The Top 8 Toughest Wet Milling Challenges and How to Solve Them
Justin Klinger, Jun 3, 2021 9:36:00 AM
Jet milling is one of the most dependable particle size reduction services manufacturers entrust to toll processors for micronizing fine powders, but it has its limits.
Typically, those limits are somewhere in the single-micron range. That’s incredibly fine, but still not fine enough for some applications.
So, when manufacturers need to accomplish sub-micron particle size ranges, wet milling is most often the answer.
Industrial, technical, and cGMP industries like pharmaceuticals have all come to demand particle sizes in this nano range, in order to harness the many potential benefits of greater surface area per particle, such as:
- Better dissolvability and product stability
- Greater chemical or active pharmaceutical ingredient (API) efficacy
- Improved performance in subsequent processes
- Increased particle packing density
- Improved sheen and color strength
- And more
These benefits often lead to safer products, better value, greater efficiency, and even cost savings on raw materials. So there’s a lot to be gained from the wet milling process, even though it may add steps to the overall manufacturing process of a product.
Just as important as target particle size are size ranges and distributions. Too many larger particles, and your material can clog equipment in subsequent processes. Too many fines, and viscosity can be a major issue if the next process is wet.
That’s why it’s vital, when working with a tolling partner on wet milling projects, to collaborate and share as much material and process knowledge as possible with their technical and scientific teams. The more data they have from the start, the better they can overcome even the toughest wet milling challenges.
How Does Wet Milling Work?
In a horizontal wet media mill, the raw feed is added to a liquid solvent — usually water, but alcohols and oils are also sometimes used — for a ratio that’s typically around 15-30% solids, though this can vary depending on a specific raw feed’s material characteristics. It can be tempting to want to push dry material to levels well above 30% in the slurry. While there are a few raw material ingredients that can be effectively milled at higher percentages, most perform most efficiently in that 15-30% range.
That slurry is then pumped through a grinding chamber that holds (typically ceramic) milling media. An agitator in the center transfers energy into the fluid, and as particles collide with the milling media, their size is reduced. The slurry is recirculated through the mill until the particle size and distribution are achieved.
Distributing the raw material in a liquid slurry helps to achieve a more uniform particle size, and other process variables — energy, time, media size, percentage of solids, and more — contribute to the efficiency of the milling process. That said, depending on the material and the size target, the wet milling process may take a few hours, a few days, or a period of time somewhere in between those extremes.
While wet milling is the process of choice for many nano scale projects, it’s not without its challenges. Let’s take a look at some situations that may arise in a wet milling project, and the approaches a team of experts might take to solve them.
Wet Milling Challenges and How to Overcome Them
1. Keeping up with demanding production volumes
Wet milling can be a more time-consuming process than most jet milling projects. It takes time to calculate solids percentages using material bulk density to properly set up equipment for an optimized process, and sometimes even to break down the particles. In addition, the end result of a wet milling process is — well, it’s wet. So, depending on the subsequent manufacturing step, the resulting material may need to be dried.
Fortunately, in most cases, this is a relatively simple challenge to overcome, with advance planning and an established relationship with a trusted toll processor. Working with a contract manufacturing partner that’s familiar with your products, processes, and supply chain can save you time when it matters most. Ask about numbers and types of wet media mills, and the size ranges available for your wet milling batches. Whether you need to mill volumes in the thousands of liters, or you’re test milling smaller batches, mill size matters.
And a responsive toll processor can be key to scaling up a process quicker than you might think possible. In some circumstances, a toller may even invest in equipment, installation, and setup expressly for your project on a timeline that’s more efficient than you could ever make possible in your own operations.
2. Smaller quantities or wet milling trials
As important as scaling up is scaling down. If you need to study a material’s feasibility, or only need around a pound or so milled to a very fine specification, you need to know your toll processing team can handle that challenge, too.
So, ask about volumes on their smallest equipment, too. Can you wet mill smaller quantities on larger equipment? Maybe — but it’s important to remember that, depending on the bulk density of your initial raw feed, your wet milling slurry often requires a range of 15-30% solids for the most efficient outcomes. Lower percentages may be possible; it’s best to collaborate closely with your technical team for guidance on what’s possible versus what’s most advisable.
Too little solid material, and you won’t achieve enough particle-on-particle contact in the milling process. That can slow things down and require a lot more energy. Too much solid material, and not enough liquid in the slurry, and you risk clogging the wet mill with sludge, which could make milling nearly impossible.
3. Unanswered material questions
First and foremost, every milling project requires a safety data sheet (SDS) on the raw material, and an SDS for the finished product. That’s no different for wet milling.
An SDS is the required starting point, and an advanced toll milling team can glean a great deal of information from it, in terms of setting up a safe process. From the starting point of material identification, their research and existing knowledge can take them well into most projects.
But you can optimize process efficiency if you already know and can share some critical information and data on processing conditions, including:
- Preferred equipment type and size
- Media type and size
- Preferred solvent
- Percentage of solids
- Flow rate
- Expected batch time
- Any other information you’ve gathered
It’s important to keep in mind that wet milling is a technical process, and your team will either build a process from scratch, or start with your established process. If it’s a first run, expect your project to be charged on a shift basis. Your first run establishes feasibility and data for cost estimating future projects.
4. Selecting the best solvent for your wet mill slurry
As you might expect, water is the most commonly used liquid in wet milling. But it’s not always the best choice — and in some cases, water isn’t even a safe choice. Some materials dissolve in water, of course.
Oxidizers, in contact with water, can generate heat and cause explosions. Your technical team should be able to offer recommendations for alternatives to water, such as oils or alcohols.
It’s important to make sure that you request a solvent that is compatible with your raw feed. If you’re new to the wet milling process, your toll processing team may be able to offer recommendations for solvents, surfactants, and dispersants, based on experience they have working with similar materials. Your prior experience and material knowledge are also essential to the decision, so the more you know and can share, the better.
Using oil or an alcohol requires knowing how to use these slurry liquids effectively and safely. If oil or moisture is already entrained in the raw feed, technicians need to make adjustments in their calculations of solids percentages. If flocculation is an issue (and with very fine gauge grinds it often can be), a dispersant or surfactant may need to be added to the slurry for the most efficient grind possible.
5. Potential dangers and safety concerns
It’s hard to overemphasize the importance of a correct, accurate SDS for your raw feed. That’s because, as part of your project’s initial review, a technical sales engineer performs a safety review of project materials and processes to ensure compatibility.
But just because your materials are compatible for milling, that doesn’t necessarily mean there’s no risk to your wet milling project. Flammable vapors and combustible materials may still be candidates for wet grinding, in expert hands.
Your tolling partner may be able to put the necessary safety measures in place, with safety engineers ensuring correct setups and processing conditions that meet NFPA (National Fire Protection Association) standards. Expect the extra precautions to add time and cost to your project; those extra steps may be all that stands between your material and your final target particle size.
6. Abrasive, hard raw materials unsuitable for ceramic milling media
Some materials are so abrasive, they can break down the ceramic milling media instead of the other way around. If you’re up against a situation like this, ask your toll processor whether they can offer autogenous wet milling.
What is autogenous wet grinding? Just what it sounds like: In this process, a coarser grit of your raw feed material is substituted for the ceramic milling media that would otherwise be worn down during the milling process. It’s a relatively simple approach, but it does require specialty equipment, and an expert tolling team.
Be advised, not all toll processors offer autogenous wet milling for highly abrasive materials. But some do, and if you need a submicron particle size, it may provide the most cost-effective path to your target specification.
7. The dreaded particle size plateau
It happens sometimes. A raw material resists further comminution in the milling process. At first, maybe the slurry circulates along and particles diminish for a while, and then, as they get closer to the target spec, they just … stay the same size. That’s why some wet milling projects take just hours, while others can take days.
What can you do? In a lot of cases, you can wait it out. Additional time and energy may be all your material needs to give up and break down in size. That time and energy have a cost.
And it turns out the process variables can be tweaked to move beyond a plateau.
That fact points back to your need to communicate closely and share as much information as possible with your technical team. Data on batch times, hardness, percent solids, and milling power all hold the potential to help your team avoid a plateau in the first place. That can help your team know which equipment to select and whether any process variables like temperature or slurry additives may make the difference — before you start.
8. Highly specialized milling services can be hard to find
Materials with extremely small particle sizes can have a tendency to agglomerate when they’re dried. So you may find, after spray-drying, that you’re looking for a contact processor to deagglomerate your final, dried powder on a jet mill.
Or it may be that your raw feed is exceptionally dense and hard, and you’re anticipating the time-energy tradeoff in your wet milling process. You may need cGMP wet milling capabilities for your APIs or special excipients.
Whatever your milling need, it’s best to think ahead about the partner you’ll trust to achieve your product specifications and start talking long before your project is at risk. The more you get to know your tolling partner, the more value they may be able to deliver to your supply chain, both upstream and downstream of your wet milling project.
You can get a fuller understanding of the different wet and dry milling methods available to you through a toll processor with our Milling Methods Comparison Guide. In addition to mechanical, jet milling, and wet grinding technologies, it introduces readers to some of the most important applications for these indispensable processes. Download a copy by clicking the link below.