Effect of Sodium Sulfide Pretreatment on Kraft Pulping, 1999 Pulping Conference Proceedings

Ildefonso Lopez, Hou-min Chang, Hasan Jameel, Wolfgang Wizani

A high sulfide concentration is very important during the transition form initial to bulk delignification. This transition occurs at approximately 150 o C. It is well known that the shortage of hydrogen sulfide ions at this point of the kraft cook leads to the formation of stable enol ether structures in the residual fiber lignin, since this type of compounds are formed from b-aryl ether linkages under alkaline conditions. This is an undesirable reaction since the formation of such structures effects bleachability and the ability to pulp to a low kappa number. The objective of this research was to study the impact of pretreatment with a range of hydrosulfide and hydroxide ions on pulping selectivity. In this study, the wood chips were treated with sodium sulfide followed by a standard kraft stage. The impact of the various treatment conditions on the amount of sulfur adsorbed on to the chips was also quantified using a sulfur mass balance. Pretreating chips with Na2S solution results in a higher pulp yield and viscosity as compared to a reference kraft cook. Optimization of the pretreatment conditions using a factorial design showed that an increase in the [NaSH] concentration in the pretreatment liquor between 0-30gpl resulted in a lower kappa number. The pulps also have a higher yield and viscosity at the same kappa number. An increase in the [NaSH]

concentration lowered the kappa number quite significantly without a severe a drop in the viscosity. An increase in the [NaOH] concentration in the pretreatment liquor resulted in a lower yield and viscosity at the same kappa number. Pretreatment times between 0.5-1.5 hr. did not have a significant impact on kappa, yield or viscosity. A shortage of sulfur during the transition phase of pulping can result in lower delignification rates. The amount of sulfur adsorbed during the pretreatment increased with the HS - concentration and temperature. Higher OH-concentration decreased the sulfur adsorbtion. Treatment with liquors containing HS-, decreases the amount of %AA required in the kraft cook. The sulfide pretreatment along with the decrease in the %AA results in higher yield and viscosity at the same kappa number. The above results can be used to optimize the pretreatment conditions in the modified batch pulping processes.