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Cleaner Technology Projects in Denmark 1997
Rensning og genanvendelse af affedterbade i galvanoindustrien
Arbejdsrapport nr. 53, 1997, Miljøstyrelsen
Large volumes of alkaline cleaning baths are every year thrown out from the Danish metal finishing industry. From the electroplating industry
alone 2,500 m3/year are disposed of. The baths are used for cleaning metal parts for oil,
grease and dirt, often as pre-treatment before the final finishing process. The cleaning
baths are continuously polluted during the operation and are normally disposed of when the
content of pollutants gets too high. The possibilities of cleaning and recycling the
cleaning baths to lower the environmental impacts are examined in this project.
When a cleaning bath is disposed of it still contains a good deal of active cleaning
chemicals. The waste, however, is reduced if the bath is continuously treated and
therefore only seldom needs replacement. Continuous purification of the baths also makes
it possible to accumulate and re-cycle the drag-out chemicals, which means even greater
savings in chemical consumption. At the same time it is possible by using this technique
to reduce or even to eliminate the discharge of cleaning chemicals, oil and grease along
with the wastewater.
In this project we have examined the possibilities of purifying the cleaning baths by
ultra- and micro-filtration. This technique is commonly applied abroad, but has only been
sporadically tested in Denmark, - with poor results. This project is focused on the
combination between applied membranes and cleaning chemicals and on other
important technical factors too.
Ultra- and micro-filtration retain large molecules like oil and grease whereas only
fine membranes retain tensides. The inorganic salts are normally not retained. In practice
this means that it is important to apply membranes, which are able to retain oil and
grease but allow tensides and other cleaning chemicals to penetrate.
Screening tests have been made with 7 different cleaning chemicals; 3 for alkaline
degreasing; 3 for electro-cleaning and 1 for pickling. Also, 1 different type of membrane
has been tested, 1 micro-filtration membrane and 3 ultra-filtration membranes. Two of the
membranes are of the hollow-fibre type while the others are of plate and frame.
The membranes are tested by filtration of a solution of pure cleaning chemicals in
normal operation concentrations in order to examine if the membranes retain tensides and
other organic chemicals. If so, the respective membranes are not suited for purification
of cleaning baths containing these chemicals. Finally, there have also been made some
tests on purification of polluted cleaning baths.
It was established during screening tests that the MF-membrane from K-flow can be
applied in connection with most cleaning chemicals. Only 1 out of the 7 investigated
chemicals was retained in a large amount. The UF-membranes can also be used for some
applications, but surfactants are often partly retained. The finest UF-membranes generally
retain more chemicals than coarse membranes. The efficiency of the membrane is obviously
first of all determined by the size of the pores, and there is no evident difference
between hollow fibres versus plate and frame membranes. All of the tested membranes are
made of polyphone/polyvinylpyrolidone (PES/PVP).
By alkaline degreasing approx. 80% of the oil is removed by each purification process.
The investigated bath was treated 5 times during the first year of operation (approx.
1,500 operational hours) and it has not been disposed of. Previously, the bath was
replaced twice a year. The average oil content has been considerably lower than before and
thus resulted in less oil drag-out in wastewater. 25% of degreasing chemicals were saved
in the first year of operation. Furthermore, our investigation indicated that additionally
30-50% of chemical savings are possible, if the content of tensides is kept at a suitable
low concentration.
By electro cleaning about 90% of the oil is removed in each purification process. The
investigated bath was treated 5 times during the first year of operation (approx. 1,450
operational hours) and it has not been disposed of. Previously, it was disposed of twice a
year. 25% of degreasing chemicals were saved in the first year of operation, as the bath
had not been disposed of. However, the content of tensides has been unnecessarily high in
this period, and therefore additional savings on 30-40% of chemicals are possible, if the
content of tensides is kept at a suitable low concentration.
Our investigations have established that the actual pickling cleaner is not polluted
appreciably with oil. One single treatment of the bath lowered the oil content from 70 to
40 mg/l. In the future it is possible to stop further purification of the pickling
cleaner, and to replace the bath only when the metal content has become too high.
Laboratory tests have confirmed that polymer MF-membranes and sometimes also
UF-membranes are applicable when the chemical composition of the bath is matching the
membrane. It is important that the surfactants are not retained and that fouling of the
membranes is avoided. Therefore, chemicals containing silicate should be avoided.
An oil-absorbing filter was tested on a degreasing bath. Mechanical filtration is done
through a filter bag of oil absorbing material. Preliminary tests show that only small
quantities of tensides are removed in the filtration. Because the items were degreased in
"tri" first, the contamination of the degreaser was very low. After this
observation, it is now possible to prolong the lifetime of the degreasing bath just like
that by factor 3. Previously the degreasing bath was disposed of routinely once a month,
today it is once every third month. This has given the company vast chemical savings and
minimised waste volume.
Summing up on the ABS-filter testing it is clear that it can be used for purification
of certain cleaning baths. However, no measurements or analyses documents the ability of
the filter to remove oil and grease, but practical experiences seem to stress the
applicability of the filter or removal of oil and dirt in alkaline cleaning baths. The
tests also show that the method cannot be used in connection with all types of cleaning
baths. The method has low initiating costs and can be used with advantage by small
companies.
Author/ institution
Flemming Dahl, Peter Hardis og Tom Hornshøj-Møller,
Miljø-Kemi, Dansk Miljø Center A/S
This report is subsidised by the National Council for Recycling and Cleaner Technology
ISSN no. 0908-9195
ISBN no. 87-7810-829-2
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