20 ton / day Pilot Plant
(5.6 MVA DC-arc facility)

20 ton / day facility consists of:

Feed system (0.1 to 3.0 t/h)
DC furnace (5.0 kA max) 'premelter'
DC furnace (10.0 kA max) 'smelter'
Lead splash condenser (350 kg Zn/h max)
Off-gas handling plant (25 000 Nm³/h)
Power supply 2 x 2.8 MVA transformer/rectifiers
Control, instrumentation and data logging systems

20 ton / day pilot plant facility

Description of the pilot plant

Furnace

Both DC-arc furnaces consist of a refractory-lined cylindrical shell, a conical roof and a flat roof positioned on top of the conical roof. The shell is lined with refractory bricks. The hearth consists of a bottom layer of crushed refractory and a top layer of ramming material. The roofs are lined with an alumina castable refractory. The furnace is equipped with water-spray cooling on the sidewalls and the roofs with water cooling jackets.

The two furnaces, the premelter and fuming furnace, can be connected via a launder and a underflow weir, for continuous transfer of hot slag. The fuming furnace can connected to the lead splash condenser via a refractory-lined duct. The off-gas from the fuming furnace can also be re-routed directly to the combustion chamber via water-cooled ducting. This is done for non-condensing testwork. The off-gas ports for both furnaces are located in the conical roofs.

Electrodes

Graphite electrodes with an outer diameter of 200 mm are used as cathodes on both furnaces. The electrodes enter the furnaces through the central ports in the flat roofs. These electrode ports consist of refractory-lined electrode seals to ensure that there is no electrical contact between the electrodes and the roofs. These seals also provide gas-tight seals around the electrodes. The anodes consists of numerous steel bars, built into the hearth refractory and connected at their lower ends to steel plates. The steel plates are connected, via radially extending arms, to the furnace shells and then to the anode cables.

Feed system

The feed system consists of several units:

the batching plant,
two final feed hoppers to the fuming furnace,
the premelter feed hopper, and
a dust feeder.

The batching plant comprises of eight storage bins (hoppers) varying in size from 0.1 to 1.5 m³, and eight dedicated horizontal belt feeders located under the bins. The belt feeders deliver material to a common conveyor belt, which elevates and discharges the feed via a splitter into either the premelter feeder or a surge hopper.

Feed material can be fed into the furnace through the centre of the electrode using a telescopic pipe or through the feed ports located in the flat or conical roof. One or both the final feeders can be used to supply material to the furnace. The final feeders only differ in capacity and feed rate capabilities. The first can deliver feed material at a rate of up to 3.0 t/h, and the second at a rate of up to 500 kg/h. Both feeders are equipped with surge bins, which deliver the feed material via pneumatically actuated slide valves (to provide gas seals). Screw feeders are used to supply the material to the furnace.

The feeder to the premelter consists of a hopper supplied with feed material directly from the batching plant via the conveyor belt. The feed material is supplied to the premelter, using a screw feeder, through the feed port in the flat roof. The feeder can deliver material at a rate of up to 1.5 t/h.

A dedicated dust feeder can also be used to supply the furnace with material either through the centre of the electrode or through the feed ports in the roof. This feeder is especially designed to handle fine feed materials (dust). It consists of a 1.5 m³ hopper with a bin activator attached to the lower portion of the conical section. The bin activator is essentially a vibrating cone suspended across the mouth of the hopper by means of hanger links with rubber anti-vibration mountings. An externally mounted motor vibrates the material being handled via the internal cone. A flexible rubber sleeve isolates the screw feeder from the bin activator, and prevents material from escaping. A vibrator is attached the screw feeder to minimize bridging of material between the screw feeder and the hopper. A rotary valve is connected between the screw feeder and the feed pipe in order to provide a gas seal.

ISP lead splash condenser

The leadsplash condenser is a smaller version of the ISP condensers that are employed, at several plants world-wide, in combination with an Imperial Smelting Furnace (ISF) for the smelting of mixed zinc and lead sinter. The condenser assembly consists of the condenser body with a single rotor, the lead circulation pump, the cooling launder with immersible cooling pins, and the zinc separation bath. The condenser body is basically a steel box, about 2 m wide, 4 m long and 2 m high, lined with refractory bricks. A variable-speed rotor is inserted through the cast refractory roof. The whole assembly occupies a space of approximately 5 m by 5 m The condenser contains 21 t (metric ton, 1000 kg) of lead, and is designed for a maximum zinc condensing rate of 350 kg/h.

Schematic presentation of the lead splash condenser

Power supply

The 5.6 MVA power supply consists of two transformers and two rectifiers, so that the facility can be operated with a single furnace at a maximum of 10 kA, or with two furnaces at 5 kA per furnace.

Control and data acquisition

The DC-arc furnaces are fully controlled using a PC-based SCADA (Supervisory Control And Data Acquisition) system, which provides a console for the configuration of operating parameters, recipe management, alarm annunciation, and data logging. Manual or semi-automatic operation is also available.

Gas handling

The off-gas system behind the condenser comprises a refractory-lined combustion chamber, water-cooled ducting, a 'trombone' gas cooler, a reverse-pulse bagfilter, a fan and a stack. The designed gas flowrate of the bagfilter is 25 000 Nm³/h. A second bagfilter with a fan capacity of 6 500 Nm³/h is connected to the premelter.

Tapping slag from the fuming furnace

Contact Information
Pyrometallurgy Division, Mintek,
200 Hans Strijdom Drive, Randburg, 2125, South Africa
Private Bag X3015, Randburg, 2125, South Africa.

Phone: +27 (11) 709-4642
Fax: +27 (11) 793-6241

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20 ton / day Pilot Plant (5.6 MVA DC-arc facility)