Comparative Numerical Study on the Weakening Effects of Microwave Irradiation and Surface Flux Heating Pretreatments in Comminution of Granite

Thermal pretreatments of rock, such as conventional heating and microwave irradiation, have received considerable attention recently as a viable method of improving the energy efficiency of mining processes that involve rock fracturing. This study presents a numerical analysis of the effects of thermal shock and microwave heating on the mechanical properties of hard, granite-like rock. More specifically, the aim is to numerically assess the reduction of uniaxial compressive strength of thermally pretreated specimens compared to intact ones. We also compare the performance of these two pretreatments (conventional heating and microwave irradiation) in terms of consumed energy and induced damage. Rock fracture is modelled by a damage-viscoplasticity model, with separate damage variables in tension and compression. A global solution strategy is developed for solving the thermo-mechanical problem (conventional heating) and the electromagnetic–thermo-mechanical problem (microwave heating). The electromagnetic part of the microwave heating problem is solved in COMSOL Multiphysics software Version 6.1 first. The electromagnetic solution is used as an input for the thermo-mechanical problem, which is finally solved by means of a staggered explicit solution method. Due to the predominance of the external thermal sources, the thermal and the mechanical parts of the problem in both cases are considered as uncoupled. Three-dimensional finite element simulations are utilized to study the damage-viscoplasticity model. An ore-shaped three-mineral numerical rock specimen is used in uniaxial compression tests.publishedVersionPeer reviewe

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope

[EN] The identification of the minerals composing rocks and their dielectric characterization is essential for the utilization of microwave energy in the rock industry. This paper describes the use of a near-field scanning microwave microscope with enhanced sensitivity for non-invasive measurements of permittivity maps of rock specimens at the micrometer scale in non-contact mode. The microwave system comprises a near-field probe, an in-house single-port vectorial reflectometer, and all circuitry and software needed to make a stand-alone, portable instrument. The relationship between the resonance parameters of the near-field probe and the dielectric properties of materials was determined by a combination of classical cavity perturbation theory and an image charge model. The accuracy of this approach was validated by a comparison study with reference materials. The device was employed to determine the permittivity maps of a couple of igneous rock specimens with low-loss and high-loss minerals. The dielectric results were correlated with the minerals comprising the samples and compared with the dielectric results reported in the literature, with excellent agreements.This paper has been financially supported through the grant reference BES-2016-077296 of the call Convocatoria de las ayudas para contratos predoctorales para la formacion de doctores de 2016 by Ministerio de Economia y Competitividad (MINECO) and by European Social Funds (ESF) of European Union, and the project SEDMICRON-TEC2015-70272-R (MINECO/FEDER) supported by Ministerio de Economia y Competitividad (MINECO) and by European Regional Development Funds (ERDF) of European Union.Gutiérrez Cano, JD.; Catalá Civera, JM.; López Buendía, ÁM.; Plaza González, PJ.; Penaranda-Foix, FL. (2022). High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope. Sensors. 22(3):1-17. https://doi.org/10.3390/s2203113811722

Numerical modelling of microwave irradiated rock fracture

Rock fracturing through microwave irradiation has received significant attention recently as a viable pretreatment for improving the energy efficiency of comminution processes. This study presents a numerical analysis on the effects of microwave heating on the mechanical properties of hard rock. In particular, the reduction of the uniaxial compressive and tensile strength of granite-like rock due to microwave irradiation induced damage is numerically assessed. Rock fracture is modelled by a damage-viscoplasticity model, with separate damage variables for tension and compression types of failure. A global solution strategy is developed where first the electromagnetic problem is solved in COMSOL multiphysics software, then its solution is used as an input for the thermomechanical problem, which is finally solved by means of a staggered explicit solution method. Due to the preeminence of the thermal radiation, the thermal and the mechanical parts of the problem are considered as uncoupled. The model behaviour is tested in 3D finite element simulations of three-mineral numerical rock specimens, with mesostructures explicitly defined, pretreated first in a microwave oven and then subjected to uniaxial compression and tension tests. The results show that the compressive and tensile strength of rock can be considerably reduced by the microwave irradiation pretreatment.Peer reviewe

An investigation on the influence of microwave energy on basic mechanical properties of hard rocks

Drilling and blasting is one of the most commonly used and convenient method for many mining and civil applications to break rocks, especially hard rocks. Because of many environmental, safety and productivity issues (i.e. Cycle time), the industry and contractors are looking for an alternative system of breaking of rocks. In underground rock breakage applications such as tunneling, continuous mechanical excavation method, normally with tunnel boring machines (TBMs) or road headers is becoming more popular and commonly used by contractors. One of the main disadvantages of such system is that, it cannot handle very hard rocks and uneconomical major disc or bit wear takes place. Today, novel explosive free rock breaking technologies (thermal, electrical, nuclear energy and microwave) are available and could be used on their own or assist mechanical conventional machines to be able to break harder rock material. This is possible because it is hoped that the microwave will reduce the strength of the rock prior to the impact of mechanical device. Three main mechanical parameters of rocks such as compressive strength, tensile strength and abrasivity index value were used to highlight the influence of microwave on to the mechanical properties of rocks. Seven different rock types were prepared and tested. Each individual specimen was exposed to the microwave energy in three power levels 800, 1250 and 3000 watts, within 0, 15, 30, 60, 120 and 240 seconds time of exposure. Different rocks due to their mineralogy behave differently when exposed to microwave energy. CERCHAR abrasivity index value of almost all samples shows reduction of about 30% as the power level of exposure increases. The tensile strength of certain samples reduces significantly as in basalt reduces of up to 80%. The unconfined compressive strength value of almost all samples reduces about 30% as well. The reduction of the strength and abrasivity of hard rock cause the penetration rate and the life time of cutter tools of a TBM to be increased by combining the microwave energy to the cutter head of the mechanical excavator

Sintering Applications

Sintering is one of the final stages of ceramics fabrication and is used to increase the strength of the compacted material. In the Sintering of Ceramics section, the fabrication of electronic ceramics and glass-ceramics were presented. Especially dielectric properties were focused on. In other chapters, sintering behaviour of ceramic tiles and nano-alumina were investigated. Apart from oxides, the sintering of non-oxide ceramics was examined. Sintering the metals in a controlled atmosphere furnace aims to bond the particles together metallurgically. In the Sintering of Metals section, two sections dealt with copper containing structures. The sintering of titanium alloys is another topic focused in this section. The chapter on lead and zinc covers the sintering in the field of extractive metallurgy. Finally two more chapter focus on the basics of sintering,i.e viscous flow and spark plasma sintering

Phase evolution and properties of a porcelain body fabricated using different rapid firing techniques

A porcelain powder was sintered using different rapid sintering techniques including direct sintering (DS), spark plasma sintering (SPS), microwave sintering (MWS), and flash sintering (FS). Densification, phase evolution, and physicomechanical properties of the fully sintered porcelain samples were investigated to improve understanding of the role of particular process parameters. Densification of the porcelains was controlled by liquid phase formation. Study of the DS revealed that formation of liquid glass is rapid even at 780 C/min heating rate, and the porcelain was fully densified within 15 min dwelling at 1175 °C. SPS enhanced densification rate about 10 times greater over that of conventional sintering, starting at 920 C. The dwelling step was negligible due to the rapid flow of the liquid glass filling the pores, assisted by the applied pressure. On using microwave energy, the sintering temperature of the porcelain was reduced by ~75 °C and dwell time from 15 min to 5 min compared to conventional sintering. Formation of the liquid phase was observed at 900-1000 °C and it was the key for the rapid densification because it promoted microwave absorbability. Densification in FS was difficult to resolve, but it could be deduced that by applying an electric potential of 1.5 kV/cm for 30 seconds, large amounts of glassy phase were produced. Various microstructures were observed as a result of the different processing routes. DS and MWS produced a typical porcelain microstructure which is dense but has numbers of residual pores embedded in the glassy phase. SPS produced a highly dense microstructure with a few residual pores assisted by applied pressure. FS, in contrast, produced a nonuniform microstructure containing under-sintered, well sintered, and over-sintered regions due to localised melting. Etched microstructures revealed special mullite morphologies. The applied pressure in SPS caused mullite orients perpendicular to the direction of the applied load. SPS also induced mullite dissolution at 1200 °C. Microwave produced fibre-like mullite via a vapour-liquid-solid mechanism having Fe(l) as a catalyst. Flash sintered samples contained dendritic mullite and some mullite needles were formed via a screw dislocation mechanism. However, mullite crystallite sizes calculated by the Scherrer equation revealed that in fully dense samples from each processing route, mullite crystallite sizes were in a narrow range of 25-40 nm. Physicomechanical of the porcelains produced using the different processing routes were similar; for example, apparent bulk density ranged 2.35-2.46 g/cm3 (MWSDS>SPS), hardness varied 5-7 GPa (MWS<SPS<DS) while fracture toughness varied from 3-6 MPa· m1/2 (MWS<DS<SPS). A comparison of energy consumption during the various sintering routes for this porcelain composition showed that DS consumed ~11 MJ/kg, MWS ~18 MJ/kg, FS ~3060 MJ/kg and SPS ~1612 MJ/Kg. While, the energy consumption listed here must be viewed with caution, the values are useful for comparison of the amounts of energy required for the different rapid sintering processes.Open Acces

Future Trends in Advanced Materials and Processes

The Special Issue “Future Trends in Advanced Materials and Processes” contains original high-quality research papers and comprehensive reviews addressing the relevant state-of-the-art topics in the area of materials focusing on relevant or innovative applications such as radiological hazard evaluations of non-metallic materials, composite materials' characterization, geopolymers, metallic biomaterials, etc

Comminution in the Minerals Industry

Size reduction processes represent a significant part of the capital as well as the operating cost in ore processing. Advancing the understanding of and improving such processes is worthwhile since any measurable enhancement may lead to benefits, which may materialize as reductions in energy consumption or wear or improved performance in downstream processes. This book contains contributions dealing with various aspects of comminution, including those intended to improve our current level of understanding and quantification of particle breakage and ore characterization techniques that are relevant to size reduction, as well as studies involving modeling and simulation techniques. The affiliations of the authors of the articles published in this book span 14 countries around the globe, which attests to the highly international nature of research in this field. The themes of the manuscripts also vary widely, from several that are more focused on experimental studies to those that deal, in greater detail, with the development and application of modeling and simulation techniques in comminution. Size reduction technologies more directly addressed in the manuscripts include jaw crushing, vertical shaft impact crushing, SAG milling, stirred milling, planetary milling, and vertical roller milling. Ores involved directly in the investigations include those of copper, lead–zinc, gold, and iron as well as coal, talc, and quartz

The measurement of the dielectric constant of concrete pipes and clay pipes

To optimize the effectiveness of the rehabilitation of underground utilities, taking in consideration limitation of available resources, there is a need for a cost effective and efficient sensing systems capable of providing effective, in real time and in situ, measurement of infrastructural characteristics. To carry out accurate non-destructive condition assessment of buried and above ground infrastructure such as sewers, bridges, pavements and dams, an advanced ultra-wideband (UWB) based radar was developed at Trenchless Technology Centre (TTC) and Centre for Applied Physics Studies (CAPS) at Louisiana Tech University (LTU). One of the major issues in designing the FCC compliant UWB radar was the contribution of the pipe wall, presence of complex soil types and moderate-to-high moisture levels on penetration depth of the electromagnetic (EM) energy. The electrical properties of the materials involved in designing the UWB radar exhibit a significant variation as a result of the moisture content, mineral content, bulk density, temperature and frequency of the electromagnetic signal propagating through it. Since no measurements of frequency dependence of the dielectric permittivity and conductivities of the pipe wall material in the FCC approved frequency range exist, in this thesis, the dielectric constant of concrete and clay pipes are measured over a microwave frequency range from 1 Ghz to 10 Ghz including the effects of moisture and chloride content. A high performance software package called MU-EPSLN™ was used for the calculations. Data reduction routines to calculate the complex permeability and permittivity of materials as well as other parameters are also provided. The results obtained in this work will be used to improve the accuracy of the numerical simulations and the performances of the UWB radar system

The effect of microwave radiation on mineral processing

Between 50% and 70% of the total energy used in the extraction process may be attributed to comminution. Microwave pre-treatment has been suggested as a means to decrease the energy requirements. A variety of mineral ores have been investigated and the effects of microwave radiation quantified in terms of the mineralogy, changes in the Bond Work Index, flotability and magnetic separation. It has been shown that microwave pre-treatment is most effective for coarse grained ores with consistent mineralogy consisting of good microwave absorbers in a transparent gangue (up to a 90% decrease in Bond work index for Palabora copper ore) whereas fine grained ores consisting predominantly of good absorbers are not affected as well (a reduction of only 25% in work index for Mambula ore). Although the mineralogy of minerals are affected by exposure to microwave radiation, flotability and magnetic separation characteristics have been shown not to be adversely affected, unless the microstructure is completely destroyed after prolonged microwave exposure. Computer simulations have shown that significant changes to comminution circuits are possible as a result of microwave induced work index reductions (three mills reduced to one). Purpose-built microwave units may hold the solution for more efficient mineral extraction in the near future