Oxidation Rate of Magnesium Sulfite Catalyzed by Cobalt Ions

Mar 3, 2014 - The general reaction orders with respect to cobalt, magnesium sulfite, and oxygen are 0.44, 0, and 0.46, respectively, and the apparent ...
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Oxidation Rate of Magnesium Sulfite Catalyzed by Cobalt Ions Li Qiangwei,† Wang Lidong,*,† Zhao Yi,† Ma Yongliang,*,‡ Cui Shuai,† Liu Shuang,† Xu Peiyao,† and Hao Jiming‡ †

School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China Department of Environmental Science and Engineering, Tsinghua University, Beijing 10084, China

‡

ABSTRACT: Oxidation of magnesium sulfite is important for recycle of byproduct in the magnesium desulfurization. The oxidation rate of magnesium sulfite, prepared by vacuum evaporation method, was investigated in a bubbling tank in presence of transition metal catalysts, which shows cobalt is the most effective. The general reaction orders with respect to cobalt, magnesium sulfite, and oxygen are 0.44, 0, and 0.46, respectively, and the apparent activity energy is 17.43 KJ·mol. The catalytic performance of cobalt compared with other metals was also analyzed employing the ion potential theory. Integrated with the threephase reaction model, we inferred that the general oxidation rate of magnesium sulfite is controlled by mass transfer of oxygen. Further, the intrinsic kinetics was predicted, indicating that the reaction orders with respect to cobalt and oxygen are 1.0 and 0, respectively. The results are helpful for the recycle of magnesium sulfite in magnesia desulfurization.

1. INTRODUCTION In China, coal constitutes about 70% of the total primary energy consumption.1 Magnesia desulfurization is an important pollution control technique, especially for the flue gas discharged from the industrial boilers.2 The magnesia hydrate is often used as an absorbent and reacts with SO2 into insoluble magnesium sulfite that could be further oxidized into magnesium sulfate, a soluble industrial material. 3 While in the practice, the desulfurization solution containing 10% of magnesium sulfate is usually abandoned to avoid the crystallizations and jams in the absorber, resulting in a serious waste of magnesium sulfate resources. Thus, some research has been carried out through evaporating and cooling the discharged solution to recycle the magnesium sulfate. However these attempts failed in many cases owing to the unsaturation of magnesium sulfate, with the solubility of about 45% at 40 °C,4 which will cause great energy consumption in the evaporating process. Another effective and feasible method for improving the concentration of magnesium sulfate is to oxidize the magnesium sulfite, which is separated and supplied into the discharged desulfurization solution outside the absorber. Nevertheless, the oxidation rate of magnesium sulfite is comparatively slow, which leads to a long operation time to increase the sulfate concentration, and the energy consumption is enlarged as well. Thus, it is much more important to improve the oxidation rate of magnesium sulfite effectively in the magnesia desulfurization process. The kinetics of sulfite oxidation has attracted much attention since it is a key industrial step in the desulfurization, especially for calcium sulfite,5,6 sodium sulfite,7 and ammonium sulfite.8 The published literatures indicated that the oxidation rate of sulfite is affected by pH, temperature, dissolved oxygen, sulfite, light, and © 2014 American Chemical Society

impurities, in which the catalytic materials involved have the most significant effect.9 As for the oxidation system of calcium sulfite, the catalytic effect of manganese ions has been investigated by Du,10 and the kinetic parameters were achieved. Further, Bronlkowska11 explained the oxidation mechanism of calcium sulfite catalyzed by manganese with employing a kinetic model. The results obtained by Semler12 also have proved the catalytic effect of ferrous ions. Moreover, we have investigated the catalytic effect of organic acids, in which the kinetic parameters were achieved in presence of peracetic acid catalyst.13 In terms of the oxidation kinetics of sodium sulfite, the catalytic effect of cupric was proved by Barron.14 Zhao9 also investigated the catalytic effect of cobalt ions. In further, The experimental results from Coichev15 and Lan16 showed a remarkable effect of manganese ions. Moreover, Zuo et al.17,18 reported that the oxidation process is accelerated by ferrous ions. As for the oxidation kinetics of ammonium sulfite, Zhou19 studied the catalytic effect of cobalt. Das20 also investigated the catalytic effect of cuprous and silver. In brief, the oxidation of sulfite is considered to be a chain reaction, which was accelerated by the initiation of catalysts. In terms of magnesium sulfite oxidation system, Shen et al.21 have identified the uncatalyzed oxidation kinetics, as well as the kinetics inhibited22 by thiosulfate. However, little information is available on the catalytic oxidation characteristics of magnesium sulfite yet, although there is great difference in the physical and chemical properties with other sulfites. Furthermore, the Received: Revised: Accepted: Published: 4145

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Table 1. Detection of the Components of Magnesium Sulfite Samples by XRF components contents, %

MgO 34.75

P2O5 0.02

Na2O 0.58

SiO2 0.09

SO3 64.17

Cl 0.30

Al2O3 0.09

there is a mass loss of about 45.96% in terms of the hydrated magnesium sulfite owing to the water mainly derived with the rise of temperature from 85 to 200 °C. In spite of the impurities in Table 1, the ratio of MgSO3·3H2O to MgSO3·6H2O, as the main components, were estimated to be about (3:7) by calculation of material balance. Furthermore there is another mass loss within the temperature range 320−480 °C, being due to the decomposition of MgSO3. Finally, 26.8% of the prepared samples, mainly being MgO, were left as a residual. 2.2. Experimental Method and Apparatus. A stirred tank reactor with bubbling, as shown in Figure 3, was used to study the oxidation rate of magnesium sulfite under the following conditions: initial cobalt concentration of 2.22 × 10−4∼1.78 × 10−3 mol·L−1, initial magnesium sulfite concentration of 0.142− 0.568 mol·L−1, oxygen partial pressure of 0.05−1.00 atm, pH 7.0−9.0, temperature of 298−328 K, gas flow rate of 30−120 L·h−1, rotation speed of 860 rpm. After 200 mL of deionized water was added, a specified quantity of catalyst solution was supplied into the reactor. The pure nitrogen, oxygen, and air were blended in a specified ratio by flow adjustment and injected into the reactor as oxidation gas. Then, the reaction was started at the same time as a specified amount of magnesium sulfite was added to the reactor, which dissolved quickly into saturation within 30 s. The hydrochloric acid and sodium hydroxide solution was employed to adjust the pH value. A trace amount of reaction solution was taken out by pipet at intervals and dissolved by hydrochloric acid, which was then diluted to the desired volume. The modification of solution volume was about 1−2% in the reaction process, which had little effect on the experimental results. The concentration of sulfate at different points of time, cS(VI),t, was determined by barium sulfate spectrophotometry. Under the given conditions, the results indicate that the sulfate concentration increases linearly with the rise of reaction time. Thus, the slope, k, is the oxidation rate of magnesium sulfite that signifies the relationship between cS(VI),t and reaction time.

published results show that the oxidation kinetics in terms of different kinds of sulfite could not been applied in each other, which might be due to the selective catalytic effect of the metals. Thus, it is much more important to reveal the catalytic oxidation kinetics, especially for magnesium sulfite, to meet the developing requirement of magnesia desulfurization.

2. EXPERIMENTAL SECTION 2.1. Preparation and Testing of Magnesium Sulfite Samples. Magnesium sulfite should be prepared on site due to its limited stability. To avoid the effects of impurities on the experimental results, the magnesium sulfite samples were prepared by vacuum evaporation method.23 Saturated solutions of sodium sulfite and magnesium chloride were prepared respectively and blended. After vigorous agitation and separation, the precipitate of magnesium sulfite was washed and shifted into the vacuum rotary evaporator (SENCO, type R201L). The vacuum evaporation conditions were as follows: 333 K, relative pressure −0.095 MPa, evaporation time 3 h. The components of magnesium sulfite samples were analyzed with wavelength-scanning X-ray fluorescence (type XRF-1700). The results are shown in Table 1. The content of magnesium ions was determined by EDTA titration, and sulfate was measured by barium sulfate spectrophotometry from beginning to end of the complete oxidation process. Taking into account oxidation of sulfite during storage, the content of hydrate magnesium sulfite is about 99%. There might be four types of hydrate magnesium sulfite according to XRD Pattern Processing and Identication software (Jade 5.0, Material Data Inc.), including of MgSO3·H2O, MgSO3· 2H2 O, MgSO3·3H2O, and MgSO3·6H2O. The prepared magnesium sulfite sample was identified by X-ray diffraction (type Bruker D8 Advance. The results in Figure 1, comparing

3. RESULTS 3.1. Comparison of Catalytic Effect of the Transition Metals. As mentioned in section 2.2, the catalytic effect of five transition metals, including Ni2+, Cu2+, Mn2+, Fe2+, and Co2+, were compared respectively with the uninhibited under the following condition: initial concentration of magnesium sulfite of 0.284 mol·L−1, oxygen partial pressure of 0.21 atm, gas flow rate of 60 L·h−1, pH 7.0, 318 K. It was noted that the initial concentration of other metal ions was 1.78 × 10−3 mol·L−1, except for Co2+ being 4.45 × 10−4 mol·L−1. The results in Figure 4 showed that Cu2+ and Ni2+ have little effect on the oxidation rate of magnesium sulfite, and the catalytic effect of Mn2+ and Fe2+ is confined yet. However, the four catalysts had been reported to be effective in the given concentration.24 In comparison, Co2+ has significant accelerating effect on the oxidation rate although its concentration is four times as small as the other metals. Thus, the order of catalytic activity is Co2+ > Fe2+ > Mn2+ > Ni2+ > Cu2+. 3.2. Effect of Cobalt Concentration on the Oxidation Rate of Magnesium Sulfite. Under the same operation conditions as detailed in Section 3.1, the initial concentration of

Figure 1. XRD analysis of magnesium sulfite samples.

with the standard pattern of hydrate magnesium sulfite, indicated that MgSO3·3H2O and MgSO3·6H2O coexists in the prepared sample, while there is no other hydrate crystal being found. We also detected the sulfite sample by thermogravimetric analysis (type Netzsch STA 449F3). It showed in Figure 2 that 4146

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Figure 2. TG analysis of magnesium sulfite samples.

Figure 3. Apparatus of catalyzed oxidation of magnesium sulfite 1-pure nitrogen, 2-pure oxygen, 3-TYW-1 air compressor, 4-buffering bottle, 5decompression valve, 6-LZB-4 glass rotameter, 7-85-2A magnetic stirrer, 8-glass reactor in volume of 500 mL, 9-thermocouple, 10-PHS-3C pH meter, 11-hydrochloric acid solution in concentration of 3 mol·L−1, 12-sodium hydroxide solution in concentration of 1 mol·L−1.

Figure 4. Catalytic effect of five transition metals on the oxidation rate of magnesium sulfite.

Figure 5. Effect of Co2+ concentration on the oxidation rate of magnesium sulfite.

Co2+ was varied between 2.22 × 10−4 mol·L−1, 4.45 × 10−4 mol·L−1, 8.90 × 10−4 mol·L−1, and 1.78 × 10−3 mol·L−1, respectively. Its effect on the oxidation rate is shown in Figure 5, indicating that the oxidation rate would increase greatly with the rise of Co2+

concentration. The concentration of Co2+ and the reaction rate were made dimensionless with respect to their initial values, and the results in Figure 6 indicate that the reaction order with respect to Co2+ is 0.44 in the intrinsic oxidation. 4147

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Figure 8. Effect of oxygen partial pressure on the oxidation rate of magnesium sulfite.

Figure 6. Reaction order of Co2+ with respect to the oxidation rate of magnesium sulfite.

3.3. Effect of Magnesium Sulfite Concentration on the Oxidation Rate. Under the same operation conditions as described in section 3.1, and an initial Co2+ of 4.45 × 10−4 mol·L−1 was applied hereafter unless otherwise specified, the initial concentration of magnesium sulfite was varied among 0.142 mol·L−1, 0.284 mol·L−1, and 0.568 mol·L−1, respectively. Its effect on the reaction rate is illustrated in Figure 7, indicating that the slopes of

Figure 9. Reaction order of oxygen with respect to the oxidation rate of magnesium sulfite.

Figure 7. Effect of magnesium sulfite concentration on the oxidation rate.

regression curves are almost the same. Thus, we inferred the concentration of magnesium sulfite is zero in the oxidation rate. 3.4. Effect of Oxygen Partial Pressure on the Oxidation Rate. The oxygen partial pressure was varied between 0.05 atm, 0.10 atm, 0.21 atm, 0.50 atm, and 1.00 atm, respectively. Its effect on the reaction rate is illustrated in Figure 8, showing that the reaction rate would increase with the rising oxygen partial pressure. According to Henry’s Law, the oxygen partial pressure is proportional to the equilibrium oxygen concentration at the gas− liquid interface. The reaction rate and oxygen partial pressure were normalized with respect to the initial values and the reaction order with respect to oxygen, shown in Figure 9, was acquired as 0.46. 3.5. Effect of Gas Flow Rate on the Oxidation Rate of Magnesium Sulfite. The gas flow rate was varied between 30 L·h−1, 60 L·h−1, 90 L·h−1, and 120 L·h−1, respectively. Its effect

Figure 10. Effect of gas flow rate on the oxidation rate of magnesium sulfite.

on the reaction rate is illustrated in Figure 10, indicating that the oxidation rate would increase with the rise of gas flow rate. 3.6. Effect of pH on the Oxidation Rate of Magnesium Sulfite. The pH value was varied between 7.0, 7.5, 8.0, 8.5, and 4148

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known as a multiphase reaction, in which the deduction of the rate controlling steps is also significant for the industrial application. 4.1. Catalytic Activity of Co2+ in the Oxidation of Magnesium Sulfite. It was assumed that the crystalline hydrate of MgSO3·xH2O consisted of H2O and the unit cells of MgSO3, in which the ions of magnesium and sulfite arrays at regular intervals. Under the given conditions, MgSO3·xH2O on the surface dissolved into the slurry prior to the inner layers and was oxidized into soluble sulfate, resulting in the stripping away from the crystal surface. In further, the newly exposed MgSO3·xH2O was oxidized. As a result, the MgSO3·xH2O crystal shrink continuously until it is oxidized completely by the dissolved oxygen. We also supposed that the magnesium ions might be replaced by the catalysts due to the similar ionic potential,26 such as cobalt ions. The substituting process, which would cause the generation of CoSO3 unit cells, was shown in Figure 13. Compared of Mg2+ with 2S22P6 outmost electron configurations, the Co2+ with 3d7 electron configurations could accommodate the electron delocalized away from SO32−, resulting a electron hole in SO32−. Thus the free radicals of ·SO3− are produced that initiate the chain reaction as follows.11,27

Figure 11. Effect of pH on the oxidation rate of magnesium sulfite.

9.0 respectively. Its effect on the oxidation rate is shown in Figure 11, indicating that the oxidation rate would increase with the rise of pH from 7.0 to 8.5, and decrease with the rise of pH from pH 8.5 to 9.0. This tendency is somewhat alike with the phenomena observed by Vidal25 in the seawater desulfurization. Overall, the oxidation rate at pH 8.5 was the highest. It might be that the catalytic activity of Co2+ is remarkable at the high value. However, it will precipitate with the rise of hydroxyl ions, resulting in a decrease of the catalytic effect. 3.7. Effect of Temperature on the Oxidation Rate of Magnesium Sulfite. The temperature was varied among 298 K, 308 K, 318 K, and 328 K. Its effect on the reaction rate is shown in Figure 12, indicating that the reaction rate will increase with the

chain initiation chain propagation

SO32 − + Co2 + → •SO−3 + Co+ •SO−3

+ O2 →

•SO−5

•SO−5 + SO32 − → SO52 − + •SO−3

(1) (2) (3)

product formation SO52 − + SO32 − → 2SO24 −

(4)

•SO−5

(5)

chain termination

+

•SO−5

→ inner product

It has been reported that most of the transition metals has catalytic effect on the oxidation of sulfite, such as Co2+, Fe2+, Mn2+, Ni2+, and Cu2+. The ionic potential of Mg2+ and other transition metals are shown in Table 2.28 Because the ionic potential of Mn2+ is different with Mg2+ in the oxidation system of magnesium sulfite, it might be difficult for Mn2+ to approach and substitute for Mg2+ owing to the steric hindrance, which results in the failure in initiation of chain reaction. We also found that Fe2+ has less catalytic effect on the oxidation of magnesium sulfite. The slurry was observed to turn to red which might be due to the prompt oxidation of Fe2+ into Fe3+ by the dissolved oxygen while Fe2+ was added. Because there are great difference in the ionic potential and valence states between Fe3+ and Mg2+, it is difficult for Fe3+ to substitute to Mg2+ and enter into the unit cells of MgSO3. Thus, the chain reaction could not be initiated as well. Compared with Co2+ with 3d7 outmost electron configurations, Ni2+ is much more stable due to its 3d8 electron configurations. Thus, Ni2+ is inactive in accelerating the oxidation process of magnesium sulfite. Likely, Cu2+ has 3d9 electron configurations, which is more stable in the solution owing to the Jahn−Teller effect. As a result, Co2+ has the most significant catalytic effect on the oxidation rate of magnesium sulfite than any other metals. 4.2. Kinetics of Catalyzed Oxidation of Magnesium Sulfite. The oxidation process in terms of magnesium sulfite comprises three steps, including an intrinsic chemical reaction between sulfite and dissolved oxygen in the liquid with the rate of RA, dissolution of magnesium sulfite from the solid into the liquid with the rate of RB, mass transfer of oxygen from the gas into the liquid with the rate of RC. The intrinsic chemical reaction takes place among the sulfite ions, dissolved oxygen, and cobalt ions. Although the kinetics of

Figure 12. Effect of temperature on the oxidation rate of magnesium sulfite.

rising temperature. Temperature dependency is in accordance with Arrhenius law and the apparent activation energy was obtained to be 17.43 kJ·mol−1.

4. DISCUSSION The catalytic effect of cobalt ions on the oxidation rate of magnesium sulfite seems much more obvious than other transition metals provided in this paper. Thus, it becomes important to reveal the mechanism of selective catalytic activity in terms of cobalt in the magnesium sulfite oxidation system. Furthermore, the general oxidation of magnesium sulfite is 4149

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Figure 13. Catalytic pathway of cobalt in the oxidation of magnesium sulfite.

Table 2. Ionic Potential of Some Metals ionic radius, pm ionic potential

Mg2+

Co2+

Mn2+

Fe2+

Fe3+

Ni2+

Cu2+

65 0.0308

74 0.0270

80 0.0250

76 0.0263

64 0.0469

72 0.0278

72 0.0278

Taking the above three steps into account, the general catalytic oxidation of magnesium sulfite is controlled by the slowest step, which is expressed as follows:

sulfite oxidation catalyzed by cobalt ions has been widely investigated,29 the published results are yet inconsistent.19 The intrinsic reaction rate was expressed as follows: l n RA = kcOm2cCo cS(IV)

R = min(RA , RB , R C)

(6)

Furthermore, the dissolution rate of magnesium sulfite can be expressed by Noyes−Whitney equation30 as follows: RB =

∂c = k pSn(cSat − cS(IV)) ∂t

We assumed that the general oxidation is controlled by the intrinsic reaction rate. Thus, the flow rate will have little effect on the general oxidation rate, which is in contradiction with the results in Figure 10. In addition, the temperature should affect the general oxidation rate obviously due to the controlling step of intrinsic reaction. However, the apparent activity energy is calculated to be 17.43 KJ·mol according to Figure 12, indicating that the general oxidation rate is insensitive to the reaction temperature. As a result, we believe that the intrinsic reaction is not the controlling step. Supposing that the dissolution rate of magnesium sulfite is the controlling step, the general reaction rate then will not be affected by the cobalt concentration or the oxygen partial pressure, which disagree with the experimental results in Figures 5 and 8. Thus, the general reaction is neither controlled by the dissolution of magnesium sulfite. In conclusion, the only possibility is that the general reaction is controlled by the mass transfer of oxygen. It also indicates that the intrinsic reaction might be much faster than the mass transfer of oxygen. Compared with the uncatalyzed reaction, the presence

(7)

In addition, the diffusion rate of oxygen and Hatta number M are expressed as follows: R C = kLcO2iE = kLcO2i(1 + M )0.5 2 D k c m − 1c l c n 2 O2 in O2 i Co S(IV) (m + 1)kL 2 = DO RA (m + 1)kL2cO2i 2

(10)

(8)

M=

(9)

Consequently for M ≤ 0.3, the intrinsic reaction is slow31 and E = 1; for 0.3 < M < 3, the intrinsic reaction rate is moderate and E = M /tanh M ; for M ≥ 3, the intrinsic reaction rate is fast and E = M . 4150

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E

of cobalt helps to improve the oxidation rate by over three times, indicating that the Hatta value is over 3. According to the mass transfer theory, the general catalyzed oxidation of magnesium sulfite is a mass transfer process accompanied with rapid chemical reaction. Thus, the mass transfer rate of oxygen and the Hatta value are expressed as follows: R = R C = kLcO2i M =

k kin kL kp l

2 l n DO2k incOm2+i 1cCo cS(IV) (m + 1) (11)

m

Further, we attempted to obtain the intrinsic reaction orders from the macroscopical oxidation results. The reaction order with respect to cobalt in the intrinsic reaction is convinced to be about 1.0, being consistent with Shaikh’s report,32 which becomes to be 0.5 in the general reaction that agrees well with Figure 6. Likely, the reaction order with respect to oxygen is zero in the intrinsic reaction, being in consistent with the published literatures.14,32 Thus, the reaction order with respect to the oxygen partial pressure is 0.5 in the macroscopical reaction, which agrees well with Figure 9. Moreover, the concentration of magnesium sulfite had little effect because it kept saturated. From the point of view of the industrial process, it would be desirable using catalyst to reduce the oxidation time of magnesium sulfite. Thus, the operation area or oxidation period, provided that the cobalt concentration being 4.45 × 10−4 mol· L−1, will be 5 times less than the uncatalyzed condition. Although the cobalt will leach out with the magnesium sulfate product, which should be supplied continuously with the investment of about $1.5 per ton of oxidation slurry, the earning of recycled magnesium sulfate will greatly overweigh the additional operation cost. Nevertheless, the cobalt might enter into the magnesium sulfate crystal and cause secondary pollution, hampering application in the magnesia desulfurization. Thus, it is necessary to develop new supported technology in order to meet the recycling requirement of magnesium sulfate.

■

n RA RB RC Sn

■

enhanced factor of oxygen diffusion from gas into the solution slope of sulfate concentration vs reaction time which represents the oxidation rate of magnesium sulfite coefficient of intrinsic reaction rate of sulfite oxidation catalyzed by cobalt mass transfer coefficient of oxygen in the liquid film dissolution rate constant of magnesium sulfite reaction orders with respect to cobalt ions in the intrinsic chemical reaction reaction orders with respect to oxygen in the intrinsic chemical reaction reaction orders with respect to sulfite in the intrinsic chemical reaction intrinsic reaction rate between sulfite and dissolved oxygen in the liquid dissolution rate of magnesium sulfite from solid into liquid diffusion rate of oxygen from gas into liquid surface area of the magnesium sulfite particles

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 312 7522023; 86-10-62771101. E-mail: wld@ tsinghua.edu.cn; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by the National Natural Science Foundation of China (No. 51178184, No. 51378204), the Program for New Century Excellent Talents in University (No. NECT-12-0847), and the Fundamental Research Funds for the Central Universities (No. 10MG35).

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NOMENCLATURE cCo concentration of cobalt ions in the reaction solution cO2 concentration of dissolved oxygen in the reaction solution cO2i equilibrium concentration of oxygen at the phase interfere cS(IV) concentration of sulfite in the reaction solution cS(VI),t concentration of sulfate at any point of time in the reaction solution cSat saturation concentration of magnesium sulfite in the reaction solution DO2 diffusion coefficient of oxygen in the liquid phase 4151

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dx.doi.org/10.1021/es404872w | Environ. Sci. Technol. 2014, 48, 4145−4152