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A cupola or cupola furnace is a melting device used in foundries that can be used to melt cast iron, ni-resist iron and some bronzes. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters and can range from 1.5 to 13 feet (0.5 to 4.0 m). 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 gases 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 lined in a similar manner but often a clay and sand mixture ("bod") may be used, as this lining is temporary.Finely divided coal ("sea coal") can be mixed with the clay lining so when heated the coal decomposes and the bod becomes slightly friable, easing the opening up of the tap holes. The bottom lining is compressed or 'rammed' against the bottom doors. Some cupolas are fitted with cooling jackets to keep the sides cool and with oxygen injection to make the coke fire burn hotter.
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. During the melting process athermodynamic reaction takes place between the fuel and the blast air. 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 metal which raises the carbon content of the iron. Silicon carbide and ferromanganese briquettes 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. Additions to the molten iron such as ferromanganese, ferrosilicon, Silicon carbide and other alloying agents are used to alter the molten iron to conform to the needs of the castings at hand.
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 grayishcolor 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.