Manufacture of Sulphur bake
• Thionation of the various aromatic amino or nitro compounds with sulfur was formerly carried out in iron pan s fitted with agitators and heated by gas fire. However, some bakes begin to stiffen up as the reaction progresses preventing further agitation; consequently, the baking process is not uniform throughout. Baking pans have been replaced by more efficient iron cylinder rotary bakers. The bakers, usually of 500–1000 kg capacity, are heated directly by gas jets or hot flue gases from a fire source and rotated at 2–10 rpm on hollow‐end trunnions supported by self‐aligning bearing rollers. The off‐gases from the reaction are led through a catch pot to a scrubber unit containing caustic soda solution.
• S and NaHS for recycling. The bakers are rotated until the raw dye is ground to a powder. It is discharged and standardized to strength and shade puri fied by solution in either caustic soda or sodium sul fide. Insoluble matter is removed by filtration. The liquors are evaporated to dryness on a steam‐heated rotating single‐ or double‐drum dryer or the dye is precipitated by the addition of acid or sodium bisul fite or by blowing air into the alkaline brew. H2S generated is absorbed in caustic soda solution. The precipitated product is filtered, washed, and air blown dry before discharge. The final drying is usually carried out in fan‐assisted steam‐heated air ovens