Optimization of sulfuric acid leaching of a Vietnamese rare earth concentrat

The modeling of Yen Phu (Vietnam) xenotime concentrate leaching by sulfuric acid was studied for the purpose
of optimizing the process. The response surface methodology (RSM) based on a central composite face-centered
(CCF) design was empirically used to model the interactive effect of the independent variables, namely leaching
temperatures of 250–450 °C, acid/concentrate (acid/conc.) mass ratios of 0.8–1.8, and leaching times of 2–6 h,
on the dependent response, namely the leaching yield. And a CCF model for the leaching of the concentrate was
proposed that exhibited good consistency with the experimental data. The shrinking core models for spherical
particles of constant size based on the Arrhenius equation were empirically used to study the kinetics of the
leaching. The activation energies calculated from the kinetic models for the chemical reaction and diffusion rate
stages have the same value of 17.3 kJ·mol−1
, which fitted well to a mixed control model of the chemical reaction
followed by a diffusion stage at leaching temperatures in the range of 473–593 K. The kinetic studies of the
leaching indicated that the leaching percent rate (or leaching yield) is controlled by the leaching temperature.
The optimization of the leaching process was estimated by analyzing the contributions of the coefficients of the
CCF model to the leaching yield. The results indicated that the effect of leaching temperature on leaching yield is
the strongest; it is five times higher than that of the acid/conc. Mass ratio and four times higher than that of the
leaching time. The effects of acid/concentration mass ratio and leaching time on leaching yield are insignificant.
In addition, the optimum data for leaching are as follows: the leaching temperature, acid/conc. Mass ratio, and
leaching time are 320 °C, 1.3, and 4 h, respectively. The proposed CCF model and kinetic study suggested that
the optimization of the Yen Phu xenotime concentrate leaching is controlled by the leaching temperature; and
the CCF model can potentially be applied in the commercial operation of Yen Phu xenotime concentrate leaching
after pilot tests on 50 kg dry concentrate per batch