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Chemical and morphological changes of millerite by mechanical activation
Int. J. Miner. Process. 66 (2002) 233 – 240 www.elsevier.com/locate/ijminpro
Chemical and morphological changes of millerite by mechanical activation W. Mulak a,*, P. Balazˇ b, M. Chojnacka a a
Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wrocław University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland b Institute of Geotechnics, Slovak Academy of Science,Watsonova 45, 043 53, Kosˇice, Slovak Republic Received 28 January 2002; received in revised form 28 May 2002; accepted 29 May 2002
Abstract The chemical and morphological changes induced on the surface of millerite by mechanical activation have been studied by means of X-ray diffraction, electron microprobe, scanning electron microscopy and infrared spectra. Leaching tests in water, dilute nitric and hydrochloric acids have been carried out. The mechanical activation of millerite grains due to their disintegration is accompanied by an increase in the number of particles and generation of fresh previously unexposed surface. The agglomeration of grains and a partial oxidation of sulphide to sulphate ions were observed. The increase in leachability of the activated millerite may well be due to the combination of mechanically induced structural defects and chemical modification of millerite surface. D 2002 Elsevier Science B.V. All rights reserved. Keywords: millerite; mechanical activation; leaching
1. Introduction In many cases, the rate of a leaching reaction can be controlled by the diffusion of reagent from solution through a layer of solid product covering the reacting solid, or the diffusion of ions from reacting solid through a layer of product to the solid – liquid interface at which reaction occurs. The diffusion coefficient will be sensitive to the degree of lattice perfection.
* Corresponding author. Tel./fax: +48-71-3284330. E-mail address: [email protected] (W. Mulak). 0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 7 - 4
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W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
Electrochemical study of anodic dissolution of millerite shows the presence of the sulphur-rich sulphide layer passivating the surface of the nickel sulphide within the potential range from 0 to 0.8 V (Power, 1981). This passive layer achieves an equilibrium thickness, and leaching process can be greatly accelerated by fine grinding. Mechanical activation involving intensive grinding produces changes in the composition and microstructure of solids. Several investigators have reported the positive influence of mechanical activation on the rate of metal sulphide leaching (Balazˇ, 2000; Bala´zˇ et al., 2000; Maurice and Hawk, 1998; Welham, 2001a,b). The aim of this work was to study the chemical and morphological changes of mechanically activated millerite by means of chemical analysis, X-ray diffraction, electron microprobe, scanning electron microscopy, infrared spectra, as well as leaching tests in water and in dilute nitric and hydrochloric acids.
2. Experimental The investigations were carried out with synthetic millerite. It was obtained by melting stoichiometric amounts of nickel powder and elemental sulphur in evacuated silica tubes at 900 jC. The product was homogenized at 360 jC for 72 h, then ground, and the fraction with grains of size < 60 Am was sieved out and used in mechanical activation. The mechanical activation of samples was accomplished in planetary mill activator URF-AGO-2 (USSR) under the following conditions: loading of mill with 110 balls of 5mm diameter; material of balls is a hard steel, rotation speed of the planet carrier: 11.7 rpm; centrifugal acceleration of thimble axes: 1000 ms 1; time of mechanical activation: 3 and 10 min, respectively (Balazˇ, 2000). The surface structure was studied by scanning electron microscope of type JEOL JSM 580 LV coupled with the link JSJS 300 X-ray microanalysis system from Oxford Instrument. The phase composition of nonactivated and activated millerite samples was examined by X-ray analysis (Philips X’PERT with CuKa radiation). Nickel in millerite and in the solution during the leaching was determined by atomic absorption method. Sulphur content as sulphide and sulphate ions in nonactivated and activated samples was established gravimetrically. Infrared spectra were recorded in Perkin-Elmer 1600 FTIR spectrophotometer in the range 400 – 1600 cm 1. The samples were prepared as KBr pellets. The compacting pressure was 20 MPa.
3. Results and discussion 3.1. Physicochemical transformation in millerite due to the mechanical activation The intensive grinding of sulphide minerals generates changes both in their surface area and bulk properties (Balazˇ, 2000). The scanning electron microscopy examination of activated millerite proved that its grains size decreases below 1 Am. The corresponding
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
235
Fig. 1. SEM photographs of activated millerite: (a) nonactivated, (b) activated (3 min), (c) activated (10 min).
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Fig. 2. X-ray microanalysis of millerite: (a) nonactivated, (b) activated (10 min).
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
237
micrographs of nonactivated and activated samples are depicted in Fig. 1a, b and c, respectively. In addition to the disintegration effect of particles, the formation of agglomerates can also be observed. These may have dimensions comparable to those of nonactivated particles. The complexity of millerite surface transformation due to mechanical activation is also documented by microprobe analysis (Fig. 2a and b). During the activation process, oxidation of millerite occurs, and the oxygen peak considerably grows. After 10 min of activation, the oxygen content reaches the level of 23%. The phase composition of nonactivated as well as activated samples are presented in Fig. 3. The diffraction peaks for mechanically activated millerite are lower than those for nonactivated one mainly because of disordering processes of millerite crystal structure by intensive grinding. The disordering process is intensified with increase in the grinding time (compare Fig. 3b and c). Two phases—characteristic diffraction peaks of millerite and NiSO46H2O—are depicted in Fig. 3. The presence of sulphate ions was also proven by infrared spectroscopy. The IR spectrum of activated and nonactivated millerite after 10min grinding is presented in Fig. 4. The bands at: 1103, 787, 670, 610 cm 1 may just correspond to NiSO46H2O (Nyquist and Kagel, 1971). Results of chemical analysis of the millerite before and after grinding process are summed up in Table 1.
.
Fig. 3. X-ray diffraction of millerite: (a) nonactivated, (b) activated (3 min), (c) activated (10 min), where ( ) denotes NiSO46H2O.
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Fig. 4. Infrared spectra of millerite: (1) nonactivated, (2) activated (10 min).
Based on Table 1, one may conclude that sulphur in both the nonactivated and activated millerite samples is present as sulphide and sulphate ions simultaneously. However, their ratio changes essentially in favour of the sulphate ions after the mechanical activation due to a partial oxidation. The activated samples also differ from the nonactivated by the presence of iron, the content of which increases in the samples ground for a longer time. The reason of this is a contact of millerite during the grinding with the steel balls. 3.2. Leaching tests One of the interesting conclusions of our previous studies (Mulak and Wawrzak, 1997) is that the dissolution rate of millerite in 2.0 M HNO3 solution at temperature 50 jC is completely inhibited after 15 min of the leaching. After 2-h leaching under above conditions, only 1.3% of Ni is extracted, whereas activated millerite sample yields recovery of 44% of Ni after 1-h leaching time. It has been found that for leaching of
Table 1 Influence of mechanical activation on chemical composition of millerite Grinding time (min)
0 3 10
Elements content (%) Ni
S (sulphate)
S (sulphide)
Fe
63.0 49.6 44.4
0.6 3.8 4.9
33.6 25.3 24.2
– 0.4 1.6
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
239
Fig. 5. Nickel extraction leaching curves of activated millerite (10 min) in various leachants at 50 jC: (1) H20, (2) 2.0 M HCl, (3) 2.0 M HNO3.
activated millerite in nitric acid, the kinetic curve of the process obeys a linear law (Fig. 5, curve 3). The constant dissolution rate seems to indicate that the reaction area remains unchanged during the leaching time. Similar results have been obtained in catalytic action of cupric and ferric ions in nitric acid leaching of NiS and Ni3S2 (Mulak and Wawrzak, 1997; Mulak, 1987). We presume that small iron content in activated millerite, which comes from the steel balls of the mill, plays a catalytic role on the dissolution process. The possibility of 8.3% Ni dissolution in water after 5-min leaching time at 50 jC is presented in Fig. 5 (curve 1). The nickel extraction from the activated millerite in 2.0 M HCl reaches only 23% (Fig. 5, curve 2). The increase in leachability of the activated millerite in water, nitric and hydrochloric acids may be due to the combination of mechanically induced structural defects and chemical modification of mineral surface.
4. Conclusions The millerite particles disintegration due to mechanical activation is accompanied by an increase in the number of particles and generation of fresh previously unexposed surface. The agglomeration of grains is evident and amorphization is also observed. Mechanical activation of millerite particles causes a partial oxidizing of sulphide ions to sulphate ones. Moreover, the content of sulphate ions rises with increase in the grinding time.
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W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
A sample of mechanically activated millerite for 10 min yields a recovery of 44% Ni after 1-h leaching time in 2.0 M HNO3 solution at 50 jC. Only 1.3% Ni is extracted from nonactivated millerite under the same conditions. The increase in leachibility of the activated millerite may well be due to the combination of mechanically induced structural defects and chemical modification of mineral surface. The thorough understanding of the kinetics and mechanism of the activated millerite requires further investigations.
References Balazˇ, P., 2000. Extractive Metallurgy of Activated Minerals. Elsevier, Amsterdam. Bala´zˇ, P., Boldizˇa´rova´, E., Achimovicˇova´, M., Kammel, R., 2000. Leaching and dissolution of pentlandite concentrate pre-treated by mechanical activation. Hydrometallurgy 57, 85 – 96. Maurice, D., Hawk, J.A., 1998. Ferric chloride leaching of mechanically activated chalcopiryte. Hydrometallurgy 49, 103 – 123. Mulak, W., 1987. The catalytic action of cupric and ferric ions in nitric acid leaching of Ni3S2. Hydrometallurgy 17, 201 – 214. Mulak, W., Wawrzak, D., 1997. Leaching behaviour of millerite (h-NiS) in moderate oxidizing and reducing media. Proc. XX Int. Mineral Processing Congr., vol. 4. GMDB Clausthal-Zellerfeld, Germany, pp. 189 – 193. Nyquist, R.A., Kagel, R.O., 1971. Infrared Spectra of Inorganic Compounds (3800 – 45 cm 1). Academic Press, New York. Power, G.P., 1981. The electrochemistry of the nickel sulphides: I. NiS. Austr. J. Chem. 34, 2287 – 2296. Welham, N.J., 2001a. Mechanochemical processing of gold-bearing sulphides. Miner. Eng. 14, 341 – 347. Welham, N.J., 2001b. Mechanochemical processing of enargite (Cu3AsS4). Hydrometallurgy 62, 165 – 173.
Chemical and morphological changes of millerite by mechanical activation W. Mulak a,*, P. Balazˇ b, M. Chojnacka a a
Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wrocław University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370 Wrocław, Poland b Institute of Geotechnics, Slovak Academy of Science,Watsonova 45, 043 53, Kosˇice, Slovak Republic Received 28 January 2002; received in revised form 28 May 2002; accepted 29 May 2002
Abstract The chemical and morphological changes induced on the surface of millerite by mechanical activation have been studied by means of X-ray diffraction, electron microprobe, scanning electron microscopy and infrared spectra. Leaching tests in water, dilute nitric and hydrochloric acids have been carried out. The mechanical activation of millerite grains due to their disintegration is accompanied by an increase in the number of particles and generation of fresh previously unexposed surface. The agglomeration of grains and a partial oxidation of sulphide to sulphate ions were observed. The increase in leachability of the activated millerite may well be due to the combination of mechanically induced structural defects and chemical modification of millerite surface. D 2002 Elsevier Science B.V. All rights reserved. Keywords: millerite; mechanical activation; leaching
1. Introduction In many cases, the rate of a leaching reaction can be controlled by the diffusion of reagent from solution through a layer of solid product covering the reacting solid, or the diffusion of ions from reacting solid through a layer of product to the solid – liquid interface at which reaction occurs. The diffusion coefficient will be sensitive to the degree of lattice perfection.
* Corresponding author. Tel./fax: +48-71-3284330. E-mail address: [email protected] (W. Mulak). 0301-7516/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 7 - 4
234
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
Electrochemical study of anodic dissolution of millerite shows the presence of the sulphur-rich sulphide layer passivating the surface of the nickel sulphide within the potential range from 0 to 0.8 V (Power, 1981). This passive layer achieves an equilibrium thickness, and leaching process can be greatly accelerated by fine grinding. Mechanical activation involving intensive grinding produces changes in the composition and microstructure of solids. Several investigators have reported the positive influence of mechanical activation on the rate of metal sulphide leaching (Balazˇ, 2000; Bala´zˇ et al., 2000; Maurice and Hawk, 1998; Welham, 2001a,b). The aim of this work was to study the chemical and morphological changes of mechanically activated millerite by means of chemical analysis, X-ray diffraction, electron microprobe, scanning electron microscopy, infrared spectra, as well as leaching tests in water and in dilute nitric and hydrochloric acids.
2. Experimental The investigations were carried out with synthetic millerite. It was obtained by melting stoichiometric amounts of nickel powder and elemental sulphur in evacuated silica tubes at 900 jC. The product was homogenized at 360 jC for 72 h, then ground, and the fraction with grains of size < 60 Am was sieved out and used in mechanical activation. The mechanical activation of samples was accomplished in planetary mill activator URF-AGO-2 (USSR) under the following conditions: loading of mill with 110 balls of 5mm diameter; material of balls is a hard steel, rotation speed of the planet carrier: 11.7 rpm; centrifugal acceleration of thimble axes: 1000 ms 1; time of mechanical activation: 3 and 10 min, respectively (Balazˇ, 2000). The surface structure was studied by scanning electron microscope of type JEOL JSM 580 LV coupled with the link JSJS 300 X-ray microanalysis system from Oxford Instrument. The phase composition of nonactivated and activated millerite samples was examined by X-ray analysis (Philips X’PERT with CuKa radiation). Nickel in millerite and in the solution during the leaching was determined by atomic absorption method. Sulphur content as sulphide and sulphate ions in nonactivated and activated samples was established gravimetrically. Infrared spectra were recorded in Perkin-Elmer 1600 FTIR spectrophotometer in the range 400 – 1600 cm 1. The samples were prepared as KBr pellets. The compacting pressure was 20 MPa.
3. Results and discussion 3.1. Physicochemical transformation in millerite due to the mechanical activation The intensive grinding of sulphide minerals generates changes both in their surface area and bulk properties (Balazˇ, 2000). The scanning electron microscopy examination of activated millerite proved that its grains size decreases below 1 Am. The corresponding
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
235
Fig. 1. SEM photographs of activated millerite: (a) nonactivated, (b) activated (3 min), (c) activated (10 min).
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Fig. 2. X-ray microanalysis of millerite: (a) nonactivated, (b) activated (10 min).
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
237
micrographs of nonactivated and activated samples are depicted in Fig. 1a, b and c, respectively. In addition to the disintegration effect of particles, the formation of agglomerates can also be observed. These may have dimensions comparable to those of nonactivated particles. The complexity of millerite surface transformation due to mechanical activation is also documented by microprobe analysis (Fig. 2a and b). During the activation process, oxidation of millerite occurs, and the oxygen peak considerably grows. After 10 min of activation, the oxygen content reaches the level of 23%. The phase composition of nonactivated as well as activated samples are presented in Fig. 3. The diffraction peaks for mechanically activated millerite are lower than those for nonactivated one mainly because of disordering processes of millerite crystal structure by intensive grinding. The disordering process is intensified with increase in the grinding time (compare Fig. 3b and c). Two phases—characteristic diffraction peaks of millerite and NiSO46H2O—are depicted in Fig. 3. The presence of sulphate ions was also proven by infrared spectroscopy. The IR spectrum of activated and nonactivated millerite after 10min grinding is presented in Fig. 4. The bands at: 1103, 787, 670, 610 cm 1 may just correspond to NiSO46H2O (Nyquist and Kagel, 1971). Results of chemical analysis of the millerite before and after grinding process are summed up in Table 1.
.
Fig. 3. X-ray diffraction of millerite: (a) nonactivated, (b) activated (3 min), (c) activated (10 min), where ( ) denotes NiSO46H2O.
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W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
Fig. 4. Infrared spectra of millerite: (1) nonactivated, (2) activated (10 min).
Based on Table 1, one may conclude that sulphur in both the nonactivated and activated millerite samples is present as sulphide and sulphate ions simultaneously. However, their ratio changes essentially in favour of the sulphate ions after the mechanical activation due to a partial oxidation. The activated samples also differ from the nonactivated by the presence of iron, the content of which increases in the samples ground for a longer time. The reason of this is a contact of millerite during the grinding with the steel balls. 3.2. Leaching tests One of the interesting conclusions of our previous studies (Mulak and Wawrzak, 1997) is that the dissolution rate of millerite in 2.0 M HNO3 solution at temperature 50 jC is completely inhibited after 15 min of the leaching. After 2-h leaching under above conditions, only 1.3% of Ni is extracted, whereas activated millerite sample yields recovery of 44% of Ni after 1-h leaching time. It has been found that for leaching of
Table 1 Influence of mechanical activation on chemical composition of millerite Grinding time (min)
0 3 10
Elements content (%) Ni
S (sulphate)
S (sulphide)
Fe
63.0 49.6 44.4
0.6 3.8 4.9
33.6 25.3 24.2
– 0.4 1.6
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
239
Fig. 5. Nickel extraction leaching curves of activated millerite (10 min) in various leachants at 50 jC: (1) H20, (2) 2.0 M HCl, (3) 2.0 M HNO3.
activated millerite in nitric acid, the kinetic curve of the process obeys a linear law (Fig. 5, curve 3). The constant dissolution rate seems to indicate that the reaction area remains unchanged during the leaching time. Similar results have been obtained in catalytic action of cupric and ferric ions in nitric acid leaching of NiS and Ni3S2 (Mulak and Wawrzak, 1997; Mulak, 1987). We presume that small iron content in activated millerite, which comes from the steel balls of the mill, plays a catalytic role on the dissolution process. The possibility of 8.3% Ni dissolution in water after 5-min leaching time at 50 jC is presented in Fig. 5 (curve 1). The nickel extraction from the activated millerite in 2.0 M HCl reaches only 23% (Fig. 5, curve 2). The increase in leachability of the activated millerite in water, nitric and hydrochloric acids may be due to the combination of mechanically induced structural defects and chemical modification of mineral surface.
4. Conclusions The millerite particles disintegration due to mechanical activation is accompanied by an increase in the number of particles and generation of fresh previously unexposed surface. The agglomeration of grains is evident and amorphization is also observed. Mechanical activation of millerite particles causes a partial oxidizing of sulphide ions to sulphate ones. Moreover, the content of sulphate ions rises with increase in the grinding time.
240
W. Mulak et al. / Int. J. Miner. Process. 66 (2002) 233–240
A sample of mechanically activated millerite for 10 min yields a recovery of 44% Ni after 1-h leaching time in 2.0 M HNO3 solution at 50 jC. Only 1.3% Ni is extracted from nonactivated millerite under the same conditions. The increase in leachibility of the activated millerite may well be due to the combination of mechanically induced structural defects and chemical modification of mineral surface. The thorough understanding of the kinetics and mechanism of the activated millerite requires further investigations.
References Balazˇ, P., 2000. Extractive Metallurgy of Activated Minerals. Elsevier, Amsterdam. Bala´zˇ, P., Boldizˇa´rova´, E., Achimovicˇova´, M., Kammel, R., 2000. Leaching and dissolution of pentlandite concentrate pre-treated by mechanical activation. Hydrometallurgy 57, 85 – 96. Maurice, D., Hawk, J.A., 1998. Ferric chloride leaching of mechanically activated chalcopiryte. Hydrometallurgy 49, 103 – 123. Mulak, W., 1987. The catalytic action of cupric and ferric ions in nitric acid leaching of Ni3S2. Hydrometallurgy 17, 201 – 214. Mulak, W., Wawrzak, D., 1997. Leaching behaviour of millerite (h-NiS) in moderate oxidizing and reducing media. Proc. XX Int. Mineral Processing Congr., vol. 4. GMDB Clausthal-Zellerfeld, Germany, pp. 189 – 193. Nyquist, R.A., Kagel, R.O., 1971. Infrared Spectra of Inorganic Compounds (3800 – 45 cm 1). Academic Press, New York. Power, G.P., 1981. The electrochemistry of the nickel sulphides: I. NiS. Austr. J. Chem. 34, 2287 – 2296. Welham, N.J., 2001a. Mechanochemical processing of gold-bearing sulphides. Miner. Eng. 14, 341 – 347. Welham, N.J., 2001b. Mechanochemical processing of enargite (Cu3AsS4). Hydrometallurgy 62, 165 – 173.