Closed-Circuit Crusher Product Analysis

The product gradation curves for screened feed, described under the preceding sub-heading, can be used as a basis for calculating approximate screen analysis of products from closed-circuit crushing stages, but the values cannot be taken directly from the curves.

For example, consider a crusher set to turn out a product 70 percent of which will pass a %-in. square opening, and in closed circuit with a screen which is equipped to remove the minus %.-in. product. Our curve shows that approximately 85 percent of the crusher product will pass the ¾ -in. square openings.

Now, suppose that we wish to know how much minus 14-in. fines we may expect from the circuit. We do not go to the curve which touches the 100 percent ordinate at the ¾ -in value; we calculate the percentage from the same curve which was used to predict the proportion of minus * in. in the crusher discharge. This curve shows approximately 29 percent of minus ¼, in. in the material as it comes from the crusher, or 29 tons of fines in each 100 tons of crusher output. But, for the circulating load, we are only interested in that fraction of the crusher output which will pass the *-in. screen, which is 85 tons. That part of the product gradation curve which lies below the 85 percent value represents the gradation of the finished product, and 29 tons out of each 85 would be minus ¼ in.

Let x equal percentage of minus ¼ in. in the finished product, then x: l00 =29:85 or x=34.1 percent of minus ¼-in. rock from the closed circuit operation. Any other size of product may be estimated in a similar manner. Note that if we had used a curve touching the 100 percent ordinate at the %-in. value, we would have arrived at a value approximately 50 percent for the minus ¼-in. fractional value which is obviously erroneous for rock of average characteristics. We will comment on closed circuit crushing, and upon certain assumptions which have to be made in closed circuit calculations, in a later discussion of reduction-crushing.