The HK Fibers Autoclave Process the next generation
in wastepaper processing

Swaroop Iyengar
Kellogg Brown and Root, 601 Jefferson Avenue, Houston, Texas 77002

Abstract

A novel process, the HK Fibers Autoclave Process, has been developed to treat whole OCC bales. Based on mill tests, using a 20 ton/day pilot plant unit, at Rand Whitney Containerboard, Montville CT., the process results in improved paper machine runnability and cost reductions in chemicals and energy. The benefits also included rapid pulping of treated OCC, thus increasing the throughput of the OCC plant. Kellogg Brown & Root has developed and engineered the first such commercial facility at the Montville plant

The Autoclave Process is covered by a series of patents by Willard Carlson and Ivar Stockel. Their original impetus was to find a suitable technique to repulp materials that are difficult to pulp in a pulper, such as OCC with wet strength (carrier stock) and food board. Subsequently the Autoclave Process has been found to have broader applications.

The Autoclave process consists of a treating whole wastepaper bales in series of evacuations and pressurisations with a dilute caustic solution. Typically this is a batch operation lasting 6 minutes.

A system to handle 600 TPD is currently being started up at Rand Whitney's facility.

Introduction

Mills that utilise OCC have traditionally been selective in the type of waste they purchase, minimising or eliminating board containing wet strength such as carrier board. There is generally a cost associated with such sorting as well as limiting sourcing options. There is also an environmental toll since this type of wastepaper, once removed, has to be sent to a landfill. A method to handle such wastepaper is described in this paper. The batch process consists of submerging whole bales of waste paper in a very dilute solution of caustic (<0.1%) and subjecting them to a series of pressure and vacuum cycles. The bales are then processed through a traditional OCC system. The cycle time is typically in the range of 4-6 minutes.

Caustic treatment (at the pulper) and soaking of OCC (after pulping or at the high density chest) has been utilised at various mills. Typically this has involved high temperature, high consistency and long retention times. Strength benefits have also been reported with the use of high levels of caustic (2-9%), however mill reports show that the use of elevated levels of caustic adversely interfered with the acid papermaking conditions used at most linerboard facilities [6].

The process described in this paper differs from these treatment and soaking methods in that it uses low levels of caustic which do not interfere with papermaking, treats whole bales, requires very short retention times for treatment and does not require high temperature.

System Design

In general, the Autoclave Processing system is located in close proximity to the existing pulper feed conveyor for example inside an existing OCC warehouse. The process equipment is in line with the existing bale conveyor operating in an automated batch sequence. Three separate conveyors are used: Conveyor No.1 will load the autoclave, conveyor No.2 will be inside the autoclave, conveyor No.3 will discharge the autoclave by receiving the bales from conveyor No.2 when treatment is finished (see Figure 1).

Figure 1

Figure 1 (for a larger image, please click here)

Mechanically the system consists of the following key process equipment each of which is described below:

  • Autoclave
  • Surge Tank
  • Holding tank or sump
  • Pressurising system including whitewater tank
  • Vacuum system

Autoclave
The carbon steel autoclave, is a horizontal cylindrical vessel of about 8' ID and 28' long (between parting faces), with hydraulically operated doors at each end capable of unlocking and opening or closing and locking in about 25 seconds. The vessel is designed to withstand full vacuum and at least 150 psig internal hydraulic pressure. The autoclave is a coded pressure vessel.

The autoclave is connected to a surge tank through a line and valve. The surge tank provides a volume for quick transfer of vacuum to the autoclave as well as a reservoir for excess fill liquor from the autoclave (see Figure 2).

Figure 2

Figure 2 (for a larger image, please click here)

A perforated steel plate inside the autoclave will allow filling liquor to completely fill the autoclave and pass into the surge tank without permitting loose pieces of cardboard to float up and plug the line and valve.

No.2 Conveyor is located inside the autoclave to move bales into and out of the autoclave. Baffles vertical and horizontal are installed inside the autoclave, providing a rectangular cross section to constrain the bales when the autoclave is full.

Sump or Holding tank
The autoclave is mounted with a slope towards the discharge end. A ball valve and vertical pipe is connected to the bottom of the autoclave at the discharge end extends into a sump under the autoclave. Filling liquor enters and discharges through this ball valve. An optional continuous self-cleaning trash screen lifts the trash from the pit into a container for periodic removal. Filling liquor level is maintained in the sump with overflow from the whitewater chest.

Pressurising system including whitewater tank
A tank containing filling liquor comprised of paper machine white water and dilute NaOH solution (supplied by makeup metering pumps) provides the autoclave with a pressurising medium. A pump connected to the whitewater tank and with a pressure control loop is used to pressurise the autoclave to a set pressure. The pressurising pump is normally recirculating into the whitewater tank, switching to the autoclave during the pressurising part of the sequence. Level is maintained in the tank so that it makes up by overflow to the sump ensuring that there is always adequate fill liquor for the autoclave.

Surge Tank
A 2000 gallon carbon steel, cylindrical, horizontal tank is installed above the autoclave as surge tank to ensure that the autoclave is full and receives excess fill liquor. The tank is also a reservoir for vacuum and is designed to withstand full vacuum.

Vacuum System
A vacuum system is installed which consists of a liquid ring vacuum pump along with an air ejector and a separator. The vacuum system is piped and instrumented to allow operation under moderate vacuum for filling the autoclave with liquor without the air ejector, or under controlled high vacuum with the air ejector.

The vacuum system and the pressurising system are packaged together on a compact skid assembly, which is pre-fabricated.

Description of operation

OCC bales are brought by clamp truck and lifted onto a loading table. The operator pushes the bales onto the inclined belt conveyor No.1. Sensors on the conveyor allow metering of successive bales. When the proper number of bales have been loaded onto the conveyor, an alarm warns the operator not to load additional bales.

The automated sequencing controls next opens the autoclave doors, raises the conveyors, and the bales are transferred onto conveyor No.2 inside the autoclave. Processed bales exit the autoclave to conveyor No.3 prior to the transfer of new bales. The autoclave doors are then closed and the conveyors lowered.

When the autoclave doors are securely closed, filling liquor (white water and caustic liquor) is drawn into the autoclave from the sump using the vacuum system, until the bales are completely submerged and the autoclave is full. The pressurising pump is then valved to the autoclave and the pressure in the autoclave is raised to a specific set point and is kept at that level for a desired length of time (15-30 seconds). The pressurising pump runs continuously and normally recirculates. After the stipulated retention time the pressure is relieved to atmospheric pressure in 10 seconds, while the pressurising pump is stopped.

When the autoclave pressure is at 0 psig, the autoclave pressure is reduced once again. When the desired absolute vacuum is reached, the pressurising pump is re-started, to reach the target control pressure. At the same time, the surge tank is brought back to atmospheric pressure and valves allow the surge tank to drain. Next the autoclave is re-pressurised. After the desired retention time the system is brought back to atmospheric pressure.

When the autoclave pressure drops to zero, valves are opened and the autoclave drains into the sump. When all liquid has been drained from the autoclave the doors are unlocked and opened and the conveyor No.2 is activated to discharge bales onto No.3 conveyor which discharges the treated bales on the OCC pulper feed conveyor.

White water makeup is accomplished by pumping continuously clear white water from the paper machine saveall to the whitewater tank. A supply pump adds a 25% solution of NaOH to the white water, from the NaOH storage tank. The NaOH is injected into a static mixer inserted in the white water pipe. A metering pump controls flow of NaOH. The ratio between white water and NaOH flows is adjusted by occasional laboratory checking of concentration in the sump and/or of samples of autoclave liquor.

The movement of liquor in and out of the autoclave may dislodge trash and occasional loose cardboard pieces, which may be carried into the sump. The sump geometry will cause these rejects to accumulate into the deepest part below the suction/discharge pipe. Intermittent cleaning of the sump is required or alternatively installation of an inclined moving screen will transfer the rejects out of the sump and dump them into a container for occasional disposal.

Mechanism and Results

Pangalos et al [2], theorise the mechanism for the Autoclave Process consist of the following steps:

1. De-bonding of fibres prior to pulping;

2. Leaching/dissolution of surface lignin;

3. Softening of the fibre walls (due to swelling and delignification).

The applied alkaline solution resolves hydrogen bonds between fibres, while also neutralising acidic end groups at the fibre surfaces. Any excess alkali can then proceed to dissolve lignin previously precipitated onto the fibre surfaces and can penetrate the fibre-walls causing swelling and delignification (mostly through extraction). These liberated, swollen fibres are more compliant and experience less damage (less fines creation) across papermachine refiners. This effect is achieved during the alternate pressure and vacuum cycles of the process. This effect is greater than that obtained by pulping with caustic or soaking under atmospheric conditions with caustic in soaking towers[3, 4].

Removal of the surface and near surface lignin improves bonding between these fibres during subsequent papermaking. The resulting pulp thus inherently allows for the production of a stronger sheet, which in turn allows for the reduction in strength additives, such as starch, or a reduction in the energy required to refine the pulp to a given target strength. The Autoclave Process efficiently produces a pulp having, on average, longer and more compliant fibres, with fewer fines.

The process has also been tested with milk cartons and other wet strength containing bleached board with similar results. This provides a tremendous improvement over the techniques of using peroxyacid which can add as much as $40/ton to the operating cost [5]

Commercial Testing

Results from mill trial data during the production of 42 lb. linerboard at Rand-Whitney's Montville, CT. mill are shown below.

Table A

The first mill trial, summarised in Table A, with a 100% impregnated OCC supply showed that a 70% reduction (from 20 lb/t to 6 lb/t) in the applied starch could be achieved along with a 45% reduction in the average specific energy consumption (from 3.05 HpD/t to 1.67 HpD/t), while the quality of the linerboard remained within 3s of the average baseline Burst Index value. The base-sheet and top-sheet refiner plates were retracted completely to their no-load power consumption level.

Table B

In another mill trial, summarised in Table B, half of the mill supply was produced with the Autoclave Process and a 65 % reduction (from 23 lb/t to 8 lb/t) in the applied starch was achieved with a 20% reduction in the average Specific Energy Consumption (from 2.66 HpD/t to 2.13 HpD/t). The quality of the linerboard remained within 3s of the average baseline Burst Index value. An 74% reduction (from 23 lb/t to 6 lb/t) in the applied starch was achieved with a 14% reduction in the average Specific Energy Consumption (from 2.66 HpD/t to 2.29 HpD/t) while the quality of the linerboard remained within 3s of the average baseline Burst Index value. The top-sheet refiner plates were retracted completely to their no-load power consumption levels.

Based on these tests Rand-Whitney has installed a full scale 600 TPD system to process all the OCC used by the mill. The system is currently in the middle of start-up and commissioning.

Conclusion

OCC users now have a unique way of upgrading the quality of fibre that they are using with the Autoclave Process developed by Rand-Whitney. Savings in starch and refining are combined with the ability to increase capacity through the pulper and the use of grades such as wet strength, which could not previously be processed.

Acknowledgements

We would like to thank Rand Whitney Containerboard for permission to present the Process and results from testing at the mill.

References

1. Carlson W.E., Stockel I.H., U.S. Patents 5,271,805 (1993), 5,496,445 (1996), 5,496,439 (1996), 5,536,373 (1996).

2. Pangalos, G., Monroe, J.E., Scogin, R.W., Carlson, W.E., Stockel, I.W. "The effect of bale impregnation on recycled linerboard mill production" 86th Annual Meeting PAPTAC, Feb 2000.

3. Freeland, S.A. Hrutfiord, B.F., "Caustic treatment of OCC for Strength Improvement during recycling" Tappi J., 77(4); 185-191 (1994)

4. Healey, E., "Alkali Soaking ups Quality, Yield of OCC Used in Boxboard Furnishes", Pulp and Paper, September: 138-139 (1990)

5. Huston, J., Babb, C., Homans, J., "Pulping High Wet strength Milk Cartons" Tappi Recycling Symposium: 403-414 (1995)

6. Freeland, S., "Recent Developments in Caustic Soaking of Old Corrugated Containers" Tappi Pulping Conference Proceedings 333-338 (1994)