Classification overview

Iain Crosley of Hosokawa Micron Ltd gives an overview of classification:

In general, powders are made up of a distribution of parameters that includes, for example, composition, particle size, shape, surface area and electrostatic charge. In order to suit its end use, the significant characteristics of a product must be controlled and specified. Doing this requires more uniform, and even engineered, particles. Better control of the distributions can improve significantly the properties of the product and its final use, giving enhanced flow characteristics or packing density, for instance. The current trend in particulate processing is towards greater product selectivity, higher capacity and yield, and increased automation.

'Classification' is the sorting of the initial distribution of particles to achieve a desired degree of uniformity, according to the chosen parameter. This might be density, shape, or size - to name but a few. 'Classification' is not to be confused with 'separation'. 'Separation' generally refers to dissimilar materials, whereas 'classification' refers to different grades of the same material.

Classification is used to control or limit the shortcomings of the previous processing stages, for example to eliminate oversize particles at the exit of a mill or spray drier. Such particles could render the final product unusable for its intended application, whether by spoiling the surface finish of coatings, causing failures in electronic components, or affecting the activity of a drug in a biological system.

Classification techniques

Many different types of classification techniques exist and a breakdown of the main equipment selection criteria will include:

Unit operation requirements:

• Capacity/throughput (kg/h)
• Feed material & final product quality (particle size distribution (PSD), shape)
• Yield
• Selectivity (sharpness of cut)
• Material characteristics (abrasive, friable, compactable)

Process requirements:

• Type of process upstream of classifier (e.g. wet mill, spray drier)
• End-use of products (e.g. slurry, pellets, dry powder)
• Automation / control requirements
• Specific requirements (GMP, validation, CIP/SIP)

Definitions of classifier terms:

Classification efficiency: Following classification, the particle size distributions of the fine and coarse fractions more or less overlap. That is, the fine fraction contains particles that are larger than the cut size, and the coarse fraction contains particles smaller than the cut size. The fewer particles of the wrong size in each fraction, the sharper the cut.

In a processing plant, high cut sharpness means the best-quality fine fraction and the most economical processing. This is because oversize in the final product is sharply limited, the fine fraction is large, and little material is lost since the coarse fraction contains few fines.

Throughput: Throughput is the mass flow rate of material (feed, fines, or coarse) through the air classifier and it strongly affects cut sharpness. Cut sharpness typically remains constant up to a certain throughput and then decreases continuously. Throughput also depends on the airflow and the classifier size, because a higher airflow can carry more material and a larger classifier can handle a greater airflow .

Energy requirements: Completely dispersing the feed will provide an efficient classification with high cut sharpness. For a very fine classification (for which the feed is typically fine), the classifier must have sufficient power to fully disperse the feed. This requires more energy. The specific energy required by each air classifier in order to produce a defined quantity of final product at a specified cut size and sharpness, is subject to wide variation.

Classification technologies

Classification technologies fall into two main categories: those that use liquid suspensions ('wet' - generally using water) and those conveyed using a carrier gas ('dry' - generally using air):

Wet

• Hydrocyclones
• Centrifuges
• Wet dynamic classifications
• Screens / sieving
• Sedimentation

Dry

• Sieving
• Static classifiers (cyclones)
• Dynamic classifiers (single stage, multi-stage)
• Cross-flow classifiers
• Counter-flow classifiers (elutriators) 

Classifiers can also be combined with other unit operations such as milling.

Market trends

A variety of factors is responsible for driving developments in powder and particulate processing, including:

• Requirement for reduced particle size (especially in areas such as toners, pharmaceuticals, minerals)
• Tighter PSDs (width of the distribution - removal of both fines and coarse tails)
• Pressure to improve yield
• More energy efficient unit operations
• Stable operation
• Reduction in waste, including start-up/shut-down
• More precise control over unit operations
• Wider domain of operation

In the past, wet techniques were necessary in order to achieve finer particle sizes. Now, however, improvements in dry particulate processing techniques, such as the development of fluid energy micronising mills, have allowed the processing of finer materials in the dry phase - indeed many new applications preclude the use of wet processing. Consequently, the majority of new materials are produced and used in the dry phase. This has taken the place of producing material in the wet phase and then drying it (which can lead to such phenomena as agglomeration). It is more energy efficient to undertake classification in the same phase.

Classifier operation is a critical but often overlooked step in any process. Optimal set-up and real-time monitoring can lead to significant process gains. Real-time control of separator speed, improved cut efficiency and the removal of bottlenecks to improve yield are all goals in optimizing the process. The following concerns apply to both wet and dry unit operations:

• High load on the grinding unit impacts throughput
• Sub-optimal operation leading to yield bottlenecks
• Many grades of powder from the same feedstock requiring different process set points
• Labor-intensive manual control of process
• Poor batch-to-batch comparability

Malvern particle size analyzers for wet and dry applications can be tailored to your process requirements. To see our range of solutions for real-time monitoring of particle size in classification processes click here

Principles of air classifier operation

Air classifier operation is based on airflow. The airflow concepts of elutriation, free vortex, and forced vortex are used either separately or in combination in the design of today's classifiers.

Elutriation is the process of separating by washing, in this case using air as the washing medium. Elutriation is generally used to separate the bulk of fines and coarse particles by introducing the feed into an airflow. The airflow raises the fine particles against gravity to a fines collector. Being too heavy to be carried upwards, the coarse particles decelerate and fall with gravity, against the flow, into the coarse fraction collector. The cut point can be adjusted by increasing or decreasing airflow velocity within the housing. Elutriation is a crude method of separation that is seldom used alone. It can, however, greatly improve the effectiveness of the vortex methods when added as a pre-separation stage.

Free vortex methods involve airflow moving in a decaying circular pattern towards an outlet, as in a cyclone. Coarse particles are thrown to the periphery and are removed to a coarse fraction collector. The fines are drawn inwards to the outlet with the airflow. Particle size is controlled by varying the airflows and the dimensions of the housing.

Using a free vortex technique results in relatively inefficient classification. The forced vortex, on the other hand, is a more complicated method that achieves more precise particle size control. As in the free vortex design, air is drawn from the outer edges of a housing, but a driven vaned rotor is added to 'force' the circular motion of the airstream and the entrained feed particles. In this design the coarse particles are thrown to the periphery, but the fine particles pass through the rotor vanes and escape to outlet. Precision in separation is gained by varying the airflow velocity and also the rotor speed. This enables an infinitely variable separation to be made within the limits of the classifier.

In a combination system, elutriation is used as a pre-separation method to improve effectiveness in the vortex. The rotor and outlet are positioned at the top of the housing. Air flows from below, to the outlet, passing through the feed material. Coarse particles fall with gravity and are discharged through the bottom. The finer particles are carried to the rotor section where accepted particles pass through the rotor to the outlet and rejected particles fall along the outer walls to the bottom discharge. The secondary air source (free vortex) picks up any acceptable fines that are settling out with the coarse, and recycles them to the rotor.

High-energy dispersion is a very effective variation on the vortex-elutriation design. High velocity air at the tip of the rotor vanes disperses or washes the fines from the coarse before the material enters the vortex. There is no opportunity for the fines to re-entrain or re-agglomerate with the coarse before the separation is effected. This design significantly increases not only the efficiency of the process, but also the fineness of separation and the sharpness index. It is usually specified for low-micron separations and fractioning.

The benefits of classifications include the following, all of which enhance the economics of the process:

• reduced load on the grinding unit
• better efficiency
• possible production of many grades of powder from the same feedstock
• capability of producing finer products with steeper particle size distributions from the same grinding step

New products may be classified for the value-added segment of the powder processing market, increasing the market value of a powder subsequent to processing.

In dry processing, sieving has reached the limits of its capabilities. No longer is it capable of producing the required particle size for a new generation of functional materials that require particle sizes below 5 m m. However, safety screens remain in use in downstream final grade lines, to prevent infrequently occurring oversize particles from contaminating the product, and for multi-separations in coarser particle size ranges.

Cyclone separation does not give sufficiently accurate particle size separations (poor steepness of cut), and because of its limited control parameters (which preclude the production of a wide range of different product grades), this too is not used as a primary cut operation.

Classifier types

There are several different types of air classifier with capabilities ranging from very coarse to extremely fine. Each unit depends on elutriation or centrifugal force or both, and can generally be categorized as:

Technology		Size range (µm)		Typical Application		Advantages		Disadvantages
Gravity air classifier		>1000		De-dusting, contaminant removal		No moving parts		Limited control
Cyclone classifiers		20 - 300		Powder coatings		Improved performance over static cyclone		Low performance cut sharpness
Spiral separators		3 - 80		Contamination free processing		Flexibility		Poor cleanability
High energy dispersion classifiers		<5		Fines removal for toners Various polymers		High aspect ratio materials		Energy consumption
Turbine classifiers (Single and Multiple wheel)		5 - 150		General purpose classification including minerals, chemicals, polymers, pharmaceuticals		Good general purpose performance and flexibility		Cleanability can be an issue
Next generation classifiers		<5		High capacity general purpose		Improved cut point and reduced energy usage		Developing technology with limited track record

Air classifier performance

Air classification that achieves cut sizes of a few microns typically follows Stokes' law. To study parameters that limit the air classifier's performance and determine the unit's theoretical cut-size, the following equations can be derived in the Stokes flow region:

.(1)

where x_t is the theoretical cut-size, µ is the air viscosity, v is the airflow velocity, p_s is particle density, and g is gravitational acceleration.

And:

.(2)

where v_r is the radial air velocity, r is the separation radius, and v_p is the particle's tangential velocity.

Equation (1) describes the theoretical cut-size based on elutriation. Increasing or decreasing the classifier's air-flow velocity (v) adjusts the cut-size. Equation (2) describes the theoretical cut-size based on centrifugal cut-size based on centrifugal-free and forced vortex airflows.

To evaluate the air classifier's cut-size and sharpness, construct a grade efficiency curve that plots size selectivity (n_D) versus particle size (D). You can calculate the relationship by analyzing the particle size distributions of the feed and final product to determine what percentage of a particle size in the feed goes into the coarse fraction.

Size selectivity is defined as:

The cut size (x₅₀) is the particle size corresponding to n_D =0.5 on the grade efficiency curve.

Cut sharpness is determined by intersecting the curve with the n_D =0.25 and n_D =0.75 lines and placing the particle sizes at the line intersections in relationship to each other.

Cut sharpness (x₂₅ /x₇₅ ) is often used to quantify air classifier performance. Cut sharpness values range from 0.0 (almost no classification) to 1.0 (ideal but not achievable classification). In a production operation, an air classifier's cut sharpness ranges between 0.3 and 0.7. At a low classifier load, the cut sharpness can reach 0.9. A good classifier has a wide adjustable range and can achieve a very fine cut size and high cut sharpness. You can also spot some undesirable classifying phenomena (such as material loss, recycling, grinding, poor dispersion, agglomeration, feed splitting, or overloading) in the classifier's operation by examining the deviation of the unit's grade efficiency curve (which passes through 0.0 at the finest particle size and 1.0 at the coarse end).

Note: x₅₀ is the equiprobable cut size; that is the particle size corresponding to the 0.5 size selectivity value. x₂₅ is the particle size corresponding to the 0.25 size selectivity value. x₇₅ is the particle size corresponding to the 0.75 size selectivity value.

References

[1] "Particle size - an introduction", A. Rawle
[2] "Sizing up air classifiers", L. Hixon, AIChE 1991
[3] "Air classifiers: How they work and how to select one", C. C. Huang, PBE March 1998
[4] "Review of air classifiers", H. Prem, AIChE 1990