Method for dissolving an oxide layer

Abstract

The invention relates to a method for dissolving an oxide layer containing chromium, iron, nickel, and radionuclides by means of an aqueous oxidative decontamination solution, which contains permanganic acid and a mineral acid and which flows in a circuit (K1), wherein the oxidative decontamination solution is set to a pH value 2.5.

Claims

1. A method for dissolving an oxide layer comprising chromium, iron, nickel, and radionuclides, the method comprising: providing an aqueous oxidative decontamination solution containing permanganic acid and sulfuric acid, flowing in a circuit, wherein the oxidative decontamination solution is adjusted to a pH 2.5; wherein, in a first process step, the oxide layer is oxidized in layers and dissolved by circulating the decontamination solution; wherein, after complete consumption of the permanganic acid, with the circulation continuing, the oxidative decontamination solution is carried, in a second process step, over a bypass line through a cation exchanger to bind divalent Fe, Ni, Zn, and Mn cations present in the decontamination solution, after which, permanganic acid is added to the decontamination solution; wherein the first and second process steps are repeated cyclically until a preset dichromic acid concentration is present in the oxidative decontamination solution, and, in a third process step, while continuing the circulation, the decontamination solution is sent over the bypass line to an anion exchanger to bind dichromate; wherein the first, second, and third process steps are repeated cyclically until a preset thickness of the oxide layer has been removed; wherein, in a fourth process step, by adding a carboxylic or dicarboxylic acid, the circulating sulfuric acid solution is conveyed over the bypass line through the cation exchanger in which Fe ions are bound with a simultaneous liberation of carbonate, dicarbonate, or oxalate and sulfate ions.

2. The method according to claim 1, wherein the dichromate is bound in the anion exchanger with simultaneous liberation of the sulfate ions.

3. The method according to claim 1, wherein a quantity of anion exchange resin used in the anion exchanger is selected based on the quantity of dichromate ions to be retained on the anion exchange resin.

4. The method according to claim 1, wherein permanganate ions concentration in the oxidative decontamination solution is set such that when a prespecified dichromate ion concentration is reached, the permanganate ions are consumed by chemical oxidation reactions, wherein the following equation applies:
total consumption of HMnO.sub.4 [kg]=Cr-III load [kg]U with 1.35U1.40.

5. The method according to claim 1, wherein, in the oxidative decontamination solution, the permanganic acid is set at a maximum concentration of 150 ppm.

6. The method according to claim 1, wherein the sulfuric acid in the oxidative decontamination solution is regenerated by removing Mn-II/Fe-II/Fe-III/Ni-II ions using the cation exchanger.

7. The method according to claim 1, wherein the dichromic acid formed during the degradation of the oxide layer is actively involved in the decontamination process.

8. The method according to claim 1, wherein oxalic acid is used as the dicarboxylic acid, and, after complete removal of the iron ions, the oxalic acid is oxidized to carbon dioxide with permanganate, and the Mn cations formed are bound on the cation exchanger.

9. The method according to claim 1, wherein, at the beginning of degradation of the oxide layer, the pH is adjusted with sulfuric acid, and no more sulfuric acid is added during the degradation of the oxide layer and performance of the further process steps.

10. The method according to claim 1, wherein the oxide layer further comprises zinc.

11. The method according to claim 1, wherein the oxide layer is an oxide layer formed on an inner surface of a coolant circuit of a nuclear power plant, or of a component of the nuclear power plant.

12. The method according to claim 1, wherein the preset dichromic acid concentration is 300 ppm, or less.

13. The method according to claim 1, wherein the preset dichromic acid concentration is 100 ppm, or less.

14. The method according to claim 1, wherein the dicarboxylic acid is oxalic acid.

15. The method according to claim 1, wherein permanganic acid is added to the oxidative decontamination solution to re-establish the initial concentration.

16. The method according to claim 1, wherein a quantity of sulfuric acid used is calculated according to the pH in the oxidative decontamination solution depending on the amount of permanganic acid used, and the permanganic acid quantity requirement is calculated based on the expected amount of chromium to be oxidized according to the equations:
pH=X[(mg/kg HMnO.sub.4 used)9E-05] with 2.0X2.2, and
mg/kg H.sub.2SO.sub.4=YpH.sup.Z with 16Y18, and 4.5Z6.5, if dissolved cations in the oxidative decontamination solution are not taken into consideration, or
mg/kg H.sub.2SO.sub.4=[YpH.sup.z]+[(K.sub.1*F.sub.1)+(K.sub.2F.sub.2+ . . . (K.sub.n*F.sub.n)] if dissolved cations in the oxidative decontamination solution are taken into consideration, wherein 16Y18, and 4.5Z6.5, and F.sub.1, F.sub.2 . . . F.sub.n is a specific factor of respective cations.

17. The method according to claim 16, wherein the specific factor (F) for the cations below is determined as follows: F1 (Fe-II) between 1.70 and 1.74, F2 (Fe-III) between 2.55 and 2.61, F3 (Ni-II) between 1.62 and 1.66, F4 (Zn-II) between 1.45 and 1.50, F5 (Mn-II) between 1.70 and 1.80.

18. The method according to claim 1, wherein the permanganic acid concentration is set such that an oxide layer with a thickness of between 0.3 m and 0.6 m is removed until the permanganic acid is completely consumed.

19. The method according to claim 18, wherein the thickness of the oxide layer to be removed is governed by the quantity of permanganic acid used.

20. The method according to claim 1, wherein the first, second, and third process steps are carried out at a temperature between 60 C. and 120 C.

21. The method according to claim 20, wherein the first, second, and third process steps are carried out at a temperature between 95 C. and 105 C.

22. The method according to claim 1, wherein the pH is adjusted with sulfuric acid to a value <2.2.

23. The method according to claim 22, wherein the pH is adjusted to 2.0.

24. The method according to claim 1, wherein, after hematite present in the oxidative decontamination solution after fixation of the dichromate in the anion exchanger, the hematite is dissolved by the addition of the carboxylic or dicarboxylic acid, the dissolved Fe ions are bound in the cation exchanger.

25. The method according to claim 24, wherein the dicarboxylic acid is oxalic acid, and wherein the oxalic acid is set at a concentration of between 50 ppm and 1000 ppm.

26. The method according to claim 25, wherein oxalic acid remaining in the oxidative decontamination solution after complete removal of Fe ions is decomposed with permanganic acid, forming CO.sub.2 and Mn.sup.2+, and the Mn.sup.2+ ions are fixed on the cation exchanger.

27. The method according to claim 24, wherein the removal of the hematite is carried out at a temperature between 60 C. and 120 C.

28. The method according to claim 27, wherein the removal of the hematite is carried out at a temperature between 95 C. and 105 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings show the following:

(2) FIG. 1 the working pH range according to the invention compared to the prior art,

(3) FIG. 2 change in the permanganic acid concentration and the cation and dichromic acid concentrations as a function of the process duration,

(4) FIG. 3 the process sequence in the oxidative decontamination step,

(5) FIG. 4 the process sequence for the hematite step including the final cleanup step,

(6) FIG. 5 sequential oxidative decontamination steps and increase in dichromic acid as a function of the number of sequential oxidative decontamination steps in the case of dichromic acid remaining in the solution,

(7) FIG. 6 theoretical representation of the decontamination circuit as well as the ion exchange cleanup circuit and

(8) FIG. 7 removal of an oxide layer as a function of the number of oxidative decontamination steps performed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) It is clear from FIG. 1 that when the pH as a function of the permanganate acid concentration lies below the oblique line drawn in FIG. 1, definitely no manganese dioxide can form. According to the prior art, the process is performed at a pH and a permanganate acid concentration that lies above the straight line. As a result, manganese dioxide forms. The straight line is therefore determined according to equations (6) and (7) or (7).

(10) FIG. 2 shows that depending on the process in time and conversion of the permanganate to Mn.sup.2+ the concentration of the cations and the dichromic acid increases.

(11) FIG. 3 shows the oxidative decontamination according to the invention in a purely theoretical aspect. In process step D1, permanganic acid is added to the solution depending on the pH established by the sulfuric acid according to equations (6, 7, 7) to dissolve the metal oxides and form readily soluble sulfates. The Cr-III oxide is oxidized to Cr-VI and is present in the solution as dichromic acid. After the permanganate has been converted completely or essentially completely to Mn.sup.2+ and the solution is substantially free from MnO.sup.4 ions, the solution flows over a bypass to the cation exchanger KIT, in which the cations are fixed. Sulfuric acid and dichromic acid remain in the solution.

(12) Then once again permanganic acid is added to the solution, which is no longer flowing through the cation exchanger, corresponding to the Cr.sup.3 oxide to be oxidized. Addition of sulfuric acid is not necessary as long as the quantity per kg solution is calculated according to equations (7) and (7). After the dichromic acid concentration has reached a predetermined value, the solution flows over the bypass through the anion exchanger AIT, in which dichromate ions are fixed in the previously described manner. Then sulfuric acid and hematite remain in the solution.

(13) The hematite is removed from the solution according to FIG. 4. For this purpose, first oxalic acid is added (process step D4). The solution flows through a cation exchanger KIT, wherein the dissolution of the hematite and the fixing of the Fe ions are performed simultaneously. This process step D4 is performed until no further iron is removed. Then permanganic acid is added in process step D5 to decompose the oxalic acid, forming carbon dioxide, and the manganese sulfate that forms is removed with cation exchangers. Then only sulfuric acid remains in the solution.

(14) FIG. 7 shows, purely theoretically, that the oxide layer can be removed layer by layer, specifically depending on the number of oxidative decontamination steps performed, thus the addition of HMnO.sub.4. It is recognized that oxide layers at thicknesses of approximately 0.3 m to 0.6 m are removed per oxidative decontamination step.

(15) In process step D1, chemical conversion of the low solubility Fe, Cr, Ni structure into readily soluble oxide forms takes place with the aid of permanganic acid. The dissolution of the converted oxide forms is achieved with sulfuric acid. In terms of process technology, this is performed in a circulating operation K1 (FIG. 6) in a sulfuric acid-permanganic acid solution. The circulating operation K1 is maintained until the permanganic acid is completely consumed and converted to Mn.sup.2+. The transformation of the permanganic acid to Mn.sup.2+ is usually 2 to 4 hours if the permanganic acid concentration at the beginning of the process is set at less than 50 ppm, especially in the range between 30 and 50 ppm. The conversion of the oxide structure and the dissolution of the converted oxides take place simultaneously. The final products of the dissolution process are sulfate salts. After the end of the D1 phase, the D2 phase begins. In this process metal cations present as sulfate salts are passed over the cation exchanger KIT and fixed there. In this exchange process the sulfate is released again and is available to the decontamination solution.

(16) During phase D2just as during phase D1the circulating operation K1 is maintained unchanged and the connection of the cation exchangers is done in bypass operation. The cleanup rate (flow rate) through the cation exchanger (m.sup.3/h) relative to the total volume of the system to be decontaminated [m.sup.3] is predetermined from the respective system design of the nuclear power plant. The bypass operation K2 with ongoing circulation operation K1 of the cation exchanger is continued until all cations are bound to the cation exchanger KIT. The total time required for this is predetermined by the available cleanup rate.

(17) After the end of phases D1 and D2, a process technology hold point is provided. The further process steps are directed toward the total oxide content of the system to be decontaminated. If large amounts of chromium are present in the oxide matrix, it is advisable to repeat phases D1 and D2. This repetition process D1+D2 can be continued until the dichromate concentration in the decontamination solution has reached a value of, for example, 100 ppm dichromate. Then process step D3 follows. At the time of phase D3, sulfuric acid and dichromic acid are present in the decontamination solution. The dichromic acid is removed from the solution by means of bypass operation of an anion exchanger. During phase D3 the circulating operation K1 of the system to be decontaminated is operated further without change. The addition of the anion exchange circuit K3 is done in bypass operation. The bypass range of the cation exchange circuit K2 can also be further operated. The bypass operation K3 of the anion exchanger is continued until the dichromate ions are bound to the anion exchanger AIT. The time required for this is determined by the available cleanup rate. The reduction of the dichromate concentration is advantageously continued up to a final concentration of less than 10 ppm. Through the persistence of small quantities of dichromate in the solution, the properties of dichromate for protecting the base material are maintained.

(18) After the end of phase D3, a second process technology hold point is programmed. In the course of the hold point 2, the further procedure is determined, including the considerations described below. The additional process steps are directed toward the total oxide load of the system to be decontaminated. If a large oxide load is present, the process sequence D1 to D3 must be repeated several times before the hematite step is initiated, wherein the number of sequences D1 to D3 is preferably limited to a maximum of 4 times. In the hematite step, designated as phase D4, the hematite Fe.sub.2O.sub.3 produced in the oxidative decontamination step is dissolved in a sulfuric acid-oxalic acid solution. At the same time, fixation of the dissolved iron on the cation exchanger KIT takes place. Sulfuric acid and oxalic acid are again released from the beginning by cation withdrawal, and are continuously available for the hematite solution process. During the total phase D4 both the circulating operation K1 of the system to be decontaminated and the cation exchange circuit K2 are operated. The connection of the cation exchange circuit K2, in which the iron is fixed, takes place in bypass operation. The hematite dissolution phase, thus phase D4, is operated until no further appreciable iron removal takes place.

(19) In the subsequent process step D5, in which sulfuric acid and oxalic acid are present, the oxalic acid is degraded oxidatively to CO.sub.2. The oxidative degradation is accomplished by means of HMnO.sub.4. In this process only the circuit K1 is operated, without the cation exchanger K2 or the anion exchanger K3 having flow through it. After degradation of the oxalic acid, sulfuric acid and Mn sulfate are present in the solution. Only after degradation has taken place will the Mn.sup.2+ be bound to the cation exchanger by connecting in circuit K2.

(20) After the end of the hematite step a process technology hold point 3 is programmed in. During the hold point 3 the further procedure is determined. The continuing process steps are based on the total oxide load of the system to be contaminated. If a large oxide load is present, process steps D1 to D5 must be repeated until the desired decontamination result (dose rate reduction) has been reached. When this occurs, the final cleanup step will be performed. Chemically this means that sulfuric acid is removed from the system. This is performed with anion exchange resins D6. During process step D6 both the large circulating operation K1 of the system to be decontaminated and the anion exchange circuit K3 are operated. The bypass operation K3 of the anion exchanger is continued until the sulfate ions are bound to the anion exchanger ATT. The total time required for this is predetermined by the available cleanup rate.

(21) Repetition of the individual phases D1 to D6 in and of themselves does not occur. Instead, process steps D1+D2 or D1+D2+D3 or D1+D2+D3+D4 or D1+D2+D3+D4+D5 are repeated several times.