Thermal and Kinetic Analysis of the Process of Thermochemical

Article pubs.acs.org/IECR

Thermal and Kinetic Analysis of the Process of Thermochemical Decomposition of Phosphogypsum with CO and Additives Liping Ma,* Yalei Du, Xuekui Niu, Shaocong Zheng, and Wei Zhang Kunming University of Science and Technology, KunmingYunnan, P.R. China 650093 S Supporting Information *

ABSTRACT: Phosphogypsum (PG) is a waste byproduct from the processing of phosphate rock by the ‘‘wet acid method’’ of fertilizer production. One of the main methods for reusing PG is to decompose and recycle Ca and sulfur contained in it. However, the decomposition reaction process is very complex because of its complicated contents, and very high temperature is needed for the reaction. In this paper, to decrease the reaction temperature, CO as a main reducer and some additives were added in the decomposition process. Results show that the decomposition temperature will decrease from 1000 to 809 °C with pure CO. When CaCl2 is used as an additive, the decomposition temperature can decrease to 790 °C, and at the same time the reaction rate will be increased, the main product being CaS at this condition. The thermal and kinetic action of this process has also been discussed.

1. INTRODUCTION Phosphogypsum (PG) is a waste byproduct from the processing of phosphate rock by the ‘‘wet acid method’’ of fertilizer production, which currently accounts for over 90% of phosphoric acid production. The wet process phosphoric acid treatment process, or “wet process”, is widely used to produce phosphoric acid and calcium sulfatemainly in dehydrate form (CaSO4·2H2O):

ability of 40kt/a sulfuric acid and 60kt/a cement. The main equipment used in the manufacture was rotary kiln.12−16 The main component in phosphogypsum is CaSO4·2H2O (calcium sulfate dihydrate), or CaSO4·1/2H2O (calcium sulfate hemihydrate), and CaSO4 (calcium sulfate anhydrite). The phosphogypsum may react as follows in a reduction atmosphere:

Ca5F(PO4 )3 + 5H 2SO4 + 10H 2O → 3H3 PO4 + 5CaSO4 · 2H 2O + HF

(1)

Over 30 million tons of phosphogypsum is produced per annum in China and less than 10% has been reused.1−3 By the end of 2008, the pile stock of phosphogypsum was almost more than 100 Mt,4 which causes various environmental and storage problems. During the last several years of the past century and until now, many researchers have devoted efforts to find methods of utilization of waste phophogypsum. One of these methods is to manufacture PG as a soil regulator or fertilizer,5−8 the other is applied in building materials, such as making plasterboard, land plaster, brick, etc.9−11 Having the advantage of being able to recycle calcium and sulfur resources in phosphogypsum and having a large capacity for treatment, decomposition of phosphogypsum for use as sulfuric acid and cement production was proposed by British and Austria during 1967−1969, and a pilot scale plant (240t/d sulfuric acid and cement, respectively) had been built by the OSW-KRUP Company in 1969. A 350t/d production line was also put into production in South Africa in 1972. In China, over the last century, there was a great development of sulfuric acid and cement production through phosphogypsum decomposition: seven sets of product lines had been built by many companies, such as Lubei Company, Luxi Chemical Engineering Company, Qindao Dongfang Chemical Engineering Limit Company etc., each having the product © 2012 American Chemical Society

CaSO4 + C → CaO + CO ↑ + SO2 ↑

(2)

CaSO4 + 4C → CaS + 4CO

(3)

CaSO4 + CO ↑ → CaO + SO2 ↑ +CO2 ↑

(4)

CaSO4 + 4CO ↑ → CaS + 4CO2 ↑

(5)

3CaSO4 + CaS → 4CaO + 4SO2 ↑

(6)

Besides the main component, there are more than 10 other components coming from phosphate rock contained in PG, which makes the process of the decomposition reaction very complex, and the reaction temperature is from 1000 to 1300 °C17−20 it needs almost more than 1300 °C in a rotary kiln.12 How to decrease the reaction temperature and cut down the energy cost are the main problems in reusing and recycling PG for sulfuric acid and cement production. Some common conditions for the decomposition process had been explored, such as adding Fe2O3, ZnO, and SiO2 as additives for the reaction.21−23 However, different reaction conditions could get the different products because of the complex components, which is the main factor of how the solid product is further used. In previous research of our group, the decomposition process of phosphogypsum in a nitrogen atmosphere at different Received: Revised: Accepted: Published: 6680

December 20, 2011 April 29, 2012 April 29, 2012 April 30, 2012 dx.doi.org/10.1021/ie2029859 | Ind. Eng. Chem. Res. 2012, 51, 6680−6685

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Composition of PG (%) composition content (%)

SO3 40.86

CaO 29.82

SiO2 9.43

Al2O3 0.236

Fe2O3 0.132

MgO 0.055

total P2O5 1.17

hydrotropic P2O5 0.87

composition content (%)

total F 0.52

hydrotropic F 0.12

Na2O 0.043

K2O 0.086

MnO 0.002

free water 5.38

crystal water 4.27

acid-insoluble material 7.006

Figure 1. Thermodynamic equilibrium composition of the system of CO and CaSO4: (left, condition a) CaSO4:CO = 1:2 and (right, condition b) CaSO4:CO = 5:1.

conditions, with the use of high sulfur concentration coal as a reducer, was studied,24−27 For this study, the decomposition temperature needed to be more than 1000 °C. To decrease the decomposition temperature, some exploring experiments with additives were performed.28 In this paper, CO was used as a reducer for the decomposition of PG and some additives were added in this process to investigate how the reaction was affected. The thermal and kinetic actions of the decomposition reaction were analyzed, and the reaction mechanism of these processes were discussed.

2. EXPERIMENT 2.1. Sample Prepared. Phosphogypsum sample used in this study comes from Yunnan Natural Gas and Chemical Engineering Company, the compounds are listed in Table 1. After drying and filtration, the size of the phosphogypsum sample is about 0.074 mm. 2.2. Experiment Equipments. In this study, XXWRT-2C TGA (Beijing Optician Plant) and SK2 tube resistance stove (Tianjing Zhonghuan Experiment Stove Company, Ltd.) were used for thermogravimetric analysis and the decomposition experiment. The gas components were analyzed by KM9106 complex fuel gas analyzer (KANE Company, British), the solid products were analyzed by D/max-3BPEX-P96 X-ray powder diffraction (Japan) 2.3. TGA Analysis. Thermogravimetric analysis was taken on XXWRT-2C TGA. The mass of phosphogypsum sample was between 10 and 20 mg, CO was used as an reducer, and nitrogen was used as a carrier, with a flow rate of 10 mL/min at different heating rate β(5, 10, 15, 20, 40 °C/min, respectively). Coal was used as a reducer for comparison; at this time phosphogypsum sample and coal were mixed with a definite proportion with mass of between 10 and 20 mg.

Figure 2. The TG/DTG curves for the system of CO and phosphogypsum at different heating rates.

with CO at different conditions is shown in Figure 1, in which the balance relationship of different compounds could be found. At 27 °C (300 K), CO2 and CaS appeared in both reaction conditions. In condition “a” (Figure 1, left panel) CO was almost reacted completely; however there is residue in condition “b” (Figure 1, right panel) from the reaction balance compounds. According to the theory of atoms conservation and the proportion of different compounds in Figure 1, it could be presumed that the reaction taking place in these two conditions may be as follows: CaSO4 + 4CO = CaS + 4CO2

3. RESULTS AND DISCUSSION 3.1. Thermal Theory Analysis for PG Decomposition with CO. The thermal balance analysis for PG decomposition

(5)

When the temperature is higher than 507 °C (780 K), in condition a, the amount of CaO and SO2 increased with the 6681

dx.doi.org/10.1021/ie2029859 | Ind. Eng. Chem. Res. 2012, 51, 6680−6685


Article

temperature, and the amount of CaSO4 and CaS decreased. The reaction may take place as follows: 3CaSO4 + CaS = 4CaO + 4SO2

From Figure 1 it is clear that when the reactant CaSO4 is excessive, there are only three kind of reactions that take place from 27 °C (300 K) to 1227 °C (1500 K). However, when the reactant CO is excessive, there are several kinds of reactions that take place at the same condition, other products such as CaCO3 appeared and the decomposition reaction of CaSO3 also took place. Therefore, it could be conjectured that the main reactions taking place between 27 and 1227 °C are reactions 5 and 6, and the reaction 5 takes place first. 3.2. TG/DTG Analysis for PG Decomposition with CO. TG/DTG analysis for PG decomposition with CO as a reducer at different heating rate β (β = 5, 10, 15, 20, 40 K·min−1) were shown in Figure 2. There is only one obvious mass loss step for the decomposition process; the mass loss mainly appears at the temperature range between 777 °C (1050 K) and 977 °C (1250 K), and this temperature range increased with the increasing of β and moved toward the high temperature area. For comparison, TGA analysis for pure PG and pure gypsum are shown in Figure 3. There are two main mass loss steps. The first took place at 150 °C caused by water loss, and the second took place at 1000 °C for PG, which is lower than the decomposition temperature (about 1250 °C) of pure gypsum. The TGA analysis also indicated that the decomposition reaction for pure gypsum is more complete than that of pure PG at this

(6)

Otherwise, at this condition, the amount of CaCO3 and CaSO3 increased with the temperature. When the temperature reached 927 °C (1200 K), the amount of CaCO3 decreased, and the amount of CaSO3 also decreased when the temperature reached 1107 °C (1380 K); therefore some reactions may take place at 507 °C as follows: CaO + CO2 = CaCO3

(7)

CaO + SO2 = CaSO3

(8)

And when the temperature is in the range of 927−1107 °C, there are some reactions that may take place in condition a: CaCO3 = CaO + CO2

(9)

CaSO3 = CaO + SO2

(10)

In condition b, less CO existed at 1107 °C, according to the theory of atoms conservation. The reaction may take place at this temperature as follows: CaS + 3CO2 = CaO + SO2 + 3CO

(11)

Figure 3. TG-DTA curves of pure gypsum and phosphogypsum.

Table 2. Samples and Reaction Conditions for Catalytic Reduction Decomposition sample A B C D E F G H I

catalyst CaCl2 Fe2O3 Fe2O3 + CaCl2 C C + CaCl2 C + Fe2O3 C + Fe2O3 + CaCl2

atmosphere

heating rate β (°C/min)

mass proportion (PG: catalyst)

initial decomposition temperature (°C)

mass loss (wt %)

CO CO CO CO N2 N2 N2 N2 N2

10 10 10 10 10 10 10 10 10

100:0 100:5 100:5 100:5 pure CaSO4·2H2O 16:1 100:6.25:5 100:6.25:5 100:6.25:2.5:2.5

809 790 810 790 990 563 515 515 559

38.74 40.64 39.60 41.36 25.17 7.66 5.95 4.18 5.27

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Figure 4. The TG/DTG curves for the decomposition of phosphogypsum with different addition.

Figure 6. XRD analysis for the decomposition production of PG with CO at β = 10 K·min−1.

the mass loss is also low. It is only 5.95%, which means the decomposition reaction is incomplete. The XRD analysis for the solid product of PG decomposition with CO at 10 °C/min heating rate is shown in Figure 6, and Figure 7 shows the XRD analysis for the solid product at the conditions of sample B (Table 2). It is clear that the main production of PG decomposition with CO is CaS, an amount of reactant CaSO4 also exists at the condition shown in Figure 6. However, the reaction is more complete when CaCl2 is added; almost all the reactant CaSO4 decomposed, and CaS is the main product (Figure.7). It is coherent with thermal analysis that reaction 5 takes place first. 3.4. Kinetic Analysis for PG Decomposition with Additive CaCl2 at CO Atmosphere. The kinetic model for a reaction is important for industry designing. For the kinetic analysis of a solid-state reaction, the nonisothermal FWO, KAS, and Friedman methods have been widely used in calculating the activation energy, because these methods could directly calculate the activation energy without an exact dynamic model.29−32 Therefore, the FWO method was used to calculate the activation energy E of the decomposition reaction at CO atmosphere with additional 5% CaCl2. The Ozawa equation is as follows:

Figure 5. The TG/DTG curves for the decomposition of phosphogypsum with different addition.

condition. A comparison of Figure 2 and Figure 3 shows that it is clear that the decomposition temperature is lower with CO as the reducer. (The mechanism of the decomposition process was analyzed in the previous section.) 3.3. TG/DTG Analysis for PG Decomposition with Additives. To investigate the effect of the catalyst for the decomposition temperature of PG, different additives were used in the decomposition process of PG. The samples proportions and reaction conditions are shown in Table 2. TG/ DTG analysis results are shown in Figure 4 and Figure 5. Table 2 also gave the decomposition temperature and mass loss for the samples decomposition. From the analysis of TG/DTG for sample A and F (Figure 4 and Figure 5) it could be concluded that the decomposition reaction is more complete with CO as a reducer than that in N2 atmosphere and carbon as a reducer. The mass loss is 38.74% in CO and it is only 7.66% with carbon (Table2, sample F). Additives could affect the decomposition temperature and mass loss. At the condition of sample B (CO as a reducer with addition 5% CaCl2), the initial decomposition temperature will be 790 °C and the mass loss will arrive at 40.64%. The mass loss is 41.36% with the addition 5% Fe2O3 + CaCl2 at the same condition with the initial decomposition temperature also at 790 °C. At N2 atmosphere, the initial decomposition temperature is low with C + CaCl2; however,

log = log[AE /RG(α)] − 2.315 − 0.4567E /(RT )

(12)

where β is the heating rate, °C/min; A is the pre-exponential factor, s; E is the activation energy, kJ/mol; α is the reaction conversion; G(α) is the dynamic model function; and R is the gas constant, 8.314J/mol·K. At the same conversion α, G(α) is fixed, therefore log(β) is linear with T−1 and the activation energy E could be obtained from the slope. The calculated results for the samples of A and B at different heating rate β are listed in Table 3 and the activation energy E for these two reactions are shown in Figure 8. For the PG decomposition with CaCl2 additive, the activation energy E is between 150 and 200 kJ/mol from the model calculations, while it is between 300 and 400 kJ/mol for the pure PG decomposition reaction. This indicates that the additive has the ability to act as a catalyzer which could decrease the activation energy of decomposition reaction and make the reaction take place more easily than that with the pure PG sample. 6683



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Figure 7. XRD analysis for the decomposition production of PG with CO + CaCl2 at β = 10 K·min−1.

■

Table 3. The Linear Fiting Results of log β and T−1 for the Reaction 5% CaCl2

* Supporting Information The simple theory proposed in the paper; all the tables and original TG/DTA analysis reports for the samples in Table 2; some figures and XRD analysis studied in this work. This material is available free of charge via the Internet at http:// pubs.acs.org.

pure PG

α%

predictive residual square

R value

predictive residual square

R value

10 20 30 40 50 60 70 80 90

0.004 2 0.007 8 0.002 9 0.004 0 0.002 0 0.006 1 0.008 6 0.009 7 0.009 1

0.967 4 0.939 1 0.977 7 0.989 2 0.984 5 0.952 7 0.933 0 0.924 4 0.929 2

0.091 3 0.037 6 0.018 3 0.007 8 0.004 2 0.001 4 0.000 3 0.001 3 0.005 1

0.290 5 0.708 1 0.857 9 0.939 4 0.967 0 0.989 5 0.997 6 0.990 0 0.959 2

ASSOCIATED CONTENT

S

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-871-5170905. Fax:86-871-5170906. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support for this project was provided by National High Technology Research and Development Plan (863 of China, 2011AA06A106) and Natural Science Foundation of China (21176108), which is greatly acknowledged.

■ Figure 8. The relationship between apparent activation energy (Eapp) and the reaction conversion (α) for the reaction: (A) pure PG; (B) 5% CaCl2+CO.

4. CONCLUSION The decomposition process of PG with pure CO needs a longer reaction time, and the initial decomposition temperature is about 809 °C. The main solid product is CaS. When the additive CaCl2 is used, the decomposition rate and mass loss are all increased, and the content of main product CaS is also increased. The additive CaCl2 acts as a catalyst to decrease the decomposition temperature from 809 to 790 °C. Further research will be taken to investigate the function of CaCl2 in the decomposition reaction.

NOMENCLATURE A = constant E = reaction active energy f(α) = differential form of reaction mechanism function G(α) = integral form of reaction mechanism function R = gas constant T = temperature of reaction, °C t = times, s

Greek Letters

■

a = the conversion of reactant β = the rate of temperature increasing, °C·min−1

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Thermal and Kinetic Analysis of the Process of Thermochemical

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