X-ray separator for separating metals from recycled materials

Abstract

An X-ray separation apparatus for separating metals from recycled materials includes an X-ray source (1) which generates a radiation beam (2) which passes through material transported by a conveyor belt (3) and reaches an array of transmission sensors (4) and is capable of generating a fluorescent signal which is further detected by an array of fluorescence sensors (5) positioned adjacent to the beam (2), thereby allowing the separation apparatus to simultaneously perform both fluorescent and transmission analysis using both phenomena caused by the single beam (2).

Description

[Technical field]

[0001] The present invention relates to the recovery of metals from scrap, and in particular to an X-ray separation device for separating non-ferrous metals. [Background technology]

[0002] As is well known, automobiles and other large industrial and domestic products of essentially metallic composition, at the end of their life, are ground in large hammer mills (so-called car shredders) to reduce the size of the ground pieces to less than 150 mm in order to obtain scrap metal. On leaving these mills, the mixed ground material is subjected to a de-ironization process using large electromagnetic drums with the aim of recovering and removing the ferromagnetic steels.

[0003] The waste produced by these magnetic drums consists primarily of plastics, rubber, polyurethane foam, glass, aluminum, copper, zinc, zamak, lead, stainless steel, electrical wire, stone residues, iron oxide, and some ferromagnetic steel parts lost during the de-ferrication process. A mixed metal concentrate is then produced using Foucault current and induction sensor separation equipment.

[0004] Further wet or dry density separation processes can separate light non-ferrous metals such as aluminum and magnesium from heavy metals such as copper, brass, stainless steel, and lead. The present invention applies to downstream processes that focus on the separation of dissimilar metals.

[0005] A special feature of this application is that the separation process to achieve a throughput of at least 1 tonne / hour, which is usually considered a minimum in the recycling industry, requires the processing of a large number of pieces per hour, as the weight of the pieces can vary from a few grams (3-4g) to over 1000g, with an average weight usually in the range of 10-50g. This means that the separator needs a few milliseconds, usually 5-50ms, to identify the chemical composition of each piece that makes up the material stream.

[0006] Separation devices capable of carrying out this task use X-ray technology, which can be of two types: fluorescent technology (X-ray fluorescence = XRF) or transmission technology (X-ray transmission = XRT).

[0007] XRF fluorescence technology allows to gather information about the chemical nature of materials through a purely superficial investigation, since the signal comes from a depth of several hundred micrometers from the surface, does not tell us anything about the internal composition of the piece being analyzed (called the "sample") and is only reliable assuming absolute homogeneity of the sample composition. In other words, if the inside of the sample, called the "bulk", has a different chemical composition than the surface layer, for example because the sample is made of a piece of a certain metal and an insert made of another metal, fluorescence cannot confirm this.

[0008] This is a common situation when considering materials obtained from car shredders as materials to be sorted for metals, since the metal pieces to be sorted may consist of a mixture of metals or metal inserts with different chemical properties, or metals coated with other metals (e.g. galvanized pieces). Another common case is tubes and wires covered with a plastic sheath. If the sheath is thick enough, it may prevent the detection of the fluorescent signal of the underlying metal, leading to the conclusion that the sample does not contain any metal in the XRF analysis. Also, materials may be contaminated, and a surface layer of dirt (usually metal powder) may change the results of the XRF analysis.

[0009] Additional drawbacks of the XRF technique include the limitations on the metals that can be sorted and the low spatial resolution: in the former case, the analysis speed imposed by the application under consideration (a few milliseconds per piece) results in a fluorescence phenomenon that is very weak for light elements such as Al or Mg, while it is clearly stronger for heavier chemical elements (e.g. from Ti onwards).

[0010] In terms of spatial resolution, a typical XRF separation device comprises a source generating a polychromatic X-ray beam projected into the material stream to be analyzed, and an adjacent array of fluorescent sensors (called SDDs = Silicon Drift Detectors) arranged equidistant from each other along a linear array transverse to the direction of the stream. Ideally, the longitudinal resolution in the flow direction is limited by the sample rate and acquisition time of the SDDs (usually in the range of 5ms-50ms), while the lateral resolution is limited by the number of SDDs and the pitch with which they are arranged along the array (usually 15mm-60mm). For example, an SDD with a pitch of 50mm, corresponding to the width of the field of view of each SDD, an acquisition time of 15ms and a material transit time of 2.5mm / ms, results in a reading "surface" of 1875 square mm (=50mm x 15ms x 2.5mm / ms), comparable to the average surface area of ​​the parts to be processed.

[0011] Given the low spatial resolution of the XRF technique, it will always be necessary to combine it with a second technique that is able to identify both the shape and the position of the sample with a higher resolution. This information is essential both to analyze the sample and to carry out the separation between the various metals identified. The techniques used for such a combination are either cameras (black and white or color) or 3D laser scanners, or both.

[0012] A typical design of an XRF separator involves feeding the material on a horizontal conveyor belt, above which the X-ray source and sensor are placed to avoid obstacles that would impede the propagation of the fluorescence signal. Typical values ​​for the source-belt distance are in the range of 100-300 mm, preferably 100-200 mm, and the anode-cathode voltage in the source must be modified according to this distance to optimize the fluorescence emission signal. The voltage used is in the range of 20-75 kV, preferably 30-40 kV, and the source power is less than 500 W.

[0013] Under these conditions, known XRF separation devices can identify the maximum percentages of Cu and Zn in an Al alloy: less than 6% Cu and less than 11% Zn, respectively. The fluorescent signal is analyzed by a linear array of SDDs whose distance from the belt can vary from 1 cm to several centimeters depending on the height of the pieces passing over the belt to avoid hitting the sensor. The state-of-the-art trend is to move the source closer to the belt and to maximize the fluorescent signal using less and less power and lower and lower voltages.

[0014] An alternative possible configuration is to perform the analysis "on the fly", i.e., to let the parts fall from a conveyor belt or chute and place the analytical element (source, SDD) close to where the drop starts (usually below the trajectory of the pieces). In this way, the distance from the bottom of the sample to be analyzed is constant, so the height of the pieces and their positioning do not affect the distance from the analytical element, as is the case when performing the analysis on a conveyor belt.

[0015] XRT transmission techniques overcome some limitations of XRF technology, differing in the type of sensor based on scintillator technology and in the placement of the sensor on the opposite side of the sample from the X-ray source, which is more powerful and placed further away than in XRF technology. The source-sensor distance is typically between 500 mm and 1200 mm and is chosen as a compromise between the need to maximize the process throughput and the need to optimize the transmission signal. The first requirement is met by increasing the source-sensor distance, since it increases the part of the belt illuminated by the beam, while the second requirement is met by bringing the source as close as possible to the sensor, since it increases the low energy components in the beam, where the absorption differences are most pronounced.

[0016] Independently of the above distances, the anode-cathode voltage that optimizes the transmitted signal for any possible configuration is 130 kV to 160 kV with a current of up to 19 mA (corresponding to a maximum power of about 3 kW). Depending on whether the application requires a somewhat unbalanced spectrum in the high energy direction, the source can be equipped with a beryllium or copper window that acts as a radiation filter.

[0017] Radiation emitted by the X-ray source passes through the entire sample and is transmitted to a sensor on the other side, which consists of a photodiode coated with a scintillator material that converts the X-ray radiation into visible radiation and then into a current signal, which is converted to a voltage, amplified, and then sampled and quantized by analog-to-digital conversion electronics.

[0018] The photodiodes are chosen to give a much higher resolution than that offered by the fluorescence sensors used in XRF technology, with sensor sizes of about 0.4-3.2 mm and acquisition times ranging from 10 μs to tens of milliseconds. In XRT technology, the sensors are also arranged side-by-side to form a linear array transverse to the direction of material feed. Successive scans consecutive in time allow an image of the passage of the sample to be reconstructed.

[0019] An alternative configuration allows for what is called "dual energy" (DE) spectroscopy, i.e. using two sensor arrays of identical size and pitch but different energy sensitivity, as described for example in US Pat. No. 5,399,633. The two arrays are overlapped such that the upstream array, relative to the direction of beam origin, produces a signal integrating the absorption at low energies emitted by the X-ray source (typically in the range 30 keV-90 keV), and the downstream array integrates the absorption at higher energies (typically in the range 70 keV-150 keV). Values ​​in the overlapping range 70-90 keV, where both arrays give a signal, are typically filtered out during readout data processing.

[0020] If the spectrum of the source is polychromatic, the transmitted radiation carries the signature of the absorption coefficients, which vary for each chemical element and its alloys. The result is two images of the sample, with a spatial resolution equal to the face of the photodiode and taking into account the different effects of the absorption coefficients at high and low energies.

[0021] The advantage of the XRT technique is the ability to obtain very detailed spatial information and to evaluate the nature of materials within a piece. The excellent lateral and longitudinal resolution allows the recognition of details of shape or characteristic morphology without the need for combination with a camera or laser scanner as in XRF techniques. Samples with local inhomogeneities, such as inserts, are easily detected and the presence of surface dirt does not impair the analysis.

[0022] The disadvantages of XRT technology are limited chemical resolution, possible complete absorption of radiation by the sample, and the inability to distinguish between surface coating and alloy materials. In the first case, the disadvantage arises from the fact that elements with similar atomic numbers are rarely recognized as distinct, while theoretical calculations show that it is possible to accurately detect a number of chemical elements equal to the number of arrays at different energies, i.e., even three or more overlapping arrays can be used in theory, whereas in DE spectroscopy there are only two. Also, the minimum percentage change in chemical ligands that can produce a change in the transmission signal differs for each element.

[0023] In the second case, if the analyte consists of a heavy alloy (e.g. steel alloy, copper alloy, brass, zinc alloy) and is of sufficient thickness, the signal transmitted to the sensor will be very weak or not transmitted at all. Finally, a sample in which metal X is used as a surface coating on a piece made of metal Y will be indistinguishable from a sample composed of an alloy of metals X+Y, since the transmitted signal is still a linear combination of the absorption by the two metals. [Prior art documents] [Patent documents]

[0024] [Patent Document 1] US Patent Application Publication No. 2007 / 030953 Summary of the Invention [Problem to be solved by the invention]

[0025] It is therefore an object of the present invention to provide an X-ray separation device which overcomes the above mentioned drawbacks. This object is achieved by a separation device having a single X-ray source which, due to the presence of both a fluorescence sensor and a transmission sensor, can perform both XRF and XRT analysis simultaneously, depending on the positional and operating parameters, as described in claim 1. Further advantages and features of this separation device are described in the dependent claims. [Means for solving the problem]

[0026] The fundamental advantage of the device is therefore to exploit two complementary physical phenomena using a single common excitation source, such that the synergistic use of the two techniques, XRF and XRT, allows the scope of application to be extended beyond that of the combination of the two techniques used individually. In other words, as will be shown below, this compensates for the inherent limitations of the techniques used when used individually, and exploits their complementarity to improve analytical accuracy and broaden the scope of application, which cannot be reproduced by applying XRF and XRT analysis separately and in succession. Moreover, such a combination overcomes technical preconceptions regarding the difficulty of combining two readings, as is the case in WO 2012 / 094568, which uses the XRT technique but with the presence of a film or coating to eliminate or reduce the fluorescence emission.

[0027] Another important advantage of the above device is its simple construction and low cost, since it does not require any analytical device components (source, sensor) that are technically different from those used in conventional devices, it is less costly than two separate XRF and XRT separators, and it offers better performance than either of them or a system in which this separator is placed in series.

[0028] Further advantages and features of the X-ray separation device according to the present invention will become apparent to those skilled in the art from the following detailed description of one embodiment thereof, taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0029] [Figure 1] 1A-1D are longitudinal cross-sectional views showing schematic diagrams of the structure of a conventional XRF separation device in two possible configurations. [Diagram 2] FIG. 1 is a vertical cross-sectional view illustrating a schematic structure of a conventional XRT separation device. [Diagram 3] FIG. 2 is a schematic longitudinal cross-sectional view of the analytical section of a combined XRF-T separator according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] With reference to FIG. 1, it can be seen that the prior art XRF separation device can be made in a first configuration (a) for "on the fly" analysis, where the material M flows down the chute S and is analyzed by an analysis system A placed below the falling trajectory as soon as it starts to fall from the chute S. Such an analysis system A includes a suitable X-ray source and an array of SDDs for analyzing the comminuted material M by XRF techniques, the readings of which are analyzed by a control unit U which commands the control electronics E of a valve that emits a jet of air to deflect the trajectory of the metals to be recovered. In this way, the metals (open circles) are directed to a more distant collection vessel, while the waste (closed circles) continues on a path that drops into a closer collection vessel.

[0031] In a second configuration (b), the material M is fed by a horizontal conveyor belt T and passes beneath a source X emitting a radiation beam capable of causing a fluorescence phenomenon that is detected by an adjacent sensor array D. As in the previous configuration, the readings from the sensor D are analyzed in a control unit U which commands the control electronics E of a valve that emits a jet of air to deflect the metal to be recovered as it falls from the conveyor belt T.

[0032] A conventional XRT separation device configuration is shown in Figure 2, which in many ways corresponds to the configuration of Figure 1(b), in which material M is fed by a horizontal conveyor belt T and passes under a source X' of suitable power, which emits a radiation beam B that can pass through the material M and the conveyor belt T and is detected by an array of sensors D' located between the upper and lower branches of the conveyor belt T. As in the previous configuration, the readings from the D' sensors are analyzed in a control unit U', which also contains the control electronics for valves that emit jets of air to deflect the metal to be recovered as it falls from the conveyor belt T.

[0033] The separation apparatus according to the present invention (the analysis section of which is shown in FIG. 3) differs from the known separation apparatus mentioned above in that it includes an X-ray source 1 which generates a radiation beam 2 which can pass through the material conveyed by a conveyor belt 3 and reach an array 4 of transmission sensors (preferably a dual array for "dual energy" analysis). The beam 2 can also generate a fluorescence signal which is detected by an array 5 of SDDs placed adjacent to the beam 2, so that the separation apparatus of the present invention uses both the fluorescence and transmission phenomena caused by the single beam 2 to perform both XRF and XRT analysis simultaneously.

[0034] For this purpose, the inventors have defined the operating parameters of anode-cathode voltage, current intensity and distance H between the source 1 and the array of transmission sensors 4 in order to successfully achieve the difficult task of balancing the conflicting requirements of two different techniques, XRF and XRT. Indeed, the following ranges are different from those conventionally used for each of the two techniques mentioned above. Anode-cathode voltage: 100 kV to 130 kV, preferably 100 kV to 110 kV, Current strength: at least 20mA, Distance H: 300-700mm, preferably 350-550mm.

[0035] The distance of the SDD array 5 from the belt 3 is determined by the maximum height of the pieces that make up the material M, with a margin of 10-20 mm to avoid collisions, this distance is usually 100-120 mm, but can generally vary between 50 mm and 150 mm.

[0036] In contrast to the aforementioned trend of reducing distance, voltage, and beam power to maximize the fluorescence signal, applicants have demonstrated that it is surprisingly possible for XRF technology to maintain its ability to effectively identify aluminum alloys while operating in the above ranges. In this regard, a comparison has been made between the fluorescence signal obtained by simply adding an SDD array to a conventional XRT separator and the fluorescence signal obtained with the XRF-T separator of the present invention.

[0037] In the first case, a typical configuration of an XRT separator was used, with a distance of 900 mm between the source and the conveyor belt, a distance of 120 mm between the belt and the SDD array, an analysis time of 20 ms to fit the requirements of the metal recycling process, and a generated power of 3 kW (this means that the product of voltage and current is constant, and as the voltage increases the current must decrease). The source has a tungsten target tilted by 20° to achieve a larger shift of the radiation spectrum to lower energies, and a beryllium window.

[0038] Within the conventional voltage range used for XRT technology, the copper fluorescence signal has a maximum value of about 190 units on average, due to some oscillations in the signal, while the background noise detected on an empty belt is on average 12 units, resulting in a signal-to-noise ratio SNR = 15.8. Under these conditions, taking into account the signal oscillations, and considering that the minimum detectable signal is equal to the average noise value plus 10 units, it turns out that the lower detection limit for copper in aluminum alloys is a percentage of about 9%. With such a sensitivity, the presence of copper cannot be detected in aluminum alloys with a percentage of copper of 6% or less.

[0039] In the second case, which is within the scope of the present invention, the source distance was reduced to 500 mm, which still provides illumination of a belt of a width large enough to meet the process throughput of the metal recycling industry. A 0.4 mm thick aluminum filter was attached to the source to reduce background noise in the signal and to remove the tungsten fluorescence radiation from the source itself. Note that the system also works without the filter, the filter was introduced only to benefit the data processing algorithms, but when the source is at 900 mm the filter cannot be used as it would attenuate the intensity of the beam too much and would not give a proper fluorescence signal.

[0040] Using the voltage range of 100-130 kV mentioned above, which is lower than that of conventional XRT separation devices, the maximum fluorescence signal is obtained at 110 kV, averaging about 450 units, while the background noise has an average value of 10 units, resulting in a signal-to-noise ratio SNR=45. Under these conditions, taking into account that the minimum detectable signal is equal to the average value of the noise plus 5 units, and not 10 units as in the first case due to the reduction of signal oscillations by the filter, it turns out that the lower detection limit of copper in aluminum alloys is about 3.3%. This value matches the proportion of copper in aluminum alloys, so that this device can be used for separation applications between aluminum alloys.

[0041] The two signals acquired by the fluorescence and transmission sensors are preferably synchronized via an external trigger signal to increase the reliability of the vision system. The result is a transmission map with precise spatial resolution for high and low energies in the case of the DE sensor, and a fluorescence map with significantly lower spatial resolution. The transmission signal conveys information about the location of the sample, its shape and its "bulk" properties, while the fluorescence signal conveys information about the chemical composition of the surface for the first few hundred micrometers. This compensates for the inherent limitations of the techniques used when considered separately, and exploiting their complementarity increases the accuracy of the analysis and broadens its scope of application. In fact, a discrepancy between the two pieces of information is a sign of heterogeneity in the chemical composition.

[0042] In addition to absorption, which depends on the thickness of the sample at each point, if the transmission signals at high and low energies are cross-correlated, compositional information is obtained that is independent of the height of the piece and contains only an indication of the volume composition of the piece. This information can be used in addition to absorption and fluorescence, especially in the case of metals where the fluorescence-only signal is weak.

[0043] As mentioned above, the special technical configuration of the separation device allows a synergistic effect to be obtained from the combination of the two techniques, XRF and XRT analysis, carried out simultaneously on the same piece. Furthermore, in a preferred embodiment of the DE type dual sensor array 4, the transmission spectrum provides additional data on the nature of the material, which is useful in the case of materials that generate weak fluorescence signals, such as aluminum alloys, magnesium alloys, and light chemical elements such as silicon. The following examples show the effectiveness and synergistic effect of the claimed combination of XRF and XRT techniques. EXAMPLES

[0044] A piece of 1xxx series aluminum alloy, 5mm thick, with a 500μm thick copper coating: the XRF technique identifies it as 100% copper, the XRT technique identifies it as an aluminum alloy of the 2xxx family based on the mixture of copper on the surface and aluminum underneath. Only by combining the two pieces of information can we detect the compositional discrepancy and deduce that it is in fact a 1xxx series alloy coated with copper. EXAMPLES

[0045] Aluminum piece with iron-based metal insert: Fluorescence signal analysis reports the chemical composition of the sample as 100 percent aluminum, but the transmission image clearly shows the presence of objects of different chemical composition within it, making it clear that this is reinforced aluminum, not an aluminum alloy. EXAMPLES

[0046] A brass piece and an aluminum piece, both covered with over 500 μm of zinc: Fluorescence signal analysis reports the chemical composition of both samples as 100 percent zinc, but the transmission image shows that the aluminum piece has a lower volume density than the brass. Combining the two pieces of information, we can detect that these are two galvanized samples made of two different materials, rather than two zinc-containing alloys.

[0047] In general, in all cases where there are compositional differences between the surface and the "bulk" of a piece, for example due to the presence of surface contamination, which is very common in the case of scrap, the combination of the two techniques, XRF and XRT, allows obtaining information that would not be obtained by using the two techniques individually. It should also be noted that it is not possible to apply the two techniques consecutively to the same piece and compare the results, not only because it would require two separators connected in series, which would significantly increase the cost and the footprint of the separation plant, but also because the material is in continuous motion and the piece rotates / shifts in the second analysis compared to the first, making it physically impossible to follow the path that the piece travels from one separator to another.

[0048] It will be apparent that the embodiment of the separation apparatus according to the invention described and illustrated above is merely one example, of which many variations are possible, and in particular the separation apparatus may also be configured for "on the fly" analysis by locating the analysis section near the start of the falling trajectory of the material from a chute or conveyor belt, and preferably locating the X-ray source and SDD array below this trajectory.

Claims

1. an X-ray source (1) configured to generate a radiation beam (2) capable of passing through pulverized material (M) including scrap metal; at least one array of transmission sensors (4) arranged transversely to the feed direction of the pulverized material (M), aligned with the radiation beam (2) and arranged on the opposite side of the flow of the pulverized material (M) to the X-ray source (1); a control unit configured to receive readings from the array of transmission sensors (4) and to control valves for emitting air jets; A metal separation apparatus comprising: an array of fluorescence sensors (5) arranged adjacent to the radiation beam (2) and transversely to the feed direction, said array being located on the same side of the flow of ground material (M) as the X-ray source (1) and operatively connected to the control unit and transmitting its read data to said control unit; the control unit is configured to analyze and compare the readings from both the transmission sensor (4) and the fluorescence sensor (5) for the same piece of ground material (M); The X-ray source (1) The anode-cathode voltage is 100 kV to 130 kV, preferably 100 kV to 110 kV; The current strength is at least 20 mA; The distance (H) of the transmission sensor (4) from the array is between 300 mm and 700 mm, preferably between 350 mm and 550 mm. A metal separation apparatus comprising:

2. 2. A metal separation device according to claim 1, characterised in that the distance between the array of fluorescence sensors (5) and the bottom of the flow of grinding material (M) is between 50 mm and 150 mm, preferably between 100 mm and 120 mm.

3. 3. The metal separating apparatus according to claim 1 or 2, characterized in that it comprises two arrays, the transmission sensors (4) of which have the same size and pitch but different energy sensitivity, and the two arrays are overlapped such that the upstream array, relative to the direction of origin of the radiation beam (2), produces a signal integrating the absorption at low energies emitted by the X-ray source (1) and the downstream array, relative to the direction of origin of the radiation beam (2), produces a signal integrating the absorption at high energies.

4. 4. The metal separation apparatus of claim 3, wherein if there are overlapping bands of energy where both arrays emit signals, the control unit is configured to filter them out when processing the readout data.

5. 5. The metal separation device according to claim 1, wherein the X-ray source (1) has a tungsten target tilted at 20°, a beryllium window and an aluminium filter, preferably with a thickness of 0.4 mm.

6. 6. The metal separating apparatus according to claim 1, wherein the control unit is configured to synchronize the two signals obtained from the fluorescence sensor (5) and the transmission sensor (4) via an external trigger signal.

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