Note: Descriptions are shown in the official language in which they were submitted.
QM 36074
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CLEAN NG OF_RTICLES
This inventlon relates to the cleaning oE
contaminated articles using organic solvents.
In the past, halogenated solvents have been in
widespread use for cleanlng industrlally manufactured
articles and, whllst such solvents are particularly
advantageous in terms of their volatility, absence of
flash point and abil ty to dissolve contaminants
typically encountered, the use of such solvents is
currently perceived as being unsatisfactory from the
environmental standpoint.
Alternative hydrocarbon solvents have been
proposed such as terpene solventsi for example, see
published European Patent Application No. 354027.
However, known alternative solvents have relatively low
flash points and are therefore potentially hazardous if
used in cleaning plant in which the solvents are heated
to elevated temperatures in the course of effecting
cleaning of the articles and/or removing the solvent from
the articles. Also, especially in the case of terpene
solvents, cleaning efficiency deteriorates rapidly with
build up of dissolved contaminants within the solvent
thereby giving rise to a need for frequent replacement of
spent solvent with fresh solvent.
According to one aspect of the invention there is
provided a process for the cleaning of contaminated
articles, comprising:
(a) contacting the articles with a non-chlorinated
organic solvent at a temperature below the flashpoint
thereof, the solvent being capable of combining with the
contaminant(s);
(b) contacting the solvent bearing articles with an
aqueous rinsing medium at a temperature below said
flashpoint to effect removal. of the solvent from the
articles;
(c) collecting the contaminated solvent;
(d) heating the collected solvent in a fire hazard
containment 70ne to produce a vapour containing the
solvent;
(e) condensing the solvent vapour; and
(f) recycling the solvent from step (e) for further
contact with the articles in step (a).
Thus, in accordance with the invention, a
non-chlorinated (usually non-halogenated) solvent is use~
for cleaning of the articles and, in the course of the
cleaning and rinsing stages, the solvent is at all times
at a temperature below its flashpoint thereby eliminating
any fire hazard, and the solvent is recovered in a
heating stage separate from the solvent contacting and
rinsing stages and forming a fire hazard containment
zone.
Preferably the temperature of the rinsing medium
in step (b) is at least 5C, and often at least 10C,
below the flashpoint (closed cup) of the solvent.
Typically the temperature at which the solvent is
contacted with the articles in step (a) is at least 5C,
often at least 10C, below the Elashpoint (closed cup) of
the solvent.
Preferably contacting of the articles with the
solvent is effected by immersion of the articles in -the
solvent and contacting of the solvent bearing articles is
conveniently effec~ed by immersion of the articles in the
rinsing medium.
Usually, solvent collected from step (b) will
contain rinsing medium; the collected mixture may either
be separated and only the solvent treated in accordance
with step (d) or it (the mixture) may be treated in
accordance with step (d) in which case step (e) may
comprise condensing the solvent and rinsing medium, the
latter being recycled for use in step (b).
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The recycled components, whether solvent or
aqueous, are conveniently re-introduced in such a way as
to displace contamlnated solvent or rinsing medium
respectively and the displaced quantities are fed to said
zone for heating in accordance with step (d).
In general, the solvent will be non-halogenated
and may comprise a hydrocarbon or mixture of hydrocarbons
which may be in admixture with a co-solvent such as an
alcohol. The organic solvent and the rinsing medium will
in general be substantially immiscible with one another.
Preferably, in the fire hazard containment zone,
oxygen is substantially excluded from contact with the
solvent by producing an atmosphere of an inert gas or
vapour which may be a liquid phase.
Usually contaminated solvent derived directly from
step (a) is also supplied to the containment zone for
treatment in accordance with steps (d) to (f).
According to a second aspect of the invention
there is provided a process for the cleaning of
contaminated articles, comprising:
(a) contacting the articles with a non-chlorinated
organic solvent at a temperature below the flashpoint
thereof, the solvent being capable of combining with the
contaminant(s);
(b) contacting the solvent bearing articles with an
aqueous rinsing medium at a temperature below said
flashpoint to effect removal of the solvent from the
articles;
(c) collecting at least the contaminated solvent from
step (a) and/or (b);
`(d) subjecting the collected solvent to steam
distillation to produce a vapour containing the solvent
and steam such that the s-team is effective to
substantially exclude oxygen thereby preventing ignition
of the organic solvent vapour;
' ~- ' ' ' '
(e) condensing the vapour; and
(f) separating the solvent ~rom the condensate.
Various other features and aspects of the
invention will become apparent from the followlng
description and the appended claims.
The invention wlll now be described by way of
example with reference to the accompanying drawings, in
which:
Figure 1 is a schematic view of cleaning plant in
accordance with the invention and embodying one form of
steam distillation unit/fire hazard containment zone;
Figures 2 to 4 are schematic vie~s of alternative forms
of steam distillation unit/ fire hazard containment zone
which may be used in place of the s-team distillation/fire
hazard containment zone shown in Figure 1.
Referring to the ~igure 1, the cleaning plant
comprises a first tank 10 containing a non-chlorinated
organic solvent (for example, a steam distillable
terpene-based compound) suitable for dlssolving the
contaminants, such as grease, on articles to be cleaned~
In a typical application, the articles comprise printed
circuit boards from which rosin flux residues remaining
after soldering processes are to be removed. The articles
are typically introduced as a batch into the tank through
the top thereof for immersion in the solvent. The solvent
is maintained at a temperature substantially below its
flashpoint, eg. about 35c in the case of a terpene based
solvent having a flashpoint of about 55C. It is to be
understood that the invention is not limited to terpene
based solvents however, Eor example, the organic solvent
may take various forms such as a mixture of
non-halogenated hydrocarbons combined with a polar
organic solvent, such as an alcohol, to increase the
solvent power.
After immersion ln the wash tank, the
solvent-washed articles whlch will have a film of the
hydrocarbon solvent on their surfaces are then immersed
in one or more water rinse tanks 12 (only one is
illustrated in the drawing) in which the temperature is
held below ~he flashpolnt of the hydrocarbon. Where more
than one rinse tank 12 is used, they will be operated in
cascade fashion with the rinse water flowing from the
final rinse tank to the first. A rinse additive may be
included in the water. Following rinsing, the articles
are subjected to hot air drying in vessel 14, typically
at a temperature of the order of 50 to 140C. The contents
of either or both of the tanks 10 and 12 may be agitated by
any suitable means such as pump agitation or ultrasonic
agitation.
During the cleaning cycle, the soil removed from
the articles will tend to collect in the wash tank 10 and
the rinse tank(s) 12 and the build up of the contaminant
affects the efficiency of cleaning. To reduce the
frequency of replacement of contaminated solvent with
fresh solvent, the system incorporates a concentrator
stage 16 which effects steam distillation. The
concentrator vessel is pre-charged with a mixture of
water and the organic solvent, the mixture containing
water and organic solvent in proportions allowing
co-distillation to be carried out at a temperature
somewhat below the boiling point of the organic solvent.
In the concentrator, the contents are heated to vaporise the
water/organic solvent mixture. The vapour is condensed in a
trough 18 equipped with cooling coils 19 or the like and the
resulting liquid phase is fed to a separator 20 for
separating the water from the organic solvent.
The water is then recycled via feed line 22 to the
rinse tank 12 (or the final rinse tank when more than one
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rinse tank is used) and is introduced at or near to the
base of the rinse tank so as to displace an equal volume
of water lrhe dlsplaced volume, which will contain the
organic solvent and any contamlnant carried over by the
organic solvent, is extracted by means of a weir
arrangement 24 and is fed via feed line 26 to the
concentrator 16.
Similarly, the organic solvent from the separator
20 is fed via feedline 28 incorporating cooler 30 into
the wash tank 10 at or adjacent the base thereof to
displace contaminated organic solvent, eg. via a weir
arrangement, and the displaced organic solvent is fed via
feedline 32 to the concentrator vessel 16 for
vaporisation. The cooler 30 is optionally provided ln order
to reduce the temperature of the organic solvent to a value
below its flashpoint. Alternatively, the cooler 30 may be
eliminated if the condenser 1~ is designed so as to ensure
that the condensed solvent is cooled to a temperature
below its flashpoint.
Since the contamination will be non-volatile, it
will remain in the concentrator vessel and only
substantially p-ure water and organic solvent will be
di.stilled off for recycle to the rinse and wash tanks
respectively. To reduce the load on the concentrator, a
separator (not shown) for separating the water and
organic solvent may be fitted to the outlet of the water
rinse tank (or final rinse tank where appropriate). The
water from this separator may be fed to the concentrator
while the organic solvent may be returned to the wash
tank 10. The cleanliness levels of the wash and rinse
tanks for a given contaminatiorl input rate and carryover
rate is a function of the amount of organic solvent/water
boiled off in the concentrator which is directly
proportional to the ~leat input to the concentrator, thus
allowing the cleanliness levels to be determined by
control of the heat input to the concentrator.
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In the concentrator, the organic solvent will be
above its flashpoint and to eliminate fire hazards, the
concentrator unit is designed accordingly and may be
encased in a suitable enclosure 34 for con~alnment of any
fire risk, the interior of the enclosure 34 being
maintained at a negative pressure by means of a fan 36.
In practice, the contents of the enclosure will be
classified as a flammable area and will be subject to
appropriate safety measures, such as the exclusion of all
potential spark-discharge producing equipment.
The cooler 30 is also enclosed within the
enclosure 34 and downstream of the cooler, the
temperature of the rluid within the feedline 28 is sensed
by a suitable sensor (not shown) to ensure that the
liquid is below its flashpoint by at least a
predetermined amount; if the temperature registered by
the sensor is above a preset level, means may be provided
to produce an alarm and/or prevent further recycling of
fluid from the enclosure 34 back to the wash tank 10.
Primary fire hazard containment is achieved by
filling the interior of the vessel 16 with an inert gas
such as nitrogen at least at start up of operation so as
to exclude oxygen. In operation of the vessel 16, steam
is generated which displaces the inert gas and the steam
so generated excludes ingress of oxygen into the vessel
16 to maintain security against fire as a result of
solvent vapour coming into contact with air at levels
above the lower explosion limit of the solvent vapour. If
desired, an inert gas atmosphere may be maintained in the
interior of the enclosure 34.
Referring now to rigure 2, the steam distillation
unit or concentrator stage shown is intended to be used
in place of that shown in Figure 1. The concentrator
vessel 50 is located within a flame proof enclosure 52
from which all potential spark discharge-generating
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equipment, such as electrical motors etc, is excluded.
Although not shown in Figure 2, solvent and water from
the tanks 10 and 12 are supplied to the vessel 50 in the
same manner as described in connection wlth Figure 1 and
the vessel is pre-charged with solvent and water in
predetermined proportions to prcduce a mixture which can
be co-distilled at a temperature somewhat below the
boiling point of the solvent; typically the mixture can
be vaporised at a temperature of the order of 100C.
The base of the vessel 50 is sloped to allow the
contaminant to collect for periodic drainage by opening of
drain valve 54. The contents of the vessel 50 are heated by
a heater comprising a charnber 56 through which hot fluid,
eg water, is circulated via inlet and outlet lines 55 and
57, the temperature of the hot fluid being sufficient to
heat the vessel contents to a temperature at which the
water and solvent co-distil.
In addition to being charged with organic solvent
and water (respectively depicted by reference numerals 58
and 60), the vessel 50 is also initially charged with an
inerting liquid 62 which has a higher density than water
and the organic solvent and a lower boiling point than
the temperature at which the water/solvent mixture boils
and also lower than the flashpoint of the solvent. Above
the liquid contents of the vessel 50, headspace 51 is
present in which vapour collects when the contents are
heated. Vapour accumulating in the vessel interior passes
through pipe 63 and into water cooled
condenser/sub-cooler 64 in which the vapour is condensed
and cooled to a temperature below the flashpoint of the
solven~t.
The lighter condensed water/solvent mixture is
separated by a weir arrangement 66 and passes via line 67
to a phase separator 6~ which is the counterpart of the
separator 20 in Figure I and which has outlets 69, 71 for
feedlng water and solvent respectlvely back to the tanks
10 and 12. The heavler inerting liquid settles to the
base of the condenser/sub-cooler 64 and is fed to a
catchpot 70, the outlet of which is provlded with a pair
of valves 72, 74. Opening and closing of the valve 72 is
controlled by a temperature controller 76 which is
responsive to the temperature prevailing in the headspace
51 of the vessel 50. When open the valve 72 allows the
inerting liquid to be returned to the vessel 50 via line
73 and, when closed, prevents such return flow so that
the inerting llquid is stored in the catchpot 70. The
valve 74 is a flow control valve which serves to control
the rate at which the inerting liquid can flow back into
the vessel 50.
In operation, at start-up the vessel 50 contains
the inerting liquid which because of its higher density
will reside at the base of the vessel. At this stage, the
valve 72 is opened either manually or automatically in
response to supply of heated fluid to the chamber 56. As
the contents of the vessel are heated by flow of the
heating fluid through the chamber 56, the inerting liquid
by virtue of its lower boiling point begins to vaporise
and fill the headspace 51 above -the upper liquid level
thereby displacing any air to atmosphere via vent 30 and
hence excludin~ substantially all oxygen initially
present in -the headspace 51 prior to the commencement of
vaporisation of the solvent/water mixture. At this stage,
the inerting compound is circulated around a closed path
comprising the vessel 50, condenser/sub-cooler 64,
catchpot 70 and back to the vessel and is in the liquid
phase as it passes through the catchpot and back to the
vessel 50.
As the contents of the vessel 50 heat up further,
the solvent/water mixture begins to vaporise, eg at a
temperature close to 100 C, and because of the inert
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vapour atmosphere already created in the headspace 51,
solvent vapour generation takes place in substantially
oxygen free conditions thereby minimising the risk of the
solvent vapour being ignited. As heating continues, the
headspace 51 gradually fllls with solvent/water vapour
which displaces the vapour generated from the inerting
liquid and the temperature within the headspace 51 and
hence the plpe 63 rises to the boiling polnt of the
solvent/water mixture. When the temperature controller 76
registers a predetermined temperature corresponding to
that at which the solvent/water mixture vaporises, eg
about 100C, the valve 72 is closed, possibly after a
short time delay to ensure that the headspace is filled
with the solvent/water vapour. Closure of the valve 72
leads to the inerting liquld being collected and stored
in the catchpot 70 where it remains until the
concentrator unit is to be shut-down.
In this manner, the solvent/water vapour gradually
displaces the inerting vapour and the inerting liquid is
removed from the vessel 50. At this stage, it will be
noted that oxygen is substantially excluded by the
atmosphere of steam and solvent vapour present in the
headspace of the vessel 50 thereby minimising any risk of
solvent vapour ignition. The separation of the
contaminant from the solvent and water then proceeds in
the fashion descri.bed in connection with ~igure 1.
When ~he operation of the concentrator is to be
discontinued, the fluid supply to chamber 56 is
terminated and the heater begins to cool down. At this
time or just prior to cessation of heating, the valve 72
is opened (eg. automatically in response to operation of
a shut-down switch) thus allowing the inerting liquid to
re-enter the vessel. 3ecause of its low boiling point,
vaporisation of the inerting liquid re-comrnences and the
inerting vapour then builds up in the headspace to
replace the solvent/water vapour which, following
cessation of heating, begins to condense. By appropriate
timing of opening of valve 72, it is possible to ensure
that oxygen continues to be excluded from the headspace
51 while the solvent is in the vapour phase. This
condition is then maintained until the contents of the
vessel have cooled sufficientlY below the flashpoint of
the solvent and hence a safe condition attained. The
inerting liquid may either be collected in the vessel 50 or
in the catchpot 70 for re-use when the concentrator is next
operational. In the latter event, it will be appreciated
that the valve 72 will need to be opened on start-up to
allow the inerting liquid to re-enter the vessel 50.
The inerting liquid may comprise any suitable
chemically inert liquid having a sufficiently low boiling
point that it will begin to vaporise before the organic
solvent; usually it will also have a density greater than
the aqueous rinsing medium and the solvent and be
substantially immiscible with the rinsing medium and the
organic solvent. Suitable compounds may be selected from
highly or fully fluorinated organic compounds comprising
predominantly carbon and fluorine atoms, optionally with
a minor proportion of hydrogen atoms with or without
oxygen or nitrogen atoms, eg a perfluoroalkane such as
that mahufactured by 3Ms under the trade name
"Fluorinert" FC72.
Referring to Figure 3, this illustrates another
form of the steam distillation or concentrator unit which
is similar to that shown Ln Figure 2 in many respects and
like parts are therefore identified by the same reference
numerals as used in Figure 2. This embodiment is adapted
to ensure that the inerting fluid is vaporised in
quantities sufficient to fill the headspace 51 rapidly.
The vessel 50 is provided adjacent its base with two
compartments 82 and 84, one (82) of which is intended to
contaln the inerting liquid 62 and the other (8g) of
which contains the main bulk of the water (layer 60) and
organic solvent (layer 58) from which the contaminants
are to be separated. In practice, the inerting liquld 62
will be covered by shallow layers of water and solvent
86, 88 but, in contrast with the embodiment of Figure 2,
the vapour generated from the inerting liquid does not
have to travel through the main bulk of the water and
solvent in order to reach the headspace 51 which would
otherwise tend to re-condense the vapour to some extent
especially on initial start-up of operation.
Heating of the contents of the vessel 50 is
effected by hot fluid passing through heat exchange
piping 90 or the like, the latter being shaped as shown
in order to provide an extended heating path within the
compartment 82 and also to heat the inerting fluid
directly at positions distributed throughout the depth of
the compartment 82. Operation of this embodiment is
generally the same as described in connection with Figure
2; during initial start-up, the inerting fluid (eg.
Fluorinert FC72) is vaporised and accumulates in the
headspace S1 to exclude oxygen and is cycled around a
path including condenser/sub-cooler 64 and the catchpot
70 and back to the compartment 82. Once the temperature
reaches a predetermined value indicative of the presence
of a substantial quantity of solvent/water vapour in the
headspace 51, the valve 72 is closed manually or
automatically and the inerting liquid is collected in
catchpot 70 and stored there until operation is to be
discontinued. At shut-down, the valve 72 is opened to
allow the inerting li~uid to flow back into the
compartment 82 so that, as the heat exchanger cools down,
inert vapour is supplied to the headspace to maintain
exclusion of oxygen until the contents of the vessel 50
have cooled to a point below the flashpoint of the
solvent and hence a safe condition achieved.
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Referring tG ,-igure 4 in which a further
modification of the steam distil.lation/concentrator unit
is shown. Agaln, where appropriate, the same reference
numerals are used to indicate components in Figure 4
which are similar and function similarly to those shown
in Figure 2. In the embodiments of Figures 2 and 3, the
inerting liquid is heated within the vessel 50 by the
same heating source as used to heat the water and
solvent. In Figure 4, the inerting liquid is stored in a
separate vessel 92 which receives, via line 94, condensed
inerting fluid from the condenser/sub-cooler 64 and is
provided with d heater 96 (which may be an electrical
resistance heater for instance) for vaporising the
inerting liquid within the vessel 92. The inerting vapour
passes to the headspace 51 within vessel 50 via valve 98
which is controlled by means of controller 76 in similar
fashion to valve 72 in Figure 2. Heating of the contents
of vessel 50 is effected by hot fluid passing through
heat exchanger 90.
In this embodiment, when operation of the
concentrator unit is initiated, the heater 96 is
energised and valve 98 is opened so that vapour is
generated in vessel 92 and supplied to the hèadspace 51
for the purpose of excluding oxygen. Condensed inerting
vapour is returned to the vessel 94 to complete the
cycle. When the temperature prevailing in the headspace
51 reaches a predetermined value indicative of the
presence of a substantial ~uantity of water/solvent
vapour in the headspace, controller closes valve 98 and
also produces a signal to switch off the heater 96. All
of the inerting fluid is then gradually returned to the
vessel where it is stored until shut-down of the
concentrator unit. At shut-down or just prior thereto,
heater 96 is energised and valve 98 opened to allow a
blanket of vaporised inerting fluid to be generated in
- 14 -
headspace 51 until the temperature wlthin the vessel 50
falls to a point below the flashpoint of the solvent. At
that stage, the heater 96 is switched off and the valve
98 is closed so that the inerting fluid can be collected
in vessel 92.
In each of the embodiments shown in Figures 2 to
4, means may be provided for preventing the contents of
the vessel being hea~ed if the inerting fluid is not
present or not present in a sufficient quantity to ensure
adequate exclusion of oxygen. Thus, for example, in the
embodiment of Figure 3, the compartment may house a float
which has a density greater than that of the solvent and
water but less than that of the inerting liquid so as to
register the level of the inerting liquid within that
compartment and means may be provided for sensing the
position of the float prior to operation of the
concentrator unit so that, if the float is below a
predetermined position within the compartment 82, supply
of heat, ie. in this case via the heat exchanger 90, is
disabled and a signal may be produced to alert an
operator to the need to top up the contents of the
compartment 82. In addition, or alternatively, the
compartment 82 may be provided with a transparent viewing
window to enable the contents oE the compartment 82 to be
inspected with or without the aid of a float so that
before initiating operation a check can be made that the
inerting li~uid content is within acceptable limits.
It will be seen that the steam distillation unit
forms a hire hazard containment zone. Although the
operating t~mperature of the steam distillation unit is
above the flashpoint of the solvent, the risk of ignition
of the solvent vapour when the distillation unit is in
its normal steady state condition of operation is
eliminated since air is precluded from entering -the unit
by the steam generated from the water which is boiling.
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Moreover, during heatlng up and cooling down of the
liquid contents of the distillation unit, even though
vapour generated fr~m the solvent/water contents will
pass through the flammable limits of the solvent (which
would otherwise represent a potentially dangerous
situation, such vapour is produced in the presence of the
inerting vapour which serves to exclude air during the
initial heating phase and during the cooling down phase
of operation. The use of an inerting fluid as described
in the embodiments of Figures 2 to 4 will be seen to be
particularly effective compared with for example a
nitrogen purging system which tends to be less safe and
requires more intensive operator monitoring.
