It is likely that an exothermic reaction was the ignition source for the fuel in the dust collector. Of the two types of fuel – steel dust, and oil – it is more likely that the oil was the fuel and also [indirectly] the ignition source. It is well-known that “partial oxidation” of hydrocarbons is exothermic – leading to an increase in temperature – and can lead to the formation of aldehydes which have low auto-ignition temperatures. For acetaldehyde, the AIT is 175 °C, and a thick layer of oily dust can prevent escape of the heat that results from the “partial oxidation”. To prevent recurrence of fire, you may consider either: (1) discharge the accumulated dust into water, or (2) minimize accumulation of oily dust by frequent removal from the dust collector and deposit the dust in water prior to storage, transport, and/or disposal.