| Show calculating THE efficiency OF classifiers by robert S lewis professor of mining university of utah A new for formula for calculating the efficiency of classifiers has been proposed by R T hancock 0 this formula is based on the that the assay of an ore concentrate or tailing expresses the metal content of the sample rather than its mineral minera 1 content the metal content is lower than the mineral content consequently the use of assays essays implies that a theoretical recovery of iper cent would mean the recovery of the of the metal itself free from any combined element various forms of the formula are given clo metallurgical efficiency cn recovery concentration waste in feed clo metallurgical concentration times enrichment ratio 1 clo waste in feed 70 metallurgical emcie ncy concentration times enrichment ratio 1 assay of feed this formula is different from the traditional method of calculating efficiencies from assays essays alone and for this reason it has not gained favor rapidly however it is theoretically cally sound and can be simplified to a form that is easy to use H A whited white and B R bates in his discussion of mr whites article give the ideas involved in the formula and illustrate its use by several problems the following is a digest of this article tahe only change lies in the use of mesh instead of 90 mesh in giving adata the 90 mesh refers to institute mining and metallurgy screens in which series the aperture is one half of the mesh thus a 90 mesh screen has an aperture of 1180 of an inch or in the nearest screen of the tyler series is the mesh screen having an aperture of in in applying the formula to calculation of the efficiency of classifiers let of classifier i of oversize in feed oio aio 0 of oversize in the classifier overflow u of oversize in the classifier underflow E ei i 0 u i then iia i il u 0 example A screening test on a classifier set to give a separation at mesh gave these results feed contained 40 of mesh material underflow 80 of mesh material overflow 10 of mesh material E the formula considers that the classifier feed is divided into ato three portions one is cleanly divided into the desired products the ratio of this portion to the original t feed expresses the efficiency of the classifier the two other portions are unaltered feed one portion going into the under flow ow and the other going into the overflow the ratio of the fhe amount going into the underflow to that going into the overflow is immaterial ss is this material does not affect the metallurgical efficiency it simply pa passes ases unchanged ed through h the me machine thus in the above example 66 3 of the feed ced is is properly separated the remaining bem arely merely split up between underflow and overflow and serves to 10 contaminate these products in calculating the efficiency of classifiers results are comparable mp arable only in case the feed is of the same compos composition and the same series of screens are used in analyzing the overflow the relative efficiency of classifiers decreases as the percentage of oversize in the feed increases classification in in open circuit grinding is likely to be more efficient than in in closed circuit grinding where there is a large load and much coarse erial is returned to the classifier if I 1 tonnage of feed in classifier 0 tonnage bonnam of overflow from classifier U tonnage of underflow from classifier U ubi 06 0 0 U UI diio I 1 i io 0 and 0 I 1 u i i u i u 05 0 u 0 this combination of efficiency and weight formulas is useful in solving problems connected with the operation of classifiers example what would be the tonnage of tinder underflow flow from a bowl classifier if it were used in place of a straight classifier conditions as given below at separation the performance of a straight classifier was i 0 u E assume 0 tons then U 2097 tons for a feed of mesh material tests had shown that a bowl classifier might be expected to give an efficiency of we now have the following data from which u and U are to be calculated i 0 E vo 0 0 tons substituting first in tile the efficiency formula 0 u from which u then U 1627 tons this calculation shows that with a bowl classifier and for grinding in a closed circuit there would be a reduction of 2097 tons in material returned ifor for grinding re example 1000 tons of feed containing of mesh material goes directly to tube mills for grinding re what would be the tonnage of underflow and overflow from a bowl classifier treating this feed if the overflow is to contain 90 of mesh size from previous tests with feed of mesh sent to a bowl classifier an efficiency of 67 should be expected but to be on the safe side an efficiency of will be assumed then i E and 1 1000 tons quantities to be determined are u 0 and U substituting in the equation for efficiency u is found to be J then 0 1000 tons U 1000 tons thus the bowl clas classifier would be expected to handle 1000 tons of feed and send tons out a as finished prod the underflow of tons conta containing inin g 82 af 1100 mesh material being returned to the tube mills to be reground the formula for efficiency can be applied to the several spigots of a hydraulic classifier but to do this the overflow from one compartment must be screen sized as it forms the feed to the next compartment of the classifier thus the efficiency for each spigot must sabe be computed separately this is a tedious operation and practically is of little value since a classifier does not operate smoothly enough to permit of adjusting it and then leaving it alone for a day or two a classifier efficiency R T hancock eng al jour press september 25 1920 b the application of formulas in ore dressing R T hancock cornish institute of engineers november 1920 c also private communication d economics of ore concentration R T hancock eng min alin journal april 10 1920 20 classification efficiency H A white journal chemical metal surgical ical and mining alining society of south africa february 1915 |
