Search

パルプの塩素漂白により発生する有機塩素化合物の多くは難分解性，蓄積性に加えて毒性を示すことが知られている。この有機塩素化合物の削減を目的として日本の製紙会社は1996年から漂白工程のECF転換を進めてきた。日本紙パルプ研究所では王子製紙，日本製紙のECF転換の進行に合わせ，排水による環境負荷を調査してきたが，その過程で明らかになったクロロホルムとクロロフェノール類の削減状況を報告する。漂白工程でのクロロホルム発生は広葉樹パルプ漂白系でパルプ1t当たり平均46g（g／t）から0．50g／tに，針葉樹パルプ漂白系で平均43g／tから0．55g／tへ95％以上減少し，漂白工程のECF転換の主要な目的であったクロロホルムの発生抑制は十分達成された。総合排水での排出濃度も発生の削減をうけ激減し，水道水質基準の0．06mg／Lをも下回るようになった。塩素漂白により発生する有害性の指摘される3塩素化以上のクロロフェノール類も，ECF転換によって発生はほぼ100％削減され，米国EPAのクラスタールールが求める漂白排水のクロロフェノール類の規制をクリアするレベルとなった。

日本の製紙産業はAOX自主規制値を設定し，漂白パルプ製造プロセスや排水処理設備を改善することによってダイオキシン問題を解決してきたが，1996年の改正大気汚染防止法を契機に，有機塩素化合物のさらなる自主的削減策として無塩素（ECF）漂白の導入が進んだ。その結果，2006年には漂白パルプの80％以上がECF漂白によって製造されるまでになった。ECF漂白を採用することにより，環境影響が懸念されているクロロホルム，クロロフェノール類，ダイオキシン類などの有機塩素化合物の漂白工程における発生量が著しく減少した。その結果，全有機塩素化合物の指標であるAOXは工場からの排出量が大幅に減少し，排出原単位は0．2kg／t以下にまで低下した。一方，二酸化塩素漂白の採用でクロレートの排出量が増加した。ECF漂白へ転換することによって排水の水生生物への影響が変化するかどうかに関しては，現在日本紙パルプ研究所で評価中である。パルプ漂白に関する有機塩素化合物の問題はECF漂白への転換によって一応の解決をみたが，製紙産業としては今後も継続的な環境対策を進めていくことが重要である。

Kraft pulp and sulfite pulp bleaching mills using chlorine, chlorine dioxide (D) and other chlorinated compounds are under surveillance of the dioxins regulation in Japan. One of questions is whether water qualities of process sewers in chlorine dioxide bleaching (Elemental Chlorine-free bleaching: ECF bleaching) mills are less than the environmental water quality (1pg-TEQ/L) or not. Objectives of this report are to review studies about the dioxins level of ECF mill sewers and model experiments from an aspect of the environmental water quality. Laboratory bleaching were carried out using a mill hardwood oxygen-bleached kraft pulp (LOKP), chlorine dioxide from a mill R 8 generator in a laboratory high-share mixer. Active chlorine factors were in the range of 0.14-1.14. In all cases, 2, 3, 7, 8-TeCDD and 2, 3, 7, 8-TeCDF were not detected for the pulps and spent liquors. Examination of process water from an ECF bleaching line in Niigata mill of Hokuetsu Paper Mills, LTD. shows that the concentrations of the dioxins in both sewers of the chlorine dioxide stage (D0) and the alkali stage (E/O) were below 1pg-TEQ/L, and that the 2, 3, 7, 8-TCDF concentrations were 0.41pg/L or less. In addition, a main source of 1, 3, 6, 8- and 1, 3, 7, 9-TeCDD in the process water seemed to be an agrochemical in water supplied from a river.In the meantime, Japan revised the Air Pollution Control Law in 1996. The main purpose of the law is to reduce discharge of toxic pollutants such as carcinogenic substances into the air. The Japan Papermaking Association estimated the nationwide chloroform discharge to air for the fiscal year 1996 at 1, 655 t on the basis of field studies, and initiated a reduction of the discharge. As a result, the estimated discharge was reduced to 1, 118 t, which was a reduction of 32%, for the fiscal year 1999. However, further reduction is required because there is still much discharge. We estimated chloroform formation and discharge in the mill having an ECF bleaching line and a chlorine bleaching line as well as an effluent treatment process. It was clarified that the chloroform formation in ECF bleaching was approximately one eightieth of that in chlorine bleaching.

With the implementation of the Cluster Rule in the United States, all bleached grade kraft pulpmills required to meet stringent levels of absorbable organic halides in the effluent. The method most used to meet the absorbable organic halide levels was to convert to 100% substitution of chlorine dioxide in the predominately C or CD first stage. As mills substituted chlorine dioxide in the first stage the bleachplant, thereby converting to elemental chlorine free (ECF) bleaching, several key parameters were impacted. Two of these were that delignification in ECF was less than partial substitution of chlorine dioxide in the chlorine stage and secondly the increase in pH in the first stage did not give the level of metals solubility as compared to chlorine or CD bleaching. Metals management has become a significant issue in ECF bleaching as metals that previously had a minor influence in bleaching create issues with inorganic deposition and efficiency reduction of peroxide bleaching. Several operating conditions have been identified that when optimized, reduced the issues associated with scale development in various stages of the bleachplant.

Scaling troubles of calcium oxalate are observed frequently in ECF (elemental chlorine free) bleaching process at a kraft pulp plant. Phenomena of the scale formation is very complicated which includes simultaneous nucleation, dissolution, growth and/or adhesion of crystals. The scale formation tendency can be predicted by a saturation index (SI) calculated by pH and concentration of scaling species.Scale inhibitor shows effects such as nucleation prevention, crystal distortion and suspended solid dispersion. For scale inhibitor treatment, a critical saturation index (CSI) is defined as a maximum SI which shows no nucleation at some inhibitor concentration.ECF bleaching process has a tendency to show higher SI compared with a conventional bleaching. At a washing filter of chlorine dioxide stage in ECF process. CSI optimized “Depoclean” treatment shows a stable operation without scaling trouble.

Since many studies have pointed out ECF (Elemental Chlorine Free) as environmentally friendly with no harmful by-products and no specific toxicity of the resulting waste waters compared to TCF (Totally Chlorine Free); ECF bleaching is now considered all over the world as a Best Available Technology. Most of the incoming projects over the world are based on ECF bleaching and world ECF bleached pulp shate is still growing since it took the leadership in 1996. About 38 Mt of pulp was ECF bleached in 1997, 33 Mt was ECU (elemental chlorine used) bleached and 4 Mt was TCF bleached.Whatever wood species considered, transfer to ECF technology is easily achievable with respect to brightness and mechanical properties goals while minimising the quantity of organohalogenated by-products. Western Europe has almost completely turned to ECF and with the new Cluster Rules regulation (November 1997), NORTH AMERICA pulp mills still using chlorine are moving to ECF.To summarize we can conclude that moving to ECF technology is the best economical way to minimize pollutant and organohalogenated emissions with low investment cost and taking care of pulp properties. ECF is also the bleaching technology which will allow us to meet the year 2000 minimum mill effluent challenge.