Brightness is the main quality parameter in the production of
ground calcium carbonate (GCC) used as filler in the paper industry.
By spiking a high quality calcite concentrate with various
iron-bearing contaminant phases prior to micronisation and subsequently
performing spectrophotometric measurements on the
resulting slurries, it was found that even very small amounts of
pyrrhotite and pyrite have a significant detrimental effect on the
brightness of the product. Product quality is compromised at contaminant
concentrations as low as 0.01–0.05 wt%.
Pyrrhotite produces a more bluish tint and reduces the brightness
of the slurry more than pyrite. The latter is probably due to
pyrrhotite’s lower inherent optical reflectance, but possibly also
due to smearing effects and differences in grindability. The brightness
is reduced as the contaminant phase becomes more finely distributed
in the calcite slurry. This factor suggests itself as the main
explanation for the deviating results found when adding magnetite.
In spite of having an inherent optical reflectance that is lower
than those of pyrite and pyrrhotite, magnetite produces a signifi-
cantly lower reduction in brightness. Results from grindability
tests strongly indicate that the sulphide phases will become more
finely distributed during micronisation.
The results show how the brightness of the slurry depends on a
number of factors, and that the colouring properties of the contaminant
phases vary. As a consequence, facing ever stricter quality
requirements, GCC producers will undoubtedly benefit from establishing
site-specific empirical data bases describing the optical response
of the various contaminating phases in their raw materials