CA1206120A - Elutriation highgrader - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This specification relates to a counter-flow sedimentation separator which is designed to separate mixed aggregate materials on either side of a preset cutoff density and to a method of separating such materials. The separator has been designed primarily to highgrade gold tailings but can be used to preform a similar function with other materials given an adequate difference in density between the materials to be separated. It employs a multi-cloumn design which coupled with screening allows density separations to be com-pleted throughout the usual spectrum of particle sizes found in tailings and can be modified according to the material to be processed in order to maximize efficiency.

Description

This invention rela~es to a process and appara~us particularly applicable to the mining industry by which pa~ticles are separated according to density.

BACKGROUND OF THE INVENTION

Typically gold ore is composed of up to a few ounces of gold per tone of host medium. Gold has a dQnsity oE about 17 gmJcm3, and silica, a typical host medium, has a density of about 3 gm/cm3. Traditionally gold ore has been crushed into fine particles in a mill, then a cyanide leaching process is used to chemically remove the gold contained in the crushed ore. Once the gold is remo~ed the crushed ore or tailings (as they are now called) are disposed of. Invaria~ly ~his method is inefficient in that not all the gold is removed. The primary reason for this inefficiency lies in the fact that not all the gold is exposed to the leaching ac~ion due to inadequate crushing. This inadequate crushing is due to interference caused by the smaller particles protecting the larger particles which require further crushing.
Forms of an elutriation techni~ue have been in use in laboratories for decades where particles are sorted according to diameter, as in a ~Cyclo-sizer~ (Trademark of Warman International)O
The other major form of density separation previously used employed a stationary high densi~y slurry into which "

material to be separated was simply dropped then vibrated.
This batch method of density separa~ion has proven to be too slow and expensive to be used on a large scale.
Two Canadian patent~ of interest relating to the use 5 o~ fluids to separate mixed material6 aLe Canadian Patent No.
102,673 of Trottier i6sued DecembQr 18, 1906 and Canadian Patent No. 373,878 o Remick issued May 17, 1938. The Trottier reference describes and illustrates a single column apparatus for separating at differe~t levels in the column mate~ial of constant size but of different density. The column contai~s a plurality of vertically spaced 60rting tables and collection vanes which act to sort materials according to density and permit higher density material6 to fall while lighter density mater;als are drawn off at appropriate heights within the column do~n-shoots. The Remick patent describes an apparatus and proce~s for separating low density materials such as coal from a higher density host material such as slate. The mixed materials are int~oduced to a tank provided wi~h an u~wardly flowing current of water which carries particles of lesser s~e~ific gravity, such as coal, upwardly and out of the upper end o~ the tank onto an overflow wier, where they are collec~ed on a screen. ~gain the Remic~ apparatu~ is inef~icient over a large range of particle sizes.
The object of the invention i~ to apply a unique, controlled, combined elutria~ion screening technique to density separation~, pArticularly applicable to gold and diamond tailings in order to economically recover previo~ly unavailable valuable ma~erials.

SUMM~RY OF THE INVENTION

According to the present invention, there i8 pro~ided a counter-flow sedimentation separator and process designed to separate mixed aggregate materials by den6ity frac~ioning.
The invention embodie~ a process which utilizes both the classifying effect of a column o~ fluid, such as wa~er, flowing vertically and standard size screening techniques used in an alternating sequence. This process begins by introducing the particulate material, to be separated, into an upward flowing column of water known as an ~lutriation Column. The rate of flow of the water will depend on ~he size and density characteristics of the particles to ba separated, bu~ a velocity of about 20 cm~s would be typical. The high density underflow is retained, and the low density ~or smaller size) overflow is introduced into the next lower velscity classifying flow which allows smaller high densi~y particles to settle out. The underflow is screened, such that the material which is underflow due only to its relatively large size ~ecomes the di~carded oversize, the undersize being of high densi~y is retained. The overflow from this second classifying column is introduced into the next Elutriation column which again has a lower flow rate~ and the underflow is sub3ec~ed to a screening ~ ~6~

process with a smaller mesh SiZ2. Again it is the undersized underflow which is retained and the overflow continues to be introduced to lower velocity flows and finer mesh screening proces~es as described until the overflow is so fine that it is believed to contain very little gold.
The invention also embodies an apparatus consisting of a series of elutriation columns with decreasing flow rates and increasing column diameters such that the overflow from each column is introduced into the next column o~ the series. This apparatus employs a constant feed and can effect separations at a high rate with little expense. In a preferred embodiment, the apparatus is further desi~ned to introduce the material to be separated into a cyclon~ mixing chamber in order to break up any large lumps of tailings which might erroneously become under10w if not broken in~o particulate componen~s.

This apparatus according to the present in~ntion allows large quantities of tailings ~o be processed and high-graded. The highgraded tails can then be returned ~o ~he extraction plant where more gold can be extracted using ~radi-tional techniques. In order to effect the se2aration across ~he spectrum ~f particle sizes. multiple elutriation columns are coupled with screening techniques such tha~ the retained material is both under~low in the elu~ria~ion column and under-size with respect to the screening process.

BRIEF D~SCRIPTION O~ TH~ DRAWINGS

In drawings which illustrate example embodiments of the invention:
FIGURE 1 is a graph illustrating typical theoretical design considerations used to set flow rates, decide how man~
columns should be employed, and decide where screening pro-cedures should be employed in the apparatus and process of the present invention;
FIGURE 2 is a cross-sectional schematic view of a typical elutriation column of an example apparatus according to the present invention; and FIGURE 3 is a schematic representation of an example four-stage elutriation process according to the present lnvention.
While the invention will be described in conjunction with an example embodiment, it will be understood that it is not intended to limit ~he invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifi-cations and equivalents as may be included within ~he spirit and scope of the invention as defined by the appended claims.

DETA~LED DESCRIPTION OF~THE IMVENTION

The elutriation process described here refers to a type of dynamic counter-flow sedimentation in which the sep-~~` ~æ~

aration medium (liquid) is moved rather than the solid matterto be separated. Aggregate materials and tailings materials of restricted si~e ranges, generally less than 20 mesh, are ideally su;ted for elutriator sep~ration. The purpose of the design is to fab~icate a heavy density concentrate (underflow material) from a mixed aggregate material; the lighter density waste material is swept away by the upward flow of a separation medium such as water or air while the heavy density valuable material i~ allowed to fall into an entrapmen chamber for collection.
Theoretically, a particle of diameter "d~ and density " Ps", will fall at a velocity "v" through any liquid o~
density "~Q " and viscosity "~", as indicated by the equation:

d 9 (PS - P~) lg~

where "g" is the acceleration due to gravity. In an elutriation process, the variable "v" is used to set ~he flow rate of the liquid column such that particles of a required density "Ps" and diameter "d" will be at terminal velo~ity within the liquid and thus su~pended. Particles of greater diameter or density will drop to the bot~om as underflow and particles of a lesser size or density will be washed away by the fluid column, as overflow. Thus this system can be used to separate particles varying in size or density.
one probable use of the present invention would be to % ~-separate industrial grade diamonds from their host material as present separation techniquas often miss many of the smaller diamonds which are still of value in industrial applications.
The apearatus and process discussed here with respect to gold tailings would most likely require somewhat higher flow rate (up to several meters per second~ and slightly larger mesh screening ope~ations based on the di~ferent size and density characteristics of diamond tailings.

Turning to FIGURE 1, ideally line "X" represents the optimum separation cutoff line. This line is set just higher than the true solids density oP the tailsO thus only host material containing other higher density elements will settle out. In practice, however. separations a6 along line "X" must be approximated as indicated by the shaded area ahove lines 1 ~hrough 4 where the areas above lines 1 through 4 depict the product placement characteristics of the underflow for each column in a series of four. The dashed lines, "A", "Bl', and "C" indicate where screening processes are employed such ~hat the oversize is discarded, and the undersize which is to the righ~ of ~hese lines i5 retained for reprocessing. Thus ~he entire shaded area in FI~URE 1 represen~s the size density characteristics of the particles which will be re~ained. The curved lines, 1 through 4, are calculated as based on the pre-viously stated equation such that flow rate "v" is set and the25 density of particles of a given size is calcula~ed. Decreasing ~he flow ra~e has the effect of broadening the range of the ~%~

curve. All the curves asymptotically approach the density of the separation medium itself. Thus as can be seen in FIGURE 1, ~he curves can be broadened to the right by decreasing the flow such that at zero flow tha parabola i5 flattened into a ~trai~ht line along the density of the separation medium.
Similarly the vert;cal lines representing the screening proc2ss can be adjusted in order to op~imize efficiency.
The number of stages ~o be used is also an important consideration. The greater the number of stages the greater the efficiency of the apparatus as a whole, howe~er, as the overflow from each column is introduced to ~he subsequent column, the inflow volume increases rapidly from one column to the next. Very quickly siæe considerations limit the number of stages to be used.
In the actual portion of the apparatus where the sep-arations are being made, the ideal flow pattern is ~hat o~ plug flow, (ie. the exact design velocîty across the entire cross-section o~ the separation column) however in ~ractice this must be app~oximated due ~o the finite viscosity of the separation medium. Thus the higher the Reynold ' 8 number describing the flow in the aolumn the more accurate the separation, and thus the greater e~ficiency of the appara~u~ as a whole. Maximizing the Reynold's number must however be weighed against restric-tions of physical size. This problem becomes especially25 cLitical when dealing with the lo~er ~low rates. However as it is improbable that the tradi~ional leaching process would erroneously miss much of the gold associa~ed with minus 270 mesh particles, separa~ions this low in the spectrum need not be carried out. Further, this lack of gold associated with the veLy fine particles allows desliming of the slurry before it is introduced to the first mixing chamber without adversely affecting column e~ficiency. Th;s desliming process is useful in that it removes the Eines and thus inhibits larger conglomerate particles from corming as these particles would be mistakenly retained.
FIGURE 2 i6 a ~chematic representation of a typical appa~atus where the overflow from each column enters the mixing chamber ~ of the next column 2. Each succe6sive column 2 also has a lower flow rate produced by water pump 5 to allow finer particles as underflow. Generally the system would be ~et to concentrate the desired element by a factor of about 100, but this can be altered.
In a further effort to break down large particles in the mixing chamber 4 the slurry is introduced such that it will cau~e a cyclone effect in the mixing chamber. The lower set of baffles 8 prevent these currents from in~erfering with the elu~riation action. The upper set of baffles 10 again reduce the turbulence in the upper part of the mixing chamber 4. This allows particles which should have become underflow, but were caught up in the turbulence of the mixing chamber, to settle back down to the elutriation column 12.
The apparatus repre6ented in FIGUR~ 3 comprises ~our 6~20 separate columns 2 like those in FIGURE 2 such that the over-flow of each column is introduced to the mixing chamber of the nex~ column in ~he series. The underElow to be screened is removed from the undeLflow valve 14, at the bottom of settling chamber 16 below each separation column 12.
As an alternative, the process may be carried out essentially in reverse, ~hat is, using a series of screening ~rocedures, then making individual counter-flow sedimentation density separations on the individual size fractions from each screening procedure.

Typically ~he process of the invention as applied to gold ore recovery employs 3 to 8 elutria~ion columns with flow rates in the range of 100 to 0.3 cm/sec., as well as 2 to 7 screening operations usually between the 20 to 270 mesh sizes.
(For diamond tailings, the flow rates of the columns would be in the range of 1000 to 1 cm/sec.~. Further, as the density difference between diamonds and host material is often relatively small, additional columns (perhaps up to ten) will be required to achieve adequate separation efficiency.
Possibly the optimum approach would embody an initial screening proee~s to separate the tailings into various size fractions, which could ~hen be introduced in~o separate apparatus, with the flow rates appropriate to that portion of the tailings. An example apparatus of this type designed to process 100 tonnes of gold tailings per hour would be comprised of four steel columns standing side- by-side, each about 10 m in height and ~:~æ~

up to two metres in diameter. A system of these dimensions would require a water supply with a capacity of about 2500 l/min and a h~ad of around 15 metres. This new appara-tus allows the economical recovery of quantities of gold or other a~propriate materials which until this time have been hidden ln worthless tailinys.
The previously described apparatus could also be used to highgrade material other than gold, given that tha required material be of a different density than i~5 host ma~erial.
FIGURE 1 graphically represents a majori~y of the design considerations pertinent to this apparatus. As stated earlier the horizontal straight line labelled "X" represents the ideal point at which the desi~ed separation is to be made. The elu~riation highgrader could also be incorporated in a standard extraction procedure on the ore itself ra~her than just the tails. Finally in order to highgrade by a very large factor, large quantities of underflow (after standard screening) could be re-introduced to the apparatus. I~ association with this system, a system for pumping and recycling the separation medium, and a slurry feed system will also be required, however the nature and design of these systems are obvious to a person skilled in the art.
The single elutriation technique ha~ been employed in the pa~t to effect classifications based on par~icle size with a great deal o~ success as the terminal velocity of a particle falling through a liquid varies directly with the square of the o6~2~

particle diameter. Further, as most aggregate materials are of very consis~ent densi~y, accurate size separations by elutriation are ~uite simple. The apparatus and process according to the prasent invention are unigue in that they perorm separations based on particle density with little regard to particle size. This system should prove to be economically successful as the tailings which are to be concentrated are abundant, pre-crushed, above-ground (and thus far more accessible than ore), and can have considerable concentrations of residual gold especially if the tailings were processed years ago when the cyanide leaching process was less refined. Furthermore, as water will probably be ~he most common separation medium, the environmental and health problems associated with the leaching proces6es used in gold e~traction can be minimi2ed.

Thus it is apparent that ~here has been provided in accordance with the invention coun~er-flow ~edimentation separator that fully satisfies the objects, aim~ and advanta~es set forth above. While the invention has been described in conjunction wi~h a specific embodiment thereof, it is evident that many alternatives, modi~ications and ~ariati~ns will ~e apparent to those skilled in the art in light of the foregoing description. Accordingly, it is in~ended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of th~ appended claims.

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for separating desired particulate high density materials from associated particulate lower density materials, comprising passing the particulate high and lower density materials through a series of elutriation columns through which an appropriate medium flows to produce underflow and overflow, with the overflow from each column being introduced to the next elutriation column in the series, the flow rate of the medium in each column being a decrease from that of the previous column and selected to have desired materials of decreasing size being contained in the underflow from each successive column.

2. A process according to claim 1 wherein the underflow from each column is screened in a screening step, the mesh sizes employed in each screening step being progressively smaller through the series and being selected according to the flow rate from the elutriation column from which the underflow passed, and wherein the desired material is the undersize from each screening column.

3. A process according to claim 1 or 2 wherein the process is particularly adapted for separating gold from associate particulate lower density materials, and wherein the flow rate of each step is selected from a rate in the range from 0.3 to 100 cm/sec..

4. A process according to claim 1 or 2 wherein the process is particularly adapted for separating diamonds from associate particulate lower density materials, and wherein the flow rate of each column is selected from a rate in the range from 1 to 1000 cm/sec..

5. A process according to claim 1 wherein the particulate high and lower density materials are based into a cyclone mixing chamber at the commencement of each elutriation column to assist in breaking down large sized materials.

6. A process according to claim 2 wherein the elutriation columns are set to concentrate the desired material by a factor of about 100.

7. An apparatus for separating particulate high density materials from associated particulate lower density materials comprising a plurality of elutriation columns, the overflow from each column constituting the inflow into the next column of the series, during operation the flow rate in each successive column decreasing from one column to the next, the bottom of each column being provided with an outlet for underflow means being provided to collect the underflow from each column.

8. An apparatus according to claim 7 having two to eight columns forming the series.

9. An apparatus according to claim 7 wherein each column comprises a cyclone mixing chamber in which the high density and lower density materials are initially fed, each column further being provided with a separation column below the cyclone mixing chamber and communicating therewith and a settling chamber beneath the separation column communicating therewith, fluid to operate the columns being introduced to the column through the settling chamber.

10. An apparatus according to claim 7 further provided with a screen for each column to receive and screen the under-flow from that column, the mesh size of each screen being pre-determined and increasing in numeral value in the series as the flow rate of the corresponding column decreases in the series.

11. An apparatus according to claim 10 wherein each screen has a mesh size selected from sizes in the range of from about 20 to about 270 mesh.

12. An apparatus according to claim 9 wherein the mixing chamber is provided with a set of lower baffles to facilitate the elutriation action in the column and an upper set of baffles to reduce turbulence in the upper part of the mixing chamber.

13. A process according to claim 1 where the high and lower density materials are screened before passing to the series of elutriation columns, and wherein the elutriation flow rates are selected according to the size fraction resulting from each screening step.

14. A process according to claim 13 wherein screening steps precede each elutriation columns

15. A process according to claim 14 wherein each size fraction is introduced for elutriation to an individual one of said columns with an appropriate flow rate, each underflow being the desired high density material.