Towards a Microwave Metal Extraction Process

Abstract

Microwave processing is a relatively new technology that has been successfully applied in a number of industries. However, in the field of metal extraction , there is a paucity of success, despite considerable hype in the early years. Increased reaction rates, lowered reaction temperatures and higher energy efficiencies were some of the advantages claimed. Even with over almost two decades of research and some pilot plant studies, microwave metal extraction remains essentially a laboratory technique. In this paper, the research performed over the last few decades will be reviewed and some reasons for the lack of success in developing microwave metal extraction processes will be discussed. Based on the knowledge gained from these experiences, a more informed view of microwave heating can lead to the realization that there is still some untapped potential for this heating technique in metal extraction processes.

Introduction

Pyrometallurgical processes utilize energy in the form of heat to extract metals from ores. Various energy sources can be employed to produce the necessary heat as shown in Fig. 1. These range from generating the energy in the material itself via in situ chemical reactions such as roasting, to utilizing radiation directly from the sun, as in a solar furnace . Conventional sources include the combustion of carbon and hydrocarbons and the utilization of electrical energy in various forms.

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Heating techniques in pyrometallurgy

The choice of the energy source is primarily determined by economic, but also increasingly, by environmental factors. The conventional heat transfer mechanisms in these processes differ, but mainly occur through convective, conductive and radiative processes. The degree of contribution of each mechanism is mainly determined by the type of heat source, the characteristics of the material being heated and both the material and the reactor geometries. It is generally accepted that these conventional energy sources provide heat in a manner, which is essentially independent of the characteristics of the material. Also the energy source does not directly affect the chemical reaction, except for perhaps in some cases, modifying the oxidation /reduction potential of the atmosphere.

More recently, various energy sources have been developed that generate heat directly within the material itself via an interaction with electromagnetic radiation . The heating efficiency of these processes is still governed to some extent by the properties of the energy source. However, in comparison to conventional processes, the heat developed depends to a much larger extent on the properties of the material. One example is induction heating, in which an alternating electromagnetic field at radio frequencies, induces eddy currents in a conductive material as well as hysteresis effects in magnetic materials. Another more recent example is microwave processing, where the interaction between the microwave radiation and the object strongly depends on the properties of the material.

Microwaves are a form of electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz and wavelengths in the range of 1 mm to 1 m. For a domestic microwave oven at a frequency of 2.45 GHz, the wavelength is 12 cm, while for industrial applications the frequency is 915 MHz and the wavelength is 33 cm. Although there are a wide range of materials that could be processed by microwaves , some materials are more conducive to microwave processing than others. Therefore, in order to determine if a particular material could potentially be treated in a microwave process it is necessary to understand the interaction between the material and the microwave radiation . However, not only does the sample interact with the field, but the field is also affected by the sample. Currently, the major commercial application of microwaves is in the food processing industry, in particular for drying. The recent industrial applications involve the processing of relatively homogeneous materials, which undergo comparatively simple reactions with only small changes in the chemical and physical properties of the material. Thus, process control is simplified. In pyrometallurgical processes, there are usually very significant changes in the chemical and physical properties of the sample, for example as a metal oxide is reduced to a metal or a metal sulphide is converted to a metal oxide or sulphate. Thus, the interaction is very complex and depends on both fundamental and empirical factors. Additionally, temperature measurement and thermal runaway are potential issues in a microwave process.

In this paper, firstly the advantages and disadvantages of microwave processing are reviewed. Secondly the important factors involved in a microwave process are discussed. Thirdly, the application of microwaves to pyrometallurgical processes is analysed. Finally, some conclusions are drawn with regards to the future application of microwaves to pyrometallurgical processes.

Advantages and Disadvantages of Microwave Heating

As with any heating technique, there are a number of advantages and disadvantages of microwave technology. The claimed advantages of microwave heating are shown in Table 1, where the advantages are divided into both thermal and non-thermal effects. Also included is an evaluation or critique of the effects. Thermal effects are the differences between conventional and microwave processing that have been attributed to the dissimilarities in the heating mechanisms. Consequently, these effects are ascribed to the temperature rise in the material due to the absorption of the microwave radiation . Non-thermal effects are the phenomena that are utilized to explain those results which supersede the thermal effects. It is purported that these arise as a result of the direct interaction and excitation of the individual molecules in the material by the microwave radiation . The division between thermal and non-thermal effects is not clearly defined and there remains considerable controversy over the validity of the non-thermal effects.

Table 1

Thermal and non-thermal advantages of microwave processing

Thermal	Advantage	Evaluation
Heating	Volumetric	Limited penetration
Heating rate	Rapid	Material dependent
Heat transfer	Improved	Material dependent
Efficiency	Higher	Hybridize
Equipment	Less	Limited
Energy	Clean	General
Selectivity	Lower energy	Material dependent
Control	Instantaneous	General

Non-thermal	Advantage	Evaluation
Reaction rates	Increased	Higher temps
Reaction temps	Lowered	Incorrect temps

The primary advantage of microwave heating is often referred to as volumetric heating, in that the heat energy is generated within the material itself, with very little energy being produced in the surroundings [1, 2]. Since all of the energy is deposited in situ , this can potentially give rise to high heating rates. In liquids where thermal diffusion can readily occur, the temperature can be relatively uniform. However, in solids this can be more difficult to achieve, due to a number of factors, particularly microwave penetration depth. In conventional processes, the heat is generated outside of the sample and is transferred by conductive, convective and radiative processes [3]. Thus, the surroundings are at a higher temperature than the sample, which can lead to overheating at the surface. Therefore, in microwave heating, the temperature gradient is inverted, to some degree, in comparison to conventional heating.

As a result of these differing heat transfer mechanisms, microwave processing would be expected to be more efficient than conventional heating. This should also lead to reduced equipment size requirements. However, the evaluation of the efficiency of microwave heating requires the inclusion of the efficiencies of all the energy transfer steps. For example, if one considers conventional heating using a hydrocarbon or carbonaceous-based in situ combustion process, then the efficiency of energy transfer from the source to the material would be about forty percent. In a microwave only process, the transfer of energy from the microwave source to the sample could be perhaps double at about eighty percent, due to the reduced heat transfer to the surroundings. Again, if one considers a hydrocarbon or carbonaceous combustion process as the initial energy source for the production of the electrical energy , then the efficiency of this process would be about forty percent. Furthermore, the conversion efficiency of the electrical energy into microwaves also has to be considered and this is typically about eighty percent. Taking these three energy conversion steps into account, the overall efficiency of the microwave process is about thirty percent [4]. Since the efficiency of the microwave process could be less than the conventional process, then for a microwave process to be utilized, the efficiency needs to be improved and/or there have to be additional motives for utilizing microwaves . Some examples of these reasons could be that the material cannot be processed or manufactured by any other technique.

The electrical energy in a microwave process can be generated from nuclear energy rather than hydrocarbon or carbonaceous combustion and therefore can be environmentally clean. Additionally, the microwaves can be applied to or withdrawn from the material almost instantaneously. Since the interaction of any given material with microwave radiation is related to its characteristic permittivities then the constituents of a mixture can heat at different rates [5–7]. Thus, it should be possible to preferentially heat a microwave hyperactive material in a non-active matrix. This effect has the potential to lower bulk temperatures and energy requirements. This is known as selective heating and is responsible for the phenomenon of differential thermal stress fracturing in minerals and also for selective reactions [8–10]. However, its application depends to a large degree on the physical and chemical properties of the materials in the mixture. In addition to the permittivities , important parameters here are the size of the particles and the thermal conductivities of the materials involved.

Non-thermal effects also known as the “microwave effect” is a term often used to describe the unusual phenomena observed in some of the microwave research, in particular some of the initial studies [11–15]. Non-thermal effects arise as a result of the direct interaction and excitation of the individual molecules in the material by the microwave radiation rather than thermal heating effects. These non-thermal effects remain controversial and it has even been argued that they are masked by the thermal effects [1]. For example, extremely high rates of reaction and/or significant decreases in the reaction temperatures have been reported under microwave irradiation [16]. In a number of studies, it was concluded that as a result of this microwave-material interaction, the thermodynamic requirements for the process were altered [17–20]. Some recent papers have explained non-thermal effects in terms of thermal effects but with more accurate temperature measurement [21, 22].

As with other heating techniques, there are also a number of disadvantages of using microwave radiation as an energy source, which are shown in Table 2. Firstly, the interaction between the electromagnetic field and the material is complex and depends upon numerous factors such as the moisture content, the chemical composition, the temperature , the sample geometry and the position in the field. Consequently, the behavior can be variable, but complete predictability may not be necessary to develop a process. Temperature uniformity and temperature measurement are two issues which need to be addressed in a microwave process [23]. Temperature uniformity depends upon both the characteristics of the material and also the electromagnetic field. Temperatures are only measured at a point and because of the non-uniformity, accurate knowledge of the temperature distribution in the sample will be difficult. To compensate for this, modelling techniques can be utilized to predict the temperature distribution in the material. For commercial applications, a frequency of 915 MHz is typically used [24] and here the largest magnetron is 100 kW, although multiple magnetrons can be employed. Only a few microwave installations have been operated and in comparison to conventional heating sources, the capital cost of a microwave installation would be much higher. Additionally, due to the complexity of microwave processing, the research and developments costs for the process would be expected to be significantly greater than for a conventional process.

Table 2

Disadvantages of microwave processing

Effect	Disadvantage	Evaluation
Field	Complex	Not critical
Temperature	Non-uniform	Levelling
Temperature	Inaccurate	Model
Magnetron	Up to 100 kW	Use multiple
Capital cost	High	Universal
R&D effort	Expensive	Universal

Factors Involved in a Microwave Process

There are a large number of factors in the development of a microwave process and these are shown in Fig. 2. These factors will be discussed in the following sections.

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Factors involved in the selection of a microwave heating process

Fundamental Factors

The fundamental properties that determine the interaction of the microwaves with the material are the permittivities (dielectric properties) and the magnetic properties (permeabilities). Most materials of interest are not magnetic and consequently the interaction is mainly determined by the permittivities . However, of particular interest in many pyrometallurgical processes is the formation of magnetite, which is a hyperactive microwave absorber because of its high permeabilities. High temperature measurement of the permittivities , and in some cases the permeabilities, are required since pyrometallurgical operating temperatures can exceed 1600 °C.

Accurate measurement of both of these properties at high temperatures and under controlled atmospheres, are required for a comprehensive understanding of the behavior of a given material under microwave radiation . Also, precise knowledge of these properties is necessary as they are used as input parameters for numerical models. These models can be employed to predict absorbed power and temperature distributions in the sample and in any surrounding materials. Generally, the permittivities and permeabilities need to be known both as a function of temperature and frequency. Although the frequency in any given microwave process may be fixed, measurement of the frequency dependency is necessary to characterize the relaxation processes in the material.

The most common methods for measuring these properties are the transmission line methods, the free space method and the cavity perturbation technique [25, 26]. Figure 3 shows the various factors involved in permittivity measurements. These techniques have severe temperature limitations for application to pyrometallurgical processes. Also at the temperatures involved in pyrometallurgical processes, the interactions between the sample and the container may become significant. If the material is a solid then measurements can be made on the bulk material, a powder or a compact. If there is liquid formation, then the sample needs to be in a non-reactive container. The choice of container may be limited by the requirements of the measurement technique itself. Additionally, for many pyrometallurgical reactions of interest, the atmosphere needs to be controlled and this is either not possible or is severely restricted.

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Factors involved in permittivity measurements

Many of the techniques use a very small sample size, which can lead to error if the material of interest is highly heterogeneous. Also, in particular, for the high temperature techniques, the capital and operating costs of the equipment are high and thus the measurements are expensive. Consequently, with the current measurement techniques, there are severe limitations for permittivity determinations. In summary, for pyrometallurgical applications there are issues with regards to maximum temperature , container reactivity, atmosphere control , sample size and cost of measurement.

Sample Factors

The interaction of a material with the electromagnetic field generated by microwaves is much more complex than in conventional heating processes. This interaction is multifaceted since not only does the sample interact with the field, but also the field is affected by the sample. For efficient heating, it is necessary that the electrical field couples with the sample such that the maximum amount of energy is transferred into the sample. In addition to the relative real and imaginary permittivities and their dependencies on temperature and frequency, the physical and chemical properties of the sample and its interaction with the radiation also have considerable influence on the coupling [3]. The relationship between the electromagnetic field and the sample is complex and dependent on a number of factors, as shown in Fig. 4. The important physical properties are mass [1], volume [3, 27], surface area [28], density [29] and sample geometry [1].

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Sample factors in microwave heating

Types of Cavities

The cavity is the device where the material of interest interacts with the electromagnetic field and is also called the applicator. The cavity is designed so as to maximize the efficiency of coupling between the material being processed and the electromagnetic field. There are two common types of cavities utilized in microwave processing: the single mode cavity and the multimode cavity [1]. In a single mode cavity, there is only one mode of the electromagnetic field and the electric field is simpler than in a multimode cavity. Here the sample and/or the reactor are situated within the waveguide itself. The field distribution within the cavity can be well-defined and the sample can be located where the maximum electric and/or magnetic field strengths are obtained. Consequently, the single mode cavity is designed for a specific application. However, in this type of system the sample size is limited to relatively small sizes.

In a multimode cavity, the electromagnetic field can exist in numerous different configurations. Thus, in comparison to a single mode cavity, the electric field is much more complex. This permits the processing of a wide variety of materials with different geometries and permittivities over an extensive range of locations in the cavity. Therefore, this technique is suitable for the treatment of bulk materials in either batch or continuous processes as would be experienced in extractive metallurgy . The typical cavity consists of a rectangular metal cuboid with inside surfaces that are highly reflective of microwave radiation . The dimensions of the cavity are related to the product dimensions and are significantly larger than the wavelength of the radiation . The major disadvantage of the multimode cavity is that uniform heating is difficult to achieve due to the time dependent variations in both the material and the process parameters [1]. Additionally, there is the potential for constructive interference of the various modes of the electromagnetic field within the sample, which can accentuate the problem. This can lead to the phenomenon known as thermal runaway [30, 31]. Consequently, numerous techniques have been devised to mitigate this effect in a multimode cavity.

Temperature Measurement

In most processes, accurate knowledge of the temperature is an important parameter. This is particularly significant in pyrometallurgy , where energy consumption is a major cost factor. For the temperatures encountered in microwave processing, there are two main sensor types: thermocouples and pyrometers. In microwave processing, temperature measurement is a substantive issue and inaccurate temperature measurements can lead to erroneous process efficiency calculations. These issues for thermocouples and pyrometers in a microwave system are summarized in Fig. 5. Both thermocouples and pyrometers are point temperature measurement techniques and thus accurate knowledge of the temperature in a sample with a temperature gradient has inherent limitations.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig5_HTML.gif — Fig. 5
Issues with thermocouples and pyrometers in a microwave system

For precise temperature measurement, the thermocouple must be in thermal equilibrium with the sample. The primary problem in a microwave system is that equilibrium is difficult to achieve since the immediate surroundings are at a different temperature than the sample. Thus the thermocouple must be in direct contact with the sample, which can react with the thermocouple or the sheath. Additionally, contact with the sample can be lost due to changes in the sample shape or movement of the sample. Furthermore, with microwave heating there is a temperature gradient in the sample and thus multiple measurements are required. The secondary problem in a microwave system is the interaction of the metallic thermocouple or the sheath with the electromagnetic field itself. This can not only affect the temperature reading but also the thermocouple can interfere with the electromagnetic field [23].

Optical pyrometry is the other temperature measuring technique utilized in pyrometallurgy and here once more there are a number of issues. Again with microwave heating, a temperature gradient is generated within the sample, with the outside of the sample being at a lower temperature than the interior. With a pyrometer it is only possible to measure the surface temperature . Additionally, pyrometer readings can be influenced by the much lower temperature above the surface of the sample [32]. Furthermore, accurate knowledge of the emissivity is required and this changes with temperature and frequency, surface morphology and variation in sample composition. Furthermore, readings are usually taken through a Pyrex or quartz window and the transmittance of the window needs to be taken into account. Moreover, gaseous species in the atmosphere or film formation on the window can affect the reading [32].

Non-Uniform Heating and Thermal Runaway

Stable and uniform heating is a common objective of most microwave heating processes. However, in a multimode cavity there a large number of variables, which fluctuate over time and any slight variation in these parameters, can result in non-uniform heating. Many materials of interest have both a low thermal conductivity and an imaginary permittivity that increases rapidly with temperature . These factors combined with the inherent inhomogeneity of most materials results in certain parts of the sample heating more quickly than others. This leads to localized overheating and can result in the phenomenon known as hot spots [9, 33, 34]. Consequently, during processing, the heat transfer mechanisms can change from microwave heating of the sample with some conduction, to microwave overheating at the hot spots and increased heat transfer due to conduction. This thermal instability makes temperature measurement even more unreliable and local overheating can potentially damage the sample.

In order to promote temperature uniformity in the sample, a number of temperature levelling techniques have been devised and are shown in Fig. 6. For uniform heating, the sample size should be less than the wavelength of the radiation . Perhaps, the most common is the movement of the sample in the field. Alternatively, the field can be modified using a mode stirrer, which smooths the power density distribution [12]. Field uniformity can also be promoted through the use of multiple magnetrons. Hybrid heating with conventional heating of the surface, to replace the losses, can result in more uniform heating [1, 3]. Pulsing or power cycling of the microwaves allows for heat dissipation from the hotter areas, which can improve temperature uniformity [35, 36]. Surface scanning can be utilized to direct more microwave power to areas of the sample at a lower temperature [32]. Increasing the number of modes by increasing the cavity size can promote more uniform heating. Finally and more recently, frequency variation is another technique which can be utilized to achieve temperature flattening [33].

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Methods to promote temperature uniformity in the sample

Typical Microwave System

A typical laboratory multimode microwave system is shown in Fig. 7. It consists of a power supply in which the power can be varied continuously from 0 to 2000 kW and the frequency is fixed at 2.45 GHz. The microwaves are generated in a magnetron and are transmitted down a waveguide to the multimode cavity where the sample is processed. The waveguide is rectangular in cross-section and is made from a metal with a high electrical conductivity, such that the microwaves are reflected and thus transmitted with a high efficiency . The cavity or the applicator is designed to contain both the sample and the electric field, such that after passing through the waveguide the electric field interacts with the sample, which is referred to as the load. The applicator is a rectangular box with reflective metallic walls and is considered to be a multimode cavity. In contrast to a single mode cavity, the multimode cavity is designed to maintain numerous high order modes of the electromagnetic field with a high strength. Typically, for pyrometallurgical applications, the sample is contained in a microwave transparent reactor and the atmosphere can be controlled. The waveguide contains a tuner, a directional coupler, a circulator and a dummy load. The tuner matches the load to the power supply so as to maximize the absorbed power and thus minimize the reflected power and this is known as impedance matching. The directional coupler separates the forward power from the reflected power such that they can be measured using two crystal detectors. The circulator acts as a one way valve, allowing the forward power to pass through, but the reflected power is absorbed and the energy is dissipated in the dummy water load. In order to promote temperature uniformity, the load can be rotated so as to homogenize the field in the sample or the standing wave patterns can be modified using a mode stirrer. Additionally, temperature measuring devices such as a pyrometer or a thermocouple could be added to the system.

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A typical multimode microwave system

Extractive Metallurgy Applications

A number of comprehensive reviews regarding the utilization of microwaves for the extraction and processing of metals have been given in the literature [11, 12, 36]. The areas covered ranged from applications in mineral processing [32, 33], to hydrometallurgical [10, 16] and pyrometallurgical processing [8, 9, 35], to the sintering of metals [2, 17, 18, 27]. In mineral processing , advantage is taken of the differing permittivities of the minerals to achieve selective heating. This can give rise to thermal stress fracturing, which can lower the energy requirements in comminution and this can also lead to improved metal recovery [37]. Additionally, the surfaces of minerals , particularly sulphides, can be modified and this can be used to advantage in flotation processes [38]. In hydrometallurgical processing, a large amount of microwave energy can be rapidly inputted to a relatively small amount of an aqueous-based solution. This high energy density leads to enhanced reaction rates and further improvements can be achieved by operating at high pressure. The microwave heating behavior of aqueous solutions is well-understood and occurs as a result of both the interaction of the microwaves with the polar water molecules and the phenomenon of ionic conduction [39].

In pyrometallurgy , both the permittivities and the mechanisms of the interaction of microwaves with both solids and liquids of interest are largely unknown. Consequently, the results are mainly empirical in nature. In general, the pyrometallurgical processing of these materials can be explained in terms of the well-established high temperature materials chemistry plus the thermal effects of microwave radiation . However, as discussed previously, steep temperature gradients and both the measurement and control of temperature are problematic. Additionally, there are usually very dramatic changes in the chemical and physical properties of the sample, for example as a metal oxide is reduced to a metal or as a metal sulphide is converted to a metal oxide or metal sulphate. This combination of both changing sample and field properties leads not only to complexity but also to unpredictability. Slight changes can lead to substantial experimental irreproducibility. As an example, Fig. 8 shows the absorbed microwave power as a function of time for six samples of similar compositions, which were compacted into a cylindrical briquette and then processed at 1100 W in a microwave system. It can be seen that each sample behaves differently. For all samples, the absorbed power initially decreases and then increases slowly, but is typically in the range of 60–70%. At longer times the absorbed power begins to increase rapidly but the time at which this rise occurs varies from sample to sample. Subsequently, strikingly dissimilar behaviours are observed for samples 1 and 4 in comparison to samples 2, 3, 5 and 6. For samples 1 and 4, after the rapid increase, the power is absorbed more slowly, reaches a maximum and then begins to decrease.

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Variation in absorbed microwave power as a function of time for six similar briquettes of 30 g of a dehydrated (150 °C) nickeliferous silicate laterite ore containing 6% charcoal processed at 1100 W

For samples 2, 3, 5 and 6, after the rapid increase, the power again is absorbed more slowly but both the maximum and subsequent decreases were not observed over the time period studied. The rapid increase in the rate is due to the increase in the permittivities of the raw materials and the production of magnetite, which is a hyperactive microwave absorber. However, as the magnetite is converted to wustite, the carbon is consumed and the rate of absorption decreases. If metallic iron is formed then an increased amount of the microwaves will be reflected. Thus, a maximum in the absorbed microwave power indicates that significant reaction has occurred. Clearly there is significant irreproducibility with some samples undergoing a greater reaction extent than others.

Various carbonaceous materials are used as reducing agents in pyrometallurgical processes and their behavior in the microwave field can vary widely, since their interaction with the field will depend on their chemical and physical properties. Additionally, various physical configurations of the carbonaceous material in the sample are possible and these are shown in Fig. 9. In each case, the heating behavior will be different. In one case, the carbonaceous material is added as layers in the sample (a) and in another, a cylinder of carbonaceous material is placed in the center of the sample (b). A large amount of carbon in the interior of the sample can promote internal heating. Yet another example shows the material distributed as a few relatively large particles (c) and another where there are a large number of small but interconnected particles (d). A small number of large particles can lead to localized heating. The volume fraction where the particles become interconnected is referred to as the percolation threshold. Here the permittivities are significantly different from those where there is matrix material between the particles. Additionally, the chemical composition and the physical properties of the carbonaceous material change on heating as it releases volatiles and carbonizes. Furthermore, in a pyrometallurgical process, where the carbonaceous material is a reducing agent, its particle size decreases as it reacts with the metal oxide. Consequently, the behavior of the carbonaceous material can change significantly during microwave processing.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig9_HTML.gif — Fig. 9
Some possible arrangements of carbon particles in the sample: a layered carbon, b centralized carbon, c large disconnected carbon particles, d and small but connected carbon particles

In pyrometallurgical processes, the raw materials typically consist of complex minerals to which various reagents are added. Chemical and physical changes can have significant impacts on the microwave absorption. Consequently, the permittivities of the materials being processed and their temperature and frequency dependencies are relevant. In a pyrometallurgical microwave process, a carbonaceous material is added as a reducing agent to a metal oxide-containing mineral. Figure 10 shows the effects of temperature and frequency on the permittivity of a typical oxide-carbon mixture. The carbon addition is the stoichiometric amount required to reduce the metal oxide(s) of interest in the mineral or concentrate. It can be seen that the relationship between these parameters is complex. At low temperatures the permittivities are low but increase with increasing temperature , reach a maximum during the reduction process but then decrease as the carbon is consumed and metal is produced. Thus, at high temperatures, the permittivities are again low. In general, the permittivities increase with decreasing frequency and this is more pronounced during the reduction process. Thus, the interaction of the microwaves with the sample changes during the process and the highest absorption would be expected during reduction .

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig10_HTML.gif — Fig. 10
Effects of temperature and frequency on the permittivities of a sample undergoing carbothermic reduction

The effects of processing time on the typical heating behaviours of the metal oxide-containing mineral, the carbonaceous reducing agent and a stoichiometric mixture are shown in Fig. 11. The metal oxide-containing mineral is a low loss material and consequently the temperature increases only slowly with time. A carbonaceous material is usually a low loss material at low temperatures, but as the temperature increases, carbonization occurs and the permittivities increase rapidly. When mixed with the low loss metal oxide-containing mineral, the carbonaceous material can act as a susceptor as the temperature increases. However, the carbon also acts as a reagent in the process and is consumed and thus its contribution to the permittivities eventually decreases.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig11_HTML.gif — Fig. 11
Temperature versus time characteristics of a typical carbon, a mineral and also a mixture of carbon plus a mineral

Additionally, reduction results in metallic particle formation. Thus, the interaction of the electric field with metals is of considerable interest. Generally, metals are excellent conductors, thus they reflect microwaves and the penetration or skin depth is very small, of the order of microns. Figure 12 shows the possible shapes of the metallic particles as the reduction process proceeds. At low reduction temperatures, the metallic particles are very fine and have an irregular shape. If the metal particles are very small, such as micron or submicron sized particles, where there is a large surface area and the particle size is close to the penetration depth, then microwave heating can occur. However, as the temperature increases, the particles spheroidize, agglomerate and grow and would be expected to become more reflective. Consequently, the permittivities would decrease. Eventually if the temperature is high enough then the metal and slag can form bulk phases and separate from each other. Again as for carbon, the shape and the amount of the metal particles will affect their interaction with the electric field and thus change the heating mechanisms, the heating behavior and the microwave absorption efficiency . Consequently, the absorption efficiency reaches a maximum during the reduction process, and the absorption will decrease as the amounts of metal oxide and carbon decrease and the amount of metal increases. An additional factor is the penetration depth into the sample itself.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig12_HTML.gif — Fig. 12
Possible arrangements of metal particles in the sample: a individual small irregular particles, b individual small spherical metal particles, c individual large spherical metal particles, d separation of metal from slag

Figure 13 shows the absorption efficiency of a mixture of a metal oxide-containing mineral plus a stoichiometric amount of carbon as well as a metal oxide-containing mineral with excess carbon for a constant power input. At low temperatures, the permittivities are low and the absorption is low. However, as the temperature increases, both the permittivities and the absorption increase due to the carbonization. Additionally, if some iron oxide is present then magnetite can form and because of its magnetic properties, this will also result in increased absorption. The absorption efficiency reaches a maximum and as the carbon and the magnetite are consumed then the absorption begins to decrease. Additionally, metallic particles are produced which are reflective and this again lowers the absorption.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig13_HTML.gif — Fig. 13
Absorption efficiency versus time for a sample with the stoichiometric carbon addition and another sample with excess carbon

Thus, it is seen that the microwave absorption efficiency is high during the reduction process, but low both prior to and after reduction . If there is excess carbon, which is not consumed, then the absorption efficiency can be higher throughout the process and also can remain high even after reduction . This facilitates enhanced melting and separation .

Figure 14 shows the top (a), side (b) and interior views (c) of a microwave reduced briquette of a nickeliferous silicate laterite ore. The sample has cracked and the interior temperature can be seen to be much higher than the exterior. The side view of the briquette shows very little reaction. However, the interior has significantly reacted and many beads of molten metal can be observed. These results clearly demonstrate the ability of microwaves to penetrate into the sample and develop considerable internal heating, with very little exterior heating.

../images/468727_1_En_84_Chapter/468727_1_En_84_Fig14_HTML.gif — Fig. 14
a Top view of reacted briquette showing internal heating, b Side view of reacted briquette showing limited reaction, c Interior view of reacted briquette showing reaction and metal droplets

Pilot Plant Studies

There have been only a limited number of semi-pilot or pilot plant studies on the use of microwaves in pyrometallurgical processes and these are summarized in Table 3. EMR microwave technologies developed a pilot scale process for the roasting of gold ores [40]. The refractory gold ores were roasted at 350–400 °C in a 2 ton per day fluidized bed. These roasting temperatures were much lower than in the conventional roasting process. High sulphur and high arsenopyrite concentrates were processed with gold recoveries of about 95%.

Table 3

Summary of pilot plant studies

Raw material	Procedure	Power/energy	Feed rate	Product	References
Gold ore	Roasting	13.9 kWh/kg	130 kg/h	Gold	[39, 43]
Magnetite ore	Reduction	20 kW	N/A	Iron	[40, 43]
Ilmenite concentrate	Reduction	20 kW	N/A	Reduced pellets	[41, 43]

It was reported that the sulphur could be recovered as elemental sulphur, rather than sulphur dioxide gas. Arsenic was also recovered. Additionally, it was claimed that the energy requirements were much lower than in the conventional roasting process. Commercialization was planned but did not materialize.

The largest scale microwave iron production process was rated at 20 kW [41]. One kilogram of magnetite ore was mixed with 18% graphite and processed for 35 min at 17.5 kW. The impurity content of the iron was lower than that from a blast furnace , except for sulphur. The impurities were lower as result of vapourization and the low oxygen potential. The higher sulphur content was due to the slag composition. A pilot plant operation has been developed for the carbothermic reduction of a titanium oxide concentrate [42]. The power level was 20 kW and 20 kg of an ilmenite concentrate were processed. The pellets contained 14% coke as a reducing agent and 5% (Na₂SO₄ + S + NaCl + Fe) and were reduced in the temperature range of 1100–1150 °C for 90 min. The metallization rate of the pellets was 92% and the reduced pellets contained about 53% TiO₂ .

Conclusions

(1)
In pyrometallurgical processes, microwaves offer numerous potential advantages such as volumetric heating, high heating rates and selective heating. In comparison to the conventional heating techniques utilized in pyrometallurgical processes, microwave processing is significantly more complicated. In conventional processes, the heat transfer is mainly dependent on convective, conductive and radiative processes. However, in microwave processing, the heating behavior is dependent not only on the changing properties of the material, but also on the changing field characteristics.
(2)
The fundamental properties involved in microwave processing are the permittivities for non-magnetic materials and the permeabilities for magnetic materials. In general, these are difficult to measure and are not known at high temperatures either for solids or liquids at gas compositions that simulate pyrometallurgical processes. Here very high temperatures up to 1600 °C may be reached and the gas compositions can be complicated, thus it is difficult to accurately determine both the permittivities and the permeabilities.
(3)
In the industrial microwave processes which have been developed, the reactions occur at relatively low temperatures and involve relatively simple changes such as dewatering of relatively homogeneous materials. In contrast, in pyrometallurgical processes, the material is naturally heterogeneous and there are very radical changes as a metal oxide is reduced or a metal sulphide is roasted. These reactions result in significant changes in both the microwave response of the material and the field, which can be difficult to reproduce experimentally and also to predict by modelling techniques.
(4)
In a pyrometallurgical process, where temperatures can reach 1600 °C, temperature measurement and control is important since the viability of the process depends on the energy efficiency . However, in microwave processes, both temperature measurement and control are difficult and thermal runaway can occur. There are a number of techniques that have been devised for temperature levelling but these have not yet been applied to high temperature pyrometallurgical systems.
(5)
A pyrometallurgical reduction process requires high temperatures for complete reaction. At low temperatures, the permittivities of the sample are low, even for samples containing carbon and thus the absorption is low. For a stoichiometric carbon addition, as the temperature increases, the absorption increases before reaching a maximum and then decreases again as the carbon is consumed and metal is formed. Thus, efficient microwave absorption occurs over only a limited range of intermediate conditions. However, the presence of excess carbon can facilitate a higher level of absorption efficiency even after the reduction process is complete.
(6)
As a result of the complexity of microwave processes, the research and development costs would be expected to be significantly higher than for a similar conventional process. Additionally, since very few large scale microwave systems have been developed, then the capital costs would be expected to be considerably higher than for a conventional process. The maximum power of a magnetron is only 100 kW and although larger scale installations can be achieved by using multiple magnetrons, these cannot accommodate the very high throughputs required in pyrometallurgical processes.

Acknowledgements

The author wishes to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for their support of this research. Also, John Forster is acknowledged for providing the data for Fig. 8 and the photographs of microwaved samples in Fig. 14.

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