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The present paper introduces the key advantages of ImhoflotTM, JamesonTM, and RefluxTM flotation cells over the conventionally used mechanical and column cells from different perspectives. The impact of slurry mean retention time, bubble size distribution, and energy input was studied for all cell types. The mean retention time of laboratory scale ImhoflotTM (V030-cell) and RefluxTM flotation cells (RFC100) were measured experimentally using KCl as a tracer. Also, initially a statistical and practical overview of previously installed ImhoflotTM, and JamesonTM cells was presented in this work. It was found that more industrial data is available for the JamesonTM cell. The diagnostic results showed that RefluxTM, JamesonTM, and ImhoflotTM functionally operate similarly based on providing intensive turbulence in the downcomer. They were initially applied to the Australian and the UK coal industries and installed in the cleaning stage of flotation circuits, while there are now more applications in a wide variety of minerals across the world in different flotation stages. First pilot trials on a Russian gold ore were reported operating both JamesonTM and ImhoflotTM cells at the rougher-scalper and cleaner stages providing superior results using the ImhoflotTM cell as rougher-scalper and the JamesonTM at the cleaner. Formation of sub-micron and micron-sized bubbles, effective hydrodynamic characteristics, and low capital and operating costs were reported as major advantages of intensified flotation cells over the conventionally used ones in improving the recoverability of ultra-fine particles. Literature data showed that these cells provide greater gas-hold-up values (40-60%) over the mechanical (5-20%) and column cells (5-25%) with substantially lower power inputs. It was indicated that low mean slurry retention time could lead to a potential enhancement in their throughputs, but further industrial measurements are required to prove this statement. The RefluxTM cell showed a plug-flow mixing regime, while ImhoflotTM V-Cell followed the trend of perfect mixing and plug-flow dispersion regimes.
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The devices most commonly used for classification in the mining industry are hydrocyclones. Hydrocyclones have been preferred over most other devices due to low operating cost and a small footprint and have not received as much attention as more expensive comminution units, and their technological progress has been incremental. A classification study using semi-inverted hydrocyclones was conducted to determine if a hydrocyclone could be operated in a way that would produce similar results to a screen. The results from the study were encouraging with substantial reduction in water split to coarse product (Rf) and much coarser cut sizes. A hydrocyclone with lower water split to underflow (Rf) will improve classification of fines which would otherwise be sent back to the grinding circuit. The ability of hydrocyclones to cut coarser would potentially expand their application in the areas where the fine screens are currently in use. Therefore, the very low Rf and coarser cut are of paramount importance for a step change in future industrial operation. Other potential benefit compared with fine screens are reduction of the footprint required for the installation and in challenges in even slurry distribution. The classification performance of a semi-inverted hydrocyclone will be presented in this paper.
Conference Paper
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Shortly after the first applications of closed circuit ball mill grinding it was realised that classification increases mill capacity. Therefore, over the years closed ball mill – cyclone circuits have become an industry standard. Although mill capacity increases with circulating load, an optimum circulating load of 250% was established due to limitations in cyclone classification efficiency. The role of classification in grinding circuit performance appears to have been neglected in the current efforts to reduce the energy consumption of grinding circuits. A simple relationship quantifying the effect of circulating load and classification efficiency on grinding circuit capacity is used in this paper to discuss possibilities for improving the capacity of ball mill circuits. The analysis considers the current practical limitations of classification equipment. Also discussed in this paper, are the potential benefits to flotation due to the reduction of overgrinding.
Screens, which provide better separation efficiencies compared to hydrocyclones, have been attracted the attention of minerals industry for many years. However, low capacity in fine screening applications has limited the use of screens to classify the ball mill product in wet grinding circuits. In recent years, the separation efficiency of fine particles in a small footprint has been largely increased by the advances in high-frequency screening technology, thus increasing the use of screens to close fine wet grinding circuits. The potential of high-frequency fine screens to improve overall circuit performance was evaluated in this paper using lead-zinc ore grinding data. Data come from circuits at the lead-zinc mine in Turkey where both high-frequency screens and hydrocyclones are used to close grinding circuits. The ball mill-hydrocyclone circuit and the ball mill-screen circuit were sampled in order to measure and compare the performance of the circuits. The ball mills are different in design. Therefore, simulation studies were carried out to eliminate the differences in factors affecting the performance of the circuits such as milling conditions. Data indicated that using high-frequency screens in place of hydrocyclones in grinding circuit improves throughput by reducing the circulating load. Lower by-pass and sharper separation provided by high-frequency screens are the major factors affecting the capacity of the circuit and the results showed that 13% more throughput is achieved with 15% less grinding energy when hydrocyclones are replaced by high-frequency fine screens.
Coal flotation is a complex multiphase process governed by different sub-processes and interphase interactions. The coal cleaning efficiency by flotation is largely affected by many different physical and chemical factors that can be roughly classified into three main group: coal feed properties, pulp chemical and rheological properties, and machine and operational properties. A great number of flotation kinetic model have been proposed in literature but a vast majority uses three parameters to describe the flotation kinetics, which are the ultimate recovery, the flotation rate constant, and flotation time. The models expand on the classical theory of flotation proposed by Zuniga (1935) that is based on the assumption that the particle-bubble collision rate is first-order with respect to the number of particles in the system, while bubble concentration remains constant. The flotation rate constant is directly proportional to available bubble surface area and probability of flotation, which is strongly dependent on particle size. Therefore, particle size is one of the most important parameters in coal flotation because it affects gas bubble mineralization and froth stability, bubble size distribution and air holdup, bubble-particle collision, attachment, and detachment rates, and reagent adsorption. Numerous researchers have studied the effect of particle size on flotation kinetics over years. This paper provides a comprehensive review of coal flotation kinetics models with a special focus on the effect of particle size on coal kinetic rate, recovery, and product quality. A particular emphasis will be put on research findings reported over the last three decades.
Cyclones in three closed circuit grinding systems were changed from conventional vertical mounting to horizontal mounting with improvement in classification efficiency. Apex plugging problems were eliminated because horizontal mounting permitted the use of larger apex orifices. Reduction in circulating load was accompanied by reduced maintenance requirements.
Dynamic separators are efficient dry classifiers because new designs during the last 50 years have reduced the by-pass and improved the partition curve. By contrast cyclones have been used as classifiers in wet grinding circuits for 50 years but without serious change and the early problems which were caused by high and variable by-pass, and different mineral specific gravities, still exist. High frequency screens are now used for wet separation and the operating characteristics of closed grinding circuits using cyclones or screens are discussed. Data come from circuits at the Condestable, Cerro Lindo, El Brocal and Catalina Huanca mines in Peru, the Eczacibasi mine in Turkey, and the Apatit mine in central Kola Peninsula in Russia. All classifier data are described by their partition curves and all mill data by their breakage rates. With cyclones the by-pass, d50, and sharpness of the partition curve change as operating conditions change but with high frequency screens the values vary little because the by-pass is low, the sharpness is high, and d50 depends on the aperture size. Data from all plants indicate that lower by-pass and sharper separation improve breakage rates and throughputs. An initial model of the screen is proposed.
Minerals processing engineers and grinding experts agree that classification efficiency and circulating load both have a major effect on the efficiency of closed circuit ball mills. However, the effect of each is difficult to quantify in practice as these two parameters are usually interrelated. Experimental work was undertaken by Metso Process Technology and Innovation (PTI) to better understand this relationship. The data obtained was compared to a simplified relationship for closed circuit grinding performance model, developed by Magdalinovic in 1991, to quantify the effect of classification efficiency and circulating load on the capacity of the circuit. The data also allowed an initial assessment of the model's accuracy to be evaluated.
From the examination of data from detailed plant surveys and associated laboratory batch testing, the principal effects of particle size in flotation have been identified. The current state of knowledge concerning the role of this variable is discussed in terms of the evidence presented. It is concluded that the minimum degree of hydrophobicity necessary for the flotation of a particle depends upon its size and as a result, recovery-size curves are a valuable diagnostic aid to the assessment of flotation performance. Entrainment is shown to be an important contributory mechanism to the recovery of fine particles which, when coupled with a low rate of genuine flotation, can account for much of the observed behaviour of such fines. The significance of particle size and its consequences in flotation research, in plant operations and in control schemes has been under-rated. The separate conditioning or flotation or both of separate size fractions seems inevitable as ores become increasingly difficult to concentrate.
The energy efficiency of comminution processes is very low based on the energy required to generate new fracture surface area relative to the mechanical (strain) energy input. However, the maximum ideal limiting efficiency ELimit against which actual efficiencies may be compared is unknown. Therefore, theoretical analyses were undertaken to determine ELimit for a compressive loading comminution machine based on the stress state in a single particle containing a central crack (flaw). The analyses show that ELimit increases with increasing Poisson’s ratio ν, having values of ∼5%, ∼7.5% and ∼9% at ν ∼0, ∼0.2 and ∼0.39, respectively. Actual energy efficiencies, EB, expressed in terms of the energy required to create new fracture surface area relative to the work input, using standard Bond work index values, are lower in the range
Mineral particle size plays a significant role on the flotation process. The flotation performance (grade and recovery) of apatite particles in a complex carbonatite ore from Brazil was investigated in a flotation column as a function of particle size and reagent concentrations. Flotation feed samples were also mineralogically and chemically characterized. For the bench scale flotation experiments, the sample was fractioned into different size intervals. The experiments were conducted according to a factorial experimental design. The experimental data were analyzed using P2O5 grade/recovery curves, response surface methodology and canonical analysis. An optimum particle size was found under the experimental conditions studied. The P2O5 grade and recovery rates obtained with a particle size between 37 μm and 105 μm were simultaneously higher than those required industrially.
Two industrial problems, energy consumption and mineral recovery, have dominated research in comminution during the past five years. Both have required new and fundamental insights into the fracture process, when brittle particles are subject to rapid impact. New experimental procedures using the ultrafast load cell, dual pendulum and high-precision image analysers have produced experimental data that is providing the basis for significantly new models of the fracture process. New generation comminution machines are exploiting the deeper understanding and theoretical models of wide applicability are emerging to provide a sound basis for design, scale-up and optimization of comminution operations.
High Frequency Vibrating Screens In Closed Grinding Circuits
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Split-Feed Circuit Design For Primary Sulfide Recovery
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