US 3237765 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
Filed May 28, 1964 arch 1, 1966 A. M. GAUDIN ET'AL.
COPPER ORE CONCENTRATION BY INDUCED RADIOACTIVITY Concenl'rale 2 Sheets-Sheet 1 Photoelectric Timer Relay High Vollage Supply Calculator Overall Activity Sealer Coincidence Oounler INVENTORS ANTOINE M. GAUDIN HARALD F. RAMDOHR United States Patent 3,237,765 COPPER ORE CONCENTRATION BY INDUCED RADIOACTIVITY Antoine M. Gaudin, Cambridge, Mass., and Harald F.
Ramdohr, Leopoldshafen, Germany, assignors to Copper Range Company, New York, N.Y., a corporation of Michigan Filed May 28, 1964, Ser. No. 370,978 7 Claims. (Cl. 209-1115) This application is a continuationin-part of our application Serial No. 234,741, filed November 1, 1962, now abandoned.
This invention relates to the sorting of pieces of copperbearing ore having varying elemental values and more particularly it relates to the method and means for separating from ore those pieces having at least a predetermined amount of copper by inducing radioactivity in the ore, and utilizing this radioactivity to effect the separating.
In recent years copper ore has become leaner in grade owing partly to the depletion of relatively rich deposits and partly to the wider use of mass mining methods. As a result barren material is taken along with the richer materials in the mining operations. Accordingly, any method of sorting ore particles to separate barren material from the richer materials to provide a rich grade feed to the mill is desirable.
Several methods have been proposed to utilize radioactive properties in treating other types of ore to effect the desired separation. These methods can be classified into two groups: one utilizes the natural radioactivity of the ore; and the other uses artificial radioactivity. In the former case, where natural radioactive ores such as uranium ores are involved, the process of sorting the coarse ore pieces generally consists of measuring the gamma radioactivity given off by each piece in a given length of time and then determining the weight of this piece to obtain the ratio of radiation to the weight of the piece. Means have been proposed to reject the piece when the ratio indicates the concentration of radio active material is lower than a predetermined value. The artificial radioactivity is usually induced into the ore pieces by absorption of suitable radionuclide from solution or by irradiation with neutrons or gamma rays to create radioactivity in situ. The difliculty in both cases is twofold: (1) how to tag the desired constituent in an ore with radioactivity without tagging all the materials; and (2) how to get rid of the radioactivity in the concentration after it has served its purpose.
It is the primary object of this invention to use artificial radioactivity as a basis effectively to sort copper ore pieces according to the concentration of the desired constituent, and to eliminate the radiation hazards from the final product. This elimination can be achieved simply by waiting as shown by the half-life of Cu 64 in Table I. For example, at the end of five days the activity of Cu 64 is reduced to one-thousandths of the initial activity.
Broadly stated, the invention provides a new method of and means for sorting copper-bearing ore pieces capable of emitting annihilation radioactivity when activated by neutrons. Annihilation radioactivity is a form of radiation generated when a positron combines with an electron to form two gamma rays having equal energy, for example of 0.511 mev., flowing off at the same instant with the velocity of light and in exactly opposite directions. In the practice of this invention we utilize this special radioactive property by first irradiating the copperbearing ore pieces to induce artificial radiation which results in annihilation radioactivity and then detecting the gamma radiations emitted from the newly generated cop per radioisotope, using a coincident count technique to measure the gamma rays due to copper, which are oo- 3,237,765 Patented Mar. 1, 1966 existent within a finite time interval. This measurement represents the annihilation radiation in the pieces which in turn is a function of the amount of positron emitter in the ore pieces. By relating this measurement to a predetermined mineral concentration of a desired constituent, the sorting of these ore pieces can be effectively conducted by directing the pieces of ore having a ratio above this predetermined value to one point and the pieces having a ratio below this predetermined value to another point.
An important feature of this process resides in the waiting time following the neutron irradiation of the pieces of copper-bearing ore. The irradiation forms short-lived isotopes of other elements in the ore and it is only after a waiting time of several minutes, say 10 to 30 minutes, sufficient to permit the short-lived isotopes to decay, that the radiation detections, measurements and determinations are made to effect sorting of the desired copper-bearing pieces of ore from other pieces.
A preferred embodiment of the invention is described hereinbelow with reference to the drawings wherein FIG. 1 is a schematic diagram of a sorting apparatus constructed according to the teachings of this invention;
FIG. 2 is a plan view of the detector shown in FIG. 1; and
FIG. 3 is a diagram showing a correlation between the trajectory of the pieces and the electric processing, with the sorting device of FIG. 1 indicating the ratio of time to distance in the apparatus.
The ore to be used in this example is a shale containing about 53% SiO 16% A1 0 7% FeO, 4% MgO, 2.5% K 0, 2% Na O as major constituents. The copper content varies from a few hundredths of a percent to several percent. A calculation carried out from a more detailed chemical analysis on an ore of 1% copper shows the amount of radioactivity of each element formed in one kilogram of ore after an irradiation time of 0.1 sec. at a flux of 5 10 neutrons cmF/sec. The results of this calculation are shown in Table I.
This table lists the amount of radioactivity in microcuries at the end of the irradiation for each radio element formed, the half life of the same and the gamma radiations emitted, expressed as a percentage of the corresponding amount of activity. Thus, for Si 31 the amount of activity formed is 1.92 ,uC., of which 0.07% only gives a gamma radiation of 1.62 m.e.v.
Table I [Calculated radioactivity of one kilogram of a 1% Cu ore sample exposed to 5 l0 neutrons/cmF/sec. for 0.1 second] Activity Isotope Obtained, Hall-life Gamma Radiation 1. 91 2. h 0.07% 1.62 rnev. 283. 2 2.30 1.78 mev.
0. 043 45 (1 53.9% 1.1 mev.45.8% 1.29 mev. 0. 092 2.9 y None. 0. 33 9.7 min 58.2% 0.84. mev.41.4% 1.02
mev. 0. 39 15.0 100% 2.75 1nev.100% 1.37
mev 0.0006 d None. 0. 027 8.7 in- 10% 4.05 mew-90% 3.1 mev. 0. 18 5.8 111' 94.4% 0.32 mev.others. 5. 3 12.5 h 18% 1.55 mev. 1. 4 2.58 h 50% 0.85 mew-others. 6. 4 12.8 h 38% 0.511 rnev.; 1% 1.34 mev. 35.0 5.1 m' 9.2% 1.04 may.
Several radionuclides are formed, but it can be seen from Table I that not all of these isotopes emit gammarays and that others are relatively short-lived. Thus, only Fe 59, Na 24, Mn 56 and K 42 will interfere with the activated copper. The rapid decay of the shortlived isotopes makes it possible to operate the process on the radiation from the copper which has an advantageous radiation life.
Two pieces with a slightly different but low copper content will however not be markedly different in overall activity from each other. Thus, an experiment Was made comparing the activity of high grade ore pieces (6% Cu) against the activity of low grade ore pieces (0.1% Cu). It was found that the overall count rate on the average of ten pieces differed only by a factor of two, while the copper ratio was about 60.
The coincidence counting method uses the special radiation properties of Cu 64. This isotope with a half-life of 12.8 hours decays by several simultaneous ways. One of the decays (39%) gives B radiation. With this we are not concerned further. 19% gives off a [3+ radiation which on contacting electrons in the surrounding matter becomes twice 19% or 38% gamma radiation of 0.511 m.e.v.:
One percent of the radiation is involved in an internal electron activation stage which then decays with a gamma of 1.34 mev. and the remaining 41% decays with no gamma to Ni 64.
The procedure used depends on the two gammas of 0.511 mev. formed as explained above. The two gammarays are in coincidence, and they travel in opposite directions.
The detector, shown in FIG. 2, consists of two halves of a cylinder of plastic scintillator material with a hole 11 drilled along the axis.
A plastic scintillator was selected for several reasons:
(1) Detectors can be cast and machined into any shape desired up to the size of a barrel.
(2) Plastic materials, in this case polyvinyl-toluene dispersed in another plastic body, are relatively inexpensive.
(3) The emission time of the light quanta created by gamma impacts is very short, of the order of 10 sec.
Over a coincidence unit the two halves are connected to a sealer. This sealer receives pulses when tWo gamma rays hit each detector in coincidence in a time interval of about 10 see. This time interval is determined by the resolving time of the electronic parts of the system. coincidences can be counted, when; one of three events happen: (1) true gamma coincidences when a pair of Cu 64 gammas gets into the scintillator; (2) a random coincidence from background occurs; and (3) a cascade near-coincidence hits both detectors.
Random coincidences occur when two unrelated gamma rays are reaching both detectors from a source in a time interval of less than 10* sec. Their numberis related to the resolving time of the device and to the activity of the background source by N Number of counts in second half of detector =Resolving time of coincidence counter It is not intended that cascade near-coincidences be counted as true gamma coincidences. Yet, cascade neareoincidences can be counted, when radionuclides are present which in decaying emit two gamma-rays per atom substantially instantaneously but one after the other, and unoriented in space. Co 60 is an example of an isotope decaying according to a cascade pattern. The number of coincidences recorded in the sealer depends on the percentage with which these cascades occur in the decay scheme of a given nuclide, of their energy, and largely upon the solid angle which the detector forms around the source. The counting system used for coarse ore such as lumps a few inches in diameter tends to maximize output of true Cu 64 coincidences and at the same time minimize counting of random and cascade coincidences. The number of random coincidences is small, if a fast circuit is used. To test the importance of random coincidences, a special experiment was made. This involved the use of a cesium 137 source, which produced a background of 7 10 c.p.m. This gave 700 coincidences per minute, or approximately 0.1 percent random coincidences.
The apparatus shown in FIGS. 1 and 2 was built on the basis of free fall of the ore pieces through it. The activation of the ore is also carried out to deal with falling ore pieces. As the speed of free fall increases from the point of release with v=gt (v velocity, g=gravitational constant, and t=time), pieces have to be evaluated as close to the releasing point as possible in order to get a maximum counting time, t out of a given length of detector. By the same token a maximum length of detector is also desirable. The third condition is maximum efficiency of counter to keep activation level and cost down. This can be done by enclosing the path of free fall within a doughnut or tubelike detector.
In our apparatus, the length of the scintillator is 13 inches. This detector stands upright on a table of laboratory table height. The actual sorting gauge is 15 inches from the ground to provide space for two bins 119 and 20 underneath. The over-all travelling length of an ore piece through the unit from a releasing point 2 inches above the detector (to provide space for a suitable re leasing mechanism) to the exit into one of the bins is 31.5 inches. The electronic part of the sorting device was arranged according to the schematic sketch in FIG. 1. The ore pieces are first irradiated in a conventional neutron irradiator 13. The pieces of copper ore 14 may vary in size, say, from one-half inch in diameter to about six inches in diameter. In a finite interval of time sufli= cient to permit the decay of the unwanted short-lived isotopes the activated pieces of ore, one-at-adime, then enter by the feeder 15, and cut the light beam from light source 16 to a photoelectric cell 16a. This cell starts the coincidence counter, which counts the radiation of each falling piece separately.
The positron emitter (Cu 64) in the falling piece gen= erates the annihilation radiation in the form of two gamma rays 21 and 21a which travel in radially opposite directions as shown in FIG. 2, and are trapped by hemicylindrical scintillators. The photomultiplier tubes 12 and 12a connected to the scintillators separately measure these two gamma rays and relay the signal to the coincidence counter.
As mentioned before, the over-all activity of the ore is proportional to the weight of the material with only very small differences for widely difiierent copper contents. Obtaining an over-all count from an ore piece thus is equivalent to weighing the piece. As illustrated in the drawing another pair of photomultiplier tubes 17 and 17a connected to an over-all activity sealer are used to measure the over-all radiation; however, it is possible to obtain the over-all count by suitable wiring of the existing tubes. An electronic division of the number of Cu 64-eoincidences by the over-all activity count is obtained from the calculator. This ratio is a function of the amount of copper present divided by the weight of the particle. By comparing this ratio with a predetermined value, which is based upon calculations from data on the activation, the copper content desired, and the counter efficiency, the calculator having reached this predetermined value will emit a pulse which operates a flipping gate 18. By this time the piece of ore has left the scintillators and is dropping through the open gate into the concentrate bin 19. After a pre-set time, a few milliseconds longer than it takes the first piece to pass the gate, the whole system is reset by a timer, and is ready to take the next piece. A suitable timer is manufactured by Industrial Timer Corp., Newark, New Jersey, the type being TDAF 115 v., 60 cycles, 1000 w. The time correlation between dropping of the piece and electric operation is given in FIG. 3.
If a second piece, starting the counter and falling through the system, should not reach the pre-set count because of its low copper content, no pulse goes to the gate and the piece drops right through into the tailing bin 20. The unit is then again automatically reset by the timer.
Various modifications of this apparatus can be made according to the teachings of this invention. For pieces having substantially the same weight, the over-all activity counter and the calculator can be eliminated. The coincidence counter counts the radiation of the falling particle until a pre-set number of coincidence is reached. This pre-set number is based upon calculations from data on the activation, the copper content desired and counter efficiency. Having reached the pre-set number a pulse leaves the counter, which operates the flipping gate 18, to eifect the separation.
Since the ratio obtained from the calculator is a function of the copper assay, by removing the tube 22 the apparatus is converted for assaying the ore concentrations.
Also instead of using the free-fall principle, conveyor belts, for example, can be used to carry the ore pieces through various Zones. The flipping gate can be replaced by controlling a blast of air to separate the ore pieces.
Table II lists nuclides which can be formed by thermal neutron irradiation, and which give off 5+ particles. These isotopes emit positrons which result from thermal neutron irradiation.
Table 11 Natural abundance of parent isotope Cross-section, as a percentage barns ot the total amount of that element Isotope Half-life (no: oHoozocuk Table II lists also the half-lites and activation cross-sections for the seven positronemitters. Coincidence counting can be used for detection and analysis of elements containing these isotopes.
1. A method of sorting copper ore pieces according to a predetermined mineral concentration which comprises irradiating said ore pieces with neutrons resulting in Cu 64 and other isotopes which are short-lived and decay at a much faster rate than Cu 64, separately detecting gamma rays from said pieces in two semi-cylindrical volumes of space surrounding each one piece, measuring said gamma rays which are coexistent within 1O seconds of each other, determining the over-all gamma radiation from said pieces, correlating the measurements of said coincident gamma rays to the determination of overall gamma radiations to obtain the ratio thereof, relating said ratio to said predetermined mineral concentration, and directing said pieces to one point when said ratio of said pieces is above said predetermined concentration and to another point when said ratio is below said concentration.
2. A method of sorting copper-bearing ore pieces according to a predetermined copper concentration, said pieces containing varying concentrations of copper, which comprises irradiating said ore with neutrons to produce ore pieces containing Cu 64 and other isotopes which are short-lived, waiting for a time sufiicient to permit selective decay of short-lived isotopes, separately detecting gamma rays travelling diametrically opposite direction from said Cu 64 .piece, measuring said gamma rays which are coexistent within a predetermined time interval of each other, individually determining the total radiation of said ore, correlating the measurement of said gamma rays to the determination of over-all radiations to obtain the ratio thereof, relating said ratio to said predetermined copper concentration, and directing said pieces to one point when said ratio is above said predetermined concentration and to another point when said ratio is below said concentration.
3. A method of sorting copper-bearing ore pieces according to a predetermined copper concentration, said pieces containing varying concentrations of copper and other metals, which comprises irradiating said ore with neutrons to produce ore pieces containing Cu 64 and isotopes of said other metals, waiting for a time sufiicient to permit selective decay of unwanted isotopes, detecting the gamma radiation from said piece of ore, separately detecting gamma rays travelling in diametrically opposite direction from said Cu 64 piece, measuring sai-d gamma rays which are coexistent within a predetermined time interval of each other, individually determining the total radiation of said ore, dividing the measurement of said gamma rays in coincidence by the total radiations to obtain the ratio thereof, relating said ratio to said predetermined copper concentration, and directing each piece of ore according to the value of said ratio in a direction to effect separation of pieces of ore having a predetermined copper concentration from other pieces.
4. A method of sorting copper-bearing ore pieces which comprises irradiating said ore pieces with neutrons forming Cu 64 and short-lived isotopes, waiting for a time suflicient to permit selective decay of unwanted shortlived isotopes, detecting the total gamma radiation from each piece of ore, also separately detecting gamma rays travelling in diametrically opposite directions from pieces of ore containing Cu 64 and accepting only gamma rays that are in coincidence, relating the abundance of gamma rays in coincidence to the total gamma radiation of each piece to obtain the ratio thereof, and directing each piece of ore according to the value of this ratio, in a direction to effect its separation from other pieces having copper below the predetermined level.
5. An apparatus for sorting neutron activated pieces capable of emitting annihilation radiation which comprises means to feed said pieces individually past the center portion of a substantially cylindrical gamma raydetecting means consisting of two hemicylindrical scintillators spaced closely apart and connected separately to two photomultiplier tubes, coincidence means connected to said two photomultiplier tubes and providing an output proportional to the number of pulses respectively substantially simultaneously occurring in each of said photomultiplier tubes, two additional photomultiplier tubes connected to said two hemicylindrical scintillators, means measuring total outputs from said two additional photomultiplier tubes, correlating means providing outputs proportional to the ratio of outputs from said coincidence means and said two additional photomultiplier tubes and flow control means responsive to outputs of said correlating means whereby pieces are separated according to the concentration.
6. An apparatus for assaying neutron activated ore pieces capable of emitting annihilation radiation which comprises means to move said pieces individually through a radioactive detecting zone, consisting of first detecting means capable of measuring the coincidence of gamma radiation and second detecting means capable of measuring total gamma radiation, correlating means receiving outputs from said first detecting means and said second detecting means and providing outputs proportional to the ratio of outputs of first and second detecting means.
7. An apparatus for assaying a particular material capable of emitting positrons when activated by neutrons References Cited by the Examiner UNITED STATES PATENTS Pritchett 250-715 X Pritchett 20911l.5 Scherbatskoy 250-71.5 X Parker 25083 X M. HENSON WOOD, ]R., Primary Examiner.
ROBERT B. REEVES, Examiner.
R. S. SCHACHER, Assistant Examiner.