Research & Innovation
Fine and coated papers session was full of air . . . and pitch!
PEOPLE ATTENDING the Thursday afternoon Fine and Coated Papers session were not disappointed: speakers tackled key problems that papermakers currently experience with fine and coated papers, namely ai...
March 1, 2000 By Pulp & Paper Canada
PEOPLE ATTENDING the Thursday afternoon Fine and Coated Papers session were not disappointed: speakers tackled key problems that papermakers currently experience with fine and coated papers, namely air in the sheet and pitch deposits. And the solutions presented stirred very positive reactions from the audience. Clearly, these challenges needed to be addressed.
The presentation on Successful Air Content Control — Benefits to the Papermaker is based on the assertion that almost all paper machine stocks carry a certain level of air content. Chafiq Belouadi, BASF Canada Inc., began with showing how entrained air in papermaking stock has a negative impact on paper machine performances and paper quality. Entrained air in paper furnish can greatly reduce the drainage rate by blocking the opening of wire fabrics. Air bubbles are also known to impair paper formation and tensile strength; the sheet formed is also more porous and rougher. Entrained air introduces variations in basis weight and reduces the initial wet strength of the sheet on the forming table, causing paper machine runnability problems.
Another negative effect of entrained air is the reduction of pump efficiency. “This is not only energy consumption that we are talking about, but this could also lead to paper machine speed limitations in cases where the fan pump is used at full capacity,” Belouadi explained. “Entrained air and foam are ideal sites for the agglomeration of hydrophobic materials, such as wood pitch, white pitch and adhesives. Those sticky chemicals agglomerate around the air bubbles to form secondary stickies. Secondary stickies detrimentally affect the paper machine runnability and paper quality. They can deposit on paper machine wires, Uhle boxes, dryers cans and calendar stacks.”
Logically, entrained air removal is necessary. Belouadi’s approach is both mechanical and chemical. The physico-mechanical deaeration in a Deculator” (Ahlstrom) is achieved by spraying and impinging heated pulp suspension under vacuum near the boiling point of suspension. While mechanical deaeration is very effective, the stock suspension can only be kept air-free as long as it does not get exposed to air again. The only thing is, the stock leaving the physico-mechanical deaeration is described as “hungry” for air. The use of chemical deaerators is thus necessary to avoid reintroduction of air in the pulp suspension.
Chemical deaerators have a similar structure to air stabilizing substances, but are more hydrophobic. Due to their lower solubility in water, they displace surfactants from the water air interface. Air bubble stabilization is interrupted; small bubbles coalesce, rise and burst.
Answering a delegate question, Belouadi emphasized on the fact that defoamers kill foam — not the air. “Always remember: no air, no foam!,” he repeatedly said. Whether a given chemical acts as defoamer or deaerator depends mostly on its chemistry and its distribution in water. Non emulsifiable oil type defoamers tend to form a separate oil phase on pulp suspensions. They are effective foam killers, but do not influence entrained air.”
Mill experiences shown by Belouadi were convincing. In one mill, the newsprint paper machine, equipped with a high-speed gap former and working from a mixture of TMP and deinked pulp, could not go beyond the 1450 m/min speed limit because the fan pump was then running at full capacity. Measurement of air content indicated an increase of entrained air in the headbox from 0.01% to 0.5%. To overcome the problem, a high temperature chemical deaerator was added to the wire flumes. Within a short period of time, the entrained air in the headbox was dramatically reduced and the fan pump speed was decreased. In addition to the 3.5% paper machine speed increase, drainage was improved and depositions on wire and felts were reduced. The two other mill experiences showed a reduction of deposition and holes in the sheet (cleaning downtime was cut by 30%), and an over 3 points increase of filler retention. They also showed an increase in total retention of four points on a standard newsprint machine using TMP and deinked pulp with a high filler content, at neutral pH.
Anton Esser, BASF Aktiengesellschaft, discussed Efficient Process Chemicals for White Pitch Treatment. The key according to Esser, is to “control the attraction between cationic polymers and wood pitch.”
Tacky detrimental substances come from many sources and are determined by the raw materials used: rosin and wood extractives introduced by virgin fibres, pressure sensitive adhesives, hot melts and binders originating from previous recycling of waste paper, and binders, co-binders and other coating color auxiliaries from coated broke. The consequences of these substances vary from premature replacement of wire and felts, to web breaks, dirt and even holes in the finished paper. Analysis of deposits usually show more than a single contaminant source. It is very common to find deposits consisting of a mixture of adhesives, wood pitch, and white pitch. Unlike wood pitch based on rosin particles, white pitch particles have only a small anionic charge. If left untreated, they tend to homoflocculate. A fixing agent reacting exclusively by a charge mechanism is often not efficient. The size of white pitch particles is also spread over a much wider range. The shear forces acting on large particles may become very large and eventually tear off fixed particles from the fibre surface.
BASF developed a new modified cationic Polyvinylamine (PVAm), displaying dual characteristics of high charge density and hydrophobicity. Used as a fixing agent, the modified PVAm shows an additional attraction to hydrophobic materials largely due to its own hydrophobic nature. Due to this additional attraction, white pitch particles are more strongly attached to the fibre surface, thus forming heteroflocs with superior strength properties. The strong heteroflocs are bound to the finished paper and subsequently removed from the machine circuit.
Compared to the polyamine, treatment with the modified PVAm leads to better results in all categories, particle reduction, deposit formation and deposition tendency. The absolute level of deposit formation was decreased to less than 2%, which is seventeen times smaller compared to polyamine and still five times smaller compared to the blank. Modified PVAm has only been introduced recently. It is now being tested in two selected mills for the treatment of coated broke. The improvement that has been achieved is reduced deposition in paper machine loops, less web breaks, and lower dirt count.
Esa Tiikkaja, Neles Automation, presented Formation Control Studies. The study was made on a fine paper machine in order to evaluate the possibilities for closed-loop formation control. Preliminary studies showed that the pulp consistency and fibre length distribution were the two pulp parameters with the most notable influence on formation. The lack of linearity of the fibre makes formation control with the headbox consistency difficult. It is thus necessary to search for the optimal consistency areas for each paper grade produced. The other parameter, fibre length distribution and specifically the average fibre length, can be used to control formation. It appears that the effect of fibre length on formation is monotonous at least in the areas of application. This parameter can be used for closed-loop control formation.
Mill trials demonstrated that by measuring the average fibre length of the paper machine furnish, the amount of softwood in softwood and hardwood blend was controlled with a conventional PID controller. The authors of the study concluded that, as expected, formation control with the furnish fibre length can stabilize the paper formation.
The session ended on a short (an unexpected, since not scheduled) “slide show” about defects most commonly seen on coated paper.
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