EP0408097A1 - Tanning process of hides - Google Patents

Abstract

The invention relates to a process for tanning acidified wet raw hides. This process consists (a) in preparing a water-insoluble organic liquid bath, (b) in suspending in this bath an inorganic tanning agent based on a metal compound, especially a chromium salt or complex, (c) in adjusting the degree of hydration of the hides above a minimum threshold of 200% (weight of water added to the dry weight of hides) and below a maximum threshold corresponding to the limit of retention of water in the hide, (d) in bringing the hides thus hydrated into contact in the organic bath containing the tanning agent for a period of at least 4 hours and in slowly introducing a base into the bath so as gradually to raise the pH of the water retained in the hides to a final value of between a minimum threshold of 3.9 in the case of a chrome tanning agent, and a maximum threshold of 4.4 in the case of this chrome agent, and (e) in removing the hides from the bath and leaving them to rest for at least 24 hours. The process of the invention makes it possible to obtain at the same time a complete exhaustion of the tanning bath, an absence of release of tanning agent in the subsequent finishing operations and a leather of good quality.

Description

The invention relates to a tanning process for acidified raw hides or waste acidified raw hides; it aims to provide leather of improved qualities, benefiting in particular from better fixation of the tanning agents. It applies in particular to the skins of sheep, cattle, pigs, mestizos, goats (preferentially striped skins but also, if necessary, woolen skins).

We know that the skins are traditionally treated to obtain leathers by subjecting them to pickling, degreasing and tanning, followed by various finishing operations (food, dyeing, drying). Degreasing is generally carried out by means of aqueous baths containing surfactants which make it possible to extract the fats; these baths are produced by means of light petroleum cuts. The tanning is then carried out in the traditional way using aqueous baths of chromium salt.

This traditional process gives good qualities of leather; however, this method has two very serious drawbacks. Firstly, it can be seen that the tanning baths are never completely exhausted in chromium, which leads to discharges of baths containing significant quantities of chromium (of the order of 30% relative to the quantities introduced). However, chromium is an extremely toxic pollutant and discharges must undergo chromium elimination treatments which are very expensive and do not allow direct recovery of chromium. In addition, during the finishing operations which follow the tanning of the skins (in particular during the dyeing operations), chromium releases are observed which pollute the baths and require elimination treatments such as tanning baths.

In addition, the publications: "PL BARTLETT et al., J. Am. Leather Chem. Ass. VOL. 75 1980, p 190-199 Use of Fluorocarbon 113 in the processing of leather" and "PL BARTLETT et al., JALCA , VOL. 72 1977, p 467-474 Energy reduction through solvent processing of leather "describe a leather manufacturing process in which tanning is carried out using a mixed bath either water / 1,1,2 trichloro-1,2,2 trihane / methane / methanol or water / 1,1 trichloro , 2 trifluoro-1,2,2 ethane, bath in which is dissolved a chromium salt As it is indicated in these publications, the presence of the trichloro-1,1,2 trifluoro-1,2,2 ethane allows to reach a greater concentration of chromium dissolved in water, with an equal amount of chromium in the entire bath; according to the authors of these publications, this greater concentration in the aqueous phase leads to an effect of acceleration of penetration of chromium into the skin No indication as to releases is given in these publications Experiments carried out by the inventors of the present invention, under the conditions indicated in these publications, have led to releases of chromium in baths of the same order as those of the traditio process This process therefore does not solve the aforementioned problem of chromium rejecta and, to the knowledge of the inventors, is not currently practiced.

Furthermore, the publication: "WEI QING-YUAN J. Soc. Leather Tech. Chem. Vol. 71 1987 p 195-198 Dry tanning in solvent media" describes a so-called "compact" tanning process, in which one mixes the chromium salt in a hydrocarbon medium (kerosene or mixture of saturated hydrocarbons with longer chains); to limit the discharge of chromium from the tanning bath, the author proposes to carry out in the same bath all the leather treatment operations following the tanning: neutralization, dyeing, food, finishing. It is said in this publication (p 197 column 1 under table III) that the tanning effect continues thus after the tanning itself and that the residual chromium from the tanning bath continues to act on the leather during this process "compact". Consequently, this process avoids producing rejects after each treatment operation, but does not solve the problem of chromium rejects: in fact, these rejects are found at the last stage and are very important; in this regard, this publication indicates (p 197 1st column, penultimate paragraph): "The results show that 45.24% of the chromium is fixed on the skin and that 54.76% remained at the bottom of the container ". In addition, the tests mentioned in this publication were carried out on small skin fragments, and it seems doubtful that the quality of such a treatment" compact "where all the reagents are mixed in the same bath can be satisfactory when whole skins are treated.

Furthermore, US Pat. No. 4,039,281 describes a tanning process consisting in preparing a tanning bath by means of an organic solvent immiscible with water into which are successively introduced a mineral tanning agent and an agent. synthetic tanning. The skins are wrung by centrifugation to reduce their hydration rate to a very low value. The contact time of the skins with the tanning bath is between 50 minutes and 105 minutes, a neutralizing agent being added in certain examples. At the end of the tanning, the organic solvent is pumped to immediately carry out the subsequent treatments in the same tank (food, dye ...). This process is therefore characterized by:
. the use of a succession of mineral and synthetic tanning agents,
. conditions of very poor skin hydration,
. a very short tanning time,
. absence of controlled basification of the tanning bath,
. further treatment of the skins immediately after tanning.

According to this patent, these conditions reduce the release of tanning agents (col. 2 line 16 to 19).

The inventors have discovered that, unexpectedly, it is possible to rigorously suppress the releases of tanning agents (both in the tanning baths and, during subsequent operations, by salting out) by using new conditions of setting. work characterized by:
- the use of a single mineral tanning agent based on a metallic compound (and not a succession of mineral and synthetic agents),
- controlled hydration of the skin, more important than in the aforementioned patent,
- a gradual and controlled basification of the skins in the tanning bath, with a much longer contact time (more than double),
- a rest period following tanning.

The process of the invention therefore proposes to carry out a tanning of acidified raw hides (or waste of acidified raw hides) by completely exhausting the tanning baths and obtaining a fixing of the tanning agent capable of preventing any subsequent release.

• (a) à préparer un bain liquide organique, insoluble à l'eau et chimiquement inerte vis-à-vis de la peau et des agents de tannage minéraux,
• (b) à mettre en suspension dans ledit bain organique un agent de tannage minéral à base d'un composé métallique,
• (c) à ajuster le taux d'hydratation des peaux dans une plage comprise entre :
  . un seuil minimal de 200 % (poids d'eau rapporté au poids sec de peau) à partir duquel l'ensemble des fibres de la peau est imprégné d'eau avec création au coeur et en surface d'une porosité homogène par solvatation et gonflement des fibres,
  . et un seuil maximal correspondant à la limite de rétention totale de l'eau dans la structure de la peau par affinité hydrophile des fibres,
• (d) à mettre en contact les peaux ainsi hydratées dans le bain organique contenant l'agent de tannage pendant une durée au moins égale à 4 heures, et au cours de cette mise en contact, à introduire lentement dans le bain une base ayant une force basique pKa comprise entre 3,5 et 7, dans des conditions propres à conserver un taux d'hydratation des peaux compris entre les seuils minimum et maximum précités, ladite base étant introduite de façon à augmenter progressivement le pH de l'eau retenue dans la peau jusqu'à une valeur finale comprise entre :
  . un seuil minimal correspondant à la limite de réticulation totale du composé métallique,
  . et un seuil maximal correspondant à l'apparition du phénomène de crispation des peaux, ou au pH de précipitation du composé métallique dans l'eau,
• (e) à retirer les peaux du bain et à les laisser reposer pendant au moins 24 heures.

The process according to the present invention consists:

• (a) preparing an organic liquid bath, insoluble in water and chemically inert with respect to the skin and mineral tanning agents,
• (b) suspending in said organic bath a mineral tanning agent based on a metallic compound,
• (c) adjusting the hydration rate of the skins in a range between:
  . a minimum threshold of 200% (weight of water relative to the dry weight of skin) from which all of the fibers of the skin are impregnated with water, creating a homogeneous porosity at the core and on the surface by solvation and swelling fibers,
  . and a maximum threshold corresponding to the limit of total retention of water in the structure of the skin by hydrophilic affinity of the fibers,
• (d) bringing the skins thus hydrated into contact in the organic bath containing the tanning agent for a duration at least equal to 4 hours, and during this bringing into contact, slowly introducing into the bath a base having a basic force pKa of between 3.5 and 7, under conditions suitable for maintaining a hydration rate of the skins between the aforementioned minimum and maximum thresholds, said base being introduced so as to gradually increase the pH of the water retained in the skin up to a final value between:
  . a minimum threshold corresponding to the total crosslinking limit of the metallic compound,
  . and a maximum threshold corresponding to the appearance of the phenomenon of skin tension, or to the pH of precipitation of the metal compound in water,
• (e) remove the skins from the bath and allow them to sit for at least 24 hours.

It has been observed that the combination of these characteristics makes it possible to obtain leathers of excellent qualities, while benefiting from a total exhaustion of the tanning baths, without any tendency to release during subsequent operations (food, dyeing, finishing ...). This result is remarkable and unexpected since, to date, no process allows to combine:
. sufficient fixing of the tanning agent to eliminate any risk of release in subsequent operations,
. total exhaustion of the tanning baths,
. obtaining good quality leather.

Thus the method of the invention provides a radical solution to the problem of pollution by the discharge of metallic tanning agents such as chromium. This result seems in part to be explained by the combination of the following phenomena. The hydration rate of the skins is sufficient to swell the fibers evenly over the entire surface, but reduced enough to avoid passage of water through the organic bath. The mineral tanning agent, alone in suspension in the organic water-free bath, is attracted to wet skin because of its affinity for water, dissolves in the water of hydration retained by each skin and diffuses to the heart of it evenly. Thus, the dissolution of this tanning agent takes place only inside the skin. The slow basification develops, in a homogeneous manner, the metal agent / protein bonds, so as to obtain a good quality binding, uniform and non-reversible; slow olification of the complexes of metal agents (increase in size) occurs and results in slow crosslinking with the proteins of the skin, which explains the complete absence of salting out.

The maximum skin hydration threshold (corresponding to the total water retention limit) depends on the species; for sheep or goat skins, this threshold is 310%.

inférieure à 0,6 unité pH par heure. La quantité de sel ou de ou de complexe de chrome mise en suspension dans le bain est avantageusement telle que le rapport pondéral chrome/peau acidifiée soit comprise entre 0,7 % et 1,5 %.The process of the invention can be carried out with a chromium salt or complex, in particular chromium sulphate. The base is then introduced into the bath so as to increase the pH to a final value between a minimum threshold of 3.9 and a maximum threshold of 4.4 corresponding to the appearance of the phenomenon of tense skin. . Preferably to ensure good crosslinking of the chromium with the protein, the base is introduced gradually so as to obtain a rate of basification

less than 0.6 pH unit per hour. The amount of salt or chromium complex or suspended in the bath is advantageously such that the chromium / acidified skin weight ratio is between 0.7% and 1.5%.

inférieure à 0,3 unité pH par heure. La quantité de sel ou complexe d'aluminium mise en suspension dans le bain est avantageusement telle que le rapport pondéral aluminium/peau acidifiée soit compris entre 0,3 % et 0,8 %.The process of the invention can also be implemented with other metallic tanning agents, in particular with an aluminum salt or complex, in particular aluminum sulphate. The base is then introduced into the bath so as to increase the pH to a final value between a minimum threshold of 3.5 and a maximum threshold of 3.9 corresponding to the precipitation pH of the aluminum. Preferably to ensure good crosslinking of the aluminum with the protein, the base is introduced gradually so as to obtain a rate of basification

less than 0.3 pH unit per hour. The amount of aluminum salt or complex suspended in the bath is advantageously such that the aluminum / acidified skin weight ratio is between 0.3% and 0.8%.

The organic bath can be prepared from a compound of the following families (anhydrous or substantially anhydrous): halogenated hydrocarbons in particular 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1-trichloroethane , tetrachlorethylene, dichloromethane; aliphatic hydrocarbons from C₆ to C₁₆.

According to a preferred embodiment, the base is introduced into the bath in the form of a crystallized powder of the following group: sodium bicarbonate, sodium formate, sodium acetate, sodium phthalate, sodium citrate.

• (d) l'on met en contact les peaux dans le bain pendant une durée comprise entre 6 heures et 10 heures,
• (e) après retrait, on laisse reposer les peaux pendant une durée comprise entre 24 et 96 heures.

The following conditions make it possible to associate with the performances already mentioned a good economic profitability of the process:

• (d) the skins are brought into contact in the bath for a period of between 6 hours and 10 hours,
• (e) after removal, the skins are left to stand for a period of between 24 and 96 hours.

In addition, it was noted that the final quality of the leathers obtained was improved (flexibility, feel ...) by first performing a degreasing of acidified wet skins by putting in the presence of a liquid bath based on trichloro- 1,1,2, trifluoro-1,2,2 ethane, free of surfactant. This degreasing allows a satisfactory reduction in the quantities of fat contained in the skin, the fatty residue remaining uniformly distributed in the skins. In addition, such a degreasing takes place in the absence of surfactants, which avoids polluting discharges of these compounds.

• - la figure 1 est un diagramme fournissant pour l'exemple 3 les courbes d'épuisement (trait continu : A) et de relargage (traits discontinus : B) en fonction du taux d'hydratation des peaux.
• - la figure 2 est une diagramme fournissant pour l'exemple 6 les courbes d'épuisement (trait continu : C) et de relargage (traits discontinus : D) en fonction du pH final de l'eau retenue dans les peaux.

The description of the examples which follow with reference to the figures of the appended drawings illustrates the process of the invention and its performance; on these drawings:

• - Figure 1 is a diagram providing for Example 3 the exhaustion curves (solid line: A) and release (broken lines: B) as a function of the hydration rate of the skins.
• - Figure 2 is a diagram providing for Example 6 the exhaustion curves (solid line: C) and release (broken lines: D) as a function of the final pH of the water retained in the skins.

EXAMPLE 1 Conditions according to the process of the invention

10 kg of wet lambskin acidified (pickled) and degreased using a liquid bath based on 1,1,2 trichloro-1,2,2 trifluoro-1,2,2 ethane is used, free of surfactant. After this degreasing, the skins contain about 8% by weight of fat relative to the dry matter.

The hydration rate of the skins is adjusted to 250% (based on the dry weight of the skin) by rewetting using a brine bath at 8 ° Baumé for about 1 hour at room temperature.

These skins are then placed in the presence of 20 liters of trichloro-1,1,2 trifluoro-1,2,2 ethane in which 6% chromium salt (relative to the weight of pickled skins) ("Chromosal" sold by " Bayer ": 33% basicity, 18% by mass of element Cr) is in suspension; this corresponds to a weight of 1.08% chromium relative to the initial weight of skins. Basified after one hour of stirring, gradually introducing 1% (by weight relative to the weight of pickled skins) of sodium formate (pKa = 3.8) over a period of 1 hour. 0.25% NaHCO₃ (pKa = 6.52) is then added every half hour for 4 hours. The formate and the baking soda are introduced in powder form. The corresponding average basification rate is approximately 0.39 pH units per hour.

The final pH of the water retained in the skins is 4.15. The skins are removed from the solvent bath and left to stand for 48 hours (extended skins stacked on top of each other).

The organic bath is then analyzed by direct observation, atomic absorption and elementary chemical analysis. It is found that it is rigorously free of water, tanning agents and basification products. This bath is recovered and recycled.

The tanned skins obtained are subject to a visual analysis and a chromium analysis after the rest period: the chromium is uniformly distributed and fixed, the skins having a shrinkage temperature greater than 100 ° C., which characterizes good tanning quality. These skins then undergo a traditional procedure in an aqueous medium of food, dyeing and drying, during which no release of Cr is observed (assay by atomic absorption of the element in successive baths).

Visual, touch and mechanical resistance tests (tearing and tearing) show that the finished leather is of equivalent mechanical quality and of superior quality to the touch compared to leather obtained by a traditional tanning process.

EXAMPLE 2 Hydration rate too low

The experimental procedure followed is identical to that of Example 1, but we start with pickled skins at 150% humidity.

After tanning, the organic bath is free of water, tanning agents and basification products.

On the other hand, during subsequent operations, a release of chromium (2% relative to the amount present in the leather) is observed in the aqueous baths (dye, food, etc.).

In addition, the quality of leather obtained is not satisfactory due to a non-uniform distribution of the chromium.

EXAMPLE 3 Variable hydration rate

A complete study on the influence of the hydration rate of the skin was carried out according to a procedure identical to that followed in Examples 1 and 2 but by varying the hydration rate. The results of this study are shown in FIG. 1: curve A of exhaustion of the tanning bath and curve B of chromium release.

It appears that there is a humidity zone (hatched zone) in which it is necessary to locate to obtain, at the same time:
- total exhaustion of the tanning bath,
- a total absence of salting out during subsequent processing operations (food, tincture, etc.),
- a quality of leather at least equivalent to that obtained by a traditional tanning process.

EXAMPLE 4 final pH too high

The experimental procedure followed is identical to that of Example 1, starting from pickled skins at 270% humidity.

These skins are placed in the presence of 20 liters of trichloro-1,1,2 trifiuoro-1,2,2 ethane in which 6% chromium salt is in suspension. After 1 hour of stirring, the basification is carried out by gradually introducing 1% of sodium formate for 1 hour then 0.375% of sodium hydrogen carbonate every half hour for 4 hours. The bases are introduced in powder form (basification rate of 0.56 pH per hour).

The final pH of the water retained in the skins is 5. The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath is free of water, tanning and basification products.

In this case, the finished leather obtained is of poor quality. Its appearance and flexibility are not correct due to a phenomenon of tightness (tightening of the skin fibers).

EXAMPLE 5 : final pH too low

The experimental procedure followed is identical to that of Example 1, starting from pickled skins at 280% humidity.

These skins are placed in the presence of 20 liters of trichloro-1,1,2 trifluoro-1,2,2 ethane in which 6% chromium salt is in suspension. After one hour of stirring, the basification is carried out by gradually introducing 1% of sodium formate for one hour and then 0.1875% of NaHCO₃ every half hour for 4 hours. The bases are introduced into powder (basification rate equal to 0.28 pH unit per hour).

The final pH of the water retained in the skins is 3.6. The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath contains a small amount of water in which a small fraction of tanning products is dissolved, resulting in a non-total exhaustion of the tanning bath (exhaustion rate of the order of 80%).

These skins then undergo a traditional procedure in an aqueous medium of food, dyeing and drying, during which a release of chromium (between 2 and 3% of the chromium contained in the skin) is observed.

EXAMPLE 6 Variation of the final pH

A complete study on the influence of the basification was carried out according to a procedure identical to that followed in Example 1, but by varying the value of the final pH. The results of this study are shown in FIG. 2: curve C of bath depletion and curve D of chromium release). It appears that there is a pH zone (hatched zone) in which it is necessary to be located in order to obtain both:
- total exhaustion of the tanning bath,
- an absence of salting out during subsequent processing operations (food, dye ...),
- a quality of leather at least equivalent to that obtained by a traditional tanning process.

To the left of the hatched area, we observe phenomena of non-exhaustion of the bath and release of chromium; on the right, the leather is tense and an unsatisfactory quality is obtained.

EXAMPLE 7 : Duration too short

The experimental procedure followed is identical to that of Example 1. We start with pickled skins at 250% humidity.

The skins are placed in the presence of 1,1,2-trichloro-1,1,2-trifluoroethane in which 6% chromium salt is in suspension. After one hour of stirring, basification is carried out by simultaneously and gradually introducing 1% of sodium formate and 2% of sodium hydrogencarbonate for 1 hour 30 minutes.

The final pH of the water retained in the skins is 4.01. The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath is free of water, agents for tanning basification products.

During the subsequent stages of food and dyeing in an aqueous bath, a slight release of chromium (2 to 3% chromium) is observed.

The leather obtained is not of very good quality. Its appearance and flexibility are not correct due to a slight phenomenon of tension.

EXAMPLE 8 : Non-optimal basification (too fast)

The experimental procedure followed is identical to that of Example 1, starting from pickled skins at 300% humidity.

The skins are placed in the presence of 20 liters of trichloro-1,1,2 trifluoro-1,2,2 ethane in which 6% chromium salt is in suspension. After one hour of stirring, the basification is carried out by introducing 1% sodium formate at one time. Stirring is continued for 1 hour and then 2% sodium hydrogen carbonate is added all at once (basification rate equal to 2 pH units per hour). The mixture is left to stir for 4 hours. The final pH of the water retained in the skins is 4.12. The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath is free of water, tanning agents and basifying agents.

The skins then undergo a traditional procedure in an aqueous medium of food, dyeing and drying, during which no release of Cr is observed.

But the leather obtained is of poor quality. Its appearance and flexibility are not entirely satisfactory due to a slight phenomenon of tension due to a sudden change in pH during basification.

EXAMPLE 9 Absence of rest

The experimental procedure followed is identical to that of Example 1, starting from pickled skins at 280% humidity.

The skins are put in the presence of 1,12,2 trichloro-1,2,2 trifluoroethane in which 6% chromium salt is in suspension. After one hour of stirring, the basification is carried out by gradually introducing 1% of sodium formate over a period of 1 hour. 0.25% of sodium hydrogen carbonate is then added every half hour for 4 hours. The bases are introduced in the form of powder.

The final pH of the water retained in the skins is 4.11.

The skins are removed from the bath and they are immediately subjected to an aqueous food, dyeing and drying procedure. During these subsequent steps, there is a significant release of chromium from the skin (10-20%).

The finished leather obtained is of poor quality. Hollow skins are obtained, the feel of which is not correct.

EXAMPLE 10 : Different solvent

The experimental procedure followed is identical to that of Example 1, starting from pickled skins at 300% humidity.

The skins are placed in the presence of 20 liters of 1,1,1-trichloroethane in which 6% of chromium salt and 1% of sodium formate are in suspension. After two hours of stirring, 0.375% of sodium phthalate (pKa = 5.51) is added every half hour for four hours. The phthalate is introduced in powder form (rate of basification: 0.4 pH unit per hour).

The final pH of the water retained in the skins is 4.13. The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath is free of water, tanning agents and basifying agents.

The tanned hides obtained, on which the chromium is uniformly distributed and completely fixed, have a shrinking temperature greater than 100 ° C. No release of Cr is observed during the subsequent steps in an aqueous medium.

The final leather is of quality at least equivalent to that obtained by a traditional tanning process.

EXAMPLE 11 Goat skins

The experimental procedure followed is identical to that of Example 1, but we start with wet, acidified and defatted goat skins with a hydration rate of the skins adjusted to 250% by rewetting using a brine bath. .

The final pH of the water retained in the skins is 4.17. The skins are left to stand for 48 hours.

The same results are obtained as for Example 1, namely:
- total exhaustion of the tanning bath,
- a total absence of salting out during subsequent processing operations (food, tincture, etc.),
- a quality of leather at least equivalent to that obtained by a traditional tanning process.

EXAMPLE 12 Tanning agent: aluminum sulphate

5 kg of wet lambskins acidified and degreased using a liquid bath based on 1,1,2 trichloro-1,2,2-trifluoro-1,2,2 ethane free of surfactant are used.

The hydration rate of the skins is adjusted to 280% relative to the dry weight by rewetting using a brine bath at 8 ° B for about 1 hour at room temperature. During this operation, the pH is brought from 2.0 to 2.9 by the addition of 1% sodium formate.

The skins are placed in the presence of 10 liters of 1,1,1-trichloroethane in which 10% of aluminum salt (sulfate at 8.6% by mass of aluminum element) is in suspension; this corresponds to a weight of 0.86% Al relative to the weights of initial skins.

Basified immediately by gradually introducing over 2 hours, 1% sodium acetate. 0.25% of sodium bicarbonate is then added every 30 minutes for 4 hours. Acetate and sodium bicarbonate are introduced in powder form. The final pH of the water in the skin is 3.9 and the average rate of basification is 0.17 pH unit per hour.

The skins are removed from the solvent bath and left to stand for 48 hours.

The organic bath is free of water, tanning agents and basifying agents.

Aluminum salting out is not observed during the subsequent operations in an aqueous bath: vegetable retanning, food, dyeing.

The finished leather is of quality at least equivalent to that obtained by a traditional tanning process.

EXAMPLE 13 : final pH too high

The experimental procedure followed is identical to that of Example 12, but an amount of 0.35% of sodium bicarbonate is added every half hour for 4 hours.

The final pH of the water retained in the skins is 4.5 (rate of basification: 0.25 pH unit per hour).

The organic bath is free of water, but the presence of a precipitate of aluminum hydroxide in suspension is noted, and the aluminum is not completely fixed on the skin.

A slight salting out of aluminum is observed during the subsequent operations.

Claims (12)

1 / - Tanning process for acidified raw hides (or waste acidified raw hides), characterized in that it consists: (a) preparing an organic liquid bath, insoluble in water and chemically inert with respect to the skin and mineral tanning agents, (b) suspending in said organic bath a mineral tanning agent based on a metallic compound, (c) adjusting the hydration rate of the skins in a range between:
. a minimum threshold of 200% (weight of water relative to the dry weight of skin) from which all of the fibers of the skin are impregnated with water, creating a homogeneous porosity at the core and on the surface by solvation and swelling fibers,
. and a maximum threshold corresponding to the limit of total retention of water in the structure of the skin by hydrophilic affinity of the fibers, (d) bringing the skins thus hydrated into contact in the organic bath containing the tanning agent for a duration at least equal to 4 hours, and during this bringing into contact, slowly introducing into the bath a base having a basic force pKa of between 3.5 and 7, under conditions suitable for maintaining a hydration rate of the skins between the aforementioned minimum and maximum thresholds, said base being introduced so as to gradually increase the pH of the water retained in the skin up to a final value between:
. a minimum threshold corresponding to the total crosslinking limit of the metallic compound,
. and a maximum threshold corresponding to the appearance of the phenomenon of skin tension, or to the pH of precipitation of the metal compound in water, (e) remove the skins from the bath and allow them to sit for at least 24 hours. 2 / - Tanning process according to claim 1, characterized in that: (b) a chromium salt or complex, in particular chromium sulfate, is suspended in the organic bath, (d) the base is introduced into the bath so as to increase the pH to a final value between a minimum threshold of 3.9 and a maximum threshold of 4.4 corresponding to the appearance of the phenomenon of tension skins. 3 / - Tanning process according to claim 2, characterized in that (b) an amount of chromium salt or complex is suspended in the organic bath such that the chromium / acidified skin weight ratio is between 0, 7% and 1.5%. 4 / - Method according to one of claims 2 or 3, characterized in that (d) the base is introduced gradually so as to obtain a basification speed

less than 0.6 pH unit per hour. 5 / - Tanning process according to claim 1, characterized in that: (b) an aluminum salt or complex, in particular aluminum sulfate, is suspended in the organic bath, (d) the base is introduced into the bath so as to increase the pH to a final value between a minimum threshold of 3.5 and a maximum threshold of 3.9 corresponding to the precipitation pH of the aluminum . 6 / - Tanning process according to Claim 5, characterized in that (b) an amount of aluminum salt or complex is suspended in the organic bath such that the weight ratio aluminum / acidified skin is between 0 , 3% and 0.8%. 7 / - Method according to one of claims 5 or 6, characterized in that (d) the base is introduced gradually so as to obtain a basification speed

less than 0.3 pH unit per hour. 8 / - Tanning method according to one of claims 1, 2, 3, 4, 5, 6 or 7, wherein (a) one prepares the organic bath from an anhydrous compound of the following families: halogenated hydrocarbons, in particular 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1-trichloroethane, tetrachlorethylene, dichloromethane; aliphatic hydrocarbons from C₆ to C₁₆. 9 / - Method according to one of claims 1 to 8 for the tanning of sheep or goat skins, characterized in that (c) the hydration rate of the skins is adjusted within a range between minimum threshold of 200% and a maximum threshold of 310% corresponding to the total water retention limit for sheep or goat skins. 10 / - Method according to one of the preceding claims, in which (d) a base is introduced into the bath in the form of crystallized powder of the following group: sodium bicarbonate, sodium formate, sodium acetate, sodium phthalate , sodium citrate. 11 / - Method according to one of claims 1 to 10, characterized in that: (d) the skins are brought into contact in the bath for a period of between 6 hours and 10 hours, (e) after removal, the skins are left to stand for a period of between 24 and 96 hours. 12 / - Method according to one of the preceding claims, characterized in that a degreasing of the acidified wet skins is carried out beforehand by putting them in the presence of a liquid bath based on 1,1,2 trichloro, trifluoro -1,2,2 ethane, free of surfactants.

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