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ENVIRONMENTAL
ISSUES IN THE CERAMIC TABLEWARE INDUSTRY
In the context of the Environment Act 1995 and the UK Government's June
1998 consultation paper "Less waste : More value", the tableware
industry, in its traditional format, does not perform well. It is the
purpose of the PROMOTE LIFE PROJECT
to demonstrate that this performance can be improved significantly. The
environmental issues which affect the industry when using conventional
materials and processes are:
Manufacturing Losses: The greatest damage to the environment occurs
when the finished product is not of saleable quality and is scrapped.
In this situation all the raw materials, energy and other resources expended
in the production of those items are irretrievably wasted since the scrap
is normally sent to landfill. By comparison with other industry sectors,
loss levels in tableware manufacture are very high with scrap levels as
high as 15-20% not uncommon.
Raw Material Waste Management: Scrap levels for green bodies (i.e.
before firing) are high (typically 5%) by comparison to other process
industries but at this stage the majority of the materials can be recycled
within the factory. However, in the average factory about 2% of body raw
materials cannot be recovered and are discharged either as water-borne
effluent or are reduced to solid and sent to landfill. Scrap body materials
that have been fired (biscuit), typically accounting for 10% of production,
can seldom be recycled and are therefore, also sent to landfill. A significant
exception to this is in Stoke-on-Trent where a local tile manufacturer
(Johnson Ceramic International) has arranged to collect all fired scraps
from the surrounding factories and incorporates them in their tile body.
Glaze materials, despite being of much higher value, are frequently wasted
to a greater degree. It is not uncommon for 10% of the glaze materials
purchased to be discharged as aqueous effluent or landfill. Many coloured
glazes and decoration materials contain toxic heavy metals, which present
an additional environmental hazard.
Plaster Usage: Traditionally, all forming methods employed moulds
made from plaster of Paris. It is estimated that 80% of tableware in the
EC is formed using such moulds. (The alternative is to use long life,
porous plastic moulds.) Each plaster mould has an average useful life
of about 100 cycles and no reliable way has yet been found for reconstituting
the plaster and reusing it to make new moulds. Many scrap moulds are sent
to landfill. Some initiatives exist for recycling the plaster in the construction
industry, for use in the manufacture of plasterboard etc.
In addition to the large volumes of material waste arising from the use
of plaster moulds, the tableware manufacturing process is a high consumer
of energy as each mould must be dried between successive cycles. Each
plaster mould requires on average 2.3 MJ of drying energy during its lifetime
of 100 cycles.
Energy Consumption: A recent survey of energy usage in the UK ceramics
industry showed that natural gas accounts for 84% of total energy consumption
in the tableware sector, with electricity providing 15% (Oil & coal together
account for only 1%). Total UK tableware consumption in 1996 was 5,444
TJ, equating to 68 GJ/tonne of product sold. The energy consumption is
divided between drying (17%), firing (57%) and power, space heating and
lighting (26%). Extrapolating these figures to all of Europe Union suggests
a total energy consumption (for tableware manufacture) of 40,800 TJ, with
a similar split between drying, firing and ancillary services.
The vast majority (81%) of dryers is fuelled by natural gas, with 5% being
electrically heated and only 7% using waste heat from the kilns. This
is still a very low number but the use of waste heat has increased in
recent years.
Kilns used for firing of tableware operate either continuously (tunnel
kilns with densely packed ware cars, single-layer roller or belt kilns)
or intermittently (the ware is static in firing and must be removed and
replaced between firing cycles.) Continuous kilns are usually gas-fired
whereas intermittent kilns normally employ electrical heating. Electricity
now accounts for only about 8% of total firing energy in the tableware
industry.
The total specific energy consumption shows a wide spread from <10 GJ/tonne
to >100 GJ/tonne in some cases. 50 GJ/tonne represents a working average.
Here energy usage is quoted in Joules per dry tonne of raw materials purchased.
Generation of Noxious & Greenhouse Gases:The use of natural gas
for drying and firing in a tableware factory causes the release of substantial
quantities of carbon dioxide (CO2) and water vapour
directly into the atmosphere. Electrical power consumption is indirectly
responsible for the release of carbon dioxide (CO2),
sulphur dioxide (SO2) and nitrogen oxides (NOx)
from the electricity power station generator. ETSU recently quoted the
following emissions for the average UK power station:
134 tonnes of CO2 per TJ electricity generated
1.12 tonnes of NOx per TJ electricity generated
0.38 tonnes of SO2 per TJ electricity generated
Water:
A significant part in the tableware manufacturing process is the use of
water. The body and glaze suspensions are prepared with solids contents
of approximately 46% by weight i.e. 54% water content. Thus for each tonne
of finished product, approximately 1 tonne of water must be extracted by
filter pressing and in ware dryers.
Traditionally, water from the ware dryer is vented to atmosphere as steam,
wasting both water and heat. Water recovered from filter pressing is contaminated
with mineral particles and dissolved chemicals and must be filtered and
chemically cleaned before reuse. The application of decoration by decals
and transfers also requires the use of relatively low quantities of water
in the process however, the levels of contamination by heavy metals and
VOC's are high. This water must also be treated before disposal or reuse.
Treatment and recycling of water is practised in some European factories
but the majority discharges their used water with the minimum degree of
treatment that is needed to comply with the relevant environmental legislation.
Hydrogen Fluoride Emissions: Fluorine is present as an integral component
in most types of clay and cannot be removed before use. This fluorine is
released from the clay, primarily as gaseous hydrogen fluoride (HF), on
firing above 750°C. Atmospheric hydrogen fluoride is now recognised
as a health hazard and as a nuisance to the environment, particularly in
urban areas where it caused the etching of window glass. The concentration
of HF emitted from the kiln chimney stack is influenced by a number of factors:
- the configuration
of the ware loading in the kiln
- the firing
temperature profile
- atmospheric
conditions in the kiln
- the kiln
design.
The recommended
upper limit on HF concentration in the kiln flue gases (soon to be a legal
requirement) is 10 mg/m3. Many European brick manufacturers have
fitted limestone scrubbers to remove HF from the kiln exhaust. However,
these are expensive to install and operate and they produce an environmental
impact themselves through their consumption of electrical power (typically
20kW for a large kiln) and the large quantity of limestone (typically 7500
tonne per year) that they consume. In addition, the limestone has to be
quarried, transported and then sent to landfill when used. Thus scrubbers
are really only transferring pollution from one location to another, not
removing it.
In some cases it has been possible to comply with local requirements on
ground concentration of HF by increasing the height of the kiln stack and
hence dispersing the emergent gases over a large area. However this should
be regarded as a short-term measure only - the main requirement is to minimise
the amount (not the concentration) of HF released to the environment.
The average concentration of HF in tableware kilns is likely to be of the
order of 20-30 mg/m3. Assuming there are 400 tableware kilns within the European
Union, with an average air mass flow rate of 5000 m3/hr, this translates
to approximately 430 tonnes of HF released to the atmosphere in Europe per
year by the tableware industry.
Volatile Organic Compounds (VOC): These occur in most forms of the
materials used in decorating tableware. The VOC's and are normally burned-off
in the firing process with the combustion products being vented to atmosphere.
There is no reliable data on the quantities of VOC's that are emitted by
the tableware industry. |
