What do we mean by saying that a ladle cover knots, elbow knots, blowpipe knots in refractory materials? What is the “knot”?
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Blowpipe is also the construction of unshaped refractory materials. Refractory material for ladle cover is light weight heat insulation refractory material. What you said that the construction of bend out refractory materials in some steel mills called "knot".
A less accurate view of the statement is probably the meaning of the “construction of refractory material" , for example, pouring castable can be called “knot”, the pounding of the construction can also be said that, so it is with spraying paint.