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Effects of intensive grinding on the dissolution of celestite in acidic chloride medium
Minerals Engineering 22 (2009) 14–24
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Effects of intensive grinding on the dissolution of celestite in acidic chloride medium Murat Erdemog˘lu a,*, Salih Aydog˘an b, Eberhard Gock c a_ b c
Inönü University, Engineering Faculty, Department of Mining Engineering, 44280 Malatya, Turkey Selcßuk University, Engineering and Architecture Faculty, Department of Mining Engineering, 42075 Konya, Turkey Clausthal University of Technology, Institute of Mineral and Waste Processing and Dumping Technology, Walther Nernst Street 9, D-3392 Clausthal Zellerfeld, Germany
a r t i c l e
i n f o
Article history: Received 12 December 2007 Accepted 14 March 2008 Available online 28 April 2008 Keywords: Celestite Dissolution Intensive grinding Planetary ball mill
a b s t r a c t Effect of intensive grinding on the dissolution of celestite in acidic barium chloride and sodium chloride solutions was studied by investigating structural changes occurred during milling. Complete dissolution of the celestite was achieved within 25 min by milling in a planetary ball mill in which ball to ore ratio is 10. But, increasing grinding time and ball to ore ratio diminished the dissolution rate. X-ray amorphous phase content and XRD breadths increased and XRD line reflection intensity decreased with increasing of grinding time, and celestite does not undergo a considerable phase transformation during milling. SEM micrographs showed that how prolonged milling results in an increasing degree of agglomeration and a reduced amount of fines. Additionally, the ground samples were heated at elevated temperatures and then re-ground under earlier grinding conditions. Structural, morphological and dissolution characteristics of the samples obtained by each of the three treatments were also evaluated. It was concluded that high energy milling for short times increases the dissolution rate of celestite. But, increase in the particle size due to agglomeration or compactness of the particles as a result of impact stress predominant in planetary ball milling decreased the leaching rate, as grinding time and ball to ore ratio in the mill were increased. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Celestite (SrSO4) is the most mined strontium mineral as starting material for the manufacture of a variety of strontium chemicals such as SrCO3, SrCl2, Sr(NO3)2 and SrO. Among these chemicals, strontium carbonate (SrCO3) has great importance: Majority of the artificially produced strontium carbonate has been consumed as an additive in the production of faceplate glass of colour television picture tubes to block X-ray transmission and improve the appearance of the glass. Because of the problems either originated from the content and grade of celestite concentrate or the problems occurred during the conversion of celestite to SrS in the black ash process (Erdemog˘lu and Canbazog˘lu, 1998; Owusu and Litz, 2000) or to SrCO3 in the double decomposition method (Iwai and Toguri, 1989; Castillejos et al., 1996), improvements for the relevant process have been investigated by many of the researchers (Suárez-Orduña et al., 2004; Xu and Zhu, 2005; Erdemog˘lu et al., 2006, 2007; Obut et al., 2006). It is expected that investigations about the conversion of celestite to strontium carbonate will not be stopped since there are still certain drawbacks of the relevant methods, such as high energy consumption in the black ash process during high temperature leaching and acid costs in
* Corresponding author. Tel.: +90 422 3410010; fax: +90 422 3410046. E-mail address: [email protected] (M. Erdemog˘lu). 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.03.004
the double decomposition process. Apart from the methods signifying conversion of celestite to strontium sulphide or carbonate, dissolution of celestite by formation of the strontium tetrachloride complex was comprehensively studied by Aydog˘an et al. (2006). They have reported that celestite easily dissolves in hydrochloride acid solutions containing barium chloride, and sodium chloride appreciably enhances the dissolution rate. However, the leaching rate is so slow that under well mixed and optimum conditions, celestite completely dissolves within 180 min. Grinding has been used to make suitable size of particles in order to control the reactivity of the raw materials. It is known that dry grinding leads to random delamination of the silicate layers, to a strong structural alteration with important particle size reduction and to increase of surface area (McCormick and Froes, 1998; Balek et al., 2007). On the other hand, mechanical activation shows a wide range of potential applications. It has been reported that mechanical activation significantly improves the leaching kinetics of sulphide and oxide minerals. The effect is attributed to the increase of specific surface area and structural disorder (Balázˇ, 1996), enhanced strain (Balázˇ, 2000), amorphization of mineral crystals (Tkácˇová et al., 1993), microtopography (Tromans and Meech, 1999), and formation of new phases amenable to leaching (Welham and Llewellyn, 1998; Welham, 2001) and thermal reduction (Welham, 2002; Pourghahramani and Forsberg, 2007a,b), but could require a considerable amount of energy (Kleiv and Thornhill, 2007). Therefore, ultrafine grinding or mechanical processing
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
a
100
Ball to Ore ratio (w/w) = 10 Grinding Time, min. 1 5 15 30 45
Extracted Sr, %
80
60
40
Crushed ore (45x38 μm) 0 0
10
20
30
40
50
60
Time, min 100
Ball to Ore ratio (w/w) = 20 Grinding Time, min.
80
Extracted Sr, %
of minerals has been studied extensively in recent years and today with the growing interest in the unusual properties of the milled material. There are no reports in the literature on the effects of intensive grinding or mechanical activation on the crystalline structure of celestite mineral. The main purpose of this study is to investigate the effects of intensive grinding on celestite by applying high energy milling in a planetary ball mill. In order to evaluate the effects of intensive grinding, dissolution of ground and unground celestite in acidic chloride solution was compared. 2. Experimental
20
b
1 5 15 30 45
60
40
The material used in the study was a celestite concentrate obtained from Barit Maden TAS ß (Sivas) in Turkey. The chemical composition of the celestite concentrate is as %: 95.50 SrSO4; 3.25 CaSO4 2H2O; 0.77 BaSO4 and 0.31 Fe2O3. Particle size of the concentrate was as cumulative undersize %: 99.60, 2 mm; 9.28, 0.106 mm. d50 and d80 values were 0.380 and 0.750 mm, respectively. A Fritsch (Idar-Oberstein, Germany) Pulverisette 6 planetary ball mill was used for milling. Ball to sample ratio was adjusted to 10, 20 and 40 by using 70, 35 and 17.5 g air-dried sample, respectively, in a 250 cm3 hard metal tungsten carbide container using 700 g hard metal tungsten carbide balls (10 mm diameter and 85 pieces). Speed of revolution for the mill was kept constant at 500 min1. Samples were ground for 1, 5, 15, 30 or 45 min. The milling experiments were kept on for 5 min at 15 min intervals to prevent temperature rise in the mill. To evaluate the grinding intensity and planetary mill efficiency, the specific grinding work (stress energy), SE, was used, which is given as Pourghahramani and Forsberg (2007a), SE ðJ=kgÞ ¼
20
Crushed ore (45x38 μm) 0 0
10
20
30
40
50
60
Time, min
c
15
mB a n tM D mS
ð1Þ
where mB, mS, a, n, tM and D refer to mass of grinding media (kg), mass of material charge (kg), theoretical acceleration in the center of the bowl in the planetary mill (m/s2), speed of revolution (1/s), grinding time (s) and mill diameter (m), respectively. Theoretical acceleration for this planetary ball mill is 26.41 m/s2, as stated by the supplier. Accordingly, relationship between grinding time and
100
100
d 80(Feed) = 0.750 mm
80
60
Ball to Ore ratio (w/w) = 40 Grinding Time, min. 1 5 15 30 45
40
20
d 80, μm
Extracted Sr, %
80
60
40
Crushed ore (45x38 μm)
Ball to Ore Ratio (w/w)
20
10
0 0
10
20
30
40
50
60
Time, min Fig. 1. The plots representing the dissolution kinetics of celestite concentrate ground at the ball to ore ratio of (a) 10, (b) 20 and (c) 40.
40 0 0
10
20
30
40
50
Grinding time, min. Fig. 2. Change of d80 value of samples as a function of grinding time, ground at different ball to ore ratios.
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
16
a
A: SrSO4 B: Ore : Gypsum
Counts, a.u.
Ball to Ore ratio (w/w) = 10 Grinding time, min. 1 5 15 30 45
A B
20
22
24
26
28
30
32
34
2Theta, degree
b
Ball to Ore ratio (w/w) = 40
A: SrSO4 B: Ore : Gypsum
Counts, a.u.
Grinding time, min. 1 5 15 30 45
A B
20
22
24
26
28
30
32
34
2Theta, degree Fig. 3. XRD patterns illustrating the effect of grinding on the celestite ground for various periods at ball to ore ratio of (a) 10 and (b) 40.
1 Ball to Ore Ratio
I/I0
F/F0
10
0.8
40
I/I0 or F/F0
stress energy transferred to the particles being ground at each ball to ore ratios of 10, 20 and 40 was established as SE = 0.165tM, SE = 0.330tM and SE = 0.660tM, respectively. For dissolution of the samples, the method inspected by Aydog˘an et al. (2006) was used. The experimental conditions during the dissolution tests were as follows: HCl, 0.5 M; NaCl, 0.5 M; BaCl2, 8.25 103 M; rate of agitation, 400 min1; weight of the sample in one liter solution, 4 g; solution temperature, 60 °C. Strontium in the solution obtained after a dissolution experiment was determined by a SensAA Model flame atomic absorption spectrophotometer (GBC Scientific Equipment, Australia). Particle size analysis was performed using Mastersizer 2000 model particle size analyzer (Malvern Instruments Ltd, UK) operates in wet mode using distilled water. Particle size measurement was allowed to continue in anticipation of a relatively steady particle size distribution was established. The X-ray diffraction (XRD) measurements were carried out using a diffractometer X’Pert (Philips, Netherlands) over the 2h range of 20–35° at a scan rate of 0.01°/min with Cu Ka radiation. The XRD lines were identified by comparing the measured patterns of the samples to the JCPDS data cards. A Leo EVO 40 model scanning electron microscope (SEM) was used to examine particle and surface morphology of the samples.
0.6
0.4
0.2
0 0
400
800
1200
1600
2000
Stress energy, kJ/kg Fig. 4. Effect of stress energy released by planetary ball milling on the relative X-ray intensities (I/I0) and peak area ratios (F/F0) of celestite for 2H = 25.94°.
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2 þ HSO 4 ¼ H þ SO4
3. Results and discussion
ð3Þ +
3.1. Dissolution of celestite For dissolution of celestite in acidic barium chloride solution, the following equilibrium reaction is proposed by Aydog˘an et al. (2006): SrSO4 ðsÞ þ BaCl2 ðaqÞ ! BaSO4 ðsÞ þ SrCl2 ðaqÞ
ð2Þ
Driving force for this reaction is the relatively low solubility of BaSO4 (log Ksp = 9.96 at 20 °C) compared with SrSO4 (log Ksp = 6.62). HCl has significant effect on the celestite dissolution by releasing further SO2 4 ions from celestite to form HSO4 ions. They have found that the pH of the solution maintains its initial value until the end of the leaching, which is explained by deprotonation of bisulphate ion to give sulphate ion precipitating with Ba2+ ion, as
From this point of view, H ion acts as a catalyst in order to increase the dissolution rate of celestite. It has been reported by Reardon and Armstrong (1987) that solubility of celestite at the 25–50 °C temperature range increases with NaCl concentration up to 3 M. The most possible reaction for celestite in NaCl solution is described as,
SrSO4 þ 4Cl ¼ ½SrCl4 2 þ SO2 4
ð4Þ
with the formation of strontium tetrachloride complex, ½SrCl4 2 . By introducing Ba2+ ions to acidic sodium chloride solution, BaSO4 immediately precipitates, according to
H3 Oþ
SrSO4 þ 4Cl þ Ba2þ ! ½SrCl4 2 þ BaSO4
ð5Þ
Fig. 5. SEM micrographs of the particles from celestite concentrate (a and b) and the samples ground at different conditions: Ground for 1 min (c and d), 30 min (e and f) and 45 min (g and h) at ball to ore ratio of 10, and ground for 1 min (i and j) and 45 min (k and l) at ball to ore ratio of 40.
18
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Fig. 1 collectively illustrates the dissolution kinetics of the samples ground under various grinding conditions, namely at different ball to ore ratios, and also the sample crushed and classified to 45 + 38 lm size fraction. Only 44% of strontium is extracted by 60 min of dissolution of the crushed sample. However, considerable rates were achieved by dissolution of the samples obtained after each of varied grinding conditions. Comprehensively, when ball to ore ratio is 10, dissolution rate increased with the grinding time up to 30 min, so that strontium was completely extracted within 25 min of leaching. Nevertheless, a slight decrease in the dissolution rate was observed for the sample ground for 45 min (Fig. 1a). When ball to ore ratio is 20, the dissolution characteristics of the samples ground for various time periods were more complicated; however they have much faster kinetics than the crushed ore. In this case, 98% of strontium was extracted within 25 min leaching of the sample which was ground for 5 min. But, prolonged grinding caused a decrease in the dissolution rate (Fig. 1b). As the
ball to ore ratio was adjusted to 40, almost all of the strontium in the sample ground for 1 min was extracted within 25 min of leaching. But increasing grinding time extensively decreased the dissolution rates (Fig. 1c). These results revealed that the ball to ore ratio directly affects the dissolution characteristics. 3.2. Particle size, XRD and SEM analyses Reaction kinetics in leaching processes largely depends on the particle size. Ground celestite samples were subjected to particle size analysis. For the feed material and each of the sample ground at different grinding condition, particle size which occupies 80% of the materials (d80) was determined. Fig. 2 shows change of d80 value with respect to grinding time and ball to ore ratio. As the grinding time increases, d80 decreases from 33.56 to 18.32 lm up to 15 min of grinding at the ball to ore ratio of 10. But it increases for further grinding times. When the ball to ore ratio is 40, d80
Fig. 5 (continued)
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
was systematically increased from 19.76 to 94.45 lm. This suggests that the impact stress which is predominant in planetary ball milling causes compactness of the particles, especially in the range of finest grains, as grinding time and ball to ore ratio increases. It is likely that the practical limit to grinding for enhanced dissolution is the point at which the particles re-aggregate. According to Gock (1977), in case of micro scale comminution processes, the effect of inhomogenity points on a particle becomes less significant as particle size decreases. In a particle size range <1 lm, the resulting free enthalpies cause agglomerations. While, in traditional ball milling, a permanent alternation between destruction and formation of agglomerates characterizes the comminution process, the predominant impact stress in planetary ball milling causes agglomerates to form states of compaction, corresponding to a briquetted substance. Torres et al. (2007) concludes that all the defects created during the mechanical activation contribute to the mass transfer necessary for sintering. As the time of mechanical activation increases the number of defects increase, leading to a more dense material.
a
To appreciate the disordering in the bulk of minerals as a consequence of mechanical activation, the method of X-ray diffractometry is frequently applied (Balázˇ, 2003). In order to identify the effect of grinding on the crystalline structure of celestite, Xray diffraction analyses were performed on the samples ground at the ball to ore ratio of 10 and 40. Fig. 3 illustrates also the XRD patterns for commercially available analytical grade SrSO4 and the concentrate used for this study. It was observed either in Fig. 3a or in Fig. 3b that as the grinding time increases, X-ray amorphous phase content and XRD breadths increase and XRD line reflection intensity decreases. Celestite does not undergo a significant phase transformation during milling. The crystal phase is still SrSO4. The reason for this may be explained by structural characteristics of celestite crystal. Miyake et al. (1978) has extensively investigated on the crystal structures of three isostructural sulphates which known as insoluble, barite, celestit and anglesite, together with anhydrite. They have calculated average cation to oxygen bond lengths for barite, celestite and anglesite (2.952 Å, 2.831 Å and 2.87 Å, respectively), the average sulphur to oxygen
Ball to Ore ratio (w/w) = 10 Grinding time = 1 min.
: CaSO4 : CaSO4.2H2O : SrSO4
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Counts, a.u.
19
Ore
20
22
24
26
28
30
32
34
2Theta, degree
b
Ball to Ore ratio (w/w) = 10 Grinding time = 45 min.
: CaSO4.2H2O : SrSO4
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
30
32
34
2Theta, degree Fig. 6. Comparison of XRD patterns of the heat treated celestite preliminarily ground for (a) 1 min and (b) 45 min at ball to ore ratio of 10, and (c) 1 min and (d) 45 min at ball to ore ratio of 40.
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20
bond lengths (1.478 Å, 1.474 Å and 1.49 Å, respectively), the stretching force constants (6.27, 6.34 and 5.98 md/A, respectively), the bending force constants (0.46, 0.47 and 0.42 md/A, respectively), the repulsive force constants (0.73, 0.75, and 0.70 md/A, respectively). According to the authors, the observed relationship between the stretching force constant within the SO4 tetrahedra and the field strength in the alkaline-earth group compounds is the result of an interaction of the coulombic force between a metal ion and an oxygen ion. As the coulombic force between a metal ion and an oxygen ion increases, the restoring force between sulphur and oxygen atoms increases. The increase of the restoring force results in an increase of the stretching force constant in the SO4 tetrahedra. The mean sulphur–oxygen bond length become shorter with increasing the stretching force constant in this order PbSO4, SnSO4, BaSO4, SrSO4 and CaSO4. As stated also by Redfern and Parker (1998), the interaction between the sulphate groups and the cations in SrSO4, like BaSO4, is bidentate through the edges of the SO4 tetrahedra, which is leading to two orientations of sulphate groups by alternating across the pattern. However, CaSO4 has both monodentate and bidentate interactions between the sulphate groups and the cations.
c
The peaks belong to gypsum disappear within 1 min of grinding either at the ball to ore ratio of 10 or 40. Zhang et al. (1996) have reported that the dehydration of gypsum occurs not only by heating but also by a mechanochemical effect during grinding and dry grinding of gypsum makes it dehydrate more easily but not completely; The reduction of diffraction peaks intensity implies the formation of amorphous material. The decrease of X-ray diffraction intensities is accompanied by a general broadening of the XRD pattern. Consequently, the increase of the line breadths is due to the plastic deformation and disintegration of the material (Pourghahramani and Forsberg, 2007b). For quantification of the degree of milling intensity, the relative intensity loss (I/I0) against the feed material (I0) is measured. Besides, in order to consider the crystallite size effect appearing in the form of peak broadening, the ratio of peak areas (F/F0) instead of the intensity ratio is more specific (Dincßer et al., 2000). Fig. 4 shows the relative X-ray intensities (I/I0) and the peak area ratios (F/F0) at the lattice plane 002 of celestite with respect to stress energy at the ball to ore ratios of 10 and 40. Both X-ray intensity and peak area ratios sharply decrease with increasing stress energy. However, an area of little variation is realized after releasing
Ball to Ore ratio (w/w) = 40 Grinding time = 1 min.
: CaSO4 : CaSO4.2H2O : SrSO4
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
30
32
34
2Theta, degree
d
: CaSO4.2H2O : SrSO4
Ball to Ore ratio (w/w) = 40 Grinding time = 45 min.
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
2Theta, degree Fig. 6 (continued)
30
32
34
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
21
Fig. 7. SEM micrographs of the samples ground for 1 min (a and b) and 45 min (c and d) at the ball to ore ratio of 10, and 1 min (e and f) and 45 min (g and h) at the ball to ore ratio of 40, after heating at 1000 °C for 24 h.
approximately 200 kJ/kg energy by grinding; meaning that increasing either the ball to ore ratio or grinding time no longer affects the
crystal structure and crystallite size of celestite. This result suggests that, rather than crystallinity, agglomeration or compaction
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
22
a
100 1' G 30' G 80 45' G
Sr extracted, %
1' G + H 60 30' G + H 45' G + H 40 1' G + H + 1' RG 30' G + H + 30' RG 20 45' G + H + 45' RG
Ball to Ore ratio (w/w) = 10
Crushed Ore
0 0
10
20
30
40
50
60
Time, min
b
100 1' G
Sr extracted, %
80
45' G 1' G + H
60 45' G + H 40
1' G + H + 1' RG 45' G + H + 45' RG
20
Ball to Ore ratio (w/w) = 40
Crushed Ore
0 0
10
20
30
40
50
60
Time, min Fig. 8. Comparison of the dissolution kinetics of the celestite samples subjected to different grinding and/or heating treatments. The ball to ore ratio is (a) 10 and (b) 40. Symbols in the legends: G, grinding; H, heating at 1000 °C for 24 h; RG, re-grinding under the previous grinding conditions.
Ball to Ore ratio (w/w) = 10 Grinding time = 45 min.
Counts, a.u.
Treatment Ground Ground and Heated at 1000 °C Ground, Heated at 1000 °C, Re-ground
20
22
24
26
28
30
32
34
2Theta, degree Fig. 9. Comparison of X-ray diffraction patterns for the sample ground for 45 min, then heat treated at 1000 °C and afterward re-ground for 45 min.
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
occurred by increased stress energy predominates in the decrease in the leaching kinetics of celestite. SEM analyses were performed to observe morphological characteristics of the ground samples. As seen in Fig. 5, the SEM micrographs show how prolonged milling results in an increasing degree of agglomeration and a reduced amount of fines. Individual agglomerates are present in the fine fractions of all ground samples. As the milling intensity increases, the material consists entirely of agglomerates. The unmilled and 1 min ground samples consist of angular particles. Massive and compact agglomerates form during intensive grinding in the planetary ball mill. The agglomeration of fine particles has been also reported for sulphide minerals (Balázˇ, 2000) and for oxide minerals (Tkácˇová, 1989). It is observed that grinding for 45 min at the ball to ore ratio of 40 produces much more distinct agglomerates causing a drastic decrease in the dissolution rate. The agglomeration of particles during extended dry grinding is common and is explained by agglomeration of the structurally modified particles following the initial reduction of particle size. It has been reported that this phenomenon occurs because of the tendency of the activated material to reduce its surface free energy (Pourghahramani and Forsberg, 2007b). As described also by Fletcher and Welham (2000), the crystallites are assumed to break apart during the milling, their number increases with increasing milling time. The increase in crystallite number is accompanied by the generation of amorphous material that effectively coats the crystallite and binds them together into clumps. 3.3. Heat treatment and re-grinding To facilitate the effects of heat treatment on the structure of extensively ground celestite, the samples which were previously
23
ground for 1 and 45 min were heated in an air-circulated muffle furnace for 24 h at 600° and 1000 °C. Seen in Fig. 6 is re-appearance of main celestite peaks with increasing intensities and shrinking breadths. Amorphous content present in the samples begins to decrease as heating temperature increases. More interesting is that CaSO4 appears in the samples ground for 1 min and heated at 600 °C (Fig. 6a and c). However, there is no trace of crystalline anhydrite in the samples ground for 45 min (Fig. 6b and d). SEM micrographs of these samples exposed that heating causes sintering, so that fine particles and agglomerates are all connected after heating (Fig. 7). According to Torres et al. (2007), in order for sintering to occur, it is necessary to lowering free energy of the system. One of the driving forces for sintering is the curvature of the particle surfaces. Surface diffusion and lattice diffusion does not cause densification, however, reduces the curvature of the neck surface. This reduction in the neck surface, reduce the driving force necessary for densification mechanisms to take place such as grain boundary diffusion. In the meanwhile, disappearance of gypsum in the samples ground for prolonged times and of anhydrite in the ground and heated samples should also be considered, however any other solid phase like (Srx,Cay)SO4 was never detected by XRD analysis. The samples ground and treated at 1000 °C were subjected to grinding tests once more. Subsequently, these re-ground samples were leached again by the same way. Heating of the ground samples drastically decreased the dissolution rate, so that it is similar to dissolution rate of the crushed ore. However, dissolution rate increased again when heat treated ground samples were re-ground. But in this case, it never reached to its primary levels (Fig. 8). Figs. 9 and 10 display change of X-ray diffraction peak intensities by different treatments and SEM micrographs for selected samples,
Fig. 10. SEM microphotographs of the samples first ground for 45 min at the ball to ore of 10 (a and b) and 40 (c and d); then heated at 1000 °C for 24 h, and then re-ground under previous grinding conditions.
24
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
respectively. X-ray diffraction peak intensity which was recovered by heat treatment decreased much more by 45 min of re-grinding of already ground and heat treated sample (Fig. 9). As it was observed in SEM micrographs (Fig. 10) size of the particles in the re-ground samples is larger than the particle size of formerly ground samples, and re-grinding distorted sintered structure of the heated particles, causing relatively higher dissolution kinetics compared to the heated celestite, possibly due to decrease in the particle size. 4. Conclusions The aim of this study was to investigate the effects of intensive grinding on a celestite concentrate by applying high energy milling and to examine the extent of intensively ground celestite dissolution in acidic chloride solution. Complete dissolution of the celestite was achieved within 25 min by milling in a planetary ball mill in which ball to ore ratio is 10. But, increasing grinding time and ball to ore ratio entirely weakened the dissolution rate. X-ray amorphous phase content and XRD breadths increase and XRD line reflection intensity decreases with increasing grinding time, but celestite does not undergo phase transformation even after 45 min of grinding at the ball to ore ratio of 40. After releasing approximately 200 kJ/kg energy by planetary ball milling, increasing either the ball to ore ratio or grinding time no longer affects the crystal structure and crystallite size. SEM micrographs showed that prolonged milling give rise to an increasing degree of agglomeration and a reduced amount of fines. As grinding time increases, d80 decreases up to 15 min of grinding at ball to ore ratio of 10 while it increases for further grinding times. When ball to ore ratio is 40, d80 was systematically increased from 19.76 to 94.45 lm. Heat treatment eliminates the structural effects of intensive grinding and sharply decreases the dissolution rate. But, re-grinding of primarily ground and then heated samples considerably cause to an increase in the rate. Consequently, it was concluded that crystal structure of celestite is not easily disturbed even under insensitive grinding conditions, and agglomeration or compaction occurred by increased stress energy dominates on the decrease in the dissolution kinetics of celestite. Acknowledgements The authors would like to thank Gökhan Ucßar, Ali Aras, Mustafa Birinci and Petra Sommer for their technical support during characterization of the materials, and Dr.-Ing. Volker Vogt and Dr.Ing. Jörg Kähler for their helpful support during study at the Clausthal University of Technology. The financial support for the project _ BAPB 2006/50 by Inönü _ under grant IÜ University is gratefully acknowledged. The research activities were also supported financially by the Scientific and Technical Research Council of Turkey _ _ (TÜBITAK-B IDEB, Turkey) and German Research Foundation (Deutsche Forschungsgemeinschaft, DFG, Germany) to whom thanks are due. References Aydog˘an, S., Erdemog˘lu, M., Aras, A., Ucßar, G., Özkan, A., 2006. Dissolution kinetics of celestite (SrSO4) in HCl solution with BaCl2. Hydrometallurgy 84 (3–4), 239– 246.
Balázˇ, P., 1996. Influence of solid state properties on ferric chloride leaching of mechanically activated galena. Hydrometallurgy 40, 359–368. Balázˇ, P., 2000. Extractive Metallurgy of Activated Minerals. Elsevier, Amsterdam. Balázˇ, P., 2003. Mechanical activation in hydrometallurgy. International Journal of Mineral Processing 72, 341–354. Balek, V., Pérez-Maqueda, L.A., Poyato, J., Cˇerny´1, Z., Ramírez-Valle, V., Buntseva, I.M., Pérez-Rodríguez, J.L., 2007. Effect of grinding on thermal reactivity of ceramic clay minerals. Journal of Thermal Analysis and Calorimetry 88 (1), 87– 91. Castillejos, E.A.H., DelaCruz, B.F.P., Uribe, S.A., 1996. The direct conversion of celestite to strontium carbonate in sodium carbonate aqueous media. Hydrometallurgy 40, 207–222. Dincßer, H., Önal, G., Gock, E., 2000. The influence of different types of grinding stress on the production of chromium chemicals. In: Özbayog˘lu, G. et al. (Eds.), Mineral Processing on the Verge of the 21st Century. Balkema, Rotterdam, pp. 47–53. Erdemog˘lu, M., Canbazog˘lu, M., 1998. The leaching of SrS with water and the precipitation of SrCO3 from leach solution by different carbonating agents. Hydrometallurgy 49, 135–150. Erdemog˘lu, M., Sarıkaya, M., Canbazog˘lu, M., 2006. Leaching of celestite with sodium sulphide. Journal of Dispersion Science and Technology 27, 439–442. Erdemog˘lu, M., Aydog˘an, S., Canbazog˘lu, M., 2007. A kinetic study on the conversion of celestite (SrSO4) to SrCO3 by mechanochemical processing. Hydrometallurgy 86 (1–2), 1–5. Fletcher, N.H., Welham, N.J., 2000. Enhanced dissolution following extended milling. AIChE Journal 46 (3), 666–669. Gock, E., 1977. Beeinflussing des Löseverhaltens sulfidischer Rohstoffe durch Festkörperreaktionen bei der schwingmahlung. Habilitattionsschrift. Technical University of Berlin, Berlin. Iwai, M., Toguri, J.M., 1989. The leaching of celestite in sodium carbonate solution. Hydrometallurgy 22, 87–100. Kleiv, R.A., Thornhill, M., 2007. Production of mechanically activated rock flour fertilizer by high intensive ultrafine grinding. Minerals Engineering 20, 334– 341. McCormick, P.G., Froes, F.H., 1998. The fundamentals of mechanochemical processing. JOM 11, 61–65. Miyake, M., Minato, I., Morikawa, H., Iwai, S., 1978. Crystal structures and sulphate force constants of barite, celestite and anglesite. American Mineralogist 63, 506–510. _ 2006. Direct mechanochemical conversion of celestite Obut, A., Balázˇ, P., Girgin, I., to SrCO3. Minerals Engineering 19 (11), 1185–1190. Owusu, G., Litz, J.E., 2000. Water leaching of SrS and precipitation of SrCO3 using carbon dioxide as the precipitating agent. Hydrometallurgy 57, 23–29. Pourghahramani, P., Forsberg, E., 2007a. Effects of mechanical activation on the reduction behaviour of hematite concentrate. International Journal of Mineral Processing 82, 96–105. Pourghahramani, P., Forsberg, E., 2007b. Changes in the structure of hematite by extended dry grinding in relation to imposed stress energy. Powder Technology 178, 30–39. Reardon, E.J., Armstrong, D.K., 1987. Celestite (SrSO4(s)) solubility in water, seawater and NaCl solution. Geochimica et Cosmochimica Acta 51 (1), 63–72. Redfern, S.E., Parker, S.C., 1998. Atomistic simulation of the effects of calcium and strontium defects on the structure and stability of BaSO4. Faraday Transactions 94 (14), 1947–1952. Suárez-Orduña, R., Rendón-Angeles, J.C., López-Cuevas, L., Yanagisawa, K., 2004. The conversion of mineral celestite to strontianite under alkaline hydrothermal conditions. Journal of Physics: Condensed Materials 16, 1331–1344. Tkácˇová, K., 1989. Mechanical activation of minerals. Elsevier, Amsterdam. Tkácˇová, K., Balázˇ, P., Mišura, B., Vigdergauz, V.E., Chanturiya, V.A., 1993. Selective leaching of zinc from mechanically activated complex Cu–Pb–Zn concentrate. Hydrometallurgy 33 (39), 291–300. Torres, J., Almanza, J., Flores, A., Castro, M.de J., Herrera, M., 2007. Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures. Materials Characterization 58, 859–863. Tromans, D., Meech, J.A., 1999. Enhanced dissolution of minerals: microtopography and mechanical activation. Minerals Engineering 12, 609–625. Welham, N.J., 2001. Enhanced dissolution of tantalite/columbite following milling. International Journal of Mineral Processing 61, 145–154. Welham, N.J., 2002. Activation of the carbothermic reduction of manganese ore. International Journal of Mineral Processing 67, 187–198. Welham, N.J., Llewellyn, D.J., 1998. Mechanical enhancement of the dissolution of ilmenite. Minerals Engineering 11 (9), 827–841. Xu, X., Zhu, T., 2005. Coupled process of reaction and solvent extraction. I. The reaction between CO2 and SrCl2 coupled with solvent extraction of HCl. Hydrometallurgy 76, 11–17. Zhang, Q., Kasai, E., Saito, F., 1996. Mechanochemical changes in gypsum when dry ground with hydrated minerals. Powder Technology 87, 67–71.
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Effects of intensive grinding on the dissolution of celestite in acidic chloride medium Murat Erdemog˘lu a,*, Salih Aydog˘an b, Eberhard Gock c a_ b c
Inönü University, Engineering Faculty, Department of Mining Engineering, 44280 Malatya, Turkey Selcßuk University, Engineering and Architecture Faculty, Department of Mining Engineering, 42075 Konya, Turkey Clausthal University of Technology, Institute of Mineral and Waste Processing and Dumping Technology, Walther Nernst Street 9, D-3392 Clausthal Zellerfeld, Germany
a r t i c l e
i n f o
Article history: Received 12 December 2007 Accepted 14 March 2008 Available online 28 April 2008 Keywords: Celestite Dissolution Intensive grinding Planetary ball mill
a b s t r a c t Effect of intensive grinding on the dissolution of celestite in acidic barium chloride and sodium chloride solutions was studied by investigating structural changes occurred during milling. Complete dissolution of the celestite was achieved within 25 min by milling in a planetary ball mill in which ball to ore ratio is 10. But, increasing grinding time and ball to ore ratio diminished the dissolution rate. X-ray amorphous phase content and XRD breadths increased and XRD line reflection intensity decreased with increasing of grinding time, and celestite does not undergo a considerable phase transformation during milling. SEM micrographs showed that how prolonged milling results in an increasing degree of agglomeration and a reduced amount of fines. Additionally, the ground samples were heated at elevated temperatures and then re-ground under earlier grinding conditions. Structural, morphological and dissolution characteristics of the samples obtained by each of the three treatments were also evaluated. It was concluded that high energy milling for short times increases the dissolution rate of celestite. But, increase in the particle size due to agglomeration or compactness of the particles as a result of impact stress predominant in planetary ball milling decreased the leaching rate, as grinding time and ball to ore ratio in the mill were increased. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Celestite (SrSO4) is the most mined strontium mineral as starting material for the manufacture of a variety of strontium chemicals such as SrCO3, SrCl2, Sr(NO3)2 and SrO. Among these chemicals, strontium carbonate (SrCO3) has great importance: Majority of the artificially produced strontium carbonate has been consumed as an additive in the production of faceplate glass of colour television picture tubes to block X-ray transmission and improve the appearance of the glass. Because of the problems either originated from the content and grade of celestite concentrate or the problems occurred during the conversion of celestite to SrS in the black ash process (Erdemog˘lu and Canbazog˘lu, 1998; Owusu and Litz, 2000) or to SrCO3 in the double decomposition method (Iwai and Toguri, 1989; Castillejos et al., 1996), improvements for the relevant process have been investigated by many of the researchers (Suárez-Orduña et al., 2004; Xu and Zhu, 2005; Erdemog˘lu et al., 2006, 2007; Obut et al., 2006). It is expected that investigations about the conversion of celestite to strontium carbonate will not be stopped since there are still certain drawbacks of the relevant methods, such as high energy consumption in the black ash process during high temperature leaching and acid costs in
* Corresponding author. Tel.: +90 422 3410010; fax: +90 422 3410046. E-mail address: [email protected] (M. Erdemog˘lu). 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.03.004
the double decomposition process. Apart from the methods signifying conversion of celestite to strontium sulphide or carbonate, dissolution of celestite by formation of the strontium tetrachloride complex was comprehensively studied by Aydog˘an et al. (2006). They have reported that celestite easily dissolves in hydrochloride acid solutions containing barium chloride, and sodium chloride appreciably enhances the dissolution rate. However, the leaching rate is so slow that under well mixed and optimum conditions, celestite completely dissolves within 180 min. Grinding has been used to make suitable size of particles in order to control the reactivity of the raw materials. It is known that dry grinding leads to random delamination of the silicate layers, to a strong structural alteration with important particle size reduction and to increase of surface area (McCormick and Froes, 1998; Balek et al., 2007). On the other hand, mechanical activation shows a wide range of potential applications. It has been reported that mechanical activation significantly improves the leaching kinetics of sulphide and oxide minerals. The effect is attributed to the increase of specific surface area and structural disorder (Balázˇ, 1996), enhanced strain (Balázˇ, 2000), amorphization of mineral crystals (Tkácˇová et al., 1993), microtopography (Tromans and Meech, 1999), and formation of new phases amenable to leaching (Welham and Llewellyn, 1998; Welham, 2001) and thermal reduction (Welham, 2002; Pourghahramani and Forsberg, 2007a,b), but could require a considerable amount of energy (Kleiv and Thornhill, 2007). Therefore, ultrafine grinding or mechanical processing
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
a
100
Ball to Ore ratio (w/w) = 10 Grinding Time, min. 1 5 15 30 45
Extracted Sr, %
80
60
40
Crushed ore (45x38 μm) 0 0
10
20
30
40
50
60
Time, min 100
Ball to Ore ratio (w/w) = 20 Grinding Time, min.
80
Extracted Sr, %
of minerals has been studied extensively in recent years and today with the growing interest in the unusual properties of the milled material. There are no reports in the literature on the effects of intensive grinding or mechanical activation on the crystalline structure of celestite mineral. The main purpose of this study is to investigate the effects of intensive grinding on celestite by applying high energy milling in a planetary ball mill. In order to evaluate the effects of intensive grinding, dissolution of ground and unground celestite in acidic chloride solution was compared. 2. Experimental
20
b
1 5 15 30 45
60
40
The material used in the study was a celestite concentrate obtained from Barit Maden TAS ß (Sivas) in Turkey. The chemical composition of the celestite concentrate is as %: 95.50 SrSO4; 3.25 CaSO4 2H2O; 0.77 BaSO4 and 0.31 Fe2O3. Particle size of the concentrate was as cumulative undersize %: 99.60, 2 mm; 9.28, 0.106 mm. d50 and d80 values were 0.380 and 0.750 mm, respectively. A Fritsch (Idar-Oberstein, Germany) Pulverisette 6 planetary ball mill was used for milling. Ball to sample ratio was adjusted to 10, 20 and 40 by using 70, 35 and 17.5 g air-dried sample, respectively, in a 250 cm3 hard metal tungsten carbide container using 700 g hard metal tungsten carbide balls (10 mm diameter and 85 pieces). Speed of revolution for the mill was kept constant at 500 min1. Samples were ground for 1, 5, 15, 30 or 45 min. The milling experiments were kept on for 5 min at 15 min intervals to prevent temperature rise in the mill. To evaluate the grinding intensity and planetary mill efficiency, the specific grinding work (stress energy), SE, was used, which is given as Pourghahramani and Forsberg (2007a), SE ðJ=kgÞ ¼
20
Crushed ore (45x38 μm) 0 0
10
20
30
40
50
60
Time, min
c
15
mB a n tM D mS
ð1Þ
where mB, mS, a, n, tM and D refer to mass of grinding media (kg), mass of material charge (kg), theoretical acceleration in the center of the bowl in the planetary mill (m/s2), speed of revolution (1/s), grinding time (s) and mill diameter (m), respectively. Theoretical acceleration for this planetary ball mill is 26.41 m/s2, as stated by the supplier. Accordingly, relationship between grinding time and
100
100
d 80(Feed) = 0.750 mm
80
60
Ball to Ore ratio (w/w) = 40 Grinding Time, min. 1 5 15 30 45
40
20
d 80, μm
Extracted Sr, %
80
60
40
Crushed ore (45x38 μm)
Ball to Ore Ratio (w/w)
20
10
0 0
10
20
30
40
50
60
Time, min Fig. 1. The plots representing the dissolution kinetics of celestite concentrate ground at the ball to ore ratio of (a) 10, (b) 20 and (c) 40.
40 0 0
10
20
30
40
50
Grinding time, min. Fig. 2. Change of d80 value of samples as a function of grinding time, ground at different ball to ore ratios.
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
16
a
A: SrSO4 B: Ore : Gypsum
Counts, a.u.
Ball to Ore ratio (w/w) = 10 Grinding time, min. 1 5 15 30 45
A B
20
22
24
26
28
30
32
34
2Theta, degree
b
Ball to Ore ratio (w/w) = 40
A: SrSO4 B: Ore : Gypsum
Counts, a.u.
Grinding time, min. 1 5 15 30 45
A B
20
22
24
26
28
30
32
34
2Theta, degree Fig. 3. XRD patterns illustrating the effect of grinding on the celestite ground for various periods at ball to ore ratio of (a) 10 and (b) 40.
1 Ball to Ore Ratio
I/I0
F/F0
10
0.8
40
I/I0 or F/F0
stress energy transferred to the particles being ground at each ball to ore ratios of 10, 20 and 40 was established as SE = 0.165tM, SE = 0.330tM and SE = 0.660tM, respectively. For dissolution of the samples, the method inspected by Aydog˘an et al. (2006) was used. The experimental conditions during the dissolution tests were as follows: HCl, 0.5 M; NaCl, 0.5 M; BaCl2, 8.25 103 M; rate of agitation, 400 min1; weight of the sample in one liter solution, 4 g; solution temperature, 60 °C. Strontium in the solution obtained after a dissolution experiment was determined by a SensAA Model flame atomic absorption spectrophotometer (GBC Scientific Equipment, Australia). Particle size analysis was performed using Mastersizer 2000 model particle size analyzer (Malvern Instruments Ltd, UK) operates in wet mode using distilled water. Particle size measurement was allowed to continue in anticipation of a relatively steady particle size distribution was established. The X-ray diffraction (XRD) measurements were carried out using a diffractometer X’Pert (Philips, Netherlands) over the 2h range of 20–35° at a scan rate of 0.01°/min with Cu Ka radiation. The XRD lines were identified by comparing the measured patterns of the samples to the JCPDS data cards. A Leo EVO 40 model scanning electron microscope (SEM) was used to examine particle and surface morphology of the samples.
0.6
0.4
0.2
0 0
400
800
1200
1600
2000
Stress energy, kJ/kg Fig. 4. Effect of stress energy released by planetary ball milling on the relative X-ray intensities (I/I0) and peak area ratios (F/F0) of celestite for 2H = 25.94°.
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17
2 þ HSO 4 ¼ H þ SO4
3. Results and discussion
ð3Þ +
3.1. Dissolution of celestite For dissolution of celestite in acidic barium chloride solution, the following equilibrium reaction is proposed by Aydog˘an et al. (2006): SrSO4 ðsÞ þ BaCl2 ðaqÞ ! BaSO4 ðsÞ þ SrCl2 ðaqÞ
ð2Þ
Driving force for this reaction is the relatively low solubility of BaSO4 (log Ksp = 9.96 at 20 °C) compared with SrSO4 (log Ksp = 6.62). HCl has significant effect on the celestite dissolution by releasing further SO2 4 ions from celestite to form HSO4 ions. They have found that the pH of the solution maintains its initial value until the end of the leaching, which is explained by deprotonation of bisulphate ion to give sulphate ion precipitating with Ba2+ ion, as
From this point of view, H ion acts as a catalyst in order to increase the dissolution rate of celestite. It has been reported by Reardon and Armstrong (1987) that solubility of celestite at the 25–50 °C temperature range increases with NaCl concentration up to 3 M. The most possible reaction for celestite in NaCl solution is described as,
SrSO4 þ 4Cl ¼ ½SrCl4 2 þ SO2 4
ð4Þ
with the formation of strontium tetrachloride complex, ½SrCl4 2 . By introducing Ba2+ ions to acidic sodium chloride solution, BaSO4 immediately precipitates, according to
H3 Oþ
SrSO4 þ 4Cl þ Ba2þ ! ½SrCl4 2 þ BaSO4
ð5Þ
Fig. 5. SEM micrographs of the particles from celestite concentrate (a and b) and the samples ground at different conditions: Ground for 1 min (c and d), 30 min (e and f) and 45 min (g and h) at ball to ore ratio of 10, and ground for 1 min (i and j) and 45 min (k and l) at ball to ore ratio of 40.
18
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
Fig. 1 collectively illustrates the dissolution kinetics of the samples ground under various grinding conditions, namely at different ball to ore ratios, and also the sample crushed and classified to 45 + 38 lm size fraction. Only 44% of strontium is extracted by 60 min of dissolution of the crushed sample. However, considerable rates were achieved by dissolution of the samples obtained after each of varied grinding conditions. Comprehensively, when ball to ore ratio is 10, dissolution rate increased with the grinding time up to 30 min, so that strontium was completely extracted within 25 min of leaching. Nevertheless, a slight decrease in the dissolution rate was observed for the sample ground for 45 min (Fig. 1a). When ball to ore ratio is 20, the dissolution characteristics of the samples ground for various time periods were more complicated; however they have much faster kinetics than the crushed ore. In this case, 98% of strontium was extracted within 25 min leaching of the sample which was ground for 5 min. But, prolonged grinding caused a decrease in the dissolution rate (Fig. 1b). As the
ball to ore ratio was adjusted to 40, almost all of the strontium in the sample ground for 1 min was extracted within 25 min of leaching. But increasing grinding time extensively decreased the dissolution rates (Fig. 1c). These results revealed that the ball to ore ratio directly affects the dissolution characteristics. 3.2. Particle size, XRD and SEM analyses Reaction kinetics in leaching processes largely depends on the particle size. Ground celestite samples were subjected to particle size analysis. For the feed material and each of the sample ground at different grinding condition, particle size which occupies 80% of the materials (d80) was determined. Fig. 2 shows change of d80 value with respect to grinding time and ball to ore ratio. As the grinding time increases, d80 decreases from 33.56 to 18.32 lm up to 15 min of grinding at the ball to ore ratio of 10. But it increases for further grinding times. When the ball to ore ratio is 40, d80
Fig. 5 (continued)
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
was systematically increased from 19.76 to 94.45 lm. This suggests that the impact stress which is predominant in planetary ball milling causes compactness of the particles, especially in the range of finest grains, as grinding time and ball to ore ratio increases. It is likely that the practical limit to grinding for enhanced dissolution is the point at which the particles re-aggregate. According to Gock (1977), in case of micro scale comminution processes, the effect of inhomogenity points on a particle becomes less significant as particle size decreases. In a particle size range <1 lm, the resulting free enthalpies cause agglomerations. While, in traditional ball milling, a permanent alternation between destruction and formation of agglomerates characterizes the comminution process, the predominant impact stress in planetary ball milling causes agglomerates to form states of compaction, corresponding to a briquetted substance. Torres et al. (2007) concludes that all the defects created during the mechanical activation contribute to the mass transfer necessary for sintering. As the time of mechanical activation increases the number of defects increase, leading to a more dense material.
a
To appreciate the disordering in the bulk of minerals as a consequence of mechanical activation, the method of X-ray diffractometry is frequently applied (Balázˇ, 2003). In order to identify the effect of grinding on the crystalline structure of celestite, Xray diffraction analyses were performed on the samples ground at the ball to ore ratio of 10 and 40. Fig. 3 illustrates also the XRD patterns for commercially available analytical grade SrSO4 and the concentrate used for this study. It was observed either in Fig. 3a or in Fig. 3b that as the grinding time increases, X-ray amorphous phase content and XRD breadths increase and XRD line reflection intensity decreases. Celestite does not undergo a significant phase transformation during milling. The crystal phase is still SrSO4. The reason for this may be explained by structural characteristics of celestite crystal. Miyake et al. (1978) has extensively investigated on the crystal structures of three isostructural sulphates which known as insoluble, barite, celestit and anglesite, together with anhydrite. They have calculated average cation to oxygen bond lengths for barite, celestite and anglesite (2.952 Å, 2.831 Å and 2.87 Å, respectively), the average sulphur to oxygen
Ball to Ore ratio (w/w) = 10 Grinding time = 1 min.
: CaSO4 : CaSO4.2H2O : SrSO4
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Counts, a.u.
19
Ore
20
22
24
26
28
30
32
34
2Theta, degree
b
Ball to Ore ratio (w/w) = 10 Grinding time = 45 min.
: CaSO4.2H2O : SrSO4
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
30
32
34
2Theta, degree Fig. 6. Comparison of XRD patterns of the heat treated celestite preliminarily ground for (a) 1 min and (b) 45 min at ball to ore ratio of 10, and (c) 1 min and (d) 45 min at ball to ore ratio of 40.
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
20
bond lengths (1.478 Å, 1.474 Å and 1.49 Å, respectively), the stretching force constants (6.27, 6.34 and 5.98 md/A, respectively), the bending force constants (0.46, 0.47 and 0.42 md/A, respectively), the repulsive force constants (0.73, 0.75, and 0.70 md/A, respectively). According to the authors, the observed relationship between the stretching force constant within the SO4 tetrahedra and the field strength in the alkaline-earth group compounds is the result of an interaction of the coulombic force between a metal ion and an oxygen ion. As the coulombic force between a metal ion and an oxygen ion increases, the restoring force between sulphur and oxygen atoms increases. The increase of the restoring force results in an increase of the stretching force constant in the SO4 tetrahedra. The mean sulphur–oxygen bond length become shorter with increasing the stretching force constant in this order PbSO4, SnSO4, BaSO4, SrSO4 and CaSO4. As stated also by Redfern and Parker (1998), the interaction between the sulphate groups and the cations in SrSO4, like BaSO4, is bidentate through the edges of the SO4 tetrahedra, which is leading to two orientations of sulphate groups by alternating across the pattern. However, CaSO4 has both monodentate and bidentate interactions between the sulphate groups and the cations.
c
The peaks belong to gypsum disappear within 1 min of grinding either at the ball to ore ratio of 10 or 40. Zhang et al. (1996) have reported that the dehydration of gypsum occurs not only by heating but also by a mechanochemical effect during grinding and dry grinding of gypsum makes it dehydrate more easily but not completely; The reduction of diffraction peaks intensity implies the formation of amorphous material. The decrease of X-ray diffraction intensities is accompanied by a general broadening of the XRD pattern. Consequently, the increase of the line breadths is due to the plastic deformation and disintegration of the material (Pourghahramani and Forsberg, 2007b). For quantification of the degree of milling intensity, the relative intensity loss (I/I0) against the feed material (I0) is measured. Besides, in order to consider the crystallite size effect appearing in the form of peak broadening, the ratio of peak areas (F/F0) instead of the intensity ratio is more specific (Dincßer et al., 2000). Fig. 4 shows the relative X-ray intensities (I/I0) and the peak area ratios (F/F0) at the lattice plane 002 of celestite with respect to stress energy at the ball to ore ratios of 10 and 40. Both X-ray intensity and peak area ratios sharply decrease with increasing stress energy. However, an area of little variation is realized after releasing
Ball to Ore ratio (w/w) = 40 Grinding time = 1 min.
: CaSO4 : CaSO4.2H2O : SrSO4
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
30
32
34
2Theta, degree
d
: CaSO4.2H2O : SrSO4
Ball to Ore ratio (w/w) = 40 Grinding time = 45 min.
Counts, a.u.
Treatment Ground Ground and heated at 600 °C Ground and heated at 1000 °C
Ore
20
22
24
26
28
2Theta, degree Fig. 6 (continued)
30
32
34
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
21
Fig. 7. SEM micrographs of the samples ground for 1 min (a and b) and 45 min (c and d) at the ball to ore ratio of 10, and 1 min (e and f) and 45 min (g and h) at the ball to ore ratio of 40, after heating at 1000 °C for 24 h.
approximately 200 kJ/kg energy by grinding; meaning that increasing either the ball to ore ratio or grinding time no longer affects the
crystal structure and crystallite size of celestite. This result suggests that, rather than crystallinity, agglomeration or compaction
M. Erdemog˘lu et al. / Minerals Engineering 22 (2009) 14–24
22
a
100 1' G 30' G 80 45' G
Sr extracted, %
1' G + H 60 30' G + H 45' G + H 40 1' G + H + 1' RG 30' G + H + 30' RG 20 45' G + H + 45' RG
Ball to Ore ratio (w/w) = 10
Crushed Ore
0 0
10
20
30
40
50
60
Time, min
b
100 1' G
Sr extracted, %
80
45' G 1' G + H
60 45' G + H 40
1' G + H + 1' RG 45' G + H + 45' RG
20
Ball to Ore ratio (w/w) = 40
Crushed Ore
0 0
10
20
30
40
50
60
Time, min Fig. 8. Comparison of the dissolution kinetics of the celestite samples subjected to different grinding and/or heating treatments. The ball to ore ratio is (a) 10 and (b) 40. Symbols in the legends: G, grinding; H, heating at 1000 °C for 24 h; RG, re-grinding under the previous grinding conditions.
Ball to Ore ratio (w/w) = 10 Grinding time = 45 min.
Counts, a.u.
Treatment Ground Ground and Heated at 1000 °C Ground, Heated at 1000 °C, Re-ground
20
22
24
26
28
30
32
34
2Theta, degree Fig. 9. Comparison of X-ray diffraction patterns for the sample ground for 45 min, then heat treated at 1000 °C and afterward re-ground for 45 min.
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occurred by increased stress energy predominates in the decrease in the leaching kinetics of celestite. SEM analyses were performed to observe morphological characteristics of the ground samples. As seen in Fig. 5, the SEM micrographs show how prolonged milling results in an increasing degree of agglomeration and a reduced amount of fines. Individual agglomerates are present in the fine fractions of all ground samples. As the milling intensity increases, the material consists entirely of agglomerates. The unmilled and 1 min ground samples consist of angular particles. Massive and compact agglomerates form during intensive grinding in the planetary ball mill. The agglomeration of fine particles has been also reported for sulphide minerals (Balázˇ, 2000) and for oxide minerals (Tkácˇová, 1989). It is observed that grinding for 45 min at the ball to ore ratio of 40 produces much more distinct agglomerates causing a drastic decrease in the dissolution rate. The agglomeration of particles during extended dry grinding is common and is explained by agglomeration of the structurally modified particles following the initial reduction of particle size. It has been reported that this phenomenon occurs because of the tendency of the activated material to reduce its surface free energy (Pourghahramani and Forsberg, 2007b). As described also by Fletcher and Welham (2000), the crystallites are assumed to break apart during the milling, their number increases with increasing milling time. The increase in crystallite number is accompanied by the generation of amorphous material that effectively coats the crystallite and binds them together into clumps. 3.3. Heat treatment and re-grinding To facilitate the effects of heat treatment on the structure of extensively ground celestite, the samples which were previously
23
ground for 1 and 45 min were heated in an air-circulated muffle furnace for 24 h at 600° and 1000 °C. Seen in Fig. 6 is re-appearance of main celestite peaks with increasing intensities and shrinking breadths. Amorphous content present in the samples begins to decrease as heating temperature increases. More interesting is that CaSO4 appears in the samples ground for 1 min and heated at 600 °C (Fig. 6a and c). However, there is no trace of crystalline anhydrite in the samples ground for 45 min (Fig. 6b and d). SEM micrographs of these samples exposed that heating causes sintering, so that fine particles and agglomerates are all connected after heating (Fig. 7). According to Torres et al. (2007), in order for sintering to occur, it is necessary to lowering free energy of the system. One of the driving forces for sintering is the curvature of the particle surfaces. Surface diffusion and lattice diffusion does not cause densification, however, reduces the curvature of the neck surface. This reduction in the neck surface, reduce the driving force necessary for densification mechanisms to take place such as grain boundary diffusion. In the meanwhile, disappearance of gypsum in the samples ground for prolonged times and of anhydrite in the ground and heated samples should also be considered, however any other solid phase like (Srx,Cay)SO4 was never detected by XRD analysis. The samples ground and treated at 1000 °C were subjected to grinding tests once more. Subsequently, these re-ground samples were leached again by the same way. Heating of the ground samples drastically decreased the dissolution rate, so that it is similar to dissolution rate of the crushed ore. However, dissolution rate increased again when heat treated ground samples were re-ground. But in this case, it never reached to its primary levels (Fig. 8). Figs. 9 and 10 display change of X-ray diffraction peak intensities by different treatments and SEM micrographs for selected samples,
Fig. 10. SEM microphotographs of the samples first ground for 45 min at the ball to ore of 10 (a and b) and 40 (c and d); then heated at 1000 °C for 24 h, and then re-ground under previous grinding conditions.
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respectively. X-ray diffraction peak intensity which was recovered by heat treatment decreased much more by 45 min of re-grinding of already ground and heat treated sample (Fig. 9). As it was observed in SEM micrographs (Fig. 10) size of the particles in the re-ground samples is larger than the particle size of formerly ground samples, and re-grinding distorted sintered structure of the heated particles, causing relatively higher dissolution kinetics compared to the heated celestite, possibly due to decrease in the particle size. 4. Conclusions The aim of this study was to investigate the effects of intensive grinding on a celestite concentrate by applying high energy milling and to examine the extent of intensively ground celestite dissolution in acidic chloride solution. Complete dissolution of the celestite was achieved within 25 min by milling in a planetary ball mill in which ball to ore ratio is 10. But, increasing grinding time and ball to ore ratio entirely weakened the dissolution rate. X-ray amorphous phase content and XRD breadths increase and XRD line reflection intensity decreases with increasing grinding time, but celestite does not undergo phase transformation even after 45 min of grinding at the ball to ore ratio of 40. After releasing approximately 200 kJ/kg energy by planetary ball milling, increasing either the ball to ore ratio or grinding time no longer affects the crystal structure and crystallite size. SEM micrographs showed that prolonged milling give rise to an increasing degree of agglomeration and a reduced amount of fines. As grinding time increases, d80 decreases up to 15 min of grinding at ball to ore ratio of 10 while it increases for further grinding times. When ball to ore ratio is 40, d80 was systematically increased from 19.76 to 94.45 lm. Heat treatment eliminates the structural effects of intensive grinding and sharply decreases the dissolution rate. But, re-grinding of primarily ground and then heated samples considerably cause to an increase in the rate. Consequently, it was concluded that crystal structure of celestite is not easily disturbed even under insensitive grinding conditions, and agglomeration or compaction occurred by increased stress energy dominates on the decrease in the dissolution kinetics of celestite. Acknowledgements The authors would like to thank Gökhan Ucßar, Ali Aras, Mustafa Birinci and Petra Sommer for their technical support during characterization of the materials, and Dr.-Ing. Volker Vogt and Dr.Ing. Jörg Kähler for their helpful support during study at the Clausthal University of Technology. The financial support for the project _ BAPB 2006/50 by Inönü _ under grant IÜ University is gratefully acknowledged. The research activities were also supported financially by the Scientific and Technical Research Council of Turkey _ _ (TÜBITAK-B IDEB, Turkey) and German Research Foundation (Deutsche Forschungsgemeinschaft, DFG, Germany) to whom thanks are due. References Aydog˘an, S., Erdemog˘lu, M., Aras, A., Ucßar, G., Özkan, A., 2006. Dissolution kinetics of celestite (SrSO4) in HCl solution with BaCl2. Hydrometallurgy 84 (3–4), 239– 246.
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