The Mekhanobr Institute’s research has highlighted a comprehensive set of measures to improve the productivity of grinding sections of processing plants, including those related to discharge grates. It has been shown that with increasing mill size, a number of unfavorable technical and technological factors emerge, leading to a series of ball mill redesigns to convert them to central discharge.

The above circumstances, as well as other cases where the mesh size of an autogenous mill grate is 60×60 mm, and no grains larger than 10 mm are observed in the mill discharge, served as the basis for conducting specialized research.

The main objective during the research period was to evaluate the influence of key grate parameters (the configuration and size of the openings, their placement along the end face, and the design of the lifters) and process factors (the content of fines in the mill) on its overall throughput and diamond throughput in particular.

• Round openings have the lowest throughput;
• The throughput of short openings (a, c) is 5-8 times higher than that of traditional conical openings (b, d);
• The throughput of radially located slots is maximum;
• The throughput of openings located downstream of the lifter (radial rib) (in the «shadow zone») is 5-6 times lower than that of openings located upstream of the lifters;
• The average particle size of the waste discharged through the screen at the periphery of the drum (belts 10, 11, 12) is 1.5-2 times lower than that at the center of the drum (belts 1, 2, 3);
• Overall, the throughput of a screen with «regular» mesh sizes is virtually independent of the «live» cross-sectional area.

   For example, when the cross-sectional area of the grate was increased by a factor of 10, the throughput remained virtually unchanged. However, the particle size distribution of the ground product changed significantly: the yield of fine particles (0.07%) decreased by a factor of 2.5 (from 67% to 27%). Therefore, by selecting the mesh size, it is possible to influence the granulometry of the finished product without reducing the grate throughput.

Based on the above, the design of the grate for an autogenous grinding mill, a sketch of which is shown in Figure 2, is quite clear. It must meet the following requirements:

• The screen openings must be slotted and located along the radius;
• The thickness of the screen plate in the area of the slot must not exceed the slot width, and the slot length must not be less than three times the width;
• The slot width must vary from the periphery to the center, depending on the granulometry task;
• The entire surface of the screens must be equipped with heels, with the most exposed part (levels 5-9) equipped with large heels, which protect the surface from large particles while allowing fine material to slide freely along the screening surface, finding its slot;
• The size of the «cross-sectional area» should not be decisive, but it is important to keep in mind that the configuration and dimensions of the slots must meet the above conditions;
• The strength characteristics of the screen are ensured by the closed-beam structure of the inner surface facing the unloading chamber and the heel structure of the outer surface facing the loading window.

   Figure 3 shows a sketch of the recommended ball mill grate. Unlike the traditional grate (Figure 4), the cross-sectional area is reduced by a factor of 3, the taper of the opening is significantly (2 times) larger, and the cell walls run parallel to a depth of 10-15 mm, allowing for a calibrated, predetermined cell width over a long period of time. Unlike the traditional grate, the working surface of the new grate is protected from the impact of the balls by heels positioned so as to ensure constant relative movement of the pulp along the grate. This is actively facilitated by the absence of any radial ribs. The strength characteristics of the new grate are ensured by a closed system of stiffening ribs located on the opposite working side (View A).

Overall, the new grate is 20% lighter than the traditional grate and therefore 20% less expensive.

To ensure the continuous discharge of «inefficient» sized balls, it is possible to select a location on the grate for the opening through which this ball size will be preferentially discharged.

Based on the above data and our own practical experience, we believe that some companies’ replacement of the screen discharge system with a drain discharge system not only reduces mill throughput but also contributes to larger overflows and, most importantly, to overgrinding of the useful component.

The screen proposed for testing, thanks to its increased (several times) throughput, will allow the grinding section to operate with greater circulation, which will further contribute to reducing overgrinding. Adding a cantilever mill with its short drum and a larger screen area (1.5-1.8 times) to this approach will further reduce overgrinding. In this regard, it’s also worth mentioning the new hydrocyclones currently being tested at TTD. Their separation efficiency down to 74 µm suggests that their use alone could increase mill productivity by 5-10%.