CA1182590A - Apparatus for continuously measuring the element content in slurries - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An apparatus for continuously measuring the elemental contents of a slurry independent of the slurry density and slurry composition, by utilizing X-ray fluorescence analysis.
The apparatus includes a measuring chamber with a slurry flow channel and a first measuring window disposed on one side of the channel and a second measuring window disposed on the other side of the channel. A first source of primary radiation and a target are disposed behind the second measuring window. A detector is disposed behind the first measuring window. An annular source of primary radiation is provided around an open passage formed by a collimator between the detector and the first measuring window. The collimator collimates the primary radiation and the target radiation as well an the X-ray radiation excited by the first source and the annular source.

Description

BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for continuously measuring the elemental content of a slurry inde-pendently of the slurry density and slurry composition by util-izing X-ray fluorescence analysis.
Due to the rising worldwide need for raw materials, it is increasingly necessary to mine deposits having a low con-tent of the desired raw material. Such deposits are primarily exploited with the use of the f]otation method. In the flota-tion rnethod, valuable mineral is obtained from an aerated sus-pension of finely ground raw materials and water, a so-called flotation slurry, with the aid of chemicals. Use of -this method requires continuous analysis of -the product streams to insure economic operation of facilities which of-ten include widely branched-out slurry conduits. Analytical devices are required which quickly indicate the mineral con-tents of interest at strategically impor-tant points in the process -to make it possible to quickly intervene in the operation of the process. This is particular]y necessary when monitoring the waste streams and the concentrate streams leaving the production facility.
Losses of ~.~

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~ valuable minerals in the waste stream result in considerable / financial erosion in the operation of such a facility.
Further, the quality requirements imposed by the processin~ industry which uses the resultant concentrates are very high and can be met only with difficulty. The industry requirement~ not only relate to providing concentrates with a specific content of valuable mineral, but also to ~ providing the concentrates with precise proportions of so-called deleterious components. Exceeding such proportions can lead to considerable financial losses and can result in having to discard the entire product.
It is customary to conduct the control of the process streams by means of wet chemical analysis, especially in relatively small flotation plants. This analysis method cannot be effected continuously and requires considerable amount of time. Using wet analysis it is initially neces-sary to withdraw samples from the product streams and to process these samples appropriately by drying, grinding, homogenizing, etc., before analysis can begin. U~ing such wet chemical analysis methods, a time delay of several hours up to a day can be expected from the time the sample is taken to the time the result of analysis is obtained. As a result, entire daily productions, on occasion, may have to be discarded.
Time-consuming wet chemical analysis is being replac-ed in part by X-ray fluorescence. In this me~hod, disper-/ sive, conventional, multichannel X-ray spectrometers / excited by an X-ray tube are usedO Although a substantial saving in time is obtained as compared with wet ~hemical analysis, use of these devices still involves an undesirable time delay between the ~aking o the sample and the analyti-cal result caused by the necessary preparation of the sample.
Only a continuous, rapid, and highly responsive quality control promotes efficient production by a controlled intervention in the process operation. In order to reduce the time delay occurring between the taking of the sample and the analytical result, devices and processes have been developed which make it possible to carry out a direct analysis at the product stream. In this context, mention is made of the on-stream analysis system Courier 300 developed by the company Outokumpu Oy of Finland. This device is, in principle, a continuously operating sample~taking system with discontinuous analysis based on X-ray fluorescence. In this proce~s, a partial product stream is drawn via pumps and pipeline~ from various sample taking points in the 10tation plant and conducted to a battery of measuring cells. A movable measuring head with X ray tube and analytical section travels at predetermined time intervals along the variou~ cells and determines in a quasi-continuous fashion the elemental contents of the individual slurry treams.
This arrangement, however, is very expensive and it is / difficult to financially justify it for use in relatively / small flota~ion plants.
So-called immer~ion probes have also been developed.
In contras~ to the conventional X-ray fluorescence method, these devices utilize excitation by an isotope sou~ce in place of excitation by an X-ray tube. The immersion probes are hung in the slurry stream. In the case of flotation plants, for example, they are hung in the so-called flotation cells. One disadvantage in this method is the inhomogeneity of the slurry usual in flotation cells. E'urther, an additional density measuring probe is necessary in all cas s. Such immersion probes have been developed by the companies Outokumpu Oy of Finland, Philips of Australia, and NUTMAQ of England.
All of the on-line analysis devices heretofore developed on the basi~ of X-ray fluorescence initially determine the element content o~ the slurry. The determination of the element content in the solid matter, however, re-quires an additional measurement of the slurry density.
Because densimeters operate accurately only in a bicomponent system of liquid/solid matter, the measurements can be in error due, for example, to air occlusions which of~en occur in the slurry in flo~ation processes. This i5 a disadvan-tage of conventional X-ray fluorescence analysis (XRFA) 25 Rrocesses.

, , In addition to the devices operatiny on the basis of X-ray fluorescence, there are also on-line analysis devices operating according to the principle of neutron activation analy-sis. In these devices, a partial slurry stream first flows continuously through an irradiation cell with the neutron source.
The slurry is "activated" at this point and then Elows via an inductive flowmeter into a measuring cell provided with a detec-tor where the induced activity is measured. During backflow, the slurry passes a densimeter. The required use o~ the densi-meter ln the neutron activation analysis process results inthe same disadvantages noted above in the X-ray fluorescence analysis process. Furthermore, in the neutron activation analy-sis process a specific slurry throughflow must be maintained constant at all times.
:tn our United S-ta-tes Patent No. 4,388,530, we disclose an apparatus utilizing X-ray fluorescence to provide for the rapid and continuous element analysis of a process stream.
In that apparatus a measuring chamber with a slurry flow channel is provided at each side wi-th a measuring window. 'l'he detec-tor is disposed behind one measuring window and a primary radia-tion source and a target are disposed behind the other measuring window. The transmitted primary and target radiation as well as excited X-ray radiation are detected by the detector. This design of the measuring chamber, however, is not sufficient when /

/ determining, for example, ~he content of Pb in the slurry / due to the low X-ray yield of Pb in that chamber geometry.

SUMMARY OF THE I~VENTION

It is an object of the present invention to p~ovide an apparatus that can be used for analysis of the content of an element in a slurry.
A further object of the present invention is to provide such an apparatus which can be used for analysis of PB in a 5 lurry.
Another object of the present invention is to provide such an apparatus which can be used for analysis of, for example, Pb in PbS/ZnS flotation slurries having varying Pb contents from 0.1% in the total recovery stream up to 84% and more in the Pb concentrate.
Additional objects and advantages of the presen-t invention will be set forth in part in the description which follows and in part will be obvious from the description or can be learned by practice o~ the invention. The objects and advantages are achieved by mean~ of the apparatus, instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing objects and in accordance with its purpose, the presen~ inven~ion provides an improved apparatus for the continuous measurement by X-ray fluores-c~nce analysis of the elemental contents of a slurry-- 7 --independent of the slurry denslty and slurry composition. The improved apparatus includes the basic arrangement of the measur-ing chamber described in our United S-tates Patent No. 4,388,530.
Thus, -the apparatus includes a measuring chamber having a slurry flow channel. The chamber has a first measuring window disposed on one side of the channel and a second measuring window dis-posed on the o-ther side of the channel. A first primary radia-tion source and a target are disposed behind the second measur-ing window and a detector is disposed behind the first measuring window. ~he first primary radiation source excites the target to emit target radiation and also excites the slurry contents to emit X--ray radiation. The primary radiation from the first source, target radiation and excited X~ray radiation are -trans-mitted -to and detected by the detector. In accordance with the improvement of the present invention a collimator means forms an open passage between the detector and the first meas-uring window, and an annular source of primary radiation for irradiating the slurry contents to excite addi-tional X--ray radiation thereErom is located between the de-tector and the first measuring window. The collimator means collimates the primary radiation, the target radiation and the X-ray radia-tion excited by the first primary radiation source and the annular source.

.; , It is to be understood that both the foregoing general description and the following detailed description are exemplary and are not intended to be restrictive of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1 and 2 are different sectional views through a measuring chamber in accordance with the invention.
Figure 3 shows an X-ray fluorescence spectrum of a slurry containing lead, produced by employing apparatus in accor-dance with the invention.
Figure 4 is an enlarged view of a portion of the spec-trum of Figure 3.
Figure 5 is a graph of experimental results obtained by employing apparatus in accordance with the present invention and showing the variation of lead K-alpha 1 X-ray lines for varying concentrations of lead in the solid material and varying sluxry densities.
Figure 6 is a graph of a regression analysis correla-ting the known lead concentration of slurries with that experi-mentally obtained by employing apparatus in accordance with the invention.
DETAILED DESCRIPTION OF TEIE PREFERRED EMBODIMENTS
The basic concept of the measuriny apparatus disclosed in our United States Patent No. 4,388,530 has not been changed. A

_ g_ representative partial slurry stream is removed from the produc-t stream and fed to a measuring unit or system such as described in ~nited States Patent No. 4,388,530 by means of a pump. The slurry stream to be analyzed continuously flows through a stir-ring vessel where it is homogenized. A representative partial slurry stream is removed frorn the homogenized slurry contained in the stirring vessel and fed by means of a pump through a flow-meter and through a measuring chamber 1 (shown in Figures 1 and 2) which is part of and which is firmly screwed to the measuring system in the pump pressure line. The partial slurry stream is returned via an outlet chamberto'he stirring vessel or discharged into the main slurry stream.
Referring to Figures 1 and 2, measuring chamber 1 includes a slurry channel 2 which can be varied in thickness by changing spacers 3 and thus can be op-timized for -the respec-tive slurry composition. In slurry channel 2, on opposite sides thereof, the walls of measurillg chamber 1 are equipped wi-th measuring windows 4 and 5 required Eor X-ray fluorescence analy-sis processes. Wlndows 4 and 5 preferably are made of 300 ~m thick Hostaphan (terephthalate) film such as tha-t manufactured by Hoechst AG. Supporting inserts 15 support windows 4 and 5 and prevent them from possible expansion or flapping during the analysis process.
A first source of primary radioactive radiation in the form of a point source 6 and a target 12 are moun-ted by a / holder 16 behind window 4. A detector 8 is provided behind window 5 and a collimator, shown generally at 9, is disposed between detector 8 and window 5. Collimator 9 is located in a mount 17 and comprises a sleeve lQ surrounded by a shield 11. Mount 17 contains a flange which mates with shield 11. Sleeve 10 defines a passage 20 between detector 8 and window 5. An annular source 7 of radioactive radia-tion is enclosed by shield 11. Annular source 7 may either comprise a single source having the shape of an annulus or a plurality of sources disposed around passage 20. Both ~ources 6 and 7 can comprise Co57.
As can be seen from Figures 1 and 2, point source 6, target 12, windows 4 and 5, collimator 9, source 7 and detector 8 are laterally aligned with each other.
Detector 8 is permanently fixed in a protective sleeve 18 and is screwed to an adjustment device (not shown). Thus it is possible to very precisely and, importantly, reproducibly set the geometry of detector 8 with respect to measuring chamber 1.
Detector 8 is preferably an intrinsic germanium planar detector, such as that made by PGT. A 30 liter coolant container which operates with liquid nitrogen i8 provided for cooling detector 8. A safety device automatically switches off the high voltage if the cooling level is too low. There is thus no danger of destruction of detector 8 as a result of cooling system malfunction. By doubling the i container volume, the replenishing intervals for liquid nitrogen are extended to about 20 days.
, Measuring chamber 1 is permanently mounted in the measuring system by means of crew connections 13. Additionally ¦ 5 provided seating pins (not shown) assure an accurate ~eat which can be reproduced at any ~ime. Moreover, all individual components are manufactured to fit. Thus, there exists no danger of changes in geometry due to installation work.
The target material employed for Pb determination 10 is mercury oxide encased in resin. The reason for the selection of mercury as the target material is the energy position of its X-ray lines.
Shield 11 is comprised of metallic tungsten powder embedded in resin and protects detector 8 against the 15 primary radiation of annular source 7. Tungsten is selected as the shielding material because, next to lead, it has the most favorable attenuation haracteristics. For ~he purpose of attenuating the tungsten inherent X~ray lines, shield ll may be provided with a silver foil in the direction toward 20 the radiation channel or detector 8. Sleeve 10 may be made from Sn, Cd, or Ag.
I~ operation, point source 6 emits prim~ry radiation.
Part of this primary radiation excites target 12 to emit X-ray radiation. Anothex part of this primary radiation 25 causes the slurry contents in measuring chamber 1 to emit X-ray radiation. The element in the slurry ~o be analyzed, /
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which in the present case i8 Pb, is thus caused to emit radiation. Primary radiation from point source 6, target radiation, and element specific X-ray radiation ex~ited by point source 6 are transmitted through the slurry and are measured by detector 8. The yield of element (Pb~ specific X-ra~ radiation resulting from point source 6, however, is very low and not sufficient for determination of the element content. In accordance with the present invention annular source 7 is utilized in addition to point source 6 to realize a better yield of elemental X-ray radiation from the slurry contents. In fact a greater portion of the slurry elemental X-ray radiation i5 generated by annular source 7 on the side of detector 8. With this geometry, transmission as well as reflection axe utilized in one measuring device.

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Detector 8 converts the primary radiation, the target radiation and the X-ray radiation, as attenuated or reflected, respectively, by the slurry in measuring chamber 1 -to electrical pulses. After preamplification and major amplification these pulses are received by a multichannel analyzer. The analyzer feeds the integrals of the spectral regions of interest for the evaluation (see F`igures 3 and 4) via an in-terface -to a pro--gramrnable computer. Thanks to the development of a novel stan-dardization technique it is now possible to omit the previously required dead time correction. Suitable programmable computers, multichannel analyzers and like electronic apparatus and cir-cuitry are described in United States Patent No. 4,388,530.
The theoretical basis for the invention is discussed below.
With the use of the X-ray K lines, the following for-mula can be set up for the measurement of -the X-ray radiation of the element to be analyzed:
IF = I~f ~p o Cp exp(X) __(X) _ _2(Y) (1) where:
IF' = X-ray radiation of the element -to be analyzed.
Kf = geometry factor as well as the yield of characteristic X-ray radiation of the desired element;
~p = sample density (g/ccm);

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Cp= weight concentration of the desired element in the sample;
H = .distance between detector and sample;
d = thickness o the sample;
X -/up~g ~, y = X~/~p ~p~d -up / Op /Ufp p = mass absorption coefficient of the sample for the primary radiation;
~fp = mass absorption coefficient of the s~mple for 1~ the elemental X-ray radiation; and E2 = exponential integral function.
The primary target radiation intensi~ies IT0 and ITl, respectively, attenuated by the slurry, and measured by detector 8, can be expressed similarly to (1) IT~ = KT~ ' exp (-/uOp ~p ) (2) : ITl = ~Tl exP ( ~iP P

The selection of the corresponaing ~ample thickness can bring the reduction of the second member of equation (1) to zero. ~nder thi5 condition equation (1) is modified as ~ follows:
IF = KE~Cp'~p (A0 ~ Al~X) (4) The product Cp ~p in ~his equation can be expressed a~ follows or the transmission values measured for the primary and target radiation-c ~ c P P ~ (a1~L0W + a2 LlW) (5 C-R
"X" is calculated in a similar manner:
X = ~ ~ ~ L0W ~ ~ LlW (6~
The lead content in total recoveries varies in a range from about 0.1% to 0.6% with a zinc content in the range from 0.5% to 0.3% and a slurry density of about 1.175 g/ccm.
A series of slurries were examined. All combinations of slurry densities (g/ccm) of 1.10, 1.15, 1.175, 1.20, 1.225 with lead concentrations in the solid material of 0.05, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.7 and 1% were covered.
The zinc concentration was 0.2% in all samples.
The excitation of X-ray lines of lead was effected by means of 122 KeV gamma energy from the approximately 0.5 MCi strong Co57 annular source 7 (reflection geometry) and from the Co57 point source 6 of the same strength (trans-misio~ geometry~. Target 12 was mercury oxide cast in resin. As noted above, the selection of mercury is bas~d on the energy position of its X-ray lines. The K-alpha 1 line (70.82 KeV) of mercury lie~ near enough to the lead K lines to assure meeting the condition ~fP/~IP = constant-However, this mercury line can nevertheless be measured separately from the lead lines with a detector ~ having good resolution. The thickness of target 12 was set so that the intensities of primary radiation ¦112 KeV) and target o~

radiation (70.82 KeV) measured at a zero sample were appro-ximately equal. This permitted measurements of both lines with a similar statistical error.
Figure 3 shows an X-ray fluorescence analysis spectrum for a particular determination in accordance with t~e above.
Figure 4 is an enlarged view of ~he spectrum of Figure 3 from 55 to 85 KeV.

The various peaks in Figures 3 and 4 represent the regions of interest in the spectrum caused by excitation of the slurry. The net peak area for Hg-K alpha 1 and Pb-K alpha 1 are shown by the cross-hatching under their respective peaksO ~sackground or reference radiation is also shown by cross-hatching. For a calculation of the net peak areas, a linear background subtraction was made. For this purpose the backgrounds from several channels to the left and right of the respective peak were calcula~ed.
Figure 5 graphically show~ the results of an experiment in which the in~enqity of the lead K-alpha 1 line (ver~ical axis) was measured for diferent concentrations of lead in the solid material (horizon~al a~is) and for different slurry den~ities (solid lines). As can be seen from Figure 3~

5~ the intensity of ~he lead K-alpha 1 lines for any given concentrating of lead in ~he slurry depends on the density of the slurry. Thu~, Figure 5 clearly shows that the density influence mus~ be eliminated in order to perform a 5 valid quantitative analysis for determination of the Pb concentration.
Figure 6 shows the resul~s of a first regression analysis made with the aid of measured values from 28 different slurries. The measured concentrations of lead are ompared with the theoretical (i.e. known) concentrations of lead in the slurries. The interrupted lines in Figure 6 delimit a region of four standard deviations (~ or - two standard deviations from the regression line). As can be seen, the coincidence between the experimental results and the known concentrations is good, which is also confirmed by the chi s~uare value of about 45.3.
It must here be considered that malfunction due to interference was noted during the first serie~ of measure-ments, explained in paxt by the electronic ~ystem employed and in part by inhomogeneities in the slurry~ Changes made to the stirring mechanlsm noticeably improved the thorou~h mixing of the slurry.

Claims (6)

WHAT IS CLAIMED IS:

1. In an apparatus for the continuous measurement by X-ray fluorescence analysis of the elemental contents of a slurry independent of the slurry density and slurry composition comprising: a measuring chamber having a slurry flow channel, a first measuring window provided at one side of the channel and a second measuring window provided at the other side of the channel, a detector disposed behind the first measuring window, a first source of primary radiation and a target disposed behind the second measuring window, the target being disposed between the second measuring window and the first source wherein the first source excites the target to emit target radiation and excites the slurry contents to emit X-ray radiation, and the primary radiation from the first source, the target radiation, and the excited X-ray radiation are transmitted to and detected by the detector, the improvement comprising:
collimator means forming an open passage between said detector and said first measuring window, and an annular source of primary radiation for irradiating the slurry contents to excite additional X-ray radiation therefrom located between said detector and said first measuring window and about said open passage, wherein said collimator means collimates the primary radiation, target radiation and X-ray radiation excited by said first source and said annular source of primary radiation.

2. An apparatus as defined in claim 1, wherein said annular source comprises a plurality of individual sources disposed about said open passage.

3. An apparatus as defined in claim 1, wherein said annular source comprises a single source having the shape of an annulus.

4. An apparatus as defined in claim 1, 2 or 3 wherein said collimator means comprises a resin mass which is embedded with tungsten and a sleeve which is enclosed by the resin mass, and wherein said annular source is held in said resin mass.

5. An apparatus as defined in claim 1, 2 or 3 wherein said collimator means comprises a resin mass which is embedded with tungsten and a sleeve which is enclosed by the resin mass, wherein said annular source is held in said resin mass, and wherein said sleeve comprises a material selected from the group consisting of Sn, Cd or Ag.

6. An apparatus as defined in claim 2, wherein said target is made of a resin mass with HgO bound therein.