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|1. What is powder
|2. Root causes of
powder caking ?
|3. How to solve
powder caking ?
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.
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.
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.
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.
Liquid bridges are one of the most common root causes of powder caking, and one of the most instictively 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 1st 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 brigde 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%.
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.
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.
The solution to a powder caking 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.
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]:
[Zafar] A review of bulk powder caking, Zafar et al, 2017, Powder
[Johanson] Understanding and solving material caking problems in dry bulk storage vessels, PBE, 2014