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Effects of physical variables on settling velocities of calcium and strontium phosphates in mother solution
OBUNWO, CC; IBOROMA, SD & BAGSHAW, AP
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
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