Re: exactly how do organic mercaptans form in paper mills

Thanks for the question, Bernard.  Here's a quick rundown of the formation 
of the organic mercaptans you asked about.  I assume you are interested 
because of the odor problems these little molecules cause.  The reduced 
sulfur compounds (mainly hydrogen sulfide and methyl mercaptan) are formed 
during the pulping process, before the bleaching process; hence the 
chlorine is not involved in the formation.  By the way, US mills are in the 
process of eliminating elemental chlorine from the bleaching sequence for 
environmental reasons.  

The mercaptans are then released into the air when the black liquor (used 
pulping chemicals) is burnt to recover the fuel value of the organic 
compounds removed from the trees.  Some of these compounds don't burn 
completely and are able to transverse the scrubbing systems in the outlet 
of the boilers and are released into the air.  

The Handbook of Pulping and Papermaking by C. Biermann (1996) reports that 
the sulfur molecules can be sensed by the human nose in the range of parts 
per billion, which makes them more of a nuisance than a hazard.  

I hope this is enough information to answer your question.