Kinetics of the reduction of sodium sulfate in kraft black liquor ash.

摘要

The kinetics of the reduction of {dollar}rm Nasb2SOsb4{dollar} by H{dollar}sb2{dollar} in a model mixture of {dollar}rm Nasb2SOsb4{dollar}-{dollar}rm Fesb2Osb3{dollar}, and by H{dollar}sb2{dollar}-CO in a model mixture of kraft black liquor ash (KBLA) as well as in actual KBLA, both mixed with {dollar}rm Fesb2Osb3{dollar}, was investigated in a thermogravimetric analysis system. {dollar}rm Fesb2Osb3{dollar} is a catalyst for the reduction of {dollar}rm Nasb2SOsb4{dollar}.; The kinetics of the reduction of {dollar}rm Nasb2SOsb4{dollar} mixed with {dollar}rm Fesb2Osb3{dollar} by H{dollar}sb2{dollar} was well described by the second-order nucleation and growth model (NGM) for X {dollar}leq{dollar} 0.80 and by the shrinking-core model subject to product layer diffusion control (SCM-PLD) for X {dollar}geq{dollar} 0.80 over a temperature range of 655-700{dollar}spcirc{dollar}C. The rate of conversion of the reduction of pure {dollar}rm Nasb2SOsb4{dollar} at 685{dollar}spcirc{dollar}C without the presence of {dollar}rm Fesb2Osb3{dollar} was smaller, and could be described by the conversion-time (X-t) expression valid for the phase boundary reaction controlled model for a solid-solid reaction of spherical particles.; The kinetics of the reduction of {dollar}rm Nasb2SOsb4{dollar} in a model mixture of KBLA was well described by the third-order NGM for 0.15 {dollar}leq{dollar} X {dollar}leq{dollar} 0.90 and by the SCM-PLD for X {dollar}geq{dollar} 0.90 over a rather narrow temperature range of 720-730{dollar}spcirc{dollar}C. The reducing H{dollar}sb2{dollar}-He feed gas contained 10% CO to suppress the decomposition of {dollar}rm Nasb2COsb3{dollar} by H{dollar}sb2{dollar}. The X-t curves in the temperature range of 675-715{dollar}spcirc{dollar}C are described by SCM-PLD kinetics during the deceleratory period.; The kinetics of the reduction of {dollar}rm Nasb2SOsb4{dollar} by H{dollar}sb2{dollar}-10% CO in actual KBLA is well described by the third-order NGM for 0.20 {dollar}leq{dollar} X {dollar}leq{dollar} 0.95 over a temperature range of 700-725{dollar}spcirc{dollar}C. The conversion range described by the NGM is smaller between 680 and 700{dollar}spcirc{dollar}C and a long induction period is observed.; The change in color of the reduced samples from red-burgundy to black-grey indicated that {dollar}rm Fesb2Osb3{dollar} had been changed chemically during the reduction. The formation of mixture of FeS and possibly FeS{dollar}sb2{dollar} during the reduction is proposed based on XPS/ESCA analysis of the actual KBLA. Although SEM micrographs showed that local sintering/fusion occurred to a certain extent in the reduced samples, complete reduction of {dollar}rm Nasb2SOsb4{dollar} could be obtained.; Based on these observations and the kinetics, the possible mechanisms for the catalytic effect of {dollar}rm Fesb2Osb3{dollar} and that of the H{dollar}sb2{dollar}/H{dollar}sb2{dollar}-CO reduction of {dollar}rm Nasb2SOsb4{dollar} involving both solid-solid and gas-solid reactions are proposed.