Ultrasonic flotation system

An ultrasound flotation unit for separation of tails from liquors obtained by floating ores, wherein the unit comprises a vertically disposed cylindrical mixing chamber that forms a bubble-particle contact region within a cylindrical flotation cell that has a bubble-pulp separation region surrounding said mixing chamber; the chamber having an air feed conduit in a lower portion thereof, an ore pulp feed conduit in a higher portion above said air feeding chamber, an ultrasonic transducer disposed above an aperture in a top portion of the mixing chamber and means for subjecting said chamber to an amount of power in kilowatt-hours per metric ton through a sonic probe to focus an amount in watt/ml of ultrasonic power to the chamber to provide a residence time of slurry within the bubble-pulp separator region that is about 30 to 100 times longer than in the ultrasonic mixing chamber.

FIELD OF THE INVENTION 
The present invention relates generally to an ultrasonic flotation system 
for use by the minerals industry for flotation of hydrophobic minerals 
particles or ions from an aqueous medium. The ultrasonic power is focused 
in a small portion of the cell (the mixing chamber) in order to 
effectively utilize the agitation energy to quickly attach hydrophobic 
particles to the bubbles. As a result of combining the ultrasonic power to 
a small portion of the cell, the ultrasonic vibrations provide an 
efficient mixing energy that creates micro-agitation within the fluid to 
effectively collide the mineral particles with the bubble. The ultrasonic 
vibrations from the flotation system generate small bubbles of air and 
disperse them quickly throughout the slurry in the mixing chamber and 
cause cavitation of the fluid at the particle surface which precipitates 
dissolved air upon the surface of the hydrophobic mineral particles. These 
small bubbles of air attach to particles faster than in the case of 
conventional flotation systems. 
DESCRIPTION OF THE PRIOR ART 
Mineral processing researchers have tried for years to utilize the micro 
agitation which can be provided by ultrasound to enhance the flotation 
separation of minerals. However, most of the literature addresses the 
effects of ultrasound on the ore conditioning per se prior to commencement 
of flotation. Some of the researchers who have reported improved flotation 
recovery using ultrasonically emulsified flotation collector reagents are: 
Glembotski et al. "Flotation of Ores," USSR, Jan. 25, 1961, 159 pp.; 
Khan et al. "Application of Ultrasound For Selection In Collective 
Concentrates", Inst. Steel and Alloys, Moscow, Vol. 7, No. 3, 1964, pp. 
27-31; and Ponteleeva et al. "Effect of Preliminary Ultrasonic Treatment 
of the Pulp on Floatability", Inst. Steel and Alloys, Moscow, Vol. 7, No. 
3, 1964, pp. 27-31. 
Glembotski et al reported improved selectivity by ultrasonically 
conditioning a complex Cu-Pb or Pb-Zn ore pulp before conventional 
flotation separation. Ultrasound has also been used to dereagentize 
concentrate pulps by ultrasonically destroying the absorption layers on 
the mineral surfaces. 
Khan et al reported that selective flotation was achieved by their 
technique on a mixed sulfide concentrate. 
These researchers have proclaimed several reasons for improved recovery 
with ultrasonic treatment, such as cleaner mineral surfaces with more 
sites available for collector attachment, better dispersion of the ore 
particles, selective flocculation of the fine hydrophobic particles, and 
more effective dispersion and distribution of the flotation reagents among 
the ore particles (Sastri et al. "Some Effects of Ultrasonics and Their 
Application in Metallic Ore Processing", Journal of Scientific and 
Industrial Research, Vol. 36(8), 1977, pp. 379-385). 
A few studies have been conducted using ultrasound during flotation. .sup.1 
Stoev et al studied the effect of ultrasonic dispersion of air bubbles 
which resulted in improved recovery of fine coal and .sup.2 Nicol et al, 
investigated fine-particle flotation in an acoustic field. Improved 
flotation kinetics were obtained with ultrasound especially for fine size 
particles (minus 10 .mu.m). These authors suggested that ultrasonic 
cavitation at the particle's surface caused dissolved air to precipitate 
out on the mineral surface. This mechanism overcomes the hydrodynamic 
fluid flow limitations that occur in fine particle flotation and allows 
improved recovery of the fine particles. However, the energy costs for the 
ultrasonic agitation were too high for economical application. 
FNT .sup.1 "Flotation with Sound Carrying Bubbles," Coke Chem. Vol. 7, USSR, 
1966, p. 12E. .sup.2 "Fine-Particle Flotation in an Acoustic Field" 
International Journal of Mineral Processing, 17, 1986, pp. 143-150. 
Recent flotation hydrodynamic research by the Bureau of Mines (Jordan et 
al. "New Flotation Technology to Recover Ultrafine Chalcopyrite," SME Fall 
Meeting, St. Louis, Mo. Preprint No. 86-335, 1987, 11 pp.) has shown the 
importance of turbulent agitation on the recovery of fine particles. The 
turbulent fluid flow contained numerous microscopic eddies that bring 
together the bubbles and particles more frequently than the more quiescent 
streamline flows. This micro-agitation increased the number of 
particle-bubble collisions, produced faster flotation kinetics, and 
resulted in higher recovery of the fine particles. However, these studies 
also show that the most effective agitation within a conventional 
flotation cell occurred in only a small portion of the flotation cell 
volume (Jordan et al, "Evaluation of a Turbulent Flow Model for Fine 
Bubble and Fine Particle Flotation," SME Annual Meeting, Las Vegas, Nev., 
Preprint No. 89-172, 1989, 12 pp.). 
To optimize flotation the micro agitation energy should be concentrated on 
the ore pulp only long enough to attach the particle to the bubble. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to advance the 
technology of flotation beneficiation by providing a continuous ultrasonic 
flotation unit designed to focus the microagitation of ultrasonic 
vibrations on the ore pulp in a small mixing chamber for short period of 
time. 
It is another object of the present invention to provide a continuous 
ultrasonic flotation unit which permits a mixture of ore, water and air to 
be ultrasonically agitated as it is passed through a small mixing chamber. 
A yet further object of the present invention is to provide a continuous 
ultrasonic flotation unit that permits the ultrasonic agitation to 
disperse the air into small bubbles that quickly attach to the hydrophobic 
minerals. 
A further object yet still of the present invention is to provide a 
ultrasonic flotation system which permits the mixture to exit from the 
agitation chamber so that the air bubbles, with the hydrophobic particles 
already attached, quickly rise to the top of the separator. 
A still further object of the present invention is to provide an ultrasonic 
flotation system which permits hydrophilic particles to settle to the 
bottom of the separator and be recoverable as tailings. 
In achieving the foregoing and other objects in accordance with the 
ultrasonic flotation system of the invention as embodied and broadly 
described herein, a continuous ultrasonic flotation system was invented to 
both generate air bubbles in the ore pulp and effectively collide the 
newly generated bubbles with the hydrophobic ore particles. Towards these 
ends, two distinct regions are provided within the ultrasonic flotation 
cells. The first region is a bubble-particle contact region which consists 
of a vertical cylindrical mixing chamber that allows air to enter through 
a conduit at it's bottom and the conditioned ore to enter the cylinder of 
the mixing chamber above the entering air feedport and conduit. An 
ultrasonic transducer is positioned at the top where the slurry exit the 
mixing chamber. As the air, water and ore particles move through the 
mixing chamber, the ultrasonic agitation breaks the air into small bubbles 
and vigorously mixes the ore particles with the newly generated bubbles. 
At this point, the hydrophobic ore particles attach to the bubbles and 
leave the mixing chamber as a bubble-particle agglomerate. The second 
region of the sonic flotation cell is the bubble-pulp separation region 
formed by the cylindrical flotation cell. As the bubble-particle mixture 
exits the mixing chamber, it disperses within the much larger and 
relatively quiescent bubble pulp separator. The air bubbles with attached 
hydrophobic minerals rise to the top of the chamber to form a froth, and 
overflow along the outer edge of the cylinder. The mineral particles that 
are not attached to the air bubbles settle out and exit through the bottom 
of the flotation cell as tailings. The residence time of the slurry within 
the bubble-pulp separator region is from 30 to 100 times longer than the 
residence time in the ultrasonic mixing chamber, and this effectively 
concentrates the ultrasonic energy for rapid bubble-particle attachment in 
from about 1 to about 3% of the cell's volume.

DETAILED DESCRIPTION OF THE INVENTION 
In general, single and two stage processing of the ultrasound floatation 
system were tested. The best P.sub.2 O.sub.5 recoveries were obtained at 
the 400 ml/minute feed rate where a 21% P.sub.2 O.sub.5 concentrate was 
produced at a 94% recovery. At the faster feed rate, 800 ml/minute, a 19% 
P.sub.2 O.sub.5 concentrate was produced with 91% recovery of the 
phosphate. In both fine and coarse phosphate flotation, two stage 
processing was more effective than single stage processing. 
Conventional laboratory batch flotation tests were conducted with a 250-g 
Denver DR flotation cell on both the coarse and fine phosphate flotation 
feeds for comparison with the ultrasonic flotation tests. For the coarse 
phosphate flotation, the combined flotation concentrate was 30% P.sub.2 
O.sub.5 and it recovered 92% of the phosphate. The flotation rate constant 
was 1.26 min.sup.-1. Kelly, E.G., D. J. Spotteswood, "Flotation and Other 
Surface Separations", Introduction to Mineral Processing, John Wiley & 
Sons, Inc., 1982, New York, N.Y., p. 317. The speed of flotation is 
expressed by a first order differential equation with a flotation 
constant. Plant flotation recoveries for the coarse phosphate feed are 
typically around 60%. Conventional laboratory flotation recovery of the 
coarse phosphate feed is typically higher than the recovery obtained in 
the plant under similar reagent dosages. Moudgil et al. "Enhanced Recovery 
of Coarse Particles During Phosphate Flotation," Annual Report--Florida 
Institute of Phosphate Research, October, 1988, 59 pp; suggested that the 
hydrodynamics of the laboratory flotation cell were much more effective 
than the plant scale flotation cells. For the fine phosphate flotation 
feed, the conventional laboratory flotation cell produced a 27% P.sub.2 
O.sub.5 concentrate and recovered 91% of the phosphate. Its flotation rate 
constant was slightly lower at 1.20 min.sup.-1. These results are 
comparable to typical phosphate results and formed the baseline for 
comparison with the test results from the ultrasonic flotation tests. The 
ultrasonic flotation tests were similar in grade and recovery to the 
conventional laboratory flotation tests. However, the flotation rate was 
2.5 times faster for the ultrasonic flotation cell than the laboratory 
conventional flotation cell. 
In selecting the best overall conditions for ultrasonic flotation, the 
concentrate grade, phosphate recovery, flotation rate and energy 
consumption must all be balanced. The best grade and phosphate recovery 
for the coarse feed was obtained at a relatively slow flotation rate and 
with a high energy consumption. As shown in table 1, 94% recovery of the 
phosphate was obtained for the coarse feed at a power consumption of 6.8 
(kilowatt-hours per metric ton) kW.h/mt. By increasing the feed rate to 
1,000 ml/minute the energy requirements decreased to 4.3 kW.h/mt with a 
corresponding decrease in phosphate recovery from 94 to 84%. This data 
shows that by sacrificing P.sub.2 O.sub.5 recovery, the energy consumption 
can be lowered and the flotation rate can be increased. For the fine feed, 
increasing the feed rate lowered the energy requirements from 7.5 to 3.0 
kW.h/mt while the phosphate recovery remained the same. 
TABLE 1 
______________________________________ 
Comparison of ultrasonic flotation results at two 
different.sub.1 power levels for coarse and fine phosphate 
flotation. 
Feed rate, ml/minute 
Coarse feed 
Fine feed 
500 1000 400 800 
______________________________________ 
Air to ore ratio ml/g 
5.5 5.5 9.5 9.5 
Concentrate grade pct P205 
30 30 19 20 
P205 recovery % 94 84 91 90 
Flotation rate constant min.sup.-1 
1.95 3.6 4.1 4.7 
Power consumption kW .multidot. h/mt 
4.3 7.5 3.0 
______________________________________ 
.sub.1 For two stages of flotation 
To optimize the system for a given ore, the factors of cost, production 
capacity, recovery and grade must all be considered. The system was tested 
with a phosphate ore, but will work just as well on other ores. The ore 
contained phosphate and quartz and ranged in particle size from 400 mesh 
(38 .mu.m) to 16 mesh (1000 .mu.m). To simulate a typical Florida 
phosphate operation, the ore was split into size fractions (Lawver, 1983), 
coarse feed (420 .mu.m to 100 .mu.m size) and fine feed (32 .mu.m to 420 
.mu.m size). Each test sample was conditioned with fatty acid at 67% pct 
solids for 5 minutes in a slow speed mixer. For the coarse phosphate 
conditioning, 0.4 g/kg fatty acid was used. The fine phosphate flotation 
feed was conditioned with 0.4 g/kg fatty acid and 0.2 g/kg sodium silicate 
to depress the fine silica. After conditioning the sample was placed in a 
feeding tank diluted with water and frother (Dowfroth 1012) to a 
concentration of 25 ppm frother. The feed slurry was pumped to the 
ultrasonic cell at a fixed feed rate until the conditioned ore sample was 
depleted. 
During testing, the effect of flotation staging was investigated by passing 
the wet tailings product through the ultrasonic flotation cell again. 
During the second pass or procesing through the ultrasonic flotation cell, 
no additional reagents were used. Timed samples of the first and second 
concentrates and the final tailings were dried and analyzed for P.sub.2 
O.sub.5. 
Several feed rates and airflow rates were tested along with the two mixing 
chambers, the three ultrasonic probe positions, and the single or two 
stage process. Factorial designs of the experiments were conducted to 
maximize the experimental data, quantify the reproducibility, and minimize 
the number of experiments. A two by three factorial design was conducted 
on the coarse phosphate feed, to test the effect of agitation chamber type 
and position of the ultrasonic probe. The 33% solids coarse phosphate was 
fed to the ultrasonic system at 500 ml/minute and at an air to ore ratio 
of 10.3 ml air per gram of ore. Mixing chamber #1 and #2 were tested with 
the ultrasonic probe positioned 10 mm below, evenwith, and 10 mm above the 
top of the mixing chamber. Statistically, there was no significant 
variation in the product phosphate grade, which averaged about 27% P.sub.2 
O.sub.5. The best results were obtained with mixing chamber #1 with the 
ultrasonic probe even with the top of the mixing chamber. The product 
grade was 27% P.sub. 2 O.sub.5, the flotation rate was 3.14 min.sup.-1, 
and the phosphate recovery was 92%. The energy consumption for both mixing 
chambers was 6.8 kW.h/mt of feed. 
A similar factorial design was conducted using the fine phosphate which was 
fed at 50% solids and 400 ml/minute with an air to ore ratio of 9.5 ml/g. 
The mixing chamber type and the position of the ultrasonic probe had no 
effect upon the product grade which averaged 22% P.sub.2 O.sub.5. The best 
flotation rate constant of 3.76 min.sup.-1 was obtained with the 
ultrasonic probe 10 mm below the top of the #1 mixing chamber. 
A three by three factorial design was conducted to study the effect of feed 
rate, air to ore ratio and flotation staging. A 33% solids coarse 
phosphate slurry was fed at 500, 1000, and 2000 ml/minute with air to ore 
ratios of 5.5, 10.3, and 23.5 ml/g. Only the #1 mixing chamber was tested 
and the ultrasonic probe was positioned even with the top of the mixing 
chamber. Each sample was passed through the system twice to determine the 
effect of flotation staging. The best conditions for the coarse phosphate 
feed occurred at a feed rate of 500 ml/minute, 5.5 ml/g air to ore ratio, 
and two stages. A 30% P.sub.2 O.sub.5 concentrate was produced that 
recovered 94% of the phosphate. The flotation rate constant was 1.95 
min.sup.-1, conventional laboratory flotation cell. 
The fine phosphate ore feed was also tested in a factorially designed 
experiment with two feed rates, three air to ore ratios, and two types of 
flotation staging using mixing chamber #1. The fine phosphate was fed at 
50% solids at 400 ml/minute and 800 ml/minute. The air to ore ratio ranged 
from 4.3 to 13.0 mL/g, and both single and two stage processing were 
tested. The best P.sub.2 O.sub.5 recoveries were obtained at the 400 
mL/min feed rate where a 21 pct P.sub.2 O.sub.5 concentrate was produced 
at a 94 pct recovery. At the faster feed rate, 800 mL/min, a 19 pct 
P.sub.2 O.sub.5 concentrate was produced with 91 pct recovery of the 
phosphate. As it was with the coarse phosphate flotation, two stage 
processing was more effective than the single stage process. The 
continuous ultrasonic flotation system is designed to both generate air 
bubbles in the ore pulp and effectively collide the newly generated 
bubbles with the hydrophobic ore particles. 
As shown in FIG. 1, there are two distinct regions within the ultrasonic 
flotation cell 10. The first region 11 is the bubble-particle contact 
region which consists of a vertical cylindrical mixing chamber. The air 
enters a conduit 12 at the bottom and the conditioned ore enters the 
cylinder mixing chamber through a conduit 13 above the air feedport. The 
ultrasonic transducer probe 14 is positioned at the top where the slurry 
exits the mixing chamber, as depicted by arrows A. As the air, water, and 
ore particles move through the mixing chamber the ultrasonic agitation 
breaks the air into small bubbles and vigorously mixes the ore particles 
with the newly generated bubbles. The hydrophobic ore particles attach to 
the bubbles and leave the mixing chamber as a bubble-particle agglomerate. 
The second region of the sonic flotation cell is the bubble-pulp 
separation region 15. This region is formed by the cylindrical flotation 
cell. As the bubble-particle mixture exits the mixing chamber, it 
disperses within the much larger and relatively quiescent bubble pulp 
separator. The air bubbles with attached hydrophobic minerals rise to the 
top of the cylinder, form a froth, and overflow along the outer edge of 
the cylinder through conduit 16. The mineral particles that are not 
attached to the air bubbles settle out and exit through the bottom of the 
flotation cell. The residence time of the slurry within the bubble-pulp 
separator region is 30 to 100 times longer than the residence time in the 
ultrasonic mixing chamber. As already mentioned, this effectively 
concentrates the ultrasonic energy for rapid bubble-particle attachment in 
1 to 3% of the cell's volume. 
For the prototype, two different mixing chambers were tested. Each mixing 
chamber being designed to provide 1 to 2 watt/ml of ultrasonic power 
within the mixing chamber. The height to diameter ratio of the mixing 
chamber was also varied between 1:1 to 3:1. The first mixing chamber (#1) 
was 25 mm diameter and 25 mm high. The air feed port was at the bottom and 
the ore feed port was 10 mm from the bottom. The second mixing chamber 
(#2) was 18 mm diameter and 60 mm high. It's air feed port was also at the 
bottom, but the feed port was 20 mm up from the bottom. The ultrasonic 
agitation was supplied by a Sonicator Ultrasonic Liquid Processor. The 
20-kHz ultrasonic vibrations from the transducer crystals were amplified 
through an acoustic horn called a probe that focussed the ultrasonic 
vibrations on a flat, 12-mm-diameter tip. This tip was centered near the 
top of the mixing chamber. Tests were conducted at three different tip 
positions, 10 mm down from the top, even with the top, and 10 mm above 
the top of the mixing chamber. The effective volumes of the mixing chamber 
depended upon the position of the ultrasonic probe tip and ranged from 6 
to 12 ml for the 1 chamber and 8 to 12 ml for the #2 chamber. The 
agitated mixture exited the mixing chamber through an aperture created by 
the top of the mixing chamber and ultrasonic probe. Power measurements 
were recorded for each test and converted to kilowatt-hours per metric ton 
(kW.h/mt) of feed. The bubble-pulp separator region was formed by a 
100-mm-diameter cylinder. At the top of the bubble-pulp separation region, 
a shallow froth developed and overflowed the top edge forming the 
flotation concentrate. The effective volume of the bubble-pulp separator 
was defined as the region from the top of the mixing chamber to the top of 
the bubble-pulp separator. These effective volumes ranged from 390 to 670 
mL depending upon the mixing chamber and the ultrasonic probe tip 
position. The region outside and below the top of the mixing chamber is 
the tailings consolidation zone 17 where the unattached particles 
descended as shown by arrow A.sub.2. These particles settled and were 
removed by a small pump (not shown) in the tailings region. The overall 
dimensions of the prototype ultrasonic flotation cell were 100 mm diameter 
and 100 mm high. A liquid overflow (not shown) was attached to the 
flotation cell to maintain a constant height of fluid within the 
ultrasonic flotation cell. The height of this overflow pipe is adjustable 
and depended upon the feedrate of the system. 
It is apparent from the foregoing that the ultrasonic flotation system 
provides several advantages over conventional flotation systems, not the 
least of which are: 
1) Focussing the ultrasonic power in a small portion of the cell (the 
mixing chamber) effectively utilizes the agitation energy to quickly 
attach the hydrophobic mineral particles to the bubbles and the mixing 
energy is efficiently used by this technique; 
2) The ultrasonic vibrations create micro-agitation within the fluid to 
effectively collide the particles with the bubbles; 
3) The ultrasonic vibrations generate small bubbles of air and disperse 
them quickly throughout the slurry in the mixing chamber; and 
4) The ultrasonic vibrations cause cavitation of the fluid at the particle 
surface which precipitates dissolved air upon the surface of the 
hydrophobic particles and these small bubbles of air attach to the 
particles faster than would be the case when using conventional flotation 
systems. 
While the foregoing description and illustrations of the present invention 
have been shown in detail with reference to preferred embodiments as well 
as alternate modifications thereof, it is to be understood by those 
skilled in the art that the foregoing and other modifications are 
illustrative only, and that equivalent changes may be employed, without 
departing from the spirit and scope of the invention, which is defined by 
the appended claims.