The Del-Tin Fiber MDF plant near El Dorado in southern Arkansas, an imposing installation, is gradually working its way up to its full 150 million ft2 annual production of its trade marked Solidium board. Currently it produces at about 80%, running four shifts on a seven-day week with a total workforce of 122, including office staff.
This is a joint venture between Deltic Timber and Temple Inland. It provides a market for Deltic’s residues which supplies nearly half the plant’s raw material from its sawmills.
The original mill study was done by Mid-South Engineering, Hot Springs, Arkansas from the ground up, which also negotiated the joint venture as well as engineering the mill. Mid-South had worked with both Temple Inland and Deltic Timber for many years.
The other half of the raw material comes from International Paper (I-P) and Weyerhaeuser mills, other sawmills, and some chip mills. It all comes from within a 100-mile radius. Another wood residue company provides fuel for the heat energy plant.
The mill began production on southern yellow pine sawdust and chips, but is now running entirely on chips. The sawdust contributed heavily to dryer loads and drying is running at full capacity. There just isn’t capacity to pre-dry sawdust.
The raw material enters at a truck dump and goes out to the Finnish BMH stacker and reclaimer in outside storage. Another dump is dedicated to fuel – a mixture of pine and hardwood bark, and ground-up pallets. This is supplemented with green sawdust. Ground-up board trim also goes for fuel. General and plant manager Gary Griffis explained: “We shut off the sawdust. We’re much more consistent as a result today.”
Material entering the mill crosses BM&M shaker screens, separating material into fuel fines, overs and acceptable. Segregated material feeds into silos. Two augers meter out of the chip silo into face and core infeed conveyors. Metal detectors are located at both the truck dump and conveyors feeding into the refiners.
Chips go into a pre-steaming bin and then to the digesters and Andritz 8,000-hp 60in refiners. Emphasis has been on additional refining to provide a much more consistent fibre and, consequently, more homogeneous boards, and a high quality board. There is a parallel operation for face and core. Standard Borden UF resin is injected into the blow line with an atomizing nozzle just beyond the refiners. A scavenger system dilutes the resin and reduces formaldehyde emissions.
Dosed fibre is blown to the two-stage M-E-C face and core dryers. The first stage dries material to an 18 – 20% moisture content while the second takes it down to 12 – 14%. The first stage is heated by a Callidus heat plant, the second by hot oil from a heat exchanger served by off-gases from the Callidus plant. GreCon meters monitor moisture content, about 12%, after the second stage dryer.
Heat is captured all along the line. Fuel is hogged and goes to a mixing bin in the Callidus system where it is split between two kilns. A plug screw forces fuel into the feed end of these kilns, the goal being to gasify the fuel as it moves toward the flame front.
Off-gases containing the gasified fuel leave through ducts which feed into the large, secondary combustion chamber which is a big cross-over loop towering above the mill.
“At that point,explained Mr Griffis, “we hit it with fresh air, providing oxygen. Then we get the combustion that produces the heat for our various uses.However, this is supplemented with natural gas burners.
As the heat comes down the big duct it passes through an air-to-air heat exchanger. The introduced air comes from the press enclosure which includes formaldehyde fumes eliminated in the process. This air stream feeds the dryers from where the exhaust air passes through a recuperator – another air-to-air heat exchanger. The oxygen introduced into the Callidus system is the dryer exhaust.
As Mr Griffis pointed out: “We’re totally utilising and destroying all of our emissions through almost a triple pass.”
The flue gas stream headed toward the stack passes through a boiler making steam and then through the hot oil heat exchanger. The boiler produces steam for refining and turbines running some pumps/fans, and also hot oil for the second- stage dryer and the press. As the flue gases continue, they pass through an electrostatic precipitator.
All this isn’t easy. The dryer operator, in controlling moisture content, must have a constant supply of heat and the Callidus system must run consistently.
One operator has computerised control of the heat plant. The wood yard, refining and dryers are controlled by one operator, as is the forming and press line. All three are located in one control room. Each has a support person out in the mill. Entering the Schenck forming system, the material dumps into separate face and core forming bins. These feed into an air sifter that removes particles, resin spots, resin chunks or fibre balls too big to go through the press.
Mr Griffis explained: “At that point we enter our heating and humidification system where we heat the air stream and also inject steam to help increase fibre temperature and control moisture again. The material goes to the infeed cyclone which dumps the fibre onto infeed conveyors. These are parallel systems.”
The face infeed cyclone has a diverter to split face material between top and bottom faces. Core material goes to the core infeed cyclone and this dumps onto the core infeed conveyor. These three infeed conveyors go to a Schenck swivel-belt conveyor that lays the fibre into the forming bin, distributing it evenly across the width. Out of the forming bin the material goes through Schenck mechanical formers which distribute the fibre evenly through the forming line belts. There are three formers: bottom face, core, and top face.
Each former has a weight scale and GreCon moisture meters on the core and top face. The mat goes through a Schenck 1900 PLI pre-compressor then another metal detector to protect the press, adjustable trim saws and Grecon density gauge. Next is a reject opening through which a mat can be recycled at full line speed out to the core forming bin. All this is automatic except for changing line width by moving the saws in and out. The Schenck line was supplied by Dieffenbacher.
The mats transfer to the 9ft wide, 38m long, Küsters continuous press equipped with automatic chain guides. It can press material from l/8in to 1in. Del-Tin runs from 7/32in to 7/8in. Berndorf of Austria supplied the continuous steel belts.
Entering temperature is about 415°F and maximum pressure is 750psi per frame. The 65 frames can be controlled independently. Each has a pressure pot row with nine 10in diameter cylinders across the press’ width. These provide the pressure. On top of it is the hot oil heating platten. On top of this is a multitude of small roller chains transferring the heat from heating platen to the stainless steel belt then to the mat in the middle. On the top is another steel belt, roller chain and heating platen. Heat can be changed by zone, while pressure can be changed per frame.
“If you are running something requiring higher density you put on a lot of pressure in the infeed,said Mr Griffis. “If you’re running thicker boards, you go further into the press before you apply higher pressures.
“Küsters is the only press with these small roller chains which provide much more flexibility in the product mix because of heat transfer.Del-Tin selected the press because of its ability to run thin boards with higher and more uniform heat transfer across the mat, he explained.
“The idea in pressing a mat is that you have to keep it in there long enough to cure it,Mr Griffis explained. “That’s getting the centre of the mat to 212°F to remove the water, turn it to steam and then you have your mat cured. That happens around frame 40 or so. Then you begin to adjust the thickness on your board. The thinner you get, press dwell time is shorter. You can run your press faster.”
Next are two Dieffenbacher flying cutoff saws. Leaving that area the pressed panels go through a thickness gauge. “We try to keep thickness variation across the panel to plus or minus three or four thousandths [of an inch],said Mr Griffis. “You don’t want the sander removing any more material than absolutely necessary.”
The outfeed side was supplied by Metso. Three star coolers allow the board to cool with some post-curing. Panels are then automatically stacked and can go directly to the sander or to the Lukki storage area. The Lukki is an automatic retrieval system. This computerised overhead hoist on rails uses steel pallets. It puts a stack into one of 54 cells and logs the products’ ID, thickness, width and length into the database. When the sander is ready for that particular product, providing the sander is not running behind the line, the operator types in a request and the Lukki goes out, picks it up and takes it to the sander.
It does the same for the large Schelling book saw system, but this can run independently.
It cuts lengthwise first, then rips and cross-cuts, feeding to two stacking lines. The computerised priority for the Lukki is press first, then sander and the saws third.
The sanding line was by Metso, except for the sander itself, a Kimwood eight-head, nine foot wide sander that has primary and secondary sections. The primary’s job is to remove most of the bulk and, secondary, is the finishing sander producing a desired finish up to 150-grit. This is the largest sander ever built by Kimwood and is followed by a Kimwood cross-belt sander which finishes and removes lengthwise marks. It produces a finish similar to 180- grit belt. The operator’s cab also houses the grader who will send to one of two grade bins: A or B. Strapping is by Signode. Some 5% to10% of shipping is railed. The rest is truck shipped.