Lesson 2/Learning Event 3
The column for the roll crusher breakdown was based on the 39 tons per hour of oversize found previously.
Using Table 9, for the roll crusher, which receives 39 tons per hour of +3/4 inch material, 25% (.13 + .12) of the
product will be larger than 1/2 inch, 15% (.15) of the product will be larger than 3/8 inch and smaller than 1/2
inch. Similarly 26% (.10 + .08 + .08) of the product will be larger than No. 4 and smaller than 3/8 inch and 34%
(.15 +.19) will be from No. 4 down to microscopic dust. Notice that adding 25% + 15% +26% + 34% = 100%.
This addition can be used to double check figures. Summing across the row, you now have the breakdown in tons
per hour for the crushing equipment.
Go back and review your top and bottom product size range and summarize your sieve analysis results as follows:
INITIAL GRADATION CHECK
Once the tonnages in each size range have been estimated, you can check the expected product for compliance
with product specification. Tabular form is the most graphic method for a gradation check. Product in this
example falls in the size category from 3/4 inch to 3/8 inch, and the reminder is byproduct. From the sieve
analysis, there are 25.80 tons of product produced each hour, and 34.20 tons of by-product. The gradation is
determined by taking the tonnage produced in each size range divided by the total product tonnage, which gives
the percent retained. This percent retained is the portion of the product which would not pass through the screen
corresponding to the specified size range. It is the percentage produced which is larger than the specified size.