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As a further illustration of the process, the following tabulation covers the calculations for three machines of the same type, in closed-circuit with screens fitted for %-in. product, and operating at the same efficiency of 90 percent.
In each of the above cases we have carried the tabulation through to a crusher setting slightly in excess of the product size, and in one case (the No. 322 crusher on 3/8 in. products) the figures indicate maximum output at this plus-product setting.
However, plus-product settings are not recommended for either standard gyratory crushers or reduction crushers of standard, or short, throw types, and figures for such settings cannot be considered as reliable. The reason this is so will be clear when we consider just what kind of particles constitute the circulating load in a closed-circuit system incorporating any crusher of the pressure type.
We have pointed out in a preceding section that a high-speed, short throw crusher, such as the Type R or the Newhouse, will reduce practically all of its product to a one-way
dimension not exceeding the close-side setting of the crusher; hence, in a closed-circuit operation, if the crusher-setting is anything less than the square-opening product size, practically all of the material will be crushed, during the
first pass, to a size which is les s than the screen opening, in at least one of its dimensions. Now, if all of the materials were broken in cubical shave, and our screen operated at 100-percent efficiency, there would not be any circulating load for any minus-product size crusher setting. It follows, therefore, that the entire circulating load, except that portion of it which is due to less-than-perfect screen efficiency, must consist of pieces that are more or less flat in shape. It follows, also, that these pieces will not be gripped by the crusher in any succeeding pass, unless they happen to fall in edgewise, and that most of them must be broken by being crushed between one of the crushing faces and some other particle of material. That they are so broken is evident; else the circuit would soon be choked with over-size pieces. |
