Principal Factors of Contact Charging of Minerals for a Successful Triboelectrostatic Separation Process – a Review

Preparation of pyrite concentrate powder from the Thackaringa mine for quantitative phase analysis using X-ray diffraction

The quantitative phase analysis using X-ray diffraction of pyrite ore concentrate samples extracted from the Thackaringa mine is problematic due to poor particle statistics, microabsorption and preferred orientation. The influence of sample preparation on these issues has been evaluated, with ball milling of the powder found most suitable for accurate and precise quantitative phase analysis. The milling duration and other aspects of sample preparation have been explored, resulting in accurate phase reflection intensities when particle sizes are below 5 µm. Quantitative phase analysis on those samples yielded precise phase fractions with standard deviations below 0.3 wt%. Some discrepancy between the elemental composition obtained using X-ray powder diffraction data and that determined using wavelength-dispersive X-ray fluorescence was found, and is thought to arise from unaccounted for crystalline phase substitution and the possible presence of an amorphous phase. This study provides a methodology for the precise and accurate quantitative phase analysis of X-ray powder diffraction data of pyrite ore concentrate from the Thackaringa mine and a discussion of the limitations of the method. The optimization process reveals the importance of confirming reproducibility on new samples, with as much prior knowledge as possible.

Forced Triboelectrification of Fine Powders in Particle Wall Collisions

Triboelectric separation as an inexpensive and environmentally friendly technique could contribute to material-specific sorting. However, the application as a widespread method is limited due to the complexity of the process. In particle wall collisions, various parameters like collision energy and angle, work function of the contact partners, humidity, surface roughness, etc. influence the particle charging in a hardly predictable way. This study investigates the possibilities of forced triboelectric particle charging by applying an electrical potential to the metal contact partner (copper/steel pipe). The variations included different pipe lengths (0.5 m–3 m), particle materials, and particle sizes for limestone. A distinction is made between the net charge of the particles and the positive, negative, and neutral mass fractions. The work functions of the investigated materials vary from about 3.2 eV to >8.5 eV for glass, limestone, artificial slag, and lithium aluminate particles. With the applied high-voltage potential, the particle net charge can be shifted linearly. For limestone, it is shown that the neutral fraction is highest at the Point of Zero Net Charge (PZNC). This observation may identify an approach for the material selective separation of one target component from a multi-material mixture.

Assessment of the Electrostatic Separation Effectiveness of Plastic Waste Using a Vision System

The work presented here describes the first results of an effective method of assessing the quality of electrostatic separation of mixtures of polymer materials. The motivation for the research was to find an effective method of mechanical separation of plastic materials and a quick assessment of the effectiveness of the method itself. The proposed method is based on the application of a dedicated vision system developed for needs of research on electrostatic separation. The effectiveness of the elaborated system has been demonstrated by evaluating the quality of the separation of mixtures of poly (methyl methacrylate) (PMMA) and polystyrene (PS). The obtained results show that the developed vision system can be successfully employed in the research on plastic separation, providing a fast and accurate method of assessing the purity and effectiveness of the separation process.

Investigation into powder tribo-charging of pharmaceuticals. Part II: Sensitivity to relative humidity

Environmental conditions can have a profound impact on the bulk behaviour of pharmaceutical powders, including their tribo-charging tendency. Typically, high relative humidity (RH) has been associated to a reduction in the electrostatic charge of the material. However, the occurrence of charge mitigation seems to be related to the quantity of water molecules at the powder surface, which depends on intrinsic material attributes (i.e., water sorption propensity), and external factors (i.e., RH level). In the present study, pharmaceutical powders (i.e., microcrystalline cellulose, D-mannitol, paracetamol and magnesium stearate) were conditioned at three levels of RH, relevant for pharmaceutical operations, and their bulk behaviour, including charging propensity, was analyzed. Depending on the material type, powders sorbed water from the humid atmosphere to different extents, resulting in different charging behaviours. Overall, the charge density of the materials was found to decrease after a certain RH or monotonically decrease with an increase of RH, except for D-mannitol. For this material, a contrasting trend of increase in charging was observed with an increase in RH. Moreover, the powders showed a distinct tribo-charging sensitivity to RH, with paracetamol being the most affected. These findings suggest that a careful consideration on solid material-moisture interactions is needed when using RH as strategy to minimize electrostatic effects in powder processing.

Photo-stimulated triboelectric generation

A new photo-stimulated triboelectric generation occurring between a metal-oxide and polyimide during friction was demonstrated. The output currents of the triboelectric nanogenerator were significantly enhanced, under light illumination, up to approximately 5 times depending on the wavelength of the light, providing a new route for energy harvesting devices as well as self-powered selective photodetectors.