Denver firefighters are trying to clean up a hazardous materials spill that sparked a flash fire at the Rock Bottom Brewery in downtown Denver.On Friday morning, around 7:15 a.m., a worker dropped a keg on the area of floor where chlorine dioxide was spilled earlier this week, firefighters said.The impact of the aluminum keg on the floor caused a spark and a flash fire occurred, said Denver Fire Capt. Craig Carter. No one was hurt, but a small toxic chlorine cloud was formed.Denver firefighters are in the restaurant trying to clear the fumes and clean the chlorine dioxide from the concrete.Apparently, chlorine dioxide was not completely cleared from the floor when it spilled on Tuesday, Denver Fire said. Chlorine dioxide is used for industrial-strength cleaning and bleaching.Firefighters have set up a blue decontamination tent outside the restaurant. Several fire engines and at least a dozen firefighters responded to the hazmat call.Rock Bottom Brewery is located on the 16th Street Mall. Curtis Street was closed from 15th to 17th streets as firefighters work to contain the spill.The 16th Street mall shuttle is still running.