Pretreatment of Metal Parts
Importance of reclaiming process water, phosphate and other metal cleaning solutions.
HAZARDOUS MATERIAL TREATMENT AND DISPOSAL
In cleaning and pretreatment of metal parts, potentially hazardous and polluting substances are present in almost every step of the process. In the U.S., metal finishers (about 15,000 companies) generate more individual wastewater discharges than any other industry. In decades past, it was common for manufacturers to discharge untreated wastewater and other industrial effluents into municipal sewer systems or directly into groundwater supplies such as streams and rivers. Now, regulations to protect water, air, and soil require that wastewater be treated before it can be discharged anywhere. Of particular concern are metals such as cadmium, chrome, copper, lead, nickel, iron, and zinc.
Parts to be cleaned are soiled with a wide range of contaminants. During manufacturing, machining, and handling, metal parts can accumulate a variety of contaminants, including shop dirt, lubricants, drawing and cutting fluids, polishing and burnishing pastes, stamping and buffing compounds, brazing and welding fluxes, stenciling inks, fibers, dust, grit, grease and grime, scale, smut, and metal wastes such as filings, chips, and dust. Machining processes such as cutting, milling, bending, grinding, drilling, and burning leave behind contaminants such as machining fluids and fragments of metal. Rusts or tarnishes may also be present on metal stock, particularly on materials that have been in storage. In many cases, more than one contaminant may be present.
These contaminants are removed with a variety of products, each of which presents challenges for safe handling and disposal:
- De-rusting or pickling agents are extremely low pH solutions that accumulate high levels of iron other dissolved metals.
- Cleaning solutions may be acid, neutral, or alkaline aqueous products; solvents are also used. Alkaline cleaning solutions contain surfactants that break down oils and greases so they can be rinsed away; the surfactants require special treatment before disposal. Alkaline tank solutions accumulate large amounts of oils and greases that may need to be gathered and treated as hazardous waste. Very strong alkaline cleaners require pH neutralization before disposal. Cleaning solutions may contain chelants; these are chemical compounds that attract and bind metal particulates so they can be rinsed away. They, too, need special handling.
- Conversion coating treatments that use iron or zinc phosphates will leave high concentrations of these metals behind in the coating baths, generally in the form of sludge that must be treated and disposed of.
- Rinse waters may be continuously overflowed or recirculated, or some combination of the two methods may be used; regardless of rinse method, the rinse waters will become contaminated with all the soils, cleaners, and additives that are present throughout the process, and may need treatment before re-use or release into sewers or public water supplies.
- Post-rinse treatments apply rust inhibitors or other final sealing rinses to the product before finishing. These may introduce any of a variety of substances into the waste stream, including chromium, zinc, and other metals, all of which may require special treatment.
Wastewater from cleaning and pretreatment processes thus contains a mix of soils, contaminants, and potentially hazardous cleaning agents and treatments.
Wastes may be addressed by and combination of:
- Removal – Waste haulers can be contracted to remove wastes, including rinse waters, for off-site treatment and disposal
- Filtration – Filters can be installed to capture sludge and other solid wastes for later treatment, disposal, or removal
- In-Plant Treatment – Typically, in-plant treatment consists of a multi-tank system where wastewater is collected and treated to remove contaminants. This processing can be done in batches or in a continuous flow, depending on the rate and volume of flow from the line operation. Individual treatment tanks may be necessary for segregation and treatment of particular wastes (e.g., hexavalent chromium), for adjusting pH in large quantities of water, or for physical filtration. For many operations, in-plant treatment is the easiest and most cost-efficient means of dealing with wastewater.
In a typical in-plant treatment system:
- Waste water from the cleaning and pretreatment line is directed into the treatment system, either in batches or in a continuous stream, as suits the operation.
- Chemicals are added to adjust pH, reduce metals, and hasten precipitation (settling) of solids.
- The settled solids are pumped out to a secondary system where the water can be removed; this secondary system could be a sludge pit or a special filtering device.
- The water from which the solids have been removed is further clarified and cleaned before being discharged.
REDUCING WATER USE
Both to contain costs and to protect natural water resources, manufacturers look for ways to use as little water as possible while maintaining process quality.
Possible approaches to reduce water use include:
- Convert parallel rinses to counterflow rinses and add more counterflow rinses where possible.
- Install a “reverse cascade,” in which the cleanest water is used for the last rinse before being circulated “upstream” to be used in the first rinse area.
- Install spray rinses above process tanks; in some cases, this can replace a separate rinse tank.
- Re-use rinse water whenever possible.
- Install flow-control devices to permit water flow only when parts are in the tank.
- Re-use pre-treated wastewater in non-critical rinses (this may require filtering to remove suspended particulates)
Reduce the “dragout” of chemical solutions between stages of the cleaning and treatment system:
- Use the lowest possible contractions possible to meet quality standards.
- Run the processing tanks at maximum temperature so as to make the cleaning solutions most effective.
- Install drain boards between process tanks and rinse tanks, and adjust the positioning of parts as needed to encourage good drainage and to minimize the amount of cleaning solution that will be transferred into the rinse tanks. Look for other ways to increase drainage, such as removing parts slowly from baths.
- Use sufficient rinsing between process tanks. Add wetting agents to improve rinsing action.
Metal finishers use several methods to recover and recycle material that used to be discarded as waste. Recycling can reduce the amount of waste that must be otherwise treated and disposed of, and can even generate revenue. For example, metals can be recovered and reused, sent back to suppliers, or sold to reclaimers.
Depending on the operation, process solutions can be returned directly to process tanks for immediate re-use, or treated for later re-use.
Your supplier can help you to find the “greenest” solutions for your operation.