Grindability Of Ore Research Articles

Enhancing Iron Ore Grindability through Hybrid Thermal-Mechanical Pretreatment

Grinding is an important process of ore beneficiation that consumes a significant amount of energy. Pretreating ore before grinding has been proposed to improve ore grindability, reduce comminution energy, and enhance downstream operations. This paper investigates hybrid thermal mechanical pretreatment to improve iron ore grinding behavior. Thermal pretreatment was performed using conventional and microwave approaches, while mechanical pretreatment was conducted with a pressure device using a piston die. Results indicate that conventional (heating rate: 10 °C; maximum temperature: 400 °C), microwave (2.45 GHz, 1.7 kW, 60 s), and mechanical (14.86 MPa, zero delay time) pretreatments improved the studied iron ore grindability by 4.6, 19.8, and 15.4%, respectively. Meanwhile, conventional-mechanical and microwave-mechanical pretreatments enhanced the studied iron ore grindability by 19.2% and 22.6%, respectively. These results suggest that stand-alone mechanical pretreatment or microwave pretreatment may be more beneficial in improving the grinding behavior of the studied fine-grain iron ore sample. The results of the mechanical pretreatment obtained in this study may be used in a simulation of the HPGR system for grinding operations of similar iron ore

Feed hardness and acoustic emissions of autogenous/semi-autogenous (AG/SAG) mills

In this study, the relationship between acoustic emissions and hardness of different rock types (model quartz, model calcite, and real iron ore) coupled with binary mix ratios of model quartz and iron ore (1:3, 1:1 and 3:1) was investigated in a laboratory-based AG/SAG mill. The acoustic emission response and sensitivity of the mill were compared, along with its product particle size distribution. Features, such as discrete wavelet transform (DWT), power spectral density estimate (PSDE), and statistical root mean square (RMS) were extracted from the mill acoustic emissions. From the results, it was evident that the mill acoustic emission response can be used to classify different rock hardness and their binary mixtures. Model quartz emitted the highest acoustic response, followed by iron ore and model calcite at the initial stages of grinding (5 min). The results further indicated that as the rock feed sizes reduced, the average mill noise also increased as a function of grinding time. Accordingly, the acoustic emission presented a contrasting effect with model calcite and model quartz emitting the highest and lowest noise emission, respectively. The proportions of different mineral types (model quartz and iron ore) were reflected well in the acoustic emissions. The study has demonstrated that the integration of acoustic sensing techniques in AG/SAG mills can serve as a proxy for online measurement of different rock/ore hardness, for example, as a fast track method to determine the hardness of ores in comparison to the traditional ore grindability procedures, such as Bond work index (BWI) and SAG power index(SPI) tests which can be laborious and time-consuming.

Evaluation of rock characterization tests as geometallurgical predictors of bond work index at the Tasiast Mine, Mauritania

This paper presents a geometallurgical study for predicting ore grindability at Tasiast Gold Mine. Drill core samples of main gold-bearing lithologies were subjected to three phases of testing for characterization of physicomechanical, geochemical, mineralogical, and textural rock properties. In phase one, a set of physiomechanical and geochemical properties were measured using rapid and portable rock characterization tests. The measured properties include surface rebound hardness (Leeb hardness test), multi-element geochemistry (portable XRF test), acoustic wave velocity, and strength index (Point load test). In the second phase, the rock samples were subjected to more time-consuming and expensive micro-scale tests including mineralogical characterization by XRD and textural classification by petrographic analysis of thin sections. Finally in the third phase, the core samples were used for Bond ball mill work index (BWI) test to assess their grinding behaviour. Geometallurgical associations were identified between grindability and the geometallurgical test predictors using principal components analytics and K-means clustering. These associations were then used for fitting predictive models for BWI using multiple linear regression. Inferential tests were applied to evaluate how well micro-scale (phase 2) and drill core-scale (phase 1) properties can predict BWI behaviour, and how these predictions capture important geometallurgical relationships to BWI. The best BWI predictive model was considered by assessing statistical fit, testing speed, relative cost, and portability and amenability of the testing tool to the field. Accordingly, at the Tasiast mine multi-element geochemistry and lithological textural characteristics are the top two predictors of BWI.

Predicting mill feed grind characteristics through acoustic measurements

The present study investigates the propensity of predicting ore grindability characteristics and varying pulp densities through acoustic measurements on the Magotteaux ball mill. Specifically, the grinding behaviour of two different mill feeds (model quartz and iron ore) together with solid loadings (50, 57, and 67 wt% solids) were correlated against measured acoustic signals. The acoustic response analysis by root mean square (RMS) and power spectral density techniques indicated that model quartz sample emits higher energies than iron ore sample during grinding, relating to their different hardness properties. RMS analysis also showed that the noise intensities of both samples depreciate considerably as a function of increasing grind time, which corresponds well with their grind calibration curves. The selected pulp densities showed marginal differences in acoustic emission, which was reflected in their product size distribution. Results from this study further show the potential of using acoustic sensors as a proxy for real-time mill feed characteristics, mill operation monitoring and optimisation.

Variability Study of Bond Work Index and Grindability Index on Various Critical Metal Ores

It is a well-known fact that the value of the Bond work index (wi) for a given ore varies along with the grinding size. In this study, a variability bysis is carried out with the Bond standard grindability tests on different critical metal ores (W, Ta), ranging from coarse grinding (rod mills) to fine grinding (ball mills). The relationship between wi and grinding size did not show a clear correlation, while the grindability index (gpr) and the grinding size showed a robust correlation, fitting in all cases to a quadratic curve with a very high regression coefficient. This result suggests that, when performing correlation studies among ore grindability and rock mechanics parameters, it is advised to use the grindability index instead of the Bond work index.

Optimization of a SAG Mill Energy System: Integrating Rock Hardness, Solar Irradiation, Climate Change, and Demand-Side Management

Integration of renewable energy into mining and processing operations is becoming necessary as part of a strategy towards sustainability in the minerals industry. A solar photovoltaic plant along with a battery energy storage system (PV-BESS) can provide a long-term solution to cope with increasing energy costs, thus reducing the tension with other societal competing needs for a key resource such as clean energy. However, sizing these systems is challenging, in face of uncertain ore grindability and solar power availability. In this paper, we present an application of an integrated model to size the PV-BESS, where the variability of ore grindability is modeled using geostatistical tools, solar irradiance variability is captured using a Markov chain simulation model, and the entire system is optimized through linear stochastic optimization, considering a fixed mine schedule for the feed of a semiautogenous grinding (SAG) mill. The main goals are to minimize the costs associated with operating the SAG mill in the presence of a PV-BESS system, understand how the sizing and costs change under the influence of stochastic drivers, and reveal the potential for demand-side management in mines. The size and costs of the necessary infrastructure are determined for a model of estimated grindability and an ensemble of 50 simulated models of grindability, and compared against the sizing and cost of the ground truth. The effect of climate change on solar energy availability is accounted for by forecasting the ratio of excellent, good, and moderate days over bad days in terms of irradiance out to the year 2030. Finally, the effects of stockpiles and feed control according to the processing plant needs, namely a demand-side management approach, are evaluated to reveal the impact on the energy requirements and the sizing of the photovoltaic and storage system. The model is optimized considering a yearly cost function, with hourly resolution for the solar irradiance and hardness models. The results show that integrating solar power into the operation of a SAG mill has potential to reduce the total energy cost by 27%. Robustness against climate change can be achievedwith an increase in total cost of 1%. Finally, use of stockpiles to manage the ore supply to the mill and minimize the energy cost to process it results in a cost reduction of around 2%, which should offset the rehandling cost of managing the stockpiles.

Microwave Treatment of Ultramafic Nickel Ores: Heating Behavior, Mineralogy, and Comminution Effects

Ultramafic nickel ores are difficult to process because they contain serpentine, an anisotropic mineral with a nonspherical morphology and multiple pH-dependent surface charges. Dehydroxylation of serpentine in ultramafic nickel ores by microwave treatment is proposed to improve the processability of these ores. Upon heating, serpentine is converted to olivine, an isotropic mineral that is benign in mineral processing circuits. The microwave heating of two ultramafic nickel ores is explored in this paper, as well as effects on mineralogy and grindability. The first ore was sourced from the Okanogan nickel deposit in Washington State, USA, while the second ore was obtained from the Vale-owned Pipe deposit located in the Thomson Nickel Belt in Manitoba, Canada. The ultramafic nickel ores were found to heat well upon exposure to microwave radiation and the heating behaviors were a function of the imaginary permittivities. The temperatures achieved during microwave treatment were sufficient to dehydroxylate serpentine, and the serpentine content in ultramafic nickel ores was reduced by 63–84%. The grindability of ore with consistent texture (OK ore) improved dramatically with microwave treatment, whereas the grindability of ore with inconsistent texture (Pipe ore) was found to decrease. Pentlandite liberation and specific surface area improved for both ores with microwave treatment. Ultimately, microwave pretreatment did not decrease the energy required for grinding under the conditions studied. However, energy savings may be realized when overall process improvements are considered (e.g., grinding, rheology, flotation, material handling, dewatering and tailings treatment).

Microwave heating of magnesium silicate minerals

Worldwide high-grade nickel sulphide deposits are being depleted and new ones are not being discovered. Therefore, the exploitation of low-grade ultramafic nickel ores is of increasing interest. Processing these deposits, however, is a challenge, due to the high serpentine content. Serpentine minerals are anisotropic, increase the viscosity of ore slurries, dilute the concentrate, and slime-coat the valuable nickel mineral pentlandite. Microwave pre-treatment of ultramafic nickel ores has been proposed to convert the serpentine to olivine, reduce the viscosity of the ore slurry, improve ore grindability and mineral liberation, and reduce the overall energy requirements of processing. To optimize the process, an understanding of the microwave heating properties of ultramafic nickel ores and constituent minerals is necessary. The microwave heating of serpentine and olivine, the primary components of ultramafic nickel ores, is explored in this paper. Microwave heating tests and high-temperature microwave properties (real and imaginary permittivity) analysis were performed. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) tests were also completed to explain the permittivity data. Serpentine responded more readily to microwaves than olivine and the maximum temperatures achieved were 216 °C and 57 °C, respectively. Serpentine did not reach the temperature required for dehydroxylation (600-700 °C). The real and imaginary permittivities for serpentine and olivine varied with temperature, but overall were low, indicating these minerals cannot be expected to heat well on their own upon exposure to microwave radiation. A microwave susceptor is required for the serpentine in ultramafic nickel ores to reach dehydroxylation temperatures. It is hypothesized that the small quantities of pyrrhotite and magnetite present in ultramafic ores play this role.

Effect of magnetic pulse treatment upon titanium magnetite ore grindability and crushability

Effect of magnetic pulse treatment upon titanium magnetite ore grindability and crushability

Effect of undersize misplacement on product size distribution of Bond’s ball mill test

The standard Bond ball mill grindability test determines the ore grindability (g rev−1) and work index on an ore which gives the ball mill power consumption. The test is carried out on a standard sample and the number of revolutions per minute (rev min−1) required for the next cycle is determined after screening out and weighing the undersize fraction from a screen of a mesh the same size as the intended product passing size. The standard procedure assumes 100% efficiency in screening out the undersize fraction. However in plant grinding and classification circuits especially those based on dry grinding and classification, the efficiency of classifiers will be less than 100%. The misplacement of undersize particles in the oversize recirculating load is a practical reality which cannot be avoided. The aim of this study was to examine the effect of misplaced undersize particles on the P80 of product. The P80 values obtained from the standard Bond ball mill procedure and those of an inefficient ball mill grinding – classifier circuit were compared. The P80 values obtained with an inefficient classification were lower than the values obtained from the standard Bond ball mill procedure. The relationship between standard P80 values and those obtained with inefficient classification can be used as a guide to adjust operating parameters such as mill load and mill speed to produce the desired product size.

Rock lithological classification using multi-scale Gabor features from sub-images, and voting with rock contour information

Estimation of rock composition in mining plants is important for determining rock size and grindability which, in turn, may improve control of the grinding process. Variations in ore grindability and size distribution directly affect a mill's power consumption and throughput. It is therefore highly desirable to develop new methods to estimate rock lithological composition remotely. In this paper, a new method for remote lithological classification based on digital image analysis is proposed. The method is based on a single digital video camera for image acquisition from rocks on a conveyor belt. Each image is broken into sub-images to extract texture information using several different spatial scales. Gabor filters are used to extract features from each sub-image for five different spatial scales and eight orientations. After feature extraction, each sub-image is classified into a lithological class using a support-vector machine (SVM). Finally, information from the contour of each rock is used to select all sub-images that fall inside the contour and vote for the rock class. The method was tested on three databases, one of them containing sub-images of 64×43pixels of five ore types assigned to three grindability classes (soft, medium and hard). The second database has seven ore types containing sub-images of 60×40pixels. The third database was captured with higher resolution cameras and controlled illumination with five ore types and containing sub-images of 64×48. The databases were divided into 2 subsets, one for training with cross validation, and the other for testing. Classification accuracy was compared with previously published results on both databases. Comparisons show that our proposed method yielded significant improvements in classification accuracy, between 8.3% and 26%, relative to previously published results.

The influence of equipment settings and rock properties on high voltage breakage

High voltage breakage is a novel comminution method that relies on highly energetic electrical pulses to weaken or fully fragment rocks. The potential of this technology to improve liberation and increase the grindability of ores has been demonstrated previously, but the fragmentation process is not fully understood. In this study a total of 20 rock types were treated in a SELFRAG Lab device to determine the influence of equipment parameters on breakage. Rock mass properties and Bond Work Index were determined for each rock type to identify their relation to breakage behaviour. Results show how, by influencing total applied energy, the number of discharges and voltage are the two major influences on the resultant product size. It has also been shown that coarser feed sizes are more amenable to high voltage breakage. Acoustic impedance, porosity and quartz content were found to relate to breakage but Bond Work Index only correlates loosely.

Iron ore grindability improvement by microwave pre-treatment

The influence of microwave pre-treatment on grindability of iron ore (Orissa, India) was investigated by grindability tests. SEM analysis characterized the micro-fractures in microwave treated sample. This may be due to the thermal stress cracking resulted from microwave energy pre-treatment. XRD analysis showed the crystalline content of the sample. It was found that the microwave treated iron ore has peak more than that of untreated ore, i.e. the crystallinity increased with the microwave exposure time. The calculation of HGGI indicated increase in the ease of grinding or decrease in grindability index of the microwave treated ore. Grindability test showed that the microwave treated iron ore grinds much more rapidly initially than the untreated ore. The results showed that the breakage function of both microwave untreated and treated iron ore is dependent of the particle size. The grindability increased significantly as a result of microwave pre-treatment with the specific rate of breakage ( S i ) increasing by an average of 50%. It was concluded that microwave assisted grinding produced good results particularly for grinding characteristics.

Detection and identification of ore grindability in a semiautogenous grinding circuit model using wavelet transform variances of measured variables

A reduced dimension dynamic model subject to random disturbances for a semiautogenous grinding (SAG) circuit is developed that is able to handle changes in the characteristics of the new feed ore. This dynamic model, which is adjusted using data from a large mineral processing plant, has been developed with the objective of being useful for design and testing of fault identification and detection (FDI) systems, soft-sensors, automatic control systems, etc. The reduced dimension is a requirement in order to be able to obtain in a reasonable time statistical results to evaluate such systems. In this paper this SAG circuit dynamic model is used to test a method for detecting changes and identifying the grindability of the ore being processed by the SAG circuit. The method used — based on the variance of the continuous wavelet transform of measured circuit variables — incorporates improvements of a previous method. Results show that a step change of new feed ore is detected in about 30 to 80 min depending on the grindability change. This result may be considered to be adequate when taking into account that the response time for the mill hold-up to attain a new equilibrium value after the ensuing transient is of about 3 h. Identification of grindability in stationary operation gives near 100% of correct classification under the analysed conditions. The sensitivity of the FDI method to changes in circuit characteristics is also assessed and acceptable results are obtained.

Study the effect of chromite ore properties on pelletisation process

Chrome ore properties play a critical role in various pelletisation subprocesses (grinding, filtering, pelletisation and sintering) and slight variation in ore properties significantly affect the whole pelletisation process. Three ore samples (sample-A, B and C) were collected from different working faces of a chromite ore mine. Experiments were carried out to find out the difference in grinding (Inherent ore granulometry, ore hardness, ore grindability and friability), pelletisation (green pellet strength, balling time and pellet size) and sintering (pellet strength and porosity) characteristics of these ore samples. Chemical analysis and microscopic studies of the ore samples and pellets were also carried out to identify the cause of difference in pelletisation characteristics of ore samples. These differences were mainly caused by ore genesis process, degree of weathering and mining method. Results of the study show that the ores collected from different working faces of a mine show the significant difference to unstabilize the process with degraded product quality. It was found that blending could be a suitable option to tailor the desired grade feed for steady process efficiency and product quality with proper natural resource utilization.

Inferential measurement of SAG mill parameters II: state estimation

This paper discusses the combined state and parameter estimation of SAG mill inventories and model parameters. Recognised simulation models are utilised for the rock and water charge state equations. New models of the ball charge and mill shell lining states are presented. The 36 state system is detectable although not completely observable. Five ore grindability and mill discharge grate parameters augment the state system. One mill weight and two discharge measurement models are presented and utilised in two state estimation formulations. Results indicate that a size by size SAG mill discharge measurement model provides superior state estimates and improved discharge grate parameter estimation compared to a bulk SAG mill discharge measurement model.

Grindability soft-sensors based on lithological composition and on-line measurements

The grinding efficiency evaluation can be performed through the comparison of the operational work index with the ore work index Wi. In this work, the development of an ore grindability soft-sensor (ESTMOL) is presented. The ore work index is estimated on the basis of its lithological composition. Also addressed is the experimental development of a lithological composition sensor (ACOLITO) for ores on a conveyor belt. The lithological composition is determined from image analysis on samples obtained by a color video camera. Finally, a global operational work index for a complete grinding section is defined here, and its on-line estimation (PREDIMOL) is addressed, including the required soft-sensors to overcome the measurement problems. The experimental work is done with samples obtained from the CODELCO - Andina grinding plant. All the sensors have given up to now good results.

Application of the ring-loaded strength (RLS) disc test to monitor the effects of thermal pre-treatments on ore grindability

The physical and mechanical properties of rock can be measured by a variety of standard tests and studies have shown that there should be a fundamental connection between breakage in comminution and rock ‘strength’. The current study aims to develop the ring-loaded strength test, used as an industry standard for the characterisation of brittle ceramics, for determination of comminution behaviour from exploration borehole core samples. Preliminary studies using model materials (well defined Cornish granites, Italian marble, fused silica glass, soda-lime-silica glass) have demonstrated the reproducibility of data from the basic test method, and these studies have been extended to testing of core samples from South Crofty and Geevor mines. The tests have also been used on core samples subjected to heat treatments to determine the effects of such pre-treatments on rock strength and to correlate the data with grindability index measures or specific comminution energy.

The influence of mineralogy on microwave assisted grinding

The influence of mineralogy on microwave assisted grinding is elucidated. Detailed mineralogical analysis of various commercially exploited ores was carried out and the textural relationships between the individual mineral phases established. Microwave heating profiles were determined for each constituent mineral. Each ore was then subjected to microwave radiation for varying time periods, and change in work index with microwave exposure time quantified. It is suggested that increases in grindability of ores after microwave treatment are related to the specific mineral species present, particle size of the specific mineral and also the degree of dissemination. Conclusions are made regarding the economic implementation of this technology.

Can blasting enhance the grindability of ores?: K. Nielsen & J. Kristiansen, Transactions — Institution of Mining & Metallurgy, Section A, 104(Sept–Dec), 1995, pp A144–A148

Can blasting enhance the grindability of ores?: K. Nielsen & J. Kristiansen, Transactions — Institution of Mining & Metallurgy, Section A, 104(Sept–Dec), 1995, pp A144–A148