Hello Folks,
January 19, 1998
In my last post I presented some information to you about DD assays. Today I am going to address three ingredients in the assay that I find to be contaminated more than any of the other fire assay reagents. This gets into actual process information that is not considered private by any of my clients and is therefore being shared with all of you. Dr. Al Holston of the Saskatchewan Research Council in Canada first introduced the filtration part of this process to me. He is the lab supervisor. The lab is at Saskatoon University in Saskatchewan.
Silver, lead and litharge are common ingredients to be contaminated with gold. Failure to correct an assay or be aware of these potential givers of gold to a total reduction fusion fire assay is a terrible thing and has caused more than one prospector to chase ghosts that came from his assay ingredients. The method for analysis of these metals has been conventionally done. All reputable assayers use ingredients that are purchased as pure. So be it for that one. Now to test these ingredients, they all must be in the form of elemental lead or silver. Litharge which is PBO and is made from Pb3O4, as I understand the process, is a component that produces lead metal in the assay. Pb3O4 is not litharge, but is red lead, the plumbic lead form. If used to substitute for litharge in the assay, it should also be tested. Both of the lead oxides have to be reduced to metal to be tested. However, in recent years I have not found the litharge and red lead to have enough contamination to discuss. The test lead I use is of a high purity also, but I have batches of test lead around my lab that fail the purity test miserably. The silver is another story. You purchase high purity silver and you expect just that.
In ores where PGMs are expected and also with cluster resources, large amounts of silver are used in the assays as collectors. As a rule of thumb, 20 times as much silver is used as the amount of platinum or cluster metal expected to be recovered in the fire assay. A common silver addition, inquart, is 1 gram. Boy, that is a lot of silver for a 29.1 gram sample. The term "lot of silver" is amplified when considering that Murray White and I usually use only .004 grams (4 mg) of silver in most of our assays. That is 1/250th the amount used by some assayers for PGM samples. Well, the label has already told us these metals are pure. So who cares! Fact is, I do. The common test.
If you dissolve the silver (parting the silver from the insoluable gold) in nitric or sulfuric acid, no residue will remain. Surely the silver is pure. Or is it?
Take a gram or two of pure silver. Dissolve it in hot nitric acid, about 11%. Do this in a white porcelain-parting cup. After dissolution, study the bottom of the cup. Is there any blackish or blackish brown residue that would indicate gold? Study it well. That is why you used a porcelain-parting cup. It is the white background that helps to see the traces of residue metals. Go to the AA or ICP (Induced Coupled Plasma) for an analysis of the acid. Still no gold higher than maybe 2 parts per billion is reported, insignificant. The silver has truly met the acid test. My test.
Now dilute the acid with about 300 ml of distilled water. Filter the acid through a 1-micron cellulose Millipore filter.
Dry the filter membrane and study it under a microscope. If your silver is impure with gold, some microscopic black bacteria looking filaments will appear on the membrane. I use 200 power for this obversation. Digest the filter membrane and its contents in Aqua Regia acid. Do an ICP analysis of the acid for gold and, oh my goodness, where did that gold come from? Now repeat the procedure with a .1 micron filter membrane. With some silver samples even more gold will appear. Sometimes much more than with the 1 micron filter. Assay that too. Remember we started with pure silver. There should be no interference that will give phony results to an ICP analysis. If the ICP is ever to be believed, it is with this test. When you do this test you can find contamination with gold in the assay silver so high as to impart many ounces of gold per ton into your assays. Usually it is quite low, but still it is common to impart substantial values when assaying using 1 gram silver inquarts.
In doing this test, please remember we never were able to get a filtration film porosity so low as to filter the largest known microclusters of gold. Still, even micro-chunks of gold failed to report accurately by the conventional assays. Another method of analyzing the filter paper is to wash it in distilled water while still in the filtering device. Follow up with another test. Fire assay of the cellulose. You need no inquarts. Usually the ratios of gold to silver will be adequate enough in favor of silver to do a reasonably accurate fire assay.
If you follow the rules of inquartia and part silver correctly, the filter cellulose will show none of the bacteria looking gold and only a few micro-specks will have missed the analysis. I don't know about the true micro-cluster gold because the gold we are referring to here is colloidal.
You can test lead by the same procedure. I do not know a procedure to test assay metals for visual non-detectable PGMs. Any ideas out there? mike |
