Calcium carbonate: evaluating particle size and surface area
Calcium carbonate is used in various fields, as an additive including the food industry for instance, as a result of its whitening and anti-acid (code E170) properties, as well as a natural source of calcium for the body.
Two different industrial processes are used to manufacture calcium carbonate: the grinding of natural calcium carbonate (Ground Calcium Carbonate, GCC) extracted from calcite deposits, or precipitation (Precipitated Calcium Carbonate, PCC) through chemical synthesis (1). PCC is preferred in the food sector because of its higher purity and surface area, the latter being essential to improve its absorption efficiency. Moreover, since it is the result of a synthesis process, it is possible to control its size (generally around one micron) and crystalline form more effectively, ensuring greater product homogeneity (2). PCC is produced using several methods, including the Solvay process (where it is considered an intermediate by-product) and carbonation reaction (3). Limestone from natural deposits is used in both methods as the initial reagent. Finally, following synthesis, PCC can be coated with additives (fatty acids, resins, surfactants) to improve its wettability and ease of dispersion (1).
Characterising the product obtained in terms of size, morphology and surface area is a fundamental step for both the producer and the end user.
In this study, we analysed food-grade calcium carbonates from different manufacturers, evaluating particle size and specific surface area. The particle size distribution was analysed using the Malvern-Panalytical Mastersizer 3000 laser diffraction system. At the same time, the specific surface area was analysed using the nitrogen physisorption technique with the Micromeritics Tristar II.
The following is the overlapping of the particle size curves obtained from analysing the various samples using the Aero S dry dispersion system (figure 1) and the overlapping of the adsorption isotherms (figure 2) that report the amount of nitrogen adsorbed by the samples in the relative 0.05 -0.3 P/P0 pressure range.
Figure 1: The overlapping of the particle size curves
The table shows the volume-weighted average diameter D[4;3], a useful parameter for describing the particle size with a single number.
Table 1: volume-weighted average diameter
Figure 2: The overlapping of the adsorption isotherms
The table shows the specific surface area data (SSA) calculated using the Multipoint BET method.
The samples have different sizes; the samples with the smaller particles are samples 4 and 6, while the sample with the larger size is sample 1. A clear correlation appears between particles size and surface area, the area increasing with the decrease of particles size.
To conclude both techniques are deeply significant for Calcium carbonate powders characterization; however the particle size distribution using laser diffraction is a faster and widespread approach. Other properties such as the specific surface area, depend on particle size data, as demonstrated in this work. There are other factors that could influence this correlation, such as the product crystallinity, morphologies, surface textures and the presence of additives.
1) R. Kieffer and F. Benesovsky, Encyclopedia of Chemical Technology, in Calcium Carbonate, ed. B.M.t. Carbon, Interscience Publishers, New York, 1964, vol. 4, p. 410.
2) What is Ground Calcium Carbonate (GCC) or Limestone?, & What is PCC, Available from: http://www.specialtyminerals. com/our-minerals/what-is-gcc-limestone/.
3) H.-P. Mattila and R. Zevenhoven, Production of precipitated calcium carbonate from steel converter slag and other calcium-containing industrial wastes and residues, Advances in Inorganic Chemistry, ed. R. van Eldik and M. Aresta, 2013, pp. 347–384.