MANUFACTURE OF PAPER AND PAPER PRODUCTS # 68

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

Oxygen delignification can be carried out using one of the following possible modifications: the HC process (high consistency), the MC process (medium consistency), or the atmospheric MC process.

In a typical medium-consistency system pulp from a brownstock washer or decker at about 10-14% consistency is charged with caustic or oxidized white liquor, is preheated in a low-pressure steam mixer and is pumped through one or more medium-consistency gas mixers to an upflow pressurised reactor. Medium-pressure steam and oxygen are added upstream of the medium-consistency mixer or are added directly into it. The reactor bottom may be conical or may have a rotary bottom, and the top is equipped with a discharger.

These features, in conjunction with a proper reactor geometry, are required to minimize channelling in the reactor. Post-oxygen filtrate may be added to the top of the reactor to facilitate discharging of the pulp. The stock is depressurized and is blown through a separator, where small amounts of by-product gases, inert gases, and oxygen are released. Depending on the temperature in the reactor, steam may also be flashed from the pulp in the separator. This steam may be vented, returned to the low-pressure steam mixer, or condensed in a heat exchanger.

MC systems are delignifying kraft pulps from 37% to 50%, and most operate in the range of 40% to 45%.

Medium-consistency reactors operating at atmospheric pressure at the top have been used to delignify sulfite pulps requiring a relatively small reduction in kappa number. It has been proposed that atmospheric medium-consistency reactors could also be used with kraft pulps, particularly hardwood pulps, to obtain a reduction in lignin content of 30-35%. So far the atmospheric MC process has not been installed in kraft pulp mills.

Magnesium compounds are normally used to preserve pulp quality in MC systems, particularly where intensive delignification is required. Some systems that process kraft hardwood pulps, sulfite pulps, and speciality pulps such as screen rejects are operating without magnesium additions.

Since the medium-consistency process has in recent years become a practical alternative to the high consistency process, pulp producers who are considering the installation of an oxygen delignification system must evaluate which type of system to install. The main factors affecting the relative economics are: capital cost of the system, the degree of delignification, and the steam, alkali oxygen and power consumption.

The largest MC system currently in operation has a capacity of 665 air dry metric tons/day. However, a MC system with a capacity of 1365 air dry metric tons/day is expected to be in operation in Brazil (since 1989).

The first commercial MC oxygen delignification plant was started up in 1980 at the Consolidated Paper Corp.'s kraft pulp mill in Wisconsin, USA. Since then some 30 MC plants have been installed in kraft pulp mills.

Material/Energy Balances and Substitutions

Environmental and Economic Benefits

Since lignin contents in pulp are reduced by 45-50% in the oxygen stage, less chlorine and chlorine dioxide are required to achieve the same brightness level. This results in less BOD, COD, and color in the subsequent bleaching stage effluent.

The following investment and maintenance costs are reported in Swedish crowns (1990) for a 600-metric ton/day kraft mill.

Constraints

Using oxygen delignification the kappa number of softwood pulp can be reduced by approximately 50% without deteriorating strength properties of the pulp. This is possible no matter if the brown stock has been produced by conventional or extended delignification. The same degree of delignification is obtained with the HC and MC process. Using the atmospheric MC process a reduction of the kappa number by 30-35% is considered viable.

When applying oxygen delignification to the production of hardwood pulp made of birch, the same reduction of the kappa number could be obtained. However, oxygen delignification extended to low kappa numbers reduces the pulp yield and therefore lower delignification degrees are applied for birch-pulp.

Contacts

Industry/Program Contact and Address

Consolidated Paper Corp.'s Kraft pulp mill

Wisconsin, USA.

Mrs Virve Tulenheimo, MSc, Research Engineer

Technical Research Center of Finland

Non-Waste Technology Research Unit

P.O.Box 205

SF-02151 Espoo Finland

Tel: +358 0 4561; Fax: +358 0 460 493

Telex: 122972 vttha sf

 

Abstractor Name and Address

Same as above.

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.