One of the feedstocks for Conform-type continuous extrusion machines is granular metal pieces, chiefly of aluminium or copper. These granules are used in e.g. a conform machine to form extruded end-products such as wire, rod, bar or special shaped sections.

In order to optimise the quality of the extruded end- products, it is desirable to use a granular feedstock material of acceptable purity. Typical contaminants, particularly for copper granules, include organic components like PVC and polyethylene sheathing from wire insulation. It is also desirable to minimise or avoid metal oxide impurities in the granules which can affect mechanical performance of extruded end-products.

Electrical wire comprising copper conductor, which has been used for various applications is eventually discarded. Because the copper can be retrieved, there is interest in recycling the material, to produce recycled copper wire for re-use. Generally the wire is granulated but there may be up to about 1% by weight of plastics insulation material remaining as contaminants.

The present invention seeks to provide a method and apparatus for treating metal granules, and more particularly a method and apparatus for thermal treatment to remove contaminants from metal granules such as copper granules.

The invention also seeks to provide such apparatus and method which can operate to reduce or minimise oxide impurities and thereby achieve a desirably low content of oxygen in the granules, for example no higher than 600 ppm of oxygen in the treated copper or aluminium granules but preferably less than 400 ppm, and even more preferably less than 300 ppm of oxygen in the treated granules, such as 250 ppm or below.

We have now devised such a method and an apparatus which can be constructed to carry the method into effect, which is based on a combined thermal and cooling treatment with controls to reduce or minimise the level of oxygen within the granules after contaminant removal.

Broadly the present invention in both its aspects is based on vaporising organic contaminants, recrystallising and annealing of the metal granules, minimising oxidation during vaporisation, recrystallisation and annealing, and using an atmosphere which can be non-oxidising or reducing in nature to control i.e. minimise formation of metal oxide.

According to a first aspect of this invention there is provided a method of removing contaminants from granular metal feedstock by heat treatment thereof and subsequent cooling within a gaseous atmosphere which is non-oxidising or reducing and which method comprises:

(i) providing a supply of metal granules,

(ii) causing said granules to be fed along a predetermined path,

(iii) subjecting said granules to a primary heating process wherein the granules are heated to a temperature within a primary temperature range,

(iv) subjecting said granules to secondary heating wherein the granules, after heating by said primary heating process are either maintained within said primary temperature range or are further heated to a temperature within a secondary temperature range,

(v) subjecting the heated granules to a cooling process wherein the heated granules are cooled to a temperature within a third temperature range,

(vi) discharging cooled granules from the cooling means, and

(vii) throughout the heating and cooling processes maintaining the granules within an inert or other non-oxidising and/or reducing atmosphere.

Optionally, after step (vii) and before use of the

treated granules in e.g. a conform processing machine, the granules can be acid treated to remove any metal oxidation which may have occurred before use, also organic and inorganic residues from any plastic contamination on the granules can be removed by such acid treatment. This may be effected by immersion in a weak acid e.g. immersing treated granules in a weak acid bath for 1-5 mins, washing with water if required and drying if required.

In a second aspect the invention provides apparatus for removing contaminants from granular metal feedstock which is constructed to apply heat treatment to said granules and subsequent cooling thereof within an atmosphere comprising inert or other non-oxidising gas and/or reducing gas which comprises:

(a) supply means adapted to supply a feed of metal granules,

(b) a generally hollow container in communication with said supply means, defining a predetermined path along said granules can be fed,

(c) a primary heating means extending over at least part of said container wherein granules within the container can be heated to a temperature within a primary temperature range,

(d) a secondary heating means also extending over at least part of said container wherein the granules which have been heated by said primary heating means can be maintained within or near said primary temperature range or can be further heated to a temperature within a secondary temperature range,

(e) a cooling means extending over part of said container or over an extension thereof in communication with said container wherein the feed of heated granules can be cooled to a temperature within a third temperature range,

(f) means adapted to discharge cooled granules from the container or extension thereof, and

(g) atmospheric control means arranged to maintain an atmosphere within the container and in any extension thereof if present within the region of said cooling means, which comprises inert or other non-oxidising gas and/or reducing gas.

Optionally the apparatus may include an acid bath containing a weak acid solution to be located in the vicinity of the means (f) for discharging treated granules.

The primary temperature range may be 700°C ± 300°C, more preferably ± 200°C most preferably ± 50°C.

The secondary temperature range may be 720°C ± 300°C, more preferably ± 200°C most preferably ± 50°C.

The third temperature range may be below 100°C, preferably below 60°C, more preferably below 50°C, most preferably below 40°C for example 35°C or below.

The supply and feed of metal granules can be continuous in the apparatus and method.

Said contaminants preferably comprise plastics material which is capable of vaporisation within the primary and/or secondary heating means. The feedstock may be aluminium or copper granules, preferably copper granules produced from cable and/or soudronic scrap.

It is preferred to employ an atmosphere which is at least mildly reducing by using a gaseous mixture as the atmosphere which comprises an inert or other non-oxidising gas, such as nitrogen, and a reducing gas such as hydrogen. The mixture may comprise N ₂ and H ₂ in a volumetric ratio of the order 5N ₂ : 1.5 H ₂ - This can be achieved using controlled flow meters delivering N ₂ and H ₂ to the interior of the container (and of any extension part thereof in communication therewith) which is most desirably sealed against the ingress of external air.

The supply means can be a pulsed feed supply hopper terminating at a control valve which communicates via a conduit to the said container. The container may be a

hollow tube, cylinder or drum, having an internal helical thread and which drum or cylinder is rotated by a drive means to ensure that a supply of granules fed at an inlet end thereof is caused to travel along the length of that container, and if present of any extension part thereof.

Such drive means can be a belt-or chain-driven motor adapted to rotate the container continuously from 0.5 rpm to about 10 rpm, but preferably no higher than 2 rpm.

The container is preferably of corrosion resistant material such as nickel- chrome alloy. An extension thereof may be connected by a suitable junction part at that end of the container remote from the supply inlet.

Overall, in use, the granules may spend some 1- 20 preferably 2 - 15 minutes within the primary and second heating means.

The primary and secondary heating means may be a combined furnace which has a hollow internal region within which the container is mounted for rotation. Such a furnace can be heated and controlled electrically, and the drive motor may be electrically operable and controlled for adjusting rotation speed of the container and hence duration of the granules within a given length of the container e.g. residence time within the primary heating means, secondary heating means and within the cooling means.

The cooling means may comprise a jacket having a hollow interior and encasing either a substantial part of the container or alternatively surrounding an extension part thereof. Coolant can be supplied and recycled to coolant spray means situated within the jacket above the said container part or an extension thereof. Coolant may collect, optionally for recycle, in a trough also within the jacket, beneath the container part which is to be cooled by the sprayed coolant.

Means for discharge may be a manifold located at that end of the container (of its extension) remote from the granule supply inlet. It may have a controllable rotary valve discharge outlet.

The gaseous atmosphere for the heating and cooling process may be provided by constructing a gaseous supply

inlet, at the discharge end of the container (or of the extension) to feed in the appropriate non-oxidising and/or reducing atmosphere so as to purge the entire interior of the container and of any extension part of air before operation of the heating and cooling process.

In order that the invention may be illustrated, more easily appreciated and readily carried into effect by those skilled in the art, an embodiment of the present apparatus constructed to put into effect the present method, will now be described by way of non-limiting example only with reference to the accompanying drawing wherein:

Figure 1 is cross sectional view of metal granule treatment apparatus, and

Figure 2 is a side elevation of the section shown in Figure 1, viewed in the direction of the heating means.

As shown in the drawings, a feed hopper 1 supply means has a funnel shaped inlet leading to a rotary control valve 2 which in turn communicates with a supply conduit 3 whose entry point 4 for supplying granules terminates within the inlet end of a hollow rotatable drum 6 having an internal continuous helical groove 7. There is a gas-tight seal 5 between conduit 3 and drum 6. The drum 6 is in communication with an extension drum 9 of similar dimension and with a continuous internal helical groove 7 too. A junction member 8 joins the container 6 to extension part 9 in a gas-tight manner and may provide support for the weight of the combined drum/extension at its location (although not shown) .

The hollow drum 6 is externally surrounded by an electric furnace defining a heating zone 10 and comprising both a primary heating means 11 (not shown separately) and a secondary heating means 12 (not shown separately) or a heat retaining means (not shown separately) . The electric furnace has a hollow interior (see Figure 2 particularly) and the container 6 is located within that hollow interior and mounted for rotation although drive means to initiate and maintain rotation are not illustrated.

Spaced from one end of the furnace is a cooling

zone 13 comprising a metal tank 14, coolant feed pipe 15, coolant sprays 16 adapted to spray coolant such as water from a trough 17 comprising a recyclable supply of coolant liquid.

The tank 14 is also hollow and the extension part 9 of the container is free to rotate therein without contacting the tank. At the outlet end 18 of the extension part 9, a granule collection manifold 19 is provided with a gas-tight seal for collecting the heat treated and subsequently cooled metal granules. The manifold 18 has a controllable rotary outlet valve 20 leading to a discharge or final granule collection point 21 at the end of the entire apparatus remote from the granule supply means 1,2,3. Granules can be collected here or fed directly into conform- type extrusion apparatus, or to 9 specially designed feed system for granules to undergo other contaminant removal steps e.g. removal of metallic contaminants.

Rotation of the drum 6 causes the extension 9 to rotate and granules thereby travel along the internal path of the drum and the extension during which travel they are subjected to the heating and cooling steps in the presence of an inert or of a reducing atmosphere.

In a more specific example, the furnace defining the heating zone 10 is designed to heat treat 1000 kg/hr of granulated Soudronic/Cable scrap at a temperature of 700°C over a cycle of 2 - 5 mins.

Electrical Rating: 150 kW total 2 zones. 415 - 3 - 50 Hz.

Heating Zone: Heat treatment is carried out in an electrically heated drum. the drum is fabricated from heat resistant steel with an internal helix cut into the internal wall. The heating chamber is lined with ceramic fibre of sufficient thickness to give a cold face temperature of 50°C. Heating is by use of silicon carbide heating elements. The elements being arranged in two zones, namely a primary heating means which is a preheat zone with high input to give a rapid heat-up period, and a secondary heating means in the form of a soak zone with lower input to maintain the charge at its treatment temperature.

Three phase multi-tapped transformers are fitted to control the elements.

The pre-heat and soak zones are temperature controlled with separate policemen over temperature controllers.

Cooling Zone: The cooling zone supplies cooled granules from an extension of the heating drum also with a co¬ operating internal helix cut into the internal wall. The cooling drum is fabricated from a corrosion resisting stainless steel. Cooling is assisted by the water sprays situated above the extension. The extension is encased by a steel jacket-like enclosure with a stainless steel tank in the base to collect the cooling water. The water is circulated using an integrated pumping system. The water could pass through a heat exchanger.

Drive: The drum is driven by a chain and sprocket arrangement via a gearbox and variable speed motor.

Charging: The charge is introduced to the furnace by a rotary valve. The valve is fitted with an adjustable vent type metering baffle to control the charge rate. The drive is speed controlled for close control of the charge rate. The valve is fitted with a stainless steel rotor and blade.

Discharging: Discharging is carried out through a rotary valve which operates at a fixed speed.

Atmosphere: A non-oxidising and reducing atmosphere comprising a gaseous mixture of nitrogen and hydrogen atmosphere is used. This is controlled from a panel, containing flow meters, flow control valves and all necessary controls and mixing arrangements (not shown) . A flow alarm controller is fitted with two alarms for high and low flows. The control system is arranged so that nitrogen is introduced before hydrogen, and in the event of a leak developing in the atmosphere supply line (not shown) , the hydrogen is shut down. The atmosphere ratio is altered to suit the charge.

Control: The furnace is controlled by a personal computer linked to all variable parameters such as temperature, cooling systems, drum rotation etc., to give standard operating conditions. All controls can be conveniently installed in a suitable panel and prewired.

Although not shown in the drawing an acid immersion bath can be located adjacent the outlet 21 for acid immersion of the cooled granules, and washing/drying before use in e.g. a conform extrusion machine.

Examples: There now follows in tabular form, details of embodiments 1 to 8, and for comparison purposes the same physical properties seen in a typical batch of fresh standard copper electrical wire.

The table below shows the results obtained from granulating and thermally treating copper cable scrap under varying conditions of cleanliness and usage of atmospheric gas control.