March, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Industrial Utilization of Leather Scrap’ By Joseph Michelman 20
SEAVER
ST., ROXBURY, MASS.
HE utilization of the large amounts of leather scrap position products. These evolved vapors, upon condensathat accumulate in the leather-working industries has tion, yield a distillate composed of an ammoniacal liquor, an heretofore been confined principally to agricultural oil, and, in the case of vegetable-tanned leathers, a tar also. purposes. However, with the advent of improved methods A charred residue of animal charcoal, which from mineralfor the fixation of atmospheric nitrogen and for the synthetic tanned leathers also contains metallic oxides, remains in production of ammonia, and with the increasing availability the retorts; the noncondensable gases--l‘leather gas”of ammonium sulfate from gas works, it is highly probable are combustible, and are therefore appropriately scrubbed that the scrap will henceforth find decreasing use in the pro- and then burned under the retorts for fuel. The Char-“Leather duction of fertilizers. AcBlack” cording to Rogers,2 many Destructive distillation of leather scrap offers promising A study of the charred states have laws against the possibilities for its profitable industrial utilization. The residue remaining in the reuse of either hair or leather scrap for this purpose is classified according to the tannage torts reveals that it possesas fertilizers, and Guillin3 or into “upper” and sole leathers. ses many of the desirable and others have pointed out The distillation process is similar to that used for wood, properties of a decolorizing that the chromium in the etc. The principal distillation products are animal charcarbon. The char from leather scrap is deleterious coal, chromic oxide, ammonium salts, pyrocatechol, vegetable-tanned leathers is to vegetation. Other prothe pyrroles, and pyrocoll. Little if any pyridine bases or a f o r d of animal charcoal posals to utilize the scrap fatty acid nitriles are formed, which is substantiated by obviously containing some are of rather diversified the work of Weidel and Ciamician on gelatin, vegetable charcoal arising nature and not of sufficient The uses of the char as a decolorizing carbon and of the from the tannin substances. importance to merit dechromic oxide as the pigment “chrome green” are disIt is composed of nearly pure tailed consideration. Treatment of the ammoniacal liquor with calcussed. carbon, liberated a t a comThe object of this investicium chloride is recommended. A method has also been paratively low temperature, gation was to find good use devised for the separation of the pyrocoll from the pyr350” to 450” C., which is for the large amounts of roles, which also permits of the purification of the latter. much lower than the critical leather scrap available toScrap leather is shown to be an excellent source for the temperature range below day. A process of general industrial preparation of the pyrroles and pyrocoll. The which active or activatable applicability to the enoruse of the pyrroles for making indoles is suggested. mous variety of leathers and carbon should be liberated to the myriad of shapes and from its compounds. The sizes of which the scrap is composed is afforded by that of anatomical structure of the hide is conducive to the formadestructive distillation. The thermal decomposition of leather tion of a char of great porosity upon carbonization. The would, of course, yield a charred residue and a m m ~ n i abut , ~ corium, which is the only portion of the hide used in leatherin this investigation an attempt was made to study more making, is composed of bundles of fibers which interlace somexhat loosely on the under side of the skin but are closely matted completely these as well as the other distillation products. Leathers for this purpose can be classified advantageously on the epidermal side. Carbonization probably causes first according to the tannage to which they have been sub- an interstitial swelling of the fibrous constituents and then an jected-i. e., vegetable-tanned, mineral-tanned, and oil- expulsion of the volatilizable interstitial substances, leaving tanned leathers. The shoe-making and the leather-working a residue of open and porous texture. industries constantly produce considerable quantities of clean, Leather and bones have in common the gelatinous content classifiable scraps and wastes, easily segregated in accord- which is the carbon-forming matter upon destructive distilance with the tannage. Another convenient classification lation. Accordingly, “leather black” will have some of the is into sole leather and “upper” leather, for nearly all of the same primary or active carbon that is found in bone black. former is vegetable-tanned and most of the latter is chrome- “Leather black” is active-it possesses decolorizing properties which were confirmed by semiquantitative tests with tanned. methylene blue-although it is not so active as some of the Procedure better grade activated decolorizing carbons. If the better grade chars are represented by 100, then the decolorizing The destructive distillation of leather scrap is carried out in a manner analogous to that used in the carbonization power of bone black is only30, according to Hortona6“Leather of wood or other organic matter. The scrap, compactly black” subjected to an activation process such as that of bound into bales or billets, is heated in retorts, preferably Chaney and othersB should yield a decolorizing carbon under diminished pressure. An evolution of vapors begins possessing unique properties. The char from chrome-tanned leathers contains 10 to a t about 100” C., and as the temperature is gradually raised a carrier gas, such as steam or flue gas-or even some of the 20 per cent of chromic oxide (Cr208). The latter can be re“leather gas” previously evolved-may be introduced into covered very easily by burning the char from chrome-tanned the retorts to promote the removal of the volatilized decom- scrap in a current of air. The char then burns readily and without odor, and the chromic oxide remains as a fine, green 1 Received October 29, 1924. ash. The char from chrome-tanned leather thus simulates
T
2 8
“Industrial Chemistry.” 3rd ed ,p. 448. J . SOC.Leather Trades’ Chem., 1, 117 (1917). Lamb, J Soc Ckem Ind., 36,986 (1917).
* THISJOURNAL, 16,519 (1923). 6
Ibrd., 16, 1244 (1923).
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Vol. 17, No. 3
bone char in that it has a mineral constituent intimately rectly with an acid would fix these basic substances as salts or associated with the carbon content. It would be highly resinify them, unless the hydrogen-ion concentration were carecontrolled. desirable to remove this valuable mineral matter and thereby fully 3-The calcium carbonate obtained, especially from leave a carbon of exceedingly Porous nature. The various tanned leathers, is recoverable as white precipitated calcium solvents, such as Rochelle salt, hydroxy organic acids, etc., carbonate. (The calcium carbonate from the ammoniacal liquor recommended for the extraction of chromium from leather of vegetable-tanned leathers is darker.) &-The availability and the cost of calcium chloride favor its have little if any effect in removing the chromium use. Ammonia in the form of the &loride is more valuable than from the char, because the chromium is now in the condition in easily the form of the sulfate, and ammonium chloride is of the oxide. CrzOn. which is rather inert chemically. The purified by sublimation. chromium,‘ which is It is an open quesp r e c i p i t a t e d in the tion whether the reDestructive Distillation of Scrap Leather -Flow Sheet SCRAPLEATHER leather in the process ported r e a c t i o n s of of tanning as the hypyrocatechol with cal4 J. CHAR DISTILLATE GAS droxide, Cr(OH)S, is cium chloride in the , I I I d e h y d r a t e d into the I presence of ammonia or Decolorizing Ammoniacal liquor Tar and oil Fuel of pyrogallol with caloxide by the higher carbon I under temperatures used in retorts c i u m hydroxide and chloride’ would occur the process of destrucin this instance. The tive distillation. It is 1 entirely possible to use presence of the carbonChromic the char from chromea t e i n the ammoniacal Carrier oxide gas tanned leather scrap as “Chrome green” liquor should prevent Into retorts the existence of cona decolorizing carbon Dis‘til also, and after it has besiderable concentration of calcium ion, unlesa come spent to burn i t an excess of calcium for the recovery of the up to A2000 c . 2000 c. up c h l o r i d e were used. chromic oxide. The N e v e r t h e 1e s s , the chromic oxide obtained darker color of the calb y e i t h e r method p i r o l eHydrocarbons s cium carbonate from should find ready use as the ammoniacal liquor the pigment “chrome of vegetable-tanned green,” and for this K p rroles Toluene, etc. f (For re-use) leathers may be attribpurpose it is in an excelJutable to some interlent state of sub-divireaction. This system sion. requires further study. The condensable disPyrroles KOH s o h . The alternative method tillation Droducts coma l w a v s r e m a i n s of prise 4O’to 50 per cent of the weight of the origiual leather scrap. Many of the neutralizing with acid the ammoniacal liquors containing constituents of the distillate are common to all kinds of the phenols. The supernatant liquid is decanted from the precipitate of leather; for instance, the hide substance will yield the same decomposition productsirrespective of the tannage. Although calcium carbonate, and it may’now be heated and evaporated it is thus possible to charge the retorts with segregated por- without any injurious effects to the phenols, whereupon much tions of scrap classified according to the tannage and to treat of the empyreumatic matter is expelled, and the ammonium the distillates from all leathers in common, it is preferable, chloride crystallizes out. Provision may be made for collecb for simplicity of subsequent operations, to carbonize the ing the initial vapors arising from the evaporating ammonium chloride liquor, for these contain some pyrrole, amines, etc. chrome- and vegetable-tanned leathers separately. The sulfite and thiosulfate in the ammoniacal liquor no doubt The Ammoniacal Liquor owe their origin to the sulfonated oils and to the sulfates. Collagen, of which the hide substance is chiefly composed, The ammoniacal liquor, which is very readily separated is one of the proteins that is rather low in sulfur content, and from the tar and the oil by decantation, contains ammonium hence the yield of sulfo-cyanide (arising from the combination salts such as the carbonate, the sulfocyanide, the sulfite, and of the sulfide with the cyanide) will be necessarily small. the thiosulfate, etc., and polyhydroxy phenols such as Pyrogallol distilling with decomposition a t normal pressure pyrogallol and pyrocatechol. These phenols are obviously may not always be present in amounts warranting recovery. the decomposition products from the vegetable tanning mat- Pyrocatechol, on the other hand, is much more stable, dister. It is not desirable to recover the ammonia by the usual tils readily a t 245” c.,is slightly more soluble than ammomethods of distillation and absorption of the evolved am- nium chloride, and thus will be found in the concentrated monia gas, because of the instability of the phenols in ammonium chloride mother liquors. alkaline solution. It is much preferable to add sufficient calcium chloride to the ammoniacal liquor to precipitate The Pyrroles and Pyrocoll the carbonate present, which method affords these marked The tarry and the oily portions of the distillate contain advantages: principally pyrrole, pyrrole derivatives, pyrocoll, hydrocar1-The hydroxyl-ion concentration is considerably diminished bon oils, and some phenols. Mineral-tanned leathers yield by the formation of ammonium chloride, which hydrolyzes very slightly acid, whereas ammonium carbonate is distinctly alkaline a dark-colored, viscous oil, lighter than the ammoniacal in reaction liquor. The crystals of pyrocoll are plainly discernible sus2-Calcium chloride does not apparently enter into combinapended between the oily and the aqueous layers. Vegetable-
+-
$
4
1
.1
-+
+
r\B y‘o\
7 \
tion with any of the basic substances that may be present from the tar or the oil. Xeutralization of the ammoniacal liquor di-
7
“Allen’s Commercial Organic Analysis.” Vol. 111, p. 537.
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March, 1925
249
tanned leathers yield a light colored oil, lighter than the ammoniacal liquor, and in addition a tar heavier than the ammoniacal liquor. I n this instance the tar contains the pyrocoll and most of the pyrroles, while the upper oil is composed principally of hydrocarbon oils. The following treatment is applicable to both cases: To the tarry and oily portions, separately or combined, is added a diluent and solvent such as benzene, toluene, solvent naphtha, etc. , which enables the crystals of pyrocoll to be filtered off very easily, The filtrate may be washed with water to remove the remaining phenols, and it is then distilled into two fractions: A , up to 200' C., and B , 200" to 310" C. The solution of the pyrroles in toluene, which constitutes the lower boiling fraction, is treated with sufficient solid potassium hydroxide to combine with the pyrroles to form potassium pyrroles, which separate from the solvent thus permitting the recovery of the latter for re-use. Sodium hydroxide is unsuitable for this purpose; sodium metal reacts with difficulty with the pyrroles; potassium metal would be satisfactory, but it is too expensive, Digestion of the potassium pyrroles with water liberates the pyrroles as an oil with the formation of aqueous potassium hydroxide solution. This alkaline solution may be used advantageously to decompose the pyrocoll into the potassium salt of a-pyrrole-carboxylic acid, or it may be used in making iodol (tetraiodopyrrole), or it may be used in the process of converting pyrrole derivatives into indole derivatives. The oil obtained from the action of water on the potassium-pyrroles is fractionated for pyrrole and its homologs, a- and @-methylpyrroles,dimethylpyrroles, etc.
There are, however, additional constituents in leather that apparently negative the foregoing conclusions when applied to finished leather, and these subtances are the fats and oils which are used in fat-liquoring and currying. But this is of little if any significance in the operation of the process, for these reasons:
Note-The filtrate from the pyrocoll separation may be distilled into fractions varying with the boiling points of the pyrroles However, the procedure involving only two fractions is preferable if i t is desired to keep the pyrroles, which are unstable in light and air, in a more stable form-namely, the potassium compound.
Scrap leather is a fruitful source for pyrroles and pyrocoll, and it offers marked advantages over bones for obtaining these substances. The yield of pyrroles per ton of scrap leather is greater than that from a similar quantity of bones, owing to the much larger content ol' gelatinous matter in the leather scrap. Moreover, the pyrroles obtained from scrap leather can be made practically free from pyridine bases and fatty acid nitriles which complicate the extraction of the pyrroles from bone oil. The conversion of the pyrroles into the corresponding indole derivatives, already referred to, offers an excellent use for these substances. This reaction has been extensively studied by Denn~tedt.'*r'~ It involves a polymeric condensation of pyrrole into tripyrrole in the presence of sulfuric acid and then the elimination of ammonia. Distillation of the mixture with steam over potassium hydroxide yields indole.
Absence of Pyridine Bases At the outset of this investigation the absence of considerable amounts of pyridine bases was observed. The addition of calcium chloride to the ammoniacal liquor yielded a liquor with an odor more characteristic of resinified pyrrole than of the pyridine bases. This observation was not fully explained until months later, when a study was made of the literature on bone oil. Weidel and CiamicianJs in order to determine the sources of the various constituents of bone oil, destructively distilled gelatin, which they presumably used as representative of the gelatinous substances present in bones. These investigators found no detectable amounts of pyridine bases in the decomposition products resulting from the destructive distillation of gelatin, and they stated also that nitriles of the fatty acids were not formed from gelatin. They then postulated that the pyridine bases in bone oil were formed by the condensation of the ammonia, methylamine, etc., with the acrolein arising from the decomposition of the glycerol of the fats, which view seems to be accepted today. Leather and gelatin are both produced from the same parent material-that is, clean, fat-freed hide substances; and they should yield thermal decomposition products common to both. Weidel and C i a m i ~ i a nfound ~ , ~ in their investigation of the decomposition products from gelatin, ammonium salts, pyrrole and pyrrole derivatives, pyrocoll, amines, phenol (C&JOH), hydrocarbons, etc. The same substances were independently found among the distillation products from leather scrap. The obvious conclusion is that hide substance to which a phenolic or a mineral-tanning agent has been added should yield the same thermal decomposition products as gelatin gives, and it should yield neither pyridine bases nor fatty acid nitriles.
* Ber 9
13,85 (1880). \lonaish , 1, 279 (1880).
1-Sole leather rarely contains more than 2 per cent of fat or oil, and according to Lob10 it is practically free from fat. 2-Hydrocarbon oils and paraffin may be used to dress and finish leathers. 3-The condition of the fats and oils in leather may further preclude the possibility of the formation of pyridine bases upon destructive distillation. As I&wkowitschll points out, grease recovered by extracting leather clippings with petroleum ether contains considerable amounts of unsaponifiable matter and free fatty acids. As these are not glycerides, they should not give rise to pyridine bases upon carbonization. &The scrap leather, particularly from "uppers," can be subjected to an extraction process for the removal and recovery of these fats and oils, prior to distillation.
The condition and the nature of leather scrap preclude an unqualified generalization that pyridine bases are not formed in the destructive distillation of the scrap. If any pyridine bases are formed they would be present in minimal amounts, and they would arise from the fats and oils present, but only if such fats and oils were glycerides. This point has been especially studied, for the statement has appeared in the literature that pyridine bases are among the thermal decomposition products of gelatin and of leather. Theoretical Discussion
HC-CH
HC-CH
HC--CH
HC-CH
' '
3 H h\N/ L + H C\N/ II d H H L\N/ C H H C\N/ # H + O - ' j H
H
H
Pyrrole
H
\N/ H Indole
Tripyrrole
+NH3+
E'
Pyrrole
The reaction with homologous p y r r ~ l e s ~is~ somewhat J* different although it is carried out under similar conditions. The reaction with a-methylpyrrole is represented below.
ri
HC--CH I1
CHajCH3 C
\N/ H
\N/ H
hfethylpyrrole
I
HC-CH I
CH HC
'&.CHa+
\N/ H
Dimethyldipyrrole
0-\E)
CH3
CHs
+ NHc
Dimethylindole
The a-pyrrole carboxylic acid obtained from the pyrocoll, as already discussed, loses carbon dioxide readily and passes into pyrrole. Pyrocoll is easily purified by sublimation, lo
II
Chem. Ztg , 30, 935 (1906). "Chemical Technology and Analysis of Oils, Fats, and Waxes," Vol.
111,p. 426. '2 Ber., 27, 4 i 9 (1894). 18 German Patent 125,489 (1901). l' Dennstedt, Ber ,21,3429 (1888).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2 50
it is stable, and it is thus a valuable source for preparing pyrrole uncontaminated by any homologs, and for obtaining a-pyrrolecarboxylic acid. It is interesting to note that pyrocoll has not been obtained from any of the substances known to yield pyrroles other than gelatin, and now scrap leather. n
Pyrocoll
a-Pyrrolecarboxylic acid
Pyrrole
Weidel and C i a m i ~ i a n , 'in ~ their investigation of gelatin, obtained in addition to the "lower pyrroles" boiling between 125" and 190' C., also some "higher pyrroles'' boiling above 200' and up to 260" C. The same fractions were also found in the distillation products from scrap leather. These higher boiling oils give many of the characteristic reactions of pyrrole, and their behavior has strongly suggested to the author that they may contain polypyrroles such as dimethyldipyrrole, of which there can be a t least three isomeric modifications. These pyrrole oils distil over with some of the higher boiling hydrocarbon oils, but the two are not miscible and separate into two distinct layers upon standing, the hydrocarbons ' fr, forming the upper layer. It is d a c u l t to determine whether the saturated aliphatic hydrocarbons obtained as by-products from the distillation of leather scrap are the original oils used in currying and dressing the leather, or whether they are the decomposition prod16
Monofsh., 1, 294 (1880).
Vol. 17, No. 3
ucts from the fatty acids present. The latter view is suggested by the consistent formation of these oils from all kinds of leather scrap, by the presence in leather of fatty acids, and by the boiling points of two of the oils, about 272" and 306" C. C1SH32, b. P. 270' c . ++C17Hae, b. p. 303" c.
Palmitic acid, CljH3iCOOH = COa Stearic acid, C17HasCOOH = COa
The mineral or hydrocarbon oils that may be used to treat leathers might, of course, contain oils of these boiling points. Some hydrocarbon oils are also formed from the animal matter present in the leather, but these oils are lower boiling and are principally aromatic. The question of the formation of the hydrocarbon oils can be settled only by the carbonization of leather of known origin and history; this is obviously not possible with scrap leather. A satisfactory solution of this question is important, for if the fatty acids decompose into hydrocarbon oils, then fatty acid nitriles are not formed; and if this is correct, then a previous extraction of the fats and oils from the scrap is not necessary. The hydrocarbons are recoverable and are members of the upper boiling fractions of petroleum used as lubricants. Substances Obtained by This Process By this process there have been obtained from scrap leather the following industrially valuable substances: (1)ani(3) ammal charcoal, (2) chromic oxide-"chrome-green," monium chloride, (4)precipitated calcium carbonate, (5) sulfocyanide, (6) pyrocatechol, (7) pyrocoll, (8) pyrrole and homopyrroles, (9) higher pyrroles, (10) a. fuel gas, (11)hydrocarbon oils, and (12) fats and oils (solvent extracted).
A Turbine Oil Deposit' By A. G. Blakeley PHILADELPHIA 8r READING COAL& IRON Co., POTTSVILLE, PA.
OMETIME ago a sample representing a material or deposit taken out of the water-cooling compartment of a Ridgeway turbine was received in this laboratory. The turbine oil used had been of quite satisfactory quality, and no lubrication trouble had been experienced. Owing to the nature of the deposit, however, it was thought desirable to report the analysis. The material as received a t the laboratory consisted of brown powder and lumps, the lumps being easily crumpled to a brown powder. Chemical analysis showed the following results :
.......................................
Moisture or water.. Oil (soluble in petroleum ether or gasoline) . . . . . . . . . . . . . . . . . . . Matter insoluble in petroleum ether b u t soluble in ethyl ether.. Asphaltic matter.. Insoluble matter ..........................................
........................................
.
Per cent 0.5 43,3 50.4 2.2 4.4
The moisture or water was determined by heating the material for an hour a t 105' C. The material melted on heating. The oil was determined by extracting the original material by means of petroleum ether or gasoline. The oil was probably turbine oil slightly darkened in color. The matter insoluble in petroleum ether but soluble in ethyl ether was a brittle, brown solid containing about 0.7 per cent 1 Presented before the Division of Petroleum Chemistry a t the 68th Meeting of the American Chemical Society, Ithaca, N. Y . , September 8 to 13. 1924.
ash. This ash contained iron oxide and copper oxide. It is probable that this brittle, brown solid consisted in part of metallic soaps. This ether-soluble portion of the turbine deposit had, no doubt, been formed by continued circulation and use of the oil, and by the formation of free fatty acids which gradually acted upon the metal with which the oil came in contact. The asphaltic matter was found by solution in chloroform. The extraction with chloroform was made on the material remaining insoluble after the extractions with petroleum ether and ethyl ether. The asphaltic matter was a black powder which did not melt a t 105" C. The insoluble matter reported in the analysis represents the matter not soluble in either petroleum ether, ethyl ether, or chloroform. The insoluble matter was mostly metallic iron, but contained some particles of wood and dirt. It is regretted that the sample obtained was too small in amount to permit a more detailed examination. This turbine deposit or material had, no doubt, been formed by the circulation and use of the turbine oil and by the collection of small amounts of dirt. Any oil in use is likely to show slight changes, and eventually some depositing out w i l l take place. The writer believes that the material was formed almost entirely from the turbine oil itself. The oil reported in the table of analysis was liquid in nature, but was held in solid form by admixture with the ethylether solubles consisting in part of metallic soap.
