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By A. A. Nourbakhsha

The main application of refractories based on andalusite in ferrous metallurgy are in air heaters of blast furnaces, mobile mixers, and hot-metal transfer, pig iron ladle, steel-casting, and intermediate ladles. Three specific properties of andalusite are very interesting for the production of high-quality shaped and unshaped mullite refractories: being free of water, its crystal a remains stable without changing its density or volume after firing and therefore has no internal porosity .Graphite additives due to its great wetting ability play an important role in raising the property of refractory bodies. In order to improve properties of andalusite graphite bodies, oxidant agent for increasing in hot strength and oxidation resistance can not be avoided. In this research, firstly 8 pig iron ladles from ESC(esfahan Steel company) with different conditions selected and monitored during usage to determine of main factors affecting on corrosion of refractories. Results showed that tap to tap time, undischarged pig iron ,penetrated pig iron are the main ones.Literature studies showed that the graphite addition help to thermal shock resistance and liquid penetration(steel,pig iron) effect. In this research particle size distribution of aggregates containing andalusite and graphite determined by andreazen equation. Then different mixes with different percentage of graphite,Si, FeSi, SiC prepared by uniaxially pressing under 1500kgf/cm2 pressure by adding 4%wt novalak resin and 0.4%wt HMTA as hardner. Samples fired at 1100-1500oC under reduction atmosphere. According to specific standards density, porosity and CCS measured. Analytical tests such as XRD, SEM were carried out on sample selected from point of view of mechanical properties at different temperatures. Results showed that optimized amount of Si and Fe-Si for _-SiC formation in body is 4 and 6 %wt respectively at 1500OC. XRD and SEM analysis on sample with Fe-Si additives showed no destructive phase through the matrix, also SiC additives not only increase oxidation resistance but also no effect on thermo-mechanical properties.

Oct 1, 2011

By Sadough vanini Ali

The silicon anisotropic wet chemical etching technique is widely employed for fabricating various microelectromechanical system (MEMS) structures for advantages such as simple etching equipment and its capability to fabricate special microstructures. In this paper, we consider a complete theoretical derivation of the analytical solution based on geometrical and kinetic aspects of step flow on any surface and with a mathematical method we derive new equation for calculating the microscopic etch rates of surface atoms for a new range of concentrations and temperatures. Furthermore, a novel equation is presented for the calculation of the microscopic activation energies for some typical surface atoms and a new comparison of activation energies between some atoms is made that divides atoms into three categories. The results demonstrate agreement with the experimental results.

Oct 1, 2011

By Mohammad Mehdi Salarirad

Fouling of activated carbon by different organic species is a great concern in carbon in pulp and carbon in leach circuits. The rate of gold adsorption onto activated carbon from synthetic gold solutions which contained different amounts of potassium ethyl xanthate (PEX), potassium isobutyl xanthate (PIBX), and methyl isobutyl carbinol (MIBC) were evaluated through batch adsorption tests. The results of these experiments showed that the gold sorption rate rapidly decreases by increasing the concentration of collectors including PIBX and PEX up to 20 mg/L and at higher concentrations, the kinetic rate constant (i.e., k value) gradually leveled off to 53 h-1 for PIBX, which is 46% lower than the 99 h-1 for PEX. At low concentrations of up to 15 mg/L of MIBC, a gradual decrease of the rate constant was observed, while at higher concentrations, the decrease was found to be more pronounced. The gold loading capacity and the rate of gold adsorption onto activated carbon from a gold cyanidation pulp in the presence of flotation reagents were also evaluated. The results of these experiments showed that the flotation reagents have no significant effect on the gold loading capacity of activated carbon at prolong adsorption period, meanwhile the rate of gold adsorption continuously decreases by increasing the concentration of flotation reagents in the leaching solution.

Oct 1, 2011

By A. A. Nourbakhsha

During the last few years, low cement castable refractories found more application in industry, because of good low and high temperature mechanical properties.In the case of alumina based castables ,usually fume silica or reactive alumina are used as superfines in the matrix to form mullite at high temperature which depends to source of alumina . Mullite network formation improves the hot mechanical properties and thermal shock resistance of bodies. The mullitisation of andalusite (so called primary mullite) also produces a certain amount of rich silica glass, part of it being expelled out of the grain. This phenomenon restrict the application of andalusite low cement castable in pig iron ladle. In this research, firstly 8 pig iron ladles from ESC(esfahan Steel company) with different conditions selected and monitored during usage to determine of main factors affecting on corrosion of refractories. Results showed that tap to tap time and undischarged pig iron are the main ones.Therefore monolithic refractories such as castables which increase thermal shock resistance could be substituted. Regarding to ternary phase diagram of Al2O3-SiO2-CaO ,fumed silica was selected as fine particle.After optimization of particle size distribution by using of modified andreasen equation and rehological properties ,different mixture were composed with andalusite ,fume silica, reactive alumina,secar 71 ,SiC and kyanite.All aggregates system components were mixed dry for 1 min in hobart mixer .After water addition wet mixing was continued until getting flowing about 4 min.The ready mixes were poured in the moulds of of 25mm X 25mm X 140mm with vibrations .After demoulding the samples were cured for 24 hrs and dried at 110OC/18hrsand then fired at 1100-1500OC. Physical ,mechanical ,phase analysis(XRD) and microstructure studies (SEM)were carried out according to ASTM standards. Results showed that by addition of kyanite ,more mullite phase formed through the matrix but,because of expansion occured during this phase transformoration, some cracks formed and mechanical properties decreased.By SiC addition the silica film on the surface of SiC lead to improvement of mechanical properties at high temperature

Oct 1, 2011

By Saeid Akhavan

Accumulative roll bonding (ARB) is a severe plastic deformation process for achieving ultrafine grains in metallic sheets without changing the sample dimensions. In this study, ARB process was carried out on 5083 aluminum alloy sheets up to 6 cycles. In this process, the surfaces of the strips to be joined are roughened and cleaned and then stacked. After stacking, the specimen is roll-bonded by rolling using a 50% reduction. The rolled specimen is cut into two halves, and the above-mentioned procedure is repeated for several times. Microstructural characterizations were analyzed by transmition electron microscopy (TEM). Tension and Vickers hardness tests were performed on produced sheets for evaluating their mechanical properties. Moreover the texture parameter of the ARB-ed samples was calculated using their XRD patterns and the related equation. Additionally, the texture evolution during the ARB process was demonstrate by analyzing the measured pole figures and orientation distribution functions (ODF) and skeleton lines. The results indicated that at the early stages of ARB process, grains were subdivided by deformation induced boundaries. After several ARB cycles continuous recrystallization resulted in a microstructure covered with small recrystallized grains with an average diameter below 100 nm at the end of four cycle. Texture analysis revealed fiber textures have been discovered with continiuose of the ARB process cupper. Brass, S and dillamor textures identify which their differences are in intensity of ARB process. Results indicated conformity between mechanical evolutions and texture evolutions during ARB process.

Oct 1, 2011

By Mahmoud Shakeri1

Various types of thin-walled structures have been frequently used as energy absorbers to mitigate the adverse effects of impact and hence protect the vehicle structure under consideration when subjected to different loading conditions. Compression tubular structures that buckle in a progressive manner, can be used to advantage, providing inexpensive and versatile, but nevertheless weight and stroke efficient energy absorbers. In this paper quasi-static nonlinear finite element simulations are performed to study the effects of number of corners of the cross-section, wall thickness, plastic hardening rate of material, foam-filler and load angle on deformation mode and mean crushing force of tube. Crush strength increases as the number of corners of the cross-section increases, though it almost saturates for the number of corners beyond 11. Higher plastic hardening rate stabilizes the collapse pattern. Foam-filled tubes seem to have higher critical load angles in oblique loading and display a more stable and gradual decrease in mean load over the transition region. Substantial decrease in the initial peak load and dynamic load at any a given deflection was observed as the load angle increases. The investigation of bitubal arrangements and Multi-cell tubes reveals that these may be preferable to monotubal specimens due to the presence of the inner profiles. In addition, a comparison is conducted between the numerical and experimental results of the mean forces of some special angle elements. Good agreement is obtained.

Oct 1, 2011

The adoption of new technologies is accompanied by risks that the stakeholders will wish to understand and minimise. The process of developing a dynamic simulation and the plant it will be used in provides a discipline and attention to detail that highlights and provides an opportunity to mitigate risks that will exist in a production environment. The simulations will enable investigation of what the outcome of process failures will be and the impact maintenance. Enumerating this information will enable designs to be optimised and operating and control strategies to be tested.The success of new technology is dependent on the operators and their understanding of it and the machines that use it. In this context dynamic simulations can provide training and experience without risk to personnel, equipment or production. This experience will include situations that would not be induced in a production plant for training purposes. This paper outlines a generic template for a dynamic simulation that can be used to evaluate the introduction of an emerging technology into an operating environment. It includes process flows and surge capacity, breakdown and maintenance, operator interactions and availability, push and pull production scheduling and system control.

Sep 26, 2011

By J Ortiz

In almost all copper mining companies, mine planning is done considering different time frames, leading to long-term (life-of-mine) (LOM), medium-term (first three years) and short-term (first year, monthly, weekly, and up to daily plans). Shorter term plans are usually associated with more abundant information, hence models with lower uncertainty. However, they capture more of the variability found on the geological attributes. In long-term models, usually estimation methods tend to hide this variability, providing smoothed interpretations.Medium and long-term plans consider average values for the parameters and optimisation techniques are applied aimed at maximising the economic profit. Short-term plans, on the other hand, are made to comply with the medium-term promises, maximising the use of the available resources (equipment and processing capacities) and dealing with the variability not characterised in the previous planning stages. This generates an important gap between the short-term and the medium and long-term plans. A case study was made using an important oxide copper mine, where the variability found in the short-term plan has a large impact in the productive process, thus reducing the daily ore grade variability is an important goal. In this paper, this planning issue is addressed by creating medium-term mine plans using mixed integer programming which are compared using the output of a simulation of the daily extraction that is aimed at satisfying the daily constraints of the processing plant, the availability of mining equipment and the variability of ore grade. The result shows the expected gap between these mine planning processes, which suggests correcting some of the parameters used in future medium and long-term plans.

Sep 26, 2011

By H Waterer, L Zheng, T Kalinowski

The Hunter Valley Coal Chain (HVCC) consists of mining companies, rail operators, rail track owners and terminal operators, together forming the worldÆs largest coal exporting facility. In 2008, the throughput of HVCC was about 92 million tonnes, or more than ten per cent of the worldÆs total trade in coal for that year. The coal export operation generates around $15 billion in annual export income for Australia. As demand has increased significantly in recent years, and is expected to increase further in the future, efficient supply chain management is crucial. The Hunter Valley Coal Chain Coordinator Limited (HVCCC) was founded to enable integrated planning and coordination of the interests of all parties, thus improving the efficiency of the system as a whole. One of the many planning challenges faced by the HVCCC is that of annual maintenance planning. Different supply chain elements, such as trains, railway track, terminal equipment and load points, must undergo regular preventive and corrective maintenance, leading to significant reductions in system capacity (up to 15 per cent). However good alignment of the maintenance tasks can reduce their impact, and the HVCCC undertakes an annual process to ensure the impact of maintenance on the supply chain capacity is as small as possible. This is achieved in an iterative negotiation process between HVCCC and individual service providers. In the past, maintenance schedule optimisation was largely manual, which for the more than 1000 tasks involved is quite labour-intensive.In this paper the authors describe an approach developed at the University of Newcastle in partnership with the HVCCC to automate this schedule optimisation process. We will discuss our experience in applying exact (mixed integer programming) and heuristic techniques from mathematics and computer science to address the problem. This work is anticipated to lead to new decision-support tools for the HVCCCÆs capacity planning team.

Sep 26, 2011

By D T. Ribeiro, J F. C L Costa, J C. Koppe

Drilling and sampling are responsible for most of the costs during mineral exploration, especially in multi-elements and polymetallic deposits, such as iron, manganese and bauxite deposits. On these deposits, often it is necessary to estimate multiple attributes at several granulometric fractions, ie distinct particle sizes. This paper investigates the effect of sampling at a close spacing grid only the head grades for the multiple chemical species in iron ore deposits. The remaining required data at the various grain sizes are sampled at a sparse grid. The derived geological model is checked for precision and accuracy comparing the output obtained against a model obtained using a complete isotopic data set including head grades and chemical analysis at each granulometric fraction analysed at all sampled positions. Among the several tested alternatives, the best procedure for dealing with the posed problem involved utilising the correlation coefficients between the grades at distinct fractions to obtain an isotopic database prior to interpolation. Cokriging was the method used, based on the intrinsic coregionalisation model. Estimates were obtained with the quality needed for the proposal of the study. Results showed that at least 50 per cent of the blocks were estimated with errors less than ten per cent relative to the grades obtained at the reference model, using only 15 per cent of the granulochemical data employed to obtain the reference model. The methodology also preserved the correlations between the attributes in their several granulometric fractions on the estimated model.

Sep 26, 2011

By L van Latum, A Koh, I Thomas

Haul truck drivers are at a high risk of developing fatigue due to the monotonous nature of the work and demanding shift rosters. In the mining industry, fatigue is identified as a major contributing factor in several fatal accidents a year. A number of fatigue detection devices are now available, that can contribute to improved fatigue risk management in the mining industry. This paper describes the technology and algorithms behind one particular product which is both predictive and un-obtrusive to the operator. The Advisory System for Tired Drivers (ASTiD), was developed by Fatigue Management International (FMI) in collaboration with the Loughborough Sleep Research Centre (LSRC), both in the United Kingdom. While this system includes a model of the human circadian rhythm as a bias for predicting expected fatigue risk by time of day, it also uses yaw data generated by a solid-state gyroscope in the horizontal plane. Algorithms analyse this data to determine the level of monotony of actual driving conditions and to detect changes in steering control caused by driver fatigue. During normal levels of attention, a haul truck driver applies continuous, small adjustments to the steering wheel. As the level of fatigue increases, steering adjustments become less frequent and more erratic. Differentiating between these modes of control in the highly variable mining environment is accomplished by chromatic processing of the gyroscope data. This paper will go into some detail explaining the principles behind chromatic processing and the specific application of these in detecting fatigue in haul truck operators.Technology in itself is not the solution to addressing operator fatigue, but it plays an important role in ongoing monitoring and maintaining awareness with mining personnel. Furthermore, the specific principles behind ASTiD technology support a risk-based approach to fatigue management, allowing optimisation of the mining operation for productivity as well as safety.

Sep 26, 2011

By E Ben-Awuah

Mine planning is the backbone of all mining enterprises and determines the profitability or otherwise of any mining investment. Providing high quality ore and tailings containment at the right time are the main drivers in oil sands mining. Recent environmental and regulatory requirements also makes waste management an integral part of mine planning in the oil sands industry (Directive 074). Due to the limitation of lease area, the pit phase advancement in oil sands mining is carried out simultaneously with the construction of tailings dykes in the mined out areas of the pit. These in-pit dykes are constructed to hold tailings that are produced during the processing of the oil sands ore after an initial external tailings facility. Most of the materials used in constructing these in-pit and ex-pit dykes come from the oil sands mining operation, making it necessary to have a plan for supplying the dyke material. The research problem here is categorised as: determining the order of extraction of ore, dyke material and waste to be removed from a predefined ultimate pit limit over the mine life that maximises the net present value of the operation û a strategic schedule; anddetermining the destination of dyke material that minimises construction cost depending on the construction requirements of the various dykes as per their designs û an operational schedule. There has been developed, implemented and verified a theoretical optimisation framework based on mixed integer goal programming (MIGP) to address the oil sands long-term production scheduling and waste management problem. The research introduced the development of multiple material types and multiple destination optimisation techniques using MIGP in the context of mine planning. This study presents an integration of mixed integer programming and goal programming in solving large-scale mine planning optimisation problems using clustering and pushback techniques.

Sep 26, 2011

Reconciliation involves the collection of tonnage, grade (quality) and contained metal (product) data from disparate and hopefully independent sources. Examples are exploration data, production sampling data from blastholes or drawpoints, and process plant data. These data may be compared by means of ratios (factors). The F1 factor usually relates short-term (ore control) model tonnages, grades and metal content to ore reserves depleted. The F2 factor usually relates received-at-mill tonnages, grades and metal content to delivered-to-mill = production tonnages, grade and metal content. The F3 factor is F1 × F2 and enables a comparison of a mineÆs ability to recover the tonnage, grade and metal content estimated in ore reserves. The F1 factor measures the accuracy of orebody knowledge in the ore reserves to the demarcation of ore and waste by ore control (short-term model). The F1 factor may be used to check and calibrate the selectivity of mineral resource models and/or planned dilution assumed in transfer from mineral resources to ore reserves. The F2 factor enables a check on unplanned dilution entering the ore stream between ore control and the mill. Using the factors it is possible to calculate a monetary value on improvements in the accuracy of orebody knowledge, selectivity and the effects of dilution and ore loss. Reconciliation should be an implicit part of the mining process, and reconciliation targets should be a key performance indicator for well-run mines.

Sep 26, 2011

By B Muller

The disposal of tailings has become a greater issue due to the changing economics of mineral extraction. The processing of lower grade deposits and tougher environmental regulations has led to the selection process for tailings disposal options becoming more complicated. Current selection methods are too time consuming or too easily manipulated for inexperienced users to use successfully. This paper examines a different method for the initial selection of a tailings disposal system, using a numerical system that was introduced into the mining method selection field. The numerical method system was compiled by reviewing previous work done on the positive and negative aspects of the tailings disposal methods used by the coal and metallurgical mining industries. Assessing the common trends and design aspects of each of the methods was undertaken and a ranking was designed to explain how well suited the tailings disposal methods are to a situation. The tool successfully analysed sites using common data to indicate methods for disposal that would have provided minimal technical problems, low environmental risk and been practically possible. The tool also proved successful in identifying critical areas that would need further investigation. This new system was tested on some disaster case studies from around the world in order to show that it could have successfully selected a different method for tailings disposal that would have prevented the disaster.

Sep 26, 2011

The Rio Tinto Iron Ore group currently estimates grade attributes for mineral resources using ordinary kriging (OK). OK produces a smoothed globally unbiased estimate, but does not capture local variability, which can have significant impact on mine planning and scheduling. The dimensions and the multivariate nature of iron ore deposits make point simulations of whole deposits impractical. With a direct block simulation method the size and speed limitations of point simulations can potentially be overcome, resulting in a viable method for capturing local variability and for providing risk measures into the mine plan and schedules. Some iron ore deposits are highly folded (post mineralisation) and require preprocessing using an unfolding algorithm to properly define the continuity parameters and directions of anisotropy before simulation can proceed. Block co-simulation proceeds in unfolded space and since mine planning require a block-model with regular block-sizes; a regular block-model is unfolded first. This paper discusses a block co-simulation approach that can be used to simulate the irregular blocks resulting from the unfolding process. A section of an iron ore deposit is used for testing the block co-simulation algorithm and the simulation results are compared against the Isatis direct block simulation method. Available blasthole data is also used to validate the behaviour of the block co-simulation algorithm.

Sep 26, 2011

By A T. Bernardes

Using the cellular automata computational technique it was possible to simulate the generation and discharge of an iron ore pellet pile inside a direct iron reduction oven, as well as to calculate the reduction degree of the pellets as they leave the oven. It allows simulation of the variation of operational parameters and verify the results of those variations in the pellet reduction degree. The simulated data was validated in two instances: for the dynamics of the pellet flow inside the oven it was used in an acrylic bi-dimensional silo; and data from direct reduction iron literature was used for validation of the pellet reduction. The simulated results were considered acceptable to describe the movement of the iron pellet inside the direct reduction iron oven and to describe the iron pellet reduction direct in the oven.

Sep 26, 2011

By M Reisi, H Waterer, M Savelsbergh, F Engineer

The Hunter Valley Coal Chain Coordinator (HVCCC) Limited is the primary planning organisation responsible for the export of coal through the Port of Newcastle, Australia. HVCCC stakeholders include producers (mining companies), rail track owners, above rail operators, and terminal operators. The HVCCC is tasked with coordinating the activities of all the corresponding parts of the coal export supply chain so as to maximise the system efficiency. The HVCCC also has a forward looking planning role: with demand for coal expected to double in the next decade, strategic capacity planning is a core activity for the HVCCC. Optimisation and simulation models are key aids in such activity. They are used to suggest and evaluate infrastructure expansions and operating policy changes. These models require input data in the form of shipping stems, which are arrival streams of ships at the port, together with their cargo types and composition, and date and time of arrival.Although everyone is familiar with the expression ægarbage in equals garbage outÆ, relatively little research is dedicated to the issues surrounding input data generation. In this research we seek to redress this, and have developed a method for generating high quality shipping data. Shipping stems used as input should be realistic, reflecting key characteristics of historical patterns. They should accurately represent future demand scenarios. Yet they should also have controllable parameters that can be used to test what-if scenarios, such as are required by the HVCCC to answer questions about future infrastructure or operational plans.Here we review our process for achieving this, and give a case study analysing the effects of different shipping stems on the performance of the coal chain, as measured by a capacity assessment tool. The latter is an optimisation model that takes shipping stem and infrastructure data as input, and calculates key supply chain performance metrics. The potential benefits of alternative demand management strategies are analysed by measuring the performance of the coal chain given different shipping stems obtained via the stem generatorÆs controllable parameters.

Sep 26, 2011

By B J. Wilde

Evaluating the resources and reserves in a mineral deposit requires many interdependent subjective decisions. The decision of stationarity is arguably the most important. This decision includes: the choice of domains that subset the entire deposit, modelling the boundaries of each domain, and understanding and considering the nature of the contacts between domains. Reasonable choices are made in consideration of the geological context, the available data, the goals of the study and the available resources for the study. This paper reviews aspects of the decision of stationarity in modern resource assessment with a focus on quantifying uncertainty and using numerical/statistical tools to help the geomodeller. The use of volume/distance functions to quantify uncertainty in domain limits is particularly important and is presented in detail. The use of contact plots provides help with the continuity of grades across domain boundaries. Realisations are not assembled as a branching combinatorial of all possibilities, but as a set of realisations that provide a measure of uncertainty. This paper amounts to a definition of stationarity for modern resource assessment.

Sep 26, 2011

By J F. C L Costa, J C. Koppe, C Diedrich

Currently mineral resources evaluation is firstly expressed in terms of volume. Next the resources are transformed into tonnes by using an average bulk density applied to the whole deposit or, in some other circumstances, various different densities to distinct geological domains. Previous work in uranium, copper and nickel deposits used linear correlation between grades and bulk density to derive the density. Geophysics methods are also another good way to obtain density indirectly.These approaches do not take into account that two samples with the same grade but with different bulk densities can characterise different sample supports, leading to severe bias in grade and tonne estimates. It is important that the systematic measurements of bulk density are included into the estimation process, particularly when density is not correlated with the grades. Additionally, the extra time and costs involved in obtaining the bulk density data have made the mining industry disregard it.This study reports the impact in resource estimates by using a methodology that includes the bulk density variability. It involves the determination of a variability model for the bulk density and another model for the composite that was weighted by density as well as by its length. Next, these variables are estimated by kriging and the final grade estimates are obtained. The tonnages are obtained with the bulk density estimates. The conventional methodology was also applied and both estimates are compared to the reconciliation data from a major copper deposit. The results show a significant difference between the tonnage and grade estimates obtained from these two methodologies.The results show that considering density variability when compositing leads to a more accurate methodology resulting in better grade and tonnage estimates locally and globally.

Sep 26, 2011

By J Araya, O Leiva

The mine evaluation process, composed of the designing, planning and valuation of a mine project, is, among other variables, based on an estimated orebody model. However, as the information from the drill holes is limited, it is impossible to know with certainty the mineral content in each block within the estimated block model. Consequently, uncertainty about the value of metal grades, or other geological attributes, arises because of the lack of information at unsampled locations. This is one of the reasons why, in any mine operation, there will always exist a divergence between estimated and real values. Cut-off grade optimisation is the process used to determine the operating mining strategy (long-term production scheduling) that will maximise the total profit value (normally referred as the net present value (NPV)) of a mine operation. This is done by defining a dynamic cut-off grade policy which is constrained by the production capacity of the mining, mill and refining.Real options is a type of contract, written on real assets such as mining, oil and other natural resource industries, that gives you the right but not the obligation to exercise the contract. It is based on operational and managerial flexibility, which allow mine decision makers to react to either good or bad operational and economic events in the future. But, what do geostatistics and cut-off grade optimisation have to do with the application of real options in a mine project evaluation process? The objective of this paper is to answer the previous question by showing how geostatistics combined with cut-off grade optimisation are key tools for corporate/managerial decision makers, which allow them not only to analyse the resource/reserves of their projects, but also to generate operational strategies that can maximise project value by minimising the downside risk due to ore tonnes and grade uncertainties, while achieving operational and physical constraints such as mining and processing capacities. A real copper grade open pit mine project is used as a case study.

Sep 26, 2011