WO1994021830A1

WO1994021830A1 - Recovery of zinc, iron, lead and silver values from zinc sulphide concentrate by a multi-stage pressure oxidation process

Info

Publication number: WO1994021830A1
Application number: PCT/CA1994/000177
Authority: WO
Inventors: Michael Edward Chalkley; Michael Joseph Collins; Nigel Edwin Tuffrey
Original assignee: Sherritt Inc.
Priority date: 1993-03-25
Filing date: 1994-03-25
Publication date: 1994-09-29
Also published as: GB9306201D0; AU6372194A

Abstract

A process for recovering zinc and iron from zinc- and iron-containing sulphidic material, which also contains lead and silver, in which the sulphidic material is leached under oxidizing conditions in a series of leach steps to produce a zinc-containing solution from which zinc can be recovered by conventional means such as electrowinning, a lead- and silver-bearing product containing substantially all of the lead and a substantial portion of the silver present in the zinc-containing sulphidic materials, and a substantially pure, saleable or environmentally acceptable iron residue in the form of hematite which contains substantially all of the soluble iron present in the zinc-containing sulphidic materials. In addition, a substantial portion of the sulphide sulphur present in the zinc-containing sulphidic material is converted to elemental sulphur which is easily physically separable from the lead-containing residue.

Description

_RE_COVERY _OF ZI_NC, IRON, LEAD AND SILVER VALUES FROM ZINC ^SUL¬ PHI_{DE CONC}E_NTR_ATE B_Y A MULTI-STAGE PRESSURE OXIDATION PROCESS

This invention relates to a process for the recovery of zinc and iron from zinc- and iron-containing sulphidic concentrate which also contains lead and silver values.

It is known to recover zinc from zinc-containing sulphidic material by leaching the material under oxidizing conditions at elevated temperature in aqueous sulphuric acid solution to provide an undissolved residue and a leach solution containing the dissolved zinc. After carrying out the necessary purification steps, the purified leach solution is electrolyzed to produce elemental zinc. Most zinc-containing sulphidic materials usually also contain iron, and it is known that the presence of iron is desirable because it assists the oxidation leaching of sulphidic material and hence assists in obtaining adequate dissolution of zinc. It is usual for the leach to be commenced with a slight stoichiometrlc excess of sulphuric acid relative to the amount of zinc in the zinc-containing material, to ensure the presence of sufficient iron in the solution. However, U.S. Patent No. 3,867,268 issued February 18, 1975 teaches that for the purpose of recovering zinc any stoichiometrlc acid excess relative to the zinc should not exceed 1.2:1, i.e. 20%, as this would result in the amount of dissolved iron and free acid in the leach end solution being undesirably high.

Thus, with a conventional stoichiometrlc excess of acid, iron is also dissolved, and is present in the leach solution. Because the subsequent zinc electrolysis step requires that the zinc-containing solution to be electrolyzed be substantially iron-free, it is necessary to remove iron in a purification step, even though the leach may be conducted in such a way that a minimal amount of iron is dissolved.

Zinc-containing sulphidic material may, in addition to iron, also contain lead, and in some cases the lead

S BS IT T S T content may be sufficiently high to warrant recovery of both lead and zinc. In zinc recovery processes such as described above, substantially all of the lead remains in the leach residue together with most of the iron. The presence of iron in the residue complicates the subsequent recovery of lead therefrom.

U.S. Patent No. 4,505,744 issued March 19, 1985, teaches that if the stoichiometric acid excess relative to the zinc content of the zinc-containing sulphidic material is from about 50 to about 100%, a substantial amount of the iron dissolves as well as zinc, without any significant dissolution of lead. Thus, the leach residue produced is relatively iron-free.

U.S. Patent No. 4,505,744 further teaches that iron in the solution from the first leach step may be precipitated by contact with further zinc-containing sulphidic material which also contains iron in a second leach step, operated with little or no stoichiometric excess of sulphuric acid relative to the zinc content of the further zinc-containing sulphidic material. The majority of the dissolved iron is precipitated and reports with the second leach residue. This method is useful, for example, if the further zinc-containing material contains minor amounts of lead and silver whose recovery is not economically desirable since the lead and silver content of the further zinc-containing material will be lost in the leach residue.

The iron residue produced in the second leach step described in U.S. Patent No. 4,505,744 is comprised primarily of jarosites and hydrated iron oxides. This residue is no longer suitable for disposal in many locations because of the current strict environmental regulations with respect to the disposal or iron-containing residues, particularly jarosite, goethite and paragoethite.

Iron in the form of hematite is a more readily disposable iron residue. Hematite also contains a higher iron content than jarosite and thus provides a smaller quantity of residue for rejection or disposal. _r The present invention overcomes two problems i associated with the treatment of zinc-containing sulphidic materials which also contain substantial amounts of iron, lead and silver, namely the generation of a saleable or environmentally acceptable iron residue in the form of hematite, and the high recovery of lead and silver in a high grade product residue.

According to the present invention, zinc-containing sulphidic materials, which also contain iron, lead and silver, are leached under oxidizing conditions in a series of leaching steps to produce a zinc-containing solution from which zinc can be recovered by conventional means such as electrowinning, a lead and silver-bearing product containing substantially all of the lead and a substantial portion of the silver present in the zinc-containing sulphidic materials, and a substantially pure iron residue in the form of hematite which contains substantially all of the soluble iron present in the zinc-containing sulphidic materials. In addition, a substantial portion of the sulphide sulphur present in the zinc-containing sulphidic material is converted to elemental sulphur which is easily physically separable from the lead-containing residue.

In a first oxidizing leach, a sufficient stoichiometric acid excess is maintained to minimize or prevent the precipitation of iron in the autoclave, thereby allowing the recovery of a high lead-silver product from the leach residue. The presence of iron in the leach solution offers the opportunity to precipitate the iron from that solution, following suitable purification steps, as a relatively pure, marketable product in the form of hematite. The purification steps prior to precipitation of iron as hematite include reduction of all ferric iron to the ferrous state and neutralization of acid in the solution. In a second oxidizing leach, which is carried out to substantially neutralize the sulphuric acid and precipitate residual iron present in the solution following the iron precipitation step, lead and silver and a substantial portion of the iron present in the zinc- containing sulphidic material treated in this leach report to the leach residue. The oxidic portion of this residue is recovered and contacted with aqueous sulphuric acid solution to substantially dissolve all of the iron and upgrade the lead and silver content of the residue. The slurry from this leach may be transferred to the liquid- solid separation of the first oxidizing leach, such that substantially all of the soluble iron entering the process reports to the first oxidizing leach solution and subsequently to the iron precipitation step, and substantially all of the lead and silver values entering the process report to a high grade lead-silver product which is recovered from the residue of the first oxidizing leach.

More particularly, the process of the invention for recovering zinc and iron from zinc- and iron-containing sulphidic material which also contains lead and silver, comprises leaching the material under oxidizing conditions in a first oxidizing leach at a temperature in the range of about 130° to about 170°C in aqueous sulphuric acid solution, conveniently spent electrolyte from electrowinning of zinc, with a stoichiometric excess of sulphuric acid relative to the zinc and lead content of the material of from about 50 to about 100% so as to provide a terminal acidity of about 50 to 80 g/L to produce elemental sulphur and lead- and silver-containing residue and a leach solution containing zinc and iron, separating the sulphur and lead- and silver-containing residue from the zinc- and iron-containing leach solution in a liquid-solid separator, treating the residue to recover elemental sulphur, lead and silver values, reacting the zinc- and iron-containing solution with zinc- and iron-containing sulphidic material under partially oxidizing conditions in a neutralization- reduction leach at a temperature in the range of about 75° to 115°C to partially neutralize excess acid and to substantially reduce ferric iron to the ferrous state to produce a residue containing unreacted zinc concentrate, elemental sulphur and lead sulphate and a leach solution containing zinc and iron values, separating the residue from the leach solution and recycling the said residue either to the first or to a second oxidizing leach, treating the leach solution under oxidizing conditions at a temperature in the range of about 170° to 230°C to precipitate iron as hematite, separating the solution containing zinc, sulphuric acid and iron from the hematite and reacting said solution with a stoichiometric excess of zinc- and iron-containing sulphidic material in a second oxidizing leach at a temperature in the range of about 130° to 170°C to neutralize the sulphuric acid and precipitate iron to produce a residue containing a sulphidic fraction containing elemental sulphur and unleached sulphides and an oxidic fraction containing precipitated iron and lead and silver values, reacting at least the oxidic fraction with aqueous sulphuric acid solution, conveniently spent electrolyte from electrowinning of zinc, to dissolve iron and upgrade the contained lead and silver values, feeding the resulting slurry to the first oxidizing leach or to the thickener of the first oxidizing leach whereby soluble iron reports to the zinc- and iron-containing solution, neutralizing the solution from the second oxidizing leach to precipitate iron and recovering zinc from said solution. Preferably, the sulphidic fraction is separated from the oxidic fraction, such as by flotation, the sulphidic fraction is recycled to the first oxidizing leach, and the oxidic fraction is reacted with spent electrolyte to dissolve iron and upgrade contained leach and silver values.

The leach solution from the neutralization- reduction leach preferably is neutralized prior to precipitation of iron as hematite. The leach solution can by neutralized to a pH in the range of 3 to 5 by the addition of an effective amount of a neutralizing agent such as limestone or lime. The elemental sulphur from the lead- and silver-containing residue from the first oxidizing leach may be purified to produce a sulphur product.

Embodiments of the process of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic flowsheet of an embodiment of the process of the invention; Figure 2 is a schematic flowsheet of another embodiment of the process of the invention; and Figures 3, 4, 5, and 6 are schematic flowsheets of further variants of the process of the invention. With reference first to Figure 1 of the drawings, a portion of the concentrate to be treated is fed to high acid leach 10 (first oxidizing leach), together with unreacted concentrate from low acid leach 12 (second oxidizing leach), to be described, at a temperature in the range of about 130° to 170°C in aqueous sulphuric acid solution with a stoichiometric excess of sulphuric acid relative to the zinc and lead contents of about 50 to 100% so as to yield a terminal acidity of from about 50 to 80 g/L to produce elemental sulphur and lead- and silver- containing residue and a leach solution containing zinc and iron. The elemental sulphur and the silver and lead containing residue are separated from the zinc and iron containing solution in thickener 14 and treated to recover the elemental sulphur, lead and silver values.

The zinc- and iron-containing leach solution is reacted with a portion of the zinc concentrate in the neutralization-reduction 16 at a temperature in the range of about 75° to 115°C and a pressure in the range of atmospheric to about 300 kPa (gauge) to partially neutralize excess acid and to reduce the ferric iron in solution to the ferrous state. Sufficient oxygen is added to provide partially oxidizing conditions and promote the consumption of excess acid by reaction with zinc concentrate, without further oxidation of ferrous iron to the ferric state once the excess acid is consumed. The neutralization-reduction residue containing unreacted zinc concentrate, elemental sulphur and lead sulphate is separated from the leach solution in liquid-solid separator 18 and recycled to first oxidizing leach 10 for the recovery of metal values and elemental sulphur. The neutralization-reduction solution, containing zinc and iron values is neutralized further to the range of pH 3 to pH 5 in neutralization step 20 by the addition of a suitable neutralizing agent such as limestone or lime and, following liquid-solid separation 22 for removal of gypsum, the solution is treated under oxidizing conditions at a temperature in the range of 170° to 230°C in hematite precipitation 24 to precipitate iron as a saleable hematite product which is recovered from liquid-solid separation 26.

The recovered solution from hematite precipitation 24, containing zinc, sulphuric acid and residual iron, is contacted with the remainder of the zinc concentrate in low acid leach 12 at a temperature in the range of about 130° to 170°C with a stoichiometric excess of zinc concentrate. Substantially all of the sulphuric acid is neutralized and the iron is precipitated to produce a solid residue containing a sulphidic fraction comprised of elemental sulphur and unleached sulphides and an oxidic fraction comprised of precipitated iron and lead and silver values from the zinc concentrate. This solid residue is separated from the leach solution by liquid-solid separator 28 and treated to separate the sulphidic fraction from the oxidic fraction such as by flotation step 30.

The sulphidic fraction from flotation step 30 is recycled to the high acid leach 10 and the oxidic fraction is subjected to iron dissolution 32 by contact with aqueous sulphuric acid solution to substantially dissolve all iron and upgrade the lead and silver content of the residue before transfer to thickener 14 to ensure that essentially all soluble iron is picked up in the liquid phase in thickener 14 for subsequent precipitation as hematite and the lead and silver are recovered as lead-silver residue.

The solution from liquid-solid separator 28 is subjected to neutralization in step 34 by the addition of a neutralizing agent such as zinc oxide fume, zinc hydroxide, basic zinc sulphate, limestone and slaked lime in an amount sufficient to neutralize the sulphuric acid and precipitate iron. Precipitated iron is removed by liquid-solid separator 36 and the solution purified in step 38 and treated by electrowinning 39 for the production of zinc cathode metal. Spent electrolyte from electrowinning can be used for the sulphuric acid solution in the first oxidizing leach 10 and iron dissolution 32.

The iron removal step 34 may conveniently be carried out in two stages with the first stage operated at about pH 3.5 and the second stage operated at about pH 5. The product liquor from the second stage proceeds to purification 38, while the second stage solids are recycled to the first stage for recovery of zinc values precipitated at the higher pH. The product solids from the first stage are combined with the neutralization solids from liquid- solid separator 22 as an iron-containing gypsum residue.

The process shown in Figure 2 differs from the process of Figure 1 in that all of the return electrolyte from electrowinning is treated in iron dissolution 46, and the iron dissolution slurry is fed directly to the high acid leach 48. Control of the terminal acidity in the iron dissolution step is no longer required.

The process shown in Figure 3 differs from that shown in Figure 2 in that zinc concentrate and bulk lead- zinc concentrate which contains significantly higher lead and usually higher silver contents than zinc concentrate,are treated separately. All of the bulk concentrate is fed to the high acid leach 90, and all of the zinc concentrate is fed to the neutralization- reduction step 92. Acid generated by hydrolysis of iron in hematite precipitation 100 is neutralized with neutralization-reduction residue in the low acid leach step 104.

The process shown in Figure 4 differs from the process of Figure 2 in that an oxidic fraction is not separated from the low acid leach residue by flotation for treatment in iron dissolution. The entire low acid leach residue is treated with return electrolyte from electrowinning in iron dissolution 60, and the resulting slurry is fed directly to high acid leach 62.

With reference to the process shown in Figure 5, the iron cake from the iron removal step 50 is processed along with the iron precipitated in the low acid leach 52 in the iron dissolution step 54. More of the dissolved iron in the process now reports to the hematite product. A further significant advantage to this variation is that a two stage iron removal process is not required. Iron removal 50 can be carried out in one stage at pH 5 and zinc that reports to the iron cake is recovered in the iron dissolution step 54. A more facile liquid-solid separation step 70 is included after the iron dissolution step, to avoid dilution of the high acid leach residue with gypsum. This liquid-solid separation step could be replaced by flotation to separate the lead and gypsum fractions following the high acid leach if the concentrate feed to low acid leach 52 contains sufficient lead and silver values to justify recovery of these values with the lead- silver product.

In the process shown in Figure 6, hematite produced is ponded rather than sold. The quality of the hematite is of less importance and the neutralization step is therefore unnecessary and has been deleted. This saves substantially on limestone or lime consumption and considerably less gypsum residue is produced.

The present invention has particular application for new zinc plants in a "greenfields" location where local environmental regulations preclude the disposal of conventional zinc plant iron residues such as jarosite, goethite or paragoethites, and/or where a high recovery of lead and silver from the zinc-containing sulphidic materials is desirable. The zinc-containing sulphidic material may be of any grade with respect to iron and lead content. The invention is demonstrated for treatment of a combination of zinc concentrate and bulk zinc-lead concentrate in the following non-limitative examples. O 94/21830

- 11 -

Example 1 Neutralization-Reduction

Seven liters of synthetic high acid leach solution, containing 17 g/L Fe³⁺, 1 g L Fe²⁺, 67 g/L H2SO₄ and 94 g L Zn was contacted with 1.2 kg zinc concentrate, containing 9.3% Fe, 3.7% Pb, 0.014% Ag, 31.3% S and 48.3% Zn, at 110'C under 100 kPa oxygen overpressure, with agitation, for two hours. Analyses for the product solution and solids are given in the table below.

Example 2 Neutralization

The product solution from example 1 was contacted with slaked lime slurry at 85^*C for one hour, to give a solution of pH 4 to 5. The composition of the products is given in the table below.

Example 3 Hematite Precipitation

The product solution from example 2 was treated at 200^*C in an autoclave with oxygen sparging to give a total pressure of 2000 kPa (gauge) for two hours. Analyses for the resulting solution and solids are given in the table below.

Example 4 Low Acid Leach

The product solution from example 3 was contacted with the product solids from example 1 in an autoclave at 150'C with oxygen sparging to give a total pressure of 1100 kPa (gauge) for two hours. Calcium lignosulphonate and quebracho additives were combined with the autoclave charge, at a rate of 1 kg each per tonne of zinc concentrate treated in example 1. Analyses for the products are given in the table below. Rate samples collected during the test indicated that very lirde reaction occurred after the first hour.

Example 5 Low Acid Leach Flotation

The leach residue from example 4 was repulped in leach liquor to a slurry containing 15% by weight solids and subjected to eighteen minutes of rougher-scavenger flotation, followed by 10 minutes of cleaner flotation of the rougher-scavenger concentrate. No reagents were added to assist the flotation. The analyses of the product streams, including elemental sulphur analyses, are given in the table below.

Example 6 Iron Dissolution

The rougher-scavenger and cleaner flotation tailings from example 5 were mixed with 7.2 L of commercial zinc spent electrolyte, containing 168 g/L H SO₄ and 51 g/L Zn for two hours at 90^*C in an atmospheric vessel. Analyses of samples of the resulting solution and solids are given in the table below. The bulk of the product was kept as a slurry for further treatment.

SUBSTITUTE SHEET Example 7 High Acid Leach

The cleaner concentrate from example 5 and the product slurry from example 6 were combined with 0.7 kg bulk zinc-lead concentrate, containing 13.3% Fe, 12.2% Pb, 0.034% Ag, 27.9% S and 33.1% Zn, in an autoclave at 150^*C with oxygen sparging to give a total pressure of 1100 kPa (gauge) for one hour. Calcium lignosuiphonate and quebracho additives were combined with the autoclave charge, at a rate of 1 kg each per tonne of bulk concentrate treated. Analyses for the products are given in the table below. The product solution was very similar in composition to the synthetic solution used in example 1.

Example 8 High Acid Leach Flotation

The leach residue resulting from a test earned out as in example 7 was repulped in leach liquor to a sluny containing 13% by weight solids and subjected to eighteen minutes of rougher-scavenger flotation, followed by 10 minutes of cleaner flotation of the rougher- scavenger concentrate. No reagents were added to assist the flotation. The analyses of the product streams are given in the table below. The flotation scavenger tailings represents the product lead-silver residue for the process.

Example 9 Sulphur Melting and Filtration

The cleaner flotation concentrate described in example 8 was melted in an oven at 150^*C and vacuum filtered to separate molten sulphur in the filtrate from a filter cake. The resulting sulphide filter cake contained 37% elemental sulphur by entrainment, representing a recovery of 81% of the elemental sulphur in the sulphur concentrate to the elemental sulphur filtrate.

S Example 10 Iron Removal

Low acid leach solution generated as described in example 4 was heated to 85^*C for one hour with neutralization to pH 3.5 by addition of slaked lime. The solution from this first stage of iron removal was maintained at 85'C for an additional hour with addition of slaked lime to give a solution of pH 5. Analyses for the product iron free solution (second stage) and solids (first stage) are given in the table below.

The overall distribution of iron, lead, silver, elemental sulphur and zinc in the feeds and products for two cycles of batch tests according to the examples described above is given in the table below.

It will be understood that changes and modifica¬ tions may be made in the embodiments of the invention with¬ out departing from the scope and purview of the appended claims.

Claims

1. A process for recovering zinc and iron from zinc- and iron-containing sulphidic material which also contains lead and silver, the process comprising leaching the material under oxidizing conditions in a first oxidizing leach at a temperature in the range of about 130° to about 170°C in aqueous sulphuric acid solution with a stoichiometric excess of sulphuric acid relative to the zinc and lead content of the material of from about 50 to about 100% effective to provide a terminal acidity of about 50 to about 80 g/L to produce elemental sulphur and lead- and silver-containing residue and a leach solution containing zinc and iron, separating the zinc- and iron- containing leach solution from the sulphur and lead- and silver-containing residue in a liquid-solid separator, reacting the zinc- and iron-containing solution from the first oxidizing leach with zinc- and iron-containing sulphidic material under partially oxidizing conditions in a neutralization-reduction leach at a temperature in the range of about 75° to 115°C to partially neutralize excess acid and to substantially reduce ferric iron in the solution to the ferrous state to produce a residue containing unreacted zinc concentrate and elemental sulphur and a leach solution containing zinc and iron values, separating the leach solution from the residue of the neutralization-reduction leach and treating the said leach solution under oxidizing conditions at a temperature in the range of about 170° to 230°C to precipitate iron as hematite and to produce a solution containing zinc, sulphuric acid and residual iron, separating the solution containing zinc, sulphuric acid and residual iron from the hematite and reacting said solution with a stoichiometric excess of zinc and iron-containing sulphidic material in a second oxidizing leach at a temperature in the range of about 130° to 170°C to neutralize the sulphuric acid and precipitate iron to produce a residue containing β sulphidic fraction containing elemental sulphur and unleached sulphides and an oxidic fraction containing precipitated iron and lead and silver values, reacting at least the oxidic fraction with aqueous sulphuric acid solution to dissolve iron and upgrade the contained lead and silver values, feeding the resulting slurry optionally to the first oxidizing leach or to the first oxidizing leach liquid-solid separator whereby soluble iron reports to the zinc and iron-containing solution, and the lead and silver values report to the lead- and silver-containing residue, neutralizing the solution from the second oxidizing leach to precipitate residual iron, and recovering zinc from said neutralized solution.

2. A process as claimed in claim 1 in which the zinc is recovered by electrowinning from neutralized solution from the second oxidizing leach thereby producing spent electrolyte, and the aqueous sulphuric acid solution used in the first oxidizing leach is spent electrolyte from zinc electrowinning.

3. A process as claimed in claim 1 in which the zinc is recovered by electrowinning of the neutralized solution from the second oxidizing leach thereby producing spent electrolyte, and reacting at least the oxidic fraction from the second oxidizing leach with spent electrolyte from zinc electrowinning to dissolve iron and upgrade lead and silver values, and feeding the resulting slurry optionally to the first oxidizing leach or to the first oxidizing leach liquid-solid separator.

4. A process as claimed in claim 3, reacting the sulphidic fraction and the oxidic fraction from the second oxidizing leach with spent electrolyte from the zinc electrowinning to dissolve iron, and feeding the resulting slurry to the first oxidizing leach.

5. A process as claimed in claim 1, separating the elemental sulphur from the lead- and silver-containing residue from the first oxidizing leach, and purifying the elemental sulphur to produce a sulphur product.

6. A process as claimed in claim 1 in which the stoichiometric excess of zinc- and iron-containing material in the second oxidizing leach is recycled residue from the neutralization-reduction leach.

7. A process as claimed in claim 1 in which the residue from the neutralization-reduction leach is recycled to the first oxidizing leach.

8. A process as claimed in claim 7 in which the leach solution from the neutralization-reduction leach is neutralized prior to precipitation of iron as hematite.

9. A process as claimed in claim 8 in which the separated solution from the neutralization-reduction leach is neutralized to a pH in the range of 3 to 5 by the addition of an effective amount of a limestone or lime neutralizing agent.

10. A process as claimed in claim 9 including separating the sulphidic fraction from the oxidic fraction of the residue from the second oxidizing leach and recycling the sulphidic fraction to the first oxidizing leach.

11. A process as claimed in claim 1 in which the leach solution from the second oxidizing leach is neutralized by the addition of an effective amount of a neutralizing agent selected from the group consisting of zinc oxide fume, zinc hydroxide, basic zinc sulphate, limestone and slaked lime for iron removal, and the said solution is purified prior to electrowinning for the recovery of zinc.

12. A process as claimed in claim 11 in which the zinc is recovered by electrowinning of the neutralized solution from the second oxidizing leach thereby producing spent electrolyte, and recycling the spent electrolyte in part to the first oxidizing leach and in part to the step for dissolution of iron with upgrade of contained lead and silver values.

13. A process as claimed in claim 3 in which the leach solution from the second oxidizing leach is neutralized by the addition of a neutralizing agent selected from the group consisting of zinc oxide fume, zinc hydroxide ,basic zinc sulphate, limestone and slaked lime for iron removal, iron precipitated from the leach solution from the second oxidizing leach during the neutralization step and precipitated iron in the residue from the second oxidizing leach are reacted in the iron dissolution with spent electrolyte from electrowinning, the resulting slurry is optionally subjected to a liquid-solid separation, and the separated solution is recycled to the first oxidizing leach liquid-solid separatoror the resulting slurry is recycled directly to the first oxidizing leach liquid-solid separator.

14. A process for recovering zinc and iron from zinc- and iron-containing sulphidic material which also contains lead and silver, the process comprising leaching the material under oxidizing conditions in a first oxidizing leach at a temperature in the range of about 130° to about 170°C in aqueous sulphuric acid solution with a stoichiometric excess of sulphuric acid relative to the zinc and lead content of the material of from about 50 to about 100% effective to provide a terminal acidity of about 50 to about 80 g/L to produce elemental sulphur and lead- and silver-containing residue and a leach solution containing zinc and iron, separating the zinc- and iron- containing leach solution from the sulphur and lead- and silver-containing residue in a liquid-solid separator, reacting the zinc- and iron-containing solution from the first oxidizing leach with zinc- and iron-containing sulphidic material under partially oxidizing conditions in a neutralization-reduction leach at a temperature in the range of about 75° to 115°C to partially neutralize excess acid and to substantially reduce ferric iron in the solution to the ferrous state to produce a residue containing unreacted zinc concentrate and elemental sulphur and a leach solution containing zinc and iron values, separating the leach solution from the residue of the neutralization-reduction leach, recycling the said residue to the first oxidizing leach, neutralizing the leach solution, separating the neutralized leach solution from the neutralization residue and treating the said neutralized leach solution under oxidizing conditions at a temperature in the range of about 170° to 230° to precipitate iron as hematite and produce a solution containing zinc, sulphuric acid and residual iron, separating the solution containing zinc, sulphuric acid and residual iron from the hematite and reacting said solution with a stoichiometric excess of zinc and iron-containing sulphidic material in a second oxidizing leach at a temperature in the range of about 130° to 170°C to neutralize the sulphuric acid and precipitate iron to produce a residue containing a sulphidic fraction containing elemental sulphur and unleached sulphides and an oxidic fraction containing precipitated iron and lead and silver values, separating the sulphidic fraction from the oxidic fraction by flotation, recycling the sulphidic fraction to the first oxidizing leach, reacting the oxidic fraction with aqueous sulphuric acid solution to dissolve iron and upgrade the contained lead and silver values in an iron dissolution step, feeding the resulting slurry to the first oxidizing leach liquid-solid separator whereby soluble iron reports to the zinc and iron-containing solution, and the lead and silver values report to the lead- and silver-containing residue, neutralizing the solution from the second oxidizing leach to precipitate residual iron, recovering zinc from said neutralized solution by electrowinning, thereby producing spent electrolyte, and recycling spent electrolyte in part to the first oxiziding leach and in part to the iron dissolution step.