Ladle (metallurgy)
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In foundry work a ladle is a container used to transport and pour out molten metals. It needs to be:
- Strong enough to contain a heavy load of metal.
- Heat-resistant like a furnace.
- Heat-insulated as much as can be managed, to avoid losing heat and overheating its surroundings.
For foundries making small castings, a hand-held ladle somewhat resembling a kitchen ladle for soup is enough, with a long handle to keep the heat of the metal away from the person holding it.
For bigger castings and in steel mills, it can run on wheels, a purpose-built carrying car or be slung from an overhead crane.
Ladles are most commonly made of fabricated steel. The most common shape is a vertical cylinder, but other shapes are possible: the most common of these is known as a 'torpedo' ladle, is shaped as a horizontal cylinder suspended between two bogies, and is commonly used to transport liquid iron from a blast furnace. Between the molten metal and the steel shape is a refractory material that may be from 1 mm (0.04") to 150 mm (6") thick or more. The refractory material protects the steel shell and acts as a thermal barrier to contain the heat.
Ladles can be "lip pour" design or "bottom pour" design:
- For lip pour design the ladle is tilted and the molten metal pours out of the ladle like water from a pitcher.
- For bottom pour ladles, a stopper rod is inserted into a tapping hole in the bottom of the ladle. To pour metal the stopper is raised vertically to allow the metal to flow out the bottom of the ladle. To stop pouring the stopper rod is inserted back into the drain hole. Large ladles in the steelmaking industry may use slide gates below the taphole.
Ladles can be either open-topped or covered. Covered ladles have a (sometimes removable) dome-shaped lid to contain radiant heat; they lose heat slower than open-topped ladles. Small ladles do not commonly have covers, although a ceramic blanket may be used instead (where available).
Medium and large ladles which are suspended from a crane have a bale which holds the ladle on bearings, called trunnions. To tilt the ladle a worm gear mechanism is used, which tilts the cylindrical shell while the bale carries the weight. The gear mechanism may be hand operated with a large wheel or may be operated by an electric motor or pneumatic motor. Internal friction brakes are used to regulate the tilting speed of the ladle. The largest ladles are poured using a special, two-winch crane, where the main winch carries the ladle while the second winch engages a lug at the bottom of the ladle. Raising the second winch then rotates the ladle on its trunnions.
Some ladles are designed for special purposes such as adding alloys to the molten metal. Ladles may also have porous plugs inserted into the base, so gases can be bubbled through the ladle to enhance alloying or metallic treatment practices.
