NON-IMMERSIVE METHOD AND APPARATUS FOR QUANTITATIVE ANALYSIS OF LIQUID METALS AND ALLOYS

Abstract

A non-contact, non-immersive method and apparatus are provided for accurately measuring quantitatively one or more elements in liquid metal or alloy samples using laser-induced breakdown spectroscopy (LIBS). The method is particularly useful for process and/or quality control within the metallurgy industry for accurately and very quickly measuring minor component or impurity elements in liquid metal in the production process, without touching the liquid metal and without the need for cooling and solidifying samples for analysis. In the method and apparatus a pre-determined distance is dynamically maintained between emission receiving optics and the surface of a liquid sample being analysed and the instrument does not come in contact with the liquid metal surface. Liquid samples are heated and/or maintained at a desired temperature. For many elements, values for limit-of-detection, measurement repeatability and accuracy about or below 1 ppm are achieved using this method.

Claims

1. A non-contact, non-immersive method of measuring quantitatively one or more elements in a liquid metal or alloy sample, comprising: obtaining a sample of the liquid metal or alloy to be analysed, maintaining or placing the sample in a sample container which is substantially upwardly open, heating or maintaining the sample at or above a desired temperature, placing an instrument head and/or the sample container such that the instrument head is above the sample surface, wherein the instrument head comprises laser excitation optics that are connected to a laser, receiving optics for receiving emission from the sample, and an open-bottom chamber providing plasma confinement and stable environmental conditions through which the laser excitation optics guide laser light and wherein the instrument head is provided with a distance sensor, positioning the receiving optics at a pre-determined distance in the range from about 5 mm to about 10 mm and preferably from about 10 mm to about 50 mm from the sample surface such that emission from a particular part of the plasma plume is collected, wherein said receiving optics are arranged at an angle relative to the sample surface, in the range of about 30° to about 75°, and measuring with the distance sensor a distance to the sample surface and automatically moving the receiving optics or sample container to position the receiving optics at a pre-determined distance from the sample surface, directing a stream of inert gas through a gas channel into the open-bottom chamber, emitting one or more laser pulse on the sample through the excitation optics, receiving emitted light through the receiving optics from the sample and transmitting to a detector for recording spectral information of the detected light, comparing one or more selected emission peaks to calibration values in order to obtain quantitative determination of one or more elements.

2. The method according to claim 1, further comprising maintaining substantially consistent and inert atmospheric conditions around the sampling point.

3. (canceled)

4. The method according to claim 1, wherein the distance sensor is dynamically operated such that the distance to a sample surface is dynamically maintained and adjusted as necessary.

5. The method according claim 1, wherein the laser excitation optics and receiving optics are fixedly arranged in a laser optics unit comprised in the instrument head, and wherein said positioning the receiving optics comprises positioning said laser optics unit.

6. (canceled)

7. The method according to claim 5, wherein the laser beam is focused at or near the sample surface when the laser optics unit is positioned.

8. The method according to claim 1, wherein the sample is heated or maintained at a temperature above at least 400° C., such as above at least 600° C. such as above at least 700° C.

9. The method according to claim 1, comprising heating the sample container with inductive heating.

10. The method according to claim 1, comprising arranging the sample container in contact with a surface of a source of liquid metal or alloy.

11. The method according to any of claim 1, wherein the sample being placed in the sample container has a volume in the range from about 1 mL to about 1000 mL and preferably in the range from about 5 mL to about 100 mL.

12. The method according to claim 1, wherein said positioning of the receiving optics is arranged by moving the sample container towards the instrument head.

13. The method according to claim 1, wherein a trough, crucible or other open-top source containing the metal or alloy to be analysed functions as the sample container.

14. The method according to claim 13, wherein the sample surface is moving horizontally.

15. The method according to claim 1, wherein said positioning positions the receiving optics at the pre-determined distance from the sample surface which is a set distance with a margin of less than ±50 μm and preferably with a margin of less than ±25 μm.

16. The method according to claim 1, for determining in a liquid metal or alloy sample the content of one or more elements selected from Aluminium, Silicon, Phosphorus, Sulphur, Calcium, Chloride, 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, Lithium, Beryllium and Boron.

17. The method according to claim 1 further comprising the step of emitting a series of laser pulses on a sampling point on the sample surface prior to said receiving emitted light.

18. The method according to claim 1, further comprising directing a stream of inert gas through a gas channel into the open-bottom chamber facing the sample surface through which chamber the laser pulses pass to the sample surface.

19. An apparatus for quantitatively measuring contact-free and without immersive probe one or more elements in a liquid metal or alloy sample, comprising an instrument head comprising laser excitation optics and receiving optics, an open-bottom chamber extending inwardly from a bottom surface of the instrument head a pulsed laser connected to said laser excitation optics, a spectrograph for resolving received emission, a detector connected to said spectrograph for recording spectral information, a gas channel or gas line for feeding a stream of inert gas to the open-bottom chamber, the receiving optics being arranged on a vertically moveable support, the receiving optics having an associated distance sensor for measuring a distance to the surface of a sample in the sample container, so that the receiving optics can be positioned at a predetermined distance from the surface of a sample in the sample container, wherein the receiving optics are arranged at a distance from the sampling point of a sampling surface in the range from about 5 mm to about 100 mm and are arranged at an angle in the range of about 30-75® with respect to the sample surface.

20. The apparatus according to claim 19, further comprising a computer or control unit with means for receiving input from said distance sensor, and a moving mechanism to automatically move said moveable support controlled by said computer or control unit based on input from said distance sensor.

21. The apparatus according to claim 20, wherein said moving mechanism is able to move said moveable platform with a precision of less than ±50 μm and preferably of less than ±25 μm.

22. The apparatus according to claim 20, wherein said distance sensor and moving mechanism are dynamically operated to continuously maintain a pre-determined distance and adjust as necessary during operation.

23. The apparatus according to claim 19 wherein the laser excitation optics and receiving optics are fixedly arranged in an optics unit comprised in the instrument head.

24. The apparatus according to claim 23, wherein said optics unit is arranged on said moveable support within said instrument head.

25. The apparatus according to claim 19, wherein the instrument head is moveable and functions as said moveable support.

26. The apparatus according to an claim 19 wherein the instrument head comprises a laser beam channel extending at least from the laser excitation optics to the open-bottom chamber, and an emission receiving channel that extends from the open-bottom chamber towards the receiving optics.

27. (canceled)

28. (canceled)

29. The apparatus according to any claim 19, comprising a sample container configured to allowing heating and/or maintaining the sample at a temperature of at least 400° C. and preferably at least 600° C.

30. The apparatus according to claim 29, wherein said sample container can hold a sample volume in the range from about 1 mL to about 1000 mL.

31. The apparatus according claim 29 wherein the sample container comprises heating means which are preferably inductive heating means.

32. The apparatus according to claim 29, wherein said sample container is arranged on a moveable support.

33. The apparatus according to claim 29, wherein the sample container is adapted and arranged on a moveable support which is configured so that the sample container can be placed in contact with the surface of a source of liquid metal or alloy, and wherein the instrument head is moveable in at least two directions, so that the sample can be measured when the sample container is positioned in contact with said surface of a source of liquid metal or alloy.

Description

BRIEF DESCRIPTION OF FIGURES

[0041] The skilled person will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

[0042] FIG. 1 shows a schematic overview of an instrument of the invention, with a heated sample container.

[0043] FIG. 2 shows another embodiment of the instrument of the invention, with a sample container heated in a source of liquid metal.

[0044] FIG. 3 shows another embodiment of the instrument of the invention, with a trough (launder) in a processing facility functioning as a sample chamber.

[0045] FIG. 4 shows a schematic overview of bottom end of the instrument head, showing the open-bottom chamber and optics.

[0046] FIG. 5 shows another embodiment of the bottom end of the instrument head, with different layout in inert gas channels.

[0047] FIG. 6 Measured concentration values for listed elements with OES system and LIBS system of the invention.

[0048] FIG. 7 shows correlation plots between data from a LIBS apparatus of the invention and comparative results from OES.

[0049] FIG. 8 shows average absolute difference in concentration measurements between the new LIBS apparatus and OES.

DETAILED DESCRIPTION

[0050] The invention is described in further detail with reference to the accompanying drawings which are not to be construed as limiting the overall scope of the general concept of the invention. In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0051] In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

[0052] FIG. 1 illustrates an embodiment of an apparatus 1 of the invention that comprises an excitation laser 2 which is arranged inside an instrument head 3. The laser emits light that is transmitted through an optical pathway 7, a mirror 6 reflects the light in this configuration where the path lies at an angle, the light beam is directed to laser excitation optics 4, which comprises a focusing lens that focuses the beam on the sample surface or in proximity thereto. Receiving optics 5 are arranged to receive emission from a sample plasma created by the interaction between the excitation laser and sample. A distance sensor 14 is arranged in fixed relation to the receiving optics and excitation optics. The receiving optics 5 transmits received emission light through an optical guide 8 to a spectrograph 9 which is connected to a CCD camera 10. A sample container 11 is heated with heating element 12. Distance sensor 14 measures a distance to the surface of a sample of liquid metal in the sample container. The distance sensor transmits signals to a control unit 13 (PLC computer). The control unit adjusts dynamically the vertical position of the instrument head with movement actuators (not shown) to maintain an exact pre-determined distance to a sample surface. In this configuration the instrument head is also moveable horizontally. Computer 15 controls the laser operation and detection components (spectrograph and CCD camera) and processes and analyses the obtained data. Sample container 11 is configured to receive and hold a sample of liquid metal. Heating element 12 ensures that the sample is maintained at a desired temperature. A sampling device 20/21 is shown above a trough 31 with a flow of liquid metal 30. A gas cylinder 16 with inert gas (preferably Argon) is connected via a gas channel 17 to transmit regulated gas flow into an emission receiving channel 19 that accommodates the receiving optics. The emission receiving channel extends from an open-bottom chamber 18 configured to contain the plasma during the measurement.

[0053] FIG. 2 illustrates an alternative configuration wherein sampling scoop 20 is used as a sample container and heated by partially immersing in the liquid metal 30 in the trough 31. In this configuration the instrument head 3 can preferably extend telescopically from the main instrument body, to be positioned above the trough and withdrawn therefrom after analysis is completed.

[0054] In FIG. 3, yet another alternative is shown where a specific sample container is not used but rather the trough in which liquid metal flows or is contained serves as a sample container. Thus, the instrument head is positioned above the surface of liquid material in the trough and the receiving optics are positioned at a pre-determined distance from the liquid surface, and preferably the pre-determined distance is dynamically maintained.

[0055] FIG. 4 shows a schematic close-up view of the open-bottom chamber 18 at the bottom of the instrument head, with the emission receiving channel 19 extending at an angle from the chamber to lens acting as the receiving optics 5. Gas channel 17 feeds Argon gas with a controlled flowrate, to maintain a slight overpressure inside the chamber 18 and emission receiving channel 19. The optical pathway 7 extending from the laser excitation optics 4 to the open-bottom chamber is shown in this configuration as substantially narrower than the open-bottom chamber, as the laser excitation optics 4 (a lens in this configuration) focuses the laser light to a narrow beam path.

[0056] FIG. 5 shows an alternative arrangement of the chamber 18 and emission receiving channel 19, where the gas channel 17 splits and extends into both the chamber 18 and emission receiving channel 19.

EXAMPLES

[0057] A comparative study was performed to evaluate the results from a LIBS apparatus of the invention and compare to results obtained for the same samples using a high-end industry-standard OES system (Optical Emission Spectroscopy), a Bruker Q8 Magellan OES system in an aluminium plant, using standard sampling and measurement protocols.

[0058] The repeatability of measurements from the LIBS apparatus was compared with measurement from the Bruker OES, using the same sample material in each case. Three measurements at each concentration were performed with the LIBS apparatus in the melt and three measurements were carried out on corresponding solid samples in the OES. In this way, 11 elements were analysed in up to 150 samples having varying concentration levels of impurity elements. Analysis runs were carried out on-site over a period of three months.

[0059] The range of concentrations for the individual elements measured in this way are indicated by black lines in FIG. 6(a).

[0060] FIG. 6 (b) shows the variability in individual measurements of samples in the lowest 10% of the concentration range (lower set of horizontal bars in FIG. 6(a)) for the Bruker OES system (open circles) and the LIBS system (black squares) according to the present invention, confirming that for most of the investigated elements, the absolute standard deviation for both the LIBS system and the OES system is below 2 ppm. The high absolute variability for Si and Fe results from the high concentration of those elements in the measured samples. In both cases, the relative standard deviation (% RSD), for the LIBS system as well as the OES system, is of the order of 1% for Si and Fe. In general, both the absolute standard deviation and the % RSD are concentration-dependent. The LOD for each element in the case of LIBS measurements, estimated using least-squares fitting of calibration data using weighted errors and taking three standard deviations at the y-axis intercept as a measure of the LOD, is found to be of the same order as the measurement variability shown in FIG. 6(b).

[0061] The different degree of variability between elements for the LIBS measurements, shown in FIG. 6(b), is mainly (although not exclusively) dependent on the relative strength of the LIBS signal for a given elemental concentration. It should be noted that while this data is representative of the current configuration, there are still many opportunities to increase the detected signal strength, signal processing techniques and/or the number of samples in each measurement in order to reduce the variability and improve the LOD in the LIBS measurements. It should be emphasized that the number of laser pulses averaged in the LIBS measurement reported here is at least one order of magnitude lower than the number of sparks used in the OES measurement.

[0062] In order to evaluate measurement accuracy, the raw LIBS output data (normalized signal strength corresponding to each element, in arbitrary units) was calibrated against OES concentration measurements on the corresponding solid samples. FIG. 7 shows correlation plots between LIBS (“EA2000”) and OES results. LIBS data was recorded over a 3-month period without recalibration of the equipment. Open symbols represent data recorded within the 1st week of the measurement period that was used for calibration of the LIBS signals.

[0063] For most of the investigated elements, an excellent correlation between the OES-measured concentration and the LIBS signal is observed. It should be emphasized that the data was collected over a period of three months and that no recalibration of the LIBS system took place in this period while the OES system was recalibrated daily, in accordance with the smelter's procedures. The increased scatter in Si data (which is much larger than the variability of individual measurements) presumably relates to uncertainties arising in the sampling process itself that depends on the exact sampling, preparation and measurement procedure and is therefore potentially substantially random and operator-dependent. For Si and similarly behaving elements, a separate independent calibration of the LIBS apparatus is required to ensure optimum performance, using calibrated reference standards that can be measured in the liquid form. Such an approach will, in general, enable the LIBS apparatus to provide accurate concentration measurements for elements that are problematic when measured in the solid phase.

[0064] The LIBS apparatus was calibrated using a set of samples measured within a 1-week interval (represented with open symbols in FIG. 7). Later readings (black symbols in FIG. 7) correlated exactly with this initial data set, confirming that no significant drift of the LIBS signal took place over the full 3-month measurement period. In order to establish the accuracy of individual LIBS measurements, the average absolute deviation between calibrated LIBS readings (black symbols), typically covering the lower third of the calibration range, and the corresponding OES concentration measurements was calculated. Results are shown in FIG. 8, showing that the LIBS and OES readings agree to within 1-3 ppm for elements Mn, V, Ti, Sn, Cr, Ni, Cu, while the accuracy is lower for elements that show a reduced correlation (Zn and Ga, cf. FIG. 7). For Si and Fe, the results agree, on average, to within 20-30 ppm, representing an agreement better than approximately 5% and 2% of the average measured concentrations, respectively.

[0065] It should be emphasized that LIBS sampling and analysis was done on-site in the casting house within an aluminium plant, next to a trough with flowing aluminium, while OES measurements were carried out in a laboratory setting. This demonstrates how the inventive apparatus and method are highly suitable for direct process and quality control in metal processing and production facilities and have the potential to replace off-line laboratory analysis.

[0066] In summary, it can be stated that the present performance of the LIBS apparatus and method of the invention in terms of measurement precision and accuracy for many of the investigated elements is comparable to the high-end OES system used as a benchmark. The LIBS readings have been shown to be stable for months, without recalibration. The LIBS apparatus and method therefore show good promise to outperform lab measurements on process samples carried out using high-end OES systems, in addition to providing faster results and the possibility of real-time monitoring of impurity concentrations that are difficult or impossible to replicate with current laboratory methods.

[0067] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Throughout the description and claims, the terms “comprising”, “including”, “having”, and “containing” and their variations should be understood as meaning “including but not limited to”, and are not intended to exclude other components.

[0068] The present invention also covers the exact terms, features, values, ranges, etc. in cases where these terms, features, values, ranges, etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., “about 3” shall also cover exactly 3 or “substantially constant” shall also cover exactly constant).

[0069] The term “at least one” should be understood as meaning “one or more”, and therefore includes both embodiments that include one and multiple respective components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”.

[0070] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0071] Use of exemplary language, such as “for instance”, “such as”, “for example” and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.

[0072] All of the features and/or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and/or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.