A new White Liquor Oxidation Technology: Experience from a recent start up at Metsa-Botnia, Kaskinen

摘要

Since the early 70's, oxidized white liquor has been the preferred source of alkali for the O_2 delignification processes, as it is less expensive than caustic soda. The white liquor stream requires however, that the sodium sulphide be preoxidized into sodium thiosulfate prior to its introduction in the pulp. Commonly, air has been used as the oxidant but progressively, more sophisticated processes based on oxygen have emerged. Higher efficiency, smaller footprint, less sulphur emissions and especially lower investment & maintenance costs explain this gradual change. Today more than 40 oxygen-based systems are operating worldwide.In large pulp mills where high amounts of O_2 are needed, a convenient way of supplying gas is with an On Site VSA production plant; the lower purity (around 93% O_2) is no longer a problem for most applications like bleaching, however the presence of inert gases represents a operational issue when it comes to O_2 based white liquor oxidation units and lower efficiencies were noticed.Recognizing that this problem could slow down the use of O_2 in the Pulp & Paper industry, Air Liquide started an investigation, which was conducted to develop a complete technology to address these concerns. This technology is now in operation in three pulp mills in Europe.This paper will cover in the first part, the main conclusions of the laboratory study: 1. Kinetics of oxidation from sodium sulphide to sodium thiosulfate ("partial oxidation") is an extremely fast reaction even at 110°C 2. Conversion to "fully oxidized " white liquor is possible at relatively low temperature, preventing metallurgical problems 3. Overall reaction is governed by gas to liquid transfer, meaning an efficient gas to liquid transfer system is essential 4. System can operate at 10 bars overall pressure. A multi-referenced existing gas transfer technology (self suction turbine) was adapted to white liquor oxidation and tested at pilot scale with success using high purity 99.5% oxygen. Additional studies helped in developing a special purge system; making possible the use of lower purity O_2 (down to 80 %), with excellent oxygen transfer efficiency. The second part of the paper will describe Metsa-Botnia, Kaskinen mill implementation within the course of a global mill modification aimed at replacing an air based system. Air Liquide was awarded the white liquor oxidation system. It has been in operation since mid 2005, and encompasses several interesting features, among them: 1. The production of 25 m~3/h of "partial oxidized" white liquor for standard use (O_2 delignification) 2. The test with "fully oxidized" white liquor in oxygen delignification to evaluate the global process enhancement 3. The future possible use of "fully oxidized" white liquor in the peroxide bleaching stage to reduce caustic requirement 4. The use of recompressed off-gas from the O_3 bleaching stage as the source of O_2, with a purity of 80 to 85% Experience and results after several months of operation are described.