Step by Step Guide to the Wet Grinding Process: Everything You Need to Know
Justin Klinger, Apr 29, 2021 9:10:00 AM
Toll processors have a range of wet and dry milling methods at their disposal to achieve incredibly fine gauge powders.
Wet grinding — also called wet media milling — is often the preferred process for particle size reduction to a very fine scale, down to the micrometer and even nanometer range.
Fine milling, micronization, and nanonization can be particularly useful for improving processability and efficacy of active ingredients, as in pharmaceuticals or agrochemicals.
It can also be used to attain an extremely small particle size and high particle surface area to achieve the greatest possible vibrancy and opacity of pigments in paints, inks, coatings, and even cosmetics and food ingredients.
In short, wet milling can help manufacturers get greater value from smaller quantities of valuable substances, which can result in considerable savings on raw materials, from metal ores and ceramics to industrial chemicals and active pharmaceutical ingredients.
Read on to learn more about the steps involved in wet grinding, and how the process can vary from one project to another.
How does a wet grinder work?
Wet milling machines can include horizontal media mills, attritor mills, cascading type ball and/or bead mills, and basket or batch mills. Unlike dry mechanical and jet milling processes, wet milling requires multiple, specific steps that can vary based on different characteristics of the raw feed.
To achieve final particle specifications, material scientists and wet milling experts select, adjust, and control multiple variables, including:
- Liquid slurry ingredients, quantities, and ratios
- Milling media types and sizes
- Mill speeds
- Time in process
Steps required in a wet milling process vary, but can include:
- Charging the media mill
- Dispersion in liquid
Step 1: Wetting
The first step to wet grinding is eliminating air from the surface of the raw feed, so that each individual particle is fully in contact with the liquid slurry. Ensuring complete contact is vital for effective milling. This requires a liquid with lower surface tension than the free surface energy of the solid.
Selecting the right liquid, whether aqueous or solvent, depends upon the material being milled. Hydrophilic raw feeds, such as titanium dioxide (TiO2) wet easily in aqueous milling media. Others can require solvent-based slurries, surfactants, or other specialty wetting agents.
Step 2: Charging the media mill
As the raw feed in the liquid slurry circulates through the mill, solid particles suspended in the liquid collide with one another as well as with the milling media. The impact and shear forces result in particle size reduction.
Milling media type and size are selected based on material characteristics of the raw feed, as well as the desired final particle size. The wet mill is charged, or partially loaded with milling media, which circulates with the slurry. Typical media mill bead materials include:
- Tungsten carbide
- Zirconium oxide
- Polymer resins
- And more
The size of grinding media also impacts final particle size and morphology. A toll processor’s technical experts can perform small-scale processing trials to determine the best combination of media, speed, and time before scaling up to full production on larger wet milling equipment.
Step 3: Deagglomeration
Many raw materials have a tendency to clump together, or agglomerate. In order to properly disperse the solid particles in the liquid milling slurry, they must first be separated from one another, or deagglomerated.
This is accomplished using mechanical energy to agitate the powdered raw feed in the liquid, whether using a blade mixer in a premix tank or in a media mill.
Step 4: Dispersion in a liquid slurry
Once the raw feed has been deagglomerated, complete dispersion of the solid particles throughout the liquid slurry ensures uniform milling. This is also accomplished either using a blade dispersion blender or a media mill for finely dispersing denser, harder raw materials. The amount of liquid slurry, relative to the solid raw feed, impacts viscosity, which can have an effect on milling efficiency.
Deagglomerated and dispersed raw feeds in liquid slurry often need to be stabilized in order to avoid sedimentation or flocculation. For materials like inorganic pigments and ores, dispersion additives usually do the job — and a trusted toll processor’s material science experts can advise on the correct stabilizers for your wet milling project. Selection and quantities of surfactants, stabilizers, and other additives can make or break a wet milling process, so expertise matters.
Step 5: Wet Grinding (and Recirculation)
In this step, the liquid slurry with dispersed solids is circulated in the media mill along with the grinding media until the final particle size, or grind gauge, and surface characteristics are achieved. This can require multiple passes and speed adjustments in the hands of an expert equipment operator. While the circulation step can be time-intensive, wet media milling typically consumes less energy than dry milling processes, and that reduced energy consumption figures into overall cost estimates.
During this step in the process, material samples may be analyzed for quality assurance and process control. A toll processing partner’s in-house analytical capabilities can help ensure timely completion of wet milling projects without having to wait for testing and analysis from outside labs.
Step 6: Separation
After circulating (and recirculating) the material and media in the mill until the target particle size specifications have been achieved, the slurry is screened to separate the milling media from the product.
Step 7: Degassing or drying
Whether the liquid slurry is degassed or dried depends upon the end use of the milled material. Degassing the liquid ensures that all air or gases entrapped within the suspension medium are released. This process is used when the final product is to remain dispersed in liquid.
If a dried powder is the desired final product, then an added drying step is needed. This is most often achieved either by drying in a fluidized bed or using flash drying equipment. While drying adds the time and cost of an additional processing step, dry powders are typically far less costly to ship than liquids.
Your trusted toll processing partner can not only point you to the best milling process, whether wet or dry, to achieve your final specifications — their teams of experts can also tweak, adjust, and perfect your milling process for optimal efficiency, product quality, and cost-effectiveness. So no matter your industry, raw material, or final particle target specifications, you can trust their decisions are made with your best interests in mind.
You’ll find an overview of wet media milling in our Milling Methods Comparison Guide, along with explanations of a wide range of milling and grinding technologies and processes. Gain a deeper understanding of the methods and capabilities toll processors can use to achieve your specifications. Just click the link below to download your own copy today.