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C. R. KINNEY and DONALD SCHWARTZ' Department of Fuel Technology, The Pennsylvania State University, University Park, Pa.
Partial Air Oxidation of Chattanooga Uraniferous Black Shale b Humic acids are formed similar to those obtained from coal and seem to have essentially a quinoid carbon structure
CHATTANOOGA
black shale is a sedimentary deposit containing trace amounts (less than 0.01%) of uranium. Because of its large lateral extent, lying under several eastern states, the quantity of uranium contained is large, and is a potential source of this material. The deposit contains organic matter, pyrite, and other minerals whose utilization or elimination should be considered in the development of an economically successful process for recovery of uranium. Therefore, the chemistry of how these substances were deposited in the shale and their chemical behavior on treatment are of particular interest. The shale is composed of mineral grains in a very fine state of subdivision cemented together by a matrix of organic matter. The percentages of components vary greatly, but average samples contain about 60% of siliceous minerals, quartz, feldspar, and clays, 15% of pyrite, 5% of a wide variety of minerals in small amounts, and 20% organic matter. A chemical analysis of the sample used in the work is given in the table at right. The sample was obtained from the Sligo adit in Tennessee. Properties of Organic Matter
The coating of organic matter on the mineral grains makes the mineralogy of the shale particularly difficult. The organic matter is insoluble in common organic solvents and cannot be recovered satisfactorily by this means. By removing the mineral constituents with boiling hydrochloric acid, and evaporating with 48Yo hydrofluoric acid and cold dilute (1 to 3) nitric acid (70), the organic matter can be largely freed from the inorganic components. Carbon and hydrogen analyses following these treatments are given in Table I. The carbon percentage of the organic matter following treatments of the shale with boiling hydrochloric acid, calculated on an ash-free basis (Table I), is markedly lower than samples 2 an8 3, from which the siliceous material had been removed by hydrofluoric acid treatment, and the hydrogen is considerPresent address, Villanova University, Villanova, Pa.
ably higher. These differences are believed to be due partly to water of hydration of the siliceous matter, which is given up during combustion, and partly to the oxidation of pyrite, which is converted to iron oxide with a smaller molecular weight. The loss of water of hydration increases the observed percentage of hydrogen because the water of hydration is weighed with the water of combustion; the decrease in weight of ash due to the oxidation of pyrite tends to lower both carbon and hydrogen percentages. Consequently the analysis of sample 3 containing but 1.8% ash is assumed to be more representative of the kerogen of the shale under study. However, treatment with strong acids and exposure to air probably had an appreciable effect upon its composition. Compared with typical oil shale kerogens (3, 9 ) the organic matter of the Chattanooga shale is low in carbon and hydrogen and high in oxygen (plus sulfur and nitrogen). For example, Green River (Colorado) oil-shale kerogen contains 76.2% carbon, 10.0% hydrogen, and 13.8% oxygen (plus sulfur and nitrogen). The low percentage of hydrogen (5.3%) in the Chattanooga kerogen is in the range of most coals and probably is the main reason for low yields of oil on distilling this shale. The high percentage of oxygen (plus sulfur and nitrogen), 30.3%, which is similar to low-rank coals, is unusual for a shale and indicates the presence of more oxygen-containing groups. This is of particular interest in attempting to produce oxygen derivatives of practical value from shale. Infrared Spectra. The structure of the acid-treated kerogen containing 1.8% ash was studied using infrared spectra obtained from the finely powered material using the pressed potassium bromide-window technique
Analysis of Chattanooga Shale" Component % ' Component % ' Si02 47.9 Fe 6.9 A1208
Pzo5 Mgo Ti02 NazO CaO
10.0
S(su1fide)
6.6
0.7 0.7
S (organic) SOa--
0.9 0.2 13.7 1.5
0.6
C
0.4 0.1
H
Total
90.2
Determined by wet methods; carbon and hydrogen determined by combustion. a
(77). Spectrogram 1 in Figure 1 shows the characteristic absorptions. The strong band a t 2.95 microns is probably due mainly to hydroxyl groups. This does not rule out the likelihood of nitrogen-hydrogen bonds which absorb also at 6.2 microns. The character of the hydroxyl absorptions in the region of 7 to 10 microns indicates that several types of alcohols are present. It is also likely that ether structures contribute to the general absorption in this region. Carbonyl groups absorb in this region too, but the weakness of the absorption at about 5.8 microns shows that the group is essentially absent. I n fact, the small absorption observed is probably due to oxidation of alcohols to carbonyl groups by the cold dilute nitric acid used to remove the pyrite. Considering the ease of oxidation, the 6.2-micron band is possibly due to carbon-carbon double bonds. The 6.6micron band that accompanies the 6.2micron band in many aromatic structures which absorb at 6.6 microns is not important. The carbon-hydrogen stretching absorptions seem to be mainly aliphatic, although a small amount of aromatic carbon-hydrogen absorption may be the cause of the breadth of the hydroxyl absorption which extends u p to 3.35 microns. The atomic hydrogen-carbon ratio is almost exactly 3 . This suggests that a part of the structure must be aromatic or highly unsaturated if aliphatic. The lack of definite absorption peaks in the 8- to 13-micron region probably is due to an insufficient concentration of definite types of olefinic or aromatic structures to give sharp peaks. As the organic matter is insoluble in the common organic solvents, it is assumed to be of a macromolecular character, and because of the high-oxygen content, it is assumed further that the unit molecules are condensed extensively by ether linkages. No definite absorption characteristic of ether linkages can be observed in the spectrum; however, the broad absorption from 7.5 to 9.5 microns covers both aromatic and aliphatic ether absorptions and several of the hydroxyl bending vibrations. Since no characteristic peaks appear in this region, it is assumed that fusion of absorptions of several oxygen-containing groups occurs and that these include the possibility of both alcohol and ether groups. VOL. 49, NO. 7
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2 COAL ACIDS, 4.3% ASH
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3SHALE ACIDS 13.470ASH
A
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Figure T .
infrared spectra of kerogen and humic acids
Low-Temperature Air Oxidation
For coriverting the organic matter of the shale into soluble products, partial air oxidation at 150" to 300' C. was studied. Apparatus and methods used previously for coals ( 4 ) were selected because there would be less attack of the mineral constituents: particularly the pyrite, than with aqueous oxidizing reagents. However, pyrite is oxidized under conditions favorable to the conversion of the organic matter to soluble products, and when the oxidized shale is treated with water a strongly acidic solution results because of the formation of sulfuric acid, as well as ferrous and ferric sulfates. This acid solution extracts a considerable part of the uranium, but a smaller part remains with the resulting water-insoluble humic acid like oxidation products and is extracted by alkali with humic acids and a rather large amount of siliceous mineral matter. Maximum yields of the humic acid p o d u c t are obtained at 200" C. At lower temperatures, the rate of oxidation is sharply decreased, and at higher temperatures oxidation to carbon dioxide decreases the yields. The effect of oxidation time is also important. At 200" C., yields a t 150, 200, and 250 hours are 14.7, 18.4, and 18.37',, respectively (Table 11). Because of the large amount of ash contained in these acids, it is better to compare them on
Table 1.
an ash-free basis, which gives 11.O, 14.3, and 14.1%, respectively. As the maximum yield was obtained a t 200 rather than at 250 hours, it is assumed that the humic acids already formed are slowly oxidized a t 200" C. and that after 200 hours more humic acids are destroyed than are produced. Stepwise oxidation might give still larger yields. Carbon Analyses. The ash-free analyses of the humic acids decreased from 63.9Y0 at 150 hours of oxidation to 54.6y0 a t 200 hours and to 52.770 a t 250 hours (Table 11). Increasing the time of oxidation increased oxygen content materially. The percentage of hydrogen appears to remain essentially constant, suggesting that the additional oxidation did not involve removal of hydrogen from the carbon structure. Although the 150-hour humic acids contained markedly more carbon, the maximum conversion of the original carbon of the shale to humic acids, 56.9%, was at 200 hours (Table 11). Smaller yields of humic acids were obtained a t higher oxidation temperatures. At 225' C. (Table II), the yields are 11.2 and 11.3% for 150 to 200 hours, respectively. At still higher temperatures the yields fell off sharply and were not included in the table. The carbon contents of the 225" C. acids were relatively high, particularly the 200-hour acids which contained 61.8yG carbon.
Analyses of Organic Matter Concentrates on Moisture-Free Basis As Received, % Ash C H
Ash-Free Basis, % 0 (by C H d8.)"
Sample 1.7 58.2 6.8 14.5 75.1 Shale treated with hot HC1 3.7 63.3 5.4 44.6 29.5 Res. 1 treated with hot HFb 5.2 64.4 5.3 63.2 Res. 2 treated, cold, dil. " 0 3 1.8 Including sulfur and nitrogen. * As-received sample also contained 17.8% total 9, 15.27, pyritic S, 2.6% organic 8.2% Fe. ~
1 126
INDUSTRIAL AND ENGINEERING CHEMISTRY
35.0 31.3 30.3 S, and
This was probably due to higher decarboxylation rate at the higher temperature, which would increase the carbon content of the 225' C. acids. The percentage of total carbon of the shale converted to humic acids a t this temperature is still over 50'%, despite the lower weight yield. Little change in the percentage of hydrogen with increased oxidation temperature, compared with the values a t 200' C., indicated that the carbon structure had been stripped of all reactive hydrogen atoms a t 200' C. The distribution of the carbon of the shale when oxidized under conditions giving the maximum yield of humic acids--i.e., 200' C. for 200 hours with -200 mesh shale and with an air space velocity of 9.3 hour-l--is shown in Table 111. Under these conditions 12.57' of the carbon was oxidized to carbon dioxide and 70.4% remained in the oxidized shale. The balance of 17.17' was converted partly to volatile acids, among which acetic acid was identified, and the remainder was no doubt carbon monoxide ( 3 ) , which is not determined quantitatively. Examination of Oxidation Products. The water-soluble products were extracted with water at 80" C. The yield amounted to 12.1y0 of the raw shale (Table 111). I n addition to sulfuric acid, iron sulfates, and a considerable part of the radioactivity, 0.57' of the total carbon was extracted as water-soluble acids. These were recovered by extracting the watersoluble products, after evaporating to dryness, with ether. After the oxidized shale was extracted with water, it was treated repeatedly with 5% sodium hydroxide, and the humic acids were recovered ( 4 ) . Thcse acids amounted to 18.4y0 of the raw shale and accounted for 56.9% of the carbon (Table 111). The alkaliinsoluble shale residue contained 67.5YG of the original shale and only 2.97' of the carbon. The losses, 3.470 of the raw shale and 10.17, of the carbon, were due mainly to loss of water-soluble acids extracted with humic acids by the alkali but not precipitated with them on acidification. As the quantity was small, no attempt was made to recover them. This method of treating the shale completely destroys the cementing action of the organic matter. I n fact, 10- to 20-mesh shale oxidized for only 100 hours a t 200' C. undergoes complete disintegration when the water-extracted shale is treated with alkali. If this mixture is allowed to settle quietly, extensive stratification of the mineral matter occurs. Probably differences in specific gravity and particle size are mainly responsible for the separation of the mineral grains, but the natural dispersing action of the sodium hurnatrs seems to have an effect. For a more detailed studv of the humic
acids, a larger quantity was prepared at 200' C. for 200 hours. This sample contained 20,6y0 ash, including a portion of the uranium, which could not be reduced materially by washing with water or with dilute hydrochloric acid which depresses peptization of the humic acids. A spectrographic analysis of the ash (before electrodialysis of the humic acids) showed that it contained large amounts of silicon, ahminum, and sodium with smaller quantities of iron, titanium, magnesium, and calcium (Table IV). Since the aluminum, iron, magnesium, and calcium were not soluble in dilute hydrochloric acid when the humic acids were precipitated, it is assumed that the ash is composed of complex silicates and that the sodium is mainly adsorbed sodium chloride. However, there appears to be some correlation between amount of sodium retained by humic acids and amount of siliceous material associated with different samples of these acids. Electrodialysis. When the acids containing 2O.6Y0 ash were subjected to electrodialysis most of the sodium ions were removed in 24 hours. From 24 to 56 hours no sodium ions were detected in the cathode chamber, although sodium was still retained, as shown by the presence of 2.6% in the ash of the humic acids, calculated as the oxide (Table IV). The humic acids after 56 hours of dialysis still retained 13.4% ash and a part of' the total uranium. Taking into account the loss of sodium, the relative composition of the main constituents was essentially unchanged. Presumably the iron, magnesium, calcium, and some sodium were sequestered by the aluminosilicate complex. Evaporation of these acids containing 13.4% ash with 48% hydrofluoric acid, followed by removal of soluble fluorides with water, reduced the ash content to 2.7%. As silicon was vaporized by this treatment, it is clear that the other ' inorganic constituents which became water soluble were originally held in a silicate complex. On dissolving the humic acids, containing 2.7% ash, in dilute sodium hydroxide and reprecipitating with hydrofluoric acid, the ash content was raised to 4.370, no doubt because of retention of sodium fluoride. Carbon and hydrogen analyses, calculated to an ash-free basis, of the humic acids (20.6Y0 ash), the electrodialyzed acids (13.4% ash), and the hydrogen fluoride-treated acids (4.3Y0 ash) gave inconsistent results (Table V). Since the percentage of carbon increases with decrease in ash and the hydrogen decreases, it seems probable that the cause was water of hydration retained by the siliceous mineral matter when the humic acids were dried at 105' C. If it is assumed that the difference between the percentage of hydrogen of the original preparation on an ash-free basis
Table II. Yields and Analyses of Humic Acids Prepared under Different Conditions and Carbon Percentage of Shale Converted to Humic Acids"
H
Shale Carbon in Humic Acids, %
200
150 200 250
12.9 18.4 18.3
14.7 21.1 22.9
54.5 42.4 40.6
2.7 2.7 2.5
11.0 14.3 14.1
63.9 54.6 52.7
3.2 3.5 3.2
51.3 56.9 54.2
225
150 200
13.0 13.3
14.2 15.0
54.8 52.6
2.9 2.6
11.2 11.3
63.9 61.8
3.4 3.1
52.0 51.1
Conditions OC. Hours
a
Yield
As Received, % Ash C
% . YieldAsh-Free, C
H
-200-mesh shale, air space velocity 9.3 hour.-'
and that of the purified acids containing 4.3% ash (0.7%) is due to water of hydration, the original acids contained 6.3% water. On this basis the ashand hydrate-free carbon and hydrogen become 63.6 and 2.6% (Table V). O n the same basis, the electrodialyzed acids with 13.4y0ash contained 64.9% carbon and 2.7% hydrogen. The higher percentage of carbon may be due to the transfer into the anode chamber, during the electrodialysis, of a small amount of water-soluble organic acids which presumably have a lower percentage of carbon. This is not borne out by the analysis of the acids further treated with hydrogen fluoride, which contained only 63.2% carbon. Possibly this lower value is due to additional change when the acids were evaporated to dryness with hydrogen fluoride. Assuming that the latter acids retained water according to their ash content (4.3%) and in the same proportion as that calculated for the original sample-Le., 6.3% for an ash content of 20.6yo-their moisture content would be 1.3% and the recalculated percentage of carbon would be 64.1%, hydrogen 2.5%, sulfur 2.8%, nitrogen 2.2QJ,, and oxygen 28.4y0 (by difference) (Table VI). This composition is similar to that of humic acids obtained by the air oxidation of coals a t 200' C. (4, 7) or by the alkaline permanganate oxidation of a shale low in hydrogen such as the St. Hilaire in France (3),but it is in marked contrast with more normal oil shales which contain more hydrogen, such as the Green River shale (9).
Table 111. Yields of Oxidation Products and Distribution of Carbon" Total Yield, Products Carbon dioxide Oxidized shale Volatile products (by diff.)
C,
%
%
6.3 101.4
12.5 70.4
...
17.1
Oxidiaed Shaleb Water-soluble products 12.1 0.5 Alkali-soluble humic acids 18.4 56.9 Alkali-insoluble residue 67.5 2.9 3.4 10.1 Losses (by diff.) Totals 101.4 70.4 a Optimum conditions: 200' C., 200 hours, -200-mesh shale, air space velocity 9.3 hour.-' Percentage of raw shale.
Table IV.
Spectrographic Analysis of Humic Acid Ashes" Humic Acid Ash, % After Before After boiling dialysis dialysisb NaOHC
Constituent 83.3 43.9 70.4 Si02 19.4 5.2 AlnOa 13.8 4.2 0.8 Fez03 2.2 1.0 1.0 1.6 Ti02 1.5 0.2 MgO 1.0 0.3 0.1 CaO 0.2 37.gd 2.6 9.4 Na2O a Trace elements not included. Actual estimations totaled 107.2%; values adjusted proportionately to make 100%. Actual estimations totaled 96.1%; values adjusted proportionately to make 100%. Adjusted t o make 100%.
Molecular and Equivalent Weights. Molecular and equivalent weights of two samples of humic acids are given in Table VI1 [methods described in (S)]. The first sample was part of the original sample containing 21.1% ash (Table 11) which had been evaporated four times with 4870 hydrofluoric acid, reducing the ash to 9.9%. This sample gave a molecular weight of 245 and an equivalent weight of 169. These values are similar to those obtained from nitric acid-oxidized bituminous coal (8) but are somewhat different from those from the second sample of shale humic acids or the air-oxidized coal
humic acids and permanganate-oxidized shale humic acids ( 3 ) shown in Table
VII. The molecular and equivalent weights of the second sample of Chattanooga shale humic acids, which was the large preparation after electrodialysis with 13.4y0 ash (Table V), were materially higher than the first (Table VII). This is probably due to less efficient oxidation during preparation of the large sample (20.6Y0 ash), as evidenced by the higher content of carbon on an ashfree basis, 58.6 and 54.670. The higher molecular weights of the VOL. 49, NO. 7
JULY 1957
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Analyses of Purified Humic Acid Samples
Table V.
Ash-Free Basis,
As Received, %" Ash C H 2.6 20.6 46.5 13.4 '53.9 2.6 2.5 4.3 60.5
Sample
%
C
H
Ash- and HydrateFree Basis, % C H 63.6 2.6 64.9 2.7 64.1' 2.5c
58.6 3.3 Original preparation Electrodialyzed* 62.2 3.0 2.6 63.2 Treated with H F a Average of duplicate samples. Contained 2.4% S and 1.8% N as received; 2,7% S and 2.1% K ash-flee; 2.5% S and 2.2% N ash- and hydrate-free. Calculated on basis that acids containing 4.3% ash retained (4.3/20.6) X 6.3y0 of water.
Analyses of Shale and Coal Humic Acids Prepared by Air and Alkaline Permanganate Oxidation"
Table VI.
Humic Acids
c, %
Shale, Chattanooga Coal, Pittsburgh
64.1 65.6
0 (by Diff.),
H, % S,% Air Oxidation at 200' C. 2.5 2.3
2.8 1.0
Alkaline Permanganate Oxidation 2.4 1.1 Shale, St. Hilaire 64.0 9.2 0.8 Shale, Green Riverb 67.5 Li Ash-free basis. * Humic acids soluble in ethyl alcohol.
Table VII.
%
2.2 1.4
28.4 29.7
1.6 1.9
30.9 20.6
Molecular and Equivalent Weights of Humic Acids Ash-Free 4sh, C,
Source and Method of Preparation Chattanooga shale, air oxidized at 200° C. Chattanooga shale, large sample electrodialyzed Freeport coal, " 0 3 oxidized Pittsburgh coal, air oxidized at 200' C. St. Hilaire shale, alkaline KMn04 Oxidized
Table VIII.
N , 70
%
%
9.9 13.4 1.3 4.3 1.0
54.6 58.6 62.2 65.6 64.0
Mol.
Kt. 245 369 234 190
198
Eq. Kt. 169 201 166 206 194
Ratio 1.5 1.8 1.4 0.9 1.0
Analysis and Distribution of Products of Hydrogen Peroxide Oxidation
Products Original humic acids Water-soluble acids Water-insoluble residue Carbon dioxide (calcd.)" Water-soluble acids Water-insoluble, alkalisoluble residue Alkali-insoluble residue Carbon dioxide (calcd.)a
Yield, As Received, % Ash-Free, % Distribution of G./IOOG. Ash C H C H c, % First Treatment with 30% HzOz
...
37.7 48.0 60.8
17.5 8.0 28.4 % .
.
49.1 44.0 36.7
2.8 2.8 2.5
.. . ...
59.5 47.8 51.2 '
s..
Second Treatment with 30% HzOsb 35.7 5.0 39.3 3.2 41.4 26.8 72.5 12.9 1.5 46.9 2.7 70.5
... ...
... ... ... ... ..
3.4 3.0 3.5
.
*.
...
33.8 35.9 30.3 (by diff.)O
3.4 5.5
13.7 3.4
...
...
I . 18.8 (by diff.)a,c Including small quantity of volatile acids. Water-insoluble residue from first oxidation was used, 35.9% of total carbon. T o make 35.9%.
Chattanooga shale humic acids suggest that the unit molecules of the original macromolecules are larger than those of coals and the St. Hilaire shale, but they may be due to chemically combined silicates. The mineral matter in these acids disperses easily in the melted catechol, used as the cryoscopic solvent for molecular weights, with no evidence of macroscopic Tyndall effects. Infrared Spectra. The shale humic acids are not only similar to the coal humic acids in composition and molecular and equivalent weights, but give similar infrared spectra. A spectrogram of coal humic acids obtained by
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oxidizing high-volatile ,4, Pittsburgh seam, bituminous coal with air at 200' C. (7) is shown in Figure 1 as No. 2. Compared with it are spectrpgrams of two samples of shale humic acids. No. 3 was obtained from electrodialyzed acids retaining 13.4y0 ash and No. 4 from acids containing 17.5y0 ash, prepared from 10- to 20-mesh shale instead offrom the -200-mesh sample. Because of the larger particle. size of the shale, oxidation was less efficient, and the yield of humic acids was only 11.970. These acids contained 59.5% carbon and 3.4% hydrogen on an ash-free basis. The similarity of the three spectra
INDUSTRIAL A N D ENGINEERING CHEMISTRY
is apparent. The strong peak at 5.8 microns is due to the introduction of carboxyl groups into the structure of the organic matter. This is further shown by increased absorption at about 3.2, 7.2, and 8.0 microns. Associated acids also absorb at about 11 microns, and the coal humic acids show some absorption at this point, but the shale acids do not, indicating that these acids are not associated through their carboxyl groups to any great extent. The increased absorption in the region of 9.7 microns, particularly in the shale humic acid, is due to the large amount of silicates present. The 9.7-micron absorption is shown more effectively in spectrograms 5 and 6. The aliphatic carbon-hydrogen stretching vibrations of the mineral-free kerogen at about 3.5 microns (No. 1) seems to be largely eliminated by oxidation to humic acid (Nos. 3 and 4). The double bond absorption at 6.2 microns remains, particularly in coal and dialyzed acid spectra (Nos. 2 and 3). I n spectrum4 it appears, from the relatively weak intensity of the 6.2-micron band compared with that of the original kerogen (KO. 1 ) or of the humic acids prepared under more efficient oxidizing conditions (No. 3), that double bonds in the original kerogen have been partly oxidized to carbonyl groups. The morc effective oxidation apparently produced new double bonds, either in aromatic structures or in quinoid molecules ( G ) . The subsequent formation of new double bonds is further substantiated by the relative strengthening of the 6.2-micron band by oxidation with hydrogen peroxide-compare spectrogram 5 with 4. The absence of fine structure in the region of 10 to 15 microns indicates that no definite type of carbon structure predominates in the shale humic acids.
Oxidation of the Humic Acids Further oxidation of the shale humic acids with hot 10% hydrogen peroxide produces water-soluble acids ( I ) and carbon dioxide. After refluxing for 2 hours the mixture was cooled and the insoluble humic acids were filtrred out. The aqueous filtrate ivas distilled, but only 0.16% of volatile acids were obtained. From Duclaux constants the acids appeared to be mainly acetic acid. The nonvolatile water-soluble acids were recovered by evaporating the distillation residue to dryness. From the yields of products and their analyses (Table VIII); the distribution of carbon in the products is calculated. When the humic acids are oxidized to water-soluble acids, nearly as much carbon is oxidized to carbon dioxide as to water-soluble acids. This type of oxidation is to be expected if the humic acids are essentially quinonelike (6)
BLACK SHALE O X I D A T I O N molecules to which carboxyl groups are attached. The water-insoluble residual humic acids were also oxidized, as shown by their carbon content of 51.2% on an ash-free basis compared with 59.5% at the start. An infrared spectrogram (No. 5, Figure 1) shows no major changes except an increase in silicate absorption at 9.7 microns which accompFnies the increase in ash from 17.5 to 28.4y0. A second oiidation of the residual humic acids with hot hydrogen peroxide converted an additional 13.770 of the original carbon to water-soluble acids (Table VII), 18.87' of the total carbon was oxidized to carbon dioxide. The water-insoluble residue was extracted by dilute sodium hydroxide, leaving a small insoluble residue. O n acidifying the alkaline extract, humic acids were precipitated which contained 72.5% ash and 3.4% of the total carbon. Infrared Spectra. An infrared spectrogram of the residual humic acids is No. 6 in Figure 1. This spectrum is dominated by strong absorption at 9.7 microns and the general dilution effect of the high mineral matter content of 72.5Y0, but the relatively greater absorption a t 6.2 microns compared with the 5.8-micron band seems significant. Thus, if the relative absorptions of these two bands are compared with those in the humic acids resulting from the first peroxide oxidation, No. 5, and in the original humic acids, No. 4, a marked increase in the relative intensity of the 6.2-micron band is observed. In spectrum 6 the relative intensities of the other carboxyl absorptions at about 3.2, 7.2, and 6.0 microns, compared with the 6.2 band, are decreased, indicating that the decrease is due to relatively fewer carboxyl groups or, conversely, to a larger concentration of the unsaturated structures absorbing at 6.2 microns. No satisfactory structural interpretation of this change has been found. The lack of a 6.6-micron band accompanying the 6.2 band again suggests a quinoid structure rather than aromatic, but this cannot be accepted as final proof. The appearance of a small aliphatic carbon-hydrogen stretching vibration at 3.45 microns in spectrogram 6 is probably due to the diminished carboxyl absorption in this region. The nonvolatile water-soluble acids obtained by the first peroxide oxidation contained 47.8% carbon and 3.0y0 hydrogen on an ash-free basis (Table VIII). As the preparation contained 8.0% of siliceous ash, it may be assumed that the acids contained less than 3.0% hydrogen because of water of hydration retained by the mineral matter. Consequently, the important change in chemical composition during the oxidation of the humic acids to water-soluble
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Figure 2.
Infrared spectra of water-soluble acids
acids is introduction of oxygen, which increases from about 37 to @yo. A study of the structure of the watersoluble acids as revealed by infrared absorption spectra suggests that the oxygen introduced is mainly in the form of carboxyl groups. In Figure 2 the first spectrogram, from water-soluble acids produced in the original air oxidation of the shale, shows strong carboxyl absorptions a t 3.2, 5.8, 7.2, 8.0, and 11.O microns. Additional bands at 3.8 to 3.9, 5.4, 5.6, 6.2, 10.9, 13.5, and 13.8 microns are noteworthy. Absorption a t 3.81 and 3.99 microns is characteristic of succinic acid, and consequently this acid may be present. The pair of absorptions at 5.4 and 5.6 microns appears to be due to an acid anhydride structure. Because of the strong absorption a t 8 rather than 9 microns and the fact that conditions were not favorable to anhydride formation, it appears that a cyclic anhydride was produced, possibly succinic or an ortho aromatic anhydride. The absorption at 6.2 microns with none a t 6.6 is similar to that of the humic acids and suggests a quinoid-type structure. The absorptions in the 10- to 15:micron range may be due to outof-plane hydrogen absorptions, but no assignments can be made at present. The second spectrogram in Figure 2 is from an acetone extract of the airoxidized humic acids amounting to 1.9%. Its similarity with No. 1 indicates that the acetone extract consists mainly of water-soluble acids absorbed (5) on the humic acids precipi-
tated from the alkaline extract by the addition of acid. The apparently low intensity of the 5.8- and 8.0-micron bands of spectrum 1 compared with 2 is due to concentration effects only. The third spectrogram in Figure 2 is from the anode chamber of the electrodialysis of air-oxidized shale humic acids during the interval of 38 to 56 hours. The water- and acetone-soluble acids were probably all dialyzed out during the first 38 hours. Consequently, the spectrogram after 38 hours (No. 3) shows important differences, particularly at 2.9 to 3.2 microns, 5.4, 5.6, 6.2, and 12 to 14 microns. This spectrogram is very similar to No. 4 obtained from the oxidation of the humic acids with hydrogen peroxide. The increased absorption at 2.9 microns in both spectra 3 and 4 suggests a larger amount of alcoholic hydroxyl, although this may be partly due to water of hydration of the siliceous mineral matter which they both contain-compare the absorption a t 9.7 microns. Hydroxyl groups also absorb in the region of 7 to 9 microns, and the broader absorption in both of these spectra indicates the presence of this structure, possibly tertiary or phenolic in character. T h e practical elimination of the anhydride absorption a t 5.4 and 5.6 microns in the peroxide-oxidized acid spectrum (No. 4) indicates that this structure is not produced by peroxide oxidation of the humic acids. The retention of the absorption at 3.8 to 4.0 microns seems to show that the absorption is not due to succinic acid. VQL. 49, NQ,. 7
JULY 1957
1129
The spectrum of the water-soluble acids produced by a second peroxide oxidation of the residual humic acids (Table VIII) is similar to that obtained from the first oxidation (No. 4) and is not reproduced. The similarity of the spectra suggests that the humic acids, oxidized in the second treatment, produced structurally similar water-soluble acids, although the latter acids contained a little less carbon, 41.4y0 compared with 47.8YG (Table VIII). These results obtained with hydrogen peroxide oxidation show that the airoxidized shale humic acids are easily oxidized by this reagent to water-soluble acids, but that it is not a satisfactory reagent because nearly 50yGof the carbon is oxidized to carbon dioxide. The ease with which the humic acids are oxidized probably is of structural significance and suggests that the humic acids are not basically aromatic acids. Alkaline Permanganate Oxidation. A less destructive oxidizing reagent for the humic acids is alkaline permanganate. Even with an excess of hot permanganate, although highly dilute, relatively little carbon dioxide is produced and the yields of water-soluble acids are high. Oxidations were made using the method of Bone and coworkers ( Z ) , except that the carbon converted to water-soluble acids was determined by oxidation to carbon dioxide with potassium dichromate in sulfuric acid. The amount of carbon dioxide was found by absorbing it in Ascarite and weighing the increase. Thus, a sample of the electrodialyzed humic acids containing 13.4YG ash, subjected to refluxing alkaline permanganate for 15 hours, used up 5.6 grams of permanganate per gram of acids. Of the total carbon, 4.4YG was oxidized to carbon dioxide, 0.9Y0to volatile acids, 28.9% to oxalic acid, and 65.0% to water-soluble acids (Table IX). The appearance of 28.9% of the carbon as oxalic acid seems to confirm the quinoid or possibly aromatic type of structure for a part a t least of the structure of the humic acids. The water-soluble acids were recovered from the alkaline oxidation filtrate by
Table IX. Yields of Products and Distribution of Carbon in Alkaline Permanganate Oxidation of Humic Acids Products Carbon dioxide Volatile acids (calcd. as acetic) Oxalic acid Other water-soluble acids Losses (by difference) Total
1 130
Yield, %
70 of Total C
16.1
4.4
2.3
84.2
0.9 28.9
... ...
65.0 0.8 100.0
acidification and evaporation to dryness. Light yellow ether-soluble acids were extracted from the residue of salt and acids in a Soxhlet. Spectrogram 5 in Figure 2 shows the infrared absorptions of this fraction. The spectrum is very similar to that of the hydrogen peroxideoxidized water-soluble acids, No. 4. The usual carboxyl absorptions at 3 . 2 , 5.8, 7.2, and 8.0 (8.15) microns are prominent. The absence of a band at 11 microns indicates that these acids are not associated. A strong band at 3.8 to 4.0 microns is in evidence, as well as a broad shoulder a t 5.4 and a weaker one a t 5.65. A strong hydroxyl absorption a t 2.97 microns indicates that hydroxyl groups have been introduced. The decrease of the oxygen absorptions in the region of 8.5 and 9.0 microns with strengthening of the band a t 8.15 microns suggests that the group is phenolic in nature. The 6.2-micron band again appears as a shoulder absorption and suggests the presence of quinoid structures. KO accompanying aromatic band a t about 6.6 microns is apparent. An unusually strong band a t 13.95 microns suggests the possibility of the cis --CH=CHstructure, which is characteristic of the outside portions of quinoid structures and absorbs strongly in this region and moderately at 7.2 microns. However. the weaker absorption at 13.0 to 13.2 microns which accompanies the 13.95 band suggests the possibility of an aromatic structure. Following ether extraction of the water-soluble acid residue, acetone-soluble acids were extracted exhaustively in the Soxhlet. The product was a chocolate brown amorphous solid. Spectrogram 6 in Figure 2 shows the characteristic absorptions of the fraction. Difficulties in preparing potassium bromide windows from this sample, because of their sticking to the die, made it necessary to use a more dilute concentration (O.Iy0 instead of l.Oyo), which resulted in relatively less intense absorption peaks. The spectrum is essentially identical with that of the ether extract, except for the bands a t about 13 and 14 microns. Consequently the material seems to be composed mainly of hydroxy acids with no carbon structure sufficiently outstanding to produce characteristic absorption peaks. The relative amount of absorption a t about 6.6 microns seems to be a little larger in this spectrogram, suggesting a little more aromaticity. The pair of shoulder bands at 5.45 and 5.62 microns indicates some anhydride structures which might well be aromatic. Conclusions
The organic matter of Chattanooga shale is a macromolecular-weight material of unusual chemical composition. The carbon and hydrogen percentages
INDUSTRIAL AND ENGINEERING CHEMISTRY
are low and the oxygen is high. The oxygen-containing groups appear to be mainly alcoholic in character. but ether linkages may be involved in the highmolecular weight structure of the material. Phenolic groups may also exist. Aliphatic carbon-hydrogen absorptions indicate the presence of aliphatic structures, but thelow atomic hydrogen-carbon ratio suggests that a large part of the carbon structure is of an unsaturated polycyclic or aromatic nature. Partial air oxidation of the organic matter destroys most of the alcoholic groups with the introduction of carboxyl groups, forming humic acids similar in composition to those obtained from coal. From the composition of these acids, their equivalent and molecular weights: infrared absorption characteristics, and behavior on oxidation with alkaline permanganate their carbon structure seems to be mainly quinoid in nature. The shale humic acids are remarkable in the large quantity of mineral matter which accompanies them into alkaline solution and on acid precipitation. The presence of large amounts of silicon, aluminum, iron, titanium, magnesium, calcium, and sodium, even after treatment with acids or electrodialysis, indicates the presence of a silicate complex. Further oxidation of the humic acids with hydrogen peroxide or alkaline permanganate destroys a large part of the color and produces water-soluble acids. The general structure of these appears to be related to that of the humic acids. Acknowledgment
The analyses were made by H. I,. Lovell. H . T. Grendon, and H. T. Darby. Literature Cited
Bailey, A. E. W., others, Fuel 33, 209 (1954); 34, 37 (1955). Bone, W. A , , Parsons, L. G. B., Sapiro, R. H., Groocock, C. M.: Proc. Roy. SOC.(London) 148A, 492 (1935). Dancy, T. E., Giedroyc, V., J . Inst. Petroleum 36, 607 (1 950). Friedman, L. D., Kinney, C. R,, IND.EKG.CHEM.42,2525 (1950). Kinney, C. R., Ockert, K. F., Zbid., 48, 328 (1956). Love, D. L., thesis, Pennsylvania State Universitv. 1955. ( 7 ) LovelI, H. L., thesis, Pennsylvania State Universitv. 1952. Polansky, T. S.‘$’ Kinney-, C . R., Fuel 31, 409 (1952). Robinson, W. E., Heady, H. H., Hubbard. A. B., IND.ENG.CHEM. 45,788 (1953). Stanton, F. M., Fieldner, A. C., Selvig, W. A., U. S. Bur. Mines, Tech. Paper 8 (1918). Stimson, M. M., O’Donnel, M. J., J . Am. Chem. SOC.74,1805 (1952). RECEIVED for review July 13, 1956 ACCEPTED December 26, 1956 Part of research performed under Contract AT(30-1)1442with the U. s. Atomic Energy Commission.
