Abstract
The reaction of citric acid with calcite was investigated using the rotating disk apparatus. The effects of disk rotational speed, system pressure, and presence of magnesium and ferric ions on this reaction were examined. Scanning electron microscope (SEM) was also used to characterize the surface of the calcite disks at the end of each experiment.
The reaction of citric acid and calcite is mass-transfer limited up to 500
rpm. The reaction rate of citric acid–calcite is limited by the precipitation of calcium citrate on the surface, especially at atmospheric pressure. Increasing the system pressure from 1000 to 1500
psi has no significant effect on the dissolution rate of calcite. The diffusion coefficient of 7.5
wt% citric acid in the presence of calcium citrate and calcium ions is
4.5
E
-
6
cm
2
/
s
at
50
∘
C
and 1000
psi. The presence of magnesium ions in citric acid solutions results in higher concentration of calcium ions in solution.
Calcium citrate forms only on the disk surface, but not in the bulk solution. The morphology of the calcium citrate layer is dependent on the rotational speed of the calcite disk. More calcium citrate precipitation occurs at high disk rotational speeds, above 500
rpm, and at system pressures less than 100
psi. Calcium citrate appears to precipitate as feathery aggregates of radiating platy crystals on the surface of calcite.