GB2197880A - Copper recovery from copper/lead alloys - Google Patents

Abstract

Copper/lead alloy is treated to give a copper-rich solid product by dipping a cooled surface 3 into the molten alloy and recovering a solid of enhanced copper content. Various preferred procedures and forms of apparatus are described. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THE RECOVERY OF COPPER FROM COPPER/LEAD ALLOYS This invention relates to the recovery of copper from copper/lead alloys produced from primary or secondary pyrometallurgical processes. Examples of primary processes are zinc/lead blast furnaces, lead blast furnaces, plasma furnaces, flash smelting furnaces, cyclone furnaces, top lance furnaces etc. Examples of secondary processes are reverberatory. furnaces, Peirce-Smith converters, short rotary furnaces etc.

The alloy to be treated may contain from 2% up to 80% by weight, of copper, with the balance substantially lead, but possibly containing up to 5% other metals e.g. tin or antimony.

This invention has as its object a method of removing an impure, but upgraded, copper from a copper/lead alloy melt of variable composition.

This invention consists in a method of recovering copper-rich products from a molten copper/lead alloy, characterised in that the molten alloy is subjected to controlled cooling by heat extraction through a cooled solid surface, either at a free surface of the molten alloy or immersed in the molten alloy, and a copper rich phase is removed, by solidification on the cooled surface.

The alloy composition and phase is defined by the phase diagram appended (as Fig.l). Initial composition between 13 and 64% copper at temperatures above 9900C will produce two melts on cooling through the range down to 9540C and the proportions of the two melts will depend upon the initial alloy composition as shown in Table 1.

Initial compositions below 13% copper will produce crystalline copper and decopperized lead alloy on cooling through the range 954 to 3270C.

Initial composition above 64% copper will produce crystalline copper and copper-lead alloy on cooling from the melting point of copper at 10830C. Ultimately at 9540C a monotectic of 64% copper remains in proportions determined by the initial composition. Further heat extraction from the monotectic at 9540C produces crystalline copper and 13% copper alloy until no further monotectic remains where upon the single melt will cool down to 3270C depositing copper crystals as described above.

Suitable apparatus for exercise of the invention consists of a device with a cooled surface or surfaces either touching the surface of the melt and/or immersed within the melt. Heat is extracted from the melt, as far as possible, solely through the cooled surface. The walls containing the melt are insulated or heated to prevent deposition of solid coppery phases.

In a first embodiment of the invention the cooling surface is a horizontal plate incorporating an internal cooling system. This plate is placed in contact with the surface of the melt whereupon, depending on the rate of cooling, melt composition in contact and movement of cooling surface, an enriched metallic copper deposit forms on the cooling surface.

For example, when in this first embodiment the initial copper/lead alloy is between 13 and 64% copper and is above about 9900C, two melts will form on cooling with the upper melt being enriched with copper and floating on the lower denuded layer. The cooling surface will then be in contact with the upper melt containing about 64% copper. If cooling is rapid the upper melt will solidify on to the cooling surface without time for lead to be expressed. The ultimate copper concentration in this case will be around 64% or similar to that which would be obtained by decanting the phases.

At slower cooling rates, crystalline copper will form on the cooling surface and 13%, or less, copper alloy will be expressed into the bulk of the upper melt at around 9540C. The purity of copper formed depends on this expression of 13% or less copper alloy to avoid inclusion of lead in the crystalline deposit. Some inclusion of lead will be unavoidable but considerable improvement in copper product grade can be achieved as shown in Table 2.

Table 1 Effect of initial melt composition on upper melt at 954"C.

Final composition of monotectic Initial melt Upper melt layer at 9540 C containing composition 64% copper % Cu Copper distribn lead distribn volume fraction 20 44.2 6.0 0.15 30 71.3 17.1 0.36 40 84.9 31.8 0.56 50 93.0 52.4 0.75 60 98.4 83.3 0.93 Table 2 Effect of lead expression on copper content of product from monotectic laver Lead expressed % Cu in product 0 64 20 69 40 75 60 82 80 90 As a further example, when in this first embodiment the initial copper/lead alloy is below 13% copper either as fresh feed or as a result of the above explained treatment of alloy greater than 13% copper, then crystalline copper will precipitate from the alloy on to the cooling surface. The cooling surface may be the same surface as that described in the first example with the process of precipitation continuing after the 64% copper monotectic has disappeared. In this example however, expression of lead is more important and defines the point at which the deposition process begins to degrade the product unacceptably.

In a second embodiment of the invention the cooling surface or surfaces are immersed in the melt in the form of discs, pins, cylinders, cones or other shapes. The cooling system can be internal within the cooling body or external by withdrawal and re-introduction of the cooling body into the melt.

The same wide range of initial alloy compositions can be treated by this second embodiment as described above for the first embodiment.

The advantages of immersed cooling surfaces include the greater cooling surface area that can be incorporated per unit volume of melt and the reduced distance the copper alloy needs to travel to the cooling surface.

In a second aspect the invention consists in apparatus for recovering copper-rich products from molten copper/lead alloys, comprising an insulated container for molten alloy, an internally-cooled shell, for collecting solid copper-rich products and means for feeding molten alloy to cooled surfaces of the shell and for removing molten alloy from the container.

A specific example of operation according to this second embodiment, will now be described, with reference to Figure 2 which is a sectional sketch of an apparatus suitable for operation of the invention.

A continuous stream of copper/ lead alloy, at a temperature at which copper-rich phases are liquid, is caused to flow from a runner (1) into a chamber (2) containing a immersed cooled surface in the form of a hollow frusto-conical shell (3). The cooled shell is held in the chamber (2) containing copper lead alloy which is fed from runner (1) into the interior of the shell and caused to flow under the lower rim (4) of the shell and into the annular body of molten alloy in chamber (2), where it rises and leaves via an overflow weir (5).

The shell may be of various geometrical shapes or forms but in the simplest embodiments it consists of an open-ended cylinder or open-ended frustum of a cone which is provided with internal cooling passages for heat-transfer media. Mechanical stirrers or mixers (6) can be incorporated ir.o cooling chamber (2) to provide effective alloy flow patterns and also to dilute incoming alloy with lower copper-content alloy from lower in the chamber.

The incoming copper/lead alloy is cooled by the inner walls (3a) of the shell (3) and high-copper-content material is deposited on those walls, with expression of lead, as described previously. The copper/lead alloy becomes progressively more dense as it deposits copper and cools. Thus it sinks gradually through the interior of the shell (3) until at the bottom it underflows into the annular body of alloy in chamber (2) qt a temperature sufficiently low to avoid difficulties of coppery accretion characteristic of higher copper content operation. This low copper/lead alloy can then be passed from the alloy chamber to a further decopperizing stage or possibly to a continuous lead refinery e.g.

as developed by the British Non Ferrous Metals Research Association (see European Patent Specifications 0042,296 and 0099,711). Alternatively it can be cast and sold as a lead bullion.

The crude copper product deposited on the inner walls of shell (3) is allowed to build-up until either the interior of the shell becomes restricted in flow area or because heat flux has been reduced to an unacceptable level, at which point shell (3) is lifted out of container (2) and replaced by a fresh shell so as to maintain continuous operation.

Deposition of copper on the outer surface of shell (3) is reduced or prevented by insulation (3b). The tendency for copper to deposit, on the inner surfaces of chamber (2), from the annular body of copper/lead alloy is reduced by internal insulation (2a) on the walls of the chamber (2).

The deposit on the removed shell is allowed to cool and will separate from the shell by differential contraction at which point the high copper mass can be removed, especially so in the case of a conic section, whereby the keying of deposit to the inside wall of the shell is easily broken. The shell is then ready for re-insertion into the alloy container for further deposition.

The correct degree of cooling is important in all embodiments to ensure that excessive lead entrainment is avoided unless removing bulk material of near-monotectic composition. Internal or external cooling methods are proposed to control the temperature driving force.

The internal cooling method will be a system of indirect heat transfer to a heat transfer fluid circulating in passages within the cooling elements. Suitable heat transfer fluids are gases, fused salts, vaporizing liquids, oils etc.

The external cooling method will be a system whereby the cooling surfaces are removed periodically from the melt to be cooled by a heat sink. Suitable heat sinks are gas streams, vaporizing liquid sprays, radiation panels etc. A suitable system could be partially submerged cooling discs rotating at constant velocity so that heat gained in passing through the melt is removed by the heat sink out of the melt.

All methods will require a system of coppery product removal. In the case of cooling elements with indirect cooling these will be withdrawn periodically to strip product from the cooling surface. In the case of cooling elements with direct cooling3 the product will be stripped from the cooling surface during the cooling cycle in the form of ribbon or flakes.

It is a feature of operation according second embodiment of the to the invention that the bulk melt circulates past the cooling surfaces to deposit product copper-rich material. This may be achieved by natural convection within the melt or it may be enhanced by mechanical or eletromagnectic assistance. For example, an induction heated melt will set up pronounced flow circulation.

To avoid excessive deposition of solid material on the walls of the melt-containing vessel the walls may be well insulated or the melt may be heated for example by the electrical induction principle whereby the melt near the walls is heated above a temperature at which deposition occurs.

The invention will now be further illustrated by reference to the following practical example of a copper plating operation.

EXAMPLE: Copper was added to batches of molten lead in a crucible and heated to form a series of homogenous copper/lead melts, containing 25% w/w of copper.

The crucible was then allowed to cool slowly to a temperature within the range 990 to 9300C. At this stage a cooling surface, preheated to above 5000C, was introduced at, or below, the surface of the melt and a deposit allowed to form for ten minutes. The cooling surface was then removed from the liquid and, after draining liquid, the adherent deposit was removed.

Cooling surfaces consisting of forced-air-cooled copper tube, copper plate and finally an 80mm diameter by 10mum thick mild steel disc were investigated. The latter two provided cooling by radiation and convention from their top surfaces and by conduction up the central rod by which they were held.

For the case of the steel disc where cooling was not so rapid as for the copper surfaces, the following results were obtained.

TABLE 3 Test Melt Temperature % Input Copper % Copper (W/W) CC Removed in deposit 1 968 - 955 45 77 2 938 - 930 53 87 3 940 - 935 - 71 These tests demonstrated that, with the correct cooling rate, casts of crude copper can be produced which are substantially upgraded from the monotectic composition (64% Cu, 36% Pb). Deposition rates of crude copper from these tests also appear favourable, with test 2 having a mean specific plating rate of 1.08 tonnes/(h.m.2) over the 10 minutes of the test.

Examination of the microstructures of the deposits clearly showed directional effects with copper crystals growing away from the nucleating surface and lead metal being periodically entrapped in fibrous or sheet form. The lower temperature gradient and more difficult nucleation conditions of the steel collecting surface are believed responsible for the observed coarser copper structures and linear growth of the deposits compared to the copper collecting surface deposits.

Claims (13)

CLAIMS:

1. A method of recovering copper-rich products from a molten copper/lead alloy characterized in that a body of molten alloy is subjected to controlled cooling by heat extraction through a cooled solid surface, either at a free surface of the molten alloy or immersed in the molten alloy, and a copper-rich phase is removed by solidification on the cooled surface.

2. A method as claimed in claims wherein the cooled surface is in the form of a horizontal plate which is placed in contact with the molten alloy.

3. A method s claimed in claims 1 or 2 wherein the body incorporating the cooled surface has internal passages adapted to carry a heat-transfer fluid.

4. A method as claimed in claims 1 or 3 wherein the body is an open-ended cylinder or open-ended frustum of a cone.

5. A method as claimed in claim 1 wherein the body incorporating the cooled surface is rotated within the molten alloy.

6. A method as claimed in any one claims 1, 2, 3 or 4 wherein the body incorporating the cooled surface is static and the molten alloy is caused to flow past the cooled surface.

7. A method as claimed in claim 4 wherein molten alloy is fed to cooled interior walls of the cylinder or frusto-conical body.

8. Apparatus for recovering copper-rich products from molten copper/lead alloys, comprising an insulated container for molten alloy, an internally-cooled shell, for collecting solid copper-rich products and means for feeding molten alloy to cooled surfaces of the shell and for removing molten alloy from the container.

9. Apparatus as claimed in claim 8 wherein the shell is cylindrical or frusto-conical.

10. Apparatus as claimed in claims 8 or 9 wherein the shell incorporates outer surfaces which are partially insulated.

11. Apparatus as claimed in claims 8, 9 or 10 wherein the container is provided with a stirrer or circulator which is adapted to stir the alloy within the shell.

12. Apparatus for the recovery of copper-rich products from molten copper/lead alloys, substantially as hereinbefore described, with reference to Figure 2.

13. A method for the recovery of copper-rich product from a molten copper/lead alloy substantially as hereinbefore described with reference to the Example.