GB2029451A

GB2029451A - Electrolytic working-up of ammonium nitrate solution

Info

Publication number: GB2029451A
Application number: GB7929786A
Authority: GB
Original assignee: Alkem GmbH
Current assignee: Alkem GmbH
Priority date: 1978-08-31
Filing date: 1979-08-28
Publication date: 1980-03-19
Also published as: DE2838007B1; GB2029451B; ES483731A1; BE878492A; FR2435109A1; DE2838007C2; FR2435109B1

Abstract

A process for working-up solutions, such as radioactive waste, containing ammonium nitrate, which solutions still contain a small amount of heavy metal ions such as uranium and plutonium, is carried out under conditions of reverse current electrolysis, whereby a spatial separation of NH4<+>-ions and NO3<->- ions is effected, NH3 being expelled by an increase in temperature of the solution, and NO3<->-ions, as HNO3, the accumulating heavy metal ions being drawn off for further processing or recycling.

Description

SPECIFICATION Process and apparatus for working-up solutions containing ammonium nitrate The present invention relates to a process and an apparatus for working up aqueous solutions containing ammonium nitrate, particularly those resulting during the conversion of uranyl- and plutonyl nitrate solutions, and which may still contain a very small amount of heavy metal ions.

The solutions concerned are, in general, the filtrates resulting from the precipitation of metal compounds which are difficult to dissolve. In the case of uranium and plutonium these filtrates are slightly radioactive.

For the disposal of radioactive waste as large as possible a reduction in volume of the same must be achieved, so that, amongst other things, the space for the final storage is as small as possible. On the other hand, it is worth striving before the final storage to extract from solutions such as these the substances which can still be utilised, such as, for example, the heavy metals uranium and plutonium as well as nitric acid, and to supply them for reuse. During the treatment of such solutions the danger of explosion of the ammonium nitrate must be taken into consideration, thus calling for a splitting up of the ammonium nitrate, before or during the reduction in volume of these solutions, into such products which, from this point of view, can be used without any problems.

The processes, of a thermochemical nature, which have become known for the decomposition of ammonium nitrate have, however, in addition to the necessary considerable apparatus costs, the disadvantage that the recovery of the heavy metals contained in the solutions requires further special processes.

The particular task which forms the basis of the present invention is to find a process which requires the smallest possible apparatus costs and allows, in addition to the decomposition up of ammonium nitrate, the recovery of the heavy metals present in the solution.

According to the present invention there is provided a process for working-up an aqueous solution containing ammonium nitrate and a quantity of heavy metal ions, wherein the solution is subjected to reverse current electrolysis so as to obtain within the solution a spatial separation of NH4±ions and of NO3-- ions; the temperature of the solution is raised so as to cause the ammonia to be expelled from the solution; and NO#-ions, in the form of nitric acid, and the accumulating heavy metal ions are drawn off from the solution.

The process may be carried out continuously, the starting solution being added constantly and the products of electrolysis being removed constantly. Alternatively, the process may be carried out intermittently, the starting solution being added periodically and the products of electrolysis being removed periodically.

The solution may contain at least two different heavy metal ions, for example uranium and plutonium, the different heavy metal ions being drawn off from different regions of the solution.

The incoming solution may contain about 500 mg of heavy metal ions, about 130 grams of nitrate ions, and about 100 grams of ammonia (as NH3 and NH4+) per litre.

The invention also relates to an apparatus for effecting the process thereinbefore defined, which apparatus comprises: an electrolysis tank for containing the solution; ~a source of direct current; electrodes connected with the current source and mounted in the tank in a spaced apart relationship so as to permit a spatial separation of the NH4±ions and of the NO3-ions; means for supplying the solution to the tank; means for drawing off the NO3- -ions, in the form of nitric acid; means for drawing off the heavy metal ions; and a a plurality of diaphragms positioned between the electrodes and transversely of the path of the electric current so as to divide the tank into a plurality of chambers from which the products of electrolysis can be removed.

The apparatus may include a temperature regulating device for regulating the temperature of the solution within the entire tank, which regulating device can be individually adjusted for the separate chambers of the tank.

The apparatus may include means for adjusting the velocity of flow of the starting solution. The permits the removal of the heavy ions from different areas of the tank because the velocity of flow of the starting solution and of reverse current electrolytic solution respectively inside the electrolytic tank can be adjusted so that it is smaller than the ion-drift velocity resulting from the applied voltage or from the current conducted through the tank. The following expression is valid in the stationary operating condition: E(x) . Ci C;~V,vO where E(x) = the field intensity at point x.

Ci = concentration of the components i to be removed VL = velocity of flow of the reverse current electrolytic solution.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawing which illustrates one embodiment of an apparatus for effecting the method according to the present invention.

In the apparatus, the quantity of reverse current electrolyte can be regulated at point 14 by means not shown and may consist, for example, of NH4NO3 solution. The solution containing ammonium nitrate could, however, be additionally or alternatively quantitatively regulated in compartment a as a branch flow, whilst the main flow consists, for example, of H20 and is likewise added at 14.

The electrolysis tank 1 is divided into individual compartments a to I by a plurality of upright walls 15 which function as diaphragms. Electrodes 11 and 22 for supplying the current are located at the ends of the tank, which electrodes are connected to a controllable direct current supply device 2. In this example the incoming solution 3 is supplied as a branch flow to the compartment a, the main flow, e.g. H2O, at 14 and the outgoing solution 4 is drawn off from the compartment L Above the compartment b, an outlet 5 is provided for the ammonia, NH3, which has been expelled. Moreover, the entire area of the tank is equipped with a temperature regulating device 6 to compensate for what could possibly be a considerable development of heat in the non-polarised area.In addition, the temperatures in the individual compartments of the tank can be adjusted separately. Extraction ducts 7 are provided in the compartments c, d, 6 for removal of the solution rich in heavy metal ions in these compartments.

The incoming solution 3 can contain, for example 500 mg heavy metal/l, approximately 130 g/l nitrate (NO3), approximatly 100 g/l total ammonia (NH3 and NH4+) and 80 g/l carbonate as CO3. This solution can be supplied to compartment a, as is illustrated in the figure, as well as to each of the other compartments. After the attainment of a state of equilibrium, the contents of the tank become alkaline in an area near the cathode 11, and the temperature increases due to the Joulean heat. Ammonium ions drifting towards the cathode 11 are split up according to the equation: NH4+ rNH3+H+ and form water according to the equation: H+ + OH- rH20 The ammonia NH3 is expelled at the increased temperature and carried away via the duct 5.

This ammonia is then fed to an absorption washer in a manner not illustrated in the figure and may, for example, be returned to the conversion process for uranium or plutonium.

The operating conditions are fixed so that in the steady state operating condition the concentration of the NH4+ ions appearing in the tank is reduced to zero in a predetermined compartment towards the anode 12. In the acidic area of the tank, which is separated from the alkaline area by a neutral zone, the NO3- ions of the ammonium nitrate can be continuously drawn off in the form of dilute KNO3, as is illustrated by duct 4, and can be returned to the process.

The heavy metal ions contained in the solution are drawn off at various places in the tank depending upon the selected operating parameters for subsequent treatment. This is indicated in the figure by the extraction ducts 7 in the compartments c, d, e. Since the iondrift velocity of uranium and plutonium is variable, when both types of ions are present the separate extraction of ions can be carried out simultaneously from different areas of the electrolysis tank with a high degree of purity.

Claims

1. A process for working-up an aqueous solution containing ammonium nitrate and a quantity of heavy metal ions, wherein the solution is subjected to reverse current electro- lysis so as to obtain within the solution a spatial separation of NH4±ions and of NO3ions; the temperature of the solution is raised so as to cause the ammonia to be expelled from the solution; and NO --ions, in the form of nitric acid, and the accumulating heavy metal ions are drawn off from the solution.

2. A process according to claim 1, wherein the process is carried out continuously, the starting solution being added constantly and the products of electrolysis being removed constantly.

3. A process according to claim 1, wherein the process is carried out intermittently, the starting solution being added periodically and the products of electrolysis being removed periodically.

4. A process according to any preceding claim and in which the solution contains at least two different heavy metal ions, wherein the different heavy metal ions are drawn off from different regions of the solution.

5. A process according to claim 4, wherein the solution contains uranium and plutonium ions.

6. A process according to any preceding claim, wherein the incoming solution contains about 500 mg of heavy metal ions, about 130 grams of nitrate ions, and about 100 grams of ammonia (as NH3 and NH4+) per litre.

7. A process for working-up an aqueous solution containing ammonium nitrate and a quantity of heavy metal ions substantially as hereinbefore described with reference to the accompanying drawing.

8. An apparatus for effecting the process according to any preceding claim, which apparatus comprises: an electrolysis tank for containing the solution; a source of direct current; electrodes connected with the current source and mounted in the tank in a spaced apart relationship so as to permit a spatial separation of the NH44-ions and of the NO3-ions; means for supplying the solution to the tank; means for drawing off the NOj -ions, in the form of nitric acid; means for drawing off the heavy metal ions; and a plurality of diaphragms positioned between the electrodes and transversely of the path of the electric current so as to divide the tank into a plurality of chambers from which the products of electrolysis can be removed.

9. An apparatus as claimed in claim 8 and including a temperature regulating device for regulating the temperature of the solution within the entire tank, which regulating device can be individually adjusted for the separate chambers of the tank.

10. An apparatus as claimed in claim 8 or 9 and including means for adjusting the velocity of flow of the starting solution.

11. An apparatus for working-up an aqueous solution containing ammonium nitrate and a quantity of heavy metal ions substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.