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Comminution
Comminution
Comminution
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Mineral crusher (left side of image, next to water wheel) used in 19th century Cornwall for communition of tin ore

Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes.[1] Comminution is related to pulverization and grinding. All use mechanical devices, and many types of mills have been invented. Concomitant with size reduction, comminution increases the surface area of the solid.

For example, a pulverizer mill is used to pulverize coal for combustion in the steam-generating furnaces of coal power plants. A cement mill produces finely ground ingredients for portland cement.[2] A hammer mill is used on farms for grinding grain and chaff for animal feed. A demolition pulverizer is an attachment for an excavator to break up large pieces of concrete. Comminution is important in mineral processing, where rocks are broken into small particles to help liberate the ore from gangue.[3] Comminution or grinding is also important in ceramics, electronics, and battery research.[4] Mechanical pulping is a traditional way for paper making from wood. The mastication of food involves comminution. From the perspective of chemical engineering, comminution is a unit operation.

In geology, comminution refers to a natural process during faulting in the upper part of the Earth's crust.[5]

Energy requirements

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The comminution of solid materials consumes energy.[6] Approximately 65% of the power for the production of cement is consumed in comminution.[7]

The comminution energy can be estimated by:

  • Rittinger's law, which assumes that the energy consumed is proportional to the newly generated surface area;[8]
  • Kick's law, which related the energy to the sizes of the feed particles and the product particles;[9]
  • Bond's law, which assumes that the total work useful in breakage is inversely proportional to the square root of the diameter of the product particles, [implying] theoretically that the work input varies as the length of the new cracks made in breakage.[10][11]
  • Holmes's law, which modifies Bond's law by substituting the square root with an exponent that depends on the material.[6]

Three forces are typically used to effect the comminution of particles: impact, shear, and compression.

Methods

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Diagram of froth flotation cell used to process ores after communition. A mixture of ore and water called pulp [1] enters the cell from a conditioner, and flows to the bottom of the cell. Air [2] or nitrogen is passed down a vertical impeller where shearing forces break the air stream into small bubbles. The mineral concentrate froth is collected from the top of the cell [3], while the pulp [4] flows to another cell.
Idealized image of chemical mechanical polishing, a kind of grinding.[12]

There are several methods of comminution. Comminution of solid materials requires various types of crushers and mills depending on the feed properties such as hardness at various size ranges and application requirements such as throughput and maintenance. The most common machines for the comminution of coarse feed material (primary crushers) are the jaw crusher (1m > P80 > 100 mm), cone crusher (P80 > 20 mm) and hammer crusher. Primary crusher products in intermediate feed particle size ranges (100mm > P80 > 20mm) can be ground in autogenous (AG) or semi-autogenous (SAG) mills depending on feed properties and application requirements. For comminution of finer particle size ranges (20mm > P80 > 30 μm) machines like the ball mill, vertical roller mill, hammer mill, roller press or high compression roller mill, vibration mill, jet mill and others are used. For yet finer grind sizes (sometimes referred to as "ultrafine grinding"), specialist mills such as the IsaMill are used.

Trituration, for instance, is comminution (or substance breakdown) by rubbing. Furthermore, methods of trituration include levigation, which is the trituration of a powder with a non-solvent liquid, and pulverization by intervention, which is trituration with a solvent that can be easily removed after the substance has been broken down.

See also

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References

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Comminution

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Comminution is the reduction of solid materials from one average to a smaller average through mechanical processes such as crushing, grinding, cutting, vibrating, or other forms of . This fundamental operation applies to break down bulk materials into finer particles, often categorized into coarse (around 5 mm), fine (down to 63 μm), and ultrafine (<63 μm) regimes, depending on the desired output. In mineral processing and mining, comminution plays a critical role in liberating valuable minerals from surrounding gangue material, enabling efficient separation and extraction; it typically involves staged processes starting with primary crushing (reducing ore from over 1 m to P80 > 100 mm) and progressing to fine grinding (P80 < 30 μm) using equipment like jaw crushers, semi-autogenous mills, and ball mills. This step consumes significant energy—often 30-50% of total mining operations—but enhances mineral recovery, reduces chemical processing costs, and increases surface area for downstream treatments. Beyond mining, comminution is essential in the pharmaceutical industry, where size reduction (also known as micronization) improves drug dissolution rates, bioavailability, and uniformity by increasing particle surface area, facilitating better absorption and therapeutic efficacy in formulations like tablets, inhalers, and topical products. It also supports applications in food processing (e.g., grinding nuts or spices), cosmetics (producing fine powders for additives), and materials engineering (creating nanomaterials via atomization down to 20 nm), though challenges include low energy efficiency, potential contamination, and agglomeration at finer scales. Forces involved—such as impact, attrition, shear, and compression—dictate the method selection, with techniques like hammer milling or stirred media mills tailored to specific industries for optimal particle size distribution and minimal waste.

Definition and Fundamentals

Definition and Scope

Comminution is the mechanical process of reducing the size of solid materials, particularly ores and rocks, from larger particles to smaller ones through methods such as breaking, crushing, or grinding. This size reduction is essential in industries like mining and materials processing, where it facilitates the handling and further treatment of raw materials by creating more manageable particle sizes. The process involves applying external forces to fracture the material along natural weaknesses, resulting in a distribution of particle sizes that can range from coarse fragments to fine powders, depending on the desired outcome. The scope of comminution encompasses both dry and wet processes, with dry methods typically used for coarser reductions to avoid moisture-related complications, while wet processes, often involving water as a medium, are preferred for finer grinding to improve efficiency and control dust. It is distinct from fragmentation, which refers to the initial, often uncontrolled breaking of materials such as in blasting, and from pulverization, which denotes extreme size reduction to achieve very fine particles, usually below 100 micrometers, for applications requiring high reactivity. Comminution bridges these concepts by systematically progressing through stages: coarse reduction via crushing for particles typically larger than approximately 10 mm (with final crushing stages down to 5-10 mm), and finer reduction via grinding for sizes below that threshold. In ore processing, a primary goal is particle liberation, where valuable minerals are separated from surrounding gangue through targeted breakage, enhancing the efficiency of downstream separation techniques. Comminution's importance lies in its role in increasing the specific surface area of materials, which accelerates chemical reactions and improves reactivity in subsequent processing steps, such as leaching or flotation. However, it is highly energy-intensive, accounting for up to 50% of the total energy consumption in mineral processing operations, making efficiency improvements a critical focus for sustainability. This energy demand underscores the need for optimized processes that balance particle size reduction with minimal input, while avoiding over-grinding that could re-lock liberated minerals.

Historical Development

Comminution practices trace back to ancient civilizations, where manual and water-powered methods were employed for size reduction in both agriculture and mining. In agriculture, hand-operated querns—rotary stone mills—were widely used from prehistoric times to grind grains into flour, representing one of the earliest forms of mechanical comminution. In mining, the Romans advanced ore processing around the 1st century BCE to 1st century CE by adopting stamp mills, which utilized trip-hammers powered by waterwheels or animals to crush extracted ore prior to further refinement. These hydraulic stamp mills, inherited from earlier Greek designs dating to the 3rd century BCE, marked a significant step toward mechanized comminution in extractive industries. The 19th century brought industrialization to comminution through the integration of steam power and innovative crusher designs, driven by expanding mining operations during the . Steam engines powered early rock crushers, enabling more efficient ore handling in underground mines and boosting productivity in regions like Michigan's copper districts. A pivotal advancement occurred in 1858 when Eli Whitney Blake patented the jaw crusher, a double-toggle mechanism that used compressive force between a fixed and movable jaw to break stone, laying the foundation for modern primary crushing equipment. In the 20th century, comminution evolved toward finer grinding and greater automation, with key inventions enhancing capacity and uniformity. Ball mills emerged in the 1870s, initially for grinding flint in pottery production, but soon adapted for mineral processing through tumbling action with steel balls as media. Rod mills followed in the early 1900s, designed to produce more uniform particle sizes than ball mills by using elongated steel rods, addressing challenges in primary grinding stages after crushing. By the 1950s, autogenous grinding gained traction, particularly with the first commercial applications in the late 1950s, where ore itself served as the grinding medium, reducing reliance on external liners and media. Post-World War II, the focus shifted to energy-efficient designs amid rising operational costs and resource demands, incorporating optimized mill geometries and process controls to minimize power consumption in large-scale operations. Recent decades have emphasized sustainable and intelligent comminution, with high-pressure grinding rolls (HPGR) introduced in the 1980s as an energy-saving alternative to traditional mills. Developed by researchers like Klaus Schönert, HPGR technology compresses ore beds between counter-rotating rolls at high pressures, achieving up to 20-30% energy reductions compared to conventional methods and first commercialized around 1988 for diamond and iron ore processing. In the 2010s and continuing into the 2020s, artificial intelligence has been integrated for process control, using machine learning algorithms to optimize mill parameters in real-time, predict wear, and enhance throughput—evidenced by advanced sensor-based systems that have improved efficiency by 10-15% in modern circuits.

Physical Principles

Mechanisms of Size Reduction

Comminution involves several primary mechanisms of size reduction, each applying distinct types of stress to fracture materials. Compression applies gradual, sustained pressure that squeezes particles until they deform and break, commonly effective for brittle minerals by propagating cracks along stress lines. Impact delivers sudden, high-velocity force to shatter particles through shock waves, ideal for friable materials as it exploits internal flaws for rapid fragmentation. Attrition reduces size via shearing or rubbing forces between particles or against surfaces, producing finer particles through surface abrasion and edge chipping. Cutting, primarily for softer or fibrous materials, severs particles with sharp-edged tools, minimizing distortion while achieving precise size control. Fracture during comminution follows concepts rooted in material behavior under stress, distinguishing brittle failure—characterized by sudden crack propagation without significant plastic deformation—from ductile failure, where materials yield and deform plastically before breaking. In brittle fracture, prevalent in hard minerals, cracks initiate and grow rapidly due to stress concentrations at microscopic flaws, as described by Griffith's theory, which posits that fracture occurs when the energy release rate from crack extension equals the surface energy required to create new crack surfaces. This theory highlights how pre-existing microcracks amplify local stresses, lowering the overall fracture strength compared to theoretical cohesive limits, thus governing breakage in non-plastic regimes. Material properties profoundly influence the efficacy of these mechanisms, with hardness—measured on the Mohs scale from 1 (talc) to 10 (diamond)—indicating resistance to deformation and scratching, thereby dictating energy needs for breakage. Toughness, the ability to absorb energy before fracturing, and brittleness, the tendency for sudden failure, further modulate outcomes; brittle materials like quartz (Mohs hardness 7) fracture cleanly under impact or compression due to low toughness, yielding sharp-edged fragments, while more ductile materials such as coal (Mohs hardness ~2.5–3) exhibit higher toughness, resisting breakage and producing irregular, rounded particles through plastic flow. These properties interact, as higher hardness often correlates with brittleness in minerals, enhancing crack propagation but increasing wear on processing surfaces. Particle interactions during comminution vary between single-particle and bed modes, altering stress distribution and breakage patterns. In single-particle comminution, isolated particles experience direct, uniform stress, maximizing energy transfer for efficient fracture but risking over-crushing of fines. Bed comminution, involving layered particle assemblies, introduces inter-particle cushioning and multi-directional forces, promoting attrition alongside compression while reducing overall energy efficiency due to energy dissipation through particle rearrangements. Moisture content modulates these interactions, as low levels enhance brittle fracture by weakening inter-particle bonds, whereas high moisture (>2–3%) forms bridges that increase cohesion, dampen impacts, and lower size reduction efficiency by promoting agglomeration over clean breaks.

Energy Laws and Requirements

The energy requirements in comminution are governed by several empirical laws that relate the input energy to the degree of size reduction achieved. These laws provide foundational models for estimating power consumption in crushing and grinding operations, though they differ in their assumptions about the underlying mechanisms of particle breakage. Kick's law, proposed in 1885, posits that the energy required for size reduction is proportional to the logarithm of the reduction ratio, making it suitable for coarse crushing where deformation and dominate over surface creation. Mathematically, it is expressed as E=Klog⁡(D1D2),E = K \log \left( \frac{D_1}{D_2} \right), where EE is the energy per unit , KK is Kick's constant (dependent on ), D1D_1 is the initial , and D2D_2 is the final . Rittinger's law, developed in , assumes that the energy is primarily used to create new surface area during fine grinding, where surface energy becomes significant. It states that energy consumption is directly proportional to the increase in specific surface area, given by E=Kr(1D2−1D1),E = K_r \left( \frac{1}{D_2} - \frac{1}{D_1} \right), with KrK_r as Rittinger's constant. This model holds best for brittle materials in fine reduction processes, as it accounts for the absorbed in forming surfaces. Bond's law, introduced in the , offers an intermediate empirical approach based on extensive mill data, linking to the of the reduction and incorporating a material-specific work index. The standard equation for the work input is E=10Wi(1P−1F),E = 10 W_i \left( \frac{1}{\sqrt{P}} - \frac{1}{\sqrt{F}} \right),
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