Numerous practices have been developed to eliminate or minimize discharges of pollutants from the metal finishing industry. Successful source reduction measures have been implemented to eliminate cyanide plating baths, as well as substitute more toxic solvents with less toxic cleaners.
In many cases, cleaning with solvents has been eliminated altogether through the use of water-based cleaning supplemented with detergents, heating, and/or agitation. Other source reduction measures have been implemented to minimize the discharges of toxic materials. For example, drain boards and splash plates have been commonly installed to prevent drips and spills. Additionally, the design of immersion racks or baskets and the positioning of parts on these racks or baskets have also been optimized to prevent trapping of solvents, acids/caustics, or plating baths.
The utilization of recycle and reuse measures has also been commonly used. Many facilities have been able to minimize water use and conserve rinsewaters and plating baths by measures including the following20,21:
1. Utilizing a dead rinse, resulting in the concentration of plating bath pollutants. This solution may be reused directly or further purified for reuse.
2. Conserving waters through countercurrent rinsing techniques.
3. Utilizing electrolytic recovery, customized resins, selective membranes, and adsorbents to separate metal impurities from plating baths, acid/caustic dips, and solvent cleaning operations.
These operations and measures not only extend the useful life of solutions, but also prevent or reduce the discharge of pollutants from these operations. Two industries have implemented best management practices that resulted in substantial cost savings and pollutant reductions. Emerson Electric implemented a program that resulted in savings of more than USD 910,000/yr (in terms of 2007 USD)22 and reductions in solvents, oxygen-demanding pollutants, and metals. Best management practices implemented by a furniture manufacturer in the Netherlands resulted in a reduction in metals discharged and a decrease in water use. A detailed discussion of these programs is provided in the following paragraphs.
Emerson Electric, a manufacturer of power tools, implemented a Waste and Energy Management Program to identify opportunities for pollution prevention. An audit resulted in the following actions5:
1. Development of an automated electroplating system that reduced process chemical usage by 25%, process batch dumps by 20%, and wastewater treatment cost by 25%.
2. Installation of a water-based electrostatic immersion painting system to replace a solvent-based painting system. The water-based system resulted in a waste solvent reduction of more than 95%.
3. Installation of an ultrafiltration system that recovers 30 kg/d (65 lb/d) of waste oil and purifies 1135 kg/d (2500 lb/d) of alkaline cleaning solution for reuse, which resulted in a reduction of 5-day biochemical oxygen demand (BOD5) loadings to the treatment system of 200 kg/ month (370 lb/month). This avoided the need for installation of additional treatment.
4. Installation of an alkaline and detergent and steam degreasing system, which resulted in a reduction in waste solvents by 80%.
In addition to the reduction of pollutants, Emerson realized annual costs savings of USD 835,000 (in terms of 2007 USD)22 in reduced raw material use, USD 2900 in reduced water use, and USD 68,500 in reduced waste disposal.
A furniture manufacturer in the Netherlands reduced metals in its effluent by switching to cyanide-free baths, allowing for longer drip times, using spray rinsing, reusing water, and implementing a closed cooling system. These best management practices, complemented by the installation of treatment technology, reduced metals in the effluent from 945 to 37 kg/yr. Water use also decreased from 330,000 to 20,000 m3/yr.
1.7.3 Waste Minimization in Electroplating
The Michigan Department of Environmental Quality recommended the following procedures for waste minimization in the electroplating industry23:
1. Slow line to an 8 s count for removal from baths, which drastically reduces the dragout and in turn reduces waste in rinsewater. Rinsewater flow can now be reduced and ultimately the amount of sludge generated can be reduced.
2. Hold the rack for a 10 s count over the bath, during which time the majority of drips will fall. By doing this you will reduce waste in rinsewater.
3. Put drip catchers between the baths to catch and return any solution to the bath. This will also eliminate most of the buildup between the baths and ultimately reduce the cleanup time and waste generated.
4. Use the rinse bath water again in a different area. For example, if there is a line with a chromic acid etch bath followed by counter-flow rinse baths and a neutralizer bath followed by counter-flow rinse baths, use the dirtiest rinse after the neutralizer bath and pipe it to the rinse baths after the chromic acid tank. This saves water and reduces sludge.
5. Spraying or aerating the rinses uses less water and does a better job. Also, counter-flow rinses will save water.
6. Assess wastewater treatment chemicals, and replace the chemicals that create large volumes of sludge with chemicals that do not.
7. If there is a three-bath rinse after a metal bath, leave the first rinse as a dead bath and use as make-up for the metal bath.
8. Cost out a dryer for the sludge to reduce the volume of sludge.
9. Look at metal recovery online and either reuse or sell it as scrap.
10. Look at sending your waste to a smelter who recovers metals from dried sludge. Separate wastewater treatments may be needed for metal separation.
1.8 primary metals
Primary metal industries include facilities involved in smelting and refining of metals from ore, pig, or scrap; rolling, drawing, extruding, and alloying metals; manufacturing castings, nails, spikes, insulated wire, and cable; and production of coke. Major subcategories include blast furnaces, steel works, rolling and finishing mills; iron and steel foundries; primary and secondary smelters and refiners of nonferrous metals such as copper, lead, zinc, aluminum, tin, and nickel; establishments engaged in rolling, drawing, and extruding nonferrous metals; and facilities involved in nonferrous castings and related fabricating operations. The main processes common to metal forming operations and the wastes that are typically generated are discussed in the following5:
1. Sintering. This process agglomerates iron-bearing materials (generally fines) with iron ore, limestone, and finely divided fuel such as coke breeze. The fine particles consist of mill scale from hot rolling operations and dust generated from basic oxygen furnaces, open hearth furnaces, electric arc furnaces, and blast furnaces. These raw materials are placed on a traveling grate of a sinter machine. The surface of the raw materials is ignited by a gas and burned. As the bed burns, carbon dioxide, cyanides, sulfur compounds, chlorides, fluorides, and oil and grease are released as gas. Sinter may be cooled by air or a water spray at the discharge end of the machine, where it is then crushed, screened, and collected for feeding into blast furnaces. Wastewater results from sinter cooling operations and air scrubbing devices that utilize water.
2. Iron making. Molten iron is produced for steel making in blast furnaces using coke, iron ore, and limestone. Blast furnace operations use water for noncontact cooling of the furnace, stoves, and ancillary facilities and to clean and cool the furnace top gases. Other water, such as floor drains and drip legs, contribute a lesser portion of the process wastewaters.
3. Steel making. Steel is an iron alloy containing less than 1% carbon. Raw materials needed to produce steel include hot metal, pig iron, steel scrap, limestone, burned lime, dolomite fluorspar, and iron ores. In steel-making operations, the furnace charge is melted and refined by oxidizing certain constituents, particularly carbon, in the molten bath, to specified levels. Processes include the open hearth furnace, the electric hearth furnace, the electric arc furnace, and the basic oxygen furnace, all of which generate fumes, smoke, and waste gases. Wastewaters are generated when semiwet or wet gas collection systems are used to cleanse the furnace off gases. Particulates and toxic metals in the gases constitute the main source of pollutants in process wastewaters.
4. Casting operations. This subcategory includes both ingot casting and continuous casting processes. Casting refers to the procedure of turning molten metal into a specified shape. Molten metal is distributed into an oscillating, water-cooled mold, where solidification takes place. As the metal solidifies into the mold, the cast product is typically cooled using water, which is subsequently discharged.
5. Forming operations. Forming is achieved by passing metal through cylindrical rollers, which apply pressure and reduce the thickness of the metal. Rolling reduces ingots to slabs or blooms. Secondary operations reduce slabs or blooms to billets, plates, shapes, strips, and other forms. Cooling and lubricating compounds are used to protect the rolls, prevent adhesion, and aid in maintaining the desired temperature. Hot rolling generates wastewaters laden with toxic organic compounds, suspended solids, metals, and oil and grease. Cold rolling operations, occurring at temperatures below the recrystallization point of the metal, require more lubrication. The lubricants used in cold rolling include more concentrated oil-water mixtures, mineral oil, kerosene-based lubricants (neat oils), or graphite-based lubricants, which are typically recycled to reduce oil use and pollutant discharges. Subsequent operations may include drawing or extrusion to manufacture tube, wire, or die casting operations. In these operations, similar pollutants are discharged. Contaminated wet scrubber wastewaters may also be generated from extrusion processes but to a lesser degree than in iron- and steel-making and sintering operations.
6. Acid pickling. Steel products are immersed in heated acid solutions to remove surface scale during pickling operations. This generates wastewater from three sources:
(a) Rinsewater used to clean the product after immersion in pickling solution
(b) Spent pickling solution or liquor
(c) Wastewater from wet fume scrubbers
The first source accounts for the largest volume of wastewater but the second source is very acidic and contains high concentrations of iron and heavy metals.
7. Alkaline cleaning. This process is used when vegetable, mineral, and animal fats and oils must be removed from the metal surface prior to further processing. Large-scale production or situations where a cleaner product is required may use electrolytic cleaning. The alkaline cleaning bath typically contains a solution of water, carbonates, alkaline silicates, phosphates, and sometimes wetting agents to aid cleaning. Alkaline cleaning results in the discharge of wastewaters from the cleaning solution tank, and subsequent rinsing steps. Potential contaminants include dissolved metals, solids, and oils.
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