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By C. Hsu

Underground coal gasification (UCG) is becoming more popular as the reserves of good quality, mineable coal are starting to diminish, and yet the global energy demand from coal is still increasing. The purpose of this research project was to investigate cavity formation within a coal block due to the combustion reactions in the context of UCG. The cavity plays a pivotal role in the UCG process, as it is essentially the gasification reactor. Cavity formation in an in situ gasification process using the forward combustion linking method (FCL) had been investigated, and a laboratory model was created to simulate the process. The experiment was performed by drilling a U-shaped tunnel into a coal block, which was then combusted internally with air that was fed through an injection hole. A heating element (at approximately 500°C) was used to supply the required heat for combustion at the base of the injection well. The coal blocks were analysed using micro-focus X-ray tomography. The tomography results showed that the coal tended to crack along the bedding plane after a short duration of combustion, due to either the formation of clinker or the expansion of swelling vitrinite along the horizontal tunnel. The deposit was thicker at the base of the injection well compared to the base of the production well; this may have been caused by the turbulence of the air flow and the relatively high oxygen concentration at the base of the injection well. A comparison of the results with work by Daggupati et al. (2010) showed the same trend, despite the slightly different methodology applied.

Jan 1, 2014

By K. Sasaki, S. Kuroiwa, H. Miki, Y. Aoki, T. Hirajima, Y. Orii, G. P. W. Suyantara

Selective flotation to separate copper minerals from, for example, Mo- and/or As-bearing minerals is a continuing problem because the selectivity of current systems is imperfect. Selective Cu-As flotation is currently difficult, and the most common Cu-Mo selective flotation method uses NaHS; this must be done in an enclosed system because, under acidic conditions, this method produces hydrogen sulfide. In this paper, some novel alternative flotation processes that use redox reactions are discussed; these methods have a much lower environmental impact. Oxidation and reduction reactions can be used to make copper minerals more hydrophilic and Mo-bearing minerals more hydrophobic. X-ray photoelectron spectroscopy (XPS) analyses indicated that the copper minerals became hydrophilic because hydrophilic iron precipitated and several hydroxides covered the copper minerals’ surfaces, whereas the molybdenum minerals remained hydrophobic because their sulfide surfaces were not covered. Flotation experiments on these minerals after redox treatments indicated that excellent separation of Cu-minerals from Mo-minerals can be achieved. Possible mechanisms for this separation were investigated using detailed surface analyses and thermodynamic calculations. The additives used to initiate the redox reactions are easy to handle and make these new flotation procedures better than current processes because the new methods do not emit effluent gas and because they are commercially viable. Keywords: Flotation; chalcopyrite; molybdenite; oxidation; reduction

Jan 1, 2020

By W. Newcomen, G. Dick

"Effective management and mitigation of pit slope instability in open pit mines begins with a comprehensive slope monitoring programme. The ability to differentiate between non-critical pit wall movements due to rebound or relaxation of the excavated slopes and movements that may be indicative of slope failure is important for maintaining a safe working environment and maximizing production. Slope failure prediction methods using velocity, acceleration, and strain criteria have been introduced and put into practice over the past few decades. Forty-eight slope failures with surface monitoring data are presented and assessed using the strain-based failure prediction approach, with consideration of the quality of the rock mass and the potential failure mechanism. The results indicate that the strain-based approach can be used to provide general guidance regarding strain thresholds for pit walls for a variety of failure modes in diverse geological environments. The advantages and potential drawbacks of the strain-based and other slope failure prediction methods are discussed. The importance of implementing a pit slope monitoring and performance evaluation system early in mine development is also emphasized. IntroductionThe successful prediction of the Manefay Slide pit slope failure at Bingham Canyon at 9:30 am on 10 April 2013 (Engineering and Mining Journal, 2014) highlighted the importance of having a comprehensive slope monitoring system that provides an accurate prediction of the time of failure of an open pit slope. Using most of the geotechnical monitoring tools available, the time of the failure was predicted within hours and equipment and personnel were evacuated from the pit well in advance of the failure. From a time to slope failure perspective, the monitoring system in place was a success.There are two general groups interested in monitoring pit walls, with slightly different objectives in mind when the monitoring system is being set up. There are those at the mine site that have to implement the pit slope design, monitoring protocols, and the action response plans, and there are those designing the slopes (often consultants) who regularly visit the mine but may not be responsible for the day-to-day activities. was a success.There are two general groups interested in monitoring pit walls, with slightly different objectives in mind when the monitoring system is being set up. There are those at the mine site that have to implement the pit slope design, monitoring protocols, and the action response plans, and there are those designing the slopes (often consultants) who regularly visit the mine but may not be responsible for the day-to-day activities."

Jan 1, 2016

By M. A. Belokrys, F. O. Milovich, D. D. Aksenova, V. A. Ignatkina, V. A. Bocharov, A. A. Kayumov

Copper remains one of the most sought-after metals in the industry. Tennantite (Cu12As4S13) and tetrahedrite (Cu12Sb4S13) that occur in pyrite copper, copper-zinc and polymetallic ores are an additional reserve for increasing technological parameters of flotation. Pyrite mass (85% and more) has similar flotation properties to other sulfide minerals, and imperfection of reagent regime is a common problem that occurs during quality copper concentrate processing. Due to tennantite's and chalcopyrite's differences in flotation properties with butyl xanthate at pH > 10, tennantite often reports to the flotation tails. Hight alkalinity of the medium increases the oxidation level of sulfide minerals, thus increasing the concentration of S2O3 2-, SO3 2- SO4 2-. At processing facilities in the Russian Federation, the concentration of thiosulfate ions in pulp is in the range of 0.5 to 1.5 g/l and higher, which leads to a decrease in flotation activity of the tennantite. The effect of butyl xanthate (as standard), thionocarbamate, dithiophosphate and M-TF (a mixture of dithiophosphate and thionocarbamate) on monomineral (tennantite and pyrite) flotation in the presence of sodium thiosulfate was studied. The variable parameters of the "thiosulfate-collector" flotation complex were thiosulfate concentration (100-900); pH (8-12,5). The scientific methods included contact angle for hydration assessment; TEM for surface studies; UV spectroscopy and iodometric titration for solubility control. Comprehensive studies have established a connection between the concentration of thiosulfate ions and tennantite flotation selectivity. Flotation activity of tennantite is higher at pH 8 with a thiosulfate concentration under 300 mg/l. Pyrite remains active with butyl xanthate under the same conditions. TEM images of the surface films of the collectors on pyrite were acquired at pH 8. The MTF provides the biggest difference in adsorption, wettability and flotation activity between tennantite and pyrite at pH 8 and thiosulfate concentration under 300 mg/l, in comparison with butyl xanthate. The results of the research will be applied to improve selective reagent regimes of tennantite minerals flotation. Keywords: Tennantite, pyrite, chalcopyrite, sphalerite, flotation, sulfhydryl collectors, thiosulfate, adsorption, mechanism, reagent regime

Jan 1, 2020

By L. Ntlhakana, K. Khoza-Shangase, G. Nelson, I. Maposa

Occupational noise-induced hearing loss (ONIHL) is associated with various risk factors in South African mines. We investigated the association between standard threshold shifts (STSs) and exposure to noise and platinum mine dust (PMD), using demographic data, five years of annual audiometry screening results, and noise and dust exposure data. Miners’ age, sex, percentage hearing loss (PLH), and dust and noise exposure data were gathered, and a linear mixed effects regression model used to predict STS. Average occupational exposure levels to noise and dust were calculated from recorded measurements. A total of 12 692 records were analysed. Most miners were male (89.6%) and more than 50% were younger than 41 years. More than 70% were exposed to > 85 dBA noise and 58% were exposed to 1.5–2.99 mg/m3 PMD. Changes in hearing levels ranged from 8.3 dBHL at baseline (2014/2015) to 10 dBHL in 2016 (STS), with no changes thereafter. The model showed that the estimated effect on STS for males was 27% and 21% higher than for females, for the left and right ear, respectively The estimated effect of age, PLH, noise exposure and years of exposure on STS was < 10% for each variable. There was no statistically significant association between PMD and STS. The combined effects of age, sex, years of exposure to noise, and noise exposure levels, and strength of associations can be used to predict STS for this group of miners. Our findings may be used to measure the efficacy of the mine’s hearing conservation programme.

Aug 1, 2021

By U. König, N. Norberg

Ore mineralogy defines the properties during processing and metal extraction. Fast and frequent monitoring of the mineral content and related process parameters brings value to the mining operation and material processing. Decreasing ore qualities and increasing prices for raw materials require a better control of processed ore and a more efficient use of energy. Traditionally quality control in mining industries has relied on time consuming wet chemistry or the analysis of the elemental composition. The mineralogy that defines the physical properties is often monitored infrequently, if at all. The use of highspeed detectors has turned X-ray diffraction (XRD) into an important tool for fast and accurate process control. XRD data and their interpretation make the difference in the identification of minerals, in describing their distribution in ore bodies and in their beneficiation during processing. The use of statistical techniques such as Partial Least-Squares Regression (PLSR) or Principal Component Analysis (PCA) on XRD raw data has been successfully trialled to determine raw materials and processed ores, in addition to well established methods for mineral identification and quantification. PLSR can be used to correlate process parameters directly from the XRD pattern without investigating the mineral content. Hidden information present in the XRD pattern but not determined with traditional methods can be used to improve the characterisation of the analysed materials. The practical use for the direct determination of process relevant parameters during exploration and ore beneficiation will be illustrated in several case studies from mining and ore processing. Keywords: Geometallurgy, mineral monitoring, X-ray diffraction, process efficiency

Jan 1, 2020

By W. A. Smith, M. C. Bekker, C. Marnewick

Projects or project-orientated approaches have become a common form of work in nearly all sectors of economies. This has led to concepts such as ‘projectified’ and ‘project orientated’ organizations. By defining projectification of a company, industry, or economy as the share of project work in total work, one can reasonably determine the impact that project management, and by default projectification, has had on that company, industry, or economy in terms of staff optimization and allocation. This paper presents the results for such a projectification study of the South African mining industry. This sector has long been a significant contributor to the country’s economy from a gross value added (GVA) and employment point of view. Understanding the impact of projectification and the project management way of work on this industry may potentially add significant value to both the mining and project management knowledge areas. We show that although the mining industry is considered by some to operate in archaic ways, the level of projectification has increased over time, and now represents approximately one third of all work conducted.

May 1, 2021

I. Book review Legal and institutional arrangements in minerals development. Edited proceedings of a workshop organized jointly by the VN. Department of Technical Co-operation for Development and the German Foundation for International Development (DSE), held in Berlin in August 1980. London, Mining Journal Books, 1982. Soft, 228 X 149 mm, xi + 223 pp. Approx. R30 + GST. Reviewer: M. J. Martinson 2. New books Strength of metals and alloys (IOSMA 6). R. C. Gifkins (Editor). New York, Pergamon, 1982. Approx. 1200 pp. 3 vols. V.S. $220. Deformation-mechanism maps. The plasticity and creep of metals and ceramics. H. J. Frost and M. F. Ashby. New York, Pergamon, 1982. Approx. 250 pp. V.S. $60. Pressure diecasting, Part 2. The technology of the casting and the die. D. F. Allsop and D. Kennedy. New York, Pergamon, 1982. $ V.S. 27.50.

Jan 1, 1982

By D. Moses, J. A. Onyango

Carbonatites are generally competent rock masses with rock mass rating class II rating 60–74. In spite of their competency, they tend to be affected by weak features like manganese-iron veins and/or in situ rock damage due to brecciation associated with carbonatite complexes. Rock slope failure in such hard rocks is complex since such structures within the rock mass form weak links that could potentially control slope instability. In this contribution, a numerical simulation using phase2 v 7.0 was carried out to investigate the influence of in situ rock damage on the stability of mine pit walls. The outcome reveals that, the existence of breccia in the competent rock mass has the capability to reduce the slope stability performance particularly at gentle dipping angles of emplacement in close range to the slope toe. However, as the emplacement position of breccia moves away from the pit wall, the stability performance increases at gentle dipping angle <50º. On the contrary, at the dipping angle of 50° the performance of slope reduced, and at steeper angles >50° the impact becomes negligible. Thus, from a series of analyses, mine design in brecciated rock masses, the ratio of 1:5 between the breccia distance from slope toe and pit depth should be implemented to counter its impact, and if the breccia is within or close to the pit limit, a deliberate effort must be made to mine it out.

Sep 1, 2025

By L. Morgan, M. Le Roux, Q. P. Campbell

Dry coal processing is becoming increasingly favoured as a fine coal beneficiation technique. The decline in suitable good-quality coal, as well as the ongoing decrease in usable process water, makes dry beneficiation of the fine coal fraction even more crucial. Dry fine coal beneficiation is a relatively young process with most of the methods still in the developmental phase. Additionally, many of the dry processing options available are better suited for coarser (+6 mm) and easy-to-beneficiate coal. Air winnowing for fine particles has been established in both agricultural and pharmaceutical industries, which indicates that the method can be used to separate smaller particles on a density basis and therefore, may prove effective in the coal industry as well. A proof of concept study was conducted using both computer-based simulations (which do not form part of this paper) and physical experiments. The initial winnowing unit consisted of a closed box with a horizontal air stream flowing through the top section. Coal particles were injected into the air stream and displaced horizontally across a distance (x), depending on the particle size and density, thus actuating a separation based on these two parameters. For this study particle sizes of 6, 4 and 2 mm were used all with a density ranging from 1200 kg·m-3 to 2000 kg·m-3. The simulation in Star CCM+ confirmed the findings of the mathematic behavioural model and provided a virtual representation of the separation using streamlines and particle tracks. The coal tests prove that size separation can be achieved at an airflow velocity of 21 m·s-1 and the density cut-point can also be predicted by using the prediction matrices constructed as part of this study. The results show efficiency values (EPM) of 0.11 and 0.09 at density cut-points of 1400 kg·m-3 and 1500 kg·m-3 respectively. The data obtained from the study indicates that air winnowing can be used to separate fine coal particles based on both size and density. However, in order to achieve an efficient density separation, a narrow size range is required. With some improvements to the model, simulation, method and design, the separation efficiency can also be improved. The next phase of testing will include the upscaling of the process to a demonstration plant, optimising the separation unit and testing the method on different qualities of coal. Keywords: Dry processing, fine coal, winnowing, simulation, tracers, demonstration plant

Jan 1, 2020

By F. Dong, F. Yang, J. Wang, Z. Wang, D. Mingxia

Antimony oxide ores are important antimony resources, but their recovery is still a problem. In this paper, the adsorption behaviour and mechanism of dodecamine and quaternary ammonium salt on the surface of cervantite and quartz were investigated. The flotation behaviour of cervantite and quartz using quaternary ammonium salts and dodecylamine as collectors was studied by single mineral flotation experiments. The adsorbed amount was measured and infrared spectroscopy was used to study the ability of several cationic collectors to float cervantite and quartz. The results show that quaternary ammonium salts can simultaneously float the cervantite and quartz. It is difficult to achieve separation of the two minerals using quaternary ammonium salts as collectors. At pH>10, dodecylamine can float quartz but not cervantite, which can achieve reverse flotation separation of the two minerals. The difference between the abilities of quaternary ammonium salts and dodecylamine to float cervantite and quartz lies in the different ionisation properties of the two reagents. Quaternary ammonium salt is a strong electrolyte that is completely ionised in the pulp and exists mainly in the form of quaternary ammonium ions. Dodecylamine is a weak electrolyte and predominantly forms molecules under alkaline conditions. Keywords: Cervantite, quartz, dodecylamine, quaternary ammonium salts, flotation

Jan 1, 2020

By N. Duda, P. Śliwiński, P. Stefaniak, B. Jachnik

Ventilation and air-conditioning are some of the most energy-consuming processes in deep underground mining. Additionally, the energy consumption of these processes systematically increases along with the expansion of the mine. Therefore, proper energy efficiency assessment and consumption forecasts of these systems are particularly important in modern mines for ongoing tracking and planning of the likely costs. In the article, a short-term prediction model for surface air–conditioning stations is described. This model takes into account dependencies that were identified, based on the data from the supervisory control and data acquisition (SCADA) systems. These dependencies between the response value and the external conditions are then incorporated into the model. This way, for example, the ambient temperature or the total number of specific days of the week (with sometimes significantly lower or higher than the weekly average energy consumption levels) are included. Taking into account these dependencies, and e.g. forecasted average monthly temperatures allow us to include the seasonality of energy consumption in the presented model. In the article, a sample energy efficiency analysis of the A/C system based on an example of the KGHM Polish Copper Polkowice-Sieroszowice mine is performed. The results of the solution proposed in the article are then evaluated, and compared to the currently functioning, more basic forecasting methods. However, given the general nature of the dependencies used in the model, similar solutions could be used in other case studies.

Sep 1, 2021

By H. M. Friede

Notes on the composition of pre- European copper and copper-alloy artefacts from the Transvaal by H. M. Friede. Ph.D. (Visitor) Seven copper artefacts (five ingots and two ornaments) found at prehistoric sites in the Transvaal were submitted to analysis. All the artefacts showed a high copper content and a relatively Iow level of impurities. A little-known ingot type from the Limpopo Valley (&apos;nail&apos; ingot) is described, and its analysis is given. The analysis of two copper ornaments that come from sites dated to the 5th Century AD. and the 11th Century AD. showed the constancy of the composition of copper articles over long periods. The well-known &apos;Blaauwberg Ingot&apos; was re-analysed, and the results showed that this ingot cannot be regarded as a true bronze. Analytical values for the composition of three brass artefacts from Bambandyanalo (Mapungubwe), Northern Transvaal, are given. Some aspects of pre-European bronze manufacture and brass smelting, and of early metal trade from the East Coast into the interior of South Africa, are discussed.

Jan 2, 1975

By F. D. Agrensa, J. Hadjigeorgiou, E. Storimans, S. J. Thorpe

Reinforcement and surface support are part of an integrated system that transfers and shares loaduntil the excavation surface is stabilized or until the ground support system fails. Steel mesh is the most popular primary surface support element in underground hard rock mines in Canada. Mesh straps are a secondary surface support that provide additional containment as they distribute the load between and across the reinforcement elements. This investigation addresses the role of steel properties, and in particular steel chemistry, in the long-term performance of mesh and mesh straps when exposed to an aggressive corrosive environment. It reports on comparative accelerated corrosion studies to compare the resistance to corrosion of different surface support elements. This has significant implications on the choice of surface support and the anticipated rehabilitation requirements.

Oct 9, 2024

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By A. M. Garbers-Craig, T. Coetsee, T. M. Nemavhola

Accretions often form in furnaces when slag and charge materials attach to the refractory wall and build up over time. Accretion formation is usually unwanted because it reduces the working volume of the reactor and hinders material flow through the reactor. However, in some instances a thin, stable accretion layer may be desirable to protect the underlying refractory material. In order to prevent and/or manage accretion formation, it is important to understand the underlying principles of this phenomenon in the particular reactor. Excessive accretion formation hampered production at the Exxaro FerroAlloys ferrosilicon melting and atomization plant. This plant uses induction furnaces in which a 15% silicon-iron alloy is produced by batch smelting a mixture of ferrosilicon of 75%Si grade and low-carbon steel. The molten ferrosilicon alloy is then gas-atomized to a powdered product for use as a dense medium in mineral processing plants. The objective of this study was to investigate the effect of different impurity levels in the ferrosilicon feed material on the extent of accretion formation as well as the effect on the accretion properties, which influence the ease of accretion removal upon furnace shut-down. Refractory and accretion samples were collected after a furnace shut-down and characterized using X-ray diffraction and scanning electron microscopy–energy dispersive spectroscopy. It was concluded that the trace elements in the FeSi-75 feed material (Al, Ca, Mn) were mostly responsible for accretion formation, but that rust on the low-carbon steel and oxidation of the steel contributed to accretion attachment to the lining. The total contaminant content, calcium to aluminium ratio in the FeSi-75 feed material, and thereby the liquid to solids ratio in the accretion at temperature determine the strength of attachment as well as growth of the accretion.

Feb 2, 2021

By E. Marè, D. R. Groot, R. Engelbrecht, T. Coetsee

Hydrogen peroxide is often used as an oxidising agent in experimental waste electronic equipment (WEE) leaching work. However, the high cost of peroxide is a negative factor in its consideration in further process development. A few studies have reported on the efficient application of peroxide addition schedules to ensure optimum reagent consumption. Large quantities of unleached substrate material contained in WEE feed material result in a large dead volume of material in the leaching reactor. This factor necessitates fast reaction rates to enable the use of a high feed material throughput reactor to enhance overall project viability via hydrometallurgical processing. In this study the effects of hydrogen peroxide addition schedules on base metal leaching efficiency and solution temperatures were tested at laboratory scale. The results showed that overall peroxide consumption can be optimised to lower levels by matching peroxide additions to the particular feed material’s leaching behaviour. A simplified integrated model for peroxide decomposition is used to explain its effect on solution temperatures and leaching rates. Keywords: Peroxide consumption, waste electronic equipment, copper leaching, integrated model

Jan 1, 2020

"Chairman: Dr N. G. W. Cook Rapporteur: Mr M. P. RobertsPapers:Planning open pit mining operations using simulation by A. Bauer and P. N. Calder Simulation for control of underground liquid flow by G. N. Pitts and P. B. CrawfordEvaluation of production strategies in a group of copper mines by linear programming by P. H. Williams, J. N. Brooke and D. M. PoulterAn equipment maintenance system by D. W. Dean and J. A. RyderTaken as a whole, the difference between the four papers in this session were perhaps more illuminating than their similarities. The titles of the first and second papers contained the word &apos;simulation&apos;, and in both papers random number generation was relied upon in the selection of inputs from empirical distributions, but the nature of the models to which these values were inputs were quite different. The third paper displayed another method of uniting the empirical and the theoretical in a linear programming formulation whose matrix coefficients were measured empirically, and here some apt remarks on computer implementation were included. This paved the way for the last paper which abandoned all theoretical pretensions in favour of a detailed description of computer implementation and perhaps as a result gave rise to the most feeling audience participation.The systems mentioned were developed for the iron, petrol, copper and gold producing industries in the US, Canada, Zambia and South Africa..As with all the parallel sessions, contributions from the floor may have been limited by the shorter time available to delegates to acquaint themselves with the papers before the session. In spite of this, and due largely to the active chairman¬ship of Dr Cook, the discussion during the session which is summarized here contributed much to the information contained in the papers themselves.In his presentation, Dr Calder summarized his paper. He expected that simulation would be used in future on an almost universal basis in open pit mining. It could contribute new knowledge more cheaply than experimentation in the flea For example, this study had shown what was previously thought to be a matter of more shovels and trucks was really a blasting problem, and improving blasting had increased production to the desired level. The simulation had shown that uniformity of truck performance was a desirable factor. It had also been used to study the advisability of a dispatching system.Dr Ramani contended that where equipment characteristics were available, deterministic standard simulation could adequately simulate systems in which interference occurred. Operator performance could be measured against perfor¬mance curves predicted by standard simulation and poor operators spotted and retrained. The present system did not take into account variability in truck loading, for instance. Dr Calder replied that if the number of variables was large, the standard approach became too complicated for operating personnel to follow. He admitted he had not tried it. The variability of truck loading could be coped with easily using the Monte Carlo approach. Operator variability was already incorporated because the time studies are actual."

Jan 1, 2014

By R. L. Setsepu, S. O. Bada, J. Abdulsalam

The utilization of biomass as a solid fuel for co-firing has received great attention from boiler manufacturers as a clean coal technology (CCT) option. This research aimed to produce biocoal pellets, as a clean energy fuel, using hydrochar from trees planted to rehabilitate acid mine drainage (AMD) water and fine coal discards. The hydrochar was synthesized by hydrothermal carbonization of Searsia lancea harvested from AMD-contaminated land at a temperature of 280°C and a residence time of 90 minutes. It was blended with discard coal (–1 mm) at ratios of 25% and 50% hydrochar to produce different forms of solid pelletized biocoal (BC). The physicochemical and mechanical properties of each of the biocoal pellet blends were determined. The 100% hydrochar had the highest calorific value of 29.99 MJ/kg, while the raw discard coal had a calorific value of 16.73 MJ/kg. The ash content decreased from 42% in the discard coal to 25% in the blend of 50% coal and 50% hydrochar biocoal pellets. Biocoal pellets comprising 25% hydrochar and 75% discard coal (BC25 HC/75 COAL) displayed the best mechanical properties (compressive strength 3.06 MPa) of all the fuels, but the physicochemical properties were inferior to the BC50 HC/50 COAL pellets. This research has demonstrated that hydrochar synthesized from a tree species planted for hydraulic control of AMD has the capability to act as a binder for improving the mechanical properties and energy characteristics of fine discard coal.

Dec 1, 2021

By M. A. El-Sayed

"Double oxide films (bifilms) are significant defects in light alloy castings that occur when, during melt handling, the oxidized surface of the melt is folded over onto itself and then entrained into the liquid metal, encapsulating a portion of the local atmosphere. These defects create crevices in the final castings and have been shown to decrease tensile and fatigue properties, and also to increase their scatter, making casting properties unreproducible and unreliable. This paper reports on two different experiments performed to study the behaviour of such defects in Al alloys. In the first experiment and as a proxy for a bifilm, air bubbles were held in an Al-5Mg alloy melt for varying periods of time before the melt was allowed to solidify. The composition of the gas content of the bubbles was subsequently analysed using mass spectrometry to ascertain how it changed with time. In the other experiment the influence of the age of bifilms on the mechanical properties of Al-5Mg castings was assessed by holding castings in the liquid state for different times before solidification, followed by mechanical property testing and SEM and EDX studies of the fracture surfaces of tensile test bars. This research could provide a better understanding of the behaviour of double oxide film defects, and may lead to the development of new techniques by which these defects could be eliminated, or at least deactivated, in light alloy castings. Introduction One of the major structural defects affecting the reproducibility of mechanical properties of aluminium castings is the double oxide film defect (El-Sayed and Essa, 2017; El-Sayed and Ghazy, 2017). This type of defect is usually created as a result of poor filling system design. Owing to the very high reactivity of liquid Al-Mg alloy with oxygen in the air, the surface of the liquid metal can be considered to be permanently coated with an oxide film (Weigel and Fromm, 1990). As a consequence of surface turbulence, the oxidized metal surface can become folded over onto itself, but not fused, and then submerged into the bulk liquid, with a portion of the mould atmosphere being entrapped within it (Campbell, 2003; Al- Sayed, 2016; El-Sayed, Hassanin, and Essa, 2016)."

Nov 1, 2018