The Mary Murphy mill
The following quotation from a paper by George E. Collins on the metallurgical practice at the Mary Murphy mill is given because it contains the experience of other operators as well as my own opinion on the subject.
" The conical end of the Hardinge mill acts, to some extent, as a device to expedite continuous discharge, in much the same way as the device recently brought out by Chalmers and Williams for improving the discharge from a cylindrical tube mill. The conical end of the Hardinge, while of limited efficiency, is simple and positive, and really works. One can obtain nearly as small a proportion of slime in a long tube mill as in a Hardinge, by crowding the feed and using a large quantity of water; but apparently there is then a slight tendency for the discharge to contain still more oversize.
"Mr. Hardinge's original contention that with his'mill the weight of blow is automatically adjusted to suit the progressively finer particles of pulp as they pass from head to discharge end, and that owing to this automatic adjustment there is practically no necessity for return of oversize, is, according to our experience, not borne out by the facts. As in the case of the tube mill, if you crowd the tonnage and increase the water sufficiently to obtain a granular product, you have to arrange for the return of a very considerable amount of oversize.
"In various publications Hardinge has illustrated this supposed zonal action in his mill by cuts in which he superimposes on a picture of the conical mill, an outline of several gravity stamps, commencing with one of heavy weight and high drop, and progressively lessening to one of small weight and low drop at the end; the stamps being supposed to represent the force of blow struck by the pebbles in the corresponding stages of the Hardinge mill as you pass from feed to discharge end.
"This is, however, largely a fallacy. It is true that the size of pebbles is automatically graded, to some extent, and obviously the force of blow struck would be thereby lessened. But the peripheral speed is at the same time lessened to such a degree that near the discharge end the pebbles merely slide on one another without cascading, so that there is no crushing action worth mentioning, and what actually occurs is a slight process of abrasion, which is just what we want to avoid. In fact, in comparing the work of the Hardinge with that of the tube mill, according to our experience, it is pretty much a question of relative first cost and operating expense. We have at Romley one 3> by 16-ft. Gates tube mill, two 6-ft. by 22-in. Hardinge mills, and one 4-ft. by 22-in. Hardinge. We find that the capacity of the tube mill and that of one 6-ft. Hardinge, using the same feed, are almost identical; and the cost of the Hardinge was about 20 per cent, greater. As to power consumption, we have no means of measuring. The Hardinge uses more pebbles, and has to be relined oftener."