Marcel Donnet

New morphology of calcium oxalate trihydrate precipitated in a segmented flow tubular reactor

Reference LTP-ARTICLE-2000-007doi:10.1023/A:1006771428827View record in Web of Science Record created on 2005-01-12, modified on 2017-05-10

High-quality nickel manganese oxalate powders synthesized in a new segmented flow tubular reactor

Abstract High-quality nickel manganese oxalates have been prepared using an innovative approach for the production of homogeneous powders, the continuous Segmented Flow Tubular Reactor (SFTR). This new reactor is mainly composed of a mixer, a segmenter, a tubular section and a decanter. Mixed Ni–Mn oxalates are synthesized in the SFTR. The powders present controlled morphology, narrow particle size distribution, high purity and desired stoichiometry. Their characteristics are compared to those of powders obtained in a batch reactor. These oxalates are precursors for nickel manganites, used as negative temperature coefficient thermistor (NTC) ceramics.

Contribution of aggregation to the growth mechanism of seeded calcium carbonate precipitation in the presence of polyacrylic acid.

Our work investigates the precipitation mechanism of a seeded calcium carbonate reaction, by using cryogenic TEM to observe the early stages of the reaction. The early precipitation of a hydrated phase is proposed as an intermediate phase before transformation into calcite. Thermodynamic modeling in conjunction with pH, surface potential measurements, and colloidal stability modeling demonstrate that calcite growth is dominated by agglomeration. This is in agreement with the cryogenic TEM observations, which suggest oriented attachment dominates early aggregation. The final stage of the reaction is described by a ripening mechanism that is significantly inhibited when high concentrations of polyacrylic acid (PAA) are used. The different concentrations of PAA lead to significant differences in the final particle substructure observed using cross section TEM. At low PAA concentrations, single crystal particles result, coherent with the proposed early oriented attachment mechanism and interfacial energy calculations. A core shell model is proposed for high PAA concentrations, whereas internal ripening of nanosized pores has been observed for low PAA concentrations, suggesting trapped solvent during the rapid initial particle formation at the relatively high supersaturations (S = 30) investigated.

A thermodynamic solution model for calcium carbonate: Towards an understanding of multi-equilibria precipitation pathways

Thermodynamic solubility calculations are normally only related to thermodynamic equilibria in solution. In this paper, we extend the use of such solubility calculations to help elucidate possible precipitation reaction pathways during the entire reaction. We also estimate the interfacial energy of particles using only solubility data by a modification of Mersmanns approach. We have carried this out by considering precipitation reactions as a succession of small quasi-equilibrium states. Thus possible equilibrium precipitation pathways can be evaluated by calculating the evolution of surface charge, particle size and/or interfacial energy during the ongoing reaction. The approach includes the use of the Kelvins law to express the influence of particle size on the solubility constant of precipitates, the use of Nernsts law to calculate surface potentials from solubility calculations and relate this to experimentally measured zeta potentials. Calcium carbonate precipitation and zeta potential measurements of well characterised high purity calcite have been used as a model system to validate the calculated values. The clarification of the change in zeta potential on titration illustrates the power of this approach as a tool for reaction pathway prediction and hence knowledge based tailoring of precipitation reactions.