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Energetics of mechanical activation – Application to ilmenite
Minerals Engineering 22 (2009) 572–574
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Technical Note
Energetics of mechanical activation – Application to ilmenite C. Sasikumar a, S. Srikanth a,*, N.K. Mukhopadhyay b, S.P. Mehrotra c a b c
National Metallurgical Laboratory – Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India National Metallurgical Laboratory, Jamshedpur 831 007, India
a r t i c l e
i n f o
Article history: Received 16 December 2008 Accepted 27 January 2009 Available online 5 March 2009 Keywords: Oxide ores Grinding Particle size Surface modification Mineral processing
a b s t r a c t An attempt is made to measure the energy stored in the material during mechanical activation and its manifestation in various forms (defects, new surfaces and interfaces, strain and structural disorder) through direct energy measurements, calorimetry, surface area and surface energy measurements. The elastic strain, grain size and extent of amorphisation is estimated from XRD analysis. The strain energy is calculated using the theory of elasticity and the amorphisation energy from thermodynamic calculations. For ilmenite, the energy transferred to the material is about 6% of the specific energy consumption in 4 h of planetary milling. A major part of the energy is stored as elastic strain energy, structural disorder and in point and line defects whereas the energy stored in additional surfaces and grain boundaries are comparatively much lower. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical activation is a non-equilibrium enhancement of the potential energy of materials using mechanical forces such as high energy milling. The enhanced energy manifests as increased surface area, vacancy and dislocation concentrations, enhanced grain boundaries, stacking faults and pores, structural disorder, metastable phases, higher oxidation states, alteration of the bond length/ angles/energy and increased energy of electrons (Tkacova, 1989; Balaz, 2000). The various non-equilibrium effects undergo relaxation with varying relaxation times (Balaz, 2000). However, depending on the time elapsed after activation, relaxation processes are incomplete and part of the energy is stored in the material. Although the stored energy can be determined from law of energy conservation through independent measurements of total work done, the internal energy change of the grinding media and the heat evolved during the process, these experiments are tedious and involve large uncertainties. Alternately, the stored energy can be deduced from individual contributions of various effects:
DEM ¼ DHdef :rel þ cS DS M þ cGB DAGB M þ e DEe þ famor DHamor þ ftrans DHtrans þ Heat loss
ð1Þ
where DHdef.rel is the enthalpy of point, line and surface defects with short relaxation times; cs is the specific surface energy of material; DS is the change in surface area during milling; M is the molar weight of the sample; cGB is the specific grain boundary energy; DAGB is the * Corresponding author. Tel.: +91 44 22542077; fax: +91 44 22541027. E-mail address: [email protected] (S. Srikanth). 0892-6875/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2009.01.013
change in grain boundary area; e is the lattice strain; DEe is the strain energy; famor is the fraction of amorphisation; DHamor is the enthalpy of amorphisation; ftrans is the extent of structural transformations and DHtrans is the associated enthalpy change. The energy contributed by defects having longer relaxation times is accounted indirectly in the strain energy. In Eq. (1), the change in enthalpy is assumed to correspond to the change in internal energy, i.e., the process is assumed isobaric and work done against external pressure is neglected. The objective of this study is to analyze the energetics of the mechanical activation process. 2. Experimental An ilmenite concentrate from Indian Rare Earths Limited (Manavalakurichi, India) region is used. A Fritsch Pulviresette-5 planetary mill with agate bowl and balls is employed for mechanical activation. The mill is coupled with a power meter (YOKOGAWA WT-3000 with sensitivity of 0.2%) to measure the energy input during milling. The samples are milled for 30, 90 and 240 min in separate experiments. The chemical composition of the ilmenite used and the conditions of milling are given earlier (Sasikumar et al., 2007). The effect of mechanical activation on lattice parameters, crystallite size, elastic strain and degree of amorphisation are analyzed by XRD and line broadening analysis using standard methods (Warren, 1990). The XRD measurement is carried out in a Siemens D-500 diffractometer using Co Ka radiation at a scan rate of 1°/min. The enthalpy of relaxation of defects is measured in an aqueous solution using an isothermal conduction calorimeter (Thermometric AB,
573
C. Sasikumar et al. / Minerals Engineering 22 (2009) 572–574
ary energy is approximately 40% of the specific surface energy (Imamura and Senna, 1982). The total grain boundary energy for various times of activation derived by combining the specific grain boundary energy with the measured grain boundary area is given in Table 1.
Sweden) with a sensitivity of 0.1 mW. The activated samples are subjected to relaxation for 7 h at 27 °C in the calorimeter. The differential power between the sample and an alumina reference is continuously recorded. The heat evolved or absorbed is determined from the integral of power vs time plot. The surface area is measured using a multipoint surface area analyzer (MICROMERETICS ASAP2020) and the particle size using a laser diffraction analyzer (CILAS 1180). The contact angles are measured by the capillary rise method (KRUSS Tensiometer K100) in five liquids (n-hexane, formamide, water, di-chloromethane and ethylene glycol). The surface energy is determined from the measured contact angles using the Young’s equation and the equation of state (Kwok and Neumann, 1999). The surface energies of the experimental liquids are taken from Kwok and Neumann (1999).
The elastic strain energy is determined from theory of elasticity (Senna, 1985):
The energy transferred to the material (DEM), deduced from energy input to the mill with and without the material is summarized in Table 1. The specific energy consumption is 11,087 w h/kg and the energy transferred to the material is 660 w h/kg in 4 h of milling. The enthalpy of relaxation of defects derived from the calorimetric results for various activation times is given in Table 1. At equilibrium, the activated material will completely relax to equilibrium state and the enthalpy of relaxation will correspond to the stored energy. However, during the timescale of the calorimetric experiment, only partial relaxation of defects occurs depending on temperature and time. The relaxation kinetics for the various micro-processes given by Balaz (2000) indicates that the point, line and surface defects relax within 10–1000 s whereas, structural and surface area relaxation requires higher times (Heinicke, 1981). The timescale of the calorimetric experiment corresponds to the energy of relaxation of the point and line defects especially at the surface. The increase in surface energy (DESE) can be estimated from the expression (Xiao et al., 2005):
EA ¼ DHF fA
DHTCrys$Amor ¼ DHMP F
Z
T
DCpdT
ð7Þ
MP
where
Z
The mean particle size and BET surface area for various milling times are tabulated in Table 2. The total surface energy for various times of activation is given in Table 1. The energy stored through enhanced grain boundaries (DEGB) can be given as:
T
DCpdT ¼
MP
Z
T
½Cpliquid Cpsolid dT
MP
1 1 T T MP liquid 1 1 aðT T MP Þ þ bðT 2 T 2MP Þ þ c T T MP solid
¼ aðT T MP Þ þ bðT 2 T 2MP Þ þ c
ð3Þ
The average crystallite size (D) deduced from X-ray line broadening measurements of the six most intense reflections using the Williamson–Hall method (1953) is tabulated in Table 2. The crystallite size decreases initially with time of activation, reaches a critical value after which it remains constant. The total grain boundary area is determined from the mean crystallite size assuming a tetrakai-decahedron configuration (Suryanarayana, 2004):
DAGB ¼ N 47:569 ðD=3Þ2
ð6Þ
where DHF is the enthalpy of fusion at the reduced temperature of activation and fA is the fraction of amorphisation. The enthalpy change associated with the amorphisation process can be written as:
ð2Þ
DEGB ¼ cGB DAGB M
ð5Þ
where e is the volumetric elastic strain; l is the shear and K is the bulk modulus of the material. The bulk and shear modulus of ilmenite are taken to be 174 and 90 GPa, respectively, ( Liebermann, 1976; Tromans and Meech, 2001). The non-uniform elastic strain is obtained from the XRD line broadening measurements (Williamson and Hall, 1953). The uniform elastic strain was determined from the lattice parameter measurements. The measured lattice parameters and elastic strains for various times of milling are tabulated in Table 2. The calculated strain energies for various milling times are given in Table 1. The amorphisation energy of the milled samples is determined using the following equation:
3. Results and discussion
DESE ¼ cS DS M2
18lK e2 4l þ 3K
E¼
where DHFMP is the enthalpy of fusion at the melting point and a, b, and c are the specific heat (Cp) coefficients of ilmenite in solid and liquid states DHFMP was taken to be 90.8 kJ/mol (Tromans and Meech, 2001). The specific heat of ilmenite in solid state is taken from the database of the FACTSAGE software (Version 5.0). Since data on the specific heat of liquid ilmenite is not available, it is derived from the Kopp–Neumann rule (Rao, 1985) using the Cp data of liquid FeO and TiO2. Strictly, the Kopp–Neumann rule cannot be applied to liquids. The enthalpy of fusion is derived at 300 K and the fraction of amorphisation is derived from the integral intensity of XRD peaks (Ohlberg and Strikler, 1962). The fraction and the energy
ð4Þ
where N is the number of grains per unit volume determined from specific volume of the material. In general, the specific grain bound-
Table 1 Specific energy consumption, energy transferred to material, stored energy and its distribution. Time of milling (min)
Energy applied to the mil (kJ/mol)
Energy transferred to material, DEM (kJ/mol)
Energy lost in breakage of bonds (kJ/mol)
30 90 240
742 2241 6053
25.6 115.6 359.7
12.2 79.4 267.2
ð8Þ
Manifestation of stored energy in different forms Surface, DESE (J/mol)
Grain boundary, DEGB (J/mol)
Elastic Strain, DEe (kJ/mol)
Amorphisation, DEA (kJ/mol)
Defects with short relaxation times, DHdef.rel (kJ/mol)
10.7 16.4 37.8
81.1 102.0 173.6
4.7 6.9 52.6
6.4 20.3 25.6
2.2 8.9 14.1
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C. Sasikumar et al. / Minerals Engineering 22 (2009) 572–574
Table 2 Summary of the measured parameters as a function of time of activation. Time of milling (min)
Mean particle size (lm)
BET specific surface area (m2/g)
Crystallite size (nm)
0 30 90 240
169.12 20.28 1.72 0.97
6.132 8.894 9.714 11.632
88.6 35.2 31.9 31.0
of amorphisation for various activation times are given in Tables 2 and 1, respectively. The stored energy is the sum of energy changes contributed by the enthalpy of relaxation, surface energy, grain boundary energy, strain energy and energy of amorphisation. These are summarized in Table 1 for various times of activation. It is observed that a major part of the energy is stored as strain energy, structural disorder and in point and line defects whereas the extent of energy stored in additional surfaces and grain boundaries are lower. A large part (50–75%) of the energy transferred to the material during milling is lost during breakage of bonds as heat. The energy stored in the mineral also manifests through a reduction in activation energy (DE) and an increase in the rate of dissolution. The decrease in activation energy for the sulfuric acid dissolution of ilmenite because of mechanical activation (for 6 h) was determined to be 12 kJ/ mol (Sasikumar et al., 2007). This value is much lower compared to the estimated stored energy (93 kJ/mol). The difference is partly due to the fast relaxation of the non-equilibrium effects at the leaching temperatures, resulting in only a part of the stored energy being available. 4. Conclusions An attempt is made to estimate the distribution of energy in various forms during the mechanical activation of ilmenite. The specific energy consumption in 4 h of planetary milling is 6053 kJ/mol of which about 360 kJ/mol is transferred to the material. More than half of the energy transferred to the material is lost as heat during the breakage of bonds and only the remaining energy is truly stored. The energy is stored in additional surfaces and interfaces, point, line and volume defects, high energy structures and strain. A large part of the stored energy is reflected as strain energy (53 kJ/mol) and structural disorder (26 kJ/mol). Part of the defect energy stored in the activated solid is found to relax
Lattice parameters (nm) a
c
0.489 0.494 0.495 0.503
1.357 1.366 1.368 1.392
Nonuniform strain
Uniform strain
Fraction amorphisation
0 0.0010 0.0015 0.0020
0 0.0165 0.0201 0.0554
0 0.115 0.363 0.457
soon (14 kJ/mol for the ilmenite milled for 4 h). The energy stored through additional surfaces and grain boundaries is much lesser. Acknowledgements The authors are thankful to Sapan Das and Ravikumar for the SEM and XRD analyzes and to CSIR, New Delhi for a SRF for Sasikumar. References Balaz, P., 2000. Extractive Metallurgy of Activated Minerals. Elsevier, Amsterdam. Heinicke, G., 1981. Recent advances on tribochemistry. In: Proceedings of the International Symposium on Powder Technology, Kyoto, 81, pp. 354–364. Imamura, K., Senna, M., 1982. Change in the phase stability of zinc blende and wurtzite on grinding. Journal of Chemical Society: Faraday Transactions 78, 1131–1140. Kwok, D.Y., Neumann, A.W., 1999. Contact angle measurement and contact angle interpretation. Advances in Colloid and Interface Science 81, 167–249. Liebermann, R.C., 1976. Elasticity of ilmenites. Physics of the Earth and Planetary Interiors 12, 5–10. Ohlberg, S.M., Strikler, D.W., 1962. Determination of percent crystallinity of partly devitrified glass by X-ray diffraction. Journal of American Ceramic Society 45, 170–171. Rao, Y.K., 1985. Stoichiometry and Thermodynamics of Metallurgical Processes. Cambridge University Press, Cambridge. Sasikumar, C., Rao, D.S., Srikanth, S., Mukhopadhyay, N.K., Mehrotra, S.P., 2007. Dissolution studies of mechanically activated Manavalakurichi ilmenite with HCl and H2SO4. Hydrometallurgy 88, 154–169. Senna, M., 1985. Problems on the mechanically induced polymorphic transformation. Crystal Research and Technology 20, 209–217. Suryanarayana, C., 2004. Mechanical Alloying and Milling. Marcel Dekker, New York. Tkacova, K., 1989. Mechanical Activation of Minerals. Elsevier, Amsterdam. Tromans, D., Meech, J.A., 2001. Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation. Minerals Engineering 14, 1359–1377. Warren, B.E., 1990. X-ray Diffraction. Dover Publishers, New York. Williamson, G.K., Hall, W.H., 1953. X-ray line broadening from filed aluminum and wolfram. Acta Metallurgy 1, 22–31. Xiao, Zhongliang, Chen, Qiyuan, Yin, Zhoulan, Hua, Huiping, Wu, Daoxin, 2005. Calorimetric investigation on mechanically activated storage energy mechanism of sphalerite and pyrite. Thermochimica Acta 436, 10–14.
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Technical Note
Energetics of mechanical activation – Application to ilmenite C. Sasikumar a, S. Srikanth a,*, N.K. Mukhopadhyay b, S.P. Mehrotra c a b c
National Metallurgical Laboratory – Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India National Metallurgical Laboratory, Jamshedpur 831 007, India
a r t i c l e
i n f o
Article history: Received 16 December 2008 Accepted 27 January 2009 Available online 5 March 2009 Keywords: Oxide ores Grinding Particle size Surface modification Mineral processing
a b s t r a c t An attempt is made to measure the energy stored in the material during mechanical activation and its manifestation in various forms (defects, new surfaces and interfaces, strain and structural disorder) through direct energy measurements, calorimetry, surface area and surface energy measurements. The elastic strain, grain size and extent of amorphisation is estimated from XRD analysis. The strain energy is calculated using the theory of elasticity and the amorphisation energy from thermodynamic calculations. For ilmenite, the energy transferred to the material is about 6% of the specific energy consumption in 4 h of planetary milling. A major part of the energy is stored as elastic strain energy, structural disorder and in point and line defects whereas the energy stored in additional surfaces and grain boundaries are comparatively much lower. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical activation is a non-equilibrium enhancement of the potential energy of materials using mechanical forces such as high energy milling. The enhanced energy manifests as increased surface area, vacancy and dislocation concentrations, enhanced grain boundaries, stacking faults and pores, structural disorder, metastable phases, higher oxidation states, alteration of the bond length/ angles/energy and increased energy of electrons (Tkacova, 1989; Balaz, 2000). The various non-equilibrium effects undergo relaxation with varying relaxation times (Balaz, 2000). However, depending on the time elapsed after activation, relaxation processes are incomplete and part of the energy is stored in the material. Although the stored energy can be determined from law of energy conservation through independent measurements of total work done, the internal energy change of the grinding media and the heat evolved during the process, these experiments are tedious and involve large uncertainties. Alternately, the stored energy can be deduced from individual contributions of various effects:
DEM ¼ DHdef :rel þ cS DS M þ cGB DAGB M þ e DEe þ famor DHamor þ ftrans DHtrans þ Heat loss
ð1Þ
where DHdef.rel is the enthalpy of point, line and surface defects with short relaxation times; cs is the specific surface energy of material; DS is the change in surface area during milling; M is the molar weight of the sample; cGB is the specific grain boundary energy; DAGB is the * Corresponding author. Tel.: +91 44 22542077; fax: +91 44 22541027. E-mail address: [email protected] (S. Srikanth). 0892-6875/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2009.01.013
change in grain boundary area; e is the lattice strain; DEe is the strain energy; famor is the fraction of amorphisation; DHamor is the enthalpy of amorphisation; ftrans is the extent of structural transformations and DHtrans is the associated enthalpy change. The energy contributed by defects having longer relaxation times is accounted indirectly in the strain energy. In Eq. (1), the change in enthalpy is assumed to correspond to the change in internal energy, i.e., the process is assumed isobaric and work done against external pressure is neglected. The objective of this study is to analyze the energetics of the mechanical activation process. 2. Experimental An ilmenite concentrate from Indian Rare Earths Limited (Manavalakurichi, India) region is used. A Fritsch Pulviresette-5 planetary mill with agate bowl and balls is employed for mechanical activation. The mill is coupled with a power meter (YOKOGAWA WT-3000 with sensitivity of 0.2%) to measure the energy input during milling. The samples are milled for 30, 90 and 240 min in separate experiments. The chemical composition of the ilmenite used and the conditions of milling are given earlier (Sasikumar et al., 2007). The effect of mechanical activation on lattice parameters, crystallite size, elastic strain and degree of amorphisation are analyzed by XRD and line broadening analysis using standard methods (Warren, 1990). The XRD measurement is carried out in a Siemens D-500 diffractometer using Co Ka radiation at a scan rate of 1°/min. The enthalpy of relaxation of defects is measured in an aqueous solution using an isothermal conduction calorimeter (Thermometric AB,
573
C. Sasikumar et al. / Minerals Engineering 22 (2009) 572–574
ary energy is approximately 40% of the specific surface energy (Imamura and Senna, 1982). The total grain boundary energy for various times of activation derived by combining the specific grain boundary energy with the measured grain boundary area is given in Table 1.
Sweden) with a sensitivity of 0.1 mW. The activated samples are subjected to relaxation for 7 h at 27 °C in the calorimeter. The differential power between the sample and an alumina reference is continuously recorded. The heat evolved or absorbed is determined from the integral of power vs time plot. The surface area is measured using a multipoint surface area analyzer (MICROMERETICS ASAP2020) and the particle size using a laser diffraction analyzer (CILAS 1180). The contact angles are measured by the capillary rise method (KRUSS Tensiometer K100) in five liquids (n-hexane, formamide, water, di-chloromethane and ethylene glycol). The surface energy is determined from the measured contact angles using the Young’s equation and the equation of state (Kwok and Neumann, 1999). The surface energies of the experimental liquids are taken from Kwok and Neumann (1999).
The elastic strain energy is determined from theory of elasticity (Senna, 1985):
The energy transferred to the material (DEM), deduced from energy input to the mill with and without the material is summarized in Table 1. The specific energy consumption is 11,087 w h/kg and the energy transferred to the material is 660 w h/kg in 4 h of milling. The enthalpy of relaxation of defects derived from the calorimetric results for various activation times is given in Table 1. At equilibrium, the activated material will completely relax to equilibrium state and the enthalpy of relaxation will correspond to the stored energy. However, during the timescale of the calorimetric experiment, only partial relaxation of defects occurs depending on temperature and time. The relaxation kinetics for the various micro-processes given by Balaz (2000) indicates that the point, line and surface defects relax within 10–1000 s whereas, structural and surface area relaxation requires higher times (Heinicke, 1981). The timescale of the calorimetric experiment corresponds to the energy of relaxation of the point and line defects especially at the surface. The increase in surface energy (DESE) can be estimated from the expression (Xiao et al., 2005):
EA ¼ DHF fA
DHTCrys$Amor ¼ DHMP F
Z
T
DCpdT
ð7Þ
MP
where
Z
The mean particle size and BET surface area for various milling times are tabulated in Table 2. The total surface energy for various times of activation is given in Table 1. The energy stored through enhanced grain boundaries (DEGB) can be given as:
T
DCpdT ¼
MP
Z
T
½Cpliquid Cpsolid dT
MP
1 1 T T MP liquid 1 1 aðT T MP Þ þ bðT 2 T 2MP Þ þ c T T MP solid
¼ aðT T MP Þ þ bðT 2 T 2MP Þ þ c
ð3Þ
The average crystallite size (D) deduced from X-ray line broadening measurements of the six most intense reflections using the Williamson–Hall method (1953) is tabulated in Table 2. The crystallite size decreases initially with time of activation, reaches a critical value after which it remains constant. The total grain boundary area is determined from the mean crystallite size assuming a tetrakai-decahedron configuration (Suryanarayana, 2004):
DAGB ¼ N 47:569 ðD=3Þ2
ð6Þ
where DHF is the enthalpy of fusion at the reduced temperature of activation and fA is the fraction of amorphisation. The enthalpy change associated with the amorphisation process can be written as:
ð2Þ
DEGB ¼ cGB DAGB M
ð5Þ
where e is the volumetric elastic strain; l is the shear and K is the bulk modulus of the material. The bulk and shear modulus of ilmenite are taken to be 174 and 90 GPa, respectively, ( Liebermann, 1976; Tromans and Meech, 2001). The non-uniform elastic strain is obtained from the XRD line broadening measurements (Williamson and Hall, 1953). The uniform elastic strain was determined from the lattice parameter measurements. The measured lattice parameters and elastic strains for various times of milling are tabulated in Table 2. The calculated strain energies for various milling times are given in Table 1. The amorphisation energy of the milled samples is determined using the following equation:
3. Results and discussion
DESE ¼ cS DS M2
18lK e2 4l þ 3K
E¼
where DHFMP is the enthalpy of fusion at the melting point and a, b, and c are the specific heat (Cp) coefficients of ilmenite in solid and liquid states DHFMP was taken to be 90.8 kJ/mol (Tromans and Meech, 2001). The specific heat of ilmenite in solid state is taken from the database of the FACTSAGE software (Version 5.0). Since data on the specific heat of liquid ilmenite is not available, it is derived from the Kopp–Neumann rule (Rao, 1985) using the Cp data of liquid FeO and TiO2. Strictly, the Kopp–Neumann rule cannot be applied to liquids. The enthalpy of fusion is derived at 300 K and the fraction of amorphisation is derived from the integral intensity of XRD peaks (Ohlberg and Strikler, 1962). The fraction and the energy
ð4Þ
where N is the number of grains per unit volume determined from specific volume of the material. In general, the specific grain bound-
Table 1 Specific energy consumption, energy transferred to material, stored energy and its distribution. Time of milling (min)
Energy applied to the mil (kJ/mol)
Energy transferred to material, DEM (kJ/mol)
Energy lost in breakage of bonds (kJ/mol)
30 90 240
742 2241 6053
25.6 115.6 359.7
12.2 79.4 267.2
ð8Þ
Manifestation of stored energy in different forms Surface, DESE (J/mol)
Grain boundary, DEGB (J/mol)
Elastic Strain, DEe (kJ/mol)
Amorphisation, DEA (kJ/mol)
Defects with short relaxation times, DHdef.rel (kJ/mol)
10.7 16.4 37.8
81.1 102.0 173.6
4.7 6.9 52.6
6.4 20.3 25.6
2.2 8.9 14.1
574
C. Sasikumar et al. / Minerals Engineering 22 (2009) 572–574
Table 2 Summary of the measured parameters as a function of time of activation. Time of milling (min)
Mean particle size (lm)
BET specific surface area (m2/g)
Crystallite size (nm)
0 30 90 240
169.12 20.28 1.72 0.97
6.132 8.894 9.714 11.632
88.6 35.2 31.9 31.0
of amorphisation for various activation times are given in Tables 2 and 1, respectively. The stored energy is the sum of energy changes contributed by the enthalpy of relaxation, surface energy, grain boundary energy, strain energy and energy of amorphisation. These are summarized in Table 1 for various times of activation. It is observed that a major part of the energy is stored as strain energy, structural disorder and in point and line defects whereas the extent of energy stored in additional surfaces and grain boundaries are lower. A large part (50–75%) of the energy transferred to the material during milling is lost during breakage of bonds as heat. The energy stored in the mineral also manifests through a reduction in activation energy (DE) and an increase in the rate of dissolution. The decrease in activation energy for the sulfuric acid dissolution of ilmenite because of mechanical activation (for 6 h) was determined to be 12 kJ/ mol (Sasikumar et al., 2007). This value is much lower compared to the estimated stored energy (93 kJ/mol). The difference is partly due to the fast relaxation of the non-equilibrium effects at the leaching temperatures, resulting in only a part of the stored energy being available. 4. Conclusions An attempt is made to estimate the distribution of energy in various forms during the mechanical activation of ilmenite. The specific energy consumption in 4 h of planetary milling is 6053 kJ/mol of which about 360 kJ/mol is transferred to the material. More than half of the energy transferred to the material is lost as heat during the breakage of bonds and only the remaining energy is truly stored. The energy is stored in additional surfaces and interfaces, point, line and volume defects, high energy structures and strain. A large part of the stored energy is reflected as strain energy (53 kJ/mol) and structural disorder (26 kJ/mol). Part of the defect energy stored in the activated solid is found to relax
Lattice parameters (nm) a
c
0.489 0.494 0.495 0.503
1.357 1.366 1.368 1.392
Nonuniform strain
Uniform strain
Fraction amorphisation
0 0.0010 0.0015 0.0020
0 0.0165 0.0201 0.0554
0 0.115 0.363 0.457
soon (14 kJ/mol for the ilmenite milled for 4 h). The energy stored through additional surfaces and grain boundaries is much lesser. Acknowledgements The authors are thankful to Sapan Das and Ravikumar for the SEM and XRD analyzes and to CSIR, New Delhi for a SRF for Sasikumar. References Balaz, P., 2000. Extractive Metallurgy of Activated Minerals. Elsevier, Amsterdam. Heinicke, G., 1981. Recent advances on tribochemistry. In: Proceedings of the International Symposium on Powder Technology, Kyoto, 81, pp. 354–364. Imamura, K., Senna, M., 1982. Change in the phase stability of zinc blende and wurtzite on grinding. Journal of Chemical Society: Faraday Transactions 78, 1131–1140. Kwok, D.Y., Neumann, A.W., 1999. Contact angle measurement and contact angle interpretation. Advances in Colloid and Interface Science 81, 167–249. Liebermann, R.C., 1976. Elasticity of ilmenites. Physics of the Earth and Planetary Interiors 12, 5–10. Ohlberg, S.M., Strikler, D.W., 1962. Determination of percent crystallinity of partly devitrified glass by X-ray diffraction. Journal of American Ceramic Society 45, 170–171. Rao, Y.K., 1985. Stoichiometry and Thermodynamics of Metallurgical Processes. Cambridge University Press, Cambridge. Sasikumar, C., Rao, D.S., Srikanth, S., Mukhopadhyay, N.K., Mehrotra, S.P., 2007. Dissolution studies of mechanically activated Manavalakurichi ilmenite with HCl and H2SO4. Hydrometallurgy 88, 154–169. Senna, M., 1985. Problems on the mechanically induced polymorphic transformation. Crystal Research and Technology 20, 209–217. Suryanarayana, C., 2004. Mechanical Alloying and Milling. Marcel Dekker, New York. Tkacova, K., 1989. Mechanical Activation of Minerals. Elsevier, Amsterdam. Tromans, D., Meech, J.A., 2001. Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation. Minerals Engineering 14, 1359–1377. Warren, B.E., 1990. X-ray Diffraction. Dover Publishers, New York. Williamson, G.K., Hall, W.H., 1953. X-ray line broadening from filed aluminum and wolfram. Acta Metallurgy 1, 22–31. Xiao, Zhongliang, Chen, Qiyuan, Yin, Zhoulan, Hua, Huiping, Wu, Daoxin, 2005. Calorimetric investigation on mechanically activated storage energy mechanism of sphalerite and pyrite. Thermochimica Acta 436, 10–14.