The designs for fine grinding mills are incredibly diverse, as these machines are engineered to process various materials for many applications. The material and end use thus determine the type of grinding machine needed for a specific process, including a mill’s distinctive configurations and the construction materials out of which it’s made. With the underlying economics behind all production processes, manufacturers must consider a machine’s technical specifications when choosing fine grinding mills for any application.
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How to Choose the Best Fine Grinding Mills
Fine grinding mills differ in construction, configuration, size, performance, and other specifications. Similar machines used to process the same material often differ depending on what company made them. Goods within the same category will often vary depending on the exact processes used, as manufacturers seek to make their end products unique from those of their competition. Understanding what features fine grinding mills should have for specific applications depends on how and to what degree the raw materials will be broken down during processing. These characteristics will help a manufacturer choose the best machine type.
Generally, fine grinding mills can be classified into these groupings:
- Attrition mills
- Ball media mills
- Impact mills
- Jet mills
- Roller mills
- Universal mills
Attrition mills generate lower temperatures and tend to have lower capacities, so they are used for heat-sensitive materials like spices or flammable materials like wood chips. Using media in their fine grinding, ball mills use both attrition and impact to reduce material into very fine powders, so are often used for mixing constituent ingredients and for processing bulk powders. Commonly used impact mills like hammer mills come in disparate configurations, designs and sizes, used for diverse applications from enabling recycling operations to activating ingredients in pharmaceutical medication.
Jet mills impact and grind particles using compressed gases to achieve extremely fine grinding for cosmetics or pigments. Compressing bulk, roller mills crush and grind materials like gravel or grain between cylindric rollers. Yet even among each of these types of fine grinding mills, the end product will differ depending on the machine’s manufacturer and its exact design specifications.
A Step-By-Step Guide to Choosing the Right Fine Grinding Mills
Particle size reduction for most industries involves equipment that crushes larger chunks into sizes typically less than an inch (approximately 25 mm) across, while fine grinding mills can reduce material beyond 400 mesh (37 microns) and even into the nanometer range. Additionally, certain properties of the processed material must be considered. These characteristics include friability (how well a product fractures when stress is applied), abrasiveness, adhesion, particle density, grain size, hardness, moisture content, particle shape, product temperature, slope angle and toxicity.
When choosing fine grinding mills, manufacturers must consider these three things:
- The raw material being processed includes properties like moisture and oil content, friability, abrasiveness, brittleness, elasticity, hardness, hygroscopicity, sensitivity to heat, and viscosity.
- Particle size distribution – such as the minimum, maximum, mean, or median size desired for the end product.
- The fine grinding mill's flow rate, capacity, dimensions, and utility requirements.
Thought must be given to the material being processed, and the result needed for a fine grinding machine design. However, mills that are part of the processing system used by a manufacturer must do more than consider material qualities. Costs and other operational limitations require consideration as well. Critical thinking must go into decision-making on which of the many fine grinding mills to use, for which this brief step-by-step guide can assist:
Consider Material Properties
Before anything else, a comprehensive assessment must be made of the material to undergo fine grinding. Mills require specific features when processing raw materials with certain qualities, especially properties like their abrasiveness or hardness. Manufacturers should choose fine grinding mills that give the desired characteristics to the end product while defining the desired particle size and dimensions of the end product. Contemplating the concentrations and amounts of specific ingredients should also be part of this analysis.
Conditions & Specifications for Fine Grinding
When deciding between various fine grinding mills, manufacturers must scrutinize the conditions under which the material will be processed. Fine grinding mills work in environments and with either dry or wet materials, affecting the type of mill used and the extent of processing. How long and quickly materials undergo processing will also determine which type of fine grinding mills to use.
Processing forces for friable (brittle), dry, and hard materials include:
- Attrition: Able to achieve very fine grinding, mills of this type use friction between two opposing surfaces or between particles themselves; often, these machines are referred to simply as fine grinding mills, though Buhrstone, disc, and long gap mills are specific types of attrition mills.
- Compression: Mainly producing coarser particle reduction with fewer fines involves the pressurized force of two solid surfaces pressed together to break apart materials used for grinding mills like jaw crushers and roller mills.
- Impact: Generating a mix of coarse and fine particles, this method for particle reduction involves impacts either by accelerated particles colliding at speed with a stationary implement or sometimes with each other, a method used by hammer and pin mills.
Processing forces for elastic, fibrous, and soft materials include:
- Cutting: These fine grinding mills act much like scissors, featuring multiple sharp-edged blades with at least one blade stationary and one in motion, a method used by cutting and knife mills.
- Shearing: Involving a force with multiple solid surfaces that move in opposite directions, this type of particle size reduction uses cutting and crushing actions to break down materials, a technique employed by rotor beater mills and ultra centrifugal mills.
Many types of fine grinding mills use combinations of these forces. For example, ball and SAG (semi-autogenous) mills use attrition and impact forces to break down materials.
Implements for Fine Grinding Mills
The implements in fine grinding mills help determine the particle size to which the raw material can be reduced, along with their distributions. Configurations, tooling, and the type of mill will also affect how the material is broken down, and the material’s hardness must also be considered. While countless mechanisms can be fitted on fine grinding mills to achieve the appropriate particle size reduction and distribution, these fall into two basic categories.
These implements are:
- Hammers: These mills are generally used for coarser materials and come with or without screens to control when particles are finished grinding. The hammers attached to the rotor can either be mobile or stationary. Used for dry products but not generally fine grinding, mills of this type can be manually, mechanically, or pneumatically gravity-fed. They can be used for fine grinding with specific setups or configurations, such as higher rotor speeds and screens with smaller mesh.
- Pins: Available with or without screens, these fine grinding mills feature a rotor with rotating pins juxtaposed against stationary pins. These mills can achieve very fine powders. Fineness can also be adjusted by altering feed rates or changing the motor spins' speed. Since they also generate considerable centrifugal force, these mills allow for wetter and more viscous products to be processed.
Blades can also be used as implements on mill rotors but aren’t used for fine grinding. Mills featuring fixed blades work like scissors and are generally used for applications like recycling, which break materials into smaller bits.
Production Testing of Fine Grinding Mills
A reputable manufacturer of fine grinding mills will have a testing facility where customers can evaluate performance before buying a machine. Seeing fine grinding mills in action will provide insight into a machine's performance under real-life milling conditions so that a manufacturer can evaluate such things as efficiency, product quality and wear rate before purchasing.
Prater Industries: Fine Grinding Mills for Specific Applications
Prater Industries has plenty of experience since its founding in the s with building fine grinding mills to meet its customers' various applications and specifications. The experienced Prater engineering team has been frequently challenged to improve upon our machines used for fine grinding. Mills are used for all manner of applications and our engineers use their creativity and experience to design them to suit our customers’ specific needs.
Fine Grinding Mills for the Grass Seed Industry
A company that processes and sells specialty grass dust to enhance the growth of lawns and other grassy areas approached Prater to see how it could improve its processes. The company was experiencing quality control issues with a specific blend of bone meal and urea made by its current toll processor. This toll processor used an air classifying mill built by one of Prater’s competitors, which unfortunately didn’t meet the particle distribution necessary for the product. There were additional time constraints, as the grass growing season was ending, so implementing a solution needed to happen quickly.
Prater did a production-level run of the blend at our onsite testing facility, using a model similar to the toll processor. After preliminary testing, Prater engineers recommended doing a test run with one of Prater's M-series fine grinding mills to see how well it performed in comparison. After watching both tests, the grass seed company was impressed with the tight particle size distribution and, after seeing the Prater’s fine grinding mill in action within another customer’s system, they decided Prater’s suggestion of equipment would fit their application well. Ultimately, a system was developed around one of Prater’s M-51 fine grinding mills within a system designed by Prater’s engineers that included Prater feeding, blending and unloading equipment.
Fine Grinding Mills for the Pet Food Industry
A top pet food producer was using one of Prater’s M-series fine grinding mills but found that the lifespan of the jaws on the machine had halved. From the typical three-month time the jaws were designed to last, they now lasted only a month and a half. Prater’s engineers considered that a change in the flow system and raw material being used, along with regular wear, were causing the fine grinding mill’s jaws to wear more quickly.
The customer used Prater’s fine grinding mill for free-flowing ingredients that were ground into a highly tight particle size distribution. The precision tolerances between the stationary grinding surface and the rotor blades—known as the jaws—allowed the customer strict control of particle size and distribution. Prater’s engineers surmised that the processed material's abrasiveness was severely shortening jaw life.
Prater’s engineers sought to change the design of the fine grinding mills to increase jaw life and reduce downtime. The engineering team needed to find a harder yet still machinable material so that the construction material wouldn’t warp when machined to meet tight tolerances. A new material was found that met the criteria, with the team’s redesign increasing the lifespan of the fine grinding mill’s jaws to four months, surpassing their objectives by a third.
Fine Grinding Mills for Processing Soy Flakes
Prater’s engineers don’t just react to problems within processing systems. Sometimes, they see a solution to problems that haven’t yet resulted in complaints. This is the case with the processing of soy flakes, which are used primarily in the animal feed industry. Though principally grown for meal, soybeans also produce soy oil as a secondary product. To remove the hulls, soybeans are cracked and then rolled into full-fat flakes, which expediates oil extraction by disrupting the oil cells. Once oil has been removed from the flakes, they’re dried into flakes without fat, which are then processed further into soybean meal for animal feed or ground into soy flour. However, they can also be ground into concentrated soy proteins that can be used for a variety of human food products. Two types of soybeans have arisen due to the variance in soybean use, one for oil and the other for food.
Prater’s Mega Mill hammer mills have been identified as ideal for grinding soy meal. Mega Mills can be easily cleaned and otherwise maintained, as the housing allows for completely removing the rotor, which can be cleaned and back in production in under twenty minutes. Unlike conventional hammer mills that take several hours to clean, this results in considerable downtime. Prater’s M-series fine grinding mills for processing soy flakes allow processors to achieve extraordinarily tight distributions. Engineers at Prater have tweaked our M-series fine grinding mills to perfect them for processing soy flakes, allowing for particle reduction to different degrees of fineness with only minor changes to our M-series fine grinding mills.