Volatilization Behavior of Fluorine in Fluoroborate Residue during

Article pubs.acs.org/est

Volatilization Behavior of Fluorine in Fluoroborate Residue during Pyrolysis Yuheng Feng,† Xuguang Jiang,*,† Yong Chi,† Xiaodong Li,† and Hongmei Zhu‡ †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Hangzhou Huanjie Environment Engineering Co. Ltd., Hangzhou 310000, China

‡

ABSTRACT: Industrial hazardous waste from the fluorine chemical industry sometimes has a high content of fluorine, and incinerating it could be very poisonous if the flue gas is not properly disposed. In this study fluoroborate residue is used to represent a typical waste from the fluorine chemical industry . Thermogravimetric analysis coupled with Fourier transform infrared analysis (TG-FTIR analysis) was used to study the evolution characteristics of gaseous products during the pyrolysis of fluoroborate residue. The pyrolysis process of fluoroborate residue could be divided into three stages according to the TG analysis: moisture loss, fast decomposition, and charring. SiF4, BF3, and HF are found as fluorine gas species evolved in the pyrolysis process. The evolution of SiF4 finishes at 600 °C. The evolution of BF3 has two peaks and most of the emission happens before 600 °C. The release of HF could be divided into two stages due to the different existence of F−. In addition, the reforming condition of different fluorine-containing gaseous substances is verified in a thermodynamic equilibrium model and the results could explain the experiment phenomenon well.

1. INTRODUCTION With the fast development of fluorine chemistry industry in China, fluorine emission during the thermal disposal of the residue from this industry has attracted more and more attention. Large amounts of toxic gases, such as HF, SiF4, and BF3, are released into the atmosphere during the incineration process, which brings damage to human health and the environment. Many researchers focus on emissions of F during thermal process of coals.1−3 Compared to the low content of fluorine in coal with a mean of 150 μg/g4 in world coals and 82 μg/g in Chinese coals,5 the fluorine content in fluorochemical industry waste could be in a wide range, depending on fluorine content in raw material and chemical process. If a waste with a high content of fluorine is not treated properly for fluorine removal, serious damage to human health and the environment could occur. Most fluorine in coal exists as inorganic matter.1 It may be in the lattice of carboxyl apatite and mica in the form of isomorphism, in the solution of internal waters or absorbed on the surface of particles, or in the form of inorganic mineral such as CaF2 and Ca5(PO4)3F. The F in solution is released in the form of HF with the evaporation of adsorptive water and constructive water at low temperature (under 700 °C). The inorganic mineral form of F is decomposed at high temperature (>800 °C). The HF released at high temperature will react with SiO2 in ash to form SiF4. At 1200 °C, about 96% of F in coal is emitted.1 Previous research has quantified the release behavior of fluorine in coal by subtracting the mass of fluorine in char from that in original coal.1−3 So the accurate forms of emitted © 2011 American Chemical Society

fluorine gases and the release trends of these gases have not materialized in previous literature. Incineration has been widely used for disposal of hazardous industrial waste in China. A two-step system is applied for most thermal disposal: a rotary kiln and a secondary combustion chamber. A reduction atmosphere is usually adopted in the rotary kiln. So it is very necessary to know the distribution of fluorine in gas phase and the release trend of fluorine gases in reduction atmosphere for the control of gaseous fluorine pollution. Thermogravimetric analysis coupled with Fourier transform infrared analysis (TG-FTIR analysis) is a very useful tool in analysis of pyrolysis and combustion process of solid fuels, as it monitors continuously both the weight of nonvolatile materials and evolution of different gases.6 Weight loss curve (TG curve) and derivative weight loss curve (DTG curve) can be obtained in the TG analysis, in which different thermal processes can be distinguished according to the peaks in DTG curve. In FTIR analysis, plenty of information is provided which can be used to identify the composition of mixed evolution gases and quantify them. It is widely used in researching the pyrolysis process of coal,7 biomass,8,9 and hazardous wastes.10−13 In this paper, a typical hazardous waste with high content of fluorine, fluoroborate residue, was studied with TG-FTIR Received: Revised: Accepted: Published: 307

August 13, 2011 November 7, 2011 December 6, 2011 December 19, 2011 dx.doi.org/10.1021/es202828k | Environ. Sci. Technol. 2012, 46, 307−311

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minimize secondary reactions, the stainless steel transfer pipe and gas cell (20-cm optical path length) were heated at 180 °C. Heating rates of 10 °C/min were adopted for the current work. For most hazardous wastes, 950 °C is high enough for most gas species to evolve from the sample,10−12 but the evolution of fluorine gas may happen over 1000 °C. So 1300 °C, which is the upper limit for the analyzers, was adopted as final temperature. Nitrogen was used as inert gas of the pyrolysis process. Resolution in FTIR was set at 4 cm−1. Spectrum scan frequency was 20 times per minute, and the spectral region was 4000−400 cm−1. Approximately 10 mg of the samples was used in this study. The sample was crushed into powder, the grain size of which was smaller than 0.2 mm. The sample had a high content of fluorine, which may cause the generation of highly corrosive gaseous products such as HF and SiF4. To avoid the reaction of these corrosive gases with the material of the crucible, a crucible made of α-alumina oxide, which is stable in corrosive atmosphere at high temperature, was used.

analysis to realize the release form of gaseous fluorine and obtain the release curve of each gas. The results were compared with the prediction from a thermodynamic equilibrium model.

2. EXPERIMENT 2.1. Materials. The fluoroborate residue used in this study was obtained from a waste incineration plant in Quzhou, Zhejiang Province. This residue comes from the production line of fluorobenzene. First, the benzidine was diazotized. Then the arenediazonium salt solution was mixed with NaBF4 solution, and the product, crystalloid diazonium fluoroborate, was separated. Diazonium fluoroborate was heated and N2 and BF3 were released. Liquid fluorobenzene was obtained. The solid residue was separated from liquid fluorobenzene in the last process, which has a high content of fluoroborate. The elemental analysis of the sample is in Table 1. From Table 1, it could be found that the content of fluorine, 5.467%, Table 1. Elemental Analysis of Fluoroborate Residue (Mass %)a moisture

C

7.31 Na

34.59 Fe

3.66

0.31

H 2.31 Ca 0.23

O

N

F

Cl

S

48.84 Si

2.20 Al

5.90 Mg

0.69 K

0.11 B

0.14

0.09

0.06

0.05

3. RESULTS AND DISCUSSION 3.1. TG Analysis. The weight loss and weight loss rate of the sample are reported in Figure 2. The DTG curve of the

0.84

a

Moisture is reported on as-received basis. Elements are reported on dry basis. The contents of C and H were determined by Liebig method. The content of N was determined by semimicro Kjeldahl method. The content of S was determined by IR spectrometry. The content of F and Cl was determined by combustion−hydrolysis/ion chromatography (IC) method. The contents of Na, Fe, Ca, Si, Al, Mg, and K were determined by X-ray fluorescence (XRF) analysis. The content of B was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the content of O was determined by difference.

is much higher compared to other industrial hazardous wastes. XRD-pattern of fluoroborate residue is shown in Figure 1. It is observed that NaBF4 is the dominant crystalline fluorine compounds in fluoroborate residue. Figure 2. TG and DTG curve for pyrolysis of fluoroborate residue.

fluoroborate residue is generally divided into three stages. The first stage (t < 130 °C) is the loss of moisture in the sample. The second stage, pyrolysis of the sample, is from 130 to 682 °C. Fifty percent of the weight loss occurs in this stage. Two peaks could be found in the DTG curve, at 288.6 and 508.2 °C, respectively. The peak at 288.6 °C is the highest in the whole heating process. Charring of the residue is from 682 °C to the ending of the heating. The total weight loss of the sample at 1300 °C is 82.9%. 3.2. Kinetic Parameters of Pyrolysis of Fluoroborate Residue. To determine the kinetics of the pyrolysis process, the second stage is subdivided into two steps according to the peaks in DTG curve. The kinetics of each step can be described as

dα = kf (α) dT

Figure 1. XRD pattern of fluoroborate residue.

(1)

where α and k represent the unreacted mass fraction and rate constant under this activation energy. T is thermodynamic temperature with the unit K. When the rate constant is in

2.2. Methods. A Nicolet-Nexus 670 spectrometer and a Mettler-Toledo TGA/SDTA851e thermo analyzer were coupled by a Thermo-Nicolet TGA special connector. To 308

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Arrhenius form,

⎛ E ⎞⎟ k = A exp⎜ − ⎝ RT ⎠

(2)

Here R is universal gas constant (R = 8.314 Jmol−1 K−1), E is activation energy, and A is pre-exponential factor. First-order Arrhenius Law iss usually used in kinetic analysis of pyrolysis of solid fuels,14 where

f (α ) = 1 − α

(3)

Equation 1 can be transformed into

dα k = dT f (α ) β

Figure 3. Infrared spectrum of pyrolysis products from fluoroborate residue at 563.0 °C.

(4)

temperature, HF reacts with B2O3 formed in eq 12 and regenerates BF3 (eq 14). A small amount of HCl is found in the spectrogram, this is in accordance with the chlorine content in the sample (0.69% as dry based).

After integration, eq 4 can be represented as

g (α ) =

∫0

α

A dα = f (α ) β

∫0

T

⎛ E ⎞⎟ exp⎜ − dT ⎝ RT ⎠

(5)

NaBF4 → NaF + BF3

where g(α) is integral form of the kinetic mechanism function. Using the Caots−Redfern method15 for integration of eq 5,

ln

g (α ) T

2

⎡ AR ⎛ E 2RT ⎟⎞⎤ = ln⎢ ⎜1 − ⎥− ⎣ βE ⎝ E ⎠⎦ RT

where

g (α ) T

2

(7)

⎛ AR ⎞ E = ln⎜ ⎟ − ⎝ βE ⎠ RT

(10)

2NaF + H2O → Na2O + 2HF

(11)

4BF3 + 3SiO2 → 3SiF4 + 2B2O3

(12)

4HF + SiO2 → SiF4 + 2H2O

(13)

6HF + B2O3 → 2BF3 + 3H2O

(14)

17

Because the term 2RT/E is much less than 1, it can be neglected. Hence,

ln

NaF + H2O → NaOH + HF 17

(6)

g(α) = − ln(1 − α)

(9)

16

18

19

(8)

To determine the activation energy E and frequency factor A, let x = 1/T and y = ln(g(α)/T2). The relation of y and x is linearity at slope −E/R and intercept ln(AR/βE), which can be obtained by linear fitting. Both E and A can be calculated from the slope and intercept of the line. The activation energies E and Arrhenius pre-exponential factors A for during pyrolysis stage are listed in Table 2. It is

19

The release characteristics of the fluorine gases are shown in Figure 4. The emission of BF3 can be divided into three stages.

Table 2. Activation Energy and Arrhenius Pre-Exponential Factors from Pyrolysis of Fluoroborate Residue series (1) first peak at pyrolysis stage (2) second peak at pyrolysis stage

activation energy (KJmol−1)

preexponential factor

R2

130−376

38.7

2

7.01 × 10

0.953

376−682

70.1

4.22 × 103

0.926

temperature range (°C)

observed that the activation energy rises as the temperature is increased. 3.3. FTIR Analysis. Figure 3 shows the spectrum products from fluoroborate residue at 563.0 °C, from which CO2, BF3, SiF4, HF, and HCl can be found. The main source of fluorine in the sample is sodium fluoroborate. The sodium fluoroborate is decomposed to form BF3 and sodium fluoride when heated (eq 9). The fluoride salt is hydrolyzed and HF (eqs 10 and 11) is formed. BF3 and HF corrode SiO2 in the sample and generate another fluorine gaseous product, SiF4 (eqs 12 and 13). At high

Figure 4. Absorptions with temperature of the fluorine gas species in pyrolysis process.

The first stage is from 122.0 to 381.9 °C. The peak of this stage is at 299.2 °C. The second stage is from 381.9 to 1079.2 °C. The evolution of BF3 increases with temperature and reaches its 309

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crest value at 525.5 °C. But it decreases rapidly above 525.5 °C, since NaBF4, the main source of BF3 has been destructed completely near this temperature. Zachara et al.16 studied the thermal decomposition of fluoroborates. After undergoing polymorphic transformations at low temperature, the fluoroborates melt and emit BF3 according to a simple acid−base mechanism. In the thermogravimetric analysis, the temperature range of the weight loss of NaBF4 is from 450 to 700 °C, while the weight loss range of Ca(BF4)2 is 170−290 °C. These two ranges were proved by the release curves of BF3 here, from which the corresponding release ranges were found. XRD pattern of the fluoroborate residue at 700 °C during pyrolysis is in Figure 5, from which it is observed that the main crystal

and constructive water, while the second peak is due to the decomposition of inorganic mineral. The two stages of the evolution of HF may be caused by the different form of fluorine in fluorochemichal industry cracking residue. Kharitonov et al.17 investigated reaction of sodium fluoride with water vapor at 900−995 °C when the H2O partial pressure was 0.03−0.45 atm. The reaction of powdered NaF with water vapor forms gaseous HF and NaOH. The activation energy of the reaction is (4.0 ± 1.0) × 104kcal/mol. 3.4. Model Simulation. To understand more deeply about the conversion mechanism of fluorine gases released from a hazardous waste with fluoroborate, a typical thermodynamic software package, Factsage, is used here. The core module of Factsage, Equilib, is set up by minimizing the Gibbs free energy, which can work out the type and proportion of each substance under chemical equilibrium conditions.20,21 The version of the software used here is Factsage 5.2. Factsage uses elements as input data. The initial forms of elements are ignored. But the initial forms would influence the results in some conditions. For example, when the input elements are the same, an element could exist in different kind of compounds. Its chemical activity is determined by the bond energy by which it is connected to the other parts of molecule. So the output data of the system might be inconsistent because of different chemical bonds. Furthermore, Factsage predicts the equilibrium of a closed system, but the evolution process of pyrolysis gases is in an open system. The released gases which exit from of the fixed bed will not affect the system. So it is not possible to predict the actual proportion of each released fluorine gas during the whole heating process. Hence, to verify the migration regularity of fluoride, actual proportion of each element is not input into the software. Transport characteristics of fluoride in NaBF4, the major source of fluoride in fluoroborate residue, is investigated by Factsage here. SiO2 and H2O are adopted as two other reactants. To satisfy complete reaction of eqs 1, 2, 4, and 5, the stoichiometric numbers of the three reactants are adjusted. An idealized proportion, NaBF4:SiO2:H2O = 1:1:1, is adopted as input to Factsage. The contents of three main gas species found in IR spectrum with temperature are in Figure 6. Some output fluorine gases whose quantities are small or not found in the FTIR analysis are neglected here. From Figure 6 it is found that the content of SiF4 is very large and decreases rapidly from 200 to 600 °C. After 600 °C, the quantity of SiF4 is very small. This trend verifies the volatile curve of SiF4 in the experiment, in which no emission of SiF4 is found after 600 °C. The release of BF3 starts from 200 °C, has its peak at 600 °C, and decreases after 600 °C. That means most of the BF3 will evolve from the sample under 600 °C which is in accordance with the experiment. The content of HF increases continuously in the whole temperature range, which means a continuous rise trend of HF in experiment. This trend is also in accordance with experiment. Some peaks of the evolution of SiF4 and HF in the experiment could not be explained by the software. This is because the principle of Factsage is minimizing the Gibbs free energy. The initial form in which an element exists is not taken into consideration. Although the output of thermodynamic software did not predict the quantity of the fluorine gases released in the experiment accurately, it could explain the release trends of these gases. Large quantity of BF3 will be released from the sample under 600 °C and the evolution of HF is in increased with the increasing of temperature in the whole heating course.

Figure 5. XRD pattern of pyrolysis products from fluoroborate residue at 700 °C.

component at this temperature is NaF. NaBF4, which is found in XRD pattern of original sample, is not found here. This means that most of NaBF4 has been decomposed and formed NaF and BF3 under 700 °C. The third stage is from 1079.2 °C to the finish of the heating course. The emission of BF3 rises a little during this stage. This is due to the increase of HF released from the sample, which reacts with the B2O3 formed by eq 4. Brynestad Jorulf et al.19 used HF to purify SiC containing oxides impurities. SiC powders are treated with gaseous HF mixed with an inert gas, e.g., Ar, at 200−650 °C. The oxides, e.g., SiO2 and B2O3, are converted to volatile fluorides, while reaction of the SiC with the HF is negligible. The emission of SiF4 ranges from 105.0 to 608 °C and reaches its highest peak at 508.8 °C. There are two shoulder peaks at 293.7 and 379.5 °C. The first shoulder peak and highest peak of the evolution of SiF4 are near the two peaks of the evolution of BF3, which are at 299.2 and 525.5 °C, respectively. This is because the SiF4 is from the reaction of BF3 with SiO2 at low temperature. The isotherms of adsorption of BF3 on SiO2 at 20−300 °C were detected by Kozorezov.18 The adsorption of BF3 on SiO2 was done through irreversible chemisorption. The adsorption centers consist of O atoms and OH groups. No more SiO2 is found above 608 °C The release of HF is divided into two stages. In the first stage, the evolution of HF starts at 464.7 °C and increases with temperature until it reaches its maximum at 663.4 °C. Then it drops gradually as the temperature raises. The second stage is from 996.8 °C to the end of the heating course, during which the release of HF presents a steep rise. The increase of HF causes an increase of BF3 as a result of eq 14. The first peak is caused by the hydrolysis of F− with evaporation of adsorptive 310

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Figure 6. Proportions of three main fluorine gases at different temperatures.

The reactions of BF3 and HF with SiO2 are under 600 °C and will not happen at higher temperature.

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AUTHOR INFORMATION Corresponding Author *Phone: +86 571 87952775; fax: +86 571 87952438; e-mail: [email protected].

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ACKNOWLEDGMENTS This project was supported by National Basic Research Program (973 Program) of China (Grant 2011CB201500), National High Technology Research and Development Program (863 Program) of China (Grant 2009AA064704), and National Project of Scientific and Technical Supporting Program (2007BAC27B04-3).

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