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Removal of ferric ions from aluminum solutions by solvent extraction, part I: Iron removal
Separation and Purification Technology 159 (2016) 18–22
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of ferric ions from aluminum solutions by solvent extraction, part I: Iron removal Xiaoxue Sun, Yuzhu Sun ⇑, Jianguo Yu ⇑ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2015 Received in revised form 23 December 2015 Accepted 29 December 2015 Available online 30 December 2015 Keywords: Ferric sulfate Synergistic extraction Stripping
a b s t r a c t This study investigated removal of ferric ions from aluminum solutions to explore the basic data for the recovery of aluminum resources from coal spoil. A novel synergic extractant, consisting of di-(2ethylhexyl) phosphoric acid (P204) and tertiary amine (N235) with sulfonated kerosene (SK) as diluent, was adopted in this research. The influences of reaction time, pH, temperature, and phase ratio, on the extraction processes were systematically investigated. Result showed the Fe removal was greater than 97% in only one contact under optimal conditions. The corresponding stripping processes were also performed focusing on the effects of phase ratio, H2SO4 concentration, and reaction time. Almost 99% Fe was easily stripped using 1 mol/L H2SO4. These findings indicated that compared with single extractants, the new synergic extractant shows significant advantages. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal spoil (CS) is a byproduct generated during mining, washing, and processing of coal. Numerous CS stacks have accumulated without proper treatment, which not only cover large areas of land but also cause many environmental problems. It is therefore imperative to develop new technologies [1–4] to promote the utilization of CS, especially in coal-based, energy-consuming countries such as China. In fact, CS is a valuable mineral resource. Its main components are SiO2 and Al2O3, and generally, the content of the latter ranges from 15% to 40%. With increasing demands for aluminum oxide and declining in bauxite reserves, CS has drawn increasing attention as a potential alternative non-bauxite aluminum resource. Although many methods have been proposed for the extraction of aluminum from CS, the previous research of the current authors found that a sulfuric acid leaching route presents more promising prospects. A proposed process is as follows: activation of coal spoils using compound activation followed by leaching in sulfuric acid, crystallization of aluminum sulfate and removal of ferric sulfate by solvent extraction, enabling the production of Al2O3 or other aluminum compounds. Removal of ferric sulfate (Fe3+) from aluminum sulfate by solvent extraction is a key step in the process due to the relatively
⇑ Corresponding authors at: P.O. Box 266, Meilong Road 130, East China University of Science and Technology, Shanghai 200237, PR China. E-mail address: [email protected] (Y. Sun). http://dx.doi.org/10.1016/j.seppur.2015.12.054 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
high iron content with an Al/Fe weight ratio of 30.27 in crystallized Al2(SO4)318H2O, which leads to poor Al2O3 quality. A variety of extractants such as P204 [5–9], OPAP [5,6], HEHEHP [10], Aliquat 336 [11], Cyanex 921 [14] and LIX860 [12] have been used for solvent extraction of Fe3+. Among them, di-(2-ethylhexyl) phosphoric acid (P204) is the most widely used extractant in hydrometallurgical process for the separation and purification of metals such as aluminum and iron. It has been proved that mechanism [13] of Fe3+ extraction with P204 is ion-exchange at low acidity and solvent effect at high acidity and P204 is a very effective extractant for Fe3+ removal. However, the extracting of Fe3+ with P204 requires a long contacting time and the stripping of Fe3+ from the loaded P204 is very difficult, which hinders the use of P204 to remove Fe3+ from acid solution. In view of the above, in order to achieve a higher Fe removal within a shorter time and to make the stripping process easier, this research proposed a synergistic solvent extraction (SSX) system, and the effects of a number of experimental parameters, on the Fe3+ removal were systematically investigated. 2. Experimental 2.1. Reagents and equipment Di-(2-ethylhexyl) phosphoric acid (P204, Shanghai Aoke Industrial Co., Ltd., China), Tertiary amine (N235, Shanghai Rare-Earth Chemical Co., Ltd., China), sulfonated kerosene (SK, Shanghai Rare-Earth Chemical Co., Ltd., China), n-pentyl alcohol (Sinopharm
X. Sun et al. / Separation and Purification Technology 159 (2016) 18–22
Chemical Reagent Co., Ltd., China), isoamyl alcohol (Sinopharm Chemical Reagent Co., Ltd., China) and cyclohexane (Sinopharm Chemical Reagent Co., Ltd., China) were used without purification. Al2(SO4)318H2O, Fe2(SO4)3 and H2SO4 (Analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. Al2(SO4)318H2O was dissolved in deionized water. The solution was filtered through a G4 sand-core funnel to eliminate residual solids, and then recrystallized to obtain a highly pure starting material. The concentrations of Al3+ and Fe3+ (denoted by Al2O3 and Fe2O3 in the standard) were determined by chemical titration and spectrophotometric method respectively according to the national standard GB/T 1574-1995. The content of Al3+ was titrated with zinc acetate standard solution utilizing ethylenediamine tetra-acetic acid (EDTA) as a masking agent; By using a 4802 UV–VIS double beam spectrophotometer (Unico (Shanghai) Instrument Co., Ltd., China), the concentration of Fe3+ was determined colorimetrically at 570 nm after adding tiron reagent. The starting solution was prepared with a mass ratio of Al3+/Fe3+ of 30.27, which was consistent with previous research results, by dissolving Al2(SO4)318H2O and Fe2(SO4)3 in deionized water, giving an initial pH of 1.5–2 and Fe3+ concentration of around 1.26 g/L. The pH of the aqueous phase was determined using a SevenMulti pH and Ion Meter (S40, Mettler Toledo) and thermal control was provided by a Huber Refrigeration bath circulator (CC-415, Huber Kältemaschinenbau GmbH). 2.2. Procedures Initial experiments using different proportions of extractants and the effects of experimental parameters on the extents of Fe3+ removal and stripping were studied in 100 mL globe-shaped funnels by mixing 20 mL each of the aqueous and organic phases for predetermined periods of time at 600 rpm. The emulsion was then allowed to stand for 10 min, after which samples of the aqueous phase were withdrawn for analysis. Fe removals were calculated using the amount of Fe3+ in aqueous phase divided by the amount of total Fe3+, while the distribution ratios (D) were calculated as the ratio of the concentration of Fe3+ in the organic phase and that in the aqueous phase at equilibrium.
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SK as the diluent could be attributed to the subsequent reasons. First, a diluent should be of low toxicity, so that it could be used in large-scale industrial processes. The median lethal dose (LD50), the dose required to kill half the members of a tested population after a specified test duration, is frequently used as a general indicator of a substance’s acute toxicity. A lower LD50 is indicative of increased toxicity. Universally used diluents in liquid–liquid extraction are SK, n-pentyl alcohol, isoamyl alcohol, and cyclohexane, with their LD50 of 36,000, 2200, 1300 and 12,705 mg/kg respectively, which implies that SK enjoys the lowest toxicity. Furthermore, Fe removals using the SSX system in SK, n-pentyl alcohol, isoamyl alcohol, and cyclohexane diluents were 88.19%, 81.38%, 78.27% and 86.62% respectively under the same experimental conditions. As a result, SK was chosen to be the diluent, owing to its low toxicity and high Fe removal ability. Infrared spectra (Magna-IR 550, 4000–400 cm1) of P204, N235 and the SSX system is presented in Fig.1. Peaks from 2800 to 3000 and 1300 to 1500 cm1 represent the ACAH symmetric and asymmetric deformation vibrations and stretching vibrations [11]. The characteristic peak at 1229.62 cm1 due to the stretching vibration of the P@O bond in P204 [14] has shifted to 1214.35 cm1 in the SSX system, while the peak at 1096.16 cm1 in N235 which is assigned to CAN stretching vibration has shifted to 1040.96 cm1 in the SSX system. This phenomenon suggests that both P204 and N235 played roles in the extraction reaction in the mixed P204 and N235 system and it is this reaction that has promoted Fe extraction [15]. Thermodynamically speaking, it seems rather difficult to explain synergistic mechanism from the viewpoint of saturation of coordination due to complexity of the system, nevertheless, it can be explained probably by a number of chemical equilibria in the solvent system. When P204 and N235 (R3N) are present in the organic phase, they can form a molecular association, particularly in a low-acidity solution as shown below:
3. Results and discussion
ð1Þ
3.1. Effect of organic compositions Solvent extraction of ferric ions from sulfate solutions was carried out with the synthetic solution containing 1.26 g/L Fe3+ and the organic systems consisting of 30% P204 and 30% N235 in diluent SK separately, and the SSX organic system consisting of 15% P204 and 15% N235 in SK at 25 °C and an A/O ratio of 1:1. The extent of Fe removal and the distribution ratios (D) are presented in Table 1. With the organic systems consisting of 30% P204 in SK, the iron removal was 66.5% and the distribution ratio D 2.0, nevertheless, with the organic system consisting of 15% P204 and 15% N235 in SK, the iron removal increased to 88.2%, and the distribution ratio to 7.5. This is the evidence of synergistic effect with the addition of N235 to the P204 system, leading to a significant improvement of iron removal. It should be pointed out that using
The formation of a molecular association compound will increase the dissociation of P204 to hydrogen ions, which will result in an increase in the extraction of Fe3+ by cation exchange. The addition of N235 to P204 also shows synergism from the viewpoint of kinetics. The kinetics of Fe extraction using P204-N235
Table 1 Fe removal using P204, N235, and their mixture. Extractant
SK
Fe removal (%)
D
30% P204 30% N235 15% P204/15% N235
70% 70% 70%
66.51 68.47 88.19
1.99 2.17 7.47
Fig. 1. IR spectra of P204, N235 and the SSX system.
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extractant and only P204 extractant were studied in our subsequent experiment and one could conclude that the addition of N235 into P204 turned the extraction process from a mixed diffusion-chemical reaction mechanism to a chemical reaction mechanism. In conclusion, the P204-N235 extractant could achieve higher Fe removal than the only P204 extractant. The effects of various volume ratios of P204, N235 and SK on Fe removal are shown in Fig. 2. The operating volume concentration of P204 was from 0% to 40%, because the viscosity was too high when the concentration of P204 exceeded 40%. At a P204:N235 volume ratio of from 2:3 to 1:1 with 20% to 50% SK, the highest Fe removal of greater than 95% was obtained. This volume ratio corresponded to P204:N235 molar ratio of from 0.8 to 1.2. One can assume that proportions of extractants could form some more stable metal coordination complex with Fe3+ and further investigations will be conducted in the future. So the subsequent experiments were conducted using a volume ratio of 3:3:4 for P204: N235:SK.
Fig. 3. Effect of reaction time on Fe removal. Experimental conditions: 25 °C, phase ratio = 1:1.
3.2. Extraction 3.2.1. Effect of reaction time Fig. 3 plots the extraction of Fe and Al against reaction time at 25 °C using the selected organic system consisting of 30% P204 and 30% N235 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1. It is shown that the equilibration time for Fe extraction is less than 20 min and only approximately 3% Al would be co-extracted into the organic phase. A reaction time of 20 min was therefore selected for the subsequent experiments. It should be mentioned that the iron extraction reached 90% in 5 min contact, indicating the residence time should be 5 min. 3.2.2. Effect of pH The effect of initial aqueous phase pH values on Fe removal at 25 °C using the selected organic system the volume ratio P204: N235 in SK of 30:30 and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1 is shown in Fig. 4. The extent of Fe removal increased with increasing pH from 0.58 to 1.7, and the Fe extraction reached almost 100%. These results are very similar to those reported by Yu and Chen [10]. 3.2.3. Effect of temperature The effect of temperature on Fe removal was investigated in the range from 10 °C to 80 °C using the selected organic system consisting of 30% P204 and 30% N235 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1. The results are also shown in Fig. 4. The Fe extraction
Fig. 4. Effects of initial aqueous pH and temperature on Fe removal. Experimental conditions: 25 °C, phase ratio = 1:1 (Effect of pH); pH = 1.8, phase ratio = 1:1 (Effect of temperature).
increased with the increase in temperature from 10 °C to 25 °C and peaked at 25 °C. It decreased significantly with a rise in temperature above 40 °C, indicating that Fe extraction is an exothermic reaction and it should be operated at around 25 °C. 3.2.4. Effect of phase ratio The organic:aqueous phase ratio (O/A) was studied in the range from 0.2 to 4 by varying the organic volume while keeping the aqueous volume unchanged using the selected organic system the volume ratio P204:N235 in SK of 30:30 and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at 25 °C. It was found that a positive correlation between phase ratio and Fe removal up to a phase ratio of unity (Fig. 5). These results suggest that a phase ratio of 1:1 is sufficient to extract most of the Fe3+ from the aqueous phase. Therefore this phase ratio was used in the subsequent experiments. To sum up, the best parameters for extraction, derived from the single-factor experiments, are as follows: reaction time greater than 20 min, initial pH of 1.5–2, 1:1 phase ratio, and room temperature. Under these conditions, the Fe removal was greater than 97%, which meets the required standard of YS/T 274-1998 in China [16]. The residence time should be 5 min in practical operation and all Fe can be removed in a counter–current operation. 3.3. Stripping
Fig. 2. Effect of relative proportions of P204-N235-SK in the organic phase on the removal of Fe. Experimental conditions: 25 °C, 20 min contact time.
Sulfuric acid [10] or hydrochloric acid [8] are widely adopted for iron stripping. Sulfuric acid with concentrations ranging from 0.2
X. Sun et al. / Separation and Purification Technology 159 (2016) 18–22
21
appeared after 5 M H2SO4 treatment (Fig. 7). This phenomenon proved the oxidation of N235. Normally, tertiary amine can be oxidized to amine oxide [17–20]. So one possible reaction of N235 and concentrated sulfuric might be as follows: þ
R3 N þ H2 SO4 ! R3 N O þSO2 " þH2 O
ð2Þ
Moreover, the infrared spectra of N235 with 5 M H2SO4 treatment shown in Fig. 7(b) proved the existence of aliphatic sulfonic acid group with bands at 1168.07 and 1008.46 cm1 assigned to sulfonic acid group [21] and 579.09 cm1 assigned to CAS stretching vibration [22], therefore, another reaction happened probably like this:
R3 N þ H2 SO4 ! Nx Oy þ SO2 " þH2 O þ R-SO3 H Fig. 5. Effect of phase ratio on Fe removal and Fe stripping. Experimental conditions: 25 °C, pH = 1.8 (Extraction); 25 °C (Stripping).
Meanwhile, bands at 3200–2500 cm1 [23] and 1065.64 cm1 [21] were attributed to amine salt, which proved amine salt generating reaction as shown below: þ
to 10 mol/L was used as the stripping agent. Other parameters considered are phase ratio and reaction time. When one factor was considered, others maintained the same as 1 mol/L of sulfuric acid, 1:1 of O/A, 25 °C of temperature and 1.209 mol/L of initial Fe. 3.3.1. Effect of phase ratio The effect of phase ratio in the range from 0.2 to 3 using a sulfuric acid concentration of 1 mol/L on Fe stripping is shown in Fig. 5. An increase in iron stripping from 3% to 99% occurred with an increase in phase ratio from 0.2 to 1; with further increase in phase ratio, the stripping decreased a little bit. The decreased Fe stripping when the phase ratio was larger than 1:1 was probably because of the reduced reaction surface of the organic phase. Increasing in phase ratio corresponded to addition of the total volume, as each experiment was conducted with the same organic volume, which in turn led to smaller power per organic phase under the same rotating speed.
ð3Þ
þ
R3 N O þ2R-SO3 H ! R3 N ðR-SO3 Þ2 þ H2 O
ð4Þ
In conclusion, Eqs. (2)–(4) above are possible reactions of N235 and concentrated sulfuric. It is well known that Fe stripping from P204 is difficult [24] and it can be found that the use of around 4 M HCl achieves highest Fe stripping with only P204 as extractant (Fig. 8). (Fig. 8 was obtained with 1 h reaction time using 30% P204 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1 and 25 °C as the extraction conditions and different concentrations of sulfuric acid and hydrochloric acid at an O/A ratio of 1:1 and 25 °C as the stripping conditions.) However iron stripping from the SSX system tends to be quite easy by 1 M H2SO4, which can not only save acid cost, but most importantly, also save base cost for neutralization. In SSX system, N235 takes effect as a phase transfer
3.3.2. Effect of sulfuric acid concentration Fig. 6 shows the effect of sulfuric acid concentration on Fe stripping at 25 °C at a 1:1 phase ratio. The iron stripping peaked at 1 mol/ L acid with almost 100% Fe stripping. Above an acid concentration of 5 mol/L the organic phase was oxidized that the recycled extractant could no longer be used. It was found that the infrared spectra of P204 with and without 5 M H2SO4 treatment were almost the same with some differences in intensities, but for N235, the band at 1096.16 cm1, which was assigned to CAN stretching vibration, dis-
Fig. 6. Effect of sulfuric acid concentration and reaction time on Fe stripping. Experimental conditions: 25 °C.
Fig. 7. IR spectra of (a) P204 and (b) N235 with and without 5 M H2SO4 treatment. Experimental conditions: 25 °C, 1:1 phase ratio, 20 min reaction time.
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Foundation of China (No. 21306053), Key Project of Scientific Research Innovation of Shanghai Municipal Education Commission (No. 14ZZ063), Fundamental Research Funds for the Central Universities (WB1213008). References
Fig. 8. Effect of acid concentration on Fe stripping with only P204 as extractant.
reagent [24] that makes H2SO4 more easily be transferred to the extract phase or the two-phase interface, which makes the stripping process easier:
2R3 NðorgÞ þ H2 SO4 ! ðR2 NHÞ2 SO4ðorgÞ
ð5Þ
3.3.3. Effect of reaction time The effect of reaction times up to 40 min on Fe stripping is also shown in Fig. 6. From the figure, one can see that the iron stripping was nearly 90% in 10 min and reached equilibrium at 20 min with 95% iron stripped. In general, the best parameters for iron stripping are 1 mol/L sulfuric acid, 1:1 phase ratio, and 20 min reaction time. To restore the extraction capacity, 0.5 M ammonium hydroxide was used to neutralize the residual sulfuric acid with 1:1 phase ratio. More than 97% Fe extraction was obtained at last. 4. Conclusions A novel SSX system consisting of P204 and N235 in SK diluent was developed to separate ferric ions from aluminum solutions. The optimal experimental conditions for both extraction and stripping were obtained using the single factor method. Fe removal was greater than 97% under the conditions of reaction time exceeding 20 min, initial aqueous pH of 1.5–2, 1:1 phase ratio, and room temperature. Fe stripping was greater than 99% using low concentration H2SO4 of 1 mol/L. Compared with traditional single extractants, this new synergistic extractant presents significant advantages and pragmatic application prospects to purify aluminum salts recovered from coal spoil. Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China (863 Program, No. 2011AA06A102), National Natural Science
[1] X.X. Sun, Y.Z. Sun, J.G. Yu, Leaching of aluminum from coal spoil by mechanothermal activation, Front. Chem. Sci. Eng. 9 (2) (2015) 216–223. [2] X.X. Sun, Y.Z. Sun, J.G. Yu, Crystal structure of aluminium sulphate hexadecahydrate and its morphology, Cryst. Res. Technol. 50 (4) (2015) 293– 298. [3] X.X. Sun, Y.Z. Sun, J.G. Yu, Cooling crystallization of aluminum sulfate in pure water, J. Cryst. Growth 419 (2015) 94–101. [4] X.X. Sun, Y.Z. Sun, J.G. Yu, Cooling crystallization of aluminum sulfate in pure water modulated by sodium dodecylbenzenesulfonate, Cryst. Res. Technol. 50 (8) (2015) 682–687. [5] S. Meleš, M.V. Proštenik, Solvent extraction of Fe(III) by di-(2-ethylhexyl) phosphoric acid from phosphoric acid solutions, Polyhedron 3 (5) (1984) 615– 617. [6] F. Principe, G.P. Demopoulos, Comparative study of iron(III) separation from zinc sulphate-sulphuric acid solutions using the organophosphorus extractants, OPAP and D2EHPA, Part I: Extraction, Hydrometallurgy 74 (2004) 93–102. [7] G.P. Demopoulos, I.O. Mihaylov, G. Pouskouleli, Synergistic extraction of iron (III) from sulphuric acid solutions with mixed kelex 100-alkyl phosphorus acid extractants, Solvent Extr. Ion Exch. 11 (1) (1993) 67–89. [8] F. Principe, G.P. Demopoulos, Comparative study of iron(III) separation from zinc sulphate-sulphuric acid solutions using the organophosphorus extractants, OPAP and D2EHPA, Part II: Stripping, Hydrometallurgy 79 (2005) 97–109. [9] Y.Q. Ma, X.W. Wang, M.Y. Wang, C.J. Jiang, X.Y. Xiang, X.L. Zhang, Separation of V(IV) and Fe(III) from the acid leach solution of stone coal by D2EHPA/TBP, Hydrometallurgy 153 (2015) 38–45. [10] S.X. Yu, J.Y. Chen, Synergistic extraction of ferric iron in sulfate solutions by tertiary amine and 2-ethylhexyl 2-ethylhexylphosphonic acid (HEHEHP) or dialkylphosphonic acid, Hydrometallurgy 22 (1989) 183–192. [11] R.K. Mishra, P.C. Rout, K. Sarangi, K.C. Nathsarma, Solvent extraction of Fe(III) from the chloride leach liquor of low grade iron ore tailings using Aliquat 336, Hydrometallurgy 108 (2011) 93–99. [12] J. Simpson, P. Navarro, F.J. Alguacil, Iron(III) extraction by LIX 860 and its influence on copped(II) extraction from sulphuric solutions, Hydrometallurgy 42 (1996) 13–20. [13] J.Y. Chen, S.Q. Yu, Z.C. Wu, Separation and Utilization of Iron in Hydrometallurgy, Metallurgical Industry Press, Beijing, 1991, pp. 192–219. [14] R.K. Mishra, P.C. Rout, K. Sarangi, K.C. Nathsarma, A comparative study on extraction of Fe (III) from chloride leach liquor using TBP, Cyanex 921 and Cyanex 923, Hydrometallurgy 104 (2010) 298–303. [15] Y. Sun, Y.L. Li, X.J. Quan, W.P. Liao, Synergistic extraction of molybdenum using acid-base coupling extractants of di-2-ethylhexyl phosphoric acid (P204) and trialkylamine N235, Chinese J. Appl. Chem. 26 (2009) 1353–1356. [16] YS/T 274-1998, Alumina, China non-ferrous metal general corporation, 1998. [17] C.C. Guo, Organic Chemistry, Science Press, Beijing, 2010, pp. 408–409. [18] G.L. Quan, B. Wang, Z.Y. Huang, Y.D. Wu, Organic Chemistry, Higher Education Press, Beijing, 2008, pp. 452–453. [19] C.X. Lu, H.G. Du, Organic Chemistry, Science Press, Beijing, 2008, pp. 583–589. [20] C.J. Sun, X.J. Wang, T. Chen, Principle and Application of Organic Oxidation Reactions, Chemical Industry Press, Beijing, 2013, pp. 427–443. [21] S.F. Weng, Fourier Transform Infrared Spectrometry, Chemical Industry Press, Beijing, 2010, pp. 377–388. [22] G.B. Rong, S.Z. Zhu, Structure Determination of Organic Compounds Tables of Spectral Data, East China University of Science and Technology Press, Shanghai, 2002, pp. 278–279. [23] N.Q. Lu, Z.H. Deng, Practical Infrared Spectrometry, Electronic Industry Press, Beijing, 1989, pp. 180–181. [24] R.J. Ma, Extractive Metallurgy, Metallurgical Industry Press, Beijing, 2009, pp. 404–436.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of ferric ions from aluminum solutions by solvent extraction, part I: Iron removal Xiaoxue Sun, Yuzhu Sun ⇑, Jianguo Yu ⇑ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2015 Received in revised form 23 December 2015 Accepted 29 December 2015 Available online 30 December 2015 Keywords: Ferric sulfate Synergistic extraction Stripping
a b s t r a c t This study investigated removal of ferric ions from aluminum solutions to explore the basic data for the recovery of aluminum resources from coal spoil. A novel synergic extractant, consisting of di-(2ethylhexyl) phosphoric acid (P204) and tertiary amine (N235) with sulfonated kerosene (SK) as diluent, was adopted in this research. The influences of reaction time, pH, temperature, and phase ratio, on the extraction processes were systematically investigated. Result showed the Fe removal was greater than 97% in only one contact under optimal conditions. The corresponding stripping processes were also performed focusing on the effects of phase ratio, H2SO4 concentration, and reaction time. Almost 99% Fe was easily stripped using 1 mol/L H2SO4. These findings indicated that compared with single extractants, the new synergic extractant shows significant advantages. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal spoil (CS) is a byproduct generated during mining, washing, and processing of coal. Numerous CS stacks have accumulated without proper treatment, which not only cover large areas of land but also cause many environmental problems. It is therefore imperative to develop new technologies [1–4] to promote the utilization of CS, especially in coal-based, energy-consuming countries such as China. In fact, CS is a valuable mineral resource. Its main components are SiO2 and Al2O3, and generally, the content of the latter ranges from 15% to 40%. With increasing demands for aluminum oxide and declining in bauxite reserves, CS has drawn increasing attention as a potential alternative non-bauxite aluminum resource. Although many methods have been proposed for the extraction of aluminum from CS, the previous research of the current authors found that a sulfuric acid leaching route presents more promising prospects. A proposed process is as follows: activation of coal spoils using compound activation followed by leaching in sulfuric acid, crystallization of aluminum sulfate and removal of ferric sulfate by solvent extraction, enabling the production of Al2O3 or other aluminum compounds. Removal of ferric sulfate (Fe3+) from aluminum sulfate by solvent extraction is a key step in the process due to the relatively
⇑ Corresponding authors at: P.O. Box 266, Meilong Road 130, East China University of Science and Technology, Shanghai 200237, PR China. E-mail address: [email protected] (Y. Sun). http://dx.doi.org/10.1016/j.seppur.2015.12.054 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
high iron content with an Al/Fe weight ratio of 30.27 in crystallized Al2(SO4)318H2O, which leads to poor Al2O3 quality. A variety of extractants such as P204 [5–9], OPAP [5,6], HEHEHP [10], Aliquat 336 [11], Cyanex 921 [14] and LIX860 [12] have been used for solvent extraction of Fe3+. Among them, di-(2-ethylhexyl) phosphoric acid (P204) is the most widely used extractant in hydrometallurgical process for the separation and purification of metals such as aluminum and iron. It has been proved that mechanism [13] of Fe3+ extraction with P204 is ion-exchange at low acidity and solvent effect at high acidity and P204 is a very effective extractant for Fe3+ removal. However, the extracting of Fe3+ with P204 requires a long contacting time and the stripping of Fe3+ from the loaded P204 is very difficult, which hinders the use of P204 to remove Fe3+ from acid solution. In view of the above, in order to achieve a higher Fe removal within a shorter time and to make the stripping process easier, this research proposed a synergistic solvent extraction (SSX) system, and the effects of a number of experimental parameters, on the Fe3+ removal were systematically investigated. 2. Experimental 2.1. Reagents and equipment Di-(2-ethylhexyl) phosphoric acid (P204, Shanghai Aoke Industrial Co., Ltd., China), Tertiary amine (N235, Shanghai Rare-Earth Chemical Co., Ltd., China), sulfonated kerosene (SK, Shanghai Rare-Earth Chemical Co., Ltd., China), n-pentyl alcohol (Sinopharm
X. Sun et al. / Separation and Purification Technology 159 (2016) 18–22
Chemical Reagent Co., Ltd., China), isoamyl alcohol (Sinopharm Chemical Reagent Co., Ltd., China) and cyclohexane (Sinopharm Chemical Reagent Co., Ltd., China) were used without purification. Al2(SO4)318H2O, Fe2(SO4)3 and H2SO4 (Analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. Al2(SO4)318H2O was dissolved in deionized water. The solution was filtered through a G4 sand-core funnel to eliminate residual solids, and then recrystallized to obtain a highly pure starting material. The concentrations of Al3+ and Fe3+ (denoted by Al2O3 and Fe2O3 in the standard) were determined by chemical titration and spectrophotometric method respectively according to the national standard GB/T 1574-1995. The content of Al3+ was titrated with zinc acetate standard solution utilizing ethylenediamine tetra-acetic acid (EDTA) as a masking agent; By using a 4802 UV–VIS double beam spectrophotometer (Unico (Shanghai) Instrument Co., Ltd., China), the concentration of Fe3+ was determined colorimetrically at 570 nm after adding tiron reagent. The starting solution was prepared with a mass ratio of Al3+/Fe3+ of 30.27, which was consistent with previous research results, by dissolving Al2(SO4)318H2O and Fe2(SO4)3 in deionized water, giving an initial pH of 1.5–2 and Fe3+ concentration of around 1.26 g/L. The pH of the aqueous phase was determined using a SevenMulti pH and Ion Meter (S40, Mettler Toledo) and thermal control was provided by a Huber Refrigeration bath circulator (CC-415, Huber Kältemaschinenbau GmbH). 2.2. Procedures Initial experiments using different proportions of extractants and the effects of experimental parameters on the extents of Fe3+ removal and stripping were studied in 100 mL globe-shaped funnels by mixing 20 mL each of the aqueous and organic phases for predetermined periods of time at 600 rpm. The emulsion was then allowed to stand for 10 min, after which samples of the aqueous phase were withdrawn for analysis. Fe removals were calculated using the amount of Fe3+ in aqueous phase divided by the amount of total Fe3+, while the distribution ratios (D) were calculated as the ratio of the concentration of Fe3+ in the organic phase and that in the aqueous phase at equilibrium.
19
SK as the diluent could be attributed to the subsequent reasons. First, a diluent should be of low toxicity, so that it could be used in large-scale industrial processes. The median lethal dose (LD50), the dose required to kill half the members of a tested population after a specified test duration, is frequently used as a general indicator of a substance’s acute toxicity. A lower LD50 is indicative of increased toxicity. Universally used diluents in liquid–liquid extraction are SK, n-pentyl alcohol, isoamyl alcohol, and cyclohexane, with their LD50 of 36,000, 2200, 1300 and 12,705 mg/kg respectively, which implies that SK enjoys the lowest toxicity. Furthermore, Fe removals using the SSX system in SK, n-pentyl alcohol, isoamyl alcohol, and cyclohexane diluents were 88.19%, 81.38%, 78.27% and 86.62% respectively under the same experimental conditions. As a result, SK was chosen to be the diluent, owing to its low toxicity and high Fe removal ability. Infrared spectra (Magna-IR 550, 4000–400 cm1) of P204, N235 and the SSX system is presented in Fig.1. Peaks from 2800 to 3000 and 1300 to 1500 cm1 represent the ACAH symmetric and asymmetric deformation vibrations and stretching vibrations [11]. The characteristic peak at 1229.62 cm1 due to the stretching vibration of the P@O bond in P204 [14] has shifted to 1214.35 cm1 in the SSX system, while the peak at 1096.16 cm1 in N235 which is assigned to CAN stretching vibration has shifted to 1040.96 cm1 in the SSX system. This phenomenon suggests that both P204 and N235 played roles in the extraction reaction in the mixed P204 and N235 system and it is this reaction that has promoted Fe extraction [15]. Thermodynamically speaking, it seems rather difficult to explain synergistic mechanism from the viewpoint of saturation of coordination due to complexity of the system, nevertheless, it can be explained probably by a number of chemical equilibria in the solvent system. When P204 and N235 (R3N) are present in the organic phase, they can form a molecular association, particularly in a low-acidity solution as shown below:
3. Results and discussion
ð1Þ
3.1. Effect of organic compositions Solvent extraction of ferric ions from sulfate solutions was carried out with the synthetic solution containing 1.26 g/L Fe3+ and the organic systems consisting of 30% P204 and 30% N235 in diluent SK separately, and the SSX organic system consisting of 15% P204 and 15% N235 in SK at 25 °C and an A/O ratio of 1:1. The extent of Fe removal and the distribution ratios (D) are presented in Table 1. With the organic systems consisting of 30% P204 in SK, the iron removal was 66.5% and the distribution ratio D 2.0, nevertheless, with the organic system consisting of 15% P204 and 15% N235 in SK, the iron removal increased to 88.2%, and the distribution ratio to 7.5. This is the evidence of synergistic effect with the addition of N235 to the P204 system, leading to a significant improvement of iron removal. It should be pointed out that using
The formation of a molecular association compound will increase the dissociation of P204 to hydrogen ions, which will result in an increase in the extraction of Fe3+ by cation exchange. The addition of N235 to P204 also shows synergism from the viewpoint of kinetics. The kinetics of Fe extraction using P204-N235
Table 1 Fe removal using P204, N235, and their mixture. Extractant
SK
Fe removal (%)
D
30% P204 30% N235 15% P204/15% N235
70% 70% 70%
66.51 68.47 88.19
1.99 2.17 7.47
Fig. 1. IR spectra of P204, N235 and the SSX system.
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extractant and only P204 extractant were studied in our subsequent experiment and one could conclude that the addition of N235 into P204 turned the extraction process from a mixed diffusion-chemical reaction mechanism to a chemical reaction mechanism. In conclusion, the P204-N235 extractant could achieve higher Fe removal than the only P204 extractant. The effects of various volume ratios of P204, N235 and SK on Fe removal are shown in Fig. 2. The operating volume concentration of P204 was from 0% to 40%, because the viscosity was too high when the concentration of P204 exceeded 40%. At a P204:N235 volume ratio of from 2:3 to 1:1 with 20% to 50% SK, the highest Fe removal of greater than 95% was obtained. This volume ratio corresponded to P204:N235 molar ratio of from 0.8 to 1.2. One can assume that proportions of extractants could form some more stable metal coordination complex with Fe3+ and further investigations will be conducted in the future. So the subsequent experiments were conducted using a volume ratio of 3:3:4 for P204: N235:SK.
Fig. 3. Effect of reaction time on Fe removal. Experimental conditions: 25 °C, phase ratio = 1:1.
3.2. Extraction 3.2.1. Effect of reaction time Fig. 3 plots the extraction of Fe and Al against reaction time at 25 °C using the selected organic system consisting of 30% P204 and 30% N235 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1. It is shown that the equilibration time for Fe extraction is less than 20 min and only approximately 3% Al would be co-extracted into the organic phase. A reaction time of 20 min was therefore selected for the subsequent experiments. It should be mentioned that the iron extraction reached 90% in 5 min contact, indicating the residence time should be 5 min. 3.2.2. Effect of pH The effect of initial aqueous phase pH values on Fe removal at 25 °C using the selected organic system the volume ratio P204: N235 in SK of 30:30 and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1 is shown in Fig. 4. The extent of Fe removal increased with increasing pH from 0.58 to 1.7, and the Fe extraction reached almost 100%. These results are very similar to those reported by Yu and Chen [10]. 3.2.3. Effect of temperature The effect of temperature on Fe removal was investigated in the range from 10 °C to 80 °C using the selected organic system consisting of 30% P204 and 30% N235 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1. The results are also shown in Fig. 4. The Fe extraction
Fig. 4. Effects of initial aqueous pH and temperature on Fe removal. Experimental conditions: 25 °C, phase ratio = 1:1 (Effect of pH); pH = 1.8, phase ratio = 1:1 (Effect of temperature).
increased with the increase in temperature from 10 °C to 25 °C and peaked at 25 °C. It decreased significantly with a rise in temperature above 40 °C, indicating that Fe extraction is an exothermic reaction and it should be operated at around 25 °C. 3.2.4. Effect of phase ratio The organic:aqueous phase ratio (O/A) was studied in the range from 0.2 to 4 by varying the organic volume while keeping the aqueous volume unchanged using the selected organic system the volume ratio P204:N235 in SK of 30:30 and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at 25 °C. It was found that a positive correlation between phase ratio and Fe removal up to a phase ratio of unity (Fig. 5). These results suggest that a phase ratio of 1:1 is sufficient to extract most of the Fe3+ from the aqueous phase. Therefore this phase ratio was used in the subsequent experiments. To sum up, the best parameters for extraction, derived from the single-factor experiments, are as follows: reaction time greater than 20 min, initial pH of 1.5–2, 1:1 phase ratio, and room temperature. Under these conditions, the Fe removal was greater than 97%, which meets the required standard of YS/T 274-1998 in China [16]. The residence time should be 5 min in practical operation and all Fe can be removed in a counter–current operation. 3.3. Stripping
Fig. 2. Effect of relative proportions of P204-N235-SK in the organic phase on the removal of Fe. Experimental conditions: 25 °C, 20 min contact time.
Sulfuric acid [10] or hydrochloric acid [8] are widely adopted for iron stripping. Sulfuric acid with concentrations ranging from 0.2
X. Sun et al. / Separation and Purification Technology 159 (2016) 18–22
21
appeared after 5 M H2SO4 treatment (Fig. 7). This phenomenon proved the oxidation of N235. Normally, tertiary amine can be oxidized to amine oxide [17–20]. So one possible reaction of N235 and concentrated sulfuric might be as follows: þ
R3 N þ H2 SO4 ! R3 N O þSO2 " þH2 O
ð2Þ
Moreover, the infrared spectra of N235 with 5 M H2SO4 treatment shown in Fig. 7(b) proved the existence of aliphatic sulfonic acid group with bands at 1168.07 and 1008.46 cm1 assigned to sulfonic acid group [21] and 579.09 cm1 assigned to CAS stretching vibration [22], therefore, another reaction happened probably like this:
R3 N þ H2 SO4 ! Nx Oy þ SO2 " þH2 O þ R-SO3 H Fig. 5. Effect of phase ratio on Fe removal and Fe stripping. Experimental conditions: 25 °C, pH = 1.8 (Extraction); 25 °C (Stripping).
Meanwhile, bands at 3200–2500 cm1 [23] and 1065.64 cm1 [21] were attributed to amine salt, which proved amine salt generating reaction as shown below: þ
to 10 mol/L was used as the stripping agent. Other parameters considered are phase ratio and reaction time. When one factor was considered, others maintained the same as 1 mol/L of sulfuric acid, 1:1 of O/A, 25 °C of temperature and 1.209 mol/L of initial Fe. 3.3.1. Effect of phase ratio The effect of phase ratio in the range from 0.2 to 3 using a sulfuric acid concentration of 1 mol/L on Fe stripping is shown in Fig. 5. An increase in iron stripping from 3% to 99% occurred with an increase in phase ratio from 0.2 to 1; with further increase in phase ratio, the stripping decreased a little bit. The decreased Fe stripping when the phase ratio was larger than 1:1 was probably because of the reduced reaction surface of the organic phase. Increasing in phase ratio corresponded to addition of the total volume, as each experiment was conducted with the same organic volume, which in turn led to smaller power per organic phase under the same rotating speed.
ð3Þ
þ
R3 N O þ2R-SO3 H ! R3 N ðR-SO3 Þ2 þ H2 O
ð4Þ
In conclusion, Eqs. (2)–(4) above are possible reactions of N235 and concentrated sulfuric. It is well known that Fe stripping from P204 is difficult [24] and it can be found that the use of around 4 M HCl achieves highest Fe stripping with only P204 as extractant (Fig. 8). (Fig. 8 was obtained with 1 h reaction time using 30% P204 in SK and the synthetic feed solution containing 1.26 g/L Fe and 38.14 g/L Al at an A/O ratio of 1:1 and 25 °C as the extraction conditions and different concentrations of sulfuric acid and hydrochloric acid at an O/A ratio of 1:1 and 25 °C as the stripping conditions.) However iron stripping from the SSX system tends to be quite easy by 1 M H2SO4, which can not only save acid cost, but most importantly, also save base cost for neutralization. In SSX system, N235 takes effect as a phase transfer
3.3.2. Effect of sulfuric acid concentration Fig. 6 shows the effect of sulfuric acid concentration on Fe stripping at 25 °C at a 1:1 phase ratio. The iron stripping peaked at 1 mol/ L acid with almost 100% Fe stripping. Above an acid concentration of 5 mol/L the organic phase was oxidized that the recycled extractant could no longer be used. It was found that the infrared spectra of P204 with and without 5 M H2SO4 treatment were almost the same with some differences in intensities, but for N235, the band at 1096.16 cm1, which was assigned to CAN stretching vibration, dis-
Fig. 6. Effect of sulfuric acid concentration and reaction time on Fe stripping. Experimental conditions: 25 °C.
Fig. 7. IR spectra of (a) P204 and (b) N235 with and without 5 M H2SO4 treatment. Experimental conditions: 25 °C, 1:1 phase ratio, 20 min reaction time.
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Foundation of China (No. 21306053), Key Project of Scientific Research Innovation of Shanghai Municipal Education Commission (No. 14ZZ063), Fundamental Research Funds for the Central Universities (WB1213008). References
Fig. 8. Effect of acid concentration on Fe stripping with only P204 as extractant.
reagent [24] that makes H2SO4 more easily be transferred to the extract phase or the two-phase interface, which makes the stripping process easier:
2R3 NðorgÞ þ H2 SO4 ! ðR2 NHÞ2 SO4ðorgÞ
ð5Þ
3.3.3. Effect of reaction time The effect of reaction times up to 40 min on Fe stripping is also shown in Fig. 6. From the figure, one can see that the iron stripping was nearly 90% in 10 min and reached equilibrium at 20 min with 95% iron stripped. In general, the best parameters for iron stripping are 1 mol/L sulfuric acid, 1:1 phase ratio, and 20 min reaction time. To restore the extraction capacity, 0.5 M ammonium hydroxide was used to neutralize the residual sulfuric acid with 1:1 phase ratio. More than 97% Fe extraction was obtained at last. 4. Conclusions A novel SSX system consisting of P204 and N235 in SK diluent was developed to separate ferric ions from aluminum solutions. The optimal experimental conditions for both extraction and stripping were obtained using the single factor method. Fe removal was greater than 97% under the conditions of reaction time exceeding 20 min, initial aqueous pH of 1.5–2, 1:1 phase ratio, and room temperature. Fe stripping was greater than 99% using low concentration H2SO4 of 1 mol/L. Compared with traditional single extractants, this new synergistic extractant presents significant advantages and pragmatic application prospects to purify aluminum salts recovered from coal spoil. Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China (863 Program, No. 2011AA06A102), National Natural Science
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