Process for deliming hides in the tanning industry and apparatus for carrying out said process

Abstract: The present invention relates to a process for the deliming of hides in the tanning industry and to an apparatus for carrying out said process. The process uses CO₂ as deliming agent and the flow-rate, in relation to a unit mass of hide to be treated, of the CO₂ which is introduced in the environment which contains the hides is increased at least once during the treatment time.

Description: The present invention relates to a process for deliming hides in the tanning industry and to an apparatus for carrying out said process. As is known, the technological cycle to which hides are subjected during tanning generally comprises a first step of desalination in which the dry hides are washed in water so as to remove the salt used for their preservation. The desalination step is followed by a soaking step in which the hides are treated with water and additives, such as for example surface-active substances, enzymes and sodium sulphide, so as to soften them and prepare them for subsequent treatments. The hides are then treated with water, sodium sulphide and slaked lime. This step, termed lime pit, relaxes the hides and depilates them, making them soft. After this step, the hides are subjected to fleshing to eliminate the fleshy and gelatinous subcutaneous parts. The fleshing step is followed by a deliming step in which the hides, after a first wash with water, are generally treated with ammonium sulphate in water to prepare them for the subsequent steeping treatment. The deliming step is performed in order to eliminate from the hides the residual lime which would prevent the successful outcome of the steeping step. The steeping step, which is again performed in a water bath with appropriate enzymes, has the effect of completely relaxing the hides and preparing them for the subsequent tanning step. The technological tanning cycle then continues with the steps of actual tanning (chrome or vegetable tanning) and of dyeing and drying of the hides. One or more of the described steps can be avoided for some types of hide. Some disadvantages are observed by analyzing the deliming step in greater detail. The ammonium sulphate, which performs an acidifying action by penetrating in the hides, has in fact the disadvantage of developing ammonia, with consequent problems of toxicity for personnel. The waste water which derives from this treatment furthermore has a high content of sulphates and ammonia and as such is highly polluting for the environment. Other types of deliming agents, such as dicarboxylic acids, formic acid, acetic acid, which do not produce the same harmful effects, have been proposed and used but have considerably higher costs. As an alternative to these deliming agents, the use of carbon dioxide to be injected into the bath in which the hides are immersed during this treatment, has been proposed. The use of carbon dioxide as deliming agent has evident advantages with respect to the use of conventional deliming agents; the specific cost of CO 2 is in fact lower than the cost of any other deliming agent, and taking furthermore into account the fact that the ecological impact of the waste water is low, its disposal is markedly less onerous. Though the use of carbon dioxide has the above described advantages with respect to conventional deliming agents, it has the disadvantage of requiring longer treatment times. On the other hand, the carrying out of deliming by introducing a constant and high flow-rate of CO 2 in the bath in which the hides are immersed does not allow to correspondingly reduce the treatment times. By introducing in the bath a CO 2 flow-rate higher than a certain limit which varies according to the type of hide and to the treatments already performed, an extremely rapid reduction of the pH is in fact caused, but a rapid acidification of the grain of the hide, i.e. of its surface portion, is also caused, and this entails a blocking effect against the penetration of CO 2 inside the hide. This blocking effect hinders the continuation of the process of neutralization of the lime which is present inside the hide. At the end of a deliming treatment performed in this manner, one would obtain satisfactory pH values but a deliming which is markedly insufficient for the successful outcome of the subsequent treatments. The aim of the present invention is to solve the above described problems by providing a process for deliming hides in the tanning industry which, by using CO 2 as deliming agent, allows to reduce the treatment time with respect to known processes which use CO 2 , thus also reducing the operating costs of the treatment facilities. Within the scope of this aim, an object of the invention is to provide a process which, at the end of the treatment, gives the hides a degree of acidity which is ideal for the successful outcome of the subsequent steeping treatment. Another object of the invention is to provide a process which allows to obtain a high yield in the use of the amount of CO 2 employed. A further object of the invention is to provide an apparatus for carrying out the process according to the invention which can be applied with modest investments to the production facilities which are currently in use in the tanning industry. This aim, these objects and others which will become apparent hereinafter are achieved by a process for deliming hides in the tanning industry, consisting of the use of CO 2 as deliming agent, wherein the flow-rate, in relation to a unit mass of hide to be treated, of the CO 2 introduced in the environment which contains the hides is increased at least once during the treatment time, as indicated in claim 1. The process according to the invention can be performed with a deliming apparatus which comprises a drum intended to contain a treatment bath and the hides to be treated, said drum being supported so as to be rotatable about its own axis, which is arranged substantially horizontally, and being actuatable with a rotary motion about said axis to mix said hides, characterized in that it comprises means for introducing CO 2 into said drum. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the process according to the invention, and of some embodiments of the apparatus for its execution, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figures 1 to 3 are schematic views of three embodiments of the apparatus for carrying out the process according to the invention; figures 4 and 5 are respective diagrams of the trend of the pH and of the amount of CO 2 introduced into the environment which contains the hides to be treated as a function of time in the process according to the invention. The deliming process according to the invention substantially consists in using CO 2 as deliming agent, similarly to known processes, but differently from said known processes the flow-rate, in relation to a unit mass of hide to be treated, of the CO 2 which is introduced into the environment which contains the hides to be treated is increased at least once during the treatment time. More particularly, the treatment preferably comprises a first step of introduction of a flow-rate of CO 2 which is substantially comprised, depending on the characteristics of the hides to be treated and on the previously undergone treatments, between 0.1 g/min per Kg of hides and 0.4 g/min per Kg of hides and a second step of introduction of a flow-rate of CO 2 which is substantially comprised, again depending on the hides to be treated and on the previously performed treatments, between 0.3 g/min per Kg of hides and 0.6 g/min per Kg of hides. The first step can have a duration comprised between 1/10 and 3/4 of the total duration of the treatment, which can vary from 30 minutes to 3 hours. The introduction of CO 2 preferably occurs by injection into a water-based bath in which the hides are immersed. Figures 4 and 5 plot, by way of example, the diagrams of the trend of the pH and of the amount of CO 2 as a function of the treatment time in the deliming of 2,500 kg of fatted calf hide with a thickness of approximately 4 mm performed according to the invention. In a first step of the treatment, a flow-rate of CO 2 which in this case is equal to 0.2 g CO 2 /min per Kg of hide is introduced in the bath which contains the hides. With this flow-rate of CO 2 , a rapid reduction of the pH is achieved though a degree of acidity such as to block deliming is not reached on the surface of the hides. After this first step, which lasts approximately 30 minutes in the example being considered, the flow-rate of CO 2 is increased to 0.3 g CO 2 /min per Kg of hides. Surprisingly, it has been observed that this increased flow-rate, which would have had the effect of blocking the deliming if it had been delivered in the first step, does not block the treatment and allows to complete it in approximately 1 hour and 45 minutes, obtaining a deliming which is qualitatively and quantitatively similar to that which can be obtained with conventional methods. As can be seen, the process according to the invention obtains a rapid reduction of the pH, but said reduction is not so rapid as to cause the blocking of the deliming. At the end of the treatment, depending on the type of hide, the pH has a value comprised between 6.5 and 8.5 to allow the successful outcome of the subsequent steeping treatment and not to damage the hide with excessively acid values. The pH value is preferably comprised between 7.0 and 7.3. In practice, for an equal final result, by means of the process according to the invention it is possible to process according to the invention it is possible to significantly reduce and in some case even halve the time required for known CO 2 deliming treatments. As illustrated in figure 1, the process according to the invention can be performed in drums 1 of a known type, which are supported so as to be rotatable about their own axis 1a which is arranged horizontally, and in which a water-based bath, with the hides 2 to be treated, is introduced. The desired amount of CO 2 is conveyed by a distributor 3 to two feed ducts 4a and 4b which are connected to ducts 5a and 5b which in turn pass through the bases of the drum at the axis 1a and feed delivery nozzles 6a and 6b which are immersed in the bath. A plurality of delivery nozzles 6a and 6b is preferably provided in the drum 1, said nozzles being distributed around the axis 1a of the drum and fixed to the walls of said drum between agitators 7 which are fixed in a known manner to the internal surface of the drum. In this case, each nozzle is fed by a duct 5a or 5b and said ducts 5a and 5b are in turn connected to the feed ducts 4a and 4b by means of rotary couplings 8a, 8b of a known type which provide a selective connection, feeding CO 2 only to the nozzles which, during the rotation of the drum, are in each instance immersed in the bath. The introduction of CO 2 can also occur above the free surface of the bath by adequately pressurizing the interior of the drums In this case the reaction between the CO 2 and the lime of the hides is performed by the continuous mixing obtained with the rotation of the drum about its own axis obtain a better result in the subsequent steeping treatment. A further advantage of the process according to the invention is that it can be performed, with easily executable modifications, in known deliming facilities. The introduction of CO 2 in the water of the bath can also be performed indirectly, as illustrated by way of example in figures 2 and 3, by adding CO 2 to a preset amount of the bath liquid drawn from the drum 1 and subsequently reintroduced in said drum with the aid of pumps 9 and 10. As illustrated in figure 2, the recirculation of the liquid of the bath can be performed with the aid of a tank 11 which collects the liquid which flows out of an appropriately perforated portion of the drum. A known CO 2 distributor 12 is arranged on the delivery duct of the pump 9 which leads into the drum. As illustrated in figure 3, the recirculation of the liquid of the bath can also be obtained by immersing the inlet of the suction duct of the pump 10 into the bath and arranging the outlet of its delivery duct inside the drum. Similarly to what is illustrated in figure 2, the CO 2 is introduced into the delivery duct of the pump 10 by means of a known distributor 13. The delivery and suction ducts of the pump 10 pass through openings which are defined on the bases of the drum at the axis 1a so as not to interfere with the rotation of the drum. In practice it has been observed that the process according to the invention fully achieves the intended aim, since together with the already known advantages which are intrinsic to the use of CO 2 as deliming agent it achieves a reduction in the treatment time and therefore a productivity increase in the deliming of hides. Another advantage which derives from the process according to the invention is that it is possible to perform the deliming with sufficiently high temperatures, so as to

Claims: Process for deliming hides in the tanning industry, using CO 2 as deliming agent, comprising: a first step of introduction of CO 2 in an environment containing hides to be treated, with a first flow rate substantially lower than 0.4 g/min for each Kg of hides to be treated ; and a second step of introduction of CO 2 with a second flow rate which is higher than said first flow rate, said second flow rate being substantially lower than 0.6 g/min for each kg of hides to be treated. Process according to claim 1, characterized in that said first flow-rate of CO 2 is substantially comprised between 0.1 and 0.4 g/min for each Kg of hides to be treated and said second flow-rate of CO 2 is substantially comprised between 0.3 and 0.6 g/min for each Kg of hides. Process according to claims 1 or 2, characterized in that said first step has a duration which is substantially comprised between 1/10 and the 3/4 of the total duration of the treatment. Process according to one or more of the preceding claims, characterized in that the hides to be treated are arranged in a bath and in that the pH of said bath, at the end of the treatment, is substantially comprised between 6.5 and 8.5. Process according to one or more of the preceding claims, characterized in that the pH of the bath at the end of the treatment is substantially comprised between 7.0 and 7.3. Process according to one or more of the preceding claims, characterized in that the overall duration of the treatment, depending on the type of hides, is substantially comprised between 30 minutes and 3 hours. Process according to one or more of the preceding claims, characterized in that the CO 2 is injected directly into the bath which contains the hides. Process according to one or more of the preceding claims, characterized in that the CO 2 is injected into a part of liquid drawn from said bath and subsequently reintroduced in said bath. Process according to one or more of the preceding claims, characterized in that the CO 2 is introduced in the environment which contains the hides above the free surface of the bath.

Inventors: BECCALLI MASSIMO, MEREGALLI EDOARDO, BECCALLI, MASSIMO, MEREGALLI, EDOARDO
Applicants: SOL S P A FORMERLY CALLED SOL, SOL S.P.A. (FORMERLY CALLED SOL INDUSTRIE S.P.A.)
Application year: 1991
Main classification: C14C1
IPC codes: C14C1, C14C15