Cement, Energy and Environment

design, increases cases strength, and reduces dust formation and the scope of maintenance. The conical casing dramatically reduces and simplifies scheduled charging and recharging of the ball fill. World practice in charging balls (Table2) advises a stagewise decrease of ball size per chambers with a constant value of Ll¢=3%. In a one– chamber TCM, this concept is implemented automatically. In doing so, the upper layer of balls is arranged horizontally over the entire casing length. This simplifies the involved and time– consuming process of additional charging of the stopped mill be setting up continuous additional automatic charging by simply dropping a maximum- diameter ball into the chute of an operating mill at intervals depending on ball wear per tonne of ground material. As the 13 12~ ball wears, it will move along the cone and be replaced with a next new one. In the CIS countries, additional ball charging is scheduled for every 9 days {this schedule, as a rule, is not observed because of time consuming operations and mill stopping). However, in only 9 days, the wear and tear of the entire ensemble of balls reduces the productivity by 13.7% (Figure 5). A critical and not yet solved problem is choosing a ball grade generally related to the diameter of the average weighted ball daw. dl·ql +d2·q,+ ··· +dn·qn do.,= . - ql +q2+...+qn (5) Where q is the weight of balls of respective diameters included in the grinding charge (t): d is ball dia. (mm). The grade of grinding bodies should ensure an effective grinding charge, which would create a big material contact surface, prevent free passage of the material through the spaces between the balls without grinding, and at the same time, ensure passage of ground size fractions along the mill. In the TCM, this task is solved by automatic self-regulation of the average weighted ball irrespective of the initially chosen ball grades. The diverse character of grinding (Table 2) over the length (of the chambers) in classical TBMs results in irregular wear of grinding bodies. Th is is especially obvious in ore rod mills. Machined rods, representing an integral fixed– length grinding body, undergo wear to a conical shape, i.e. the wear intensity in the end of the mill is higher than its beginning. The conical casing, which ensures optimal ball operation Figure 8. Process diagram of a closed-cycle grinding plant with a 4413.5 m mill. ! 1 - bag collector; 2 - separator with external cyclones (N rotor = 180 kW, fan Q = 1SO 000 m'/hr and N = 250 kW); 3 - elevator (Q = 470 tph, l N = 75 kW); 4 - air chutes; 5 - 4 x 13.5 m mills, N = 3200 kW; 6 - aspiration column; 7 - 6 x 800 mm dia. cyclones; 8 - bag fllter; 9- fan (N = 125 kW, Q = 60 000 m'/hr); 10 - expander-tube pumps (Q = 1SO tph); 11 - belt conveyor ; 12 - clinker and additives batchers; 13 - fan (N = 55 kW, Q = 25 000 m 1 /hr). 19