Oxidative cleavage of lignin aromatics during chlorine bleaching of kraft pulp
Methanol liberation and methoxyl loss during chlorine bleaching of softwood kraft pulp were quantitatively investigated to estimate the degree of structural modification of lignin aromatics. An increase in the chlorine multiple led to enhanced methoxyl loss from lignin. Our result, using pH-adjusted chlorine water (pH 5.7), by which chlorination under oxidation-favorable conditions was achieved, strongly supported the importance of the oxidation reaction by chlorine during delignification and lignin degradation. It was also suggested that methanol can be produced not only via catalytic hydrolysis by chlorine but via oxidative cleavage of the ether bond as well. The infrared spectrum of chlorolignins suggested that chlorine oxidation can open aromatic rings to muconic acid derivatives without cleaving ether bonding of the methoxyl group. No straight relation between the methoxyl content and the kappa number of chlorinated pulps was shown. The methoxyl content of bleached kraft pulps subjected to successive chlorination and alkali extraction showed a good relation with the kappa number. This means that almost all the portions of the oxidatively modified lignin structure were successfully removed during these treatments, and the aromatic structures of residual lignin in chlorinated and alkali-extracted pulps were thought to remain intact.
Evaluation of the extent of the oxidation reaction during chlorine bleaching of pulp
A modified method was developed to evaluate how much chlorine is consumed by the oxidation reaction during the chlorine bleaching process. This evaluation is, in principle, based on the sum of chloride produced during the chlorination stage (C-stage) and produced during alkali treatment of both the C-stage effluent and the chlorinated pulp. Results obtained by this method proved that about 50%–75% of chlorine was consumed by the oxidation reaction during chlorine bleaching, depending on the reaction condition of chlorination. Even under a reaction condition that is not favorable to an oxidation reaction (low pH), approximately three electrons were abstracted from one lignin structural unit by chlorine bleaching. This result provides additional evidence for our recent observation that lignin is extensively oxidized during chlorine bleaching even when pure chlorine without any chlorine dioxide substitution was used.
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.