Class Particle Distribution Particle Size L/D Ratio Bulk Density Compressibility Angle of Repose Adshesive/Cohesive Property
Class I

Granular, Free Flowing

Uniform 1/16 – 3/4″ Small Uniform 5% or less 20° or less None
Class II


May or may not be uniform 20 – 200 mesh 1 – 2 5 – 15% 35° Slightly cohesive
Class III


Uniform 200 – 400 mesh 1 – 2 Slightly permissible 2 – 30% 30 – 40° Moderate
Class IV 


Uniform 100 – 400 mesh (149-37 micron) Agglomerated “Chains”, can be 4 – 5:1 20 – 30% or more 40° or more Adhesive
Class V

Fibrous, Flakey, Flocculent

Non-uniform 1/2″ – 1/8″ 3:1 or more Permeable Varies from 2 – 10% Varies upto 90° Interlocking tendencies


Class I — Granular, Free Flowing:  These are the easiest materials to store, discharge and convey. Very uniform particle size with consistent, low
length / diameter L/D ratio. Most often ” hard” granular shapes, from 1/16″ – 3/4″ ( 1.58 – 19mm) are not subject to degradation. They exhibit no discernible adhesive or cohesive properties. Uniform Bulk Density, Low Compressibility, ( less than 5%) and repose angle of 20° or less. Typical examples are plastic pellets, coated prills, silica sand, and aggregates, palletized /cured feed, ion exchange resin and dry salt.

Class II — Powder Sluggish:  These sluggish powders are comprised of individual particles which cannot be readily discerned. Running in size from 20 -200 mesh ( 841-74microns). May or may not be uniformly distributed, L/D ratio between 1-2. They posses sligh cohesive properties due to non – uniform irregular particle configuration. Slightly compressible ( 5 -15%) and repose angle upto 35°. Typical examples are baking flour, Limestone, Fluorspar, pulverized mineral ore, soda ash, fine silica sand, ground coke and medium grade sugar.

Class III — Powder Sluggish/ Fludizeable:  These powders ( 200 – 400 mesh / 74 – 37 micron ) are sluggish but fluidize able. Their slight permeability is defined as the degree to which air, or another gas, may be passed through the void spaces between the individual particulates of the material mass. Gas entrained during conveying or processing does not readily “percolate our” Gas retention capability can vary between zero to several hours, depending upon other physical properties. The entrained gas gives the powder a ” liquid” or fluidized appearance. L/D ratios of 1-2. The posses moderate cohesive properties which aid in gas retention, also possibly due to large surface area. Compressibility varies from 5 – 15% a natural repose angel of 30 – 40°. Typical examples : hydrated lime, cement, silica gel, starch, fly ash, clay, polymers, carbon black.

Class IV — Powder, Sluggish , Adhesive:  These sluggish, adhesive, or cohesive powders ( 100 – 400) mesh / 149-37micron) seldom exhibit particle segregation. The particle shape may be agglomerated or dendritic (having a branched, crystalline shape with the ” branches” extending from the faces of the main body). This particle configuration may cause both internal and external adhesion. L/D ratios of agglomerated ” chains” can be 4-5:1. Compressibility varies from 20 – 30% ( or more). Natural repose angle is excess of 40°. Class IV is often associated with electro-static charge potential. Typical examples are pigments (organic or inorganic) metallic oxides ( Ti02, iron oxide, zinc oxide, lead oxide, chromium / nickel oxide), centrifuge cake , fluorocarbons, sludges, vegetables products such as Soya bean meal, cottonseed meal, high fat content bakery products, calcium carbonate and dispersion resin.

Class V — Fibrous, Flake, Flocculent:  These materials are generally the toughest to handle due to their non-uniform particle size and shape, non – uniform length to dismeter ratios (in excess of 3:1)and their mechanical (physical) interlocking tendencies. These may be fibrous ( regular or irregular, “thread like” with flexible structure), flakes (“plate like”) flocculent (amorphous solids). Class V will interlock, mat or form large agglomerates. Particle size and shape exceeds 1/2″ X 1/8″ ( 12.7-3mm). Physical compressibility could vary from 2-10% highly permeable. The natural repose angle can vary, upto 90°. Typical examples are wood chips, sawdust, plastic regrind, asbestos fibers, fiberglass strand, chopped paper, steel/brass/aluminum chips and e.g. residues such as bagasse.