The essence of electrical enrichment methods
Electrical enrichment methods are based on the difference in the electrical properties of the separated minerals. Differing in electrical conductivity, dielectric permittivity, contact potential, triboelectric, pyroelectric or piezoelectric effect, they acquire a different value or sign of charge during charging and, as a result, a different trajectory of movement in an electric field, providing separation of particles according to their electrical properties or electrical separation of minerals.
The particles of the separated material can be charged by contact with a charged electrode, ionization in the electric field of a corona discharge, electrification by friction, changes in temperature, pressure, and other methods. The choice of particle charging method provides the greatest difference in the electrical properties of the main minerals to be separated, and thus the maximum efficiency of electrical separation.
Each charged mineral particle during separation in an electric field is affected by:
electric Coulomb force F e, due to the attraction of a particle to an oppositely charged electrode and its repulsion from a similarly charged electrode both in a uniform and in a non-uniform field. Influence R e on the trajectory of particle motion is practically leveled only in a field of variable polarity due to the mechanical inertia of the particles;
mirror image strength F 3 , due to the interaction of the residual charge of the particle and the equal inductive charge caused by this charge on the surface of the electrode. The force is directed towards the electrode. In absolute terms, it is much less R e and its effect is noticeable only near the electrode or in contact with it;
ponderomotive force F n due to the difference between the values of the permittivity of the particle ε h and Wednesday ε where the separation takes place. It tends to push the particle into weaker parts of the field if ε h< ε s, and vice versa retract at ε h > ε with. Force manifests itself only in an inhomogeneous field, including, in contrast to F uh, and in fields of variable polarity. It is very small in air compared to F e and reaches high values in liquids with high dielectric constant;
mechanical force, the main of which are the force of gravitational attraction, F G centrifugal force F u resistance forces of the medium F s.
The forces of molecular adhesion of particles between themselves and with electrodes, the friction force between the particles and the electrode for particles larger than 0.1 mm, as well as the inertial forces acting at the final stage of separation, are relatively small and are usually not taken into account.
The separation of differently charged particles occurs as a result of the action of electrical and mechanical forces on them in the working area of the separator. The ratio of forces and the efficiency of separation in this case will depend on the difference in the electrical properties of the separated minerals, changes in the electric field strength in time (constant or variable) and space (homogeneous or variable), the presence of moving charge carriers (ions, electrons), the type of separation medium (gas or liquid) and the nature of the movement of the material in the working space of electric separators.
In separators with a curved drum-type transport electrode (Fig. 6.1, a) the process of separation of minerals occurs in the air.
Rice. 6.1. Vector diagrams of forces acting on particles in separators: a, b- drum electrostatic; in- planar electrostatic; G- chamber electrostatic; d- dielectric; one- positively charged particle; 2- negatively charged particle
An inhomogeneous electrostatic or electric field of constant polarity with a strength of up to 10 kV/cm is created between the drum and the second electrode or electrode system spaced from it at some distance. electrical force F e will press against the drum particles that have a charge sign opposite to the polarity of the drum, and repel like-charged particles from it. The power of mirroring F 3 , directed towards the center of the drum, keeping the particles on its surface. Centrifugal force F c , on the contrary, it tends to detach particles from the surface. Gravitational force F r acts vertically down, its components depend on the angle of rotation of the drum. ponderomotive force F P
is directed from the center of the drum, since the dielectric constant of minerals is greater than that of air, and the concentration of the field lines of force increases towards the second electrode. However, the strength F P , as well as the air resistance force F with for granular particles in the working area of the separator, is relatively small and can be ignored.
Resultant force F, which determines the trajectory of particles in the electric field of the separator, is the vector sum of the main interacting forces:
In separators with a flat transport electrode (Fig. 6.1, in) between it and the second electrode located on top or a system of electrodes, an electric or electrostatic field with a strength of 2- 4 kV/cm Resultant force F, which determines the trajectory of the separated particles, is the sum of the electric force F uh , mirror image powers F h , and gravitational force F G , causing the movement of particles along the plane and significantly affecting the separation of minerals that differ sharply in shape:
By the forces F with and F P , as in the first case, can be neglected.
In chamber separators (Fig. 6.1, G) an electrostatic field of constant polarity with a strength of 2 - 4 kV / cm is created between the plate electrodes. The separation of particles with different charges is carried out in the process of their free fall between the electrodes. In this case, the movement of particles in the horizontal direction is determined mainly by the electric force F uh , causing the attraction of particles to the oppositely charged electrode and their repulsion from the electrode of the same name. Force F 3 begins to appear only when the particles approach one of them, therefore, like the force F P , practically does not affect their separation. In the vertical direction, multidirectional gravity forces will act on each particle F G and medium resistance F P.
Separation of minerals in a non-conductive liquid in dielectric separators (Fig. 6.1, e) occurs in a sharply inhomogeneous electric field of variable polarity with a strength of up to 5 kV/cm. The process-determining force under these conditions is the ponderomotive force F n. Under its action, particles with a permittivity ε 2 , greater ε s, are drawn into the region of the field of greatest strength near the electrode with a small radius of curvature, while particles with ε 2, smaller ε s, pushed out of this area. Of the mechanical forces affect the separation of particles, the force of gravity F G and the resistance of the medium as in the vertical F c, as well as horizontal F" with direction.
Independent work No. 4 On the subject of GTR of the Student group 14 OCA Khaidarova Malokhat. TOPIC: Rare types of enrichment. Electrical enrichment. Electrical enrichment is a process of separation of mineral particles in an electric field, based on the difference in their electrical properties. Electrical enrichment methods are used to enrich non-metallic minerals (coal, kaolin, quartz sand, etc.) The electrical enrichment method is based on mechanical and electrical forces acting on various components of the processed material (ore) when moving them in an electric field. The electric beneficiation method is commonly used to refine other beneficiation processes, and it requires fine material (grains) ranging in size from 2 to 0.1 mm. An electric charge can also be formed on a mineral particle by the action of an electric field on it at a certain distance.
When moving in an electric field, mineral grains receive charges, resulting in attractive or repulsive forces that affect the trajectory of particle movement.
By selectively acting on the charged particles of various minerals, the electric field allows them to be separated into separate products. For electrical enrichment, the most important characteristics of minerals are electrical conductivity and dielectric constant. The efficiency of electrical enrichment in some cases can be increased by heating the ore to a temperature of 50°C and above in order to dry it.
In particular, it has been found that surface moisture not only has a negative effect on the enrichment process, but, when maintained within optimal limits, it contributes to an increase in the difference in the electrical conductivity of the separated minerals and thereby improves the selection. Electrical enrichment is a mineral separation process based on the difference in the value and the sign of the charges of mineral particles that acquire an electric charge as a result of friction against another body; in this case, different bodies acquire charges that differ in magnitude and sign.
When electrified by friction due to the transition of electrons, friction charges (triboelectric charges) arise on the particles, sometimes reaching a large value. The sign of the charge depends on the nature of the particles and the material of the tray along which they move, as well as on the state of their surface, etc. If different minerals enriched product acquire different signs and sufficiently large triboelectric charges, this product can be divided in an electric field into separate mineral fractions.
For example: when moving along a duralumin plate, quartz acquires a large negative charge, and disthene - less, after which the mixture of these minerals can be separated in an electric field: quartz deviates towards the positively charged electrode more than disthene. When particles are charged by means of direct contact with a charged electrode, the particles on the contact side receive charges that are opposite in sign to the charge of the electrode.
In this case, the dielectric charge due to its polarization cannot be transferred to the electrode, and the particle remains electrically neutral. At the same time, due to the good electrical conductivity of the conductor, the charge that has arisen is neutralized, as a result, the conductor acquires the charge of a charged electrode and is repelled from it as a similarly charged one.
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Electrical enrichment methods are based on the difference in the electrical properties of minerals, namely the difference in electrical conductivity and dielectric constant.
In many substances there are free charged microparticles. A free particle differs from a "bound" particle in that it can move a long distance under the action of an arbitrarily small force. For a charged particle, this means that it must move under the action of an arbitrarily weak electric field. This is exactly what is observed, for example, in metals: an electric current in a metal wire is caused by an arbitrarily small voltage applied to its ends. This indicates the presence of free charged particles in the metal.
Characteristically, the carriers are free only inside the conductor, that is, they cannot freely go beyond its boundary.
Conductors are metals, electrolytic liquids. In metals, electrons are carriers; in electrolytic liquids, ions are carriers (they can have a positive and negative charge).
Under the action of an external electric field, positive carriers move along the field, and negative carriers move against the field. This leads to the appearance of a current directed along the field.
The ordered movement of charge carriers, leading to the transfer of charge, is called an electric current in a substance. Electric current occurs under the influence of an electric field. The property of a substance to conduct an electric current is called electrical conductivity.
According to the electrical conductivity, all minerals are divided into three groups:
1. Conductors with electrical conductivity 10 2 - 10 3 S/m
Siemens (Cm) - the conductivity of such a conductor in which a current of 1A passes at a voltage at the ends of the conductor of 1V.
2. Semiconductors with electrical conductivity 10 - 10 -8 S/m
3. Nonconductors (dielectrics) with electrical conductivity
< 10 -8 См/м
For example, graphite, all sulfide minerals are good conductors. Wolframite (Fe, Mn) WO 4 (10 -2 -10 -7) and cassiterite SnO 4 (10 -2 -10 2 or 10 -14 -10 -12) have moderate electrical conductivity, and silicate and carbonate minerals conduct electricity very poorly .
Electrical methods are used in the enrichment of titanium-zirconium, titanium-niobium, tin-tungsten collective concentrates, as well as in the enrichment of phosphorites, coal, sulfur, asbestos and many other minerals, the processing of which by other methods (gravitational, flotation, magnetic) is not effective.
The physical essence of the electrical separation process is the interaction of the electric field and a mineral particle with a certain charge.
In an electric field, charged particles move along various trajectories under the action of electrical and mechanical forces.
This property is used to separate mineral grains in apparatus called electric separators.
The electrical forces acting on mineral particles are proportional to the magnitude of the charge and the strength of the electric field, since
where is the permittivity equal to ,
E is the tension in the given environment.
Mechanical forces are proportional to mass:
Gravity:
Centrifugal force:
For small particles, electrical forces are greater than mechanical ones, and for large particles, mechanical forces prevail over electrical ones, which limits the particle size of the material smaller than 3 mm, enriched in electrical separators.
An electric field arises in the space around an electrically charged particle or between two charged particles.
Using the electrical properties of minerals during enrichment, the following types of separation are used: by electrical conductivity (Fig. 14.8), by dielectric constant, by triboelectrostatic and pyroelectric effect.
Rice. 14.8 Conductivity separators
a. Electrostatic separator; b. Electric corona separator;
in. Crown - electrostatic separator
1- bunker; 2 - drum; 3 - brush for removing the conductive fraction; 4, 5, 6 - receivers for products; 7 - electrode; 8 - cutter; 9 - corona electrode; 10 - deflecting electrode.
These processes are used in finishing concentrates of rare metals, diamond and others, but they can also be used in the enrichment of coal, manganese ores, foundry sands, etc. These methods enrich only dry fine-grained materials (with a moisture content of not more than 1% for ore minerals and no more than 4-5% for coals).
According to the conductivity of electricity, all bodies are divided into conductors, semiconductors and dielectrics - non-conductors.
Electrical methods are based on the difference in the behavior of charged particles in an electric field or on a charged electrode.
If particles move along a charged electrode, then charges are induced on the surface of the IC; on the one facing the electrode - of the opposite sign, and on the one farthest from the electrode - of the same sign. A charge of the opposite sign from the conductor particle passes to the electrode, a charge of the same name as the electrode charge remains on it, and the particle is repelled from the electrode. The charge does not transfer from the dielectric and the particle is attracted to the electrode.
Usually the electrode has the form of a rotating grounded drum (Fig. 24, a).
To improve the separation and increase the deflection trajectory of the conductor particles, a roller with a charge is placed, the sign of which is opposite to the sign of the drum charge. This enrichment is called electrostatic.
Separation will be improved if, before entering the drum, the particles are charged with a charge opposite to the sign of the drum charge.
In industrial separators, the drums are located one below the other; instead of drums there can be plates (Fig. 24, b).
When particles rub against each other or against some specific surface, for example, the surface of a vibrating transpoter, particles of different minerals can be charged with charges of different signs, and when passing between two drums or planes with opposite signs of charge, they will deviate in different directions according to their charge. This type of separation, based on electrification by friction, is called triboelectric. It is of little practical importance.
If two electrodes, one on which has a small radius of curvature (point, thin wire), and the other has a large radius of curvature (drum, plane), impose a significant potential difference of up to 30 kv. then a corona discharge will occur near the thin electrode - air ionization. A flow of ions is created from the corona electrode to the ground electrode: this flow charges all mineral particles in the interelectrode space. The charged mineral particles will also move towards the grounded electrode and settle on it. As a result of this, the conductors will give up their charge, receive the charge of the electrode and repel or become neutral, while the non-conductors will remain on the electrode. The corona electrode is usually negatively charged, since in this case a higher breakdown voltage is created.
The charge of particles depends on the strength of the electric field, the radius of the particles and their permittivity. The behavior of particles on a grounded electrode depends mainly on their electrical conductivity.
In corona separators, non-conductors and semiconductors retain their charge better when moving towards the electrode, and separation occurs more clearly on these separators than on purely electrostatic ones. Therefore, crown and combination separators are becoming more and more common. Combined separators are designed in Irgiredmet.
Electrical enrichment makes it possible to obtain low-ash coal with a size of -2 to 0.05 mm and remove most of the sulfur from it; wolframite - to separate from waste rock, ilmenite, feldspar - from quartz, cassiterite - from scheelite (obtain cassiterite in concentrate up to 97%), iron oxides - to separate from quartz sand, etc.
Corona plate separators, which create an "electric wind" of charged particles, can be used for dry classification. IGDAN has developed classifiers with a capacity of up to 30 g per hour.
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Electrical enrichment methods are based on differences in the electrical properties of the separated minerals and are carried out under the influence of an electric field.
Electrical methods are used for small (-5 mm) dry bulk materials, the enrichment of which by other methods is difficult or unacceptable for economic or environmental reasons.
Of the many electrical properties of minerals, industrial separators are based on two: electrical conductivity and the triboelectric effect. In laboratory conditions, the difference in permittivity, the pyroelectric effect, can also be used.
A measure of the electrical conductivity of a substance is the specific electrical conductivity (l), numerically equal to the electrical conductivity of a conductor 1 cm long with a cross section of 1 cm 2, measured in ohms to the minus first degree per centimeter to the minus first degree. Depending on the electrical conductivity, all minerals are conventionally divided into three groups: conductors, semiconductors and non-conductors (dielectrics).
Conductive minerals are characterized by high electrical conductivity (l = 10 6 ¸10 ohm - 1 × cm - 1). These include native metals, graphite, all sulfide minerals. Semiconductors have a lower electrical conductivity (l = 10¸10 - 6 ohm - 1 × cm - 1), they include hematite, magnetite, garnet, etc. Dielectrics, unlike conductors, have a very high electrical resistance. Their electrical conductivity is negligible (l< 10 - 6 ом - 1 ×см - 1), они практически не проводят электрический ток. К диэлектрикам относится большое число минералов, в том числе алмаз, кварц, слюда, самородная сера и др.
The triboelectric effect is the appearance of an electric charge on the surface of a particle during its collision and friction with another particle or with the walls of the apparatus.
Dielectric separation is based on the difference in the trajectories of movement of particles with different dielectric constants in a non-uniform electric field in a dielectric medium with a dielectric constant intermediate between the permeabilities of the separated minerals. During pyroelectric separation, heated mixtures are cooled by contact with a cold drum (electrode). Some components of the mixture are polarized, while others remain uncharged.
The essence of the electric method of enrichment is that particles with different charges in an electric field are affected by a different force, so they move along different trajectories. The main force acting in electrical methods is the Coulomb force:
where Q is the charge of the particle, E is the field strength.
The electrical separation process can be conditionally divided into three stages: preparing the material for separation, charging the particles, and separating the charged particles.
Charging (electrification) of particles can be carried out in different ways: a) contact electrification is carried out by direct contact of mineral particles with a charged electrode; b) ionization charging consists in exposing particles to mobile ions; the most common source of ions is corona discharge; c) particle charging due to the triboelectric effect.
To separate materials by electrical conductivity, electrostatic, corona and corona-electrostatic separators are used. By design, drum separators are most widely used.
In drum electrostatic separators (Fig. 2.21, a) an electric field is created between the working drum 1 (which is the electrode) and the opposed cylindrical electrode 4. The material is fed into the working area by the feeder 3. Electrification of particles is carried out due to contact with the working drum. The conductors receive a charge of the same name as that of the drum and repel it. Dielectrics are practically not charged and fall along a trajectory determined by mechanical forces. Particles are collected in a special receiver 5, which is divided by means of movable partitions into compartments for conductors (pr), non-conductors (np) and particles with intermediate properties (pp). In the upper zone of the crown separator (Fig. 2.21, b) all particles (both conductors and dielectrics) acquire the same charge, sorbing ions formed due to the corona discharge of the corona electrode 6. Getting on the working electrode, the conductor particles are instantly recharged and acquire the charge of the working electrode. They are repelled from the drum and fall into the receiver of the conductors. Dielectrics do not actually discharge. Due to the residual charge, they are retained on the drum, they are removed from it using a cleaning device 2.
The most common corona electrostatic separator (Fig. 2.21, in) differs from the corona electrode by an additional cylindrical electrode 4, which is supplied with the same voltage as the corona electrode. (The radius of curvature of the cylindrical electrode is much greater than that of the corona electrode, but less than the working drum - electrode.) The cylindrical electrode contributes to an earlier separation of conductive particles and allows you to "stretch" the dielectric conductors over a greater horizontal distance.
If the difference in the electrical conductivities of the particles is negligible, separation on the aforementioned separators is not possible, and then a triboelectrostatic separator is used. Here, too, the drum separator is most widely used (Figure 2.22). Structurally, this apparatus is very close to an electrostatic separator, but has an additional element - an electrolyzer, manufactured either in the form of a rotating drum or a vibrating tray. Here, the particles of minerals rub against each other and against the surface of the electrizer. In this case, the particles of different minerals acquire opposite charges.
Methods of electrical enrichment based on the difference in the dielectric constant and on the pyrocharge of particles (charging by heating) have not received industrial application.
Electrical enrichment methods are relatively widely used in the processing of ores of rare metals, they are especially promising in arid regions, since they do not require water. Also, electrical methods can be used to separate materials by size (electrical classification) and to clean gases from dust.
