Solvent extraction of phosphoric acid
This work investigated the purification of phosphoric acid using
a suitable organic solvent, followed by re-extraction of the acid from
the solvent using water. The work consisted of practical batch and
continuous studies and the economics and design of a full scale plant,
based on the experimental data.
A comprehensive literature survey on the purification of wet
process phosphoric acid by organic solvents is presented and the
literature describing the design and operation of mixer-settlers has
also been reviewed.
In batch studies, the equilibrium and distribution curves for
the systems water-phosphoric acid-solvent for Benzaldehyde, Cyclohexanol
and Methylisobutylketone (MIBK) were determined together with hydrodynamic
characteristics for both pure and impure systems. The settling time
increased with acid concentration, but power input had no effect. Drop
size was found to reduce with acid concentration and power input.
For the continuous studies a novel horizontal mixer~settler
cascade was designed, constructed and operated using pure and impure
acid with MIBK as the solvent. The cascade incorporates three air
turbine agitated, cylindrical 900 ml mixers, and three cylindrical
200 ml settlers with air-lift solvent interstage transfer.
Mean drop size in the fully baffled mixer was correlated. Drop
size distributions were log-normal and size decreased with acid
concentration and power input and increased with dispersed phase hold-up.
Phase inversion studies showed that the width of the ambivalent
region depended upon rotor speed, hold-up and acid concentration.
Settler characteristics were investigated by measuring wedge
Distribution coefficients of impurities and acid were also
investigated. The following optimum extraction conditions were found:
initial acid concentration 63%, phase ratio of solvent to acid 1:1
(v/v), impeller speed recommended 900 r.p.m. In the washing step
the maximum phase ratio of solvent to water was 8:1 (v/v). Work on
phosphoric acid concentration involved constructing distillation
equipment consisting of a 10£ spherical still ..
A 100 T/d scale detailed process design including capital cost,
operating cost and profitability was also completed. A profit model
for phosphoric acid extraction was developed and maximised.
Recommendations are made for both the application of the results
to a practical design and for extensions of the study.