Improved method for realizing accurate measurement of high vapor pressure elements in molten metal by using laser-induced breakdown spectroscopy

Abstract

A method allows for reproducible, precise and substantially temperature-independent quantitative measurement of one or more high vapor pressure elements in a liquid metal or alloy sample using laser-induced breakdown spectroscopy (LIBS) or other similar methods. The method introduces one or more selectively reactive substances and a substantially inert carrier gas to alter the local environment at and around the sampling point of the liquid metal or alloy. This results in a significant and rapid reduction in the vapor effect of the LIBS signal on the high vapor pressure species in the metal or alloy melt, partially eliminating the temperature dependency of the high vapor pressure elements and their dependency on false environmental factors, thereby reducing the associated uncertainty in LIBS measurements.

Description

Technical Field

[0001] This disclosure relates to the field of spectroscopy, or more specifically to laser-induced breakdown spectroscopy (LIBS) methods for achieving precise and (near) temperature-independent quantitative elemental analysis of volatile elements in liquid metals and alloys. Background Technology

[0002] Laser-induced breakdown spectroscopy (LIBS) is a well-established technique for the chemical analysis of solid, liquid, and gaseous samples. With proper sample preparation and suitable plasma radiation analysis, this method can be used for quantitative elemental analysis. LIBS has been applied to the chemical analysis of a wide variety of materials, primarily in solid form.

[0003] The potential for using laser-induced plasma chemical analysis of liquid metals was discovered as early as 1966 [Runge, Bonfiglio, Bryan, Spectrochimica Acta 22, 1678-1680 (1966)]. With advancements in optical technology, efforts to develop LIBS systems for analyzing molten metals, including zinc, aluminum, and steel, have increased significantly over the past 25 years due to the industrial significance of this measurement.

[0004] Previously proposed LIBS systems for molten metal analysis typically involve immersion probes or non-contact measurements, where the measuring device can be located near or far from the surface to be analyzed. The applicant's earlier application EP4009037 discloses a non-immersion LIBS apparatus for high-precision elemental analysis of major non-volatile elements in liquid metals or alloys. This apparatus includes a measurement chamber with a bottom opening, through which a laminar flow of, preferably, inert gas is arranged. It is known that in LIBS measurements, the atmosphere near the measurement point is controlled. This is typically achieved by introducing an inert gas around the measurement point, most commonly to provide an environment that ensures a strong and stable LIBS signal. Inert or substantially inert gases with different physical properties (such as atomic / molecular weight, thermal conductivity) have been used to influence plasma expansion. Furthermore, helium has been used to promote energy transfer between individual atomic species in the plasma through collisional processes. In some cases, gas mixtures have been used to obtain the most suitable parameters for enhancing the LIBS signal and improving accuracy. CN110132943A discloses a method for using a gas mixture of Ar, Ne, and He as the ambient gas around the measurement point, wherein the composition of the mixture is optimized to slightly improve the repeatability of LIBS spectra on solid alloy samples.

[0005] In the prior art, gas flow or pressure in LIBS measurements is also used for secondary purposes. US 6,909,505 discloses a LIBS probe in which a gas flow is used to generate bubbles inside molten metal and to perform laser excitation on the inner surface of the bubbles. US 6,762,835 discloses a LIBS probe for liquid metal in which an inert gas flow (nitrogen, argon, helium, or a mixture of the three) and thus the pressure inside the probe are modulated to position the metal surface inside the probe at a specific distance from the excitation and collection optics. Therefore, in the prior art describing LIBS analysis of liquid metals for quantification of trace elements present in the metal, the purpose of introducing a flow of gas or gas mixture around the sampling point or controlling the pressure of the gas or gas mixture around the sampling point is to achieve one or more of the following effects: 1) increasing the overall signal strength; 2) increasing the reproducibility of plasma emission by using a chemically inert gas with suitable physical parameters; and 3) applying positive or negative pressure on the surface of the liquid metal to control the metal level or generate bubbles.

[0006] As a non-inert gas, oxygen is known to play a role in the development of plasma emission through the so-called oxygen quenching effect, which shortens the lifetime of plasma emission. For aluminum targets, the presence of oxygen in the surrounding gas also induces significant molecular emission from AlO (K. Chartig, B. Brumfield, M. C. Phillips, S. S. Sarilal, *Acta Spectrochimica Acta Part B: Atomic Spectroscopy*, 135, 2017, 54-62). Therefore, the presence of oxygen leads to a reduction in the time-integrated LIBS signal and potential spectral interference from molecular emission, both of which are detrimental to quantitative chemical analysis. Furthermore, in the case of liquid metal analysis via LIBS, the presence of oxygen in the environment surrounding the measurement point leads to rapid oxidation of the ablated metal, and the resulting oxides accumulate around the measurement point, further increasing measurement instability.

[0007] In LIBS analysis of liquid metals, it has been recognized that quantifying trace or alloying elements with high vapor pressures is problematic, where the partial pressure of the element in the melt leads to a significant amount of the element present in the gas phase above the liquid metal surface. Studies have shown that for such volatile elements, the measured LIBS signal includes contributions from the liquid metal or alloy evaporated by the laser pulse, as well as contributions from the gas phase present above the melt surface. In this case, the gas phase contribution to the LIBS signal depends exponentially on the melt surface temperature and is proportional to the activity of the element in the melt. This is recognized, for example, in CN107328760, which discloses an empirical melt temperature correction formula to explain the temperature dependence of LIBS data for trace elements in liquid steel. However, applying this formula requires accurate knowledge of the melt surface temperature, which is not always readily available. Furthermore, as is common in LIBS analysis, the use of multiple laser pulses introduces additional evaporation around the sampling point, which cannot be explained by this correction formula. Finally, the proposed formula does not account for the interaction effects of other elements in the melt, which can be significant even for very low concentrations of interacting substances. Therefore, this effect must be corrected for separately, requiring prior knowledge of the melt's chemical composition. In some applications, such as metal recycling, it is impractical to consider the presence of multiple potential impurities and alloying elements to correctly interpret LIBS signals generated by specific volatile substances.

[0008] In addition to the above, further complications arise due to the potential absorption of plasma emission by vapor surrounding the emitting material and which is typically at a lower temperature. This is well known in the art to limit the applicability of certain emission lines to quantification [Rezaei et al., Acta Spectrochimica Sinica B, 169, 105878 (2020)], particularly those associated with atomic transitions where the absorbing gas has high low-level occupation and high transition probabilities. Although such self-absorption can occur purely by gas or plasma formed by the ablation of a laser pulse without any significant vapor pressure, the presence of vapor above the melt surface exacerbates the problem. Summary of the Invention

[0009] The present invention seeks to overcome the above-mentioned drawbacks and / or provide an improved LIBS measurement method that can perform quantitative elemental analysis of both major and minor components in molten metal or alloy, regardless of whether these components are volatile.

[0010] In a broader sense, this invention provides an improved method using laser-induced breakdown spectroscopy (LIBS) technology, which enables reproducible, accurate, and substantially or nearly temperature-independent quantitative concentration measurements of liquid metal or alloy samples that may contain one or more high vapor pressure elements.

[0011] In this article, elements in molten metals or alloys that exhibit a vapor pressure significantly higher than that of the host metal in the melt are referred to as volatile or high vapor pressure elements, such as, but not limited to, magnesium, zinc, or sodium in aluminum melt.

[0012] This invention is particularly applicable to process control and / or quality control in the metallurgical industry, such as, but not limited to, primary and recycled aluminum production plants, aluminum recycling facilities, aluminum casting facilities, and virtually any other industry requiring precise quantitative elemental analysis of high vapor pressure elements present in liquid metals or alloys. Furthermore, this invention is especially useful under conditions where the temperature of the metal being measured cannot be precisely controlled and / or measured. An advantage of this invention is that it substantially reduces or eliminates the influence of the vapor pressure of one or more volatile elements on the LIBS signal. This is achieved by introducing one or more selectively reactive gaseous substances in precisely controlled proportions into a continuous stream of other substantially inert carrier gases (such as, but not limited to, argon, neon, helium, or mixtures thereof). The stream of the combined gas mixture (referred to herein as the selectively reactive gas mixture) is then directed to the surface of the molten metal or alloy sample, particularly around the intended measurement point (or sampling point), such that the environment around the measurement point comprises at least a substantially inert carrier gas and a mixture of one or more selectively reactive components. Therefore, by providing a (continuous) supply of a selective reactive gas mixture comprising one or more reactive components that specifically interact with one or more volatile components, the effects of the vapor pressure of one or more volatile substances (from molten metal or alloy), such as the interaction of carbon dioxide with evaporated magnesium, can be reduced or eliminated. This gas flow can be conveniently achieved in a bottom-opening chamber, such as the one disclosed in EP4009037, where a continuous and unobstructed flow above the measurement point can be ensured.

[0013] The purpose of a selectively reactive gas mixture is to balance the evaporation of selected volatiles that occur continuously from the exposed surface of the molten metal in an open chamber. In this document, the terms "measurement point" and "sampling point" are used interchangeably and refer to a point or area on the surface of the molten metal or alloy to be measured.

[0014] The application of selectively reactive gas mixtures described in this paper differs fundamentally from the application of gases or gas mixtures in prior art LIBS measurements. Applying a continuous flow of a selectively reactive gas mixture, containing a carrier gas of a defined proportion of specific selectively reactive gas components, to the sample surface at the sampling point, compared to a substantially inert gas environment, results in a significant reduction in the strong temperature dependence and vapor-related effects of the LIBS signal for reactive volatiles in molten metals or alloys, while maintaining high and stable signal levels for the same substances and other elements from liquid metal ablation. Therefore, this effect differs from the oxygen quenching effect, which affects the overall plasma emission from liquid metal in a substantially non-selective manner. By essentially eliminating the temperature dependence of the LIBS signal from high vapor pressure elements, the associated uncertainties in its quantification are greatly reduced.

[0015] Another advantage of this invention is that the use of a selectively reactive gas mixture ensures repeatable measurements of multiple laser pulses in a rapid pulse sequence incident on the surface of a liquid metal. Yet another advantage is that it reduces or substantially eliminates the influence of other elements in the melt on the recorded signal of the volatile material. Furthermore, it significantly reduces additional self-absorption of light from the emitting material, which occurs in the gas phase of the same material formed by evaporation from the melt surface.

[0016] Depending on conditions such as melt temperature and the vapor pressure and electronic structure of the volatile substances, the application of a selectively reactive gas mixture can lead to an increase or decrease in the observed LIBS signal. However, in both cases, when using a series of repeatedly incident laser pulses, it ensures consistent and accurate measurement of volatile elements and allows for quantification even when the temperature and presence of other elements in the molten metal are not clearly known. This invention can be used with any LIBS measuring apparatus that includes suitable components for quantitative elemental analysis of molten metal or alloy samples, regardless of whether the sample contains volatile or non-volatile elements or mixtures thereof, and that the LIBS measuring apparatus includes, or can be connected to, a suitable measuring chamber that allows for the introduction of a continuous flow of the selectively reactive gas mixture at and around the sampling point of the liquid metal sample. The method is not limited to any sample holding component, as it can operate with various sample containers, such as sample crucibles, and can also be operated where a suitable measuring chamber can be arranged adjacent to the surface of the liquid metal in a furnace opening, trough / tank, etc.

[0017] According to one aspect of the invention, a method is provided for the quantitative analysis of one or more elements in a molten metal or alloy by LIBS, wherein the negative impact of the gaseous phase of volatile trace elements or alloying elements in the molten metal or alloy on the analysis of volatile elements is reduced or substantially eliminated.

[0018] The method includes the steps of arranging a measurement chamber facing a surface of a molten metal or alloy, wherein the measurement chamber includes one or more openings or windows to provide a path for a focused laser beam from a LIBS-excited laser to enter the chamber and reach a sampling point on the surface, and to provide a path for emitted light from the generated plasma to exit the chamber and be collected and transmitted to a detector; the step of continuously transmitting a selectively reactive gas mixture to the sampling point within the measurement chamber, wherein the selectively reactive gas mixture includes one or more substantially inert carrier gas components and one or more selectively reactive gas components, and wherein one or more selectively reactive gas components of the selectively reactive gas mixture have the property of preferentially reacting with one or more volatile elements present in the molten metal or alloy; the step of ablating a portion of the sample by emitting one or more laser pulses with sufficient light energy at the sampling point to generate plasma above the sampling point to perform LIBS measurements on the sample surface; the step of receiving emitted light from the generated plasma and transmitting the emitted light to a detector for recording spectral data of the detected light; and the step of analyzing the spectral data to obtain a quantitative determination of one or more elements in the molten metal or alloy.

[0019] The molten metal or alloy can be the main component of the molten metal or alloy.

[0020] In one embodiment, the transmittance of the selectively reactive gas mixture flowing (or fed) to (or toward) the sampling point can be performed continuously. In another embodiment, the transmittance of the selectively reactive gas mixture can be performed before and during LIBS measurements on the sample surface.

[0021] In one embodiment, the selectively reactive gas mixture is mixed before being conveyed to the measuring chamber and is conveyed to the measuring chamber through one or more gas channels. In such an embodiment, the selectively reactive gas mixture may be provided by a premixed gas source (e.g., a compressed gas container) connected, for example, via a feed line to one or more gas channels, which in turn are connected to the measuring chamber.

[0022] In some embodiments, the instrument head may include one or more gas channels, such as those embedded within the instrument head. In some embodiments, a laser excitation channel and / or a receive / emit channel may be connected to or serve as one or more gas channels. The one or more gas channels are configured to facilitate the transmission of gas into the measurement chamber.

[0023] In one embodiment, the selectively reactive gas mixture may be prepared within a first feed line, which carries and is controllably connected to a gas source of a substantially inert gas or a gas source comprising one or more substantially inert gas components. The first feed line is connected to one or more second feed lines, wherein the second feed lines are used to controllably deliver one or more selectively reactive gas components into the first feed line, and allow, for example, the substantially inert gas or gas mixture to be mixed with one or more selectively reactive gas components to form the selectively reactive gas mixture by using a gas mixer and / or flow control components, and then the selectively reactive gas mixture is delivered to a measuring chamber through one or more gas channels.

[0024] In one embodiment, a selectively reactive gas mixture can be prepared within a measuring chamber, wherein one or more selectively reactive gas components are transported into the measuring chamber through one or more first gas channels, and one or more carrier gas components are transported into the measuring chamber through one or more second gas channels. In such an embodiment, a substantially inert carrier gas or gas mixture can be controllably transported directly into the measuring chamber through the first gas channels, and one or more selectively reactive gas components can be controllably transported directly into the measuring chamber through one or more second gas channels, wherein mixing of the carrier gas and the selectively reactive gas components is performed within the measuring chamber to form the selectively reactive gas mixture.

[0025] In some embodiments, wherein the selectively reactive gas mixture is mixed before being transferred to the measuring chamber or is mixed within the measuring chamber, the method may include the step of controlling the flow of one or more carrier gas components and the flow of one or more selectively reactive gas components to provide a suitable proportion of the components before mixing the selectively reactive gas mixture.

[0026] The steps of controlling the flow of one or more carrier gas components may include using a combination of at least one flow control valve and at least one flow meter.

[0027] The steps of controlling the flow of one or more selective carrier gas components may include using a combination of at least one flow control valve and at least one flow meter.

[0028] In one embodiment, flow control components are provided for both one or more carrier gas components and one or more selectively reactive gas components. These flow control components may include linkage mechanisms, such as, but not limited to, a direct connection between two flow control components to regulate flow, or a connection from each flow control component to an external control unit that regulates the flow of the selectively reactive gas components such that the flow of the selectively reactive gas components is proportional to the flow of the carrier gas. This can be particularly useful if the mixing of one or more carrier gas components and one or more selectively reactive gas components to form a selectively reactive gas mixture occurs within the measuring chamber, i.e., after the selectively reactive gas components and the carrier gas are transferred to the measuring chamber, or before being transferred to the measuring chamber, in an additional chamber or container for premixing. In one embodiment, a gas mixer may be used to mix one or more selectively reactive components and one or more carrier gas components to form a selectively reactive gas mixture.

[0029] In one embodiment, the total flow rate of the selectively reactive gas mixture is controlled. In one such embodiment, the total flow rate of the selectively reactive gas mixture is in the range of 0.5-5 L / min. In another embodiment, the total flow rate of the selectively reactive gas mixture can be in the range of 1-5 L / min. In yet another embodiment, the total flow rate of the selectively reactive gas mixture can be in the range of 1-2 L / min.

[0030] In one embodiment, one or more substantially inert carrier gas components may be selected from the rare gas group. In one embodiment, one or more substantially inert carrier gas components include helium, neon, argon, or mixtures thereof. In one embodiment, one or more carrier gas components include argon. The selection of (multiple) substantially inert carrier gas components is generally based on suitable properties for obtaining the gaseous substance or mixtures thereof, such as, but not limited to, reactivity, atomic / molecular weight, energy level structure, diffusivity, and thermal conductivity.

[0031] In one embodiment, one or more selectively reactive gas components are selected based on their chemical properties that allow one or more selectively reactive gas components to selectively react with one or more volatile elements in a molten metal or alloy. For the reasons discussed above, gas mixtures containing a large proportion of oxygen (such as, but not limited to, air or air-like gas mixtures), or any mixture having a substantial proportion of oxygen (e.g., greater than 5%, greater than 2%, or greater than 1%), are excluded from being selected as one or more selectively reactive gas components, or as a selectively reactive gas mixture.

[0032] In one embodiment, one or more selectively reactive gas components are carbon dioxide (CO2).

[0033] In one embodiment, one or more selectively reactive gaseous components are sulfur hexafluoride (SF6).

[0034] In one embodiment, the proportion of one or more selectively reactive gas components in the selectively reactive gas mixture is in the range of 1% to 20% relative to the total volume of the selectively reactive gas mixture, preferably in the range of 2% to 10%, more preferably in the range of 4% to 8%, such as in the range of 1%, or 2%, or 3% or 4%, or 20% or 10% or 8% or 6%.

[0035] In one embodiment, the proportion of one or more selectively reactive gas components in the selectively reactive gas mixture is in the range of 1% to 20% relative to the total number of moles in the selectively reactive gas mixture, preferably in the range of 2% to 10%, more preferably in the range of 4% to 8%, such as in the range of 1%, or 2% or 3% or 4%, up to 20% or up to 10% or up to 8% or up to 6%.

[0036] In one embodiment, feeding / transferring a selectively reactive gas mixture or individual components of a selectively reactive gas mixture into the measurement chamber can be performed at a controlled and time-varying flow rate to achieve a balance between the gas flow within the measurement chamber and the evaporation of the volatile components of the liquid metal, and to maintain this balance at least during the duration of the LIBS measurement.

[0037] The measuring chamber needs to provide stable conditions by allowing controlled flow of selectively reactive gas components at the sampling point and in the immediate vicinity of the sampling point, such that one or more selectively reactive components of the selectively reactive gas mixture selectively react with at least one evaporated component of the molten metal.

[0038] The measurement chamber allows a continuous flow of selectively reactive gas mixtures at the sampling point and the generated plasma, as well as in the immediate vicinity of the sampling point and the generated plasma. In one such embodiment, the measurement chamber can be formed by configuring a LIBS instrument head (i.e., a non-immersion instrument head) above the surface of the molten metal or alloy. In an alternative embodiment, the measurement chamber can be formed by immersing a LIBS instrument head or a portion thereof into the molten metal or alloy (i.e., an immersion instrument head). In these embodiments, the measurement chamber can be arranged in a suitable position and orientation relative to the liquid metal surface, allowing LIBS measurements to be readily performed at the sampling point on the surface. The measurement chamber can have any suitable size and shape for accommodating measurements of a portion of the surface of the liquid metal to be tested, i.e., for containing a plasma plume (where a portion of the sample is ablated and excited to form plasma during measurement), thereby providing confined and stable environmental conditions for the sampling point and the plasma plume.

[0039] In one embodiment, the measurement chamber is formed by a non-immersion instrument head. The measurement chamber may be a chamber with a suitably shaped bottom opening to accommodate measurements of the sample via an ablation portion (typically in the form of a plasma plume) and to allow a continuous flow of gas through the chamber while still maintaining confined and stable environmental conditions around the sampling point. In such an embodiment, a space is formed between the distal end of the instrument head and the surface of the molten metal or alloy, thereby allowing a continuous flow of a selectively reactive gas mixture at and around the sampling point and further through this space, and out of the measurement chamber.

[0040] In one embodiment, the measurement chamber is formed by an immersion instrument head such that at least the distal end of the instrument head is below the surface of the molten metal or alloy. The measurement chamber includes one or more openings positioned above the surface of the molten metal or alloy to allow a continuous flow of the selectively reactive gas mixture at the sampling point and the generated plasma, as well as in the immediate vicinity of the sampling point and the generated plasma. In such an embodiment, the measurement chamber is a substantially closed volumetric chamber, thereby protecting the interior of the measurement chamber from external environmental conditions (or atmosphere). However, to ensure a continuous flow of the selectively reactive gas mixture around the measurement point at the surface of the liquid metal sample, the substantially closed volumetric chamber may be made of a perforated or porous material to allow a continuous gas flow through the measurement point without significantly increasing the internal gas pressure of the measurement chamber (significantly increasing the internal gas pressure of the measurement chamber would affect the surface of the liquid metal). In one embodiment, the instrument head may be configured and arranged such that one or more openings for allowing a continuous flow of the selectively reactive gas mixture are positioned less than 10 mm above the surface of the molten metal or alloy.

[0041] In some embodiments, the LIBS instrument head used in this invention may include: ● The distal end of the instrument head faces the surface of the liquid metal during measurement. During measurement, the distal end of the instrument head is preferably positioned within 1-10 mm above the surface of the molten metal or alloy sample. In alternative embodiments, the distal end may be immersed in the molten metal or alloy. ● A bottom-opening measurement chamber extending upward from the distal end of the instrument head, allowing gas to flow through the chamber, past the sampling point, and into the space between the distal end of the instrument head and the surface of the molten metal or alloy. In an alternative embodiment, wherein the instrument head is at least partially immersed in the molten metal or alloy, the measurement chamber includes one or more openings allowing gas to flow through the chamber, past the sampling point, and out of one or more openings. ● A laser excitation channel that extends from a pulsed laser to a cavity with a bottom opening. The laser excitation channel can also be used as a gas channel. ● A laser excitation optics arranged in a laser excitation channel, the laser excitation optics including a focusing element for focusing the laser. ● Transmit / receive channel, which extends from the bottom-opening chamber to the laser receiving optics. The transmit / receive channel can also be used as a gas channel. ● Receiving optics, which are used to receive the emission of plasma generated by the excitation laser at the sample surface. ● Components that measure and maintain a substantially consistent relationship between the measuring chamber and the surface of the liquid metal during measurement. ● Gas passage for feeding (multiple) gases or (multiple) gas mixtures into a chamber with a bottom opening.

[0042] In some embodiments, a gas or mixture of gases, such as one or more selectively reactive gas components and / or one or more carrier gas components and / or a selectively reactive gas mixture, is fed from at least one gas source or gas mixer into the measurement chamber through one or more gas channels and / or laser excitation channels and / or emitter-receiver channels to obtain and / or maintain desired environmental conditions, preferably defined by the selectively reactive gas mixture, at and immediately above the sample surface within the measurement chamber, wherein the gas flow does not significantly disturb the sample surface. This results in the plasma's surrounding environment being characterized by a constant flow of substantially homogeneous gases, thereby balancing the flow of atoms of volatile substances from the liquid metal sample surface. Therefore, the number, size, and shape of the gas channels, the flow rate of the gas(s) or gas(s) mixture, and the distance from the distal end of the bottom-opening measurement chamber to the sample surface, or alternatively, the number, size, and shape of one or more openings, are preferably and carefully selected to maintain a stable and continuous flow through the chamber, particularly around the sampling point.

[0043] After establishing the desired conditions in the measurement chamber by providing a flow of at least a sufficient selective reactive gas mixture to balance the evaporation of the volatile components of the molten metal, a portion of the sample is ablated by emitting one or more laser pulses with sufficient light energy at the sampling point to generate plasma above the liquid metal surface to perform LIBS measurements on the sample surface. Subsequently, the emitted light from the generated plasma is received and transmitted to a detector for recording the spectral data of the detected light. The spectral data is then analyzed, for example, by using a control unit, to obtain a quantitative determination of one or more elements.

[0044] As those skilled in the art will recognize, the excitation and detection components used to generate and receive atomic emissions from molten metal or alloy samples can include, but are not limited to, conventional LIBS methods, LIBS with bicollinear or noncollinear pulses, combined LIBS / discharge methods, fiber-coupled laser excitation, or any other LIBS measurement technique or apparatus known in the art. In one embodiment, the method is used to measure the concentration of one or more major components and / or the concentration of one or more trace elements in the molten metal or alloy. This includes both nonvolatile elements and one or more volatile elements affected by selectively reactive gas mixtures, wherein the measurement produces concentrations that are both accurate and substantially independent of temperature.

[0045] In one embodiment, the method is used to analyze various metals and metal alloys, such as, but not limited to, aluminum and aluminum alloys, steel and steel alloys, iron and iron alloys, copper, zinc, lead, and other metals, as well as metal alloys in their liquid state. As mentioned above, the method is particularly useful in industrial environments and applications where the melt contains at least one volatile element. The method is not limited to any specific element and can be used to measure the concentration of major components in an alloy sample as well as trace elements that may be present in the metal or alloy. Therefore, in some embodiments, the method can be used for LIBS measurements of molten metals or alloys that may include one or more elements selected from the group consisting of: aluminum, silicon, phosphorus, sulfur, chlorine, calcium, magnesium, sodium, lithium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, tin, antimony, tungsten, rhenium, iridium, platinum, gold, mercury, lead, and bismuth.

[0046] As described above, this method can be advantageously used to measure molten metal or alloy samples containing elements with high vapor pressure. These include, but are not limited to, magnesium, sodium, calcium, zinc, strontium, lithium, mercury, and cadmium, at typical process temperatures of aluminum and aluminum alloy melts.

[0047] In one embodiment, the method can be used to measure the magnesium concentration in molten metal or alloy. The method can also be advantageously used when the temperature of the liquid metal is uncontrollable and / or cannot be precisely known. Attached Figure Description

[0048] The foregoing and other embodiments, features, and advantages of the present invention will become apparent from the following more detailed description of specific embodiments of the invention, as illustrated in the accompanying drawings, in which the same reference numerals refer to the same parts throughout different views. The drawings are not necessarily to scale; rather, the emphasis is on illustrating the principles of the invention.

[0049] Figure 1A schematic diagram according to an embodiment of the invention is shown, wherein a selectively reactive gas mixture is conveyed into a measuring chamber with a bottom opening disposed above the surface of a molten metal or alloy, wherein the selectively reactive components of the selectively reactive gas mixture react with evaporated volatile chemical components from the melt.

[0050] Figure 2(a) schematically illustrates, with reference to the accompanying drawings, the accumulation of volatile chemical components in the bottom-opening measurement chamber when a substantially inert carrier gas mixture is fed into the measurement chamber for LIBS measurements of molten metals or alloys.

[0051] Figure 2(b) schematically illustrates, with reference to the accompanying drawings, how the accumulation of one or more volatile chemical components is reduced in a bottom-opening measurement chamber when a selectively reactive gas mixture is conveyed to the measurement chamber (e.g., for LIBS measurement of molten metals or alloys) according to the present invention.

[0052] Figure 3 A schematic diagram according to an embodiment of the invention is shown, wherein a selectively reactive gas mixture is conveyed into a measuring chamber having an immersion tubular structure including perforations (or holes) to allow a continuous flow of the selectively reactive gas mixture through the measuring chamber.

[0053] Figure 4(a) shows a schematic diagram according to an embodiment of the invention, wherein a selectively reactive gas mixture can be transferred from a premixed source of the selectively reactive gas mixture to a measuring chamber.

[0054] Figure 4(b) illustrates an embodiment in which the selectively reactive gas mixture is formed by mixing a first gas source comprising a substantially inert carrier gas and a second gas source comprising at least one selectively reactive gas component before the mixed selectively reactive gas mixture is transferred to the measuring chamber.

[0055] Figure 5 shows two schematic diagrams of embodiments of the invention, wherein an automated moving member is used to configure the LIBS instrument head to form a measurement chamber and perform LIBS elemental analysis measurements on molten metals or alloys. Figure 5(a) shows an embodiment of the invention in which some components related to the mixing of the protective gas mixture are arranged as separate moving members. Figure 4(b) shows an embodiment of the invention in which some components can be attached and connected to the automated moving member.

[0056] Figure 6The temperature dependence of the LIBS signal as a function of melt temperature (°C) is shown when using (i) a carrier gas that is only substantially inert (argon) and (ii) a mixture of selectively reactive gases (a mixture of argon and carbon dioxide). The LIBS signal originates from the volatile component (magnesium) in the molten alloy (the main metal is aluminum).

[0057] Figure 7 The graphs show the variation of the LIBS signal of magnesium in a molten alloy as a function of the number of successive sequences of laser pulses incident on the surface of the molten alloy for (i) argon gas and (ii) a mixture of argon and carbon dioxide.

[0058] Figure 8 The graph shows the LIBS signal of magnesium measured in two molten metal or alloy samples (i.e., aluminum melts containing 1% Mg and 4% Mg) as a function of the proportion of the selectively reactive gas component in a selectively reactive gas mixture consisting of argon and carbon dioxide. Detailed Implementation

[0059] Exemplary embodiments of the invention have been described above and below. These embodiments are provided to provide a further understanding of the invention, but are not intended to limit its scope.

[0060] In the following description, a series of steps may be described. Those skilled in the art will understand that, unless the context requires, the order of the steps is not critical to the resulting configuration and its effects. Furthermore, those skilled in the art will appreciate that, regardless of the order of the steps, the presence or absence of time delays between steps may exist between some or all of the described steps.

[0061] As used herein (including in the claims), the singular form of a term should be interpreted to include the plural form as well, and the plural form of a term should also be interpreted to include the singular form, unless the context otherwise indicates. Therefore, it should be noted that, unless the context explicitly states otherwise, as used herein, the singular forms “a / an” and “the” include plural referents.

[0062] Throughout the specification and claims, the terms “comprise,” “including,” “having,” and “contain,” and their variations, shall be understood to mean “including but not limited to,” and are not intended to exclude other components.

[0063] The present invention also covers the use of precise terms, features, values ​​and ranges in combination with terms such as about, around, generally, substantially, essentially, at least, etc. (i.e., “about 3” should also cover the exact 3 or “substantially constant” should also cover the exact constant).

[0064] The term "at least one" should be understood to mean "one or more," and therefore includes all embodiments that include one or more components. Furthermore, dependent claims referencing an independent claim describing that feature using "at least one" have the same meaning, regardless of whether the feature is referred to as "the one" or "the at least one."

[0065] It should be understood that modifications can be made to the foregoing and following embodiments of the present invention without departing from the scope of the invention. Unless otherwise stated, the features disclosed in the specification may be replaced by alternative features for the same, equivalent, or similar purposes. Therefore, unless otherwise stated, each disclosed feature represents an example of an equivalent or similar feature in a general series.

[0066] The use of exemplary language, such as "for instance," "such as," "for example," etc., is intended only to better illustrate the invention and does not imply any limitation on the scope of the invention unless required otherwise. Unless the context clearly indicates otherwise, any steps described herein may be performed in any order or simultaneously.

[0067] Except for at least some mutually exclusive combinations of features and / or steps, all features and / or steps disclosed in the specification can be combined in any combination. The features of this invention apply to all aspects of the invention and can be used in any combination.

[0068] Figure 1 A bottom-opening measurement chamber (1) according to the invention is schematically shown for use in conjunction with LIBS analysis (or other suitable methods). The bottom-opening measurement chamber (1) is formed when the LIBS instrument head (2) is configured adjacent to the surface (3) of a liquid metal or alloy, which may include volatile chemical components (4) exhibiting continuous atomic fluxes away from the surface and may be present in large quantities in the gas phase above the molten metal or alloy surface. Figure 1A selectively reactive gas mixture (5), comprising at least one substantially inert carrier gas component and at least one selectively reactive gas component (6), shown in both dashed and solid lines, is transported through a laser excitation channel (7) into a bottom-opening chamber. In some embodiments, the selectively reactive gas mixture (5) may additionally or alternatively be transported into the measurement chamber through a transmit-receive channel (20) or one or more gas channels (30). The selectively reactive gas mixture (5) flows through the bottom-opening chamber (1) toward the liquid metal or alloy surface (3) and exits the bottom-opening chamber (1) through an opening between the distal end (8) of the instrument head (2) and the sample surface (3), preferably providing a continuous flow of the selectively reactive gas mixture (5) through the measurement chamber (1). The selectively reactive gas component (6) of the selectively reactive gas mixture (5) reacts with the evaporated chemical component (4). This is advantageously performed before and during LIBS measurements for quantitative determination of the chemical composition of the liquid metal. Note that the separation of at least one selectively reactive gas component (6) within the selectively reactive gas mixture (5) is for clarity and illustrative purposes only. In practice, the selectively reactive gas mixture (5) comprising at least one selectively reactive gas component (6) may be substantially mixed as it enters the measuring chamber (1) with the bottom opening or may be mixed within the measuring chamber (1).

[0069] Figure 2 schematically compares two cases in which (i) a pure inert carrier gas or gas mixture (9) (shown by dashed lines) and (ii) a selectively reactive gas mixture (5) are directed toward the surface of the liquid metal or alloy (3) through a bottom-opening measurement chamber (1) during or before LIBS measurement, where a focused pulsed laser beam (10) is directed to the surface of the liquid metal or alloy (3). More specifically, Figure 2(a) shows how the volatile components (4) of the liquid metal or alloy accumulate in the bottom-opening chamber due to direct evaporation and laser-induced evaporation. The measured LIBS signal will include contributions from the volatile substances (4) dissolved in the melt and from the gas phase of the same substance, which includes evaporated material ablated by the previous laser pulse (10). Furthermore, the gas phase contributes to the self-absorption of emitted light from the volatile substances. For comparison, Figure 2(b) highlights how the selectively reactive gas component (6) of the selectively reactive gas mixture (5) ensures that the volatile component (4) does not exist as atomic matter in the gas phase in a significant amount within the bottom-opening chamber (1), at least around the sampling point (11), and further ensures that the volatile component does not accumulate during measurement due to the action of the incident laser pulse. In this case, the measured LIBS signal will primarily contain contributions from the liquid metal, and the additional self-absorption of the surrounding gas phase is reduced or essentially eliminated.

[0070] Figure 3 The diagram schematically illustrates how a selectively reactive gas mixture (5), comprising a selectively reactive gas component (6), can be passed to a liquid metal surface within a tube (12) or similar material partially immersed in liquid metal or alloy (3), provided that the tube is formed with perforations (13) or other components to allow the selectively reactive gas mixture (5) to continuously pass through the liquid metal surface to react with the volatile chemical components (4) of the liquid metal or alloy before the gas mixture (5) leaves the tube (12).

[0071] Figure 4 schematically illustrates how a selectively reactive gas mixture (5) with desired components can be prepared, for example, before being fed into the measuring chamber via one or more gas channels and / or laser excitation channels and / or emitter-receiver channels. The measuring chamber (1) can be a bottom-open measuring chamber (1a) or a generally closed volume measuring chamber (1b), as shown in Figures 4(a) and 4(b), respectively.

[0072] In Figure 4(a), the selectively reactive gas mixture (5) is supplied from a single premixed gas source (13) containing (multiple) substantially inert carrier gases and (multiple) selectively reactive gas components. The selectively reactive gas mixture is delivered to the bottom-opening measurement chamber (1) via a feed line (14) connected to the instrument head (2). It can be connected to a laser excitation path / channel (not shown) and / or a transmit / receive channel (not shown) and / or at least one separate gas channel configured in the instrument head (2) and transmitted through the laser excitation path / channel (not shown) and / or a transmit / receive channel (not shown) and / or at least one separate gas channel in the instrument head (2). The gas flow is advantageously regulated by a gas flow controller (15) to provide a stable, continuous flow of the selectively reactive gas mixture (5) to and through the measurement point (11).

[0073] In Figure 4(b), a selectively reactive gas mixture (5) from two different gas sources is mixed. In this document, the term "gas source" may refer to a compartment or container for containing a substantially pure gas or a mixture of prepared gases, or any other suitable structure for storing a gas or gas mixture. The gas or gas mixture may be pressurized within the compartment or container. A first gas source (16) may contain a substantially inert background gas A, such as, but not limited to, Ar, He, Ne, or mixtures thereof, and a second gas source (17) may contain a selectively reactive gas B, such as, but not limited to, CO2 and SF6. Both gases A and B are fed from the gas source compartments (16) and (17) to the gas mixer (18) via two separate feed lines (14a, 14b), whereby the gas mixer will prepare a selectively reactive gas mixture with a desired composition, such as, but not limited to, 5% gas B and 95% gas A. In some embodiments, mixing can be advantageously controlled to provide a constant flow of gas A and gas B, or flow A can be measured to provide a proportional flow of gas B via a link (19) between a flow meter (15a) and a flow controller (15b). More generally, in some embodiments, the gas mixture can also be prepared from three or more gas sources, with appropriate variations in the number of feed lines, gas controllers, gas mixers, etc.

[0074] Gas preparation can be used in conjunction with any type of suitable LIBS measurement device and / or instrument head, such as a bottom-opening chamber (Figure 4(a)) or an immersion perforated tube (Figure 4(b)), as described in embodiments of the invention, such that the local environment at the LIBS sampling point is controlled by a selectively reactive gas mixture. In some embodiments, one or more feed lines may include a dehumidifier and / or a filter.

[0075] Figures 5(a) and 5(b) schematically illustrate how the apparatus of the present invention can be mounted / arranged on an automated moving component, such as, but not limited to, a controllable robotic arm (21), to properly align and configure the instrument head (2) above the sample surface of the molten metal or alloy (2) and perform quantitative elemental analysis of the molten metal or alloy. The robotic arm can be controlled manually (22) or by a control unit such as a computer (not shown). Alternatively, the robotic arm can be controlled remotely or can be programmed to automatically configure the instrument head (2) and perform LIBS measurements using the apparatus of the present invention, wherein the programming ensures proper alignment of the instrument head (2) above or immersed in the surface of the molten metal or alloy (2). In some embodiments, alignment of the instrument head (2) with the surface (3) of the molten metal or alloy sample may require distance sensing by one or more distance sensors, wherein the distance sensing measures distances such as from at least one reference point on the device to the surface (3) of the metal or alloy sample, and wherein at least one reference point may be a point on the bottom of a laser excitation system or a transmitter-receiver system or a measurement chamber with a bottom opening.

[0076] Figure 5(a) illustrates an embodiment of the invention, wherein the measuring chamber is formed by arranging a non-immersion LIBS instrument head above the surface of molten metal or alloy in a tank (24). In this embodiment, a substantially inert carrier gas source (16), a selectively reactive gas component source (17), and two feed lines (14a, 14b) are configured as separate movable components relative to the robotic arm (21) and connected to the instrument head (2) for mixing one or more selectively reactive gas components and one or more substantially inert carrier gas components together to form a selectively reactive gas mixture within the measuring chamber when it is arranged above the surface of the molten metal or alloy.

[0077] Figure 5(b) illustrates an embodiment of the invention, wherein a gas mixer (18) and feed lines (14a, 14b), at least one filter (23) and / or at least one dehumidifier (23) are arranged as part of a robotic arm (21) and on top of the robotic arm (21). In this embodiment, before and / or during LIBS measurements of molten metal or alloy arranged in an (open) metal furnace (25), a selectively reactive gas mixture is prepared in the gas mixer (18) and transmitted from the feed line (14c) through at least one gas channel and / or through a laser excitation channel and / or through a transmit-receive channel to the instrument head (2).

[0078] Example

[0079] Example 1

[0080] The temperature dependence of the LIBS signal of the volatile component (magnesium) in a liquid alloy (containing 1 wt% magnesium) relative to the normalized LIBS signal of the base metal (aluminum) was investigated using both inert gas and a mixture of selectively reactive gases. The results are shown in... Figure 6 In a pure argon atmosphere, the Mg signal increases exponentially with melt temperature, which can be explained by the increased presence of Mg in the gas phase. It is equally evident that measurement uncertainty increases significantly at higher melt temperatures. Clearly, in a substantially inert environment, accurate melting temperature is necessary to quantify volatile components when performing LIBS analysis. However, temperature correction only partially addresses this issue, as the amount of material in the gas phase also depends on the presence of other elements in the melt and surface reactions, making precise quantification impractical. However, if the substantially inert carrier gas contains selectively reactive gas components, in this case carbon dioxide, the temperature dependence of the magnesium signal is essentially eliminated, and... Figure 6 Measurement uncertainty over the shown melting temperature range (690–790 °C) did not increase significantly. Therefore, precise knowledge of the melt temperature is no longer required for quantification, and the absence of a gas phase contribution means that the intensity of the measured LIBS signal is no longer substantially affected by other elements or surface effects in the melt.

[0081] Example 2

[0082] The LIBS signal of the volatile component (magnesium) in the molten metal or alloy (an aluminum alloy containing approximately 1% magnesium by weight) was recorded as six consecutive series of 300 laser pulses each. The laser pulses were directed onto the surface of the molten metal or alloy, in which an inert gas (argon) flowed through a bottom-opening chamber, and a selectively reactive gas mixture comprised both the inert gas (argon) and a selectively reactive gas component (carbon dioxide). The experimental results are as follows: Figure 7 As shown. During measurement, the bottom-opening chamber maintains a fixed relationship with the molten metal surface. In the case of a pure inert gas, the measured signal is high, exhibiting large variance between measurements, and increasing with consecutive measurements. When the gas contains selectively reactive components, the measured LIBS signal exhibits small variability between measurements (error bars smaller than the symbols in the figure) and does not change with consecutive measurements. Therefore, only in the presence of selectively reactive gas components can increased accuracy in quantification be obtained by increasing the number of consecutive measurements.

[0083] Example 3

[0084] The measured LIBS signals of the volatile component (in this case, magnesium) in two molten metal or alloy samples (i.e., aluminum alloys comprising approximately 1% Mg and 4% Mg, respectively) were investigated as a function of the proportion of the selectively reactive gas component (in this case, carbon dioxide) in the selectively reactive gas mixture. The results are summarized in... Figure 8 In this case, increasing the concentration of the selectively reactive gaseous component (in this case, carbon dioxide) in the essentially inert carrier gas (argon) leads to a rapid decrease in the LIBS signal from the volatile components present in the molten metal (in this case, magnesium). This is consistent with the gradual elimination of the gas-phase contribution of the volatile components to the LIBS signal by the gas-phase reaction. Clearly, without precise control, a small amount of the selectively reactive gaseous component (<2%) will produce an unstable LIBS signal from the volatile components. The minimum amount of the selectively reactive gaseous component required according to the invention generally depends on the concentration of the volatile components and the melting temperature. Figure 8 As shown, a larger amount of selectively reactive gaseous components is needed to compensate for the evaporation of higher concentrations of volatile components from the molten metal. For Figure 8 The concentration and melt temperature shown indicate that the signal is completely stable at a CO2 concentration of approximately 4-6%, and the stable CO2 concentration depends on the Mg concentration.

Claims

1. A method for quantitative analysis of one or more elements in a molten metal or alloy by LIBS, wherein the negative impact of the gaseous phase of the volatile trace element or alloying element in the molten metal or alloy on the analysis of the volatile element is reduced or substantially eliminated, the method comprising: a. A measurement chamber is arranged facing the surface of the molten metal or alloy, wherein the measurement chamber includes one or more openings or one or more windows to provide a path for a focused laser beam from a LIBS-excited laser to enter the chamber and reach a sampling point on the surface, and to provide a path for emitted light from the generated plasma to leave the chamber and be collected and transmitted to a detector; b. A continuous flow of a selectively reactive gas mixture is delivered to the sampling point within the measurement chamber, wherein the selectively reactive gas mixture comprises one or more substantially inert carrier gas components and one or more selectively reactive gas components, and wherein the one or more selectively reactive gas components of the selectively reactive gas mixture have the property of preferentially reacting with one or more volatile elements present in the molten metal or alloy; c. Perform LIBS measurements on the surface of the molten metal or alloy by emitting one or more laser pulses with sufficient light energy at the sampling point to ablate a portion of the sample and generate plasma above the sampling point; d. Receive the emitted light from the generated plasma and transmit the emitted light to a detector for recording the spectral data of the detected light; as well as e. Analyze the spectral data to obtain a quantitative determination of one or more elements in the molten metal or alloy.

2. The method as described in claim 1, wherein, The selectively reactive gas mixture is mixed before being transferred to the measurement chamber and is transferred to the measurement chamber through one or more gas channels and / or through a laser excitation channel and / or through a transmit-receive channel.

3. The method of claim 1, wherein the selectively reactive gas mixture is mixed in the measuring chamber, wherein one or more selectively reactive gas components are transported to the measuring chamber through one or more first gas channels, and the one or more carrier gas components are transported to the measuring chamber through one or more second gas channels.

4. The method of claim 2 or 3, wherein the method further comprises the step of controlling the flow of the one or more carrier gas components and the one or more selectively reactive gas components to provide a suitable proportion of the components before mixing the selectively reactive gas mixture.

5. The method of claim 1, wherein the total flow rate of the selectively reactive gas mixture is in the range of 1-5 L / min.

6. The method of any of the preceding claims, wherein the one or more carrier gas components are selected from the rare gas group.

7. The method as described in any of the preceding claims, wherein the one or more carrier gas components comprise argon.

8. The method as described in any of the preceding claims, wherein the one or more selectively reactive gaseous components are carbon dioxide (CO2) or sulfur hexafluoride (SF6).

9. The method as claimed in any of the preceding claims, wherein the proportion of the one or more selectively reactive gas components in the selectively reactive gas mixture is in the range of 1% to 20%, preferably in the range of 2% to 10%, and more preferably in the range of 4% to 8%, relative to the total volume of the selectively reactive gas mixture.

10. The method of any of the preceding claims, wherein the measurement chamber allows a continuous flow of the selectively reactive gas mixture at the sampling point and the generated plasma, and in the neighborhood of the sampling point and the generated plasma.

11. The method of claim 10, wherein, The measuring chamber is formed by arranging a non-immersion LIBS instrument head above the surface of the molten metal or alloy.

12. The method of claim 10, wherein the measurement chamber is formed by immersing a LIBS instrument head or a portion thereof into the molten metal or alloy, wherein the measurement chamber includes one or more openings positioned above the surface of the molten metal or alloy to allow a continuous flow of the selectively reactive gas mixture at the sampling point and the generated plasma, and in the neighborhood immediately adjacent to the sampling point and the generated plasma.

13. The method of the preceding claims, wherein the one or more openings for allowing the flow of the selectively reactive gas mixture are located less than 10 mm above the surface of the molten metal or alloy.

14. The method as described in any of the preceding claims, wherein the step of arranging the measuring chamber is performed automatically.

15. The method of any of the preceding claims, wherein the method is used to measure the concentration of one or more major components of a molten metal or alloy and / or further measure the concentration of one or more trace elements in the molten metal or alloy.

16. The method of the preceding claims, wherein the method is used to measure the concentration of magnesium in the molten metal or alloy.

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