Casimiro da Silva Santos, global program manager, Nalco
Aug. 31, 2008
In the past three decades, retention and drainage programs, which have used an inorganic microparticle component, such as colloidal silica and bentonite, have been widely practiced. Generally microparticles are added in combination with an organic high molecular weight polymer or starch, after these have been added to the papermaking stock and after a high shear point (e.g. pressure screen before the headbox). Typical benefits of microparticle based programs are superior filler retention, improved drainage and improved sheet properties such as formation and porosity, when compared to single or dual flocculant programs. However, in modern paper machines, such as gap formers and hybrid formers that are used in the production of different paper grades and operate at over 1,000 m/min, these typically expose papermaking furnish to very high levels of hydrodynamic shear forces, making retention more difficult where traditional single and dual polymer programs cannot achieve acceptable results.
|Type of Machine
||STFI roll–blade former
||TMP (57%), SWD (28%) and Filler (15%)
|Jet–wire ratio (m/min)
|Slice Opening (mm)
|Basis Weight (g/m²)
|Press Loads (kN/m)
||60, 500, 700
The need for increased drainage at higher retention levels in paper grades with higher ash content has led to the development of microparticle-based retention programs. However, the trade-offs between retention, drainage and sheet properties are further accentuated in gap former machines. For optimal performance, gap formers need to have a comprehensive retention system that is able to provide the highest retention possible with a lower flocculation (small flocs), while improving drainage in a controllable manner, as the amount of water drained is crucial to achieve enhanced sheet quality. In gap former machines, the fiber suspension is directly injected into the wedge of two forming wires, with the subsequent formation of the paper web between two forming rolls. Strong and abrupt drainage is created on both sides of the paper web due to the centrifugal forces and the application of vacuum. Subsequently, the web travels through a series of pressure static blades that promote drainage in both sides of the paper web forming the sheet. In this section of the gap former, the high shear forces that promote drainage place significant strain or demand in the retention system performance, as retention can be limited by formation. The correct ratio between drainage at the forming rolls and drainage at the blades is essential for the quality of the sheet, as too much drainage in the forming rolls impacts negatively on formation, z-filler distribution and z-directional strength.
It is this balance between two drainage areas and retention that has challenged several companies over the years to modify their microparticle retention programs to cope with the high shear forces of gap formers. However, some companies have decided to re-think their approach to retention and drainage in modern paper machines, and in some cases new chemistries have been developed and new concepts have been tested. These new concepts break away from the traditional concepts typically seen in the paper industry.
This article focuses on the implementation of Nalco’s ELLIPSIS Cationic Micropolymer Technology as part of a comprehensive retention and drainage program as a viable solution for papermakers operating high speed machines to have at their disposal to get more drainage in a controllable way, while increasing filler and fines retention without detrimental effects on formation and other sheet properties.
||Total Filler Count per Side
||Distribution# per Side²
|ELLIPSIS @ 9% Ash
New retention and drainage system
Paper producers are under tremendous pressure to improve the cost-competitiveness of their operations by improving On-Machine Efficiency (OME) and paper quality. Requirements for porosity, surface roughness, ink receptivity, and formation for good printability necessitate higher filler loadings. Higher filler retention and controllable drainage without adversely affecting sheet properties created the need for developing a new retention system based on micropolymer technology that is able to slow down drainage without affecting paper machine runnability while increasing filler retention and provide even filler distribution within the paper web. The functionality of new chemistry and the new retention and drainage program has been verified with laboratory tests, pilot machine trials, mill scale tests and commercial applications. These will be discussed in detail throughout this article.
Pilot paper machine trials
A micropolymer trial was run on the EuroFEX Experimental Paper Machine at the Swedish Pulp and Paper Research Institute (STFI) in Stockholm, Sweden, to evaluate the performance of the Ellipsis technology retention based program relative to existing single flocculant and microparticle-based retention systems. Another objective of the trial was to explore the potential benefits of the new micropolymer based program on sheet properties (e.g. formation).
The stock used in the trial was a typical publication paper furnish consisting of 85% total fiber (57% peroxide bleached TMP and 28% softwood kraft, from Södra Skogsägarna, Sweden) and 15% filler from which 58% was kaolin clay and 42% was ground calcium carbonate. The retention programs evaluated were a single flocculant program, a bentonite-based microparticle program in combination with a cationic flocculant and a micropolymer-based program in combination with a cationic flocculant.
Figure 1 illustrates a schematic diagram of the short circulation of the EuroFEX Pilot Paper Machine with the indication of the flocculant and microparticle/flocculant addition points. The machine conditions that were used during the trial are detailed in Table 1. The forming unit was a roll-blade gap former. After the initial roll dewatering, the papermaking furnish is drained over a blade section with blades on both sides so that drainage is symmetrical. Both addition points and paper machine conditions were maintained constant during the trial, which lasted 6 days with several runs with different dosing curves and feeding strategies. One of the trial conditions was to evaluate the performance of chemicals without the influence of paper machine-related factors.
The pilot paper machine trials demonstrated improved normalized formation (ß-formation) at higher retention levels for the micropolymer based retention program, when compared to traditional single flocculant and bentonite based programs (Figure 2). Other benefits included slower early drainage rate and preferential ash retention.
Preferential ash retention can be analyzed through the slope variation on the first pass ash retention (FPAR)-first pass retention (FPR) ratio in Figure 3. As the angle or slope of the line FPAR/FPR versus dosage increases when compared to a 45-degree angle and unitary slope line, the higher the contribution of filler and fines into the total retention value. Through analysis of Figure 3, it can be concluded that an Ellipsis technology-based retention program shows higher preferential filler retention than traditional microparticle and single-flocculant programs.
These advantages are extremely important in today’s papermaking operations where paper producers tend to sacrifice retention levels (FPAR and FPR) in order to achieve formation specifications demanded by their customers. This is a consequence of the high-speeds paper machines are now able to typically achieve. In order to reach speeds at above 1,000 m/min, a gap former machine needs to drain all the water in the papermaking stock very quickly in the early stages of sheet formation. This abrupt drainage, especially in the forming roll before the loadable blade section, causes fines and filler to be rushed to the surface of the sheet while the fibers stay within the middle of the fiber matt entrapped together with water. This bounded water creates high hydrodynamic pressure within the forming sheet, causing the fines and ash to get loose of the fiber matt. Ideally, fines and ash should be thickened together with the fiber in order to have a balance between retention, drainage and formation. It is clear that to achieve this, early drainage in the forming roll needs to be decreased enough to create a good entrapment of the fiber and filler within the paper web, however it should not be a such a drastic reduction that it will cause the paper machine to stall and slow down in order to avoid sheet breaks, thus reducing OME and productivity. This is another one of the benefits the micropolymer-based retention program, with the ability to smooth down drainage in the early stages, about 15%, while increasing ash retention by 21% when compared to traditional bentonite based programs.
Case studies of commercial applications
The following examples outline the efficiency and quality issues that were faced at mill sites, and how the Ellipsis technology retention based program was a viable solution to overcome them.
Case Study 1: This example illustrates how a North American newsprint producer improved paper machine efficiency by improving production and reducing draws.
A standard newsprint hybrid former paper machine producing 170,000 tonnes/yr, with an operating speed of 1,200 m/min using thermo-mechanical pulp (TMP) and recycled newsprint pulp (RNP) as furnish, desired to improve OME by increasing paper machine speed through more uniform drainage and improve runnability by strengthening the RDF program.
The previous program was single component (cationic flocculant) and a comprehensive system survey was conducted to better understand the customer’s productivity issues. The mill’s current retention and drainage program was evaluated in the laboratory along with several Nalco programs in order to identify the optimal choice. As a result of these evaluations, (verified in conjunction with the customer) Nalco’s proposal was to use the new Ellipsis technology. This program was designed based on a modified retention and drainage platform. This was a dual-component program utilizing cationic micropolymer and high molecular weight flocculant.
The application of Nalco’s RDF program resulted in the following benefits that resulted in improved OME:
• 6 m/min draw reduction.
• A 7% increase on first paper retention (FPR) from 50 to 53.5% on average.
• A 14% tray solids reduction and a 9% headbox consistency reduction.
• A 2 tonne/day lower fiber loss from the uhle-boxes.
• An 8% production increase on average
Case Study 2: A European paper manufacturer was able to reduce operating costs by replacing filler type, improving ash retention while reducing linting tendency of the paper produced in one of its paper machines.
The paper machine, a hybrid former producing 230,000 tonnes/yr of improved newsprint and book grades (40-65 g/m2) from groundwood (GWD), thermomechanical (TMP) and kraft furnish running at 1,200 m/min was trying to improve ash content from 6% to 9% in the final paper while replacing precipitated calcium carbonate (PCC) by ground calcium carbonate (GCC) as a way of reducing its operational costs. Previous attempts with a traditional retention system failed, as the final paper produced presented severe linting tendency when printed in offset printing presses.
Following a comprehensive system survey, which included the evaluation in the laboratory of several retention programs in order to identify the optimal choice, Nalco’s proposal was to use Ellipsis technology. The implementation of the new program was based on a two-step approach in order not to disturb paper machine runnability and evaluate the benefits in a consistent manner. In the first step, the amount of ash was maintained at 6% while replacing the filler type from PCC to GCC. In the second step the filler content was increased from 6% to 9% using GCC as filler. The implementation was successful as the mill was able to replace and improve filler type and content.
Printing tests were performed at the Future Printing Centre in Raisio, Finland in order to verify if the paper produced with the new conditions presented linting tendency. As a result of these, it was observed that the linting tendency was reduced by 29% and 32% at 15,000 and 45,000 imprints, respectively (Figure 4). The pressroom operators considered this reduction in linting as good and acceptable for pressroom operation without affecting printing quality and press runnability.
Using analytical tools such as Scanning Electron Microscope (SEM) and sheet splitting to determine the filler distribution on the surface and z-direction of the sheet, it can be concluded that the linting reduction is due to a reduction of filler agglomerates on the surface of the sheet of the most filled side, thus resulting in a more evenly distributed filler in the surface of the sheet. The correlation found between the total average linting and the filler distribution number for the most filled side demonstrates this (Figures 5a and 5b, respectively). Thus it can be inferred that as filler is agglomerated onto the surface of the fiber due to a strong retention program, linting tendency increases.
While filler evenness was substantially improved by 26% and 58%, when sheet ash was increased from 6% to 9%, respectively, sheet two-sidedness was reduced by two percentage points and increased by four percentage points, respectively (Table 2). Generally higher filler retention levels mean lower two-sidedness, however z-filler distribution (Figure 6) indicates that the mechanical setting of the paper machine former was incorrect, causing a predominant fourdrinier type of behavior of the former, which influenced sheet two-sidedness at higher filler retention levels.
The new retention program was able to improve filler retention and avoid filler agglomeration of the surface of the sheet, thus reducing the linting tendency of the paper when printed in an offset press.
In high speed machines operating at 1,000 m/min, if fibers and fines are mixed with the wrong combination of mechanical forces, large porous stable flocs are created, which are very difficult to drain in the forming section, resulting in papers with irregular z-filler distribution, poor formation, low strength and poor printability. Moreover, this is further accentuated by the use of high doses of high molecular weight flocculants in combination with microparticles to achieve higher filler retention and increased drainage. In recent years, companies have tried to modify and optimize their microparticle based retention programs to cope with the shear forces present in the forming section of high-speed machines with limited success. Papermakers became used to operating their gap formers and hybrid former machines with low retention levels in order to maintain or improve formation. The implementation of an Ellipsis technology retention program based on the combination of a high molecular weight flocculant and a micropolymer on high-speed machines provides papermakers with viable solution to retain selectively fines and filler while providing increased drainage in a controllable way. Using this technology, papermakers can optimize mechanically their gap and hybrid former to achieve new levels of filler retention without sacrificing z-filler filler distribution, formation, roughness and strength.
Casimiro da Silva Santos is global program manager, Expertise Center RDF & Functionals, Paper Services Division, Nalco
Pulp & Paper International is FREE to qualified subscribers.
Click here to find out more.