Firefighters say a passerby noticed smoke coming from the building, which has been vacant for several years.
Authorities say demolition crews had been doing hot work inside the building Wednesday.
According to Bucksport's Fire Captain Pam Payson, the hot work produced some sparks which smoldered in one of the dust collectors before eventually catching fire.
Payson says workers helped firefighters extinguish the flames.
"They had heavy equipment that they could actually pull the dust collector off the side of the building, which kept the fire in that main spot," Payson said. "Most of it, you know, the problem, it's old dust that's been collected and compacted."
While the property currently is vacant, there are several proposals for redevelopment of the land, including a planned salmon farm.