Conventional chrome tanning methods employed in the leather processing industry subject the hides and skins to treatment with a wide variety of chemicals and passage through various unit operations. All this involves an enormous amount of time and they contribute to an increase in COD, chlorides, sulfates and other mineral salts, which end up as effluent. But, perhaps more alarmingly, the process uses profuse quantities of water in areas where there is rapid depletion of ground water.
To overcome this, a process has been explored to reduce water usage, vis-a-vis deliming, pickle and basification-free chrome tanning for the stabilisation of cow, buffalo and goat skins. Apart from that, associated washings which follow conventional liming and deliming operations have also been removed in this innovative process.
Limed collagen matrices can be transferred to the chrome tanning system directly after fleshing. This leads to a substantial decrease in chemical consumption; 20% for chrome tanning alone. Consumption of water can be cut by 37- 40%, while the total tanning process from raw hide to chrome tanned can be decreased by l2-15%.
The performance of the leathers is at a par with, or sometimes better than, conventionally processed leathers, which is substantiated by an estimation of physical properties and hand evaluation by experienced tanners. The process also enjoys a reduction in COD loads of 80-85%, a total solids content reduction of 82% and a decrease in TDS in the effluent of 84%. This process is also economically viable when compared with a conventional tannage due to less power consumption and less consumption of chemicals. Good quality, fuller, thick tanned leather can be obtained from this formulated process which indirectly results in a far better yield. Thus the new eco-friendly chrome tanning process leads to a unique tanning system with the resultant cleaner environment.
Leather processing has been an important industrial activity which has gained significant economic relevance in India. However, the industry has been labelled with the stigma of a highly polluting industry and there are general concerns that leathermaking has adverse effects on the environment.
In the conversion of limed pelt to chrome tanned leather, an intermediate deliming and pickling along with associated washings are normally performed. These enable even penetration, without precipitation, of chromium into the collagen matrix.
Traditionally, deliming is usually performed by using a single variety ammonium salt of strong mineral acid such as ammonium sulfate or ammonium chloride or a mixture of the two. Whereas, pickling or acidification is achieved using a mineral acid such as sulfuric or an organic acid such as formic or a mixture the two. Salt, usually sodium chloride, is added in appreciable quantities to restrict swelling of the collagen in the acidic medium.
Two intermediate washing operations, using copious amounts of water, follow the liming and deliming operations for the removal of free lime and soluble salts of calcium respectively. Both the deliming and pickling processes are inexpensive and very effective but suffer in that high ammonium, sodium, chloride and sulfate ion concentrations in the tannery effluent cause serious environmental problems, in particular where the tannery cannot discharge its effluent to a marine environment directly. Moreover, high dissolved solid contents in the effluent poses a serious threat to that marine environment and also requires a tedious treatment for the disposal of waste legitimately within limits.
Over the decades, several studies have shown that in the absence of salt, organic sulfonic acids can suppress the swelling that occurs in pickling. The research has culminated in the commercial availability of several preparations as salt-free pickling agents, but most of these formulations still require deliming as a compulsory unit operation, which can impart undesirable characteristics to the resultant pickled stock and slow penetration within the collagen matrix, leading to process problems.
It is also known that commercial conventional chrome tanning has poor chromium uptake, only about 55-60% (average). So, constant innovative process modifications for cleaner technology have been of the utmost importance in the leather-processing sector to safeguard our environment.
Pollution prevention, waste minimisation, product empowerment and process innovation are the key words used by the scientists and industrialists globally. However, these approaches are very much confined within recycling of wastewater to a maximum number of cycles followed by discharging or use of environmentally-friendly chemicals.
Discharge of minimum pollution loads or a zero-discharge concept, in principle, should be the topic of the day and reduce pollution completely. Several chrome management methods are established, eg high exhaust chrome tanning, pickle and basification free chrome tanning and closed pickle-tan loop systems and a method of producing leather in a narrow pH range of 4-8 has also been reported. In the present study, pretanning methodology has been changed after liming and has been integrated with the concept of a deliming, pickling and basification free chrome tanning process.
The scorn towards the industry – sometimes rhetorically misnamed as the byproduct of the effluent industry – can also be healed because the process uses limited amounts of water. Thus, the rapid depletion of the ground water table can be safeguarded from the tanners’ end.
Another major problem of the industry is the non-availability of raw hides and skins with good substance. Tanned leathers produced in this eco-friendly chrome tanning methodology have been much fuller and flatter – producing greater yield due to a reduction in wrinkles.
Experimental
Materials: Eight wet-salted 30-35sq ft cow hides were selected as the raw materials. All the chemicals used for the processing were of commercial grade. The chemicals used for the analysis of leather and spent liquors were of analytical grade. Each hide was halved along the backbone, with the left half processed conventionally, and the right half subjected to the experimental tannage.
Conventionally, basic chromium (III) sulfate exhibits tanning potency through the trivalent chromium cation. To accomplish tanning with basic chrome powder in its conventional state, the charge behaviour of the pelt has to be altered to accommodate the tannage, eg through pickling.
The present study takes the opposite view, and has concentrated its attention on modifying the electro-chemical behaviour of basic chrome sulfate to enable it to tan limed pelts.
Care has been taken to avoid the use of external reactants to facilitate this charge conversion and the intrinsic properties of basic chromium sulfate have been activated to serve the purpose instead.
Different percentages of basic chrome sulfate powder (percentages based on fleshed weight) were added to the limed pelt (after fleshing) maintaining an identical concentration of tanning float (involving added float and bound water in pelt) in all the cases.
The volume of added float depends on the tanner, the condition of tanning, eg drum size, size and type of pelt, speed of drum, driving system etc.
Bound water has been calculated to be 40% of the fleshed weight in this case. The results obtained are shown in Table 1 and Figure 1.
It is to be noted that this optimisation is true with the variety of BCS powder used here and with the type of limed pelt (ie its liming or unhairing condition, thickness). If the two variables change, optimisation of the process will once again be needed. But, the authors are determining if a trend can be ascertained with different case studies, which are on-going.
Each side was treated with a different dosage of basic chromium sulfate powder in the above stated manner just after the fleshing operation. The fleshed hides were run for eight hours and the degree of penetration of the chrome tanning salt in the cross- section for each case verified. Then, the leathers were tested for their apparent hydrothermal stability by immersing a small piece of tanned leather in boiling water for three minutes. Thereafter, the area of the sample pieces is superimposed on the outline of pieces drawn before immersion. The tanned leathers are washed, and the chromium (III) content in the leather is measured using the standard procedure. Evaluations of apparent properties of tanned leathers have been carried out with the help of experienced tanners.
Conventional tanning process
The eight wet-salted left sides were soaked conventionally using 5% sodium sulfide, 8% lime and 400% water in a wooden drum (all percentages based on soaked weight). The hides were run for two hours, and then for ten minutes every hour for six hours before being left overnight in the drum.
Next day the hides were run intermittently as before. On the third day they were fleshed, scudded, and then weighed. The pelts are then washed with 300% water (percentage based on fleshed weight) for 30 minutes. Subsequently, the hides were delimed by treating with 150% water and 1.5% ammonium sulfate, 0.75% sodium bisulfite (w/w) for 90 minutes.
Completion of the deliming operation was ascertained by checking the cross-section of the delimed pelt was colourless on a phenolphthalein indicator. Bating was carried out in the same bath for 30 minutes by the addition of 0.5% commercial alkali bate. The bated pelts were washed with 200% water for ten minutes. Pickling was carried out by treating the pelts initially with 100% water and 8% sodium chloride for ten minutes followed by the addition of 1.7% sulfuric acid in three instalments at intervals of 15 minutes and subsequently run for three hours until the pH reached 2.5-2.8 throughout the cross-section.
Chrome tanning was then initiated in the same float by adding 8% basic chromium sulfate after draining 50% of the pickle float. The drum was run for six hours and then left overnight until the cross-section showed full penetration.
It was run for 20 minutes the next day and basified using a mixture of 1% sodium formate, 1.5% sodium bicarbonate and 50% water (based on fleshed weight) in three instalments at an interval of ten minutes.
The drum was run for three hours and the pH was checked to be 3.8. Tanned leathers were subjected to a gross assessment of hydrothermal stability by means of a boil test. The wet-blue was then washed with 250% water and piled.
Experimental tanning process
The eight right sides of the cow hides were soaked using a conventional method. The sides were given the same liming as the controls. On the third day the hides were fleshed, scudded and the fleshed weight was noted.
The fleshed pelts were subjected directly to chrome tan instead of conventionally being followed by washing, deliming, washing and pickling.
Figure 2 and Figure 3 show the variation of pH in each unit operation for conventional and experimental chrome tanning process respectively. The pelts (without any wash) are taken into the drum and 12% chrome (III) sulfate powder (33% basic; 18.03% Cr) based on fleshed weight was added.
The skins were run for six hours until complete penetration is ascertained. Then the float is increased by 260% based on fleshed weight and run for three hours. End pH of the float is checked to be 3.1. The chrome tanned leathers were lightly rinsed and subjected to shrinkage temperature measurement using standard procedure. Then the leathers were piled for 48 hours.
All the wet-blue (conventional and experimental) are sammed, split and shaved to 1.1-1.2mm to be processed separately into shoe upper leather (up to dyed crust).
Analysis of chromium
Samples from exhaust chrome liquor were collected for both experimental and conventional processes. The liquors were analysed for chromium as per the standard procedures. Leathers were also analysed for the chromium content using standard procedures. Samples are taken from the butt portions as per official methods for control and experimental wet leather samples, and were estimated for moisture before analytical estimation.
Determination of shrinkage temperature
Hydrothermal stability of the tanned leather was measured using the Theis shrinkage meter.
Analysis of wastewater
Liquors from all unit operations up to tanning were analysed for COD, TS and TDS values (dried at 103-105°C for one hour) as per the standard procedures. Procedures have been followed for evaluation of both conventional and experimental sides.
Input audit
A comprehensive input-output audit for raw materials, water, chemicals and other products were determined for both conventional and experimental tanning processes excluding soaking, liming and post-tanning operations. A mass balance for the effluent collected was calculated for a particular unit operation.
Physical properties and hand evaluation of leathers
Using IULTCS standards, samples were conditioned at 80+- 4°F and 65+-2% relative humidity over 48 hours. Physical properties (tensile strength, percentage elongation at break and tear strength) were evaluated as per standard procedures. Other properties (softness, fullness, grain tightness and smoothness) plus general appearance tanned wet-blue have been assessed by hand and have been symbolised as + or – as shown in Table 1. The numbers of symbols emphasise a better property or vice versa.
Physical properties
Figure 4 and Figure 5 show tissue structure of both conventional and experimental tanned wet-blue respectively. A more open structure is found with the experimental one compared with the conventional one. Table 2 lists the physical properties, eg tensile strength, stitch tear strength, grain crack strength and ball bursting strength, all of which were evaluated on the crust leathers, both parallel and perpendicular to the backbone and the mean value calculated thereafter. Grain crack index also have been taken as mean values.
Assessment of material balance
An audit for material input and output was conducted to assess comparative efficacy of the experimental process. All calculations were carried out on one ton of raw hide (Table 3). All the unit operations were followed identically up to and including the liming. In the case of the conventional process, this required 174kg of chemicals. Chrome exhaustion was 65% of the applied basic chrome sulfate; the exhaustion level for the experimental tanning process was 90%.
Conventional chrome tannage requires 22kg of mild alkali salts for basification. Hence conventional chrome tanning basification discharges 42.2kg of solids for one ton of raw cow hide. But this solid discharge is reduced to 10.2kg in the case of the experimental chrome tannage. Total amount of chemicals consumed for conventional and experimental processing methods are 314kg and 223kg respectively.
The amount of water required for both control and experimental processing is tabulated in Table 4. It is evident from these values that the experimental chrome tanning process enjoys a reduction in water consumption of 30% compared with conventional methods. The lower volume of consumption of water results in a low volume of effluent going to the effluent treatment plant (ETP) which means a reduction in treatment costs.
It has been estimated that by 2025 AD, 1.8 billion people will live in countries or regions with absolute water scarcity. So, this achievement of reducing water consumption by 6.76l/kg of raw hides and skins is encouraging.
Chromium uptake
The chromium uptake was 60% and 94% of the added basic chromium sulfate for the conventional and experimental processes, respectively, which is shown in Table 6.
The most important point to be highlighted is that residual chrome liquor can be recycled to the next batch after replenishment of the consumed chrome to the required degree of concentration. Such recycling had been repeated for several times without any treatment required.
It is evident from Table 6 that the chrome content in the wet-blue leathers from the conventional process is much less when compared with the experimental process.
This is because the experimental process involves chrome tanning at a much higher pH than the pH of acidic amino acids in collagen when all the carboxylic groups of acidic amino acids are in an ionised state.
Furthermore, chrome tanning in the limed stage, when the collagen is opened up, offers minimum physical resistance to chromium molecules from tanning liquors.
Chromium (VI) estimation
Both the wet-blue and crust leathers were subjected to chromium (VI) assessment in the leather. Assessment procedures followed the IUC 18 standard, and Table 7 shows that leather produced from the control process develops chromium (VI) after 30 days of storage whereas leathers produced in experimental process do not develop any detectable amounts of chromium (VI). All the crusts were dried under shade with the neutralisation pH maintained at 5.5-6.0 for minimum interference of the processing parameters usually responsible for development of chromium (VI) in leather.
Conclusion
Qualitative and quantitative assessment of conventional and experimental leather proved the efficacy of the experimental process and resultant leather. Material audit and assessment of water consumption indicates less chemical and water consumption respectively. Physical properties of the experimental leather are also better in the case of experimental leather. Fibre opening is also comparatively better than that of conventional leather. Huge salinity discharged in the effluent of conventional chrome tan is also absent in the effluent of the experimental process. Simultaneously, TS, TDS and COD values are reduced greatly in the effluent of the process.
Thus, this experiment has led to the development of a highly environmentally-friendly chrome tannage, which can be followed for every type of leather.
Acknowledgement
The authors heartily thank Arnab Kumar Jha, proprietor, Weblec India, Kolkata, for his continuous motivation and persistent rectification of flaws during the study.
The authors also thank Pallab Kumar Biswas and Prabal Goswami, students of the Government College of Engineering and Leather Technology, Kolkata, for their tireless cooperation in implementing the work.
