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PULPING OF HIGHLY CONTAMINATED RECYCLED PAPER
Greater demands are being placed on the performance of recycled paper pulpers due to a significant deterioration in the quality of recycled paper delivered to paper mills.
In particular, the contamination levels in the recycled paper have substantially increased over the past few years.
The function of the pulper is to gently slush the recycled paper raw material into a pumpable pulp, suitable for processing in stock preparation whilst facilitating the efficient removal of contaminants from the pulp.
The design of the pulping and detrashing equipment must therefore satisfy a number of process requirements:
To meet these process requirements, a number of pulping and detrashing systems have been developed and are successfully in operation.
LOW CONSISTENCY CONTINUOUS PULPING
This graph (Fig. 1) mirrors the situation of a pulping system without and with a detrashing system:
This situation applies for all recycled paper qualities
when slushing at low consistency (approx. 4-5 % bone dry.
The TwinPulp pulping system (Fig. 2) of a modern stock preparation line for packaging grades using OCC coming either from local sources (LOCC) or from the US (AOCC) consists of
The coarse heavy particles are discharged via a junktrap. Stock accepts are fed forward in the main process line or back to the pulper.
The washed light rejects are discharged to a screen drum, the accepts from here return to the pulper.
Existing stock preparation plants can be retrofitted with the TwinPulp detrashing system, resulting in production capacity increases of up to 25 %, lower fibre losses, reduced flake contents after pulping to below 12 % and more efficient reject removal. Measurements on existing plants have shown average contaminant removals of more than 75 % at this early stage (Fig. 6).
HIGH CONSISTENCY BATCH PULPING
As already mentioned, pulping systems aim to slush the raw material as completely as possible without damaging fibres and without breaking down non-paper components.
This is particularly required in deinking plants slushing raw materials such as:
as well as for pulping systems for packaging paper grades such as:
High consistency pulping at consistencies of 12-18 % bone dry with intensive but gentle fibre- to -fibre friction in maintains the original fibre properties and minimises fragmentation of contaminants.
With relatively clean recycled paper furnishes, slushing and detrashing can be undertaken with the PreClean I – pulping and detrashing system (Fig.7).
With highly contaminated recycled paper furnishes, pulping and detrashing is carried out with the PreClean II pulping and detrashing system (Fig. 8).
The results are:
A part from contaminant dewatering the stock is deflaked in the bottom part of the T-Rex for recovery of useful fibres.
PreClean II pulping systems are successfully in operation for deinking plants as well as for OCC preparation systems. The world's biggest HD pulper with 160 m³ has been in operation for years, handling OCC in the USA. Others are operating in Japan and in Australia with the same raw material.
An optimized helix design (Fig. 13) introduced in existing plants has resulted in a reduction in slushing time of up to 20 % as well as an increase in slushing consistency. This represents an increase in production for a very low increase in specific slushing energy.
Besides these advantages, a reduction in flake content (Fig. 14) and an improvement in ink particle detachment was observed.
Before feeding the raw material into the pulper all wires have to be removed. The wires can be automatically wound into compact coils in a bale wire coiler (Fig. 15) and sold as scrap metal.
HIGH CONSISTENCY CONTINUOUS PULPING
Combining the advantages of gentle high consistency slushing (Fig. 16) with a continuous operating process ensures not only optimum pulping conditions but also preservation of the original fibre properties of the furnish.
The Voith TwinDrum pulping and screening system combines all these advantages including those of conventional drum pulpers, in a completely new and successfully proven pulping principle. We have patented this process.
The TwinDrum concept consists of two drums (Fig. 17) – one for pulping and one for screening.
In a conventional drum pulper (Fig. 18) the slushing is achieved by carrying the stock upwards and letting it fall down in a relatively high speed rotating drum. This is not very efficient.
Voith's drum pulper (Fig. 19) consists of a drum rotating around a D-shaped stationary displacement core. The drum and displacement core walls form a semi-annular channel in which the furnish is carried upwards from the collection zone at the bottom of the pulper. Shear forces in this displacement channel generate intensive fibre to fibre friction, ensuring effective kneading and slushing of the furnish. The furnish then drops over the top edge of the displacement core and back down into the collection zone. Here, the large available volume provides a sufficiently long dwelltime. This ensures complete wetting down and swelling of the furnish, - essential conditions for facilitating the pulping process.
This continuous wetting, softening, kneading and fibre to fibre friction process at consistencies up to 26 % bone dry along the length of the drum ensures gentle, yet efficient pulping of the recovered paper, without breaking down the non-paper components, such as plastic foils, stickies, etc.
The special features of the TwinDrumTM pulping and screening system are:
For deinking plants this innovative displacement principle provides highly efficient, yet gentle slushing, effective blending and mixing/reaction of deinking chemicals and optimum ink detachment at optimum low rotational speeds of 1.5 m/sec for the pulping drum.
For best screening efficiency the drum screen (Fig. 22) operates at a speed of 2.5 m/sec. With a controlled consistency of 4 – 8 % bone dry right from the inlet,
Even with wide variations in furnish input and consistency, the screening results are constantly good. The drum screen (Fig. 22) is highly effective, with extremely low, or even no residual fibre content at all in the drum screen rejects (Fig. 23).
This impressive result confirms the process advantages of this advanced development with separate pulper and drum screen. The importance of individual adjustment of operating parameters for both drums to suit the individual operating conditions should not be under estimated.
The paper industry nowadays is forced to recycle raw materials with a constantly growing amount and variety of contaminants and also with recycled paper qualities not long ago considered not feasible to use.
The main factors for correct selection of efficient equipment are: