The smelting of zinc concentrates has traditionally been carried out in a roast-reduction two stage process in which a roasting stage produces a solid calcine or sinter which is then fed to a reduction stage in which zinc is produced as a vapour and collected in a condenser.

In one method these operations were carried out completely in the solid state in externally heated - retorts. Coke acted as the reductant and, in a continuous version of the process, zinc was collected in a liquid zinc splash condenser. This process was thermally inefficient because of the indirect heating of the calcine and because the maximum throughput of a unit was small owing to the need to provide for small heating paths for reasonably rapid heat transfer. For these reasons the process became uneconomic compared with the alternative technologies for both smelting and for electrolytic zinc production. The process is now seldom used.

The Imperial Smelting Process (ISP) uses a blast furnace to smelt zinc sinter. Coke is used as both fuel and reductant in this process, and the gases pass into a lead splash condenser where zinc is collected at a low concentration in a stream of lead recirculating through a cooler where a zinc rich phase is ^• collected. This process suffers from the need to use expensive coke as both fuel and reductant and the feed materials and air must be preheated to high temperatures in order to ensure that reaction gases

remain at high temperatures to avoid back oxidation of Zn to ZnO before reaching the condenser. A very large moving grate sinter machine is required to ensure effectively complete sulphur elimination from sinter and this imposes the need for mechanical maintenance as well as providing difficult conditions for capture of all of the roaster gases. In spite of these problems the ISP remains commercially viable because the reactor allows for ready collection of a wide range of other metals in a bullion product. The ISP has become an acceptable and economic means for producing both zinc and lead from complex materials.

It is an object of the present invention to overcome the aforementioned problems of the ISP and retort processes.

In one aspect the present invention provides a novel apparatus comprising a furnace for smelting zinc sulphide material having a liquid slag layer and a gas space, and including an upwardly extending, preferably substantially vertical, fluid-cooled wall which extends into the slag layer and divides the gas space into a first and a second zone; each zone- having at least one top submerged lance extending downwardly, preferably substantially vertically, through the gas space, into the slag layer therein; and at least one flue gas offtake from each zone; the furnace being adapted to enable circulation and mixing of the slag between the two zones.

The invention also provides a process for recovery of zinc from sulphide materials characterized by

(a) establishing a slag layer in a furnace;

(b) introducing zinc sulphide material together with an oxidizing gas into a first of two laterally adjacent zones

formed by a gas space of the furnace being divided by an upwardly extending fluid-cooled wall which extends into the slag layer, thereby producing zinc oxide dissolved in the slag

4 and oxides of sulphur;

(c) removing oxides of sulphur with flue gases from the first

-1 zone;

(d) maintaining conditions such that circulation and mixing of the slag occurs between the two zones;

(e) introducing a reductant into the second zone whereby zinc 10 oxide in the slag is reduced to zinc metal vapour, and

(f) removing zinc metal vapour from the second zone with reducing flue gases.

It is essential that circulation and mixing of the slag occurs between the two zones, as this transfers zinc oxide from the first zone to the second zone. This is permitted by the fluid-cooled wall not closing off a basal portion of the furnace, below the slag layer, .-such as by that wall having a lower edge spaced from the furnace base.

The oxidizing gas injected into the slag in the 20 first zone through, at least one top submerged lance, may be air, air enriched with oxygen, or oxygen. Fuel, for example coal or natural gas may be injected together with the oxidizing gas.

Reductant, for example coal or natural gas, is preferably injected into the slag in the second zone through at least one top submerged lance, together with a carrier gas. The carrier gas is preferably an oxidizing gas which may be for example air, air enriched with oxygen or oxygen.

Zinc metal vapour removed from the second zone may 30 be recovered in a splash condenser.

When it is desired to recover lead as well as zinc, lead fume produced in the first zone is separated from the flue gases and recycled to the slag in the first zone, thereby producing a lead bullion which is tapped from the furnace.

Reference now is made to the accompanying drawings, in which:

Fig. 1 is a diagrammatic representation of a cross-section of a furnace according to the invention; Fig. 2 is a sectional view on line II-II of Figure 1; Fig. 3 is a flow-sheet representing continuous sulphide smelting of low-lead zinc sulphide concentrate according to the invention; and

Fig. 4 is a flow-sheet illustrating continuous sulphide smelting of high-lead zinc sulphide concentrate according to the invention.

Figures 1 and 2 illustrate the relatively simple direct smelting system which we have devised for zinc sulphide containing feed material which overcomes the main problems of the ISP and retort processes. The process uses a new system for a reactor 10, having a peripheral wall 12 which, in horizontal section as in Figure 2, is formed of two inter-connected penannular portions 13,14. Across the junction of portions 13,14 the interior of reactor 10 is divided by a liquid-cooled/ preferably water-cooled, wall 16 which separates two gas streams of two zones 17,18 of reactor 10 as shown in Figure 1. Coolant for wall 16 is supplied by a inlet pipe (not shown) and leaves via an outlet pipe (not shown). In the first, oxidation zone 17 of the reactor 10, zinc concentrate feed is fed continuously into the reactor in dry powdered form, via at least one lance

20. Alternatively, the feed may be wet, as with filter cake, and fed directly through the roof of the furnace by suitable means. Air, or an oxygen-air mixture, is blown into liquid slag bath 22 of oxidation zone 17, via such lance 20, at a suitable rate to oxidise all of the sulphides in the feed to oxides. Any fuel requirements are supplied by coal injected into the bath with the air. The air and oxygen and coal are injected through top submerged lances 20 so as to avoid the impingement of the reacting gases on a refractory lining of wall 12 or of hearth 24 of the furnace, and thus to limit the refractory erosion.

Smelting reactions in oxidation zone 17 produce a slag, of composition determined by the feed and flux material fed to the furnace and by the quantity of slag recirculating between oxidation zone 17 and the second, reduction zone 18. Some volatile constituents of the feed such as lead, cadmium, arsenic and antimony can be volatilised in the smelting conditions in zone 17 to produce a fume, removed via offtake 26, which may contain most of these volatile materials; almost all of the zinc remaining in the slag. This fume is collected in a baghouse or electrostatic precipitator before the gases, containing sulphur dioxide, are passed to a suitable conventional system for disposal or dispersal and thereby enable production of less impure, or substantially pure, zinc by reduction of the slag, depending on feed composition.

While wall 16 divides zones 17,18 and extends into the slag bath ^' 22, its lower edge is spaced from hearth 24; wall 16, for example, being supported on laterally spaced legs 28. As a consequence of that spacing, zinc rich slag

circulates underneath the water cooled wall 16, between oxidation zone 17 and reduction zone 18. In zone 18, at least one top submerged lance 30 is used to inject coal, with air or oxygen, to product strongly reducing conditions. These conditions result in reduction of ZnO dissolved in the slag to Zn vapour, which is condensed from flue gases removed via offtake 32. The flue gases then pass through conventional dust separation systems and are burnt to remove their CO and Km content before dispersal. This combustion can be usefully used to produce steam or generate electricity.

Slag is able to be tapped off from the reactor at intervals, through a tapping system 34. Bullion or speiss, which can also be produced, are able to be tapped at intervals through a tapping system 36. The composition of the slag is controlled to produce a suitable viscosity at the operating temperature for any feed composition by the flux additions and the tapping frequency.

The process of this invention can be operated in a number of ways. In one version, illustrated by the flowsheet in Figure 3, reactor 10 of Figure 1 is operated with a low- lead zinc sulphide feed from source 40 to produce all of the lead and other volatile constituents as fume, which is passed from oxidation zone 17 to dust collector 42 and treated separately for recovery of values. The reduction zone 18 is operated under conditions of excess coal feeding as lump material fed to the slag surface, or as fine material injected down the lance, to generate a gas very rich in CO and H_ and containing low levels of CO- and H-O. The zinc vapour in this gas is collected in a zinc condenser 44 to produce zinc for market. Gases from condenser 44 pass to dust

collector 46, with solids from the latter being recycled to the line from source 40.

In another version, illustrated in the flowsheet of Figure 4, the reactor 10 of Figure 1 is operated with a high lead zinc sulphide feed from source 50. The fume solids from collector 52 are recycled to oxidation zone 17 to force all the lead to report to the slag. The lead in the slag is reduced to metal in the reducion zone 18 and is tapped off for refining. The conditions in zone 18 are maintained just sufficiently reducing with injected coal to produce zinc as vapour which is collected in a lead splash condenser 54. The lead may be maintained at about 525°C in the condenser 54 where its solubility for zinc is about 2.5%. It then recirculates to a cooling system 56 where liquid zinc with about 1.2% lead separates at about 425°C. The cooled lead containing about 2.25% zinc then recirculates to splash condenser 54. Gases from system 56 pass to dust collector 58, from which the solids are recycled, via the line from source 50, to zone 17. These process variations allow for cheap production of high purity zinc from low lead concentrates or for cheap production of zinc containing low levels of lead and coproduction of bullion from high lead concentrates. By using coal for combustion and reduction in a compact reactor the system overcomes the major disadvantages of the ISP process. Feed preparation is minimised so that capital as well as operating costs are limited. The invention will be further illustrated by the

following non-limiting example, based on use of a reactor as shown in Figures 1 and 2. EXAMPLE:

This example illustrates the operation of the process to produce high-grade zinc using the flow sheet as illustrated in Figure - 3. Thirty-two tons per hour of zinc concentrate containing 63.0% zinc, 0.30% lead, 0.12% cadmium, 1.8% iron 0.40% Si0 ₂ , 0.90% CaO and 30.80% sulphur was injected together with 0.5 tph of silica flux, 2 tph fume, 3

₃ tph fine coal and 70,000 Nm /h of air down 3 Sirosmelt lances 20 into the slag in the oxidation zone 17 of the furnace 10. Smelting in zone 17 took place at a temperature of 1330°C. Gases from offtake 26 were rich in S0 ₂ and were cleaned of fume, rich in lead and cadmium, by passage through a baghouse. After the baghouse the gases entered a plant for the production of sulphuric acid before the remaining clean gases were able to be dispersed to atmosphere via a tall stack.

The slag in the bath recirculated under the water- cooled wall 16 into the reduction zone 18 of the furnace 10.

Two lances 30 were used to inject 10 tph of fine coal and 48,000 Nm 3/h of air into the bath at zone 18. This produced gas containing 20 tph zinc metal which passed to a lead splash condenser in which zinc and lead were dissolved in a spray of lead-zinc alloy. The alloy produced flowed to a cooling and separating section where zinc with about 1.2% lead was collected. The lead-zinc alloy was then recirculated to the splash condenser to take up more zinc.

The gases from the splash condenser passed through an electrostatic precipitator, a combustion chamber, and

boiler and then were able to be dispersed as clean gas to the atmosphere through a stack. Slag was tapped from system 34 at 4-hourly intervals from the reduction zone 18 of the furnace in batches of 6.4 tons of slag containing 2.8% zinc, 33.9% iron, 35.3% Si0 ₂ and 17.54% CaO.

There are a number of advantages of the process over other means for producing zinc. These include the following:

1. The plant is small for its production rate because of the intense nature of smelting and reduction operations. This results in low fuel requirements because of the small heat losses. It also gives a reduction in capital costs and more easy control of emission.

2. Feed preparation for the processes is minimal so that the capital cost of the plant is low, operations are simple and there are savings in labour and maintenance costs and most ancillary feed materials can be fed to the reactor in an untreated form.

3. Since sinter is not required, it is not necessary to add the excessive quantities of limestone needed for maintaining a high melting point sinter. Minimal fluxing for a fluid slag is required and this can be achieved in the example with no limestone addition, and a small addition of cheap and easily smelted silica flux.

4. The avoidance of sintering results in low maintenance costs and simple operation, and avoids an environmental problem.

5. The use of coal as fuel results in low costs for this component of the operating costs, compared with coke fuel or electric heating. 6. Oxygen can be substituted partially for air to operate

the process with only that coal required for reduction.

7. The process is flexible in that it can be used to treat a wide variety of feed-types without modification

The reactor is described as being formed of two sections of penannular form, between which a fluid cooled wall extends. This arrangement, in which the reactor has a horizontal section approximating a figure eight form, is highly useful as it facilitates the circulation of slag within, and between the oxidation and reduction zones under the action of solid and gaseous feed to each zone. However, the reactor can be of other forms. Thus, the reactor can be circular or elliptical in overall form, with similar benefit. Also, the reactor can be of square or rectangular section in overall form, although such forms provide less than optimum slag circulation.

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.