Calculating circulating load around screen and crusher

Assume, for example, that we are to crush a material, in a closed-circuit crushing stage, to pass a ¾ -in. square opening (as determined by a flat testing sieve); that our screen, which forms a part of the circuit, will be fitted with the proper size of openings to make this ¾ -in. product, and will be capable of performing at 90 percent efficiency. Also assume that the material is such that the crusher selected will crush it to a size, 70 percent of which will pass a square-opening test sieve equivalent to the close-side crusher setting. (We determine this either from tests made on the material, or from the table of approximate values presented in a foregoing section. Using this 70-percent value as a basis, we first set up, from the product curves for screened feed, herewith, a column of percentages that can be expected to pass the ¾ -in. opening, using as many trial crusher-settings as we wish (usually three or four will suffice). This column should be set up opposite the corresponding crusher settings, along with another column showing the capacity ratings, of the particular machine under consideration, at each of these settings.

Having tabulated this information, we consult our table of circulating loads-or use formula (2)-and list a fourth column of figures, showing the percentages of circulating loads which can be expected on the basis of the values in column 3, and for 90 percent screen efficiency.

A final calculation is now performed to arrive at the amount of original feed which, when added to the circulating load, will equal the capacity rating of the crusher at each trial setting. The formula for this calculation is:

 (3)   Tons for original feed =  Capacity rating of crusher      
                                1 plus (Circulating load
                                expressed as a decimal)

The following tabulation shows how the problem works out for two sizes of the Type "R" crusher. It is interesting to note that the settings for maximum output, i.e., tons of original feed, are not the same for these two machines.

These figures indicate that maximum output may be expected from the No. 636 crusher at a setting equal to the product size, whereas, the No. 322 machine shows up best at ½ in setting. If we wish to obtain this maximum performance from the larger machine, or from a battery of them, we must provide screening area sufficient to handle 1.50 times the original feed rate. For the smaller machine we need provide only 1.06 times the original feed rate.