Headbox

The slurry is pumped iato another stock chest, where wax ia emulsion form, usually about 0.5—1.0% wax-to-fiber weight, and 1—3% PF resia are added. PF resia is also added on the basis of resia soHds-to-dry fiber. Thea a small amouat of alum is added, which changes the pH (acidity) of the slurry, causiag the resia to precipitate from solutioa and deposit on the fibers. Resia is required ia greater quantity than ia the Masonite process because only light bonding occurs between fibers prepared ia a refiner. The fiber slurry is thea pumped to the headbox of a Fourdrioier mat former, and from this poiat the process is similar to the Masonite process. 

Fig. 4. Fourdrinier paper machine A, headbox B, Fourdrinier wet end with foil boxes C, wet and D, dry suction boxes, pickup, and closed transfer of web...

Subsequent to stock preparation and proper dilution, the paper furnish usually is fed to the paper machine through one or more screens or other devices to remove dirt and fiber bundles. It then enters a flow spreader which provides a uniform flowing stream and which is the width of the paper machine. The flow spreader, or manifold, discharges the slurry into a headbox, where fiber flocculation is minimised by microturbulence and where the proper pressure head is provided to cause the slurry to flow at the proper velocity through the slice and onto the moving Fourdrinier wire. 

Fig. 6. (a) The Bel Bale II twin-wke former. Stock from A, the headbox, is formed into sheet between B, wke number 1 and C, wke number 2. The web is removed from wke 2 by D, a suction pickup roU. (b) A roU-type twin-wke tissue former. Stock from A, the headbox, passes between B, wke number 1 and C, wke number 2 around D, a forming roU, and the web is removed from wke 2 by E, a suction pickup roU. 

A rolling mill drive motor failed in service. Investigation revealed the motor was short circuited by water in-leakage. This forced a prolonged shutdown, as the motor could not be replaced on site. High humidity was initially assumed to be the problem. Upon close inspection of the motor air coolers, however, it was determined that water leaked from headboxes into the air plenum spaces. Severe corrosion at tube rolls was observed (Fig. 2.9). 

Components in which water temperature changes abruptly with distance, such as heat exchangers, tend to accumulate precipitates. Heater surfaces also accumulate precipitates if the dissolved species have inverse temperature solubilities. Systems in which pH excursions are frequent may accumulate deposits due to precipitation processes. Plenum regions, such as heat exchanger headboxes, tend to collect deposits. 

HDPE melt, 20 160 Head-area meters, 77 664-666 Headboxes, in paper manufacture, 73 106 Header design, 73 271-276 Head group/substrate interaction, 24 139 Headspace analysis, 77 518... 

The dispersion of fibers in the headbox is brought about by subjecting the slurry or suspension to shear stresses, usually with turbulence. Various designs have been developed to accomplish this. 

The stock is pumped through a manifold into the headbox of the paper machine, where the stock flow is decelerated and distributed over the width of the machine. Various baffles and step diffusors are used to avoid vortex flow and stagnation zones. The furnish leaves the headbox through the slice, a narrow gap with controlled profile, and impacts on one or two endless screens, the so-called papermakers wire. Water is removed from the fiber mat by the action of foils and vacuum. 

In connection with a program at the National Bureau of Standards, sponsored by National Archives and Records Service (33), on the development of specifications for permanent record papers, some handsheets were made from a pulp in which the carboxyls were covered with aluminum. Just before the sheet was formed, some calcium carbonate was added to the headbox. Thus, the cellulose had aluminum on the carboxyls, a characteristic of instability, and the paper had a calcium carbonate filler, a characteristic of stability. 

The furnish is fed to the headbox of a paper machine (Fig. 15.12), about the length of a city block. Sufficient additional fresh water (white water) is added to bring the pulp concentration in the headbox down to about 0.5%. White water, the water removed from the fibers as the paper sheet is formed on the machine, is continuously recycled. This low-concentration of pulp in water is necessary to obtain an even distribution of fiber (a smooth formation ) as the sheet is formed. The diluted furnish is fed through an adjustable horizontal slot (the slice) in the headbox onto an endless wire screen (the Fourdrinier wire), which moves rapidly away from the headbox. Direct drain-... 

At the end of the Fourdrinier wire a couch roll removes more water from the web before it is transferred to the press section. At this point the moisture content of the web is about 80% water (20% consistency, 200g/L). The white water from the Fourdrinier is continuously recycled 1000 kg of stock at 0.5% consistency is reduced to a 25 kg mat (5 kg fibre, 20 kg water) by the time it leaves the wire and 975 kg of white water is recycled to the make-up chest and thence back to the headbox. A 500 tonne a day papermachine recycles some 100 000 m of water a day. 

If there were a subliminal message in this seeond edition, it would be the intereormeetivity of the subjeet matter, linking the fundamentals of eell ehemistry and ultrastrueture, the physies of instability etc. to the headbox, the log seanner etc. Consequently the intelleetual infusion from elsewhere - the east-offs from broader diseiplines - together with metieulous attention to detail - the subtlety of thought -sustains every design eoneept. The eapaeity to innovate is quite astonishing. 

Hamalainen, J., Miettinen, K., Tarvainen, P. and Toivanen, J. (2003). Interactive solution approach to a multiobjective optimization problem in paper machine headbox design. Journal of Optimization Theory and Applications 116, 2, pp. 265-281. 

Not all surfaces can be engineered to avoid deposits. Although pipes and headboxes can be polished, these too will eventually become the nexus for a layer of scale. Other areas of the machine such as a machine wire will attract a different type of deposit... 

Retention, drainage and formation were one of the most difficult balances to obtain in papermaking when only one product was used to attain these properties. When a few hundred grams of polyacrylamide were used to flocculate the stock just before the headbox, both the drainage and fines retention increased as more was added. Similarly, paper formation deteriorated with increasing retention aid addition. 

Properties of surfaces deteriorate as deposits build up. A thin invisible film of pitch can transform the original almost-perfect surface of a press roll to give much more inferior properties. The smooth surface of an electiopolished headbox is transformed to a material with high roughness through the deposition of a thin layer of scale. 

This, you may think, is only of theoretical significance and possibly relevant comments for small machines in countries that are not considered to be at the forefront of technology . Take a look at the two photos below taken of the headbox of a state-of-the-art paper machine in the Nordic countries before and after a cleaning (Fig. 2.1). Remember that a headbox should ideally be of high smoothness. 

Barium sulphate is also becoming a rather common scale, particularly in closed pulp mills. When it occurs, the problem of its formation is exbemely troublesome as it is a salt with very low solubility. Because barium sulphate is a salt of such low solubility, barium sulphate scale may form even if barium concenbation, relative to other cations, is low, particularly when concentration in the system of sulphate ions is high. Barium is dissolved from the wood in the digester. The sulphate ion comes into the system through additives such as alum, and high sulphate ion concenbation may develop as a result of accumulation of the sulphate ion from papermaker s alum (used in the rosin sizing process) or from sulphuric acid used for pH conbol. A particular problem is Barium sulphate deposition in the headbox of Kraft liner machines. 

Formation - acceptable formation, better formation with lower headbox solids... 

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