EP3020483A1

EP3020483A1 - Method for grinding materials in rotating drum using ferromagnetic grinding bodies

Info

Publication number: EP3020483A1
Application number: EP14823549.2A
Authority: EP
Inventors: Andrey Vladimirovich SMOTRITSKIY; Aleksandr Andreyevich SMOTRITSKIY; Fyodor Fyodorovich BORISKOV; Sergey Alekseevich CHERVYAKOV
Original assignee: Obshchestvo S Ogranichennoy Otvetstvennostyu "byuro Sovremennykh Tekhnology"
Current assignee: Obshchestvo S Ogranichennoy Otvetstvennostyu "byuro Sovremennykh Tekhnology"
Priority date: 2013-07-08
Filing date: 2014-07-04
Publication date: 2016-05-18
Anticipated expiration: 2034-07-04
Also published as: EP3020483B1; RU2536886C1; WO2015005828A1; EP3020483A4

Abstract

The invention relates to a method for grinding materials in a rotating drum by ferromagnetic grinding bodies and can be used in the processes of the preparation of raw material for beneficiation, as well as in construction, chemical and others industries.

The essence of the invention is that the grinding of the materials is carried out in a rotating drum, partially filled with ferromagnetic grinding bodies, wherein the grinding bodies are affected by the magnetic force of at least one electromagnet, mounted on the drum, and the magnetic field is formed by separate pulses in the quantity of not less than two, wherein the first pulse is divided into a number of shorter pulses. In contrast to the prototype, the first pulse is characterized by alternating-sign magnetic field, it is fed at an angular position of the electromagnet from -60° to -30°, where the central angle begins from a vector directed from the axis of rotation of the housing vertically downwards, the second pulse is characterized by constant magnetic field, it is fed at an angular position of the electromagnet from 0° to +30°, and its feeding is stopped at an angular position of the electromagnet from 110° to 165°.

The technical result is the improvement of the energy efficiency of the process of grinding materials in ball mills.

Description

The invention relates to a method for grinding materials in a rotating drum by ferromagnetic grinding bodies and can be used in the processes of the preparation of raw material for beneficiation, as well as in construction, chemical and others industries.
The efficiency of grinding in case of using conventional ball mills does not exceed 1-2%. Low efficiency is related to the fact that a significant portion of the energy supplied to the mill shaft is spent on the friction between grinding bodies. A greater number of the interactions between particles and between particles and grinding bodies have a nature of friction with a low specific energy, rather than impact. In this case the energy is spent mainly on heating and the wear of grinding bodies, and only a small portion of the energy is used to grind the material. The trajectory of the movement of materials in a ball mill is such that only a small number of interactions occur with high energies. It is these collisions which result in the advantageous effect - the destruction of rocks. A significant portion of the energy supplied to the mill shaft is used for the friction of materials in the "dead zone", which results in the heating of rocks, the wear of grinding bodies and lining. Currently, there are several technical solutions aimed at improving the efficiency of grinding in ball mills.
A method is known, which is implemented in a ball mill, comprising a cylindrical chamber, mounted for the rotation around the horizontal axis of the cylinder, a number of steel balls in the chamber, wherein the chamber is made of a paramagnetic material, the balls are made of a ferromagnetic material, on the outside of the chamber at least one magnet is installed, the magnetic force lines of which are directed into the chamber and which is movable in relation to the chamber and can have a number of positions on the corresponding arc, the center of curvature of which is on the axis of rotation of the chamber [ US 5,383,615 , IPC B02C 17/00, January 24, 1995].
The disadvantages of this method is the complexity of the manufacturing of the construction required for its implementation, as well as the impossibility of eliminating "dead zones" due to the fact that the electromagnet during the rotation of the mill is static with respect to the housing.
Another known method is implemented in an electromagnetic ball mill, comprising a tubular housing and a gear mechanism, wherein on the outside of the tubular housing from 2 to 3 electromagnets are installed, one of the electromagnets is directly connected to the power source, while the other electromagnets are connected via power relays [ CN101708477 , IPC B02C17/00, May 19, 2010].
This method is aimed at creating additional kinetic energy of grinding bodies by raising them to a great height by the action of electromagnets, but the disadvantage of this method, related to the static position of electromagnets, is the impossibility to eliminate "dead zones", the absence of which primarily determines the energy efficiency of the plant.
Another known method involves grinding the materials in a rotating drum of a mill by magnetically soft grinding bodies, for example, balls, wherein the balls are captured and raised to an optimal height by the magnetic force of magnets mounted on the drum, after which the balls are dropped into the drum cavity, when the electromagnets are disconnected from the power source, wherein the electromagnets are mounted on the drum along the helical line with a shift by the angle α, wherein the electromagnets, moving down to the lower part of the mill, are connected to a power source in the range of the central angle β = 10÷40° when measuring the angle from a vertical ray with a vertex coinciding with the axis of rotation of the drum against the direction of movement of the drum, and the electromagnets, which are rising up, are disconnected from the power source in the range of the central angle y = 90÷135° when measuring the angle from a vertical beam in the direction of rotation of the drum, where the angle α is determined by the expression: α = k(0.95÷1.05)Π/n-1, where k is an odd integer, n is the number of electromagnets.
The disadvantage of this method is low energy efficiency resulting from the lack of the opportunity to use the energy accumulated in the electromagnets. Besides, low intensity of grinding rocks in the lower part of the mill is caused by the absence of processing in this zone and the capture of a small amount of grinding bodies, resulting from the presence of only one long constant magnetic pulse.
The closest in terms of technical essence and purpose is the method of magnetomechanical grinding of materials by ferromagnetic grinding bodies, involving the rotation of the mill drum with n = 20/√D, where n is the number of revolutions per minute; D is the diameter of the drum, m, with two electromagnets diametrically mounted on it (through 180°), wherein the poles of magnetic fields of electromagnets in the process of movement in the zone of the lower hemisphere of the drum are directed into the cavity of the mill drum to ensure additional to earthly gravitation acceleration of balls during their falling onto the material and the lifting of the balls to an optimal height, wherein the magnetic field of each electromagnet is formed by separate pulses in the quantity of not less than two, wherein the duration of the first pulse is equal to the duration of the movement of the electromagnet with the mill drum from the left end of the horizontal (the beginning of angular coordinates = 0°) down to the point with a coordinate from 30 to 50°, and the second pulse is given the duration equal to the time of the movement of the electromagnet up from the point with a coordinate, which is within the range of 100-140°, to the right end of the horizontal (coordinate 180°), and each of these magnetic pulses is divided into a number of shorter pulses, the length of which is equal to, for example, the period between the end and the excitation of the short pulse [ RU2319546 , IPC B02C19/00, November 8, 2005]. This solution is selected as a prototype.
The disadvantage of this method is low efficiency caused by incorrectly selected limits of switching the magnetic field of electromagnets on and off. Besides, the splitting of the second pulse, generated by the electromagnets, into a number of shorter pulses makes it impossible to capture and raise the grinding bodies to the required height, which calls the efficiency and implementability of the method into question.
The common disadvantage of all known methods is the absence of the opportunity to use the transformation of the energy of magnetic field into the kinetic energy of grinding bodies, which leads to a relatively disordered movement of grinding bodies and the dissipation of the energy that determines the low efficiency of the operation of ball mills.
The purpose of this invention is to improve the energy efficiency of the process of grinding materials in ball mills.
This task is solved by grinding the materials in a rotating drum, partially filled with ferromagnetic grinding bodies, wherein the grinding bodies are affected by the magnetic force of at least one electromagnet, mounted on the drum, and the magnetic field is formed by separate pulses in the quantity of not less than two, wherein the first pulse is divided into a number of shorter pulses. In contrast to the prototype, the first pulse is characterized by alternating-sign magnetic field, it is fed at an angular position of the electromagnet from -60° to -30°, where the central angle begins from a vector directed from the axis of rotation of the housing vertically downwards, the second pulse is characterized by constant magnetic field, it is fed at an angular position of the electromagnet from 0° to +30°, and its feeding is stopped at an angular position of the electromagnet from 110° to 165°.
The switching on and off of the magnetic field of the first pulse and the switching on of the magnetic field of the second pulse are implemented in a time not exceeding 0.21√D sec, where D is the diameter of the housing, m, to use the recovery of the energy of the magnetic field into the kinetic energy of the grinding bodies. Positive limiting ensures the timely receipt of the pulse by a grinding body; negative limiting is imposed by electronic equipment in the process of the physical implementation of the method.
The end of the first pulse can be considered to be the beginning of the second pulse, although this is not a prerequisite for the implementation of the method.
The limit of the beginning of the first pulse is determined by the degree to which the mill is filled with the material. The slope angle is usually close to 45°, the optimal fill factor is 40-50%, therefore, the angle of the beginning of the first pulse is -45°. Given the possible overload operation of the mill, the variability of slope angle for different rocks, as well as the need for reserve, let us determine the start of the operating range to be at -60°. The second limit of the range of the beginning of the operation is determined by the smallest possible slope angle for a ball mill with the minimal load as -30°.
The feeding of the second pulse (retention mode) is implemented near the range from 0° to 30°. The main criterion for choosing the angle of retention is getting into the beginning of the formation of the "dead zone". The choice of this angle is finally determined in the process of mill setting for a specific rock to be ground and also depends on the ratio of mill dimensions, dimensions of electromagnets, and the number of grinding bodies.
The magnetic field of the second pulse is switched off through a series of stepwise reductions of the value of the magnetic field by 6-15% of the original value of the magnetic field. The total time of switching off is not less than 0.3√D sec, where D is the diameter of the housing, m. The described law of changing the magnetic field of the second pulse ensures the elimination of simultaneous dumping of all grinding bodies captured by an electromagnet. Dumping becomes stretched in time, which ensures the protection of the suspension of the mill. Besides, time-stretched flow of grinding bodies significantly increases the intensity of grinding, while the specific energy consumption for grinding is significantly reduced.
The number of electromagnets depends on their characteristics, on the diameter of the mill, and on the quantity and composition of the rocks in the mill. Even installing a single electromagnet in each section of the mill will lead to a positive result. To achieve the maximum effect, such a device should have effective coverage close to the radius of curvature of the mill housing. Fulfilling these requirements results in the high cost and increased dimensions of the device. Placing N electromagnets over the whole circumference of the mill housing leads to a reduction of the requirements to the size of the magnetic field of each electromagnet, and, consequently, to a reduction of the dimensions, cost and energy consumption. For example, if the number of electromagnets is equal to twelve, the size of the effective field of each electromagnet is close to R/2. The maximum number of electromagnets is not limited. It is preferably to place 3-4 electromagnets in each cross section of the mill.
The claimed method as compared to the prototype is characterized by a number of new essential features:

new modes of switching the magnetic field of the electromagnet on and off, ensuring an effective impact on the grinding bodies and the mill feed material;
using alternating-sign magnetic field to generate the first pulse;

The comparison of the claimed method with the known method makes it possible to conclude that the claimed solution meets the criterion of "novelty".
Earlier, in the process of grinding materials in ball mills electromagnets were used solely for the purpose of changing the trajectory of grinding bodies and eliminating the "dead zone" formed in the lower part of the mill. The present invention makes it possible to improve the efficiency of ball mills due to the recovery of the energy of the magnetic field, accumulated in electromagnets during each cycle of their operation, into the kinetic energy of grinding bodies, which contributes to the most efficient grinding of rocks. The application of alternating-sign magnetic field makes it possible to provide conditions for the development of defects in the pieces of magnetic rocks along the boundary lines between various media, as well as the mechanical impulse impact due to the alternating-sign forces (attraction and repulsion), acting on the grinding bodies in the alternating-sign magnetic field. Using alternating-sign magnetic field, ensuring the recovery of the energy of the magnetic field into the kinetic energy of grinding bodies, for the purpose of improving the energy efficiency of ball mills, is new and ensures achieving the claimed result. Besides, using alternating-sign magnetic field solves the problem of the residual magnetization of the core, the presence of which means the persistence of the force of attraction of grinding bodies to the electromagnet, which amounts to 30-80% of the initial force of attraction, which in turn reduces the kinetic energy at the point of the collision of a grinding body with the ore.
The foregoing makes it possible to draw a conclusion that the claimed technical solution meets the criterion of "inventive level".
The applied method can be successfully used in the processes of the preparation of raw materials for beneficiation, as well as in construction, chemical and other industries. The method is implementable under actual operation conditions, with the use of known materials and devices.
It makes it possible to conclude that the claimed solution meets the criterion of "industrial applicability".
The claimed method is implemented in the following way. Electromagnets revolve together with the movable mill housing at a certain speed. The operation of electromagnets is synchronized with their angular position in relation to the gravitational vector, because it is one of the main forces, on which the operation of a ball mill is based. To be definite, let us assume that the reference point (0°) is the angular position of an electromagnet, coincident with the extreme lower position of the movable housing. A radius, drawn to this point from the axis of rotation, coincides with the gravitational vector of an arbitrary body, located at a point of the axis of rotation of the movable housing. The positive reference direction of the angular position is in the direction of travel of the housing.
The first period of the operation of an electromagnet is aimed at processing the material by intensive alternating-sign magnetic field to provide conditions for the development of defects in the pieces of magnetic rocks along the boundary lines between various media, as well as the mechanical impulse impact due to the alternating-sign forces (attraction and repulsion), acting on the grinding bodies in the alternating-sign magnetic field. This objective can be achieved only if there are rocks and grinding bodies in the electromagnet coverage area, so the beginning of the first period of the operation of the electromagnet is its angular position from -60° to -30° depending on the properties of the raw material (various slope angle). The time of completion of this period coincides with the beginning of the second period.
When an electromagnet moves into the zone where the thickness of the rocks is significant, the impulses of the forces acting on the grinding bodies are inefficient, while an important goal is the prevention of the emergence of the "dead zone". To achieve this objective, the grinding bodies and the rocks, clamped between them, should be drawn to the electromagnet. This period begins within the range of electromagnet positions from 0° to +30°. In this mode the electromagnet creates a constant magnetic field with a maximum intensity.
The completion of the second period is determined by the selected speed of rotation. At low speed, the switching off of the electromagnet is carried out at high altitude (in an angular position up to 165°), at high speeds of rotation - from 110°. After switching off the magnetic field, rocks move along a parabola, while grinding bodies and conductive pieces of rocks, in addition to the forces of inertia, receive an additional momentum due to the interaction of currents, generated inside the grinding bodies by the specified law of the decrease of the magnetic field with this field.
In previously known ball mills, the energy is primarily consumed by abrasion (the process is characterized by high energy consumption and low degree of the destruction of rocks), and only a small portion of energy is used to destroy the rocks. The efficiency of the grinding process is in this case equal to 1-2%. The claimed method makes it possible to increase the efficiency of the grinding process to 2-3%, in other words, 1.5-3 times.
For the purpose of debugging of hardware modes and studying the effectiveness of the mill equipped with electromagnets according to the claimed method, a model of ball mill with a housing having a diameter of 1 m was made. In the course of the experiments the rotational speed of the mill housing ranged from 0.05 to 0.5 rps. The switching on and off of the magnetic field of the first pulse is implemented during 0.05 sec. The switching off of the magnetic field of the second pulse is implemented through a series of partial stepwise reductions of the magnetic field, wherein the total turn-off time was 0.3 sec. Two diametrically opposed electromagnets are used. The length of the mill is 0.8 m. The mill was filled with steel grinding bodies 050 mm in the amount of 1.2 t. During the experiments titanium magnetite ore from the Kachkanar deposit with a grain size of 5 mm and copper-zinc ore from the Karabash deposit were processed. The time of processing in all the experiments amounted to 30 minutes.
The comparison of the effectiveness of the operation of the mill was carried out by three experiments for each of the two types of raw material:

in the first experiment (experiment 1) electromagnets were disconnected, and the mill worked in a cascade mode.
in another experiment (experiment 2) the method described in the prototype was implemented.
in the third experiment (experiment 3) electromagnets worked in accordance with the claimed method, and the modes of operation were optimized for specific raw material.

The results of the comparison are shown in Table 1, from which it can be seen that the implementation of the proposed method makes it possible to increase the energy efficiency of a ball mill 1.68 times as compared to the prototype, in case of grinding the ore from the Kachkanar deposit, and 2.22 times in case of grinding the ore from the Karabash deposit.

Table 1

Method for grinding materials in a rotating drum by ferromagnetic grinding bodies
Raw material	No. of experiment	Energy consumpt ion, kWh	Yield, class 74 µm,			Specific energy consumption , kW/kg
Raw material	No. of experiment	Energy consumpt ion, kWh	kg	-	%	Specific energy consumption , kW/kg
Kachkana r ore	Experiment 1	5.1	108	-	27%	0.047 kW/kg
	Experiment 2	7.8	210	-	52%	0.037 kW/kg
	Experiment 3	7.4	340	-	85%	0.022 kW/kg
Karabash ore	Experiment 1	4.9	188	-	47%	0.026 kW/kg
	Experiment 2	6.9	170	-	42%	0.040 kW/kg
	Experiment 3	6.5	360	-	91%	0.018 kW/kg

Claims

A method for grinding materials in a rotating drum by ferromagnetic grinding bodies, which are affected by the magnetic field of at least one electromagnet, the magnetic field of which is formed by separate pulses in the quantity of not less than two, where the first pulse is divided into a number of shorter pulses, wherein the first pulse is characterized by alternating-sign magnetic field and is fed at an angular position of the electromagnet from -60° to - 30°, where the central angle begins from a vector directed from the axis of rotation of the housing vertically downwards, the second pulse is characterized by constant magnetic field and is fed at an angular position of the electromagnet from 0° to +30 °, and its feeding is stopped at an angular position of the electromagnet from 110° to 165°.
The method of claim 1, wherein the switching on and off of the magnetic field of the first pulse and the switching on of the magnetic field of the second pulse are implemented in a time not exceeding 0.2√D sec, where D is the diameter of the housing, m.
The method of claim 1, wherein the end of the first pulse is considered to be the beginning of the second pulse.
The method of claim 1, wherein the magnetic field of the second pulse is turned off through a series of stepwise reductions of the value of the magnetic field by 6-15% of the original value of the magnetic field, wherein the total time of switching off is not less than 0.3√D sec, where D is the diameter of the housing, m.
The method of claim 1, wherein the grinding bodies in each cross section of the mill are affected by the magnetic field of twelve electromagnets.
The method of claim 1, wherein the grinding bodies in each cross section of the mill are affected by the magnetic field of three or four electromagnets.