GB2275815A

GB2275815A - Recovery of radioactively contaminated mixed metal scrap

Info

Publication number: GB2275815A
Application number: GB9403300A
Authority: GB
Inventors: Hans-Heinrich Westermann; Manfred Sappok; Wolfgang Dumpelmann
Original assignee: KERNKRAFTWERKE GUNDREMMINGEN; Siempelkamp Giesserei KG
Current assignee: KERNKRAFTWERKE GUNDREMMINGEN; Siempelkamp Giesserei KG
Priority date: 1993-02-25
Filing date: 1994-02-22
Publication date: 1994-09-07
Anticipated expiration: 2014-02-22
Also published as: DE4405022A1; SE9400518D0; SE507554C2; GB2275815B; SE9400518L; IT1273317B; GB9403300D0; FR2701962A1; FR2701962B1; DE4405022C2; ITMI940308A1; ITMI940308A0

Abstract

A process for the recovery of metals from radioactively-contaminated mixed metal scrap which arises on the shutdown of nuclear engineering installations and has a significant content of non-metallic constituents is carried out with separate melting stages for the metals present. After casting, the radioactivity of the metallic components still amounts to about 5% of the initial radioactivity. The waste materials from the melt (slag, fly-ash) are disposed of according to their radioactivity.

Description

RECOVERY OF RADIOACTIVELY CONTAMINATED MIXED METAL SCRAP This invention relates to a process for the recovery of metals which are utilisable without hazard, at least for an industrial field of application in the sense of the legal regulations, from radioactively contaminated mixed metal scrap which arises on the shutdown of nuclear engineering installations and which comprises a plurality of non-ferrous metal components as well as steel, a total metal content of 50 to 80 weight % and up to 100 weight % of non-metallic constituents in the remainder. In the sense of the legal regulations the term "utilisable without hazard" means that the average specific radioactivity is reduced to such an extent that radiation hazards are not to be feared within the context of the use in accordance with the regulations of these metals or of components manufactured from them. In the Federal Republic of Germany these regulations are firmly established in the Atomgesetz [Atomic Energy Act].

In order to recover metals which are determined as utilisable without hazard in the sense of the legal regulations from radioactively contaminated mixed metal scrap, it is necessary within the context of the processes carried out in large-scale industrial practice to separate the mixed metal scrap into the different metals by sorting and to decontaminate the components consisting of a given metal in a separate process. This is often technically impossible, and is expensive. In particular, the necessity for sorting is frequently troublesome. If this is the case, the volume of the radioactively contaminated mixed metal scrap is reduced by press compaction and the mixed metal scrap compacted in this manner is supplied, generally in separate containers, to an intermediate or final storage stage. In the course of this procedure, valuable residual materials are lost, e.g. the contents of aluminium and copper, which are usually considerable, in the mixed metal scrap arising from nuclear engineering installations.

A basic object of the present invention is to provide a process by means of which metals which are utilisable without hazard, at least for an industrial field of application in the sense of the legal regulations, can be recovered in a simple manner from radioactively contaminated mixed metal scrap, without costly sorting and decontamination measures.

According to the present invention there is provided a process for the recovery of metals which are utilisable without hazard, at least for an industrial field of application in the sense of the legal regulations, from radioactively contaminated mixed metal scrap which arises on the shutdown of nuclear engineering installations and which comprises a plurality of non-ferrous metal components as well as steel, a total metal content of 50 to 80 weight % and up to 100 weight % of non-metallic constituents in the remainder, wherein the separation melting of metals with fractionated casting of the melt is employed, with the following measures: the mixed metal scrap with its proportion of non-metallic components is charged unsorted into a metal melting furnace, the mixed metal scrap with the non-metallic components is first heated to the casting temperature of the non-ferrous metal with the lowest melting point and this non-ferrous metal is cast, wherein thereafter the remaining mixed metal scrap is heated to the casting temperature of the non-ferrous metal with the next highest melting point and this is cast, and so on as far as the steel, insofar as the metals of the fractionated cast components have a specific radioactivity which permits their utilisation without hazard, they are supplied for utilisation, insofar as the metals of the fractionated cast components have a specific radioactivity which does not permit their utilisation without hazard, they are supplied reduced in volume to an intermediate storage stage, wherein the waste materials from the melt (slag, fly-ash) are disposed of according to their radioactivity.

The fact that the cast metal can be supplied directly and without difficulty for hazard-free utilisation means that appropriate uses for the recovered metal are provided within the context of the legal regulations. In the Example below it is explained that certain residual radioactivity levels are permissible in the above context. The metals recovered by the process according to the invention may also of course be further processed and practically freed from radioactive contamination in a simple manner. Separation melting for the preparation of mixed metal scrap from scrap iron with nonferrous metal constituents is known in the art (DE-OS 15 33 118). In this known process the mixed metal scrap contains practically no non-metallic constituents. It is known (DE 32 13 764 Al) that a melting and processing crucible which is disposed on the upper side of a vehicle and forms a split mould can be employed for the melting and treatment of metal wastes, including radioactive metal wastes. The metal wastes are introduced into the crucible in a closed system and are melted by the electroslag melting process with the use of a non-consumable electrode. In this procedure a slag flux and slag are introduced into the crucible in layers and electrical current is passed through by means of the non-consumable electrode in order thereby to heat the slag flux. The metal wastes processed here also contain practically no non-metallic constituents. Contaminated non-metallic constituents are unavoidable in radioactively contaminated mixed metal scrap which arises on the shutdown of nuclear engineering installations, and in practice are present in amounts from 20 to 50 weight %. They cause a particular problem within the scope of the measures described initially which embody a compulsory sorting stage.

The present invention stems from the surprising recognition and ascertainment that on the application of the process according to the invention only a small percentage of the total radioactivity present in the mixed metal scrap is contained in the cast metals. If the mixed metal scrap originates from the repair or shutdown of nuclear engineering installations, the residual radioactivity in the metals recovered by the process according to the invention still only amounts to about 5 % of the initial radioactivity contained in the mixed metal scrap. The measures according to the invention have a separating effect. The radioactivity, particularly from the non-metallic constituents, passes into the melt waste material, and 20 to 50 % of non-metallic constituents can be disposed of without problems. The costly sorting stage described initially is no longer necessary.

Consequently, mixed metal scrap which has not been pretreated, and which arises on the shutdown of and during the repair of nuclear engineering installations, can be processed without difficulty within the scope of the process according to the invention to provide utilisable metals. The surprising reduction in radioactivity achieved according to the invention permits utilisation without hazard, in the sense of the legal regulations, without difficulty. It should be understood that the legal safety regulations are complied with by the process according to the invention. This applies to the handling of the mixed metal scrap, to the charging and operation of the metal melting furnace and to the casting stage, and also applies to the treatment of the off-gases, the scrubbing of which produces fly-ash which has to be disposed of according to its radioactivity. In particular, numerous possibilities exist within the scope of the invention for further fashioning and development. Thus uncontaminated metals may be admixed with the contaminated mixed metal scrap before introduction into the melting furnace and/or on charging the melting furnace in order to reduce the residual activity of the recovered metals by dilution to form a mixture. In this procedure the amount of uncontaminated metals is preferably selected so that all cast components can be supplied for utilisation without hazard. If the mixed metal scrap contains proportions of aluminium and copper, the invention teaches that the mixed metal scrap is first heated in the metal melting furnace to the casting temperature of the aluminium and this is cast, and the remaining mixed metal scrap is heated thereafter to the casting temperature of the copper and this is cast.

The process according to the invention may be carried out using widely differing metal melting furnaces. An induction furnace is preferably employed as the metal melting furnace. It is then recommended that the components which occur in the mixed metal scrap are packed as tightly as possible in the induction furnace. The metal melting furnace is sealed so that contaminated off-gases cannot escape into the atmosphere, but instead are subjected to an off-gas scrubbing procedure.

In the process according to the invention, a very extensive separation takes place, in the metal melting furnace, of the phase which has become liquid due to melting and the remaining solid phase containing the other metals.

The separation can be effected to a very considerable extent if, after reaching a casting temperature for a non-ferrous metal and before this non-ferrous metal is cast, the contents of the melting furnace are subjected to a separation treatment for a solid/liquid separation, e.g. by vibration. Even though a very wide range of metal melting furnaces can be employed within the scope of the invention, for reliable operation of the process precautions should always be taken to ensure that the charge in the metal melting furnace is heated over its entire height.

EXAMPLE After acceptance and goods inwards inspection, delivered radioactive mixed metal scrap was placed in intermediate storage and was later taken to a closed area for further processing. The mixed metal scrap was assembled to form melting charges. A total of four melting charges was prepared. No pretreatment was carried out. Rather, it was apparent that the scrap comprised quite normal dismantled parts of installations which had not even been subjected to the slightest pretreatment in the power station.

The delivered mixed metal scrap contained a large proportion of non-ferrous metals. For these reason the parts were charged to the furnace individually. This resulted in a higher packing density, thus ensuring better coupling on inductive heating. After the furnace had been filled, a ladle burner was introduced in the usual manner and an encapsulated system was thus produced. Since the components charged here comprised complete motors, amongst other items, a maximum filling weight of 580 kg could be achieved.

In order reliably to capture any possible gases produced, the upper, tilting extraction hood was moved over the furnace as far as possible. Following this the material was preheated/heated throughout to 7480C by means of the ladle burner. This operation was accompanied by the production of blue-black smoke, which was reliably captured by the tilting extraction hood. After this pretreatment the furnace was switched on at its lowest setting, which resulted in complete, uniform heating throughout over the entire height of the furnace. The temperature in the furnace was increased to about 7550C. The burner could now be removed, and 100 kg aluminium was cast.

The temperature was raised by induction to the casting temperature of copper using the furnace. After this temperature had been reached, the contained fraction of 190 kg copper could be cast. The steel/cast steel fraction of 290 kg could then be melted and cast in the same manner. All materials were cast into existing casting moulds which were coated with a water-based graphite wash.

The average specific activity over all the days on which melting was carried out was determined as follows: aluminium = 0.10 Bq/g copper - 0.53 Bq/g steel = 0.73 Bq/g.

The recovered aluminium could be released unconditionally, at least for utilisation in the industrial field. Release to the industrial field is possible for the recovered copper. The steel fraction with a specific activity of 0.7 Bq/g can be released for use in industry or can be used for the manufacture of nuclear engineering products. If no use were found in the nuclear engineering field, e.g. due to the analysis determined, an optimum reduction in volume for intermediate or final storage would be ensured.

Neither the aluminium, nor the copper, nor the steel/casting steel were subjected to further treatment in the furnace or in the moulds after casting.

Due to the non-metallic components in the mixed metal scrap, e.g. in the form of electrical insulators, an increased volume of waste material must be taken into account. After the tests it was shown that the proportion of waste materials arising was 16.5 % of the amount charged. In steel melting of the usual type the proportion of waste is < 5%.

An average activity distribution was ascertained for the melts and the waste materials from the activities determined. According to this only 5.5% of the total activity determined was still present in the cast materials. In detail, the results obtained were as follows: Total activity (melt and waste materials) = 19563.05 kBq Aluminium: 0.17% of the total activity determined Copper: 1.45% of the total activity determined Steel: 3.898 of the total activity determined

Claims

1. A process for the recovery of metals which are utilisable without hazard, at least for an industrial field of application in the sense of the legal regulations, from radioactively contaminated mixed metal scrap which arises on the shutdown of nuclear engineering installations and which comprises a plurality of non-ferrous metal components as well as steel, a total metal content of 50 to 80 weight % and up to 100 weight % of non-metallic constituents in the remainder, wherein the separation melting of metals with fractionated casting of the melt is employed, with the following measures: the mixed metal scrap with its proportion of non-metallic components is charged unsorted into a metal melting furnace, the mixed metal scrap with the non-metallic components is first heated to the casting temperature of the non-ferrous metal with the lowest melting point and this non-ferrous metal is cast, wherein thereafter the remaining mixed metal scrap is heated to the casting temperature of the non-ferrous metal with the next highest melting point and this is cast, and so on as far as the steel, insofar as the metals of the fractionated cast components have a specific radioactivity which permits their utilisation without hazard, they are supplied for utilisation, insofar as the metals of the fractionated cast components have a specific radioactivity which does not permit their utilisation without hazard, they are supplied reduced in volume to an intermediate storage stage, wherein the waste materials from the melt (slag, fly-ash) are disposed of according to their radioactivity.

2. A process according to claim 1, with the additional measure that uncontaminated metals are admixed with the contaminated mixed metal scrap before introduction into the melting furnace and/or on charging the melting furnace in order to reduce the residual activity of the recovered metals by dilution to form a mixture.

3. A process according to claim 2, with the additional measure that the amount of uncontaminated metals is selected so that all cast components can be supplied for utilisation without hazard.

4. A process according to any one of claims 1 to 3, wherein the mixed metal scrap contains proportions of aluminium and copper, with the additional measure that the mixed metal scrap is first heated in the metal melting furnace to the casting temperature of the aluminium and this is cast, and wherein the remaining mixed metal scrap is heated thereafter to the casting temperature of the copper and this is cast.

5. A process according to any one of claims 1 to 4, with the proviso that an induction furnace is employed as the metal melting furnace.

6. A process according to any one of claims 1 to 5, with the proviso that after reaching a casting temperature for a non-ferrous metal and before this non-ferrous metal is cast the contents of the melting furnace are subjected to a separation treatment for a solid/liquid separation, e.g. by vibration.

7. A process according to any one of claims 1 to 6, with the proviso that the charge in the metal melting furnace is heated over its entire height.