THE GRAPHICAL REPRESENTATION OF ASH LIBERATION IN MILLED COAL

The objective in many cases of industrial grinding is not simply to pr oduce fine sizes but to liberate one component from another by breakag e of particles which contain both components locked together. Thus, it is not sufficient to know only the product size distribution, but it is also necessary to know also the range of interlocking within each s ize, called the liberation function for that size. This paper describe s a technique for calculating the function from data where float-sink analysis can be used to separate different compositions and shows how the results can be represented graphically. Various size fractions of a hammer-milled coal and the same coal ground in a ball mill were exam ined to determine the liberation function M(C) for each size. It was f ound to be necessary to fit a three parameter function to points on th e Mayer curve of ash (mineral matter) units floating versus mass float ing. The function was A(M) = k(1)M + k(2)M(n). Since A(M) = integral(0 )(M)CdM(C), then C = dA(M)/dM and the liberation function is M(C) = [( C - k(1))/nk2](1/(n-1)). This technique worked well for the hammer-mil led coal data taken at 1.4, 1.5 and 1.6 specific gravities, and enable d the calculation of the locking index for each product size (root 2 s creen intervals). However, the corresponding data for ball-milled coal were inconsistent, although the change of liberation with increased g rinding could be shown qualitatively from plots of M/(1 - phi) versus C/phi, where phi is the ash fraction of each size.