Arriving at the pulp mill
The pulp used for Sappi paper comes from well-managed tree farms. It arrives at the pulp mill in several forms - as whole tree trunks, as wood chips and as paper pulp from other mills. We also make pulp from secondary fibre sources such as waste paper.
Sappi mills produce different types of paper to meet the exact needs of the customer and the desired characteristics of the end product.
At the pulp mill
Here the logs first have to be debarked. The debarked trunks are either ground to fibres for mechanical wood pulp or processed to chips for chemical pulp. Recycled pulp is made using waste paper.
With the mechanical pulping process, the wood is processed into fibres by adding water and grinding it.
During the chemical pulping process, lignin, the natural 'glue' that holds the wood fibres together, is dissolved, freeing the wood fibres. Chemical pulping creates either a sulphate or sulphite pulp, depending on the chemicals used. The fibre obtained in this way is quite clean and undamaged. The paper made from chemical pulp is called coated fine paper (or woodfree paper).
In the course of the recycled pulping process, newspapers, cardboard boxes and magazines are de-inked at some of Sappi's mills. End-uses for paper made during this process include linerboard and fluting.
Whitening the pulp
Untreated wood pulp has a brown or brownish colour and has to be bleached before it can be used to make white papers.
Pulp can be bleached with chlorine or chlorine compounds, as well as with oxygen or hydrogen peroxide. Chlorine-based processes have fallen out of favour because of their environmental impact. Consequently, elemental- and totally-chlorine free processes, such as those at Ngodwana Mill, are now more frequently used.
Sappi pioneered the Sapoxal oxygen bleaching process - a world first, now an industry standard. Sappi was also the first paper manufacturer to make woodfree paper from elementally chlorine-free pulp.
Refining the pulp
The bleached pulp is now refined to give it the exact properties required for the particular type of paper. At the refiner, the pulp passes through a system of rotating and stationary blades which fibrillate the fibres to enhance the way they mesh together and cause the bonding properties of the fibres to be increased.
This operation allows a strong, tenacious paper sheet to be fabricated for the manufacture of boxes and sacks and provides the surface properties for printing grades according to customer specification.
Preparing the 'mix' or 'furnish'
The furnish consists of a blend of pulp - usually hardwood and softwood with some machine broke according to a recipe for a particular grade of paper. After refining, the pulp is processed to define the final characteristics of the paper.
Various chemicals are added depending on the particular specifications of the paper to be made. For example, fillers such as clay and calcium carbonate (chalk) are added for opacity, brightness and smoothness; dyes are added for shade control; optical brighteners are added for whiteness and sizing agents are added to control liquid penetration.
Sizing agents coat the surface of the individual fibres and fill the spaces between them in order to minimise the ability of liquids to 'wet' the paper, thus forming a barrier to moisture.
When all the components are correctly prepared - all dissolved in water - they are mixed in their required proportions and this 'mix', more correctly termed the 'furnish', is ready for conversion on the paper machine into a continuous sheet of paper.
By far the most important component added at this stage is water. About 100 litres of water are needed to make every kilogram of paper. This usage can only be economically and ecologically justified through recycling this water. Thanks to closed water circulation and sophisticated water treatment plants, Sappi's paper machines recycle about 90% of their water.
Paper mill digester where lignin and other components of wood are removed from the cellulose fibres
At the paper machine
The primary function of the paper machine is to create a uniform web of paper. The paper machine has three major components - the base sheet forming section, the press section and the drying section.
The headbox continually agitates the furnish to prevent the fibres from 'clumping' together. The paper machine has a continuous forming wire moving at high speed. When it leaves the headbox, the furnish is only about 1% fibre and 99% water. The furnish is now rapidly dewatered along the length of the moving wire.
The fibres begin to bond and a mat is formed. The mat remains on the surface of the wire, while water continues to be removed from both sides as the furnish is sandwiched between the top and bottom wires.
From the wire, where the sheet is now about 80% water, the new paper web passes into the press section. Here it is squeezed between a series of pressure rollers that reduce the water content to about 65%. Then the web goes into the dryer section of the paper machine where each side is passed in turn over a series of steam heated drying cylinders.
Depending on its intended end-use, the paper may need further embellishment such as a surface coating with starch in a size press, either integrated into the paper machine (on-line coating - not discussed here) or as a separate process (off-line coating).
Finally, at the end of the machine, the paper is wound into a large reel.
A base sheet of paper has now been manufactured.
To optimise the printing characteristics of the paper, special coatings can be applied in an off-line coater.
There are several types of coating processes available. The most frequently used systems are the film size press and blade coating processes.
Film press coating provides a uniformly thick coating layer which follows the surface of the paper and is known as contour coating.
In the blade coating process, excess coating is applied onto the paper and afterwards 'doctored' or scraped off by a steel blade. Due to the pressure of the doctor blade, a uniform paper surface is created as the irregularities of the paper are coated.
Each side is coated and dried in sequence - with drying being performed by either infrared heat, hot air folds or drying cylinders. Coating weight and humidity are constantly monitored and adjusted.
The number of coating applications and drying systems on the coating machine depends on whether one, two or three coatings on the base are required.
Coating mainly consists of pigments (chalk, clay or talk) which covers the surface of the base paper and is bound there by binding agents either having a natural base (casein or starol) or synthetic compounds (synthetic dispersal).
After its manufacture as base paper, the coated paper surface is optimised for its end-use, again either by off-line or on-line machine calendering.
A calender or super-calender can be used to develop the smoothness and gloss on the surface of the paper. The paper passes through up to 16 rolls which apply pressure and temperature to the coated paper surface.
These rolls have different surfaces. Steel rolls and elastic rolls achieve the various glazing and surface treatments.
At the end of the paper machine line, the paper is wound at a controlled tension onto a reel, ready for finishing and dispatch.
The paper now has all the characteristics required for its end-use, but must be cut to the size required by the customer.
The jumbo reels are transported to a finishing department, where they are cut into narrower reels by a slitter/rewinder. These reels can be delivered directly to the customer, or can be further processed into specific paper sheet sizes on a sheeter.
Modern sheeters not only cut the paper reel to the desired size, but also check paper surface quality, sort out faulty sheets, count the sheets electronically, automatically slip in counting strips and provide automatic pallet exchange so that the sheeter does not need to be stopped between pallets.
Guillotine-type cutters cut smaller quantities of sheets to special sizes.
They can also trim all four sides of the sheet ready for certain specialist printing processes.
The final step is the packing of the paper. It can be sold as reels, reams, or sheets packed in bulk according to customer requirements. Effective packaging avoids transport damage and provides protection against moisture.
To avoid spoilage, all the paper stock is carefully stored in light- and water-protected areas. The pallets awaiting delivery to the customer are sorted and stacked by computer-controlled warehouse machinery