DE3617448A1 - Continuous unhairing of animal hides - Google Patents

Abstract

The invention relates to a multi-stage process for unhairing (dehairing) animal hides, starting from soaked hides, comprising a) a hair impregnation stage in an appropriately equipped spatial unit E wherein the soaked hides are moistened with an aqueous-alkaline reducing solution (S-red) of a reducing agent R for 2-30 minutes within the temperature range 20 to 50 degrees Celsius by spraying or pouring, the spatial unit E being constructed in such a way that the hides are hanging or lying and the S-red run-off can be collected and recirculated, and subsequently b) a hair preserving stage in which the hide is treated with a 2 +/- 1.5% strength solution (S-ox) of at least one oxidising agent ox by casting or spraying, the temperature of the solution being within the temperature range 20 to 50 degrees C and the treatment time being from 1 to 30 minutes, depending on the oxidising agent, and subsequently c) a hair loosening stage in which the hide is treated with a dilute alkaline solution (S-alk) by spraying or casting so that the pH rises to 11-13, followed by d) hair removal wherein the hair, which is in a slightly pulped (gelatinised) state, is mechanically removed from the hide and, if necessary after a treatment with an acid solution, separated off.

Description

Field of the Invention

The invention relates to a method for depilation animal hides in a so-called "continuous process", i.e. in one for the automated process intended, multi-stage process, being softened Skins are free of sulfide or low in sulfide.

State of the art

There are two trends in modern leather technology become decisive: the constraint, the processes to be designed in such a way that it complies with the applicable provisions and meet ecological demands and second, the tendency to automate the Workflows. There are several on this line recently proposed procedures. since about 1972 was by E. Heidemann and co-workers a very fast, low-waste liming process developed the "Darmstadt continuous process" has become known. [See. E. Heidemann: The leather 35, 143-150, (1984)].

The process is for processing fresh ones Skins designed. These are using rods scars hanging out on chains through cabins transported. After that, they are continually with a solution of 0.5-1.0% sodium hydroxide solution and 3.0% Sprayed sulfur sodium. After about 20 minutes The hair mud is treated mechanically stripped. The mud must be in one Hair sludge reactor to be worked up to the Reconstitute hair by acidifying and sulfide to remove.  

Spraying can also be done with sulphide-free liming agents (ERHAVIT DU from Röhm GmbH). The surprising result was found that also with solutions from mercapto compounds in Recycling can be done with improvement.

Cranston developed a new method of depilation, Davis and Scroggie developed in Australia ("CSIRO-LIME-process", see R.W. Cranston et.al., Leather, 186, 229-231 (1984); ibidem February 1986, pp. 33-37). The process can be in the barrel or in the mixer be performed. First, the soaked Skin impregnated with sodium sulfate hydrate. After that the excess of the not included Washed off sodium sulfohydrates. The one received Fleet can be reused in recycling. Then the excess present in the hair becomes Sodium sulfhydrate by treatment with Calcium hypochlorite oxidatively destroyed in a short liquor. Now hydrated lime is added and agitated for one hour. Then the hair is removed. The resulting fleet can also be reused in recycling. With these processes you get a hair-free, sheer nakedness. After hair removal, the skin is opened in a fresh bath with hydrated lime.

However, the only one seems problematic Use of sodium sulfate hydrate in the impregnation, because of the development of hydrogen sulfide cannot be excluded.

To get a pure, hair-free nakedness, may the oxidation of the excess sodium sulfhydrate performed only for a short time (movement, 10 minutes) will. It seems questionable whether the today's batch sizes of 5 t salt weight and more  in such a short time an even distribution of the Calcium hypochlorite can be achieved.

Task and solution

The constraints described above, with the Keywords "ecological harmlessness" and "Automation of work processes" characterized can be the state of the art Technology in need of improvement in several ways appear.

It has now become an improved multi-step process found for depilation of animal hides, the starting from softened skins in automatic Operation can be carried out. The improved Procedure includes

• a) a hair impregnation stage in a spatial unit E set up for this, in which the softened skins with an aqueous alkaline reducing solution L-Red, preferably with a pH in the range 4-12, especially 8-9, of a reducing agent R , optionally with the addition of hydrotropics , during 2-30 minutes, preferably 6-15 minutes in the temperature range 20 to 50 degrees Celsius, preferably 25 to 35 degrees Celsius, are moistened by spraying or pouring, the spatial unit E being set up in such a way that the skins hang or lie and the running solution L-Red can be collected and recirculated, and then
• b) a hair preservation level
  in which the skin is treated with a 2 ± 1.5%, preferably one percent, solution L-Ox of at least one oxidizing agent Ox by pouring or spraying, the temperature of the solution being in the range 20 to 50 degrees Celsius, preferably 20 to 35 degrees Celsius lies and the exposure time depending on the oxidizing agent is 1 to 30 minutes, and then
• c) a level of hair loosening
  in which the skin is treated with a dilute alkaline solution (L-alk) by spraying or pouring in such a way that the pH increases to 11-13, followed by
• d) hair removal
  wherein the hair, which is in a slightly silted state, is mechanically removed from the skin and, if appropriate, is separated off after treatment with an acid solution, preferably at pH 4 to 6. The structure of the hair obtained according to the invention is largely intact. The depilated hides can be further processed into leather in a manner known per se. (Cf. F. Stather "Tanning chemistry and tanning technology, 4th edition, Akademie-Verlag, Berlin, 1967).
• For further skin disruption, the hairless skins are, for example - in a limestone scrubber, approximately with 200% of a liquor containing 3% by weight of lime, or
  • - with a 2-3% by weight lime, 0.5% by weight caustic soda and a sulfide-free liming aid such as ERHAVIT F® (product of Röhm GmbH)
  • - Treated with an enzyme liming agent containing alkaline proteases (eg ERHA PH 3790; product of Röhm GmbH) at a pH value of around 10 for one to 12 hours.

Can be advantageous in the implementation of the invention Proceed as follows:

The process is designed to be automated Operation can be carried out and that at least the aqueous reducing solution L-Red is collected again and can be recirculated.

• a) Hair impregnation level In this stage, at a pH between 4 and 12, preferably at pH 8-9, the hair or Slightly reduced keratin. Immunization of the Hair due to lanthionine formation occurs in this pH does not rise. At the same time, that diffuses Reducing agent in the hair follicle where it is in activated in the subsequent stage of hair loosening is attacking the hair root. The Reduction solution L-Red contains an effective one Quantity of known, harmless Reducing agent (Red).
• The reducing agents (Red) are inorganic and organic, sulfur-containing reducing agents, for example sulfides such as NaHS, Na2S or sulfide-free reagents, e.g. on the base organic thio compounds, such as mercaptoalkanols, preferably a combination of both types of reduction in question. Continue to be considered as Reducing agents, acid derivatives, such as sulfites,  Bisulfites, dithonites and thiosulfates, Hydroxymethanesulfinates, neutral compounds such as Thiourea, thiourea dioxide.
• Nitrogen compounds such as cyanides, Rhodanides, nitrites, phosphorus compounds such as Phosphites applicable. Hydrazine is also effective and derivatives, against their use but speak of ecological-toxicological concerns. As a reducing agent are also harmless Metal compounds such as iron II and tin salts possible.
• Organic reducing agents are also of interest such as. reducing carbohydrates like glucose, Dextrin and ascorbic acid, formic acid, optionally reduced quinoid compounds. Combinations of Use reducing agents.
• Reducing agents such as boranates (cf. Ullmann, 4th ed., Vol. 20, pg. 137ff, publisher Chemie, 1981) can be used advantageously.
• In general, the reducing agents should be Reduction potential from -0.1 to -2 volts below possess the conditions to be applied (whereby to note that reduction potentials are strong are pH dependent. See K. Rauscher, J. Voigt,
• I. Wilke, K .Th. Wilke, Chemical Tables and Calculating Tables, 5th edition, Harri Deutsch Verlag, Frankfurt / M. and Zurich, 1972).
• The content of the applied aqueous alkaline Reduction solution L-Red on reducing agents (Red) is usually in the range of 10 to 50 wt .-%.  
• To improve the hair removal effect the reduction solution L-Red, in particular Known nitrogenous active ingredients hair-attracting effect or hydrotropes in the recommended quantities, generally between 0.01 and 50% by weight, preferably 0.5 to 20 % By weight, based on the red, are added. These include e.g. the one in the so-called Amina applicable substances, especially alkylamines with C1-C6-alkyl substituents, especially primary amines, but also secondary and tertiary, optionally also cyclic amines, hydroxy (alkyl) - and diamines. (See DE-PS 4 82 418; F. Stather in "Tanning chemistry and tanning technology", pp. 177-178, Akademie Verlag, Berlin, 1967).
• For example, Ethylamine, ethanol and Diethanolamine, ethylenediamine, piperidine.
• Urea and. Are advantageously used as hydrotropes Derivatives of the same, guanidinium compounds such as e.g. the hydrochloride and / or suitable detergents added. The exposure time of the Reducing agent is 2-30 minutes, preferably 6-15 minutes. The temperature of the reducing solution is between 20 and 50 degrees Celsius, preferably between 25 and 35 degrees Celsius.
• The aqueous reducing solution L-Red is sprayed or poured onto a hanging or lying skin in a suitably equipped unit E using a suitable device.
• The reducing solution can also under increased Pressure sprayed or poured. Expiration The solution is collected and recirculated. After the exposure time of the reducing agent  becomes the adherent, existing in excess Agent via a mechanical device from the yet well-preserved hair squeezed and into the Original spray solution recirculated. As a guide is the consumption, based on the soft weight, e.g. with 6-8% of depilatory agent from about Type ERHA EM 3918 1.2-1.8% ERHAVIT DU the Röhm GmbH. You can choose between this level and the next stage, a washing process of approx. 1 Minutes.
• b) Hair maintenance level
  After the hair has been impregnated with the reducing solution L-Red, the skin is treated with a 2 ± 1.5%, preferably 1%, aqueous solution of an oxidizing agent L-ox, for example with Ca (OCl) 2, by spraying or pouring. The temperature of the oxidation solution (L-ox) is between 20 and 50 degrees Celsius, preferably 20 to 35 degrees Celsius. The pH of the solution in the case of Ca (OCl) ₂ is between 6 and 8. Depending on the oxidizing agent, the exposure time is 1-30 minutes, preferably Ca (OCl ₂ ) 1-5 minutes. Instead of Ca (OCl) ₂ , oxidizing agents, in particular inorganic oxidizing agents with a redox potential of + 0.2 to + 2 volts, can be used in the same way. (See K. Rauscher et al., Loc.cit.). Examples include peroxides, permanganates, persulfates, H ₂ O ₂ , peroxyacids, chlorites, hypochlorites, etc. The oxidizing agent inactivates excess reducing agent on the top of the hair and thus largely prevents hair contamination.
• c) Loosening level
  After the hair-preserving treatment using an oxidizing agent, the pH is brought to 11- by spraying / pouring with a dilute alkaline solution L-alk, preferably with a 1% hydrated lime or 2% NaOH solution or a mixture of both alkalis. 14 increased. If sulfides, hydrogen sulfides or mercaptans are used, these are activated and cause further hair loosening on the hair root side. The swelling of the skin, which would result in mechanical fixation of the loosened hair, is prevented by brief spraying or watering (5-15 minutes), and there is no swelling water due to a lack of liquor.
• The temperature of the alkali solution L-alk is between 20 and 40 degrees Celsius, preferably between 20 and 30 degrees Celsius.
• The hair can be slightly bulged mechanically remove from the skin and can after Treatment in an acid solution at pH 4-5 without Greater odor nuisance easily separated will. As acids contained in the acid solution are, for example, inorganic acids, such as hydrochloric acid, sulfuric acid or organic Acids such as formic acid can be used.

In the interest of the greatest possible use of the chemicals or fleets used a modified embodiment of the invention Process advantageous.

The skins are initially based on the Step a) shown above with an aqueous alkaline Reducing solution L'Red, preferably in the pH range 11-14, specifically 12.5-13, being the alkalinity  expediently adjusted with sodium hydroxide solution, sprayed. The reducing agents preferably comprise organic ones Thio compounds and inorganic thio compounds, derived from the Rongalites, especially in form their alkali or ammonium salts, thiourea and Thiourea dioxide and boranates. The Reduction solutions L'Red can (analog LRed) still Hydrotropics or detergents in the specified Amounts included. The temperature of the spray solution is advantageously at 25-35 degrees Celsius, which Spray duration including exposure time at 5-20 Minutes.

The mechanical stage takes place in the subsequent stage Separation of the cut hair, the separation the adhering reducing solution preferably by Centrifugation, for example in a sludge centrifuge and returning the reducing solution to the first Process stage. This way you can at least 30-35% of the sludge weight in the form of Recover reduction solution. The hairless hides can be analogous to those described above Methods are further processed, for example in a post-cleaner.

As a quantitative example, about one Solution like the commercial hair removal solution ERHA EM 3918 (product of Röhm GmbH) of which 3-4% in terms of salt weight be consumed (with recirculation).

The separated, preferably centrifuged Hair mud is appropriately used with pertinent Treated oxidizing agents. (May be mentioned e.g.  Peroxides, permanganate, persulfate, peroxyacids, Hypochlorite, chlorite, bisulfite, Fe III salts). It is then advantageous to use acids (see above) adjusted to a pH range 4-5, where Restructuring of the affected hair takes place; finally it is filtered off and dried.

The method variant explained above retains essentially the beneficial effects of first mentioned variant, but requires less equipment expenditure for spraying / pouring etc. as they contains fewer process stages. Above all, it yields better usability of the reduction solution L′red than in the former variant.

However, the latter variant sets a higher one mechanical effort in hair mud treatment, e.g. because of the continuous sludge centrifugation and advance in oxidation.

Example for process variant 1: 1) Hair impregnation level

5 kg pieces of softened and split cowhide (3-4 mm) are hanging on bars in one automatic continuous system in a corresponding Room with a reducing solution for 10 minutes sprayed. The reducing solution consists of organic thio compounds (15% by weight of a 50% solution of the sodium salt of mercaptoethanol, 4 wt .-% thioglycolic acid, balance water).

The pH is adjusted to 9.5 with caustic soda. (1 g of the reduction solution has iodometric Titration an iodine consumption of 130 mg). The Temperature is 25 degrees C.

At the end of the spraying process, there will be excess Reduction solution withered and to the original solution returned. By fresher improvement Reduction solution and NaOH (50%) Reduction and pH of the solution by means of iodometric control kept constant.

Laxity does not occur yet. The averaged Reduction solution consumption is approx. 6-8% in terms of soft weight (with recirculation).

2) hair preservation

The reduced skins are also on Hanging rods for 2 minutes with a 0.4% Sodium hypochlorite solution (155 g active chlorine per liter of stock solution).

Consumption: 0.7 g active chlorine per kg skin.  

3) hair removal

After the excess oxidation solution has wiped off the hanging cattle skins are 10 minutes long sprayed with a 2% sodium hydroxide solution. After a further 5 minutes of exposure, excess will become Lye wilted and recirculated. The sult Hair is mechanically applied using a doctor roller away.

By improving fresh caustic soda a must Guaranteed minimum pH of 12.8-13 be (pH statistical control).

The time from the first spray contact to the depilated skin is 28 minutes. The stripped hair mud is collected and acidified to pH 5-6 with 2N H ₂ SO ₄ . No toxic hydrogen sulfide escapes, nor does volatile organic thio compounds. As a result of the restructuring, the hair can be easily removed in a chamber filter press or a sludge centrifuge (dryness of the hair sludge 15-20%). The treated pelts are hairless and show no scarring. They are in a liming limer (200% liquor, 3% hydrated lime, 0.5% NaOH (33%), 1% of a liming aid, e.g. ERHAVIT F from Röhm GmbH, Darmstadt; running time 5-6 hours at 25 degrees C) or by treatment with an alkaline proteinase (0.1% ERHA PH 3790 from Röhm, Darmstadt, in 100% liquor, 25 degrees C., pH 10, running time 5 hours). After the skin has been disrupted, descaling and pickling can be continued in a known manner.

Process variant 1 is analogous applicable with L-Red solutions of the following reducing agents both individually and in combination

Sulfites
Bisulfites
Dithionite
Thiosulfates
Hydroxymethanesulfinates

in the form of their alkali salts, especially the sodium salts or as ammonium salts and thiourea and Thiourea dioxide, continue with solutions of boranates. In the case of thiourea dioxide, hair is preserved, no staining occurs.

Example for process variant 2: 1) Hair loosening and hair removal

5 kg pieces of softened and split cowhide (3-4 mm gap thickness) are hanging on poles in an automated continuous machine with sprayed with a reducing solution. (1 g Reduction solution reduces 130 mg of iodine iodometric titration).

The reducing solution consists of organic Thio compounds (e.g. ERHA EM 3918 from the company Röhm GmbH, Darmstadt). The pH of the solution is at least 13. The spraying time is 10 minutes, the temperature of the spray solution is 25 degrees C. After spraying is complete excess reduction solution is wiped off and fed to the original solution. With improvement fresh reducing solution and sodium hydroxide solution (50%) is the volume of the depilatory solution, the reduction value (see above) and the pH value 13 kept constant. The hair is loosened  and reduced that it is mechanically easy can be removed by means of scraper rollers. The time from the first contact of the skin with reducing solution until she was completely hairless 13 minutes.

consumption

From the above reducing solution is after Processing of 25 pieces of skin a medium one Consumption of 2-4% in terms of soft weight calculated. The prerequisite is that excess Reduction solution after treatment of the Hair sludge in a slurry centrifuge is recovered. From 1 kg of the accruing Hair sludge could 300-350 g Reduction solution are isolated. The recovered reducing solution has nothing lost in reducing power.

Hair mud processing

1 kg of hair sludge freed from the reducing solution is foamed with 30-40% water and with 1-2 wt .-% Fe (III) chloride fullfat (techn.) Added. The pH of the sludge solution is noticeable from pH 11 pH 8; by further adding dilute hydrochloric acid (2n) pH 4 is adjusted.

Sullied hair gets so far by adding acid restructured that it is easily over a Press off the chamber filter press and then dry leaves.

Instead of Fe (III) chloride sulfate can also other oxidizing agents are used, e.g. 1% chlorine lime, with correspondingly more hydrochloric acid for pH 4 adjustment is required.  

Process variant 2 is also analog Way applicable with the sulfur-containing reducing agents individually or in combination:

Sulfites
Bisulfites
Dithionites
Thiosulfates
Hydroxymethanesulfinates,

in the form of their alkali salts, especially the sodium salts or as ammonium salts and thiourea and Thiourea dioxide. When using boranates for hair flocculation in the acidic area to forego obvious reasons.

In the case of thiourea dioxide, the hair stays received, there is no hair staining.

Claims (10)

1. Multi-stage process for the depilation of animal hides, starting from softened hides, comprising

• a) a hair impregnation stage in a dedicated spatial unit E , in which the softened skins with an aqueous alkaline reducing solution (L-Red) of a reducing agent R , for 2-30 minutes, in the temperature range 20 to 50 degrees Celsius, by spraying or pouring are moistened, the spatial unit E being set up in such a way that the skins hang or lie and the running solution L-Red can be collected and recirculated, and then
• b) a hair preservation stage in which the skin is treated with a 2 ± 1.5% solution (L-Ox) of at least one oxidizing agent Ox by pouring or spraying, the temperature of the solution being in the temperature range 20 to 50 degrees Celsius and the duration of exposure depending on the oxidizing agent is 1 to 30 minutes, and then
• c) a hair loosening step in which the skin is treated with a dilute alkaline solution (L-alk) by spraying or pouring in such a way that the pH increases to 11-14, followed by
• d) hair removal wherein the hair, which is in a slightly bulged or loosened state, is mechanically removed from the skin and, if appropriate, is separated off after treatment with an acid solution.

2. Multi-stage depilation procedure Claim 1, characterized in that in the Step a) the aqueous alkaline reducing solution (L-Red) has a pH in the range 7-14. 3. Multi-stage depilation process according to Claim 2, characterized in that the aqueous alkaline reducing solution (L-Red) has a pH in the range 4-9. 4. Multi-stage process according to the claims 1-3, characterized in that in the Step a) the aqueous alkaline reducing solution (L-Red) still contains hydrotropes. 5. Multi-stage process according to the claims 1-4, characterized in that in the Stage a) the softened skins in the temperature range 20-35 degrees Celsius by spraying or pouring be moistened. 6. Multi-stage process according to the claims 1-5, characterized in that in the Stage b) the temperature of the solution (L-Ox) in Temperature range is 20-35 degrees Celsius.   7. Multi-stage process according to the claims 1-6, characterized in that for Hair removal d) with an acid solution in the pH range 3-6 is worked. 8. Multi-stage process according to the claims 1-7, characterized in that it is in automated operation is carried out. 9. Multi-stage process according to claims 1 to 8, characterized in that in the aqueous alkaline reducing solution L-Red sulfur-containing reducing agents or hair solvents come into use. 10. Multi-stage process according to claims 1 to 8, characterized in that in the aqueous alkaline reducing solution L-Red Boranates are used.