WO2011158096A2

WO2011158096A2 - A method for treating hides

Info

Publication number: WO2011158096A2
Application number: PCT/IB2011/001337
Authority: WO
Inventors: Rinaldo Cogolo
Original assignee: Rinaldo Cogolo
Priority date: 2010-06-14
Filing date: 2011-06-08
Publication date: 2011-12-22
Also published as: WO2011158096A3; ITRE20100049A1; IT1404926B1

Abstract

A method for treating hides (100), comprising an unhairing stage in which the hides (100) are immersed for a period in an unhairing bath, and a subsequent liming stage in which the hides (100) are immersed for a period in a liming bath, the unhairing bath comprising at least a quantity of water, a quantity of at least a reducing agent and a quantity of at least a base substance, the liming bath being obtained by adding to the unhairing bath a quantity of liming liquor which has been recovered from a previous liming stage, and a further quantity of water.

Description

A METHOD FOR TREATING HIDES

Technical Field

This invention concerns a method for treating hides, typically bovine hides, in a tanning process.

Background Art

As is known, before hides are tanned, they undergo a soaking stage which is then followed by an unhairing and liming stage.

The aim of the soaking stage is to clean the hides, in order to remove the salt with which they have been preserved after skinning, and to restore to the hides the water which was lost during preservation.

In the unhairing and liming stage, the hides are treated with suitable chemical products to eliminate pelt, to instigate break-up of the epidermis, and to cause the dermis to swell, the aim being to relax the dermis in order to make it more reactive to all the subsequent stages of the tanning process.

Conventionally, unhairing and liming are performed simultaneously, by immersing the hides to be treated in a bath, known as the liming bath, which comprises water, reducing agents, typically sodium sulphide and sodium hydrosulphide, and a basic substance, typically calcium hydroxide.

This operation is performed inside a large rotating cylindrical vessel, known as a liming drum, which contains the liming bath and the hides to be treated, and which is periodically rotated on its axis in such a way that the hides are tumbled around, thus ensuring that the hides receive uniform treatment.

Liming baths based on sodium sulphide and calcium hydroxide are universally the most frequently used baths, because they are relatively inexpensive and provide optimal results as regards certain tactile and aesthetic qualities of the finished leather (surface handling, softness, a fine grain and the like), which determine the commercial value of the product.

However the disposal of used liming liquor constitutes a contraindication for this technology. In fact the presence of sodium sulphide and hydrosulphide makes it necessary for the liming liquor to undergo a series of preliminary treatments before the wastewater can be subjected to the usual biological purification processes, and this increases the overall cost of disposal.

Sodium sulphide and hydrosulphide can also release toxic gases in sewers and drains, especially if they mix with spent acid liquors from other tanning processes.

However, perhaps the most serious problem is that the sodium sulphide and hydrosulphide in the liming liquor are the main causes of the foul-smelling gas which is typical of the tannery sector (the characteristic smell of rotten eggs). Emissions of foul-smelling gas are released not only inside the tannery during the tanning process, but also and above all during the process of disposing of the liming liquor, particularly in purification plants which are installed in the tannery, or in consortium or public purification plants.

As a result, the bad odour often becomes a bone of contention between companies operating in the tanning industry and local communities and institutions, and also the main reason for the tanning industry's bad reputation among the public at large.

In the past, a number of strategies for recovering liming liquor have been put forward, with the aim of keeping processing and waste disposal costs under control and reducing the environmental impact and bad odour caused by the processing of hides.

These strategies usually include collecting the liming liquor when processing is complete, filtering out the sludge and hairs, and reusing the liquor for a subsequent unhairing and liming stage, after having topped up the quantities of sodium sulphide, sodium hydrosulphide and calcium hydroxide to the required level.

One of these strategies is described in Chinese patent CN 101319254, which in particular comprises performing Unhairing and liming of the hides in two successive stages, one distinct from the other.

The unhairing stage is performed by immersing the hides to be treated in an unhairing bath which contains a quantity of water which is 300% the weight of the hides to be treated, a quantity of soda of 0.8%-1.2% of the weight of the hides, and smaller quantities of other enzymes and additives.

This stage of unhairing is performed internally of a liming drum and over a very long time, from 18 to 24 hours.

Owing to the composition of the unhairing bath this depilation stage is only able to attack the bulb of the pelt, so that the pelt detaches from the hide, while remaining whole and in suspension in the unhairing bath.

On completion of the unhairing stage, the hides are dripped so that they become separated from the unhairing bath, which is completely discharged, possibly after having been filtered to collect the pelt.

The unhaired and dripped hides are then subjected to the liming stage.

The stage of liming is performed by immersing the unhaired and dripped hides in a liming liquor bath which comprises a quantity of 300% of the weight of the hides, the liming bath having been recovered from a previous liming stage, to which suitable quantities of liming agent and sodium-based compounds are added to return it to full strength.

In practice, each liming stage is always performed using the same liming bath, which is cyclically left to sediment, is filtered and reinforced with enzymes and other active substances.

Thanks to this recycling of the liming, this strategy enables reducing the quantities of pollutants overall released into the wastewaters, especially with regard to ammonium nitrates.

The adoption of these strategies however does not provide standardised results. Since the liming liquor is periodically reused and topped up to return it to the desired concentration of sodium and calcium sulphide, its content of salts, proteins, amino acids, ammonia and other substances increases, thus impeding accurate control over the process, and influencing the outcome of the unhairing and liming process.

In particular, it has been noted experimentally that strategies of this type enable performing an extremely small number of liming stages, after which the results from the liming will be so poor that the liming bath has to be definitively discharged and completely re-prepared, without in fact obtaining great benefits in terms of reduction of emissions.

A further disadvantage of the above-mentioned strategy is that the quantity of water used in the unhairing stage is extremely high, to the point that it does not enable disgregation of the epidermis and in general does not enable obtaining an effective swelling of the dermis structure. This strategy further causes a significant increase in production costs, both for the amounts of water needed and for the consequent costs for disposing of the water each time the unhairing bath is discharged.

An aim of the invention is to obviate the above-mentioned drawbacks in the prior art, within the ambit of a simple, rational and low-cost solution.

This aim is achieved by the characteristics of the invention disclosed in the independent claims. The dependent claims delineate preferred aspects and/or particularly advantageous aspects of the invention.

Disclosure of Invention

In particular the invention discloses a method for treating hides, comprising an unhairing stage in which the hides are immersed for a period in an unhairing bath, and a subsequent liming stage in which the hides are immersed for a period in a liming bath.

In this way, the new method introduces a clear separation between the unhairing process and the subsequent liming process, thus providing a process which ensures an optimal result which is constant over time.

In an important aspect of the invention, the unhairing bath comprises at least a quantity of water, a quantity of at least a reducing agent, and a quantity of at least a base substance, while the liming bath is obtained by adding a quantity of liming liquor which has been recovered from a previous liming process, and a further quantity of water, to the unhairing bath.

In general the word "water" is taken as meaning "technologically purified water", i.e. water containing only traces of any other chemical substances, for example, water taken directly from the mains supply or from other sources of supply.

In practice, in the strategy of the invention, for each treatment cycle (which includes subjecting a determined quantity of hides to both the unhairing and the subsequent liming stages), the liming bath is prepared as a mixture of the unhairing bath prepared for the same cycle and a liming liquor recuperated from a previous cycle.

This operating strategy is very different from the known methods which comprise completely discharging the unhairing bath and then using, for the liming bath, only a recuperated liming liquor, simply filtered and reinforced with additivies.

Differently to these known methods, by proceeding in this way, after several recycling operations and thanks to the addition of the water contained each time in the unhairing bath and the further quantity of water, the recuperated liming bath will reach a constant equilibrium of proteins and salts, thus standardising the unhairing and liming stages over long periods of time, while still permitting complete recovery of the liming liquor.

In this matter, it is observed that during the liming process the hides absorb a part of the liming liquor, so that the total quantity of liming liquor recovered is always less than the initial amount. Therefore, the recuperated liming liquor can effectively be re-used completely in a successive cycle, notwithstanding the fact that it is added to a new unhairing bath.

This recovery of liming liquor also means that the wastewater discharged by tanneries will not contain sodium sulphide or sodium hydrosulphide, except in negligible quantities, thus obviating or at least significantly reducing the serious problem posed by the foul-smelling gas emissions which are typical of the tannery sector.

The method of the invention also achieves water consumption savings, reduces energy costs, lowers the consumption of chemicals and cuts wastewater purification costs.

In particular, the method of the invention does not require discharging the unhairing baths, which are always reutilised in the lime bath, affording a significant saving in terms of water and elimination costs thereof, with respect to the known recycling methods. In an aspect of the invention, the water which is used to obtain the unhairing bath and the liming bath is preferably heated water, for example heated to a temperature of between 27°C and 28°C.

Using heated water improves the effectiveness of the unhairing and liming stages.

In another aspect of the invention, the reducing agent is preferably chosen from between sodium sulphide and sodium hydrosulphide.

The use of sodium sulphide and/or hydrosulphide is advantageous both from an economic viewpoint, since these substances are generally inexpensive, and from the viewpoint of the effectiveness of treatment, since these substances endow hides with good chemical and physical qualities.

The quantity of the reducing agent is preferably between 1% and 3% of the weight of the hides, for example between 1.4% and 2.5% or between 2% and 3% of the weight of the hides.

In this way, after several recycling operations, the appropriate concentration of reducing agent is obtained to make the unhairing and liming stages effective. In particular, thanks to this quantity of reducing agent in the unhairing bath, at each cycle an unhairing bath is obtained which enables complete destruction of the hide, not only in the sense of detaching it as happens in the method of the above-cited Chinese patent. In other words, the pelt practically dissolves and assumes the form of a protein paste which dissolves in the unhairing bath, thus guaranteeing a complete removal of the pelt even from the depth of the pores of the hide, and thus ensures a final product which is overall superior and finer. In another aspect of the invention, the base substance is preferably calcium hydroxide.

It is advisable to use calcium hydroxide because it is inexpensive and capable, in association with the above-mentioned reducing agents, of providing good end results.

The quantity of the basic substance is preferably between 1% and 3% of the weight of the hides, for example between 1.6% and 3%, or between 2% and 2.5% of the weight of the hides. In this way, after several recycling operations, the appropriate concentration of basic substance is obtained to achieve an effective unhairing and subsequent liming stage.

In particular, thanks to this quantity of base substance contained in the unhairing bath, the liming bath subsequently obtained starting from the unhairing bath will have the correct concentration of substance which enables completion of the liming in very short times and with excellent results.

In a further aspect of the invention, the unhairing bath can also comprise a first quantity of liming liquor recovered from a previous liming stage,

This initial quantity of recovered liming liquor, which is used to realise the unhairing bath, is preferably between 7% and 30% of the weight of the hides, for example between 7% and 10%, or between 20% and 30% of the weight of the hides.

In this case, the quantity of water which is used to constitute the unhairing bath is also preferably between 7% and 20% of the weight of the hides, for example between 7% and 10% of the weight of the hides.

These quantities of recovered liming liquor and water enable an effective unhairing bath to be obtained.

Further, since the quantities of water and of recovered liming liquor are substantially balanced, the hides swell less.

If the unhairing bath contains no recovered liming liquor, the quantity of water in the unhairing bath will preferably be between 15% and 30% of the weight of the hides, for example between 15% and 20% of the weight of the hides.

The quantities of water mentioned above have the advantage of obtaining an unhairing bath which enables completion of the stage of unhairing in a very modest time with respect to the prior art, such as for example the prior art of the above-mentioned Chinese patent.

Further, in synergy with the prescribed quantities of base substance and reducing agent, the above-cited quantities of water also enable subsequently obtaining a lime liquor having an optimal composition. In a preferred aspect of the invention, the period of time for which the hides remain in the unhairing bath is between 3 and 7 hours, for example between 3 and 5 hours.

In this way, the reducing agent can penetrate deeply into the full substance of the hides undergoing treatment, thus improving the quality of the treatment, thanks to which a good level of unhairing and cleaning of the grain can be achieved.

In another aspect of the invention, the quantity of recovered liming liquor which is subsequently added to the unhairing bath to obtain the liming bath is preferably between 60% and 100% of the weight of the hides, for example between 60% and 80% of the weight of the hides.

The further quantity of water which is added to the unhairing bath to obtain the liming bath, is preferably between 50% and 70% of the weight of the hides. These quantities of recovered liming liquor and of water which are added to the unhairing bath enable an effective liming bath to be obtained, notwithstanding the use of a significantly lower quantity of water with respect to the prior art, such as for example the method described in the above-cited Chinese patent. If necessary, the liming bath can also comprise a certain quantity of suitable enzymes, typically proteins, surfactants and degreasers, which are added to improve the maturation effect of the liming bath, ensuring smoothing of wrinkles in the leather and a well-cleaned grain, and completing the destruction and elimination of hair bulbs.

In another preferred aspect of the invention, the hides remain in the liming bath for between 8 and 15 hours, for example between 12 and 15 hours.

This ensures complete penetration of the liming bath into the full substance of the hides, thanks to which, at the end of the liming stage, the hides will be fully saturated by the bath liquor, well-cleaned and swollen to the desired extent. In an embodiment of the invention, the unhairing and liming stages are carried out with the hides kept inside a vessel, for example a common drum, in which the unhairing bath and the liming liquor are prepared. This solution means that the method can be implemented using drums and equipment which are already available in the tanning industry, thus reducing plant costs.

In an aspect of the invention, the unhairing and liming stages include tumbling the hides inside the unhairing bath and the liming bath, typically by rotating the drum in which they are contained.

Thanks to the tumbling action, the unhairing and the liming liquor penetrate better into the full substance of the hides, thus improving the treatment's effectiveness.

The invention can also be implemented in the form of a plant for treating hides comprising a vessel for containing the hides, for example a common drum, means for supplying water into the vessel, means for feeding a reducing agent into the vessel, means for feeding a base substance into the vessel, a holding tank for liming liquor, means for extracting the liming liquor from the vessel and supplying it to the holding tank, means for extracting the liming liquor from the holding tank and introducing the liming liquor into the vessel, and a plant control apparatus comprising a program for implementing a method comprising all the previously-described stages.

Brief description of the Drawings

Further characteristics and advantages of the invention will emerge from the description provided herein below as a non-limiting example, with the aid of the figures in the appended drawings.

Figure 1 is a schematic view of a plant for implementing a method of the invention.

Best Mode for Carrying Out the Invention

The plant 1 comprises a usual liming drum 10, which essentially consists of a large cylindrical vessel exhibiting a horizontal axis X and is designed to rotate around the horizontal axis X.

The drum 10 comprises an openable hatch 11 for introducing the hides 100 and the fluids which are destined to constitute the unhairing and liming baths.

The drum 10 further contains two truncoconical perforated screens 12, which are arranged at the edges between the cylindrical side wall and the oppositely- positioned base surfaces of the drum 10, in such a way as to divide the inner space into three distinct chambers: a central chamber 13 for containing the hides 100, and two oppositely-positioned ring-shaped chambers, 14 and 15, for draining off the process fluids.

Each drainage chamber, 14 and 15, is provided with a respective discharge mouth, 16 and 17, which is afforded in the side wall of the drum 10.

Each discharge mouth, 16 and 17, is closed by an openable cover, and is designed to be aligned with a respective discharge manifold, 18 and 19, which is installed in a fixed position under the drum 10.

The discharge manifolds, 18 and 19, are hydraulically connected to a single inclined discharge pipe 20, which is sub-divided, downstream from the discharge manifolds, 18 and 19, into a first branch 21 which leads into a holding tank 22, and a second branch 23 which leads directly to a drainage gully 24 of a usual disposal plant.

Both branches 21 and 23 are provided with respective flow adjustment valves, 25 and 26.

The plant 1 further comprises a holding tank 27 for the liming liquor, which a tower 28 supports at a higher level than the drum 10, and a submersible pump 29, which sucks up the liming liquor contained inside the drum 10 and conveys liming liquor into the holding tank 27 via a supply conduit 30.

The holding tank 27 is preferably coated with thermal insulating material which is designed to reduce heat dispersal from the recovered liming liquor.

The holding tank 27 can also be placed in communication with the drum 10 via a feed conduit 31 , via which feed conduit 31 the liming liquor, which has been recovered and is stored in the holding tank 27, can be re-introduced into the drum 10.

Lastly, the holding tank 27 is provided with an openable nozzle 32 for discharging the sludge which can settle on the bottom of the holding tank 27. In operation, the hides 100 to be treated are introduced into the drum 10 via the hatch 11.

Typically, the drum 10 can contain between 5 and 10 tonnes of hides. After the hides 100 have been introduced, a usual soaking process commences with the aim of cleaning the hides 100, in such a way as to remove the salt with which the hides 100 were preserved after skinning, and of restoring the water which the hides 100 lost while being preserved.

This soaking stage is implemented by introducing warm water at a temperature of around 24-26°C into the drum 10 and then rotating the drum 10.

The water is heated and introduced into the drum 10 via a suitable feed system which is not shown in figure 1 , since it is well known in the prior art.

At the end of the soaking stage, the drum 10 is halted in a position in which the discharge mouths, 16 and 17, are aligned with their respective discharge manifolds, 18 and 19.

While the perforated screens 12 retain the hides 100 inside the drum 10, the discharge mouths, 16 and 17, are opened to allow the soaking water to run off from the drainage chambers, 14 and 15, into the discharge pipe 20.

During this stage, the valve 25 is a closed position while the valve 26 is left open, thus the soaking water runs off exclusively into the second branch 23 towards the drainage gully 24.

When this stage is over, the discharge mouths, 16 and 17, are closed and the unhairing stage commences.

The unhairing stage is implemented by introducing a first quantity of technologically purified water, pre-heated to about 27-28°C, into the drum 10.

The quantity by weight of this water is about 7-20%, for example 7-10%, of the overall weight of the hides 100 which were initially introduced into the drum 10.

The water can be heated and introduced into the drum 10 using the same feed system mentioned herein above.

At the same time, an initial quantity of liming liquor is introduced into the drum

10, which liming liquor has been recovered from an earlier liming process, as will be clarified herein below.

The quantity by weight of this recovered liming liquor is approximately the same as the quantity of water, i.e. around 7-30%, for example 7-10%, of the overall weight of the hides 100 which were initially introduced into the drum 10. The recovered liming liquor is extracted from the holding tank 27 and introduced into the drum 10 via the feed conduit 31.

A quantity of sodium sulphide and a quantity of calcium hydroxide in powder are then added to the thus-obtained mixture of water and liming liquor, in such a way as to obtain the actual unhairing bath.

The quantity by weight of the calcium hydroxide in powder is approximately 1- 3%, or 1-1.6%, or 2-2.5% of the overall weight of the hides 100 which were initially introduced into the drum 10.

The quantity by weight of the sodium sulphide corresponds to approximately 1- 3%, for example 1.4-2.5%, or 2-3% of the overall weight of the hides 100 which were introduced into the drum 10, in such a way as to provide a bath with a high concentration of this reducing agent.

The quantity of sodium sulphide required to totally eliminate hair and epidermis, and to ensure the grain is well-cleaned, can however vary significantly according to the type of hides 100 being treated.

Note also that the unhairing bath could be obtained in an alternative way without the above-mentioned quantity of recovered liming liquor, and correspondingly increasing the quantity of water to an amount of between 15% and 30% if the weight of the hides, for example between 15% and 20% of the weight of the hides.

When the unhairing bath is ready, the drum 10 is set in rotation at a constant speed of about 2 revolutions per minute for around 30 minutes, in such a way as to distribute the calcium hydroxide uniformly in the unhairing bath. Then the drum 10 is made to rotate intermittently with five-minute rotation phases which are implemented once per hour.

After two hours in the unhairing bath, a good level of unhairing and cleaning of the grain of the hides 100 can already be seen.

It is however preferable to prolong the period of permanence in the unhairing bath to reach an overall time of between 3 and 7 hours, for example between 3 and 5 hours, according to the type of hides being treated.

In this way, the full substance of the hides 100 is completely penetrated by the unhairing bath, thus providing an optimal treatment. When the unhairing stage has been completed, the liming stage is implemented.

The liming stage is implemented by adding a further quantity of technologically purified water, previously heated to a temperature of around 27-28°C, to the unhairing bath in the drum 10.

The weight of this further quantity of water is around 50-70% of the overall weight of the hides 100 which were initially introduced into the drum 10.

The warm water is introduced via the same previously-used feed system.

When the further quantity of water has been introduced, the drum 0 is set in rotation at a constant speed of about 2 revolutions per minute for around 30 minutes.

At this point, a further quantity of liming liquor, which has been recovered from a previous liming stage, is added to the drum 10.

The weight of this further quantity of recovered liming liquor is the same as, or slightly greater than the quantity of added water, i.e. around 60-80% of the overall weight of the hides 100 which were introduced into the drum 10.

In this stage too, the recovered liming liquor is extracted from the holding tank

27 and introduced into the drum via the feed conduit 31.

After the introduction of this further quantity of recovered liming liquor, the drum is set in rotation again at a constant speed of about 2 revolutions per minute for around 30 minutes.

Then the drum 10 is made to rotate intermittently, with once-hourly rotation phases lasting five minutes for the next 8-15 hours, for example 12-15 hours. This completes the liming stage: the hides 100 are well-cleaned, have swollen to the extent desired, reaching the fullness desired and have been completely soaked, throughout their thickness, by the liming liquor.

By appropriately varying the percentages by weight of the further quantities of water and of recovered liming liquor, it is obviously possible to obtain different degrees of swelling and fullness of the treated hides.

During the liming stage, the hides absorb a significant quantity of liming liquor, such that at the end of thie stage the residual quantity of liming liquor is less than the original quantity. After the liming stage, the drum 10 is halted in a position in which the discharge mouths, 16 and 17, are aligned with the respective discharge manifolds, 18 and 19.

The drum is then kept stationary for a certain rest period, thanks to which the pelt hair and the sludge produced by the chemical reactions during the unhairing and liming stages are left to decant on the bottom of the drum 10. Subsequently the submersible pump 29 is lowered into the drum 10 and suspended at a level which is higher than the level of the decanted sludge, in such a way as to be in a position to suck up only the liquid phase of the used liming liquor.

The submersible pump 29 is then activated to pump the liming liquor into the holding tank 27, in order for the liming liquor to be reusable for a subsequent unhairing and liming stage, as described herein above.

In this way, the liming liquor is always recovered and is not conveyed to the purification plant, thus reducing the tannery's pollution load by up to 60-70%, and drastically reducing the emissions of foul-smelling gas caused by the disposal of liquor containing sodium sulphide.

As the quantity of liming liquor recovered at the end of each cycle is lower than the initial quantity (due to the absorption of the hides), the liming liquor can be entirely reutilised in the following cycle, being added to a new unhairing bath. When the liming liquor recovery is complete, the discharge mouths, 16 and 17, of the drum 10 are opened, in such a way as to allow the sludge to flow out of the drainage chambers, 14 and 15, into the discharge pipe 20, while the hides 100 are retained inside the drum 10 by the perforated screens 12.

In this stage, the valve 26 is in a closed position while the valve 25 is left open, thus the sludge flows exclusively through the first branch 21 towards the holding tank 22.

The sludge can then be sent from the holding tank 22 to a filter press (not illustrated), to form cakes which are then incinerated.

The treated hides 100 are then washed with water, and removed from the drum 10, in order to be subjected to subsequent stages of the tanning process. In an aspect of the invention, the plant 1 can be provided with a control apparatus comprising an electronic processor unit, a memory unit connected to the processor unit, and software stored in the processor's memory unit, in such a way that when the processor unit launches the program, the stages of unhairing and liming previously described are performed automatically.

Note that the operation of the plant 1 has been described herein above as operating in a normal interim cycle. Naturally, when the unhairing and liming stages are performed for the first time, recovered liming liquor is not yet available, therefore the unhairing and liming baths will be carried out using only water, sodium sulphide and calcium hydroxide, and possibly enzymes, surfactants and degreasers, in quantities which are appropriately greater than those described herein above.

Obviously a person skilled in the art could introduce numerous technical and applicational modifications to the plant 1 and the relative method of operation, without thereby forsaking the ambit of the invention as claimed herein below.

Claims

1 ) . A method for treating hides (100), characterised in that it comprises an unhairing stage in which the hides (100) are immersed for a period in an unhairing bath, and a subsequent liming stage in which the hides (100) are immersed for a period in a liming bath, the unhairing bath comprising at least a quantity of water, a quantity of at least a reducing agent and a quantity of at least a base substance, the liming bath being obtained by adding to the unhairing bath a quantity of liming liquor which has been recovered from a previous liming stage and a further quantity of water.

2) . The method of claim 1 , characterised in that the water which is used to obtain the unhairing bath and the liming bath is heated.

3) . The method of claim 1 , characterised in that the reducing agent is chosen from between sodium sulphide and sodium hydrosulphide.

4) . The method of claim 1 , characterised in that the quantity of the reducing agent is between 1 % and 3% of the weight of the hides (100).

5) . The method of claim 1 , characterised in that the base substance is calcium hydroxide.

6) . The method of claim 1 , characterised in that the quantity of the base substance is between 1% and 3% of the weight of the hides (100).

7) . The method of claim 1 , characterised in that the quantity of water in the unhairing bath is between 15% and 30% of the weight of the hides (100).

8) . The method of claim 1 , characterised in that the unhairing bath further comprises an initial quantity of liming liquor which has been recovered from a previous liming stage.

9) . The method of claim 8, characterised in that the initial quantity of recovered liming liquor is between 7% and 30% of the weight of the hides (100).

10) . The method of claim 8, characterised in that the quantity of water in the unhairing bath is between 7% and 20% of the weight of the hides (100). 11 ) . The method of claim 1 , characterised in that the time the hides (100) remain in the unhairing bath is between 3 and 7 hours.

12) . The method of claim 1 , characterised in that the quantity of recovered liming liquor which is added to obtain the liming bath is between 60% and 100% of the weight of the hides (100).

13) . The method of claim 1 , characterised in that the further quantity of water which is added to obtain the liming bath is between 50% and 70% of the weight of hides (100).

14) . The method of claim 1 , characterised in that the period of time for which the hides (100) remain in the liming bath is between 8 and 15 hours

15) . The method of claim 1 , characterised in that the unhairing and liming stages are implemented while keeping the hides (100) inside a vessel (10) in which the unhairing bath and the liming bath are prepared.

16) . The method of claim 1 , characterised in that the unhairing and liming stages include tumbling the hides (100) in the unhairing bath and the liming bath

17) . A plant (1 ) for treating hides (100), comprising a vessel (10) for containing hides (100), means for supplying water into the vessel (10), means for feeding a reducing agent into the vessel (10), means for feeding a base substance into the vessel (10), a holding tank (27) for liming liquor, means (29, 30) for extracting the liming liquor from the vessel (10) and feeding the liming liquor into the holding tank (27), means (31 ) for extracting the liming liquor from the holding tank (27) and introducing the liming liquor into the vessel (10), and a plant control apparatus (1 ) comprising a program for implementing the method of any of the preceding claims.