Grinding & dispersing
Although most energy in inks and coatings manufacture is consumed in the fine grinding process, pre-dispersing is the area where the most significant efficiency and sustainability gains can be made. Replacing High Speed Dissolver technology with bead-milling in the pre-dispersing stage can reduce the bead size required in fine-grinding, reducing overall energy demand by as much 50 percent, while cutting process times and costs.
Frank Tabellion
Post-pandemic, much has changed in everyone’s lives. With more time to consider how we live and work, combined with growing media attention on the environment and a rise in the frequency of extreme weather conditions attributed to climate change, consumers are more conscious than ever about the environmental consequences of what they buy and how it is packaged.
Before the pandemic, in 2018, a survey of 7,000 consumers across seven countries – France, Germany, Italy, Poland, Spain, Turkey and the United Kingdom – found that 68 percent said that being environmentally friendly was now more important or very important to them. However, a post-pandemic study from McKinsey & Company with US consumers found that whilst environmental impact of packaging plays an important role in a purchasing decision (14%), by far the most important buying decision was price (61%).
So how can the coatings industry drive costs down whilst improving overall sustainability?
One clear target has to be reducing energy consumption in the manufacture of coatings and inks. This has benefits for the planet and can also provide significant cost and efficiency benefits for manufacturers.
Traditionally, the route towards achieving energy savings in ink and coatings production has been focused on the fine grinding process. Given that this is where 80 percent of the energy input of the wet processing is required, that’s a logical decision.
But looking at the science of the overall process, there is a critical process that whilst consuming little energy itself, has a significant impact on the relative energy required to achieve the color, strength, gloss, transparency, rheology/viscosity and stability/shelf life the industry demands. That process is ‘pre-dispersing’ or ‘mixing’.
Indeed, laboratory testing followed by commercial trials and now product applications has shown that taking a new approach to pre-dispersing in combination with the newest generation of vertical high-power density mills can save up to 50 percent of energy input across the process.
This equates to savings of approximately 200 kg CO₂ equivalents per ton of ink. For a 5,000t/annum production, this would be the equivalent of the CO₂ capture of up to 40,000 trees.
As well as sustainability, it’s important to note that by making improvements in the pre-dispersing process, producers could also benefit from higher process stability, less down time and less human interaction, leading to a more efficient production with economic benefits beyond the energy savings.
Most ink and coatings manufacturers use a two-part process to incorporate pigments, binders, additives and solvents into a consistent and homogeneous product. Step one is pre-dispersing. Step two is fine grinding (see figure 1).
The theory for energy efficiency in the overall process is clear. The smaller the bead you can use in the final milling process, the less mass-specific energy is required to create a quality color dispersion. And that means a more sustainable process and thus, end product.
However, the optimum bead size in the fine grinding process is dictated by the largest particle size in the raw materials. Even small fractions of oversized particles left in the raw material after a pre-dispersing process will result in inefficient fine grinding.
What can we do to re-imagine this traditional process with an energy-saving focus?
In most industrial and laboratory applications, the pre-dispersing step uses High-Speed Dissolver (HSD) technology. This features a serrated tooth disk that rotates at high peripheral speed to cause centrifugal displacement of the base fluid, resulting in a powerful vortex. Powder is drawn down into the vortex and subjected to intense shear at the tip of the tooth disk, resulting in rapid dispersion.
This methodology is capable of breaking down loose particles in a pre-mix solution, but the shearing force applied by this mechanism isn’t enough to break down harder agglomerates or aggregates that are part of the coarse tail of the distribution. Typically, it is the last 10 to 15 percent of the particle size distribution that is the hardest to break down and shearing simply can’t apply enough force in most cases. Given that even a small percentage of larger particles left in the pre-mix will compromise the amount of specific energy required for the fine grinding process, this is a significant issue for sustainability.
So, what’s the solution? Bühler’s Grinding and Dispersing research and development scientists have proved that replacing HSD shearing at the pre-dispersion stage with a new low-energy bead milling application can provide the step-change in efficiency and sustainability that the ink and coatings industry has been looking for.
Taking a holistic view of the whole process and looking at the fine-grinding stage, there are two further factors that stand out. Increasing the flow rate of the slurry could result in a lower specific energy demand. And optimizing the bead activation and bead distribution in the grinding chamber could potentially reduce the amount of energy being wasted by heat dissipation and wear.
The two fundamental aspects of bead milling are the intensity of a unit milling event and the number of bead-particle interactions.
The bead size needs to be big enough to break down the largest particles in the pre-mix. But the paradox is that the smaller the bead we deploy, the more bead-to-particle interactions we see, resulting in less energy usage. When it comes to sustainable production, reducing bead size is the most effective strategy, but only if we can do that without leaving lumps or unacceptable deviations in viscosity.
Since the number of beads scales inversely with the diameter of the beads to the power of three, reducing the bead size by one order of magnitude results in three orders of magnitudes more beads in the same volume (see figure 2).
Even cutting the size of beads in half results in an eightfold increase in the number of beads (see figure 3).
Since many beads introduce the milling energy more efficiently, using smaller beads results in a lower energy demand for a given milling task, which in turn can be accomplished in a shorter time, with less energy.
The goal in the pre-dispersing step is to cut off the tail in the particle size distribution, which represents the large-oversized particles that compromise the use of smaller beads in fine grinding and consequently result in a higher energy demand (see figure 4). The physical contact of beads in the pre-dispersing step is much more efficient than the weak shear forces introduced by HSD technology. The theory is sound, but how can it be applied?
Laboratory tests confirmed the theory. The next step was to develop a commercial bead-based pre-dispersing unit that could be tested on industrial applications.
The result is the “MacroMedia” pre-dispersing unit (see figure 5 – item 2), which is connected to the premixing tank for high flow rate recirculation operation.
1) Dosing of solid and liquid materials in the mixing tank
2) Circulation between pre-dispersing unit and the mixing tank
3) Transfer from the mixing tank to the recirculation tank via pre-dispersing unit
4) Circulation between high-power density bead mill and the recirculation tank
5) Transfer from the recirculation tank to the let-down tank via the high-power density bead mill
6) Addition of liquid components and transfer to the next production step
Compared to the standard HSD technology, the tank is only equipped with a low energy stirrer with a small motor of 7.5-11 kW depending on the tank size, because the pre-dispersing is done inside the pre-dispersing unit.
The new bead-based process in the unit delivers a particle size of 150 micron max compared to around 300-400+ micron with the HSD process. This will enable the use of 0.3 mm beads in the fine grinding step, instead of 0.8 mm that would be required with HSD.
This approach is also more sustainable in its own right. The energy requirement of the pre-dispersing is approximately 20 percent less than a traditional HSD.
Further efficiencies are delivered by clever design. The pre-dispersing unit consists of a compact 6 liter grinding chamber with an integrated pump mounted on the same drive shaft, enabling flowrates of up to 15m³/h. Inside the beads are activated by a pin counter-pin system with a unique, multi gap bead separator system, which is resistant to blocking (see figure 6).
1) Product feed
2) Stator with pin geometry
3) Multi-gap separation
4) Rotor design with centrifugal relief
These high flow rates and the multiple gap separation system allow the use of 3-5 mm beads in the pre-dispersing process, which are the perfect size to reduce oversized particles using heavy-duty bead particle interaction.
Having reduced the bead size required – by far the largest lever available in reducing energy in the grinding process – there were two other significant aspects that affect energy consumption to tackle in the fine griding process.
Firstly, increasing the frequency that the material can be passed through the fine grinding mill was a goal. Increasing the number of particles that are efficiently stressed by the beads, results in a lower specific energy demand.
A new design for the fine grinding unit, called the “MicroMedia Invicta” (see figure 5, item 4), features an improved bead separation system with a reduced gap size between screen and centrifugal cage (see figure 7). In combination with an enlarged screen, it enables high flow rates up to 8,000l/h (compared to 4,000/h previously achieved), without pressure problems.
1) Screen
2) Slots
3) Pin / counter-pin
4) Cooling
To optimize bead activation and distribution to enable more efficient power usage, the unit uses a pin counter-pin system with pins on the rotor and stator. The double cylindrical annular mill design in combination with the centrifugal bead separation system allows the beads to freely flow around with the product inside the chamber, from the outer to the inner annulus, maximizing the number of milling interactions. This reduces the amount of energy being wasted by bead compression. An additional design benefit is the high cooling capacity provided via the rotor and stator surfaces, protecting temperature-sensitive products.
This newest generation of high-power density bead mills features a 25 percent enlarged active grinding chamber volume vs. its predecessors, which results in 25 percent more beads, further improving energy efficiency. The intelligent pin arrangement enhances bead activation and distribution in the chamber, as well as further reducing potential hydraulic packing on the bottom. This delivers 50 percent higher productivity and 100 percent higher flow rate compared to the standard bead mill technology.
The new process was evaluated for sub-micron grinding in the laboratory. In order to satisfy the requirements for an ink-jet application, the target particle size d90 was < 140 nm which requires the use of small media of max. 0.3 mm and high-efficiency grinding equipment in order to reach the target quality with a useful productivity rate.
The lab strived to use 0.1 mm media in a high-performance bead mill operated in re-circulation for this application, which required installation of a 0.05 mm (50 μm) gap size screen as a bead separator.
As a consequence, the pre-dispersion process had to deliver a particle size distribution with a d100 < 50 μm. This goal was achieved by introducing 100 kWh/t at a net productivity rate of 100 kg/h, as shown by laser diffraction and also a wet-sieve test, where no residue was found when passing the slurry through a 50 μm filter bag.
This was confirmed by the subsequent fine grinding which was carried out with 0.1 mm media in re-circulation without any productivity or consistency issues. Fine-grinding required an additional 800 kWh/t to reach the final target of d90 < 140 nm, which for a 15 liter mill equates to a productivity of approx. 35 kg/h.
Another sample of the pre-dispersed slurry was milled with 0.2 mm media under similar conditions. Due to the larger media, this process required 1,200 kWh/t at a rate of 20 kg/h. This showed that the fine-grinding process using 0.1 mm media was 33 percent more energy efficient and 75 percent more productive.
Dr. Frank Tabellion is the Global Director of Product Management and Process Technology in the Grinding & Dispersing business area at Bühler AG. Born in 1968, he has more than 20 years of experience in the wet grinding and dispersing industry. He has worked in various functions including R&D, Sales, Product and Process Development and was General Manager of Partec GmbH. He is currently based in Switzerland, at Bühler’s HQ in Uzwil.
Dr. Frank Tabellion
Bühler AG
Global Director of Product Management & Process Technology
frank.tabellion@buhlergroup.com
+41 71 955 39 62
Gupfenstrasse 5
Uzwil
9240
Switzerland
