Methods and apparatus for quantitative chemical analysis of liquid metals and alloys

Abstract

An improved method and apparatus for measuring elements in a liquid metal or alloy sample using laser-induced breakdown spectroscopy (LIBS) is provided. The apparatus includes an apparatus head having a pulsed excitation laser and a laser path channel, laser excitation optics, light-receiving optics for receiving light emission from plasma generated by interaction of the laser with the sample, an open-bottom chamber extending from the flat bottom surface of the apparatus head to the top, and preferably a gas channel for supplying gas to the open-bottom chamber. The laser and laser excitation optics are configured such that when the apparatus head is located at a distance in the range of 1 mm to 10 mm from the sample surface, the focal point of the pulsed excitation laser is located at a distance greater than one Rayleigh length of the focused excitation laser beam below the sample surface.

Description

Technical Field

[0001] The present invention relates to the field of spectral analysis, and more particularly, to a laser-induced breakdown spectroscopy (LIBS) method and apparatus for analyzing liquid metals and alloys using a non-immersion type device for high-precision analysis.

Background Art

[0002] Direct chemical analysis of molten metals is of great concern in the production, casting, and recycling of metals and alloys. With current technology, analysis of a few specific impurities can be performed in large amounts of liquid metal using a solid-state electrochemistry probe, but a more extensive chemical analysis of alloying or trace elements can be obtained using spectral measurements performed on the surface of the molten metal. One such technique is laser-induced breakdown spectroscopy, LIBS, which is often proposed as being suitable for the analysis of liquid metals. LIBS is an atomic emission spectroscopy technique in which a laser pulse is used to excite a sample, generating a plasma composed of the excited energy state of the material, and then detecting the light emitted from the plasma during subsequent relaxation, which includes narrow spectral emission lines characteristic of the elements in the sample.

[0003] Laser excitation of small volumes of material following atomic emission spectral analysis provides a rapid and versatile method for elemental analysis of solid, liquid, or gas phase samples. Such techniques are generally considered to require little to no sample preparation. Nevertheless, in terms of quantitative analysis, LIBS has had limited success in industrial analysis to date and has generally been considered inferior to other elemental analysis techniques such as spark emission spectrometers (spark OES or arc-spark OES), inductively coupled plasma emission spectroscopy (ICP-AES), and inductively coupled plasma mass spectrometry (ICP-MS). The first technique mentioned is particularly applied to the surface of solid conductive samples, but in some cases, it has been processed to a specific depth to obtain the best match between the surface analysis and the actual chemical composition of the original molten material. When analyzing metals, the other two methods typically involve chemically dissolving a large amount of solid sample and then introducing it into an inductively coupled plasma.

[0004] Various approaches to determining the chemical composition of liquid metals using LIBS have been proposed in the prior art. EP184590 discloses a method and apparatus for analyzing fluids by measuring the emission spectrum from the irradiated surface of a fluid that may be moving vertically, using a high-power pulsed laser. The problem of non-stationary surfaces is addressed by using a focusing lens with a sufficiently long focal length such that the relationship 0.95 ≤ L ≤ 1.05 applies, where L represents the varying distance between the lens and the surface being measured. That is, the surface is kept within the so-called Rayleigh length of the focused laser beam, which defines the depth of focus in the incident direction. In the example provided in EP184590, a lens with a focal length of 170 cm is used, and the literature states that the use of lenses with focal lengths of 100 cm to 200 cm would allow for vertical movement of the surface of the substance being measured in the range of 10 cm to 20 cm. For the same reason, a concave mirror with a focal length similar to that of a concave mirror in a laser focusing optical system is used to focus the light. The analysis results are disclosed for unspecified samples, analyzing the content of elements (Si, Mn, Cr, Ni) in the concentration range of 0.018% to 0.40%, and expressing relative deviations of 8%, 11%, 10%, and 12% from "conventional methods."

[0005] Aragon et al. (Applied Spectroscopy, 53(10), 1999, 1259-1267) disclose the analysis of carbon content in solid steel samples using LIBS, and state that liquid and gaseous samples can also be analyzed with this technique. The reported accuracy is 10% for carbon content in the range of 150 ppm to 1100 ppm, and the detection limit is 250 ppm. The effect of changing the position of the excitation laser focal point relative to the sampling point on the LIBS signal is discussed. The conclusion in this paper is that to optimize the intensity of the LIBS signal, the beam focal point should be slightly below the sample surface but still within the Rayleigh length.

[0006] Rye et al. (Review of Scientific Instruments, 73(10), 2002, 3589-3599) disclose a fiber optic LIBS sensor for in-situ online measurement of the elemental composition of molten alloys in a furnace. The sensor is based on the transmission of laser energy through a multimode optical fiber. Laser radiation from the fiber is collimated by a specially designed stainless steel holder that holds a collimating lens and a focusing lens, and is ultimately focused into the aluminum molten material in the furnace. Atomic emission is collected by the same stainless steel holder. The setup is intended to hold the molten surface at the focus of the focusing lens.

[0007] WO2007 / 012440 discloses a LIBS-based method for analyzing liquid metal using a probe equipped with a ceramic tube to protect the probe from the liquid metal. Gas pressure is applied into the tube by a gas flow supplied through the probe inlet, and the gas is also used to remove impurities from the surface of the tank by purging with a purge gas flow. A liquid-submerged piping system extending into the tank of liquid metal is used, which causes a convex meniscus of molten metal inside the casing of the liquid-submerged tube. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] EP184590 [Patent Document 2] WO2007 / 012440 [Non-patent literature]

[0009] [Non-Patent Document 1] Aragon et al., Applied Spectroscopy, (USA), 1999, Vol.53, No.10, p.1259-1267 [Non-Patent Document 2] Rai et al., Review of Scientific Instruments, (US), 2002, Vol.73, No.10, p.3589-3599 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Currently, accurate elemental analysis in metal production industries such as aluminum mills and steel smelters generally involves obtaining liquid metal samples at different stages of the production process, solidifying the metal, and analyzing the solid samples using research equipment. However, it would be highly advantageous to analyze the elemental composition of liquid metal in situ with high-precision quantification that accurately represents the correct chemical composition of the molten metal. However, prior art describing spectroscopic measurements performed on liquid metal surfaces, including those involving LIBS analysis, has not demonstrated a sufficiently high level of measurement accuracy to rival the aforementioned research techniques. [Means for solving the problem]

[0011] The present invention is defined in the claims and the following description. The present invention describes an apparatus and method for the quantitative measurement of multiple elements in a liquid metal or alloy sample, which can be obtained using surface measurements with small sample volumes, such as LIBS. A feature of the present invention is to ensure that the measurement results provide values ​​for the concentration of trace impurities or alloying elements that represent the bulk concentration in the sample. The method is particularly useful for process control and / or quality control in metallurgical industries such as aluminum plants, steel mills, silicon and ferrosilicon plants, etc., and other essential industries where accurate quantitative analysis of liquid metals or alloys is desired, without requiring, for example, casting of solid metal samples for laboratory analysis.

[0012] The present invention provides an improved method and apparatus for ensuring appropriate measurement conditions when performing quantitative analysis of elements on a molten metal surface, representing bulk concentrations. This provides results with significantly greater accuracy than those achieved in liquid metal analysis using methods and apparatus described in the prior art. The application of the present invention has been shown to provide a correlation coefficient (Pearson r) greater than 0.9999 between surface analysis of minute elements in liquid metal and bulk analysis of the corresponding solid sample, a low limit of detection (LOD), and high measurement reproducibility. The features and detailed embodiments described below refer generally to both the method and apparatus of the present invention.

[0013] The method of the present invention is based on a novel combination of features that improves upon conventional methods applying LIBS technology, and this combination significantly enhances the accuracy and precision (collectively, "precision") previously achieved and reported by LIBS-based methods.

[0014] A main aspect of the present invention is a method for measuring one or more elements in a liquid metal or alloy sample using laser-induced breakdown spectroscopy (LIBS), - Prepare a sample of the liquid metal or alloy to be analyzed, Placing the device head on the sample surface, wherein the device head is provided with a laser excitation optical system arranged to receive and focus light transmitted from a pulsed excitation laser, a light receiving optical system for receiving light emission from the sample, and an open-bottom chamber through which the laser excitation optical system guides laser light, said placing comprising the open-bottom chamber extending upward from a substantially flat bottom surface of the device head, - arranging the device head at a distance from the sample surface such that the laser excitation optical system focuses a pulse from the excitation laser at a focal point below the sample surface and at a distance longer than the Rayleigh length of the laser pulse, effectively placing the sample surface outside the proper focus of the excitation laser beam; - irradiating the sample with one or more laser pulses passing through the excitation optical system using sufficient light energy to ablate a portion of the sample volume and generate a plasma on the liquid metal surface; - receiving light emitted from the generated plasma through the light receiving optical system, transmitting it to a detector, and recording data of the spectrum of the detected light; - analyzing the spectral data to obtain a determination of the amount of one or more elements; A method comprising the above steps.

[0015] Another aspect provides an apparatus for measuring one or more elements in an alloy sample or liquid metal using LIBS, for example, an apparatus particularly for implementing the method of the present invention, - comprising a pulsed excitation laser, - comprising a device head, the device head a laser path channel, a laser excitation optical system arranged within the laser path channel, a light receiving optical system for receiving light emission from a plasma generated on the sample surface, An open-bottom chamber extending upward from a substantially flat bottom surface of the device head, wherein a laser path channel extends into the open-bottom chamber, It comprises at least one gas channel for supplying gas to the open-bottom chamber through at least the laser path channel, The pulsed excitation laser and laser excitation optical system, when the apparatus head is at a distance of 1 mm to 10 mm from the sample surface, have a focal point below the sample surface, and the focused laser has a Rayleigh length of Z R、 It is configured to be at a depth greater than (fZ). In other words, the distance d from the laser excitation optical system to the sample surface is the focal length of the excitation optical system, expressed as f, which is the distance greater than or equal to the Rayleigh length, where d < (fZ). R ) is less than but generally greater than half the focal length, d>(f / 2). This feature of the device is particularly useful for stabilizing plasma generation and the Rayleigh length (±Z) of the focused beam. R This serves the purpose of preventing harmful interactions between the focused laser and the surrounding environment, which commonly occur near or within the vicinity of the sample, from occurring before the laser pulse reaches the sample surface. The Rayleigh length for the selected laser and excitation optics can be calculated from well-known physical equations and / or measured by measuring the cross-sectional profile of the pulsed laser beam along the beam path. Generally, the Rayleigh length, and therefore the distance from the sample surface to the appropriate beam focus as specified by this invention, depends on the characteristics of both the excitation optics and the laser beam.

[0016] The influence on the LIBS signal by changing the focal position of the excitation laser with respect to the sampling point has been discussed in the prior art, for example, including argon etc. (described above). In this paper, in order to optimize the intensity of the LIBS signal, it is concluded that the beam focus should be slightly below the sample surface but still within the Rayleigh length. The same general conclusion can be widely seen in the prior art, that is, the excitation beam focus should exactly coincide with the sample surface. The need to maximize the overall LIBS signal in this way is, in some cases, imposed by the inefficient detection of the emitted radiation, for example, when the detection optical system is arranged away from the plasma excitation point.

[0017] Conversely, according to the present invention, it is advantageous to be placed near the sampling point in order to separate the detection optical system from the excitation optical system, to counter the influence of the reduced signal, and to realize the advantage of increased measurement stability provided by placing the beam focus further below the sample surface. This can be particularly important when measuring high-temperature liquid metals.

[0018] The prior art describes a measurement probe, where the excitation laser is arranged separately from the probe and connected to it using a multimode optical fiber. This requires, for example, as in the probe described by Rai et al. (described above), that the pulsed laser light be coupled to the fiber without ablating the input end of the fiber, where the facet (surface) of the fiber is intentionally placed outside the focus range of the lens for this reason. As a result, this also limits the energy that can be physically coupled to the fiber and transmitted to the probe. Furthermore, multimode interference causes a degradation in beam quality, and in order to exceed the threshold for ablation at the sample surface, a strong focusing or condensing of the excitation beam within the probe is required. Low pulse energy, mode interference, and strong focusing all contribute to the instability of the fiber-coupled probe method, regardless of whether the probe is immersion type or non-immersion type.

[0019] The methods of the present invention are not limited to these but can be applied to a variety of metals and metal alloys such as liquid aluminum, aluminum alloys, steel, steel alloys, iron, iron alloys, copper, zinc, lead, and other metals and metal alloys, and are extremely useful in the aforementioned industrial environments and applications.

[0020] The methods and apparatus of the present invention are not limited to the analysis of specific elements, but can be used to determine the concentrations of both major components or trace components, including dissolved gases, in a metal or alloy sample. Accordingly, in some embodiments, the methods and / or apparatus are for determining the true bulk concentration of one or more elements selected from aluminum, silicon, phosphorus, sulfur, chloride, calcium, magnesium, sodium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, tin, antimony, wolfram, rhenium, iridium, platinum, gold, mercury, lead, and bismuth in a liquid metal or alloy sample. The methods of the present invention are also suitable for quantifying very light impurity elements such as hydrogen, lithium, beryllium, boron, and carbon, which are difficult to detect by certain other analytical methods.

[0021] This measuring device is equipped with appropriate excitation and detection means for generating and receiving atomic emission from a sample. This includes, but is not limited to, all variations of plasma excitation methodologies known in this technology, including, but not limited to, the use of dual-collinear or non-collinear pulses, composite LIBS / discharge methods, and conventional LIBS methods.

[0022] In a preferred embodiment, spectral analysis is based on the LIBS method, in which one or more laser pulses are sequentially guided through the excitation optical system to the sample surface, the light emitted from the sample is received by the photodetector optical system, and transmitted to a detection unit for recording spectral information of the detected light. Optimal optical detection methods and subsequent processing of the detected radiation are known to those skilled in the art. Then, one or more emission peaks are analyzed from the spectral information and are generally compared to calibration values ​​to obtain quantitative measurements of one or more elements. Preferably, the apparatus includes a computer for receiving and processing spectral data from the detector. The computer is advantageously programmed to process the data, including normalizing the spectrum, assigning or selecting relevant peaks, calculating signal intensity, and calibrating by comparison with reference values.

[0023] In any given embodiment, the excitation optical system and the photodetector optical system may be completely separate or comprise partially identical optical elements. An important feature of the present invention is that the excitation means and the photodetector optical system are precisely positioned at a predetermined distance from the sample surface for each individual excitation event, preferably while maintaining a substantially constant sample temperature and ensuring sufficient internal motion of the liquid metal to ensure homogeneity of the sample, such that a portion of the sample ablated by the plasma represents the entire volume of the sample.

[0024] The pulsed excitation laser used in the preferred embodiment is a conventional type, such as those commonly used in current LIBS configurations. Therefore, in typical embodiments, the laser has a wavelength in the range of approximately 200 nm to approximately 3000 nm. According to the present invention, the optical excitation is configured such that stable excitation conditions are ensured, a sufficiently large and reproducible volume of the liquid metal sample is ablated during excitation, and the chemical composition of the ablated portion of the sample represents the composition of the entire sample. In addition to other aspects of the present invention, this requires that for laser pulses having a duration of 1 ns to 20 ns, the energy of each pulse is in the range of 10 mJ to 500 mJ. Generally, the energy of the laser pulse must be selected such that the laser fluence or flux on the sample surface exceeds a threshold for ablation, and the precise value of the pulse energy depends on a number of factors, including the excitation wavelength, pulse duration, distance from the excitation optical system, and the material being ablated.

[0025] The apparatus head can be appropriately configured such that the laser excitation channel extends generally perpendicularly from the pulsed excitation laser to an open-bottom chamber. The emission-receiving channel can extend from the open-bottom chamber to the laser receiving optics. It has been found useful to position the emission-receiving channel at an angle such as approximately 45° from the vertical (and parallel to the optical axis of the emission-receiving optics). In some embodiments, a flow of inert gas is delivered through a gas channel to openings located both within the laser excitation channel and the emission-receiving channel. Thus, the receiving optics may include lenses positioned to receive optical cones centered at angles in the range of approximately 30° to 75° with respect to the sample surface, such as approximately 30°, approximately 35°, approximately 40°, approximately 45°, approximately 75°, approximately 70°, approximately 65°, approximately 60°, approximately 55°, and approximately 50°.

[0026] The light-receiving optical system is preferably appropriately configured to be positioned at an appropriate distance from the sample surface, such as in the range of approximately 5 mm to approximately 100 mm, such as from approximately 5 mm, or from approximately 10 mm, or from approximately 15 mm, or from approximately 20 mm, or from approximately 25 mm, or from approximately 30 mm to approximately 100 mm, or from approximately 90 mm, or from approximately 80 mm, or from approximately 75 mm, or from approximately 60 mm, or from approximately 50 mm, or from approximately 40 mm, when the apparatus head is precisely positioned.

[0027] In some embodiments, the light-receiving optical system consists of one or more lenses, which are preferably arranged radially around the point of contact between the laser pulse and the sample surface. Thus, the light-receiving optical system can consist of a number of lenses arranged at the same or different angles with respect to the sample surface. Light focused by one or more light-receiving optical systems can be transmitted via fiber optics or other light transmission means to the same or different spectrometers (for example, each lens in the number of lenses can transmit light to its respective spectrometer). In some embodiments, such a number of spectrometers is configured such that each spectrometer collects radiation in a limited wavelength range, and the number of spectrometers as a whole covers all desired wavelength ranges. In some embodiments, the spectral detection may also include detection of a selected wavelength band using one or more suitable band filters and optical sensors.

[0028] The instrument head has a bottom surface that faces the sample surface during measurement, and this surface has an open-bottom chamber as described above, which is a chamber for containing a portion of the sample to be detected, generally in the form of a plasma plume, providing containment and stable environmental conditions surrounding the sampling point.

[0029] In some embodiments, a flow of preferably inert gas is supplied from a source through one or more gas channels to an open-bottom chamber, maintaining an inert atmosphere and preferably a slight excess pressure within the aforementioned chamber, and preferably a substantially laminar flow of gas between the sample surface and the bottom surface of the apparatus head from and near the open-bottom chamber. This means that there is a constant and substantially laminar flow of gas such as argon, helium, or nitrogen in and immediately adjacent to the plasma region. Accordingly, the gas channels are preferably sized and shaped to maintain a laminar flow of gas through them, and are configured, for example, with a flow rate and a Reynolds number of about 2300 or less.

[0030] Preferably, one or more gas channels have openings in the chamber to carry a flow of preferably inert gas from a gas supply source. This maintains a slight excess pressure in the chamber and gas channels relative to the ambient atmosphere, ensures a substantially continuous, consistent, and non-reactive gas environment over the sampling point, and protects the optical path to the detection optics (which may also be a common channel) from dust, smoke, and gases, particles, droplets, etc., from the optical path of the excitation light and laser pulse collisions on the sample surface. The bottom surface of the apparatus head must withstand heat from adjacent sample surfaces and preferably comprises ceramic or other heat-resistant material having low thermal conductivity. Cooling means such as channels for coolant or cooling gas can be applied within the apparatus head and particularly near the bottom, but are not limited to these.

[0031] The open-bottom chamber is not limited to a specific geometric shape, and in some embodiments, for example, has a diameter in the range of 10 mm to 20 mm or 1 cm. 2 ~3cm 2The chamber has a generally cylindrical shape with a horizontal cross-section in the range of [specify range]. The chamber may have a square, hexagonal, octagonal, or other cross-sectional shape, but is not limited to these. The height of the chamber is such that it can accommodate the plasma generated by the laser. In some embodiments, the height of the chamber is in the range of 5 mm to 15 mm, such as 5 mm to 12 mm or 5 mm to 10 mm, for example, 5 mm, 6 mm, 8 mm, or 10 mm.

[0032] The open bottom (inlet) of the chamber can have the same cross-section as the chamber wall, or the bottom (hole in the chamber) can have a smaller cross-sectional area than the chamber body itself. Therefore, in one embodiment, the chamber may have a cross-section with a diameter of 10 mm to 15 mm, such as 10 mm, 12 mm, or 15 mm, and the bottom layer of the device head may have a concentric hole with a diameter that is 2 mm to 4 mm smaller, such as 2 mm, 3 mm, or 4 mm smaller. The thickness of this bottom layer may be in the range of 1 mm to 6 mm, or in the range of 1 mm to 10 mm, such as approximately 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, or 4 mm.

[0033] In some embodiments, the bottom surface of the apparatus head is preferably provided with an insulating layer, which is a heat-resistant layer, such as a high-temperature resistant ceramic layer. The bottom of the bottom surface has a hole concentric with an open-bottom chamber, the hole being the same size and shape as the cross-section of the chamber, or smaller or larger, and can be configured as a selectively narrow bottom hole of the open-bottom chamber. Thus, a hole in such a heat-resistant layer can be configured to define an inlet that is narrower than the main chamber itself in part. The inlet opening can be the same thickness as or less than the heat-resistant layer, for example the layer can be in the range of 4 mm to 12 mm thickness, while the inlet opening to the chamber can be in the range of 1 mm to 6 mm thickness, for example 1, 2, 3, 4, or 5 mm, and above the inlet opening, the heat-resistant layer forms part of the main chamber of this embodiment.

[0034] Sample handling can be advantageously performed, for example, by using a robotic arm adjacent to the source of the liquid metal or alloy to be analyzed, where the robotic arm holds a crucible or other liquid container and obtains a portion of the material by scooping up an appropriate amount of sample. The crucible held by the robotic arm may be a crucible that can then be used as a sample crucible, or, in other embodiments, the robotic arm can be used to pour the acquired sample into another designated sample crucible. A sample of liquid metal may also be manually introduced into a sample crucible, for example, by a human operator using a sample ladle for extracting metal from a reduction cell, mixing furnace, holding furnace, etc., and the sample ladle may, in some embodiments, be used as a sample crucible to hold the sample during measurement.

[0035] In some embodiments, the sample crucible is advantageously brought near an electromagnetic induction means, which simultaneously provides heating and stirring effects. In such embodiments, a key feature of the present invention is that the coupling between the sample and the heating and stirring means is configured and controlled to minimize surface movement while maintaining a desired temperature and ensuring homogenization of the liquid metal sample during a period suitable for performing the elemental analysis. The period during which the measurement is performed is referred here to as the measurement period. During the measurement period, the liquid metal sample is kept in close proximity to the source of the induction field, ensuring proper coupling between the two. In a preferred embodiment, the source comprises a substantially flat, circular induction element positioned beneath the sample crucible.

[0036] For example, to protect against spills of liquid metal, the heating element can be appropriately placed on a top plate or similar flat surface, such as a ceramic surface or a surface made of other suitable heat-resistant material. The minimum distance from the induction heating element to the bulk metal solution during the measurement period depends on the precise configuration of the induction heating element and its driving force, and can be in the range of, for example, 10 mm to 50 mm.

[0037] In other embodiments, heating can be supplied by methods other than induction heating, such as conduction heating, and is advantageously configured to provide a considerable degree of convection in the sample.

[0038] In some embodiments, the sample crucible is positioned as a reservoir, and samples can be supplied to and from the reservoir via connected ducts or openings. Such a reservoir may be positioned on a dedicated sample loop that can guide molten metal, for example, from a flow within a production facility or as a conduit from a furnace. Preferably, the ducts or openings can be closed with gate valves or the like to stop the flow of metal through the reservoir during analysis.

[0039] As used herein, the term “sample crucible” generally refers to the situations described above and similar cases, namely any container that can withstand high temperatures and hold liquid metal, and can refer to a sample crucible of any size. Furthermore, the sample crucible is not limited to a specific shape or material, however its dimensions must be selected to allow for precise handling and positioning of the liquid metal sample with respect to the measuring instrument. It is advantageous that the sample crucible is resistant to thermal shock and non-wetting for the specific liquid metal or alloy being analyzed.

[0040] In a preferred embodiment of the present invention, the LIBS measurement is performed on the sample surface using a non-contact measuring device positioned relative to the sample container, by moving the measuring device as a whole, or by moving the movable part of the device referred to herein as the device head, or by moving the sample container, or both, such that the device head is positioned at a predetermined distance above the surface of the liquid metal sample. The device should be configured to allow a substantially laminar flow of an inert gas around and over the measurement point during the measurement period.

[0041] The sample crucible is substantially open upward, meaning that a sufficient portion of the sample surface within the crucible should be exposed to the excitation means, and some of the light emitted from the sample resulting from the excitation should reach the photodetector optical system. Depending on the size and shape of the sample container, in some embodiments the top of the container may be fully open so that substantially the entire sample surface is exposed, while in other embodiments the top is partially closed, but not limited to, for example, 30%, 40%, 50%, 60%, or 75%, as long as the above criterion of necessary access to the sample surface is satisfied.

[0042] A prerequisite for this method is that a consistent and substantially constant temperature of the liquid metal sample is guaranteed during the analysis, and that this temperature is higher than the melting point of the metal or alloy. In preferred embodiments, this involves actively heating the sample container continuously or periodically. In some embodiments, the active heating is stopped immediately before the sample analysis, i.e., before the emission spectrum from the sample plasma is perceived / recorded. In some embodiments of the present invention, the sample is heated and maintained at a temperature of at least 400°C, such as at least 600°C, at least 700°C, at least 800°C, at least 1000°C, or at least 1200°C or higher, or other temperatures such as about 1600°C. The optimal temperature for sample consumption during analysis depends, in particular, on the melting point of the specific type of metal or alloy being analyzed.

[0043] As a non-limiting example, pure aluminum has a melting point of 660°C, and thus, in the context of the present invention, the optimal temperature of the sample may be in the range of about 660°C to about 960°C, such as from about 680°C or from about 700°C to about 780°C, up to about 760°C, or up to about 750°C. For other metals and alloys, higher temperatures are required to maintain the sample in a molten state; for example, steel has a melting point in the range of 1370°C to 1540°C depending on the type and grade, and therefore, in some non-limiting embodiments of the present invention, the sample is heated to a temperature within or above the above range, such as above about 1400°C, above about 1500°C, or above about 1600°C. In a preferred embodiment, the temperature of the molten metal is monitored in the sample container itself, for example, using a sheathed thermocouple. The temperature of the liquid metal can also be monitored indirectly or by using non-contact optical thermometers.

[0044] Sample containers can have different sizes to accommodate samples of appropriate volumes depending on the configuration of the heating means, the method used to obtain the sample, etc. In some embodiments, the volume of the sample placed in the sample container ranges from about 50 mL or about 100 mL to about 1000 mL, up to about 500 mL, up to about 300 mL, up to about 250 mL, up to about 200 mL, or up to about 100 mL, for example, in the range of 50 mL to 500 mL, 100 mL to 300 mL, for example, about 100 mL, about 150 mL, or about 200 mL. Even if the actual volume of the liquid metal to be analyzed spectroscopically is relatively small (i.e., the actual volume / area that emits light to be irradiated and detected), a larger overall sample volume is important for maintaining a stable sample temperature and, in embodiments involving induction heating, ensuring optimal interaction between the heating and stirring means while simultaneously minimizing internal motion due to volume forces generated by the induction field and surface instability caused by sample deformation. The sample container is preferably sized to accommodate the desired maximum amount of sample to be contained therein. Furthermore, the shape of the sample container can be appropriately selected and configured to block potentially harmful laser radiation and plasma radiation escaping from the sampling point during measurement.

[0045] In one embodiment, heating of the sample container is provided by placing the container in contact with the surface of an adjacent source of liquid metal or alloy, for example, within a trough from which the sample is taken. This means that the sample container is placed on a movable platform (e.g., an extendable movable arm, hook, etc.) that is appropriately configured to transmit such a heating effect and to move the container toward the surface and / or maintain the container in a desired contact state with the surface for at least a certain period of time. In such embodiments, it is advantageous that the heating means introduce a temperature gradient into the sample sufficient to cause some degree of convection in the liquid metal sufficient to maintain substantially chemical homogeneity of the sample.

[0046] In preferred embodiments, an advantage of the method and apparatus of the present invention is that the accuracy and precision of chemical analysis are optimized. To ensure the accuracy of the analysis, it is preferable that the steps, including sampling, sample preparation, and analysis, be carried out in substantially the same manner for the analysis of different samples, that is, the same or substantially the same amount of liquid metal is taken and placed in the sample container, the samples are kept at the same or substantially the same temperature, and the internal motion induced during or immediately before measurement is substantially the same. Furthermore, the location of the sampling point and its surrounding environment, i.e., the local atmosphere at and directly above the sampling point and along the photoexcitation path, are maintained substantially the same. Maintaining consistency also requires that continuous analysis be performed at the same or substantially the same predetermined distance between the excitation and detection means and the sample surface, both in repeated analyses of the same sample and between analyses of different samples. To achieve this, the steps described above are preferably automated as described herein for preferred embodiments of the present invention.

[0047] In some embodiments, the method for preparing a liquid metal sample for analysis additionally includes a step of removing the outermost layer of the sample, in particular, the sample needs to wait between sampling and analysis so that a crust or film forms on the surface. In some embodiments, this is preferably done by a mechanical scraper of non-wettable material, in particular an automatic scraper that moves along the surface of the metal immediately before analysis, or by scooping the surface by moving the sample against a fixed scraper. The scraper may be provided as part of the measuring head, extending into the liquid metal sample from a substantially flat bottom, and the scraping function may be performed by moving the measuring head horizontally before performing the measurement. The scraper may also be arranged as a separate component. In some embodiments, scraping is performed by moving the sample crucible over the scraper. In some embodiments, the removal of the surface layer may be achieved alone or in addition to mechanical skimming using an excitation method such as a laser pulse that strikes the sample surface at the measurement point prior to the actual analysis. The emission from such a "cleaning pulse" can also be advantageously monitored to determine the presence and chemical nature of surface contamination. The "cleaning pulse" can be configured to have a different pulse energy from the plasma excitation pulse used to perform the LIBS measurement.

[0048] A key feature of the present invention is ensuring a predetermined distance between a specific portion of one excitation means and / or photodetector optical system and the other sample surface. However, this is technically feasible only when, in the case of a liquid sample, the movement of the liquid surface can be minimized at the same time. The present invention teaches that it is important to maintain internal motion and a substantially constant sample temperature during the analysis in order to ensure representative measurements. An advantage of the method of the present invention is that these conditions can be satisfied simultaneously, while ensuring that the sample surface is maintained in a fixed relationship with the excitation and / or detection means throughout the measurement process, with a variation of less than ±100 μm, preferably less than ±50 μm, and more preferably less than ±25 μm.

[0049] To further ensure an appropriate distance to the sample surface, the photodetecting optics should be precisely positioned at a predetermined distance from the sample surface for each measurement. This is preferably achieved by having a distance sensor in the apparatus head directed towards the sample surface through the opening of the sample container. The output from the distance sensor is transmitted to a control unit that controls the movement of the apparatus head, the photodetecting optics, and / or the sample container by a moving mechanism to adjust the distance to a desired value. In some embodiments, precise positioning of the apparatus head is achieved by using a servo motor in cooperation with a suitable gear mechanism for moving the apparatus head or its movable internal unit or movable platform. Electromagnetic coils or other translational motion means, including stepper motors, may also be used. The distance sensor and control unit can advantageously be configured to operate dynamically, for example, during the handling, processing, and analysis of the sample, the sensor operates continuously to measure the distance to the sample surface, and based on feedback from the sensor, the control unit adjusts the distance substantially continuously as needed.

[0050] In one embodiment, the measuring head features one or more pins or similar discrete structures extending from the substantially flat bottom surface of the measuring head and contacting or penetrating the surface of the liquid metal sample when the measuring head is positioned at the measurement location, which is intended to reduce vertical waves on the surface of the liquid metal sample (for example, in the case of pure aluminum near its melting point, it may exhibit high surface tension of up to 1000 mN / m), and such waves are related, for example, to the induced motion of the liquid metal sample, shock waves propagating from a laser pulse that has previously struck the sample surface, gas flow from an open-bottom chamber, or external mechanical vibrations.

[0051] The sampling location (i.e., the location on the liquid surface where the sample is excited) and the local environment / atmosphere directly above it are substantially consistent throughout repeated measurements. In LIBS measurements, this is achieved, for example, by ensuring consistent energy of the laser pulse when interacting with the sample, and, in preferred embodiments, by ensuring that inert atmospheric conditions are maintained at and around the sampling point. Thus, in preferred embodiments, a flow of an inert gas, most commonly such as argon, is applied to the sampling point through an open-bottom space such as a chamber, as described below. Within the framework of the present invention, it is important that any gas flow from the apparatus head is substantially laminar and not significantly perturb the surface of the liquid metal, and not disturb the conditions ensured by the present invention.

[0052] In summary, the present invention comprises a measuring device and means for acting on a liquid metal sample as follows: - The outermost layer can be removed from the metal sample before analysis. The excitation optical system is configured as follows: - The sample surface is positioned at least 1 Rayleigh length in front of the appropriate beam focus. - The sample surface is precisely positioned and placed relative to the substantially flat bottom of the measuring head. During the analysis, we guarantee the following: - The temperature of the sample is substantially constant. - The surface motion of the liquid metal during analysis is minimized. - A substantially laminar gas flow is provided to maintain a stable state within the light beam path and for the development of laser-induced plasma. Furthermore, in some embodiments, the following is guaranteed: -Internal motion in the liquid metal is sufficiently maintained. Furthermore, the sampling and processing of the metal are preferably automated with respect to heating and stirring means for the liquid metal and measuring devices for consistent and accurate positioning of the sample. In some embodiments, the apparatus is configured for use in a fixed location, such as near an open trough, holding furnace, alloy furnace, or melting crucible in a metal manufacturing or processing plant, but is not limited thereto. In other embodiments, the apparatus is configured to be easily transportable or portable, for example, on a platform that can be moved from one sampling point to another.

[0053] Those skilled in the art will understand that the drawings described below are for illustrative purposes only. The drawings are not intended to limit the scope of the teachings of this invention in any way. [Brief explanation of the drawing]

[0054] [Figure 1] Figure 1 shows the geometric arrangement of the optical system and laser beam relative to the sample. [Figure 2] Figure 2 shows how laminar gas flow is provided through the apparatus head of the present invention and between the apparatus head and the sample surface. [Figure 3] Figure 3 shows an embodiment with a gas flow arranged in the laser beam path and the path for light emission detection. [Figure 4] Figure 4 shows an embodiment in which induction heating is applied to the sample to improve its homogeneity. [Figure 5] Figure 5 shows measurement results illustrating how surface instability is reduced by decreasing the coupling to the induction heating means. [Figure 6] Figure 6 shows an embodiment in which the distance measuring sensor is positioned on the device head. [Figure 7] Figure 7 shows how the heat-resistant base plate is positioned at the bottom of the device head. [Figure 8] Figure 8 shows an embodiment having a pin extending from the apparatus head that touches the sample surface to reduce vertical wave motion on the sample surface. [Figure 9] Figure 9 shows a schematic diagram of an embodiment of the present invention, which has a flat induction heating element placed below the sample crucible. [Figure 10] Figure 10 shows an embodiment in which the sample crucible is heated by being placed in a flow of liquid metal. [Figure 11] Figure 11 shows an embodiment of the present invention in which a liquid metal sample is collected in a reservoir within a channel having inlet and outlet valves. [Figure 12] Figure 12 shows the movement of a mechanical scraper used to remove the outermost layer before measurement. [Figure 13] Figure 13 shows the correlation between concentration measurements obtained with an OES system calibrated with traceable standards and LIBS measurements performing the features of the present invention. [Figure 14] Figure 14 shows the relative standard deviation (%RSD) of repeated measurements using the features of the present invention for several trace and alloying elements in liquid aluminum, compared to the relative standard deviation of measurements in an OES system for solid aluminum sampled from the same molten material. [Modes for carrying out the invention]

[0055] The present invention will be described in further detail with reference to the accompanying drawings, which should not be construed as limiting the overall scope of the general concepts of the invention. Illustrative embodiments of the invention will be described below with reference to the drawings. These examples are provided to provide a further understanding of the invention without limiting its scope.

[0056] The following description illustrates several specific features that are useful in illustrating embodiments of the present invention, which are defined separately by the claims. Similarly, a series of steps are described, and those skilled in the art will understand that, unless required by context, the order of the steps is not important to the resulting configuration and its effects. Furthermore, regardless of the order of the steps, it will be apparent to those skilled in the art that there may or may not be time delays between some or all of the steps described.

[0057] Figure 1 shows the essential geometric configuration of the present invention, illustrating the interaction between a pulsed laser beam 1 and a laser excitation optical system comprising a focusing element 2 that defines the focal length f of the focused laser beam. This (virtual) focus is shown located inside sample 23, and for an ideal beam, 1 Rayleigh length Z is present in both directions from the focus. R It has a Rayleigh range that extends by only a certain amount. Experimentally, the Rayleigh length has conventionally been defined as the distance along the direction of incidence of the beam from the beam waist to a point (on both sides of the focal point) where the cross-sectional area of ​​the beam is twice that of its waist.

[0058] The laser pulse interacts with the sample, ablating a portion of the sample volume, which is shown here as a dashed line (not an exact proportion), and generates a plasma plume 3 (not an exact proportion) on the liquid surface.

[0059] Figure 2 schematically illustrates how the gas flow 4 (typically an inert gas) flows through the laser beam path 10 and the open-bottom chamber 5, passing over the plasma plume 3 and the bottom surface 7 of the apparatus head 6 near the chamber 5. The bottom surface is preferably maintained at a distance of about 2 mm to 3 mm from the surface of the sample 23 held in the crucible 8.

[0060] Figure 3 shows a possible configuration of a separate light emission detection path 11 leading to a detection optical system (not shown). Preferably, an inert gas is also guided through a gas path 12 to the light emission detection path 11 and further to the laser light emission path 10.

[0061] Figure 4 shows an embodiment in which the induction heating element 9 is positioned beneath the crucible 8 holding the sample. The induction heating element is shown as a cross-section of concentric coils forming a substantially flat element. Inductive coupling with the sample 23 in the crucible generates a stirring effect within the sample, schematically shown, and enhances the homogeneity of the sample. Placing a substantially flat element beneath the sample, and appropriately adjusting the induction power and the distance between the sample and the flat element, ensures that the stirring effect does not cause excessive vertical movement of the sample surface during measurement.

[0062] Figure 5 is a graph showing the effect of inductively heating the sample, illustrating how surface instability is reduced by decreasing the coupling with the induction heating means. For illustrative purposes, the LIBS apparatus is arranged according to the sample preparation method according to the present invention, and the sample crucible is heated by induction heating using a flat induction heating element placed beneath the sample crucible, thereby changing the distance between the crucible and the heating element.

[0063] The graph shows the measured results of surface instability and temperature drop as a function of the coupling between the liquid aluminum sample and the inductive heating element. The derivative is set to maintain a constant fixed temperature above the melting point of the metal. The crucible holding the sample is then moved to a certain distance away from the derivative at the start of each spectroscopic measurement. The measurement interval lasts 30 seconds, during which vertical surface motion (shown as a standard deviation from the mean) and temperature drop due to the decrease in coupling with the inductive electromagnetic field are recorded. When the distance between the bottom of the sample crucible and the top of the derivative is small (5 mm or less), the sample surface is more disturbed, with surface fluctuations observed from 40 μm to 90 μm-100 μm for the closest measured distance (1 mm). Conversely, the sample temperature remains relatively stable, decreasing by approximately 10°C for every 5 mm during the measurement period. As the distance increases further, vertical surface motion decreases, but the temperature decreases more during the measurement period, reaching nearly 20°C at the maximum distance of 15 mm. The shaded area defines the appropriate operating range for a specific heating element and crucible configuration, ensuring that the present invention provides optimal measurement conditions that simultaneously minimize vertical surface motion and temperature drop.

[0064] Figure 6 shows an embodiment of the present invention in which distance measurement is applied to ensure an accurate and precise distance between one light-emitting element and the other excitation and receiving optical system. A distance measuring sensor 20 is located in the apparatus head 6 and detects the distance between the apparatus head and the sample surface. The distance sensor transmits a signal to a control unit that adjusts the vertical position of the apparatus head using a moving actuator (not shown) to set or maintain an accurate predetermined distance to the sample surface.

[0065] Figure 7 is a simplified diagram showing how the heat-resistant bottom plate 35 is positioned at the bottom of the apparatus head 6. The plate forms an inlet opening 36 to the open-bottom chamber 5, which is concentric with the chamber but has a diameter / cross-section slightly smaller than the chamber wall.

[0066] Figure 8 shows a pin 15, an optional feature of the present invention, extending from the bottom of the apparatus head and contacting or penetrating the surface of the liquid metal when the apparatus head is in the measurement position. In actual embodiments, the number and shape of such features may vary. The purpose of this feature is to act on the vertical motion of waves on the surface of the liquid metal sample. Similar features can be used to perform the function of a scraper to remove oxide or slag film from the surface immediately before measurement by contacting or immersing them in the liquid metal surface and utilizing the simultaneous horizontal motion of the measuring head and the sample crucible relative to each other.

[0067] Figure 9 shows a schematic example of a system according to the present invention, in which a sample crucible 8 has a handle 24 and is shown in the basic shape of a cylindrical ladle for containing a sample of liquid metal 23. Below the crucible, a circular and flat induction heating element 9 is shown at a certain distance from the crucible. An apparatus head 6 is shown on the sample crucible and is provided with a supply path 12 for an inert gas. The apparatus head surrounds an optical system for focusing and directing a laser beam to the sample surface and for receiving and transmitting radiation emitted from the plasma generated on the sample surface. A thermocouple 30 measures the temperature of the sample.

[0068] Figure 10 shows an embodiment in which a sample crucible 8 is heated by being placed in a flow of liquid metal 40. This can be done, for example, by collecting a sample of liquid metal, then extracting it, and attaching a sample ladle to a moving means (not shown) adjacent to a trough of liquid metal for immersing the crucible in the flow in order to hold the crucible in a stationary position during measurement. Preferably, the apparatus head 6 is also positioned on a movable arm or movable platform so that the head can be appropriately positioned above the sample and then away from the trough.

[0069] Figure 11 shows an alternative embodiment in which the sample crucible 8 is configured as a reservoir, into which the sample can be supplied through ducts 41, 42. The ducts can be advantageously connected to and positioned in a trough or furnace that holds liquid metal, such as in a manufacturing facility. Duct 42 is shown with two flaps 43, 44, one on each side of the reservoir, which represent gates in the form of gate valves, respectively, that can close the ducts on both sides of the reservoir to stop the flow through the reservoir before and during measurement. Below the reservoir crucible, an induction heating element 9 is shown.

[0070] Figure 12 illustrates the principle of using a scraper 50 to scoop the surface of a liquid metal sample 23 immediately before measurement. The scraper is moved across the surface, or alternatively, the sample crucible is moved, so that there is movement of the scraper relative to the sample surface. The scraper can be mounted on the apparatus or configured as a separate part.

[0071] Figure 13 shows the correlation between LIBS measurements of three trace elements in liquid aluminum performed using the present invention and the corresponding OES measurements of solid samples cast from the same molten sample. In all cases, the correlation coefficient (Pearson r) is greater than 0.9995, suggesting that the correlation is limited only by random measurement errors in the LIBS and OES measurements on the order of 1% of the measured concentration. In both cases, the measurement standard deviation is approximately the same as or smaller than the size of the symbols in the figure.

[0072] Figure 14 shows the relative measurement standard deviations of 10 elements (silicon, iron, copper, manganese, nickel, chromium, titanium, gallium, and antimony or magnesium) at indicated concentrations measured in liquid aluminum (panel (a)) and AlSi7Mg0.3 aluminum alloy (panel (b)) using the present invention (white circles). For comparison, the figure shows the random errors observed in OES measurements of corresponding solid samples (black squares) prepared from the same melt, according to ASTM sampling and measurement standard E1251. For most elements, the random errors in the liquid metal measurements are smaller than those observed in the corresponding solid samples. This difference is more pronounced in the case of alloys, where segregation during solidification is known to significantly affect the homogeneity of the sample.

Claims

1. A method for measuring one or more elements in a liquid metal or alloy sample using laser-induced breakdown spectroscopy (LIBS), - Prepare a sample of the liquid metal or alloy to be analyzed, - The arrangement involves arranging a device head on the surface of a sample, wherein the device head comprises a laser excitation optical system arranged to receive and focus light transmitted from a pulsed excitation laser, a light-receiving optical system for receiving light emitted from the sample, and an open-bottom chamber through which the laser excitation optical system guides the laser light, the open-bottom chamber extending upward from the flat bottom surface of the device head, and the arrangement involves - The apparatus head is positioned at a distance from the sample surface such that the laser excitation optical system focuses the pulse from the excitation laser to a focal point located below the sample surface, at a distance longer than one Rayleigh length of the focused laser beam. - To ablate a portion of the sample volume and generate plasma on the liquid metal, one or more laser pulses are emitted onto the sample using sufficient light energy to pass through the excitation optical system, - The system receives light emitted from the generated plasma through the photodetector optical system, transmits it to a detector, and records the spectral data of the detected light. - Analyzing the spectral data to determine the amount of one or more elements, A method for providing this.

2. The method according to claim 1, wherein the laser excitation optical system is positioned within the apparatus head such that when the laser focus is appropriately positioned, the bottom surface of the apparatus head is at a distance of 1 mm to 10 mm from the sample surface.

3. The method according to claim 1 or 2, comprising supplying a gas flow through the open-bottom chamber to maintain an inert atmosphere in the chamber and a laminar flow of gas between the sample surface and the bottom surface of the apparatus head near the open-bottom chamber.

4. The method according to any one of claims 1 to 3, wherein a laser excitation channel extends from the pulsed excitation laser to the open-bottom chamber, and an emission-receiving channel extends from the open-bottom chamber to the photodetector optical system.

5. The method according to claim 4, dependent on claim 3, wherein a gas flow is supplied through the gas inlets of the laser excitation channel and the light emission receiving channel, forming a gas flow that passes through the channels and toward and through the open-bottom chamber.

6. The method according to any one of claims 1 to 5, comprising providing a heat source near the sample and heating the sample using the heat source for a period including at least the period prior to the analysis, wherein heat is applied over the measurement period.

7. The method according to claim 6, wherein the heat source comprises a flat induction heating element placed beneath the sample crucible.

8. The method according to any one of claims 1 to 7, wherein the step of scooping the surface of the sample is performed before measurement.

9. An apparatus for measuring one or more elements in a liquid metal or alloy sample using laser-induced breakdown spectroscopy (LIBS), - Pulsed excitation laser and, - The device comprises a device head, and the device head is Laser path channel and A laser excitation optical system arranged within the laser path channel, A light-receiving optical system for receiving light emitted from a sample plasma, An open-bottom chamber extending upward from the flat bottom surface of the device head, wherein the laser path channel extends into the open-bottom chamber, The chamber comprises at least one gas channel for supplying gas to the open bottom chamber, The pulsed excitation laser and laser excitation optical system are configured such that, when the apparatus head is at a distance of 1 mm to 10 mm from the sample surface, the focal point of the pulsed excitation laser is below the sample surface and at a distance longer than one Rayleigh length of the focused excitation laser beam. A device that is composed of such devices.

10. The apparatus according to claim 9, wherein the open-bottom chamber has a cross-sectional area in the range of 1 cm² to 3 cm² and a height in the range of 5 mm to 15 mm.

11. The apparatus according to claim 9, wherein when the apparatus is in the measurement position, it comprises one or more pins or other separate structural members extending downward from the flat bottom surface and contacting or penetrating the surface of the liquid metal.

12. The apparatus according to claim 9, further comprising a scraper for scooping up the surface of the sample.

13. The apparatus according to claim 9, further comprising a sample crucible for receiving and holding the aforementioned sample.

14. The apparatus according to claim 9, comprising a heat source for supplying heat to the sample, wherein the heat source is an induction heat source comprising a flat induction heating element positioned below the sample crucible.

15. The apparatus according to any one of claims 9 to 14, wherein the flat bottom surface is provided with a heat-resistant layer, the heat-resistant layer has a hole concentric with the open bottom chamber, and the hole has the same size and shape as the cross-section of the chamber, or is smaller or larger.

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