phenomenon of Alumina pick up by the liquid mould flux during Continuous Casting and how it affects the quality of cast strands?

The pickup of alumina in the mold causes the formation of calcium aluminates. Since there are a variety of different calcium aluminate morphs(each one with a higher melting temp.) over time the mold flux can and will begin to thicken up and the fluid lubrication on the strand mold faces can be interrupted causing surface defects & breakouts. Old timers used to throw Calcium flourides (spar) into the mold to liquify it but each time you do that you also increase the CA content which in turn will eventually thicken back up. Spar is a bad actor in the mold. In order to stay fluid there is a delicate chemical balance between CA & Al.