An integrated study of CaCO3 formation and inhibition

The management of mineral scale is a challenge in many industrial fields and environmental issues relating to eutrophication are leading industry to develop
environmentally-friendly solutions to scale control. The determination of the strategy of inhibition is a real challenge as many factors influence the formation of calcium carbonate formation and affect the performance of scale inhibitors.

The processes of CaC03 formation on a surface have been studied. The mechanisms of inhibition of three green (Carboxymethyl Inulin, CMI, Polymaleic acid, PMA and
Polyaspartic acid, PA) and one conventional (Polyphosphinocarboxylic acid, PPCA) scale inhibitors of calcium carbonate nucleation and growth have been investigated. The interactions between the chemicals and the surface have been studied by chronoamperometry.
The use of surface analysis enabled these interactions to be characterised. The in-situ electrochemical flow cell permitted the characterisation of the crystals formed (size, number and surface coverage of the electrode) and the affinity of the chemicals to inhibit the nucleation and/or the growth to be determined. Important information on the different effects of scale inhibitors on CaC03 polymorphs has been found by use of Synchrotron X-Ray
Diffraction (SXRD) as an in-situ method for crystal characterisation.

The inhibitors partly inhibit the deposition of CaC03 by forming a film on the deposition surface. The interactions between the inhibitors and the metal surface occur via the
cations Ca2+ and/or Mg2+. The adsorption of the chemicals on the crystals ofCaC03 has been investigated and it appears that the PPCA, PMA and PA act by blocking the growth sites of the crystals. Each inhibitor exhibits an affinity to the surface of deposition and with the
CaC03 crystals but to a different extent resulting in different inhibition efficiencies. The PPCA affected greatly the deposition of CaC03 and PA and PMA exhibited similar effects. Nevertheless the PA is more efficient when it is present during the nucleation step or early
crystallisation whereas PMA acts more during the growth process. The SXRD permitted three steps of growth to be detected in the non-inhibited case and PMA, PA, CMI act by reducing the growth of the different planes whereas the PPCA totally inhibited two main crystal planes
(the aragonite (032) and the vaterite (300» and an acceleration of the growth of the calcite
C(006) and of the aragonite A( Ill) was observed.