Note: Descriptions are shown in the official language in which they were submitted.
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CA 02319360 2000-07-24
"Automatic checking and regulation of cleaner baths by determination of
alkalinity"
This invention relates to a process for the automatic checking and regulation
of cleaner baths,
whereby the free alkalinity and/or the total alkalinity of the cleaner bath is
determined by
way of a measuring and regulating parameter and, where necessary, adjusted by
metering
measures. The process has been conceived in particular for technical cleaner
baths in the
metalworking industry, such as in automobile manufacture. It permits the
functionality of
the cleaner bath, characterised by the parameter "alkalinity", to be monitored
automatically
and, where necessary, enables the cleaner bath to be replenished
automatically, or by external
request, or enables other bath-maintenance measures to be initiated. In
particular, the
0 process is conceived in such-a way that the results of the determinations
are transmitted to
a location remote from the cleaner bath. In addition, it is possible for
interventions to be
made in the automatic measuring sequence from a location remote from the
cleaner bath or
for make-up metering or other bath-maintenance measures to be initiated. The
"location
remote from the cleaner bath" may be situated, for example, in a higher-level
process-control
system, in a central control station of the plant in which the cleaner bath is
located, or even
outside the plant.
The cleaning of metallic parts prior to continued processing thereof
constitutes a standard
task in the metalworking industry. The metallic parts may, for example, be
contaminated
with pigment grime, dust, abraded metal, anti-corrosion oils, cooling
lubricants or reshaping
0 aids. Prior to continued processing, in particular prior to an anti-
corrosion treatment (for
example, phosphatisation, chromatisation, anodisation, reaction with complex
t7uorides, etc)
or prior to lacquering, such contaminants have to be removed by a suitable
cleaner solution.
Spray processes, immersion processes or combined processes are considered for
this purpose.
Industrial cleaners in the metalworking industry are, as a rule, alkaline (pH
values in the
?5 range above 7, for example between 9 and 12). The basic constituents
thereof are alkalis
(alkali metal hydroxides, carbonates, silicates, phosphates, borates) and also
non-ionic and/or
anionic surfactants. The cleaners frequently contain, as additional auxiliary
components,
complexing agents (gluconates, polyphosphates, salts of amino carboxylic
acids, such as
ethylenediamine tetraacetate or nitrilotriacetate, salts of phosphonic acids,
such as salts of ~'
CA 02319360 2000-07-24
' ' _ ~ _
hydroxyethane diphosphonic acid, phosphonobutane tricarboxylic acid, or other
phosphonic
or phosphonocarboxylic acids), anti-corrosive agents, such as salts of
carboxylic acids having
6 to 12 carbon atoms, alkanolamines and foam inhibitors, such as alkoxylates,
closed at the
terminal groups thereof, of alcohols having 6 to 16 carbon atoms in the alkyl
residue.
Provided that the cleaner baths contain no anionic surfactants, cationic
surfactants may also
be used.
By way of non-ionic surfactants, the cleaners as a rule contain ethoxylates,
propoxylates
and/or ethoxylates/ propoxylates of alcohols or alkylamines having 6 to 16
carbon atoms in
the alkyl residue, which may also be closed at the terminal groups thereof. By
way of
0 anionic surfactants, alkyl sulfates and alkyl sulfonates are widely used.
Alkyl benzene
sulfonates are also encountered, but they are disadvantageous from the
environmental point
of view. By way of cationic surfactants, cationic alkylammonium compounds
having at least
one alkyl residue having eight or more carbon atoms are particularly
preferred.
The alkalis in the cleaner bath contribute to the cleaning capacity thereof.
For instance, they
saponify saponifiable contaminants, such as fats and thereby make the latter
soluble in water.
In addition, they contribute to the detachment of insoluble dirt from the
surface of the metal
by electrostatic repulsion, as a result of adsorption of OH ions rendering the
surfaces
negatively charged. As a result of reactions of this type, optionally also as
a result of
removal by entrainment, alkalinity is consumed, so that the cleaning effect
diminishes with
0 time. It is therefore conventional to check the alkalinity of the cleaning
baths at certain times
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and, where necessary, to replenish the solution with new active substances or
to renew it
entirely. This examination is carried out either manually or by means of an
automatic titrator
locally. In this connection the alkalinity is generally checked by titration
with a strong acid.
':
The operating personnel assess the alkalinity on the basis of the acid
consumption and take
''S the necessary measures, such as bath replenishment or bath renewal. This
currently i
conventional process assumes that operating personnel are present in the
vicinity of the
cleaning bath at the requisite checking times.. The shorter the checking
intervals that are
desired, the greater are the demands made on the operating personnel for the
check
measurements.
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' CA 02319360 2000-07-24
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From EP-A-806 244 a process is known for determining the pH of a solution
automatically
and for make-up metering of acid or lye automatically in the event of
anomalies. The object
thereof involves maintaining the pH of a stream of liquid at a predetermined
value. Acid-
base titration is not carried out using this process. In this case, it is
necessary to check the
functionality of the plant in situ. It is not possible to intervene from a
remote location in the
sequence of pH measurements and metering measures.
On the other hand, the present invention sets itself as an object the
automating and
documenting of the checking of cleaner baths by determination of the
alkalinity in such a way
that at least the results of the determination of alkalinity are stored on a
data carrier and/or
0 are output. The measuring device employed should preferably examine and
calibrate itself
and in the event of malfunction transmit an alarm signal to a remote point.
Furthermore, it
should preferably be possible to examine the functionality of the measuring
device and the
results of measurement from a remote point. Moreover, it should be possible
far
interventions to be made in the measuring sequence and in the bath-maintenance
measures
from a remote point. By virtue of the remote control desired, the effort
expended by.
personnel on checking and regulation of the cleaner baths is to be reduced:
This object is achieved by means of a process for automatic determination of
the alkalinity
of one or more cleaning baths containing surfactant by acid-base reaction
using an acid,
whereby, subject to the use of a suitable measuring device under program
control,
0 (a) a sample of specified volume is drawn from a cleaning bath,
(b) if desired, the sample is freed of solids,
(c) a selection is made as to whether free alkalinity and/or total alkalinity
is to be
determined,
(d) the sample is titrated by addition of an acid, or an acid is submitted and
the latter is
2~ titrated with the sample,
(e) the result of the titration is output and/or stored on a data carrier
and/or utilised as
the basis for further evaluations.
The sample volume drawn in step (a) may be permanently programmed into the
control part
of the measuring device to be employed for the process. The magnitude of the
sample
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' CA 02319360 2000-07-24
volume may preferably be changed from a remote location. Furthermore, the
control
program may be so designed that it makes the sample volume to be used
dependent on the
result of a preceding measurement. For instance, the sample volume may be
selected to be
larger, the lower the alkalinity of the cleaner bath. The accuracy of the
determination of
alkalinity may be optimised by this means.
When reference is made to a "remote location", for the present purposes this
is to be taken
to mean a location that is not in direct contact, or at least not in optical
contact, with the
cleaner bath. The remote location may be, for example, a central process-
control system that
checks and regulates the cleaner bath as a sub-task within the context of an
overall process
0 for surface treatment of the metallic parts. The "remote location" may also
be a central
control and observation point from which the overall process is checked and
regulated and
which is located, for example, in a room differing from that of the cleaner
bath. However,
a point outside the plant in which the cleaner bath is located also comes into
consideration
by way of "remote location". By this means it becomes possible for specialists
who remain
outside the plant in which the cleaner bath is located to examine and regulate
the cleaner
bath. As a result, it is necessary for specialist personnel to be detained
substantially less
frequently at the location of the cleaner bath.
Suitable data lines with which the results of the determinations of alkalinity
and also control
commands may be transmitted are known.
ZO Between the drawing of the sample and the actual measurement, it may be
desirable to free
the sample of solids in the optional step (b). In the case of a cleaner bath
that is only slightly
charged with soiids, this is not necessary. However, in the case of a solids
contemt of the
cleaner bath that is too high, valves penaining to the measuring device may
become clogged
and sensors, such as electrodes may be contaminated. It is therefore advisable
to remove a
solids from the sample. This may be effected automatically by filtration or by
using a
cyclone separator or a centrifuge.
In step (c) a selection is made as to whether the free alkalinity and/or the
total alkalinity is
to be determined. This may be input permanently into the program control
sequence. For
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' CA 02319360 2000-07-24
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instance, both the free alkalinity and the total alkalinity may be determined
in one
determination cycle. However, the program may also decide to determine one of
these two
values more frequently than the other. This may be the case, for example, when
determinations carried out previously have shown that one of the two values
changes more
rapidly than the other. Of course, the choice of whether tree alkalinity or
total alkalinity is
to be determined may also be made by means of an external request. For the
present
purposes, the expression "external request" is to be understood to mean that
it is possible for
interventions to be made in the automated determination sequence either by a
higher-level
process-control system or manually via a data line.
0 The terms "free alkalinity" and "total alkalinity" are not unambiguously
defined and are
handled differently by the various users. For instance, particular pH values
may be defined,
up to which titration has to be effected in order to determine either the free
alkalinity or the
total alkalinity, for example pH = 8 for free alkalinity, pH = 4.5 for total
alkalinity. These
preselected pH values have to be input into the control system for the
automatic
determination process. As an alternative to particular pH values, the
transition points of
certain indicators may also be selected with a view to establishing the free
alkalinity and the
total alkalinity. Alternatively, turning-points in the curve of pH values may
be selected and
defined as end points for the free alkalinity or the total alkalinity.
With a view to actual determination of the alkalinity in step (d), use is made
of the acid-base
0 reaction with an acid. A strong acid is preferably selected for this
purpose. In this
connection, the sample may be titrated by addition of an acid until the
specified criteria either
for free alkalinity or for total alkalinity are attained. Alteratively, the
acid may be submitted
and titrated with the sample.
The result of the titration is subsequently output and/or stored on a data
carrier (step (e)).
'_'S In this connection, the data carrier may be situated at the location of
the determination or in
a remote arithmetic logic unit. The expression "output of the result of the
titration" is to be
understood to mean that the result is either passed on to a higher-level
process-control system
or is displayed on a screen or printed out in a recognisable manner. In this
connection, the
location of the display or the output of the result may be the "remote
location" defined
' CA 02319360 2000-07-24
above. Preferably, the results of the individual determinations are stored on
a data carrier at
least for a specified time interval, so that they may subsequently be
evaluated, in the sense
of quality assurance, for example. However, the results of the determinations
of alkalinity
do not have to be output or stored on data carriers immediately. Instead, they
may also be
utilised directly as the basis for further calculations. the results of these
further calculations
being displayed or stored. For instance, instead of the current alkalinity
value in each case,
the trend of the alkalinity values andlor the relative change thereof may be
displayed.
Alternatively, the current alkalinity values may be converted into "% of
nominal content".
In the simplest case, the process according to the present invention operates
in such a way
0 that steps (a) to (e) are repeated after a specified time interval. This
specified time interval
depends on the requests of the operator of the cleaning bath and may comprise
any time
interval in the range from about tive minutes to several days. For quality
assurance it is
preferable that the specified time intervals lie, for example, in the range
between five minutes
and two hours. For instance, a measurement may be carried out every 15
minutes.
However, the process according to the present invention may also be
implemented in such
a way that steps (a) to (e) are repeated after time intervals that are
shorter, the more the
results of two consecutive determinations differ. The control system for the
process
according to the present invention may also decide itself whether the time
intervals between
the individual determinations are to be shortened or lengthened. Of course,
the control
0 system has to be provided in advance with an instruction as to which time
intervals are to
be selected for which differences between the results of consecutive
determinations.
Moreover, the process according to the present invention may be implemented in
such a way
that steps (a) to (e) may be carried out at any time by means of an external
request. As a
result, immediate checking, for example of the alkalinity content of the
cleaner bath, may
be undertaken if quality problems are established in subsequent process steps.
Measurement
of the alkalinity may also take place in a time-controlled manner (in
accordance with fixed
time intervals) or in an event-controlled manner (in the event of established
changes or as a
result of external requests).
CA 02319360 2000-07-24
The process according to the present invention is preferably implemented in
such a way that
the measuring device employed checks itself and, where necessary, recalibrates
itself. To
this end, provision may be made such that, after a specified time interval or
after a specified
number of determinations or by reason of an external request, the
functionality of the
measuring device employed is examined by check measurements of one or more
standard
solutions. For the purpose of examination, a standard solution having known
contents of tree
and total alkalinity is titrated. This examination is closest to reality if a
standard cleaner
solution is employed by way of standard solution, the composition of which
approximates as
closely as possible to the cleaner solution to be examined. The standard
solutions are
preferably maintained subject to exclusion of air or under a protective gas
(nitrogen, for
example).
A central issue when examining the functionality of the measuring device is
constituted by
the checking of the sensor employed. For instance, the latter may be a pH-
sensitive
electrode, in particular a glass electrode. With the aid of a buffer solution
by way of
standard solution, it is possible to examine whether the electrode is
supplying the expected
voltage, whether it responds within the expected time and whether the slope
thereof (_
change in voltage as a function of the change in pH) lies within the nominal
range. If this
is not the case, the measuring device outputs an alarm signal locally or,
preferably, at a
remote location. This alarm signal may contain a suggestion for intervention
that is selected
by the control program of the measuring device or by the higher-level process-
control
system. For instance, it may be suggested that the electrode be cleaned or
exchanged.
In the process according to the present invention there may also be provision
such that the
functionality of the measuring device employed is examined by check
measurement of one
or more standard solutions if the results of two consecutive measurements
differ by a
specified amount. By this means it is possible to distinguish whether
established anomalies
in the alkalinity of the cleaner bath are real and require bath-maintenance
measures or
whether they are being simulated by an error in the measuring system.
Depending on the result of the examination of the measuring device employed,
the
determinations of alkalinity that have taken place between the current check
measurement and
CA 02319360 2000-07-24
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the preceding one may be provided with a status identitier that denotes the
reliability of these
determinations of alkalinity. If, for example, consecutive check measurements
for the
purpose of examining the measuring device employed have shown that the latter
is operating
correctly, the determinations of the alkalinity may be provided with a status
identifier "OK".
If the results of the check measurements differ by a specified minimum amount,
the
determinations of the alkalinity that have taken place in the meantime may,
for example, be
provided with the status identifier "dubious".
Furthermore, provision may be made such that, depending on the result of the
examination
of the measuring device employed, automatic determination of the alkalinity is
continued
.0 and/or one or more of the following actions are carried out: analysis of
established
anomalies, correction of the measuring device, termination of the
determination of alkalinity,
transmission of a status signal or an alarm signal to a higher-level process-
control system or
to a monitoring device - that is to say, to a remote location. Accordingly the
measuring
device may, if desired, decide, in accordance with specitied criteria, whether
it is operational
1.5 to such an extent that the determinations of alkalinity may be continued
or whether anomalies
are established that necessitate manual intervention.
Various sensors are suitable for tracking the acid-base reaction of the
cleaner solution with
the acid employed for the titration. In accordance with the present state of
the art, use will
preferably be made of a pH-sensitive electrode, such as a glass electrode. The
latter supplies
0 a pH-dependent voltage signal which may be evaluated further. The use of an
electrode of
this type is particularly straightforward in terms of apparatus and is
therefore preferred.
However, with a view to tracking the acid-base reaction of step (d), use may
also be made
of an indicator, the pH-dependent interaction of which with electromagnetic
radiation is
measured. For instance, this indicator may be a classical color indicator, the
change in color
of which is measured photometrically. Alternatively, use may be made of an
optical sensor.
In this connection it is, for example, a layer of an inorganic or organic
polymer having a
fixed dyestuff that changes color at a certain pH. The change in color is
based, as in the
case of a classical color indicator, on the fact that hydrogen ions or
hydroxide ions that are
able to diffuse into the layer react with the dyestuff molecules. The change
in the optical
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CA 02319360 2000-07-24
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properties of the layer may be determined photumetrically. Alternatively,
tilms, such as
organic polymers, may be used, the retractive index of which changes as a
function of the
pH. If, for example, a light guide is coated with such a polymer, it is
possible to ensure that
total reflection occurs in the light guide on one side of a threshold value of
the refractive
index, so that a ray of light is transmitted. On the other side of the
threshold value of the
refractive index, however, total reflection no longer occurs, so that the ray
of light exits the
light guide. At the end of the light guide it is then possible to detect
whether the light is
being propagated by the light guide or not. A device of this type is known as
an "optrode".
Furthermore, inorganic or organic solids, the electrical properties of which
change with the
.0 pH of the surrounding solution, may be employed as sensors. For instance,
use may be
made of an ion conductor, the conductivity of which depends on the
concentration of the H''
or OH' ions. By measurement of the d.c. or a.c. conductivity of the sensor, it
is then
possible for the pH of the surrounding medium to be inferred.
The measuring s stem em to ed in the rocess according to the
Y P Y p ~ present invention is .
1_S preferably designed such that it automatically monitors the tilling levels
and/or the
consumption of the reagents employed (acids, standard solutions and test
solutions, possibly
auxiliary solutions) and, in the event of a specitied minimum tilling level
not being attained,
outputs a warning signal. By this means it is possible to prevent the
measuring device from
becoming non-functional as a result of lacking the necessary chemicals.
Monitoring of the
.0 filling levels may be effected using known methods. For instance, the
vessels with the
chemicals may be situated on a balance that registers the respective weight of
the chemicals.
Or use may be made of a float. Alternatively, a minimum filling level may be
examined by
means of a conductivity electrode that is immersed in the vessel containing
the chemical.
i..
The warning signal to be output by the measuring device is preferably
transmitted to the
remote location, so that the appropriate measures may be initiated from there.
In general,
provision is preferably made in the process according to the present invention
such that the
results of the determinations and/or of the check measurements and/or of the
calibrations
and/or the status signals are transmitted to a remote location continuously or
at specified time
intervals and/or on request. By this means, monitoring personnel that do not
have to be 1
s0 present at the location of the cleaner bath are kept constantly informed of
the current
' CA 02319360 2000-07-24
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alkalinity content thereof. Depending on the results of the determinations and
of the check
measurements, necessary corrective measures may be taken either automatically
via- a
process-guidance system or as a result of manual intervention.
In the event of a specified minimum value of the alkalinity (free and/or total
alkalinity) not
being attained or on external request, the simplest corrective measure
involves a device being
activated that meters one or more replenishing components (solution or powder)
into the
cleaning bath. This may be effected, for example, in an automated manner in
such a way
that, depending on the alkalinity content ascertained, a certain quantity of
replenishing
solution or replenishing powder is supplied to the cleaning bath. In this
regard, the
0 magnitude of the added portion itself or, in the case of tirmly specified
added portions, the
time intervals between the individual additions may be varied. This may be
effected, for
example, with the aid of metering pumps or in a weight-controlled manner. In
the process
according to the present invention there is provision, on the one hand, such
that in the event
of certain deviations from the nominal value (particularly if the
functionality of the measuring
device is established by the check measurements), a certain quantity of
replenishing
component is metered into the cleaning bath in order to make it up. On' the
other hand,
however, this make-up metering may also be undertaken by reason of an external
request,
from a remote location for example, irrespective of the current alkalinity
content.
In another embodiment of the present invention the cleaning bath is
replenished in a
.0 throughput-dependent manner using a specitied quantity of replenishing
component per unit
passed through (basic metering). For instance, in the case of a cleaning bath
for automobile
bodies the quantity of replenishing component that is added per cleaned body
may be
established. The checking, in accordance with the present invention, of the
alkalinity then
serves to check and to document the success of this specified addition and
also, by means of
?5 additional event-dependent fine metering (additional metering in the event
of the nominal
values not being attained, suspension of the basic metering in the event of
the nominal values
being exceeded), to achieve a more constant mode of operation of the cleaning
bath. By this
means, fluctuations in quality are reduced.
Of course, the process according to the present invention assumes that the
appropriate device
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CA 02319360 2000-07-24
is made available. The latter includes a control system, preferably a computer
control
system, which controls the course of measuring in a time-dependent and/or
event-dependent
manner. It must furthermore include the necessary reagent vessels, pipelines,
valves.
metering and measuring devices etc for regulation and measurement of the
sample streams.
The materials should be adapted to the intended use, for example they should
consist of high-
grade steel and/or synthetic material. The control electronics of the
measuring device should
comprise an appropriate input-output interface, in order to be able to
communicate with a
remote location.
The process according to the present invention makes it possible, on the one
hand, to check
.0 the alkalinity of cleaning baths in si a and to initiate specified
corrective measures without
manual intervention. By this means the process safety is enhanced and a
constantly reliable
cleaning result is achieved. Deviations from the nominal values may be
detected at an early
stage and corrected automatically or manually before the cleaning result is
impaired. On the
other hand, the measured data are preferably transmitted to a remote location,
so that
operating or supervising personnel are kept constantly informed of the state
of the cleaning
bath also when it is not located in the immediate vicinity thereof. The effort
expended by
personnel on checking and regulation of the cleaning bath may be considerably
reduced by
this means. By virtue of the documentation of the data collected in the
process according to
the present invention, the requirements of modern quality assurance are taken
into account.
The consumption of chemicals may be documented and optimised.
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