Ward Brothers Steel was convicted by magistrates in Bishop Auckland last Friday and ordered to pay £1,200 costs to the Environment Agency (EA).
The firm admitted two breaches of the waste management licence it holds for the site in Albert Road, Darlington, and two charges of not reporting a fire.
Durham Fire Brigade was called to a blaze at the site in December 2003 and said vapour from leaking aerosol cans had been ignited.
The EA investigated this and found that staff had not inspected the waste responsible for the fire to see if it was hazardous.
Ward Brothers Steel then compacted the aerosols in clear breach of its waste-management licence.
Firefighters were called out again in March 2004 when an EA officer noticed a second fire at the same site. The company gave no explanation for this.
An EA spokesman said: "This case shows just how dangerous waste can be and how vital it is that licence holders comply with the proper procedures when accepting waste.
"It took several hours for the fire brigade to put out the first fire, and this, combined with the EA's investigation, has taken its toll on the public purse."