Method for separating technetium from a nitric solution

Details Method for separating technetium from a nitric solution Method for separating technetium from a nitric solution

1999-06-07

2001-01-30

Valentine, Donald R. (Department: 1741)

Electrolysis: processes, compositions used therein, and methods

Electrolytic process involving actinide series elements or...

C205S560000

Reexamination Certificate

active

06179981

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for separating technetium from a nitric solution of technetium through electrolysis.
More particularly, the invention relates to the separation of Technetium-99 having the chemical formula TcO
4
—, also called Tc(VII), or pertechnetate, from a nitric solution through electrodeposition of metal technetium, corresponding to Tc(0) also called Tc
met
, and of TcO
2
, H
2
O, corresponding to Tc(IV).
Nitric solutions of technetium are for example solutions derived from the reprocessing of irradiated nuclear fuel, and more generally from the processing of radioactive waste. Also, with the process of the invention, it is possible to reduce the &bgr; activity of these nitric solutions.
This process of separation may be followed by a vitrifying process to stock the technetium extracted from these solutions.
The process of the invention finds application for example in the separation of technetium-99, from solutions derived from the counter-flow liquid-liquid “PUREX” extraction process for reprocessing irradiated nuclear fuel. This process uses a solution of concentrated nitric acid as extraction solution, and the technetium-99 collecting in this solution may reach concentrations of 150 to 200 mg/l, for nitric concentrations which may be as high as 3.5 to 4.5 mol/l. This extraction solution may also, in trace form, contain other elements derived from nuclear combustion such as
106
Ru,
134
Cs,
137
Cs,
144
Ce,
154
Eui,
125
Sb.
Table 1 below shows one example of the analysis results of the different chemical species present in an extraction solution of the “PUREX” process.
TABLE I
Analysis of a nitric solution derived from a
“PUREX” process
Component
Unit
Value
HNO
3
mol/l
3.5-4.5
Technetium (VII)
mg/l
150-200
Ruthenium-106
&mgr;Ci/l
50
Antimony-125
&mgr;Ci/l
2.5
Caesium-134*
&mgr;Ci/l
40
Caesium-137
&mgr;Ci/l
50
Cerium-144
&mgr;Ci/l
40
Europium-154
&mgr;Ci/l
5
The technetium contained in these solutions cannot be collected by an evaporation process of the aqueous phase since such process would lead to loss of the most of the technetium in the form of volatile
Technetium-99 in solution, through its ionic structure, has the properties of an electrolyte, that is to say that under the effect of an electric field in an electrolytic cell, it will migrate towards the cathode on which it will be reduced. The chemical reaction equations (1) and (2) below illustrate an electrolysis of an aqueous solution of technetium-99 or TcO
4
−
:
TcO
4
−
+8H
+
+4e
−
→Tc
3+
+4H
2
O Tc
3+
+3e
−
→Tc
met
  (1)
TcO
4
−
+8H
+
+3e
31
→Tc
4+
+4H
2
O Tc
4+
+40H
−
cathodic
→Tc
2
, 2H
2
O  (2)
These equations (1) and (2) show that, in theory, a deposit of a mixture forms on the cathode, or a mixed deposit of metal Tc represented as Tc
met
in equation (I), and of TcO
2
, 2H
2
O according to equation (2).
Two types of yield may be calculated to assess this electrolysis:
a chemical electrolysis yield defined as the ratio between the quantity in mg of technetium Tc
met
and/or TcO
2
, 2H
2
O, deposited on the cathode, and the quantity in mg of technetium-99 present in the solution before electrolysis;
a faradic electrolysis yield defined as the ratio between the number of coulombs passed through the electrolytic cell and the quantity of Tc deposited on the cathode surface.
The two preceding reaction equations (1) and (2) show that the quantity of Tc
met
and TcO
2
, 2H
2
O deposited on the cathode, and therefore the chemical electrolysis yield, relates firstly to the technetium concentration at the start of electrolysis and, secondly, to the pH of the aqueous solution of technetium. When the technetium concentration increases, the chemical and faradic yields of metal Tc increase, and when the pH increases the chemical and faradic yields of Tc
met
are reduced.
The technetium concentration and pH of the electrolyte solution also have an influence on the stability of the chemical forms of technetium with reduced (II) and (IV) valences in respect of the hydrolysis reaction. Any variation in the Tc concentration of the solution does not alter the chemical and faradic yields of electrodeposition. On the other hand, when the pH of the solution increases, the hydrolysed chemical forms Tc (III, IV), including TcO(OH) and TcO(OH)
2
, increase in concentration leading to a reduction in the chemical yield of the process.
Also, in respect of pH, it has been shown that when the H+ ion concentration of the electrolysis solution is higher than 0.1 M, that is to say when the pH of this solution is less than 1, the electrodeposition of TcO
2
, 2H
2
O is greatly reduced owing to its extensive solubility in an acid medium.
Also, the electrodeposition of metal Tc in an aqueous solution on the cathode modifies the electrochemical properties of the latter, in particular it may cause a reduction in the hydrogen overvoltage, that is to say an increase in the rate of electrochemical decomposition of the water molecules, leading to a rise in the pH of the solution.
Hydrogen overvoltage being defined as the difference between the thermodynamic value of the potential of the H
+
/H
2
(E=O) couple and the value of the potential over and above which the formation of hydrogen is effective is a real system. The hydrogen overvoltage value characterises the rate of electrochemical decomposition of water during the electrolysis of aqueous solutions.
A decrease in the hydrogen overvoltage, that is to say an acceleration in the electrochemical decomposition of water, can be seen during electrolysis accompanied by the formation of the cathode deposit of a metal.
Such electrochemical modification may cause a fast hydrolysis reaction of the electrodeposited species.
PRIOR ART
Document U.S. Pat. No. 3,374,157 describes a process for the electrodeposition of metal technetium on a metal substrate to prepare a source of technetium-99.
Electrodeposition of metal technetium is conducted using 150 ml of a sulphuric acid solution containing technetium-99 in the form of ammonium pertechnetate, and a completing agent stabilizing the pertechnetate ions. The complexing agents described are oxalic acid, citric acid, tartaric acid, glutaric acid, malonic acid, succinic acid and their ammonium salts. These complexing agents are intended to increase the chemical yield of metal Tc formation. The pH of this solution lies between 1 and 2 and the metal technetium is electrodeposited on a metal substrate such as copper, nickel, aluminium, silver, gold, stainless steel and platinum.
One of the drawbacks of this process is that the complexing agent stabilzing the pertechnetate ions slows down the rate of hydrolysis of the Tc(III) and Tc (IV) ions and at the same time moves the electrodeposition potential of the technetium towards negative values thereby causing a drop in the faradic yield of the technetium and an increase in TcO
2
, 2H
2
O in the deposit on the metal substrate.
Further, this document describes a quantitative electrodeposition of metal Tc on a metal substrate, but does not describe the separation of technetium-99 from a nitric solution.
In a nitric solution, the presence of nitrate ions complicates the electrochemical reduction mechanism of technetium-99.
The reaction equations (3), (4), (5) and (6) below illustrate the different possible electrochemical reactions during the electrolysis of an electrolyte solution containing technetium-99 in the presence of nitrate ions:
NO
3
−
+3H
+
+2e
−
→HNO
2
+H
2
O  (3)
Tc(III)+NO
2
−
+2H
+
→Tc(IV)+NO+H
2
O  (4)
Tc(IV)+NO
2
−
+2H
+
→Tc(V)+NO+H
2
O  (5)
Tc(III)+NO
3
−
+2H
+
→Tc(VII)+NO
2
−
+H
2
O  (6)
The reaction equation (3) illustrates a cathodic reduction of the nitrate ions in nitrous acid HNO
2
at the time of

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