Crews came under attack while dealing with a scrap wood and sawdust fire at Bloomfield Recycling in Tipton on Wednesday evening.
A fire spokesman said police were attending and were dealing with the bystanders who were "making life difficult" for crews.
Rail services have been affected by smoke across the nearby railway line.
Smoke from the blaze could be seen from miles around and reached about 50ft (15.2m) into the air.
Trains diverted
A West Midlands Fire and Rescue spokesman said: "There has been some difficulty with bystanders.
"Youths have been making life difficult for some of our firefighters by stoning them."
Network Rail said train services were being disrupted between Coseley and Tipton with services currently at a standstill within the area.
Virgin Trains on the West Coast Mainline were being diverted and not calling at Sandwell and Dudley.
It is not yet known what has caused the fire. Ambulances are on standby at the scene as a precaution.
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