Pollution Prevention & Best Management Practices

Dry Machining
Dry machining eliminates fluid management issues such as need for fluid purchase and management costs, the need for continuous treatment of fluid and maintenance of fluid composition, and the use of biocides. Evaluate whether fluids are necessary. If parts arrive already coated with fluid, application of more fluid may be unnecessary. Evaluate whether operating conditions are severe enough to warrant the use of fluids. Process changes that make dry machining possible are:

• Keep tool clean, sharp, and within tolerances.
• Change tool bits to alternative materials such as low friction coatings and high-temperature wear resistant materials.
• Employ compressed air to remove chips from the work area.

Metalworking Fluid Management
Metalworking fluids are a significant source of waste from metal cutting and shaping processes and often contaminate subsequent process baths. This section provides an overview of input substitutions, process modifications, and operation and maintenance changes that can reduce the generation of waste associated with the metalworking fluids.

Fluid Selection
The first question that needs to be resolved before beginning the fluid selection process is whether a fluid is required.

Many firms lack a formal process for selecting the metalworking fluids they use. Often fluid selections are made based only on the fluids functionality in the machining process. The fluid can have a significant pollution impact on later processes if these processes are not considered during the selection process. Other things to consider in the selection process are:

• environmental issues such as the recyclability and are they from renewable resources (vegetable-based fluids)
• healthy and safety implications of the fluid, such as toxicity and exposure
• is the fluid chlorine-free, as chlorine has an impact on the waste designation of spent fluids

Fluid application
Firms can minimize waste generation associated with metalworking fluid use by reducing the amount of fluid applied in any given process. Firms can greatly reduce fluid use and associated waste issues by modernizing and optimizing application equipment. Firms should reduce the volume of fluid applied to the greatest extent possible by:

• analyzing the lubrication and cooling requirements of the machining process
• using bulk fluid delivery systems only when absolutely necessary
• using micro-dispersion and other low- volume fluid applicators with metalworking fluids that have higher lubricating qualities

Fluid Maintenance
Metalworking fluids require constant and periodic maintenance to monitor fluid properties, remove contaminants, and add make-up fluid. A well-crafted maintenance regimen can greatly extend the life of metalworking fluids or extend the fluid life indefinitely. Elements of a successful maintenance regimen include:

• testing of the fluid for pH, concentration, and microbial growth
• adding make-up fluid, water, biocide, or other additives where needed
• filtering the fluid to remove both oil and particulate contaminants

Removing particles and tramp oils by filtering reduces sites for microbial growth. Filtering equipment vary greatly in scale, complexity, and cost. Selecting the right type of filtering equipment for a given application is dependent on a number of factors including:

• the type of fluid used
• the type of material being machined
• the machining process
• the size of the particles formed
• the rate of contaminant loading
• the cleanliness requirements of the metalworking fluid for the specific application

Sump Maintenance
Metalworking fluid sumps should be covered to prevent airborne microorganisms out of the fluid. Covers also keep out trash such as dust, cigarette butts and food which foul fluids.

Periodical cleaning of metalworking fluid (MWF) sumps and trenches keep them free of solid matter that can hamper fluid flow. Solid materials provide excellent sites for microbe growth and may clog MWF delivery lines. Sumps and trenches should be disinfected when MWF is removed. Disinfectant options include bleach, alcohol or steam cleaning. Without disinfecting, new MWF fluid will be inoculated with leftover bacteria when it is added to the sump, causing the fluid to prematurely degrade.

Distilled or Deionized Water
Water quality affects the performance of a coolant more than any other factor (Willa 1997). Water is mixed with soluble, semi-synthetic, and synthetic fluids, and make-up water is added on a regular basis to replace water lost to evaporation. Minerals present in this water can cause numerous problems if not removed before the water is combined with the oil. Hardness, calcium and magnesium ions, can promote the formation of insoluble soaps that may clog lines and filters or even plate out on machines. Hard ions may also react with fluid additives, such as surfactants and wetting agents, making them unavailable.

The three most common treatment technologies used to remove impurities from incoming water are de-ionized water (DI), reverse osmosis (RO), and ultra-filtration (UF). Water softening chemicals are not recommended as a means of removing minerals from water that is being used in coolants because these chemicals do not remove corrosive sulfate and chlorine ions that are present.

Machine Maintenance & Spill/Leak Control
Hydraulic and metalworking fluid leaks are a source of pollution in the metal fabricating process. Over time, machine gaskets, seals, and wipers become worn and cracked, causing fluid to drip onto the floor, machine parts, or in the case of hydraulic fluids, into the metalworking fluid baths. Drips to floors and machine parts require cleanup that is usually performed using mops, granular absorbent, rags, or absorbent pads. In the case of hydraulic fluids dripping into metalworking fluid baths, the tramp oil acts as a site for bacterial growth, and also causes smoke and odor problems, all of which result in the need to change the fluid.

To eliminate these problems firms should design a gasket, seal, and wiper maintenance program that is appropriate to the application. Wear on the parts will depend upon the severity of the application and the fluid environment, and the program should be tailored appropriately. A maintenance program might include:

• a list of all plant equipment, including equipment location
• an operating schedule for each piece of equipment
• a service history (days since last tune up)
• a maintenance history or log (days since last breakdown)
• maintenance manuals for all equipment (MA TURI 1996)

The use of pumps, spigots and funnels for transferring MWF will reduce the amount of lost fluid and the risk of spilling fluids. Using absorbent pads when spills do occur will cut down on the amount of absorbent material that must be discarded as hazardous waste, and save money in fresh absorbent and waste handling costs since the pads can be wrung out and reused. The use of mops for gross clean-up of leaks and spills should be discouraged because a small volume of spilt metalworking fluid added to a large volume of water will greatly increase the volume of waste that must be treated as oily contaminated waste or hazardous waste.

Recycling Systems
Metalworking fluids require constant and periodic maintenance to monitor fluid properties, remove contaminants, and add make-up fluid. Elements of a successful maintenance regimen include:

• testing of the fluid for pH, concentration, and microbial growth
• adding make-up fluid, water, biocide, or other additives where needed
• filtering the fluid to remove both oil and particulate contaminants

A well-crafted maintenance regimen can greatly extend the life of metalworking fluids or extend the fluid life indefinitely. By testing the fluid continuously or periodically, maintaining the appropriate proportion of ingredients, and removing metal particles and tramp oils using filtering equipment, degradation of fluid by microbes may be prevented. If particles and tramp oils are not removed by filters, they provide sites for microbial growth and may damage the tool and fluid circulating system.

Filtering equipment vary greatly in scale, complexity, and cost. Selecting the right type of filtering equipment for a given application is dependent on a number of factors including:

• the type of fluid used
• the type of material being machined
• the machining process
• the size of the particles formed
• the rate of contaminant loading
• the cleanliness requirements of the metalworking fluid for the specific application

Chips Management For metal wastes in the form of chips or swarf, firms should employ a means of removing metalworking fluid from the metal wastes. Chips and swarf can entrain significant amounts of fluid, which makes scrap dealers reluctant to accept these types of scraps. These wastes may entrain as much fluid as metal particles. Fluids pose potential environmental and health and safety risks and potential liabilities for scrap dealers and also cause problems with processing the scrap.

The mechanism employed to remove excess fluid depends largely on the size of the particles and the type of metalworking fluid that is present. At a minimum, chips and swarf should be initially stored in a container that allows for drainage of fluid. Centrifuges can remove mineral oil-based metalworking fluids from chips and swarf. Hydroclones are effective at separating very fine particles from water-based fluids. Compaction is another way of removing metal working fluids from fine particles. These processes ensure the recovery of as much of the metalworking fluid as possible and increase the value of the metal waste.

Air Handling Systems
Air handling systems can be a significant source of inefficiency in metal fabrication and machining shops due to the energy required to run these systems. Lines should be inspected periodically for leaks and shop personnel should be encouraged to eliminate frivolous uses of compressed air.

Metal Waste Reduction
Metal parts are fabricated from metal stock that comes in the form of blanks, billets, or sheets. When the parts have been formed or cut from stock, there is often bulk scrap left over that is then sold to a scrap dealer for a small fraction of the initial stock purchase price.

The blanks, billets, or other forms of metal stock should be selected specific to each work order to reduce the generation of unnecessary scrap. Often firms are required to perform unnecessary cutting steps due to stock being larger than needed. Firms should work with the suppliers when this is the case so that scrap from rough stock cutting can be minimized. Optimizing pattern layout is another way of reducing metal scrap waste and can be performed using existing software packages, such as CAD/CAM.

Pollution Prevention Technologies

Laser Drilling and Cutting
Lasers are beams of monochromatic light focused precisely to produce a very intense energy beam. Lasers are very flexible and can be easily and rapidly controlled in intensity and direction using computer control systems. Lasers may be used for drilling and cutting, as well as welding, heat treating, and engraving. They can replace mechanical metal removal processes as well as arc and gas welding, and induction, flame, and plasma hardening. Wastes reduced by using laser processing include metal cutting fluids, wastewater, slag, and scale.

Water Jet Cutting
Waterjet cutting is used to replace conventional mechanical cutting methods as well as laser, plasma, or oxyfuel cutting. Waterjet systems use special pumps, pressure nozzles to create a thin, high velocity, high pressure stream of water. For very hard materials, abrasives may be added to enhance cutting action of the water stream.

Although there is limited information available on the overall environmental performance of waterjet cutting, it is known that switching to waterjet cutting may reduce or eliminate certain wastes, including: metal cutting fluids, contaminated wastewater, and slag and scale from oxyfuel cutting. Capital costs of waterjet cutting systems are in the range of $165,000 to $600,000, and therefore are probably not a good option for small shops.

EDM
EDM can be used to cut various complex shapes, particularly in hard materials, such as tool steel. Metal is removed by a series of rapidly recurring electrical discharges between the cutting tool (electrode) and the workpiece in the presence of a dielectric fluid. EDM eliminates the need for heat treating metal products and parts, and possible subsequent distortion associated with heat treating. It is particularly effective on extremely hard materials, parts with complicated geometries, or very thin materials.

EDM may be used for cutting, drilling, die-making, punching, and mold-making. It may be used as a replacement for mechanical milling, cutting, and drilling as well as laser cutting and drilling. The major waste stream reduced by using EDM is broken cutting and drilling tools. This can be significant in applications where tool breakage is high. Capital costs for EDM equipment is in the range of $100,000 to $200,000 per unit and hence it is usually not an option for small shops.

Replace Salt Bath Furnaces
Fabricators should replace barium and cyanide salt heat treating with carbonate/chloride carbon mixtures or with furnace heat treating. The various types of furnace heat treating are preferable to all types of salt bath heat treating because they do not generate the spent salt baths that must then be handled as hazardous waste. The sections below discuss a few alternatives.

Adhesives

Water based adhesives
Water-based adhesives are used to a limited extent in the joining of metals due to their lower peel and shear strength. They are formulated from rubber components with water as the carrier fluid. Curing may be performed in ovens or under ambient conditions. Water-based adhesives may be applied using existing equipment, if the equipment is compatible.

For facilities that have a lot of application equipment, water-based adhesives are preferable to other alternatives because they may use the same application equipment, eliminating the need to train employees on new application equipment.

Costs of purchasing water-based adhesives are 15 to 20 percent lower than solvent-based. Overall operating costs for water-based are estimated to be 33 percent lower than solvent-based systems (1993 dollars). Other cost issues associated with switching to water-based adhesives include:

• capital and operating costs of pollution control equipment are eliminated
• if adhesive formulated onsite, may not need to upgrade equipment
• handling costs for new aqueous waste stream if switching from a solvent-based system

Hot melt adhesives
Hot melt adhesives are solvent-free and eliminate VOCs. Standard hot melts are applied with a slot die or roll coater, and PUR hot melts may be applied in dots or thin glue lines, allowing them to replace mechanical fasteners in various applications (PPRC 1998b).

Application equipment used for solvent-based systems are incompatible with hot melt adhesives due to the temperatures of hot melt systems. Employees would require training on the operation of new application equipment if a switch to hot melt adhesives is made.

Hot melt adhesives should be compared to solvent-based adhesives on the basis of dry solids applied and coating thickness yielded per pound. Capital equipment costs for hot melt systems are much less than solvent-based systems when control equipment is included in the analysis. Overall, operating costs are less for hot melt systems because of decreased processing time.

Radiation cured adhesives
The two most widely used radiation-cured adhesives are ultraviolet (UV) and electron beam (EB) systems. UV adhesives are best suited for small-scale applications, and EB adhesives are better suited for high-volume operations. EB adhesives have higher installation costs, but unlike UV, may cure the area between two substrates. UV adhesives may be applied on heat sensitive substrates, and are unaffected by ambient temperature or humidity (PPRC 1998c). Speed and feed rates of radiation-cured adhesives can be increased and production line lengths may be significantly decreased.

Like hot melt adhesives, radiation-cured adhesives should be compared to solvent-based adhesives on the basis of dry solids applied and coating thickness yielded per pound. Capital costs for radiation-cured systems are 27 percent less than solvent-based systems (based on 1994 dollars), not including pollution control equipment required for solvent-based systems. Regulatory, hazardous waste management and disposal costs associated with radiation-cured adhesives are much less than solvent-based adhesives.