Keeping non-ferrous foundry furnaces clean and well maintained so they operate as intended promotes factors that manufacturers care about the most: product quality, process and foundry efficiency, long life of capital equipment and the bottom line.
However, cleaning and maintenance tend to get lost when longtime workers retire, the remaining staff has high turnover and production volume is high—a time when a clean, well-maintained furnace is most critical. rock wool blanket 1
Responsibility for metal quality falls directly on management’s shoulders, says David W. White, co-owner of D and S Consulting LLC, Springboro, Ohio. Leadership must ensure their workers have the proper training and resources to deliver clean aluminum that’s free of hydrogen and inclusions, on time and at temperature. Customer demand is for high-quality castings, White continued, so if managers are lax in training and resources, then scrap rates tend to creep upward. When foundries are slammed with work, as they have been for the last few years, that upward trend in the number of rejects gets worse.
There’s been an uptick in demand in recent years especially for aluminum, the most commonly processed metal in non-ferrous foundries, due to the electric vehicle market and housing boom, White explains.
“If you look at your house, it probably has an average of 22 electric motors in it,” White said. “Look around and see all the fans and everything that works with an electric motor. Those motor rotors are all poured out of aluminum.”
Further prompting the need for good industrial hygiene is that aluminum baths are characterized by a problem related to cleanliness and maintenance.
“One of the biggest factors, especially with aluminum melting, is they’re prone to build aluminum oxide on the furnace refractory (lining), and it will creep through seams,” says Kelley Shreve, general manager of foundry furnace manufacturer Thermal Product Solutions LLC, Riverside, Mich., which operates as Lindberg/MPH. “We’ve had to replace furnaces because we couldn’t get the aluminum oxide out even with a jackhammer.”
White calls aluminum oxide a “cancer” because of its growth and spread in a furnace’s lining.
“Relining is one of the most expensive items in a foundry or diecasting operation budget,” White wrote in a recent article in Foundry Management & Technology magazine. “Some spend hundreds of thousands of dollars on refractory relines but will not hire one more person to clean the furnace every day–which is necessary for aluminum melting furnaces.”
The cost of a relining can be 50-60% of a furnace’s initial price. A foundry melter can range from five to seven figures.
To keep cleaning on track for melting and holding furnaces, and the launder that connects them, White suggests creating an incentive program based on every dollar saved from melt loss and refractory relining. This not only will aid cleanliness, he said, it also should yield longer-lasting furnaces.
However, there are other points of view.
“Some places will shut their furnace down every year, no matter what, to do what they would call a cold clean,” notes Ross Lee, president of Detroit-based Schad Refractory Construction Co. “And there are different philosophies: ‘Hey, let’s just produce for longer. We know that we’re causing these problems and eating up our linings but we just want to get as many tons of metal out of this thing as we possibly can. We don’t want to shut down every single year for this and spend money on it, let’s just abuse this thing for a few years and know that it’s not going to last as long and then we’re going to replace it.’”
For those who don’t heed White’s advice, visibly damaged or broken refractory, oxide buildup and failure to get melt break are signs a relining is due.
One tipoff may even be visible on the outer shell of the furnace. This indicates that some of the refractory is missing,” according to Don Kublick, engineering manager at Lindberg/MPH. “Either large chunks of refractory or large cracks that opened up create hotspots that are even to the point of being red in color.”
White recommends taking temperature readings at the same locations on the furnace’s outer shell every three months. If the temperature readings start to tick up, it’s time to check the condition of the refractory.
The relining process starts with a tear-out team armed with pneumatic, handheld jackhammers to break up the furnace lining. For oxide buildup that a jackhammer can’t chip away mechanically, Kublick says an oxygen or magnesium lance may be used, which employs a thermo-chemical reaction.
Once the refractory is carted away and the furnace’s structure is inspected and any repairs are made, new refractory installation can start. For the cold face, or insulating layer, this can include layers of a blanket of nonwoven fiber, board and insulating fire brick (IFB), all made of ceramic. The board can also be made of vermiculite.
In addition to making the most efficient use of heat generated by a gas or electric burner, the cold face also enhance safety, Kublick pointed out. Premium insulating materials are available and may offer better insulating properties, a higher upfront cost that can save money in energy outlays.
“That’s definitely a discussion that’s had, as far as premium materials for either the hot face or to help reduce wear and oxide growth,” Kublick says. “There are some higher-end, micro-porous refractory boards, which will give you better insulating properties.”
Once the cold face is installed, temporary forms are inserted to create the hot face. The forms can be made of steel that are reusable, or constructed of lumber on site. A castable, alumina-based mixture of dry material and water in ratios to make it the consistency of cement is then poured or pumped into the mold. This mixture, or monolithic, has a curing time of 75-125 hours.
While the monolithic is the consistency of cement before it cures, “It’s a lot more high tech than a traditional cement that you would use for architectural (purposes),” Kublick says.
Lee agrees on the high-tech nature of the materials. Competition drives material manufacturers to “research, develop and bring to the marketplace products that cost less, last longer, can be installed faster and can be dried out faster,” he notes.
A walk-through of Schad’s giant storage and fabrication sheds with Lee reveals the extent of variance among refractory products. Pallets are stacked with bags of different monolithic mixtures, a wide range of blankets and bricks, and board of various densities.
Although refractory brick use in non-ferrous furnaces is diminishing, Lee still sees advantages to employing them.
“The properties are much more consistent, because they’re made in a factory,” he says. “When you get the brick, you know that it was mixed properly, it’s the proper chemistry and it’s been dried out or fired. If you’re mixing the monolithic, the water is added at the plant (foundry), the temperature of the material is variable, the temperature of the water is variable, the set time could be a variable.”
Some of Schad’s customers ask for a probe or thermocouple to be inserted in the wet monolithic so they can see how heat travels through the lining as it dries.
As an alternative to installing a mold and using wet monolithic that has to dry on-site, the hot face can be made off-site to minimize the interruption to production in a foundry, as two of Schad’s workers were doing during Lee’s walk-through with a reporter.
“When you’re working on process equipment for a company, even a couple of hours (of downtime) is not convenient to them,” Lee explains. “It’s all about getting the downtime as small as possible.”
Kublick adds: “That could mean a customer has to gear up production to make time in their schedule, then the lining materials brought in to be stored in a warm, dry place. And the relining has to be done in a fairly warm environment. You don’t want to expose the lining materials to below freezing temperatures.”
To shrink the time it takes for relining, demo can start on a furnace that’s still warm because a robot does the job. Bricks may take longer to install, but they have no need to dry out like monolithic does. Heat from an external force or a furnace’s pilot light can be used to dry out the monolithic, but the timing is tricky.
“You can’t heat it up too fast,” Lee warns. “Or you’ll have spalls or explosions as the steam finds its way out.”
Kublick explains, “You can use either an outside heat source, which would be basically a burner. There are some companies that provide dry-out services so they’ll have a separate burner that we use to slowly heat up the furnace. Or the furnace burners themselves can be used to start out with just pilots only for those that have smaller pilot flames, then switch over to the main flame and proceed up to a higher (setting) as needed.”
The total thickness of the hot and cold faces can be 20" [508 mm] or more in a melting furnace, while in a lower-temperature holding furnace an 8" [203 mm] lining is fairly common, Kublick says.
To delay a relining as long as possible and keep the refractory in top shape, Bill Walter, of McEnglevan Industrial Furnace Company Inc., Danville, Illinois, recommends painting the refractory “every couple weeks” with sealer. Walter, who said he’s been building furnaces since 1938, said the paint job is needed less often for aluminum melters and more often for those that melt brass.
White, who was in the furnace business for 40+ years with the Schaefer Group Inc., Dayton, Ohio, and has had his own consulting company for three years, reminisces that in the old days refractory sealer was to prevent the formation of aluminum oxide. Now most refractory used in aluminum furnaces resists oxide formation.
“However, aluminum oxide will penetrate all refractories eventually,” he says.
For good industrial hygiene, White advises cleaning a melting furnace every day and a holding furnace every two days, “except for the electric-immersion type, which can go about five days between cleanings,” according to his article.
Molten-metal launder systems should be skimmed and their sidewalls scraped every day and the bottom lining scraped every week, White wrote. If a buildup occurs in these systems, an overflow may result.
“Your combustion system can get out of adjustment, and if it starts to go fuel rich, that can cause your furnace to have a cold flame and have to work harder to produce the same results,” White added in an interview for this article. “If it goes air rich, now you’ve got more oxygen in there. And that can create more oxide. So you want it to be what we call stochiometric, which is in ratio with the air-to-gas mixture.”
Running a furnace out of ratio can use more energy and increase utility costs, too.
Furnace manufacturers such as Lindberg provide on-site training and a manual detailing cleaning and maintenance with every furnace they sell.
“It truly has to be a partnership with the with the end user so that they fully understand starting on day one what has to be done as far as how to clean the furnace, how to inspect the furnace to make sure that you’re heating it thoroughly enough that you’re not building up any oxide growth,” Kublick says. “If you do see oxide growth, schedule a more stringent cleaning or an outside cleaning company to come in and clean it so you get back to refractory walls without any oxide growth.”
Mike Grande thinks having one of his company’s ovens as part of the Industrial Internet of Things (IIoT) is better than having an on-site technician. The tech can troubleshoot a problem that arises and fix it, but the IIoT has the data to predict a failure or malfunction.
“If a parameter goes outside the bounds, we get a notification and we call them to see if they’re having a problem,” explains Grande, vice president of sales at Wisconsin Oven Co., East Troy, Wisc. “That allows them to do preventive maintenance ahead of time, so that they’re not down unexpectedly.”
Data analysis may even detect glitches in a foundry’s processes.
A Wisconsin Oven engineer may log in and see that every hour a customer’s burner fires at full blast for 10-15 minutes and uses a lot of energy to do so.
“That’s telling us, most likely, the customer’s opening the doors while they pull their load cart out and keeping them open while unloading and reloading the cart,” he said. “If they’re not trained properly, they just leave the oven doors open for the 10 minutes it takes to get the parts on it and put it back in.”
A deviation alarm can sound an alert in the same situation, but it’s an extra cost and not every oven has one.
“We can identify really simple, obvious things like that, which are causing a lot of energy loss and loss of efficiency,” Grande adds. “With the Industrial IoT, it has drastically changed the landscape, and allows us to provide a much higher level of responsiveness.”
All of Wisconsin Ovens’ higher-end machines are part of the IIoT, but having the data monitored and analyzed is an upcharge.
While the ovens aren’t used for melting metal, many foundries have one or more to preheat dies and to dry molds and cores before molten metal is poured into them. They ensure the release coating is dry, and residual moisture is removed.
Routine maintenance on Wisconsin Oven’s equipment is more straightforward than that for foundry furnaces. The maintenance includes cleaning the burner on its gas-heated equipment and taking amp draw readings on the heating elements in its electric equipment. If the oven is equipped with a conveyor belt, a maintenance technician should inspect the belt and slide bed for wear, check the tension on the belt and the drive chain, check the oil in the gear reducer, lubricate the bearings, and he’s done. All of the V-belts on the blowers should be inspected periodically, again, for wear.
rendered calcium silicate board Whether a furnace or oven, setting up and following a cleaning schedule and doing the maintenance outlined in the equipment’s manual should keep it in top shape to pump out BTUs and produce “the cleanest metal and longest-lasting furnaces in the business,” for as long as possible, as White wrote.