Patent Number: 050200844
Section: summary

This invention relates to a method and apparatus for the analysis of samples of materials in which a finely divided heavy metal is dispersed in a non-metallic matrix, especially mineral ores containing gold, uranium, lead or platinum, using the technique of X-ray fluorescence (XRF). The expression "heavy metal" used herein means tungsten and metals of higher atomic number. A "non-metallic matrix" means a matrix consisting predominantly of non-metallic elements of low atomic number and/or their compounds, such as silica, but may also contain metals which are not "heavy metals" as defined above, either in elemental or in combined form, such as iron or barium. The phenomenon of energy-dispersive X-ray fluorescence (XRF) is well-known. A sample, for example, of a mineral ore, is bombarded by high-energy X-rays and the fluorescence spectrum is analysed by counting the rate of emission of photons over a range of photon energies. Assay laboratories of gold mines have a daily chore of analysing large numbers of samples of gold ore; these include both exploration samples taken to locate the gold-bearing ore, and random samples of ore actually mined, for accounting purposes. The total number of samples to be analysed per day typically ranges from a few hundred to several thousand depending on the size of the mine. A high throughput of samples is therefore highly desirable, in the order of say one sample per hundred seconds, and the only practicable assay method which can achieve this is the traditional fire assay method. This method, however, is expensive in time, labour, capital and running costs, and less accurate than desirable. It is essentially a manual process requiring large quantities of electricity and other consumables and in a typical laboratory dealing with 1,000-2,000 samples per day it is difficult to keep track of individual samples. The use of X-ray fluorescence techniques to analyse ores for heavy metals is known, and is described for example in U.S. Pat. No. 3,404,275 (Martinelli) UK Patent Patent Specification 1080346 (VEB Vacutronic), German Offenlegungschriften 2046606 and 2140794 (Siemens AG), U.S. Pat. No. 4,224,517 (Lubecki) and European Patent Application 0166914 (Kernforschungzentrum Karlsruhe GmbH). Most of these disclosures contemplate the use of the characteristic K-emission peaks of the elements of interest. Such fluorescent emission may be stimulated by various sources, principally X-ray tubes (as in UK Patent Specifications 1070337 (Laurila) and 1017595 (UKAEA), and German Offenlegungsschriften 2054464) and radionucleides (as in U.S. Pat. No. 3,404,275). None of these disclosures however uses high energy X-radiation within the meaning of the present disclosure. None of the above prior art discloses the particular technique of counting the emitted fluorescence photons in energy bands selected in relation to the K-emission bands of the elements under analysis according to the present invention. The use of metallic filters to shape the radiation incident upon the sample under analysis is known, for example, from U.S. Pat. No. 3,404,275 where a cadmium filter is used, but not for the particular purpose relevant to the present invention; this U.S. Patent, for example, uses for exciting the sample not high-energy X-rays, but .delta.-rays. Again, UK Patent 1070337 discloses the use of nickel or cobalt filters but in the context of analysis for lighter metals than those contemplated by the present invention. It is especially significant that none of the prior art discloses the preferred method according to the invention of producing the stimulating radiation--that is the exploitation of the broad bremsstrahlung peaks, filtered through a suitable metallic filter to excite the characteristic K-emission peaks of the metals under analysis. The use of germanium detectors, both of high purity (European Patent Application 0166 914 and UK Patent 4224571) and silicon or lithium-drifted (Offenlegungsschriften 2046606 and 2054464) is known but not in the context of the particular energy profile of the fluorescence photons analysed according to the invention. It will be noted that none of the extensive prior art mentioned above is specifically directed to the analysis of ores for gold. X-ray fluorescence techniques have been employed to analyze gold ores but these generally use the gold L band which does not give the sensitivity required for measurement of the one part per million level of gold which is critical. Small samples effectively of a few grams are the maximum that can be used in these known machines. The sample must be ground to an extremely fine powder and a binding resin or glaze added. On the other hand some of the prior disclosures mentioned above (e.g. U.S. Pat. No. 4,224,517) are specifically concerned with analysis for uranium, but do not disclose the particular use of filtered high-energy bremsstrahlung radiation from a high-energy X-ray source according to the invention. Metallic ores when bombarded by high energy X-radiation (which for the purposes of this description means radiation of by energy greater than 80 keV) emit fluorescence photons which are grouped into characteristic peaks. The K X-ray fluorescence emission spectrum of gold is characterised by two peaks known respectively as the gold K.alpha..sub.1 and the gold K.alpha..sub.2 peaks respectively at approximately 68.8 keV and 67.0 keV. According to the invention we have discovered a method of rapidly analyzing the ores of gold and other heavy metals which gives significant advantages over the known fire assay and L X-ray fluorescence techniques mentioned above. We are able, using preferred embodiments of the invention, to detect a concentration of 1 ppm by mass of gold at a confidence level of 95% in a counting time of 100 seconds. The invention is applicable not only to gold but also to other heavy elements such as uranium, platinum and lead. According to the invention we provide a method for analyzing a sample of ore for at least one heavy metal comprising exciting the ore with high energy X-rays to produce a fluorescence emission spectrum and measuring the intensity of the K-emission bands of the said metal or metals in the spectrum, characterised in that: (a) The X-rays are produced by an X-ray tube. PA1 (b) That most of the high energy bremsstrahlung peak is eliminated by a metallic filter interposed between the source and the sample to give high energy bremsstrahlung radiation of 100 to 130 kev incident upon the sample and PA1 (c) The number of fluorescence photons emitted in each of a plurality of energy bands is counted and compared, the width and energy of the bands being chosen in relation to the K-emission peaks of the metal or metals in the sample. PA1 (1) the gold K.alpha..sub.2 peak on both sides of the maximum PA1 (2) the trough between the gold K.alpha..sub.2 and gold K.alpha..sub.1 peaks PA1 (3) the slope of the gold K.alpha..sub.1 peak below its maximum PA1 (4) the slope of the gold K.alpha..sub.1 peak above its maximum PA1 (5) the trough between the gold K.alpha..sub.1 peak and the mercury K.alpha..sub.1 peak PA1 (6) the mercury K.alpha..sub.1 peak on both sides of the maximum PA1 (1) the region immediately below band (2) described below PA1 (2) the thorium K.beta..sub.2 peak PA1 (3) the uranium K.beta..sub.1 peak PA1 (4) the uranium K.beta..sub.3 peak PA1 (5) the trough between the uranium K.beta..sub.2 and K.beta..sub.1+3 peaks PA1 (6) the uranium K.beta..sub.2 peak PA1 (a) The X-rays are produced by an X-ray tube with a plutonium or uranium anode or secondary target. PA1 (b) The ore sample is excited with the characteristic K X-rays of the material of the anode or secondary target and PA1 (c) The fluorescence photons emitted by the sample are passed through an iridium filter. PA1 (2) Passing the X-ray fluorescence spectrum emitted by the sample at right angles to the exciting rays through a metallic iridium or platinum filter PA1 (3) Detecting the fluorescence photons by a germanium detector PA1 (4) Measuring the intensity of the K.alpha..sub.1 emission bands of the gold content of the sample. PA1 (1) A source of high-energy X-rays PA1 (2) Means to hold a sample of ore in the path of the X-rays PA1 (3) Detector means to count the fluorescence photons emitted by the sample and characterised by PA1 (4) Means to compare the counts of emitted photons in selected energy bands. PA1 (a) With falling excitation voltage and increasing filter thickness the photon flux falls. We can compensate up to a point by reducing the distance between the X-ray tube anode and the sample, and between the sample and the detectors, but this introduces a number of other problems. PA1 (b) It is advisable to restrict the effective photon energy to above 100 keV to minimise particle size effects. PA1 (c) Simultaneous measurement of gold and uranium content requires a significant number of incident photons to be of higher energy than 115.6 keV, the K-edge of uranium. The anode material generally preferred for use in the method according to the invention is tungsten but the use of other anode materials is within the scope of the invention. The method of the invention is especially relevant to the analysis of powdered gold ores containing below 10,000 ppm of gold and of ores containing both gold and uranium. Typically, in gold mining applications, 90% of the ore samples will be of concentration up to 10 ppm gold by mass. (All ppm data in this description are by mass). The metallic filter interposed between the source and the sample is preferably tin metal at least 4 mm thick. As will be described in more detail below, it is a preferred feature of the invention to pass the fluorescence photons emitted by the sample through a heavy metal filter to reduce the bremsstrahlung energy peak and thereby to enhance the relative number of counts in the K-bands of the metals under analysis. This filter is preferably of iridium or platinum when the ore contains gold, and of osmium when the ore contains platinum. A preferred feature of the method according to the invention is that the photons emitted by the sample are counted by at least one detector of high purity germanium, preferably in the form of a disc of active thickness 2-4 mm. In a preferred method according to the invention photons are counted in each of two background energy bands lying either side of the K.alpha..sub.1 peak and in each of two signal energy bands lying between the background bands either side of the peak maximum and the total counts of photons in the background bands and in the signal bands compared, the two background bands being substantially equal in energy width and the two signal bands also being substantially equal in energy width. This "symmetry" of the energy bands counted is an important preferred feature of the invention. The bands are chosen so as to show symmetry about the K.alpha..sub.1 peak maximum, the object being to reduce errors due to shifts in the incident energy peak due to external causes such as supply voltage variations and temperature sensitivity of components. It is advantageous to compress the width of the signal bands and increase that of the background bands thus achieving a better signal:background count ratio. This "compression" of the peak is achieved by better detector resolution which in turn results from improvement in the physical configuration of the detector and its following electronics, reduction of noise and the like. A further preferred method according to the invention uses not four but six energy bands. Using this method, as will be more particularly described hereinafter, it is possible to detect and eliminate interference from certain metals present in the heavy metal ore, for example thorium, mercury and tungsten. In one such preferred method, applicable to the analysis for gold of ore samples containing mercury and/or tungsten, photons are counted in each of six adjacent energy bands embracing respectively: In another such preferred method, applicable to the analysis for uranium and gold of ore samples containing thorium, photons are counted in each of six adjacent energy bands embracing respectively: An important preferred method according to the invention for the analysis of gold in an ore is characterised in that: According to the invention, a particular method of analysing a sample of ore, especially a gold ore or an ore containing gold and uranium, comprises: (1) Exciting the sample with high-energy bremsstrahlung X-rays having their maximum energy at about 115 keV produced by an X-ray tube with a tungsten anode and filtration through a metallic tin filter The invention further provides an apparatus for analysing the heavy metal content of an ore comprising: Characterised in that the X-ray source is a tube and a metallic filter is interposed between the source and the sample which eliminates part of the high-energy bremsstrahlung peak whereby high energy bremsstrahlung radiation of 100-130 keV is incident upon the sample. In the apparatus according to the invention, the anode is preferably of tungsten and the filter of tin metal; the filter is perferably of tin 4-5 mm thick. Preferably, the apparatus also comprises a heavy metal filter interposed between the sample and the detecting means, preferably of osmium, iridium or platinum. Preferably the detector means is at least one body of high purity germanium, especially a plurality of high purity germanium discs each 2-4 mm thick. Also in the apparatus according to the invention, the fluorescence spectrum is preferably viewed at a scattering angle of from 80-100 degrees to the exciting radiation. In an important preferred emodiment of the apparatus according to the invention, the X-ray source is a tube with a plutonium or uranium anode or secondary target and an iridium filter is interposed between the sample and the detecting means. A preferred form of the apparatus according to the invention, associated with the preferred method set forth above, comprises means to count the emitted fluorescence photons in each of two background energy bands lying either side of the K.alpha..sub.1 peak of the heavy metal under analysis and in each of two signal energy bands lying between the background bands and either side of the said K.alpha..sub.1 peak maximum and to compare the total counts of photons in the background bands with the total counts of photons in the signal bands, the two background bands being substantially equal in energy width and the two signal bands also being substantially equal in signal width. A particular aspect of the apparatus according to the invention is an apparatus for analyzing the gold and/or uranium content of a sample of ore by X-ray fluorescence, comprising an X-ray tube with a tungsten anode, a metallic tin filter, means to hold and retain the sample in the path of the X-rays emitted from the source and passed through the filter, an X-ray detector and means to detect the emission of photons of various energies from the sample, characterised by the interposition of a metallic platinum or iridium filter between the sample and the detectors and the use of germanium detectors. An iridium filter is preferred. It is an important feature of the invention that the fluorescence radiation emitted by the sample is viewed at a scattering angle of about 90.degree. to the exciting radiation. However, the scattering angle in any application is a matter of compromise between conflicting requirements as described below and the invention is not limited to any particular scattering angle. Generally it is found that an angle within the range 80.degree.-100.degree. is suitable. For purely mechanical considerations, an angle of exactly 90.degree. is easiest to work with. However, the angle must also be chosen to minimise scattered non-parallel rays which give rise to background counts and from this point of view is preferably set at the Compton scattering minimum angle. Further, there is a correlation between peak energies and scattering angle--generally a particular peak shifts to a lower energy with an increase in scattering angle. A compromise between these three factors must be reached and it is generally found for the applications described herein that best results are obtained at a scattering angle of about 100.degree.. The X-ray tube may be any known type that produces high energy X-rays (i.e. photons) of the appropriate energy (as to which, see below) and is preferably water-cooled in a closed circuit cooling system with an integral radiator or chiller. In this description the expression "high energy" is used to indicate an exciting energy of 80 keV or more; the energy region 80-160 keV being of principal interest. However, a preferred method of producing high energy X-rays for use in the method according to the invention uses an X-ray tube with a tungsten anode. Such tubes emit, in addition to the tungsten characteristic peaks, the highest of which lies at 69 keV and is too low to be of practical value in analysis for gold (the K-edge of which lies at 80.7 keV), a bremsstrahlung peak at 65 keV the upper edge of which lies at about 130 keV. The bremsstrahlung peak is generated by the slowing down of electrons in the tube anode. In this preferred method the tungsten characteristic peaks and most of the bremsstrahlung peak are eliminated by a metallic filter interposed between the source and the sample to give high energy bremsstrahlung radiation of 100-130 keV, preferably with a maximum at about 115 keV, incident upon the sample. This is in the form of a semi-Gaussian peak. The metallic filter is preferably of tin metal and at least 4 mm, generally from 4-5 mm, thick. The excitation voltage for the X-ray tube is optimised at about 130 kV when a 4 mm tin filter is used. The optimal voltage decreases as the tin thickness is increased, for example 125 kV with 5 mm tin. The effective incident photon energy can be varied by altering the excitation voltage on the X-ray tube and the thickness of tin filter in front of it. Thicker filtration will both harden the energy (i.e. increase the average energy) and narrow the bremsstrahlung peak, but of course reduces the number of photons striking the sample. As the average energy moves downwards towards the K-edge of gold at 80.7 keV, the probability of exciting the gold will increase. However, in practice the resulting increase in signal will be partially counteracted from a signal to noise point of view by an increase in background and attenuation effects. Most of the peak is in the energy range 100-130 keV but the low energy tail does extend lower than this and, after single or multiple scattering by the sample contributes significantly to the background counts in the gold energy measurement region. There are a number of practical limitations which restrict the ranges of excitation voltages and tin filter thicknesses one can use. The invention is not however limited to the use of highly filtered bremsstrahlung radiation as described above. It is also possible to achieve greater sensitivity by the use of an X-ray tube with uranium or (preferably) plutonium anode or secondary target, to generate uranium or plutonium characteristic K X-rays as well as bremsstrahlung. In this instance the characteristic radiation of the anode or secondary target material used to excite the sample and the bremsstrahlung radiation has its maximum at 150 keV, well above the energy region of interest. Some filtration of the bremsstrahlung radiation is still required, however, for example by the use of a tin filter as described above, to attenuate substantially the bremsstrahlung "tail" below 80 keV relative to the characteristic K-bands. The secondary target may be internal or external; it will be appreciated that the use of a secondary external target has advantages when tubes with anodes of these metals are unavailable. The lowest energy plutonium K X-ray, the K.alpha..sub.2, is at 99.2 keV. Being characteristic radiation, this is monoenergetic, and there is no low energy tail, as with bremsstrahlung radiation. Similarly, the lowest energy uranium K X-ray is 94.6 keV as compared with the gold K absorbtion edge at 80.7 keV. The relative attenuation of the low energy tail significantly reduces the background counts in the gold measurement and allows detection of lower gold levels. Another advantage is that the scattered characteristic radiation would be attenuated better by an iridium filter than the present bremsstrahlung, which would result in a higher sensitivity. Plutonium K X-rays will excite both gold and uranium, whereas uranium K X-rays will excite only gold. This is one reason for preferring plutonium. A second reason is that the plutonium K X-rays are several keV higher in energy and, when scattered from the sample, will not contribute so much to the background counts in the gold energy measurement region. The operating voltage of the uranium or plutonium anode X-ray tube is ideally higher than the tungsten anode tube, possibly as high as 300 kV or thereabouts. The higher the voltage, the higher the efficiency of production of characteristic radiation from the anode material. Also, the higher the energy of the bremsstrahlung, which is produced at the same time. As stated above some tin filtration is still needed to reduce the low energy tail of the bremsstrahlung but the latter should be a much lower fraction of the total exciting intensity. An alternative to an X-ray tube using electrons for production of characteristic K X-rays of plutonium or uranium, is to bombard the target material with protons or alpha-particles. These produce much less bremsstrahlung when they are slowed down in the target material. The ore samples must be finely ground but the average grain size, which should preferably be less than 100 microns, is not critical, although it has been found that with larger gold grain sizes in the ore better results are obtained by fine grinding. With fine gold grain sizes the sample may be ground to sub-millimetric size only. The sample is preferably contained in a cylindrical thin-walled container of plastic material such as acetal plastic, certain features of which are, as more particularly described below, preferred according to the invention. A preferred feature of the invention is the use of a heavy metal filter interposed between the fluorescent sample and the detectors. For gold analysis using X-rays from a tungsten anode tube the heavy metal is preferably platinum or iridium; for platinum analysis an osmium filter is preferred. As will be more particularly described later with reference to the drawings, the use of a filter of one of these metals reduces the scattered bremsstrahlung peak at about 100 keV and enables the characteristic bands to be more readily detected by allowing the detection of higher count rates in the energy region of interest. Apart from considerations of cost, a platinum filter should be used where the content of lead in the sample is to be measured, as the iridium K.beta. band overlaps the lead K.alpha. band. The invention is not, however, restricted to the use of a filter of iridium or platinum, or indeed of any metal. As described herein, these particular metals are especially useful in analysis for gold and certain other elements using particular sources of exciting radiation. For example, platinum may be detected and analysed using an osmium filter. However, it is possible using other sources (for example radioisotopes) to analyse for low concentrations within the range contemplated by the invention and this is of particular relevance for the detection and analysis of lighter elements such as silver and copper as well as for that of gold, using the same detectors as are contemplated herein; such methods are, however, not within the scope of the present invention. A heavy metal filter cannot be used for the simultaneous measurement of light and heavy elements in similar dilutions. Nevertheless, whatever the excitation source, the use of an appropriate filter such as those described herein will generally make possible analysis at lower concentrations than is possible without such filter. The above principles may be summarised by the statement that the filter medium is chosen to reduce the bremsstrahlung peak so as to enhance the count rate in the energy region of interest. The invention is not limited to the use of filters consisting of a foil or sheet of the metal in question. For example, the filter may be made from finely divided metal dispersed in an epoxy or other resin matrix. Similar considerations also apply to the tin or other metal filter which, as mentioned above, is to be interposed between the radiation source and the sample. The fluorescence photons passing through the iridium or platinum filter are preferably counted by at least one detector consisting substantially of metallic germanium. It is found that circular germanium detectors arranged in a regular array and orientated axially to the incident radiation are preferable. The combined use of germanium detectors and an iridium filter in preferred embodiments of the invention is an important feature which gives the capacity to detect gold in ore samples down to a concentration of 1 ppm at a confidence level of 95% in a counting time of 100 seconds. The detector unit is preferably cooled to cryogenic temperatures by liquid nitrogen from a small cryostat automatically topped up from a large cryostatic reservoir. The bombardment of the detectors by the high energy fluorescence photons emitted from the sample passing through an iridium filter generates a signal the intensity of which is amplified by pre-amplifiers (generally, one for each detector) and processed to give only the readings for the metals to be analysed. The invention includes the analysis of sample containing other heavy elements such as uranium, platinum and lead. Indeed these elements are commonly found in gold ores. It is possible using the invention to measure two or more of these metals simultaneously, for example, especially uranium and gold. In principle it is possible to adapt the apparatus and method according to the invention to detect and analyze for elements in the periodic table down to, say, tungsten. A method and apparatus corresponding to those of the invention may be used for measuring heavy metals such as gold in materials other than mineral ores; of especial importance is the measurement of gold in carbon from carbon-in-pulp plants. The drawings illustrate the underlying physic and the layout of an apparatus according to preferred embodiments of the invention.