The following article appears in the journal JOM, 51 (12) (1999), pp. 23-25.

Iron Control in Zinc Pressure Leach Processes

The occurrence of zinc in sulfide ore deposits is generally accompanied by various iron minerals. Hence, even the most efficient concentrators generally produce a zinc concentrate with significant iron content. The efficient recovery of zinc metal from zinc concentrates requires the rejection of iron residue in a form that minimizes the zinc entrainment. Careful control of the iron precipitation step is important, so that the iron residue produced is amenable to efficient liquid-solid separation in order to obtain high zinc recoveries. In hydrometallurgical zinc processes, the coprecipitation of minor impurities along with iron precipitation is also important in producing zinc-sulfate solution from which high-purity zinc cathode can be electrowon. The integration of Dynatec's zinc pressure leach process with existing roast-leach-electrowin plants employing various methods of iron rejection is briefly described in this article, along with the application of two-stage pressure leaching in stand-alone processes.

INTRODUCTION

Conventional zinc concentrates typically contain 5-10 percent iron. The iron commonly associated with zinc concentrates can be present as either a replacement for zinc in sphalerite or marmatite or as separate minerals, such as pyrite, pyrrhotite, or chalcopyrite. Consequently, the disposal of iron residues is an integral part of the design and operation of zinc refineries.

Zinc has been recovered from sulfide concentrates by hydrometallurgical routes for almost a century. The disposal of iron was a major difficulty for the industry for many decades and was directly responsible for low overall zinc recoveries. The introduction of the jarosite process in the mid-1960s allowed the precipitation of iron in an easily filterable form, thereby increasing overall zinc recovery. Subsequently, three additional iron precipitation processes have found commercial application in the zinc industry the goethite process, the paragoethite process, and the hematite process.

Meeting process criteria as well as complying with environmental legislation are two important issues that must be addressed when designing the flow sheet for a zinc refinery. Impurity deportment must also be considered. Potential feeds must be evaluated, and consideration must be given to the iron removal and purification stages to ensure that the zinc-bearing solution is amenable to producing high-quality zinc cathode in the cellhouse. Additionally, if the feed concentrates contain impurity elements in sufficient quantities such that recovery is economically justified, this should also be incorporated into the process design. The volume of the iron residue and its stability are factors that must be addressed in the disposal of tailings and long-term storage.

Throughout the 1980s and the 1990s, the application of the Dynatec zinc pressure leach process to the treatment of a wide variety of zinc-bearing concentrates has been extensively studied. While the major objective of this work was to maximize zinc recovery, much effort has been focused on the behavior of iron in the process.

PROCESS CHEMISTRY

The zinc pressure leach process depends upon the following simple reactions in which zinc sulfide, pyrrhotite, or iron in sphalerite, galena, and chalcopyrite react with sulfuric acid and oxygen to produce metal sulfates and elemental sulfur.

ZnS + H₂SO₄ + 0.5O₂ —> ZnSO₄ + H₂O + S°

FeS + H₂SO₄ + 0.5O₂ —> FeSO₄ + H₂O + S°

CuFeS₂ + 2H₂SO₄ + O₂ —> CuSO₄ + FeSO₄ + 2H₂O + 2S°

These reactions are slow in the absence of a species that will facilitate oxygen transfer. One such species is dissolved iron. The net reaction for leaching zinc sulfide, as shown above, is the sum of the following two reactions:

ZnS + Fe₂(SO₄)₃ —> ZnSO₄ + 2FeSO₄ + S°

2FeSO₄ + H₂SO₄ + 0.5O₂ —> Fe₂(SO₄)₃ + H₂O

Normally, there is sufficient acid-soluble iron in the concentrate to supply the needs of the leach.

Pyrite is present in most zinc concentrates, and the extent of pyrite oxidation in the pressure leach depends on a number of leaching parameters. Under normal zinc pressure leaching conditions, a limited portion of the sulfide content of pyrite is oxidized directly to sulfate as shown below, with little or no elemental sulfur production.

FeS₂ + H₂O + 3.5O₂ — > FeSO₄ + H₂SO₄

Since all zinc concentrates contain iron, and the iron minerals are leached during pressure leaching, the behavior of iron is controlled by the acid content of the pressure-leach solution. The pressure leach of zinc concentrate is normally designated as a low-acid leach or a high-acid leach.

In a low-acid leach, the quantity of acid provided is in slight excess to that required to leach all of the zinc present in the concentrate. Since the iron minerals, with the exception of pyrite, are in competition with the zinc minerals for the acid, acid concentrations fall rapidly in the autoclave, and the hydrolysis and precipitation of iron is favored. The following precipitation reactions, which regenerate sulfuric acid, are commonly observed during the pressure leaching of zinc concentrates.

Fe₂(SO₄)₃ + (x + 3) H₂O —> Fe₂O₃·xH₂O + 3H₂SO₄

Fe₂(SO₄)₃ + 2H₂O —> 2FeOHSO₄ + H₂SO₄

3Fe₂(SO₄)₃ + 14H₂O —> 2H₃OFe₃(SO₄)₂ (OH)₆ + 5H₂SO₄

PbSO₄ + 3Fe₂(SO₄)₃ + 12H₂O —> PbFe₆(SO₄)₄(OH) ₁₂ + 6H₂SO₄

If sodium, potassium, or ammonium ions are present in the leach solution, the corresponding jarosite species will be precipitated in the low-acid leach.

The precipitation of iron in the pressure-leach autoclave may be minimized by maintaining a relatively high residual acid concentration in the pressure-leach solution. In situations where the lead and silver contents of the feed concentrates are significant, it may be desirable to limit the precipitation of iron during the zinc extraction process in order to separate a high-grade lead-silver product from the pressure-leach residue. Maintaining the sulfuric acid concentration in solution above 50 g/l is sufficient to limit the iron precipitation to a minor amount.

INTEGRATING ZINC PRESSURE LEACHING WITH ROAST-LEACH OPERATIONS

Pressure-leach technology has been successfully integrated with existing roast-leach operations. The first commercial integration of the Dynatec zinc pressure leach process was at the Cominco Zinc Refinery in Trail, British Columbia, in 1980. The expansion and ensuing operation are well documented.^1-3 In the process as practiced by Cominco, the bulk of the iron contained in the zinc concentrate fed to pressure leaching is initially extracted to solution, with a portion of the dissolved iron reprecipitating in the autoclave as plumbojarosite. Following removal of sulfur by flotation, the pressure-leach slurry is directed to the calcine leach circuit. Dissolved iron is precipitated in the neutral leach. The neutral leach residue reports to the acid leach step. The acid leach residue is transferred to the lead smelter, where it is converted into a disposable slag.

A low-acid pressure leach circuit was also chosen for integration with the zinc plant at Kidd Creek to expand zinc production capacity. The original plant, constructed in 1972, was based on conventional electrolytic zinc plant technology, including iron precipitation as sodium jarosite. The pressure-leach expansion was commissioned in 1983, and the operation has been described in detail.⁴ Since iron is precipitated as sodium jarosite, sodium ions are present in the spent electrolyte, and some iron is precipitated as sodium jarosite in the low-acid pressure leach. The pressure leach residue is washed and impounded along with the jarosite residue produced in the main plant.

The Ruhr Zink Refinery at Datteln, Germany, also operates a roast-leach-electrowin circuit. For many years, iron was rejected as a marketable hematite product. The Dynatec zinc pressure leach process was selected in 1989 for integration with the existing circuit for a refinery expansion,⁵ and the zinc pressure-leach plant was constructed and operated in the early 1990s.⁶ Since all of the iron present in the zinc concentrates was to be converted to hematite, a high-acid zinc pressure leach was selected for the integration. Spent electrolyte was added in the pressure-leaching stage to give an acid-to-zinc molar ratio of about 1.75:1. The resultant free-acid concentration of the pressure-leach discharge solution was greater than 50 g/l, and iron precipitation was minimal. Following treatment in a reduction step, iron in the pressure-leach solution was precipitated as hematite in separate autoclaves. The pressure-leach autoclave discharge solids were processed in by-product recovery steps for separation of an elemental sulfur flotation concentrate. Lead and silver in the zinc-concentrate feeds were recovered as flotation tailings. Unreacted sulfides were recovered by hot filtration of the sulfur concentrate and transferred to roasting operations.

DYNATEC'S TWO-STAGE ZINC PRESSURE LEACH PROCESSES

Dynatec's two-stage zinc pressure leach processes are applicable where complete replacement of the roast-leach section is required or for a greenfield zinc plant. In the two-stage pressure leach, the second-stage pressure leach liquor is contacted with fresh zinc concentrate in the first stage of pressure leaching to produce a pregnant zinc solution low in residual acid and iron. The pregnant zinc solution from the first-stage leach reports to a neutralization circuit in which the remaining acid is neutralized and the residual iron is precipitated. The neutralized solution then reports to conventional purification with zinc dust followed by electrowinning. The second stage of pressure leaching is run at higher acidity to achieve overall high zinc extraction.

Countercurrent Two-Stage Pressure Leaching

The first two-stage zinc pressure leach plant was commissioned in Flin Flon in the summer of 1993.⁷ This plant, employing Dynatec zinc pressure leach technology, is operated by Hudson Bay Mining and Smelting Company, Ltd.

Figure 1. The two-stage countercurrent zinc pressure leach flow sheet.

The countercurrent zinc pressure leach flow sheet is outlined in Figure 1. All of the fresh zinc concentrate and second-stage pressure leach liquor are fed to the first-stage autoclave. Spent electrolyte is split between the first- and second-stage autoclaves. Zinc extraction in the first stage leach is 75% to 85%. Iron initially extracted in the autoclave is largely reprecipitated, due to the low acidity, as hydrated iron oxides and jarosite. The discharge solution from the first-stage leach, typically containing about 8 g/l free acid and 2 g/l iron, is transferred to solution treatment steps.

The partially leached first-stage residue, typically containing about 15% zinc and about double the iron concentration of the fresh zinc concentrate, is leached with the remainder of the spent electrolyte in the second-stage leach. The acidity in the second-stage leach is maintained at greater than 30 g/l, and the resultant overall zinc extraction is generally greater than 99%. The second-stage residue solids, typically containing 0.3% Zn, 20% Fe, and 60% S, are impounded in a tailings pond.

The second-stage residue may be further treated for precious metals and elemental sulfur recovery if there is sufficient precious metals value in the zinc concentrate feed. To recover precious metals, the second-stage leach residue is subjected to froth flotation for the separation of an elemental sulfur concentrate. Elemental sulfur is then recovered by melting and hot filtration of the sulfur concentrate. In the zinc refinery at Flin Flon, for a period shortly after startup, there was sufficient gold contained in the zinc-concentrate feed to warrant operation of the flotation and sulfur melt and filtration circuits. During this time, the produced sulfide cake was fed to the copper smelter for the recovery of copper and precious metals values. The flotation tailings containing the bulk of the iron in the zinc-concentrate feeds was sent to impoundment.

Cocurrent Two-Stage Pressure Leaching

Figure 2. The two-stage cocurrent zinc pressure leach flow sheet.

Integrated miniplant campaigns piloting a cocurrent two-stage zinc pressure leach flow sheet have also been conducted by Dynatec in Fort Saskatchewan in the 1980s and 1990s. The cocur-rent flow sheet, presented in Figure 2, was chosen to maximize the recovery of silver contained in the zinc-concentrate feeds. The major differences of the cocurrent flow sheet in comparison to the countercurrent flow sheet are that fresh concentrates are fed to both stages, and essentially all of the spent electrolyte is fed to one stage (high acid). Product liquor from this stage is the principal lixiviant in the other (low acid) stage, although additional spent may be directed to the low-acid leach to maintain the overall plant balance.

In the cocurrent process, the first-stage (low acid) leach is run under similar conditions as the first-stage leach in the countercurrent flow sheet. As a result, the first-stage leach discharge solution compositions are similar. However, only a portion of the fresh zinc concentrate, the feed with the lower silver values, is fed to the first-stage leach in the cocurrent flow sheet. The residue produced in the first stage is subjected to froth flotation. The flotation tailings, containing the bulk of the iron in all of the feed concentrates, is impounded.

The first-stage leach flotation concentrate, containing unreacted sulfides and elemental sulfur, is directed to the sec ond-stage (high acid) leach along with the fresh zinc concentrate containing the higher silver values and spent electrolyte. The acidity in the second-stage leach is maintained greater than 50 g/l to ensure minimal precipitation of iron. The second-stage residue is separated by flotation to recover an elemental sulfur concentrate and a flotation tailings product high in lead and silver content. The bulk of the iron remains in solution and reports to the first-stage leach where it is precipitated as hydrated iron oxides and jarosite and rejected.

Fluoride Deportment

Fluoride is a common contaminant in zinc concentrates, and controlling fluoride is an important consideration where zinc is recovered by electrowinning. High fluoride levels in solution can result in the sticking of electrowon zinc to the aluminum cathodes. The generally acceptable maximum concentration of fluoride in zinc electrowinning is 10-20 mg/l, and modern cellhouses employing jumbo cathodes and automated stripping prefer a maximum fluoride concentration of 10 mg/l to ensure uninterrupted stripping.

The Hudson Bay zinc refinery, where zinc pressure leaching has completely replaced roasting, has successfully demonstrated the pressure-leach process for the treatment of zinc concentrates containing typical fluoride concentrations of 50-90 g/t. Fluoride concentrations in the spent electrolyte are 5-7 mg/l, comparable to about 8 mg/l prior to the implementation of direct pressure leaching. The deportment of minor elements, including fluoride, at the Hudson Bay facility has been discussed in detail in recent papers.^8-10

Zinc concentrates recently tested in Fort Saskatchewan contained relatively high levels of fluoride, with the low-silver and highsilver zinc concentrate feeds containing 376 g/t and 791 g/t fluoride, respectively. Preleaching of the concentrates with spent electrolyte successfully removed a large fraction of the fluoride, with minimal zinc extraction, and the preleached low-silver and high-silver feeds contained 109 g/t and 34 g/t fluoride, respectively. The pre-leach liquor was neutralized with lime, and approximately 80 percent of the contained fluoride was precipitated along with the iron and aluminum, leaving most of the zinc in solution for subsequent recovery. This work is reported in detail elsewhere.¹¹

After preleaching, the zinc concentrates were treated in a cocurrent zinc pressure leach miniplant campaign. Fluoride pick-up to solution in the second-stage leach was about 9 mg/l, producing a solution containing 20 mg/l fluoride. Despite the relatively high concentration of fluoride in the first-stage leach feed, there was a net precipitation of fluoride in the first-stage leach, which can be attributed to coprecipitation of fluoride with iron in this autoclave. The fluoride level in the first-stage solution was about 13 mg/l, compared with 11 mg/l fluoride in the spent electrolyte fed to the second-stage leach. Additional tests following the miniplant campaign identified means to decrease the fluoride concentration in the first-stage pressure leach solution to between 5 mg/l and 10 mg/l.¹¹

CONCLUSIONS

Iron behavior is readily controlled in the pressure leaching of zinc concentrates. Depending on overall requirements for the zinc plant (e.g., en-vironmental legislation, impurities deportment, or by-product rcovery options), iron may be precipitated at various locations in the flow sheet and in a variety of forms.

K.R. Buban, M.J. Collins, and I.M. Masters are currently with the Metallurgical Technologies Division of Dynatec Corporation.