Technical Field

The present invention relates to a method and an apparatus for casting brittle metals, especially ferro alloys and silicon, by rapid cooling directly to saleable size.

Background Art

Ferro alloys and silicon are traditionally cast in rather large blocks which after cooling are mechanically crushed to produce particle sizes which are suitable for sale. When ferro alloys and silicon are used as additives for molten iron and iron alloys and as additives for other molten metals, they are usually added in a unitary size which varies from some tenth of a gram to a few kilograms. The mentioned mechanical crushing of cast blocks of ferro alloys and silicon generates a substantial amount of material having a particle size below the specified saleable size. This amount which normally is referred to as undersize, can amount to 10-15 percent by weight of the produced ferro alloy and silicon, and represents a severe problem for ferro alloys and silicon producers, as the undersized material either has to be remelted or sold at reduced prize. In addition to the undersized material generated during crushing of cast blocks, further undersized material is generated during transport and handling of the crushed ferro alloys and silicon. Finally mechanical crushing and sieving equipment represent a large capital investment, high operating costs and environmental problems.

It has been tried to solve the above mentioned problems by casting ferro alloys directly to saleable size and by using high cooling rates which gives a stronger alloy and thereby a reduced amount of undersized material during crushing. Thus it has been tried to cast ferro alloys in the form of ingots by using conventional casting machines of the kind used for casting pig iron ingots. Such casting machines have, however, the disadvantage that they consume a lot of space and are very costly to buy and install. It has further been found that consumption of casting moulds is very high when such casting machines are used for casting ferro alloys. This kind of casting machines have therefore found a limited use for casting of ferro alloys and silicon.

In Norwegian patent No. 163124 it is disclosed a method and an apparatus for casting ferro alloys where molten ferro alloy is cast directly to a saleable size in cooled copper moulds. Each copper mould comprises two halves which can be opened and closed and which contains a plurality of mould cavities connected to each other

through channels having small cross-section. During casting the two halves of the mould are kept together and molten ferro alloy is poured into the mould cavities. After solidification and cooling to a temperature of 300°C below the solidus temperature, the mould is opened and the cast ingots are removed. The two halves of the mould must thereafter be cooled and closed before a new casting cycle can be started. In the Norwegian patent it is stated that the time needed for each casting cycle is 90 seconds. The method and the apparatus according to Norwegian patent No. 163124 have, however, a number of disadvantages which is the reason why the method has not been put into use. Thus the casting process has to be interrupted during the time needed for cooling of the ferro alloy in the copper mould, opening and emptying of the mould, cooling the mould, closing the mould and moving the mould into position for pouring of ferro alloy. Further, it may be difficult to fill all the mould cavities as molten ferro alloy may solidify in the channels between each of the mould cavities before all the cavities have been filled with ferro alloy.

From Norwegian patent No. 166355 it is known to cast ferrosilicon by using horizontal continuous casting. This method has, however, not found any practical use.

From US patent Nos. 1,255,102 and 1,129,159 it is known to cast soft metals such as lead on cooled casting wheels which are equipped with moulds on their periphery. These casting wheels are used to cast blocks or bars of a rather large size. These moulds have a low ratio between the length and the longest diagonal in the cross- section at the moulds. The methods and apparatus disclosed in these patents are not useful for casting brittle metals like ferro alloys and silicon to a final shape.

Disclosure of Invention

It is an object of the present invention to provide a method and an apparatus for casting molten metals, especially ferro alloys and silicon, whereby rods of the metal or alloy are continuously cast which rods in two dimensions have a size that corresponding to a pre-set final size and which rods easily can be split into final size in the third dimension after casting.

Thus, according to a first aspect, the present invention relates to a method for casting molten metals, especially ferro alloys and silicon, wherein molten metal is continuously supplied from a tundish to a plurality of open moulds arranged along the outer periphery of at least one substantially horizontally arranged casting cylinder which is being rotated about its length axis, which moulds have a cross-section which in two dimensions corresponds to a pre-set final size of the cast metal and where the

length of the moulds is at least ten times longer than the longest diagonal in the cross- section of the moulds, that the metal is solidified in the course of 10 to 25 seconds, and that the solidified metal rods are caused to fall out of the moulds during rotation of the casting cylinder whereafter the rods are cooled to ambient temperature by natural or controlled cooling and split into final size in the third dimension.

According to a preferred embodiment the molten metal is supplied to moulds having a length between 20 and 50 times the length of the longest diagonal in the cross- section of the moulds.

According to another preferred embodiment the molten metal is supplied to one end of the moulds, whereby the moulds are filled with flow of molten metal from one end of the mould.

According to a second aspect, the present invention relates to an apparatus for continuously casting molten metals, especially ferro alloys and silicon, which apparatus comprises a tundish for molten metal, at least one oudet opening for molten metal arranged in one of the sidewalls of the tundish, at least one substantially horizontally arranged casting cylinder which can be rotated about its length axis and which cylinder is equipped with a plurality of elongated moulds on its periphery for receiving molten metal form the tundish, said moulds having a length which is at least 10 times longer man the longest diagonal in the cross-section of the mould.

According to a preferred embodiment the moulds on the periphery of the casting cylinder have cross-sections that in two dimensions correspond to the final size of the metal to be cast. It is further preferred that the moulds have such cross-sections that the longest diagonal in the cross-section is between 20 and 50 mm.

The shape of the cross-section of the moulds is such that the cast rods due to their weight, fall out of the moulds during rotation of the casting cylinder. In order to achieve this it is preferred that the moulds have a substantially V-shaped cross-section with rounded bottom. It is further preferred that the angle towards the horizontal plane is maximum 75°.

According to another preferred embodiment the casting cylinder is equipped with internal channels for circulation of a cooling medium, preferably water.

In order to obtain a rapid solidification of the molten metal supplied to the moulds, the moulds are made from a material having a high thermal conductivity, such as for

example copper or brass. The time for solidification will typically be in the range between 10 and 25 seconds and the total retention time for the metal in the moulds will typically be in the range of 10 to 60 seconds.

By the method and the apparatus according to the present invention ferro alloys and silicon and other brittle metals can be cast directly to a saleable size in a simple and efficient way. The casting process is continuous with a rapid cooling to temperature below the solidus temperature. The rapid solidification gives a homogeneous material with a fine grained structure, which is a good starting point for further heat treatment and/or cooling to room temperature. The metals produced by the present invention are very clean and strong and generate almost no fines during handling. Typical product size is 10 - 30 mm.

The apparatus according to the present invention occupies little space and represents a relatively small investment. Further the capacity is very flexible as the numbers of casting cylinders can be varied. The apparatus according to the present invention can therefore without any problems be installed in existing smelting plants for ferro alloys and silicon.

Brief Description of the Drawings

Figure 1 shows a side view of the apparatus according to the present invention,

Figure 2 shows the apparatus in Figure 1 seen from above,

Figure 3 shows a cut along line I - 1 in Figure 2, and,

Figure 4 shows a cut along line II - II in Figure 3.

Detailed Description of preferred embodiments

In Figures 1 through 3 there are shown a ladle 1 containing molten ferro alloy or silicon. The ladle 1 can be replaced by a holding furnace or by the tapping spout of a smelting furnace for production of ferro alloy or silicon. From the ladle 1 the molten ferro alloy or silicon is poured or bottom tapped into a tundish 2. The tundish 2 shown in Figure 3 is mounted at 3 and can be lifted and lowered by means of an hydraulic or pneumatic cylinder 4 or by an electromechanical device or the like. The tundish 2 can further be equipped with a lid 5 for preventing heat losses and a heating device can be arranged as indicated by reference numeral 6 in order to supply heat to the melt

contained in the tundish 2. The tundish 2 can be rectangular, as shown in Figure 1 and 2, or be built as a circular induction furnace with outlet openings in bottom.

The tundish 2 is equipped with defined outlet openings 7 in its sidewalls or bottom for molten ferro alloy or silicon. On the outside of each outlet opening 7 there is arranged a casting cylinder 8 which will be further described below.

It should be emphasised that the apparatus according to the present invention has at least one outlet opening 7 and at least one casting cylinder 8, but that advantageously more than one outlet opening 7 and more than one casting cylinder 8 are arranged. In the embodiment shown in the drawings it is thus arranged five outlet openings 7 and five casting cylinders 8.

As shown in Figure 3 the casting cylinder 8 is substantially horizontally mounted in one end at 9 on an axis that runs through the length axis of the casting cylinder 8. The casting cylinder 8 is further equipped with conventional means (not shown) for rotation of the casting cylinder 8 about its length axis. At the surface of the casting cylinder 8 there is arranged a plurality of elongated moulds 10 which are parallel and runs in the entire length of the casting cylinder 8. The casting cylinder 8 is mounted in such a way that a vertical plane running through the length axis of the casting cylinder 8 also runs through the centre of one of the outlet openings 7 in the tundish 2. The upper level of the casting cylinder 8 is arranged below the outlet opening 7 in the tundish 2.

The material in which the moulds are made is preferably a material having a high thermal conductivity, such as for example copper, brass, silicon carbide, steel and cast iron. Fu v.er the casting cylinder 8 is equipped with internal channels 15 for circulation of a cooling medium, such as water. The moulds 10 have cross-sections which in two dimension correspond to the final size of the cast product. For example, for casting ferrosilicon the moulds 10 have such cross-sections that the largest diagonal is between 20 and 50 mm. The cross-section of the moulds has such a shape that the solidified metal rods contained in the moulds 10, due to their weight will fall out of the moulds 10 during rotation of the casting cylinder 8. Preferably the cross- section of the moulds 10 is V-shaped with rounded bottom and has an angle towards the horizontal plane of maximum 75°. The length of the moulds 10 and thereby the length of the casting cylinder 8 is at least 10 times, and typically between 20 and 50 times ;• -ater than the longest diagonal in the cross-section of the moulds 10.

Below the casting cylinder 8 there is arranged a conveyor belt 11 or other suitable means for transporting cast rods as they fall out of the moulds 10 during rotation of the casting cylinder 8. When the metal rods are falling down on the conveyor 11, the rods will partly break crosswise up in several pieces. Along the conveyor belt 11 there can be arranged a zone 16 for consolidating the temperature of the cast rods and a zone 13 for final cooling of the cast rods. Natural and forced cooling with air has been found to give a very strong final product of ferrosilicon and manganese alloys. In the end of the conveyor belt 1 1 there is preferably arranged one or more terminals 14 where the cast rods are allowed to drop in order to split the rods into shorter pieces, typical with length 0.5 to 3 times larger cross section of cast metal rods. The terminal 14 can be a rotating cylinder with sieve, where the metal pieces breakes up crosswise and are thumbeled in such a way that the metal do not generate more fines during additional handling and transportation.

In order to prevent spillage of molten ferro alloy or silicon when the casting cylinder 10 is being rotated to bring a new mould into position below the outlet opening 7, it is, as shown in Figure 4, preferably arranged knife-formed edges between each of the moulds 10.

By use of die apparatus according to the present invention the tundish 2 is filled with molten ferro alloy or molten silicon while one of the moulds 10 on the periphery of die casting cylinder 8 is being kept in position below the outlet opening 7 in the tundish 2. When the level of the melt in the tundish 2 reaches the outlet opening 7, molten metal or alloy will start to flow through the oudet opening 7 and down into the mould 10. When the first mould 10 has been filled, the casting cylinder 8 is rotated in order to bring the next mould 10 into position below the outlet opening 7. This sequence is then continued . The diameter of the casting cylinder 8 is such that when the first mould 10 that was filled with molten metal or alloy has been rotated to such a position that the metal or alloy contained in that mould falls out of the mould due to gravity, the metal or alloy in that mould has been cooled to a temperature below the solidus temperature and formed a solid rod.

The casting cylinder 8, and thereby the moulds 10, are continuously cooled during rotation of casting cylinder 8. The cast rods fall down onto the conveyor belt 11 and are cooled to ambient temperature by natural or controlled cooling, for example by air fans. Finally the cooled rods are split into final size in the third dimension by subjecting the rods to free fall form defined heights or the rods being thumbeled in a rotating cylinder.