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Recovery and recycling of lithium value from spent lithium titanate (Li2TiO3) pebbles
Journal of Nuclear Materials 440 (2013) 104–109
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Recovery and recycling of lithium value from spent lithium titanate (Li2TiO3) pebbles D. Mandal ⇑ Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
g r a p h i c a l a b s t r a c t Effects of various process parameters on the recovery of Li-from spent Li2TiO3 pebbles were investigated. From the experimental results it was observed that the leaching rate increases with speed of stirring till 450 rpm and then above 450 rpm; the increase in speed of stirring does not have any significant effect on the leaching rate as shown in the following figure. Effects of other parameters on the Li-recovery from spent Li2TiO3 pebbles are discussed in this paper.
a r t i c l e
i n f o
Article history: Received 21 November 2012 Accepted 9 April 2013 Available online 27 April 2013
a b s t r a c t In the first generation fusion reactors the fusion of deuterium (D) and tritium (T) is considered to produce energy to meet the future energy demand. Deuterium is available in nature whereas, tritium is not. Lithium-6 (Li6) isotope has the ability to produce tritium in the n, a nuclear reaction with neutrons. Thus lithium-based ceramics enriched by Li6 isotope are considered for the tritium generation for its use in future fusion reactors. Lithium titanate is one such Li-based ceramic material being considered for its some attractive properties viz., high thermal and chemical stability, high thermal conductivity, and low tritium solubility. It is reported in the literature, that the burn up of these pebbles in the fusion reactor will be limited to only 15–17 atomic percentage. At the end of life, the pebbles will contain more than 45% unused Li6 isotope. Due to the high cost of enriched Li6 and the waste disposal considerations, it is necessary to recover the unused Li from the spent lithium titanate pebbles. Till date, only the feasibilities of different processes are reported, but no process details are available. Experiments were carried out for the recovery of Li from simulated Li2TiO3 pebbles and to reuse of lithium in lithium titanate pebble fabrication. The details of the experiments and results are discussed in this paper. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Tel.: +91 022 25593938; fax: +91 022 2550515. E-mail addresses: [email protected], [email protected] 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.028
In the first generation fusion reactors two hydrogen isotopes viz., deuterium (D) and tritium (T) will be introduced at high
D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
105
Nomenclature A E k
pre-exponential factor ((mol m3)1n sl) n is the order of reaction activation energy (kJ mol1 K1) rate constant ((mol m3)1n sl), n is the order of reaction, k(T) is the temperature-dependent rate constant
temperature plasma to fuse to produce thermal energy according to the nuclear fusion reaction: D þ T ! He þ n þ 17:6 MeV [1,2]. Naturally occurring hydrogen contains 0.014% (140 ppm) of deuterium [3] and different technologies are available to separate deuterium from the compounds of hydrogen viz., water (H2O), ammonia (NH3), hydrogen sulphide (H2S) etc. [3], whereas, natural hydrogen contains only 7.0 1016% tritium, which is negligible and the technology to separate tritium from natural hydrogen has not yet developed. Tritium can be produced by scattering Li6 isotope with thermal neutrons through (n, a) reaction; Li6 + n ? He + T [4]. The in situ generation of tritium is preferred as its half life is 12.3 years [5] and its storage is difficult. Li based ceramics; enriched by Li6 isotope viz., lithium titanate (Li2TiO3) and lithium-ortho-silicate (Li4SiO4) are considered for the generation of tritium [1,2]. Lithium has variety of applications as discussed above, but its abundance in the earth’s crust is only 0.0018% by weight [4]. The burn up of lithium based ceramics enriched by Li6, for the use for tritium generation is up to 15% [6,7]. Moreover, the enriched Li6 is very costly, which is comparable to the cost of gold. Hence, it is necessary to recover and recycle lithium from the spent solid breeder materials. Due to the wide use of lithium in various fields, a lot of works were carried out to recover Li from various sources. Tsuchiya et al. [8] studied the dissolution and recovery pattern of Li form four Li-based ceramics viz., Li2O, LiAlO2, Li2ZrO3 and Li4SiO4. Different acids viz., HCI, HNO3, H2SO4, HF and aqua regia were used to dissolve. Li was recovered from the solution by precipitating as carbonate by adding ammonium carbonate ((NH4)2CO3). Recovery of Li was 82–84% [8]. Alvani et al. [9] recovered Li-value form Li2TiO3 pebbles by dissolving the pebbles in aqueous solution hydrogen peroxide (H2O2) and citric acid. Sintered pebbles take 1– 2 days to dissolve while non-sintered pebbles take few hours. Li2TiO3 pebbles were fabricated form this dissolved solution either by citric acid or sol–gel route. In another process Alvani et al. [9] used nitric acid (HNO3) in addition to the aqueous solution of H2O2 and citric acid. Lagos and Becerra [10] proposed a conceptual methodology to recover lithium in form of carbonate from lithium titanate pebbles partially burnt by neutron irradiation by a hydrometallurgical process. They leached Li2TiO3 pebbles under boiling
m R t T
mass (suffix o stands for initial, t for mass at time t, and 1 for after infinite time) (kg) universal gas constant (kJ mol1 K1) time (h) temperature (K)
reflux of 4 M hydrochloric acid for 6 h. Li2TiO3 dissolved in hydrochloric acid forming titanium dioxide and lithium chloride according to the Reaction (1).
Li2 TiO3 þ 2HCl ! 2 LiCl þ TiO2 ðsÞ þ H2 O
ð1Þ
The aqueous solution of lithium chloride after removing the precipitate of TiO2 was evaporated to dryness and dissolved in water. Successive evaporations and the dissolution of the residue in water were repeated several times till the pH of solution reach near nine. The dried residue after final stage was then dissolved in ethanol. The alcoholic solution was filtered, washed and a carbonating agent was added to precipitate lithium as carbonate. The recovery of Li by this process was reported up to 83% [9]. Some of these processes either consume high energy or huge amounts of reagents or even both. Thus different researchers put their efforts and focused to establish a suitable hydrometallurgical route primarily for lithium extraction. These processes mainly involve leaching in acid or alkaline medium to dissolve the lithium content, purification to separate the other elements and precipitation by controlling the pH of the solution or by adding a reagent to recover lithium that could be used by the industry. In all the previous studies the effect of particle size of Li2TiO3, degree of agitation and effect of temperature on dissolution etc. were not studied and in all the cases Li-recovery was up to only 85%. An improved process has been developed to recover and recycle lithium value of spent lithium titante pebbles in which the Li-recovery was more than 90%. In this study the Li2TiO3 pebbles were dissolved in dilute HCl at low temperature with constant stirring. Experimental details and results are discussed in this paper.
Table 1 Properties of lithium titanate (Li2TiO3) pebbles used in the study. Properties
Values/ranges
Size (mm) Sphericity (largest diameter/smallest diameter) Theoretical density (TD) (kg m3) Density (TD%) (%) Lithium density (kg m3) Open porosity (%) Closed porosity (%) Grain size (lm) Surface area (BET) (m2 kg1) Thermal conductivity at 300 K (Wm1 K1) Coefficient of thermal expansion; at 500 °C (DL/Lo) (–) Crushing load (N)
1.0 1.01 3420 81.42 400 7.0 5.0 2–6 180 2.5 0.8 25–30
Notes: 1. TD-Theoretical density i.e., density of material without any void. 2. DL – increase in length, Lo – initial length.
Fig. 1. Schematic diagram of the experimental setup for study of leaching of Li from lithium titanate pebbles.
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D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
2. Experimental 2.1. Materials (i) Simulated lithium titanate (Li2TiO3) pebbles. The objective of the study is to develop a process which can be used to recover lithium value form the spent Li2TiO3 pebbles from future fusion reactor. The Li2TiO3 pebbles used in the study were synthesized and fabricated by the solid state reaction process developed by Mandal et al. described in details somewhere else [1,2]. Spherical Li2TiO3 pebbles of size 1.0 mm were used and the properties of the Li2TiO3 pebbles used in the study are shown in Table 1. (ii) Hydrochloric acid (HCl), of 99.8% purity, purchased from Merck and Loba Chemicals, Mumbai, India. To leach lithium from Li2TiO3 Hydrochloric acid was used. The reasons to use hydrochloric acid are discussed below. (iii) Sodium carbonate (Na2CO3) analytical grade, procured form Merck Chemicals, Mumbai, India. To precipitate lithium as lithium carbonate from lithium hydroxide solution sodium carbonate was used. (iv) Distilled water. Distilled was used in the experiments, primarily to dilute hydrochloric acid to the desired molar solution. 2.2. Methods
Table 2 Range of operating parameters and step increased values used in the study. Operating parameters
Range
Step increase
Minimum
Maximum
Concentration of HCl Temperature Time of leaching Speed of stirring
4M
6M
1M
60 °C 30 min 200
90 °C 4h 900
Solid/liquid (g/ml)
0.05
0.2
10 °C 30 min 50 till 300 and 150 above 300 0.05
on a hot plate with a temperature controller to heat the slurry at constant temperature. The temperature of the solution was controlled within ±3 °C and the temperature of the slurry was noted at an interval of 5 min and the average temperature of each run is determined by time average of the noted readings. A known of volume of HCl solution with known concentration was added to the flux. After the desired stirring speed and reaction temperature were attained, the solid sample of 5 g was added to the solution in reactor. 5 ml solution was withdrawn and filtered after specific time for analyzing the concentration of lithium in the solution by Atomic Absorption Spectrophotometer (AAS) and
(a)
100 o
Leaching temperature : 60 C
2.2.1. Leaching The experiments were designed with the main objectives with the (a) selective leaching of lithium and (b) the separation of lithium from titanium dioxide to the extent possible. The extent of separation i.e., leaching efficiency of lithium is affected by the following parameters.
90 80
% of Lithium leached [mass %]
The schematic diagram of the experimental apparatus used is shown Fig. 1. The apparatus consists of a three-necked 1000 ml round bottom flask with reflux setup, a magnetic stirrer and a hotplate. The experimental method consists of process steps include (a) leaching and (b) precipitation, discussed in details below.
LEGENDS 200 rpm, 600 rpm,
250 rpm, 750 rpm,
300 rpm, 900 rpm
450 rpm,
70 60 50 40 30 20 10 0
(i) (ii) (iii) (iv) (v) (vi)
Leaching agent. Concentration of the leaching agent. Temperature. Speed of agitation. Solid to liquid ratio, and Particle Size.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h]
(b)
100 o
Leaching temperature (T) : 90 C 90
% Lithium leached [mass %]
80
In the experimental work spherical Li2TiO3 pebbles of size 1.0 was used as mentioned above. To study the effect of particle size on the recovery of lithium from fine Li2TiO3 particles of size range 100–200 lm were used. These fines were obtained by pulverizing 1.0 mm Li2TiO3 pebbles in a planetary ball mill and classified standard sieves. It is reported that both HNO3 and HCl give relatively more recovery of lithium compared to H2SO4 [11–13]. Though the handling of HCl is difficulties due to the chloride corrosion, it is preferred to HNO3 because the deposal of nitrate waste which will generate due to the latter’s use viz. sodium nitrate is a problem as per the norms of pollution control standard [11,12]. The leaching of Li2TiO3 pebbles were carried out in a 1000 ml three necked and flat bottom glass reactor. The flux was fitted with a reflux condenser to reduce the loss of solution by evaporation and a thermometer. The solid was suspended in the solution by stirring the solution using a magnetic stirrer. The flux was kept
70 60 50 40 30 20 LEGENDS 200 rpm, 600 rpm,
10
250 rpm, 750 rpm,
300 rpm, 900 rpm
450 rpm,
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 2. Effect of speed of agitation on the leaching of lithium with initial concentration of hydrochloric acid 4M and solid liquid ratio, 1:6, (a) at T = 60 °C and (b) at T = 90 °C.
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D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
(a) 40
(a) 100 o
After 4 hrs at 90 C
o
98
36
96
Li - recovery [mass % ]
Li-Recovery [mass % ]
After 4 hrs at 60 C 38
34
32
30
28
92
90
88
26 100
94
86
200
300
400
500
600
700
800
900
1000
100
200
300
-1
500
600
700
800
900
1000
Speed (in rpm) of agitation [min ]
(b) 100 90
400
-1
Speed (in rpm) of agitation [min ]
(b) 100 o
T=60 C:
1 hr,
2 hrs,
3 hrs,
4 hrs)
90 80
Li - recovery [mass % ]
Li - Recovery [mass % ]
80 70 60 50 40 30
70 60 50 40
20
30
o
1 hr,
400
500
T=90 C:
2 hrs,
3 hrs,
4 hrs)
10 100
100
200
300
400
500
600
700
800
900
200
300
1000
600
700
800
900
1000
-1
Speed (in rpm) of agitation [min ]
-1
Speed (in rpm) of agitation [min ] Fig. 3. Effect of speed of agitation on the recovery of lithium at 60 °C after different durations.
Fig. 4. Effect of speed of agitation on the recovery of lithium at 90 °C after different durations.
5 ml fresh lixiviant was added into the reactor immediately to maintain the volume of the solution constant. To obtain the optimum conditions, leaching experiments were tested under various conditions, i.e. changing speed of agitation, temperature, S/L ratio and concentration of the acid.
90
% Lithium extracted [mass %]
2.2.2. Precipitation The lithium that is leached into the aqueous solution is precipitated as lithium carbonate by using a carbonating agent as the main objectives of the process are, (i) to recover the Li value present in the leached solution in the form of solid lithium carbonate (Li2CO3) by precipitation, and (ii) to minimise the solubility of Li2CO3 in the solution so as to obtain the maximum recovery of Li from the solution. The solubility of lithium carbonate in water is retrograde i.e. solubility of lithium carbonate in aqueous solution decreases with increase in temperature. Thus, the reactants were heated at 90 °C and filtered in hot condition so as to obtain maximum recovery of the lithium. The precipitated Li2CO3 was separated by filtration, dried and weighed to know the quantity of Li2CO3 precipitated. The XRD analysis of the sample was carried out to confirm the presence of single phase Li2CO3.
100
Agitator Speed: 450 rpm o o 60 C, 70 C o o 80 C, 90 C
80 70 60 50 40 30 20 10 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (T) [h] Fig. 5. Effect of temperature on the extraction of lithium at agitator speed of 450 rpm.
108
D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
3. Results and discussion
100
S:L=1:6 S:L=1:9 S:L=1:12 S:L=1:15
80
The effects of various parameters viz., speed of agitation, temperature, solid to liquid ratio and acid concentration on leaching are discussed below.
70
3.1. Time
% of Lithium extracted [mass %]
90
Fig. 2a and b shows the recovery of lithium with reaction time at 60 °C and 90 °C respectively at different speeds of agitator. It is observed that recovery of lithium increases with the reaction time of leaching at any speed of agitation at 60 °C as well as at 90 °C. Lithium recovery also increases with time for any particular value of other process parameters i.e., speed of agitation, solid to liquid ratio, concentration of acid etc., as discussed in the following sections (also see Figs. 5–7).
60
50
40
30
20 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 6. Effect of solid to liquid (S/L) ratio on the extraction of lithium at agitator speed of 450 rpm, initial 4M HCl solution and at 90 °C.
4M HCl 5M HCl 6M HCl
100
% Lithium extracted [ mass % ]
90
80
70
60
3.2. Speed of agitation The effect of stirring speed on the lithium extraction from Li2TiO3 pebbles was investigated in a solution of 4M HCl at 60 °C with a solid–liquid ratio of 1:6 in the range of 200–900 rpm for 4 h. Fig. 2a and b shows the recovery of lithium with reaction time at 60 °C and 90 °C respectively at different rpm of agitator. It is observed that Li-recovery increases with the time of leaching as well as with the speed of agitation. Fig. 3a shows the percentage of recovery of lithium with speed of agitation after 4 h of leaching and Fig. 3b shows the same after different time of leaching at temperature 60 °C. Similarly Fig. 4a shows the percentage of recovery of lithium with speed of agitation after 4 h of leaching and Fig. 4b shows the same after different time of leaching at temperature 90 °C. These figures (Figs. 3 and 4) shows that the leaching rate increases with speed of stirring till 450 rpm and then above 450 rpm; the increase in speed of stirring does not have any significant effect on the leaching rate. To investigate the effect of speed of stirring at elevated temperature, the experiments were carried out with 4M HCl at 90 °C with speed of stirring in the range of 200–900 rpm.
50
3.3. Effect of temperature 40
30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 7. Effect of concentration of hydraulic acid on the extraction of lithium from Li2TiO3 pebbles.
2.3. Operating parameters In each experiment amount of 5 g Li2TiO3 pebbles was used. The various operating parameters and their ranges used in the experiments are shown in Table 2.
The effect of temperature on the extraction lithium from lithium titanate pebbles was studied with the process parameters as: (i) the initial concentration of hydrochloric acid solution 4M, (ii) agitator speed 450 rpm and (iii) solid–liquid ratio of 1:6 and the experiments were carried out at 60 °C, 70 °C, 80 °C and 90 °C for 4 h. It was found that as the temperature increases, the extraction of lithium increases as shown in Fig. 5. After 4 h, the extraction of lithium increases from 42.45% at 60 °C to 94.8% at 90 °C. In general the rate of mass transfer and the rate of chemical reaction of an endothermic reaction are increases with temperature. The diffusivity of liquids increases linearly with temperature where as the reaction rate increases exponentially as predicted by Arrhenius law (Eq. (2)), which fits well over wide temperature ranges and is strongly suggested from various standpoints as being a very good approximation to the true temperature dependency [13].
k ¼ AeðRT Þ E
ð2Þ
Table 3 Solubility of Li2CO3 in NaCl solution of different concentration due to addition of Na2CO3 at different concentration. % Na2CO3
10 20 30
Solubility of Li2CO3 at different concentration of NaCl due the addition of Na2CO3 at different concentrations. Stoichiometric
10% excess
20% excess
30% excess
10.2 17.48 22.82
9.33 15.99 21.07
8.61 14.7 19.56
7.98 13.22 18.25
D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
Fig. 8. Photograph of (a) Li2TiO3 pebbles used in the study and (b) precipitated and dried Li2CO3 particles.
where, k is the reaction rate constant at temperature T, A is called the frequency or pre-exponential factor, E is called the activation energy of the reaction and R is the universal gas constant. The experimental results show the increase in the fraction of extraction with temperature is not exponential. This indicates that the increase in extraction rate is not significant not due to increase in chemical reaction rate but due to improved mass transfer. Thus, subsequent experiments were carried out at a temperature of 90 °C. 3.4. Solid–liquid ratio The effect of solid (S) to liquid (L) ratio on the extraction of lithium from lithium titanate pebbles was studied with 4M initial concentration of HCl at 450 rpm agitator speed at 90 °C for 4 h and the S/L ratio varied from 1:6 to 1:15. The results presented in the Fig. 6 shows that the leaching rate increases with increase in S/L ratio up to 1:12 and beyond this ratio no significant improvement on leaching rate was observed. Dispersion of the reaction mixture is better at higher S/L ratio as this increases the rate of mass transfer. Thus, leaching rate increases with increase in S/L ratio. Thus, the subsequent experiments were carried out with S to L ratio of 1:12. 3.5. Acid concentration The effect of hydraulic acid concentration on the lithium extraction from lithium titanate pebbles was investigated in a solution of HCl with concentration varying from 4M to 6M at 450 rpm, 90 °C for 4 h with the solid–liquid ratio of 1:12. The results presented in Fig. 7 shows that the leaching efficiency increases with increase in concentration of hydraulic acid during the initial period of reaction. As the reaction proceeds further, at the end of 4 h, there is no significant increase in the fraction of lithium leached. This indicates that the reaction is a first order reaction. 3.6. Particle size Some of the experiments were repeated by using fine Li2TiO3 particles of size range 100–200 lm as mentioned in Section 2.2. No significant improvement in the degree of recovery of lithium at either temperature and at any particular speed of agitation was observed. This is due to the porous nature of the pebbles as the pebbles have 7% open porosity (see Table 1) through which hydrochloric acid penetrate easily. So, in the process of Li-recovery and recycle from spent Li2TiO3 pebbles the operation of size reduction is not necessary.
carbonate. From the study, it was found that the recovery of lithium is the maximum when an excess of 30% sodium carbonate is used. Table 3 shows the solubility of Li2CO3 in NaCl solution of different concentration due to addition of Na2CO3 of different concentrations. It is known that presence of NaCl at a concentration >15% in the solution decreases the solubility of Li2CO3 due to salting effect. Assuming the reaction to reach completion, the concentration of sodium carbonate that gives >15% concentration of NaCl in the solution after reaction was estimated. The precipitated Li2CO3 was separated by filtration, dried and kept in desiccators overnight. The quantity of Li2CO3 precipitated was determined by weighing and XRD analysis of the sample was carries out. The XRD analysis confirmed the presence of single phase Li2CO3. Fig. 8a shows a photograph of Li2CO3 particles used in the study and Fig. 8b shows a photograph of precipitated and dried Li2CO3 particles. The recovered Li-value in the form of Li2CO3 can be recycled to synthesize and fabricate Li2TiO3 pebbles by solid state reaction process as described by Mandal et al. [1,2] 4. Conclusions An advanced process has been developed for the recovery and recycle of lithium value of spent lithium titanate pebbles. Effects of various process parameters on the extraction of lithium from spent lithium titanate pebbles were investigated. From the study, it was found that the recovery of lithium is the maximum when an excess of 30% sodium carbonate is used. A recovery of more than 98% is obtained by carrying out the precipitation of lithium from the LiCl solution obtained after leaching by using 40% excess of 30% sodium carbonate and heating the solution to a temperature of 95 °C. The recovered Li-value in the form of Li2CO3 can be easily recycled for the synthesis and fabrication of Li2TiO3 pebbles for reuse in the fusion reactors. The developed process is scalable and moreover, the operations in the process can be remotely handled if required to avoid the hazards involved in the tritium release form the spent pebbles. Acknowledgments The author is thankful to Mr. S.K. Ghosh of BARC for his valuable suggestions in carrying out this work and the author is thankful to Mrs. Samyukta G., Mr. M.C. Jadeja and Mr. B.K. Chougule for their help in carrying out the experiments. References [1] [2] [3] [4] [5]
[6]
[7] [8]
3.7. Precipitation of lithium carbonate When sodium carbonate solution was added to the solution obtained after leaching, a white precipitate was obtained. The white precipitate gives an indication that the product obtained is lithium carbonate. This precipitate was analyzed and identified as lithium
109
[9] [10] [11] [12] [13]
D. Mandal, M.R.K. Shenoi, S.K. Ghosh, Fusion Eng. Des. 85 (2010) 819–823. D. Mandal, D. Sathiyamoorthy, V.G. Rao, Fusion Eng. Des. 87 (2012) 7–12. J.D. Lee, Concise Inorganic Chemistry, fifth ed., Blackwell Science Ltd., 1996. S. Glasstone, A. Senonske, Nuclear Reactor Engineering, 3rd Ed., CBS Publisher & Distributor, Delhi, India, 1986. D. Mandal, D. Sen, S. Mazumder, M.R.K. Shenoi, S. Ramnathan, D. Sathiyamoorthy, Sintering behaviour of lithium-titanate pebbles: modifications of microstructure and pore morphology, in: D. Singh, J. Salem, S. Widjaja (Eds.), vol. 32, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2011. K. Tsuchiya, H. Kawamura, Fabrication and Characterizationn of 6Li-enriched Li2TiO3 Pebbles for a High Li-burnup Irradiation Tests, Japan Atomic Energy Agency, 2006. C. Alvani, P.L. Carconi, S. Casadio, V. Conitini, A. Dibartolomeo, F. Pierdominici, A. Deptula, S. Lagos, C.A. Nannetti, J. Nucl. Mater. 289 (2001) 303–307. K. Tsuchiya, H. Kawamura, M. Saito, K. Tatenuma, M. Kainose, J. Nucl. Mater. 219 (1995) 246–249. C. Alvani, S. Casadio, V. Conitini, A.D. Bartolomeo, J.D. Lulewicz, N. Roux, J. Nucl. Mater. 307–311 (2002) 837–841. S. Lagos, R. Becerra, Fusion Eng. Des. 347 (2005) 134–139. D. Mandal, S.K. Ghosh, Environ. Geochem. 8 (2005) 244–249. D. Mandal, Ind. Chem. Eng. 45 (3) (2003) 203–206. O. Levenspiel, Chemical Reaction Engineering, third ed., John Wiley & Sons, New York, 1999.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Recovery and recycling of lithium value from spent lithium titanate (Li2TiO3) pebbles D. Mandal ⇑ Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
g r a p h i c a l a b s t r a c t Effects of various process parameters on the recovery of Li-from spent Li2TiO3 pebbles were investigated. From the experimental results it was observed that the leaching rate increases with speed of stirring till 450 rpm and then above 450 rpm; the increase in speed of stirring does not have any significant effect on the leaching rate as shown in the following figure. Effects of other parameters on the Li-recovery from spent Li2TiO3 pebbles are discussed in this paper.
a r t i c l e
i n f o
Article history: Received 21 November 2012 Accepted 9 April 2013 Available online 27 April 2013
a b s t r a c t In the first generation fusion reactors the fusion of deuterium (D) and tritium (T) is considered to produce energy to meet the future energy demand. Deuterium is available in nature whereas, tritium is not. Lithium-6 (Li6) isotope has the ability to produce tritium in the n, a nuclear reaction with neutrons. Thus lithium-based ceramics enriched by Li6 isotope are considered for the tritium generation for its use in future fusion reactors. Lithium titanate is one such Li-based ceramic material being considered for its some attractive properties viz., high thermal and chemical stability, high thermal conductivity, and low tritium solubility. It is reported in the literature, that the burn up of these pebbles in the fusion reactor will be limited to only 15–17 atomic percentage. At the end of life, the pebbles will contain more than 45% unused Li6 isotope. Due to the high cost of enriched Li6 and the waste disposal considerations, it is necessary to recover the unused Li from the spent lithium titanate pebbles. Till date, only the feasibilities of different processes are reported, but no process details are available. Experiments were carried out for the recovery of Li from simulated Li2TiO3 pebbles and to reuse of lithium in lithium titanate pebble fabrication. The details of the experiments and results are discussed in this paper. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ⇑ Tel.: +91 022 25593938; fax: +91 022 2550515. E-mail addresses: [email protected], [email protected] 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.028
In the first generation fusion reactors two hydrogen isotopes viz., deuterium (D) and tritium (T) will be introduced at high
D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
105
Nomenclature A E k
pre-exponential factor ((mol m3)1n sl) n is the order of reaction activation energy (kJ mol1 K1) rate constant ((mol m3)1n sl), n is the order of reaction, k(T) is the temperature-dependent rate constant
temperature plasma to fuse to produce thermal energy according to the nuclear fusion reaction: D þ T ! He þ n þ 17:6 MeV [1,2]. Naturally occurring hydrogen contains 0.014% (140 ppm) of deuterium [3] and different technologies are available to separate deuterium from the compounds of hydrogen viz., water (H2O), ammonia (NH3), hydrogen sulphide (H2S) etc. [3], whereas, natural hydrogen contains only 7.0 1016% tritium, which is negligible and the technology to separate tritium from natural hydrogen has not yet developed. Tritium can be produced by scattering Li6 isotope with thermal neutrons through (n, a) reaction; Li6 + n ? He + T [4]. The in situ generation of tritium is preferred as its half life is 12.3 years [5] and its storage is difficult. Li based ceramics; enriched by Li6 isotope viz., lithium titanate (Li2TiO3) and lithium-ortho-silicate (Li4SiO4) are considered for the generation of tritium [1,2]. Lithium has variety of applications as discussed above, but its abundance in the earth’s crust is only 0.0018% by weight [4]. The burn up of lithium based ceramics enriched by Li6, for the use for tritium generation is up to 15% [6,7]. Moreover, the enriched Li6 is very costly, which is comparable to the cost of gold. Hence, it is necessary to recover and recycle lithium from the spent solid breeder materials. Due to the wide use of lithium in various fields, a lot of works were carried out to recover Li from various sources. Tsuchiya et al. [8] studied the dissolution and recovery pattern of Li form four Li-based ceramics viz., Li2O, LiAlO2, Li2ZrO3 and Li4SiO4. Different acids viz., HCI, HNO3, H2SO4, HF and aqua regia were used to dissolve. Li was recovered from the solution by precipitating as carbonate by adding ammonium carbonate ((NH4)2CO3). Recovery of Li was 82–84% [8]. Alvani et al. [9] recovered Li-value form Li2TiO3 pebbles by dissolving the pebbles in aqueous solution hydrogen peroxide (H2O2) and citric acid. Sintered pebbles take 1– 2 days to dissolve while non-sintered pebbles take few hours. Li2TiO3 pebbles were fabricated form this dissolved solution either by citric acid or sol–gel route. In another process Alvani et al. [9] used nitric acid (HNO3) in addition to the aqueous solution of H2O2 and citric acid. Lagos and Becerra [10] proposed a conceptual methodology to recover lithium in form of carbonate from lithium titanate pebbles partially burnt by neutron irradiation by a hydrometallurgical process. They leached Li2TiO3 pebbles under boiling
m R t T
mass (suffix o stands for initial, t for mass at time t, and 1 for after infinite time) (kg) universal gas constant (kJ mol1 K1) time (h) temperature (K)
reflux of 4 M hydrochloric acid for 6 h. Li2TiO3 dissolved in hydrochloric acid forming titanium dioxide and lithium chloride according to the Reaction (1).
Li2 TiO3 þ 2HCl ! 2 LiCl þ TiO2 ðsÞ þ H2 O
ð1Þ
The aqueous solution of lithium chloride after removing the precipitate of TiO2 was evaporated to dryness and dissolved in water. Successive evaporations and the dissolution of the residue in water were repeated several times till the pH of solution reach near nine. The dried residue after final stage was then dissolved in ethanol. The alcoholic solution was filtered, washed and a carbonating agent was added to precipitate lithium as carbonate. The recovery of Li by this process was reported up to 83% [9]. Some of these processes either consume high energy or huge amounts of reagents or even both. Thus different researchers put their efforts and focused to establish a suitable hydrometallurgical route primarily for lithium extraction. These processes mainly involve leaching in acid or alkaline medium to dissolve the lithium content, purification to separate the other elements and precipitation by controlling the pH of the solution or by adding a reagent to recover lithium that could be used by the industry. In all the previous studies the effect of particle size of Li2TiO3, degree of agitation and effect of temperature on dissolution etc. were not studied and in all the cases Li-recovery was up to only 85%. An improved process has been developed to recover and recycle lithium value of spent lithium titante pebbles in which the Li-recovery was more than 90%. In this study the Li2TiO3 pebbles were dissolved in dilute HCl at low temperature with constant stirring. Experimental details and results are discussed in this paper.
Table 1 Properties of lithium titanate (Li2TiO3) pebbles used in the study. Properties
Values/ranges
Size (mm) Sphericity (largest diameter/smallest diameter) Theoretical density (TD) (kg m3) Density (TD%) (%) Lithium density (kg m3) Open porosity (%) Closed porosity (%) Grain size (lm) Surface area (BET) (m2 kg1) Thermal conductivity at 300 K (Wm1 K1) Coefficient of thermal expansion; at 500 °C (DL/Lo) (–) Crushing load (N)
1.0 1.01 3420 81.42 400 7.0 5.0 2–6 180 2.5 0.8 25–30
Notes: 1. TD-Theoretical density i.e., density of material without any void. 2. DL – increase in length, Lo – initial length.
Fig. 1. Schematic diagram of the experimental setup for study of leaching of Li from lithium titanate pebbles.
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D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
2. Experimental 2.1. Materials (i) Simulated lithium titanate (Li2TiO3) pebbles. The objective of the study is to develop a process which can be used to recover lithium value form the spent Li2TiO3 pebbles from future fusion reactor. The Li2TiO3 pebbles used in the study were synthesized and fabricated by the solid state reaction process developed by Mandal et al. described in details somewhere else [1,2]. Spherical Li2TiO3 pebbles of size 1.0 mm were used and the properties of the Li2TiO3 pebbles used in the study are shown in Table 1. (ii) Hydrochloric acid (HCl), of 99.8% purity, purchased from Merck and Loba Chemicals, Mumbai, India. To leach lithium from Li2TiO3 Hydrochloric acid was used. The reasons to use hydrochloric acid are discussed below. (iii) Sodium carbonate (Na2CO3) analytical grade, procured form Merck Chemicals, Mumbai, India. To precipitate lithium as lithium carbonate from lithium hydroxide solution sodium carbonate was used. (iv) Distilled water. Distilled was used in the experiments, primarily to dilute hydrochloric acid to the desired molar solution. 2.2. Methods
Table 2 Range of operating parameters and step increased values used in the study. Operating parameters
Range
Step increase
Minimum
Maximum
Concentration of HCl Temperature Time of leaching Speed of stirring
4M
6M
1M
60 °C 30 min 200
90 °C 4h 900
Solid/liquid (g/ml)
0.05
0.2
10 °C 30 min 50 till 300 and 150 above 300 0.05
on a hot plate with a temperature controller to heat the slurry at constant temperature. The temperature of the solution was controlled within ±3 °C and the temperature of the slurry was noted at an interval of 5 min and the average temperature of each run is determined by time average of the noted readings. A known of volume of HCl solution with known concentration was added to the flux. After the desired stirring speed and reaction temperature were attained, the solid sample of 5 g was added to the solution in reactor. 5 ml solution was withdrawn and filtered after specific time for analyzing the concentration of lithium in the solution by Atomic Absorption Spectrophotometer (AAS) and
(a)
100 o
Leaching temperature : 60 C
2.2.1. Leaching The experiments were designed with the main objectives with the (a) selective leaching of lithium and (b) the separation of lithium from titanium dioxide to the extent possible. The extent of separation i.e., leaching efficiency of lithium is affected by the following parameters.
90 80
% of Lithium leached [mass %]
The schematic diagram of the experimental apparatus used is shown Fig. 1. The apparatus consists of a three-necked 1000 ml round bottom flask with reflux setup, a magnetic stirrer and a hotplate. The experimental method consists of process steps include (a) leaching and (b) precipitation, discussed in details below.
LEGENDS 200 rpm, 600 rpm,
250 rpm, 750 rpm,
300 rpm, 900 rpm
450 rpm,
70 60 50 40 30 20 10 0
(i) (ii) (iii) (iv) (v) (vi)
Leaching agent. Concentration of the leaching agent. Temperature. Speed of agitation. Solid to liquid ratio, and Particle Size.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h]
(b)
100 o
Leaching temperature (T) : 90 C 90
% Lithium leached [mass %]
80
In the experimental work spherical Li2TiO3 pebbles of size 1.0 was used as mentioned above. To study the effect of particle size on the recovery of lithium from fine Li2TiO3 particles of size range 100–200 lm were used. These fines were obtained by pulverizing 1.0 mm Li2TiO3 pebbles in a planetary ball mill and classified standard sieves. It is reported that both HNO3 and HCl give relatively more recovery of lithium compared to H2SO4 [11–13]. Though the handling of HCl is difficulties due to the chloride corrosion, it is preferred to HNO3 because the deposal of nitrate waste which will generate due to the latter’s use viz. sodium nitrate is a problem as per the norms of pollution control standard [11,12]. The leaching of Li2TiO3 pebbles were carried out in a 1000 ml three necked and flat bottom glass reactor. The flux was fitted with a reflux condenser to reduce the loss of solution by evaporation and a thermometer. The solid was suspended in the solution by stirring the solution using a magnetic stirrer. The flux was kept
70 60 50 40 30 20 LEGENDS 200 rpm, 600 rpm,
10
250 rpm, 750 rpm,
300 rpm, 900 rpm
450 rpm,
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 2. Effect of speed of agitation on the leaching of lithium with initial concentration of hydrochloric acid 4M and solid liquid ratio, 1:6, (a) at T = 60 °C and (b) at T = 90 °C.
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D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
(a) 40
(a) 100 o
After 4 hrs at 90 C
o
98
36
96
Li - recovery [mass % ]
Li-Recovery [mass % ]
After 4 hrs at 60 C 38
34
32
30
28
92
90
88
26 100
94
86
200
300
400
500
600
700
800
900
1000
100
200
300
-1
500
600
700
800
900
1000
Speed (in rpm) of agitation [min ]
(b) 100 90
400
-1
Speed (in rpm) of agitation [min ]
(b) 100 o
T=60 C:
1 hr,
2 hrs,
3 hrs,
4 hrs)
90 80
Li - recovery [mass % ]
Li - Recovery [mass % ]
80 70 60 50 40 30
70 60 50 40
20
30
o
1 hr,
400
500
T=90 C:
2 hrs,
3 hrs,
4 hrs)
10 100
100
200
300
400
500
600
700
800
900
200
300
1000
600
700
800
900
1000
-1
Speed (in rpm) of agitation [min ]
-1
Speed (in rpm) of agitation [min ] Fig. 3. Effect of speed of agitation on the recovery of lithium at 60 °C after different durations.
Fig. 4. Effect of speed of agitation on the recovery of lithium at 90 °C after different durations.
5 ml fresh lixiviant was added into the reactor immediately to maintain the volume of the solution constant. To obtain the optimum conditions, leaching experiments were tested under various conditions, i.e. changing speed of agitation, temperature, S/L ratio and concentration of the acid.
90
% Lithium extracted [mass %]
2.2.2. Precipitation The lithium that is leached into the aqueous solution is precipitated as lithium carbonate by using a carbonating agent as the main objectives of the process are, (i) to recover the Li value present in the leached solution in the form of solid lithium carbonate (Li2CO3) by precipitation, and (ii) to minimise the solubility of Li2CO3 in the solution so as to obtain the maximum recovery of Li from the solution. The solubility of lithium carbonate in water is retrograde i.e. solubility of lithium carbonate in aqueous solution decreases with increase in temperature. Thus, the reactants were heated at 90 °C and filtered in hot condition so as to obtain maximum recovery of the lithium. The precipitated Li2CO3 was separated by filtration, dried and weighed to know the quantity of Li2CO3 precipitated. The XRD analysis of the sample was carried out to confirm the presence of single phase Li2CO3.
100
Agitator Speed: 450 rpm o o 60 C, 70 C o o 80 C, 90 C
80 70 60 50 40 30 20 10 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (T) [h] Fig. 5. Effect of temperature on the extraction of lithium at agitator speed of 450 rpm.
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3. Results and discussion
100
S:L=1:6 S:L=1:9 S:L=1:12 S:L=1:15
80
The effects of various parameters viz., speed of agitation, temperature, solid to liquid ratio and acid concentration on leaching are discussed below.
70
3.1. Time
% of Lithium extracted [mass %]
90
Fig. 2a and b shows the recovery of lithium with reaction time at 60 °C and 90 °C respectively at different speeds of agitator. It is observed that recovery of lithium increases with the reaction time of leaching at any speed of agitation at 60 °C as well as at 90 °C. Lithium recovery also increases with time for any particular value of other process parameters i.e., speed of agitation, solid to liquid ratio, concentration of acid etc., as discussed in the following sections (also see Figs. 5–7).
60
50
40
30
20 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 6. Effect of solid to liquid (S/L) ratio on the extraction of lithium at agitator speed of 450 rpm, initial 4M HCl solution and at 90 °C.
4M HCl 5M HCl 6M HCl
100
% Lithium extracted [ mass % ]
90
80
70
60
3.2. Speed of agitation The effect of stirring speed on the lithium extraction from Li2TiO3 pebbles was investigated in a solution of 4M HCl at 60 °C with a solid–liquid ratio of 1:6 in the range of 200–900 rpm for 4 h. Fig. 2a and b shows the recovery of lithium with reaction time at 60 °C and 90 °C respectively at different rpm of agitator. It is observed that Li-recovery increases with the time of leaching as well as with the speed of agitation. Fig. 3a shows the percentage of recovery of lithium with speed of agitation after 4 h of leaching and Fig. 3b shows the same after different time of leaching at temperature 60 °C. Similarly Fig. 4a shows the percentage of recovery of lithium with speed of agitation after 4 h of leaching and Fig. 4b shows the same after different time of leaching at temperature 90 °C. These figures (Figs. 3 and 4) shows that the leaching rate increases with speed of stirring till 450 rpm and then above 450 rpm; the increase in speed of stirring does not have any significant effect on the leaching rate. To investigate the effect of speed of stirring at elevated temperature, the experiments were carried out with 4M HCl at 90 °C with speed of stirring in the range of 200–900 rpm.
50
3.3. Effect of temperature 40
30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time [h] Fig. 7. Effect of concentration of hydraulic acid on the extraction of lithium from Li2TiO3 pebbles.
2.3. Operating parameters In each experiment amount of 5 g Li2TiO3 pebbles was used. The various operating parameters and their ranges used in the experiments are shown in Table 2.
The effect of temperature on the extraction lithium from lithium titanate pebbles was studied with the process parameters as: (i) the initial concentration of hydrochloric acid solution 4M, (ii) agitator speed 450 rpm and (iii) solid–liquid ratio of 1:6 and the experiments were carried out at 60 °C, 70 °C, 80 °C and 90 °C for 4 h. It was found that as the temperature increases, the extraction of lithium increases as shown in Fig. 5. After 4 h, the extraction of lithium increases from 42.45% at 60 °C to 94.8% at 90 °C. In general the rate of mass transfer and the rate of chemical reaction of an endothermic reaction are increases with temperature. The diffusivity of liquids increases linearly with temperature where as the reaction rate increases exponentially as predicted by Arrhenius law (Eq. (2)), which fits well over wide temperature ranges and is strongly suggested from various standpoints as being a very good approximation to the true temperature dependency [13].
k ¼ AeðRT Þ E
ð2Þ
Table 3 Solubility of Li2CO3 in NaCl solution of different concentration due to addition of Na2CO3 at different concentration. % Na2CO3
10 20 30
Solubility of Li2CO3 at different concentration of NaCl due the addition of Na2CO3 at different concentrations. Stoichiometric
10% excess
20% excess
30% excess
10.2 17.48 22.82
9.33 15.99 21.07
8.61 14.7 19.56
7.98 13.22 18.25
D. Mandal / Journal of Nuclear Materials 440 (2013) 104–109
Fig. 8. Photograph of (a) Li2TiO3 pebbles used in the study and (b) precipitated and dried Li2CO3 particles.
where, k is the reaction rate constant at temperature T, A is called the frequency or pre-exponential factor, E is called the activation energy of the reaction and R is the universal gas constant. The experimental results show the increase in the fraction of extraction with temperature is not exponential. This indicates that the increase in extraction rate is not significant not due to increase in chemical reaction rate but due to improved mass transfer. Thus, subsequent experiments were carried out at a temperature of 90 °C. 3.4. Solid–liquid ratio The effect of solid (S) to liquid (L) ratio on the extraction of lithium from lithium titanate pebbles was studied with 4M initial concentration of HCl at 450 rpm agitator speed at 90 °C for 4 h and the S/L ratio varied from 1:6 to 1:15. The results presented in the Fig. 6 shows that the leaching rate increases with increase in S/L ratio up to 1:12 and beyond this ratio no significant improvement on leaching rate was observed. Dispersion of the reaction mixture is better at higher S/L ratio as this increases the rate of mass transfer. Thus, leaching rate increases with increase in S/L ratio. Thus, the subsequent experiments were carried out with S to L ratio of 1:12. 3.5. Acid concentration The effect of hydraulic acid concentration on the lithium extraction from lithium titanate pebbles was investigated in a solution of HCl with concentration varying from 4M to 6M at 450 rpm, 90 °C for 4 h with the solid–liquid ratio of 1:12. The results presented in Fig. 7 shows that the leaching efficiency increases with increase in concentration of hydraulic acid during the initial period of reaction. As the reaction proceeds further, at the end of 4 h, there is no significant increase in the fraction of lithium leached. This indicates that the reaction is a first order reaction. 3.6. Particle size Some of the experiments were repeated by using fine Li2TiO3 particles of size range 100–200 lm as mentioned in Section 2.2. No significant improvement in the degree of recovery of lithium at either temperature and at any particular speed of agitation was observed. This is due to the porous nature of the pebbles as the pebbles have 7% open porosity (see Table 1) through which hydrochloric acid penetrate easily. So, in the process of Li-recovery and recycle from spent Li2TiO3 pebbles the operation of size reduction is not necessary.
carbonate. From the study, it was found that the recovery of lithium is the maximum when an excess of 30% sodium carbonate is used. Table 3 shows the solubility of Li2CO3 in NaCl solution of different concentration due to addition of Na2CO3 of different concentrations. It is known that presence of NaCl at a concentration >15% in the solution decreases the solubility of Li2CO3 due to salting effect. Assuming the reaction to reach completion, the concentration of sodium carbonate that gives >15% concentration of NaCl in the solution after reaction was estimated. The precipitated Li2CO3 was separated by filtration, dried and kept in desiccators overnight. The quantity of Li2CO3 precipitated was determined by weighing and XRD analysis of the sample was carries out. The XRD analysis confirmed the presence of single phase Li2CO3. Fig. 8a shows a photograph of Li2CO3 particles used in the study and Fig. 8b shows a photograph of precipitated and dried Li2CO3 particles. The recovered Li-value in the form of Li2CO3 can be recycled to synthesize and fabricate Li2TiO3 pebbles by solid state reaction process as described by Mandal et al. [1,2] 4. Conclusions An advanced process has been developed for the recovery and recycle of lithium value of spent lithium titanate pebbles. Effects of various process parameters on the extraction of lithium from spent lithium titanate pebbles were investigated. From the study, it was found that the recovery of lithium is the maximum when an excess of 30% sodium carbonate is used. A recovery of more than 98% is obtained by carrying out the precipitation of lithium from the LiCl solution obtained after leaching by using 40% excess of 30% sodium carbonate and heating the solution to a temperature of 95 °C. The recovered Li-value in the form of Li2CO3 can be easily recycled for the synthesis and fabrication of Li2TiO3 pebbles for reuse in the fusion reactors. The developed process is scalable and moreover, the operations in the process can be remotely handled if required to avoid the hazards involved in the tritium release form the spent pebbles. Acknowledgments The author is thankful to Mr. S.K. Ghosh of BARC for his valuable suggestions in carrying out this work and the author is thankful to Mrs. Samyukta G., Mr. M.C. Jadeja and Mr. B.K. Chougule for their help in carrying out the experiments. References [1] [2] [3] [4] [5]
[6]
[7] [8]
3.7. Precipitation of lithium carbonate When sodium carbonate solution was added to the solution obtained after leaching, a white precipitate was obtained. The white precipitate gives an indication that the product obtained is lithium carbonate. This precipitate was analyzed and identified as lithium
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[9] [10] [11] [12] [13]
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