SHAW, Raymond (79 MCilwraith Street, Princes Hill, Victoria 3054, AU)
A process for formation of gas bubble particle aggregates within a slurry and separating the formed aggregates from the slurry including using first and second flotation cells wherein the first and second cells operate with different levels of shear and including transfer of externally formed gas bubble hydrophobic particle aggregates to either or both of the first and second cells.
A process for formation of gas bubble hydrophobic particle aggregates within a slurry and separating the formed aggregates from the slurry including using first and second flotation cells wherein the first and second flotation cells operate with different levels of shear.
A process according to claim 1 or claim 2 wherein the first flotation cell operates with a low shear rate to form and separate weakly bonded gas bubble particle aggregates .
A process according to claim 3 where the first flotation cell includes an internal draft tube with a downwards pumping impellor through which slurry incorporating gas bubbles passes and from which gas bubble hydrophobic particle aggregates flow in a low turbulence predominantly laminar flow pattern to enter a separation zone.
A process according to claim 4 where the first flotation cell includes a venturi to draw gas into the slurry to form the slurry gas bubble mix which passes into the draft tube and flows downward driven by the impellor rotation.
A process according to claim 5 where a slurry of fine particles are collided with gas bubbles in an external high shear sparger before entering the first flotation cell. 2
A process according to claims 1 to 6 where the first flotation cell includes at least 3 offtakes to allow separation of the gas bubble hydrophobic particles aggregates from the other particles where the other particles are classified into different streams based on their size and/or density.
A process according to claims 1 to 7 wherein the low shear flotation cell includes sloped blades to impart a swirling motion on the slurry to give improved separation of the gas bubble particle aggregates from the free particles within the slurry .
A process according to claim 1 and 2 where the first flotation cells is a low shear fluidised particle bed device where the gas bubbles collide with the mineral particles within a fluid bed.
A process according to claim 9 where the first cell includes a means of imparting a swirling motion on the fluidised particle bed.
A process according to claims 1 and 2 wherein the second flotation cell operates with high shear to enable collision of fine particles with gas bubbles to form gas bubble hydrophobic particle aggregates .
A process according to claim 10 where the high shear is achieved using a high shear rate mechanical impellor turning at high speeds .
A process according to claim 11 where the high shear is achieved using a high pressure sparger either external to or within the cell .
A process according to claims 1 and 2 where a third flotation cell is included which is a
conventional mechanical cell operating at medium shear .
A process to separate gas particle bubble
aggregates formed within the slurry within a
flotation cell by collecting them in a froth
including transferring externally formed gas bubble hydrophobic particle aggregates to the flotation cell .
A process according to claims 1 and 15 wherein the gas bubble hydrophobic particle aggregates is predominantly composed of particles > 75um.
A process according to claims 1 and 15 wherein the gas bubble hydrophobic particle aggregates is predominantly composed of particles <45um.
A process according to claims 1 and 15 where the transferred gas bubble hydrophobic particle
aggregates are obtained from other flotation cells within the plant.
A process according to claim 17 where the gas bubble fine hydrophobic particles being recycled are obtained from high shear flotation cells .
A process according to claim 16 where the gas bubble hydrophobic particle aggregates being
recycled are obtained from low shear flotation cells .
Summary - The present invention relates to a novel
approach to the use of gas bubble hydrophobic particle aggregates to give improved separation and recovery of valuable minerals from a wide range of ore types . The invention is particularly applicable to heterogeneous ores where one or more of the minerals is either naturally hydrophobic, or can be selectively made hydrophobic through chemical additions, and substantial segregation (liberation) of the minerals occurs when the ore is crushed and/or ground to a size below 3mm.
Background - Flotation is currently used to separate many minerals from unwanted gangue. This is commonly achieved by crushing and grinding the ore to quite fine sizes
(typically predominantly <150um) , adding chemical (s) to render one or more minerals hydrophobic and forming and separating gas bubble hydrophobic particle aggregates using multiple flotation cells.
Current flotation circuits are commonly composed of mechanical flotation cells used to recover the mineral containing particles from the ground ore (rougher cells) and either more mechanical flotation cells and/or column flotation cells used to "clean" the initial concentrate by more selective flotation of the valuable minerals .
The overall recovery of the valuable minerals is mainly controlled by the performance of the rougher cells as the tails from these, and subsequent scavenger cells, is discarded. The feed ore is commonly ground sufficiently to enable good recovery through the rougher section of the flotation plant. This grinding is very energy intensive and is the main cost in processing many ores and therefore the recovery achieved is normally a balance between finer grinding versus how much material is lost through the circuit. The performance of the rougher section of the flotation circuit is therefore critical to the overall economics of the operation and also to the total amount of energy and water required in the processing.
The "cleaning" often also involves further grinding of the initial concentrate to increase the liberation of the different minerals to improve their separation. The recovery of the valuable minerals is normally very high through the cleaning process and the grinding costs are moderate as although only the initial concentrate from the first stage separation needs to be ground this is usually ground much finer to improve separation .
The presence of excessive unwanted entrained particles of gangue which contain little or no valuable mineral is therefore undesirable as it increases the amount of material needing to be treated. Although the costs of the cleaning are moderate the ability to separate gangue from the valuable minerals is important as this determines the quality of the product and hence the value as feed to further processing. The particles fed to the rougher flotation cells typically have a wide range of particle sizes as neither the grinding mills nor the hydrocyclones used to classify the output from the grinding mills can produce narrow size distributions. The rougher section of the plant should ideally be capable of recovering particles with a wide size range. In many copper plants for example the
particles range from as fine as lOum up to around 250um with the valuable mineral being present in particles of all of these sizes.
The particles in the feed slurry also have a range of compositions from just being composed of the valuable or waste mineral through to composites which have varying amounts of each. The chemical treatment improves the hydrophobicity of all particles which have some of the target mineral exposed at the surface but the level of hydrophobicity achieved varies with the composition. The flotation cells therefore also need to be capable of recovering particles with different levels of
hydrophobicity as well as with different sizes. The current mechanical cells commonly used in most flotation plants are primarily designed to collect strongly hydrophobic particles which are in the size range of 30-lOOum. The finer particles, even if strongly
hydrophobic, are not always collected because it is difficult to efficiently collide them with gas bubbles and achieve attachment .
Conventional tank flotation cells typically use radial flow turbine type impellers to disperse air bubbles and create turbulence and high shear to force the
particles and bubbles to collide and for the hydrophobic ones to become attached and form particle-gas bubble aggregates. The strength of the attachment depends upon both the particle size and hydrophobicity and for the coarser particles and the less hydrophobic particles the strength is insufficient to survive the turbulence and most of these particles then detach and largely behave the same as the unattached hydrophililic particles . Significantly reducing the speed and/or size of these impellers in conventional cells to reduce the turbulence and shear is not practical as this leads to inefficient gas dispersion and also poor solids suspension as these impellers are not highly efficient at retaining solids in suspension especially the coarser particles which are more prone to settling. Collision efficiency is generally improved by having high shear environments and/or using finer gas bubbles but even with those the collision efficiency for finer particles is much lower than for coarser particles . The coarser particles collide readily but are often detached from the bubbles by being exposed to high shear due to the turbulent mixing within the cell and/or detach from the bubbles in the froth zone and are not recovered. The partially hydrophobic particles are particularly prone to detachment in the pulp and the froth as their adhesion to bubbles is weaker than that of the highly hydrophobic particles. The particles with 20-50% hydrophobic surface are more difficult to recover than those with >50% hydrophobic surface and are often lost in flotation circuits when their particle size is >75um.
The difficulty in recovering even the strongly hydrophobic fine particles has been well known for many years and specialist high shear equipment has been
developed to treat slurries containing only fine particles such as are found in concentrates fed to cleaner circuits but this equipment is not commonly used for the wider size distributions in the ground ore.
In many conventional plants increasing the speed of the impellor within the mechanical cells to impart more energy and shear has been found to improve the recovery of these fine particles as has decreasing the size of the gas bubble either through changes in the way they are
dispersed and/or through adding chemicals such as
increased amounts of frother to change bubble behaviour and give finer bubbles .
This increase in the energy and impellor speed improves the fines recovery but is detrimental to the recovery of the coarser and less hydrophobic particles. The coarser particles readily collide with gas bubbles and if they are sufficiently hydrophobic will attach but if then subjected to shear forces such as in the turbulent region of the mechanical cells they detach and therefore are not collected as desired.
Typically highly hydrophobic particles above lOOum can detach in conventional float cells with those above 150um being very prone to detachment as evidenced by the relatively poor recovery of those particles in most flotation cells. The detachment is also more likely for particles of moderate hydrophobicity as measured by the contact angle with the coarser particles (>75um) having a contact angle below 60° being quite at risk. The second major contributor to poor recovery of coarse and/or less hydrophobic particles is the difficulty of removing them from the flotation cell even if they remain attached to the bubbles and rise to the top of the pulp. Typically a froth layer is used to hold the bubble- particle aggregates above the slurry in the flotation cell and this froth is removed either by flowing off the surface and/or by being mechanically raked off. The coarser particles are prone to become detached from the bubbles within this froth layer before they can be removed and fall back into the pulp below and are lost.
The amount of particles lost depends upon their size, hydrophobicity and the stability of the froth layer.
Particles with moderate hydrophobicity above 75um are most at risk with those above 150um even if strongly
hydrophobic being very difficult to collect unless the froth is highly stable. The stability of the froth depends primarily on the amount of hydrophobic fine particles present and modifying the surface tension using chemicals .
The amount of material collected in the froth depends upon the stability of the individual bubbles and the amount of time the froth spends in the flotation tank. The hydrophobic fine particles stabilise the bubbles such that the amount of coalescence is low and therefore more material is collected in the froth as few particles fall off during coalescence and more liquid with entrained solids is retained in the froth as the total liquid content in the plateau borders between the bubbles is dependant on the bubble size. This type of fine bubble froth helps maximise the recovery of all size particles but leads to more unwanted gangue particles being
collected than is usually desirable.
In contrast the larger hydrophobic particles can promote coalescence leading to larger bubbles in the froth and less entrained liquid and correspondingly less
unwanted gangue. The current flotation circuits are not designed to have a controlled balance of coarse and fine particles present in individual cells as the high shear conditions that favour the presence of fine particles are directly opposite to the low shear conditions which allow the hydrophobic large particles to enter the froth.
These issues with losses of fine and coarse particles and excessive collection of unwanted gangue in the froth have been well understood within the industry for many years but despite that there is still no generally
acceptable way of overcoming them apart from to grind all of the ore sufficiently fine that there are minimal coarse particles remaining to be lost and using additional
"cleaning" stages to separate out the gangue particles. This is costly and many plants that are unable to grind the ore sufficiently fine accept the coarse particle losses as a process inefficiency. Neither of these outcomes is satisfactory as finer grinding is very expensive financially, uses large amounts of energy with corresponding greenhouse implications , and also generated fine tailings which are difficult to dispose of and also increase the amount of water used in the process. The losses in coarse particles are also an economic loss as well as adding to the specific energy needed in that the lost particles have consumed
considerable energy through the mining and initial
Invention - This invention provides a process for
formation of gas bubble particle aggregates within a slurry and separating the formed aggregates from the slurry including using first and second flotation cells wherein the first and second cells operate with different levels of shear and including transfer of externally formed gas bubble hydrophobic particle aggregates to either or both of the first and second cells.
The present invention relates particularly, although by no means exclusively, to preparing a slurry of particles that are less than 3mm and which substantially contains a mixture of particles that are well wetted by water (hydrophilic) and others that are non wetted
(hydrophobic) and composites with a heterogeneous surface which are partially hydrophobic and partially hydrophilic such that the hydrophobic particles and a desired amount of the partially hydrophobic particles are separated from the fully and predominantly hydrophilic particles using flotation and/or gravity assisted separation much more efficiently than is achieved using currently available commercial processes.
This separation depends upon the particles colliding with gas bubbles, normally but not always air, wherein the gas attaches to hydrophobic surfaces on the particles to form particle-gas bubble aggregates. These aggregates have a lower density than the unattached particles and can therefore be separated based on their different flow properties within the slurry. Most commonly they are allowed to float to the surface of a flotation cell where they form a froth which flows over the top of the tank and is collected.
The hydrophobic and/or partially hydrophobic particles may arise naturally such as is the case for some carbonaceous materials and some clays , or more commonly arises from chemical treatment where "collector" chemicals are added to the slurry which preferentially attach themselves to target minerals in the areas where they are exposed at the surface to render the particles containing significant amounts of these minerals partially
The use of gas bubble particle aggregates to achieve separation is applicable to a wide range of materials particularly minerals such as the base metal sulphides , oxides and silicates such as with iron ores , halides such as potash and hydrocarbon containing
materials such as coal, oil shale and the Canadian Oil sands. The main criteria is that at least some of the particles within the slurry are more hydrophobic than others either as they occur naturally or after further treatment such as by adding chemical reagents, oxidation, reduction or reacting with chemicals such as sulphides such that differential attachment of gas bubbles is possible . The amount of mineral exposure and consequent surface coverage by the chemical depends upon the size, density and overall composition of the particle but is preferably over 10%, more preferably over 25% and even more preferably over 50% to give sufficient hydrophobicity that gas bubbles can be attached sufficiently strongly to enable separation of the gas bubble aggregate. The size to which the particles are ground depends upon the size and distribution of the different minerals within the ore and is selected such that the majority of the target valuable mineral is contained within particles that have at least 5% valuable mineral and preferably within particles that over 15% of valuable mineral and more preferably with 25% of the valuable mineral and a substantial portion preferably over 50% and more preferably over 70% and even more preferably over 90% of the particles contain little or no valuable mineral.
The size needed can be determined by crushing and grinding the ore to a range of sizes and analysing the particles within each fraction using either chemical analysis and/or using X-Ray or electron microscopy
techniques such as X-Ray Tomography, The MLA or QEM-SCA . More preferably the size required is determined by
measuring the grain size and spatial distribution of the minerals within coarse ore particles (typically 5-50mm) using a technique such as X-Ray Tomography and then calculating the distribution of minerals within particles of each size as the ore is crushed.
This process is particularly suited to recovering coarser mineral particles in the size range 100-lOOOum which are partially or fully hydrophobic whilst also achieving high recovery of the more readily floated partially or fully hydrophobic particles in the size range 30-lOOum and of the more difficult to float partially or fully hydrophobic fine particles that are less than 30um.
The process uses flotation cells operating with two or more different levels of shear within the primary stage of the circuit where the ground ore is processed in the rougher and scavenger cells. The first type of
flotation cell operates with low shear rates and low turbulence sufficient to enable collisions between gas bubbles and coarse particles typically over 75um but with shear rates low enough that the aggregates that form are not disrupted within the cell . The second type of flotation cell operates with a high shear rate and high turbulence to enable collision of fine (< 30um) particles with gas bubbles .
A third type of flotation cell operates with a medium shear rate and medium turbulence with sufficient shear to enable collision of medium size particles (30- lOOum) but not such high shear rates that the gas bubble particle aggregates formed within the cell are disrupted before they can be separated and collected from the cell .
The actual shear rates present in any mixed systems are difficult to specify accurately because there is usually a wide range of shear rates especially in cells which have mechanical impellors where the highest shear found at the impellor tip is much larger than that within other regions of the cell . A more common approach used for these cells is to relate the shear to the amount of energy fed into the cell . For mechanically agitated cells the energy input for high shear cells would typically be > 5W/kg with that for low shear cells being < 0.5W/kg and the medium shear cells falling between these in the range 0.5W/kg to 5W/kg. The turbulence within the cells is linked to the shear rates in that high shear rates cause high turbulence whereas medium shear rates create medium levels of
turbulence and low shear rates create low levels of turbulence. These terms are often interchanged when describing flotation cells and that is also the case in this patent with the level of turbulence described also describing the level of shear and vice versa. For non conventional cells and external spargers the shear rates are set by the duty requirement with the energy and related shear for the low shear cells being set as the minimum required to maintain the coarse solids sufficiently suspended to make contact with the bubbles and to not settle in the cell and build a stationary layer within the cell and prevent operation.
For the high shear cells the energy and related shear is set by the need to give sufficient collision frequency of the fine particles and bubbles to collect the hydrophobic particles . The medium shear cells typically have sufficient energy and related shear to maintain a well mixed collision zone for particles in the 30-lOOum range and sufficient gas dispersion that the hydrophobic and partially hydrophobic particles collide with and attach to bubbles without significant detachment.
This technology is ideally suited to the use of non conventional low shear flotation cells specifically designed for maximising the recovery of weakly bonded particle gas bubble aggregates such as occur with coarse particles and/or composite particles with only a limited surface exposure of the minerals which give the
The non conventional low shear flotation cells can be of the fluid bed type as proposed by Jameson in patent application WO2010/135760 (Al) or by Barbee
(US4822493) or can use novel designs as shown in the drawings given as Figures 5 to 9 of this application.
The high shear rate cells can be mechanical cells where the impellor is designed to give high shear rates or where existing cells have the impellor speed increased to give more shear or can be cells which incorporate a sparger wherein the slurry is contacted with the gas bubbles outside of the main section of the tank using a pressurised sparger and the gas bubble particle mix with pre-existing gas bubble particle aggregates is fed into the main body of the flotation cell for separation.
The medium shear rate cells are commonly
conventional mechanical cells as are widely used
throughout the industry. The separation of the gas bubble particle aggregates is commonly achieved by allowing them to float to the surface and form a froth which is then mechanically removed from the flotation cell. The stability of the froth formed is therefore of critical importance to achieving high recovery of the aggregates.
The low shear rate cells do not give efficient collision of fine particles with gas bubbles and therefore predominantly form weakly bonded aggregates with coarse and/or partially hydrophobic particles. These aggregates are not capable of forming even moderately stable froths and most of the aggregates break up and the particles fall back into the slurry and are lost. One method of
overcoming this is to deliberately operate the low shear flotation cells without a froth and include a removal method at the surface such as is used in the HydroFloat teetered bed cells from Eriez Manufacturing.
An alternative approach is to provide aggregates composed of fine hydrophobic particles and gas bubbles to the cell such that these aggregates also enter the froth. Fine hydrophobic particles are known to stabilise
flotation froths so the addition of these aggregates assists in forming a froth with sufficient stability to collect the weakly bonded aggregates. The fine hydrophobic particles need to be formed external to the cell but are sufficiently strongly bonded that they can be added into the mixing zone of the flotation cell from where they can float to the surface along with the weakly bonded
aggregates . The fine particle gas bubble aggregates can be formed by passing the feed slurry through a high shear sparger prior to it entering the cell or can be
transferred from another flotation cell where the
flotation concentrate is predominantly made up of
hydrophobic fine particles such as would be produced in a high shear rate cell .
The degree of separation of the hydrophobic particles entering the froth in gas bubble particle aggregates from the hydrophilic free particles present in the slurry below the froth also depends upon the froth stability. Some slurry is entrained in the froth and carried over with the valuable minerals which lowers the quality of the concentrate produced. The amount of slurry is primarily dependent on the bubble size within the froth with froths composed of finer bubbles entraining more slurry than those with coarser bubbles .
With very stable froths where there is limited coalescence of the bubbles coming from the pulp phase there is considerable unwanted entrainment. The presence of some coarse hydrophobic particles is beneficial to these froths as they promote coalescence of the bubbles and decrease the entrainment. Hydrophobic coarse particles are generally not able to form stable gas bubble particle aggregates in the high shear rate cells as they are disrupted by the high shear rates in the mixing zone and therefore if desired are best added into the cell either directly into the froth and/or the more quiescent upper part of the flotation cell as preformed aggregates transferred from another cell within the circuit. Hydrophobic coarse particles can more readily be added to column cells of the type used in cleaner circuits where the contacting of the slurry with the slurry is done using external spargers as the shear rates within the cells themselves are quite low. In these cells the
hydrophobic coarse particles can be added directly into the cell preferably as aggregates or alternatively into the upper section of the cell such that they collide with rising gas bubbles to form aggregates . The coarse
hydrophobic particles can increase coalescence in the froth which tends to be stabilised by the presence of fine hydrophobic particles to give larger bubbles and less entrapped water and hence less entrainment of hydrophilic particles .
The drawings included in this patent application show how this technology can be applied for separating minerals such as copper sulphides from waste silicate and oxide gangue and the same types of arrangements can be used for the others systems included in the patent decription.
Figure 1 shows one means of using varied shear level cells within a flotation plant. The classified feed stream from the comminution circuit (1) which is typically a cyclone overflow is first fed into medium shear
flotation cells (10) to collect the strongly hydrophobic mid sized particles which are typically in the range 30- lOOum as a concentrate (2) . The tailings from these cells (3) then pass to lower shear flotation cells where the coarser hydrophobic and partially hydrophobic particles are collected as concentrate (4) and the tailings are removed and fed to high shear flotation cells (12) . The fine hydrophobic particles are recovered into froth in these cells and removed as concentrate (6) and the tailings which are substantially depleted of hydprophobic particles are removed and either discarded or sent to additional cells such as in a scavenger circuit to collect any remaining valuable minerals . If these low shear cells desire to operate with a stabilised froth then part (8) of the concentrate
containing fine hydrophobic particles (6) is recycled from the high shear cells to the low shear cells (12) to improve the stability of the froth formed in those cells and improve the recovery of the coarse particles. In this arrangement the sequence of the cells can be varied with the high shear cells precede the low shear cells such that the more weakly bonded aggregates are removed in the final cells .
Figure 2 shows another arrangement of the
different shear cells where the feed material (1) is classified using conventional means such as hydrocyclones (13) into coarse (14) and fine (15) particle streams prior to being fed to the flotation cells. With this arrangement the coarse particles are within a higher solids stream with lower overall flow than is the case when the
classification is not used and this can assist with the recovery of the hydrophobic particles in the low shear cells (11) .
None, part or all of the fine particle stream from the classification (15) can be aerated in a high shear sparger to contact the fine hydrophobic particles with bubbles prior to feeding part or all of this stream into the low shear flotation cells (11) . Where
insufficient fine particles are available from the feed slurry stream some fine concentrate (8) can be recycled to improve recovery in the froth. The improved recovery and lower flow to the low shear cells improve performance sufficient to offset the added costs of the extra step. Figure 3 shows the same basic configuration as in Figure 2 with the addition of a centrifugal separator (16) to give enhanced gravity separation of the fine particle concentrate (6) into a clean concentrate (17) and a lower grade stream with a mix of hydrophobic and hydrophilic fine particles (18) which is recycled to the low shear cells to assist with froth stability. This also enables the direct production of a higher grade concentrate (17) without mineral losses than could normally be achieved.
Figure 4 shows the use of low shear float cells for flash flotation to treat the discharge from the comminution circuit (19) at a much earlier stage where a high solids classified stream containing coarse particles (21) is fed directly to low shear flotation cells (11) to recover coarse particles before this stream is recycled to the grinding circuit. This feed stream can be produced using conventional grinding mills and/or using alternative breakage methods such as HPGR' s , high energy pulsed microwaves and/or electric pulse breakage.
The tailings (22) from the cell(s) are returned to the grinding circuit. The finer fraction (1) from the classification unit (20) are typically fed into the flotation circuit and treated as per any of the other examples given. If required part of the fine concentrate (8) can be transferred to the low shear flash float cell to assist in stabilising the froth and improving the recovery of the coarse particles .
Figure 5 shows a novel design for a low shear, low turbulence flotation cell as shown. This design uses a cylindrical tank (7a) containing an inner draft tube (9a) open at both ends and which has an impeller (6a) fitted part way down driven from the top to cause flow of the slurry as indicated by the arrows within the diagram. The feed slurry (la) containing a mix of desired and undesired particles is fed into the tank through a pipe (12a) located such that the flow is into the upper part of the draft tube above the impeller. The upper section of this pipe contains a venturi (13a) to draw gas bubbles (2a) into the slurry to collide with the particles and form gas bubble particle aggregates .
This aerated slurry (14a) is then passed into the draft tube system being injected above the impeller such that as it passes through the impeller it is mixed with recycled flow from the remainder of the tank. The level of flow of the two streams, and the amount of gas injected, are controlled to give stable operation of the impeller, and avoid cavitation and excessive turbulence, and good mixing of the two streams as they pass down the draft tube and out into the main volume of the tank. Normally the mixing achieved by passing the slurry through the venturi will cause sufficient collisions between the coarse particles and the gas bubbles to form the desired particle - gas bubble aggregates and achieve separation of the hydrophobic and hydrophilic particles .
The directed flow of the slurry within the cell also enables additional particle gas bubble collisions to occur as particles that are dislodged from gas particle bubble aggregates that are either not able to make the transition from the liquid slurry into the froth or are disrupted by coalescence in the froth phase are able to collide with gas bubbles and reform aggregates . This mechanism is particularly suited for the coarser particles which require only modest collison energy to form
If desired a sparge ring can also be fitted to the cell (10a) to provide additional air with this sparge ring (10a) located such that the flow down the draft tube below the impeller is forced past the bubbles coming from the sparge ring. The desired particle - gas bubble
aggregates substantially separate from the bulk of the slurry near the top of the cell and instead of being drawn back into the draft tube and recirculated form a froth (3a) on top of the cell and flow over the lip into a launder (8a) for recovery of a concentrate (4a) . A portion of the slurry is removed from a separate offtake (11a) normally designed and placed so as to minimise the amount of particle-gas bubble aggregates entrained. This slurry is then either disposed of as tails (5a) or is fed to another cell or other equipment for further treatment.
The impeller to force the slurry past the sparger at a sufficiently high flow that the gas bubbles become entrained in the slurry. The majority of the gas bubbles are entrained rather than flowing upwards under their buoyancy towards the impeller. As they become entrained, and during their flow within the slurry, the bubbles and particles are able to collide and attachment of the hydrophobic particles to the gas bubbles occurs.
In some cases where there are not sufficient collisions especially for the particles typically below 200 microns additional collisions can be promoted by adding an in line mixer (not shown) into the system
between the venturi and its outlet into the draft tube. This mixer is not as high shear as is often used for very fine particles in column cells but rather is carefully designed to give increased mixing without creating such high turbulence and high shear that there is excessive detachment of the coarse particles from gas bubbles .
Figure 6 shows a further modification of this design where this novel cell is also used to directly recover fine particles within the slurry. For feed
slurries with significant numbers of valuable particles below the coarse range improved separation and recovery is possible by incorporating separate air spargers (16a) of the type used on column cells such as the Metso Minerals CISA Microcel sparger to enable collision between the fine particles and air bubbles under turbulent conditions .
Other slurry streams containing fine hydrophobic such as concentrate recycles can also be added to the cell either directly or through a sparger of this type.
This slurry from the external sparger can be fed directly into the cell as the fine particle-gas bubble aggregates formed in the external spargers are quite stable and are not substantially disrupted by passage through the impeller, venturi or lower turbulence in line mixer. These gas-particle aggregates flow down the draft tube and then up the main tank and enter the froth which they help stabilise as they are recovered. Any free bubbles in this aerated slurry can undergo further collisions with hydrophobic particles and improve the overall recovery within the flotation cell .
Figure 7 shows a modified arrangement of this cell where part of the slurry is withdrawn from within the tank and recycled through one or air contacting systems. This enables both more collisions to occur (particularly important for fines) and more air to be drawn in for low energy collisions (preferred for coarse) . Figure 8 shows three offtakes from the tank which is operated such that the flow imparted by the impeller is such that there is some size segregation within the tank whereby a stream enriched in coarser particles can be withdrawn using an offtake (18a) near the bottom of the tank for recycle (20a) and/or transfer to the next processing step or disposal (19a) and a stream enriched in finer particles can be withdrawn from a higher level offtake (11a) and recycled separately, normally through the high pressure sparger, and/or transferred for further treatment. Further modifications can be included to the flotation cell design to achieve higher levels of
efficiency for specific needs. This use of three off takes can enable separation of the pulp into the concentrate product flowing over the top lip, a fine particle slurry stream taken from an outlet part way up the side of the tank and a coarse particle stream taken from an outlet placed near the bottom of the tank can be enhanced by deliberately inducing a swirl within the tank such as can be achieved by using horizontal baffles (21a) such as is done with Toroidal gas solid reactors of the type marketed by Torftech as Torbed.
The different flow properties of the particles which have bubbles attached and the unattached fine and coarse particles makes this possible. The three streams can then be treated separately with if desired some of the fine unattached particles being passed through a sparger to achieve more gas particle collisions and some of the coarse being recycled to the top of the draft tube most probably through the venturi and possibly an additional mixer. Alternatively these streams can be taken to other equipment in the plant for further processing or one or other disposed of as waste.
Figure 8 shows an alternative design for a low shear rate cell in which the particles separation is enhanced by imparting centrifugal forces on them. The feed slurry (lc) which has previously been contacted with gas in a separate external contactor contains a mix of gas bubble - particle aggregates and individual hydrophilic particles is fed into the tank through a bottom inlet and distributed across the tank or part thereof such that it enters the main section of the tank (2c) by passing through a set of angled blades (3c) which impart a swirl motion onto the flow. The slurry stream passes up through the tank in a spiral motion as suggested by the flow direction arrows (8c) . As the slurry moves upwards the heavier isolated hydrophilic particles move towards the tank wall and are removed through one or more outlets on the side of the tank. The lighter bubble-particle aggregates continue to move up the tank until they move into the froth layer (6c) for collection as a concentrate (7c) . The upper section of the tank contains a suitable system to ensure that the slurry level in the tank does not become high enough for this to flow out of the tank with the froth.
Figure 9 shows a further alternative for a low shear rate cell designed primarily to separate and recover quite coarse particles (>150um) . This cell has separate entry points for the high solids coarse particles stream (9c) and the lower or even solids free aerated stream
(10c) . The high solids stream enters from the top of the cell and is distributed around the cell via a conical distributor (11c) such that it contacts the aerated stream (10c) which imparts a swirl on the entire stream.
The form of the slurry in the cell depends upon the relative flow of the two streams and their solids loadings with one preferred approach being to have the solids form a toroidal shaped fluidised bed (12c) in which the hydrophobic and partially hydrophobic particles being fed in the high solids stream (9c) are brought into contact with the gas bubbles in the aerated stream (10c) to form bubble particle aggregates . These aggregates , and the finer individual hydrophilic particles then move upwards in the tank in a spiral motion and are removed separately. The larger hydrophilic particles move to the side of the tank within the fluid bed and are removed from the upper section of this fluid bed through one or more side outlets in this area of the tank (4c) .
Figure 10 shows the arrangement where coarse
hydrophobic particles in the concentrate from the low shear cells (4) are fed to the column cleaner cell (27) with the fine concentrate from the high shear cells (6) which gives a high grade concentrate (28) and a tailings stream (29) which is commonly recycled to regrinding and further treatment .
Concentrators configured to take advantage of this use of staged collection of the valuable minerals using different shear levels for contacting the gas bubbles and particles combined with selective transfer of particles between cells to maximise their recovery via the froth phase can achieve improved recovery with less grinding energy required and in some situations the ability to make a higher quality product.
Preferably these concentrators include the use of one or more new cell designs with much lower shear levels and turbulence than existing mechanical cells but with
sufficient agitation to avoid settling of the coarser particles causing inefficient collisions and/or in extreme cases sanding up and blockages in the circuit.
For new plants these changes should allow the ore grinding circuit to produce coarser product than is currently the case hence reducing the energy and water usage. The primary stages of size reduction of the ore may be able to just use the more energy efficient devices such as High Pressure Grinding Rolls. The staged shear is also particularly beneficial for plants which use new breakage equipment such as that based on microwave and/or electric pulse fragmentation, which have been shown to give
improved segregation of minerals at particles sizes around 200-800um which is too coarse for conventional circuits and equipment but which can be treated using the equipment and circuit configuration of this invention.