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Fundamental Study on Separation of Light Rare Earth Elements in P204-HCl-H3AOH System

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Tutor: TuGanFeng WuWenYuan
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: P204-HCl-H3AOH system,distribution ratio,separation coefficient,extracting capac
CLC: O658.2
Type: PhD thesis
Year:  2008
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Abstract:
Since the rare earth (RE) products have particular physical and chemical properties, they are widely used in metallurgy, petrochemical industry, various functional materials and glass ceramic field etc. In order to obtain individual rare earth element or mixed several rare earth elements from mixed rare earth feed, P204 or P507 as extractant saponified by ammonia liquor or ammonium acid carbonate are used to separate rare earth elements, but this method produces drain water contained high concentration NH4+, which heavily give environment pollution.In this thesis, the coordination agent citric acid (H3AOH) is added to rare earth and the unsaponified P204 is used as extractant in order to improve the process of separating light rare earth. The distribution ratio of rare earth elements in rare earth chloride solution and P204 extractant, separation coefficient, saturated extraction capacity and acidity, citric acid concentration, rare earth concentration were studied. The mechanics and the kinetics of the light rare earth extraction were studied.Under the conditions of different rare earth feed, acidity and citric acid concentration, the distribution ratio of the light rare earths and extraction saturated capacity in P204-HCl-H3AOH system were studied by single stage extraction experiments. The ternary linear regression equation of feed acidity, citric acid concentration and rare earth concentration was obtained, and the effect of them on the distribution ratio and separation coefficient were analyzed. The results show that, when feed acidity pH 1.0, citric acid concentration 0.25mol¡¤L-1 and rare earth concentration 0.25mol¡¤L-1, the max distribution ratio(D) comes up to DLa=0.1767, DCe=0.7353, DPr=1.5221 and DNd=2.4201 respectively, the max separation coefficient(¦Â) are¦ÂCe/La=4.16,¦Âpr/Ce=2.07 and¦ÂNd/Pr=1-59 respectively. By continuous stage extraction, the average separation coefficient of are¦ÂCe/La=3.50,¦Âpr/Ce=2.05 and¦ÂNd/Pr=1-35 respectively at the same experiments conditions as single stage extraction, which are higher compare to the saponified P204 system¦ÂCe/La=2.14,¦ÂPr/Ce=1-67 and¦ÂNd/Pr=1-33. Moreover, the capacity of unsaponified P204 extracting rare earth increases with citric acid concentration, its maxium is 29.71 g¡¤L-1, which increases nearly 50% compare to soaponified P204.The extraction equilibrium constants(K) of unsaponified P204 in the citric acid system are 1gKLa=-1.77, 1gKCe=-1.63, 1gKPr=-1.26, 1gKNd=-0.86 when pH>1; and 1gKLa=-0.61,1gKCe =0.13,1gKpr=0.40,1gKNd=0.80 when 0<pH¡Ü1. The extraction equilibrium constants increase with atomic number, which indicates P204 extracting the light rare earth ions in P204-HCl-H3AOH system is "positive extraction". The difference value of extraction equilibrium constants between P204 and every light rare earth element is bigger than the difference value of the coordination equilibrium constants between of citric acid and every light rare earth element. This causes the separation coefficient of the light rare earths increase in P204-HCl-H3AOH system.The mechanics of extracting rare earth ion in P204-HCl-H3 AOH system were determined by infrared spectrum analysis. The results show that, extraction reaction are the replacement of H+of P-OH with rare earth ions according to cation exchange mechanics when pH>1. When 0<pH<1, the reaction process is complex, and P=O band and P-OH band participate the coordination with rare earths simultaneously, and its coordination mechanics is cation exchange and neutral complexation. When pH<0, the coordination mechanics is the same as that of 0<pH<l, but coordination reaction of P=O band and is primary. The above indicates that, when 0<pH¡Ü1 and pH¡Ü0, the existing of citric acid impeds the formation of P204 dipolymer, and double coordination mechanics strengthen the extracting ability of P204, therefore enhance extracting capacity. The infrared spectrogram of P204 coordinating with the different rare earth elements shows that the coordination ability increases gradually from La to Nd. Moreover, the variation of frequency and relative peak intensity of P=O band in P204-HCl-H3AOH system are bigger than that in P204-HCl system when extracting the same rare earth ion, which further explains the cause of the increase of rare earth separation coefficient when existing citric acid.The kinetics of extracting the light rare earth in P204-HCl-H3AOH system was investigated by constant interfacial area method. The results show that, the rate of P204 extracting light rare earths increases with stirring speed, temperature, specific interfacial area. According to Arrhennius equation, the apparent activation energy of extracting light rare earths are ELa=19.11KJ¡¤mol-1, ECe=13.13 KJ¡¤mol-1, EPr=10.22 KJ¡¤mol-1 and ENd= 8.39 KJ¡¤mol-1 respectively, which illustrates that this process has obvious diffusion velocity control characteristic. By studying the effect of acidity and extractant concentration on extraction rate, the positive reaction rate equations of extracting light rare earths in P204-HCl-H3AOH system are determined as fellows:
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