Microwave-assisted metallurgy

Pretreatment of ironmaking materials

Grinding

Microwave-assisted grinding of iron ores containing haematite, magnetite and goethite has been investigated since late 1980s. ^71,72 Microwave heating is useful for improving grindability of ores and mineral liberation because it offers the potential of decreasing energy consumption required during grinding iron ores by inducing thermal stress cracking. The iron ores could be heated to average maximum temperatures between 840 and 940°C in a 3-kW microwave oven operating at 2·45 GHz. ⁷² Owing to the stress cracking occurring at a lower temperature compared with conventional heating, less energy was consumed during the process. Microwave heating reduced the work index of iron ores by 10–24%. Cleaner liberation of the ore mineral at a larger particle size could reduce the grinding energy requirements, improve the concentrate grade and increase metal recovery after beneficiation. An experimental study on the enhancement effect of microwave pretreatment on grindability of iron ores revealed that after microwave treatment, the grindability increased significantly with the specific rate of breakage (S _i) (increased by an average of 50%). ⁷³ S _i is defined as ⁷⁴ (10) where x ₀ is the standard size (usually taken as 1 mm), x _i is the upper size of screen interval indexed by i, A and α are constants that depend on the material and the milling condition, and Q _i is a correction factor, which is 1 for small particles (normal breakage) and less than 1 (abnormal breakage) for too large particles to be nipped and fractured properly by the ball size in the mill.

The experimental findings in a very recent study demonstrated that breakage of microwave-treated iron ore followed a first-order behaviour for fine feed sizes and deviated from first order for coarse feed sizes. ⁷⁴ The microwave-treated ore produced more coarse-sized material and broke faster than untreated iron ore. It was recognised that microwave treatment created new cracks and developed network cracks at the borders between iron oxide minerals (good microwave absorbers) and gangue minerals (poor microwave absorbers). As a result of the treatment, the breakage rate of iron ores and the throughput of grinding with a minimum proportion of fines were increased. Table 5 shows the values of specific rate of breakage of untreated and microwave-treated feeds. ⁷⁴

Drying

It is well known that some iron ores contain a substantial amount of water that should be removed for ironmaking. As water is a strong microwave absorber, rapid dehydration (drying) of iron ores with a high water/moisture content can be easily achieved. This feasibility has been demonstrated by experimental observations on drying of agglomerates (e.g. iron ore pellets) using microwave energy. ¹² It was suggested that the dehydration rate could be very high without crack and burst generated in pellets. A large amount of dehydration energy could be saved compared to traditional heating. During the process, the most effective factor influencing the microwave dehydration was microwave power, followed by drying time and sample weight. To speed up dehydration of iron ores, additives, such as carbon, are extensively used. A recent experimental study on the dehydration behaviour of goethite–graphite pellets by microwave heating has shown that carbon content in pellets controlled the dehydration process. ⁷⁵ The efficiency of microwave heating depended on the ratio of FeO(OH):C. With an appropriate ratio (e.g. 1∶1), sufficiently large distance between graphite particles allowed microwaves to penetrate into the core of pellets, resulting in volumetric heating and rapid dehydration. The schematic in Fig. 7 illustrates microwave absorption behaviours of sample pellets with different amounts of carbon. ⁷⁵ Microwaves might penetrate into the pellets or reflect at the pellet surface, depending upon the carbon addition.

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7.

Microwave absorption behaviours of goethite–graphite pellets with different amounts of carbon ⁷⁵ (reproduced with permission of the Iron and Steel Institute of Japan)

Sintering

Conventional ironmaking usually involves a step known as sintering in which iron ore fines (dust) with other fine materials are agglomerated at high temperatures to create a product that can be used in a blast furnace (BF). Among various sintering approaches, microwave-assisted carbon-less sintering of iron ores was believed to be advantageous to energy conservation and environmental protection because of less CO₂ emissions. The advantages were demonstrated by carbon-less sintering of magnetite concentrates under microwave irradiation. ¹² The entire process was dominated by rapid heating of Fe₃O₄ having less liquid phase generated in sintered magnetite concentrates when the magnetite content was increased. To assist with iron ore sintering, hot airflow ignition of sinter mixtures (iron ores, coke, etc.) using microwave energy was recently proposed. ⁷⁶ The ignition system consists of a power control cabinet, a microwave heating device, a sinter pot and a preheating furnace (shaft furnace). Airflow was first preheated in the shaft furnace to approximately 300°C and then directed to a microwave heating reactor in which porous ceramics were heated by microwaves at 2·45 GHz. After heat exchange with the porous ceramics, high-temperature airflow would conduct heat to the sinter pot where sintering of the mixture proceeded. It was found that the ignition energy consumption for sintering using microwave power was 43·77 MJ m⁻², far less than 185·13 MJ m⁻² by the traditional ignition process. Other advantages of this new ignition technique included sufficient coke combustion, better mineralised product (sinter) and less air pollutants.

Oxidation

Aiming at separating iron from titanium-bearing minerals, microwave oxidation of ilmenite ore using 28 GHz microwave irradiation has been studied. ⁷⁷ Unlike aforementioned low-frequency microwaves (915 MHz and 2·45 GHz), which were produced by magnetrons, 28 GHz microwaves were generated from a gyrotron. Using a waveguide, high-frequency microwaves were directed to a multimode microwave applicator in which samples were irradiated at various power levels (0·8, 1·0, and 1·5 kW) and temperature was measured by thermocouple and pyrometer, as shown in Fig. 8. ⁷⁷ After the ilmenite ore sample was exposed to microwave irradiation for given time periods, the original FeTiO₃ phase in the ore was oxidised to rutile (TiO₂) and pseudobrookite (Fe₂TiO₅). It was found that both ilmenite and pseudobrookite were good microwave absorbers. The grain growth rate of pseudobrookite and rutile under microwave irradiation was much faster than that obtained in the resistance furnace. The rapid grain growth of pseudobrookite led to a considerable difference between the grain sizes of pseudobrookite and rutile, which was favourable for subsequent separation steps, e.g. magnetic separation and dilute acid leaching.

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8.

Schematic of multimode 28 GHz microwave processing system ⁷⁷ (reproduced with permission of the Iron and Steel Institute of Japan)

Dephosphorisation

In general, iron ores containing high phosphorus cannot be directly used for steelmaking because of the detrimental effects of phosphorus on steel properties, such as various forms of embrittlement that reduce the toughness and ductility of steel. Although dephosphorisation of iron ores has been studied for a long time, efficient separation of the Fe- and P-containing phases at reasonable cost still remains a main obstacle in the dephosphorisation process. Recently, an integrated process, which combines microwave carbothermic reduction (coal as a reductant) with magnetic separation, for dephosphorisation of high phosphorous iron ores was proposed. ⁷⁸ It was found that the reductant has a weak effect on the dephosphorisation rate and iron recovery when its addition was varied from 18·7 to 21·3 wt-% (Fig. 9). The dephosphorisation rate decreased with increasing microwave irradiation time, while the iron recovery ratio showed the opposite tendency. It was revealed that efficient separation of the reduced Fe and unreduced P-containing gangues determined the dephosphorisation rate. Although the desired dephosphorisation rate was not obtained, microwave treatment was considered an effective approach for enhancing iron recovery.

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9.

Effects of carbon addition and microwave irradiation time on the iron recovery ratio and dephosphorisation rate ⁷⁸ (reproduced with permission of the Iron and Steel Institute of Japan)

Reduction of iron oxides/ores

Laboratory-scale experimental progress

Many efforts have been expended to assess laboratory-scale reduction of iron oxides/ores using microwave energy starting from pioneering investigations to temperature distribution in the spherical composites (pellets) of Fe₂O₃ and carbon subjected to 2·45 GHz microwaves. ⁸ It was revealed during heating that the temperature of the pellet decreased from the centre to surface, which was contrary to that of conventional heating. The experimental results were compared with numerical ones that were based on a simplified model. As the temperature dependence of dielectric properties, thermal conductivity and specific heat was not available at the time of the publication, the effects of these factors were not considered in the proposed numerical model. Despite these disadvantages with the model, which eventually caused some discrepancies between the results of numerical simulation and experimental observations, the basic trend suggested by the simulation was found reliable. Based on these findings, a follow-up study on the reduction of magnetohaematite ore particles in CO atmosphere with microwave irradiation was performed. ⁹ It was pointed out that the reducibility and post-reduction strength of the sample were enhanced by microwave heating and the optimal values were achieved after microwave treatment (1300 W) for 6 min. During the reduction process, magnetite was the component dominating the temperature rise of the sample. Another analogous work confirmed the superiority of microwave heating over conventional heating in the carbothermic reduction of iron ores. ¹⁰ Phase transformation from Fe₂O₃ to Fe₃O₄ enhanced the heating process. As a hyperactive microwave absorber, Fe₃O₄ heated rapidly. However, it was surprising to find out a weak microwave input power dependence of heating rate, probably because of the heavy heat radiation from the sample to surroundings. The work also indicated that common solid reductants, such as charcoal and coke, were effective for microwave reduction of iron oxides. Compared with coke, charcoal exhibited stronger microwave absorption. In addition, the microwave reduction process was improved by adding lime and limestone as fluxing agents although their microwave absorption capabilities were inferior to those of iron oxides.

The application of microwave energy may significantly influence the carbothermic reduction kinetics of iron ore (mainly haematite) pellets. To examine this possibility, a conventional microwave oven with an 800-W power operating at 2·45 GHz was used for the reduction of Fe₂O₃ pellets mixed with graphite powders. ⁷⁹ It was found that reduction started from the outer part of pellets because carbon was in contact with the surface of iron oxide pellets. The pellet surface was covered by molten metallic iron, preventing diffusion of CO to the pellet centre. It was concluded that reduction was topochemically controlled by diffusion, resulting in an unreacted centre. The advantage of volumetric heating of microwaves was not clearly demonstrated in this study. In fact, this unexpected phenomenon can be explained by the faster heating rate of graphite (compared to that of iron oxide) and the relatively large size of iron ore pellets (0·6 cm of radius) used in the experiment, making reduction take place at the contacting surface first. This work also found the load position-dependent heating characteristics. The best available location of sample, surprisingly, was found to be 26 cm in front of the microwave aperture and 6 cm from the bottom of the applicator. It can be concluded with confidence that microwave heating relies on the load position in microwave applicator and on sample size.

Microwave heating characteristics of ironmaking materials have been extensively studied because of their significance in microwave reduction of iron oxides/ores. The microwave absorptivity of Fe₃O₄ powder, indicated by temperature increase, was found to decrease with increasing relative density but was independent of particle size. It was believed to be caused by the relatively large penetration depth of Fe₃O₄ compared to metal. ⁸⁰ A series of experiments on the reduction of iron ore concentrates showed that a binder containing water could increase the heating rate of iron ore pellets containing coal. ¹² Powdered coal was suggested to be more effective for heating and the reduction process compared with those of particled coal. Another study on the carbothermic reduction of iron ore/cement/carbonaceous material (coke and charcoal) composite pellets exposed to microwave irradiation (2·45 GHz, 1100 W) revealed that Portland cement had better microwave absorption than iron oxides. ⁸¹ The particle size of carbonaceous material must be strictly controlled and optimised for the highest reaction rate. The results also confirmed more rapid heating of charcoal than coke, which was attributed to a greater content of volatile matter in charcoal. An experimental test of carbothermic reduction of oxidised ilmenite/coke composites (mainly FeTiO₃) with a microwave power level of 1·5 kW operating at 2·45 GHz found that ilmenite ore particles selectively coupled with microwaves that created hot spots in the composite pellets that consequently contributed to a lower reduction temperature and shorter reaction time. ⁸² A recent study also suggested that the reduction reaction between iron ore and petroleum coke was highly dependent on microwave heating characteristics of the materials, whose effects could be evaluated by simultaneously measuring temperature and mass of materials during reduction. ⁸³ Figure 10 shows a schematic of the microwave heating system for iron ore reduction in argon, in which two directional couplers were used. By using the couplers (‘1’ and ‘2’ in Fig. 10), the irradiated microwave power and the reflected power were measured. The variations of temperature and mass loss of the materials (pellets) throughout the reduction processes were monitored by a pyrometer and a semi-analytical balance, respectively. This type of system was able to determine the kinetic curves of iron ore reduction and energy consumption during the process in either maximum E or H fields.

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10.

Schematic of microwave iron ore reduction system ⁸³ (reproduced with permission of John Wiley and Sons)

The temperature dependence of microwave carbothermic reduction of iron oxides has been investigated extensively. The work on the microwave reduction characteristics of mixed powders of Fe₃O₄ with carbon black (2·45 GHz, 2·8 kW) demonstrated that Fe₃O₄ absorbed microwaves below approximately 650°C while carbon black absorbed microwave energy in a broader temperature up to 1200°C. ⁸⁴ The reduction of magnetite to wüstite started at least 400°C, with subsequent reduction of wüstite to iron initiated at 800°C. Inhomogeneous reactions were found throughout the mixture heating and reduction process. As a step towards better understanding of microwave carbothermic reduction, magnetite iron ore–coal composite pellets were irradiated with a gradually increased microwave power level from 0·2 to 2 kW (in 2 min intervals) at 2·45 GHz. ⁸⁵ It was established that the reduction of Fe₃O₄ to FeO occurred between 800 and 1000°C while the subsequent reduction of FeO to Fe started at temperatures ranging from 1000 to 1250°C. Meanwhile, carbon was considered both a reductant and heating agent, contributing to localised microwave reduction of the composite samples. It was interesting to note that at all temperatures during the reduction process, the proportion of reduced iron was larger near the sample pellet surface (out part) than in the centre, as indicated by the white regions in Fig. 11. ⁸⁵ This phenomenon was partly attributed to the fact that microwave absorption capability of magnetite decreased at temperatures above 650°C because of the Curie point effect. No microwave magnetic loss would contribute to microwave heating of the sample, ¹⁸ which was demonstrated by permittivity and permeability measurements of Fe₃O₄ discussed previously. Hence, samples received more external radiant heat from the exothermic auxiliary substance (SiC) used in the experiment, making reactions proceed faster near the sample surface.

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11.

Backscattered electron (BSE) images of the initial and irradiated magnetite iron ore–coal composites at different positions showing the growth in proportion of reduced iron during microwave reduction ⁸⁵ (reproduced with permission of the Iron and Steel Institute of Japan)

It is believed that microwave heating affects the entire process of iron oxide reduction, which has been demonstrated by the reactions between Fe₃O₄ and carbon black powders with 2·8 kW microwaves at 2·45 GHz. ⁸⁶ It was shown that the reduction of magnetite to wüstite was accomplished below 651°C. A three-stage reduction process was identified based on the analysis of heating time dependence of reduction rate of magnetite–carbon black powder mixtures irradiated by microwaves up to 800, 1000, and 1200°C. As shown in Fig. 12a , the stages of the reduction process were determined by different average maximum reduction rates: 0·017 g s⁻¹ (stage I, vary fast), 0·0017 g s⁻¹ (stage II, very low), and 0·0045 g s⁻¹ (stage III). ⁸⁶ The initial rapid mass reduction in stage I was attributed to multiple reactions (Table 6), ⁸⁶ indicated by the peaks in Fig. 12b–f . It is noteworthy that the area of each peak represented the mass of C+O loss resulting from different reactions. Considering strong microwave absorption capabilities of magnetite and carbon, the reactions between iron oxide and carbon might dominate the microwave iron and steelmaking process. Energy requirements for the reduction process could be lowered by a thermal non-equilibrium state induced by microwave heating.

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12.

Carbon plus oxygen (C+O) weight reduction rate versus microwave irradiation time for magnetite–carbon black mixtures: a total irradiation time showing the three stages and b–f amplification of a showing only stage I during which various reactions in Table 6 were identified, as indicated by the peaks 1–6 ⁸⁶ (reproduced with permission of the Iron and Steel Institute of Japan)

Because microwave heating also exhibits a strong frequency dependence, several experimental tests have focused on microwave reduction of iron oxides at frequencies other than 2·45 GHz. For instance, an investigation was undertaken for the reduction of magnetite with carbon at both 2·45 and 30 GHz. ⁸⁷ It was shown that high-quality pig iron could be produced in air with a 30 GHz heating system, whereas FeO was generated as the main phase in reduction of magnetite using 2·45 GHz microwave heating. Magnetite was found more efficiently reduced at higher frequencies, probably because of the effect of cathodoluminescence. Another recent study illustrated the chemical behaviour of reaction between Fe₃O₄ and C under electric and magnetic fields of microwaves at 915 MHz. ⁸⁸ It was concluded that the combined effect of thermal energy and microwave magnetic field enhanced the high-temperature reduction with microwaves acting as a dynamic catalyst.

Scale-up of microwave iron- and steelmaking systems

As mentioned above, microwave reduction of iron oxides/ores has been extensively explored for a long time. However, to date, little work has been done on the scale-up of heating systems, primarily because of the difficulty in furnace design which requires multidisciplinary knowledge to achieve uniform heating ⁸⁹ with suppression of microwave thermal runaway. ^90,91

To establish a microwave heating system for continuous pig iron production, a microwave furnace having maximum power of 12·5 kW operating at 2·45 GHz was constructed. ^87,92 Mixed powders of magnetite ore and carbonaceous materials including coal, coke and graphite were used for the experiment. It was concluded that several requirements should be met for constructing a successful microwave heating furnace that is capable of continuous pig ironmaking: (1) high microwave energy density in reaction chamber, (2) low temperatures of insulating materials (<c.a. 1000°C), (3) no pig iron solidification in tapping hole, (4) no feed pipe clogging, and (5) minimum heat loss of applicator. In a successive study, the maximum power of the furnace was scaled up to 20 kW for pig iron production. ⁹³ It was found that pig iron can be produced in 35 min from 1 kg of mixed powder consisting of magnetite ore and 18% graphite with a microwave power level of 17·5 kW. The contents of impurities such as phosphorus and silicon in pig iron were much lower than pig iron produced by a BF because of vaporisation and low oxygen potential during reduction. The content of sulfur in pig iron was a little higher than that in BF pig iron owing to the slag formed. The results showed that approximately 60% of energy was used for heating materials and reduction of magnetite to wüstite and 15% for the reduction of wüstite to iron and molten pig iron.

Regarding the variation of microwave absorption capabilities of materials during the iron oxide reduction process (e.g. Fe₂O₃—Fe₃O₄—FeO—Fe), a direct-steelmaking technology by combination of microwave, electric-arc, and exothermal heating was developed and used for construction of a 225-kW microwave steelmaking furnace at 915 MHz. ^94–98 It was shown that a significant reduction in CO₂ emissions and energy consumption (by 25%) could be achieved by using this technology compared to that of conventional basic oxygen furnace steelmaking methods. One of the most distinguishing advantages in this technology was attributed to the application of electric arc heating for the final stage of reduction (from FeO to Fe), which avoided poor microwave absorption of metallic iron in the final step of steel production. This technology provided a method that combines diverse heating techniques, making either batch or continuous molten steel production possible.

Recycling and remediation of ferrous metallurgical wastes

It is expected that the production of crude steel will increase in the future. ⁹⁹ Along with the growth in steel production, the amount of iron- and steelmaking industrial waste has increased considerably. Microwave heating has been proposed as an alternative to traditional methods for recycling and remediation of ferrous metallurgical wastes including slag, dust, sludge, and steel pickling liquors.

Slag

Each year a large amount of slag is generated during iron and steel production. Although some slag is used for land filling or road construction, most of the slag has not been used efficiently. ¹⁰⁰ To recycle metallic components from slag, a substantial amount of work has focused on the slag treatment using microwave energy to modify the physical characteristics of iron- and steelmaking slag. The pioneering work in the field centred on the microwave heating characteristics of a synthetic CaO–SiO₂–Fe_tO slag. ¹⁰¹ The heating rate of the slag subjected to 1·6 kW microwave power operating at 2·45 GHz strongly depended on the ratio of Fe ³⁺/(Fe²⁺+Fe³⁺) in the slag. When the ratio approached approximately 0·16, the slag had the largest dielectric loss resulting from the coexistence of CaFe₃O₅ and Fe₃O₄. The experimental results suggested that the maximum heating rate of slag might be obtained by optimising the ionic species of Fe. In another study, recovery of Fe and P from CaO–SiO₂–Fe_tO–P₂O₅ slag by heating slag–graphite mixtures with microwave power (1·7 kW, 2·45 GHz) was explored. ¹⁰⁰ The heating rates of the examined materials followed the order: graphite≫Fe₃O₄≫Fe₂O₃≫synthesised CaO–SiO₂–Fe_tO–P₂O₅ slag>industrial slag. It was found that the heating process was dominated by microwave absorption of graphite. The recovery ratios of Fe and P reached 0·97 and 0·89, respectively, when the carbon equivalent C _eq was 1·69.

Apart from those studies on iron recovery, microwave carbothermic reduction (graphite as a reductant) of Cr-containing steelmaking slag and pickling sludge was investigated with the goal of recycling Cr effectively. ¹⁰² Test results demonstrated that the major Cr-bearing phase of the slag, Cr₂O₃, was preferentially heated in the electric field of microwaves, followed by rapid generation of metallic Cr during reduction. The heated sample showed locally enhanced reduction kinetics and was reduced at lower (average) reduction temperatures than the expected value from thermodynamics. It was probably caused by inhomogeneous temperature distribution resulting from the large temperature dependence of Cr₂O₃ permittivity and by the existence of local arcing. Figure 13 shows a schematic of models of enhanced reaction kinetics and formation of metal pieces. ¹⁰² It is observed from Fig. 13a that the permittivity of Cr₂O₃ might increase rapidly with increasing temperature, resulting in hot spots that had locally much higher temperatures than other positions. The oxide could then be reduced to metallic Cr in these hot spot areas where the thermodynamic conditions were satisfied. The enhanced reduction kinetics might also be attributed to occurrence of local arcing, which was associated with thermal plasma discharge during microwave heating, as illustrated in Fig. 13b . However, this possibility was not confirmed in the work because of lack of intimate experimental techniques for the heating process.

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13.

Schematic of models of microwave-enhanced reaction kinetics and formation of metal pieces ¹⁰² (reproduced with permission of the Iron and Steel Institute of the Japan)

To improve physical properties of slag, crystallisation of BF slag induced by microwave heating at different frequencies (28 and 2·45 GHz) was examined. ¹⁰³ It was shown that crystallisation took place at a lower temperature (800°C) at which the crystalline phases (e.g. CaO⋅SiO₂, 2CaO⋅Al₂O₃⋅SiO₂, and CaO⋅Al₂O₃⋅2SiO₂) were not generated by conventional electric resistance furnace. Additionally, the crystallisation process of slag was significantly accelerated using microwaves at 28 GHz, while exposure to 2·45 GHz microwave radiation had little effect on the samples. On the other hand, it is known that changes in physical properties of slag caused by microwaves may facilitate some subsequent treatments, such as slag grinding. Regarding this effect, microwave cyclic heating was applied to induce cracks in BF slag bearing high titanium. ¹⁰⁴ It was shown that the pervoskite phases (mainly CaTiO₃) in the slag were responsible for heating. The slag compressive strength decreased with increasing cyclic number and heating time. Approximately 35% of decrease in strength was obtained after 10 cycles of heating for 5 min. Another study on microwave treatment of BF slag bearing titanium demonstrated that the major Ti-bearing phase, CaTiO₃, could be heated to 868°C in 150 s under 750 W, 2·45 GHz microwave irradiation. ¹⁰⁵ By increasing the power to 2 kW, the maximum temperature reached was 1280°C. The rapid heating subsequently led to an increase in thermal stress at the interface of the slag specimen. On the contrary, microwave irradiation promoted the densification of BF slag compact after microwave-assisted hydrothermal treatment of the slag using an adjustable microwave power level up to 1·6 kW operating at 2·45 GHz. ¹⁰⁶ Tobermorite (Ca₅Si₆O₁₆(OH)₂⋅4H₂O) was identified as the major phase formed in the slag after the treatment, contributing to the increased reaction kinetics. The hydrothermal reaction rate was increased by 1–2 orders of magnitude using microwaves as the heating source.

Dust

Electric-arc furnace (EAF) dust is a hazardous waste containing substantial amounts of zinc, lead, and iron. ¹⁰⁷ The first attempt on pyrometallurgical recovery of metallic elements from EAF dust using microwave heating centred upon processing synthetic dust–carbon composites. ¹⁰⁸ The reduction of ZnO was found slow compared to that of Fe₂O₃. Simultaneous recovery of Zn and Fe from EAF dust mixed with graphite powders could also be achieved using microwave energy. ¹⁰⁹ This is because Zn and Fe were obtained as vapours and metal drops, respectively, in 300 s under 2·45 GHz, 1·7 kW microwave irradiation. Meanwhile, the slag produced was found non-hazardous and safe for disposal. These results were consistent with another similar study in which microwave heating succeeded in recovering metallic Zn and Fe from EAF dust. ¹¹⁰ Besides these pyrometallurgical processes for dust treatment, microwave energy was employed as the heating source for Zn and Pb leaching in caustic solution. ¹¹¹ It was shown that the leaching kinetics of Zn and Pb was considerably improved by microwave heating which was, according to the authors, probably because of super heating of liquid during violent interactions between microwaves and the EAF dust solids suspended in the solution. Analogous findings were obtained in microwave-assisted leaching of Zn from basic oxygen furnace dust with sulfuric acid. ¹¹² It was found that microwave heating enhanced the leaching process, leading to a substantially higher zinc recovery than that obtained by conventional process. There were two distinct stages of the leaching process: an early stage controlled by diffusion from 22 to 50°C with the apparent activation energy (E _A) of 1·15 kJ mol⁻¹, followed by a second stage controlled by a surface chemical reaction above 50°C with activation energy of 22·01 kJ mol⁻¹. ¹¹³ The remarkable difference between these two stages was likely attributed to the unstable reaction equilibrium caused by microwave non-thermal effects.

Sludge

Generally, steelmaking sludge contains a substantial amount of water that should be removed before further treatment. In a recent study, the effect of microwave irradiation on dehydration behaviours of industrial sludge from stainless steel plants was investigated. ¹¹⁴ Samples containing a mixture of a common industrial sludge (30 wt-% water, 24·3 wt-%Fe) and graphite were treated in a 0·67 kW (maximum power) microwave multimode applicator. Figure 14 shows the weight loss of the sludge mixture subjected to microwave heating up to 150°C. ¹¹⁴ It was seen that weight loss caused by dehydration started at 120°C with 80% of total loss accomplished in the first 9 min. The results clearly demonstrated the high efficiency of microwave irradiation in dehydration of the sludge. In addition, there was a significant difference between the dehydration rates of sludges heated by microwave energy and by conventional electric furnace heating. Their weight losses were found to be 34 and 7%, respectively. The superiority of microwave dehydration could make the commercialisation of this technology viable. However, details of scale-up were not provided in the work.

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14.

Weight loss of sludge by microwave treatment with the arrow pointing temperature drop along with weight gain taking place at 285 s. This extraordinary behaviour was associated with active dehydration, which led to water drops precipitated on the ceiling of the microwave heating apparatus ¹¹⁴ (reproduced with permission of the Iron and Steel Institute of Japan)

Steel pickling liquors

The disposal of pickling liquors containing significant amounts of impurities, such as Fe, Pb, Mn, Cu, Co, Ni, Zn, Cr and Cd, remains a key environmental concern for steel industry. In industrial practices, these liquors require treatments before discharge. Compared with a conventional/autoclave hydrothermal process for treatment of steel pickling liquors, microwave-based methods are generally faster, cleaner, and more economical. In view of these benefits, microwave-hydrothermal processing of chloride pickling liquors, with different pH values ranging from 6 to 13 (adjusted with NaOH), was carried out. ¹¹⁵ Microwave treatment was found capable of eliminating heavy metal contamination from the pickling liquors with appropriate PH values (>9). Figure 15 compares the metal concentrations in the original liquor and after microwave-hydrothermal treatment at 110°C for 5 min (pH = 13) with regard to levels established by local environmental legislation (Class 3 effluent, Brazil, 1987) for some environmentally hazardous elements. ¹¹⁵ It can be seen that the metal concentrations were considerably reduced after the treatment. The microwave-hydrothermal technology demonstrated the potential to treat industrial effluents for production of impurity-free water. Further, some value-added products, such as magnetite, goethite, β-FeOOH and haematite, were obtained along with the purification of steel picking liquors, depending on the pH value. The microwave-hydrothermal technology is thus important from both environmental and economic points of view as it is capable of combining the removal of various types of aqueous pollutants with the production of valuable materials.

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15.

Ratio (log scaled) of selected metal concentration to the maximum permitted level (class 3 effluent) for the original liquor and for the one microwave treated at 110°C for 5 min (pH = 13) ¹¹⁵ (reproduced with permission of Elsevier)

Microwave-assisted direct leaching/reduction

Nickel in a laterite ore could be leached with a 25 vol.-% sulfuric acid solution under microwave irradiation (2·45 GHz, 600 W) within 1·5 h. ¹¹⁸ When the leaching temperature was 90°C, approximately 90·8% of nickel was recovered. The kinetic analysis showed that the leaching process was controlled by chemical reaction occurring on the surface of samples and the apparent activation energy was 38·9 kJ mol⁻¹. Microwave-assisted leaching promoted dissolution of laterite ores compared with conventional thermal conductive heating, such as water bath heating. As expected, the leaching process depended on the leaching agents and temperature. When microwave radiation was applied directly to the leaching slurry containing sulfuric and hydrochloric acids for recovering nickel and cobalt from their ores with high content of magnesium, the metal extraction increased substantially in comparison with conventional leaching at temperatures between 230 and 250°C. ¹¹⁹

As previously mentioned, the addition of carbonaceous materials to laterite ores will promote the microwave heating process. This effect has been clearly demonstrated in carbothermic reduction of laterite ores under microwave irradiation. A mixture of laterite and lignite was found susceptible to microwave irradiation at 2·45 GHz, and its temperature reached 900°C after 120 s of heating using an input power of 800 W. ¹²⁰ The reaction between laterite and lignite occurred rapidly, forming a metallic iron–nickel alloy, as shown in Fig. 16a . ¹²⁰ By using twice the stoichiometric addition of carbon, a reduction degree of 70% was achieved after 480 s of heating at 800 W. It was also observed that the reduction degree strongly depended on the sample mass. The reduction degree decreased from about 70 to 25% when the mass of the laterite–lignite mixture increased from 13·5 to 493·5 g, as shown in Fig. 16b . The sample mass (or volume) dependence of reduction yield was considered to be the most important characteristic of the microwave carbothermic reduction.

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16.

a A spherical Fe(Ni) alloy particle formed after 240 s of microwave reduction of laterite using lignite and b logarithmic scale diagram of reduction degree as a function of mass of a laterite–lignite mixture with two times the stoichiometric carbon addition after 480 s of heating at 800 W ¹²⁰ (reproduced with permission of Elsevier)

Microwave preheating followed by leaching

Like microwave-assisted direct reduction, microwave preheating takes advantage of microwave coupling effect of carbonaceous materials, which act as reducing agents simultaneously. Microwave energy may be utilised to cause pre-reduction, which enables recovery of the metals by conventional leaching. For instance, laterite ores mixed with active carbon were found susceptible to microwaves and the mixture temperature reached approximately 1000°C in 6·5 min. ¹²¹ The rapid heating led to the formation of an intermediate reduction product that was then subjected to sulfuric acid leaching. The recovery of nickel was dependent on the iron reduction degree after microwave preheating, and the nickel recovery reached about 90% when the reduction degree of iron was controlled at approximately 60%.

In addition to carbonaceous materials, leaching agents can also be added to laterite ores during microwave preheating to enhance efficiency of subsequent leaching operations. For this purpose, chlorides and sulfuric acid have been extensively used. Microwave energy was used in conjunction with chlorination to produce metal chlorides, which were separated from the gangue and then processed by conventional approaches to obtain high metal yield. An early patent disclosed that the mixture of laterite, ferric chloride and sodium chloride resulted in the formation of nickel- and cobalt-bearing chlorides after being microwaved for 4–8 min. ¹²² These intermediate products were subsequently leached in water from which nickel and cobalt could be recovered. In a similar study, ammonium chloride was mixed with laterite ores containing nickel, cobalt, iron and magnesium oxides. The mixture was then irradiated at 1200 W for 4–5 min under a nitrogen atmosphere, producing a nickel–cobalt-bearing intermediate product that was then leached in water at 80°C for 30 min. ¹²³ It was shown that the nickel and cobalt extraction rates could reach up to 66 and 78%, respectively. For comparison and prevention of thermal runaway during leaching, microwave pulses were applied to irradiate a similar mixture with a total pulsing time of 5 min in air. It revealed that 70% of nickel and 85% of cobalt were extracted after only 15 min of water leaching, which were comparable to leaching rates obtained through conventional treatment in a rotary kiln at 300°C for 2 h. The extraction of nickel can be further enhanced by applying microwave radiation to the mixture of nickel laterite ores and sulfuric acid (mass ratio: 2). The mixture was first irradiated by microwaves at a power level of 800 W for 6 min, and 92·76% of nickel in the ores was then extracted by water leaching at 90°C for 90 min. ¹²⁴ During the process, the liquid–solid reaction was promoted by the migration of ionic species and/or excitation of the dipolar species because of the increased contact area of reactants under microwave irradiation.

Conventional pre-roasting followed by microwave-assisted hydrothermal leaching

As an environmentally friendly process, which simultaneously improves the yield of extracted metals within a shorter reaction time, pre-roasting laterite ores followed by microwave-assisted hydrothermal leaching was proposed and applied for extracting nickel and cobalt from a saprolite laterite ore having low iron and high magnesium contents. ¹²⁵ It was found that 89·19% of nickel and 61·89% of cobalt could be extracted from the ore pre-roasted at 300°C when sulfuric leaching under microwave irradiation was conducted at 70°C for 60 min.

Microwave-assisted direct leaching

Microwave-assisted direct leaching of copper sulphides and other copper-bearing concentrates has attracted increasing attention in recent years. Microwaves demonstrated positive effects on copper recovery from chalcopyrite containing approximately 34 wt-% copper in a mixed leaching solution consisting of 0·5M H₂SO₄ and 0·25M Fe₂(SO₄)₃. ¹²⁹ Figure 17 shows a comparison of fractions of copper reacted (Cu extraction) by microwave leaching and standard (conventional) leaching of chalcopyrite at various temperatures as a function of time. ¹²⁹ The results revealed that higher copper extraction could be achieved by microwave leaching and the difference between copper extractions by microwave and conventional leaching increased with leaching temperature and time. After 3 h of microwave leaching, the amount of copper extracted at 91°C was approximately 17% compared to 13·5% conventionally. Higher recovery and leaching kinetics in microwave systems were associated with selective heating of mineral particles in the solution. Owing to high conductivity, the chalcopyrite exhibited a shorter microwave penetration depth, causing the mineral to heat by conduction. A superheated layer because of high-loss leaching solutions created a localised heating environment compared with the measured bulk solution temperature, which improved the reaction rate. Similar phenomena were found in the microwave-assisted leaching of chalcocite containing 47·7 wt-%Cu, 21·4 wt.-%S and 11·5 wt.-%Fe with a mixed solution of CuCl₂, NaCl and HCl. ¹³⁰ The fast leaching was attributed to the microwave-induced super heating of copper ions, which became more significant with increasing temperature. The super heating caused non-uniform temperature distribution in the solution, which eventually influenced the leaching efficiency.

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17.

Comparison between microwave leaching and conventional leaching of chalcopyrite at various temperatures as a function of time (C_Fe2(SO4)3: 0·25M, particle size: <38 μm) ¹²⁹ (reproduced with permission of Elsevier)

It is believed that a uniform temperature distribution during microwave-assisted leaching can be achieved provided the reactor size is optimised. A recent study showed the effect of reactor size on microwave leaching of copper from chalcopyrite in a Fe₂(SO₄)₃⋅H₂SO₄ solution. ¹³¹ The leaching experiments were carried out in a single-mode cavity (modified WR340 waveguide) using reactors of two sizes (20 and 50 mm in diameter). The copper recovery was higher when leached in the small reactor while the recovery obtained in the large reactor under microwave conditions was comparable to that obtained conventionally, as shown in Fig. 18. ¹³¹ The enhanced recovery in the small reactor was because of the selective heating of chalcopyrite coupled with the effect of microwave penetration depth. This work reveals that microwave penetration depth is one of the most important factors for heating and deserves more attention when investigating hydrometallurgical systems under microwave conditions.

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18.

Comparison between copper recoveries from chalcopyrite when leached in a single-mode microwave cavity using reactors of different sizes and in a standard reactor (C_Fe2(SO4)3: 0·25M, particle size: <38 μm, temperature: 91°C, time: 3 h) ¹³¹ (reproduced with permission of Elsevier)

The characteristics of microwave heating also contributed to extraction of copper from a molybdenite concentrate (42 wt-%Mo, 3·6 wt-%Cu) with sulfuric acid in a bath-type process at ambient pressure (2·45 GHz, maximum power: 850 W). ¹³² A minimum energy consumption of about 0·9 MW h/t was required to decrease the copper content of the concentrate from 3·6 wt-% to less than 0·1 wt-% in 15 min. Microwave-assisted leaching exhibited a faster heating rate and consequently a more rapid dissolution rate than conventional heating. The control mechanisms during the leaching process were verified by the diffusion-controlled shrinking core model.

Microwave pretreatment followed by leaching/reduction

Microwave pretreatment plays an important role in copper extraction. The application of microwave heating to the oxidative roasting of copper sulfate concentrate in the presence of CaCO₃ could accelerate the roasting process and eliminate SO₂ emissions. ¹³³ The reaction rate with microwave roasting was 3·97–17·15 times faster than that with conventional roasting. The calcine obtained was leached with a NH₃–(NH₄)₂CO₃ solution for copper extraction. For a 33-g sample irradiated by microwaves at a power level of 575 W for 10 min, the copper extraction was as high as 96·6%. When CuFeS₂–CaCO₃ pellets were roasted with conventional heating at 600–900°C for 60 min, the copper extraction was only 71–77·4%. The temperatures for microwave roasting should be controlled at 400°C or less to avoid sintering of samples, which leads to remarkably lower copper extraction. A similar microwave pretreatment was applied to a chalcopyrite concentrate mixed with an acidified FeCl₃ solution, which promoted copper yield by subsequent leaching in cold water. After the originated intermediate product was leached in cold water for 5 min, the copper leaching rate reached 55·2%. ¹³⁴

Microwave pretreatment also aids in bioleaching of copper minerals. The influence of microwave irradiation on bioleaching behaviour and mechanisms of a low-grade complex copper sulphide ore, subjected to microwave heating in a domestic microwave oven (1100 W, 5 min), was assessed in a mixed mesophilic bacterial culture using electrochemical techniques. ¹³⁵ Microwave treatment improved the bioleaching behaviour of the ore, with greater effects on copper and iron dissolutions than on zinc and lead. Microwaved samples displayed higher reactivity, dissolution rate, dissolution current and current density with decreased polarisation resistance. The effect of microwave heating can be indicated by comparing the elemental compositions of the sulphide ore before and after microwave pretreatment, as summarised in Table 7. ¹³⁵

Microwave energy can be applied to heap leaching of copper sulfide ores. This is because application of high-power microwave energy introduces micro-fractures and cracks into the ore, leading to an improved leaching efficiency. In particular, pulsed microwave pretreatment displayed strong effects on copper extraction during a diagnostic leach. ¹³⁶ For a low-grade ore containing finely disseminated sulphide mineral phases, the extraction was improved because of the pretreatment of ore particles in a high-intensity microwave field. There was a greater internal surface area of ore particles as a result of microwave-induced fractures along grain boundaries, exposing locked Cu-bearing mineral phases to the leach solution. The larger particle size fractions were more susceptible to microwave pretreatment than finer particle size fractions, because of an increase in the volume percentage of microwave-absorbing phases in larger particles. This result is consistent with another study in which more energy was required to induce thermal stresses that generated physical damage within ore particles when the particle size was decreased. ¹³⁷

An early patent described a leaching or smelting precursor method of drying and heating ore particulates, or concentrates, which had been previously mixed with carbonaceous materials (e.g. active form of carbon and other carbon-containing material). Because these carbonaceous materials were capable of being readily dried and heated to charring temperatures by microwave energy, rapid heating of the composite material with microwave energy was possible. ¹³⁸ In fact, microwave energy has been found to rapidly dry and heat these mixtures so that reduction reactions within mixtures were initiated. This method has been used for pretreatment of copper concentrates containing 24 wt-% copper. After smelting of the pretreated concentrates, a reduction product containing 93% copper was obtained.

Microwave-assisted direct reduction

Compared with the intensive work on application of microwave energy to extraction of copper from sulphide ores, limited studies reported the use of microwave energy in recovering Cu from copper oxide ores. Recently, microwave irradiation was applied to carbothermic reduction of copper oxide (CuO) and a malachite [Cu₂CO₃(OH)₂] concentrate. ¹²⁶ Using an 800-W power supply, and with addition of lignite as a reducing agent having two times the stoichiometric carbon content, almost complete reduction of 10 g of CuO was achieved in 4 min. When lignite was replaced by graphite, isolated spherical and laminar metallic copper particles were formed in the first 2 min of reduction, as shown in Fig. 19a and b . ¹²⁶ These copper particles having dimensions of a few micrometres were surrounded by unreacted CuO and graphite particles of spherical shape. On the contrary, poor microwave absorption of the malachite concentrate–lignite mixture produced a maximum temperature of only 200°C. By adding 5% graphite powders to the mixture, rapid heating of the malachite concentrate–lignite–graphite mixture (800°C in 2 min) was observed with sequential malachite calcination and CuO reduction. This phenomenon indicated the importance of susceptors in microwave reduction of copper oxide ores. After being irradiated at a power level of 800 W for 440 s, the reduction degree of CuO produced by the calcination of malachite was approximately 90%.

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19.

Formations of a spherical and b laminar metallic copper at early stages (2 min) of microwave carbothermic reduction of CuO (power: 650 W, reducing agent: graphite, carbon content: two times the stoichiometric C content) ¹²⁶ (reproduced with permission of Elsevier)

Microwave-assisted direct reduction/leaching

In microwave carbothermic reduction of zinc oxide, the reactions rely on the microwave absorption capabilities of reducing agents because pure ZnO rarely absorbs microwave energy at low temperatures. ¹⁴⁰ Among common carbonaceous materials used as reducing agents, coke and graphite exhibit better microwave absorption than charcoal and carbon black. This finding has been confirmed by a study on microwave carbothermic reduction of a zinc oxide concentrate. ¹⁴⁰ When coke was used as the reducing agent, the reduction degree reached approximately 70% within 12 min using a 2·45-GHz, 1000-W domestic microwave oven. The reduction started from the sample core and propagated outward, demonstrating an opposite direction of heat conduction to that of conventional heating.

Aside from application to carbothermic reduction of zinc-bearing materials, use of microwave energy in zinc extraction via hydrometallurgical route was widely reported. When microwave leaching (2·45 GHz, 650 W) of sphalerite ore (−98+76 μm) was carried out in the solution of 1·0M FeCl₃ and 0·1M HCl at 95°C for 30 min, the extraction rate of Zn reached 59·3% opposed to 28·4% under conventional conditions. ¹⁴¹ With leaching of sphalerite ore in H₂SO₄ and Fe₂(SO₄)₃ solutions, the microwave leaching kinetics was much faster than that produced by conventional heating. The leaching rate of zinc by microwave irradiation reached 92%, whereas by conventional heating it only amounted to 41%. ¹⁴² To further understanding of microwave effects on leaching of sphalerite, the leaching kinetics of pure sphalerite crystals in acidic ferric chloride under conventional- and microwave-heated conditions was examined. ¹³⁹ As shown in Fig. 20, zinc recovery from microwave-heated sphalerite was higher than that from conventional-heated sphalerite under both stagnant and agitated conditions. ¹³⁹ In particular, the increase in zinc recovery after microwave-assisted leaching was more obvious under stagnant conditions as compared to that in conventionally heated systems. This phenomenon was partly because of ‘skin’ heating, which caused higher temperatures at the walls of the leaching vessel where the particles would be located because of centrifugal forces developed during stirring.

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20.

Effect of agitation on microwave and conventional leaching of sphalerite in ferric chloride (C_FeCl3: 0·5M, particle size: 38–53 μm, temperature: 91°C) ¹³⁹ (reproduced with permission of Elsevier). MW: microwave leaching; Conv.: conventional leaching

Microwave energy was also applied to the extraction of zinc from zinc silicate ore using sulfuric acid in a 2·45 GHz, 750 W microwave oven. ¹⁴³ After being irradiated for 15 min, the extraction rate of zinc was 99·08% and those of silica and iron were 0·30 and 0·10%. The results suggested that microwaves can be used to selectively eliminate silica and iron from the leaching solution.

Microwave leaching has been used for extraction of zinc and lead from a variety of wastes. Electric-arc furnace dust is a hazardous waste product containing large amounts of zinc and lead. By using microwave radiation at 2·45 GHz as the heating source for caustic leaching of EAF dust, the rates of zinc recovery were 5–10% higher under microwave conditions as compared to those observed with conventional leaching. ¹¹¹ This result was assumed to be because of the super heating of liquids, extremely violent behaviour and interaction of microwaves with the EAF dust in the solution. More zinc was dissolved than lead in the microwave leaching process, which was attributed to a difference in behaviour between zinc and lead compounds under microwave irradiation. Another similar study used a modified home microwave oven for reduction of EAF dust containing petroleum coke. The recovery of zinc and metallization reached 99·23 and 100% within 15 min of microwave irradiation with the addition of 20% carbon. ¹⁴⁴ The main phases in the dust consisted of ZnFeO₄ (franklinite) and ZnO (zincite). Besides EAF dust, BF sludge containing zinc is another type of waste that was processed using microwave energy. Microwave-assisted leaching of BF sludge resulted in a very rapid dissolution of the zinc-bearing phases in a 0·5M sulphuric acid solution. ¹⁴⁵ A maximum Zn recovery of 92% was achieved within 3 min at microwave power of 90 W, compared with 88% zinc recovery after 7 min of conventional leaching at 65°C.

Microwave pretreatment followed by leaching

Microwave pretreatment may significantly enhance the recovery of zinc from wastes. This possibility was demonstrated by leaching zinc using HCl solution from a microwave-pretreated spent catalyst mainly composed of activated carbon saturated with zinc acetate containing 8·76 wt-%Zn. ¹⁴⁶ The blocked pores of spent catalyst were unlocked by microwave pretreatment at 950°C for 12 min, increasing the contact area between leaching reagent and zinc source. It was primarily caused by different microwave absorption capabilities of activated carbon and zinc acetate. The pretreatment induced a temperature gradient in the catalyst, forming cracks in the contact face that assist in leaching.

Microwave pretreatment of titanium-bearing materials

Microwave heating is finding increasing use in ore grinding for liberation of valuable minerals. ¹⁵⁸ Microwave energy induces boundary fractures inside the ore because of differential expansion between minerals and matrix, facilitating the liberation of minerals. The selective heating characteristic of microwave energy has found uses in grinding of an ilmenite ore. ¹⁵⁹ It was shown that the ilmenite ore could be effectively heated to a temperature of 350°C in 1 min. The recovery of magnetite from the ore was increased to 72% for the ore irradiated with 3 kW microwaves for 30 s before water quenching. On the contrary, the recovery of magnetite from the untreated ore was only 44%.

Microwave reduction of titanium-bearing materials

Direct reduction of ilmenite usually requires a high reductive temperature or additives for improving its reactivity. Before thermal reduction, pre-oxidation of ilmenite is usually adopted in the processing of ilmenite ore for production of TiO₂ pigment and metallic titanium. Microwave energy was proposed as a heat source for partial reduction of oxidised ilmenite concentrates. ¹⁶⁰ The microwave-absorbing characteristics of mixtures of oxidised ilmenite and different carbonaceous reducing agents, including coconut-based activated carbon, coke and graphite, were assessed based on the measurement of microwave attenuation (dielectric loss factor) and frequency shift (dielectric constant) of the materials. The optimum ratios of coconut-based activated carbon, coke, and graphite to oxidised ilmenite were found to be 30, 30 and 10%, respectively. The ionic species of iron in ilmenite concentrates, between the ferrous and ferric states, significantly enhanced the chemical activity. With a certain reduction degree, iron can be preferentially extracted to yield a titanium-rich beneficiate suitable for use as a feed for production of titanium dioxide pigments via the chloride process. ¹⁶¹ Based on these results, the feasibility of pilot-scale production of titanium-rich (mainly TiO₂) materials by carbothermic reduction of ilmenite concentrates was examined. ¹⁶² Microwave power of 20 kW was used in the work for treating feed pellets of 20 kg with 14% reducing agent (coke) in the temperature range 1100–1150°C for 90 min. The metallisation rate of pellets was 92·1%, and the TiO₂ grade of the primary titanium-rich materials was 72·01%. The yield of primary titanium-rich materials reached 67·5%, and the recovery of TiO₂ approached 90·1%.

Apart from preparation of TiO₂, production of metallic titanium from various sources in the presence of microwave radiation has been widely reported. A recent study suggested that Ti could be recovered from vanadium titanomagnetite ores by microwave carbothermic reduction, followed by smelting in an EAF. ¹⁶³ Rapid reduction of vanadium titanomagnetite concentrate with coke (C_fixed/O_{bonded with Fe} = 1·2) was achieved because the heating rate reached 60°C min⁻¹ in the first 10 minutes for a 1 kg mixture of pellets irradiated by microwaves at a power level of 10 kW. It was evident that the extent of the reduction was largely dependent on the reducing agent used. However, it was also surprising that, in vacuum, microwave irradiation might not only act as a heat source but also promote the reduction of TiO₂ without any reducing agent. ¹⁶⁴ This feature was attributed to the fact that microwave irradiation caused field electron currents inside TiO₂ under vacuum, which then enabled the atomic oxygen plasma emission that resulted in the direct reduction of TiO₂. In fact, there are other advantages of microwave-vacuum reduction of titanium oxides. The vacuum reduction of titanium oxides under microwave irradiation at the maximum point of the magnetic field in a single-mode cavity showed a higher reduction degree of TiO_2−x and a lower activation energy for reduction compared with conventional heating. ¹⁶⁵ Furthermore, the morphologies of TiO_2−x after conventional heating and microwave magnetic field irradiation were distinct. Figure 21a shows that TiO_2−x was crystallised with round-shaped Laue spots after conventional heating. ¹⁶⁵ However, under microwave magnetic field irradiation it became decrystallised with some nano-domain crystalline structures, as indicated by the encircled areas in Fig. 21b . The Laue spots also appeared as concentric arcs. It was found that microwave energy could convert into not only thermal energy but also chemical and kinetic energies. In particular, the distribution of the kinetic energy was about 4·4 times greater than that of the thermal energy and the energy distributed as activation energy was 10 ³ times less than both the kinetic and thermal energies.

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21.

TEM images of TiO_2−x; a conventional heating and b microwave magnetic field irradiation ¹⁶⁵ (reproduced with permission of the Chemical Society of Japan)

Microwave pretreatment of gold-bearing materials followed by leaching

Microwave pretreatment is used to augment the grinding of complex gold ores before cyanide leaching. The pretreatment usually enhances access to the mineral of interest and percolation/migration of lixiviants by micro-crack formation within the host minerals. The formation of micro-cracks also allows size reduction and liberation of gold at a lower stress level because of thermal stress cracking associated with selective heating of the different mineral components under microwave irradiation. This strategy is expected to maximise recovery and man hours spent at processing plants. Recently, the effect of microwave pretreatment on the grindability and leachability of a free-milling gold ore containing quartz, silicates and iron oxides was examined. ¹⁷³ As shown in Fig. 22a , the crushing strength (average) of 50 ore particles in the size range between 0·25 and 3·4 mm was reduced by 31·2% after microwave pretreatment (2·45 GHz, 800 W). ¹⁷³ The presence of micro-cracks improved the extraction rate of gold, which exceeded 95% within 12 h opposed to 22 h for the non-microwaved sample. The results are presented in Fig. 22b .

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22.

Scatter plot showing a the crushing strength of 50 as-received and microwaved gold ore particles (heated to 735°C) and; b gold extractions from as-received and microwaved gold ores ¹⁷³ (reproduced with permission of Elsevier)

Extraction of gold from gold ores containing carbonaceous matter and sulphidic minerals (double refractory) usually requires an oxidation step to liberate the ultra-fine gold from the matrix of the sulphides and to remove carbon that pre-robs the dissolved gold. The use of microwave energy was found useful in roasting a double refractory flotation concentrate for removal of sulphidic and carbonaceous matters. ¹⁷⁴ The sample temperature increased with increasing incident microwave power, processing time and sample mass. As a result of hyperactive response of the concentrate to microwaves, a low incident power of 600 W was found to be suitable for roasting and higher power levels resulted in sintering and melting of the concentrate. Although the gold extractions exceeded 96% for both microwave roasting and conventional roasting, microwave roasting resulted in a higher total carbon removal rate, a greater heating rate and a lower specific energy consumption. In another study, microwave energy found applications in sulfur removal through the oxidative roasting (neutral atmosphere) of a refractory pyrite–arsenopyrite flotation concentrate, which gave a 99% gold recovery with a significant reduction in processing time and energy consumption. ¹⁷⁵

The performance of microwave roasting and conventional roasting (700°C with constant oxygenation) in recovery of gold from refractory gold ores has been compared recently. ¹⁶⁸ Both approaches were found to be able to transform sulphidic matrix minerals into oxides, allowing easier trapping of gold by leach liquors. After applying microwave energy, the roasting time was reduced by 77–83%. On average the consumption of electrical energy was decreased by about 88%, while the gold recovery obtained was about 90% of that achieved through traditional roasting. It was also found that too high temperature should be avoided because sintering of the load material had detrimental effects on the recovery stages (leaching or smelting). In addition, two measures were proposed to improve gold recovery: varying the applicator dimensions and applying microwave heating cycles. In principle, the former is sometimes necessary for maximising energy peaks, higher heating rates and more rational use of energy. This is because any load that enters the applicator will distort the electromagnetic field distribution due to the variation of the material dielectric constant. The latter has a positive effect on the recovery margin of gold ores because it is possible to roast more material without sintering.

For treatment of double refractory gold ores having relatively poor microwave absorption capabilities, the way the microwave energy is applied has impact on gold recovery. In a study on extracting gold from a double refractory gold ore containing 1·56 wt-% sulfur and 5·96 wt-% total carbon, direct microwave roasting, indirect microwave roasting (using magnetite as a susceptor) and conventional roasting were performed to remove carbonaceous matters. ¹⁷⁶ After direct microwave roasting, the room temperature permittivity of the gold ore decreased to extremely low values because of dehydration, a decrease in the inorganic carbonaceous matter and the conversion of pyrite (FeS₂) to haematite. Although the temperature of the gold ore increased with mass, time and microwave energy input, the decreased microwave absorption in the ore made oxidation of remaining organic carbon difficult. In contrast to direct microwave heating, effective removal of carbon was obtained for indirect microwaved samples with a high gold extraction, essentially because of the hyperactive microwave response of the susceptor. Figure 23 compares gold recoveries for conventional and indirect microwaved roasted samples after cyanide leaching. ¹⁷⁶ It is revealed that the recovery of gold from indirect microwaved samples could be completed much faster than that from conventional processed samples. To extract 98% of the gold, only 40 min of indirect microwave roasting was needed for the samples. For conventional roasting, however, the time must be extended to more than 20 h (26 h at 580°C and 24 h at 710°C).

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23.

Gold recoveries for conventional and indirect microwaved roasted samples after cyanide leaching (frequency: 2·45 GHz, power: 700 W) ¹⁷⁶ (reproduced with permission of Elsevier)

Gold recovery is expected to be improved by examining different process schemes with microwave pretreatment. For example, three process schemes were used for processing a gold concentrate in which finely dispersed gold inclusions were originally inaccessible by cyanide solution. They included (1) calcination at 360°C, (2) oxidative roasting at about 600°C, and (3) sintering with KOH at about 380°C. ¹⁷⁷ In all cases, the final operation was the recovery of gold from calcine by cyaniding. Calcination at 360°C in a microwave field gave higher values of the indices in comparison with the other two schemes. In oxidative roasting, the lower recovery of gold was a consequence of haematite formation. The poor recovery of gold when the concentrate was sintered with KOH was probably because of the formation of oxide films on the particle surface (possibly dense haematite and insoluble compounds) that hindered access of cyanide. Pyrite was identified as the main mineral contributing to heating of the gold-containing concentrate by selective absorption of microwave energy. The processing scheme consisting of calcination, followed by cyaniding, gave a 94·8% gold recovery from the concentrate.

Gold may be economically recovered from gold-bearing ores if the process does not require flotation and roasting steps. A promising process proposed in the early 1990s included steps to expose pre-crushed ore (up to 95% of the ore passing a 2 mm screen) to pulses of microwave energy of 1–30 s durations with intervals of 10–120 s between pulses for a total treatment time of up to 1 h to condition the ore. Then, gold was leached from the conditioned ore, which had been ground to a size of below 76 μm. ¹⁷⁸ It was thought that improvement in gold recovery of the crushed ore was not because of oxidation of sulphides to any significant extent but was caused by micro-cracking in the crushed ore as a result of exposure to pulsed microwave energy. This speculation was confirmed by an experimental investigation of gold recovery from an Ora Banda gold ore using pulsed microwave energy. When the crushed ore was treated with 1300 W microwave energy for 3 s every 30 s for 10 min, an increase of up to 62% in the amount of gold could be recovered from the crushed ore compared with the unprocessed ore. During the pretreatment, there was little change in composition except for a small amount of oxidation. Additionally, the entire process did not depend on the frequency of microwave energy.

Microwave recovery of gold from wastes

Microwave energy was utilised in gold ore processing plants to combust waste activated carbon containing very high gold values. ¹⁷⁹ The resulting ash was treated by conventional cyanide leaching to recover any gold. The dielectric property measurement of the carbon showed that it had good microwave absorptivity with extremely high permittivity values. The sample temperature increased with increasing incident microwave power, sample mass and decreasing particle size. A maximum sample temperature of 1010°C was attained in 5 min for a 3-g sample with a particle size of below 90 μm at an incident microwave power of 1000 W. By introducing air into the microwave combustion chamber, carbon could be completely oxidised, forming a residue of gold-containing ash that was treated by the cyanide leaching process for gold recovery. Over 95% gold extraction could be achieved within 8 h from the ash by leaching. Microwave combustion of gold-containing waste activated carbon is a novel and technically viable process for gold recovery.