| The MILOX Process for Eliminating Chlorine and Sulfur from Pulping | Finland | - | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 26
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
New technology
Cleaner Production Application
The Milox process is a sulfur-free peroxyformic acid pulping method, which was the first process to produce fully bleached chemical pulp both from hardwood and softwood totally without sulfur and chlorine chemicals.
Peroxyformic acid has the ability selectively to delignify wood. The lignin is oxidized and broken down, and together with extractives and hemicelluloses, dissolves in the pulping liquor. The sulfur-free lignin can be precipitated from the spent liquor.
A three-stage cooking process allows peroxide consumption to be reduced. The first stage involves acid cooking in the presence of small amounts of peroxide at 80oC; this is followed by refluxing in formic acid without peroxide, at 100oC The process ends with oxidative peroxyformic acid cooking, again at 80oC.
This method converts both birch and pine into pulps with well separated fibers in yields of 40-50% depending on the Kappa number. Some pulping experiments have been made with Eucalyptus Globulus. The viscosity of the pulp is high, indicating little chemical damage to the cellulose.
At the end of the cooking process, birch pulp reaches a brightness of 47%, while pine pulp achieves 35%, with Kappa numbers of 3,5 and 9,0 respectively. Both birch and pine peroxyacid pulps can be bleached to over 90 brightness by means of an alkaline hydrogen peroxide.
Viscosity remains high and the low lignin content of the fibers keep peroxide consumption down. The strength properties of Milox birch pulps are close to those of birch kraft pulp. However the strength characteristics of pine peroxyacid pulps are much poorer than those of the corresponding sulfate pulps.
The kraft pulps pre-bleached with peroxyformic acid reach a brightness of 86% with alkaline hydrogen peroxide only. In a final stage using oxygen, ozone and peroxide, kraft can be bleached to 90 brightness.
Both birch and pine Milox bleached kraft pulps show strength characteristics and optical properties similar to those of conventional kraft pulps.
Pulps produced using the Milox method and kraft pulps bleached with chorine-free Milox method are all compatible with grades where kraft pulp is used today. Hardwoods can also be made into dissolving pulp.
Environmental and Economic Benefits
The cost of the first phase (cooking and bleaching) pilot will be around FMK 10 million. If the second phase is built the figure will be around doubled.
So far the effluents have not been characterized. The recovery systems of the pilot plant should provide important data. The greatest advantage is, however, that the effluent contains no chlorinated organic compounds.
Constraints
Use for the precipitated lignin has not yet been found.
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.height="6"