Ore particle size has a significant effect on the magnetic properties of the iron ore. The figure below shows the relationship between the specific magnetic susceptibility, coercivity and particle size of magnetite.
As can be seen from the figure below, as the particle size decreases, the specific magnetization coefficient x of magnetite decreases correspondingly, and the coercive force increases. This relationship is evident when the particle size is d < 40 microns, and is more pronounced at particle sizes d < 20 to 30 microns.
Since the magnetite particle size reduction, susceptibility lower than that, the magnetic decreased, increase in the production of metal loss; however, a large coercive force of the magnetite fine particles, there is a large residual magnetization, remanence agglomerates, ore The finer the particles, the more pronounced the phenomenon of magnetic agglomeration, which can make the fine ore become a " magnet " or " magnetic flux " . This magnetic agglomeration is beneficial to reduce metal loss. However, magnetic agglomeration is not good for improving the grade of concentrate, which brings trouble to the stage grinding and needs to take demagnetization measures.
   The shape of the ore also has a certain influence on the magnetism of the magnetite. Because the ore particles of the magnetite are magnetized in the magnetic field, the magnetic domains inside the ore particles are regularly arranged along the direction of the external magnetic field, and an additional magnetic field opposite to the direction of the external magnetic field is generated, so that the magnetization magnetic field strength inside the ore particles is weakened. This additional magnetic field is called the demagnetizing field, or the degaussing field, using H. Said, as shown below.
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   Due to the existence of the demagnetizing field, the effective magnetizing magnetic field strength acting inside the ore should be the difference between the external magnetic field strength and the demagnetizing field strength, ie
H effective = H outside - H retreat
In the formula, H is effective - the effective magnetization magnetic field strength inside the ore, an / m;
H outside - external magnetic field strength, safety / meter;
H retreat - the strength of the demagnetizing field produced by the ore itself, An / m.
The research shows that under the action of the same external magnetic field, the specific magnetic susceptibility after the mineralization of the long strips is larger than that of the spherical ore particles, and the longer the value is, the larger the value is. Because the distance between the N pole and the S pole formed at both ends of the elongated strip is large, the strength of the demagnetizing field inside the ore is reduced, so the effective magnetic field strength inside the ore is large, and the degree of magnetization is high. The magnetic properties shown are stronger than the relatively short spherical ore particles.
According to the practice, when a uniform magnetic field in the magnetic mineral particles generated demagnetizing field intensity H. And the magnetization is proportional to H ore particles back. which is:
H = -NJ back
Where N is called the demagnetization coefficient or degaussing coefficient, which is the proportional coefficient (or degaussing coefficient) related to the shape of the ore. The longer the ore, the smaller the N value.
A negative sign indicates that the direction of the H pass is opposite to the direction of J.
How the demagnetization coefficient N is determined? The T researcher has done a lot of work to measure and calculate the demagnetization coefficient values ​​of some different shapes of objects. The real side values ​​of the demagnetization coefficients for objects of different shapes are listed in the table below.
Demagnetization coefficient values ​​of different shapes | |||||
Relative length | Â Demagnetization coefficient value N | ||||
  l   | Ellipsoid | Cylinder | Prism, the bottom is | ||
√s | 1:01 | 1:02 | 1:04 | ||
10 | 0.255 | 0.23 | 0.225 | 0.215 | 0.2 |
8 | 0.42 | 0.305 | 0.295 | 0.29 | 0.275 |
6 | 0.64 | 0.47 | 0.45 | 0.43 | 0.405 |
4 | 1.08 | 0.785 | 0.75 | 0.72 | 0.68 |
3 | 1.3 | 1.08 | 1.04 | 1 | 0.94 |
2 | 2.18 | 1.6 | 　 | 　 | 　 |
1 | 4.19 | 3.5 | 　 | 　 | 　 |
S - the cross-sectional area perpendicular to the direction of the external magnetic field. [next]
( 1 ) along with the size ratio m ( =
The increase in demagnetization coefficient is gradually reduced. When the size is small, the geometry of mineral particles has a great influence on the value of the ratio of demagnetizing factor N m. This effect increases as the particle size ratio of m ore is gradually reduced. For example, when m>10 , the demagnetization coefficient N values ​​of the ellipsoid, the cylinder and various prisms are very similar. Therefore, when When the value is large, it can be considered as N ≈ 0 . Therefore, it can be explained that the factor affecting the demagnetization coefficient N of the ore is firstly the size ratio of the ore particles, not the shape of the ore particles.       ( 2 ) When
When the values ​​are the same, the magnitude of the demagnetization coefficient N values ​​of different geometries is: ellipsoid > cylinder > prism.    It should be noted that the data listed in the above table is the demagnetization coefficient of the regular geometry. But in fact the shape of the ore is irregular. In addition, the data in the above table is measured in a uniform magnetic field, and in fact the magnetic field of the magnetic separator is an inhomogeneous magnetic field. In the ore dressing production, the mineral grain l
Or the nuggets generally have a slightly longer direction, so √ s ≈ 2 can be taken ; therefore, when calculating the demagnetization coefficient of the ore particles, N ≈ 0.16 can be taken .
l l
The data listed in the above table is the demagnetization coefficient N value as a function of √ s . √ s is called the size ratio and is expressed in m .
The influence of the shape of the ore particles is not considered in the above-mentioned volume magnetic susceptibility x 0 of the object or the specific magnetic susceptibility x of the object. In order to eliminate the influence of the shape of the ore particles on the magnetic properties, the ratio of the magnetization to the effective magnetic field strength acting inside the ore particles is used instead of the ratio of the magnetization to the external magnetic field strength when comparing and evaluating the magnetic properties of the mineral. This ratio is called the material susceptibility. It also divides the volume susceptibility K of the substance and the specific susceptibility of the substance x , respectively:
   It can be seen from the above formula that when the demagnetization coefficient of the ore particles is N<<1 , or the material magnetization coefficient K and x of the ore particles are small, that is, the magnetic properties of the mineral particles of the ore particles are weak (such as weak magnetic minerals) . 0 , x 0 and the susceptibility of the material are approximately equal in value. The strong magnetic mineral with a certain shape has a large difference between K and x and K 0 and X 0 . The magnetization coefficients of the ferromagnetic minerals listed in the literature generally refer to the material ratio magnetization coefficient.
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