US3669546A

US3669546A - Device for spectrographic analysis of a liquid metal

Info

Publication number: US3669546A
Application number: US83801A
Authority: US
Inventors: Jean Marcel Virloget
Original assignee: INSTR DE CONTROLE ET D ANALYSE
Current assignee: INSTR DE CONTROLE ET D ANALYSE; SOC FRANCAISE D'INSTRUMENTS DE CONTROLE ET D'ANALYSES; EIDP Inc
Priority date: 1969-10-28
Filing date: 1970-10-26
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: DE2052669B2; GB1292085A; DE2052669A1; DE2052669C3; FR2063713A5

Abstract

APPARATUS FOR RECEIVING A LIGHT BEAM FROM A SELECTED REGION OF AN ELECTRIC ARC AND TRANSMITTING IT WITHOUT ALTERATION INTO A SPECTROGRAPH, COMPRISES A SEALED ENCLOSURE HAVING A CONTROLLED ATMOSPHERE, A CONCAVE RECEIVING MIRROR WHICH IS ADJUSTABLE IN POSITION RELATIVE TO TWO PERPENDICULAR AXES, A FLAT MIRROR, A FLAT DIVIDING MIRROR WHICH DIVIDES THE BEAM INTO A MAIN FRACTION, WHICH FORMS A REAL IMAGE AT THE SPARK POINT OF THE SPECTROGRAPH, AND A SECONDARY FRACTION WHICH CONTROLS THE POSITION OF THE RECEIVING MIRROR.

Description

June 13, 1972 J. M. VIRLOGET 3,669,546

DEVICE FOR SPECTROGRAPHIC- ANALYSIS OF A LIQUID METAL 5 Sheets-Sheet 1 Filed Oct. 26, 1970 mm E @N & R a A. mm

June 13, 1972 J. M. VIRLOGET DEVICE FOR SPEOTROGRAPHIC ANALYSIS OF A LIQUID METAL 5 Sheets-Sheet 2 Filed Oct. 26, 1970 mm mm June 13, 1972 J. M. VIRLOGET 3,669,546

DEVICE FOR SPECTROGRAPHIC ANALYSIS OF A LIQUID METAL 3 Sheets-Sheet 5 Filed Oct. 26, 1970 :v 2 mm mm vw United States Patent 3,669,546 DEVICE FOR SPECTROGRAPHIC ANALYSIS OF A LIQUID METAL Jean Marcel Virloget, Le Mesnil-Saint-Denis, France, as-

signor to Socit Francaise dlnstruments de Controle et dAnalyses, Yvelines, France Filed Oct. 26, 1970, Ser. No. 83,801 Claims priority, application France, Oct. 28, 1969, 6936939 Int. Cl. G01 3/30, 3/00 US. Cl. 356-86 13 Claims ABSTRACT OF THE DISCLOSURE The invention is applicable to the spectrographic analysis of any molten metal in a furnace.

It is an object of the present invention to provide a device which enables spectrographic analysis of a liquid metal contained in a melting or manufacturing furnace. More particularly, the device according to the invention serves to collect and to transmit, without alteration, a light beam emitted in a furnace at the surface of a liquid metal to a spectrograph of known type.

It is also an object of the present invention to provide an assembly comprising the above device and forming a functional combination of a spectrograph and a furnace particularly for metals or alloys at elevated melting temperatures.

The word furnace is used here in its widest sense to refer to all enclosures, particularly with refractory walls, adapted to contain a liquid metal, whether a known type of furnace, a convertor, a mixer or the like. Likewise, the word metal services to indicate all materials, particularly metallic alloys, of which it is required to known at least partially the composition while they are in a liquid state under the influence of heat. It will be understood that the invention is particularly applicable to iron metallurgy and to metallurgy in general in which molten metals are produced in a temperature range of 300 to 7800 C. The application of the invention is, however, not limited to these possibilities.

It is known that it is extremely useful, particularly in electric steel furnaces and with continuous flow installations, to be able to know in as short a time as possible the composition, or at least the content of certain elements, of a molten metal in a furnace. The production of steel has, with the use of oxygen, become a rapid process of the order of twenty minutes in all, which in return requires rapid correctional measures, and, consequently, analysis made as quickly as possible during the production. This requirement tends to render impractical, because it is too slow, the classical method, which consists in removing a sample, allowing it to solidify, polishing it roughly on one face and using it as an electrode at the spark point of a spectrograph, which is frequently situated in a laboratory some distance from the furnace. In a number of countries attempts have already been made to collect a light beam emitted by an electric are created at the surface of a fusion bath and to analyse this beam by means of a spectrograph. However, until now the various solutions proposed have not enabled a correct and constant analysis which is industrially utilisable to be obtained. It is evident, in fact, that anlysis graphs should be obtained and reproduced at will by arcing on the metal liquid which possess a constant correlation with the graph of the true composition of hte metal such as would be provided, for example, by chemical analysis.

The difficulties to be overcome, which are numerous, include:

The elevated temperatures (1600 C. for steels) which prevail at the interior of the furnace and the high sensitivity of spectrographs to the ambient temperature (they are usually maintained at a stabilised temperature of A 0.);

The poor quality, from the optical point of View, of the atmosphere in the vicinity of the molten bath (fumes metallic vapours-varying presence of C0, C0 S0 0 etc.) and the high sensitivity of the light beam to variations in this atmosphere. For steels, it should be possible to collect and transmit without alteration, to the analysis spectrograph rays the wavelength of which are between 1650 and 2800 A., metalloids emitting rays mainly between 1650 and 2000 A. and metals mainly between 2200 and 28 A. In certain cases, it should be possible to detect luminoous rays from certain bodies up to 3500 A. and even up to 8000 A. (7765 A. for the detection of potassium).

To give a single example, oxygen totally absorbs rays of wavelength less than 1860 A.

It is an object of the present invention to mitigate the difficulties briefly outlined above and also others which are discussed below.

An important object of the invention is to transmit without alteration to a spectrograph sufi'iciently spaced from a furnace a light beam provided by an electric arc created at the surface of molten metal at the interior of an industrial furnace.

A further object of the invention is to collect a light beam emitted by a selected region of an electric arc and to conserve to a satisfactory extent the beam coming from this same region, in spite of the relative instability of the electric arc in space.

A still further object of the present invention is to provide a device suitable for transmitting a light beam including a widely differing range of wavelengths, e.g. of 1600 to 8000 A. or more.

Another important object of the present invention is to provide a complete industrially utilisable installation capable of providing in a brief time of the order of one minute a correct and reproducable spectrograph analvsis of a molten metal in a furnace.

According to the present invention a device for receiving and transmitting a light beam emitted by a selected region of an electric are produced in a zone of elevated temperature, for example a furnace containing a molten metal, to an analysis apparatus such as a spectrograph having a spark point, an entry opening and an inlet slot for a luminous beam, comprising a hollow and closed elongate enclosure having an orifice communicating with means for controlling the atmosphere in the interior of the enclosure, an inlet opening for an incident light beam disposed in one end wall and closed by a suitable transparent material, a first outlet opening for a main fraction of the light beam disposed in the opposite end wall and communicating with the inlet of the spectrograph by a sealed connecting element, an optical system disposed within the enclosure and comprising at least a receiving mirror which receives the light beam from the inlet opening and reflects it, the receiving mirror being mounted so as to be adjustable in position with respect to at least one axis, a mirror which has an opening and which divides the light beam into a main fraction which passes to the spectrograph and a secondary fraction, a mechanism for correcting the position of the receiving mirror, this mechanism being connected to the receiving mirror, and a control elment connected to the mechanism and having a beam detector disposed in the path of the secondary fraction of the beam in such a manner that the position of the receiving mirror is controlled in accordance with the quantity of light received by the beam detector, the optical system producing a real image of the selected region of the electric arc at the spark point of the spectrograph.

In one embodiment of the invention, the elongate enclosure surrounds the spark point of the spectrograph, the element connecting the first outlet opening of this enclosure to the inlet of the spectrograph being formed by a hollow tubular sealing element containing, preferably, an optical condenser.

In another embodiment of the invention, the sealed connecting element is an optical fibre having one end disposed as the spark point and a second end, preferably of rectangular section, adapted and fixed to the inlet slot of the spectrograph.

According to another aspect of the invention, it is an object of the inventionto provide an assembly for spectrographic analysis of a metal infusion comprising; a

. furnace having a refractory wall with a lateral opening,

a molten metal contained by the wall of the furnace at a level below the lateral opening, a generator producing an electric are between an electrode and the surface of the molten metal, a hollow connecting element secured at one end to the wall of the furnace around the lateral opening, a device as defined above secured to the second extremity of the said hollow connecting element around the inlet opening of this device, and a spectrograph connected to the first outlet opening of this device.

The invention will be more readily understood from the following description of two embodiments thereof given by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a general schematic partial view in section of an assembly embodying the invention;

FIG. 2 shows a partial schematic view supplementing FIG. 1;

FIG. 3 is a partial view in section of a further assembly according to a second embodiment of the invention.

'Before beginning the description, it is noted that the assemblies illustrated, which embody the invention, may be provided with numerous modifications, which can be made by those skilled in the art, required for adaption to the operation of the type of furnace and the nature of the liquid metal. 'It is accordingly to be understood that the embodiments given here are in no way limiting.

'Referring to FIG. 1, there is shown a furnace 1 having a thick refractory wall 2 in which there is formed a lateral opening 3. This furnace contains a liquid alloy 4 which is maintained molten by means of heating, which is not illustrated. A spark generator 5 is mounted close to the furnace 1 and is connected to an electrode 6 which extends into the liquid bath 6 and to an electrode 7 which is maintained at a small distance above the surface of the liquid metal facing and in the vicinity of the opening 3.

Means other than the generator 5 illustrated could be employed provided that they create an electric are 8 at the surface of the liquid metal 4, which performs the role of an electrode.

The are 8 is relatively unstable in the sense that it varies in shape and position, the surface of the bath 4 not being not perfectly still and causing displacement of the arc. Only the central region of the arc is relevant.

Around the opening 3 on the wall 2 of the furnace 1 there is secured one end of a flexible hollow connecting element which is expansible, such as a light bellows 9, which can support large variation of temperature without its length varying and which can absorb deformations in 4 the lateral and longitudinal directions. The other end of the bellows 9 is fixed to the front face of the front wall 11 of a device indicated generally by reference numeral 10, which will be described in greater detail below.

The device 10 has, opposite the front wall 11, a rear wall 12 to which is secured one end of a sealed connecting element such as a flanged tube 13, the other end of which is secured to a spectrograph 14 of known type.

The device 10 comprises an elongate enclosure which is sealed and which has a connecting orifice 16 by means of which it can be connected to a means (not shown) for allowing the atmosphere prevailing in the interior of the enclosure 15 to be controlled. A vacuum may be produced in the interior of the enclosure 15, and then at a predetermined pressure a gas may be emitted which is transparent to certain wavelengths (nitrogen, argon, helium etc.).

The bellows 9 is secured to the wall 11 around an inlet opening 17 which is closed by a suitable material having the required transparency, such as quartz disc 18.

In addition, within the thickness of the wall 11 thereis provided part of a refrigerating circuit 11a which is connected to a fluid circulation system and a passage 11b which opens outside the enclosure 15 through a circular orifice close to the quartz disc 18 and which communicates with the interior of the bellows 9. This passage 11b is connected to a source of gas under pressure; in this way, a permanent flow of gas from the inlet opening 17 through the bellows 9 towards the furnace 1 can be established. The tube 13 is fixed to the wall 12 around a first outlet opening 19, which may be formed either by a quartz disc analogous to the quartz disc 18 or, as in the present embodiment, by an optical condenser 20 mounted in the tube 13.

The enclosure 15 has a second, lateral, outlet opening 21 closed by a quartz disc 22,

The enclosure 15 also contains an optical system which comprises:

A mirror 23, referred to as the receiving mirror, which in this example is a concave mirror of magnification 1 and which is disposed so as to receive a light beam from the selected region of the are 8;

A fiat mirror 24 disposed to receive the beam reflected by the receiving mirror 23;

A flat mirror 25 having a central opening 26 disposed in the path of the beam reflected by the mirror 24 and referred to as the dividing mirror.

Reference is now made to FIG. 2, which supplements FIG. 1 and in which the enclosure 15 has been omitted.

The receiving mirror 23 is mounted so as to be adjustable in position with respect to two perpendicular axes. In this example, it is supported by a flexible spindle 27 at one point of its periphery; a rigid spindle 28 fixed to the opposite point connects it to a permanent magnet 29 which is mounted so as to be displaceable in opposite directions in the interior of a winding 30 supported in the interior of the enclosure 15. This forms a mechanism for correcting the position of the receiving mirror 23. An identical assembly (not shown) disposed in another plane and connected to the mirror 23 at another point allows correction of the position of the mirror around another axis. The mirror may be mounted on a ball and socket joint instead of being supported by the flexible spindle 27.

Facing the mirror 25 there is disposed a flat mirror 31 which is associated with a lens 32 and with a control member indicated generally by reference numeral 33. The control member 33 comprises a beam detector and is connected to the mechanism for correcting the position of the receiving mirror 23.

In this embodiment, there are two groups of two

photoelectric cells

34, 35 disposed in opposite pairs in two perpendicular directions (a single group is shown in FIG. 2). The photo-

electric cells

34 and 35 of one group are connected to a differential amplifier 36 the output of which is connected to a power amplifier 37. The output of the power amplifier 37 is in turn connected to the correcting mechanism, more particularly to the winding 30.

The above-described asembly shown in FIG. 2 can be placed in the interior of the enclosure 15. In the case of FIG. 1, it is placed on the exterior, the mirror 31 being, as can be seen, disposed facing the second outlet opening 21 of the enclosure 15.

Returning again to FIG. 1, attention is again directed to the spectrograph 14. The spectrograph 14 will not be described in detail because it is of a well known type, but it is noted that the spectrograph 14 has a spark point 38 where an electric spark is produced when a sample of solidified metal is to be analysed, an entry opening 39 and an entry slot 40.

The expression spark point is used here since it relates to the particular case of an electric spark. It should however be understood that in general this expression refers to the region where the atoms of the material to be analysed are excited by means of an electric are or otherwise.

As shown in FIG. 1, the spark point 38 is located in the interior of the enclosure 15 and the closure of the second outlet opening 19 is ensured, with respect to sealing, by the condenser 20. This arrangement is not essential. The spark point could be on the exterior of the enclosure 15, for example outside the second outlet opening 19. In this case, the tube 13 should be more elongated. In addition, a quartz disc can also be placed in the outlet opening 19. The arrangement illustrated is preferable because it avoids the use of this supplementary quartz disc and thus it enables the spark point 38 to be located in the controlled atmosphere of the enclosure 15.

The operation of the arrangement described above will now be explained.

An electric arc 8 having been created at the surface of the molten metal, the device 10 is disposed so that the receiving mirror 23 receives through the inlet opening 17 a beam enamating from the region of the are which is of most interest. For convenience of explanation, it will be assumed that it is a beam of visible light although it could be composed of certain wavelengths outside the visible spectrum.

The mirror 23, by way of the mirror 24, reflects the beam onto the mirror 25 which, in turn, divides the beam which it receives into a main fraction 41 and a secondary fraction 42. The secondary fraction 42 passes through the second outlet opening 21 and is reflected by the mirror 31 and, by means of the lens 32 (which is preferably of variable focal length) a real image of the arc 8 is formed at the centre of the four photo-

electric cells

34, 35. If the are 8 is displaced, its image formed by the lens 32 is also displaced; the relative illumination of the photoelectric cells is modified and the differential amplifier 36 transmits a correcting signal. This signal is converted into a modification of the intensity of the current passing through the winding 30. The position of the magnet .29 changes and so does that of the receiving mirror 23 in such a manner that the latter always receives a light beam coming from the same selected region of the are 8.

The main fraction 41 coming from the concave mirror 23 forms another real image of the are 8 at the spark point 38. It will be noted that the central fraction of the beam received by the receiving mirror 23 is employed for analysis by the spectrograph while the annular fraction which is less homogenous and more variable, is employed for correcting the position of this mirror.

It is important that the beam should undergo the fewest possible refractions and reflections between the are 8 and the spectrograph 14. The invention is notable for the simplicity of the means used to receive the beam issuing from the same region of the arc 8 and transmitting it with the least alteration possible.

Other mirrors could of course be employed for elongating the path of the beam and thus further spacing the spectrograph from the furnace, for example, by forming successive intermediate images before the beam reaches the spectrograph. In any case, as has been mentioned, it is desirable to limit the number. It will also be noted that the flat mirror 24 could be replaced by a dividing mirror having a central reflecting part and an annular opening surrounding this reflecting part. The main fraction 41 would then be directed directly towards the spark point 38, as in FIG. 1, and the secondary fraction 42 which passes through this new dividing mirror could be reflected from the enclosure by one or more other mirrors.

In practice, in many cases, as soon as the temperature of the molten metal to be analysed reaches a fairly elevated value, the device 10 cannot be used by itself. It should be joined to the furnace 1 by means of the bellows 9. Cold water is then supplied through the refrigerating circuit 11a and a gas under pressure is supplied to the passage 11b. This gas is selected in accordance with its transparency to the wavelengths employed, as is that in the enclosure 15. It passes through the bellows 9 and enters the furnace 1, into which it is discharged. In this way a controlled atmosphere is provided between the are 8 and the entry opening 17. At the same time, the metallic vapours which would have a tendency to deposit on the quartz disc 18 are kept away. The quartz disc 18 could be provided with a closure which would protect it while the arrangement is not in operation.

It is useful for the connecting element between the furnace and the arrangement 10 to be capable of supporting large difference of temperature and also to be deformable sufliciently easily in its lateral and longitudinal directions. It is thus possible to effect a first centring of the receiving mirror 23 on the electric discharge arc by relative displacement of the spectrograph and the arrangement 10, making use of the flexibility of the bellows 9. The angular amplitude of the adjustment of the mirror 23 is, in effect, limited.

Proceeding as described above, one encounters a second aspect of the invention which comprises a complete assembly from the furnace to the spectrograph. This assembly can be modified in numerous different Ways depending upon the furnace employed. "FIG. 3 shows one example. In this figure there is shown an installation in which parts which also appear in FIG. 1 have been indicated by the same reference numerals as in FIG. 1.

FIG. 3 shows the constnuction of the Wall 11 with the refrigeration circuit 11a and the passage 11b. Also, it shows that the exterior mirror 31 is mounted with the required inclination at the base of a casing 43 which is secured along the enclosure 15..

The main difference with respect to the embodiment of FIG. 1 is that, the furnace being of a different, smaller type, the bellows 9 is not secured directly to the wall 2 of this furnace but to an intermediate support 44 which is cooled by an internal circulation of water. The intermediate support 44 is adapted to receive, by means of screws and joints, a nose-piece 45 which forms a con!- nection with the lateral opening 3 of furnace 1. To simplify the description, it is mentioned the support 4 is receivable in the external face of the wall of a furnace of which the element 2 is the internal face, while the nose-piece 45 limits the lateral opening 3 formed in this wall.

A further embodiment of the invention, which is not illustrated in the drawings because it is simple to understand, will also be mentioned because it can be used with advantage for light alloys, in particular when wavelengths of more than 2000 A. are to be transmitted.

According to this modification, the tube 13 ('FIG. 1) is omitted, and as a sealed connecting element an optical fibre is employed which is of known type formed of elementary filaments. One end of this fibre is disposed at the spark point 38 and the other has an elongate rectangular section corresponding to the inlet slot 40 of the spectrograph and is secured at this slot. The sealing of the enclosure 15 is preserved by gripping the optical fibre by means of a packing where it passes through the first outlet opening 19.

This embodiment of the invention provides a gain in light with respect to the embodiments of FIGS. 1 and 2.

By employing elementary fibres of about 15 1. diameter, they can be arranged in a single line and adjusted to the inlet slot 40. The limit of employment for wavelengths greater than 2.000 A. is due to the actual nature of the optical fibres; new materials could increase the possibility of employing them. In any case, at present, with the restriction mentioned, one can employ fibres having several metres length which enables the spectrograph -14 to be further spaced from the device 10 if necessary.

In the above description, for the purposes of explaining the apparatus, it has been stated that a lateral opening of the furnace is employed. The invention is however not limited to this arrangement. One could, as a modification, employ an upper opening in the furnace through which an optical rod is pushed downwardly. This rod could be arranged to receive a beam emitted from the electric arc and to direct it upwardly and then to one side, when the situation would 'be as described above but at a different level. As in the case of the lateral opening in the furnace, this optical rod could be traversed by a flow of gas under pressure.

The results of an experiment carried out with an assembly such as that shown in FIG. 3 will now be given. The quantity of molten metal was about three kilogrammes, and the spectrograph was of known type suitable for wavelengths of 1700 to 3700 A.

The results obtained by analysis of the liquid metal are given on the table below in comparison with the results of the analysis of the same metal in its solid state obtained with the same spectrograph by the normal sampling method.

The results are expressed in the form of relative error in both cases with respect to the content measured by analytical chemistry.

1 Or analysis impossible.

It will be noted that only for carbon between 0.1 and 0.7% and for phosphorus is the error occurring with the assembly according to the invention greater, in a sufficiently clear manner, than the error obtained by spectrographic analysis of solid metal; but these errors and 4%) are of an amount which is acceptable in practice.

It will also be noted that for a high carbon content (2 to 5%) the spectrographic analysis of solid metal is impossible because of the preferential path followed by the spark across the carbon. This awkward phenomenon is less likely to occur with molten metal so that the present invention, in these conditions, enables an analysis to be carried out which cannot be carried out by normal spectrographic analysis.

I claim:

1. A device for receiving and transmitting a light beam emitted from a selected region of an electric are produced in a zone of elevated temperature, for example a furnace containing a molten metal, to an analysis apparatus such as a spectrograph having a spark point, an entry opening and an inlet slot for a luminous beam, comprising a hollow and closed elongate enclosure having an orifice communicating with means for controlling the atmosphere in the interior of the enclosure, an inlet opening for an incident light beam disposed in one end wall and closed by a suitable transparent material, a first out-let opening for a main fraction of the light beam disposed in the opposite end wall and communicating with the inlet of the spectrograph by a sealed connecting element, an optical system disposed within the enclosure and comprising at least a receiving mirror which receives the light beam from the inlet opening and reflects it, the receiving mirror being mounted so as to be adjustable in position with respect to at least one axis, a mirror which has an opening and which divides the light beam into a main fraction which passes to the spectrograph and a secondary fraction, a mechanism for correcting the position of the receiving mirror, this mechanism being connected to the receiving mirror, and a control element connected to the mechanism and having a beam detector disposed in the path of the secondary fraction of the beam in such a manner that the position of the receiving mirror is controlled in accordance with the quantity of light received by the beam detector, the optical system producing a real image of the selected region of the electric are at the spark point of the spectrograph.

2. A device as claimed in claim 1, wherein, the elongate enclosure encloses the spark point of the spectrograph, and the element connecting the first outlet opening of the enclosure to the inlet of the spectrograph being formed of a hollow sealed tubular element containing an optical condenser.

3. A device as claimed in claim 1, wherein the sealed connecting element comprises an optical fibre extending between the elongate enclosure and the inlet of the spectrograph.

4. A device as claimed in claim 3, wherein the optical fibre has one end disposed at the sparking point and a second end of rectangular section disposed at the inlet slot of the spectrograph and fixed to the said slot.

5. A device as claimed in claim 1, wherein the elongate enclosure has a second outlet opening closed by a suitable transparent material disposed in the path of the secondary fraction of the light beam, the control member of the beam detector being disposed on the exterior of the elongate enclosure and the correcting mechanism coupled with the receiving member being disposed within the elongate enclosure.

6. A device as claimed in claim 5, wherein the control member of the beam detector comprises, in the path of the secondary fraction of the beam, a lens which produces an image of at least a part of the initial electric are, at least two photo-electric cells disposed in opposition in the plane of the image, and electronic circuit connected 'to the photo-electric cells, the circuit emitting a signal to the correction mechanism coupled to the receiving mirror for correcting the position of this mirror in accordance with the intensity of the beam received by one or the other of the photo-electric cells.

7. A device as claimed in claim 6, wherein the correcting mechanism coupled to the receiving mirror comprises a winding connected to the electric circuit, a permanent magnet freely movable in the interior of the winding, a spindle connecting the permanent magnet to a first point on the mirror, the mirror being supported at a point spaced from the first point by a flexible spindle.

8. A device as claimed in claim 6, wherein the receiving mirror is mounted so as to be adjustable in position in the interior of the enclosure in at least two orthogonal planes, while two groups of two photo-electric cells are disposed in opposition in a perpendicular direction, two correction mechanisms being connected respectively to the two groups of photo-electric cells.

9. A device as claimed in claim 5, wherein the optical system disposed in the enclosure comprises a receiving mirror inthe form of a concave mirror of magnification substantially equal to 1 receiving the incident beam, a first flat mirror receiving the beam reflected by the receiving mirror and reflecting it towards the first outlet opening and towards the spark point, a second flat mirror having a central opening and receiving the beam reflected by the first fiat mirror and dividing it into a main fraction which passes through the central opening and a secondary fraction which is reflected through the second outlet opening.

10. An assembly for the spectrographic analysis of a molten metal comprising a furnace having a refractory wall with an opening, a molten metal contained by the wall of the furnace, a generator producing an electric are between an electrode and the surface of the molten metal, a hollow connecting element secured at one end thereof to the wall of the furnace around the opening, a device as claimed in claim 1 secured to the second end of said hollow connecting element around the inlet opening of this device, and a spectrograph connected to the first outlet opening of the device.

11. An assembly as claimed in claim 10, wherein the wall of the enclosure of the device provides in the vicinity of the transparent material enclosing the inlet opening at least one orifice which is open to the interior of the hollow connecting element, this orifice communicating with a passage which is connected to a source of gas under pressure in such a manner that a permanent flow of gas exists from the periphery of the inlet opening of the enclosure through the hollow connecting element to the interior of the furnace.

12. An assembly as claimed in claim 10, wherein the wall of the closed elongate enclosure contains, at least around the inlet opening, an internal cooling circuit for the circulation of fluid.

13. An assembly as claimed in claim 11, wherein the opening is disposed in the upper wall of the furnace and a hollow optical rod which is sealed is introduced through this opening, the said rod being connected to the inlet of the device and a stream of gas under pressure for flowing through this rod.

References Cited UNITED STATES PATENTS 3,090,278 5/1963 Saunderson 356-80 RONALD L. WIBERT, Primary Examiner V. P. McGRAW, Assistant Examiner US. Cl. X.R.