"kokylna" is a colloquial or steel industry slang for "blast furnace", or perhaps even "blast furnace plant/workshop". However, in the context it just referss to a blast furnace. I believe the term originated in the term "coking", meaning (as extensively explained on the webpage referred below):
"the heating of coal in coking chambers, closed off from the outside air, in the course of which the volatile constituents such as coke oven gas, tar, benzol, hydrogen sulphide and ammonia are expelled, collected and recycled for other uses"
Personally, I am not surprised anyone working in those types of plants for more than a few weeks would have suffered some damage...