PROCESS FOR THE TREATMENT OF HIDES

Abstract: The disclosure relates to a method of treating hide for leather manufacture, the method comprising the steps of treating softened hide with lime, deliming the hide by treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning. In the method according to the present invention, use is made of a gas containing carbon dioxide as the lime-dissolving substance. The gas is added to the treatment bath continuously or at intervals in that it is dissolved in the treatment bath substantially throughout the entire deliming process. A further characterizing feature of the present invention is that it is possible to carry out the bating process in the treatment bath for deliming without removing ions dissolved therein.

Description: TITLE OF INVENTION: P EOCESS FOR THE TREATMENT OF HIDES. TECHNICAL FIELD The present invention relates to a method in the treatment of hides for leather manufacture, comprising the treatment of softened hides or skins with lime, deiiming of the hides by 5 treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning. BACKGROUND ART The corium of the hide is that part of the hide which is 10 processed into leather in the tanning operation. The epidermis, hair and the fatty tissue on the hypodermis, the hide layer located most proximal to the carcass, must be removed by special treatment processes. At the same time, the corium is rendered more suitable as a material for leather, certain undesirable components 15 are removed and the fibre tissue of the hide is modified in several respects in order to render it better suited for the subsequent tanning process. This preparatory work is essentially carried out by means of a process called "liming". The vast majority of hides and skins enter the tannery in the 0 wet-salted or "cured" state. After sorting, splitting, weighing and stamping of the hides, these are collected into large batches. The first operation is to free the hide material from preservatives, i.e. salt, impurities, blood and certain aqueous proteins by treatment in water. In the so-called softening 5 process, salt, interfibrillary water-soluble albumins and globulines soluble in brine are dissolved out, as well as dirt, other impurities and blood. Since, in the original curing process, the hide has lost a considerable amount of its original water content, the primary purpose of the softening process is to 0 restore the hide to its normal water content. After this conditioning of the hide, it is subjected to the liming treatment. In this operation, between 3 and 10 per cent of calcium hydroxide (calculated on the weight of the hide) is employed, being introduced into the water with the hides. To facilitate the removal of hair, up to 3 per cent of disodium sulphide (calculated on the weight of the hide) is also added, or other applicable compound possessing a depilatory effect. The limed hide is strongly alkaline and swollen. The generally employed tanning agents - vegetable tanning agents and chrome salts - react acidically, are sensitive to alkali and form compounds insoluble with lime. Direct treatment of the strongly alkaline hides with tanning agents will result in a leather of poor quality and, moreover, the considerable swelling of the hide in the initial stages of the tanning process is not desirable. The limed hides must therefore be freed of their alkalinity and their swelling. By neutralization of the alkalinity, the calcium hydroxide present in the hide (which is but sparingly soluble) will normally be removed only from the outer regions of the hide. Insufficient deiiming will result in stains on the finished leather which may derive from the deposition of calcium carbonate (by reaction with carbon dioxide in the air or because of hard water), calcium sulphate (sulphuric acid from the chrome-tanning), calcium tannate (tannic acid from vegetable tanning agents). By flushing with water, substances absorbed by capillary action and absorbed lime can be, removed. Even after thorough flushing with water for several hours, about 70 per cent at most of the lime can be removed from the hide. Consequently, treatment with l me-dissolving substances is necessary. In the deiiming of the limed hides, the pH should be reduced at most from a level of between 12 and 14, usually around 12, to a level of around 8. This lowering of the alkalinity (or increase of the acidity) must be achieved with extreme care, since the outermost regions of the hide may swell even in the briefest presence of surplus acid in the aqueous treatment liquid. Acid- swollen collagen is particularly sensitive, which may cause cracks to occur in the finished leather. For this reason, no powerfully dissociating acids are employed, but use is made instead of weak organic acids, for example aliphatic or aromatic dicarboxylic acids, or weakly acidic salts, for example ammonium sulphate, ammonium chloride or polyphosphates. Of particular advantage in this context are acidic agents with a great capacity to dissolve lime, for example sulphophthalic acid or complexing agents. Ready-to-use mixtures are also available on the market for deiiming, these compounds containing, for example, a mixture of citric acid and formic acid. The deiiming process is normally terminated by bating, i.e. the action of proteolytic enzymes on the basis of pancreal or bacterial proteases. Their effect has not been clearly established. In all probability, they promote by a fibre-loosening action uniform and deep penetration of deiiming agents and the dissolution of lime. These enzymes are active within a pH range of between 6.5 to 9, the most active range being between 7 and 8.5. Bating preparations therefore usually contain ammonium sulphate and pH buffer substances. Between the deiiming stage with aci ' d to essentially neutral pH and the bating stage, the hides are normally washed up to 3-4 times to remove calcium compounds and other substances which, during later treatment stages, may disturb the process and/or be deposited on the surface of the hide. The deiiming process is normally carried out in large rotating drums with a horizontal axis of rotation, in which drums there may be introduced, for example, 3000 litres of water and 3000 kg of limed hides. In addition, deiiming agents are added, these normally being in liquid or solid form, for example the commercially available mixture of formic acid and citric acid. A common proportion of acid is 1 per cent of the dry weight of the hides, i.e. 30 kg of acid in the present case. The amount of acid employed is also adapted to the thickness of the hides, thick hides being more difficult to delime and hence requiring a larger amount of acid. This amount of acid is added in a single operation and as a sole batch on initiation of the deiiming process. This large amount of acid may lead to locally high acid concentrations, with the aforesaid acidic swelling, and thus poorer leather quality, as a consequence. Typical neutralization curves for the deiiming process are reproduced in Fig. 4, Fig. 4a showing the pH as a function of the time for continuous, constant addition of 1.5 per cent carbon dioxide during 30 minutes and Fig. 4b showing the pH as a function of the time for addition of a corresponding amount of pulverous, weak acid according to the technique used today, where the entire amount of acid is added in one lot at zero time. Moreover, the treatment liquid is of optimum acidity in the initial phase of the deiiming process and the amount of acid decreases in proportion to the dissolution of lime from the hide. It is difficult to achieve continuous supply of the acid in these horizontally rotating drums. OBJECTS OF THE PRESENT INVENTION One object of the present invention is to realize a method of rendering the deiiming process more effective, so as to obtain improved leather quality. A further object of the present invention is to substitute the relatively expensive prior-art dei ming agents by a cheaper agent. Yet a further object of the present invention is to real ze a method in which it is possible to execute the bating process concurrently with or directly after deiiming, with no intermediate washing stage. Another further object of the present invention is to enable deiiming and bating to be carried out in such a manner that smaller volumes of liquid are used, which liquids have to be cleaned before being discharged to a recipient. Yet another object of the present invention is to carry out the deiiming with more environmentally acceptable chemicals than have hitherto been used. SUMMARY OF THE INVENTION These objects will be attained according to the present invention by a method in the treatment of hides for leather manufacture, the method comprising the treatment of softened hide with lime and possibly with disodium sulphide, deiiming of the hide by treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning, the method being characterized in that as lime- dissolving substance a gas containing carbon dioxide is dissolved in the aqueous treatment bath until the pH of the treatment bath has been reduced to a predetermined value which lies in a range suitable for bating, the carbon dioxide being the lime-dissolving substance. Preferred embodiments of the method according to the present invention are disclosed in the dependent claims. According to the present invention the hides are softened and lime-treated in the usual manner. Thereafter, the hides are freed of the existing layer of fat on the meat side and depilated. In many cases, thick or heavy hides are skived parallel with the grain layer of the hide. The upper skiver is the more valuable, as it contains the grain layer that is characteristic for different leathers. The hide freed of hair and fat is called a bare hide. Bare hides and water are placed in the treatment vessel, which is usually a rotating drum with a horizontal axis of rotation. The bare hides may possibly be washed with this water, whereafter the vessel is refilled with fresh water. According to the present -state of the art the bare hides are washed with water 3-4 times before the deiiming acid is added. This i-s not required according to the present method, according to which washing is normally carried out not more than once. When the bare hides and treatment liquid, i.e. water, have been introduced into the treatment vessel the latter is caused to rotate and gas containing carbon dioxide or preferably pure carbon dioxide is introduced into the vessel. The addition of the gas containing carbon dioxide can take place directly to the liquid or to the gas phase existing above the liquid. The latter method gives a less effective solution of carbon dioxide in the treatment liquid but is suitable for supplying to existing equipment. The pressure of the gas phase existing above the liquid can be approximately atmospheric pressure, i.e. ambient pressure, or a positive pressure. Normally the treatment vessels are designed so that they do not permit or allow establishment of a positive pressure in the treatment vessel, which pressure differs essentially from the ambient pressure. The temperature of the treatment liquid is regulated in a known manner within the wanted range. Preferably the amount of added carbon dioxide is such that the treatment liquid is substantially saturated, at least during the initial phase of the deiiming and as far as the concluding phase. The carbon dioxide supply can be continuous or intermittent. According to one embodiment the pH of the treatment liquid is lowered to 8.3 or some other lower value at which the bating agent has great activity, whereafter the bating agent is added. The supply of carbon dioxide can then temporarily be discontinued. The pH of the treatment liquid will then rise slightly. Thereafter carbon dioxide can again be supplied. Alternatively, the supply of carbon dioxide can proceed without interruption also during the addition of the bating agent. According to another embodiment the supply of carbon dioxide can continue until a lower pH has been attained in the- treatment bath and the bating agent can then be added to this without any intermediate ' washing stage. Normally, when a pH of approximately 6.8 has been attained in the treatment liquid a relatively stable level will be attained which is attributable to the buffer effect of the carbonic acid at this value. For further lowering of the pH value large amounts of carbon dioxide are required. Especially in the deiiming of thick, non-skived hides, deiiming can be carried out in two stages, in which case the first stage involves deiiming with carbon dioxide and the second stage deiiming with a conventional organic acid or an ammonium salt. This procedure true enough involves the use of less environmentally acceptable chemicals than carbon dioxide, but in recompense gives faster deiiming than with only carbon dioxide. A major benefit is however that the total used amount of conventional deiiming agent is significantly lower in implementation of occurring two-stage procedures. When a conventional deiiming agent is used it is nevertheless not possible to supply the deiiming agent directly to the treatment liquid and the bare hides have to be washed before the deiiming agent is added. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The nature of the present invention and its aspects will be more readily understood from the following brief description of the accompanying drawings, and discussion relating thereto. In the accompanying drawings: Fig. 1 shows the change in pH as a function of time on the addition of carbon dioxide; Fig. 2 shows the amount of carbon dioxide as a function of time in the same experiment as illustrated in Fig. 1; Fig. 3 shows the quotient of the amount of carbon dioxide and the amount of hide in per cent as a function of time in the same experiment as illustrated in Figs. 1 and 2; and Figs. 4a and 4b show a comparison between the pH of the treatment liquid as a function of time for treatment with carbon dioxide according to the present invention and in a conventional manner. EXAMPLE 1 A deiiming experiment was carried out in a rotary, laboratory-scale drum. Four lime-treated bovine hides of a weight of approx. 50 kg and roughly the same amount of water were placed in the drum. Carbon dioxide was supplied to a water circulation pipe and dissolved in the water of the water circulation pipe by an injector disposed in the circulation conduit. The volume of carbon dioxide was approximately 8 litres per minute, calculated at atmospheric pressure. The temperature of the treatment bath was approximately 25 °C. At time 0, the pH of the treatment bath was 12 and, there- after, carbon dioxide was continually supplied at the above- disclosed volume per minute. After 10 minutes, the pH of the treatment bath had fallen to 8.4. After 50 minutes, the supply of carbon dioxide was shut off. At that point, the pH in the treatment bath was 7.3. The surface of the hide was now clean, but the inner regions of the hide required further treatment. After 85 minutes from the beginning of the experiment, the supply of carbon dioxide was restarted. Immediately prior to the commencement of the gas supply, the pH of the treatment bath was 8.2. Five minutes later, i.e. 90 minutes after commencement of the experiment, the supply of carbon dioxide was terminated. At that point, the pH of the bath was 7.5. The hide was now lime-free and the treatment was discontinued. The total carbon dioxide consumption was between 0.5 and 1 kg, i.e. between 1 and 2 weight per cent of the weight of the hide. The pressure in the rotary drum was equal to or insignif cantly higher than the ambient atmospheric pressure. The hides were then subjected to bating, i.e. enzjuiatic treatment, without any intermediate washing of the hides in the deiiming bath. Thereafter, the hides were chrome-tanned and dyed in the conventional manner. The leather thus obtained was of superior quality compared to leather obtained in a conventional process -. EXAMPLE 2 In a factory-scale experiment with 3 000 kg of hides and 3000 litres of water, carbon dioxide was continually supplied for 180 minutes. During this experiment, such an amount of carbon dioxide was supplied to the treatment bath that the bath was saturated with respect to this gas throughout the entire length of the experiment. The pressure in the rotary drum was substantially equal to atmospheric. The amount of supplied carbon dioxide in kg as a function of time in minutes is apparent from Fig. 2. The pH •- in the treatment bath was measured continuously for 210 minutes, as will be apparent from Fig. 1. After 180 minutes of carbon dioxide supply, the pH had fallen to a level of approximately 7.1. Thereafter, the pH rose, for 30 minutes without carbon dioxide supply, to approximately 7.4. The carbon dioxide supply per kg of hide as a function of time is apparent from Fig. 3. In this experiment, the carbon dioxide consumption was also between 1 and 2 per cent of the dry weight of the hides. After bating in the treatment liquid resulting from the deiiming process and without any intermediate washing, pickling and chrome-tanning were carried out. The thus obtained leather was assessed to be of superior quality as compared with the leather from the same initial raw materials which had been delimed in a conventional manner. It has also been found that after deiiming with carbon dioxide more uniform dye distribution is obtained upon dyeing, which is ascribable to a cleaner leather surface. The surprising feature of the present invention is that carbon dioxide is employed for deiiming. Carbon dioxide has been viewed by persons skilled in this art as a chemical which must be avoided under any circumstances in the manufacture of leather, since calcium and carbonic acid form sparingly ' soluble calcium carbonate which leads to stains on the surface of the leather. For example it is evident from Fritz Stather, Gerbereichemie und Gerbereitechnologie (tanning chemistry and tanning technology), Akademie-Verlag, Berlin, 1951, second edition, page 153 "Lime stains" that: "Such stains arise when the hides, with their high lime content, come into prolonged contact with the carbon dioxide in the atmosphere owing to insufficient immersion in the lime liquid or by being left lying around in contravention of good practice, or when they are rinsed in water with a high content of carbon dioxide or in water having a relatively high level of bicarbonate hardness." In the same reference on page 584 under b) "Lime stains", it is stated that: "In similar fashion, lime stains may appear when the process water used for rinsing delimed hides contains excessive quantities of free carbon dioxide or excessive carbonate hardness." On pages 110 and 111 of a book published as recently as in 1987 in the series Bibliothek des Leders (the leather library), band 5 (volume 5), Das Farben von Leder (the dyeing of leather), Umschau Verlag, Frankfurt am Main, under subheading 2.1 Das Betriebswasser (the process water) it is stated that: "In the operation of liming, the presence of calcium and magnesium bicarbonates, especially in the rinsing and/or washing, can cause lime 'shadows' that lead to light-coloured stains and to a dyeing result that is marred by cloudiness." It is evident from Table 20 on page 111 that for carbon dioxide the limit value for good water quality is zero. A further wholly unexpected outcome was that it is possible to dispense with the previously obligatory washing in 3-4 stages after the dissolution of lime from the limed hide and before the bating process. According to a hypothesis which remains to be confirmed, it is believed that, according to the present invention, there are formed calcium complexes which are stable during the continued treatment, and, in this instance, calcium ions do not interfere with the treatment chemicals in later stages. The carbon dioxide supply to the treatment bath for deiiming is preferably regulated such that the treatment bath is saturated in respect of carbon dioxide. It is not necessary to supply a gas comprising solely carbon dioxide. Instead, carbon dioxide may be supplied together with other gases which do not have any disturbing effect under the deiiming process proper. In such an event, the volume of gas supplied must be increased so that the volume of carbon dioxide per se will be at the contemplated level . According to the present invention, the supply of carbon dioxide and the time between discontinuation of the gas supply and the bating process is controlled such that the bating is initiated when the pH level in the treatment bath between approximately 7.5 and 8, since, at 25 °C, the enzymes which are used in the bating -process are at their most effective at pH levels of between 7.5 and 8.0. The lower value is to be preferred. It is also evident from the preceding that in respect of the time for deiiming and bating it is advantageous that the bating take place concurrently with further addition of carbon dioxide when the pH of the bath has fallen to around 8.3 or lower. An essential advantage of the present method is that hide delimed with carbon dioxide can be chrome-tanned with up to 20 per cent lower consumption of chromeous chemicals in comparison with hide delimed in the conventional manner.

Claims: WHAT WE CLAIM AND DESIRE TO SECURE BY LETTERS PATENT IS: 1. In the treatment of hides for leather manufacture, a method comprising the treatment of softened hide with lime and possibly with disodium sulphide, deiiming of the hide by treatment in an aqueous treatment bath under agitation and with the addition of a lime-dissolving substance, bating, pickling and tanning, characterized in that as lime-dissolving substance a gas containing carbon dioxide is dissolved in the aqueous treatment bath until such time as the pH of the hide and the treatment bath has been reduced to a predetermined level which lies in a suitable range for bating, the carbon dioxide being the lime-dissolving substance. 2. The method as recited in Claim 1, characterized in that the predetermined pH is the same as or lower than 8.3. 3. The method as recited in Claim 1 or 2, characterized in that the aqueous treatment bath is kept substantially saturated with carbon dioxide until such time as a pH level in the treatment bath has been attained which is at most 1.5 pH units, specially at most 0.5 pH units, lower than the predetermined level. 4. The method as recited in one or more of Claims 1-3, characterized in that the supply of carbon dioxide is discontinued when the pH of the treatment bath is 7.5 or lower. 5. The method as recited in one or more of Claims 1-4, characterized in that the bating agent is supplied when the pH level of the treatment bath has fallen to a second predetermined pH level, which lies within the range suitable for bating. 6. The method as recited in Claim 5, characterized in that after completion of bating either a strong mineral acid intended for pickling is added to the treatment bath or else the treatment liquid is replaced with water and the strong mineral acid is added to this water. 7. The method as recited in Claim 1, characterized in that in the deiiming of thick, non-skived hides, the treatment liquid is kept substantially saturated with carbon dioxide until such time as the pH of the treatment liquid has fallen to a third pH level that is lower than 8.3, and in that a different weak acid than carbonic acid is subsequently added. 8. The method as recited in Claim 7, characterized in that the weak acid is an acid which is normally used in deiiming, e.g. lactic acid, boric acid, acetic acid, citric acid, or an ammonium salt. 9. The method as recited in one or more, of Claims 1-8, in which after liming of the softened hides fat and hair are removed and skiving possibly performed, characterized in that the hides are washed not more than once before the gas-containing carbon dioxide is supplied. 10. The method as recited in one or more of Claims 1-9, characterized in that the deiiming is carried out at substantially atmospheric pressure. 11. The method as recited in one or more of Claims 1-9, characterized in that a positive pressure is maintained of the gas containing carbon dioxide in the treatment vessel.

Inventors: TUOHIMAA TIMO, TUOHIMAA, TIMO
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1988
Main classification: C14C1
IPC codes: C14C1