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If our problem had covered a crushing condition calling for 80 instead of 85 percent passing the opening equivalent to the crusher setting, we would have found that our point fell exactly on a curve, regardless of what crusher setting we had selected. This is because all of the family of curves are based on the 80 percent line. Obviously a group of curves might be based on any percentage line, but it is usual practice to choose the 80 or 85 percent values.
It will be noted that the curves bend upward in very marked fashion above the 75-85 percent region. This simply reflects the tendency of practically all materials to slab, or spall, to some extent in the crusher. As a matter of fact, product gradation in this upper range (above the openside setting of the crusher) is of a distinctly uncertain and variable nature, and about all that a group of curves can do is to reflect the general tendency. Fortunately the exact screen analysis in this fraction of the primary crusher output is recrushed in succeeding stages, and all that is required is to know approximately how much of it there will be to recrush.
Although the groups of curves we have been considering are intended for calculations involving primary crushing operations, they may also be used for secondary crusher products in those cases where no screening is performed
between primary and secondary stages. Such an arrangement is seldom encountered in modern plant design, except where large jaw crushers, set very wide, are followed by a secondary, usually of the standard gyratory type, to reduce further the very coarse output of the jaw crusher to a size which can be handled by the recrushing, screening, and elevating equipment in the balance of the planto In such cases it is simplest to consider the two-stage set-up as a single machine with discharge opening equal to that of the secondary crusher. |
