The air jet milling (commonly known as a molino de chorro) is arguably the industrial equipment that best harnesses the power of “air.” It utilizes high-pressure gas accelerated through special nozzles to create a supersonic energy field. This drives material particles to collide with one another and undergo friction and shearing during high-speed fluidization. As a result, it achieves ultra-fine grinding at the micron and even submicron levels.
As a media-free powder processing system, its core principle lies in “self-grinding of materials.” This completely eliminates media contamination caused by traditional mechanical wear. Combining fully enclosed negative-pressure operation with unique low-temperature grinding capabilities, the air jet milling has become an indispensable “high-tech” piece of equipment in cutting-edge fields such as pharmaceuticals, new energy, and fine chemicals.
Working Principle and Process Flow
The core process of the air jet milling follows a closed-loop logic of “high-efficiency grinding—precision clasificación—closed-circuit collection”:
Supersonic acceleration (grinding stage):
High-pressure propellant gas (compressed air or inert gas) passes through a Laval nozzle, converting pressure energy into kinetic energy. This instantly accelerates the gas flow to supersonic speeds of 1.5 to 3.5 Mach.
Fluidized Self-Grinding (Impact Section):
Driven by the intense jet stream, the material becomes suspended in a fluidized state. Particles collide, rub against each other, and experience shear forces with extremely high kinetic energy, achieving self-grinding without direct contact with the equipment’s metal lining.
Forced Turbine Classification (Classification Section):
The gas-solid mixture enters the classification zone driven by negative pressure. The high-speed rotating fine turbine clasificador generates powerful centrifugal force, which balances the drag force created by the airflow.
Non-conforming coarse particles:
The centrifugal force exceeds the drag force, causing them to be flung against the chamber wall, where they settle and return to the grinding chamber for reprocessing.
Conforming ultrafine product:
The drag force exceeds the centrifugal force, allowing the particles to pass smoothly through the classifier blades and enter the downstream cyclone separator and dust collector for efficient collection.
Technical Evaluation of Four Main Air Jet Milling Types
Different chamber designs determine the efficiency of airflow energy utilization and the types of materials they are suitable for. The following is a comparison of the current mainstream models:
| Equipment Category | Core Milling Principle | Core Technical Advantages | Limitations & Drawbacks | Golden Application Scenarios |
| Molino de chorro de lecho fluidizado | Multiple opposing nozzles at the base; materials collide in a centralized fluidized state. | High energy efficiency, minimal liner wear, superior classification precision, supports large-scale capacities. | Relatively complex structure; higher equipment capital expenditure (CapEx) and mantenimiento costos. | Industrial large-scale production, high-purity, and high-value materials (e.g., lithium battery cathodes). |
| Horizontal Spiral Molino de chorro (Flat/Pan Type) | Gas enters tangentially; particles rotate and collide in a flat, circular trajectory. | Extremely compact structure, no dead zones (easy to clean), moderate cost, quick disassembly. | Significant wear on the milling chamber walls, risking contamination from metal or liner debris. | Pharmaceutical APIs, conventional fine chemicals, low-to-medium value non-metallic minerals. |
| Loop Molino de chorro (Circulatory Pipe Type) | Materials are accelerated and collide inside a loop pipe system. | High milling intensity; can perform surface modification or drying concurrently in continuous operations. | High energy consumption; pronounced pipeline wear at the curved sections. | R&D laboratory pilot scales, highly abrasive special materials, small-batch surface modification. |
| Molino de chorro de objetivo | Particle streams are carried by gas to impact a fixed target plate at high speed. | Extremely simple structure; lowest equipment cost. | Severe target plate wear, heavy impurity contamination, wide particle size distribution, lacks sub-micron capability. | Primary crushing of brittle materials, university laboratory pre-milling. |
Four Core Advantages
Low-Temperature Grinding
By utilizing the Joule-Thomson effect during gas injection, the gas undergoes adiabatic expansion at the nozzle outlet, causing the system temperature to naturally drop by 20°C to 40°C. This “self-cooling” feature eliminates the need for an additional refrigeration system, providing optimal protection for heat-sensitive components (such as biopharmaceuticals, aromatic traditional Chinese medicines, and low-melting-point resins) against denaturation or melting.
High-Purity Assurance
Since the process relies on particle-to-particle collision for grinding, no grinding media (such as zirconia beads or steel balls) are used. The equipment lining, classification wheels, and piping can be fully customized with wear-resistant, contamination-resistant coatings made of ceramics (such as alumina, silicon carbide, and silicon nitride) or polymers (such as polytetrafluoroethylene, PTFE). This fully meets the iron impurity control requirements for injection-grade active pharmaceutical ingredients (APIs) and electronic-grade materials.
Precise and Controllable “Narrow Distribution” Particle Size
Equipped with a high-precision turbine classification system featuring variable-frequency speed control, the finished product’s particle size can be freely adjusted within the range of D97 = 2–150 μm. In high-end models, optimized flow field design enables the stable production of submicron (quasi-nanoscale) products with D50 < 1 μm. The particle size distribution curve is extremely narrow, with no coarse particle contamination.
Robust “Intrinsically Safe” Explosion-Proof System
For special powders that are flammable, explosive, or prone to oxidation (such as graphite for lithium-ion battery anodes, metal powders, and sulfur), the air jet mill system can be integrated with a closed-loop system using inert gases (such as nitrogen or argon). Fully automatic oxygen sensors strictly control oxygen levels below the safety threshold, ensuring safe and worry-free processing.
Typical Industry Applications and Selection Recommendations
Air jet mills currently play a pivotal role in transforming materials into high-value products across various high-end segments of the industrial supply chain:
Pharmaceuticals and Biologics:
Typical Materials: Active pharmaceutical ingredients (APIs), ultrafine powders from cell-broken traditional Chinese medicines, and inhalation formulations (e.g., inhaled insulin powders).
Selection Challenges: Extremely high requirements for sterility, ease of cleaning (CIP/SIP), and cross-contamination prevention. The preferred choice is a flat-bed air jet mill with ceramic lining or made entirely of stainless steel.
New Energy Battery Materials:
Typical Materials: Lithium iron phosphate, ternary cathode materials, synthetic/natural graphite anodes, solid-state electrolytes.
Selection Challenges: Magnetic substances (iron, nickel, chromium) must be controlled to the ppb level, and high production capacity is required. The preferred choice is a fully ceramic-lined fluidized-bed counter-current air jet mill.
Fine Chemicals and Advanced Materials:
Typical Materials: High-end automotive pigments, catalysts, specialty flame retardants, aerospace-grade ceramic powders (silicon carbide, boron nitride).
Selection Challenges: Extremely high material hardness (Mohs hardness 7–9 or higher), making equipment wear resistance the top priority. The preferred choice is a fluidized-bed counter-current air mill, with high-grade ceramic coating applied to wear-prone components.
Conclusión
When selecting an air jet milling, it is crucial to avoid blindly pursuing low prices or focusing solely on a single performance metric. Instead, priority should be given to evaluating the material’s Mohs hardness, heat sensitivity, flammability and explosiveness, as well as the finished product’s tolerance for iron impurities. For most modern production lines requiring high purity and high throughput, a fluidized-bed counter-current jet mill combined with an inert gas recirculation system is typically the “golden combination” for ensuring both quality and safety.
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— Publicado por Emily Chen
