مقالات سلامت در آبکاری

environmental controls

WASTE MINIMIZATION AND RECOVERY

TECHNOLOGIES

BY W. J. MCLAY

DEDIETRICH PROCESS SYSTEMS INC., UNION, N.J.; WWW.DDPSINC.COM

AND F. P. REINHARD

CH2M HILL, EAGAN, MINN.

The surface-finishing industry is a chemical-intensive industry. A special categoryof chemical processes, characterized primarily as electrochemical processes,are used to treat and condition, or “finish,” the surfaces of a variety ofmanufactured goods and components to either enhance visual appeal, improvecorrosion resistance, or to increase product durability or serviceability.Some providers of finishing services, and most manufacturers with in-housefinishing operations, are understandably inclined to view themselves as purveyorsof finishing services for the end products that they process or as producers ofthe products that are manufactured, rather than as operators of chemical plantand chemical producing processes.Surface-finishing processes certainly fall under the definition of chemicalprocesses. As such, they are no less subject to the limitations and laws of chemistryand physics and to good process design and chemical engineering practice.The similarity of chemical production processes and surface-finishing processesis strong. At the heart of electroplating and waste-treatment operations,one finds many of the classic chemical unit operations and process techniquescommon to chemical production: mass and energy transfer, fluid flow, mixing,evaporation, reaction, sorption, crystallization, concentration/dilution, solid/liquid separation, etc.A broad variety of chemicals is used by the finishing industry; however, onlya small fraction of the chemicals purchased for bath make-up and operation isultimately incorporated in the finished goods. While chemical manufacturingprocesses generate more hazardous waste on a tonnage basis, surface-finishingprocesses lose a disproportionate quantity of purchased chemicals as byproducthazardous waste. The value associated with this wastage, plus the added cost oftreatment and disposal, constitute major pressure on operating margins andprofit.In addition, finishing operations also require equally disproportionate quantitiesof process water per unit of production for parts cleaning and preparation,for bath make-up and maintenance and, of course, for rinsing. In many parts ofthe country the availability of quality process water is becoming a major concernto the finishing industry.The price and conditioning costs of raw water are also increasing. Manyfinishers are looking for practical ways to limit water usage and to recover andreuse as much process water as possible.Some firms have achieved, or are approaching, the elusive goal of zero liquiddischarge. Also, the added incentive of potentially not requiring an effluentdischarge permit has strong appeal.In addition, finishing processes cannot be operated with the same degree ofcontrol common to many chemical production processes. By definition, manychemical processes are essentially steady-state processes and lend themselvesto tight statistical control. In comparison, finishing processes are more readilycategorized as unsteady-state processes that are relatively chaotic from a processstandpoint and, as a consequence, are more difficult to monitor and control.This characteristic has nourished the relatively straightforward “lime-and-settle”method of treating toxic wastes and has hindered the acceptance and applicationof what are now a well-documented set of chemical process techniquesfor reducing the high level of waste generated by surface-finishing processes.In an ideal finishing process, there would be no bath drag-out. Chemicallosses would be restricted only to those chemicals that are consumed in cleaningand preconditioning surfaces and to those portions of the plating baths, whichproduce the desired surface coating or condition.In the real world, bath drag-out is, of course, unavoidable. Drag-out can bereduced to some extent by instituting such mechanisms as increasing dwelltime over baths, decreasing bath surface tension, forward pumped spray rinses,air knives, etc. Despite such efforts, substantial quantities of bath can still belost to the rinse system. The net result is that bath drag-out continues to be theprimary contributor to the extraordinary quantity of chemical waste generatedby the surface-finishing industry.This article reviews a number of well-demonstrated and proven chemical recoverymethods, collectively known as separation technologies, for reducing or insome cases reversing bath drag-out. When properly selected and applied, one ormore of these technologies in combination can be confidently used to separateand recover dragged-out bath or specific chemical components or values of certainbaths or solutions and to separate and condition rinsewaters for recycle and reusein the plating process.Each technology separates the constituents of a solution differently. Forexample, evaporation separates the solvent (water) from the rest of the bathconstituents. All other techniques affect separation on either a molecular oran ionic level. The choice of technology, or combination of technologies, isdetermined by both bath chemistry (what the chemistry lets you do) and by theunderlying operating economics.

ECONOMICS OF RECOVERY VERSUS TREATMENT

There are essentially four approaches that can be taken to evaluate point-sourcerecovery potential in given metal-finishing operations.

Operating Savings

Plating facilities with existing and adequate waste treatment systems can readilyassess operating savings for a candidate recovery technology. A given recoverytechnology is evaluated on the basis of savings on purchased process chemicalsand associated waste treatment chemicals plus any resultant savings in sludgehandling and disposal cost. If the payback on invested capital is attractive, therecovery system should be installed.

Avoidance of Waste Treatment Capital Cost

Operating cost is the primary consideration for a new plant or for existing plantswith an inadequate treatment system. In this case the economic evaluationincorporates an added factor; the avoidance of additional capital investmentfor waste treatment capacity.

Improvement of Manufacturing Operations

The implementation of recovery and quality maintenance methods and systemsfor both process water and process baths can help improve the performance ofplating and surface-finishing baths and, in turn, the quality of the finish and theproducts that are produced. Such action will also help to reduce the amount ofrejects and reworking of parts. Both aspects benefit production and quality controland will reduce operating costs and increase the value of fabricated products.

Total Avoidance of Sludge Disposal

For this scenario, justification for investment in recovery is based on the obviousdesirability of eliminating generations of hazardous waste residuals. Stringent economicquantification is difficult in this case because of the uncertainty associatedwith determining long-term liability costs for future landfill disposal; nevertheless,there is powerful emotional appeal attached to the avoidance or minimization oflong-term liability.

Evaluating Strategies

The first of these strategies is clearly the most conservative. It is easily applied andis the strategic analytical technique, which has traditionally been used by manymetal finishers. The rapid escalation of sludge disposal costs makes point sourcerecovery techniques, which were unattractive a few years ago, very enticing now.The second strategy is legitimate but must be analyzed and applied with caution.There is a tendency to assume that recovery can be a complete substitute fortreatment. Careful consideration must be given to potential downtime of recoveryequipment; the generation of excess waste if the units are overloaded; the treatmentof side streams such as regenerate waste or blowdown from the recoveryprocess; accidents such as tank overflow, heat exchanger failure, spills or drips ofchemicals, etc., plus unanticipated sources of regulated pollutants.An example of the last-mentioned caution would be the presence of zinc ioncontamination in the drag-out from alkaline cleaners, acid dips, and chromatedips in a zinc plating line. Too often attention is focused on recovery of the dragoutfrom the main plating tank, with no recognition that effluent quality maybe unsatisfactory simply as a result of minor contributions from various othersources. When considering this strategy, the absolute minimum provision forunrecovered waste should be the determination of the minimum holding andtreatment capacity needed to cope with the volume of unanticipated accidentsor upsets.The third strategy is the most efficient and productive way of converting wastetreatment capital into waste minimization and production control efforts. Manyexamples today prove that the incorporation of pollution control and maintenanceequipment into plating operations helps to significantly reduce batchdumps of process baths. Controlled bath maintenance limits bath impuritiesthat cause plating quality problems and thus improve fabrication while reducingmanufacturing cost. In many cases, short duration ROI objectives can be realized.The fourth strategy is the most risky and the most difficult to support by facts.It is a rare situation where the generation of sludge can be completely eliminated,even in a theoretical sense, especially if such unanticipated occurrences as justdiscussed are considered.In summary an investment in recovery technology and equipment should besupported by a hard, quantifiable economic analysis and supported by adequateoperator and maintenance training. There is constant activity in the marketplacewith new developments and promising breakthroughs in technology. Marketingclaims can often make the situation bewildering, but it is appropriate to bearin mind that the laws of chemistry, physics, and economics will prevail. Thefundamental law of ecology teaches that there is no free lunch.Mother Nature is a tough task mistress. She has made it much easier and lesscostly to mix things together than to take them apart.

SOURCES OF WASTE

There are three categories of waste that must be considered when formulatinga waste minimization program.

Bath Drag-Out to Rinses

This is the carryover of concentrated process baths on the workpieces, whichis removed by stagnant and flowing water rinses.

Bath Dumps

Most of the process baths used in metal finishing are expendable and must beperiodically discarded when their chemical activity is below a level acceptablefor production purposes.

Floor Spills

This is a catch-all category including both accidental and purposeful incidentalwaste sources such as tank overflows, drips from workpieces, leaking tanks orpipes, spills of chemicals, salt encrustations, equipment and floor wash-downwater, oil drips, or spills from gear boxes, etc.Historically, most of the emphasis on recovery technologies has focused onrinsewater since it constitutes the majority of the flow leaving an operation andnecessitates expensive waste treatment. Bath dumps are usually infrequent andare low in volume. Often, dumped baths can be hauled to a distant location by awaste service provider for final treatment and disposal. A subsequent section ofthis article will discuss the possibility of regeneration for certain of these baths toeliminate the need for periodic dumping.Floor spills are nearly impossible to manage by the application of recoverytechnologies due to their unpredictable and intermittent nature and to the factthat they are so heterogeneous in composition. The primary attack on floor spillsis tight operating and process control, adequate operator and safety training, programsto eliminate accidents, and, of course, good housekeeping.The following sections will deal with the techniques applied to rinsewater. Thesecan be divided into those that return a concentrated solution back to the originatingprocess and those that aim to recover metals or chemicals for use elsewhere.

CONCENTRATE RECOVERY METHODS

There are a number of important factors that should be considered in regard toreturning concentrate to the originating process. First, the majority of metalfinishingprocess baths is ultimately expendable. They have a finite life and areperiodically discarded. Recycling of drag-out simply accelerates this processand will give no net gain unless some regeneration scheme is employed on theprocess bath itself. Thus, recovery of drag-out is most often considered only forthe baths that operate in a reasonably balanced condition, primarily the processbaths. A general recovery schematic for return methods is pictured in Figure 1. In the case of those electroplating baths wherereturn of drag-out seems practical,two factors should be examined:

1. In most cases there is a tendency for harmful impurities to accumulate overtime from drag-out return. These impurities can be metals or other cationsor anions dragged into the bath. Or, they can be electrolytic breakdownproducts normally generated during bath operation. Examples of the latterwould be the formation of carbonate through anodic oxidation of cyanide orthe generation of undesirable organic breakdown products formed throughthe electrolytic breakdown of brighteners, wetting agents, grain refiners, etc.

2. In baths that use soluble anodes, the primary metal generally has a tendencyto “grow” or to accumulate in the bath. This generally occurs because the electrochemicalefficiency for anodic dissolution is higher than is the efficiencyof cathodic deposition and/or because the bath itself has a solubilizing effecton the anodes during periods of inactivity.In many cases both of these effects are fortunately minimized or controlledby the routine loss of bath through drag-out, filtration, purification, and by theremoval of suspended solids and sludge. In some baths, however, such as brightnickel, the accumulation of impurities can be a problem in spite of the normallosses from maintenance and purification procedures.When a high percentage of drag-out is returned by any of the technologies thatwill be reviewed, it may mean that the accumulation of cationic contaminants willbecome evident more quickly or more frequently, requiring a purposeful bleedoffof plating bath that is obviously somewhat counterproductive. In regard toimpurity accumulation, complete return of drag-out necessitates purification/maintenance operations or may increase the frequency of those already practiced.Since virtually every such operation creates loss of bath this is again an offsettingconsideration to any recovery that is being gained.A proper analysis of the optimum scheme should include all losses from theoperation and the impact the recovery of drag-out will have on other sources of loss.

Evaporation

Evaporation is the oldest and most broadly applied of the separation technologiesand has an extensive operating history. In the surface-finishing industry,evaporative recovery is classified as a concentrate and return technology and itstrack record and benefits are well demonstrated.Evaporation is routinely used for point source separation and recovery of platingbaths and their associated rinsewaters for recycle to the finishing system.Evaporation is also being used successfully to minimize liquid discharges frommanufacturing plants by concentrating certain pretreated wastewaters, or brines,for haul-away and disposal while recovering additional process water for recycleto the process.Compared to other separation methods, evaporation is more energy intensive;however, it is the only recovery technology that can treat plating rinsewaters toseparate the solvent (water) from the dissolved chemicals and concentrate theremaining solution back to, or even beyond, bath strength. To minimize energyconsumption recovery rinsewater volume can be minimized by the applicationof counter-current rinse hydraulics.On the positive side, evaporation is a straightforward, rugged, reliable, broadlyapplicable, and widely practiced recovery technique. Materials of constructionEvaporation separates volatile from nonvolatile constituents of a solution bymeans of heat-energy-driven phase change (converting liquid to vapor) resultingin a recovered concentrate. In the case of using a vapor condensation technique,atmospheric and vacuum evaporation generate a distillate that can be recoveredin most cases as process water. Compared to other separation and recovery techniquesevaporation can easily concentrate back to, and in some cases well beyond,bath concentration.Heat energy is required to evaporate water from an aqueous solution. Theamount of energy required is roughly 1,000 Btu/lb mass of water evaporated,regardless of whether the evaporation is conducted at atmospheric pressure orunder vacuum. There is no exception to this rule! It can be called the rule of 1,000.To evaporate a pound of water, this quantity of heat energy must be supplied fromsome energy source. With the possible exception of an unlimited supply of hot,dry desert air, or of waste process heat that could be captured for use, vaporizationenergy is rarely “free.”Atmospheric evaporators are essentially simple scrubbing devices that use an airstream to strip water as vapor from a liquid solution. In essence, an atmosphericevaporator is an air stream humidifier. They have been widely used by industrybecause of their low cost and operating simplicity. Atmospheric units are generallyapplied singly (Fig. 2) or in multiples to dewater various plating rinse watersto recover bath concentrate.Atmospheric evaporators operate by either pushing or pulling an air streamthrough a mesh bed or grid-work over which rinsewater, or in some cases, the bathitself, is circulated. Either the air stream or the bath, or both, must be heated toprovide the necessary 1,000 Btu of heat energy needed to evaporate each pound ofwater. Heat must be supplied from somewhere or the unit won’t function.The amount of water removed with each pass is a function of the mass, temperature,and humidity of the air stream, and of the temperature of the liquidbeing circulated through the unit. Heat energy is usually supplied by an externalheat exchanger. If a normally hot plating bath is being circulated through theevaporator, the total heat energy required may be provided entirely by the bathitself, which, of course, will have to be reheated.The amount of water an air stream can remove from an aqueous solution isa function of a number of factors including the relative humidity of the air atthe process environment; the temperature of both the air stream and the liquidsolution; the relative mass velocities of both streams through the evaporator; thedegree of effective contact between both streams; and the concentration of theliquid solution being evaporated. The necessary 1,000 Btu/lb of water vaporizedstill must be provided.In most atmospheric evaporator designs, the vaporized rinsewater is not captured.Instead, the humid air stream is vented to atmosphere. To avoid possiblecarryout and discharge of hazardous substances, the air stream may require additionalscrubbing through a neutralizing or water-irrigated vent scrubber beforefinal discharge.One recent atmospheric evaporator design has added a condenser and closedthe air circuit to eliminate or minimize potential exhaust emissions. A much largercondenser is required to condense water vapor from a stream of air than wouldbe required if air was not present. The presence of an inert gas, such as air, in theexhaust vapor stream reduces normal condensing coefficients by 90% or more.An interesting application, which is well suited to atmospheric evaporation involves the recovery and simultaneous cooling of hard chrome baths thatoften require external cooling to remove excess heat created by high operatingamperage during plating.In such circumstances, both rinsewater and bath may be blended for dewateringby the evaporator. In cases where the quantity of heat generated by theelectric power demand of the bath is not adequate for the evaporation duty, theaddition of external trim heat may be required.Atmospheric evaporators are not considered to be energy efficient. At minimum,several pumps are required to introduce feed, to circulate the solution tobe concentrated and, depending onsystem hydraulics, to remove concentrate.There are inherent inefficiencies in moving and heating large volumes of air.Spray temperatures must be high. Solution boiling points are higher at atmosphericpressure than under vacuum operation, which results in a lower effectivetemperature differential or thermal driving force.Despite the simplicity of design and lower initial capital cost, these factorsconspire toward higher energy consumption, by an estimated factor of at least10% beyond the theoretical requirement per pound of water evaporated whencompared to single-stage vacuum evaporation. Vacuum evaporators have beenused successfully for more than 30 years by the surface-finishing industry forpoint source recovery of plating baths and rinsewaters. They are somewhatmore complex and require a higher initial capital investment than single- stage,noncondensing atmospheric units.Vacuum evaporators are instrumented for push-button, fail-safe operationand provide close and consistent control of the recovered bath concentration.There are three main categories of vacuum evaporator used in the surfacefinishingindustry to recover dragged out plating bath and rinsewater: (1) single-effect (single-stage) designs, which are usually the most simple and easy to operate(Fig. 3); (2) multiple-effect (multistage) designs, which are more complex butare more energy efficient; and (3) some special designs for such applications asbrine concentration. All vacuum designs are devices for distilling a liquid phaseat reduced temperatures in the absence of air and for producing a concentrate.Water distillate is also recovered as a by-product.Vacuum evaporators, as employed by the plating industry for bath and rinsewaterrecovery, are usually the more simple, less complex, single-stage designsconsisting of a heated boiler section, a vapor/liquid separator section, a watervapor condenser, a vacuum circuit, and a control system. The boiler and condensersections may be arranged horizontally or vertically. The most commonheating source is clean, low-pressure, saturated steam, which is ideal because itis a demand energy source and requires a minimum of control. When the supplypressure is regulated, the steam temperature is automatically established anddoes not require further control. Units are available to accommodate hot waterand electrically driven heat pumps.Some of the benefits of operating under vacuumare that it reduces the boilingtemperature of the bath being concentrated, which lessens or eliminatesthe potential for thermal damage to heat-sensitive constituents or additives;increases the temperature differential (the thermal driving force) between theheat source and the liquid being concentrated resulting in smaller, more efficientand less costly boiler and condenser designs; extracts resident air from the systemupon startup and eliminates any possibility of carry-over of hazardous chemicalsto a vent stream; excludes air from the system, which eliminates the potentialfor air oxidation of recovered chemicals or bath; recovers high-quality waterdistillate for return to the plating line; desensitizes the system to fluctuations infeed concentration when operated in a concentrate recycle mode; eliminates thepotential for hazardous air emissions; lessens the tendency for scale to form onheating or other surfaces by operating at reduced temperatures; provides bettermanagement of foam; reduces the number of pumps required to one, the vacuumpump or eductor circulating pump, whichever is used; and provides tightprocess control by recovering bath at an adjustable and repeatable concentration.The operating vacuum selected or recommended by the evaporator supplieris generally a function of the chemistry of the particular bath being recovered.Baths containing heat-sensitive constituents, such as expensive organic brightenersor additives, are usually concentrated under higher vacuum and lower boilingtemperatures than are baths that do not require such constituents.High vacuum operation requires physically larger evaporators to accommodatethe higher specific vapor volumes encountered under those conditionsand to maintain vaporvelocities and system pressure drop within design ranges.The level of vacuum, and thus the boiling point, can be varied within a specificrange of vacuum for any given evaporator capacity. But, if an evaporatordesigned for optimum performance at 11 in. of mercury vacuum is operatedbelow its design vacuum, say at 26 in. of mercury vacuum, vapor velocities willincrease substantially and both the output capacity and product quality willdeteriorate.To satisfy the range of vacuum required by the widely differing bath chemistriesused in the surface-finishing industry, suppliers of vacuum units havedeveloped a series of standard, off-the-shelf, corrosion-resistant evaporatordesigns to accommodate most bath chemistries and operating requirements.The energy demand of a single-stage vacuum evaporator is roughly 1,000Btu/lb water evaporated, or roughly 9,000 Btu/gal of water evaporated (allowingfor losses), the same as the theoretical energy requirement for atmosphericoperation.Because a high percentage of drag-out is usually returned with either atmosphericor vacuum evaporation, impurity removal and management may berequired. Such purification techniques are well established. In the case of chromebaths, and thanks to the fact that chromium is present as an anionic complex,cation exchange or electropurification systems can be easily applied in a separatehydraulic loop around the rinse system to remove and control any cationicimpurities that may accumulate. For chromium etch systems, electrolytic reoxidationof trivalent chromium or electropurification, should be considered. Inthis application, electropurification will produce less discharge than would acation exchanger by its associated reagent waste stream.Contaminant removal or purification techniques normally used with otherbaths, such as carbon filtration or dummying for nickel baths, membrane electrolysisfor metal impurity control, or carbonate removal from cyanide baths,can continue to be applied to the process baths as required.Vacuum evaporation has been successfully and dependably used for manyyears to recover a wide variety of plating baths including such difficult chemistriesas encountered in chromic acid plating and chromic/sulfuric acid etchbaths. Associated rinsewaters are also recovered for reuse in the plating process.An application for vacuum evaporation of some increasing interest is brineconcentration. In some localities, the discharge of pretreated metal-finishingeffluent is being restricted because the effluent still has a high salt concentration.Salt is the unfortunate and unavoidable byproduct of chemical treatmentof metal-bearing wastewater.Usually, pretreated wastewater effluent is further processed by membranesystems to further separate and consolidate the mixed salt solution. The rejectfrom this step can then be processed by any of several types of vacuum evaporatorto concentrate the brine either to a level slightly below the limit of solubilityof the salt mixture or slightly beyond to produce a concentrate discharge fromwhich the salt slurry can settle and be discharged. The supernatant liquor canbe returned to the feed circuit where it will mix with the incoming feed forreprocessing through the evaporator.

Reverse Osmosis

After evaporation, reverse osmosis (RO) has the longest operating history. Mostcommercial recovery installations have been on nickel plating operations.On the positive side RO is a relatively mature technology and uses considerablyless energy than evaporation for the same rinsewater feed rate. A typical recoveryscheme is given in Figure 4.On the negative side, the degree of concentration of the separated bath by RO islimited. If maintaining appropriate permeate quality [10-100 ppm total dissolvedsolids (TDS)], the practical maximum concentration of the reject (or concentrate)is 10,000 ppm (1.4 oz/gal) TDS. If permeate quality is not an issue, then 50,000 to80,000 ppm (6.7-10.7 oz/gal) TDS reject concentration can be achieved. In manycases, if the recovered solution is returned directly to the plating bath, there maynot be sufficient natural water evaporation from the bath to accommodate thevolume of recovered RO concentrate. Similar to evaporation, RO returns essentiallyall of the undesirable impurities.RO has gained favor in recent years as a pretreatment for incoming processwater, which has high TDS, and in some cases, for clean up of contaminatedprocess water for recycle to the process.RO is a pressure-driven membrane process. The driving force of this process,the hydrostatic pressure gradient, is the difference in hydrostatic pressure betweentwo liquid phases separated by a membrane.In reverse osmosis, particulates, macromolecules, and low molecular masscompounds, such as salts and sugars, are separated from a solvent, usually water.This is accomplished by applying a hydrostatic pressure greater than the osmoticpressure of the feed solution. The osmotic pressure of a particular feed solutionvaries directly with the concentration of the solution. In typical applications feedsolution have a significant osmotic pressure, which must be overcome by thehydrostatic pressure applied as the driving force. This pressure requirement limitsthe practical application of this technology.The transmembrane flux (permeate flow) is a function of hydrodynamic permeabilityand the net pressure difference—the hydrostatic pressure difference betweenfeed and filtrate solutions minus the difference in osmotic pressure between thesesolutions. The osmotic pressure of a solution containing low molecular masssolutes can be rather high, even at relatively low solution concentrations.In practice, it is practical to use RO to separate water (solvent) from all othersubstances of a solution in order to concentrate the solution and/or to generate orrecover clean water for process reuse. The applied pressure is generally between 200and 700 psig. In some cases, such as advanced reverse osmosis and high-pressureapplications, the pressure may be as high as 1,000 to 2,000 psig.Depending on both the characteristics of the dissolved constituents and onthe practical operation of the equipment, the dissolved constituents are rejecteddifferently. This phenomenon is called the membrane rejection rate. The fractionof nonrejected substances is called leakage. The leakage of the various salts isdependent on the following parameters: size of dissolved molecules, ion radiuselectrical load of the ions, and interacting forces between ions and solvents.The rejection of organic substances is mainly dependent on the molecularweight and size of the molecules.RO has seen limited application to nickel rinsewater. RO can separate andreturn clean nickel bath, but usually at too low a concentration for total returnto the process bath. Also, with RO, boric acid is partially transported across themembrane requiring monitoring and make-up as required.Membrane performance decreases with operating time resulting in adecreased permeate flow rate (flux), which can be reasonably restored by periodiccleaning of the membrane. Over time, the membranes will likely require replacementdue to damage from (1) hard water constituents; (2) fouling by organics;(3) general deterioration by acids or alkalis; (4) normal membrane compactionwith use; and (5) destruction by oxidizing chemicals such as peroxides, hypochlorite,or chromic acid.

Electrodialysis

Electrodialysis (ED) uses a “stack” of closely spaced ion exchange membranesthrough which ionic components of a solution are selectively transported. Thedriving force is a rectifier-generated voltage imposed on electrodes at the twoends of the stack. Ionic components are pulled out of a relatively dilute rinsestream (the first flowing rinse station) and accumulated in a highly concentratedstream, which can be either returned to the process, as shown in Figure 5, orotherwise recovered.The advantages of ED include low energy consumption, the ability to producea highly concentrated stream for recovery, and the fact that only ionic materialsarerecovered, so that many undesirable impurities are retarded and rejected. Onthe negative side, ED is a membrane process,which requires clean feed, carefuloperation, and periodic maintenance to avoid damage to the stack, which isusually reconditioned by the manufacturer when required. ED units can besuccessfully used to recover gold, silver, nickel, and tin electrolytes as well asselected acids and rinsewater.An interesting feature of this technology is that a bright nickel electroplatingbath can be circulated at a slow rate through the unit, thus providing a continuousremoval of organic impurities, essentially eliminating the need for batchpurification with its associated major losses of nickel metal.

Membrane Electrolysis

Membrane electrolysis (ME) is a membrane process driven by an electrolyticpotential. It is mainly used to remove metallic impurities from plating, anodizing,etching, stripping, and other metal-finishing process solutions. Thistechnology utilizes a diaphragm or an ion exchange membrane and an electricalpotential applied across the diaphragm or membrane. Compared to electrodialysis,most membrane electrolysis systems utilize only a single membrane ordiaphragm positioned between two electrodes.The use of ion exchange membranes is advantageous because higher iontransfer rates can be achieved in comparison to inorganic- or organic-based diaphragms.Ion exchange membranes are ion permeable and selective, permittingions of a given electrical charge to pass through. Cation exchange membranesallow only cations, such as copper or aluminum, to pass through. Similarly,anion exchange membranes allow only anions, such as sulfates or chlorides, topass through.The efficiency of ME depends on the migration rate of ions through theion exchange membranes. The energy required is the sum of two terms: (1) theelectrical energy required to transfer the ionic components from one solutionthrough the membrane into another solution, and (2) the energy required topump the solutions through the unit.Electrochemical reactions at the electrodes are other energy-consuming processes,but the energy consumed for electrode reactions is generally less than1.0% of the total energy used for ion transfer.The total electrical potential drop across an ME cell includes the concentrationpolarization and the electrical potential required to overcome the electricalresistance of the cell itself. This resistance is caused by the friction between ions,membranes, and water during transfer from one solution to another, all of whichresults in an irreversible energy dissipation in the form of heat. Because of theheat generated, the total energy required in practice is significantly higher thanthe theoretical minimum energy required.The energy necessary to remove metals from a solution is directly proportionalto the total current flowing through the cell and the voltage drop betweenthe two electrodes. The electric current required to remove metals from a solutionis directly proportional to the number of ions transferred through the ionexchange membrane from the anolyte to the catholyte. The electrical energyrequired in ME is directly proportional to the quantity of metal (cations) thatmust be removed from a certain volume of anolyte to achieve the desired productquality.Energy consumption is also a function of the electrical resistance of a cellpair. The electrical resistance of a cell pair is a function of the individual resistancesof the membrane and the solution in the cell. Furthermore, because theresistance of the solution is directly proportional to its ionic concentration, theoverall resistance of a cell is usually determined by the resistance of the weakerelectrolyte. Figure 6 is a schematic of the ME cell.ME can be utilized to remove metal impurities from process baths, such asetch and stripping baths, as well as conversion coating, chemical milling, andsealing solutions. An effective membrane surface area between anolyte or processsolution and catholyte of 0.07 m2 or 0.75 ft2 allows a maximum amperage of 60to 100 A for process solution purification. This membrane electrolysis processdoes not only remove metals from process solutions but also helps to maintainthese solutions at certain activity levels.When applied for the purification of a very corrosive solution that can dissolvemetal electrodes, a three-compartment ME system must be used. A centercompartment is utilized for the corrosive process solution and the adjacentcompartments, which are separated by ion exchange membranes from the centercompartment, operate as catholyte and anolyte compartments. During operation,anolyte/catholyte-maintenance solutions are recirculated through theircorresponding cells and storage tanks. The purified process solution is pumpedvia a designated pump from the process tank back into the process bath.Depending on the chemistry and the specific application, ME systems aredesigned either with cation or with anion exchange membranes. Typicalapplicationsfor the ME technology in surface-finishing operations include regenerationof etching and stripping solutions; purification and regeneration of chromiumplating baths; recycling and maintenance of chrome conversion coating solutions;and reactivation and metal removal from deoxidizing solutions.Benefits of the ME technology are consistent performance and quality of etchingagents and acids; constant production speed; accurate high-quality etchingand chrome conversion coating results; reduced reject rate (no costly refinishing)reduced manpower requirement because of process automation; and reducedwastewater treatment and waste disposal result in lower operating cost.

Diffusion Dialysis

Diffusion Dialysis (DD) is also a membrane technology for separating andrecovering clean acid from used or spent acid solutions. Compared to electrodialysisor ME, DD does not require an electrical potential across the membraneto effect separation. A flow schematic of a typical DD system is illustrated inFigure 7.The separation mechanism utilizes the concentration gradient between twoliquids—deionized (DI) water and the used process acid—separated by a specificanion exchange membrane, which allows natural diffusion ofhighly dissociatedacid (anions) through the polymeric membrane structure while cations (metals)are rejected because of their positive electrical charge. The mechanism of freeacid diffusion through the membrane, due to the concentration differencebetween the free acid and DI water, is known as Donnan diffusion.Multiple layers of membrane are arranged in a filter-press-like stack throughwhich both DI water and spent acid flow by gravity. Clean acid is separatedfrom the feed stream by the concentration-driven transport mechanism acrossthe membrane stack to effect a partition and recovery of an acid stream (diffusate)in conjunction with the generation and discharge of a waste stream(dialysate).DD is being utilized for the following applications: recycle of hydrofluoric/nitric acids for etching stainless steel; recovery of sulfuric/nitric and sulfuric/hydrochloric acids for etching nonferrous metal; reclamation of sulfuric andhydrochloric acids for etching of steel-based materials; recuperation of sulfuricacid from anodizing processes; and regeneration of battery acids.On the positive side DD is a low-energy, low-pressure, continuous processthat requires no additional reagent or regeneration chemicals, resulting inlessTDS in the plant discharges.On the negative side, for every volume of acid recovered (diffusate), an equalvolume of acidic waste (dialysate) is generated for further processing for recoveryor for waste treatment. While the recovered, clean acid is generally reusable,the operating principle imposes a limit to the achievable concentration forthe recovered acid, which can be fortified with concentrated acid as required.Typical maintenance procedures for DD systems include: filtration of thefeed stream to remove total suspended solids and to avoid deposition of suspendedsolids on the membranes; temperature regulation of the feed liquor andDI water supply within a prescribed temperature range to maintain recoveryefficiency; and protection of the membranes against exposure to oxidizingagents such as chromic and nitric acids and to organic solvents, lubricants,inhibitors and surfactants.With efficient feed filtration, membrane cleaning is generally requiredapproximately twice per year. With observance of the above operating andmaintenance practices, experience indicates membrane life can be about 5 years.

Ion Exchange

Ion exchange is a chemically driven separation process. It is an ideal and usefulseparation method for collecting low concentrations of ionic materials, such asmetal salts, from dilute rinsewater. This characteristic differentiates it from allof the previously discussed methods where relatively low flow rates and highconcentrations of recoverable materials must be maintained.From a recovery standpoint, ion exchange is not capable of producing a“highly” concentrated stream for recycle (20-25 g/L is a practical limit). It isalso difficult to optimize the split between recovered metal salts and excessregenerant acid, which is intolerable in the plating bath. Also noteworthy isthe fact that a waste stream containing excess regenerant must be dealt with,as shown in Figure 8.

NONRECOVERY METHODS

Nonrecovery or indirect recovery methods do not return concentrate to the originatingprocess; thus, they obviate any concern over accumulation of impuritiesor the primary metal in the bath. The result is a “decoupling” of the recoveryprocess from the basic manufacturing operation, which may be a considerablebenefit if downtime or process upsets cannot be tolerated. A general schematicis given in Figure 9.In certain instances, these nonreturn processes may also allow recovery fromprocess bath losses other than drag-out (i.e., purification losses or plating bathdesludging waste). This is in sharp contrast to the previous category of recoverymethods, which can actually increase losses to purification or sludge removaloperations by increasing the frequency with which they must be performed.

Electrolytic Metal Recovery

In the metal-finishing industry electrolytic metal recovery (EMR) is both a usefuland a familiar electrochemical process technique that applies special electroplatingequipment to reduce the concentration of dissolved metals in many typesof process solutions such as plating rinse water and dumped baths. Removingmetal in solid form avoids the need to treat and convert the metal content ofsuch process solutions to sludge. In the mining industry, EMR is referred to aselectrowinning.Recent advances in EMR cell design now make it possible to reduce the metalconcentration of spent electroless baths and rinsewater prior to waste treatmentand to recover metal from chloride or ammoniacal etch solutions while concurrentlyregenerating the etch baths.There are three common embodiments of EMR in commercial use in theplating industry:

1. “Extractive” methods, which aim primarily to remove the metal fromthe recovery rinse but with little regard to byproduct value, are depictedin Figure 10. One of these deposits the metal on a sacrificial plasticstarter cathode. The cost of the starter cathode and the undesirabilityof introducing plastic to a smelter or secondary recovery operation area significant offset to any resale value of the metal.Another type of extractive cell produces a spongy or powdery deposit,which is removed as a sludgelike material (usually from thebottom ofthe recovery cell) and is usually of little or no value. The high surfacearea of the powder exposes a significant portion of the metal to oxida-tion. The powder also entrains mother liquor, which is virtually impossibleto rinse out completely. This results in an acidic, wet powder, oftencontaminated with halite ions, which in turn render the recovered metalpowder difficult or impossible to reuse or sell.

2. High-surface-area recovery cells deposit the metal on some type offibrous or filamentous substrate. In some cases, the plated metal is discardedor sold as a low- volume residue, while in others, the depositedmetal is stripped chemically or electrochemically so that the end resultis a concentrated solution of the metal that was recovered.

3. True EMR or electrowinning approaches recover a solid slab or sheetof relatively high-purity metal, that can be easily handled, weighed,assayed, or transported and sold for the best available price in thesecondary metal markets. In certain recovery applications or circumstances,the electrodeposited metal is pure enough to be reused as anodematerial in the originating plating process. This type of cell usuallyapplies some type of moving or rotating cathode, or alternatively, a highsolution velocity over fixed cathodes.To reduce the effect of electrode polarization common to low metal ionconcentrations and to increase ion diffusion rates at the electrodes, it is recommendedthe solution be heated. Otherwise, plate-out of metal from these lowconcentration solutions will be hindered. Strong air agitation is anothermethodfor providing adequate mechanical mixing, but it removes heat from the system,thus reducing operating rates. Air agitation may also add to the load on air pollutioncontrol equipment.

Ion Exchange

In addition to the use discussed earlier under concentrate recovery methods, ionexchange can be used for several other applications, which include recuperation ofnoble metals, recovery of metals from rinsewater in combination with electrolyticmetal recovery, and the purification of some process solutions such as chromatebaths.In gold recovery, ion exchange is effective in collecting essentially at tracesfrom a dilute rinse stream. Historically, such gold-laden ion exchange resinswere burned by a gold refiner who recovered the ash. Currently some companiesare offering a tolling service to regenerate the ion exchange resin chemically andreturn it to the user.In either case the primary disadvantages are the difficulty in assaying a heterogeneousmass of metal-laden ion exchange beads and the high tolling chargesfrom the refiner or processor. Both of these factors preclude recovery of maximumgold value.A second emerging application involves linking two recovery techniques; ionexchange and EMR. In this scheme, as shown in Figure 11, the ion exchange bedis used to collect metal ions from dilute rinsewater and the acid formed in theelectrowinning operation serves to regenerate the ion exchange resin.

SLUDGES AS BYPRODUCTS

There has been a steady increase in the number of companies interested in usingmetal- bearing waste treatment sludge as a feedstock in their manufacturing processes;nevertheless, most mixed sludge has no value. In fact, the generator oftenhas to pay freight costs plus a fee to the processor for removal and treatment. Atypical example would be a sludge containing 5 to 10% copper or nickel, whichcan be used as a feedstock for a pyrometallurgical operation (a smelter). Suchmetal-finishing sludge is a richer source of feedstock than the typical ore minedfrom the ground.On the other hand metal-finishing sludge is typically highly variable in compositionand can contain a significant amount of inorganic salt in the entrainedwater. Halides can be particularly troublesome in a smelting operation. From thestandpoint of long-term liability, the metal finisher needs to consider that 90 to95% of such sludge will not be turned into product at the smelter but will windup in the smelter’s residues. Although such recycling may appear advantageousunder today’s regulations, the long-term environmental significance of smelterresidue needs to be factored into the decision.A more promising situation exists if a metal finisher generates a segregatedsludge that consists essentially of a single metal. Single metal sludges containingonly tin, nickel, cadmium, copper, or zinc have excellent potential for being usedas feedstock for reclaiming operations, which can operate in an environmentally“clean” manner, producing little or no residue. Furthermore, the metal contentof such segregated sludge may be a candidate for in-house recovery by the metalfinisher by redissolving the sludge and applying EMR. Segregated sludge is thenatural by-product of the closed-loop or integrated rinse treatment method,which has been successfully practiced for decades in both the U.S. and Europe.

REGENERATION OF BATHS

Historically, most of the effort on recovery was focused on drag-out; however,most of the chemical load from a metal-finishing operation will usually befound in the dumps of expendable process baths and the losses from purificationof plating solutions or sludge removal of the process tank. Operations,such as cleaning, pickling, bright dipping, etching, and chemical milling, areworth being investigated for recovery potential. Some of these applications arediscussed in the following.

Copper and Its Alloys

EMR as described earlier is highly effective on many copper pickling and millingsolutions including sulfuric acid, cupric chloride, and ammonium chloridesolutions. Solutions based on hydrogen peroxide are generally best regeneratedby crystallization and removal of copper sulfate with the crystals being sold asa byproduct or redissolved for EMR.Bright dipping in highly concentrated nitric/sulfuric acid is a difficult challengefor regeneration because the solution volumes involved are usually quitesmall (5-25 gal) and the drag-out losses are very high. Regeneration is theoreticallypossible by distillation of the nitric acid and removal of copper sulfatebut the economics are not likely to be attractive for most metal finishers. Thisapproach does have potential for larger plating plants or for large-scale, centralizedrecovery facilities, which serve a number of plants.

Aluminum and Its Alloys

The caustic etch used in many aluminum finishing lines and the chemical millingsolution used for aircraft components can be regenerated by crystallizationand removal of aluminum trihydrate; however, the process must be carefullycontrolled and maintained. The economics currently favor only relatively largeinstallations but development of lower cost approaches is likely.Sulfuric acid anodize solution and phosphoric acid bright dip bath can bothbe regenerated using DD or acid retardation, which is a sorption process usingion exchange resins. The cost and complexity of such recovery operations requireeconomic evaluation on a case by case basis.Chromic acid anodizing solutions can be regenerated by the use of cationexchange or ME. Both technologies can be used to remove the accumulatingaluminum together with other metal impurities such as copper and zinc. The lifeexpectancy of the resin is shorter than on normal waste treatment applications,but the method is still practical and economical. The use of ME has shown effectivepurification and maintenance capabilities of these baths.

Iron and Steel

Pickling is commonly used in steel mills for the surface finishing of steel productsor as a pretreatment operation for a galvanizing process. Large volumes of spentacid containing metal contaminants are generated. Among the various methodsavailable for acid purification and recovery, DD is very useful for the recovery offree acid from spent pickling baths.Both sulfuric and hydrochloric acids are commonly used for cleaning steel.Sulfuric acid can be regenerated by crystallization of ferrous sulfate. Hydrochloricacid can be recovered by distilling off the acid and leaving behind the iron oxide.These technologies have been used for many years in large installations and bytolling reclaimers but are not likely to ever be economical for small metal-finishingor galvanizing plants where the production cannot justify the capital investment.

Plastic Etching

Concentrated chromic acid solutions are used to etch plastic surfaces prior toplating. These operations consume very high quantities of chemicals and generatelarge quantities of sludge. Standard practice today is to reclaim essentially allof the chromium from such anoperation through a combination of evaporationand electrochemical oxidation of the trivalent chromium.Today, a combination of evaporation and ME can be used to extend the operatingtime of a chromic acid etch indefinitely.

Alkaline Cleaners

Alkaline cleaners are probably the most widely used process baths in all of metalfinishing. Treatment significance will increase as water recycling becomes a moreprevalent practice. Most cleaner formulations are antagonistic to good treatmentof a metal-finishing effluent because they are chemically formulated to keep dirtand oil in suspension. If their concentration is high enough in an effluent thissame effect prevents efficient removal of the precipitated metals.Dumps of alkaline cleaners, passing through a treatment system, are a notorioussource of upsets and a high contributor to the TDS in a metal-finishingeffluent. In addition there are certain cases where large finishing operations onsmall sewer systems, or small receiving streams, may have a problem meetingrequirements for the organic content due to wetting agents and detergents.The cleaning of parts in surface-finishing operations generates a lot ofimpurities in the cleaner bath. These impurities, such as oils, dirt, and soil,wear out the cleaner baths and have to be removed to extend the life of thecleaner. Free or tramp oil is usually removed with a skimmer. Emulsified oilwill usually build up in the bath, with some of it splitting into a floating layerwhere it will be removed by the skimmer. Most of the aqueous and semiaqueousbath formulations contain an inhibitor to provide rust protection for steelparts. Surfactants displace oil from the parts to be cleaned and form a stableemulsion. The life of the bath is dependent upon how much soil is brought inwith the parts and how much drag-out occurs as the parts are moved from thecleaning bath into the rinse tank.For many installations in surface-finishing operations continuous micro- andultrafiltration systems using inorganic or organic membranes are successfullyused to remove oils, grease, lubricants, soils, and solids from alkaline cleanersand can give the bath essentially indefinite life. An additional benefit is thesteady-state condition of the cleaner, which will improve control over the processand the quality of the product being manufactured. The selection of the membranesis not only important regarding the operating temperature of the bathbut also for the pore size or macromolecular structure. Elevated temperature candeteriorate organic-based membranes and too small a pore size can cause therejection of valuable chemicals such as surfactants or inhibitors.

Phosphating Baths

Precipitates are formed continuously in phosphating operations presenting maintenanceheadaches and often resulting in the solution being discarded. Usually,the precipitates accumulate in the process tank, primarily on the heating coils.When the solution is removed from the tank this accumulation of sludgecan be manually removed. The solution should be decanted back into the tankto minimize wastage but this consumes space and time so the solution is oftendiscarded and replaced.It is far more efficient to install a continuous recirculation system through aclarifier with gentle agitation in the sludge blanket zone. This allows the solutionto be used indefinitely, reduces the labor for manual clean-out of sludge, andallows a dewatered sludge to be easily removed from the bottom of the clarifier.

Chromating Solutions

Both ion exchange and electrochemical methods have been demonstrated to beeffective for regeneration of spent chromates; however, in almost all cases, themetal finisher relies upon the proprietary chemical supplier to be responsiblefor the appropriate balance in the chromating bath. Either of these regeneratingtechnologies makes the metal finisher responsible for the overall chemicalmaintenance of all constituents in the bath. It is possible that proprietary supplierswill provide a service to assist the finisher in maintaining a proper balancewhen one of the applicable techniques is applied. Economics are not likely to beattractive except in the case of high production operations using the more concentratedchromates, which give high salt spray resistance against “white rust.”

RECOVERY AND RECYCLING OF PRETREATED WASTEWATER

Conventional techniques for water conservation (countercurrent rinsing, conductivitycontrols, etc.) are used extensively in the industry; however, the unavoidableend product of all waste treatment methodologies is a “salt” containing effluent,or brine. Effluent TDS from such a system can be sufficiently high to limitpotential for recycle and reuse as process water without desalination.Clearly, achieving the minimumconsumption and discharge of water necessitatessegregated handling of concentrated solution dumps since they will carrymore TDS over a given period of time than bath drag-out. In a similar fashion theuse of segregated closed-loop treatment rinses allows the first station of the rinsesystem (drag-out tank) to be as high as 10 to 15% of the TDS of the process bath,greatly extending the opportunity to recycle subsequent higher quality rinses.There is increasing interest in this country to further close the loop by desalinatinga treated effluent for maximum recycle and reuse. A number of large plantshave been constructed with all of the TDS being concentrated into a small volumeof brine, which is hauled from the plant.While this may be necessary and economical in some cases it is not logical formost cases. Unless the plant is located near a seacoast, disposal of the brine islikely to be problematic. It is highly corrosive to concrete and steel structures andmore difficult to assimilate in the environment than a high volume effluent at1,000 mg/L TDS. The real answer lies in reducing the consumption of chemicalsin the metal-finishing operation and thus the quantity of TDS requiring discharge.For situations where desalination and recycling of a treated effluent is desirableor necessary the following treatment technologies can be considered.

Ion Exchange

Recycling of metal-finishing wastewater through ion exchange equipment hasbeen practiced for decades in Germany and for many years in Japan. Practicalexperience shows the need for segregated collection and treatment of not onlybatch dumps but also the first rinse after each process that flows at a rate to takeaway approximately 90% of the chemical load.Secondary and/or tertiary rinses can then be recirculated through ion exchangeequipment after very thorough particulate filtration and carbon filtration. Cyanideand hexavalent chromium are problematic because they are poorly released fromthe anion exchange resins and tend to exist as perpetual low-level contaminantsthroughout the plant’s rinsewater system.Aside from high cost, the major drawback of this approach is that it actuallyincreases the TDS discharge from the plant. In theory, if regeneration of ionexchange resins could be perfectly efficient, the process would multiply the TDSremoved from the recirculated water by a factor of two. In practice, however, a 100to 300% excess of regenerant chemical is typically required. This can be reducedto the range of 50 to 100% excess by holding and reusing certain fractions of theregenerant waste stream at the cost of additional capital investment and operatingcomplexity. As a result of this need for excess regenerant, the TDS removed fromthe recirculating rinsewater is multiplied by a factor of three to six.Since it is the TDS that presents the problem for the environment and not thewater, this approach does not hold long-term promise for the metal-finishingindustry. In Germany, the population density has exacerbated the problem withTDS accumulating in the rivers. Practicing water chemists now recognize thecounterproductive nature of this treatment process.

Evaporation/Distillation

Where either waste heat or reliable solar energy are available, vacuum evaporationor multistage vacuum distillation can be an attractive alternative for producingclean water. Capital costs are high but the ability to concentrate the brine is virtuallyunlimited and the equipment is rugged and reliable.

Reverse Osmosis

RO technology has been refined and extensively applied to the desalination of seawater and brackish waters. Metal-finishing wastewater requires a relatively highdegree of pretreatment and filtration to protect RO membranes from fouling.Pretreatment processes can be designed so that soluble compounds, such as metalsilicates and oxides, can be removed as precipitates by a filtration stage to such ahigh degree that membrane fouling can be significantly avoided; however, becauseof the wide variety of chemicals used in metal finishing, the water chemistry canbe complex, highly variable over time, and difficult to accurately predict.The large commercial scale installations have had mixed results. Success onone plant effluent is not assurance that the next will be workable. In addition theconcentration of brine that can be produced is relatively low so that large quantitiesof low-concentration brine require disposal.

Electrodialysis

ED has also found extensive commercial applications for desalination of brackishwater; however, the efficiency of the process falls off unacceptably if the productwater is not in the range of 500 to 600 mg/L TDS or higher. The process canproduce a rather high concentration of brine and the water quality limitationcan be overcome by using RO or ion exchange for high purity applications withinthe plant.Since ED is also a membrane process, similar concerns apply as mentionedfor RO; however, ED is likely to prove somewhat more tolerant of varying waterchemistry. This is due to the ability to frequently reverse the electrical potentialacross the membrane stack, which helps offset the fouling tendency, albeit at asacrifice in capacity.

Zero Liquid Discharge Systems

Some firms, because of their location in small towns with small municipal treatmentplants or because of discharge restrictions or other circumstances, haveimplemented treatment and recovery programs geared to recover all possible processwater for recycle and reuse within the plant. Only solid sludge or brine slurryis produced for haul-away and disposal. These firms come as close as practical tohaving a zero discharge operation.While any of the foregoing methods can be applied individually to conditionraw water, the recovery and conditioning of pretreated effluent requires a multistepprocess. It is not uncommon for a pretreated effluent to still have high TDS,mostly as sodium sulfate or sodium chloride. Some firms have successfully appliedall or some of the following process steps to further process pretreated, high-TDS effluent to recover clean, reusable process water and to achieve zero liquiddischarge: sand filtration, carbon filtration, single- or two-stage RO followed bymixed bed ion exchange (if necessary).The reject from the RO system, which may still represent a considerable volumeof dilute brine, can be further processed by vacuum evaporation to achieve a concentrationclose to the limit of solubility of the brine mixture, which is dischargedfrom the evaporator at an elevated temperature. Upon cooling, salt crystals willseparate and settle. The supernatant liquor can be mixed with the RO reject feedstream and circulated back through the evaporator. Meanwhile, the resulting saltslurry can be removed from the settling tank for further dewatering, which is notusually necessary, and readied for haul-away.A process of this nature is probably not economically viable unless the total dailyvolume of process water used in the plant is in the order of 50,000 gpd or more.