An industrial mixer is, from a fire protection standpoint, a pressure vessel that somebody has deliberately filled with a fuel and air mixture and then agitated. Whether the charge is a combustible powder, a flammable solvent, or both, the mixing action is specifically designed to achieve intimate contact between the fuel and the air in the headspace. That is precisely the condition required for a deflagration.
The ignition source in a powder blending operation is almost always electrostatic. Charging dry material into a vessel generates charge on the particulate, on the charging equipment, and on any conductive object in the transfer path. If any of those is not bonded to ground, it accumulates charge until it discharges into a headspace that may well be within its explosible range. Mechanical friction from a bearing failure or a foreign object in the agitator is the secondary source.
What distinguishes vessel protection from open-area protection is isolation. A deflagration inside a mixer does not stay in the mixer. It propagates through the charging chute, the discharge line, the dust collection duct, and any connected vessel, and the pressure piling effect in connected volumes can produce far more severe conditions downstream than in the vessel where it started. Protecting the mixer without isolating its connections leaves the propagation path fully open.