Elimination of salt from pickling28 March 2004
Abstract The possibility of carrying out the pickling process in the absence of sodium chloride has been investigated. The results show that this is possible to achieve. In fact, the resulting swelling is almost completely suppressed in the following phase of chrome tanning. The leathers, after retanning, dyeing, fatliquoring and staking, present chemical, physical and organoleptic properties which are as good as standard pickled pelts. An improved alternative of the studied process is also shown together with the acids with small amounts of chromium salts. In this case, swelling was reduced and the samples were perfectly comparable with the control treated with sodium chloride. The resultant leathers showed chemical, physical and organoleptic characteristics equal to those of the standard. The results were confirmed by semi-industrial trials under tannery conditions. Ongoing investigations at SSIPMC are looking into applying the salt free method to hide and skin preservation. Introduction The elimination of sodium chloride from wastewaters represents one of the most difficult problems which the tanner has to resolve. The extreme solubility of common salt and its great inactivity to precipitation could ultimately compromise the tanning industry. Unfortunately, actual technology and knowledge do not allow us to avoid its use because of the irreversible damages which would be produced on the hides during pickling. Acid treatment promotes the deactivation of negative charges on the carboxylic groups of the collagen side chains. As a result, it unbalances the equilibrium in favour of the positive charges of the side-aminic sites. Repulsive forces are then produced within the structure which keep the polypeptide chains at a distance from one another. Therefore, spaces are generated where the water quickly penetrates, producing the swelling. Under these conditions, osmotic pressures, up to 400 atms, are developed, which can cause the complete breaking of the bonds among the protofibrils (elevated state of swelling) leading to a serious destabilisation of the structure1. The presence of sodium chloride with the acid avoids the development of this destructive phenomenon. The addition in the bath of sodium chloride increases the ionic concentration of the external solution by increasing the osmotic tension. Such an increase makes the external solution hypertonic along with the water contained inside the fibre structure. A transfer of water, from the inside towards the outside, is caused with evident dilution. The dilution produces a great dissociation of the acid and the salt, whose ions will be forced to separate through the interfibrillar spaces to the aminic and carboxylic groups of the side chains. The process is completed when the concentration inside and outside the fibrils results in perfect equilibrium. When the hides are adequately dehydrated, the carboxylic functions are completely uncharged and the aminic sites will be present in the form of salts. The hide will be ready for mineral tanning. If there is a considerable increase in the ionic concentration of the external solution, the same result can be reached by the employment of products different from sodium chloride. It is known that a notable increase of the acid concentration in the pickling reduces the swelling until it corresponds to the isoelectric point. Water has a substantial role in the swelling. In fact, the water flow, or its outflow, between the inside and the outside of the hide may or may not produce this phenomenon. Water controls the ionic concentration and, therefore, the employment of sufficient water quantities in the pickling is significant. These discussions leave out the role developed by the so-called 'non-swelling' acids. Many have an aromatic structure, which produces bipolar forms, useful for a stabilisation of the structure by crosslinking. Such links increase the effect of the repulsive strengths between the chains and so repress the swelling2,3,4 in the same way as aldehydes act5. The final study explores the employment of a mixture of non swelling acids together with reduced amounts of sodium chloride (solution at 2Bé for sodium chloride) and formate, or a strongly complexed chrome salt and formate mixture. At the end of pickling, the addition of chrome sulfate at 33% Bs Schorlemmer6 is added. This work is similar to research performed at SSIPMC some years ago in order to eliminate the deliming and pickling steps7,8. In this investigation, the effect produced on the swelling of the hides in the pickling process was studied. The acids used (sulfuric and formic acid) in this process were either in the presence of varying quantities of water or small amounts (2%) of 33% basic chromium sulfate. In the latter studies, the high acidity of the bath and the presence of the sulfuric and formic acids during the complexing action, prevent the Cr (III) from forming crosslinks within the collagen structure and, therefore, the salt operates as a deswelling agent. Such a function is created by the peculiar activity of the sulfate ions, which show some tendency to generate crosslinks between the protofibrillar units, making them stable. At the end of pickling, the remaining amount of chromium sulfate is added and the tanning process completed. In acidic conditions, chrome tanning is performed. In both cases the level of swelling was studied after tanning. The observations and results obtained are described as follows. Experimental As mentioned, two experimental possibilities have been considered. The first had the aim of varying the quantity of water added to the bath in order to study the swelling variations as a function of the ionic concentration. The second investigated improvements that could be obtained by adding small quantities of chromium salt to provide a deswelling and not a fixing action. In both cases, after studying the swelling action during pickling, followed by chrome tanning in the same bath as pickling, the sides were retanned, dyed, fatliquored and finished. On the final leathers, chemical, physical and organoleptic characteristics were evaluated compared with the control. Post tanning operations were only performed on samples which showed organoleptic properties which were more or less satisfactory with commercial leathers. The method of pickling that gave the best results was transferred to a semi-industrial scale at a tannery in Naples. Materials Hides: For all trials, bovine hides in the limed state were used. Chemicals: All the chemicals used in this study are commercially available. The chrome salt used was chrome sulfate at Bs 33% Sch. Swelling variation study The limed hides were delimed and then pickled using diluted sulfuric and formic acid. The pickling operation with and without chrome was carried out using a typical overnight process with further additions of acid in order to achieve a hide pH of around 3. Pickling in the presence of increasing amounts of water The quantity and type of acids were the same. The amounts of water varied in the two initial phases of addition, shown as follows: At each step, the sample weight, a measurement of hide swelling, the pH of the solution, a cross-section of the hide and the hide appearance were evaluated. The results were compared with the control process, which were 80% water and 6% sodium chloride, with the same acid additions. Following pickling, chrome tanning was carried out by addition of the chrome salt to the pickle bath. The final appearance and the swelling of the leathers was evaluated and compared with the control. The wastewaters were analysed for the presence of nitrogen. The method that used the highest quantity of water, strangely, resulted as giving the best results as it produced a wet-blue material similar to the control. In this case, the distribution of the chrome inside the cross-section of the hide was analysed. The chromed tanned leathers were then shaved, retanned, dyed and fatliquored using the same tannery that provided the limed hides. The tannery technicians noted that the physical and handling properties of the finished leathers were a good comparison with their own production. Chemical and physical analyses were also carried out. Pickling in the presence of small quantities of chromium salts Additionally, the effect on the swelling produced by the addition of reduced amounts of chrome salt in the bath of pickling was analysed. The treatment process can be seen in Table 3. As the experimental method evolved, studies of the levels of swelling and pH of the solution and hides were evaluated. Such evaluation, in particular the swelling development, was extended to the tanning and the basifying process steps. For basifying, it would be more suitable to talk about the weight increase as the hides have absorbed the tanning and basifying products. Following pickling and tanning, the condition of the hides was observed and compared with the pickled control that was tanned in the presence of sodium chloride. The wastewaters were analysed to determine the eventual presence of nitrogen as this indicates possible structural damage. On the chrome tanned leather, analyses were performed in order to monitor the chrome distribution through the cross-section. Subsequently, this technology was applied to the pickling and tanning of the three matched-side hides. Following shaving, the sides were retanned, dyed and fatliquored at SSIMPC and, finally, finished by the tannery. The finished leathers were subjected to chemical, physical and organoleptic analysis. The technical staff at the tannery evaluated the handle and appearance. Results and discussion Figure 1 shows the weight increase of the hides (or swelling), during the process of pickling as a function of the percentage of water used. The results showed that the swelling, for the same pH value, increased when the quantity of water was increased to reach the maximum value of 30%. Above this value, the swelling is practically independent from the dilution. This phenomenon shows that starting with a certain quantity of water, the inflow of water directly into the inside of the hide is balanced by the outflow directed toward the external solution. Such an outflow increases the dissociation of the ions and favours their entrance into the hide, producing a neutralisation of the charges or the activation of the positive charges. As soon as the equilibrium is reached, any dilution up to 60% and over, does not influence the swelling mechanism, ie the entrance of water. Clearly swelling results for an equal amount of water are dependent on the final pH, reaching the maximum value at the end of the pickling, pH3.0. This can be explained by considering that by lowering the pH, it produces an increase of positive charges into the hide. This promotes a stronger repulsion between the polypeptide chains with the formation of larger spaces where the water flows. In every case, the presence of the sodium chloride, (Figure 1), completely eliminates the phenomenon of swelling, independent of the value of the final pH and the overall water quantity used. The strong increase of the ionic concentration makes the external solution hypertonic compared with the water contained inside the fibre structure of the hide. This causes a release of water from the inside toward the outside, preventing swelling. The quantity of water absorbed does have consequences for the final physical state of the hide. Table 4 shows the aspect of the hides at the end of the pickling. It was predicted that the samples, treated with 30 and 50% water in the absence of salt are more firm and rigid than those pickled in the presence of the lowest quantities of water. These samples have a soft touch similar to that of the hides pickled in presence of sodium chloride. The study of further variations, induced by the swelling, in the following process of tanning gave interesting results. Figure 2 shows that the modifications to the same pickled samples with the tanning and basifying process are represented together with the swelling variations in pickling. The tanning and subsequent basifying decreased the weight of the pickled hides. The swelling results were reduced and were closer to the leathers treated in the presence of sodium chloride (control with 80% water). In the case of pickling with only 10% water, the weight of the hides was, after tanning, practically identical to the control. This means that water outflow, produced by the addition of the chrome salt and the bicarbonate, is much more consistent when pickling is carried out in the absence of sodium chloride. The pickled samples with sodium chloride show a strong increase in weight after tanning and basifying. Nevertheless the differences mentioned can be considered as the result of the absorption of the tanning and basifying products rather than water, which always results in an outflow of water. This means that the water causing swelling, which is effectively present at the end of the tanning, is decidedly less important than that which is calculated by the weight differences. If we consider that the weight increase of the hides - in a traditional pickling process followed by chrome tanning - depends exclusively on the absorbed substances from processing. The real swelling in the absence of salt corresponds, after tanning, to no more than 20-25%, using the maximum water quantity during pickling. In such a case, we could not show that the increases are only due to the water absorbed, because the percentages of chrome found in the hide are decidedly higher than the control (Table 5). This shows that the acid swelling is not an irreversible process but reversed as much as alkaline swelling that is produced in the unhairing and liming processes. Table 4 shows that the only crosslinking during tanning completely modifies the appearance of the pickled hides. The hides pickled with the maximum amount of water, which were previously swollen and rigid after tanning and basifying, were similar to the standard with an absence of rigidity and with adequate flexibility. These samples were analysed to verify the distribution of the chrome. Figure 3 shows the values of nitrogen in the final wastewaters following tanning and basifying. The results show that the nitrogen levels are very low and suggest that the acid attack during pickling without salt has not produced any structural destabilisation. The small amounts found can be attributed to the last traces of impurities caused by bating. This is confirmed by small quantities of nitrogen found in the final wastewaters of the control hides. Differences are observed on the distribution of the chrome along the cross-section of the leathers in comparison with the control (Table 6). From the data in Table 6, it emerges that pickling in the presence of sodium chloride guarantees a more homogeneous distribution of the tanning agents in comparison with the sample treated with the new experimental method. Considering the results and in particularly, the appearance of the hides after tanning, the experiment on three match-sided hides used a new pickling technology without salt in presence of 50% of water. The obtained wet-blue was shaved, retanned, dyed and fatliquored according to processes used by the tannery which supplied the limed hides. Then the hides were dried, staked, finished and finally tannery technicians assessed the appearance of the leathers. According to their judgement, the leathers were comparable enough with those normally produced by the tannery. The final side leathers were analysed to assess their chrome and fat content and physical properties. Table 7 shows the chemical analysis. According to the results, there are no great differences between the leathers treated with the new technology and the standard procedure. Many positive considerations can be made by the comparison of physical properties (Table 8). Despite the less even distribution of chromium throughout the cross-sections of the experimental samples, as shown in Table 6, the new pickling method can still be used according to our overall results. Better results can be obtained by applying another method which uses lower quantities of chromium in the pickle without salt. Figure 4 shows the increase in weight (swelling) of the hide during the pickling until a pH of about 3.0. In such a case, the water used is constant and the swelling results only as a function of the pH. The differences in the weight increase between the control and the sample treated without salt are lower than the treatments previously mentioned. This is caused by a repressing action induced by the small amounts of chrome salts, which strains the water contained in the hide, limiting the capacity to swell. After pickling, the increase in hide weight corresponds to about 15%, while the control value is around 6%. This difference is almost cancelled out after tanning and basifying. Results in Figure 5 consider that the increases in weight in comparison with the control are only the consequence of the higher absorption of the chrome salt and bicarbonate (Table 9). The appearance of the hides after pickling and basifying results in equal values to the control. The final condition of the grain gave excellent results without wrinkles and adequate flexibility. The quantity of nitrogen in the bath at the end of tanning was also negligible. Analysis of chromium in the leather (Table 9) shows a distribution slightly inferior to the control sample. As mentioned, the three sided hides were treated according to a standard control and the new pickling technology. The organoleptic aspect, chemical and physical characteristics of the final materials, were evaluated. Once more the hides were tanned, shaved, dyed, retanned and fatliquored using the same tannery recipe. The leathers were then staked and finished. Technical tannery staff evaluated the handle and appearance of the leathers. According to their judgement, the experimental leathers corresponded fully to their production standard and were perfectly acceptable. The results of chemical and physical analyses are seen in Tables 10 and 11. It can shown that the actual process of pickling, with the employment of suitable quantities of sodium chloride, can be replaced using this experimental technology to obtain final leathers with the same chemical, physical and organoleptic characteristics as the control. The application of the two studied methodologies and, in particular, the second which includes small quantities of chromium salt, provides a dramatic reduction in NaCl from the wastewaters. Semi-industrial trials carried out in tannery conditions according to the second method have fully confirmed the results according to studies at the Stazione Sperimentale Pelli. Conclusion This paper shows a possibility of eliminating the sodium chloride from the process of pickling and tanning, without varying the type of acids and the process. In spite of all the reported literature, pickling can also be performed in the absence of sodium chloride, or any other salt, without producing any destabilisation. In fact, acid swelling is reversible in a similar way to alkaline swelling in deliming. Acid swelling is subsequently reduced in the following process of chrome tanning. The final leathers, apart from a less uniform chromium distribution, are comparable with those of the traditional process for chemical, physical and organoleptic properties. An improvement can be made by the employment of small quantities of chrome salt during pickling. In these conditions, the chrome salt provides a strong deswelling action as it makes the external solution hypertonic. This forces water to flow out from the inside toward the outside of the hide. The final consequence is a very reduced swelling similar to the standard method. Tanning follows in the same bath as the pickle with the addition of the remaining quantity of chrome salts. The final leathers show chemical, physical and handling characteristics similar to the control standard. This final method has been applied to semi-industrial trials, which have confirmed the results mentioned previously. Therefore, it is possible to eliminate sodium chloride from the pickling process without any major variations to the final characteristics of the leather. Stazione Sperimentale Pelli is now looking at how to eliminate sodium chloride from the preservation of raw hides and skins.