Metallurgical apparatus – Means for melting or vaporizing metal or treating liquefied... – By separating metal in a molten mass from undesired...
Reexamination Certificate
1999-11-11
2001-09-04
Andrews, Melvyn (Department: 1742)
Metallurgical apparatus
Means for melting or vaporizing metal or treating liquefied...
By separating metal in a molten mass from undesired...
C420S590000
Reexamination Certificate
active
06284190
ABSTRACT:
BACKGROUND TO THE INVENTION
1. Field of the Invention
This invention relates to the recovery of metal from dross.
2. Description of the Prior Art
It is well known that when many molten metals are in contact with the atmosphere, compounds of those metals, primarily oxides, are formed. Especially when there is some movement of the molten metal, the metal and the metal oxide combine to form a material known as dross. Dross consists of droplets of the molten metal which are encrusted with the oxide which forms a sponge-like network. The dross of lead-tin solder, for example, appears like rather fibrous demerara sugar. The dross floats, due to surface tension and/or buoyancy, on the molten metal or sticks to the container holding the metal.
In many industrial processes where dross is formed, it is necessary to remove the dross from time to time. For example, in a wave soldering machine, solder in a bath is pumped to create a static wave in the surface of the solder, and printed circuit boards are fed across the crest of the wave so that the solder kisses the leads of the components and the tracks of the circuit board. If the dross is allowed to build up, it can become entrained in the wave and adversely affect the quality of the soldering, causing low product yields.
The dross could simply be ladled out from the molten metal and disposed of. However, the metal content of the dross can be high and typically, according to one estimate, is in the range of 30 to 90%. The cost of disposing of the raw dross and replacing it with “clean” metal can be a significant.
Attempts have been made in the past to recover the metal content of the dross. For example, patent documents U.S. Pat. No. 4,119,136 and U.S. Pat. No. 4,334,664 describe the recovery of tin from tin dross by draining the tin out at high temperature. The recovery of aluminium from aluminium dross is described in, for example: U.S. Pat. No. 4,772,320 (squeezing hot dross between rollers under pressure); U.S. Pat. No. Re. 31,028 (rolling and milling of cold dross); U.S. Pat. No. 4,394,978 (grinding and screening of cold dross); and U.S. Pat. No. 4,386,956, U.S. Pat. No. 4,540,163, U.S. Pat. No. 4,565,572, WO-A-82/01,895 and WO-A-84/03,719 (squeezing of hot dross between a ram or piston and a trough or cylinder). The recovery of lead-tin alloy from solder dross is described in WO-A-95/25,823, and this also involves squeezing the hot dross with a piston in a cylinder. These latter “hot-squeeze” techniques require the use of a large and substantial machine in order to apply the required pressure, and this in itself is expensive. In existing soldering production lines, there is often not much spare space near the soldering bath, and therefore it may be necessary to site such a large machine away from the soldering bath, as a result of which the dross needs to be reheated before it can be squeezed. Also, the hot-squeeze machines, as exemplified by WO-A-95/25,823, operate on an essentially five-phase cycle: (1) adding the dross to the machine; (2) raising the dross to the required temperature; (3) squeezing the dross; (4) allowing the metal to drain from the dross; and (5) removing the oxide. (Also, it may periodically be necessary to clean a grille or sieve through which the recovered metal or oxide passes.) Accordingly, dross cannot be added to the machine as and when required, only during phase “1” of the cycle.
OBJECT AND SUMMARY OF THE INVENTION
The aims of the present invention, or at least of some specific embodiments of it, are: to enable the recovery of a large or significant proportion of the metal from the dross; to do so without requiring the exertion of high pressures and therefore obviating the need for a large and substantial machine so that there is greater flexibility in the siting of the machine near the source of the dross; and to enable dross to be added as and when required, without needing to wait for a particular phase of operation (subject, of course, to the machine not becoming overloaded).
In accordance with a first aspect of the present invention, there is provided an apparatus for recovering metal from dross, the apparatus comprising: a chamber, means to allow dross to be placed in the chamber, an agitating member mounted in the chamber, means for driving the agitating member so that it agitates the dross in the chamber so as to release the metal from the oxide of the dross, and means to allow the released metal and the oxide to be removed from the chamber.
It has been found that agitating the dross is effective in separating the metal from the oxide. It would appear that it breaks down the sponge-like network of the oxide which encrusts the metal by “knocking it about a lot”, rather than by crushing or squeezing it, as in the prior art.
In order to increase the effectiveness of the apparatus, there is preferably a plurality of such agitating members each arranged to be driven by the driving means.
For mechanical simplicity, the driving means is preferably operable to rotate the, or each, agitating member in the chamber, and, in one embodiment, the driving means includes a rotary shaft extending through the chamber (and driven for example by an electric motor), and the, or each, agitating member is provided by a vane on the shaft.
The, or each, vane is preferably inclined relative to the axis of the shaft so as to urge the dross, metal and oxide in the direction of the shaft. The vaned shaft therefore acts not only to break down the dross but also to feed the material through the chamber so that a continuous, or quasi-continuous, operation can be achieved.
The agitating members or vanes are, for reasons of constructional simplicity and high effectiveness, preferably arranged in groups spaced apart along the shaft, the vanes in each group being distributed around the shaft.
The axis of the shaft is preferably generally horizontal.
Also, the chamber is preferably provided by a tubular member, with the means to allow dross to be placed in the chamber and the means to allow metal and oxide to be removed from the chamber being provided by inlet and outlet openings in the tubular member spaced apart therealong. In the case where the vane or vanes are mounted on a shaft, its axis preferably extends longitudinally of the tubular member. When various of the above features are provided in a single apparatus, dross can simply be ladled into the inlet opening, whereupon it is broken down into metal and oxide by, and fed along the tubular member by, the rotating vaned shaft, so that metal and oxide comes out of the outlet opening.
The apparatus preferably further includes means for heating the tubular member, preferably also with thermostatic control.
The apparatus also preferably further includes a receptacle for receiving metal and oxide which has been removed from the chamber, and means for separating out an oxide layer from the metal on which the oxide floats in the receptacle. Preferably, the receptacle has a first compartment for receiving the metal and oxide, a second compartment, and a passageway between the two compartments and below the level of the oxide layer in the first compartment, so that the metal can flow from the first compartment to the second compartment. The second compartment preferably has a weir over which the metal can flow into a metal collection container, and the first compartment preferably has a weir over which oxide can pass into an oxide collection container. The apparatus preferably further includes a member which is drivable to urge the oxide towards the oxide weir, and/or a member which is drivable to cause vertical stirring between the oxide layer and the metal. It has been found that a screw rotatable about its axis can be employed to perform both of these functions. The apparatus preferably further includes means for introducing air bubbles into the metal in the receptacle, so that the air bubbles rising from the metal aerate the oxide layer on the metal. The aeration of the oxide makes it more fluid so that it can be more easily be moved toward the oxide weir, and also makes it les
van der Lugt Nicholas Stuart
Willmore John
Andrews Melvyn
Sheridan Rose P.C.
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