GB2153262A

GB2153262A - Froth flotation apparatus

Info

Publication number: GB2153262A
Application number: GB8500890A
Authority: GB
Inventors: Francis George Miller
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-01-30
Filing date: 1985-01-14
Publication date: 1985-08-21
Also published as: AU3762285A; GB8500890D0

Abstract

A froth flotation installation for separating solid particles from an aqueous slurry includes a tank (30) divided by baffles (139, 140, 141) into a plurality of zones. The direction of flow of aqueous slurry of unrecovered solid particles is changed by these baffles at least twice between the first froth flotation zone and the last froth flotation zone to improve solids- froth contact and to minimize the possibility of particles adopting a short circuit path through the installation. A plurality of such froth flotation zones is serviced by a single slurry pump 148. The first froth flotation zone is larger than any other so that the mean residence time of the aqueous slurry is greater in the first froth flotation zone. Agitation and aeration of the slurry is performed by vortex chambers (135-7) having the construction shown in Fig. 7. <IMAGE>

Description

SPECIFICATION Froth flotation separation method and apparatus THIS INVENTION relates to froth flotation separation installations and more particularly those froth flotation separation installations wherein a mixture of solid particles is separated into a float product and a non-float (or sink) product during transit as an aqueous slurry through sequential flotation zones in which the aqueous slurry is repeatedly agitated and in which gaseous bubbles are introduced adjacent to the bottom of each flotation zone. The float product passes upwardly through the aqueous slurry with the gaseous bubbles and is collected as a froth above the upper surface of the aqueous slurry. The aqueous slurry which is not recovered as a float product is recovered as a non-float (or sink) product.

Froth flotation installations are widely used in the mineral separation industries for separating solid raw materials into a useful product and a waste product according to the difference in the physical properties of the materials, especially the surface properties of the raw materials. Froth flotation is extensively used to concentrate coal or mineral sulfides and oxides. Finely ground ores or coal have particles with different surface properties with respect to water, i.e. some particles are hydrophilic and some particles are hydrophobic. In some ores, all of the particles are hydrophilic in varying degrees. The differential in hydrophilic characteristics permits separation of the more hydrophilic particles from the less hydrophilic particles. The finely divided fresh particles are agitated in water with air bubbles.

The bubbles and the particles combine and rise to the surface of the aqueous slurry as a frothy concentrate which can be skimmed for collection above the level of the aqueous slurry of unrecovered particles. The more hydrophilic particles remain in the aqueous slurry and are recovered as a sink product.

Various chemical reagents are added to the froth flotation installations to improve the recovery. These reagents are: frothing agents which alter the surface tension of the water and thus promote small bubble formation; collectors which improve the attachment of particles to bubbles and assist in forming a stable froth; activators which improve the performance of the collectors; depressants which selectively interfere with the effectiveness of the collectors.

While the present invention is applicable to separation of mineral ores, its application to coal separation will be discussed in detail for simplicity. Coal which is to be separated in froth flotation equipment is customarily ground to fine particle size, for example, 0.75 millimetres. The fine particles of coal are delivered as an aqueous slurry as raw fine coal or obtained from prior separation equipment (.e.g centrifugal separators such as hydrocyclones, screens, etc.). The function of the froth flotation process is to recover two distinct products. The float product contains most of the combustible ingredients of the raw coal and generally has a reduced sulfur content and a reduced ash content when compared with the raw coal. The non-float (or sink) product contains less combustible ingredients, more ash ingredients and generally more sulfur ingredients than the raw coal.

Typically the froth flotation process is carried out in a number of sequential flotation cells wherein an aqueous slurry of raw coal solids is introduced into a first froth flotation cell and subjected to agitation with rising gas bubbles to permit flotation of the more hydrophobic particles for recovery as a froth above the liquid surface of the aqueous slurry of unrecovered solids. The aqueous slurry of unrecovered solids moves from the first flotation zone to a second flotation zone where the slurry is again agitated with freshly created upwardly rising gas bubbles to effect further seoparation of the more hydrophobic particles.

An aqueous slurry of unrecovered, non-float solids passes from the second flotation zone through succeeding intermediate flotation zones, if any, where the agitation of the aqueous slurry of unrecovered solids with upwardly rising freshly created gas bubbles is repeated and froth containing the more hydrophobic particles is recovered above the level of the slurry of unrecovered, non-float solids.

The aqueous slurry of unrecovered, nonfloat solids from the last of the intermediate flotation zones is delivered to the last flotation zone where a final agitation of the aqueous slurry with freshly created rising gas bubbles is carried out. The more hydrophobic remaining particles rise upwardly along with gas bubbles in the last flotation zone. An aqueous slurry of unrecovered, non-float solid particles from the last flotation zone is separately recovered as the non-float (or sink) product of the process. One of the shortcomings of the sequential froth flotation separation process is the amount of energy required to agitate the aqueous slurry of unrecovered solids and to generate fresh gas bubbles near the bottom of each individual flotation zone.A typical agitation/bubble formation involves a motor-driven mechanical agitator in the central region of each individual flotation zone for creating agitation and aeration in the aqueous slurry. The energy required in each of the flotation zones is appreciable and significantly affects the cost of the separation process.

Another phenomenon associated with froth flotation is the increasing difficulty of establishing efficient separation in succeeding froth flotation zones. In the initial froth flotation zone, the incoming raw coal solids have a substantial fraction of particles which will enter into the float product. However as these float product particles are removed from the first flotation zone, there are fewer float product particles remaining in the aqueous slurry of unrecovered solids which is delivered to each succeeding flotation zone. Each succeeding flotation zone requires greater energy to create additional agitation and aeration to achieve the more difficult separations. The recent history of froth flotation installations shows that the energy of the agitation apparatus is not increased in response to this need for progressively higher energy from feed to tailings.Instead, common practice has been to install long lines of smaller agitation apparatus, each with about equal energy requirements. By providing more froth flotation zones, each with a smaller agitator apparatus, high separation efficiencies can be maintained but at a sacrifice of greately increased length of the froth flotation installation with resulting higher cost. Hence, to reduce the higher costs, the current practice is to increase the size of the agitator-aerators, and to increase the size of the flotation zones, but to shorten the length of the froth flotation installation.

The reduction in overall length decreases investment expenses and reduces building requirements and energy consumption. However, separation efficiency, resulting from shorter length and lower energy, may be adversely affected.

One of the devices heretofore employed in froth flotation units is a vortex chamber which receives pressurized liquid from a tangential entry pipe and delivers a single bottom liquid product with great turbulance. The vortex chamber also functions as an aspirator for flotation gas and hence comprises a single unit which achieves the requisite agitation and flotation gas bubble formation within a froth flotation cell. The aqueous slurry introduced into such vortex chambers heretofore is a side stream of aqueous slurry of unrecovered solids drawn from one of the intermediate froth flotation zones or from the last froth flotation zone.

A vortex chamber has a cylindrical body, an inverted conical frustum base, a top central pipe extending into the interior and at least one tangential feed conduit. Liquids at elevated pressure are delivered through the tangential feed conduit into the cylindrical body to create a vortex therein. All of the liquids are discharged through an opening at the bottom of the conical frustum base. Gases are aspirated into the vortex through the top control pipe so that the discharged liquids contain dispersed gas bubbles. Vortex chambers also are called aeration chambers or aeration and agitation chambers. Multiple tangential feed conduits might be employed for the vortex chamber.

Heretofore the aqueous slurry of raw coal solids has been delivered to the first froth flotation zone from a collector box which is reasonably non-turbulent. The flow velocity of the incoming aqueous slurry of raw solids has been intentionally dissipated in the relatively non-turbulent collector box.

The expression ''raw coal" in this specification includes freshly mixed coal, also coal which has received some preliminary separation processing and also coal which is recovered from silt ponds and simiiar accumulations of fine coal previously considered to be unsuitable for recovery.

It is among the objects of the present invention to provide an improved method and apparatus for froth flotation and more particularly for froth flotation in multiple sequential froth flotation zones.

According to one aspect of the invention, there is provided a froth flotation installation for separating solids into a float product and a non-float product, the installation including a plurality of sequential flotation zones, including a first froth flotation zone, a last froth flotation zone and at least two intermediate froth flotation zones, each said froth flotation zone including means for agitating said aqueous slurry of unrecovered solids and means for introducing flotation gas; means for collecting float product; means associated with the said last zone for withdrawing the said non-float product; said installation including baffle means to define a flow path for aqueous slurry through said installation between said first both flotation zone and said last froth flotation zone, whereby the aqueous slurry flowing through the said intermediate froth flotation zones experiences at least two changes in direction prior to entering the last froth flotation zone.

In use of such an installation, the aqueous slurry moves from the first froth flotation zone through the intermediate froth flotation zones and through the last froth flotation zone from which the non-float (or sink) product is recovered. The direction of flow of the aqueous slurry through the intermediate froth flotation zones changes at least twice between the first froth flotation zone and the last froth flotation zone. By the use of baffles, the length of the flow path from the first flotation zone to the last can be increased, while the cross-sectional flow area available to the aqueous slurry is correspondingly reduced in transit through the intermediate froth flotation zones whereby a plurality of froth flotation zones can be arranged between a first froth flotation zone and a last froth flotation zone without greatly increasing the linear distance between the first froth flotation zone and the last froth flotation zone. Hence the overall length of the froth flotation installation can be appreciably reduced in comparison to the length of corresponding multi-zone froth flotation installations of the prior art. By utilizing this novel flow direction change, lower overall energy input into the froth flotation installation can be achieved.Also improved mixing and increased gas bubble contacts with solid particles can be achieved. Furthermore, the agitation and gas flotation bubble-forming devices in the intermediate froth flotation zones can be manifolded compactly whereby the overall system can be operated efficiently with a single slurry recycle pump. The invention also permits a simplified controlled application of energy to each of the sequential froth flotation zones to achieve optimum separation effectiveness in each froth flotation zone.

According to anotfher aspect of the invention, there is provided a froth flotation installation for separating solids into a float product and a non-float product, the installation including three sequential flotation zones, each containing aqueous slurry of unrecovered solids, each of said froth flotation zones containing a vortex chamber as a means for agitating the said aqueous slurry and means for introducing flotation gas and means for forming bubbles of said flotation gas; means for delivering aqueous slurry of unrecovered solids sequentially through the said froth flotation zones; each said vortex chamber having a cylindrical portion and therebelow an inverted conical frustum and at least one tangential feed inlet communicating with the said cylindrical portion, conduit means for introducing an aqueous slurry of said solids into said feed inlet; said vortex chambers being disposed about a central distribution chamber; the conduit means of each said vortex chamber communicating with said distribution chamber; pump means for pressurizing aqueous slurry of unrecovered solids; slurry delivery means for delivering said aqueous slurry under pressure to said distribution chamber whereby a single pump delivers aqueous slurry through said distribution chamber to each of said vortex chambers.

According to yet another aspect of the invention, there is provided a method of separating solids from an aqueous slurry by froth flotation into a float product and a non-float product including providing a plurality of sequential flotation zones, including a first froth flotation zone, a last froth flotation zone and at least two intermediate froth flotation zones, agitating said aqueous slurry of unrecovered solids in each said froth flotation zone and introducing flotation gas in each said froth flotation zone and wherein the direction of the flow path of said aqueous slurry through said installation is changed at least twice between said first froth flotation zone and said last froth flotation zone, whereby the aqueous slurry flowing through the said intermediate froth flotation zones experiences at least two changes in direction prior to entering the last froth flotation zone.

Embodiments of the invention are described below, by way of example, reference being had to the accompanying drawings in which: Figure 1 is a schematic side elevation view of a multi-zone froth flotation installation of the prior art; Figure 2 is a schematic plan view of the prior art multi-zone froth flotation installation of Fig. 1, Figure 3 is a schematic side elevation view of a first froth flotation zone in a froth flotation installation forming one embodiment of the present invention, Figure 4 is a schematic plan view of the first froth flotation zone illustrated in Fig. 3, Figure 4A is a schematic plan view of a variant first froth flotation zone employing more than one vortex chamber, Figure 5 is a cross-sectional view of the vortex chamber of Fig. 3 taken along line 5-5 in Fig. 3, Figure 6 is a cross-sectional view of the vortex chamber of Fig. 3 taken along the lines 6-6 of Fig. 3, Figure 7 is a vertical sectional view of a vortex chamber of the type which is employed in the present invention as a combination agitator and flotation gas bubble generator, Figure 8 is a schematic plane view of a multi-zone froth flotation installation forming another embodiment of the invention, Figure 9 is a schematic plan view of a multi-zone froth flotation installation forming yet another embodiment, Figure 10 is a plan view of a multistage froth flotation installation forming yet another embodiment of the invention, Figure Ii is a side elevational view taken along the line of 11-11 of Fig. 10, and Figure 12 is an end view of the multi-zone froth flotation installation Fig. 10 taken along the line 12-12 of Fig. 11.

Figs 1 and 2 illustrate a typical multi-zone froth flotation installation 10 of the prior art including a first froth flotation zone 11, a last froth flotation zone 1 2 and intermediate froth flotation zones 1 3a, 1 3b, 1 3c, 1 3d. A feed collector zone 1 4 precedes the first froth flotation zone 11. A non-float (or sink) collector 1 5 follows the last froth flotation zone 1 2. A froth collecting trough 16, 1 7 is provided on each side of the installation 10 to receive froth overflowing from each of the froth flotation zones 11, 1 2, 1 3. Each of the froth flotation zones 11, 12, 1 3 is provided with an agitator/bubble generator device 1 8.

A vortex chamber as defined herein is the preferred agitator/bubble generator device.

Mechanical powered impellers with appropriate gas supply may be used as the agitator/bubble generator device. Onr or more gas nozzles may be employed to creat turbulence and gas bubbles; such systems are known as pneumatic cells.

A gas inlet pipe 1 9 is provided for introducing flotation gas, normally air, into the agita tor/bubble generators 1 8. An inlet pipe 20 is provided to introduce an aqueous slurry of raw coal into the feed collector zone 14 where any kinetic energy of the aqueous slurry of raw coal is dissipated.

A baffle 21 separates the collector 14 from the first froth flotation zone 11 to permit the aqueous slurry of raw coal to enter from the feed collector 1 4 into the first froth flotation zone 11 along the bottom of the first froth flotation zone 11 into proximity with the agitator/bubble generator 1 8 in the first froth flotation zone 11. The agitator/bubble generator 1 8 creates liquid turbulence within the first froth flotation zone 11 and creates great quantities of upwardly rising gas bubbles. The least hydrophilic solids from the slurry in the first froth flotation zone 11 tend to combine with bubbles and float upwardly to form a blanket of froth 22 above all of the froth flotation zones 11, 12, 1 3.

The first froth flotation zone 11 is separated from its succeeding intermediate froth flotation zone 1 3a by means of a baffle 23 which is open at the bottom to permit flow of water and solid particles which did not rise upwardly into the froth blanket 22. Similar baffles 24, 26, 27 are provided to define the intermediate froth flotation zones 13a, 13b, 13c, 13d.

Each of baffles 24, 25, 26, 27 is open at the bottom to permit underflow of aqueous slurry of solid particles which have not risen into the froth blanket 22. In some installations, some of the baffles 21, 23, 24, 25, 25, 26, 27 are not provided and the aqueous slurry moves unobstructedly between froth flotation zones.

The the underflow slurry encounters an agitator/bubble generator 1 8 in each of the intermediate froth flotation zones 13, 13b, 13c, 13d. The underflow from the last intermediate froth flotation zone 1 3d moves beneath the baffle 27 into proximity with the agitator/bubble generator 1 8 in the last froth collection zone 1 2. An aqueous slurry of solid particles which have not entered into the froth blanket 22 is withdrawn beneath a baffle 28 into the non-float (or sink) collector 15, from which the aqueous stream of sink product is recovered through a conduit 29.

The froth blanket flows over the sides of the froth flotation zones 11, 12, 1 3 into the froth collection troughs 16, 1 7 from which the froth is collected through troughs 30, 31 respectively.

According to the prior art as developed in Fig. 1 and 2, an aqueous slurry of raw coal introduced through the feed conduit 20 is separated in a typical froth flotation installation 10 into a float product at the collecting troughs 30, 31 and a non-float (or sink) product in the conduit 29. The float product contains a lower ash content and generally a lower sulfur content than the non-float (or sink) product. The float product also contains more combustible ingredients than the nonfloat (sink) product.

Each of the agitator/bubble generators 1 8 requires energy for operation. As shown in Fig. 1 and 2, there are six sequential, in-line froth flotation zones 11, 1 3a, 1 3b, 1 3c, 1 3d.

12, each of which requires energy inputs to operate an agitator/bubble generator 1 8 therein.

In order to minimize the number of intermediate froth flotation zones 13, the trend in the froth flotation art is to employ larger agitator/bubble generators 1 8 which have significantly larger and more expensive energy requirements than each of the smaller agitator/bubble generators 1 8. However the totai energy requirements of the large agitators/bubble generators is less than the energy requirement of the greater number of smaller agitator/bubble generators. As a result overall energy requirement is reduced, but because of the shorter length of the installations having larger machines, the larger-short installations tend to be somewhat less efficient than the smaller-long installations.

It is an object of this invention to provide the separation efficiency associated with the smaller-long machine installations and to retain the dimensions and the energy efficiency of the larger-short installations.

A first embodiment of the present invention is illustrated in Figs. 3, 4, 4A, 5, and 6 wherein a first froth flotation zone 41 of a froth flotation installation 40 is provided with a vortex chamber 42 which functions as a combined agitator/bubble generator. Such vortex chamber devices 42 have been manufactured and used as agitator/bubble generators heretofore. One such device is illustrated more fully in Fig. 7.

The aqueous slurry of raw coal solids is introduced through an inlet conduit 43 tangentially into a vortex chamber 42 with sufficient velocity to create turbulence and to aspirate bubble-forming gases from a gas inlet conduit 44 containing gas at about 2 psig.

The aqueous slurry of raw coal solids along with small bubbles is discharged through the base of the vortex chamber 42 whence the bubbles rise upwardly toward the surface of the first froth flotation zone 41. The vortex chamber 42 is more fully illustrated in Fig. 7 as having a generally cylindrical body portion 45 connected to an inverted conical frustum 46 having a bottom opening 47. The gas inlet conduit 44 extends downwardly into the cylindrical portion 45 at the center thereof. A conical frustum-shaped shroud 48 is secured around the opening 47. A conical annular gas chamber 49 surrrounds the outer surface of the conical frustum portion 46 to serve as a manifold for introducing gas through bottom openings 50 beneath the narrow upper surface of the conical frustum-shaped shroud 48.

Bubble forming gas is introduced into the conical annular passageway 49 through a gas inlet pipe 51.

The conical frustum-shaped shroud 48 is perforated at its widest portion and is preferably imperforate at its narrow portion.

A typical vortex chamber has a cylindrical portion 45 about 1 2 inches inner diameter and about 8 inches top-to-bottom. A corresponding conical frustum portion 46 is about 6 inches top-to-bottom. The inlet conduit 43 is about 4 inches inner diameter. The bottom opening 47 is about 4 inches inner diameter and is outwardly flared at approximately 1 5 degrees. The shroud 48 has a major diameter of about 36 inches and a cone angle of about 140 degrees. The cylindrical portion 45 and the conical frustum portion 46 are fabricated from metals, usually stainless steel. The shroud 48 may be fabricated from glass fiber reinforced plastics such as polyester or polyurethane.

In operation, an aqueous slurry of raw coal solids is introduced at a suitable pressure, e.g.

10 to 36 psig, from the inlet conduit 43 tangentially into the cylindrical portion 45 and thereupon downwardly through the conical frustum 46 and outwardly through the bottom opening 47. The reduced pressures within the vortex chamber 42 will cause aspiration of bubble forming gases from the gas inlet pipe 44. The energy released from the vortex chamber 42 will create small gas bubbles.

Some of the gas bubbles pass upwardly through the openings in the perforated conical frustum-shaped shroud 48. Other gas bubbles continue downwardly with the rapidly agitated aqueous slurry of raw coal solids and rise upwardly outside the perimeter of the conical frustum-shaped shroud 48. Additional bubble forming gas may be introduced through the gas inlet pipe 51 to a conical, annular manifold chamber 49 whence the gas is delivered downwardly through openings 50 to create additional gas bubbles for froth separation.

Preferably, the agitator/bubble generator apparatus 1 8 which appear in intermediate froth flotation zones and in the last froth flotation zone will have the same appearance as the vortex chamber 42 except that the incoming stream of aqueous slurry will be a stream which is drawn from one or more of the intermediate flotation zones or from the last froth flotation zone and pressurized in an appropriate pump hereinafter to be described.

It may be desirable in some installations to employ more than one vortex chamber in the first froth flotation zone, each of which receives aqueous slurry of raw coal under pressure, preferably from an elevated location.

This alternative system which employs multiple vortex chambers in the first froth flotation zone is illustrated in Fig. 4A wherein the first froth flotation 41A contains three vortex chambers 42a, 42b, 42c, each of which receives an aqueous slurry of raw coal from a coal feed inlet pipe 43 through inlet pipes 43a, 43b, 43c respectively. Each of the vortex chambers receives bubble forming gas from a pipe 44 through separate conduits 44a, 44b, 44c. One of the advantages of employing multiple vortex chambers in the first froth flotation zone is that smaller vortex chambers can be produced and installed at substantial cost savings as compared to the cost of large vortex chambers.While three vortex chambers are illustrated in Fig. 4A, it should be apparent that two such vortex chambers or four or more such vortex chambers might be employed in the first froth flotation zone 41a in order to take advantage of the present invention.

Referring to Figs. 8 and 9, there is illustrated in plan view an improved froth flotation installaion according to another embodiment of this invention. Fig. 8 illustrates a four stage separation system contained in a generally rectangular tank 60 having long sides 61 and short sides 62, 62'. Vertical baffles 63, 64, 65, 66, 67, 68 divide the interior of the tank 60 into a defined flow path having characteristics which will hereinafter be described. The tank 60 includes a first froth flotation zone 69, intermediate froth flotation zones 70, 71 and a last froth flotation zone 72. The firstf froth flotation zone 69 includes a first agitator/bubble generator 73. The intermediate froth flotation zones 70, 71 include intermediate agitator/bubble generators 74, 75. The last froth flotation zone 72 includes a last agitator/bubble generator 76.The first agitator/bubble generator 73 preferably is operated by means of incioming aqueous slurry of raw coal solids introduced through a conduit 77 although other sources of aqueous energy may be employed.

A pump 78 withdraws aqueous slurry of unfrothed solids from one or more locations in the tank 60 through conduit 79 and introduces pressurized aqueous slurry through conduits 80, 81, 82 to agitator/bubble generators 74, 75, 76, respectively. Appropriate froth collector troughs 83, 84, 85 are provided along the sides 61 and 62' of the tank 60. An appropriate non-float (sink) product collector 86 communicates with the last froth flotation zone 72.

It will be observed from Fig. 8 that incoming raw coal passes sequentially along the heavy dash/dot line 87 through the tank 60 and sequentially through the froth flotation zones 69, 70, 71, 72. The flow path 87 experiences directional changes between the first froth flotation zone 69 and the last froth flotation zone 72. The flow path 87 has a reduced cross-sectional flow area between the first froth flotation zone 69 and the last froth flotation zone 72. This unique flow direction change permits use of relatively smaller agitator/bubble generators 74, 75, 76 without increasing the length of the side 61 of the froth flotation tank 60. Small baffles 88 may be provided to eliminate sharp corners where objectionable solids accumulations might otherwise develop.

It will be observed that the non-float (sink) collector 86 in Fig. 8 is presented along one of the sidewalls 61. Accordingly a froth collector trough 85 is provided along the end wall 62'. As an alternative the non-float (sink) product collector 86 could be positioned along the end wall 62' remote from the agitator/bubble generator 76. In such alternative installation, the froth collector trough 83 would extend for the entire length of the sidewall 61.

A further embodiment in Fig. 9 includes a tank 90 having long edges 91 and short edges 92, 92'. Baffles 93, 94, 95, 96, 97, 98, 99. 100, 101, 102, and 103 divide the interior of the tank 90. A first froth flotation zone 104 includes a first agitator/bubble generator 105 which is preferably powered by kinetic energy of an aqueous slurry of raw coal solids through a conduit 1 06. A last froth flotation zone 107 includes a last agitator/bubble generator 1 08. Intermediate froth flotation zones 109, 110, 111, 112, 113 are provided with corresponding agitator/bubble generators 114, 115, 116, 117, 118.A mean flow path for aqueous slurry through the froth flotation installation of Fig. 9 is shown by the heavy dash/dot line 11 9. The mean flow path 11 9 experiences numerous directional changes between the first froth flotation zone 104 and the last froth flotation zone 107 whereby, as shown in Fig. 9, five intermediate froth flotation zones are accommodated in the length of the long side 91.

Froth collecting troughs 120, 121 are provided along each of the long sides 91. Each of the froth flotation zones 104, 109, 110, 111, 112, 113 and 107 is in contact with at least one of the froth collector troughs 120, 121. A sink product collector 122 communicates with the last froth flotation zone 107.

Vertical planes 84 (Fig. 8) and 120 (Fig. 9) are midway between sidewalls 61 (Fig. 8) and 91 (Fig. 9). Note that the baffles 63, 60, and 65, 66, 67 of Fig. 8 extend from a sidewall toward the opposite sidewall past the vertical midplane 84. Similarly, the baffles 93, 94 and 95, 96, 97 and 99, 100, 101 of Fig. 9 extend from a sidewall 91 toward the opposite sidewall past the vertical midplane 1 20.

It will be observed by inspecting Figs. 8 and 9 that the flow reversal through the froth flotation installation permits the use of multiple agitator/bubble generator devices in a relatively short length 61 (of tank 60) or 91 (of tank 90). In addition to increasing the contact of aqueous slurry with agitator/bubble generator devices, the improved flow pattern permits economies in consolidating the froth flotation installation, particularly where the agitator/bubble generator devices are vortex chambers of the type already described herein. By advancing aqueous slurry of unrecovered particles through the intermediate froth flotation zones with the directional changes, improved mixing is achieved and the bubble contact with suspended solids is improved.

The first froth flotation zone 69 (Fig. 8) and 104 (Fig. 9) is larger than the succeeding froth flotation zones whereby the mean residence time of the aqueous slurry is greater in the first froth flotation zone than in the succeeding froth flotation zones.

Note that the flow passageway for the aqueous slurry through the intermediate froth flotation zones is less than one-half the distance between the sidewalls 61 (Fig. 8) and 91 (Fig. 9).

While the improved flow pattern for a froth flotation installation of Figs. 8 and 9 has been described with vortex chambers as the agitator/bubble generator devices, it should be apparent that the improved froth flotation installation also can employ other types of agitators and bubble generators with corresponding benefits. Other types of useful agitator/bubble generators include mechanical impellers and air nozzles.

When the present invention employs the vortex chambers according to the preferred embodiment, further substantial economies can be achieved as illustrated in Figs 10, 11, 1 2. Fig. 10 illustrates a froth flotation installation 1 30 including (insofar as shown in Fig.

10) two intermediate froth flotation zones 131, 132 and a last froth flotation zone 133.

A respective vortex chamber 135, 136, 1 37 is provided in the froth flotation zones 131, 132, 133, respectively. Appropriate baffles 139, 140, 141 provide a desired flow pattern as shown in the heavy dash/dot line 1 34.

Each of the vortex chambers 135, 136, 1 37 has a slurry inlet pipe 142, a bottom skirt 143, and a gas inlet pipe 144.

The slurry inlet pipes 142 communicate with a distributor box 145 which is positioned beneath a bottom wall 146 of the froth flotation installation 1 30. The distributor box 145 is connected to an outlet conduit 147 from a pump 148 which draws aqueous slurry from selected locations in the froth flotation installation 1 30 through a deaerator chamber 149.

Thus with a single pump 148, an appropriate aqueous slurry can be delivered through the slurry inlet pipes 142 to three vortex chambers 135, 136, 137 and, if desired, to an additional vortex chamber (not shown) through a slurry inlet pipe 142' which is illustrated in phantom outline in Fig. 10.

A vertical plane 1 53 in Fig. 10 is midway between sidewalls 1 54. The baffle 139, 140, 141 extends from one sidewall 1 54 toward the opposite sidewall past the vertical midplane 53.

By combining the multiple vortex chambers in the manner illustrated in Figs. 10, 11, 12, significant economies in equipment cost and operating expenses can be achieved. It is possible to alter the flow rate of aqueous slurry entering each of the vortex chambers 135, 136, 1 37 and thereby provide an appropriate energy input for each of the vortex chambers. Flow control means, such as an orifice plate or other valve device can be installed in the slurry inlet pipes 142. Preferably the last vortex chamber 1 37 will receive the greatest energy input, i.e. the greatest flow velocity of aqueous slurry. The intermediate vortex chamber 1 36 will receive more energy than the preceding vortex chamber 135.

As shown in Fig. 12, the gas inlet pipes 144 preferably are connected to a hollow manifold chamber 1 50 which is positioned at the level of a froth-slurry interface 151. The manifold chamber 1 50 preferably is a part of a structure known as a crowd-board which has been employed in froth flotation installations of the prior art to preclude accumulation of froth in the quiescent space in the centre of a froth flotation unit. The crowd-board is a hollow inverted pyramid which directs flow of froth toward the edges of the froth flotation unit. In the preferred embodiment of Fig. 12, the manifold chamber 1 50 functions not only as a crowd-board but also as a plenum chamber for the bubble forming gases, usually air.

Supplemental bubble forming gas may be introduced through pipes 1 52.

By way of example, the froth flotation installation of Figs. 10, 11, 1 2 may be confined between sidewalls 1 54 about 1 2 feet long and end walls 1 55 about 1 2 feet long. The vertical height of the sidewalls 1 54 may be about 5 feet.

Claims

1. A froth flotation installation for separating solids into a float product and a non-float product, the installation including a plurality of sequential flotation zones, including a first froth flotation zone, a last froth flotation zone and at least two intermediate froth flotation zones, each said froth flotation zone including means for agitating said aqueous slurry of unrecovered solids and means for introducing flotation gas; means for collecting float product; means associated with the said last zone for withdrawing the said non-float product; said installation including baffle means to define a flow path for aqueous slurry through said installation between said first froth flotation zone and said last froth flotation zone, whereby the aqueous slurry flowing through the said intermediate froth flotation zones experiences at last two changes in direction prior to entering the last froth flotation zone.

2. An installation according to claim 1 wherein the mean residence time of the said aqueous slurry in said first froth flotation zone is greater than the main residence time of the said aqueous slurry in any of the said intermediate froth flotation zones.

3. An installation according to claim 1 which has a generally rectangular shape including two sidewalls and two end walls, said first froth flotation zone being defined in part by the first of said end walls; said last froth flotation zone being defined in part by the second of said end walls; said baffle means defining a flow passageway for said aqueous slurry, said flow passageway having a width which is less than one-half that of the distance between the said sidewalls.

4. An installation according to claim 3 wherein said baffle means includes a plurality of baffles and some of said baffles extend from one sidewall toward the opposite sidewall past a vertical midplane between said sidewalls.

5. A froth flotation installation for separating solids into a float product and a non-float product, the installation including three sequential flotation zones, each containing aqueous slurry of unrecovered solids, each of said froth flotation zones containing a vortex chamber as a means for agitating the said aqueous slurry and means for introducing flotation gas and means for forming bubbles of said flotation gas; means for delivering aqueous slurry of unrecovered solids sequentially through the said froth flotation zones; each said vortex chamber having a cylindrical portion and there-below an inverted conical frustum and at least one tangential feed inlet communicating with the said cylindrical portion, conduit means for introducing an aqueous slurry of said solids into said feed inlet; said vortex chambers being disposed about a central distribution chamber; the conduit means of each said vortex chamber communicating with said distribution chamber; pump means for pressurizing aqueous slurry of unrecovered solids; slurry delivery means for delivering said aqueous slurry under pressure to said distribution chamber whereby a single pump delivers aqueous slurry through said distribution chamber to each of said vortex chambers.

6. An installation according to claim 5 wherein flow control means are provided between said distribution chamber and each of said vortex chambers to regulate the flow rate of aqueous slurry entering each of said vortex chambers.

7. An installation according to claim 6 wherein the flow velocity of said aqueous slurry is increased in each sequential vortex chamber.

8. An installation according to claim 5 wherein a froth flotation gas distributor chamber is positioned within the said froth flotation installation centrally of said vortex chambers and extends below the level of aqueous slurry in said froth flotation zones; said froth flotation gas distribution chamber geing connected by pipes to said vortex chambers to provide at least in part the froth flotation gas requirements of each said vortex chamber.

9. A method of separating solids from an aqueous slurry by froth flotation into a float product and a non-float product including providing a plurality of sequential flotation zones, including a first froth flotation zone, a last froth flotation zone and at least two intermediate froth flotation zones, agitating said aqueous slurry of unrecovered solids in each said froth flotation zone and introducing flotation gas in each said froth flotation zone and wherein the direction of the flow path of said aqueous slurry through said installation is changed at least twice between said first froth flotation zone and said last froth flotation zone, whereby the aqueous slurry flowing through the said intermediate froth flotation zones experiences at least two changes in direction prior to entering the last froth flotation zone.

10. A method according to claim 9 wherein the mean residence time of the said aqueous slurry in said first froth flotation zone is greater than the mean residence time of the said aqueous slurry in any of the said intermediate froth flotation zones.

11. A method according to claim 9 wherein the said flow path of said aqueous slurry through a vertical midplane between opposed sidewalls of said froth flotation installation is diverted at least twice between said first froth flotation zone and said last froth flotation zone.

1 2. A method according to any preceding claim wherein aqueous slurry of unrecovered solids is withdrawn from said froth flotation installation, and the pressurized aqueous slurry of unrecovered solids returned to said froth flotation zones through a distributor chamber.

13. A method according to claim 9 including establishing the flow rate of said aqueous slurry between said distribution chamber and each of said froth flotation zones to regulate the relative flow rate to each said froth flotation zone.

14. A method according to to claim 1 3 wherein said aqueous slurry is supplied at successively higher flow rates to successive said froth flotation zones, as reckoned from the first said zone to the last zone.

1 5. A froth flotation installation substantially as hereinbefore described with reference to, and as shown in Figs. 3 to 7 of the accompanying drawings.

1 6. A froth flotation installation substantially as hereinbefore described with reference to, and as shown in Fig. 8 of the accompanying drawings.

1 7. A froth flotation installation substantially as hereinbefore described with reference to, and as shown in Fig. 9 of the accompanying drawings.

18. A froth flotation installation substantially as hereinbefore described with reference to, and as shown in Figs. 10, 11 and 1 2 of the accompanying drawings.

1 9. A method of separating solids from an aqueous slurry by froth flotation, substantially as hereinbefore described with reference to any one or more of Figs. 3 to 1 2 of the accompanying drawings.

20. Any novel feature or combination of features disclosed herein