Technical Note

Technical
Note
FIRE ASSAYING
Art or Science?
The pyro-metallurgical recovery of gold from precious metal ores has been practiced for over
two thousand years. The basic fire assaying technique for the determination of gold has changed
little over the centuries. The only major scientific advance probably occurred in the early 1960s
with the introduction of atomic absorption spectroscopy (AAS). This provided an alternative to the
traditional and time consuming gravimetric technique for the determination of gold. There was
no change to the chemistry involved in fire assaying but AAS simply provided assayers with the
ability to process more samples and achieve lower detection limits. This was the beginning of the
high capacity fire assay facilities that we see operating today.
Commercial laboratories have made significant productivity advances over the past forty years to
keep pace with the demand for fire assay determinations. However the basic chemistry remains
the same. The nexus between productivity and chemistry is important as it explains many of the problems that geologists and
metallurgists experience when dealing with commercial fire assay data. Because gold is rarely homogeneously distributed in nature
and usually present in quite low concentrations, the frequent approach to fire assaying is to request the largest test sample possible
to obtain a more statistically valid result. Unfortunately, what charge weight is requested for assay and what can be realistically
analysed may bear little similarity. A basic knowledge of the chemistry and practice of fire assaying can be helpful in understanding
why this divergence may occur.
BASIC PRINCIPLES
The fire assay technique uses high temperature and flux to ‘melt’ the rock and allow the gold
to be collected. Lead formed from the reduction of litharge (PbO), is traditionally used as the
collecting medium for silver and gold. The test sample is intimately mixed with a suitable flux
that will fuse at high temperature with the gangue minerals present in the sample to produce a
slag that is liquid at the fusion temperature. The liberated precious metals are scavenged by the
molten lead and gravitate to the bottom of the fusion crucible.
Upon cooling, the lead button is separated from the slag and processed in a separate furnace
for a high temperature oxidation (cupellation) where the lead is removed, leaving the precious
metals behind as a metallic bead called a prill. Traditionally this prill was then partially dissolved in nitric acid (parted) to remove silver
and the remaining gold determined by weighing (gravimetry). Alternatively, the prill can be dissolved in a mixture of hydrochloric
and nitric acid (aqua regia) and the concentration determined by spectroscopic methods (AAS, ICPAES or ICPMS). The concentration
is normally expressed as parts per million (ppm), which is equivalent to grams per tonne (g/t).
FLUXES
The following chemicals are normally found in, or added to, a commercial fire assay flux. Some of these re-agents play multiple
roles during fire assay, depending on the sample matrix being analysed. The flux composition may have to be supplemented by
increasing the proportion of one or more of these components to produce a successful fusion
REAGENT
REASONING
Litharge
Used to provide lead to collect the precious metals. It is also a strong basic flux and reacts with metallic
oxides and silica to form a slag. By far the most expensive component of a fire assay flux.
Soda Ash
A powerful basic flux that is usually the principal component of fire assay flux. It reacts with silicates to form
a slag.
Borax
An acidic flux that lowers the fusing point of all slags. It forms fusible complexes with limestone and magnesite
Silica
An acidic flux that forms the principal component of many samples. Small amounts are present in the flux to
prevent attack on the fire assay crucibles when assaying samples deficient in silica.
Nitre
A powerful oxidising agent added to the flux when assaying samples containing sulfides.
Flour
A source of carbon used to reduce the litharge to lead.
Silver
A small amount is added to the flux to provide a collection medium for the precious metals.
PRACTICE VERSUS THEORY
In theory, every different sample matrix will require a unique flux to ensure complete recovery of
the gold and other precious metals. A mine laboratory that analyses more than one ore type may
well use a separate flux formulation for each. A commercial laboratory on the other hand will be
expected to process many different samples types, often present in the same work order.
Most commercial laboratories will use one or two multi-purpose fluxes to process all of their
samples, with minor adjustments made to compensate for the perceived sample composition
(usually unknown to the fire assayer). This has resulted in laboratories opting to use a relatively
high flux to sample weight ratio to compensate for matrix variations and to provide a consistent,
successful fusion.
A successful fusion will result in the formation of only two phases – a lead button and a clean slag. The lead button should be
relatively bright and soft, indicating the absence of other base metals, and it should weigh approximately 30 grams. A small lead
button or the presence of additional phases indicates that the fusion has been unsuccessful and the precious metal recovery will be
low. The assay will then have to be repeated using either a different flux composition or a lower fire assay charge weight.
A minimum flux to sample ratio of 4:1 is usually accepted as being adequate to produce a good fusion for a non-complex sample
matrix (defined as an alumino-silicate containing relatively low concentrations of base metals, sulfides and carbonaceous materials).
Most commercial laboratories traditionally use a high soda ash, low litharge flux for routine fire assay determinations as this provides
a good compromise between cost and effectiveness. One disadvantage is that the lower density of this material compared to
a high litharge flux results in a significant volume of flux being required. The soda ash in the flux evolves carbon dioxide during
decomposition, which provides the boiling action necessary to ensure the circulation of the molten lead globules through the melt to
scavenge the precious metals. Too large a volume in the fire assay crucible will result in boil over and subsequent loss of sample, as
well as damage to the furnace floor.
THE FIRE ASSAY CHARGE WEIGHT
All fire assay texts quote an “assay ton” (approximately 30 grams) as the charge weight to be
used for a fire assay determination on a simple sample matrix. As the sample matrix becomes
more complex due to the presence of base metal oxides and sulfides, this charge weight must
be reduced (in some cases dramatically) thereby allowing the assayer to achieve the large flux to
sample ratio required for a satisfactory fusion.
To cope with the ever increasing demand for gold assays, commercial laboratories have focused on increasing productivity through
the introduction of larger furnaces, mechanical sample/flux mixing equipment and multi-pour systems. This has resulted in the
standardisation of fire assay crucibles to a shape that allows efficient use of the furnace floor space. This, however, limits the ability
of the laboratory to increase the amount of flux used where the higher flux to sample ratio is required.
Most commercial laboratories recommend the use of a 30 gram fire assay charge weight as this provides the best compromise
between optimum gold recoveries and the potential for a poor fusion. ALS Minerals typically performs fire assay determinations
using a flux to sample ratio of 5 – 6:1 to reduce the chance of a poor fusion and the resulting low precious metal recoveries.
Clients requesting higher charge weights on the grounds that this will provide a more statistically valid sample may well find that
assay results on their internal QC samples will tend to have a negative bias, a bias that will be more pronounced when assaying a
‘difficult’ sample matrix. The more ‘difficult’ the sample matrix, the greater the chance of a failed assay resulting in the laboratory
having to repeat the assay at its own cost. This, combined with the need to maintain efficient turnaround times, drives the laboratory
to always use its experience in estimating a fire assay charge weight that will maximise its chance of achieving a successful fusion.
DIFFICULT SAMPLES
There are a significant number of elements and compounds that create difficulties for fire
assayers. Some of these difficulties can be overcome by modifying the fire assay flux. In other
cases, the only available option is to reduce the charge weight to limit the amount of impurities
that can concentrate in the lead button. A brief list of the main offenders follows:
Procedures are available to successfully fire assay nearly all sample types provided the composition
of the material is known. Unfortunately, this knowledge is rarely available for a commercial
laboratory. Fire assayers must therefore hazard a guess at the composition and either modify
the flux, reduce the sample weight, or both. Accordingly, the failure rate for certain sample types
may initially be quite high, leading to re-assays and a delay in releasing the results.
ELEMENT
COMMENT
Copper
May be reduced to the metal during fusion and reports in the lead button. This can then inhibit cupellation, making it impossible to recover the precious metals. Alternatively, it may react with pyrite to form
a matte that will preferentially absorb gold.
Nickel
Reacts similarly to copper with regards to cupellation, but will create problems at far lower concentrations (> 0.5% in the lead button). A combination of nickel and copper will create far greater problems
than either of the elements individually.
Antimony
Completely miscible with molten lead. More than 2% antimony in the lead button may cause cracking of the cupel, resulting in low gold recovery. It will also form antimonides with copper, nickel or iron
(speiss) which will preferentially absorb gold.
Arsenic
Will form arsenides with copper, nickel or iron (speiss), resulting in low gold recovery.
Tellurium
Extremely detrimental to the recovery of precious metals during cupellation when present in amounts
of > 0.5% in the lead button. It lowers the surface tension of the precious metal prill, allowing some of
the doré to be absorbed by the cupel.
Selenium
Behaves similarly to tellurium.
Sulfur (Sulfides)
Can cause problems by forming mattes with copper, nickel or iron compounds, resulting in low gold
recoveries. Will also produce large lead buttons if using a high litharge flux. This can be controlled by
adding a calculated amount of oxidant.
Carbon (organic
matter)
Can cause major problems during fire assaying due to the formation of lead shot within the slag, leading to low lead button weights. This will result in low gold recoveries.
Another difficult sample type, but one that most geologists would love to have, are those that contain very high
concentrations of gold, but very little silver. The end result is a large precious metal prill that cannot be parted until
additional silver is added (inquarting) and the cupellation step repeated. High gold samples will also increase the risk of
contamination throughout the fire assay circuit.
ACCURACY AND PRECISION
A fire assay determination will recover approximately 99% of the gold present in the sample, provided the fusion and cupellation
steps are not compromised. A small amount of gold will always remain in the fire assay crucible (usually adhering to the walls) after
the melt has been poured. In addition, the following conditions can further impact on the efficiency of gold recovery:
□□ If the fusion process is less than ideal, the amount of gold remaining in the crucible and the slag increases.
□□ If the flux composition is inadequate for this particular sample type and a matte or speiss is formed during the fusion, gold losses
can be quite considerable.
□□ If the lead button is contaminated with unacceptable levels of certain base metals, further losses can occur due to the formation
of scoria that may inhibit cupellation.
Having raised all of these potential problems, fire assaying is still the most accurate and readily available technique for the
determination of gold and platinum group metals. Laboratories that analyse concentrates containing finely disseminated gold can
achieve a precision of better than two percent using fire assay with a gravimetric finish. High quality analysis always includes the
re-treatment of the slag and the cupel to recover any gold lost during the initial fire assay step. However, for a high quality analysis
of this type, the customer can pay hundreds of dollars per sample.
The routine fire assay determination offered by most commercial laboratories should provide accuracy and precision better than 10%
at a concentration level >50 times the limit of reporting. This of course is dependant upon the laboratory achieving a satisfactory
fusion and cupellation and the absence of ‘coarse’ gold in the sample.
The presence of ‘coarse’ gold is fortunately not all that common, even though this reason is often used to explain poor laboratory
precision. It may well be that the imprecision is due to poor gold recoveries during the fire assay procedure and that after reducing
the sample weight or modifying the flux, the ‘coarse’ gold problem is no longer evident.
SCREEN FIRE ASSAYS AND GRAVIMETRIC FINISHES
If a sample is identified to contain ‘coarse’ gold, a screen fire assay is the best way to obtain a definitive result. This procedure
involves screening a large pulverised sample (commonly 1 kg) at 75 microns. The entire oversize (including the disposable screen) is
fire assayed as this contains the ‘coarse’ gold and a duplicate determination is made on the ‘minus’ 75 micron fraction. A calculation
can then be made to determine the total weight of gold in the sample. This procedure is equivalent to assaying a large sample to
extinction and averaging the results.
If a client requires high accuracy for samples containing significant gold concentrations (>15 ppm), a gravimetric finish is recommended.
Weighing a precious metal prill is inherently a more accurate procedure than the routine dissolution and spectrometric finish provided
by most laboratories, mostly due to the reduced number of critical steps involved. Problems can arise if the concentration of gold is
low as it is difficult to see, let alone collect and weigh a particle of less than 50 micrograms. The accuracy of this process depends
upon the ability of the operator to remove all of the silver during parting process while, at the same time, retaining all of the gold
for weighing.
SUMMARY
This brief description of fire assaying will hopefully provide customers with an understanding of the problems and pitfalls that exist
and why laboratories follow certain practices to obtain a satisfactory outcome. Fire assaying is not a simple materials handling
procedure that will always provide a satisfactory result just by following a recipe. There is a significant knowledge requirement that
can only be gained through many years of experience in handling the myriad of sample matrices that may contain gold.
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