Microwave Heating Behavior Research Articles

Microwave roasting of a carbonaceous sulphidic gold concentrate

Gold ores containing carbonaceous matter and sulphidic minerals are generally referred to as double refractory. Extraction of the gold from such ores usually requires an oxidation step to both liberate the ultra-fine gold from the matrix of the sulphides and remove the carbon that preg-robs the dissolved gold. In this investigation, a double refractory flotation concentrate was microwave roasted to oxidize both the sulphides and the carbonaceous matter. The concentrate was characterized by thermogravimetric and infrared analysis and the microwave absorbtion characteristics were quantified by determining the permittivities. The microwave heating behaviour studies showed that the sample temperature increased with increasing incident microwave power, processing time and sample mass. Due to the hyperactive response of the concentrate to the microwaves, a low incident power of 600 W was found to be suitable for roasting, as higher powers resulted in sintering and melting of the concentrate. The gold extraction values after cyanidation were over 96% and these were similar to those obtained by conventional roasting. The main advantages of microwave roasting were that both the total carbon removal rates and the heating rates were higher and the specific energy consumptions were lower than in conventional roasting.

Microwave heating behavior of nanocrystalline Au thin films in single-mode cavity

In this work, microwave heating curves and microstructural evolution of nanocrystalline Au thin films were studied so the heating behavior of metals could be better understood. Films with different thickness were irradiated by the H and E maximums of microwave in a single-mode cavity. These samples were more efficiently heated by H field than by E field. With the increase of film thickness, the attained highest temperature increased. This phenomenon was compared with the theory of the transmission line analysis. Peaks appearing at the initial stage of the heating curves were also compared with similar heating behaviors observed in metal powder compacts, and the possible mechanisms related to the heating behavior were explored.

Microwave heating of soda-lime glass by addition of iron powder

Experimental studies were conducted to investigate the microwave (MW) heating behavior of soda-lime glass beads with added iron powder. These studies were intended to obtain fundamental knowledge for vitrification solidification and for the fabrication of metal-reinforced glass-matrix composites. The glass beads (0.2 mm diameter) did not heat very well by themselves at temperatures greater than 200 °C within 600 s in a multimode applicator at a power of 0.67 W. The addition of iron powder (average 70 μm, volume fraction 18%) made it possible to heat the glass beads above 700 °C within 60 s. At lower fractions of 3–11 vol%, however, a sudden temperature rise [thermal runaway (TRW)] occurred after the incubation time period. A single-mode MW applicator was used for clarifying the electric (E)-field and magnetic (H)-field contributions to the heating of each material and their mixtures. The results of this study demonstrated that the H-field contributed to the heating of the iron and then triggered the heating of the glass. The E-field component is necessary for heating the glass to a temperature higher than 800 °C. The factors determining the threshold values of the volume fraction causing TRW are discussed.

Microwave pretreatment of a double refractory gold ore

Double refractory gold ores, such as Barrick’s Goldstrike ore, contain both pyrite and carbonaceous matter and the latter results in the loss of gold from solution during cyanide leaching. One possible remedy is to roast the ore and thus oxidize the pyrite and remove the organic carbon. In this research, microwave roasting of a double refractory gold ore was investigated as an alternative method and the results were compared to those obtained by conventional roasting. The compositional changes of the ore during roasting were determined by thermogravimetric analysis (TGA). Also, both the real and the imaginary permittivities, which determine the amount of energy absorbed by the ore and the heating rate of the ore respectively, were evaluated. In addition, the microwave heating behaviour was studied. Conventional and both direct and indirect microwave roasting tests were performed and in all the cases, the pyrite was readily converted into hematite. Direct microwave roasting could not remove the organic carbon. Indirect microwave roasting was conducted using magnetite as a susceptor and preg-robbing was eliminated when about 94% of the organic carbon was removed. For both conventional and indirect microwave roasting, gold recoveries of over about 98% were achieved after cyanide leaching. For microwave roasting, both the total carbon removal rates and the heating rates were higher and the specific energy consumptions were lower than the corresponding values for conventional roasting.

Heating of Metal Particles in a Single-Mode Microwave Applicator

Microwave (MW) heating behavior of various metal particles was investigated using a single-mode applicator. Considering the distributions of the electromagnetic fields in the wave guide, specimens were placed at four specific positions with respect to the electric and the magnetic fields of MW. They were heated at conditions of constant power input.It was demonstrated that iron particles were heated well in the magnetic field, and that ferro-magnetic metal particles having the higher Curie point was heated the better. It was possible to heat iron bulk particles (∼3 mm) in a magnetic field without occurrence of electric discharge. In the range of nickel particle size between 45 and 150 μm, the particles with the smaller size were heated the better.Nickel oxide (NiO) was heated well only in the position of large electric field, which indicates that the heating was caused by the different (dielectric heating) mechanism from the metal particles.From these results, contribution of magnetic field to heating metal particles was discussed, considering the heating mechanisms of the magnetic loss and the eddy current loss. The dependence of the heating rate of metal particles on their size was discussed in terms of the heat transfer rate.

Dry comminution and liberation with microwave assistance

This paper presents the experimental results obtained from microwave‐assisted comminution/liberation tests on various minerals. The effects of microwave energy intensity and exposure time on microwave heating behaviour and grind‐ability of materials are discussed. The experimental results indicated that particle size has a significant effect. The coarser particles (−9.50 + 4.75 mm) of some minerals (limestone and quartz) were affected by microwave heating to varying degrees, resulting in an increased fineness of the ground product in a subsequent dry ball milling. It was observed that the silicate and carbonate minerals are difficult to heat at lower microwave energy intensities (<7 kW) during a short exposure (<30 min) due to their principally transparent or reflection to microwave energy. The thermal stress fractures in a copper ore in microwave exposure occurred readily, resulting in a better and cleaner liberation of sulphide minerals from the ore matrix.

Microwave Heating Behavior of Blast Furnace Slag Bearing High Titanium

Blast furnace (BF) slag in south-western part of China contains large amount of titanium. For the purpose of beneficial utilization of titanium in the slag, microwave (MW) processing of the slag has been proposed. In this study, a fundamental research on heating behavior of the slag by application of 2.45 GHz MW was performed.The slag specimens can be heated in a domestic MW oven at constant input power (0.5 kW) and sudden increase of the temperature is observed after several heating cycles. A phenomenon of so-called thermal runaway (TRW) occurred. A part of the specimen melted, and the XRD analysis indicated that this area became amorphous. Crystallization of the CaTiO3 phase occurred by MW heating of the melted slag at 600°C.A sintered CaTiO3 compact was heated by MW much more than the synthetic Chinese slag. The synthetic slag without containing Ti was not heated very well. Permittivity of CaTiO3 (loss factor) was measured and demonstrated to be much larger than the other oxides in the slag. Therefore, it is concluded that CaTiO3 phase is responsible for heating of the whole slag.

Gold recovery by microwave augmented ashing of waste activated carbon

Gold ore processing plants that utilize carbon adsorption technologies generate a waste activated carbon, which contains very high gold values, and the recovery of this gold represents a significant source of extra revenue. In this research, microwave energy was utilized to combust the waste activated carbon and the resulting ash was treated by conventional cyanide leaching to recover the gold. Firstly, the real and the imaginary permittivities of the activated carbon were measured by the cavity perturbation technique and it was found that they were extremely high and thus the carbon would be expected to be a good microwave absorber. Secondly, the microwave heating behaviour of the activated carbon was studied as a function of incident microwave power, processing time, sample mass and particle size. The results showed that the sample temperature increased with increasing incident microwave power, processing time, sample mass and decreasing particle size. For a 3 g sample with a particle size of −90 μm, temperatures of over 1000 °C could be attained at an incident microwave power of 1000 W and for 5 min processing time. Finally, the effects of air flowrate, microwave power and processing time on the combustion of the activated carbon were studied. By introducing air into the microwave combustion chamber, the carbon could be completely oxidized and the residue was a gold-containing ash, which was treated for gold recovery by the conventional cyanide leaching process. Over 95% gold extraction could be achieved within 8 h. The results of this research demonstrate that the microwave combustion of gold-containing waste activated carbon is a technically viable and novel process for gold recovery.

Microwave heating behaviour of nickeliferous limonitic laterite ores

The nickeliferous laterite ores, in which the nickel occurs in oxide form, represent a significant potential resource of metallic nickel. However, in comparison to the nickel-containing sulfide ores, the extraction costs are relatively high and thus it will be necessary to develop new processing techniques, which are both technically and economically viable. In the present research, the potential application of microwaves for the heating of a nickeliferous limonitic laterite ore ((Fe,Ni)O(OH) · nH2O) was investigated. Firstly, since the nickeliferous limonitic laterite ore contains considerable moisture, both free and combined, then thermogravimetric analysis (TGA) was performed in order to characterize the changes, which result from the dehydration processes. Derivative thermogravimetric analysis (DTGA) curves were calculated from the TGA data. Secondly, the real (ε′) and imaginary (ε″) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 °C using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent (tanδ=ε″/ε′) was calculated from the permittivity data. Finally, the microwave heating behaviour of the nickeliferous limonitic laterite ore was determined at 2460 MHz. The results show that the both the real (ε′) and imaginary permittivities (ε″) and the loss tangent (tanδ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities (ε′ and ε″) increased with increasing slope of the TGA curve and vice versa during the goethite to hematite dehydroxylation reaction, where there was a maximum in the permittivities (ε′ and ε″). It is proposed that these changes, which occur during the dehydroxylation reaction, are a result of the liberation of hydroxyl units from the goethite structure. Furthermore, some hydroxyl units were retained in the hematite structure (i.e. hydrohematite (α-Fe2−x/3O3−xOHx)) even after the goethite to hematite transition and thus the permittivities of the dehydroxylated ore were higher than that of normal hematite. With regards, to their microwave heating behaviour, it is shown that despite the relatively low permittivities (ε′ and ε″) of these materials at low temperatures, they can be heated using microwaves. The microwave heating process can be improved by conventional preheating of the sample or by the addition of charcoal as a susceptor or by selecting a crucible material, which acts as a susceptor. There is a rapid increase in the permittivities over the temperature range of about 600–800 °C and this combined with the low thermal conductivities of these oxides, can result in rapid internal heating of the sample and thermal runaway.

Microwave heating behaviour of nickeliferous limonitic laterite ores

The nickeliferous laterite ores, in which the nickel occurs in oxide form, represent a significant potential resource of metallic nickel. However, in comparison to the nickel-containing sulfide ores, the extraction costs are relatively high and thus it will be necessary to develop new processing techniques, which are both technically and economically viable. In the present research, the potential application of microwaves for the heating of a nickeliferous limonitic laterite ore ((Fe,Ni)O(OH) · nH 2O) was investigated. Firstly, since the nickeliferous limonitic laterite ore contains considerable moisture, both free and combined, then thermogravimetric analysis (TGA) was performed in order to characterize the changes, which result from the dehydration processes. Derivative thermogravimetric analysis (DTGA) curves were calculated from the TGA data. Secondly, the real ( ε ′) and imaginary ( ε ″) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 °C using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent (tan δ= ε ″/ ε ′) was calculated from the permittivity data. Finally, the microwave heating behaviour of the nickeliferous limonitic laterite ore was determined at 2460 MHz. The results show that the both the real ( ε ′) and imaginary permittivities ( ε ″) and the loss tangent (tan δ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities ( ε ′ and ε ″) increased with increasing slope of the TGA curve and vice versa during the goethite to hematite dehydroxylation reaction, where there was a maximum in the permittivities ( ε ′ and ε ″). It is proposed that these changes, which occur during the dehydroxylation reaction, are a result of the liberation of hydroxyl units from the goethite structure. Furthermore, some hydroxyl units were retained in the hematite structure (i.e. hydrohematite (α-Fe 2− x/3 O 3− x OH x )) even after the goethite to hematite transition and thus the permittivities of the dehydroxylated ore were higher than that of normal hematite. With regards, to their microwave heating behaviour, it is shown that despite the relatively low permittivities ( ε ′ and ε ″) of these materials at low temperatures, they can be heated using microwaves. The microwave heating process can be improved by conventional preheating of the sample or by the addition of charcoal as a susceptor or by selecting a crucible material, which acts as a susceptor. There is a rapid increase in the permittivities over the temperature range of about 600–800 °C and this combined with the low thermal conductivities of these oxides, can result in rapid internal heating of the sample and thermal runaway.

Pilot Scale Continuous Microwave Dry-Media Reactor – Part 1: Design and Modeling

This work presents a new pilot plant continuous microwave dry-media reactor (CMDR) for industrial chemical applications. The CMDR consists of a 6 kW conveyor microwave oven with a subsequent hot air holding section. This microwave reactor has been designed for dry media or solvent-free reactions and can treat through-put in the range of 10–100 kg/h. The microwave heating behavior on the small scale is analyzed and the results are used to estimate the electromagnetic field requirements on the large scale. The temperature and the electric field distribution in the reactor are modeled and experimentally validated. In the second part of this study, a “waxy” esterification reaction was investigated with the CMDR. The reaction time needed for 95% yield was reduced by a factor of 20–30 compared to conventional industrial reactors. This was due to the more homogeneous heat transfer of microwaves, which allows a higher bulk temperature to be reached.

Pilot Scale Continuous Microwave Dry-Media Reactor - Part 1: Design and Modeling

This work presents a new pilot plant continuous microwave dry-media reactor (CMDR) for industrial chemical applications. The CMDR consists of a 6 kW conveyor microwave oven with a subsequent hot air holding section. This microwave reactor has been designed for dry media or solvent-free reactions and can treat through-put in the range of 10–100 kg/h. The microwave heating behavior on the small scale is analyzed and the results are used to estimate the electromagnetic field requirements on the large scale. The temperature and the electric field distribution in the reactor are modeled and experimentally validated. In the second part of this study, a “waxy” esterification reaction was investigated with the CMDR. The reaction time needed for 95% yield was reduced by a factor of 20–30 compared to conventional industrial reactors. This was due to the more homogeneous heat transfer of microwaves, which allows a higher bulk temperature to be reached.

Formation of Reaction‐Bonded Silicon Nitride Using Microwave Heating

Rod‐shaped silicon compacts were nitrided in a microwave applicator using a minimum of insulation in order to maximize the temperature gradients to effect an inside‐out reaction. These specimens exhibited very nonuniform conversion to Si3N4, and fully nitrided areas had poor microstructures consisting of alternating regions of high and low density. The key factor was found to be sintering of the silicon powder during initial heating which caused the specimens to be electrically conducting during the early stages of the reaction and led to large changes in the microwave heating behavior of the specimens as they nitrided. The temperature and composition profiles in the specimens were simulated using a numerical model. The results of the model indicated that the observed microstructures were caused by high temperatures and temperature gradients in the areas of maximum nitridation rate which caused silicon vapor to diffuse within the specimens. Some compacts were made from a mixture of silicon powder and silicon nitride powder to avoid sintering of the silicon particles. These specimens nitrided uniformly with inside‐out composition profiles, indicating that microwave heating would be beneficial for the nitridation of pure silicon powder compacts if sintering could be avoided.

Modeling Microwave Heating Characteristics of Granular Hygroscopic Solids

A model that predicts the microwave heating behavior of granular hygroscopic solids with low spatial dimension-topenetration depth ratio was derived. The model consists of an energy transport equation based on a macroscopic application of the law of energy conservation, the series solution of Fick’s diffusion equation, and a modified Gallaher’s equation. The model gave a good prediction of the microwave heating characteristics of white beans. A sensitivity analysis of the model indicates that the microwave heating characteristics of a moist hygroscopic dielectric is sensitive to initial moisture content, size and heat transfer coefficient.

Dielectric Properties of Selected Tungsten Compounds from 60 MHz to 9.44 GHz

Crystalline powders of tungsten compounds ammonium paratungstate, ammonium metatungstate, tungsten trioxide hydrate (WO3 · H2O), and tungsten blue oxide have been studied. The complex permittivity of the pulverized materials has been measured at room temperature from 60 MHz to 9.44 GHz. Intrinsic permittivity values of the crystalline solids have been derived from the apparent dielectric data of the powder material. Heating rate plots of powder samples of about 30 g at 2.45 GHz and 850 W radiation have also been taken. The slopes of the heating rate plots and the estimated dielectric loss factors of the crystalline solids are correlated. The experimentally observed microwave heating behavior and decomposition properties under microwave irradiation have been explained.

Experimental Observations of Thermal Spikes in Microwave Processing of Ceramic Oxide Fibers

ABSTRACTMicrowave heating of alumina/silica fiber tows in a single-mode microwave cavity at 2.45 GHz has produced a surprising thermal spike behavior on the fiber bundles. During a thermal spike, a “hot spot” on the tow brightens rapidly, persists for a few seconds, and then rapidly extinguishes. A hot spot can encompass the entire tow in the cavity or just a localized portion of the tow. Some local hot spots propagate along the fiber. Thermal spikes are triggered by relatively small (<15%) increases in power, thus having obvious implications for the development of practical microwave fiber processing systems. A tow can be heated through several successive thermal spikes, after which the tow is left substantially cooler than it was originally, although the applied microwave electric field is much larger. X-ray diffraction studies show that after each temperature spike there is a partial phase transformation of the tow material into mullite. After several excursions the tow has been largely transformed to the new, less lossy phase and is more difficult to heat. Heating experiments with Nextel 550 tows are examined for a plausible explanation of this microwave heating behavior.

Microwave Sintering of Metal-Ceramic and Ceramic-Ceramic Composites

ABSTRACTPolyphase ceramic and metal ceramic composites with at least one high dielectric loss component are potential candidates for application of microwave sintering, because of the high effectiveness of power dissipation and short sintering times. In order to identify material parameters essential for effective microwave sintering, different mixtures of conductive, semiconductive and insulating phases were investigated. When only a limited amount of a highly dissipative component is present, a modification of the grain size and morphology of the dissipative phase has been identified as major factor leading to optimized microwave heating behavior. Surprisingly, microwave sintering is also possible in the presence of a metallic liquid phase.

The Microwave Heating Behaviour of Metallic-Insulator Composite Systes

ABSTRACTThe microwave heating behaviour of composite systems is of considerable interest for theprocessing of ceramics or the creation of materials with interesting dielectric properties. In this investigation, the heating response of different mixtures of metallic and insulating powders is described. The heating rate as a function of metal type, mass fraction and powder types will be presented for pressed pellet samples.From the results, three distinct heating regimes have been identified. At low temperatures. induced electric currents cause the metal particles to heat. As the temperature increases, exothermic oxidation of the metal powder particles increases the heating rate. Eventually, the insulating powder (alumina) begins to absorb microwave energy more efficiently and this dominates the high temperature behaviour. The relative importance of thesethree effects for different temperatures depends on the particular powders which are used.