The Karioi process

Ruapehu Bulletin, Volume 9, Issue 424, 18 February 1992, Page 12

The Karioi process

Raw Materials About 25 percent of the wood for the mill is slabwood Radiata pine chips which come from local sawmills. The remainder of the wood comes in log form from: mature and medium density Radiata pine; Radiata young crop of thinnings; and minor species including Pinus Pondersoa, Pinus Nigra and Pinus Contorta. The minor species wood comes from Karioi forest while the Radiata pine comes from other local forests and farms. Woodroom The logs are cut into 1.5m to 2m lengths then fed into a drum debarker. Bark is detrimental to the finished product as it leaves small dark specks in the pulp and also lowers the strength of the pulp. Bark is later used as fuel providing

heat which is used to dry the pulp. The logs are reduced to chip size in a chipper, by butting up against a rotating disc with knives attached. A vibrating screen is used to segregate the oversize chips, which are rechipped, and undersize chips (or 'fines') which are used as fuel in the bark bumer. The chips are separated into three piles of different types and later fed into the pulp process, with different mixtures depending on the finished pulp type required. Chipwash Blowlines transfer the chips to the mill and into a presteaming vessel which heats the chips before washing them by using steam produced by the refiners. Turnpage 13

""

Frompage 12 Hot process water is then added in a mixing chamber and the chips are carried upward on a screw drainer. Impregnation/ preheating The chips then travel through a compaction screw which forces out excess water before being carried upward in a chamber by two upfeed screws. This chamber contains sodium sulphite which penetrates the chip. Another compaction screw carries the chips to the preheater where they are held under pressure at 120°C for 5 minutes before going into the refiners. This impregnationpreheater stage softens the lignin (glue) binding the individual fibres, which allows for better fibre separation in the refiners. Refining The refiners have a stationary and a revolving plate with the chips passing between the two plates. The refiner plates have a raised pattern on them with the bars becoming increasingly finer towards the outside. Refining is a very energy intensive process using up to 80% of the mill's total electricity of approximately 850 megawatt hours per day., Primary refiners act in

separating chips into individual fibres. The plates on the secondary refiners have a finer bar pattern which roughens up the outer layers of individual fibres to develop inter-fibre bonding characteristics, thereby improving the quality of the pulp. Secondary refined pulp is around 25% pulp and 75% water. The refined pulp is then diluted to a consistency of 3% pulp and 97% water. Screening, cleaning and storage From a storage chest the pulp undergoes a two stage screening and cleaning process which removes any fibres which are not fully developed. The reject fibres are then re-refined. The first stage involves pressure screening the pulp, this forces the pulp through a basket with fine holes in it. The acceptable pulp which passes through the holes is collected in another stockchest for further dilution. In the second stage the pulp is separated by centrifugal action. This requires a consistency of 0.5% pulp and 99.5% water. The rejects from both stages are collected in a stock chest and then thickened to 15-20% consistency.

Dewatering The pulp is then passed to one of three dewatering units. A drum filter is a large revolving drum covered by a fine mesh cloth where vacuum is applied to the inside of the drum causing the pulp to stick to the outside thereby drawing the water out. A pulp disc filter operates on the same principle but uses a series of disc segments arranged around a central shaft. The pulp is formed, under vacuum, onto bags which cover each segment as they rotate through the wet pulp. Jets of water separate the dewatered pulp from the filter bag and then drop it down a chute. The pulp is diluted to 5% for ease of pumping before being transported to the buffer storage. Bleaching Up to one third of the mill's pulp production can be bleached. Bleaching takes the colour out of the lignin (glue) to increase pulp brightness (or 'whiteness') making the end product more suitable for making printing paper. Hydrogen peroxide is used here in one process. For bleaching the, pulp consistency becomes 30% pulp and 70% water. Turnpage 14

Winstone s IMiii j

After mixing, the pulp I is carried to a high den- | sity storage tower I allowing the bleaching I reaction to occur. The I bleached pulp is then diI luted and sent on to the I drying stages. I Drying I There are two separate I drying lines using press I machines. One takes the consistency of the pulp to 20% pulp 80% water, the second section i squeezes out the remaining water to achieve a 50-55% consistency. The pulp mat is then shredded into small lumps in a revolving shredder then fluffed up into individual fibres, to increase the surface area for drying. A two stage flash drying process using hot air provides the final drying. The air is heated in three ways: by heat exchangers using steam from the refiners; through an oil heat exchanger from the bark

burner (burning the waste bark); by naked flame using an oil gun to heat the air. The latter provides the heat control. There are two successive heating sections followed by a cooling section before the pulp is blown to the baling presses for compaction into bales. Finishing Line Each drying line has its own finishing line, with the capacity to handle the total mill production. The pulp bales are firstly put through a compaction press which presses the pulp into large bales each weighing around 250kgs. They are wrapped and tied with wires, then stacked in piles of three before both lines pass to a common unitiser (which binds six bales together forming 'a unit') and loaded on to railway carriages for transportation to the Port of Napier.