Monitoring pyrogenic processes

Abstract: Pyrogenic processes in which gas or fume is evolved are monitored by impinging a beam of X-rays (3) on the gas or fume in an exit-duct. The radiation produced by the X-ray fluorescence is detected and analysed for the contribution of various elements. In this way the progress of the metallurgical process can be monitored and the information used for process control.

Description: This invention relates to monitoring pyrogenic processes, and in particular to a method and to apparatus for monitoring pyrogenic processes such as combustion or pyrometallurgical processes, the latter being processes wherein metal compounds are processed at relatively high temperatures, so as to recover metal values from the raw materials, while producing process off-gases, and includes distillation of metals and metal compounds. Although process modelling is established for a limited number of pyrometallurgical processes, e.g. the blast furnace for iron making, such models rely heavily on constant feed rates and composition, idealisation of conditions and a knowledge of fundamental kinetic data. Without the latter, models are worthless in their ability to predict the state of reaction. If no process model exists, reliance must be placed on either the experience of the process operators or the monitoring or sampling of the process or process streams. Accurate sampling of the phases in the furnace itself is often difficult and sometimes impossible. However, where a pyrometallurgical process generates gases, vapour, fume or dust, its composition and the way in which this changes over time can be used as an indication as to the extent or state of reaction within the furnace. Manual sampling of the fume and subsequent chemical analysis is sufficiently time consuming and labour intensive as to be an impractical on-stream method for process monitoring. DE-A-2226393 discloses apparatus for analysing a molten metal bath by X-ray fluorescence. The apparatus comprises a housing, which is to be immersed in the bath, and an X-ray detector window mounted between two X-ray sources in one side of the housing. Both the detector window and the sources are of high-temperature-resistant material. X-rays impinging on the detector window are counted by a counter mounted in the housing, a gas being continuously passed through the counter. P.F. Hahn-Weinheimer et al., "Grundlagen und praktische Anwendung der Roentgenfluorezenzanalyse (RFA)", Braunschweig, 1984, pages 162-164, discusses the analysis of blast furnace slag and copper slag by X-ray fluorescence. GB-A-2049 170 discloses X-ray fluorescence analysis of a slurry which passes through an enclosed duct between an X-ray source and a detector which receives both X-rays from the source and X-rays emitted by the slurry. D.A. Close, "X-ray Fluorescence Determination of Uranium in the Gaseous Phase", in Nuclear Instruments and Methods in Physics Research, Vol. A234, Amsterdam, 1985, pages 556-561, discloses apparatus for measuring the amount of uranium in gaseous UF₆ inside a cylinder using X-ray fluorescence. This requires a highly collimated source, a highly collimated detector, and a very rigid geometry. The radiation to be detected has to pass through the cylinder wall. The problem to be solved is that of applying the known technique of X-ray fluorescence analysis to the hostile environment of a gas-stream leaving a pyrogenic reactor, with sufficient sensitivity and adaptability. The present invention provides apparatus for monitoring a pyrogenic process by detecting X-ray fluorescence in a gas-stream leaving a pyrogenic reactor, the gas-stream passing through a duct having a wall (4), the apparatus comprising: a housing (5) having a front wall adapted to be mounted on the duct wall (4); a primary X-ray source (1) accommodated in the housing (5), the source (1) directing an X-ray beam (3) through an aperture in the housing (5) and an opening in the duct wall (4), the source (1) being mounted in a portion (2) which is swivellable so as to direct the beam (3) at a predetermined angle; an X-ray detector (7) accommodated in the housing (5), radiation produced by X-ray fluorescence in the gas-stream being received by the detector (7) through said opening in the duct wall (4) and an aperture in the housing (5) over a restricted field of view which intersects the X-ray beam (3), the region in which the said field of view and the X-ray beam (3) coincide being spaced from the duct wall (4) so that the detector (7) only detects radiation from the gas-stream in the said region and does not receive radiation from any deposits on the duct wall (4); water cooling means and air purging means (6) for protecting the source (1) and the detector (7) from heat, fume, and dust carried by the gas-stream in the duct; and analysing means for analysing the radiation received by the detector (7) so as to allow the contribution made by individual elements to be determined. The preferred pyrogenic process is a pyrometallurgical one. The pyrometallurgical process is suitably the fuming of volatile metals or their compounds from slags or other molten metallurgical phases. Alternatively, the process may be suitably a primary or secondary smelting or refining process, or may be a roasting or other solid state oxidation or reduction process. The pyrogenic process may alternatively be a combustion process. In accordance with the preferred embodiment described below, a primary X-ray source set into a specially designed mounting produces a beam of X-rays that are directed into a duct which may form part of the furnace itself or may be a flue carrying the off-gases from a kiln, furnace or other type of reactor used in pyrometallurgical processing. An X-ray detector offset from the X-ray source in the same mounting detects any fluorescence produced by excitation of the elements present in the gases, vapour, fume or dust. By careful arrangement of the angle of inclination of the X-ray tube and the detector, and by providing collimation on both the X-ray tube and the detector, it is possible to ensure that the detector does not pick up any excitation from either deposited fume or condensate on the walls of the duct or the materials constituting the walls. The detector only senses the fluorescent excitation produced by the gases, vapour, fume and dust components in the off-gases leaving the high temperature reactor. As the relative sensitivity of the detector to a given element is a function of that element's characteristic energy, it is necessary to ensure that the X-ray source has sufficient energy to produce the desired degree of excitation. A voltage source of 50 kV would be capable of producing a detectable response from elements in the atomic table, typically from sulphur upwards. The output from the detector is amplified and the signal may be fed to a remote multi-channel analyser from which the required X-ray intensities can be computed and displayed. The multi-channel analyser suitably comprises a large number of channels each covering a small voltage range. An incoming X-ray energy related voltage pulse from the detector is input as a count into the appropriate channel relative to that voltage level. The process is repeated for all incoming pulses to build up a spectrum of counts in each of the levels. By calibration with known X-ray sources, the resulting spectrum can be calibrated into energies and thus the peaks present in the spectrum can be identified. Typically it might be necessary to record the detector output for a period of 50-100 seconds to obtain sufficient counts to build up a spectrum. Consequently, although the detector is operating on-line, only one spectrum and therefore only one measurement of concentration is obtained every counting period. Alternatively it is possible to set up one or more single channel analysers to which the detector signals are fed. The single channel analyser is essentially an adjustable voltage window detector which will only pass voltage pulses between defined levels. The levels are adjusted to specific particular X-ray peaks. The output from the single channel analyser thus relates to a specified element and this can be passed to a rate meter to provide an analogue indication in counts per second for any element of interest. It is then a continuous analytical system for any particular element or elements present in the gases, vapour, fume or dust. The process is applicable to batch or continuous pyrometallurgical processes. Examples of batch processes to which the invention is applicable include fuming of tin and zinc from slags and other metal-bearing materials, cupellation processes and refining of primary and secondary materials e.g. copper, lead, tin. Examples of continuous processes to which the invention is applicable include shaft-furnace smelting, fluidised-bed roasting and high intensity smelting processes, e.g. plasma arc furnaces. Advantages of the use of apparatus according to the invention are as follows: The apparatus can monitor the composition of the gases, vapour, fume or dust for all elements typically from sulphur upwards in the atomic table, qualitatively and quantitatively. The apparatus can be used to determine reaction end-points in batch processing by measuring the changing composition of a component or components present in the gases, vapour, fume or dust. Thus it can be used to prevent excessive processing times and unnecessary losses of metal values. The apparatus can also be used at elevated temperatures where existing physical sampling methods are impractical. This is seen as a major advantage of the system over other methods as it allows the gases, vapour, fume or dust to be analysed before it is affected by down-stream processing e.g. quenching, dilution, physical dropout, mixing. The apparatus can also detect low levels of a component present in a gas, vapour, fume or dust containing a high concentration of other elements. It can thus detect and record a number of elements over a wide range of concentrations. This can be important when checking for losses of valuable metal components. The apparatus is sensitive enough for it to be used for monitoring stack emissions for environmental purposes. For example gases from the combustion of fossil fuels may be monitored. The apparatus is not restricted to use in batch processing. Where the process is of a continuous nature, the analyser can detect departures from steady-state conditions which produce changes in the gas, vapour, fume or dust composition, e.g. in fluid bed roasting of sulphidic ores it can be used to monitor the total sulphur as sulphide, sulphate and sulphur dioxide in the off-gases, in fuming processes it can be used to control selectively fuming rates, and in the combustion of fossil fuels it can be used to monitor the sulphur dioxide content in the off gases. The invention will be further described with reference to the following illustrative examples which relate to various pyrometallurgical processes using apparatus according to the invention. A suitable apparatus is illustrated in the accompanying drawing, which is a partly sectional side view of such apparatus. The apparatus shown in the drawing comprises a source of X-rays, generally designated 1, mounted in a swivel mounting block 2 which is fixed externally to a furnace duct or flue and is adjustable so as to direct a beam of X-rays 3 into the furnace duct or flue bounded by a wall 4 at a pre-determined angle through an aperture in a housing 5 and an opening in the wall 4. The water-cooled housing 5 and an air purge system 6 serve to protect the X-ray source from the heat, fume and dust of the gases carried by the duct. An X-ray detector 7 is mounted in a main block 8 in a position off-set from X-ray source 1 so as to receive (through the opening in the wall 4 and an aperture in the housing 5) radiation produced by fluorescence of the elements irradiated by the X-rays passing into the duct from the source 1. The region of coincidence of the X-ray beam 3 and the field of view of the detector 7 is shown hatched in the drawing. The output from the detector 7 is passed to a multi-channel analyser, a rate meter or other counting device. EXAMPLE 1 - FUMING OF TIN FROM SLAG 100 kg of a tin slag containing approximately 6% Sn was charged to a pilot-scale top blown rotary furnace and melted down. Iron pyrites was added to fume the tin as sulphide. The tin sulphide fume was oxidised at the mouth of the furnace, by dilution air, and the resultant tin oxide collected in a bag filter plant. The X-ray fluorescence (XRF) apparatus as shown in the drawing was mounted on the duct work between the furnace mouth and the bag filter plant. The response of the XRF system to the presence of the tin in fume was monitored throughout the fuming cycle, and samples of the tin slag were taken from the furnace at regular intervals as well as iso-kinetic fume samples. The samples of tin slag were analysed and the XRF response to tin in the fume was correlated with the residual tin level in the slag and tin in the fume samples. From this, the fuming rate at the commercial reaction end-point of 0.3% Sn was, for this furnace arrangement, determined to be equivalent to 15 counts per second of tin on the read-out from the XRF apparatus. On three subsequent tin fuming cycles, the process was terminated when the XRF response fell to this predetermined level of 15 counts per second. The assays of final slags were found to be 0.20%, 0.19% and 0.20% Sn respectively. Thus the XRF apparatus had accurately predicted on-line the commercial reaction end-point. EXAMPLE 2 - REFINING OF BLACK COPPER Copper bearing scrap is recycled by smelting to produce a copper-based alloy containing iron, zinc, nickel, tin and lead. This impure copper, known as black copper, is refined in converters using air or oxygen-enriched air in two stages. In the first, iron and zinc are removed as a slag phase. Zinc fuming also occurs. In the second blow any residual zinc plus tin and lead are removed as a second slag or by fuming. The XRF apparatus was mounted on the ductwork between a copper converter and its associated bag filter gas cleaning plant. The XRF system was then used over a period of weeks to determine the variation in the fuming levels of zinc, tin and lead. The variations seen indicated that it was possible to use the XRF response to contol the process and effect a separation of the zinc and tin fume produced. As a result, tin losses to the first slag can be reduced and the tin content of the second fume increased. EXAMPLE 3 - FUMING OF ZINC FROM SLAGS Zinc-bearing slags from the smelting of primary copper and lead concentrates are fumed at temperatures of 1100°-1300°C. Reductant is required to produce zinc-bearing vapour and this is usually blown into the molten slag with air through tuyeres. Secondary air is admitted above the bath to oxidise the zinc-bearing vapour which is subsequently collected as an oxide fume. During blowing, other elements also volatilise. The XRF apparatus was mounted on the wall of a zinc fumer above the point where secondary air was admitted. The temperature of the gas phase being monitored was in excess of 1200°C. At this temperature the equipment was capable of showing continuously on-line variations in fuming rates of zinc, lead and arsenic. The performance of the XRF apparatus long term was unaffected by the very high levels of radiation from the gas phase. EXAMPLE 4 - MONITORING OF SULPHUR IN PROCESS GAS STREAMS During the converting of metal mattes, sulphur dioxide is produced by blowing with air or oxygen-enriched air. A knowledge of the sulphur dioxide content of the converter offgas or measure of the total sulphur removed can be used by furnace operators to determine when to terminate the blowing period. Conventional techniques for monitoring the sulphur dioxide content of gases, such as infra-red analysis, all involve sampling of the hot, often dirty, gases containing condensibles. These sample streams must then be thoroughly cooled and cleaned to avoid damage to the analytical equipment. The XRF apparatus was mounted on the wall of a perspex vessel which could be filled with gas mixtures containing various levels of sulphur dioxide. The XRF apparatus was found to be capable of detecting sulphur dioxide at concentrations of 100 ppm by volume under these ideal conditions. The XRF apparatus was then transferred to a duct containing a hot, dirty gas stream which was known to contain sulphur as sulphur dioxide, metallic sulphides and sulphates. The temperature of the duct was 500°-600°C with fume loadings in excess of 100 g m⁻³ at total sulphur levels of up to 5%. At these temperatures the fume was sticky, due to incipient melting, and thus conventional sampling was not possible. In this environment a positive response to the presence of sulphur was recorded which could be distinguished readily from other elements present in high concentrations. In a similar way, the sulphur content of gases from the combustion of fossil fuels can be monitored.

Claims: Apparatus for monitoring a pyrogenic process by detecting X-ray fluorescence in a gas-stream leaving a pyrogenic reactor, the gas-stream passing through a duct having a wall (4), the apparatus comprising: a housing (5) having a front wall adapted to be mounted on the duct wall (4); a primary X-ray source (1) accommodated in the housing (5), the source (1) directing an X-ray beam (3) through an aperture in the housing (5) and an opening in the duct wall (4), the source (1) being mounted in a portion (2) which is swivellable so as to direct the beam (3) at a predetermined angle; an X-ray detector (7) accommodated in the housing (5), radiation produced by X-ray fluorescence in the gas-stream being received by the detector (7) through said opening in the duct wall (4) and an aperture in the housing (5) over a restricted field of view which intersects the X-ray beam (3), the region in which the said field of view and the X-ray beam (3) coincide being spaced from the duct wall (4) so that the detector (7) only detects radiation from the gas-stream in the said region and does not receive radiation from any deposits on the duct wall (4); water cooling means and air purging means (6) for protecting the source (1) and the detector (7) from heat, fume, and dust carried by the gas-stream in the duct; and analysing means for analysing the radiation received by the detector (7) so as to allow the contribution made by individual elements to be determined. Apparatus as claimed in claim 1, in which the analysing means comprises a multi-channel analyser for detection of a range of elements. Apparatus as claimed in claim 1, in which the analysing means comprises a single-channel analyser for measurement of a particular element, or a multiplicity of single channel analysers for measurement of a range of elements. Apparatus as claimed in any preceding claim, in which the axis of the detector (7) is perpendicular to the front wall of the housing (5). A method of monitoring a pyrogenic process, using an apparatus according to any preceding claim, the method comprising directing an X-ray beam (3) from the source (1) through an aperture in the housing (5) and an opening in the duct wall (4) into a gas-stream leaving a pyrogenic reactor through the duct and receiving radiation produced by X-ray fluorescence in the gas-stream, through said opening in the duct wall (4) and an aperture in the housing (5), by means of the detector (7); operating the cooling means and the purging means (6); and passing the output of the detector (7) to the analyser. A method as claimed in claim 5, in which the pyrogenic process is a pyrometallurgical process, such as the fuming of volatile metals or their compounds from slags or other molten metallurgical phases, or a primary or secondary smelting or refining process, or roasting or another solid state oxidation or reduction process. A method as claimed in claim 5, in which the pyrogenic process is a combustion process.

Inventors: FLETT DOUGLAS, HOLT GARY, TILY PETER JOHN, FLETT,DOUGLAS, HOLT,GARY, TILY,PETER JOHN
Applicants: BRITISH STEEL PLC
Application year: 1988
Main classification: G01N23
IPC codes: G01N23, F27D21, F23M11