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Microwave radiation leaching of colemanite in sulfuric acid solutions
Separation and Purification Technology 105 (2013) 24–32
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Microwave radiation leaching of colemanite in sulfuric acid solutions Salih U. Bayca ⇑ Celal Bayar University, Soma Vocational School, Soma, 45500 Manisa, Turkey
a r t i c l e
i n f o
Article history: Received 22 June 2012 Received in revised form 8 November 2012 Accepted 9 November 2012 Available online 6 December 2012 Keywords: Microwave: leaching Colemanite Boric acid Gypsum crystallization
a b s t r a c t Two leaching methods were used to study colemanite leaching reactions. First, the conventional acid leaching method was performed using a glass reactor at atmospheric pressure, leaching in a water bath. Second, microwave acid leaching was carried out using the glass reactor in a modified microwave oven. The characterization of ground colemanite was determined by X-ray diffraction (XRD) analysis, X-ray Fluorescence (XRF) analysis and chemical analysis. An investigation was made of the influence of the solid/liquid ratio, stirring speed, acid concentration, microwave power, reaction time and reaction temperature on the leaching recovery of boron oxide. The results of the conventional acid leaching (CVAL) method were compared to the microwave leaching (MWAL) method. The crystallization of gypsum was investigated with regard to obtaining high-purity boric acid at different acid concentrations using microwave leaching, in which all of the calcium ions in the liquid phase were incorporated into the gypsum crystals. The optimum leaching efficiency for the CVAL and MVAL methods was 99.82% and 99.9% respectively. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Boric acid, B(OH)3, is currently produced in Turkey by the acid leaching process. In this process, colemanite concentrate is leached with a hot sulfuric acid solution, and calcium ions result. The calcium ions react with sulfate ions and gypsum forms as a byproduct. To produce high purity boric acid, gypsum is separated from the reaction solution by filtration. Bayca [1] found that the colemanite leaching reaction was very fast and required only 30 min for completion. However, 30 min was an insufficient time for the crystallization of the gypsum. Gypsum crystallization needed a minimum of 120 min. Becker [2] reported that the crystallization of the gypsum formed extended the process by 2–8 h. Colemanite is a calcium borate mineral (theoretically, 50.8% B2O3) with a monoclinic crystal structure and a chemical formula of Ca2B6O115H2O. The common states of mineral boron in nature are colemanite, ulexite, and tincal. These are commercially produced in Turkey by open-pit mining. Colemanite ore is produced in open mines in Bigadic, Balikesir province. Colemanite ore in Bigadic has 32% B2O3. The colemanite is enriched at Bigadic by way of physical processes such as crushing, wetting in water, washing in a tumbler, sieving, triage and classification. The enriched coarse product, in different grain sizes and chemical compositions, is offered for sale as concentrated colemanite. The majority of concentrated colemanite is exported [3]. The concentrate products are: very coarse particle size of 25–125 mm with 42 ± 1% B2O3; coarse ⇑ Fax: +90 236 6122002. E-mail address: [email protected] 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.014
particle size of 3–25 mm with 35 ± 2% B2O3; medium particle size of 1–3 mm with 28 ± 2% B2O3; and fine particle size of – 1 mm with 17 ± 5% B2O3. The finer particles are pumped into three waste dams. The dissolution rate increases with increasing temperature for colemanite in acetic acid [4], for colemanite in phosphoric acid [5], for ulexite in sulfuric acid [6], for colemanite in oxalic acid [7], for ulexite in oxalic acid [8], for tincal in oxalic acid [9], for ulexite in perchloric acid [10], for tincal in phosphoric acid [11], and for ulexite in acetic acid [12]. Recently, microwave technology has been applied to a number of mineral processing areas such as microwave leaching of copper [13] and nickel ores [14,15], and the desulfurisation of coal [16– 18]. Microwave radiation is a part of the electromagnetic spectrum, with wavelengths ranging between approximately 1 cm and 1 m. Microwave energy is a non-ionizing electromagnetic radiation, with frequencies in the range of 300 MHz to 300 GHz. Within this range, microwaves have been extensively used in communication, especially in radar, cellular phones, television and satellite applications [19]. Because electromagnetic radiation does not ionize matter, the structure of matter is not decomposed [14]. The frequencies extensively used for heating purposes are 915 MHz and 2.45 GHz, which correspond to wavelengths of 33.5 and 12.2 cm respectively [20]. Microwaves have lower energy and higher wavelength than ultraviolet, visible and infrared radiation [21]. The interaction of dielectric materials with electromagnetic radiation in the microwave range results in energy absorbance. The ability of a material to absorb energy while in a microwave
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cavity is related to the loss tangent of the material. This depends on the relaxation times of the molecules in the material, which, in turn, depends on the nature of the functional groups and the volume of the molecule. The dielectric properties of a material are related to temperature, moisture content, density and material geometry [22]. The ability of materials to absorb microwave energy is dependent on their dielectric properties.
Digital Thermometer
Mechanically controlled stirrer
Programmable Circulator
Reflux condenser
e ¼ e0 je00 where e is relative complex permittivity, e0 is permittivity (dielectric constant), e00 is the dielectric loss factor and j is the imaginary component, and eo is the permittivity of free space (8.85 10–12 F/m). The dielectric loss factor is related to two other dielectric properties by the equation:
e00 tan d ¼ 0 e
Glass reactor
Water bath Copper pipe
where tand is loss tangent. At a given frequency the power absorbed is
P ¼ 2pf eo e00 jEj2
Fig. 1. The leaching experiment test set-up.
where P is energy developed per unit volume (W/m3), f is frequency (2450 106 Hz), and E is electric field strength within the product (V/m). The speed of microwave heating is due to the deep penetration of microwaves into the material, and the dielectric properties can be used to determine the extent of penetration. The term penetration depth is defined as the depth at which the microwave power level is reduced to 36.8% (or 1/e) of its value.
2
00 2 !1=2
4 1þ e d¼ e0 2pf ð2e0 Þ1=2 c
31=2 15
where d is the penetration depth, c is the the speed of light (3 108 m/s) of incident radiation [23–27]. In this study, the aim was to shorten the leaching time, to shorten the crystallization time of gypsum, to investigate the effects of leaching parameters, to compare the effects of temperature as the result of conventional heating methods and the microwave radiation heating method, to completely remove the gypsum, which reduces the purity of boric acid, from solution, to determine the parameters affecting the crystallization of gypsum which affect the efficiency of the separation of gypsum from the solution, to determine the effects of this parameter on the crystallization time of gypsum and the purity of boric acid, and to determine the optimum conditions for the microwave leaching process by which high-purity boric acid solution is obtained. 2. Materials and methods 2.1. Materials The ground colemanite sample in this study was obtained from Bigadic Boron of Eti Mine, Turkey. The laboratory tests were carried out in DEFAM. The sieve analysis of the sample was performed by sieving. 2.2. Conventional acid leaching method (CVAL) The conventional acid leaching test setup design is shown in Fig. 1. The leaching tests were carried out in a 500-mL three-neck glass reactor at atmospheric pressure. A mechanical stirrer (IKA RW20) with a digital display was used to agitate the solution. A programmable cryostat bath/circulator (Polyscience 9106) was used for heating or cooling to keep a constant reaction temperature. A spiral condenser was set up on the glass reactor to prevent loss of solution by evaporation. The temperature of the solution in
the reactor was measured with a portable probe digital temperature controller with a range of between 0 and 150 ± 0.1 °C. The samples were weighed to 0.1 mg using a Denver Instrument SI 234 analytical balance. The leaching time was measured by digital chronometer. During the leaching process, 50 mL of sulfuric acid solution was placed in the reactor and stirred, and the digital chronometer was started. After the desired reaction temperature was reached, a certain amount of the dried colemanite sample was added to the solution. After a certain period of time, the stirrer and chronometer were stopped. The hot solution was filtered using blue filter paper and vacuum without any change in temperature. Each experiment was repeated at least twice. The percentage of B2O3 in the filtrate was determined by volumetric methods [28]. 2.3. Microwave leaching method (MWAL) The experimental setup used for the Microwave Leaching Process is shown in Fig. 2. A domestic microwave oven was modified for this leaching study. The modified microwave oven produced power of 900 W at a frequency of 2.45 GHz. A three-neck 500-mL glass reactor was put into the MW oven. A spiral condenser was set up on the glass reactor to prevent loss of solution by evaporation. The temperature of the solution in the reactor was measured by digital infrared thermometer with a range of between 60 and +1500 ± 0.1 °C (Ebro TFI 650). 50 mL or 100 mL of sulfuric acid solution and a certain amount of the dried sample were added into the reactor. After being placed into the microwave oven, the reactor and glass pipes were connected to each other. The leaching solution was not stirred. After a certain period of time, the oven was turned off. The temperature of the glass reactor was measured remotely with the infrared thermometer. The hot solution was filtered using blue filter paper and vacuum without any change in temperature. Each experiment was repeated at least twice. The amounts of CaO in the filtrate were determined by volumetric methods. 2 mL the sample solution was added to Erlenmeyer flask. NaOH solution was dropped for adjusting pH 13. Murexide indicator was added and stirred. EDTA (ethylene diamine tetra acetic acid) solution was added drop by drop to the solution in the flask, which was being stirred at a stable rate, until the contents of the flask changed in colour from red to orchid purple. The burette was read to determine the volume of EDTA. The leaching (dissolution) efficiency was percent
Leaching efficiency of B2 O3 ¼
M1 100 Mo
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Open Atmosphere
Glass stopper Water outlet Reflux Condenser Reflux condenser
Water inlet
Stainless steel pipe
Magnetron
Glass pipe
Infrared thermomet Laser light Parallel Necked Glass Reactor PTFE
Fig. 2. The microwave acid leaching experiment test set-up.
where M1 is the percent of dissolved B2O3 in the solution, Mo is the percent of B2O3 in the original sample. 3. Results and discussion 3.1. Characterization of the sample The results of the particle size analysis of the dried samples are given in Fig. 3. According to the cumulative percent passing curve of ground colemanite particles, the sizes obtained were 98 lm 80%.
The chemical analysis of the colemanite sample (except B2O3) with Shimadzu XRF (X-ray fluorescence) is given in Table 1. It was shown that the sample consisted of 40.32% B2O3, 28.95% CaO, 2.5% SiO2 and 2.33% SrO. The XRD analysis of the sample was carried out on a Rigaku DMAX 2200 Series X-ray diffractometer with Cu Ka radiation. As seen in Fig. 4, the sample was mainly composed of colemanite (79%) and small amounts of calcite (CaCO3). 3.2. Effect of microwave power on boron oxide extraction When evaluating the effect of microwave power on boron oxide extraction by the MWAL method, microwave power values of 360– 900 W were selected. Further, tests were performed under typical conditions: microwave irradiation was 5 min, the ratio of solid-toliquid was 10%, and acid concentration was 10%. The effect of microwave power is shown in Fig. 5. The leaching efficiency of bor-
Table 1 Chemical analysis of ground colemanite.
Fig. 3. The particle size distribution of the colemanite.
Component
Colemanite (%)
SiO2 Al2O3 Fe2O3 B2O3 CaO MgO SrO LOI
2.50 0.09 0.21 40.32 28.95 0.90 2.33 24.17
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110 A
100
A: Colemanite B: Calcite
90 Intensity (Counts)
80 70 A
60 50 A B
40
B
A A
30
B
20 10 0 10
20
30
40
50
60
70
80
90
Two-Theta (Degree) Fig. 4. X-ray diffraction analysis of colemanite.
on oxide slowly decreased as the microwave power was increased in the range of 360–540 W. However, the efficiency increased with an increase in microwave power in the range of 540–720 W. The interaction of molecules is proportional to the microwave power. The kinetic energy of the molecules is converted into heat energy due to the interaction of molecules and as a result the temperature increases. As the colemanite mineral is soluble at high temperatures, the leaching efficiency is increased with increasing microwave power. Xia and Pickles [29] reported that higher power had a remarkable influence on the increase of metal dissolution. The optimum leaching rate of boron oxide was reached at 720 W. When the microwave power was increased from 720 to 900 W, the dissolution efficiency of boron oxide remained stable. 3.3. Effect of leaching time on boron oxide extraction Experiments were carried out according to the conditions of the CVAL method. Tests were performed on CVAL to determine the effect of time on boron oxide extraction under typical conditions as follows. Acid concentration was 10%, stirring speed was 500 rpm, and temperature was 80 °C. The time range selected was 5– 40 min. It can be seen from Fig. 6 that leaching efficiency increased with an increase in time during leaching. According to the CVAL
tests, the leaching rate of boron oxide from colemanite reached 99.82% when the time period was 40 min. Experiments were conducted to determine the effect of the length of time of microwave irradiation on the leaching rate of boron oxide in MWAL. The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid-to-liquid was 10%, and acid concentration of sulfuric acid was 10%. The time was between 1 and 5 min in MWAL tests, and it was shown that the leaching of boron oxide slowly increased with the increase of time. The optimum leaching rate was obtained with a time of leaching of 5 min. The leaching time of MWAL was shorter than that of CVAL. This can be explained as follows: heating begins throughout the solution when microwave radiation is applied. The ions are caused to vibrate and move as an effect of the microwave radiation. The dissolution rate increases, and this leads to a shorter time for leaching. The effects of leaching time on the solution temperature are shown in Fig. 7. The solution temperature reached 160 °C in the first minute of the experiment. The maximum solution temperature (183 °C) was reached in the 3 min of leaching time.
100
Leaching recovery of B2O3, %
Leaching efficiency of B2O3 , %
100
90 80 70 60 50
40 30 20 10 0 200
90
80
70 MWAL 60
CVAL
50 0 300
400
500
600
700
800
900
1000
5
10
15
20
25
30
35
40
45
Leaching time, min
Microwave power, W Fig. 5. Effect of microwave power on boron oxide recovery (acid concentration of 10%, leaching time of 5 min).
Fig. 6. Effect of leaching time on boron oxide recovery (CVAL: acid concentration of 10%, solid to liquid ratio of 10%, speed of 500 rpm, at 80 °C and MWAL: acid concentration of 10%, solid to liquid ratio of 10%, Power 720 W).
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3.4. Effect of the ratio of solid to liquid on boron oxide extraction
80
Figure 8 shows the time taken for the B2O3 to reach its maximum dissolution efficiency of 99.9% under optimum conditions and the variable condition of 5, 10, 20 and 30% solid/liquid ratio. The method described above was used in both CVAL and MWAL experiments. Tests were performed by CVAL to determine the time effect of boron oxide extraction under typical conditions as follows. Acid concentration was 10%, stirring speed was 500 rpm, and temperature was 80 °C. It can be observed from Fig. 8 that in the CVAL method the leaching time increased slowly as the solid-to-liquid ratio increased from 5 to 20%, but a solid-to-liquid ratio of 30% led to a sharp increase in the leaching time. This may result from the evaporation of water while the acid concentration of suspension was constant (1.748 g acid/g colemanite). Thus, the ratio of solid to liquid had a significant effect on the leaching of boron oxide in the CVAL method, and for this reason the reaction rate decreased. The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid to liquid was 10%, and acid concentration of sulfuric acid was 10%. There was no significant increase in leaching time even with an increase from 5% to 30% in the solidto-liquid ratio of the solution in the MWAL method. The maximum leaching efficiency in the CVAL method was reached at 40 min. with a ratio of solid to liquid of 30%. However, the same efficiency was reached with MWAL at 5 min. with a solid-to-liquid ratio of 10%. The ratio of solid to liquid had a significant effect on leaching time in the CVAL method and was proportional to the leaching time, but an increase in the same parameter had no remarkable influence on leaching time in the MWAL method. The experimental temperature was 80 °C in the CVAL method while it was 183 °C in the MWAL method. In the CVAL method, the increase in the kinetic energy of molecules depends on temperature over a long time. The migration of ionic species and/or rotation of dipolar species promotes the liquid–solid reaction process due to the increased contact area of reactants [15]. On the other hand, when a microwave field is applied to a solution containing metal/ions, they move due to their inherent charge as a result of the force of the electric and magnetic field components of the microwave radiation. Charged particles collide, and the collisions lead to the conversion of kinetic energy to thermal energy. More collisions occur in solution depending on the increase in the concentration of ions/atoms. As a consequence, a faster heating occurs in solution [30]. Jafarifar et al. [31] examined microwave assisted leaching followed by
70
CVAL
Leaching time, min
60
MWAL
50 40 30
20 10 0 0
5
10
15
20
25
30
35
Solid/liquid ratio, % Fig. 8. Effect of solid-to-liquid ratio on leaching time (CVAL: 80 °C, 1.748 g acid/g colemanite, 500 rpm, 99.9% B2O3 recovery and MWAL: Power 720 W, 1.748 g acid/g colemanite, 99.9% B2O3 recovery).
chemical precipitation to recover platinum. They found that the maximum leaching efficiency of the conventional acid leaching method was 150 min while the maximum leaching efficiency of the microwave leaching method was 5 min.
3.5. Effect of temperature on boron oxide extraction The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid to liquid was 10%, and the acid concentration of sulfuric acid was 10%. It can be seen in Fig. 9 that the leaching rate varied linearly with temperature. Colemanite minerals, pure water and sulfuric acid have a high dielectric loss factor which increases with increasing microwave power. The results of the comparison between conventional leaching and microwave leaching at different dissolution time are summarized in Table 2. The maximum leaching efficiency in the conventional acid leaching experiment was achieved at 80 °C. In contrast, the microwave leaching experiment reached maximum leaching efficiency at 183 °C. The temperature of the leaching solution increased from 160 to 183 °C with the increase in microwave power.
190
195
180
190
170
Leaching temperature, °C
Solution temperature, °C
200
160 150 140 130 120 110
185 180 175 170 165 160 155
100
150 0
1
2
3
4
5
6
Microwave leaching time, min Fig. 7. Effect of leaching time on solution temperature (acid concentration of 10%, solid-to-liquid ratio of 10%, 720 W).
300
400
500
600 700 800 Microwave power, W
900
1000
Fig. 9. Effect of microwave power on leaching temperature (acid concentration of 10%, solid-to-liquid ratio of 10%, 5 min).
S.U. Bayca / Separation and Purification Technology 105 (2013) 24–32
3.6. Characterization of leaching products Colemanite dissolved completely in hot sulfuric acid solutions under the optimum conditions. The leaching products were determined as being solid or liquid phase. Borate ions and calcium ions were detected in the liquid leaching product obtained by filtration, and gypsum crystals were determined in the solid product. Boric acid crystals were not observed in the solid leaching product. This is because boron oxide has high solubility at high temperatures, and only crystallizes at lower temperatures as its solubility is reduced. Kuskay and Bulutcu [32] found that boric acid crystallizes when the solution is cooled to around 35 °C. Colemanite reacts with sulfuric acid according to Eq. (1). The borate ions, sulfate ions and calcium ions are released from PLS (Pregnant Leach Solution).
Ca2 B6 O11 5H2 OðsÞ þ H2 SO4ðaqÞ þ 10H2 O 2 ¼ 6BðOHÞ4ðaqÞ þ 4Hþ þ 2Ca2þ ðaqÞ þ SO4ðaqÞ þ 2H2 O
ð1Þ
The CVAL tests were conducted under typical conditions, as follows. The ratio of solid-to-liquid was 10%, the temperature was 80 °C, acid concentration was 10%, the stirring speed was 500 rpm, the particle size was –98 lm, and the leaching time was 40 min. The leaching efficiency of boron oxide by CVAL was 99.82%. X-ray diffraction analysis (Philips PW3710/1830) of leaching solid product of CVAL is given in Fig. 10. The major phase was anhydrite (CaSO4) in the XRD analysis. SEM analysis of (FEI Quanta 400 MK2) samples of solid residue from CVAL leaching is given in Fig. 11. It can be seen that only anhydrite crystals of needle type were formed. The crystals were approximately 50 lm in size and regular in shape. This can be attributed to the 40-min crystallization period. The optimum conditions of MWAL were a ratio of solid to liquid of 10%, a temperature of 183 °C, an acid concentration of 10%, a particle size of 98 lm, microwave power of 720 W and a leaching time of 5 min. The leaching efficiency of boron oxide by MWAL was 99.9%.
As a result of the reaction between calcium and sulfate ions, gypsum crystals formed according to Eq. (2). The gypsum crystals precipitated in PLS.
Ca2 B6 O11 5H2 OðsÞ þ H2 SO4ðaqÞ þ 16H2 O ¼ 6BðOHÞ4ðaqÞ þ 12HþðaqÞ þ 2ðCaSO4 2H2 OðsÞ Þ
ð2Þ
Boric acid crystals formed in solution cooled to 35 °C (Eq. (3)).
Ca2 B6 O11 5H2 OðsÞ þ 2H2 SO4ðaqÞ þ 6H2 O ¼ 6BðOHÞ3ðsÞ þ 2ðCaSO4 2H2 OðsÞ Þ
ð3Þ
Table 2 The comparison of conventional and microwave acid leaching methods. Parameter
Conventional acid leaching, CVAL
MW leaching, MWAL
Leaching time, min Solid/liquid ratio, % g acid/g colemanite Temperature, °C Microwave power, W Stirring speed, rpm Particle size (d80), lm Recovery of B2O3, %
40 10 1.748 80 No 500 98 99.82
5 10 1.748 183 720 No 98 99.90
(a) 100µm CVAL
(b) 50 µm CVAL Fig. 10. X-ray diffraction analysis of leaching solid product of CVAL.
29
Fig. 11. SEM analysis of leaching solid product of CVAL.
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Fig. 12. X-ray diffraction analysis of leaching solid product of MWAL.
(a) 100 µm MWAL
X-ray diffraction analysis of solid leaching product of MWAL is given in Fig. 12. The major phase was anhydrite (CaSO4) in the XRD analysis. This is in agreement with Eq. (3). In Eq. (3) anhydrite was observed in the XRD analysis despite the formation of gypsum. This leaching reaction may be attributed to the rise in temperature. The SEM analysis of MWAL anhydride crystals is given in Fig. 13. The morphology of the anhydrite crystals was needle type. Crystals were approximately 50 lm in size. However, their shape was broken and irregular. This may be not enough time was given for crystals to grow in this solution. Crystallisation and leaching time were both 5 min in duration. 3.7. Chemical precipitation of gypsum The effect of acid concentration on the extraction of boron oxide was investigated by single-stage MWAL and multi-stage MWAL methods. A leaching efficiency of 99.9% B2O3 was achieved under conditions of single-stage MWAL as shown in Table 3, as follows. The microwave power was 720 W; the leaching time was 8 min. and acid concentration was in the range of 5–25%. The calcium content of the liquid phase, which was obtained by separating the solid and liquid phases in the PLS, was decreased by increasing the acid concentration, and was determined as 1.083% Ca. The results of the multi-stage MWAL experiment are given in Table 4. The leaching rate of boron oxide in multi-stage MWAL was found to be 99.9%, and the calcium content was 0.017%. The calcium content of the liquid phase decreased as the acid concentration increased. As seen in Fig. 14, the calcium content of the liquid phase of single-stage MWAL was higher than the calcium content of the liquid phase of multi-stage MWAL. It was found that the calcium content of the liquid phase varied according to the leaching methods. The calcium contents of the liquid phase which were obtained by applying single stage MWAL, CVAL and multi stage MWAL methods at an acid concentration of 5% were measured as 3.365%, 2.690% and 2.231% Ca respectively (Fig. 15). It was determined by XRD analysis that the solid phase was composed of a large amount of gypsum (anhydride) crystals. The gypsum was formed by a reaction between calcium and sulfate ions which were released during the dissolution of colemanite in sulfuric acid solution. A large number of calcium ions, which reduced the purity of the boric acid, was determined in the liquid phase. The calcium ions pass to the boric acid solution (liquid phase) by filtration of the leaching solution. The Ca2+ ions must be removed from PLS to obtain high purity and high yield of boric
(b) 50 µm MWAL Fig. 13. SEM analysis of leaching solid product of MWAL.
acid. Conversion of all calcium ions in PLS to gypsum (anhydride) crystals is achieved using two methods. In the first of these methods, the acid concentration of the solution is increased. In second method, the time of crystallization is increased. In this study, the first option was preferred in order to decrease the time of the boric acid production process. It was determined that, as a result of an increase from 5% to 25% concentration of sul-
Table 3 The experimental conditions of the Fig. 11 for the single stage MWAL. Parameters
A
B
C
D
E
Microwave power, W Acid concentration, % Leaching time, min Solid/liquid ratio, %
900 5 8 5
900 10 8 5
900 15 8 5
900 20 8 5
900 25 8 5
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S.U. Bayca / Separation and Purification Technology 105 (2013) 24–32 Table 4 The experimental conditions of the Fig. 11 for the multi-stage MWAL. Parameters
F1
F2
G1
G2
H1
H2
K1
K2
L1
L2
Microwave power, W Acid concentration, % Leaching time, min Solid/liquid ratio, %
900 5 3 50
900 – 5 5
900 10 3 33.33
900 – 5 5
900 15 3 25
900 – 5 5
900 20 3 20
900 – 5 5
900 25 3 16.67
900 – 5 5
3.5
Ca in solution, %
3.0 2.5 2.0 1.5 1.0 Multi satge MWAL
0.5
Single stage MWAL
0.0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
Sulphuric acid, % Fig. 14. Effect of sulfuric acid concentration on Ca in solution.
furic acid, the pH of the solution of sulfuric acid was decreased from 1.21 to 0.36. The calcium content in the liquid phase was decreased by increasing acid concentration in both single stage MWAL and multi stage MWAL methods. The calcium content of the liquid phase obtained at an acid concentration of 25% was 1.083% Ca for the single stage MWAL method and 0.017% Ca for the multi stage MWAL method. The lowest calcium level was obtained from the multi stage MWAL method at 25% acid concentration. The liquid phase containing very low calcium content can be attributed to the formation of a supersaturated solution of gypsum. The supersaturated solution of gypsum can be defined as a solution containing more
Ca2+ and SO2 ions than a saturated solution. It was found that 4 the calcium content of the liquid phase of the single stage MWAL method was higher than that of the multi stage MWAL method. This may be attributed to the fact that the multi stage MWAL method accelerates gypsum formation. The dissolution of colemanite and precipitation of gypsum crystals occurs simultaneously in acidic media [33]. The solubility of gypsum decreases with an increasing content of sulfuric acid solution. The size of gypsum crystals increases with time due to crystal growth [34]. Supersaturation was controlled by adding sulfuric acid solution. Mersmann [35] reported that supersaturation was achieved by increasing acid concentration. Abdel Aal [36] found that the morphology of gypsum crystals is affected by free sulfate content and supersaturation ratio. Large crystals are obtained at high sulfate levels and a low supersaturation ratio gives thick crystals. Rashad et al. [37] reported that gypsum nucleation rate increased with the addition of 1% MgO. Calcium sulfate crystallization occurs in parallel with leaching. According to the process adopted, calcium sulfate dihydrate (gypsum) (CaSO42H2O) or calcium sulfate hemihydrate (CaSO40.5H2O) is crystallized [2,38]. The results indicate that 31%, 25%, and 23% increases in filtration rate were achieved at low, medium, and high sulfate contents, respectively. The observed improvements are attributed to changes in crystal size distribution [39]. 4. Conclusions In this study, colemanite mineral was leached with sulfuric acid solution at atmospheric pressure at laboratory scale to obtain high purity boric acid and a shorter process time. The leaching conditions of colemanite mineral in sulfuric acid solution were optimized using both conventional acid leaching (CVAL) and single stage and multi stage microwave leaching methods (MWAL).
5 % acid 3.5
Ca in solution, %
3
25 % acid
2.5 2 1.5 1 0.5 0 CVAL
single stage MWAL
Multi-stage MWAL
Fig. 15. Effect of microwave acid leaching sulfuric acid concentration on Ca in solution.
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The optimum leaching efficiency of the CVAL method was 99.82%, under typical conditions where acid concentration was 10%, stirring speed was 500 rpm, temperature was 80 °C, the leaching time was 40 min and the solid/liquid ratio was 10%. The optimum leaching efficiency of the MVAL method was 99.9%, under typical conditions where microwave power was 720 W, acid concentration was 10%, the time was 5 min, the solid/liquid ratio was 10% and temperature was 183 °C. A leaching efficiency of 99.82% was achieved in 40 min using the CVAL method, and the calcium content of the liquid phase was determined as 2.69% Ca. However, maximum leaching efficiency (99.90%) of boron oxide was reached by the MWAL method in 5 min. Despite this decrease in leaching time, the liquid phase contained 3.335% Ca. In the multi-stage leaching method carried out at an acid concentration of 25%, the calcium content of the boric acid solution (the liquid phase) was 0.017%. The efficiency of the removal of calcium ions from the boric acid solution was 99%. The rate of crystallization increased with the pH of the solution. A very high increase in the leaching time was observed with an increase in the solid-to-liquid ratio in CVAL, but there was no significant increase in leaching time in MWAL when the solid-to-liquid ratio was increased. This means that the solid-to-liquid ratio of the leaching process can be as high as 30%, which increases the plant capacity. The cost of production of the plant will be reduced because of the increase in the plant capacity. The removal of calcium ions was investigated using single-stage and multi-stage microwave leaching methods. The calcium content of the liquid phase in single stage MWAL was higher than the calcium content of the liquid phase in multi-stage MWAL. The reason for this is that the formation of gypsum accelerates the rate of reaction in multi-stage MWAL. The optimum conditions for multi-stage microwave leaching were found to be a microwave power of 900 W, an acid concentration of 25%, a leaching time of 8 min, a solid/liquid ratio of 5%, a temperature of 191 °C, and a calcium content of 0.017% Ca. Maximum leaching efficiency was obtained by the conventional acid leaching method in 40 min, while maximum leaching efficiency was obtained in 8 min using the multi-stage microwave leaching method. The removal efficiency of calcium ions from boric acid solution was found as 99% with acid concentration of 25% using the multi-stage microwave leaching method. Acknowledgments The author would like to thank Eti Mine Works General Management for the XRD and XRF analysis and for providing samples. References [1] S.U. Bayca, The extraction of boric acid from colemanite process waste by attrition scrubbing and leaching, 2012 (unpublished). [2] P. Becker, Phosphate and Phosphoric Acid, Marcel Decker Inc., New york, 1989. [3] S.U. Bayca, Effects of the addition of ulexite to the sintering behavior of a ceramic body, Journal of Ceramic Processing Research 10 (2009) 162–166. [4] C. Ozmetin, M.M. Kocakerim, S. Yapici, A. Yartasi, A semiempirical kinetic model for dissolution of colemanite in aqueous acetic acid solutions, Industrial and Engineering Chemistry Research 35 (1996) 2355–2359. [5] H. Temur, A. Yartasi, M. Copur, M.M. Kocakerim, The kinetics of dissolution of colemanite in H3PO4 solutions, Industrial and Engineering Chemistry Research 39 (2000) 4114–4119. [6] M. Tunc, S. Yapici, M.M. Kocakerim, A. Yartasi, The dissolution kinetics of ulexite in sulfuric acid solutions, Chemical and Biochemical Engineering Quarterly 15 (2001) 175–180.
[7] M. Alkan, M. Dogan, Dissolution kinetics of colemanite in oxalic acid solutions, Chemical Engineering and Processing 43 (2004) 867–872. [8] M. Alkan, M. Dogan, H. Namli, Dissolution kinetics and mechanism of ulexite oxalic acid solutions, Industrial and Engineering Chemistry Research 43 (2004) 1591–1598. [9] Y. Abali, S.U. Bayca, E. Mistincik, Kinetics of oxalic acid leaching of tincal, Chemical Engineering Journal 123 (2006) 25–30. [10] N. Demirkiran, A. Kunkul, Dissolution kinetics of ulexite in perchloric acid solutions, International Journal of Mineral Processing 83 (2007) 76–80. [11] Y. Abali, S.U. Bayca, E.A. Guler, The dissolution kinetics of tincal in phosphoric acid solutions, Chemical Engineering Journal 5 (2007) 1–10. [12] A. Ekmekyapar, N. Demirkiran, A. Kunkul, Dissolution kinetics of ulexite in acetic acid solutions, Chemical Engineering Research and Design 86 (2008) 1011–1016. [13] M. Lovas, I. Murova, A. Mockovciakova, N. Rowson, S. Jakabsky, Intensification of magnetic separation and leaching of Cu-ores by microwave radiation, Separation and Purification Technology 31 (2003) 291–299. [14] Y.-F. Chang, X.-J. Zhai, Y. Fu, Phase transformation in reductive roasting of laterite ore with microwave leaching, Transactions of Nonferrous Metals Society of China 20 (2008) 969–973. [15] X. Zhai, Q. Wu, Y. Fu, L. Ma, C. Fan, N. Li, Leaching of nickel laterite ore assisted by microwave technique, Transactions of Nonferrous Metals Society of China 20 (2010) 77–81. [16] N. A. Rowson, Desulphurisation of coal by microwave energy, Ph.D. Thesis, The University of Leeds, UK, 1986. [17] N.A. Rowson, N.M. Rice, Magnetic enhancement of pyrite by caustic microwave treatment, Minerals Engineering 3 (1990) 355–361. [18] N.A. Rowson, N.M. Rice, Desulphurization of coal using low power microwave energy, Minerals Engineering 3 (1990) 363–368. [19] E.T. Thostenson, T.W. Chou, Microwave processing: fundamental and applications, Composites, Applied Science and Manufacturing 3 (1999) 1055–1071. [20] M. Al-Harahsheh, S.W. Kingman, Microwave-assisted leaching, Hydrometallurgy 73 (2004) 189–203. [21] F. Langa, P.L. Cruz, A. Hoz, A.D. De La Ortiz, E.D. Barra, Microwave irradiation, Contemporary Organic Synthesis (1996). [22] P.E. Stott, G.E. Zaikov, V.F. Kablov, Chemical and Biochemical Physics, Kinetics and Thermodynamics, Nova Science Publishers, Newyork, USA, 2008. [23] P.G. Smith, Introduction to Food Process Engineering, Springer, New york, 2011. [24] J.N. Lalena, D.A. Cleary, E. Carpenter, N.F. Dean, Inorganic Materials Synthesis and Fabrication, Wiley, USA, 1996. [25] P.O. Risman, Terminology and notation of microwave power and electromagnetic energy, Journal of Microwave Power and Electromagnetic Energy 26 (1991) 243–250. [26] C.R. Buffler, Microwave Cooking and Processing, Van Nostrand Reinhold, New York, 1993. [27] K.L. Yam, The Wiley Encyclopedia of Packaging Technology, Wiley, USA, 2009. [28] W.W. Scott, Standard Methods of Chemical Analysis, D. Van Nostrand, New York, 1963. [29] D.K. Xia, C.A. Pickles, Microwave caustic leaching of electric arc furnace dust, Minerals Engineering 13 (2000) 79–94. [30] M. D. Schaefer, Microwave Tempering of Shrimp with Susceptors, Master thesis, Virginia Polytechnic Institute and State Universit, USA, 1999. [31] D. Jafarifar, M.R. Daryanavard, S. Sheibani, Ultra fast microwave-ass, sted leaching for recovery of platinium from spent catalyst, Hydrometallurgy 78 (2005) 166–171. [32] B. Kuskay, A.N. Bulutcu, Design parameters of boric acid production process from colemanite ore in the presence of propionic acid, Chemical Engineering and Processing: Process Intensification 50 (2011) 377–383. [33] N. Taylan, H. Gurbuz, A.N. Bulutcu, Effects of ultrasound on the reaction step of boric acid production process from colemanite, Ultrasonics Sonochemistry 14 (2007) 633–638. [34] A. Al-Othman, G.P. Demopoulos, Gypsum crystallization and hydrochloric acid regeneration by reaction of calcium chloric solution with sulfuric acid, Hydrometallurgy 96 (2009) 95–102. [35] A. Mersmann, Supersaturation and nucleation, Chemical Engineering Research and Design 74 (1996) 812. [36] E.A. Abdel-Aal, M.M. Rashad, H. El-Shall, Crystallization of calcium sulfate dehydrate at different supersaturation ratios and different free sulfate concentrations, Crystal Research and Technology 39 (2004) 313–321. [37] M.M. Rashad, M.H.H. Mahmoud, I.A. Ibrahim, E.A. Abdel-Aal, Crystallization of calcium sulfate dihydrate under simulated conditions of phosphoric acid production in the presence of aluminum and magnesium ions, Journal of Crystal Growth 267 (2004) 372–379. [38] E.A. Abdel-Aal, Crystallization of phosphogypsum in continuous phosphoric acid industrial plant, Crystal Research and Technology 39 (2004) 123–130. [39] H. El-Shall, E.A. Abdel-Aal, B.M. Moudgil, Effect of surfactants on phosphogypsum crystallization and filtration during wet-process phosphoric acid production, Separation Science and Technology 35 (2000) 395–410.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Microwave radiation leaching of colemanite in sulfuric acid solutions Salih U. Bayca ⇑ Celal Bayar University, Soma Vocational School, Soma, 45500 Manisa, Turkey
a r t i c l e
i n f o
Article history: Received 22 June 2012 Received in revised form 8 November 2012 Accepted 9 November 2012 Available online 6 December 2012 Keywords: Microwave: leaching Colemanite Boric acid Gypsum crystallization
a b s t r a c t Two leaching methods were used to study colemanite leaching reactions. First, the conventional acid leaching method was performed using a glass reactor at atmospheric pressure, leaching in a water bath. Second, microwave acid leaching was carried out using the glass reactor in a modified microwave oven. The characterization of ground colemanite was determined by X-ray diffraction (XRD) analysis, X-ray Fluorescence (XRF) analysis and chemical analysis. An investigation was made of the influence of the solid/liquid ratio, stirring speed, acid concentration, microwave power, reaction time and reaction temperature on the leaching recovery of boron oxide. The results of the conventional acid leaching (CVAL) method were compared to the microwave leaching (MWAL) method. The crystallization of gypsum was investigated with regard to obtaining high-purity boric acid at different acid concentrations using microwave leaching, in which all of the calcium ions in the liquid phase were incorporated into the gypsum crystals. The optimum leaching efficiency for the CVAL and MVAL methods was 99.82% and 99.9% respectively. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Boric acid, B(OH)3, is currently produced in Turkey by the acid leaching process. In this process, colemanite concentrate is leached with a hot sulfuric acid solution, and calcium ions result. The calcium ions react with sulfate ions and gypsum forms as a byproduct. To produce high purity boric acid, gypsum is separated from the reaction solution by filtration. Bayca [1] found that the colemanite leaching reaction was very fast and required only 30 min for completion. However, 30 min was an insufficient time for the crystallization of the gypsum. Gypsum crystallization needed a minimum of 120 min. Becker [2] reported that the crystallization of the gypsum formed extended the process by 2–8 h. Colemanite is a calcium borate mineral (theoretically, 50.8% B2O3) with a monoclinic crystal structure and a chemical formula of Ca2B6O115H2O. The common states of mineral boron in nature are colemanite, ulexite, and tincal. These are commercially produced in Turkey by open-pit mining. Colemanite ore is produced in open mines in Bigadic, Balikesir province. Colemanite ore in Bigadic has 32% B2O3. The colemanite is enriched at Bigadic by way of physical processes such as crushing, wetting in water, washing in a tumbler, sieving, triage and classification. The enriched coarse product, in different grain sizes and chemical compositions, is offered for sale as concentrated colemanite. The majority of concentrated colemanite is exported [3]. The concentrate products are: very coarse particle size of 25–125 mm with 42 ± 1% B2O3; coarse ⇑ Fax: +90 236 6122002. E-mail address: [email protected] 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.014
particle size of 3–25 mm with 35 ± 2% B2O3; medium particle size of 1–3 mm with 28 ± 2% B2O3; and fine particle size of – 1 mm with 17 ± 5% B2O3. The finer particles are pumped into three waste dams. The dissolution rate increases with increasing temperature for colemanite in acetic acid [4], for colemanite in phosphoric acid [5], for ulexite in sulfuric acid [6], for colemanite in oxalic acid [7], for ulexite in oxalic acid [8], for tincal in oxalic acid [9], for ulexite in perchloric acid [10], for tincal in phosphoric acid [11], and for ulexite in acetic acid [12]. Recently, microwave technology has been applied to a number of mineral processing areas such as microwave leaching of copper [13] and nickel ores [14,15], and the desulfurisation of coal [16– 18]. Microwave radiation is a part of the electromagnetic spectrum, with wavelengths ranging between approximately 1 cm and 1 m. Microwave energy is a non-ionizing electromagnetic radiation, with frequencies in the range of 300 MHz to 300 GHz. Within this range, microwaves have been extensively used in communication, especially in radar, cellular phones, television and satellite applications [19]. Because electromagnetic radiation does not ionize matter, the structure of matter is not decomposed [14]. The frequencies extensively used for heating purposes are 915 MHz and 2.45 GHz, which correspond to wavelengths of 33.5 and 12.2 cm respectively [20]. Microwaves have lower energy and higher wavelength than ultraviolet, visible and infrared radiation [21]. The interaction of dielectric materials with electromagnetic radiation in the microwave range results in energy absorbance. The ability of a material to absorb energy while in a microwave
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cavity is related to the loss tangent of the material. This depends on the relaxation times of the molecules in the material, which, in turn, depends on the nature of the functional groups and the volume of the molecule. The dielectric properties of a material are related to temperature, moisture content, density and material geometry [22]. The ability of materials to absorb microwave energy is dependent on their dielectric properties.
Digital Thermometer
Mechanically controlled stirrer
Programmable Circulator
Reflux condenser
e ¼ e0 je00 where e is relative complex permittivity, e0 is permittivity (dielectric constant), e00 is the dielectric loss factor and j is the imaginary component, and eo is the permittivity of free space (8.85 10–12 F/m). The dielectric loss factor is related to two other dielectric properties by the equation:
e00 tan d ¼ 0 e
Glass reactor
Water bath Copper pipe
where tand is loss tangent. At a given frequency the power absorbed is
P ¼ 2pf eo e00 jEj2
Fig. 1. The leaching experiment test set-up.
where P is energy developed per unit volume (W/m3), f is frequency (2450 106 Hz), and E is electric field strength within the product (V/m). The speed of microwave heating is due to the deep penetration of microwaves into the material, and the dielectric properties can be used to determine the extent of penetration. The term penetration depth is defined as the depth at which the microwave power level is reduced to 36.8% (or 1/e) of its value.
2
00 2 !1=2
4 1þ e d¼ e0 2pf ð2e0 Þ1=2 c
31=2 15
where d is the penetration depth, c is the the speed of light (3 108 m/s) of incident radiation [23–27]. In this study, the aim was to shorten the leaching time, to shorten the crystallization time of gypsum, to investigate the effects of leaching parameters, to compare the effects of temperature as the result of conventional heating methods and the microwave radiation heating method, to completely remove the gypsum, which reduces the purity of boric acid, from solution, to determine the parameters affecting the crystallization of gypsum which affect the efficiency of the separation of gypsum from the solution, to determine the effects of this parameter on the crystallization time of gypsum and the purity of boric acid, and to determine the optimum conditions for the microwave leaching process by which high-purity boric acid solution is obtained. 2. Materials and methods 2.1. Materials The ground colemanite sample in this study was obtained from Bigadic Boron of Eti Mine, Turkey. The laboratory tests were carried out in DEFAM. The sieve analysis of the sample was performed by sieving. 2.2. Conventional acid leaching method (CVAL) The conventional acid leaching test setup design is shown in Fig. 1. The leaching tests were carried out in a 500-mL three-neck glass reactor at atmospheric pressure. A mechanical stirrer (IKA RW20) with a digital display was used to agitate the solution. A programmable cryostat bath/circulator (Polyscience 9106) was used for heating or cooling to keep a constant reaction temperature. A spiral condenser was set up on the glass reactor to prevent loss of solution by evaporation. The temperature of the solution in
the reactor was measured with a portable probe digital temperature controller with a range of between 0 and 150 ± 0.1 °C. The samples were weighed to 0.1 mg using a Denver Instrument SI 234 analytical balance. The leaching time was measured by digital chronometer. During the leaching process, 50 mL of sulfuric acid solution was placed in the reactor and stirred, and the digital chronometer was started. After the desired reaction temperature was reached, a certain amount of the dried colemanite sample was added to the solution. After a certain period of time, the stirrer and chronometer were stopped. The hot solution was filtered using blue filter paper and vacuum without any change in temperature. Each experiment was repeated at least twice. The percentage of B2O3 in the filtrate was determined by volumetric methods [28]. 2.3. Microwave leaching method (MWAL) The experimental setup used for the Microwave Leaching Process is shown in Fig. 2. A domestic microwave oven was modified for this leaching study. The modified microwave oven produced power of 900 W at a frequency of 2.45 GHz. A three-neck 500-mL glass reactor was put into the MW oven. A spiral condenser was set up on the glass reactor to prevent loss of solution by evaporation. The temperature of the solution in the reactor was measured by digital infrared thermometer with a range of between 60 and +1500 ± 0.1 °C (Ebro TFI 650). 50 mL or 100 mL of sulfuric acid solution and a certain amount of the dried sample were added into the reactor. After being placed into the microwave oven, the reactor and glass pipes were connected to each other. The leaching solution was not stirred. After a certain period of time, the oven was turned off. The temperature of the glass reactor was measured remotely with the infrared thermometer. The hot solution was filtered using blue filter paper and vacuum without any change in temperature. Each experiment was repeated at least twice. The amounts of CaO in the filtrate were determined by volumetric methods. 2 mL the sample solution was added to Erlenmeyer flask. NaOH solution was dropped for adjusting pH 13. Murexide indicator was added and stirred. EDTA (ethylene diamine tetra acetic acid) solution was added drop by drop to the solution in the flask, which was being stirred at a stable rate, until the contents of the flask changed in colour from red to orchid purple. The burette was read to determine the volume of EDTA. The leaching (dissolution) efficiency was percent
Leaching efficiency of B2 O3 ¼
M1 100 Mo
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Open Atmosphere
Glass stopper Water outlet Reflux Condenser Reflux condenser
Water inlet
Stainless steel pipe
Magnetron
Glass pipe
Infrared thermomet Laser light Parallel Necked Glass Reactor PTFE
Fig. 2. The microwave acid leaching experiment test set-up.
where M1 is the percent of dissolved B2O3 in the solution, Mo is the percent of B2O3 in the original sample. 3. Results and discussion 3.1. Characterization of the sample The results of the particle size analysis of the dried samples are given in Fig. 3. According to the cumulative percent passing curve of ground colemanite particles, the sizes obtained were 98 lm 80%.
The chemical analysis of the colemanite sample (except B2O3) with Shimadzu XRF (X-ray fluorescence) is given in Table 1. It was shown that the sample consisted of 40.32% B2O3, 28.95% CaO, 2.5% SiO2 and 2.33% SrO. The XRD analysis of the sample was carried out on a Rigaku DMAX 2200 Series X-ray diffractometer with Cu Ka radiation. As seen in Fig. 4, the sample was mainly composed of colemanite (79%) and small amounts of calcite (CaCO3). 3.2. Effect of microwave power on boron oxide extraction When evaluating the effect of microwave power on boron oxide extraction by the MWAL method, microwave power values of 360– 900 W were selected. Further, tests were performed under typical conditions: microwave irradiation was 5 min, the ratio of solid-toliquid was 10%, and acid concentration was 10%. The effect of microwave power is shown in Fig. 5. The leaching efficiency of bor-
Table 1 Chemical analysis of ground colemanite.
Fig. 3. The particle size distribution of the colemanite.
Component
Colemanite (%)
SiO2 Al2O3 Fe2O3 B2O3 CaO MgO SrO LOI
2.50 0.09 0.21 40.32 28.95 0.90 2.33 24.17
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110 A
100
A: Colemanite B: Calcite
90 Intensity (Counts)
80 70 A
60 50 A B
40
B
A A
30
B
20 10 0 10
20
30
40
50
60
70
80
90
Two-Theta (Degree) Fig. 4. X-ray diffraction analysis of colemanite.
on oxide slowly decreased as the microwave power was increased in the range of 360–540 W. However, the efficiency increased with an increase in microwave power in the range of 540–720 W. The interaction of molecules is proportional to the microwave power. The kinetic energy of the molecules is converted into heat energy due to the interaction of molecules and as a result the temperature increases. As the colemanite mineral is soluble at high temperatures, the leaching efficiency is increased with increasing microwave power. Xia and Pickles [29] reported that higher power had a remarkable influence on the increase of metal dissolution. The optimum leaching rate of boron oxide was reached at 720 W. When the microwave power was increased from 720 to 900 W, the dissolution efficiency of boron oxide remained stable. 3.3. Effect of leaching time on boron oxide extraction Experiments were carried out according to the conditions of the CVAL method. Tests were performed on CVAL to determine the effect of time on boron oxide extraction under typical conditions as follows. Acid concentration was 10%, stirring speed was 500 rpm, and temperature was 80 °C. The time range selected was 5– 40 min. It can be seen from Fig. 6 that leaching efficiency increased with an increase in time during leaching. According to the CVAL
tests, the leaching rate of boron oxide from colemanite reached 99.82% when the time period was 40 min. Experiments were conducted to determine the effect of the length of time of microwave irradiation on the leaching rate of boron oxide in MWAL. The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid-to-liquid was 10%, and acid concentration of sulfuric acid was 10%. The time was between 1 and 5 min in MWAL tests, and it was shown that the leaching of boron oxide slowly increased with the increase of time. The optimum leaching rate was obtained with a time of leaching of 5 min. The leaching time of MWAL was shorter than that of CVAL. This can be explained as follows: heating begins throughout the solution when microwave radiation is applied. The ions are caused to vibrate and move as an effect of the microwave radiation. The dissolution rate increases, and this leads to a shorter time for leaching. The effects of leaching time on the solution temperature are shown in Fig. 7. The solution temperature reached 160 °C in the first minute of the experiment. The maximum solution temperature (183 °C) was reached in the 3 min of leaching time.
100
Leaching recovery of B2O3, %
Leaching efficiency of B2O3 , %
100
90 80 70 60 50
40 30 20 10 0 200
90
80
70 MWAL 60
CVAL
50 0 300
400
500
600
700
800
900
1000
5
10
15
20
25
30
35
40
45
Leaching time, min
Microwave power, W Fig. 5. Effect of microwave power on boron oxide recovery (acid concentration of 10%, leaching time of 5 min).
Fig. 6. Effect of leaching time on boron oxide recovery (CVAL: acid concentration of 10%, solid to liquid ratio of 10%, speed of 500 rpm, at 80 °C and MWAL: acid concentration of 10%, solid to liquid ratio of 10%, Power 720 W).
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3.4. Effect of the ratio of solid to liquid on boron oxide extraction
80
Figure 8 shows the time taken for the B2O3 to reach its maximum dissolution efficiency of 99.9% under optimum conditions and the variable condition of 5, 10, 20 and 30% solid/liquid ratio. The method described above was used in both CVAL and MWAL experiments. Tests were performed by CVAL to determine the time effect of boron oxide extraction under typical conditions as follows. Acid concentration was 10%, stirring speed was 500 rpm, and temperature was 80 °C. It can be observed from Fig. 8 that in the CVAL method the leaching time increased slowly as the solid-to-liquid ratio increased from 5 to 20%, but a solid-to-liquid ratio of 30% led to a sharp increase in the leaching time. This may result from the evaporation of water while the acid concentration of suspension was constant (1.748 g acid/g colemanite). Thus, the ratio of solid to liquid had a significant effect on the leaching of boron oxide in the CVAL method, and for this reason the reaction rate decreased. The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid to liquid was 10%, and acid concentration of sulfuric acid was 10%. There was no significant increase in leaching time even with an increase from 5% to 30% in the solidto-liquid ratio of the solution in the MWAL method. The maximum leaching efficiency in the CVAL method was reached at 40 min. with a ratio of solid to liquid of 30%. However, the same efficiency was reached with MWAL at 5 min. with a solid-to-liquid ratio of 10%. The ratio of solid to liquid had a significant effect on leaching time in the CVAL method and was proportional to the leaching time, but an increase in the same parameter had no remarkable influence on leaching time in the MWAL method. The experimental temperature was 80 °C in the CVAL method while it was 183 °C in the MWAL method. In the CVAL method, the increase in the kinetic energy of molecules depends on temperature over a long time. The migration of ionic species and/or rotation of dipolar species promotes the liquid–solid reaction process due to the increased contact area of reactants [15]. On the other hand, when a microwave field is applied to a solution containing metal/ions, they move due to their inherent charge as a result of the force of the electric and magnetic field components of the microwave radiation. Charged particles collide, and the collisions lead to the conversion of kinetic energy to thermal energy. More collisions occur in solution depending on the increase in the concentration of ions/atoms. As a consequence, a faster heating occurs in solution [30]. Jafarifar et al. [31] examined microwave assisted leaching followed by
70
CVAL
Leaching time, min
60
MWAL
50 40 30
20 10 0 0
5
10
15
20
25
30
35
Solid/liquid ratio, % Fig. 8. Effect of solid-to-liquid ratio on leaching time (CVAL: 80 °C, 1.748 g acid/g colemanite, 500 rpm, 99.9% B2O3 recovery and MWAL: Power 720 W, 1.748 g acid/g colemanite, 99.9% B2O3 recovery).
chemical precipitation to recover platinum. They found that the maximum leaching efficiency of the conventional acid leaching method was 150 min while the maximum leaching efficiency of the microwave leaching method was 5 min.
3.5. Effect of temperature on boron oxide extraction The typical conditions for MWAL were that microwave power was 720 W, the ratio of solid to liquid was 10%, and the acid concentration of sulfuric acid was 10%. It can be seen in Fig. 9 that the leaching rate varied linearly with temperature. Colemanite minerals, pure water and sulfuric acid have a high dielectric loss factor which increases with increasing microwave power. The results of the comparison between conventional leaching and microwave leaching at different dissolution time are summarized in Table 2. The maximum leaching efficiency in the conventional acid leaching experiment was achieved at 80 °C. In contrast, the microwave leaching experiment reached maximum leaching efficiency at 183 °C. The temperature of the leaching solution increased from 160 to 183 °C with the increase in microwave power.
190
195
180
190
170
Leaching temperature, °C
Solution temperature, °C
200
160 150 140 130 120 110
185 180 175 170 165 160 155
100
150 0
1
2
3
4
5
6
Microwave leaching time, min Fig. 7. Effect of leaching time on solution temperature (acid concentration of 10%, solid-to-liquid ratio of 10%, 720 W).
300
400
500
600 700 800 Microwave power, W
900
1000
Fig. 9. Effect of microwave power on leaching temperature (acid concentration of 10%, solid-to-liquid ratio of 10%, 5 min).
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3.6. Characterization of leaching products Colemanite dissolved completely in hot sulfuric acid solutions under the optimum conditions. The leaching products were determined as being solid or liquid phase. Borate ions and calcium ions were detected in the liquid leaching product obtained by filtration, and gypsum crystals were determined in the solid product. Boric acid crystals were not observed in the solid leaching product. This is because boron oxide has high solubility at high temperatures, and only crystallizes at lower temperatures as its solubility is reduced. Kuskay and Bulutcu [32] found that boric acid crystallizes when the solution is cooled to around 35 °C. Colemanite reacts with sulfuric acid according to Eq. (1). The borate ions, sulfate ions and calcium ions are released from PLS (Pregnant Leach Solution).
Ca2 B6 O11 5H2 OðsÞ þ H2 SO4ðaqÞ þ 10H2 O 2 ¼ 6BðOHÞ4ðaqÞ þ 4Hþ þ 2Ca2þ ðaqÞ þ SO4ðaqÞ þ 2H2 O
ð1Þ
The CVAL tests were conducted under typical conditions, as follows. The ratio of solid-to-liquid was 10%, the temperature was 80 °C, acid concentration was 10%, the stirring speed was 500 rpm, the particle size was –98 lm, and the leaching time was 40 min. The leaching efficiency of boron oxide by CVAL was 99.82%. X-ray diffraction analysis (Philips PW3710/1830) of leaching solid product of CVAL is given in Fig. 10. The major phase was anhydrite (CaSO4) in the XRD analysis. SEM analysis of (FEI Quanta 400 MK2) samples of solid residue from CVAL leaching is given in Fig. 11. It can be seen that only anhydrite crystals of needle type were formed. The crystals were approximately 50 lm in size and regular in shape. This can be attributed to the 40-min crystallization period. The optimum conditions of MWAL were a ratio of solid to liquid of 10%, a temperature of 183 °C, an acid concentration of 10%, a particle size of 98 lm, microwave power of 720 W and a leaching time of 5 min. The leaching efficiency of boron oxide by MWAL was 99.9%.
As a result of the reaction between calcium and sulfate ions, gypsum crystals formed according to Eq. (2). The gypsum crystals precipitated in PLS.
Ca2 B6 O11 5H2 OðsÞ þ H2 SO4ðaqÞ þ 16H2 O ¼ 6BðOHÞ4ðaqÞ þ 12HþðaqÞ þ 2ðCaSO4 2H2 OðsÞ Þ
ð2Þ
Boric acid crystals formed in solution cooled to 35 °C (Eq. (3)).
Ca2 B6 O11 5H2 OðsÞ þ 2H2 SO4ðaqÞ þ 6H2 O ¼ 6BðOHÞ3ðsÞ þ 2ðCaSO4 2H2 OðsÞ Þ
ð3Þ
Table 2 The comparison of conventional and microwave acid leaching methods. Parameter
Conventional acid leaching, CVAL
MW leaching, MWAL
Leaching time, min Solid/liquid ratio, % g acid/g colemanite Temperature, °C Microwave power, W Stirring speed, rpm Particle size (d80), lm Recovery of B2O3, %
40 10 1.748 80 No 500 98 99.82
5 10 1.748 183 720 No 98 99.90
(a) 100µm CVAL
(b) 50 µm CVAL Fig. 10. X-ray diffraction analysis of leaching solid product of CVAL.
29
Fig. 11. SEM analysis of leaching solid product of CVAL.
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S.U. Bayca / Separation and Purification Technology 105 (2013) 24–32
Fig. 12. X-ray diffraction analysis of leaching solid product of MWAL.
(a) 100 µm MWAL
X-ray diffraction analysis of solid leaching product of MWAL is given in Fig. 12. The major phase was anhydrite (CaSO4) in the XRD analysis. This is in agreement with Eq. (3). In Eq. (3) anhydrite was observed in the XRD analysis despite the formation of gypsum. This leaching reaction may be attributed to the rise in temperature. The SEM analysis of MWAL anhydride crystals is given in Fig. 13. The morphology of the anhydrite crystals was needle type. Crystals were approximately 50 lm in size. However, their shape was broken and irregular. This may be not enough time was given for crystals to grow in this solution. Crystallisation and leaching time were both 5 min in duration. 3.7. Chemical precipitation of gypsum The effect of acid concentration on the extraction of boron oxide was investigated by single-stage MWAL and multi-stage MWAL methods. A leaching efficiency of 99.9% B2O3 was achieved under conditions of single-stage MWAL as shown in Table 3, as follows. The microwave power was 720 W; the leaching time was 8 min. and acid concentration was in the range of 5–25%. The calcium content of the liquid phase, which was obtained by separating the solid and liquid phases in the PLS, was decreased by increasing the acid concentration, and was determined as 1.083% Ca. The results of the multi-stage MWAL experiment are given in Table 4. The leaching rate of boron oxide in multi-stage MWAL was found to be 99.9%, and the calcium content was 0.017%. The calcium content of the liquid phase decreased as the acid concentration increased. As seen in Fig. 14, the calcium content of the liquid phase of single-stage MWAL was higher than the calcium content of the liquid phase of multi-stage MWAL. It was found that the calcium content of the liquid phase varied according to the leaching methods. The calcium contents of the liquid phase which were obtained by applying single stage MWAL, CVAL and multi stage MWAL methods at an acid concentration of 5% were measured as 3.365%, 2.690% and 2.231% Ca respectively (Fig. 15). It was determined by XRD analysis that the solid phase was composed of a large amount of gypsum (anhydride) crystals. The gypsum was formed by a reaction between calcium and sulfate ions which were released during the dissolution of colemanite in sulfuric acid solution. A large number of calcium ions, which reduced the purity of the boric acid, was determined in the liquid phase. The calcium ions pass to the boric acid solution (liquid phase) by filtration of the leaching solution. The Ca2+ ions must be removed from PLS to obtain high purity and high yield of boric
(b) 50 µm MWAL Fig. 13. SEM analysis of leaching solid product of MWAL.
acid. Conversion of all calcium ions in PLS to gypsum (anhydride) crystals is achieved using two methods. In the first of these methods, the acid concentration of the solution is increased. In second method, the time of crystallization is increased. In this study, the first option was preferred in order to decrease the time of the boric acid production process. It was determined that, as a result of an increase from 5% to 25% concentration of sul-
Table 3 The experimental conditions of the Fig. 11 for the single stage MWAL. Parameters
A
B
C
D
E
Microwave power, W Acid concentration, % Leaching time, min Solid/liquid ratio, %
900 5 8 5
900 10 8 5
900 15 8 5
900 20 8 5
900 25 8 5
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S.U. Bayca / Separation and Purification Technology 105 (2013) 24–32 Table 4 The experimental conditions of the Fig. 11 for the multi-stage MWAL. Parameters
F1
F2
G1
G2
H1
H2
K1
K2
L1
L2
Microwave power, W Acid concentration, % Leaching time, min Solid/liquid ratio, %
900 5 3 50
900 – 5 5
900 10 3 33.33
900 – 5 5
900 15 3 25
900 – 5 5
900 20 3 20
900 – 5 5
900 25 3 16.67
900 – 5 5
3.5
Ca in solution, %
3.0 2.5 2.0 1.5 1.0 Multi satge MWAL
0.5
Single stage MWAL
0.0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
Sulphuric acid, % Fig. 14. Effect of sulfuric acid concentration on Ca in solution.
furic acid, the pH of the solution of sulfuric acid was decreased from 1.21 to 0.36. The calcium content in the liquid phase was decreased by increasing acid concentration in both single stage MWAL and multi stage MWAL methods. The calcium content of the liquid phase obtained at an acid concentration of 25% was 1.083% Ca for the single stage MWAL method and 0.017% Ca for the multi stage MWAL method. The lowest calcium level was obtained from the multi stage MWAL method at 25% acid concentration. The liquid phase containing very low calcium content can be attributed to the formation of a supersaturated solution of gypsum. The supersaturated solution of gypsum can be defined as a solution containing more
Ca2+ and SO2 ions than a saturated solution. It was found that 4 the calcium content of the liquid phase of the single stage MWAL method was higher than that of the multi stage MWAL method. This may be attributed to the fact that the multi stage MWAL method accelerates gypsum formation. The dissolution of colemanite and precipitation of gypsum crystals occurs simultaneously in acidic media [33]. The solubility of gypsum decreases with an increasing content of sulfuric acid solution. The size of gypsum crystals increases with time due to crystal growth [34]. Supersaturation was controlled by adding sulfuric acid solution. Mersmann [35] reported that supersaturation was achieved by increasing acid concentration. Abdel Aal [36] found that the morphology of gypsum crystals is affected by free sulfate content and supersaturation ratio. Large crystals are obtained at high sulfate levels and a low supersaturation ratio gives thick crystals. Rashad et al. [37] reported that gypsum nucleation rate increased with the addition of 1% MgO. Calcium sulfate crystallization occurs in parallel with leaching. According to the process adopted, calcium sulfate dihydrate (gypsum) (CaSO42H2O) or calcium sulfate hemihydrate (CaSO40.5H2O) is crystallized [2,38]. The results indicate that 31%, 25%, and 23% increases in filtration rate were achieved at low, medium, and high sulfate contents, respectively. The observed improvements are attributed to changes in crystal size distribution [39]. 4. Conclusions In this study, colemanite mineral was leached with sulfuric acid solution at atmospheric pressure at laboratory scale to obtain high purity boric acid and a shorter process time. The leaching conditions of colemanite mineral in sulfuric acid solution were optimized using both conventional acid leaching (CVAL) and single stage and multi stage microwave leaching methods (MWAL).
5 % acid 3.5
Ca in solution, %
3
25 % acid
2.5 2 1.5 1 0.5 0 CVAL
single stage MWAL
Multi-stage MWAL
Fig. 15. Effect of microwave acid leaching sulfuric acid concentration on Ca in solution.
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S.U. Bayca / Separation and Purification Technology 105 (2013) 24–32
The optimum leaching efficiency of the CVAL method was 99.82%, under typical conditions where acid concentration was 10%, stirring speed was 500 rpm, temperature was 80 °C, the leaching time was 40 min and the solid/liquid ratio was 10%. The optimum leaching efficiency of the MVAL method was 99.9%, under typical conditions where microwave power was 720 W, acid concentration was 10%, the time was 5 min, the solid/liquid ratio was 10% and temperature was 183 °C. A leaching efficiency of 99.82% was achieved in 40 min using the CVAL method, and the calcium content of the liquid phase was determined as 2.69% Ca. However, maximum leaching efficiency (99.90%) of boron oxide was reached by the MWAL method in 5 min. Despite this decrease in leaching time, the liquid phase contained 3.335% Ca. In the multi-stage leaching method carried out at an acid concentration of 25%, the calcium content of the boric acid solution (the liquid phase) was 0.017%. The efficiency of the removal of calcium ions from the boric acid solution was 99%. The rate of crystallization increased with the pH of the solution. A very high increase in the leaching time was observed with an increase in the solid-to-liquid ratio in CVAL, but there was no significant increase in leaching time in MWAL when the solid-to-liquid ratio was increased. This means that the solid-to-liquid ratio of the leaching process can be as high as 30%, which increases the plant capacity. The cost of production of the plant will be reduced because of the increase in the plant capacity. The removal of calcium ions was investigated using single-stage and multi-stage microwave leaching methods. The calcium content of the liquid phase in single stage MWAL was higher than the calcium content of the liquid phase in multi-stage MWAL. The reason for this is that the formation of gypsum accelerates the rate of reaction in multi-stage MWAL. The optimum conditions for multi-stage microwave leaching were found to be a microwave power of 900 W, an acid concentration of 25%, a leaching time of 8 min, a solid/liquid ratio of 5%, a temperature of 191 °C, and a calcium content of 0.017% Ca. Maximum leaching efficiency was obtained by the conventional acid leaching method in 40 min, while maximum leaching efficiency was obtained in 8 min using the multi-stage microwave leaching method. The removal efficiency of calcium ions from boric acid solution was found as 99% with acid concentration of 25% using the multi-stage microwave leaching method. Acknowledgments The author would like to thank Eti Mine Works General Management for the XRD and XRF analysis and for providing samples. References [1] S.U. Bayca, The extraction of boric acid from colemanite process waste by attrition scrubbing and leaching, 2012 (unpublished). [2] P. Becker, Phosphate and Phosphoric Acid, Marcel Decker Inc., New york, 1989. [3] S.U. Bayca, Effects of the addition of ulexite to the sintering behavior of a ceramic body, Journal of Ceramic Processing Research 10 (2009) 162–166. [4] C. Ozmetin, M.M. Kocakerim, S. Yapici, A. Yartasi, A semiempirical kinetic model for dissolution of colemanite in aqueous acetic acid solutions, Industrial and Engineering Chemistry Research 35 (1996) 2355–2359. [5] H. Temur, A. Yartasi, M. Copur, M.M. Kocakerim, The kinetics of dissolution of colemanite in H3PO4 solutions, Industrial and Engineering Chemistry Research 39 (2000) 4114–4119. [6] M. Tunc, S. Yapici, M.M. Kocakerim, A. Yartasi, The dissolution kinetics of ulexite in sulfuric acid solutions, Chemical and Biochemical Engineering Quarterly 15 (2001) 175–180.
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