Table of Contents
Translation of laboratory results into terms of plant/mill-scale operations is, in the usual case, less difficult in flotation than in gravity concentration, and in all cases more certain than where chemical reactions such as occur in leaching and precipitation operations are concerned. Any flotation result that can be obtained in a laboratory machine can be obtained in mill operation, if the essential laboratory conditions are duplicated. The converse of this statement is also true, except for the fact that the mill-sized machine is capable of handling a somewhat coarser feed than can be handled in the laboratory machine. Considering the essential elements of pulp treatment in detail, the translation from laboratory results to mill results will be as follows:
Average size of feed may be slightly coarser in the mill than in the laboratory or, if the grinding in the mill is carried to the same extent as in the laboratory, a somewhat better result, other conditions being equal, may be expected in the mill than in the laboratory.
Water may make a considerable difference between laboratory results and mill results and this difference may be either in favor of or to the detriment of the mill. The former will ordinarily be the case if a portion of the mill water is reclaimed and re-used. Under these circumstances it will ordinarily be found that the flotation agent brought back by the mill water will lessen, to a considerable extent, the amount of new flotation agent that it is necessary to add, and that froth will be more easily obtained with this reclaimed water mixed in. If, however, there is any considerable amount of soluble salts in the ore, or if the settling ponds are of considerable area and in an arid region and there is any considerable amount of dissolved solids in the new water, then the salts in the water may have a harmful effect on flotation.
Flotation agents in the mill will be the same as in the laboratory except that it will generally be possible in the mill to lessen, to some extent, the proportion of so-called frothing oil in the mixture.
The peripheral speed of the agitators in agitation-type machines may, in general, be somewhat less in the mill than in the laboratory.
The air consumption per cubic foot of pulp treated in pneumatic machines will usually be less in the mill than in the laboratory. The pressure on the under side of the blanket will be necessarily higher in the mill machine than in the laboratory machines described, on account of the greater head on the pulp side of the blanket.
Time of treatment necessary in the mill will be very closely the same for a given recovery and grade of concentrate as in the laboratory.
The grade of final concentrate obtained in the mill will be close to that obtained in the laboratory. The recovery will come close to the indicated extraction calculated by the formula (page 166) from laboratory results, if, in the calculation, the figure for grade of concentrate is that obtained from the cleaner operation, the figure for rougher tailing is that obtained from the rougher operation, and the middling or cleaner tailing obtained in the laboratory is disregarded, provided that the grade of this middling product is not more than twice the grade of the original heads, and that the mineralogical character of the middling is not markedly different from that of the original feed.
Concentrator Plant/Mill Tests
It cannot be too strongly urged that before a mill is erected, some testing work be done on mill-sized flotation machinery. This work should be done in a test mill at the mine, on ore whose prior handling corresponds as closely as possible with the scheme to be followed in the finished mill, and the water used should be as near as possible of the character of the water that is to be used in the operating plant. If such a test does no more than confirm the laboratory results, it will pay for itself in the information that it gives concerning mill operation on the ore and it may be that the test will bring up conditions which were overlooked in the laboratory testing work. Some of the equipment used in such a test can ordinarily be utilized in the final plant so that it need not all be charged against the testing work.
Equipment and Processes
Skin Flotation Machines
The Wood machine
As described in U. S. patent 1,088,050, is illustrated diagrammatically in Fig. 24. It consists of two tanks (A) and (P), in which water is maintained at the desired level by means of regulating valves. A roller (C) covered with corrugated rubber belting and submerged with its center well below the surface of water in the tank (A) is caused to rotate in the direction indicated by the arrow. As the roller emerges from the body of liquid, it carries with it, covering its upper surface, a thin layer of water. Dry ore from the hopper (H) is fed in a thin sheet by means of the shaking feeder (G) onto the surface of the revolving roller. As this sheet of dry ore strikes the surface film on the layer of water on the roller (C), the gangue minerals are wetted and sink beneath the surface of the water and settle in the grooves, while the minerals of metallic luster tend to float. When the floating and submerged minerals are carried over to the point where the surface of liquid in the tank intersects the surface of the roller, the floating mineral rides out onto this surface because of the fact that the surface film is continuous over the tank and the roller, while the gangue minerals, being already submerged, remain beneath the surface. As this portion of the roller passes down to the lowest point in its revolution, the submerged gangue minerals fall off and settle to the bottom of the tank. At the side of the tank (A) opposite from the roller (C) is another roller (I) supported with its axis above the surface of the liquid in the tank. Over this roller passes an endless rubber belt (K) which passes in turn over the pulley (R), similarly submerged in the tank (P), and the guide roller (M). The purpose of this combination of pulleys and belt is to remove the surface film with its load of metallic mineral from the tank (A) and to transfer the same to the tank (P). This is accomplished as shown in the large-scale sketch of this part of the apparatus in the figure. A gentle current from (C) toward (I) is maintained by reason of the constant addition to the surface film at (C) and a constant removal of surface film at (C) by the traveling belt (K). It will be noted that the surface film is again continuous from the liquid in the tank (A) to the surface of the liquid in the tank (P) over the surface of the belt (K). Due to the disturbance at the point where the belt (K) passes below the surface of the liquid in the tank (P) the less tightly held material in the film is shaken out and settles to the bottom of the tank (P). This material constitutes the middling of the process and is re-treated on gravity concentration apparatus. The tailing of the process is discharged at the valve (B). The floating concentrate overflows from the tank (P) at the lip (W), the level of liquid in tank (P) being maintained so as to overflow a thin sheet of liquid at this point. In an earlier patent, 984,633, issued in 1911, Wood states that in certain instances a small quantity of oil “will render the film more characteristically selective with regard to the particular particles which it will convey upon its moving surface, and thus extend the range, and permit of better control of its selectivity.” The machine as built has a feed roller three feet wide, requires about 0.25 h.p. to operate and is said * to have a capacity of from 1000 to 2000 lb. per hour, the higher figure corresponding to an ore with which the ratio of concentration is high.
The Macquisten tube
The Macquisten tube is described in U. S. patents No’s 865,194, 865,195 and 865,260 to A. P. S. Macquisten. A diagrammatic sketch of the apparatus is presented in Fig. 25. It consists of a tube (b), one foot internal diameter by six feet long, closed at the inlet end with the exception of a small central opening, and open at the discharge end. The tube is supported horizontally by means of tires on rollers (c) and is caused to rotate at about 30 r.p.m. At the discharge end a water-tight joint is made with a pointed box (d), which is fitted with an overflow lip (e). The interior of the tube is fitted with a helix of about 1 1/2-in. pitch. Various forms of inside surface are represented in the tube sections shown in the figure. The overflow lip of the discharge box is at an elevation of about three inches above the bottom of the tube and the pulp level in the box and tube are maintained at such a level that a film of water about 1/32 in. deep overflows the discharge lip. Liquid pulp is introduced into the machine through the feed trough (a). Due to the revolution of the tube, the solids in the pulp are raised above the surface of the pulp in the tube. Water drains away most rapidly and completely from the particles of metallic luster. When the solid material lifted above the pulp surface has attained such a height that the angle of repose is exceeded, it slides back. The dried minerals of metallic luster tend to, and do, in part, float, while the wet gangue minerals submerge. This operation is repeated many times during the passage of the pulp through the machine for every particle of solid that settles at a sufficient rate to bring it in contact with the inner surface of the tube or with the mass of settled solids thereon The finest material or slime tends, in large part, to pass through the tube in suspension and thus to get no chance to separate. A gentle current (about 10 ft. per min.) of the surface is maintained through the tube from feed to discharge end by reason of the incoming stream of pulp. The movement of the surface layer may be accelerated by air jets directed toward the discharge end. When the submerged solids reach the discharge tank they sink to the bottom and are withdrawn as tailing, while the floating concentrate passes over the lip of the tank into the concentrate launder and into collecting tanks through a pipe (g). The pipe (h) serves for the removal of tailing for re- treatment in another tube of the same variety. The capacity of a tube is said to be about five tons per 24 hr., but the capacity actually is determined more by the liquid surface available for flotation, and the amount of floatable material in the feed than by the quantity of solids that the tube will convey. At the Morning mill * at Wallace, Idaho, 175 to 200 lb. of zinc concentrate can be floated per tube per 24 hr. At this mill feed pulp is passed, through four tubes in series, three tons per 24 hr. being passed to each series. This makes the average capacity per tube 0.75 ton per 24 hr. The feed is granular having about 9 per cent, on 40-mesh and 11 per cent, through 200-mesh. The feed pulp carries from 14 to 20 per cent, solids.
Macquisten states in Patent 865,194 as follows: “Usually water would be employed as the separating agent in the case of metalliferous ores, but obviously any liquid may be substituted therefor, which has the suitable constitution or properties to effect the separation in the manner herein described, or the properties of the water or other liquid with respect to its surface tension or capillarity may be modified by the addition of a suitable acid, or alkali or soluble salt or other substance. The surface condition of the particles to be separated may be modified or altered by suitable treatment with active chemicals which will attack the surface of the particles.” In practice, petroleum oils and acids have been premixed with the pulp before flotation is attempted.
The DeBavay process
The DeBavay process of skin flotation is described in U. S. patent 864,597 issued to A. J. F DeBavay. A process called the DeBavay and practiced on a considerable scale in Australia, is described by Hoover (“Concentrating Ores by Flotation,” 3rd edition, page 114). This method bears little, if any, resemblance to the method described in the patent. The patented method is based upon a conception of DeBavay’s, stated by him in his patent as follows: “I have found that the sulfids of zinc, lead and silver as they exist in the ore are generally coated with carbonates of zinc, iron and manganese and other matter and that it is impossible to separate by flotation in such a condition more than a very small proportion of the zinc blende particles from the gangue and even then the floatable portion contains a large proportion of the gangue, and hence it becomes necessary to treat the ore as herein described to prepare it for separation by flotation.” The method which he describes consists in subjecting the pulp to the action of an equal volume of a weak aqueous solution of carbonates of ammonia, bi-carbonates of sodium or bi-carbonates of potassium or of carbonic acid gas, or of any other reagent which will bring about the separation of the coating or covering from the zinc blende particles, or to trituration, and subsequently delivering the ore, after washing out this solution and the slimes, as a thin paste upon the upper end of an inclined table and washing down this table with a thin sheet of water onto the surface of water in a tank. In this tank the sulphide minerals were supposed to float by skin flotation and the tailings to sink. He further specified that by the term “water” he included any other liquid in which particles of zinc blende are capable of flotation. In the process described by Hoover the tailing from gravity concentration, crushed to pass about 40-mesh, was first deslimed, then fed into a mixing tank and agitated with a cold acid solution of about 0.2 strength, the proportion of acid solution to solid being four or five to one. After treatment for a considerable time in this tank the solid was allowed to settle, the acid solution drawn off and the settled solid was washed twice to remove acid. The solid was next placed in an “oiling vat” in which it was thoroughly agitated with water and from two to three pounds per ton of a mixture of one part of castor oil and four parts of low-grade kerosene. A small amount of chlorine gas (about 0.02 per cent, on the water) was also passed into the mixture in this tank. The oiled pulp was elevated from this machine by means of a montejus and fed into a series of separating cones of the variety shown in Fig. 26. The pulp flowing down over the corrugated conical surface, upon meeting the liquid surface of the pulp in the separating cone at (A), divided into sulphide-rich material which floated and was removed over the periphery, and gangue which sank and was removed through the spigot. A large number of these cones were used, the capacity of each being small.
In carrying out this process in the laboratory it is to be observed that the floating material consists principally of agglomerates of sulphide minerals and air bubbles and that the process is rather one of pulp-body froth flotation than of skin flotation.
Oil-flotation Processes
The Everson process
As described in U. S. patent 348,157, consists essentially in mixing dry, finely-powdered ore with oil and subsequently diluting with acidulated water to form a freely-flowing pulp, and agitating and separating the sulphide mineral.
The relative proportions of ore and oil in the mixing operation are such as will form a pasty mass. The next step is described in the patent as follows: “the mass
[should] be opened out or broken up and thoroughly stirred in the water in order that the sand or quartz may be freed and carried away.” There is a further instruction to remove the concentrate by constant overflow of water or by other devices. The result of the treatment as above outlined is to produce a mass consisting essentially of oil, sulphide mineral and air, which is lighter than the gangue and can be separated therefrom partly by buoyancy alone and partly through the aid of a rising current of water. The disclosure of the patent as to reagents is broad. In general it is:—“a fat or an oil, either animal, mineral, or vegetable, or a fatty constituent or acid of an animal or vegetable fat or oil, or any constituent of a mineral oil, together with an acid, either mineral or vegetable, or a soluble neutral or acid salt” Further:—“any fat or oil, and any acid, either mineral or vegetable, or any soluble neutral or acid salt, or any compound of fats and oils with appropriate acids” Specifically:— “petroleum paraffine-oils tallow, (melted,) lard, lard-oil, red-oil, (impure oleic acid,) cotton-seed oil, castor-oil, sperm-oil, and linseed-oil, and some combinations of these with each other. The acids are sulphuric, hydrochloric, nitric, phosphoric, acetic, oxalic, tannic, and gallic the following salts, to wit: the sulphates and chlorides of sodium, zinc, and copper, and the double sulphate of potash and alumina.” A method of making and using sulphonated oils is disclosed.
The Robson process
As described in U. S. patent 575,669, consists essentially in mixing powdered ore in a moist or pasty state, i.e. containing from 25 to 35 per cent, water with a considerable bulk of an oily liquid, and subsequently to permit the mixture to stratify, when the oil carrying the minerals of metallic luster will float above the balance of the mixture and may be drawn off with its mineral load.
The Elmore process
Also as described in U. S. patents 676,679 and 689,070 and further described as to apparatus in patents 653,340 and 692,643 consists in first producing a freely-flowing pulp by mixing pulverized ore and water in proportions of 6 to 1 to 10 to 1 by weight, adding thereto a relatively large quantity of oil, up to more than a ton per ton of solids, adding also sulphuric acid, mixing the ingredients together in a trough in which rotates a horizontal shaft with blades, and then passing the mixture to a separating box of the nature of a spitzkasten. The pulp level in the separating box should be kept at such a height that slight overflow of pulp liquor is allowed. Under these conditions the oil layer on the surface of the pulp will be not in excess of one half inch in thickness. Elmore specifies that the mixing should be so limited in violence as not to break the oil up into minute globules, i.e. not to emulsify the oil in the pulp. In the mill applications of the Elmore process it has been found that a considerably greater quantity of sulphide is floated than can be accounted for by the buoyant force that could, under the most favorable circumstances, be applied due to the difference in specific gravity between the active oil and the pulp, and examination of a floating concentrate reveals the fact that a considerable amount of air in the form of minute bubbles is present. In practice, the actual oil consumption per ton of ore treated is relatively small and is said to be between 10 and 20 lb. per ton of ore.
Scammell and Wolf Process
In U. S. patents 770,659 and 787,814 respectively, describe methods of oil flotation in the presence of a freely flowing pulp in which the viscosity of the oil is increased by various methods. The principal method described is treatment of the oil with chloride of sulphur before introduction into the pulp.
None of these oil-flotation methods is in present use.
Mill Data P. 146-160