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Handheld XRF for Gold (6 replies)
Gold is a problem even in concentrates and especially if it occurs as coarse particles. I tested a unit a few years ago on concentrate (50 to 150 ppm Au) and there was a very poor correlation between the estimate from the unit and a true fire assay result. Multiple readings were more scattered than fire assay results and the averages were in poor agreement as well. While I have heard of these units being used in processing plants as an alternative to true assays, I don't think think this is appropriate for gold. I also tried it with arsenic and antimony which showed similarly poor correlation against more conventional assay techniques. These units are much more suitable for exploration where semi-quantitative results are acceptable. Just my own personal opinion.
Samples in the range of 80% passing 40-50 microns which should have been fine enough. There was very little free coarse gold. The vast majority is in solid solution in pyrite so homogeneity should not have been an issue. There was high arsenic however.
I have been very interested in XRF and the potential to use it for gold assaying for a long time. Unfortunately the detection limit is too high for most practical applications. Part of it has to do with the size of the gold atom, and the amount of energy that it takes to knock an inner shell electron free, and the other part, is as you described, interference from other elements. Another issue relates to the nugget nature of gold and the spherical attenuation of X-Ray radiation --a small amount of gold nearby has the same response as a much larger amount of gold only a short distance further away.
Your comment about the size of the Gold atom is certainly interesting, I wasn't aware that was a factor. You make an excellent point about the exponential decay of the X-Ray radiation with distance. Attenuation is certainly going to be an issue with non-homogeneous or complex matrix ores. Have you or your team had any successes assaying doré bars or concentrated eluate solutions with confidence?
I have been interested in use of XRF for a long time, but my interest has largely been in potential use for assaying of RC cuttings, drill core, rock faces, and perhaps for grade control. I have heard mention of its use for assaying gold bars, but can't comment specifically on it. Also, I believe that there may be potential use for assaying loaded carbon.
Although XRF has limited uses in exploration and mine geology for gold values, it is rapidly becoming an accepted tool for other elements, and can be really useful in helping to distinguish between lithologic units in otherwise very similar looking altered rock.
They are going to do X-ray diffraction as well as XRF. It also mentions that there will be a sample preparation unit at the rig which will dry and prepare samples. We know that XRF works best if it is analyzing prepared samples of powdered material, compressed into little uniform flat pellets. That takes some time to do in a prep lab. In that form the effective detection limit decreases, also, a desktop unit can be more effective that the small handheld XRF. I'd be surprised if the lower detection limit for gold was going to be lower than 5 ppm. It could be really good for a series of other elements such as copper, silver, or nickel. It might also be highly effective in aiding in identifying litho units based on trace elements.
Portable XRF use with Coarse Gold: We recently bought a handheld XRF unit for our exploration geologists. I want to use it in the mill when it's not being used for exploration and I want to see how others have utilized them in processing plants. The XRF salerep noted that the detection limits for Au were typically around 5 ppm, rendering them less useful for most feed and tail samples but still useful in concentrate and stripping circuits. Is anyone willing to share their experiences with similar handheld or portable XRF devices, pulverizing samples or their use in gravity circuits and gold rooms? Of importance is pulverizing the solids as a means to improve homogeneity?
I wonder if the poor correlations were due to complex, heterogeneous samples or the presence of similar elements? Lead and Arsenic have similar excitation spectra which can confuse the sensor leading to under-reporting gold content. I have heard that there are different algorithms on the device that are targeted specifically for different concentrations, perhaps another avenue to explore but the precision issue still remains.