3218
Ind. Eng. Chem. Res. 1999, 38, 3218-3224
Thermal and Mechanochemical Self-Propagating Degradation of Chloro-organic Compounds: The Case of Hexachlorobenzene over Calcium Hydride Giacomo Cao,*,†,‡ Stefania Doppiu,§ Marzio Monagheddu,§ Roberto Orru ` ,† Mariella Sannia,† and Giorgio Cocco*,§,| Dipartimento di Ingegneria Chimica e Materiali, Universita` degli Studi di Cagliari, Piazza d’Armi, 09123 Cagliari, Italy, and Dipartimento di Chimica, Universita` degli Studi di Sassari, Via Vienna 2, I-07100 Sassari, Italy
We report on the highly exothermic solid-state reaction between hexachlorobenzene and calcium hydride. Once ignited by a thermal spike, the reaction displays a self-sustaining character in the CaH2/C6Cl6 molar ratio of 3:18. The high temperatures reached, i.e., 2550-2900 K, ensure a complete breakdown of the aromatic molecule and of undesired chloro-organic congeners, with only inorganic halide salts being found among the end-products. Combustive-like reactions were also observed when reactant powders were subjected to intensive mechanical treatment by ball milling. The combustive range of mechanically driven processes falls within that found in the true self-sustaining regime even if the activation and the extinction of the reaction were ruled by completely different mechanisms. A neat correlation was worked out relating the temperatures at the combustion front to the total heat evolved in the reaction carried out in the mechanochemical mode. The same end-products were also found. The practical exploitation of a self-sustaining methodology for the disposal of hazardous organochlorine compounds seems feasible. Introduction This study is a part of a long-term research project on the reduction of toxic organic compounds by nonconventional reductive methods. It has been shown recently that mechanochemical treatments by high-energy impact ball milling (BM) of powder mixtures composed of chloro-organics and reactive substrates can lead to a complete degradation of the organic compounds. Ca and Mg and their oxides were employed as reductive agents under either inert or hydrogen atmospheres.1-3 The specific use of calcium hydride as substrate was found to be more effective in determining highly selective and rapid dechlorination results3. In addition, by increasing the milling intensity beyond a well-defined impact energy threshold, an explosive-like reaction was observed, leading to gaseous hydrogen, graphite, and calcium chloride salts as end products.4 Successful tests were also carried out on real polychlorobiphenyls (PCBs) and dioxin-contaminated powders with a conversion yield in the organic chlorine content greater than 99.9999%.5 The BM methodology is highly attractive since it allows the process to be carried out in a confined and strictly controlled environment. The lack of oxygen in the reaction atmosphere in particular prevents the formation of hazardous oxidized congeners. However, there are some inherent limitations to its practical exploitation. A problem is represented by the scaling up of a laboratory milling reactor by several orders of * To whom correspondence should be addressed. † Universita ` degli Studi di Cagliari. ‡ E-mail:
[email protected]. § Universita ` degli Studi di Sassari. | E-mail:
[email protected].
magnitude to a size of interest to industry. The difficulties seem mainly linked to the development of a leakproof technology owing to the dynamic operating conditions of the grinding tools. In addition, the complete breakdown of the organic molecules is achieved after relatively long milling times, usually on the order of hours, requiring a conspicuous consumption of energy. These shortcomings and other related problems prompted us to search for alternative activation methods to the mechanochemical route. Bearing in mind the strongly exothermic characteristics of the reactions involved, we thought it is of interest to verify the possibility of triggering dehalogenation processes by conventional ignition methods. It is well-known that exothermic combustion chemical reactions can be initiated in the “self-propagating” mode by local heating of the sample (with a hot wire or an electrical spark or even a laser or ion beam) or in the “thermal explosion” mode by adiabatically heating the whole mass of the reactants to the onset of the spontaneous transformation event.6-8 The former mode, which is known in the literature as self-propagating hightemperature synthesis (SHS), is exploited in this work to test the reaction between hexachlorobenzene and calcium hydride powders ignited by means of a thermal radiation pulse. It is worth noting that although self-propagating reactions of the thermite type have been proposed in the literature for the fixation of high-level radioactive wastes,9 the use of this type of reactions for environmental protection purposes has received renewed attention only recently.10-12 The extension of SHS to organic systems is a promising field to explore.13 Likewise, the intensification of diffusion reaction by milling to a spontaneous propagation stage is a relatively new
10.1021/ie980790+ CCC: $18.00 © 1999 American Chemical Society Published on Web 08/13/1999
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3219
Figure 1. Schematic representation of the reactor chamber and the assembly employed in the SHS trials: (1) ignition source, (2) starting mixture, (3) reaction front, and (4) final products.
topic in the field. It has been referred to as SHS mechanochemistry (SHSM).13 Indeed, to the best of our knowledge, no parallel SHS and SHSM experiment have been carried out at this writing. The reaction mechanisms, the effects of processing variables, and the threshold conditions for reaction initiation remain poorly understood. In an attempt to shed light on these phenomena, we trace here a comparison between the results obtained with the two different techniques above. Aside our primary interest in the disposal of toxic chlorinated aromatics, the aim of finding possible cross-correlation adds further motives to this study. Experimental Section A diagram of the SHS setup is depicted in Figure 1. It consists of a stainless steel reaction chamber, a power supply (Belotti), an infrared pyrometer (Land Instrument, Cyclops 152A), a video camera, and a video recorder, which allows one to observe and record the reaction evolution. A computer system equipped with a data acquisition board (model PCI-MIO-16XE-50, National Instruments) drives the power supply to produce a well-defined energy pulse. Sample temperatures were also measured using W-Re thermocouples. Calcium hydride (Aldrich, 95% purity, -4 + 40 mesh) and hexachlorobenzene (Aldrich, 99%) were first mixed at different molar ratios (CaH2/C6Cl6 ) 3, 4, 6, 9, 12, 15, and 18) and pressed into cylindrical pellets (10 mm in diameter and 10-20 mm length) up to a green density ranging from 2.7 to 3.2 g/cm3. Each pellet was introduced into the reaction vessel, which was evacuated and then filled with argon at 1 atm. The latter operation was repeated four times to guarantee an inert environment. The reaction was initiated at one base of the sample by means of a tungsten coil (R. D. Mathis Company, U.S.A.) connected to the power supply programmed to produce an energy pulse of 20 V for about 3 s. This interval was selected so that the energy source turned off as soon as the reaction was initiated. The reproducibility of the ex-
perimental runs was verified by repeating each of them at least twice. SHSM runs were carried out with a Spex 8000 mill under argon atmosphere. This commercially available machine drives a hardened steel cylinder (reactor vial), 5.7 cm high and 3.5 in diameter, through a periodic three-dimensional course recurring 875 times per minute. One grinding ball was used, and constant powder batches of 8.8 g were run. Provided collisions are inelastic, the ball, loose inside the vial, follows a linear trajectory and collides with a vial base transferring entirely its kinetic energy to the trapped powder. In the present experimental conditions, a collision frequency, N, of 29 hit/s was set. The impact energy, E equal to 0.118 J, was obtained as 1/2 mv2 where m is the ball mass and v is the collision velocity. The most representative parameter of the mechanical treatment is the reduced intensity, IM (W/g), given by the product of E and N and normalizing the result to the powder load. A constant IM value of 0.40 W/g was employed. Full details on the ball-mill dynamics and the experimental procedures to determine the milling parameters have been reported elsewhere.14 The X-ray diffraction analysis of combusted powders was carried out with a Siemens D500 diffractometer equipped with a graphite monochromator in the diffracted beam. Cu KR radiation was employed. Patterns were collected in the step-scanning mode under helium flux by making use of a special sample holder. The identification of the diffraction phases was accomplished by the peak analysis routine on the basis of data index powder diffraction files.15 Gas products were sampled from the reactor’s headspace through leak-proof valves and analyzed by a PE 8500 gas chromatograph (GC) equipped with a flame ionization detector. To control the presence of residual hexachlorobenzene or possible organic compounds the reacted powders were treated in n-pentane and analyzed by a Fisons HRGS 5300 GC and a Finnigam TRACKER mass spectrometer GC/MS PE 8420. We caution the reader that sample handling, i.e., mixing and pelletizing of reacting mixture, should be carried out in a strictly controlled inert environment. Possible organic byproducts at incomplete stages of the reaction can be dangerous, and calcium hydride becomes very reactive and will burn at contact with air. Furthermore, milling experiments must be carried out with the greatest care since the reactor can burst under milling. Results and Discussion A precise CaH2-C6Cl6 stoichiometry is required for the complete reduction of organic chlorine. Assuming a straight decomposition route,
3CaH2 + C6Cl6 f 3CaCl2 + 6C + 3H2 (∆H° ) -1709.6 kJ/mol) (1) the CaH2/C6Cl6 molar ratio remains set at 3, which represents the lowest limit of the composition range explored in this work. By increasing the CaH2 content above a molar ratio of 6, the formation of the mixed hydride chloride is favored, according to the following transformation path
6CaH2 + C6Cl6 f 6CaHCl + 6C + 3H2 (∆H° ) -1804.6 kJ/mol) (2)
3220 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
From quoted thermodynamic data,16-17 an enthalpy value of -1709.6 kJ/mol of hexachlorobenzene was calculated for the reaction shown in eq 1. This value then increases to -1804.6 kJ/mol in the case of the CaHCl end-product. As expected from these high exothermic qualities, a self-sustaining behavior was observed in the whole interval of composition tested, extending from a molar ratio of 3:18. This interval is wider than the combustive range observed in SHSM runs,4 (i.e., a molar ratio of 4.3:15). We will return on this point later in the discussion. The reaction evolution was monitored through the video recorder, and regardless of the mixture composition, ignition occurred within 2-4 s after the thermal spike, with release of a large amount of gaseous products. A typical photographic sequence of the selfdegradation process is shown in Figure 2. The combustion front propagation velocity was found in the range 0.5-1.0 cm/s. Note that these values are well within the range of 0.1-25 cm/s typically found in the combustion synthesis of inorganic materials.6-8 To pursue this argument somewhat further, let us consider a typical temperature profile of the reaction front, registered by a thermocouple embedded in the pellet, as shown in Figure 3. A peak temperature of about 1850 K was recorded. To obtain an estimate of the combustion wave temperature under adiabatic conditions, the descending temperature trend was extrapolated back to the time of the rising temperature edge. We obtained a value of about 2400 ( 200 K, which, despite the rough procedure involved, approaches remarkably well the adiabatic temperature, Tad, of about 2500 K relevant to the reaction at the molar ratio of 6 employed in the trial. Tad was calculated as
∆H° )
(Tm - Ti) + ∑∆Hm + ∑Csolid p (Tad - Tm) + Chydrogen (Tad -Ti) ∑Cliquid p p
(3)
where the Cp’s are the thermal capacities of the solid and liquid end-product and of hydrogen, Ti and Tm are the initial and melting temperatures, and ∆Hm is the melting enthalpy. Thus, in this range of compositions, experimental and calculated Tad are well above 1800 K, which, on an empirical basis, is recognized to be the lowtemperature limit for a self-sustaining behavior in inorganic systems.6 It can be noted that the melting temperature of C6Cl6, which is only 505 K, is far below the combustion temperature, and it is probably safe to say that melting occurs below the ignition temperature of the reaction, i.e., ahead of the combustion front. Front propagation rates and wave temperatures cannot be obtained in the case of SHSM. A milling reactor is much like a black box, and at the present stage of development, it is not possible to monitor the inside powder condition directly under milling. It is also not known if the reaction runs under a true self-sustaining regime or occurs in a thermal explosion mode. Nevertheless, additional information was gained from the milling runs. The occurrence of explosive-like phenomena in the course of the mechanical treatment is usually inferred from a sudden increase of the external temperature of the milling container. The milling time at which one marks the temperature jump is referred to as ignition time, tig. One such recording is presented in the inset of Figure 3. The trace shows some of the substantial differences between SHS and SHSM re-
gimes. One notes that long ignition times are required to trigger a combustive reaction by milling -tig values across the combustive range decrease from 34 625 (molar ratio of 4.34) to 12 225 s (molar ratio of 15). Obviously, either this incubation period does not exist in the case of conventional SHS or it pertains to a completely different time scale. This item deserves further comments. It is well-known that in solids undergoing the disruptive action of repetitive mechanical pulses, structural changes are caused and highly defective conditions develop, providing an excess of short-circuit diffusion pathways.18-19 A lessening of the diffusional constraints inevitably determines an enhanced atomic mobility and an intensification of the chemical interactions occurs at each collision event. The result is that, under continuative milling, the free energy of the reacting system increases, progressively approaching the activation energy of the reaction. A particular configuration of the reactants is eventually reached where interdiffusion processes speed up to a critical rate. It can be surmised that the reaction starts initially in the powder trapped at the impact and that the powder self-heating provides for the spontaneous spread of the reaction outward from the immediate impact area. Thus, the precombustion milling time cannot be regarded as a simple pretreatment period during which an optimal thorough mixing of the reactant is attained. tig is a characteristic parameter of the process that defines the total amount of mechanical energy delivered to the reacting system. This quantity, referred to as the dose,20 DM, is equal to IMtig where IM is an intensive quantity and tig is the related extensive factor. In this direction, it has also been reported that a combustive reaction takes place only when a critical dose of mechanical energy has been supplied to the reacting system, irrespective of the IM parameter.4 Therefore, DM results are proportional to the mechanochemical activation energy of the process.14 From these considerations, a substantial difference rises between the two running modes of the reaction. The activation energy of the process and other kinetic constraints control the SHS reaction. These features seem to play a limited role in the mechanochemical regime. A further consequence is that low ignition temperatures are required under milling. One has to consider here the local temperature rises in the powder during the impact, resulting from the amount of kinetic energy dissipated as heat.21-22 For the case in hand, where the collision velocity was of about 3.8 m/s, corresponding to an impact energy of 0.118 J, the local powder temperature increase was evaluated to reach only some 10 K. By also considering that the average temperature of the reactor at the combustion event is usually close to the room temperature (a plateau at about 320 K can be ascertained in the temperature trace shown in the inset of Figure 3), we found that ignition temperatures in the present SHSM experiments are definitely below those needed to trigger the reaction by conventional methods. Unlike the SHS temperatures, they are certainly below the melting point of hexachlorobenzene. As for the combustive range, some comments now seem possible. Obviously, a large excess of one reactant increases the heat capacity of the reacting mixture but does not contribute to the released reaction heat. The reaction loses its high exothermic quality and, once ignited, is unable to self-propagate. In addition to this,
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3221
Figure 2. Propagation snapshots of the SHS reaction in a CaH2 and C6Cl6 compact with a molar composition ratio of 6.
however, a quite different mechanism becomes operative in SHSM. As a rule, the impact energy is shared among
the reactants in a direct ratio with their mass. In these runs, where we set a constant impact energy and used
3222 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
Figure 3. Temperature rise in the middle of the pellet during the reaction advancement. A very rough estimate of the combustion wave temperature under adiabatic conditions was obtained by drawing a straight line through the points beyond the maximum and extrapolating back to the time of peak onset. (inset) Temperature trace of the milling reactor as a function of the milling time. The spike marks the occurrence of a combustionlike reaction. Both traces refer to the case of a reactants molar ratio equal to 6.
the same mixture loads, the CaH2/C6Cl6 weight ratio increased through the interval from 0.65 to 2.23; for the case of a molar ratio equal to 18, for which we did not observe a combustive reaction, the weight ratio is 2.7. Larger amounts of mechanical energy were therefore progressively dissipated into the excess of calcium hydride, leaving the whole mixture below the ultimate limit of structural and thermodynamic stability required to initiate the reaction. It appears that different mechanisms rule the tendency toward fading out of combustion in mixture far from the reaction stoichiometry. In SHSM, the system cannot reach the mechanochemical activation threshold, whereas, in thermally ignited processes, the reaction extinguishes because the heat released is inadequate to sustain the reaction. On the opposite side of the combustion range, safety reasons suggested that we stop the milling experiments at a molar ratio around 4. As already noted in the Experimental Section, these kinds of problems pose some restrictions when carrying out a combustion reaction by milling. Safety problems seem of minor concern in the case of SHS reaction, and it appears as a further merit of this methodology. Nevertheless, the progressively explosive character of the reaction also became apparent in the SHS case, when moving toward higher C6Cl6 contents. Due to the violent evolution of gaseous products, growing amounts of powder were spread out in the reaction chamber. The ratio between the mass of the residual pellet core, P1, and the total mass of the original pellet, P0, is shown in Figure 4 as a function of the reactant molar ratio. The behavior is manifest for the increasingly severe reaction conditions when approaching the stoichiometric range. Let us go back to the inset of Figure 3 to consider another distinctive parameter of a SHSM experiment. Now, we refer to the vial temperature jump, ∆T. Regarding the milling reactor as a bomb calorimeter and taking into account its heat capacity, CMR, the total heat evolved in the course of the reaction can be obtained as ∆TCMR. Adiabatic conditions can safely be assumed owing to the short time scale of the combustion event. The reaction heat per mole of hexachlorobenzene, ∆QV, was then obtained by normalizing ∆TCMR data to the
Figure 4. Weight fraction, P1/P0, of the end products observed in compacted form at the end of the SHS reaction. Results are shown across the whole combustive range.
Figure 5. Experimental reaction heat, ∆QV, as a function of the reactant molar ratio. Lower and upper lines mark the reaction enthalpy limits for the formation of CaCl2 and CaHCl, respectively. Table 1. Survey of the SHSM Main Parameters CaH2/C6Cl6 stoichiometry
C6Cl6 (×102 mol)
∆T (K)
∆QV (kJ/mol)
∆QV/∑Cp (K)
4.34 4.93 6.02 9.02 11.48 14.98
1.87 1.79 1.63 1.32 1.14 0.96
79.6 79.5 69.9 56.8 50.7 42.3
1731 1806 1745 1750 1809 1791
4141 4107 3584 2858 2519 2081
actual C6Cl6 content in the mixture. Further details on this methodological approach are reported in our previous paper.4 ∆T and ∆QV are summarized in Table 1. ∆QV fit remarkably well within the enthalpy range provided by the reaction 1 and 2 above. Furthermore, as shown in Figure 5, an ascending trend is recognizable that, at the upper limit of the combustion range, approaches 1805 kJ/mol, which represents the higher enthalpy benchmark for the formation of CaHCl. Indeed, the mixed hydride-chloride compound is expected to form at a high CaH2 content. In a strict sense, ∆QV expresses the reaction heat at constant volume and cannot directly be compared to the reaction enthalpy. However, in the present case, the ∆PV contribution to the reaction heat is negligible. Let us still consider the experimental reaction heats. In searching for possible connections with SHS results, ∆QV values were divided by the mean heat capacity of the end products at room temperature, ∑Cp, which also includes the unreacted excess of CaH2. The results are
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Figure 6. Ratio of the experimental reaction heat and the mean heat capacity of the products, ∆QV/∑Cp, vs SHS combustion front temperatures. A linear relationship exists between the two data series. Due to a nonexact correspondence of the mixture compositions in the SHSM and SHS trials, the quoted temperatures are interpolated values. Error bars are also shown.
quoted as ∆QV/∑Cp in Table 1. Since the procedure neglects the enthalpy variation terms and the latent heats in eq 3, unrealistically high temperatures were obtained. Nevertheless, the results provide an immediate self-consistent comparison of the powder self-heating across the combustion range and are therefore representative of the reaction adiabatic temperature.23-24 Since ∆QV/∑Cp values characterize the exothermal behavior of the process, it seems possible to relate these quantities directly to the combustion front temperature registered in SHS experiments. The comparison is presented in Figure 6, where ∆QV/∑Cp data are plotted versus the propagation wave temperatures acquired, for practical reasons, with an infrared pyrometer. It should be emphasized, however, that the measured temperatures are those pertaining to the surface of the sample and can be different from the temperatures in the middle (bulk) of the samples by as much as 100-200 K. The correlation is really quite a good one, considering the experimental sources of the two data series. It appears that the transformation paths are ruled by a similar mechanism despite the rather different conditions under which the reaction proceeds. Similar conclusions can be inferred from the X-ray analysis of the solid end-products carried out in parallel at the end of both SHS and SHSM processes. Only CaCl2 (12-0056), CaHCl (14-1079), graphite (26-1079), and unreacted CaH2, were indexed in the diffraction patterns according to the quoted diffraction files.15 An example is shown in Figure 7 that refers to a reactant mixture with a molar ratio equal to 12. It is seen that we did not find residual chlorinated organic compounds in the solid phase. In fact, their content is at the ppm level, as it may be seen in Table 2 where the GC/MS results of the solid-phase analysis are reported. In Figure 7, we also report the pattern of the SHScombusted powders after chemical passivation under an argon-air stream in order to expose the final powders to X-ray analysis without resorting to special conditions. Ca(ClO)24H2O (1-1165), CaClOH (36-0983) and CaCl24H2O (1-1184) formed. The analysis of the gas phase sampled at the end of the self-propagating degradation revealed the presence of hydrogen, carbon monoxide, methane, benzene, toluene, and chloro-benzene, whose content is reported in
Figure 7. Cu KR X-ray diffraction patterns of starting mixture and combusted powders for a CaH2/C6Cl6 molar ratio of about 12. Lower and intermediate patterns refer to the powder reacted under milling and thermally ignited, respectively. The same compounds and similar compositions are observed. Samples were scanned under an helium flux. The upper pattern refers to the SHS end products after oxygen passivation. The more intense peaks of the quoted compounds are marked. Table 2. Organic Compounds and Organochlorine Found in Solid End-Products
a
compound
CaH2/C6Cl6 ) 12a
acetylene benzene toluene naphthalene methylnaphthalene cyclohexylbenzene biphenyl methylbiphenyl ethylbiphenyl chlorobenzene ∑ dichlorobenzene isomers ∑ trichlorobenzene isomers ∑ tetrachlorobenzene isomers pentachlorobenzene hexachlorobenzene anthracene and/or phenantrene terphenyl
718
