Production of metal lumps and apparatus therefor

Metal lumps or pebbles are produced by introducing a molten metal stream into a stream of water in a direction which is substantially the same as the direction of the water stream and at a velocity which is substantially the same or slightly less than the velocity of the water stream.

FIELD OF INVENTION
 This invention relates to the production of lumps of metal from the
 corresponding liquid of the metal, and more specifically to the casting of
 iron, steel, slag, ferroalloys, and other metals and their alloys into
 biscuit-shaped lumps where the longest dimension is typically of the order
 of 20 to 100 mm. These lumps are significantly larger than those produced
 by existing granulation methods. As used herein "metal" or "material",
 depending on the context, includes substantially pure metals, metallic
 alloys, and slags produced by or from metallic processes.
 BACKGROUND OF THE INVENTION
 In the metallurgical industry, there are a number of processes in which a
 product has to be temporarily cooled down, stored and possibly
 transported, and later remelted. Such a product is defined herein as a
 "product for remelting" (PFR).
 The most common PFRs are the ferroalloys like ferro-chromium,
 ferro-manganese, ferro-nickel and ferro-silicon, which are used as a
 source of alloying elements during the manufacture of certain types of
 steels. The furnaces that produce these PFRs are often geographically
 distant from the site of their end use. There are also some other metals
 like aluminium, copper and zinc, and some of their alloys, that are
 similarly produced in a different place from where they are used. These
 materials therefore need to be converted from the liquid form to some type
 of solid form that can be handled and transported.
 Another type of PFR arises and is later consumed in the same plant. This
 typically occurs when a downstream production unit is taken off line for
 maintenance, but the upstream unit continues to produce. The hot metal
 that continues to come from the upstream unit cannot be held molten in
 storage until the downstream unit comes back on line, and consequently
 must be converted into a solid form that can later be remelted or blended
 in. The PFR is then effectively a buffer between stages. An example of a
 plant where this could occur is an integrated iron and steel works, where
 a blast furnace produces pig iron that is then fed to a steel plant for
 conversion to steel, that goes on in turn to a continuous caster. In this
 case, if the steel plant stops the pig iron must be taken elsewhere, while
 if the continuous caster stops the steel must be handled in some other
 way.
 Existing methods of handling PFRs are mainly the following.
 Bed Casting, and Pooling
 Here, the molten material is poured into moulds on the ground, and after
 cooling is broken up into lumps of the required size. A problem here is
 the unavoidable production of a certain amount of unwanted fines.
 Ingot Casting, Including Casting Strands and "Chocolate Moulds"
 In this process, the liquid material is poured into moulds. These may be
 either individual moulds, or may be assembled in a continuous loop as a
 casting strand. It is a relatively expensive process, tends to be labour
 intensive, and requires careful operation.
 Granulating
 In essence, this involves breaking up a stream of molten material either by
 means of a water jet or on a target, with the material then falling into a
 tank of water. The particles produced tend to be smaller than desired by
 end users, and the product is usually wet when it comes from the process,
 but the product is suitable for easy mechanical handling.
 There are of course many other methods for casting hot materials, but these
 are of rather marginal relevance to PFRs. One such process is atomising,
 in which the molten material is converted to a fine powder by means of a
 high pressure jet of water or gas. This powdered product is too fine for
 remelting, and is typically used for powder metallurgical processing, for
 welding electrodes or as a heavy-medium for mineral separation.
 Existing Types of Granulation
 In one version of this process, a strong jet of water at a speed of between
 5 and 15 m/s is directed to collide with a falling stream of material.
 This breaks up the material into droplets between about 1 and 20 mm in
 size which fall into a bath of water and solidify. In another
 implementation, a stream of molten material is broken up by a refractory
 target placed in its path, and the resulting droplets, varying up to about
 25 mm in size, then fall into a bath of water. The former process is
 widely known in the industry as the Showa Denko process, and the latter as
 the Granshot process. Another process, which is generally used in the
 granulation of slag, has a near-vertical stream of molten material
 colliding with strong horizontal jets of water, with the mixture being
 swept along a near-horizontal launder filled with rapidly-flowing water.
 Lastly, lead shot is made by allowing droplets of molten material to fall
 about 45 meters through air in a device known as a shotting tower. The
 resulting droplets, which are usually a millimeter or two in diameter,
 solidify as they fall through the air.
 The techniques used in the aforementioned processes have by now entered the
 public domain, --see for example the Granshot process patented in 1975 in
 U.S. Pat. No. 3,888,956. However, there are some new variations that have
 been patented more recently. For example, South African patent ZA
 90/4005A, describes a scheme like an extension to the Granshot process, in
 which the refractory element on which the molten metal stream impacts is
 oscillated vertically. Other patents, ZA 91/2653 and U.S. Pat. No.
 5,258,053 (1993), describe a process in which molten metal is run onto a
 refractory target shaped like a launder and then into a tank of water. The
 outlet of this target is close to the surface of the water, and the water
 within the tank is kept reasonably still, with a gentle and uniform flow
 of less than 0.1 m/s being directed at right angles to the submerged metal
 stream.
 U.S. Pat. No. 4,192,673 addresses the problem of particles, of ferro-nickel
 in their specific case, that form flat wrinkled shapes during granulation,
 because of the generation of carbon monoxide (CO) gas as the ferro-alloy
 cools. The inventors claim that this can be prevented by the addition of
 deoxidising agents such as particularly aluminium, but also ferro-silicon,
 ferro-manganese and the like.
 An example of a newer development for the granulation of slag is disclosed
 in U.S. Pat. No. 4,374,645. Here, the molten slag is first contacted with
 a high-speed jet of warmer water to break it up, after which it falls into
 a slower cooler stream of water.
 Deficiencies of the Prior Art
 The following are some of the main deficiencies.
 The bed-casting and mould-casting processes require labour to be present in
 the vicinity of the casting operation. Molten metal, particularly in the
 quantities employed in iron, steel and ferro-alloy production, is
 exceedingly dangerous.
 The exposure of hot metal to the air often generates fumes. Large pools of
 hot metal therefore tend to be associated with rather more pollution than
 is desirable.
 As mentioned previously, the process of breaking up a block of cast alloy
 generates a portion of fines which have a lesser commercial value. The
 granulation process lessens the problem of fines, but the dimensions of
 the granules produced by the existing processes remain somewhat smaller
 than those that the end users consider optimum.
 The granulation process can sometimes produce "corn flakes", which are
 light fluffy paper-like particles, instead of normal granules. These may
 subsequently break up into smaller particles, which then create similar
 problems to the fines from casting.
 Existing granulation processes are susceptible to occasional explosions,
 often associated with an accumulation of a large mass of hot metal under
 the water.
 Granulated material is normally wet when it comes from the granulator. This
 wetness can give problems when the material is used subsequently and such
 material must usually be dried.
 Identification of the Need
 Most users would seem to prefer lumps of ferro-alloy in about the 20 to 100
 mm size range. This is said to be because lumps of that size range will
 fall rapidly through the slag layers covering a typical bath of molten
 metal. It is also a requirement that the material should feed easily
 through the existing materials handing systems. The material should also
 be dry. The existing granulated materials feed easily, but the particles
 tend to be too small. Lumps of broken-up cast ferro-alloy would seem to be
 able to meet the size requirement, but there is then an unavoidable loss
 in the form of fines. Some users would also seem to have a preference for
 the form of granules over that of broken-up material. There appears to be
 no prior art which can produce, without significant disadvantages, a
 granular material with the form and size preferred by the users.
 There is therefore, notwithstanding the efforts of others, still a specific
 need for a reliable, safe, convenient and cheap process to convert molten
 metal by direct solidification, without intervening crushing, into solid
 pieces as lumps of a size and form that are acceptable to the end users.
 These lumps should preferably be substantially globular or biscuit shaped,
 where the longest dimension is typically between 20 and 100 mm. Besides
 the requirements mentioned, these lumps should ideally be capable of
 withstanding the rigours of storage, transport and handling without
 degrading to fines. The technique to produce these lumps should not be
 more hazardous nor require more human labour and maintenance than the
 presently-used methods. It is obviously a requirement of such a process
 that it should not introduce excessive quantities of undesirable
 impurities into the ferro-alloy. The process should also be simple to
 construct and operate, particularly relative to existing methods.
 SUMMARY OF THE INVENTION
 The invention provides, in the first instance, a method of producing lumps
 or pebbles wherein a stream of molten metal is introduced in a co-current
 configuration into a stable flow of cooling fluid. (In other words the
 metal stream is introduced in a direction which is substantially the same
 as the direction of the cooling fluid stream). The mixture is possibly but
 not necessarily contained in a flume, with a small and controlled velocity
 mismatch between metal and coolant. This velocity mismatch should be less
 than 5 m/s and preferably less than 2 m/s in order that large lumps of the
 solid material are produced. The metal and fluid streams may be arranged
 to be lamellar and stable.
 The words "lumps" and "pebbles" are used interchangeably herein.
 The fluid may be:
 water;
 an organic or inorganic liquid;
 a slurry (for example a suspension of dense medium, graphite or other fine
 substances);
 an emulsion or a solution, containing salts (e.g. brine), surface active
 agents or liquids (organic or inorganic);
 a fluidised bed of fine, solid particles.
 The important properties of the cooling fluid include its density, boiling
 point, heat capacity, heat transfer ability, viscosity and its chemical
 reactivity with the surface of the hot lumps. Although water is generally
 preferred on account of its availability, cleanliness and heat capacity,
 other liquids or mixtures of substances may offer benefits. For example,
 the addition of a soluble salt to water will increase its boiling point
 and accelerate its ability to transfer heat out of the hot metal or slag.
 The density and viscosity of water can also be altered by preparing a
 water-based slurry, for example of ferro-silicon, magnetite or graphite
 powders in water. Densities of as high as 3.5 g/cm.sup.3 can be achieved
 by the addition of ferro-silicon powder. The addition of graphite will
 improve the lubrication between solid lumps and floor of the flume and
 will also change the oxygen potential of the coolant. A similar change to
 the oxygen potential of the coolant can be achieved by the addition of
 higher alcohols such as isopropyl alcohol. The system can be rendered
 moderately oxidising, if desired, by the addition of a nitrate salt.
 Conversely, reducing conditions can be assured by adding a nitrite salt.
 In the case of special, high value metals, there could be an advantage to
 using an organic liquid, such as oil, or a silicone-based liquid, as the
 coolant. The addition of surfactants, oxidants or reductants, or other
 trace chemicals which can modify the surface chemical reactions between
 the hot lumps and coolant is also advantageous. A fluidised bed offers the
 prospect of extremely high densities.
 The fluid may be unsupported and may be permitted to fall freely. In this
 case, the process involves a gentle co-current introduction of metal into
 the fluid stream and is different from the Showa Denko granulation process
 where an essentially vertical stream of metal is shattered by a
 fast-flowing horizontal stream of liquid.
 Alternatively the fluid stream may be guided for movement along a
 predetermined path by means of a suitable structure, such as a flume. When
 use is made of a structure to guide the flow of the fluid stream then the
 inclination, length and shape of the structure can be arranged or varied
 according to requirement so that the molten metal stream slides down the
 structure while submerged in the fluid stream, while simultaneously
 ensuring that adequate cooling and control of the shape of the lumps are
 achieved.
 The shape of the product may be controlled to some extent by the shape of
 the channels in the flume. The floor of the flume may have a large number
 of parallel channels, effectively creating parallel paths down which a
 number of streams of hot metal are swept simultaneously.
 An on-line assessment of the shape of the lumps may be used to control the
 position of a tundish, from which the molten metal is supplied, in a
 feedback system.
 The flume may have a complex shape. As one example, this may include an
 initial region of a relatively steep inclination and a secondary region of
 a relatively shallow inclination, which may be substantially linear. The
 curvature in this initial region may be such that the trajectories of the
 cooling fluid and the metal stream are matched so that the effective
 vertical acceleration of the metal stream is reduced below that normally
 due to gravity. Under these conditions, the fluid and the metal streams
 may be made to accelerate downwards at close to or even beyond free fall
 conditions. The flume may alternatively have a straight path inclined at
 whatever slope is considered convenient. Another possibility is to have
 undulations along a region of the flume. As a further option, when viewed
 in plan, the flume may be straight or it may follow a curved path, for
 example a spiral flume. The optimum profile may depend on the nature of
 the material to be processed, and a different profile may be needed for
 each type of material.
 The aspect ratio, shape and size of the resulting pebbles may be influenced
 by one or more of the following: the inclination of the supporting
 structure for the fluid stream; the cross-sectional profile of the
 supporting structure for the fluid stream, the amount by which the
 temperature of the metal stream exceeds the liquidus temperature, also
 known as the "superheat"; the angle of impingement of the metal stream
 onto the cooling fluid or onto a floor of the supporting structure used
 for guiding the fluid stream; the temperature and composition of the
 cooling fluid stream; and the rate of flow of the cooling fluid or of the
 metal stream, or both, and the inherent turbulent flow patterns within the
 cooling fluid and metal.
 An important aspect of the invention is that the lumps, after they have
 formed in the cooling fluid, should be allowed to solidify sufficiently
 with a thick enough skin before any impact is experienced to avoid a
 distortion of their shapes. The time needed for sufficient solidification
 is a function of a number of parameters. These include the rate of heat
 transfer from the lumps, the amount of energy that needs to be removed,
 the time in contact with the cooling fluid, the type of cooling fluid, the
 size and shape of the lumps, the mechanical and thermal properties of the
 lumps at elevated temperatures, and the surface tension of the liquid
 lumps. It is important that the metal stream should be submerged in the
 fluid stream for long enough to ensure that sufficient heat is extracted
 from the metal so that the metal is rigid when it is separated from the
 fluid stream.
 Separation of the metal from the fluid stream may be effected by ejecting
 the metal lumps from the cooling fluid into a holding or collecting tank
 or on to a fluid/metal separator such as a chain grate or a vibrating
 deck. The apparatus should be such that a pile-up of the rigid but hot
 lumps of material cannot occur. This is required in order to prevent steam
 or hydrogen explosions.
 The pieces of metal may be removed either by an apparatus similar to a
 continuous grate conveyer or by a vibratory conveyor or other apparatus.
 If a soluble material forms part of the fluid, then a spray and wash
 station may be used at this stage.
 The material may be cooled further after separation and transported to a
 convenient storage place or a standard arrangement to screen and sort the
 lumps. A means of cooling the lumps while moving them may also be
 provided. For example the lumps may be collected or otherwise positioned
 on a heat resistant conveyor such as a grate conveyor, and they may be
 dried by means of air which is directed on to the lumps.
 The invention also provides for the apparatus to produce a stream of a
 coolant fluid and for introducing a stream of molten metal into the
 coolant stream in a substantially co-current manner.
 Means may be provided for varying the flow rates of the coolant and the
 metal. For example, use may be made of a variable speed pump, or control
 valves, to vary the velocity and flow rate of the coolant.
 The ratio of the flow rate of the molten metal to the flow rate of the
 coolant may be between 1:5 and 1:15, and typically is of the order of
 1:10, on a mass basis.
 The rate of flow of the metal may also be controlled in any appropriate
 manner and for example may be controlled by varying the head of metal in a
 tundish which is positioned to discharge into the fluid stream. The cross
 section of an exit aperture of the tundish may be varied to alter the
 velocity and flow rate of the metal stream, for example by changing the
 diameter dynamically during the pour or prior to the pour or by using a
 conical plug. The position of the tundish may be adjustable so that it can
 be moved in a horizontal or in a vertical plane in order that the metal
 stream may fall into the coolant at an optimum angle and at an optimum
 position. Tilting mechanisms for the pouring of the metal from the ladle
 to the tundish and for controlling the flow rate of the metal may also be
 included in the apparatus. Emergency overflows for excess metal may also
 form a part of the control of the metal flow rate.
 The apparatus may include a spout or spouts of appropriate geometry to lead
 the metal from the tundish into the coolant at the appropriate velocity
 and inclination.
 The coolant will be unavoidably turbulent because of the high Reynolds
 number, but it should be smooth and stable. Excessive turbulence is to be
 avoided as this may affect the shape and size of the lumps. To achieve
 this characteristic, the apparatus may include a stilling well into which
 the coolant is fed, and a weir that the coolant spills over to pass from
 the stilling well into the flume. An initial region of the flume before
 the metal is added may be used to allow any excessive turbulence to
 dissipate. A header tank may also be provided so that in the event of a
 power cut, the coolant would continue for a further given time period.
 Because heat is dissipated in the fluid, equipment to cool the fluid may
 be required.

THEORETICAL ANALYSIS
 The present invention was suggested by the results of a theoretical
 analysis of the processes acting on a blob of molten metal or slag coming
 into contact with a coolant liquid such as water. Therefore, the reasoning
 employed to arrive at the claimed invention will be briefly described.
 The sizes of the lumps resulting from the granulation process depend on the
 way the liquid metal is handled while it is being cooled to
 solidification. There are a number of forces that influence the shape of a
 lump during such a process, and the eventual sizes and shapes are
 determined by the ways in which, and the extents to which, these forces
 are brought to bear on the lumps. The forces of relevance are:
 Surface tension. The surface-tension force tries to pull the lump into a
 ball, but is relatively weak. This is the main force that has to be relied
 upon to hold a large lump together while it is still liquid.
 Hydrodynamic drag forces. Any object moving through a fluid will experience
 drag forces. In the case of a blob of liquid metal flowing through a
 coolant, the drag forces will tend to rip the surface away, and so break
 up the blob.
 Forces of motion. Flows of either the liquid metal or the coolant will try
 to keep moving because of their momentum. A stream of liquid impacting on
 a surface will flatten and spread out, and may then break up into blobs or
 droplets. Even the presence of strong flows inside a blob of liquid are
 capable of breaking up the blob.
 Gravity and containment forces. Gravity is relatively strong compared to
 the other forces acting on a blob, and in particular over a relatively
 short distance it can accelerate a blob to speeds at which the other
 forces that are then incurred cause the blob to break up. Gravity also
 causes a liquid that is held in a vessel to take the shape of the vessel.
 However, if the liquid does not wet the material of the floor of the
 vessel, it will tend to be pulled into a ball by the surface tension
 forces while simultaneously being flattened by gravity.
 Friction forces. A lump of metal sliding down a channel will experience a
 friction drag from its rubbing against the floor of the channel. If the
 lump is only partly solidified, this friction force may be enough to
 distort the shape of the lump or even to tear the lump apart.
 The invention is based on the use of apparatus which is designed to produce
 these forces in a combination which acts to form large lumps of metal or
 slag, instead of the relatively smaller lumps which are formed in other
 granulators. The large lumps of metal must be formed under conditions
 which are relatively safer than, for example, simply pouring a stream of
 hot metal into water. To achieve these objectives it has been established
 that the stream of hot liquid metal must not be subjected to drag forces
 or forces of motion that exceed the surface tension forces. Secondly the
 stream must be split into blobs of the required size and shape. Lastly the
 blobs must not be subsequently subjected to excessive forces of any type
 until they have solidified sufficiently.
 Modelling and simulation of the formation of individual blobs by
 finite-element methods is almost impossible because the process is
 essentially random. However, quantitative studies of the basic mechanisms
 can yield some insight, and there are also other approaches that can be
 used, such as dimensional analysis and free energy. The following analysis
 makes use of these concepts. It will be shown that the breakup of a metal
 stream into the desired sequence of blobs can be achieved by addressing
 especially the interplay between surface tension and drag, the amount of
 material charged to the flume at any particular instant, and the kinetic
 energy imparted to the metal or slag stream. The observations described
 hereinafter were verified with the assistance of water modelling.
 Ratio of drag to surface tension forces.
 Consider a spherical blob moving through a fluid. The drag force is given
 by:
EQU F.sub.drag =C.sub.D.(r.sigma..sup.2).(.rho..nu..sup.2 /2) (1)
 while the surface-tension force that holds any two halves of the blob
 together is given by:
EQU F.sub.surften =.sigma..2.pi.r (2)
 where:
 C.sub.D is the drag coefficient (dimensionless)
 r is the radius of the blob (meters)
 .rho. is the density of the fluid surrounding the blob (kilograms per cubic
 meter)
 .nu. is the velocity of the blob relative to the fluid (meters per second),
 and
 .sigma. is the surface tension of the blob's interface with the fluid
 (newtons per meter).
 The ratio of these two forces is therefore:
 ##EQU1##
 The first bracket in equation 3 is essentially constant for a given
 geometry. Therefore, the more important term in terms of the practical
 problem at hand is the second bracket, which may be defined here as the
 blob number, N.sub.blob :
 ##EQU2##
 This dimensionless number is also called the Weber number, but as there are
 other definitions of the Weber number, the specific name "blob number" has
 been used to avoid confusion.
 A blob will be torn apart when N.sub.blob exceeds a certain critical value.
 Conversely, the blob will stay intact if the blob number remains below the
 critical value. In equation 4, the parameters .sigma. and .rho. depend
 only on the substance of the blobs, so for a given desired size of the
 blobs, i.e. given r, only .nu. can be varied to keep the blob number below
 the critical value. Furthermore, if the velocity .nu. goes up then the
 size r will go down. In practical terms, this means that the velocity of
 the blobs must be kept relatively similar to the velocity of the fluid if
 large lumps are to be obtained.
 The departure from the prior art is to achieve this by bringing together
 the streams of hot metal and water co-currently and at similar velocities.
 Splitting up of the Stream of Hot Metal
 A ribbon of liquid metal in a channel is characterized by a free energy,
 which is a combination of the surface energy and the potential energy.
 However, in some cases, such a ribbon can achieve a lower free energy by
 spontaneously breaking up into blobs. It can be shown theoretically that
 there is a minimum free energy for such a stream at a certain mass per
 unit of length (kilograms per meter), which is referred to herein as the
 critical loading. At this critical loading, a ribbon of liquid metal will
 stay as a continuous ribbon and will not break up into blobs, because the
 free energy is at its minimum and cannot go any lower. If the ribbon
 starts off with less mass per unit length than the critical loading, the
 extra free energy will spontaneously drive the system to break up the
 ribbon into segments so that within each segment the mass per unit of
 length becomes approximately equal to the critical loading. Conversely, if
 there is more mass per length than the critical loading, the excess mass
 will attempt to flow out of the ends of the ribbon, to get back to the
 critical loading.
 In practical terms, this means that the apparatus must be run with a flow
 of hot metal that produces a loading just below the critical value, so
 that the ribbon will break up. For iron, steel, ferrous alloys, and other
 materials that have similar surface tensions and densities, it can be
 calculated that for typical flume designs this critical loading is of the
 order of about 1.5 kilograms per meter, although this value does depend on
 parameters that can vary, such as surface tension, density, and curvature
 of the channel. If, for example, the velocity of the metal is of the order
 of 2.0 meters per second, then the absolute maximum throughput is
 approximately 1.5 kilograms per meter.times.2.0 meters per second=3.0
 kilograms per second.
 Forces of Motion
 Because the surface-tension force is relatively weak, the surface energy is
 relatively small by comparison with the typical kinetic energies and
 potential energies. Hence, if a largish blob of liquid metal is dropped
 more than just a small amount onto a surface, it will tend to splatter and
 so break up into smaller drops.
 Some typical comparative values are as follows. Consider a blob of mass 0.1
 kilograms, with a surface area of 0.003 square meters. If the surface
 tension is 1.0 newtons per meter, then the surface energy per unit of mass
 is 0.03 joules per kilogram (- calculated from 0.003 square
 meters.times.1.0 newtons per meter.div.0.1 kilograms). For the kinetic
 energy of the blob to match this needs a velocity of only about 0.25
 meters per second (- calculated from (2.times.0.03 joules per kilogram)).
 Alternatively, equating this to potential energy requires an elevation of
 only 3 millimeters (- calculated from 0.03 joules per kilogram.div.9.8
 meters per second squared). Not all of a blob's potential or kinetic
 energy will go into overcoming the surface energy, but it should be
 evident from these values why it is necessary to introduce the hot liquid
 metal into the stream of water very gently, specifically being careful to
 make the flows co-current and of a similar velocity, and not to drop the
 stream of hot metal too far before it meets the water.
 Calculation of the Transient Thermal Field
 The previous section has explained why it is important that a blob is not
 subjected to impact or other external forces until it has solidified. In
 this section the length of time that the blob must remain in the coolant
 stream before it is solid is addressed. This parameter controls the length
 of the flume. Since such information is difficult to measure with
 accuracy, these temperature distributions were calculated.
 The published explicit solutions to transient heat flow through spheres and
 slabs were harnessed, together with the available thermophysical data, and
 together used as input into a custom-designed computer program. An
 assessment of the dimensionless number known as the Biot number, N.sub.Bi,
 revealed that the temperature gradients inside a volume of liquid
 ferro-alloy are less steep than those between lump and environment, and
 thereby indicated that an explicit series solution was required in order
 to calculate the temperatures. The most relevant part of the heat transfer
 calculation for the case of granulation applies to the first few seconds,
 and therefore up to 80 terms were required in the series calculations in
 order to provide reasonable accuracy.
 The various physical parameters necessary to undertake these calculations
 are listed in Table 1. The values were obtained from the literature where
 available and cross-checked by means of crude calorimetric and heat
 transfer experiments.
 TABLE 1
 Data used to model the temperature distribution within spheres
 and slabs of ferrochromium.
 Property Values
 Temperature of melt, .degree. C. 1600
 Liquidus temperature, .degree. C. 1560
 Temperature at which stiff, .degree. C. 1500
 Temperature of fluid, .degree. C. 15
 Emissivity 0.2 to 0.4
 Thermal conductivity, W/m/k 20 (porous) to 50 (solid)
 Effective heat capacity*, J/kg/K 838
 Density at 1500.degree. C., kg/m.sup.3 6600
 hc in air, W/m.sup.2 /K 5 (still air) to 80 (forced air)
 hc in water (film boiling), 300 to 600
 W/m.sup.2 /K
 Radiant h, W/m.sup.2 /K 90 to 200
 Combined h, W/m.sup.2 /K 650 to 850
 *includes latent heat of solidification
 The heat transfer from a blob of molten ferro-alloy will initially be by a
 combination of convection and radiation. However, the explicit analytical
 expressions referred to only consider convective heat transfer across a
 boundary layer. Nevertheless, since radiant heat transfer is also
 important in the case of very hot metals or slags it was accounted for in
 the form of an equivalent heat transfer coefficient, h.sub.r where
 h.sub.r =.sigma...epsilon..(T.sub.s +T.sub.a).(T.sub.s.sup.2
 +T.sub.2.sup.a)
 where .sigma. is the Stefan-Boltzmann constant and .epsilon. is the
 emissivity of the metal.
 The total amount of heat transferred to the environment is therefore
 approximately
EQU q=A.(h.sub.r +h.sub.c).(T.sub.s -T.sub.a)
 The heat transfer calculations were combined with a knowledge of the
 temperature at which the metal becomes sufficiently solid to resist impact
 deformation (determined as described in the section to follow), to yield
 an estimate of the minimum time necessary to stabilize the desired shape,
 and hence the required length of the flume.
 Determination of the Temperature at Which Rigidity is Established
 It may be assumed that metals will not be able to withstand a shear stress
 when above their liquidus temperatures, and that they will be solid below
 their solidus temperatures. Clearly, therefore, the critical temperature
 at which a solidifying blob of metal becomes rigid will lie somewhere
 between the liquidus and solidus temperatures.
 Since the precise values of the liquidus and solidus temperatures have an
 influence on the pebble casting process, the relevant temperatures for the
 materials used in the experimental trials were determined from the phase
 diagrams and confirmed in some cases by differential thermal analysis
 (DTA).
 The temperature at which rigidity is established depends on how an alloy
 solidifies and reference should be made to FIG. 2. In the case of charge
 chrome, a significant proportion of refractory Cr.sub.7 C.sub.3 needles is
 formed quite rapidly, and in considerable quantities, in the temperature
 range from liquidus to about 50.degree. C. below the liquidus. These
 needles were observed in later metallographic examinations to interlock.
 Although the last of the liquid only solidifies at around 1200.degree. C.,
 it was found that a bulk sample of charge chrome was already rigid at
 about 1500.degree. C. Similar behaviour, but over other ranges of
 temperature, are expected for other metals.
 Determination of the Critical Time to Reach Rigidity for Different Sized
 Blobs
 The time for a blob of liquid material to become rigid depends on a number
 of factors, including the rate of heat transfer, the size and shape of the
 blob, and the temperature and composition of the medium in which it
 solidifies. A number of different cooling fluids could be used, as has
 been mentioned earlier. To demonstrate this, in the calculations that
 follow, it has been assumed that rigidity in a sphere of high carbon
 ferrochromium is achieved when a skin of material at 1500.degree. C. or
 less has extended approximately 20% of the distance towards the centre of
 the sphere. Similar calculations are possible for other metals.
 Calculations for a 10 mm diameter blob show that it will take an
 impracticably long time to solidify in air. However, when water is the
 quenching medium the blob is effectively rigid in less than one second. In
 the present work, it is desired to produce pebbles with a characteristic
 dimension of about 20 to 100 mm. This leads to the requirement that heat
 must be extracted by a medium such as water for 21/2 to 31/2 seconds
 before the blob will be rigid.
 Practical Implementation
 Various configurations of the apparatus were tested. A flume of 2 m length
 was found to be too short, resulting in still-liquid blobs being ejected.
 A 10 m flume produced solid material. For the channel, three radii of
 curvature were tried, namely 50 mm, 75 mm and 100 mm. All three worked,
 but the smallest radius of curvature tended to produce blobs that were
 rather narrow. The largest radius of curvature, on the other hand, was too
 flat as the stream of metal tended to meander from side to side and
 collide with the side walls of the channel.
 Fluid flow in a channel is well analyzed in the literature. The velocity of
 the water flowing down the flume depends on the flow rate, the slope and
 the hydraulic radius. In the apparatus of the invention, as shown in FIGS.
 3 and 6, the water velocity was about 2 to 3 meters per second with a
 slope of from about 1 in 7 to 1 in 13 and a flow rate of about 10 to 25
 liters per second per channel. Steep slopes created excessive turbulence
 which adversely affected the shapes of the blobs. Shallower slopes and
 lower flow rates occasionally caused a blob to get stuck in the flume. In
 all cases, a settling distance of about 2 meters was provided to allow the
 initial rough liquid flow to settle down, before the metal was added.
 FIG. 3 illustrates in enlarged detail a portion of the apparatus shown in
 FIG. 6. Molten metal 10 is contained in a tundish 12 and is discharged
 through one or more holes 14 onto a short refractory lined channel or
 spout 16. The metal discharge rate is regulated by the size of the hole in
 the tundish.
 The spout 16 guides the stream of hot metal from the tundish 12 and leads
 it gently into the water stream 18 in a launder or flume 20.
 The flow rates of metal are typically about 1.5 to 2.5 kilograms per second
 per flume channel. High flow rates tend to encourage strings of "sausages"
 rather than discrete blobs, although the exact limit depends on the type
 of metal. It has experimentally been determined that a loading of 1.8
 kilograms of mild steel per meter of channel length produces a continuous
 "sausage". There is no particular disadvantage with a lower metal flow
 rate except for the likelihood of the metal freezing up at very low flow
 rates and the fact that a lower flow rate implies a lower throughput which
 affects the economic viability of the process.
 FIG. 6 is a schematic perspective illustration of apparatus 22 according to
 the invention. Like reference numerals to those employed in FIG. 3 are
 used to indicate like components.
 The flume 20 may be a single or multi-channel device and is supported on a
 suitable structure 24 to give the required flume inclination. The flume
 discharges into a catching tank 26 and water is circulated from this tank
 by means of a pump 28, through a pipeline 30 to a header tank 32. The
 header tank discharges into a stilling well 34 at the upper end of the
 flume and overflow from the well is directed into an upper portion 36 of
 the flume which allows the liquid flow to stabilise.
 The tundish is charged with molten metal from a ladle 38 which is supported
 by means of a suitable crane, not shown. Standby ladles 40 and 42 are
 safety receiving vessels that can take any molten metal overflows that
 might occur. Molten metal from the tundish flows into a cross channel 44
 which discharges into the spout 16, if there is a single channel in the
 flume, or into a number of spouts if there are multiple channels in the
 flume.
 The flow rates of the cooling water stream and of the molten metal stream
 may be controlled to ensure an optimal production of metal lumps. The
 cooling water flow rate can be controlled by varying the speed of the pump
 28, or by using control valves (not shown), to vary the velocity and flow
 rate of the water.
 The rate of flow of the molten metal may be controlled for example by
 varying the head of metal in the tundish or the cross-section of the exit
 aperture of the tundish through which the molten metal is discharged. The
 position of the tundish and cross channel assembly may also be adjusted.
 For example the assembly can be moved horizontally or vertically, to
 ensure that the metal stream falls into the water stream at an optimum
 angle and at an optimum position.
 A vibratory separator 46 is mounted above the catching tank. The separator
 traps the lumps of solid metal and allows the liquid to flow through to
 the tank. The separator advances the metal lumps towards its discharge end
 48 and the lumps falling from the separator are collected in a heap 50, or
 may be fed to a cooler and dryer.
 Known granulating processes produce wet or damp granules. The introduction
 of such granules into a furnace can produce explosive results. It is
 therefore desirable to ensure that the lumps are dried and this may be
 achieved, for example, by using a separator such as a chain grate, or any
 other suitable heat resistant conveyor, to separate the liquid from the
 metal lumps. As shown in FIG. 6 the separator 46, which may be of a
 considerable length, is then used to transport the lumps past one or more
 air blowers 51 which direct streams of air onto the lumps, from different
 directions if necessary, to ensure that the lumps are at least partly
 dried and, at least to some extent, are cooled.
 As an alternative to the vibratory separator a chain grate may be used to
 separate the liquid from the metal lumps.
 Safety is an important consideration in the operation of the apparatus. As
 with conventional granulators the contacting of molten metal with water
 occasionally produces explosions. In the apparatus of the invention
 however the amount of metal in contact with the water at any given time is
 relatively small.
 FIG. 1 illustrates some possible different cross-sectional profiles of the
 flume.
 FIG. 1(a) illustrates a flume with a relatively small radius of curvature
 while FIG. 1(b) illustrates a relatively large radius of curvature. FIG.
 1(c) illustrates the concept of a water jacket 52 conforming to the inner
 cross-sectional shape of the flume.
 FIG. 1(d) shows a flume with two side-by-side channels each of which
 accommodates a fluid stream into which a respective stream of molten metal
 is directed.
 FIG. 1(e) shows a flume with a central channel 54 in which a molten metal
 stream is concentrated and which is flanked by outer channels 56 which
 allow for a relatively greater volume of water flow. The last mentioned
 design tends to limit the meandering effect of the liquid metal, referred
 to earlier, when the channel radius is too large.
 Trials with Molten Metals
 Equipment
 An induction furnace was used to remelt up to 50 kg of metal which was
 tapped and transferred to a tundish, from where it flowed into the flume.
 The tapping temperatures of the metal were recorded with a dip
 thermocouple or a pyrometer or both.
 Procedure
 Several runs were carried out on this apparatus using a number of alloys
 with a variety of different set-up configurations. The nominal
 compositions of some of the alloys used are given in Table 2 below.
 TABLE 2
 Compositions of the ferro-alloys used for the pebble casting
 trials
 Melting
 Material Fe Cr Mn Si C range, .degree. C.
 charge chrome 38 52 -- 3 7 1200-1570
 0.5% carbon 44 54 -- 1.4 0.5 1500-1600
 ferrochromium
 medium carbon ferro- 17 -- 80 1 2 1180-1220
 manganese
 ferro-silicon 25 -- -- 71 0.4 1215-1370
 Results
 As predicted by the theoretical analysis, it was found that too great a
 degree of coolant turbulence produced irregular shaped particles, and too
 low an inclination of the flume or too great a metal flow caused the
 formation of long sausages. The best-shaped product was obtained with a
 flume length of 10 m, an inclination in the range 1 in 8 to 1 in 12, a
 metal feeding rate of about 1.5 kg per second per channel, and a
 relatively smooth stream of water, flowing at about 15 liters per second
 per channel. Thus the ratio of the metal to water flow rates is of the
 order of 1:10, on a mass basis.
 Some of the products obtained using different configurations and metals are
 shown in FIGS. 5, 5(a), (b), (c) and (d), while FIG. 4 shows the size
 distribution of lumps which were produced.
 The experiments were conducted on a plant capable of processing only 0.15
 tons of liquid metal per minute. A full-scale plant would be required to
 process molten metal at a rate of at up to about 3 tons per minute, and
 would have to run without interruption for up to 30 minutes.