NH4OH–KOH pulping mechanisms and kinetics of rice straw

The mechanisms and kinetics of NH4OH–KOH mixture pulping rice straw were studied. When aqueous ammonia was mixed with a small amount of caustic potash (ratio of 1:5), three distinct delignification phases were observed in the pulping process: a bulk delignification phase from the beginning of the cooking period to 100 °C, a supplementary delignification phase from 100 °C to 155 °C lasting a further 45 min, and a residual delignification phase until the end of the cooking period. There were two silica removal phases; the first phase was from the beginning of the cooking period to 100 °C and the second phase was from 100 °C to the end of the cooking period. The rate of delignification reaction was first order with respect to residual lignin and 0.3 order with respect to [OH−]. The silica removal was pseudo-first-order with respect to residual silica and 0.6 order with respect to [OH−]. The activation energies of the delignification and removal of silica reactions were 35.6 and 30.9 kJ/mol, respectively.