Environment-Dependent Breakage Rates in Ball Milling

Breakage rates of particles in a ball mill change with instantaneous particle size distribution in the mill. Slurry density and the presence of a grinding aid also affect breakage rates substantially. The effect of these variables, which constitute the mill environment, on breakage rates has been quantified with a unique estimation method known as the G-H method. This method enables the estimation of breakage rates of all size intervals by a simple linear graphical scheme. In general, breakage rates of coarse particles increase in the presence of fine particles, while the rates of fine particles remain relatively unaffected. Grinding aid restores the fluidity of the solid-liquid mixture even at very high percent solids and so the rate of grinding returns to ‘normal’ from ‘erratic’ behavior

Master of Science

thesisThe following research fundamentally deals with the cause and implications of nonlinearities in breakage rates of materials in wet grinding systems. The innate dependence of such nonlinearities on fines content and the milling environment during wet grinding operations is also tested and observed. Preferential breakage of coarser size fractions as compared to the finer size fractions in a particle population were observed and discussed. The classification action of the pulp was deemed to be the probable cause for such a peculiarity. Ores with varying degrees of hardness and brittleness were used for wet grinding experiments, primarily to test the variations in specific breakage rates as a function of varying hardness. For this research, limestone, quartzite, and gold ore were used. The degree of hardness is of the order of: limestone, quartzite, gold ore. Selection and breakage function parameters were determined in the course of this research. Functional forms of these expressions were used to compare experimentally derived parameter estimates. Force-fitting of parameters was not done in order to examine the realtime behavior of particle populations in wet grinding systems. Breakage functions were established as being invariant with respect to such operating variables like ball load, mill speed, particle load, and particle size distribution of the mill. It was also determined that specific selection functions were inherently dependent on the particle size distribution in wet grinding systems. Also, they were consistent with inputs of specific energy, according to grind time. Nonlinearity trends were observed for 1st order specific selection functions which illustrated variations in breakage rates with incremental inputs of grind time and specific energy. A mean particle size called the fulcrum was noted below which the nonlinearities in the breakage trends were observed. This magnitude of the fulcrum value varied with percent solids and slurry filling, indicating that breakage rates were being influenced by the milling environment as a whole. Primarily, there was always an increase in the breakage rates of coarser fractions with an increase in the amount of fines in the particle population. Consequently, the breakage rates of the finer size fractions were observed to decrease with an increase in grind time. Similar trends were noticed for 2nd order specific selection functions, where incremental inputs of specific energy were provided to observe realtime trends in the nonlinearity of breakage rates closely. Although the breakage rates for coarser size fractions increase with an increase in the amount of fines, the nature of nonlinearities varied with extended grind times. 1st order and 2nd order energy-specific breakage rates were observed to notice the variation in trends with extended grind times. Implications of such nonlinearities in specific breakage rates of various materials were tested on predictive simulation techniques, using the normalized linear population balance model and compared with an incremental methodology of specific energy input

Grinding and Concentration Technology of Critical Metals

The production and supply of raw materials is a basic ecosystem service, the origin of multiple production value chains. In 2011, the European Union elaborated a list of critical raw materials (CRMs), taking the economic and strategic importance for the European economy and the supply risk. Although focused mainly on the energy sector, the USA, Canada, and other countries took recently similar steps. Despite the great inertia characterising the mineral raw materials sector, some steps towards the Industry 4.0 paradigm can be envisaged. Significant challenges to the mining sector are the appropriate process design using the best available technologies; the increase in energy efficiency; the responsible use of water and handling of mining wastes; the social acceptance of the activity; and the digitalisation challenge. This book aims to propose strategies that can help face those challenges, especially in increasing energy efficiency in comminution operations

Research in the synthesis and characterization of magnetic fluids, phase 2 Quarterly report, Jun. - Sep. 1967

Synthesis and characterization of colloids of thermally stable ferrofluids with higher magnetization and susceptibilit

Online SAG mill pulse measurement and optimization semi-annual technical progress report

ReportThe grinding efficiency of semi autogenous milling or ball milling depends on the tumbling motion of the total charge within the mill. Utilization of this tumbling motion for efficient breakage of particles depends on the conditions inside the mill. However, any kind of monitoring device to measure the conditions inside the mill shell during operation is virtually impossible due to the sever environment presented by the tumbling charge. An instrumented grinding ball, which is capable of surviving a few hours and transmitting the impacts it experiences, is proposed here. The spectrum of impacts collected over 100 revolutions of the mills presents the signature of the grinding environment inside mill. This signature could be effectively used to optimize the milling performance by investigating this signature's relation to mill product size, mill throughput, make-up ball size, mill speed, liner profile and ball addition rates. At the same time, it can also be used to design balls and liner systems that can survive longer in the mill. The technological advances made in electronics and communication makes this leap in instrumentation certainly viable. Hence, the instrumented grinding ball offers the ability to qualitatively observe and optimize the milling environment

Hexagonal Boron Nitride as Filler for Silica-Based Elastomer Nanocomposites

Two-dimensional hexagonal boron nitride (hBN) has attracted tremendous attention over the last few years, thanks to its stable structure and its outstanding properties, such as mechanical strength, thermal conductivity, electrical insulation, and lubricant behavior. This work demonstrates that hBN can also improve the rheological and mechanical properties of elastomer composites when used to partially replace silica. In this work, commercially available pristine hBN (hBN-p) was exfoliated and ball-mill treated in air for different durations (2.5, 5, and 10 h milling). Functionalization occurred with the -NH and -OH groups (hBN-OH). The functional groups were detected using Fourier-Transform Infrared pectroscopy (FT-IR) and were estimated to be up to about 7% through thermogravimetric analysis. The presence of an increased amount of oxygen in hBN-OH was confirmed using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy. (SEM-EDS). The number of stacked layers, estimated using WAXD analysis, decreased to 8–9 in hBN-OH (10 h milling) from about 130 in hBN-p. High-resolution transmission electron microscopy (HR-TEM) and SEM-EDS revealed the increase in disorder in hBN-OH. hBN-p and hBN-OH were used to partially replace silica by 15% and 30%, respectively, by volume, in elastomer composites based on poly(styrene-co-butadiene) from solution anionic polymerization (S-SBR) and poly(1,4-cisisoprene) from Hevea Brasiliensis (natural rubber, NR) as the elastomers (volume (mm3) of composites released by the instrument). The use of both hBNs in substitution of 30% of silica led to a lower Payne effect, a higher dynamic rigidity, and an increase in E0 of up to about 15% at 70 C, with similar/lower hysteresis. Indeed, the composites with hBN-OH revealed a better balance of tan delta (higher at low temperatures and lower at high temperatures) and better ultimate properties. The functional groups reasonably promote the interaction of hBN with silica and with the silica’s coupling agent, sulfur-based silane, and thus promoted the interaction with the elastomer chains. The volume of the composite, measured using a high-pressure capillary viscometer, increased by about 500% and 400% after one week of storage in the presence of hBN-p and hBN-OH. Hence, both hBNs improved the processability and the shelf life of the composites. Composites obtained using hBN-OH had even filler dispersion without the detachments of the filler from the elastomer matrix, as shown through TEM micrographs. These results pave the way for substantial improvements in the important properties of silica-based composites for tire compounds, used to reduce rolling resistance and thus the improve environmental impacts

Dynamic simulator for a grinding circuit

Thesis (M.S.) University of Alaska Fairbanks, 2017The grinding circuit is a primary and indispensable unit of a mineral processing plant. The product from a grinding circuit affects the recovery rate of minerals in subsequent downstream processes and governs the amount of concentrate produced. Because of the huge amount of energy required during the grinding operation, they contribute to a major portion of the concentrator cost. This makes grinding a crucial process to be considered for optimization and control. There are numerous process variables that are monitored and controlled during a grinding operation. The variables in a grinding circuit are highly inter-related and the intricate interaction among them makes the process difficult to understand from an operational viewpoint. Modeling and simulation of grinding circuits have been used by past researchers for circuit design and pre-flowsheet optimization in terms of processing capacity, recovery rate, and product size distribution. However, these models were solved under steady approximation and did not provide any information on the system in real time. Hence, they cannot be used for real time optimization and control purposes. Therefore, this research focuses on developing a dynamic simulator for a grinding circuit. The Matlab/Simulink environment was used to program the models of the process units that were interlinked to produce the flowsheet of a grinding circuit of a local gold mine operating in Alaska. The flowsheet was simulated under different operating conditions to understand the behavior of the circuit. The explanation for such changes has also been discussed. The dynamic simulator was then used in designing a neural network based controller for the semi-autogenous mill (SAG). A two-layer non-linear autoregressive (NARX) neural network with feed to the mill as exogenous input was designed using data generated by the simulator for a range of operating conditions. Levenberg-Marquardt (LM) and Bayesian Regularization (BR) training algorithms were used to train the network. Comparison of both algorithms showed LM performed better provided the number of parameters in the network were chosen in a prudent manner. Finally, the implementation of the controller for maintaining SAG mill power to a reference point is discussed

Circulation rate modelling of tumbling mill charge using positron emission particle tracking (PEPT)

Includes abstract.Includes bibliographical references.This research is focused on developing theoretical understandings of charge circulation trends as observed in tumbling mills at different operating conditions. Of particular interest is the underlying assumptions being made by many mill models that a particle imparts energy for potential breakage only once per revolution of the mill to the charge body – that is, that the circulation rate of mill charge can be assumed to be constant irrespective of the speed at which the mill is run. The trajectory data used in this thesis is derived from positron emission particle tracking (PEPT) experiments conducted at the University of Birmingham positron imaging centre and further experiments were conducted at the iThemba LABS in Cape Town. The experimental approach is highly suited to allow the effective examination of the assumption that the grinding charge in these mills circulates at a constant rate of unity

Discrete element method based scale-up model for material synthesis using ball milling

Mechanical milling is a widely used technique for powder processing in various areas. In this work, a scale-up model for describing this ball milling process is developed. The thesis is a combination of experimental and modeling efforts. Initially, Discrete Element Model (DEM) is used to describe energy transfer from milling tools to the milled powder for shaker, planetary, and attritor mills. The rolling and static friction coefficients are determined experimentally. Computations predict a quasi- steady rate of energy dissipation, Ed, for each experimental configuration. It is proposed that the milling dose defined as a product of Ed and milling time, t, divided by the mass of milled powder, mp characterizes the milling progress independently of the milling device or milling conditions used. Once the milling dose is determined for one experimental configuration, it can be used to predict the milling time required to prepare the same material in any milling configuration, for which Ed is calculated. The concept is validated experimentally for DEM describing planetary and shaker mills. For attritor, the predicted Ed includes substantial contribution from milling tool interaction events with abnormally high forces (\u3e103 N). The energy in such events is likely dissipated to heat or plastically deform milling tools rather than refine material. Indeed, DEM predictions for the attritor correlate with experiments when such events are ignored in the analysis. With an objective of obtaining real-time indicators of milling progress, power, torque, and rotation speed of the impeller of an attritor mill are measured during preparation of metal matrix composite powders in the subsequent portion of this thesis. Two material systems are selected and comparisons made between in-situ parameters and experimental milling progress indicators. It is established that real-time measurements can certainly be used to describe milling progress. However, they need to be interpreted carefully depending on hardness of brittle component relative to milling media. To improve the DEM model of the attritor mill, it is desired to avoid the removal of unrealistic, high-force events using an approach that would not predict such events in the first place. It is observed that during experiments in attritor, balls may jam causing an increased resistance to the impeller’s rotation. The impeller may instantaneously slow down, quickly returning to its pre-set rotation rate. Previous DEM models did not account for such rapid changes in the impeller’s rotation. In this work, this relationship between impeller’s torque and rotation rate is obtained experimentally and introduced in DEM. As a result, predicted Ed, are shown to correlate well with the experimental data. Finally, a methodology is proposed combining an experiment and its DEM description enabling one to identify the appropriate interaction parameters for powder systems. The experiment uses a miniature vibrating hopper and can be applied to characterize the powder flow for variety of materials. The hopper is designed to hold up to 20,000 particles of 50-tam diameter, which can be directly described in DEM. Based on comparison of discharge rate from experiments and model, all 6 interaction parameters were analyzed and the ideal conditions identified for Zirconia beads. The values of these parameters for powders are generally not the same as those established for macroscopic bodies. In addition, effects of some other experimental parameters such as particle size distribution and amplitude of vibration are also investigated