Patent Number: 053405056
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS A lead plate of a thickness of 0.25 mm and with an area of 2.times.88 cm.sup.2 was used in the performance of the experiments described below. To remove any covering of the lead plate with a protective film of grease, it was degreased with acetone prior to insertion into the treating solution. Each use of fluoboric acid HBF.sub.4 was based on 50% pure acid and the various degrees of dilution were obtained by adding de-ionized water. The lead plate was weighed before and after each treatment. In a first test run the weight loss of a standardized lead plate of the above mentioned type in various HBF.sub.4 concentrations was determined as a function of time. This resulted in the graphs shown in FIG. 1B. Using HBF.sub.4 acid without added H.sub.2 O.sub.2, there were very small relevant differences after 200 minutes in the various concentrations between 5 and 50%. Different weight loss of the lead plates was shown only after approximately 400 minutes, where lead plates subjected to HBF.sub.4 acid at higher concentrations showed greater lead losses. After approximately 200 minutes the weight loss per plate at all concentrations of HBF.sub.4 acid was approximately 0.05 grams. Similar tests were repeated with the addition of 0.5% by volume of H.sub.2 O.sub.2, again as a function of various concentrations of HBF.sub.4 acid. The new graphs shown in FIG. 1A indicate a greatly improved dissolution of lead from the plates. A weight loss of approximately 15 grams was measured after approximately 100 minutes on all plates, regardless of the concentration of HBF.sub.4 acid. Accordingly it was shown that the dissolution of lead had been increased by a factor of 300 within half the time. In contrast to the tests without the addition of hydrogen peroxide, it was shown that the increase in the concentration of HBF.sub.4 acid above 5% did not obtain an improvement in the results. Accordingly, it was shown that the decomposition of the oxide layer took place immediately and the dissolution of lead started quickly because of the addition of 0.5% by volume of H.sub.2 O.sub.2. Initially dissolution was fast and afterwards slowed. Dissolution ceased once a concentration of 55 grams of lead per liter had been attained. Analogous observations have been shown following tests with Ni, Cu, Ag, Hg and steel. Subsequently the tests, so far made at room temperature, were repeated at a temperature of 60.degree. C. Here, again, it was shown, that the decomposition rate steeply increased as a result of the addition of 0.5% of H.sub.2 O.sub.2, however, no increase in lead dissolution over the performance of tests at room temperature was noted. ______________________________________ Dissolution Kinetics Metal in [mg/cm.sup.2 h] ______________________________________ Ag approx. 1.0 Cu 1.0 Hg 0.8 Ni 3.0 Inocel 600 0.5 ______________________________________ These data refer to a reagent of 5% HBF.sub.4 with 0.5% H.sub.2 O.sub.2 at a temperature of 25.degree. C. Thus, the result of the work up to here is that an optimum result is achieved with 5% HBF.sub.4 acid. Now, the rate of solubility of lead in 5% HBF.sub.4 acid was determined as a function of the concentration of hydrogen peroxide contained therein. FIGS. 2A and B show the result. With increasing H.sub.2 O.sub.2 concentration a steady increase of the speed of dissolution of the lead was noted, this within a range from 0.05 to 2% by volume. In every case lead dissolution was initially fast and slowed after 60 minutes. With hydrogen peroxide concentrations between 0.5 and 1.0%, the solution attained a maximum lead concentration of 80 grams per liter towards the end of the process. At this concentration a white sediment formed in the solution and on the surface of the lead. At higher concentrations of H.sub.2 O.sub.2 the dissolution reaction was strongly exothermic. Using the test arrangement with 50 milliliters of solution, the latter started to boil immediately and a white sediment formed almost simultaneously in the solution. The maximum lead concentration in a 10% HBF.sub.4 solution leveled out at approximately 120 grams per liter. Although this concentration is greater by approximately 50% than in the previously measured cases, such dissolution conditions are unacceptable in a process on the industrial scale. The result of all of the work described was that the preferred reagent for dissolving the surfaces of oxidized or non-oxidized lead plates takes place most advantageously in a solution of about 5% HBF.sub.4 acid and about 0.5% by volume of hydrogen peroxide. The work in connection with the process for the decontamination of radioactively contaminated articles of lead or lead-containing alloys was performed using this solution. A few tests to replace hydrogen peroxide by other oxidation agents have also resulted in useful solutions. Tests using permanganate-HBF.sub.4 solutions have also shown acceptable results. The best results were, surprisingly, achieved with a combination of different oxidation agents, together with 5% fluoboric acid. In particular, a mixture where 0.5 to 2% by volume of hydrogen peroxide and 0.1 to 2% of potassium permanganate were added to 5% fluoboric acid, resulted in considerable increase in the values shown in the above table regarding dissolution kinetics. The oxidation agent, potassium permanganate KMnO.sub.4, oxidizes the metals or their oxides and transforms them into a form which is particularly readily dissolvable in the acid. Such a solution of metals and metal oxides containing radioactivity is, for example: EQU MnO.sub.4.sup.- +2H.sub.2 O+3e.sup.- .fwdarw.MnO.sub.2 +4OH.sup.- In contrast to the known AP-Citrox decontamination process, no manganese dioxide MnO.sub.2 is deposited on the surface of the metal. The contaminated articles must be degreased in a first step (1), as shown in FIG. 3. They are placed in a solution bath (2) thereafter. This already contains the described reagent, 5% HBF.sub.4 acid and 0.5% by volume hydrogen peroxide. After the reagent has been allowed to act on the lead plates for approximately 60 minutes, depending on the required removal depth, and the now decontaminated lead plates are removed (3) from the solution bath (2). The solution, which is now contaminated, is passed (4) to an electrolysis bath, for performing electrolysis (5). The contaminated lead or lead oxide is now deposited on the anode or cathode. The concentrated, radioactively contaminated material (6) is now present in a highly concentrated form and nuclear disposal in a known manner is now possible. The remaining HBF.sub.4 acid is taken from the electrolysis cell by stream (7) and recycled by stream (9) to solution bath (2). This is done with the addition (8) of H.sub.2 O.sub.2 until the desired concentration has again been attained. When all articles have been decontaminated, the process can be stopped by neutralizing the acid after electrolysis has been performed by the addition of potassium hydroxide or by regenerating it in a cationic ion exchanger into a pure, non-contaminated acid. A sediment is formed in a known manner in the course of this, which can be filtered out or sedimented. The remaining, contaminated filter cake can be solidified and nuclear disposal in a known manner is now possible. The remaining filtrate is free of activity and also no longer contains lead. It can therefore be disposed of without any additional precautions, for example by placing it in the sewage disposal system. In further test runs it was determined under what conditions the electrolysis of the 5% HBF.sub.4 acid should be performed in order to obtain as efficient as possible a precipitation of the lead or lead oxide. The tests were performed at room temperature and with the use of stainless steel at the cathode and with a graphite anode. The electrolyte consisted of 5% HBF.sub.4 acid with a Pb.sup.2+ content of approximately 30 grams per liter. The electrolyte was prepared by dissolving lead in 5% HBF.sub.4 acid with a 0.5% H.sub.2 O.sub.2 content by volume. The initial pH value was approximately 0. Lead electrolysis was started at a potential of approximately 2.0 Volts. Bubbles were initially formed on the anode surface. They disappeared as soon as lead oxide had been formed. During electrolysis the voltage remained stable with a current density of 30 as well as 45 milli-Ampere per cm.sup.2, until the lead concentration was approximately 5 grams per liter. Starting at this point, the voltage began to increase, while simultaneously bubble formation could be seen, particularly on the anode, accompanied by a rapid deterioration of the coulombic efficiency. With a density of the electrolysis current of 30 mA per cm.sup.2, the coulombic efficiency was a little more than 80%, while with an increase of the current density to 45 mA per cm.sup.2 the coulombic efficiency was nearly 100%. The coulombic efficiency depends upon whether it is calculated before or after the moment of voltage increase. FIGS. 5A and 5B show two examples of lead electrolysis. In both cases the current was maintained at a fixed value. It was noted that the voltage remained stable as long as the lead concentration was below 5 to 6 grams per liter. As soon as this concentration had been achieved, the voltage began to increase and the coulombic efficiency decreased. An increase in the voltage also led to the formation of oxygen bubbles on the surface of the anode. It therefore seems advantageous to perform electrolysis while controlling the voltage in order to prevent the formation of oxygen. It follows from the tests that the dissolution of metallic lead in HBF.sub.4 acid of less than 50% with a content of less than 2% by volume of H.sub.2 O.sub.2 caused considerably improved dissolution. Particularly good results were obtained with 5% HBF.sub.4 acid with a content of 0.5% H.sub.2 O.sub.2 by volume. It was possible to dissolve in this solution 35 grams of lead per liter in approximately 90 to 120 minutes. Following the dissolution of the lead, the solution was used without any additional modification directly as an electrolyte for the recovery of lead. Electrolysis resulted in homogenous lead at the steel cathode and, correspondingly, in lead dioxide PbO.sub.2 at the graphite anode. Coulombic efficiency was more than 90% as long as the electrolysis voltage was maintained at a potential where there was almost no O.sub.2 formed. Various additional methods of use can be realized when a reagent is used which comprises a mixture of 5% HBF.sub.4 as well as 0.5 to 2% by volume H.sub.2 O.sub.2 and 0.1 to 2% KMnO.sub.2. Since with use of this reagent nothing but water-soluble components accumulate, the decontaminated articles can be simply rinsed clean with water at the end. With the high speed of dissolution it has also been shown, that this reagent can also be pumped directly into a closed pipe system, for example the heat exchanger of a nuclear power plant, recirculated in it for a number of hours and subsequently pumped out in the form of a radioactive reagent and electrolytically regenerated. Since the solution is wholly water-soluble, the pipe system can subsequently by rinsed with water. An alternative to this is that the reagent is kept in the pipe system, and then passed through an ion exchanger after some time, by means of which all radioactive portions can be removed from the system. Regeneration by means of an ion exchanger is a known technology, which need not be further discussed here. A possible alternative comprises first exposing the articles to be decontaminated to an oxidizing agent and only then placing them into a pure HBF.sub.4 acid bath or spraying them with HBF.sub.4 acid. This operation can be repeated several times until the metal surface to be decontaminated shows radioactivity below the easily measured limits. Finally, it is also possible to perform the first oxidation with the aid of an oxidizing agent and only after this to execute the method already previously described and to place the metal articles which are to be radioactively decontaminated into a reagent of HBF.sub.4 and an oxidizing agent. While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.