The extraction equilibrium relations of the system TbCl_3-DyCl_3-HCl-H_2O-HEH (EHP)-kerosine were studied. The distribution ratio is a function of three variables: the total rare earth concentrations (x_1), the acidities (x_2)and the mole fractions of Tb or Dy(x_3). Different forms of models were established:(1) Models with total rare earth concentration separating into two sectionsD=e~(a_1+a_2x_2+a_4x_2x_3+a_6x_3)X_1~(a_5+a_3x_2)(2) Models with mole fractions separating into three sectionsD=a_1x_1~(a_2+a_3lnx_1+a_4x_3)e~(a_5x_2+a_6x_3)... The extraction equilibrium relations of the system TbCl_3-DyCl_3-HCl-H_2O-HEH (EHP)-kerosine were studied. The distribution ratio is a function of three variables: the total rare earth concentrations (x_1), the acidities (x_2)and the mole fractions of Tb or Dy(x_3). Different forms of models were established:(1) Models with total rare earth concentration separating into two sectionsD=e~(a_1+a_2x_2+a_4x_2x_3+a_6x_3)X_1~(a_5+a_3x_2)(2) Models with mole fractions separating into three sectionsD=a_1x_1~(a_2+a_3lnx_1+a_4x_3)e~(a_5x_2+a_6x_3) for middle section of Tb and DyD=a_1x_1~(a_2+a_3lnx1+a_4x_2)e~(a_5x_2) for high Tb and high Dy sections(3) Model of individual Tb or DyD=a_1x_1~(a_2+a_3lnx_1+a_4x_2)e~(a_5x_2)The relative errors of these models are 3.18～5.16% and the correlation coefficients are 0.9928～0.999. It is interesting that the models of hith Tb or high Dy (mole fraction of Tb or Dy>0.9) are in the same form as that of individual Tb or Dy.The extraction equilibrium curves of the two phases, the different curves of distribution ratios of Tb-Dy binary and mono systems with the change of acidities, and the curves of the organic rare earth concentrations with the change of mole fractions were drawn by calculating with these models. It is observed that the presence of Dy in Tb decreases D_(Tb), and Tb in Dy increases D_(Dy). The curves of D in high Tb or high Dy with change of, aqueous acidities coincide with that of individual D_(Tb) or D_(Dy)·D_(Dy) curve of low Dy in high Tb are much higher than that of individual D_(Dy), and D_(Tb) curve of low Tb in high Dy are lower than that of individual D_(Tb). It means that in binary rare earth system, the presence of small quantity of other element has only very little effect to the D of main element, and it can be neglected, but the D of element in small quantity is affected by the main element. When there's large quantity of Tb, as D_(Tb) is smaller than D_(Dy), the quantity extracted is decreased as compared with individual Dy, the acidity of equilibrium aqueous phase is lower and the free extractant increased, more Dy can be extracted, D_(Dy) is increased. On the contrary, D_(Tb) is decreased by the same reason. That is why the separation factor in binary system is larger than that calculated from mono system.Some valuable informations were investigated for developing an optimization Process:(1) Factors affecting the extraction processes. The effects of phase ratios, feed concentrations and the position of feed stage were studied. It is observed that in feed stage the rare earth concentration in organic phase must be the highest and the acidity in aqueous phase must be the lowest. The feed stage must be the stage where the composition of the entering aqueous phase is equal or near to the feed composition.(2) The relation between the extraction factor and the separation effect. Assuming that the extraction factors of the extraction and scrub sections are equal, and the distribution ratios are unchanged, L. Alders indicated that the mininum number of stages occurs when EB=1/K (i.e.E_A.E_B=1). But in practical extraction process, it is impossible to obtain the equal extraction factors. Assuming the distribution ratios are unchanged, calculating by L. Alders formula for non-extracted fraction, in certain quantity of feed, the mininum number of stages for certain separation occurs when E_A·E_B of both sections deviate from 1 in the same degree. In practical extraction processes as simulated with these models of the system studied. The separation effect is the best when the deviations of E_A·E_B from 1 are in the same degree in both sections. The separation effect is better when E_B·E_B in scrub section equals or closes to 1. If both E_A and E_B in extraction section are<1, the separation effect is poor.An optimization process for the separation of Tb-Dy for the system studied was suggested according to these informations. |