| The Modified Continuous Cooking Process for Producing Pulp with High Viscosity | Sweden | 1989 | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 58
Background
This case study was submitted on the part of the Working Group on Cleaner Production in Pulp and Paper Industries in the framework of the UNEP IE/PAC Cleaner Production Program with the support of the Technical Research Center of Finland's Non-Waste Technology Research Unit.
Cleaner Production Principle
Process modification
Cleaner Production Application
The modified continuous cooking (MCC) process is a process that produces pulps of high viscosity suitable for oxygen delignification.
The MCC process rectifies the drawbacks in a conventional, continuous kraft cook. The effective alkali concentration through the duration of the cook is evened out by decreasing it at the beginning of the cook and increasing it at the end. This makes up for the high effective alkali concentration at the beginning and low concentration at the end of a conventional kraft cook.
In a conventional cook, the high bulk lignin concentration at the end of the cook hinders diffusion of dissolved lignin out of the chips, causing reprecipitation. This is tackled by reducing the concentration of bulk lignin from the digester and adding fresh chemicals, accomplished by carrying out the final cooking stage countercurrently, where white liquor is also added.
The MCC process has been accomplished through the modification of two-vessel Kamyr continuous digesters to allow both the introduction of white liquor at several points and the countercurrent flow of the liquor during the final cooking phase.
A non-chlorine fiber line, which has been commissioned, consists of a digester for medium-consistency cooking and a medium-consistency, oxygen-delignification stage, followed by an efficient post-oxygen washing stage with pressure diffuser and a wash press. Bleaching is carried out with the sequence (C+D)(EOP)DD with a pressurized extraction stage and a pressure diffuser for efficient washing. If required, the first stage can be carried out without chlorine.
The MCC process has been successfully introduced in the pulp industry. Today the method has been applied in about ten mills. A laboratory study was conducted to show the bleachability of MCC pulp. The bleaching sequences were (C+D)(EO)DD and D(EO)DD.
Environmental and Economic Benefits
At 100% substitution of chlorine dioxide for chlorine in the first stage AOX is less than 1,0 kg/ton in the effluent before external treatment, chlorinated phenols are 98% eliminated, and the dioxin content of the bleached pulp is close to the limit of detection.
The above mentioned fiber line, which have been commissioned in a Scandinavian mill, can be regarded as one of the most advanced in the world today, meeting current and future requirements for the environment while still producing fully competitive pulp. It is possible to produce bleached pulp without the use of chlorine, achieving brightness even above 90% ISO, the level required for the most demanding market pulps.
Compared with conventional bleaching, a non-chlorine alternative has some significant effects on the environment.
The extra chemical costs for D(EOP)DD bleaching are about $10/ton (1989) or less, a reasonable cost when compared with conventional alternatives.
Constraints
The absence of chlorine requires extra efforts in all process stages from cooking to final bleaching. For final bleaching, the sequence D(EOP)DD is suitable, with some modifications of normal practice. In the first stage, the chemical charge should be generous. Low-multiple alternatives are not relevant when chlorine is absent. Also, in the enforced extraction stage, both temperature and oxygen pressure should be high. Magnesium is a good viscosity protector in such an EOP stage and when the hydrogen peroxide is absent.
Contacts
Review Status
This case study was submitted by the UNEP Working Group on Cleaner Production in the Pulp and Paper Industries, based at the Technical Research Center of Finland (address above) in 1992, as part of a contract for UNEP IE. Before submission, the case studies were reviewed at the Center. They were edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.