Modelling and experimental validation of tribocharging for space resource utilisation (SRU)

Space Resource Utilisation (SRU) technology will enable further exploration and habitation of space by humankind. For example, oxygen produced \textit{in situ} can be used as the oxidiser in rocket propellant, or for life support systems. The production of oxygen on the Moon can be achieved through the thermo-chemical reduction of the lunar soil, also known as regolith. All reduction techniques require a consistent feedstock from this mix of fine mineral particles to produce oxygen reliably and consistently. The preparation of this feedstock, known as beneficiation, is a critical intermediate stage of the SRU flowsheet, however it has received little research attention relative to the preceding excavation, and the subsequent oxygen production stages. Triboelectric charging and free-fall separation are attractive technologies for mineral beneficiation as they offers low mass, low power, and low mechanical complexity compared to other approaches. Tribocharging is a process by which particles (conductors, semi-conductors, and insulators) acquire charge through frictional rubbing and subsequent separation. Previous experimental studies have tested different designs of tribocharging apparatuses for terrestrial and space applications, however charge transfer modelling methods have not been employed to optimise design parameters. Furthermore, whilst modelling of the triboelectrification process has been presented in the literature using the discrete element method (DEM), these models often depend on poorly quantified or ill-defined parameters, such as an effective work function for insulating materials. Previous studies have also been restricted to either 2D or 3D domains and have not considered the impact of this on the performance of the models. To address these knowledge and research gaps, the objectives of this thesis are as follows: \begin{enumerate} \item Develop a novel tribocharge modelling approach based on the discrete element method that de-emphasises the poorly-defined quantities found in the high-density limit approach that has been demonstrated previously; \item Determine the suitability of modelling tribocharging in 2D and 3D; \item Validate this novel tribocharge modelling method by comparing simulation outputs and experimental data; \item Present and validate a new DEM-based method for tribocharger design optimisation; and, \item Evaluate experimentally the impact of an optimised tribocharger design on the performance of an electrostatic separator using standard mineral processing criteria. \end{enumerate} A straightforward experimental method to quantify key tribocharging model parameters, namely the charge transfer limit, $\Gamma$, and the charging efficiency, $\kappa_c$, is presented herein. These parameters are then used in both 2D and 3D DEM charge transfer simulations (particle-particle and particle-wall interactions; single and multiple particles and contacts) to evaluate the suitability of faster 2D models. Both the 2D and 3D models were found to perform well against the experimental data for single-contact and single-particle, multi-contact systems, however 2D models failed to produce good agreement with multi-particle, multi-contact systems. A novel DEM-based approach for tribocharger design optimisation using particle-wall and particle-particle contact areas as proxies for charge transfer is demonstrated. This optimisation method is used to design an optimal tribocharger for use under terrestrial conditions. The novel tribocharge modelling approach was then applied to the optimised charger design. This design was then built and validated experimentally, with good agreement found between the model outputs and experimental data. The optimised terrestrial design was then employed to study the charging behaviour of pure silica and ilmenite, as well as binary mixtures of silica and ilmenite, and samples of lunar regolith simulant JSC-1. Ilmenite was used because it is a target mineral for oxygen production from the lunar regolith, and silica was used because of its position in the triboelectric series relative to ilmenite. The optimised tribocharger design affected significantly the movement of pure ilmenite in the electrostatic field, despite a negligible change in bulk charge. Experimental results from the binary mixtures indicate that ilmenite recovery is independent of initial ilmenite concentration and can be predicted from the mass distribution of pure ilmenite samples. For JSC-1, the tribocharger was found to increase the density of the material in certain collectors. This thesis presents new modelling approaches for both tribocharging and tribocharger design optimisation. These techniques will facilitate ultimately the development of beneficiation technologies for SRU. The use of these modelling methods should increase confidence in the performance of tribocharger designs proposed for future SRU missions to the Moon.Open Acces

FUNDAMENTAL IMPROVEMENT IN THE TRIBOCHARGING SEPARATION PROCESS FOR UPGRADING COAL

Triboelectrostatic separation is a physical separation technique that is based on surface electronic property differences among minerals to achieve a separation. Minerals have different surface conductivities and electron affinities. They are charged differently in quantity and/or polarity after a tribocharging process. Particles with different surface charges move discretely under external electric field produce a separation. Electrostatic separation is a dry mineral processing method that does not require any water or chemical reagents. It can greatly simplify the processing circuit and reduce operating cost. Additionally, problems caused by water in conventional wet mineral processing such as water freezing, dewatering, water pollution and water treatment are eliminated. Electrostatic separation has great potential as a fine particle separator (i.e. \u3c 1mm) in industrial minerals processing application, especially in arid areas where water supply is limited. In the current study, particle tribocharging kinetics was evaluated using a model system comprised of copper, pure coal, silica and ceramic. The results of the tribocharging process were recorded and analyzed using an oscilloscope and a signal processing technique. Charge exchange, charge separation and charge relaxation corresponding to tribocharging processes were studied using the generated pulsing signals. The signals provided a method to quantify the charge penetration into the conductor bulk during tribocharging. A new method to measure the particle surface charge using the pulsing was proposed and assessed, which was extremely useful for subtle surface charge measurements which effectively eliminated environmental noise. The interactive forces at the contacting interface, relative displacement, material electronic properties and ambient relative humidity were found to impact particle surface charge. The silica surface sites are 69 times more chargeable than the coal surface, which provides a fundamental explanation for upgrading that is achievable for silica-rich coal using triboelectrostatic separation. The influences of operating and environmental parameters were quantified and compared using an environment controlled chamber. Energy consumption at the interface was found to be positively correlated with the particle charge. Relative humidity has dual effects on the particle tribocharging, excessively low or high humidity levels do not favor particle tribocharging. Finally, a semi-empirical mathematical model of particle tribocharging was developed from the basic tribocharging compression model utilizing the parametric experiment study results. The model provides a more accurate method to predict particle surface charge under exact tribocharging conditions. A novel rotary triboelectrostatic separator (RTS) using the tribocharging mechanism was tested for upgrading fine coal. The particle size influencing the RTS tribocharging and separation process is investigated. A practical method to quantify the particle charging distribution was developed based on the direct particle charge measurement and a Gaussian distribution assumption. The smaller particles were found to have a higher average surface charge and wider surface charge distribution, which provided an opportunity to separate the high grade and the low grade coal particles. However, particles that are too small have weak particle-charger tribocharging effect that reduces particle tribocharging efficiency. The particle separation process was analyzed considering the exact experimental hydrodynamic separating conditions. Smaller particles were found to be more sensitive to the airflow that used to transport the particles as a result of the effect on residence time in the separation chamber. A method combining mathematical and statistical analysis was proposed to theoretically predict RTS separation efficiency based on the particle charging conditions and particle separation conditions. The particle horizontal displacement probability distribution was ultimately derived from this method. The model predictions indicate that a wider horizontal displacement distribution provides improved separation efficiency for the RTS unit. The theoretical analysis indicates that a particle size range between 0.105 and 0.21 mm has widest horizontal displacement distribution and thus represents an optimum particle size range which is in agreement with experimental results. The influences of the RTS operating parameters on separation performance achieved on a pure coal-silica mixture were investigated using a parametric study. The optimum operating conditions were identified. Using the optimum conditions, a five-stage separation process was conducted using the RTS unit to obtain the necessary data for the development of an ideal performance curve. Two stages of RTS separation were found to generate good quality clean coal with acceptable recovery. Particle tribocharging tests were performed using pure coal, pure silica and the coal-silica mixture as model feed materials. The test result found that mixing the pure coal with the sand reduced the particle charge distribution of the coal while increasing the charge distribution of the pure silica particle. The finding explains the inability to produce clean coal products containing ultra-low ash contents. However, the rejection of silica to the tailings stream is very high. The RTS upgrading of low-ash coal sample was tried using experiment design method, which revealed that feed rate was the most significant while the applied charger voltage and the injection air rate were the least significant in regards to product quality. Feed mass flow rate and the co-flow air rate have a significant interactive effect. Considering the theoretical findings, the impact of high feed rates is due to the negative effect on particle tribocharging efficiency resulting from an increase in the particle-particle surface charge relaxation. Under the optimum test conditions, an ultraclean coal was produced with an ash content of 3.85±0.08% with a combustible recovery of 62.97±1.11% using the RTS unit

Reduction of Wind Tunnel Contamination During Flow Visualization Experiments Using Polystyrene Microspheres

Evaluation of novel methods and materials for seeding tracer particles for particle image velocimetry (PIV) was carried out in the Basic Aerodynamic Research Tunnel (BART) at NASAs Langley Research Center (LaRC). Seeding of polystyrene latex microspheres (PSLs) from ethanol/water suspensions and from the dry state was carried out using custom built seeders. PIV data generated using the novel methods were found to be in general agreement with data collected using the current seeding methods. Techniques for assessing PSL fouling of wind tunnel surfaces were identified and refined. Initial results suggest that dry seeding PSLs may allow comparable data quality to wet seeding while reducing wind tunnel screen fouling. Results also indicate that further developments to the dry seeding system should focus on increasing single particle flux into the wind tunnel. Modifications to PSLs and seeding equipment to achieve this have been identified and are discussed

A Review of Sorting and Separating Technologies Suitable for Compostable and Biodegradable Plastic Packaging

As a result of public pressure and government legislation to reduce plastic waste there has been a sharp rise in the manufacture and use of alternatives to conventional plastics including compostable and biodegradable plastics. If these plastics are not collected separately, they can contaminate plastic recycling, organic waste streams, and the environment. To deal with this contamination requires effective identification and sorting of these different polymer types to ensure they are separated and composted at end of life. This review provides the comprehensive overview of the identification and sorting technologies that can be applied to sort compostable and biodegradable plastics including gravity-based sorting, flotation sorting, triboelectrostatic sorting, image-based sorting, spectral based sorting, hyperspectral imaging and tracer-based sorting. The advantages and limitations of each sorting approach are discussed within a circular economy framework

Development of a Testbed for the Beneficiation of Lunar Regolith - Concentrating an Ilmenite-Rich Feedstock for In-Situ Oxygen Production on the Moon

The utilization of extraterrestrial resources may one day enable mankind’s further exploration and sustainable colonization of the Solar System. An easily accessible and very versatile resource found on Earth’s celestial neighbor, the Moon, is lunar regolith. This unconsolidated mixture of soil and rocks contains large quantities of oxygen, which in return can be used to produce consumables for propulsion and life support systems. However, the oxygen is chemically bound to minerals and must, thus, be extracted. The preparation of a feedstock that is chemically and physically suited for the extraction is termed beneficiation and depicts a vital stage in the context of in-situ oxygen production. Developing a test stand that demonstrates the technical feasibility of lunar mineral beneficiation in a laboratory setting is the purpose of this master’s thesis. The testbed’s main function is to concentrate the target mineral ilmenite (a titanium-iron oxide), to reject unwanted gangue minerals (like silicates), and to remove unfavored size fractions (e.g., dedusting and oversize grain removal). To ensure that the end product fulfills this function in a satisfactory manner, a systematic engineering design process consisting of seven work packages is applied. This involves a review of existing studies and an investigation of available processes, the definition of requirements and specification, as well as various conceptualization activities (process and setup selection plus sketching). Moreover, results of design calculations and data of methodically selected components are integrated into a 3D model, to be created using computer-aided design software. Production planning activities like the preparation of procurement-related documentation completes the development. The outcome of this thesis is a well-engineered and methodically mature beneficiation system that encompasses three dry separation stages: Particle size separation, magnetic separation, and electrostatic separation. This multi-stage approach guarantees the reliable and efficient enrichment of ilmenite from lunar regolith simulant. Hence, it is ready to be brought into being through assembly, integration, and test and can eventually be used for beneficiation-related experiments

Study on Developments of Processes for Powder Coatings

Powder coating, which utilizes polymer resin based powder, is a relatively young technology for decorative and protective purposes. It was first developed in the early 1950s and gradually grew into automotive finishes. Compared to conventional liquid coating, powder coating is an environmentally friendly technology, given that there is no emission of harmful solvent throughout the entire operating process. It is also much more efficient, economical and energy saving as a result of the capability to reuse all the powder. However, there are still some problems in the powder coating process limiting its application. This thesis focused on studying some key aspects of the manufacturing and application processes of both coarse powder and fine powder. Several new techniques were developed during the process. The study started with an investigation of powder coating manufacturing processes. Most of the production is aiming at maximizing the productivity, while the produced powder has a wide particle size distribution, leading to a poor surface condition. In order to narrow the particle size distribution, a new design of classifying cyclone for an air classifying mill (ACM) with a reversed air inlet was studied. Experimental results indicated that this novel cyclone design could effectively reduce the span of the particle size distribution, without deteriorating the collection efficiency compared to conventional cyclones. In addition, the spraying properties of powder coatings were investigated during electrostatic spraying. In order to overcome the Faraday Cage effect, a modified corona spray gun with a multi-electrode design supplied by an alternating charging pattern was invented. Experimental results demonstrated that the Faraday Cage effect could be significantly mitigated by using the new spray gun at various gun voltages. Besides, the new design of spray gun could provide higher transfer efficiency compared to its conventional counterpart. Moreover, this study also improved the powder coating processing technique involving metallic pigments. To achieve better bonding between metallic pigments and powder coating particles, a liquid bonding agent (bonder) was introduced into the blending process. It was found that the concentration of metallic pigments changed minutely from the pre-sprayed and post-sprayed powder, indicating that the recycled powder could be reused. Therefore, this new bonding method of utilizing liquid bonder could provide a strong affinity between the powder coating and metallic pigments so as to prevent separation of the two materials during spraying. At the end, a pre-heating method of applying powder coating onto thermoplastic substrates, which helps powder deposition and adhesion onto plastic substrates, was studied. Three widely used powder coatings have been tested for this method. The coating films were evaluated by both visual inspection and instrumental measurements. It was found that this new processing method could provide a smooth surface as well as a strong adhesion to the difficult substrate. Furthermore, for the purpose of protecting the thermoplastic substrate from deformation during the curing process, a UWO low-cure method was applied to these three powder coatings. Compared with commercial low-cure powder, the UWO low-cure coatings perform better in a few aspects. The discoveries and analysis in this thesis work are contributing to powder coating development. Several original techniques have been invented in this study and they could provide a good guideline for future work of modern powder coating technology

電磁粒体の力学とその月・火星・小惑星に存在するレゴリスの操作技術に関する研究

早大学位記番号:新7464早稲田大