Silver determination of Gold Bullion

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fgorduna

New member
Joined
Jul 27, 2021
Messages
1
We employ cupellation for gold and silver determination where:
for gold - we add 2.75 as much silver to 0.49 - 0.50 g samples then part the silver from resulting bead
for silver - separate proof (0.5g * pre-assay + balance silver); no silver added to samples

For the gold part, our issue is how the surcharge/correction factor is computed:
TRIAL 1
99.98% Wt. In Wt. Bk Diff. Average
STD-1 502107 503506 1500 1490
STD-2 497390 498862 1572
STD-3 497731 499100 1469
STD-4 498649 499969 1420
The average difference is then subtracted from the weigh back of the samples to be able to get the % Au:
TRIAL 1
Sample Wt. IN Wt. BK Assay %Au
07152101 497108 492889 98.85 98.84
498678 494392 98.84
07152102 499527 499091 99.61 99.60
500033 499543 99.60
07152103 499533 500011 99.80 99.80
496176 496707 99.81
Is this the correct way of using the surcharge/correction factor?

For the silver part, we have an issue with the cupellation part wherein the weigh back for samples with high purity gold (>99%) are higher than the weigh in. Our parameters are 1050C with 1-2 minutes opening phase, 25 minutes driving phase (T=980C), and 1-3 minutes finishing phase (T=1050C) using a Carbolite Gero CF 60 furnace. For lower grade samples we are able to get good results but for high grade samples we always get a higher weigh back. What seems to be the issue here? BTW, we are using mabor magnesia 14HB cupels.
Silver is computed from the difference of total precious metal (dore) weight and the gold from the gold part.
 
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