Cupola furnace
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A Cupola or Cupola furnace is a melting device used in foundries. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters. The overall shape is cylindrical and the equipment is arranged vertically, usually supported by four legs. The overall look is similar to a large smokestack.
The bottom of the cylinder is fitted with doors which swing down and out to 'drop bottom'. The top where gases escape can be open or fitted with a cap to prevent rain from entering the cupola. To control emissions a cupola may be fitted with a cap that is designed to pull the gases into a device to cool the gasses and remove particulate matter.
The shell of the cupola, being usually made of steel, has refractory brick and refractory patching material lining it. The bottom is lining in a similar manner but often a clay and sand mixture may be used, as this lining is temporary. The bottom lining is compressed or 'rammed' against the bottom doors.
To begin a production run, called a 'cupola campaign' the furnace is filled with layers of coke and ignited with torches. Some smaller cupolas may be ignited with wood to start the coke burning. When the coke is ignited, air is introduced to the coke bed through ports in the sides called tuyeres.
When the coke is very hot, solid pieces of metal are charged into the furnace through an opening in the top. The metal is alternated with additional layers of fresh coke. Limestone is added to act as a flux. As the heat rises within the stack the metal is melted. It drips down through the coke bed to collect in a pool at the bottom, just above the bottom doors. A thermodynamic reaction takes place. The carbon in the coke combines with the oxygen in the air to form carbon monoxide. The carbon monoxide further burns to form carbon dioxide. Some of the carbon is picked up by the falling droplets of molten steel and iron which raises the carbon content of the iron. Silicon carbide and ferromanganese briquets may also be added to the charge materials. The silicon carbide dissociates and carbon and silicon enters into the molten metal. Likewise the ferromanganese melts and is combined into the pool of liquid iron in the 'well' at the bottom of the cupola.
The operator of the cupola, the cupola tender, observes the amount of iron rising in the well of the cupola. When the metal level is sufficiently high, the cupola tender opens the taphole to let the metal flow into a ladle or other container to hold the molten metal. When enough metal is drawn off the taphole is plugged with a refractory plug.
The cupola tender observes the iron through the sight glass for signs of slag formation, which is normal. Most slags will rise to the top of the pool of iron being formed. A slag tap hole, located higher up on the cylinder, and usually to the rear or side of the iron taphole, is opened to let the slag flow out. The viscosity is low (with proper fluxing) and the red hot molten slag will flow easily. Sometimes the slag which runs out the slaghole is collected in a small cup shaped tool, allowed to cool and harden. It is fractured and visually examined. With acid refractory lined cuploas a greenish colored slag means the fluxing is proper and adequate.
During the production, samples may be taken from the metal and poured into small molds. A chill wedge is often poured to monitor the iron quality. These small, approx 18 mm (3/4") wide x 38 mm (1-12") tall triangular shaped pieces are allowed to cool until the metal has solidified. They are then extracted from the sand mold and quenched in water, wide end first. After cooling in the manned the wedges are fractured and the metal coloration is assessed. A typical fracture will have a whitish color towards the thin area of the wedge and grayish color towards the wide end. The width of the wedge at the point of demarcation between the white and gray areas is measured and compared to normal results for particular iron tensile strengths. This visual method serves as a control measurement.
After the cupola has produced enough metal to supply the foundry with its needs, the bottom is opened, or 'dropped' and the remaining materials fall to the floor between the legs. This material is allowed to cool and subsequently removed. The cupola can be used over and over. A 'campaign' may last a few hours, a day, weeks or even months.
Some cupolas are fitted with cooling jackets to keep the sides cool and with oxygen injection to make the coke fire burn hotter.
Cupolas can be used to melt iron, ni-resist iron and some bronzes.
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