Dev Testing, Sep 15, 2021 12:00:00 AM
The processes of wet and dry grinding touch countless products that we use daily.
Everything from breakfast cereal and cold medicine to house paint and cement requires at least one grinding step in its production.
Wet and dry grinding are the two most common and effective milling methods, and both have advantages, disadvantages, and specific challenges that can complicate processes.
Whether a product goes through a wet grinder or a dry grinding process depends on multiple factors, including raw material, target particle size, particle surface and shape requirements, final application, and more.
In this article, we’ll break down the differences between wet grinding and dry grinding, and discuss examples of each milling process.
What is Wet Grinding? What is Dry Grinding?
Before we dive into the applications, advantages, and disadvantages of wet and dry grinding, let’s make sure we’re on the same page about what we actually mean.
First, the terms grinding and milling are interchangeable. If you know a piece of equipment called a jet mill is used, you may already be used to calling the process milling. Others who may be less familiar with milling equipment and processes may use grinding as a general way to describe the process of particle size reduction.
There’s no need to split hairs. (That would require a different process.)
The terms wet milling and dry milling are overarching concepts that do little to actually describe the processes and forces involved. But some methods are only possible under wet conditions, and some require dry milling conditions.
Although the goal of both grinding methods is to reduce particle sizes to a target range, the processes are different. In general, dry grinding uses particle-on-particle impacts to reduce size, while wet grinding involves dispersing the raw material in a liquid and circulating the resulting slurry to smash particles against solid grinding media to reduce their size.
Similarities and Differences Between Wet Grinding & Dry Grinding
Dry grinding is a relatively simple process. Within any number of specialized machines, the raw material travels within a contained area and either collides with other particles or strikes against machine components (such as rotors) until the raw feed breaks down to the desired size.
In a wet mill, on the other hand, particles are dispersed in a liquid slurry and pumped through a grinding chamber. The particles ride along in the liquid and are crushed among the grinding media.
Dry grinding, as in a jet mill, uses a single pass process; material enters the mill, passes through, and is expelled, reduced in size. In contrast, wet grinding uses a process of recirculation. The slurry is exposed to the grinding media over and over, for hours if necessary, until the desired particle size is achieved.
Although the two types of grinding methods are different, one challenge is common to both dry grinding and wet grinding: the potential for wear on milling equipment that, over time, can damage components and threaten to contaminate the product.
In both wet and dry grinding, the raw feed material can be abrasive and damage equipment as a result of repeated collisions with machine parts — and in the case of wet grinding, with milling media. In addition, the carrier fluid used in a wet process needs to be chemically compatible with the raw feed. Whether that includes water, oil, solvents, or surfactants, the liquid can eat away at grinding media, agitators, or other components.
Dry or wet, it’s critical that equipment particles and grinding media stay out of the end product.
How to Choose Between Wet Milling & Dry Milling
When deciding between wet and dry grinders, the required characteristics for the material’s final application serve as the most important determining factor.
For example, a process on a dry grinder, when used on one particular pyrotechnic material, typically results in superior characteristics compared to that same material after wet milling. That information alone may be enough to choose the dry process.
Target particle size is always the next consideration. The small target particle sizes in jet milling and wet milling lead us to sometimes use specialized particle technology terms when we refer to the milling processes.
Micronization refers to reducing particle size down into the sub-10μ (micron) range. (For reference, that’s about the size of a water droplet in fog.) All micronizing is grinding, but not all grinding processes can achieve micron-level particle size reduction.
What about sub-1μ particles? Sizes smaller than 1 micron in diameter are referred to as the nanometer level. One micron is equal to 1,000 nanometers. Typically, nano sized particles range from 200-500nm. The process of achieving particles this small is sometimes called nanonization.
In general, dry grinding can reduce particles to single-digit micron sizes. The uses of a wet grinder, on the other hand, often involve reducing particles to the nanometer range. (Remember, 1nm = 1,000μ.) While dry grinding’s simplicity often makes it the first method considered, if extremely small size is the goal, wet media milling 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.
- The end material must be wet, and the size goal is 10 microns (small, but not extremely so). Therefore, dry grinding in a jet mill is the best method, with the material then requiring blending into a wet dispersion.
- The end material must be dry, and the size goal is 500 nanometers (very small). In this case, a wet media mill must be used, and the material needs to be dried after particle size reduction.
The second example leads us to the next consideration when choosing between wet and dry milling: cost.
If wet grinding is used to reduce particles to 500 nanometers, as in the example above, but that product is not intended for use in a liquid base, a drying step is required.
Why? First, dry milling can’t achieve the particle size needed, so it’s not an option. Second, shipping the material in a liquid slurry that's 75% water — water that will then be removed via a spray dryer — adds significant freight and handling expenses.
The good news is that wet grinding is more energy efficient than dry milling methods, using up to 30% less power to drive a wet mill. Over time, that energy savings can add up and displace some of the cost of the added drying step.
Other Considerations When Choosing Between Wet & Dry Milling
If application, size, and cost don’t lock a project into dry grinding or wet grinding, it may come down to a simple question of capacity. Can one grinding method or piece of equipment better handle the required particle size in the quantities needed?
Along that same line of thinking, is packaging a factor? Since grinding is a continuous process, packaging formats like bags, barrels, or super sacks need to be considered in terms of whether they can be emptied at a rate that keeps up with the equipment speed.
Sometimes formulation can be a consideration. Additives may be needed to facilitate more efficient wet or dry milling. Material knowledge based on trials and past experience inform the choice of additives. Obviously, it’s most efficient to include as much raw material particle solid into the mix as possible.
Experienced providers of wet and dry grinding have quality assurance processes in-house, including analytical lab testing capabilities. While this may not determine one milling method over another, it’s important to have these services available to ensure an end product that meets specifications and quality standards.
The list of materials that can be ground in wet or dry processes is incredibly long, from inkjet printer inks and paint pigments to personal care products and nutraceuticals.
So, whether you call it grinding or milling, it takes the upfront guidance and dedicated capabilities of an experienced toll processing partner to help you choose the right particle size reduction technologies and appropriate post-processing services to meet your product goals.
Get your own introduction to expert guidance when you download our Grinding Project Application Checklist. It can help you get started evaluating your needs, project scope, specifications, and other details that can impact processes and outcomes. Just click the link below to access and download your own copy, to reference whenever you need or share with colleagues.
