US3508857A - Process for removing keratin-containing parts from animal hides and pelts and the keratin-containing parts adhering to such hides and pelts - Google Patents

Description

Int. c1. c14c. N06

US. Cl. 894.16 Claims ABSTRACT OF THE DISCLOSURE Process for removing keratin-containing parts of animal hides and pelts by exposing the hides and pelts to the action of chlorine dioxide until the keratin-containing parts have jellified and softened, and by then treating the hides and pelts with an alkaline bath.

The present invention relates to a new process for removing the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts, for example hairs or scales.

By unhairing, there is understood in the tanning industry an operation in 'which the hair of hides and pelts is loosened to such an extent that it can easily be mechanically removed from the dermis together with the keratin-containing parts of the hides such as the dermis, hairs, and scales.

A process for unhairing and degrading hides and pelts has been proposed in US. Patent 3,294,479, in which process the hides and pelts are exposed to the action of chlorine dioxide. This prior art process for removing the keratin-containing parts of animal hides and pelts is based on oxidative splitting of the sulfur bridges of the keratin to form sulfhydryl groups and subsequent oxidation of the sulfhydryl compound to water-soluble keratinsulfonic acid.

In the unhairing process by oxidation, chlorine is formed as a by-product. The chlorine fulfills two functions which are of importance in the production of leather, namely, on the one hand, chlorination of the natural fat and, on the other hand, chlorination of the collagen. The chlorinated natural fat is easily emulsified by the keratin= sulfonic acids formed in the course of the process, so that the hides are automatically degreased in this process. On the other hand, the fixation of the chlorine onto the collagen considerably strengthens the structure of the leather and thereby suppresses the formation of loose grain in the upper layer of the leather, which occurs often when unhairing is effected according to the usual methods with lime and sulfide.

When the prior art oxidation process is carried out in a closed rotating drum, the hair and keratin-containing parts of hides and pelts loosen and separate from the hides and pelts after a sufiiciently long time of action of the chlorine dioxide. The unhairing process is usually carried out in the presence of water and can be effected by introducing into an aqueous bath either chlorine dioxide gas or substances which, in an aqueous, preferably acid, medium, give off chlorine dioxide. As such sub stances, alkali metal chlorites, preferably sodium or potassium chlorite, are especially suitable. The alkali metal chlorites are preferably used in an aqueous acid solution at temperatures of up to about 45 C. and at. pH-values up to 7, preferably up to 6.

With this prior art method, separation of the hair or ted States Patent 3,508,857 Patented Apr. 28, 1970 ice of the keratin-containing substances of the hides or pelts, or decomposition of the hair or keratin-containing parts, takes place only when a. sufficient amount of keratinsulfonic acid has formed.

That is, in order completely to remove the hair and keratin-containing substances of hides and pelts with chlorine dioxide according to this method, it is necessary that the total sulfur content of the keratin be converted into keratin-sulfonic acid. The end of this reaction can best be observed by means of the known sulfhydryl reaction of keratin with lead acetate resulting in a deep black coloration. When the black coloration no longer appears, the reaction is complete. Total oxidation of the keratin sulfur generally requires about 10-30 hours and about l2 kg. of chlorine dioxide per kg. of hair or keratin-containing substances. An acceleration of this process is not possible: temperatures exceeding about 4045 C. cannot be used because the collagen is then destroyed by the acid present.

The present invention is based on the discovery that it is not necessary to convert the total keratin sulfur into the sulfonic acid form, but that even the conversion of a considerably smaller portion, about half, of the sulfur of the keratin to sulfonic acid is suflicient for dissolving the hair or other keratin-containing parts in water if the afore said treatment is followed by a treatment with an inorganic or organic base at pH-values above 7.5, preferably above 8.5. This new method permits the removal of the keratin-containing parts such as epidermis, hairs or scales from the hides in substantially shorter periods of time, in general after about 28 hours and in most cases within 3-5 hours. In addition, this new method also permits a considerable reduction in the necessary amount of chlorine dioxide so that only 0.3-1 kg. of chlorine dioxide per kg. of keratin-containing substance are required.

Thus, the present invention particularly relates to a process for removing keratin-containing parts of animal hides and pelts and keratin-containing parts adhering to such hides and pelts by first exposing the hides and pelts to the oxidizing action of chlorine dioxide in the presence of Water, the time of chlorine dioxide exposure and the quantity of chlorine dioxide used being considerably smaller than in the prior art process described in US. Patent 3,294,479, and then exposing the hides and pelts so treated to inorganic or organic bases.

In the oxidation step of;the present invention, which step is considerably shorter than for the prior art process described earlier, the chemical and physical structure of the keratin-containing parts of the hides and pelts are modified so that the keratin-containing parts, especially the hair, become gelatinous and soft. The fraction of keratin which at this stage has passed into solution is still very small and the lead acetate reaction is clearly positive because of the sulfhydryl groups still contained in the undissolved keratin parts. The keratin-containing parts such as epidermis and hair loosen and separate from the hides and pelts only during a subsequent treatment with inorganic or organic bases, and then dissolve in the alkaline bath employed.

For treating hides and pelts according to the present invention, chlorine dioxide in gaseous form, suitably with exclusion of oxygen in order to reduce the risk of an explosion and preferably in admixture with an inert gas such as carbon dioxide or nitrogen, can be allowed to act on the wet hides and pelts. In practice, however, chlorine dioxide is applied to the hides and pelts from an aqueous bath. For carrying out this process in practice, chlorine dioxide gas may be introduced into an aqueous medium which contains the hides and pelts to be treated. The chlorine dioxide gas can be produced in a suitable apparatus in an easy and simple manner, for example by unsrmtvun BIU'UIIE acidification of an alkali metal chlorite solution, or by reduction of chlorous acid or of the salts thereof with suitable reducing agents such as nitrous acid or oxalic acid, or by the action of concentrated sulfuric acid on alkali metal chlorates. The production of chlorine dioxide from chlorous acid and oxalic acid is of particular advantage, because carbon dioxide is formed as an oxida tion product of oxalic acid and dilutes the gaseous explosive chlorine dioxide so that it can be handled without risk. The stream of chlorine dioxide gas is then intro duced, if desired together with an inert gas as a diluent, into the unhairing bath through a distributing line. The unhairing bath should not be alkaline and should have a pH-value below 7.0, generally a value up to 6.0.

For a simpler handling when carrying out the oxidation process in an aqueous medium, it is in many cases more suitable to use aqueous solutions of substances which give off chlorine dioxide instead of gaseous chlorine dioxide. .As substances which give off chlorine dioxide, the water-soluble salts of chlorous acid, particularly the alkali metal chlorites, are especially suitable. For reasons of economy, sodium chlorite or potassium chlorite are usually used. Upon acidification to pH-values below 6.5, particularly below pH 6.0, aqueous solutions of these substances generate more chlorine dioxide the lower their pH-value is.

In the chlorine dioxide treatment step in the unhairing process of the present invention, an acid bath having a pH-value of up to about 6.5, preferably in. the range of pH 1.5 to 3.5, is generally used in order to secure con tinuous and not too violent generation of gas. In this man ner, the action of chlorine dioxide over a prolonged period of time on the hides and pelts to be treated is ob-= tained. In principle, however, it is also possible to adjust the pH of the treating baths to acid pH-values outside the iadicated pI-I-range. If higher pH-values are used, a reduction of the pH-value generally takes place because of hydrochloric acid and chlorous acid which are formed in the course of the treatment. pH-values lower than 1.5, on the other hand, involve the risk that the generation of chlorine dioxide will proceed too rapidly and that the chlorine dioxide will escape.

For adjusting the pH of the treating bath to an acid pH-value, any desired inorganic or organic acids which are intert toward chlorine dioxide may be used. For reasons of economy, inorganic acids such as sulfuric acid, hydrochloric acid, or phosphoric acid are preferably used. Organic acids such as formic acid, acetic acid, lactic acid, glycollic acid, ben'zene-sulfonic acid, or naphthalene-sulfonic acid may also be used. Furthermore, mixtures of different acids can be employed. If strong inorganic acids are used, the generation of chlorine dioxide can be slowed down by the addition of butter salts. If desired or re quired, acid salts such as primary sodium phosphate or primary sodium sulfate, or acid buffer mixtures, for example a mixture of a mineral acid and a salt such as sodium acetate, may be employed,

The quantity of aqueous liquor to be used when carry-= ing out the oxidation is generally about l500% by weight of the soaked hides and pelts. The optimum quantity of liquor depends on the manner of carrying out the treatment. When unhairing is carried out in a pit, relatively large quantities of water must usually be used since the material must be completely covered by the unhairing solution. In this case, the quantity of water depends considerably on the size and shape of the pit. When oxidation is effected in a closed drum, which is the preferred method in practice, only a relatively small quantity of bath is required, viz about l00%, preferably 3070%, by the weight of the soaked hides and pelts.

In the present invention, chlorine dioxide is allowed to act on hides and pelts for several hours, preferably at room temperature or even at lower temperatures or at slightly elevated temperatures, with or without movement of the hides and pelts. The temperature used influences the treatment time in the oxidation process. If very low temperatures, for example below 8 C., are used, the oxidation proceeds too slowly and economical operation is no longer possible. The use of too high temperatures, for example temperatures exceeding 50 C., involves the risk that the collagen of the hides will be damaged. For this reason, the use of temperatures above about 45 C. is to be excluded. In practice, temperatures in the range of from about 10 to 40 C. are generally used. In principle, however, unhairing with chlorine dioxide may also be effected at temperatures exceeding 50 C., provided that the collagen has been previously treated with tanning agents according to known methods and has thereby been made insensitive to heat.

The time of exposure to chlorine dioxide required for effecting partial oxidation according to the unhairing process of the present invention depends in the first instance on the type of the hides and pelts to be treated, and, furthermore, on the concentration in which the chlorine dioxide is used in the aqueous bath. If the chlorine dioxide is used in a high concentration, oxidation proceeds faster than if the chlorine dioxide is used in diluted state. When the treatment is carried out under conditions which are preferably used in practice, for example at temperatures of about 2538 C., with a quantity of liquor of about 3070% (referred to the weight of the soaked hides and pelts), a chlorine dioxide amount of 0.33% (referred to the weight of the soaked hides and pelts), and an average rotation of the drum of about 5-12 revolutions per minute, the oxidation treatment will generally be concluded after about 2-8 hours, depending on the type of the hides and pelts to be unhaired. In practice, the oxidation conditions, i.e., the temperature, quantity of water, quantity of chlorine dioxide, and number of revolutions of the drum will be so adjusted that the treatment is com plete within about 4 hours.

The above values apply to the ordinary conditions met in practice. These values, however, do by no means and in no way represent a limitation of the invention, since in view of the varying nature of natural products, it is impossible to establish generally valid process conditions for their treatment. The conditions must be adapted to the requirements of practice, or tested in a particular case by a preliminary test. Thus, for example, in the cases of extremely strong hairs or dead hairs, it is possible and may even become necessary to use a period of oxidation exceeding the above described oxidation time range. Such an extended oxidation neither deteriorates nor causes damage to the hides and pelts treated. But, since it is an object of the invention to shorten the unhairing process, it will be endeavored to make the oxidation treatment as short as possible.

The oxidation treatment can be terminated when the hairs and other keratin-containing substances to be removed can be easily and smoothly separated from the hides and pelts after a following relatively short treatment with mild alkaline agents. The oxidation treatment with chlorine dioxide should be discontinued before observing that the black coloration of the sulfhydryl reaction with lead acetate no longer appears. In general, the oxidation treatment requires 2-8 hours. In most cases, a treatment for a period of about 35 hours is sutficient.

When the partial oxidation operation in the unhairing process of the present invention is carried out with aqueous solutions containing substances which give off. chlorine dioxide, it is in general sufiicient to use these substances in an amount ranging from about 0.5 to 5% by weight of the untreated hides or pelts. However, this should not be considered a limitation on the invention. Rather, it is quite possible to use larger quantities, but for reasons of economy the quantity of the substances which give off chlorine dioxide will be held at a minimum. In principle, it is also possible to carry out the oxidation operation in the unhairing process of the present invention according to the so-called Schwoede-process." wherein an aqueous paste containing the substance yielding chlorine dioxide is applied to the pelts or hides. In this case, it may be necessary to use larger quantities of up to about 30% or more, by weight of the hides and pelts, of substances yielding chlorine dioxide.

After termination of the oxidation treatment with chlorine dioxide, the hides or pelts are exposed to the action of inorganic or organic bases. For this purpose, any desired inorganic bases may be used, for example the alkali metal and alkaline earth metal hydroxides and soluble carbonates and bicarbonates such as calcium hydroxide, sodium hydroxide, or potassium hydroxide or, preferably, sodium carbonate or potassium carbonate, or the corresponding bicarbonates, ammonia, or organic bases such, for example as methylamine, ethylamine, mono-, dior triethanolamine, and similar primary, secondary, and tertiary alkylamine and hydroxyalkylamine bases can be employed. In principle, it is also possible to use reducing or oxidizing alkaline agents for the alkaline after-treatment, for example alkali metal sulfides or alkali metal hypochlorites. However, because the hides and pelts swell at higher pH-values, the use of too strongly alkaline baths will generally be avoided.

When, the oxidative treatment is effected in a drum (which is the most widely used method in the industry), or even when the treatment is carried out in a pit, the pH of the bath for the alkaline after-treatment is adjusted, after termination of the oxidation, to pH-values of at least 7.5, advantageously to values above 8.0, by the use of. inorganic or organic bases. If it is not intended to prevent the swelling of the hides which occurs in stronger alkaline media, there is no upper limit on the pH-value. Advantageously, the treatment with inorganic or organic bases is carried out in the alkaline range up to about pH 11. It is preferred to adjust the pH of the bath to pH- values in the range of between 8.5 and 9.5.

If oxidation treatment has been effected according to the Schwoede-process, a solution of inorganic or organic bases can be applied to the hides or pelts. If desired or required, the hides or pelts may also be treated with gaseous ammonia, or they may be introduced into a bath containing inorganic or organic bases. The period of time during which the hides and pelts are treated with organic or inorganic bases may be relatively short. Care must be taken, however, that the keratin-containing parts of the hides and pelts are completely soaked with the alkaline solution. In general, the keratin-containing parts loosen and separate from the hides and pelts, or the keratin-containing parts such as hairs, scales and similar substances dissolve in the aqueous alkaline solution after only a short. period of time, about -20 minutes.

Although completely oxidized keratin-sulfdhic acids, as are formed in the prior art C10 unhairing process described earlier herein, cannot be precipitated from their aqueous solutions by acids, the major portion of all separated keratin parts dissolved as keratin sulfates by the process of the present invention can easily be precipitated by acidifying the waste water to a pH-value of below 6. For this reason it is very easy to remove the major portion of dissolved organic substances from the waste water of the unhairing process.

In the prior art unhairing process using chlorine dioxide, waste water conditions are already substantially more favorable than are those in conventional unhairing process using lime-sulfide. The waste waters obtained after unhairing with chlorine dioxide are non-toxic and are innocuous to fish. In addition, the quantities of waste water in such a process are considerably smaller than are those in the unhairing process with lime and sulfide.

In addition, the process of the present invention per= mits precipitation, to a large extent and with simple means, not only of the keratin components of the unhairing solution, but of other chemicals which remain behind in tanning waste waters as well. This is best achieved in a very eflicient manner by combination of the acid waste waters from pickling, tanning, dyeing, and greasing with the alkaline unhairing solution obtained in the proo ess of the present invention. The dissolved keratin sulfate, in its precipitated acid form, has a distinct cationic character and a spongy structure with anion-exchanging properties. Thus, it can absorb all anionic chemicals and thereby remove them from the waste water.

The bleaching effect produced by chlorine dioxide on the phenolic melanine color pigments of hairs and roots of hairs is greater with full oxidation of keratin-sulfur than with partial oxidation as in the method of the present invention. In addition, the phenolic melanines turn dark in an alkaline solution. In order to remove this coloration of the hides, and also to remove any hairs still remaining on the hides, it is suitable to add a small amount of chlorine dioxide, or of a salt which yields chlorine dioxide, to the pickling bath which is usually used prior to chrome tanning, and to drum the hides further for about one hour in the tanning drum. Such baths in general comprise an acid in an amount of about 0.52%, common salt (NaCl) in an amount of about 5-15 and water in an amount of about 30-200% (by weight of the soaked hides or pelts). For this after-treatment with chlorine dioxide, an amount of about 0.10.4% (by weight of the soaked hides and pelts), or the corresponding quantity of a salt yielding chlorine dioxide, is generally sufi'icient. This small amount of chlorine dioxide is very rapidly consumed and restores the light color of the hides. Of course, this after-treatment with small amounts of chlorine dioxide or of substances which yield chlorine dioxide may also be carried out in an operation separate from pickling. After bleaching and pickling, chrome tanning can be carried out as usual, for example by adding a basic chromium sulfate in an amount of about 2-6% by weight, calculated as Cr O to the pickling bath. The tanning operation can then be completed as usual.

The process of the present invention permits, for the first time, the production by a simple method of a completely tanned leather from a salted green hide in only about 10-15 hours. When the usual green hides which have been fleshed and salted in a slaughter-hoouse are used, all these operations can be performed during this time in the same drum, without it being necessary to transfer the hides to another container. In the new process, it is only necessary to change the bath twice. All chemicals and additives can be added in liquid form during the rotation of the drum. The aqueous baths which are to be removed between the individual operations can be drained through a slide valve which can be controlled electronically. This permits automation of the total process with the aid of a program control.

The following examples illustrate the invention but they are not intended to limit it thereto:

EXAMPLE 1 4 black and white bovine hides of North American origin, having a weight. of 102 kg. in salted state, were soaked, dehaired and tanned in a drum within 14 hours in the following 3 process steps:

(a) Acid soak The salted hides were introduced into a drum and soaked for 3 hours in 300 l. of water at 20 C. and containing 2.5 kg. of concentrated sulfuric acid, the drum revolving with 3 revolutions per minute. After this time the bath was discharged.

(b) Unhairing 50 l. of water, 5 kg. of common salt (NaCl), 0.3 kg. of concentrated sulfuric acid, and 3 kg. of a sodium chlorite solution having a strength of 30% were introduced into the drum and drumming was continued for 3 hours during which time the chlorine dioxide and the chlorine which developed were completely consumed. Thereafter, the free space in the drum was free from gas and the hairs were gelatinized and showed a brown color, but still adhered strongly to the hides. The cystein reaction with lead acetate still showed a distinct black coloration. The bath. dyed potassium iodide starch paper only Weakly blue. The starting pH-value of 2.0 of the oxidizing bath had increased during the treatment with chlorine dioxide to pH 2.8 and the temperature had risen to 38 C.

In order to remove the partially oxidized hairs, 50 l. of water at a temperature of C. and containing 2 kg. of soda were added through the hollow axle of the drum and drumming was continued for 30 minutes. During that time epidermis and hair separated from the corium. The hides took a light brown color. During this separation process, the starting pH of 8.5 of the alkaline bath fell to 8.0, because a small amount of acid diffused outwards from the interior of the hides.

(c) Pickle and tanning After draining of the unhairing bath, 100 l. of water, 10 kg. of common salt, 0.2 kg. of formic acid having a strength of 85%, 0.6 kg. of concentrated sulfuric acid, and 1 kg. of a sodium chlorite solution having a strength of 30% were introduced into the drum and the hides were treated for 1 hours with this solution. At the end of this treatment, all chlorine and chlorine dioxide was con sumed and the hides had again assumed a light color. The pH-value of the bath was 2.5 and the temperature was 28 C.

10% of a self-neutralizing chromium sulfate, containing 30% of Cr- O were added to this solution and drumming was continued for 6 hours. After this time, the hides were well tanned. and fast to boiling. The pH-value of the solution at the end of this treatment was 3.2 and the final temperature was 25 C.

EXAMPLE 2 3 air-dried snake hides having a dry weight of 752 g. were soaked in a standing drum for 24 hours with 7.5 liters of water with addition of g. of soda as soaking accelerator until the hides were soaked uniformly and showed a supple touch.

The hides were then drummed in a revolving drum with 3 l. of water while introducing 21 g. of chlorine dioxide gas through the hollow axle into the free space of the drum. The gaseous chlorine dioxide may be produced either from an aqueous 5% chlorine dioxide solution, obtained as an intermediate product in chlorite produc tion, by slow heating to 50 C. while simultaneously passing through a stream of carbon dioxide, or it may be pro duced according to known methods by acidification of an alkali metal chlorite solution or reduction of an alkali metal chlorate solution with oxalic acid.

During rotation of the drum, the air space was continuously traversed by the path so that the latter rapidly saturated with chlorine dioxide.

Drumming of the hides was continued for 24 hours at 25 C. The design of the snake, which design consists of melanines, was thereby bleached to a large extent, so that it appeared below the undissolved keratin scales as a light brown residual pigmentation.

About 10 g. of a sodium hydroxide solution having a strength of 40% were then added and drumming was continued for minutes. The scales, which were slightly sulfonated, were rapidly dissolved and the design appeared again in deeper tones.

In the production of large size reptile leather for clothing, it is in many cases desired that the design be completely removed. For this purpose, snake hides which have been descaled by oxidation as above can be exposed again to chlorine dioxide.

In order to effect this treatment, 4.5 l. of water, 5 g. of sulfuric acid and 150 g. of common salt are added to the snake hides after draining 01f the alkaline solution which contains the scales, while introducing 20 g. of chlorine dioxide gas through the hollow axle in the course of 8 hours as described above. The keratin-containing pig ments of the hide are thereby completely removed and the snake hides take on a pure white color.

After this treatment, the snake hides can be tanned according to known methods with chromium or alum tanning agents, dyed and dressed.

EXAMPLE 3 3 sheep hide swere dehaired by means of a wet-shearing machine to a shearing depth of 3 mm. The wool so obtained can be sold as particularly fine sheared wool.

The residual wool, which is fixed to the corium, and the keratin-containing epidermis were removed from the hides as follows.

The hides were drummed for 5 hours with 5 l. of water, 50 g. of sulfuric acid, 500 g. of common salt (NaCl), and 230 g. of sodium chlorite. The temperature of the bath thereby rose from 20 to 37 C., and the pH-avlue was 2.5. After this time, the solution was free of chlorine di oxide and the residual hairs were gelatinized to a high degree. The hairs and the epidermis separated almost quantitatively from the hides by the addition of g. of mono-ethanolamine and upon further drumming for 15 minutes. The sheep hides were well degreased and their otherwise normally spongy structure was considerably solidified.

In order to destroy the few short hairs which still remained, the greasy first unhairing bath, from which the hairs can be flocked out by acidification, was drained. The hides were then subjected in the same drum to a 2-hour after-treatment with 30 g. of sodium chlorite in S l. of water, 25 g. of sulfuric acid, and 250 g. of common salt. The hides thus dehaired were pure white, completely de haired, and also showed an excellent hard grain.

Because of chlorination of the natural fat which has taken place as a side effect of dehairing, the fat content of the hides was reduced from originally 28-30% to 8.5-- 11%. This may be due to better emulsifiability of chlorinated fats.

The hides can be tanned, oiled and dressed according to known methods. In this respect, the good polishability of the grain side and flesh side of the hides in the wet state, which sheep hides which have been 'unhaired by the lime sulfide method do not show, is of particular advantage.

EXAMPLE 4 A. bovine hide having a weight of about 20 kg. was introduced into a pit filled with 500% by the weight of the hide of water at 30 C. The bath was adjusted to a pH-value of 3.54.0, and gaseous chlorine dioxide was introduced through a distributing tube during a period of 4 to 6 hours. Taking into account the consumption of 50-60% of the chlorine dioxide by the keratin of the hair, the amount of chlorine dioxide introduced should not be more than 11 liters per hour in order not to exceed the solubility of chlorine dioxide in water which is about 5 g./l. The chlorine dioxide was obtained from an acidified sodium chlorite solution by the addition of an acid and was passed over with the aid of a continuous stream of gaseous nitrogen, serving as inert diluent. It is also pos= sible to use in the same manner a stream of a gas mixture of chlorine dioxide and carbon dioxide, produced by the action of oxalic acid on sodium chlorite.

The bleaching effect of the chlorine dioxide became apparent after a period of exposure of 3 hours. After 4-6 hours, the hairs were strongly damaged. At this stage, a small cutting of the treated hide was introduced into a solution of an alkali, preferably sodium carbonate, having a strength of 1%. The oxidized hairs must dissolve within 15 minutes, otherwise the treatment with chlorine dioxide is continued for 12 hours and the test is then repeated. When the test was positive, introduction of chlorine dioxide was discontinued and the bath. was adjusted to a pI-Lvalue of 7.5-8.5 by means of an alkali, preferably sodium carbonate. The hairs dissolved completely within 15-30 minutes. It is advantageous to move the hides slightly during this time. After this treatment with the weak alkali solution, the hide had a light yellow to brown color which is due to the phenolic groups of the pigments of the air. For preparing dark or covered leather, the hide can be further treated in the usual manner. For pre paring a pure white hide, for example as those required in the manufacture of light aniline leather, the bath is acidified again to pH 3.5-4 and the hide is exposed for 1 hour to the action of chlorine dioxide. A pure white and well degreased hide is then obtained, the fat wrinkles of which are suppressed to a large extent.

EXAMPLE 5 A calf hide soaked in an acid soak and having a weight in soaked state of about 8 kg. was hung up in wet state in a chamber which can be closed in air-tight manner and which is provided with a gas inlet and outlet tube. After closing the chamber, the air contained in it was displaced by nitrogen. A uniform stream of a gas mixture, consisting of 1 part by volume of chlorine dioxide and 1 part by volume of nitrogen, was passed through for 4 hours at 20 C. The quantity of chlorine dioxide should not exceed 6 liters per hour. The chlorine dioxide was produced from an acidified sodium chlorite solution and was passed over with the aid of nitrogen serving as inert gas. After 4 hours, the hair was very strongly bleached and strongly damaged. For controlling the degree to which the hair was damaged, a small cutting of the hide was introduced into a sodium bicarbonate solution having a strength of 2%. The hair must dissolve completely within minutes, otherwise the treatment with chlorine dioxide is continued for another hour. When the test was positive, the gas mixture in the chamber was replaced by nitrogen; the hide was then treated at C. in a drum with 100% by the weight of the hide in soaked state of a bath which had been adjusted to pH 7.5-8.5 by means of sodium bicarbonate. The hairs dissolved completely within 10-20 minutes. The hide thus treated with alkali had a slightly yellow color which was due to the hair pigments. The hide can then be further treated by the usual methods, for example, pickling, tanning, etc. either in the unhairing bath or in a fresh bath. For preparing a pure white hide, for example, as that required in the manufacture of aniline leather, the hide can subsequently be treated with a pickling bath with the addition of 0.5-1% of sodium chlorite. It is also possible to introduce the hide into the chamber described above and to bleach it therein for about /2 to 1 hour, after displacement of the air by nitrogen, by the action of about 6 liters of gaseous chlorine dioxide. A pure white hide is then obtained which can be used for the manufacture of very light colored aniline leather.

We claim:

1. A method for removing the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts, which method comprises contacting said hides and pelts with chlorine dioxide in an aqueous bath at a pH of up to 6 for 2 to 8 hours until said keratin-containing parts have jellified and softened, and then treating said hides and pelts with an alkaline bath having a pH-value of at least 7.5 until said keratincontaining parts are loosened and removed.

2. A method as in claim 1 wherein chlorine dioxide is generated in situ in said aqueous bath from a substance which yields chlorine dioxide.

3. A method as in claim 2, wherein said substance is an alkali metal chlorite.

4. A method as claimed in claim 1, wherein said aqueous bath has a pH-value from 1.5 to 3.5.

5. A method as in claim 1 wherein chlorine dioxide is contacted with said hides and pelts in an amount from 0.3 to 3 by weight of the soaked hides and pelts.

6. A method as in claim 1, wherein contact with chlo- [1;18 dioxide is at a temperature between 10 C. and 4 C.

7. A method as in claim 1 wherein said alkaline bath has a pH-value from 8.5 to 9.5.

8. A method as in claim 1 wherein said hides and pelts treated with said alkaline bath are then contacted with a sinall amount of chlorine dioxide.

9. A method as in claim 1 wherein said hides and pelts treated with said alkaline bath are then contacted with from 0.1 to 0.4% of chlorine dioxide by weight of the soaked hides and pelts.

10. A process as claimed in claim 1, wherein the keratin-containing parts which are removed from the hides afid pelts and dissolved in form of keratin sulfate in the alkaline bath are precipitated by acidifying the waste water of this process to a pH below 6.

11. A method for removing the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts which method comprises contacting water-soaked hides and pelts in the absence of a bath with gaseous chlorine dioxide for 2 to 8 hours until said keratin-containing parts have jellified and softened and then treating said hides and pelts with an alkaline bath having a pH-value of at least 7.0 until said keratin-containing parts are loosened and removed.

12. A method as claimed in claim 11, wherein chlorine dioxide is contacted with said hides and pelts in an amount from 0.3 to 3% by weight of the soaked hides and pelts.

13. A method as claimed in claim 11, wherein the said alkaline bath has a pH-value from 8.5 to 9.5.

14. A method as claimed in claim 11, wherein said hides and pelts treated with said alkaline bath are then contacted with a small amount of chlorine dioxide.

15. A method as claimed in claim 11, wherein said hides and pelts treated with said alkaline bath are then contacted with from 0.1 to 0.4% of chlorine dioxide by weight of the soaked hides and pelts.

References Cited UNITED STATES PATENTS 3,194,865 7/1965 Pagan et a1. 8--94.16 3,294,479 12/ 1966 Rosenbusch et al 8-94.l6

OTHER REFERENCES Journal of the American leather Chemists Association, 1966, vol. 61, pp. 414-419.

GEORGE F. LESMES, Primary Examiner I. R. MILLER, Assistant Examiner US. Cl. KR.