Achieving consistent cleaning performance. . .
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For product finishers there are few issues that cause as many problems as trying to achieve consistent cleaning performance while extending bath life. Whether your evaluation relates to quality or cost, cleaning operations are critical to high yields. Failure to clean effectively, particularly parts with surface soils, grease, general dirt and debris, results in expensive re-work, or worse, high levels of rejects if the contamination is not detected before painting/coating.
Manufacturers frequently advise finishers to "dump and replace" cleaner baths as the best available option, regardless of the cost and consequences of "dump and replace." The consequence of this approach continuously presents a "cleanliness/quality vs. cost" trade-off.
One of today's manufacturing goals is source reduction, or P2, as pollution prevention is referred to. Given the conflict of product quality vs. trying to survive in a highly competitive business environment, a "dump and replace" approach poses some of the toughest material consumption/waste generation problems a plant manager wrestles with daily. cleaner baths, bath life, oil-contaminant control,
Cleaning metals involves not only the selection of the cleaner(s), but also the process equipment and proper cleaning cycle required to produce an acceptable parts/hour rate. Industry counterparts suggest an equation for a cleaning operation where the burden for providing an economical operation could be written as:
Higher production throughput and/or source reduction is achieved by improving one or more of those factors, which includes equipment or technology modifications, particularly in the arena of minimizing or eliminating facility discharge and disposal. However, its relationship to process cost reduction is where the focus on source reduction is altering most operational practices. If you accept in the above equation that "Acceptable Parts/Hour" = Unit Cost, then any process cost reduction (e.g., Acceptable Parts/Hour × 115% or Unit Cost × 85%), has to involve one or more of the following changes: process equipment and upgrade; process cycle throughput improvement; or more cost-effective cleaner use.
When making any changes in manufacturing operations, improved Return on Investment (ROI), and better standards of finished parts quality are the main concerns.
In Figure 1, oil separation is shown as just one step in the cleaning process rather than one of the primary elements involved. These four elements are temperature, agitation, chemistry and dwell time.
Because of this, oil separation and removal has been relegated as a post-manufacturing waste treatment process.
Treating oil control as a fifth element in this "Circle of Four" enables manufacturers to exercise greater flexibility over the other process elements. And, in doing so, they achieve both a higher and more consistent performance from the detergent and extended cleaner bath life.
After many years devoid of technical innovation, conventional mechanical separation has been superceded by a revolutionary thin-film fluid-recovery principle. It allows plant managers to revert to cleaners and lubricants that are more easily separated and re-used, bringing in-line real-time contaminant control efficiency.
The only way to make your cleaning process as "lean and mean" as your other manufacturing processes is to eliminate waste and optimize performance. Integrating continuous, high-efficiency oil removal eliminates wasteful consumption of materials and labor and allows you to operate everything else at maximum performance levels at the lowest cost.
Sincethe Montreal Protocol, aqueous cleaning has emerged as the dominant cleaning method. Although aqueous cleaning may require longer drying periods, an advantage is that when used properly alkaline cleaners, or neutral pH cleaners, will leave the surface in a water-break-free condition. Vapor degreasing does not.
Until now, conventional wisdom has advocated that four factors play an important role in the process. Compensating adjustments made within two or more elements in this "Circle of Four" can improve the parameters and goals for each process. Oil and grease are the main contaminants. In removing them, the cleaner may become contaminated with suspended solids (dirt).
Water and the switch to aqueous cleaning. Divergent opinions exist as to how to achieve a totally clean surface. Industry experts advise close and constant monitoring of the cleaner bath, with frequent dumping to ensure the integrity of the cleaning process. Others stress the importance of rinsing. This suggests that it is better to use large volumes of rinse water and dump the rinse tanks constantly, since water is the least costly chemical in the process.
Neither approach recognizes the changing realities of the situation. Tightening effluent discharge standards under the Clean Water Act legislation have increased the importance of selecting the best in-line pretreatment system for your process. With increased water use, emphasis must be on capturing and removing oil where it is generated, within the manufacturing process, and not as a part of the general treatment of mixed wastes. To leave it as a post-production wastewater treatment process misses the opportunity to significantly reduce costs.
Many of us know the high costs of waste treatment or off-site disposal. Also, with U.S. demand for industrial and institutional cleaning chemicals forecast to rise to $7.5 billion annually by the year 2,002, the impact on material consumption and waste generation issues gain greater importance. And how long before pricing is used as an economic tool to force better water consumption, conservation and discharge?
Manufacturers have found that alkaline solutions tend to be more viable because of their capacity to remove a broader range of soils. Typically, these water-based solutions are formulated with regard to the particular cleaning task, (material being cleaned, type of soil, required cleanliness, etc), and parts cleaned.
Detergents use both chemical and physical action to clean. They contain a number of chemical components such as detergents, surfactants, saponifiers, chelators, corrosion inhibitors and anti-re-deposition additives.
The first requirement for removing soil is "wetting," which loosens the soil/surface bond by reducing surface tension. The ability of the solution to wet a particular surface, especially in penetrating tight spaces, is why cleaners contain surfactants, which lift the soils from the surface.
Every cleaner has a certain capacity to absorb oil and dirt. However, it is in the chemical displacement mechanism where you begin to see the differences between neutral and alkaline degreasers. While a clean surface is the common goal of both, their mechanism for cleaning is different.
Because saponification is a permanent chemical reaction, it is unsuitable for most oil-recovery methods, especially where cleaner recycling and oil reclamation are the primary source reduction/pollution prevention goals. Here, the oil is chemically attacked by the alkalinity of the solution to make it partially water-soluble, and easily removed from the part's surface. Unfortunately, the oil remains suspended in the solution and, because it has been chemically changed, will not collect at the surface; therefore, traditional oil-recovery methods are not effective. The cleaner's alkalinity is constantly consumed and requires fresh chemical additions once or twice a day. Without these constant alkalinity additions, the cleaning solution becomes ineffective quickly.
Since no permanent chemical reaction occurs, this type of cleaning is more mechanical in nature and, therefore, reversible. This makes it more conducive to the application of effective oil-recovery and recycling technologies.
The surfactants and wetting agents in the cleaner break the surface tension at the oil/water interface. As the oil is lifted off, a thin-film of surfactant encapsulates the oil droplet, making it temporarily soluble and preventing it from re-depositing on the metal surface. After a short period, the surfactant releases the oil and it floats to the surface. From there it can be easily recovered. The oil release process is not totally complete, but slight loading of oil in the cleaner should not be detrimental to the process. Improvements to accelerate the detachment of the surfactants from the oil are often referred to as oil-splitting or oil-ejecting properties.
Once it releases the oil, the surfactant can again remove, emulsify and then release additional oil during the cleaning process. Under ideal operating and oil-removal conditions, fresh cleaner need be added periodically rather than daily.
In emulsifying the oil, these cleaners act much like solvent degreasers. Unfortunately, unless it almost completely releases all the oil, emulsification forces the oil contamination to become an integral part of the cleaner. The cleaner's performance curve continually degrades as it loads with, or irreversibly absorbs, more and more oil (see Figure 2). This can cause oil to spray or be dipped back on the workpiece as free oil.
To overcome this recontamination problem, chemical manufacturers formulate the cleaner to emulsify the oil. This way, its saturation point is effectively extended to the point where the oil is homogeneously and chemically stabilized throughout a volume of aqueous cleaner. This avoids a film developing on the surface of the bath. But in doing so, the applications of effective recycling technologies are impeded and, consequently, total cleaning costs rise.
The more oil entrained in the bath, the more the surfactants are occupied. This reduces the availability of surfactants for the parts cleaning process, eventually occupying all of the surfactant in the aqueous cleaner.
Other problems caused by emulsification include:
Simple changes can be introduced into your cleaner system, making it a more cost-effective cleaning process.
Regardless of the agitation methods used, contaminant control is a constant. Most mechanical separation devices commonly used (belt skimmers, disk skimmers, overflow weirs, etc.) were originally developed for out-of-process, end-of-pipe (EOP) wastewater treatment processes. These systems were not designed and are unable to deliver the performance needed in a continuous manufacturing operation that directly impacts the quality of finished parts. By bringing EOP methods to the plant floor, their inefficiency leads to a performance degradation that over time impacts on production quality and operating costs, including product re-work, frequent dumping, washer/maintenance downtime, off-site disposal costs and transportation liability.
Also, because rinsing is important in the aqueous cleaning process, the tendency has been to recommend the use of large volumes of water. This exacerbates the problems and increases wastewater treatment costs.
Some of the operational features of more familiar conventional mechanical separation methods make them less effective than modern aqueous processes demand. Until now, one of two common principles of mechanical separation have been used, either adhesion techniques (such as an oil wheel/disk, hose or belt skimmer), or skimming (weir skimmers, tank overflow to drain). There are built-in disadvantages to using either of these methods in the aqueous cleaning process.
With adhesion techniques, a floating oil layer will always be on the cleaning bath surface. This is due to surfactants, which prevent the oil from adhering to the oliophilic surface of the wheel or belt. Subsequent cleaning will be compromised because uncollected oil will be reintroduced to the workpiece. This becomes more apparent in volume production where the low separation/ removal capacity of oliophilic devices results in oil accumulations in the cleaner reservoir.
Because of construction features, skimmer users have two options: 1) Let a thick layer accumulate and skim only oil, which means a constant surface layer of oil; or 2) Overflow or skim off the top oil/water layer, which removes significant amounts of cleaner.
There are some not so good aspects of using overflow weirs. In overflowing a top layer, containing both oil and cleaner, users not only lose the composition of the cleaner, they also force the treatment plant to deal with the separation of these often difficult oil/cleaner emulsions. If the level in the cleaner or soak tank drops below the level of the knife edge weir, the oil-skimming operation stops dead with the floating oil on the wrong side of the process. This greatly increases the risk of recontamination.
To avoid this, the tendency is to increase the rate of overflow, which increases the volume of oil/water wastes that must be processed.
In addition to these oliophilic removal devices and the overflow weirs, manufacturers have also employed coalescers and microfiltration and ultrafiltration systems. Typically coalescers are used where the solution tends toward a greater emulsification of the oil. The plates or media present in the coalescer collect the dispersed oil (micelles) until droplets large enough to float to the surface are formed. While this overcomes one problem (emulsification) the removal of the then floating oil still poses another problem.
Membrane separation technologies such as microfiltration and ultrafiltration use thermoplastic polymers or certain inorganic materials to remove insoluble particulate materials. Basically, microfiltra-tion is used to remove suspended solids, and ultrafiltration is used for dissolved non-ionic materials. Because particulate and free oils are present in aqueous cleaning applications, this can create fouling problems. So most process engineers configure microfiltration or ultrafiltration systems downstream of an oil/water separator to avoid the reduction in microfilter flow that would make it difficult to keep up with soil loading in the washer.
Even advocates of ultrafiltration conclude that although it reduces oil concentration to less than 0.01%, it also removes active ingredients, because the membranes inhibit passage of desirable cleaner components. The authors of that review noted that these ingredients can be replaced in the permeate to regenerate the cleaner fully, although the amount of non-ionic surfactant removed would have to be determined first.
It is no surprise that the driving force behind R&D efforts to find a more efficient and cost-effective mechanical component within the complete cleaning/pretreatment process was the separation performance of these conventional mechanical separation methods. And the alternative of employing more sophisticated and expensive membrane separation technologies and then expecting them to perform a primary separation that they were not designed for is the same overkill as taking a sledgehammer to crack a nutshell.
A new concept in mechanical separation of oil and dirt is continuous in-line precision thin-film oil-control technology. This thin-film separation technology provides the high-efficiency oil and dirt removal required by both small and large volume manufacturers, without affecting the composition of the aqueous cleaner.
Unlike the disadvantages of conventional separation methods, the principle on which this device functions enables users to separate and recover thin films of oil. It has no moving parts and can recover molecular-thin sheen, the same as the iridescent rainbow sheen you see on a puddle in a parking lot after a rainstorm.
An innovative adaptation of Bernoulli's1 principle of fluid pressure differential is applied to the separation of two liquids having different specific gravities. Exploiting the specific gravity differential between the two liquids, the `wing' shaped module collects and concentrates the floating oil without affecting the cleaner's composition. This allows efficient removal of the contaminants. Contaminant removal is 99% water-free.
In aqueous cleaning applications, oil separation and removal is achieved without removing cleaner components critical to soil removal. Because its design forces the detaching of the surfactants from the oil prior to its removal from the device, the cleaner is recycled to the process with its composition and effectiveness intact. How are these surfactants preserved? By concentrating the oil/water interface present within the thin-film oil control unit, surfactant is "over-loaded" by the concentrated pure oil layer. As a result, it migrates to the bottom of that layer and is reintroduced into the cleaner flowing out of the bottom valve. The surfactant-free oil is automatically removed.
Pulses of oil do not affect the efficiency of the technology. Oil removal is a function of loading, the more oil, the faster its removal. And because the technology is scaleable, systems can be configured to match the process parameters of any cleaning operation.
Typically, this thin-film oil-control device is installed as an `upgrade' to an existing wash process. A patented level-following, flow-sensitive, pick-up head is installed in the existing cleaner tank.
The installation takes about two hours. The thin-film recovery device is mounted in a tank placed next to the bath and, using gravity feed, cleaner is fed from bath to separator. Oil-free cleaner is returned to the process and made available for extended re-use.
With 100% floating oil separation right on the factory floor, any manufacturer involved in the aqueous cleaning of parts, whether metal, plastics or glass can profit.
Zinc plater. On the soak clean tank of a zinc plating operation, the oil and grease removal results from a belt skimming operation showed recovery rates of only 1.4%. Oil removal was complicated due to the foaming nature of the chemistry used.
Without any other changes, the introduction of continuous thin-film separation immediately improved the oil and grease removal to 94.7%. Projections show that this will deliver a $30,000 cost reduction in oil disposal from these two plating lines, without factoring in any other process cost savings.
Part carbonizing in drying oven—problem caused by poor oil control. A heavy-equipment manufacturer found that when just cleaned parts were placed in a drying oven, a residual oil film on the parts caused a carbonized stain problem.
The oil was not removed quickly and completely and re-deposited on the parts as they moved out of the cleaning process. Those parts had to be re-washed. On some, the carbonization was particularly bad and the re-work took more than just re-processing the parts through the washer.
The existing skimmer was replaced and the cleaner tank was upgraded with the thin-film oil control. As quickly as the oil was washed off the parts, the new separation system was removing the floating oil and returning stripped cleaner back to the cleaning process. Because of the rapidity of the removal process, there was no build-up of oil in the cleaner. Subsequently, the manufacturer determined that it had cut its cleaner consumption costs by 50%.
Matching recovery system to a cleaner. Having switched to an oil-splitting neutral cleaner, this cookware manufacturer found its belt skimmers could not handle the oil volumes quickly released by the chemistry. To avoid recontamination the manufacturer was using a shop-vacuum to suction the floating contaminant off the surface of the cleaner tanks. Given the number of washers, this was time-consuming and labor intensive.
The thin-film separation technology was run on a washer that removed oils from a sanding operation. Bath life was typically four weeks. In this case, the bath was five weeks old and looked very dirty. The solution was sampled prior, and the FOG content (fats, oil and grease) was recorded at 101,000 ppm. The new technology was operated for three days before the solution was re-sampled. The FOG was now showing 85 ppm. As a process note, when the bath gets very contaminated, support personnel tend to reduce the amount of cleaner solution they add. At this stage, due to the pending washer dump, the solution had very little of the detergent concentrate in it.
The bath was recharged with a 10% by volume concentration of the surfactant-based neutral cleaner, where the solution shows an initial FOG reading of 1,200 ppm. After six days of production using the thin-film separation system the FOG reading was maintained at 1,270 ppm, or 1.27%. Unlike the definite reduction in flow due to the filter clogging in the ceramic microfilter that was tested on a similar wash operation, the thin-film unit had no problem keeping up with the oil loading. Oil removal efficiency was not effected by total water flow. And the chemistry supplier who had introduced the system into the process noted that the heavy metals seemed to stay with the oil that was removed.
Prior to the adaptation of the thin-film technology, the switch to a neutral cleaner had eliminated 600,000 lbs of alkaline hazardous materials, 5,616 lbs of hydrochloric acid, and racked up savings in cleaner consumption, clean-out labor and energy of $27,735. Operational findings show that the new technology is adding to those savings, extending cleaner bath life from four weeks to 13-plus weeks.
In a typical aqueous cleaning process where conventional oil-recovery methods are used, the cleaner becomes increasingly saturated with the oil, and the performance degrades. This requires that the cleaner solution be titrated, often once or twice daily. When this happens, the process is described as having a saw-tooth performance curve.
As performance drops to a point below the minimal level, the cleaner tank then needs to be dumped and replaced. And it is the frequency of this "dump and replace" cycle that causes cleaning process costs to escalate (Fig. 5 and 6).
However, once the thin-film cleaner recycling and oil-recovery system is deployed, and it has loaded slightly with oil, the detergent then maintains a consistent cleaning performance well above the minimal cleaning performance represented by the dotted line in Figures 7 and 8. Because of this performance extension, cleaner bath life is doubled as a minimum.
One of the Big 3 automotive manufacturers was using the thin-film technology as the basis of its oil-recovery system. By introducing an oil-splitting chemistry at the same time as it lowered the temperature of the cleaning process, it was able to clean 130,000 rear axle cover plates in three weeks and reported that the detergent was as clean on day 21 as on day one.
From the visual impressions gained, the impact of this thin-film cleaner recycling and oil-reclamation technology on ALL aspects prompts serious examination of the total aqueous cleaning system. In particular, the increased availability of the cleaner and the washers themselves, both from the longer in-service period (i.e., the maintaining of a constant flat performance curve) and the reduced system downtime offer manufacturers with a clear route toward higher productivity.
The value of this will vary from application to application, but there are industry analysts who equate a bottom-line benefit of a 1% decrease in maintenance downtime as being equal to a 15% increase in sales.
1 Bernoulli's theorem. At any point in a tube through which a liquid is flowing the sum of the pressure, potential energy and kinetic energy is a constant.