PRODUCT REVIEW
Characteristics of Bone Clear Carbon Chung Chi Chou’ and Kenneth R. Hanson Research and Dewlovmeni Division. Amstar Corp.,
CHUNGCHI CHOU, a research chemist at Amstar Corp., receiued a B.S. in chemical engineering from Chen-Kung University, Taiwan, Republic of China and a Ph. D. i n physical chemistrv from Bavlor University (1968). His current research interests are i n the area of solid-gas and solid-liquid interface systems. H e is also active in the area involving the application of physical chemical principles to the sugar refining process. His past experience includes work in project and process engineering i n sugar manufacturing process. ”
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KENNETHR. HANSONis Director of Research and Deuelopment for the Amstar Corp. Before joining this diuision he worked cn seueral o f the company’s cane sugar refineries as a refining technologist and as a superuisor. Later, he uas a project engineer with the Central Engineering Department. H e receiued his B.S. in chemical engineering from T u f t s University in
ed decolorizing and aeasnmg aasoroenr m sugar rennmg processes. I t is a product of the destructive distillation of animal bones. During this process, collagen macromolecules undergo pyrolysis and carbonaceous material gradually “accumulates” on the surface of hydroxyapatite crystals according to a mechanism which possibly involves various degradation, condensation, and hydrogen elimination reactions. After a period of use, hone char becomes exhausted because of the accumulation of organic matter and ash on its surfaces and/or in its pores. It is regenerated by washing and kilning. A hone char research program was conducted from 1939 to 1963 by the National Bureau of Standards under the sponsorship of the Bone Char Research Project, Inc. Much of the previous work was devoted to the development of experimental methods for the measurement of the characteristics of hone chars, and the study of the fundamental aspects of the effects of operational variables on the performances of the adsorbent. Fundamental investigation into the nature and the characteristics of the adsorbent has received less study. In the present work, several methods such as differential thermal analysis, infrared analysis, oxidation studies, and electron microscopical examination were employed in an attempt to define the fundamental nature of the carbon of bone chars and thus to indicate the direction for the improvement of bone char performance in the sugar refining process. Experimental
Material. A. A new bone char made by a British supplier with approximately 80% of calcium phosphate in the hydroxyapatite form, 10% carbon, and impurities (other inorganic salts). B. A stock hone char from a sugar refinery of the Amstar Corp. with about 150 service cycles in the adsorption processes. C. Extracted carbon from new bone char A. The carbon in tlie hone char was isolated for direct examination hv leaching of the hone char, by dissolving acid-soluble ash with hot concentrated HC1 solution and base-soluble ash with KOH solution a t room temperature. I
7.
Prod. Res. Develop.,Vol. 10, No. 1, 1971
To whom correspondence should be addressed
Differential thermal analysis, oxidation studies, infrared analysis, and electron microscopical studies were employed to define the nature of the carbon of bone chars. DTA studies indicate that the carbon phase consists mostly of amorphous carbon with small amounts of hydrogen-rich carbon and pseudographite. Infrared analysis shows that the carbon bears an aromatic skeletal structure. Oxidation studies and electron microscopical examination indicate that the carbon phase is well dispersed throughout the entire structure of bone chars and that the inorganic phase of bone chars provides support for a thin film of carbon with some degree of aggregates. A possible bonding between the x electron of the basal plane carbon and the inorganic surface of bone chars is deduced from these studies.
Table I. Analysis of a Dried Bone Char A Wt
Carbon Silicon dioxide Phosphorus pentoxide Calcium Magnesium Alkaline salts Ferric oxide Carbonate Sulfate
Yo
10.5 1.28 33.45 32.68 0.58 0.2 0.11 3.6 0.22
Table II. Some Structural Parameters of Bone Char A Surface area, m’ta
Total by BET method
Cumulative, pore radius greater than 9A
Cumulative, pore rodius greater thon 20A
105.6
97.5
65.2
Pore volume, m l / g Cumulative, pore radius greater than 20A
Av pore radius,
Total pore
Cumulative, pore rodius greater than 9A
0.22
0.21
0.18
42.3A
2V/A
x 10’
D. A mixture of 105 extracted carbon C and 9 0 5 decarbonized bone char A. Bone char A was decarbonized by burning finely ground bone char A in an oven a t 500” C in air atmosphere for 10 hours. E. A mixture of 10‘; finely ground CAL carbon. an adsorbent made from a special grade of coal and containing about 90‘; carbon, and 9 0 5 decarbonized bone char A. The analysis of a typical new bone char is shown in Table I and some structural parameters of bone char A are shown in Table 11. The cumulative pore volumes and surface area were calculated from the low temperature nitrogen desorption isotherm by Barrett and Joyner’s method (1951). The surface area from the nitrogen adsorption isotherm was computed by Brunauer. Emmett. and Teller’s method (1938). The nitrogen sorption isotherm was obtained by Isorpta Analyzer Model I1 of Engelhard Industries. Inc., Newark, N . J. A detailed study of the physical structure of various bone chars using the low temperature nitrogen sorption technique and the adsorption from solution technique, as well as the significance of the findings in sugar refining operations, will be published elsewhere (Chou, 1970). Differential Thermal Analysis. The DTA were performed with an Aminco 4-4442 Thermoanalyzer in an oxygen atmosphere. Oxygen flowed through the sample at 50 cc per minute at 1-atm pressure. The temperature was increased @C per minute from room temperature to
800” C. The reference material was alpha-alumina (48to 100-mesh). Samples were ground to pass 100 mesh. The amount of sample used was about 0.16 gram. Oxidation Studies. The material chosen was bone char A. The sample was preheated a t 550°C a t 10 ’ mm of Hg pressure for 1 hour to remove as much “carbonaceous” carbon-e.g., hydrogen-rich carbon-as possible. The schematic diagram of the reaction system is shown in Figure 1. The system is designed for oxidation studies in a temperature range of 200” to 800’C,pressure from 2.5 to 50 psig, and a flow rate range from 50 to 500 cc per minute. An oxygen-nitrogen mixture was used as the oxidizing gas. Helium and the oxygen-nitrogen mixture were passed through Mg(ClO,)? and Ascarite to remove water and carbon dioxide before entering the reaction system. The reaction chamber design included a coil with an inside diameter of 1.5 mm, so that the gases would be preheated to reaction temperatures before passing over the sample. The gas pressure in the reaction tube is controlled by a pressure regulator. A gas flow regulator is provided to obtain the desired flow rate. The furnace temperature is maintained within 1 0 . 5 ”C of the desired reaction temperature by a temperature controller. Since the gas is preheated in a small coil inside the furnace, it is assumed that the sample temperature is close to that of the furnace temperature, although during the oxidation period a rise in temperature of as much as 2” C was observed. ‘The oxidation runs were carried out a t temperatures of 300” and 350@Cand pressures of 5 and 10 psig, corresponding to oxygen partial pressures of 3.94 and 4.94 psia. respectively. The samples, usually 2.5 grams. were carefully packed inside the reaction tube with glass wool in the bottom, top, and center of the tube so as to eliminate “channeling” of the oxidizing gas through the samples.
Figure 1 . Schematic diagram of flow reaction system A. Helium tank. B. Oxygen-nitrogen mixture tank. C. Mg(CIO,)I. D . Ascarite. E. Pressure regulator. F. Thermometer. G. Thermocouple. H . Furnace. I . Gas preheoter. J. Temperature controller. K. Gos outlet pressure regulator. I . Flowmeter. M . Reaction tube. N . Flow regulator. P. Pressure gage
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
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The reaction tube was stainless steel with a n inside diameter of 10 mm and a length of 13 cm. The sample was first heated in a helium stream t o prevent oxidation. When the furnace temperature reached the desired temperature and a steady state obtained, the oxidizing gas was substituted for the helium a t a flow rate of 50 cc per minute. After oxidation for the desired time, the oxidizing gas was replaced by helium and the reaction tube cooled to room temperature. The weight loss was obtained by weighing the sample before and after oxidation. I n this flow-type reaction system, the bulk diffusion effect is minimized. Infrared Analysis. For infrared analysis, the finely ground sample was thoroughly mixed with KBr by grinding and pressed into a KBr pellet (0.5 to 2 mg of sample per 300 mg of KBr). Infrared spectra were obtained with a Perkin-Elmer Model 337 infrared spectrophotometer. Electron Microscopical Examination. The electron micrographs were made by the Material Research Laboratory. Washington University, St. Louis, Mo. Three samplesdecarbonized bone char A, stock char B , and new bone char A-were pulverized and mixed separately with 80-100mesh poly (bisphenol-A-carbonate) powder. The separate mixtures were molded a t 190°C (40;C above 150”C, the glass transition temperature of polycarbonate) a t 1000 psi. After molding, each specimen was thin-sectioned t o 1400-A thickness by means of a MT-2 microtome obtained from Ivan Sorvall, Inc., Nonvalk, Conn. Electron micrographs were taken of each thin-sectioned specimen using a Hitachi 11-B electron microscope operated a t 75 kv. No distortion of either the polymer substrate or the embedded bone char particles was observed during the operation. An electron micrograph of new bone char A was also taken using a dusting method. A copper grid coated with evaporated graphite film was used as a support.
by a broad exotherm caused by the oxidation of amorphous carbon. The small amount of crystalline pseudographite was oxidized last, giving the shoulder peak attached to the broad exotherm. I n similar cases, Haldeman and Botty (1959) studied the oxidation of carbon deposits on cracking catalysts and showed that the hydrogen-rich area of the coke deposits was attacked preferentially. Rode and Balandin (1958) obtained a bifurcation of t h e peak on the DTA curves of a coked catalyst which had been used for 1 2 hours. They interpreted the two peaks as further indication of the existence of two types of carbonaceous deposits on the chromia catalyst. Our DTA data reveal that most of the carbon in the adsorbents is amorphous carbon. This is not surprising, since the adsorbents are manufactured a t a temperature not exceeding 1500’F. Under this condition. the carbon produced is expected t o be mostly amorphous in nature. The pseudographite would be expected t o grow and the “carbonaceous” carbon to decrease in quantity with increased heating time and temperature. The oxidation of the three forms of carbon was not resolved on the DTA curves of the bone chars, possibly because the amounts of carbonaceous carbon and pseudographite in the adsorbents are too small to be detected by the DTA under the present experimental conditions. Comparison of Figure 2a and b shows that the oxidation peak temperature (TD14)of extracted carbon was about I
1
Results and Discussion
Differential Thermal Analysis. A typical DTA curve of bone char A is shown in Figure 2a. The endothermic peak below about 175°C is attributed to the desorption of physically adsorbed water from the adsorbent. The exothermic peak a t 380°C is due to the oxidation of the carbon phase in the bone char. T o characterize the carbon phase of the bone char directly. the carbon phase was extracted from the inorganic phase, mostly hydroxyapatite, using the method previously described. Figure 2 b is the DTA curve of extracted carbon C. The broad endothermic peak reveals t h a t the extracted carbon adsorbed an appreciable amount of water. The oxidation exotherm of the extracted carbon centering a t about 480°C is remarkably different from t h a t of the carbon in bone char A (Figure 2 a ) . The exotherms of the extracted carbon show a large exothermic peak with a well defined peak preceding a broad exothermal peak which is followed by an unresolved shoulder peak. The results suggest the presence of three types of carbon in the carbon phase, possibly “carbonaceous” carbon (more or less organic in nature). amorphous carbon, and pseudographite. The amorphous carbon may be visualized as a microcrystalline carbon of graphite-like structure with x electron resonating in a fused polynuclear aromatic ring system. Carbonaceous carbon. having more hydrogen content, was oxidized first. giving rise to a sharp peak, followed 4
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
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0
I
100
200
300
400
TEMPERATURE,
500
600
71
‘C
Figure 2. Differential thermal analysis curves a. British new bane char (somple A )
b. Extracted carbon from bone char A (sample C) c . Mixture of
10% extracted corbon C and 90% decarbonized bane
char A (sample D)
d. Mixture of 10% CAL carbon and 90% decarbonized bane char A (sample E)
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Table Ill. Distribution of Carbon on Bone Char
Carbon occluded by inorganic phase Carbon aggregates Carbon distributed as thin film
I
5.6'~ 14.0' c 80.4'c
100°C higher than that of the carbon in bone char A. This seems to indicate that the carbon in bone char A is more reactive to oxygen than the extracted carbon, possibly because of the catalytic effect of the inorganic phase of the adsorbent. Two possible factors may account for the catalytic effect of the inorganic phase on the oxidation of carbon. First, the possible interaction between the T electrons of the basal plane carbon and the inorganic surface may weaken the bonds between carbon atoms and thus increase the rate of oxidation. The second factor is the effects caused by impurities. Certain impurities in small amounts can strongly catalyze the reaction of carbon with oxidizing gases (Walker et al., 1939). For example, in the study of the oxidation of coke on aluminosilicates, Goldstein (1966) found that the oxidation peaks shift to lower temperatures with in DTA curves addition of certain cations into the coked catalysts. I t is known that the bone char, in addition to 80$ hydroxyapatite, contains many other cations (Table I ) , which might serve as catalysts in the oxidation of carbon. T o study the possibility of the catalytic effect of impurities in carbon oxidation, a sample of bone char A was decarbonized to obtain the inorganic phase of the bone char. Extracted carbon, equivalent to the carbon content of the bone char. was then added to the decarbonized bone char (inorganic phase) to make a physically combined mixture (sample D ) . The DTA curve of the mixture was obtained (Figure 2c). The oxidation peak temperature (To.ra)of the carbon in the physically combined mixture is about 507'C, which is close to the TITI., of the extracted carbon (Figure 2 b ) but more than 100°C higher than the TIyr,4of carbon in the original bone char (Figure 2 a ) . The result seems to discount the possibility that the impurities in bone char participate as catalysts in the oxidation of carbon. If the impurities alone in the bone char had a catalytic effect in the physical mixture, the T I Y I . ,of ~ the carbon in the extracted carbondecarbonized char mixture would be close to that of carbon in bone char. T o check further the effect of impurities in bone char on the oxidation of carbon, a DTA curve was obtained for a mixture of decarbonized bone char and CAL carbon equivalent to the original carbon content of the bone char. The oxidation temperature for the CAL carbon mixture (T13T,l,528"C),as shown in Figure 2 d , is only slightly higher than that for the extracted carbon mixture ( T D . I . 507" , ~ , C) but about 150" C higher than that of carbon in bone char. The results favor the explanation that the speculated catalytic effect of the inorganic phase on the oxidation of carbon is initiated through the possible interaction between the 7i electron of the basal plane carbon and the inorganic phase rather than through the specific effect of impurities alone. Alternatively, the results may indicate that the impurities in a merely physical mixture of carbon-decarbonized char do not cause catalysis; rather, the catalysis must depend on the intimate association between carbon and impurities in the original bone char. From the above discussion, a general conclusion may
-a
/
$ 1 UI rn
b---
-TEMPERATURE,30O0C, PRESS.lOpsig TEMPERATURE,35O0C.PRESS.iOpsig
501
0
J
I
1
1 I
301 ri 20 0
5
I
I
10
15
TIME,
I
20
25
30
min.
Figure 3. Effect of time, pressure, and temperature on oxidation of carbon on bone chars
be reached. The proposed catalytic effect of the inorganic phase (hydroxyapatite and/or impurities) on the oxidation of the carbon is most likely due to the close association between the carbon and inorganic phase in the original bone char, possibly through the interaction of 7i electrons of basal plane carbon with the inorganic phase. Oxidation Studies. The study of the oxidation of carbon in bone char furnishes some information on the distribution of carbon in bone chars. A new bone char A was used in this study. Figure 3 shows the effects of time, pressure, and temperature on the oxidation of carbon. For the samples studied, the total carbon burnoff leveled out after 13 minutes. The total weight loss a t the plateau of the curves is taken as the maximum accessible carbon. This increased with increased pressure of the oxidizing gas (Figure 3b and c ) . The result is not surprising. In the oxidation experiment, the sample was heated in a helium atmosphere to the oxidation temperature for 45 minutes before replacing the helium with oxidizing gas. I t is expected that the helium in the "dead end" pores and! or relatively small pores would not be completely replaced and the partial pressure of the oxygen inside the pores would be proportional to that outside the pores. Since the amount of carbon oxidized in these pores depends on the partial pressure of oxygen inside the pores. the maximum accessible carbon is expected to increase with increasing pressure of the oxidizing gas. The result indicates that the diffusion of oxygen into these micropores is significant in the oxidation of carbon in the bone char studies. and that the carbon in bone char is distributed throughout the entire structure of the bone char. The effect of temperature on the maximum accessible carbon was also studied (Figure 3 a ) . The increase in maximum accessible carbon with increase in temperature is appreciable, implying that there is some carbon which can be oxidized only a t a higher temperature. The increase in maximum carbon burnoff is not likely due to a higher gas diffusion rate a t a higher temperature. This possibility Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
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ADSORPTION
- - - - - - - DESORPTION n W
g
100-
0 v)
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z
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-
W
W 0 L
t
” z
50-
w
’
l
25
01 0
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RELATIVE
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I
0.4
0.2
0.8
0.6 PRESSURE,
1.0
p/p,
Figure 4. Nitrogen sorption isotherms at -195°C for bone chars a . British new bone char A
b. Stock bone chor B
is ruled out by the fact that the maximum carbon burnoff is independent of time (Figure 3). If the rate of gas diffusion was important, the maximum carbon burnoff should have increased with time of oxidation. The result could be explained in terms of the activated entry of oxygen molecules into the “ink bottle” type pores which have “neck” diameters of about the same size as oxygen molecules. When a bone char pore contains a constriction slightly wider than the diameter of an oxygen molecule. each molecule must pass an energy barrier of height E in order to enter the constriction. The number of molecules which enter the constriction (and thus are available for the oxidation of carbon inside the pore) are proportional to Ae ’ “, which increases exponentially with rise in temperature. This “activated entry” effect may also be important for the transport of the reaction product out of the ink bottle type pores. The result confirms the 7
E
LL
0
0 I
09
0.8
0.7
CARBON
0.6
0.5
0.4
FRACTION REMAINING,
0.3
0.2
0.1
c/ce
Figure 5 . Rate of carbon oxidation vs. carbon fraction remaining 6
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
presence of ink bottle type pores in bone char as found in the low temperature nitrogen sorption studies and the adsorption from solution work (Chou, 1970). I n the nitrogen sorption isotherms of bone chars, the hysteresis loop as shown in Figure 4 was attributed to the ink bottle type pores in the relatively larger pore region ( > 2 0 A ) . The present work indicates the presence of ink bottle type pores in the micropore region ( < 2 0 A). The disappearance of the hysteresis loop below a radius of 20 A (Figure 4) does not necessarily imply the absence of ink bottle type pores in this region, but rather the limited applicability of the Kelvin equation in the micropore region. Further information on the distribution of carbon in bone chars can be elucidated from the rate of oxidation data. The carbon of a new bone char A was oxidized using the method previously described. The rate of oxidation was calculated as a function of the carbon fraction remaining (Figure 5 ) . The rate of oxidation at an early stage of combustion is relatively low, then increases as the carbon fraction remaining decreases, and finally remains fairly constant over the range of 0.65 to 0.25 C,lCo, where C is the carbon remaining and Co the maximum accessible carbon at a temperature of 350°C and pressure of 10 psig (Figure 3). I n the later stages of oxidation, the decreasing rate presumably is due to the oxidation of carbon by intradiffusion of oxygen into the micropores. The interpretation of the shape of the oxidation rate curve may be related t o the distribution of carbon in the bone char. I n a similar study of the oxidation of coke deposits on cracking catalysts, Haldeman and Botty (1959) obtained a burning rate curve with a relatively high initial burning rate, and showed that the hydrogenrich sites of the coke were attacked preferentially. The low initial burning rate in the present work suggests the absence of carbonaceous carbon (presumably hydrogenrich carbon) in the bone char as expected, because the samples were heat-treated a t 550°C in vacuum for 1 hour before the oxidation runs in an effort to remove the carbonaceous carbon.
-0.6
-
is supported by the fact that the B E T surface area of bone char was found to be relatively constant during the decarbonization of bone char from about 9 to 1‘ carbon (Bennett and Abram, 1967). The implication that the inorganic phase of the bone char provides a support for a thin film of carbon with some degree of aggregates. Assuming the initial 15% of carbon burned is carbon aggregates, and using the maximum accessible carbon obtained from Figure 3, the distribution of carbon in bone char is calculated and listed in Table 111. The total carbon content of bone char used in the calculation is 8 . 9 7 , as determined by a combustion method (Chou, 1969). The average thickness of the carbon film was calculated to be 2.51 layers, using an interplaner distance of 3.6 A and a carbon surface area of 62 m’ per gram of char, obtained by the technique of adsorption from solution (Chou, 1970). The kinetics of the oxidation of carbon of bone char was studied in the temperature range of 350“ to 400°C with an oxygen partial pressure of 0.42 psia (in an oxygennitrogen mixture with 2.8‘; O1).using a flow reaction system. The activation energy was found to be 2 1 kcal per mole as obtained from Figure 6. This value is considerably lower than the activation energy for the oxidation of coke or carbon found in the literature. For example, Gulbransen and Andrew (1952) found a value of 37 kea1 per mole, Blyholder and Eyring (1955) obtained 42 kcal per mole for graphite oxidation, and Massoth (1967) employs 40 kea1 per mole in the oxidation study of coked silica-alumina catalysts. If the breaking of the carbon-carbon bond is the ratelimiting step in the oxidation of carbon, the low value of activation energy obtained in the present work may be attributed to the catalytic effect of the inorganic phase as previously proposed. However. under the experimental condition using 2.SCc O? in N ? atmosphere, the diffusion effect may prevail, which would then also give a low activation energy. Research on this phase is in progress. Infrared Analysis. A typical spectrum of bone char A is shown in Figure 7a. The strong bands in the 1100-
-0.8
c E
;;-0.9 w . 0
‘Z : O S, 0
-1.0
G
I
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1.60
1.55
[
1.50
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Activation energy for oxidation of carbon of bone char A
Based on the DTA study, it has been proposed that the oxidation of carbon in the bone char may be catalyzed by the inorganic phase of the bone char. I t has been further shown that the catalytic effect of the inorganic phase is likely to be initiated through the interaction between the T electron of the basal plane carbon and the inorganic surface. Therefore, the catalytic effect would be expected to prevail for the carbon “bonded” to the inorganic surface. The argument suggests that the low initial burning rate may be attributed to the noncatalytic oxidation of carbon aggregates which are not directly “bonded” to the inorganic surface. The rate of oxidation increases as the carbon aggregates are consumed, exposing the “inner” carbon, which is closely associated with the inorganic phase. The high burning rate in the range 0.65 to 0.25 C Co is ascribed to the catalytic oxidation of the “inner” carbon, The increase in burning rate in the range from about 0.9 to 0.6 CiCo (Figure 5) may not be attributed to the development of new surface area as the result of the decarbonization of bone char. This I
I
-c 3 z
W
z
Figure 7. Infrared spectra
t-4
o . British new bone char A
2
W
a
b. Decarbonized bone char A
w
c. Extracted carbon from bone char A (sample C)
V
z a
m U 0 e7
m
a
t v 1
4000
3000
,
I
2000
1800
1600
FREQUENCY,
, 1400
, 1200
I
1000
800
600
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CM-’
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
7
water in the extracted carbon. T h e two small hands between 700 and 850 em-' may be attributed 'to the aromatic hydrogen out of plane bending. T h e CH stretching region from 2800 to 3150 cm-' is not resolved, indicating a low hydrogen content in the carbon. Incidentally, comparison of the infrared curve ,of hone char A (Figure l a ) and that of the extracted carbon (Figure 7 c ) shows that no detectable inorganic phase remains in the extracted carbon. The infrared analysis indicates that the carbon in hone char has an aromatic skeletal structure with low hydrogen content. I t is interesting to point out the behavior of the hand at 632 cm-'. The spectra of all new chars from various sources we have studied show no hand a t 632 cm-'. New hands, however, appeared at 632 cm-' in the spectra of all stock chars of various origins and decarbonized new hone char A (Figure I b ) , and increased in intensity with length of service. Fowler and his coworkers (Fowler et al., 19661, in the infrared study of hydroxyapatite and its related compounds, reassigned the band at 631 cm." to the librational mode of hydroxyapatite. The fact that the band at 631 cm-' appeared only in the spectra of stock chars and decarbonized char suggests that the assignment of the band a t 632 em-' demands further study. I n practical application, the behavior of the hand a t 632 em-' seems to be of importance in connection with the hone char regeneration process. Electron Micrographs of Bone Char. Several hone chars were examined electron micrographically. Figure 8 shows .I . me m e general appearance 01 a new Done cnar using dusting method. idost of the inorganic salts and carbon are not resolved in'to separate phases, and the aggregates -:"h+ +., Cn,. L....n" have the appearanc.,.~ LLi.h..b nlllulrlluuu phase. I n some areas, the inorganic crystal can he seen on the edg e of the aggregate particles. The mi(Zrographs of thin-sectioned samples are shown in Figure
