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Free of water tanning using CO2 as process additive—An overview on the process development
J. of Supercritical Fluids 66 (2012) 291–296
Contents lists available at SciVerse ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Free of water tanning using CO2 as process additive—An overview on the process development Renner Manfred ∗ , Weidner Eckhard, Jochems Björn, Geihsler Helmut Fraunhofer Institute UMSICHT, Osterfelder Str. 3, 46047 Oberhausen, Germany
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Article history: Received 2 November 2011 Received in revised form 16 January 2012 Accepted 16 January 2012 Keywords: Free of water Tanning Animal skin Carbon dioxide Process intensification
a b s t r a c t Leather is a product traded worldwide. The majority of leathers are cattle skins tanned by using chromium-III salt, which are in the focus of this work. The article describes the shortening of tanning time by using compressed carbon dioxide as process additive from 30 to 5 h in lab-scale (63 mL autoclave). In pilot-scale (20 L autoclave) a tanning process practically free of wastewater is demonstrated. Compared to conventional processes less chromium-III salt is used and the process time can be reduced to 2.5 h. Some underlying principles for the process intensification caused by carbon dioxide are described. The article concludes with first results from a demonstration plant of technical size with a volume of 1700 L, which is able to tan a mass of up to 700 kg (>100 m2 ) per batch in a rotating drum. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Tanning of leather is one of the oldest human technologies. In 100,000 B.C. the utilization of leather and fur allowed the Neanderthals to survive in climatically not acceptable regions [1,2]. At that time the animal hides were conserved by using smoke. Later, humans chewed fats into the skin. In 2010 a 5500-year-old laceup shoe tanned by vegetable tanning agents was found [3]. In 3000 B.C. the Egyptians imported aluminum salts over thousands of kilometers for tanning for example loincloths of priests [4]. In 1858 tanning with chromium salts was developed by Knapp [5]. This method revolutionized the tanning industry due to the high quality of chromium tanned leather [6]. Nowadays over 2000 km2 of leather are produced every year. To produce this amount, about 7 million tons of skin, 500,000 tons of salt, 500,000 tons of chromium-III salt are needed. The turnover of leather as an intermediate product is approx. US $45 billion. Approximately 90% of all leather is tanned by using chromiumIII salt. After China the EU is the second largest producer in the world. Chinas production capacity has doubled since 1998 and was 613 million m2 in 2006. In 2007 the European tanneries produced approx. 325 million m2 in 1650 tanneries with 26,000 employees [7–9]. Leather manufacturing is extremely intensive with respect to raw materials and work. The raw materials account for 50–70% of the production costs, wages for 7–15%, chemicals for 10% and
∗ Corresponding author. Tel.: +49 208 8598 1411; fax: +49 208 8598 1424. E-mail address: [email protected] (R. Manfred). 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2012.01.007
energy for 3%. An overall amount of about 14 million m3 of wastewater is generated worldwide. According to estimations the costs of environmental protection measures in European tanneries account for approx. 5% of their turnover [10]. The high amount of consumables and chemicals used illustrates the enormous demand for an intensified and environmentally friendly process. The high consumption of fresh water, mostly drinking water, and the strong contamination of water strongly influence on the perception of leather as a “natural product”. The new process principle described in the following combines advantages of the conventional method, mainly high quality, with the possibility of considerable savings in process time, chemicals and water. The following section provides a brief overview on the most relevant process steps of the conventional tanning process. Tanning methods can be categorized into three groups regarding the tanning agents. It is possible to tan with metal salts like chromium and aluminum, vegetable tanning agents like quebracho and tara and synthetic tanning agents like syntane. During the process the skin has to be prepared for an optimal absorption of the tanning agent into the skin prior to cross-bonding between skin collagen and tanning agent [11–14]. 1.1. Conventional tanning process For conservation purposes, skins are usually salted and dried for the transport to tanneries or they are transported in chilled or frozen form. In a tannery, the skins are washed and softened at the beginning of the production cycle by washing out salt and adjusting moisture. This so-called softening is carried out at
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equipment described in Section 2.4. In initial experiments, a surplus of tanning solution was applied, similar to conventional tanning. The dependency of chromium content and shrinkage temperature from the time of tanning and pressure was investigated. In the course of the development an optimized method was found, where the amount of water was minimized, resulting in a process practically free of wastewater. For that purpose a part of the water present in the skin after the pickling step was removed. This water was replaced by tanning solution 2. The skin thus pretreated was introduced into high pressure equipment and treated under different pressures at varying contact times. The quality was assessed either by measuring the chromium content and/or shrinkage temperature. For both procedures it was found that leather of very high quality can be obtained at a drastically reduced time of contact by carrying out the tanning step above a certain pressure of CO2 . Additionally, wastewater, containing chromium can be reduced to almost zero. 2. Materials and methods 2.1. Tanning solution
Fig. 1. Chromium complex bound to skin collagen [25].
Similar to conventional tanning, chromium-III salt was used as tanning agent for all experiments. The tanning solution consisted of water, chromium-III salt, salt, formic and sulfuric acid. In the tanning industry all weights are related to the weight of the wet skin before the tanning step (example: 150 g water on 100 g wet skin means 1:1.5).
pH-values of 7–9. In the next step, the liming – i.e. the fatty top skin – is destroyed and the hair is removed from the skin by adding lime and sulfur compounds. Proteins are washed out and breaking of polypeptide groups leads to higher fibre mobility. During liming collagen is “exposed” and the penetration of tanning agents into the skin is facilitated. The high alkalinity of the lime (pH > 10) is strongly reduced for the next working step, the pickling process. This is essential because the skin swells at high pH values and swelling can lead to the destruction of collagen chains. During the pickle, the skin is prepared for the tanning process at a pH value of 2.5 (8–20 h). Lime residues are thus neutralized and removed and the diffusion capability for the tanning agents is increased further [15]. The next step is tanning, taking place in rotating tanning drums over a period of 12–20 h. The skins are brought into intensive contact with liquid containing a tanning agent, usually a solution of water and chromium-III salts. During this stage, the pH value is adjusted from 2.5 at the beginning of the process to 3.8 at the end of the process [16–18]. This permits optimum penetration of the tanning agent at a low pH value and by steadily raising the pH value the tanning agent bonds with the reactive groups of the skin collagen, the carboxyl groups [19,20]. This complexation is shown in Fig. 1. If chromium interlinks collagen molecules via chemical bonds this results in a high mechanical strength and cooking stability of the leather [23]. The tanning can be called successful if 3.8 to 5% of Cr2 O3 by weight are absorbed in the leather [21]. This concentration is used as a first criterion, comparatively easy to monitor, to define high leather quality for trials on laboratory scale [22]. Due to its color, the intermediate product generated in this step is called “wet-blue”. Subsequently finishing wet blue with colors, oils and specific chemicals brings about different forms of appearance like smooth leather for clothes, stiff leather for shoes or soft leathers for furniture [17,24].
An average cattle skin has a surface area of 7–9 m2 . It is divided into several parts. The part with the highest quality and the most regular structure is the so-called “croupon”. The neck (described in Sections 3.1 and 3.2) and the croupon (described in Section 3.3) were taken for this work. The neck had a thickness of 2–2.4 mm and the croupon one of 2.6–3 mm. Both were obtained by splitting the whole skin with a band knife. The neck was taken as one part per trial (area of about 0.25 m2 ). The croupon was divided into 5 parts (with an area of about 0.2 m2 per part). For the experiments shown in Section 3.3 the parts 1–5 (see Fig. 2) were divided further into 4 equally sized parts.
1.2. CO2 -intensified tanning process
2.3. Quality assessment
The basic idea of the new process is to carry out the tanning step under the pressure of CO2 . Limed and pickled skins were provided by a tannery as a whole or in parts as described in Section 2.2. The skins were put into contact with either of two tanning solutions (Section 2.1). For comparison the tanning was performed at ambient pressure and at elevated pressures in a CO2 -atmosphere in the
Chromium tanned leather has to be resistant against boiling water without degeneration of the collagenous structure. As cooking stability is most commonly used in industry, the quality assessment of wet-blue in this work was carried out by measuring the shrinkage temperature. This criterion was assessed by using the equipment shown in Fig. 3. After the tanning process a defined part
2.1.1. Tanning solution 1 Tanning solution 1 consisted of 1:10 water, 1:0.09 chromiumIII salt (Baychrom F), 1:0.005 formic acid, 1:0.008 sulfuric acid and 1:0.12 sodium chloride. That means that a mass of wet skin of 1 kg is contacted with 10 kg of water, 0.09 kg of Baychrom F, 0.005 kg of formic acid, 0.008 kg of sulfuric acid and 0.12 kg of salt. 2.1.2. Tanning solution 2 Tanning solution 2 consisted of 1:0.3 water (previously removed from the wet skin), 1:0.05 chromium-III salt (Baychrom F), 1:0.0003 formic acid and 1:0.0003 sulfuric acid. That means that from 1 kg mass of wet skin a mass of 0.3 kg of water is removed and replaced by 0.3 kg of water, 0.09 kg of Baychrom F, 0.0003 kg of formic acid and 0.0003 kg of sulfuric acid. 2.2. Skin
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Fig. 5. Reduction of process time using CO2 as process intensifier in lab-scale [26].
horizontal autoclave was used for stirring the tanning solution at a speed of 40 rpm. Fig. 2. Parts of cattle skin.
2.4.2. 20 L high pressure tanning bin The PLC-controlled “pilot-scale plant” was especially build for high pressure tanning. Inside the horizontal autoclave a rotatable cage can be used for tanning comparable to industrial procedures. In order to ensure that the skin moves around continuously inside the cage some metal pins were screwed in vertically. The motion is relevant for mass transport and regularity of the chromium distribution. The maximum working parameters were 32 MPa, 80 ◦ C and 20 rpm.
of the leather was punched out. This sample was placed firmly with an interlock on one side of the apparatus. The other side was connected by a yarn using a hook. A weight tightened the system. The sample was subsequently submerged into water. The water was heated up until it boiled. When the leather sample shrinked at a temperature below 100 ◦ C the quality was not sufficient.
2.4.3. 1700 L high pressure tanning bin To carry out test series in preindustrial scale a “demonstration plant” with a volume of 1700 L was build. The autoclave is also positioned horizontally. A mass of up to 700 kg (>100 m2 leather) can be tanned per batch in a rotating drum. The design of the drum is comparable to conventional pressure-less tanning bins. The use of pins, mounted to the inner side of the rotating drum, intensifies the motion of the skins and thus a good contact of gas and tanning agent. The equipment allows the feeding of tanning liquids against pressures of up to 26 MPa.
2.4. High pressure equipment
3. Results
2.4.1. 63 mL high pressure view cell A conventional high pressure cell was used for carrying out the experiments presented in Fig. 4. The autoclave has a volume of 63 mL and a maximum of 250 ◦ C and 35 MPa could be reached. A height adjustable magnetic stirrer mounted in the middle of the
3.1. Chromium tanning under CO2 -pressure as function of pressure and contact time
Fig. 3. Measurement of shrinkage temperature.
The results demonstrated in Fig. 5 (63 mL autoclave) and Fig. 6 (pilot-scale plant) were carried out comparable to conventional
Fig. 4. Left: lab-scale equipment; middle: 20 L pilot-scale plant; right: 1700 L demonstration plant.
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Fig. 6. Process time using CO2 as process intensifier in pilot-scale.
processing using an aqueous phase as reservoir for the mass transport of chromium ions (tanning solution 1). A detailed description of the processing is given by Renner et al. [26]. Fig. 5 shows the comparison of conventional processing and processing under the influence of compressed carbon dioxide at 10 MPa. The skin samples had a diameter of 2 cm. Mass ratios between skin and solution 1 were identical in all experiments. During tanning each skin sample was tanned individually. The samples were wetted by the tanning solution, not submerged. The quality of the leathers produced was assessed using emission spectrometry, by measuring the reduction of the chromium content of the tanning solution during tanning. With 3 wt% of chromium in skin (equal to 4 wt% of Cr2 O3 ) a high quality is reached. A shortening of process time from 30 to 5 h using carbon dioxide at 10 MPa could be proved. Chromium content and maximum uptake of 3.6 wt% chromium are comparable to industrial values [27,28]. Similar observations in lab-scale have been published by other workgroups [29–31]. The results were transferred into pilot-scale. The skin samples weighted about 700–900 g. The tanning sessions were carried out at the same tanning solution to hide mass ratios as in laboratory scale. The basket rotation was set to 10 rpm and the hide was pulled continuously through the tanning solution as is done in the conventional process. Tanning sessions that produced high leather quality are shown in the form of plus signs, and those that produced lower quality in the form of minus signs. Fig. 6 shows that tanning time can be shortened to 2.5 h using 20 and 30 MPa and to 3 h using 10 MPa. The main focus of the investigation was on whether the shrinkage criterion of cooking stability was reached or not. It became apparent that at 30 MPa, no significant reduction of the tanning time versus 10 MPa can be achieved. 3.2. Tanning free of chromium contaminated wastewater One of the most important problems of the tanning industry is the wastewater after the tanning step, which is highly charged with chromium and salt. The objective of the investigation presented in this section was the reduction of chemicals and chromium contaminated wastewater. The results shown in Fig. 7 were obtained in a modified process. The skins were partly dewetted mechanically before tanning. This removed mass was substituted by tanning solution 2, which was soaked up into the skin in 30 min.
Fig. 7. Process time using CO2 at different pressures in pilot-scale.
Tanning those skins in our equipment without applying gas pressure, results in bad quality (not shown in Fig. 7). For carrying out experiments under pressure, the soaked skin was introduced into the pressure vessel and CO2 -pressure was built up in some minutes. During the high-pressure process the chromium ions diffuse to the reactive sites (carboxyl groups) of the skin collagen. Fig. 7 shows the influence of pressure and time. The values for 10 MPa were taken from Fig. 6 and are used to compare the results obtained with solution 1 and solution 2. It can be seen that the tanning time can be further reduced, even at lower pressures. The trials carried out at 2 MPa of carbon dioxide pressure failed to produce high leather quality. A threshold pressure could be identified at 3 MPa. At a time of 2 h at 3 MPa and at 5 MPa positive and negative results were achieved, indicated by overlapping of plus and minus in Fig. 7. As mentioned in Section 2.2 different parts of the skin have been tanned. Some parts were of high quality even after 2 h, while others did not reach sufficient cooking stability. At such low times, variations in skin properties (pore distribution, fiber compactness) are of relevance for the tanning results. At contact times of 2.5 h and beyond, all parts (either from croupon or neck) were of high quality. A wastewater reduction of more than 90% was achieved with this approach. With a conventional tanning process, approx. 1–2 tons of chromium loaded water is generated during the production of one ton of leather. The new method produces less than 10 kg of wastewater per ton of leather. The amount of chromium-III salt used can be reduced by about 45%, the amount of sulfuric and formic acid by about 95%. The further reduced process time can be attributed to the greater surface area available for the diffusion of carbon dioxide. In the approach applied in lab-scale experiments, lower parts of the hide were immersed in the tanning solution, while the upper side was in direct contact with CO2 . The gas could predominantly diffuse into the skin via the upper side. In pilot-scale, experiments (Fig. 6) with “free-flowing tanning” solution 1 were performed. In difference to lab-scale, the skins were moved by means of the rotating drum, thus repeatedly exposing both upper and lower side of the skin surface to CO2 . By this effect, the required contact time for a good quality was reduced from 5 h in lab-scale to 3 h in pilot scale. In a second approach, no free flowing liquid (tanning solution 2) was present in the autoclave. The samples were moved mechanically. Due to the lack of free liquid, they have been in continuous contact with the gas phase. This method resulted in a further (slight) reduction of tanning time (3 h to 2.5 h). Even more important than time saving
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Fig. 8. Leather shrinkage by variation of pressure and time.
is the avoidance of waste water and the reduction of the required amount of chromium and salt by applying this modified tanning process. 3.3. Influence of pressure using a minimum of chemicals for different parts of skin The aim of this investigation was to identify conditions for the tanning of all parts of skin with varying cross sections and fiber compactness. The ratio of masses was calculated as shown in Section 2.1.2. For each point in Fig. 8 four samples were produced and assessed. The average weight of the samples was around 600 g. A process time of 2 h results in leathers with a shrinkage temperature (TS ) between 86 and 92 ◦ C. For 30 and 4 MPa TS does not increase above 93 ◦ C at a tanning of 4 h. Using 5 MPa TS can be increased from 95 to 97 ◦ C at a tanning time between 3 and 6 h. A shrinkage temperature of 100 ◦ C can be obtained after 2.5 h and 6 MPa for all investigated parts of the skin. All 20 samples showed cooking stability. 4. Discussion and conclusions The results demonstrate the effectiveness of tanning, which is considerably intensified by application of pressurized CO2 . In labscale and pilot-scale tanning time could be shortened by factors of 3–6 using CO2 as process intensifier at 10 MPa. Trials in pilotscale showed that increasing pressure from 10 to 30 MPa has no significant influence. Regarding the tanning time the mass of the liquid phase in the autoclave looms large. The larger the area for the diffusion of compressed carbon dioxide into the skin the shorter the time needed for tanning is. In a new approach it was found that a surplus of (free) tanning liquid in the autoclave could be avoided if tanning is carried out under CO2 -pressure. In that case the tanning time could be shortened to 2.5–3 h. Furthermore it was found that the pressure required for obtaining a good quality can be as low as 3 MPa. Reducing the overall mass of water in the tanning bins requires an increase of the concentration of chromium salts in the tanning solution, to achieve a sufficient quality of the leather. Absorbing water with high chromium content into partly dewetted skins is resulting in a higher concentration gradient of chromium. Thus diffusion is accelerated. Additionally the skin matrix is most likely widened by CO2 . This behavior was investigated for several complex solids, including certain biopolymers [32–37]. Regarding the sizes of chromium
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polynucleate complexes diffusing into the skin, even a slight widening has a great influence on penetration. According to Nishad [39], Covington [40] and Braeumer [41], penetration is not the time limiting step, but most of the process time is taken by the diffusion inside the collagen microfibrils [41]. The chromium complexes have a size of about 0.75 nm and 1.29 nm [38]. The polynucleate complexes have to diffuse between the fibrils for reaching reactive binding sites [42]. The gap between the microfibrils is estimated to be 1.4 nm [43]. The “diffusion tunnels” in the fibrils can take up a length of several centimeters [44–47]. Having no (free) liquid phase outside the leather CO2 can penetrate directly over the surface into the skin. This way the mass transport is influenced positively. First trials carried out at Fraunhofer UMSICHT using a demonstration plant (1700 L autoclave volume) with a rotating drum showed very promising results. An area of over 100 m2 per batch can be treated now. It was found that lower pressure than in pilot scale resulted in very high leather quality, probably because of an intensified flexing of the material. Flexing is improved from smaller to bigger skins which, due to their size and shape, are able to twist. Torsion seems to improve the mass transport of tanning agents and CO2 . This assumption is confirmed by know-how from conventional tanning [48]. To sum up it can be noted that the newly introduced tanning process bears a great potential for environmental protection. The process time for tanning can be considerably reduced. Chromium contaminated wastewater can be reduced by over 95%. Theoretically, this could result in savings of 14 billion liters per year. 160,000 tons of chromium can be saved per year because now it is only necessary to use just as much chromium as is needed for producing high leather quality. Overdosing can be reduced considerably. This reduction of the tanning agent may lead to saving another 6 billion liters of water which would be necessary for the production of the chromium.
Acknowledgments The authors would like to thank the German Ministry of Education and Research for the financial support. Furthermore, the authors would like to thank Helmut Geihsler and the students Jan Rech, Björn Jochems and Anna Oelbermann for their assistance with the investigations.
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Free of water tanning using CO2 as process additive—An overview on the process development Renner Manfred ∗ , Weidner Eckhard, Jochems Björn, Geihsler Helmut Fraunhofer Institute UMSICHT, Osterfelder Str. 3, 46047 Oberhausen, Germany
a r t i c l e
i n f o
Article history: Received 2 November 2011 Received in revised form 16 January 2012 Accepted 16 January 2012 Keywords: Free of water Tanning Animal skin Carbon dioxide Process intensification
a b s t r a c t Leather is a product traded worldwide. The majority of leathers are cattle skins tanned by using chromium-III salt, which are in the focus of this work. The article describes the shortening of tanning time by using compressed carbon dioxide as process additive from 30 to 5 h in lab-scale (63 mL autoclave). In pilot-scale (20 L autoclave) a tanning process practically free of wastewater is demonstrated. Compared to conventional processes less chromium-III salt is used and the process time can be reduced to 2.5 h. Some underlying principles for the process intensification caused by carbon dioxide are described. The article concludes with first results from a demonstration plant of technical size with a volume of 1700 L, which is able to tan a mass of up to 700 kg (>100 m2 ) per batch in a rotating drum. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Tanning of leather is one of the oldest human technologies. In 100,000 B.C. the utilization of leather and fur allowed the Neanderthals to survive in climatically not acceptable regions [1,2]. At that time the animal hides were conserved by using smoke. Later, humans chewed fats into the skin. In 2010 a 5500-year-old laceup shoe tanned by vegetable tanning agents was found [3]. In 3000 B.C. the Egyptians imported aluminum salts over thousands of kilometers for tanning for example loincloths of priests [4]. In 1858 tanning with chromium salts was developed by Knapp [5]. This method revolutionized the tanning industry due to the high quality of chromium tanned leather [6]. Nowadays over 2000 km2 of leather are produced every year. To produce this amount, about 7 million tons of skin, 500,000 tons of salt, 500,000 tons of chromium-III salt are needed. The turnover of leather as an intermediate product is approx. US $45 billion. Approximately 90% of all leather is tanned by using chromiumIII salt. After China the EU is the second largest producer in the world. Chinas production capacity has doubled since 1998 and was 613 million m2 in 2006. In 2007 the European tanneries produced approx. 325 million m2 in 1650 tanneries with 26,000 employees [7–9]. Leather manufacturing is extremely intensive with respect to raw materials and work. The raw materials account for 50–70% of the production costs, wages for 7–15%, chemicals for 10% and
∗ Corresponding author. Tel.: +49 208 8598 1411; fax: +49 208 8598 1424. E-mail address: [email protected] (R. Manfred). 0896-8446/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2012.01.007
energy for 3%. An overall amount of about 14 million m3 of wastewater is generated worldwide. According to estimations the costs of environmental protection measures in European tanneries account for approx. 5% of their turnover [10]. The high amount of consumables and chemicals used illustrates the enormous demand for an intensified and environmentally friendly process. The high consumption of fresh water, mostly drinking water, and the strong contamination of water strongly influence on the perception of leather as a “natural product”. The new process principle described in the following combines advantages of the conventional method, mainly high quality, with the possibility of considerable savings in process time, chemicals and water. The following section provides a brief overview on the most relevant process steps of the conventional tanning process. Tanning methods can be categorized into three groups regarding the tanning agents. It is possible to tan with metal salts like chromium and aluminum, vegetable tanning agents like quebracho and tara and synthetic tanning agents like syntane. During the process the skin has to be prepared for an optimal absorption of the tanning agent into the skin prior to cross-bonding between skin collagen and tanning agent [11–14]. 1.1. Conventional tanning process For conservation purposes, skins are usually salted and dried for the transport to tanneries or they are transported in chilled or frozen form. In a tannery, the skins are washed and softened at the beginning of the production cycle by washing out salt and adjusting moisture. This so-called softening is carried out at
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equipment described in Section 2.4. In initial experiments, a surplus of tanning solution was applied, similar to conventional tanning. The dependency of chromium content and shrinkage temperature from the time of tanning and pressure was investigated. In the course of the development an optimized method was found, where the amount of water was minimized, resulting in a process practically free of wastewater. For that purpose a part of the water present in the skin after the pickling step was removed. This water was replaced by tanning solution 2. The skin thus pretreated was introduced into high pressure equipment and treated under different pressures at varying contact times. The quality was assessed either by measuring the chromium content and/or shrinkage temperature. For both procedures it was found that leather of very high quality can be obtained at a drastically reduced time of contact by carrying out the tanning step above a certain pressure of CO2 . Additionally, wastewater, containing chromium can be reduced to almost zero. 2. Materials and methods 2.1. Tanning solution
Fig. 1. Chromium complex bound to skin collagen [25].
Similar to conventional tanning, chromium-III salt was used as tanning agent for all experiments. The tanning solution consisted of water, chromium-III salt, salt, formic and sulfuric acid. In the tanning industry all weights are related to the weight of the wet skin before the tanning step (example: 150 g water on 100 g wet skin means 1:1.5).
pH-values of 7–9. In the next step, the liming – i.e. the fatty top skin – is destroyed and the hair is removed from the skin by adding lime and sulfur compounds. Proteins are washed out and breaking of polypeptide groups leads to higher fibre mobility. During liming collagen is “exposed” and the penetration of tanning agents into the skin is facilitated. The high alkalinity of the lime (pH > 10) is strongly reduced for the next working step, the pickling process. This is essential because the skin swells at high pH values and swelling can lead to the destruction of collagen chains. During the pickle, the skin is prepared for the tanning process at a pH value of 2.5 (8–20 h). Lime residues are thus neutralized and removed and the diffusion capability for the tanning agents is increased further [15]. The next step is tanning, taking place in rotating tanning drums over a period of 12–20 h. The skins are brought into intensive contact with liquid containing a tanning agent, usually a solution of water and chromium-III salts. During this stage, the pH value is adjusted from 2.5 at the beginning of the process to 3.8 at the end of the process [16–18]. This permits optimum penetration of the tanning agent at a low pH value and by steadily raising the pH value the tanning agent bonds with the reactive groups of the skin collagen, the carboxyl groups [19,20]. This complexation is shown in Fig. 1. If chromium interlinks collagen molecules via chemical bonds this results in a high mechanical strength and cooking stability of the leather [23]. The tanning can be called successful if 3.8 to 5% of Cr2 O3 by weight are absorbed in the leather [21]. This concentration is used as a first criterion, comparatively easy to monitor, to define high leather quality for trials on laboratory scale [22]. Due to its color, the intermediate product generated in this step is called “wet-blue”. Subsequently finishing wet blue with colors, oils and specific chemicals brings about different forms of appearance like smooth leather for clothes, stiff leather for shoes or soft leathers for furniture [17,24].
An average cattle skin has a surface area of 7–9 m2 . It is divided into several parts. The part with the highest quality and the most regular structure is the so-called “croupon”. The neck (described in Sections 3.1 and 3.2) and the croupon (described in Section 3.3) were taken for this work. The neck had a thickness of 2–2.4 mm and the croupon one of 2.6–3 mm. Both were obtained by splitting the whole skin with a band knife. The neck was taken as one part per trial (area of about 0.25 m2 ). The croupon was divided into 5 parts (with an area of about 0.2 m2 per part). For the experiments shown in Section 3.3 the parts 1–5 (see Fig. 2) were divided further into 4 equally sized parts.
1.2. CO2 -intensified tanning process
2.3. Quality assessment
The basic idea of the new process is to carry out the tanning step under the pressure of CO2 . Limed and pickled skins were provided by a tannery as a whole or in parts as described in Section 2.2. The skins were put into contact with either of two tanning solutions (Section 2.1). For comparison the tanning was performed at ambient pressure and at elevated pressures in a CO2 -atmosphere in the
Chromium tanned leather has to be resistant against boiling water without degeneration of the collagenous structure. As cooking stability is most commonly used in industry, the quality assessment of wet-blue in this work was carried out by measuring the shrinkage temperature. This criterion was assessed by using the equipment shown in Fig. 3. After the tanning process a defined part
2.1.1. Tanning solution 1 Tanning solution 1 consisted of 1:10 water, 1:0.09 chromiumIII salt (Baychrom F), 1:0.005 formic acid, 1:0.008 sulfuric acid and 1:0.12 sodium chloride. That means that a mass of wet skin of 1 kg is contacted with 10 kg of water, 0.09 kg of Baychrom F, 0.005 kg of formic acid, 0.008 kg of sulfuric acid and 0.12 kg of salt. 2.1.2. Tanning solution 2 Tanning solution 2 consisted of 1:0.3 water (previously removed from the wet skin), 1:0.05 chromium-III salt (Baychrom F), 1:0.0003 formic acid and 1:0.0003 sulfuric acid. That means that from 1 kg mass of wet skin a mass of 0.3 kg of water is removed and replaced by 0.3 kg of water, 0.09 kg of Baychrom F, 0.0003 kg of formic acid and 0.0003 kg of sulfuric acid. 2.2. Skin
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Fig. 5. Reduction of process time using CO2 as process intensifier in lab-scale [26].
horizontal autoclave was used for stirring the tanning solution at a speed of 40 rpm. Fig. 2. Parts of cattle skin.
2.4.2. 20 L high pressure tanning bin The PLC-controlled “pilot-scale plant” was especially build for high pressure tanning. Inside the horizontal autoclave a rotatable cage can be used for tanning comparable to industrial procedures. In order to ensure that the skin moves around continuously inside the cage some metal pins were screwed in vertically. The motion is relevant for mass transport and regularity of the chromium distribution. The maximum working parameters were 32 MPa, 80 ◦ C and 20 rpm.
of the leather was punched out. This sample was placed firmly with an interlock on one side of the apparatus. The other side was connected by a yarn using a hook. A weight tightened the system. The sample was subsequently submerged into water. The water was heated up until it boiled. When the leather sample shrinked at a temperature below 100 ◦ C the quality was not sufficient.
2.4.3. 1700 L high pressure tanning bin To carry out test series in preindustrial scale a “demonstration plant” with a volume of 1700 L was build. The autoclave is also positioned horizontally. A mass of up to 700 kg (>100 m2 leather) can be tanned per batch in a rotating drum. The design of the drum is comparable to conventional pressure-less tanning bins. The use of pins, mounted to the inner side of the rotating drum, intensifies the motion of the skins and thus a good contact of gas and tanning agent. The equipment allows the feeding of tanning liquids against pressures of up to 26 MPa.
2.4. High pressure equipment
3. Results
2.4.1. 63 mL high pressure view cell A conventional high pressure cell was used for carrying out the experiments presented in Fig. 4. The autoclave has a volume of 63 mL and a maximum of 250 ◦ C and 35 MPa could be reached. A height adjustable magnetic stirrer mounted in the middle of the
3.1. Chromium tanning under CO2 -pressure as function of pressure and contact time
Fig. 3. Measurement of shrinkage temperature.
The results demonstrated in Fig. 5 (63 mL autoclave) and Fig. 6 (pilot-scale plant) were carried out comparable to conventional
Fig. 4. Left: lab-scale equipment; middle: 20 L pilot-scale plant; right: 1700 L demonstration plant.
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Fig. 6. Process time using CO2 as process intensifier in pilot-scale.
processing using an aqueous phase as reservoir for the mass transport of chromium ions (tanning solution 1). A detailed description of the processing is given by Renner et al. [26]. Fig. 5 shows the comparison of conventional processing and processing under the influence of compressed carbon dioxide at 10 MPa. The skin samples had a diameter of 2 cm. Mass ratios between skin and solution 1 were identical in all experiments. During tanning each skin sample was tanned individually. The samples were wetted by the tanning solution, not submerged. The quality of the leathers produced was assessed using emission spectrometry, by measuring the reduction of the chromium content of the tanning solution during tanning. With 3 wt% of chromium in skin (equal to 4 wt% of Cr2 O3 ) a high quality is reached. A shortening of process time from 30 to 5 h using carbon dioxide at 10 MPa could be proved. Chromium content and maximum uptake of 3.6 wt% chromium are comparable to industrial values [27,28]. Similar observations in lab-scale have been published by other workgroups [29–31]. The results were transferred into pilot-scale. The skin samples weighted about 700–900 g. The tanning sessions were carried out at the same tanning solution to hide mass ratios as in laboratory scale. The basket rotation was set to 10 rpm and the hide was pulled continuously through the tanning solution as is done in the conventional process. Tanning sessions that produced high leather quality are shown in the form of plus signs, and those that produced lower quality in the form of minus signs. Fig. 6 shows that tanning time can be shortened to 2.5 h using 20 and 30 MPa and to 3 h using 10 MPa. The main focus of the investigation was on whether the shrinkage criterion of cooking stability was reached or not. It became apparent that at 30 MPa, no significant reduction of the tanning time versus 10 MPa can be achieved. 3.2. Tanning free of chromium contaminated wastewater One of the most important problems of the tanning industry is the wastewater after the tanning step, which is highly charged with chromium and salt. The objective of the investigation presented in this section was the reduction of chemicals and chromium contaminated wastewater. The results shown in Fig. 7 were obtained in a modified process. The skins were partly dewetted mechanically before tanning. This removed mass was substituted by tanning solution 2, which was soaked up into the skin in 30 min.
Fig. 7. Process time using CO2 at different pressures in pilot-scale.
Tanning those skins in our equipment without applying gas pressure, results in bad quality (not shown in Fig. 7). For carrying out experiments under pressure, the soaked skin was introduced into the pressure vessel and CO2 -pressure was built up in some minutes. During the high-pressure process the chromium ions diffuse to the reactive sites (carboxyl groups) of the skin collagen. Fig. 7 shows the influence of pressure and time. The values for 10 MPa were taken from Fig. 6 and are used to compare the results obtained with solution 1 and solution 2. It can be seen that the tanning time can be further reduced, even at lower pressures. The trials carried out at 2 MPa of carbon dioxide pressure failed to produce high leather quality. A threshold pressure could be identified at 3 MPa. At a time of 2 h at 3 MPa and at 5 MPa positive and negative results were achieved, indicated by overlapping of plus and minus in Fig. 7. As mentioned in Section 2.2 different parts of the skin have been tanned. Some parts were of high quality even after 2 h, while others did not reach sufficient cooking stability. At such low times, variations in skin properties (pore distribution, fiber compactness) are of relevance for the tanning results. At contact times of 2.5 h and beyond, all parts (either from croupon or neck) were of high quality. A wastewater reduction of more than 90% was achieved with this approach. With a conventional tanning process, approx. 1–2 tons of chromium loaded water is generated during the production of one ton of leather. The new method produces less than 10 kg of wastewater per ton of leather. The amount of chromium-III salt used can be reduced by about 45%, the amount of sulfuric and formic acid by about 95%. The further reduced process time can be attributed to the greater surface area available for the diffusion of carbon dioxide. In the approach applied in lab-scale experiments, lower parts of the hide were immersed in the tanning solution, while the upper side was in direct contact with CO2 . The gas could predominantly diffuse into the skin via the upper side. In pilot-scale, experiments (Fig. 6) with “free-flowing tanning” solution 1 were performed. In difference to lab-scale, the skins were moved by means of the rotating drum, thus repeatedly exposing both upper and lower side of the skin surface to CO2 . By this effect, the required contact time for a good quality was reduced from 5 h in lab-scale to 3 h in pilot scale. In a second approach, no free flowing liquid (tanning solution 2) was present in the autoclave. The samples were moved mechanically. Due to the lack of free liquid, they have been in continuous contact with the gas phase. This method resulted in a further (slight) reduction of tanning time (3 h to 2.5 h). Even more important than time saving
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Fig. 8. Leather shrinkage by variation of pressure and time.
is the avoidance of waste water and the reduction of the required amount of chromium and salt by applying this modified tanning process. 3.3. Influence of pressure using a minimum of chemicals for different parts of skin The aim of this investigation was to identify conditions for the tanning of all parts of skin with varying cross sections and fiber compactness. The ratio of masses was calculated as shown in Section 2.1.2. For each point in Fig. 8 four samples were produced and assessed. The average weight of the samples was around 600 g. A process time of 2 h results in leathers with a shrinkage temperature (TS ) between 86 and 92 ◦ C. For 30 and 4 MPa TS does not increase above 93 ◦ C at a tanning of 4 h. Using 5 MPa TS can be increased from 95 to 97 ◦ C at a tanning time between 3 and 6 h. A shrinkage temperature of 100 ◦ C can be obtained after 2.5 h and 6 MPa for all investigated parts of the skin. All 20 samples showed cooking stability. 4. Discussion and conclusions The results demonstrate the effectiveness of tanning, which is considerably intensified by application of pressurized CO2 . In labscale and pilot-scale tanning time could be shortened by factors of 3–6 using CO2 as process intensifier at 10 MPa. Trials in pilotscale showed that increasing pressure from 10 to 30 MPa has no significant influence. Regarding the tanning time the mass of the liquid phase in the autoclave looms large. The larger the area for the diffusion of compressed carbon dioxide into the skin the shorter the time needed for tanning is. In a new approach it was found that a surplus of (free) tanning liquid in the autoclave could be avoided if tanning is carried out under CO2 -pressure. In that case the tanning time could be shortened to 2.5–3 h. Furthermore it was found that the pressure required for obtaining a good quality can be as low as 3 MPa. Reducing the overall mass of water in the tanning bins requires an increase of the concentration of chromium salts in the tanning solution, to achieve a sufficient quality of the leather. Absorbing water with high chromium content into partly dewetted skins is resulting in a higher concentration gradient of chromium. Thus diffusion is accelerated. Additionally the skin matrix is most likely widened by CO2 . This behavior was investigated for several complex solids, including certain biopolymers [32–37]. Regarding the sizes of chromium
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polynucleate complexes diffusing into the skin, even a slight widening has a great influence on penetration. According to Nishad [39], Covington [40] and Braeumer [41], penetration is not the time limiting step, but most of the process time is taken by the diffusion inside the collagen microfibrils [41]. The chromium complexes have a size of about 0.75 nm and 1.29 nm [38]. The polynucleate complexes have to diffuse between the fibrils for reaching reactive binding sites [42]. The gap between the microfibrils is estimated to be 1.4 nm [43]. The “diffusion tunnels” in the fibrils can take up a length of several centimeters [44–47]. Having no (free) liquid phase outside the leather CO2 can penetrate directly over the surface into the skin. This way the mass transport is influenced positively. First trials carried out at Fraunhofer UMSICHT using a demonstration plant (1700 L autoclave volume) with a rotating drum showed very promising results. An area of over 100 m2 per batch can be treated now. It was found that lower pressure than in pilot scale resulted in very high leather quality, probably because of an intensified flexing of the material. Flexing is improved from smaller to bigger skins which, due to their size and shape, are able to twist. Torsion seems to improve the mass transport of tanning agents and CO2 . This assumption is confirmed by know-how from conventional tanning [48]. To sum up it can be noted that the newly introduced tanning process bears a great potential for environmental protection. The process time for tanning can be considerably reduced. Chromium contaminated wastewater can be reduced by over 95%. Theoretically, this could result in savings of 14 billion liters per year. 160,000 tons of chromium can be saved per year because now it is only necessary to use just as much chromium as is needed for producing high leather quality. Overdosing can be reduced considerably. This reduction of the tanning agent may lead to saving another 6 billion liters of water which would be necessary for the production of the chromium.
Acknowledgments The authors would like to thank the German Ministry of Education and Research for the financial support. Furthermore, the authors would like to thank Helmut Geihsler and the students Jan Rech, Björn Jochems and Anna Oelbermann for their assistance with the investigations.
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