From the analysis of numerous theoretical studies and experimental data summarized in the work of Andreev S.E., Zverevich V.V. and Perov V.A. «Crushing, grinding and screening of minerals,» it is known that, due to the colossal difference in the content of large class in the loading and unloading parts of the drum mill, the efficiency of grinding varies from maximum to minimum. Therefore, according to the authors, «it would be ideal to grind in mills so short that the material would be subjected to the minimum required number of shocks and all the resulting finished product would be immediately removed from the mill.» It would seem an obvious statement, but in practice it is practically not used. As you know, the most characteristic indicator — the ratio of the diameter of the mill to its length (D: L) in practice ranges from 0.3 to 5. Moreover, the ratio of more than one is mainly used for self-grinding or semi-self-grinding mills in Russia, Canada, the USA, for the Scandinavian countries it is characteristic — 0.9-1.1, for South Africa — 0.3-0.5.

The proposed new development largely eliminates the above drawbacks of drum mills, namely, provides a short residence time of the material in the mill and a high throughput — more than 5-8 times that of traditional ones.

Below are sketches of the new Cantilever mill

Pic. 1. Cantilever mill — side view

Piс. 2. Cantilever mill — top view

Pic. 3. Drum section

It can be seen from the figures that the mill consists of a cylindrical drum 1 with end covers 2 and 3, a drive shaft 4 installed in supports 5 and 6, a mill drive consisting of an electric motor 7 and a gearbox 8, as well as a supporting frame 9. In the center of the end cover 2 there is a loading hole made in the form of a conical neck 10. End cover 3 is carrying and is made three-dimensional of two disks 11 and 13. Grid 12 is arranged in disk 11. Cylindrical sleeve 16 is arranged in center of cover 3 to fit drum 1 on shaft 4.

The pulp lifting discharge chambers 14 entering the discharge chute 15 are rigidly connected to the discs 11, 13 and the cup 16, forming a strong structure similar to that of the truss, which is known to be the most rigid. Supports 5 and 6 are fixed and centered on the supporting frame 9, which eliminates any alignment work directly on site.

The mill operates as follows. Rotation of electric motor 7 through reduction gear 8 and shaft 4 rotating in rolling bearings 5 and 6 is transmitted to drum 1 cantilevered by cover 3 at the end of shaft 4. Material and water are continuously fed into the drum through the neck 10 in the end cover 2. The ground product passes through grids 12 into pulp lifting chambers 14, from where it flows into stationary chute 15.

The main advantages of the cantilever mill design over traditional ones are:

1) The use of rolling bearings instead of sliding bearings significantly increases the reliability of mills, significantly reduces operating costs, since their maintenance is reduced only to periodic replacement of grease (these mills do not have oil stations);

2) The drum shaft bearings and the drive are mounted on a single frame with factory alignment, so that installation and commissioning takes no more than 5-7 days. In addition, due to the supporting frame, cantilever mills do not require massive foundations;

3) Cantilever mills have a weight of 30-50%, and the occupied area is 40-50% less than traditional ones, they do not require multi-ton cranes for maintenance, since the most difficult repair can only consist in replacing the lining;

4) Thanks to easy assembly and disassembly, cantilever mills can be easily delivered to fields located in hard-to-reach places and quickly put into operation. Since these mills can replace all crushing stages and the ball grinding stage (in coarse grinding), fast delivery and quick commissioning make them simply indispensable in the development of small deposits;

5) Cantilever mills, for example ball mills, due to the increased diameter, and therefore the increased living area of the lattice, have a throughput 5-8 times higher than traditional ones;

6) Due to the increased diameter of the mill, and therefore the higher kinetic energy of the balls, their size can be significantly smaller than for traditional mills and, accordingly, of higher quality;

7) Cantilever mills, due to the smaller internal volume compared to the traditional one and the same capacity, have a specific productivity 1.4-2 times higher;

8) The absolute rotation speed of the cantilever mill drum is 15-20% lower than the traditional one, which is a positive factor in terms of reliability;

9) Cantilever mills belong to a low-maintenance type of equipment, since the main units are designed for long-term maintenance, for example, drum support bearings — for 50 years, shaft — for 50 years, gear box bearings — for 12-15 years.

10) The use of a special design lining (heel) with heeled elevators increases the productivity of the mill, especially on soft ores, by 40-70%, while energy consumption is reduced by 30-40%.

11) The new grating has a mass of 10-15% less, and, therefore, the cost is lower, it is stronger, has increased throughput, which helps to reduce grinding.

The new ore preparation technology, coupled with new lining and gratings, can give an increase in productivity of up to 80% and energy savings of up to 50%, depending on the structure of the ore.

In general, improving the mill itself, its replaceable elements, technologies (and the reserve here is huge), you can achieve great results in terms of economic benefits. In support of these theses, we offer brief information on the whole range of work carried out in the company «Technique and Technology of Disintegration» (TTD), on improving the design of the mill itself, its internal elements, as well as on the technological technique of combating critical size.

Table 1
Comparative characteristics of self-grinding mills of cantilever and traditional type with equal performance

1. Comparative technical and economic parameters
2. Conventional MMC-7.0х2.3
3. Cantilever MMSK-8.3х2.2
4. Cantilever mill parameters: greater than (+) or less than (-) relative to traditional,%

Table 2
Comparative characteristics of ball mills of cantilever and traditional types with equal performance

1. Comparative technical and economic parameters
2. Conventional MShR-3,6х5,0
3. Cantilever MSHRK-5.7×2.0
4. Cantilever mill parameters: greater than (+) or less than (-) relative to traditional,%