A material derived from a type of fossilized algae, diatomaceous earth is a soft sedimentary rock that was used by the ancient Greeks for making bricks and pottery. It’s essentially made up of the skeletons of single-celled organisms called diatoms that lived in prehistoric oceans, lakes, rivers, and streams millions of years ago. Also referred to as diatomite, deposits of diatomaceous earth consist mostly of silica dioxide, as silica reacts with oxygen.
What makes diatomaceous earth so useful is its many beneficial properties, which include abrasiveness, absorptive capabilities, brightness, inertness, insulating properties, large surface area of diatomite particles, porosity, and purity of its silica content. Though many other naturally available materials contain silica, diatomaceous earth is typically 80-90 percent silica. A key aspect of products that utilize diatomaceous earth, particle sizes vary according to the application for which it’s used.
Why Diatomaceous Earth Particle Size Matters?
For most applications, diatomaceous earth takes the form of a fine white to off-white powdery material. For diatomaceous earth, particle sizes occur naturally from 3 microns to about a millimeter in size, though these particles usually range from 10-200 microns. Though its exact texture depends on its granularity, diatomaceous earth has the abrasive feel of pumice powder, with a low density due to its porosity.
Diatomaceous Earth: Particle Size for Specific Applications
The silicon dioxide within diatomaceous earth particles is mostly amorphous, meaning the particles have no definite shape. However, the size of these particles differs according to the application for which it’s used. Though they’re the same material, diatomaceous earth works better at certain granulations than others in certain applications.
Generally, particles of diatomaceous earth are reduced for applications as follows:
- Powdered diatomaceous earth (particle sizes typically from 2-30 microns) is used to control insects in grain bins, as an anti-caking agent, and for animal feed.
- Fine diatomaceous earth (particle size averaging 2.5 microns) can be mixed with animal feed.
- Granular diatomaceous earth (particle size from 50-1000 microns) is mixed into animal feed and used for odor control.
A key use for diatomaceous earth is as an insecticide, though diatomaceous earth particle size must be under 10 microns to be most effective against insects. Since amorphous silica isn’t toxic to humans or livestock, utilizing it to protect stored grain makes it particularly useful. Additionally, diatomaceous earth in animal feed improves its flowability by helping to keep it dry, while it also keeps livestock from developing worms.
Filtering media for liquids and beverages:
- Absorbent material for liquids
- Acting as support for chemical catalysts
- Cat litter boxes to reduce smell
- Coating matting agent
- Filler to reinforce plastics and rubber
- Healthcare products
- Insulating against heat
- Mild abrasive in products like toothpaste or metal polishes
- Soil additive for potted plants
- Stabilizing dynamite
Yet one of the most important uses that are affected by diatomaceous earth particle size involves insect control.
Diatomaceous Earth Particle Size for Insecticides
Diatomaceous earth particle size plays an important role in protecting stored grains and other foodstuffs against pests like mealworms and certain beetles, moths, and weevils. Generally, the smaller the particle size, the more effective it was as an insecticide. One Turkish study looked at diatomaceous earth particle size from 1-30 microns, finding that it was less effective on more active insects or those with thicker epidermises.
Its absorbent properties is how diatomaceous earth works against insects. In essence, it causes insects to dehydrate, leading to their deaths. It becomes even more effective when combined with certain fungi or plant-based oils. As such, it can replace chemical pesticides as a more environmentally-friendly to control insect pests.
Impact Strength & Diatomaceous Earth Particle Size
Another application affected by the particle size of diatomaceous earth involves its use in composites. A recent MDPI study looked at how diatomaceous earth particle size affects the impact strength of composites. The study found that composites containing particles with larger diameters had lower impact strength than those with smaller diameters.
Garden Grade Vs. Food Grade & Crystalline Vs. Amorphous
The grade and shape of particles are another key aspect when choosing the best type of diatomaceous earth. As small as a single micron, particle sizes can be filtered out using diatomaceous earth for filtering liquids, making it particularly useful for water filtration. Garden grade is sometimes referred to as pool grade, as it filters water in pools. It’s also calcined, a type of heat treatment that improves the filtering capabilities of diatomaceous earth.
Garden grade also has a higher proportion of structured crystalline silica than the amorphous particles in food grade diatomaceous earth. Particles are largely amorphous, with less than one percent crystalline, in food-grade diatomaceous earth. The food grade can thus be used in animal feed, for dietary supplements, and as a pesticide that keeps insects from destroying grain in storage. Particle size doesn’t make a difference in this case, but rather how diatomaceous earth particles are shaped.
Equipment to Reduce Diatomaceous Earth Particle Size
Two pieces of equipment made by Prater Industries are ideal for particle size reduction of diatomaceous earth. These are the Rotormill, a type of attrition mill produced by IPEC (International Process Equipment Company), and Prater’s own Mega Mill Hammer Mill. Acquired by Prater in 2021, IPEC is now a Prater subsidiary that manufactures the Rotormill.
The Rotormill has a long gap design that makes screens unnecessary, thus eliminating a source of wear and choking. With an option for hardened internal grinding implements, the Rotormill works well for abrasive materials like diatomaceous earth. Particle size reduction occurs primarily through attrition, as particles collide with each other, and is capable of continuous milling into fines at a high throughput rate. Additionally, the Rotormill is more efficient than other mills, as it conducts several operations simultaneously, including surface coating and deagglomeration.
Prater’s Mega Mill promotes a uniform grind without causing equipment to get excessively hot, while its cantilevered rotor for grinding allows it to operate quietly. Like the Rotormill, it can handle continuous operation, requiring less airflow than traditional hammer mills. The Mega Mill performs diatomaceous earth particle size reduction via both attrition and hammers within the grinding chamber. Additionally, its design produces higher-quality material while requiring lower power inputs and reducing the need for maintenance.
The Rotormill pulverizes through collisions between particles within its grinding chamber, where air turbulence is generated. An internal spinning rotor made up of two sections assists with pulverization, with heavy-duty bearings providing stability. The rotor has a lower section that functions as a fan to distribute diatomaceous earth. Particle size is further reduced into a finely ground material as it goes through several stages in the upper section.
Grinding plates speed airflow against the mill’s grooved interior lining, which reduces the size of each diatomaceous earth particle. Size reduction is further augmented via high-velocity turbulence, where miniature pockets of rotating air move through the chamber at high speeds. The continuous airflow also minimizes heat buildup in the machine. The Rotormill’s unique design allows for variable airflows and grinding plate adjustments, which allow it to produce various diatomaceous earth particle sizes.
Prater’s Mega Mill
The Mega Mill uses impact force to crush diatomaceous earth. Particle size reduction involves hammers affixed to support pins on a highspeed rotor, pivoting in a specific pattern within the grinding chamber. Since precision bearings support the rotor, allowing for extremely close tolerances with the screens attached to the machine’s frame. While the primary means to reduce diatomaceous earth particle size comes from the impact of hammers, it also reduces material when these particles collide with each other, the surface of the screen, and the interrupter plates.
Specific features of Prater’s Mega Mill include:
- Accessible rotor and screens make cleaning and maintenance easy.
- Improves grinding operations with well-placed interrupter plates.
- Limits dust from production, so is ideal for pneumatic conveyancing.
- Offers uniform grinding at a high capacity with little buildup of heat.
- Robust design makes it more durable.
With the Mega Mill, diatomaceous earth particle size can be adjusted in several ways, including by changing hammer tip speed, clearance between screens and hammers, number and type of hammers, along with the number of and size of holes in the screen.