Effects of physical variables on settling velocities of calcium and strontium phosphates in mother solution

Abstract

Settling velocity is an important parameter used in modeling solid-liquid flow operations and for evaluating tank volumes in water treatment technology. In the present study, series of bench-scale batch-wise precipitation and settling tests were performed to evaluate the effect of some physical variables such as precipitate concentration, temperature and stirring on settling velocity of tricalcium diphosphate (TCDP) and tristrontium diphosphate (TSDP) in mother solution. Various amounts of TCDP and TSDP precipitates were generated in reaction mixtures, by mixing sub-saturated equi-molar solutions (0.1M) of the respective ions in various metal to phosphate (M/P) volume ratios (10:90ml; 20:80ml; 30:70ml; 40:60ml; 50:50ml; 60:40ml; 70:30ml; 80:20ml; 90:10ml). Settling velocity diagrams of TCDP and TSDP were obtained as functions of M/P volume ratio, temperature, and repeat stirring. The results showed that whereas temperature and stirring variables had a linear relationship with precipitate settling velocity, a non-linear relationship was observed between the precipitate concentration variable and settling velocity. When the temperature of the mixtures were raised from 30oC to 50oC, 0.76- and 0.45- fold increases in settling rate were recorded for TCDP and TSDP, respectively. On the other hand, with repeated stirring of TSDP precipitate, 83% increase in settling velocity was recorded between the first and third stirrings. The results may be employed in settling tank designs for the removal of phosphates in water treatment process. 

Keywords: settling velocity, mother solution, batch-wise precipitation, temperature, amount of precipitates, Ca₃(PO₄)₂, Sr₃(PO₄)₂.