Technical Design

Bleached Chemi-Thermo Mechanical Pulp (BCTMP)

The Penola Pulp Mill will produce BCTMP from plantation raised Eucalyptus globulus for paper and cardboard.

BCTMP is a mechanical pulp that tends to have both high bulk and opacity properties. Bulk measures paper thickness. Opacity measures transparency. High bulk pulps lend themselves readily to board grade applications but can also be used to reduce weight and/or input costs in paper grade applications. BCTMP has become a popular component within the fibre furnish of high-end mailings and commercial magazine papers.

Mill Flexibility

Featuring the latest BCTMP manufacturing technology, the Penola Pulp Mill will not only be able to make a range of products that will suit the needs of a diverse papermaking industry, but also have the horsepower to drive the production rates to its maximum capability. At the heart of the production process is the latest in refining technology; Twin 68 refiners. These refiners enable the mill to produce clean pulp for the complete range of freenesses required in the market. Freeness is a measure of the ‘drainability’ of the pulp matrix, i.e. how freely water passes through it.

To change from a low freeness pulp to a high freeness pulp requires adjustments in feed rate and energy to obtain the new grade within hours. Modern instrumentation tracks the changes through the mill, applying appropriate chemicals and operating conditions at each point in the process to achieve the desired new grade and meet all of the predetermined customer specifications. Automated controls, trained operations personnel, precise application of chemical and other inputs allows the Penola Pulp Mill to change grades quickly to meet the diverse requirement for specifications, such as brightness, bulk and strength properties, while minimising or eliminating debris, shives and other contaminants that are a detriment to the papermaker. Penola Pulp Mill will be able to respond to all new grades and new specifications provided they match the inherent morphological characteristics of the eucalyptus fibre.

Process Description

The pulp mill process is detailed below.

Woodchip Island

Wood chips will be delivered to the site by road from regional plantations in trucks; either single trailer units or B-Doubles. The site road system allows a one-way traffic loop with lay-bys to accommodate any backlog in unloading wood chip trucks.  The timber will be de-barked and chipped in the plantations to avoid large-scale wood handling facilities at the pulp mill site.  The wood chip storage area caters for a five to 10 day demand based on the pulp mill production peak of 2,300 air dried metric tonnes per day (admt/day) of printing and writing or 2,300 admt/day of board grade pulp.

The wood chips are conveyed from the truck unloading station to a rotating distribution arm. Chips are conveyed to the pulping process 24 hours, seven days per week.  Since no debarking is necessary only small amounts of residues from the wood handling system will be generated.

Pulping Fibreline Process Steps

Chip washing and chemical impregnation	Two lines of two stage impregnation
Refining	Three lines of 2 stage refining, each for 767 admt/d
Bleaching with hydrogen peroxide	Three lines of MC/HC bleaching with residual recycle
Washing	Two stages after final stage of bleach
Dewatering	Six twin wire presses in parallel
Pulp drying	Three lines of three stage flash drying each for 767 admt/d
Bale forming	Six slab presses
Baling	Three lines

Chip Washing and Chemical Impregnation

The chips from the wood yard will be screened, metered and conveyed to a common pre-steaming bin and a chip washing system.  Oversize chips will be rejected and re-chipped.  The lines will include multiple chip washers, a trash separator, a wash water tank, a dewatering screw, wash water cleaning and a wash water storage/sedimentation tank.  The chips are steamed with return steam in a silo before being washed and dewatered in a screw type dewaterer.

In the manufacture of BCTMP, the wood chips are impregnated with small amounts of hydrogen peroxide (H₂O₂) and caustic soda (NaOH).  Combined, these chemicals form the ‘chip preparation liquor’, which softens the structure of the chip in preparation for refining.  ‘Impregnation liquor’ chemicals are added to the chips as they are being fed into the impregnation vessel.  The impregnated chips are then fed to the primary refiners.

Refining

Refiner mechanical pulp is an umbrella name for pulps produced by grinding wood chips in a disc refiner.  A refiner has grinding discs with patterned surfaces and a chip feeding device.  The chips are fed into the refiner and the fibres separated as the chips are heated and abraded on their way towards the periphery.

During the refining, electric power is converted into heat and a large amount of low quality steam is generated from the moisture in the chips.  Some of this steam is used internally in the pulping process to steam and preheat the chips but there is a large surplus of steam that is used in the evaporation and drying stages.

The main line refining concept is two stage high-consistency pressurised refining with Twin 68 refiners in the first stage followed by S2070 refiners in the second stage.  A third low-consistency refining stage consisting of TwinFlo refiners is used for final freeness control.  Reject refining is also done at low-consistency, using TwinFlo refiners.

The refined pulp is pumped to a multistage screen room.  To separate impurities and long stiff fibres, the pulp is screened through rotor driven laser cut screen baskets and cleaned in a vortex cleaner (hydrocyclone). The screened accepts are dewatered on two disc filters to handle the volume and range of pulp freeness and the rejected screenings are sent to the low consistency reject refiners.

Removal of latency (the tendency for fibres to curl) is achieved by storage of the screened pulp at low stock consistency for 30 minutes at high temperature.  This also provides process storage prior to bleaching.

Bleaching with Hydrogen Peroxide

Mechanical pulp from green wood has a brightness of 45- 55% ISO. Pulp is bleached to 85% ISO brightness utilising a two stage MC (medium consistency) and HC (high consistency) bleach system. Hydrogen peroxide and sodium hydroxide are the basic components of the bleach liquor make-up. The liquor is added to the two bleach stages in fixed quantities under specific conditions to achieve high brightness at low dosage rates. As well, residual chemical from the second stage tower will be used as make-up for the first stage to improve efficiency. For different grades of pulp, the bleach formulations are changed to achieve various levels of brightness and other specific pulp characteristics. Advanced control systems are used to control the amount of chemicals used to obtain the desired grade.

Screw presses are used to remove water and clean the pulp prior to the MC stage, in between the MC and HC stages and finally after the HC tower.  The screw press is ideal for deresination and removal of unwanted contaminants that affect good bleach performance.  A high consistency mixer thoroughly mixes bleaching chemicals into the pulp prior to the HC stage.

Pulp Washing

Dissolved materials in the prepared pulp are removed in three washing

stages (two stages with screw presses and one stage with twin wire presses) to provide a clean product acceptable for the pulp market.  The final dewatering is done in twin wire presses where a dryness of more than 45% is achieved prior to flash drying.

Pulp Dewatering

Twin wire presses are used to obtain the final water content of the pulp before thermal drying. The twin wire press operates by forming a pulp mat in between two travelling wires, which is then pressed through a series of pressure loaded rolls causing the water to extrude through the wires and out of the system to achieve a dryness of more than 45% prior to flash drying. There will be six twin wire presses in parallel to accommodate the production capabilities of the mill as well as meeting stringent COD limits in the final product. The dewatered pulp is conveyed out of the press section to the flash dryers while the filtrate is returned to the process.

Pulp Drying

Mechanical pulp produced as market pulp is dried in flash dryers.  Prior to drying, the pulp is fluffed in high consistency fluffers.  These are designed to maximise the surface area of the pulp and save energy in the dryers.  The pulp is pressed, finely shredded and sucked into a warm current of air and into the first of a series of drying towers.  The moisture quickly evaporates and is carried away as outgoing wet air.  In order to minimise the particulate emission, a combination of high efficiency cyclones and a secondary scrubber is used.

In three lines of three stage flash dryers, a final dryness of around 88% is achieved.  The flash dryer air will be heated using natural gas as the primary source of heat generation and excess steam from the refiners as a secondary source.

This configuration for the flash dryer is an integral part of the total optimised mill-wide energy concept; integrated with the entire fibreline (refining, bleaching, final dewatering, etc.) and the effluent treatment.

Baling and Storage

After the flash dryer, pulp bales (900 x 630 mm, 500-520 mm high, approximately 200 kg) are produced in six slab presses operating in parallel.  When baled the pulp can have final dryness of 88-90% depending upon end-user preference.  The bales are wrapped, stacked and labelled in the baling line and strapping is used to hold the bale together for lifting during shipping operations.  The bales are consolidated into units of eight and held in storage on site ready for transport.

Loading Area

At full operating capacity the pulp mill will produce 750,000 tonnes of pulp each year.  To minimise impacts on the community caused by transporting the pulp, Penola Pulp Mill will utilise the regional rail infrastructure to move the pulp to the Port of Adelaide.  A warehouse and loading area on the eastern side of the mill has been designed to handle the required numbers of shipping containers.  From the end of the production line the pulp bale units will be loaded into containers, which will be stock piled in the container yard ready for the daily train service.

Water Recycling Technology

Despite the massive resources available in Australia, it is recognised to be the driest continent in the world. Regional Australia has found it particularly difficult to attract investment from any industry requiring water as a resource. The Penola Pulp Mill will demonstrate world’s best practice for industrial water use and recycling and is anticipated to be the global benchmark.

The mill will utilise world leading Zero Liquid Discharge (ZLD) water recycling technology. While conventional pulp mills average 35 to 45 mega litres consumption per day, the Penola mill will use just five to seven mega litres per day. This water is make-up water for losses via steam in the manufacturing process. Four other mills in the world harness this technology; however the methods they employ are overall less environmentally and technically advanced than for Penola.

Water Recovery

Penola’s advanced water recovery process will utilise best available leading technology to recover clean pure water from raw effluent. Upon leaving the pulp mill the raw effluent will be treated to remove suspended particles, such as pulp fibres and fines, to facilitate efficient evaporation. The evaporator plant will feature MVR (Mechanical Vapor Recompression) evaporators followed by MEE’s (Multiple Effect Evaporators) to concentrate the effluent from 1.5% solids up to 65% solids. The evaporator design features innovative technological advances for clean-in-place non-interruptible operation, utilising low draw electrical fans on the MVR’s and clean recycled steam from the refiners on the MEE’s. The evaporators effectively become the kidneys of the process taking a contaminated effluent and producing clean condensate to be reused directly back in the pulp mill, while concentrating all the waste products for eventual oxidative destruction.

Once the liquor has been concentrated to 60 – 65% solids, it is fired in a recovery boiler for thermal destruction of the organic based solids and recovery of the inorganic chemicals. Clean burning of the organic portion of the liquor produces a further supply of steam that is used for evaporation as well as an additional heat input to the flash dryers for final drying of the pulp. Because of this effective use of “free” steam the water recovery process at Penola will be extremely cost effective and environmentally friendly.

The inorganic portion of the concentrated liquor that is fired in the Recovery Boiler is converted into a liquid smelt that leaves via the bottom of the boiler and is mixed with a mild caustic solution to create a new liquor that is green in colour. The green liquor is filtered and reacted with lime to produce sodium hydroxide, which is recycled back to the pulp mill for impregnation and bleaching. This in fact creates another recycle loop in the process further enhancing the economics and environmental stewardship of the Penola Pulp Mill.