Thiourea dioxide is abbreviated as FAS in papermaking. FAS can be bleached because sulfoxylic acid and dilute sodium sulfate generated by decomposition in alkaline solution have reducing properties. During bleaching, the structure of chromophoric groups in pulp can be changed and the absorption of light can be reduced. Improve pulp whiteness. FAS bleaching is easy to operate, has good bleaching effect, and has no pollution to waste water, which can be directly discharged into farmland.

Factors that affect the results of FAS bleaching are as follows:

①FAS dosage. At different bleaching temperatures, the brightness of deinked pulp from waste paper increases with the increase of FAS dosage. When the amount of FAS is less than 0.3%, the whiteness increases significantly; when it exceeds 0.6%, the whiteness increases slowly.

② NaOH and FAS dosage ratio. Considering the chemical cost and bleaching effect, it is more suitable when the dosage ratio of NaOH and FAS is 2.

③Bleached pulp concentration. Generally speaking, when the concentration of bleached pulp increases, the effective concentration of chemicals during bleaching increases accordingly, which is beneficial to the improvement of bleaching efficiency. But as long as the mixing is uniform, FAS bleaching at medium or high concentrations is feasible.

④ bleaching time and temperature. Bleaching time and temperature In the process of FAS bleaching pulp, the effective bleaching time is the shortest when the process condition is 80℃. When the bleaching temperature is low, it can generally be compensated by prolonging the bleaching time.