WO2000064588A1

WO2000064588A1 - The magnetic separation of particles

Info

Publication number: WO2000064588A1
Application number: PCT/IB2000/000482
Authority: WO
Inventors: Jane Elizabeth Ennis; Fredre Dunn
Original assignee: De Beers Consolidated Mines Limited
Priority date: 1999-04-21
Filing date: 2000-04-19
Publication date: 2000-11-02
Also published as: AU3831600A; AU762433B2; CA2370900A1

Abstract

A set of magnetic tracers (48) is provided for the magnetic separation of ore particles into different fractions based on the magnetic susceptibility thereof. The set comprises a plurality of tracer groups (48.1 to 48.7), with each group comprising a plurality of tracers, each of which have the same magnetic susceptibility and a common coloration (yellow, pink, green, cinnamon, orange, brown and grey) by means of which the tracers in one group can be readily distinguished over tracers in another group. Separate groups in the set cover a predetermined range or spectrum of magnetic susceptibility values. The invention extends to a method of sorting a body of particulate ore material, such as particulate diamond-containing gangue and gangue material, into at least two different fractions by determining a desired cut or split point between the two different fractions based on the magnetic properties, thereof selecting a set of magnetic tracers of the type described above, passing the tracers through a permroll having an output with an adjustable splitter for splitting the particulate ore material into the two fractions, and using the magnetic tracers heuristically to adjust the splitter to the desired cut point.

Description

THE MAGNETIC SEPARATION OF PARTICLES

BACKGROUND OF THE INVENTION

THIS invention relates to the magnetic separation of particles having different magnetic properties, and in particular to the magnetic separation of minerals.

Magnetic separators such as permrolls are used in plants to separate minerals having specific magnetic characteristics from those which do not. A number of minerals contain ferromagnetic components, either as a major recoverable element or as a contaminant. Permrolls are used to isolate such ferromagnetic components, either for recovery or for removal, and take the form of a large cylindrical drum which rotates about its axis. A magnetic field is set up on the drum either by permanent magnets or by electromagnets. Material which is passed over the drum experiences a number of different forces, including a radially outward centrifugal force, a downward gravitational force and a radially inward magnetic force which essentially counteracts the centrifugal force.

Generally, the greater the magnetic susceptibility of a particle, the closer the trajectory of the particle will be to the drum due to the dominant effect of the magnetic force, whilst the particle having low or zero magnetic susceptibility, such as a diamond particle, will have a trajectory further away from the drum, due to the predominant influence of the centrifugal force on that particle.

In the magnetic separation of non-magnetic diamond from magnetic gangue material, one or more splitter plates are set up to take advantage of the different trajectories arising from the different magnetic properties of the material to separate diamonds and diamond-containing gangue material from ferrous gangue material which is barren. The position of this plate is adjusted by trial and error. This is a time-consuming process. During the adjustment or setting up phase, diamonds or gangue typically end up being separated at least initially into the wrong fraction, resulting either in initial low recovery of diamond or over-recovery, as a result of which the recovered fraction needs to undergo additional processing. This problem may be perpetuated in the event of the position of the splitter plate not finally being optimized.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of sorting a body of particulate material into at least two different fractions comprising the steps of determining a desired cut or split point between the two different fractions based on their magnetic properties, selecting a set of magnetic tracers divided into at least two groups, each group having differing magnetic properties which span the split point and corresponding identifiable characteristics enabling one group to be distinguished from the other, passing the tracers through a magnetic separator having an output with an adjustable splitter for splitting the particulate material into the two fractions, and using the magnetic tracers heuristically to adjust the splitter to the desired cut point, and subsequently passing the body of particulate material through the magnetic separator to sort it into the two different fractions using the pre-adjusted splitter.

Preferably, the magnetic properties include magnetic susceptibility.

Conveniently, the magnetic separator is a permroll.

Advantageously, the two different fractions include desired or recoverable and undesired or non-recoverable fractions, and the desired cut or split point is determined by initially analyzing the body of particulate material to determine the particle magnetic property distribution range and identifying that point along the distribution range which represents the optimum percentage recovery to concentration in respect of the desired fraction.

Typically, the body of particulate material is an ore body and the two different fractions include a recoverable mineral fraction having specific magnetic properties and a mineral fraction having magnetic properties which differ from the recoverable fraction.

In one preferred form of the invention, the recoverable fraction includes diamonds and diamond-containing gangue material and the non-recoverable fraction includes ferrous barren gangue material.

The invention extends to a set of magnetic tracers for use in the magnetic separation of particles into different fractions based on the magnetic susceptibility thereof, the set comprising a plurality of tracer groups, with each group comprising a plurality of tracers, each of which have the same magnetic susceptibility and at least one other common identifiable characteristic by means of which the tracers in one group can be readily distinguished over tracers in another group, with the separate groups in the set covering a predetermined range or spectrum of magnetic susceptibility values.

The at least one common identifiable characteristic is preferably colour, with each group of tracers being coloured differently. The at least one common identifiable characteristic may also include shape, with each group or set of tracers being shaped differently.

Conveniently, all of the tracers have substantially the same density, which is equivalent to at least one fraction of the particles being separated. Typically, the tracers in each of the tracer groups have a plurality of different sizes or size ranges, which correspond to the particle size distribution of at least one fraction of the ^"particles being separated.

In one form of the invention, the predetermined magnetic susceptibility values fall within or include a range from 20x10^"6cm³g^"1 to 300x10^"6cm³g^"\ and are adapted for use with a NdFeB permroll.

Alternatively, or in addition, the predetermined magnetic susceptibility values fall within or include a range from 400x10^"6cm³g^"1 to 4000x10^"6cm³g \ and are adapted for use with a BaFe permroll.

The magnetic tracers may have a range of magnetic susceptibility values arranged to mimic diamond, diamond-containing gangue and gangue.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a highly schematic diagram of a permroll and split plate arrangement utilized in the sorting method of the invention;

Figure 2 shows a schematic graphical representation of the magnetic susceptibility of a set of tracers of the invention against percentage recovery to concentration;

Figure 3 shows a schematic diagram of a first set of cubic tracers of the invention for use with a NdFeB permroll; and

Figure 4 shows a schematic diagram of a second set of octahedral tracers of the invention for use with a BaFe permroll. DESCRIPTION OF EMBODIMENTS

Referring first to Figure 1 , a highly schematic diagram is shown of a round cylindrical permroll 10 viewed along its central axis of rotation in a clockwise direction indicated by arrow 12. The permroll may take the form of an axial permroll or a radial permroll. The axial permroll is formed from rod magnets which are placed axially around the inner circumference of a drum, with the magnets being positioned such that their north and south poles face one another, as a result of which they are mutually repelling. The radial permroll comprises a plurality of circular permanent magnetic discs stacked in a mutually repelling configuration along the central axis of the perm roll, with thin steel washers or spaces interposed between adjacent magnets.

A magnetic particle 14 passing over the permroll 10, regardless of whether it is an axial, a radial or an induced permroll, experiences essentially the same set of forces. These include a radially outwardly directed centrifugal force F_cen indicated by vector 16, and given by the equation:

_ _ mv² cen ^{— *"} ~

(1)

where:

F_Cen = centrifugal force (N) m = mass of particle (kg) v = tangential velocity of particle (m/s) r = radius of drum (m)

As the material passes over the drum, the particles experience a downward force due to gravity described by the following equation (2), and indicated by vector 18: Fgra = mg

(2)

where:

Fg_rav = gravitational force (N) m = mass of particle (kg) g = gravitatonal acceleration (m/s²)

The magnetic particles in the feed experience a third force of magnetic attraction, namely F_mag, due to the magnets in the drum. This is a radially inwardly directed force indicated by vector 20 which directly counteracts the centrifugal force F_cen, and is described by equation (3) as follows:

Fmag = χmHVH

(3)

where:

F_mag = magnetic force (N) χ = mass magnetic susceptibility (cm³/g) m = mass of particle (kg)

H = magnetic field strength (Gauss)

VH = magnetic field gradient (Gauss/cm)

As the particle 14 passes over the permroll 10, it is subjected to all three of the above forces. For a non-magnetic particle such as diamond, F_mag = zero, and the resultant force R-* is depicted by a vector 22, which is the vector sum of F_cen (vector 16) and F_grav (vector 18). For a magnetic particle, the resultant force is the vector sum of all three of the forces F_cen, F_grav and F_mag (vector 20), with the resultant force R₂ being depicted by vector 24. The amplitude and direction of the vectors 22 and 24 will tend to change, depending, for a particular permroll, on the magnetic susceptibility of the particle, which will tend to change the magnetic force, as is indicated by broken outline vector 20.1 , and on the speed of rotation of the permroll, which will affect the centrifugal force, as is indicated by broken outline vector 16.1. As a result of the vectors 22 and 24, nonmagnetic particles will tend to fall further away, and magnetic particles will tend to fall closer to the roll.

The placing of a splitter plate 26 between the vectors 22 and 24 will thus tend to separate magnetic from non-magnetic particles. A greater spread is achieved by increasing the velocity of the drum 10, which will in turn increase the tangential velocity of particles to the extent that a non-magnetic particle having a high tangential velocity will follow, say, trajectory 28, whereas a nonmagnetic particle having a lower tangential velocity will follow, say, trajectory 30. An increase in horizontal distance 32 that a particle falls from the drum increases the region for the magnetic discrimination of the particles, thus decreasing the number of particles per unit volume in the discrimination region, and reducing the probability of a particle hitting the splitter plate 26. A reduction in the number of particles that hits the splitter plate tends to reduce the chances of particles being misplaced or wrongly classified.

At present, the proportion of gangue material in the feed to the plant is estimated, and the splitter plate is adjusted by eye in an on-line trial-and-error or heuristic process so that non-magnetic diamonds and substantially nonmagnetic diamond-containing gangue fall on the right hand side of the splitter plates, as is indicated by trajectories 28 and 30, and magnetic gangue particles fall to the left hand side of the splitter 26, as is indicated by trajectory 34. As was described previously, this rough-and-ready method does not make for efficient recovery during the initial setting up procedure, as the splitter plate invariably needs to be adjusted. A non-optimal recovery situation may also result in the event of the splitter plate not ultimately being set up in exactly the right position.

Referring now to Figure 2, a graph 40 is shown of magnetic susceptibility plotted against percentage recovery to concentration. The magnetic properties of the incoming ore at a processing plant are analyzed separately to ascertain the magnetic characteristics of the ore, and in particular the full range of magnetic susceptibility of the ore. A cut point which provides the optimum percentage recovery to concentration is then determined on the basis of the magnetic distribution of the particles given by the graph 40.

A set of magnetic tracers is then selected in which the magnetic susceptibility value corresponding to the cut point falls squarely within the set. The magnetic tracers are manufactured with a density of 3.53SG, which is equivalent to the density of diamond, so that the tracers can simulate the trajectory of a diamond passing over the permroll. The different magnetic susceptibilities are achieved by adding prescribed amounts of crushed magnetite to a standard density tracer which has an SG of 3.53. A colour coding is used to differentiate tracers with different magnetic susceptibilities. The following table indicates the two different sets of tracers which were manufactured.

Table 1

The first set of tracers are intended for use with relatively strong magnetic permrolls formed from Neodymium Iron Boron (NdFeB). These tracers are cubic in shape, to minimize the effect of the shape factor when calculating the magnetic field interaction. Groups of tracers having magnetic susceptibilities χ (x 10^"6cm³g^"1) of 20, 40, 60, 80, 100, 200 and 300 are provided, with each group being coloured in the manner indicated in the table. The choice of colours is limited by the pigment of the high density filler material used to obtain the 3.53SG. In the case of a relatively weak magnetic permroll such as a Barium Ferrite (BaFe) permroll, the magnetic susceptibility values are significantly higher, ranging from 400 to 4000, with the same colour coding being provided. This set of tracers is octahedral in shape, similarly to minimize the effect of the shape factor when calculating the magnetic field interaction.

Each group of tracers included three different sizes of tracers, namely 12mm tracers (coarse), 6mm tracers (middle) and 3mm tracers (fine). The size distribution of the tracers is designed to simulate the size distribution of the particles which the tracers are imitating.

In the particular example above, 25 tracers were provided per colour per size range, with the result that there were 1050 tracers per set (25 tracers x 14 colours x 3 sizes). In Figure 3, the first set of cubic tracers 44 is shown, with the differently coloured tracers 44.1 to 44.7 corresponding to the susceptibilities indicated in Table 1 , and each colour tracer in turn being grouped into the aforementioned coarse (12m), middle (6mm) and fine (3mm) groups 46.1 , 46.2 and 46.3 respectively. Figure 4 shows the set of tracers 48 which are arranged to be used in conjunction with the BaFe permroll. The tracers are octahedral in form, and similarly are arranged^" in seven colours 48.1 to 48.7 identifying the susceptibility values listed in Table 1 with each colour being divided into the fine (3mm), middle (6mm) and coarse (12mm) size groupings 50.1 to 50.3 respectively.

In the magnetic separation of non-magnetic diamond from magnetic gangue material, the relatively weak BaFe permroll is used in the initial upstream recovery process on which there is still a relatively high percentage of highly magnetic crushed gangue material present. The splitter plate is adjusted using the octahedral set of tracers 48 having magnetic susceptibility values ranging from 400 to 4000, which are arranged to mimic the relatively high magnetic susceptibilities of barren gangue material having a high magnetic content. This plate is then adjusted, say, to separate the lower magnetic susceptibility yellow and pink tracers from the progressively higher magnetic susceptibility green, cinnamon, orange, brown and grey tracers. In this way, all crushed ore material having a magnetic susceptibility higher than around 500 x 10^"6cm³g^"1 is separated from the moderately susceptible material having a magnetic susceptibility of less than 500 x 10^"6cm³g^"1. All diamondiferous material thus reports to the non-magnetic section and only the strongly magnetic material is removed. Further downstream in the process, either immediately downstream or after additional crushing steps have taken place, the more highly magnetic NdFeB permroll is utilized in conjunction with the first set of cubic tracers 44. In this separating step, the middle group 46.2 of tracers could, for example, be utilized.

As is clear from the graph, the optimum percentage recovered to concentration point is chosen to correspond to the magnetic susceptibility χ of 60 x 10^"6cm³g^"1, which is that displayed by the cubic green tracers. During the setting up process, the splitter plate was adjusted heuristically (i.e. by trial and error) to a point in which all of the yellow and pink tracers, together with half of the green tracers, lay on the left hand side of the splitter plate, and the other half of the green tracers^" together with the cinnamon, orange, brown and grey tracers, lay to the right of the splitter plate. After the splitter plate had been adjusted to achieve this end result, the permroll could then be used to process the material. In this way, the actual position of the splitter plate could be adjusted to a point where it was coincident with the predetermined desired split point, where all of the desired particles had a magnetic susceptibility χ of less than 60 x 10^"6cm³g^"1. In this way it is ensured that close to 100% (say in excess of 98% or 99%) of the desired particles may be recovered, in conjunction with a smaller quantity of gangue material, with the resultant concentration of diamondiferous material being significantly greater than the equivalent concentration after the BaFe permroll separation

In the case of diamond, which is weakly magnetic, a cut point will be set such that as close to 100% as possible of the all diamond particles, including encased diamond particles, report to the non-magnetic fraction (left of the splitter plate, and below point 42 on the graph). This corresponds to a magnetic susceptibility of 60 x 10^"6cm³g^"1 for a NdFeB permroll.

A major advantage of the method of magnetic separation of the invention and the tracers used therein is that they can be used to optimize the magnetic separation of minerals, and in particular of essentially non-magnetic diamond from magnetic gangue material. The separation process is optimized from the start by the pre-adjustment of the one or more splitter plates to ensure that under- or over-recovery does not occur, both initially and during the separation process.

Claims

1. A method of sorting a body of particulate material into at least two different fractions comprising the steps of determining a desired cut or split point between the two different fractions based on their magnetic properties, selecting a set of magnetic tracers divided into at least two groups, each group having differing magnetic properties which span the split point and corresponding identifiable characteristics enabling one group to be distinguished from the other, passing the tracers through a magnetic separator having an output with an adjustable splitter for splitting the particulate material into the two fractions, using the magnetic tracers heuristically to adjust the splitter to the desired cut point, and subsequently passing the body of particulate material through the magnetic separator to sort it into the two different fractions using the pre-adjusted splitter.

2. A method according to claim 1 in which the magnetic properties include magnetic susceptibility.

3. A method according to either one of the preceding claims 1 or 2 in which the magnetic separator is a permroll.

4. A method according to any one of the preceding claims in which the two different fractions include desired or recoverable and undesired or non-recoverable fractions, and the desired cut or split point is determined by initially analyzing the body of particulate material to determine the particle magnetic property distribution range and identifying that point along the distribution range which represents the optimum percentage recovery to concentration in respect of the desired fraction.

5. A method according to any one of the preceding claims in which the body of particulate material is an ore body and the two different fractions include a recoverable mineral fraction having specific magnetic properties and a mineral fraction having magnetic properties which differ from the recoverable fraction.

6. A method according to claim 5 in which the recoverable fraction includes diamonds and diamond-containing gangue material and the non-recoverable fraction includes ferrous barren gangue material.

7. A set of magnetic tracers for use in the magnetic separation of particles into different fractions based on the magnetic susceptibility thereof, the set comprising a plurality of tracer groups, with each group comprising a plurality of tracers, each of which have the same magnetic susceptibility and at least one other common identifiable characteristic by means of which the tracers in one group can be readily distinguished over tracers in another group, with the separate groups in the set covering a predetermined range or spectrum of magnetic susceptibility values.

8. A set of magnetic tracers according to claim 7 in which the at least one common identifiable characteristic is colour, with each group of tracers being coloured differently.

9. A set of magnetic tracers according to either one of the preceding claims 7 or 8 in which the at least one common identifiable characteristic includes shape, with each group or set of tracers being shaped differently.

10. A set of magnetic tracers according to any one of claims 7 to 9 in which all of the tracers have substantially the same density, which is equivalent to at least one fraction of the particles being separated.

11. A set of magnetic tracers according to any one of the preceding claims 7 to 10 in which the tracers in each of the tracer groups have a plurality of different sizes or size ranges, which correspond to the particle size distribution of at least one fraction of the particles being separated.

12. A set of magnetic tracers according to any one of the preceding claims 7 to 11 in which the predetermined magnetic susceptibility values fall within or include a range from 20x10^"6cm³g^"1 to 300x10^"6cm³g^"1, and are adapted for use with a NdFeB permroll.

13. A set of magnetic tracers according to any one of the preceding claims 7 to 11 in which the predetermined magnetic susceptibility values fall within or include a range from 400x10^"6cm³g^"1 to 4000x10^"6cm³g^"1, and are adapted for use with a BaFe permroll.

14. A set of magnetic tracers according to any one of the preceding claims 7 to 13 which have a range of magnetic susceptibility values arranged to mimic diamond, diamond-containing gangue and gangue.