ANTI-CAKING AGENTS FOR SODIUM CHLORIDE AS A ... - Aidic

ANTI-CAKING AGENTS FOR SODIUM CHLORIDE AS A CRYSTAL
GROWTH INHIBITOR: INTERACTIONS STUDIED USING VARIOUS
TECHNIQUES
A.A.C. Bode a , V. Vonk a , D.J. Kok a , M. Steensma b , S. Jiang b , J.A.M. Meijer b ,
W.J.P. van Enckevort a , E. Vlieg a
a
IMM Solid state chemistry, Radboud University Nijmegen, Nijmegen, the Netherlands
(a.bode@science.ru.nl)
b
AkzoNobel Industrial Chemicals, Salt and Crystallization, Deventer, the Netherlands
Keywords: Crystal growth, nucleation, agglomeration
When left untreated, powders of sodium chloride (NaCl) will cake. Caking causes
problems when handling large quantities and must therefore be prevented. An anticaking
agent is applied to prevent caking. One of the traditional agents used in industry
is potassium ferrocyanide. Although the influence of this anti-caking agent on the
crystallisation of NaCl was investigated in detail in 1965 [1] , the fundamental interaction
between the ferrocyanide ion and the sodium chloride crystal surface remained
unknown. Anti-caking agents are generally assumed to be crystal growth inhibitors,
preventing crystals from growing together and agglomerating [2] .
Recently, Akzo Nobel, a large salt producer in the Netherlands, introduced a new anticaking
agent for sodium chloride: iron(III)-meso tartaric acid (Fe-mTA) [3] . The anticaking
properties of this agent are similar to those of ferrocyanide. It has the advantage
that iron can easily be removed from a salt solution prior to electrolysis. Ferrocyanide
cannot be removed by ion-exchange filtration nor by adding lye, so the remaining iron
causes degradation of the electrodes and deposition of Fe(OH)3 in and/or on the
membrane, leading to a rise in energy consumption and maintenance costs. Another
advantage of Fe-mTA is that it does not contain nitrogen, which means that the
explosive nitrogen trichloride (NCl3) is not formed during electrolysis.
We investigated the influence of Fe-mTA on the crystal growth of NaCl, studying its
influence on crystal morphology, critical supersaturation for nucleation and growth rate.
We also determined the amount of anti-caking agent incorporated into the crystal. Like
other anti-caking agents [1,2] , Fe-mTA retards crystal growth and changes the
morphology of NaCl crystals. Interestingly, it does not inhibit nucleation, as we
observed no significant critical supersaturation (figure 1), whereas all other known anticaking
agents for salt do show nucleation inhibition [1] .
We used several techniques to determine the fundamental interactions between these
anti-caking agents and the sodium chloride crystal surface. In the literature, two
mechanisms have been proposed for the function of anti-caking agents during
crystallisation. In one mechanism, the anti-caking agent forms an insulating monolayer
on the crystal surface, which prevents the crystal from growing or dissolving [4] . In the
other mechanism, individual or small clusters of molecules of the anti-caking agent

Figure 1. Critical supersaturation for nucleation of NaCl crystals as a function of the initial
concentration of the anti-caking agents (ferrocyanide and Fe-mTA). The boxes show the morphology of
the NaCl-crystals caused by Fe-mTA.
adsorb onto the crystal surface at step edges. The adsorbed molecules slow down step
flow and retard crystal growth [5] .
We studied the influence of the anti-caking agents ferrocyanide and Fe-mTA on the
growth of NaCl crystals by monitoring the step flow of monatomic steps on the {100}faces
of these crystals using atomic force microscopy (AFM). Without anti-caking
agents present, monatomic steps on these crystals are known to become mobile at a
relative humidity of 60% [6] . We confirmed these findings and investigated the influence
of various concentrations of the anti-caking agents on the movement of steps. Figure 2
shows two AFM pictures of step movement on a blank (100) surface.
Figure 2. AFM images of step movement on a blank sodium chloride (100) surface: a) step position at t =
0. b) step position at t = 4 hours. Step movement rate approximately 250 nm/hour.

No step movement is observed when an anti-caking agent is applied. By finding the
minimum amount needed to stop the step movement, we can determine if a full
monolayer of the anti-caking agent is needed or if dispersed single molecules can retard
the step movement.
We used surface X-ray diffraction (SXRD) to determine the chemical interaction
between the ferrocyanide ion and the sodium chloride crystal surface. This technique
can be used to detect atoms adsorbed on a crystal surface. In 2004 this technique was
used to determine the structure of the adsorbed water layer on NaCl [7] . We measured
data sets for a blank (100)-NaCl crystal surface and a (100)-NaCl surface treated with
ferrocyanide. Results show that the ferrocyanide ion, Fe(CN)6 4- , replaces a NaCl6 5-
cluster at the surface. An occupancy of 30% to 50% of a full monolayer was found,
indicating that ferrocyanide does not pin steps. Instead, a (partial) monolayer is needed
to block crystal growth. The ferrocyanide ions have to be removed for growth to
continue. Otherwise, they have to be overgrown, leaving vacancies in the crystal lattice
to counter the charge difference between the cluster and the ferrocyanide ion. This
blocks further growth and explains its anti-caking activity. New experiments are
planned to measure diffraction data of Fe-mTA on the sodium chloride crystal surface.
Finally, we want to determine if the anti-caking agents adsorb preferentially at step or
kink sites on the surface or if they adsorb on the whole surface. To study this, ToF-
SIMS experiments will be performed in imaging mode. This technique can detect iron
atoms with a very high resolution. These measurements are underway.
Combining these different surface science techniques, we acquired a picture of the
fundamental chemical interactions between the anti-caking agents and the crystal
surface. These mechanisms are not yet well understood. However, understanding these
interactions will help to develop anti-caking agents systematically, replacing the trial
and error methods applied up to now.
Acknowledgements:
We thank the Dutch ministry of economic affairs (EOS-KTO programme, Agentschap
NL) for funding.
References:
[1] van Damme-van Weele, M.A. Influence of additives on the growth an dissolution
of sodium chloride crystals, Ph.D thesis: Enschede, the Netherlands, 1965.
[2] Yeong-Lin, C.; Jane-Yu, C. Selection of anti-caking agents through
crystallization. Powder Technology, 1993, 77, 1-6.
[3] Geertman, RM. Use of carbohydrate-based metal complexes in non-caking salt
compositions; US patent WO 00/59828: Arnhem, the Netherlands, 2006
[4] Sangwal, K. Additives and Crystallization Processes, Wiley: New York, 2007
[5] Cabrera, N and Vermilyea, D.A. Growth and Perfection of Crystals, Wiley: New
York, 1958.
[6] Shindo, H.; Ohashi, M.; Baba, K.; Seo, A. AFM observation of monatomic step
movements on NaCl(001) with the help of adsorbed water. Surface Science 1996,
357-358, 111-114.
[7] Arsic, J.; Kaminski, D.M.; Radenovic, N.; Poodt, P.; Graswinckel, W.S.; Cuppen,
H.M.; Vlieg, E. Thickness-dependent ordering of water layers at the NaCl(100)
surface. Journal of Chemical Physics 2004, 120, 9720-9724.