Optimised cooling water treatment for the avoidance of deposits during strand cooling

The continuously cast strand must be cooled in a defined way as the cooling strongly influences the quality of the product. In the secondary cooling zone the heat is extracted from the material exclusively by water sprayed onto the strand surface through spray nozzles. The reaction of casting powder substances with the cooling water on the hot strand surface leads to the formation of hardly soluble compounds. The main reaction product is calcium fluoride which primarily causes an undesired, hard layer of deposits on the spray nozzles. Depending on the layer thickness, this can affect the spraying pattern and thus cause insufficient cooling, and can in severe cases lead to major plant malfunctions. Accompanied by a large series of investigations, a concept for the treatment of cooling water has been developed that reliably avoids the formation of these hard, mainly CaF2-containing deposits, using a practice-oriented laboratory testing method. In addition to the systematic selection of cooling water conditioning agents for the stabilization of CaF2, also the mechanism of deposit formation, microbiological aspects, a highly efficient method for the removal of the suspended solids in the spray cooling water and plant specific conditions had to be considered. Very positive operating results have been achieved with this cooling water treatment method.