Herein\'agglomeration\'is taken to include the process of binding together smaller particles to form lager particles (e. g. granules) by a process involving at least partial melting of the smaller particles.

Known inoculant production involves casting slabs of inoculant material, and then crushing them to produce correctly sized particles. The chrushing process produces powder fines as a by-product, typically in a significant quantity, and commonly these are ferrosilicon fines. As fines, they are undersized for use as an inoculant, and this by-product must thus be agglomerated in order to be usable. There are several known methods for agglomerating fine powder particles into larger particles, including using liquid binding agents to bind the particles together, or sintering the particles under heat. The use of binding agents can be disadvantageous because drying of the larger particles subsequent to agglomeration is generally necessary, and this can be time-consuming. Furthermore, the presence of the binding agent in the agglomerated particles may be undesirable, for example if the agglomerated particles are to be added to another substance (e. g. a metallurgical melt), since the binding agent may cause unwanted contamination. In this respect, sintering of the smaller particles to form larger particles may be preferable, but there are also problems with conventional sintering techniques. In particular, with sintering there may be insufficient control over the size of the larger particles formed, for example a very wide range of particle sizes may be produced. Another problem which frequently occurs with conventional sintering techniques, is that the particles may fuse to the wall of the kiln or other vessel in which the sintering takes place. This problem is particularly prevalent in rotary kiln sintering, for example.

In our European Patent Application No. EP 1092470 Al there is disclosed a sintering technique utilising a laser beam to heat the powder particles, which overcomes such problems.

It is an object of the invention to provide an improved granular inoculant.

According to the present invention there is provided a granular inoculant wherein the granules are spherical or substantially so, and comprise one of ferrosilicon material and a different metallurgical additive component encapsulated within the other thereof.

As used herein,\'metallurgical additive\'includes any element or compound which has, or contributes to, an inoculating effect, in use.

The encapsulation is preferably by the solidification of partly melted ferrosilicon material around the additive component.

The term\'melted\'and as used throughout this specification embraces melting or softening of substances which may, for example, have sharp melting temperatures, or a series of melting temperatures, or no sharp melting temperature.

The fine powder particles are agglomerated to form a plurality of granules. The agglomeration may, if desired, be accomplished by heating the particles to a temperature at which they start to sinter (hereinafter referred to as the\'sintering temperature\') or by heating the particles to higher temperatures. For example the temperature may range from the sintering temperature through to the temperature (s) at which the component (s) may become totally fluid, or to even higher temperatures.

The powder particles may, for example, have a maximum diameter (or other maximum dimension) of a fraction of a millimetre, and the spherical or substantially spherical inoculant granules formed by the agglomeration of the powder particles may have a maximum diameter (or other maximum dimension) of a larger fraction of a millimetre, or of one or more millimetres. For example the particles comprise so-called\'fines\' of an inoculant, namely a ferrosilicon mixture, the largest of which have a maximum dimension of approximately 0.3 mm (with many of the fines being significantly smaller than this). After agglomeration into granules, the smallest granules which are formed preferably have a minimum dimension of at least 0.2 mm, and the largest granules preferably have a maximum dimension of 0.7 mm. In an alternative embodiment, the granules have a size range of approximately 2.0-6.0 mm, for example, other sizes of particle may, of course, be used, and the sizes of the granules produced may be larger or smaller than those of this example.

The energy stream employed to heat the particles can be, for example, a laser beam, as in EP 1092470 Al, a stream of hot gas or plasma, a flame, or an electric discharge directed at the particles to produce the desired melting or partial melting of the particles. When oxidisable materials are present as metallurgical additive components of the powder, it is preferred to conduct the process in a reducing atmosphere or in an inert gas atmosphere. In theory the process could be operated in vacuo, but generally speaking, this would be expensive to realise. Provided the process can tolerate oxidising conditions, the energy stream can be a flame produced by oxidation of combustible material, for example, acetylene/oxygen, hydrogen/oxygen, and aluminium powder/oxygen.

Hot gas produced by subjecting gas to electrical discharge is also a suitable energy stream. For example, a stream of hot gas or plasma can be generated by passing gas between two or more electrodes, and striking an arc between the electrodes. Also suitable for use are devices which employ the interaction of substances with electricity to achieve an energy stream capable of heating the particles to a high temperature, for example the so-called atomic hydrogen torch which can produce extremely high temperatures. Devices which provide an energy stream by a combination of electrical heating and combustion of a gas can also be employed. A number of known welding techniques produce an energy stream suitable for use to agglomerate the particles, for example gas tungsten-arc welding (GTAW), atomic hydrogen welding (AHW), plasma-arc welding (PAW), Thermit welding, electron-beam welding (EBW), metal inert gas welding (MIG) and oxy-hydrogen flame welding. Any of these welding techniques can also be supplemented with laser welding technology.

Preferred methods of generating and utilising the energy stream use the technology of welding processes conducted in a reducing or inert gas environment, for example, tungsten inert gas (TIG) and plasma welding processes. Plasma welding technology is particularly preferred.

If desired, the melting or partial melting effect produced by the energy stream can be augmented by the use of supplementary heating, for example, by carrying out the process in an oven or furnace, by directing electromagnetic energy at the particles, or by using a laser beam, for example as employed in laser-beam welding (LBW) technology.

The quantity of heat provided by the energy is suitably at least sufficient to cause melting or softening of areas of the surface of the particles so that agglomeration of one or more adjacent particles may be facilitated.

If desired a proportion, or all, of the particles may undergo total melting.

Under these circumstances the molten or partially molten particles can agglomerate, e. g. by the effect of surface tension, and subsequently cool to form the granules. However, the application of too high a temperature, or heating for too long a period, may cause the production of undesirably large agglomerate, or even total fusion of the whole mass of particles.

Relative movement is provided between the powder particles and the energy stream such that at least some of the particles are brought into the path of the stream for a predetermined time and are thereby melted or partly melted and are then conducted away from the stream and cooled to form a plurality of solid granules. The relative movement can be provided by moving the particles and maintaining the source of the energy stream stationary, or by moving said source and maintaining the particles stationary, or by a combination of both these methods.

The source of the energy stream is maintained substantially stationary, i. e. the particles to be agglomerated are arranged to move with respect to a fixed source of the energy stream. For example, one or more plasma welding torches directed towards the particles are maintained substantially stationary. Although this latter arrangement is preferred, it is possible, in alternative arrangements, for the source of the energy stream to be moved, for example by moving suitable welding devices so that the produced energy streams impinge on the particles. In this way, the energy stream or streams may\'scan\'the particles to be agglomerated.

In the interests of efficiency, however, an arrangement in which the particles are moved relative to a stationary source of the energy stream or streams is normally preferred.

There is a support body which supports the particles during at least part of (preferably the whole of) the heating by the energy stream. The support body preferably includes at least one groove in which the particles being heated by the energy steam are contained. The shape and depth of the groove will normally be chosen according to the size of the particles being agglomerated and the size of the granules to be formed by the agglomeration. The groove preferably has a depth of at least 2 mm, more preferably at least 4 mm, even more preferably at least 8 mm, for example approximately 10 mm. The groove may be substantially square or rectangular in cross-section, but other cross-sectional shapes are possible, for example substantially U-shaped or V-shaped.

The support body preferably moves with respect to the source of the energy stream, thereby causing or assisting the powder particles to move with respect thereto; i. e. the moving support body preferably carries the particles through the energy stream. For example, the support body may comprise a conveyor belt or a system of rollers, or a single wheel or roller. When a wheel or roller rotating with respect to the source of the energy stream provides the support body, the speed of rotation employed will depend on the desired residence time of the particles in the energy stream. Similarly, if the support body comprises a conveyor belt or like device the rate of movement of the belt will be determined by the desired residence time. The chosen residence time itself will depend on a variety of factors including the heating power of the energy stream, the melting properties of the powder, and the desired particle size of the produced granules.

The support body is a roller which preferably rotates about an axis which is substantially perpendicular to the direction of travel of the energy stream.

The powder particles are preferably moved towards the energy stream by means of a chute, the chute preferably supplying the powder particles to the support body. If desired, a hopper or other source of the powder particles can be employed to feed the particles to the top of the chute.

Where the support body for the particles does not move, the particles may move with respect to the support, for example they may be guided by the support. The particles may, for example, move under gravity, e. g. preferably on an inclined support, or they mat be blown by pressurized gas. Movement along an inclined support can be assisted by vibration if desired.

The relative movement of the particles to be agglomerated may be continuous or discontinuous. For example, the particles may be moved into the energy stream and then held substantially stationary (or at least significantly slowed down) for a predetermined duration while they heat up and melt or partially melt, before being moved out of the energy stream when the desired amount of melting has occurred. It is preferred to move the particles continuously whilst maintaining the source of the energy stream stationary.

In order to produce discrete granules of agglomerated inoculant particles from a process in which the particles are moved continuously, one method is to cause the energy stream to be emitted from the source in a series of discrete pulses. The duration of the pulses of the energy stream, and the intervals between the pulses, together with the speed with which the particles move with respect to the stream, may be selected (e. g. by trial and error) so as to produce discrete granules of the desired size.

To assist in the formation of discrete granules rather than large chunks of agglomerated powder, it is preferred to accelerate the agglomerated powder away from the heating zone (i. e. the zone where the energy stream heats the particles). Any loose bonds (e. g. resulting from surface tension effects) which may have formed between adjacent particles may be caused to break by the acceleration effect before the particles cool and set solid. Thus the movement of the support body may cause some aggregations of the powder particles to become separated from each other as they move away from the heating zone. Such acceleration may, for example, be provided at least in part by gravity, for example, the particles may be allowed to fall under gravity (with or without additional velocity or acceleration provided by the support body) subsequent to being heated by the energy stream. In another embodiment the particles may be accelerated away from the heating zone by means of a stream of high velocity gas.

After the energy stream has heated the particles and agglomeration has occurred, the produced inoculant granules are cooled. For example, they can be allowed to cool in air or another fluid substantially without contacting any solid objects. This may conveniently be achieved by allowing the hot granules to fall a distance through the air after being heated by the energy stream. Additionally or alternatively, the hot granules may be quenched in a liquid, e. g. water.

The powder particles comprise ferrosilicon material mixed with one or more metallurgical additive components, for example, metallic, semi- metallic, ceramic, cermet, or other inorganic components. Examples of suitable components used with the ferrosilicon material are one or more materials selected from calcium, sulphur, oxygen, magnesium, zirconium, magnesium fluoride, magnesium fluorosilicate, calcium fluoride and iron alloys. In particular the components could be sulphides, carbides, oxides, silicon carbide, calcium/silicon alloy or compounds, iron metal (e. g. as a powder), iron/silicon/barium alloy, rare earth (e. g. cerium) metal alloys or compounds with silicon, pyrites, iron oxides (e. g. Fe203), barium carbonate, bismuth metal or its compounds, titanium metal, barium sulphate, alumina, silica, and carbon (e. g. graphite). Preferably the granules are produced by the use of powder particles, i. e. fines, comprising a ferrosilicon-based inoculant containing 60-80% silicon by weight, together with at least one other component selected from several suitable metallurgical additives.

In another embodiment of the present invention, silicon carbide is included in the powder particles to improve the granulometry of the final granules. Thus, for example, the inclusion of 1 to 10 wt%, preferably about 3 wt% of silicon carbide in the powder particles can be beneficial.

The spherical nature of the granules ensure that they are free flowing at rates as low as 1 g/s, and they are preferably in the size grading 0.2-0.7 mm. Different components are sintered with the ferrosilicon to provide suitable inoculants for use with grey cast iron, ductile iron and for general use respectively.

Granules of the invention have one or more components encapsulated in one or more different components, i. e. encapsulation in the manner of particles in a block of ice. For example the granules could comprise refractory oxide in ferrosilicon (iron silicide or iron/silicon alloy).

Generally the encapsulation occurs due to the use of a mixture of at least two or more components having widely differing melting (or softening) temperatures, wherein during the heating stage the lower melting (or softening) component may undergo partial or complete melting (or softening) and the higher melting (or softening) component may remain totally or partially in the solid state.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a process and apparatus for producing inoculant of the invention, Figure 2 is a detail of a part of the apparatus of Figure 1, Figure 3 is a graph showing comparative inoculation measurements for two prior art inoculants and an inoculant of the invention, with ductile iron, and Figure 4 is a table showing comparative inoculation results with ductile iron, for the two prior art inoculants of Figure 3 and eight different inoculants of the invention.

Figure 1 is a schematic representation of one process and apparatus for producing inoculant according to the invention. The apparatus comprises a supply 1 of fines or powder particles 3, the supply including a hopper 5 containing the powder particles. The apparatus also includes a source for the energy stream which, in the present example, is a plasma arc torch 7 (shown in more detail in Figure 2 of the Drawings). The plasma arc torch 7 emits a plasma jet 9 to heat and melt or partially melt some or all of the powder particles 3. A support body 11 is provided in the form of a rotatable wheel. The granules 15 formed by the agglomeration of the powder particles 3 pass into a receptacle 13. The support body wheel 11 has a circumferential groove (not shown) which caries the powder particles 3 which are supplied to the wheel.

The process is conducted as follows. A stream 17 of powder particles 3 is supplied to the top of the support body wheel 11 (in the groove), which is rotating at approximately 10 rpm. The stream of powder particles will normally be supplied to the rotating wheel down a feeder tube or chute from the hopper 5. The source 7 emits a high temperature plasma jet 9 which impinges on the particles as they travel with the movement of the wheel 11. The powder particles 3 supplied to the wheel pass through the plasma jet and are heated by it such that they melt, or partially melt. At least some of the molten, partially molten or softened particles fully encapsulate other particles which may themselves be molten, partially molten or solid (i. e. totally un-melted). The overall effect is that at least some agglomeration of the powder particles occurs to form encapsulated granules, normally having an average particle size greater than the original powder particles. At this stage the encapsulated granules may already be spherical or generally so.

The powder particles which have been heated, and which are at least beginning to agglomerate with neighbouring particles, fall from the rotating wheel 11 through the air into the receptacle 13. As the particles fall into the receptacle, some separation of the agglomerated material can occur, and surface tension effects can result in the particles assuming a more, or even more, spherical shape. Before the particles reach the receptacle, they have preferably cooled sufficiently to ensure that they remain as discrete granules and do not agglomerate further.

The receptacle 13 preferably contains a sieve 19, or a series of sieves (not shown), and the granules 15 of the required maximum size which pass through the sieve are poured into packaging 21 for distribution. Typically sieve sizes are chosen to separate the granules into desired\'fractions\'i. e. proportions of granules having desired particle size distribution.

Undersize granules can, if desired, be recycled to the powder particle supply. Oversize granules can be sold as such, or crushed and reprocessed.

Figure 2 of the Drawings shows a plasma arc torch 7 (well known in the art as a\'non-transferred plasma arc\'torch) comprising a tungsten cathode 25 surrounded by a copper nozzle 27 which also acts as the anode. An electric arc 8 is formed between the tip 10 of the cathode and the anode close to the orifice 35. The copper nozzle is water-cooled (not shown) to prevent it overheating when the arc is initiated. Inert gas, for example argon, is injected into the nozzle so that it travels downwardly through the annular passage 29 between the cathode 25 and the copper nozzle 27 and emerges at the nozzle orifice 35 as plasma jet 9. A suitable DC voltage is applied across the cathode/anode combination from source 31 with current availability sufficient to produce the required temperature and density of the arc. The arc is initiated, and stability thereof is maintained, by the application of a suitable high frequency source of electricity 33. The DC current flowing in the arc may lie, for example, in the range 5 to 200 amperes, and the DC voltage across the anode/cathode combination may lie, for example, in the range 60 to 120 volts. When the arc is initiated between the anode and cathode the argon gas becomes highly ionised (to form so-called\'plasma\') and emerges from the nozzle orifice 35 in the form of a narrow cylindrical jet of plasma 9 having a temperature of up to 10,000°C or even higher. The small diameter of the nozzle orifice constrains the plasma jet to the form of a narrow stream which is eminently suitable for heating and so causing agglomeration of the powder particles.

Instead of the energy stream being a plasma, with the plasma arc heating (sintering) the particles, any other suitable alternative can be employed.

Various such alternatives have been referred to earlier herein. Moreover, as disclosed in EP 1092470 Al, a laser beam can provide the energy stream, and reference is made thereto for details of the method and apparatus.

To assess the improvement in performance of inoculants of the invention, these have been compared in use with known inoculants which are conventionally produced, i. e. have not been produced as spherical agglomerated encapsulated granules. One such known inoculant of the Applicant is sold under the Registered Trade Mark INOCULIN 90, this being a specially graded (i. e. fine powder) version of INOCULIN 25 which is a ferroalloy containing 67% Si, Ca, Al, Zr and Mn as active constituents. INOCULIN 25 is a general purpose inoculant for use with all types of grey cast, ductile and CG irons, having high solubility even in low temperature metal. Another such known inoculant is that of the Company Pechiney sold under the Trade Mark SPHERIX. This is a ferroalloy which contains 70-75% Si, 0.7-1.4% Al, 1-2% Ca, 0.8-1.3% Bi and 0.4-0.7% TR.

Accordingly Figure 3 is a graph showing a comparison of the performances of INOCULIN 90 and SPHERIX with the performance of an inoculant of the invention, referred to as NEOPERL-MIX 10, which is composed of the constituents of INOCULIN 90 together with 3.69% Ca, 2.0% 02 and 2.5% S. The Ca, 02 and S can be provided by means of any suitable compounds. The comparison is for the use of these inoculants with ductile iron, the inoculation effect being measured by the change (, a) in the lower eutectic temperature (TE low) relative to the percentage of inoculant addition. In this example the inoculant was produced by laser beam sintering.

From the Figure 3 graph, it will be seen that from between approximately 0.06% inoculant addition and 0.1% inoculant addition (where the graphs flatten out, showing that the inoculant effects have maximised), the inoculant of the invention provides increased inoculation as compared to the two prior art inoculants.

Figure 4 is a table showing comparative results for the inoculation affects of the three inoculants shown in the graph of Figure 3 along with seven other inoculants of the invention, each of which is a mixture of INOCULIN 90 with the additional constituents listed. From the table it can be seen that several of the Mixes and Batches listed produce improved inoculation as compared to the prior art inoculants which are not in accordance with the invention. Improved inoculation is identified by a lower percentage of inoculant being required to raise the (lower) eutectic temperature by the amount indicated. A dash in the table indicates that the effect has maximised, as mentioned above for the graph of Figure 3. It is also believed that sintering of the mixture of components which constitute the Applicant\'s various INOCULIN products (presently agglomerated by the use of one or more binders), namely INOCULIN 10, 25,80,90 and 98, would also produce improved inoculants according to the invention, and the same applies for sintering of the mixture of SPHERIX components.

Other possible sintered compositions are INOCULIN + S + 02 and INOCULIN + S with Ca, Rare Earth elements or Mg. Again the elements can be added by means of any suitable compound containing them.

From further tests it has been found that for use with grey cast iron the following further granular sintered mixtures of the invention produce better inoculation than INOCULIN 90: MIX 5: INOCULIN 90 + 2.0% 02 MIX 6: INOCULIN 90 + 2.0% 02 + 2.5% S For ductile iron, a corresponding better performing inoculant of the invention is : MIX 5: INOCULIN 90 + 2.0% 02 Although the inoculants of the invention of Figures 3 and 4 have been produced by sintering using a laser beam, the use of an alternative energy stream such as plasma or TIG welding produces inoculants with similar improved inoculant properties.