BACKGROUND OF THE INVENTION Hydrometallurgical processes typically involve leaching of a metal containing material with an aqueous solution containing a chemical additive reactive with the metal values present within the material or, in the case of a biohydro-metallurgical process, an aqueous solution containing microorganisms such as bacteria.

The leaching process may be conducted in a number of ways: in situ, heap and reactor. Reactors may take the form of tanks which are fed with a slurry containing a finely ground mineral concentrate or ore. A number of such tanks may be included in a hydrometallurgical plant. The slurry is usually maintained in suspension by a mechanical agitation means which is a notable consumer of power, particularly in large reactors necessary to contact large volumes of leachant with low grade minerals.

A challenge in hydrometallurgical treatment of iron bearing minerals is the simple economic oxidation of iron present, for example, in the form of a sulphide.

The common sulphides of this kind are pyrite, pyrrhotite and chalcopyrite which may be in association with, or containing, valuable base metals such as nickel, copper. The treatment of nickeliferous pyrrhotites is a particular challenge. Iron present in these minerals is susceptible to oxidation into the ferric state. Ferric ion itself acts as a strong oxidant of metalliferous minerals present within the slurry, liberating metal values into the solution in ionic form and releasing elemental sulphur in a typically non-molten state.

The oxidation of minerals by ferric ion is a predominantly exothermic reaction and liberates heat. The reaction also reduces the ferric ions to ferrous ions so it is necessary for ferric ion to be regenerated after oxidising minerals from ferrous ions produced by the oxidation of minerals so that the reaction may be continued under favourable conditions without the need for addition of supplemental ferric ion. Ferric ion regeneration often proceeds optimally at a different temperature range, or under different conditions, from the oxidation reaction.

In a typical reactor system, employing a number of tank reactors, the oxidative leach takes place in one stage followed by thickening and separation of the mineral bearing material. The underflow from the thickening stage may be disposed of as tailings. The overflow requires treatment for regeneration of ferric ion and may require heat exchange for heating prior to return to the leach stage.

The overflow or the underflow may require processing for recovery of metals of economic value. It will be understood that the leach itself should be conducted under closely controlled temperature conditions.

SUMMARY OF THE INVENTION It is the object of the present invention to provide a reactor and reactor system suitable for the conduct of hydrometallurgical reactions which enables the operations of reaction and solid-liquid separation to be accomplished in a process efficient and cost effective manner.

It is a further object of the present invention to provide a reactor and reactor system suitable for the conduct of hydrometallurgical reactions involving the agency of ferric ion as oxidant which enables the operations of reaction, solid- liquid separation and regeneration to be accomplished in a process efficient and cost effective manner.

With these objects in view, the present invention provides, in a first aspect, a reactor comprising an inlet for an input mineral containing slurry; a settling portion comprising a settling zone and a compression zone in which the mineral containing slurry is thickened by gravitation; a leaching agent inlet located to deliver leaching agent to the settling portion; an underflow for removal of thickened leached slurry from the reactor; and an overflow for removal of a substantially clarified liquor from the reactor; wherein the leaching agent inlet delivers leaching agent to at least one of the settling zone and compression zone of the settling portion in a substantially counter-current direction to the direction of flow of input slurry such that slurry in the settling zone of the settling portion is subject to reaction with the leaching agent.

Generally, the reactor may be in the form of a thickener, preferably of rakeless type, in which an input slurry is delivered as a feed to the thickener through an inlet located in an upper portion of the reactor. Settling is conducted in a conventional manner so that the reacted slurry may be passed to tailings, disposal or a further process stage via the underflow located at the bottom of the

thickener. The compression zone is that relatively high pulp density zone proximate the underflow. Reaction in this zone is less desirable as bypassing may be problematic. In a bottom portion of the reactor, advantageously in proximity to the bottom of the thickener, above the underflow, an inlet or a number of inlets may be arranged to deliver the leaching agent to either or both of the settling zone and compression zone. Multiple inlets may deliver to each zone.

The leaching agent will be delivered through the inlet (s) to flow towards the top of the thickener volume in a direction generally counter-current to the flow of input slurry. Provision may also be made for introduction to the reactor of gases such as oxygen for use in a metallurgical process though ordinarily the reaction will proceed in the absence of oxygen.

The flow of leaching agent must be controlled at a rate less than the settling rate of the input slurry to the reactor. This assists in maintaining a clear overflow from the thickener. This overflow may be supplied to other process stages for metal recovery as well as to ferric ion regeneration as described below.

The leaching agent is to be broadly understood as any chemical or biological agent that, when introduced to the reactor under appropriate conditions, will react to economically acceptable extent with minerals present in the slurry.

Thus the nature of the process occurring within the reactor may be varied widely from application to application.

It follows from the reactor design that the reaction will generally occur in the settling portion where the bulk of the solids present within the input slurry will be located. Particularly, reaction will occur in the settling zone, a transitional region of relatively less pulp density than the compression zone. Means for delivering a flocculating agent to the reactor to aid in settling may also be provided. If the reactor is to be used for bioleaching, the flocculating agent must be non-toxic to the leaching micro-organisms.

The reactor is to be distinguished from those of fluidised bed type since the flow pattern typically desired in the present reactor is a form of plug flow and not a turbulent regime that would cause pulp level instability. Generally, the reactor will be employed for an oxidation type reaction by ferric ions or other like chemical oxidants of non-gaseous nature.

Where appropriate, the ferric ion regeneration reaction-which may be accomplished in any conventional manner that allows oxidation of ferrous ions

generated in the leach process back to ferric ions by chemical reaction with gaseous oxidants such as air or oxygen or mixtures of such gases; or biological means with temperature and pH sensitive bacteria-may be conducted at low temperature compared to that of the leach. Therefore, cooling devices may be provided to cool the overflow for regeneration and re-supply of at least a portion thereof to the reactor by recirculation means at a temperature suitable for conducting the biological reaction. The remaining portion of the overflow is passed to leached metal recovery.

Oxidative leaching processes, especially those involving ferric ion, may be successfully and economically applied to a wide range of mineral types and the reactor of the invention may have applicability in a wide range of operations.

Thus, in a further aspect of the invention, there is provided a metallurgical process plant comprising one or more reactors as above described. The metallurgical process plant may also include regeneration reactors for regenerating ferric ion from overflow or other reactor streams.

Significant of oxidative leaching processes suitable to be carried out in the reactor may include leaching of refractory ores and concentrates, base metal ores, and concentrates especially nickel, lead, zinc and copper ores and concentrates, uranium ores and concentrates and coal for removal of pyrite. The reactor may also be used to separate lead and zinc minerals within complex ores and concentrates. The zinc mineral is selectively dissolved while the lead sulphide remains substantially inert.

Advantages of the present invention may include the integration of the reaction and solid/liquid separation steps which may further facilitate subsequent operations such as regeneration, heat transfer and so on. Also, because recirculation of large volumes of leaching liquor is possible, the size of the reactor may be substantially reduced in comparison to conventional reactors and power consumption may likewise be reduced. As power consumption is an important economic factor, particularly in treatment processes involving oxidative leaching of minerals and materials by ferric ion this is a most advantageous potential saving arising from the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following description of a preferred embodiment thereof made with reference to the accompanying

drawings in which: Figure 1 is a schematic view of one embodiment of the reactor of the following invention; and Figure 2 is a schematic diagram of a process involving the reactor of Figure 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION There is shown in Figure 1 a reactor 10 having generally the construction of a conventional thickener having important modifications as will be described below. That is, reactor 10 comprises a generally cylindrical upper vessel portion 12 with a generally conical bottom vessel 14 in which compression of the thickener bed primarily takes place. Both vessel portions 12 and 14 are fabricated from a material inert to liquors within the reactor 10. Such liquors may be expected to be corrosive. Either special alloys or glass/polymer linings for corrosible materials may be employed to address this problem. The object of the reactor 10 is to enable reaction of a mineral containing slurry introduced to it with a leaching agent while allowing settling to occur. Accordingly, provision is made toward the bottom of the reactor 10, though advantageously above the level of the underflow port 30, for the leaching agent to be introduced to the reactor 10.

One or a number of leaching reagent inlet (s) 20 may be located in the bottom portion 11 of the reactor 10 for delivering leaching agent to the settling portion 25 which includes both a settling zone 15 and a compression zone 17..

Leaching agent inlet (s) 20 may be placed at single or multiple locations and/or depths within the reactor 10 at or above the level of the underflow port 30.

In the embodiment shown in Figure 1, leaching agent is introduced into both the compression zone 17 and settling/thickening zone 15. A distributor ring may be used for the purpose of evenly distributing leaching agent inflow over a cross- section of reactor 10.

Leaching agent is introduced through inlet (s) 20 at a velocity less than or about equal with the settling velocity of particles in the reactor 10 so that a settling zone 15 constitutes also a reaction or"fluidised"zone in which the leaching agent may contact and react with the mineral containing particles. Pulp density in this transitional zone may typically be 10% to 40% solids. Countercurrent plug flow of leachant and solid particles is thus promoted within the settling zone 15. The

leaching agent inlets 20 may be in the form of jets, nozzles and valve control may be employed. Input and output flows are to be balanced. Even distribution of leaching agent upward flow is encouraged.

Provision may be made in the reactor 10 for the leaching agent flow to have a generally constant velocity across the cross-sectional area of the reactor 10 at about the level it is introduced. The object is to avoid channeling or introduction of leaching agent in a manner that will cause instability of the pulp level which is determined in a manner known in the art.

In this respect, it will be understood that the reactor 10 has a number of distinct zones through which pulp density varies from substantially zero (clarification zone) to a transitional settling zone to the desired underflow pulp density (compression zone). It is not intended that the reactor 10 function other than as would a typical thickener in this respect except that the settling zone 15 may be deeper. There is an important distinction between the proposed reactor 10 and a conventional fluidised bed in that concentration of solids in the settling zone 15 throughout its depth may vary quite significantly in contrast to substantial homogeneity conveniently maintained in the latter. However, achievement of settling zone 15 homogeneity is not precluded by the present invention.

Thus slurry which may be flocculated using a suitable flocculating agent, is introduced at the top of the reactor by line 40 through conventional feedwell 45 or similar suitable arrangement, the direction of flow being substantially downwards toward the underflow port 30. The concentration of the slurry may be at any convenient value but may be less than about 30 % solids, preferably less than about 20% solids. Introduction of leaching agent allows the settling zone 15 to become a reaction zone and product liquor may be recovered as a substantially clear solution containing less than about 100 mg/l solids.

Settling may be promoted by the addition of flocculants of conventional type with the proviso that, if biological regeneration is considered, the levels of floculant in overflow 80 must be maintained at microbiologically non-toxic levels.

A floculant non-toxic to the preferred bacteria may be preferred. Generally, bacteria will be of sulphur/iron oxidising type such as thiobacillus, sulpholobus, ferrobacillus and the like.

The underflow stream 31 removed from the reactor 10 may have pulp density typically in the range 30 to 60 % and may be pumped, for example by a

diaphragm pump or other pump suitable for pumping of high pulp density slurries to further processing stages.

Dependent upon process chemistry, the underflow solid stream 31 will contain, in addition to unleached gangue minerals, elemental sulphur and other by products of the leaching process. Washing in counter current decantation (CCD) circuits 200, as well as appropriate solid/liquid separation steps 300 may be interposed between the leach and/or further processing stages.

The overflow liquor stream 80 from a ferric ion oxidative leach of a bse metal concentrate may contain ferrous ions and base metal ions. The base metal ions may be recovered from the liquor by solvent extraction or any other suitable metal recovery process known in the art. In the case of a treatment process for nickeliferous pyrrhotite, the solvent may advantageously have selectivity for nickel ions.

Provision may be made for recirculation of a portion of the overflow 80 or underflow 31 to the reactor 10 through lines 60 and/or 70. The underflow 31 may be pumped to further reactors operating in similar manner to reactor 10 shown in Figure 1. Alternatively, the underflow 31 may be directed to another reaction circuit operating on the basis of different process chemistry.

For example, one or more reactor/thickeners 10 may be conveniently employed in a metallurgical process plant for the treatment of base metal sulphides in association with pyrite, pyrrhotite or other iron sulphides.

Alternatively, the reactor 10 may be employed to leach iron sulphides to liberate gold entrapped in pyrite, pyrrhotite, and other iron sulphides. The gold will remain in the underflow 31, solids from which may require treatment such as, but not limited to, cyanidation 500. Figure 2 shows a process involving these steps.

The reactor 10 and the leaching step would be used in preference to roasting which produces sulphur dioxide, and pressure leaching. The former process is carried out at 500-600°C, pressure oxidation is carried out at high pressure and at 200°C. Advantages of the invention over these processes are operation at atmospheric pressure and temperatures slightly above ambient.

Ferric ion is utilised in the reaction to leach the sulphides according to the following half-reaction: Fe3+ + e--------> Fe2+

The ratio of ferric to ferrous ion in the settling or reaction zone 15 may be critical to the achievement of optimal leaching conditions and accordingly it may be desirable to have a ferric ion regeneration stage 100.

The oxidation reaction is typically an exothermic one and optimal temperature may attain in excess of 60°C, a higher temperature than optimal for ferric ion regeneration by some conventional methods. So provision may be made for cooling of the overflow 80, where the ferrous ion concentration is most conducive to regeneration, prior to entry to the regeneration reactor 100 which typically operates in the range 40°C to 50°C. Ferrous ion reports in greatest quantity in the overflow 80 which may be passed to regeneration reactor 100.

Overflow 80 filtration or solid/liquid separation ought not to be necessary due to good clarity with low suspended solids.

A counter-current heat exchanger or heat exchanger module 400 may be employed for cooling purposes with overflow 80 being cooled as return leaching agent is heated, being used-at least in part-as a coolant. Thus temperature may be controlled in a straightforward manner, the reactor 10 dimensions and input flows being selected to achieve the desired objectives. In rare cases, the heat exchanger 400 may be required to have heating function and this possibility is provided for.

Regeneration then occurs in any manner known in the art by chemical or biological means, the latter being preferred. Biological regeneration simply involves one of ion oxidising bacteria of the above description. Regeneration also allows environmental benefits to be achieved because ferrous sulphate is an undesirable waste product. Its treatment to recover ferric ions that may be employed in the leaching reaction avoids most environmental problems.

In the case where the regeneration process involves bacteria, it may be necessary to carefully control pH and nutrient conditions to those most optimal for bacterial viability. Oxygen containing gases such as air/oxygen or mixtures of these may also require to be introduced perhaps by diffusers as disclosed in the Applicant's co-pending Australian Provisional Patent Application No PP 7180 entitled"PROCESS"filed 18th November, 1998.

Other metals may be present in the overflow 80 in the form of sulphates and the overflow may be treated for recovery of any metal values.

Processes may be carried out with a wide range of minerals, namely base

metal sulphides such as nickel, copper, zinc sulphides; pyrite; refractory gold ores and concentrates; uranium ores and so on, especially those containing significant quantities of iron. Without wishing to be bound by any theory, it is considered that the reactor/thickener 10 may conveniently be employed for treatment of materials containing up to 50%-60% iron in the form of sulphide.

The reactor 10 may be instrumented and controlled from the point of view of temperature and level in a convenient manner known in the art. The reactor 10 may be provided with heating/cooling means as appropriate to maintain reactor 10 temperature with desired limits recalling that oxidative reactions are exothermic, creating the risk of temperature excursions. Bundles or coils may be used for this heat exchange purpose and are conveniently used in the absence of strong currents within the reactor 10.

A number of reactors 10 may be employed in series or parallel and, in such case, the design and operation is as described herein.

The following example is intended to show that a reactor 10 of the type above described may be operated to produce a clarified overflow suitable for delivery to further metallurgical process stages is possible.

EXAMPLE The hydro-dynamic testing was conducted in a 300 milimetre diameter, 5 metre tall column reactor. A flocculated 3.5 wt% calcium carbonate slurry was introduced near the top of the column at the same time water was introduced into the column near its base through a distributor ring with an upwardly directed flow.

With a constant inlet slurry flow rate of 10 litres per minute and uniform floculant addition, the overflow remained clear for flow rates through the distributor ring up to 20 litres per minute. Under steady state operation, the solids concentration in the fluidised zone above the distributor ring was essentially uniform.

At a slurry flow rate of 10 litres per minute and a distributor ring flow of 20 litres per minute the solids concentration in the column was the same as the slurry feed concentration (3.5 wt%). The liquid throughput at the overflow was 25 litres per hour which is in the same range as ultra high rate thickeners. With lower distributor ring flow rates, the solids concentration in the fluidised bed increased. At a distributor ring flow of 10 litres per minute and 3.5 litres per minute, the steady state solids concentration was about 7 wt% and 18 wt%, respectively. With an 18 wt% solids concentration and a bed working height of 4.4 metres, the average solids residence time was nearly 3 hours.

Modifications and variations to the reactor of the disclosure may be made by one skilled in the art and such modifications and variations are intended to fall within the scope of the present invention