The so-called electrical properties of mineral deposits refer to the resistance, dielectric constant, specific conductivity and rectification of the mineral deposits, which are the basis for judging whether electrical separation can be used for separation. Because the composition of various mineral deposits is different, the electrical properties reflected are also significantly different. Even if it is attributed to the same type of mineral deposit, because of the different impurities contained, the electrical properties are also different, but no matter what, there is always a certain scale of change. For our reference.
Resistance refers to the ohmic value measured when the particle size of the mineral deposit is d=1 mm. According to the measured resistance values of various mineral deposits, mineral deposits are often divided into the following three types, namely:
Conductor-the resistance is less than 106 ohms, indicating that this type of mineral deposits have better conductivity. , Can make it separate from the conductor part.
Non-conductor-the resistance is greater than 107 ohms, this kind of minerals have very poor conductivity, and can only be separated as non-conductors in general electric selection.
Medium conductor—that is, the conductivity is between conductor and non-conductor, and the resistance is greater than 106 ohms but less than 107 ohms. Such mineral deposits are often separated as electro-selected ore.
The concept of electrically selected conductors and non-conductors is very different from the physics of conductors, semiconductors, and insulators. The conductor mineral deposit referred to in this article means that after electrons are adsorbed in an electric field, the electrons can move freely on the mineral particles, or they can generate positive and negative charges after being induced by electrodes in a high-voltage electrostatic field. This kind of positive and negative charges can also Move freely. The opposite is true for non-conductors. After it absorbs charges in a corona field, the charges cannot move or conduct freely on its surface. It can only be polarized in a high-voltage electrostatic field. The center of the positive and negative charges only violates, and this positive and negative charge is a constraint. The charge cannot be removed, and once it leaves the electric field, it returns to its original state without showing positive and negative electrical properties. Mineral deposits with medium conductivity (or semiconductors) are those between conductors and non-conductors. Except for some of these deposits, in the practice of electric separation, there are generally contiguous bodies.
constant The dielectric constant refers to the ratio of a capacitor with a dielectric material to a capacitor without a dielectric material (referring to vacuum or air). Under the same voltage, if a dielectric is inserted between the two plates of the capacitor, the capacitance of the capacitor will definitely increase. The dielectric constant ε can be expressed by the following formula:
In the formula, Cm—the capacitance of mineral deposits or materials, F;
Co—the capacitance of air, F.
The size of the dielectric constant is now an important criterion for measuring and judging whether a mineral deposit can be selected for electrical separation. The larger the dielectric constant, the better the conductivity, and vice versa, the poor conductivity. Under normal circumstances, those with a dielectric constant ε>12 are classified as conductors, and are separated by conventional electrical selection as conductors; for those below 12, if the dielectric constants of the two minerals still have a large difference, then conflict electrical selection can be selected. Make it separate.
According to the research results, the size of the dielectric constant is not determined by the size of the electric field strength, but depends on the frequency of the AC power used for the measurement, and is related to the temperature. RM Fouss’s research has reached a conclusion that the dielectric constant at low frequencies The constant is large, and the dielectric constant is small at high frequency. The dielectric constants of mineral deposits listed in books and periodicals are all values measured under alternating current conditions of 50 or 60 Hz. In the MKS system, the vacuum dielectric constant ε0=8.85×10-12 farads/meter or coulomb 2 Cattle/•m2.
The measurement of permittivity includes the plate capacitance method and the dielectric liquid method. The former is a dry method and the latter is a wet method.
A. In the plate capacitance method
, a pure mineral deposit piece to be tested is placed between two parallel metal plates. The mineral deposit piece is sliced and polished to the size of the metal plate. The appearance of measuring capacitance can be selected, and the difference frequency capacitance meter can also be selected. The method and measurement method are shown in Figure 1. The size of the two metal plates is completely the same, and the area A is much larger than the space d between the plates. If it is not put in the mineral deposit, the capacitance of the two plates is C0.
Under the same conditions, after being put into the mineral deposit to be tested, its capacitance must be many times larger than that of air, that is, Cm>C0, so the dielectric constant of the mineral deposit is:
The units of Cm, C0 and εm are the same as before, but the unit of capacitance is too big, so choose the light method (μμf), 1μμf=10-12F. This method is only suitable for large crystalline pure mineral deposits or gangue mines. Not suitable for granular mineral deposits.
B. In the
practice of measuring the dielectric constant by wet method , most of the mineral deposits are granular, and there are many fine particles, so the flat method is not applicable. The principle of this method is to use electrodes to attract or squeeze the mineral particles to be measured in a dielectric liquid to determine the dielectric constant of the mineral deposit. The schematic structure is shown in Figure 2. That is to say, in a container, put two very thin steel needles from the bakelite cover on the upper part, with a distance of about 1 mm, add a certain amount of dielectric liquid into the container, and then pass a single phase to the two needle poles. (50 or .60 Hz) alternating current. Put the mineral particles to be tested into the liquid in the container. At this moment, the mineral particles with a higher dielectric constant than the dielectric liquid are attracted to the needle electrode, and those with a lower dielectric constant are squeezed away from the electrode.
According to the needs, the size of the dielectric constant of the dielectric liquid is adjusted in advance and then adjusted continuously. For example, when measuring the dielectric constant of quartz, add 5 ml of methanol and 0.5 ml of methanol in the container. After mixing, the dielectric constant εh=5.1. After adding a few quartz particles, if the quartz particles are attracted to the electrode, it proves that the liquid The dielectric constant of is still small. Add 0.1 ml of methanol. At this moment, the dielectric constant ε of the dielectric liquid has increased to 5.63. If you see quartz particles just being pushed out, the dielectric constant of quartz is somewhere between these two , So εQ=(5.1+5.63)/2=5.36. This method is more laborious, but more accurate, suitable for granular mineral deposits.
(3) Specific conductivity The
specific conductivity refers to the ratio of the degree of difficulty for electrons to flow in or out. The degree of difficulty is related to the resistance of the touch interface between the mineral particles and the electrode, and the interface resistance is related to the potential difference between the mineral particle and the electrode’s touch surface or point, that is, the voltage. If the voltage is too low, the electrons cannot flow into or out of those mineral particles with poor electrical properties. When the voltage increases, the electrons can flow in or out well. At this moment, the conductive mineral particles are embodied as a conductor. Non-conductors have different behaviors and trajectories in the electric field. Figure 3 shows the minimum voltage equipment required to determine various mineral deposits. The measured mineral deposits are given to the drum, if the voltage reaches
When the value reaches a certain value, the mineral particles are attracted by the electrode 3, and the falling orbit is violated, and the voltage at this moment is the lowest voltage. On the contrary, if the voltage is low, the mineral particles do not reflect the violating effect of the conductor, and fall along the ordinary track. For this reason, different voltages and different polarities (positive or negative) can be selected to determine the minimum voltage required for various mineral deposits. Graphite is a good conductor, and the lowest voltage required is only 2800 volts. It is used as a standard in the world to compare the lowest voltage required for various mineral deposits with it. This ratio is defined as the specific conductivity. For example, if the minimum voltage required for ilmenite is 7800 volts, the total specific conductivity is 2.51 (that is, the ratio of 7800 to 2800), and so on.
It is necessary to clarify that these measured and calibrated voltages are the lowest voltages, not the best sorting voltages. The practical sorting voltages are often much higher than those listed in the table.
Because of the different electrical properties of various mineral deposits and the different polarity (positive or negative) of the charged electrodes, they exhibit different behaviors in the electric field. It is manifested as a conductor, otherwise it is a non-conductor. When sorting quartz, only when the electrode polarity is positive and the voltage is 8892 to 14820 volts, it becomes a conductor, and when the electrode is negative, it becomes a non-conductor. When sorting magnetite and ilmenite, it is contrary to the above two situations. No matter the polarity of the electrode is positive or negative, only the voltage reaches a certain value, it will be reflected as a conductor. This is reflected in various mineral deposits. This electrical property is called rectification. For this reason, mineral deposits that only obtain positive electricity are called positive rectifying minerals. For example, the above calcite, the electrode is negatively charged at this moment; quartz only obtains negative electricity, so this type of mineral is called negative rectifying minerals, and the electrode is positively charged at this moment; while magnetite Regardless of whether the charged electrode is positive or negative, it is embodied as a conductor, which is called full rectification.
The minerals we want to sort are classified as conductor or non-conductor, and its specific conductivity can be found, and the lowest sorting voltage can be determined. According to its rectification, it can be determined to choose positive or negative electricity. In most cases, the electrodes are all negative electricity instead of Use positive electricity.
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