Powder that ‘cake’ present serious problems in bulk storage and handling systems. The most common cause is moisture migration due to temperature changes with materials that are soluble. Caking creates resistance to flow in bins, hoppers and silos and can result is build-up of residue on walls and the surfaces of equipment. Not all materials are affected, and some are much more vulnerable than others, but steps can usually be taken to avoid conditions that develop caking processes or disturb the effects. Several mechanisms cause bulk materials to ‘cake’ by forming strong bonds between the constituent particles.
The phenomenon should not be confused with cohesive, electrostatic, surface tension forces and interlocking characteristics that may develop strong bulk strength, but are not rigid or brittle. Caking is essentially related to the nature of the contact surfaces involved and how these develop firm continuity between the constituent particles.
The basic methods to counter caking are: -
- Avoid process conditions that allow caking mechanisms to develop
- Modify the material to prevent caking
- (as a last resort), Disturb the caked structure
Avoiding caking conditions means controlling the site conditions such that the active mechanisms that cause caking of the particles are frustrated. These mechanisms include: -
- Chemical processes, Such as oxidation and hydration
- Thermal processes, Causing sintering or fusion of contact surfaces.
- ‘Adhesive’ processes Due to binding agents, such as solvents or oil coatings drying out.
- Particle deformation Elastic or plastic media that forms close contact, flat surfaces that molecularly bond.
- Crystal growth: Water-soluble materials or chemicals or chemicals that dissolve in solvent vapours can form crystals bridges on moistened surfaces between points of particle contact.
Typical products: Sugars, salts, fertilisers, gravy granules, acids, pigments, detergents, soup and food mixes, plaster, cement, starches, whey powders, egg, mixes, flour, milk powder, coffee, flavourings, spices, chocolate, powders, powdered drink mixes, cosmetics, seasonings and materials, soluble in water or solvents.
Oxidation: Oxidation is sensitive to small moisture changes and increases with temperature. The rate increases also with time
Typical Products: Materials that are ferrous based or contain copper, aluminium, magnesium, carbon or coal.
Hydration: Minerals hydrate by converting an oxide to a double hydroxide or incorporation water molecules directly into the crystalline structure of a new mineral, as in the hydration of feldspars to clay minerals, garnet to chlorite or kyanite to muscovite. Some mineral structures, for example, montmorillonite, are capable of including a variable amount of water without significant change to the mineral structure.
Typical Products: Portland cement, phosphates
Precautions: Ensure dry material remains dry and not exposed to humid or vapour laden atmosphere, conditions that promote condensation or moisture ingress. Keep moist materials moist to prevent the particle surfaces from drying. Use dry air for pneumatic conveying, Avoid extended storage and exposure to moisture, heat and temperature variations Use Mass Flow hoppers, avoid static storage areas, promote bulk deformation, if practical; e.g. by cross inserts in parallel bodies of hoppers. Prevent condensation and moist atmospheres.. Inert the local atmosphere with nitrogen, carbon dioxide or argon for long-term storage.
Prevention: Use anti-caking agents, such as stearates, talc, dicalcium phosphate, calcium silicate, fumed silica, sodium aluminosilicate, potato starch, kaplin and micro crystalline collelde.
Or: store in dried or inert atmosphere and avoid temperature changes.
Or: Enlarge particle size and/or change particle shape to reduce bonding areas.
Drying: Dried bulk products are rarely completely devoid of moisture because, even if the constituent particles are dry, air in the voids tends to have high humidity. As the material cools, moisture is expelled from the combined gas of air and water vapour to condense on the nearest colder surface. Drying damp products can also activate thermal softening of the particles to deform and present surfaces in close proximity to bond or sinter. The presence of oils or salts can develop sticky firm contacts or lead to crystal growth. Cohesion can increase many orders of magnitude during drying. Heat and exposure to dry air exacerbates such caking.
Precaution: Prevent condensation or provide a local, non-contact, condensing surface with drainage to remove excess water.
Elastic deformation: Plastic or elastic changes allow contact points to become flattened. This attains a much more stable structure and molecular and electrostatic forces become significant between the close surface proximities.
Typical products: pressure sensitive materials in the form of flakes, stringy lengths or thin chips of polymers, elastomers and noodle-like extrudates.
Viscous deformation:
Typical products: Pitch, adhesives and elastomers that soften with temperature and applied pressure.
Precautions: Minimise solids contact pressure with inserts and design to promote mass flow, eliminate temperature fluctuations and avoid impact loading of susceptible materials. Allow expansion at extraction point; Decrease storage temperatures below the critical softening temperature; Limit storage time and re-circulate stored material.
Oil deposits:
Physical process: High temperatures can draw out natural oils in particles. A decrease in temperature can allow the material to dry and stick particles together.
Typical products: Materials containing natural oils such as soybean meal, or products with oil content, such as cakes mixes.
Precautions: Reduce storage temperatures, promote interparticle motion during storage and design vessels to promote flow at the walls to prevent hang-ups.
Solvent fusion: Chemical activity that fuse contact surfaces.
Typical products: Adipic acid and other loose solids derived from a slurry process and subsequently dried.
Precautions : Dry the product thoroughly and agitate during cooling. Prevent temperature fluctuations until the solvent is completely dissipated.
Release of chemically bound water:
Physical process: Some materials will hydrate with moisture from the atmosphere. If chemically bound water migrates to the surface of the particles and the temperature drops, the water can combine with the material and form a strong bond between particles
Typical products: Gypsum, limestone, Kernite and materials that have hydration temperatures close to room temperature.
Precautions: Keep the material dry, convey in dry air and do not place hot material in cold storage vessels. Purge storage vessels with dry air or nitrogen and avoid temperature cycles that transverse hydration temperatures.
There is no guaranteed way to predict exactly how any one bulk solid will behave based on associated experience alone. Even research and development, process calculations and pilot plant studies may not prevent material handling problems in production systems subject to variability in product, production and environment. The only effective way to predict and avoid processing problems is by measuring the material’s flow properties at simulated plant conditions and using this data to design or recommend equipment. This means duplicating temperature, storage time, chemical reactions and solids contact pressures, while taking into consideration the construction materials used in the process or storage vessel.
Mechanical disturbance of caked products
A mass of caked bulk material usually create a firm block that is difficult to break up. The bonds are weaker at an early stage of caking and these can be disturbed by flow that induces shear into the bulk. Note that shear does not normally occur in a mass flow hopper until the material reaches the converging region, but then the total cross section is put under stress by the superimposed mass.
These can place an exceptionally high load on the hip junction, whereas a protrusion from the wall at a slightly higher location can apply concentrated loads on portions of the boundary and encourage progressive breakdown of the mass. Higher stresses can be generating in ‘V’ shaped hoppers than cone shaped to upset light caking. However, regions that are caked at a smaller span have proportionally less gravitational force available to break the firm structure. Regions near the outlet are more susceptible to fracture by vibration if the outlet is unconfined. Mechanical agitators can incur extreme loads and are not recommended. Jenike and Johanson patented an alternating converging/diverging hopper shape to impart bulk restructuring of the mass every time that discharge occurs to disturb contact points that may have caked. Cross inserts in the body of the hopper can achieve a similar effect, but account must be taken of the high loads that may be involved to avoid damage or failure of the bin.
Anti-Caking additives:
Additives prevent caking by coating the host particles with smaller, inert ones that prevent intimate contact taking place between the particles prone to develop firm connections or absorb moisture that would lead to caking processes. The quantity that may be added to foodstuffs is constrained by regulations of the Ministry of Health and Welfare in terms of section 15(1) of the Foodstuffs, Cosmetics and Disinfectant Act 54 of1972 under Government Notice R.2507 in 1982 and amendments R.2354 and R.3408. Whereas the primary functions of these anti-caking agents are to separating susceptible particles or, either absorb moisture or prevent it from contacting the particle surfaces, wider objectives of these additives are to: -
- Improve flow and increase packaging rates, Aid the consistency of bulk density
- Reduce or prevent build up in spray drying operations. Reduce adhesion to contact
surfaces,
- Decrease Dusting
- Prevent the formation of lumps
- Absorb liquids, fats, and oils for powdered food applications
- Decrease clogging and bridging during production,
- May also be added to improve taste, texture, reduce mould, improve colour or
appearance
- Some anti-caking other organic solvents are soluble in water. Others are soluble in
alcohol or function, either by adsorbing excess moisture, or by coating particles and
making them water repellent.
Calcium silicate, (CaSiO3),is a commonly used anti-caking agent that is added to such as table salt. It absorbs both water and oil. Anti-caking agents are also used in non-food items such as road salt, fertilisers, cosmetics synthetic detergents, and in manufacturing applications.
Amongst the additives used to prevent caking are: -
Tricalcium Phosphate > Sodium ferrocyanide > Sodium silicate
Powdered cellulose > Sodium bicarbonate > Potassium ferrocyanide
Calcium ferrocyanide > Bone phosphate > Silicon dioxide
Calcium silicate > Magnesium trisilicate > Talcum powder
Sodium aluminosilicate Potassium > Aluminium silicate
Calcium aluminosilicate Bentonite > Aluminium silicate
Stearic acid > Polydimethylsiloxane > Powdered Rice
Magnesium stearate
