18 Sep Commercial breakthrough in MFC processing
PTS, Paper and Board Symposium, presentation in Munich September 18th-14th
Turning cellulosic pulp into fibrils is an energy intensive process. A lot of research has focused on reducing the amount of energy needed to make Microfibrillated Cellulose (MFC). The primary line of thought has been to soften up the pulp by means of chemical or enzymatic pre-treatment. Significant progress in reducing mechanical energy demand has been reported. However, the equipment used to apply the mechanical energy hasn´t developed very much over the years. The standard procedure is still to use colloid mills or homogenizers. These work well in lab and pilot plants but scaling them up for industrial use poses a huge challenge. So, despite the significant advances that have been made in MFC processing it is generally recognized that there is a need to find more robust processing equipment in order to make MFC industrially viable.
Why is a mineral supplier venturing into MFC?
For a minerals processing company, grinding is a core competence. Most minerals need to be ground as part of the process of making them suitable for the many and diverse applications they are sold into. Our parent company’s experience ranges from vacuum grinding soft graphite to extreme fineness and a specific surface area approaching 1 000 m2 /g to preparing some of the hardest materials on earth for making abrasives. Grinding of marble, limestone, chalk, talc and kaolin to make coating and filling minerals for the paper industry is perhaps a little bit more straightforward but still they tend to be ground to an extreme fineness and customer demands for low cost and quality are very high.
The Printing & Writing paper segment is a big consumer of minerals and in recent times there has been a worrying lack of paper market growth to help drive demand for minerals. As a major supplier of minerals to the Printing & Writing paper segment we had good reason to consider what possibilities there could be to mitigate the effect of the market decline on the minerals business.
Global P&W Paper production, year on year change in % (RISI):
So, it is no coincidence that 2008 was a good year to start a MFC project aimed at increasing the use of minerals in paper making. The timing was also good to respond to a growing need for paper makers to find ways of reducing cost. With falling paper demand came oversupply, poor prices and a paper industry focused on cost cutting as a recipe for survival.
The FiberLean project:
Project time line:
The FiberLean project started out in relatively small scale back in 2008. From then it has evolved to become one of Imerys’ biggest development projects ever. We are now over 40 people, 100% dedicated to FiberLean. Together we have developed the process, the plants, the products, and a number of applications. We have 3 000 dry ton/year of MFC capacity in operation today. Our ambition is to have 50 000 dry ton/year capacity within 5 years’ time.
Example of typical FiberLean on-site plant:
Example of on site installation with 2 000 dry ton per year capacity.
We are NOT using colloid mills. If you are used to crushing and grinding rock, a colloid mill comes across as an engineering equivalent of a jet engine. We all know a jet engine is capable of accidentally grinding birds, but for reliability and safety this is not really to be recommended. When making FiberLean we use grinders where tolerances are measured in cm rather than in microns. Our co-grinding concept is what makes this possible. We grind cellulose to MFC with the help of minerals, such as kaolin or calcium carbonate. The mineral is present in our process for a very good reason, as it facilitates transfer of mechanical energy from the moving parts of the grinder to the fibers. Fibrils are ripped out from the fiber surface when the fibers are hit and rubbed by a huge number of moving mineral particles.
To a mineral processing engineer it is well known that to grind to a fine size you need a fine grinding media. The challenge comes when the grinding media is too fine to be separated out from the product in a cost efficient manner. The thought step with the FiberLean process is that the grinding media, i.e. the filler, does not need to be removed as it is an integral part of the desired product.
With the FiberLean process we have come up with a way of making MFC in a truly industrial scale with the reliability and safety required for on-site installations and 24/7 operation. This process is also designed to use regular pulp and mineral as the sole raw materials. We need no pre-treatment of the fibers and no chemical additives during the grinding.
The FiberLean product coming out of the process is a composite of MFC with mineral. The impact on the mineral going through the grinding process is very small. The fiber, being the softest component, is ground to MFC while the filler remains virtually unchanged. Moreover, the MFC-mineral composite is easy to handle. It is obviously viscous, even at the 5-6% solids coming out of the process, but it can be pumped and it mixes in easily when dosed into the paper making system.
Raw materials input (top two pictures) and FiberLean product coming out of the process:
The FiberLean MFC:
The FiberLean MFC is optimized to be a wet end strength additive. It is obvious from the above picture that this is a MFC made up of bundles of fibrils. It is important to maintain good length of the fibrils as this is essential to allow the MFC to create a network with filler and also between fibers. We can grind finer in our process but this actually results in lower strength performance. So, the FiberLean MFC is an optimum product for strength increase in wet end application. The relative coarseness also limits the extent of dewatering resistance. The water holding ability of the MFC is outweighed by the positive dewatering impact of the filler increase made possible from the increased strength. A finer MFC would lead to drainage issues.
Close up picture of paper with FiberLean MFC:
The FiberLean found in the paper (as pictured) is very similar to the original product, in that it still retains the high level of entanglement between the MFC and the filler. This is a result of the co-grinding process providing an extreme level of blending of the mineral and the MFC.
What is not visible in the pictures is the low surface charge of the FiberLean MFC. This is due to the absence of fiber pre-treatment. The FiberLean MFC surface charge is similar to that of ordinary pulp but obviously with increased specific surface area.
Impact of FiberLean MFC on white water cationic demand:
It is this balance of properties that makes the FiberLean MFC well suited for use to facilitate filler increase in paper.
FiberLean MFC application:
MFC is known to provide good strength. By processing a minor part of the normal fiber to MFC it is possible to increase the paper strength, such as tensile, or to trade the strength increase for higher filler content. The MFC will also have a major impact on tensile elongation to break. Combined with the higher tensile strength this makes the paper containing MFC significantly better able to handle mechanical stress. This is a good basis for a filler increase concept where the aim is not only to maintain paper quality but also the operational efficiency of the paper making process.
Impact of FiberLean MFC on Tensile Index:
Impact of FiberLean MFC on Breaking Elongation:
Impact of FiberLean MFC on Tensile Energy Absorption:
Before going to paper mills to propose full scale trials we felt there were some other obvious questions around the impact of MFC on paper machine runnability we needed to have the answers to. We were conscious of the ability of MFC to hold water and that this could represent a big challenge to drainage on a paper machine. We also saw the challenge to maintain retention when radically increasing filler loading, especially when done in combination with very fine MFC.
To evaluate these potential show-stoppers we ran trials on pilot paper machines, initially small and slow ones and later at Innventia´s Fex machine. Here we ran a trial using a standard 80/20 Eucalyptus/NBSK fiber blend and GCC filler, on its own and in combination with 1,7% MFC, at 800 m/min. From this we took some key learning that has later proven to be accurate and repeatable in full scale.
Impact of filler increase and FiberLean MFC on first pass ash retention:
Web solids after the couch roll:
Web solids after press section:
Initial wet strength after press section:
Based on this knowledge we felt confident to go into full scale trials without risking upsetting the wet end chemistry or having to slow down the paper machine.
We have now run close to 40 full scale trials and through this we have learnt to master a number of different applications. With a high degree of confidence we can now describe the general performance of MFC in filler increase applications.
Application of the FiberLean concept, will give,
- Exceptional performance stability
- Increased initial wet strength, even at elevated filler content
- Minimal impact on wet end chemistry
- Overall positive impact on drainage (less moisture to dry off)
- The potential for significant filler increase
Filler increase vs. FiberLean MFC dose from commercial trials in wood free papers:
The target performance is to use 2% MFC to allow 10% extra filler. It is obviously possible to go for a smaller filler increase and a lower MFC dose but also to go substantially higher. In pilot scale we have made FiberLean paper with over 60% filler content. The 10% filler increase target is a compromise where we deem it possible to still retain the general feel, looks and properties of an established paper
At 10% filler increase from 2% MFC one can also expect some positive side effects:
- Increased initial wet strength
- Increased bond strength
- Higher opacity
- A much tighter sheet (reduced porosity)
- Improved coating hold out
- Better smoothness
The ability of MFC to provide initial wet strength is a key differentiator when compared to alternative chemistry-based filler increase systems. The radical increase in cellulose surface area and the resulting increase in hydrogen bonding means strength is improved as early as the sheet has formed. The result is improved runnability.
Maintaining bulk when fiber is replaced by higher density mineral is a challenge but this can be managed through optimization of the papermaking process.
- Reducing calandering to constant smoothness and/or gloss.
- Use coarse CTMP or reduce fiber refining as improved bond strength permits
- Use a bulkier mineral such as scalenohedral PCC
- Reduced pressing to constant press moisture
Many of our trials have been in mills making coated wood free paper. Using MFC to allow increasing filler content of coated wood free base paper made from market pulp is very attractive. Not only is there a significant saving to be made from replacing pulp with filler but there are also several ways to benefit from the coating hold out effect of the MFC. Some mills have seen reduction in binder demand in the coating colour. In the case shown below the improvements in quality of the coating were traded for a 40% reduction in calandering intensity. This allowed maintained bulk, despite going for a rather aggressive increase in filler loading.
Change in properties of base paper and final coated paper with and without FiberLean:
The base concept for FiberLean is to provide cost savings without sacrificing current paper specification or paper machine efficiency. Perhaps more exciting are the prospects of making bigger changes to the way a paper product is made or to make entirely new grades of papers.
FiberLean for paper product development:
The White Top Liner example is tried in practice, and it is possible to make the white top layer at higher filler loading without sacrificing the strength properties. The higher loading gives a boost in opacity to allow a reduction in the white top layer basis weight and still maintain coverage of the dark base. Our experience shows an overall reduction in white fiber consumption of close to 20% without compromising WTL quality.
Our first commercial operation for FiberLean is with Verso Bucksport mill in Maine, USA. We ran trials in 2010-2012 to evaluate the FiberLean technology in Printing & Writing paper. The plant was built in 2013 and commercial production began in Q1 2014. We are currently running 24/7 and see, among other things, the paper machine operating with very good runnability.
Trials and the future:
Running full scale trials with MFC is a challenge due to the low solids of the product and the large volumes that thus need to be shipped, stored and dosed. It has not been uncommon for trials to require 50 m3/hour of product to be delivered just-in-time into a small temporary tank. To facilitate running more and longer trials we have invested in a pilot plant where we can turn the low solids FiberLean product coming out of the grinding process to a solid granulate product form packed in big bags. This materials still holds a lot of water but it can be handled and stored like any other big bag supplied product, such as starch.
FiberLean pilot plant in Trebal, UK:
Before dosing the FiberLean to the paper machine the product needs to be returned to slurry form. This can be done using one of our mobile make down units. The make down units have capacity to turn FiberLean from Big Bags into slurry at a rate to keep pace with a 300 000 ton/year paper machine at 2% MFC dose.
It is well known that adding MFC offers some very attractive technical advantages to paper makers. To allow making use of these advantages in full scale and to make them commercially interesting there was a need for a processing step-change to reach a sensible scale and cost efficiency of MFC production. We claim that this kind of step change is exactly what FiberLean represents.
Some paper is already made at lower cost and better quality with MFC.
Commercial Director, FiberLean