Particulate material separation

Field of the invention

The invention relates to separating particulate material and in particular to the dry separation of particulate material.

Background of the invention

In order to meet customer demands, ore producers are required to meet tight size restrictions on particulate material. This is particularly the case with fine particles less than a particle size of about 100 microns. By meeting these tight restrictions, companies can command higher prices for their product. Additionally in a number of mining and mineral processing applications, a higher percentage of impurities or undesirable ore types can be found in the finer fractions of material. Hence, if these finer fractions can be effectively separated from the bulk material, then the grade of the ore can be increased, and/or the amount of deleterious impurities reduced, by a simple separation process.

Particulate material can be separated by a variety of techniques which can be broadly classified as wet techniques and dry techniques. Wet techniques use water as a medium to assist in the separation and are particular useful where water is required for subsequent processing. However, where this is not the case, the introduction of water into a separation process is undesirable, because it drastically increases costs associated with materials handling, thickening, filtration, drying, etc. Additionally, water is a scarce resource at many mining and industrial sites and processes which are as effective and less dependant on water are preferred.

Dry separation is therefore increasingly becoming the preferred method of performing particle separation and such techniques generally involve agitating a bed of material to be separated to produce stratification of the bed of material. Separate fractions can then be removed from different heights of the material.

Another method involves using an air permeable table and supplying a quantity of air sufficient to fluidise the material. These tables receive a feed from the top of the table and discharge the fines from the upper end of the table and the coarse particles from the lower end of the table. These tables rely on a delicate balance between the incline of the table and the air flow rate and prove to be difficult to maintain in operation when the feed material changes. Hence, continuous removal of ore fines proves to be difficult.

Although not well known, UK Patent application number GB2221172 describes a different approach. The document describes a gravitational separator having a housing 1 with an inclined perforated bottom underlying three perforated grids 13 which in turn underlie a series of vertical chutes 3. In use, feed material is fed onto an upper edge of the inclined perforated bottom and air is supplied via the perforated bottom in a quantity sufficient to fluidise the feed material. Air is drawn upwardly through the grids and through the chutes to carry away a fine fraction of the feed material. The heavier fraction is removed via the lower edge of the inclined perforated bottom.

Summary of the invention

It is object of the present invention to provide an improved separator for separating mining products or at least provide an alternative device in the market place.

Accordingly, in a first aspect of the invention, a separator is provided for the dry separation of a particulate material including:

(i) a fluid permeable support surface for supporting a body of particulate material,

(ii) a fluid supply configured to supply fluid to the support surface, the surface permitting the flow of fluid therethrough to entrain a selected particle size fraction of the particulate material;

(iii) a hood communicating with an extraction means to draw fluid and the entrained fraction from the support surface into the hood and

(iv) an auxiliary fluid inlet intermediate to the support surface and the hood for the supply of fluid above the support surface.

The preferred fluid is air, but of course other fluids, such as nitrogen for example, could be supplied to the body of particulate material in other applications.

Advantageously, the hood may be fitted with a screen configured to selectively permit the selected particle size fraction to pass therethrough. Those particles which are entrained, but outside the allowed particle size range and unable to pass through the screen, return to the vicinity of the support surface and are processed with the remaining material.

In a preferred arrangement, the separator includes a vibrator operable to vibrate the support surface for improved separation efficiency.

The support surface may be part of a conveyor continuously supplying particulate material to a region under the hood. The support surface is preferably inclined so that feed material received on an upper region of the support surface falls toward a lower region of the support surface, the selected particle size fraction being entrained in the fluid stream and removed leaving a remaining fraction in the lower region. Preferably the inclination is at least 30° from horizontal to prevent fines retreating upwardly along the support surface.

In some applications where the fine fraction is of interest for further processing, the hood may be operably connected to a further separating means or a collection device such as a bag house or a cyclone. In other applications, the fluid and the entrained fraction may simply be discharged. Preferably, the selected fraction is of a particle size less than about 200 micron. More preferably, the selected fraction is of a particle size less than about 100 micron.

The separator is most advantageously deployed for separating iron ore. Other applications envisioned include the removal of:

• fine chromite from mineral concentrates;

• fine deleterious clays from mineral sands;

• fines from any dry feed or other industrial flow stream.

Brief description of the drawings

Figure 1 is a schematic cross sectional view of a portion of an embodiment of the present invention;

Figure 2 is a performance chart showing the particle size distribution for the product, fine fraction and course fraction; and

Figures 3(a) to (d) illustrate the improvements in the composition of the iron ore after the removal of the fines.

Detailed description of the embodiments

As shown in Figure 1 , the separator 10 includes an air permeable table 20 and a hood 30 with screen 50 spanning the hood 30. The hood 30 is spaced from the support surface 20 leaving openings 100. A vibrator 90 (shown only in schematic form) is operably connected to the support surface 20. An extraction fan (not shown) may be arranged to extract air and the selected fraction 70 through hood 30 as indicated by arrow B. A pressurised air supply (not shown) provides an upward air flow from under the support surface 20 as indicated by arrows A.

In use, feed material 60 is supplied to the separator 10 toward the upper edge of support surface 20. The support surface is inclined by an angle a to the horizontal such that the feed material tends to flow downwardly across the support surface 20.

The magnitude and character of the vibration from vibrator 90 and the volume of airflow from the air supply A is selected to optimally fluidise the feed material and entrain a selected fraction of the material. The amount of airflow B is selected to be greater than the air supplied from A such that the entrained faction 110 is more effectively upwardly drawn towards the screen 50. The difference in airflows is made up by inwardly flowing

airstreams C, which flow through openings 100 that are provided by the spacing of hood 30 from the support surface 20. This effectively creates a negative pressure within the hood. Thus air entering through C effectively works as a barrier or seal maximising the amount of particulate material entrained in B.

In this embodiment, the airflows are selected to optimally fluidise the bed of material and entrain a fraction of material smaller than 100 microns. The airflow B is selected to more effectively draw the less than 100 micron fraction 70 of the material upwardly. The screen 50 has an aperture size of 100 microns such that larger particles included in the entrained fraction 110 are prevented from passing through. In this way, there is a more efficient separation of the particles.

By spacing the hood from the support surface 20, it is possible to independently vary the airflows A and B and thereby optimise the fluidisation and entrainment of the particles and the subsequent carrying of the particles through the hood. This is an advantage over prior art devices that include support surfaces spanning closed duct like systems where it is difficult to set the airflow from the blower at a level that is optimum for both purposes. This is especially so when there are changes in the feed size due to plant fluctuations and feed sources or changes in product size which is often requested by purchasers of mining products.

It has been found that the use of a screen 50 instead of baffles as in some prior art devices (such as that described in UK patent application number GB2221172) allows more precise control over the fraction of material to be separated. This arrangement also reduces the height of the machine significantly, thus resulting in reduced construction costs.

The use of vibrator 90 in addition to air supply A enables better separation efficiency by producing a more dilated and less dense fluidised particle bed, which can be optimised according to the feed particle size by changing airflow A, the vibration frequency, the vibration amplitude and the character of the bed. The more dilated particle bed also leads to a higher throughput.

A disadvantage of the use of vibration is the tendency of fine material to move upwardly along the support surface 20 due to particle-to-particle interaction. This is substantially overcome by inclining the support surface 20 at an angle a that is 30 degrees or more to the horizontal. In this embodiment the angle a is 30 degrees.

To demonstrate the effectiveness of the invention, iron ore having a specific gravity of 4.2 was fed into a prototype of the invention with a capacity of 600 kg/hr/m ² . In this example, the support surface was 0.3 x 0.6 metres in size and had a 0.5 mm rectangular slots and the hood was fitted with a mesh screen having a 0.1 mm aperture size.

Figure 2 illustrates the composition of the feed and the separated coarse and fine fractions. The graph illustrates the weight percentage (Y-axis) of particles in the respective streams that are less than a particular particle size (X-axis). It can be seen that 22 wt% of the particles in the feed were less than 100 microns in size. After separation using the prototype of the invention described above, the course product stream, ie, the stream exiting the support surface at 80 in Figure 1 , contained only 2 wt% of particles less than 100 microns in size, whereas the fine product stream, ie, stream 70 in Figure 1 , contained 75 wt% of particles less than 100 microns in size. During the operation of the separator to generate this data, screen 50 in Figure 1 was not present. The fine product stream consisted of 28 wt% of the feed stream, whereas the course stream consisted of 72 wt% of the feed stream. The results also show that all of the fines fraction was less than about 300 microns. As illustrated, the separator provides precise control over the separation and was able to remove a substantial amount of the fine particles less than 100 microns from the feed stream.

The coarse product was then analysed for Fe, Siθ ₂ , AI ₂ O ₃ and P content and compared with the respective contents of the feed material before separation. It can be seen that not only are there improvements in the Fe content of the ore after treatment for the size fractions removed, but also the levels of the impurities AI ₂ O ₃ , SiO ₂ and P are also reduced. This is related to the differing densities of the ore containing fine particles relative to the impurity containing particles and the greater proportion of impurities present in the fine ore fractions.