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IUE 1 IUE RECOMMENDATIONS ON CLEANER TECHNOLOGIES FOR LEATHER PRODUCTION 2004 Updated Document Introduction This renewable source of raw material is used for such items as shoes and upholstery, which might otherwise have to be manufactured from alternative non-renewable products such as synthetic materials and other petrochemical based products. The condition of the raw stock received by the tanning industry has a direct effect on the resulting cleaner technologies that can be applied. Good farming practices are encouraged so that hides and skins do not suffer from ectoparasite infestation or damage inflicted by barbed wire, horns or other outside influences. Such damage has to be masked by the tanners involving extra processes using additional material resources and often creating added waste disposal problems (e.g. buffing dust, shavings etc.) The amount of dung attached to an animal hide or skin as a direct result of poor farming practices also uses more natural resources and creates additional loadings on the effluent and the solid wastes which becomes the responsibility of the tanner. Damage to the hide or skins, such as poor flaying practice at the abattoir, may also create increased waste disposal problems for the tanner. All these factors have to be taken into account when
considering the application of cleaner or clean technologies. The International Union Environment (IUE) Commission
is concerned to take into account the technologies currently applied
by the most advanced tanneries and not just to consider the latest
developments from research units. Fresh or uncured rawstock is available to tanneries in many countries.
Whenever possible, treatment of fresh hides and skins is the best
solution to reduce salt pollution. The time elapsing between slaughter
and further treatment (whether curing itself or the initiating of
wet processing in the tannery) must not exceed a few hours. When an
abattoir and a tannery are operationally linked, fresh rawstock may
be used, but excess above the capacity of the tannery must be handled
differently. Beyond this short period, it is necessary to cool the hides and skins,
either in ice or cold air. Cold air is necessary if hides are to be
transported over long distance. Storage below 4°C can extend preservation
for up to three weeks, under ideal conditions, although some dehydration
is to be expected. This system of retaining rawstock quality is used
in Europe, by transporting rawstock in refrigerated lorries, but it
is recognised that this may not be feasible or economical in developing
economies. Rawstock may be preserved in ice, but storage is more problematical
than chilling, due to melting of the ice, run-off of water and the
potential for bacterial growth on wetted pelt. Shade drying of small skins is a low cost and environmentally acceptable
process in some climates. Controlled air-drying using heat pump or
other system is suitable for any climate. Dry salting, combining salt curing and shade drying, can minimise
the amount of salt used for preservation of skins and hides. The use of antiseptics with low environment impact and toxicity can
help to increase storage time of fresh or chilled hides and skins.
Suitable preservatives that are used around the world include: TCMTB,
isothiazolones, potassium dimethyl dithiocarbamate, sodium chlorite,
benzalkonium chloride, sodium fluoride and boric acid. Their use must
be regularly reviewed, to reflect changing legislation, because they
will be discharged in the effluent. It is possible to eliminate up to 10% of the salt added to hides and
skins for preservation, by using hand shaking, mechanical brushes
or a suitable drum. The salt can be reused for pickle processes after
dissolution and removal of solids, but it must not be used for curing
purposes because it is too contaminated with bacteria, particularly
halophilic or halotolerant bacteria, which can cause so called red
heat. This method of salt recovery gives a partial answer to the salt pollution
problem. Neither brine curing nor salt curing can be considered as
cleaner technologies, even if pre-fleshing green hides reduces this
waste. It is recognised that salt curing is one of the greater contributors
to the environmental impact of tannery operations. Even recovering
some of it has limited benefits, because its reuse is extremely limited,
its ecological disposal is difficult impossible and the cost of fresh
salt is so low. The new generation of drums and processors facilitate efficient draining
and washing, and allow the routine use of low floats for processing,
thereby resulting in significant savings in water consumption. The consumption of fresh water can be minimised by using a countercurrent
system of washing, to concentrate the salt (if present) and the other
soluble materials, such as dirt and blood. Additional cleaner technology that can be applied at this stage is
the fleshing of green hides after soaking. It yields a lower quantity
of fleshings, with a neutral pH. Green fleshings are more valuable
than limed fleshings with regard to tallow recovery, because the green
fleshings are not subjected to the hydrolysing liming process. In
this way, the amount of recovered tallow is greater and the content
of undesirable free fatty acid is much lower, so the quality is better. An associated problem with this approach is the presence of dung on hides, which causes the fleshing blade to cut into hide, thereby damaging the pelt in an economically unacceptable way. Removal of dried-on dung by methods other than soaking is difficult. However, dung removal is a pre-requisite to processing. The problems associated with dung contamination may be pre-empted by utilisation of hides and skins, where available, from animals that have been reared through a quality assurance or clean hide scheme. These schemes generally require animal husbandry practices that minimise dung contamination. 2.2 Classical unhairing-liming process The enzymatic treatment of hides and skins can be considered as a cleaner technology only if the amount of sodium sulphide is reduced substantially. However it is not yet possible to replace totally sodium sulphide in processing skins and hides. There are other agents available that reduce the amount of sulphide in liming, e.g. organic sulphur compounds (mercaptoethanol, salts of thioglycolic acid, formamidinesulphinic acid) and amines based proprietary products. However, it should be borne in mind that all hair dissolving
processes will contribute to the COD/BOD of tannery effluents. For traditional skin production, painting and sweating may be considered
cleaner technologies. Recovery of hair before dissolution, either
when it is separated during the liming, or at the end of a hair saving
process, can lead to a COD reduction of 15-20% for the mixed tannery
effluent, and a total nitrogen decrease of 25-30%. There are several established methods of hair saving, routinely used
in industry. However, is it recognised that they do not provide a
complete effect, since each incorporates a hair dissolving step, to
deal with residual short hairs. Direct recycling can be applied when there is a good control level
in the tannery. Resulting advantages are savings in sodium sulphide
(up to 40%) and in lime (up to 50%). It can give a decrease of 30-40%
of the COD and 35% of the nitrogen for the mixed effluent. The quality of the leather produced can be affected negatively through
this recycling process, unless the unhairing and opening up processes
are used in two steps. This is because the suspended melanin and undissolved
cuticle fragments from the dissolved hair (referred to as scud) are
driven into the grain by mechanical action, making it dirty. This cleaner technology is industrialised in several large bovine
tanneries for shoe upper leather. The success depends on how the hair
is removed and how well the recycled liquors are cleaned up before
they are recycled. Faced with the difficulties of upgrading the chromium-tanned split
waste, splitting in the lime can be considered as a cleaner technology,
as it saves chromium and yields a by-product that can be used for
food casings or for the production of gelatine. Up to 40% of a tannery's production of ammoniacal nitrogen comes from
the use of ammonium salts during the deliming process. Carbon dioxide
deliming can be considered as a cleaner technology giving good results
on light bovine pelts (thickness less than 3 mm). For thicker hides,
it is necessary to increase float temperature (up to 35°C) and/or
process duration and/or to add small amounts of deliming auxiliaries.
In order to effectively eliminate the creation of hydrogen sulphide
as the pH of the deliming solution falls, 0.1% hydrogen peroxide can
be used to scavenge residual sulphide. The grain enamel should be
allowed to delime for perhaps 5 minutes, to guard against oxidation
damage, then the peroxide can be added safely. If the pH of CO2 deliming float is lower compared to common procedure,
special bates can be used. Also, bates with a lower content of ammonium
are available. Ammonium-free deliming agents, such as weak acids or esters, can totally
or partially replace ammonium salts used for conventional deliming.
However, in comparison with CO2 deliming the resulting COD is often
higher, due to the contribution from the reagent. Cost and slowness
of reaction make them less viable. Chromium tanning salts are used today in 85% of tanning processes
around the world. Only the trivalent form is used for tanning operations
and this chemical cannot be replaced by another to give the same quality
of leather. An argument for continuing to use basic chromium(III) sulphate is
the ease of managing its discharge into the environment and its low
environmental impact. Chromium(VI), a recognised carcinogen, is not
used in leather manufacturing processes. When pickling and tanning steps are separated, the recycling of pickling
floats can save up to 80% of normal salt used and 20 to 25% of the
pickling acid. When they are conducted as one step, the neutral electrolyte
can still be recycled in the spent liquor and reused for pickling.
However, in the absence of analytical data, it must be assumed that
much of the formate in the system will be bound to chromium, either
on the leather or in solution. For wool-on sheepskins, using long floats over 150%, recycling of
pickling and tanning liquors is current and routine practice. It is
also feasible to recycle bating floats. Solvent degreasing is still in use. This practice can lead to a cleaner
technology when the solvent is recovered, the extraction brines are
recycled and the natural grease is recovered for commercial use. Discharge
of solvents is unavoidable with solvent degreasing, but alternative
technologies can be applied for high quality skin production. On wool-on lambskins, it is a common practice to undertake dry solvent
extraction when crusted. The use of non-solvent methods implies the use of higher amounts of surfactants. Ethoxylated fatty alcohols are recommended instead of the more widely used ethoxylated alkylphenols, because they are more easily degraded and the use of the latter are to be restricted in the EU. The COD from aqueous treatment may amount as much as 200,000 mg/l, due to the content of natural grease and surfactants (1g/l of natural grease is about 2,900mg/l COD, and 1g/l ethoxylated alkylphenol is about 2,300mg/l COD). To ensure complete mobilisation, aqueous degreasing would, ideally, be carried out at a temperature above the melting point of the grease. However, the melting point of the grease is normally very close to the shrinkage temperature of the skin. For example, the melting point of sheepskin grease is approximately 42°C, whereas the shrinkage temperature of sheepskin pickled pelt is approximately 50°C. Therefore, the risk of heat damage to the pelt precludes the use of temperatures above the melting point of the grease. The grease may also be contained within lipocytes, further limiting its dispersal. The aqueous degreasing of pigskins may be assisted by the use of proteolytic enzymes to degrade the lipocyte and, thus, mobilise the grease. However, this may not be possible for sheepskins where the fibre structure is more susceptible to the proteolytic activity of the enzyme. 3.3 Wet-white pre-tanning This process can be considered as a cleaner technology if the chemicals
used are neither toxic nor cause adverse environmental impact. Aluminium(III),
titanium(IV) and zirconium(IV) have been suggested for this role:
they are not listed as hazardous, although restricted in several countries,
but their degree of reversibility depends on how they have been applied.
Aldehydic tanning agents can be considered as leading to a cleaner
process, according to local regulations, but their reactions are completely
irreversible, so contribute to a different character in the leather.
Syntans are an option, because their action is more reversible. The alternative approach is to change the properties of the pelt,
to make it less prone to distort when the surface is struck by the
shaving blade. This can be achieved by reducing the ability of the
fibre structure to slip over itself: this is best achieved with hydrated
silica, used in the fabric industry for the same purpose. Silica interacts
weakly with collagen, in a non-tanning manner, and the effect can
be reversed: any discharged silica has negligible environmental impact. 3.4 Direct recycling of chromium tanning floats When large quantities of chromium bearing floats are recovered, recycling
after precipitation is another solution for chromium recovery. Precipitants
that might be used include sodium carbonate, sodium hydroxide, and
magnesium oxide. The difference between them is the effect they have
on the precipitate: the faster the basifying reaction, which is dependent
on the alkalinity and the solubility, the more voluminous is the precipitate
and the slower is the settling rate. Therefore, the greatest sludge
density is obtained using magnesium oxide. The addition of polyelectrolyte
can improve flocculation.
Sludge obtained after sedimentation and optional filtration is re-dissolved
in sulphuric acid, to control the desired basicity in the product.
In order to ensure complete solubilisation of the chrome sludge, the
reaction should be conducted at >70oC. For conventional tanning,
it is possible, with this process, to obtain a clarified effluent,
with less than 10 mg/l of chromium expressed in Cr, which might be
reused for the next pickling or tanning float. The clarified effluent
can also be reused for first soaking float.
Using recovered chrome for tanning results in wet blue that is slightly
paler than conventional production. Further the re-use of precipitated
chromium will lead to an increase in the neutral salts in the effluent. In order to reduce chromium concentration in the waste float, high
exhaustion chromium salts, adapted basification products and/or temperature
increase can be used. In essence, all proprietary options are based
on higher astringency, by employing higher pH in basification, but
most importantly elevated temperature. In most cases, chromium tanning should be considered as the best available
technology. Many alternative formulations have been proposed, but
none can exhibit the versatility of chromium(III) for making a wide
variety of leathers. Also, the high hydrothermal stability of chrome
leather is a prerequisite for many modern applications of leather.
Vegetable tanning is the traditional alternative to chrome tanning:
conducted by a dry drum process, or in closed circuit vats, it can
minimise waste and must be included in these considerations. Due to
the high pollution load and slow biodegradability, conventional vegetable
tanning cannot be considered more environmentally friendly than chrome
tanning. Vegetable tanning has limited applications, because of the
low hydrothermal stability, the filling effect and the hydrophilicity
of the resulting leathers. Recovery of vegetable tanning floats by
ultrafiltration is used in several European tanneries and the recovered
tannins may be used in the tanning process. Semi-metal tanning can produce chrome-free leather, with equally high
hydrothermal stability. It is a combination of a metal salt, preferably
but not exclusively aluminium(III), and a plant polyphenol containing
pyrogallol groups, often in the form of hydrolysable tannins. When the use of chromium is required for retanning operations, the
same consideration should be given as for chrome tanning. Absence
of environmentally unsound dyestuffs, especially those containing
benzidine and other banned aromatic amines, and of halogenated oils
in fatliquors, form essential elements of cleaner processing. High
level of exhaustion for syntans, dyes and fatliquors are also to be
considered: in each case, the chemical principles and conditions for
reaction with the leather must be optimised. The use of water-based finishes is fundamental for a cleaner process,
but the inherent need to use crosslinkers should be kept in mind.
Chemicals used in finishing must not contain any environmentally undesirable
heavy metals or other restricted products. Water based formulations
(containing low quantities of solvent) are available for spray dyeing.
Finishing products have to meet the current limits imposed by environmental
and workers health regulations. The equipment used is extensive. Roller
coating or curtain coating machines are more desirable from an environmental
point of view, but they cannot be used for all types of leather. For
other types, spraying units with economisers and high volume low pressure
(HVLP) spray guns can reduce discharges to the environment. Recycling typically means a second utilisation for the
same purpose, reuse may mean utilisation for different purposes and
recovery incorporates an isolation step. Recovered material can then
be recycled or reused.
Recycling technologies have been used for long time in vegetable tanning
processes, indeed the conventional counter-current method incorporates
recycling as a fundamental element of the technology. To reduce the volume of saline effluents, particularly if this segregated
float needs to be evaporated or specifically processed, it is possible
to reuse soaking floats in a counter current method, analogous to
vegetable tanning. Here, the pelts progress into cleaner float and
the contaminated floats move towards the dirt soak. Only the dirt
soak liquor, in which dirt and salt are accumulated, are discharged
to waste and treatment. This decreases the amount of water to be evaporated,
when salinity is restricted, and reduces the presence of biocides
in effluent. However, it does not solve the problem of what to do
with the dirt soak solution. Lagooning where feasible reduces the
volume, but the salt remains. When sheepskins are solvent degreased, recycling of the residual solvent
after distillation is currently operated. Furthermore, the extraction
brine is also easy to reuse, to save sodium chloride. Recycling of pickling float has been proven to be highly satisfactory
in terms of salt savings and partly for acid savings. There is no
great difficulty if density and acidity of the float can be regularly
controlled. The most common practice is to collect the residual tanning float,
to filter it, to adjust its acidity, then to reuse it as a new tanning
float before adding fresh chromium salt. The recovered volume may
be more than required for subsequent tanning operations, but it is
possible to reuse the liquor in post tanning. It is much less feasible to recycle post-tanning floats, since the
chemical condition required for the steps may be different and steps
tend to be conducted sequentially in the same float. Therefore the
problem of contamination is compounded, especially since these steps
vary greatly, even in a single factory. Thus, recycling technology
cannot be recommended. Leather production is a water intensive industry, therefore measurement
and control of consumption are important and essential points of water
management In many countries water has become a scarce commodity and the costs
for the consumption and discharge of water increase regularly. Water
has to be managed properly and several options are available to minimise
the overall consumption of water. Reduction: The first step is reduction of water consumption, with
strict measurement and control of consumption. Low float processing,
batch-type washing instead of rinsing and combining processes (compact
recipes) are practical examples of technologies to reduce water consumption
by 30% or more. However, lower volume of water will result in higher
pollutants concentration, but that will be partially offset by the
greater efficiency of shorter float process steps. Limits to reducing
float length must be borne in mind, since not all processes benefit
from reduced float length. Recycling: Certain specific processes are suitable for recycling of
floats, although in most cases installations for treatment are necessary.
Examples are; soaking, liming, unhairing, pickling and chrome tanning
liquors, which can reduce the overall water consumption by 20-40%. Re-use: Biologically treated effluent offers the opportunity of replacing
a certain amount of the process floats, such as the beamhouse process
floats, with treated water. Depending on the type and efficiency of
the treatment process additional operations might be necessary, such
as filtration and disinfection, to meet the required water quality
standards. Membrane systems provide the possibility of reusing treated effluents,
provided that most of the residual organic matter is removed and disposal
of the concentrate is achievable.. Processes should be optimised with regard to chemical use to minimise waste. Reduced floats allow reduction in chemical use (liming, deliming and pickling). However, due regard should be placed on the chemical and biochemical principles of processing, in order to avoid the unnecessary excessive chemical use, for example, lime, sulfide, salt, chrome, dyes, lubricants, etc. Updated by
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