Kraft pulping is considered as most dominant process to obtain lignin from the lignocellulosic biomass. In pulping process via kraft method, aqueous sodium or potassium hydroxide (Lye) along with sulfides at a temperature range of 423-453 K for 2 hours to separate lignin from the cellulose under high pH condition.

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i dont know if both Hydroxide & Sulphur are added to the Kraft Process.

it might not require that much Sulphur, but may need to be part of the ingredients.