In our SLC kiln, we frequently found melting material during kiln start up. What are the possible causes such as insulation brick after shut down, ash from co-fuel used in kiln heat up period?

It is very difficult to say what material is melting without seeing the situation on the ground. However you must remember that the thermal energy must be much higher during start-up as there is no exothermic contribution from the formation of cement clinker. For this reason it is entirely possible to have some melting when feed is first applied to the kiln.