EPA/625/7-90/006July 1990

GUIDES TO POLLUTION PREVENTION:THE FABRICATED METAL PRODUCTS INDUSTRY

RISK REDUCTION ENGINEERING LABORATORYCENTER FOR ENVIRONMENTAL RESEARCH INFORMATION

OFFICE OF RESEARCH AND DEVELOPMENTU.S. ENVIRONMENTAL PROTECTION AGENCY

CINCINNATI, OHIO 45268

Printed on Recycled Paper

NOTICE

This guide has been subjected to U.S. Environmental Protection Agencyspeer and administrative review, and approved for publication. Approval does notsignify that the contents necessarily reflect the views and policies of the U.S.Environmental Protection Agency, nor does mention of trade names or commercialproducts constitute endorsement or recommendation for use. This document isintended as advisory guidance only to the fabricated metal products industry indeveloping approaches for pollution prevention. Compliance with environmentaland occupational safety and health laws is the responsibility of each individualbusiness and is not the focus of this document.

Worksheets are provided for conducting waste minimization assessments ofmetal fabrication facilities. Users are encouraged to duplicate portions of thispublication as needed to implement a waste minimization program.

Reprinted with permission of EPA.NC Division of Pollution Prevention and Environmental Assistance

PO BOX 29569RALEIGH NC 27626-9569

http://www.p2pays.org/919-715-6500

50 copies printed on recycled paper at a cost of $60.95 or $1.22 each.

FOREWORD

This guide provides an overview of the metal fabrication processes andoperations that generate waste and presents options for minimizing wastegeneration through source reduction and recycling. Such processes are an integralpart of aerospace, electronic, defense, automotive, furniture, domestic appliance,and many other industries. Fabricated metal processes generate various hazardouswaste streams, including oily wastes from machining operations, heavy metal-bearing streams from surface treatment and plating operations, and additionalwastes related to paint application.

Reducing the generation of these wastes at the source or recycling the wasteson- or off-site will benefit the metal fabricating industry by reducing raw materialneeds, reducing disposal costs, and lowering the liabilities associated with hazardouswaste disposal.

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ACKNOWLEDGMENTS

This guide is based in part on waste minimization assessments conducted byJacobs Engineering Group, Inc. Pasadena, California for the California Departmentof Health Services (DHS). Contributors to these assessments include: David Leu,Benjamin Fries, Kim Wilhelm, and Jan Radimsky of the Alternative TechnologySection of DHS. Much of the information in this guide that provides a nationalperspective on the issues of waste generation and minimization for metal fabricatorswas provided originally to the U.S. Environmental Protection Agency by Versar,Inc. and Jacobs Engineering Group, Inc. in Waste Minimization - Issues andOptions, Volume II, Report No. PB87- 114369 (1986). Jacobs Engineering GroupInc. edited and developed this version of the waste minimization assessment guide,under subcontract to Radian Corporation (USEPA Contract 68-02-4286). Jacobspersonnel contributing to this guide include: Carl Fromm, project manager;Michael Meltzer, principal author; Michael Callahan, contributing author; andSally Lawrence, technical and production editor.

Lisa M. Brown of the U.S. Environmental Protection Agency, Office ofResearch and Development, Risk Reduction Engineering Laboratory, was theproject officer responsible for the preparation and review of this guide. Othercontributors and reviewers include: Dr. Marvin Fleischman, chemical engineeringprofessor, University of Louisville, Kentucky; Larry Foss, Foss Plating Co., SantaFe Springs, California; Rick Slaney, Ronlo Engineering Ltd., Camarillo, California;and Dennis Bronk , plant manager, Hansens Laboratory Furniture Industries, Inc.,Newbury Park, California.

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SECTION 1INTRODUCTION

This guide was prepared to provide plant operatorsor environmental engineers of commercial fabricated metal

facilities with guidelines and options to minimize bothhazardous and non-hazardous wastes. Others who mayfind this document useful are regulatory agencyrepresentatives and consultants.

The worksheets and the list of waste minimizationoptions were developed through assessments of Los Angelesarea firms commissioned by the California Department ofHealth Services (DHS 1989). The firms operations,manufacturing processes, and waste generation andmanagement practices were surveyed, and their existingand potential waste minimization options werecharacterized. Economic analyses were performed onselected options.

Four types of processes used in metal fabrication areexamined in this guide: machining operations, parts cleaningand stripping, metal surface treatment and plating, andpaintapplication. These processes use a variety of hazardousmaterials, including metal-working fluids,solvents,alkalineand acid cleaning solutions, treatment and plating solutionsthat contain hazardous metals such as chromium andcadmium, as well as cyanide and other chemicals, andpaints containing solvents and heavy metals. Many ofthose hazardous substances are being phased out in someapplications, in favor of more benign compounds.

Waste minimization is a policy specifically mandatedby the U.S. Congress in the 1984 Hazardous and SolidWastes Amendments to the Resource Conservation andRecovery Act (RCRA). As the federal agency responsiblefor writing regulations under RCRA , the U.S EnvironmentalProtection Agency (EPA) has an interest in ensuring thatnew methods and approaches are developed for minimizinghazardous waste and that such information is made availableto the industries concerned. This guide is one of theapproaches EPA is using to provide industry-specificinformation about hazardous waste minimization.

EPA has also developed a general manual for wasteminimization in industry. The Waste Minimization Oppor-tunity Assessment Manual (USEPA 1988) tells how toconduct a waste minimization assessment and developoptions for reducing hazardous waste generation at a

facility. It explains the management strategies needed toincorporate waste minimization into company policies andstructure, how to establish a company-wide waste minimi-zation program, conduct assessments, implement options,and make the program an on-going one. The elements ofwaste minimization assessment are explained in the Over-view, next section.

In the following sections of this manual you will find:

An overview of the fabricated metal industryand the processes used in it (Section Two);

Waste minimization options for the industry(Section Three);

Waste Minimization Assessment Guidelinesand Worksheets (Section Four)

An Appendix, containing:

- Case studies of waste generation andwaste minimization practices of threefacilities;

- Where to get help: Sources of usefultechnical and regulatory information

Overview of Waste MinimizationAssessment

In the working definition used by EPA, wasteminimization consists of source reduction and recycling.Of the two approaches, source reduction is usuallyconsidered preferable to recycling from an environmentalperspective. Treatment of hazardous waste is consideredan approach to waste minimization by some states but notby others, and thus is not addressed in this guide.

A Waste Minimization Opportunity Assessment(WMOA), sometimes called a waste minimization audit, isa systematic procedure for identifying ways to reduce oreliminate waste. The steps involved in conducting a wasteminimization assessment are outlined in Figure 1 andpresentedin more detail in the next paragraphs. Briefly, theassessmentconsists of a careful review of a plants operationsand waste streams and the selection of specific areas toassess. After a particular waste stream or area is established

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ciwHighlight

as the WMOA focus, a number of options with the potentialto minimize waste are developed and screened. Thetechnical and economic feasibility of the selected optionsare then evaluated. Finally, the most promising options areselected for implementation.

To determine whether a WMOA would be useful inyour circumstances, you should first read this sectiondescribing the aims and essentials of the WMOA process.For more detailed information on conducting a WMOA,consult The Waste Minimization Opportunity AssessmentManual.

The four phases of a waste minimization opportunityassessment are:

Planning and organization

Assessment phase

Feasibility analysis phase

Implementation

PLANNING AND ORGANIZATIONEssential elements of planning and organization for a

waste minimization program are: getting managementcommitment for the program; setting waste minimizationgoals; and organizing an assessment program task force.

ASSESSMENT PHASEThe assessment phase involves a number of steps:.

.

.

Collect process and facility data

Prioritize and select assessment targets

Select assessment team

Review data and inspect site

Generate options

Screen and select options for feasibility study

Collect process andfacility data. The waste streamsat a facility should be identified and characterized.Information about waste streams may be available onhazardous waste manifests, National Pollutant DischargeElimination System (NPDES) reports, routine samplingprograms and other sources.

Developing a basic understanding of the processesthat generate waste at a facility is essential to the WMOAprocess. Flow diagrams should be prepared to identify thequantity, types and rates of waste generating processes.Also, preparing material balances for various processescan be useful in tracking various process components andidentifying losses or emissions that may have been

unaccounted for previously.

Prioritize and select assessment targets. Ideally, allwaste streams in a facility should be evaluated for potentialwaste minimization opportunities. With limited resources,however, a plant manager may need to concentrate wasteminimization efforts in a specific area. Such considerationsas quantity of waste, hazardous properties of the waste,regulations, safety of employees, economics, and othercharacteristics need to be evaluated in selecting a targetstream.

Select assessment team. The team should includepeople with direct responsibility and knowledge of theparticular waste stream or area of the plant.

Review data and inspect site. The assessment teamevaluates process data in advance of the inspection. Theinspection should follow the target process from the pointwhere raw materials enter the facility to the points whereproducts and wastes leave. The team should identify thesuspected sources of waste. This may include the productionprocess; maintenance operations; and storage areas for rawmaterials, finished product, and work in progress. Theinspection may result in the formation of preliminaryconclusions about waste minimization opportunities. Fullconfirmation of these conclusions may require additionaldata collection, analysis, and/or site visits.

Generate options. The objective of this step is togenerate a comprehensive set of waste minimization optionsfor further consideration. Since technical and economicconcerns will be considered in the later feasibility step, nooptions are ruled out at this time. Information from the siteinspection, as well as trade associations, governmentagencies, technical and trade reports, equipment vendors,consultants, and plant engineers and operators may serveas sources of ideas for waste minimization options.

Both source reduction and recycling options should beconsidered. Source reduction may be accomplished through:

Good operating practices

Technology changes

Input material changes

Product changes

Recycling includes:

Use and reuse of waste

l Reclamation

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Screen and select options for further study. Thisscreening process is intended to select the most promisingoptions for full technical and economic feasibility study.Through either an informal review or a quantitative deci-sion-making process, options that appear marginal, im-practical or inferior are eliminated from consideration.

An economic evaluation is carried out using standardmeasures of profitability, such as payback period, return oninvestment, and net present value. As in any project, thecost elements of a waste minimization project can bebroken down into capital costs and economic costs. Savingsand changes in revenue also need to be considered.

FEASIBILITY ANALYSIS IMPLEMENTATIONAn option must be shown to be technically and

economically feasible in order to merit serious considerationfor adoption at a facility. A technical evaluation determineswhether a proposed option will work in a specificapplication. Both process and equipment changes need tobe assessed for their overall effects on waste quantity andproduct quality. Also, any new products developed throughprocess and/or raw material changes need to be tested formarket acceptance.

An option that passes both technical and economicfeasibility reviews should then be implemented at a facility.It is then up to the WMOA team, with managementsupport, to continue the process of tracking wastes andidentifying opportunities for waste minimization,throughout a facility and by way of periodic reassessments.Either such ongoingreassessments or an initial investigationof waste minimization opportunities can be conductedusing this manual.

SECTION 2FABRICATED METAL INDUSTRY PROFILE

Industry DescriptionFabricated metal products are classified under Standard

Industrial Classification (SIC) 34, and include industriesengaged in processes that machine, treat, coat, plate, paintand clean metal parts. There are two major segments of theindustry: job shops that process materials owned by otherparties on a contractual basis, and captive shops that areowned and part of larger manufacturing facilities. Metalfabrication processes are integral parts of aerospace,electronic, defense, automotive, furniture, domesticappliance, and many other industries. Fabricated metalprocesses generate various hazardous waste streams,including oily wastes from machining operations, heavymetal-bearing streams from surface treatment and platingoperations, and solvents, alkaline and acid solutions frommetal cleaning and stripping operations, and additionalwastes related to paint application. Bach of the major wastegenerating processes is profiled below.

Machining OperationsMachining operations involve various metal cutting

processes that include:

l turning

l drilling

l milling

l reaming

l threading

l broaching

l grinding

l polishing

l planing

l cutting and shaping

Machining processes use cutting tools of some sortthat travel along the surface of the workpiece, shearingaway the metal ahead of it. Most of the power consumedin cutting is transformed into heat, the major portion ofwhich is carried away by the metal chips, while the remainder

is divided between the tool and workpiece. Interfacetemperatures of up to 200F have been measured (Baumeis-ter 1967).

Turning processes and some drilling are done onlathes, which hold and rapidly spin the workpiece againstthe edge of thecutting tool. Drilling machines are intendednot only for making holes, but for reaming (enlarging orfinishing) existing holes. This process is also carried out byreaming machines using multiple cutting edge tools. Mill-ing machines also use multiple edge cutters, in contrastwith the single point tools of a lathe. While drilling cuts acircular hole, milling can cut unusual or irregular shapesinto the workpiece.

Broaching is a process whereby internal surfaces suchas holes of circular, square or irregular shapes, or externalsurfaces like keyways are finished. A many-toothed cuttingtool called a broach is used in this process. The broachsteeth are graded in size in such a way that each one cuts asmall chip from the workpiece as the tool is pushed orpulled either past the workpiece surface, or through aleader hole (Baumeister 1967). Broaching of round holesoften gives greater accuracy and better finish than reaming.

METALWORKING FLUIDSMetalworking fluids are those liquids (or sometimes

gases) that are applied to the workpiece and cutting tool inorder to facilitate the cutting operation. A metalworkingfluid is used:

1) to keep tool temperature down,preventing premature wear and damage;

2) to keep workpiece temperature down,preventing it from being machined to a warpedshape or within inaccurate dimensions;

3) to provide a good finish on the workpiece;

4) to wash away chips; and

5) to inhibit corrosion or surface oxidation ofthe workpiece.

Also, and very important, metalworking fluids arefrequently used to lubricate the tool-workpiece interface,in addition to simply cooling it.

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Metalworking fluids can be air-blasted, sprayed ordrawn through suction onto the tool-workpiece interface.Types of fluids include water (either plain or containing analkali); emulsions of a soluble oil or paste; and straightoils (those that are not water-based) such as mineral,sulphurized, or chlorinated oil.

Air drafts are often used with grinding, polishing andboring operations to remove dust and chips, and to cool toa certain extent. Aqueous solutions containingapproximately one percent by weight of an alkali such asborax, sodium carbonate or trisodium phosphate exhibithigh cooling properties and also provide corrosionprevention for some materials. These solutions areinexpensive and sometimes are used for grinding, drilling,sawing, and light milling and turning operations (Baumeister1967).

Emulsions consist of a suspension of oil or paste inwater, typically at the ratio of one part oil to 10 to 100 partswater, depending on the application. Rich mixtures of oilto water are used for broaching, threading and gear cutting,while a 1:20 ratio suffices for most lathe work, drilling andscrew machine work.

Oil are used for metal cutting where lubrication ratherthan cooling is essential for tool life and/or work quality.

WASTE STREAMSThe major wastes from machining operations are

spoiled or contaminated metalworking fluids which aretreated as hazardous wastes because of their oil content, aswell as other chemical additives that some contain such aschlorine, sulfur and phosphorus compounds, phenols,creosols and alkalies. While fresh metalworking fluidscontain varying degrees of oil depending on their function,tramp hydraulic and lubricating oils also find their wayinto the fluids during the course of operations. Spentmetalworking fluids are at present either disposed of orrecycled on- or off-site. Recycling typically consists ofseparating the oils through such methods as centrifugingand refining them or using them as fuel.

Solvent wastes resulting from cleaning of parts andequipment also comprise a sizable waste stream. Thisstream is examined in the Metal Parts Cleaning andStripping section.

Many fabricated metal industries generate cuttingsand other scrap metal. Scrap that is destined for reclamationis not regulated as hazardous waste. If metal chips frommachining operations are mixed with hazardousmetalworking fluid wastes, however, the waste stream istreated as hazardous.

While metalworking fluid purchases typically accountfor less than 0.5 percent of the cost of operating a machinetool (Schaffer 1978), the problems that contaminated anddegraded fluids can cause can be expensive and troublesome.Proper coolant and cutting oil maintenance is necessary toprevent excessive machine tool downtime, corrosion, andrancidity problems.

Metalworking fluid rancidity, perhaps the mostcommon problem, can affect productivity and operatormorale. Rancidity odors are produced in contaminatedfluids due to bacterial action. The odors are especiallystrong when machines are started up after periods ofdowntime. The odors are frequently unpleasant enoughthat the fluid must be changed.

Insufficient maintenance of cutting fluids, especiallywater-based fluids, can result in workpiece and machinetool corrosion. Many cutting fluids are relied upon toprotect in-process parts from corrosion, but they will notoffer this protection if they have deteriorated due to rancidity,or if they are not maintained at the recommendedconcentrations. Cutting fluids also must not be allowed topenetrate into gear boxes or into lubricating oil reservoirs,or internal damage to machines can result.

Contamination of water miscible metalworking fluidsby tramp lubricating and hydraulic oils constitutes one ofthe major causes of fluid deterioration. The tramp oilsinterfere with the cooling effect of the fluids, promotebacterial growth, and contribute to oil mist and smoke inthe shop environment. Tramp oils impair the filterabilityof metalworking fluids through both disposable andpermanent media filters, and thus inhibit recycling. Trampoils also contribute to unwanted residues on cutting toolsand machine parts (Sluhan, W. A.)

The most serious problem caused by tramp oils is thepromotion of bacterial growth, primarily Pseudomonasoleovorans, in the metalworking fluid. Such bacteriadegrade lubricants, emulsifiers and corrosion inhibitors inthe metalworking fluids, and liberate gases, acids and saltsas byproducts of their growth (Sluhan, W.A.). Bacterialgrowth also interferes with the cooling effect ofmetalworking fluids.

The tramp oils that most contribute to bacteria growthare hydraulic oils (used in hydraulic assist systems), due totheir high water miscibility compared to lubricating oils,and to the phosphorus antiwear compounds they contain,which catalyze microbe growth. Lubricating and machineramp oils create less problems, because their lowermiscibility causes them to float to the surface of thecoolant.

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Metal Parts Cleaning And StrippingCleaning and stripping operations are integral to

numerous processes in industries involved with themanufacture of metal parts and equipment. Virtually allfabricated metal objects require some form of cleaning.Machined parts are cleaned with solvents; paint, oxidationand old plating is stripped from workpieces using causticsand abrasives; and workpieces in plating lines are cleanedseveral times using water, acids, caustics and detergents.Implementation of proper, environmentally sound cleaningand stripping techniques will markedly reduce toxicitiesand volumes of wastewater, as well as reducing processchemical requirements.

PROCESS DESCRIPTIONFive types of metal cleaning media are utilized by

industry: 1) solvents (both halogenated andnonhalogenated); 2) alkaline cleaners; 3) acid cleaners: 4)nonchemical, abrasive materials; and 5) water. Alkalineand acid cleaners are usually referred to as aqueous cleaners.Mixtures of solvents and alkalines are frequently used.Mixtures where water-immiscible solvent is emulsified inwater (often containing other additives) are termed emulsioncleaners.

Although metal parts cleaning is frequently thought ofas a simple operation requiring little more than washing apart in solvent, many metal parts require sophisticated andrather complex sequences of cleaning steps. The design ofa cleaning operation is generally dependent upon threeinterrelated factors:

l The nature of the contamination. It is importantto know the composition as well as the historyof contaminants on the metals surface, in orderto design the proper cleaning system andsequence of baths or other operations. Alkalinecleaners are often used to remove heavy soilsand some solid oils, while caustics are goodpaint stripping agents. Acid cleaners andabrasives are employed to remove oxidationscale and rust. When parts have beencontaminated with several materials,sequencing of cleaning operations can beimportant. For instance, a layer of oilycontamination might be removed by an alkalinecleaner before abrasives are used to remove arust layer.

. The metal substrate. The contaminant must beremoved to the required degree withoutadversely affecting the metal substrate.Reactivity of different metals with alkaline andacids varies, and thus cleaners that are

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appropriate for one metal may not be for another.

The degree of cleanliness required. The cleaningthat a metal surface requires varies dependingupon the particular surface treatment, platingor coating operations it will be subjected to.For instance, parts going to a cyanide-zincplating bath do not usually need to first receivea high level of cleaning since cyanide-basedplating solutions exhibit strong cleaning actionsof their own. For a nickel plate to adhere to ametal surface, on the other hand, the surfacemust be extremely clean. Thus, thorough andrigorous cleaning operations are needed priorto the nickel plating.

It is frequently the case that no one cleaning operationcan be specified as best based simply on reviewing theabove factors. Several cleaning methods often appearappropriate, and only through experiment can the best onebe selected.

Cleaners, except for abrasives, are normally containedin large open tanks, with the parts to be cleaned mountedon racks or in perforated horizontal barrels. The decisionto use racks or barrels depends on the size and shape of thepart as well as the type of coating it requires.

SOLVENT CLEANINGSolvents are the most widely used class of cleaners.

They are employed for removing oil-based contaminants,in either cold cleaning, diphase cleaning, or vapor phasecleaning operations.

Cold cleaning generally employs unheated or slightlyheated nonhalogenated solvents, and is the most commontype of cleaning. The four categories of cold cleaning are:1) wipe cleaning; 2) soak cleaning; 3) ultrasonic cleaning;and 4) steam gun stripping. Wipe cleaning consists ofsoaking a rag in solvent and wiping the metal part clean.Soak cleaning involves the immersion of the parts in asolvent tank. Ultrasonic cleaning is identical to soakcleaning, except that an ultrasonic unit is added to the tank,which provides a vigorous cleaning action throughout thetank. The main application of steam gun stripping is forpaint removal from metal objects. A stripper made up ofnonhalogenated solvents is fed into a steam line, throughan adjustable valve, mixed with the steam and ejected athigh speed from a nozzle.

Diphase cleaning systems are so named because theyuse both water and solvent phases for cleaning. Parts to becleaned first pass through a water bath, then a solventspray. Vapor phase cleaning, also called vapor degreasing,consists of a tank of halogenated solvent heated to itsboiling point. Parts to be cleaned are placed in the vapor

zone above the liquid solvent. The vapor that condenses onthe cooler part dissolves oil-based contamination andrinsesthe part clean. Since the potential exists for considerablygreater air emissions from vapor phase cleaning than fromcold cleaning tanks, special recovery equipment is installed,consisting of cooling jackets and/or finned coil condensers.By cooling the air above the vapor, a dense cool air blanketis formed which helps suppress vapor from escaping. Thesecond unit, a finned coil condenser, is installed inside thetank and condenses any vapor that reaches it.

AQUEOUS (ALKALINE AND ACID) CLEANINGAND STRIPPING

The cleaning action of aqueous cleaners relies mainlyon displacement of soils rather than on their dissolution, asis the case with organic solvent. Since both alkaline andacid aqueous cleaners and strippers use the same equipment,they are discussed together. Alkaline cleaning solutionscontain builders (sodium salts of phosphates, carbonates,silicates, and hydroxides) and surfactants (detergents andsoaps). Other additives may include anti-oxidants andstabilizers as well as small amount of solvents. Alkalinecleaners and strippers are employed to remove soil frommetal parts, as well as old plating and paint Acidiccleaning solutions may contain mineral acids (nitric, sulfuricand hydrochloric), organic acids (sulfarnic, acetic, oxalicor cresylic), detergents, chelating agents and occasionallysmall amount of solvents. Acid cleaners remove rust,scale, and smut, which is formed from electrocleaning.Very strong alkaline cleaners containing cyanide andcleaning agents have recently been formulated to replaceacid cleaners. No matter what type of aqueous cleaner isused, soak tanks similar to those used for solvents are themost common cleaning method employed. Some aqueouscleaners, however, are used in electrochemical cleaning, inwhich the workpiece is connected to a source of current. Indirect current electrochemical cleaning, the workpiece isattached to the cathode, causing hydrogen gas to be formedat the parts surface that provides a scrubbing action. Smutformation (the plating of metal contaminants in the solutiononto the workpiece) does sometimes occur, however, aswell as hydrogen embrittlement of the metal. Thesedisadvantages are avoided in reverse current cleaning, orelectropolishing, in which the workpiece is attached to theanode. Metal substrate is dissolved electrolytically,liberating the surface contaminant.

ABRASIVE CLEANING AND STRIPPINGAbrasive cleaners are designed for removing rust,

oxides and burrs, old plating and paint, and to create asmooth surface. Typical abrasives are aluminum oxide orsilicon carbide mixed with an oil or water based binder.The abrasive-binder mixture is applied to a buffing wheelmade from an absorbent material such as cloth. The metal

part is held against the spinning wheel. Vibratory finishingis another method of abrasive cleaning in which a load ofmetal parts is immersed in a vibrating tank containingabrasive material and water. Similar cleaning methodsemploy tumbling barrels and centrifugal barrel finishing.

WATER CLEANINGWater cleaning is an integral part of every parts

cleaningprocess. Most of the cleaning operations mentionedabove require that a water wash be performed before andafter each operation. The washing is generally done eitherin a soak tank, or using a spray unit. Because rinse watergenerally comprises the largest waste stream in metalfabrication processes, measures for reducing the amount ofwater required (such as extending water bath life bypreventing its contamination by other cleaning media) arevery important in reducing the overall volume of wastes.

WASTE STREAMSThe primary wastes associated with metal parts

cleaning are listed in Table 1, along with their sources. Thecomposition of the waste depends on the cleaning mediaused, type of substrate, and the type of soil removed (oils,greases, waxes, metallic particles, oxides, etc.). If a facilityhas a wastewater treatment system, primary rinse water,alkaline and acid cleaning solutions can be mixed together(one acts to neutralize the other) and then treated.

Secondary rinse water (if secondary rinse is employed)is usually used to replace discarded primary rinse waterand/or used as a makeup for cleaning solutions. Forfacilities using small amounts of cleaner, the tendency is todrum the material for disposal. Solvent waste can be sentto an off-site recycler or recycled on-site using distillationequipment.

Metal Surface Treatment And PlatingOperations

Metal surface treatment and plating are practiced bymost industries engaged in forming and finishing metalproducts, and involve the alteration of the metal workpiecessurfaceproperties, in order to increase corrosion orabrasionresistance, alter appearance, or in some other way enhancethe utility of the product. Plating and surface treatmentoperations are typically batch operations, in which metalobjects are dipped into and then removed from bathscontaining various reagents for achieving the requiredsurface condition. The processes involve moving theobject to be coated (the workpiece) through a series ofbaths designed to produce the desired end product.Workpieces can be carried on racks or in barrels. Largeworkpieces are mounted on racks that carry the parts frombath to bath. A set of small parts can be contained in barrelsthat rotate in the plating bath.

Table 1. Metal Parts Cleaning Wastes

No.1.

WasteDescriptionAbrasive

2. Solvents

Alkalines

Acids

5. Rinsewater

ProcessOrginRemoval of rust,scale polishingof metalRemoval of oil-based soils

Removal oforganic soils,descalingRemoval ofscale, smutRemoval ofpreviouscleaningmaterial

PROCESS DESCRIPTIONPlating operations can becategorized as electroplating

and electroless plating processes. Surface treatment includeschemical and electrochemical conversion, case hardening,metallic coating, and chemical coating. Most metal surfacetreatment and plating processes have three basic steps:surface cleaning or preparation (which was examined inthe previous action); the actual modification of the surface,involving some change in its properties (e.g. case hardening,or the application of a metal layer); and rinsing or otherworkpiece finishing operations.

Chemical and Electrochemical ConversionChemical and electrochemical conversion treatments

are designed to deposit a coating on a metal surface thatperforms a corrosion protection and/or decorative function,and in some instances is a preparation for painting. Processesinclude phosphating, chromating, anodizing, passivation,and metal coloring. Phosphating treatments provide acoating of insoluble metal phosphate crystals that adherestrongly to the base metal. The coatings provide somecorrosion resistance, but their main function, due to theirabsorptivity,is as a base for the adhesion of paints, lacquers,and oils to the metal surface. Chromate coatings areapplied to minimize rust formation and to guarantee paintadhesion. Chromating baths ingredients include hexavalentchromium, one or two mineral acids (e.g. sulfuric or nitric),and often several organic or inorganic activatingcompounds.

Anodizing employs electrochemical means to developa surface oxide film on the workpiece, enhancing itscorrosion resistance. Passivation is a process by which

CompositionAluminum oxide, silicametal, water, grease

Halogenated and non-halogenated solvents,oil-based contaminantsAlkaline salts,additives, organicsoils, waterAcids, additives,dissolved metal salt, waterWater with traces ofcleaners and additives

protective films are formed through immersion in an acidsolution. In stainless steel passivation, embedded ionparticles are dissolved and a thin oxide coat is formed byimmersion in nitric acid, sometimes containing sodiumdichromate.

Case HardeningCase hardening produces a hard surface (the case)

over a metal core that remains relatively soft. The case iswear-resistant and durable, while the core is left strong andductile. Case hardening methodologies include carburizing,carbonitriding, nitriding, microcasing, and hardening usinglocalized heating and quenching operations.

Carburizing, the most widely used case hardeningoperation, involves diffusion of carbon into a steel surfaceat temperatures of 8450 to 9550C, producing a hard case inthe high carbon areas. Nitriding processes diffuse nascentnitrogen into a steel surface to produce case-hardening.Nitriding is accomplished using either a nitrogenous gas,(usually ammonia), or a liquid salt bath, typically consistingof 60 to 70 percent sodium salts, mainly sodium cyanide,and 30 to 40 percent potassium salts, mainly potassiumcyanide. Carbonitriding and cyaniding involves thediffusion of both carbon and nitrogen simultaneously intoa steel surface.

Applied energy methods are those that generate a casethrough localized heat and quenching, rather than throughuse of chemicals. Very rapid heat application results insurface hardening with little heat conducted inward. Sinceno carbon or nitrogen is diffused into the workpiece, it isthe existing carbon content of the ferrous metal that

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determines hardness response. Heating can be accomplishedthrough electromagnetic induction, high temperature flamesor high velocity combustion product gases.

Metallic CoatingsMetallic coatings provide a layer that changes the

surface properties of the workpiece to those of the metalbeing applied. The workpiecebecomes acomposite materialwith properties generally not achievable by either materialsingly. The coatings function is usually as a durable,corrosion resistant protective layer, while the core materialprovides the load-bearing function. Metallic coatings asdefined here refer to diffusion coatings (in which the basemetal is brought into contact with the coating metal atelevated temperatures allowing lattice interdiffusion of thetwo materials); spraying techniques; cladding (applicationusing mechanical techniques); vapor deposition and vacuumcoating.

Hot dipping is a diffusion process that involves partialor complete immersion of the workpiece in a molten metalbath. Common coating materials include aluminum, coatedlead, tin, zinc, and combinations of the above. The coatingmetal in a cementation diffusion process is applied inpowdered form at a high temperature (800 to 11000 C), ina mixture with inert particles such as alumina or sand, anda halide activator. The main applications of sprayeddiffusion coatings are for workpieces difficult to coat byother means due to their size and shape, or that aredamageable by the high temperature heating required ofother methods. Vapor deposition and vacuum coatingproduce high quality, pure metallic layers, and cansometimes be used in place of plating processes. A layerof metal cladding can be bonded to the workpiece usinghigh pressure welding or casting techniques. Cladding canoffer an alternative to plating in some situations.

ElectroplatingElectroplating is achieved by passing an electric current

through a solution containing dissolved metal ions as wellas the metal object to be plated. The metal object acts as acathode in an electrochemical cell, attracting metal ionsfrom the solution. Ferrous and nonferrous metal objectsare typically electroplated with aluminum, brass, bronze,cadmium, chromium, copper, iron, lead, nickel, tin, andzinc, as well as precious metals such as gold, platinum, andsilver. Common electroplating bath solutions are listed inTable 2.

The sequence of unit operations in an electroplatingoperation is very similar when either racks or barrels areused to carry parts. A typical sequence involves varioustypes of cleaning steps, stripping of old plating or paint, theactual electroplating steps, and rinsing steps between andafter each of the above operations.

Table 2. Common Electroplating BathCompositions

Electroplating Bath NameBrass and Bronze

CompositionCopper cyanideZinc cyanideSodium cyanideSodium carbonateAmmoniaRochelle saltCadmium cyanideCadmium oxideSodium cyanideSodium hydroxideCadmium fluoroborateFluoroboric acidBoric acidAmmonium fluoroborateLicoriceCopper cyanideSodium cyanideSodium carbonateSodium hydroxideRochelle saltCopper fluoroborateFluoroboric acidCopper sulfateSulfuric acidCopper pyrophosphatePotassium hydroxideAmmoniaCopper cyanidePotassium cyanide

Potassium fluorideChromic acidSulfuric acidChromic acidSulfateFluoride

Cadmium Cyanide

Cadmium Fluoroborate

Copper Cyanide

Copper Fluoroborate

Acid Copper Sulfate

Copper Pyrophosphate

Fluoride-ModifiedCopper Cyanide

Chromium

Chromium withFluoride Catalyst

Electroless plating uses similar steps, but involves thedeposition of metal on a metallic or non-metallic surfacewithout the use of external electrical energy.

WASTE STREAMSCommon plating and surface treatment process wastes

are listed in Table 3. Two of the waste streams, spentalkaline cleaning solutions and spent acid cleaningsolutions, are generated by periodic replacement ofcontaminated solutions. Rinse waters are generated fromoverflow of rinse tanks and contamination by drag-outfrom cleaning baths. Waste removed from plating tanks bythe continuous filtering of the baths results in filter sludges.

10

Table 3. Process Wastes

Waste ProcessDescription Origin

Spent processsolutionsFilter sludges

Plating and chemicalconversionPlating and chemicalconversion

Quench oilsand quench oil tankcleanup wastesSpent salt bath

Wastewater treatmentsludgeVent scrubber wastes

Ion exchange resin

Case hardening

Carburizing, nitrid-ing, cyaniding

Wastewater treatment

Vent scrubbing

Demineralization ofreagents process water

Wastes produced at a particular facility will be similarto those listed, but their precise composition will depend onthe specific process. Some or all of the waste types listedmay be combined into a single stream before treatment anddisposal. It is common to combine concentrated cyanidewastes from plating and cleaning solutions, for instance,with filter sludges. These are generally kept separate,however, from acidic wastes and from the dilute cyanidesolutions.

On a volume basis, contaminated rinsewater accountsfor the majority of plating process waste. As shown in theprevious sections, plating processes can involve manyrinsing steps. Rinsewater is used to wash off the drag-outfrom a work piece after it is removed from a bath. Drag-outrefers to the excess solution that adheres to the workpiecesurface and gets carried out of the solution bath uponwithdrawal of the workpiece from the bath. In general, theuse of small part barrels in the plating process (barrelplating) produces more drag-out than rack plating. This isbecause a barrel carries in it more plating solution uponwithdrawal from the bath than a rack does, and becausedrainage of the drag-out back into the bath is more difficultwith barrels. If the drag-out from one bath is carried intothe next bath in the sequence due to incomplete rinsing, itis referred to as drag-in, and is considered a contaminantin the latter bath.

Spent cleaning and plating solutions are another sourceof plating wastes. Several types of cleaning solutions areused to prepare a metal surface for electroplating. Shipping

CompositionSee Table 2.2

Silica, silicides,carbides, ash, platingbath consituentsOils, metal fines,combustion products

Sodium cyanide andcyanate. Potassiumcyanide and cyanate.Metal hydroxides,sulfides, carbonatesSimilar to processsolution compositionBrine, HCI, NaOH

wastes are a special type of cleaning waste. They resultfrom the stripping off of the old plated deposit prior to thedeposition of anew metal plate. Cleaning solutions may beacidic or basic, and may contain organics. Heavy metalsare usually not present, although some cleaning solutionscontain cyanide. Spent plating solutions contain highconcentrations of metals. These solutions are not regularlydiscarded like cleaning solutions, but may require purgingif impurities build up.

Wastes produced from spills and leaks are usuallypresent to some extent in an electroplating process. Wateris used to wash away floor spills, and the resultingwastewater contains all of the contaminants present in theoriginal solutions. Wastewater is also produced from thewet scrubbing of ventilation exhaust air.

Wastewater produced in an electroplating processmay contain a variety of heavy metals and cyanide. Themetals are typically removed by adding lime or otherprecipitating agents, and precipitated under alkaline pH.The resulting metal hydroxide precipitate forms a dilutesludge, which is thickened and then disposed of bylandfilling.

Paint ApplicationThe application of paint is practiced within most

fabricated metal industries. Surface coatings are usedwherever it is desired to provide decoration, protection,and/or safety marking to a product or item. Most paintcoatings for fabricated metal products are solvent based

11

although many shops are replacing these with water basedmaterials.

PROCESS DESCRIPTIONBefore a product coating can be applied to a surface,

the surface must be free from contamination. As describedabove, many different types of abrasives, alkalines, acids,and solvents, as well as water, are used by industry to cleanmetal surfaces. Once a part is cleaned, surface treatmentsuch as phosphate coating can be applied if desired. Thepurpose of surface treatment is to condition or prepare thesurface so that the paint forms a better bond with the metalsurface.

After the item has been cleaned and treated, paint canbe applied. Depending on the size, shape, complexity, andquantity of items to be painted, different application methodscan be employed. When it is desired to paint a large numberof very small items, the most commonly used methods aretumbling, barreling, or centrifuging. For all three methods,the parts are placed inside a barrel, solvent-based paint ispoured onto the items, and the barrel is then rotated. Aftera short time and at the correct point of tackiness, the partsare transferred to an oven in a wire basket. While paintconsumption using these methods is very small, theempirical nature of the operation requires that the operatorbe highly experienced to achieve reliable results.

For cylindrical items, a commonly used method isdipping. Here the paint is held in a large tank and the objectto be painted is slowly lowered into the tank and thenwithdrawn. Many complex items can be dip paintedprovided that the drainage points (the places where theexcess paint drips off), can be located where they are notnoticeable.

Flow coating is often employed for items that wouldbe difficult to dip because of their size or shape, or as ameans of avoiding the installation and operation of largedip tanks. A flow coating system operates by using highpressure sprays to flood the item with solvent-based paint.After spraying, the item is allowed to drain and the excesspaint is recirculated. Since a considerable amount ofbubbling occurs due to spraying, the item is then passedthrough a solvent chamber where the solvent vapors allowthe paint to reflow. Following this operation, the item isthen oven-dried. The main disadvantage of flow coating ishigh solvent loss, which can be three times as large as fordipping and twice as great as for spraying.

For relatively flat items of large area, roller coatingand curtain coating machines are used. Roller coating isused extensively by the canning industry for painting flat

metal sheets that are then fabricated into cans. It is also usedfor spreading or applying glue to wood in the manufacturingof plywood. A roller coating machine operates by meteringpaint or coating material onto a roller and then transportingthe item past the roller by means of a conveyor belt. Acurtain coating machine consists of a pressurized containeralong the bottom of which is an adjustable slit that allowsthe coating to flow and form a vertical curtain. A conveyorbelt is placed on each side of the curtain so that work itemsare passed through the curtain and coated without theconveyor belts being coated.

While all of the above-mentioned methods have foundwidespread acceptance by industry, the most widely usedmethod for applying paint is still the spray gun. A spraygun operates by using compressed air, to atomize the paintand produce a fan or circular cone spray pattern. Manyinstallations are automated so that a fixed gun is turned onwhen an object passes in front of it. In its simplest use, thegun is hand-held and the object remains stationary. Someof the variations on spray gun painting are airless sprayguns and electrostatic spray guns. Airless spray guns forcethe paint out at high pressure so that air is not require foratomization. By eliminating the use of compressed air,operating costs are lower, spray mists are not produced,and expensive exhaust systems are not required.Electrostatic spray units are designed so that the atomizedpaint leaving the gun has a positive charge. This positivecharge causes the paint to be attracted to the object whichis connected to ground. Since more of the paint reaches itstarget (thereby reducing overspray), less waste is generated.

Following the application of paint, the item is passedthrough a drying or curing oven. The curing methodsemployed, infrared or ultraviolet, will depend on the typeof paints being used. Once dried, the items are sent toinspection and final packaging or assembly. If a part failsinspection because of a bad finish, it is usually reworked bystripping off the paint and returning it to the cleaningoperation.

WASTE STREAMSThe primary wastes associated with product coating

applications consist of empty paint containers, spentcleaning solutions, paint overspray (including paintcollected by air pollution control equipment), spent strippingsolutions, and equipment cleaning wastes. Wasteminimization methods for stripping and cleaning areexamined under the Parts Cleaning heading in SectionThree; source reduction and recycling methods for theother waste streams are examined under Paint Wastes inSection Three.

12

SECTION 3WASTE MINIMIZATION OPTIONS

FOR FABRICATED METAL PRODUCT FACILITIES

IntroductionThe list of individual primary fabricated metal industry

waste streams and their sources along with a list of sourcereduction and recycling methods is presented in Table 4.Recommended waste reduction methods and identifiedprocedures are discussed in the following sections. Thesemethods came from industry contacts and publishedaccounts in the open literature.

In addition to the waste reduction measures that areclassified as process changes or material/productsubstitutions, a variety of waste reducing measures labeledas good operating practices has also been included. Goodoperating practices are defined as procedures or institutionalpolicies that result in a reduction of waste. The followingdescribes the scope of good operating practices:

Waste stream segregation

Personnel practices

- Management initiatives

- Employee training

Procedural measures

- Documentation

- Material handling and storage

- Material tracking and inventory control

- Scheduling

Loss prevention practices

- Spill prevention

- Preventive maintenance

- Emergency preparedness

Good operating practices apply to all waste streams.

Material Handling And StorageImproper storage and handling can result in spoilage

and obsolescence of raw materials, resulting in thegeneration of hazardous and other waste streams. Efficient

operating practices can reduce or eliminate waste resultingfrom obsolescence and improper storage. Source reductionmethods for reducing waste include:

Material PreinspectionMaterials should be inspected before being accepted

and unacceptable or damaged materials returned to themanufacturer or supplier. This avoids both disposal of anearly full container of unusable material and printing anunacceptable product.

Proper Storage of MaterialsMany chemicals are sensitive to temperature and

humidity. Much waste can result from improper storage.Chemical containers list the recommended storageconditions. Meeting the recommended conditions willincrease their shelf life.

Restrict Traffic through Storage AreaTo prevent raw material contamination, the storage

area should be kept clean. Also, the storage area should notbe open to through traffice. Through traffic will increasedust and dirt in the storage area, increasing possiblecontamination. In addition, spills in the storage area willbe easier to contain if traffic is restricted.

Inventory ControlInventories should be kept using the first-in, first-

out practice. This will reduce the possibility of expiredshelf life. This practice may not work for specialty materialsthat are seldom used. Computerized inventory systemscantrack the amounts and ages of the raw materials.

Purchase Quantifies According to NeedsRaw material order quantities should be matched to

usage. This avoids having a large, partly used container ofink going bad in storage because it wasnt properly sealed.Large shops should order materials in large containers,which may be returnable, thereby eliminating or reducingthe need to clean them. It takes less time to scrape out thelarge single container than several small ones. Orderingmaterials in returnable tote bins may maximize theseadvantages.

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Table 4. Waste Minimization Methods for Fabricated Metal Industry Processes

ProcessMaterial Handling

and Storage

Waste Stream

Machining Wastes MetalworkingFluid

Parts Cleaning Solvents

Aqueous Cleaners

Abrasives

Rinsewater

Surface Treatment Process Solutionsand Plating

Rinsewater

Source Reduction Options Recycling Options

Material PreinspectionProper Storage of MaterialsRestrict Traffic through

Storage AreaInventory ControlPurchase Quantities

According to NeedsUse of High Quality

Metalworking FluidDemineralized Water UseConcentration ControlSump and Machine CleaningGasket, Wiper and Seal

MaintenanceCleaning of Metalworking FluidAssigning Fluid Control

.ResponsibilityTank Lid InstallationIncrease in Freeboard SpaceInstallation of Freeboard ChillersCross-Contamination AvoidanceAppropriate Makeup SolutionsSolvent StandardizationConsolidating OperationsMedia Substitution

Test Expired MaterialUsefulness

Filtration of MetalworkingFluids

SkimmingCoalescingHydrocycloningCentrifugingPasteurizationDowngrading

Gravity SeparationFiltrationBatch DistillationFractional DistillationUse as FuelOn- and off -site Recycling

Sludge RemovalUse of Dry Cleaning and

Stripping MethodsMedia SubstitutionUse of Greaseless or Water-Based

BindersUse of Liquid SpraysWater Level ControlSynthetic AbrasivesRack and Barrel System DesignRinse System DesignSpray and Fog RinsesChemical RinsingDeionized Water

Oil SeparationPickling Bath Recycling

Increasing Solution LifeMaterial SubstitutionProcess SubstitutionChemical CoatingMechanical Cladding

and Coating

Reduction in Drag-Out ofProcess Chemicals:Speed of WithdrawalSurface TreatmentPlating Bath Concentrations

Use of Cleaning Baths as pHAdjusters

Metal RecoveryEvaporationReverse OsmosisIon Exchange

Electrolytic RecoveryElectrodialysis

Rinsewater Reuse

I4

Table 4. Waste Minimization Methods for Fabricated Metal Industry Processes (contd)Process Waste Stream Source Reduction Options Recycling Options

Surfactant UseSolution TemperatureWorkpiece PositioningDrag-Out Recovery

System Design Considerations:Rinsetank DesignMultiple Rinsing TanksReactive RinsingFog Nozzles and SpraysAutomatic Flow ControlsRinse Bath Agitation

Precipitating Agents and OtherTreatment Chemicals

Trivalent Chromium UseWaste SegregationSludge DewateringSelection of Clean Processes

Surface Treatmentand Plating(contd)

Paint Application

Treatment Wastes

Case HardeningWastes

Empty Containers

Paint ApplicationWaste

Waste SegregationBulk PurchasingMinimizing ResidualsOverspray Reduction: Reusing Solvent Paint Mixtures

Equipment Modifications Recovery through DistillationOperator Training Recovery through Filtration

Material Substitution ReplacingSolvent-Based Coatings with:Water-Based CoatingsRadiation-Curable CoatingsPowder Coatings

15

Test Expired Material for UsefulnessMaterials having expired shelf-life should not

automatically be thrown out. Instead, this material shouldbe tested for effectiveness. The material may be usable,rather than becoming a waste. Arecycling outlet should befound for left over raw material that is no longer wanted.

Machining WastesSOURCE REDUCTION METHODOLOGIES

A primary problem in metalworking fluid managementis contamination with tramp oil and the problems that resultfrom this. While the best solution for tramp oil problemsis to prevent the oils from entering the metalworking fluid,some contamination will occur as the machines and theiroil seals and wipers wear. This can be reduced throughpreventive maintenance such as periodic seal and wiperreplacement. Optional metalworking fluid performancestarts with a preventive maintenance program that includes:

use of high quality, stable cutting and grindingfluids:

use of demineralized water for mixing purposes:

fluid concentration control;

control of fluid chemistry (pH, dissolvedoxygen, etc.);

fluid contamination prevention;

periodic sump and machine cleaning;

periodic gasket, wiper and seal inspections andreplacements to minimize tramp oilcontamination;

regular cleaning of metalworking fluid throughfiltering or centrifugation, in order to minimizemicrobe growth by controlling tramp oilbuildup; and

assignment of responsibility for fluid control toone person.

Fluid cleaning can be accomplished through filtrationand clarification, using bag, cartridge or disc filters, chipwringers and centrifuges. Periodic addition of specializedbiocides into the metal working fluid can also extend itslife, by combating microbe growth (Zabik 1987; Porter1988).

An irritating problem in many shops is thecontamination of fluids with trash such as cigarette butts,food or food wrappers that find their way into sumps.Better housekeeping procedures, including operator trainingand coverage of sumps with screens or solid covers, canhelp reduce this ongoing problem.

Because water constitutes 90 to 99 percent of watersoluble cutting and grinding fluids, high mineral contentwater can adversely affect fluid performance bydeteriorating emulsions, causing corrosion and enhancingmicrobial growth. Purification of water throughdeionization or reverse osmosis before it is mixed with thefluid can help reduce these problems.

It is important to carefully select the metalworkingfluid most suitable for the particular application, in order tomaximize performance and long fluid life. Fluid selectionshould be done from an overall, plant-wide perspective, inorder to find the best products as well as to minimize thenumber of different fluids in use. With the broad applicationsof some high quality fluids, it is sometimes possible toemploy only one type in an entire plant, although differentapplications in the plant may require different proportionsof water and concentrate.

A periodic schedule of metalworking fluid testing canalert plant staff to deteriorating fluid qualities in time toprevent failure of the fluid. Tests might include analysesfor pH, specific component concentration includingadditives, particulate matter, trampoil, rust inhibitor, biocideconcentrate, and dissolved oxygen. Low pH values indicatelow product concentrations, and thus related problemssuch as increases in metal fines or other suspended solids,and heightened vulnerability to microbe growth and trampoil contamination (Porter 1988).

In order to make informed choices of fluids, it isimportant to know not only about the fluids cutting andgrinding abilities, but also about factors such as theirresistance to bacterial attack, the residues they leave onmachine tools and workpieces, the corrosion protectionthey offer, the health dangers they present, such as skin orrespiratory irritation, and the environmentally hazardouschemicals they contain. Chemically active lubricants areoften used, for instance, that contain chlorine, sulfur orphosphorus (Centrico 1986). Fluids can also containphenols, creosols and harsh alkalies. Tramp oils oftencarry other hazardous contaminants into metalworkingfluid, and can lead to breakdown of the fluid and formationof hydrogen sulfide.

Use of synthetic metalworking fluids can sometimesresult in dramatically increased fluid life. Synthetic fluidsare made up of chemicals such as nitrites, nitrates,phosphates and borates. Synthetic fluids contain only zeroto one percent soluble oils in the fluid concentrate, comparedto 30 to 90 percent soluble oil in non-synthetic metalworkingfluid concentrates. While the lubricity of synthetic fluidsis lower than many non-synthetic fluids, an advantage ofsynthetic fluids is that tramp oils are not able to contaminatethem as easily as non-synthetic fluids, for they are not ableto readily enter the fluidemulsion, which leads to breakdown

16

of the fluids qualities (EERC 1988). Many synthetic fluidsoffer greater thermal stability at high temperatures, resistingoxidation better than non-synthetic fluids.

Gases can sometimes be used in place of coolants,because they offer cooling of workpieces and tools with noworkpiece contamination. Air is the most frequently usedgas, and is employed both in dry cutting or with otherfluids. Nitrogen and carbon dioxide are occasionally usedas well, but their cost is high and therefore their applicationsare limited (Porter 1988).

RECYCLING METALWORKING FLUIDS

In coalescing techniques, the fluid is brought intocontact with an oleophilic (oil loving) medium formedinto a high surface area shape such as corrugated plates orvertical tubes. Oil droplets impinge on the media and clingto it, eventually coalescing to form large droplets that floatto the surface of the fluid and are skimmed off by adjustableweirs. Coalescers are not effective for removing water-miscible hydraulic oils or emulsified lubricating oils, forthese do not readily separate from the metalworking fluid.

By recycling deteriorated or contaminated fluids, costly A hydrocyclone uses centrifugal force to separatehauling and disposal can be reduced. Also, recycling will solid contaminants from the fluid. Waste fluid is pumpedminimize the need for purchase of high priced fluid under pressure into the top of a cone-shaped compartmentconcentrates. While many shops engage off-site recycling in which a vortex is set up. As the spinning fluid acceleratescompanies to handle their spent fluids, it is very feasible for down the cone, solids are forced to the outer wall. Thelarger shops to recycle in-house. The processes recyclers solids move downward and are discharged, while the cleanemploy separate oily wastes from water. The water is fluid is forced by back pressure to move upward throughreleased to the sewer while the oil is refined or used as fuel. the center of the cone. Hydrocyclones can remove particlesIn-house recycling typically has a different focus than off- down to about 5 microns; they cannot, however, efficientlysite: to extend the usable life of metalworking fluids, rather remove small quantities of tramp oil. The advantage of thisthan to separate and refine the oils it contains. Continuous type of system is that it is mechanically very simple andin-house filtration of fluids in machine sumps reduces the relatively easy to operate.requirements for new fluids, avoids recyclers charges, andsaves money by reducing machine downtime for cleaning

Centrifuging involves mechanical rotation of the

and coolant recharge.metalworking fluid, providing several thousand Gs ofseparating force. Centrifugation is able to remove hydraulic

Methodologies for recycling metalworking fluids oils and other emulsified tramp oils as well as free oils.include filtration, ultrafiltration for water removal, Low RPM centrifuges are also used as chip wringers toskimming, flotation, coalescing, hydrocycloning, separate reusable oil clinging to metal chips.centrifuging, pasteurization and downgrading. In gravitypressure and vacuum filtration technologies, the waste

One recycling method gaining popularity is a

collant is passed through a disposable filter to removecombination of pasteurization and low speed centrifuging.

solid particles. Diatomaceous earth filters are also used atWhile this method is promising for certain applications,

times, but their adsorptive properties are so high that theypasteurization is a tremendously energy intensive process,

can actually remove additives from a metalworking fluid.and is only marginally successful in controlling microbe

In skimming separations, the metal working fluid is allowedgrowth. Pseudomonas aeruginosa and Pseudomonasoleovorans are two coolant-attacking bacteria that are

to sit motionless until immiscible tramp oil floats to the notoriously hard to kill. Pasteurization can also cause de-surface, where it is manually removed or skimmed emulsification of oils, and if the metalworking fluid hasautomatically using oil-attracting belts, floating ropes orwheels. If the oil contaminants are fairly miscible, as is the

degraded to the point where it has a gray color and emits a

case with hydraulic oils, or if the coolants in the fluid havehydrogen sulfide odor, pasteurization and centrifugation

emulsified the oils, they will not rise to the surface on theircan only remove the odor and color, but often cannot

own, and other separation techniques must be used.restore the fluids lubricity and corrosion inhibition.

Separation of oil contaminants can sometimes be enhanced Used high performance hydraulic fluids that no longerthrough dissolved air flotation. In this method, the fulfill exacting specifications can often be downgradedmetalworking fluid waste stream is put under high pressure and employed as cutting oils. For instance, certain mil specand air is injected. When the pressure is released, the air hydraulic oils cannot be employed in their originalcomes out of the solution, attaches to the oil and grit in the application once their viscosity has dropped due to polymer

fluid, and floats it to the surface, where it can be skimmedoff.

17

shearing, but if the oils have been kept clean, additives canbe mixed into them to make excellent metalworking fluids.

Parts CleaningSOURCE REDUCTION METHODOLOGIESSolvents

The most common piece of equipment used for solventcleaning is the soak tank, followed by the vapor degreaser.The main methods for reducing waste from both types ofequipment are the same. The two most important sourcereduction goals are to minimize evaporation vapor loss andto maintain solvent quality. By reducing evaporation loss,the composition of the solvent will be maintained as closeas possible to its original composition. By maintainingsolvent quality, the need for replacement is reduced.Halogenated solvents contain chemical stabilizers thathelp prevent acid formation and remove acid contaminantsfrom the bulk of solution. As a solvent is used, its abilityto neutralize or prevent acid formation lessens. Unlessmeasures are taken to maintain quality and prevent asolvent from going acid, the entire quantity of solventwill have to be replaced more often. Measures that areconsidered helpful in maintaining quality and minimizingvapor

.

.

.

.

.

.

.

loss include:

Installation of tank lids

Increase in freeboard space

Installation of freeboard chillers

Avoidance of cross-contamination

Sludge removal

Use of appropriate makeup solutions

Solvent standardization

l Consolidating operations

Besides maintaining quality and minimizing loss,substitutions of hazardous solvents with other media isoften a very effective means of source reduction. Mediasubstitution is discussed at the end of this Source Reductionsection.

Installation of lids on tanks. Lids should be placed onall tanks when they are not in use. By installing a coverduring periods of non-use, vapor degreaser solvent loss canbe reduced by 24 to 50 percent. Additional reductions havebeen achieved by installing covers designed to allow thebath to be used even while the cleaning operation is inprogress. Known as silhouette entries, the openings inthe covers have shapes that match the shapes of the partsbeing degreased and therefore minimize the area for vaporloss. Covers should be designed to slideclosed horizontally

across the open surface. This disturbs the vapor zone lessthan covers that are hinged.

Increase infreeboard space. Freeboard is the distancebetween the top of the vapor zone and the top of the tank.EPA regulations recommend a vapor degreaser freeboardof 75 percent of the tanks width. For shops where airturbulence is present, increasing the freeboard to 100percent can provide an additional reduction. Increasing thefreeboard on other tanks containing volatile solvents aswell as on degreasers is effective in reducing solvent usageand solvent waste stream volumes.

Installation offreeboard chillers in addition to coolingjackets. In this approach, a second set of refrigerated coilsis installed above the vapor degreasers condenser coils.These coils chill the air above the vapor zone and create asecondary barrier to vapor loss. Therefore, special watercollection equipment is also frequently required, due towater contamination of the solvent from frost build-up onthe coils.

Avoidance of cross-contamination of solvent is anissue to be addressed, especially when the types of solventsbeing used have similar sounding names. As little as one-tenth of one percent l,l,l-trichloroethane mixed into atank of trichloroethylene can cause an acid condition andrender the bath unusable for many applications (Smith1981).

Water contamination as well as solvent cross-contamination can lead to acid formation. In addition,water contamination increases diffusion of solventsincreasing evaporative loss. To avoid water contamination,the water separator should be cleaned and checkedfrequently for proper drainage. Next, the temperature ofthe water exiting the condenser coils should be maintainedat 90 to 100F. Finally, parts should be checked to see thatthey do not enter the degreaser while wet. This may call forusing oil-based abrasives and cutting oils in productionsteps prior to cleaning.

Sludge that collects in the bottom of the tank shouldalways be removed promptly. Contaminants such as paintabsorb solvent, dissolve into solution, and reduce cleaningefficiency. Zinc and aluminum fines, which are particularlyreactive in chlorinated solvents, can lead to acid formation,if allowed to collect. Organic soil contamination shouldnot be allowed to exceed 10 percent for cold cleaningoperations and 25 percent for vapor degreasers. Whenthese levels are exceeded, acid formation can occur.

Using appropriate makeup solutions for the solventbath. As solvents are used, their ability to neutralize acidlessens. Often, when an acid acceptance test indicates thata solvent is close to going acid, fresh solvent is added to

18

boost the level of stabilizers in the tank. This, however, is Frequent removal of sludge. Separator units designeda poor practice, since the level of stabilizers in the tank can to remove sludge and particulate matter continuously fromnever be made equal to the level of stabilizers in fresh alkaline or acid baths can reduce waste stream volumes andsolvent. The proper technique is to analyze the solvent and save on disposal costs. One typical unit for alkaline bathsadd specific components rather than fresh solvent. Usually, consisting of a pump, hydrocyclone and sludge retentionthe expense of analysis will be offset by the savings in tank has reduced replacement chemical costs in a steelsolvent for tanks of 500 gallons or more (Dumey 1984). cabinet manufacturing shop by 20 percent, and the timeThis method should also be useful for facilities that recycle interval between dumping and total cleanout of the systemtheir solvent, since distillation removes most stabilizers has been lengthened from four to thirteen weeks (Report tofrom the solvent. Congress 1986).

Solvent standardization. For facilities using a largenumber of cold cleaning tanks, standardizing the solventused would help by increasing the potential for recyclingand minimizing the chances of cross-contamination fromother solvents. Standardizing in this context implies usinga minimum number (preferably one) of types of solvents inall operations in the plant.

Consolidating operations. Once solventstandardization has been implemented, the next step is toconsider consolidating cold cleaning operations into acentralized vapor degreasing operation. While cold cleaningsolvents must usually be discarded when the level ofcontamination exceeds 10 percent, vapor degreasers canoperate up to a level of 25 to 30 percent contamination. Inaddition, vapor degreasers provide much better cleaning,and the parts leave the unit dry.

Use of dry cleaning and stripping methods. Cleaningand stripping of parts can often be accomplished byemploying sand or bead blasting techniques. In onedecorative plating shop, hazardous sludge production wasreduced 75 percent by replacing alkaline stripping of oldpaint and plating layers with sand blasting. The dry wastesproduced were minimal, and were much less expensive todispose of than the sludge (Jacobs 1986).

Abrasives

Other waste reduction techniques based on betteroperating practices include locating cold cleaning tanksaway from heat sources, controlling the amount of heatsupplied to vapor degreasers, avoiding spraying partsabove the vapor zone or cooling jacket, and avoidingsolvent vapor drag-out.

Abrasive powders are usually mixed with an oil-basedor water-based binder and are then applied to a polishing orbuffing wheel. Waste from this operation consists of wornout cloth wheels saturated with abrasive, metal particles,binder, and various oxides. Wastes from vibratory or massfinishing operations consist of abrasive, metal particles,water, and oxides dispersed in a slurry. Alkaline or acidcleaners are some times added to the slurry so that additionalcleaning action is provided. Usually slurries are discardedwhen the abrasive has undergone a given amount of attritionor breakdown. The following source reduction methodsare applicable for abrasive cleaners:

Solvent vapor drag-out from a tank occurs when aworkpiece is inserted or withdrawn from the tank tooquickly. The speed of withdrawal of the work should notexceed 11 feet per minute. In addition, the geometry of theworkpiece can affect drag-out. If the space between thewall of the tank and the workpiece is too narrow, then apiston effect will force solvent vapor out of the tank. As ageneral measure, the cross-sectional area of the work loadshould not exceed 50 percent of the tanks open area.

Use of greaseless or water-based binders for polishingor buffing. When oil-based binders are used, the frictionalheat generated during buffing can cause the binders toburn. This in turn leads to the need for additional workpiececleaning such as alkaline soaking. When properly used,greaseless compounds leave the buffing wheel clean anddry (Dumey 1984). Also, greaseless compositions adherewell to the surface of the wheel, extending wheel life.

Aqueous CleanersAqueous cleaners include, as mentioned above, alkaline

and acid cleaning solutions. Alkaline cleaners are employedto remove organic contaminants from metal surfaces andcan replace solvent cleaners in many applications. Acidcleaners are used to remove oxidation, scale, and rust frommetal surfaces. Aqueous cleaners are most commonlyapplied in heated soak tanks, often with spray units installedif alkaline solutions are being employed. Source reductionmethods for reducing aqueous cleaning wastes include:

Use of liquid spray compositions. Most abrasivecompositions are formulated for use in bar form (the bar isheld against the wheel to apply abrasive). With a liquidspray system, a spray gun applies the compound to thewheel automatically. Since the optimum quantity ofcompound can be maintained on the wheel more easilywith a spray system, wheel wear due to compound deficiencyand compound waste due to over-application are minimized.Also, since spray compounds are usually water-based,there should be no need for subsequent cleaning due toburned binder material deposits on the workpiece.

19

Careful control of water level in mass finishingequipment. If insufficient water is used in mass finishingoperations, work items leaving the equipment will be dirtyand the attrition rate of the abrasive and its replacementfrequency will increase (Durney 1984). Water levels mustthus be carefully metered in these operations.

Synthetic abrasives. Abrasive cleaning and deburringof workpieces is sometimes accomplished by putting bothworkpieces and abrasive grit into a tumbling barrel androtating until the parts are finished. Beach sand and riverrocks are often used as abrasives. These will grind down,however, into a large volume of fine silt mixed with metalfines that must be treated as a hazardous waste. Thisproblem can be reduced by using aluminum oxide grit inplace of beach sand, and ceramic abrasive deburring materialin place of river rock.

R i n s e w a t e rWater is used to remove or dilute cleaning solutions

that are dragged out with cleaned parts. If these cleaningsolutions are notremoved, they can affect the quality of thework and contaminate subsequent cleaning and processingoperations. Since rinsing is essentially a process of dilution,the general trend in the past was to use large volumes ofwater. Today, however, efficient rinsing is required toachieve the proper level of dragout dilution and alsoconserve water. By conserving water, capital and operatingcosts for waste treatment units are minimized. Sourcereduction methods forreducing cleaning solution drag-outand the amount of water required include:

Proper design and operation of rack system

Proper design and operation of barrel system

Proper design and operation of rinse system

Installation of spray rinses

Installation of fog nozzles

Chemical rinsing

Deionized water use

Proper Design and Operation of Rack SystemThrough proper design and operation of a rack system,

solution drag-out can be significantly reduced. Partsshould be racked so that the surface is nearly vertical andthe longest dimension is horizontal. Also, the lower edgeshould be tilted from the horizontal (this allows run-off tooccur at a comer rather than the entire edge). Withdrawalfrom the cleaning solution should be made slowly and thepart allowed to drain over the tank for a minute or two.Additional drainage time can be provided by installingsloped dram boards at the end of the tank.

For items with cup-shaped recesses, drainage can be adifficult problem. If the part cannot be positioned to allowfor drainage, special measures must be taken. Some ofthese measures include drilling or repositioning drainholes in the part, tilting the rack as it is removed from thebath, and/or installing air jets to blow off cleaner solutionfrom the part.

Rack maintenance is also important. If rack insulationis cracked, solution can be carried out in the gaps andfissures. In addition, exposure of the rack metal duringelectroplating operations can lead to contaminated bathsolutions. Uninsulated racks used for cleaning should bestripped regularly since the rough surface will hold solutionby capillary action.

Proper Design and Operation of Barrel SystemWhile barrels are normally fully immersed during

electroplating operations, maximum rinsing efficiencyoccurs when the barrel is only immersed partially. Theproper depth and rate of rotation depends on many factorsbut normally occurs when the barrel is immersed to about38 percent of its diameter. In most plants, rinse tanks forbarrel operations are designed and operated the same wayas electroplating baths (i.e., the barrel is fully immersed).After immersing the barrel, it is raised over the tank whilerotating and is allowed to drain. At a minimum, twocounterflow cold rinses and a final hot rinse should alwaysbe used.

Proper Design and Operation of Rinse SystemSince the process of rinsing is one of dilution, good

practice should assure that the rinse tank is well mixed atall times. Tanks can be mixed by agitating the water withoil-free air, introducing fresh water at the bottom of thetank, and by other mechanical means. The water in the tankshould exhibit a rolling turbulence without undue splashing.For cleaners that are not very easy to rinse off, heated rinsetanks are often employed.

Once agitation and complete mixing in the tank havebeen assured, the next concern is the number of rinses.Pinkerton and Graham (1984) presented evidence that useof a second rinse can cut water requirements by over 90percent. For facilities with limited floor space, single rinsetanks can often be converted to multiple rinses by weldingone or more dams across the tank. Most modem facilitiesare designed with multiple rinse tanks after each cleaningoperation.

If the spent rinse water is used as makeup for evaporationfrom the process bath, hazardous chemicals are preventedfrom entering the waste streams, reducing the risk to theenvironment that they pose. If evaporation from theprocess bath is not great enough for the bath toaccommodate

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the quantity of spent rinsewater produced, an evaporator (PCE) and trichloroethylene (ICE) are currently beingcan be added to the process bath. Alternatively, some replaced by 1,l,1,-trichlotoethane in many applications.shops raise the temperatures of their process baths at night, Benzene and other toxic aromatics are often replaced byadding the spent rinsewater as makeup the next morning. aliphatic solvents such as Stoddard naphthas.

Installation of water sprays on rinse tanks. Forinstallations with single rinse tanks and limited space,rinsing efficiency can easily be increased by installing aspray system. By spraying work items with fresh water asthey are raised above the rinse, the equivalent of an extraone-half counterflow bath is obtained. Sprays should beproperly designed to provide uniform coverage on the partand not produce undue splashing. Spray rinsing is alsobeneficial on multitank counterflow systems where eachspray unit is fed water from the succeeding rinse tank.

Other substitutes may include dibasic acid esters,terpenes, amines, or alcohols. Prerequisites include lowflammability, low vapor pressure, low toxicity, highsolvency and low cost.

Installation of fog nozzle on heated aqueous cleanertanks. A fog nozzle is a special high pressure water sprayunit that produces a finely atomized mist of water or fog.Since the water is so finely dispersed, only a small amountof water is used compared to a normal spray unit. Therefore,fog nozzles can be used over heated cleaner tanks forrinsing work items without introducing a surplus of water.Two main benefits of using fog nozzles are that: 1) theyhelp cool the part so that the cleaning solution has lesschance of drying on the part: and 2) they reduce drag-outby diluting the solution retained on the part.

Terpenes, essential oils isolated from plants throughgentle heating or steam distillation, are especially promisingas potential substitutes for many solvents as well as aqueouscleaners. Terpenes are less toxic and more biodegradablethan most solvents. Limonene cleaners, commerciallyimportant terpenes made from oils of lemon or orange, arelisted as GRAS (Generally Recognized As Safe) substancesin the Code of Federal Regulation (Hayes 1987). Limoneneshave tested favorably against solvents, solvent emulsions,and alkaline cleaners for removal of heavy greases,carbonized oils and oily deposits.

Reported disadvantages of terpenes include difficultyin separating oily wastes from them in order to recycle thecleaning solution. Ultrafiltration is being tested as onemeans to recover the cleaning solution. In addition, becauseof their low volatility, terpenes are not usable in vapordegreasing operations.

Chemical rinsing. In some facilities, rinse water froman alkaline cleaning operation is reused to rinse parts froman acidcleaning operation. The basic premise is to combinerinsing and waste treatment in one operation. While thisprocedure reduces the amount of waster rinse water generatedand the degree of wastewater treatment required, thepotential for contaminating the parts with metal hydroxideprecipitates is increased. Therefore, this method should belimited to those parts not requiring rigorous cleaning.

Use of deionized water for rinsing. Use of regular tapwater is a major source of impurities in any closed loopsystem. By employing deionized water, many rinses canbe reclaimed using a simple evaporation system. In addition,use of deionized water can extend plating bath life byreducing impurity drag-in as well as the number of rejectsproduced. Many packaged systems commercially availablecan supply deionized water of adequate quality becausemost electroplaters do not require extremely high puritywater.

Surfactants added to terpenes forms emulsifiablecleaning compounds that are water rinseable. Products onthe market include BIOACT (manufactured by PetrotemInc.), a substitute for chlorinated solvents andchlorofluorocarbons (BIOACTs atmospheric ozonedepletion factor is zero since it contains no chlorine orbromine) and Citrikleen, a limonene manufactured byPenetone (Tenafly ,New Jersey) that can be sprayed, foamedor brushed on workpieces, or used in immersion baths.

Replacement ofsolvents with aqueous cleaners. Thereis currently a major effort to switch from cold tanks usingnon-halogenated solvents and vapor degreasers usinghalogenated solvents to cold tank alkaline and emulsioncleaners. This changeover is largely driven by regulatorypressure to reduce air emissions from degreasing operations.

Numerous examples exist of successful substitutionof aqueous cleaners for solvents. In one case, an electronicmanufacturing facility that originally cleaned printed circuitboards with solvents found that by switching from asolvent-based cleaning system to an aqueous-based system,the same operating conditions and workloads could bemaintained. The aqueous-based system cleaned 6 timesmore effectively. This resulted in a lower product rejectrate, and eliminated a hazardous waste (USEPA 1983a).The Torrington Company in Walhalla, S.C. also reduced

Media SubstitutionReplacement of solventswith less toxic solvents. Since

the choice of which cleaning medium to use is seldom clearcut, many opportunities exist for substituting one cleaningmedium with another. Toxic solvents can often be replacedwith safer alternatives. For instance, perchloroethylene

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the hazard posed by its waste streams by replacing 1,l,l-TCA used to clean metal bearings with a considerably lessexpensive alkaline degreaser that employed a two-stagewasher and hot air drier (Kohl, Moses and Triplett 1984).

Emulsion cleaners combine solvent cleaning withaqueous cleaning so that water-immiscible solvent isdispersed in the aqueous phase with the aid of emulsifiers,surfactants and coupling agents. The large surface area ofthe dispersed solvent phase can sometimes attain resultsachievable with pure solvent. Solvent vapor pressure andevaporation losses are suppressed in emulsion cleaners.Disadvantages include residual oil film on the part (whichnecessitates an additional cleaning step in applicationswhere a high degree of cleanliness is required), relativelylow saturation capacity, and difficulties in recycling byseparation of oil and reconstituting the cleaner.

Replacement of solvents with mechanical and thermalalternatives. Solvents are often used to dry parts followinga waterrinse operation. As an alternative, air blast systemsutilizing a high velocity air jet can blow water droplets andother contaminants from glass, metal, or wood parts. Drystripping and cleaning using a plastic or sand blast mediato clean and strip parts can reduce disposal costs and waterusage and has been shown to significantly reduce laborcosts. The blasting media can also be recycled. Hill AirForce Base in Ogden, Utah, has successfully employedplastic beads propelled by high pressure air jets to removepaint from aircraft exteriors. Besides eliminating generationof hazardous waste, the use of bead blasting improvedpersonnel working conditions, was easier to perform thansolvent paint stripping, cost less and used less raw material.Other abrasive blasting materials, such as sand and CO,pellets, are also used for paint stripping.

Replacement of aqueous cleaners and strippers withabrasive media. Use of abrasivescan eliminate the needfor aqueous cleaners in some situations. In one example,a manufacturer that cleaned nickel and titanium wire in analkaline chemical bath installed a mechanical abrasivesystem. In another situation, a decorative electroplatingshop reduced its hazardous sludge buildu