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.
In this article, we’ll break down the differences between wet grinding and dry grinding, and discuss examples of each milling process.
In milling, wet and dry grinding are the primary methods, each with its own set of advantages, disadvantages, and challenges, making them crucial to understand for efficient milling processes.
It’s important to note that 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.
Whether a product goes through a wet grinder or a drying and grinding process depends primarily on the required characteristics for the material’s final application. Other factors also guide decision making, including raw material, target particle size, particle surface and shape requirements, and more.
Wet grinding uses a process of recirculation. In a wet mill, 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.The slurry is exposed to the grinding media over and over, for hours if necessary, until the desired particle size is achieved. The uses of a wet grinder often involve reducing particles to the nanometer range, with 1nm equaling 1,000μ.
Dry grinding uses a single pass process to reduce particles to single-digit micron sizes. In 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 particle size.
While dry grinding and wet grinding are different methodologies, they share a common challenge: the potential for wear on milling equipment that, over time, can damage components and threaten to contaminate the product.
Abrasive raw feed material can damage equipment as a result of repeated collisions with machine parts in dry milling — 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. Water, oil, solvents, or surfactants can eat away at grinding media, agitators, or other components.
Whether dry or wet, it’s essential to prevent equipment particles and grinding media from contaminating the final product.
When deciding between wet and dry grinders, 3 factors must be taken into consideration:
The required characteristics for the material’s final application is the most important determining factor in choosing wet or dry grinding. 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 suffice to opt for the dry process.
Dry grinding’s simplicity often makes it the first method considered. However, if extremely small size is the goal, wet media milling is the only way to get there.
Target particle size in jet milling and wet milling is categorized into two milling processes:
Typically, dry grinding can reduce particles to single-digit micron sizes. A wet grinder, on the other hand, often reduces particles to nanometer sizes, with 1nm equaling 1,000μ.
But, it’s not always that simple. Specifications can alter the grinding method of choice, as in these examples:
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.
If application, size, and cost do not clearly define a project as needing dry or wet grinding, it might ultimately boil down to a straightforward 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. Materials 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 distinguish 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.
Selecting the appropriate particle size reduction technologies and post-processing services to align with your product goals requires the expert guidance and dedicated capabilities of an experienced toll processing partner.
Our Wet Grinding & Dry Milling Methods eBook helps you assess needs, project scope, specifications, and more. Click the link below to download your copy of this valuable guide.