Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-06-14
2001-05-01
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C266S088000, C266S228000, C266S233000, C266S236000
Reexamination Certificate
active
06224648
ABSTRACT:
BACKGROUND OF THE INVENTION
This application is a National Stage application that claims the priority of PCT/NL97/00678 filed Dec. 9, 1997 and Netherlands application No. 1004824 filed Dec. 18, 1996.
The invention relates to a method for separating metals and/or metal alloys of different melting points, a mixture of the said metals and/or metal alloys being brought to a temperature at which the metal or the metal alloy of relatively low melting point is molten and the metal or the metal alloy of relatively high melting point is in the solid state, and finally separating the solid constituents from the melt.
The suspension of solid constituents in a melt can be made, on the one hand, by heating a solid substance so that only the low-melting components melt and, on the other hand, by cooling a melt so that a fraction of the melt crystallizes out.
In practice, aluminium is produced using an electrolysis process which uses considerable amounts of energy. Also known is a process as indicated in the introduction in which metals, including aluminium, are refined by fractional crystallization from the melt (cf. journal “Metall” No.
7-8
/95, pp. 491-495). In this process, the crystals and the melt are separated inefficiently, for example by pressing the crystals downwards using a compression means to form a compact mass.
SUMMARY OF THE INVENTION
The object of the invention is to be able to separate melt and solid (unmelted or crystallized) pieces which, depending on the process and the phase diagrams, may be the product stream or the residual stream of the process, extremely efficiently and with a relatively low power consumption.
According to the invention, a washing column having one or more filters which can withstand the temperature of molten metal is used to separate off the solid constituents, each of these filters being arranged in the wall of a separate pipe which extends through the washing column, the mixture being fed to one end of the washing column and moves along the pipe or pipes towards means arranged at the opposite side of the washing column for removing solid crystalline material, the melt passing via the filter or filters into the pipes and flowing, in countercurrent to the mixture situated on the outside of the pipe or pipes, towards a melt discharge, and some of the crystalline material removed being melted and then being conveyed to the bed of crystals as washing liquid, in order to wash the said bed of crystals with its own melt, in counter current.
The method according to the invention is particularly suitable for refining aluminium, for example separating low-alloy aluminium (wrought alloys) and high-alloy aluminium (cast alloy). However, the method may also be used for refining other metals, such as copper, lead, tin and zinc.
It should be noted that Swiss patent 168,784 has disclosed a method for purifying light metals or their alloys. In a vat there is situated a melt, the temperature of which is set in such a manner that the impurities have solidified and the pure metal is discharged in the liquid state through a filter situated in the vat. There is no mention of a washing column, nor of the melt which is discharged via the filter and the mixture situated on the outside of the filter being in countercurrent. Furthermore, this publication does not disclose that part of the removed crystalline material is melted and then fed to a bed of crystals as washing liquid, in order to wash the bed of crystals with its own melt, in countercurrent.
Furthermore, U.S. Pat. No. 4,043,802 has disclosed a method for the continuous purification of impure metals. In this method, a mixture of metal crystals and liquid is situated in a column which has a relatively cool zone and a relatively warm zone. There is a continuous temperature gradient between these zones. The temperature of the cool zone is set in such a manner that there can be both liquid and crystalline metal. The temperature of the warm zone is sufficiently high to melt the crystals. Part of the column in the said cool zone is cooled by the formation of a coherent mass of metal crystals. Incipient melting is then brought about, in order to release crystals in the mass, from the mass in the crystal column, to the liquid metal in the cool zone of the column. The crystals are then conveyed, via the temperature gradient, into the warm zone, where the crystals are melted and the warm zone is enriched with the pure, liquid metal. Here again, there is no mention of a washing column in which filters are arranged in the wall of a separate pipe which extends through the washing column, the mixture is fed to one end of the washing column and moves along the pipe or pipes towards means arranged at the opposite side of the washing column for removing solid crystalline material. Furthermore, this American patent does not disclose that the melt passes via filters into a pipe or pipes and flows, in countercurrent to the mixture situated on the outside of the pipe or pipes, towards a melt discharge. Another new feature compared to this American patent is that part of the removed crystalline material is melted and then fed as washing liquid to the bed of crystals, in order to wash the bed of crystals with its own melt, in countercurrent.
U.S. Pat. No. 4,511,398 describes a method for purifying aluminium which is contaminated with metals, using sodium. In this case, use is made of a separating column, and the temperature range lies between the melting point of the aluminium or the aluminium alloy and the boiling point of sodium. The preheated sodium is introduced into the separating column from a circuit provided with a heat exchanger, means for precipitating solid particles, a pump and a storage tank, in quantities at least five times greater than the mass of the contaminated metal. The sodium flows in countercurrent to the contaminated aluminium, which is likewise introduced into the separating column. The purified aluminium, which contains a small residual quantity of contaminated metal and a sodium content of not more than 500 ppm, is removed from the separating column. Sodium with a low concentration of contaminated metal is returned to the circuit.
Preferably, the crystalline material is removed by a scraping knife.
In order to wash solid particles and to prevent the contaminants from reaching the stream of melt, some of the stream of melt is returned to the column as washing liquid.
This can be achieved by using a valve, which controls the discharge of melt (maybe the product), to regulate the pressure in such a manner that a portion of the melt penetrates into the bed of crystals and washes the said bed in countercurrent. If the flow rate of washing liquid is set correctly, the washing liquid will crystallize out before reaching the filters. On the one hand, this means that there is no loss of washing liquid and, on the other hand, the filters are prevented from freezing up. The level at which the washing liquid crystallizes—the so-called washing front—can be established on the basis of the sharp change in temperature above and below the washing front.
The pipes, each with their own filter, are arranged uniformly over the cross-section of the cylindrical washing column.
It may be advantageous to generate a pulsating stream of liquid in the discharge of the pipe or pipes. This makes it possible, in particular, to reduce the friction along the wall at the level of the filters.
The device which is suitable for carrying out the method comprises, according to the invention, a melting or crystallization vat with temperature-regulating means for keeping the temperature of high-melting and low-melting metals or alloys situated in the said vat at a value between the melting or crystallization temperature of the said high-melting and low-melting metals or alloys, a washing column with one or more pipes, which extend therein, are closed at one end and in part of the wall of which a filter is arranged, forming the only direct connection for the melt between the space outside the pipes and the space inside the pipes, means for guiding
Verdoes Dirk
Visscher Hendrik
King Roy
McGuthry-Banks Tima
Nederlandse Organisatie voor Toegepast-Natuurwetenschappelijk On
Young & Thompson
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