1 Jun 2017

XRF pellet preparation for mined mineral analysis

Sample heterogeneity affects the accuracy and repeatability of many X-Ray Fluorescence (XRF) measurements. Consistent sample preparation is required for measurements that are fully representative of the whole sample composition. The mineral mining industry has a need for several different measurements at different stages of the recovery process, and the sample preparation required will vary. However, high tonnage hydraulic presses are a popular consistent and powerful method of XRF pellet preparation.

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In XRF spectroscopy, the fluorescence signal attenuates rapidly with increasing distance from the atoms within the sample. As a result, the measured result is strongly biased towards the surface composition of the sample. This effect is amplified with lighter elements (e.g. Na, Ca, Mg, P), whose fluorescence signals attenuate more rapidly than for heavier elements.

Exploration

Exploration of iron deposits involves determining the type of mineral ore (hematite, magnetite, taconite) the likely yield of iron (Fe), manganese (Mn), and other profitable elements, as well as determining so-called ‘penalty’ elements that might negatively affect the quality of steel produced using the ore. These include phosphorus (P), silicon (Si), aluminum (Al), and sulfur (S).

After a new mineral deposit has been identified it must be mapped to determine the value of the deposit, its composition, and the ease (cost) of its recovery. An exploration project will be undertaken to determine these facts. Mining geologists recover surface rock samples and take deeper rock cores which are then analyzed in the field or back in the laboratory.

Grade Control

Once the mine is operational, the produced ores need to be accurately characterized before shipment. Quantification of a few elements, such as Fe and Mn plus the important penalty elements that may be present can be achieved with Wavelength Dispersive XRF, while full major and minor element analysis can be quickly gathered using an Energy Dispersive XRF instrument.

For analysis in the laboratory, the raw samples are crushed and pulverized to form a powder. These powders may either be fused with a flux material at high temperature (1000-1200˚C) to form a fused bead, or mixed with a binder material and pressed into a pellet using a hydraulic press. Fused beads take longer to prepare and dilute the sample (reducing signal intensity), but the method is free from grain size and mineralogical effects. Pressed pellets are faster to prepare, but the aforementioned effects are not removed.

Excavation control

Another important function of XRF in the mining industry is detecting changes in the ore composition during excavation. As more of the deposit is uncovered the levels of iron and penalty element content might change, and the operation may have to be modified to react to this, for example, by excavating in a different location, or changing downstream processing.

Low grade ores may require benefication, various crushing and milling processes aimed at removing non-valuable materials from the ore and improving the percentage of iron. XRF can be used again to measure the results.

XRF sample preparation for mineral mining analysis

Pellet pressing is a common sample preparation method for XRF spectroscopy analysis. By first milling a sample to a powder, a sample can be placed into a die which is then pressed. Depending what the sample is and how easy it will bind together will determine the load required. 

A mineral ore may require up to 40 tons. If the sample does not bind during pressing, a wax binder can be used to assist which is added at the milling stage. Specac produce high end evacuable pellet die of various diameters, which work consistently with the Atlas range of top level hydraulic XRF pellet presses.

The Autotouch automatic hydraulic XRF pellet press is ideal for this area of XRF pellet preparation, as it can operate with up to 40 tons of pressure. It has easy, multi-lingual user controls and includes a PETG safety guard which protects the user.

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