GB2311517A

GB2311517A - Metal extraction process

Info

Publication number: GB2311517A
Application number: GB9704890A
Authority: GB
Inventors: Stuart David Brown
Original assignee: Tioxide Group Services Ltd
Current assignee: Tioxide Group Services Ltd
Priority date: 1996-03-28
Filing date: 1997-03-10
Publication date: 1997-10-01
Also published as: NZ314483A; GB9704890D0; AU1623497A; GB9606570D0

Description

METAL EXTRACTION PROCESS This invention relates to a process for extracting metals from a metal containing material, in particular from a complex sulphide ore.

Although the world demand for non-ferrous metals such as zinc and copper can currently still be met by conventional treatment of simple ores, such resources are finite and as reserves become depleted it will be necessary to utilise more complex sulphide deposits, processing of which is uneconomic at present.

Complex sulphide ores generally comprise an intergrown association of the zinc ore sphalerite and the copper ores tetrahedrite and chalcopyrite, possibly in conjunction with the lead ore galena, within a pyritic, pyrrhotitic or arsenopyritic matrix; silicates carbonates and micaceous rocks may also be major gangue constituents. Typically, such ores contain 5-10% zinc and 9-22% copper.

Several process are known for treating such complex sulphide concentrates but they tend to be uneconomic or have the disadvantage that the zinc and copper are dissolved simultaneously.

It is an object of this invention to provide a process which gives selective leaching of the metals in a complex sulphide concentrate.

In co-pending PCT Patent Application No PCT/AU 96/00612 there is disclosed a process of extracting metals using an aqueous acid solution containing an acid and one or more transition element containing species and introducing an oxidising agent into the solution in order to oxidise the transition element containing species to a higher oxidative state.

Also, in co-pending PCT Application No PCT/AU95/00643 there is disclosed a process for extracting metals using an aqueous acid solution comprising the co-product solution from the manufacture of titanium dioxide by the sulphate process.

It has now been found that by treating the complex sulphide concentrate with the acidic solutions mentioned above in a two-stage process using different strengths, selective leaching of different metals, particularly zinc and copper is possible.

According to one aspect of the present invention, there is provided a process for selectively extracting metals from a complex sulphide concentrate comprising the steps of: a) treating said concentrate in a first stage with an aqueous, acidic solution containing an acid and one or more transition element containing species; b) introducing an oxidising agent into said solution in order to oxidise the transition element ion in said transition element containing species to a higher oxidation state; c) oxidising said metal containing material with the oxidised transition element containing species to effect release of a first metal from said concentrate, into the solution; d) separating the solid residue from the solution; and e) treating the solid residue in a second stage by repeating steps (a) to (c) using a solution having a different acid and transition element containing species content than the solution used in steps (a) to (c) to effect release of a second metal.

Preferably, the solution used in the second stage has a higher acid and transition element containing species content than the solution used in the first step.

The acid is preferably a non-oxidising acid, although oxidising acids, such as nitric acid, may be used if desired. The acid is more preferably a mineral acid. A suitable non-ondising acid is sulphuric acid which is generally the most economical acid to use. However, other acids which can be used include, but are not limited to hydrochloric acid and phosphoric acid.

Preferably the acid content is 1 to 60 g/l for the first stage and 50 to 300 g/l for the second stage.

The transition element containing species may be any aqueous species containing one or more transition elements capable of having variable valence. As used herein, the term transition element is intended to include the main transition elements, or first transition series, from Sc to Cu; the lanthanide elements, or second series from Y to Ag and the actinide elements or third transition series, from Hf to Au. Preferably, however, the transition element is selected from the main transition elements. For economic reasons the most preferred transition element is iron.

Preferably, the concentration of the transition element is 0.1 to 12 gA for the first stage and 8 to 80 g/l for the second stage.

The process of the invention includes the step of oxidising the one or more transition elements to a higher oxidation state. Preferably, both before and after oxidation, the transition element is present in solution in an ionic state, such as in a simple or complex ion. The invention enables use of inexpensive oxidising agents, such as air or oxygen gas. However, other oxidising agents, such as hydrogen peroxide or ozone, may be used instead, if desired.

The use of oxidising agents such as oxygen gas or air has the further advantage of avoiding the introduction of contaminants into the reaction mixture, as might be the case if other oxidising agents were used, which therefore avoids the need for a subsequent step of removing the contaminant.

Where the oxidising agent is gaseous, such as air and oxygen, it is preferably introduced into solution by bubbling the gas through the acidic solution. A preferred means of introducing the oxidising gas into the solution is by using an aeration tube or a glass flit type aeration tube. In general, the finer the bubbles of oxidising gas introduced into the solution, the faster the oxidation reaction. The bubbles of oxidising gas may be as high as 3 millimetres in diameter. Preferably the bubble diameter is less than 0.8 millimetres. More preferably, the bubble diameter is in the range of from 0.01 to 0.1 millimetres.

The amount of oxidising agent introduced to the aqueous acidic solution is preferably 1 to 2 times the stoichiometric amount needed to achieve the desired reaction rate. Where oxygen gas is the oxidising agent, it is preferably introduced into the solution at a rate of from 0.001 to 2.0 grams of oxygen per litre of leaching solution per minute. For example, in the case of material containing 5% Zn, it may be necessary in the first stage to feed oxygen at a rate of between 0.05 and 0.10 grams of oxygentlitretminute into a leaching solution containing 20% w/v of metal containing material in order to achieve 90% or above reaction in 60 minutes.

It is further preferred to bubble the gaseous oxidising agent through the reaction mixture in a continuous or semi-continuous stream.

The process may be conducted over a wide pressure range.

Preferably, however, the process is conducted at atmospheric pressure.

It is preferred that the acidic solution is reacted with the concentrate at an elevated solution temperature. For most process conditions, reaction rate increases with increasing temperature. Preferably, the solution temperature is at least ambient temperature. More preferably, the solution temperature is at least 60"C. For some process conditions, reaction rate increases sharply at 700C and above.

The pH of the aqueous acidic solution is of course acidic.

Preferably, the pH of the solution is no higher than 6.5. More preferably, solution pH is in the range of 0 to 5.

Oxidation rate can be improved by agitating the acidic solution during introduction of the oxidising agent. Agitation is effected to ensure the metal containing material is adequately suspended in the solution and the oxidising species is adequately dispersed in the acidic solution. Agitation may be effected by a rotating impeller or the like within the acidic solution.

The following agitation rates are for an impeller having a diameter of 10.3 cm. The "tip speed" of a rotating agitator is the speed of a point on the periphery of the agitator and is independent of the diameter of the agitator.

To convert revolution rate to tip speed (in metres/minute) it is necessary to multiply by approximately 0.32. Tip speeds are given in brackets after each agitation rate. Preferably the impeller rotates at a speed greater than 200 rpm (64.74m/min). More preferably, the rate of rotation of the impeller is 400 rpm (129.49m/min) or higher. While agitation rates may be as high as 1700 rpm (550.3 im/min) for many applications, optimum results are achieved at rates no higher than 750 rpm (242.79m/min).

Agitation during the process can have the disadvantage of causing foaming or frothing of the solution. Foaming can entrain solids and physically separate them from the acidic solution, thereby making the handling of the solution difficult. An effective amount of a foam control agent may therefore be advantageously added to the acidic solution. One such foam control agent is calcium lignosulphonate. It may be present at a concentration of up to 1% w/v. However, for most applications, the calcium lignosulphonate has a maximum concentration of 0.05% w/v, such as around 0.025% w/v. The minimum concentration of calcium lignosulphonate is typically around 0.0001% w/v.

The presence of sulphide in the ore means that the leaching process can result in formation of free sulphur and/or sulphur compounds which may coat unreacted particles. This phenomenon can prevent or reduce reaction of the coated particles with the leaching solution, thereby adversely affecting the leaching rate. Coating by sulphur-containing material is particularly problematic where the material being leached contains chalcopyrite. This problem can be alleviated by including the step of attrition of the metal containing material. This may be affected by addition of an attrition agent to the leaching solution during agitation thereof. The attriting agent assists to physically remove the sulphur containing coating by attrition, thereby exposing the surface of the unreacted particles to the leaching solution. A suitable attriting agent is particulate SiO2, such as sand. Where an attriting agent is used, it is preferably present in an amount which is approximately equal to the amount of ore. Thus the ratio of sand to ore is preferably 0.2:1 to 1.5:1. Attrition can also be effected by increasing the volume of solids in the reaction mixture.

Calcium ligonsulphonate, in addition to its defoaming properties, also acts as a dispersant of free sulphur and/or sulphur compounds. Thus, the addition of both sand and calcium lignosulphonate to the solution further enhances leaching rate.

The acidic solution may additionally contain one or more reaction promoters. Such promoters include copper ions and/or derived from carboxylic acids, such as acetic acid. Copper is principally used as a promoter when it is not the transition element containing species. The copper may be added to solution such as by adding copper sulphate, CuSO4.SH2O. Alternatively, the copper may already be present in the acidic solution, such as where copper has been released into the solution as a result of leaching copper containing materials, e.g.. tailings, mineral concentrates etc. Acetate ions may be added as acetic acid. The preferred concentration of copper ions is about 0.6 g/l. Acetate ions, if present, are preferably present at a concentration of about 1.25 g/l, expressed as equivalent amount of acetic acid.

The aqueous acidic solution may also include chloride species.

Chloride may be present at a concentration of up to 20 g/l. However, in some embodiments, it is present at a concentration of 10 g/l or less.

Typically, the minimum chloride concentration is around 0.5 gull.

Reaction rate is also dependent on the chemical and physical form of metal in the ore, such as particle size, chemistry and percentage of constituent particles and overall metal content. Preferably the particles have a P80 of 20 microns.

In a preferred embodiment of the first aspect of the, invention, the aqueous acidic solution contains sulphuric acid and iron-containing species. Oxygen gas is introduced into solution as a continuous stream of fine bubbles while the reaction mixture is agitated by means of a rotating impeller. The reactions believed to be occurring are: O2(g) 2Fe2+(aq) - > 2 Fe3+(aq) 2 Fe3+(aq) + MeS e 2 Fe3+(aq)+Me2+ + S o2(g) 2Fe2+(aq) - > 2 Fe3+(aq) where MeS is a sulphide of a divalent metal ion denoted as Me.

Thus, in the above mentioned preferred embodiment, the transition element is iron, present in solution as Fe2+(aq) ions. The Fe2+(aq) ions react with the oxidising agent, oxygen gas, as it bubbled through the solution to produce Fe3+(aq) ions. The oxidation of iron ions is enhanced by agitation of the reaction mixture which assists in dispersing the oxygen gas and suspending the concentrate in the solution. The ferric ions thus produced themselves become an oxidising agent for the metal sulphide.

Accordingly, the ferric ions react with the metal sulphide MeS to give Me2+(aq) ions and to oxidise S2 in the sulphide to elemental sulphur. In the process, the ferric ions are reduced to ferrous ions which then become available to commence the cycle again. In this manner, a continuous, cyclic oxidation process can be effected.

It is to be noted that the above preferred embodiment of the process of the invention has as a by-product elemental sulphur. This is advantageous in that the production of sulphate is minimised which avoids the need for expensive sulphate removal treatment.

According to a second aspect of the invention, there is provided a process for selectively extracting metals from a complex sulphide ore comprising the steps of a) treating the concentrate in a first stage with an acidic solution containing effective concentrations of iron-containing species, sulphuric acid and one or more leach rate accelerants present in the co-product solution from the manufacture of titanium dioxide by the sulphate process, whether such leach rate accelerants are present as a result of using a co-product solution or are separately compounded, to effect release of a first metal from said concentrate into the solution; b) separating the solid residue from the solution; and c) treating the solid residue in a second stage by repeating step (a), in which the co-product solution used in the first stage is at least 25% more dilute than the solution used in the second stage.

Preferably, an oxidising agent is introduced into the solution in the same manner as for the process of the first aspect of the invention as mentioned above.

Other conditions and additives also may be the same as for the process of the first aspect of the invention The present invention also provides an apparatus for use in a process for extracting metal from a metal containing material, said apparatus including: agitating means for agitating an aqueous acidic solution to which has been added a metal containing material; a conduit for introducing an oxidising agent to said aqueous acidic solution, said conduit having an outlet close to, preferably adjacent or below, said agitating means; and operating means for operating said agitating means.

The agitating means may comprise a blade, paddle or impeller, or the like, rotatable about a drive shaft. Preferably, the agitating means comprises a rotating disk having teeth extending downwardly from the periphery thereof. The agitating means is operated by an operating means which may comprise a motor driving the drive shaft. In use, the agitating means is vertically positioned in the aqueous solution so as to achieve maximum agitation with minimal frothing of solution.

The apparatus flirther includes a conduit through which an oxidising agent, preferably an oxidising gas, is introduced to the aqueous solution.

The conduit may comprise a tube made from any suitable material compatible with the aqueous acidic solution, such as glass, rubber, polythene, etc. The outlet of the conduct is located in close proximity to, such as adjacent, below or within, the agitating means. Such an arrangement assists in minimising the bubble size of oxidising gas fed into solution and maximising the dispersion of the oxidising gas through solution.

In order to further maximise dispersion of the oxidising gas through the solution, the outlet of the conduct may comprise a plurality of fine holes or perforations. Such an outlet may comprise a glass frit or a "weep hose".

The apparatus may further include a second agitating means located above the first agitating means in a position suitable for minimising froth formation. Typically, the second agitation means, if present, is positioned so that in use, it is in the upper part of the reaction mixture. The second agitating means also may comprise a blade, paddle, disk or impeller.

Preferably, the second agitating means is an impeller rotatable about the same shaft as for the first agitating means.

The apparatus may further comprise a vessel for containing the reaction mixture of aqueous acidic solution and ore. Preferably, the vessel is made from corrosion resistant or other compatible material. The vessel further preferably includes within its interior, means for increasing the turbulence and shear of the solution during agitation thereof, thereby increasing dispersion of the oxidising agent in solution. Such means preferably comprises projecting structures within the interior of the vessel, such as baffles. The projecting structures prevent the reaction mixture from merely "circulating" the vessel during agitation which can prevent adequate mixing of the reactants. Accordingly, the projecting structures enhance turbulence and dispersion of the oxidising gas in solution.

The apparatus may further include heating means for heating the reaction mixture, if required, to the appropriate temperature. The heating means may comprise a gas burner, resistance heater, or direct or indirect steam.

The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings and Examples.

Figure 1 illustrates a first embodiment of apparatus for carrying out the leaching steps. Apparatus 10 includes a glass vessel 12 having baffles 14. A conduit 16 feeds air or oxygen from a compressor (not shown) or other suitable source to a rubber weep hose 18 including a number of perforations 20. An agitating means comprising an impeller 22 mounted on a rotatable shaft 24 is operable by a drive motor 26. The impeller 22 comprises a disc 28 having peripheral teeth 30 extending from the base thereof. The weep hose 18 is positioned below the impeller 22.

During operation of the apparatus 10, air or oxygen fed into the conduit 16 is expelled from the weep hose 18 via the perforations 20 as fine bubbles which are dispersed through the reaction mixture by the impeller 22.

Figure 2 illustrates a second apparatus embodiment in which like reference numerals refer to like parts of the embodiment of Figure 1. One difference between the first and second embodiments is that the outlet of the conduit 116 is a glass fit instead of a weep hose. A second difference is that the agitating means comprises a triangular stirrer 122 mountable on a rotatable shaft 124 instead of a disc- shaped impeller with peripheral teeth.

Figure 3 illustrates a third apparatus embodiment in which, like reference numerals again refer to like parts of the first and second embodiments. One difference between the third embodiment and each of the first and second embodiments is that the outlet of conduit 216 comprises an open ended tube positioned so that air is expelled under the centre of the impeller 222 just above teeth 230. A second difference is that mounted on the rotatable shaft 224 above the impeller 222 is a second impeller 232.

The impeller 232 includes vanes 234 extending therefrom. The action of the second impeller 232 in agitating the upper part of the reaction mixture is to reduce foam formation.

EXAMPLE 1 320g of a complex sulphide concentrate containing 1.6% zinc and 8.6% copper was mixed with 1600ml of an acidic solution containing 30 g/l sulphuric acid and 8 g/l iron (as Fe ) in the apparatus of Figure 1.

Oxygen was fed to the mixture at a rate of 501/h and the mixture was maintained at a temperature of 90"C. The mixture was agitated by an impeller rotating at 750 rpm (corresponding to a tip speed of 242.79 m/min). The results are shown in Figure 4 from which it can be seen that 90% of the zinc was recovered in 150 min.. After this time, 10% of the copper was also removed.

The residue was separated from the solution by flotation and filtration and mixed with an acidic solution containing 250 g/l sulphuric acid and 16 g/l iron (as Foe") and treated as described above. The results are shown in Figure 5 from which it can be seen that almost all of the remaining copper was recovered after 180 min. It was established that all of the tetrahedrite copper was recovered in 60 min.. The copper present in the solution resulting from the zinc leach was recovered as copper cementate by the addition of zinc scrap.

EXAMPLE 2 The same concentrate was treated in the same way as in Example 1 except that co-product effluent from the manufacture of titanium dioxide from ilmenite feedstock as disclosed in Example 3 of PCT Application AU/ 95/00643 was used at 50% dilution for the zinc leach and full strength for the copper leach. Similar results were obtained.

Claims

1. A process for selectively extracting metals from a complex sulphide concentrate comprising the steps of: a) treating said concentrate in a first stage with an aqueous, acidic solution containing an acid and one or more transition element containing species; b) introducing an oxidising agent into said solution in order to oxidise the transition element ion in said transition element containing species to a higher oxidation state; c) oxidising said metal containing material with the oxidised transition element containing species to effect release of a first metal from said concentrate, into the solution; d) separating the solid residue from the solution; and e) treating the solid residue in a second stage by repeating steps (a) to (c) using a solution having a different acid and transition element containing species content than the solution used in the first stage to effect release of a second metal.

2. A process according to claim 1 in which the solution used in the second stage has a higher acid and transition element containing species content than the solution used in the first stage.

3. A process according to claim 1 or 2, in which the acid is sulphuric acid and the acid content is 1 to 60 g/l for the first stage and 50 to 300 g/l for the second stage.

4. A process according to claim 1 in which the transition element is iron and the concentration ofthe transition element is 0.1 to 12 g/l for the first stage and 8 to 80 g/l for the second stage.

5. A process for selectively extracting metals from a complex sulphide ore comprising the steps of a) treating the concentrate in a first stage with an acidic solution containing effective concentrations of iron-containing species, sulphuric acid and one or more leach rate accelerants present in the co-product solution from the manufacture of titanium dioxide by the sulphate process, whether such leach rate accelerants are present as a result of using a co-product solution or are separately compounded, to effect release of a first metal from said concentrate into the solution; b) separating the solid residue from the solution; and c) treating the solid residue in a second stage by repeating step (a), in which the co-product solution used in the first stage is at least 25% more dilute than the solution used in the second stage.

6. Apparatus for use in a process for extracting metal from a metal containing material, said apparatus including: agitating means for agitating an aqueous acidic solution to which has been added a metal containing material; a conduit for introducing an oxidising agent to said aqueous acidic solution, said conduit having an outlet close to, preferably adjacent or below, said agitating means; and operating means for operating said agitating means.