CS276444B6 - Process for pickle-cured hides tanning - 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 present invention relates to a process for making raw pleated (acidified) skins or waste from raw pleated skins. It is an object of the invention to recover leather of improved quality, in particular leather with improved tanning fixation. In particular, the invention can be used for tanning sheepskin, cowhide, pigskin, crossbreed and goatskin (preferably wool-free, but optionally also skin from which wool has not been removed).

It is known that ββ skins are traditionally processed to obtain leather by pilling, defatting and tanning, followed by various leather finishing operations (lubrication, dyeing, drying). The defatted skin is usually carried out in aqueous baths containing surfactants which allow fat extraction. These defatting baths are prepared using light petroleum fractions. The tanning is then carried out in a traditional manner using chromium salt water baths 3.

This traditional processing of the skin yields leather of good quality. However, this traditional skin treatment method has two serious drawbacks. The first of these drawbacks is that virtually never tanning the chromium content of the tanning bath is completely depleted, which results in the waste tanning bath still containing significant amounts of chromium and is therefore ecologically very harmful (these tanning baths contain aei 30% of the original chromium content). Since chromium is a substance that is very toxic from an ecological point of view, exhausted waste tanning baths must be subjected to cleaning operations in order to remove the chromium from these baths before they are discharged into the stream, which is expensive. Moreover, these operations do not directly recover the chromium, which regeneration would allow the immediate reuse of chromium in the tanning baths.

The second is the fact that chromium is desalted during the uenl finishing operation (especially during the dyeing operation), in particular during the dyeing operations, resulting in soiling of these chromium baths, so that these baths have to be - as well as the tanning bath - subjected to purification operations to remove chromium.

Publikaco P.L. BARTLETT et al., 3rd Am. Leather Chem. Ass. ev. 75 1980, pp. 190-199, U3e of Fluorocarbon 113 in the process of leathor and P.L. BARTLETT et al., 3.A.L. C.A., ev. 72, 1977, pp. 467-474, Energy Reduction through Solvent Proceesing of Leather, describes a method of making leather in which tanning is performed using a mixed tanning bath. This mixed tanning bath is a cell of water (1,1,2-trichloro-1,2,2-trifluoroethane / methanol or water)

1,1,2-trichloro-1,2,2-trifluoroethane and the chromium salt dissolved in this tanning bath. Oak reported in these publications, allows the presence of 1,1,2-trlchloro-1,2,2-trifluoroethane phase to achieve higher concentrations of the chromium salt dissolved in water with the same amount of chromium present throughout the tanning bath. According to the authors of the publications, the defect is higher in the concentration of chromium in the aqueous phase to accelerate the penetration of chromium into the skin.

However, there is no reference in these publications to the exhausted waste tanning bath. However, the attempted skin treatments carried out by the inventors under the conditions disclosed in these publications have resulted in exhausted waste tanning baths containing substantially the same amount of residual chromium. Which contain the exhausted tanning bath resulting from the traditional tanning process. Therefore, this proposed method of tanning does not solve the aforementioned drawback deficiencies in view of the knowledge obtained by the authors of the present invention. due to the high content of ecologically harmful chromium in the exhausted tanning baths, it is not currently without a demanding cleaning operation intended to eliminate chromium.

In WEI QXNG-YUAN O. Soc. Leather Tech. Chem., Vol. 71, 1987, pp. 195-198, "Dry tanning in solvent media", describes a tanning process, referred to herein as a compact process, wherein a tanning bath is prepared by mixing a chromium salt and a hydrocarbon medium (kerosene or a mixture of saturated hydrocarbons with a longer carbon string). In order to reduce the chromium content of the exhausted waste tanning bath, the author proposes to carry out in one and the same bath all other skin treatment operations following tanning, i. neutraCS 276 444 BS Lysing, Dyeing, Lubrication and Finishing. In this publication it is noted (atr. 197, column 1 below Table III) that the tanning effect occurs after the tanning process itself and that the residual chromium contained in the tanning bath after the tanning process is complete! on the skin throughout the compact process; during the following operations performed in the same bath.

Although this avoids the formation of individual waste baths falling off after each individual treatment of the skin and the contaminated salted chromium, the problem associated with the formation of an exhausted waste bath containing a considerable amount of chromium is not solved in this way. On the contrary, even in this tanning process, a spent waste bath containing a significant amount of environmentally harmful chromium is produced after the last processing operation. The results show that 45.24% of chromium is bound to the skin, while 54.76% of chromium remains at the bottom of the container.

In addition, the experimental leather treatments carried out in this publication were carried out with small pieces of leather, and such a compact treatment in which all the reactants are mixed in a single bath does not seem to give satisfactory results even if treated with it. whole skin.

Finally, in U.S. Pat. No. 4,039,281 DC, it describes a method of tanning a skin by preparing a DC tanning bath using an organic water-immiscible solvent into which a mineral tanning agent and a synthetic tanning agent are successively introduced. The skin is dried by centrifugation to a very low moisture content. The contact time between the skin and the tanning agent is between 105 and 50 minutes, with the addition of a neutralizing agent in some cases. After the tanning is complete, the organic solvent is drained off so that further processing operations carried out in the same vessel (lubrication, staining ...) can be carried out immediately.

This method is therefore characterized by:

- using a sequence of mineral and synthetic agents,

- very low degree of skin moisture,

- very short tanning time,

releasing regulated alkalinization of the tanning bath, and

- further processing the skin immediately after tanning.

Under the tanning conditions set forth in the cited US patent, the amount of waste tanning bath waste is reduced (see column 2, lines 16 and 19).

It has now been unexpectedly found that it is possible to significantly suppress the formation of ecologically depleted waste tanning baths (but also of baths from subsequent treatments that are contaminated with chromium due to their salting out) by applying new tanning conditions. These new conditions include:

- use of a single mineral compound based on a metal compound (and therefore not a sequence of mineral and synthetic agents),

- controlled skin hydration, more significant than in the aforementioned patent,

- progressive and controlled alkalinization of the tanning bath skin with a much longer contact time (more than twice) of the tanning bath skin, and

- the resting period after tanning.

SUMMARY OF THE INVENTION The present invention is directed to a process for making raw peeled skins nobo waste from raw peeled skins in which the tanning bath is completely depleted in terms of its chromium content and such fixation of the tanning agent to the skin is avoided. .

CS 276 444 B6

The essence of the method according to the invention consists in the fact that

a) prepare a liquid organic, water-insoluble bath that is chemically inert to tanned skin and mineral tanning substances,

(b) whereupon a mineral compound based on a metal compound is suspended in said organic bath;

c) and the degree of skin moisture is adjusted! to the range between:

- a minimum value of 200% (water mass, not dry skin weight), starting from which all skin fibers are impregnated with water to produce homogeneous porosity both inside and on the surface of the solvation and swelling of the fibers, and

- the maximum value corresponding to the total amount of water retained in the skin structure by the hydrophilic affinity of the fibers,

d) and the thus hydrated skin ee is contacted with said organic bath containing? a tanning agent for at least 4 hours, during which contact a base having a pKa value of 3.5 to 7 is slowly introduced into the bath under conditions maintaining a degree of humidity between the above minimum and maximum values, said base being introduced into the bath by gradually increasing the pH of the water retained in the skin to a final value between:

a minimum value corresponding to the value at which complete cross - linking of the metal compound occurs, and

- the maximum value corresponding to the value at which the leather is twisted or the metal compound precipitates in water,

(e) whereupon the βθ skin is removed from said bath and left for at least 24 hours.

The skin tanning process according to the invention is preferably characterized in that e) b) suspends a salt or chromium complex, in particular chromium sulphate, in the organic bath, and d) introduces a base into the bath so as to increase the pH to a final value between a minimum value of 3, 9 and a maximum value of 4.4 corresponding to the value at which the leather begins to twist.

The tanning process according to the invention is preferably characterized in that an amount of a chromium salt or chromium complex is suspended in the organic bath such that the weight ratio of chromium / peeled leather lies within the range of 1: 1. 0.7 to 1.5%.

The skin tanning process according to the invention is preferably characterized in that the base d) is introduced into the bath stepwise so as to achieve a pH alkalization rate of less than 0.6 pH units per hour.

The skin tanning process according to the invention is preferably characterized in that ee b) suspends an aluminum salt or complex, in particular aluminum sulphate, in the organic bath, and d) introducing a base into the bath so as to increase the pH to a final value between 3.5 and a maximum value of 3.9 corresponding to the pH at which the aluminum precipitates.

The tanning process according to the invention is preferably characterized in that b) an amount of aluminum salt or complex is suspended in the organic bath such that the weight ratio of aluminum / peeled leather is in the range of 0.3 to 0.9%.

The tanning process according to the invention is preferably characterized in that d) the base is introduced into the bath stepwise so that an alkalization rate of less than 0.3 pH per hour is achieved.

The tanning process of the invention is preferably characterized in that ee a) the organic bath is prepared from an anhydrous compound selected from the group consisting of halogenated hydrocarbons, in particular 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1- trichloroethane, tetrachlorethylene and dichloromethane; and aliphatic hydrocarbons having a total carbon number of 6 to 16.

The tanning process of the invention is preferably characterized in that it is used for tanning sheep and goat skins, c) adjusting the degree of skin moisture in the range between a miniCS 276 444 B6 of 200% and a maximum of 310% corresponding to the total amount of water retained. or skin skin.

The tanning process according to the invention is preferably characterized in that the base is introduced into the bath in the form of a crystalline powder selected from the group consisting of sodium bicarbonate, sodium formate, sodium acetate, sodium phthalate and sodium citrate.

The tanning process according to the invention is preferably characterized in that d) is brought into contact with the skin for 6 to 10 hours and e) after removal from the bath, the skin is left to rest for 24 to 96 hours.

The skin tanning process according to the invention is preferably characterized in that the pre-defatted wet skin is contacted with a surfactant-free liquid bath based on 1,1,2-trichloro-1,2,2-trifluoroethane.

From the results obtained it could be concluded that the combination of working conditions used in the tanning process according to the invention makes it possible to obtain leather of excellent quality, in which the tanning bath is completely exhausted and does not occur during subsequent processing operations (lubrication, dyeing, finishing) ...) for the salting out of chromium from the skin and thus for the contamination of these partial processing baths with ecologically harmful chromium. This result is remarkable and unexpected, since at the date of filing the present invention, no skin tanning process has been achieved simultaneously:

- fixation of the tanning agent to the extent that no chromium is salted out of the skin during the operation following the tanning of the skin,

complete exhaustion of the tanning bath, and

- obtaining good quality skin.

In this regard, the tanning process of the invention represents a radical solution to the problem of environmental pollution by waste spent tanning baths containing metal compounds, for example chromium compounds. The result achieved may be partially explained by a combination of the following phenomena. The degree of moisture in the skin is sufficient to cause homogeneous swelling of the fibers over the entire surface, but on the other hand, ζοββ too low to allow water to pass from the skin to the organic tanning bath. A mineral tanning agent which is suspended in a single tanning water organic tanning bath is attracted to moist skin due to its affinity for water and thus solubilizes and diffuses homogeneously throughout the skin mass in the moisturized water retained in the skin. The tanning agent dissolves exclusively inside the skin.

Gradual alkalization gives the conditions for homogeneous metal reagent / protein bonding to the extent that good quality is achieved. Uniform and irreversible fixation of the tanning agent in the skin. The slow lysis of the metal boat complexes that occurs in this way causes them to crosslink slowly with the skin proteins, which explains the fact that there is no salting out of the tanning agents from the skin, which otherwise usually occurs during subsequent processing operations.

The maximum degree of skin moisture (corresponding to the total amount of water the skin is able to hold) depends on the skin type. For sheep or goat skins this maximum moisture level is 310%.

A salt or a complex of chromium, in particular chromium sulfate, may be used in the process of the invention. In this case, the base is introduced into the bath so as to increase the pH up to a final pH, comprised between a lower limit of 3.9 and an upper limit of 4.4, which corresponds to the pH at which torsion begins to occur. leather. In order to ensure good chromium crosslinking with the skin protein, the ee base is gradually introduced into the bath to maintain an alkalization rate of ΔρΗ / Δt of less than 0.6 pH units per hour. The amount of the chromium salt or chromium complex suspended in the bath is preferably such that a chromium / peeled (acidified) skin weight ratio of 0.7 to 1.5 is achieved

EN 276 444 05

The process according to the invention can also be carried out using other metal tanning agents, in particular an aluminum salt or complex, in particular aluminum sulphate. In this case, the base is introduced into the bath by raising the pH up to a final value in the range between the lower limit of 3.5 and the upper limit of 3.9, which corresponds to the pH at which aluminum precipitates from the tanning bath. In order to ensure good loading of the aluminum with the skin protein, the base is preferably gradually added to the bath so that an alkalization rate ΔρΔ / Δ t of less than 0.3 pH units per hour is achieved. The amount of the salt or aluminum complex suspended in the tanning bath is preferably such that the weight ratio of aluminum / peeled skin is in the range of 0.3 to 0.8%.

The organic bath may be prepared using a compound (anhydrous or substantially anhydrous) selected from the group consisting of halogenated hydrocarbons, in particular 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1-trichloroethane, tetrachlorathylene and dichloromethane and aliphatic hydrocarbons having a total carbon number of 6 to 16.

In a preferred embodiment of the process of the invention, the base is introduced into the bath in the form of a crystalline powder selected from the group consisting of sodium bicarbonate, sodium formate, sodium acetate, sodium phthalate and sodium citrate.

The combination of the results described above with the good economic profitability of the process according to the invention guarantees the following conditions:

(d) maintain the skin in contact with the tanning bath for 6 to 10 hours; and

e) after removal of the skin from the tanning bath, the skin is left to rest for 24 to 96 hours.

In addition, it has been found that a better final leather quality (flexibility, flexibility and feel ...) is achieved when defatting damp-peeled skins by contacting the skins with a 1.1-based liquid bath prior to tanning. 2-trichloro-1,2,2-trifluoroethane, said defatting bath being free of surfactants.

This defatting allows a sufficient reduction in the amount of fats contained in the skin, the fats remaining in the skin being evenly distributed throughout the skin. In addition, the defatting is carried out in the absence of surfactants, which eliminates the problems arising from the infestation of the water stream by these ecologically harmful surfactants.

In the following, the process of the invention will be explained in more detail with reference to the drawings in which Figure 1 shows a graph of the results of Example 3 in the form of a curve representing the degree of bath exhaustion (solid line A) and curves representing the percentage of chromium salted out of skin (dashed line B). depending on the degree of skin hydration, and Figure 2 shows a graph of the results of Example 6 in the form of a curve representing the degree of bath exhaustion (solid Line C) and a curve representing the percentage of chromium salted out of the skin (dashed line D). skin.

Example 1

The processing conditions are consistent with the process conditions of the invention

Use 10 kg of wet peeled (acidified) lamb which was defatted with a 1,1,2-trichloro-1,2,2-trifluoroethane-based liquid bath free of surfactants. At the exit of the defatting stage, the skins contain about 8 wt. % fat, based on dry skin weight.

The degree of hydration of said skins is adjusted to 250% (based on dry skin weight) by soaking in an 8 ° (8µm) brine bath for about one hour at ambient temperature.

The treated skins are then contacted with 20 liters of 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% chromium salt (based on the weight of the peeled skins) is suspended (Chromosal marketed by Bayer: 33 % base and 18 wt.% elemental chromium); this corresponds to a chromium content of 1.08% based on the starting weight of the skin.

CS 276 444 85

After stirring for 1 hour, the bath is gradually introduced to make 1% (by weight of the peeled skin) sodium formate (pKa > 3.8) alkaline for one hour. Then, 0.25% sodium bicarbonate (pKa > 6.52) was added to the bath every half hour for a total of 4 hours. Oak sodium formate and sodium bicarbonate are introduced into the bath! in the form of a powder. The average alkallization rate corresponds to up to 0.39 pH units per hour.

The final pH of the water retained in the skins is about 4.15. Then, the skins are removed from the solvent bath and allowed to stand for 48 hours (spread on top of each other in a pile).

The organic bath is then analyzed by direct observation and by atomic absorption and elemental analysis. It was thus determined that the bath was completely free of water, tanning agent and alkali products. This bath is regenerated and then recycled.

Also, tanned leather is subjected to visual analysis and analysis to determine: presence and distribution of chromium in leather. The chromium is divided and fixed uniformly in the leather mass, the leather thus treated having a shrinkage temperature of more than 100 ° C, which characterizes the good tanning quality. The leather is then subjected to a traditional treatment in an aqueous lubricating and dyeing bath and is then dried, during which no conventional chromium deposition into the treatment baths has been observed. This finding was supported by atomic absorption of the samples of the downstream processing baths.

Visual tests, touch tests and tests to find out! the mechanical resistance of the leather thus obtained (tearing and tearing) has shown that the leather thus treated has the same mechanical resistance and a better hand quality compared to that obtained using the traditional tanning process.

Example 2

Hydration level too low

The same procedure as described in Example 1 was used, but in this case starting from peeled skins with a moisture of 150 ° C.

At the exit of the tanning stage, the organic bath is free of water, tanning agents and alkalization products.

In contrast to the previous results, however, chromium salting out (2% based on the amount of chromium contained in the skin) in aqueous processing baths (dyeing, lubrication, etc.) can be observed during subsequent processing operations.

In addition, the quality of the leather obtained is not satisfactory. This is because chromium is distributed uniformly in leather.

Example 3

Variable degree of skin hydration

This study of the effect of the degree of skin hydration on the quality of the leather obtained was performed using the procedure used in the previous Examples 1 and 2, varying the degree of hydration of the treated skin. The results of this study are plotted in Figure 1, where curve A represents the degree of depletion of the tanning bath versus the degree of hydration and curve B represents the deposition of chromium salinity to the degree of hydration.

The results show that there is a zone with a defined degree of skin hydration in which:

- complete exhaustion of the tanning bath,

complete elimination of chromium salting out into subsequent processing baths (lubrication, dyeing ...), and

- a quality of 3-skin which is at least equivalent to that of a traditional tanning process.

CS 278 444 BS

Example 4

Final pH too high

The procedure is the same as that described in Example 1, starting from peeled skins having a moisture content of 270%.

These skins are contacted with 20 liters of 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% of the chromium salt is suspended. After stirring for 1 hour, the bath was basified by sequentially introducing 1% sodium formate over one hour and then 0.375% sodium bicarbonate every half hour for 4 hours. Said base is introduced in the form of a powder (a bar alkalization rate of 0.56 pH unit per hour).

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

The organic bath is free of water, reagents and alkalization products.

In this case, the finished leather obtained is of poor quality. The appearance of the leather and its suppleness are not good, since the leather is twisted (shrinkage of the skin fibers).

Example 5

Final pH too low

The same procedure as in Example 1 was used, starting from peeled skin having a moisture content of 280%.

These skins are contacted with 20 liters of 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% of the chromium salt is suspended. After stirring for 1 hour, the bath was basified by sequentially introducing 1% sodium formate over one hour and then 0.1875% sodium bicarbonate every half hour for a total of 4 hours. The bases are introduced into the bath in the form of powders (alkalization rate equal to 0.28 pH units per hour).

the final pH of the water retained in the skins is 3.6. The skins are then removed from the tanning bath and allowed to rest for 48 hours.

The organic tanning bath contains a small amount of water in which a small fraction of the reagents is dissolved, resulting in incomplete depletion of the tanning bath (the tanning degree of exhaustion being about 80%).

These skins are then subjected to a subsequent traditional treatment in an aqueous lubricating and dyeing bath and then dried. During these subsequent processing operations it is possible to observe the chromium deposition in the said baths (about 2 to 3% of chromium, based on the chromium content of the skin).

Example 6

Variable final pH

This alkalization study was performed using the procedure described in Example 1, varying the final pH. The results of this study are plotted in Figure 2. Curve C represents the depletion of tanning bath depletion on the final pH of the water retained in the skin, and Curve D represents the dependence of chromium salinity on the final pH of the water retained in the skin. 2 of the graph it is clear that there is a pH zone (shaded zone) in which it is possible to simultaneously achieve:

- complete exhaustion of the tanning bath,

- elimination of chromium salting-out in the after-treatment bath (lubrication, dyeing ...),

- a leather quality at least equivalent to that of a traditional tanning process.

CS 276 444 BS

To the left of the hatched zone, we can observe the tanning bath and the chromium salting out. To the right of the hatched zone, the leather is twisted and the leather quality is therefore unsatisfactory.

Example 7

Too short time

The procedure described in Example 1 was also used in this test. Starting from plated skin having a moisture content of 250%.

These skins are contacted with 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% of the chromium salt is suspended. After stirring for 1 hour, the bath is made basic by the simultaneous and sequential addition of 1% sodium formate and 2% sodium bicarbonate over one hour and thirty minutes.

The final pH of the water retained in the skins is 4.01. The skins are then removed from the tanning bath and allowed to rest for 48 hours.

Organic tanning bath It is free of water, detergent and alkalizing products.

During subsequent lubrication and dyeing of the skin, slight salting of chromium (2-3% of chromium) into the aqueous lubricating and dyeing bath can be observed. The leather thus obtained is of very good quality. The leather does not have the correct appearance and flexibility because it is slightly twisted.

Example 8

Non-optimal alkalization (very fast)

Also in this test, ee will use the procedure described in Example 1 starting from peeled skin having a moisture content of 300%.

These skins are contacted with 20 liters of 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% of the chromium salt is suspended. After stirring for 1 hour, the bath was made basic by the single addition of 1% sodium formate. The micban is continued for one hour after which 2% sodium bicarbonate is added in one portion (alkalization rate equal to 2 pH units per hour). The bath was stirred for 4 hours. The final pH of the water retained in the skins is 4.12. The skins are removed from the tanning bath and allowed to rest for 48 hours.

The organic bath is free of water, tanning agent and alkalizing agents. These skins are then traditionally treated in an aqueous lubricating and dyeing bath and then dried. During this post-treatment, chromium is not salted into the treatment bath.

However, the leather obtained is of poor quality. Their appearance and flexibility are not entirely satisfactory due to the slight twisting of the leather as a result of the rapid pH change during alkalizing of the tanning bath.

Example 9

Leave the rest time between tanning and subsequent skin processing

Also in this test, the procedure described in Example 1 is used starting from nickel plated skins having a moisture content of 200%.

These skins are contacted with 1,1,2-trichloro-1,2,2-trifluoroethane in which 6% of the chromium salt is suspended. After stirring for one hour, the tanning bath was basified by gradually introducing 1% sodium formate over one hour. Then 0.25% sodium bicarbonate is added every half hour for a total of 4 hours. Said base is introduced into the tanning bath in the form of a powder. The final pH of the water retained in the skins is 4.11.

CS 276 444 B6

The skin is then removed from the bath and immediately afterwards subjected to a subsequent treatment in an aqueous lubricant and dye! bath, then dried. During this post-treatment, significant salting out of the chromium contained in the skin (10 to 20%) can be observed. The leather thus obtained is of poor quality. Oe twisted and does not feel right.

Example 10

Various solvents

The test is carried out as in Example 1, using a peeled skin with a moisture content of 300%.

These peeled skins are contacted with 20 liters of 1,1,1-trichlorothane in which 6% of the chromium salt and 1% of sodium formate are suspended. After two hours of stirring, 0.375% sodium phthalate (pKa > 5.51) is added to the bath every half hour over a total of four hours. The sodium phthalate is introduced into the bath in powder form (alkalization rate: 0.4 pH units per hour).

The final pH of the water retained in the skin is 4.13. The skins are then removed from the tanning bath and allowed to rest for 48 hours. The organic bath is free of water, tanning agents and alkalizing products.

Such tanned leather, in which the chromium is uniformly divided and fully fixed, has a shrinkage temperature of more than 100 ° C. During the subsequent treatment of the skin in the aqueous treatment baths, no salting out of the chromium into these baths was observed. The leather thus obtained is of a quality at least equal to that of a traditional tanning process. ·

Example 11

Goatskin

The procedure is the same as in Example 1, but starting from wet peeled and defatted goats with a moisture level set to 250% by moistening in a brine bath. The final pH of the water retained in the skins is 4.17. The skin is allowed to rest for 48 hours after removal from the tanning bath.

The same results as in Example 1 are obtained, namely:

- complete exhaustion of the tanning bath,

elimination of chromium salting out during subsequent processing operations (lubrication, staining ...), and

a quality of leather which is at least equal to the quality of leather obtained by the traditional tanning process.

Example 12

Reagent: aluminum sulphate

Use 5 kilograms of lamb skins which are wet, plated and defatted in a surfactant-free 1,1,2-trichloro-1,2,2-trifluorothane bath. The degree of hydration of these skins is set to 280% (based on dry skin weight) by moistening in an 8 ° (Baumé) brine bath for about one hour at ambient temperature. During this operation, the pH is adjusted to 2.0-2.9 by the addition of 1% sodium formate.

These skins are then contacted with 10 liters of 1,1,1-trichloroethane in which 10% of an aluminum salt (sulphate containing 8.6% by weight of elemental aluminum) is suspended; this corresponds to 0.80 wt. J aluminum, based on the original weight of the skin.

The tanning bath was made alkaline by sequential introduction of 1.0 M sodium acetate over 2 hours. After

CS 275 444 BS is then added with 0.25% sodium bicarbonate every 30 minutes over a total of 4 hours. Sodium acetate and sodium bicarbonate are introduced into the bath as powders. The final pH of the water retained in the skins is 3.9 and the (average) alkalization rate is

0.17 pH units per hour.

The skins are then removed from the bath and allowed to rest for 48 hours. The organic bath is free of water, reagents and alkalizing products. During the post-treatment of the skin in the aqueous treatment baths (vegal finishing, lubrication, dyeing) no aluminum salting out to said baths was observed.

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

Example 13

Final pH too high

The same procedure as in Example 12 was followed except that 0.35% sodium bicarbonate was added every half hour over a total of 4 hours. The final pH of the water retained in the skins is 4.5 (alkalization rate: 0.25 pH units per hour). The organic bath is free of water, but the presence of an aluminum hydroxide precipitate suspended in the reagent bath was observed. Nor is aluminum completely fixed to the skin.

Therefore, during the post-treatment a slight salting-out of aluminum into the treatment bath was observed.

Claims (10)

1.‘Procedure for the treatment of raw piked skins or waste of raw piked skins, characterized by: a) prepare a liquid organic water-insoluble bath that is chemically inert to tanned skin and mineral tanning agents, (b) whereupon a mineral reagent based on a metal compound is suspended in the organic bath, ... c) and the degree of dampness of the skin is set within the range! between: - a minimum value of 200% (water weight based on dry skin weight), starting from which all skin fibers are impregnated with water to produce homogeneous porosity both inside and on the surface by solvation and swelling of the fibers, and a maximum value corresponding to the total amount of water retained in the skin structure by hydrophilic affinity fibers, d) and the hydrated skin is contacted with said organic bath containing the tanning agent for at least 4 hours, during which contact a base having a pKa value of 3.5 to 7 is slowly introduced into the bath under conditions maintaining a degree of humidity between the aforementioned minimum and maximum values, wherein said base is introduced into the bath so as to gradually increase the pH value of the water retained in the skin to a final value between: - a minimum value corresponding to the value at which complete cross-linking of the metal compound occurs, and -maximum value corresponding to the value at which the leather is twisted or the metal compound precipitates in water, (e) whereupon the skin is removed from said bath and left for at least 24 hours. CS 275 444 2. The method according to claim 1, characterized in that: (b) suspends a chromium salt or complex, in particular temple sulphate, in the organic bath; and d) The base is introduced into the bath by raising the pH to a final value between a minimum value of 3.9 and a maximum value of 4.4 corresponding to the value at which the leather begins to twist. 3. A method according to claim 2, characterized in that: b) suspending in the organic bath an amount of a salt or a chromium complex such that the weight ratio of chrome / peeled skin lies in the range of 0.7 to 1.5%. 4. A method according to claim 2 or 3, characterized in that: d) introducing the base into the bath gradually so as to achieve an alkalization rate of ApH / At of less than 0.6 pH units per hour. 5. A method according to claim 1, characterized in that it is (b) suspending an aluminum salt or complex, in particular aluminum sulphate, in the organic bath; and d) The base is introduced into the bath so that the pH is raised up to a final value in the range between a minimum value of 3.5 and a maximum value of 3.9 corresponding to the pH at which the aluminum precipitates. 5. The method of claim 5, wherein 3Θ b) suspending in the organic bath such an amount of aluminum salt or complex that the weight ratio of aluminum / peeled skin lies in the range of 0.3 to 0.8%. 7. Method according to claim 5 or 6, characterized in that: d) introducing the base into the bath gradually so that an alkalization rate of ApH / A t of less than 0.3 pH units per hour is achieved. Method according to any one of Claims 1 to 7, characterized in that: (a) the organic bath is prepared from an anhydrous compound selected from the group consisting of halogenated hydrocarbons, in particular 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1-trichloroethane, tetrachlorethylene and dichloromethane, and aliphatic hydrocarbons having a total carbon number atoms of 6 to 16. Method according to one of Claims 1 to 8, characterized in that it is used for tanning sheep and goat skins, (c) adjusts the degree of moisture of the skin between a minimum value of 200% and a maximum value of 310% corresponding to the total amount of water retained by the sheep or goat skin. Method according to any one of the preceding claims, characterized in that 3e d) introducing the base into the bath in the form of a crystalline powder from the group comprising sodium bicarbonate, sodium formate sodium acetate, sodium phthalate sodium citrate. 11. A method according to any one of the preceding claims 1 to 10, characterized in that: (d) contact the skin with the bath for a period of 5 to 10 hours; and (e) after removal from the bath, the skin is left to rest for 24 to 96 hours. Method according to any one of the preceding claims, characterized in that a pre-fattening of the wet plated crosses is carried out by contacting them with a liquid bath based on 1,1,2-trichloro-1,2,2-trifluoroethane free of surface treatment. active substances.