A review of the applications of the JK size-dependent breakage model Part 1: ore and coal breakage characterisation

It has been 10 years since the JK size-dependent breakage model was developed (Shi and Kojovic, 2007). This paper reviews 20 applications of the model for the mineral and coal industries in the past 10 years to encourage its use in future applications. The review is divided into three parts: Part 1 for ore and coal breakage characterisation, Part 2 for assessment of material strength and energy requirement for size reduction, and Part 3 for modelling comminution equipment. Part 1 of the review covers the model structure, its applications in high energy single impact data reduction, low energy incremental breakage modelling, reduced breakage testing method, a new impact breakage testing method using mixed particles in a wide size range, fine particle breakage characterisation (JKFBC), and a multi-component breakage model for coal grinding. It has been proved through these applications that the JK size-dependent breakage model is a useful tool for ore and coal breakage characterisation

A review of the applications of the JK size-dependent breakage model Part 3: comminution equipment modelling.

It has been 10 years since the JK size-dependent breakage model was developed (Shi and Kojovic, 2007). This series of papers present 20 applications of the model for the mineral and coal industries. Part 3 of this series reviews its application to modelling comminution equipment. It is demonstrated that the JK size-dependent breakage model not only can be used for modelling particle breakage, but also can be employed as a basic structure for modelling comminution equipment. This is different to the traditional equipment modelling approaches, in which the population balance models are dominant. In this energy-size reduction modelling approach, the JK size-dependent breakage model provides a key mathematical template to rationally link the size reduction as the equipment model output with ore breakage property and specific energy as the two model inputs. Five case studies are reviewed, which includes modelling of a hammer mill for coke feed preparation, a vertical spindle mill for coal pulverised fuel grinding, a ball mill for batch grinding and continuous operation, HPGR simulations based on piston press testing data, and high voltage pulse disintegration of ores. A common feature of these models is that they all incorporate the measured material breakage characteristic parameters and machine operational conditions that are represented by the size-specific energy relation. The energy-size reduction modelling approach permits the simulations of the effects of changes in ore/coal and machine operating conditions on comminution product size distribution

Effect of flotation conditions on froth rheology

Froth flotation has been widely used for separating and concentrating valuable minerals in an ore. Froth rheology is an important parameter that affects flotation performance by affecting froth transportation. This paper reports the results of an investigation into the effect of flotation conditions on the froth rheology, in particular the influence of frother dosage and feed grade. Atwo-factor factorial experiment program was performed in a 20 L continuously operated flotation cell. It was found that, within the boundaries of this work, varying the frother dosage has no significant influence on the froth rheology, while the feed grade exhibits a positive correlation with the froth viscosity. In line with the previous study, the froth rheology has a negative correlation with the bubble size, and a positive correlation with the fraction of the bubble lamellae covered by solid particles. The froth rheology model which has been previously developed by the authors (as a function of froth properties) was also validated in the current work

Determining the significance of flotation variables on froth rheology using a central composite rotatable design

Froth performance in a flotation cell is expected to be affected by froth rheology due to change in the froth transportation rate. However, very little study has been performed to investigate how froth rheology responds to flotation variables. This paper presents an experimental program performed to study the effects of flotation variables (i.e. feed grade, feed particle size, froth height, superficial gas velocity and impeller speed) on the froth rheology. These conditions were varied using a central composite rotatable design (CCRD).Froth rheology was found to change significantly with a variation in flotation conditions and exhibited shear-thinning behaviour. Assuming the froth moving towards the flotation lip is an open channel flow, the shear rate in the froth was calculated to be less than 4s. Results of the CCRD experiments showed that the flotation variables have different effects on the froth rheology. The interactions between these flotation variables in determining froth rheology were also analysed. A shear rate specific empirical model was developed to relate froth rheology to the flotation variables and their interactions

Comparison of energy efficiency between E and MPS type vertical spindle pulverizer based on the experimental and industrial sampling tests

0.5%–2% gross power generation of coal power plant is consumed by vertical spindle pulverizer (VSP), and it is essential to select a VSP with better operational performance. Simulated studies of lab-scale mills, which show the similar breakage mechanism with VSP, and industrial sampling on VSPs are conducted to compare energy efficiencies of E and MPS type VSPs (with the grinding media of balls and tread rollers, respectively). The classical energy-size reduction model is modified with the addition of particle size in the exponential form to compare the grinding energy efficiency (product fineness for the certain specific energy) of two lab-scale mills. Also, differences in structure and operational parameters of lab-scale mills are considered. For the industrial sampling tests of two VSPs, recorded data and size distribution of sampled materials are preliminarily compared. Product t10 is selected as the bridge to connect the specific grinding energy and size distribution of products. The modified breakage model is combined with the King's equation to compare the energy efficiency on the premise of feed in the same fineness. Comprehensive comparison of the results obtained from both lab-scale and industrial-scale VSPs suggests that the MPS type VSP shows the higher grinding energy efficiency and lower total energy consumption

Photoconductivity of biased graphene

Graphene is a promising candidate for optoelectronic applications such as photodetectors, terahertz imagers, and plasmonic devices. The origin of photoresponse in graphene junctions has been studied extensively and is attributed to either thermoelectric or photovoltaic effects. In addition, hot carrier transport and carrier multiplication are thought to play an important role. Here we report the intrinsic photoresponse in biased but otherwise homogeneous graphene. In this classic photoconductivity experiment, the thermoelectric effects are insignificant. Instead, the photovoltaic and a photo-induced bolometric effect dominate the photoresponse due to hot photocarrier generation and subsequent lattice heating through electron-phonon cooling channels respectively. The measured photocurrent displays polarity reversal as it alternates between these two mechanisms in a backgate voltage sweep. Our analysis yields elevated electron and phonon temperatures, with the former an order higher than the latter, confirming that hot electrons drive the photovoltaic response of homogeneous graphene near the Dirac point

Strong light-matter coupling in two-dimensional atomic crystals

Two dimensional (2D) atomic crystals of graphene, and transition metal dichalcogenides have emerged as a class of materials that show strong light-matter interaction. This interaction can be further controlled by embedding such materials into optical microcavities. When the interaction is engineered to be stronger than the dissipation of light and matter entities, one approaches the strong coupling regime resulting in the formation of half-light half-matter bosonic quasiparticles called microcavity polaritons. Here we report the evidence of strong light-matter coupling and formation of microcavity polaritons in a two dimensional atomic crystal of molybdenum disulphide (MoS2) embedded inside a dielectric microcavity at room temperature. A Rabi splitting of 46 meV and highly directional emission is observed from the MoS2 microcavity owing to the coupling between the 2D excitons and the cavity photons. Realizing strong coupling effects at room temperature in a disorder free potential landscape is central to the development of practical polaritonic circuits and switches.Comment: 25 pages, 7 figure

An overfilling indicator for wet overflow ball mills

The lack of constraints in ball mill capacity in the published ball mill models may result in unrealistic predictions of mill throughput. This paper presents an overfilling indicator for wet overflow discharge ball mills. The overfilling indicator is based on the slurry residence time in a given mill and given operational conditions. Mathematical descriptions of the method to estimate the volume-based residence time of slurry are presented. A database consisting of 121 sets of industrial overflow ball mill surveys worldwide was used to establish the pattern of the slurry residence time in the full scale operational overflow ball mills. According to the pattern, the residence time thresholds beyond which overfilling a ball mill is likely to occur were defined. For a ball mill with an internal diameter smaller than 5.9 m, the volume-based residence time threshold is set at 2 min; and for a ball mill larger than 5.9 m in diameter, the threshold is set at 1 min. In addition to being incorporated in ball mill models to warn of any unrealistic simulations, the overfilling indicator can also be utilised at ball mill operation sites to guide the mill throughput control and optimisation

Coal breakage characterisation. Part 2: multi-component breakage modelling

A multi-component breakage model has been developed at the Julius Kruttschnitt Mineral Research Centre (JKMRC) to describe the energy-size reduction in relation to particle size and density for coal breakage characterisation. The model takes the following form: t=M/(RD/ RDmin)c ·{1-exp[-f·x·E]}. The model incorporates four parameters and fits 60 JKFBC (JK Fine-particle Breakage Characteriser) test data for each of the Australian coal and Chinese coal samples, with R = 0.982 and 0.978 respectively. The multi-component model can be switched into a single component model by setting c = 0. A set of t-family of curves for coals ground in the JKFBC are presented. It was found that the data from various particle sizes and densities of the two coal samples, collected from the Australian and Chinese power stations, all fall on similar t-curve trend lines. These t-family of curves can be used in the multi-component model to estimate the product size distribution from the predicted t values. A procedure has been developed to calibrate the multi-component model with seven tests based on a combination of various particle sizes, coal densities and grinding energy levels, using the JKFBC device. Over 100 sets of data have been used to validate the calibration procedure

Coal breakage characterisation. Part 3: applications of the multi-component model for HGI prediction and breakage simulations

A new method was developed to infer the HGI (Hardgrove Grindability Index) values from the product fineness indicator, t, predicted by the multi-component breakage model. A total of 41 sets of JKFBC (JK Fine-particle Breakage Characteriser) tests, on coals from Australia and China, plus ores containing various minerals and a clinker, were used to validate this method. The JKFBC and the multi-component breakage model have the potential to improve or replace the traditional HGI test. A new coal grindability index can be generated from the multi-component model parameters. As the major error sources associated with the traditional HGI test have been removed in this new approach, superior repeatability and reproducibility can be expected. More work is recommended in this area to validate this novel approach and to identify its limitations. Simulations were conducted by employing the multi-component breakage model to demonstrate the effects of particle size and density on pulverised fuel (PF) grinding; to troubleshoot the PF production problems related to the coal properties; and to elucidate the observed trend of the non-linear phenomenon of the HGI effect on the PF milling capacity; whereby a 10 unit reduction in HGI from 50 to 40 requires four times more energy than a 10 unit HGI reduction from 90 to 80 in order to achieve the same PF production rate