Solubility and mechanisms of precipitation of calcium oxalate in D (chlorine dioxide stage) filtrates

The apparent solubility of calcium oxalate in bleach plant D filtrates and infiltrate-free solutions, i.e. in solutions with no organic substance except oxalate ions, increased when the pH was decreased below 4, when the temperature was raised, and when the ionic strength and the magnesium ion concentration were increased. The apparent solubility, however was greater in D filtrates than in filtrate-free solutions. This was due to a difference in the composition of the precipitates formed. The calcium oxalate formed from D filtrates consisted of a mixture of the monohydrate and the dihydrate of calcium oxalate, whereas the precipitate formed from a filtrate-free solution consisted only of the monohydrate. The formation of the dihydrate at the expense of the monohydrate was probably a consequence of the inhibiting effect on the monohydrate crystal growth of some organic compounds in the D filtrate.