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Dissolution of magnesium from calcined serpentinite in hydrochloric acid
Minerals Engineering 32 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Technical Note
Dissolution of magnesium from calcined serpentinite in hydrochloric acid Alena Fedorocˇková, Martin Hreus, Pavel Raschman ⇑, Gabriel Sucˇik Technical University of Košice, Faculty of Metallurgy, Letná 9, Košice, Slovak Republic
a r t i c l e
i n f o
Article history: Received 12 November 2011 Accepted 7 March 2012 Available online 22 April 2012 Keywords: Serpentinite Serpentine Calcination Magnesium Reactivity Dissolution
a b s t r a c t In the production of pure magnesium compounds from serpentinite, acid leaching is usually the first stage of the overall process. However, faster magnesium dissolution can be achieved and the size of a potential leaching reactor can be reduced if serpentinite is calcined prior to leaching. Moreover, use of calcined serpentinite can reduce problems relating to corrosion of the reactor (lower leaching temperatures and pressures can be applied) and foam formation (chemically bonded water, which forms bubbles when released in the reactor, can be removed by calcining). This paper examines how calcination temperature and time influence the amount of magnesium dissolved during the initial period of leaching of calcined serpentine in hydrochloric acid. Fine-grained serpentinite calcined between 640 °C and 700 °C displayed the highest reactivity. The fraction of magnesium dissolved was up to 30-times higher as compared to leaching of uncalcined serpentine under identical reaction conditions. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Serpentinite is a rock which consists predominantly of one or more serpentine group minerals. The name serpentine is mainly applied to three hydrated magnesium silicate minerals: antigorite, chrysotile, and lizardite. They all have essentially the same chemical composition [(Mg,Fe)3Si2O5(OH)4] but different crystal structures. Currently serpentinites are used as railway ballasts, building materials, or thermal/electrical insulation. Large serpentinite reserves have a potential to find use also in more sophisticated applications, such as carbon dioxide capture and storage (Metz et al., 2005), as a potential source of nickel (McDonald and Whittington, 2008a,b) or as a raw material in the production of magnesium metal and/or pure magnesium compounds. In the production of pure magnesium compounds, acid leaching of serpentine is usually the first stage of the overall process. Leaching generates a soluble magnesium salt, which builds up in solution and is then separated from the insoluble residue, and later refined. Commonly used lixiviants are hydrochloric acid (Dutrizac et al., 2000; Taubert, 2000; Nagamori and Boivin, 2001), and other inorganic and organic acids. Serpentines consist of acidic silicate layers and basic brucitelike layers, alternating each other. This double-layered structure results in the acid–base character of serpentines, which causes them to be resistant to acidic and alkaline solutions. To accelerate
⇑ Corresponding author. E-mail address: [email protected] (P. Raschman). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.03.006
leaching, Zhang et al. (1997) applied mechanochemical treatment to the serpentine. An alternative approach could be calcination. During calcination, water is released according to Eq. (1), thus causing the compact, double-layered structure of serpentine to become disordered (Zulumyan et al., 2007). The intermediate Mg3Si2O7(s), formed in Eq. (1) and defined as dehydroxylate I (MacKenzie and Meinhold, 1994), is therefore expected to be more reactive than the original (uncalcined) serpentine, thus displaying faster magnesium dissolution in acidic solutions. The calculated equilibrium temperature for the thermal decomposition of serpentine (i.e., when the water vapour partial pressure reaches 101,325 Pa) is 539 °C (Roine, 2002). At temperatures above 800 °C, dehydroxylate I is converted into forsterite (Eq. (2)). Conversion of serpentine to forsterite as a main product, accompanied by amorphous silica, at temperatures between 800 °C and 1000 °C (Brindley and Hayami, 1965) is described by Eq. (3).
Mg3 Si2 O5 ðOHÞ4 ðsÞ ! Mg3 Si2 O7 ðsÞ þ 2H2 OðgÞ
ð1Þ
2Mg3 Si2 O7 ðsÞ ! 3Mg2 SiO4 þ SiO2 ðsÞ
ð2Þ
2Mg3 Si2 O5 ðOHÞ4 ðsÞ ! 3Mg2 SiO4 ðsÞ þ SiO2 ðsÞ þ 4H2 OðgÞ
ð3Þ
It is hypothesized that calcination of serpentine could accelerate magnesium dissolution, reduce the leaching time and, consequently, the size of the vessel. Moreover, use of calcined serpentinite could reduce problems relating to the corrosion of the vessel since lower leaching temperatures and pressures can be used during leaching. Foam formation in the reactor could be minimized because the chemically bonded water that would be released as bubbles in the vessel, could otherwise be released during calcination.
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
2
Fig. 1. 3D in situ high-temperature X-ray powder diffraction sequence illustrating the phase transformation of serpentine to forsterite during heating the serpentinite.
Herein, we explore how calcination of serpentinite can enhance the dissolution of magnesium in hydrochloric acid. The literature reports on the effects of hydrochloric acid concentration, leaching temperature and serpentinite particle size on the dissolution of magnesium (Dutrizac et al., 2000; Teir et al., 2007), but there were no publications reporting on the effects of calcination temperature and calcination time. Results showing the relationship between the conditions of calcining and the amount of magnesium dissolved are briefly discussed.
2. Experimental procedures 2.1. Materials Bulk material obtained from the mining and processing of serpentinite in Dobšiná, Slovakia was used in our laboratory tests. Two test samples with particle sizes up to 315 lm, or ranging from 315 to 500 lm (referred to as SED 0–315 and SED 315–500, respectively) were prepared by dry-screening. The samples contained 22.0–22.2% magnesium, 17.4–17.9% silicon, 5.8–5.9% iron, 12.8–13.0% chemically bonded water and carbon dioxide, and minor elements (calcium, aluminium, and nickel). Mineral phases identified were: lizardite, chrysotile, calcite, magnetite, and orthopyroxene.
2.2. Calcination of serpentinite First, to find the calcination temperature, thermal decomposition of serpentine was studied by differential thermal analysis (DTA) and thermogravimetric analysis (TGA) as well as by hightemperature in situ X-ray powder diffraction between 100 °C and 1000 °C. Then, calcined samples were prepared as follows: ten grams of serpentinite was placed in a crucible with a lid and inserted in a muffle furnace where the sample was heated at a predefined temperature (between 600 °C and 830 °C) for a selected time (10–150 min). After calcination, the weight of each sample was measured to calculate the weight loss.
Mineralogical composition and crystallinity of uncalcined and calcined serpentinite were determined by X-ray powder diffraction (XRD). 2.3. Leaching tests The reactivity of uncalcined or calcined serpentinite was determined as follows: one gram of sample was dissolved at 20 °C in 250 ml of 0.25 M hydrochloric acid. The suspension was stirred at 500 min1. Aliquots of the slurry were withdrawn after 5 min, filtered and the filtrate analyzed. In the present study, the reactivity of calcined serpentinite was expressed in terms of the fraction of magnesium dissolved according to the following equation:
MgOðsÞ þ 2HClðaq:Þ ! Mg2þ ðaq:Þ þ 2Cl ðaq:Þ þ H2 OðlÞ
ð4Þ
The fraction of magnesium dissolved, XL, was calculated from the following equation:
XL ¼
25V KIII cKIII M MgO m wMgO
ð5Þ
where VKIII and cKIII equals the volume and concentration of the Komplexon III solution consumed for the titration of magnesium that was present in 10 ml of filtrate, respectively, m is the weight of the sample leached (equal to 1 g), MMgO = 40.3 g mol1 is the molar weight of MgO, and wMgO is the weight fraction of MgO in the uncalcined or calcined serpentinite tested. 3. Results and discussion 3.1. Thermal decomposition of serpentine DTA showed that the thermal decomposition of serpentine started at 640 °C and the maximum endothermic effect was observed at 693 °C. Thermal decomposition of serpentine caused a weight loss due to the release of water (Eq. (1)). TGA revealed a maximum rate of thermal decomposition at 693 °C. An exothermic effect with an onset at 810 °C was also determined by DTA, which indicates that forsterite is formed in accordance with Eq. (2).
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
(a) Fraction of magnesium dissolved, XL (-)
100
is expected to yield magnesium of lower reactivity as a result of forsterite recrystallization and/or formation.
80
3.2. Hydrochloric acid leaching of calcined serpentine The influence of the calcination temperature and time on the amount of magnesium dissolved during leaching is illustrated in Fig. 2. While the reactivity XL (defined in Eq. (5)) increases gradually with the calcination time at temperatures below 700 °C, a different situation was observed at temperatures above 700 °C: XL increased with the calcination time at constant temperature until it reached a maximum, and then it decreased. For example, the maximum reactivity of fine-grained serpentinite calcined at 720 °C and 760 °C was achieved after 20 and 10 min, respectively (Fig. 2a). A similar trend was observed for coarse-grained serpentinite for which the maximum reactivity at 760 °C was achieved after 15 min, and after 7 min at 830 °C (Fig. 2b). The results shown in Fig. 2 suggest the following:
60
40
20
0
0
20
40
60
Calcination time (min)
(b)
600°C
640°C
720°C
760°C
800°C
uncalcined
Fraction of magnesium dissolved, X L (-)
100
80
60
40
20
0
3
0
40
80
120
Calcination time (min) 640°C
760°C
830°C
uncalcined
Fig. 2. Influence of calcination temperature and time on fraction of magnesium dissolved after 5 min of leaching for (a) sample SED 0–315 and (b) sample SED 315–500.
(a) The non-calcined serpentine leached only 2.2% of the magnesium present. Under optimum conditions of calcination, the calcined serpentine leached close to 70% magnesium. (b) About 10% less magnesium was leached from coarser serpentinite particles. (c) Increasing the calcination time increased reactivity when the calcination temperature was less than 700 °C. Above that, increasing the calcination time decreased the reactivity due potentially to the formation of forsterite. There is thus an interaction between calcination time and temperature. One cannot postulate about the effect of increasing the temperature on the leaching behaviour without considering the calcination time.
The initial magnesium dissolution rate was estimated to be 1–2 104 mol m2 s1 and 6–9 108 mol m2 s1 for calcined serpentinite of the highest and lowest reactivity, respectively (in these calculations, a specific surface area between 6 m2 g1 and 8 m2 g1 for calcined serpentinite samples was used, as determined using the B.E.T. nitrogen adsorption technique). The published value of the rate of forsterite dissolution in hydrochloric acid at 25 °C and pH = 1 is 6.3 108 mol m2 s1 (Pokrovsky and Schott, 2000). Such a rate is up to 4 orders of magnitude lower than the maximum dissolution rate of magnesium observed in the present work, but it is comparable to the lowest one. Hence, the assumption that calcined serpentinite is deactivated due to crystalline forsterite formation seems to be reasonable. (d) The optimum conditions appear to be a calcination temperature less than 700 °C with calcination time of 60 min. (e) The effects of calcination time and temperature are the same on coarse-grained and fine-grained serpentine. 4. Conclusions
The influence of the calcination temperature between 400 °C and 1000 °C on phase composition and crystallinity is illustrated in Fig. 1. Starting from 600 °C, the XRD count from the main phases originally present, lizardite and chrysotile, is getting progressively lower. Between 600 °C and 800 °C, these minerals gradually disappear and the structure of calcined sample appears to be highly disordered. At temperatures above 800 °C, a new phase, forsterite, is formed. DTA, TGA and XRD analyses suggest that the samples should be calcined at temperatures between 600 °C and 800 °C to produce a reactive form of magnesium. Calcination at higher temperatures
The effects of calcination temperature and calcination time on the dissolution of magnesium from calcined serpentinite in hydrochloric acid were examined in this study. Sixty minutes exposure to temperatures between 640 °C and 700 °C produced the most reactive calcine. The fraction of magnesium dissolved after 5 min of leaching was found to be up to 70%, compared to 2.2% for uncalcined serpentinite. Temperatures above 700 °C for less than 20 min produced slightly less reactive magnesium. Longer calcination time caused deactivation of calcined serpentinite.
4
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
Acknowledgements This study was financially supported by the Grant Agency of the Ministry of Education of the Slovak Republic and Slovak Academy of Sciences (Grant No. 1/0267/09). Suggestions and recommendations of the two reviewers are greatly acknowledged. References Brindley, G.W., Hayami, R., 1965. Mechanisms of formation of forsterite and enstatite from serpentine. Miner. Mag. 35, 189–195. Dutrizac, J.E., Chen, T.T., White, C.W. 2000. Fundamentals of serpentine leaching in hydrochloric acid media. In: Kaplan H.I., Hryn J.N., Clow B.B. (Eds.), Magnesium Technology. The Minerals, Metals & Materials Society, Nashville. TMS Annual Meeting, pp. 41–51. MacKenzie, K.J.D., Meinhold, R.H., 1994. Thermal reaction of chrysotile revisited: a 29 Si and 25Mg MAS NMR study. Am. Miner. 79, 43–50. McDonald, R.G., Whittington, B.I., 2008a. Atmospheric acid leaching of nickel laterites review Part I. Sulphuric acid technologies. Hydrometallurgy 91, 35–55. McDonald, R.G., Whittington, B.I., 2008b. Atmospheric acid leaching of nickel laterites review Part II. Chloride and bio-technologies. Hydrometallurgy 91, 56– 69.
Metz, B., Davidson, O., de Coninck, H.C., Loos, M., Meyer, L.A. (Eds.), 2005. Special report on carbon dioxide capture and storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Nagamori, M., Boivin, J.A., 2001. Technico-economic simulation for the HCl-leaching of hybrid serpentine and magnesite feeds. Canadian Metallurgical Quarterly 40 (1), 47–60. Pokrovsky, O.S., Schott, J., 2000. Kinetics and mechanism of forsterite dissolution at 25°C and pH from 1 to 12. Geochim. Cosmochim. Acta 64 (19), 3313–3325. Roine, A., 2002. HSC Chemistry v. 5.11, Calculation Software for Thermodynamics. Outokumpu Research Oy, Pori, Finland. Taubert, L., 2000. Hydrochloric attack of serpentinites: Mg2+ leaching from serpentinites. Magnesium Res. 13 (3), 167–173. Teir, S., Revitzer, H., Eloneva, S., Fogelholm, C.-J., Zevenhoven, R., 2007. Dissolution of natural serpentinite in mineral and organic acid. Int. J. Miner. Process. 83 (1– 2), 36–46. Zhang, Q., Sugyiama, K., Saito, F., 1997. Enhancement of acid extraction of magnesium and silicon from serpentine by mechanochemical treatment. Hydrometallurgy 45 (3), 323–331. Zulumyan, N.O., Isaakyan, A.R., Oganesyan, Z.G., 2007. A new promising method for processing of serpentinites. Russ. J. Appl. Chem. 80 (6), 1020–1022.
Contents lists available at SciVerse ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Technical Note
Dissolution of magnesium from calcined serpentinite in hydrochloric acid Alena Fedorocˇková, Martin Hreus, Pavel Raschman ⇑, Gabriel Sucˇik Technical University of Košice, Faculty of Metallurgy, Letná 9, Košice, Slovak Republic
a r t i c l e
i n f o
Article history: Received 12 November 2011 Accepted 7 March 2012 Available online 22 April 2012 Keywords: Serpentinite Serpentine Calcination Magnesium Reactivity Dissolution
a b s t r a c t In the production of pure magnesium compounds from serpentinite, acid leaching is usually the first stage of the overall process. However, faster magnesium dissolution can be achieved and the size of a potential leaching reactor can be reduced if serpentinite is calcined prior to leaching. Moreover, use of calcined serpentinite can reduce problems relating to corrosion of the reactor (lower leaching temperatures and pressures can be applied) and foam formation (chemically bonded water, which forms bubbles when released in the reactor, can be removed by calcining). This paper examines how calcination temperature and time influence the amount of magnesium dissolved during the initial period of leaching of calcined serpentine in hydrochloric acid. Fine-grained serpentinite calcined between 640 °C and 700 °C displayed the highest reactivity. The fraction of magnesium dissolved was up to 30-times higher as compared to leaching of uncalcined serpentine under identical reaction conditions. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Serpentinite is a rock which consists predominantly of one or more serpentine group minerals. The name serpentine is mainly applied to three hydrated magnesium silicate minerals: antigorite, chrysotile, and lizardite. They all have essentially the same chemical composition [(Mg,Fe)3Si2O5(OH)4] but different crystal structures. Currently serpentinites are used as railway ballasts, building materials, or thermal/electrical insulation. Large serpentinite reserves have a potential to find use also in more sophisticated applications, such as carbon dioxide capture and storage (Metz et al., 2005), as a potential source of nickel (McDonald and Whittington, 2008a,b) or as a raw material in the production of magnesium metal and/or pure magnesium compounds. In the production of pure magnesium compounds, acid leaching of serpentine is usually the first stage of the overall process. Leaching generates a soluble magnesium salt, which builds up in solution and is then separated from the insoluble residue, and later refined. Commonly used lixiviants are hydrochloric acid (Dutrizac et al., 2000; Taubert, 2000; Nagamori and Boivin, 2001), and other inorganic and organic acids. Serpentines consist of acidic silicate layers and basic brucitelike layers, alternating each other. This double-layered structure results in the acid–base character of serpentines, which causes them to be resistant to acidic and alkaline solutions. To accelerate
⇑ Corresponding author. E-mail address: [email protected] (P. Raschman). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.03.006
leaching, Zhang et al. (1997) applied mechanochemical treatment to the serpentine. An alternative approach could be calcination. During calcination, water is released according to Eq. (1), thus causing the compact, double-layered structure of serpentine to become disordered (Zulumyan et al., 2007). The intermediate Mg3Si2O7(s), formed in Eq. (1) and defined as dehydroxylate I (MacKenzie and Meinhold, 1994), is therefore expected to be more reactive than the original (uncalcined) serpentine, thus displaying faster magnesium dissolution in acidic solutions. The calculated equilibrium temperature for the thermal decomposition of serpentine (i.e., when the water vapour partial pressure reaches 101,325 Pa) is 539 °C (Roine, 2002). At temperatures above 800 °C, dehydroxylate I is converted into forsterite (Eq. (2)). Conversion of serpentine to forsterite as a main product, accompanied by amorphous silica, at temperatures between 800 °C and 1000 °C (Brindley and Hayami, 1965) is described by Eq. (3).
Mg3 Si2 O5 ðOHÞ4 ðsÞ ! Mg3 Si2 O7 ðsÞ þ 2H2 OðgÞ
ð1Þ
2Mg3 Si2 O7 ðsÞ ! 3Mg2 SiO4 þ SiO2 ðsÞ
ð2Þ
2Mg3 Si2 O5 ðOHÞ4 ðsÞ ! 3Mg2 SiO4 ðsÞ þ SiO2 ðsÞ þ 4H2 OðgÞ
ð3Þ
It is hypothesized that calcination of serpentine could accelerate magnesium dissolution, reduce the leaching time and, consequently, the size of the vessel. Moreover, use of calcined serpentinite could reduce problems relating to the corrosion of the vessel since lower leaching temperatures and pressures can be used during leaching. Foam formation in the reactor could be minimized because the chemically bonded water that would be released as bubbles in the vessel, could otherwise be released during calcination.
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
2
Fig. 1. 3D in situ high-temperature X-ray powder diffraction sequence illustrating the phase transformation of serpentine to forsterite during heating the serpentinite.
Herein, we explore how calcination of serpentinite can enhance the dissolution of magnesium in hydrochloric acid. The literature reports on the effects of hydrochloric acid concentration, leaching temperature and serpentinite particle size on the dissolution of magnesium (Dutrizac et al., 2000; Teir et al., 2007), but there were no publications reporting on the effects of calcination temperature and calcination time. Results showing the relationship between the conditions of calcining and the amount of magnesium dissolved are briefly discussed.
2. Experimental procedures 2.1. Materials Bulk material obtained from the mining and processing of serpentinite in Dobšiná, Slovakia was used in our laboratory tests. Two test samples with particle sizes up to 315 lm, or ranging from 315 to 500 lm (referred to as SED 0–315 and SED 315–500, respectively) were prepared by dry-screening. The samples contained 22.0–22.2% magnesium, 17.4–17.9% silicon, 5.8–5.9% iron, 12.8–13.0% chemically bonded water and carbon dioxide, and minor elements (calcium, aluminium, and nickel). Mineral phases identified were: lizardite, chrysotile, calcite, magnetite, and orthopyroxene.
2.2. Calcination of serpentinite First, to find the calcination temperature, thermal decomposition of serpentine was studied by differential thermal analysis (DTA) and thermogravimetric analysis (TGA) as well as by hightemperature in situ X-ray powder diffraction between 100 °C and 1000 °C. Then, calcined samples were prepared as follows: ten grams of serpentinite was placed in a crucible with a lid and inserted in a muffle furnace where the sample was heated at a predefined temperature (between 600 °C and 830 °C) for a selected time (10–150 min). After calcination, the weight of each sample was measured to calculate the weight loss.
Mineralogical composition and crystallinity of uncalcined and calcined serpentinite were determined by X-ray powder diffraction (XRD). 2.3. Leaching tests The reactivity of uncalcined or calcined serpentinite was determined as follows: one gram of sample was dissolved at 20 °C in 250 ml of 0.25 M hydrochloric acid. The suspension was stirred at 500 min1. Aliquots of the slurry were withdrawn after 5 min, filtered and the filtrate analyzed. In the present study, the reactivity of calcined serpentinite was expressed in terms of the fraction of magnesium dissolved according to the following equation:
MgOðsÞ þ 2HClðaq:Þ ! Mg2þ ðaq:Þ þ 2Cl ðaq:Þ þ H2 OðlÞ
ð4Þ
The fraction of magnesium dissolved, XL, was calculated from the following equation:
XL ¼
25V KIII cKIII M MgO m wMgO
ð5Þ
where VKIII and cKIII equals the volume and concentration of the Komplexon III solution consumed for the titration of magnesium that was present in 10 ml of filtrate, respectively, m is the weight of the sample leached (equal to 1 g), MMgO = 40.3 g mol1 is the molar weight of MgO, and wMgO is the weight fraction of MgO in the uncalcined or calcined serpentinite tested. 3. Results and discussion 3.1. Thermal decomposition of serpentine DTA showed that the thermal decomposition of serpentine started at 640 °C and the maximum endothermic effect was observed at 693 °C. Thermal decomposition of serpentine caused a weight loss due to the release of water (Eq. (1)). TGA revealed a maximum rate of thermal decomposition at 693 °C. An exothermic effect with an onset at 810 °C was also determined by DTA, which indicates that forsterite is formed in accordance with Eq. (2).
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
(a) Fraction of magnesium dissolved, XL (-)
100
is expected to yield magnesium of lower reactivity as a result of forsterite recrystallization and/or formation.
80
3.2. Hydrochloric acid leaching of calcined serpentine The influence of the calcination temperature and time on the amount of magnesium dissolved during leaching is illustrated in Fig. 2. While the reactivity XL (defined in Eq. (5)) increases gradually with the calcination time at temperatures below 700 °C, a different situation was observed at temperatures above 700 °C: XL increased with the calcination time at constant temperature until it reached a maximum, and then it decreased. For example, the maximum reactivity of fine-grained serpentinite calcined at 720 °C and 760 °C was achieved after 20 and 10 min, respectively (Fig. 2a). A similar trend was observed for coarse-grained serpentinite for which the maximum reactivity at 760 °C was achieved after 15 min, and after 7 min at 830 °C (Fig. 2b). The results shown in Fig. 2 suggest the following:
60
40
20
0
0
20
40
60
Calcination time (min)
(b)
600°C
640°C
720°C
760°C
800°C
uncalcined
Fraction of magnesium dissolved, X L (-)
100
80
60
40
20
0
3
0
40
80
120
Calcination time (min) 640°C
760°C
830°C
uncalcined
Fig. 2. Influence of calcination temperature and time on fraction of magnesium dissolved after 5 min of leaching for (a) sample SED 0–315 and (b) sample SED 315–500.
(a) The non-calcined serpentine leached only 2.2% of the magnesium present. Under optimum conditions of calcination, the calcined serpentine leached close to 70% magnesium. (b) About 10% less magnesium was leached from coarser serpentinite particles. (c) Increasing the calcination time increased reactivity when the calcination temperature was less than 700 °C. Above that, increasing the calcination time decreased the reactivity due potentially to the formation of forsterite. There is thus an interaction between calcination time and temperature. One cannot postulate about the effect of increasing the temperature on the leaching behaviour without considering the calcination time.
The initial magnesium dissolution rate was estimated to be 1–2 104 mol m2 s1 and 6–9 108 mol m2 s1 for calcined serpentinite of the highest and lowest reactivity, respectively (in these calculations, a specific surface area between 6 m2 g1 and 8 m2 g1 for calcined serpentinite samples was used, as determined using the B.E.T. nitrogen adsorption technique). The published value of the rate of forsterite dissolution in hydrochloric acid at 25 °C and pH = 1 is 6.3 108 mol m2 s1 (Pokrovsky and Schott, 2000). Such a rate is up to 4 orders of magnitude lower than the maximum dissolution rate of magnesium observed in the present work, but it is comparable to the lowest one. Hence, the assumption that calcined serpentinite is deactivated due to crystalline forsterite formation seems to be reasonable. (d) The optimum conditions appear to be a calcination temperature less than 700 °C with calcination time of 60 min. (e) The effects of calcination time and temperature are the same on coarse-grained and fine-grained serpentine. 4. Conclusions
The influence of the calcination temperature between 400 °C and 1000 °C on phase composition and crystallinity is illustrated in Fig. 1. Starting from 600 °C, the XRD count from the main phases originally present, lizardite and chrysotile, is getting progressively lower. Between 600 °C and 800 °C, these minerals gradually disappear and the structure of calcined sample appears to be highly disordered. At temperatures above 800 °C, a new phase, forsterite, is formed. DTA, TGA and XRD analyses suggest that the samples should be calcined at temperatures between 600 °C and 800 °C to produce a reactive form of magnesium. Calcination at higher temperatures
The effects of calcination temperature and calcination time on the dissolution of magnesium from calcined serpentinite in hydrochloric acid were examined in this study. Sixty minutes exposure to temperatures between 640 °C and 700 °C produced the most reactive calcine. The fraction of magnesium dissolved after 5 min of leaching was found to be up to 70%, compared to 2.2% for uncalcined serpentinite. Temperatures above 700 °C for less than 20 min produced slightly less reactive magnesium. Longer calcination time caused deactivation of calcined serpentinite.
4
A. Fedorocˇková et al. / Minerals Engineering 32 (2012) 1–4
Acknowledgements This study was financially supported by the Grant Agency of the Ministry of Education of the Slovak Republic and Slovak Academy of Sciences (Grant No. 1/0267/09). Suggestions and recommendations of the two reviewers are greatly acknowledged. References Brindley, G.W., Hayami, R., 1965. Mechanisms of formation of forsterite and enstatite from serpentine. Miner. Mag. 35, 189–195. Dutrizac, J.E., Chen, T.T., White, C.W. 2000. Fundamentals of serpentine leaching in hydrochloric acid media. In: Kaplan H.I., Hryn J.N., Clow B.B. (Eds.), Magnesium Technology. The Minerals, Metals & Materials Society, Nashville. TMS Annual Meeting, pp. 41–51. MacKenzie, K.J.D., Meinhold, R.H., 1994. Thermal reaction of chrysotile revisited: a 29 Si and 25Mg MAS NMR study. Am. Miner. 79, 43–50. McDonald, R.G., Whittington, B.I., 2008a. Atmospheric acid leaching of nickel laterites review Part I. Sulphuric acid technologies. Hydrometallurgy 91, 35–55. McDonald, R.G., Whittington, B.I., 2008b. Atmospheric acid leaching of nickel laterites review Part II. Chloride and bio-technologies. Hydrometallurgy 91, 56– 69.
Metz, B., Davidson, O., de Coninck, H.C., Loos, M., Meyer, L.A. (Eds.), 2005. Special report on carbon dioxide capture and storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Nagamori, M., Boivin, J.A., 2001. Technico-economic simulation for the HCl-leaching of hybrid serpentine and magnesite feeds. Canadian Metallurgical Quarterly 40 (1), 47–60. Pokrovsky, O.S., Schott, J., 2000. Kinetics and mechanism of forsterite dissolution at 25°C and pH from 1 to 12. Geochim. Cosmochim. Acta 64 (19), 3313–3325. Roine, A., 2002. HSC Chemistry v. 5.11, Calculation Software for Thermodynamics. Outokumpu Research Oy, Pori, Finland. Taubert, L., 2000. Hydrochloric attack of serpentinites: Mg2+ leaching from serpentinites. Magnesium Res. 13 (3), 167–173. Teir, S., Revitzer, H., Eloneva, S., Fogelholm, C.-J., Zevenhoven, R., 2007. Dissolution of natural serpentinite in mineral and organic acid. Int. J. Miner. Process. 83 (1– 2), 36–46. Zhang, Q., Sugyiama, K., Saito, F., 1997. Enhancement of acid extraction of magnesium and silicon from serpentine by mechanochemical treatment. Hydrometallurgy 45 (3), 323–331. Zulumyan, N.O., Isaakyan, A.R., Oganesyan, Z.G., 2007. A new promising method for processing of serpentinites. Russ. J. Appl. Chem. 80 (6), 1020–1022.