Copyright © 2005 by Ana Minerva Bonilla Updated: 9/8/06 17:07
THE WEEKEND MINER
Rediscovering American Mineral Resources
THOUGHTS FROM THE "OLD MINER"
Description of the Lead Fire Assay Process
I was surprised at the reaction to the last column on The Dominance of the Lead Fire Assay. We have our counter to set to identify the number of unique (individual computers that have logged on) visits vs. the normal "hits", which, as of this writing is almost 1,400. It seems an awful lot of you have questions as to just what a fire assay is and how it is done. As such, the timing for this column has been accelerated, since this is the first significant feedback we have received. I am not a certified assayer but I will try to explain it as best I can.
The first step of any test is getting a good and representative sample. This is one of the major issues with Fire Assay. Since the Fire Assay approach was developed in relatively ancient times, it was probably developed for use on gold and maybe silver, which is probably why it works better for them than it does for platinum group metals. Since the sample size is so small, an "Assay Ton" or 29.166 grams of ore is generally used, a good sampling technique is absolutely the key to getting a reasonable assay. Also, a single assay is probably not terribly meaningful, so a group of assays will probably be required for any commercial analysis. Curt Freeman of Avalon Development, reminds us of the South African saying of "think centimetric". This means a man walking normally could step right over the Merensky Reef in a single stride, which he cites as less than a meter wide. Yet, the Reef contains many billions of dollars in Platinum Group Metals. Also, with gold and PGMs, grain size can have a big impact on results, because of the small sample size used. A single large grain can cause a disproportionately high reading in the assay, or they may be totally lost in the splitting technique. With gold you can often actually see the gold, which helps in this instance. This is not as easy for PGMs and silver, since they look like a lot of other metals and are found in a much greater number of mineral forms. Gold usually either looks like gold or is found as a telluride (probably black) or as "electrum", as an alloy of silver. PGMs are found in over 200 different mineral forms, 40 to 50 of which are fairly common. Densities and colors also get you with PGMs, as one PGM-bearing mineral only has a specific gravity of 5.71, and the colors range from silver to pink to black to tan and a lot of others in between.
So start with a fairly large sample that is thought to be representative. A very important thing is to keep good records of where you got the sample; the surrounding mineralogy and the geology of the area the sample was taken from. This may prove to be important when trying to interpret the results. The sample has to be crushed or ground down to 100 mesh or less (I usually try to get it to about 200+ mesh). This may prove to be difficult if you have Native or elemental material in the sample. Precious metals are fairly ductile and may smear when ground. If you grind the sample check the apparatus for evidence of residual metal on your cutter or grinding surfaces. Once ground, carefully split the sample down until you get about a 30-gram sample. This is the "Assay Ton" cited above.
Add to this sample a mixture of "litharge" (lead oxide), a reducing agent or agents and your fluxes. This is where the real "art" of the assay comes in: the selection of the reducing agent(s) and the fluxes.
The reducing agent(s) is (are) typically carbon, in any form. The last part of this statement often leads to spirited discussions among assayers. Flour, just plain old wheat flour like you make bread from, is often used. Other assayers use purified carbon sources such as charcoal, often medicinal grade charcoals for their assays. The function of a reducing agent in any chemical reaction is pretty straight forward so I tend to be a generalist here. However, fluxes are totally a different matter. The choice of fluxes is probably the most complex part of an assay and a main reason you need to make the assayer aware of the surrounding mineralogy and geology of the sample location. The presence of chalcogenides (tellurides, selenides and sulfur), pictagenides (elements like bismuth, phosphorus, arsenic and antimony), copper, nickel, magnesium oxide, chromites and others will all influence the flux mixture. The impact of the flux and how it may be adjusted for suspected constituents will be the subject of my next column or two as this is too big of a subject to cover today.
This whole mixture is then placed into the furnace, usually a muffle-type. This process is called the "firing". Here the contents are melted, totally. The reducing agent causes the lead oxide to be smelted into lead, which "collects" the gold, silver and most of the PGMs that may have been in the sample. (By the way solder, used for electrical connections does the same thing. Don't sell it as junk lead or otherwise dispose of it, until you have it checked for precious. A lot of scrap dealers get rich because "gold, silver, platinum and palladium go into solder like sugar into hot coffee".) The molten mixture is removed and swirled or mixed to make sure everything has gone into the lead solution. It is then poured into a cone-shaped mold. Here the lead, carrying the precious metals, will sink to the bottom. The cone-shape is used to more clearly show the break point between the lead, or "button" and the slag, which will probably appear glassy. This glassy material, or slag, is generally the non-metallic elements in the ore and the flux materials.
After cooling this cone-shaped product is separated from the mold and the glassy slag is broken or removed from the metallic lead "button". Traditional wisdom is to discard the slag, however, I would suggest retaining and labeling it for reasons that will be identified in the column where we will talk about the impact of constituent mineralogy and geology.
This "button is placed into a cupel. A cupel was historically a small dish made of bone ash and clay, but may also be of other materials today. The cupel is designed to absorb lead, in its molten form, but not the precious metals present. The material in the cupel is placed into an "Assay" or "Cupellation" furnace and heated to 2,000 degrees Fahrenheit. The lead will turn back into an oxide, which then volatizes away from the precious metals and is absorbed into the cupel. Any precious metals present should be delivered as a small speck of metal, called the "bead", on the bottom of the cupel. The cupel is removed from the furnace and allowed to cool.
Once cooled, the bead is removed from the cupel and weighed. Historically this was done on a microbalance or the "best scales available at the time". That statement in itself should cause you to question some of the older, historical, fire assays. Today, very accurate, electronic scales are often used. This weight should give you a reading for the precious contained in the sample. If an "Assay Ton" sample size was used (29.166g), a milligram of material in the bead equals I troy ounce of precious in the ore.
If the bead is thought to contain only gold and silver a simple "parting" process is performed. The bead is placed in hot nitric acid. The silver is soluble in nitric acid, so the silver dissolves - or "parts"- leaving the gold. The remaining solids are then weighed to determine the amount of gold in the bead. If PGMs are present the situation becomes much more complex and the assayer has to go through a number of steps to identify each metal present. One method is to remove the lead and dissolve the entire bead in aqua regia. The solution is analyzed by atomic absorption spectrometry or other method to allow the precious content to be calculated.
As you can tell accuracy in the sampling, preparation, handling and weighing is very important. Even a small error will have drastic impact on the results - when you are looking for amounts in the milligram range.
Well, that's it. Not a real scientific description, but it hopefully gives you an idea of what is going on when you send your samples out to be assayed. Next column we will cover the influence of some of the potential minerals and elements in the ore that may cause you problems and some methods of dealing with them. Until then, remember - be careful out there!