Cleaner technology refers to measures taken to reduce or eliminate at the source of production any pollution, or waste and to help save raw materials, natural resources, and energy, thereby increasing performance, productivity, or efficiency by minimizing negative effects on the environment. In leather marketing, the two terms ‘eco-leather’ and ‘bio-leather’ are used to signalize environmental sustainability [1]. Ethiopia has about 57.83 million cattle; 28.04 million sheep and 28.61 million heads of goat livestock population. About 8 million of cattle hide, 12 million of sheep skin and 8 million of goat skin are available annually. There are 27 tanneries in Ethiopia and 21 of them are operating currently. Bahir Dar tannery is among the tanneries presently operating with its full capacity. Turning non treated perishable skins and hides into leather of good quality requires the use of chemicals which react with the skins in aqueous phase. Hence it can be assumed that high concentrations of the used chemicals as well as compounds coming from the skin can be found in the wastewater. Wet salting is one of the most commonly used hide and skin curing practices in the world. Salt is generally cheap and widely available having good dehydrating properties. Curing salt is removed during soaking and discharged in waste soak streams. From the production process the amount of waste water generated is 12,500 m3 and 150 tons of solid waste (of which 54.5 tones is dusted salt [2]. Based on laboratory result the wastewater is characterized by high load of pollutants such as chloride, COD, BOD, TDS etc. The salt discharged in the soak liquor increases the total dissolved solids content of ground water [3].  Pickling is carried out to reduce the pH of the pelt before to mineral tanning and some organic tannage (e.g. chrome tanning, and vegetable tanning), thereby sterilizing the skin, ending the bating action, and improving the penetration of the subsequent tanning material. However, this process also discharges high amount of float to the environment in addition of the salt discharged due to leather curing. The low level of cleaning technology standard has given rise to environmental issues activating the people to think in a new way for the creation of green sustainable process. Bahir Dar tannery pickles its pelts with that of conventional pickling method of 8% salt, and 1% sulfuric acid and the curing salt is not recycled and used in pickling operation. However, the salt discharged due to pickling and curing operation have negative impact on the environment due to the liberation of high amount wastewater with high chloride content. Thus, the purpose of this study is to pickle the pelt by using recycled dusted salt and non-swelling sulfonic acid as a cleaner technology approach, and Bahir Dar tannery is selected as a case.

Generally as the statement of the Problem of the current study, the leather tannery-industry sector has been hugely contributing to the socio-economic development of the nation in terms of foreign currency earning, job creation and tax generation. However, this sector in general is among the most polluting industrial sectors, being very pollution-intensive [4]. Sodium chloride is a common salt used during curing for preservation and leather pickling to prevent excessive acidic swelling and finally discharged with in waste water after processing. However, the presence of salt at large amount in the discharge waste water of tanneries highly affects the salinity of the fertile soil which intern reduces the agricultural productivity of the land. In addition to the decrease in productivity of the soil, when the discharged waste water enters to the nearby river, the salinity of the river will increase which results in aquatic life disruption and the salt discharged in the soak liquor increases the total dissolved solids content of ground water [5].

The extreme solubility of common salt and its great inactivity to precipitation could negatively affect the tanning industry. Thus, the elimination of sodium chloride from wastewaters represents one of the most difficult problems which the tanner has to resolve. On the other hand, in salt free pickling process a rather larger amount of aromatic sulfonic acid or syntan were used to lower the pH of float to around 3 in order to uptake the chrome due to its lower reaction rate with the pelt, and usually takes about 24 hours. Moreover, sulfonic acid is much more expensive than NaCl and other acids used in pickling process; and the properties such as color, light fastness, softness and handling performance of leather produced are negatively affected by the use of high amount of sulfonic acid due to its corrosive nature [6].Curing salt is not recovered in Bahir Dar tannery and left on the soaking process which discharges high amount of waste water with high chloride content. The fresh salt is also used in leather pickling and discharged after pickling in waste water which intern causes different environmental problems.  Therefore, the main objective of this research is to introduce cleaner technology approach in leather pickling using curing salt and sulphonic acids. To reduce all the above problems associated with the concentration of salt, the chloride load and the problems faced due to salt free pickling; a combination recycled curing salt and sulfonic acid will be used in leather processing as a cleaner technology approach.

The finding of this study will enhance knowledge of understanding on the potential utilization of curing salt and sulphonic acids in leather pickling in leather manufacturing. Generally, the significances of this study are helps to reduce the amount of fresh common salt required for pickling processes, helps to reduce the quantity of soak liquor discharged, helps to minimize the pollution load, will minimize the significant amount of polluting chemicals at the source instead of treating the discharged waste, decrease waste treatment cost of the tannery and help as a baseline for other researchers who further studies on this area beyond the scope of this study.

Cleaner Production in Leather Manufacturing; Cleaner Production as pollution prevention approach does play a huge role in reducing the quality (type of pollutants) and quantity of waste water generated in leather processing. Material balance in beam house processing section (per processing one ton of raw hide) indicated that cleaner production practice in that section enabled a pollution reduction from that section of 50%, 50%, 31%, 52%, 52%, 60%, 30% and 100% for parameters of effluent volume, total lime, inorganic solids, COD, BOD, salt, sulfide and NH3-N respectively. Around 60% of total chloride in the tannery effluent is due to the curing process using salt, which is released in the soaking effluent, the rest contributed by pickling and a few by the tanning and dyeing operations. Then, Cleaner Production in this case to reduce chlorides in the effluent involves natural drying of skins; chilling/ice preservation for short term storage; antiseptics usage for long-term storage; trimming before curing and other operations; mechanical/manual removal of salt to save it before soaking; salt free pickling [7].

Amount of Salt Recovered and Purified From Cured Skin

Since the scope for green processing is limited, sodium chloride to a high volume of total dissolved solids (TDS) in the soak waste liquor. During soaking operation, no commercially viable technology for threating effluent has been developed to date. A large amount of salt stacked to the hide and skin surface can be removed by shaking the surface of the skin by mechanically or manually. In this research, the salt is collected from ten sheep skin selected by purposive method of sampling by shaking and de-dusting. Based on purposive method of sampling technique as stated above a sample of ten pieces of wet salted sheep skin being processed to produce finished leather were taken peace by peace from beam house processes which were expected to generate the de-dusted salt and material balance analysis techniques were applied (weighing the sample before and after the operation).

Table 1: The amount of salt recovered and purified from cured sheep skins.

The average desalted salt is the average of the salt desalted from the above skin.

This amount of salt can be de-dusted salt manually though the desalted salt from desalting operation contains various impurities. Finally, clear salt solution can be obtained by dissolving the salt recovered in water simply through screening it and clarifying it using activated charcoal. Then finally the threated salt solution was left to direct sun light and weighed after the recovery.

Thus, the amounts of salt recovered were weighed to be 0.0592kg per skin. Percent of salt recovered and purified can be calculated as

Thus, it is possible to recycle 7.346% of salt on the weight of the raw skin taken for soaking operation. Moreover, after applying the necessary treatment it is possible to obtain 5.92% on the weight of the skin. This amount of salt can affect the environment to a greater extent if not managed properly by different ways like recycling and treating it with different additives for making it usable for the subsequent pickling operation prior to chrome tanning. Finally, this much of salt can be used for pickling operation since it is effectively treated and it is ready for use.

Experimental Pickling Process Using Purified Salt and Sulfonic Acid

Pickling refers to the treatment of bated pelts with a solution of salt and acid to bring them to an acidic condition. The objectives of pickling are to bring the skin to the desired pH for the preservation of the pelt for a longer time before tanning and to bring the skin to the desired pH for tanning-specially in chrome tanning etc. The pH desired will depend upon the tanning to be used and the time between bating and the start of tanning. 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 favor of the positive charges of the side-aminic sites. Repulsive forces are then produced within the structure which keeps the polypeptide chains at a distance from one another. Therefore, spaces are generated where the water quickly penetrates, producing the swelling. In this study the pickling process was done in the same procedure as stated in the methodology.

The first sample was pickled with proportion of 50% purified salt and 50% sulfonic acid.These sample has a good resistance to swelling and the PH of required for leather tanning was achieved in arrange of 2.8 – 3 as these PH is the desirable PH for the subsequent chrome tanning. The green structure and other defects of leather were assessed after dyeing operation. These samples have given good grain structure and which is not damaged by acidic swelling and defects like dyeing uniformity and impurities were not visible in these samples of leather.

When we come to the second experimental sample leather pickled with 50% of recycled and purified salt and 3.5% of sulfonic acid, it has almost comparable properties with the first sample obtained by pickling with 50% of purified salt and 3 % sulfonic acid. Moreover, the grain structure was slightly different from the first sample in such a way that it was slightly coarser.

The third sample was obtained by pickling 50 % of salt which is purified after being recycled and 2.5 % of sulfonic acid. These sample leather have good grain structure the swelling resistance was also surprising, meaning highly resistant to swelling, but the time it takes together to rich the right PH range of 2.8 – 3 is longer than that of the first sample.  The pickling time for completion for completing pickling operation for the second sample was 3 hours. 

The fourth experimental sample leather was obtained by pickling the sample leather by 45% of purified salt and 3% of sulfonic acid. These sample leather have resulted almost similar property with the second experimental sample leather in such a way that the duration of pickling operation was lengthen to 2 hours and 15 minutes to reach the required PH for chrome tanning. Moreover, the grain blemishes were visible after post tanning processes specifically after dyeing processes.

The final experimental leather sample was obtained by pickling the sample leather with that of 55% of purified salt and 3% of sulfonic acid. These sample leather has resulted good grain structure with the overall short pickling duration since, the right range of pickling liquor was obtained in 2 hours and 15 minutes. All detail physical properties like shrinkage, temperature, tensile strength, elongation and tear strength were also measured and conducted for further comparison of those experimental sample leathers.

Shrinkage Temperature of Experimental Sample Leathers

Shrinkage temperature is one of the most important parameters in characterizing the thermal stability of leather. It provides information about the degree of tanning because the better the cross-linking reactions between the collagen fibers and the tannins, the higher the shrinkage temperature. Good quality leather should have a minimum shrinkage temperature which should satisfy the minimum standard 102 degree centigrade. Shrinkage temperature results of sample leathers can be presented in the Table below.

Table 2: Shrinkage temperature of sample leather.

As it can be seen in the above experimental result the first sample which is pickled with the proportion of 4% salt and 3% sulphonic acid, the second sample having pickling proportion of 4% salt and 3.5% sulphonic acid, the last (fifth) sample having a pickling. proportion of 3.5% salt and 3% sulphonic acid have the minimum requirement of shrinkage temperature of the standard which is greater than 95°C. this indicates that the chrome fixation of these thee samples is higher than the other two, because as chrome fixation is increased the shrinkage temperature of samples will be proportionally raised. Thus, the three samples mentioned above meet the minimum requirement of the standard for shrinkage temperature.

Tensile Strength Test Result of Experimental Leather

Leather standards are instrumental in the determination, testing, and evaluation of the various physical and chemical properties of different forms of leather. These standards help users and producers of leather goods all over the world in assessing their materials for good quality and workmanship towards satisfactory use According to tensile Strength is the force per unit of the original cross-sectional area of the un stretched test piece which is applied at the time of rupture of the test piece [8]. It is calculated by dividing the breaking force in kilograms-force by the cross-section of the upstretched test piece in square centimeters. It is a reliable indication of the quality of the leather for development, control, specification acceptance, and service evaluation of leather.

Table 3: Tensile strength of sample leather.

Among experimental sample leather results in the table mentioned above, the first and the second samples with salt and acid proportion of 4% and 3%, and 4% and 3.5% respectively meet the minimum standard requirement of chrome tanned leather. Tus, among other experimental sample tests the first and the second trial have meet the standard requirement for tensile strength of upper leathers tanned with chrome tanned.

Elongation of leather is the measure of the stretchiness of leather in the direction of force applied and is the indication of the leathers resistance to before breakage. In this study the first sample having a percentage of elongation of 45.7 is more stereachaible than the other trials performed with different concentration of salt and sulphonic acid.

Tear Strength Test Result

Tear strength is the load required to tear the leather between two tongues formed by splitting the leather perpendicular to its surface. Two tongues, formed by cutting through a strip of material along much of its centerline, are pulled apart to tear through the remainder of the strip. With some materials, the force needed to start a tear is not the same as that needed to continue it. this point is checked by having the centerline cut end in a small circular hole so that the tear must start at some point around the edge of the hole during the test, and the corresponding load is measured. Tear Strength is the load in kilograms required to tear leather having a thickness of one centimeter [9]. In this study the tear strength results of sample leathers pickled with different purified salt and sulphonic acid are presented in the Table below.

Table 4: Tear strength of sample leather.

As mentioned in the table above the minimum requirement for tear strength tanned with basic chromium sulphate 51 N/mm. but all the experimental samples except for sample one does not meet the minimum recommended standard for leathers tanned with chrome tanning. Thus, the first sample having an over load tear strength only meets the minimum required standard, since the maximum load the tear strength tester can operate is 64n/mm and the readings for the results of the first sample is over load with 5% percentage remained un teared.

That means:

Thus the total tear strength result for this experimental sample leather is

Tear total = 64N/mm + 3.2N/mm = 67.2N/mm. Thus, only the the first sample having a total tear strength result of  67. 2 N/mm meets the standard requirement for leathers tanned with basic chromium sulphate.

Estimated Amount of Salt Replaced With Sulphonic Acid

According to the results of the physical properties obtained, it is possible to approximate or estimate the amount of fresh salt that can be substituted with the subsequent chemical, which is more ecofriendly and can be used to produce eco leather by reducing the contents of the chloride in the pickling effluent of leather making. Thus, from all above trials for experimental sample leathers, the first trial or sample performed with pickling proportion of 4% purified salt and 3% sulphonic acid is an ideal experimental sample having a comparable physical property with that of the standards required with the conventional pickling process of leather of leather making. Thus, as 4% salt and 3% sulphonic acid is desirable, the amount of fresh salt that can be replaced with sulphonic acid is 4% approximately [10-15].

Eco-friend leather processing has an important effect on a healthier environment and human health. Cleaner technology approach in leather pickling process is one method of Eco-friend leather processing and it can be done by using curing salt and sulfonic acid. In this research work 10 samples were treated with different proportion of purified salt and sulphonic acid. Among these trials The sample pickled 4% purified salt 3% sulphonic acid has better physical properties in terms of shrinkage temperature, tensile strength, tear strength, and elongation than other experimental samples produced with different proportion purified salt and sulphonic acid [16-20]. That was basically because of whenever the concentration of sulphonic acid increases the chrome fixation of experimental leather becomes slightly lower and lower physical properties of leather is always increased with the increase of chrome fixation [21-25].

Pickling process using purified salt and sulphonic acid can give rise to good physical properties, which can make it suitable for longer use [26-30]. Based on pickled skin weight, 7.346% and 5.92% salt can be de-dusted and purified from cured sheep skin on the weight of the skin cured. Therefore, the use of this cleaner technology approach using purified dusted salt and sulphonic acid is effective for pickling a pelt which is feasible for the economic and environmental effects of our country with better physical properties. Finally it is recommended that dusted curing salt can be reused in pickling.

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