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Paper Manufacture

Introduction

Paper manufacture has its origins dating back more than 5000 years when the Egyptians produced papyrus made from the stem of the papyrus plant, Cyperus papyrus. The modern-day type of paper originates from China around AD105 and is very similar to how paper is manufactured today. The basic raw material is wood pulp which apart from water contains three main ingredients cellulose, lignin and hemicelluloses. The action of pulping is to break down the bulk structure into constituent fibres. This can be achieved by chemical means, mechanical processes or a combination of all and are described below.

Chemical Pulping The two main techniques used are the Kraft process and the Sulphite process, with the Kraft process being essential, accounting for around 80% of the produced chemical pulp. Both of these processes remove the bulk of the lignin and hemicelluloses, with the Kraft process causing less damage to the cellulose fibres, hence producing stronger fibres. The sulphite process, however, makes the pulp easier to bleach. Kraft Pulp Process A flow diagram showing the important parts of this process:



Wood Chips – These are small pieces of wood chip which get stored in a silo.

Digester & Blow Tank – The digester is a large tank containing a combination of chemicals, a solution of sodium hydroxide and sodium sulphide (white liquor), and heat and pressure of around 150 psi to dissolve the lignin to start the process of converting the chips to the pulp. After the digester, the pulp passes to the blow tank causing the wood to separate into individual fibres.

Screening & Washing – The larger fibre pieces are screened out and reprocessed. The pulp has a washing process to remove chemicals and dissolved lignin. This process takes several hours then the lignin binder dissolves. The cellulose fibres, called brown stock, are separated from the spent cooking liquor or black liquor. The brown stock is then stored to await bleaching.

Bleaching – This part is usually done with chlorine and then a sodium hydroxide extraction. Then this is followed using sodium hypochlorite and finally a chlorine dioxide treatment.

Pressing & Drying – The pulp slurry gets converted into sheets where the water starts draining via gravity and suction. Finally, the water content gets reduced to less than 10% using the steam-heated jets.

Pulp Bales - Immediately after the drying stage, the sheets are cooled to keep their colour. The sheets are then cut into smaller sheets and baled. Sulphite Pulp Process In chemical pulp manufacturing, this accounts for less than 20%. The main issue is that the effluent cannot be recycled. This process produces pulp which is virtually pure cellulose fibres.

The main chemicals used are sulphur or iron sulphide and calcium carbonate (limestone). These help form sulphur dioxide gas and calcium bisulphite, the key chemicals in the digestion process. They are produced by spraying water down a tower containing the limestone whilst sulphur dioxide is blown up the tower.

Woodchips are steam-heated under high pressure with this mix. The acidic mix dissolves the lignin but also attacks the outer wall of the fibre and forms a weaker paper. However, this process does form a brighter pulp. Mechanical Pulping

In its simple terms, the logs are debarked and then mechanically ground with various grindstones to separate the fibres. The heat generated by this action softens the lignin and allows the fibres to separate. This pulp is then washed clear of these grindstones by showers of water. However, this mechanical nature damages many fibres that give paper low strength. Also, as the lignin is retained in the pulp, the resulting paper discolours and becomes brittle with time.

There are benefits to this pulping process in that the pulp yield is very high, the cost of the paper is cheap, and it avoids using those costly chemicals.


Modern methods of mechanical pulping tend to use two different types of processes, called refiner mechanical pulp (RMP) and thermochemical pulp (TMP).

Refiner mechanical pulp (RMP) In this process, wood shavings or washed chips get fed into grooved discs of a refiner. Here compresses the wood that generates heat, softening the lignin and forcing the fibres apart. It helps produce longer, unbroken fibres giving a stronger paper.

Thermomechanical pulp (TMP) In this similar process, the wood chips get preheated in steam then a two-stage refining process is used. Both frictions from the refiner discs and the steam soften the lignin causing the fibres to separate. In both techniques, processed wood pulps are passed through screens to remove the unseparated fibres. There are also processes which combine both chemical and mechanical methods. They tend to be intermediate in quality of the two individual techniques. Defoamers in Paper Manufacture

The conditions used in a paper mill are ideal for the generation of foam. These are water, sources of surface-active materials, air and mechanical energy. Air is the main problem in the generation of foam that causes various issues like circular marks on the paper, pin-holing etc. In paper manufacturing, there are many pumps, agitators etc. Here can lead to the formation of foam. Several defoamers are used in this area like, silicone-based, oil-based, water-based, powder defoamers and polymer types.

In the Kraft process, the foam gets generated in the black liquor. This is one of the most challenging foams to control, and in this process, the conditions are highly alkaline. In an ideal world, only one defoamer would need to be used. The various factors mentioned above require different products to combat the foam issue.

The main addition area is in the screening and washing stage.

At present Lankem, have two oil-based defoamers that have the potential to be utilised in the paper manufacturing process. These are Dfoam AX1 and Dfoam AR2. Both of these products are powerful defoamers.



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