Elective new process technology

After the minerals are charged by various methods, they are sorted in an electric field, mainly because the ore particles are subjected to various electric and mechanical forces, and the resulting trajectories are also different, so that they can be separated from each other. The theory of the electrification process mainly involves three aspects: the first is to generate an electric field suitable for electrification; the second is how to obtain a certain amount of electric charge; and the third is to receive various electric and mechanical forces after being charged. Make it match and achieve sorting.
The most widely used electro-optic practice is the high-voltage corona electric field and electrostatic field, and most of them are non-uniform electric fields, and the drum type electric sorter is the most common and representative one.
First, the factors affecting corona discharge
Corona discharge is a self-sustaining discharge, and electrical selection requires stable discharge, that is, DC discharge that does not change with time, and the negative electrode is most widely used. This type of discharge uses a corona pole with a small diameter and a large curvature, with a high voltage negative or positive charge, and the other pole is a ground electrode, which is opposite to the strip electrode and has a large diameter and a small curvature. The distance between the positive and negative electrodes (called the pole pitch) is very small, often 60 to 70 mm, so that after the combination, it is easy to generate corona discharge. Nowadays, the drum type electric separator widely used in various countries in the world has a corona wire diameter of only 0.2 to 0.5 mm, and a drum diameter of 250 to 350 mm. The ratio of the diameter of the two is 1:1250 or 1:1750. Obviously, the difference between the two is extremely great. If the grounding is extremely flat, the difference is even greater.
From the point of view of mineral processing , this kind of continuous discharge is required to be stable and reliable, that is, a space near the drum or various grounding electrodes can generate a stable space body charge, and it is forbidden to generate spark discharge, which is mainly caused by disorder of space body charge during spark discharge. In practice, it has been repeatedly proved that when spark discharge is generated, the beneficiation index must be reduced. The most prominent factors influencing corona discharge are supply voltage, pole distance, and air humidity.
Voltage: Under the same conditions, the higher the voltage, the greater the corona current on the grounding pole (drum surface or plane), and the relationship curve is shown in Figure 1.

Figure 1 voltage vs. corona current curve

Pole distance: The pole distance is also an important factor that seriously affects the corona discharge current. Under the same voltage, the smaller the pole pitch is, the larger the current is, that is, the current decreases with the increase of the pole pitch. The relationship curve is shown in Fig. 2. Show.

Figure 2 Effect of pole pitch on corona current distribution

Too small and can not be used when the selected electrical pole pitch, though it is subject to a corona discharge, but once the power supply voltage is unstable and contains a small amount of iron ore quality, easy to produce the spark discharge, resulting in disruption of normal electrical selected from . In the production, 60-70 mm pole pitch is often used, and in the laboratory, 50 mm is commonly used.
Air humidity: Air humidity is also a factor affecting corona discharge. Under the same voltage, the greater the humidity of the air, the smaller the current, which is mainly due to the increase in humidity, and the air molecules are not easily ionized by electrons.
Figure 3 is a plot of voltage vs. current for different air humidity.

Fig. 3 Curve of voltage and current of different air humidity 1. Humidity is 12g/m ³ ; 2. Temperature is 29g/m ³ ; 3. Humidity is 75g/m ³ ; 4. Humidity is 120g/m ³

When corona discharges, a light purple glow is produced on the surface of the electrode, like dew drops, while ozone is released, and a sound like a leak is emitted, and the magnitude of the corona current can be read from the milliamp or microampere. Alternatively, the distribution of the corona current can also be measured from the surface of the drum. In short, the higher the voltage, the more obvious these phenomena and situations. [next]
Second, the calculation of corona discharge electric field
The calculation of the corona electric field is more complicated, and no accurate formula is available. This is because the pole distance is small and there are many factors affecting the discharge. At present, there is only a standard cylindrical shape, that is, the charged electrode is just in the circle. The form of the center of the cylinder; the other is that the form of the corona pole and the plane pole can be calculated, and other forms cannot be calculated.
1. Calculation of cylindrical electric field strength and discharge current The calculation of the initial corona discharge electric field strength of this form can be performed by the following simple calculation method.

Where E _K - the initial field strength of the cylindrical corona discharge, kV / m;
δ —— the relative density of air, equal to 1 at standard pressure and 25 °C;
r ₀ ——corona discharge electrode radius, m
If a more accurate calculation method is used, the following formula is used.

Where E is the electric field strength, kV/m;
r ₀ ——corona discharge electrode radius, m;
I - line current density, mA / m;
K - ion mobility, m ² /V•s.
Calculation of the corona pole discharge current in a cylindrical shape:

Where R - cylinder inner diameter, m
u —— applied to the center of the cylinder corona voltage, kV;
u _k - corona discharge starting voltage, kV.

2. Calculation of the electric field strength and discharge current of the surface pole and the corona pole. This form is that the discharge electrode is a sharp-pointed pole or a filament pole, and the grounding is extremely planar, and the electric field strength is calculated as:

Where x is the distance from the center of the corona pole, m;
l - the distance between the two poles, m;
Other symbols are the same as before.
When the plane pole is combined with the corona pole, the current I is calculated as

Its configuration is shown in Figure 4.
Among the above various formulas, except for the calculation of the cylindrical shape is in line with reality, the rest belong to some empirical formulas, and the field strength and current of the commonly used electric sorter are difficult to calculate, so they are determined by the measurement method.

Figure 4 Plane pole and corona pole configuration diagram
1. corona pole; 2. plane pole [next]

Third, the probe method to determine the electric field strength
The probe method introduces a detecting electrode at a point to be tested, and the power source is another high voltage power source. When the high voltage applied to the detecting electrode is changed, the current on the detecting electrode changes, and the detecting electrode is made. After the volt-ampere characteristic curve, the straight line portion of the curve is extended to intersect the abscissa, and the potential value of the intersection point is the potential of the measuring point. Figure 5 shows the volt-ampere characteristic curve of the probe method. After the voltage is obtained, the field strength can be calculated from the above formula.

Figure 5 Detecting the volt-ampere characteristic curve by the probe method

Fourth, the measurement of corona current
The corona pole discharges to the grounding pole, and microcurrent is inevitably generated on the drum surface or the plane pole and other forms of the grounding pole. The magnitude and distribution of such current have a very important relationship with the electrical selection. Because it is related to the amount of charge in the area of ​​the ore particles, in this state of very small pole distance, it can not be calculated at all, only by the method of measurement.
The evaluation is made in the ground electrode paste a copper foil surface, and insulated from the ground electrode, and then this foil microamp meter is connected with a high voltage corona electrode then, emitted from the electrode The electrons will fall on the copper foil and be reflected on the ammeter. The magnitude of the current is the magnitude of the current at the point on the drumhead or plane. Turning the grounding pole can measure the corona or corona. The entire distribution curve of the pole to the ground pole, and so on, can be used to measure the current distribution curve of various electrode structures. The simple circuit is shown in Fig. 6.

Figure 6 shows the grounding pole surface corona current distribution device diagram
1. drum; 2. copper foil; 3. insulating paper; 4. micro-ampere meter

The size and distribution reflected on the surface of the grounding electrode are closely related to the voltage between the electrodes, the pole pitch, the electrode structure, the air humidity, and the size of the corona pole diameter. The voltage is the same, the small polar distance corona current is large, and vice versa; the same pole distance, the higher the voltage, the greater the corona current, and vice versa. The higher the voltage, the greater the distribution of corona current on the drum surface. Fig. 7 is a distribution curve of the corona current on the drum surface when the pole pitch is the same, the voltage is different, and one corona pole is discharged.

Figure 7 Corona current distribution curve with different voltages and the same pole pitch

Obviously, the current at 20 kV is several times higher than the current of 12 kV (the highest point in the center), and the area of ​​action is also significantly increased. The higher the voltage, the more prominent.
The electrode structure is different, and the corona current distribution on the ground electrode surface is also very different at the same voltage and the same pole pitch. Figure 8 shows the curves measured for two different forms of electrode structure. The A electrode structure in the above figure has a static electrode, the corona current (drum surface) is weakened, and the B electrode structure reduces the area of ​​the corona electric field. These are important for us to determine whether an electrode is reasonably applicable.

Fig. 8 The surface curve of corona current on the drum surface with different electrode structure 1. Corona pole; 2. Static electrode; 3. Grounding pole

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