Method and apparatus for the production of metal granules

Method and apparatus for the production of metal granules from a molten metal are disclosed. A molten metal stream is directed against an impact element located above the surface of water in a water tank. The impact of the molten metal upon the impact element causes the molten metal to disintegrate into drops which spread out radially from the impact element. The drops fall down into the water below the impact element in an annular region a certain radial distance from the impact element. The radial distance is varied by varying the velocity of the molten metal stream relative to the impact element at the instant of impact, and/or by varying the height of the impact element above the water surface, in order to substantially continuously vary the radius of the annular region in which the molten metal drops hit the water surface. By using the method and apparatus of the present invention it is possible to granulate metals and metal alloys having a low sinking rate in water and a high enthalpy.

TECHNICAL FIELD 
This invention relates to the production of metal granules starting from a 
molten metal which in the form of a stream is caused to fall against an 
impact element provided above the surface of a volume of water in a water 
tank, so that the stream of molten metal by impact against the impact 
element is disintegrated into drops which are spread out in all radial 
directions from the impact element. The drops fall down into the water 
provided beneath the impact element in an annular region at a certain 
radial distance from the impact element, said distance being determined, 
i.a. by the velocity of the stream of molten metal relative to the impact 
element at impact against said impact element and by the height of the 
element above the water surface. The drops of molten metal, as they sink 
towards the bottom of said tank, successively solidify so that said drops 
reach the bottom of the tank in the form of granules which are completely 
solidified or at least solidified on the surface. 
BACKGROUND ART 
U.S. Pat. No. 3,888,956 describes a method of producing metal granules. The 
method of this patent is widely used, particularly for the production of 
crude iron, ferro nickel, ferro chromium, etc. The method has also been 
used for the granulation of ferro silicon. However, certain problems are 
involved in the latter application. One of these problems is due to the 
fact that silicon has a comparatively low density. Moreover, during the 
solidification, pores are formed in the ferro silicon granules, which 
further reduce the effect of gravity upon the granules. The granules 
therefore sink comparatively slowly through the water, with the result 
that the water at the surface of the water is heated more than in the case 
when granulating heavier metals and more homogeneous granules, 
respectively. Further, the heat energy concentration in silicon is very 
high as compared to many other metals and alloys. The enthalpy per unit of 
weight of silicon is for example 2.3 times as high as that of iron. A 
granulation rate of 1000 kg/min of silicon thus, in terms of the amount of 
heat energy that has been drawn off, corresponds to the granulation of 
2300 kg iron/min. 
The combination of the low sinking rate and the high enthalpy of silicon 
and ferro silicon gives rise to very high heat concentrations and the 
formation of steam in the surface layer of the water when using the 
described granulation technique. This problem cannot be solved by 
increasing the intake of cooling water into the water tank, and even heavy 
circulation of the water will only give a minimal improvement. Therefore, 
in order to be able to produce granules with desired shapes and sizes, and 
also to prevent the risk of stream explosions, it is necessary to operate 
with a granulation rate which in many respects is undesirably low for the 
granulation of silicon, ferro silicon and the like. 
BRIEF DISCLOSURE OF THE INVENTION 
It is an object of the present invention to improve the granulation method 
referred to above, in order to make the method more suitable for the 
granulation of silicon, ferro silicon and other comparatively low density 
and/or heavily heat developing metals or metal alloys. 
It is also an object of the invention to make it possible to easily 
increase the granulation capacity of existing plants. 
The fact that the improved method of the present invention is adapted to 
certain requirements particularly relating to the granulation of silicon, 
ferro silicon and other metals, which have a comparatively low density and 
which have a high enthalpy content, does not mean that the method is less 
suitable for the granulation of more "usual" products like iron, ferro 
nickel, nickel, ferro chromium, steel, etc. To the contrary, it is also an 
object of the invention to improve the conditions for the granulation of 
these products as well. Thus any metals (including alloys), which can be 
granulated with an impact element may be used in the practice of the 
present invention. 
These and other objects can be achieved when the velocity of the molten 
metal stream relative to the impact element at the instant of impact 
and/or the height of impact element above the water surface are 
periodically varied in order to substantially continuously vary the radius 
of the annular region within which the majority of the drops hit the water 
surface. 
Further features and aspects of the invention will be apparent from the 
appended claims and from the following description of the preferred 
embodiment of the method and the apparatus, and from calculations for some 
conceived cases.

DESCRIPTION OF PREFERRED EMBODIMENT 
The apparatus which is schematically shown in FIG. 1 comprises a 
cylindrical tank 1 which is filled with a volume of water 2 to a level 3. 
The bottom of the tank is conical and converges downward toward a 
discharge conduit 5 for discharging granules produced together with a 
certain quantity of water. 
Methods known per se can be used to speed up the velocity of the water in 
the discharge conduit in order to obtain a desired elevation of the 
granules, e.g. the method described in British Patent No. 2 030 181 or the 
method described in Swedish Patent No. 7805088-7. Also other methods for 
lifting the granules can be used, e.g. endless elevators such as described 
in U.S. Pat. No. 3,888,956. This part of the system will therefore not be 
described in any detail. A feeding-in conduit for cooling water has been 
designated 7. Surplus water is supplied through this conduit during the 
granulation, so that the water level, in combination with a spillway or 
weir, is maintained at a constant level. 
An impact element 8 is located in the centre of the tank at a height h 
above the water level 3, which height is periodically varied during the 
granulation between a lower position h.sub.e and an upper position h.sub.u 
by means of a motion means 9. 
The impact element or sprayhead 8 consists in a manner known per se of a 
round brick of refractory material. The brick has a flat top and is 
connected with the motion means 9 through a vertical rod 10. The motion 
means 9, according to the preferred embodiment, consists of a hydraulic 
cylinder with a piston in the cylinder connected with the rod 10, which in 
other words defines or is an extension of the piston rod. The hydraulic 
cylinder 9 is provided in a housing 11 which is supported by supports 12. 
The housing 11 can be filled with water. A passage for the rod 10 has been 
designated 13. Conduits 14 for the feeding of hydraulic oil to and from 
the hydraulic cylinder 9 extend through the housing 11 and through the 
bottom part 4 of the water tank. Means 15 for the regulation of the flow 
of oil to and from the hydraulic cylinder 9 are schematically shown. 
A tundish 16 with a chute 17 for supplying molten metal to the tundish 16 
is provided above the impact element/sprayhead/brick 8. A casting hole 18 
is located exactly above the brick 8. The stream of molten metal which 
hits the brick 8 has been designated 19. The total fall of the molten 
metal, in other words the level of the molten metal in the tundish 16 
above the water level 3, has been designated H. 
When the stream of molten metal 19 hits the brick 8, the molten metal is 
disintegrated into drops 20, which are distributed over the surface of the 
water in all radial directions along path-ways which more or less have the 
form of flat parables. If the total fall H and the height h of the brick 8 
above the water level 3 is constant, all the drops 20 will hit the water 
surface 3 within a restricted annular zone at a certain radial distance 
from the brick 8. When the brick 8 is raised at a comparatively high rate 
by means of the hydraulic cylinder 9, the falling speed of the stream 19 
is added to the vertical velocity of the brick 8, so that the impact 
energy and hence the distribution radius of the drops 20 will increase. It 
is realized that certain functional correlations exist between the stroke 
length S of the brick, its end positions h.sub.e and h.sub.u the total 
fall H, the velocity of the brick and the period of the motion. 
CALCULATIONS 
FIGS. 2-11 illustrate five different examples, in which the above mentioned 
functional correlations have been analyzed theoretically. In Table 1, the 
numerical values of the lowest height of the sprayhead 8 above the water 
level, the stroke length, the total fall, the period, and the maximal 
velocity of the sprayhead in the upward direction have been set forth for 
the five cases. 
TABLE 1 
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h.sub.e S H P V max 
Example Figure cm cm cm sec cm/sec 
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1 2 and 7 10 30 100 0.4 125 
2 3 and 8 15 30 100 0.4 125 
3 4 and 9 20 30 100 0.4 125 
4 5 and 10 
10 30 70 0.4 125 
5 6 and 11 
10 70 100 1.0 105 
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h.sub.e : The lowest height of the sprayhead above the water level 
S: The stroke length of sprayhead 
H: The total fall of the molten metal 
P: The period 
V max: The maximal speed of the upward directed motion of the sprayhead 
The graph illustrating the rate of the sprayhead was identical in examples 
1-4. Starting from the speed 0 at the beginning of each period, the upward 
directed movement of the sprayhead 8 was first accelerated, so that the 
speed reached a maximum of 125 cm/sec after a time period of 0.18 second. 
Thereafter the motion was retarded to 0 when the sprayhead 8 reached its 
upper position, when the height h.sub.u above the water level 3 was 40, 
45, 50, and 40 cm, respectively, which occurred after 0.36 sec. At the 
instant when the sprayhead had its highest upward directed speed V max, it 
just passed the first half of its stroke length, which means that the 
height h in the first four examples in this instant was 25, 30, 35, and 25 
cm, respectively. When the sprayhead 8 had reached its highest point--the 
height h.sub.u above the water level 3--the sprayhead was rapidly brought 
back to its starting position with the height h.sub.e =10 cm above the 
water level 3 during the 0.04 second which remain of the period. 
The height h of the sprayhead above the water level 3 expressed in meters, 
its upward directed speed v expressed in meters/sec and the distribution r 
of the granules expressed in meters (mean value of the radial distance 
where the drops hit the water surface) as a function of time during a 
cycle are illustrated in FIGS. 2-6 in the form of the graphs h1, h2 . . . 
h5; v1, v2 . . . v5; and r1, r2 . . . r5 in the five examples, 
respectively. 
In all the examples, the largest distribution, r max, was achieved 
immediately after the instant when the sprayhead 8 had passed half of its 
total stroke length. The smallest distribution in all the examples was 
achieved in the starting position, when the sprayhead 8 was located in its 
lowest position h.sub.e above the water level. 
It is desirable that the drops 20 be distributed substantially evenly over 
the water surface during each cycle of operation, which means that a 
larger amount of drops should land in the outmost annular region, since 
the drops in that region can be distributed over a larger surface than for 
annular regions which are closer to the centre. Moreover, the cooling is 
more efficient in the outer parts, because of the proximity of the 
entrance of cooling water through conduit 7, which also is favourable for 
a more dense distribution of drops of molten metal in the outer regions. 
The best chart of distribution, FIG. 7, was achieved in example 1. In 
examples 2 and 3 the central parts of the tank were not efficiently 
utilized for the granulation. In example 4, when the total fall was lower 
than in the other examples, the peripheral or outer parts of the tank were 
not used, which is not good, since there is excess capacity for a large 
tank. On the other hand, such a distribution may be desirable in those 
cases when there is available only a relatively small tank. This to some 
extent also concerns example 5, where, however, the general character of 
the distribution chart, FIG. 11, approaches closer to the ideal.