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Gold/Mining/Energy : Gold Price Monitor -- Ignore unavailable to you. Want to Upgrade?

To: IngotWeTrust who wrote (74355)	8/2/2001 10:33:25 AM
From: E. Charters	Read Replies (1) \| Respond to of 116815

Mossbauer AAS was invented for gold assaying by the boys in Noranda PQ. It had about 25 papers of note by them and other practical investigators by the time I had fallen afoul of it and had to run 10,000 "test" assays through one unfortunate mine's lab. (Early 80's) These following difficulties have not yet been solved and a couple of them are not well known by the AA--hole chemical pundits of br. phd extraction. 1. The iron absorption curve for gold I is larger and "overamps" the gold curve and can mask it. New Cinch. Basically the large iron curve make gold a blip on its shoulder. Area calcs below the curve are therefore fraught with error in the low-gold high iron regions. There are other metals too if present that can mask gold in this way. Lead interferes too. 2. In the flame, the metals that can interfere with gold's light absorption and mask it are manifold. Calcium and other metals have as yet unknown quantitatively but observed "matrix" interference effects in varying ways. Mixes of present metals have also not logged or preventable total effects. This is very difficult to control for. Polarized Light Zeeman Spec methods may have some hope here or using emission as plasma arc may lessen this. In fact if you use emission and can calibrate well you might eliminate it entirely. It takes deep research and I don't see the papers yet. I know a lot of people are using DC plasma arc. Expensive setup and it may not cure some other dissolution problem we shall see. 3. Ketone or other organic double stage or single stage extraction is necessary from the aqua regia. Straight acid aspiration is a bad idea. Ketone has problems of its own. For one thing the ketone "eats up" differentially in different Ph samples that reflect the different times of dissolution and components of the sample. So you always end up with varying Ph samples creating varying volumetric reagant and this make things all over the map. You can even "burn" ketone with hot samples with unpredictable results as well. 4. Co precipitation, co dissolutiuon and sequential dissolution of elements in the samples, once acidulated lead to uncertain times for each sample to a complete dissolution. As well, the hydronium ion gold-grabber from king water has a life of 15 minutes. If some gold releasing iron and other silicate combos are not broken down for 1.5 to 3 hours in hot acid, it may well be that addition of aqua regia at late stage (be careful! it could cost you your life if you do not remove the beakers from the hote plate and do this with extreme care!)is necessary to get the gold. 20 minutes cannot be enough time to dissolve AU when it is the most insoluble of elements, unless there is very little calcium, iron or other stuff that eats acid, in the ore. 5. In addition to above the calibration sample must be the same Ph as the ore samples at finish. They won't be. They should also have a mix of all interfering elements for each sample grade. This make about 30 calibration samples for each different type or ore at each mine. You won't get it right. They must also stagger the ore grades from .02 to 1.0 ounces for ever .03 ounces to make a good curve. This curve is actually not one-inflection well-behaved. Near 0.02 in most machines it is vertical, like above one ounce. How could this happen? I don't know but I think it is in the sensitivity of the gold lamp and the detetectors. Anyway do about 500 assays with carefully staggered large and small samples near the detection limit and you will discover low is high not very low. (How you discover this is worth money) (There is also an elementary stat/math reason for this as well.) Near the detection limit there is a tendency for machine to err low rather than high. It is trying to build an average in a "flicker" situation. It will therefore average low. 6. Grind, low grade, and sample size are critical factors trying to get mine averages right. Most get that wrong. Sample mixing and splitting is also critical. AA samples tend to be small (ten grams) and poorly calibrated. They are very poor arbiters above one ounce and below .02. 7. Ketone aspiration is quite often halted by fine quartz which collects in the reagant. This can be solved in part by using wash of hydrofluoric. Be careful. Bad stuff. It also cause problems for Ph and acid eat up. 8. As you dissolve longer you may run into production of a not tested for gold species (II or III) in an oxidizing environment. This must be controlled for. Problem areas. 1. sample size. You need 30 grams for ores. Perhaps 60 for tails. Handling is a problem, dissolution times etc.. 2. calibration difficulties - curve staggering. 3. aspiration problems, clogging. 4. dissolution times 20 minutes not enough. Must add acid. 5. matrix effects - interference in absorption. NOe easy solution. 6. PH changes for different ores. (Most gold mines have several) 7. Volumetric reagant problems. Acid eatup. see 6. 8. oxidizing/reducing gold specie problems with necessary dissolution times. must stay reducing. 9. Near 0 curve changes, see calibration. Tails assays inaccurate. 10. Flame variances with air pressure and type of propane. 11. Absorption of light by aging lamp, with sputtering variances of flame and lamp. The whole AAS method is no more "atomic" or fundamentally accurate than fusion, or fire assay. It is also far less mine and mill calibrated. All assay methods are atomic. But only fire ends up with a recovery of the element and is therefore positive. Thus, its mimicry of gold mining and recovery has a high degree of parallelism, and its positivity cannot be exceeded. Nor can its accuracy be improved on as it is subject to far fewer interferences chemically, process wise, and physically. It gets 90% of the gold most of the time. AA gets only a small portion of the target element comparitively, and has about 5 or more collection and process comparision stages that are fraught with machine error, sample treatment variance error, and aging error of gold lamps as well. AA is the comparision of many complex elements. Fire assay is the recovery of gold that compares very well with what a mine does. It is a fraud to say AA or any other chemical extraction and non-positive analysis technique is more accurate than fire assay. That is as ludicrous as saying the metric system of measurement is more accurate or decimal number bases are better or count numbers or fractions more accurately. All such systems are eually arbitrary and not one exceeds the other in any kind of fundamental accuracy. But when it comes to AA and fire assay there is a very fundamental difference. In 1700 the most accurate method of chemical analysis that would exist for the next 300 years was gold fire assaying. Gold fire assaying, then as now, routinely ACCURATELY measure quantities of a substance to parts per million to 90% accuracy. No method of chemical analysis would attempt to get within 50% accuracy with 100 times that concentration for anything for 200 years. And they have yet to prove accuracy is better than 50% yet for most atomic methods as chemical analysis on small concentrations. Gold assaying by fire, if done carefully, is simply the most accurate form of determination on the planet at the lowest concentration that has every been devised. That is why they call it assaying. AAS is SEMI-Quantitive COMPARITIVE chemical analysis. It is not assaying. Assaying is the QUANTITATIVE POSITIVE determination of the AMOUNT of elements CONTAINED to a certain accuracy. Fire assaying is a 100% EXTRACTIVE technique, not in most cases, a COMPARATIVE method. In the end philosophically I will admit that ALL methods are comparitive. But only fire assay attempts to be completely extractive. Nothing can be more accurate than taking the gold out of the sample in its entirety and weighing it. Burning a partial extraction and eyeing its molecules for a few seconds and making an extrapolation does not compare. EC<:-}