Cleaning processes using cleaners exhibiting cloud point behavior

A cleaning process to remove soil using cleaners exhibiting cloud point behavior (i.e., have a cloud point) and/or in which the cleaner can be recycled is disclosed. The cleaning mixture can be an aqueous cleaning mixture. The temperature can be adjusted so that the cleaning mixture is contacted with the object to be cleaned at a temperature at or above the cloud point temperature, and the temperature and/or composition of spent cleaning mixture can be adjusted so that it is processed to form a soil-enriched stream and a soil-depleted stream while it is below its cloud point temperature. Processing to form the soil-enriched and soil-depleted streams can involve adjusting the system so that at steady state ##EQU1## where C.sub.Washer is the concentration of cleaner in the spent cleaning mixture and C.sub.Washer Target is the desired concentration of cleaner in the cleaning mixture prior to contacting the object to be cleaned. Desirably a hyperhydrophilic membrane is used in a filtration process to form the soil-enriched and soil-depleted streams from the spent cleaning solution.

BACKGROUND OF THE INVENTION 
This invention concerns the use of cleaners and particularly aqueous 
cleaners. More specifically, in one aspect this invention concerns the use 
of certain cleaners exhibiting the desired cloud point behavior (i.e., 
have a cloud point that occurs as the temperature rises) and in another 
aspect concerns the recycling of cleaners regardless of whether they 
exhibit cloud point behavior. Preferably, the cleaners are aqueous 
cleaners. 
The use of aqueous cleaning processes (alkaline and acidic) for parts 
cleaning and degreasing is increasing as ozone-depleting solvents (e.g., 
trichloroethylene, methylene chloride) are being legislated out of use 
(phase out date of 1995). Using aqueous cleaners is considered safer and 
environmentally more acceptable than solvent degreasing. Also, aqueous 
cleaning agents are reported to produce cleaner surfaces than those 
resulting from solvent degreasing because solvent cleaning often leaves 
residual soils. 
Aqueous cleaners, however, are not free from drawbacks. Unlike vapor 
degreasing methods, which recycle the active solvents, aqueous cleaners 
have rarely been recycled. The aqueous cleaning process typically is a 
batch operation, ending when the cleaning solution capacity has been 
exhausted by accumulated oils and other soils. That results in both high 
cleaner replacement cost and a cost for oily waste disposal. The disposal 
problem also occurs with biodegradable cleaners, because once they are 
contaminated with waste oils and dissolved metals they cannot be 
discharged to a drain. 
Alkaline cleaning is the dominant aqueous cleaning method for primary soils 
removal. Alkaline cleaners are effective for removing unpigmented oils, 
greases, and cutting or grinding fluids from the objects to be cleaned in 
both immersion and spray washer cleaning systems. Alkaline cleaners are 
moderately effective for removing soils containing pigmented drawing 
compounds such as zinc oxide, flour, graphite, and stearates. Alkaline 
cleaners employ both physical and chemical cleaning mechanisms. Whether a 
cleaner is alkaline or acidic, a cleaner's physical action reduces the 
surface tension of the soils, thereby emulsifying and lifting them away 
from the object. An alkaline cleaner's chemical action (i.e., via its 
hydroxide groups) saponifies or hydrolyzes certain soils (e.g., lard) to 
produce soaps. 
In general, an alkaline cleaner is composed of a mixture of three major 
components: (1) builders, (2) organic or inorganic additives, and (3) 
surfactants. The most common builders include sodium metasilicate, sodium 
hydroxide, sodium bicarbonate, and different forms of sodium phosphate. 
Hydroxides and bicarbonates are inexpensive forms of alkalinity. The 
silicates act to disperse soil and protect certain metals (e.g., aluminum) 
from attack by alkali salts Additives soften the water (the solvent or 
carrier) by binding metal ions. Additives include chelating agents such as 
sodium gluconate, sodium citrate, and tetrasodium ethylenediamine 
tetraacetate (EDTA). Typically, anionic and nonionic surfactants are used 
in formulations for immersion cleaning baths, In formulations for spray 
cleaners, nonionic surfactants are used almost exclusively because of 
their low-foaming characteristics. 
Acid cleaning is used less often than alkaline cleaning for degreasing 
metal parts, because acid cleaners capable of removing heavy oil and 
grease deposits are expensive. Acid cleaning typically is used in 
combination with alkaline cleaning to pre-treat parts before finishing and 
is effective at removing oxides and rust from metal surfaces without the 
application of heat. In some cases, acid cleaning has been found to be 
even more effective than alkaline cleaning for removing soils contaminated 
with pigmented drawing compounds. As with alkaline cleaners, the 
mechanisms involved in acid cleaning are both chemical and physical, 
depending upon the composition of the cleaner. The chemical action of the 
acid cleaner reduces oxide films and rust. 
Simple acid cleaners can be composed of mineral acids (e.g., hydrochloric 
and sulfuric), organic acids (e.g., citric, acetic, gluconic, and oxalic), 
and other acid salts (e.g., sodium phosphates). More complex forms of acid 
cleaners contain surfactants (e.g., polyether alcohols), inhibitors (e.g., 
thiourea, wheat flour), and solvents (e.g., glycol ethers). 
To illustrate the economic advantage of recycling if it could be 
successfully accomplished, assume that a given washer is operated at a 
detergent concentration of 5%v/v (volume/volume), disposed of semiweekly, 
and operated 48 weeks per year. Assuming a typical cleaner cost of $10 per 
gallon of concentrate and a waste disposal cost of $1 per gallon of waste, 
the yearly operating cost for this washer is $36 per gallon of washer sump 
capacity per year. For a 300 gallon washer, the net operational expenses 
exceed $10,000, excluding the costs of capital and labor required to 
operate and maintain the system. By recycling this cleaner, the annual 
operating costs could be reduced to less than $1,000 while improving 
overall cleaning quality and consistency. Similarly, the annual savings 
generated by recycling 500- and 5,000-gallon washers would exceed $16,000 
and $160,000, respectively. 
J. K. Liou, "The Technical And Economical Feasibility Of Ultrafiltration 
With CARBOSEP For The Regeneration Of Degreasing Baths," 8 pages (ca. 
1990); E. Park et al., "Vibratory Solution Recycling: A Case Study In 
Pollution Prevention," 4 pages (ca. 1991); H. J. Weltman et al., 
"Replacement Of Halogenated Solvent Degreasing With Regenerable Aqueous 
Cleaners," Proceedings of the 46th Annual Purdue Industrial Waste 
Conference (May 15, 1991); T. C. Lindsey et al., "Recovery Of An Aqueous 
Iron Phosphating/Degreasing Bath By Ultrafiltration," Air & Waste, volume 
44, pages 697-701 (May 1994); D. Wright, "Minimizing Waste From A Spray 
Washer," Finishers' Management, pages 22-32 (October 1993); H. Schwering 
et al., "Crossflow Microfiltration For Extending The Service Life Of 
Aqueous Alkali Degreasing Solutions," Plating And Surface Finishing (April 
1993); Prosys Corporation, "Microfiltration With Periodic Backpulse," 
Technical Bulletin Issue No. 91-02A (1992); Prosys Corporation, "General 
Membrane Filtration," Technical Bulletin Issue No. 91-04 (1992); Prosys 
Corporation, "Aqueous Cleaning: The PROSYS Aqueous Cleaning Recycle 
system," 4 pages (ca. 1993); United States Filter Corporation, "MEMBRALOX 
Ceramic Filters For Metal Cleaner Recovery And Reuse," 5 pages (ca. 1993); 
MICRODYN Technologies, Inc., "Recovery Of Toxic Waste From Process Water," 
1 page (ca. 1993); ECO Resources, Inc., "Cleaner Recycling System," 
Technical Bulletin ECO-MFO 3 (ca. 1993); Membrex, Inc., "Application 
Focus: Aqueous Cleaner Recycling By Ultrafiltration," 4 pages (1994); and 
U.S. Pat. Nos. 5,205,937; 5,207,917; and 5,350,457, concern the use and/or 
recycle of cleaners, including aqueous cleaners. (Each and every document 
discussed, referenced, or otherwise mentioned herein, whether or not prior 
art, is hereby incorporated herein in its entirety for all purposes.) 
Membrex, Inc., "Alkaline Cleaner Recycle Handbook," 20 pages (ca. August 
1994), was published by Membrex, Inc., the assignee of this application, 
and is also relevant but is not statutory art. 
U.S. Pat. Nos. 4,790,942; 4,867,878; 4,876,013; 4,906,379; 4,911,847; 
5,000,848; 5,143,630; and 5,254,250, all owned by Membrex, Inc., concern 
filtration equipment, membranes (e.g., Membrex, Inc.'s UltraFilic.RTM. 
membranes), and/or methods, which may be useful with the present 
invention. 
A major difficulty in developing a satisfactory cleaner recycling method 
that is widely applicable arises in part from the numerous cleaners used, 
which differ in composition and chemical and physical properties. One 
approach to developing improved methods for recycling cleaners is to 
formulate cleaners that pass freely through a separatory membrane and form 
oil-surfactant (soil-cleaner) micelles large enough to be retained by the 
membrane. However, the overwhelming majority of cleaners on the market 
today are not easily recycled. Many cleaners are formulated to form 
surfactant micelles of nearly the same size as the surfactant-oil 
micelles. The similarity in size between the surfactant micelles and the 
surfactant-oil micelles makes recycling by a filter nearly impossible. In 
other cases, such surfactant micelles and surfactant-oil micelles can be 
size differentiated by microfilters with pore sizes ranging from 0.2 to 1 
micron. 
Use of such filters, however, has many disadvantages. Microfilters are 
known to foul because of entrapment of solids and other colloidal 
materials deep within the filter matrix. Expensive and non-reliable 
methods such as back-pulsing permeate back through the membrane into the 
feed stream are used to combat such fouling mechanisms. Additionally, 
microfilters have pore sizes large enough to allow oils to pass through 
the membrane back into the washer. 
Clearly, a need exists for filtration processes capable of effectively 
recycling cleaners and particularly aqueous cleaners. Despite the known 
aqueous cleaner recycling methods, there is still a need for methods to 
recycle aqueous cleaners that accommodate different cleaners, that recycle 
with high efficiency, that are easy to operate, and that are cost 
effective. 
SUMMARY OF THE INVENTION 
A method satisfying that need and having additional advantages that will be 
apparent to those skilled in the art has now been developed. Those 
additional advantages include cost savings through waste minimization; 
lower operating costs through a reduced rate of net cleaner usage; higher 
quality cleaning (which reduces the need to rework parts and thus the 
cost); and better cleaning, which makes possible higher quality subsequent 
surface treatment (e.g., electroplating and painting). 
Broadly speaking, in one aspect the method of this invention is a process 
for cleaning objects that are contaminated with soil utilizing a cleaning 
mixture containing a cleaner, the cleaning mixture having a cloud point 
that occurs as the temperature rises, the process comprising the steps: 
(a) contacting the object to be cleaned with the cleaning mixture at a 
temperature at or above the cloud point temperature to remove soil from 
the object and suspend at least some of the removed soil in the cleaning 
mixture, thereby forming spent cleaning mixture; (b) adjusting the 
temperature of spent cleaning mixture or adjusting the composition of 
spent cleaning mixture or both so that the temperature of the resulting 
spent cleaning mixture is below its cloud point temperature; and (c) 
processing resulting spent cleaning mixture while its temperature is below 
its cloud point temperature to form a soil-depleted stream and a 
soil-enriched stream. 
In another aspect, the method of this invention is a process for cleaning 
objects that are contaminated with soil utilizing an aqueous cleaning 
mixture containing a cleaner, the cleaning mixture having a cloud point 
that occurs as the temperature rises, the process comprising the steps: 
(a) contacting the object to be cleaned with the aqueous cleaning mixture 
at a temperature at or above the cloud point temperature to remove soil 
from the object and suspend at least some of the removed soil in the 
cleaning mixture, thereby forming spent cleaning mixture; (b) adjusting 
the temperature of spent cleaning mixture so that the temperature of the 
cooled spent cleaning mixture is below its cloud point temperature; and 
(c) processing cooled spent cleaning mixture while its temperature is 
below its cloud point temperature to form a soil-depleted stream and a 
soil-enriched stream. 
In still another aspect, the method of this invention is a process for 
cleaning objects that are contaminated with soil utilizing a cleaning 
mixture containing a cleaner, the process comprising the steps (a) 
contacting the object to be cleaned with a cleaning mixture containing a 
cleaner in which the desired cleaner concentration is C.sub.Washer Target 
to remove soil from the object and suspend at least some of the removed 
soil in the cleaning mixture, thereby forming spent cleaning mixture in 
which the cleaner concentration is C.sub.Washer, and (b) processing the 
spent cleaning mixture to form a soil-depleted stream and a soil-enriched 
stream, the concentration of cleaner in the feed stream processed to form 
the soil-depleted and soil-enriched streams being C.sub.Working Tank, the 
processing step including adjusting the system so that at steady state 
##EQU2## 
In preferred embodiments, the cleaning mixture is an aqueous cleaning 
mixture and the cleaning mixture displays the desired cloud point behavior 
(whether or not it is an aqueous cleaning mixture). In other preferred 
embodiments, the cleaning mixture is an aqueous cleaning mixture that 
displays the desired cloud point behavior (exhibition of a cloud point as 
the temperature rises) and the system is adjusted by adjusting cleaner 
concentration so that at steady state 
##EQU3## 
In other preferred embodiments the cleaner concentration that is adjusted 
is C.sub.Working Tank and it is adjusted to be approximately equal to 
C.sub.Washer Target divided by transmission coefficient .gamma.. In other 
preferred embodiments, forming soil-depleted and soil-enriched streams 
from the spent cleaning mixture is accomplished using one or more 
filtration steps employing a membrane that has the appropriate pore sizes, 
pore size distributions, and surface chemistry for the substances present 
in the cleaning system (i.e., the one or more cleaners, the carrier, the 
other substances in the cleaning mixture, the soil, etc.). 
The cleaning methods of this invention provide numerous advantages, which 
will be apparent to those skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION 
It has now been discovered that for cleaning mixtures displaying certain 
cloud point behavior, controlling the temperatures makes possible a 
cleaning process using recycling with the benefits noted herein. It has 
also been discovered that regardless of whether a cleaning mixture 
displays cloud point behavior, the cleaner concentrations and/or other 
parameters can be controlled to make possible a cleaning process using 
recycling with the benefits noted herein. Finally, it has also been 
discovered that use of membranes having the appropriate pore sizes, pore 
size distributions, and surface chemistries for the substances present in 
the cleaning system makes possible a cleaning process using recycling with 
the benefits noted herein. 
Before further discussing the benefits of controlling temperature, cloud 
point behavior will first be explained. Depending on the particular 
carrier (e.g., water, organics), on the one or more cleaners, additives, 
and other substances present, a cleaning mixture may exhibit cloud point 
behavior. Without wishing to be bound by any theory, for many cleaners it 
appears that the cloud point is the temperature at or above which 
aggregates of cleaner (and possibly other substances present in the 
mixture) form. Those aggregates may scatter visible light, thereby making 
the mixture seem cloudy. For cleaning mixtures displaying this behavior, 
if the temperature is below the cloud point temperature, the "cloudiness" 
does not occur. In other words, it is believed that below the cloud point 
temperature, the aggregates causing the cloudiness do not form to any 
noticeable degree. As used herein, the term "cleaning mixture having a 
cloud point that occurs as the temperature rises" is used to refer to 
cleaning mixtures that have a temperature at or above which the cloudiness 
occurs and persists. 
For some cleaners, as the temperature rises, the tendency to form 
aggregates is reduced so that at or above a certain temperature, 
aggregates tend not to form in any appreciable quantity and cloudiness is 
not observed whereas below that temperature the cloudiness is observed. 
For yet other cleaners, the aggregates do not form and the cloudiness is 
not observed at any normal processing temperature. 
One practical effect of the presence of aggregates is that, broadly 
speaking, the more and the larger the aggregates, the lower the cleaning 
efficacy for a given overall bulk cleaner concentration. Without wishing 
to be bound by any theory, the decrease in cleaning efficacy may arise 
from what may be thought of as a decrease in the effective local 
concentration and/or effective surface area of cleaner at the molecular 
level because the individual cleaner molecules have "clumped together" 
into fewer and larger aggregates. If substantially all of the cleaner 
forms large aggregates, there may be an "oiling out" of the cleaner, that 
is, the cleaner may substantially only self-aggregate and form a separate 
layer. 
Another practical effect of the formation of aggregates is that, depending 
on the size of the aggregates, the use of a membrane and its pore sizes, 
pore size distribution, and chemistry, and on other factors, the membrane 
may not allow any significant portion of the aggregates to pass through 
it. 
Broadly speaking, the soils to be removed from an object by the cleaning 
process will tend to be more fluid at higher temperatures. Thus, in 
general, higher temperatures are preferred for cleaning. However, if a 
cleaning mixture having a cloud point that occurs as the temperature rises 
is used at too high a temperature, cleaner efficacy may be severely 
reduced and oiling out of the cleaner may possibly occur. Accordingly, it 
is desired to use as high a temperature as possible without forming so 
many aggregates that cleaning efficacy is impaired too much. 
Whether a particular cleaning mixture to be used has a cloud point may be 
readily determined merely by adjusting the temperature of the mixture and 
observing whether cloudiness occurs or disappears. Furthermore, the 
transmission coefficient for various cleaning mixtures below and above the 
cloud point temperature may also be readily determined. 
For example, the cloud point temperature was determined for each of the 
following commercially available cleaning mixtures listed in TABLE 1 
(below) and then the transmission of cleaner through a membrane was 
determined at a temperature below the cloud point temperature and at a 
temperature at or not more than 5 Fahrenheit degrees (5.degree. F.) above 
the cloud point temperature, as set forth in TABLE 1. The membrane used 
was Membrex, Inc.'s UltraFilic.RTM. MX-500 membrane, a membrane having a 
500,000-dalton molecular weight cutoff. The membrane was mounted in a 
Membrex, Inc. BENCHMARK.RTM. rotary filtration device, cleaning mixture 
was placed in the annular feed region under 10-15 psig pressure, and the 
device was normally operated (by rotating the inner cylinder) for about 2 
hours. At the end of the prescribed period, the concentration of cleaner 
in the permeate was determined and the percentage transmission determined. 
Transmission coefficient .gamma. will be defined as the fraction of cleaner 
presented to the upstream or feed side of the membrane or filter 
("membrane" and "filter" are used synonymously herein) that passes through 
the membrane to the downstream or permeate side of the membrane. In other 
words, .gamma. is the concentration of cleaner in the permeate divided by 
the concentration of cleaner in the feed, or 
##EQU4## 
TABLE 1 
______________________________________ 
CLEANER TRANSMISSION ABOVE AND 
BELOW CLOUD POINT TEMPERATURE 
TRANSMISSION 
TRANSMISSION 
BELOW ABOVE 
CLOUD POINT CLOUD POINT 
CLEANER TEMPERATURE TEMPERATURE 
______________________________________ 
Genspray 202 99% 83% 
Mountaineer 502-LB 
90% 65% 
Oak Kleen 309 97% 76% 
Alkalume Cleaner 143 
98% 67% 
TM-943 (Vermont America) 
99% 79% 
Parker Amchem TD-1208-LP 
92% 64% 
Parker Amchem TD-1208-OX 
99% 60% 
Parker Amchem TD-1383-BK 
52% 7% 
Parker Amchem TD-1414-FI 
93% 51% 
Parker Amchem TD-1308-TT 
82% 60% 
Betz Kleen 4000 99% 57% 
Betz Kleen 4005 96% 67% 
Betz Kleen 4010 96% 80% 
Oakite Inpro-Clean 3800 
99% 87% 
Oakite Inpro-Clean 2500 
99% 89% 
Oakite Inpro-Clean 1300 
99% 59% 
______________________________________ 
In FIG. 1 membrane 10 separates upstream or feed side F from downstream or 
permeate side P. Membrane 10 has pores 12. Water (carrier) 14 is present 
on both sides of the membrane. Particles of soil 16 (in this case oil) are 
present on feed side F of membrane 10. As used herein, the term "soil" 
means any type of soil, including oil. Free cleaner (detergent) particles 
18 are also present on feed side F of the membrane as are soil-cleaner 
particles 24 (soil particles coated/emulsified with cleaner). This cleaner 
is one that tends to self-aggregate (i.e., cloud) as the temperature 
rises. Arrow 26 indicates the pressure commonly applied to the fluid on 
the feed side of the membrane, and arrow 28 indicates the fluid flow on 
the feed side of the membrane. That flow (e.g., a recirculation flow) may 
be adjusted to be high enough to help keep the feed side of the membrane 
clean (or at least discourage any significant formation of a stagnant or 
boundary layer). If high shear devices (as opposed to conventional 
filters) are used, e.g., Membrex, Inc.'s rotary cylinder and rotary disc 
devices, a high recirculation flowrate will generally not be needed 
(because the high shear provides enough cleaning action). Because of the 
combination of cleaner particle size, membrane pore size, and membrane 
chemistry, free cleaner particles 18 can pass through pores 12 from feed 
side F (cleaner particles 20 are in the process of passing through pores 
12) to become free cleaner particles 22 on the permeate side P of membrane 
10. Self-aggregates of particles of cleaner 18 are not shown because the 
temperature is below the cloud point temperature. Particles 16 of soil and 
particles 24 of soil coated/emulsified with cleaner are too large to pass 
through pores 12. Water molecules, which are not individually depicted, 
are small enough to pass through pores 12. Simply put, the membrane acts 
as a barrier to the free-floating and emulsified oil phases but passes the 
free detergents (cleaners) and water. 
In FIG. 2 soiled objects 30 to be cleaned are located in washing tank 
(washer) 32. The fluid present in washer 32 is a combination of recycled 
soil-depleted stream 34 from membrane separation 58, any make-up 36 of 
cleaning mixture necessary to replenish cleaner lost (e.g., cleaner lost 
along with soil purge stream 52), and soil carried into the washer by 
parts 30 to be cleaned. Washer effluent (spent cleaning mixture) 38 is 
moved by pump 40 through solids removal 42, Spent cleaning mixture 38 
after any solid removal is denominated spent cleaning mixture stream 44. 
Working tank 50 receives soil-contaminated stream 44, any cleaning mixture 
make-up 46 that is desired to be added to the working tank (cleaning 
mixture make-up can be added at almost place in the flowscheme), and 
recycled soil-enriched stream 48. The effect of recycling soil-enriched 
stream 48 is to cause the soil concentration in working tank 50 to 
increase. The steady state soil concentration depends on a number of 
factors, including the soil concentration in recycle stream 48 and the 
relative flow rates of working tank effluent stream 54 and recycle stream 
48. 
After the soil has been concentrated sufficiently in tank 50, to keep the 
system at steady state (if the process is not being run purely in a batch 
mode) stream 52 is purged to remove from the system an amount of soil 
equal to the net soil being added to the system by parts 30 being washed 
in tank 32. 
Working tank effluent 54 is sent by pump 56 to membrane separation 58. 
Although spent cleaning mixture 38 has had some of the entrained solids 
(if present) removed in unit 42 and has been combined with recycle 48 and 
cleaning mixture make-up 46, stream 54 is still considered to be spent 
cleaning mixture. Thus, as used in the claims, the "spent cleaning 
mixture" resulting from cleaning the objects to be cleaned should be 
understood to indicate stream 38 as well as streams produced from it (for 
example, streams 44 and streams 54) prior to processing stream 38 into 
soil-enriched and soil-depleted streams. 
Processing device 58 may comprise one or more unit operations of any type 
so long as it or they result in processing stream 54 into soil-enriched 
and soil-depleted streams. Device 58 will usually be one or more 
filtration devices. If processing device 58, which splits the spent 
cleaning mixture into soil-enriched and soil-depleted streams, is not the 
type of device that inherently can provide enough cleaning action on the 
feed side of the membrane (e.g., a rotary disc device using a hydrodynamic 
phenomenon such as Taylor vortices), pump 56 may need to circulate a 
substantially higher amount of fluid across the feed side of the membrane 
(cross-flow) to provide the required membrane cleaning action. In that 
case, one of the functions of working tank 50 is to provide hold-up and 
mixing volume for the large recycle stream 48, which may be significantly 
larger in flow rate than stream 44. On the other hand, if a separation 
device is used that does not require the high flow rate on the feed side 
to provide the desired cleaning, tank 50 may be eliminated. In that case 
the presumably smaller recycle stream 48 could be recycled to the feed 
side of pump 56, where stream 48 could be combined with stream 44. If a 
separation device is used that requires a large cross-flow rate, the 
working tank may still be eliminated by recirculating the soil-enriched 
phase back to the inlet (or suction) of pump 56 and withdrawing a 
continuous or semi-continuous bleed stream (or purge) from the recycled 
soil-enriched stream. Adjusting the ratio of the flowrate of the bleed 
stream to the flowrate of the soil-depleted stream allows the cleaner 
concentration of the spent cleaning mixture delivered to the membrane to 
be manipulated (adjusted). 
It should be understood that FIG. 2 is schematic and does not Show the 
instrumentation, wiring, or heat exchange equipment that might be 
necessary to adjust the temperature of any of the streams. Because the 
cleaning mixture has a cloud point that occurs as the temperature rises, 
the system desirably is run so that soil-depleted stream 34 is contacted 
with the objects to be cleaned at a temperature above the cloud point 
temperature and stream 54 enters separation unit 58 at a temperature below 
the cloud point temperature so that the cleaner does not self-aggregate to 
any significant extent, If it does self-aggregate to any significant 
extent, rejection of cleaner by membrane 10 will increase, which is 
undesirable. In other words, if the cleaner does self-aggregate to any 
significant extent, relatively more of the cleaner will be unable to pass 
through membrane 10 (because the aggregates are too large for the membrane 
pores) and relatively more of the cleaner will be present in soil-enriched 
stream 48 being recycled to working tank 50 and relatively less of the 
cleaner will be present in soil-depleted stream 34 being recycled to 
washing tank 32. Therefore, cooling and heating may be required to adjust 
the temperature of the streams to the desired levels. Instead of adjusting 
the temperature, other parameters (e.g., the presence and/or concentration 
of various substances) may provide the means for causing a stream to be 
below its cloud point (in which case, there is no significant cloudiness) 
or at or above its cloud point (in which case, there is significant 
cloudiness). 
Generally, the cleaning mixture will be used to clean the objects to be 
cleaned at a temperature not more than 10.degree. F. above its cloud point 
temperature, desirably not more than 7.degree. F. above its cloud point 
temperature, preferably not more than 5.degree. F. above its cloud point 
temperature, and most preferably not more than 2.degree. F. above its 
cloud point temperature. Thus, the cleaning mixture will be used to clean 
at a temperature of from its cloud point temperature to not more than 
10.degree. F. above its cloud point temperature, desirably of from its 
cloud point temperature to not more than 7.degree. F. above its cloud 
point temperature, preferably of from its cloud point temperature to not 
more than 5.degree. F. above its cloud point temperature, and most 
preferably of from its cloud point temperature to not more than 2.degree. 
F. above its cloud point temperature. 
Generally, the spent cleaning mixture will be processed to form the 
soil-enriched stream and the soil-depleted stream at a temperature below 
its cloud point temperature and at a temperature not more than 10.degree. 
F. below its cloud point temperature, desirably not more than 7.degree. F. 
below its cloud point temperature, preferably not more than 5.degree. F. 
below its cloud point temperature, and most preferably not more than 
2.degree. F. below its cloud point temperature. 
If the membrane and system were perfect, no soil would pass through 
membrane 10, no aggregates would form or be present in separator 58, and 
transmission of cleaning mixture through the membrane (for recycle to the 
washing tank in stream 34) would be 100%, that is, transmission 
coefficient .gamma. would be unity. 
As shown in the system in FIG. 2, the membrane continuously traps the 
soiled (e.g., oily) phases in the working tank while recycling regenerated 
cleaner (stream 34) back to the parts washer sump (washing tank 32). By 
performing a mass balance on the oil levels in the washing tank, 
time-dependent oil concentration profiles can be obtained. According to 
the analysis, as shown in FIG. 3, low steady state oil levels can be 
maintained by filtering as little as half the washer volume per day. This 
indicates that relatively small, economical filtration systems can be used 
for recycle and achieve a significant improvement in washer performance. 
The implications of this analysis are significant. Use of recycle systems 
will significantly affect process cost and part quality. By regenerating 
the cleaning fluid, savings are realized through reductions in both the 
usage of virgin cleaner and the disposal rate of spent cleaner. To further 
reduce the disposal volume of oily waste, the contents in working tank 50 
(FIG. 2) can be intermittently batch processed. System payback with these 
savings may be achieved in less than one year. 
Because the recycle systems allow part washers to operate at low steady 
state oil levels (FIG. 3), parts can be consistently cleaned to a higher 
level. Consequently, secondary surface finishing operations (painting, 
electroplating, etc.) produce higher quality parts that have lower rework 
rates. These benefits can result in even greater cost savings than those 
realized only from minimizing cleaner usage and disposal rates. 
To ensure cleaner integrity after regeneration by a given recycling method, 
the cleaner composition must be validated. Because cleaning mixture 
chemistries are so diverse, this analysis can be quite complicated. 
Detailed knowledge of the cleaning mixture's composition is necessary to 
make the validation process efficient. A number of tests are used 
routinely in the industry to verify cleaner activity. For an alkaline 
cleaning mixture, such tests include pH titration to measure free and 
total alkalinity, extraction and titration to measure surfactants, 
chemical oxygen demand (COD) analysis to measure the general organic 
surfactant concentration level, and HPLC and Fourier Transform Infrared 
Spectroscopy (FTIR) to qualitatively "fingerprint" the broad chemical 
composition of the cleaning mixture. Methods similar to those used for 
validating the integrity of alkaline cleaners are used to validate acid 
cleaners. 
Whether or not a cleaner having a cloud point is used, manipulation of 
cleaner distribution within the recycle process can be used to speed the 
approach to steady state and can, even if the cleaner transmission 
characteristics are only fair, be used to improve washer performance. 
Following is a mass balance on a closed loop washer system (FIG. 2): 
##EQU5## 
where C.sub.Permeate =C.sub.Working Tank .times..gamma.. 
The time dependent solution to this equation is as follows: 
##EQU6## 
where N.sub.T equals the number of turnovers (volume divided by flowrate) 
and t is time. 
The steady state solution to this equation is as follows: 
##EQU7## 
From these equations it can be shown that the rate of approach to steady 
state can be expedited by setting the working tank cleaner concentration 
equal to C.sub.Washer Target /.gamma.. It can also be shown that the 
target washer concentration can be achieved in the case of less than 100% 
cleaner transmission by increasing the working tank concentration to a 
value of C.sub.Washer Target /.gamma.. This approach to recycling 
compensates for poorly recyclable cleaners, that is, cleaners whose 
transmission coefficient .gamma. is not as high as would otherwise be 
desired and which might not be used at all if it were not for the present 
invention. (It should be understood that "C.sub.Working Tank " refers to 
the concentration of cleaner in the stream that is fed to separator 58 to 
be processed into the soil-enriched and soil-depleted streams, whether or 
not a working tank per se is used.) 
FIGS. 5 and 6 illustrate results from experiments where transmission 
coefficient .gamma. was only approximately 1/3. This test was performed at 
40.degree. C. with the a 500,000-dalton molecular weight cutoff 
ultrafiltration membrane. (As used herein, the term "ultrafiltration" 
refers to a filtration process using a filter whose largest pore size is 
about 0.1 microns (an "ultrafilter") and the term "microfiltration" refers 
to a filtration process using a filter whose largest pore size is about 1 
micron (a "microfilter").) As can be seen from FIG. 6, the cleaner 
concentration in the working tank was spiked twice (at about 92 hours and 
again at about 98 hours). Following each addition of cleaner, the cleaner 
level in the washing tank increased (FIG. 5). When the cleaner level in 
the working tank was increased to 3 times the target level of cleaner in 
the wash tank (at about 100 hours in FIG. 6), the wash tank cleaner 
concentration rapidly approached the target level (i.e., the y-axis value 
in FIG. 5 jumped from about 0.82 to about 1). 
It has also been found that recycle of cleaners can be enhanced by use of a 
membrane having the appropriate surface chemistry and chemical 
composition. For example, Membrex, Inc. systems (using Membrex, Inc.'s 
patented (U.S. Pat. No. 4,906,379) UltraFilic.RTM. ultrafiltration 
membrane) can be used for recycling rinse waters and for recycling and 
recovery of semi-aqueous cleaners such as Dupont's Axarel-32.RTM.. 
Membrex, Inc.'s UltraFilic.RTM. membrane is made of modified 
polyacrylonitrile (PAN) and it has physicochemical properties that allow 
it to be used in the normally harsh process environment of parts washing 
facilities. Desirable membrane properties include: (1) surface chemistry 
that helps the membrane resist fouling (plugging) from both free-floating 
and emulsified oils; (2) a chemically cross-linked matrix to enhance 
physical and chemical stability toward pH and aggressive solvents; and (3) 
pore size and morphology designed to ensure complete passage of all 
cleaner components while retaining greater than 99.9% of the oils. 
The surface of Membrex, Inc.'s UltraFilic.RTM. PAN membrane was chemically 
modified (see U.S. Pat. No. 4,906,379) to be extremely hydrophilic 
("hyperhydrophilic"). In practice, this hyperhydrophilic surface resists 
fouling by oils, emulsions, and other hydrophobic solutes. Consequently, 
efficient rates of filtration can be achieved for extended periods of 
time. The ability of this membrane to efficiently process waste streams 
containing free-floating as well as emulsified oils is a key performance 
advantage over other ultrafiltration membranes. Conventional membranes 
made of more hydrophobic materials such as polyvinylidene difluoride 
(PVDF), polysulfone (PS), or unmodified polyacrylonitrile (PAN) foul 
readily in the presence of even low concentrations of free oils. 
The difference in surface hydrophilicity of these membranes can be shown 
quantitatively by examining the contact angles formed by a water droplet 
on each membrane's surface (contact angle theta for water droplet 60, as 
shown in FIG. 7). Smaller contact angles indicate greater driving forces 
for water to adsorb to the membrane and for oil to be rejected. Membrex's 
UltraFilic.RTM. modified PAN membrane has a contact angle that is many 
times lower than that for unmodified PAN, PS, or PVDF membranes. 
Membrex, Inc.'s UltraFilic.RTM. membrane matrix is chemically cross-linked 
to preserve its performance properties (e.g., porosity and pore size 
distribution) over a broad range of pH (pH 1 to pH 13). The membrane is 
even resistant to a wide range of aggressive solvents (e.g., chlorinated 
hydrocarbons, ketones, amides), including those that normally dissolve 
PAN. 
Membrane pore size and morphology are important elements in choosing 
ultrafiltration membranes for aqueous cleaner recycling. The pore size of 
a membrane must be selected such that all active components in the 
cleaning mixture flow through the membrane while the oils are filtered out 
(i.e., are rejected). Additionally, the membrane morphology must be 
designed to inhibit the physical plugging of the filter by suspended 
colloidal materials. The morphology of the UltraFilic.RTM. filters is such 
that the separation of the micelles occurs at the surface of the filter, 
thus eliminating the internal pore plugging fouling mechanism common to 
microfilters. 
The most important factor in selecting a cleaning mixture is whether it can 
provide the required cleaning under the proposed conditions of use to 
clean the soil from the objects in question. Whether a cleaning mixture is 
recyclable is desirably determined by testing virgin cleaning mixture. 
Major factors in the determination are (1) whether the cleaning mixture 
turns cloudy or forms large micelles (i.e., does the cleaner--or do the 
cleaners--self-aggregate and form particles large enough to scatter light) 
at its--or their--proposed use concentration and (2) whether the cleaning 
mixture is above its cloud point temperature at its proposed use 
temperature. The cleaning mixture should be filtered at least 10 times 
above and below the cloud point temperature, alkalinity (for an alkaline 
cleaning mixture) should be quantified (free and total alkalinity 
analysis), and cleaner transmission should be determined (determining 
.gamma.) using, e.g., chemical oxygen demand analysis. If .gamma. is 
greater than 0.75 (75%), the cleaner should be considered recyclable, 
assuming the other criteria are met. 
The tests run on the virgin cleaning mixture include surfactant titration 
(total amount of surfactant is determined by an extraction/titration 
method; in general, each surfactant requires a special method); free 
alkalinity (to measure free, unreacted alkalinity builder in the cleaning 
mixture); total alkalinity (to measure free alkalinity plus the alkalinity 
consumed in the cleaning process); conductivity (to measure the total 
ionic content); refractometry (to indirectly measure the concentration of 
dissolved components that influence the refractive index); FTIR (to 
measure the presence of all surfactants using infrared spectroscopy); HPLC 
(to separate all the surfactants using various chromatographic 
techniques); Total Organic Carbon (to measure total organic content, 
including surfactant and oil in contaminated samples); and Chemical Oxygen 
Demand (to measure organic content and content of certain metals, 
including surfactant and oil in contaminated samples). 
Variations and modifications will be apparent to those skilled in the art 
and the following claims are intended to cover all variations and 
modifications falling within the true spirit and scope of the invention. 
For example, if different components of the cleaning mixture have 
different transmission coefficients .gamma. through the membrane or 
membranes that are used for processing spent cleaning mixture into 
soil-enriched and soil-depleted streams, the one or more components may be 
selectively concentrated in one or the other of those two streams. That 
may be advantageous in some systems, where different components of the 
cleaning mixture are desirably used at different concentrations in 
different washing tanks and/or at different temperatures.