Patent Number: 043615341
Section: summary

This invention concerns the simultaneous measurement of the chemical concentrations of the silicon and aluminium constituents of materials. It provides both a method and apparatus for that purpose. There are many possible applications of the present invention, including the measurement of silicon and aluminium in coal and in iron ores. However, the invention was developed primarily to permit the monitoring of the chemical concentrations of aluminium and silicon in bauxite ores, as part of a quality control process for the mineral industry, and it is this application of the invention that will be described in detail in this specification. There are two particular areas in the bauxite industry where the present invention will be used. One is the monitoring of ore quality during ship-loading operations of bauxite for export, where monitoring of aluminium grade and silicon impurity concentrations are essential to ensure that the ore satisfies export contract specifications. The other use is the monitoring of ore quality whilst sorting the bauxite into stockpiles of different specified chemical concentrations of silicon and aluminium. Depending on the way in which they have been formed, these stockpiles (a) may contain ore which has been blended for ore treatment plants or (b) may be used in subsequent blending operations. In both of these situations, the current practice in monitoring the ore quality involves the periodic sampling of the ore from the bulk supply, which is usually moving on a conveyor belt when the sample is taken. The samples are moderately large (several kilogrammes) and are either subsampled immediately, or are mixed with other samples, taken by a predetermined number of automatic sampling cycles to form a representative bulk sample, which is then sub-sampled. Sub-sampling and crushing proceeds until a small specimen (of about 1 g) is ultimately available for chemical analysis by wet chemical assaying or by X-ray fluorescence analysis procedures. These sample preparation procedures are particularly time-consuming if good representivity of the bulk is required in the sample. The analysis is also time-consuming. It has been found that in some situations (for instance during ship loading), when variations of ore quality occur these analytical methods are not fast enough to permit steps to be taken to correct the chemical concentrations of aluminium and silicon (for example, by further blending measures). If it were possible to apply the prior art techniques to on-stream analysis of bauxite on a moving belt, a more rapid analysis of the constituents and hence more rapid corrective blending measures should, in principle, be possible. Unfortunately, wet chemical methods cannot be applied to an on-stream situation, and on-stream X-ray fluorescence methods are inapplicable to lump-flow measurement. The X-ray fluorescence method is also unsuitable for the analysis of untreated bulk samples, due to the low penetration of X-radiation (less than 1 mm), and the fact that the ore is heterogeneous as regards moisture and particle size. Neutron activation analysis, which is the basis of the present invention, does not have the problems associated with wet chemical assaying or the X-ray fluorescence technique, noted above, when applied to the analysis of large bulk samples. Indeed, activation analysis methods are directly applicable to large bulk samples and require minimal sample preparation in terms of crushing and drying. They also avoid most of the heterogeneity problems associated with the application of X-ray fluorescence analysis to bulk samples because the neutrons and gamma rays involved have a much deeper penetration than X-rays. For this reason, the monitoring of bauxite ore quality (and the aluminium and silicon content of other materials) on moving belts is amenable to the neutron activation method of the present invention. Neutron activation methods have previously been applied to the analysis of silicon and aluminium in small samples (less than 150 g). For example, they have been described in the paper by F. Dugain and J. Tatar in Ann. Inst. Geol. Publici Hungary, Volume 54, p 375 (1970), and in the papers by L. Alaerts, J. P. Op de Beeck, and J. Hoste, in Anal. Chim. Acta, Volume 70, p 253 (1974) and in Anal. Chim. Acta, Volume 78, p 329 (1975). These methods depend on two interactions with the constituent chemical elements. One interaction is that occurring between fast neutrons and .sup.28 Si, producing .sup.28 Al by the reaction .sup.28 Si(n,p).sup.28 Al. The product, .sup.28 Al, decays with a 2.3 minute half life, emitting 1.78 MeV gamma radiation. Similarly, when the aluminium constituent of the sample is irradiated with slow neutrons, the same radioactive isotope, .sup.28 Al, is produced with the consequent emission of 1.78 MeV gamma radiation. Since there is negligible interaction between the fast neutrons and the material in small samples, the chemical concentrations of silicon and aluminium can be calibrated directly against the number of 1.78 MeV gamma ray counts observed within a given time interval after irradiation first with fast neutrons and then with slow neutrons. With bulk samples of bauxite, particularly samples having a significant water content, special allowance must be made for the slowing down of the fast neutrons used for the silicon analysis and the associated production of .sup.28 Al due to the capture of slow neutrons by the aluminium during the same irradiation. One method of allowing for this effect is described below, in the description of the operation of the present invention. For the currently used neutron activation analysis techniques, because two irradiations are necessary (with fast, then slow neutrons), there is a considerable capital outlay on the two neutron sources and their respective shielding assemblies. If the analysis is applied to a moving stream of ore on a belt, two spectrometric gamma ray detectors [for example, 127.times.127 mm NaI(T1)] are also required. If the analysis is performed on bulk samples contained in bins or boxes, although only one gamma ray detector is necessary, the analysis procedures for silicon and aluminium must duplicate each other, which doubles the necessary time and effort for analysis. In addition, for the measurements to be useful to the analyst, it is essential that the fast and slow neutron flux should be constant and reproducible from one measurement to the next. The present invention offers appreciable savings in time and equipment cost, compared with current technology, by providing a method of analysis which is based on a single sample irradiation followed by a single measurement procedure. The nuclear reactions providing the basis of the present invention are: .sup.27 Al(n,p).sup.27 Mg (used for the determination of the aluminium constituent), and .sup.28 Si(n,p).sup.28 Al (for the silicon determination). The energies of neutrons effective in these reactions are greater than 4.5 MeV. The radioactive nucleus .sup.27 Mg decays with a half life of 9.46 minutes and emits two gamma rays during its decay, which have energies of 0.844 MeV and 1.055 MeV respectively. The emission and half life of the other radioactive nucleus, .sup.28 Al, have been described above. As previously mentioned, a third nuclear reaction is important with all bulk samples having significant water content as well as aluminium and silicon constituents. This reaction, .sup.27 Al(n,.gamma.).sup.28 Al, which entails the capture of slow neutrons in aluminium, results in the emission of 1.78 MeV gamma radiation which is additional to the 1.78 MeV gamma radiation resulting from the fast neutron reaction with the silicon constituent of the sample. (Note that even with sources emitting only fast neutrons for sample irradiations, appreciable numbers of fast neutrons are slowed down to thermal energies within the sample by their collision with the hydrogen nuclei associated with the water content of the sample). Applications of the above nuclear reactions for the fast neutron activation analysis of aluminium and silicon have been described in the scientific literature. For example, reference can be made to the paper by R. H. Gijbels and J. Hertogen in Pure Appl. Chem., Volume 49, p 1555, (1977), and the paper by J. Kuusi in Nucl. Appl. Technol., Volume 8, p 465 (1970). However, these applications are either for small samples, or for larger samples that contain little hydrogen and therefore cause negligible moderation of the fast neutrons within the samples. The present invention overcomes the problem of interference by aluminium with silicon from the 1.78 MeV gamma radiation in the following way. After the fast neutron irradiation, the 1.78 MeV gamma rays from the sample are measured concurrently with those emitted by .sup.27 Mg at 0.844 MeV and 1.015 MeV for a preset time interval. (If the sample container is fabricated from a material such as copper, which produces gamma radiation interfering with the 1.015 MeV gamma rays of the sample, the 1.015 MeV gamma rays are excluded from the analysis). Since the number of counts from .sup.27 Mg are due only to aluminium, the chemical concentration of aluminium can be related directly to these recorded counts, given a knowledge of the mass of the sample. With a knowledge of the aluminium content of the sample, provided the slow neutron flux in the material is also known, the component of the 1.78 MeV gamma radiation due to slow neutron reactions with the aluminium can be subtracted from the total 1.78 MeV gamma radiation count to provide the gamma radiation at 1.78 MeV resulting from fast neutron activation of the silicon in the sample. Because the thermal neutron flux within the bulk sample is sensitive to water content, it is essential to measure the number of thermal neutrons in a given time interval during the neutron irradiation. For this purpose, a suitable neutron detector will be located adjacent to the sample. The number of neutrons recorded by the detector is proportional to the thermal neutron flux within the sample. Thus, according to the present invention, a method of simultaneously analysing the aluminium and silicon content of a sample of material comprises the steps of: (a) irradiating the sample with fast neutrons; (b) monitoring the thermal neutron flux within the sample; (c) monitoring the gamma radiation from the irradiated sample at energies of 1.78 MeV and 1.015 and/or 0.844 MeV; (d) using the monitored gamma radiation at 1.015 and/or 0.844 MeV to estimate the aluminium content of the sample; and (e) using the monitored gamma radiation at 1.78 MeV, compensated by the gamma radiation at 1.78 MeV due to the thermal neutron reaction with the estimated aluminium in the sample, to estimate the silicon content of the sample. Also according to the present invention, apparatus for the simultaneous analysis of aluminium and silicon content of a sample of material comprises: (a) a fast neutron source, adapted to irradiate the sample of material; (b) a thermal neutron detector, located to monitor the thermal neutron flux in the irradiated sample; and (c) a gamma ray detector separated from the neutron source and shielded therefrom, adapted to monitor the gamma spectrum from the irradiated sample, at least at energies of 1.78 MeV and of 0.844 and/or 1.015 MeV.