Fast temperature programmed gas chromatography coupled to supercritical fluid chromatography (SFC×GC)

The topic of this thesis is the development of comprehensively coupled (supercritical fluid × gas) chromatography and its application to the chemical analysis of biodiesel and biodiesel blends. A future low-carbon economy might still have a need for large, high-efficiency diesel engines fueled with a high-quality carbon neutral fuel such as biodiesel. The quality of fuels are judged according to technical standards: documents that detail the requirements of compliance. Liquid fuels are complex mixtures that challenge the separation science used to ensure compliance. Chromatographic separations that use different separation mechanisms can be combined to meet those challenges, culminating in comprehensive coupling, where every fraction of a first separation is subjected to a second separation. The fundamental difference between the separation mechanisms of supercritical fluid chromatography (SFC) and gas chromatography (GC) allows for a powerful, technically feasible, coupled technique. To make the coupling practical the GC separation must be fast and temperature-programmed. A coaxial resistive heater for short capillary columns with active cooling by liquid carbon dioxide was constructed, with which hundreds of consecutive fast temperature-programmed GC separations were performed at a rate of four per minute. These chromatograms of fractions of SFC separations were combined to construct two-dimensional SFC×GC chromatograms. When biodiesel and biodiesel blends are analysed by SFC×GC, separation in the first dimension is by polarity and degree of unsaturation and in the second dimension by volatility. The resulting chromatograms contain powerful patterns of peaks, with the aromatic hydrocarbons separated from the alkanes, and the fatty acid methyl esters (FAMEs) of the biodiesel separated from the hydrocarbons of the petrodiesel. Because the flame ionization detector (FID) is used, quantification should be straightforward and reliable. The FID remains compatible with SFC×GC even when organic modifiers are added to the SFC mobile phase, because the volatile modifiers elute on the GC as a solvent peak, separate from the less-volatile analytes of interest. SFC×GC can be used for research and quality control in the liquid fuels and vegetable oil industries.Thesis (PhD (Chemistry))--University of Pretoria, 2020.ChemistryPhD (Chemistry)Unrestricte

Journal impact factors - the good, the bad, and the ugly

This paper provides an overview of the concepts of citations and journal impact factors, and the implications of these metrics for the Journal of the Southern African Institute of Mining and Metallurgy (JSAIMM). Two key research literature databases publish journal impact factors; namely, Web of Science and Scopus. Different equations are used to calculate journal impact factors and care should be exercised when comparing different journals. The JSAIMM has a low impact factor compared with some of the more prestigious journals. It nevertheless compares well with journals serving other mining sectors, such as the Canadian CIM Journal. The problems associated with journal impact factors are discussed. These include questionable editorial practices, the negative impact of this concept on good research, and the problem of a few highly cited papers distorting the journal impact factor. As a consequence, there is growing resistance to the use of journal impact factors to measure research excellence. The San Francisco Declaration on Research Assessment is a global movement striving for an alternative assessment of research quality. As a recommendation, the Editorial Board of the JSAIMM should adopt a pragmatic approach and not alter good journal policies simply to increase the journal impact factor. The focus should remain on publishing excellent quality papers. Marketing of the Journal, the quality of the published papers, and its open access policy should be used to counter the perception that journals with high impact factors are better options in which to publish good research material.https://journals.co.za/journal/saimmam2023Mining Engineerin

Rock engineering aspects of a modified mining sequence in a dip pillar layout at a deep gold mine

Scattered mining was practised on Kusasalethu Mine (previously Elandsrand Gold Mine) prior to 1998, but at deeper mining levels it was no longer feasible, since it would have resulted in unacceptably high stress levels and energy release rates. Longwall mining was not adopted at these depths as the mine required a more flexible mining method owing to the highly variable grade and the presence of geological structures. A mining method was developed that consisted of dip stabilizing pillars for regional support, as well as bracket pillars to clamp geological structures. A strict sequence of extraction was followed and this, together with the particular layout, was called the ‘sequential grid mining method’. This method addressed two key problems, namely negotiating adverse geology and the erratic grade of the Ventersdorp Contact Reef (VCR) orebody. However, a significant drop in production rates resulted in the need for alternatives and improvements to the original mine design. Modifications to the design were proposed in order to increase production rates, and an investigation to consider the rock engineering implications of these modifications was conducted. The study indicated that the modified method, called the ‘multi-raise mining method’, appears to be feasible and might address some of the production problems that were experienced with the original sequential grid design. An analysis of actual seismic data showed no significant differences between the original sequential grid mining and the implemented multi-raise mining. The numerical modelling of the mining layouts showed slightly higher interim energy release rates (ERRs) and average pillar stress (APS) levels during the extraction process. The final values are nevertheless identical to that of the original sequence. The study also investigated the use of a modelled moment method to analyse future seismic trends. The study illustrated that the expected seismic trends will be very similar for the multi-raise method compared to the original sequential grid mining method. This study is nevertheless considered of a preliminary nature and ongoing monitoring and analysis of seismic data at the mine is required to verify the response of the rock mass to the modified sequence and increased extraction rate. In particular, future work needs to investigate the effect of mining rate (advance rates in individual panels as well as volume of mining in particular raise lines) on the level of seismicity.http://www.saimm.co.za/journal-papersam2016Mining Engineerin

Numerical computation of average pillar stress and implications for pillar design

A number of issues relating to the computational aspects of pillar design are addressed in this paper. The computation of average pillar stress values is important when attempting to establish criteria for pillar design and in the analysis of the stability of tabular pillar layouts. One of the default ‘classic’ numerical methods that are used to determine pillar stresses is the displacement discontinuity method. In many instances it is not clearly understood that this approach does suffer from some limitations, particularly in relation to the fact that in coarse element simulations, the simulated average pillar stress (APS) can depend on the chosen mesh size. The nature of this error is highlighted in this paper and some strategies are suggested to bound the magnitude of these errors. It is demonstrated as well that the popular linear stiffness approximation to pillar or seam compressibility does appear to allow reasonably accurate estimates of the average pillar stress when either the pillar height is varied or when the seam modulus differs from the host rock modulus. A practical implication of this study is that if the seam modulus is noticeably lower than that of the host rock, such as for coal seams, it is important to use a linear stiffness constitutive model for the pillars rather than a ‘rigid’ pillar assumption. This added complexity seems unnecessary, however, when simulating hard rock pillars in mines where the seam modulus is very similar to that of the surrounding rock.http://www.saimm.co.za/ai201

Crush pillar support - designing for controlled pillar failure

The aim of any mine design is to ensure that the excavations remain stable for the period they will be in use. Various pillar systems are used to ensure that underground stopes remain stable and that mining activities do not affect the surface infrastructure through either surface subsidence or seismicity. Intermediate-depth platinum mines make use of in-stope pillars designed to fail while the pillars are being cut at the mining face. The pillar stress exceeds the loading capacity and the pillars crush as a result. The aim of the paper is to provide an overview of in-stope crush pillars. This will include the application, behaviour, function, mechanism, impact, and design of a crush pillar system.This paper was first presented at the, Platinum Conference 2014, 20–24 October 2014, Sun City South Africa.The work described in this paper forms part of the PhD study of Michael du Plessis at the University of Pretoria.http://www.saimm.co.za/journal-papersam201

Rockburst support in shallow-dipping tabular stopes at great depth

This paper investigates the unique problems associated with the design of rockburst support for shallow-dipping tabular excavations. These designs are particularly problematic when the stoping width is very small. In steep dipping orebodies, the layouts and mining methods can be selected to ensure that miners never enter the stopes. Only the access drives need to be protected by rockburst resistant support. In shallow-dipping orebodies, this problem is more difficult as miners enter the stopes and the entire hanging wall needs to be supported. A simple analytical model is used to investigate the implications for support design as a result of the convergence associated with the tabular geometry and the possibility of rocks being ejected during a rockburst. This illustrates that a support system is required that is initially stiff, but it should also be yieldable to survive the convergence in the back areas. By trial and error, the historic support solutions in the South African gold mines evolved into a system of timber packs and elongates to meet these requirements. When considering the three accepted key functions of modern rockburst support methodology namely, reinforce, retain and hold, the typical support design for these shallow-dipping orebodies does not meet all these requirements. The rock is highly fractured and fallouts occur between roofbolts during rockbursts as areal support is difficult to implement. Steel mesh is not used as it is often destroyed during cleaning operations. Solving this problem is of critical importance to ensure the viability of deep South African gold mines in the future.http://www.elsevier.com/locate/ijrmms2019-12-01hj2018Mining Engineerin

Sekuriteit vir koste deur maatskappye : terug na 1909 in die Transvaal, of nie?

Die Maatskappywet 71 van 2008 bevat nie ’n bepaling soortgelyk aan artikel 13 van die Maatskappywet 61 van 1973 rakende sekuriteit vir koste deur plaaslike maatskappye nie. Dit is kennelik ’n oorsig aan die kant van die wetgewer. Gevolglik kan daar nie van ’n incola maatskappy qua maatskappy sekuriteit vir koste in siviele verrigtinge geverg word nie. Dit is ’n terugkeer na die gemeenregtelike posisie in die Transvaal voor 1909. Alhoewel Hooggeregshowe oor inherente magte beskik om die gemenereg te wysig, word aan die hand gedoen dat ingryping deur die wetgewer noodsaaklik is om die oorsig reg te stel. Landddroshowe beskik nie oor sulke inherente magte nie en ingryping deur die wetgewer sal nie alleen tot regsekerheid lei nie maar ’n eenvormige posisie in hooggeregshof- en landdroshofprosedure meebring.http://www.lexisnexis.co.zaam201

A limit equilibrium fracture zone model to investigate seismicity in coal mines

This paper explores possible synergies between techniques used to minimise seismicity in deep South African gold mines and their applicability to control coal bumps. The paper gives a summary of the techniques used in the deep gold mines and a critical appraisal if these are useful in coal mines. The techniques typically include control of mining rate, preconditioning, optimisation of extraction sequences and centralised blasting. Of particular interest to the coal bump problem is an experimental limit equilibrium fracture zone model implemented in a displacement discontinuity code. This was recently developed for the gold mines to enable the interactive analysis of complex tabular mine layout extraction sequences. The model specifically accommodates energy dissipation computations in the developing fracture zone near the edges of these excavations. This allows the released energy to be used as a surrogate measure of ongoing seismic activity and addresses a number of the weaknesses in the traditional usage of this quantity as a criterion for the design of seismically active layouts. This paper investigates the application of the model to a hypothetical coal longwall layout and the specific problem of coal bumps.http://www.elsevier.com/locate/ijmstam2018Mining Engineerin

Simulation of time-dependent crush pillar behaviour in tabular platinum mines

It has been established that significant time-dependent stope convergence may occur over time periods of hours and days in certain hard-rock gold and platinum mines. The source of this time-dependent behaviour appears to be associated with both preexisting discontinuities and with mining-induced fractures that form near the stope face. These induced fractures may be associated with blasting processes and may also be formed in response to high stress concentrations in the unmined regions immediately ahead of the stope face. In shallower platinum mining operations, time-dependent behaviour is, however, observed to be much less marked unless some form of specific mining-induced fracturing occurs. One particular case of considerable interest is the time-dependent behaviour that is found to be associated with the formation and deployment of crush pillars. The purpose of the paper is to present a simple limit-equilibrium computational model of this behaviour that is sensitive to both the formation sequence and the size of planned crush pillars in a mine layout. This model provides a useful means to optimize the sizing of crush pillars, and at the same time may be used to identify potentially hazardous circumstances in which pillars may not crush in a stable manner.http://www.saimm.co.za/am2013ai201

A case study of geotechnical conditions affecting mining-induced seismicity in a deep tabular mine

This work forms part of the MSc study of Lourens Scheepers at the University of Pretoria. (http://hdl.handle.net/2263/78882)Seismic risk in the deep gold mines of South Africa has been studied for many decades. A clear understanding of the effect of geotechnical conditions on the seismic hazard nevertheless remains elusive. Certain reef types seem to be associated with a higher risk of rockbursts. The stability and deformation behaviour of excavations on the different reef horizons are affected by the rock types and the varying strength properties. The seismic response to mining is therefore also expected to differ according to the geotechnical conditions. As a case study of this behaviour, the seismicity at Mponeng mine was investigated. On the VCR (Ventersdorp Contact Reef) horizon, two areas can be delineated. On the eastern side of the mine, the footwall is shale, and on the western side it is a strong brittle quartzite. More large-magnitude events occur in the area with the shale footwall than the area with the quartzite footwall. Moment tensor analyses indicated that the majority of the large-magnitude events are not related to geological structures, but are face-related, implying that shear failure of intact rock is occurring ahead of the mining front. Preliminary modelling indicated that the closure volume for the shale footwall may be higher than that for the quartzite footwall, providing a possible explanation for the observed difference in seismic response.https://journals.co.za/journal/saimmam2023Mining Engineerin