MoDoCrown®

1995

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Spruce wood

MoDoCrown® sulphite pulp, both the paper - and dissolving grades, is made from spruce ( Picea Abies ) a softwood species with long and thin flexible fibres. The wood substance in softwoods is composed of two different cells, tracheids ( 90-95 % ) and ray cells ( 5-10 % ). The tracheids give the mechanical strength required, and provide for water transport. The fibre dimension depend on several factors; genetics, the region where the tree is grown and the actual position in the tree. The average length of Scandinavian softwood tracheids is 2-4 mm and the width is 0.02-0.04 mm. The ray cells consists of parenchyma cells and transport liquid in the radial direction of the tree. The fibres give a paper grade pulp which is easily beaten and with good strength characteristics and a dissolving pulp with high alfa-cellulose content.

Pulping process

A flow sheet of the sulphite process is shown in figure 1.(or in figure 1a)

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The raw material used is 100% spruce for the dissolving grade, but since we use a two-stage cooking process, up to 10% pine may also be used for the paper grade production. The logs are debarked, chipped and the chips are then fed sequentially to the thirteen batch digesters. The first stage is a neutral impregnation stage with sodium sulphite, and the second stage where the lignin is dissolved. During dissolving production, both stages are performed under acid conditions. This enables both the lignin and hemicellulose to be dissolved and a high alfacellulose content is obtained.

Easy to bleach

The cook gives an even pulp quality as a result of computerized process control. Control is based on the analysis of the concentration of active cooking chemicals in the cooking liquor. The kappa number after the cook is approximately 8 -10 for the paper grade and 3 - 4 for the dissolving grade, which means that both grades are very easy to bleach. The two stage cook gives a paper pulp grade with high yield and high hemicellulose content. This results in an easily beaten pulp. The two stage process also makes it possible to adjust different pulp characteristics such as tensile and tear strength. After the cook the pulp is washed in one of the twelve batch diffusers as screening the resin content of the pulp is decreased by controlled internal deresination, where the resin is solubilized by alkali. To ensure a low resin content in the system as well as in the pulp, process back water is also deresinated. After deresination the pulp is bleached in two stages. The prebleaching is an alkali stage which is reinforced with oxygen during the production of the paper grade pulp. Peroxide only is used in the second and final bleaching stage

Closed loop bleaching

Since 1991 both the paper and dissolving grades from the Domsjö Sulphite Mill are bleached in a completely closed bleach loop. This unique technical break-through is possible due to the fact that the bleaching process uses peroxide only and it has resulted in low emissions of BOD (biological oxygen demand) and COD (chemical oxygen demand). The emission of AOX is 0 for both pulp grades. These reductions are on the same level for both the paper and dissolving grades. Both the paper grade and the dissolving grade can be fully bleached, i.e. MoDoCrown® ECO bright > 89% ISO, MoDoCrown® about 85% ISO and MoDoCrown® Dissolving >91 % ISO respectively. After bleaching the pulp is screened before it is dried on a drying machine. The pulp is then sheeted, baled and packed in units of six bales.

Recovery

All the spent liquor from the bleaching and washing stages is recovered. The remaining effluents, such as condensates and losses, are treated in a highly efficient biological treatment plant. The spent liquor is burnt in one of the two recovery boilers. From the melt resulting from combustion in the boiler chemicals and energy are recovered and new cooking liquor is prepared. Flue gases from the recovery boilers are treated in flue gas cleaners developed by MoDo.

The paper grade

MoDoCrown® is an easily beaten pulp. It also develops strength rapidly with beating. These two facts can be seen in figure 2.
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Due to the thin flexible fibres it is easy to obtain a high density ( figure 3 ),

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which is desirable in the case of greaseproof papers. However, the MoDoCrown® sulphite pulp has to be beaten. more carefully than a kraft pulp. By so doing the pulp will maintain its unique fibre characteristics and its strength will not be lost due to fibre cutting.

Characteristics and applications

MoDoCrown® sulphite pulp paper grade is characterized by
- very good beatability
- low beating energy demand
- high hemicellulose content
- high initial wet strength
- extremely good formation compared to other softwood pulps
- low adhesion to the yankee cylinder
- low resin content
- no emission of AOX
- very low emission of oxygen consuming organic matter due to closed loop bleaching
MoDoCrown® is very well suited for the following paper products:

Printing and writing paper


Coated paper

The thin and flexible sulphite fibres give extremely good information in paper. They also contribute to a smooth surface, for both coated papers as well as high-quality printing and writing papers.

Folding box board

The elastic modulus of MoDoCrown® is high. ( figure 4 )

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Pulp with high elastic modulus gives stiffness to a board when used in the outer layer. MoDoCrown® is therefore well suited to be used in the tops and the backs in folding box board.

Greaseproof, transparent and high density papers

Sulphite fibres are easily beaten. Highly beaten MoDoCrown® has a dense structure and low opacity at very high drainage resistances, thus making it very suitable in greaseproof, transparent and high density papers.

Coloured paper

When the paper grade pulp MoDoCrown® is a major component in the furnish, it has shown to give an even and efficient dying in the production of coloured paper. This is one of the reasons that the pulp is preferred in many different types of papers of different shades and colours.

Tissue

The two stage-stage cooking process makes it possible to obtain properties important for tissue manufacture, such as good strength at low beating rates, low adhesion to the yankee cylinder and softness in the finished product. MoDoCrown® is thus an excellent component in tissue.

The dissolving grade[23kB-firewo.gif]

The modified two-stage cooking process makes it possible to obtain dissolving pulps with a high degree of uniformity. The most important requirement for a viscose pulp is an even viscosity. This is obtained by controlling the viscosity in the cooking and bleaching processes so it will suit the customer requirements.

Characteristics and applications

MoDoCrown® Dissolving is characterized by


- even viscosity ( controlled to any viscosity of 400-900 ml/g )
- high brightness
- high alfacellulose content
- low resin content
- low metal content
- no AOX emissions
- very low emission of oxygen consuming organic matter due to closed loop bleaching
- very good chemical reactivity in the viscose process
- excellent viscose filterability ( figure 5 )

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- possibilities of running with extreme high viscosity of the viscose solution due to very good filtration characteristics
- optimum distribution of the molecule weight of cellulose in the pulp requires low charges of carbon disulphide during xanthation

MoDoCrown® Dissolving is used for the production of rayon staple fibre, cellulose fibre and CMC.

Domsjö Sulphite Mill and the environment

Thanks to two-stage digestion and chlorine-free bleaching, the Domsjö Sulphite Mill has a closed bleach plant.
This has greatly reduced the AOX emissions.

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The effluent from other parts of the mill undergoes biological treatment where bacteria break down the organic material into e.g. methane gas which we use as fuel.
We minimize the release of fibres in sedimentation basins where the fibres are recovered before the water is discharged.
The fibres, too, are used as fuel. Thanks to these environmental investments,which have resulted in an improved oxygen saturation of the surrounding Örnsköldsviks bay to 90 %, salmon are once again migrating past the mill on their way up the Mo river to spawn.

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With electrostatic precipitators and flue gas cleaning we have also reduced the emissions of sulphurous substances into the air by more than 70 % in recent years. What now remains is to finetune the mill even further. This is where personal involvement is so important.
Together with new equipment and new environmental technologies, we also invest heavily in information and training of our personnel in environmental questions.

Research and development

The Mo och Domsjö Research and Development laboratory and the technical staffs of our mills have a long tradition in the development of new processes for pulping and this work continues both in the fields of more environmentally compatible products and overall product improvement.The MoDo technical staff together with the R & D laboratory is very keen to assist and advise customers on problem solving and product development.

End

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