ON THE ISSUE OF FILTRATION UNITS AT MINERAL PROCESSING PLANTS

B. A. ZEMLYAKOV

(Presented by the scientific seminar of the Department of Mineral Processing)

In many technological processes, the filtration process is widely used; it is an intermediate operation between thickening and drying [1], and most often this process is carried out on continuous vacuum filters.

The arrangement of the equipment of a filtration unit (vacuum filter, receiver, vacuum pumps, and air traps) is usually implemented according to two schemes, depending on whether the vacuum filters are placed on the upper or lower floors of the plants (Fig. 1) [2].

Fig. 1. Diagram of filtration units: a — arrangement of the vacuum filter and receiver at lower elevations; b — at the upper elevations of the plant. 1 — vacuum filter; 2 — vacuum collector (receiver); 3 — centrifugal pump; 4 — filtrate collection tank; 5 — vacuum pump; 6 — pipeline for removal of filtrate; 7 — trap (water separator).

In order that the filtrate, together with the air, does not enter from the receiver into the vacuum pump and put it out of service, the indicated filtration schemes...

...this includes a water seal. When the vacuum filter and receiver are located on the upper floors of a plant, the water seal, in the form of a 10-meter pipe, is lowered directly from the receiver into the filtrate tank (Fig. 1b). When the receiver and vacuum filter are located at the lower elevations of the plant, the water seal is lowered from a special water separator installed above the filtrate tank (Fig. 1a).

In the present work, a scheme is proposed for a filtering installation with a float valve and without a water seal (Fig. 2). According to this scheme, the filtrate from receiver 2 is pumped into filtrate tank 4 by centrifugal pump 3, equipped with a check valve on the discharge line, or by a float-valve apparatus for automatic filtrate discharge of the design of A. N. Stekachev (Institute of Mining, Academy of Sciences of the USSR) [3].

Fig. 2. Scheme of a filtering installation with a float valve and without a water seal:
1—vacuum filter; 2—vacuum tank (receiver); 3—centrifugal pump; 4—filtrate tank; 5—vacuum pump; 6—float valve.

To protect vacuum pump 5 from filtrate entering it together with the air, float valve 6 is installed in the receiver; as the receiver fills with filtrate, the valve rises upward and closes off the vacuum line. After the receiver is freed of filtrate, valve 6 must open the vacuum line, which can be accomplished by selecting the proper weight of the float valve, taking into account the possible maximum pressure drop before and after the valve and the cross-sectional area of the inlet opening closed by the valve.

Another approach may be chosen, whereby the operation of the valve should be interlocked with the operation of vacuum pump 5, so that when valve 6 closes the vacuum line, the vacuum pump is automatically disconnected from it and is switched on only when valve 6 fully opens the vacuum line, i.e., when the receiver is freed of filtrate.

The use of the filtration scheme described makes it possible to install vacuum filters at any elevation in the plant and to avoid installing a water seal. This is a particular advantage for enterprises with shops arranged on one level and on mountain slopes, where, in order to install a water seal, long main pipelines have to be run to higher-level shops or water separators have to be installed high above the filtering-department premises; in the former case this leads to a reduction of vacuum in the system, and in both cases it worsens the operating and repair conditions of the filtering installation.

For dewatering flotation concentrates and slurry at coal-preparation plants, disk vacuum filters are usually used.

Each filter disk consists of metal sectors inserted into the openings of a horizontal hollow shaft.

Usually the sectors are made of steel and consist of a shell covered on both sides with stamped screens 4–5 mm thick, with openings of 5–6 mm. Sometimes these shells are equipped with solid internal partitions for uniform blowing of cake from both sides of the disk. Each sector is covered with a filter screen.

The main shortcomings of metal sectors are: the small open area of the stamped screen beneath the sieve frame, the large volume of the sector cavity, which significantly reduces the effect of blowing, their shortage, the high cost and rapid wear of the stamped screens and filter cloth, and also the great weight of the sector.

Recently, at a number of concentration plants, wooden inserts have begun to be used, inserted into the shell of the sector [4, 5]; these are plates with grooves planed along their surfaces (Fig. 3).

One of the shortcomings of wooden inserts is likewise, as in the case of metal sectors, the small open area of the screen through which the filtration process takes place. This occurs because part of the screen lies on the ribs of the wooden sector and is not subjected to the action of the vacuum.

The width of the ribs is made approximately equal to the width of the grooves or somewhat narrower than them. Therefore the area occupied by the ribs amounts to approximately half the area of the sector.

If the sectors of vacuum filters are made

Fig. 3. Insert of a vacuum-filter sector made of wood.

Fig. 4. Vacuum-filter sector made of plastic with triangular ribbing.

entirely of plastic with triangular ribbing (Fig. 4), then, in the manufacture of each six-disk vacuum filter (with disk diameter up to 1800 mm), this will make it possible to save approximately about one ton of metal and, in combination with capron filter cloth—whose use promotes good cleaning of the sectors and of the screen itself from cake as a result of deformation (stretching) of the cloth during blowing—will lead to a significant increase in the service life of the filtering elements of the filter. The latter circumstance will play a positive role in the process of automating the operation of filtration units.

Conclusions

The use of a filtration-unit scheme with a float valve and without a hydraulic seal makes it possible:

1) to install vacuum filters at any elevation of the plant according to the course of the technological process;

2) to prevent filtrate from entering the vacuum pump together with air;

3) to reduce the consumption of piping, metal for the water separator, and the costs of their purchase and installation;

4) the sectors of vacuum filters should be made of plastics with a triangular form of corrugation, and capron cloth should be used as the filtering fabric;

5) the triangular form of corrugation will make it possible to increase substantially the open area of the screen used for filtration, which will significantly improve the course of the filtration process.

REFERENCES

1. K. A. Razumov. Design of mineral-processing plants. Metallurgizdat, Moscow, 1952.

2. S. P. Artyushin. Operation and adjustment of vacuum filters. Ugletekhizdat, Moscow–Kharkov, 1951.

3. N. A. Saldakov. Float-valve apparatus for automatic start-up of a filter. Gornyi zhurnal, 4, Moscow, 1955.

4. A. P. Zheodev, A. G. Livshits. Use of wooden vacuum filters for dewatering flotation concentrates. TsITI, Moscow, 1954.

5. A. S. Aminov. Possibility of improving the operation of vacuum filters. Coke and Chemistry, No. 1, 1961.