Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
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Dore Weight Discrepancy (9 replies)

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

What can cause weight discrepancy in a Dore bar (Gold alloy) between the initial value (after refining and quenching) and the value after shipping from 312 m elevation to 218 m elevation.

Discrepancy is about 3.5 g out of 24 kg. Sources considered:

Gravity which would cause weigh reduction since g = GM/r^2 (Should not cause this much discrepancy)
Gold alloy tarnishing, stress corrosion, stress cracking, formation of silver sulphide (Unlikely since gold and silver have high electrode potential, corrosion unlikely to happen in short period of time)
Balance calibration. The balances were checked, calibrated; they're working great
Since gold and silver have high thermal conductivity, it is possible that the temperature difference is causing the discrepancy, by converting energy into mass (quantum particle exchange)
Pressure difference between two locations, which might cause vapour to condense and build in cracks (When going at higher elevation "lower pressure"). Or maybe moisture absorption (When going at lower elevation "higher pressure")

Please correct me if I am wrong in any of these assumptions (elaborate) and comment on possible causes of the discrepancy.

1 year ago

If the bars contain blow holes or irregularities, ensure that they are properly dried in an oven before weighing.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

If I understand well you are using two different balances. Maybe this difference (0.015% relative) is only due to measurement uncertainty. Is your discrepancy resulting from statistical study (mean value of differences) or just coming from one measurement?
How the 24 kg have been weighed? Which kinds of balance have been used?

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

I will check if micro-porosity or casting defect is a factor.
Yes, two different scales are being used. The scale at the first location is Mettler Toledo XP64001l. Both scales were calibrated with 25 kg OIML Class F1 check weight. In addition, repeatability tests as well as eccentricity tests was done to confirm the accuracy of the scales.

The discrepancy result is a result of a long term study to give 10 g discrepancy of about 69.8 kg (weighted average) I should also clarify that:
Value at 218 m (elevation) > Value at 312 m (elevation) So the weight is actually increasing from the original value.

Sandeep Bisht
1 year ago
Sandeep Bisht 1 year ago

A few pointers:

per earlier suggestion, heat the bars at -100C for several hours; cool to AMBIENT & then weigh on a calibrated scale; you might want to include the same calibration weight(s) in the shipment to lower elevations for crosscheck against the secondary balance;
Ensure that there are no slag particles in crevices that might dislodge during shipment &cause the weight aberration;
The elevation difference of 300-ft will create a weight difference of -0.7-g for a 24-kg bar.

1 year ago
Gruppen 1 year ago

I never had this problem with high gold content Dore, but with high silver content dore, I observed this slight weight gain with time and believe it may be due to the silver absorbing oxygen from the air (or perhaps sulphur from the traces of H2S commonly found in the air). Heating as suggested would probably drive off the oxygen and sulphur that was absorbed, but you would very likely need to heat well above 100C.

1 year ago

It happened to me several times and it was because of the difference of temperature. Make sure the bars are naturally cold before weight them. It can take you one day, depending on the size of the bars.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

The bar dryness and porosity go hand in hand more so when you have a lower purity bar sometimes a bit of a matt layer even- moisture gets trapped there. Then again 3 grams in 24000 Is that accuracy necessary?

You can Vacuum pack your bar- that will solve all the moisture issues as that will evaporate off compensate for the 3 grams in the weight declaration?
Atmospheric pressure may also play a roll. But this can be calculated.

Alan Carter
1 year ago
Alan Carter 1 year ago

Remember the weight you ship is used for insurance purpose and for initial payment from the refinery. The refinery will provide you with the weight and assay. You can assay a duplicate pin to obtain a settled assay of the bar.

I would concentrate on other plant issues than this.

1 year ago
Oberfuhrer 1 year ago

This difference is caused by encrusted water in small pores, look at the appearance of the bars Dore, and if these have too many pores, you must first remove the water with a compressor will come out the biggest drops and if the bar has cracks, it is best heated in an oven at 150 degrees Celsius, then let the bar is cold and dry, measure the relative humidity of the place is doing the weighing.

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