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Recent developments in microwave-assisted comminution
Int. J. Miner. Process. 74 (2004) 71 – 83 www.elsevier.com/locate/ijminpro
Recent developments in microwave-assisted comminution S.W. Kingman a,*, K. Jackson a, A. Cumbane a, S.M. Bradshaw b, N.A. Rowson c, R. Greenwood c a
School of Chemical Environmental and Mining Engineering, University of Nottingham, University Road, Nottingham, NG7 2RD, UK b Department of Chemical Engineering, University of Stellenbosch, Stellenbosch, South Africa c School of Chemical Engineering, University of Birmingham, Birmingham, B15 2TT, UK Received 12 May 2003; received in revised form 10 September 2003; accepted 10 September 2003
Abstract The influence of high electric field strength microwave energy on copper carbonatite ore has been elucidated. It has been shown that very short exposures times can lead to significant reductions in ore strength as determined by point load tests. Comparative drop weight tests were carried out to determine any potential change in required comminution energy for microwave-treated material. It was shown that reductions in required comminution energy of over 30% could be achieved for microwave energy inputs of less than 1 kW h t 1. Comparative specific rate of breakage grinding studies showed that significant increases in grindability could also be achieved after short microwave exposure times, changes being related to particle size. QEM*SEM liberation studies showed that the amount of locked and middling copper sulphides was reduced from 69.2% to 31.8% in the + 500 Am size fraction. Conclusions are made regarding the potential future development of this technology. D 2003 Elsevier B.V. All rights reserved. Keywords: energy requirements; grindability; impact energy; mineral liberation
1. Introduction The mineral resource and environmental industries are major employers and represent a significant proportion of gross domestic product for many countries worldwide. World consumption of minerals is increasing, a trend likely to continue if the substantial and rising expectations of countries in the Pacific Rim, Central and South America, and Africa are to be met. * Corresponding author. Tel.: +44-115-951-4081; fax: +44-115951-4115. E-mail address: [email protected] (S.W. Kingman). 0301-7516/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2003.09.006
The sustainable development and use of raw resources requires new, better and more efficient processes. The recent UK Government Technology Foresight document, from the Natural Resources and Environment Panel (1999), stated that new developments were required to improve and adapt existing unit processes. In particular, improvements in rock breaking and mineral liberation techniques and reductions in energy requirements for metal production processes were required. Various approaches have been taken in an attempt to improve the efficiency of comminution processes. These have included steady state comminution circuit modelling and the development of novel comminution
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devices such as the high pressure grinding rolls by Scho¨nert (1988). Whilst both of these techniques have offered improvements, the improvements have been incremental. The ability to alter the properties of the ore to reduce strength and improve liberation is ultimately the only way that step changes in the efficiency of comminution processes will be made. One such way of changing the fundamental properties of the ore has been microwave pre-treatment. Pioneering work was reported by Walkiewicz et al. (1988, 1991). Other workers in the field include Tavares and King (1995). Although promising reductions in grindability or ore strength were obtained, the economics of the process were considered unfavourable at the time. More recent work confirmed that microwave radiation is extremely effective in inducing fracture in mineral specimens. Kingman et al. (1999) exposed massive Norwegian ilmenite ores to microwave radiation for varying times and showed reductions in Work Index of up to 90%. The influence of the microwave treatment on downstream magnetic separation was also quantified and increased recovery of ilmenite was demonstrated for microwave-treated samples in comparison to untreated samples. It was concluded that short, high-power treatments were most effective because over-exposure of the sample led to reductions in downstream processing efficiency. Kingman and Rowson (2000) extended this work to investigate the reasons for possible increases in recovery of valuable mineral after microwave treatment. It was shown that increased recovery was due not only to increases in liberation but also to enhancement of the magnetic properties of the material. Kingman et al. (2000a) also carried out a qualitative study of the influence of mineralogy on the response of ores to microwave radiation. It was concluded that samples with a mixture of ‘good heaters’ and ‘medium heaters’ in a lattice of ‘poor heaters’ with a coarse grain size gave greatest reduction in work index after microwave treatment. Poorest response could be expected from ores containing highly disseminated, fine-grained minerals. A further conclusion from the work was that whilst the technical benefits were attractive, the economics were poor, with significant microwave energy inputs being used to give the reductions in comminution energy. The influence of microwave treatment on flowsheet design has also been investigated by Kingman et
al. (1999). Rod mill feed from a South African copper plant was subjected to microwave treatment at various power levels for various time-periods. The comparative Bond Work Index determined for each sample was used to investigate the performance and design of the actual grinding circuit for the ore in question. Using flowsheet-modelling software, it was shown that reducing the Work Index meant that a conventional system, e.g., an open-circuit rod mill followed by a closed-circuit ball mill, could be replaced by a single, open-circuit, rod mill. It was appreciated that this might not be the best operational procedure but was carried out as an indication of the possible influence of microwave treatment on flowsheet design. However, the conclusion drawn from the work was that, again, the technical benefits were attractive but the economics were poor, as the reduction in work index was achieved at high microwave power input. This effectively implies a high microwave energy cost that could not be justified based on the work index reduction. In order to improve the efficiency of microwave-assisted comminution to the point where it is potentially economically viable, improved understanding of the physics of microwave heating is required. The majority of the early test work involving microwave treatment of ores was carried out in multimode cavities. This is by far the most widely used type of microwave applicator, used in almost all domestic ovens and a large number of industrial units. The major benefit of this type of cavity is its mechanical simplicity and versatility. A basic, multimode cavity consists of a metal box, at least several wavelengths long in two dimensions. In simple terms, when the microwaves are launched into the box, they form a complex pattern of areas of high and low electric field strength, otherwise known as modes. In most materials, the electric field rather than the magnetic field is responsible for heating and it has been shown that the power density (or volumetric absorption of microwave energy) is proportional to the square of the strength of the electric field. Due to the complex distribution of the areas of high field strength, ensuring uniform high power density within a sample and achieving rapid heating rates is very difficult. This helps explain why previous work has reported poor economics as extended residence times have been required to induce the desired effects in the treated ore samples.
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The influence of power density on change in ore strength is also of great importance and has recently been investigated by Whittles et al. (2003). These workers carried out a series of comparative, twodimensional finite-difference simulations to examine the thermo-mechanical response of ores to microwave treatment. The simulations were carried out on a simulated ore of microwave-responsive 1-mm2 grains of pyrite within a microwave-transparent calcite matrix. After the simulation of heating and thermal stress generation at various power densities over a range of exposure times, uniaxial compressive strength tests were simulated. The results clearly showed that high power densities or electric field strengths were vital to rapid stress generation within the ore matrix. Simulation of samples treated at a power density of 1 109 W m 3 showed a theoretical reduction in strength from 125 to 80 MPa in 30 s microwave exposure. However, simulations of samples treated at a power density of 1 1011 W m 3 showed a theoretical reduction in strength from 125 to 60 MPa in 0.05 s. This reduction in time is suggested as being a result of reduced thermal conduction through the sample and greater thermal gradients across the grain boundaries of the adsorbing and non-adsorbing phases within the model. It is also important to note that the greater strength reduction was achieved using only 1/6th of the energy. This has important implications for the economic viability of the process. The production of high-strength electric fields and, hence power densities, in a multimode cavity is difficult. Therefore, it is necessary to move to a different type of cavity. A single mode cavity comprises a metallic enclosure into which a microwave signal of the correct electromagnetic field polarisation will undergo multiple reflections. The superposition of the reflected and incident waves gives rise to a standing wave pattern that is very well defined in space. The precise knowledge of electromagnetic field configurations enables the ore to be placed in the position of maximum electric-field strength allowing maximum heating-rates to be achieved at all times. In general, the heating zone in single mode cavities is small, which means that very high power density can easily be achieved. In the early evolution of microwave heating, such cavities saw little industrial use. This was mainly
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because they lacked the versatility offered by multimode cavities and are typically best suited to heating filamentous materials. However, they can offer extremely rapid heating rates and the ability to heat materials that appear transparent to microwaves in multimode cavities using similar input power levels. The aim of the work described in this paper was to investigate the comminution behaviour of ore treated at high microwave power but for short residence times. Initial characterisation of comminution behaviour of the treated ore was done in terms of ore crushing parameter, grindability and QEM*SEM liberation data.
2. Experimental 2.1. Materials The ore used for this investigation was a South African copper carbonatite utilised in previous studies (Kingman, 1999). Representative portions of the sample were analysed by qualitative, backscattered SEM to determine the major gangue and ore minerals and their relative abundances. This analysis revealed that the sample consisted of various proportions of calcite (CaCO3), dolomite (CaMg(CO3)2), apatite (Ca 5 (PO 4 ) 3 (F,Cl,OH), olivine (Mg 3 Si 2 O 5 (OH) 4 ), phlogophite (KMg 3 AlSiO 10 (F,OH) 2 ), serpentine (Mg3Si2O5(OH)4), pyroxene and plagioclase feldspar. Carbonate minerals were found to be particularly abundant and consisted generally of calcite and dolomite. Magnetite was found to be abundant within this sample (approx. 20%). It occurred as discrete, liberated grains (often >300 Am) and to a lesser extent as intergrowths with transparent gangue. EDAX examination of the magnetite confirmed that it contained predominately of Fe and O with Mg and Ti being present in smaller amounts. The principal copperbearing sulphide minerals were found to include cubanite (CuFe2S3), chalcopyrite (CuFeS2), bornite (Cu5FeS4) and chalcocite (Cu2S). 2.2. Microwave treatment Samples were irradiated in a variable power (3 –15 kW), microwave heater manufactured by Sairem of France. A schematic of the apparatus is shown in Fig. 1. Essentially the apparatus consists of a power supply
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Fig. 1. Schematic of microwave apparatus.
and water-cooled 2.45 GHz magnetron for generation of the microwave energy, a single mode TE10n applicator and a short-circuit tuner to produce the standing wave. The applicator consisted of 82-mm ID tube mounted in the broad face of WR430 waveguide and extended 125 mm on either side of the waveguide. The apparatus also included an automatic E-H tuner to match the impedance of the generator to the impedance of the applicator, ensuring a minimum amount of reflected power. Samples were exposed in the cavity for varying time-periods and at different power levels as seen in Table 1. To achieve very short exposure times, the residence time was controlled by using a pneumatic piston in which the speed of the return stroke could be controlled. A schematic of the apparatus is shown in Fig. 2. Samples were placed in 75-mm ID cylindrical glass tubes, which were then passed vertically down through the single mode cavity to give the appropriate cavity residence time. In all tests, there was approximately 1 kg of material within the microwave-heating zone, the actual mass being dependent on particle size. Table 1 Matrix for point load tests Particle size (mm)
Applied power level (kW)
Exposure time (s)
53 + 45 53 + 45 53 + 45 53 + 45 37 + 31 37 + 31 37 + 31 37 + 31
untreated 5 10 15 untreated 5 10 15
N/A 0.1, 0.5, 0.1, 0.5, 0.1, 0.5, N/A 0.1, 0.5, 0.1, 0.5, 0.1, 0.5,
1 1 1 1 1 1
Fig. 2. Cross section of ore presentation system.
2.3. Point load testing Point load tests were developed to give a quick but reproducible method of determining uniaxial compressive strength of samples without the need for large amounts of sample preparation. The test has recently been adapted by Bearman et al. (1997) for prediction of comminution behaviour in rocks. The point load test has the advantage of requiring no sample preparation so that a statistically sound test sample can be rapidly processed and an assessment made of the strength. The test is very simple, a particle is measured and placed between a pair of specially shaped, hardenedsteel tips. Force is then applied between the tips and the maximum force sustained by the particle is recorded. A schematic of the test principle is shown in Fig. 3. An approximation to the uniaxial compressive strength, known as Is(50) can then be calculated (Broch and Franklin, 1972). Several empirical equations have been developed to convert the test data to Is(50) and then to an approximation of compressive strength (Hoek and Brown, 1980). As only comparative data were required, only the Is(50) value was used and the equation used in this work is given below: 2 3 Depth 0:45 Force 7 6 50 7 Is ð50Þ ¼ 6 4 4 Width Depth 5 p
ð1Þ
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Fig. 3. Principle of operation of point load apparatus.
Samples of ore were subjected to point load tests, both as untreated material and after each type of treatment, the matrix of tests being shown in Table 1. For each test, 10 pieces of ore were treated and the Is(50) value for each sample calculated. The value reported is the median of the 10 tests, also given for completeness is the mean standard deviation and range. 2.4. Drop weight testing Whilst changes in point load index are useful for comparative purposes, they are difficult to interpret in terms of changes in process efficiency. A standard test has been developed by the Julius Kruttschnitt Mineral Research Centre (JKMRC), University of Queensland, Australia (Napier-Munn et al., 1996). It characterises ores in terms of constants known as ‘‘impact breakage parameters’’. In the drop weight test, a known mass falls through a given height onto a single particle providing an
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event that allows characterisation of the ore under impact breakage. Although the test is physically simple, it is supported by detailed data analysis. Although the drop weight test has advantages over the point load test in terms of statistical reliability and the potential use of the data from the analysis, it has a number of disadvantages, particularly the length of time taken to carry out a test. For each drop weight test, 15 samples were tested in 5 size fractions at 3 levels of energy input. Details of the recommended size fractions, number of particles and energy levels are given in Table 2. After each test has been completed, the resulting fragments are collected and sieved to give a product size distribution. The measured size distribution is then used to determine breakage functions for the material. The analytical procedure is based on the assumption that product size distributions are a function of input energy or specific comminution energy (Ecs kW h t 1). To model the breakage process the JKMRC developed a method of relating energy to geometric size reduction. The basic principle of the method is as follows. If a single particle is broken, the size distribution of the daughter particles may be considered as a M2 series and a cumulative size distribution graph plotted. The graph is then replotted after dividing the xaxis by the original particle size. A series of marker points are then used to describe the size distribution. These are defined as a percentage passing t, a fraction of the original particle size. Thus, t2 is the percentage passing half of the original size, etc. The value of t10, i.e., the amount passing 10% the original mean size, is used as a characteristic of size reduction and may be considered a fineness index. To make use of this technique, the marker points t2, t4, t25, t50 and t75 are stored in matrix form against t10. This information is then used to calculate the values of A and b, which are Table 2 Nominal energy input particle size combinations for JKMRC drop weight tests Particle size Number of Energy 1 Energy 2 Energy 3 range (mm) particles (kW h t 1) (kW h t 1) (kW h t 1) 63 53 45 37.5 31.5 26.5 22.4 19 16 13.2
10 15 30 30 30
0.2 0.5 1.25 1.25 1.25
0.125 0.125 0.5 0.5 0.5
0.05 0.05 0.125 0.125 0.125
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defined as the ore impact breakage parameters. A and b are related to ECS and t10 by the following Eq. (2): t10 ¼ A½1 eðbECSÞ
ð2Þ
The drop weight test provides effective characterisation of an ore for crushing. It should be remembered that chipping and abrasion occur in tumbling mills and that additional tests would be required to obtain a comprehensive assessment of the benefits of microwave treatment. The impact parameter A b has been correlated, although not well, to the Bond Work Index using Eq. (3) (Napier-Munn et al., 1996): A b ¼ 3:5WI þ 117
2.6. Liberation tests Samples of treated and untreated material (15 kW, 2.45 GHz, 0.2 s) produced in drop weight tests were screened into four size fractions viz. + 500 Am, + 150/ 500 Am, + 38/ 150 m and 38 Am. Representative sub-splits of the samples were mounted in chlorinated epoxy resin, polished and carbon coated for analysis by QEM*SEM (Napier-Munn et al., 1996). In each size fraction results were reported in terms of % locked, middling and liberated where these were defined as < 30% liberated, 30– 80% liberated and >80% liberated, respectively. Liberation characteristics were determined for copper sulphides only.
ð3Þ 3. Results and discussion
To enable comparison, drop weight tests were carried out both on untreated material and on material treated at 15 kW for 0.2 s. From the results, breakage parameters specific comminution energy were calculated. The energy levels used in this investigation were half those recommended in the standard method as it was found that if the recommended energy levels were used no discrimination between treated and untreated material was produced despite the appearance of significant visible fracture in the microwavetreated samples. 2.5. Grindability testing To understand the grindability behaviour of the material, 2 500 g samples of the following size fractions were produced: 19 + 16, 16 + 13.2, 13.2 + 9.6, 9.6 + 6.7, 6.7 + 4.75 and 4.75 + 3.35 mm. One sample was left untreated and the other was exposed to microwave energy in the single mode cavity for 0.2 s at 15 kW. The samples were then ground for increasing lengths of time (1– 5 min) in a 12-in. Bico Braun ball mill with a standard Bond mill charge at a speed at 70 rpm. The mass left in the original fraction after a specific time being determined by sieving. The data were evaluated using a standard population balance method (Austin et al., 1984; Wang and Forrsberg, 2000). The breakage function was evaluated by plotting the natural logarithm of mass remaining in a specific size fraction as a function of cumulative grinding time.
3.1. Point load tests Fig. 4 shows the change in Is(50) as a function of microwave exposure time for material in the size range 53 + 45 mm. It can be clearly seen that microwave treatment has a significant effect on the strength of the ore, with exposures as short as 0.1 s giving reductions in Is(50) of approximately 40%. Table 3 shows the full data set for the experiment. It can be seen that the results for microwave-treated material show a larger standard deviation than the untreated. Examination of the upper and lower limits revels why this is the case. Indeed, for most of the microwave-treated material, the strength of at least one (or often more) particles was close to zero. Naturally, this gives rise to the large standard deviation for treated samples because the untreated samples all had reasonable strength. Previous work (Kingman, 1999) has shown reduction in ore strength to be related to microwave power level. This is also shown here with significant reductions in strength noted for 10 and 15 kW but not for 5 kW. It is apparent that a critical power density exists above which the damage is significant. For this ore within this cavity, this is shown to be produced by at least 10 kW. After the initial drop in strength, little benefit is gained by increased exposure time. This was suggested by Whittles et al. (2003) in their paper. For all exposure times, 5 kW gives the smallest reduction in strength. However, in all cases, these
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Fig. 4. Is(50) as a function of microwave exposure time for material 53 + 45 mm.
results are highly significant, as previous work showed that at least 10 and often 180 s were required to achieve small reductions in strength. In this study, significant reductions in strength are demonstrated for an exposure time of 0.1 s, up to 1800 times less than that previously used. Fig. 5 shows a similar plot for 37 + 31 mm material. It can be clearly seen that significant reductions in strength can be achieved in short exposure times of less than 0.1 s. The plot is broadly similar to that shown in Fig. 4 with the complete data for this size fraction is shown in Table 4.
Table 3 Is(50) as a function of microwave exposure time for material 53 + 45 mm showing statistical measures Treatment
Untreated 15 kW
10 kW
5 kW
Time
– 0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
Is(50) 10 6 MPa Mean
Standard deviation
Median
Upper limit
Lower limit
3.32 2.06 0.93 1.29 1.82 1.86 1.98 1.82 2.34 2.29
0.88 1.73 1.02 1.06 1.30 1.74 1.48 1.30 1.07 1.07
3.28 2.06 0.62 0.88 1.28 0.83 1.59 3.16 2.89 2.19
4.67 5.47 3.18 3.26 3.83 4.89 4.36 4.64 3.44 4.13
1.95 0.53 0.06 0.06 0.16 0.06 0.07 1.36 0.06 0.74
3.2. Drop weight tests Drop weight tests were carried out on microwaveand non-microwave-treated samples using the energy levels shown in Table 2. After completion of the tests, the appearance function and breakage parameters were calculated. Before breakage in the drop weight tester the microwave-treated sample exhibited significant amounts of visible fractures and many pieces could be crumbled by hand. The untreated sample, by contrast, was wholly competent. Table 5 shows the breakage parameters for the treated and untreated material. The value of A is the theoretical maximum value of t10, whilst A b is the slope of a plot of Ecs versus t10 at Ecs = 0. For most hard rocks (for a reasonable range of b values), there is a limiting value for A, the maximum value that t10 can achieve as Ecs is increased. This value is approx. A = 50. Note, for crusher breakage, t10 is typically 10 – 20% (Napier-Munn et al., 1996). The impact breakage parameter b is actually an indicator of the amenability of the ore to size reduction by impact. It can be seen that the value of b is significantly higher for the microwave treated material, suggesting a much softer ore. Figs. 6 and 7 show the effect of specific comminution energy on the breakage index t10 for different size classes. It can be clearly seen that the microwavetreated ore shows reduced values of Ecs over almost
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Fig. 5. Is(50) as a function of microwave exposure time for material 37 + 31 mm.
the entire range of t10 values, indicating that the material is softer because of microwave treatment. This is particularly true for the larger particle sizes indeed for the smallest size fractions the data is similar. The role of particle size in microwave assisted comminution is discussed later in this paper. 3.3. Grinding tests Fig. 8 shows a plot of the specific rate of breakage versus particle size for treated and untreated material. It can be seen that the difference in specific rate of breakage decreases with particle size. The largest Table 4 Is(50) as a function of microwave exposure time for material 37 + 31 mm showing statistical measures Treatment
Untreated 15 kW
10 kW
5 kW
Time
– 0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
Is(50) MPa 10 6 Mean
Standard deviation
Median
Upper limit
Lower limit
2.88 1.67 1.52 1.30 1.50 1.59 1.35 2.28 0.75 1.87
1.06 1.31 1.43 1.10 0.91 0.91 0.45 0.81 0.60 0.78
2.75 1.30 1.08 1.15 1.37 0.89 1.28 2.36 0.71 1.54
4.38 3.44 4.22 3.44 3.01 4.95 2.11 3.47 1.24 2.89
1.05 0.10 0.07 0.07 0.10 0.10 1.00 1.15 0.10 1.07
particle size fraction ( 19 + 16 mm) is most influenced with over 70% difference between the nontreated and treated samples. For the smallest size fraction ( 4.75 + 3.35 mm), the percentage change is 11%, whilst not as much as the larger fractions is still significant. The reason for the reduction with particle size is almost certainly related to the mineralogy or texture of the samples. The major microwave adsorbing phase within the material is magnetite. The magnetite is observed to occur in large aggregates sometimes greater than 10 mm in size. In order for differential expansion as a result of microwave heating to occur the heated phase must be entirely surrounded by transparent material. As the particle size decreases, the amount of magnetite, which is not totally surrounded by transparent gangue, will increase and thus the percentage change in specific rate of breakage will drop. It is likely therefore that more greater changes in specific rate of breakage could be achieved in ores, which have a finer texture. It can also be seen that the specific rate of breakage in the larger size fractions is low. This is expected for
Table 5 Breakage parameters for treated and non-treated ore Treatment
A
b
Ab
15 kW 0.2 s NON
52.5 54.3
2.35 1.61
123.4 87.4
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Fig. 6. Plot of t10 versus Ecs for different sized untreated ore.
laboratory mills and has previously been noted (King, 2001). 3.4. QEM*SEM liberation results Results of the QEM*SEM study for copper sulphide minerals are shown in Fig. 9a and b. It can be seen that microwave treatment has had the most significant influence on the + 500 Am fraction. The amount of liberated material in this fraction has increased from 31.8% in the untreated fraction to 69.2% in the treated fraction. The amounts of liber-
ated, middlings and locked copper sulphides in the other size fractions remain relatively similar. The significant increase in liberation of the copper sulphide mineral in the coarse size fraction is potentially important as could increase the effective liberation size of the copper sulphide within the ore. This would have the effect of reducing grinding energy consumption as the required grind size would be coarser and it also would also potentially increase the total recovery as less material would be overground and lost to slimes. Fig. 10 shows typical images obtained from the QEM*SEM analysis, they
Fig. 7. Plot of t10 versus Ecs for different sized treated ore.
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Fig. 8. Plot of specific rate of breakage versus mean particle size for treated and untreated samples.
indicate the nature of the classification, i.e., locked, middlings and liberated. 3.5. Explanation of mechanism of strength reduction The impressive strength reductions achieved in the present work with very short, high power microwave exposure can be explained by considering the temperature gradients produced across grain boundaries. If a very high microwave power density is generated within a microwave absorbing phase for a short residence time, then significant temperature gradients would be expected to develop between the absorbing and non-absorbing phases, leading to large thermal stresses. On the other hand, if the ore were heated slowly at low power, as was carried out in previous work, thermal conduction would reduce the size of the thermal gradients across the grain boundaries between the absorbing and non-absorbing phases. In turn, this would lead to small thermal stresses and less weakening of the ore. Recent thermo-elastic simulations by Whittles et al. (2003) showed that this is true. High power densities (1011 W m 3) maintained for short times (0.1 s) produced thermal gradients that in turn gave rise to significant thermo-mechanical stresses. These could reduce the material strength significantly. Lower power densities (109 W m 3) achieved lower strength reductions at total energy inputs almost an order of magnitude larger.
Fig. 9. (a) Liberation spectrum for copper sulphide minerals for untreated copper ore. (b) Liberation spectrum for copper sulphide minerals for ore treated 15 kW 0.1 s.
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Fig. 10. Examples of copper ore liberation characteristics, i.e., liberated, locked and middlings.
3.6. Industrial significance As stated earlier, the major disadvantage of previous work on microwave assisted comminution has been the amount of energy required to achieve the desired effects compared with the potential savings. Energy balances presented in previous work (Kingman et al., 2000a,b) showed that, for ore similar to that used in this test programme, a microwave energy input of over 43 kW h t 1 was required to achieve a reduction in work index of over 70% from 14 to 4 kW h t 1. In the present work, a 30% reduction in impact breakage parameters has been achieved at much lower microwave energy input. For material treated at 15 kW for 0.2 s, assuming 1 kg in cavity, it can be shown that the microwave energy input was: 0:2 1000 Energy ¼ 15 ¼ 0:83 kW h t1 3600 1 The result of increasing the electric field strength (or power density) is clear; significant strength reduction or reductions in grinding resistance can be achieved at low microwave energy inputs. Furthermore, these values suggest that scale-up is potentially feasible in terms of energy added to the process. Reductions in specific comminution energy of 30% coupled with significant improvements in ore grind-
ability are attractive in themselves; however, it must be remembered that the benefit of a softer ore is not only in the reduced crushing energy, but also in reduced mill size (or greater throughput for the same size mill) and also the very significant reduction in the circulating load (Kingman, 1999). It should also be remembered that potentially the most important potential benefit of microwave assisted comminution is increased liberation at larger sizes. This was clearly demonstrated by the QEM*SEM liberation data, in which the liberation in the + 500 Am fraction was doubled. Other benefits of microwave-assisted comminution include:
Reduced plant size for set throughput Potentially reduced wear costs per tonne
Lower water consumption (less mills required)
Smaller downstream recovery circuits (liberation at higher sizes)
The TE10n (electric field orientation transverse to the direction of propagation of the microwaves) cavity used for this test programme was ‘an off the shelf’ model designed for general heating applications. No attempt was made to optimise the cavity design with regard to the particle size distribution of the feed material or the dielectric properties of the material. This process will undoubtedly lead to higher power
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densities and lower residence time. This is of particular importance, as it will enable the input microwave power to be kept to a minimum and the throughput to be maximised. This will be vital if plant implementation is to be achieved. In the same way optimised cavity design and electric field strength should enable similar if not better results to be obtained with ore of different mineralogy. This study was completed on a material with a relatively coarse liberation size whose major microwave heated phase (magnetite) was also relatively large. It would obviously be desirable to achieve similar results with much finer grained ores. This work has shown that the degree of failure is related to microwave power densities giving rise to thermally induced stresses and strains. It is highly likely therefore that if ores of finer mineralogy are to be treated then higher power densities would be required than those used in this work. This is because faster heating rates are required to produce the same level of stress in a smaller particle than a large one. It is well known within the microwave-engineering field that staged development is vital for successful development. The next stage of this work will therefore be to develop a continuous pilot scale processing system that will process up to 1 t/h of ore. The facility will be designed to handle several specific ore types, which will allow some degree of optimisation in the presentation of the microwave energy to the ore. This test facility will also allow determination of the above benefits at reasonable scale.
4. Conclusions For the first time, significant changes in ore breakage and liberation characteristics have been demonstrated at microwave powers inputs that potentially may be economic. The influence of microwave radiation of high electric field strength has been quantified. It has been shown that significant reductions in strength can be achieved in very short microwave exposure times. There was some suggestion from the experimental data that the reduction in strength up to a certain level is related to applied power level. However, there appears to be a certain power level at which little more breakage can be induced.
Drop weight tests were carried out and the influence of microwave pre-treatment quantified. It was shown that reductions in required breakage energy of over 30% could be achieved for modest microwave energy inputs of less than 1 kW h t 1. Further work is suggested at the tonne-per-hour level to allow more scale up data to be produced and a more detailed economic analysis to be carried out. Grindability tests have shown that significant reductions in specific rate of brewage of over 40% can be obtained in a ball mill. These reductions, however, were shown to be intimately related to particle size. QEM*SEM data showed that the liberation of copper sulphide minerals in the + 500 Am size fraction increased by over 100% in the microwave treated material. References Anon, 1999. Technology Foresight—Natural Resources and Environment, Office of Science and Technology, London, UK. Austin, L.G., Klimpel, R.R., Luckie, P.T., 1984. Process Engineering of Size Reduction: Ball Milling SME, New York. ISBN 0-89520-421-5. Bearman, R.A., Briggs, C.A., Kojovic, T., 1997. The application of rock mechanics parameters to the prediction of comminution behaviour. Min. Eng. 10 (3), 255 – 264. Broch, E., Franklin, J.A., 1972. The point load strength test. Int. J. Rock Mech. Min. Sci. 9, 669 – 697. Hoek, E., Brown, E.T., 1980. Underground Excavations in Rock The Inst. Min. and Metall., London. King, R.P., 2001. Modelling and Simulation of Mineral Processing Systems, Butterworth Heinemann, Oxford, UK. Kingman, S.W., 1999. The Influence of Microwave Radiation on the Comminution and Beneficiation of Minerals. PhD thesis, University of Birmingham, UK. Kingman, S.W., Rowson, N.A., 2000. The effect of microwave radiation on the magnetic properties of minerals. J. Microw. Power Electromagn. Energy 35 (2), 141 – 150. Kingman, S.W., Corfield, G., Rowson, N.A., 1999. Effect of microwave radiation on the mineralogy and magnetic processing of ilmenite. Magn. Electr. Sep. 9, 131 – 148. Kingman, S.W., Vorster, W., Rowson, N.A., 2000a. The influence of mineralogy on microwave assisted grinding. Min. Eng. 13 (3), 313 – 327. Kingman, S.W., Vorster, W., Rowson, N.A., 2000b. The effect of microwave radiation on the processing of palabora copper ore. Trans. S. Afr. Inst. Min. Metall., 197 – 204 (May/June). Napier-Munn, T.J., Morell, S., Morrison, R.D., Kojovic, T., 1996. Mineral Comminution Circuits. Their Operation and Optimisation. JKMRC Monograph Series in Mining and Mineral Processing, vol. 2. University of Queensland, Australia.
S.W. Kingman et al. / Int. J. Miner. Process. 74 (2004) 71–83 Scho¨nert, K., 1988. A first survey of grinding with high-compression roller mills. Int. J. Miner. Process. 22, 401 – 412. Tavares, L.M., King, R.P., 1995. Microscale investigation of thermally assisted comminution. Proceedings XIX International Mineral Processing Congress, San Francisco, vol. 1, pp. 203 – 208. Walkiewicz, J.W., Kazonich, G., McGill, S.L., 1988. Microwave heating characteristics of selected minerals and compounds. Miner. Metall. Process. 5 (1), 39 – 42.
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Walkiewicz, J.W., Lindroth, D.P., McGill, S.L., 1991. Microwave assisted grinding. IEEE Trans. Ind. Appl. 27 (2), 239 – 242. Wang, Y., Forrsberg, E., 2000. Microwave assisted comminution and liberation of minerals. In: Ozbayoglu, G., et al. (Ed.), Mineral Processing on the Verge of the 21st Century. Balkema, Rotterdam, pp. 3 – 9. ISBN 905809 1724. Whittles, D., Kingman, S.W., Reddish, D., 2003. The influence of power density on microwave assisted breakage. Int. J. Miner. Process. 68, 71 – 91.
Recent developments in microwave-assisted comminution S.W. Kingman a,*, K. Jackson a, A. Cumbane a, S.M. Bradshaw b, N.A. Rowson c, R. Greenwood c a
School of Chemical Environmental and Mining Engineering, University of Nottingham, University Road, Nottingham, NG7 2RD, UK b Department of Chemical Engineering, University of Stellenbosch, Stellenbosch, South Africa c School of Chemical Engineering, University of Birmingham, Birmingham, B15 2TT, UK Received 12 May 2003; received in revised form 10 September 2003; accepted 10 September 2003
Abstract The influence of high electric field strength microwave energy on copper carbonatite ore has been elucidated. It has been shown that very short exposures times can lead to significant reductions in ore strength as determined by point load tests. Comparative drop weight tests were carried out to determine any potential change in required comminution energy for microwave-treated material. It was shown that reductions in required comminution energy of over 30% could be achieved for microwave energy inputs of less than 1 kW h t 1. Comparative specific rate of breakage grinding studies showed that significant increases in grindability could also be achieved after short microwave exposure times, changes being related to particle size. QEM*SEM liberation studies showed that the amount of locked and middling copper sulphides was reduced from 69.2% to 31.8% in the + 500 Am size fraction. Conclusions are made regarding the potential future development of this technology. D 2003 Elsevier B.V. All rights reserved. Keywords: energy requirements; grindability; impact energy; mineral liberation
1. Introduction The mineral resource and environmental industries are major employers and represent a significant proportion of gross domestic product for many countries worldwide. World consumption of minerals is increasing, a trend likely to continue if the substantial and rising expectations of countries in the Pacific Rim, Central and South America, and Africa are to be met. * Corresponding author. Tel.: +44-115-951-4081; fax: +44-115951-4115. E-mail address: [email protected] (S.W. Kingman). 0301-7516/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2003.09.006
The sustainable development and use of raw resources requires new, better and more efficient processes. The recent UK Government Technology Foresight document, from the Natural Resources and Environment Panel (1999), stated that new developments were required to improve and adapt existing unit processes. In particular, improvements in rock breaking and mineral liberation techniques and reductions in energy requirements for metal production processes were required. Various approaches have been taken in an attempt to improve the efficiency of comminution processes. These have included steady state comminution circuit modelling and the development of novel comminution
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devices such as the high pressure grinding rolls by Scho¨nert (1988). Whilst both of these techniques have offered improvements, the improvements have been incremental. The ability to alter the properties of the ore to reduce strength and improve liberation is ultimately the only way that step changes in the efficiency of comminution processes will be made. One such way of changing the fundamental properties of the ore has been microwave pre-treatment. Pioneering work was reported by Walkiewicz et al. (1988, 1991). Other workers in the field include Tavares and King (1995). Although promising reductions in grindability or ore strength were obtained, the economics of the process were considered unfavourable at the time. More recent work confirmed that microwave radiation is extremely effective in inducing fracture in mineral specimens. Kingman et al. (1999) exposed massive Norwegian ilmenite ores to microwave radiation for varying times and showed reductions in Work Index of up to 90%. The influence of the microwave treatment on downstream magnetic separation was also quantified and increased recovery of ilmenite was demonstrated for microwave-treated samples in comparison to untreated samples. It was concluded that short, high-power treatments were most effective because over-exposure of the sample led to reductions in downstream processing efficiency. Kingman and Rowson (2000) extended this work to investigate the reasons for possible increases in recovery of valuable mineral after microwave treatment. It was shown that increased recovery was due not only to increases in liberation but also to enhancement of the magnetic properties of the material. Kingman et al. (2000a) also carried out a qualitative study of the influence of mineralogy on the response of ores to microwave radiation. It was concluded that samples with a mixture of ‘good heaters’ and ‘medium heaters’ in a lattice of ‘poor heaters’ with a coarse grain size gave greatest reduction in work index after microwave treatment. Poorest response could be expected from ores containing highly disseminated, fine-grained minerals. A further conclusion from the work was that whilst the technical benefits were attractive, the economics were poor, with significant microwave energy inputs being used to give the reductions in comminution energy. The influence of microwave treatment on flowsheet design has also been investigated by Kingman et
al. (1999). Rod mill feed from a South African copper plant was subjected to microwave treatment at various power levels for various time-periods. The comparative Bond Work Index determined for each sample was used to investigate the performance and design of the actual grinding circuit for the ore in question. Using flowsheet-modelling software, it was shown that reducing the Work Index meant that a conventional system, e.g., an open-circuit rod mill followed by a closed-circuit ball mill, could be replaced by a single, open-circuit, rod mill. It was appreciated that this might not be the best operational procedure but was carried out as an indication of the possible influence of microwave treatment on flowsheet design. However, the conclusion drawn from the work was that, again, the technical benefits were attractive but the economics were poor, as the reduction in work index was achieved at high microwave power input. This effectively implies a high microwave energy cost that could not be justified based on the work index reduction. In order to improve the efficiency of microwave-assisted comminution to the point where it is potentially economically viable, improved understanding of the physics of microwave heating is required. The majority of the early test work involving microwave treatment of ores was carried out in multimode cavities. This is by far the most widely used type of microwave applicator, used in almost all domestic ovens and a large number of industrial units. The major benefit of this type of cavity is its mechanical simplicity and versatility. A basic, multimode cavity consists of a metal box, at least several wavelengths long in two dimensions. In simple terms, when the microwaves are launched into the box, they form a complex pattern of areas of high and low electric field strength, otherwise known as modes. In most materials, the electric field rather than the magnetic field is responsible for heating and it has been shown that the power density (or volumetric absorption of microwave energy) is proportional to the square of the strength of the electric field. Due to the complex distribution of the areas of high field strength, ensuring uniform high power density within a sample and achieving rapid heating rates is very difficult. This helps explain why previous work has reported poor economics as extended residence times have been required to induce the desired effects in the treated ore samples.
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The influence of power density on change in ore strength is also of great importance and has recently been investigated by Whittles et al. (2003). These workers carried out a series of comparative, twodimensional finite-difference simulations to examine the thermo-mechanical response of ores to microwave treatment. The simulations were carried out on a simulated ore of microwave-responsive 1-mm2 grains of pyrite within a microwave-transparent calcite matrix. After the simulation of heating and thermal stress generation at various power densities over a range of exposure times, uniaxial compressive strength tests were simulated. The results clearly showed that high power densities or electric field strengths were vital to rapid stress generation within the ore matrix. Simulation of samples treated at a power density of 1 109 W m 3 showed a theoretical reduction in strength from 125 to 80 MPa in 30 s microwave exposure. However, simulations of samples treated at a power density of 1 1011 W m 3 showed a theoretical reduction in strength from 125 to 60 MPa in 0.05 s. This reduction in time is suggested as being a result of reduced thermal conduction through the sample and greater thermal gradients across the grain boundaries of the adsorbing and non-adsorbing phases within the model. It is also important to note that the greater strength reduction was achieved using only 1/6th of the energy. This has important implications for the economic viability of the process. The production of high-strength electric fields and, hence power densities, in a multimode cavity is difficult. Therefore, it is necessary to move to a different type of cavity. A single mode cavity comprises a metallic enclosure into which a microwave signal of the correct electromagnetic field polarisation will undergo multiple reflections. The superposition of the reflected and incident waves gives rise to a standing wave pattern that is very well defined in space. The precise knowledge of electromagnetic field configurations enables the ore to be placed in the position of maximum electric-field strength allowing maximum heating-rates to be achieved at all times. In general, the heating zone in single mode cavities is small, which means that very high power density can easily be achieved. In the early evolution of microwave heating, such cavities saw little industrial use. This was mainly
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because they lacked the versatility offered by multimode cavities and are typically best suited to heating filamentous materials. However, they can offer extremely rapid heating rates and the ability to heat materials that appear transparent to microwaves in multimode cavities using similar input power levels. The aim of the work described in this paper was to investigate the comminution behaviour of ore treated at high microwave power but for short residence times. Initial characterisation of comminution behaviour of the treated ore was done in terms of ore crushing parameter, grindability and QEM*SEM liberation data.
2. Experimental 2.1. Materials The ore used for this investigation was a South African copper carbonatite utilised in previous studies (Kingman, 1999). Representative portions of the sample were analysed by qualitative, backscattered SEM to determine the major gangue and ore minerals and their relative abundances. This analysis revealed that the sample consisted of various proportions of calcite (CaCO3), dolomite (CaMg(CO3)2), apatite (Ca 5 (PO 4 ) 3 (F,Cl,OH), olivine (Mg 3 Si 2 O 5 (OH) 4 ), phlogophite (KMg 3 AlSiO 10 (F,OH) 2 ), serpentine (Mg3Si2O5(OH)4), pyroxene and plagioclase feldspar. Carbonate minerals were found to be particularly abundant and consisted generally of calcite and dolomite. Magnetite was found to be abundant within this sample (approx. 20%). It occurred as discrete, liberated grains (often >300 Am) and to a lesser extent as intergrowths with transparent gangue. EDAX examination of the magnetite confirmed that it contained predominately of Fe and O with Mg and Ti being present in smaller amounts. The principal copperbearing sulphide minerals were found to include cubanite (CuFe2S3), chalcopyrite (CuFeS2), bornite (Cu5FeS4) and chalcocite (Cu2S). 2.2. Microwave treatment Samples were irradiated in a variable power (3 –15 kW), microwave heater manufactured by Sairem of France. A schematic of the apparatus is shown in Fig. 1. Essentially the apparatus consists of a power supply
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Fig. 1. Schematic of microwave apparatus.
and water-cooled 2.45 GHz magnetron for generation of the microwave energy, a single mode TE10n applicator and a short-circuit tuner to produce the standing wave. The applicator consisted of 82-mm ID tube mounted in the broad face of WR430 waveguide and extended 125 mm on either side of the waveguide. The apparatus also included an automatic E-H tuner to match the impedance of the generator to the impedance of the applicator, ensuring a minimum amount of reflected power. Samples were exposed in the cavity for varying time-periods and at different power levels as seen in Table 1. To achieve very short exposure times, the residence time was controlled by using a pneumatic piston in which the speed of the return stroke could be controlled. A schematic of the apparatus is shown in Fig. 2. Samples were placed in 75-mm ID cylindrical glass tubes, which were then passed vertically down through the single mode cavity to give the appropriate cavity residence time. In all tests, there was approximately 1 kg of material within the microwave-heating zone, the actual mass being dependent on particle size. Table 1 Matrix for point load tests Particle size (mm)
Applied power level (kW)
Exposure time (s)
53 + 45 53 + 45 53 + 45 53 + 45 37 + 31 37 + 31 37 + 31 37 + 31
untreated 5 10 15 untreated 5 10 15
N/A 0.1, 0.5, 0.1, 0.5, 0.1, 0.5, N/A 0.1, 0.5, 0.1, 0.5, 0.1, 0.5,
1 1 1 1 1 1
Fig. 2. Cross section of ore presentation system.
2.3. Point load testing Point load tests were developed to give a quick but reproducible method of determining uniaxial compressive strength of samples without the need for large amounts of sample preparation. The test has recently been adapted by Bearman et al. (1997) for prediction of comminution behaviour in rocks. The point load test has the advantage of requiring no sample preparation so that a statistically sound test sample can be rapidly processed and an assessment made of the strength. The test is very simple, a particle is measured and placed between a pair of specially shaped, hardenedsteel tips. Force is then applied between the tips and the maximum force sustained by the particle is recorded. A schematic of the test principle is shown in Fig. 3. An approximation to the uniaxial compressive strength, known as Is(50) can then be calculated (Broch and Franklin, 1972). Several empirical equations have been developed to convert the test data to Is(50) and then to an approximation of compressive strength (Hoek and Brown, 1980). As only comparative data were required, only the Is(50) value was used and the equation used in this work is given below: 2 3 Depth 0:45 Force 7 6 50 7 Is ð50Þ ¼ 6 4 4 Width Depth 5 p
ð1Þ
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Fig. 3. Principle of operation of point load apparatus.
Samples of ore were subjected to point load tests, both as untreated material and after each type of treatment, the matrix of tests being shown in Table 1. For each test, 10 pieces of ore were treated and the Is(50) value for each sample calculated. The value reported is the median of the 10 tests, also given for completeness is the mean standard deviation and range. 2.4. Drop weight testing Whilst changes in point load index are useful for comparative purposes, they are difficult to interpret in terms of changes in process efficiency. A standard test has been developed by the Julius Kruttschnitt Mineral Research Centre (JKMRC), University of Queensland, Australia (Napier-Munn et al., 1996). It characterises ores in terms of constants known as ‘‘impact breakage parameters’’. In the drop weight test, a known mass falls through a given height onto a single particle providing an
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event that allows characterisation of the ore under impact breakage. Although the test is physically simple, it is supported by detailed data analysis. Although the drop weight test has advantages over the point load test in terms of statistical reliability and the potential use of the data from the analysis, it has a number of disadvantages, particularly the length of time taken to carry out a test. For each drop weight test, 15 samples were tested in 5 size fractions at 3 levels of energy input. Details of the recommended size fractions, number of particles and energy levels are given in Table 2. After each test has been completed, the resulting fragments are collected and sieved to give a product size distribution. The measured size distribution is then used to determine breakage functions for the material. The analytical procedure is based on the assumption that product size distributions are a function of input energy or specific comminution energy (Ecs kW h t 1). To model the breakage process the JKMRC developed a method of relating energy to geometric size reduction. The basic principle of the method is as follows. If a single particle is broken, the size distribution of the daughter particles may be considered as a M2 series and a cumulative size distribution graph plotted. The graph is then replotted after dividing the xaxis by the original particle size. A series of marker points are then used to describe the size distribution. These are defined as a percentage passing t, a fraction of the original particle size. Thus, t2 is the percentage passing half of the original size, etc. The value of t10, i.e., the amount passing 10% the original mean size, is used as a characteristic of size reduction and may be considered a fineness index. To make use of this technique, the marker points t2, t4, t25, t50 and t75 are stored in matrix form against t10. This information is then used to calculate the values of A and b, which are Table 2 Nominal energy input particle size combinations for JKMRC drop weight tests Particle size Number of Energy 1 Energy 2 Energy 3 range (mm) particles (kW h t 1) (kW h t 1) (kW h t 1) 63 53 45 37.5 31.5 26.5 22.4 19 16 13.2
10 15 30 30 30
0.2 0.5 1.25 1.25 1.25
0.125 0.125 0.5 0.5 0.5
0.05 0.05 0.125 0.125 0.125
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defined as the ore impact breakage parameters. A and b are related to ECS and t10 by the following Eq. (2): t10 ¼ A½1 eðbECSÞ
ð2Þ
The drop weight test provides effective characterisation of an ore for crushing. It should be remembered that chipping and abrasion occur in tumbling mills and that additional tests would be required to obtain a comprehensive assessment of the benefits of microwave treatment. The impact parameter A b has been correlated, although not well, to the Bond Work Index using Eq. (3) (Napier-Munn et al., 1996): A b ¼ 3:5WI þ 117
2.6. Liberation tests Samples of treated and untreated material (15 kW, 2.45 GHz, 0.2 s) produced in drop weight tests were screened into four size fractions viz. + 500 Am, + 150/ 500 Am, + 38/ 150 m and 38 Am. Representative sub-splits of the samples were mounted in chlorinated epoxy resin, polished and carbon coated for analysis by QEM*SEM (Napier-Munn et al., 1996). In each size fraction results were reported in terms of % locked, middling and liberated where these were defined as < 30% liberated, 30– 80% liberated and >80% liberated, respectively. Liberation characteristics were determined for copper sulphides only.
ð3Þ 3. Results and discussion
To enable comparison, drop weight tests were carried out both on untreated material and on material treated at 15 kW for 0.2 s. From the results, breakage parameters specific comminution energy were calculated. The energy levels used in this investigation were half those recommended in the standard method as it was found that if the recommended energy levels were used no discrimination between treated and untreated material was produced despite the appearance of significant visible fracture in the microwavetreated samples. 2.5. Grindability testing To understand the grindability behaviour of the material, 2 500 g samples of the following size fractions were produced: 19 + 16, 16 + 13.2, 13.2 + 9.6, 9.6 + 6.7, 6.7 + 4.75 and 4.75 + 3.35 mm. One sample was left untreated and the other was exposed to microwave energy in the single mode cavity for 0.2 s at 15 kW. The samples were then ground for increasing lengths of time (1– 5 min) in a 12-in. Bico Braun ball mill with a standard Bond mill charge at a speed at 70 rpm. The mass left in the original fraction after a specific time being determined by sieving. The data were evaluated using a standard population balance method (Austin et al., 1984; Wang and Forrsberg, 2000). The breakage function was evaluated by plotting the natural logarithm of mass remaining in a specific size fraction as a function of cumulative grinding time.
3.1. Point load tests Fig. 4 shows the change in Is(50) as a function of microwave exposure time for material in the size range 53 + 45 mm. It can be clearly seen that microwave treatment has a significant effect on the strength of the ore, with exposures as short as 0.1 s giving reductions in Is(50) of approximately 40%. Table 3 shows the full data set for the experiment. It can be seen that the results for microwave-treated material show a larger standard deviation than the untreated. Examination of the upper and lower limits revels why this is the case. Indeed, for most of the microwave-treated material, the strength of at least one (or often more) particles was close to zero. Naturally, this gives rise to the large standard deviation for treated samples because the untreated samples all had reasonable strength. Previous work (Kingman, 1999) has shown reduction in ore strength to be related to microwave power level. This is also shown here with significant reductions in strength noted for 10 and 15 kW but not for 5 kW. It is apparent that a critical power density exists above which the damage is significant. For this ore within this cavity, this is shown to be produced by at least 10 kW. After the initial drop in strength, little benefit is gained by increased exposure time. This was suggested by Whittles et al. (2003) in their paper. For all exposure times, 5 kW gives the smallest reduction in strength. However, in all cases, these
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Fig. 4. Is(50) as a function of microwave exposure time for material 53 + 45 mm.
results are highly significant, as previous work showed that at least 10 and often 180 s were required to achieve small reductions in strength. In this study, significant reductions in strength are demonstrated for an exposure time of 0.1 s, up to 1800 times less than that previously used. Fig. 5 shows a similar plot for 37 + 31 mm material. It can be clearly seen that significant reductions in strength can be achieved in short exposure times of less than 0.1 s. The plot is broadly similar to that shown in Fig. 4 with the complete data for this size fraction is shown in Table 4.
Table 3 Is(50) as a function of microwave exposure time for material 53 + 45 mm showing statistical measures Treatment
Untreated 15 kW
10 kW
5 kW
Time
– 0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
Is(50) 10 6 MPa Mean
Standard deviation
Median
Upper limit
Lower limit
3.32 2.06 0.93 1.29 1.82 1.86 1.98 1.82 2.34 2.29
0.88 1.73 1.02 1.06 1.30 1.74 1.48 1.30 1.07 1.07
3.28 2.06 0.62 0.88 1.28 0.83 1.59 3.16 2.89 2.19
4.67 5.47 3.18 3.26 3.83 4.89 4.36 4.64 3.44 4.13
1.95 0.53 0.06 0.06 0.16 0.06 0.07 1.36 0.06 0.74
3.2. Drop weight tests Drop weight tests were carried out on microwaveand non-microwave-treated samples using the energy levels shown in Table 2. After completion of the tests, the appearance function and breakage parameters were calculated. Before breakage in the drop weight tester the microwave-treated sample exhibited significant amounts of visible fractures and many pieces could be crumbled by hand. The untreated sample, by contrast, was wholly competent. Table 5 shows the breakage parameters for the treated and untreated material. The value of A is the theoretical maximum value of t10, whilst A b is the slope of a plot of Ecs versus t10 at Ecs = 0. For most hard rocks (for a reasonable range of b values), there is a limiting value for A, the maximum value that t10 can achieve as Ecs is increased. This value is approx. A = 50. Note, for crusher breakage, t10 is typically 10 – 20% (Napier-Munn et al., 1996). The impact breakage parameter b is actually an indicator of the amenability of the ore to size reduction by impact. It can be seen that the value of b is significantly higher for the microwave treated material, suggesting a much softer ore. Figs. 6 and 7 show the effect of specific comminution energy on the breakage index t10 for different size classes. It can be clearly seen that the microwavetreated ore shows reduced values of Ecs over almost
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Fig. 5. Is(50) as a function of microwave exposure time for material 37 + 31 mm.
the entire range of t10 values, indicating that the material is softer because of microwave treatment. This is particularly true for the larger particle sizes indeed for the smallest size fractions the data is similar. The role of particle size in microwave assisted comminution is discussed later in this paper. 3.3. Grinding tests Fig. 8 shows a plot of the specific rate of breakage versus particle size for treated and untreated material. It can be seen that the difference in specific rate of breakage decreases with particle size. The largest Table 4 Is(50) as a function of microwave exposure time for material 37 + 31 mm showing statistical measures Treatment
Untreated 15 kW
10 kW
5 kW
Time
– 0.1 0.5 1.0 0.1 0.5 1.0 0.1 0.5 1.0
Is(50) MPa 10 6 Mean
Standard deviation
Median
Upper limit
Lower limit
2.88 1.67 1.52 1.30 1.50 1.59 1.35 2.28 0.75 1.87
1.06 1.31 1.43 1.10 0.91 0.91 0.45 0.81 0.60 0.78
2.75 1.30 1.08 1.15 1.37 0.89 1.28 2.36 0.71 1.54
4.38 3.44 4.22 3.44 3.01 4.95 2.11 3.47 1.24 2.89
1.05 0.10 0.07 0.07 0.10 0.10 1.00 1.15 0.10 1.07
particle size fraction ( 19 + 16 mm) is most influenced with over 70% difference between the nontreated and treated samples. For the smallest size fraction ( 4.75 + 3.35 mm), the percentage change is 11%, whilst not as much as the larger fractions is still significant. The reason for the reduction with particle size is almost certainly related to the mineralogy or texture of the samples. The major microwave adsorbing phase within the material is magnetite. The magnetite is observed to occur in large aggregates sometimes greater than 10 mm in size. In order for differential expansion as a result of microwave heating to occur the heated phase must be entirely surrounded by transparent material. As the particle size decreases, the amount of magnetite, which is not totally surrounded by transparent gangue, will increase and thus the percentage change in specific rate of breakage will drop. It is likely therefore that more greater changes in specific rate of breakage could be achieved in ores, which have a finer texture. It can also be seen that the specific rate of breakage in the larger size fractions is low. This is expected for
Table 5 Breakage parameters for treated and non-treated ore Treatment
A
b
Ab
15 kW 0.2 s NON
52.5 54.3
2.35 1.61
123.4 87.4
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Fig. 6. Plot of t10 versus Ecs for different sized untreated ore.
laboratory mills and has previously been noted (King, 2001). 3.4. QEM*SEM liberation results Results of the QEM*SEM study for copper sulphide minerals are shown in Fig. 9a and b. It can be seen that microwave treatment has had the most significant influence on the + 500 Am fraction. The amount of liberated material in this fraction has increased from 31.8% in the untreated fraction to 69.2% in the treated fraction. The amounts of liber-
ated, middlings and locked copper sulphides in the other size fractions remain relatively similar. The significant increase in liberation of the copper sulphide mineral in the coarse size fraction is potentially important as could increase the effective liberation size of the copper sulphide within the ore. This would have the effect of reducing grinding energy consumption as the required grind size would be coarser and it also would also potentially increase the total recovery as less material would be overground and lost to slimes. Fig. 10 shows typical images obtained from the QEM*SEM analysis, they
Fig. 7. Plot of t10 versus Ecs for different sized treated ore.
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Fig. 8. Plot of specific rate of breakage versus mean particle size for treated and untreated samples.
indicate the nature of the classification, i.e., locked, middlings and liberated. 3.5. Explanation of mechanism of strength reduction The impressive strength reductions achieved in the present work with very short, high power microwave exposure can be explained by considering the temperature gradients produced across grain boundaries. If a very high microwave power density is generated within a microwave absorbing phase for a short residence time, then significant temperature gradients would be expected to develop between the absorbing and non-absorbing phases, leading to large thermal stresses. On the other hand, if the ore were heated slowly at low power, as was carried out in previous work, thermal conduction would reduce the size of the thermal gradients across the grain boundaries between the absorbing and non-absorbing phases. In turn, this would lead to small thermal stresses and less weakening of the ore. Recent thermo-elastic simulations by Whittles et al. (2003) showed that this is true. High power densities (1011 W m 3) maintained for short times (0.1 s) produced thermal gradients that in turn gave rise to significant thermo-mechanical stresses. These could reduce the material strength significantly. Lower power densities (109 W m 3) achieved lower strength reductions at total energy inputs almost an order of magnitude larger.
Fig. 9. (a) Liberation spectrum for copper sulphide minerals for untreated copper ore. (b) Liberation spectrum for copper sulphide minerals for ore treated 15 kW 0.1 s.
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Fig. 10. Examples of copper ore liberation characteristics, i.e., liberated, locked and middlings.
3.6. Industrial significance As stated earlier, the major disadvantage of previous work on microwave assisted comminution has been the amount of energy required to achieve the desired effects compared with the potential savings. Energy balances presented in previous work (Kingman et al., 2000a,b) showed that, for ore similar to that used in this test programme, a microwave energy input of over 43 kW h t 1 was required to achieve a reduction in work index of over 70% from 14 to 4 kW h t 1. In the present work, a 30% reduction in impact breakage parameters has been achieved at much lower microwave energy input. For material treated at 15 kW for 0.2 s, assuming 1 kg in cavity, it can be shown that the microwave energy input was: 0:2 1000 Energy ¼ 15 ¼ 0:83 kW h t1 3600 1 The result of increasing the electric field strength (or power density) is clear; significant strength reduction or reductions in grinding resistance can be achieved at low microwave energy inputs. Furthermore, these values suggest that scale-up is potentially feasible in terms of energy added to the process. Reductions in specific comminution energy of 30% coupled with significant improvements in ore grind-
ability are attractive in themselves; however, it must be remembered that the benefit of a softer ore is not only in the reduced crushing energy, but also in reduced mill size (or greater throughput for the same size mill) and also the very significant reduction in the circulating load (Kingman, 1999). It should also be remembered that potentially the most important potential benefit of microwave assisted comminution is increased liberation at larger sizes. This was clearly demonstrated by the QEM*SEM liberation data, in which the liberation in the + 500 Am fraction was doubled. Other benefits of microwave-assisted comminution include:
Reduced plant size for set throughput Potentially reduced wear costs per tonne
Lower water consumption (less mills required)
Smaller downstream recovery circuits (liberation at higher sizes)
The TE10n (electric field orientation transverse to the direction of propagation of the microwaves) cavity used for this test programme was ‘an off the shelf’ model designed for general heating applications. No attempt was made to optimise the cavity design with regard to the particle size distribution of the feed material or the dielectric properties of the material. This process will undoubtedly lead to higher power
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densities and lower residence time. This is of particular importance, as it will enable the input microwave power to be kept to a minimum and the throughput to be maximised. This will be vital if plant implementation is to be achieved. In the same way optimised cavity design and electric field strength should enable similar if not better results to be obtained with ore of different mineralogy. This study was completed on a material with a relatively coarse liberation size whose major microwave heated phase (magnetite) was also relatively large. It would obviously be desirable to achieve similar results with much finer grained ores. This work has shown that the degree of failure is related to microwave power densities giving rise to thermally induced stresses and strains. It is highly likely therefore that if ores of finer mineralogy are to be treated then higher power densities would be required than those used in this work. This is because faster heating rates are required to produce the same level of stress in a smaller particle than a large one. It is well known within the microwave-engineering field that staged development is vital for successful development. The next stage of this work will therefore be to develop a continuous pilot scale processing system that will process up to 1 t/h of ore. The facility will be designed to handle several specific ore types, which will allow some degree of optimisation in the presentation of the microwave energy to the ore. This test facility will also allow determination of the above benefits at reasonable scale.
4. Conclusions For the first time, significant changes in ore breakage and liberation characteristics have been demonstrated at microwave powers inputs that potentially may be economic. The influence of microwave radiation of high electric field strength has been quantified. It has been shown that significant reductions in strength can be achieved in very short microwave exposure times. There was some suggestion from the experimental data that the reduction in strength up to a certain level is related to applied power level. However, there appears to be a certain power level at which little more breakage can be induced.
Drop weight tests were carried out and the influence of microwave pre-treatment quantified. It was shown that reductions in required breakage energy of over 30% could be achieved for modest microwave energy inputs of less than 1 kW h t 1. Further work is suggested at the tonne-per-hour level to allow more scale up data to be produced and a more detailed economic analysis to be carried out. Grindability tests have shown that significant reductions in specific rate of brewage of over 40% can be obtained in a ball mill. These reductions, however, were shown to be intimately related to particle size. QEM*SEM data showed that the liberation of copper sulphide minerals in the + 500 Am size fraction increased by over 100% in the microwave treated material. References Anon, 1999. Technology Foresight—Natural Resources and Environment, Office of Science and Technology, London, UK. Austin, L.G., Klimpel, R.R., Luckie, P.T., 1984. Process Engineering of Size Reduction: Ball Milling SME, New York. ISBN 0-89520-421-5. Bearman, R.A., Briggs, C.A., Kojovic, T., 1997. The application of rock mechanics parameters to the prediction of comminution behaviour. Min. Eng. 10 (3), 255 – 264. Broch, E., Franklin, J.A., 1972. The point load strength test. Int. J. Rock Mech. Min. Sci. 9, 669 – 697. Hoek, E., Brown, E.T., 1980. Underground Excavations in Rock The Inst. Min. and Metall., London. King, R.P., 2001. Modelling and Simulation of Mineral Processing Systems, Butterworth Heinemann, Oxford, UK. Kingman, S.W., 1999. The Influence of Microwave Radiation on the Comminution and Beneficiation of Minerals. PhD thesis, University of Birmingham, UK. Kingman, S.W., Rowson, N.A., 2000. The effect of microwave radiation on the magnetic properties of minerals. J. Microw. Power Electromagn. Energy 35 (2), 141 – 150. Kingman, S.W., Corfield, G., Rowson, N.A., 1999. Effect of microwave radiation on the mineralogy and magnetic processing of ilmenite. Magn. Electr. Sep. 9, 131 – 148. Kingman, S.W., Vorster, W., Rowson, N.A., 2000a. The influence of mineralogy on microwave assisted grinding. Min. Eng. 13 (3), 313 – 327. Kingman, S.W., Vorster, W., Rowson, N.A., 2000b. The effect of microwave radiation on the processing of palabora copper ore. Trans. S. Afr. Inst. Min. Metall., 197 – 204 (May/June). Napier-Munn, T.J., Morell, S., Morrison, R.D., Kojovic, T., 1996. Mineral Comminution Circuits. Their Operation and Optimisation. JKMRC Monograph Series in Mining and Mineral Processing, vol. 2. University of Queensland, Australia.
S.W. Kingman et al. / Int. J. Miner. Process. 74 (2004) 71–83 Scho¨nert, K., 1988. A first survey of grinding with high-compression roller mills. Int. J. Miner. Process. 22, 401 – 412. Tavares, L.M., King, R.P., 1995. Microscale investigation of thermally assisted comminution. Proceedings XIX International Mineral Processing Congress, San Francisco, vol. 1, pp. 203 – 208. Walkiewicz, J.W., Kazonich, G., McGill, S.L., 1988. Microwave heating characteristics of selected minerals and compounds. Miner. Metall. Process. 5 (1), 39 – 42.
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Walkiewicz, J.W., Lindroth, D.P., McGill, S.L., 1991. Microwave assisted grinding. IEEE Trans. Ind. Appl. 27 (2), 239 – 242. Wang, Y., Forrsberg, E., 2000. Microwave assisted comminution and liberation of minerals. In: Ozbayoglu, G., et al. (Ed.), Mineral Processing on the Verge of the 21st Century. Balkema, Rotterdam, pp. 3 – 9. ISBN 905809 1724. Whittles, D., Kingman, S.W., Reddish, D., 2003. The influence of power density on microwave assisted breakage. Int. J. Miner. Process. 68, 71 – 91.