In the beginning, pulp was beaten with sticks. This method is only used today for certain exotic non-wood fibres, or for historical interest.
Commercial beaters, developed c. 1680 in the Netherlands (which came to be known as a “Hollander” beater), contain a wheel with bars attached, that rotates in an oval-shaped trough full of pulp. The gap between the bars and the bottom of the trough can be varied to modify the beating intensity. As well as mixing the pulp and separating fibre bundles, beating and its successor, refining, first developed and patented in 1858 in the U.S., can remove the primary wall of fibres, enable the swelling of fibres through water absorption, increase fibre flexibility, and externally fibrillate the fibres. This results in a higher surface area, leading to increased hydrogen bonding and increased strength.
If you joined the pulp and paper industry in the 21st century and work in a large facility, you may have never seen a beater. Today, refining of chemical, semi-chemical, and recycled pulps is done by low-consistency disk refiners or conical refiners operating at 3 to 5% consistency. The pulp is pumped between rotating surfaces containing bars, which is a much more energy-efficient method of developing pulp strength than beating.
With industry trends such as high recycled content and low basis weights, together with the importance of energy efficiency and cost savings, your low-consistency refiner system could help to achieve these goals, by applying some recent technology developments.
Low-intensity refining
Many theories have been developed to explain fibre development in a refiner. In 1958, the concept of Specific Edge Load (SEL) was developed, which is a measurement of refining intensity, or the energy per unit length of bar crossings in the refiner. Although this theory does not account for many factors, such as one-pass energy input, bar width, gap clearance, refining consistency, and wear of the refiner plates, SEL can be calculated in order to compare the effect of different plate patterns. This has led to the development of ultra-low-intensity refining, where the idea is to develop tensile strength with minimal damage to fibre length and tear strength.
Ultra-low-intensity refining is carried out by providing a higher number of bar crossings per revolution of the refining plates. One way to achieve this is using “overhung” refiner plates, i.e., larger-diameter refiner plates within the same refiner, but this is limited by the diameter of the refiner housing and the achievable stock flow. The other major means of providing more bar crossings is with finer bars. This is a limited opportunity when using conventional cast plates made by pouring molten metal into molds, because of the difficulty of obtaining the fine structure required.
In the late 1990s, a new way of making refiner plates was developed and patented by AFT, assembling precision-cut stainless-steel plates and fusing them together to manufacture a refiner plate. This not only enables very fine bars to be produced but results in stronger, longer-lasting plates. Compared to higher-intensity refining, these plates can often result in stronger pulp, bulkier pulp at the same drainage level, reduced energy requirement to reach a freeness target, and longer plate life, but not necessarily all at the same time. Other cost savings may be achievable owing to the ability to use less reinforcing pulp or wet-end additives such as starch.
A downside of developing an ultra-low-intensity refining strategy is the amount of patience and planning required. Although plates can be custom-designed, it can be difficult to compare refiner plate operation under identical operating conditions. Plate life is measured in months, so progress can be very slow.
Control of low-consistency refining
Refining is typically controlled to a freeness target by varying the specific energy, but manual freeness testing is typically carried out only every few hours, too infrequent for closed-loop control. Variations in incoming pulp quality, such as different batches of recycled fibre, wood species mix, pulping conditions, and bleaching parameters, are usually much faster than this, so the refining energy can only be adjusted in response to longer-term trends. In recent years, online freeness testers have been developed that can provide more frequent freeness results for tighter control.
While freeness control is important for consistent drainage behaviour on the paper machine and for machine runnability, it is a poor way to measure what is actually happening to the pulp, since low freeness can be achieved either by fibrillating the fibre (desirable) or by reducing fibre length (undesirable for tear strength). Advanced control of low-consistency refining can be achieved by a combination of online sampling, measurement of multiple fibre quality parameters, and model-predictive control using the data generated from fibre quality analysis.
In summary, paper and board mills that have low-consistency refiners may be able to apply cutting-edge technology (pun intended) to achieve product quality improvement and cost reduction, but it requires a long-term strategy and lots of patience.
Martin Fairbank, Ph.D. Martin Fairbank has worked in the forest products industry for 31 years,
including many years for a pulp and paper producer and two years with
Natural Resources Canada. With a Ph.D. in chemistry and experience in
process improvement, product development, energy management and lean
manufacturing, Martin currently works as an independent consultant,
based in Montreal. He is also an author, having recently published
Resolute Roots, a history of Resolute Forest Products and its
predecessors over the last 200 years.
Martin Fairbank Consulting
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