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.
WHAT IS WET GRINDING AND DRY GRINDING?
In wet grinding, raw materials are suspended in a liquid slurry that circulates through the milling chamber. The slurry is usually made up of water with additives such as dispersants and surfactants to help keep the particles separated from each other and prevent clogging on the grinding media. The slurry enters through an inlet port in the bottom of the milling chamber and flows upward along with any fines generated by the grinding process. As it rises, it passes over a cascade of grinding media (often balls or rods) inside the milling chamber before exiting through an outlet port at the top of the milling chamber. The grinding media act like hammers smashing chunks of solid material into smaller pieces as they fall through each pass across them in turn.
Dry grinding uses particle-on-particle impacts to reduce size. It’s most commonly used in industries that require large volumes of material to be milled, such as mining or construction. Some of the most common dry grinders include ball mills, rod mills, and attritors.
SIMILARITIES AND DIFFERENCES BETWEEN WET GRINDING & DRY GRINDING
The primary difference between dry and wet grinding is that one uses a dry powder and the other uses a liquid slurry.
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.
HOW TO CHOOSE BETWEEN WET MILLING & DRY MILLING
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.
The difference between wet and dry micro grinder is simple: In wet milling, material is suspended in a fluid for processing. In dry milling, material is processed without a fluid suspension.
Although there are several different types of dry micro grinding processes available today — including jet milling and ball milling — both processes have one thing in common: They produce very small powder particles with high surface area relative to volume. This characteristic makes dry micro grinding an ideal choice for applications like fireworks when maximizing combustion efficiency is critical.