ICP spectrometry is an excellent method for measuring PGMs in solution. It is an emission technique, which requires the sample to pass through a nebuliser to create a fine mist. This is then swept into the plasma via a spray chamber and a torch. The plasma itself is a high-energy source that causes the atoms and ions within the sample to absorb energy and reach high energy states, thus giving off energy in the form of light, which is then measured by the spectrometer. |
