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Enzyme application in Leather Industry

Introduction

Leather industry is a very old manufacturing sector producing a broad range of goods such as leather footwear, leather bags, leather garments, and so on. The raw material used in leather industry is derived from the waste product of food industry, specifically from meat processing. This waste product is converted into desirable and useful leather products. Leather and its products are one of the most traded products globally. They are produced from renewable and readily available resource. The leather industry trade is presently exceeding US$ 80 billion in a year and this is expected to grow as there is increase in population and urbanization of the countries.The consumption of leather products by the humans is very common and used almost every day. The primary raw material for any leather processing industry is derived from slaughter houses and waste from the meat industry. This raw material is processed and converted into usable leather in tanneries. Hence, tanning industry is considered as one of the primary leather processing units in the entire leather industry.Although the leather tanning industry primarily utilizes the waste from the meat industry, it also involves the usage of many chemicals to convert the raw material into finished product. Thus, leather industry consumes resources and produces pollutants which are toxic and hazardous to the environment. For instance, in leather processing one metric ton of raw material is converted into only 200 kg of usable leather product (comprising 3 kg of chromium). The solid and liquid waste includes about 250 kg of non tanned solid waste, 200 kg of tanned waste (comprising 3 kg of chromium), and 50,000 kg of wastewater effluent (comprising 5 kg of chromium). Altogether, one metric ton of raw material yields only 20% as finished leather product and more than 60% as solid and liquid waste including the highly carcinogenic heavy metal “chromium”.

Leather manufacturing and processing leads to the formation of solid waste and wastewater effluent containing toxic and hazardous chemicals; the waste from leather industry leads to harmful effects on living beings and environment. The waste includes used animal skin and effluent containing chemicals such as chlorides, sulfates, hydrocarbons, amines, aldehydes, and so on, along with heavy metals including arsenic and chromium used in leather tanning process. These toxic wastes have the potential to be utilized in energy production to reduce the negative impact on the environment. The two methods, namely upflow anaerobic sludge blanket (UASB) reaction and biomethanation, through which the toxic waste may be utilized to generate heat and energy are also stated.

Cleaner Leather Processing: Existing Enzyme Application

Biotechnology in the broad sense has already been used in the tanning industry for a number of years now, since the inception of the use of enzymes. The utilization of enzymes in the tanning industry is possible in all stages of production, expect perhaps the actual tanning. At the moment the processes in which enzymes are already being used with a fair degree of success are Soaking (cleansing and rehydration) , Unhairing (removal of hair) , Liming( fibre opening), Bating (removal of unwanted proteins) and in part degreasing (removal of fat). It should be mentioned that the use of enzymes in all these unit processes is not necessarily to achieve cleaner processing. Especially in soaking and bathing processes, Enzymes are used to achieve desired action on certain noncollagenous proteins in preference to chemical and not aimed to reduce the pollution load. Although several biotechnological options are available for handling effluents as well as proteinous solid wastes, the review focuses only on in-plant biotechnological leather processing.

Soaking

Soaking enzymes are mainly used to shorten the production time by disintegrating solidified noncollagenous proteins and fats that interpose themselves between the fibres and sometimes cover the external surfaces of the hide making contact between collagen fibres and water difficult. Use of enzymes is not aimed for a specific action but rather a broad- spectrum of action in order to obtain both the solubilization and removal of the interfibrillar proteins for easy rehydration of the skin. Carbohydrases and proteases are the types of enzymes used in the soaking of hides. The optimum pH of operation can range from 4.0 to 10.5. The advantage of an enzyme soak include short wetting, loosening the scud. initiation of the opening of the fiber structure, and when used at an alkaline pH of less than 10.5, production of a product with less wrinkled grain. Grimm reported a soaking process using proteolytic enzymes and carbohydrates at pH 5.5-10.0. Toshev and Esaulenko wet back nonsalted, preserved sheepskin using the enzyme from A.oryzae in 4 to 5 hrs. Botev and coworkers used 0.5- 0.6% of bacterial alpha amylase for soaking dried wool lambskins and found that is resulted in stronger amylolytic activity, much weaker proteolytic activity and no lipolytic activity.

Liming and Reliming

The conventional liming process employs lime and sulphide in high proportions. These materials form a source of pollution in the spent lime liquors. Lime being a poorly soluble alkali, there is an advantage from limited availability of dissolved alkali. Although sulphide is a toxicant, it is the prime depilant in the unhairing process.Reduction of sulphide at source is now possible using enzyme assisted processes. Enzymes used in unhairing are generally of the proteolytic type that catalyze the breakdown of noncollagenous proteins. The origin can be animal such as bovine or porcine pancreas etc. Enzyme assisted unhairing causes loosening of hair by selective breakdown of cementing substances and presents a hair approach. Modification were made by pretreating the hides or skin either with bases or disulfide cleaving compounds for accelerating the process.

Bathing

In bathing, the hides and skin are treated with proteolytic enzymes to remove certain undesirable noncollagenous proteins. Mclaughlin and coworkers established that the bathing enzymes bring about physio-chemical changes in the skin. They concluded that the most vital function of the enzymes in the bate is the removal of the coagulate or coagulated protein of skin. Gustavson developed a new method for investigating the nature of the bating process, particularly the alterations of the hide substance by the proteolytic enzymes of the bate. Moore demonstrated that the pancreatic enzymes based bate is superior to the fungal and bacterial enzyme bate, although the difference is not large, at higher temperature, bating enzymes are more efficient than at lower temperatures.

Degreasing

Degreasing is another stage of the tanning process in which the use of enzymes is foreseeable and feasible. The quantities of natural fats are very high, especially in some types of sheepskins wherein they can represent 20-30% of the total weight of the skins. The normal fat removal methods involve the use of anionic or non-ionic surface active agents or solvents. Trabitzsch evaluated the potential of lipases in degreasing skins. Papp et al patented a process in which animal skins were treated with lipases and amylase in the presence of deoxycholic acid catalyst to remove lipids and noncollogenous proteins. Jareckasemplyed a combination of two enzymes,Protosubtilin G2X and Lipavamorin G3X, for 18 hours at 39-40 degree C at pH 7.5 and succeeded in unhairing and degreasing pigskin. Subsequently, Jareckas and Shehtakova substituited Protofradin G3X for the Protosubtilin G2X and found that this process improved the texture of pigskin,removed fat, mucosaccharides and substances other than collagen. It was found that the structure became more open, retained more fat during fatliquoring and produced more elastic and fuller leather than pigskin unhaired by conventional method.

Progress in Recent past

In the past two decades, significant improvements in the production and application of enzymes in various leather processing steps have occurred. One such area is the use of enzymes for the removal of dung from hides during soaking process. Dung, which contains cellulose, hemicellulose and lignin upto 80% poses severe problems for tanners in the production of high quality leather. Attempts have been made to design an ideal enzymes unhairing, in which enzymatic activity is oriented towards the epidermis thereby their action on collagen is minimized. Also attempts have been made to develop alkaline proteases from Alcaligenus faecalis, Aspergillus tamarii and Streptomyces griseus capable of loosening the hair without any chemical assistance. An alkaline protease has been produced from Rhizopus oryzae through solid state fermentation which dehair the skins completely. Commercial exploitation of such enzymes would depend on the cost effective production and secured method of application at industrial level.

Scale up studies of the production of proteases for bating and possible use of local materials such as cow pancreas for bating have also been carried out. The new development in this field lies in the use of enzymatic products capable of effecting the bating action even at acidic pH. Acid protease produced by Aspergillus usamii has been shown to be an ideal bating agent for sheep-pelt. Studies tofind the efficiency of degreasing using enzymes reveal that pickling with acid proteases and lipases increases the effectiveness of degreasing while neutral lipases increase the efficiency during degreasing at neutral pH.

Application of enzymes in leather processing: a comparison between chemical and co enzymatic processes

The scarcity of natural resources and the accumulation of pollution caused by human activity have required the development of production technology that is less harmful to the environment. The concept of cleaner production has been used in tanneries in order to mitigate their impact and reduce the loss of chemicals, water and raw materials. According to Rajamani et al. (2009), the world capacity of leather processing is 15 million tons of hides and skins per year. The average wastewater discharge is more than 15,000 million liters/day. Solid waste generation from the tannery process is estimated at 6 million tons/year. The disposal of large quantities of sludge, approximately 4.5 million tons/year, and effluent from treatment plants is a major issue.

The amount and type of waste generated during leather production is variable and depends on numerous factors such as breed, slaughtering procedure, conservation of hides, and the technology used for hair removal and tanning. Lime/sulfide is widely used in hair removal because it is more efficient and cheaper than other currently available technologies. Chromium salts are the most common tanning agents.

The sulfur present in the effluent comes from organic matter (especially hair) and from compounds used in the processing of hides including surfactants and unhairing agents, such as sodium sulfide (Na₂S). Sulfur is found in effluents in the form of sulfates and sulfides. The risk of hydrogen sulfide (H₂S) formation during effluent treatment poses a serious environmental problem. To avoid generating hydrogen sulfide from the effluent, the sulfide should be oxidized, which requires an additional step in wastewater treatment. The unhairing process can be performed using bacteria or chemicals such as hydrogen peroxide and sodium hypochlorite to oxidize substances.

Traditionally, tanneries apply enzymes in the bate step to achieve deep cleaning of the hide. However, enzymes were also used in the hair removal process at the beginning of the last century before the development of chemical processes for hair removal. These proteins are gaining more prominence because they are considered to be environmentally friendly technologies and because of advancements made in the purification, development and improvement of enzymes. Enzymes are currently applied at various stages of leather processing, from beamhouse operations until the final stages.

DEHAIRING AND FIBER OPENING PROCESS FOR COMPLETE ELIMINATION OF LIME AND SODIUM SULFIDE

The use of lime and Sodium Sulfide in leather making creates a lot of environmental concern. However, there is no commercial beam house process that could totally eliminate the use of lime and Sodium Sulfide. In this invention, a novel bio-chemical process has been Standardized employing specific enzymes and non-toxic chemical that could totally eliminate the use of lime and Sodium Sulfide in leather processing. It has been found that the extent of hair removal and opening up of fiber bundles is comparable to that of the conventional limed leathers. Performance of the leathers is shown to be on par with conventionally leathers. The process also enjoys reduction in chemical oxygen demand and total Solids load compared to conventional process.

Conventional leather processing involves four important operations, Viz., pre-tanning, tanning, post tanning and finishing. It includes a combination of single and multi-step processes that employs as well as expels various biological, organic and inorganic materials. Beam house processes (liming and reliming) employ lime and Sodium Sulfide and purifies the skin matrix by the removal of hair, flesh and other unwanted materials. Various application methods include pit, paddle, drum and painting on flesh side. After this stage, the hide/skin is termed as pelt. Deliming, bating and pickling processes prepare the skin for subsequent tanning. Tanned skin matrix is further retanned to gain substance of required softness and dyed to preferred shades. Generally, liming-reliming process liquors contribute to 50–70% of the total biochemical oxygen demand (BOD) and chemical oxygen demand (COD) load from a tannery waste water and 15-20% in the case of total solids (TS) load as reported by Aloy et al (Tannery and Pollution, Centre Technique Du Cuir: Lyon, France, 1976). Apart from this, a great deal of Solid wastes containing lime sludge, fleshings, and hair are generated. The extensive use of Sulfide bears unfavorable consequences on environment and the efficacy of effluent treatment plants as reported by Colleran et al (Antonie van Leeuwenhoek, 67,29, 1995). Several lime and sulfide free liming methods have evolved during the past century. Bose and Dhar (Leather Science) reviewed the use of enzymes Such as proteolytic, amylolytic, etc from various sources namely animal, mold, bacterial and plant for dehairing hides and skins. However, these methods include the use of lime. Rosenbusch (Das Leder, 16, 237, 1965) has reported the use of chlorine dioxide for dehairing. Morera et al (Journal of the Society of Leather Technologists and Chemists, 81, 70, 1997) have studied the use of hydrogen peroxide in alkaline medium for dehairing by oxidation mechanism. However, the reduction in pollution load especially COD is not significant. Sehgal et al (Journal of the Society of Leather Technologists and Chemists, 80, 91, 1996) have developed a non-enzymatic sulfide free dehairing process using 1% nickel carbonate, 1% Sodium hydroxide, 5% lime and kaolin along with water by painting. However, disposal or recovery of nickel compounds poses serious health problems. Schlosser et al (Journal of the Society of Leather Technologists and Chemists, 70, 163, 1986) have reported the use of Lactobacillus based enzymes at acidic conditions for dehairing. This method leads to the solubilization of collagen at the experimental conditions. Valeika etal (Journal of the Society of Leather Technologists and Chemists, 81, 65, 1997; 82,95, 1998) have attempted to replace lime for dehairing using Sodium hydroxide and Sodium sulfide. They also found that the addition of salts such as Sodium chloride, Sodium Sulfate, Sodium formate or Sodium hydrogen phosphate influence the extent of hair removal as well as opening up of the dermis Structure. Commercial application of these methods is not popular in the global leather sector. Thanikaivelan et al (Journal of the Society of Leather Technologists and Chemists 84, 276, 2000) have developed a lime free enzymatic dehairing process along with reduced amount of Sodium Sulfide, which ensures complete dehairing within 18 hrs. However, enzyme-assisted lime-Sulfide dehairing is being followed in some parts of the world. All the methods are applicable for only dehairing of skins/hides in leather processing. The dehaired pelts require fibre opening. Conventionally the fibre opening is obtained by treatment with lime through osmotic swelling. Liming removes all the interfibrous materials especially proteoglycans and produces a system of fibres and fibrils of collagen which are clean as described by Campbell et al (Journal of American Leather Chemists Association, 68, 96, 1973). This is achieved by the alkali action as well as osmotic pressure built up in the skin matrix. Thanikaivelan et al (Environmental Science & Technology, 36,4187, 2002) have successfully developed lime free fibre opening process employing X-amylase. However, no Successful attempt has been made to eliminate lime and Sodium Sulfide completely in leather processing.

OBJECTS OF THE INVENTION

The main objective of the present invention is to provide a novel dehairing and fiber opening process for complete elimination of lime and Sodium Sulfide, which obviates the major drawbacks. Another objective of the present invention is to provide a complete set of beam-house processes that employs only enzyme and non-toxic chemical also another objective of the present invention is to provide a bio-chemical based beam-house process that provides softer and smoother leathers. Few other objectives like providing a biochemical based beam-house process that leads to significant reduction in chemical oxygen demand and total Solids load and providing a lime and Sodium Sulfide free beam-house process that totally obviates the formation of dry sludge.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the present invention provides a novel dehairing and fibre opening process for complete elimination of lime and Sodium Sulfide, which comprises i) adding 5-10% w/w, based on the weight of soaked hides/skins of water to 0.5-1.5% w/w based on the weight of soaked hides/skins of proteolytic enzyme, exhibiting optimum activity at pH 7.5-11.0 and temperature 25-40 C, optionally in the presence of not more than 1.5% w/w, based on the weight of soaked hides/skins, of silicate salt, to prepare a paste, ii) applying the paste, as formed in Step (i) on the flesh or grain side of the hides/skins by known method, iii) piling the pasted hides/skins grain to grain for a period of not less than 12 hours followed by removing the hair by known method to get dehaired hides/skins, iv) treating the dehaired hides/skins, as obtained in Step (iii) with 5-10% w/w, based on the weight of dehaired hides/skins, of silicate salt in presence of 50-250% w/w of water, preferably under stirring condition, for a period of not less than 3 hrs, followed by fleshing by known method to get pelt for subsequent post fibre opening processes. In an embodiment of the present invention, the proteolytic enzyme used may be selected from bacterial protease, fungal protease, either individually or in any combination. In another embodiment of the present invention, the silicate salt used may be selected from Sodium metasilicate, water glass, Sodium orthosilicate, either individually or in combination. The process of the present invention is described below in detail. A dehairing paste is prepared by mixing proteolytic enzyme in the range of 0.5-1.5% w/w, on the weight of soaked skins or hides with 5-10% w/w of water, optionally in the presence of not more than 1.5% w/w of silicate salt. The dehairing paste, thus prepared is applied on the flesh or grain side of the soaked skins or hides by known manual or mechanical method and the pasted hides/skins are piled grain to grain for a period of not less than 12 hrs. The skins or hides are then dehaired by conventional method. The dehaired skins or hides are mixed with 50-250% w/w of water on the weight of dehaired skins or hides and treated with 5-10% w/w of silicate salt preferably under stirring condition, for a period of not less than 3 hrs followed by fleshing by known method to get pelt for subsequent post fibre opening processes. The novelty and non obviousness of the present development lies in using proteolytic enzymes and non-toxic silicate salt for dehairing and fibre opening, thereby providing an Eco-benign bio-chemical based beam house process that totally eliminates the use of lime and Sodium Sulfide.

ECO-FRIENDLY BIO-PROCESS FOR LEATHER PROCESSING

The present invention relates to an eco-friendly beam house process for dehairing of leather using commercially available enzymes that obviates the use of lime and other alkalis. The present invention also provides an alternate process for opening the fibers of leather using commercially available enzymes. The present invention further provides a three-step process for tanning hide or skin in a narrow pH range of from about 7.5 to about 8.5.

Conventional leather processing involves a number of unit processes and operations. It includes a combination of single and multi-step processes that employs as well as expels various biological, organic and inorganic materials as described by Germann (Science and Technology for Leather into the Next Millennium, Tata McGraw-Hill Publishing Company Ltd., New Delhi, 1999, p. 283). Conventional method of pre-tanning and tanning processes involve 7-8 Steps comprising Soaking, liming, reliming, deliming, bating, pickling, chrome tanning and basification and discharge enormous amount of pollutants. This accounts for nearly 90% of the total pollution from a tannery as analyzed by Aloy et al (Tannery and Pollution, Centre Technique Du Cuir, Lyon, France, 1976). This includes biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), Sulphides, chlorides, sulphates, chromium, etc.[3] This is primarily due to the fact that the conventional leather processing employs “do-undo process schemes such as Swell-deswell (liming-deliming); pickle depickle (pickling-basification) as described by Bienkewicz (Physical Chemistry of Leather Making, Krieger Publishing, Malabar, Fla., 1983) [4]. In other words, conventional methods employed in, leather processing subject the skin/hide to wide variations in pH. Such pH changes demand the usage of acids and alkalis, which leads to the generation of salts. This results in a net increase in COD, TDS, chlorides, Sulphates and other minerals in tannery wastewaters as reported by Thanikaivelan et al (Journal of the Society of Leather Technologists and Chemists, 84, 276, 2000). Further, toxic gases like ammonia and hydrogen Sulphide are also emitted. Apart from this, a great deal of solid wastes like lime sludge from tannery and chrome sludge from effluent treatment plant are being generated. This happens to be a major stumbling block for many of the tanners around the World due to the stringent environmental regulations.

OBJECTS OF THE INVENTION

The main object of the present invention is to provide a novel tanning technique for processing leather which obviates the major drawbacks. Another objective of the present invention is to provide a bio based beam-house process that leads to significant reduction in chemical oxygen demand and total Solids load. Yet another objective is to provide a bio based beam-house process that totally obviates the formation of dry sludge and also to provide a three-step process that provides leathers matching the properties of leathers from conventional leather processing steps.

Steps Involved

The raw material, skins or hides, is soaked in water conventionally. The weight of the soaked skins or hides is noted after draining for 10–15 minutes. A dehairing enzyme or mixture of enzymes in the range of 0.5-2.0% on the weight of soaked skins or hides is mixed with a chemical that assists the enzyme in the range of 0-1.5% or hides in 4-8% water to form a paste. The prepared paste is applied on the flesh or grain side of the soaked skins or hides and piled for 8-20 hrs. The skins or hides are then dehaired using a conventional procedure and the weight of the skin or hide is noted down. The dehaired skins or hides are mixed with 50-350% water conventionally. An enzyme or mixture of enzymes that capable of opening up of fibre bundles is added in the range of 0.5-2.0%. Alternatively an alkali in the range of 0.3-1.25% on the weight of dehaired skins or hides can be used for the same purpose. The duration of fibre opening treatment is in the range of 2-24 hrs. The opened up skins/hides are fleshed conventionally. The weight of the pelts (skin or hide without hair and flesh) is noted. The cross section pH of the pelts is found to be 7.5-8.0. The pelts are tanned using basic chromium sulphate, vegetable tannins, chrome syntan, aluminium syntan, chromium-iron tanning agent, chromium Silica tanning agent in the range of 4-10% on the fleshed weight of skins or hides either alone or in combination with polymeric syntan in the range of 0.5-2.5% or other tanning agents in 50-150% water. The duration of tanning is in the range of 2-10 hrs. The tanned leathers are then followed by conventional leather processing sequence for different end uses. The novelty and non obviousness of the present development lies in using enzymes for dehairing that target hair as well as specific enzymes or alkali for fibre opening that target the cementing substances such as proteoglycans, which facilitates fibre opening. Also, it should be noted that all the aforesaid steps have been accomplished without pH adjustment. Thus this approach forms an eco-friendly bio based beam house process that aims towards Zero waste criteria and more particularly, eliminates the use of lime totally.[5]

AN INTEGRATED ECO-FRIENDLY TANNING METHOD FOR THE MANUFACTURE OF UPPER LEATHERS FROM GOATSKINS

The leather industry, especially in developing countries, provides economic benefits but suffers from a negative image of the environmental impact of the tannery wastes. Conventional methods of pre-tanning and tanning processes can discharge large amounts of pollutants, accounting for nearly 90% of the total tannery pollution.1,2 Conventional methods of tanning involve ‘do-undo’ operations like, curing (dehydration) - soaking (rehydration), liming (swelling) - deliming (deswelling), pickling (acidification) - depickling (basification). This typical process method subjects the hide to wide variations in pH.3 Such changes in pH demands the use of acids and alkalis, which generates salts. Questions have already been raised about the negative consequence of pollution from wastewaters, careless disposal of solid wastes and gaseous emission. The wide variations in pH during the various steps of leather making are also bound to impair the bulk as well as the surface properties of the skin matrix.6 Beamhouse processes are known to contribute 60-70% of the total pollution load in leather processing. The unhairing process is one of the most polluting operations in leather processing. Lime has a potential to drive swelling in a gradual manner due to its low solubility but the large amount of lime sludge is its main drawback. The use of ammonium salts for deliming leads to emission of ammonia. The role of salt in pickling is to suppress the acid swelling through ionic strength effects, but it contributes to TDS. Present commercial chrome tanning methods may only give about 50-70% chromium uptake. This poor uptake results in material wastage and ecological imbalances - the international specification for the discharge of chromium bearing streams is less than 2 ppm. Chrome recovery/reuse methods can be employed using a variety of equipment. The present work modifies the conventional process sequence and employs lime-free enzyme unhairing, NaOH for opening up fibres and pickle-less chrome tanning. The net benefits from this approach are reduction in the use and cost of chemicals, with a reduction in utilities. Elimination of liming, reliming, deliming, pickling and basification is possible with subsequent reduction in process time, wastes and better exhaustion of chromium. An enzyme assisted dehairing process has been established using lime and a low amount of sodium sulphide in a flesh-side application. A similar process with a reduced amount of sodium sulphide has been standardised for cowhides ensuring complete dehairing within 18 hrs. The dehaired pelt is clean, white and free from scud. However, this grain-side application is difficult on goatskins due to the dense hair. Hence, preliminary trials have been performed to optimise the application as well as the composition of the depilatory. The opening up of the fibre bundles was inadequate and the pelt required reliming. This use of lime in reliming however, creates lime sludge so a sodium hydroxide based opening up has been standardised for cowhides. However, the optimised concentration of sodium hydroxide causes problems on skins, since it is a strong alkali combined with the differences in grain to corium thickness ratio. Thus, in this investigation, preliminary experiments have been carried out on goatskins to optimise the concentration of the alkali required to open up the fibre bundles comparable to that of conventional lime treated pelts. Salt free pickling using naphthalene sulphonic acid is well known. A pickle-less tanning system using chrome syntan and modified BCS has been used. Pickle and basification free tanning system using commercial BCS has been developed for cow sides.23 In the present approach, pickle-basification free chrome tanning has been employed for tanning goatskins that were dehaired and opened-up using a standardised process. The entire pre-tanning and tanning processes have been modified to achieve a low volume of effluent.

Summary Present leather-making processes cause difficulties, with regard to their perceived environmental impact. Pretanning and tanning processes contribute more than 90% of the total pollution load from leather processing. Conventional process methods subject the skin to wide variations in pH. Such pH changes, lead to salt formation which results in a net increase in COD, TDS, chlorides, sulphates and chromium levels in tannery wastewaters. In this study, a three-step process sequence has been explored to limit the pH range in leather processing to between 4.0-8.0. A sequence of operations, viz. enzyme unhairing, NaOH based fibre opening and pickle-less chrome tanning produces leathers matching the functional performance of conventionally processed leathers but leading to substantial environmental benefits in the reduction of COD and total solids by 43 and 70%, respectively. Water consumption for processing is reduced to 17.81 / Kg raw hide. The process sequence explored appears to be economically viable. The physical characteristics, hand evaluation and softness of both experiment and control leathers were measured. Composite liquors from both processes were analysed for COD, TS, and chromium

Integrated three step tanning process: An appraisal

Softness measurements :The three step tanning process employs enzyme for dehairing and an optimized percentage of sodium hydroxide for fibre opening. Hence, it is important to evaluate the softness of the final leather. Quantitative assessment of softness for both control and experimental leathers through compressibility measurements is given in Table III. The log thickness v. log load plots for the control and experimental leathers exhibit a linear fit. The corresponding equation of the line enabled us to calculate the negative slope angles, the values are given in the Table III. Higher negative angles imply greater softness. It is apparent that the experimental leather (E) exhibits slightly higher negative slope angles (compressibility index, CI) compared to the control leather (C). This shows that the extent of opening up of the fibre bundles is comparable to the conventional liming-reliming process.

Scanning electron microscopic analysis : Scanning electron photomicrographs of crust leather samples from control and experimental leather processes showing the grain surface of the leathers at a magnification of x100 are shown in Figs. 2a and 2b. The grain surface of the control leathers seems to be flat / smooth without any wrinkles compared to the experimental leathers. This could be due to the rapid reaction of the chromium with the grain surface as against a mild and gradual fixation of chromium in control leather processing. Both the control and experimental leather samples exhibit clean grain surfaces, which indicates that there is no physical deposition of chromium. Higher magnification scanning electron microphotographs (x500) are shown in Fig. 3a and 3b, which confirm the above observation. The hair follicles look clean without any foreign materials in both cases. Scanning electron photomicrographs showing the cross section of both control and experimental leathers at a magnification of 500 are depicted in Fig. 4a and 4b. The fibre bundles are evenly dispersed (separation of fibres) in both cases. This shows that the opening up of fibre bundles of both control and experimental samples is comparable.

Chromium in leather : Liming-reliming processes are known to remove noncollagenous proteins and loosen the matrix. Loosening makes it easier for the tanning agent, dyestuff, fatliquors and other substances to diffuse into the skin. Hence, while examining the extent of opening up of fibre bundles it is imperative to look at the chromium content in leather. The chromic oxide content in wet blue leathers from both control and experimental processes are given in the Table IV. Experimental leathers contain higher percentages of chromic oxide compared to control leathers and also reveal better exhaustion of chromium. This is due to the presence of carboxyl groups of collagen in ionised form during the entire course of pickle and basification free chrome tanning.