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Recovering heavy metals.

Environmental restrictions placed on heavy metals are driving the design of systems to recover these materials for reuse or safe disposal. Equipment and processes to recover the metals are continually under development.

Heavy metals are essential in the manufacture of such products as automotive batteries and fluorescent lights and in many industrial processes. As a result, the presence of these materials in factory waste, scrapped products, or contaminated waste sites poses a major disposal problem for manufacturers.

"The problem with heavy metals is that they are indestructible. Even in a dilute form, metals such as lead are toxic to humans. If they are ingested by aquatic life in contaminated water they will continue up the food chain to human beings," said Subhas Sikdar, director of water and hazardous waste treatment research at the Risk Reduction Engineering Laboratory (Cincinnati), an arm of the Environmental Protection Agency (EPA).

Equipment and processes to recover heavy metals from post-consumer trash, factory waste streams, and waste sites are continually under development by environmental engineers. Heavy metals are recycled, rendered nonhazardous, or concentrated for landfilling, depending on the cost of disposal.

A Lot of Lead

Scrapped automotive batteries constitute one of the more visible sources of lead contamination. Between 70 million and 80 million automobile batteries are discarded in the United States annually, according to the Association of Battery Recyclers Inc. (Washington, D.C.). However, of those, approximately 95 percent are recycled. This is due mainly to the cash incentives offered to motorists by battery retailers.

East Penn Manufacturing Co. Inc. (Lyon Station, Pa.), a maker of automotive and industrial batteries, uses the batteries that have been returned by distributors to generate raw materials to make new batteries. East Penn makes use of its own recycling equipment and machinery made by M.A. Industries Inc. (Peachtree City, Ga.) at its Lyon Station secondary lead smelter in a system that can recycle up to 4 million batteries a year.

After used batteries are offloaded onto a conveyor at East Penn's secondary lead smelter, an M.A. battery turner positions them on their sides. A slow-speed saw then cuts off the battery tops, which are further processed along with the battery cases to recover the polypropylene, which is recycled by outside suppliers into new battery tops and cases. The battery cases are separated from the lead-bearing portions in an M.A. group extractor.

The spent electrolyte is collected and drained to a settling sump and then pumped to storage tanks. "The sulfuric acid is subjected to a proprietary reclamation process and is used to cut concentrated sulfuric acid to make electrolyte for new batteries," said Rick Leiby, manager of the metals division at East Penn.

The lead-bearing portions of the batteries are conveyed into a material storage room in the Lyon Station plant. "Lead is fed through an FMC Corp. (Chicago) vibratory conveyor that shakes material into a bucket equipped with a strain gauge weighing mechanism," said Leiby. A pusher feeder sends the lead into the plant's reverberatory furnace, fired with oxy-fuel burners built by American Combustion Inc. (Norcross, Ga.).

"All of the lead recovered from the furnace is used for battery manufacture," explained Leiby. He said that the slag generated from the reverberatory furnace is high in lead, antimony, arsenic, and tin. The slag is fed to a cupola furnace that reduces all of these metals into the bullion that is used to make the posts and connectors in the batteries. Slag from the cupola furnace is further processed to render it non-hazardous--and thus disposable in a residual waste landfill, Leiby added.

The recovery of battery lead requires that furnace gases be subjected to rigorous controls. Furnace gases are routed to an afterburner at temperatures in excess of 1400[degrees]F and are cooled to 250[degrees]F by a combination of water, air-cooling loops, and ventilation air from the furnace area. The cooled gases are sent to a Wheelabrator Air Pollution Control (Pittsburgh) baghouse that contains fabric bags and membranes to trap particulates. These solids are sent to the reverberatory furnace. After passing through the baghouse, gases are sent through a diffusion of ammonia and water. The resulting bisulfite material is sold as a raw material to fertilizer manufacturers.

Recouping Mercury

Another scrap source of heavy metals is fluorescent light bulbs, which contain mercury. This has been of particular concern in Sweden since the 1970s, when the country's environmental authority encouraged manufacturers of mercury products to develop ways to recover the heavy metal. In 1978, Swedish lighting manufacturer Lumalampan AB (Karlskrona) devised and patented a mercury-distillation technology that has been refined and marketed by MRT-System (Karlskrona) in Sweden and abroad since the early 1980s. American firms using the MRT mercury-distillation system are Sylvania Lighting Products (Manchester, N.H.), Recyclights (Minneapolis), and, scheduled to begin using the system next year, Marpan Supply Co. Inc. (Tallahassee, Fla.).

In the first stage of the MRT process, fluorescent lighting tubes are collected and crushed. The resulting material is fed into a sieving mechanism that separates the metal, glass, and fluorescent powder. The metal and glass portions can be recycled separately since they are virtually free of mercury, which is contained in the fluorescent powder that is fed into metal distilling barrels.

These barrels are placed in a vacuum chamber for the distillation process, which is computer controlled. The barrels are heated to a point where the mercury vaporizes and organic particles in the gases oxidize in the system's afterburners. The process gases continue to a cooling trap, where the mercury is condensed for reuse. Residual mercury is captured in a carbon filter downstream of the cooling trap.

To extend the technology's reach beyond fluorescent lighting, MRT engineers equipped their standard distiller with a larger post-combustion chamber and larger vertically installed cooling traps. The MRT Superior distiller can be used to treat such high-mercury-containing waste as mercury batteries, switches, electrical relays, and dental amalgam. An MRT Superior distiller in Germany recovers approximately 1.5 tons of mercury from the 7 tons of mercury button-cell batteries it treats each month.

Polishing Factory Effluent

For many manufacturers, heavy-metal-extraction techniques that were state of the art when installed must be retrofit to meet stricter environmental standards. This was the case at the Texas Instruments plant in Attleboro, Mass., which produces 600 gallons of effluent per minute from 40 different plating operations containing heavy metals such as cadmium, chromium, copper, lead, silver, tin, and zinc.

In July 1977, Texas Instruments received a five-year National Pollution Discharge Elimination System (NPDES) permit from the EPA to discharge its treated industrial waste-water into a pond. Plant operators used a precipitation system on their waste stream and extracted heavy metals in the form of hydroxides. Residual heavy metal concentrations in the effluent were reduced to the parts-per-million range, which was within the limits set by the permit.

According to Don Mikutel, manager of hazardous materials at Texas Instruments' Attleboro plant, the metal- rich hydroxide sludge is sent to World Resource Corp. (WRC) in Pottsville, Pa., a waste-treatment company. "WRC blends sludges from different plating operations to get a desired metal content, say, 10 percent copper, then dries the sludge and packs it into nylon containers," explained Mikutel. This product is then sold to refiners, who smelt it to recover the metal.

By 1982, the NPDES permit limits of heavy metals were down to several parts per billion. As at many manufacturers, the management at Texas Instruments decided to augment its existing heavy-metal-removal system at Attleboro and further "polish" discharges with systems like the Sulfex process developed by Permutit Co. Inc. (Warren, N.J.).

The Sulfex system at Attleboro was placed downstream of the plant's hydroxide-removal system and began operating in 1986. "The Sulfex system consists of two rectangular steel reacting vessels. These are parallel so that should one unit break down or require maintenance the other is available," said Bob Nagiel, product manager at Permutit. Influent enters the mixing zone of the vessel, where it is blended with a slurry of iron sulfide reagent from a feed tank and some polymer from a separate feed tank to help facilitate sulfide formation.

"The amount of reagent is metered proportional to flow, based on the type and concentration of metals in the waste stream," said Nagiel. Typically, 15 to 20 parts per million of reagent and less than 1 part per million of polymer are sufficient. Texas Instruments' programmable controllers oversee the addition of reagent and polymer.

Each reactive vessel is kept approximately three-quarters filled with a combination of sludge deposited by the influent and the iron sulfide reagent. "This serves as an upflow filter to maximize contact between the residual heavy metals and the reagent," explained Nagiel. Settled product made up of the metal sulfides is blown from the reactive vessel into a dewatering filter press that produces a cake-like material.

Texas Instruments sends this sulfide sludge to the York, Pa., processing facility of Envirite Corp. (Plymouth Meeting, Pa.). "Using a proprietary chemical process, Envirite converts this hazardous waste into nonhazardous material that can be safely disposed in their land fill," said Mikutel of Texas Instruments.

Clarified effluent is discharged from the reacting vessel and piped into an air scour polishing filter for the final stage of the Sulfex process. The filter is a steel gravity or pressure vessel containing sand and anthracite filtering material coated with unreacted iron sulfide to capture traces of heavy metals that may have escaped the previous removal stages. Like the reacting vessels, metal sulfides formed in the filter are removed and sent back to the front of the treatment system for further refining and recovery. An air scour breaks up any particulates caught on the filters.

"The clarified effluent emerging from the treatment system has reduced the heavy metal concentrations to near-drinking-water standards, permitting the Attleboro plant's water to be discharged into Cooper's Pond, which flows into Ten Mile River, and ultimately into Narrangansett Bay without further treatment," said Nagiel. Texas Instruments has achieved parts-per-billion levels on most metals including less than 1 part per billion of cadmium and 6 of silver.

While metal-plating operations remain the primary market for the Sulfex process, the technique has spread to other processors concerned with heavy metals in their waste-water. These include Kelly Air Force Base (San Antonio, Tex.), where a Sulfex processing system treats more than 1700 gallons per minute of wastewater containing heavy metals from aircraft maintenance.

"However, the biggest potential for the Sulfex process is cleaning the water from industrial and utility cooling towers and boiler blowdowns," said Naigel. Plant operators have increased the use of recycled water in their cooling towers to cut down the makeup water required. This concentrates metals in the water, particularly copper. When water evaporates in boilers at refineries and power plants, the toxic metals in the water remain. These materials can be removed by the Sulfex process, as is done by Koch Refining Co. (Corpus Christi, Tex.). The firm uses Sulfex equipment to clean up to 1200 gallons per minute of water from its cooling towers and boiler blowdowns.

Modern-Day Alchemists

Ancient alchemists labored to turn lead into gold. Their modern-day counterparts at Inorganic Recycling Corp. (Dublin, Ohio) have developed a technique to convert electroplating residues and steel mill dust, both waste streams that contain heavy metals, into ceramic feedstocks safe enough to make sinks and countertops.

Mechanical and chemical engineers at Inorganic Recycling designed their process to accept industrial waste in the form of dust, liquid, or sludge. These wastes are piped to a reactor vessel, basically a ribbon blender. Measured amounts of silicates are added to the vessel to bind with the heavy metals present. "We usually add the silicates manually, but at our larger operations, silicates will be added automatically," said Alan Sarko, marketing director at Inorganic Recycling.

When the chemical reaction occurs, the treated wastes form a slurry that is piped into a surge tank. Technicians draw samples for a chemical "fingerprint test" to ensure that all the reactions have taken place and that the correct chemical mixture has been achieved. "If an error is detected, that material is pumped back to the front of the process line to be! corrected," Sarko explained.

After the slurry passes the finger-print test, it is pumped to a vitrification unit. This vessel is typically fueled by natural gas--sometimes with oxygen to ensure cleanliness--at temperatures between 2500[degrees] and 2900[degrees]F. Sarko described this phase of the recycling process as a "mini-volcano" that causes the slurry to pass from a liquid to a solid form and finally to a molten state similar to lava.

Molten material forms a pool on the bottom of the vitrification unit and is piped to a water bath. This step is designed to thermally shock the process material so that it will form granules that are easier to handle than large chunks, which would otherwise result as the substance cools.

Gases from the vitrification unit are sent to a packing tower that contains a mesh to capture particulates and a cooling spray. Heavy metals will drop out of the gases as solids and form a sludge on the floor of the packing tower. This is removed, sent through a filter press, and routed back to the front of the recycling process. Water squeezed out of the sludge is reused as process water.

As an extra safeguard against toxicity, the granules are tested to see if they meet EPA standards for toxic constituents and are returned to the front of the process if they do not. "More than 99 percent pass, since the fingerprint test already established whether the correct chemical recipe was met, so the granules are collected into 2000-pound bulk bags," said Sarko.

Truckloads of these bags are sent to ceramic purchasers. Sarko said the granules are generally crushed down further so they can be poured into molds. Inorganic Recycling granules are used to make kitchenware, including sinks, countertops, and tiles. The abrasiveness of the granules allows them to be used for sandblasting, to make scouring pads, and on cutting and grinding wheels.

Inorganic Recycling commercially launched its technology three years ago. At its Delco chassis plant in Kettering, Ohio, General Motors Corp. (Detroit) uses the system to process 3000 tons of liquid electroplating waste laden with chromium. An Inorganic Recycling system that can process up to 15,000 tons of electric arc furnace dust containing lead and cadmium, among other metals, is being constructed at the Nucor Corp. (Charlotte, N.C.) steel plant in Hickman, Ark.

According to Sarko, his company views treating European incinerator ash as an opportunity for their recycling technique. "This is because, unlike in the United States, incinerator ash is classified as a hazardous waste," Sarko said. He added that his company is negotiating with the German municipal authorities in Essen to install an Inorganic Recycling system to treat waste from that city.

Getting the Lead Out

In the years before environmental regulation of industrial waste, heavy metals accumulated at sites throughout the country. As a result, soil remediation represents another application for heavy metal recovery. One technique for treating soil contaminated with wastes, including heavy metals, is soil washing. This volume-reduction process involves extracting hazardous contaminants and concentrating them into a small residual portion of their original volume. Cleaned soil is usually redeposited on site.

Soil washing was pioneered in the Netherlands in the early 1980s and is used extensively in Europe. For example, the soil-washing system developed by Bergmann B.V. (Berkel en Rodenrijs, Netherlands) is used to remove cadmium, lead, zinc, and arsenic from river-bottom sediments in both Holland and neighboring Belgium.

In the United States, companies such as Brice Environmental Services Corp. (Fairbanks, Alaska) now use the soil-washing technique developed by the Europeans.

Bergmann B.V. entered the American soil-remediation market through its sister company, Bergmann USA (Gallatin, Tenn.). Demonstrations conducted by Bergmann USA were successfully evaluated under the auspices of the EPA's Superfund Innovative Technology Evaluation program and under the U.S. Army Corps of Engineers' Assessment and Remediation of Contaminated Sediments plan. The demonstrations involved removing contaminants from the soil and sediments of the Saginaw River in Michigan and the Toronto Harbor Front in Ontario.

Bergmann USA is currently fabricating a 10-ton-per-hour soil-washing facility to treat lead-contaminated soil from a Superfund site. This plant is scheduled to begin operating in March 1993.

Richard Traver, general manager of Bergmann USA, explained that while each Bergmann system is site-specific, the basic design of the process remains the same. Excavated earth or sediment must be reduced to a nominal particle size distribution of 6 millimeter by 0, or 1/4 inch by 0. This is done by sending the contaminated soil into a screening machine whose vibrating screen removes rocks and debris larger than 2 to 4 inches. Sometimes a crushing unit is used to reduce oversize rock material.

Bergmann designers fitted wash solution sprays to the screening machine to add wash water to the feed system. This additional water provides the extraction medium for the heavy metals being removed. "Slurry is pumped to cyclone separators where the initial 'desliming' [separation of material smaller than 63 microns] from coarse fractions takes place," said Traver. Thickened underflow from the cyclone separators (70 to 75 percent solids) is fed directly to a multistage attrition scrubbing module. Surfactants or chemical wash additives are introduced in the feed box of the attrition scrubbing module.

Rotating impellers within the machine's attrition cells cause the granular soil particles to rub together, causing mechanical and fluid shear stress. This detaches contaminated silts and clay from the particles so that they will enter the wash water.

After attrition scrubbing, the Bergmann process stream enters either single- or multistage hydro-cyclone classifiers or an inclined screw-type classifier. This treatment produces two product streams. The first consists principally of scrubbed sand and solid organic matter such as coal or wood. The other flow stream is wash water, contaminated mineral silt and clay particles, and fine-sized organic matter. There may also be solubilized heavy metal ions that can be removed later by standard industrial wastewater treatment methods such as precipitation sedimentation or ion exchange.

Due to their high rate of contaminant absorption, the solid organic matter such as leaves, twigs, roots, bark, grass, and wood chips must be removed from the granular soil component. Bergmann engineers accomplish this using a density media separator, which uses a rising current of water in a fluidized bed of material to separate the lighter-specific-gravity solid organic material. Once isolated, the solid organic material can be mechanically dewatered and incinerated. The decontaminated sand fraction from the density separator can be backfilled to its excavation or sold to road material manufacturers or as concrete or asphalt aggregate.

Polyelectrolytes are added to the wash water to cause the contaminated mineral silts and clays to coagulate. These are concentrated by gravity into a sludge that can be dewatered by filter press or other mechanical means. Heavy metals that have dissolved into the wash water stream are precipitated out of the solution as hydroxide salts. These can be removed by flocculation and sedimentation or dissolved air flotation. The salts are dewatered by filtration and sent to a secondary smelter for recovery. Depending on their economic value, heavy metals undergoing the Bergmann process can be recovered, recycled, or prepared for disposal in a landfill, Traver said.

Limiting Toxics

"Environmental laws strongly encourage managing heavy metals from cradle to grave," said Sikdar of the EPA's Risk Reduction Engineering Laboratory. One way to make products and processes less hazardous to the environment is to eliminate toxic metals where possible, he added. "For example, nonhazardous materials have been substituted for mercury in many types of batteries."

Sikdar recommended restricting use of those heavy metals that cannot be replaced to applications that will not contaminate the environment. "For example, chromium is needed to produce steel, but steel is infinitely recyclable," he said. It is easier to supervise the manufacture of the steel used to make car parts than that in smaller consumer products. "In general, I see a lot more metal recycling being done," Sikdar said.

Faster On-Site Screening

Chemists at Tufts University (Medford, Mass.) are developing a sensor that can spot concentrations of heavy metals in groundwater on site within minutes. This is part of a project cofunded by the U.S. Environmental Protection Agency's Northeast Hazardous Substance Research Center at the New Jersey Insititute of Technology (Newark).

Samuel Kounaves, assistant professor of chemistry, and doctoral students Wen Deng and Zhaohui Liu, designed their device with cooperation from the Center for Integrated Systems at Stanford Univerity (Stanford, Calif.). The Tufts team tested a prototype version of the device on tap-water solutions and was able to detect concentrations of lead, cadmium, and copper in parts per billion.

The heart of the heavy metal sensor is a mercury-coated iridium disk measuring 10 microns in diameter that is contained in a cigar-shaped steel housing. When the sensor is brought in contact with the water sample, a -1-volt charge is induced that will reduce heavy metal ions into the mercury tip. "After the ions are concentrated into the tip, we gradually reduce the negative charge to zero," said Kounaves. As the charge drops, the metals will oxidize out of the mercury.

Different metals have different oxidation points, thus the distinct electrical peaks in the sensor's current indicate both the type of heavy metal (such as lead or cadmium) and its concentration. These signals are sent from the sensor tip to an analog-to-digital converter. After digitization, the signals are fed to a laptop computer that displays heavy metal concentrations in a spectrum graph.

With prototype testing completed, Kounaves and his student colleagues will be field testing the sensor in 1993. They hope to extend their instrument's capability beyond heavy metals to detecting organic contamination in water. Knowing what contaminants are present in ground-water and in what quantities is the first step to removing them.
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Title Annotation:includes related article
Author:Valenti, Michael
Publication:Mechanical Engineering-CIME
Date:Dec 1, 1992
Words:3651
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