In order to avoid heat losses through radiation, it is known to place a granular covering material on the top layer of a casting mold or ladle. The material used may be sawdust, glass, sodium silicate or Vermiculite. Recovered material such as aluminum oxide (Al2O3), calcium oxide (CaO) or aluminum drops may also be used.
While the pouring of granular or powdered material onto the top layer of a casting ladle was initially carried out in a rather rough fashion by means of a hoisting mechanism and a chute, it was soon realized that it was necessary to find a more accurate solution. It is indeed desired to obtain accuracy in the metering of the desired quantity in order to obtain a predetermined layer thickness and also an even distribution of the bulk material over the whole surface to be covered.
An improved spreading device was suggested in EP 0 389 918, wherein the device comprises a fixed part from which a cover with scrapers is suspended, and a rotating part fitted below the scrapers. The rotating part comprises a series of slits that are uniformly distributed around a central shaft. The slits are radially widening in the direction away from the central shaft. Through the rotating movement of the rotating part and the levelling action of the fixed scrapers, the bulk material is uniformly distributed over the slits of the distribution bin. A rising and falling cover makes it possible to process varying volumes of bulk material.
While this device achieves a homogeneous, protective and insulating covering layer on the top layer of a casting ladle containing molten steel it also has a number of disadvantages. Indeed, due to the central shaft, no material can be deposited directly to the center of the surface to be covered. This problem may however be partially solved by the use of deflectors for the bulk material.
Another disadvantage of the device is that after each operation, the distribution bin must again be filled with bulk material, which may cause some delay between two filling operations. Such a delay may e.g. cause unnecessary cooling of the molten steel or metal contained in the next ladle to be covered.
Furthermore, it should also be noted that the construction of the device is rather cumbersome and necessitates a non-negligible amount of preventive and corrective maintenance work.
Although, the device of EP 0 389 918 provides a homogeneous layer on the top layer of a casting ladle containing molten steel, there is still room for improvement.