OTHER FLOTATION PROCESSES
MACQUISTEN PROCESS.
One of the most important of the notation processes is that invented by A. P. S. Macquisten. The apparatus is a tube so designed as to bring the sulphide particles in the ore on to the surface of the water in the most advantageous manner to insure that the force of surface tension and the inherent quality of the sulphides to resist wetting would insure their flotation.
The tube, Fig. 51, is made of cast iron, 6ft. long and ift. in internal diameter.* Two tyres, Cast with the outside, run upon rollers, upon which the tube rests. The discharge end of the tube is open, while the feed end is closed, except for a hole in the centre to admit the feed-pipe. On the interior of the tube is cast a helical groove of ijin. pitch. The discharge end of the tube, at the right of Fig. 51, connects with a vessel called the ' separating box,' the joint between the box and the tube being watertight, although the tube is free to revolve. The box has an opening for the overflow of water, the lip of this opening being of such a height that about 3in. of water stands in the tube. The feed of pulp to the tube and the discharge of tailing at the bottom of the box are so regulated that the water passing over the lip of the box is about 1-32 in. deep. The floating concentrate passes over this lip into a launder, and the tailing passes out at the hole at the bottom of the box, going to a second tube for further treatment, if necessary, or to the tailing-flume. In operation the tube is rotated 30 r.p.m. in the direction of the helix. The pulp is thus screwed through the tube, and in its advance at every revolution the sulphides are given an opportunity to slide upon the surface of the water and float. As the tube revolves the pulp is carried up to the surface, and gently rolls over. The angle of emergence is such that some particles of sulphide are at once drained of the adhering water, take on airfilms, and float. The gangue simply slides back into the water. This action is taking place at every corrugation, whereby the sulphide particles, which are capable of being held up by the force of surface tension, are given many chances to slide out upon the surface of the water at just the angle at which there is the least likelihood of penetrating the surface.
In any given tube revolving clock-wise (looking toward the feed end) the flotation takes place chiefly at the left-hand side, where the pulp emerges from the water. The bulk of the pulp in any corrugation either slides back into the water, or floats off ; however, a certain amount always adheres to the tube, and is carried over to the water on the right-hand side, where upon meeting the water a further flotation takes place, but naturally this is less than on the left-hand side. Consequently, from the time the pulp enters the first tube there are floating sulphides streaming off from the sides of the tube, which by the time the discharge end is reached form a glistening film, covering the entire surface of the water both in the tube and in the separating-box. This is quite different in appearance from the black frothy scum which rises to the surface in the acid-flotation processes. In separating the chalcopyrite and pyrite at Golconda, the surface of the water is covered with the sparkling bronze mineral, the water looking as if it had been sprinkled with the bronze powder that is employed for decorative purposes. In treating a lead ore, there comes off a film of steel-grey galena ; in treating a zinc ore from Joplin, Mo., there was a film of mustard-yellow blende. The flotation is persistent. Once it has been effected, the mineral remains on the water until the latter has overflowed into the launder, which delivers it into the collecting- tank, and even in the latter more or less mineral can be observed floating on the surface. One treatment is not generally sufficient, and the gangue-tailing, with the remaining sulphide, is passed through a second, third, and fourth tube. By far the largest amount of concentrate is secured from the first tube.
The capacity of a single tube is 5 tons of ore per day, but as the material must be put through several tubes, the actual capacity of a set would be, say, 5 tons. The ore at the Adelaide mine assays from 2.7 to 3.2% copper, and the percentage of sulphides is low. It is important to note that on a heavy sulphide ore like that at Broken Hill it would probably be necessary to treat the material through a great many tubes, or else cut the capacity down to a very low point. These tubes have an effective flotation surface of 6 sq. ft. per tube ; and, therefore, must be limited in capacity as compared with those processes which increase the available surface by froth production.
The accounts of this process so far published seem to indicate a lack of facility in the treatment of slime.
