[0001 ] The present invention generally relates to granulation of slag from the metal industry and more particularly from the iron industry.

Background Art

[0002] Conventionally, metallurgical slag is granulated in water.

[0003] Water quenching ensures fast solidification of the metallurgical slag, which, in the case of blast furnace slag, is a necessary condition for obtaining a valuable product. A water jet is firstly used to fragmentize the hot liquid slag stream into very small particles and to transfer them into a water bath. The energy form the hot slag is withdrawn through direct contact between the hot liquid slag and the water. As this has to happen at ambient pressure, the temperature of the slag is immediately lowered to a temperature level of below 100°C.

[0004] However, a main disadvantage of the water granulation process is that sulphur contained in the hot liquid slag reacts with the water and generates sulphur dioxide (SO ₂ ) and hydrogen sulphide (H ₂ S).

[0005] Furthermore it must be noted that care must be taken to lower the temperature of the slag sufficiently rapidly and far enough to obtain a vitrified or amorphous slag rather than a (partly) crystallized slag, which is a lot less advantageously priced (about 15 times) in the market.

[0006] It would also be desirable to recover at least some of the heat from the slag in a usable form. To use this potential in an efficient way, it is necessary to rapidly cool down the slag to a temperature level, which is low enough to make the treatment of the material easier but high enough to preserve the energy at a useable level.

[0007] One possibility to overcome the disadvantage of producing noxious gases and to recover at least part of the heat comprises the mixing of liquid slag with cold slag granulates of the same chemistry. The slag can then be subject to heat recovery in a heat exchanger. However, it has been found that due to the high viscosity of the liquid slag, the cold slag granulates and the liquid slag do not mix easily and thus it is not possible to cool the liquid slag fast enough to obtain vitrified slag.

[0008] A further solution has been proposed in which (cold) solid metallic particles are added to the hot liquid slag (WO 2012/034897, WO 2012/080364). The effect is that the slag rapidly solidifies in a vitreous state without producing noxious gases. Furthermore the solid metallic particles are chemically inert and may be easily separated, recovered and reused (see WO 2012/034897). Finally, the heat from both solidified slag and metallic particles may be recovered in an appropriate device, such as a heat exchanger (see WO 2012/080364).

[0009] In practice, however, the efficiency of the above solution is highly dependent on the appropriate pouring of the slag and the correct dosing of the metallic particles. This is even aggravated by the fact that in general the incoming flow of liquid slag cannot be controlled, unless technically complex or energetically demanding additional equipment, such as actual slag filling level determination in the freshly filled mould, heated or refractory lined transfer ladles and/or equivalent are foreseen. Without such additional measures, any proper metering of the metallic particles becomes complicated and even erratic.

Technical problem

[0010] It is an object of the present invention to provide method for dry slag granulation, as well as a corresponding device, which allow to benefit from the advantages of reduced environmental impact and increased energy recovery potential, while being simple to implement, and both safe and efficient to use.

General Description of the Invention

[001 1 ] To achieve this object, the present invention proposes a method for dry slag granulation of hot liquid slag using a casting apparatus comprising an endless conveyor having a plurality of casting moulds, which endless conveyor is arranged to move said casting moulds in a first section from a slag pouring zone through a cooling zone to a discharge zone and in a second section back to the slag pouring zone, comprising the continuous steps of: a) pouring an amount of hot liquid slag into a casting mould in a position N in the slag pouring zone, b) moving the hot liquid slag containing casting mould in a position N+n within the cooling zone,

c) adding solid metallic particles to the hot liquid slag containing casting mould in position N+n by dropping a determined amount of said metallic particles into the mould from a dispensing device arranged above said mould and comprising at least one hopper for storing said solid metallic particles,

d) discharging the cooled solidified slag from the mould in the discharge zone,

wherein the actual amount of hot liquid slag poured in the casting mould in step (a) is measured in a position between N and N+n, wherein n is a number between 1 and 10, preferably between 1 and 4, most preferably n is 2, wherein the speed V _cas ter of movement in step (b) is adapted based on the measured amount of hot liquid slag, wherein the amount of metallic particles necessary for step (c) is determined based on a fixed slag/metallic particle ratio λ, wherein the amount of metallic particles added by the dispensing device in step (c) is controlled by opening of the at least one actuated sliding gate arranged at the hopper's outlet during a time t determined based on the speed V _cas ter and wherein the opening stroke x of the at least one actuated sliding gate is determined based on the determined amount of metallic particles, the determined opening time t and characteristics of the at least one sliding gate.

[0012] As already mentioned, the main cause of the difficulties in practice is not only the uncontrollable, but also variable flow of slag to cope with, while it is important to have a certain ratio between slag and metallic particles.

[0013] A major advantage of the present method is that by simply measuring the quantity of hot liquid slag within the casting mould after step (a) and before step (c), the entire process may be controlled.

[0014] First, this measured value is used to control the amount of liquid slag in the moulds. Indeed, it seems clear that the moulds of the endless conveyor should be filled with an appropriate (maximum) amount of slag, which amount should be such that there is enough room left in the casting mould to add an amount of metallic particles in accordance with the fixed slag/metallic particle ratio λ. Hence, the measurement of the amount of hot liquid slag allows to control said amount by adapting the speed of the conveyor, such as by slowing it down if the amount is below the expected (appropriate) amount and by accelerating the conveyor if the amount of liquid slag is in excess of the expected (appropriate) amount. If the amount of liquid slag is approximately the one expected, no speed adaptation is necessary.

[0015] Second, this same measurement of the amount of liquid slag is used to determine the actually needed amount of metallic particles for that particular casting mould.

[0016] In a further aspect, the invention also proposes an apparatus for dry slag granulation of hot liquid slag comprising an endless conveyor having a plurality of casting moulds, which endless conveyor is arranged to move said casting moulds in a first section from a slag pouring zone through a cooling zone to a discharge zone and in a second section back to the slag pouring zone, and wherein the apparatus further comprises

• a dispensing device arranged in the cooling zone above said moulds at a position N+n and comprises at least one hopper for storing solid metallic particles, said dispensing comprising at least one sliding gate at the hopper's outlet, the actuating of said sliding gate(s) allowing the control of the amount of metallic particles dispensed by said dispensing device,

• at least one sensor capable of measuring the actual amount of hot liquid slag poured inside a casting mould in a position between the slag pouring position N and the metallic particle addition position N+n, wherein n is a number between 1 and 10, preferably between 1 and 4,

• a control unit to adapt the speed V _cas ter of movement of the conveyor based on a measured amount of hot liquid slag, to determine the necessary amount of metallic particles to be added by the dispensing device based on a fixed slag/metallic particle ratio λ, to control the amount of metallic particles to be added by the dispensing device by opening of the at least one actuated sliding gate arranged at the hopper's outlet during a time t determined based on the speed V _cas ter and to determine the opening stroke x of the at least one actuated sliding gate based on the determined amount of metallic particles, the determined opening time t and characteristics of the at least one sliding gate.

[0017] In the context of the present invention, it shall be noted that the conveying of the moulds in the pouring zone and in the cooling zone is preferably essentially linear, i.e. the slope angle(s) is/are essentially constant within the pouring zone and the cooling zone. Furthermore, in the present context, the positions indicated relative to the position N of the mould being filled in the slag pouring zone are illustrative only. In fact the actual position of the dispensing device in a position indicated by N+n in the present document may be located at a distance d which is not a multiple of the length I of a mould, i.e. the filling of the metallic particles in a mould in position N+n does not necessarily occur when the mould in position N is located under the slag runner. Hence, the distance d between the mould being filled in the slag pouring zone and the dispensing device may be any distance between 0.1 I and 9 I, preferably 0.5 I and 3 I, in particular about 1 I (i.e. n is about 2). In practice, the lower value for distance d is limited only in order to allow for the measurement of the actual amount of hot liquid slag poured inside a casting mould. The upper limit is generally dependent on the fact that the slag should still be sufficiently liquid to allow appropriate penetration of the metallic particles into the slag.

[0018] In a further aspect of the invention, the method or the apparatus further comprises (means for) the measurement of the actual combined amount of slag and metallic particles, i.e. the amount of the slag/particles mix, in the casting mould after step (c) in a position downstream of position N+n, calculating the amount of metallic particles actually added in step (c) based on the measured combined amount of slag and metallic particles and the previously determined amount of hot liquid slag and, if the calculated amount of actually added metallic particles does not correspond to the previously determined amount of metallic particles, adapting the characteristics of the at least one sliding gate.

[0019] The main advantage of this further preferred embodiment is that it allows using the actual measurement of the amount of the resulting slag/particles mix as a feedback for adapting (or correcting) the amount of metallic particles by acting on the dispensing device, in particular on the sliding gate(s) regulation.

[0020] Advantageously, the characteristics of the at least one sliding gate comprise the mass flow of particles particles as a function of the opening stroke x ( particies/x curve), wherein the opening stroke is the distance by which the sliding gate must be moved between open and closed state (or essentially closed state, see below). In fact, this parameter basically defines the amount of particles flowing through the sliding gate per unit of time depending on the opening of the gate. This parameter can be represented as a discrete curve and may be determined experimentally depending on the particular type of sliding gate and the particular type of particles (see details below).

[0021 ] In a still further embodiment, the actual amount of hot liquid slag poured in the casting mould in step (a) and/or the actual combined amount of slag and metallic particles in the casting mould after step (c) is/are measured by determining the height of slag h _s i _ag , respectively of slag/metallic particles h _mi x in said mould, and by calculating the corresponding volume V _s i _ag and/or V _mi x based on the known shape of the casting mould, as well as the determined mass of slag M _s iag and/or the determined mass of metallic particles M _pa rticies' based on the difference between V _mi x and V _s i _ag .

[0022] Preferably, the measurement of the amount is effected by measuring the height of the slag or of the slag/metallic particles mix within the casting mould contactlessly, such by means of one or more laser range finders. Other means such as radar, acoustical or optical detection etc. may also be used.

[0023] In a still further aspect of the invention, the conveyor preferably is a so- called dual slope caster, in which the amount of liquid slag in the casting mould can be further controlled by arranging the slag pouring zone in the first section of the endless conveyor to convey the moulds with a first slope angle a which limits the effective maximum filling volume of the mould in the slag pouring zone to a value VQ, any excess slag cascading back to an upstream casting mould in position N-1 , N-2, etc. and wherein the cooling zone is arranged with a second slope angle β < a such that the effective maximum filling volume of the mould in the cooling zone has a value V _P > V _A . [0024] Hence, a further advantage of embodiments integrating such a dual slope conveyor is that the moulds of the endless conveyor are filled with a defined amount of slag. This defined amount of slag is dependent, for a given mould, on the inclination angle of the mould when it is being filled: the steeper the slope the less slag can be filled in the mould until the slag overflows or cascades to the next mould(s) (i.e. the next mould(s) upstream of that being filled). Hence, at a slope angle a, the effective maximum filling volume, i.e. the useful volume, can be defined as V _a . If the slope angle is decreased (less steep or even horizontal), the actual filling level in the mould decreases and the effective maximum filling volume, i.e. the useful volume of the mould increases to V _p thereby providing room for the solid metallic particles to be added in the cooling zone. Hence, if the slope angle β in the cooling zone is lower than the angle a in the pouring zone, a theoretical maximum amount of metallic particles corresponding to V _p - V _a may be added to the mould without risking an overflow or spilling.

[0025] In a preferred embodiment of the invention, the slope angle a in the slag pouring zone and the slope angle β in the cooling zone are chosen such that the effective maximum filling volume of the mould in the slag pouring zone V _a is between 0.25 and 0.75, preferably between 0.30 and 0.60, even more preferably 0.45 to 0.55 times the effective maximum filling volume of the mould in the cooling zone V _p . In particular, the slope angles a and β are chosen such that V _Q is about ½ v _P .

[0026] Although largely dependent on the geometry of the mould, an appropriate angle β is in practice generally situated between 0° (horizontal) and 50°, more preferably between 10° and 40°, whereas angle a is usually 5 to 20° greater than angle β.

[0027] In the case a dual slope conveyor is used, the moving of the casting moulds may be further controlled by monitoring the temperature of at least two (or more) casting moulds immediately upstream (positions N-1 and N-2) of the casting mould being filled and by varying the speed of the movement in step (b).

[0028] It is particularly preferred to control the apparatus by decreasing the speed of movement in step (b) if the temperature in the casting mould in position N-1 does not significantly increase due to a lack of backflow of hot liquid slag from position N (pouring position) and by increasing the speed of movement in step (b) if the temperature in the casting mould in position N-2 increases due to the backflow of hot liquid slag from the casting mould in position N-1 . So, in other words, if the temperature in the mould immediately upstream (N-1 ) of the mould being filled does not increase to a value near the temperature of the hot liquid slag, i.e. has a temperature well below this value, no slag is cascading back, hence the filling of the mould is at best sufficient, probably however incomplete: the conveyor is slowed down. If on the other hand, only the mould in N-1 is hot (contains liquid slag) and not mould N-2, the speed of the conveyor is deemed appropriate, the speed is not changed. Finally, if not only the mould N-1 , but also the mould N-2 becomes hot, the speed of the conveyor is too low and must be increased.

[0029] The monitoring of the temperature may be done by any appropriate means, such as with a thermocouple or similar; preferably however the monitoring is effected contactlessly, such as by means of pyrometers.

[0030] A further general problem associated with the uncontrollable and variable slag flow is that the addition of the (determined amount of) metallic particles must be done within the time frame imparted by the slag flow. The resulting difficulty is thus the limited time available to introduce the required amount addition of metallic particles to the partly filled casting mould, while the conveyor needs to continuously move the moulds further. Hence, an important advantage of the present method is that the use of a sliding gate for metering the amount of solid metallic particles allows for a rapid opening and closing, thereby greatly shortening the time necessary to bring in the correct amount of particles.

[0031 ] An appropriate sliding gate (valve) generally comprises a sliding plate arranged to close an opening by pushing and/or rotating it, generally along a sliding frame, in front of the opening (in this case the outlet of the hopper). The sliding of the plate may be effected in parallel to the conveying direction of the moulds or preferably perpendicularly to this direction. Hence, in the context of the present invention the sliding movement may be translational (e.g. linear sliding gate), rotational (e.g. rotary sliding gate; the axis of rotation being preferably parallel to a plane formed by the opening or perpendicular to that plane) or even a combination thereof (e.g. curve sliding gate). The related term "opening stroke" as it is used herein shall be interpreted accordingly as being the amplitude of sliding movement (pushing and/or rotating) necessary to open the sliding gate.

[0032] In a preferred embodiment of the sliding gate, a stationary plate having a plurality of spaced apart apertures is placed in front of the opening (outlet of the hopper) and thus (only) partly obstructs the outlet. A corresponding movable sliding plate presents corresponding apertures which may be slid in front of the apertures of the stationary plate for opening the gate and slid in a different position to close the apertures of the stationary plate. Preferably these apertures are of roughly rectangular or square shape for linear sliding gates or of roughly sector shape for rotary sliding gates with a rotation axis perpendicular to the opening and the spaces between adjacent apertures in the sliding plate are dimensioned to essentially close the corresponding apertures of the stationary plate. A major advantage of a plurality of apertures is that the stroke of the sliding gate, i.e. the sliding distance between open and closed state, is substantially reduced, which is beneficial in view of the limited time available to effect the action.

[0033] In a further preferred aspect, the sliding gate comprises two or more rows of a plurality of apertures. In such a case, it is even more preferred that the apertures of adjacent rows are provided in a staggered (zigzag) fashion to further improve the uniform distribution of the particles in the slag.

[0034] In a still further aspect, the dispensing device comprises two or more separately controllable actuated sliding gates, each sliding gate being connected to the outlet of the same hopper or of a separate hopper. Advantageously, the dispensing device comprises one (or two) hopper(s) with two separately controllable actuated sliding gates, each sliding gate covering essentially half of the casting mould's surface. The provision of two (or more) independently actuatable sliding gates allows to be more flexible in terms of opening time as the moulds are moving below the dispensing device and in terms of redundancy should one of the gates fail or become clogged.

[0035] The sliding gate or gates is/are actuated by any appropriate means, such as pneumatic means, hydraulic means, and/or by one or more electric motors. Preferably, the actuation of the sliding gate(s) is effected by one or more hydraulic cylinders.

[0036] In a still further aspect, the stroke of the sliding gate(s) is limited, such that the apertures of the stationary plate are not completely closed. The advantage of this practice is double in that the actuation is even faster and that the wear of the sliding gate and the stationary plate are substantially reduced. Indeed, it has been noted that for a given diameter of the metallic particles, it is sufficient to close the sliding gate(s) in order to leave a residual opening of each aperture which generally represents 0.3 to 1 .5, preferably about 0.8 to 1 .3 times, most preferably between about 1 .1 and 1 .25 times the diameter of the particles or of the smallest particles if particles of different diameters are used concomitantly.

[0037] In a further aspect, the method and apparatus of the invention also integrates means to precisely determine the position of (some of) the moulds, in particular the mould beneath the dispensing device. Any conventional means can be used to this purpose, in particular contact and/or contactless devices such as switches, laser, induction, etc.

[0038] The solid metallic particles are preferably dropped from a height of about 0.1 to 3 m, preferably about 0.2 to 2 m, to obtain a quick and efficient mixing of the slag and the solid metallic particles. The exact height i.e. the exact amount of energy required for the particles to penetrate the liquid slag to the desired depth depends on the composition of the slag, the temperature of the slag, the density and the diameter of the solid metallic particles etc. As the dropping of the metallic particles into the hot liquid slag may cause some spilling or splashing, the dispensing device preferably comprises splash boards arranged at least on the lateral sides below the hopper(s) and sliding gate(s) and which extend approximately to the upper tops of the moulds.

[0039] The solid metallic particles advantageously have a density of at least 2.5 g/cm ³ . Due to the difference of densities between the slag and the metallic particles, the metallic particles and the slag mix thoroughly.

[0040] The solid metallic particles are preferably spherical so as to have good mixing properties and to assure a rapid and efficient cooling of the slag. [0041 ] The solid metallic particles preferably have a diameter of at least 2 mm preferably more than 5 mm and most preferably more than 10 mm. The solid metallic particles advantageously have a diameter of less than 80 mm, preferably less than 50 mm and most preferably less than 25 mm.

[0042] The solid metallic particles are preferably made of a metal chosen amongst the group consisting of iron, steel, aluminium, copper, chrome, nickel, their alloys, as well as their alloys with other metals.

[0043] In practice, it is preferred to use steel balls because they are readily available in different diameters.

[0044] During and/or after the discharge of the slag cake from the conveyor, the cake is preferably crushed into particles of a size of about 40 - 120 mm and a bulk density of about 2 - 5 g/cm ³ , preferably of a size of about 40 - 90 mm and a bulk density of about 2 - 5 g/cm ³ .

[0045] Thereafter, the still hot slag particles and heated solid metallic particles are preferably charged into a heat exchanger, cooled with a countercurrent flow of cooling gas and discharged from the heat exchanger.

[0046] According to a preferred embodiment, the heat exchanger is subdivided in a plurality of subunits, each of said subunits having a particle inlet port, a particle outlet port, a cooling gas inlet port and a cooling gas outlet port, wherein at least one of the subunits is charged with hot slag particles and heated solid metallic particles through the particle inlet port, cooled slag particles and cooled solid metallic particles are discharged through said particle outlet port from said at least one of the subunits, said cooling gas inlet port and said cooling gas outlet port being closed during the charging and discharging of particles and wherein, simultaneously to the charging and discharging of particles, at least one of the other subunits is cooled by injecting a flow of cooling gas through the cooling gas inlet port and withdrawing a flow of heated cooling gas from said cooling gas outlet port, said particle inlet port and said particle outlet port being closed during the cooling of particles and wherein the heated cooling gas is used for energy recovery. [0047] Accordingly, the process according to a preferred embodiment of the invention proposes to use heat exchangers comprising multiple subunits, which are operated discontinuously. As it is advantageous to obtain a constant hot gas flow at the exit of the heat exchanger in order to guarantee the most efficient use of electric power generation cycles, the multiple heat exchanger subunits are operated alternately in a way that an essentially constant hot gas flow is guaranteed. By this, it is possible to obtain an essentially continuous gas handling which is decoupled from the batch type material handling.

[0048] At each moment in time, where one of the heat exchanger subunits is in emptying/filling stage, no cooling gas is flowing through this heat exchanger subunit during emptying/filling.

[0049] The same quantity of particles is filled into and extracted from the exchanger. Meanwhile, no material is entering or leaving the other heat exchanger subunits; they can thus be completely sealed off from the environment during cooling.

[0050] Preferably, one of the subunits is charged with hot slag particles and heated solid metallic particles through the particle inlet port while cooled slag particles and cooled solid metallic particles are discharged simultaneously through the particle outlet port of the same subunit.

[0051 ] Once the heat exchanger subunit is filled up, the particle inlet and the particle outlet ports are sealed and the subunit is reconnected to the cooling gas stream while another heat exchanger subunit may be disconnected. The cooling gas flow through these heat exchanger subunits does thus not encounter any leakage, thereby preventing dust and energy leaving the system. The heat exchanger subunits thus only need to be depressurized during charging and discharging of the slag.

[0052] According to a preferred embodiment, the hot slag particles and heated solid metallic particles are first charged into an insulated pre-chamber before they are charged into one of the heat exchanger subunits. The pre-chamber is preferably insulated either by refractory lining or material stone box. The low thermal conductivity of slag gives excellent insulation properties. [0053] The slag particles and solid metallic particles may also be charged in a post-chamber after cooling and after being discharged from the heat exchanger subunit. In other words, the cycle time and the quantity of particles charged may thus be chosen in such a way that the heat transfer inside the heat exchanger subunits may be controlled and kept quasi-stationary. The outlet gas temperature fluctuation caused by charging/discharging of the heat exchanger subunits will thus be minimized by choosing cycle times accordingly.

[0054] According to a further preferred embodiment, hot liquid slag is solidified in a slag cake and cooled down to about 650°C to 750°C by mixing it with solid metallic particles. Advantageously, the hot liquid slag is mixed with about the same volume, preferably resulting in a mixture containing from about 40% to about 60% volume of solid metallic particles. The required volume of metallic particles depends on the desired target temperature, the density and the heat capacity of the metallic particles, ... For steel spheres, 40% to 60% (volume percentage of total volume) are preferable.

[0055] Preferably, the heat exchanger subunits are operated under a pressure from 1 .2 bar to 4 bar i.e. the absolute pressure measured at the bottom of the slag layer in the subunit.

Brief Description of the Drawings

[0056] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1 is a cross sectional schematic view of a preferred embodiment of a dry slag granulation apparatus;

Fig. 2 is a schematic view of some of the components of a sliding gate valve useful for the dispensing device;

Fig. 3 is a perspective top view of an embodiment of a double sliding gate (hopper not represented) with staggered apertures;

Fig. 4 is a cross sectional view of a part of the dry slag granulation apparatus of Fig. 1 ; Fig. 5 is a sketch of a further preferred embodiment of a method according to the invention;

Fig. 6 is a sketch of a still further preferred embodiment of a method according to the invention; and

Fig. 7 (a) and (b) illustrate a preferred embodiment a self-correcting method of operating an apparatus as described herein.

[0057] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings.

Description of Preferred Embodiments

[0058] Fig. 1 schematically represents a cross section of a preferred embodiment of a dry slag granulation apparatus comprising a slag caster 1 with a linear endless conveyor 10 comprising a plurality of casting moulds 1 1 . It is to be noted that a so-called dual slope linear slag caster may be used as well (see e.g. Fig. 4).

[0059] The moulds 1 1 are conveyed in a first section from a slag pouring zone A through cooling zone B to a discharging zone C where the moulds are emptied from the solidified slag cakes contained therein. In a second section the now empty moulds are conveyed back to the slag pouring zone.

[0060] The hot liquid slag 21 from slag runner 20 is poured in a casting mould 1 1 passing beneath the slag runner 20.

[0061 ] In the particular case of a dual slope conveyor (as in Fig. 4), the slope angle a is relatively steep, such that the maximum effective volume V _a , i.e. the volume of slag in the mould when the slag overflows to the upstream mould N-1 , is smaller than the useful volume V _p of the mould when the mould reaches a position (> N+2 in Fig. 1 ) in the cooling zone B where the slope angle β is smaller. Preferably, angles a and β are chosen such that V _Q is about ½ V _p .

[0062] When the mould containing hot liquid slag reaches a position between the pouring position N and the position N+2 of the dispensing device 30 (i.e. N+1 in Fig. 1 and 4), the height of the hot liquid slag in the mould (h _s i _ag ) is measured by means of a sensor, preferably by means of a laser range finder (Sh _s i _ag )-

[0063] The result of this measurement is used on the one hand to adapt the speed of the conveyor (V _cas ter) and on the other hand to determine the amount of particles to be subsequently added to the hot slag in the casting mould.

[0064] When the mould containing hot liquid slag reaches a position (e.g. N+2 in Fig. 1 and 4) below the dispensing device 30, an amount of the solid metallic particles 33, such as steel balls, contained in storage hopper 31 (the amount being determined based on the measured amount of slag and the predefined slag/particles ratio) is introduced in the mould by opening sliding gate 35 mounted on the hopper's outlet 32 for a predetermined time t. The particles 33 fall into the mould and mix with the hot liquid slag 21 by the action of gravity.

[0065] Splash boards 36 may be provided to prevent splashing of liquid slag and steel balls from the mould during charging.

[0066] During introduction of the metallic particles 33 into the hot liquid slag 21 , the slag rapidly cools down and solidifies in an amorphous state. The mould now containing a solidified cake of slag and added metallic particles, such as steel balls, then further cools down while proceeding to the discharge zone C where the mould is emptied by turning it upside down. The solidified slag cake is then crushed to pieces and may proceed to a thermal recovering unit (not represented).

[0067] In a still further preferred variant of the method, the height of solidified slag/particles mix in the mould (h _mix ) is measured using a further sensor, preferably also by means of a laser range finder (Sh _mix ). Using this value as a feedback on the actual addition of particles, the process may be adapted (self-correcting process). Further details are given below.

[0068] Fig. 2 presents a schematic view of parts of a linear sliding gate valve 35. A preferred sliding gate valve for use in the present invention comprises a stationary plate 353 with a plurality of spaced apart apertures 354. The corresponding sliding plate 351 also presents spaced apart apertures 352, but the location of these apertures is such that apertures 352 may be brought by linear sliding from a closed position wherein all apertures 354 of the stationary plate are (essentially, see above) closed by those parts of the sliding plate without apertures to an open position (by sliding in the direction indicated by the arrow in Fig. 2) where apertures 352 are essentially aligned with apertures 354.

[0069] Fig. 3 represents a top perspective view of a preferred embodiment of a double (linear) sliding gate valve 35 without the surmounting hopper(s) for temporarily storing the metallic particles. For illustration purposes, only one of the valves further comprises a splash board 36 arranged below the valve. The sliding plates 351 of each valve are actuated with respect to stationary plates 353 by separate hydraulic cylinders 355.

[0070] Fig. 4 essentially corresponds to the lower part of the embodiment of Fig. 1 during operation. Again, it is recalled that the dual slope conveyor represented in Fig. 4 may also be a single slope conveyor for the purpose of the present invention.

[0071 ] As can be seen in Fig. 4, the casting mould in position N is been filled with hot liquid slag 21 from slag runner 20.

[0072] In the case of a single slope conveyor, the speed of the caster (V _cas ter) is adjusted based on the amount of liquid slag in the mould in position N+1 by measuring the height of the slag (h _s i _ag ) using a sensor (Sh _s i _ag ) as already described above.

[0073] In the case of a dual slope conveyor (as represented in Fig. 4), the control of the amount of liquid slag may be additionally controlled by the particular shape of the conveyor itself, as well as optionally with one or more temperature sensors. If the amount of slag from the runner exceeds the (local) capacity of the mould (VQ), the excess slag overflows to the immediately adjacent mould in position N-1 by gravity. If the amount (flow) from the runner is even higher, the slag from N-1 cascades to mould N-2 (this is actually the situation represented in Fig. 4). Knowing that the moulds arriving from the second section of the conveyor 10 have a temperature which is far from that of the liquid slag (to simplify this temperature will also be called "ambient temperature", although this temperature will generally be situated between 50 and 300°C or even above), it has been found that by controlling the mould temperature (T _N- i and T _N- 2) of these two positions N-1 and N-2, a simple, yet efficient control of the casting process can be provided (see also below). It is to be understood that more than two temperatures can of course be monitored if deemed necessary or desirable.

[0074] In Fig. 4, mould N+2 is located beneath the dispensing device 30 and is ready to be filled with metallic particles 33. The amount of metallic particles to be added is determined as described in detail below.

[0075] The casting mould in position N+3 in Fig. 4 shows a mould wherein the calculated amount of metallic particles have been added to the liquid slag. At this point, the slag essentially solidified in an amorphous state due to the instant adding of a substantial quantity of cold particles. It seems clear that the position for inserting the particles indicated by N+2 in Fig. 4 does not necessarily need to be the second position after the filling position as far as the slag is still hot and liquid.

[0076] Optionally, the amount of the slag/particles mix is thereafter measured to verify if the result matches the expected amount. If not the value is used as a feedback to adapt the process as further described below.

[0077] The exact position of a mould within the conveyor, and in particular its position below the slag runner and/or the dispensing device, can be determined by any known contact or contactless means, such as switches, induction, laser, etc.

[0078] Description of the process - purpose of the regulation

[0079] In the dry slag granulation process, liquid slag 21 is cooled by adding cold metallic particles 33, such as steel balls. The purpose of this cooling is to achieve

• quick cooling of the slag

• a high mix temperature for further heat recovery

[0080] Therefore, the dosing of the steel balls has to be precise and a steel/slag ratio has to be respected.

[0081 ] To achieve material transportation, a slag caster 10 technology has been chosen. The slag 21 is poured upstream, in the pouring zone A at the start of the conveyor 10 and then the steel balls 33 are added in the cooling zone B, as shown e.g. in Fig. 1 and 4. [0082] The dosing of the steel balls 33 in the cooling zone B, as well as the slag filling of the mould 1 1 in position N should be regulated. In the presently described regulation mode, the regulation is done by measuring the amount of slag in the mould in position N+1 and by adding a calculated amount of steel balls 33 in said mould as it passes below the dispensing device 30. The amount of hot liquid slag in the mould is controlled by varying the speed of the caster 10.

[0083] Because the charging time is limited, the dosing of the steel balls 33 is done using a sliding gate valve 35, as shown e.g. in Fig. 2 and 3.

[0084] Description of a preferred regulation mode

[0085] As already mentioned above, the metallic particles are preferably steel balls. The dosing of such steel balls, as well as the slag filling of the mould has to be regulated. This regulation is preferably done by varying two parameters:

• the speed of the caster

• the quantity of metallic particles (steel balls) dropped in each mould.

[0086] As the charging time of metallic particles is limited, the dosing is done using one or more gate valves, as shown e.g. in Fig. 2 and 3.

[0087] The charging is done in each mould only when the mould is located below the gate, so the mould in which the particles are charged is not undetermined.

[0088] The material charged by this gate is depending on two parameters

• opening stroke of the gate (x)

• gate opening time (t)

[0089] As it can be seen in Fig. 5, the basic regulation steps may be described as follows:

1 . Incoming of the liquid slag (in an unknown quantity) in the mould in position N

2. Once the mould has reached position N+1 , measuring of the slag level (hsiag) using a laser a) Transposition from that height {h _s i _ag ) into a slag volume in the mould ( Vsiag) and then a slag mass in the mould (M _s/ag ). b) Determination of the steel balls necessary for this mould (M _stee i), based on a slag/steel ratio (λ) c) Adaptation of the caster speed ( V _cas t _er ) based on the recorded slag height (h _s i _ag )

3. Once the mould has reached position N+2, charging of the steel balls a) The opening time (t) of the sliding gate is determined based on the caster speed (V _cas ter) b) The opening stroke of the gate (x) is determined based on i. the required steel balls mass M _sfe e/ determined above ii. the opening time (t) iii. the characteristics of the sliding gate { i _stee i /x curve)

[0090] This first regulation method can be summarized as shown in Fig. 5.

[0091 ] In a still further preferred embodiment, the method further comprises the following steps:

4. Once the mould has reached position N+3, measuring of the steel balls/slag mix height (h _mix ), using a laser a) Transposition from that height (h _mix ) into a mix volume in the mould ( V _mix ). Determination of the steel balls actually added in the mould

{Msteei).

5. Feedback - adaptation of the gate characteristic a) if Msteei = Msteei, the right quantity of steel balls have been dropped -> no adaptation of the steei /x curve b) if Msteei ≠ Msteei ^' , the Msteei /x curve is adapted in order to fit to the measured mix height. This adaptation is described below.

[0092] This further preferred regulation method can be summarized as shown in Fig. 6. [0093] Adaptation of the gate characteristics

[0094] If the measured quantity of steel balls doesn't fit with the theoretical amount (M _stee i <> M _stee iy, the curve λί _5ίθθ / /x curve should be adapted. A difference

AMsteei is determined, as the charging time t is known, the difference between calculated flow rate and real flow rate is calculated (ΔΑ).

[0095] If Mf > 0.2 x A then no correction is applied and an alarm is issued. If Mf < 0.2 x A then correction of the mass flow characteristic curve of the dosing valve will be performed.

[0096] The curve λί _5ίθθ / /x curve is a discrete curve. The correction of this curve may be done by adjusting the individual opening lengths x _n (see Fig. 7 (a)) for each flow value in the table (see Fig. 7 (b)). The more the position is distant from x the smaller the correction will be.

[0097] The correction applied to % (position comprised between x _{ and x, ₊₁ ) in order to reach the expected flow rate is Ax. The corrected opening length is x' = x + Ax .

, , ^x i+l ^{— x} i

ax = Am ·

m _i+1 - m,

[0098] Now that the correction Ax for x has been calculated, the position values x _n of the material flow table will be replaced by the corrected values x _n ' . The update is done by using the following formulas: [0099] For x _n values above %

, , &x / '- η ,

x„ = x„ 4 I 1 ; n > i

[00100] For x _n values below x: [00101 ] Limits of calculation:

m.n ^■ — ^■ ¾ — ^■ ma^ x _min = minimum allowable opening length of dosing valve x _max = maximal allowable opening length of dosing valve

Where:

x _n ' : new value in [mm] x _n : old value in [mm]

N : is the total numbers of values in the material flow table n : is the considered position in the material flow table (n = K _± : constant factor to prevent overcorrection {¾_ > 2}

Legend:

1 casting apparatus

1 0 endless conveyor

1 1 casting mould

20 slag runner

21 hot liquid slag

30 dispensing device

31 hopper

32 hopper outlet

33 solid metallic particles, e.g. steel balls

35 sliding gate valve

351 sliding plate

352 aperture in sliding plate

353 stationary plate

354 aperture in stationary plate

355 hydraulic cylinders

36 splash board

A slag pouring zone

B cooling zone

C discharge zone

Shslag sensor for measuring height of the hot liquid slag

Sh _m ix sensor for measuring height of the slag/particles mix

a slope angle of the conveyor in the slag pouring zone A

β slope angle of the conveyor in the cooling zone B

V _a effective maximum filling volume of the mould in the slag pouring zone A v _P effective maximum filling volume of the mould in the cooling zone B

TN-1 temperature sensor for mould position N-1

TN-2 temperature sensor for mould position N-2