The effect of hydrodynamic and thermodynamic factors and the addition of citric acid on the precipitation of calcium oxalate dihydrate

This paper reports on the investigation of experimental conditions relevant for spontaneous precipitation of significant amount of pure calcium oxalate dihydrate (COD). For this purpose, the hydrodynamic and thermodynamic parameters, such as mode of agitation, temperature, supersaturation and concentration of additives (citrate ions), have been studied. The results show that in the model systems, without the citrate addition and applied mechanical stirring, calcium oxalate monohydrate (COM) was observed as dominant modification after 20 min of aging, while the magnetic stirring resulted in a formation of a mixture of COM and calcium oxalate trihydrate (COT), regardless of the temperature applied. In the mechanically stirred systems, the addition of citrate ions in the range of concentrations, 0.001 mol dm⁻³ < c _i (Na₃C₆H₅O₇) < 0.012 mol dm⁻³, caused the formation of COM and COD mixture at all temperatures. At the same conditions and in the magnetically stirred systems formation of COD, in a mixture with COT or COM, has been observed. The highest COD content in the mechanically stirred system was obtained at 45 °C and c _i (Na₃C₆H₅O₇) = 0.001 mol dm⁻³ (w = 89.5%), while in the magnetically stirred system almost pure COD was obtained at 37 °C and c _i (Na₃C₆H₅O₇) = 0.008 mol dm⁻³ (w = 96.5%).