Reversing the Conventional Leather Processing Sequence for Cleaner Leather Production
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Reversing the Conventional LeatherProcessing Sequence for CleanerLeather ProductionS U B R A M A N I S A R A V A N A B H A V A N ,
P A L A N I S A M Y T H A N I K A I V E L A N ,
J O N N A L A G A D D A R A G H A V A R A O , * ,
B A L A C H A N D R A N U N N I N A I R , A N DT H I R U M A L A C H A R I R A M A S A M I
Chemical Laboratory, Centre for Leather Apparels &Accessories Development, Central Leather Research Institute,Adyar, Chennai 600 020, India
Conventional leather processing generally involves acombination of single and multistep processes that employsas well as expels various biological, inorganic, andorganic materials. It involves nearly 14-15 steps anddischarges a huge amount of pollutants. This is primarilydue to the fact that conventional leather processing employsa do-undo process logic. In this study, the conventionalleather processing steps have been reversed to overcomethe problems associated with the conventional method. Thecharges of the skin matrix and of the chemicals and pHprofiles of the process have been judiciously used for reversingthe process steps. This reversed process eventuallyavoids several acidification and basification/neutralizationsteps used in conventional leather processing. Thedeveloped process has been validated through variousanalyses such as chromium content, shrinkage temperature,softness measurements, scanning electron microscopy,and physical testing of the leathers. Further, the performanceof the leathers is shown to be on par with conventionallyprocessed leathers through bulk property evaluation.The process enjoys a significant reduction in COD and TSby 53 and 79%, respectively. Water consumption anddischarge is reduced by 65 and 64%, respectively. Also,the process benefits from significant reduction in chemicals,time, power, and cost compared to the conventionalprocess.
IntroductionConventional leather processing involves four important setsof processes, viz., pre-tanning, tanning, post-tanning, andfinishing. It includes a combination of single and multistepprocesses that employs as well as expels various organicand inorganic materials (1). The conventional method ofleather making involves 14-15 steps comprising soak-ing, liming, reliming, deliming, bating, pickling, chrometanning, basification, rechroming, basification, neutraliza-tion, washing, retanning, dyeing, fatliquoring, and fixing. Thisconventional technique discharges enormous amounts ofwastewater along with pollutants (2). This includes BOD,COD, TDS, sulfides, chlorides, sulfates, chromium, etc.
This is primarily due to the fact that the conventionalleather processing employs do-undo process schemessuch as swell-deswell (liming-deliming), pickle-depickle(pickling-basification), rechroming-basification (acidifica-tion-basification), and neutralization-fixing (basification-acidification) (3). In other words, conventional methodsemployed in leather processing subject the skin/hide to widevariations in pH (4). Such pH changes demand the use ofacids and alkalis, which results in the generation of salts ofcalcium, sodium, and chromium ions. This results in a netincrease in COD, TDS, chlorides, sulfates, and other mineralsin tannery wastewaters (5).
Conventional chrome tanning generally involves pickling,tanning using basic chromium sulfate (BCS), and followedby basification processes. Spent pickle liquor has high TDSand a considerable amount of COD, since pickling involvesthe use of 8-10% sodium chloride salt along with sulfuricacid (2). Spent chrome liquor contains significant amountof chromium, sulfates, and TDS. The conventional methodof post-tanning involves 7-8 major steps comprising re-chroming, basification, neutralization, washing, retanning,dyeing, fatliquoring, and fixing. Post-tanning processesemploy a pH range of 4.0-6.5 and a variety of chemicals.The post-tanning processes contribute significantly to TDS,COD, and heavy metal pollution, as reported by Simonciniand Sammarco (6). Several attempts have been made torender the leather processing steps cleaner (7, 8). However,these improvements are specific to a unit operation. Imple-mentation of all the advanced technologies and eco-friendlychemicals involves financial input and machinery require-ments as well. This calls for the development of integratedleather processing methods and revamping the processsequence.
Very few attempts have been made to revamp the wholeor part of leather processing steps. Thanikaivelan et al. haveattempted to process leather in a narrow pH range from 4to 8.0 (5, 9). Later, a three-step tanning process was developedwhich involves enzymatic dehairing, fiber opening usingenzyme or alkali, and pickleless chrome tanning at pH 8.0(10, 11). Recently, integrated one-step wet finishing processeshave been developed (12, 13). Further, process integrationhas been attempted by combining tanning and post-tanningsteps in one bath (14).
In this study, an attempt has been made to reverse theconventional leather processing steps. This is by treating thedelimed pelts with post-tanning chemicals such as syntans,dye, and fatliquors, followed by chrome tanning at pH 5.0-5.2 (15). The percentage of post-tanning chemicals have beencarefully designed and calculated, taking into account theshaved weight parameters. The performance of the finalleathers has been evaluated in terms of physical as well asorganoleptic properties. Softness of the leathers has beenquantified and compared with that of conventionally pro-cessed leathers. The pollution parameters, such as CODand TS, have been quantified and analyzed. Techno-economic viability of the developed process has also beendiscussed.
Experimental MethodsMaterials. Conventionally delimed/bated goatskins werechosen as the raw material. The chemicals employed forleather processing were of commercial grade. The chemicalsused for analytical techniques were of laboratory grade.
Process Chemistry of Conventional and Reversed LeatherProcesses. In this study, delimed goatskin was chosen as thestarting material for conventional and reversed leather
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Chemical Laboratory. Centre for Leather Apparels & Accessories Development.
Environ. Sci. Technol. 2006, 40, 1069-1075
10.1021/es051385u CCC: $33.50 2006 American Chemical Society VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1069Published on Web 12/23/2005
processing. The delimed goatskin is partially anionic incharge. In the conventional leather process, the delimedgoatskin is treated with sulfuric acid in order to convert thematrix charge into a cationic charge, prior to chrome tanningto avoid surface deposition as shown in eq 1. Salt is used tosuppress the swelling by maintaining the ionic balance ofthe skin matrix.
During chrome tanning, chromium is irreversibly boundwith the collagen matrix by cross linking with collagencarboxylic groups through coordinate covalent linkage, asillustrated in eq 2. Chrome-tanned leather is positive incharge.
Chrome-tanned leathers are neutralized in order to avoidthe surface fixation of negatively charged post-tanningchemicals on the positively charged chromium cross linkedmatrix. This is achieved by using mild alkalis such as sodiumbicarbonate. During neutralization, the pH of the chromiumcross linked matrix is raised to 5.0-5.2 (eq 3). Hence, thepenetration of negatively charged post-tanning chemicals isachieved on a neutral matrix (eq 4).
To fix the post-tanning chemicals, the amino group ofthe chromium cross-linked leather matrix is ionized bybringing down the pH to 3.5-4.0 using formic acid (eq 5).The positively charged amino groups form an electrostaticlinkage with the negatively charged post-tanning chemicals,such as syntans (R(S)-), dyes (R(D)-), and fatliquors (R(F)-),as shown in eq 5.
The modified leather making process reverses the con-ventional process sequence by making use of the chargecharacter of the delimed pelt. The charge of the delimed peltis partially negative. Interestingly, the chemicals used forpost-tanning are also negative in charge. Hence, the post-tanning chemicals are treated with the delimed pelt withoutany problem in penetration, as shown in eq 6.
After treating with post-tanning chemicals, the pH isbrought down to 5.0-5.2 (eq 7); this would not only facilitatethe fixation of the post-tanning chemicals but also providesproper conditions for the application of basic chromiumsulfate salt for pickleless tanning (16). The mechanism ofchrome tanning at this condition is similar to that of picklelesstanning (16). In other words, a simultaneous penetrationcum fixation of chromium molecules would take place (16).The final pH of leather as well as the spent liquor is around3.8-4.0, due to the hydrolysis of chromium molecules. Hence,the amino groups of the collagen matrix, if any, are ionizedand form electro static linkage with the negatively chargedpost-tanning chemicals (eq 8).
Hence, it is seen that the crust leathers processed fromconventional and modified methods are basically similar incharacter (see eqs 5 and 8).
Process Description. Twenty delimed/bated goatskinswere converted into shoe softy upper leathers throughconventional and reversed processes. Ten skins were usedfor each process (for process details and comparativeflowchart for conventional and reversed leather process, seethe Supporting Information).
Objective Assessment of Softness Through Compress-ibility Measurements. Softness of leathers can be numericallymeasured based on their compressibility (17). Circular leatherpieces (2 cm2 area) from experimental and control crustleathers were obtained per IUP method (18) and conditionedat 80 ( 4 F and 65 ( 2% relative humidity over a periodof 48 h. The samples were spread uniformly over the solidbase of the C & R (compressibility and resilience) tester. Theinitial load acting on the grain surface was 100 g. The thicknessat this load was measured 60 s after the load was applied.Subsequent loads were added and the change in thicknesswas recorded one minute after the addition of each load.Logarithm of change in leather thickness (Y axis) was plottedagainst logarithm of load (X axis).
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Physical Testing and Hand Evaluation of Leathers.Samples for various physical tests from experimental andcontrol crust leathers (five each) were obtained per IUPmethod (18). Specimens were conditioned at 80 ( 4F and65 ( 2% relative humidity over a period of 48 h. Physicalproperties such as tensile strength, % elongation at break,tear strength, and grain crack strength were examined as perthe standard procedures (19-21). Five crust leathers fromthe control and five from the experiment were assessed forsoftness, fullness, grain tightness, grain smoothness, andgeneral appearance by hand and visual examination. Theleathers were rated on a scale of 0-10 for each functionalproperty by two experienced tanners; higher scores indicatea better property.
Chromium Content and Shrinkage Temperature ofLeathers. Samples from the official butt portion (18) ofexperimental (wet processed stage) and control wet blueleathers were taken for chromium estimation. A known weight(1 g) of the sample was taken, and the amount of chromiumwas estimated as per standard procedures (22). Samples wereinitially analyzed for moisture content (23), and chromecontent was expressed on the dry weight basis of leather.The shrinkage temperature of the leathers was measuredusing a Theis shrinkage tester (24).
Scanning Electron Microscopic Analysis. Samples fromexperimental and control leathers after crust stage were cutfrom the official sampling position (18). Samples were cutinto specimens with uniform thickness. All specimens werethen coated with gold using an Edwards E306 sputter coater.A JEOL JSM-840A scanning electron microscope (SEM) wasused for the analysis. The micrographs for the grain surfaceand cross section were obtained by operating the SEM athigh vacuum with an accelerating voltage of 15 KV in differentlower and higher magnification levels.
Chromium Exhaustion. Chrome liquor collected fromthe control chrome tanning process was analyzed forchromium content per the standard procedure, and uptakeof chromium was calculated (25). In the case of the reversedleather process, the final liquor was collected and used forthe analysis.
Analysis of Composite Waste Liquor. Composite liquorsfrom control and experimental processes were collected fromall the unit operations except pre-tanning processes (soakingto deliming) and analyzed for COD and TS (dried at 103-105C for 1 h) per the standard procedures (26). From this,emission loads were calculated by multiplying concentration(mg/L) with volume of effluent (L) per metric ton of rawskins processed.
Results and DiscussionChromium in Leather and Spent Tan Liquor. The amountof chromium present in the leather and spent tan liquor hasbeen analyzed to assess the chromium uptake behavior ofthe reversed process. The amount of chromium present inthe leathers is given in Table 1. The leathers from the reversedprocess possess a higher amount of chromium compared tothe control leathers. This is due to the presence of carboxylgroups of collagen in ionized form during the entire courseof chrome tanning of the reversed process, as shown in eqs
7-8. The mechanism of this process is similar to that of apickle-basification-free chrome tanning process (16). Thechromium uptake values of conventional and reversedprocesses are presented in Table 1. It is seen that the uptakeof chromium is significantly increased in the reversed processcompared to the conventional process. This is in accordancewith the trend observed in the chrome content of leathers.The chromium concentration of the spent tan liquor fromconventional and reversed process is 2286 and 528 ppm,respectively. The shrinkage temperature of leathers from bothcontrol and reversed processes is more than 120 C.
Softness Measurement. Objective assessment of softnessfor both control and experimental leathers has been madethrough compressibility measurements. Softness is directlyproportional to compressibility of the leather. Hence, thelogarithm of change in thickness was plotted againstlogarithm of change in load for the control and experimentalleathers, which exhibited a linear fit (17). The plots are shownin Figure 1. The line equation was obtained from the fit. Thenegative slope angles were calculated from the line equationand the values are 8.40 and 8.46 for control and experimentalleathers. It has been reported that the negative slope angleof a very soft sheep-based glove leather and for a soft graingarment leather was 8.56 and 5.69, respectively (17). Highernegative slope angles imply more softness in the leather. Inthis study, it is seen that both the control and the experimentalleathers exhibit a higher negative slope angle comparable tothe values of very soft leathers. In other words, t...