This can be a technical challenge, however. Two presentations during the Papermaking sessions at PaperWeek Canada in Montreal this past February discussed how to help close the loop and recover these raw materials. A third presentation brought news about an innovation in the dryer section.
Maximizing re-use of white water
A presentation given by David McGowan of Kadant dealt with best practices for fibre recovery and reuse of paper mill white water. White water solids from the forming section are easily recycled by using the water to dilute pulp coming to the machine, but white water from other parts of the mill are more difficult to recycle. White water from the press section can contain felt hairs that are undesirable in the paper. And dilute white water can be used in showers, but any residual solids can plug or erode shower nozzles.
Many mills sewer white water that is difficult to recycle, and make up this volume with fresh water. This results in some loss of solids, but the cost of heating that cold fresh water by 30 to 40 C is likely higher. The installation of equipment to further remove solids from, or “polish,” this white water to make it reusable in applications such as showers can often result in a good payback.
A disk saveall is the primary equipment for recovering fibre and recycling white water, and the clear leg of this saveall is hot water that can be re-used for showers if sufficient care is taken to ensure residual solids are removed. This solids removal should take place in two stages: first a strainer designed to remove several tonnes/day of fibre that can handle typical inlet solids content of 1,000-10,000 ppm, then a polishing step that can handle a maximum solids content of 200 ppm. The polishing step is designed primarily to protect shower nozzles from plugging or eroding, and will likely only remove several kilograms/day of fibre. A good rule of thumb for nozzle protection is to divide the nozzle size by six to choose the pore size of the filter medium in the polishing filter.
Types of polishing filter
But before choosing capital equipment to do this, the existing equipment should be surveyed to make sure it is being operated according to best practices. A recent survey of 21 showers on four pieces of equipment at a tissue mill reported that optimizing the flow rate of those showers would result in 2,200 litres per minute of water savings. It is also important to replace worn nozzles, especially in high-pressure showers, since this results in excessive water use.
In summary, to reduce loss of fibre and hot water from the process, it’s important to look at opportunities to replace fresh water showers with a reliable source of white water that has been strained to remove most of the fibres and polished to prevent damage to nozzles. The energy cost of using fresh water and sewering white water can contribute up to 75 per cent of the payback to such a project.
A presentation by Mark Nelson of Kemira (based on a paper by Jaakko Ekkman) highlighted a raw material recovery opportunity that should be of great interest to manufacturers of recycled paperboard. Old corrugated containers (OCC) can contain a lot of starch, which is not only used in manufacturing the layers of linerboard and the corrugating medium between them, but in the glue that holds them together. Kemira has developed a patented technology that has the potential to recover some of this starch, providing not only a financial benefit, but also opportunities for improvements in strength and chemical oxygen demand.
The normal fate of the starch coming in with OCC is to end up as dissolved organic material in the effluent treatment plant. Dissolved starch can also lead to issues in the process when amylase enzymes present in the white water degrade it to sugars, which are a readily available carbon source for bacterial processes. In turn, this can produce bad odours from volatile fatty acid production, pH depression and increased dissolved calcium levels, as well as potential runnability problems due to reduced efficiency of chemical additives. The traditional approach is to use high levels of biocides to prevent this bacterial action, and/or more use of fresh water and less-contaminated OCC.
Kemira’s patented concept, called KemRevive, employs a two-step process to recycle the starch in the process. First, it protects starch from degradation by using a specific inhibitor for amylase enzymes, called FennoSpec 1200. Secondly, a polymeric component improves retention of the protected starch on the machine. Normal levels of biocides are used at the same time.
In one case study, a European recycled linerboard mill used the KemRevive system, which resulted in a decrease in fresh starch addition from 30 kg/t to 14 kg/t, with no reduction in strength and an energy savings of 6 kWh/t from less stock refining.
In a second case study with a European mill making both linerboard and fluting stock from 100 per cent recycled fibres, the starch savings were only five per cent but enabled an increase in ash content while meeting the strength specifications with an overall 5€/t savings. A third mill used the KemRevive system to enable speeding up the machine by five per cent.
This new technology is an innovative and promising way to make recycled board products more economical while adopting a more sustainable approach at the same time. Instead of allowing all that recycled starch to become food for bacteria, it can be re-used, lowering the amount of corn that has to be grown to produce that starch and reducing the number of deliveries of starch and the carbon footprint associated with those deliveries.
Safer, faster replacement of dryer felts
While the first two topics in this article describe how to close the loop around a paper machine, this last topic involves the physical closure of another loop: the dryer felt that loops around the dryers. It highlights an important innovation for papermaking that offers improvement in productivity as well as safety. Blake Farmer of AstenJohnson introduced his company’s new laser-bonded dryer felts at the conference.
On many machines, dryer felts are only changed about once a year, and often the replacement process is unscheduled and unplanned. Because it is such a rare event, it may be the first time for many employees involved with a dryer felt change. Operating with an inexperienced crew in the hot and humid environment of the dryer section raises safety concerns.
Traditional dryer felts have woven seams, which are thicker than the felts themselves and can result in marks on the paper. This can be a problem in certain grades such as TAD tissue.
In response to all of these issues, AstenJohnson has developed a laser-bonded dryer felt, with a zipper-like seam bonded by a laser onto the two ends of the felt. The heat created by the laser beam at the interface creates a molten weld seam that fuses the felt and the seam together. Once installed, the two ends of the seam are connected by inserting a machine-width “pin” through the seam, as shown in the accompanying photo.
This innovation not only results in seams with no caliper difference and less likelihood of wear than a woven seam, but it can save time, since the pin can be pushed through the seam across the machine in as little as three minutes, likely involving less people than a traditional woven seam.
Furthermore, the felts are designed to be reversible, i.e. no sheet side and wire side, which means one less thing to check for during installation. These felts have been available for less than two years, and so far there are about 40 mills around the world using them.
Martin Fairbank has worked in the pulp and paper industry for over 30 years and is currently an independent consultant and technical writer.
This article originally appeared in the Fall 2018 issue of Pulp & Paper Canada.
This article originally appeared in the Fall 2018 issue of Pulp & Paper Canada.