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Thermally assisted liberation - a review
M#1erals Eng#leer#ig, Vol. 3, No. I/2. pp. 181 185, 1990 Printed in Great Britain
0892-6875/90 $3.00 + 0.00 Pergamon Press plc
T H E R M A L L Y ASSISTED L I B E R A T I O N - A REVIEW
K. E. FITZGIBBON and T. J. VEASEY School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B I5 2TT, England
ABSTRACT Thermally assisted liberation ( T A L ) has been suggested as a possible method for improving recovery of minerals from appropriate ores. In some cases heat treatment has been shown to reduce grinding resistance by significant amounts and has improved liberation by promotion o f intergranular fracture rather than transgranular fracture. Unfortunately, in most studies reported to date, the process is not economically viable unless the downstream recovery o f valuable mineral species can be increased to offset the extra costs involved in heating. I f cheaper, and more efficient, methods o f heating can be employed the process might become profitable. The current paper reviews the work carried out on T A L and surveys current research in the area. Keywords Thermally assisted liberation (TAL); comminution; liberation INTRODUCTION Thermally assisted liberation is the process of heating and quenching to aid rock failure with the prime objective of promoting intergranular fracture to enhance liberation of selected mineral compounds. The process, of course, is not new as the ancients used "fire setting" to fracture rock before explosives became available. In fact, the use of heat treatment as an aid in comminution has attracted sporadic interest over many years and there are many examples of studies of the topic in the technical literature. Such studies range from practical studies of particular mineral deposits, for example the early work of Holman [1] and Yates [2] in the 1920's concerned with Cornish tin ores and quartzites, to more detailed studies of theoretical models of stresses induced by thermal processing, exemplified by the work of Brown et all. [3] and Geller et al. [4] in the 1950's and 1970's. More recently there has been a renewed interest in the topic and there is likely to be a continued research effort in view of developments in related areas of comminution methods or more selective heating methods. The earlier work is briefly reviewed here and the current and future work on TAL is discussed. 181
182
K . E . FITZGIBBON and T. J. VEASEY
T H E O R E T I C A L STUDIES OF T.A.L. Geller [5] modelled the stresses induced in rock samples when heat was applied by considering the simplified case of convective heat transfer at the surface of perfectly elastic and isotropic spheres. The assumption of sphericity was justified since, of the basic shapes, spheres are most resistant to thermal shock and therefore if quenching conditions were severe enough to cause failure in spheres, then other shapes will also fail. The relationship he derived can be used to predict numerical solutions to problems including determination of preheat temperature, maximum induced stress, minimum particle size and heat transfer rate. It is difficult to evaluate whether or not a crack would be sufficiently deep to cause fracture and Geller suggested that additional information was necessary before physical parameters could be accurately determined. Geller and Tervo [4] discussed the relationship between dimensionless maximum stress and dimensionless heat transfer rate (the Biot number) and used this to illustrate the effects of preheat temperature, quench severity and particle size on induced stress. The studies indicated that higher preheat temperatures induced larger stresses, greater quench severity induced higher stress and higher probability of failure and that the greater the particle size the greater the failure probability. STUDIES OF H E A T T R E A T M E N T ON G R I N D I N G RESISTANCE, LIBERATION AND SIZE DISTRIBUTION Brown et al. [3] discussed intergranular comminution by heating and in their introduction pointed out that if the object of size reduction is to liberate the valuable mineral component of an ore then, ideally, fracture should follow intergranular paths to the exclusion of transgranular ones. The result would be liberation with minimum size reduction, i.e. ideal comminution, and cited the earlier work by Brown and Gaudin [6] to determine the extent to which this could be achieved by heat treatment. Holman [1 ], Yates [2] and Meyers [7] have shown that heating certain rocks prior to crushing them did improve comminution characteristics in that fewer fines were produced and that grinding resistance was lowered. Specimen size and shape were also studied by Holman [1] and Myers [7] and these authors found that the effectiveness of heat treatment decreased with smaller particle sizes. Djingheuzian [8] studied the influence of temperature on efficiency in grinding and produced similar results to show that grinding efficiency was a function of particle size. An extensive study by Geller and Tervo [4] also investigated the influence of particle size on heat treatment and the results confirmed earlier work and substantiated Geller's theoretical model. Temperature and heating rates have been controlled in these studies, for example Holman [1] recommended rapid heat treatments for thermal weakening of quartz and Brown and Gaudin [3] found rapid heating to be effective for treatment of marble. A very useful study was made by Kanellopoulos and Ball [9] to determine tensile strengths of quartzite after heat treatment at different temperatures. These authors' results were in general agreement with the Geller model. Hariharan and Venkatachalam [10] also looked at the influence of heat treatment on quartz grindability and showed that the distribution modulus increased with increased temperature whilst the size modulus of ground quartz decreased. In general, the workers who studied quartz or quartzites concluded that the minimum Bond work index occurred after heating and quenching from temperatures above the quartz o~-B inversion temperature, and several studies have attributed the weakening of the rock quartz matrix to anisotropic expansion during the transition, e.g. Drost and Mahan [11 ] Kanellopoulos and Ball [9] and Hariharan
Thermally assisted liberation--a review
and Venkatachalam possible when high immersed in water. boundaries at high appropriate sites.
183
[10]. Jones and Fullard [12] found that almost ideal liberation was carbonate rocks were heated, cooled to room temperature and then This was attributed to evolution of carbon dioxide along the grain temperatures, creating internal pressure and exerting stresses at
RECENT AND CURRENT STUDIES OF T.A.L. Scheding et al [13] initiated work to study the effect of thermal pre-treatment on grinding characteristics using a typical lode tin ore consisting of cassiterite principally locked with quartz, chlorite, tourmaline and hematite. They reported on the effect of heating to 650°C followed by quenching, i.e. exploiting the a=13 quartz inversion and accompanying volume change. They found a significant decrease in the Bond work index (reduced by a factor of 0.45 after heating) and decreased fines production. An economic evaluation showed that the cost of heat treatment would be larger than that of conventional grinding along (by a factor of 6), but that enhanced liberation could lead to better overall grade/recovery results. The work was extended to cover the use of a dye-penetration technique for image analysis of the fracture network and of the effect of TAL on the mineralogy of the materials [14]. The main conclusions were that (i)
heating to 650°C followed by quenching would reduce grinding energy requirements by 55%,
(ii)
at the time it was estimated that only a 1.0% improvement in tin recovery was needed to offset heat energy costs and
(iii)
there was evidence of intergranular fracture between quartz and cassiterite. It was further suggested that enhanced liberation might be better achieved by alternative milling methods such as autogeneous or pebble mills instead of rod or ball mills.
Another example of current TAL research was reported by Veasey [15]. This programme targetted a low grade tungsten/tin ore which was expected to respond favourably. Thermal pretreatment could be expected to induce weakening at grain boundaries, particularly between quartz and wolframite, and it was anticipated that feeding pretreated ore to an established mill circuit would increase mill capacity, reduce mill wear and enable better control of product size. Energy considerations, again, were not favourable unless factors other than heat input were considered. Such factors include improved liberation, reduction of slimes (a high loss factor in wolframite processing) and beneficial alteration of properties of constituent minerals to enable higher recoveries. The study showed that grindability resistance was significantly reduced, liberation of wolframite was improved but detrimental side effects were produced if flotation or magnetic separation were employed. The latter effect has been studied in detail and reported elsewhere [ 16]. Although this study did not conclusively establish that TAL was appropriate for the target ore a further assessment has been made and the most recent phase of investigation has looked at gravity concentration by spiral separation [17]. The results of this study have indicated that TAL reduced the work index of the ore by 18%, reduced fines production during grinding, improved liberation and value distribution and showed that recoveries could be improved by up to 5%. There is still a need to scale up the work on other downstream process effects and then a full economic analysis can be made. An example of the successful application of TAL comes from the field of secondary metal recovery. Increased amounts of copper/plastic scrap are being presented as feed for the low grade fraction of material treated in the secondary copper blast furnace. The plastics content
184
K . E . FITZGIBBON and T. J. VEASEY
can cause both economic and environmental problems which affect the smelting operation. T A L at relatively low temperature (250°C) has been used to pretreat such materials so that low cost comminution can be employed to liberate metal and provide an improved feedstock [181. F U T U R E D E V E L O P M E N T OF T A L Current studies have shown that T A L can be effective for certain types of target ores and that further research is needed to fully evaluate the process. One area that deserves particular attention is optimisation of heat transfer to maximise the effect of matrix weakening to induce improved intergranular fracture. In some cases the heating stage may be made more efficient by using techniques such as fluidized bed roasters but there may be limitations due to particle size distributions of the feed materials. A particularly attractive alternative could be the use of microwave radiation to produce heating effects [15]. Microwave energy transmits heat to the centre and surface of a material simultaneously i.e. independence of thermal conductivity on heating rate is overcome and the time required to heat certain materials can be shortened. Chen et al.[19] investigated the relative transparency of forty minerals to microwave radiation and found that two types of mineral could be classified: (i)
those in which little or no heat was generated and material properties were unaffected because of transparency or total reflectivity to the microwaves and
(ii)
those in which heat was generated. They observed that ore minerals respond favourably to microwave heating whereas gangue minerals do not.
Walkiewicz et al. [20] carried out further tests of mineral receptivity to microwave heating and commented on extractive applications. In particular, stress fracturing at mineral grain boundaries in a gangue matrix was demonstrated and they concluded that this would significantly influence grindability and liberation. This work is complemented by extensive studies of dielectric properties of low-loss minerals undertaken by the USBM [21] which will enable targetting of ores for microwave TAL. In conclusion, evaluation of the economics of T A L still need to be extended and all the factors involved in downstream processing must be quantified to assess the suitability of such pre-treatment for any given system. T A L could become viable on a commercial scale if comminution techniques directing energy to grain boundaries can be developed to augment the induced weakening of the boundaries [22]. REFERENCES .
Holman, B.W. Heat treatment as an agent in rock breaking, Trans I M M 26 219 (1926-27).
.
Yates, A. Effect of heating and quenching Cornish tin ores before crushing, Trans I M M 28 41 (1918-19).
.
Brown, J.H., Gaudin, A.H. & Loeb, C.M. Intergranular comminution by heating, Mining Eng 10 490 (1958). Geller, L.B. & Tervo, R.O. Grinding of preheated rocks, Trans. I M M (Sec C) 84 25 (1975).
.
.
Geller, L.B. Thermal stresses in spheres - a basis for studying the grinding of preheated rocks, Int. J Rock Mech. Min. Sci. 9 213 (1972).
Thermally assisted liberation--a review
.
.
185
Brown, J.H. & Gaudin, A.H. Mechanisms of intergranular comminution by heating, Trans. AIME 217 423 (1960). Myers, W.M. Calcining as an aid to grinding, J.Am.Ceram.Soc. $ 839 (1925).
8.
Djingheuzian, L.E. The influence of temperature on efficiency of grinding, Trans. CIM L V H 157 (1954).
.
Kanellopoulos, A. & Ball, A. The fracture and thermal weakening of quartize in relation to comminution, J.S.A.I.M.M. 76 42 (1975).
10.
Hariharan, K. & Venkatachalam, S. Influence of thermal treatment upon grindability of quartz, Min. Mag. 105 (Feb. 1977).
11.
Drost, J.J. & Mahan, W.M. Effects of thermal treatments upon concentratability of a non-magnetic iron ore, USBM RI7797 (1973).
12.
Jones, M.P. & Fullard, R.J. Mineral liberation by thermal decomposition of a carbonate rock, Trans. IMM (Sec C) 75 127 (1966).
13.
Scheding, W.M., Sherring, A.J., Binns, D., Parker, R.H. & Wills, B.A. The effect of thermal pretreatment on grinding characteristics, Camborne School o f Mines Journal, 81 43 (1981).
14.
Wills, B.A., Parker, R.H. & Binns, D.G. Thermally assisted liberation of cassiterite, Minerals and Metallurgical Processing, 86 94 (May 1987).
15.
Veasey, T.J. Thermally assisted liberation of non-sulphide ores, 1st Int. Mineral Proc. Symp., Izmir, Turkey, 2 557 (1986).
16.
Blackburn, S. & Veasey, T.J. Effect of heat treatment in air at 820-960°C on magnetic properties of wolframite, Trans IMM 93 C55 (1984).
17.
Azizli, K. Operating characteristics of spiral separator, Ph.D. Thesis, University of Birmingham (1989).
18.
Garner, F.A., Rowson, N.A. & Veasey, T.J. Thermally assisted liberation for reclamation of copper from copper/plastic scrap, Internal Report, University of Birmingham (1987).
19.
Chen, T.T., Dutrizac, J.E., Hague, K.E., Wyslovzil, W. & Kashyap, S. The relative transparency of minerals to microwave radiation, Canadian Metallurgical Quarterly, 23 (3) 349 (1984).
20.
Walkiewicz, J.W., Kazonich, G. & McGill, S.L. Microwave heating characteristics of selected minerals and compounds. Minerals & Metallurgical Processing, 39 (Feb. 1988).
21.
Church, R.H., Webb, W.E. & Salsman, J.B. Dielectric properties of low-loss minerals, USBM RI9194 (1988).
22.
Wills, B.A. & Hall, S.T. New methods for reducing losses of fine cassiterite, Mineral Processing in the United Kingdom, (ed. P.A. Dowd), IMM, London, 143-147 (1989)
0892-6875/90 $3.00 + 0.00 Pergamon Press plc
T H E R M A L L Y ASSISTED L I B E R A T I O N - A REVIEW
K. E. FITZGIBBON and T. J. VEASEY School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B I5 2TT, England
ABSTRACT Thermally assisted liberation ( T A L ) has been suggested as a possible method for improving recovery of minerals from appropriate ores. In some cases heat treatment has been shown to reduce grinding resistance by significant amounts and has improved liberation by promotion o f intergranular fracture rather than transgranular fracture. Unfortunately, in most studies reported to date, the process is not economically viable unless the downstream recovery o f valuable mineral species can be increased to offset the extra costs involved in heating. I f cheaper, and more efficient, methods o f heating can be employed the process might become profitable. The current paper reviews the work carried out on T A L and surveys current research in the area. Keywords Thermally assisted liberation (TAL); comminution; liberation INTRODUCTION Thermally assisted liberation is the process of heating and quenching to aid rock failure with the prime objective of promoting intergranular fracture to enhance liberation of selected mineral compounds. The process, of course, is not new as the ancients used "fire setting" to fracture rock before explosives became available. In fact, the use of heat treatment as an aid in comminution has attracted sporadic interest over many years and there are many examples of studies of the topic in the technical literature. Such studies range from practical studies of particular mineral deposits, for example the early work of Holman [1] and Yates [2] in the 1920's concerned with Cornish tin ores and quartzites, to more detailed studies of theoretical models of stresses induced by thermal processing, exemplified by the work of Brown et all. [3] and Geller et al. [4] in the 1950's and 1970's. More recently there has been a renewed interest in the topic and there is likely to be a continued research effort in view of developments in related areas of comminution methods or more selective heating methods. The earlier work is briefly reviewed here and the current and future work on TAL is discussed. 181
182
K . E . FITZGIBBON and T. J. VEASEY
T H E O R E T I C A L STUDIES OF T.A.L. Geller [5] modelled the stresses induced in rock samples when heat was applied by considering the simplified case of convective heat transfer at the surface of perfectly elastic and isotropic spheres. The assumption of sphericity was justified since, of the basic shapes, spheres are most resistant to thermal shock and therefore if quenching conditions were severe enough to cause failure in spheres, then other shapes will also fail. The relationship he derived can be used to predict numerical solutions to problems including determination of preheat temperature, maximum induced stress, minimum particle size and heat transfer rate. It is difficult to evaluate whether or not a crack would be sufficiently deep to cause fracture and Geller suggested that additional information was necessary before physical parameters could be accurately determined. Geller and Tervo [4] discussed the relationship between dimensionless maximum stress and dimensionless heat transfer rate (the Biot number) and used this to illustrate the effects of preheat temperature, quench severity and particle size on induced stress. The studies indicated that higher preheat temperatures induced larger stresses, greater quench severity induced higher stress and higher probability of failure and that the greater the particle size the greater the failure probability. STUDIES OF H E A T T R E A T M E N T ON G R I N D I N G RESISTANCE, LIBERATION AND SIZE DISTRIBUTION Brown et al. [3] discussed intergranular comminution by heating and in their introduction pointed out that if the object of size reduction is to liberate the valuable mineral component of an ore then, ideally, fracture should follow intergranular paths to the exclusion of transgranular ones. The result would be liberation with minimum size reduction, i.e. ideal comminution, and cited the earlier work by Brown and Gaudin [6] to determine the extent to which this could be achieved by heat treatment. Holman [1 ], Yates [2] and Meyers [7] have shown that heating certain rocks prior to crushing them did improve comminution characteristics in that fewer fines were produced and that grinding resistance was lowered. Specimen size and shape were also studied by Holman [1] and Myers [7] and these authors found that the effectiveness of heat treatment decreased with smaller particle sizes. Djingheuzian [8] studied the influence of temperature on efficiency in grinding and produced similar results to show that grinding efficiency was a function of particle size. An extensive study by Geller and Tervo [4] also investigated the influence of particle size on heat treatment and the results confirmed earlier work and substantiated Geller's theoretical model. Temperature and heating rates have been controlled in these studies, for example Holman [1] recommended rapid heat treatments for thermal weakening of quartz and Brown and Gaudin [3] found rapid heating to be effective for treatment of marble. A very useful study was made by Kanellopoulos and Ball [9] to determine tensile strengths of quartzite after heat treatment at different temperatures. These authors' results were in general agreement with the Geller model. Hariharan and Venkatachalam [10] also looked at the influence of heat treatment on quartz grindability and showed that the distribution modulus increased with increased temperature whilst the size modulus of ground quartz decreased. In general, the workers who studied quartz or quartzites concluded that the minimum Bond work index occurred after heating and quenching from temperatures above the quartz o~-B inversion temperature, and several studies have attributed the weakening of the rock quartz matrix to anisotropic expansion during the transition, e.g. Drost and Mahan [11 ] Kanellopoulos and Ball [9] and Hariharan
Thermally assisted liberation--a review
and Venkatachalam possible when high immersed in water. boundaries at high appropriate sites.
183
[10]. Jones and Fullard [12] found that almost ideal liberation was carbonate rocks were heated, cooled to room temperature and then This was attributed to evolution of carbon dioxide along the grain temperatures, creating internal pressure and exerting stresses at
RECENT AND CURRENT STUDIES OF T.A.L. Scheding et al [13] initiated work to study the effect of thermal pre-treatment on grinding characteristics using a typical lode tin ore consisting of cassiterite principally locked with quartz, chlorite, tourmaline and hematite. They reported on the effect of heating to 650°C followed by quenching, i.e. exploiting the a=13 quartz inversion and accompanying volume change. They found a significant decrease in the Bond work index (reduced by a factor of 0.45 after heating) and decreased fines production. An economic evaluation showed that the cost of heat treatment would be larger than that of conventional grinding along (by a factor of 6), but that enhanced liberation could lead to better overall grade/recovery results. The work was extended to cover the use of a dye-penetration technique for image analysis of the fracture network and of the effect of TAL on the mineralogy of the materials [14]. The main conclusions were that (i)
heating to 650°C followed by quenching would reduce grinding energy requirements by 55%,
(ii)
at the time it was estimated that only a 1.0% improvement in tin recovery was needed to offset heat energy costs and
(iii)
there was evidence of intergranular fracture between quartz and cassiterite. It was further suggested that enhanced liberation might be better achieved by alternative milling methods such as autogeneous or pebble mills instead of rod or ball mills.
Another example of current TAL research was reported by Veasey [15]. This programme targetted a low grade tungsten/tin ore which was expected to respond favourably. Thermal pretreatment could be expected to induce weakening at grain boundaries, particularly between quartz and wolframite, and it was anticipated that feeding pretreated ore to an established mill circuit would increase mill capacity, reduce mill wear and enable better control of product size. Energy considerations, again, were not favourable unless factors other than heat input were considered. Such factors include improved liberation, reduction of slimes (a high loss factor in wolframite processing) and beneficial alteration of properties of constituent minerals to enable higher recoveries. The study showed that grindability resistance was significantly reduced, liberation of wolframite was improved but detrimental side effects were produced if flotation or magnetic separation were employed. The latter effect has been studied in detail and reported elsewhere [ 16]. Although this study did not conclusively establish that TAL was appropriate for the target ore a further assessment has been made and the most recent phase of investigation has looked at gravity concentration by spiral separation [17]. The results of this study have indicated that TAL reduced the work index of the ore by 18%, reduced fines production during grinding, improved liberation and value distribution and showed that recoveries could be improved by up to 5%. There is still a need to scale up the work on other downstream process effects and then a full economic analysis can be made. An example of the successful application of TAL comes from the field of secondary metal recovery. Increased amounts of copper/plastic scrap are being presented as feed for the low grade fraction of material treated in the secondary copper blast furnace. The plastics content
184
K . E . FITZGIBBON and T. J. VEASEY
can cause both economic and environmental problems which affect the smelting operation. T A L at relatively low temperature (250°C) has been used to pretreat such materials so that low cost comminution can be employed to liberate metal and provide an improved feedstock [181. F U T U R E D E V E L O P M E N T OF T A L Current studies have shown that T A L can be effective for certain types of target ores and that further research is needed to fully evaluate the process. One area that deserves particular attention is optimisation of heat transfer to maximise the effect of matrix weakening to induce improved intergranular fracture. In some cases the heating stage may be made more efficient by using techniques such as fluidized bed roasters but there may be limitations due to particle size distributions of the feed materials. A particularly attractive alternative could be the use of microwave radiation to produce heating effects [15]. Microwave energy transmits heat to the centre and surface of a material simultaneously i.e. independence of thermal conductivity on heating rate is overcome and the time required to heat certain materials can be shortened. Chen et al.[19] investigated the relative transparency of forty minerals to microwave radiation and found that two types of mineral could be classified: (i)
those in which little or no heat was generated and material properties were unaffected because of transparency or total reflectivity to the microwaves and
(ii)
those in which heat was generated. They observed that ore minerals respond favourably to microwave heating whereas gangue minerals do not.
Walkiewicz et al. [20] carried out further tests of mineral receptivity to microwave heating and commented on extractive applications. In particular, stress fracturing at mineral grain boundaries in a gangue matrix was demonstrated and they concluded that this would significantly influence grindability and liberation. This work is complemented by extensive studies of dielectric properties of low-loss minerals undertaken by the USBM [21] which will enable targetting of ores for microwave TAL. In conclusion, evaluation of the economics of T A L still need to be extended and all the factors involved in downstream processing must be quantified to assess the suitability of such pre-treatment for any given system. T A L could become viable on a commercial scale if comminution techniques directing energy to grain boundaries can be developed to augment the induced weakening of the boundaries [22]. REFERENCES .
Holman, B.W. Heat treatment as an agent in rock breaking, Trans I M M 26 219 (1926-27).
.
Yates, A. Effect of heating and quenching Cornish tin ores before crushing, Trans I M M 28 41 (1918-19).
.
Brown, J.H., Gaudin, A.H. & Loeb, C.M. Intergranular comminution by heating, Mining Eng 10 490 (1958). Geller, L.B. & Tervo, R.O. Grinding of preheated rocks, Trans. I M M (Sec C) 84 25 (1975).
.
.
Geller, L.B. Thermal stresses in spheres - a basis for studying the grinding of preheated rocks, Int. J Rock Mech. Min. Sci. 9 213 (1972).
Thermally assisted liberation--a review
.
.
185
Brown, J.H. & Gaudin, A.H. Mechanisms of intergranular comminution by heating, Trans. AIME 217 423 (1960). Myers, W.M. Calcining as an aid to grinding, J.Am.Ceram.Soc. $ 839 (1925).
8.
Djingheuzian, L.E. The influence of temperature on efficiency of grinding, Trans. CIM L V H 157 (1954).
.
Kanellopoulos, A. & Ball, A. The fracture and thermal weakening of quartize in relation to comminution, J.S.A.I.M.M. 76 42 (1975).
10.
Hariharan, K. & Venkatachalam, S. Influence of thermal treatment upon grindability of quartz, Min. Mag. 105 (Feb. 1977).
11.
Drost, J.J. & Mahan, W.M. Effects of thermal treatments upon concentratability of a non-magnetic iron ore, USBM RI7797 (1973).
12.
Jones, M.P. & Fullard, R.J. Mineral liberation by thermal decomposition of a carbonate rock, Trans. IMM (Sec C) 75 127 (1966).
13.
Scheding, W.M., Sherring, A.J., Binns, D., Parker, R.H. & Wills, B.A. The effect of thermal pretreatment on grinding characteristics, Camborne School o f Mines Journal, 81 43 (1981).
14.
Wills, B.A., Parker, R.H. & Binns, D.G. Thermally assisted liberation of cassiterite, Minerals and Metallurgical Processing, 86 94 (May 1987).
15.
Veasey, T.J. Thermally assisted liberation of non-sulphide ores, 1st Int. Mineral Proc. Symp., Izmir, Turkey, 2 557 (1986).
16.
Blackburn, S. & Veasey, T.J. Effect of heat treatment in air at 820-960°C on magnetic properties of wolframite, Trans IMM 93 C55 (1984).
17.
Azizli, K. Operating characteristics of spiral separator, Ph.D. Thesis, University of Birmingham (1989).
18.
Garner, F.A., Rowson, N.A. & Veasey, T.J. Thermally assisted liberation for reclamation of copper from copper/plastic scrap, Internal Report, University of Birmingham (1987).
19.
Chen, T.T., Dutrizac, J.E., Hague, K.E., Wyslovzil, W. & Kashyap, S. The relative transparency of minerals to microwave radiation, Canadian Metallurgical Quarterly, 23 (3) 349 (1984).
20.
Walkiewicz, J.W., Kazonich, G. & McGill, S.L. Microwave heating characteristics of selected minerals and compounds. Minerals & Metallurgical Processing, 39 (Feb. 1988).
21.
Church, R.H., Webb, W.E. & Salsman, J.B. Dielectric properties of low-loss minerals, USBM RI9194 (1988).
22.
Wills, B.A. & Hall, S.T. New methods for reducing losses of fine cassiterite, Mineral Processing in the United Kingdom, (ed. P.A. Dowd), IMM, London, 143-147 (1989)