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RECYCLING: Finnish mill first to use new flotation concept

Finland’s first deinking mill, Keryskuitu, in Sunila, is also the first to try a new flotation system, Metso Paper’s OptiBright concept. The installation of the multi-stage cells was part of a large…

Finland’s first deinking mill, Keryskuitu, in Sunila, is also the first to try a new flotation system, Metso Paper’s OptiBright concept. The installation of the multi-stage cells was part of a large modernization project at the mill, which opened in 1978. The two Metso MC 105 units, which run in parallel, and the common secondary MC 30 unit replaced Outukompu cells. There are air control boxes on each unit.

Affiliated with Stora Enso since 1996 (Myllykoski and Metsa Serla are minority shareholders), Keryskuitu can now produce 75 000 tonnes per year (t/y) or 340 t/d of DIP from a furnish of old newspapers (60%), old magazines (30%) and 10% mixed, mainly printing house waste. The mill tries to keep these same ratios in the pulp it produces.

The mill’s main customers are the Stora Enso mills in Summa (70% of production, newsprint) and Varkaus (15%, newsprint). Other customers include Myllykoski (8%, SC paper), Metso Tissue (4%), Fort James Suomi (1%, tissue) and UPM-Kymmene Tervasaari (2%, envelopes). As far as the customers’ use of DIP, newsprint makers will use from 30% all the way up to 60% DIP in their furnish. SC papermakers use 10 to 20% DIP.

As could be expected with its largest customer, Keryskuitu has a close working relationship with the nearby Summa mill, with cooperation in the areas of production planning, quality management, R&D, human resources and the environment. There is also working relationship with the adjacent Sunila pulp mill for wastewater treatment, electricity, fresh water and fire services.

Keryskuitu, as well as other Finnish DIP mills, is supplied by collection companies that are, for the most part, owned by the large pulp and paper firms. The recovery rate in Finland is about 70% and rising. The mill’s recovered paper is very clean and fresh; yield is about 86%. The biggest problem the mill faces is envelopes. There are discussions underway to remove envelopes from the collection process.

The mill can tailor the pulp for clients to include as much ONP or OMG as desired, but its tries to keep its “recipe” standard. Paper is trucked to the mill and dumped. Front end loaders then scoop the paper into the pulper.

Along with the flotation units, the mill installed a new tramp pulper, primary and fine screens and replaced part of the cleaning plant. It also installed a Neles (now Metso) Automation DNA process control system. This made perhaps the biggest difference to the mill as all previous controls were manually operated.

Age and capacity were the reasons behind the project. The original machinery was showing its age and the mill wanted to increase its capacity. The project was fast tracked. The decision to upgrade was made in March 2000 and start-up took place just nine months later, in December 2000. Keryskuitu chose the OptiBright units in May 2000. The mill was shut on December 10-11, 2000, to remove the old flotation units. The new cells were installed beginning on December 18 and were ready for water tests by Christmas. By December 30, stock was being fed to the units and a 60 brightness level was achieved four days later. Incoming brightness is around 48 to 50. Since start-up, some R&D has been done on the rotor design to increase efficiency. New rotors were installed in summer 2001 to compare results with the originals.

The basic operating principle of the flotation units is the use of physical phenomena (density differences) between the aeration sector and the separation sector of the flotation cell to transport pulp from one aeration stage to the next. This is said to save pumping costs.

After being fed to the cell, the pulp flows downward in the separation sector. At the bottom, it flows into corresponding aeration sector. The flow is induced by the pulp density difference between the separation part (pulp suspension) and the aeration part (pulp suspension diluted with air).

In the lower part of the aeration sector, the air is dispersed into the pulp with a high-efficiency, low-energy air dispersion rotor. The air flow and rotor speed and, thus, the average bubble size can be adjusted. Each aeration sector has its own rotor. Low-pressure air is fed to the centre of the rotor through the air holes.

Impurities are attached to the air bubbles in the high-turbulence zone of the aeration sector (around the rotor). Ink-bubble agglomerates rise with the flow caused by the density difference to the upper part of the aeration sector, where the flow is directed toward the cell wall. The pulp flows to the separation sector where it again flows downward.

The cell structure causes the main flow to proceed according to a spiral-shaped trajectory. Due to the cell structure, the number of aeration times during cell retention exceeds the number of aeration stages.

The ability to control flotation parameters such as air-to-pulp ratio, average bubble size and reject properties as well as the ability to optimize the flotation process according to the targetted results are the advantages claimed by the system.

Preliminary results from Keryskuitu show improved brightness stability, the possibility of high ash removal (more than 50%), less ash in water circulation and lower fibre losses.

Typical operating conditions for the OptiBright system are 7000 to 8000 L/min/cell, a feed consistency of 1% to 1.2%, a feed ash of 16 to 18%, a temperature of 45C, ash removal of 30 to 50% and a brightness gain of 10 to 12 units.

Prior to buying the new MC units, the mill ran a multi-stage pilot unit for several months. The concept was developed about five years ago with work led by Metso’s general manager, technology, Jukki Heimonen. It has been Juha Pousi’s “baby” for the last 4.5 years. Pousi is Metso paper’s process manager, technology centre, mechanical pulping lines, in Anjalankoski, Finland. The project with Keryskuitu really started in 1998. The mill realized it had to replace the flotation cells. It did trials at the Metso facility in summer and autumn 1998 with conventional technology. In 1999, Keryskuitu had access to the new flotation concept and did more trials. That autumn, the mill had to make a decision and Metso got “serious” about the proposal, Pousi said.

This is Metso’s first reference and, as with any installation of this type, it is a popular “tourist” location. Since start-up the mill has hosted groups once or twice a month.