Coking coal is loaded into coking ovens to make coke and to to collect volatile chemicals from which products such as nylon are made. The coke is used in blast furnaces in which the coke is burnt to generate heat and carbon monoxide which reduces iron oxide to iron which is collected in the lower reaches of the blast furnace. As the molten flows through the heated coke some carbon from the coke is dissolved.
Molten iron is tapped from the blast furnace is cast to provide pigs which can be used in foundries or is sent to further processing in open hearth furnaces or other refining processes AOD crucibles to produce steels of controlled carbon contents and to adjust other alloying elements.
My experience goes back to the '70s, travelling north on I-75 through Saginaw MI. I spent many hours waiting for the draw bridge across the expressway, watching smoke billow from GMs Central Foundry smokestacks. The bridge was up so freighters could bring coal to the foundry.
Finally, after more than enough fits and starts, the tall new Zilwaukee bridge opened across the bay. Except, by then, the smoke stacks were gone, replaced by electric furnaces, and the river was empty.