On my layout, they'll be headed off layout.
In real life, some of dust would be taken to a place called a sintering plant. This plant would collect iron particles from the dust and send it back to the furnace for melting. I've been told the sintering plant was a cut above the rest in terms of filth but I don't know much about sintering or the plant.
I know blast furnace waste products went to concrete/cement factories to be used in making concrete/cemet. I just don't know if the slag or dust was used in that process.
I would think the rest of the dust went to a landfill but I'm not sure. That wasn't in the book haha