General purpose welding rods (KS Standard E4303), depends on which welding rod standard numbering system you use......we tend to use standard AWS E6013.
PS. Your rods (I suspect) are cheap Chinese rods, the quality isn't always the best available, also damp, incorrect current, rod angle and travel speed can affect slag release...
Somewhere here there is a thread were TechicAl (a member who knows these things) explains about the rod covering/flux/slag its purpose/make up, maybe someone can remember the thread and post a link...
Looks like zzr is right, they are probably Hyundai. There is an importer in Kings Lynn (Finlex). Hyundai are Korean and actually their products are usually as good as European.
I dont know this rod so I cant really comment but it appears to be a general purpose but they describe it as a lime titania coating. This is a sort of hybrid 6013/7018 so im not surprised that you are having problems with the slag.
Then again I could be assuming too much and making the wrong conclusions