To a recycler, an object such as an automobile transmission resembles a piece of candy in which the wrapper is more desirable than what’s inside. The aluminum of the casing is more valuable than the steel gears and other contents it holds. But most recyclers find that manually separating such transmission cases from their contents is usually prohibitively costly.

For these recyclers, sweat furnaces offer an effective and affordable way to separate non-ferrous metals such as aluminum from less-valuable iron and steel. Sweat furnaces heat commingled recyclable metals to a temperature that causes the non-ferrous metals to melt and run off, leaving behind steel and other materials that melt at higher temperatures.

At sweat furnace maker EnviroAir, Inc. in Eagle, Wisconsin, CORECO Product Manager Dean Lesch said modern machines like those his company makes provide several appealing features. Compared to traditional sloping hearth sweat furnaces, recyclers don’t need to manually rake off iron and other non-melting contaminants. They also avoid impinging flames directly on molten aluminum, which creates dross. Other designs may also allow iron to dissolve into aluminum. Most importantly, up-to-date equipment meets ever-tougher environmental regulations.

CORECO designs employ a continuous rotary sweat furnace. “Material is constantly fed into the rotary hearth while molten aluminum is constantly being sweated out,” Lesch explained. “Any non-melting metals such as iron, brass or copper flow out the discharge end of the rotary hearth clean of any aluminum. The rotating action of the tube self cleans the tube by constantly creating a scouring action from the non-melting metal that prevents a buildup of material on the tube.”

Indirectly heating the CORECO furnace avoids impinging flame on the materials, reducing the loss of aluminum and other saleable non-melting metal through oxidation. “The rotary hearth is heated from the outside thereby separating the combustion system from the aluminum sweating process,” Lesch said.

Also, molten aluminum quickly runs down the angled rotary hearth and into a mold or holding furnace so that remaining iron won’t dissolve into the aluminum. “The aluminum does not pool inside the rotary hearth eliminating the opportunity for the iron to rest in the molten aluminum and be dissolved,” Lesch said.

CORECO has two standard models. The Model 1231 rotary sweat furnace handles 2,000 lbs. per hour of input. The Model 1848 rotary sweat furnace handles 5,000 lbs per hour of input.

Most customers are scrap processors, foundries, die casters and part manufacturers, which use the furnaces for in-plant recovery of mixed metal scrap.

Orders have held up well, Lesch said. “Since our equipment makes money, lowers operating cost and is good for the environment, many companies see the value of our equipment,” he said. “This has allowed us continued growth and the development of new equipment and processes to further help our customers.”

At Recycling Services International in Cohoes, New York, owner David Conway said the ability to satisfy environmental regulations drives much of his business. “It’s our pollution controls and the operational controls for efficiency and pollution control efficiency,” he said. “We use the latest technology.” Recycling Services employs solid state controls to help maintain temperature in furnace oxidizers, while also getting the maximum fuel efficiency.

That helps his customers rest easy. “With my customers one of the biggest fears is the Environmental Protection Agency,” said Conway. “But over the last four years, we’ve permitted over 35 furnaces and we’ve never been denied a permit.” Today Conway is designing furnaces that meet twice the minimum required pollution control requirements. “The reason we build them that way is, if EPA does change the regulations, we’re already ahead of the game,” Conway said.

EPA limits the amount of hazardous air pollutants in tonnage per year that a furnace can emit. They also have a minimum requirement of eight tenths of a second of retention time in the afterburner, at a minimum temperature of 1,600 degrees in the afterburner chamber. “Our units have twice that retention time and are capable of maintaining a temp of 2,000 degrees,” Conway said. “We overbuild that part of the unit just in case of something coming down the pike so we’re always a step ahead.”

Conway’s MAX-4000 aluminum sweat furnace is his most popular model. These are typically used for melting aluminum castings, such as transmissions and engines. The furnaces are large enough to hold approximately 15 transmissions at a time.

After aluminum melts in the MAX-4000, it flows to a separate holding chamber that is kept at a lower regulated temperature. “We have that separate holding chamber because to hold that molten aluminum we only need 1,400 degrees, but we need 1,600 to melt it,” Conway said. “And the more temperature you have in that holding chamber, the more metal loss.”

Conway also builds furnaces with special continuously monitored high-temperature afterburners that run at 2,200 degrees for processing scrap such as electrical transformers that have been contaminated with PCBs. He also builds furnaces that reclaim non-ferrous precious metals such as gold from computer circuit boards.

After a very busy three years, sales slowed almost to a halt for the last year, Conway said. He is optimistic about growth in sales of furnaces that employ alternative fuel sources, such as landfill methane gas. Heat recovery systems also promise to help spur the market, as furnace waste heat is redirected to preheating combustion air, heating existing buildings as other uses. The availability of government grant money is driving much of that demand, he said. “It’s really picking up now. We’ve sold two aluminum furnaces in the last three months and a transformer furnace.”