Environ. Sci. Technol. 2002, 36, 1344-1348
Estimation of Mechanochemical Dechlorination Rate of Poly(vinyl chloride) HIROSHI MIO,* SHU SAEKI, JUNYA KANO, AND FUMIO SAITO Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Poly(vinyl chloride) (PVC) was ground in air with CaO in the presence of quartz powder as a grinding aid by a smallscale planetary ball mill to investigate the relation of the dechlorination rate of PVC with the impact energy of the balls calculated from a computer simulation based on the Discrete Element Method under various conditions. Mechanochemical dechlorination proceeds as the grinding progresses and is improved with an increase in both the mill speed and the amount of balls introduced into the mill. The same trend can be seen in the relation between the specific normal impact energy of the balls and the rotational speed. The relationship between the observed dechlorination rate and the computed normal impact energy of the balls is linear, with a correlation coefficient of 0.965. This relationship can be used to estimate the dechlorination rate of PVC in a large-scale planetary ball mill.
dechlorinating reagent such as CaO in the grinding operation, it does not require any heating operation. To support use of the MC dechlorination method, it would be useful to have a correlation between the dechlorination rate of PVC and the grinding (MC) conditions to determine the optimum condition in a scaled-up MC reactor. There are a few reports for the relationship between MC effects and grinding conditions (7, 8). However, there is no theoretical method for scaling the ball mill up, and it is difficult to estimate the dechlorination rate. On the other hand, a numerical approach aided by computer simulation techniques such as the Discrete Element Method (DEM) or Particle Element Method (PEM) would give useful information on ball motion during the milling (9-11). We have proposed that the grinding rate of inorganic materials ground by planetary and tumbling ball mills can be correlated with the impact energy of the balls calculated from the simulated result obtained on the basis of the DEM (12-15). This has been shown to be applicable for the relationship between the yield of vanadium extracted in water leaching of electrical precipitation (EP) dust after dry MC treatment and the impact energy of balls using three different types of mills. The relationship was able to be expressed by a single curve, irrespective of the type of mill (16). The main purpose of this paper is to provide information on the correlation of the dechlorination rate of PVC with CaO by co-grinding them using a small-scale mill. The impact energy of the balls was simulated by DEM, and a relationship for dechlorination rate was developed that can be applied to a large-scale mill by computer simulation of its grinding conditions.
Experimental Section Introduction Poly(vinyl chloride) (PVC) has a unique chemical property of being highly stable both in its chemical composition and its behavior when heated, and it has a potential to exhibit a wide variety of plastic-elastic properties from flexible to rigid PVC products when it is mixed with plasticizer and additives. These characteristics enable the use of a large amount of PVC as a raw material in various commodities, including industrial, agricultural, and medical goods. These products give considerable benefits and have contributed to modern comfort and convenience. However, after such PVC products have been used, they become wastes, which have to be disposed of mainly by combustion or in landfills. The combustion has been carried out at waste disposal facilities for a long period of time, but it is now prohibited in many parts of the world because of the emission of harmful substances, such as a group of dioxins and HCl gas. The landfills require a large amount of land area, which is not always available. Recently several disposal methods such as the blast furnace method and the liquefying method have been proposed (1-4). These methods are very effective but employ a dechlorination process that requires high-temperature facilities. On the other hand, our group has proposed the mechanochemical (MC) method for dechlorinating PVC by grinding using a planetary ball mill followed by washing with water (5, 6). This method has a high potential to dispose of PVC wastes at any desired locations with flexible operation due to its use of a portable facility composed of a mill and a washing tank with a filter. Although this method needs a * Corresponding author phone and fax: e-mail:
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 6, 2002
The PVC sample used in this experiment was a chemical reagent (Wako Chemical Co., Ltd., Japan), and its initial mean particle diameter was about 133 µm. The degree of polymerization of the PVC sample was about 1100. CaO powder was used as a dechlorinating reagent, and it was prepared by heating calcium hydroxide [Ca(OH)2, Wako Chemical Co., Ltd., Japan] at 800 °C for 2 h. The molar ratio of CaO to PVC was fixed so as to be 2 in the ratio of Ca in CaO to Cl in PVC samples. Quartz powder (SiO2, Wako Chemical Co., Ltd., Japan) was used as a grinding aid in order to initiate the MC reaction and was added to the PVC-CaO mixture at 16.7 wt %. Two kinds of planetary ball mills were used as MC reactors for dechlorinating PVC mechanochemically: One is a smallscale mill (Pulverisette-7, Fritsch, Germany) and the other a large-scale one (Pulverisette-5, Fritsch, Germany). As for the small mill, the pot was made of stainless steel and its inner diameter, dM, and depth, h, were 40 mm and 38 mm, respectively, so that its volume, VM, was about 0.048 dm3. The media introduced into the pot were steel balls having 15.9 mm diameter, dB, and their numbers were set at 4 and 7. The weight of the powder mixture of PVC, CaO, and SiO2 was kept at 1.5 and 3.0 g. The rotational speed of the mill was varied at 8.33, 10.00, and 11.67 rps. On the large mill, the pot had dM ) 75 mm and h ) 67 mm so that its volume was approximately 0.290 dm3. The mill was introduced with 1560 balls having dB ) 4.8 mm diameter, 333 balls having dB ) 7.9 mm, or 42 balls having dB ) 15.9 mm and 10 g of the sample mixture, and it was operated in air at a rotational speed of 5.00 rps for different periods of time. Both the mills were operated in air for 15 min and stopped for 15 min so as to avoid excess temperature increase. Other operational conditions are tabulated in Table 1. In the grinding by both mills, 10.1021/es0106220 CCC: $22.00
2002 American Chemical Society Published on Web 02/14/2002
TABLE 1. Operational Conditions Pulverisette-7 mill diameter mill depth mill volume revolution radius rotation-to-revolution speed ratio ball diameter no. of balls ball-filling ratio sample weight rotational speed
Pulverisette-5
dM (mm) h (mm) VM (dm3) R (mm) r (-)
40 38 0.048 70 1.00
75 67 0.290 120 1.18
dB (mm) nB (-) J (%) W (g) N (rps)
15.9 4, 7 35, 60 1.5, 3.0 8.33, 10.00, 11.67
4.8, 7.9, 15.9 1560, 333, 42 60 10.0 5.00
TABLE 2. Material Properties and Physical Constants for the DEM 7.8 × 103 2.0 × 1011 0.30 0.16 0.47 1.0 × 10-6
density of balls Young’s modulus Poisson’s ratio coefficient of restitution frictional coefficient time step
(kg/m3) (Pa) (-) (-) (-) (s)
FIGURE 1. Relation between chlorine remaining and treatment time for the small mill operated at 11.67 rps.
the ground product was removed from the pot after grinding at the prescribed time for characterization. A total of 0.5 g of the ground product was dispersed in 0.250 dm3 of distilled water for 1 h to extract soluble compounds, mainly chlorides. Subsequently, the suspension was filtered to separate the solid and the filtrate, for which chlorine ion was measured by an ion chromatograph (LC10 series, Shimadzu Co., Ltd., Japan).
Simulation of Ball Motion Conditions of the Simulation Work. The three-dimensional motion of each ball during the milling was simulated by using the DEM, which takes account of the effect of the presence of the powder sample (17). Physical properties such as Young’s modulus and Poisson’s ratio of the ball and the time step in the simulation of the present work are tabulated in Table 2. The frictional coefficient between two balls and/or the ball-wall interface in the simulation was assumed to be 0.47 by a parameter fitting method (17). The simulated motion of the balls was reproduced from the start of the milling to 3.0 s into the operation. Other conditions in the simulation were the same as those in the experiment. Impact Energy of Balls. The colliding velocity of a ball against another ball or the mill wall is calculated by the computer simulation. The specific impact energy of the balls, EW, is calculated from the relative velocity between two colliding balls or a ball colliding against the mill wall, vj () xvN2+vT2 where vN is the normal component and vT is the tangential component), as given by n
EW )
1
∑ 2W mv
2 j
(1)
j)1
where m is the mass of a grinding media, n denotes the number of collisions within a second, and W is the weight of the sample charged into the mill pot.
Results and Discussion Correlation between Dechlorination Rate and Impact Energy for the Small Mill. Figure 1 shows the normalized chlorine remaining, Cr, in the mixture ground by the small mill at 11.67 rps as a function of the MC treatment time, t, depending on the number of balls, nB, and the sample weight, W. Plotted values are the experimental data, and the solid
FIGURE 2. Relation between dechlorination rate and rotational speed for the small mill. line denotes calculated results obtained by the empirical equation given by
Cr ) exp(-Kdt)
(2)
where Kd is the dechlorination rate of PVC. Most of the experimental data are found to be scattered along or near the calculated lines, and the correlation coefficients between the experimental results and the data calculated by eq 2 for these experimental conditions are over 0.96. Therefore, it is implied that the data can be fairly well expressed by eq 2. It is found that Cr decreases with an increase in t as well as nB and with a decrease in W. This means that the dechlorination reaction proceeds with an increase in both t and nB and is inversely proportional to W. This trend is confirmed by Figure 2, which shows Kd as a function of the rotational speed, N, depending on nB and W. That is, Kd increases with an increase in both N and nB and with a decrease in W. In this figure, Kd at 7 balls/3 g is larger than at 4 balls/1.5 g, although the ball to powder weight ratio are almost the same for these cases. VOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. (a) Relation between specific impact energy and rotational speed for the small mill. (b) Relation between specific normal impact energy and rotational speed for the small mill. On the other hand, Figure 3a,b shows the specific impact energy with panel a showing EW and the specific normal impact energy and panel b showing EN, as a function of N, depending on nB and W. The specific impact energy, EW, can be calculated from the ball motion simulated by the DEM and given by eq 1. EW is based on both the normal and the tangential velocity components of colliding balls; EN is based on the normal one. As can be seen from Figure 3, EW and EN increase with an increase in both N and nB but are inversely proportional to W. The order of magnitude for EW at N is especially inconsistent with that for Kd at N for the data at 7 balls/3.0 g and the data at 4 balls/1.5 g shown in Figure 2, while in Figure 3b, EN at 7 balls/3.0 g is larger than 4 balls/1.5 g whose order of magnitude for EN at N is quite consistent with that for Kd at N shown in Figure 2. Figure 4a,b shows Kd as a function of the impact energy (panel a, EW; panel b, EN) for different conditions. In both panels, the data are scattered near the straight line regardless of the MC conditions. However, as described earlier, it is 1346
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FIGURE 4. (a) Relation between dechlorination rate and specific impact energy for the small mill. (b) Relation between dechlorination rate and specific normal impact energy for the small mill.
TABLE 3. Dechlorination Rate of PVC Estimated by the Simulation and Experimental Results ball diameter
dB (mm)
4.8
7.9
15.9
dechlorination Kd (s-1) 17.94 × 10-5 15.34 × 10-5 9.65 × 10-5 rate from EW dechlorination Kd (s-1) 7.55 × 10-5 8.28 × 10-5 7.46 × 10-5 rate from EN dechlorination Kd (s-1) 6.35 × 10-5 8.00 × 10-5 9.71 × 10-5 rate of experiment
found that the correlation of Kd with EN shown in Figure 4b is superior to that with EW shown in Figure 4a. The correlation coefficients between Kd and EW (Figure 4a) and between Kd and EN (Figure 4b) are 0.927 and 0.965, respectively. Therefore, it is concluded that the dechlorination rate of PVC is better correlated with the specific normal impact energy of the balls rather than the specific impact energy. The gradient of the straight line in both panels, A, implies an ability of dechlo-
rination that depends on the additive or molar ratio (6) and indicates that E0 is the minimum specific impact energy of the balls for the dechlorination by the MC treatment under present conditions. The dechlorination rate for the small mill can be determined by
Kd ) A(E - E0)
(3)
In the case of Figure 4a, AW and EW,0 are measured to be 2.24 × 10-5 g/J and 2.52 J/(s g), while AN and EN,0 are 4.54 × 10-5 g/J and 0.957 J/(s g), respectively. In both relationships, the dechlorination rate can be estimated from eq 3 by substituting the above data and E (EW or EN) for the large mill, and this will be discussed in the following section. Estimation of the Dechlorination Rate for the Large Mill. The specific impact energies of the balls for the three conditions of the large mill were calculated from the computer simulation. The dechlorination rate was therefore calculated by substituting E in the large mill into the relation (eq 3) obtained in the small mill and tabulated in Table 3. Figure 5a-c shows Cr as a function of t for the large mill (panel a, dB ) 4.8 mm; panel b, dB ) 7.9 mm; panel c, dB ) 15.9 mm). The curves in these figures are obtained by substituting the value of Kd obtained from eq 3 into eq 2; the plotted values are the experimental data. It is found from Figure 5c that the experimental values are fairly consistent with both estimated lines. However, in the case of Figure 5a,b, the line estimated from EW is much farther away from the experimental values than that estimated from EN. The correlation coefficients between the estimated line from EN and the experimental results are quite close to 1.0. Therefore, it is concluded that the dechlorination rate of PVC would be correlated more precisely with EN rather than EW, irrespective of dB. This means that the dechlorination and the reaction rate for a large mill can be estimated by substituting EN in the large mill into the relation (eq 3) obtained in the small mill. Thus, the dechlorination rate of PVC by a planetary ball mill can be estimated from the relation (eq 3) and the DEM. It is expected that such a method can be developed and applied for the MC treatment of other compounds as well.
Nomenclature
FIGURE 5. (a) Correlation between estimated lines and experimental values for the large mill using the 4.8-mm balls. (b) Correlation between estimated lines and experimental values for the large mill using the 7.9-mm balls. (c) Correlation between estimated lines and experimental values for the large mill using the 15.9-mm balls.
A
gradient (g/J)
Cr
chlorine remaining (-)
dB
ball diameter (mm)
dM
mill diameter (mm)
EN
specific normal impact energy [J/(s g)]
EW
specific impact energy [J/(s g)]
E0
minimum specific impact energy for dechlorinating PVC [J/(s g)]
h
mill depth (mm)
Kd
dechlorination rate (s-1)
m
mass of a grinding media (g)
n
number of collisions of balls per second (s-1)
nB
number of balls charged into the mill (-)
N
rotational speed of the mill (rps)
R
revolution radius (mm)
t
treatment time (h)
vj
relative speed of colliding balls (m/s)
VM
mill volume (dm3)
W
sample weight charged into the mill (g)
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(10) Watanabe, R.; Hashimoto, H.; Lee, G. G. Mater. Trans., JIM 1995, 36, 102. (11) Rajamani, R. K.; Mishra, B. K.; Venugopal, R.; Datta, A. Powder Technol. 2000, 109, 105. (12) Kano, J.; Saito, F. Powder Technol. 1998, 98, 166. (13) Kano, J.; Mio, H.; Saito, F. J. Chem. Eng. Jpn. 1999, 32, 445. (14) Kano, J.; Mio, H.; Saito, F.; Tanjo, M. J. Chem. Eng. Jpn. 1999, 32, 747. (15) Kano, J.; Mio, H.; Saito, F. AIChE J. 2000, 46, 1694. (16) Kano, J.; Saito, F. J. Chem. Eng. Jpn. 1998, 31, 1014. (17) Kano, J.; Chujo, N.; Saito, F. Adv. Powder Technol. 1997, 8, 39.
Received for review February 8, 2001. Revised manuscript received December 7, 2001. Accepted December 11, 2001. ES0106220
