GB2330321A - Two stage HGMS with low intensity first stage field - Google Patents

Abstract

The high gradient magnetic separation process for separating particles is carried out in two stages, in which the first stage is at a lower intensity than the second stage. Two HGMs units may be used or the inactive canister of a reciprocating HGMs under the influence of the weak edge of the main field can be used.

Description

2330321 I-FIGH GRADIENT MAGNETIC SEPARATION The present invention is

concerned with improvements to a process known as High Gradient Magnetic Separation often abbreviated to HGMS.

The HGMS method is the most powerful of all magnetic separation techniques which has led to its application in the difficult process of removing very fine weakly,' magnetic impurities from industrial minerals such as Kaolin, calcium, carbonate, talc, mica, silica, wollastonite and nepheline syenite.

One of the most important and well established applications of HGMS is in the benefaction of Kaolin slurries where the method is used to remove iron and titania bearing mineral particles, generally smaller that 10 micron and sometimes in the submicron range. In order to achieve this, large very high intensity magnets, (typically, operating at 2 Tesla or above) are required and a key development of this technology over the last decade has been in the direction of increasing magnetic intensities up to 5 Tesla by the use of superconducting technology. Indeed, a number of Kaolin producers have exchanged their magnet copper coils to superconducting coils providing and increase in field strength from -3 Tesla to 2.5 Tesla to achieve enhanced separator throughout andlor greater impurity, removed.

All magnet separation methods operate by subjecting a mixture of magnetic and non magnetic particles to at least two forces, one of which is a magnetic force and therefore acts only on the magnetic particles. Providing the magnetic force experienced by, the magnetic particles is greater than the second force which illay he gravity, fluid drag, centripetal force or some combination of these, then a separation will take place.

The magnetic force (Fin) experienced by, a particle rnay be described in a reasonable 2 approximation by the equation:

Fm = M(d/dx)1-1 (1) where:

M = Induced Magnetisation of the particle (d/dx)H = Magnetic field gradient

For ferromagnetic particles the magnetisation (M) is a strong but non linear function of applied field with saturation typically occurring at the equivalent of approximately 0.4 to 0.5 Tesla background. For paramagnetic particles the magnetisation is much weaker but a linear function of applied field. This allows equation (1) to be rewritten as:

Fm = Vs(d/dx)H (2) where:

V particle volume s particle magnetic susceptibility 11 magnetic field intensity,'

The majority of particles requiring removal in Kaolin processing are paramagnetic.

The main competing force for HGMS of Kaolin slurries is fluid drag described approximately by; the equation.

Fd orndV (3) where:

Fd Fluid drag force n Slurry viscosity d Particle diameter v Slurry, velocity, 3 It turns out in practice that typical slurry flow velocities which allow separation to take place are in the region of I em 1 second for 2 Testa magnets and 2-3 em per second for 5 Tesla magnets. At these very slow fluid velocities gravity (or particle setting) begins to have noticeable effect. For example, the natural settling velocity of a 40 micron particle with a density of 2.5 gmlcm' (this is a typical value for many, minerals including Kaolin) is 1 mm/sec and for 100 micron particles the settling rate is 1 em/sec.

In order to provide the necessary combination of high background field and high field gradient (as required by equations (1) and (2)) the basic HGMS system comprises a solenoidal electromagnet providing a reasonably, homogenous magnetic field in a cylindrical volurne and this volume in turn houses a canister packed with fine ferrornagnetic material generally referred to as the matrix. This matrix Call take the form of wire woo), steel balls, expanded metal or some other porous ferrornagnetic body. For Kaolin processing the preferred matrix is a ferromagnetic stainless steel wool which is available in a range of fibre diameters to suit die size distribution of (lie Kaolin feed. It is the matrix which generates the high field gradient as it concentrates the magnetic lines of flux in its vicinity. This is illustrated the Fig 1. Consequently; the magnetic capture force is located in the two opposite regions on or near die surface of the matrix fibre.

Despite the relative simplicity of the HGMS process, its discovery, and subsequent development for Kaolin only, came about in the late 1960's and early, 1970's when it was realised that there were three basic requirements. These were; operating magnet with high field intensities (typically 2 Testa); the use of very fline matrices of steel wool with typical diameters of 30 to 100 micron and very low process velocities in tile order of 1 cm/see.

The high field requirement was the rnost demanding task and all of the basic patent

4 literature refers to magnetic fields in the region of 1.5 to 2.0 Tesla as being a key parameter.

The force of the magnetic particle is highly dependant on the distance from, as well as the diameter of, the wire. As a result, the chance of a particle being captured and stuck to the wire is strongly dependent on its position as well as its magnetic properties and to fluid drag effects. This means the system is not good at separating two materials of modestly different magnetic properties i.e it is not very selective. Moreover, once strongly magnetic material has been pulled on to the matrix it can be difficult to remove.

The standard HGMS system in use today is illustrated in Fig 2. The coil can he either water cooled copper or a superconducting solenoid. The latter, although more complex, enables higher fields to be generated (up to 3 or 3.5 Tesla) at significantly lower power consumption. A heavy iron franie surrounds the coil and matrix canister to niaxiniise the inagnetic efficiency thereby reducing the demand on the coil current requirement. In operation, the HGMS process follows the following steps involving sequential control of the valves and magnet power supply:-

1) Magnet field:

2) Displace water:

3) Process:

With a clean matrix the magnet is energised.

Kaolin slurry is allowed to flow up through the matrix canister. When the Kaolin level reaches the top of the canister soon after the product valve opens and svaster valve closes.

Kaolin slurry continues to pass through the magnetised matrix with capture of magnetic particles. However, the available capture sites are being used up and product quality degrades to the minil-nuill acceptable level. At this point the Kaolin feed valve closes and the water valve opens.

4) Displace Kaolin:

5) Magnet-off: 6) Flush:

Water flows through at a flow rate near to or equal the Kaolin flow so as to displace the last remaining canister volume of Kaolin without dislodging the captured magnetics. When most of the Kaolin has been displaced the product valve closes.

The magnet is de-energised.

The matrix is flushed clean with a high velocity (up to 7 or 8 em per second) flow of water. There are different flush sequences used by different operators, the most common being reverse, forward, reverse flow relative to the original kaolin flow direction.

There are certain varieties on this stepwise process preferred by, different operators. For example steps (1) and (21) are often combined because as with any inductor the bulk of the field is generated in less than half the time for full field and so the losses in product quality are niinirnal. Sometimes an air blast (sparge) is used to assist the flushing process. Also some operators have considered downward flow of the Kaolin slurry.

An alternative HGMS method which has gained some acceptance is based oil the reciprocating canister principle and illustrated in Figs 3a and 3b. With this method the magnet is left permanently, energised and separation takes place alternatively, between two mechanically linked matrix canisters. It is possible to move froill one canister to the other in a comparatively short tirne (10-220 seconds), also, because flushing takes place whilst the other canister is processing, there is some improvement in process efficiency. Since magnet energisation time has no influence on the processing cycle it is possible to use very powerful magnets and these units presentl operate at 5 Tesla. However.. the reciprocating concept places certain restrictions and 6 compromises are required, one of these is the use of multiple very short bed depth matrix canisters which results in short process of times.

In most major respects the process stages 2 to 4 are the same for the conventional switch on/off and reciprocator HGMS systems although the actual timings are in practice quite different. Table 1 gives typical timings for each of the stages for the switched mode HGMS units.

TABLE 1. Typical Process times for switched - Mode UGMIS Step: Description Elapsed Time

1 +21 Energise Magnet and displace water 1 rnin 3 Process 4-15 min 4 Displace Kaolin 1 -2 inin Magnet off 1 min 6 Flush 1.5-2.5 min In essence the lIGMS separator acts as sort of magnetic filter which traps and retains magnetic impurities in the Kaolin or other feed. Periodically when the filter is saturated it i-nust be flushed and the process restarted.

For a conventional filter one would expect that magnetic products would be captured at the front of the filter. As this became saturated particles would be captured further down until the whole bed was saturated with magnetics at which point the separation quality would rapidly deteriorate.

For Kaolin production it is normal to plot brightness of the product or purity Le magnetics removal as a function of time or product passing through the filter. For a 7 conventional filter the expected curve would appear as in Fig. 4A. In practice however, the curve found for HGMS on Kaolin is very different.

The typical HGMS process decay curve or family of decay curves at different flow velocities for Kaolin is as shown in Fig. 4B. There is a rapid initial decay in a product quality,'. The ordinate shows product quality. in terms of brightness of the product. A sirrillar curve would be obtained if this were replaced by the percentage impurity (Fe,0,+TiO). In order to improve on product quality the Kaolin producer can either reduce flow velocity or shorten process time, both of these however, result in a reduced production rate.

There have been numerous attempts to explain the shape of the decay curve froill a consideration of the particle dynamics and forces involved. The most successful model is that proposed by Professor J.11.P Watson as described in World Filtration Congress IV, Ostend, Belgium, 22-25 April 1986: "Magnetic filtration of Polydisperse Particle Systenis". In this model, Watson has shown that captured strong magnetic particles provide enhanced capture sites for very, fine and very weakly,' magnetic impurities. The model shown that analysing the UGMS process must take into account the fact that the slurry undergoing separation will contain a distribution of particle sizes and the magnetic particles will be composed of a range of particle sizes and magnetic susceptibilities. The presence of relatively; coarse strongly,' magnetic particles is believed to enhance the removal of weakly magnetic particles by providing effective trapping sites between the coarse particles. The model is the first to explain that matrix canister depth plays an important role not just in the separator duty. factor but also in actual separation efficiency,, the effect of coarse strongly magnetic particles providing "enhanced" capture sites is reinforced by,' proposals for seeding Kaolin slurries with strongly ferromagnetic particles as a method of enhancing removal of weekly, magnetic impurities by HGMS.

8 The present invention seeks to provide improved high gradient magnetic separation.

According to an aspect of the present invention there is provided an HGMS process including at least two stages, in which the first stage separation is carried out at a lower intensity, than the second stage.

The teaching herein are based on the discover), that, contrary to expectations, a low magnetic intensity separation stage prior to the nominal high intensity treatment not only improves the overall separation achieved and/or allows higher separator throughout but also has numerous beneficial effects on the operation of the high intensity (second stage) process. These benefits include: improving energy efficiency. reducing wear of components, reducing maintenance costs, reducing water consumption and reducing product dilution. Consequently, this rnethod provides an elegant solution to enhancing the capacity of existing separators. Furthermore, tile two stage process enable a new design of HGMS plant which reduces the cornplexityr of the high intensity, separator (2nd stage) design and operation.

An embodiment of the present invention is described below, by, way of example only, with reference to the accornpany,ing drawings, in which Figure 1 is an illustration of magnetic flux concentration by a ferromagnetic matrix fibre; Figure 2 is a schematic view in cross-section of a prior art switched- mode HGMS system;

Figures 3A and 3B are schematic views in cross-section of a prior art reciprocating HGMS sy,steni;

Figures 4A and 413 are graphs illustrating the perforniance of prior art systems; and Figure 5 is a graph illustrating die performance of the prepared embodiment of HGMS systems.

9 It has been shown in the above that the current know-how on Kaolin processing requires high magnetic fields, typically at 2 Tesla or higher, and that the separation is enhanced by the presence relatively coarse magnetic particles. The preferred embodiment is based on the discovery that a low magnetic intensity (at nominally, 0.75 Tesla) HGMS treatment of Kaolin slurries has a profound effect on the subsequent decay curve for a second HGMS pass at high intensities. In-line with earlier reported observations, the product from a low intensity first pass shows minimal improvement in brightness over the feed material. However, when this product is re-treated at the normal high magnetic intensities of 2 to 5 Tesla, the decay curve can be almost flat.

By way, of example, the results of treating a fine paper coating grade Kaolin of (lie type being produced in the middle Georgia (USA) or Brazil are shown n Fig 5. Cun,e A shows the decay. curve from a single pass a 5 Tesia and 1 ciii/sec slurry, \,eloclty,. This same feed when processed at the sarne velocity at 0.75 Tesla only: gave a 1.6 point brightness gain which would fall the i-ninii-nuiii requirement for a paper coating Kaolin. However, when this low intensity product was re-treated at 5 Tesia the almost flat decay, curve (A2) was obtained. The same effect was observed with a 1.7 cm/sec process velocity, as shown in curves A I and A2.

It is evident from these results that the two stage process riot only enables a higher quality, product to be achieved than can be obtained by a single high intensity treatment but by virtue of enabling the process tinie to he increased whilst keeping tile remaining process constant gives further benefits which can be listed as follows:

A longer processing tinie results in a higher duty, factor for the high field Separator, A conservative estimate indicates an increase in duty factor froill 67W to 85 % which is equivalent to almost 217 % increase in production rate.

2. The Usplace Water' and 'Displace Kaolin' steps in the HGMS cycle produce a dilution of the product because the interface between the Kaolin slurry and water is not sharp. For the reciprocating type of HGMS product, dilution is a problem that can at present only be minimised by loss of Kaolin product into the waste stream. For either type of separator using a low intensity pretreatment can reduce product by 50% or rnore. For greatest benefit the flushing stage for the two magnets should conclude with an air blast to expel most of the water so that the Kaolin slurry commences passage through a fairly dry matrix.

3. Valve operations are reduced by half or more with corresponding doubling of their lifetime.

There are number of ways in which the two stage process can be implemented, each di 1 1 1 1 -nerits depending on the local situation. For example, with switched fliode lIGMS systems the first (low intensity) stage may,' operate in- line and in sequence with the second stage. Alternatively the first stage can be operated as a separate unit. This latter option enables greater freedom in the cycle timings but is less efficient because separate pumps and tanks are required.

It Is also possible to employ a hybrid of the two, whereby the two stages operate inline (and in sequence) for part or parts of the cycle and operate separately, at other times. For example, at the displace Kaolin stage the first stage may he isolated and the Kaolin allowed to drain frorn the magnet and be pumped back into the feed tank whereas the second stage would undergo the Kaolin displacement with water as standard. This modification would reduce product dilution associated with the first stage JAGMS. As noted earlier product dilution can also be reduced at the displace water stage if water is ejected by, air blast at the end of the flush cycle.

For completeness we note [fiat regardless of whether the second stage us a switched 11 mode or reciprocating HGMS the first stage can be either switched mode or reciprocating HGMS.

Having established the advantages of two stage separation (where the first stage occurs at lower intensities than the second) it is now possible to improve on the design and operation of reciprocating HGMS. In the reciprocating HGMS systems the two active rnatrix canisters are sandwiched between the three magnetically balanced inactive canisters sometimes called "dummy canisters". These inactive canisters at present sen,e only,' to enable matrix reciprocation with minimum drive force and to reduce the magnetic imbalance stresses that would he experienced by, the magnet solenoid in their balance.

As mentioned earlier, reciprocating HGMS presently in operation utilise fields of 5 Tesla. This means that the inactive canisters occupy; a volume where the field extends from near zero up to 5 Tesla. At present this magnetised volume is riot being used for separation. Bearing in rnind that the magnetised region extends for some distance typically between 50 ci-n and 1 metre beyond the active canister undergoing processing. It becomes apparent that these inactive canisters can he configured to provide a useful first stage process.

As a continuing part of this development a series of two stage HGMS tests were carried out on a primary Kaolin presently being exploited in a European Kaolin plant. As before the first stage separation was carried out at 0.75 Tesla and the second stage at 5 Tesla. The analytical data is presently awaited, however, the test was carried out with one modification where the second stage flushing was attempted with the magnet fully, etiergised. It was found that a standard flush composing water flow was at nominally, 6 ci-n/sec combined with air blast resulted in complete flushing of the matrix. A second flush sequence with the magnet de-energised showed no material emanating from the matrix canister proving that the flush at full field (5 Tesla) was

12 completely successful. Attempts to flush the matrix at 5 Tesla without the first (low intensity) pass failed to produce a clean rnatrix even with highly aggressive flushing. The results of these tests provide the basis for further improvements to HGMS process.

As noted above the trend in HGMS development has been in the direction of higher and higher magnetic fields. For switched mode superconducting HGMS the extra energy extended in high speed energisation and deenergisation of large solenoids calls for complicated cryogenic design involving liquid nitrogen and helium liquefiers. With the need for frequent inagnet rainping eliminated, the cryogenic complexity can be reduced substantially. In essence, the two stage process as proposed has the cryogenic benefits of the reciprocator design (where the magnet is pennanently energised) without the disadvantage of the complex reciprocating double canister des ign.

13

Claims (8)

1. An HGMS process including at least two stages, in which the first stage separation is carried out at a lower intensity than the second stage.

2. An HGMS process according to claim 1, in which the first stage separation is carried out at 1 Tesla or less.

3. An HGMS process according to clairn 1 or 2, in which the first stage separation is carried out at half the field intensity or less than that used for the second stage.

4. An LIGNIS process according to clairn 1, -2 or 3 in which the second stage does not involve urgent de-energisation for a flushing process thereof.

5. An HGMS process according to any. preceding claim, in which the second stage processing tirne is extended from x to at least 1.3x whilst gi-ving equivalent or higher magnetIcs rernoval when compared with a single stage process with a processing time of' x and all other parameters for the second stage remaining the same.

6. A separation canister for use in reciprocating IIGMS systems including magnetic balancing canisters which engage in separation as a pre-cursor to separation in active canisters.

7. An HGMS process substantially as hereinbefore described.

8. A separation canister substantially, as hereinbefore described.