Fire Assay Principle, Method And Equipment

The mysterious, ancient, irreplaceable fire assay

Tiandao Research Institute 2018-01-26 Author: Zhou Xing Yu

The fire assay method is a classic analytical method that applies the principles and techniques of metallurgy to analytical chemistry and is one of the oldest methods in analytical chemistry.

The fire assay method is to quantitatively determine the content of precious metals by adding flux to smelt ores and metallurgical products. This method has the advantages of good sampling representativeness, wide applicability, and good enrichment effect. It is an important means for the chemical analysis of gold, silver, and precious metals.

Features of The Fire Assay

Fire assay is not only an ancient way of enriching gold and silver, but also an important way of gold and silver analysis. Geology, mines, gold and silver smelters use it as the most reliable analysis method and widely used in production. Many countries have set this method as a national standard method for the determination of gold in gold concentrates, copper concentrates, jewelry gold, and alloy gold. With the development of science and technology, there are more and more new techniques for analyzing gold and silver, and analytical instruments are becoming more and more advanced. Compared with other methods, the fire assay has more operating procedures and requires skills. There are many analysts who try to use other analysis methods instead of fire assay. However, the fire assay method is irreplaceable. For the determination of gold content in high-content gold raw materials or pure gold, its accuracy are inferior to other direct determination methods. In the arbitration analysis of gold and silver content, fire Assay analysis can give convincing results for all parties to the dispute. This is because the fire assay has many unique advantages that other analytical methods do not have:

(1) Good sampling representativeness. Gold and silver are often present in samples unevenly in the order of less than gram per ton. The fire assay requires a large sample amount, generally 20-40g, and even samples up to 100g or more can be taken. Therefore, the good representativeness of the sample can reduce the sampling error to a minimum.

(2) Wide adaptability. It can adapt to almost all samples, from ore, gold concentrate to alloy gold, the fire assay method can accurately determine gold and silver, including those antimonites that cannot be solved by wet analysis. For the analysis of the main components of pure gold, the fire assay can also obtain satisfactory results. Except for very few samples, this method can be adapted to almost all minerals.

(3) The enrichment efficiency is high, reaching more than ten thousand times. Fire assay can quantitatively enrich a small amount of gold and silver from dozens of grams of samples containing a large amount of matrix elements into the assay button. Even if micrograms of gold and silver are enriched, the loss is very small, generally only a few percent. Due to the simple composition of the compound granule (or enrichment slag), it is beneficial to use various test methods to determine in the future.

(4) The analysis result is reliable and accurate. South Africa Rand Company's routine analysis of pure gold (>99.9%), 74 analysis results of the same sample, standard deviation (S) 0.0058%. The S of the 10 analysis results of similar domestic products is also about 0.005%. Over the years, some scholars at home and abroad have tried to completely replace the fire assay with new wet chemical analysis or instrumental analysis, but they have not succeeded so far. Werbicki et al. compared the three analytical methods of Au in solution-AAS, ICP-AES and the assay method, and gave the standard deviation S of each method analyzed by 18 laboratories. The result is ICP-AES and AAS. Basically the same, but they are all slightly worse than the trial gold method. Wall pointed out that the fire assay is suitable for samples with a gold content of <1μg～1g, and its accuracy and precision are better than other instrumental analysis.

The Principle of Fire Assay

5.2 Basic Principles of Fire Assay
Fire assay analysis is actually a gold assay method with crucible or cupel as a container. There are many types and different operating procedures. There are lead assays, bismuth assays, tin assays, antimony assays, nickel sulfide assays, Copper sulfide test, copper-iron-nickel test, copper test, iron test, etc. However, the smelting principles and reactions in the various new assay methods still have many similarities with the lead assay method. Among all the fire assays, the most commonly used and most important is the lead assay, which has the advantage that the resulting lead can be cupellation. The combination of lead assay and cupellation technology can enrich the precious metals in dozens of grams of samples in aggregates weighing several milligrams. In the lead assay, the collection rate of Au is >99%, and the recovery rate of Au as low as 0.2～0.3g/t is still very high. The analysis accuracy of the lead assay for both macro and trace precious metals is very high. The following is a brief description of the principle of the fire assay with the lead test method as an example.
The lead fire assay is mainly divided into 3 stages:
(1) Smelting. It uses solid reagents to mix with rocks, ore or smelted products, heat and melt in a crucible, and use lead to capture gold, silver and precious metals in a molten state to form lead alloys (generally called lead buttons, also called precious lead). Due to the high specific gravity of the lead alloy, it sinks to the bottom of the crucible. At the same time, the base metal oxides and gangue in the sample react with fluxes such as silica, borax, and sodium carbonate to form slag such as silicate or borate, which floats on it due to its small specific gravity. This separates the gold and silver from the sample. Therefore, during the fire assay process, both functions of decomposing samples and enriching precious metals are played.
(2) Cupellation. Put the obtained lead alloy in a cupel at an appropriate temperature for cupellation to remove lead. During cupellation, the lead is oxidized to lead oxide and penetrates into the porous cupel, thereby removing the lead in the lead button and a small amount of base metal. Gold silver and precious metals are not oxidized and remain in the cupel to form gold and silver particles.
(3) Divide. Use nitric acid to dissolve the gold and silver alloy particles to dissolve the silver while the gold remains solid. After quenching, the obtained gold particles are weighed to calculate the gold content. The silver content can be calculated according to the difference between the quality of the gold and silver particles and the quality of the gold.
After the fire assay method completes the separation and enrichment of gold, silver and precious metals, in addition to the above-mentioned gravimetric method for determining gold and silver, after dissolving the gold and silver particles with aqua regia, a variety of chemical analysis methods can be used to determine gold, silver and other precious metals.

The theoretical basis of fire assay can be summarized into five aspects.
(1) the correct use of chemical reagents reduces the melting point and ensures that minerals with good fluidity can be obtained at the temperature reached by the assaying electric furnace.
(2) metallic lead melted at a high temperature has a great ability to collect gold and silver and precious metals, and can completely melt the gold and silver exposed in the molten state in the lead.
(3) the specific gravity of metallic lead and molten slag are different. The lead sinks to the bottom during melting to form a lead button, and the slag floats on it, realizing a good separation between the lead button and the molten slag.
(4) lead is easily oxidized at a certain temperature, and at the same time, lead oxide can be absorbed by the dense and porous cupel, and gold and silver cannot be oxidized to form composite particles and remain in the ash pan.
(5) the difference in the solubility of gold and silver in nitric acid is used to separate gold and silver. Silver forms silver nitrate and enters the solution. The gold grade can be calculated by weighing the gold.
Utensils and equipment commonly used in fire assays
(1) Assay crucible
The crucible used for assault smelting is generally called the assault crucible, and the material is refractory clay. The general requirement for the assay crucible is: it has sufficient refractory degree, that is, the crucible does not soften or collapse when heated at high temperature; it can still maintain sufficient pressure when heated, and it will not break when it is picked up or fork out. ; It can resist the chemical action of the melt, and will not be corroded by various melts including strong acid, strong alkali or containing a large amount of lead oxide, causing the crucible to leak.
(2) Cupel
Cupel is a porous refractory ware used to absorb lead oxide (or bismuth oxide) when cupellation lead buttons (or bismuth buttons). There are three commonly used cupels: cement cupel, ashes-cement cupel and magnesia cupel.
①Cement cupel uses 400, 500 Portland cement, add 8-12% water, mix well, and press on the cupel machine. The composition of Portland cement is CaO 60～70%, A12O3 4～7%, SiO2 19～24%, Fe2O3 2～6%. Cement is an inexpensive common material. The cement cupel is hard and not easy to crack, but the loss of precious metals during ash blowing is larger than the latter two.
②Cream ashes and ashes-cement ashes. The ashes are obtained by burning, grinding and then burning the bones of cattle and sheep, and all organic matter must be removed. Its composition is 90% calcium phosphate, 5.65% calcium oxide, 1% magnesium oxide, and 3.1% calcium fluoride. The fineness of ashes should be less than 0.147mm, of which 0.088mm should account for more than 50%. The cupel made of pure ashes is looser and can be used for cupellation of crude gold and alloyed gold. Assay analysis generally uses a mixed cupel of ashes and cement. The ashes and cement are mixed in different proportions, and 8-12% water is added and pressed on the cupel machine. Different people have different results. Some think that 3:7 is good, and some think that 4:6 or 5:5 is good. The ashes-cement ashes are harder than the pure ashes, but softer than the cement ashes. Using ashes-cement cupel to cupellation, the loss of gold and silver is smaller than that of cement cupel. The preparation of ashes is more troublesome and requires several processes of burning and grinding.
③ Magnesia cupel Grind the calcined magnesia finely, and it is required that more than 63% pass through a 0.074mm sieve, and the particles with a particle size of 0.2～0.1mm should not exceed 20%. The finely ground magnesia should be pressed within a few days, otherwise it will agglomerate after being left for a long time. Take 85 parts of finely ground magnesia and 15 parts of No. 500 cement, mix them, add 8-12% water and press to make a cupel. The loss of precious metals during ash blowing in the cupel made of magnesia is smaller than that of the first two.
The main component of magnesia is magnesium oxide, which is a good refractory material and can resist corrosion by alkaline flux. The lead oxide produced during lead ash blowing is a very strong alkaline flux. At high temperatures, lead oxide has a strong affinity with silicon dioxide and can invade the silicate in the cupel. The ashes-cement cupel contains more silicates. After blowing with this cupel, small pits will appear on the surface of the cupel, and precious metals will be lost due to this. Using magnesia cupel, there is no such phenomenon after ash blowing, and the surface is very smooth.
Gold and silver are blown in three kinds of cupels. The weight method is compared in the literature [23], which proves that the loss of magnesia cupel is the smallest, followed by pure ashes and ashes-cement (1+1) cupels, and cement cupels have the largest loss. . In recent years, some people have made more intuitive experiments with Ag110 and Au198 isotopes. Literature [24] reported that using Ag110 isotope and 5mg of non-radioactive silver in ashes and magnesia cupels (895 ℃), measuring Ag110 in the ashes, the results are shown in Table 5-1, the loss of silver in ashes is more than magnesium The sand cupel is 25% larger.
Literature [25] reported the use of Au198 isotope as a test to compare the loss of gold in magnesia and ashes. Cupellation at 960°C, the results obtained show that the loss of gold in the cupel is much greater than the loss in the magnesia cupel. The results are shown in Table 5-2.