At the powder scale, the anticaking agents behave more or less as
expected. The dosage required for good anticaking corresponds well
to the industrial experience. For ferricyanide and Fe-mTA (at pH 4.5),
the anticaking effect is found to decrease at lower dosage. For ferrocyanide
and Fe-mTA, we found that increasing the dosage above the
industrially applied amount (1 ppm and 5 ppm respectively) does not
improve the anticaking effectivity significantly. Also ferricyanide is
found to be a good anticaking agent, only slightly weaker than
ferrocyanide.
The fact that Fe-mTA is not effective at pH 2 is probably caused by
a difference in the complexation, since the colour of the solution
changes from yellow (pH 2) to green (pH 4–5). The colour of the
Fe-LTA and Fe-LDTA also changes, namely from yellow (pH 2) to red
(pH 4–5), but both complexes are almost inactive as an anticaking
agent at each pH condition. The relation between the complexation
of the iron tartrates and their anticaking activity will be elaborated
elsewhere [27].
At the two-crystal scale, the results deviate from those at the
powder scale. Ferro- and ferricyanide do reduce the caking bond
strength, but they require a much higher concentration than in the
powder scale experiments: 10–20 ppm versus 1 ppm at the powder
scale. Fe-mTA does not seem to work at all. This difference can be
explained from the surface roughness, since it increases the number
of contact points between the crystals. The more contact points,
the stronger the crystals will cake [28]. At the powder scale, the
number of contact points/lines will be approximately one per side
face, as the crystals are expected to form mainly vertex/edge-surface
contacts. This also explains that the particle size distribution is very
important in preventing caking, since small particles will have the
same amount of contact point per particle, but more contact points
per unit of mass.
In the two-crystal experiments, the crystal surfaces are in nearperfect
contact and therefore will have much more contact points,
depending on the surface roughness. The surfaces roughened by the
anticaking agents ferro- and ferricyanide will have contact points at
Therefore, even more contact points are present than in the case
of ferro- and ferricyanide, requiring an even higher concentration of
Fe-mTA. Unfortunately, this amount is above the solubility of Fe-mTA
in water at pH 4.5, sowewere not able to apply Fe-mTA in this amount