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Thermally assisted liberation of high phosphorus oolitic iron ore: A comparison between microwave and conventional furnaces
Powder Technology 269 (2015) 7–14
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Thermally assisted liberation of high phosphorus oolitic iron ore: A comparison between microwave and conventional furnaces Mamdouh Omran a,b,⁎, Timo Fabritius a, Riku Mattila a a b
Laboratory of Process Metallurgy Research Group, Process and Environmental Engineering Department, University of Oulu, Finland Mineral Processing and Agglomeration Laboratory, Central Metallurgical Research and Development Institute, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 28 April 2014 Received in revised form 1 July 2014 Accepted 13 August 2014 Available online 16 September 2014 Keywords: Microwave treatment Conventional heating High phosphorus oolitic iron ore Liberation
a b s t r a c t This paper aims to investigate the effects of microwave, and conventional heating pretreatment on the liberation of iron bearing minerals from high phosphorus oolitic iron ore, specifically iron ore from the Aswan region of Egypt. These effects were analyzed by examining intergranular fractures generated between the oolitic/matrix and in the oolitic layers. Grindability and energy consumption were also measured after both microwave and conventional heating pretreatments. Scanning electron microscope (SEM) photomicrographs indicated that intergranular fractures are formed between the gangues (fluoroapatite and chamosite) and hematite after microwave treatment (resulting in improved liberation of the iron ore) while only a small number of micro-cracks were observed between the oolitic/matrix and in the oolitic layers after conventional heating of iron ore. Grindability tests indicated that microwave treated iron ore could be more easily ground compared with iron ore treated in a conventional furnace. This improved grindability is attributable to the large amount of intergranular fractures which are formed on the oolitic iron ore after treatment with microwave radiation. Energy consumption measurements also revealed that microwave treatment consumes much smaller quantities of energy compared with conventional heating ovens. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Deposits of high phosphorus oolitic iron ores are distributed across the world. In some areas, such as the Wadi Fatima mine in Saudi Arabia [1], Lorraine mine in France [2], Bell Island mine in Canada [3], Dilband mine in Pakistan [4], Xuanhua region in China [5] and the Aswan region in Egypt [6,7], enormous deposits have been located. The main obstacle associated with exploiting these deposits is the fine dissemination of silica and aluminum minerals, and the high levels of phosphorus content in particular, mainly due to the poor liberation of iron minerals from oolitic gangues. Song et al. [8] observed that fine grinding (commonly 1–5 μm) is required to liberate iron minerals from associated gangue minerals. Such fine particles are very difficult to beneficiate via conventional mineral processing processes (e.g., flotation and magnetic separation). There are two main challenges regarding mineral comminution; energy consumption and mineral recovery [9]. Approximately 1.5%–2%
⁎ Corresponding author at: Laboratory of Process Metallurgy Research Group, Process and Environmental Engineering Department, University of Oulu, P.O. Box 4300, Finland. E-mail addresses: [email protected], mamdouh.omran@oulu.fi (M. Omran).
http://dx.doi.org/10.1016/j.powtec.2014.08.073 0032-5910/© 2014 Elsevier B.V. All rights reserved.
of industrial mining countries' total national energy consumption can be attributed to comminution [10]. Wang et al. [11] advocated two main reasons for investigating the improvement of liberation: first, the liberation of larger particles reduces the amount of energy consumed during grinding, and second, because a very fine grain size is extremely difficult to physically separate during separation processes, this results in increased energy consumption during grinding. Physical separation techniques, such as flotation or magnetic separation, are suitable for particle liberation at coarse grain size. When discussing physical separation techniques, it is critical to focus on techniques that consume minimum amounts of power while still offering maximum particle size in relation to particle liberation. Since the early 20th century, the use of thermal pretreatment has been proposed as a way of decreasing the costs associated with size reduction while simultaneously improving the physical separation process, supporting the liberation of valuable minerals in ores. Later discoveries have shown that the use of microwave treatment has many advantages over more conventional heat treatments [12]. Microwave energy is a non-ionizing electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz. Microwave frequencies include three bands: ultra-high frequency (UHF: 300 MHz to 3 GHz), super high frequency (SHF: 3 GHz to 30 GHz) and extremely high frequency (EHF: 30 GHz to 300 GHz) [13,14]. Microwave
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Fig. 1. X-ray diffraction pattern for high phosphorus oolitic iron ore.
heating is fundamentally different from conventional heating because microwaves take the form of electromagnetic energy and can penetrate deep into the sample. This allows sample heating to be initiated volumetrically, as opposed to conventional thermal processing which heats the sample from the outside inward via standard heat transfer mechanisms, i.e., through convection, conduction, and radiation [15]. Compared with conventional heating techniques, the main advantages of microwave heating in terms of mineral processes are: non-contact heating, energy transfer rather than heat transfer, rapid heating, material selective heating and volumetric heating [14,15]. A growing interest in microwave heating in mineral treatment has emerged in recent years, and a number of potential applications regarding microwave processing have been investigated. These include microwave assisted ore grinding, microwave assisted carbothermic reduction of metal oxides, microwave-assisted drying and anhydration, microwave-assisted mineral leaching, microwave-assisted roasting and smelting of sulfide concentrate, microwave-assisted pretreatment of refractory gold concentrate, microwave-assisted spent carbon regeneration and microwave-assisted waste management [16,17]. Microwave treatment improves the liberation of high phosphorus oolitic iron ores by generating intergranular fractures in oolitic iron ores [8]. The difference in the absorption of microwave energy, thermal expansion and the dielectric properties of iron and gangue minerals leads to the generation of intergranular fractures between iron and gangue minerals [12,16,18–20].
Table 1 Chemical composition of studied high phosphorus oolitic iron ore. Oxides
Weight %
FeO SiO2 P2O5 CaO Al2O3 MnO MgO Na2O K2O F
74.96 7.48 3.24 5.44 4.47 0.54 1.26 0.37 0.05 0.19
High phosphorus oolitic iron ores are usually composed of hematite, dolomite, clinochlore, quartz and apatite (fluoroapatite or hydroxyl fluoroapatite). Microwave radiation has a significant influence on the microstructure of the oolitic units [21]. Hematite, phosphorite, silicate minerals and other gangues contained within the ore differ in how they absorb microwave energy. These minerals have different reactions regarding thermal expansion, and thus thermal stresses are generated on the boundaries between them. When these thermal stresses reach a critical level, cracks and fissures are formed at the boundaries [22]. Jones et al. [18] state that after microwave radiation, intergranular fractures occur around the grain boundaries between absorbent and transparent phases. Amankwah et al. [23] observed that the differential heating of different minerals' phases in an ore results in thermal stress cracking, making the ore more amenable to size reduction and resulting in a decrease in the work index. The effect of microwave radiation on the grindability of iron ore has been examined by Walkiewicz et al. [24]. In their study, iron ore was subjected to radiation levels of 3 kW, 2.45 GHz while raising the temperature between 840 and 940 °C. SEM photomicrographs were then used to confirm fractures along grain boundaries and throughout the gangue matrix. Standard bond grindability tests showed that the microwave treatment reduced the work index of the iron ores by between 10 and 24%. Kingman et al. [25] studied the influence of microwave radiation on Norwegian ilmenite ores. Kingman et al. concluded that short, highpower treatments were most effective, leading to a reduction in the Work Index of up to 90%, along with increased recovery of ilmenite. Guo Chen et al. [26,27] investigated the influence of microwave pretreatment on the grindability of ilmenite ores, and concluded that the microwave irradiation process has the potential to provide a new and
Table 2 Matrix for microwave tests (900 W, 2.45 GHz). Exposure time (s)
Sample temperature (°C)
Energy consumption (kWh)
Percentages of intergranular fractures (%)
30 40 50 60
235 308 418 546
0.0117 0.0158 0.0202 0.0237
≈15% ≈30% ≈60% N80%
M. Omran et al. / Powder Technology 269 (2015) 7–14 Table 3 Matrix for conventional furnace experiments. Experiment time (s)
Heating temperature (°C)
Energy consumption (kWh)
Percentages of intergranular fractures (%)
60 × 60 60 × 60 60 × 60
400 500 600
4.85 5.03 5.33
b10% ≈20% ≈30%
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energy-efficient, less time consuming, and more environmentally friendly. The purpose of this investigation is to compare the effects of microwave and conventional heating pretreatment on the liberation of ironbearing minerals from high phosphorus oolitic iron ore. 2. Experimental and analytical methods 2.1. Iron ore sample
highly efficient method of treating ilmenite with low energy consumption. The effects of microwave irradiation on the leachability of refractory gold ores have also been studied [28]. Compared to conventional processes, microwave leaching processes were found to be highly
The high phosphorous oolitic iron ore used in this study was collected from the Aswan region of Egypt. The area east of Aswan represents the main manifestation of the Cretaceous oolitic ironstone bands of southern Egypt, which are confined to clastic successions belonging to
Fig. 2. SEM photomicrographs of the high phosphorus oolitic iron ore. A. SEM photomicrograph showing the oolitic structure. B. SEM photomicrograph showing the matrix between ooids. C, D and E. EDX analyses of the squared area (i, ii, iii) in (B), respectively.
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the “Nubian” sandstones, or “Nubia facies” [6]. Fig. 1 and Table 1 show the XRD pattern and chemical analysis, respectively, of iron ore used in the tests.
in real time. The use of an RJ45-USB data cable and SEMS software facilitated the viewing of energy consumption data on a standard computer (Table 3).
2.2. Microwave treatment
2.4. Analytical methods
Samples were treated using a 2.45 GHz microwave oven with a maximum output power of 900 W. Iron ore samples were treated in the oven with varying exposure times, as shown in Table 2. One hundred gram samples were used in the tests as representative samples. The temperatures of the test samples were measured by quickly inserting a thermocouple into the sample after the power was turned off and were monitored via a temperature controller with a digital display. The measured temperatures shown are the bulk temperatures of the test samples. The samples were allowed to cool to room temperature in the microwave oven. The amounts of energy consumed during the tests were measured using a CLM1000 Professional (Plus) energy meter connected directly to the microwave, shown in Table (2).
The bulk mineralogical composition and crystallinity of the iron ores were performed on powdered samples using a Siemens D5000 XRD powder diffractometer. Chemical analyses were performed on whole rock powders by X-ray fluorescence (Bruker AXS S4 Pioneer). The micro-morphological characteristics of the iron ore before and after treatment with microwaves and in a conventional furnace were investigated using a Zeiss ULTRA plus field emission scanning electron microscope (FESEM), which was attached to an energy-dispersive X-ray spectroscopy (EDS) unit for chemical analysis. The mineral chemistry of the iron minerals and the element distribution maps within ooids and interstitial spaces between the ferruginous oolites were determined using electron probe microanalyses (EPMA). The EPMA were performed on a Jeol JXA-8200 device with WDS/EDS microanalyzer.
2.3. Conventional heating 3. Results and discussion One hundred gram representative samples of iron ore were placed in a furnace and heated at 400, 500 and 600 °C for a period of 1 h. The amounts of energy consumed during these experiments were measured using a wireless electricity monitor (RCS-S22A). This wireless monitoring system (RCS-S22A) consists of three sensor clamps that are connected to a transmitter and then attached to the live wires of the furnace. The sensor clamps immediately begin to monitor the current and the energy monitoring system begins to work, while the LCD screen of the display unit displays the power consumption of the monitored object
3.1. Mineralogy and chemistry of the high phosphorous oolitic iron ore XRD analysis indicated that hematite is the main iron-bearing mineral, whereas quartz, fluoroapatite and chamosite are the main gangue minerals (Fig. 1). XRF analysis of the original sample indicated that FeO and P2O5 grade are 74.96% and 3.24%, respectively, as shown in Table 1. P2O5, CaO and F content are related to fluoroapatite, whereas Al2O3, MgO and MnO content are related to chamosite. SiO2 content is related
Fig. 3. Back-scattered image of the oolitic structure and element maps showing the distribution of Fe, P and Ca inside oolites and in the interstitial spaces between oolites. Scale bar is 100 μm.
M. Omran et al. / Powder Technology 269 (2015) 7–14
to quartz and chamosite. SEM images of the high phosphorous iron ores show that Fe-bearing minerals occur as oolitic hematite, shown in Fig. 2A. Fluoroapatite (phosphorus bearing mineral) occurs mainly as a fine-grained, cement-like material mixed with iron that fills the spaces between ooid grains, shown in Fig. 2B and C. Chamosite occurs as a rim surrounding the ooid grains, and as thin laminae between iron laminae in the oolitic structure (Fig. 2B and D). Fig. 3 shows element maps made in the ferruginous ooids and spaces between ooids for Fe, P and Ca. The EDS distribution map of iron shows that iron has a higher concentration inside ooids than in the spaces between ooids (Fig. 3). It can be observed that phosphorus and calcium are closely related (particles with high phosphorus content also contained high calcium content) and are concentrated in the spaces between ooids. The distribution of P and Ca is related to fluoroapatite (Fig. 3), which indicates that fluoroapatite is concentrated mainly in the interstitial spaces between ooids.
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Tables 2 and 3 show the percentages of intergranular fractures generated between the oolites/matrix and in the oolite layers after both microwave and conventional heating pretreatment. The percentages of intergranular fractures were determined based on SEM images of different areas taken from each sample. Initially, the samples were observed with the SEM, after which they were treated by microwave and conventional heating. This was followed by a second SEM observation. The two images of the same area were compared to identify the changes between fractures on the sample surfaces before and after treatment [8]. Fig. 4A shows oolitic iron ore exposed to 900 W microwave power at 40 s exposure time, and Fig. 4B shows oolitic iron ore heated in a conventional furnace at 400 °C for 1 h. Significant damage was observed
for the microwave treated sample, shown in Fig. 4A, while no damage was observed regarding the conventionally heated sample, shown in Fig. 4B. The percentages of intergranular fractures generated in oolitic iron ore treated for 40 s were approximately 30%, as shown in Table 2. On the other hand, less than 10% of intergranular fractures were generated in iron ore after being heated at 400 °C for 1 h, as shown in Table 3. When the exposure time was increased to 50 s at 900 W microwave power, fractures appeared between oolites and matrix (intergranular fractures between oolites and matrix) (Fig. 5A). When the same sample was heated at 500 °C for 1 h, shown in Fig. 5B, no significant damage was observed between oolites and matrix, and only a few microcracks were present in the oolitic layers, shown in Fig. 6. The percentages of intergranular fractures generated in oolitic iron ore treated for 50 s were approximately 60%, as shown in Table 2. Only approximately 20% of intergranular fractures were generated in oolitic iron ore that had been heated at 500 °C for 1 h, as shown in Table 3. When the exposure time was increased to 60 s at a microwave power of 900 W, cracks were more localized around the oolites' boundaries, with almost no damage observed in the oolites' grains (Fig. 7). At this stage, oolites are mostly liberated from the matrix, which means that most of the phosphorus can be removed (Fig. 3). These localized damages would effectively facilitate the liberation of oolites at a coarser size and reduce overgrinding and slimes losse. With conventional heating at 600 °C for 1 h, shown in Fig. 8, few micro-cracks and fractures were observed between the oolites/matrix and in the oolitic layers. The percentages of intergranular fractures generated in oolitic iron ore treated for 60 s were in excess of 80%, shown in Table 2. Approximately 30% of intergranular fractures were found to have been generated in oolitic iron ore after being heated at 600 °C for 1 h, as shown in Table 3. There is a noticeable difference in the shape of the fractures generated by microwave and conventional oven treatments. Microwave treatment
Fig. 4. BSE images of oolitic iron ore (A) exposed to 900 W microwave power for 40 s (B) heated in a conventional furnace at 400 °C for 1 h.
Fig. 5. BSE images of oolitic iron ore (A) exposed to 900 W microwave power for 50 s (B) heated in a conventional furnace at 500 °C for 1 h.
3.2. Quantifying ore fracture
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Fig. 8. BSE image of oolitic iron ore heated in a conventional furnace at 600 °C for 1 h.
Fig. 6. BSE image of oolitic iron ore heated in a conventional furnace at 500 °C for 1 h, showing micro-cracks in oolitic layers.
generated wide and deep fractures, while narrow and surficial fractures (vein like) were generated as a result of conventional oven heating. This is because microwave heating takes the form of electromagnetic energy and can penetrate the sample deeply, allowing heating to be initiated volumetrically, while conventional thermal processing heats the sample from the outside in through standard heat transfer mechanisms [15]. There are numerous reasons why microwave treatment improves the liberation of high phosphorus oolitic iron ore over conventional heat treatments. Hematite is an active material during microwave heating, while gangues are inactive. When iron ore is exposed to microwave radiation, hematite expands more than gangues, and the difference caused by this expansion results in the formation of intergranular fractures [8]. Table 4 lists the heating properties of hematite and gangues minerals under microwave treatment [29,30]. Microwave treatment improves the liberation of high phosphorus oolitic iron by generating intergranular fractures in oolitic iron ore [8]. Microwaves heat faster than conventional heating methods. The speed at which materials heat is important in both conventional and microwave thermally assisted liberation. In the case of microwave heating, the transfer to the absorbing grain is very rapid, as microwave energy is delivered directly to materials through molecular interaction with the electromagnetic field. In contrast, in conventional thermal processing,
energy is transferred to the material through convection at the particle surface and conduction through the particle, with convective heat also drawn from the surface. Clearly this relatively slower and more uniform heat transfer process will generate smaller temperature gradients and lower thermal stresses [31]. 3.3. Grindability test To measure changes in grindability, microwave-, conventional furnace treated and untreated samples were ground for 30 s. One hundred grams of the crushed ore samples was first treated in the microwave oven at different exposure times (30, 40, 50 and 60 s) at a maximum output microwave power of 900 W. One hundred grams of the crushed ore was also heated in a conventional furnace at 400, 500 and 600 °C for 1 h. After grinding, fractions of less than 0.125 mm of the ground specimen were determined via sieve analysis for both untreated and treated iron ore samples. Then, to calculate the grindability % = wt. of undersize fraction (−0.125) / total wt. before grinding × 100. Fig. 9 shows the weight percentages of untreated and microwavetreated samples for − 0.125 mm size fractions. It is clear from Fig. 9 that the weight percentages of − 0.125 mm increased from 46.6% for the untreated sample to 59.76% for the 60 s microwave treated sample. Fig. 10 shows the weight percentages of untreated and conventional furnace heated samples for −0.125 mm size fractions. The weight percentages of − 0.125 mm increased to 50.80% after being heated at 600 °C for 1 h, compared to 46.6% for the untreated sample. Microwave treatment displayed more cracks and intergranular fractures in iron ore, with these fractures occurring around the grain boundaries between iron and gangues minerals. A reduction in comminution energy is possible after microwave treatment. 3.4. Energy consideration
Fig. 7. BSE image of oolitic iron ore exposed to 900 W microwave power for 60 s.
The major disadvantage of thermally assisted liberation is that the energy input required to reduce the strength of the ore is greater than the reduction in grinding energy. With microwave assisted liberation, however, the energy input required to induce significant damage within the ore is quite low when compared with the energy input required by conventional thermally assisted liberation [31]. Tables 2 and 3 present the amounts of energy consumed during microwave and conventional furnace experiments. There is a substantial difference between the amounts of energy consumed during microwave and conventional furnace treatments. For example, during conventional heating at 600 °C for 1 h, approximately 5.33 kWh of energy was required to increase the grindability of iron ore from 46.60% to 50.80%. With microwave treatment, approximately 0.0237 kWh of energy was required to increase the
M. Omran et al. / Powder Technology 269 (2015) 7–14
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Table 4 Heating properties of minerals with microwave radiation. Mineral
Formula
Microwave heating
Hematite
Fe2O3
Quartz Fluroapatite
SiO2 Ca5(PO4,CO3)3F
Chamosite
(Fe++,Mg,Fe+++)5Al(Si3Al)O10(OH,O)8
Heat readily, but no mineral phase change (active) Does not heat (inactive) Very little or no heat generated Very little or no heat generated
grindability from 46.60% to 59.76% (microwave power uses 224 times less power than a conventional oven). This large difference in the amount of energy consumption between the microwave and the conventional furnace was due to the microwave heating only the “responsive” or “active” phases; therefore, no energy is wasted in heating the entire sample [32]. Another important factor is the time spent on the experiments. Microwave treatment takes only a few seconds, whereas conventional heating takes hours. Rapid heating by microwave leads to greater savings in time (time = money). This work was carried out on a laboratory scale using a multimode microwave. The energy efficiency of this multimode type applicator is approximately 40%, compared to an efficiency of N80% for pilot and full scale monomode type systems [32]. As the iron and steel sector is the second largest industrial user of energy after the chemical and petrochemical sector [33], thermal or energy efficiency is a very important factor in using energy economically. Thermal efficiency is the ratio of energy output to energy input [34,35]: Eout =Ein The energy output Eout is the useful effect and the energy input Ein is the necessary expense. Thermal efficiency expresses the amount of energy input that is converted into useful energy by the process. The difference between input and output corresponds to the total losses of the system. Kingman et al. [36] compared the amounts of microwave energy required to treat samples in single-mode cavities and multimode cavities. Assuming a 1-kg load in each case, approximately 13.88 kWh/t of microwave energy is required to treat a sample in a multimode cavity at microwave power of 10 W for a duration of 5 s. In a single mode cavity, the microwave power level is increased to 15 kW and the length of time is reduced to 0.05 s, meaning that only 0.21 kWh/t of microwave energy would be used. The result of increasing microwave power density is
Fig. 10. The weight percentages of untreated and conventional furnace heated samples for −0.125 mm size fractions.
clear; significant reductions in grinding resistance can be achieved at low microwave energy inputs. In addition to a reduction in comminution energy, there are many additional benefits of microwave treatment. The most important benefits of microwave assisted liberation are increased liberation at larger sizes, increases in plant throughput, decreases in plant wear and possible changes in process flowsheet design as a result of microwave treatment [36,37]. For example, work carried out on carbonatites ore showed that a 6% increase in recovery of FeO was possible after 2 stage wet drum magnetic separation tests for 10 second microwave treated ore in comparison to non-treated ore. Similarly, 2% increases in copper recovery after froth flotation were reported for 10 second microwave treated ore in comparison to non-treated ore [32,38]. Compared with conventional ore pre-treatment methods, microwave pretreatment consumes much less energy, improves liberation, and reduces the processing time [39].
4. Conclusions This paper studied the influence of microwave and conventional heating pretreatment on the liberation of high phosphorus oolitic iron ore. The main conclusion that can be drawn based on the results of the experiments is that microwave radiation has a significant effect on the liberation of high phosphorus oolitic iron ore. SEM photomicrographs showed that intergranular fractures formed between the gangues (fluoroapatite and chamosite) and hematite after microwave treatment, leading to improved liberation of iron ore. Only a few micro-cracks occurred between oolites/matrix and in the oolitic layers after conventional heating of iron ore. Grindability tests indicated that microwave pretreatment of iron ore can be applied effectively to enhance the grindability of iron ore. This suggests that the liberation of iron minerals can be improved, and a reduction in comminution energy is possible after microwave treatment. In summary, the findings of this paper indicate that microwave treatment possesses multiple advantages over conventional thermal treatment. When compared with conventional thermal treatment, microwave treatment consumes considerably less energy, improves liberation and reduces the processing time.
Acknowledgment
Fig. 9. The weight percentages of untreated and microwave treated samples for −0.125 mm size fractions.
The authors are very thankful to the Cultural Affairs and Missions Sector, Egypt and CIMO (Center for International Mobility, Finland) for their generous financial support of the present research study.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Thermally assisted liberation of high phosphorus oolitic iron ore: A comparison between microwave and conventional furnaces Mamdouh Omran a,b,⁎, Timo Fabritius a, Riku Mattila a a b
Laboratory of Process Metallurgy Research Group, Process and Environmental Engineering Department, University of Oulu, Finland Mineral Processing and Agglomeration Laboratory, Central Metallurgical Research and Development Institute, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 28 April 2014 Received in revised form 1 July 2014 Accepted 13 August 2014 Available online 16 September 2014 Keywords: Microwave treatment Conventional heating High phosphorus oolitic iron ore Liberation
a b s t r a c t This paper aims to investigate the effects of microwave, and conventional heating pretreatment on the liberation of iron bearing minerals from high phosphorus oolitic iron ore, specifically iron ore from the Aswan region of Egypt. These effects were analyzed by examining intergranular fractures generated between the oolitic/matrix and in the oolitic layers. Grindability and energy consumption were also measured after both microwave and conventional heating pretreatments. Scanning electron microscope (SEM) photomicrographs indicated that intergranular fractures are formed between the gangues (fluoroapatite and chamosite) and hematite after microwave treatment (resulting in improved liberation of the iron ore) while only a small number of micro-cracks were observed between the oolitic/matrix and in the oolitic layers after conventional heating of iron ore. Grindability tests indicated that microwave treated iron ore could be more easily ground compared with iron ore treated in a conventional furnace. This improved grindability is attributable to the large amount of intergranular fractures which are formed on the oolitic iron ore after treatment with microwave radiation. Energy consumption measurements also revealed that microwave treatment consumes much smaller quantities of energy compared with conventional heating ovens. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Deposits of high phosphorus oolitic iron ores are distributed across the world. In some areas, such as the Wadi Fatima mine in Saudi Arabia [1], Lorraine mine in France [2], Bell Island mine in Canada [3], Dilband mine in Pakistan [4], Xuanhua region in China [5] and the Aswan region in Egypt [6,7], enormous deposits have been located. The main obstacle associated with exploiting these deposits is the fine dissemination of silica and aluminum minerals, and the high levels of phosphorus content in particular, mainly due to the poor liberation of iron minerals from oolitic gangues. Song et al. [8] observed that fine grinding (commonly 1–5 μm) is required to liberate iron minerals from associated gangue minerals. Such fine particles are very difficult to beneficiate via conventional mineral processing processes (e.g., flotation and magnetic separation). There are two main challenges regarding mineral comminution; energy consumption and mineral recovery [9]. Approximately 1.5%–2%
⁎ Corresponding author at: Laboratory of Process Metallurgy Research Group, Process and Environmental Engineering Department, University of Oulu, P.O. Box 4300, Finland. E-mail addresses: [email protected], mamdouh.omran@oulu.fi (M. Omran).
http://dx.doi.org/10.1016/j.powtec.2014.08.073 0032-5910/© 2014 Elsevier B.V. All rights reserved.
of industrial mining countries' total national energy consumption can be attributed to comminution [10]. Wang et al. [11] advocated two main reasons for investigating the improvement of liberation: first, the liberation of larger particles reduces the amount of energy consumed during grinding, and second, because a very fine grain size is extremely difficult to physically separate during separation processes, this results in increased energy consumption during grinding. Physical separation techniques, such as flotation or magnetic separation, are suitable for particle liberation at coarse grain size. When discussing physical separation techniques, it is critical to focus on techniques that consume minimum amounts of power while still offering maximum particle size in relation to particle liberation. Since the early 20th century, the use of thermal pretreatment has been proposed as a way of decreasing the costs associated with size reduction while simultaneously improving the physical separation process, supporting the liberation of valuable minerals in ores. Later discoveries have shown that the use of microwave treatment has many advantages over more conventional heat treatments [12]. Microwave energy is a non-ionizing electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz. Microwave frequencies include three bands: ultra-high frequency (UHF: 300 MHz to 3 GHz), super high frequency (SHF: 3 GHz to 30 GHz) and extremely high frequency (EHF: 30 GHz to 300 GHz) [13,14]. Microwave
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Fig. 1. X-ray diffraction pattern for high phosphorus oolitic iron ore.
heating is fundamentally different from conventional heating because microwaves take the form of electromagnetic energy and can penetrate deep into the sample. This allows sample heating to be initiated volumetrically, as opposed to conventional thermal processing which heats the sample from the outside inward via standard heat transfer mechanisms, i.e., through convection, conduction, and radiation [15]. Compared with conventional heating techniques, the main advantages of microwave heating in terms of mineral processes are: non-contact heating, energy transfer rather than heat transfer, rapid heating, material selective heating and volumetric heating [14,15]. A growing interest in microwave heating in mineral treatment has emerged in recent years, and a number of potential applications regarding microwave processing have been investigated. These include microwave assisted ore grinding, microwave assisted carbothermic reduction of metal oxides, microwave-assisted drying and anhydration, microwave-assisted mineral leaching, microwave-assisted roasting and smelting of sulfide concentrate, microwave-assisted pretreatment of refractory gold concentrate, microwave-assisted spent carbon regeneration and microwave-assisted waste management [16,17]. Microwave treatment improves the liberation of high phosphorus oolitic iron ores by generating intergranular fractures in oolitic iron ores [8]. The difference in the absorption of microwave energy, thermal expansion and the dielectric properties of iron and gangue minerals leads to the generation of intergranular fractures between iron and gangue minerals [12,16,18–20].
Table 1 Chemical composition of studied high phosphorus oolitic iron ore. Oxides
Weight %
FeO SiO2 P2O5 CaO Al2O3 MnO MgO Na2O K2O F
74.96 7.48 3.24 5.44 4.47 0.54 1.26 0.37 0.05 0.19
High phosphorus oolitic iron ores are usually composed of hematite, dolomite, clinochlore, quartz and apatite (fluoroapatite or hydroxyl fluoroapatite). Microwave radiation has a significant influence on the microstructure of the oolitic units [21]. Hematite, phosphorite, silicate minerals and other gangues contained within the ore differ in how they absorb microwave energy. These minerals have different reactions regarding thermal expansion, and thus thermal stresses are generated on the boundaries between them. When these thermal stresses reach a critical level, cracks and fissures are formed at the boundaries [22]. Jones et al. [18] state that after microwave radiation, intergranular fractures occur around the grain boundaries between absorbent and transparent phases. Amankwah et al. [23] observed that the differential heating of different minerals' phases in an ore results in thermal stress cracking, making the ore more amenable to size reduction and resulting in a decrease in the work index. The effect of microwave radiation on the grindability of iron ore has been examined by Walkiewicz et al. [24]. In their study, iron ore was subjected to radiation levels of 3 kW, 2.45 GHz while raising the temperature between 840 and 940 °C. SEM photomicrographs were then used to confirm fractures along grain boundaries and throughout the gangue matrix. Standard bond grindability tests showed that the microwave treatment reduced the work index of the iron ores by between 10 and 24%. Kingman et al. [25] studied the influence of microwave radiation on Norwegian ilmenite ores. Kingman et al. concluded that short, highpower treatments were most effective, leading to a reduction in the Work Index of up to 90%, along with increased recovery of ilmenite. Guo Chen et al. [26,27] investigated the influence of microwave pretreatment on the grindability of ilmenite ores, and concluded that the microwave irradiation process has the potential to provide a new and
Table 2 Matrix for microwave tests (900 W, 2.45 GHz). Exposure time (s)
Sample temperature (°C)
Energy consumption (kWh)
Percentages of intergranular fractures (%)
30 40 50 60
235 308 418 546
0.0117 0.0158 0.0202 0.0237
≈15% ≈30% ≈60% N80%
M. Omran et al. / Powder Technology 269 (2015) 7–14 Table 3 Matrix for conventional furnace experiments. Experiment time (s)
Heating temperature (°C)
Energy consumption (kWh)
Percentages of intergranular fractures (%)
60 × 60 60 × 60 60 × 60
400 500 600
4.85 5.03 5.33
b10% ≈20% ≈30%
9
energy-efficient, less time consuming, and more environmentally friendly. The purpose of this investigation is to compare the effects of microwave and conventional heating pretreatment on the liberation of ironbearing minerals from high phosphorus oolitic iron ore. 2. Experimental and analytical methods 2.1. Iron ore sample
highly efficient method of treating ilmenite with low energy consumption. The effects of microwave irradiation on the leachability of refractory gold ores have also been studied [28]. Compared to conventional processes, microwave leaching processes were found to be highly
The high phosphorous oolitic iron ore used in this study was collected from the Aswan region of Egypt. The area east of Aswan represents the main manifestation of the Cretaceous oolitic ironstone bands of southern Egypt, which are confined to clastic successions belonging to
Fig. 2. SEM photomicrographs of the high phosphorus oolitic iron ore. A. SEM photomicrograph showing the oolitic structure. B. SEM photomicrograph showing the matrix between ooids. C, D and E. EDX analyses of the squared area (i, ii, iii) in (B), respectively.
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the “Nubian” sandstones, or “Nubia facies” [6]. Fig. 1 and Table 1 show the XRD pattern and chemical analysis, respectively, of iron ore used in the tests.
in real time. The use of an RJ45-USB data cable and SEMS software facilitated the viewing of energy consumption data on a standard computer (Table 3).
2.2. Microwave treatment
2.4. Analytical methods
Samples were treated using a 2.45 GHz microwave oven with a maximum output power of 900 W. Iron ore samples were treated in the oven with varying exposure times, as shown in Table 2. One hundred gram samples were used in the tests as representative samples. The temperatures of the test samples were measured by quickly inserting a thermocouple into the sample after the power was turned off and were monitored via a temperature controller with a digital display. The measured temperatures shown are the bulk temperatures of the test samples. The samples were allowed to cool to room temperature in the microwave oven. The amounts of energy consumed during the tests were measured using a CLM1000 Professional (Plus) energy meter connected directly to the microwave, shown in Table (2).
The bulk mineralogical composition and crystallinity of the iron ores were performed on powdered samples using a Siemens D5000 XRD powder diffractometer. Chemical analyses were performed on whole rock powders by X-ray fluorescence (Bruker AXS S4 Pioneer). The micro-morphological characteristics of the iron ore before and after treatment with microwaves and in a conventional furnace were investigated using a Zeiss ULTRA plus field emission scanning electron microscope (FESEM), which was attached to an energy-dispersive X-ray spectroscopy (EDS) unit for chemical analysis. The mineral chemistry of the iron minerals and the element distribution maps within ooids and interstitial spaces between the ferruginous oolites were determined using electron probe microanalyses (EPMA). The EPMA were performed on a Jeol JXA-8200 device with WDS/EDS microanalyzer.
2.3. Conventional heating 3. Results and discussion One hundred gram representative samples of iron ore were placed in a furnace and heated at 400, 500 and 600 °C for a period of 1 h. The amounts of energy consumed during these experiments were measured using a wireless electricity monitor (RCS-S22A). This wireless monitoring system (RCS-S22A) consists of three sensor clamps that are connected to a transmitter and then attached to the live wires of the furnace. The sensor clamps immediately begin to monitor the current and the energy monitoring system begins to work, while the LCD screen of the display unit displays the power consumption of the monitored object
3.1. Mineralogy and chemistry of the high phosphorous oolitic iron ore XRD analysis indicated that hematite is the main iron-bearing mineral, whereas quartz, fluoroapatite and chamosite are the main gangue minerals (Fig. 1). XRF analysis of the original sample indicated that FeO and P2O5 grade are 74.96% and 3.24%, respectively, as shown in Table 1. P2O5, CaO and F content are related to fluoroapatite, whereas Al2O3, MgO and MnO content are related to chamosite. SiO2 content is related
Fig. 3. Back-scattered image of the oolitic structure and element maps showing the distribution of Fe, P and Ca inside oolites and in the interstitial spaces between oolites. Scale bar is 100 μm.
M. Omran et al. / Powder Technology 269 (2015) 7–14
to quartz and chamosite. SEM images of the high phosphorous iron ores show that Fe-bearing minerals occur as oolitic hematite, shown in Fig. 2A. Fluoroapatite (phosphorus bearing mineral) occurs mainly as a fine-grained, cement-like material mixed with iron that fills the spaces between ooid grains, shown in Fig. 2B and C. Chamosite occurs as a rim surrounding the ooid grains, and as thin laminae between iron laminae in the oolitic structure (Fig. 2B and D). Fig. 3 shows element maps made in the ferruginous ooids and spaces between ooids for Fe, P and Ca. The EDS distribution map of iron shows that iron has a higher concentration inside ooids than in the spaces between ooids (Fig. 3). It can be observed that phosphorus and calcium are closely related (particles with high phosphorus content also contained high calcium content) and are concentrated in the spaces between ooids. The distribution of P and Ca is related to fluoroapatite (Fig. 3), which indicates that fluoroapatite is concentrated mainly in the interstitial spaces between ooids.
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Tables 2 and 3 show the percentages of intergranular fractures generated between the oolites/matrix and in the oolite layers after both microwave and conventional heating pretreatment. The percentages of intergranular fractures were determined based on SEM images of different areas taken from each sample. Initially, the samples were observed with the SEM, after which they were treated by microwave and conventional heating. This was followed by a second SEM observation. The two images of the same area were compared to identify the changes between fractures on the sample surfaces before and after treatment [8]. Fig. 4A shows oolitic iron ore exposed to 900 W microwave power at 40 s exposure time, and Fig. 4B shows oolitic iron ore heated in a conventional furnace at 400 °C for 1 h. Significant damage was observed
for the microwave treated sample, shown in Fig. 4A, while no damage was observed regarding the conventionally heated sample, shown in Fig. 4B. The percentages of intergranular fractures generated in oolitic iron ore treated for 40 s were approximately 30%, as shown in Table 2. On the other hand, less than 10% of intergranular fractures were generated in iron ore after being heated at 400 °C for 1 h, as shown in Table 3. When the exposure time was increased to 50 s at 900 W microwave power, fractures appeared between oolites and matrix (intergranular fractures between oolites and matrix) (Fig. 5A). When the same sample was heated at 500 °C for 1 h, shown in Fig. 5B, no significant damage was observed between oolites and matrix, and only a few microcracks were present in the oolitic layers, shown in Fig. 6. The percentages of intergranular fractures generated in oolitic iron ore treated for 50 s were approximately 60%, as shown in Table 2. Only approximately 20% of intergranular fractures were generated in oolitic iron ore that had been heated at 500 °C for 1 h, as shown in Table 3. When the exposure time was increased to 60 s at a microwave power of 900 W, cracks were more localized around the oolites' boundaries, with almost no damage observed in the oolites' grains (Fig. 7). At this stage, oolites are mostly liberated from the matrix, which means that most of the phosphorus can be removed (Fig. 3). These localized damages would effectively facilitate the liberation of oolites at a coarser size and reduce overgrinding and slimes losse. With conventional heating at 600 °C for 1 h, shown in Fig. 8, few micro-cracks and fractures were observed between the oolites/matrix and in the oolitic layers. The percentages of intergranular fractures generated in oolitic iron ore treated for 60 s were in excess of 80%, shown in Table 2. Approximately 30% of intergranular fractures were found to have been generated in oolitic iron ore after being heated at 600 °C for 1 h, as shown in Table 3. There is a noticeable difference in the shape of the fractures generated by microwave and conventional oven treatments. Microwave treatment
Fig. 4. BSE images of oolitic iron ore (A) exposed to 900 W microwave power for 40 s (B) heated in a conventional furnace at 400 °C for 1 h.
Fig. 5. BSE images of oolitic iron ore (A) exposed to 900 W microwave power for 50 s (B) heated in a conventional furnace at 500 °C for 1 h.
3.2. Quantifying ore fracture
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Fig. 8. BSE image of oolitic iron ore heated in a conventional furnace at 600 °C for 1 h.
Fig. 6. BSE image of oolitic iron ore heated in a conventional furnace at 500 °C for 1 h, showing micro-cracks in oolitic layers.
generated wide and deep fractures, while narrow and surficial fractures (vein like) were generated as a result of conventional oven heating. This is because microwave heating takes the form of electromagnetic energy and can penetrate the sample deeply, allowing heating to be initiated volumetrically, while conventional thermal processing heats the sample from the outside in through standard heat transfer mechanisms [15]. There are numerous reasons why microwave treatment improves the liberation of high phosphorus oolitic iron ore over conventional heat treatments. Hematite is an active material during microwave heating, while gangues are inactive. When iron ore is exposed to microwave radiation, hematite expands more than gangues, and the difference caused by this expansion results in the formation of intergranular fractures [8]. Table 4 lists the heating properties of hematite and gangues minerals under microwave treatment [29,30]. Microwave treatment improves the liberation of high phosphorus oolitic iron by generating intergranular fractures in oolitic iron ore [8]. Microwaves heat faster than conventional heating methods. The speed at which materials heat is important in both conventional and microwave thermally assisted liberation. In the case of microwave heating, the transfer to the absorbing grain is very rapid, as microwave energy is delivered directly to materials through molecular interaction with the electromagnetic field. In contrast, in conventional thermal processing,
energy is transferred to the material through convection at the particle surface and conduction through the particle, with convective heat also drawn from the surface. Clearly this relatively slower and more uniform heat transfer process will generate smaller temperature gradients and lower thermal stresses [31]. 3.3. Grindability test To measure changes in grindability, microwave-, conventional furnace treated and untreated samples were ground for 30 s. One hundred grams of the crushed ore samples was first treated in the microwave oven at different exposure times (30, 40, 50 and 60 s) at a maximum output microwave power of 900 W. One hundred grams of the crushed ore was also heated in a conventional furnace at 400, 500 and 600 °C for 1 h. After grinding, fractions of less than 0.125 mm of the ground specimen were determined via sieve analysis for both untreated and treated iron ore samples. Then, to calculate the grindability % = wt. of undersize fraction (−0.125) / total wt. before grinding × 100. Fig. 9 shows the weight percentages of untreated and microwavetreated samples for − 0.125 mm size fractions. It is clear from Fig. 9 that the weight percentages of − 0.125 mm increased from 46.6% for the untreated sample to 59.76% for the 60 s microwave treated sample. Fig. 10 shows the weight percentages of untreated and conventional furnace heated samples for −0.125 mm size fractions. The weight percentages of − 0.125 mm increased to 50.80% after being heated at 600 °C for 1 h, compared to 46.6% for the untreated sample. Microwave treatment displayed more cracks and intergranular fractures in iron ore, with these fractures occurring around the grain boundaries between iron and gangues minerals. A reduction in comminution energy is possible after microwave treatment. 3.4. Energy consideration
Fig. 7. BSE image of oolitic iron ore exposed to 900 W microwave power for 60 s.
The major disadvantage of thermally assisted liberation is that the energy input required to reduce the strength of the ore is greater than the reduction in grinding energy. With microwave assisted liberation, however, the energy input required to induce significant damage within the ore is quite low when compared with the energy input required by conventional thermally assisted liberation [31]. Tables 2 and 3 present the amounts of energy consumed during microwave and conventional furnace experiments. There is a substantial difference between the amounts of energy consumed during microwave and conventional furnace treatments. For example, during conventional heating at 600 °C for 1 h, approximately 5.33 kWh of energy was required to increase the grindability of iron ore from 46.60% to 50.80%. With microwave treatment, approximately 0.0237 kWh of energy was required to increase the
M. Omran et al. / Powder Technology 269 (2015) 7–14
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Table 4 Heating properties of minerals with microwave radiation. Mineral
Formula
Microwave heating
Hematite
Fe2O3
Quartz Fluroapatite
SiO2 Ca5(PO4,CO3)3F
Chamosite
(Fe++,Mg,Fe+++)5Al(Si3Al)O10(OH,O)8
Heat readily, but no mineral phase change (active) Does not heat (inactive) Very little or no heat generated Very little or no heat generated
grindability from 46.60% to 59.76% (microwave power uses 224 times less power than a conventional oven). This large difference in the amount of energy consumption between the microwave and the conventional furnace was due to the microwave heating only the “responsive” or “active” phases; therefore, no energy is wasted in heating the entire sample [32]. Another important factor is the time spent on the experiments. Microwave treatment takes only a few seconds, whereas conventional heating takes hours. Rapid heating by microwave leads to greater savings in time (time = money). This work was carried out on a laboratory scale using a multimode microwave. The energy efficiency of this multimode type applicator is approximately 40%, compared to an efficiency of N80% for pilot and full scale monomode type systems [32]. As the iron and steel sector is the second largest industrial user of energy after the chemical and petrochemical sector [33], thermal or energy efficiency is a very important factor in using energy economically. Thermal efficiency is the ratio of energy output to energy input [34,35]: Eout =Ein The energy output Eout is the useful effect and the energy input Ein is the necessary expense. Thermal efficiency expresses the amount of energy input that is converted into useful energy by the process. The difference between input and output corresponds to the total losses of the system. Kingman et al. [36] compared the amounts of microwave energy required to treat samples in single-mode cavities and multimode cavities. Assuming a 1-kg load in each case, approximately 13.88 kWh/t of microwave energy is required to treat a sample in a multimode cavity at microwave power of 10 W for a duration of 5 s. In a single mode cavity, the microwave power level is increased to 15 kW and the length of time is reduced to 0.05 s, meaning that only 0.21 kWh/t of microwave energy would be used. The result of increasing microwave power density is
Fig. 10. The weight percentages of untreated and conventional furnace heated samples for −0.125 mm size fractions.
clear; significant reductions in grinding resistance can be achieved at low microwave energy inputs. In addition to a reduction in comminution energy, there are many additional benefits of microwave treatment. The most important benefits of microwave assisted liberation are increased liberation at larger sizes, increases in plant throughput, decreases in plant wear and possible changes in process flowsheet design as a result of microwave treatment [36,37]. For example, work carried out on carbonatites ore showed that a 6% increase in recovery of FeO was possible after 2 stage wet drum magnetic separation tests for 10 second microwave treated ore in comparison to non-treated ore. Similarly, 2% increases in copper recovery after froth flotation were reported for 10 second microwave treated ore in comparison to non-treated ore [32,38]. Compared with conventional ore pre-treatment methods, microwave pretreatment consumes much less energy, improves liberation, and reduces the processing time [39].
4. Conclusions This paper studied the influence of microwave and conventional heating pretreatment on the liberation of high phosphorus oolitic iron ore. The main conclusion that can be drawn based on the results of the experiments is that microwave radiation has a significant effect on the liberation of high phosphorus oolitic iron ore. SEM photomicrographs showed that intergranular fractures formed between the gangues (fluoroapatite and chamosite) and hematite after microwave treatment, leading to improved liberation of iron ore. Only a few micro-cracks occurred between oolites/matrix and in the oolitic layers after conventional heating of iron ore. Grindability tests indicated that microwave pretreatment of iron ore can be applied effectively to enhance the grindability of iron ore. This suggests that the liberation of iron minerals can be improved, and a reduction in comminution energy is possible after microwave treatment. In summary, the findings of this paper indicate that microwave treatment possesses multiple advantages over conventional thermal treatment. When compared with conventional thermal treatment, microwave treatment consumes considerably less energy, improves liberation and reduces the processing time.
Acknowledgment
Fig. 9. The weight percentages of untreated and microwave treated samples for −0.125 mm size fractions.
The authors are very thankful to the Cultural Affairs and Missions Sector, Egypt and CIMO (Center for International Mobility, Finland) for their generous financial support of the present research study.
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