Patent Number: 
Section: description

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/317,257, filed Sep. 4, 2001, the disclosure of which is incorporated herein by reference. In one aspect, this invention relates to measuring elements present in coal, ores, and other substances using energy dispersive X-ray fluorescence (XRF) spectroscopy. In another aspect, the invention relates to different apparatuses and methods for use in conjunction with elemental analyzers. Techniques for analyzing or measuring the elemental composition of a substance, such as coal, using X-ray fluorescence (XRF), are well-known in the art. An example of one technique is disclosed in U.S. Pat. No. 6,130,931, the disclosure of which is incorporated herein by reference. While these XRF techniques work extraordinary well for measuring certain elements, such as sulfur, the ability to measure “trace elements” (e.g., vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, and molybdenum) has previously been limited to laboratory techniques involving extensive preparation using pulverized samples. For instance, ASTM Standard Test Method D4606 for the determination of arsenic and selenium in coal by the Hydride Generation/Atomic Absorption Method analyzes a 1.0 gram sample of coal pulverized to pass a 250 mm standard sieve. ASTM Standard Test Method D6357 for the Determination of Trace Elements in Coal, Coke, and Combustion Residues from Coal Utilizations Processes by Inductively Coupled Plasma Mass Spectrometry and Graphite Furnace Atomic Absorption Spectrometry analyzes a 0.5 gram sample of coal ash ground to pass 150 μm. The wet chemistry methods dictated by standard laboratory methods are time consuming and can only produce a single analysis in a matter of hours. Turn around time in commercial laboratories is often days or weeks and the analysis is very expensive. U.S. Pat. No. 5,020,084 to Robertson, which is also incorporated herein by reference, proposes the use of X-ray energy at a level of 100–130 kilo-electron-volts (KeV) to measure a finely divided heavy metal (gold) dispersed in a non-metallic matrix using K emission bands. However, this patent does not mention the use of low energy XRF to measure trace elements, including gold. Moreover, it dismisses L emission XRF techniques as inaccurate. Furthermore, high energy XRF cannot be used to detect the K emission bands of the lighter trace elements with atomic numbers less than or equal to 48 (Cadmium). Thus, the approach taught in the Robertson patent is not a solution to the problems identified in the foregoing paragraph. In one aspect, the present invention is an on-line sensor or sensing device for measuring (monitoring, detecting, sensing, etc.) one or more elements in a material, including the presence of trace elements down to levels of less than 1 part per million (ppm) or μg/g with relatively short analysis times (possibly as short as 2–6 minutes). This is accomplished using low energy (less than 80 KeV and, more preferably, less than 65 KeV) X-rays to bombard the stream of with X-rays in a bandwidth designed to optimally excite the characteristic K or L emission bands of the element(s) of interest. Using this system, it is possible to detect fluoresced emissions with energies as low as 1.0 KeV. Consequently, trace elements present in coal (defined by ASTM as those elements whose individual concentrations are generally less than 0.01%) such as vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, and molybdenum each have Kα or Kβ emission bands that can be readily measured by this XRF technique. Mercury and lead Lα emission bands can also be measured using this technique. Other metals dispersed in mineral ores in small or trace quantities, such as platinum and gold, can also be measured with low energy XRF, again using the Lα emission bands. In one embodiment, an adjustable voltage X-ray tube is used as the source. This allows for the incident X-ray energies to be adjusted, preferably to within a range of 1.5–3.0 times the energy of the characteristic emission bands from the elements of interest to maximize the efficiency of emission. It is also possible to use filters to narrow the band of incident X-rays, which further reduces the amount of interference, as well as a slotted collimator for collimating the X-ray energy emitted from the source. Providing an optimal X-ray source:detector (sensor) geometry and positioning the sensor as close as possible to the surface of the material also enhances the results. In accordance with another aspect of the invention, a sled for supporting a sensor, such as an XRF sensor, adjacent to a moving stream of material is disclosed. The sled is mounted so as to be capable of swinging to and fro in response to changes in the profile of the material. It may also be designed to aid in further compacting the material to help ensure that an accurate reading is taken by the sensor. Other manners of mounting a sensor and, in particular, and XRF sensor are also disclosed, including: (1) mounting the sensor in a probe for positioning in a borehole; and (2) mounting two sensors inline along a moving stream of material, with one sensing trace elements only and the other sensing the “lighter” elements. Reference is now made to FIGS. 1a and 1b, which show one embodiment of an XRF trace element sensor 10 mounted adjacent to an endless transfer conveyor belt 12 carrying a substance or material, such as coal C. The belt 12 and sensor 10 in combination may form part of a mechanical sampling system 14 for measuring the elemental composition of a sample of material, such as ore or coal C delivered from a chute H or the like. The sample may be supplied from a main conveyor line (not shown), and is preferably crushed or pulverized to have a particle size of approximately less than or equal to 10 millimeters (⅜ths of an inch) prior to being delivered to the sampling system 14. In this system 14, a leveling structure and skirting (not shown) along the sides of the belt 10 together help to assure that a constant or substantially constant geometry of coal C or other substance is presented to the sensor 10 forming part of the system 14. In FIG. 1a, the leveling structure is shown as a rotatable drum 16 capable of being moved toward and away from the surface of the belt 12 (note action arrow A), depending on the profile of the material being conveyed. However, the leveling structure of FIG. 1a could also be considered a stationary cylinder that is also movable toward and away from the belt 12. Instead of a rotatable drum or stationary cylinder, a leveling plow 18 could also be used to compact the material, as shown in FIG. 1b. Any combination of these structures could also be used, as could structures not disclosed herein, as long as the function of assuring a level, constant or substantially constant profile is achieved. A material sensor 20 may also be provided upstream of the elemental sensor 10 for indicating the presence of material on the belt 12. Opposed microwave moisture sensors 22a, 22b may also be positioned adjacent to the belt 12 for providing moisture readings, if desired. As discussed in detail further below, outputs from each of the sensors, as well as from the system 10, may be fed to a remote computer or controller through suitable transmission lines (see FIG. 2) for further use, display, or processing, as necessary or desired to measuring the elemental composition or another characteristic of the material sample. FIG. 2 provides a generally schematic view of the overall arrangement of the sensors 10, 20, 22a, and 22b, with portions of the coal C and the belt 12 cut away for clarity. The elemental sensor 10 includes an X-ray source 24 (typically an X-ray tube) and an X-ray detector 26 (typically an Si-PIN diode) positioned in a backscattering configuration adjacent to the surface of the coal C. The source and detector 24, 26 may be positioned in an instrument enclosure or box 28 having an opening covered by a window (not shown) through which the X-ray energy passes (not shown). The window may be thin (such as 0.9 mil polypropylene) to seal the enclosure 28 from fugitive dust. Since the window will absorb a fraction of the low energy X-rays, the opening may be left open to maximize the transmission of X-rays to the detector 26. When no window is employed, a positive gas pressure (air or other gas, such as helium) may be applied to the instrument enclosure, sufficient to prevent dust from entering. In a most preferred embodiment, an adjustable voltage X-ray tube is used as the source 24. This allows for the incident X-ray energies to be adjusted, preferably to within the range of 1.5–3.0 times the energy of the characteristic emission bands from the elements of interest to maximize the efficiency of emission. The X-ray source 24 is connected to a high voltage power supply 30, which may be included as part of a remote “power box” 32 also including power supply 34. As is known in the art, an interlock 36 including a warning light X and a switch may also be associated with the high voltage power supply 30 for safety and security reasons. The power box 32 may also enclose or include the device for receiving an output signal from the X-ray detector 26, such as a multi-channel analyzer (MCA) 38. A pre-amplifier and power source, identified collectively by reference numeral 37 may also be connected to the MCA 38, preferably in the instrument enclosure 28. The MCA 38 may also be coupled to and receive a signal from the material sensor 20. The outputs from the moisture sensors 22a, 22b may be connected to a separate moisture processor 40 capable of receiving and processing the analog signals. Both the MCA 38 and the moisture processor 40 may be coupled to a remote computer 42 for providing an indication of the measurements taken by the sensors (e.g., elemental composition, moisture content, etc.), such as using a monitor or display. FIG. 3a shows a preferred geometry of the X-ray source 24 and detector 26 of the present invention (which are shown in the reversed positions, as compared to FIG. 2). The source 24 is mounted adjacent to the detector 26 such that both are generally directed toward the material for which the analysis is desired, preferably about two inches from the surface thereof (and in some cases, such as when sodium is being measured, less than 0.5 inches). Preferably, the angle between a transmission axis T of the source 24 and a detection axis D of the sensor or detector 26 is an acute angle, preferably between about 65° and 90°, and, most preferably about 78°, while the angle between the transmission axis T and a plane parallel to the sample surface is also an acute angle, most preferably about 57°. To facilitate changing the position (height, spacing, or angle) of either the source 24 and the detector 26, both are independently mounted in an adjustable fashion on stable mounting structures, such as using slotted brackets 44 and corresponding fasteners 46 (e.g., nut and bolt combinations). The particular adjustable mounting used is not considered critical to the invention, as long as the desired geometry is achieved. When conveying bulk materials, such as coal, the particles shift laterally across the conveyor 12. Thus, the shape of the interrogation (measuring) zone is more distorted. To reduce the effects of varying profile in the material conveyed past the sensor 10, a collimator 48 may optionally be positioned adjacent to the source 24. Specifically, the collimator 48 is used to collimate the X-rays emitted from the source 24. Preferably, as shown in FIG. 3b, the collimator 48 includes an elongated slot 50 having a major dimension M oriented in the same direction as the material is being conveyed (that is, parallel to the direction in which the material is traveling). The collimator 48 may include openings 51 for facilitating mounting to the X-ray source 24. To further narrow the bandwidth of incident X-rays in order to excite a particular element(s) with a high degree of efficiency, a filter 52 may also be employed between the X-ray source and the material to be measured. Filter 52 may be comprised of metal and may have a thickness of 10 μm to 4 mm, depending on the energy and intensity of the incident X-rays. As perhaps best shown in FIG. 3c, the filter 52 is preferably interposed between the window 24a on the X-ray source 24 (which is normally made of beryllium) and the collimator 48, if present. Common filter materials are copper, zinc, nickel, zirconium, niobium, molybdenum or any other materials that can eliminate or reduce the X-rays in a particular energy range emanating from an X-ray source. Alloys such as brass can also prove to be effective filters as well. Preferably, the collimator 48 is fabricated of aluminum or the same material as the filter 52. Direct measurement of the K and L emission bands from a number of trace elements is possible with the sensor 10 described above. As an example, FIG. 4a shows a broad spectrum generated with the present invention using an X-ray tube as the source 24 with a molybdenum filament, a copper filter 52 mounted next to the tube window 24a, as shown in FIG. 3. The trace element of interest in this case is arsenic (As). FIG. 4b is an enlargement of the same spectrum showing the resolution of the Kα and Kβ emission bands for arsenic. In this sample, the concentration of arsenic was 23 ppm. Using this sensor 10 with different X-ray energies at less than 80 KeV, preferably less than 65 KeV, still more preferably between 20–65 KeV, and most preferably around 40–45 KeV, it is possible to measure trace elements including vanadium, chromium, manganese, cobalt, nickel, copper, zinc, and molybdenum using the Kα and Kβ emission bands. Mercury and lead Lα emission bands can also be measured using this technique (which could be of great benefit when measuring trace quantities of these metals in water). Other metals, such as platinum and gold dispersed in mineral ores in small or trace quantities can also be measured with low energy (less than 80 KeV) XRF, again using the Lα emission bands. Instead of the arrangements shown in FIGS. 1a and 1b, the XRF trace element sensor 10 described above can be mounted directly adjacent to a moving stream of material on a conveyor (not shown) using a sled 56, as depicted in FIG. 5a. The sled 56 includes a base 58 for assisting in leveling and compacting the material passing underneath the sensor 10 so the profile is substantially constant. The base 58 is sized for supporting the sensor 10 (source/detector) adjacent to an opening 58a (see FIG. 5c), as well as the instrument enclosure 28. The base 58 may also include an opening 58b that may be associated with a sensor 20 for detecting the presence of material adjacent to the sled 56. The sled 56 further includes a pair of elongated runners 60, 62. Each runner 60, 62 may be attached directly to one side of the base 58 (FIG. 5b). As perhaps best shown in FIG. 5c, the runners 60, 62 are preferably converging or narrowing relative to one another along the direction in which the material is traveling (note direction of material travel G in FIG. 5a). As should be appreciated, this further helps to compact the material as it moves towards the sensor 10 positioned downstream from the leading edge of the sled 56. Consequently, the sled 56 is particularly useful with moving streams of material that have a particle size of up to about 50 millimeters, or 2 inches, in size. The sled 56 is preferably supported by a stable support structure 64 and mounted such that it is capable of moving in response to changes in the geometry or profile of the material being conveyed. In one embodiment, as shown in FIG. 5a, the sled 56 is mounted so as to be capable of swinging to an fro along a generally arcuate path. Specifically, at least two swing arms, and preferably two pairs of swing arms 66 support the sled 56 from the stable support structure 64. The swing arms 66 are mounted to pivot structures 70 at each end such that the sled 56 is capable of swinging to and fro along a generally arcuate path. The mounting is preferably of a type that prevents the ends of the swing arms 66 adjacent to the support structure from moving in the vertical direction (as opposed to the smooth arcuate movement allowed by the pivoting of swing arms 66), which keeps vibrations to a minimum. A shock absorber or damper, such as a spring 72 (represented schematically in FIG. 5a), may also be associated with the sled 56 to resist the swinging movement. For example, the spring 72 may extend between one or both pairs of swing arms 66 on each side of the sled 56, as shown in FIG. 5a. Alternatively, it could be provided between individual swing arms and a stable structure, between the base and a stable structure, or between one or more of the swing arms and the base. The sled 56 may also include a leveling and smoothing structure 74. Preferably, this structure 74 is mounted along the leading edge of the base 58 and is designed to help compact and smooth the upper surface of the material as it is presented to the onboard sensor 10. It is shown as having a generally arcuate cross-section when viewed from the side, but may have any shape that accomplishes the desired function. A separate smoothing or leveling structure, such as a drum, roller, bar, or the like (not shown) may also be provided upstream of the leading edge of the base 56 of the sled 58. This structure maybe the only leveling structure, or it may be used in combination with structure 74. As shown in FIG. 5b, the sled 56 may hang freely above the conveyor or other structure along which the material is traveling. Preferably, the sled 56 is positioned such that, in a nominal or free hanging position, it is about 6 inches above the conveyor where the normal bed depth is 8–10 inches. Thus, when the material is on the conveyor at the normal depth, the engagement with the leveling structure 74 causes the sled 56 to move or pivot along an arcuate path, generally in the direction of travel, as shown in FIG. 5a. In this position, it should be appreciated that the weight of the sled 56 still helps to compact the material as it moves toward the sensor 10. This is true even if the spring 72 and counterweight 78 are present. To facilitate adjusting the position of the sled 56 toward or away from the conveyor, adjustable length arms 66 may be provided. Alternatively, the position of the support structure may be made adjustable, such as by using an adjustable height support frame. In addition, one or both of the leading pair of swing arms 66 may be extended beyond the pivot structure 70 on the distal end (identified with reference numeral 66′) and carry or support an adjustable counterweight 78. Consequently, the position of the counterweight 78 along the arm(s) 66′ may be adjusted to help counterbalance the weight of the sled 56 (which may be around 200 pounds) to help keep it in intimate contact with the upper surface of the moving stream of material without severely disrupting the flow. This helps to ensure that a more accurate reading is achieved by the sensor 10. The sensor 10 used with the sled 56 need not be for measuring trace elements, but instead could be used for measuring lighter elements, as described in U.S. Pat. No. 6,130,931. The sensor 10 may also be used in other possible arrangements, including configurations where the material is stationary. For example, as shown in FIG. 6, the sensor 10 may be in the form of a probe 80 adapted for being positioned or inserted in a borehole E for measuring the elemental composition of the adjacent wall W. Specifically, the X-ray source 24 and X-ray detector 26 are shown as being positioned in a backscatter configuration adjacent to a window 82 formed in the sidewall of the probe 80. The detector 26 may be coupled to an onboard MCA and amplifier 86, while the source 24 is coupled to an onboard power supply 88. The MCA/amplifier 86 and power supply 88 may in turn be coupled to data and power lines L, respectively, emanating from a remote location outside to borehole E. In the case where the borehole E is oriented vertically, a support line, such as a steel rope R, cable, or the like may also be secured to the probe 80 to assist in raising and lowering it. A sensor 10 similar to that shown in FIGS. 1a and 1b could also be mounted on the underside of a chute (not shown) carrying the material past the sensor in a fixed geometry, or it could be mounted to a flow cell to measure concentrations of trace elements in liquids or slurries. The sensor 10 could also be mounted on or adjacent to the cutter head or shearer on a mining machine, such as a highwall miner (not shown) to take measurements from the cutting face. In all cases, the sensor 10 need not be used for measuring trace elements, but instead could be used for measuring lighter elements, as described in U.S. Pat. No. 6,130,931. Correlation of the element of interest with other elements in the material to correct for matrix effects may be done using a multiple linear regression calibration relationship of the form:E=K0+K1C1+K2C2+K3C1C2+K4C12+K5C5+ . . . +KxCx+KyCy . . .Where,                E=Element of interest        K=Constant        C=Count rate under a region of interest (peak).Therefore, combining multiple X-ray sources wherein one is at a first level (e.g., <15 KeV) to cause efficient Kα emission of the “lighter” elements (atomic numbers <30) and a second X-ray source is at a second, higher level (e.g., >15 KeV) to cause efficient Kα emission of the “heavier” elements (atomic numbers >30) provides a system by which matrix effects can be corrected and relationships developed between various elements occurring in a mineral matrix together. This can dramatically improve the accuracy of the measurement. A similar improvement may be gained by using an adjustable voltage X-ray tube, as described above, controlled by a computer or a programmable logic controller (PLC). It is also possible to measure each element in a range of elements, including trace elements, from sodium (atomic number 11) through krypton (atomic number 36) using a properly configured single source.         An example of a sampling system 90 including two X-ray sources and detectors for measuring both lighter and heavier elements in the material is shown in FIG. 7 in use adjacent to a moving belt 12. Specifically, the system 90 includes a leveling device, such as drum 16, a first sensor 10 constructed substantially as described in FIG. 1, and moisture sensors 22a, 22b connected to a processor 40. The sensor 10 includes an X-ray source 24 for projecting X-ray energy greater than from about 15 KeV and up to about 65 KeV. Emissions detected by the corresponding detector are processed and sent to a computer 42 to measure the trace elements in the passing stream of material. A substantially identical sensor 100 is positioned in the same or an adjacent instrument enclosure 128, and includes an X-ray source 124 for directing X-ray energy in the range of 3–15 KeV towards the material. The source 124 may be coupled to a high voltage power supply 130 including an interlock 136, and a power supply 134 may also be provided. A second detector 126 detects the fluorescence and sends a second output signal, preferably through a pre-amplifier 137 to an analyzer, such as MCA 138 (which may be powered by a power supply associated with the pre-amplifier). The computer 42 then displays the measured elements corresponding to the range of energies emitted by the second source 124. The second sensor 100 may be substantially identical to the one shown in U.S. Pat. No. 6,130,931. The foregoing description of several embodiments of the invention have been presented for purposes of illustration and description. The description is not intended to be exhaustive or to limit the invention to the precise form disclosed. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.