What Are the Advantages of Wet Grinding vs. Dry Grinding?
Carl Fiorini, Apr 2, 2020 7:30:00 AM
It’s amazing how many products in our daily lives have gone through the grinding (or milling) process. From breakfast cereal to cold medicine to paint to cement, grinding is a key step to getting essential products to market in their correct form.
There are challenges and advantages of wet grinding and dry grinding, the two most common and effective grinding methods. In this article, we’ll break them down as well as discuss some examples of each.
“Grinding,” “Milling,” “Micronization,” “Wet/Dry” … Simple Definitions to Get Us on the Same Page
The terms “grinding” and “milling” are really interchangeable. If you know a piece of equipment called a “jet mill” is used, you may regularly refer to the process as “milling.” Others who may not be as familiar with the process or the equipment, may use “grinding” as a general way to describe it. Don’t get caught up; there are no differences between the two.
So, what is “micronization” within particle technology? That term is somewhat more specific, typically referring to particle reduction in the sub-10 micron size range. So, all micronizing is grinding, but not all grinding gets down to the micronizing particle level.
Speaking of microns, what is “nanometer” level particle reduction? Nano refers to sizes less than 1 micron, which is equal to 1,000 nanometers. The average nano size in particle reduction is approximately 200-500 nanometers. More on this later.
The terms “wet” and “dry” grinding are overarching words that basically describe each of those two processes. Although the goals are the same — make large particles smaller — the processes are different. In general, dry grinding uses particle-on-particle contact to reduce size, while wet grinding involves dispersing the material in a liquid and using solid, grinding media to reduce size.
Similarities and Differences of Wet Grinding and Dry Grinding
Dry grinding is a relatively simple process. Within any number of specialized machines, material travels within a contained area and impacts other particles or rotors until it breaks into a desired size. During wet grinding, particles dispersed in the liquid slurry are crushed by the grinding media.
Dry grinding, in a jet mill for instance, uses a single pass process; material goes in and is expelled once. Wet grinding is a recirculation process, with the slurry exposed to the grinding media over and over, for hours if necessary, until the desired particle size is achieved.
Although the grinding methods are different for each, one challenge is common to both dry grinding and wet grinding: the potential for wear on the grinding equipment that’s used to break down the material, or “materials construction.”
In dry grinding, there may be abrasives within the material that could, over time, damage the equipment. Wet grinding’s carrier fluid, or the material itself, must be chemically compatible; water, oil, surfactants, solvents can eat away at the equipment’s grinding media or parts used to agitate that media.
Dry or wet, it’s critical that the equipment’s grinding media doesn’t end up in the product.
Selecting Wet Grinding vs. Dry Grinding
When deciding between the two methods, the material’s final application (and its performance) is the number one determining factor.
For example, if a particular pyrotechnic material goes through dry grinding, the product has superior characteristics when compared to wet grinding. That alone may be enough to choose dry.
Target particle size is always the next consideration. In general, dry grinding gets particles down to single digit micron size. Wet grinding regularly reduces particles down to the nanometer range. While dry grinding’s simplicity often makes it the first method considered, if extremely small size is the goal, wet is the only way to get there.
But it’s not always that simple. Here are two examples of how size considerations can affect the process.
- Example 1: The end material must be wet, and the size goal is 10 microns (not especially small). So, dry grinding is the best method to use, with the material then needing to be mixed into a wet dispersion.
- Example 2: The end material must be dry, and the size goal is 500 nanometers (quite small). So, wet grinding must be used, and the material must then be dried after reduction.
Example 2 leads us into the next consideration: cost.
If wet grinding is used to reduce particles to 500 nanometers, like above, but that product is NOT going to be used in a liquid base, it requires drying. Shipping a slurry that’s 75% water only to remove that water via a spray drier adds significant freight and handling expenses.
However, wet grinding is more energy efficient, with up to 30% less power used to drive a wet mill. That kind of savings will add up over time.
If application, size, and cost don’t lock a project into dry grinding or wet grinding, it may come down to simple capacity. Can one grinding method (and equipment) better handle the required particle size in the quantities needed?
On that same line of thinking, is packaging a factor? Since grinding is continuous, packaging needs to be able to keep up at a determined number of bags, barrels, etc. per hour.
The need to formulate for dry or wet grinding may come into play. Obviously, it’s most efficient to include as much particle solid into the mix as possible. However, there may be need for additional additives, which can be determined based on past experience and trials.
Experienced providers of wet and dry grinding have quality assurance processes in-house, including analytical lab testing capabilities. While this may not determine one method over the other, it’s important to have these services available to ensure a quality end product.
The list of materials that can be wet or dry grinded is incredibly long, from inkjet printer inks to personal care products to nutraceuticals to essential oils and on and on.
So, call it what you will — grinding or milling — one thing applies to both proven methods: each requires expert upfront guidance and dedicated capabilities.
Want to learn more about micronization services? Our recent article offers 10 things to consider before selecting a micronization service supplier.