The burning of ibuprofen and sulfamethoxane powders in your vibratory sifter could be attributed to electrostatic “spark” discharges if the following conditions were simultaneously present in the sifter:
• A conductive (metal) section of the sifter that comes into contact with the powder such as the sieves/screens or the inlet or outlet conveying chutes is electrically isolated from ground
• The electrical capacitance of the ungrounded section of the sifter is sufficiently large to accumulate enough electrostatic energy to ignite/burn the dust particles, when released as a spark
• The movement (flow/vibration) of the powder particles over the ungrounded metal section gives rise to a sufficiently high level of electrostatic charge buildup on that section
• The isolated metal section is close (2-4 mm) to a grounded part of the sifter so that the spark can jump from the charged section to the grounded/uncharged section
• Dust particles are present within the spark gap
To prevent “spark” discharges I suggest that all the metal sections of the vibratory sifter be electrically bonded together and connected to ground so that the resistance to ground is less than 10 ohm. Please note that I have assumed that the vibratory sifter does not comprise of any plastic sections such as plastic pipes.
I also suggest that other sources of ignition such as frictional heating, frictional sparks, electrical arcs (from electric motors, etc), and self-heating be investigated and eliminated/ruled out.