PULPING OF HIGHLY CONTAMINATED RECYCLED PAPER

Author Kai Wellhäusser Company Voith Paper Fiber Systems, Ravensburg/Germany Keywords recycled fibre, pulpers, trash removal, contaminants, minimise fragmentation, early removal

INTRODUCTION

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:

• Slush down the recycled paper to individual fibres, avoiding fibre damage and minimize fragmentation of contaminants to enable efficient removal in the downstream cleaning and screening processes.
• An efficient detrashing system for removing contaminants in the first stage of the stock preparation process.
• Undertake ink detachment and effective blending of chemicals.

To meet these process requirements, a number of pulping and detrashing systems have been developed and are successfully in operation.

LOW CONSISTENCY CONTINUOUS PULPING

Fig. 1

This graph (Fig. 1) mirrors the situation of a pulping system without and with a detrashing system:

• Without a detrashing system the pulper soon had to be shut for cleaning, resulting in not only intensive cleaning work but also significant production losses.

This situation applies for all recycled paper qualities

• OCC, MOW, ONP, liquid packaging board etc.

when slushing at low consistency (approx. 4-5 % bone dry.

Fig. 2

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

• Low consistency (approx. 4-5 % bone dry) continuously operating UniPulper.

Fig. 3

• Ragger for removing stringing contaminants such as strings, wires etc. (Fig 3).

Fig. 4

• The Junkomat (Fig. 4) for removing coarse heavy contaminants which otherwise would cause serious wear or even severe damage in the following
• Contaminex (CM) operating as secondary pulper for gentle pulp deflaking and transporting
• Contaminex vertical screen (CMV). This operates intermittently with wash cycles to facilitate the separation of heavy - and lightweight contaminants.

 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.

Fig 5

• All rejects are free of fibres (Fig. 5), thus helping minimize fibre losses of the overall plant whilst also reducing rejects disposal charges.

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).

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:

• ONP, OMG, MOW, white or coloured ledger, etc.

as well as for pulping systems for packaging paper grades such as:

• OCC, mixed recycled papers, liquid packaging boards, etc.

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.

Fig. 7

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).

Fig. 8

The results are:

• Gentle fibre-to-fibre friction at consistencies up to 18 % bone dry defibers stock and minimizes size reduction of contaminants.
• Improved stock quality thanks to gentle pulping action and early contaminant removal.
• Highly effective blending of chemicals and raw materials.
• Fibre-to-fibre action also results in ink detachment from the fibres.
• Contaminex (CMS) or Fiberizer provides additional defibering, screening, contaminant separation and light rejects pumping to the drum screen.

Fig. 9

• Drum screen provides light reject screening and washing (Fig. 9). Good fibres are flushed back pulper.
• Heavyweight rejects (Fig. 10) are discharged from the Contaminex or Fiberizer into a Screenex (Fig. 11) or T-Rex (Fig. 12) for reject dewatering.

A part from contaminant dewatering the stock is deflaked in the bottom part of the T-Rex for recovery of useful fibres.

Fig. 10

Fig. 11

Fig. 12

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.

Fig. 13

Besides these advantages, a reduction in flake content (Fig. 14) and an improvement in ink particle detachment was observed.

Fig. 14

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.

Fig. 15

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.

Fig. 16

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.

Fig. 17

The TwinDrum concept consists of two drums (Fig. 17) – one for pulping and one for screening.

Fig. 18

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.

Fig. 19

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 TwinDrum^TM pulping and screening system are:

• The displacement channel provides efficient upward transport of the furnish at lower peripheral speeds than conventional drum pulpers. This provides more intensive furnish friction, i.e. a higher pulping energy.

Fig. 20

• The TwinDrum's displacement principle (Fig. 20) ensures a significantly higher capacity than with conventional drum pulpers. This can be up to 60 % of total drum volume. This high filling level, combined with efficient application of pulping energy, means the TwinDrum pulper is shorter in length, and thus more compact than conventional drum pulping systems.
• By varying the amount of furnish, dwelltime in the drum can be adjusted to suit the particular furnish whilst maintaining the same throughput rate. The filling level is easily adjusted via a discharge device on the pulper discharge side.
• Pulper power consumption remains virtually constant, independent of the filling level.
• With this intensive pulping principle, even difficult furnishes, such as liquid packaging board or recovered brown grades, present no problem. This has been successfully proven in installations handling 240 t/24 h liquid packaging board as well as OCC 1100 t/24 h (Fig. 21).

Fig. 21

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.

Fig. 22

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,
- another advantage of the separate screening drum - the whole length of the screening drum can be effectively used for the separation of contaminants.

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).

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.

Fig. 24

SUMMARY

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:

• Effective slushing at low or high stock consistencies.
• Maintaining original fibre properties.
• Minimizing fragmentation of contaminants .
• Efficient removal of contaminants at a very early stage in the stock preparation process.

BACK TO TOP