Free of Water Tanning – Intensified by CO2 Parameter ... - ISSF 2012

Free of Water Tanning – Intensified by CO2
Parameter and Scale Up Study
Renner, M. 1 , Weidner, E. 1 , Geihsler, H. 1 , Heinen, T. 2
1 Fraunhofer Institute UMSICHT, Oberhausen, Germany
2 Josef Heinen GmbH & Co KG, Wegberg, Germany
Corresponding author: manfred.renner@umsicht.fraunhofer.de; Phone: (+49) 208 85981411;
Fax: (+49) 208 85981424
ABSTRACT
The tanning industry produces 2 000 km 2 of leather every year. In Europe 26 000 employees work in 1 633 tanneries.
The number of tanneries decreases every year because of the high environmental pollution they cause. To produce this
area of leather a volume of 14 million liters of water is used. Over 90 % of the leather is tanned with chrome-III-salts
because of the high performance quality required of leather. After the tanning step the water is contaminated by
chrome and salt. This sewage water has to be cleaned at high costs and the chrome is hazardous waste and cannot be
recycled.
Fraunhofer UMSICHT has developed a new process principle to reduce the chrome contaminated sewage water, save
tanning agents and reduce the process time. The lecture will give a short survey of what tanning is about. Afterwards
the influence of pressure, process time and pH value will be shown in detail. By using for example a pressure of 60 bar
over 3 h it is possible to produce high leather quality without having sewage water. The use of chrome can be
controlled precisely and it is possible to use only as much chrome as can be fixed in the collagenic matrix.
Using carbon dioxide 14 million liters of sewage water in the tanning process, 270 million liters of sewage water to
produce the tanning agents, 80 000 tons of chrome and 500 000 tons of salt could be saved. The lecture will show
results achieved with technical (20 L)- and preindustrial scale (1 700 L) high pressure equipment.
INTRODUCTION
The usage of leather and fur allowed the Neanderthals to survive in climatically not acceptable regions [1, 2]. The furs
were conserved by using smoke. Later, humans chewed fats into the skin. In 2010 a 5 500 year old lace-up shoe tanned
by vegetable tanning agents was found [3]. In 3 000 B.C. the Egyptians imported aluminium salts over thousands of
kilometers for tanning for example loincloths of priests [4]. In 1858 the tanning with chromium salts was developed
by F. Knapp [5]. This method revolutionized the tanning industry due to the high performance quality required of
leather [6].
Nowadays over 2 000 km 2 of leather are produced every year. Approximately 90 % of all leathers are tanned by using
chrome-III-salts. To produce this amount 7 million tons of skin, 500 000 tons of salt, 500 000 tons of chrome-III-salt
are needed. The turnover of the intermediate product leather is approx. US $ 45 billion. After China the EU is the
second biggest producer in the world. Chinas production capacity has doubled since 1998 and was 613 million m 2 in

2006. In 2007 the European tanneries produced approx. 325 million square meters in 1 650 tanneries with 26 000
employees. [7, 8, 9,10]
The following chapter provides a brief overview on the most important process steps. The tanning methods can be
divided into three groups regarding the tanning agents. It is possible to tan with metal salts like chromium and
aluminium, 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 and a cross-bonding
between skin collagen and tanning agent. [11, 12, 13, 14] The first important processstep is the so called pickle. The
skin is prepared for the tanning process at a pH value of 2.5 in a water bath [15]. The next step is the tanning, taking
place in rotating tanning tanks over a period of 12 to 20 hours. During the tanning process, the skins are brought into
intensive contact with liquid containing a tanning agent, usually a solution of water and chrome-III salts. During this
stage, the pH value is elevated from 2.5 at the beginning of the process to 3.8 at the end of the process [16, 17, 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, in the skin [19, 20]. The
leather is stabilized by a strong complex-binding on the one side and with a not so strong but very effective threedimensional
binding between the complexes over the water and sulphate groups on the other side. The tanning can be
called successful if 3.8 to 5 % of Cr2O3 by weight are enriched within the leather [21]. This concentration is used as
the fundamental criterion to define high leather quality for the trials on a laboratory scale [22]. Cooking stability of the
leather results [23]. The leather coming out of these process steps is called “wet-blue”. Wet-blue is an intermediate
product. The finishing with colors, oils and different chemicals afterwards brings about different forms of appearance
like smooth leather for clothes, stiff leather for shoes or soft leathers for furniture. [24, 25]
Past research of the workgroup has shown that tanning time can be reduced significantly. A detailed description of the
processing is given by Renner et al. [27]. Figure 1 shows the comparison of conventional processing and processing
under the influence of compressed carbon dioxide. The mass ratios of all trials were equal. The sample size of the skin
samples was 2 cm in diameter and tanning solution was used at a ratio of 1 to 10 (gskin/gtanning solution). 8 weight percent
(measured on skin weight) of chrome-III-salt was dissolved in water. During tanning each skin sample was tanned
individually. The samples were wetted by the tanning solution, not submerged. The quality of the produced leathers
was assessed using emission spectrometry. With 3 weight percent of chromium in skin (equal to 4 weight percent of
Cr2O3) a high quality is reached. The chrome-content of the tanning solution was measured after the process. The
loosening of chrome was calculated on the uptake of chrome in the leather and on the 3 weight percent criterion. The
results demonstrate that process time can be reduced from 30 to 5 hours using carbon dioxide at 100 bar as process
gas. This process feature is very important. The chrome content approaches a threshold at 130 %. This corresponds to
a chrome content of 3.6 %. This measured value is also confirmed in literature and demonstrates the comparability
between laboratory scale and tanning on an industrial scale. [28, 29] Other workgroups have also published
comparable results in lab-scale [30, 31, 32].
Percental approach of 3 weight percent [%]
120
100
80
60
40
20
Tanning, 100 bar
0
Tanning, 0 bar
0 5 10 15 20 25 30
Time [h]
Figure 1. Reduction of process time using CO2 as process intensifier in lab-scale [27]

MATERIALS AND METHODS
Skin
The cattle skin is divided into several parts. The part with the highest quality and the most regular structure is the
croupon; the backpart. An average skin has a surface area of 7 to 9 m 2 . The neck and the croupon were taken for this
work. The thickness of the neck measured 2 to 2.4 mm and the croupon measured 2.6 to 3 mm. Both were achieved by
splitting the hold skin using a band knife. The neck was taken as one part per trial (area about 0.25 m 2 ). The croupon
was divided into 5 parts (with an area of about 0.2 m 2 per part). With these parts the experiments were carried out. For
the experiments shown in figure 5 the parts were divided once more into 4 equally sized parts.
Tanning solution
For all experiments chrome-III was used as tanning agent. The solution consisted of water, chrome-III-salt, salt,
formic and sulphuric acid. In the next chapter the two different process principles are described. The results
demonstrated in figure 3 were carried out comparable to conventional processing using an aqueous phase for the mass
transport of chrome ions. The chrome content was calculated with 8 wt.-% of chrome regarding the skin weight. For
the experiments shown in figure 4 and 5 just as much solution was taken as the skin could soak up. The chrome
content of the solution was calculated on the dry weight of the used skin to reach 4 to 5 weight percent of Cr2O3.
Quality assessment
The quality assessment of wet-blue was carried out by measuring the shrinkage temperature. Chrome tanned leather
has to be resistant against boiling water without degeneration of the skin. This criterion can be assessed by using the
equipment illustrated in figure 2. After the tanning process a defined part of the leather has to be punched out. This
sample is placed firmly with an interlock on one side. The other side is connected by a yarn using a hook. A weight
tightens the system. The sample is subsequently submerged into water. The water is heated up until it boils. When the
leather sample shrinks the criterion for high leather quality is not reached.
Figure 2. Measurement of shrinkage temperature
High pressure equipment
The experiments (Fig. 4 to 5) were carried out in a 20 L equipment. The equipment was build especially for high
pressure tanning. For the tanning it was essentially to use a horizontal autoclave with a rotating drum. The motion is
important for the mass transport and the regularity of the chrome distribution. A maximum of 320 bar and 70 °C can
be set. Recently a second plant with a volume of 1 700 L has been put into operation in our lab (see abstract). The
autoclave is also positioned horizontally. A mass of up to 500 kg (> 150 m 2 ) can be tanned per batch in a rotating
drum. The design of the drum is comparable to conventional pressure-less tanning bins. Using pins, mounted to the
inner side of the rotating drum, causes a high motion of the skins and thus a good penetration of gas and tanning
agent. The equipment allows to feed tanning liquids pressure-less and against pressures of up to 260 bar. First results
will be presented.
RESULTS
The trials on a laboratory scale have shown that tanning is considerably accelerated under the influence of compressed
carbon dioxide. In order to implement a process industrially, material samples must be produced that are of

epresentative size. Particularly in the very traditional tanning trade, potential customers can be convinced only via
product samples.
Investigation of the function of pressure-time
Preliminary tests at Fraunhofer UMSICHT in 20 L scale were made to test different tanning solutions and process
parameters. For the investigation pickled skins were taken. The goal of the trials was, on the one hand, to confirm the
results achieved on laboratory scale and, on the other hand, to determine optimum parameters for the tanning process.
On the x-axis the process time is depicted and on the y-axis the pressure. The 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.
The tanning sessions were carried out at the same tanning solution to hide mass ratios as on laboratory scale. The
basket rotation was set to 10 revolutions per minute and the hide was pulled continuously through the tanning solution
as in the conventional process. The graph shows that high leather quality can be produced within 2.5 h at 200 and
300 bar. Three hours were needed at 100 bar. Quality assessment was made by shrinkage temperature. By way of these
investigations a threshold time was determined that could not be shortened in this process. It also became apparent
that, at 300 bar, no significant reduction of the tanning time versus 100 bar can be achieved.
Pressure [bar]
300
250
200
150
100
50
0
Ts > 100 °C
Ts < 100 °C
2,0 2,5 3,0 3,5 4,0
Time [h]
Figure 3. Process time using CO2 as process intensifier in technical-scale
Free of chrome-contaminated sewage water tanning
One of the most important problems of tanning industry is the sewage water after the tanning step. The sewage water
is highly charged with chromium and salt. The new process principle will avoid over 90 % of waste products
comparing to industrial standard. The measurement results shown in figure 4 were determined in a modified process.
For this new method, the hides are partly dewetted before tanning and then are brought into contact with only as much
liquid as they can absorb. The liquid contains a very high chrome concentration. During the high-pressure process, the
liquid is absorbed by the hide and the chrome ions diffuse into the reactive points (carboxyl groups) of the skin
collagen. Figure 4 shows the influence of pressure and time on that diffusion. The tanning time in hours is shown on
the x-axis and the process pressure on the y-axis. The values for 100 bar were taken from figure 3 and are intended to
illustrate the different results. With parameters being otherwise identical and using the same plant technology, tanning
according to the described procedure at ambient pressure results in bad quality. The trials carried out at 20 bar of
carbon dioxide pressure also failed to produce high leather quality. A threshold pressure could be identified at 30 bar.
The overlapping of small and large points at a tanning time of 2 h and 30 bar and 50 bar shows that those trials were
strongly influenced, e.g. by the starting material. Depending on whether the hide stems from a neck or loin section, for
example, the qualities can vary. Tanning success can be assured at a tanning time of 2.5 h.
A wastewater reduction of more than 90 % was achieved with this approach. With a conventional tanning process,
approx. 1.5 to 2 tonnes of chrome-loaded water are needed to produce one tonne of leather. The new method produces
less than 100 kg of wastewater per tonne of leather.
The further reduced process time can be attributed to the greater surface area, resp. working surface, for the carbon
dioxide. Major parts of the hide were immersed in the tanning solution. The pore structures of the hide could not be
diffused as quickly by the carbon dioxide to the extent that occurs with the approach as described in this section.

Pressure [bar]
140 Ts > 100 °C
Ts < 100 °C
120
inconclusive
100
80
60
40
20
0
1,5 2,0 2,5 3,0
Time [h]
Figure 4. Process time using CO2 at different pressures in technical-scale
Influence of pressure using a minimum of chemicals for varying parts of skin
The aim of this investigation was to identify conditions for tanning of all parts of skin with varying cross sections. The
mass of chrome-III used was calculated on having 4 wt.-% of chrome bound in skin. For each point in figure 5
4 samples were produced and assessed.
Shrinkage temperature [°C]
100
98
96
94
92
90
88
86
84
2 3 4 5 6
Time [h]
60 bar
50 bar
40 bar
30 bar
Figure 5. Leather shrinkage behavior by variation of pressure and time
A process time of 2 h results in leathers with a shrinkage temperature (TS) between 86 and 92 °C. For 30 and 40 bar
TS does not increase above 93 °C at a tanning of 4 h. Using 50 bar TS can be increased to 95 to 97 °C at a tanning time
between 3 and 6 h. 100 °C shrinkage temperature can be obtained after 2.5 h and 60 bar. Increasing the time to 6 h
does not seem to result in further improvement. All samples (20 samples) are boil-proof.
DISCUSSION AND CONCLUSIONS
The results presented demonstrate the effectiveness of the process principle. In lab-scale tanning time could be
shortened by about factor 6 using CO2 as process intensifier at 100 bar. Trials in technical scale show that increasing
pressure from 100 to 300 bar has no significant influence. The mass of the liquid phase looms large. In a new
approach it was found that a surplus of liquid in the autoclave could be avoided, if tanning is carried out under CO2pressure.
In that case tanning time could be shortened to 2.5 to 3 h. On the one hand reducing the water content
requires a higher concentration of chromium-salts, to achieve sufficient quality of the leather. Absorbing water with a
high chromium content into partly dewetted skins result in a higher concentration gradient of chromium. Thus
diffusion is accelerated. On the other hand the skin matrix is widened by CO2. This behavior was investigated in
several publications [33, 34, 35, 36, 37, 38]. Regarding the sizes of diffusing chromium binucleate complexes in skin a
slight widening has a great influence on penetration. The chromium complexes have a size of about 7.5 Å and

1.29 nm [39]. The penetration is not the time limiting step [40, 41]. The diffusion inside the collagen microfibrils
takes most of the process time [42]. The binucleate complexes have to diffuse between the fibrils for reaching reactive
binding sites [43]. The gap between the microfibrils is 1.4 nm [44]. The “diffusion tunnels” in the fibrils are up to a
length of several centimeters [45, 46, 47, 48]. Having no liquid phase inside the autoclave the CO2 can directly
penetrate over the surface into the skin. In this way mass transport is influenced very positively.
A result of the comparison of figure 4 and 5 is a difference in pressure recommendable to tan pieces of skin with
varying skin properties. The collagen structure of the croupon is more dense and regular compared to the neck.
Additionally the skin samples taken for the trials of figure 5 were thinnish. 60 bar are an optimum pressure for
tanning skin with varying properties in technical scale. Referring to tanning in pilot scale at Fraunhofer UMSICHT
pressure can supposably be lowered because of intensive flexing of the material. This flexing is improved between
bigger skins which are able to twist. The torsion caused advances the mass transport additionally. This knowledge can
be transferred from conventional tanning [49]. First trials carried out at UMSICHT using a 1 700 L high pressure
tanning equipment with a rotating drum showed excellent results. An area of over 100 m 2 per batch can be produced
now.
Summing up the results a great potential for environmental protection can be expected. The process time for tanning
can be reduced from 12 to 3 h. Chrome-contaminated sewage water can be reduced by over 95 %. Theoretically, this
could result in savings of 14 billion liters per year. 160 000 t 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.
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