Chemistry of the Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems at 25 °C

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Hydrometallurgy 89 (2007) 346 – 356 www.elsevier.com/locate/hydromet

Chemistry of the Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems at 25 °C Tadahisa Nishimura a,⁎, Ryosuke Hata b a

Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 1,1 Katahira, 2-chome, Aobaku, Sendai 980-8577, Japan b Graduate School of Student, Tohoku University, Currently Ebara Co. Ltd., Tokyo, Japan Received 1 May 2007; received in revised form 20 August 2007; accepted 20 August 2007 Available online 24 August 2007

Abstract The Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems were studied by contacting either selenious acid or selenic acid solution with calcium oxide to attain equilibrium at 25 °C for one month. Analysis of the final solid phases and the associated solution, together with X-ray diffraction analysis and a study into the graphed relationships, showed the existence of three calcium selenites in the Ca–Se(IV)–H2O system — Ca2SeO3(OH)2·2H2O (Se(IV) = 4.8 × 10− 5–2.8 × 10− 4 M); CaSeO3·H2O (Se(IV) = 2.8 × 10− 4– 0.86 M) and Ca(HSeO3)2·H2O (Se(IV) N 0.86 M). It also showed four calcium selenates in the Ca–Se(VI)–H2O system — Ca2SeO4(OH)2 (Se(VI) = 0.21–0.39 M); CaSeO4·2H2O (Se(VI) = 0.40–9.1 M); CaSeO4 (Se(VI) = 10.2 M) and CaSe2O7 (Se(VI) N 10.8 M). The X-ray diffraction analyses reported and SEM analyses indicate a high degree of crystallinity of all seven compounds. The stability and solubility regions for these compounds were defined versus pH, and the conventional solubility constants and conditional free energies of formation for the less soluble CaSeO3·H2O, Ca2SeO3(OH)2·2H2O, CaSeO4·2H2O and Ca2SeO4(OH)2 were calculated from solubility data obtained. © 2007 Elsevier B.V. All rights reserved. Keywords: Calcium selenite; Calcium selenate; Ternary phase diagram; Stability; Solubility

1. Introduction In the hydrometallurgical and mining waste water processes, calcium hydroxide and calcium carbonate have been used often as a neutralizing agent or pH adjustor. Sulfate ion is usually removed as CaSO4·2H2O in nonferrous hydrometallurgy. Since selenium has similar properties to sulfur, it is possible of selenium to co-precipitate with gypsum. Also, selenium and sulfur may be incorporated into the sludge produced in ⁎ Corresponding author. E-mail address: [email protected] (T. Nishimura). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.08.006

the waste gas-desulfurization stage. The Ca–Se(IV)– H2O and Ca–Se(VI)–H2O systems are of interest in the treatment of selenium-contaminated process liquors in hydrometallurgical applications and in the mineralogy relating to the occurrence of calcium selenites and calcium selenates. In 1997, Dumm and Brown investigated the phase equilibria in the CaO–SeO2–H2O system at 23 and 80 °C. They confirmed the existence of two stable compounds Ca(HSeO 3 ) 2 ·H 2 O (pH 0.4–3.7) and CaSeO3·H2O (pH 3.7–12.5) at 23 °C. At 80 °C, CaSeO3·H2O, Ca2(SeO3)(Se2O5) and Ca2(HSeO3)2 (Se2O5) were found to exist as stable phases. In the

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JCPDS (Joint Committee on Powder Diffraction Standards) Index, Ca(HSeO3)2·H2O, Ca(SeO3)2·H2O, CaS2O5 and Ca2Se3O8 are listed as No.36-467, No.35883, No.36-468 and No.35-886, respectively, based on the report by Evert (1981). Valkonen and Losoi (1985) synthesized Ca(HSeO3)2·H2O and Ca2(HSeO3)2(Se2O5) and determined their crystal structure. They also found that Ca2(HSeO3)2(Se2O5) formed at 350 °C. Valkonen (1986) determined the crystal structure of Ca (SeO3)2·H2O. Although the X-ray diffraction data for calcium selenates have been reported, no systematic study has been conducted on the CaO–SeO3–H2O system. Snyman and Pistorius (1963) published X-ray diffraction data for CaSeO4·1/2 H2O which is listed in the JCPDS Index (No.16-481). CaSeO4·2H2O found by Quast (1988) is also listed as No.40-2345. Recently, the book entitled “Chemical Thermodynamics of Selenium”, compiled by Olin et al. (2005), gives much useful information about the behavior of selenium in aqueous solutions. In 1956, Chkhlantsev synthesized CaSeO3 by mixing the equimolar solutions of calcium chloride and sodium selenite at 50–60 °C. He measured its solubility by contacting the solid with a solution of hydrochloric acid or nitric acid (initial pH= 2–3) at 20 °C for 8 h and calculated an average value of 10− 5.53 for the solubility product constant (Kso). Ripan and Vericeanu (1968) determined the solubility of same compound in water and calculated an average value of 10− 5.74 for Kso. In the NBS Tables of Chemical Thermodynamic Properties (Wagman et al., 1982), the standard free energies of formation of CaSeO3·2H2O (−1428.7 kJ/mol) and CaSeO4·2H2O (−1486.8 kJ/mol) have been listed, and based on their values, the values of 10− 5.44 and 10− 3.09 have been calculated for Kso, respectively. Also, in the compilation on Chemical Thermodynamic of Selenium, the values of Kso for CaSeO3·H2O (10− 6.40 ± 0.25) and CaSeO4·2H2O (10− 2.68 ± 0.25) have been presented. All these solubility data have been determined in a very narrow pH range. The Thermodynamics data for compounds formed over a wide range of pH are indispensable information for evaluating alternative process options. Many processes are applicable to the removal of selenium(IV) from waste water, but they are not effective for removing selenium(VI) under moderate pH conditions. In this study, the equilibrium experiments for the Ca– Se(IV)–H2O and Ca–Se(VI)–H2O systems were performed to clarify the stability and solubility conditions for calcium selenites and calcium selenates at 25 °C. Based on the observed concentrations at equilibrium, the conventional solubility constants and conditional free energies of formation for each compound were calculated.

347

2. Experimental The experimental procedure adopted did not introduce other ions than calcium, selenium(IV), and selenium(VI) into the systems investigated. Nor was a supporting electrolyte used. 2.1. Materials All chemicals were analytical reagent grade. CaO was prepared as a source of calcium by calcining Ca(OH)2 at 1000 °C and storing the product in a dessicator until used. A selenium(IV) solution (0.10–14.6 mol/L) was prepared by dilution of a 16.3 mol/L Se(IV) solution (which was prepared by dissolving H2SeO3 in distilled water) with distilled water. A selenium(VI) solution (0.60–16.8 mol/L) was prepared by dilution of 16.8 mol/L H2SeO4 solution with distilled water. 2.2. Experimental procedure For the phase study in the neutral and alkaline ranges, CaO of various predetermined weights was placed in 100 mL sealable glass bottles each containing 50 mL of selenium(IV) solution (0.10 mol/L) or selenium(VI) solution (0.60 mol/L) in an atmosphere of nitrogen to exclude atmospheric carbon dioxide. For the phase study of the Ca–Se(IV)–H2O and Ca– Se(VI)–H2O systems in the acidic range, CaO was dissolved in a series of Se(IV) or Se(VI) solutions (25 mL) of set concentrations until the solutions were saturated, and then an excess of CaO was added to produce a fine suspension. The bottles were stoppered and then shaken in an air bath kept at 25 °C for one month to allow equilibrium to be established between the solid and liquid phases. For some selected experiments, the change in the concentrations of calcium and selenium in the solution with shaking time was determined for a period of time up to three months. It was confirmed that the concentrations of calcium and selenium decreased fairly rapidly and after two weeks, further changes in the concentrations of calcium and selenium were not detected. Based on these results, one month was adopted as shaking time in this study. After equilibration, the solutions with a suspension of precipitate were then filtered through a glass fiber filter (ADVANTAC GB-100R) after measurement of pH of the slurry. The precipitates formed at pH levels above 5.88 in the Ca–Se(IV)–H2O system, and those formed at pH above 6.92 in the system Ca–Se(VI)–H2O were washed quickly several times with deaerated distilled water and dried in vacuum at room temperature and then analysed for Ca and Se using the chemical analysis method (CAM) described. A part of the damp precipitate from each experiment in both systems was submitted to X-ray diffraction (XRD) analysis. The precipitates formed at pH below 5.59 in the Ca–Se(IV)–H2O system and all the precipitates formed in the Ca–Se(VI)–H2O system were filtered and the wet materials immediately submitted to chemical analysis and X-ray diffraction analysis. The filtrates were refiltered through membrane filter (pore size: 0.65 μm) to remove a still remaining suspension of precipitate, and then subjected to measurement of specific gravity and to chemical

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Table 1 Analyses of solutions and wet residues in the Ca–Se(IV)–H2O experiments Sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Solution

Wet residue

Specific gravity 2.281 2.139 1.966 1.811 1.650 1.462 1.376 1.290 1.198 1.155 1.119 1.106 1.104 1.100 1.098 1.073 1.057 1.012

pH

Ca (g/L)

CaO (wt.%)

Se(IV) (g/L)

SeO2 (wt.%)

CaO (wt.%)

SeO2 (wt.%)

0.39 0.65 0.95 1.34 1.56 1.80 2.14 2.43 2.81 3.22 3.48 3.63 3.68 3.85 4.09 5.59

41.1 37.9 36.0 34.0 31.2 27.3 24.6 21.9 19.0 17.7 16.3 15.9 16.1 16.2 16.0 11.8 9.22 1.72

2.51 2.48 2.57 2.63 2.64 2.61 2.50 2.38 2.23 2.14 2.04 2.01 2.04 2.06 2.05 1.54 1.22 0.24

1138 1018 843 708 545 382 313 238 155 118 86.7 68.9 67.8 67.6 67.4 50.1 39.1 7.18

70.11 66.88 60.22 54.93 46.34 36.75 31.95 25.90 18.21 14.36 10.88 8.76 8.62 8.65 8.63 6.56 5.20 1.02

14.90 13.75 15.17 15.79 13.87 16.52 15.33 15.24 16.08 16.11 16.25 14.60 14.39 15.74 17.81 19.07 18.64 15.88

70.59 69.61 69.47 68.37 64.20 67.47 67.43 67.08 67.28 66.55 63.66 58.97 56.63 39.45 38.16 40.06 39.21 32.62

Solid phase A⁎⁎ A⁎⁎ A⁎,⁎⁎ A⁎⁎ A⁎⁎ A⁎,⁎⁎ A⁎⁎ A⁎⁎ A⁎⁎ A⁎⁎ A⁎,⁎⁎ A⁎⁎ A⁎⁎ B⁎,⁎⁎ B⁎⁎ B⁎,⁎⁎ B⁎⁎ B⁎

A = Ca(HSeO3)2·H2O B = CaSeO3·H2O. ⁎XRD. ⁎⁎WRM.

analysis. Selected solid samples were examined by scanning electron microscopy (SEM). The graphical procedure known as Schreinemaker's Wet Residue Method (WRM) adopted in previous studies (Nishi-

mura and Robins, 1996; Hata et al., 2004; Nishimura et al., 2005) was used for determining the composition of the solid phases formed in the high acid range. This method relies on plotting the compositions of liquid phase and the wet residue

Table 2 Analyses of solutions and precipitates in the Ca–Se(IV)–H2O experiments Sample no.

Solution

Precipitate

pH

Ca (g/L)

Se(IV) (g/L)

CaO (wt.%)

SeO2 (wt.%)

H2O (wt.%)

Ca/Se mole ratio

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

5.88 6.21 6.83 6.99 8.09 8.35 10.60 11.59 11.99 12.10 12.21 12.30 12.34 12.49 12.50 12.51 12.50 12.51 12.51 12.51

1.00 0.628 0.277 0.213 0.082 0.075 0.044 0.092 0.161 0.225 0.318 0.441 0.527 0.807 0.810 0.803 0.814 0.819 0.819 0.821

4.13 2.52 1.06 0.815 0.226 0.193 0.061 0.027 0.022 0.025 0.024 0.021 0.011 0.0040 0.0038 0.0040 0.0037 0.0039 0.0039 0.0035

30.55 30.49 30.80 30.39 30.65 30.45 30.58 30.69 30.63 31.42 35.29 38.91 40.65 40.56 40.99 42.14 42.61 43.80 47.60 52.08

61.47 61.50 60.46 61.81 61.71 61.37 60.35 61.24 61.29 59.43 52.31 44.99 39.82 39.89 39.44 37.14 36.37 34.58 30.03 24.24

7.98 8.01 8.74 7.80 7.64 8.18 9.06 8.07 8.08 9.15 12.40 16.10 19.53 19.55 19.57 20.72 21.02 21.62 22.47 23.68

0.98 0.98 1.01 0.97 0.98 0.98 1.00 0.99 0.99 1.04 1.33 1.71 2.02 2.01 2.06 2.25 2.32 2.51 3.13 4.25

B = CaSeO3·H2O. C = Ca2SeO3(OH)2·2H2O. D = Ca(OH)2. ⁎XRD. ⁎⁎⁎CAM.

Solid phase B⁎⁎⁎ B⁎⁎⁎ B⁎,⁎⁎⁎ B⁎⁎⁎ B⁎⁎⁎ B⁎⁎⁎ B⁎⁎⁎ B⁎⁎⁎ B⁎,⁎⁎ B⁎, C⁎ C⁎,⁎⁎⁎ C⁎,⁎⁎⁎ C⁎ C⁎, D⁎ C⁎, D⁎

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349

Fig. 1. Ternary phase diagram for the CaO–SeO2–H2O system at 25 °C.

determined by chemical analysis, on a straight line on triangular coordinates. The series of lines from a number of analyses passes through a point which represents the composition of the solid phase. The degree of hydration of the solid is also then easily determined graphically.

2.3. Analysis The specific gravity of the solution was measured using a specific-gravity bottle at 20 °C. Calcium in the filtrates and precipitates was determined by titration with a standard EDTA solution using Eriochrome Black T solution as indicator. For analysis of low levels of calcium in the filtrates (below 0.1 g/L), the inductively coupled plasma spectroscopy (ICP) was used. Selenium(IV) in the filtrates and precipitates was determined by a back titration method of the unreacted thiosulfate with a standard iodine solution after reduction of selenium(IV) to selenium(III) with an excess of sodium thiosulfate. Selenium(VI) in the filtrates and precipitates was determined in the same manner after reduction of selenium (VI) to selenium(IV) with hydrochloric acid in a boiled water Table 3 X-ray diffraction data for Ca2SeO3(OH)2·2H2O

Fig. 2. X-ray diffraction patterns of the calcium selenites found in this work.

d-spacing

I / I1

d-spacing

I / I1

9.350 3.871 3.719 3.263 3.129 2.985 2.891 2.680 2.615 2.516 2.402 2.333

100 4 2 3 3 31 11 4 4 2 3 4

2.279 2.132 2.083 1.945 1.795 1.764 1.720 1.658 1.558 1.506 1.337

12 3 3 2 2 9 5 2 2 2 2

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numbered 30–32 gave a mole ratio of Ca/Se(V) of 2.0 corresponding to the Ca2SeO3(OH)2·2H2O with CaO = 40.47 wt.%, SeO2 = 40.04 wt.% and H2O = 19.49 wt.%. This result and the graphical procedure based on the wet residue method prove that there are three solid phases of Ca(HSeO3)2·H2O, CaSeO3·H2O and Ca2SeO3(OH)2·2H2O, given by the points A, B and C, and three invariant points. The first invariant point lies at point D (CaO = 2.1 wt.%, SeO2 = 8.6 wt.%) where Ca(HSeO3)2·H2O and CaSeO3·H2O coexist. The second invariant point lies at point E (CaO = 0.06 wt.%, SeO2 = 0.003 wt.%) where CaSeO3·H2O and Ca2SeO3(OH)2·2H2O coexist. The third point lies at point F (CaO = 0.08 wt.%, SeO2 = 0.00004 wt.%) where Ca2SeO3 (OH)2·2H2O and Ca(OH)2 coexist. Point G (SeO2 = 74.7 wt.%) represents a saturated solution of selenious acid. The stable regions are bounded for Ca(HSeO3)2·H2O by ADG, for CaSeO3·H2O by BED and for Ca2SeO3(OH)2·2H2O by CFE. The stability regions for two compounds of Ca(HSeO3)2·H2O and CaSeO3·H2O in selenious acid solution found at 25 °C in this work are in agreement with the results by Dumm and Brown (1997) at 23 °C. However, Dumm and Brown (1997) did not report on the formation of Ca2SeO3(OH)2·2H2O in the CaO– SeO2–H2O system. The selected wet residue samples (which are marked with an asterisk in Tables 1 and 2) were analysed by X-ray diffraction. The typical X-ray diffraction patterns of the three compounds obtained in this work are shown in Fig. 2. The X-ray diffraction pattern for Ca2SeO3(OH)2·2H2O formed in alkaline solutions could not be matched with any pattern in the JCPDS Index, and so the data for the compound is listed in Table 3. The X-ray diffraction patterns for compounds A and B formed in high and

Fig. 3. SEM images of the calcium selenites found in this work. (a) Ca (HSeO3)2·H2O, (b) CaSeO3·H2O, (c) Ca2SeO3(OH)2·2H2O.

bath. For analysis of low levels of selenium in the filtrates (below 0.5 g/L), the ICP method was used. 3. Results and discussion 3.1. The Ca–Se(IV)–H2O system All of the results from the analysis of solutions and wet residues, and solutions and precipitates (dried in vacuum) are presented in Tables 1 and 2, respectively. The original analytical data were converted to weight percent values and presented graphically in Fig. 1 as the ternary diagram for the CaO–SeO2– H2O system. In this figure, the compositions of solution, wet residue and precipitate are expressed by ○, ● and △, respectively. Chemical analyses of the precipitates of sample

Fig. 4. Stability and solubility regions for calcium selenites as a function of pH at 25 °C.

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Table 4 Analyses of solutions, wet residues and precipitates in the Ca–Se(VI)–H2O experiments Sample no.

Solution

1

2.498

2

Specific gravity

pH

Precipitate CaO (wt.%)

CaO (wt.%)

Se(VI) (g/L)

SeO3 (wt.%)

CaO (wt.%)

SeO3 (wt.%)

3.90

0.22

1314

84.60

2.285

1.69

0.10

1109

78.07

3

2.110

1.38

0.09

939

71.56

4 5

2.012 1.940

1.39 1.38

0.09 0.10

850 804

67.92 66.62

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

1.943 1.860 1.795 1.753 1.579 1.366 1.193 1.102 1.092 1.083 1.072 1.068 1.067 1.068 1.072 1.070 1.076 1.067 1.056 1.039 1.032 1.028 1.025

1.35 1.33 1.34 1.30 1.28 3.87 13.4 17.3 17.0 16.8 16.6 16.5 16.6 16.4 16.6 17.0 16.9 15.8 13.7 8.84 7.29 7.03 5.71

0.09 0.08 0.08 0.07 0.08 0.28 1.12 2.19 2.18 2.16 2.17 2.15 2.19 2.16 2.17 2.22 2.20 2.07 1.81 1.19 0.99 0.93 0.78

789 719 663 633 483 296 139 61.3 48.9 35.6 35.5 33.8 33.2 33.0 32.5 31.7 31.5 30.6 27.4 16.2 12.5 13.6 11.0

65.33 62.16 59.37 58.09 49.21 34.83 18.76 8.97 7.20 5.29 5.32 5.08 5.01 4.98 4.87 4.76 4.70 4.61 4.17 2.51 1.95 2.12 1.73

0.43 1.40 2.76 1.88 4.13 4.81 1.79 7.89 8.86 1.31 15.68 17.32 15.02 18.02 18.99 21.23 22.62 23.36 18.29 20.02 19.05 20.93 18.89 17.02 18.20 16.99 16.89 30.52 33.33 33.58 34.02 34.21 35.22 34.13

84.97 84.75 84.53 78.14 78.75 78.46 72.94 75.39 71.71 66.64 68.41 66.51 60.39 28.08 58.77 57.91 55.80 57.29 41.41 46.41 44.18 47.64 41.14 37.76 39.33 36.82 34.69 36.30 37.46 37.60 37.70 34.30 19.51 13.33

0.61 0.84 1.36 1.74 2.59 6.61 6.92 8.44 11.83 11.98 12.01 12.02 12.04 12.15 12.16 12.19

Ca (g/L)

Wet residue

SeO3 (wt.%)

Ca/Se mole ratio

Solid phase A⁎,⁎⁎ A⁎,⁎⁎ A⁎,⁎⁎ B⁎,⁎⁎

25.48

58.27

0.99

26.38 40.05 43.52 43.83 44.26 44.73 52.41 56.79

57.51 50.85 49.01 49.13 48.96 44.47 30.11 22.34

1.04 1.78 2.01 2.02 2.05 2.28 3.97 5.76

B⁎,C⁎ C⁎,⁎⁎ C⁎,⁎⁎ C⁎,⁎⁎ C⁎,⁎⁎ C⁎⁎ C⁎⁎ C⁎,⁎⁎ C⁎⁎ C⁎⁎ C⁎⁎ C⁎⁎ C⁎⁎ C⁎⁎,⁎⁎⁎ C⁎⁎ C⁎,⁎⁎⁎ C⁎,D⁎ D⁎,⁎⁎⁎ D⁎,⁎⁎⁎ D⁎,⁎⁎⁎ D⁎,⁎⁎⁎ D⁎,H⁎ D⁎,H⁎

A = CaSe2O7. B = CaSeO4. C = CaSeO4·2H2O. D = Ca2SeO4(OH)2. H = Ca(OH)2. ⁎XRD. ⁎⁎WRM. ⁎⁎⁎CAM.

low selenious acid solutions were identical to that in JCPDS Index No.36-467 for Ca(HSeO3)2·H2O and No.35-883 for CaSeO3·H2O, respectively. These three compounds were examined by scanning electron microscopy, and are shown in Fig. 3. All compounds have a high degree of crystallinity. The particles of Ca(HSeO3)2·H2O are composed of large rectangular pieces, while those of Ca2SeO3(OH)2·2H2O are thin and rhombic plate in shape. Fig. 4 shows a dual plot of the Ca/Se(IV) mole ratio in the precipitate and the log concentration in the solution, as a function of pH. It is obvious from Fig. 4(a) that Ca(HSeO3)2·H2O with Ca/ Se(IV) mole ratio of 0.5 is stable at pH below 3.5, CaSeO3·H2O with Ca/Se(IV) mole ratio of 1.0 in the pH range 3.5 to 12.3 and Ca2SeO3(OH)2·2H2O with Ca/Se(IV) mole ratio of 2.0 in the pH range 12.3 to 12.5. In this system, there are three invariant points at pH 3.5, 12.3 and 12.5, at each point, two solids phases, Ca

(HSeO3)2·H2O and CaSeO3·H2O, CaSeO3·H2O and Ca2SeO3 (OH)2·2H2O, and Ca2SeO3(OH)2·2H2O and Ca(OH)2 coexist. Fig. 4(b) shows that the calcium concentration in solutions in equilibrium with the calcium selenites has a minimum value of 1.1× 10− 3 mol/L at pH 10.6, while the selenium(IV) concentration has a minimum value of 4.4× 10− 5 mol/L at pH 12.5 where Ca2SeO3(OH)2·2H2O is in equilibrium with the solution of high calcium concentration. It should be noted that this lowest concentration for selenium (IV) (3.5 mg/L) is still much higher than the limit of industrial waste water regulation for selenium (0.1 mg/L). 3.2. The Ca–Se(VI)–H2O system The analytical data for solutions, wet residues and precipitates are presented in Table 4, and in Fig. 5 as a ternary

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Fig. 5. Ternary phase diagram for the CaO–SeO3–H2O system at 25 °C.

diagram for the CaO–SeO3–H2O system. The chemical analyses of the precipitates of sample numbered 23–25 showed a mole ratio of Ca/Se(VI) of 2.0 corresponding to the Ca2SeO4(OH)2 with CaO = 43.62 wt.%, SeO3 = 49.38 wt.% and H2O = 7.00 wt.%. From these results, it can be seen that there are four calcium selenates which are represented by the formulae CaSe2O7, CaSeO4, CaSeO4·2H2O and Ca2SeO4 (OH)2, given by the points A, B, C and D and four invariant points, given by the points E (CaO = 0.09 wt.%, SeO3 = 66.62– 67.93 wt.%), F (CaO = 0.09 wt.%, SeO3 = 65.55 wt.%), G (CaO = 2.20 wt.%, 4.70 wt.%) and H (CaO = 0.99 wt.%, SeO3 = 1.95 wt.%). The selected wet residue and precipitate samples were analyzed by X-ray diffraction. The typical X-ray diffraction patterns of the four compounds are shown in Fig. 6. The X-ray diffraction patterns for three compounds A, B, and D, that is, CaSe2O7, CaSeO4 and Ca2SeO4(OH)2 are not listed in the JCPDS Index and so the diffraction data obtained in this work are presented in Table 5. The X-ray diffraction pattern for compound C was matched with to that in JCPDS Index No.40235 for CaSeO4·2H2O. The SEM images for the four compounds produced in this work are shown in Fig. 7. The particle size and crystallinity are very well developed. Fig. 8 shows a dual plot of the Ca/Se(VI) mole ratio in the precipitate and the log concentration in the solution, both versus pH. It is noted from Fig. 8(a) that CaSeO4·2H2O having Ca/Se(VI) mole ratio of 1.0 is stable over the wide range of pH, while Ca2SeO4(OH) having Ca/Se(VI) mole ratio of 2.0 is stable in the very narrow range of pH 12.0 to 12.2. Also, this diagram shows two invariant points where CaSeO4·2H2O and

Fig. 6. X-ray diffraction patterns of the calcium selenates found in this work.

T. Nishimura, R. Hata / Hydrometallurgy 89 (2007) 346–356 Table 5 X-ray diffraction data for the three calcium selenates found in this work CaSe2O7

CaSeO4

Ca2SeO4(OH)2

d-spacing

I / I1

d-spacing

I / I1

d-spacing

I / I1

7.688 7.189 5.483 4.318 4.082 3.806 3.586 3.443 3.323 3.217 2.822 2.775 2.529 2.430 2.094 2.008 1.969 1.884 1.848 1.807 1.776 1.692 1.639 1.616 1.588 1.491 1.321 1.198 1.132

27 12 12 13 48 87 10 100 15 12 28 40 7 20 5 20 19 28 18 8 6 6 6 12 6 12 19 6 7

4.654 3.050 2.923 2.526 2.318 2.262 2.217 1.953 1.908 1.784 1.705 1.539 1.521 1.390 1.264 1.235 1.167 1.157 1.130

22 100 3 30 3 3 12 2 24 20 7 40 12 3 3 5 6 4 5

5.240 3.663 3.524 3.305 3.035 2.961 2.909 2.814 2.688 2.608 2.456 2.348 2.318 2.228 1.937 1.825 1.800 1.779 1.761 1.706 1.659 1.620 1.374

100 58 51 24 29 31 50 7 10 35 19 46 53 7 9 7 11 15 12 21 26 14 8

353

were added, selenium(IV) is precipitated as ferric selenite (Fe2(SeSO3)3·5H2O) and a majority of sulfate ion also precipitated as calcium sulfate at pH around 4. Here, the residual selenium(IV) could be co-precipitated with ferric hydroxide, when an excess of ferric sulfate was added. In the second stage, selenium(VI) is removed to a concentration lower than 50 mg/L as barium selenate (BaSeO4) by adding barium chloride at pH 5–6 and also the residual sulfate ion is precipitated as barium sulfate. In the third stage, the residual selenium(VI) is removed below the level of 0.1 mg/L by reduction– precipitation of selenium(VI) to elemental selenium with ferrous hydroxide pH 9 at 70 °C in an atmosphere of nitrogen (Nishimura et al., 2000) or else by an adsorption process using the polyamine-type weakly basic ion exchange resin (Eporasu K-6) at pH 3–12 (Nishimura et al., submitted for publication).

4. Calculation of solubility product constants for calcium selenites and calcium selenates The chemical equilibria reactions and conventional solubility product constants for the two less soluble calcium selenites, CaSeO3·H2O and Ca2SeO3(OH)2·2H2O, can be expressed by the following equations where −log[H+] = pH is introduced and the activities of all species are assumed to be unity, and where square brackets denote concentrations: CaSeO3
H2 OðsÞ ¼ Ca2þ þ SeO2− 3 þ H2 O ⁎

logc K so ¼ log½Ca2þ  þ log½SeO2− 3  Ca2 SeO3 ðOHÞ2
2H2 OðsÞ þ 2H ¼ 2Ca2þ þ SeO2− 3 þ 4H2 O

ð2Þ

þ

⁎

Logc K so ¼ 2log½Ca2þ  þ log½SeO2− 3  þ 2pH Ca2SeO4(OH)2 coexist at pH 12.0 and Ca2SeO4(OH)2 and Ca (OH)2 coexist at pH 12.2. As shown in Fig. 8(b), the calcium concentration in solutions in equilibrium with CaSeO4·2H2O keeps constant at 0.42 mol/L in the pH range 0.6 to12.0 and then decreases to 0.14 mol/L. The selenium(VI) concentration slightly decreases with increasing pH from 0.6 to 1.4 and then remains constant at same value of calcium concentration in the same pH range. A further increase in pH leads again to decreased selenium(VI) concentration until it is reduced to 11.1 g/L (0.14 mol/L). In this study, it was found that the basic compounds, Ca2SeO3(OH)2·2H2O in the Ca–Se(IV)–H2O system and Ca2SeO4(OH)2 in the Ca–Se(VI)–H2O system have the lowest solubility. Both values are much higher than the limit of industrial waste water regulation for selenium (0.1 mg/L), as well as that for Fe2(SeO3)3·5H2O (Nishimura et al., 2005) as well as BeSeO3 and BaSeO4 (Hata et al. 2004). Based on a series of our investigations for selenium in aqueous solutions, three-stage processes were proposed for removing selenium (IV) and selenium(VI) from waste water (Se b 1500 mg/L) containing sulfuric acid, coming from anode slime treatment. In the first stage, where ferric sulfate and calcium hydroxide

ð1Þ

ð3Þ ð4Þ

The concentrations, [Ca2+] and [SeO32−] can be calculated from pH and the total (analytical) concentrations of calcium and selenium(IV) in the solutions from each experiment by taking into account appropriate solution equilibria and mass balance equations. Aqueous equilibria and equilibrium constants which were calculated from the standard free energies of formation listed in the NBS Tables of Chemical Thermodynamic Properties (Wagman et al., 1982) are: Ca2þ þ H2 O ¼ CaOHþ þ Hþ K 1 ¼ ½CaOHþ ½Hþ =½Ca2þ  ¼ 10−12:68

ð5Þ

CaOHþ þ H2 O ¼ CaðOHÞ02 þ Hþ

K 2 ¼ ½CaðOHÞ02 ½Hþ =½CaOHþ  ¼10−15:32

H2 SeO3 ¼ HSeO−3 þ Hþ K 3 ¼ ½HSeO−3 ½Hþ =½H2 SeO3  ¼ 10−2:57

ð6Þ ð7Þ

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T. Nishimura, R. Hata / Hydrometallurgy 89 (2007) 346–356

þ HSeO−3 ¼ SeO2− 3 þH

K4 ¼

þ − ½SeO2− 3 ½H =½HSeO3 

¼ 10

−7:40

ð8Þ

The mass balances for the Ca and Se(IV) species are: þ

½Ca ¼ ½Ca  þ ½CaOH  þ 2þ

½CaðOHÞ02 

ð9Þ

½SeðIVÞ ¼ ½H2 SeO3  þ ½HSeO−2  þ ½SeO2− 3 

ð10Þ

Combining Eqs. (5), (6) and (9) gives: ½Ca2þ  ¼ ½Hþ 2 ½Ca=ð½Hþ 2 þ K 1 ½Hþ  þ K 1 K 2 Þ

ð11Þ

þ 2

þ

¼ K 3 K 4 ½SeðIVÞ=ð½H  þ K 3 ½H  þ K3K4Þ

Δ

f G298:15 CaSeO3
H2 O ¼ −1189:1 kJ=mol

Δ

f G298:15 CaSeO3 ðOHÞ2
2H2 O ¼ −233:01 kJ=mol:

φ φ

The chemical equilibrium equations and conventional solubility product constants for the calcium selenates, CaSeO4·2H2O and Ca2SeO4(OH)2, can be expressed by: CaSeO4
2H2 OðsÞ ¼ Ca2þ þ SeO2− 4 þ H2 O

and combining Eqs. (7), (8) and (10) gives: ½SeO2− 3 

The conditional free energies of formation for the calcium selenites estimated from the solubility constants are:

⁎

ð12Þ

The concentrations, [Ca 2+ ] and [SeO32− ] were calculated by substituting the analytical values for Ca and Se(IV) into Eqs. (11) and (12), respectively. The solubility constants for CaSeO3·H2O and Ca2SeO3 (OH)2·2H2O were then calculated by substituting these values into Eqs. (2) and (4), and are listed in Table 6. The averages of the solubility constant values for each compound are also shown in Table 6.

ð13Þ

logc K so ¼ log½Ca2þ  þ log½SeO2− 4 

ð14Þ

Ca2 SeO4 ðOHÞ2 ðsÞ þ 2Hþ ¼ 2Ca2þ þ SeO2− 4 þ 2H2 O

ð15Þ

⁎

logc K so ¼ 2log½Ca2þ  þ log½SeO2− 4  þ 2pH

ð16Þ

The conventional solubility constants for these reactions were calculated from the experimental solubility data and the standard free energies of formation

Fig. 7. SEM images of the calcium selenates found in this work. (a) CaSe2O7, (b) CaSeO4, (c) CaSeO4·2H2O, (d) Ca2SeO4(OH)2.

T. Nishimura, R. Hata / Hydrometallurgy 89 (2007) 346–356

355

Table 6 Solubility constants for the two calcium selenites No

pH 2+

+ SeO2− 3 + H2O

(a) CaSeO3·H2O(s) = Ca 16 3.85 17 4.09 18 5.59 19 5.88 20 6.21 21 6.83 22 6.99 23 8.09 24 8.35 25 10.60 26 11.59 27 11.99

log

c⁎

Kso

−4.18 −4.16 −4.13 −4.38 −4.43 −4.63 −4.75 −5.30 −5.38 −6.07 −6.14 −6.04 Av.−4.97 ± 1.17

(b) Ca2SeO3(OH)2·2H2O(s) + 2H+ = 2Ca2+ + SeO2− 3 + 4H2O 31 12.34 16.74 32 12.49 16.82 Av. 16.78 ± 0.04

Fig. 8. Stability and solubility regions for calcium selenates as a function of pH at 25 °C.

for appropriate species in a similar manner as was described above for the calcium selenites. The constants obtained from each experiment and the average of these values are listed in Table 7. The conditional free energies of formation for calcium selenates were estimated from the average of solubility product constants: Δ Δ

f Gφ298:15 CaSeO4
2H2 O ¼ −1475:9kJ=mol φ

f G298:15 Ca2 SeO4 ðOHÞ2 ¼ −1895:9kJ=mol

In this study, the conventional solubility product constants and conditional free energies of formation for the calcium selenite, Ca(HSeO3)2·H2O, and calcium selenates, CaSe2O7 and CaSeO4, are not considered here, because they are of high solubility and their derivation cannot be treated in the same way as the low solubility compounds. 5. Conclusions The chemistry of the Ca–Se(IV)–H2O and Ca–Se (VI)–H2O systems was studied by equilibrating mix-

tures of calcium oxide and selenic acid or selenium acid solution at 25 °C. The stability regions for three calcium selenites (Ca(HSeO3)2·H2O, CaSeO3·H2O and Ca2SeO3 (OH)2·2H2O) and four calcium selenates (CaSe2O7, CaSeO4, CaSeO4·2H2O and Ca2SeO4(OH)2) were defined. The solubilities of selenium for calcium selenites and calcium selenates are higher than the regulatory limit of industrial waste water for selenium, and so the removal of selenium by using calcium is unsuitable. The X-ray diffraction data for Ca2SeO3 (OH)2·2H2O, CaSe2O7, CaSeO4 and Ca2SeO4(OH)2 found in this work are shown in Tables 3 and 5, respectively. The conventional solubility product Table 7 Solubility constants for the two calcium selenates No

pH 2+

+ SeO2− 4 + 2H2O

(a) CaSeO4·2H2O(s) = Ca 13 0.61 14 0.84 15 1.36 16 1.74 17 2.59 18 6.61 19 6.92 20 8.44 21 11.83

log

c⁎

Kso

−1.79 −1.68 −1.39 −1.12 −0.85 −0.76 −0.77 −0.77 −0.83 Av.−1.11 ± 0.68

(b) Ca2SeO4(OH)2(s) + 2H+ = 2Ca2+ + SeO2− 4 + 2H2O 23 11.81 22.31 24 12.02 22.48 25 12.04 21.95 Av. 22.25 ± 0.30

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constants and conditional free energies of formation for the less soluble CaSeO3·H2O, Ca2SeO3(OH)2·2H2O, CaSeO4·2H2O and Ca2SeO4(OH)2 were calculated from the solubility data obtained. They are not standard state values because allowance for species of Ca(II)–Se(IV) or Ca(II)–Se(VI) complexes were not included in the thermodynamic calculation, however, they may be useful as conditional values. Acknowledgement The authors wish to express appreciation to Prof. Larry G. Twidwell for critical reading of the manuscript. References Chkhlantsev, V.G., 1956. Solubility products of the selenites of some metals. Zh. Neorg. Khim. 1, 2300–2305. Dumm, J.Q., Brown, P.W., 1997. Phase formation in the system CaO– SeO2–H2O. J. Am. Ceram. Soc. 80 (10), 2488–2494. Evert, M.H., 1981. Calcium selenites. Collect. Czechoslov. Chem. Commun. 46, 1740. Hata, R., Nishimura, T., Umetsu, Y., 2004. Solubility and stability regions of barium selenites and barium selenates in aqueous solution at 25 °C. Can. Metall. Q. 43, 57–65. Nishimura, T., Robins, R.G., 1996. Crystalline phases in the system Fe (III)–As(V)–H2O at 25 °C. In: Dutrizac, J.E., Harris, G.B. (Eds.),

Iron Control and Disposal, The Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, pp. 521–534. Nishimura, T., Umetsu, Y., Hata, R., 2000. Removal of selenium from industrial waste water. In: Young, C. (Ed.), Minor Elements 2000: Processing and Environmental Aspects of As, Sb, Se, Te, and Bi, SME, Littleton, pp. 355–362. Nishimura, T., Hata, R., Umetsu, Y., 2005. Phase equilibria in the Fe2O3–SeO2–H2O system. Hydrometallurgy 79, 110–120. Nishimura, T., Hashimoto, H., Nakayama, M., submitted for publication, Removal of selenium(VI) with polyamine-type weakly basic ion exchange resin. Sep. Sci. Technol. Olin, A., Nolang, B., Osadchii, E.G., Ohman, L.-O., Rosen, E., 2005. Chemical Thermodynamics of Selenium. Chem. Thermodyn., vol. 7. Elsevier, B.V., pp. 60. Quast, E., 1988. Mineralogisch-Petrographisches Institute, Univ. Heidelberg, West Germany, JCPDS grant-in-aid Report. Ripan, R., Vericeanu, G., 1968. Stud. Univ. Babes-Bolyai, Ser. Chem. 13, 31–37. Snyman, H.C., Pistorius, C.W.F.T., 1963. Some crystallographic properties of CaSO4 and its hydrates. Z. Kristallogr. 119, 151–154. Valkonen, J., Losoi, T., 1985. Structure of calcium selenite(IV) monohydrate, CaSeO3.H2O. Acta crystallogr. C41, 652–654. Valkonen, J., 1986. Crystal structures, infrared-spectra, and thermal behavior of calcium hydrogenselenite monohydrate, Ca(HSeO3)2·H2O, and dicalcium diselenite bis(hydrogenselenite), Ca2(HSeO3)2(Se2O5). J. Solid State Chem. 65, 363–369. Wagman, D.D.et al., 1982. The NBS Tables of Chemical Thermodynamic Properties, J. Phys. Chem. Ref. Data, 11, (Suppl. 2). pp. 28, pp. 61, pp. 267, pp. 270.