A method for extending your cupola campaign.

With the proper equipment, refractories and maintenance, you can avoid headaches while maximizing melt capacity.

As more tons are melted in a cupola, dimensional changes occur as a result of refractory wear. Traditionally, to avoid problems associated with lining wear, foundries dropped their cupola bottom daily or - at the most - weekly to repair the refractory. Doing so, however, places the cupola in a nonproductive mode.

But as years have passed and demands have increased, many foundries have made modifications to their cupola to enable them to melt for weeks without shutting down for repairs. Extended melt campaigns provide multiple benefits, ranging from reduced cupola operating costs to boosting operating equipment efficiency (OEE) of the molding centers. And increasing OEE through improved cupola operations increases sales potential without large capital investment.

In the melt shop alone, benefits include reduced costs of materials and labor per melt ton. Safety is also enhanced, as employees spend far less time in the hazardous environment of the cupola.

Recently, many foundries discovered that they can dramatically reduce cupola downtime by using higher quality refractories and making dimensional adjustments "on the run." By shutting down the cupola only for short periods of time on a weekly basis, foundries can still ensure consistent performance and minimize risk of premature failure while "getting more" out of their cupolas.

Presented here is Grede's view of factors involved in successfully extending melt campaigns. This article describes the steps along the way, from a freshly lined cupola at week one to the final bottom drop at week five.

Grede's Cupola

Producing 100% ductile iron, Grede-Reedsburg operates a 96 in. water-cooled "liningless/bare shell" cupola. Other melt shop notables include two holding furnaces and external desulferization. This cupola typically operates 5.5 days/week for 5 weeks prior to dropping bottom. It is normally blasted at 14,000 CFM and is complemented by supersonic oxygen injection and 1000F (538C) dirty air recuperative hotblast. During its five-week melt campaign, this cupola's operating abilities change little from the first day to the last. This is due mainly to its liningless nature, along with good, scheduled maintenance based on accumulated melt tons.

This cupola typically melts 4250 ton/week, with a capacity of 5520 ton/week, based on a five-day week. When demand dictates the need, this cupola can operate comfortably 6.5 days/week with a 12-hr cooldown, repair and startup period referred to as a "non-dropbottom" weekend. Every five weeks, when bottom drop takes place, a 24-hr period is needed for cleaning, repairs and startup.

Does it Make Sense?

Certain conditions must exist to make extended melt campaigns practical. Obviously, there must be enough molding capacity and iron demand to consistently absorb the increased melt.

Properly sized holding furnaces are real assets to the cupola's operation. Holders have two main advantages: they allow the cupola to keep melting during reasonable molding delays; and smooth out iron chemistry variations inherent to cupola melting.

The need for premium quality refractories is a must with extended campaigns. Don't let the increased cost/lb scare you; it will be offset by many other benefits downstream. Also, a topnotch proactive maintenance program is needed for all support equipment. Without a preemptive maintenance program, even the finest cupola operators and refractory technicians won't succeed.

Finally, and probably the most important for a consistently successful operation, is the need for clear, detailed procedures. Employees and supervisors at the floor level should develop such procedures - they have the necessary experience and technical knowledge.

Refractories

Grede's cupola differs from others in that the carbon well area refractory is cast. The main reasons for using a cast carbon well vs. blocks are the castable well's lower replacement cost and less time requirement, as well as a longer life. When operating for multiple weeks without looking at the inside, there must be a high level of confidence that the refractory in this area won't wear thin and result in a failure. Figure 1 shows the key factors and chemical compositions.

Each time the bottom is dropped, about 3 in. of carbon spray is applied to the castable. The taphole/breast area and frontspout are rammed in with plastic.

The area just under the bottom brickshelf has a fillet of 80% alumina spray. This prevents the bottom shelf from being exposed and melting off. Behind the fillet are several stainless steel refractory hangers that hold this fillet in place. The first half of the lined upper stack is bricked with a 9-in. cupola block. This block contains about 83% alumina and 10% silica.

The upper remaining portion of this stack is bricked in with an iron cupola block made by Grede. This is a hollow block filled in with insulating castable. Again, because these bricks wouldn't be maintained or replaced for several weeks in a row, a more durable brick was needed for this upper area. These bricks have been in use for more than a year and continue to look like new.

The cupola bottom material consists of a plastic material for sealing seams and a ramming mix that actually makes up the working bottom.

Putting the Cupola Together

To aid in sealing, the first door set in place has an upper lip that the second door (with a lower lip) will close against. Once the first door is in place, the dummy pin is wedged in place to secure this door in the closed position. Now the seal is placed on top of the first closed door, "minus the wrap" for later use. Now, the second door is raised into the closed position with the main pin being wedged in place to secure this door in the closed position.

Next, the cupola repairman climbs down a chain ladder from the charge door to ram the plastic seal in place. This is done with a small-headed rammer to seal all seams and holes.

That job accomplished, the person exits the cupola and adds one-third of the total bottom ramming material. Re-entering the cupola with a shovel, he then levels the material across the cupola bottom to form a final 8-in. slope upward away from the taphole. Once leveled, the material is thoroughly rammed with a floor rammer. After this, the second and third portions are added and the same process is followed as the first one-third. [ILLUSTRATION FOR FIGURE 2 OMITTED].

Prior to leaving the cupola a final time, the worker rams a small bott of moist molding sand over the inside of the taphole. This closes off the taphole, keeping it clean for an easy tap [ILLUSTRATION FOR FIGURE 2 OMITTED].

Next, paper and cardboard are added to the cupola until they reach a level above the tuyeres [ILLUSTRATION FOR FIGURE 3 OMITTED]. This has three purposes: it protects the fresh refractory bottom from the first coke entering the cupola; protects the protruding tuyeres from the coke; and aids in a quick, effective light-off of the new coke bed.

The Bed Burn-in Process

Prior to lighting the coke bed, a thorough check is done of the cooling system. Once the coke bed has been lit off, it is burned-in using a consistent process keying off time, temperatures and set points that have been developed from past experience. Repeating exact practices will deliver similar start-up results. When the desired bed height is verified by measuring through the charge with a premeasured, weighted cable, the stack can be charged full with the predetermined charge mix.

Starting the Charge Mix

If the regular charge mix includes silicon carbide (SIC), we recommend that a proper addition of ferrosilicon (FeSi) be added to the first few charges to raise silicon (Si) until the SiC has seen enough time and temperature to get the job done.

We've found a great advantage in charging SiC during extended melt campaigns because it protects and prolongs the life of the carbon well area - probably because there is ample Si and C to keep the iron from trying to take it from the refractory. We also believe the SiC, along with extra graphite, more effectively ties up free oxygen. It is also thought that the SiC actually creates a physical coating over the well area as it waits to disassociate.

The first few charges also contain a higher percentage of steel to control carbon at tap time. Also, ensure that the safety tuyere is fully open and functional.

Melt-in/Tap-in

With a full stack and all settings (including CFM) at their set points, the cupola can be turned on. After a predetermined amount of time, the cupola is tapped by ramming a steel bar into the taphole. Make sure caution is used should it be a dry tap, allowing iron to blow out. Also, keep burning bars and/or lance rods nearby should oxygen be needed to assist in the tap [ILLUSTRATION FOR FIGURE 4 OMITTED].

Adjusting Dam Height During Week

After tapout and chemistry stabilization, the cupola should be ready to operate steadily for the entire week. By now, you will already know where you want your dam height to start the week. As experience is gained, you can predict when and how much to raise the dam height throughout the week by monitoring the tons melted. If the dam is raised to slowly allow even small amounts of blast air to escape through the taphole, the taphole will wear quickly. At its earliest stage, this leakage will appear as waves in front of the spout, not actual blowing.

In-Operation Tuyere Replacement

Occasionally, a tuyere might be lost during operation. Traditionally, when this occurred, operators would run the cupola the rest of the shift and then drop bottom. With a continuous extended melt campaign, however, that practice becomes costly.

Prior to shutting the water off to the bad tuyere, we add water with a directed hose to the area around that tuyere's flange to prevent hotspots. With the water shut off, the bad tuyere is then allowed to completely burn off. Then, over the next several hours, the tuyere will be pulled and a new one will be installed. Caution must be taken, but this process works well. Prior to the new tuyere being put into the hot cokebed, it must have water hooked to it, which sometimes requires a temporary hose.

Melt-down and Drain 'Non-drop bottom Week'

At the end of the week, the cupola stack contents are fully melted and the iron stream is stopped in a traditional manner. Be sure to reduce the windblast as the charge level comes down. This minimizes the extreme heat introduced to the iron brick in the upper stack, preventing the possibility of melting the face off the iron bricks.

For a good, clean drain, it is best to melt the entire stack full without shutting off. Then, drain the cupola in a traditional manner. Once it has drained, all tuyeres must be kept sealed (including the safety tuyere) so the cupola won't continue to melt through a chimney-type draft effect. Allowed to occur, this can create a real mess in the taphole area. The other area draft can occur is the taphole, and this is addressed by pushing it full of green sand. This seals the taphole so there's no draft and allows the coke bed to cool off, and also makes it easy to scrape the sand out of the breast area that will later be cleaned up and repaired. If the windbox/tuyere design doesn't stop the draft, stuff an insulating fiber blanket into the tuyere.

Refractory Repairs

The breast area must be cleaned from the outside using a chipping hammer and chisel. Once this area is free of slag and only clean refractory is visible, the taphole is ready to repair.

Repair consists of laying a hollow metal taphole pipe in the breast at the right measurement and thoroughly ramming the proper refractory around the pipe. We use a 4-in. square steel tube of the length needed. Prior to setting the pipe in place, 3-4 in. of molding sand is rammed into the end that lays to the inside of the cupola. This keeps the taphole from filling full of slag and iron during bed burn-in.

All other refractory repairs should be handled in a normal manner. If you have any doubt about a certain refractory area lasting the full campaign, replace it. As soon as the refractories have all been rammed in place, put them under torch to cure prior to restarting.

Now, the cupola is ready to startup again. If the tuyeres are showing, cover them with paper and cardboard and duplicate the same process described earlier.

During each week, the exact same things must be done as described, with the exception of draining and bottom drop.

Dropping Bottom

Again, the cupola stack must be melted empty and drained in the same traditional manner. There are some minor differences, however.

For example, when the cupola is finishing draining, a hole is chipped up through the bottom while the doors are still up. This hole is located about one-fourth of the way across the bottom from the taphole. If a heel develops during the extended campaign, this hole ensures that all the iron and slag will be drained.

During this time, the blast is left on to burn the coke to tuyere level for easier quench and cleanup. Once operators are certain that no molten material remains in the cupola, the taphole is rammed full of refractory and then the cupola coke is quenched with water through the tuyeres. Water is added until it reaches tuyere level. This cools the refractories and after 30 min, allows for a safer cold drop.

At this time, the support pins are pulled out and the doors will swing open, exposing the refractory bottom. This bottom is then poked out with a ram mounted on the tines of a forktruck.

Refractory Repairs 'Drop Bottom Week'

With the bottom down, all refractory surfaces are cleaned of slag and foreign materials. During this time, the breast receives a better cleaning, as it is easier to access. Then, a larger portion of the breast is replaced with new refractory and the taphole pipe and other forms are properly located and measured.

At this time, the safety tuyere is repaired from inside and out, if needed.

During the drop weekend, the water-cooled tuyeres are closely inspected and, if needed, action is taken.

Once the refractory repairs are made with the rammers, a 3-4 in. layer of carbon spray is applied over the inside well area to seal and offer some protection. The bottom 12 in. of the well area doesn't get sprayed. Remaining rough and unsprayed, it offers a good surface for the new bottom to bind into and thus prevents possible leaks.

Now that all refractory work is done, the torch stand that holds four torches is moved under the cupola and is started up for a good cureout of the refractories, plus one in the frontspout.

From this point on, week after week, all practices should be followed as described earlier.

This article was adapted from a presentation at the 1997 AFS Wisconsin Regional Conference.