The invention is based on the discovery that the reason why big hydrocyclones which are positioned vertical need a comparatively high feed pressure is that, owing to their great length, these hydrocyclones contain such a tall liquid column that gravity becomes an important factor and influences the operation of the hydrocyclone. If however, the hydrocyclone is given a sufficient tilt this phenomenon cannot occur.
The patent refers to cyclones having an axial length of from 0.7 to 9 meters, and the Model D20B and D26B cyclones referred to above have lengths of approximately 2.1 and 2.3 meters, respectively. Development work was apparently coincidental to the investigation of magnetite heavy medium cleaning of coal in cyclones, and example results show improved separation of clean coal and refuse solids with the heavy medium cyclone in the horizontal position. Licensees of the Dutch State Mines patents from that time forward mounted heavy medium cyclones in the horizontal position, while this positioning was adopted for relatively few metallurgical applications.
Cyclone Experience at Lithium Corp.
The operation, located near Bessemer City, Morth Carolina, separates the lithium mineral Spodumene from a pegmatite ore by flotation. Similar to the conditions described for the flotation of Apatite in the Brazilian operation, excessive production of fines jeopardizes the results that can be achieved by flotation. Falling back on the Brazilian experience, the installation of a horizontally mounted D26 cyclone was recommended.
It is significant that the results with the 660mm cyclone were achieved using a 165mm (6½-inch) apex insert, a size much larger than can usually be used with a vertically mounted cyclone under similar conditions. The relatively large apex used at the Lithium Corporation of America demonstrates that apex diameter is much less critical for large cyclones mounted horizontally. With approximately 175 STPH of underflow solids, an apex of 115mm (4½-inch) diameter would probably be used for a vertically mounted cyclone.
Cyclone Experience at St. Joe Minerals
At St. Joe Minerals Corporation in Viburnum, Missouri, Galena and Sphalerite are recovered by flotation from a dolomitic limestone. More of the sulfide values are liberated at a relatively coarse size and a grind that is approximately 52 pct. finer than 200 mesh is maintained. Each of three rod mill/ball mill circuits was using three 500mm cyclones in the vertical position and was experiencing apex plugging.
The percent passing 200 mesh had decreased from 52.3 pct. to 51.3 pct. While cyclone feed solids concentration with the 500mm cyclones was maintained at 62/63 pct. solids, the slightly coarser grind at 180 STPH new feed was achieved at 65.5 pct. solids feed. Thus, except for other circuit limitations it would have been possible to increase cyclone feed dilution modestly, increase the circulating load further, and in this manner restore the finer grind. Table III summarizes the data obtained from these tests.
Evaluating Horizontal Mounting of Closed Circuit Cyclones
As tonnage of new feed increases and the tonnage of classifier sands correspondingly grows, the classifier sands become coarser, but the composite mill feed becomes finer because of the higher content of circulating load. At the same time the mill discharge becomes coarser because of decreased time-per-pass, so that the spread in size between composite feed and mill discharge decreases.
The changes in conditions inside a mill which correlate with its improved performance in closed circuit, and which must, therefore, hold the key to the explanation, are: (1) reduction in mean size of feed; (2) marked increase in near-finished sizes; (4) more rapid travel, i.e., a shorter time per pass; (5) closer approach to a uniform ratio of ball mill size to average particle size throughout the length; (6) some increase in quantity of interstitial pulp.
Visualization of physical conditions in a tumbling load indicates that maximum availability of grindable material to active grinding zones occurs when the grinding faces are in substantial contact and the interstices are completely filled with a pulp as thick as will permit ready flow.
The three installations above confirm that horizontal mounting makes it possible to operate cyclones at the normal low pressure drop without the increase in underflow volume that is likely to result from vertical mounting. Since the separation made by cyclones in closed circuit applications is modestly affected by the cyclone feed nozzle size and vortex finder diameter, but is largely dominated by the feed solids concentration, it should be possible in most horizontal installations to select relatively large fittings that will give the required capacity at low pressure drop.