HK1102831B - Process for treating animal skins - Google Patents

Description

Method for treating animal skins

The invention relates to a method for treating animal skins. More particularly, the invention relates to a method for degreasing and/or drying skins, hides or leather. Such treated animal skins can be used in a conventional manner, for example for making tanned leathers.

One of the most common methods of degreasing wet skins, other than using perchlorinated or trichlorinated solvents, is to treat these skins in a tumbler in an aqueous environment with organic solvents and non-ionic and/or anionic detergents to emulsify the fats.

DE-OS 2522902, for example, describes degreasing compositions which contain nonionic and/or anionic surfactants as auxiliaries in alcohol-based solvents.

WO 93/18188 describes the degreasing of skins, hides and leather using degreasing agents based on non-ionic emulsifiers of the fatty alcohol alkoxylate type. The nonionic emulsifier contains (a) C having an average of more than 6 EO groups in the molecule₁₂-C₁₈A mixture of a fatty alcohol ethoxylate and (b) a primary fatty alcohol ethoxylate having more than 3 EO groups in the molecule, wherein (i) the fatty alcohol on which components (a) and (b) are based has an iodine value of less than 10 and (ii) component (b) is present in an amount of 2 to 10% by weight relative to the total amount of (a) and (b).

However, a major disadvantage of these processes is that the resulting wastewater contains detergents, fats and salts that are difficult to biodegrade and/or recycle. Another disadvantage is that this process does not give as effective results as the process using perchlorinated or trichlorinated solvents.

Another degreasing method for wet skins is to treat these skins with hydrocarbons such as petroleum derivatives, white spirit and nonylphenol. A disadvantage of these degreasing methods is that water is insoluble in the solvent used. The skins must therefore be dried in a separate process step. Furthermore, highly contaminated waste water is produced in the degreasing process, which waste water contains, inter alia, fat, wool, meat residues, salt and hydrocarbons. On the other hand, dried skins are generally degreased by using chlorinated solvents such as PER (tetrachloroethylene) and TRI (trichloroethylene). However, these solvents are known to be extremely undesirable from a human health and environmental point of view.

A method for drying pelts is described, for example, in US 3,444,625. This document relates to a process in which water is removed from aqueous hides and skins by extraction with organic solvents such as formal, acetone, chloroform or dichloromethane. However, these methods have the disadvantage that the skins have to be degreased in a separate step.

The object of the present invention is to provide an effective alternative to the prior art for removing natural fats contained in dry or wet skins, which is less toxic and/or causes less environmental pollution, while having the advantage that the water contained in the skins can be extracted together with the fat.

The object of the invention is achieved by providing a method for degreasing and/or drying animal skins, comprising the step of contacting the skins with one or more extractor solvents. The term "extraction solvent" refers to an organic solvent capable of dissolving fat and water simultaneously. It should be noted that the term "pelt" generally refers to the pelts of smaller animals such as pigs, calves or sheep, whereas "big skin" generally refers to the pelts of larger animals such as cows or horses. However, the term "pelt" is also used in the prior art to describe pelts of all animals in general, and also hereinafter.

The extraction solvent of the present invention preferably comprises at least one solvent selected from the group consisting of dimethyl ether (DME), methylal, dioxolane, diethyl ether and methyl ethyl ketone. Because of their ease of recovery and for safety reasons, it is preferred to use those solvents which are gaseous at room temperature and atmospheric pressure. It is preferred to use an extractor solvent mixture comprising at least 10 wt.%, more preferably at least 15 wt.%, most preferably at least 25 wt.% DME, based on the total amount of extractor solvent. Preferably, the mixture of extractive solvents comprises, in addition to DME, at least one extractive solvent selected from the group consisting of methylal, dioxolane, diethyl ether, methyl ethyl ketone, ethanol, propanol, isopropanol, more preferably at least one solvent selected from the group consisting of methylal, dioxolane, diethyl ether and methyl ethyl ketone. Even more preferably, a mixture of methylal and dimethyl ether is used. However, dimethyl ether is most preferably used as the extraction solvent.

Although less preferred, the extraction solvent or extraction solvent mixture described above may also be used in admixture with one or more fat-miscible solvents, such as esters, including methyl acetate, ethyl acetate, and propyl acetate; hydrocarbons including n-pentane, isopentane, cyclopentane, hexane, cyclohexane, heptane, white spirit and white ether; glycols, including 2-ethoxyethanol and 2-butoxyethanol; or halogenated hydrocarbons, including CHF₂CH₂CF₃、CF₃CHFCF₃、CF₃Br and CF₃CH₂F. Preferably the mixture contains at least 35 wt.%, more preferably at least 50 wt.%, most preferably at least 70 wt.% of the extraction solvent of the invention based on the total amount of solvent used. More preferably, the mixture comprises at least 10 wt.% DME, based on the total amount of solvent used.

Dimethyl ether (DME) is gaseous at standard atmospheric conditions. It can be liquefied by cooling to below-25 ℃ at atmospheric pressure or by compression above about 5 atmospheres at room temperature. The advantage of liquefying dimethyl ether is that it readily dissolves most fats and about 6.3 wt% water at 20 ℃. It has a specific gravity of 0.661 and a latent heat of vaporization of 96.6cal/g, wherein the specific gravity is the density of the substance divided by the density of water.

The process of the present invention can be used as an initial step for removing fat and/or water from fresh or wet skins, but it can also be used as a degreasing step in a subsequent tanning process. Thus, the skins which can be used in the process of the present invention can be any skins which are also suitable for conventional degreasing and/or drying processes.

It should be noted that the term "pelt" as used throughout this document is a generic term for fresh, wet and dry animal pelts. The term "fresh skins" denotes skins obtained directly from slaughter houses, whereas the term "wet skins" denotes skins which have been subjected to one or more processes for preparing tanned leathers, such as pickling, slaking, liming, unhairing, washing, shearing, unhairing or fleshing. The term "dried skins" means skins which have been subjected to a chemical preservation treatment such as salting and/or a physical preservation treatment such as drying, cooling or freezing.

Thus, skins which are suitable for treatment by the process of the invention include, for example, unsalted or treated skins obtained from animal skins or leather conversion processes; fresh salted skins, semi-cured skins, i.e. skins which are partially dried and optionally cooled, or semi-processed skins, i.e. skins which have been subjected to some process for the preparation of tanned leathers; processed skins, i.e. skins that have undergone all tanning processes except degreasing and/or drying steps; finished hides that should be subjected to a process similar to that known as dry cleaning; and skins from sheepskin tanneries which may degrease the skins before or after pickling.

The term "tanning" as used in this specification is used in its conventional sense, i.e. to denote the general process of converting animal skins to leather.

The skins to be degreased according to the method of the invention are preferably selected from the group consisting of sheep skins, cowhides, goat skins and pigskins. The proportion of sheep skin, goat skin or pigskin to be degreased is greater than that of cow skin. It should be noted that the skilled person will understand that the term cowhide also includes cowhide of bull.

The method of treating fresh, dried, semi-finished or finished skins is similar. The fact that the skins may have chemicals such as salt, acid or alkaline detergents in them generally does not negatively affect the degreasing and/or drying process using the above-mentioned extractor solvents, and therefore these skins can be used in the degreasing and/or drying process of the present invention. In addition, the person skilled in the art will recognize that the process according to the invention can be subjected to the usual variations required for the particular case in the tannery in which it is carried out or for the treatment of the particular skins to be degreased and/or dried.

The extraction process of the present invention may be carried out in any reactor or vessel conventionally used in the degreasing process of skins, hides or leather. In a preferred embodiment, the process is carried out under pressure in a conventional reactor, the contents of which are static and which has a suitable system for wetting the skins. In so-called static reactors, the skins can be folded and placed at the bottom of the reactor or in baskets which are subsequently introduced into the reactor. However, some of the dirt originating from reactor parts or pipes which are not cleaned at all may adhere to the skins. In addition, some water and/or dirt may accumulate in the folds. Thus, in a preferred embodiment of the invention, the skins are placed in a vertical position in the static reactor while their surfaces are wetted from top to bottom with the one or more extractor solvents, or in another position where they can be completely wetted with the one or more extractor solvents.

In another preferred embodiment, the process is carried out in a conventional rotary reactor. The advantage of a rotating reactor is that the interface where the contents of the skins and the extractor solvent interact is improved by rotating, swinging or vibrating, thereby reducing the extraction time and activating the process.

The total amount of extraction solvent used in the degreasing and/or drying process of the invention, the pressure applied and the temperature in the reactor are selected on the basis of the total weight of the skins, the fat content to be removed, the analytical content of the water contained in the skins, whether a fat-miscible solvent is used and possible pre-treatments. For example, the optimum pressure to be applied depends on the manner of dosing of the extraction solvent and the amount and type of extraction solvent used. However, generally pressures of from 1 to 12 bar, preferably less than 10 bar, most preferably less than 8 bar are applied.

In order to remove fat in the most efficient manner, the temperature in the reactor is preferably controlled to be less than 50 ℃, more preferably less than 45 ℃, most preferably less than 40 ℃. The temperature used is preferably above 5 deg.C, more preferably above 10 deg.C, most preferably above 15 deg.C. This can be conveniently achieved by controlling the temperature of the stream recycled to the reactor or by heating or cooling the reactor itself. The high extraction efficiency of the one or more extraction solvents of the present invention allows for operation at a wide range of temperatures, but preferably temperatures above room temperature are applied. Most preferably in the range of 25 ℃ to 35 ℃. To obtain a reproducible process, the temperature is preferably controlled to within 5% of the set point temperature.

Before introducing the one or more extractor solvents into the reactor, in order to ensure safety, it is preferred to take reasonable measures such as introducing an inert atmosphere into the reactor before the skins are contacted with the one or more extractor solvents, optionally in combination with one or more fat-miscible solvents. More preferably, the reactor is maintained under an inert atmosphere during most or all of the degreasing and/or drying process. In a preferred embodiment, CO₂Or N₂Or other inert or fire extinguishing gases may be used for this purpose. Maintaining a constant pressure by an inert gas during the extraction process provides additional safety and adds energy to the process.

During the degreasing and/or drying treatment, the one or more extraction solvents, optionally combined with the one or more fat-miscible solvents of the invention, may be dosed continuously into the reactor, meaning that the combined solvents are added continuously to the reactor over a specified time. The dosing of the extraction solvent into the reactor can also be carried out intermittently during operation, in which case the skilled person is able to select, by routine experimentation, the optimum time interval and the optimum amount of extraction solvent, and the optimum amount of optional fat-miscible solvent. Combinations of these techniques are also possible. Examples of combinations of these techniques include, for example, where the extraction solvent and optionally the fat-miscible solvent are added first continuously, then the addition is stopped, and then they are added again continuously. However, continuous dosing operations are most preferred. In a particularly preferred embodiment, a liquid comprising the extraction solvent, optionally fat-miscible solvent, water and fat, hereinafter referred to as extraction liquid, is withdrawn from the reactor simultaneously with the dosing of the extraction solvent and optionally fat-miscible solvent. In an even more preferred embodiment, the extraction liquid thus separated is subjected to purification. Most preferably, the resulting extraction solvent and optional fat-miscible solvent are subsequently recycled to the reactor.

If it is desired to maintain a certain degree of humidity in the skins, for example for the purpose of flexibility of the skins or when it is advantageous for the subsequent steps of the tanning process, a small amount of water may be added to the extractor solvent and/or the fat-miscible solvent (if used). In a particularly preferred embodiment, the desired amount of water is added to the extraction solvent that is recycled to the reactor. Thus, the degree of drying can be easily adjusted by varying the amount of water in the solvent, so that a pelt with the desired moisture content is obtained.

Once the skins have been degreased and dried to the desired extent, the extraction liquid is preferably taken from the reactor in a closed loop. Typically, the liquid contains inter alia fat and protein residues. The extraction solvent is preferably separated from the liquid and most preferably recycled to the process. The obtained skins will contain absorbed therein an amount of the extractor solvent and optionally the fat-miscible solvent. The solvent may be removed using temperature and/or vacuum control or by inert gas stripping. The solvent absorbed in the skins is preferably evaporated and subsequently recovered. Most preferably, the combined solvent thus recovered is recycled to the reactor. The process is considered to be finished when the total amount of solvent still absorbed in the skins is less than 5g/kg of treated skins, more preferably less than 2g, even more preferably less than 1g, most preferably less than 0.5g/kg of treated skins, as determined by sampling the evaporated gas.

When DME is used as the extraction solvent or one of the extraction solvents used in the degreasing/drying process of the present invention, the presence of water and its solubility in DME improves the process safety in view of flammability, since the minimum and maximum values of flammability are reduced. Thanks to its high diffusivity, DME easily penetrates the pelt, dissolving the water and fats contained therein, forming an extract liquid mainly containing proteins in addition to water and fats, without polluting residues.

The skins are preferably contacted with one or more extractor solvents of the invention and optionally a fat-miscible solvent for a fixed period of time. The extraction time required depends on the fat and water content in the skins, the difficulty of extracting them and the desired result. Typically, the extraction time is less than 10 hours, more preferably less than 8 hours, even more preferably less than 4 hours, and most preferably less than 1 hour. Preferably the skins are contacted with the extractor solvent and optionally the fat-miscible solvent for more than 10 minutes, more preferably more than 20 minutes, most preferably more than 30 minutes. In a preferred embodiment, the extraction liquid can be sampled during the extraction process, and the point in time at which the skins are sufficiently degreased and/or dried can be determined by conventional techniques, such as by using the Karl Fisher method, which determines the percentage of water, or by determining the fat and water content by evaporating the extraction solvent and subsequently weighing the resulting residue. In general, the skilled person will readily know the fat content contained in pelts, when the type of pelts to be treated is known, i.e. the species of animal and the area from which the animal originates and preferably its feed. For example, it is known to the skilled person that spanish merino sheep skins generally contain about 12% fat, whereas australian and british sheep skins generally contain more than 30% fat. In addition, pigskin is known to have a high fat content.

With the method according to the invention, a substantially dry and completely clean pelt is obtained. The process of the invention allows the degreasing removal of skins, preferably to a percentage of more than 50% by weight, more preferably more than 75% by weight, even more preferably more than 90% by weight, most preferably more than 98% by weight, based on the total amount of fat originally contained in the fresh skins. The process of the invention allows the drying-off of pelts which have not been subjected to a preliminary drying step, preferably to a percentage of more than 30% by weight, more preferably more than 50% by weight, even more preferably more than 75% by weight and most preferably more than 90% by weight, based on the total amount of water originally present in the fresh pelts. For some purposes, however, it is preferred to maintain a certain moisture level in the skins (see above).

As discussed above, the amount of extractor solvent required to achieve a satisfactory percentage of degreasing and/or drying depends on the type of extractor solvent used, the fat and water content of the skins to be treated, whether or not the extractor solvent is used in combination with the fat-miscible solvent of the invention, and the desired degree of degreasing and/or drying of the skins. In general, however, the use of 40 litres of extractor solvent per kg of pelt to be degreased and/or dried, more preferably 20 litres, most preferably less than 5 litres of extractor solvent per kg of pelt to be degreased and/or dried, is sufficient to obtain a substantially dry and completely clean pelt, wherein substantially dry means that more than 90 wt.% of water has been removed based on the moisture content of the fresh pelt and completely clean means that more than 98 wt.% of fat has been removed based on the amount of fat contained in the fresh pelt. However, it should be noted that the longer the extraction time, the less solvent is needed to achieve the desired result.

Preferably, at least 0.5 l of extraction solvent per kg of skins to be degreased and/or dried is used, more preferably at least 0.75 l of extraction solvent is used, most preferably at least 1 l of extraction solvent per kg of skins to be degreased and/or dried is used.

The invention is illustrated by the following non-limiting examples. In addition, preferred embodiments of suitable configurations of the reactor and accompanying recirculation system for carrying out the process of the invention are illustrated in the accompanying drawings and will be described below.

Example 1

A fresh piece of sheep skin and a fresh piece of goatskin obtained from a slaughterhouse, recently peeled, with wool residues and dirt adhered (unwashed or treated), were folded at the bottom of a metal basket supporting the sample, the fold size being 30 x 30 cm. The basket was then introduced into a static reactor having a capacity of 140 l and a pressure of 4.1 bar. Subsequently, CO is introduced₂Is introduced into the reactor. The reactor is purged with dimethyl ether (DME) until air and CO are purged₂And a vacuum is established. The entire apparatus was weighed. The equipment weighs 357kg, the groupThe weight of the resultant skin was calculated to be 2,240 g. Subsequently, 53kg of DME was added in the liquid phase at a rate of 17 kg/min. The temperature was measured and the reactor was heated to 27 ℃ and held at this temperature for 1 hour. Over this 1 hour, DME-containing extraction liquid was withdrawn from the bottom of the reactor at about 17 kg/min and recycled to the top of the reactor by means of a pump with a flow rate of 17 kg/min. Thus, the skins were repeatedly washed with DME. After 1 hour, the reactor was purged of DME and placed in CO₂The combined gases are pushed into the exhaust area in the atmosphere. The skins were then weighed. The skins weighed 1,790g on the first weighing due to the presence of absorbed DME. After six hours, the absorbed DME had evaporated and the skins weighed 1,640 g. This means that a total of 640g of water and fat were lost, representing 28.6% of the weight of the untreated pelt.

By analyzing the extracted liquid during processing, not only the amount of dehydration but also the amount of defatting can be calculated. It has been found that fat can be extracted efficiently, i.e. more than 99% of the fat contained in the skins is removed.

Example 2

The so-called "double-sided" wet sheepskins and wet goatskins (clean, fleshless, untanned) for the wool cut determination were placed on the bottom of a metal basket supporting the sample in folds of 30X 30cm and subsequently introduced into a 140 liter static reactor. The total weight of the skins was 2,180 g. The procedure was followed analogously as described in example 1. However, no recycling is performed. Instead, four washes with 15kg of DME were carried out in a pump loop. The DME injected was pumped at a rate of 17 kg/min each time. After 60 minutes the reactor was purged of DME and the skins weighed. The first weighing gave a pelt weight of 1,630g due to the presence of absorbed DME. After 12 hours the absorbed DME had evaporated and the skins weighed 1,270 g. This means that a total of 910g of water and fat were lost, representing 41.7% of the weight of the untreated pelt.

The dried and degreased pelts obtained by the procedure of example 1 or 2 were essentially dry, but because some moisture was still present in the folds, examples 3 and 4 hung the pelts and wetted their surfaces from top to bottom.

Example 3

Two pieces of dried "double-sided" sheep and goat skin (clean, fleshless, dry, untanned) were introduced into a 140 liter static reactor with wool cut off. They are suspended in a vertical position and supported by the rim of the gabion. There are no wrinkles on which dirt and/or water can accumulate. The total weight of the skins was 1,230 g. The procedure as described for example 2 was followed. Thus, four purges with a pumped loop were carried out each time with 15kg of DME, each time replacing DME present in the reactor. Likewise, the DME injected was pumped at a rate of 17 kg/min. Fresh DME was introduced approximately every 15 minutes and the recycled DME was sampled. After 60 minutes (pumping time), the reactor was purged of DME. A vacuum was then established for two hours and the skins weighed. The skins weighed 1,030g on the first weighing due to the presence of absorbed DME. After 2 hours the absorbed DME had evaporated and the skins weighed 1,040g, absorbing some atmospheric moisture because of their hygroscopic properties. This means that a total of 190g of water and fat were lost, representing 15.44% of the weight of the untreated pelt. The skins are substantially dry.

Analysis of these skins by the soxhlet method using methylene chloride according to standard IUC-4/ISO 4048 showed that the fat content of the treated goat and sheep skins was below 1% and 2%, respectively.

A higher extraction efficiency was observed for these dry skins than for the wet skins of example 2.

Example 4

Two small pieces of wet cow hide, i.e. pickled cow hide and cow hide (clean, fleshless, wet, unhaired, untanned) weighing 560g and 700g respectively, were hung in a vertical position, close to the bottom of the gabion and supported by the edge of the basket due to their small size. Subsequently, the metallic basket was introduced into a 140 liter static reactor. There are no wrinkles on which dirt and/or water can accumulate. The procedure as described for example 2 was followed. Thus, four purges with a pumped loop of 15kg of DME are used each time, with each replacement of DME, as introduced from a possible recycle device. Likewise, the DME injected was pumped at a rate of 17 kg/min. After 60 minutes (pumping time), the reactor was purged of DME. A vacuum was then established for 10 minutes and the skins weighed. The first weighing gave a pelt weight of 270g and 340 g. The weight of the two pelts was unchanged after 2 hours. This means that a total of 290g and 360g of water and fat, respectively, were lost, 48.2% and 51.4%, respectively.

Analysis of these skins by the soxhlet method using methylene chloride according to standard IUC-4/ISO 4048 showed that the treated cow hide fat content was below 1%.

A preferred embodiment of a suitable configuration of the reactor and the accompanying recirculation system for carrying out the process of the invention is illustrated in the accompanying drawing.

The construction comprises a static or rotary reactor 1 for containing the skins to be degreased and/or dried, which is known per se, sometimes also referred to as "extractor". The reactor 1 comprises an inlet 2 for fresh extraction solvent and an outlet 3 for extraction liquid containing e.g. fat and protein residues and water. The outlet 3 is connected by a conduit to a reboiler 4 provided with a heating element (not shown). The bottom of the reboiler 4 is connected to a flash drum 5, while the top of the reboiler 4 is connected to the top of a collector/condenser 6 for recovering the extraction solvent, which will be explained below. Although the top of the flash drum 5 may in principle be connected directly to the top of the reboiler 4 or to the top of the collector 6, it is preferably connected to another vessel, in this embodiment a recovery vessel 7. This additional vessel can be used, for example, to check whether any water or residue is still present in the extraction solvent from flash drum 5. To close the loop, the bottom of the collector 6 is connected to the inlet 2 of the reactor 1. In addition, each of the containers 6 and 7 is provided with a drain for taking out the recovered solvent that should not be recycled.

If the extraction liquid contains DME, the recycle system is typically operated as follows: the skins are degreased and/or dried in the reactor 1 at about 25 ℃ and 4 bar; the liquid contents of reactor 1 are pumped to reboiler 4 where the solvent is distilled off at slightly elevated temperature and pressure, e.g. about 35 c and 6 bar; the gas phase resulting from this distillation flows to the collector 6, where it is condensed into a liquid at about 25 ℃ and 4 bar; the recovered extraction solvent is fed to the reactor 1.

If the amount of extracted fat and withdrawn water in the reboiler 4 exceeds a pre-selected limit, for example 50% of the reboiler 4 capacity, the extracted fat and water is pumped to the flash drum 5. In the flash drum 5, the extraction solvent is distilled off or flashed off, for example at about 45 ℃ and 8 bar, and the resulting vapor phase flows to a recovery vessel 7, where it is condensed under conditions similar to those of the collector 6. The condensed extraction solvent is pumped to a collector 6, and fat residue and water are removed from the bottom of the flash tank 5 and collected in a tank.

The above process has been found to be particularly effective in recovering and recycling DME and other extractor solvents.

Claims (15)

1. A process for degreasing and/or drying skins which comprises contacting the skins to be degreased and/or dried with one or more extractor solvents, wherein the extractor solvent is dimethyl ether or a solvent mixture comprising dimethyl ether.

2. The process according to claim 1, wherein the solvent mixture comprises at least 10% by weight of dimethyl ether, based on the total amount of solvent.

3. A method according to claim 1 or 2 wherein the skins are selected from the group consisting of sheep skins, cowhides, goat skins and pigskins.

4. The process according to claim 1 or 2, wherein one or more extraction solvents are used in combination with one or more fat-miscible solvents selected from the group consisting of esters, hydrocarbons, glycols and halogenated hydrocarbons.

5. The process according to claim 1 or 2, which is carried out in a static or rotating reactor.

6. The process according to claim 1 or 2, which is carried out at a pressure of from 1 to 12 bar and a temperature of from 5 to 40 ℃.

7. A process according to claim 1 or 2 wherein the skins are contacted with the one or more extractor solvents for a period of time from 10 minutes to 10 hours.

8. A process according to claim 1 or 2, wherein the skins are degreased to remove a percentage of more than 50 wt% based on the total amount of fat originally contained in the fresh skins and/or dried to remove a percentage of more than 30 wt% based on the total amount of water originally contained in the fresh skins.

9. A method according to claim 1 or 2, comprising the steps of:

-the skins to be degreased and/or dried are placed in a reactor,

-subjecting the reactor to an inert atmosphere, and

-contacting the skins with one or more extractor solvents by dosing the extractor solvent to the reactor continuously, intermittently or a combination thereof.

10. A process according to claim 1 or 2, wherein the one or more extractor solvents are separated after contact with the skins and subsequently recycled.

11. The method according to claim 10, comprising the steps of:

-removing the extraction solvent containing fat residues and/or water from the reactor and transferring the solvent to a first vessel,

-heating the solvent in a first vessel,

-transferring the gaseous phase resulting from the heating into a second vessel,

-condensing the gas phase in a second vessel, and

-feeding the solvent thus recovered to the reactor.

12. The method of claim 11, further comprising the steps of:

-taking the liquid phase from the first container and transferring it to a third container,

-distilling off the extraction solvent contained in the liquid phase,

-feeding the extraction solvent directly or via a fourth vessel to the second vessel.

13. The method of claim 11, further comprising the steps of:

-taking the liquid phase from the first container and transferring it to a third container,

-flashing off the extraction solvent contained in the liquid phase,

-feeding the extraction solvent directly or via a fourth vessel to the second vessel.

14. A process according to claim 1 or 2, wherein 0.5-40 litres of extractor solvent is used per kilogram of skins.

15. The method of claim 4, wherein the ester is methyl acetate, ethyl acetate, propyl acetate, the hydrocarbon is n-pentane, isopentane, cyclopentane, hexane, cyclohexane, heptane, petroleum solvent, petroleum ether, the glycol is 2-ethoxyethanol, 2-butoxyethanol, and the halogenated hydrocarbon is CHF₂CH₂CF₃、CF₃CHFCF₃、CF₃Br、CF₃CH₂F。