**2.5. Microwave assisted pretreatment of refractory gold concentrate**

Gold is considered to be refractory when it cannot be easily recovered by alkaline cyanide leaching. The vast majority of refractory gold occurs in sulphidic minerals such as pyrite (FeS2), arsenopyrite (FeAsS) and pyrrhotite (FeS). Generally, refractory gold concentrate or ore is pretreated by roasting, O2-pressure leaching or bacterial leaching, to render it amenable to gold recovery by alkaline cyanide leaching (Haque, 1987a, b)51, 52. Because microwaves in general heat sulphidic minerals easily, it should be possible to pretreat sulphidic refractory gold concentrate by microwave energy. (Haque 1987a, b)51, 52 conducted laboratory-scale microwave pretreatment tests in air on a typical arsenopyritic refractory gold concentrate. More than 80% of As and S were volatilized as As2O3 and SO2, whereas iron was oxidized into hematite (Fe2O3) at 550℃ (uncorrected). Alkaline cyanide leaching of the calcine yielded 98% Au and 60% Ag extractions.

To avoid the formation of As2O3 and SO2 this author conducted microwave calcination tests on the concentrate in a nitrogen atmosphere in a sealed silica tube. The major products were FeS, arsenious sulphide (As2S3) and sulphur (S). In addition, this author conducted microwave heating tests on a mixture of this concentrate and (NaOH), No SO2 and As2O3 evolved during microwave heating of this mixture; instead water soluble products such as; Na3AsO4 , Na2SO4, FeSO4 were formed. The microwaved solids were leached with water at 75℃. After phase separation, the residue was leached with alkaline cyanide solution, and

yielded 99% Au and 79% Ag extractions. These results opened up a wide range of possibilities for investigation (Woodcock et al., 198953). Non typical refractory gold ore, such as carbonaceous gold ore, some goethite bearing gold tailings, etc., have also been successfully pretreated by microwave heating (Author's unpublished results). Currently, EMR Microwave Technology, Fredericton, N.B., Canada is conducting pilot-scale microwave pretreatment tests on various kinds of refractory gold concentrates, ores and tailing.

Al-Harahsheh et al., 200554, have investigated the leaching kinetics of chalcopyrite under the influence of microwave treatment. Comparison of the amount of copper recovered from chalcopyrite under conventional and microwave heat treatment show marginal, but consistent, improvements in copper recovery when using microwave treatment as opposed to conventional treatment. It was suggested that the increase in copper recovery with microwave leaching was due to localized higher temperatures around the outer shell of the leaching solution as a result of the high dielectric loss factor (and thus low penetration depth) of the solution, and also selective heating of the outer skin of the chalcopyrite particles due to the high conductivity of this material.

Amankwah et al., 200555, performed tests on samples of a gold ore containing quartz, silicates and iron oxides with a head grade of 6.4 g/t of gold, using 2 kW of power in a multimode cavity. It was seen that the microwave treatment resulted in a maximum reduction of 31.2% in crushing strength and a reduction of 18.5% in work index. SEM analysis clearly showed that microwave induced fractures were occurring in the ore, and an improvement of 12% in gold recovery from gravity separation tests showed that this resulted in the liberation of the gold at coarser particles sizes during comminution.

Nanthakumar et al., 200756, investigated microwave roasting of a double refractory gold ore 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 thermo gravimetric analysis (TGA). In addition, 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 behavior 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.

Amankwah et al., 200857, studied microwave roasting of a double refractory flotation concentrate to oxidize both the sulfides and the carbonaceous matter. The concentrate was characterized by thermo gravimetric and infrared analysis and the microwave absorption characteristics were quantified by determining the permittivities. The microwave heating behavior 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.

Ma et al., 200858, investigated removal of sulfur and arsenic from refractory flotation gold concentrates, bearing with 14.95% of As and 27.85% of S, by microwave roasting. Cooling patterns of the roasted products obviously affected the removal effects under oxygen-free roasting atmosphere. The highest removal occurred by crucible-uncapped cooling pattern, followed by the so-called half-open cooling pattern, and the crucible-capped cooling pattern attained the lowest removal. The mid pattern would be preferred because it could avoid spontaneous ignition compared to the crucible-uncapped cooling one. Roasting temperature showed obvious effect only above 450°C, increasing with the roasting temperature. However, desulfur was much more difficult than de-arsenic. Under oxygen-free roasting atmosphere, 95% of arsenic was removed when roasted for 40 minutes at 550°C, while the desulfur rate was only about 40%. Comparatively, the removal of sulfur dramatically reached above 90% in oxidizing atmosphere. Additionally the roasted products were analyzed by XRD.

#### **2.6. Dielectric properties of minerals in microwave fields**

92 The Development and Application of Microwave Heating

particles due to the high conductivity of this material.

tailing.

yielded 99% Au and 79% Ag extractions. These results opened up a wide range of possibilities for investigation (Woodcock et al., 198953). Non typical refractory gold ore, such as carbonaceous gold ore, some goethite bearing gold tailings, etc., have also been successfully pretreated by microwave heating (Author's unpublished results). Currently, EMR Microwave Technology, Fredericton, N.B., Canada is conducting pilot-scale microwave pretreatment tests on various kinds of refractory gold concentrates, ores and

Al-Harahsheh et al., 200554, have investigated the leaching kinetics of chalcopyrite under the influence of microwave treatment. Comparison of the amount of copper recovered from chalcopyrite under conventional and microwave heat treatment show marginal, but consistent, improvements in copper recovery when using microwave treatment as opposed to conventional treatment. It was suggested that the increase in copper recovery with microwave leaching was due to localized higher temperatures around the outer shell of the leaching solution as a result of the high dielectric loss factor (and thus low penetration depth) of the solution, and also selective heating of the outer skin of the chalcopyrite

Amankwah et al., 200555, performed tests on samples of a gold ore containing quartz, silicates and iron oxides with a head grade of 6.4 g/t of gold, using 2 kW of power in a multimode cavity. It was seen that the microwave treatment resulted in a maximum reduction of 31.2% in crushing strength and a reduction of 18.5% in work index. SEM analysis clearly showed that microwave induced fractures were occurring in the ore, and an improvement of 12% in gold recovery from gravity separation tests showed that this

Nanthakumar et al., 200756, investigated microwave roasting of a double refractory gold ore 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 thermo gravimetric analysis (TGA). In addition, 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 behavior 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.

Amankwah et al., 200857, studied microwave roasting of a double refractory flotation concentrate to oxidize both the sulfides and the carbonaceous matter. The concentrate was characterized by thermo gravimetric and infrared analysis and the microwave absorption characteristics were quantified by determining the permittivities. The microwave heating

resulted in the liberation of the gold at coarser particles sizes during comminution.

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. Pickles et al., 200459, investigated 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 ( ) and imaginary ( ) permittivities of the ore were measured at frequencies of 912 and 2460 MHz at temperatures up to about 1000 ℃ using the cavity perturbation technique and these results were related to the DTGA curves. Also, the loss tangent ( tan ) 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 ( ) and imaginary permittivities ( ) and the loss tangent ( tan ) increase with temperature and change as both the free and the combined moisture are removed. The permittivities ( and ) 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 ( and ). 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.

Cumbane et al., 200860, has been used a measurement system, comprising a circular cylindrical TM0n0 cavity and based on a perturbation technique, for the determination of dielectric properties of five powdered sulphide minerals, which were measured at frequencies of 615MHz, 1410 MHz and 2210 MHz. The complex permittivity was measured from ambient temperature to 650 °C. The dielectric properties of galena and sphalerite exhibit little variation with temperature up to 500 °C. The dielectric properties of pyrite, chalcocite and chalcopyrite, show significant variation with temperature. These are related to composition and phase transformations during heating and were demonstrated by thermo-gravimetric analysis.

### **2.7. Microwave assisted carbothermic reduction of metal oxide**

The vast majority of heavy metals oxides and carbon, as charcoal or coke, respond to microwave heating. Therefore, the microwave assisted carbothermic reduction of metaloxides is possible. If the metal oxide is low lousy (i.e., poor receptor to microwave energy) then added carbon plays the role of microwave heating accelerator. Various researchers have demonstrated that iron oxides (hematite Fe2 O3, magnetite Fe3 O4) mixed with carbon (charcoal or coke) could be reduced to metallic iron (Standish and Worner, 199161, Gomez and Aguilar, 199562).

To compare conventional and microwave reduction, Standish et al. (1990, 1991)63, <sup>61</sup> conducted reduction tests on identical sample mixtures of hematite ore fines, coke and lime powder. A sample of each mixture was heated in an electrically heated muffle furnace at 1000℃, and in a 2450 MHz microwave oven at a power of 1.3 kW. The sample temperature was measured with a thermocouple inserted in the sample and the test was terminated when the temperature reached 1000℃. The results showed the microwave heating rate was much higher than the conventional heating rate. Some phase changes were observed and these might have enhanced the heating rate. Standish et al. (1991, 1990)61, 63 concluded based on rational assumptions for capital and operating costs, that a microwave reduction process could save 15% to 50% over a conventional operation. Chunpeng et al., 199064, also conducted microwave assisted carbothermic reduction on titanomagnetite concentrate. A powdered sample of titanomagnetite concentrate mixed with lignite powder and CaCO3 was heated by microwave power of 500 W at 2450 3 MHz. These results, compared with those generated by conventional heating test, confirmed that the reduction rate of metal oxide by microwave heating was faster than by conventional heating. Beside the carbothermic reduction of iron oxides the researchers used microwaves to smelt rare earth magnet alloys, a high value product difficult to produce by conventional techniques. Although these alloys could be produced in a microwave furnace, the furnace needed design changes to eliminate the formation of gas plasma over the melt. Moreover, a suitable microwave transparent material was needed to contain the smelt at high temperature.

#### **2.8. Microwave assisted drying and anhydration**

94 The Development and Application of Microwave Heating

of hydroxyl units from the goethite structure.

thermo-gravimetric analysis.

and Aguilar, 199562).

reaction, where there was a maximum in the permittivities (

**2.7. Microwave assisted carbothermic reduction of metal oxide** 

these changes, which occur during the dehydroxylation reaction, are a result of the liberation

Cumbane et al., 200860, has been used a measurement system, comprising a circular cylindrical TM0n0 cavity and based on a perturbation technique, for the determination of dielectric properties of five powdered sulphide minerals, which were measured at frequencies of 615MHz, 1410 MHz and 2210 MHz. The complex permittivity was measured from ambient temperature to 650 °C. The dielectric properties of galena and sphalerite exhibit little variation with temperature up to 500 °C. The dielectric properties of pyrite, chalcocite and chalcopyrite, show significant variation with temperature. These are related to composition and phase transformations during heating and were demonstrated by

The vast majority of heavy metals oxides and carbon, as charcoal or coke, respond to microwave heating. Therefore, the microwave assisted carbothermic reduction of metaloxides is possible. If the metal oxide is low lousy (i.e., poor receptor to microwave energy) then added carbon plays the role of microwave heating accelerator. Various researchers have demonstrated that iron oxides (hematite Fe2 O3, magnetite Fe3 O4) mixed with carbon (charcoal or coke) could be reduced to metallic iron (Standish and Worner, 199161, Gomez

To compare conventional and microwave reduction, Standish et al. (1990, 1991)63, <sup>61</sup> conducted reduction tests on identical sample mixtures of hematite ore fines, coke and lime powder. A sample of each mixture was heated in an electrically heated muffle furnace at 1000℃, and in a 2450 MHz microwave oven at a power of 1.3 kW. The sample temperature was measured with a thermocouple inserted in the sample and the test was terminated when the temperature reached 1000℃. The results showed the microwave heating rate was much higher than the conventional heating rate. Some phase changes were observed and these might have enhanced the heating rate. Standish et al. (1991, 1990)61, 63 concluded based on rational assumptions for capital and operating costs, that a microwave reduction process could save 15% to 50% over a conventional operation. Chunpeng et al., 199064, also conducted microwave assisted carbothermic reduction on titanomagnetite concentrate. A powdered sample of titanomagnetite concentrate mixed with lignite powder and CaCO3 was heated by microwave power of 500 W at 2450 3 MHz. These results, compared with those generated by conventional heating test, confirmed that the reduction rate of metal oxide by microwave heating was faster than by conventional heating. Beside the carbothermic reduction of iron oxides the researchers used microwaves to smelt rare earth magnet alloys, a high value product difficult to produce by conventional techniques. Although these alloys could be produced in a microwave furnace, the furnace needed design changes to eliminate the formation of gas plasma over the melt. Moreover, a suitable microwave transparent material was needed to contain the smelt at high temperature.

 and 

). It is proposed that

Materials and products such as agricultural, chemical and food product, textile, paper, lumber and many more (Cook, 198665, Schiffmann, 198766, Doelling et al., 199267).Generally, drying refers to the removal of physically adsorbed solvent such as water, acid or high vapour pressure organic substance (e.g., alcohol, acetone, ether, halogenated hydrocarbons, aromatics, etc.). Anhydration refers to the removal of water chemically bound to a substance present intermolecularly as well as to the intramolecular elimination of water from hydroxy or carboxylic compounds. It was observed that the dielectric loss factor of a material to be dried often decreases with the loss of solvent (Schiffmann, 198768). Unpublished results indicate that microwave heating can remove water from hydrated magnesium chloride (MgCl2.7H2O) and convert goethite (O=Fe–OH) into hematite (Fe2O3), (Haque, 199868).

If both the solvent and the substance to be dried are transparent to microwaves (i.e., no heating by microwave energy), then a suitable microwave heat accelerator, such as carbon, magnetite or silicon carbide must be added to the system to heat the added material to volatilize the solvent. This heating concept may be applied in the removal of volatile contaminants from soil, or even ore material (George et al., 199469).

#### **2.9. Microwave assisted minerals leaching**

Analytical chemists have used microwave heating devices routinely for the dissolution of metals, minerals and various chemical products in chemical analysis (Matthes et al., 198370, Kingston and Jassie, 198571). As mentioned earlier, microwave heating is material specific, offers a faster heating rate and consequently a faster dissolution rate than conventional heating. In fact, the principle of the dissolution of analytical samples has been applied to the leaching of various minerals contained in an ore or concentrate sample. Kruesi and Frahm, 198272 and Kruesi,198673 conducted microwave assisted leaching of lateritic ores containing oxides of nickel, cobalt, and iron. The metals of these mineral components were converted into their chlorides by microwave heating (1200 W, 2450 MHz, N2 atmosphere) a mixture of the ore and ammonium chloride between 177℃ and 312℃ for 4–5 min, followed by water leaching at 80℃ for 30 min. Nickel and cobalt extractions were 70% and 85%, respectively, and are comparable with roasting at 300℃ in a conventional rotary kiln for 2 h. Similarly, copper ores or concentrates containing oxidic and/or sulphidic minerals were solubilized by microwave heating a mixture of the ore or concentrate and ferric or ferrous chloride between 350℃ and 700℃, followed by hot brine leaching. Copper extraction was 96% (Kruesi and Frahm, 198274). To study nickel extraction, Chunpeng et al., 199024, conducted dry way chloridization of pentlandite concentrate with ferric chloride by microwave heating (500 W, 2450 MHz) in a chlorine atmosphere for 8–23 min, followed by aqueous leaching at pH 2 for 30 min. The maximum nickel recovery (~99%) was obtained from the sample heated for 14–17 min.

Peng and Liu, 1992a74, 1992b75, applied microwave energy in the leaching of sphalerite with acidic ferric chloride (FeCl3 –HCl). Various leach parameters; such as 3 temperature, particle

size and ferric chloride concentration were studied. Test results demonstrated that the leaching rate of zinc increased with temperature in both microwave and conventional heating systems. They reported 90% zinc extraction when the leach conditions were at 0.1 M HCl, 1.0 M FeCl3, 60 min microwave heating at 95℃. Under similar conditions conventional leaching yielded only 50% zinc extraction. Weian, 199774, conducted microwave assisted acidic ferric chloride leaching of a copper sulphide concentrate. The principal copper minerals in this concentrate were chalcocite (Cu2 S) and chalcopyrite (CuFeS2). The leach slurry was heated directly by microwaves (700 W, 2450 MHz) for various lengths of time. Copper recovery reached 99% after 40–45 min of microwave heating whereas conventional heating required 2h to reach the same level of extraction. This author concluded that microwave assisted leaching provided a faster dissolution rate of copper and overcame the detrimental effect from elemental sulphur build-up on the mineral surface during the leaching of the copper sulphide concentrate.

In the recovery of copper from a chalcopyritic concentrate (30.1–30.3% copper) Antonucci and Correa,199575, conducted a sulfation reaction by microwave heating (2450 MHz for laboratory tests and 915 MHz for semi pilot scale tests) a paste-like mixture of the concentrate and sulphuric acid followed by water leaching at 60℃ and pH 1.6. Semi pilot scale tests were conducted in a 35 L capacity Teflon-lined cylindrical rotatory reactor. The whole setup was placed in a multimode applicator and microwaves (915 MHz) were applied. There was an opening on top of the reactor through which the charge (conc. + H2 SO4) was fed and which also served as the outlet for the recovery of gas and elemental sulphur. The test results indicated that higher copper extraction could be achieved at higher sulphuric acid dosage. These authors concluded that copper extraction>96% could be achieved by microwaving a mixture containing 1.80 kg acid/kg conc. The process gave elemental sulphur and cupric sulphate. Antonucci and Correa, 199579, also commented that although this process demanded more energy than the conventional smelting process the production of elemental sulphur was advantageous.

#### **2.10. Microwave assisted spent carbon regeneration**

Currently, more and more gold ore processing industries are using activated carbon in CIP (carbon in pulp) or CIL (carbon in leach) operation. The carbon is regenerated after each cycle of adsorption and desorption of gold cyanocomplex. Usually, this spent carbon is regenerated by washing with a mineral acid followed by heating at high temperature (600℃ to 750℃) in an externally heated rotary kiln (Avraamides et al., 198776).

Haque et al., (199177, 199378), conducted laboratory scale carbon regeneration tests by microwave (2450 MHz) heating and confirmed the feasibility of spent carbon regeneration by microwave heating. Subsequent pilot scale carbon regeneration tests data (915 MHz) demonstrated that microwave regenerated carbon performed well or better than conventionally regenerated carbon (Bradshaw et al., 199777). Currently, Ontario Hydro, Toronto, Ontario, Canada is marketing this technology.

#### **2.11. Microwave assisted waste management**

96 The Development and Application of Microwave Heating

leaching of the copper sulphide concentrate.

production of elemental sulphur was advantageous.

**2.10. Microwave assisted spent carbon regeneration** 

Toronto, Ontario, Canada is marketing this technology.

to 750℃) in an externally heated rotary kiln (Avraamides et al., 198776).

size and ferric chloride concentration were studied. Test results demonstrated that the leaching rate of zinc increased with temperature in both microwave and conventional heating systems. They reported 90% zinc extraction when the leach conditions were at 0.1 M HCl, 1.0 M FeCl3, 60 min microwave heating at 95℃. Under similar conditions conventional leaching yielded only 50% zinc extraction. Weian, 199774, conducted microwave assisted acidic ferric chloride leaching of a copper sulphide concentrate. The principal copper minerals in this concentrate were chalcocite (Cu2 S) and chalcopyrite (CuFeS2). The leach slurry was heated directly by microwaves (700 W, 2450 MHz) for various lengths of time. Copper recovery reached 99% after 40–45 min of microwave heating whereas conventional heating required 2h to reach the same level of extraction. This author concluded that microwave assisted leaching provided a faster dissolution rate of copper and overcame the detrimental effect from elemental sulphur build-up on the mineral surface during the

In the recovery of copper from a chalcopyritic concentrate (30.1–30.3% copper) Antonucci and Correa,199575, conducted a sulfation reaction by microwave heating (2450 MHz for laboratory tests and 915 MHz for semi pilot scale tests) a paste-like mixture of the concentrate and sulphuric acid followed by water leaching at 60℃ and pH 1.6. Semi pilot scale tests were conducted in a 35 L capacity Teflon-lined cylindrical rotatory reactor. The whole setup was placed in a multimode applicator and microwaves (915 MHz) were applied. There was an opening on top of the reactor through which the charge (conc. + H2 SO4) was fed and which also served as the outlet for the recovery of gas and elemental sulphur. The test results indicated that higher copper extraction could be achieved at higher sulphuric acid dosage. These authors concluded that copper extraction>96% could be achieved by microwaving a mixture containing 1.80 kg acid/kg conc. The process gave elemental sulphur and cupric sulphate. Antonucci and Correa, 199579, also commented that although this process demanded more energy than the conventional smelting process the

Currently, more and more gold ore processing industries are using activated carbon in CIP (carbon in pulp) or CIL (carbon in leach) operation. The carbon is regenerated after each cycle of adsorption and desorption of gold cyanocomplex. Usually, this spent carbon is regenerated by washing with a mineral acid followed by heating at high temperature (600℃

Haque et al., (199177, 199378), conducted laboratory scale carbon regeneration tests by microwave (2450 MHz) heating and confirmed the feasibility of spent carbon regeneration by microwave heating. Subsequent pilot scale carbon regeneration tests data (915 MHz) demonstrated that microwave regenerated carbon performed well or better than conventionally regenerated carbon (Bradshaw et al., 199777). Currently, Ontario Hydro, Processing industries invariably generate waste material; mine-milling industries are no exception. To mitigate the danger presented by the constituents of the waste technologies are being investigated to minimize the waste generated and to provide safe handling, transportation, storage, destruction, removal or disposal of the hazardous waste. Currently, microwave energy is showing considerable potential in the management of a vast array of gaseous, liquid and solid wastes (Wicks et al., 199578). Mine milling operations generate large volumes of solid waste with acid generation potential, liquid waste containing acid, toxic heavy metals and non-metals, cyanide, ammonia, organics etc., and gaseous wastes such as, sulphur dioxide (SO2), hydrogen sulphide (H2S), ammonia (NH3), oxides of nitrogen (NOx).

Cha (1993) demonstrated in laboratory scale tests that SO2 and NOx in industrial off-gas can be decomposed into elemental nitrogen and sulphur, and a mixture of carbon dioxide and carbon monoxide. The first step of the process involves passing the off-gas stream containing SO2 and/or NOx through a column packed with activated carbon to adsorb the toxic gases. The loaded carbon column is then heated by microwaves and the resulting CO, CO2 and N2 are released into atmosphere. Sulphur is cooled in a spray chamber and collected for sale.

H2S is a very toxic gas produced during refining crude petroleum. Generally, hydrogen sulphide waste gas streams are treated by the Claus process, which is based on partial oxidation of hydrogen sulphide into sulphur and water. The Claus oxidation process requires a suitable oxidant mixture. The Kurchatov Institute in Moscow, Russia, developed a process for the decomposition of hydrogen sulphide into hydrogen and sulphur by applying a microwave plasma (plasmatron). The Argon National Laboratory (ANL) of the USA developed a 'plasma-chemical waste treatment process' in which a hydrogen sulphide waste stream is passed through a microwave-generated plasma reactor where it decomposes into hydrogen and sulphur. ANL test results indicated that this decomposition ranged from 65% to 80% per single pass. Preliminary energy and economic analysis data suggest that the plasma-chemical waste treatment process has the potential for annual energy savings of 40 to 70 trillion Btu or \$500 to \$1000 million for the refining industries. Further details of the process are available from ANL (Harkness, 199479).

Steel making furnaces generate metallic dust (Electric arc furnaces ,EAF), which use galvanized scrap metals, generate dust which often contains water leachable lead (Pb), cadmium (Cd), chromium (Cr) and zinc (Zn). This kind of dust is classified hazardous and needs to be treated prior to disposal. Currently, combined EAF dust production in Canada and the USA is 677,000 tons per year (Ionescu et al., 199780). A current dust treatment process becomes economical if the treatment scale is 40,000 tons/year or above (Xia and Pickles, 199616).

A large number of EAF mills are mini-mill type operations which need a treatment process that is a small scale, on-site and economic. Ghoreshy and Pickles, 199481, Chose microwave

energy (900 W, 2450 MHz) for the heating a typical EAF dust mixed with powdered carbon for various length of time. Over 90% zinc was volatilized as ZnO (zinc oxide), which was condensed and collected on an alumina plate placed on top of the reaction crucible. The laboratory scale test results demonstrated that zinc removal was rapid and selective. The iron rich residue can be recycled in an iron or steel making furnace. Steel making slag usually contains 20 wt. % iron. To modify the physical characteristics of and to recover iron from the slag. Hatton and Pickles, 199482, Conducted laboratory scale microwave heating tests (1000 W, 2450 MHz). The heating behavior of the steel making slag was investigated with and without the addition of carbon or magnetite. Test results demonstrated that both carbon and magnetite addition increased the heating rate of the slag; 1000℃ with carbon, 800℃ with magnetite, compared to 650℃ without any addition. The amount of iron recovered increased with heating time and reached as high as 90%. Microwave heating altered the physical and chemical properties of the slag.

#### **2.12. Latest developments in microwave processing of minerals ores**

The mechanical size reduction of solids is an energy intensive and highly inefficient process. Therefore, there is great incentive to improve the efficiency of size reduction and mineral separation processes. Over several decades, this has promoted significant amounts of research, unfortunately, this has only led to small, incremental improvements in efficiency. One area, which has shown significant promise for improving the efficiency of mineral comminution and separation processes, is microwave assisted grinding.

Until recently, the majority of test work carried out concerning microwave treatment of minerals utilized standard multi-mode cavities, similar to that found in a conventional kitchen microwave oven. The multimode cavity whilst mechanically simple suffers from poor efficiencies and low electric field strengths, vital to high power adsorption. Whilst the influence of microwave energy from this type of cavity has been shown to have a significant influence on ores and minerals, the inefficiencies of the application method have led to conclusions that at present, microwave treatment of minerals (despite the numerous process benefits) is not viable.

More recent studies have presented studies describing the influence of high electric field strength microwave energy on minerals and ores. It is well known that microwave power density in a material (or volumetric power absorption) is directly proportional to the square of the electric field strength within the material. Therefore, it was shown that if local electric field strength can be magnified energy adsorption or heating rate can be amplified many times without the use of further energy. In turn, this lead to reduced cavity residence times and reductions in the required microwave energy input per ton. Detailed tests at the University of Nottingham have shown that in cavities with high electric field strengths the microwave energy consumption to achieve a desired reduction in strength can be as little as 2% of that required in previous work.

Investigations have been carried out on several economically important ores utilizing a high electric field strength cavity for microwave treatment. A systematic approach was used in order to establish the influence of applied power level and exposure duration on each ore sample. Assessment was made of the influence of particle size on heating rate (this will have an effect on the delivery and presentation methods). During sample treatment, assessment was made of electrical energy consumption, the efficiencies of the system being calculated by standard methods. To support the test programme, results of numerical finite difference simulations are presented which illustrate the importance of microwave and ore variables on post treatment ore properties. The results of the simulations showed that if the microwave energy can be supplied to the sample very rapidly (in order of microseconds) then thermal conduction from the heated phase into the bulk ore can be minimised and thermally induced stress is maximised.

In order to validate the simulation predictions a series of experiments are reported which utilise a pulsed microwave energy delivery system on several ore types. Samples were exposed in a multimode cavity connected to a high voltage solid state modulator and pulse generator. The experimental set up was able to deliver pulses at applied power levels of 1- 5MW and pulse durations of 1-4μs at frequency of 2.8GHz.

Scanning electron microscopy and image analysis were used to map the pattern of induced fracture across the pulsed treated samples. It was shown that peak power level was the major influence on the degree of fracture with the highest powers giving greater effects. It was also shown that the fracture induced was predominately grain boundary related and the fractures did not seem to run into each other causing weakening of the bulk ore structure as found in samples treated in continuous wave microwave systems. Treated and untreated samples were then processed by appropriate separation techniques to determine if more valuable mineral could be recovered as a result of treatment. It is shown that pulsed treatment positively influences the recovery of valuable minerals from the different ore types investigated.
