One of the most impressive spectacles on steel industry is bleeding the blast furnace, as featured on the videoclip.
Bleeding the furnace is a very rare event, and occurs only perhaps once or twice per blast furnace campaign. It happens when the column of ore, coke and limestone inside the furnace clogs and no more descends downwards. Suddenly the gas pressure inside increases.
To prevent a destructive explosion and shards of the furnace, hot metal, slag and materials ricocheting and spilling around, the bleeding valve on top of the blast furnace is opened. The blast furnace gas burns immediately into carbon dioxide when contacted with the outside air, and a cloud of carbon (read: soot) spills around. While this may look like a catastrophe, the furnaces have been designed with emergency bleeding in mind, and the furnace can return operational fairly quickly after the bleeding.
This particular event happened at Corus plant, IJmuiden, Netherlands.