environmental controls

WATER POLLUTION CONTROL

FOR PAINT BOOTHS

BY ALAN MONKEN

As a nonrenewable resource, water and its conservation are of prime importancein the metal finishing industry. The reduction and control of water pollution inthe manufacturing process is an important area for improvement; one of the mostcritical areas of industrial water usage is the paint shop. Current technology canbe utilized to reduce water consumption and improve the efficiency of water use.To put this technology in perspective, it is necessary to explore the use of water inthe paint shop, the available chemical means to deal with water pollutants, andthe mechanical means of removing these pollutants.

THE PAINT SHOP

The function of the paint shop is to apply an organic coating (i.e., paint) to a substrate(i.e., metal or plastic) for protective and decorative reasons. The paint canbe applied in various forms, including dry powder, solvent-diluted formulations,and waterborne formulations. The application method can vary widely, two ofthe most common being through spraying or through immersion. In the case ofimmersion or dip-paint systems, very little paint waste is generated. The drawbackto immersion painting, however, is that variations in paint colors and types areseverely limited within the same operation. Spray systems allow a great deal offlexibility in the types and colors of coatings that can be applied.The downside of spray systems is that not all of the paint sprayed comes intocontact with the work surface. The paint that misses the production part is commonlyknown as overspray. If the process being used is powder painting, the oversprayedpaint can most typically be collected and reused (the ability for capturingthis excess powder typically is designed into the powder paint spraying system orbooth). If the process involves liquid paint, however, the paint overspray quicklychanges from an asset to a liability as it becomes paint waste. Although it is possibleto collect this oversprayed paint on dry-filter media, the most common collection/removal method is the use of the waterwash paint booth.

WATERWASH PAINT BOOTHS

The primary function of a paint booth, whether wet or dry, is to remove thepaint overspray from the air of the work environment; secondarily, it functionsto remove the paint solids from the air stream, allowing any volatile solventvapors to be expelled from the work area. Dry-filter booths make use of mediasometimes resembling conventional furnace dust filters to screen out the tackypaint solids,which are actually the organic portion of the paint responsible forproviding the coating. This media can quickly plug, reducing the effectiveness ofthe removal process. The media, once saturated with paint, is disposed of, typicallyas flammable waste. Waterwash booths perform the same function but use wateras the medium of capturing the paint overspray and the resultant waste material.Although there is a wide variety of variations in waterwash paint booth stylesand types, the two basic categories of design are side-draft and downdraft booths.Draft here refers to the way the air movement is directed through the system todraw the paint overspray in for capture. Side-draft booths, most common insmall noncontinuous metal finishing and manufacturing operations, typicallyfunction by pulling a mixture of paint overspray and air through a mobile watercurtain, using the action of the water to “scrub” the paint solids from the water.This water is recirculated from a holding tank and continuously is cascadeddown the “waterfall” wall.A similar mechanism can be used in the downdraft paint booth, which is mostcommon in larger continuous operations, such as automotive assembly plants.The downdraft system makes use of a downward airflow, most typically througha steel floor grating, into a mobile flood sheet (much like a horizontal “waterfall”)or pit of water. Downdraft systems normally contain much larger volumes of waterthan side-draft systems, making the treatment and conservation of this water evenmore critical.

COMMON PAINT-RELATED WATER POLLUTANTS

In the case of either side-draft or downdraft systems, the recirculated water comesinto contact with a wide variety of potential pollutants from the paint overspray.Many of the materials in the paint, particularly in solvent-based formulations, arenot particularly compatible in water systems. The solvents, which would includexylene, toluene, and methylene chloride, are typically not water soluble but can bewater miscible (i.e., mixable). At any rate, most of these solvents are volatile andwill evaporate over time to exit through the air exhaust. The organic resins makingup the bulk of the paint coating are insoluble in water and tend to stay tacky if nottreated with some additional material introduced into the water.If left untreated, tacky sludge can plug up recirculation pipes and pumps (aswell as adhere to any and all surfaces of the booth), reducing overall efficiency.Other additives in the paint formulation, such as film-forming/wetting agents,may or may not be soluble in the water and will be present in varying degrees. Somepigments or other inorganic components, such as zinc or chromate compounds,may be partially or completely soluble in water. These inorganics, especially thezinc and chromium salts found in many primers, can pose major problems indisposal. The key to reducing or removing any of these pollutants is to find a wayto either solubilize or detackify the paint solids and to collect and remove the dissolvedsolids (if possible).Water-based paints, unlike solvent-based formulations, dissolve or dispersereadily in water. Because of this dispersibility, caused impart by the relatively smallparticle size of the waterborne pigments and resins, water-based paints can often bedifficult to physically remove from the system. The problem then is one of solidsconcentration and removal, rather than detackification.

PAINT DETACKIFICATION

A number of methods have been developed to chemically treat the sticky organicpaint sludge collecting in waterwash paint booths. Reviewing these detackificationsystems both historically and in terms of increasing effectiveness, they includecaustic/hydroxide treatments,metal salts programs, clay-based programs, andacid colloid programs.

Caustic/Hydroxide Treatments

The use of caustic-based treatments (most commonly in the form of sodiumhydroxide) represents the oldest chemically based treatment for detackification.These products work well with lacquers (paints cured by solvent loss), which madeup many low-solids paints in the past. The basic principle of detackification is thecleavage of ester linkages found in the fatty acid-based resin by hydroxide. Thisresults in the formation of a metal-based soap, which emulsifies any remainingsolvents in the paint. The remaining material, with no solvent present, cures andhardens into a mass for “easy” removal.The drawback of the caustic-based program is that, as paint technology hasadvanced, with changes to higher solids levels (primarily for reasons of environmentalcompliance) and catalyzed curing, this type of treatment no longer fullyreacts with the components of the paint. This results in only partially killed paint,which causes most of the same problems as “live” paint. To combat this problem,caustic-based programs containing some insoluble inorganic material (such aslime) were developed. The insoluble material helps to capture some of the unkilledmass of the paint, essentially embedding it. Even these modified programs are inefficient,however, as the solids level of the paint increases past 25%.

Metal Salts Programs

Metal salts products primarily make use of aluminum and zinc salts combinedwith a source of alkalinity to form either catalyzed insoluble metal soaps (somewhatsimilar to the treatment with caustic) or suspended metal complexes, whichcan be removed by treatment with an additional polymer.The limitation of this type of treatment is that, except in the case of alkyd-based,air-cured paints, the pH control of the system is very crucial to proper operation.Fluctuations in pH level can easily cause disruption of the program, resulting inlive paint and settling solids.

Clay-Based Programs

Clay-based products primarily represent a physical, rather than chemical, methodof paint detackification. As bentonite clay absorbs water, it swells to a largeirregularly surfaced material. Sticky paint particles entering the water containingthis clay adhere to the outside surface and are, in turn, covered by otherclay particles. This results in a large detackified mass. An amine is often fed toincrease the paint’s tendency to disperse prior to contact.Although the clay itself is relatively inexpensive as treatments go, large amounts ofclay or clay slurry are often required to maintain good detackification. This producesvoluminous amounts of sludge, as compared with other treatment types. In addition,both water and solvent are often trapped in the clay matrix, making it difficult forlandfill and limiting the ability to dewater to a range of 20% to 25% maximum. Clayprograms also typically have problems with foaming and biological contamination,due to the entrapment of paint and water in the clay sludge.

Acid Colloid Programs

The acid colloid treatments function on the principle that certain mixtures ofhydrophilic (water loving) and hydrophobic (water hating) materials can formstable suspensions under acidic conditions but precipitate as associated complexesas the pH increases. There are three detackification programs currently used basedon this principle: silicate amine programs, silica amine programs, and melamineformaldehydeprograms. The basic principle is to feed th e product into the systemat a raised alkalinity level to form the associated complex. The hydrophobic endorients onto the hydrophobic paint particle, with the hydrophilic end sticking intothe water phase.This effectively allows the paint particle to be coated with a thin filmof water that prevents its surface from adhering to other surfaces.Silicate AminesIn this treatment, the hydrophilic portion is a polyamine and the hydrophobe is asilicate (usually sodium metasilicate). These materials are fed separately to the boothin a fixed ratio based on the paint overspray rate. (Most typically 4 to 13 parts ofsilicate to 1 part of polyamine and both at 5% to 15% based upon overspray.) The pHlevel is critical, since too high a pH can cause the complex to redissolve.The main drawback in silicate amine treatments is that they do not dispersepaints very well, nor do they provide instantaneous detackification. Because of this,it is not uncommon to find sticky deposits in the back sections of spray boothswhere good mixing does not occur. Better detackification usually takes place asthe system runs longer.Silica AminesThe silica amine program is very similar to the silicate amine treatment. The primarydifference is that it utilizes an aqueous colloidal silica sol as the hydrophobe.Colloidal silica can be thought of somewhat as a nonswelling clay. The silica solis fed at a ratio of 1 to 3 parts to each part of polyamine. Since the pH of thesematerials is essentially neutral, an alkalinity source (usually potassium hydroxide)is fed to bring the system pH to 8.0 to 9.0. The primary drawback of this programis that under conditions of high shear, such as might take place with a centrifugeseparator, the small size of the silica might not allow itself to fully embed onto thepaint, resulting in partially killed sludge.

Melamine Formaldehyde

This copolymer was originally developed by Du Pont in the early 1940s. It makes useof its unique organic structure to act to detackify paint. The alternating melamineand formaldehyde in the polymer chain form a two dimensional netlike structure,the melamine portion acting as the hydrophobe and the formaldehyde functioningas the hydrophile. Under alkaline conditions, the compound forms an associatedcomplex.The melamines orient on the surface of the paint while the formaldehydegroups attract the water layer that prevents the paint from sticking. Because both ofthese groups are on the same molecule, the effect of detackification is nearly instantaneous.Also, because the size of the groups is small relative to that of silicate or silicaamine, the melamine formaldehyde coats the paint particle much more effectively.One of the drawbacks of melamine formaldehyde treatment is the relativefragility of the coating. Because of this, it is necessary to disperse the paint well.Under conditions of high shear the coating can be ruptured, releasing sticky paint.The other fact to consider is that because of the sensitivity of this treatment towaterborne particulates, the cleaner the system, the more effective the melamineformaldehyde is in killing the paint. As the solids loading increases, the level ofdetackification decreases and the ability to form a good floc is affected.

SLUDGE REMOVAL METHODS AND EQUIPMENT

Once the paint sludge has been detackified or otherwise concentrated, it is necessaryto use some mechanical means to remove it from the water. The methods usedto remove the captured paint overspray from paint booths vary widely in type,effectiveness, and cost. A great deal of the choice as to which method is selected isdependent upon the type of booth, the amount of paint sprayed, the desired endresults of the sludge removal, and the money available for equipment. Optionsavailable for side- draft and downdraft systems will be examined separately, interms of both manual and automatic methods.

Side-Draft Systems

In smaller booths, the most common method of sludge removal has historicallybeen skimming. Some portion of most solvent-based paints will usually float ifuntreated; caustic-based treatments will typically result in partial float/partial sinkon a continuous basis, especially when a flotation aid is used. Many users of smallbooths were, therefore, accustomed to continual skimming of floating materialfrom their systems.With the advent of paint-dispersing polymer treatments, continuous manualskimming is unnecessary. Elimination of this process reduces much of the dailylabor and its associated costs. In side-draft systems, use of a polymer paint detackifiernormally keeps paint in a suspended, dispersed state, allowing for flocculationand flotation on a batch (periodic) basis. Manual skimming, with screens or rakes,is still possible at this point. Manual skimming has the next-to-lowest capital cost(the lowest being passive settling, which will be discussed in detail in the downdraftsection) but is also labor intensive.The next level of sophistication in side-draft sludge removal would be the useof semiautomatic or automatic equipment to remove the floating waste. Oneway of reducing labor and eliminating manual skimming in batch flocculation clean outs would be to usea wet-vacuum filtration system.This basically consistsof an industrial wet-vacuumhead on a steel drum containinga burlap (or othercoarse filter cloth) bag. Thefloating sludge (and somewater) is vacuumed from thetop of the booth tank. Thepaint sludge should collectin the bag, while the water isdrained (or pumped) fromthe bottom of the drumback into the booth. This method can also be used for sludge settling out onthe booth tank bottom, although the settled sludge must be completely detackified.Another method for removing periodically produced floating sludge is the useof a tank-side weir (see Fig. 1) In essence, a small weir is welded onto the side ofthe booth tank, allowing floating material to overflow from the booth and bepumped to a filtering tank (or other system) for dewatering.Side-draft booths can also be equipped for automatic continuous removal of floatingsludge, using equipment generically referred to as a consolidator (see Fig. 2).This type of system pumps water from the booth into a separate tank. As the wateris pumped in, a flocculating polymer is injected into the water, causing the detackifiedpaint sludge to float to the top where it is skimmed off by a continuously movingblade. The clean water is cycled back into the booth. Paint sludge can also beremoved continuously without flocculation/floating using filtration methods. Thesimplest filtration equipment consists of filter beds utilizing paper or cloth media.These systems allow the solid material to settle out on the filter media,with the waterdraining to some collection unit where it can be returned to the booth. Although thistype of system has low labor and capital requirements, it is often very cumbersome,which can be a problem sincespace around a painting area is usually at a premium.Gravity filtration systems are also slow and restricted as to through put volume,which makes them suitable for only low levels of water or sludge to be processed.Vacuum filtration, such as that done using diatomaceous earth filters, is effectiveon completely detackified materials, but can add to the overall volume of wasteproduced due to the contributions of the disposable media itself.Centrifugal methods of sludge removal/dewatering are somewhat more expensiveto purchase and install than skimming or filtration equipment but can make up thedifference in cost with their performance. The two most commonly encountered centrifugal separator types are the hydrocyclone and the centrifuge. Hydrocyclones