Abstract: fire assay, is one of the oldest methods of chemical analysis. Compared with other methods, its operating procedures are more and requires certain skills, but for the high gold content of the raw materials or the determination of gold in gold component, its precision and accuracy as other direct assays are less. So far in reference laboratories abroad still an important position.
Keywords: fire assay; flowering; Procedure; cupel
We assay analysis, ash blown up into a very important role, the main sources of error and loss of gold and silver were produced in the operation step. In my many years of testing experience, summarized in the following way, the accuracy of the measurement results of the process to improve the precision and better.
2.1 cupellation concrete steps are as follows:
2.1.1 will be clean and free of ash in the ash pan burning 900 ℃ -920 ℃ 20 minutes or so, remove the ash pan in organic matter and gaseous substances. These organisms do not go out, it will cause bubbling and loss of gold and silver in the ash blowing process.
2.1.2 The weighed sample into the ash pan, make the cupellation process. Since silver has a strong oxygen-absorbing, at the end of the cupellation stage, the temperature drops, oxygen is released from the sample, thus creating a flowering phenomenon. The reason is that silver liquid at high temperatures, absorb oxygen in the air, absorb up to 20 times the amount of silver volume in grains combined with increasing gold content, decreased absorption, gold silver ratio of about 2 or less no longer absorb, general fire assay gold bullion component analysis, gold silver ratio of 2.5 to 2.7, when there is still some liquid oxygen capacity. Oxygen absorption liquid silver, the solidification time and re-released. After the fire assay cupellation, co-release tablets oxygen are generally three forms: First, focus on a sudden discharge, forming sputtering; one is on one point more slowly released, carrying liquid metal to form a small bud-like projections ; there is a whole grain expansion so close together in the grain surface cracks. These phenomena we referred to as "bloom." Who have a "bloom" phenomenon synthetic particle can cause weight loss. The largest in three types, resulting in the analysis results invalid.
Flowering There are three types:
22.214.171.124 splash: quick release from one place.
126.96.36.199 Stubbs thorn: slow release from one place.
188.8.131.52 Surface Crack: overall surface cracks caused by expansion.
High fineness, the general use plus copper to reduce and eliminate flowering phenomenon, the principle is the Cu portion cupellation, 20% of the copper remaining in the gold and silver together grains, remaining in close grain copper and cooperation grains in the liquid (in this case lead oxide oxygen has been completed) to become absorbed by the combination of oxide, since less oxygen Cu, resulting cuprous oxide. Note, do not add too much copper, when the copper content exceeds 2%, can be formed in the late blow matte gray ash leaving the blow can not continue. Copper is usually added 40mg in 1g of gold (plus copper ratio in various countries are not the same).
2.2 The main cause of the loss of gold and silver
Silver cupellation process of loss may be mainly oxide and lead oxide inclusions, due to the loss increases with increasing temperature, it should blow the ash at a temperature as low as possible, in order to reduce the minimum loss. Gold and silver oxide into the ash pan, which is the main cause of the loss of gold and silver, which accounts for about 90% of total losses, another reason is the evaporation of gold and silver.
2.3 "feather strong" formation
Lead oxide is an exothermic reaction, especially when it is in the ash pan containing bone meal carried out, the ashes powder used as an insulator, lead is much brighter than the other parts of the furnace. Some evaporation may lead oxide crystal shape in the form of branches at the top of the condensation of lead dust pan, these "feather strong" marks the emergence of silver losses to a minimum.
2.3.1 When approaching the end of blowing ash, ash blowing slightly elevated temperature, which is when rich alloy containing silver amount of time, the alloy remains molten, to remove the remaining lead oxide, lead oxide layer when the side faces become thin, This leads to optical interference produces a "flashing colors", and the emergence of the rotating beads in the ash pan, since lead was removed after leaving the liquid metal is too cold, and when it is the latent heat of temperature , after which, while silver beads have become very bright. After the "flare", the short cupel parked in a furnace, and then removed, such as access to large Eun-joo, the ash pan to move the front portion of the stove and put a hot ash pan, when the object is solidified in the lower part of the silver, thawing state holding surface, which is to avoid expansion, are affected by the expansion of gas in solution caused forced to escape.
2.3.2 cupellation near the end, we need to heat for 2 minutes, which is to get rid of excess Pb.
2.4 ratio of lead
During the experiment should be carried out for different ratio of gold and silver content of Pb, Pb in an amount based on the amount of impurities should be added because the high purity, blowing ash longer, more serious loss of gold and silver, thus affecting the results. After blowing ash, should wait furnace temperature cooled sufficiently before opening the door, otherwise there will be flowering mentioned above, several seriously affect the measurement of open instant.
Experimental results show that different concentrations of gold and silver ratio, under the same conditions of blowing ash, have different loss rates. The higher the silver content, the total amount of gold and silver lost more serious.
2.5 Effect of PGE and gold quality
When crude gold containing platinum group metals, the ash is blown can not be removed, the presence of platinum group metals, the ash may blow after the closing grain dark, not smooth after the pearling and may be severe in the pearling process burrs, bifurcation cases, this has a serious impact on the measurement. For crude gold of Pd element, which can be separated by the gold stars and gold, (sub-gold solution is red) does not affect the measurement results. While the other is not easy to separate platinum group metals, gold and cause high. We rolled gold can largely determine the presence of a platinum group metal, and if there is something black on the volume of gold, indicating the presence of a platinum group metal, then fire assay methods for measuring results will be wrong.
For crude gold in the case of Pd-containing element, we have our own set of methods, of course, this method of gold and silver loss rate is too large, but in order to prove that we meet and the customer is responsible, we will ask the customer to observe the scene, in order to demonstrate does contain Pd element.
Three types of ash pan
Fire assay analysis of the ash pan there are two, one animal ashes made into a kind of magnesia. We generally use the gray ashes dish, but because fewer and fewer animal ashes, therefore, the application of magnesite cupel is a kind of trend.
3.1 Results and Discussion
The effects were very magnesite cupel ashes ash pan for gold content of the sample (Table 1)
Category gold / gold theoretical% /% deviation /%
1st 2nd average
Ashes 74.94 74.94 74.94 75.00 0.06
The results in Table 1 were analyzed, the results showed that deviations of gold and gold content of the theory were less than 0.1% in line with the national standard ,, use magnesite cupel a result closer to the theoretical value.
4.1 Discussion ash blown by fire assay analysis, can effectively correct our steps to reduce the error results.
4.2 By comparing the ashes ash pan and ash pan of magnesia, magnesite cupel absorption of the gold standard is small, the measurement result is slightly higher than the ashes ash pan, but the results of the analysis and the theory deviation of gold both are within the scope of national standards.
4.3 When crude gold containing Pd element, which can be separated by the gold stars and gold, (sub-gold solution is red) does not affect the measurement results.
Post time: Jul-22-2016