A Pollution Prevention Resource Manual

for Metal Finishers

A Competitive Advantage Manual

Offered by:

IAMS Logo

The Institute of Advanced Manufacturing Sciences, Inc.

(513) 948-2000

Funding provided by:

OEEF - Ohio Environmental Education Fund 


Acknowledgements

In preparing this manual intensive use has been made of the published literature, including Product Finishing, Pollution Engineering, The National Environmental Journal, the Electroplating Engineer’s Handbook, the Minnesota Association of Metal Finishers’ Metal Finishing Guide to Pollution Prevention, the U.S. EPA’s Guide to Pollution Prevention for the Metal Finishing Industry, Joseph and Arthur Kushner’s Water and Waste Control for the Plating Shop and many others. The contribution of these publications to this manual is gratefully acknowledged. Special thanks go to the metal finishing facilities where pollution prevention assessments were conducted and to the authors J. Randolph Earle and Roger C. Hoogerheide, graduate students at the Institute of Environmental Sciences at Miami University, Oxford, Ohio.

The information presented herein is believed to be accurate; however, the Institute of Advanced Manufacturing Sciences and the Ohio Environmental Education Fund jointly and severally disclaim any liability for inaccuracies or incompleteness.

The Institute of Advanced Manufacturing Sciences, Inc. (the Institute) is a not-for-profit organization whose mission is to improve the competitiveness of industry through technology transfer, training and applied research. It receives funding from the Ohio Department of Development as an Edison Technology Center, and grants from public and private institutions, and revenue from membership fees, industrial projects, software product sales, and seminars.

Several years ago the Institute created The Center for Applied Environmental Technologies (CAET) to help companies with potential environmental problems. The Center specializes in waste reduction and pollution prevention, both cost effective methods to reduce environmental problems and increase competitiveness. The Center conducts pollution prevention assessments and has co-sponsored successful pollution prevention seminars.

This project was made possible through a grant from the Ohio Environmental Education Fund (OEEF). The OEEF, administered by the Ohio Environmental Protection Agency (OEPA), sponsors environmental education projects throughout the state of Ohio through grants. The Center for Applied Environmental Technologies (CAET) received a one year grant (7/93 - 7/94) from OEEF to provide technical assistance in Industrial Pollution Prevention for the metal finishing industry in Southwest Ohio.

The purpose of this project is to demonstrate environmental and economic benefits of pollution prevention (P2) to metal finishers in the Greater Cincinnati Metropolitan Area by introducing cost effective changes in materials, processes, or management practices.

The project has several objectives:

This project has been conducted in cooperation with the Greater Cincinnati Chamber of Commerce, the City of Cincinnati, the National Association of Metal Finishers, and the American Society of Electroplaters and Surface Finishers. Extensive progress has been made in recent years in improving metal finishing processes as regulations regarding discharges into the environment have become more stringent. This industry needs to continue to meet new standards which further decrease the amount and type of wastes it may discharge. Increased costs of inputs and discharges have cut profit margins and have forced businesses to search for new ways to reduce raw materials and discharges.

In future years there will be continuing pressure to meet more stringent effluent guidelines on top of increased waste disposal costs, while the pressure to minimize manufacturing costs will be relentless. Pollution prevention has emerged as an effective method of attaining compliance and reducing operating costs. Widespread success has been achieved using simple methods and techniques.

Many of the guidelines regarding current pollution prevention technological advances will be described in this manual. Some of the technologies have involved simple installations, such as retrofitted pipes or drainboards, that are cost effective and can be implemented in house. Other innovations require capital expenditures with significant pay back periods. These include metal removal from liquids and total recycling in a closed system. Although neither total recycling nor zero discharge has been mandated and may not be financially viable, planning new equipment choices with these goals in mind should prove to be cost effective in the long run.

The loss of raw materials in the wastewater can result in five distinct cost items that need to be considered in an economic evaluation:

It is difficult to make blanket judgements as to economic feasibility of the solutions presented since the conditions in each facility vary and the characteristics and quantities of the platables are so diverse. Taking advantage of their own expertise and knowledge of local conditions, operators of each facility must determine whether a particular technology can be implemented economically. 

Regulatory Environment

The Federal Water Pollution Control Act of 1972, which established the legal framework for dealing with water pollution on a national scale, called for the establishment of pretreatment programs with standards and effluent guidelines. Standards for the metal finishing industry, first promulgated in the 1970’s, were subject to litigation and revision. Final promulgation took place in the early 1980’s. The pretreatment regulations establish a complex program of standards, monitoring, and enforcement. To achieve compliance metal finishers may incur significant capital costs, ongoing operating costs, administrative costs of obtaining and keeping discharge permits, and the risk of substantial fines and costly citizen suits for incidents of non-compliance. By eliminating the discharge of process wastewaters (i.e., by "closing the loop") the metal finisher can eliminate the administrative costs and risks of non-compliance while substantially enhancing its public image. Attainment of a 100% waste and pollution-free operation may appear remote at this time. However, efforts in that direction can only bring platers closer to an era of greatly reduced operating risk and continuing compliance with federal, state and local regulations at more reasonable costs.


 
 

Waste Reduction Techniques

The remainder of this manual will discuss various pollution prevention technologies. Included at the end of the text is a matrix table of vendors and the services which they provide. This table provides a reference to further investigate feasibility and costs of pollution prevention options. The following sections address reduction in waste generation through source reduction techniques such as improved process control and substitute chemistries. Alternative recovery and treatment measures are also addressed.

Reduce Drag-out

Chemical drag-out is the primary source of contaminated rinsewater, which requires treatment to remove harmful or regulated pollutants before the water is reused or discharged. If the drag-out is reduced, the rinse waters will be less contaminated and will last longer. Reduction of drag-out should minimize the amount of process chemical additions to the bath and the mass of contaminants that need to be removed from the rinsewater. Several process modifications that affect minimizing drag-out are listed below.

Work Piece Withdrawal Rate

The speed with which work pieces are removed from a process solution can have the greatest impact of any single factor on drag-out volume. Work piece withdrawal rate will influence the amount of solution remaining on the work piece. The more slowly a work piece is withdrawn from a process bath, the thinner the chemical film on the work piece and the less chemical contaminants in rinsewaters. 

Drain Time (Hang Time)

A drain time of at least 10 seconds has been demonstrated to reduce drag-out by 40+%, compared to the three second industry average. Longer drain time over the process solution allows more drag-out to be returned to the bath. Drain times can be controlled by posting drain times at tanks as a reminder to employees on manual process lines or by building in delays on automatic process lines.

Implementing Company
ILSCO - Cincinnati, OH

Racking

Some work pieces being plated have re-entrant cavities which trap process water when the work piece is lifted from the plating bath. Racking these work pieces so that the cavities open downward will promote draining, thereby reducing the amount of drag-out.

Implementing Company
Micro Metal Finishing - Cincinnati, OH

Work Piece Modification

Consultation with work piece manufacturers to adjust design to provide better bath liquid release, can reduce drag-out. Example: a plater asked his customer whether he could drill four holes in the work piece to improve drainage. The customer agreed and this pollution prevention technique was successfully implemented.

Implementing Company
Green Industries - Cincinnati, OH

Drain Boards

Drain boards are used to collect drag-out/drippings from work pieces on racks, and direct the solution back into its previous bath. Drainboards save solutions and keep them off the floor. These boards may be constructed of any compatible material. It is important that they be oriented to direct drips to the correct tank. Use of drain boards is a cost effective technique which will reduce chemical consumption as well as the amount of rinsewater.

Angled Barrels

For large, horizontal double hung type barrels, no producer/vendor of angled barrel technology has been found. However, one of the following modifications of existing barrels may prove useful in minimizing drag-out. Prior to implementing an angled or sloped barrel technique, the design of all tanks must be checked to assure proper clearance for the modified system.

Efficient Rinsing

Spray Rinses and Air Knives

These two measures are fairly similar in their application. When mounted on the edge of a process tank, a pipe directing air or water at the work pieces through appropriate nozzles can reduce the amount of solution which is carried over to subsequent process tanks. Either fixed or movable nozzles can be used. This procedure enables keeping a solution in its tank of origin, rather than passing into another tank as a contaminant. Not all process operations can accept this solution removal procedure; neither spray rinses nor air knives are easily applied to barrel operations. One vendor offers an internal spray rinse system for barrel operations.

Spray rinses can be used above any heated process tank to recover drag-out before it is transferred to the next tank. The spray flushes drag-out from the work pieces as they are withdrawn from the process tank. In rinsing efficacy a spray rinse is equivalent to approximately one-half a dip rinse. In many cases spray nozzles can be sized and water flow rates adjusted such that the spray rinsewater balances the evaporative losses.

Air knives are used to blow air across the surface of the work pieces as they are withdrawn from the process or rinse solution, physically pushing liquid off the work piece and into its tank of origin, thereby reclaiming drag-out, reducing the volume of rinsewater required, and enhancing the drying process. In some applications this rapid dry method may cause quality problems.

Implementing companies
Leonhardt Plating - Cincinnati, OH
NCR Corporation - Cambridge, OH

Flow Restrictors

Flow restrictors are placed directly in the rinse water inlet of a rinse tank in order to restrict the flow of water to a predetermined acceptable level. Flow restrictors work well to maintain flow rates at their predetermined values. However, because restrictors are non-adjustable in use, they may be less suitable in job shops where the variety of materials being plated typically requires variable flow rates. Good practice entails turning off flowing rinses when not in use and educating operating personnel in the advantages of water conservation.

Implementing Companies
Lancaster Electroplating - Lancaster, OH
ILSCO - Cincinnati, OH

Conductivity Cells

Flow controllers utilizing conductivity cells can solve the problem of adjusting rinsewater flow rates to variable production rates. These sensors give an indication of contamination in the rinsewater (the higher the contaminant concentration, the higher the rinsewater conductivity). The sensors trigger the inflow of clean water when the tank water becomes contaminated, the excess contaminated water overflowing to the drain. Water is added only to lower the concentration of contaminants to a predetermined level, thus reducing the amount of overflow water requiring treatment. However, many of these systems incur significant maintenance costs, as most sensing probes must be cleaned and adjusted regularly.

Contact Time

Contact time specifically refers to the length of time the work pieces are in the tank. For a given work piece and tank size the efficacy of rinsing varies with contact time. Production rate, on the other hand, varies inversely with contact time. Through experimentation the operator must find the contact time that satisfies production requirements while providing high rinse efficiency. For the same rinse efficiency, agitation of rinsewaters may shorten required contact time. 

Counter-Current or Counter Flow Rinsing

This procedure substantially reduces the amount of water required for rinsing, thereby reducing the amount of wastewater to be treated. Implementation of the procedure is most efficient when several rinse tanks are used in series. Fresh water is supplied to the rinse tank farthest from the process tank. The flow of rinsewater is from this farthest tank toward the process tank, countercurrent to the flow of plated work pieces. The counter-current rinse overflows to the preceding rinse tanks until it reaches the tank immediately following the process tank. Often the overflow from this first rinse tank discharges to the drain. However, if the process tank operates at a temperature high enough to cause sufficient evaporation, the overflow may enter the process tank, thereby reclaiming much of the drag-out. Overflow to the process tank is only practical when deionized (DI) water is used for rinsing.

Implementing Companies
PG Products - Cincinnati, OH
Leonhardt Plating - Cincinnati, OH
Lancaster Electroplating - Lancaster, OH
Micro Metal Finishing - Cincinnati, OH
ILSCO - Cincinnati, OH

Static Rinsing (Recovery Rinsing)

If direct counter-current rinsewater overflow to the process tank is not possible, the first rinse tank after a process bath may be a static rinse that builds up a concentration of “drag-in.” Using the static rinsewater to replenish the process bath reclaims much of the drag-out. Good practice requires that the static rinse tank initially be filled with deionized water. Periodically the static rinsewater should be concentrated for recycle/reclaim into the plating bath. 

Rinse Bath Agitation

Agitation of rinse water baths reduces required contact time and improves rinsing efficiency. Experience has shown that air sparging is often not an efficient agitation technique. Recirculation of a sidestream from the rinse tank is reasonably effective, but use of a propeller type agitator results in the highest efficiency. Selection of the optimum method of agitation entails balancing capital and operating costs against revenues from increased production rates. 

Solution Purification

Ion Exchange

In this process rinsewaters are pumped through beds of resin. Platable cations in the rinsewater attach to the resin by displacing hydrogen or alkali/alkaline earth cations which leave in the effluent. When these resin beds become saturated, regenerating solutions restore them to usefulness by stripping the platable cations and recharging the resin with hydrogen or alkali/alkaline earth cations. Platable cations, concentrated in the spent regenerating solution, may be treated, recycled or reclaimed. The ion exchange process can produce treated water with very low concentrations of pollutants. Platable cation concentrations of 0.1 mg/l are attained with typical installations. With some additional investment, significantly lower values can be achieved. This characteristic identifies ion exchange as a technology suitable for use in achieving 100% recycle of wastewater.

Implementing Companies
PG Products - Cincinnati, OH
Leonhardt Plating - Cincinnati, OH
Lucas Sumitomo - Lebanon, OH

Electrolytic Recovery

Electrowinning

If solution concentrations become high enough, platable metal ions may be removed and the solution partially purified, by simply plating the metal ion onto a stub electrode. The recovered metal is either sold or recycled to the plating process. Because the plating process becomes inefficient at low metal ion concentrations, this technique usually is not suitable by itself for producing wastewater that complies with discharge regulations. In conjunction with another technique, however, such as ion exchange, plating out may allow reclaim/recycle at lower capital costs. The technique may be used on spent plating bath solutions, recovered spills, discharge from static rinse tanks, and regeneration solutions from ion exchangers.

Implementing Company
Carolina Galvanizing - Aberdeen, NC

Porous Pots

For hexavalent chromium plating baths the technique of porous pot plating has been used to extend bath life, thereby reducing the discharge of pollutants. During plating, the concentrations of iron and other cationic impurities build up in a hexavalent chromium bath to the extent that plating becomes unsatisfactory. If this bath is placed in a porous pot, in which a semipermeable membrane separates cathode from anode, and power is applied, the iron and other contaminant metal ions pass through the membrane and accumulate in the cathode chamber, from which they are periodically removed for disposal. Chromate ion remains in the anode compartment as part of an anolyte which, after purification, may be returned to the plating tank for further use. Less chromium is wasted by using this technique. The liquid in the cathode compartment must be handled as waste.

Implementing Companies
Lancaster Plating - Lancaster, OH
Hadronics - Cincinnati, OH

Membrane Processes

The Ultrafiltration (UF), Nanofiltration (NF), and Reverse Osmosis (RO) processes all operate on the principle of separating suspended or dissolved solids in solutions by use of a membrane surface. The passage of liquid (called permeate) and the blocking of solids is a function of the size of the openings in the membrane, the size of the impurities, and the magnitude of the pressure applied. The following table indicates usage and pressure for each process.

UF allows dissolved solids to pass through. NF blocks some dissolved solids but allows others to pass through. RO blocks almost all dissolved solids. UF is very effective in separating water from emulsified oil, and is used in some cases in the metal finishing industry to separate suspended metals from rinse waters. NF and RO are being used in the plating industry to provide a similar separation for dissolved solids. Depending on the type of system (UF-NF-RO) and the characteristics of contaminated solution being processed, permeates may be reused. During operation of any of these systems, the contaminated liquid is recirculated many times and becomes increasingly concentrated. In some cases it is also possible to reuse this concentrated portion.

UF-NF-RO systems have proved to be economical methods to clean up waste streams, to recover resources and to meet discharge limits. Operating costs for UF systems used to clean aqueous machining coolants run between $0.013/gal (70,000 gallons per day) to $0.10/gal (1,000 gallons per day). Operating costs for NF and RO systems are higher because of the higher operating pressures required. Nevertheless, metal finishers moving toward zero aqueous discharge or total recycling should consider these systems carefully.

Implementing Companies
Acme-United Corporation (RO) - Fremont, NC
Stanley Tools (RO) - Cheraw, SC
 
 


Evaporation

Evaporation may be successfully used to recover various plating bath chemicals. This technology is based on physical separation of water from dissolved solids. Water is evaporated from the collected rinsewater to allow the chemical concentrate to be returned and reused in the process bath. The water vapor may be condensed and reused in the rinse system. Evaporation units are operated either at atmospheric pressure or under vacuum. Evaporation is often used in conjunction with rinsewater minimization techniques as the process is energy intensive and becomes expensive for facilities with large flow rates.

Atmospheric Evaporation

Atmospheric evaporation is more commonly used than vacuum evaporation due to its lower capital cost. The evaporation unit either operates at the normal boiling point of the solution with added heat, or relies on evaporation at bath temperature. These units are useful in reducing the volume of a liquid waste for treatment or disposal, or for concentrate recycling. However, the constituents of the solution can be degraded by high temperatures.

Implementing Company
Lucas Sumitomo - Lebanon, OH

Vacuum Evaporation

Evaporation under less than atmospheric pressure serves to lower the boiling point of the solution. Solutions with heat sensitive constituents can be evaporated using the vacuum technology with little or no degradation to the solution. These units can substantially reduce the boiling points of solutions. The capital cost for vacuum evaporation units is higher than the cost of atmospheric units. Use of heat pumps in evaporation and condensation can lower operating costs.

Changing Bath Composition or Bath Chemistry

Proper control of bath operating parameters will result in more consistent work piece quality as well as longer bath life. The strategy is simple: determine critical operating parameters and maintain them within the established acceptable limits. After values for these parameters have been determined it is essential to regularly monitor bath chemistry to confirm stability or to establish the need for corrective actions. For many platables, simple field test kits are available which can be used to establish bath concentrations. Suppliers sometimes set specifications for concentrations at levels higher than required for effective operation. Experimentation with lowering levels to just above the concentration at which defects will occur can lead to reduced chemical costs. Examples of operating parameters follow.

pH

In many cases optimum plating occurs only in a relatively narrow pH range. Plating tanks can be equipped with pH monitors and a control system with a means of adding acid or base as needed. Savings in bath chemicals and improved work piece quality usually will rapidly defray the associated capital costs.

Temperature

In several cases optimum plating occurs only over a relatively narrow range of temperatures (i.e., hexavalent chromium). Heating elements and a control system are a normal part of the plating equipment, so no additional capital expenditures are involved in securing good performance, but rather good practices. Management encouragement and provision of periodic training for operating personnel will help promote these good practices.

Platable Ion Concentration

The concentration of the platable ion typically specified may not correspond to the concentration which minimizes drag-out while enabling good plating efficiency. As previously mentioned, experimentation to achieve good plating at lower ion concentrations not only can save chemical cost, but can also reduce drag-out.

Additive Concentration

Chemicals such as brighteners and surfactants may be added to plating baths to improve quality or reduce drag-out. Keeping the concentration of additive in the optimum range will prevent pollution by reducing drag-out, and the amount of rework or scrap.

Surface Tension

The tendency of liquid to cling to a work piece being withdrawn from the body of liquid can be reduced by using an additive to lower the liquid surface tension. This technique may be applied to rinsewaters as well as to plating baths. Experimentation will show which surfactant is most compatible with process fluids, and at what frequency additional surfactant must be added to maintain low drag-out. 

Hazardous Chemical Replacement

Chemical replacement is a technique supported by continuing plating research on a number of fronts. Many manufacturers of plating bath chemicals have active research programs. EPA regulations spur the development of substitute materials and alternate plating processes. The following discussion focuses on some chemical replacements currently proving successful. 

Chromium

Hexavalent chromium is used extensively in decorative and functional plating applications as well as conversion coatings. Approximately 80% of the available power supplied to a hexavalent chromium bath generates hydrogen gas. Evolution of the gas produces a mist of fine water particles with entrained hexavalent chromium. As hexavalent chromium is a carcinogen and a designated hazardous air pollutant, protection of worker health and safety as well as the environment requires control of its emission. Wastewater treatment incurs the cost of an added step to reduce hexavalent to trivalent chromium.

In some applications, especially decorative plating, the use of trivalent chromium has proven successful. Use of trivalent chrome eliminates both misting and the added step in wastewater treatment. Adherence, throw and coverage are all improved, and higher rack densities can be achieved. Because bath concentration is much lower than for hexavalent chromium, drag-out is less and the amount of sludge produced by wastewater treatment is substantially reduced. Plating thickness is limited to 0.1 mil; thicker coatings exhibit cracking and spalling. Thus the technique is usually unsuitable for hard chromium coatings, which may be 20 mils or more in thickness. Although the color tones of trivalent chromium coatings are different from those of hexavalent chromium, additives to the trivalent chromium bath can often ameliorate the difference.

Implementing Companies
PG Products - Cincinnati, OH
Pioneer Metal Finishing - Franklinville, NJ
Harshaw Filtrol Company - Winston-Salem, NC 

Cyanide

The cyanide anion is an excellent complexer and exhibits wide tolerance to impurities and variations in bath composition. It has long been used in baths for plating copper, zinc, and other metals. Its principal disadvantage is its high toxicity. Another disadvantage is the cost of treating cyanide in wastewater. Because of its toxicity and adverse environmental impacts, EPA regulations severely limit the discharge of cyanide.For plating copper, certain non-cyanide alkaline baths of proprietary composition have been developed. An alkaline pyrophosphate bath is in use which exhibits good throwing power and coating ductility. It is used in printed circuit board manufacture. A strike is required if plating over steel or zinc. This bath is expensive and its wastewaters hard to treat. An acidic copper plating bath using the sulfate ion has proved versatile; however, the low pH permits substrate attack and increasing iron concentrations in the bath. Fluoboric acid is the basis for another copper plating bath which affords enhanced solubility and conductivity, as well as high plating speeds. This bath is more costly, has fewer additive systems available, and is more hazardous to use. Treatment of its wastewater is more costly.

A non-cyanide alkaline bath based on sodium hydroxide is in use for plating zinc. When operated with concentrations and other parameters in control it performs as well as cyanide-based baths, and is the least expensive of all zinc plating baths. Zinc may also be plated from an acid chloride system, of which there are three types. Acid baths offer higher cathode efficiencies and lower voltages. Throwing power is less, but deposits are brilliant and levelling.

Implementing Company
Wolverine Plating Corp. - Roseville, MI 

Stripping

As the baths used in stripping are similar to those used in plating, similar techniques of pollution prevention and waste minimization are applicable. Attention to cleanliness and process control are important in reducing stripping wastes. Stripping is usually accomplished either by chemical immersion or by electrolytic process. Although mineral acids, suitably inhibited, are useful for stripping some coatings, they are aggressive to substrates and thus limited in application.

The majority of alkaline immersion stripping is accomplished with cyanide, which does not attack steel substrates but dissolves a number of other metals used as coatings. However, health and safety as well as environmental regulations discourage the continued use of cyanide.

Several non-cyanide alkaline immersion stripping baths are available to remove copper or nickel from various substrates. These baths typically employ either the ammonium ion or an amine to provide complexing. Persulfate or chlorite anions may be used, as well as proprietary formulations. 

Reduce sludge generation

The amount of sludge generated by treating plating wastewater is proportional to the concentration of platables and other ions that precipitate at alkaline pH. Any of the techniques for reducing drag-out will also reduce sludge generation. As the hardness in the water supply will also form sludge, the use of deionized water for rinsing will reduce sludge generation. The use of magnesium hydroxide, instead of sodium hydroxide, to raise the pH and precipitate heavy metal hydroxides, will also reduce sludge generation because a lesser mass of precipitant is required and because the resultant sludge cake will dewater more easily. In some cases use of the sulfide ion to precipitate the heavy metals as sulfides will result in a lesser mass of sludge. The use of ion exchange to capture and recover platable metals can eliminate the generation of sludge, but may not be economical in every case.

Conclusion

This manual is intended to provide a practical guide to pollution prevention and waste reduction for the metal finishing industry. It should be used to help metal finishing industries identify, assess, and implement waste minimization options. Economic feasibility is a crucial concern in this process. It is recommended that metal finishers conduct further research to investigate and evaluate the technical and economic feasibility of any pollution prevention technologies. The vendor matrix table on the next page will aid in this process.

Reducing generation of wastes is most likely to provide substantial benefits to the metal finishing industry by reducing raw material costs, reducing disposal costs, and lowering the liability associated with waste disposal. 

Go to Vendor Matrix

Copyright �1996, Institute for Advanced Manufacturing Sciences/GRC International, Inc.