DK3110978T3

DK3110978T3 - Method for tanning animal skins and thus tanned leather

Info

Publication number: DK3110978T3
Application number: DK15720592.3T
Authority: DK
Inventors: Manfred Renner; Eckhard Weidner; Helmut Geihsler
Original assignee: Fraunhofer Ges Zur Foerderung Der Angewand
Priority date: 2014-02-25
Filing date: 2015-02-25
Publication date: 2018-05-28
Also published as: DE112015000953A5; ES2662614T3; EP3110978B1; WO2015127935A1; EP3110978A1

Abstract

The invention relates to a method for tanning hides, comprising the following steps: softening the hides in liquid at a pH value in a range of approximately 7 to approximately 10, unhairing the hides in liquid at a pH value of up to 13, dewatering the dripping-wet hides by 5 to 40 wt%, treating the moist hides in a pressure vessel at a pressure of 5 to 60 bar for a pressure treatment time of 60 to 360 minutes, wherein the pressure is built up by feeding carbon dioxide, introducing a tanning solution into the pressure vessel, wherein the mass of the tanning solution preferably corresponds at least substantially to the mass by which the dripping-wet hides have been dewatered in the dewatering step, lowering the pressure in the pressure vessel at least substantially continuously to ambient pressure over a pressure-relief time period of 15 to 90 minutes. Consequently, the method according to the invention leads to significantly reduced wastewater, and thus significantly reduced environmental impact, in relation to known tanning methods while achieving at least the same quality of the tanned leather.

Description

Method for tanning animal skins and thus-obtained tanned leather

The application is related to a method to tan animal skins. The method can be used for every type of tanning (mineral, vegetable or synthetic tanning) and is suitable for all types of skins to be tanned (especially deer, goat, sheep, buffalo, ostrich, crocodile, pig, snake, etc.).

The usual manufacture of leather according to prior art is divided into work in so-called water workshops (because all steps are performed in aqueous solution, so-called liquor), the actual tanning and subsequent dressing of the leather. The water workshop is subdivided into the steps of softening, hair removal (liming), deliming, pickling and acidification, whereby liming and pickling can each take place in a washing procedure.

The softening releases the raw material from dirt and preservation salt, and regains its original water contents. Softening is conducted at a pH between 7 and 10. In this, water-soluble protein materials are released.

Depilation is performed by the addition of chalk and sulphur compounds. The hair is dissolved and the strong water- and fat-containing outer skin is destroyed. Non-collagen proteins are dissolved by hydrolysis during depilation, then washed out. In addition, the internal peptide chains are broken. This process is described as skin digestion. The pH for liming can be up to 13. The skin swells a great deal due to high alkalinity.

The untanned leather skin is described as skin. The bare skin is characterized as having a water content of 60 to 80%, i.e. 10 kg bare skin contains 6-8 kg water. The dry mass of the bare skin consists of up to 98% collagen.

When deliming, the skin components and lime scale removed during depilation are removed from the skin. Various acids can be used for this. Frequently, deliming is performed with ammonia salts. Deliming with organic acids or carbon dioxide and further chemicals which reduce the pH in aqueous solutions, can also be used.

When the pH is reduced, the skin reduces its heavy swelling. The deliming is usually concluded in the pH range in which the following work step, pickling, the enzymes used have high activity. This is in most cases between pH 8 and 9. Deliming at lower pH levels is possible, however.

Pickling removes further proteins which are not suitable for leather tanning, at least enzymatically.

An acid medium must be set before the actual tanning process, i.e. the aqueous solution used for treatment (liquor) must have a pH of less than 3.5, most less than 3, so that the tanning process cannot be stopped directly after adding tanning materials, which would lead to unequal and locally too-strong tanning (so-called dead tanning). This setting of the aqueous solution is called ‘acidification’, and is usually achieved by adding sulphuric and formic acid. The acidification time is about 3 to 14 hours, and the pH can lower to 2.5.

To avoid acid swelling of the skin during pickling, salt is added to the acidification bath. The salt contents of a solution are characterised by the Baume value. The resulting Baume value is set to 5 to 9 with prior pickling procedures. The amount of salt which is needed to reach the desired Baume value depends upon the water contents of the bare skin, and on the volume of the liquor used during acidification. To reach a Baume value of 6, usually 6 % by weight of NaCl is used, related to the weight of the raw material. The electrolyte concentration produced by the addition of salt in the free liquor leads to a reduction in osmotic swelling of the skin cells, as the relative difference of the electrolyte concentration between the skin and the surrounding liquor is reduced.

In contrast to skin swelling with lime, which is developed through lyotropic effects and loading effects, pickling depends less on the type of electrolytes used, but above all on the correct concentration. If one wants to avoid adding salt to prevent osmotic swelling, so-called ‘nonswelling acids’ can be used instead of sulphuric acid. These are aromatic sulphonic acids which have a tanning effect. This tanning effect strengthens the collagen structure, and thus prevents insertion of additional water which is in the end responsible for the swelling of the skin cells. The skin soaked after pickling, soaked in aqueous solution, is referred to as pickled skin.

The central, subsequent step following pickling is tanning. Tanning lasts 12 to 48 hours. The so-called liquor ratio (ratio of the tanning solution weight to the weight of the pickled skin) is usually between 2:1 to 0.5:1. Tanning is typically done in rolling barrels. In addition to water and tannins, various alkalis, such as sodium hydrogen carbonate or magnesium oxide can be used. The pH is held between 3.6 and 4 to start the tanning process and let it run its course. During tanning, the bondable groups of collagens cross-link with the tannins.

After the tanning process, the remaining tannin solution is in the wastewater. Worldwide, 160 to 200 million tonnes of wastewater are generated annually through tanning. The amount of salt which is completely contained in wastewater, is about 500,000 tonnes per year. This causes great environmental pollution, if no wastewater cleaning techniques are used.

If wastewater is treated, usually several steps are required. First the wastewater is clarified either through decanting to separate solid materials. It is then subjected to biological clarification, or is subjected to pure biological clarification. Costs are high with either approach. Decanting is used by tanners to reduce wastewater pollution. Special fees for subsequent clarification must be paid, depending upon the legal situation. In addition, if the tanning uses chromium-containing tannins, the chrome mud must be disposed of as special waste. Purely biological clarification is expensive, and is seldom done today.

The acids which are contained in the leather and come from tanning are first neutralised. Various post-tanning or dressing steps are performed, depending upon the desired type of leather. These are known to one schooled in the art, and are therefore not explained in detail.

In WO 2010/121804 Al, a tanning procedure is described in which handling animal skin with tanning solution is done in a pressure vessel in the presence of compressed gases. This procedure makes it possible to work for the most part without “free” water, i.e. the volume of the liquid normally used in tanning is significantly reduced. This makes it possible to significantly reduce the wastewater generated through tanning or even to work without any wastewater at all.

The task of the present invention is to show an improved tanning procedure, and especially to provide a tanning process in which the use of chemicals can be further reduced, the process duration can be reduced and/or the environmental pollution caused by the entire tanning process is reduced.

At least one of the tasks of this invention is solved by a procedure to tan animal skins; this includes the following steps: - Soften the animal skins in a liquid with a pH in a range of about 7 to about 10, - Removing hair from animal skins in a liquid with a pH of about 13, - Drain the dripping wet skins by 5 to 40% by weight, - Treat the wet animal skins in a pressure vessel with a pressure of 5 to 60 bar for a pressure treatment time (pressure holding time) of 60 to 360 minutes, whereby the pressure is formed by the addition of carbon dioxide, - introducing a tanning solution into the pressure vessel, whereby the weight of the tanning solution preferably corresponds at least substantially to the weight by which the dripping wet animal skins had been dewatered in the dewatering step, - at least substantially continuous reduction of the pressure in the pressure vessel reduced to ambient pressure over a pressure relaxation period of 15 to 90 minutes.

The method according to the invention is based on the knowledge that, through treating the wet animal skins with compressed carbon dioxide, it is possible to replace the previously customary steps of deliming, subsequent pickling and pickling using pressure according to the invention by means of carbon dioxide, i.e. A pressure treatment by means of carbon dioxide already takes place after the depilation step and directly the tanning step taking place under the carbon dioxide pressure can be skipped if the dewatering step takes place before the pressure treatment step. This is very surprising for experts in the field of tanning procedures, as it was unknown before that pressure treatment with carbon dioxide is suitable for deliming and pickling, and, on the other hand, each experienced tanner would avoid under all circumstances, to treat wet animal skins which, after the depilation step, have a strong alkaline pH value, without conducting an interim step, which leads to the tanning itself; he would expect that the animal skins would be destroyed without recourse due to dead tanning procedures.

This is not the case, however. Rather, the procedure in this invention leads to tanned leather which quality is equal in comparison to usual tanned leather. At the same time, environmental pollution is significantly less due to significantly reduced wastewater volumes. In addition, the procedure in the invention saves time.

The success of the method according to the method is because carbon dioxide under pressure, in combination with the low liquid contents of wet animal skins after the dewatering step, can take the tasks of deliming, acid cleaning and pickling, and in the subsequent tanning step, the compressed carbon dioxide can sufficiently reduce the pH, so that the time needed for distribution of the tannin and the tannins into the animal skins through diffusion can largely be avoided, and on the other hand, the acid swelling described in the state of the art can be avoided, so that no aqueous acids are used, but rather a gaseous inorganic acid anhydride is used under pressure. Without requiring limitation, this is explained in the invention that acid swelling is reduced due to the reduced water volumes, and that the swelling can at least be significantly slowed. Thus, the addition of salts or salt replacement materials to prevent acid swelling is not needed. Preferably, the pressure treatment step takes place without the presence of free liquid in the pressure vessel, i.e. only the liquid is present which is found in the moist animal skins. In any case, the presence of a low volume of free liquid during the pressure handling step does not inhibit the process; it only increases the wastewater volume to be removed later.

In an embodiment of the method according to the invention, the deliming of animal skins takes place in liquid at a pH in a range of about 6 to about 9.5, and is only transferred afterwards to pressure treatment using carbon dioxide, whereby the dewatering step is performed before pressure treatment.

In another embodiment of the method according to the invention, the deliming and pickling are conducted conventionally in liquid, and only then transferred after to pressure treatment with carbon dioxide, whereby the dewatering step is performed before pressure treatment.

Also, this embodiment and design leads to a still significantly reduced wastewater production as compared to known tanning procedures and therefore to less environmental pollution.

The method according to the invention additionally allows performing the acid setting (acidification) and tanning without the use of salt or salt replacement materials. The essential to success, important reduction in tanning (to prevent a tanning reaction) and increase (to start and perform the tanning reaction) of the pH during the pickling and acidification steps can be achieved solely by compressing and decompressing the carbon dioxide found in the pressure vessel. Thus, the selection of carbon dioxide pressure would result in a reduction of the pH to at least pH 3.5 to 3. The setting of acidity (acidification) in the context of pressure treatment with carbon dioxide is performed in a period of at least 2 minutes to 3 hours (longer times are possible, but not necessary), as a rule for 15 to 120 minutes.

Before setting acidity, the animal skin is, as a rule, at a pH of 4.5 to 9.5, depending upon whether the pressure treatment step as described in the invention takes place after pickling, after deliming or even after depilation.

The pH of the skins being tanned is caused by the dissolution of the carbon dioxide in the liquid held in the skins. The skins’ pH can also be lowered without adding chemicals. At a pressure of 30 to 50 bar, the pH of water is reduced to less than pH 3.5 (much higher pressure leads to a much lower pH). The pH can thus be controlled by pressure. Such a process for tanning has been hitherto unknown. An optimum is a pH setting before the acidification step to a range between pH 8 to 9. Skins with a pH up to 12 are tanned with the method as described in the invention. The tanning liquid is added into the vessel, and then lowers the pH in this liquid. After sufficient diffusion of chrome ions (the use of a chromium-containing tanning material is avoided here) during a period of about 1 to 3 hours, the portion of the entire pressure treatment time, the pressure is relaxed, and the pH increases again. The targeted pH at the end of the tanning process is about pH 3.6 -4 for a standard Wet Blue or up to over pH 5 if tanning is performed in one step, and at the same time sub-tasks can be assumed by posttanning.

The addition of a tanning solution can take place under CO2 pressure or in a pressureless autoclave. Variants are possible in which the pH of the skin is, for example, selected to be higher, and the pH of the tanning solution lower, whereby a pH of 4 or 5 results from this combination.

Basically, therefore, two processes are possible. Both processes include a reduction of the liquid contents of the skin, e.g. through mechanical squeezing, as described in WO 2010/121804 AI, to which reference is made in this regard, and with respect to the tanning step in general. In the tanning step, the liquor ratio is about 0.1:1 to 0.7:1, preferably 0.25 to 0.65.

In process version 1, the skins to be tanned are placed in autoclaves, layered in a pressureless state with the tanning solution, and finally carbon dioxide pressure is increased. The addition of tanning solution should not exceed a period of 30 minutes. The pressure increase should also not exceed a period of 30 minutes. Otherwise, this would result in a so-called ‘dead tanning,’ i.e. a chemical reaction which takes place on the surface, which prevents further diffusion of chromium ions in the fibrillar structure of the skin, and therefore to tanning failure. A pressure increase to 15 bar, ideally between 15 and 30 bar, causes then needed, sufficient reduction of the skin’s pH. Through later pressure release and the subsequent removal of the carbolic acid from water and skin, the pH value again increases, and the tanning reaction can run its course as desired.

The targeted pH value for tanned leather (Wet Blue) is achieved through a combination of the skin's pH and the tanning solution before tanning. Depending upon the requirements, one can achieve a pH between 3.5 and 5.5.

In the present, preferred process version 2, the skins to be tanned are also laid in autoclaves, but the inside of the autoclaves are subjected to pressure before tanning solution is added to the autoclaves to create the pressure-induced setting of acidity (acidification) before addition of the tanning solution. The minimum CO2 acidification period is 2 minutes; it is optimally 15 to 120 minutes; also, a duration of 3 to 10 minutes can be advantageous in specific cases. In the autoclaves which are subjected to pressure, i.e. after the desired CO2 acidification duration, the tanning solution is added. Thus, dead tanning is securely prevented.

In each procedure in the state of the art which works with conventional chromium III salt (or other mineral, synthetic or vegetable tannins), one must add salt or salt replacement materials to counteract acid swelling of the skins in the acidification and tanning step, to equalise the electrolyte concentration between the skin and free liquor. The salt contents are measured in Baume, and usually are at Baume 6. The water which causes the undesired swelling, diffuses into the skin from the high liquid excess which is present in conventional procedures in conventional acidification/tanning barrels. Although so-called nonswelling acids can be used as auxiliary syntenes as salt replacements, these substance classes are hardly ever used due to their high costs in the tanning market.

The method according to the invention can completely forego the addition of such salts or salt replacement materials.

According to the invention, it is known that, different from usual tanning processes, a starting pH in the liquor no longer needs to be less than 3, if in any case the setting of acidity and the tanning takes place as recommended using carbon dioxide under pressure. Rather, the pH of the skin and tanning solution is set to a range between 3.5 and 9.5, then a pressure-induced setting of acidity takes place using carbon dioxide without the addition of salts or non-swelling acids or other chemicals. A person skilled in the art would exclude success in the tanning process, as, based on his prior experience, tanning animal skins cannot be accomplished due to too-early 35 binding of the chromium ions.

As compared to the state of the art, the method according to the invention also aims to achieve the following improvements: • Reduction of the acidification time to reduce the pH in the skins to be tanned from up to 14 hours, usually from 8 now to 2 hours or even less. • Complete exclusion of salts or salt replacement chemicals such as so-called ‘non-swelling acids.’ Usually, 6 percent by weight and up to a maximum of 20 percent by weight of salt as compared to the wet weight of the skin are used. Thus, on average, 60 kg of salt can be saved per tonne of skin to be tanned. • Complete exclusion of bases and acids to reduce and increase the pH during the tanning process. Before adding chromium salts or other metallic, synthetic or vegetable tannins, usually the pH is reduced by the addition of acids to below pH 3.5, and with further chromium addition even under pH 3. During tanning, mostly in the last third of the tanning period, then the pH of the tanning solution and the skin to be tanned is increased by the addition of bases to over 3.5. The chemical volumes needed for this are about 2 to 6 percent by weight of the wet weight of the skin. Thus, the method according to the invention can save 20 to 60 kg of bases and acids per tonne of skins to be tanned.

The invention is further explained below with the use of examples of embodiments.

Embodiment examples

In all embodiment examples, no salt are used in the wash baths or tanning liquors.

Embodiment example 1 100 kg split cow skins are delimed after depilation using CO2 and pickled at the end of deliming. The pickled skins are washed for 20 minutes, and show a pH of 8.4 after washing. 30 Weight percent of the water contained in the skins is then separated (dewatered). Then the dewatered skins are put into a pressure vessel. The pressure vessel is filled with compressed carbon dioxide until a pressure of 30 bar is achieved in the pressure vessel. The pressure is held constant for 2 to 5 hours to reduce the pH in the skins without adding more chemicals. Then a tanning solution (liquor) is added into the vessels under pressure, whose weight is 30 percent by weight of the weighed wet skins (therefore liquid volumes are added which had been previously dewatered). The tannin used is a conventional chromium tannin, and the tannin volume is 4.5 kg. The pressure vessel time lasts an additional 2.5 hours, i.e. the entire pressure treatment time is 4.5 hours. The pH in Wet Blue is increased to 4.9 through the final one-hour reduction in pressure to ambient pressure. This basification is achieved without the addition of further chemicals. A chromium dioxide contents of 4.4 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved in the water bath. These are standard values for Wet Blue.

Embodiment example 2

The same method as with embodiment example 1, however, the deliming is conducted with ammonium sulphate. The skins have a pH of 9.1 before pressure treatment. After pressure reduction, the pH in Wet Blue is 5. A chromium dioxide contents of 4.6 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

Embodiment example 3

The same method as with embodiment example 1, however, the tanning weight is reduced to 2.7 kg. After pressure reduction, the pH in Wet Blue is 5.2. A chromium dioxide contents of 3.1 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved. Embodiment example 4

Same method as with embodiment example 3, however, only 20 percent by weight of liquid is dewatered, and accordingly tanned with 20 percent by weight of liquor. After pressure reduction, the pH in Wet Blue is 5. A position chromium dioxide contents of 3 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

Embodiment example 5

The same method as with embodiment example 3, however only 10 percent by weight is dewatered, and accordingly tanned with 10 percent by weight of liquor. After pressure reduction, the pH in Wet Blue is 5.3. A chromium dioxide contents of 3.1 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

Embodiment example 6

Same method as with embodiment example 3, but the CO2 acidification duration is shortened to 90 minutes with a pressure of 50 bar. After pressure.reduction, the pH in Wet Blue is 4.9. A chromium dioxide contents of 3 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

Embodiment example 7

Same method as with embodiment example 3, but the CO2 acidification duration is shortened to 60 minutes with a pressure of 100 bar. The pressure relaxation period is 2 hours. After pressure reduction, the pH in Wet Blue is 5.1. A chromium dioxide contents of 3.2 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

In the following examples, after pickling in a pH range between 8 and_9, the pH of the skin is reduced by the addition of 0.2 to 0.5 percent by weight of formic acid related to the split weight in the wash liquor.

Embodiment example 8 100 kg spit cow skins (cross-section 2.7 mm at pH 7) are washed_after pickling for one hour. 0.2 percent by weight of formic acid are added to the wash liquor. 30 percent by weight of the water contained in the skins_is dewatered after washing. Then the dewatered skins are put into a pressure vessel. The pressure vessel is flooded with compressed carbon dioxide until a pressure of 30 bar is set. At this pressure, the skins are kept for 45 minutes and the pH is reduced by the compressed carbon dioxide without adding further chemicals (acidification). Thus, the tanning solution (30 percent by weight of the weighted wet skins) is added into the vessels which are under pressure. The tannin used is a conventional chromium tannin, and the tannin volume is 4 kg. The pressure holding time is 2.5 hours, i.e. the entire pressure treatment time is 195 minutes. The subsequent one-hour pressure reduction to ambient pressure finds the pH increased to 4.3 in Wet Blue. This basification is achieved without the addition of further chemicals. A chromium dioxide contents of 4 percent by weight and a shrinkage temperature of over 100 °C is achieved in the water bath.

Embodiment example 9 300 kg of split cow skins are washed after pickling in a wash bath at pH 6. 0.4 percent by weight of formic acid are added to the wash bath. The acidification period under carbon dioxide atmospheres is 30 minutes. The entire process is performed as described in embodiment example 8, but the volume of tannin used is 12 kg of conventional chromium tannin. A chromium dioxide contents of 4.2 percent by weight and a shrinkage temperature of over 100 °C is achieved in the water bath. The pH in Wet Blue is 4.

Embodiment example 10

The same procedure as in embodiment example 9, but washing is performed in a wash bath at pH 5 by the addition of 0.5 percent by weight of formic acid. A chromium dioxide contents of 4 percent by weight and a shrinkage temperature of over 100 °C is achieved in the water bath. The pH in Wet Blue achieves 3.7.

Embodiment example 11

The same method as with embodiment example 9, however, the tanning is conducted at 50 bar. Results as with embodiment example 9.

Embodiment example 12

The same method as with embodiment example 9, however, the tanning is conducted at 100 bar and the pressure relaxation period is 2 hours. Results as in embodiment example 9.

Embodiment example 13

The same method as with embodiment example 9, however only 20 percent by weight is dewatered, and accordingly tanned with 20 percent by weight of liquor. Results as with embodiment example 9.

Embodiment example 14

The same method as with embodiment example 9, however only 10 percent by weight is dewatered, and accordingly tanned with 10 percent by weight of liquor. Results as with embodiment example 9.

Embodiment example 15

The same method as with embodiment example 1, however, instead of cow skins, calf skins are used, and there is no reduction to pH 6 in the wash bath; rather, the skins are placed directly after pickling at pH 8.4 into the high-pressure vessel. Results as with embodiment example 1.

Embodiment example 16

The same method as with embodiment example 1, however, instead of cow skins, sheepskins are used, and there is no reduction to pH 6 in the wash bath; rather, the skins are placed directly after pickling at pH 8.1 into the high-pressure vessel. Results as with embodiment example 1.

Embodiment example 17

The same method as with embodiment example 1, however, instead of cow skins, goat skins are used, and there is no reduction to pH 6 in the wash bath; rather, the skins are placed directly after pickling at pH 8.3 into the high-pressure vessel. Results as with embodiment example 1.

Embodiment example 18 50 kg of skins are used, which are dewatered after depilation. The skins have a pH of 12 before pressure treatment and 20 percent by weight of liquid are pressed out. After maintaining pressure for 3 hours, the pressure is relaxed over 1 hour. The pH after tanning is 4.8. A chromium dioxide contents of 4.5 percent by weight related to the dry skin substance and a shrinkage temperature of over 100°C is achieved.

Claims

A method of tanning animal skins, comprising the steps of: - soaking the animal skins in liquid at a pH between about 7 to about 10, - removing the hairs from the animal skins in liquid at a pH of up to 13, - to dehydrate the wet animal skins by 5 to 40% by weight, - to treat the moist animal skins in a pressure vessel at a pressure of 5 to 60 bar for a pressure treatment period of 60 to 360 minutes, where the pressure builds up by the supply of carbon dioxide, - introducing a tanning solution into the pressure vessel, wherein the mass of the tanning solution preferably corresponds at least substantially to the mass that has been dewatered from the drift wet animal skins in the dewatering step; pressure relaxation time of 15 to 90 minutes.

Process according to claim 1, characterized in that the tanning solution is introduced into the pressure vessel prior to the carbon dioxide supply-induced pressure build-up.

Method according to claim 1, characterized in that the tanning solution is introduced into the pressure vessel during the pressure treatment period.

Process according to one of claims 1 to 3, characterized in that the treatment of the moist animal skins is carried out in the pressure vessel at a pressure of 15 to 50 bar, preferably 15 to 40 bar and especially preferably 15 to 30 bar.

Process according to one of the preceding claims, characterized in that the pressure treatment time is 100 to 300 minutes, preferably 120 to 250 minutes and especially preferably 160 to 220 minutes.

Process according to claim 5, characterized in that the proportion of the pressure treatment time used for acidification is 5 to 120 minutes, preferably 15 to 120 minutes and especially preferably 15 to 60 minutes.

Method according to one of the preceding claims, characterized in that the pressure relaxation time is 15 to 60 minutes, preferably 15 to 45 minutes and especially preferably 30 to 45 minutes.

Process according to one of the preceding claims, characterized in that prior to the dewatering step, a conventional descaling of the animal skins in liquid occurs at a pH between about 6 and about 9.5.

Process according to claim 8, characterized in that, after the descaling and before the dewatering step, a conventional staining of the animal skins takes place in liquid.

Tanned leather obtained by performing the method according to any one of the preceding claims.