Rock sample analysis for minerals by XRF and flame AAS.
In 2008 I worked at Amdel mineral laboratories in Mount Isa testing rock samples. I learned a lot about processing and multitasking.
The laboratory served local mining operations, the primary mineral in Mount Isa being copper. The zinc mine was somewhat smaller and yielded valuable quantities of silver and lead. There was exploration work for a prospective uranium mine and I was involved in the samples for this customer. This service allowed the mines to follow an ore body and decide where next to blast. In some cases, the customer was waiting only for our results, so timely turn around was important. Some customers paid extra for faster results.
High throughput analysis of rocks for minerals was carried out by X-ray fluorescence spectroscopy (XRF) and flame atomic absorption spectroscopy (flame AAS) involved the following steps:
Collect and label rocks
Profile each job
Grind and mill samples
Drying and weighing of samples
Produce of fused beads for XRF analysis
Running samples through XRF
Digestion of samples for AAS analysis
Run samples through AAS
Data collection and quality control
Making pressed powder samples to troubleshoot (sometimes)
Report results to client
File away completed samples
These tasks were carried by teams of workers on 12 hour shifts, with both the milling shed and lab running 24 hour per day.
Collect and label rocks Samples were delivered to the site during the day on the back of a ute. A yard master would unload them with a forklift and give a receipt to the driver. As new jobs arrived they were written on a job board in the lab with their time of arrival. Each sample was a rock of approximately 3-4Kg, wrapped in a hemp bag and tagged by the mine with a serial number in sequential order. It was noted when a sample or tag was found to be absent.
Profile each job Each job came from a specific mine. The origin would be noted and a plan to test each rock, along with a number of standards was drawn up. A job could have anywhere from 1-250 samples, but typically, they contained 80-200. Profiles were created using spreadsheet software with quality checks inserted into the sequence including standards, blanks (no sample), and repeats of earlier samples in the same job. This allowed a check to see that concentrations were precise, samples had not been measured out of sequence and that a zero was possible. Each sample had a printed envelope created for deposit of a fine powder.
Grind and mill samples Each sample was placed into a drying oven which was large enough to walk into and looked like a bank safe. They typically needed 4-8 hours to dry but rocks from some mines became known to be more hygroscopic than others and required 12-14 hours. Once dried, the samples were placed into a grinder where they were
crushed into small pebbles. They were then milled into a fine powder.
A very small particle size powder was required to ensure easy
digestion and fusion in the laboratory. Once milled the powders were transferred to envelopes and labeled boxes then brought to the laboratory Preparation shed workers wore masks and
dust coats and adhered to strict safety conditions. All staff underwent
blood lead tests every 3 months.
Drying and weighing of samples Boxes of powder sample envelopes were placed into a small oven for further drying. Samples had to be dry before weighing. Flux and standard samples were stored in desiccators with fresh desiccant in the weigh room ans staff sat at specially designed chairs in front of balances. Here each sample was weighed twice at; Into disposable glass test tubes for AAS digest and into disposable plastic vials for XRF fusion. The sample masses were weighed and the masses saved onto a computer system using “CCLASS” software. The tubes and vials were labeled systematically so that they could be understood when other staff took over part way through a job. A common task for a new employee was to sit and weigh flux into 5.0000g portions for future use.
Molten flux poured from a platinum crucible into a platinum mould
Produce fused beads for XRF analysis Fused beads were the most efficient method of sample throughput. XRF analysis is slow and expensive so it was important to minimize instrument downtime. Fused beads were made by addition of a flux made up of a silica powder that contained lithium bromide and some lithium borides to lower the melting temperature. 5.0000 grams of flux was added to each sample and these were taken to the fusion room. Typically, there was one staff member in the fusion room whose job was to keep the platinum-ware clean and dry, and to ensure the correct labeling of beads. As some customers paid for faster results, it was important to begin work on the most urgent job first. This sometimes meant putting one job on hold and starting work on another. Samples were shaken and poured into a series of dried platinum crucibles. Stickers were made up and kept in a safe place to keep track of each sample. In iron racks, the crucibles were placed into a 1000 °C oven for 15 min. Using tongs and gloves (sometimes 2 pairs of gloves), the samples were transferred from the drying racks into the cup holders on an auto fusing machine. Two of these machines were operated simultaneously which each made 6 fused beads at a time. Each run took 15 min, in which the crucibles were heated, agitated, poured onto platinum moulds (also heated), and allowed to cool and set (see image). If a bead had bubbling or a crack, it may have to be remade. The cup holders and other fittings on the fusion machine required a lot of attention, as heating changed their shape. Tools including pliers, allen keys, spare parts and screwdrivers were kept on hand and used as required. The fusion machines ran on air and natural gas, which sometimes needed to be replenished. Another task at this station was to clean the platinum-ware in hot sonic baths containing citric acid. They were then dried and sometimes re-straightened. As beads were produced successfully, they were labeled with a sticker, removed from their mold, placed into labeled plastic jars and taken to the XRF room. This task was a continuous process and the employee would travel between a wet lab for cleaning and the fusion room for bead making which had a dedicated air conditioner.
Running samples through XRF As newly made fused beads arrived at the XRF room, they were placed in order for analysis. Priority jobs were pushed ahead, which was achieved by reprogramming the computer. As most samples were fused beads, they could be systematically placed into steel chambers for continual analysis. XRF analysis is carried out under very low pressure so the samples were each blown dry and checked for cracks and debris by a staff member as dust or dirt could block the vacuum chamber of the instrument. Periodic maintenance of the vacuum chamber was normal, but became more frequent with careless sampling of dirty/dusty beads. Samples added to the instruments for analysis were manually added to the computer by entering sample number, job number, name of technician, type of sample (fused bead/pressed powder/pressed powder for Uranium) and mass of sample. The technician had to be aware of the timing of both instruments and know when to open the door to place samples. There was no risk of X-ray exposure as a robotic arm carried the sample containers to the X-ray source, however the robotic arm could become jammed if the door was opened at the wrong time. The sample containers were made of stainless steel and had lids with a small exposure hole for fused beads and a larger one for pressed powders. Lids had to be screwed on properly, or the containers would not fit through the vacuum chamber port. Each sample took 4-5 min for analysis. It was possible to program the next 2 hours of samples by placing 2 dozen or more samples into each instrument and marking them for analysis. Quick checks from time to time were then all that was needed and the person could go on with other tasks. Troubleshooting was nearly always a matter of taking apart and cleaning the vacuum chamber which typically took 30-40 min. In this laboratory there were two XRF instruments, both manufactured by Rigaku. The other competitor in the sale of XRF instruments is Phillips. Both companies are based in Japan and each instrument has its advantages and disadvantages. The Phillips models require less frequent cleaning of the vacuum chamber (about once every 3 months, instead of once every 2 weeks), but they are more expensive than the Rigakus and taking apart their vacuum chamber is more tedious. The Rigaku models have an easily accessible vacuum chamber which can be taken apart with minimal training.
Digestion of samples for AAS analysis As useful as XRF analysis is in testing for minerals, it was necessary to test the samples simultaneously with a different technique to catch if and when a sample became out of order. Flame AAS testing for silver concentration was a convenient quality assurance test. Before analysis on this instrument, samples needed to be oxidized in acid. After being weighed, they were transferred to the wet lab for an acid digestion treatment with. This involved addition of perchloric acid, exposure to heat for 1.5 h, cooling, addition of hydrochloric acid, more heat, more cooling, then addition of reverse osmosis water up to a volume of 15 mL. The racks were mixed, transferred to plastic fixtures and allowed to sit for 4 h to let the silica drift to the bottom. This was important to prevent silica from blocking the nebulizer tube of the flame AAS. The disposal of acids from this process was a challenge in a desert town. A 1000 L intermediate bulk container (IBC) was placed outside the lab connected to a dedicated sink inside. This IBC was fitted with a pH meter and NaOH solution pump to neutralize the acid for disposal.
Run samples through AAS The instrument, while capable of use with a nitrous oxide flame, was in this case operated using an acetylene/air mixture. It was calibrated and then the burner run for 5 min to warm up before use. Therefore it was preferred to carry out several jobs sequentially to conserve gases. The computer and software were started first, followed by turning on the taps on the gas bottles (located outside the building) and checking their gas levels. The inside gas taps were then turned on and the fume hood activated. If the burner was dirty it was be cleaned by placing in a citric acid bath for 2 h (after removal of rubber o-ring). It was then dried and residual solids removed from it using a business card. The emission lamp was turned on using the software. A calibration card was used to ensure that the lamp was aimed directly across the flame and the angle of the burner adjusted accordingly. It was extremely important to ensure that the burner was attached correctly and that the o-ring was snugly fitted otherwise the acetylene might escape and cause an explosion. The flame was ignited and the standard tubes filled with pre-made standard solution. It was necessary from to time to time to prepare more standard silver solution (I did this on a number of occasions). A waste tube ran from the bottom of the burner and its bottle had to be emptied periodically into the acid waste in the wet lab. As long as the auto-sampler was functioning and the teflon tube attached was not blocked, the instrument would create a calibration curve, against which to compare absorbance of unknown samples and calculate concentration. It was important to know what to expect for a calibration curve, as a low absorbance could indicate a partially blocked tube. Sometimes a piece of fine wire could be used to unblock the tube. Other times, the tube had to be replaced. Some spare tubes were always on hand. A partially blocked tube could appear normal but produce a low reading. One time the auto-sampler broke down due to a software problem the samples had to be run manually which was tedious and time consuming! Running the AAS smoothly required a good eye for detail and persistent troubleshooting. A quick glance at the colour of the flame, or the sound it made could indicate if there was a problem. Any break in the blue bar at the bottom of the flame had to be cleared. The acetylene flow was adjusted with a valve to be at a level were there was 'only just' no yellow in the flame. Some bright colours could be seen in the flame from some time to time but these did not indicate the presence of silver, which emits at 328 nm (not in the visible part of the spectrum). When samples were completed correctly, a quick glance at the printed job sheet showed the expected values for repeats and standards. If these matched the data was saved and transferred to the laboratory manager’s computer where they could be adjusted, checked by pressed powders, or reported right away if the AAS and XRF data matched correctly. Typically, AAS analysis was quicker and XRF was the limiting factor in turnaround.
Data collection and quality control Most of the data collection was done on computer, with very little paperwork. Good use of software was a must. Comparison of data was done on every job before reporting. In some cases, if all AAS data was out by 10% or so, it could be adjusted. If small adjustments like this could not fix the problem, it was possible that an error had been made in the weigh room. Determining where the error occurred often involved re-testing a dozen or so samples in the affected region by pressed powder analysis (quick for a small number of samples).
A pressed powder sample in a steel cylinder for XRF
Making pressed powder samples to troubleshoot To make a pressed powder, the samples would be retrieved and taken put into aluminium caps. These were pressed lightly with the bottom of another cap. They were then placed in a hydraulic press which applied 50 tonnes of pressure. It took some practice to know how much to fill each cap. Too much or not enough and the cap would be crumbly and might block the vacuum chamber on the XRF. Uranium tests exclusively used this type of pressed powders.
Report results to client An email containing data was sent to the client by the laboratory supervisor or the general manager. If there was a delay, the client would be notified and an alternative timeframe would be given.
File away completed samples Once analyzed and reported, the samples were retained in a storage shed for a period of time.