Magnetic separation column | Grade elevator

Introduction of magnetic separation column

Magnetic gravity separators have been used since the 1960s and 1970s, and have been used as equipment for rough separation ocoarse grinding products of magnetite and fine grinding products for desludging. Early magnetic gravity separators were uppermagnetic electromagnetic dehydration (desludging) tanks. In the 1960s and 1970s, ferrite magnets (barium ferrite and strontium ferrite)appeared, and the magnetic source was changed from electromagnetic excitation coil to magnet pile source. There are two types of magnet pile sources: upper magnetic type and lower magnetic type. In the 1980s, the "magnetic agglomerator" appeared, which improved the lower magnetic magnet pile (magnetic tower) into a multi-circle and multi-layer annular magnet pile, and the trough body was also improved from a single inverted frustum trough body to a composite trough body combining a straight cylinder and a cone. The water supply method was also improved from the upward flushing water supply pipe water supply method with a water-incoming cap at the bottom to the lower cone multi-tube tangential water supply method, which produced water line rising fluid mechanics, and the separation performance was greatly improved. The original magnetic dewatering trough was only able to separate ore mud and a small amount of fine-grained gangue, and it was improved to not only be able to separate ore mud and a small amount of fine-grained gangue, but also to be able to separate not only ore mud but also a considerable part of fine-grained gangue. The reason is that the distribution space of the magnetic source is larger and has changed, and the degree of magnetic agglomeration dispersion has been improved. The combination of magnetic agglomerator and high-frequency fine screen controls the particle size, which greatly improves the grade of the concentrate.

2. Classification and use of magnetic separation columns

In order to meet the needs of magnetite separation, magnetic separation columns are divided into desliming magnetic separation 

columns, concentrating magnetic separation columns and concentrating magnetic separation columns.

1. BT desliming magnetic separation column

BT desliming magnetic separation column is used for desliming and removing medium and fine-grained gangue from coarse-ground 

magnetite and fine-ground magnetite with high mud content. It is mainly used for roughing of coarse-ground magnetite and the 

first or second magnetic separation of fine-ground magnetite.

2. BT concentrating magnetic separation column

The main function of BT concentrating magnetic separation column is to increase iron and reduce impurities (Si, P, S). BT series 

concentrating magnetic separation columns are used for concentrating operations of fine-ground magnetite. Because it can not 

only efficiently separate ore mud and monomer gangue, but also efficiently separate poor intergrowth. It can be used for the 

following

 purposes:

1) Produce super magnetite concentrate with a grade of 70%~72% from easy-to-select magnetite;

2) Produce high-grade concentrate (65%~68%) from low-grade concentrate (58%~63%)

3) Combine fine screening to take out some qualified concentrate from coarsely ground magnetite concentrate in advance;

4) When the conventional magnetic separation concentrate is selected by the selected magnetic separation column, the final 

grinding particle size can be appropriately coarsened to achieve increased production when the concentrate grade is higher than 

the required grade.

Since the selected magnetic separation column can efficiently separate the conjoined bodies at a coarser grinding particle size, the 

final grinding particle size can be appropriately coarsened when the magnetic separation column is used for selection compared 

with conventional magnetic separation. The coarsening range of -200 mesh or -325 mesh can reach 5~15 percentage points, so 

that the purpose of increasing the final concentrate can be achieved by grinding the raw ore more, and the increase in production 

can reach 10%~30%. This is equivalent to achieving the purpose of capacity expansion without increasing the grinding volume, 

which greatly reduces the cost of concentrate.

3. BT Concentration Magnetic Separation Column

When the magnetic separation column is supplied with less or no water, the underflow concentration of the magnetic separation 

column can reach 60%~75%. Therefore, the concentration magnetic separation column can be used as the concentration 

equipment for the final concentrate of the magnetite beneficiation plant before filtration or high-concentration transportation. 

It has better concentration effect than the magnetic separator, smaller footprint than the thickener, less investment, and lower 

operating costs.

3. Structure, separation principle and characteristics of magnetic separation column

1. Structure of magnetic separation column

Take the fine magnetic separation column as an example. It is mainly composed of a separation cylinder and multiple DC excitation coils with a certain interval outside the separation cylinder, a supporting jacket, an underflow concentration sensor, an automatic electric valve for concentrate discharge, a middle and lower rotating rising water supply device and an intelligent electric control system.

图1—B中：1—给矿管  2—给矿斗  3—溢流槽  4—分选筒  5—上电磁系

        6—外筒  7—支撑板  8—下电磁系  9—给水装置  10—接线盒

        11—传感器  12—电动阀门  13—供电控制系统

2. The separation principle and structural characteristics of the magnetic column

The BT-CXZ magnetic column developed by BATA is an electromagnetic low-weak magnetic field high-efficiency magnetic gravity separator. Because of its shape like a pillar, it is named as a magnetic column after the flotation column.

The magnetic column is controlled by the intelligent electric control system to supply and disconnect power to multiple groups of DC excitation coils on the outside of the separation cylinder. The power supply mode is to supply and disconnect power from top to bottom in sequence, and the cycle is repeated, so that a magnetic field and a magnetic field force that move downward in sequence from top to bottom are generated in the separation chamber. The magnetic field is sometimes present and sometimes absent, sometimes large and sometimes small. At the same time, a high-speed rotating rising water flow is introduced in the middle and lower part of the magnetic column to wash upward with ore mud, single gangue and intergrowth, especially poor intergrowth. 

The overflow trough at the top of the magnetic separation column overflows the magnetic separation column tailings (or middlings) which are mainly composed of ore slime and single gangue, supplemented by intertwined bodies. At the same time, the magnetic field that moves downward intermittently makes the magnetic mineral particles aggregate and disperse in the form of magnetic chains in a sometimes strong and sometimes weak form; when there is no magnetic field or the magnetic field is weak, the magnetic chains are broken up by the high-speed rotating rising water flow, and the ore slime and single gangue mixed in them are dispersed and carried upward by the rising water flow, and the tailings of the magnetic separation column are produced at the top. This process is repeated many times. Each time the magnetic chain is formed, part of the ore slime, single gangue, and poor intertwined bodies are placed in the gap of the magnetic chain and carried upward by the rising water flow to form tailings, so that the downward magnetic materials are refined once, and the grade is improved once, and finally high-grade magnetite concentrate or even super magnetite concentrate is produced at the bottom of the magnetic separation column. The grade of the magnetic separation column is greatly improved, and the grade improvement can reach 2~15 percentage points according to the situation of the selected materials.

The magnetic column has a simple structure, no moving parts, and almost zero maintenance and repair workload. It is easy to operate, and the slurry water flow basically moves vertically with almost no wear. The power consumption is low, only 0.1-0.2kwh/t, and the degree of automation is high. The concentration signal of magnetite can be picked up by the concentration sensor, and the system issues a command to adjust the opening of the concentrate valve by adjusting the electronic actuator, so as to achieve highly autom aticcontrol, strong adaptability to small fluctuations in the feed rate, and high and stable indicators. Including current intensity, power supply cycle, underflow concentration, valve opening, etc. are all displayed on the display screen. The operating status of the magnetic column can be easily perceived by observing the parameters on the display screen.

4. Main technical parameters and calculation of magnetic separation column

There are 6 main technical parameters of magnetic separation column, namely magnetic field intensity, magnetic field conversion cycle, rising water volume (determines rising water speed), concentrate discharge concentration, feed concentration and feed volume.

1. Magnetic field intensity (H)

The magnetic field in the separation chamber of the magnetic separation column is generated by the excitation coil on the outside of the separation cylinder. The magnetic field intensity in the excitation coil with limited axial height is uneven along the axial and radial points. The axial coil height midpoint is the strongest, and it decreases rapidly with the increase of distance upward and downward from the midpoint; the radial magnetic field radiates from the axis to the surroundings within the coil height range with the increase of distance (close to the inner wall of the coil), and the magnetic field at the inner wall of the coil is the strongest. The magnetic field on both sides of the upper and lower ends of the coil decreases with the increase of distance upward or downward, and the magnetic field at half the radius of the coil is higher.

The distribution of isomagnetic field lines of the magnetic separation column is shown in Figure 2, taking the BT600 magnetic separation column as an example.

The magnetic field strength at a point on the axis of the coil is calculated by the following formula:

Where: H—magnetic field strength at a point on the axis of the multilayer solenoid coil, Oersted;

n—number of turns per unit length of the coil axis, turns/cm

i—current strength of the coil, ampere

r—inner radius of the coil, cm

R—outer radius of the coil, cm

L1—distance from a point (measurement point) on the axis of the coil to the upper end of the axis of the coil, cm

L2—distance from a point on the axis of the coil to the lower end of the coil, cm

The magnetic field at each point on the axis of the coil is symmetrical up and down with the midpoint as the boundary. It is only necessary to calculate the magnetic field values of a dozen above the midpoint of the axis to understand its magnetic field distribution.

2. Magnetic field conversion cycle (T)

The so-called magnetic field conversion cycle is the time taken for multiple groups of coils to be powered on and off from top to bottom for one cycle, measured in seconds, and represented by T. The height of T is related to the specifications of the magnetic separation column. The large-size magnetic separation column has a long cycle, and the small-size magnetic separation column has a short cycle. The cycle of industrial application magnetic separation column (Φ500～Φ3000mm) is 5～10 seconds.

3. Rising water velocity (V) and water supply (W)

The magnetic separation column is supplied with water by the middle and lower water supply device. The water supply to the magnetic separation column is divided into two streams, one of which is upward to generate rising water velocity, and the other is downward and discharged from the concentrate discharge valve together with the concentrate. The total water supply is the sum of the upper and lower streams.

The amount of water going upward is determined by the required rising water velocity and the water-passing cross-sectional area of the magnetic separation column:

Wup = 3600VS/106, m3/h

The amount of water going downward is determined by the amount of concentrate and the concentration of concentrate discharge:

Wdown = Qk (1-C)/C, m3/h

W= Wup + Wdown, m3/h

Where: Wup—the amount of water that forms the rising water velocity upward;

Wdown—the amount of water carried out by the concentrate;

W——total water consumption of the magnetic separation column;

V——average rising water velocity, cm/s;

S——water-passing cross-sectional area of the magnetic separation column, cm2;

C——concentrate discharge concentration;

Qk——concentrate discharge, t/h

The concentrate discharge concentration generally takes a high value for coarse particle size and a low value for fine particle size.

The water-passing cross-sectional area of the magnetic separation column is consistent with the cross-sectional area of the inner cavity of the separation cylinder.

4. Feeding capacity (processing capacity) of magnetic separation columnThe processing capacity of magnetic separation column is related to the particle size and magnetic strength of the feed ore. Generally, the processing capacity is high for ore with strong magnetism and coarse particle size, and low for ore with fine particle size and weak magnetism. It should be pointed out that when the middling ore of magnetic separation column needs to be re-grinded and re-selected, the processing capacity also includes the return amount of middling ore.

5. Selection and specifications of magnetic separation columns

1. Selection principles and key points of magnetic separation columnsMagnetic separation columns should be selected according to their purpose. Generally, the materials for roughing and coarse selection have more mud and more individual gangue. At this time, it is better to select desludging magnetic separation columns; 

if the fine grinding magnetic separation concentrate with less mud and relatively less individual gangue is mainly used to separate lean intergrowths, it is appropriate to select fine magnetic separation columns. The unit processing capacity of coarse particle size and strong magnetism is selected as the high value, the fine particle size (-200 mesh>90%~95%) is selected as the middle value, and the fine particle size (-325 mesh>70%~95%) is selected as the small value.

The processing capacity is calculated according to the following formula:

Q place = qs, t/h

Where, Q place——the processing capacity of magnetic separation column (including the return of medium ore), t/h;

q —the processing capacity per unit area (cm2) of magnetic separation column.

For example: a finely ground magnetic concentrate has medium magnetism but fine particle size, -200 mesh content ≥95%, the tailing volume of the small test magnetic separation column is about 10%, and when the new feed volume is 30t/h, the type and number of magnetic separation columns are selected.

① Since the feed is finely ground magnetic concentrate with fine particle size, BT type magnetic separation column is selected;

② The feed rate is 30t/h. Considering the return of medium ore for re-grinding and re-selection, the processing capacity is calculated as follows according to the small test medium ore volume of about 10%:

Q = 30 × 1.2 = 36t/h

③ Selection of specification unitsSince the particle size is fine and the magnetism is medium, q = 0.004t/cm2h is determined, and the cross-sectional area of the required magnetic separation column is:

S = Q / q = 36 / 0.004 = 9000cm2

Calculate the specifications of the magnetic separation column from the required cross-sectional area of the magnetic separation column:

S = πR2

S = πR2 = 9000 c㎡→D=2R→R=53.54cm→D=107.075cm

According to the required cross-sectional area of the magnetic separation column, there are two options for the specifications of the magnetic separation column:

A: Select a BT120 magnetic separation column with a diameter of Φ1200mm, whose cross-sectional area is 11304C㎡, which is 1.256 times the required one.

B: Select two BT80 magnetic separation columns with a diameter of Φ800mm, whose cross-sectional area is 5024×2=10048 c㎡, which is 1.12 times the required one.

Both options are acceptable.

6. Specifications and performance of BT selected magnetic separation column

Specifications and performance of BT selected magnetic separation column are shown in the table below

Note: ① In the column of processing capacity: the low value is selected when the feed particle size is fine, the high value is selected when the particle size is coarse, and the middle value is selected when the particle size is about 85% of -200 mesh;

② The height of the main engine in brackets is the dwarf design height.