FR2558086A1 - METHOD FOR PRODUCING ACICULAR OR EQUIAXIAL IRON OR IRON ALLOY PARTICLES BY DISSOLUTION OF A METAL STRIP CONTAINING SUCH PARTICLES - Google Patents

Abstract

THE INVENTION PROVIDES A METHOD FOR PRODUCING IRON OR IRON ALLOY PARTICLES. A BAND OF COPPER OR COPPER ALLOY 12 HAVING FINE PARTICLES OF IRON OR IRON ALLOY SENSITIVELY EQUIAXIAL AND DISTRIBUTED THROUGHOUT ITS MASS is PREPARED. THE TAPE IS SELECTIVELY DISSOLVED WITHOUT SENSITIVE DISSOLUTION OF THE IRON OR IRON ALLOY PARTICLES IN ORDER TO RECOVER THE PARTICLES.

Description

Although it extends to a wide range of applications, the invention is

  particularly relates to a relatively inexpensive apparatus and method for producing substantially equiaxial or acicular iron or iron alloy particles intended to be

  used for magnetic recording.

  Until now, various magnetic powder materials have been proposed for use

  in the preparation of the magnetic recording means

  tick; for example Fe203y, Fe203 doped with Co, Fe304, Fe304 doped with Co, FeO4-Fe203, CrO2 etc. Preparation 34 Fe4F20, C02

  of these powders requires a fairly long and costly process

  teux. For example, acicular iron particles can be produced by reduction in a fluidized bed of Fe 2 O 3. These iron particles are extremely pyrophoric and require significant treatment to

their passivation.

  A number of different methods have been proposed for producing powdered ferromagnetic metal alloy materials, such as that disclosed in United States Patent 4,274,865. Besides that it discloses a process for preparing

  a magnetic powder suitable for magnetic recording

  tick, consisting mainly of iron, this patent describes other techniques for producing particles

  ferromagnetic aciculars. However, no information

  mation does not relate to the unique process for the manufacture of equiaxial or acicular particles of iron or iron alloy as communicated by the

present invention.

  U.S. Patent 4,290,799 describes, for example, "a ferromagnetic metal pigment for magnetic recording applications, which consists mainly of iron and is distinguished by acicular particles which are well

2558086.

  developed and by superior properties as a recording medium, and a process for the production of this material ". The process for preparing metal powders, as known by this patent, is quite different from the present invention U.S. Patent 3,556,962 to Pryor et al describes a method for recovering copper waste containing iron, and to U.S. Patent 4,264,419 to Pryor et al. ;, we

  describes a method for electro-tinning

  chemical of copper-based alloys. In these two patents, neither does it describe or indicate the production of a strip having fine iron particles fully distributed or the dissolution of the strip in order to recover the particles. So the present invention stands out

  clearly from previous descriptions.

  The object of the invention is to provide

  a method for producing ferromagnetic particles

  substantially equiaxial or acicular ticks having the magnetic anisotropy or of desired shape, which is suitable for incorporation into the media

  classic magnetic recording.

  An advantage of the present invention is that it provides a method for producing substantially equiaxial particles of iron or iron alloy which remedies one or more of the

  limitations and disadvantages of the prior implementation

which has been described.

  Another advantage of the present invention is to provide a relatively inexpensive method for producing fine particles of iron or

of iron alloy.

  A method and apparatus are therefore proposed for producing substantially equiaxial particles of iron or iron alloy. A metal or metal alloy strip having fine

  equiaxial particles of iron or iron alloy repaired

  ties throughout the mass. This metal strip is selectively dissolved without any appreciable dissolution of the iron or iron alloy particles in order to recover the particles. The method may include the use of the apparatus which facilitates the gathering of particles. The weldable particles, which are collected, have a length of between about 0.05 and about 0.5> m and a length / width ratio ("aspect ratio") which ranges from about 4: 1 to about 15: 1 . The

  particles collected can be approximately equal

  axial with dimensions in the range

  ranging from about 0.05 to about 0.5 µm.

  The invention and other developments of the invention will be better understood on reading the

  description which will follow of exemplary embodiments

  preferred, with reference to the accompanying drawings showing them, o Figure 1 is a schematic representation of an apparatus for forming a strip having substantially equiaxial iron particles according to the invention; Figure 2 is a graph of the anodic dissolution of copper and iron in a sodium sulfate solution; Figure 3 is a schematic diagram of an apparatus for practicing the present invention; and figure 4 is a representation

  schematic of an electromagnetic tank according to the pre-

invention.

  The present invention relates to a method and an apparatus for producing substantially equiaxial acicular ferromagnetic particles,

2558086 -

  The method requires a metal or a metallic strip having fine particles approximately equiaxial of iron or alloy of iron, completely distributed. A strip of copper-based alloy, containing ferromagnetic particles, can be prepared by rapid solidification in such a way that the particles of iron or of iron alloy substantially equiaxial, having dimensions comprised between approximately 0.05 and approximately 0, 5 microns, are distributed substantially homogeneously or isotropically throughout the matrix of the solidified base metal. The equiaxial particles can be of either spherical or cubic morphology. The resulting copper alloy strip has particles

  substantially-equiaxial ferromagnetic having the aniso-

  magnetic tropie or desired shape. If desired, these substantially equiaxial fine particles can be elongated by cold rolling to produce particles whose length / width ratio is between about 4: 1 and about 15: 1 and preferably between about 5: 1 and about 7: 1. Alloy strip

  resulting copper has ferromagnetic particles

  acicular ticks with magnetic anisotropy or

desired shape.

  Specifically, a base metal is

  melted by any classic technique desired.

  The base metal preferably comprises copper, a copper alloy, gold or a gold alloy. It is furthermore within the scope of the present invention to propose small additions of transition metals, as will be described below. The iron is mixed into the molten base metal to form a substantially homogeneous single phase melt. Although the iron may represent more than approximately 20% by weight of the total mixture, the iron will preferably represent approximately 20% to approximately

  60% by weight of the mixture. Iron is preferably

  so pure, although it may contain some

  impurities or doping elements.

Although the present description

  relates firstly to a metal strip consisting of copper and iron, it is within the scope of the present invention to add to the molten bath some

  other components desired in order to modify the composition

  sition of ferromagnetic particles. Additions of transition metal, which increase the magnetic efficiency of the needle-like iron alloy particles obtained, can be incorporated by alloying the melt. To this end, nickel, cobalt, manganese and other transition elements, in an effective amount up to a maximum of about 10% by weight and preferably between about 2% and about 7% are advantageous and fall within the framework of the techniques

of classic alloy.

  The strip is preferably prepared by rapid solidification according to any desired mode, such as spinning in the liquid state ("melt spinning"). Other applicable techniques such as atomization, are exposed in an article by Grant entitled "Rapid Solidification of Metallic Particulates" in "Journal of Metals" of January 1983. If one uses

  these other techniques, the "non-particles" ("non-

  particles ") can be distributed in rotating discs or non-continuous copper alloy matrix parts. In general, the process of separation of the desired iron particles is carried out from the

  as described for band dissolution.

  Figure 1 shows by way of example an apparatus 10 for producing a continuous, thin and long strip 12 of copper or copper alloy dispersed with iron or an iron alloy. The mixture of metal based on molten copper and iron 14 can be introduced into a heat-resistant tube 16 made of a material such as quartz. The tube 16 can be provided at one end with a nozzle 18 having a diameter of between about 0.3 and about 1.5 mm. The molten metal 14 is preferably maintained at a temperature slightly above the liquidus point of the melt by any suitable means such as a heating resistor 20. Although the temperature may be at most not more than about 200 C above the liquidus point, it is preferably not more than about 100 C above the

  no liquidus. Notwithstanding the temperature limitations

  rature mentioned above, the molten product can be maintained at any desired temperature. A cooling substrate 22, such as a cooling wheel, can be rotatably mounted below

  of the heat-resistant tube 16. The cooling wheel

  Dissement can have any desired diameter and can be rotated at a peripheral speed of between about 320 and 256 m / min and, preferably, between about 640 and 128 m / min. However, without departing from the scope of the present invention,

  spin the wheel at any sub speed

  hated. The open end 18 of the nozzle is preferably placed less than about 5 mm and preferably less than 2 mm from the smooth surface 24 of the wheel 22. The molten metal is ejected from the tube 16 on the rotating surface 24 at a pressure of between about and about 280 kPa and preferably between about

  105 and about 175 kPa which is exerted on the melt.

  As soon as the molten metal comes into contact with the rotating surface 24, the melt rapidly cools and solidifies into a continuous thin strip 12 in which the iron particles are distributed in a substantially homogeneous or isotropic manner through the matrix

copper-based metal.

  The thickness and the width of the thin strip 12 obtained can be determined by a certain

  number of factors. For example, the super tension

  between the molten metal and the surface 24 of the cooling wheel 22 in motion determines the shape of the strip 12. As the surface tension of the melt increases relative to the wheel, the strip tends to become thicker and narrower. Increasing the rotational speed of the cooling wheel results in a thinner, wider strip. The ejection pressure of the melt 14 also determines the shape of the strip. As the pressure increases, the width of the strip increases while its thickness decreases. The nozzle diameter of between about 0.3 and 1.5 mm and preferably between about 0.8 and about 1.2 mm is also a factor. The smaller the diameter of the nozzle, the thinner and narrower the strip. Of course, the ejection temperature and the viscosity

  of the melt are also essential factors.

  The warmer the melt and the lower its viscosity, the thinner and wider the strip. It is considered that the viscosity should be between approximately 1 and approximately

mPa.s.

  The choice of the material constituting the cooling wheel must take account of the wettability between the molten thin strip and the surface 24. This wettability is mainly determined by the surface tensions of the melt and of the substrate. It has been found that a cooling wheel made of copper can be successfully used to make a strip made of the materials listed above. However, it is also possible within the framework of the present invention. use other materials such as, for example, copper alloy, aluminum,

  aluminum alloy, steel, alloy steel or graphite.

  The temperature of the melt or the melt is preferably slightly higher than the liquidus point thereof. As mentioned above, although the temperature may not be more than about 200 C above the liquidus point, it will preferably be no more than about 100 C above the liquidus point. If the temperature were below the liquidus point, the mixture would contain some solid particles and would not form properly. Conversely, if the temperature rose too high above the melting point, the melt could either spread over the cooling surface of the wheel in such a way that the strip would become too thin, or be projected beyond the front wheel to be able to solidify into a strip. As a result, the preferred temperature is slightly higher than the liquidus point so that the cooling wheel can extract enough heat to immediately make the strip slightly solid and give it some stability or mechanical strength. Depending on the particular composition of the melt and the other operating parameters of the process, the cooling rate can range from approximately

2 8

  at 10 K per second and it is preferably understood

2 6

  between about 10 and about 10 K per second.

  Although a cooling wheel is described as a preferred device for forming the band, it is also possible, without departing from the scope of the present invention, to form the band by any means

desired classics.

  The present invention requires the formation of a continuous band or pieces of metal alloy which serve as an intermediate material to produce particles of iron or iron alloy substantially equiaxial. Most of the particles

2558G86

  ferromagnets are distributed homogeneously or isotropically across the strip, and have a substantially equiaxial shape and, preferably, dimensions such that each particle constitutes a distinct magnetic domain, that is to say that it

  or of the order of approximately 0.05 to approximately 0.5 μm.

  During solidification, there are two modes of precipitation of iron for a rapidly solidifying copper-iron melt. Primary solidification tends to be relatively coarse and flaky and the iron particles have

  generally larger than about 2> m.

  Secondary solidification occurs towards the final solidification phase and gives most of the particles a substantially equiaxial morphology with dimensions varying from approximately 0.02 to approximately 0.5 "m. The equiaxial particles can

  have a cubic or spherical shape.

  The particle size is determined

  undermined by the solidification time, which in turn is determined by factors such as the casting speed, the thickness of the casting and the thermal conductivity of the alloy. Thicker sections of casting result in larger particles while thinner sections of casting result in smaller particles. Likewise, a faster cooling rate results in the formation of smaller particles. The final strip of copper or copper alloy to be produced preferably has, first

  instead, particles of iron or iron alloy feel

  substantially equiaxial, dispersed isotropically or homogeneously throughout the matrix. However, it may be desirable that the final strip of copper or copper alloy to be produced preferably has acicular ferromagnetic particles dispersed

  isotropically or homogeneously across the entire matrix.

  So far, the process described has produced parts

  homogeneously or isotropically spaced equiaxial cells.

  The next step may therefore have the objective of lengthening the particles. For this purpose, the cast strip is preferably rolled to obtain the desired length / width ratio. This rolling can be carried out cold or hot depending on the resistance of the iron particles. If the rolling is carried out hot, it must be carried out at a temperature which does not exceed the margin of between approximately 3000 C and approximately 9000 C. The length / width ratio (aspect ratio) of the particles is preferably between approximately 5: 1 and about 7: 1 although it can go

  from about 4: 1 to about 15: 1. The band has many

  of weldable iron or iron alloy particles,

  acicular, formed during the rolling phase.

  It may be desirable to anneal the laminated strip

  cold to soften the particles as required.

  To soften the iron particles, annealing would require temperatures in the range of about 4000 C to about 900O C. It should be noted that if the iron particle is acicular, it will not change shape

during annealing.

  The process continues with the dissolution of the matrix of the strip or of the parts without any appreciable dissolution of the particles of iron or of iron alloy contained therein. The particles can then be recovered. The matrix is preferably dissolved by electrolysis; however, dissolving the matrix by any other desired method

  is within the scope of the present invention.

  According to the preferred method of manufacture, a metal strip containing particles of iron or iron alloy substantially equiaxial or acicular, as specified above, is immersed in an aqueous electrolyte. The specific electrolyte is chosen to passivate the particles of iron or of iron alloy while allowing by electrolysis an aggressive anodic dissolution of the matrix of metal or metal alloy. The adjustment of the electrical potential at which the strip is maintained is of capital importance and will be described more fully

  below. Sodium sulfate at neutral pH is an electro-

  lyte preferred for this purpose. The concentration of sodium sulfate is not absolutely essential, although concentrations of between about 0.05 N and about 4.0 N are preferred. Other electrolytes suitable for this application include alkali metal sulfates. After immersing the typical continuous or non-continuous strip in an electrolytic bath of the type mentioned above, an electric current is passed between an electrode as a counter-electrode.

  or cathode and the strip as active electrode or anode.

  The strip is preferably contained in a container

  current conductor (also serving as active electrode) to which the external current can be applied, as described below. As the strip dissolves, the particles of iron or iron alloy are collected in the platform tank, from which they can easily be recovered. The external potential is maintained in the passive potential range of the iron or iron alloy particles in the strip. The result is the anodic dissolution of the copper matrix

  and recovering the passivated iron particles.

For example, we submerge the tape

  in a sodium sulfate electrolyte and we keep it

  is due to a critical potential between approximately 0, OVEsH

  (Standard Hydrogen Electrode) and approximately 1.5VESH.

  In addition, the preferred range of this electrical potential is from about 0.25 ESH to about 1VSH Maximum voltages are specified so that a high anode current, on the order of about 2 A / cm ( 2.104A / m2), either supplied by the copper or copper alloy matrix. Figure 2, which shows the anodic dissolution of copper and iron in a sodium sulfate solution, highlights the fact that a low current of the order of less than a few mA / cm is provided by the passive particles of iron or iron alloy when the potential is established as

it has been described above.

  The apparatus for applying the process

  of this invention is shown in Figure 3.

  The active electrode 30 essentially consists of an electroconductive bearing surface 31 and of the strip 12 connected by the supply wire 32 to the positive terminal 34 of a potentiostat 36. The surface 31 must be passive in the electrolyte. The negative terminal 38 of the potentiostat is raised by the ammeter 44 to the counter-electrode 46 by means of the connecting wires 42 and 40. A reference electrode 48 is connected to the terminal 49 of the potentiostat by a connecting wire 52. A potentiostat 54 is connected by junction wires 32 and 52 and wires 56 and 58 to control the potential difference between the active electrode 30 and the reference electrode 48. As mentioned

  above, a tank 60 contains the electrolyte bath 50.

  The ammeter 44 controls the current while the potentiostat 54 allows the pptentiostatic adjustment of

  the active electrode 30 with the potentiostat 36.

  The bearing surface can surround a tank 33 to hold the strip 12 as it is dissolved by the electrochemical process which takes place in the tank 60. The tank 33 preferably has a box-like structure, the upper part being .nt open, with side walls 62 and a floor 64. The tank 33 may have feet 65 to support the platform on the bottom of the tank 60. The tank, which is the active electrode, is preferably made of an inert material that will not dissolve during the electrochemical process. In the context of this invention, it is possible to use inert materials such as

  nickel, stainless steel, platinum or palladium.

  As the copper matrix of

  the strip 12 dissolves, it is deposited on the counter

  electrode 46 and most of the iron particles are released. Some of the iron particles can still be difficult to recover for several reasons. First, the process releases pieces

  of copper which still contain iron particles.

  These pieces of copper containing iron can float in the electrolyte and therefore remain outside electrical contact with the inner surface of the tank 33 or with the strip 12. This prevents the copper from

  dissolve and release the trapped iron particles.

  Second, the free iron particles, which have been

  gathered in the tank 33, can spread through

  above the side walls 62 either during the process,

  either when they are collected on the platform

shape of the tank.

  To obtain a maximum yield of particles from this process, a magnet 70 can be placed under the bearing surface 31. The magnet preferably attracts iron particles during the process

  which are against the floor 64 of the tank 61.

  The magnet also attracts the pieces of copper still containing iron particles, in contact with the undissolved floor or strip, so that the copper can continue to dissolve in order to release the remaining iron particles. Preferably, the magnet is placed outside the tank 60 so as not to be subjected to corrosion coming from the electrolyte 50. The magnet can be either a permanent magnet or an electromagnetic magnet. It must produce a magnetic field capable of attracting the particles of iron to retain them on the floor 64 or on the walls of the tank. It is also possible, within the framework of the present invention, to place the magnet between the bottom of the tank 60 and the bearing surface or the tank 31. The placement of the magnet in the electrolyte may require certain precautions, such as the use of

  inert coatings or the use of a ferrous material

  inert magnetic, to prevent it from corroding. It is also within the scope of the present invention to place the magnet inside the floor 64 or the side walls of the tank 33 and to encapsulate it with a non-corrosive material such as the metal of the active electrode. Another possibility, represented in FIG. 4, is to constitute the active electrode in a metal of ferromagnetic alloy such as, for example, an alloy of iron-nickel-chromium, and to wind a coil of wire 80 around this electrode. When we pass

  a current exciting the coil, the electrode becomes magnetic

  tick and will attract the iron particles in the re-

  quise. With this embodiment, the electro-

  magnetic can be exercised as desired. For example, while removing the tank 33 from the tank 60 to collect the iron particles, the field can be exerted to attract the particles towards the walls of the tank. Then the magnetic field can be interrupted so that particles can be easily removed from the tank. The coils can be placed next to or around the tank in any desired position so that the field is exerted at any desired location on the tank. During electrochemical treatment, metallic copper dissolves separating from the metal of the anode strip and can be easily recovered

  in an operation which is an integral part of the processing.

  It is extremely advantageous to use a counter

  copper electrode or cathode 48. The entire cathode can then be melted without contamination and reused as required. However, other metals for the counter electrode can be used, such as platinum, lead, iron, stainless steel etc. and the copper which has deposited by electrolysis can subsequently be detached mechanically. At the copper counter electrode, the electrode potential must be lowered sufficiently so that the copper ions passing into solution anodically deposit in the form of metallic copper on the cathode. In general, temperature limits of

  about 20 C to about 60 C are pre-

  but the process will be operated economically between

about O and 100 C approximately.

  Once an iron particle is dried

  adorned with electrical contact with the bearing surface 31, it will quickly lose the passivity caused by its anodic treatment. The particles of iron or iron alloy are slightly protected by a thin outer film which can be iron oxide or iron hydroxide. However, care must be taken to prevent corrosion of the iron particles in order to maximize the utility of the process. Protection of the separated iron particles can be accomplished in several ways. The electrolytic medium can be deaerated by rapid sweeping with an inert gas such as, for example, nitrogen. Deaeration has the effect

  prevent corrosion of free iron particles.

  A corrosion inhibitor for the iron or iron alloy phase can be incorporated

  in the electrolyte provided it does not reduce sensitive-

  not the anode current carried by the copper or copper alloy matrix. Among these inhibitors is sodium molybdate at a concentration of about 5 x 10 5 to about 10-3 N and sodium tungstate at a concentration of about 10 4 to about O3 N. Other inhibitors can be added adsorption which have no specific influence on the anodic corrosion of the copper matrix. For example, copper can be anodically corroded when the current density exceeds about 10 mA / cm in a sodium sulfate solution containing either a concentration of sodium molybdate or sodium tungstate of about 0.005 N. These inhibitors corrosion do not adversely affect the anode current which can be produced by dissolving copper with potential

exceeding approximately 0.45 VEsH.

  After the electrochemical separation of the particles and their inhibition against corrosion in the sodium sulphate solution, the particles are preferably quickly filtered and washed with water to which an oxidizing corrosion inhibitor has been added. These inhibitors are chosen from sodium chromate, sodium nitrite, sodium tungstate and sodium molybdate, at a concentration of approximately 0.001 N to approximately 0.1 N. Washing is followed by rapid drying and dry storage to prevent corrosion. An important advantage of the present invention is that the iron particles

  are not pyrophoric and can be manipulated

  are not pyrophoric and can be manipulated

  pulped or easily treated. For additional protection against corrosion, the particles can be stored in an inert gas such as nitrogen. Iron particles can also be separated by size using any conventional technique, such as passing them through

  a sieve or through a fluidized filter bed.

  The object of the present invention is also to protect the final particles of iron or of iron alloy which have been collected, by coating them with a layer of a metal such as copper or cobalt. The thickness of the coating can be from approximately 0.25 mm to approximately 1 mm and will preferably be between approximately 0.50 mm and approximately 0.75 mm. The coating can

  be applied in any desired manner in use

  sant, for example, the classic technology of deposition

non-electrolytic.

  The resulting needle or equiaxial particles of iron or iron alloy can be used using any of the methods

  common methods for preparing means of

  magnetic recording such as magnetic tapes, discs, flexible disks, magnetic cards

or identification systems.

  The patents and the document presented in this application are intended to be incorporated

by the reference here made.

RE V E N D I C A T I 0 N S

  1. Method for producing particles of iron or iron alloy substantially equiaxial, characterized by the following steps, o: a strip of copper or copper alloy having fine particles of iron or iron alloy is prepared distributed, substantially equiaxial; and said copper or copper alloy is selectively dissolved without any substantial dissolution of said iron or iron alloy particles in order to recover said substantially equiaxial particles, said dissolution step comprising immersion of said copper strip or of copper alloy in a solution of an electrolyte chosen from alkali metal sulfates in such a way that the particles of iron or of iron alloy are passivated while allowing the electrolytic dissolution of the

  strip of copper or copper alloy.

  2. Method according to claim 1,

  characterized in that said dissolving step con-

  siste) in addition, to collect said iron particles

or iron alloy.

  3. Method according to claim 2, further characterized by the step of protection against

  corrosion of the collected particles.

  4. Method according to claim 3, characterized in that the corrosion protection step is characterized in that a metallic coating having a thickness is applied to the particles

  between about 2.5 µm and about 12.7 µm.

  5. Method according to claim 4, characterized in that said coating consists

mainly copper.

  6. Method according to claim 4, characterized in that said coating consists

mainly cobalt.

  7. Particles of iron or iron alloy

  free, in the raw state of casting, substantially equal

  axial and having dimensions in the range included

  between about 0.05 and about 0.5 µm.

  8. Particles of iron or alloy

  free iron according to claim 7, being characterized

  laughed at by a substantially spherical morphology.

  9. The particles of free iron or iron alloy, according to claim 7, being

  characterized by a substantially cubic morphology.

  10. The particles according to claim 7 being characterized by a metallic coating having a thickness of between approximately 2.5 µm and approximately

12.7 pm.

  11. The particles according to claim

  being characterized in that said metal coating

  lique consists mainly of copper.

  12. The particles according to claim

  being characterized in that said metal coating

  lique consists mainly of cobalt.

  13. Substantially equiaxial particles of iron or iron alloy, characterized in that they are produced according to the method set out in

claim 1.

  14. Particles of iron or iron alloy substantially equiaxial, characterized in that they

  are produced using the method set out in the claim

cation 3.

  15. Apparatus for collecting ferromagnetic equiaxial particles distributed in a metal or metal alloy material, characterized in that it comprises: a device for electrochemically dissolving said metal or metal alloy material without dissolution sensitive of said

  particles, said electro- dissolution device

  chemical comprising a tank (60) containing an electrolyte, a bearing surface (31) inside said electrolyte for carrying the metal or metal alloy material (12) in dissolution and collecting the undissolved particles and for holding the active electrode in connection with the metal or metal alloy material; and a device (70) for producing a magnetic field, associated with said bearing surface to make the undissolved particles adhere by magnetic effect to said bearing surface (31) so as to obtain

  maximum yield of said equiaxial particles.

  16. Apparatus according to claim 15, characterized in that said device for producing

  the magnetic field is located outside of said tank.