RU2193185C2 - Method of detection of diamonds on conveyer in flow or in specimen of diamond-bearing rock - Google Patents

Abstract

FIELD: technological processes at mining and concentrating plants, mining diamonds and conducting geological exploration work. SUBSTANCE: method is based on technology of layer-by-layer X-radiation computer tomography of diamond-bearing rock flow. For automatic detection and separation of diamonds from flow, use is made of double set of tomographic data pertaining to each layer being analyzed. Sets of data differ from each other in energy or spectrum of X-radiation energy used for their generation. Effective mass number and mass density are determined for each small volume of layer being analyzed. Subsequent effective detection and separation of diamonds is ensured due to unique feature of this pair of parameters for diamond crystals average mass density of diamond is equal to 3.52 g/cu.cm at effective atom number equal to 6. EFFECT: enhanced efficiency; increased yield of intact diamonds.

Description

 The invention relates to the diamond mining industry and can be used both in technological processes at mining and processing enterprises in the extraction of diamonds, and during geological exploration.

 There are several known methods for detecting diamonds used in technological processes for their separation from the host rocks.

 A sticky method for detecting diamonds during enrichment is known, based on the different adhesion ability of diamonds and diamond-bearing rocks with respect to hydrophobic substances: fat, oil, kerosene, etc. materials [1].

 Disadvantages - the need for an open diamond surface.

 A known method based on the difference in conductivity of diamond-bearing rocks and diamond. Diamond is a dielectric, but the rock has weak conductivity [1].

 Disadvantages: low selectivity of the method.

 Known flotation method for detecting and separating diamonds from diamondiferous rocks [1].

 Disadvantages: the need for fine grinding of the rock and, as a result, the destruction of diamonds during grinding.

 As the closest technical solution is considered / prototype [1] / X-ray fluorescence method for detecting diamonds in diamond-bearing rocks, which is widely used in industrial diamond mining. The method is based on the fact that the afterglow time of diamond after x-ray irradiation significantly exceeds this characteristic for diamond-bearing rocks [1].

 When using this method, diamonds are detected by recording the fluorescence intensity of the test rock several milliseconds after irradiation with a flux of X-ray quanta with energies in the range of 5-55 keV.

Disadvantages:
1) Since fluorescence of shielded diamonds cannot be detected, diamond detection requires preliminary grinding of the rock to ensure that there is a free surface of diamonds on the conveyor. Such grinding is carried out using various kinds of crushers and in the technological process always precedes the actual diamond detection operation by this method.

 2) The consequence of such grinding is the receipt of additional damage by the diamonds in the rock and / or their destruction into smaller ones, which leads to a decrease in the cost and yield of mined rough.

 The purpose of the invention is to increase the efficiency of the diamond detection process and increase the yield of undamaged diamonds.

 Achieving this goal is ensured by the use of a modified method of x-ray computed tomography [2] for radiometric determination of the distribution of mass density and effective atomic number in a diamond-bearing rock.

 The essence of the invention lies in the fact that the studied rock flow is subjected to x-ray tomography in planes perpendicular to the direction of the ore flow and spaced apart from each other by distances less than the minimum linear size of the diamonds to be detected. For tomography, an X-ray or gamma-ray source (s) is used that has at least two different energy values (ranges) such that for one of these values (lying in the region of 60-110 keV) the intensity attenuation (linear attenuation coefficient, hereinafter LKO ) of probe radiation in the rock under study is determined mainly by the photoelectric effect, and for another (lying in the region of 120-500 keV) the LCO is determined mainly by the Compton scattering effect. Knowing the distribution of the LKO value for at least two such energy values (ranges) inside the volume under investigation allows us to calculate the distribution of the effective atomic number and mass density of the substance inside the volume under study.

The subsequent effective detection and separation of diamonds is ensured by the uniqueness of this pair of parameters for diamond crystals - the average mass density of diamond is 3.52 g / cm ³ with an effective atomic number of 6. Any minerals or their combination in a diamondiferous rock having a mass density of about 3, 5 g / cm ³ have a significantly higher effective atomic number. For example, if minerals based on compounds of beryllium, boron, carbon and nitrogen are excluded from consideration (diamond-containing rocks practically do not contain such minerals), the pyrope mineral Mg ₃ Al ₂ (SiO ₄ ) ₃ and corundum are closest to the diamond by atomic number and other minerals based on aluminum.

The effective atomic number of the pyrope is 8.8 at a mass density of about 3.5 g / cm ³ . Thus, even in the most unfavorable case (close numerical values of mass density), the effective atomic number of a small volume of the analyzed rock differs from that for diamond by no less than 45%. This difference allows you to confidently separate the diamond crystals from the host rock of the proposed method.

According to [3], for the total attenuation coefficient of x-rays with high accuracy, we can set
μ (ε) = τ (ε) + σ (ε) + χ (ε),
where ε is the energy of quanta, and τ, σ, and χ are the absorption coefficients for the photoelectric effect, Compton effect, and pair formation, respectively. In this case, absorption due to the formation of pairs occurs only at gamma-ray energies exceeding twice the electron’s resting energy. Thus, when the energies of the probe quanta are less than 1024 keV, the above formula takes the form:
μ (ε) = τ (ε) + σ (ε).
Moreover, the expression for σ (ε), the absorption coefficient for the Compton effect, has the form:
σ (ε) = N • ρ • Z _e • ∑ _knt (ε) / A = K _s • ∑ _knt (ε),
where ∑ _knt (ε) is the Klein – Nishina – Tamm theoretical formula for the total cross section of incoherent scattering by a free electron of a quantum with energy ε;
A is the average atomic mass;
Z _e , is the average atomic number (average number of atomic electrons);
ρ is the density of the substance;
N is the Avogadro number.

The expression for τ (ε), the absorption coefficient for the photoelectric effect, has the form:
τ (ε) = K _f • ∑ _{N photo} (ε),
where ∑ _Nphoto (ε) is the theoretical formula for the dependence of the photoelectric effect cross section for a hydrogen atom on the quantum energy, and the coefficient K _{f is} individual for each substance and can be represented as
K _f = N • ρ • (Z _eff ) ⁵ / A,
where N, ρ, and A are given above, and Z _eff is the average effective charge of the nucleus, taking into account the screening of the nucleus by electrons.

 Probing a rock containing diamonds with sounding geometry adopted in standard tomographic methods [2] by photon beams of at least two different energies (beams having two different spectra), one can obtain two (or more) sets of tomographic data (one for each from the used spectra).

 Then, having processed each such set using the usual algorithms of computational tomography [2], it is possible to calculate two or more distributions of the μ value inside the diamond-bearing rock under study - one distribution for each spectrum used.

где

- энергетически независимые томографические лучевые интегралы по лучу L от характеризующих каждый малый объем исследуемого объема величин K_s и K_f;
S(ε) - экспериментально определяемая аппаратная спектральная функция томографического тракта.If Φ ₁ (ε) and Φ ₂ (ε) are the spectra of the probing quanta, then for the intensities of the probing radiation I ₁ and I ₂ that have passed along any path inside the test sample of diamond-containing rock, we can write the following expression

Where

- energy-independent tomographic ray integrals over the ray L from the quantities K _s and K _f characterizing each small volume of the volume under study;
S (ε) is the experimentally determined hardware spectral function of the tomographic path.

In one device, several such functions can be implemented. This can be achieved, for example, by placing several X-ray detectors in series, one after the other, so that the detectors located closer to the radiation source are spectral filters for detectors located farther, as well as placing special filters between such detectors. In this case, for each such function, its own set of projection data I _ki will be obtained.

Using all the projection data sets I _k and / or I _ki obtained by such methods, it is possible to calculate (possibly using statistical methods of the “maximum likelihood” type) estimates for the values of Q _s and Q _f . And then, using standard algorithms of computational tomography [2], find the distribution of K _s and K _f in the volume under study using these “spectral” projection data.

Here it is necessary to indicate that on the plane of these quantities K _s , K _f each substance (mineral) has its own point. Its position is determined by the individual physical characteristics of a substance. Namely (see above) such as:
A is the average atomic mass;
Z _e - average atomic number (average number of atomic electrons);
ρ is the density of the substance;
Z _eff is the average effective charge of the nucleus, taking into account the screening of the nucleus by electrons.

In this case, diamond, in the sense of detection capabilities of the method proposed in the present invention, has extreme values of these characteristics. All diamond-bearing rocks (kimberlites), even if they come across areas with a close density ρ, have significantly higher values of A, Z _e and Z _eff .

The proposed method was tested by mathematical modeling. During the simulation, the studied rock sample was taken in the form of a cylinder with a diameter of 150 mm, made in one case from calcium carbonate, and in the other from a pyrope of the composition Mg ₃ Al ₂ (SiO ₄ ) ₃ with a density of 3.5 g / cm ³ . The delta functions were taken as the probing radiation spectra. The first spectrum corresponded to a quantum energy of 100 keV, and the second to 150 keV. The detected diamond was presented in the form of a cube with an edge equal to 5 mm. The detected diamond was placed both on the axis of the cylinder and at different distances from it.

 Modeling (calculation of projection data) was carried out in accordance with [4]. The set of directions along which, during the simulation, the studied sample was probed with a fan beam of quanta overlapped the angle of 360 degrees.

 The projection data obtained by such a numerical simulation were processed according to the proposed method. After such processing, the volumes occupied by diamonds, according to the physical characteristics obtained during processing, were clearly distinguished from the host rock.

Thus, if the density and effective atomic number lying in the ranges of 3-4 g / cm ³ and 5.5-6.5, respectively, are calculated from the results of the computational processing of the measured projection data for a volume inside the studied piece of rock, then such volume is a diamond. In this case, the computing device should give a signal to the actuator to separate the rock sample with such characteristics from the rock stream.

 The above ranges of permissible values of the parameters found for small elements of the flow volume are conditional.

 It is known that any detection method or device (including the proposed one) is characterized by three main interrelated parameters - the probability of detection, the probability of skipping, and the probability of false positives. Therefore, the actual values of the ranges of permissible values of mass density and effective atomic number, allowing to make a decision on the detection of diamond, are determined when setting up a device that implements the proposed method. These values depend on the actually used energies (spectra) of radiation sources, physicomechanical properties of diamondiferous rocks, etc. and are chosen experimentally to achieve optimal ratios of the above probabilistic characteristics and maximum economic effect.

This method of detecting diamonds has the following advantages over the prototype method:
- the claimed solution allows you to detect diamonds in rock samples of a larger size than the prototype;
- no free surface of diamond is required for its detection, therefore, at the stage of using the proposed method in the technological process, crushing of diamondiferous rock into pieces up to 150 mm or more in size can be dispensed with, unlike the prototype method, where preliminary crushing is carried out to sizes 25-30 mm;
- a consequence of these advantages is a decrease in the destruction and damage of large diamonds during crushing of diamond-bearing rocks during the enrichment process and, consequently, a greater output of the production of rough diamonds in value terms.

SOURCES OF INFORMATION
[1] - V.I. Epifanov, A.Ya. Pesina, L.V. Zykov / Technology for processing diamonds into diamonds, M., Higher School, 1987, p. 14.

 [2] - Hermen G. Projection image restoration: Fundamentals of reconstructive tomography. Per. from English - M.: Mir, 1983.

 [3] - Physical Encyclopedic Dictionary, Volume 4, p. 230, Publishing House "Soviet Encyclopedia", M., 1965.

 [4] - A.N. Tikhonov, V.Ya. Arsenin, A.A. Timonov / Mathematical problems of computed tomography, M., Science, 1987, Ch. III, paragraph 3.

Claims (1)

A method for detecting diamonds on a conveyor, in a stream or sample of diamondiferous rock, based on irradiating the rock layers with X-rays in the direction transverse to the direction of rock movement relative to the radiation source, characterized in that the thickness of each irradiated layer and the size of the small volume elements lying inside such a layer are selected , no more than the minimum linear size of the diamonds to be detected, and to detect diamonds inside each such layer, this layer is irradiated with X-ray quanta with at least two different energy values, such that at least one of these values lies in the range of 60-110 keV and the other in the range of 120-500 keV, the intensities of the transmitted quantum fluxes are recorded for each of the energies used in the propagation of these flows in different directions inside the irradiated layer, and the set of directions is chosen so that through an arbitrary small element of the volume of the irradiated layer, the recorded quantum flows passed in several different directions lying in the plane angle of at least 180 ^o , the recorded intensities of the transmitted quantum fluxes are recalculated into sets of attenuation coefficients of radiation intensity for all used energies along each direction, using these sets of attenuation coefficients linear attenuation coefficients (LCO) of radiation for each of the used energies in each of the small elements are calculated and from the expression defining the LCO as the weighted sum of the photoelectric effect cross section and the Compton scattering cross section, find the effective atomic number and mass density awn for each small element of volume of the irradiated layer is determined for each layer of irradiated total volume of the small elements of this layer, which is found by the mass density in the range 3,52 ± K1 g / cm ^3, with the proviso that found effective atomic number is within 6 ± K2, a signal is issued to the actuator for separating that layer or sample of diamondiferous rock for which the resulting total volume exceeds a predetermined threshold K3, and the numerical values of the coefficients K1, K2 and K3 are selected experimentally to obtain desired detection probability and false alarm passes when setting apparatus implementing the method of detection of diamonds.