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Online charge control translates into big savings for market leader Mayr-Melnhof

A comprehensive wet-end survey and installation of a sophisticated BTG Mütek charge control system is saving Switzerland’s Mayr-Melnhof a bundle while improving output quality.

By cutting chemical additives and boosting productivity, Mayr-Melnhof’s BTG Mütek system paid for itself in just 3 months.

Installation of an advanced new online charge control system based on BTG Mütek instruments has delivered substantial benefits at Mayr-Melnhof’s Deisswil mill, saving money on chemical additives, improving machine runnability, and boosting production capacity on the company’s KM 6 board machine by 1%.

The world's largest producer of recycled fibre-based cartonboard, Mayr-Melnhof is also Europe's leading manufacturer of folding cartons for a huge range of packaging applications. The company’s Deisswil mill, located a few kilometers from the Swiss capital Bern, currently produces 165,000 tonnes per year (tpy) of coated duplex board and white-lined chipboard.

With demand for its products strong and still growing, the company sought to address a number of chronic problems, including increasing consumption of costly chemicals, less-than-optimum output quality and poor runnability of its KM 6 board machine.

Fast cost savings

To resolve these and other issues, Mayr-Melnhof turned to BTG, commissioning an in-depth wet-end survey and installing a new closed-loop fixative control system that’s boosted productivity, reduced the need for additives, and decreased volumes of broke.

Implementation of the new online charge control system on KM6, the mill’s newest, highest volume machine, has allowed the company to cut resin/alum additions by one third and dosage requirements of grease-barrier products by 25%. Mass starch additions are now a thing of the past, and spray starch is required only in isolated cases. Improved filler retention has also enabled a 2g/m² reduction in fibre content in the top layer.

In addition to a 150mg/l decrease in COD outflow levels, improved machine runnability has translated into shorter adaptation times after grade changes as well as significantly reduced depositions and broke volumes. All this, combined with an overall 1% increase in machine productivity, saw Mayr-Melnhof amortize the cost of the survey and new charge control system in just three months.

Getting to grips with the problem

FIGURE 1
A comprehensive wet-end survey using a BTG Mütek SZP-06 System Zeta Potential and Mütek PCD-03 Particle Charge Detector measured the zeta potential and cationic demand at five different measuring positions along the production process.

In addition to addressing rising chemical costs, output quality problems and excessive machine downtime, Mayr-Melnhof sought to limit the use of high-cost chemicals such as grease barriers to specialty board production only.

To determine the best solution, trainee paper engineer Jan Weihs from Munich University of Applied Sciences worked with the BTG Mütek team and specialists from BASF Ludwigshafen, which undertook the selection of chemicals for optimum charge control.

To begin, lab measurements were taken to ascertain the zeta potential and cationic demand of the raw materials used in the top layer. An analysis using BTG’s Mütek SZP-06 System Zeta Potential and Mütek PCD-03 Particle Charge Detector revealed fluctuating charge levels in the four primary raw materials (Fig. 1) :

• mixed and recovered paper and board
• short-fibre and long-fibre chemical pulp
• unprinted white wood-free shavings (WI)
• lightly printed white wood-free shavings (WII)

Since the unprinted white wood-free shavings were found to be particularly highly loaded with anionic trash, this material was partly substituted by lightly printed white wood-free shavings and chemical pulp.

Wet-end survey

Step 2 involved a comprehensive wet-end survey of the KM 6 machine to look at the top layer circuit in detail.

Five measuring positions were set, and measurements at Position 1 (the fresh fibre mix) indicated that the fibres used provided a large number of free bonding sites (zeta potential -12.4mV), and that the level of anionic trash was also comparatively high (PCD value = 1.8ml poly-DADMAC 10-3N).

Position 2, after the addition of a grease barrier product, showed a decrease in both fibre charge (zeta potential -11.0mV) and trash levels (PCD value = 0.6ml), suggesting that the grease barrier product was primarily reacting with the anionic trash instead of the fibres.

After dosing with mass starch at Position 3, fibre charge had fallen even further (zeta potential -8.5mV), while anionic trash levels remained more or less constant (PCD value = 0.4ml), indicating that the starch had been absorbed into the fibres.

The 1% capacity increase on KM 6 made possible by the BTG system has prompted plans to implement online charge control on KM 5 as well.

Measurements taken at Position 4, located after the dosing point for resin/alum, showed largely unchanged levels of fibre charge (zeta potential -8.3mV) and anionic trash (PCD value = 0.2ml), indicating very poor absorption of resin/alum – possibly because the reaction time with the fibres was too short.

Poor absorption entails a risk of additive accumulation in the circuit, resulting in deposits, quality inconsistencies and increased COD levels, and creating a vicious cycle that necessitates the addition of even more costly chemicals. The use of a retention aid (Polymin) was shown to stabilize the charge level, causing the zeta potential to diminish slightly.

The wet-end survey also revealed that the very costly grease barrier product used by Mayr-Melnhof primarily served to reduce the amount of anionic trash. In many cases, it took several hours for the mix to reach desired performance and quality criteria. As a result, fibre charge had to be stabilized at a lower level before the introduction of additives.

It was clear that, by compensating more effectively for fluctuating raw material compositions, Mayr-Melnhof could substantially boost quality and optimize chemical dosages to match quality levels.

Choosing the right product

FIGURE 2a, 2b, 2c
A pitch counter was used to identify the level and particle size distribution of anionic trash. Source: BASF Ludwigshafen.

The next step in optimizing KM 6 performance was thus to choose an additive that best suited the stock system.

Since the primary function of this product – usually a short-chained, highly charged polymer – was to control the charge level without reacting in other ways with the fibres, it was important to ensure that the chosen additive did not provoke agglomeration of disturbing substances and the formation of secondary stickies.

To achieve this, a pitch counter – a measuring system identifying the level and particle size distribution of anionic trash – was used. Trials of various alternatives showed the best performer to be Catiofast VFH, a short-chained, highly charged polyvinylamine (Fig. 2a, 2b, 2c) .

Online charge measurement

FIGURE 3
Measurements over an extended time period showed a far greater charge variation than simple daytime tests had indicated.

Another three weeks of intensive lab tests led to the installation of BTG’s Mütek TSS-70 Filtration Sampler and Mütek PCT-20 Charge Analyzer downstream of the machine chest for online charge measurement and control.

Subsequent measurements taken over a 24-hour period revealed a far greater charge variation in the top layer circuit than lab tests conducted during the day had indicated (Fig. 3).

The benefits of the new BTG system have been striking – dramatically reduced chemical costs, better quality and substantially higher machine productivity.

Most notably, cationic demand was sometimes extremely high, reaching 12ml at the beginning of the morning shift – exactly the time at which production was normally switched to greaseproof qualities. On-spec quality was not reached until the expensive greaseproofing agent was added to decrease anionic trash levels – but since the cost of this additive represented a substantial overhead for Mayr-Melnhof, this result was dearly bought.

In light of analytical results, it was clear that Catiofast VFH additions would be necessary to reduce and stabilize the charge level at the target value of 1ml.

Closed-loop control

FIGURE 4
Dosing with Catiofast VFH helped quickly stabilize the system at target values.

FIGURE 6
Dosing with Catiofast VFH helped quickly stabilize the system at target values.

FIGURE 5
Dosing with Catiofast VFH helped quickly stabilize the system at target values.

Once the dosing system and controllers for Catiofast VFH had been installed, closed-loop fixative control was introduced, with Catiofast VFH being dosed directly into the machine chest. Dwell time in the chest was 30 minutes.

At morning start-up, an extreme cationic demand value of over 10ml was recorded, but levels were reduced to the target of 1ml within two hours (Fig. 4). Following further optimization of the controllers, charge level in the machine chest stabilized as early as 12 hours after dosing, as did whitewater charge.

At the onset of closed-loop control, the Catiofast pump delivered the maximum dosage of 32l/h (Fig. 5). Some eight hours later, when the level of anionic trash in the system began to decrease, dosages were reduced to 16l/h, and after 12 hours, stabilized at 2 - 3l/h. Depending on the furnish composition, additions of 2 - 4l/h of Catiofast are now sufficient to stabilize the system at the target value (Fig. 6).

Impressive results

FIGURE 7
Closed-loop fixative control increased retention as charge levels were lowered and stabilized.

Mayr-Melnhof’s closed-loop fixative control system has delivered an impressive number of benefits.

As charge levels were reduced and stabilized, retention increased (Fig. 7). As a result, the mass of the top layer increased – initially prompting the unwanted side-effect of blistering caused by reduced porosity of the layer due to higher filler levels. But since higher filler retention favourably affected formation, Mayr-Melnhof found that top layer weight could be reduced by 2g/m², counteracting blistering and effectively resolving the problem.

As an additional benefit, the increased ash content and correspondingly reduced fibre share in the top layer translated into substantial cost savings. Similarly, deposits and clusters were drastically reduced, which in turn reduced broke volumes (Fig. 8).

FIGURE 9
The ability to cut starch dosages has also dramatically reduced COD loads.

FIGURE 8
Online fixative control also dramatically reduced deposits and clusters.

The use of Catiofast also obviated the need for the addition of mass starch – which, BTG Mütek system revealed, had in any case only really served as a trash collector. In addition, the use of spray starch between the top layer and the protective layer was no longer needed for 90% of KM 6 production, with water spraying now sufficient for all but some specialty production. Lower starch dosages have, in turn, drastically decreased COD levels (Fig. 9).

Mayr-Melnhof’s engineers are so pleased with the 1% capacity increase on KM 6 made possible by the BTG online fixative control system that a strategy is now being developed to install a charge control system for the plant’s KM 5 machine as well.

To learn more about Mayr-Melnhof, visit www.mayr-melnhof.com/welcome