This topic needs to be addressed because we often ask the question "What is the assay?" and all assayers hope that the sample is representative of the entire lot. So it is imperative to do a good job sampling.
It is my hope that this thread can become an informative collaboration from some of our members to help both to explain some of the science behind the sampling and detail the different methods to effectively sample as many different scrap types as we can.
This thread grew out of some comments made on a thread about gathering data from yields into a database. If there is any chance that such a database can be successful, it will be necessary to assure ourselves that the materials are being sampled properly.
I started this by re reading an old text named The Sampling and Assay of the Precious Metals by E.A. Smith. Written in 1913, second edition in 1947 and reprinted in 1987 by Met-Chem Research.
I mentioned this in the previous thread as did Chris (GSP) and had to chuckle when I read this in the textbook.
One of the nice things about a fire assay and even an AA analysis is they are done in duplicate. For valuable lots I prefer running samples in triplicate. What this does for the assayer is it assures him or her that the sample is homogeneous if there is good agreement between the different duplicates of the same sample lot.
To put this all into some perspective to get us all on the same page, let's start off by discussing an easy material. Jewelers sweeps. Generally processed and blended into samples which are representative of anywhere from a 5 gallon bucket up to 5 full 55 gallon drums of material. Based on high grade sweeps that I have assayed, I preferred to use a 5 gram sample of the powder. A multiple drum blend of the prepared sweeps can represent as many as 2000 ounces of gold. So this lot would be run in triplicate. So every individual crucible would be loaded with 5 grams of sample and it would be done three times. Or to put this differently, every 5 gram fraction that i chose out of the 1 ounce package of powdered sample that I received for that lot had to be very very close to each other and if all samples are within close agreement, the sample is considered representative and the assay result is used. To take this analogy 1 step further, every individual ounce of the material in the entire lot has to be essentially identical to every other ounce.
So how is that accomplished?
The material has been incinerated and ball milled and passed through a screen so the largest particles are -40 mesh. I always sifted to -60 mesh because my in house assays were in better agreement for settlements with individual customers. For settling large multiple drums blended together to be sold by a primary refiner, I always liked the grab sample taken after the blend with a sample thief to be further milled in a puck mill to -80 mesh.
View attachment 34958
The material I brought for sampling was blended in a large double cone blender which effectively mixes the contents of all 5 drums into a powder that is thoroughly blended and homogenous. This material is returned to empty drums by dumping it from the blender filling one connected drum at a time until the contents are back in the drums. Once back in the drums a thief sampler is used to draw a sample from multiple locations in each drum. The sampler is inserted into the powder and pushed to the bottom of the drum. Then the sampler is opened up by twisting the inner sleeve, and powder will flow into the hollow tube from every level of the drum full of powder. The sampler is twisted closed and removed from the drum and what is inside is a sample of every level in the drum, top to bottom. This sample is dumped out of the sampler and the process is repeated 2 or 3 times per drum. All of these samples are mixed together in the same bucket and taken to the lab to prepare.
in the lab, the sample is either spread on a tray, leveled and quartered and manually mixed by an age old process known as cone and quartering. Another quicker option is to pass the powder through a splitter which divides all of the powder into equal portions. Which ever method is used, the purpose is to break down a large sample into more manageable samples without producing a systematic bias. The smaller lot now weighs about a pound and it should be the same as any other pound in the lot being sampled.
View attachment 34959
The last thing that is done is the pound sample is sifted through a -80 mesh screen. Most of the powder is small enough to pass through the screen already, remember every screen classifies the material and holds back the bigger materials passing the smaller through. Whatever does not pass through is pulverized in a puck mill. The goal is to have all of the material pass through the -80 mesh screen.
Particle size is very important in producing a homogeneous powder sample for precious metal assay. For prepared jewelers sweeps in this example I have found the agreement between assays is much closer with smaller particle size. A lot of these ratio's of particle size to sample weights were worked out for mining ores and successful sampling and assaying. And most of the work was done in the 1800's.
With high grade polishing sweeps the results at the final weighing can vary by 1 milligram and the results are considered an acceptable standard deviation.
But nobody is going to take down a mountain and grind it into dust just to assay to see if it is worth processing. So over the years standards have evolved using larger particle sizes and allowing for greater standard deviations because the larger the particle the more probability that a single high yield particle will skew the entire sample higher. For the high yield powders we eliminate that possibility be crushing it significantly small enough that every fraction of a gram contains 42 begillion particles (translation = very very many) so a lot of small and evenly dispersed values are much less likely to cause a high standard deviation. We can apply those allowable deviations from hands on practical experience to develop sampling protocols for many different materials.
It is my hope that this thread can become an informative collaboration from some of our members to help both to explain some of the science behind the sampling and detail the different methods to effectively sample as many different scrap types as we can.
This thread grew out of some comments made on a thread about gathering data from yields into a database. If there is any chance that such a database can be successful, it will be necessary to assure ourselves that the materials are being sampled properly.
I started this by re reading an old text named The Sampling and Assay of the Precious Metals by E.A. Smith. Written in 1913, second edition in 1947 and reprinted in 1987 by Met-Chem Research.
I mentioned this in the previous thread as did Chris (GSP) and had to chuckle when I read this in the textbook.
E.A. Smith in 1913 said:
One of the nice things about a fire assay and even an AA analysis is they are done in duplicate. For valuable lots I prefer running samples in triplicate. What this does for the assayer is it assures him or her that the sample is homogeneous if there is good agreement between the different duplicates of the same sample lot.
To put this all into some perspective to get us all on the same page, let's start off by discussing an easy material. Jewelers sweeps. Generally processed and blended into samples which are representative of anywhere from a 5 gallon bucket up to 5 full 55 gallon drums of material. Based on high grade sweeps that I have assayed, I preferred to use a 5 gram sample of the powder. A multiple drum blend of the prepared sweeps can represent as many as 2000 ounces of gold. So this lot would be run in triplicate. So every individual crucible would be loaded with 5 grams of sample and it would be done three times. Or to put this differently, every 5 gram fraction that i chose out of the 1 ounce package of powdered sample that I received for that lot had to be very very close to each other and if all samples are within close agreement, the sample is considered representative and the assay result is used. To take this analogy 1 step further, every individual ounce of the material in the entire lot has to be essentially identical to every other ounce.
So how is that accomplished?
The material has been incinerated and ball milled and passed through a screen so the largest particles are -40 mesh. I always sifted to -60 mesh because my in house assays were in better agreement for settlements with individual customers. For settling large multiple drums blended together to be sold by a primary refiner, I always liked the grab sample taken after the blend with a sample thief to be further milled in a puck mill to -80 mesh.
View attachment 34958
The material I brought for sampling was blended in a large double cone blender which effectively mixes the contents of all 5 drums into a powder that is thoroughly blended and homogenous. This material is returned to empty drums by dumping it from the blender filling one connected drum at a time until the contents are back in the drums. Once back in the drums a thief sampler is used to draw a sample from multiple locations in each drum. The sampler is inserted into the powder and pushed to the bottom of the drum. Then the sampler is opened up by twisting the inner sleeve, and powder will flow into the hollow tube from every level of the drum full of powder. The sampler is twisted closed and removed from the drum and what is inside is a sample of every level in the drum, top to bottom. This sample is dumped out of the sampler and the process is repeated 2 or 3 times per drum. All of these samples are mixed together in the same bucket and taken to the lab to prepare.
in the lab, the sample is either spread on a tray, leveled and quartered and manually mixed by an age old process known as cone and quartering. Another quicker option is to pass the powder through a splitter which divides all of the powder into equal portions. Which ever method is used, the purpose is to break down a large sample into more manageable samples without producing a systematic bias. The smaller lot now weighs about a pound and it should be the same as any other pound in the lot being sampled.
View attachment 34959
The last thing that is done is the pound sample is sifted through a -80 mesh screen. Most of the powder is small enough to pass through the screen already, remember every screen classifies the material and holds back the bigger materials passing the smaller through. Whatever does not pass through is pulverized in a puck mill. The goal is to have all of the material pass through the -80 mesh screen.
Particle size is very important in producing a homogeneous powder sample for precious metal assay. For prepared jewelers sweeps in this example I have found the agreement between assays is much closer with smaller particle size. A lot of these ratio's of particle size to sample weights were worked out for mining ores and successful sampling and assaying. And most of the work was done in the 1800's.
With high grade polishing sweeps the results at the final weighing can vary by 1 milligram and the results are considered an acceptable standard deviation.
But nobody is going to take down a mountain and grind it into dust just to assay to see if it is worth processing. So over the years standards have evolved using larger particle sizes and allowing for greater standard deviations because the larger the particle the more probability that a single high yield particle will skew the entire sample higher. For the high yield powders we eliminate that possibility be crushing it significantly small enough that every fraction of a gram contains 42 begillion particles (translation = very very many) so a lot of small and evenly dispersed values are much less likely to cause a high standard deviation. We can apply those allowable deviations from hands on practical experience to develop sampling protocols for many different materials.
