Picking theory (3)

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The neutron absorption method is a sorting method that separates the thermal neutrons from the absorption capacity of the nuggets and waste stones.
When a thermal neutron passes through a substance, the neutron flux is attenuated by being captured and scattered by the nucleus of the chemical element. The degree of attenuation of the original ray is related to the nuclear nature of the elements in the absorber and its content. Ore containing elements that absorb neutrons, such as boron , can be sorted by neutron absorption. The neutron ray is weakened by absorption, and its weakening law is also in the form of an exponential function like γ-ray and χ-ray (see Equation 8).
Table 5 shows the elastic scattering cross sections and trapping cross sections of some chemical elements and their isotopes for thermal neutrons. As can be seen from the table, minerals containing boron, lithium , cadmium or rare earth have larger neutron capture cross sections than those of rock-forming minerals, which can be sorted by neutron absorption.
(7) Photoelectric method Photoelectric sorting method is a sorting method that separates ore from waste rock by utilizing the difference in the ability of mineral to reflect, transmit or refract visible light.
Visible light is an electromagnetic wave having a wavelength of 380 to 760 nm.
Optical properties such as diffuse reflection, color, transparency, and translucency of minerals can be used for photoelectric picking. One of the most widely used optical properties is diffuse reflection. The main basis for evaluating whether a mineral can be photoelectrically sorted by diffuse reflection is the reflectivity of the mineral. Under normal circumstances, the difference in reflectance of the two minerals is greater than 5~10% for photoelectric picking. However, due to the complexity of the material composition and surface chemistry of mineral raw materials, their reflection of light is also different in the visible spectrum region, so the method of photoelectric picking of mineral raw materials is also different. Full-spectral sorting utilizes the full spectral reflectance difference of mineral raw materials (see Figure 6); single-spectral sorting utilizes the difference in reflectance of a mineral over a range of wavelengths in the visible region (see Figure 7); The sorting performed in different colors is essentially a two-spectral sorting, that is, using a mineral raw material in the visible spectrum region, and the difference in reflectance between two different wavelength ranges is sorted and sorted (see Fig. 8). [next]

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Photoelectrically selected illumination sources include incandescent lamps, "fluorescent lamps, quartz halogen lamps, and lasers. The spectral characteristics of the illumination source have no significant effect on the full-spectral sorting. As long as the source is stable and uniform, incandescent and fluorescent lamps can be used. Single spectrum In the case of dual-spectrum photoelectric sorting, the spectral characteristics of the source should be adapted to the spectral characteristics of the material being sorted.
The background choice during optoelectronic picking is also important. First, the selected background color should be consistent with the spectral characteristics of most minerals in the material. Secondly, the background can eliminate the influence of particle size within a certain range and improve the sorting effect. The background is generally divided into two series: color series and black and white series. Full-spectrum photoelectric sorting generally uses only black and white series backgrounds. Commonly used photodetecting elements are various types of photocells, and in recent years, the use of solid-state cameras has greatly promoted the development of photovoltaic beneficiation . [next]
After the scanning technology is applied to the photoelectric picking, especially the combination of electronic scanning and electronic computer, the spot with a diameter of 2 mm on the ore block can be distinguished. When picking on such a device, a photodetector (such as a solid-state camera) first measures the area of ​​the nugget that is larger than a certain reflectivity, and then calculates its percentage of the entire nugget area. This value is in advance. By comparing the determined cut-off values, two products, such as concentrates and tailings, can be sorted.
Optical sorting was first used in sorting metal ore, non-metallic minerals and construction materials used in the sorting. In our country has been widely used in sorting tungsten ore sorting tests in plaster, phosphorus and boron ore in stone, also achieved good results.
(8) Infrared method The method of separating the ore and waste rock by measuring the difference in the intensity of the emission or reflection of the nugget is called infrared sorting. After the solid material is heated, as the temperature increases, the vibration frequency and amplitude of the atom gradually increase, and it becomes an emission source that can emit low-frequency rays--infrared rays. Some minerals and surrounding rocks have different heat transfer coefficients and heat capacities. Under the same heating conditions, the absorbed heat energy is different, so the emitted rays are different. This feature can be used for sorting. This method has been applied to the separation of asbestos . After the asbestos-containing ore is heated for a short time, the asbestos fiber emits infrared rays with a shorter wavelength, and the surrounding rock emits a longer wavelength of radiation. According to this feature, asbestos can be sorted out using an infrared detector. The schematic diagram of the operation principle is shown in Figure 9.

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In recent years, the ore has been sorted by photoelectric method, which not only uses visible light, but also extends to the range of near-infrared and mid-infrared light, because the spectral range of some light sources includes visible light and infrared light, and the peak of the photosensitive characteristics of some solid-state cameras is also in infrared. Area. Therefore, some minerals that cannot be distinguished by the naked eye can be sorted by using different infrared rays reflected by them.
(9) Radio resonance method (conductance-magnetic method)
After placing the nugget in an electromagnetic field that generates radio waves, the electromagnetic field interacts with the nugget, and its function is different due to the difference in electrical and magnetic properties of the nugget. If the nugget is a conductor, an induced current is generated in the nugget. If it is a dielectric, the nugget is polarized. The electrical and magnetic properties of the ore can change the parameters of the oscillating circuit (such as the phase, amplitude and frequency of the current and voltage). Therefore, the method of separating the ore from the waste rock according to the change of the parameter values ​​in the circuit is called radio resonance sorting. . Since the size of the circuit parameters varies depending on the conductivity and magnetic properties of the ore (mine), this sorting method is also called conductance-magnetic method.
The conductivity of ore is related to magnetism but independent of each other. Depending on the nature of the ore, the detector can detect the conductivity or magnetism of the nugget alone, or both.
The ore conductivity is represented by the resistivity ρ and the dielectric constant ε, and the magnetic properties thereof are represented by the magnetic permeability μ or the magnetic susceptibility κ. The conductivity and magnetic properties of ore and surrounding rock are related to many factors, such as the chemical composition, mineral composition, structure, structure, humidity, porosity, etc. of the ore. In addition, it is also related to the frequency of the electromagnetic field. The resistivity ρ of the ore varies between 10 ^-6 and 10 ¹⁶ ohms [m], and the dielectric constant ε varies from 2 to 300 [pulses] per meter, and the magnetic susceptibility K is between 10 ^-7 and 10 ^{-2 .} Change between. The electrical properties of the ore at different frequencies were studied, and it was found that the frequency range that can be used for ore sorting was very wide, ranging from tens of Hz to several hundred megahertz.
When the conductivity difference between the ore and the gangue is large, an inductive self-excited oscillator is used as the radio wave generator. If the dielectric characteristics are different, a capacitive self-excited oscillator is used. Both inductive and capacitive radio wave generators can take different forms, see Figure 10.

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