During 1994, 1996 and 1997, I visited 17 captive Latin American plating shops varying in size from small to medium. I was selected by the client and CESO to act as an advisor on improving finishing and plating processes as well as optimizing pollution prevention control. Also, the client asked that I provide guidance on a variety of existing and newer pollution prevention technologies as well as conventional destructive treatment methods such as cyanide oxidation and chromium reduction
During the last assignment of 1997, I visited 13 captive shops in one city during a four-week period. Previously, in another six-week assignment, three captive plating factories were assessed and analyzed. In the third project that lasted four weeks, only one captive company was evaluated.
Table I summarizes finishing and plating processes involved in the 17 shops. Both rack and barrel plating systems are used in some shops. In addition, one company does automatic vacuum metallizing over plastic and zinc die-cast parts. Another company has a modern automatic electrostatic painting department for finishing plated steel wire products.
All the companies have hand-operated plating lines except one large hot-dip galvanizing company, which has a self-designed automatic zinc cyanide plating line for steel tubes. Two other companies have manual hoist lines for zinc, nickel and chromium plating. All but one of these lines had improper tank arrangements with cross contamination. All of the 17 plating departments, except three shops, had shops resembling those of the 1940's to 1970's, with aged or tapped rectifiers, dark rooms, little or no ventilation, no proper tank arrangement, broken or rotten wooden catwalks and water covering the floor.
None of the 17 enterprises met metal finishing industry regulations for hazardous waste discharge. All discharge numbers were greater than their country's and/or U.S. or Canadian environmental laws and regulations. The 13 captive enterprises discharged mixed wastewater streams into the sewer, which runs into the city's river. The remaining shops discharge into the municipal sewer system. None of the companies (except one) use simple dragin and dragout technology, except electrowinning on three gold-plating operations because of the value of gold.
Some of the major problems in the facilities are as follows:
- Nearly every aspect of the plating process is antiquated.
- The cleaning and plating cycle/flow is not adequate in all the 17 companies. Floating oil in the baths is a problem in a few companies because oil, grease, finger marks and smut removal are not effective and overall plating quality is not satisfactory.
- All companies use well water for rinsing; however, they try to use city tap water in the solution makeup. None use DI water.
- Fourteen companies are using cyanide solutions for copper, brass, zinc, gold, silver and stripping metals without efficient ventilation.
- Nine companies are using hexavalent chromium. Not one has an effective exhaust system, scrubber or mist eliminator for recovery of chromic acid.
Other essential information about these companies includes:
- Years in business: 25 to 48 years, except two companies seven and 11 years.
- Shifts: one to two.
- Plating employees: 1 to 50.
- Captive employees: 25 to 650.
- Shop appearance and equipment: All but one have a 1940's to 1970's style setup.
- Tank size: 300 to 3,000 liters (max); some rinse tanks as small as 150 liters.
- Degreasers: Some are using solvents such as trichloroethylene, diesel or gasoline in a drum or pail. All are not using a 60-65C hot alkaline cleaner. They are using it as a degreaser only.
- Plating chemicals: Most are locally made/mixed with a German formula, using German and Italian technology but American application methods.
- Wastewater flow rate (mixed): One to 60 gpm.
- Inhouse laboratory: Seven out of 17 companies send samples outside occasionally, or plate without analysis.
Products plated. A wide range of styles and products are plated for household, commercial or industrial use. Substrates include steel, plastic and zinc die cast and items such as locks, boxes, keys, screws, frames, electrical metal parts, appliance parts, wire fabricated items, bicycle and motorcycle parts and hot-dip galvanized parts such as tubes, wire bundles, cable trays, beams, large plates, ladders and telephone poles.
Problems. The major concernsfaced by the plating shops were inadequate or improper housekeeping; basic oil removal; inadequate cleaning and pretreatment; little or no training/education; little or no knowledge of health and safety aspects; poor or unsatisfactory plating practices; no close process controls; no preventive maintenance, including no or inadequate filtration; no temperature control; no waste segregation; no environmental equipment; improper tank arrangements; poor connections; and a build-up of deposits on the home-made racks and hooks.
Because of time constraints, I could not sample and analyze (before and after laboratory filtration) the wastewater streams before they entered the sewer. However, a shop representative committee, including a project coordinator and two chemical engineers from a university industrial development agency, analyzed the samples before my visit. The reported results are shown in Table II. Some data were not clarified, and some were found inaccurate.
It should be noted that this Latin American country's three levels of Government (Federal, Provincial and Local) have analyzed 60 different effluent river wastes from small streams that flow into the main river, and have long- and short-term projects (more than $400 million dollars) for cleaning this river through collectors and interceptors. The removal of many houses in nearby tent towns located along the river is a big obstacle facing the governments.
Solutions and recommendations. The proposed suggestions were made to the various companies as follows:
Set-up, cleaning and pretreatment.
- Rearrange the plating process, placing dragin/dragout and rinse tanks in line to prevent cross-use and contamination. Add tanks. Use dragin/dragout system for chemical recuperation using the still rinse tanks and third countercurrent (cascade) rinse tank or a spray in an empty tank. Use counterflow (cascade) rinsing rather than a parallel rinse. Install drain boards between tanks to return drainage.
- Use DI or city tap water for all process plating bath make-up and use dragin/dragout and still rinse tanks to recover chemicals.
- Consider a closed-loop system for purifying and recycling rinse water. Try ion exchange and reverse osmosis as a "roughing" before ion exchange, if the water contains dissolved solids over 500 ppm. Ion exchange also is useful for metal-bearing dilute rinses and less rework.
- Prior to racking, ensure that all racks are properly designed, with good solid contact hooks made of proper materials. Do not use hooks and bus bars made only of iron or steel. Copper is the preferred conductor.
- Remove all deposit build-up both on the hooks and racks to avoid possible contamination and DC power, current density loss.
- Remove dropped parts from each tank at each shift.
- Clean contact for solid current density flow through parts.
- Install water flow regulators on rinse water tanks with conductivity water meter for the plating shop.
- Allow enough time to drain completely.
- Use a foam blanket to suppress cyanide fumes and use mechanical exhaust or fume/vapor extraction system.
- Use poly-balls in lacquering solutions.
- Use temperature controllers on all hot process tanks. Keep alkaline/degreaser cleaner and all nickel baths between 60 to 65C.
- Consider continuous filtration using a simple four-in-one filter system/pump over the tank rather than a cumbersome, large filter system outside small plating tanks.
- Do not place greasy/oily parts directly into the alkaline cleaner. First, remove oil in a solvent of kerosene in a stainless steel tank and along with Emulkleen-23 (MacDermid). Do not use trichloroethylene, gasoline or diesel for removing oil and grease.
- Skim out floating emulsified oil every day.
- Do not dump spent acids and alkaline cleaners into the sewer/river. Keep them for pH control in destructive waste treatment system; heavy metals recycling or recovery; or removal of high levels of chlorine, iron, calcium and sulfate.
- Consider recycling or using spent process solutions to save money and prevent pollution.
- Create reclamation off-site services that are not presently available in the country.
- Separate all mixed waste streams before using any waste treatment recovery equipment to recycle chemicals or rinse water and/or recover metals for reuse.
- Route all cyanide waste streams into a separate room for conventional cyanide-oxidation process; segregate chromium wastes into another tank for chromium-reduction.
- Install a screen-tray for zinc chromating tank to reduce chromate.
Processes change. Consider alternative plating chemistries to reduce wastes and avoid conventional cyanide destruction and chromium reduction treatments. For steel parts, use one of the following:
- An alkaline non-cyanide zinc and/or acid chloride zinc process instead of cyanide alkaline zinc.
- Woods nickel strike before single or duplex nickel plating rather than copper cyanide flash.
- Alkaline non-cyanide copper process.
- Non-cyanide gold and silver processes.
- Trivalent chromium for decorative plating processes instead of hexavalent chromium.
- Use cyanide copper flash for zinc die cast parts before nickel, brass, chromium, gold or silver plating.
- Consider an alternative pickling process.
- Use a solvent of kerosene in a stainless steel tank and add additive Emulkleen-23 first for removing oil and grease instead of trichloroethylene, gasoline or diesel, then use hot alkaline/degreaser.
Equipment and technology for pollution control. First, evaluate pollution control methods before implementing them. These were some of the suggestions made:
- Use a coalescing oil separator to save time, money, chemicals and for better cleaning and adhesion.
- Consider an ultrafiltration oil separator, because floating heavy emulsified oil and smut contaminate every tank. This was also recommended because the company was dumping more than 3,000 gal of solution every two weeks.
- Use temperature controls on all hot process tanks. Keep alkaline degreaser cleaner and all nickel baths between 60 to 65C.
- Install a pulse spray with an atmospheric evaporator on chromium plating tanks to reduce chromic acid wastes.
- Use atmospheric evaporation and/or advanced ion exchange for acid copper salt, nickel salt and chromic acid salt recovery.
- Recover nickel metal using reverse osmosis.
- Recover/recycle zinc and chromium from chromating solution possibly using electrodialysis.
- Consider a combination of both technologies, such as low-temperature evaporative recovery and reverse osmosis to recover nickel from dilute concentration waste streams. Combined technologies provide the best process alternative for such dilute wastes.
- Use ion-exchange, a closed-loop system for reuse of rinse water.
It appeared to me that only a couple of enterprises were interested in making changes and improvements. Many were not willing to spend or invest the money. However, it is our Volunteer Advisors' duty to provide the necessary information and advise. Whether to implement or not depends upon the CESO's clients.
|Table I — Finishing Processes at Latin American Plating Shops|
|Neutralizer Pickle||Copper Acid||Copper Cyanide||Zinc Acid||Zinc Cyanide||Chromating yel/blue/cl||Bright Nickel|
|Table I — Finishing Processes at Latin American Plating Shops (continued from above)|
|Company Name||Brass Cyanide||Chrome Hexavalent||Bright Tin||Gold Cyanide||Silver Cyanide||Arsenic As||Phosph-
|Yellow Iriditing||Hot Dip Galvanizing|
|Table II — Reported Industrial Wastewater Discharge Stream Analysis|
|Company||ph||Grease and Oil||Total Solids||Suspended Solids||Solid Sediment||CN||Cu||Pb||Cr||Zn||Ni|
|IW: Industrial Wastewater|