Unlocking the Fundamentals of Size Reduction Methods

Impact These are impact mills like air classifier mills, hammer mills, and universal mills, which use sharp, instantaneous blows from hammers mounted on the rotor of the grinding mill striking the material.

Compression or crushing: This is generally used for mining and in mineral processing applications, it includes crushing or grinding machines, such as ball mills, flake crushers, jaw crushers, cone mills, and roller mills.

Shearing or cutting: These are chopper mills and hammer mills with knife type or bar type hammers which uses a cutting or cleaving action for size reduction.

Attrition: In this particle size is reduced by rubbing action commonly known as autogenous grinding caused by friction between particles or by grinding material against a surface. Example – Ball mills.

Did you know that size reduction process is very inefficient. Hence it is crucial to use energy as efficiently as possible. The mechanism and factors affecting size reduction process and the laws governing these operations are used to select best suitable grinding technique for each application. The stress-strain behaviour of materials during mechanical failure plays an important role during size reduction. To evaluate the effectiveness of size reduction operation, analysis of newly formed surfaces and energy involved is important. An understanding of equipment and operation parameters can minimize the overall input energy.

Commination is very inefficient process and thus it is important to use energy as efficiently as possible. Following laws are used in approximately calculating energy requirements.

• Kicks Law states that the energy required to reduce an amount of material is directly proportional to its the size reduction ratio, the energy required for grinding will be the same regardless of the original size of its particles.
• Rittinger’s Law states that the energy required for size reduction is directly proportional to the change in surface area.
• Bond’s Law notes that energy used for size reduction of the particle is proportional to the square root of the diameter of the particle produced.

Spice and Herbs Grinding Chili and herbs are fibrous in nature, hence shear + impact grinding technique is used for size reduction of the same. The heat generated during size reduction is always a concern in processing spices and herbs. Hence, advanced size reduction operations in Pulvionex and Micronex Mills with water jackets and cryogenic grinding are used to protect the aromatic and volatile components. Pulvionex mills are generally used for course grinding of spices and herbs. Whereas Micronex mills are used for fine grinding in 2nd or 3rd stage.

Limestone, Cement and Minerals application Limestone powder is brittle in nature, hence compression + impact grinding technique in Rieco’s Micronizers in collaboration with Cimma Ing. Morandotti & Company, Italy. Also, Heavy duty crushers and De-lumpers are equipment used for crushing the big size lumps of limestone into small or desired size by using various screen sizes.

Sugar and Pigment Grinding Since sugar and pigments have high calorific value, it can catch fire readily during size reduction process. Hence, sugar is ground using impact grinding technique in Air Classifying Mill. ACM is an air classifying mill with integrated grinding, classifying, conveying, and collecting operations for achieving ultra-fine grinding (up to 2 microns*) depending upon the nature of the product.

Pigments Pigments used in paint and cosmetic industry are ground using impact grinding technique in the controlled temperature using water jacketed ACM and Universal mills for maintaining colour and texture of the powder. Rieco’s universal mill has 5 different interchangeable types of rotor and status arrangements which enables us to grind different types of pigments in the single machine.

To conclude, there are numerous grinding machines using various size reduction techniques and combinations that can convert a coarse particle it into a fine or super-fine particle based on application requirements. Understanding how each piece of equipment exerts force on the particle and what range it is the most efficient is essential to selecting the right process. Further study is required to determine the pre-crushing options and combinations gives the most cost effective and efficient work within the desired particle ranges.

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