environmental controls

FILTRATION AND PURIFICATION OF

PLATING AND RELATED SOLUTIONS AND EFFLUENTS

BY JACK H. BERG

SERFILCO LTD., NORTHBROOK, ILL.; WWW.SERFILCO.COM

This introduction reflects the response needed by platers for quality control, tomeet just-in-time deliveries, and to achieve zero rejects. It also addresses the needfor platers to continue to reduce solid waste after neutralization and employfiltration wherever possible to recycle or lengthen the service life of cleaners,etchants, and rinses.Filtration usually includes the use of carbon for undesirable organic impurityremoval, which years ago also doubled as a filter media along with other formsof filter aids.Today’s acceptance of granular carbon in many situations has lessened theneed for powdered carbon and almost eliminated the weekly or monthly batchpurification treatment. There are, however, some occasions when powdered carbonmay be the only answer, and for that reason a separate piece of equipmentheld aside for such a need should be considered.Platers who appreciate the value of filtration must first understand that it isnot as much an art as it is a science. The requirement of a science is to have anorderly body of facts, facts that can be correlated and anticipated results yielded.Although there has been some work done in this area over the last 5-10 years,platers must still rely on experience to a great extent.In the past, it has been suggested that the plater decide the level of qualitysought and, using statistical quality control, determine if this goal has beenachieved. It is further recommended that the plater needs to know the partsper million of contamination (solids) so that the necessary size or dirt-holding(solids) capacity of the filter could be established. The plater must also knowthe nature of the solids, which would be critical to success. Slimy, stringy, or oilycontaminants blind a dense filter media surface quickly, whereas coarse, grainy,sandlike particles build a thick cake and still allow solution to pass, which providesfor continued solid/liquid separation.By first assessing these factors, platers can ascertain what results can beachieved. For example, slimy solids would require more surface area, whereasgritty particles could get by with less area (i.e., less solids-holding capacity).However, all filter media are not manufactured in the same manner, forinstance, filter paper, cloth, and plastic membranes provide a single junction tostop solids. Filter aids can enhance the ability of the filter media by creating aporous cake, which improves surface flow, but to really be successful a continuousmixing of filter aid and solids must be coordinated to maintain suitable porosity.Other types of filter media canprovide the necessary junction to stop solidsbut are built in such a manner as to achieve results from a combination ofsurfaces or juncture points, which achieve the solids retention by impedance.Thus, it is possible for continuous solid/liquid separation to be maintained overa longer period of time.Most filter media are rated according to the size of particles that they arecapable of stopping. Such a rating is based on laboratory tests and expressed inmicrometers. A coarse media would be 100 μm; a dense media would be 10, 5, or1 μm. The number suggests that at an efficiency level of 85 to 99%, all such particleswould be stopped, whereas if the micrometer retention level is expressed in“absolute” ratings, 100% of the stated micrometer size and larger sizes would beremoved. It further stands to reason that the coarser media will offer more solidsholdingcapacity, and the denser media will offer less solids-holding capacity.Next we discuss where these troublesome solids come from and how they canbe most effectively removed.

DIRT LOAD

The “dirt” (impurities) in a working plating bath can come from drag-in, anodes,water, and airborne sources. For their efficient removal, the system must bedesigned for the amount and type of contaminants present in the plating tank;these vary for each installation. Even without prior operating experience, anestimate of the dirt load can be made by reviewing the cleaning and platingprocesses to select and size the equipment needed.A filter with insufficient dirt-holding capacity will require frequent cleaningor servicing. The rapid pressurebuildup in the system as solids are retainedincreases the stress and wear of pump seals. By minimizing the dirt load, maintenanceof the filter and pump can be reduced considerably. Even after thoroughcleaning and rinsing, some solids and contaminants cling to parts, racks, andbarrels. Thus, they are dragged into the plating solution. The amount of drag-incontamination depends primarily on the type of parts, plating method (rack orbarrel), cleaning efficiency and rinsing cycles.In most plating plants, the type and amount of parts being processed mayvary considerably. For trouble-free operation, the filtration system should bedesigned for the heaviest work load and most difficult-to-clean parts. Drag-incontamination with barrels is high, due to incomplete draining of cleaners anddifficulty in rinsing of loads. Filtration and purification on automatic barrellines must be continuous, and equipment must be of sufficient size to minimizeservicing and work interruption.The amount of drag-in can often be reduced by improving the pretreatment.With the conversion of many vapor degreasing processes to aqueouscleaning, proper maintenance of cleaners and electrocleaners is of greaterimportance, particularly with machined or buffed parts carrying oil and lubricants.Recirculation and coalescing with an overflow weir on cleaner tanks willeffectively skim off oil and scum, which would quickly foul the filter mediumand carbon. More effective descaling will minimize the dirt load. Several countercurrentrinse tanks and a final spray rinse with clean water will also reducethe drag-in contamination. Due to the nature of the cleaning process, contaminationof the solution with organic soil (oil, wetting agents) and/or inorganic(metallic) compounds is sometimes unavoidable. These can generally be controlledby carbon treatment at the rinse tank before plating.Filterability depends on the nature, amount, and size of suspended particles,which, in turn, are contingent upon the type and chemistry of the plating solution.Generally, alkaline solutions, such as cyanide baths, have slimy or flocculentdifficult-to-filter insolubles, whereas most acid baths contain more grittysolids, which are relatively easy to filter even with a dense filter media. A quicktest of a representative sample with filter paper in a funnel will determine thenature and amount of solids present. This test will also indicate the most suitablefilter medium. Bagging of soluble anodes will materially reduce the amountof sludge entering the plating bath. Airborne dirt from ceiling blowers, motorfans, hoists, or nearby polishing or buffing operations may fall into the platingtank and cause defective plating. Good housekeeping and maintenance will, ofcourse, reduce dirt load and contamination of the plating solution.Prevention of deposit roughness is perhaps the foremost reason for filteringplating solutions. Better covering power with less chance of burning isalso achieved with a clean bath. In addition to suspended solids, the plateralso has to contend with organic and inorganic (metallic) impurities, whichare introduced into the solution primarily by drag-in. If this contamination isallowed to build up, it will affect deposit appearance. Continuous or periodicpurification of the solution with activated carbon and/or low-current-densityelectrolysis (dummying) will often remove these impurities before a shutdownof the plating line becomes necessary.The trend of Environmental Protection Agency (EPA) regulations is toseverely restrict the amount of suspended solids and dissolved metal impuritiesin wastewater discharged to sewers and streams. To comply, platingplants have had to resort to some chemical treatment of their effluents toprecipitate the metals as hydroxides. The filtration of these hydrated sludgesis difficult and requires special separation equipment. Closed-loop systems,recycling, and recovery are being employed and require greater attention tofiltration and purification.Most filtration systems consist of a filter chamber containing the filter mediaand a motor-driven pump to transfer or circulate the solution from the platingtank through the filter. The many filters and pumps on the market today makeit possible to select and justify a cost-effective filter system for each and everysolution, regardless of volume.When engineering a filter system for a plating installation, it is necessaryto first establish the main objectives, such as: high quality finish—maximumsmoothness and brightness; optimum physical properties—grain size, corrosion,

and wear resistance; or maximum process efficiency and control—coveringpower, plating rate, purification, and clarification.Then the following factors must be considered before selecting the size andmaterials needed for the filter media, chamber, pump, and motor:

1. Dirt load—suspended solids, size, kind, and amount; also soluble organicand inorganic impurities.

2. Flow rate—turnovers per hour for a given volume of solution necessaryto maintain clarity.

3. Frequency of filtration and purification—batch, intermittent, or continuousrequired to remove dirt and contamination and filter servicinginterval desired.When agitating solutions with air, a low-pressure blower is usually employed.This makes it virtually impossible to achieve good filtration of the air while keepingthe solution clean, because the plating solution then acts like a fume scrubber.If effluent regulations make it necessary to remove or reduce total suspendedsolids (TSS) from wastewater, the amount discharged per hour or shift can bereadily determined. For instance, a 100 gal/min (gpm) effluent containing 100ppm TSS (100 mg/L) will generate 5 lb of solids per hour, as calculated below:100 gpm 3.79 L/gal 100 mg/L 60 min/hr (1000 mg/g 454 g/lb) = 5 lb/hr (2.3 kg/hr)Therefore, the filter must have sufficient capacity to hold approximately40 lb of solids/8 hr of operation. A horizontal gravity filter would be the mostcost efficient for this dirt load and would operate automatically; however, ifdryness of the retained solids is to be achieved, then a filter press would berecommended.Filtration and/or purification during nonproductive hours makes it possibleto remove dirt at a time when no additional contaminants are being introducedinto the tank, such as insolubles from anodes, chemical additions, plus thatwhich would otherwise be dragged in from improper cleaning of the work.Again, individual tank operating characteristics and economics will determinethe ultimate level of acceptable quality.This brings up an important consideration. Contamination by organic compounds,inorganic salts, wetting agents, and oils is not removed by filtration,but by adsorption on activated carbon. Some plating solutions, such as brightnickel baths, generate organic byproducts during plating. It cannot be assumedthat both types of contamination increase at the same rate. A batch treatment,therefore, may eventually become necessary, either because of insoluble or solubleimpurities. A check of clarity, flow rate, and work appearance and a Hullcell test will indicate the need for transfer filtration and/or carbon treatment.If analysis shows that the concentration of insolubles (in ppm) has increased,it would indicate that the solution is not being adequately filtered. Therefore,transfer pumping of the solution through the filter should be employed as thequickest way of getting all the solids out at once and returning the clean solutionto the plating tank. Soluble impurities can be detected by inspection of thework on a Hull cell panel. Pitting, poor adhesion, or spotty appearance indicatesthe need for fresh carbon. Here again, it may be desirable to completely batchtreat the solution to restore it to good plating quality; however, since this necessitatesshutting down the plating line and requires considerable labor, everyeffort should be made to maintain solution clarity and purity continuously,without having to resort to such batch treatment.

FREQUENCY OF FILTRATION AND PURIFICATION

Since it is desirable to plate with a solution as free of suspended solids as possible,the quickest way to achieve clarification is by transfer pumping all of thesolution from one tank, through a filter, to another tank (batch treatment); however,to maintain both clarity and uniform deposit quality, continuous recirculationthrough a filter is most effective. Although continuous filtration is moredesirable, there are some plating installations that require only intermittentfiltration, because relatively small amounts of solids are present. In other cases,it is necessary to filter and purify the bath continuously, even when not plating.A high flow rate is essential to bring the particles to the filter as quicklyas possible and to prevent settling of dirt on parts being plated. Althoughplating in a solution completely free of solids would be best, this ideal can beapproached only in the laboratory. Some contamination always exists, and mustbe accepted. Continuous filtrationat a high flow rate can maintain a high levelof product quality by keeping suspended solids to a minimum. As Figure 1 indicates,four to five complete tank turnovers effectively remove 97% of all filterablematerials if no additional solids are introduced. Since, in many installations, therate at which contamination is introduced is higher than the rate at which it isremoved, the impurities and solids gradually increase with time unless filtrationis continued even during nonplating periods. The greater the turnover rate, the longer the plating bath can be operated

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Fig. 3. Typical flow versus pressure curve. Q represents the maximum open pumping against no

restriction, whereas P represents the pressure that the pump can develop at zero flow. A might

indicate the pressure drop across a depth type media or a bare support membrane, whereas points B

and C indicate the reduction in flow caused by the addition of filter aid and carbon, respectively.

before the reject rate becomes too high and batch (transfer) filtration is necessary.In practice, contaminants are not introduced at a steady rate; for instance,most are introduced with the parts to be plated and, therefore, at the momentof immersion the degree of contamination is sharply increased until it is againreduced by the action of the filters. It then increases again when more parts areput into the tank for plating.Figure 2 indicates the reduction in flow caused by the dirt buildup in thefilter on a day-to-day basis, where one week’s filtration would be effectedbefore service of the filter becomes necessary. This reduction in flow ratecould also have been representative of a longer time interval between filtercleanings. Graphically, it indicates why platers may experience roughness atvarying intervals in the plating filtration cycle. The amount of solids increasesin the tank as the flow rate decreases to a level that may cause rejects. After thefilter is serviced, the increased flow rate agitates any settled solids. Therefore,it is advisable to delay plating of parts until the contaminant level is againreduced by filtration to within tolerable limits. This phenomenon generallyoccurs in a still tank, since the dirt has more chance to settle. For this reason,when the solution is pumped into a treatment tank, sludge may be found onthe bottom of the plating tank.Dirt in an air-agitated tank can settle any time after the air is shut off. Ifcarbon and/or a filter aid is used in the filter during the continuous filtrationcycle, it should be borne in mind that, as these solids are collected on the media,the pressure increases appreciably, reducing the initial flow rate by almost 25%and the overall volume pumped through the filter by as much as 50% beforeservicing isnecessary (Fig. 3). Frequent laboratory checks will verify the amountof insolubles in the plating tank, which will tell whether a uniform degree ofclarity is being maintained or whether it is increasing slowly toward the rejectlevel. More frequent servicing of the existing filtration equipment will increasethe total volume pumped and, in turn, maintain the lowest possible level ofcontamination and minimize the need for batch treatment.It is, therefore, necessary for the plater to determine the particle size to be removed and then select themedia that provides the most solids-holdingcapacity. Then, knowing the efficiency of the media, multiply it by flow rateso that all of the solution passes through the filter in a certain period oftime, such as 1 hr or 1 min. Note the small amount of solution that is filteredin 5 min if a rate of one turnover per hour is used (Fig. 4) as compared withthe amount that would pass through at a rate of ten turnovers per hour(assume a 100-gallon solution):At one turnover per hour,At ten turnovers per hour,The point here is that if nearly the entire solutionis turned over every 5 min,then the plating bath will exhibit a high degree of clarity and purity. The netresult should be fewer rejects caused by occlusion of particulate matter in thedeposit.In modern electroplating, no area that can result in improved quality shouldbe overlooked. The plater can use the principles of high tank turnover and solutionvelocity to his advantage in his quest for zero rejects.During recent years the flow rate through the filter, or tank turnover as it isreferred to, has increased to two or three per hour or higher for most platingsolutions (see Table I). This means that 1,000 gallons require a flow rate of atleast 2,000 to 3,000 gallons per hour (7.6-11.5 m3/hr); however, platers shouldrecognize the need and employ turnovers of 10 or even 20 times per hour whenall solids must be removed (see Fig. 1).Alkaline solutions may require even higher flow rates for more effective solidsremoval by recirculation. Depending on the filter medium and its retentionefficiency, flow rates in the range of 0.5 to 2 gpm (2 to 8 Lpm) per square footof filter surface area are obtainable. Although 5 gpm per 10-in. (25-cm) cartridgeis permissible, flow rates under 1.5 gpm per cartridge offer better economy. Infact, at a given flow rate with a cartridge filter, servicing, cartridge cleaning, orreplacement can be reduced significantly by increasing the size of the filter.For example, if the size of the filter was multiplied by four the annual amount of filtercartridges consumed would be cut in half and the filter itself would operate unattended forat least four times as long before cartridge cleaning or replacement was necessary. This isan important consideration to reduce media consumption. It has also been found that the effective life of surface filters may often betripled by doubling the surface. By increasing the dirt-holding capacity andreducing the frequency of filter servicing and replacement, the cost of filtrationon a per month or per year basis is substantially reduced.

TYPES OF FILTER SYSTEMS

After estimating the dirt load and determining the flow rate and filtration frequencyrequired, a choice of filter method and medium must now be made. Themost common types of filters used in the plating industry are discussed below.These filters may be placed inside or outside the tank.In-Tank Considerations:Tank spaceMotors located over fumesLimited size of filter (less service life of media if used on pump suction)Out-of-Tank Considerations:Remote possibility for easy serviceEmploy sealless magnetically coupled pumps or direct-drive with singleor double water-flushed sealMore suitable for use with slurry tank for chemical or filter aid/carbonaddition or backwashingLarger dirt holding and flow capacity from cartridges or surface media

Cartridge Filters

Cartridges offer both surface and depth-type filtration characteristics, providingvarious levels of particle retention at different efficiencies (nominal andabsolute), manufactured in natural and synthetic (plastic) materials to providea wide range of chemical resistance, flow rates, and particle retention capacities.Pleated-surface media offer initially higher flow rates, are available with a choiceof porosities (usually in the denser range), and are sometimes given an absoluteparticle-retention rating.Depth-type media are available in 1- to 100-μm particle retention and, becauseof the variety of porosities available, they are sometimes best suited to handlehigh-dirt-load conditions. This is a result of the manner in which the depth-typecartridge filter is manufactured. Basically, it consists of a series of layers, whichare formed by winding a twisted yarn around a core to form a diamond opening.The fibers, which are stretched across the diamond opening, become thefilter media. Succeeding layers lock the previously brushed fibers in place and,. since there is the same number of diamond openings on each layer, the openingsbecome larger due to the increase in circumference; other fiber-bonded types alsoincrease density across the depth of the media.During filtration, the larger particles are retained on the outer layers of thecartridge where the openings are large, whereas the smaller particles are retainedselectively by the smaller openings on succeeding inner layers. This, then, makesit possible for an individual cartridge to have a dirt-holding capacity equal to 3.5ft2 of surface filter area of the same density. Cartridges having a 15- to 30-μmretention will often hold 6 to 8 oz of dry solids before replacement is necessary,whereas cartridges of 10 μm downto 1 μm will have a dirt-holding capacity ofperhaps 3 oz to less than 0.5 oz. These figures merely indicate that the coarsercartridges have greater dirt-holding capacity, are more economical to use, andcan be used longer before replacement.Also, as pointed out earlier, dirt loads vary from tank to tank, and cartridgesshould be selected according to the individual requirements. A dense cartridgehaving less dirt-holding capacity will load up more quickly, increasing the pressuredifferential and, therefore, reducing the flow (Fig. 5). Using coarser cartridges(greater than 30 μm on zinc, for example) that have greater dirt-holdingcapacity and a longer service life may make it possible to clarify the plating tankmore quickly because of the high obtainable flow rate. This will be accomplishedat less cost. Usually two cartridges (three on zinc, tin, and cadmium) are recommendedfor each 100 gal of tank capacity.The pump should provide a pumping rate of at least 100 gph (two tankturnovers per hour) for each cartridge. Usually, a cartridge life of 6 weeks onnickel or 4 weeks on zinc can be expected, with some tanks running as longas 12 weeks; however, much depends upon dirt load, hours of plating, and soon. With cartridges, a higher dirt load can be retained in the filter chamberbecause of the coarseness of the filter media. Higher flow rates can usually beemployed during the entire lifespan of the cartridge. This is due, in part, to thehigher head pressures of pumps employed without chancing the rupture of acartridge. Since all of the dirt is retained on and in the cartridge, the cartridgefilter can be turned off and on at will, unless the cartridges are precoated.Cartridges are changed with very little maintenance expense and no solutionloss; however, simplicity of use is perhaps the most predominant single factorin their selection.

Precoat Filters

Precoated filters consist of a membrane (leaf, sleeve, or screen) such as paper,cloth, ceramic, sintered metal, wire mesh, or wound cartridges. These membranessupport the diatomite or fibrous-type filter aid, which has been mixed ina slurry of water or plating solution and picked up by the membrane openings.The dirt is retained on the outer surface of the cake. When the pressure hasincreased and the flow rate has decreased to a point where filtration is no longerefficient, the dirt and cake are washed from the membrane. Paper membranesare discarded and replaced.The ability to obtain long runs is dependent upon proper selection of thefoundation media, coupled with a coarser-than-usual nonfibrous-type filteraid (to be used where possible). Periodic (daily, if necessary) additions of smallquantities of filter aid should be made to lengthen the cycle between servicing.The dirt-holding capacity of this type of filter is usually measured in squarefeet of filter surface. (If the standard 2.5 x 10-in. long cartridge is used, its outersurface when precoated would beequivalent to about 0.50 to 0.67 ft2 of area.)Flow rate and dirt-holding capacity of the various precoated membranes orcartridges would be about equal.Before precoating, the operator should know or determine the filtrationarea to be covered. The amount of filter aid used depends on its type and onthe solution being filtered. Generally, 0.5 to 2 oz/ft2 of filter is sufficient. Themanufacturer’s recommendations for type and amount of filter aid should befollowed if optimum results are to be obtained. A slurry of filter aid and platingsolution or water is mixed in a separate container or in a slurry tank, which maybe an integral part of the filtration system. The slurry is then caused to flowthrough the filter media and create a filter cake.Usual flow rates range from 0.5 to 2 gpm/ft2 of filter surface. A lower flowrate improves particle retention and smaller particles will be removed. It shouldbe pointed out that, although there may be a wide range in flow rate, the rangeof selectivity of particles being removed is between 0.5 and 5 μm, which is themost significant difference between precoat and depth-type cartridges and offersa wider choice of porosity.Buildup of cake should be gradual, and recirculation should continue untilthe solution runs clear. Cake should be dispersed uniformly across the mediabefore the plating solution is allowed to flow across the filter. A slurry tankpiped and valved into the filtration system becomes a convenient and versatilepiece of equipment. The slurry may be prepared with plating solution, ratherthan water, to avoid diluting critical mixtures. Via valving, the solution isdrawn into the slurry tank for sampling, preparation of slurry, and chemicaladditions. Similarly, the solution is returned to the plating tank. This methodeliminates the necessity of transfer hoses between tanks, and the subsequentrisk of loosening the cake or losing pump prime. The integral slurry tank isalso a convenient storage for backwash water.

 Precoat Backwash FiltersThese filters operate the same as, and have the same functional purpose as, ordinaryfilters with the further advantage that they can be cleaned quickly by reversingthe flow through the filter media. Backwashing the filter aid and dirt awaymakes the media available for prompt repeat precoating. The basic advantage isthat the filter chamber need not be opened each time the filter requires cleaning.Finer grades of filter aid may be precoated on top of the coarse filter aid whenfine powdered carbon is to be used continuously. Here again, periodic (daily, ifnecessary) additions of small quantities of filter aid should be made to lengthenthe cycle between backwashing. The media may be cleaned automatically withsluicing or using other devices. Iron hydroxide sludges can be dissolved by circulatingdilute hydrochloric acid from the slurry tank; additional manual cleaningmay also be required occasionally.Some disadvantages of precoat and backwashing are the possible loss of solution,increased waste treatment loading, and the possibility of migration of filteraid and carbon into the plating tank. The use of rinse water for backflushing willreduce waste treatment loading; however, if evaporation is used to control dragout,this may interfere with evaporator operation and the economies achievedby using this equipment.

Sand Filters

Using sand as the filter media, the pump and filter operate like a precoat surfacefilter and backwash like a precoat without the need of additional aid to achievefine particle retention. Performance can be acceptable based on recirculationturnover rates, with the basic disadvantage coming from a smaller surface area,which increases the need for frequent backwashing and resulting solution lossto maintain the desired flow rate (turnover required).

Horizontal Fabric and Screen Filters

These filters are especially well suited for the continuous dewatering of hydratedmetal sludges resulting from the neutralization of plating wastewater prior to

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Fig. 7. Skimmer, pump, and prefilter with carbon or free oil separator.

sewer discharge. They are also effective in removing accumulated iron sludgefrom phosphating tanks.In one such system (Fig. 6), the waste containing 1 to 3% solids is first allowedto settle in a cone-shaped tank. The supernatant liquid drains into a head box,which directs the flow across the filter medium (paper or plastic) supported bya motor-driven conveyor belt. The liquid passes through the disposable fabricby gravity flow into a receiving tank below. When the pores of the media becomeclogged, the liquid level rises and a float switch activates the belt drive. Freshmedia is fed over the tank and filtration is continuous. The cake on the fabricis allowed to drain before it is dumped into the sludge box. Gravity drain or animmersion pump empties the filtered water from the tank. Cycling and indexingof the filter are automatic. The occasional replacement of the filter fabric rollis the only labor required. The sediment in the bottom of the cone can also bedewatered periodically by filtration on the fabric. Other systems feature pressureor vacuum filtration. The sludge cake contains from 5 to 35% solids, dependingupon the equipment and type of cake. Cakes can be further treated by airevaporation or with heat for dry disposal. The filtrate can be discharged to thesewer if it meets local effluent regulations or can be recycled through the system.The performance of the unit can be improved greatly by the addition ofcoagulants and flocculating agents, such as polyelectrolytes, which increase theamount of solids, particle size, and settling rate. The flow rate is approximately1 gpm/ft2 with 90 to 95% solids retention; with coarse filter media, flow ratesincrease up to 10 gpm/ft2. Filter aid can also be precoated to improve retention.The filter media is available in porosities of 1 to 125 μm and rolls 500 yd long.Carbon-impregnated paper is used for purification and removal of organiccontaminants. The unit must be sized properly for each application to operateefficiently and with a minimum media cost. Steel, coated, stainless steel, orplastic units are available for corrosive solutions.

BATCH AND CONTINUOUS ACTIVATED-CARBON PURIFICATION

Virtually all plating solutions and some cleaners or rinses at some time will requirepurification via the adsorption of impurities on activated carbon. Those solutionsthat contain wetting agents require the most carbon; when oil is introduced intothe bath, the carbon is dispersed throughout the solution and clings to the parts,causing peeling or spotty work. Solutions that do not contain wetting agentshave a tendency to float oil to one corner, depending on the recirculation set upby the pump, and in this case the oil may be removed with a skimmer or coalescer(see Fig. 7).The choice of purification method depends on the size of tank and amountof carbon required and also on other available auxiliary equipment. Generally,carbon cartridges are used on small tanks (up to a few hundred gallons), andthe bulk or canister type or the precoat method is used for the very largest tanks.The canister type is also used on the larger tanks supplemental to surface ordepth-type cartridges or on certain automatic filters to supplement the amountof carbon.

Batch Treatment

The quality of the carbon is important and special sulfur-free grades areavailable. The average dosage is 10 lb of carbon to treat 500 to 1,000 gal ofwarm plating solution. At least sixty minutes contact time with agitationshould be allowed, followed by some settling before transfer clarificationcan be achieved.

Continuous PurificationA separate purification chamber holding bulk granular carbon, a carboncanister, or cartridges offers the most flexibility in purification treatment. Bymeans of bypass valving, the amount and rate of flow through the carbon canbe regulated to achieve optimum adsorption of impurities without completedepletion of wetting agents and brighteners in the plating bath. It providesfor uninterrupted production and fewer rejects. When necessary, the carboncan be changed without stopping filtration of the bath. Filtration shouldalways precede carbon treatment, to prevent dirt particles from covering thecarbon surfaces.

CONTINUOUS CARBON TREATMENT METHODS

Carbon Cartridges

Cartridges containing up to 8 oz of either powdered or granular carbon for every10 in. of cartridge length are available and will fit moststandard replaceablefilters that employ this type of media. They may include an outer layer, whichserves as a prefilter, and an inner layer, which serves as a trap filter. These handycartridges are ideal for small filter chambers because of the ease and convenienceof quickly replacing a conventional depth tube with the carbon tube when necessary.They may also be used with submersible filter systems, but in this case theflow rate could be greatly reduced.

Carbon Canister

Granular carbon may be used in ready-to-use chambers, each with a number ofcanisters holding up to 10 lb of granular carbon, and placed in line to the tank. Abuilt-in trap filter eliminates migration of the carbon. Prefiltration ahead of thepurification chamber will prevent solids from coating the surface of the carbonin the canister, assuring maximum adsorbency. The carbon in the canister can bereplaced when its adsorption capacity has been reached. This method of separatepurification offers the most flexibility. Any portion or all of the filtrate can betreated as needed by means of a bypass valve after the filter.

Bulk Carbon Method

Granular or bulk carbon is poured looselyaround standard depth-type cartridge filtersor sleeves, is poured into specific chambersdesigned for carbon, or is pumped betweenthe plates or disks of other surface media.Since no filter aid is used, fines breakingoff from the piece of carbon will have to bestopped by the surface media. Therefore, aninitial recirculation cycle without enteringthe plating tank or recirculation on the platingtank prior to plating is desirable. Thismethod does not alter the solids-holdingcapacity of depth-type cartridges, as mostof the carbon will stay on the outer surfacelayer; however, carbon removal is not easily accomplished.

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Fig. 8. Suction or dispersion piping

system with strainer and siphon

breaker. Drill a hole 2 in. below working

solution level as a siphon breaker

to prevent solution loss due to

unforeseen damage to piping, pump,

and so on. Chlorinated polyvinyl

chloride with screwed connections

offers maximum flexibility and ease

in installation and may also be used

on the return line by eliminating the

strainer and replacing it with a longer

length of pipe that is open along the

full length.

TIPS ON FILTER INSTALLATION

Filtration equipment should be installedas close to the plating tank as possiblein an area that affords access for servicing.Equipment that is not easy to servicewill not be attended to as frequently asrequired, and the benefits of filtration willnot be maximized. The suction line shouldalways have a larger diameter than the dischargeto avoid starving the pump (e.g.,1 in. versus in. or 2 in. versus 1.5 in.) Where it isnecessary to install theequipment more than 10 to 20 ft away, check the pump suction capabilitiesand increase the size of the suction piping (1.5 in. instead of 1 in., or 2 in.instead of 1.5 in.) to offset the pressure loss.Hoses made of rubber or plastic should be checked for compatibility withthe different solutions. Strong, hot alkaline and certain acid solutions such aschromium are especially aggressive. The use of chlorinated polyvinyl chloride(CPVC), polypropylene, or other molded plastic piping for permanent installationis becoming more common. Some plastics are available with socket-type fittings,which are joined with solvents. Their chemical inertness and temperaturecapabilities are excellent. Iron piping, lined with either rubber or plastic, is idealbut usually limited to use on a larger tank capable of justifying the investment.It should be pointed out that whenever permanent piping can be used in andout of the tank a more reliable installation will exist, since there is no shiftingto loosen fittings, and collapsing or sharp bending of hoses is eliminated. Thesuction should be located away from anode bags, to avoid their being drawn intothe line and causing cavitation. Strainers on the suction are always advisable.It is also desirable to drill a small opening into the suction pipe below thenormal solution operating level on permanent installations so that, shouldany damage occur to the system, the siphon action or suction of the pump willbe broken when the level reaches the hold (Fig. 8). This provides added safetyduring unattended operation. Whenever automatic equipment is operated,some provision must be made to protect against unforeseeable events thatcould cause severe losses. This includes some form of barrier or removablestrainer to prevent the suction of parts into the pump. The addition of a pressuregauge is strongly recommended to determine the initial pressure requiredto force the solution through the filter and also to determine when the filtermedia needs to be replaced.When starting up a new filter system, or after servicing an existing system,it is advisable to completely close the valve on the downstream side of thefilter; in this way, the pump will develop its maximum pressure, and one canimmediately determine whether thesystem is secure. Sometimes filtrationsystems are tested on a cold solution and, in turn, will leak on a hot solutionand vice versa. Therefore, a further tightening of cover bolts, flange bolts,and so on may be necessary after the filter has been operating at productiontemperature and pressure. If pump curves are not available, one may wishto check the flow at different pressure readings to determine a reasonabletime for servicing the equipment before the flow rate has dropped too low toaccomplish good dirt removal.