Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD

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Fire Assay Aque Regea (2 replies and 1 comment)

2 years ago
Han 2 years ago

First of all,for the processing of gold and tellurium ores,how exactly does the pos-roasting(calsine ores contain % 60- 62 Fe ) process work,what is the way to obtain tellurium. Second,which do you prefer for chemical analysis,aquea regea or fire assay testing for gold-tellurirate ores (VMS mine deposit for in anlyser )?

In the world The company which kind of prefer  to methods for gold analysers?

1 year ago
Jorge 1 year ago

There are two main gold-telluride minerals, calaverite and sylvanite. Regarding the calaverite, if the ore is heated to around 510 oC, then, the gold and tellurium are separated producing tellurium dioxide and metallic gold. If the temperature is higher than 800 oC, the tellurium dioxide is sublimed.

Gold-Telluride ores are a challenging material to perform the fire assayer. There are several reasons, for instance, the content of gold and silver is lower than expected because the cupellating stage is not efficient. For that reason, is highly recommended to use strong oxidizing conditions. In this way, the tellurium is reported in the lead phase. Other important variable is the sample/flux ratio. In some cases, it will be necessary to determine the appropriate ratio according to the gold-tellurium ore.

Follow the link below to get information on fire assay,

1 year ago

Hello Jorge, I am new to all of this and have some challenges regarding some gold bearing telluride ore. I wonder if you would have some insight as to how best to process. If it is not feasible to process myself perhaps you or someone else could help me find a reputable refiner who is versed in such processing? My results have been mixed and most of my buttons are coming out silverish, roasting seemed to help when done a certain way but I cant help but think my buttons likely are approximately half or even a third what they should be. Thanks for any insight.

1 year ago
Jorge 1 year ago
1 like by David

The presence of impurities in lead buttons usually influences adversely the accuracy of assaying. In general, the errors caused by impurities in cupellation may be either positive, due to retention of impurities in the bead, or negative, due to increased cupel absorption and to a slight extent to increased vaporization loss. Increased absorption and volatilization losses are usually due to the necessity for higher cupellation temperature to finish the process, but increased absorption of gold and silver may be caused by lowered surface tension of the button (as with tellurium and selenium), and increased volatilization loss may be caused by lowered vapor pressure of the precious metals or by mechanical losses if the impurity burns violently.

Elements below lead in the electromotive series, such as bismuth, copper, and tellurium, tend to become concentrated in the lead during cupellation and to be retained to a certain extent by the bead. Elements such as copper, nickel, tin, and zinc, whose oxides are infusible at cupellation temperature and are not greatly soluble in litharge, tend to form a crust over the cupelling bead and inhibit the driving. Even if the buttons can be finished without freezing, the higher cupellation temperature required to finish such buttons increases the loss of silver and gold.

Tellurium is extremely detrimental to the accurate recovery of gold and silver in cupellation, when present in amounts greater than 1% in lead buttons. Tellurium lower the surface tension of the lead button and allow some of the alloy to be absorbed by the cupel; the rest of the alloy may divide into separate particles, giving a number of individual beads. This effect is minimized by the use of fine textured hard-finished cupels. Comparatively small amounts of tellurium leave a pink stain on the hot cupel, which disappears on cooling. If more than 15% of tellurium is present in the bead, it is dull and frosted. Gold losses in the cupellation of buttons containing tellurium are much higher than with other impurities mentioned herein, which affect silver far more than gold.

The total loss of gold in the cupellation of buttons containing tellurium is decreased by the presence of silver; it is therefore advisable to add silver, if necessary, to ensure at least three times as much silver as gold being present, which ratio is necessary for parting in any event.

The concentration of tellurium in the lead button can be lowered by the use of an excess of litharge in the assay slag. In any event a certain fraction of the impurity present in the charge will be reduced and will contaminate the lead button. Hence, in assaying impure ores, large samples cause much more difficulty than do small samples. Occasionally a sample smaller than would ordinarily be used must be taken, in order to reduce the concentration of some impurity in the lead button. In some cases, the ratio of litharge to tellurium could be 500 to 1.

Other possible solution is to roast the material before cupelling. The roasting method consists of heating a weighed amount of the material to be assayed in a shallow fireclay dish in the presence of air in order to oxidize the metals present and to eliminate certain impurities. The impurities that can be eliminated by this oxidizing roast are carbon, sulfur, arsenic, antimony, and tellurium. After roasting, the roasted product (calcine) is treated in a crucible fusion in the same manner as an ore with no reducing power. Gradually raise the temperature to a maximum of 600 to 700 oC., stirring at 15 to 20 minutes. intervals, and hold at the final temperature for 25 to 30 min. Fumes could be a problem.

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