Download Metal Surface Finishing: Processes, Wastes, and Sustainability and more Study notes Literature in PDF only on Docsity!
1. PROCESS: METAL SURFACE FINISHING
ā 2. SIC CODE: 3471
3. INDUSTRY DESCFUPTION
Metal finishing is a p a r t of t h e plating and polishing industry classified under Standard
Industrial Classification (SIC) 3471. This industry is comprised of establishments
primarily engaged in all types of electroplating, plating, anodizing, coloring, and finishing of m e t a l and formed products f o r t h e trade. Most of t h e work performed by this industry is done on materials owned by others. Though m e t a l surface finishing is
classified under SIC 3471, m e t a l finishing operations a r e performed by many o t h e r
industries included in SIC groups 34 through 39 (USEPA 1980). The metal finishing
process includes some 44 unit operations and only t h e m e t a l surface finishing
operations t h a t use chemical means a r e discussed in this report.
3.1 Company Size Distribution
Since metal s u r f a c e finishing operations a r e performed by various industries classified
under many SIC codes, company size distribution d a t a f o r m e t a l s u r f a c e finishing a s an
industry was not separately available. In 1980, t h e r e were approximately 160, manufacturing facilities in t h e U.S. which were covered by t h e metal finishing category (USEPA 1980). These facilities varied greatly in size, age, and number of employees. They ranged from very small independent job shops with less than ten employees, t o small shops within large corporations, t o large facilities employing large work forces.
3.2 Principal Producers
The m e t a l surface finishing industry is dominated by small job shops employing less than 20 employees each. There a r e no major producers who control a large share of t h e market.
3.3 Geoqraphical Distribution
The geographical distribution of t h e m e t a l surface finishing industry was not available due t o the reasons s t a t e d above.
- PRODUCTS AND THEIR USE
The m e t a l surface finishing industry deals mostly w i t h the treatment o f m e t a l l i c or non-metallic products manufactured by others. Each product requires a specific
process sequence t o obtain the desired physical, chemical, or aesthetic properties
desired by the user. The principal products of the m e t a l surface finishing industry include:
Printed c i r c u i t boards C o i l coating Automotive parts
K i t c h e n utensils Jewelry Mechanical (non-automotive) parts
5. RAW MATERIALS
Reagents phosphoric acid, secondary or t e r t i a r y m e t a l phosphates, sodium dichromate, sodium nitrate, sodium cyanide, barium chloride, sodium chloride, sodium carbonate, sodium cyanate, ammonia, silicon t e t r a - chloride, zinc oxide, chromic acid (^) )
Accelerators quinoline, toluidine, nitrophenols, various oxidizing agents such as peroxides, and sulfites
Metals zinc, aluminum, chromium, cadmium, magnesium, iron, nickel, copper, silver, molybdenum, vanadium, tungsten
Alloys tin, lead-tin alloys, bronze, brass
- PROCESS DESCRIPTION
M e t a l surface treatment consists of various processes such as electroplating, electroless plating, anodizing, chemical conversion coating, cleaning, etc. (USEPA 1980, BCL 1976, Schneberger 1981, Durney 1984). Since electroplating and m e t a l surface cleaning are discussed in separate studies in this appendix, this study considers only chemical surface treatments such as electroless plating, chemical
Table 6-1 B a t h Constituents for Capper and Nickel Electroless P l a t i n q
~~ Function Copper P l a t i n q Operation N i c k e l P l a t i n q Operation
M e t a l ion source
Complexinq agent
S e d u c i n q agent o
c^ I
pH adjustor S tabil izers
0 ther additives
Cupric sulfate (3-15 9/11 N i c k e l chloride, nickel sulfate, nickel sulfarnate, nickel hydmphosphite Rochelle s a l t (sodium potassium tartarate, eth ylenedi aminetetracetic acid (EDTA), sodium salts of EDTA, n i t r i l o t r i a c e t i c acid, qluconic acid, gluconates, triethanol amine, n-hydroxy ethylene diamine tetracetate (20-50 q/l).
L a c t i c acid, dicarhoxylates
Formaldehyde, paraformaldehyde, t ri oxene, dimethylhydantoin, sodium and potassium borohydride.
Sodium hyposphosphi t e
Sodium or potassium hydroxide Sodi urn or potassium hydroxi de 2 -mercaptobenzo thiazole, thiourea, methanol Molybdic acid'anhydride,^ arsenious acid, hydroxyl amino sulfate, hvdrazine. Water soluble m e t a l cyanides, polysiloxanes, m e t h y l dichloro silane
Thiourea, soluble fluorides, elcohol sulfonates, ethylene oxide derivatives, sodium sulfate. ~~ Source: Development Document^ for^ E f f l u e n t L i m i t a t i o n Guidelines^ (USEPA^1980 ).
-) of^ stabilizers.^ The bath^ is^ eventually^ dumped^ thereby^ generating a^ waste^ stream. Other waste s t r e a m s a r e generated by rinsing operations and periodic cleaning o f t h e process equipment. Other electroless plating operations, such as vapor deposition, a r e purely physical operations and are not considered in this report.
6.2 Chemical Conversion Coating
This operation includes phosphating, chromating, m e t a l coloring, and passivating. The coating deposited on metal objects is for decorative or corrosion protection purposes, and in some instances t o prepare t h e surface for painting. The mode of operation and waste generation a r e similar t o t h e electroless plating operation described in Section 6.1. The following sections discuss t h e four different chemical conversion coating methods listed above.
6. 2. 1 Phosphating
Phosphate coatings a r e formed on t h e surfaces of iron, steel, galvanized s t e e l , aluminum, and electrodeposited zinc and cadmium t o p r o m o t e adhesion of organic coatings, t o r e t a r d interfacial corrosion, t o retain and enhance t h e performance of corrosion resistant oils, and t o assist in cold deformation processes. Small p a r t s a r e coated in barrels immersed in t h e phosphating solution a n d ā l a r g e p a r t s a r e spray c o a t e d or continuously passed through t h e phosphating solution. The object t o b e coated may b e dipped successively in a series o f processing tanks.
3
The phosphating solution consists of a phosphoric acid solution of m e t a l dihydrogen phosphate. The coating t i m e and t e m p e r a t u r e depends on t h e t y p e o f metal t o b e c o a t e d and whether a spray or immersion coating s c h e m e is used. Sometimes a c c e l e r a t o r s ( t o improve quality), stabilizers ( t o prolong bath life) and oxidizing a g e n t s ( t o control the coating r a t e ) a r e added t o t h e phosphating solution.
There a r e c e r t a i n parameters, such a s t h e r a t i o of free t o combined phosphoric acid, t o t a l acid, metal-ion concentration, a c c e l e r a t o r concentration, and t h e process t e m p e r a t u r e t h a t must be controlled t o achieve a suitable coating and maintain t h e
-3 Many^ formulations^ a r e^ available^ for^ t h e^ coloring^ of^ m e t a l s and^ most^ of^ t h e m^ a r e proprietary. The major coloring process f o r steel uses a t r e a t m e n t solution of sodium hydroxide and sodium n i t r a t e in water. The processing t e m p e r a t u r e may vary from 275-320OF, and t h e immersion t i m e may vary f r o m 5 t o 30 minutes. The coating color and characteristics are largely a function of t h e alloy being treated, s u r f a c e characteristics, concentration o f t h e bath, t e m p e r a t u r e , and immersion time.
6.2.4 Passivation
Passivation r e f e r s t o forming a protective film on metal, particularly stainless s t e e l and copper, by immersion in an acid solution. Stainless s t e e l is passivated t o dissolve embedded iron particles and t o form a thin oxide film on its surface. A typical
t r e a t m e n t solution f o r stainless steel is nitric acid or nitric acid with sodium
dichromate. Copper is passivated using a solution of ammonium s u l f a t e and copper sulfate.
6. 3 Chemical Etchinq
Chemical etcMng is used t o produce specific design configurations and tolerances on metallic or metal-clad plastic (printed circuit boards) by controlled dissolution of t h e m e t a l with chemical etchants. Typical etching solutions are f e r r i c chloride, nitric acid, ammonium persulfate, chromic acid, cupric chloride, hydrochloric acid, etc. "Bright dipping" is a special form of chemical etching used t o remove oxide layers from ferrous and non-ferrous materials.
1
6.4 Cyaniding
Cyaniding is a type of c a s e hardening t h a t produces a hard surface on a m e t a l whose c o r e remains relatively soft. The product is a hard, wear-resistant surface backed by a strong, ductile, and tough core. Carbon and alloy s t e e l s a r e usually immersed in t h e cyaniding bath f o r a specific period of t i m e t o achieve t h e required degree of s u r f a c e hardening.
The most common cyaniding solution consists of 30 percent sodium cyanide, 40 percent sodium carbonate, and 30 p e r c e n t sodium chloride. Baths containing 97,^ 75,^ and^^45 percent sodium cyanide a r e also used. Oxygen f r o m t h e a i r oxidizes the sodium
cyanide t o sodium c y a n a t e which, a t high temperatures, decomposes to form nascent carbon and nitrogen. The carbon and nitrogen a r e absorbed by the steel, which increases surface hardness. The processing temperatures may vary from 1200-135OOF f o r low penetration t o 1650-1725OF for high penetration. A combination t r e a t m e n t using high t e m p e r a t u r e immersion followed by low t e m p e r a t u r e immersion is also used.
The depth of surface t h a t is hardened is controlled by t h e t e m p e r a t u r e and t h e cyanide content. As drag-out and carbon depletion occur, special s a l t compositions a r e added t o replenish and r e g e n e r a t e t h e bath. At the end of the t r e a t m e n t , t h e objects are immersed in a w a t e r or oil bath, where quenching and rinsing is accomplished simultaneously. The quench w a t e r is potentially hazardous and is t r e a t e d for cyanide destruction followed by clarification prior t o discharge. The quench oil is also potentially hazardous and is disposed of by incineration.
7. WASTE DESCRIPTiON
The primary wastes associated with metal surface finishing, along with t h e i r process sources, are listed in Table 7-1. The wastes produced in m e t a l surface finishing operations come mainly f r o m two sources: dumping of process tanks, and rinse w a t e r s used to wash off process solutions adhering to t h e product surface or entrapped in t h e crevices due to t h e shape of t h e product piece (Durney 1984, AESI 1981, CP S t a f f 1984). The process solutions a r e periodically filtered t o remove precipitated m e t a l s and a r e reused. These filtered solids a r e mixed with solids.removed from t h e rinse w a t e r s and a r e either landfilled or sold for m e t a l reclamation.
Additional w a s t e is generated a s a result of process solution filtering.
O t h e r wastestreams include spills and leaks plus stripping wastes.
Spent Bath Solution
The activity of t h e plating solution decreases with time due t o t h e precipitation of salts and depletion o f constituents. A f t e r 3-6 regeneration cycles, t h e b a t h is eventually discharged*. This waste s t r e a m usually contains cyanides or m e t a l l i c
~ ~~~ *National Association of Metal Finishers 1985: Personal communication.
I complexes. I n the recent past, this stream was treated together w i t h the rinse water stream. However, the need for obtaining the necessary p e r m i t s and also the perceived regulatory compliance d i f f i c u l t i e s have discouraged treatment o f spent baths*. This stream i s often sent off-site f o r disposal.
Waste Rinse Water
Rinse water i s used to wash o f f process solutions adhering t o the product surface or entrapped in i t s crevices due to i t s geometrical shape. The rinse water stream is usually discharged to municipal treatment facilities w i t h or without any treatment, depending on i t s^ composition.^ The treatment procedure includes oxidative destruction of cyanides reduction o f chromates, neutralization, and solids removal.
F i l t e r Waste.
The f i l t r a t i o n step in the regeneration o f plating solution and the solids removal in rinse water treatment each generate a solid waste. These solids contain oxides or complexes o f metals and are either landfilled or sent off-site f o r m e t a l reclamation.
S p i l l s and Leaks
The overflow and leaks f r o m various process equipment are usually mixed w i t h the rinse water stream and disposed o f as explained above.
Strippinq Waste
Before coating an object, the previous coatings on it are removed by a striping operation. This i s also done t o remove coatings f r o m an improperly coated object. In small job shops, the same stripping solution could be used f o r removing d i f f e r e n t types o f coating. The disposal o f the spent b a t h f r o m such operation is similar t o t h a t o f the spent baths discussed earlier. The rinse waters are usually discharged t o municipal treatment facilities w i t h or^ without treatment.^ The untreated streams may^ contain various cyanides and cyanide complexes, hexavalent chrome, copper, nickel, zinc,
- Westinghouse E l e c t r i c Corporation, 1985: Personal communication
I
3 cadmium and other metals. The t r e a t m e n t includes oxidative destruction of cyanides, The t r e a t m e n t sludge is Spills and overflows t h a t occur c a n be mixed and t r e a t e d
reduction of chromates, neutralization, and solids removal. expected t o be landfilled. with o t h e r liquid wastes and t r e a t e d as discussed.
8. WASTE GENERATION RATES
Since metal finishing operations a r e o f t e n performed along with electroplating and o t h e r operations, the waste generation r a t e s specifically attributable t o m e t a l finishing a r e difficult t o determine. N o waste generation d a t a were in evidence a t t h e t i m e of t h e final document preparation. While no specific waste generation r a t e s were reported, fractional r a t e s were e s t i m a t e d by project s t a f f based on t h e available information and engineering judgements. These values a r e shown in Table 9-1.
9. WASTE REDUCTION THROUGH SOURCE CONTROL
9.1 Description of Techniques
The list of individual waste s t r e a m s and sources and their corresponding source reduction methods is presented in Table 9-1. The following sections discuss t h e various waste reduction methods based on a l i t e r a t u r e survey and industry contacts.
In addition t o the waste reduction measures classified a s being process changes or material/product substitutions, a variety of waste reducing measures labeled as "good operating practices" has also been included. Good operating practices a r e defined a s being procedural or institutional policies which result in a reduction of waste. The following items highlight t h e scope of good operating practice: o Waste s t r e a m segregation o Personnel practices
- management initiatives
- employee training o Procedural measures
- documentation
- material handling and s t o r a g e
- scheduling
material tracking and inventory control
can s o m e t i m e s be removed by temporarily lowering t h e b a t h t e m p e r a t u r e so as t o f o r m solid crystals. In t h e c a s e of electroless nickel plating, t h e sodium s u l f a t e t h a t forms can be crystallized by lowering t h e bath t e m p e r a t u r e t o 41-500F (Durney 1984). The crystals c a n then be removed by filtration.
Another process, patented by t h e U.S.^ Army,^ involves^ t h e^ removal^ of sodium c a r b o n a t e f r o m sodium cyanide-based plating b a t h s by cooling. Dry ice is used t o cool t h e plating bath, thereby causing t h e precipitation of t h e carbonates. The plating solution, free of carbonates, c a n then b e reused. So f a r , t h i s method has not found widespread use (Arienti 1985, Versar 1985).
Use o f a n e l e c t r o l y t i c diaphragm cell for regenerating spent chromic acid used in etching operations has been reported (AESI 1981). The process uses electrolytic diaphragm cell t o oxidize trivalent chromium t o hexavalent chromium and remove contaminants. The quality of t h e regenerated e t c h a n t was reported t o b e equal t o or b e t t e r than fresh etchant. This process, which was still in t h e developmental stage, would have a g r e a t potential f o r reducing spent chromic acid waste. In one such application, extensively t e s t e d a t t h e U.S. Bureau o f Mines in Rolla, Mo., copper etching solution was regenerated and metallic copper recovered a t t h e s a m e time. Recovery was accomplished by depositing t h e copper onto t h e cathode of t h e e l e c t r o l y t i c diaphragm cell (Basta 1983).
O t h e r measures t h a t can extend bath life a r e t o use a u t o m a t i c control systems, maintain a l l rack and barrel systems, and p r a c t i c e good house- keeping a t all times. Automatic control devices can b e used t o maintain t h e concentration by conductivity measurement. A significant change in conductivity would i n i t i a t e pumping of fresh c o n c e n t r a t e into t h e tank. This type of a u t o m a t i c control is used by large facilities especially f o r c h r o m a t e conversion coating (Durney 1984). If t h e racks or barrels (used for transferring objects t o be coated) do not have proper protective coatings, t h e bath could become contaminated. The process solution c a n a t t a c k t h e weak spots in t h e racks or barrels, causing t h e formation of metallic s a l t s in t h e bath and thereby lower its activity. Fluorocarbon
coatings applied t o the racks or barrels have been found t o b e e f f e c t i v e in (^1) minimizing such contamination (Lane 1985). Such a coating will also be helpful in lowering dragout since less bath solution t h a t remains in t h e corroded crevices on the racks or barrels. Special measures t h a t c o n s t i t u t e good operating practices a r e discussed a t t h e end of this section.
o Metal/acid recovery from spent bath solutions. By using electrolysis on t h e spent bath solutions, useful m e t a l s c a n be recovered and a t the s a m e time t h e hazardous n a t u r e of t h e spent b a t h c a n be reduced (Lewis 1980, Campbell and Glenn 1982). This method was implemented in one facility by insertion of electrodes directly into t h e cyanide destruction tank*. However, electrolysis is used only t o a limited e x t e n t by t h e industry (USEPA 1982).
In addition t o recovering metals from t h e spent bath, spent acid can also b e recovered and recycled by means of ion-exchange (Basta 19831. Eco-Tec Ltd., in Ontario, Canada, markets an acid purification s y s t e m t h a t uses a proprietary resin t h a t recovers mineral acids. The m e t a l s are recovered in a concentrated (but still dissolved) form. The c o n c e n t r a t e d m e t a l s c a n than be recovered by electrolytic means. This process is used by Modine Manufacturing, in Trenton, Mo., t o t r e a t copper-contaminated sulfuric acid/hydrogen peroxide solution which was used t o brighten brass (Basta 1983). Sodium phosphate salts, formed in nickel/copper electroless plating, can be converted into useful hypophosphite salts by using ion exchange resins activated with hypophosphorous acid. The use of ion exchange resins for regeneration, however, suffers from t h e disadvantage of generating additional wastes such a s spent resins and resin regenera tion solutions.
1
Another nonelectrolytic means of m e t a l reclamation, still in developmental stages, is t h e use of b a c t e r i a (Basta 1983). Here, t h e microbes form complexes with t h e metals in solution, creating a biomass. The biomass c a n then b e burned t o recover e l e m e n t a l metal. Laboratory studies in this a r e a a r e in progress a t Polbac Corp. (Allentown, Pa.) and t h e O'Kelly Companies (Tulsa, Ok.).
- National Association of Metal Finishers 1985: Personal communication B6-
generated by spent bath solutions can be eliminated totally. Zyaniding, however, does have several advantages over gas carboni triding (Schneberger 1981). These include more flexible opera tion illustrated b y the ability t o simultaneously t r e a t many small batches which require different cycle times, and a higher heating rate. This is important for cases where t h e t i m e needed f o r t h e object t o reach the conditioning temperature may b e a large f r a c t i o n of t h e t o t a l cycle time.
Vacuum evaporation methods for coating nickel, aluminum, and o t h e r metals have been developed. Here, m e t a l s a r e evaporated a t l o w pressure using an electron beam and the vapor condenses, a s a coating, onto t h e substrate. This method could be a viable a l t e r n a t i v e t o electroless nickel plating, which generates spent bath wastes. However, the vacuum evaporation method has several disadvantages, e.g., it requires relatively expensive equipment which becomes effective only when a large number of substrates a r e t o be coated, and the uniformity of coating thickness is generally very difficult t o control.
Chromium and cadmium can b e ,deposited on s t e e l using ion plating methods instead of electrodeposition. Ion plating uses high-energy ions t h a t bombard the depositing m e t a l which e v a p o r a t e s and then condenses on the substrate. In t h e U.S., this method is used only when other techniques a r e found inadequate. However, ion plating is in wide use in Japan where it successfully competes with electrodeposition (Durney 1984).
Chemical vapor deposition (where a chemical reaction decomposes t h e r e a c t a n t gases t o produce t h e desired coating material which then condenses on t h e substrate) c a n b e used for almost any coating operation (Durney 1984). However, its use has been limited t o t h e semiconductor industry, and commercial systems for o t h e r applications are not available a t the present time.
Ion beam processing techniques, s t i l l in t h e development stage, will b e a n excellent alternative t o case-hardening t r e a t m e n t s (Anon 1984). Here, a high energy ion beam is used to harden t h e s u r f a c e by implanting t h e ions in the material. 1
1 ) o Use o f less t o x i c solutions. Whenever possible, the use of less t o x i c solutions for m e t a l finishing w i l l lower the hazard o f spent solutions. In the case of electroless copper plating, water soluble cyanide compounds of many metals are added t o eliminate or minimize the internal stress of the deposit. It was found t h a t polysiloxanes, such as General E l e c t r i c silicon f l u i d SF-96, are also e f f e c t i v e stress relievers (Durney 1984). B y substituting cyanides w i t h polysiloxanes, the hazardous nature o f the spent b a t h solution can be reduced, Use o f trivalent chrome instead o f hexavalent chrome in chromate conversion coating can eliminate the t o x i c i t y o f the spent electroplating baths. Though some manufacturers use t r i v a l e n t chrome+, i t s use i s not growing rapidly because the quality o f t r i v a l e n t chromium coatings i s not as good as t h a t o f hexavalent coatings in many applications.+ Currently, there were a t least f i v e companies t h a t o f f e r t r i v a l e n t systems (Chementator 1982). Trivalent chromium baths can also use lower m e t a l concentrations. One such solution, developed by W. Canning Materials Ltd. in Birmingham, England, uses only 3. 5 gms/liter of t o t a l chromium compared t o the level o f 130 gms/ l i t e r s used in traditional hexavalent baths.
o More dilute process solutions. The use o f dilute bath solutions, whenever possible, would also reduce the hazardous nature of the dumped bath. In the case o f cyaniding, a t y p i c a l b a t h solution composition i s 30 percent sodium cyanide, though some f a c i l i t i e s use 45, 7 5 , and 92 percent solutions. By using a 30 percent solution instead o f a solution o f higher concentration, substantial reduction in the cyanide content o f the spent bath solution can be achieved. In electroless copper plating for printed c i r c u i t board manufacture, dilute solutions have been tried successfully by many manufacturers (USEP 1981). The use o f dilute b a t h solution could also lower subsequent rinse water requirements and m e t a l dragout i n t o rinsewater.
J * Nationa! Association o f fvletai Finishers 1985: Personal communication.
B 6 - 1 7
3
of t h e t o t a l waste volume g e r e r a t e d (AESI 1981). This s t r e a m , containing cyanides and
cyanide complexes along with o t h e r m e t a l complexes, is t r e a t e d for cyanide destruction and discharged t o the sewer. The following source control methods a r e suggested:
o Reduction of drag-out.
In an immersion-type t r e a t m e n t process, small objects a r e placed inside barrels and bigger objects are supported on racks for immersion into t h e bath. When t h e object is removed from t h e bath, t h e rack or barrel (and object) c a r r y s o m e r e a g e n t s with it, called "drag-out". Methods t h a t c a n b e used t o reduce drag-out and subsequently lower rinse w a t e r requirements are: proper racking o f t h e parts; keeping t h e racks free of m e t a l buildup and corrosion; increasing drainage t i m e above t h e process tank; using stationary recovery rinses by installing save rinse or drip rinse tanks; using air blowoff or^ tumbling t o ensure drainage; and using drainage agents (Cheremisinoff, Peina, and Ciancia 1976, AESI 1981, and Cook et. al. 1984).
1 o Effective rinsing methods. By using an e f f i c e n t rinsing sequence, t h e quantity of rinse water required can be reduced substantially. Rinsing efficiency can be improved by using properly designed rinse tanks, using a i r agitation in t h e rinse tanks, using f o g sprays, using a u t o m a t i c valves t h a t control flow r a t e s based on t h e movement of p a r t s through t h e processing line, and by using counter- c u r r e n t rinsing (Cheremisinoff, Peina, and Ciancia 1976, AESI 1981, and
USC 1983).
An e s t i m a t e d 90 p e r c e n t reduction in rinse water c a n b e achieved by using a countercurrent rinse instead of a single running rinse (AESI 1981). Converting t o a c o u n t e r c u r r e n t rinse requires only t h e addition of one or more tanks, appropriate plumbing, and an a i r agitation system. Because of space limitations or t h e use of preprogrammed hoist lines, installation of an additional rinse tank may not be possible a t many job shops. However, many facilities have reported substantial savings by converting t o countercurrent rinsing.
The use of f l o w control valves can reduce rinse water use by 50 percent w i t h m i n i m a l capital costs (AESI 1981). I n many instances, excess water i s used indiscriminately t o ensure t o t a l rinse to p r o t e c t product quality. Though many facilities have installed flow control valves, concerns about reduced product quality have contributed t o opposition t o such measures (AESI 1981).
F o g sprays, though efficient, are not suitable f o r a l l applications. In instances where the coating has l i t t l e strength i n i t i a l l y (as in the case o f chromate coatings), fog sprays are generally not used. Use o f a i r agitation in the rinse tanks promotes turbulence in the tank which increases the rinsing efficiency. The use o f a i r agitation in tanks is f a i r l y widespread (AESI 1981).
o Use o f immiscible rinses. The use o f non-aqueous immiscible solvent f o r rinsing would allow the rinsed solution t o either sink or rise during decantation, and the solution could then be returned t o the surface t r e a t m e n t b a t h f o r reuse without any pretreatment. The rinsing solvent could also be recycled. This process, i f feasible, could reduce or eliminate rinse water wastes. Tests w i t h f i v e solvents f o r use in the chromatin9 process were conducted a t the U n i t e d Technologies Research Center (AESI 1981). A major disadvantage of this method would be the potential f o r increased a i r emissions and the need t o dispose of spent solvent.
o The use of no-rinse coating processes. As the name suggests, no-rinse coatings do n o t require rinsing a f t e r a coating is formed and dried, as there are no residuals l e f t t o i n t e r f e r e w i t h the subsequent treatment. Recent developments in chromate conversion coating f o r the c o i l coating industry have resulted in a solution t h a t can be applied t o steel, galvanized steel, or aluminum, without the need for any subsequent rinsing (USEPA 1982). A f t e r the coating is formed, it is dried in a i r a t about 15OoF. This no-rinse process, though used by only a f e w c o i l coating facilities, can be used f o r other coating applications (USEPA 1982).
