Powder caking

1. What is powder caking ?

Do you have a powder caking problem ?

Powder caking, sometimes called powder clumping, is the physical transformation of a bulk powder from the free flowing state to a partial or total solidification, and with different degrees of extent. The caking of a powder can very weak, the powder being broken down in the subsequent process stages, which is not too problematic, or can be much harder, with the lumps being persistent and causing some operational problems in factories, or even consumer complaints if the material has been conditioned for being sold. Caking is experienced with all sorts of powders provided the conditions are favorable, for instance sugar but also salts or detergents.

The whole bulk powder can cake or only part of it with some lumps being randomly found. In any case a physical phenomenon, which can be of various nature, is holding the once free flowing powder particles together. The objective of this page is to review what could be the root causes explaining powder clumping and what can be done to solve the issue.

Note that other flow issues exist, showing different symptoms but are somehow less common and not detailed in this page. Among this issue, we may highlight the phenomena of flushing, which is actually the exact contrary of the problems listed above : the fluidization of the powder ends up in an uncontrollable flow outside of the silo.

2. Root causes of powder caking ?

Why do powder cake ? Analysis of the main causes of caking of powders

There are many possible root causes to explain the lumping of powder [Zafar], they can be from mechanical, chemical, plastic-flow or electrical origin. In each category there is a multitude of phenomenon that can lead to powder caking at diverse degrees, but the most important phenomena found in the industry are listed below.

2.1 Solid bridges

The strongest link in between particles, leading to a very persistent caking phenomena is to have a solid bridge in between particles. Following this phenomena, the particles are actually merging which means that the cake is not anymore formed by individual particles loosely connected but is actually a single particle.

Such solid bridges can be created due to a mechanical action in between particles : the particles are submitted to a stress, long enough so that a solid bond is formed. It is however reported that such a bond is quite weak.

A stronger link can be formed through solvent evaporation. If a solvent is present in sufficiently high quantities so that it can dissolve part of the material and form bridges in between particles, then, when evaporating, the dissolved solid will end up in between particles creating a solid bound.

2.2 Liquid bridges

Liquid bridges are one of the most common root causes of powder caking, and one of the most instinctively understood phenomena as it is very often involving water (humidity). In this mode of caking, a liquid is present on the surface of the powder, it fills at 1^st some porosity but above a certain threshold it can start creating bridges in between the particles. The bridges then hold the particles together and create a lump. This kind of bridge in between particles is not that strong and can only happen if the particles are small enough and the liquid proportion is high enough.

To give some perspective, [Zafar] mentions that the capillary force that defines the strength of the liquid bridge can be calculated for particles < 1 mm in diameter, while [Modugno] is given for lactose particles a threshold of d50<400 microns and / or water content > 3%.

2.3 Glass transition

Glass transition is a phenomenon that is happening with amorphous materials. Past a certain temperature, the glass transition temperature, the material will soften, becomes sticky and thus tends to cake. It must be noted  that the glass transition temperature is heavily influenced by the humidity of the material. Water acts indeed as a plasticizer which means that it will lower the glass transition temperature.

For amorphous material it is important to be able to determine the glass transition temperature as a function of the moisture content of the material. It allows then to verify at which humidity the glass transition temperature may be below the processing, or storage temperature, in such an event, the material will soften and starts to cake.

2.4 Measurement

Sorption isotherm, shear cells testing, differential scanning calorimetry (DSC) can be used in order to confirm the root cause of caking, measure the extent of caking, and try to establish specifications for operating parameters where lumping will not happen.

• Sorption isotherm : this measure allows to determine the moisture content of a powder as a function of the water activity (= Relative Humidity). The shape of the isotherm can already give some information about the mechanisms at play in caking (for example, it can show some capillary condensation at some stage, sign of creation of liquid bridges)
• Shear cells / powder rheometer : these instruments can help to determine how severe is the caking (is a lot of shear required to break the lump) and can also give some information on the kinetics of caking (for example a sample can be measured after different time of exposure to humidity, load, temperature...)
• Differential Scanning Calorimetry : this measure is relevant for materials caking due to the crossing of their glass transition temperature. The Tg can then be measured at different humidity (moisture content) and the data can then be coupled with the sorption isotherm in order to determine a variation of the Tg as function of the relative humidity. This can help checking if in certain process conditions for which the relative humidity and the temperature are know, if the material is at risk of caking.

3. How to solve powder caking ?

Solutions to the caking of powders

How to prevent caking of bulk solids ?

The solution to a powder clumping problem is actually dependent on the phenomena that is creating the lumping of the bulk solids. However, in practice, for factory operators, it is difficult to quickly identify such a root cause. The following solutions (proposed by [Zafar])  are thus to take more as indication and either a deeper study must be done, or some trials and error testing can be done to identify an efficient solution to powder caking.

• Have a product less fine / have less fines in the PSD (as mentionned above, small particles are more prone to caking than bigger ones)
• Reducing moisture content (this will avoid creation of liquid bridges, formation of solid bridges if the water dissolves some product before evaporating, and will avoid to lower the glass transition temperature of some materials)
• Remove from the composition the major caking contributor
• Avoid temperature and humidity cycles (this will avoid creation of liquid bridges, formation of solid bridges if the water dissolves some product before evaporating, and will avoid to lower the glass transition temperature of some materials)
• Reduce the consolidation load which is often an aggravating factor
• Add an additive, typically a flow agent that is also providing some anti caking activity : such additives can help sucking humidity for example, or can help in avoiding the contact in between particles by being in between.

Note that one particular case for additives is in the case of spray drying, the addition of a high molecular weight component (such as maltodextrin) will help raise the glass transition temperature (Tg) of the component being sprayed.

If not possible to prevent bulk powder caking, one can then think of mitigating the effect of lumping in the process. Some possibilities to think about are [Johanson]:

• Reducing the storage time : caking is not an instantaneous phenomenon, it takes some time (very variable depending on the material and the mechanisms involved), thus it is possible to reduce the risks of caking in making sure the material is not staying for too long in a hopper
• Avoiding dead areas in hoppers : mass flows hoppers will be less prone than funnel flow for caking, although it may not be sufficient to avoid totally caking
• In some cases, for the materials whose caking mechanism is related to moisture, it is possible to condition the inside of a silo with dry air in order to avoid water to reach an activity sufficient to create solid or liquid bridges, or reduce the Tg. In the case of solid bridges and influence on Tg this can be coupled by a control of the temperature for example by insulating the silo, thus avoiding cycles of temperature and humidity
• If the caking starts to happen but is not leading to a hard material, some discharging aids like fluidizing pads, vibrating bottom can help to overcome the shear rate required to break the lumps and help the product flow

Source

[Zafar] A review of bulk powder caking, Zafar et al, 2017, Powder Technology
[Johanson] Understanding and solving material caking problems in dry bulk storage vessels, PBE, 2014