CA2074746C

CA2074746C - Separation method and apparatus

Info

Publication number: CA2074746C
Application number: CA002074746A
Authority: CA
Inventors: Victor E. Ross
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1991-08-20
Filing date: 1992-07-28
Publication date: 2002-07-30
Anticipated expiration: 2012-07-28
Also published as: GB9216456D0; GB2258829A; CA2074746A1; AU2055292A; GB2258829B; AU652267B2; RU2067031C1

Abstract

A particulate mass is separated into different classes according to the oleophilicity of the particles. This is achieved by pretreating the particles in a manner to selectively render one class of particles more hydrophobic than another class of particles and introducing the treated particles into a body of aqueous froth (37). The particles are recovered in fractions according to the rate at which the particles sink in the froth (37).

Description

2'74746 "SEPARATION METHOD AND APPARATUS"
BACKGROUND TO THE INVENTION
THIS invention relates to a separation method and apparatus. In one application of the invention, it may be used to separate diamond particles from associated barren particles in a diamondiferous gravel.
Froth flotation is one method commonly used to separate particles with different surface characteristics from one another. However, in the case of diamond particles in the size range -2,8mm+ lmm (i.e. material capable of passing a screen with a 2,8mm mesh aperture but not of passing a screen with a 1mm mesh aperture), conventional froth flotation techniques have proved inefficient, with an excessive proportion of diamond particles reporting with the barren gravel and accordingly going to waste.

SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a method of separating a particulate mass into different classes of particles, the method comprising the steps of pretreating the particles in a manner to selectively render one class of particles more hydrophobic than another class of particles, introducing the treated particles into a body of aqueous froth, and recovering the particles from the body of froth according to the rate at which the particles sink in the froth.
The method may include causing relative movement to take place between the body of froth and a splitter means so that the splitter means splits the body of froth into upper and lower froth streams with the selected particles reporting in the lower stream and the non-selected particles reporting in the upper stream. Conveniently, the body of froth is formed on the surface of a body of water and is caused to move past the splitter means.
preferred steps in the method include restraining the upper surface of the body of froth with a screen deck and of depositing particulate material which is to be separated onto the screen deck. In a case where the screen deck includes flexible, parallel rods, the method may comprise the step of individually vibrating the rods to disagglomerate particle agglomerations in the particulate material.
Typically, the particles are pretreated in such a manner that particles of higher oleophilicity are rendered more hydrophobic than particles of lower oleophilicity.

~~'~4746 The pretreatment step may involve contacting the particles with an oil-containing substance.
According to another aspect of the invention, there is provided an apparatus for separating a particulate mass into different classes of particles, the apparatus comprising means for forming a body of aqueous froth, means for introducing into the body of froth particles of the mass which have been pretreated to selectively render same particles more hydrophobic than others, and means for recovering the particles from the body of froth according to the rate at which the particles sink in the froth.
The apparatus can include splitter means, past which the froth is caused to move, for splitting the body of froth into upper and lower streams. In a preferred embodiment, the apparatus comprises first, second and third tanks arranged alongside one another and means for forming the body of froth on the water in the first tank and far causing the body of froth to move in a direction from the first tank towards the second tank, the splitter means being arranged to divert a lower froth stream into the second tank and an upper froth stream into the third tank.
In this embodiment, means are provided for introducing air into a body of water containing a frothing agent in the first tank. The embodiment also includes a screen deck for restraining the upper surface of the body of froth, and means for depositing the particles onto the screen deck.
For disagglomerating any particle agglomerations, the screen deck may comprise a series of flexible, parallel rods, the apparatus including means for causing the rods to vibrate individually.

2fl~4746 In one application, the method and apparatus summarised above may be used to separate diamond particles from non-diamond particles.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:
Figure 1 diagrammatically illustrates an apparatus of the invention;
Figure 2 shows a diagrammatic plan view of the screen used in the apparatus of Figure 1; and Figure 3 shows in a diagrammatic plan view, how the individual rods of the screen are flexed.
DESCRIPTION OF THE DRAWINGS
Figure 1 shows an apparatus 10 of the invention which has an arrangement of tanks 12, 14 and 16 alongside one another. The tank 12 is subdivided, by a wall 18, into compartments 20 and 22. The three tanks 12, 14 and 16 have conical bottoms leading to outlet conduits 24, 26 and 28 respectively. The conduits 24 and 26 feed into a single conduit 30.

~~'~4'~4~

The apparatus seen in Figure 1 also includes an air feed conduit 32 connected to two porous pipes 34, typically made of Teflon or stainless steel and having pore sizes in the range 10 x 10~m to 150 x 10~m. As illustrated, the pipes 34 are located in the compartments 20 and 22 of the tank 12 arid are submerged in water 36 in those compartments. In use, air fed through the air feed conduit 32 and porous pipes 34 is sparged as fine bubbles through the water 36. The water contains a frathing agent so that a froth 37 is formed on the surface of the water.
Frothing agents such as Montan LIC40 or MIBC
(methylisobutylcarbinol) have been found to be suitable for the purpose.
As an alternative to the frothing system just described, it would also be possible to contact air with water containing the frothing agent in a spray nozzle. In this case, an air/water mixture is passed downwardly via a spray nozzle in a vertical pipe so as to exit in froth form through the bottom end of the pipe, beneath the surface of the water.
Located over the otherwise open top of the compartment 22 is a packing 35 of fine wire mesh. Froth formed on the surface of the water in the compartment 20 flows over the packing towards the tank 14. It will be appreciated that this arrangement creates a moving layer of froth that moves from left to right in Figure 1, as indicated by the arrows.
The upper surface of the moving layer of froth is restrained by a screen deck 38 composed of parallel, closely spaced, flexible rods 39 (see Figures 2 and 3). The rods 39 span from left to right in Figure 1. The rods may be made of any suitable flexible material and they are spaced apart so as to create a clear 2mm gap between adjacent rods.

Referring to Figure 2, each rod 39 in the rod deck 38 has one end fixed to a stringer 37. The opposite end of each rod passes loosely through an oversize hole in another stringer 41, so that the extremities 43 of the rods project past the stringer 41 as illustrated.
Located over the upstream end of the screen deck 38 is a hopper 40 containing a charge of diamondiferous material 42 from which it is desired to separate the diamonds. The feed material 42 has been pretreated in the manner described below. The hopper discharges the material 42 under gravity onto the screen deck 38.
The apparatus also includes a rod flexing device in the form of a belt 46 which carries a series of downwardly projecting rubber knuckles 48. The belt is driven continuously in a direction transverse to the rods 39, i.e.
into the plane of the paper in Figure 1, with the tips of the knuckles brushing over the extremities 43 of the rods. The loose passage of the rods through the stringer 41 enables them to flex in a transverse direction as exemplified diagrammatically by the rod 39A in Figure 2.
The arrangement is such that the gap SO between adjacent rods at the point of greatest flexure is about 3mm wide, so that particles of the diamondiferous feed material having a size in the range -2,8+ 1mm can fall through the screen deck.
The flexibility of the rods 39 causes them to vibrate each time their extremities 43 are contacted by a knuckle and the knuckle then breaks contact as it passes by. This vibration is an important feature of the invention, bearing mind that the feed material 42 may contain agglomerations of particles, rather than discrete, individual particles.

2(~'~4~46 _g_ As the agglomerations land on the screen deck, the vibration of the rods will tend to disagglomerate them into the discrete particulate form which is preferred.
As illustrated by Figure 3 in particular, the restraining effect of the screen deck 38 on the froth 37 is such that the upper surface 54 of the froth coincides with the level of the rods in the screen deck.
Disagglomerated particles that are able to fall through the screen deck by virtue of the flexing of the rods 39 enter the froth 37 and are conveyed downstream, in the direction from left to right in Figure 1, towards a splitter 56.
The splitter S6 is formed by the curved upper end of a wall 58 that separates the tank 14 from the tank 16, and presents a splitting edge 59 located roughly midway up the height of the layer of froth 37. The splitter 56 splits the froth layer into lower and upper portions 60 and 62 respectively, with the lower portion 60 being directed into the tank 14 and the upper portion 62 flowing over the splitter and into the tank 16 as illustrated.
Earlier on, it was mentioned that the feed material, typically diamondiferous gravel recovered from alluvial diamond deposits or kimberlite, is pretreated. The objective of the pretreatment step is to render the diamonds in the material highly hydrophobic. To achieve this, use is made of the oleophilicity of diamond. As an initial step, the feed material is properly cleaned to expose the surfaces of the diamond particles. This can be performed by attritioning of the gravel material in a ferrosilicon suspension.

_y_ Thereafter, the gravel material is conditioned in an aqueous mixture containing oily and volatile components, such as pine-oil. Alternatively, the conditioning mixture may contain oily and hydrophobic components, such as molten petroleum wax. This step in the pretreatment process can be achieved by settling the gravel particles through a layer of the relevant mixture. As a result of the oleophilicity of their surfaces, the diamond particles are selectively coated with the oily component, but non-diamond particles are not. Thereafter, the pretreated particles are allowed to settle through water to wash off any excess conditioning mixture.
It will be appreciated that the dampness of the non-diamond particles after the pretreatment process described above increases the potential for particle agglomeration, and further underlines the importance of the disagglomeration function of the screen deck 38.
Because the diamond particles have now selectively been rendered highly hydrophobic, surface tension effects in the froth layer 37 are unable to support the diamond particles against gravity. The diamond particles therefore drop through the froth layer more quickly than the non-diamond particles, which are supported at a high level in the froth layer by surface tension effects.
Some of the diamond particles fall right through the froth layer well before they even reach the splitter 56. Such particles collect at the bottom of the tank 12. Other diamond particles which have riot yet had time to fall right through the froth layer upon reaching the sputter 56 will occupy the lower portion 60 of the froth layer and are directed by the sputter into the tank 14. Non-diamond particles that occupy the upper portion 62 of the froth layer are directed to the tank 16.
Periodically a diamond-rich concentrate is withdrawn from the tanks 12 and 14 through the conduit 30 and non-diamond particles are withdrawn, for subsequent disposal as tailings, through the conduit 28. The frothing agent which is used to produce the froth layer 37 should be one which produces the required, voluminous froth to start with, but which breaks down easily once it collects in the bottom of the tanks 14 and 16, thus making it easier subsequently to recover the diamond-rich concentrate.
It will be appreciated that the efficiency of the separation process is time-dependent, in the sense that the splitting operation performed by the splitter 56 is conducted after a period of time sufficient for diamond particles to have a sunk to a clearly definable lower level in the froth layer than the non-diamond particles. On the other hand, the splitting operation should not be conducted after too long a delay, lest undesired non-diamond particles are able to sink into the lower portion 60 before the sputter 56 is reached.
Testwork on a laboratory scale indicates that the process and apparatus described above enable diamond particles to be separated from non-diamond particles with a high degree of reliability.

Claims

1. A method of separating particles of a particulate, diamondiferous mass into diamond and non-diamond particles, the method comprising the steps of pretreating the particles of the mass with an oil-containing substance, thereby to selectively render diamond particles of the mass more hydrophobic than non-diamond particles, introducing the treated particles into a body of aqueous froth with the result that diamond particles sink more rapidly through the body of froth than non-diamond particles, moving the body of froth past a splitter which is arranged to split the body of froth into upper and lower froth streams after a period of time sufficient for diamond particles to have sunk through the body of froth into the lower froth stream while non-diamond particles remain in the upper froth stream, and recovering the diamond particles from the lower froth stream.

2. A method according to claim 1 and comprising the step of subjecting the particulate mass to vibration to break up particle agglomerations in the mass before introducing the particles into the body of froth.

3. A method according to claim 2 comprising the steps of restraining the upper surface of the body of froth by means of a screen deck, depositing the particles of the particulate mass which is to be separated onto the screen deck, and allowing disagglomerated particles to fall through the screen deck into the body of froth.

4. A separating method according to claim 3 wherein the screen deck includes flexible, parallel steel rods and the method comprises the step of individually vibrating the rods to break up particle agglomerations.

5. An apparatus for separating particles of a particulate, diamondiferous mass into diamond particles and non-diamond particles, the apparatus comprising a splitter, means for forming a body of aqueous froth and moving that body past the splitter so that the body of froth is split by the splitter into upper and lower froth streams, a screen deck, means for depositing the particulate mass on the screen deck after pretreatment thereof with an oil-containing substance to selectively render diamond particles more hydrophobic than non-diamond particles, the screen deck being arranged to apply vibration to the particulate mass deposited thereon so as to break up particle agglomerations and to allow disagglomerated particles to pass downwardly into the body of froth at a position located sufficiently upstream of the splitter for diamond particles to have sunk through the body of froth into the lower froth stream, while non-diamond particles remain in the upper froth stream, by the time the body of froth is split by the splitter, and a collection vessel for collecting the lower froth stream and diamond particles therein.

6. An apparatus according to claim 5 wherein the screen deck is arranged to restrain an upper surface of the body of froth and includes a series of flexible, parallel rods, the apparatus comprising means for causing the rods to vibrate individually.

7. An apparatus according to either one of claims 5 or 6 and comprising first, second and third tanks arranged alongside one another, the froth forming means being arranged to form the body of froth on the surface of a body of water in the first tank and to cause the body of froth to move towards the splitter, and the splitter being arranged to divert the lower froth stream into the second tank for recovering of diamonds from that tank and the upper froth stream into the third tank.

8. An apparatus according to claim 7 and comprising means for introducing air into a body of water containing a frothing agent in the first tank.