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MICROWAVE ENHANCED RECOVERY OF NICKEL-COPPER ORE: COMMUNITION AND FLOATABILITY ASPECTS R. Hendaa, *, A. Hermasa, R. Gedyeb and M.R. Islamc a Laurentian University School of Engineering / b Department of Chemistry and Biochemistry, Sudbury, Ontario P3E 2C6, Canada c Faculty of Engineering/Dalhousie University, Halifax, Nova Scotia B3J 2X4, Canada A study describing the effect of microwave radiation, at a frequency of 2,450 MHz, on the processes of communition and flotation of a complex sulphide nickel-copper ore is presented. Ore communition has been investigated under standard radiation-free conditions and after ore treatment in a radiated environment as a function of ore size, exposure time to radiation, and microwave power. The findings show that communition is tremendously improved by microwave radiation with values of the relative work index as low as 23% at a microwave power of 1.406 kW and after 10 s of exposure time. Communition is affected by exposure time and microwave power in a nontrivial manner. In terms of ore floatability, the experimental tests have been carried out on a sample of 75 μm in size under different exposure times. The results show that both ore concentrate recoveries and grades of nickel and copper are significantly enhanced after microwave treatment of the ore with relative increases in recovered concentrate, grade of nickel, and grade of copper of 26 wt%, 15 wt%, and 27%, respectively, at a microwave power of 1,330 kW and after 30 s of exposure time. Submission Date: March 2005 Acceptance Date: January 2006 INTRODUCTION Microwave radiation as a source of energy has been developed primarily for communications and is finding applications in many fields such as chemical synthesis, environmental engineering, and materials processing. For instance, in the area of chemical synthesis it is largely documented that many chemical reactions experience an increase in the kinetics when subjected to microwave radiation [Gedye et al, 1988; Whittaker and Mingos, 1994; Gabriel et al., 1998]. Microwave radiation has also been effectively used in sintering [Ramkumar et al., 2001], leaching [Hua et al., 2002; Lovas et al., 2002], and catalyst selectivity [Ohgushi et al., 1999; Turner et al., 2000]. Several reviews on the applications of microwave energy have been recently published [Whittaker and Mingos, Keywords: microwave energy, mineral separation, communition, flotation, process engineering. International Microwave Power Institute 1994; Thostenson and Chou, 1999; Haque, 1999; Clark et al., 2000; Jones et al., 2002]. Microwave radiation interacts with “lossy” materials to achieve volumetric heating, in contrast to conventional heating where energy is transferred due to thermal gradients. The ability of a material to dissipate and uniformly distribute microwave energy is defined in terms of the complex dielectric permittivity of the material [Von Hippel, 1954]. The amount of energy dissipated per unit volume of material increases with the imaginary part of the permittivity known as the loss factor, ε”, and the power penetration depth, Dp. The loss factor is a function of wavelength, temperature, density, electric field orientation, etc., and Dp, is proportional to the wavelength of radiation and inversely proportional to a complex function of the loss factor [Metaxas and Meredith, 1983]. While the physics of microwave radiation is well established, devising better process parameters and operating conditions is crucial for microwave energy to be beneficial to process engineering related operations. For instance, 7 the magnitudes of both microwave power and exposure time to microwave radiation or, alternatively, their product must be kept to a minimum. A judicious choice of process conditions close to those commonly used in the mineral industry will also facilitate process scale-up at a later time [Bradshaw, 1999]. Microwave heating systems come in multi- or single-mode cavity designs. The distinction between cavity types is determined more so by the design characteristics than by the processes that can be performed inside the cavity. A single-mode cavity is a metallic box whose dimensions must be carefully controlled to correspond in a systematic way to the characteristic wavelength of the microwaves. The wave pattern is such that the intensity of the microwave field is highest in a localised and small volume of the cavity. While this feature allows the generation of a high electric field in the material, it limits the amount of material to be treated. Most importantly there is no practical way of scaling up processes. A multi-mode cavity is the most versatile type, and is typically a large box with dimensions several space wavelengths long. The microwave field is distributed throughout the cavity, but not as strong as in a single-mode cavity, thus absorbing material can be heated anywhere inside the cavity. Furthermore, the multi-mode cavity design allows easy scale up of processes. It is clear that a new type of cavity is necessary to increase cavity efficiency, as recently suggested by Kingman et al. [2004], and to allow for easy process scale up. Communition is known to be one of the least energyefficient of all separation processes. While about 60% of the total energy required in mineral separation is used during the process of communition, only about 1% of the energy delivered to the material is used to create a new surface, according to the best estimates [McCabe et al., 2001]. In composites and complex multi-phase materials such as ores, microwave radiation can be used in selective heating of the material. High loss phases (absorbing materials) will readily couple with microwave energy while low loss phases (transparent materials) will not. Therefore, the rapid heating of microwave radiation absorbing minerals in a non-absorbing gangue phase is likely to cause some differential heat effects at the interface between the different phases of the complex material, as shown in a theoretical study by Salsman et al. [1996]. Early investigations into materials heating have focused on establishing the heating rate characteristic [Ford and Pei, 1967; Chen et al., 1984]. The phenomenon of selective heating can cause thermal stresses within the material, and has been recently utilized to enhance the potential for mineral separation [Kingman and Rowson, 1998; Bradshaw, 1999; Vorster et al., 2001]. Unfortunately, in most of these studies it was concluded that energy required to improve communition by microwave radiation was greater than that required by conventional communition. The extent of structural 8 weakening, e.g., formation of micro-cracks at the phase boundaries, is crucial in communition and depends on the composition of the mineral, and the individual dielectric constants and thermal expansion coefficients of the phases. The time response of structural weakening to microwave radiation is equally important from the viewpoint of energy requirements. The process of flotation is based on the differences in chemical properties of mineral surfaces, and is designed to separate hydrophobic and hydrophilic minerals from each other [Willis, 1981]. Hydrophobic minerals attach to the air bubbles and are lifted up to the surface of the pulp, from where they are skimmed off to form the valuable product (concentrate), while hydrophilic minerals remain in the pulp, forming the waste product (tailing). Although this concept appears to be simple and straightforward, complications arise mainly due to the fact that minerals in their natural state are all hydrophilic, with only a few exceptions. Minerals are made hydrophobic by modifying their surfaces through chemical treatment, i.e., use of collectors. However, such treatment needs to be selective, in other words while some minerals in the ore pulp acquire a hydrophobic property, others must remain hydrophilic. The success of flotation relies largely on achieving such selectivity, and although the flotation process has been around for more than a century, this issue still poses a challenge in many applications. The use of benign (relatively safe to use) sources of energy such as microwaves, with their potential ability to affect the chemistry of flotation, namely, increase in reaction rate, and thus to improve the separation of useful minerals, can have a drastic impact on the mineral industry. This paper focuses on process engineering aspects of the effect of microwave radiation on mineral separation of a nickel-copper ore obtained from the Sudbury basin. The present work is aimed at investigating the effect of microwave radiation, at a frequency of 2,450 MHz, on the communition and floatability of the ore. The experimental runs have been carried out under different operating conditions, such as ore size, exposure time, and microwave power. EXPERIMENTAL PROCEDURES Material The nickel-copper sulphide ore used in this work has been obtained from the Sudbury basin. The ore “as received” was previously crushed to about 18 mm in a cone crusher. The chemical composition of representative samples of the ore was analyzed by X-ray powder diffraction (XRD), and the corresponding quantification results are shown in Figure 1. The major minerals present in the Journal of Microwave Power & Electromagnetic Energy Vol. 40, No.1, 2005 Pentlandite 1.7% Biotite 3.8% Chalcopyrite 6.1% Quartz 20.1% Pyrrotite 21.6% Amphibole 7.9% Plagioclase 38.7% Figure 1: The chemical composition of the ore as analyzed by XRD. ore are plagioclase (silicate), pyrrhotite (iron sulphide), quartz (silicate), amphibole (silicate), chalcopyrite (iron copper sulphide), biotite (silicate), and pentlandite (iron nickel sulphide). While silicates are generally transparent to microwave radiation, metals are excellent reflectors thereof. Grinding Tests The ore used in the test has been divided into four classes according to ore size: that is, ore as received with a 80% passing size, d80, of 18 mm, and cone crushed to d80 of 9.2 mm, 5.5 mm, and 1.2 mm. Standard samples were prepared by using a riffling procedure to obtain representative samples of 300 grams from each ore size. The sampling was achieved by mixing the same amount for each size fraction as the standard so that each R. Henda et al., Figuresample, 1 sample will have the same d 80 and the same size distribution as the standard sample. The method of Berry and Bruce [1966], has been used to compare between grindability power requirements of radiated samples and non-treated samples. The relative work index, RWI, can be expressed as 10 10 RWI = − Fr Pr 10 10 − Ft , Pt (1) where P and F are the d80 of the product and feed in µm, respectively, r is the reference ore sample (non-treated), and t is the test ore (radiated). International Microwave Power Institute To determine the relative work index, a reference sample was ground without microwave treatment, and the values of the d80 of the feed, Fr, and of the ground product, Pr, were determined. Then the same weight of the reference sample was radiated at different exposure times and constant power in a microwave oven with an air atmosphere. The oven, BP-210 from Microwave Research and Application, Inc., USA, is shown in Figure 2. It consists of a stainless steel cavity, a power supply in the range 0 to 1,900 W (maximum power delivered by the magnetron), a 2,450 MHz magnetron for generation of microwaves, and a control panel. The accuracy of power measurement was assessed using a calorimetric method – using 3 litres of water subjected to microwave power for 62 s. The estimated error was found to be less than 10%. The samples were placed in a highalumina tray and positioned at the center of the oven cavity. The oven is furnished with a built-in thermocouple for in-situ measurements of sample temperature (meter accuracy is±8 ºC at 1000 ºC). The thermocouple and temperature measurement system were calibrated in ice water and boiling water (a two-point calibration). During the experiments, the tip of the thermocouple is made to come in contact with the center of the sample. In each experiment the d80 of the feed, Ft, and product, Pt, for the test ore were measured. Each experiment was repeated three times, and the corresponding standard deviation was estimated, and reported in the figures. In addition to sample size and exposure time, the relative work index was also determined as a function of micro- 9 Figure 2: Snapshot of the microwave oven BP-210. wave power. The morphology of the radiated and non-treated samples was analyzed by means of scanning electron microscopy (JEOL 6400 SEM, BSE, 2nA, 20 kV, 38 mm). dard deviation was calculated. Nickel and copper contents of the concentrate and tailing were then determined by atomic absorption spectrophotometry (λCu = 324.8 nm, λNi = 232 nm). Flotation Tests RESULTS AND DISCUSSION The ore was ground to a d80 of 75 µm in a ball mill using the following grinding conditions: 70% pulp density (pH = 11), ball mill charge of 45% of the mill volume, and mill speed of 65 rpm at 70% of the critical speed. Eight minutes are needed to recover the entire slurry from the mill before the slurry is transferred to the 1.2-l flotation cell (Denver D12, impeller speed 1,100 rpm) at a pulp density of approximately 25%, followed by two minutes of pH adjustment to 9.3. After the addition of the collector, 0.1% sodium isobutyl xanthate, the sample was transferred to the microwave oven for treatment under the following conditions of exposure time and microwave power: 30 s, 60 s, 90 s, and 120 s at 70% (i.e., 0.7 x 1,900 kW = 1,330 W) of maximum microwave power. The sample was placed in a Plexiglas container and positioned at the center of the oven cavity. During the test the sample was mixed by means of the built-in mechanical stirrer. After sample conditioning for 10 minutes, nine ml of the frother, 0.1% Dowfroth 250, was added to the flotation cell and conditioned for one minute. Finally, the air valve (13 scfh) was turned on and the sample was floated over a period of five minutes (froth thickness = 15 mm). The floated (concentrate) and gangue (tailing) materials were filtered and dried for 24 hours at 100°C. Each experiment has been repeated three times and the corresponding stan- Heating Rate Characteristic 10 The heating rate, dT/dt, is related to the dissipation of microwave energy that flows into the material. It can be expressed through an energy balance on a control volume of the material as follows dT 2 π f ε oε E 2 = , dt ρCp (2) assuming a uniform electric field intensity, E. The parameters ρ and Cp are the density and heat capacity of the mineral, respectively, f is the frequency of microwave radiation, and εo is the permittivity of vacuum (8.854 x 10-14 F/cm). Equation (2) is not necessarily linear as the loss factor is known to depend on temperature in an intricate way. In the tests three samples of 300 grams were heated in the microwave oven at 70% of maximum power (1,900 kW) and their temperature, T, recorded in-situ as a function of exposure time, t. Experimental results, corresponding to the three samples, are depicted in Figure 3 (standard deviation data are given in the inset). The samples differ in size, with d80 of 18 mm, 9.2 mm, and 5.5 mm for samples #1, #2, and #3, respectively. For a given sample size the Journal of Microwave Power & Electromagnetic Energy Vol. 40, No.1, 2005 900 800 Temperature (°C) 700 600 500 400 300 200 18.0 mm 9.2 mm 100 5.5 mm 0 0 20 40 60 80 100 Exposure Time (s) Figure 3: Temperature versus microwave exposure time at 70% of maximum power (1,900 kW) for samples of different size as indicated in the inset. (The fitting lines are a guide to the eye). temperature increases rapidly (rapid regime) with exposure time then reaches a quasi-steady state regime (slow regime). The nontrivial relationship between temperature and exposure time correlates well with the relationship between the loss factor, ε”, and temperature of sulphidebearing minerals [AECL and Voss, 1990]. The other finding worth noticing in Figure 3 is that the heating rate, which is defined as the average slope of the rapid regime, seems to drastically decrease with decreasing sample size. The calculated heating rates for samples #1, #2, and #3 are ~11oC/s, 8oC/s, and 6oC/s, respectively. The reason behind this behavior is not R. Henda et al., Figure 2 quite clear, however, it may be attributed to an efficient absorption/dissipation of microwave radiation, a function of the penetration depth (Dp), by large ore particles compared to small ore particles. Furthermore, the smaller a particle is, the larger the ratio (particle surface)/(particle volume) becomes, and, consequently, more microwave radiation is transferred to the gangue material from the mineral particles by heat transfer. The other more plausible cause for this behavior may be due to a change in the dielectric property, which appears in Equation (2), of the ore with sample size, morphology, and phase change. This has been noticed in some preliminary results obtained from dielectric characterizations of the ore. This aspect of the International Microwave Power Institute heating rate is being investigated in a separate study. Relative Work Index Determination The relative work index is an indicator of how much energy is needed to grind a sample after microwave treatment compared to grinding without microwave treatment. It was determined from Equation (1) for four samples of different sizes. Samples #1-3 are similar to those of the previous section, and sample #4 has a d80 of 1.2 mm. The typical variation of the relative work index of the samples with exposure time to microwave radiation is shown in Figure 4, corresponding to 70% of maximum microwave power (1,900 kW). The grindability of the fine samples, i.e., samples # 3 and #4, does not seem to be much affected by microwave radiation over time, but shows large variations for the coarse samples, i.e., samples # 1 and #2. For the latter samples the values of RWI vary in an intricate way with exposure time: an initial decrease, then an increase, and finally a decrease in RWI. The minimum values of RWI are obtained after 10 s exposure to microwave radiation as shown in Figure 4. Similar trends have been noticed in the variation of the relative work index with microwave power at a given exposure time. A typical example is depicted in Figure 5 for an exposure time of 10 s. In this case, the minimum values of RWI are obtained at 74% of maximum microwave power (1,900 kW). These findings seem to suggest that ore grindability improves with the increase in the magnitude of microwave power to 11 120 100 RWI (%) 80 60 40 18.0 mm 9.2 mm 20 5.5 mm 1.2 mm 0 0 10 20 30 40 50 60 Time (s) Figure 4: Relative work index versus microwave exposure time at 70% of maximum microwave power (1,900 kW) for samples of different size as indicated in the inset. (The fitting lines are a guide to the eye). 120 100 RWI (%) 80 60 R. Henda et al., Figure 3 40 18.0 mm 9.2 mm 20 5.5 mm 1.2 mm 0 70 75 80 85 90 Power (%) Figure 5: Relative work index versus microwave power at 10 s for samples of different size as indicated in the inset. (The fitting lines are a guide to the eye). 12 Journal of Microwave Power & Electromagnetic Energy Vol. 40, No.1, 2005 (a) (b) (c) (d) Figure 6: SEM micrographs of the ore under different conditions: without microwave treatment (a), at 10s (b), at 40s (c), and at 50s of microwave radiation (d). Microwave power set to 70% of maximum power (1,900 kW). a certain level only (i.e., 74% in this case) when exposure time to microwave radiation is kept constant. This aspect R. Hendalater et al., 5 is further discussed inFigure this section. The non-monotonic variation of RWI with exposure time and microwave energy is attributed to morphological considerations, viz., to the density of fractures as can be noticed in the SEM monographs shown in Figure 6, at exposure time values of 0 s, 10 s, 40 s, and 50 s, and at a microwave power of 70% (1,900 kW). From an energy-saving viewpoint, the amount of energy input after 10 s exposure to 74% of microwave power (1,900 kW) is roughly evaluated as follows (.74 x 1.9) (kW) x (10/3600) (h)/(0.3/1000) (t) = 13.0 kWh/t, (3) which is smaller than the work index, ~18.5 kWh/t, of the International Microwave Power Institute tested ore. The calculation is, of course, a rough estimate on tests carried out on a laboratory scale, standard multimode microwave oven of low energy efficiency [Hermas, 2004]. As per Eq. (3), we expect that energy efficiency of the process can be further improved, if exposure times are reduced and microwave power is slightly increased, keeping in mind that their product must be lowered. An increase in the electric field strength is necessary to impart more damage to the ore (i.e., to reduce ore strength and facilitate liberation of ore particles). For instance, by reducing the exposure time by 100 and doubling the magnitude of the electric field strength, the resulting microwave energy input becomes 0.26 kWh/t. To reach such energy reductions further developments (not within the scope of the present study) in cavity designs with higher powers and shorter exposure times are neces- 13 Table 1: Standard flotation test results for the sample with d80 = 75 µm. Weight Component Mass (g) Component % Dist. Product Mass (g) % Dist. Cu Ni Cu Ni Concentrate 22.9 6.4 2.16 2.41 51.04 60.97 Tailing 335.6 93.6 2.07 1.54 48.96 39.03 Feed (calculated) 358.5 100.0 4.23 3.96 100.00 100.00 Feed (actual) 360 Table 2: Flotation results for the sample with d80 = 75 µm after 30 s of microwave treatment at 70% of maximum power (1,900 kW). Weight Component Mass (g) Product Mass (g) % Dist. Cu Ni Cu Ni Concentrate 28.9 8.1 3.15 3.26 65.11 69.94 Tailing 328.9 91.9 1.69 1.39 34.89 30.06 Feed (calculated) 357.8 100.0 4.84 4.65 100.00 100.00 Feed (actual) 360 sary, as suggested in recent work [Kingman et al., 2004]. Floatability Study The results of the flotation tests for the ore sample with a d80 of 75 µm are shown in Tables 1 and 2. The data in both tables correspond to flotation results without treatment (Table 1) and with microwave treatment (Table 2) after 30 s exposure to 70% of microwave power (1,900 kW), as described in the previous section. The findings refer to the recovered fractions of floated material (concentrate) and tailing, in addition to the composition distribution of nickel and copper, measured using atomic absorption spectrophotometry, in both fractions. As it can be seen from the data the use of microwave energy results in an increase (decrease) in the recovered concentrate (tailing), and in the grades of the useful minerals, i.e., nickel and copper. After microwave treatment of the ore, the amount of recovered concentrate increases from ~23 g to 29 g, corresponding to a relative increase of ~26 wt%. The component distribution also improves with microwave treatment and goes from ~51 wt% (standard test) to 65 wt% for copper, and from ~61 wt% (standard 14 Component % Dist. test) to ~70 wt% for nickel. It is to be noted that under the present operating conditions, the process of flotation does not seem to be much affected by exposure time to microwave radiation as depicted in Figure 7. Improvements in the floatability of the ore may be attributed to kinetic effects, viz., an increase in the rate of interaction between the collector and the solid surface of mineral particles. Of course, in order to draw conclusive observations a detailed investigation of the effect of microwave radiation on the reaction kinetics is required. CONCLUSIONS In this work the effect of microwave energy on the mineral separation, viz., the processes of communition and flotation, of a complex nickel-copper ore obtained from the Sudbury basin has been investigated. In communition, the heating rate is found to be a function of ore size and increases with sample size in a nontrivial manner. The results have shown that ore communition, expressed in terms of the relative work index, under microwave radiation is tremendously improved for coarse Journal of Microwave Power & Electromagnetic Energy Vol. 40, No.1, 2005 80 70 60 %; g 50 40 30 20 -.-.- Cu ----- Ni 10 -..-..- Conc. 0 0 20 40 60 80 100 Time (s) Figure 7: Flotation results versus exposure time for the sample with d80 = 75 µm after 30 s of microwave treatment at 70% of maximum power (1,900 kW). (#3) A comparison between the standard results (dashed lines) and the results after microwave treatment is also shown. Units: % for Cu and Ni (standard deviation shown in the inset), and gram for Concentrate (Conc.). ore samples (d80 ≥ 9 mm), but experiences little change for fine samples. The values of the relative work index vary in a non-monotonic way with microwave exposure time and power. The lowest values of the work index are obtained at very short times (~10 s) making the use of miet al., Figure 6 crowave radiationR.toHenda enhance communition economically attractive. The preliminary results from the flotation tests show substantial increase in the amount of recovered ore (~26 wt%) and in the grades of nickel (relative increase of ~15 wt%) and copper (relative increase of ~27 wt%) in the concentrate, under the present process conditions. In a forthcoming paper we will elaborate on dielectric, chemical and morphological characterization of the ore as it undergoes microwave treatment. ACKNOWLEDGMENTS Funding provided by LURF (to R.H.), and by Atlantic Innovation Fund, NSERC and Petroleum Research Atlantic Canada (to M.R.I.) is acknowledged and appreciated. International Microwave Power Institute REFERENCES AECL and Voss Assoc. Eng. Ltd. 1990. Microwaves and Minerals, I. Technology Review. Ontario Ministry of Northern Development and Mines, Background Paper 14, p. 16. Berry, T.F. and Bruce, R.W. 1966. “A Simple Method of Determining the Grindability of Ores,” Canadian Mining Journal 7: 63-65. Bradshaw, S.M. 1999. “Applications of Microwave Heating in Mineral Processing.” South African Journal of Science 95: 394-396. Chen, T.T., Dutrizac, J.E., Haque, K.E., Wyslouzil, W. and Kashyap, S. 1984. “The Relative Transparency of Minerals to Microwave Radiation.” Canadian Metallurgical Quarterly 23(1): 349–351. Clark, D.E., Folz, D.C. and West, J.K. 2000. “Processing of Materials with Microwave Energy.” Materials Science and Engineering A 287: 153-158. 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