Oxalate precipitation for separation of lanthanide and actinide from radioactive waste was examined. The solubilities of rare earth(Y, La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxalates were investigated in nitric acid and oxalic acid media. The oxalate precipitation of americium/neptunium and neodymium with oxalic acid was investigated in the simulated radwaste, which was composed of 10 elements in nitric acid solution. The decomposition of oxalate by hydrogen peroxide was investigated.
The rare earth oxalate was precipitated by addition of oxalic acid to rare earth nitrate in nitric acid solution. The concentration of rare earth in filtrate was determined by ICP. The experiment revealed that the solubility decreased as the oxalic acid concentration increased and the nitric acid concentration decreased. Based on the assumption, rare earth ions can react with oxalate ion to form $RE(C_{2}O_{4})_{n}^{3-2n}(n=1,2,3)$ complex, we have proposed a solubility model of rare earth oxalate. In order to estimate the solubility of rare earth oxalate, oxalic acid, hydrogen ion concentration and ionic strength were calculated by stoichiometric material balances of chemicals in precipitation reaction. The activity coefficients of ions were calculated by the modified Debye-Huckel equation. Within the experimental range of oxalate ion activity $1 × 10^{-8}M to $1 × 10^{-5}M, solubility products $K_{sp}$ and equilibrium constants $β_{1}$ for rare earth oxalates, were obtained.
The oxalate precipitation of americium and neodymium by oxalic acid was investigated in the simulated radwaste, which was composed of 10 elements(Cs, Sr, Fe, Ni, Pd, Ru, Mo, Zr, Nd and Am) of alkali, alkaline earth, and transition metals in nitric acid solution. The effect of concentrations of oxalic acid and nitric acid in the simulated radwaste on the precipitation yield and purity of Am and Nd was examined. The precipitated fraction of each element increased with increasing concentration of oxalic acid and decreasing concentration of nitric acid. At an initial concentration of 0.5M nitric acid and 0.5M oxalic acid, both Am and Nd were precipitated over 99% and other elements almost remained in the solution. It was also found that Am was completely coprecipitated with neodymium oxalate precipitates, and Zr caused the coprecipitation of Cs, Sr, and Pd.
The oxalate precipitation of neptunium and neodymium with oxalic acid and ascorbic acid was investigated in the simulated radwaste, which was composed of 10 elements in nitric acid solution. The effect of concentrations of ascorbic acid and nitric acid on the precipitation yield and the purity of Np and Nd in the simulated radwaste was examined at 0.5M oxalic acid. Neptunium was coprecipitated with Nd as a carrier and its precipitation yield was increased with of ascorbic acid and nitric acid concentrations. It was also found that the initial concentrations of 0.25M ascorbic acid, 2M nitric acid and 0.5M oxalic acid were desirable as an optimum condition for oxalate precipitation of Np and Nd from the simulated radwaste if Pd could be removed in the simulated radwaste prior to oxalate precipitation. Precipitation yield of Pd increased with increasing ascorbic acid. This is due to the reduction of $Pd^{+2}$ into Pd metal by ascorbic acid.
The formation of precipitates by hydrazine was experimentally examined in the simulated waste, which was composed of 9 elements (Nd, Fe, Ni, Mo, Zr, Pd, Ru, Cs, Sr). Palladium was precipitated over 90% above 0.05M of hydrazine concentration and at 2M $HNO_{3}$, while all of the other elements were hardly precipitated. The elements of Pd and Zr were precipitated 93% and 70% in the simulated solution in which concentrations of Zr and Mo were decreased from 0.069M to $3.45 × 10^{-3}$M and $6.9 × 10^{-3}$M and the acid concentration was decreased to about 0.5M after denitration. In a Pd solution of 0.5M and 2M $HNO_{3}$, the precipitation yield of Pd increased with hydrazine concentration and reached over 98% at 0.1M. The precipitation yield of Pd at 0.5M $HNO_{3}$ was higher than at 2M $HNO_{3}$. The Pd precipitates, formed by the adding of hydrazine to an acidified solution, were an amorphous compound consisting of Pd, hydrazine, nitrate and hydrate.
The decomposition rate of oxalate by hydrogen peroxide was investigated by $KMnO_{4}$ titration method. The rate equation for decomposition of hydrogen peroxide in the aqueous phase was obtained; ln$(\frac{[H_{2}O_{2}]}{[H_{2}O_{2}]_{0}})=-k_{1}t$, where $k_{1}=0.2$, for $[H^{+}] ＜ 2M, k_{1}=0.2+0.34([H^{+}]-2)$, for $[H^{+}] ＞ 2M$. As acid increase over 2M, the acid-catalytic effect appeared. A new rate equation for the decomposition of oxalate by hydrogen peroxide was proposed by;
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The rate constant for decomposition of oxalate, $k_{2}$ increased with nitric acid concentration and the effect of hydrogen ion concentration was expressed as a following expression; $k_{2}=a[H^{+}]^{n}$, where the values for a and n were obtained; a=1.54, n=0.3 at $[H^{+}] ＜ 2M$, a=0.31, n=2.5 at $[H^{+}] ＞ 2M$, respectively.
질산매질의 방사성폐액으로부터 란타나이드와 악티나이드를 분리하기 위한 옥살산염 침전 연구로서 희토류 옥살산염의 용해도 및 10개 원소로 구성된 모의 방사성용액에서 원소들의 침전특성이 조사되었으며, 과산화수소에 의한 옥살레이트 분해연구를 수행하였다.
질산과 옥살산 용액에서 희토류원소(Y, La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb)의 옥살산염 용해도를 희토류옥살레이트를 침전시킨 후, 여과액의 희토류원소 농도를 ICP로 분석함으로써 구하였다. 용해도는 옥산산농도 증가와 질산농도 감소에 의해 증가하였다. 수용액상에서 희토류 원소이온이 자유 옥살레이트 이온과 $RE(C_2O_4)_n^{3-2n}(n=1,2,3)$ 형태의 착물이온을 형성한다고 하여 이를 기초로한 용해도모델을 사용하였으며, 용해도 예측을 위해 침전후 용액의 이온강도, 옥살산농도, 산농도등을 침전반응의 화학양론적 물질수지를 이용하여 계산하였다. 이 때 각 이온들의 활동도계수는 수정된 Debye-Huckel식을 이용하여 구하였다. 제시된 용해도 모델 및 계산방법을 이용하여 침전후 희토류옥살레이트의 용해도적과 평형상수를 구하였다.
10개 원소(Cs, Sr, Fe, Ni, Pd, Ru, Mo, Zr, Nd, Am)로 구성된 모의 방사성용액에서 아메리슘과 네오디뮴의 회수율 및 정제도에 미치는 옥살산과 질산농도 영향을 조사하였다. 각 원소들의 침전율은 옥살산농도 증가와 질산농도 감소에 따라 증가하였다. 질산농도 0.5M과 옥살산농도 0.5M에서 아메리슘과 네오디뮴의 회수율은 99% 이상이었으며, 기타 원소들은 용액속에 대부분 잔류하였다. 지르코늄의 존재는 세슘, 스트론튬 및 팔라듐의 공침전을 야기시켰다.
넵튜늄과 네오디뮴등 10개 원소(Cs, Sr, Fe, Ni, Pd, Ru, Mo, Zr, Nd, Np)로 구성된 모의 방사성용액의 옥살산염 침전 및 아스코빅산 첨가영향을 조사하였다. 옥살산농도 0.5M일 때 아스코빅산 첨가와 질산농도가 넵튜늄과 네오디뮴의 침전율 및 정제도에 미치는 영향을 조사하였다. 질산농도 증가와 아스코빅산 첨가가 넵튜늄의 네오디뮴과의 공침전을 증가시켰는 데, 이는 넵튜늄 4가로의 환원에 의한 것으로 예측되었다. 넵튜늄과 네오디뮴의 공침전 회수를 위한 최적조건으로 아스코빅산 0.25M, 질산농도 2M, 옥살산 0.5M를 선정하였다. 팔라듐이 아스코빅산에 의해 침전되었으며, 이는 아스코빅산에 의해 $Pd^{+2}$이 Pd 메탈로서 환원침전됨에 의한 것임을 확인하였다.
9 성분(Nd,Fe,Ni,Mo,Zr,Pd,Ru,Cs,Sr)으로 구성된 모의용액중에 하이드라진 첨가에 의한 침전형성 연구가 수행되었다. 하이드라진 0.05M과 질산2M 조건에서 Pd은 90%이상 침전되었으나 기타 원소들은 침전되지 않았다. 질산농도 0.5M, Zr과 Mo농도 $3.45 × 10^{-3}M$과 $6.9 × 10^{-3}M$ 용액에 하이드라진을 첨가하는 경우 Pd과 Zr이 각각 93%와 70% 침전되었다. 질산농도 0.5M과 2M인 Pd 수용액에 하이드라진 첨가에서 Pd 침전율은 하이드라진 농도 증가에 따라 증가하여 0.1M에서 98%에 이르렀다. 침전율은 2M 질산에서 보다는 0.5M에서 더 크게 나타났다. 산용액에서 하이드라진 첨가에 의해 생성된 Pd 침전물은 무정형이었으며 Pd, 하이드라진, 질산염, 수화물로 구성된 화합물로 추정되었다.
과산화수소에 의한 옥살레이트 분해속도를 $KMnO_4$ 적정법을 이용하여 연구하였다. 수용액상에서 과산화수소 분해속도식이 얻어졌으며; $ln([H_2O_2]/[H_2O_2]_o)=-k_1ㆍt$, 여기서 $[H^+] ＜ 2M$에서 $k_1=0.2, [H^+] ＞ 2M$에서 $k_1=0.2+0.34([H^+]-2)$이었다. 질산농도 2M 이상에서 산촉매효과가 나타났으며, 아래와 같은 옥살레이트 분해속도식이 제시되었다.
◁수식 삽입▷(원문을 참조하세요)
옥살레이트의 분해속도상수는 질산농도에 따라 증가하였으며, 질산농도에 따라 $k_2=a[H^+]^n$의 상관식으로 표현되었다. a와 n은 $[H^+] ＜ 2M$에서 $a=1.54, n=0.3, [H^+] > 2M$에서 a=0.31, n=2.5의 값으로 나타났다.
