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 magne- site 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 cupella- tion, 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 concentra- tions (> 0.5% in the lead button). A combination of nickel and copper will create far greater problems than either of the elements individually.