SOLV-109

Study shows ultrasonic cleaning with aqueous detergents can replace chlorinated solvents in many parts-cleaning operations

hlorinated solvents have long been used by industry for cleaning purposes. Non- flammable and higldy effi- cient, these compounds

originally were thought to be of low toxicity. More recently, they have been identified as causes of environ- mental and health concerns, and many are being phased out, leaving industry and government agencies scrambling for viable alternatives.

One such alternative for metal parts is ultrasonic cleaning with aqueous detergents, which was the subject of a recent study conducted by Oak Ridge National Laboratory (ORNL; Oak Ridge, Term.) for the US. Army. The study, begun in 1992 with laboratory-scale tests, ended recently with the installation of a full-scale ultrasonic cleaning facility at the Corpus Christi Army Depot (CCAD).

Ultrasonic Cleaning

quency sound waves to cavitate a liquid medium. Cavitation - the

Ultrasonic cleaners use high-fre-

formation and collapse of low-pres- sure bubbles - in a liquid deter- gent provides both mechanical and chemical cleaning actions.

Several factors influence the cavi- tational intensity of the ultrasonics and, thus, their cleaning effectiveness.

Frequency of the sound waves. Cavitating a liquid requires a mini- mum frequency of 18 kilohertz (kHz). Today’s ultrasonic cleaners typically use frequencies between 18 and 100 kHz. Those in the met- als-cleaning industry usually oper- ate in the 20- to 40-kHz range.

electrical energy driving the trans- ducers that generate the sound waves, the greater the cavitational intensity. However, there is a maxi- mum amount of energy that can be transmitted to the liquid. Energy supplied above that amount is un- necessary and can have detrimental effects, such as surface cavitation, in which cavitation only occurs at the surface of the transducer, thus pre- venting the ultrasonic wave from radiating throughout the liquid.

Electrical energy. The greater the

t Transducers. The paclung, or 3

areal density of the transducers in 2 the treatment tank also affects clean- E

2

duces nodes, or dead zones, that in- $ hibit cleaning efficiency Studies z have shown that ultrasonic equip- ment operating at 1.53 watts per square centimeter and 20 kHz is ef- fective for cleaning large metal parts.

The liquid medium. Fluids cavi- tate at different intensities, depend- ing on such properties as surface ten- sion and density. Water offers excellent cavitational properties, so aqueous-based cleaners are a good choice for use in ultrasonic systems. Other properties, such as the liquids ability to emulslfy and disperse cont- aminants, also are important. These properties prevent contaminants from being re-deposited on the part’s surface. For these reasons, many aqueous-based detergents are com- posed of surfactants and alkaline salts. Some also contain small amounts of solvents to enhance their ability to dissolve oils.

Aqueous detergents usually are disposed in sanitary sewer systems, so local requirements should be con- sidered when selecting one. A deter- gent also must be compatible with the materials being cleaned. Alu- minum and other soft metals, for ex- ample, react quite violently with

ing efficiency. Dense packing re- I

54 Anay/June 1995

ples. These devices, which can be equipped with a number of different detectors, work well when used to an- alyze a large number of samples for a few specific compounds. However, portable GCs, particularly field units, tend to drift out of calibration with changes in ambient air and need frequent r e a other drawback is that

in the same calibra- assurance rcquire-

ts are on the order of parts per bil- lion. The practical detection limits for VOC analysis using laboratory-grade GCs are sufficiently low to allow in- formed, onsite decisions for most chlorinated and aromatic VOCs.

Analyzing final costs. Analyzing the subsurface for pollutants tradition- ally has involved collecting soil or water samples and comparing the an- alytical results with regulatory action guidelines. Drilling, collecting and an- alyzing soil samples and disposing the resulting wastes usually is time-con- suming and costly.

crews using conventional drilling techniques can collect 15 to 16 soil vapor samples from a depth of 10 feet. A typical 4-inch-diameter, 10-foot- deep soil boring produces about 15 drums of soil cuttings, which can be expensive to dispose if contaminated. The drums cost between $20 and $40 each, and disposal costs $50 to $75 per drum if the soil is contaminated. Elim- inating 15 drums, therefore, would save between $1,050 and $1,750. Dur- ing soil vapor sampling, soil is pushed aside and none is brought to surface, so there are no investigation-derived wastes to dispose.

The labor and equipment costs as- sociated with a one-day soil vapor survey are about the same as those for a one-day drilling and sampling pro- gram. However, if the soil samples are analyzed for VOCs using EPA meth-

During an average work day, field

May/Jvne 1995

ods 8010,8015 and 8020, project costs could increase $220 to $250 per Sam- ple. Because sample analysis is in- cluded as part of the soil vapor sur- vey, a cost savings between $3,500 and

20 minutes. This allows project man- agers to modify the Work Plan or Sampling and Analysis Plan on the spot, which helps minimize the need to return to a site to collect additional samples from ”hot spots” discovered during analysis.

the overall effi-

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coolant, respectively. After sitting overnight, two samples of each combination were cleaned either ul- trasonically in aqueous detergent or by vapor degreasing using per- chloroethylene (perc), trichlorotri- fluoroethane (CFC-113) or 1,1- trichloroethane (TCA). Samples cleaned ultrasonically were processed for 15 minutes at 54 de- grees Celsius, rinsed in flowing demineralized water and blown dry with argon. Vapor-degreased sam- ples also were processed for 15 min- utes, the normal operating time for degreasers in the plant’s production areas. All samples were analyzed after cleaning using XPS.

Ultrasonic cleaning consistently yielded the cleanest surfaces and was found to be effective in clean- ing a part’s “hidden” areas. CFC- 113 was the worst in removing ma- chining coolant and lapping oil, while TCA was the worst in remov- ing rust preventive oil.

Based on these results, Martin Marietta Energy Systems Inc., which runs the ORNL lab and Y-12 Plant, installed ultrasonic cleaning tanks throughout the plant. This method has been used successfully to clean various-sized parts, from small-di- ameter tubing to large metal parts. A variety of metal parts, including steel, stainless steels, aluminum and uranium, have been cleaned without compatibility or corrosion problems.

Cleaning Army Depot Parts

Y-12 program, the Army Environ- mental Center decided to fund a pro- ject to investigate the use of ultra- sonic cleaning as a substitute for chlorinated solvents in Army depot operations. To conduct this study, a variety of parts were shipped to the Y-12 Plant from CCAD and Anniston Army Depot (ANAD). The parts rep-

Encouraged by the success of the

highly alkaline materials. Energy transfer. Coupling (the

transfer of energy between the trans- ducers and the liquid) is critical. The aluminum foil erosion test can be used to determine if the cavitational intensity is adequate to provide ag- gressive cleaning action. To conduct this quick test, simply submerge a 0.003-centimeter-thick piece of alu- minum foil into an operating tank for 30 seconds. A good aggressive ultrasonic tank will cause holes to form in the foil. This test also is use- ful for identifying dead zones. (Ro- tating parts during the cleaning process ensures that they are not stuck in a dead zone.)

peratures help to melt waxes and oils for easier removal and increase the solubility of oil in the detergent. However, be aware of the deter- gent’s “cloud point,” when phase separation of the detergent occurs and it becomes less effective. Tem- perature also affects cavitational in- tensity. ORNL researchers have found that a temperature of 55 de- grees Celsius typically is effective.

Like all technology, ultrasonic cleaning has some disadvantages: it requires an initial capital investment in the equipment, and parts must be rinsed to remove the detergent, a step that is not needed in vapor de- greasing applications. A hot water rinse followed by forced-air drying is suggested to prevent water spot- ting and corrosion. Demineralized

Bath temperature. Higher tem-

May/June 1995

water is recommended to prevent minerals from depositing on the parts and causing corrosion.

initial Studies Researchers conducted several

initial studies at the Department of Energy’s Y-12 Plant (Oak Ridge, Tenn.) to explore the feasibility of ul- trasonic cleaning. X-ray photoelec- tron spectroscopy (XPS) was used to analyze the results of bench-scale tests. This technique involves bom- barding the surface of a part with X- rays and measuring the energy of the electrons released.

The binding energies of electrons from different elements or elements in different binding states vary, whch means elements present on the surface can be identified based on the binding energies of the elec- trons released. The peak heights at various energy levels show how much of a particular element is pre- sent. Carbon is the most common el- ement associated with contamina- tion. The carbon-to-base metal peak-height ratio shows the level of contamination. Lower ratios indi- cate cleaner surfaces.

To establish a sample baseline, metal samples were cleaned ultra- sonically in aqueous detergent, rinsed and allowed to dry One sam- ple of each metal was retained as a control. The metals -304L stainless steel, 4330V steel and 15-5PH steel - were coated with rust preventive oil, lapping oil and machming

55

&lodged lubricant floated on &e bath and could not be removed, be- cause the tank did not have either a filtration system to filter them out or an overflow weir to skim excess oil or grease from the top of the bath. Other possible solutions in- clude a spray to knock out the lubri- cant clumps; fixtures to rotate the bearings and better expose the dirty areas to the surface; or a solvent presoak to loosen and dissolve ex- cess lubricant. Researchers tried several solvents of different chem- istry types to dissolve the lubricant and found that straight-chain and branched hydrocarbons, such as Solvent 140, and cyclic hydrocar- bons, such as decalin and d- limonene, worked best.

Tests also were conducted on gears, a mount, a case and a squirrel cage fan from CCAD.

The gears were cleaned ultrasoni- cally for one hour at 55 degrees Cel- sius in 5 volume-percent detergent. Most of the grease was removed in 15 minutes. All of the spots were cleaned in one hour. Tn a production operation, the options mentioned above, such as an overflow weir and

56 May/June 1995

filtration system, would be useful to extend the bath life when cleaning extremely greasy parts. A solvent presoak also could reduce cleaning times and extend bath life.

ultrasonically for 25 minutes in 5 volume-percent detergent at 55 de- grees Celsius. All of the black soot was removed from the squirrel cage. Both the case and the mount were cleaned ultrasonically after being soaked in N-methylpyrrolidone (NMP) to help remove the paint on them. The black paint on the mount came off after a 25-minute soak in NMP and 65 minutes of ultrasonic cleaning in 5 volume-percent deter- gent. The silver paint came off the case easily using the NMP, but por- tions of the yellow paint remained after soaking two hours in NMP and being cleaned ultrasonically for 40 minutes in detergent.

and repairs such large tactical vehi- cles as tanks, sent bushings, thrust plates and metering valves. The bushings and thrust plates had baked-on carbon, which typically is removed with a commercial product containing amines and ethers. These particular ethers are being regulated under the Clean Air Act, and both the ethers and amines present some concern about toxicity. Some conta- mination also plugged four small holes in the metering valves.

The thrust plates were cleaned ultrasonically for two hours in 5 volume-percent detergent at 55 de- grees Celsius. Ultrasonic cleaning removed thin areas of carbon buildup but could not remove most of the carbon deposits, so re- searchers tested several presoak so- lutions. Several solvents - NMP, a blend of NMP and ethyl-ethoxypro- prionate, tetrahydrofurfuryl, anisole and dibasic esters -were evaluated at room temperature and 50 degrees Celsius. None of the sol- vents were effective in removing the carbon deposits.

Researchers then evaluated more- alkaline solutions, whch also proved inefficient. Because the current car- bon removal method used at CCAD involves potassium permanganate, which is a strong oxidizing agent, re- searchers tried other, less-toxic oxi-

The squirrel cage fan was cleaned

ANAD. ANAD, which maintains

May/June 1995

dizing agents, including ammonium persulfate and ammonium persul- fate with a small amount of sodium nitrite (known for speeding up oxi- dation processes). These showed slight effects but still left most of the carbon deposits.

remove, researchers believe that a compound was being formed with the metal base and decided to try re- moving it using formulations for etching steel surfaces, including glu- conic acid and a mixture of oxalic acid, sulfuric acid and hydrogen per- oxide (also an oxidizing agent). Sub- merging the parts in gluconic acid at 50 degrees Celsius for 15 minutes and cleaning them ultrasonically with an aqueous detergent removed portions of the carbon but not all of it.

The oxalic acid formulation was found to be the most effective. Only a few small areas of carbon re- mained after submerging the part in the oxalic acid formulation for six hours (half that time included ultra-

Because the carbon was difficult to

sonic agitation) at approximately 70 degrees Celsius. Increasing the amount of hydrogen peroxide in the solution should help to decrease the cleaning times.

Researchers also investigated the use of malonic acid, which is less toxic than oxalic acid, and sodium hydroxide. Malonic acid worked about the same as oxalic acid; sodium hydroxide worked the best.

Based on the results of these test, the Army decided to install a demonstration ultrasonic facility at CCAD. The facility has since been demonstrated successfully and has replaced the depot’s vapor degreas- ing system. It also serves as a model for other depots. E

Lisa M. Thompson is a develop- ment engineer at Martin Marietta En- ergy Systems Inc. (Oak Ridge, Tenn.), and Richard L. Eichholtz is pollution prevention technology team leader at the U S . Army Environmental Center (Aberdeen Proving Ground, Md.).

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Where Compliance Software May Boldly Go

Tomorrow’s software will be a

environmental compliance sophisticated management

tool, say indust y experts, but compatibility and adaptability are today’s concerns

By Laura Bridgewater

lthough today’s comput- ers and software fall short of what can be found on the Star Treko Enterprise, that level of sophistication

may be closer than the 24th century. Originally, compliance software

programs were on mainframes or minicomputers, and little to no com- munication was possible between programs offered by different compa- nies. Today, software is more likely to use Microsoft Windows@ on a local area network (LAN) made up of IBM- compatible personal computers or minicomputers.

These changes have opened the door to enhanced graphics and in- creased communication among soft- ware packages, whch now number more than 2,000, according to Donley Technology (Garrisonville, Va.), a publishing company that tracks the environmental software indusq. Software packages, which fall into three general categories - regulatory databases, recordkeeping and report- ing software and process monitoring programs - cover a range of cus- tomer needs.

Commercial environmental recordkeeping and compliance re- porting software products, for ex- ample, range from simple spread-

sheet programs to comprehensive management information systems.

Datastream@, which now is sup- ported by Domer Products (Milwau- kee, Wis.), is one of the more basic

May/June 1995