US 3701421 A
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Description (OCR text may contain errors)
United States Patent nsi v 3,701,421
Maxwell Oct. 31, 1972  METHOD OF MINERAL SEPARATION OTHER PUBLICATIONS 1 BY FROTH FLOATATION 'r. A. lrnme, Tech Pub. No. 1702, Mar. 1944,- Rose  Inventor: 1 John Russell Maxwell, Chapais, pp. 1- 8 Quebec, Canada Chem. Abst. 66, 1967, 77330c Chem Abst, 68, 1968, ll6522e  Asslgnee' g gf g Weston on Denver Condensed Catalog No. 63-8100, p. 13 Filed! P 1 1970 Primary Examiner-Frank W. Lutter  Appl. No.: 32,906 I Assistant Examiner-Robert I-Ialper Attorney-Rogers, Bereskin & Parr Related US. Application Data  Continuation-impart of Ser. No. 735,168, June  ABSTRACT 7. 1968, abafldoned- A method of mineral separation by froth floatation in which pulp is fed into a cylindrical tank of height ap- U-Su to is  Int. Cl. ..B03d 1/02, B03d 1/22 I mechanically agitated and air at low pressure is fed 0i 1 t k t k I 162-165 QT in suspension and to float the desired fraction of the pulp. The feature of the invention is that a low flow of  References cued air is used, in the range between 0.05 and 0.30 cubic UNITED STATES PATENTS foot per minute per cuhic foot of pulp, and low power mechanical agitation is used, the total consumed 2,41 Morse power for both air and the mechanical agitation 2,423,456 7/1947 Logue ..209/ 164 being between 0 10 and 0035 horsepower cubic 2,012,358 9/1952 Daman ..209/l68 X f t f Pulp 2,792,939 5/1957 Myers ..209/ 168 X 3,420,370 1/ 1969 lsewnardt "209/169 3 Claims, 5 Drawing Figures I N V ENTOR. JOHN RUSSELL MAXWELL f 3%, ,zzw
SHEET 2' BF 3 INVENTOR. 4 JOHN RUSSELL MAXWELL FIG-2' P'ATENTEDncm m2 3. 701.421
SHEET 3 BF 3 FIG. 3
INVENTOR. JOHN RUSSELL MAXWELL FIG. 5
fm wJ w METHOD OF MINERAL SEPARATION BY FROTH FLOATATION This application is a continuation-in-part of my application Ser. No. 735,168 now abandoned filed June 7, 1968 for Methods of Mineral Separation by Froth Flotation.
This invention relates to an improved process of mineral separation by froth flotation. In particular, it relates to a froth floatation. In particular, it relates to a froth flotation process in which much less power is used than has previously been the practice in processes having a comparable treatment capacity.
At the present time, mineral froth flotation processes are commonly conducted in cells or tanks having rotors or impellers to agitate the pulp mass, and with air introduced either from an external compressor, or from the atmosphere if the impeller can create a vortex or reduced pressure,to float the desired particles to the surface of the tank where they can be collected. It is invariably the practice to employ substantial power intensities in such cells. The power consumption for such cells ranges between 0.08 and 0.25 horsepower (for the mechanical agitation and air combined) per cubic foot of pulp volume, with a common figure being between about 0.1 l and 0.15 horsepower per cubic foot.
For example, a typical 200 cubic foot prior art cell has typically required installation of about 25 horsepower (excluding the air blower) and has consumed about 18 horsepower for mechanical agitation, while also consuming about 1% cubic feet of air per minute per cubic foot of pulp, the air consumption requiring about horsepower.
According to the invention, a low air flow is used, in the range between 0.05 and 0.30 cubic foot of air per minute per cubic foot of pulp, the air being supplied at very low pressure (just sufficient to overcome the static pressure head in the cell). In addition, very low power mechanical agitation us used. The total power consumed, for both air and for mechanical agitation, is
between 0.010 and 0.035 horsepower per cubic foot of pulp. Thus, for example, a 300 cubic foot cell according to the invention requires a total horsepower (for both the mechanical agitation and for the air) of about 6.5, while a 2,000 cubic cell requires a total horsepower of about 45, which is only slightly more than double the horsepower required for a 200 foot cell of the prior art. It has been discovered that the pulp can be maintained in suspension, and settling prevented, even at the lower powers mentioned, and that substantial tonnages of material (of the. same order of magnitude as in the much higher power) art processes can be processed efficiently.
Further objects and advantages of the invention will appear from the following description, taken with the accompanying drawings, in which FIG. 1 is a top view of a cell in which the process of the invention may be conducted;
FIG. 2 is a side sectional view along lines 2-2 of FIG. 1, with a baffle plate and part of the froth launder shown in dotted lines;
FIG. 3 is a sectional view along lines 33 of FIG. 2, showing a gangue outlet;
FIG. 4 is a sectional view along lines 4-4 of FIG. 2; and
FIG. 5 is a side view ofa portion of the cell of FIG. 1, showing a feed inlet.
The flotation cell shown in the drawings includes a tank 10 having a cylindrical sidewall 12 and a circular bottom 14 supported on four beams 16 arranged in the form of a cross. Pulp is fed into the tank through a downwardly slanting inlet pipe 18 located in the sidewall approximately half way between the top and bottom of the tank. The pulp is agitated by an impeller 20 located in the center of the tank, about 10 to 25 percent of the height of the tank from the bottom. The impeller 20 is suspended by a shaft 22 which depends from a bearing box 24 supported over the top of the tank by cross beams 26. The shaft 22 is driven in conventional manners by belts 28 protected by a guard 29, the belts being driven by a motor 30. A gear drive can be used instead of the belts.) I
Air is introduced into the bottom of the tank at a single location directly beneath the impeller, by a pipe 32 and outlet 33. The air is admitted through a check valve 34 which consists of a pair of rectangular rubber flaps 36 arranged in an upside down V shape and held in a metal box diagrammatically indicated at 38. The ends of the box are closed by metal plates, not shown, so that the arrangement forms a watertight chamber around the air outlet 33 and prevents the pulp from contacting the outlet. The flaps 36 simply separate when the air pressure at outlet 33 exceeds the pulp pressure head, and at other times the flaps 36 are pressed together by the pressure of the pulp.
The air escaping from the flaps 36 is dispersed through the tank by the impeller 20, which directs the pulp downwardly and outwardly as indicated by arrows 40 in FIG. 2. (If desired, an impeller which discharges radially outwardly only can be used.) A rubber surfaced wear plate 42 protects the bottom of the tank from abrasion and can be replaced when it is worn thin. Four vertical baffies 44, arranged at 90 intervals, extend from the tank walls to prevent swirling of the tank contents.
As the air bubbles rise in the tank, they. select the desired particles and float these particles to the surface to form a froth, which is collected by a doughnut shaped froth launder 46. (If desired, several concentric launders can be used). The launder 46 is supported by four brackets 47 attached to the baffle plates 44. A water pipe 48 extends above the froth launder and directs a water spray into it to rinse the collected froth down to a froth outlet 50.
The waste or gangue exits from the tank 10 via an outlet 51 (see FIGS. 2 and 3) which is located near the top of the tank, but below the surface 52 of the agitated pulp. The outlet 51 is defined by a baffle plate 53 welded to the side of the tank. From the outlet 51, the gangue flows upwardly through an opening 54 in the side of the tank, and over darn bars 55 to an outlet pipe 56. The dam bars 55 are set in guides 57 in a box 58 welded to the side of the tank. Depending on the input feed rate, the number of dam bars used will control the level of the pulp in the tank, and the depth of the froth 59.
The baffle plate 53, which defines an outlet below the surface of the pulp, serves toprevent froth from being removed with the gangue. The baffle plate 53 also reduces the amount of enriched material near the top of the tank tending to exit with the gangue.
The above described cell is preferably of height approximately equal to its diameter. By height is meant the distance from the bottom 14 to the top of the tank. The top of the froth launder is normally about 4 to 5 inches below the top of the tank (allowing a slight freeboard), and the top of the pulp is usually about 8 to 10 inches below the top of the froth launder, depending on the dam bars.
The air introduced into the bottom of the tank via pipe 32 is at a pressure just high enough to overcome the pressure head in the tank, so the air bubbles into the tank at very low pressure. The rate of air flow is between 0.05 and 0.30 cubic foot of air per minute per cubic foot of pulp, and in most cases the air flow will be between 0.1 and 0.3 cubic foot per minute per cubic feet of pulp. The total power used to provide the air flow and to operate the impeller is between 0.010 and 0.035 horsepower per cubic foot of pulp (usually between 0.010 and 0.030), of which typically between 0.005 and 0.012 horsepower is used for the air.
By way of example, table 1 below compares three prior art processes, listed as A, B and C, with the process according to the invention, which is listed as D.
1n the processes of the above table the pulp contained ore having a specific gravity of about 2.7, and the pulp contained about 35 percent by weight of solids. Referring to the table:
The type A method represents a commercially well known machine known for its smaller space requirement. It has been mainly used on pulp volumes 'of about 60 cubic feet. The structure embodies complex pulp agitating and selfinduced air flow features.
The type B method represents a very well known widely used form of machine of complex agitating and pulp flow features.
The type C method again employs special baffle and agitation features but is of shallow depth similar to type B.
The type D method of the invention is ordinarily applied to cells of at least 300 cubic feet, and preferably to much larger cells, e.g. up to 2,100 cubic feet and beyond. In a typical 700 cubic foot cell according to the invention, the motor horsepower was 15, the consumed horsepower was 12, the mechanical horsepower per cubic foot of pulp was 0.017, the air flow in cubic feet per minute per cubic foot of pulp was 0.18, the power required to supply the air was 0.007 horsepower per cubic foot of pulp, the total power was 0.024 hon sepower per cubic foot of pulp, and the floor area required for each cubic foot of pulp was 0.1 square feet.
TABLE 11 Dimensions Pulp Consumed Air Total (diameter Volume Mcchanhp consumed height) re ical hp hp TPD TPD/Ft 8' X 6' 300 5 4.0 9.0 600 2.0 10 X 10' 700 15 4.9 19.9 1200 1.7 14' X 14' 2000 30 10 40 3000 1.5
of 38 to 40 percent by weight solids, with ore of 2.7
specific gravity (a pulp density of 1.30 of 1.33). The difference in power requirements between the 700 cubic foot cell of table 11 and the 700v cubic foot cell referred to previously was because the table 11 cell uses a six bladed impeller, whereas the previously mentioned 700 cubic foot cell used a 4 bladed impeller.
The powerfigures listed represent a drastic departure from prior art processes. In the past, in flotation processes where it has been desired to maintain the pulp in suspension and float the desired particles to the surface, it has invariably been the practice to use far higher powers. For example, R. J. Brison and E. H. Gates, writing in the Jan., 1970 issue of Mining Congress Journal, list at p. 52, 53 the horsepower required for typical large flotation cells as of Sept. 1, 1969. The cells described are typically 200 to 300 cubic feet, the installed motor horsepower for such cells is between 25 and 50, and the air horsepower for such cells is typically between about 2 and 5. The authors also write (at p. 53)'that the formerly large differences among flotation cells of different manufacturers in power per unit volume have almost disappeared in the new large machines.
Brison and Gates refer in their article to Froth Flotation, 50th Anniversary Issue, an authoritative text published by the American Institute of Mining, Metal-- lurgical, and Petroleum Engineers and edited by D.W. Fuerstenau. Ch. 14 of the text deals with flotation machines and points out at p. 352, that many experts believe that the power intensity (hp per cubic foot of cell volume) is the most important index of machine performance, and that the capacity is roughly proportional to cell power intensity. The text also lists at p. 354 the power intensities for several different kinds and sizes of cells; the listed figures vary between 0.080 and 1.08 horsepower per cubic foot. At p. 358, the text points out produce a uniform suspension of a coarsely ground copper sulphide ore, and that a power density of 0.25 hp per cubic foot was more satisfactory.
Surprisingly, the process of the invention, although it uses one-third or less of the power intensity formerly thought necessary, operates just as efficiently in recovering minerals as the prior art suspension flotationcells. For example table 111 below compares the efficiency of the present process with that of a typical TABLE 111 SCAVENGER CIRCUIT Type D TYPE 8 x10 Mill Tail From lnvention Tail Conventional Flotation DATE Cu. Cu.
Aug. 30/67 0.11 0.10 Aug.3l/67 0.11 1.10 Sept. 1/67 0.07 0.12 Sept. 2/67 0.10 0.13 Sept. 3/67 0.10 0.12 Sept. 4/67 0.11 0.13 Sept. 6/67 0.10 0.14 Sept. 7/67 0.13 0.13 Sept. 8/67 0.14 0.14 Sept. 9/67 0.14 0.15 Sept. 10/67 0.12 0.12 Sept. 11/67 0.12- 0.13 Sept. 12/67 0.17 0.14 Sept. 13/67 0.11 0.11 Sept. 14/67 0.10 0.12 Sept. 15/67 0.11 0.11 Sept. 16/67 0.12 0.13 Sept. 17/67 0.11 0.11 Sept. 18/67 0.11 0.12
prior art methods of type B, the results compared favorably. The installation and one year operating cost that it would require enormous floor space to operate commercially, and thus the capital cost of a plant using such cells would be enormous.
Another alternative prior art process is shown in U.S. Pat. No. 2,130,144 issued Sept. .13, 1938 to J. M. Mc-
' Clave. In this process, feed is aerated in a separate tank of type B was 4.8 times the corresponding cost of method D.
The use of very low agitating and air power according to the invention provides in general a substantial saving in capital cost of a flotation plant, particularly where a substantial number of cells is used. The invention also provides in general a substantial saving in operating costs, since the power consumption is less than one-third of that normally used in the past, and since there is less wear of parts at the lower powers used.
The invention also compares favorably with various flotation processes which have been suggested in the past as alternatives to the type of process with which the invention is involved (i.e. processes requiring suspension of the pulp). For example, U.S. Pat. No. 3,455,451 issued July 15, 1969 to P. R. Smith discloses a process in which a froth layer is established by bubbling air through an inclined porous plate, and a feed layer is fed into the froth, so that the desired particles remain floated and the gangue sinks onto the inclined porous plate and then slides off the plate.
The Smith cell requires no power for mechanical agitation, but it still requires a great deal .of power because it uses very large quantities of air (and pumping air is generally a less efficient process than pumping a liquid). Specifically, the Smith cell has an average depth of 6 inches above the perforated plate (which is inclined at an angle of 30 or more), and the air flow is given as 0.004 to 0.05 cubic feet per minute per square inch of plate. This is equal to between about 1.33 and 14 cubic feet of air per minute per cubic foot of pulp. More importantly, the Smith cell, which does not operate by keeping all of the pulp in suspension, must be very shallow increased depth confers no benefit), so
and is then introduced into a rectangular flotation cell having an inclined bottom. Desired particles float to the surface, while gangue sinks to the bottom and is carried off by an upwardly sloping screw conveyor extending along the bottom. The difficulty with this apparatus is that it will not operate at an acceptable feed rate where substantial quantities of fine particles are present (as is always the case in mineral separation), because it depends on settlement of the gangue, rather than keeping all of the pulp in suspension.
Specifically, in the McClave process, the screw con- .veyor removes primarily solids, and very little liquid, from the bottom of the tank, so most of the liquid introduced by the feed must rise to the surface of the tank and exit with the desired concentrate. At commercially acceptable feed rates, the rising liquid prevents fine particles from settling, and such fine particles are therefore swept out with the concentrate. The resultant concentrate will therefore be hopelessly contaminated if any substantial quantity of undesired fine particles is present. 1
1n mineral flotation processes to which the present invention is applicable, the ore is usually ground in a crusher, e.g. a ball mill, until the largest particles are no larger than a predetermined size. The crushing process forms many particles much smaller than that size, and typically 20 percent of the solids in the pulp feed may be less than 10 microns in diameter. (1,000 microns equal 1 millimeter). Such fine gangue particles will not readily settle, and therefore it is important where fine gangue particles are present, to employ a process of the type according to the invention, where nothing is allowed to settle to the bottom of the: tank but instead the entire pulp mass is kept in suspension and the gangue or waste is removed from a location well above the bottom of the tank.
The process of the invention will normally be used with feeds having a solid content of between 7 and 50 percent, and more usually between 20 and 40 percent. The process will normally be used with metallic ores, although it can be used for other ores (e.g. potash), and the ores when ground will normally contain at least 20 percent by weight of fine particles less than 10 microns in diameter. The process of the invention will of course suspend such fine particles and will float them when desired, and it will also float much larger particles. For example, in a feed consisting primarily of water and particles of a copper sulphide ore (specific gravity, about 2.70), particles as large as about 1,000 microns can be floated. Where the feed consists primarily of brine (having a specific gravity of about 1.24) and the particles to be floated are relatively light, e.g. potash particles (specific gravity, 2.04), much larger particles, up to about 3,400 microns, can be floated.
The process of theinvention can be operated at feed rates comparable to those of much higher power prior art processes. The feed rate will of course depend on the total volume of all the cells employed in the circuit and on the retention time needed for the particular mineral being separated. Some minerals are ore difficult to float than others and therefore require a longer retention time, and in addition, high grade ores are normally given a longer retention time than are low grade ores. The retention time is normally adjusted so that at least 80 percent of the desired mineral content of the pulp is recovered in the complete flotation circuit. Some circuits according to the invention have been operated at 96 to 97 percent overall recovery, processing high grade copper ore.
In order to achieve the above recovery efficiencies, it is found that with cells operated according to the invention, the required retention times are the same as for much higher power cells, i.e. typically between 10 minutes and 1 hour, depending on the type and grade ofore being processed. A common retention time for many ores is between 10 and 20 minutes. This retention time is divided among the cells forming the circuit, so that if the flotation circuit includes three equal size cells (normally a circuit will include at least three cells, to prevent loss of mineral values by short circuiting of the feed, i.e. direct travel of feed from the inlet to the outlet of a cell), then the retention time in each cell will be about one third of the total retention time.
From the total retention time needed, and the total volume of all the cells in the circuit, the tons per day per cubic foot of total cell capacity can be simply derived. If more cells are added, the feed rate can be increased, but since the total cell capacity also increases, the overall TPD per cubic foot remains the same. Typically cells in which the process of the invention is conducted will process at least 0.5 TPD per cubic foot of total cell pulp volume, more usually at least 0.75 TPD, with recovery efficiencies of 80 percent or more, and the tonnages can be much higher, e.g. 5 or 6 TPD per cubic foot of total cell volume, depending on the kind and grade of ore.
The tank 10 may vary in size, as indicated by table ll, but it will preferably have a capacity of at least 300 cubic feet of pulp and will commonly have a much larger capacity, e.g. 2,000 cubic feet or more. The size of the tank will however be limited by the mill capacity, since it is not desirable to have the entire flotation operation performed in one tank because of the risk of shortcircuiting the feed, as previously discussed. Usually at least three cells in series will be used in a flotation circuit. In smaller cells such as the 300-1,500 cubic foot capacity, the air flow will commonly not be less than .1 cubic feet per minute per cubic foot of pulp, but in larger cells such as those of 2,000 cubic foot capacity, an air flow of 0.075 cubic feet per minute per cubic foot of pulp was used with excellent results.
The shape and proportions of the tank 10 can be changed butpreferably the tank 10 is of substantial height, i.e. of height which is at least a substantial fraction of its minimum lateral dimension, to reduce the amount of floor space needed for the flotation machines. The tank height will not normally be less than about 75 percent of the cell diameter and will not usually exceed the cell diameter. Preferably the tank is cylindrical, and of height approximately equal to its diameter. It is found that conducting the process in such a tank yields particulafly good results, while control of the cell (e.g. of the amount of air introduced) is very simple, and the floor area required is much reduced as compared with conventional much shallower cells. It is found that the cylindrical tank used is far superior to a square or rectangular tank, since there are no corners at which boiling of the pulp can occur.
It will be appreciated that although there has been described a process in which the mineral portion of the pulp has been floated, the invention is equally applicable to the numerous processes in which the waste or gangue is floated and the mineral portion is removed from below the surface of the pulp.
What is claimed as my invention is:
1. A froth flotation process for a mineral pulp having a first portion to be floated and a second portion to be separated, one of said portions being a desired mineral portion, said process comprising a. introducing said pulp into a cylindrical tank having a volume of at least 300 cubic feet and having a depth substantially equal to its diameter, said pulp containing between 7 and 50 percent by weight solids, at least 20 percent by weight of said solids being particles having a diameter less than 10 microns,
b. mechanically agitating said pulp by means of an impeller centered in said tank and located at a distance above the bottom of said tank equal to between 10 and 25 percent of the height of said tank, and introducing air into said pulp at a location below said impeller and at a pressure just sufficient to overcome the pressure of the pulp in said tank, to maintain all of the particles of said pulp in suspension and to prevent their settling to the bottom of said tank, while floating at least a part of said first portion, said agitation and aeration being sufficient to float particles of said first portion of up to 1,000 microns diameter,
0. the total power input to said pulp from said impeller and through said air being in the range between 0.010 and 0.035 horsepower per cubic foot of pulp, and the flow of said air being in the range between 0.05 and 0.30 cubic feet per minute per cubic foot of pulp,
d. removing the floated part of said first portion from the surface of said pulp,
e. and removing said second portion at a location substantially above said impeller but below the surface of said pulp,
f. the feed rate of said pulp being at least 0.5 tons per day for each cubic foot of volume of said tank and the recovery of said mineral portion being at least percent.
2. A process according to claim 1 wherein said air is introduced at a single central location directly below said impeller.
3. A process according to claim 1 wherein said tank has a volume of at least 2,000 cubic feet and said total power input is in the range from 0.010 to 0.030 horsepower per cubic foot of pulp in said tank.