Method for granulating molten metal

Abstract: A method for granulating a stream of molten metal (7) which is caused to fall from a launder or the like down into a liquid cooling bath (2) in a tank (1). The metal stream is divided into droplets in the liquid cooling bath, which solidify and form solid granules. A substantially uniform flow of cooling liquid is caused to flow from one of the side walls in the tank substantially perpendicularly against the falling metal stream, said flow of cooling liquid having a velocity of less than 0.1 m/second. The distance from the outlet of the launder to the surface of the liquid cooling bath is kept less than 100 times the diameter of the metal stream measured as the metal stream leaves the launder.

Description: The present invention relates to a method for production of granules from molten metal forming by cooling and solidifying droplets of the metal in a liquid cooling bath. US patent No. 3888956 discloses a method for the production of granules from a melt, especially from molten iron, where a stream of molten iron is caused to fall against a horizontal, fixed member. Due to its own kinetic energy, the melt is crushed against the member and forms into irregular shaped droplets which move upwards and outwards and fall down into a liquid bath of cooling medium situated below the member. By this known method it is possible to produce metal granules, but the method has a number of drawbacks and disadvantages. It is not possible to control the particle size and the particle size distribution to any significant extent, as the droplets which are formed when the molten metal hits the member will vary from being very small to rather large. When granules are manufactured from ferroalloy melts, such as for example FeCr, FeSi, SiMn, a substantial number of granules with a particle size below 5mm are produced. In the case of ferrosilicon granules, the proportion of particles having a particles size below 5mm is typically in the range of 22 - 35% by weight of the melt granulated and the mean particle size is about 7mm. Ferrosilicon particles having a size below 5mm are undesirable and particles having a particle size below 1mm are even more undesirable since they will remain suspended in the liquid cooling medium which will therefore require continuous cleaning. Swedish patent No. 439783 describes the granulation for example of FeCr by allowing a stream of molten FeCr to fall down into a water-containing bath in which stream is split into granules by means of a concentrated water jet located immediately below the surface of the water bath. This method produces a rather high proportion of small particles. In addition, the risk of explosion is increased due to the possibility of entrapping water inside the molten metal droplets. Due to the very turbulent conditions created by this method of granulation, the number of collisions between the formed granules will be high and this also increases the risk of explosion. US Patent No. 4168967 discloses a method for the production of granules of a composition defined in Claim 1, where a stream of molten alloy falls into a pool of cooling water. The pool is agitated either by mechanical stirring or by injection of a stream of water just below the surface of the pool. This agitation is essential to provide a shearing action which promotes granule formation. As in Swedish Patent No. 439783, the turbulent conditions in the pool caused by the injection of the water stream increases the number of collisions between formed granules and so increases the risk of explosion. It is an object of the present invention to provide an improved method for the granulation of molten metals which makes it possible to overcome the drawbacks and disadvantages of the known methods. According to the present invention, there is provided a method for granulating molten metals in which at least one continuous stream of molten metal is caused to fall down into a liquid cooling bath, cooling liquid is caused to flow laterally in the bath against the falling metal stream in a direction substantially perpendicular to the direction of the falling metal stream, and the metal stream is divided into granules in the bath which solidify, characterised in that the flow of cooling liquid has an average velocity of less than 0.1 m/second and is substantially even, and the cooling liquid has a vertical extent extending from the surface of the liquid cooling bath downwards to a depth where the granules at least have an outer solidified shell and/or has such a horizontal extent that the flow extends on both sides of the metal stream or the metal streams. Preferably, the flow of cooling liquid is caused to flow from one of the sidewalls of the tank substantially perpendicularly against the falling metal stream with an average velocity of preferably less than 0.05 m/second. The vertical distance from the outlet of the launder to the surface of the liquid cooling bath preferably is less than 100 times the diameter of the molten metal stream, measured at the point where the metal stream leaves the launder. It is more preferred to keep this vertical distance of the metal stream between 5 and 30 times the diameter of the metal stream, while especially good results have been obtained by keeping this vertical distance between 10 and 20 times the diameter of the metal stream. By keeping the above mentioned ratios between the vertical distances of the metal stream and the diameter of the metal stream within the above mentioned ranges, it can be ensured that the metal stream will be continuous, even as it hits the surface of the cooling liquid bath. The formation of droplets will then take place within the cooling liquid bath. As a cooling liquid, water is preferably used. In order to stabilise the film vapour which forms about the individual granules in the cooling liquid bath, it is preferred to add up till 500 ppm of tenside to the cooling water. Up to 10% of an anti-freezing agent, such as glycol, can also be added to the water. In order to adjust the pH-value the water, 0 - 5% of NaOH may be added. In order to adjust the surface tension and the viscosity of the water, water soluble oils may be added. When water is used as a cooling liquid, the temperature of the water supplied to the cooling liquid tank preferably is between 5 and 95°C. For the granulation of ferrosilicon, it is especially preferred to supply cooling water having a temperature between 10 and 60°C, as this seems to improve the mechanical properties of the produced granules. When one wishes to produce oxygen free granules, it is preferred to use a liquid hydrocarbon, preferably kerosene, as a cooling liquid. When the metal stream falls into the cooling liquid bath, constrictions will form on the continuous stream of molten metal due to self induced oscillations in the stream. These oscillations cause constrictions which increase with time and finally lead to the formation of droplets. The droplets of molten metal solidify and fall further downwards to the bottom of the tank and are transported out of the tank by means of conventional devices, such as conveyors of pumps. By causing the cooling liquid to flow continually at a low velocity of less than 0.1 m/second, substantially perpendicularly against the falling metal stream while the metal stream is falling downwards in the cooling liquid bath and is divided into droplets, the flow of cooling liquid will have little or not effect on the droplet formation. The falling metal stream will, however, continuously be surrounded by "fresh" cooling liquid, causing the temperature in the cooling liquid bath in the area of the falling metal stream to reach a steady state condition. It is thus an important feature of the present invention that the dividing of the metal stream takes place via self-induced constrictions in the stream. The cooling liquid bath thus does not contribute in the dividing of the metal stream into droplets, but is caused to flow at a low velocity solely for cooling of the metal stream. The method according to the present invention gives a substantially lower risk of explosion than the method of the prior art. The smooth conditions in the cooling liquid bath cause a low frequency of collisions between individual granules and thereby a reduced possibility for collapsing of the vapour layer which is formed about each of the granules during solidification. The method according to the present invention can be used for a plurality of metals and metal alloys such as ferrosilicon with a varying silicon content, manganese, ferromanganese, silicomanganese, chromium, ferrochromium, nickel, iron, silicon and others. By the method according to the present invention it is possibile to obtain a substantial increase in the mean granule size, and a substantial reduction in the percentage of granules having a particle size below 5mm. By using the present invention for 75% ferrosilicon, a mean granule diameter of about 12mm has been obtained and the proportion of granules having a diameter of less than 5mm is typically 10% or less. In laboratory tests, a mean granule diameter of 17mm has been obtained and the proportion of granules having a diameter less than 5mm was in the range of 3 - 4%. The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a vertical section through an apparatus for carrying out a method of granulating a molten metal according to the invention; and Figure 2 is a section along the line II-II of Figure 1. Figures 1 and 2 show a cooling liquid tank 1 filled with a liquid cooling medium 2, for example water. In the tank 1 there is a device in the form of a conveyor 3 for the removal of solidified granules from the tank 1. A tundish 4 for molten metal is arranged at a distance above the level 5 of the cooling liquid in the tank 1. Molten metal is continuously poured from a ladle 6 into the tundish 4. From the tundish 4 a continuous metal stream 7 flows through a defined opening or slit down to the surface 5 of the cooling liquid 2 and falls downwards in the cooling liquid bath while still in the form of a continuous stream. In one of the sidewalls 8 of the tank 1 there is a cooling liquid supply means. The supply 9 has an opening facing tank 1, the opening extending from the surface of the cooling liquid bath 2 downwards in the tank 1 to a level where the produced granules have at least developed an outer layer of solidified metal. Horizontally, the opening in the supply 9 has such an extent that the flow of cooling liquid will extend substantially supplied continuously via a supply pipe 10 to a manifold 11 located within the supply 9. The manifold 11 has a plurality of openings 12. The pressure in the supply pipe 10 is adjusted so that a water flow into the tank 1 is formed having a maximum average velocity of 0.1 m/second. The velocity of the water flow is substantially constant across the cross-section of the opening of the supply 9 in the sidewall 8 of the tank 2. The cooling liquid flowing out of the supply 9 is indicated by arrows in Figures 1 and 2. The metal stream inside the cooling water bath 2 will thus always be surrounded by a smooth flow of "fresh" water from the supply 9. This flow of water has a velocity which is not sufficient to break up the metal stream 7 into droplets. The metal stream 7 will be divided into droplets 13 due to self-induced oscillations which start when the stream 7 falls downwards in the cooling liquid bath. A regular droplet formation is thereby obtained which results in droplets with a substantially even particle size and a small fraction of droplets having a particle size below 5mm. The droplets 13 solidify as they are falling downwards in the cooling liquid bath 2 and are then removed from the bath by means of the conveyor 13 or by other known means. An amount of cooling liquid corresponding to the amount of cooling liquid supplied is removed from the tank 1, via an overflow or by pumping equipment (not shown). The invention will be further illustrated by the following non-limiting examples. Example 1 In a laboratory apparatus, 75% ferrosilicon was granulated in batches of 6.5 kg molten alloy. The apparatus was as described above in connection with Figures 1 and 2. In all the tests, water was used as a cooling liquid. The velocity of the water flow was kept below 0.05 m/second for all the tests. The test conditions and the results are shown in Table 1. Test No. L/D Water temp.°C D50 % <5mm 1 15 8 17 8 2 30 50 15 9 3 70 90 15 10 Example 2 In an industrial plant using an apparatus as described in connection with Figures 1 and 2, batches of 75% FeSi were granulated. Each batch consisted of a minimum of 2 tons of molten alloy. Water was used as the cooling liquid in all the tests. The velocity of the water was kept between 0.01 and 0.03 m/second. The test conditions and the results are shown in Table II. Test No. L/D Water temp.°C D50 % <5mm 4 7 25 12 9 5 15 15 11 10 6 7 40 12 10 The results show that by the method of the present invention for the granulation of ferrosilicon, a substantial increase in the mean granule size was obtained and a reduction of the fraction of granules having a particle size less than 5mm from 22 - 35% to a maximum of 10%. Example 3 In a laboratory apparatus, silicomanganese was granulated in batches of 11kg of molten alloy. The apparatus was as described in connection with Figures 1 and 2. In all tests, water containing varying amounts of glycol was used as a cooling liquid. The velocity of the water flow was kept below 0.05 m/second for all the tests and the temperature of the water supplied was kept at 60°C. The test conditions and the results are shown in Table III Test No. L/D % Glycol D50 % <5mm 1 13 10 11 4 2 8 3.4 10 6 3 13 1 9 12 The results show that for silicomanganese, a main granule size of about 80mm was obtained and that the proportion of granules below 5mm decreases with an increasing amount of glycol in the cooling water.

Claims: A method for granulating molten metals in which at least one continuous stream of molten metal is caused to fall down into a liquid cooling bath, cooling liquid is caused to flow laterally in the bath against the falling metal stream in a direction substantially perpendicular to the direction of the falling metal stream, and the metal stream is divided into granules in the bath which solidify, characterised in that the flow of cooling liquid has an average velocity of less than 0.1 m/second and is substantially even, and the cooling liquid has a vertical extent extending from the surface of the liquid cooling bath downwards to a depth where the granules at least have an outer solidified shell and/or has such a horizontal extent that the flow extends on both sides of the metal stream or the metal streams. A method as claimed in Claim 1, characterised in that the average velocity of the flow of cooling liquid is less than 0.05 m/second. A method as claimed in any preceding Claim, characterised in that the vertical distance from the outlet of a launder from which the molten metal falls to the surface of the liquid cooling bath is less than 100 times the diameter of the molten metal stream measured at the point where the metal stream leaves the launder. A method as claimed in Claim 3, characterised in that the vertical distance of the metal stream is between 5 and 30 times the diameter of the metal stream, preferably between 10 and 20 times the diameter of the metal stream. A method as claimed in any preceding Claim, characterised in that the cooling liquid is water. A method as claimed in Claim 5, characterised in that a tensid is added to the water in an amount of up to 500 ppm. A method as claimed in Claim 5, characterised in that a freezing point reducing agent is added to the water in an amount of 0 - 10%. A method as claimed in Claim 5, characterised in that 0 - 5% NaOH is added to the water. A method as claimed in Claim 5, characterised in that agents are added to the water for modifying the surface tension and the viscosity. A method as claimed in any of Claims 5 to 9, characterised in that the water added to the cooling liquid bath has a temperature between 5 and 95°C, preferably between 10 and 60°C. A method as claimed in any of Claims 1 to 4, characterised in that a liquid hydrocarbon, preferably kerosene, is used as the cooling agent.

Inventors: FORWALD KARL, FOSSHEIM RUNE, KJELLAND TORBJORN, FORWALD, KARL, FOSSHEIM, RUNE, KJELLAND, TORBJORN
Applicants: ELKEM AS, ELKEM ASA
Application year: 1992
Main classification: B22F9
IPC codes: B22F9, B01J2