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IUE 2 2004 Updated Document Aim of the document: The International Union Environment 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. The reuse of untanned solid waste is restricted or carefully controlled
for human and animal food. The general recommendations collected by the Commission have to be adapted to local conditions and under the supervision of an expert. This document will be updated periodically, to reflect changing and developing technologies.
Please note that any untanned hide by-product (e.g.
fleshings, trimmings, splits) from BSE (Bovine Spongiform Encephalopathy)
regulated animals must be totally destroyed. The dusted salts may be reused for pickling after dissolution in water
and clarification or filtration. Alternatively, the recovered salts
could be used for a number of applications including foundry casting
(in the mould), hypochlorite production and de-icing of roads. Dusted
salt can be reused for curing but a preliminary heat treatment is
required to reduce bacterial impact and to limit the presence of organic
matter in recovered salt. Green fleshings can be used in rendering plants for the recovery of
grease and meat meal. These products must be clean, and contain minimal
quantities of minerals. More importantly, green fleshings are a valuable source of high quality
tallow, a basic commodity with added value. In contrast to limed fleshings,
green fleshings need little pH adjustment prior to enzyme processing.
They produce much higher yield and the quality is good, because the
fleshings are not previously subjected to prolonged alkaline treatment. The green and limed trimmings can be used with limed splits for tallow
or gelatine production (see below). Untanned wastes mixed with farming, domestic and fish wastes can be
used for methane production; full-scale plants are in operation in
Denmark and Sweden. Limed fleshings mixed with tannery sludge are digested to produce methane by grinding to 10 mm and warming to allow microbiological activity, with increased fat or grease content resulting in increased methane production. The volume of gas evolved (comprising 75% methane) is estimated to be 615 litres per kg of organic material introduced into the digester, after 25 to 30 days at 35°C. The residual solid phase is suitable for composting according to chromium content and can be applied directly to agricultural land as a soil improver. This technique is especially suited to warmer countries, where the necessary heat input is minimal. The input mix material for this system must have at least 70% of organic matter content to operate successfully. An industrial scale plant is in operation in India. In Denmark, ferrous metal salts are added directly to the reaction vessel of the bio-gas reactor to avoid the generation of noxious and corrosive gases. 4.2 Grease and protein recovery Limed fleshings must be acidified before enzyme treatment. They produce a low yield of tallow, because it considerably hydrolysed by the liming process; in addition the quality is low because of the high content of free fatty acid from that hydrolytic reaction. Gaseous by-products of the process are hydrogen sulphide,
mercaptans and odour, and it is therefore essential to exhaust these
via a water wash or a scrubber system containing sodium hydroxide
and sodium hypochlorite. However, in some places, the exhaust gases
are passed into the air intakes of the boilers used for energy production,
thereby eliminating the need for a scrubber. In some cases, a ferrous
metal scrubber may be needed prior to the boiler to capture sulphides.
In respect of the capital and running costs, it is estimated that
for economic viability, 10 tonnes of material must be processed per
day. A second process technology involves treatment with hydrogen peroxide
and sulphuric acid at 35-40°C. For this, the fleshings must first
be chopped to a particle size of 50-200 mm. The process produces two
phases that can be separated by mechanical de-watering; grease separates
from the liquid phase, the yield being 10 to 12.5% of the original
fleshings mass. A protein phase (20-25% dry solids) is also obtained
and this can be used either as animal foodstuff after drying, or as
fertiliser. Again, a minimum quantity of 10 tonnes per day needs to
be processed for economic viability. A mixture of waste fleshings and an appropriate bulking agent (also
carbon source), with aeration, leads to compost production There are a number of reported promising uses for the recovered hair
from hair-save processes. These include: felt production, slow degrading
plant containers, keratin hydrolysate, cosmetics and pharmaceutical
products (i.e. shampoo, amino acids, etc.) Hair recovered through a hair-save process can be incorporated into
existing composting processes, as it is a valuable source of nitrogen
and organic carbon. Hair can be directly used as slow release source of organic nitrogen
and carbon for fertilising purposes Gelatine production by a specialised, purpose built process facility
represents a major utilisation opportunity for lime splits, not suitable
for tanning. The process involves lime hydrolysis; soluble gelatine
is extracted in a series of hot water batches of increasing temperature
at controlled pH. Different stages of purification, demineralisation,
concentration and sterilisation are then required prior to final drying.
The gelatine product is used by the food, photographic and pharmaceutical
industries. Lower quality gelatine or glue can be produced by acid
hydrolysis and hot water extraction. Specialised manufacturers use limed splits to produce high quality
sausage casings. The casing manufacturer will impose restrictions
on the process chemicals used in the beamhouse. Delimed hide splits can be dried in moulds of various shapes, to produce
dog chews. Limed splits, high in nitrogen, low in carbon, will compost readily,
particularly if combined with fleshings. Grease from the degreasing process can be used as a component of low
grade fatliquors through a sulphitation process. As for lime splits, the wet white process produces splits that can
be partially denatured to produce gelatine or collagen additives.
However their use in human food production is restricted. Wet white chemistry options can create environmentally friendly tanned
waste; aldehyde tanned, syntan tanned, marginally vegetable tanned
materials have little associated hazard. These shavings are particularly
suitable for use as fertiliser or as a source for collagen hydrolysate.
Aluminium containing shavings can be applied to non-acidic agricultural
land, according to local regulations. Companies are producing leatherboard from bovine chrome and vegetable
shavings and splittings in several countries, although only shavings
satisfying strict quality requirements are accepted for processing.
The leather fibres are mixed with latex, and after coagulation, the
mixture is de-watered, pressed and dried. The final product is obtained
either as separated sheets or as a continuous material. One industrial gelatine manufacturing process blends the shavings
with magnesium oxide and subsequently extracts 50% of the gelatine
content with boiling water. A chromium containing slurry ('scutch')
is generated as a waste. Protein extraction can be improved with magnesium oxide assisted by
enzymes. Liquid proteins can be used for industrial applications.
The chrome cake can be recovered for chrome liquor production. Other
alkaline agents, such as lime and sodium hydroxide are used industrially. Acid hydrolysis utilises concentrated sulphuric acid with steam injection.
The hydrolysate is neutralised with phosphates and supplemented with
organic additives to produce a fertiliser. The hydrolysate can also be used for different industrial applications,
such as in retanning operations in tanneries, as a coagulating agent
in the rubber industry, as complementary products for surfactants
and as plasticiser in concrete production. Various laboratory and industrial trials have demonstrated that chromium
containing leather waste may be thermally treated to produce an ash
containing approximately 50% chrome oxide, which is similar in nature
to the mineral ore feedstock, sodium chromite, used by the chromium
chemicals manufacturing industry. Sodium chromite, converted into
chromate, is the precursor of most chromium chemicals, including chrome
tanning agents. Enzymatic digestion of shavings results in a high quality and valuable
hydrolysate or gelatable protein and a protein contaminated chromium
sludge. The hydrolysate can be used in retanning agents, as foam stabilisers,
in the chipwood industry and gypsum industry. The chromium sludge
can be reused in a dichromate reduction plant for the production of
chromium sulphate. Full-scale factories processing shavings have been
in operation in the Czech Republic and USA. Mixing of limited amounts of chrome shaving into clay for brick making
is carried out in South America. Wherever possible, the chromium from spent tanning liquors should
be recovered and reused or used in other industry (e.g. steel). Alternatively,
high exhaustion chrome tanning systems should be used. Either method
will minimise the mass of chromium discharged. The organic content of a soaking sludge can be reduced by 65 % in
a UASB (upflow anaerobic sludge blanket) process. Usual incineration of sludge (with or without leather waste), although
technically feasible, will have limited application due to the economy
of scale, and due to associated environmental problems (air pollution
and possibility of chromium oxidation). There is no risk-based justification for banning the application of
chromium containing wastes to agricultural land. However, the chromium
content of sludge applied to land must be limited in order to comply
with existing regulations and requirements. Mixing the sludge with clay and bricketting, solidification with fly
ash and cement would minimise leaching of chromium. Viable uses for all the above wastes need to be established. 14. ALL SOLID WASTE One option for dealing with all solid waste is incineration, combustion
of the organic content of feed material, with controlled availability
of oxygen. However, this technology has a bad reputation for producing
odour and the possibility of producing dangerous fly ash makes it
less environmentally acceptable. While these options are viable, the costs and environmental compliance implications may limit their application. IUE Commission
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