sustainability-driven r&d in zinc hydrometallurgy ... · sustainability-driven r&d in zinc...

Sustainability-driven R&D in Zinc Hydrometallurgy: Remembering Lucy Rosato

George P. Demopoulos

Department of Mining and Materials Engineering McGill University

3610 University Street Montreal, QC, H3A0C5

[email protected]

ABSTRACT For more than 20 years the author had the privilege of working closely on a number of R&D projects with Lucy Rosato and her team at CEZinc. Several of these projects were driven from the sustainability agenda that Lucy Rosato championed as an innovation leader that supported research collaborations. Highlights from this highly rewarding collaboration are to be presented in this paper. In particular we will deal with the following works-topics: (a) iron control via solvent extraction as alternative to jarosite precipitation; (b) production of high-density gypsum by controlled neutralization of simulated zinc plant process/waste waters; (c) selenium removal from weak acid solutions; (d) electrolyte purification by cementation; and (e) manganese control by oxidative precipitation with SO2/O2.

Keywords: Hydrometallurgy, iron control, effluent treatment, manganese control, selenium

removal, zinc industry

PREFACE: A PERSONAL TESTIMONIAL IN MEMORY OF LUCY ROSATO

Since I first met Lucy Rosato in mid eighties, when she was a research hydrometallurgist at the Noranda Technology Centre in Pointe-Claire, Quebec, I remember her for her dynamism, openness, and enthusiasm by which she carried out her work and engaged in discussions. Her natural and effective leadership would become immediately evident to everyone. In 1988-90, I had the opportunity to witness first hand how she put together as conference chairman (that’s how we called her as up to that point no woman had chaired a conference or the Hydrometallurgy Section) the most successful Hydrometallurgy Meeting that was held in Montreal in October 1990 with over 220 attendees- one of the best-attended meetings ever; and after the hard work she would celebrate by hosting the members of the organizing committee with smoked salmon and champagne! I still have from that event the personalized notebook case that she offered to each of us as a gift. Lucy was a true trailblazer with extraordinary vision, competency, and influence that makes her loss at the height of her career even more tragic. It was a privilege to have met her and work on a number of projects with her.

COM 2015 | THE CONFERENCE OF METALLURGISTS hosting AMCAA | America's Conference on Aluminum Alloys ISBN: 978-1-926872-32-2

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I. INTRODUCTION

Hydrometallurgy in the 70’s was heralded as clean technology because at that time air pollution from old high stack smelters was a serious problem something that hydrometallurgical plants could avoid. In this context zinc hydrometallurgy came to dominate zinc extraction from concentrates leaving way behind processes like the older carbon-based calcine reducing furnaces or even the Imperial Smelting process. The Roast-Leach-Electrowin extractive process became the norm producing > 85% of world zinc metal output, in this including the more recent 100% hydro processes as is ZPL (but also direct atmospheric leaching). However, hydrometallurgy is not without its own bag of problems especially when it comes to generation of large tonnage of waste solids like tailings and residues or the release of soluble toxic elements that calls for effective effluent treatment processes. In this context it is important to recall that the economic feasibility of a modern zinc RLE plant depends among other on having access to a sulphuric acid market, on employing an environmentally viable iron disposal technology, on achieving high EW current efficiency that in turn depends on tight impurity control and low zinc consumption during purification but also on valuable by-product metal recovery, such as Ag, In, or Ge. For each tonne of zinc metal produced typically two tonnes of acid are produced, which explains the drive in developing and commercializing the last 30 years of direct zinc concentrate leaching technologies. Further, depending on the iron content of the concentrate and the method of iron disposal, namely jarosite, goethite or hematite some 0.2-1.0 tonnes of iron residues are generated in a zinc plant that constitute an environmental burden [1]. At the same time due to sensitivity of the EW process to the presence of impurities that can electrocatalyze the evolution of hydrogen, impurity control in zinc hydrometallurgical plants is of paramount importance [2]. Thus environmental compliance, process performance and economic viability are critical in achieving sustainability in a highly competitive business, as is the metal commodity sector.

Lucy Rosato as true visionary and innovation leader saw the importance of research in promoting the sustainability-driven modernization of CEZinc initiating and executing/sponsoring a series of major R&D projects. In this context she welcomed and encouraged collaborations both within the Noranda corporate system by having metallurgists from CEZinc working with researchers from Noranda Technology Centre but also with researchers external to the company in academia and government (e.g. CANMET in Ottawa). The author acknowledges with gratitude the support he received from Lucy Rosato and Noranda/CEZinc in general over the years working on a multitude of collaborative projects. I am sure the same can be testified by several of my colleagues at McGill or other universities. Unfortunately the loss of major Canadian-owned companies like Noranda, Falconbridge or Inco and the general trend in reduction of R&D spending by the metal industry [3] has seriously constrained opportunities for strategic collaborative R&D partnerships [4]. It is only hoped that symposia as the present one honouring Lucy Rosato − a passionate advocate of technical excellence, research collaboration and sustainability − can mobilize and inspire all parties concerned for a true paradigm shift in developing and adopting clean and efficient innovative hydrometallurgical processes.



Table 1: McGill hydrometallurgy projects supported by CEZinc/NTC.

1. 1989-94: “Kinetics and Modelling of the Hot Acid Leaching of Zinc Ferrites”. Lucy Rosato initiated and sponsored this project. At the time CEZinc was looking into increasing the leaching circuit throughput by increasing the acid concentration, a measure that proved counter-productive! Dimitios Filippou as part of his PhD thesis work [5] carried out a number of fundamental studies with this process including: (a) the dependency of reactivity of zinc ferrites on their magnetic properties [6]; (b) development of a grain leaching model [7]; (c) a chemical model estimating proton activities in mixed sulphate solutions [8]; and (d) development of a continuous CSTR model for the hot-acid leaching circuit [9]. Via this work it was shown that increasing acid concentration does not necessarily translate into acid leaching power as the activity of free protons that drives the reaction is reduced as the metal sulphate concentration increases hence explaining why simple acid addition resulted in lower zinc extraction efficiency. Dimitrios was hired by NTC after his PhD and stayed with the company till NTC’s closure. Now he is Senior Research Engineer with Rio Tinto Iron and Titanium, Tracy, QC and Chair of MetSoc’s Hydrometallurgy Section.

2. 1988; 1996-99: “Solvent Extraction of Iron from Zinc Process Solutions”. This project that was initiated and supported by Lucy Rosato, was part of Noranda/CEZinc’s program into finding alternatives to jarosite iron disposal technologies. Here in collaboration with CANMET (Ron Molnar and George Pouskouleli) we were asked to evaluate the feasibility of iron SX and the generation of iron chloride liquor that could be pyrohydrolyzed [10]. Work at McGill was done by BEng student Ken Quon initially and later by MEng student Frank Principe [11]. Frank initially with NTC after his studies joined Cowan Personnel Ltd.

3. 1995-2000: “Controlled neutralization of zinc plant wastewaters”. The work was initiated and supported by CEZinc’s Technical Services department directed by Lucy Rosato. The work related to CEZinc’s in-house effluent treatment improvement project led by Gary Monteigh. The research was carried out by MEng students Sidney Omelon [12] and Niels Verbaan [13]. Now Sidney is Assistant Professor with Dept. Chemical Eng., University of Ottawa and Niels, Hydrometallurgy manager with SGS-Lakefield Research.

4. 1995-1999: “Electrolyte purification by cementation”. In this project with the support of Lucy Rosato and the active involvement of Georges Houlachi and Dimitrios Filippou from NTC, we focused on the factors controlling cobalt removal and zinc dust consumption [2,14]. The work was carried out by BEng student Anne Giove (1995), MEng student Amy Nelson [15] and PDF (post-doctoral fellow) Trina Dreher [16]. Among the other things the project determined that Cd plays a critical role in Co cementation-a finding that helped CEZinc in optimizing their purification circuit. Anne Giove was hired after graduation by NTC while Amy found employment with Ballard Power in Vancouver and Trina returned to Australia as Research Associate in Melbourne. Later on in 2004, BEng student Linda Restrepo worked on another project led by Liana Centomo at CEZinc that focused on recovery of copper by cementation.

5. 1998-2004: “Use of SO2/O2 to Control Manganese in Zinc Leach Solutions”. This project led to a patented process that was piloted at CEZinc. Lucy Rosato was the initiator and sponsor of the project and co-inventor of the new process [17]. The work was executed initially by PDF (post-doctoral fellow) and co-inventor Qiankun Wang and later by MEng student Vincent Menard [18]. The pilot plant testing at CEZinc was led by NTC’s Yeonuk Choi. Qiankun after leaving McGill worked as hydrometallurgist with Inco (Vale) and then Barrick’s Technology Centre in Vancouver as for Vincent he is now with SNC-Lavalin.

6. 1998-2004: “Production of Alpha Calcium Sulphate Hemihydrate by Reaction of Sulphuric Acid with Lime”. Lucy Rosato initiated and sponsored this project. At the time the sulphuric acid market was not favourable and Lucy asked us to investigate the feasibility of producing a saleable high value construction material, alpha-hemihydrate. PhD student Yuanbing Ling worked on the project [19], while in-house testing at NTC was carried out by Anne Giove and Dimitrios Filippou. Yuanbing till recently he was working as hydrometallurgist at Areva’s Research Centre in France.

7. 2000-2001: “Silver Recovery in Zinc Calcine Leaching”. At the time CEZinc was considering the processing of silver-rich zinc concentrate and Lucy Rosato asked us to look into the behaviour and possible recovery of silver during calcine leaching. The work was led by Peter Kondos of NTC. The project was interrupted unfortunately with the closure of NTC. The work was performed by PDF Yongfeng Jia [20], who now is Professor of environmental engineering in China.

8. 2007-2011: “Selenium Elimination from Weak Acid Effluent Solutions”. This work was initiated by Elyse Benguerel and Liana Centomo of CEZinc’s Technical Services department. At the time the Se removal process developed by CEZinc/NTC under Lucy Rosato’s direction with the involvement among others of Georges Houlachi and Garry Monteigh [21] exhibited erratic performance. The research carried out by PhD student Nicolas Geoffroy [22] determined that the selenium precipitate if not filtered right away has the tendency to re-oxidize and go into solution- a finding that led to elimination of the problem by appropriate circuit intervention. Nicolas is now Consulting Engineer with Experts-Conseils CEP, Montreal.



II. McGILL-NORANDA-CEZinc COLLABORATIVE HYDROMETALURGY PROJECTS

In Table 1 a list of zinc-related projects carried out at McGill’s HydroMET laboratory with the support of CEZinc/Noranda –highlighting Lucy Rosato’s role- is provided along with the names of students/post-doctoral fellows and company collaborators involved. In the Appendix a list of non-zinc projects that were also supported by Noranda is given as well. I decided to mention the names and projects as a powerful example of the great impact Noranda and CEZinc had on university-based research and training of “Highly-Qualified-Personnel” (HQP as labeled by NSERC-the Natural Sciences & Engineering Research Council of Canada) that was unfortunately seriously curtailed after the corporate ownership and direction changes the company went through. But more importantly in making reference to the key role Lucy Rosato played in identifying the research needs and empowering her team and collaborators in addressing them. Corporate leaders, as remarkably practiced by Lucy Rosato, should see research support to universities as the assured way of creating the innovative technical minds that will contribute tomorrow to their sustainable operation, transformation and competitiveness. When it comes to supporting training of HQP through collaborative research projects just in the case of McGill HydroMET over a period of ~24 years (1988-2011), CEZinc/Noranda supported the training of 9 PhD students, 8 MEng students, 5 BEng students and 4 PDFs (refer to Table 1 and the Appendix). Among these are 8 women for whom Lucy Rosato became a powerful role model. We only hope that the mineral/metal/material companies will emulate this paradigm and keep supporting research and training in Canadian university labs.

In the following pages an overview of some of these projects is presented to highlight the influence and vision of Lucy Rosato in zinc hydrometallurgy sustainability R&D.

III. ENVIRONMENTAL-DRIVEN RESEARCH

A. Iron precipitation processes

In hydrometallurgical processes, iron is removed from solution by precipitation. Depending on the precipitated iron products, there are three main types of iron removal processes, namely: (a) the Jarosite process, (b) the Goethite process, and (c) the Hematite process. The respective theoretical formulae of the precipitated iron compounds are: MFe3(SO4)2(OH)6 (jarosite), α-FeOOH (goethite), and α-Fe2O3 (hematite). Several characteristics of these iron precipitates are listed in Table 2[1].

Table 2: The characteristics of the three major iron residues produced in the zinc industry.



The technologies for these iron removal processes have been available since the late sixties, but only the Jarosite process is widely used by the zinc industry due to its lower cost and operational simplicity. The author over the years had the opportunity to work on different aspects of iron precipitation from simple neutralization circuits [23] to hematite in collaboration with Akita Zinc company of Japan [24] to ferric arsenate [25] and scorodite [26] in supersaturation-controlled circuits [27] or autoclaves [28].

Some 25 years ago the generation and disposal of large tonnages of iron residues as is the case for example of those produced by the zinc industry started receiving increasing environmental pressures that raised serious concerns about its sustainability. For example, because of the leachability of zinc, cadmium, and copper, the jarosite residue produced by CEZinc in Valleyfield, QC was characterized as toxic waste by the Quebec Ministry of Environment (MEF) (see Table 3); similarly, the goethite residue from Vieille-Montagne, Belgium and the hematite residue from Akita Zinc, Japan were also found by the same study [29] to be leachable according to the USEPA Toxicity Characteristic Leaching Procedure (TCLP). Under these environmental pressures the global zinc industry looked into either develop stabilization technologies for the produced iron residues, or move towards “waste-free” iron removal technologies altogether [30].

Table 3: Leachability test results on the jarosite residue produced in CEZinc prior to introduction of the Jarofix process (data extracted from [29]).

Under the leadership of Lucy Rosato, CEZinc/Noranda considered a great variety of iron management options that culminated to the development and implementation of a groundbreaking stabilization technology for the zinc industry [29]. The process called “Jarofix” was a technical and economic success that continues after >25 years providing a sustainable solution to the CEZinc iron disposal problem. This was recognized with the awarding of the MetSoc Innovation Award to Lucy Rosato and her team at CEZinc in 2003. The process involves thorough washing of the jarosite residue and mixing with 15% Portand Type X cement. The stabilized jarosite product meets the TCLP test requirements [29] and since its introduction at CEZinc, the Jarofix technology has been licensed to other zinc refineries in the world proving the great impact Lucy Rosato had in this field.

B. Iron solvent extraction

As part of Noranda/CEZinc’s jarosite R&D program, CANMET and McGill’s HydroMET group carried out bench scale and mini pilot-plant trials (in late 80’s/early 90’s) to evaluate the



effectiveness of mono (2-ethylhexyl) phosphoric acid (M2EHPA) in removing iron(III) from industrial zinc process solutions supplied by CEZinc. This work [10] sought to obtain a preliminary assessment of the following conceptual process scheme as possible “waste-free” iron technology: (i) selective extraction of iron from the zinc plant leach liquor; (ii) strong HCI stripping of the loaded organic; and (iii) pyrohydrolysis of the chloride solution to recover iron as saleable iron oxide with simultaneous acid regeneration. The particular extractant was selected because of its low pH functionality, excellent Fe(III)/Zn selectivity, and high loading capacity [31]. The respective extraction and stripping isotherms are reproduced in Figure 1 [10].

The operation of the pilot plant was partially successful meeting the target iron raffinate concentration and Fe/Zn selectivity levels. Most of the impurities could be controlled with an acid scrub. However, the sulphate uptake was significant as it was chloride back transfer during HCl stripping of iron. Both were shown not to be chemically bound allowing for their removal by proper solvent conditioning and scrubbing procedures. A Strip liquor of 75 g/L Fe (as FeCl3) could be consistently produced but this was lower of the target required for pyrohydrolysis. However, the pilot plant study revealed the relatively high solubility and degradation rate of M2EHPA that called for an alternative extractant to be identified.

Figure 1. Extraction isotherms with 40vol.% M2EHPA (plus tridecanol) at 50°C and stripping isotherms with 6 N HCl at 23°C [10].

This prompted further studies focusing on an alternative extractant of better stability and functionality (vis-à-vis M2EHPA and D2EHPA), octylphenyl acid phosphate (OPAP) as well as the development of a reductive stripping method [11,32,33]. OPAP was found to be an excellent extractant for the preparation of concentrated iron chloride strip solutions. Its major advantages included very low sulphate carry-over, manageable zinc carry-over, very low chloride back-extraction, and economic iron build-up and acid balance, when a moderate strength HCl–FeCl2 strip feed (3.5 N HCl– 1 M Fe(II)) was used. By employing a mixed HCl-FeCl2 strip solution the proton activity increases thereby boosting solution stripping power. In addition, a high iron(II) background furthers the possibility of adequate iron build-up in terms of total iron(II,III) since simple iron(III) build-up is not possible. Also, FeCl2 presence was found to lower zinc and sulphate carry-over from loaded OPAP (and D2EHPA) during stripping. The material balance analysis considered a conceptual flowsheet, which integrated “electro-reductive” stripping (use of scrap iron is another option) as an iron concentration step between



the solvent extraction stripping stage and pyrohydrolysis, the final step for iron by-product formation. Figure 2 shows a conceptual flowsheet for a 200,000 tpy capacity zinc plant, which simulates an acid-balanced circuit utilizing the HCl/FeCl2 strip solution. Although such process option was not further pursued it was a very productive project generating a wealth of ideas and processing data that can prove useful to future efforts aiming to extract iron from concentrated sulphate solutions.

Fig. 2. Mass balance flowsheet involving a closed acid loop iron SX integrated 200,000 tpy capacity zinc plant (HAL O/F=hot acid leach overflow;

LO=loaded organic; Rec.Org=recycled organic; SF=strip feed; SL=strip solution; redSL=reduced strip solution; SO=stripped organic). [33]

C. Controlled Neutralization Processes

In a zinc plant various acidic solutions/waste water streams are generated that are treated by lime neutralization (and occasionaly other alkaline reagents) to remove/recover metals prior to discharging water to the environment or recycling it within the plant. Separate neutralization circuits are operated for Zn recovery (as done at CEZinc-BZS circuit-more on this later) or for sulphate removal from non-jarosite leaching circuits as is the case of hematite process operated by Akita Zinc or the 2-stage zinc pressure leaching process. Optimum design of such neutralization processes calls for the application of crystallization principles in particular supersaturation control to achieve well grown precipitate particles for filtration/dewatering ease and minimized metal losses [27]. This is exemplified by referring to the development of the basic zinc sulphate (BZS) process by CEZinc to treat the zinc-rich jarosite wash waters following the introduction of the Jarofix process [34] and to the production of well grown and clean gypsum crystals by lime neutralization of zinc plant effluents (McGill-CEZinc joint research). The CEZinc BZS process The BZS precipitation circuit and its integration with the jarosite washing circuit are shown in Figure 3.



Figure 3: The CEZinc BZS process: Integrated circuit (left) and BZS circuit (right) [34]

The circuit involves hot (80°C) lime neutralization in a 3-tank cascade. The pH (initial pH 3) is raised progressively from one reactor to the other while at the same time up to 1/3 of the precipitate is recycled to provide nucleation seed for crystal growth. Step-wise increase in pH ensures the maintenance of a low supersaturation environment, which is a necessary condition for producing well grown and easily filterable precipitated solids as we demonstrated with research done at McGill since 25 years ago [27]. Depending on the chosen terminal pH (≤6.5≤) Mn may or may not co-precipitate (25 < %Mn < 95). The bulk precipitate has bi-modal size distribution: > 30 μm gypsum crystals; 5 μm BZS particles. The gypsum/BZS precipitate is repulped and returned to the jarosite residue filter where Zn is recovered in the filtrate. In other situations the produced BZS/Zn(OH)2 precipitate may be used as neutralizing agent as done in the 2-stage HBMS pressure leach plant [36,37]. Because of the staged neutralization approach [27] the CEZinc BZS precipitate exhibits improved vacuum filtration properties, in terms of %solids from 30% to 50% and filtration rate.

Production of high-density and clean gypsum The production of clean gypsum (suitable for wallboard product or cement manufacturing) needs to be considered in the context of neutralization of acidic liquors in the zinc industry. Clean gypsum may be produced by either controlling the precipitation conditions (usually maintaining a low pH < 3) or by leaching the metal-contaminated gypsum product to remove/recover the metals. Industrial examples of the production of gypsum (sold to the cement industry) are: (i) the pre-neutralization of the hot-acid leach liquors with limestone to pH 1 in a staged circuit incorporating product recycling/seeding, and pH control as is done at the Iijima Refinery in Japan [38] (first approach); and (ii) the "Basics" Process in the paragoethite leaching circuit at the Hobart Plant in Australia [39] (second approach); clean gypsum (> 95% gypsum and < 0.05 % Zn) is produced by leaching for 2 hours at pH 1 with spent electrolyte. In the context of effluent treatment, clean gypsum is produced (via the second approach, i.e. leaching of Zn-containing gypsum precipitate with spent electrolyte) at the Clarksville Zn plant in Tennessee.

The production of clean and high-density gypsum from zinc plant acidic liquors via the first approach, i.e. controlled precipitation at low pH has been investigated at McGill with the support of CEZinc back in 1995-2000 period [12,13,35,40]. This research has focused on elaborating a neutralization technique that promotes the production of well grown and clean gypsum material via controlled supersaturation (achieved via the step-wise elevation of pH



[27]) and product recycling. An important finding of this work was that, in addition to controlling supersaturation and recycling, the choice of starting seed material is critical. Thus, simple recycling of freshly prepared gypsum material proved ineffective even when all other controls were applied. It was only with the use of synthetic or natural gypsum as seed material [12,40] that high-density gypsum could be produced. The impact of proper seeding is illustrated with the data plotted in Figure 4 [12]. As it can be seen simple recycling of batch made gypsum did not result on high-density gypsum even in the case of staged neutralization. By contrast use of good quality (natural) gypsum during start-up leads to 5-fold increase in density and full realization of the staging effect.

Figure 4: Effect of staging and use of starting seed material on gypsum density; continuous lime neutralization from pH 1 to 5 [12].

The drastic different effect of method of seeding and supersaturation (S) control on gypsum crystallization can be appreciated by examining the SEM photos of Fig. 5. The unseeded/S-controlled system produced gypsum exhibiting the so-called “rosette” morphology characterized by high specific surface area and thus prone to high impurity uptake and poor settling behaviour. The material produced by simple recycling of the freshly (homogeneously) prepared seed (no S-control) was very fine in size that again gave poor settling behaviour. However, the material produced with good quality seed (natural gypsum) and under controlled (low) supersaturation environment proved to be superior to the other ones. The attainment of low supersaturation via staged neutralization is exemplified with the data of Fig. 5 (top-right). Finally the combined use of supersaturation control and proper seeding not only regulated crystal growth and morphology but also impurity uptake as it is exemplified with the data of Fig. 5 (bottom-right) (13). It can be clearly seen that up to one order of magnitude lower impurity uptake occurs when neutralization is conducted in such a way favouring the production of smooth and large gypsum crystals (Fig. 5-top left).

D. Selenium Elimination

In the late 1990s, a chemical process to remove mercury and selenium from weak sulphuric acid solutions was patented and implemented in the CEZinc refinery-another innovation led by Lucy Rosato [21].

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Fig. 5: SEM photos of gypsum produced with start-up seed (top-left) and without seed (middle-left) by controlled neutralization at low S level; and by uncontrolled neutralization and simple product recycling (bottom-left). Supersaturation level (Ca conc; top-right) and impurity uptake

(bottom-right) in single-stage/unseeded vs. 2-stage/seeded neutralization circuits (reproduced from [12,13]).

The CEZinc process comprises two stages (Figure 6 (left)). In the first one, sodium sulphide is added to the weak acid solution in order to precipitate the soluble mercury. The solution is then treated with sodium dithionite in order to reduce the selenium to elemental form (see reaction below). The precipitate is then filtered and disposed of while the weak acid solution is neutralized before being released in the environment [41].

H2SeO3 + 2H2SO4 + 2Na2S2O4 → Se(s) + 4SO2 + 2Na2SO4 + 3H2O

While mercury removal has been consistently effective in maintaining well below the permissible level of 50 ppb the control of selenium about 10 years ago started showing rather erratic behavior with occasional releases above the target level, a problem that needed to be dealt with as concentrates with progressively higher Se content were expected to be processed. A joint CEZinc-McGill project sought to elucidate the underlying chemistry of this process and optimize the process. The most important finding of this research has been that the reaction that occurs between sodium dithionite and selenious acid produces a precipitate that is unstable in the presence of the decomposition products of sodium dithionite [42]. This was verified with observations made by CEZinc. Figure 6 (right) shows the selenium concentration curve as a function of time when the Se precipitate is left in contact with the mother liquor [43]. The redissolution of selenium may be thought to be due to reoxidation by air catalyzed by



dithionite decomposition products. While the exact mechanism behind the redissolution reaction is not fully understood, it is postulated that the SO2* and S2O4* radicals, generated during dithionite decomposition, play a role, by acting as powerful oxidizing agents, in combination with oxygen (air) [42]. As a result of this discovery- another powerful example of the importance of industry-driven university research that Lucy Rosato and her team promoted, CEZinc brought changes to its Se elimination process circuit minimizing the retention of precipitate in contact with the mother liquor during filtration and adding a polishing step for 100% compliance [43].

Figure 6: The CEZinc Hg-Se elimination process (left) [41] and redissolution of selenium precipitate in contact with the mother solution [43].

IV. IMPURITY CONTROL RESEARCH

Here we will briefly refer to two other projects undertaken at McGill with Lucy Rosato’s involvement that aimed to optimized impurity control strategies. The first one focused on Co cementation and in particular in reducing zinc dust consumption, while the second one to a new method of Mn removal in anticipation of zinc feedstocks with increased Mn content.

A. Cobalt Cementation

The zinc electrowinning process is unusual from a thermodynamic point of view because zinc metal has a more negative reduction potential than hydrogen. One would therefore expect hydrogen gas to evolve at the expense of zinc deposition. However, zinc metal is electrowon economically from acidic zinc sulphate solution because hydrogen evolution has a high overpotential on zinc metal [2]. In order to maintain this large overpotential, almost all impurities in the leach solution must be completely removed [14]. Any remaining impurities act as catalysts for hydrogen evolution causing large drops in current efficiency as it can be verified with the indicative data presented in Figure 7.

In a collaborative CEZinc-McGill project in the second half of the 90’s initiated with Lucy Rosato’s support [2,14], we worked to understand the factors affecting cobalt removal and zinc dust consumption as cobalt is the most difficult impurity to remove [44]. Cobalt cementation is very slow due to kinetic barriers and requires the use of activators. Industry uses two activation methods to effect cobalt cementation: activation with arsenic/copper or with Sb/Cu [2].

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Figure 7: Effect of impurities on Zn EW current efficiency. The flowchart of the generic 2-stage antimony activation purification process is shown in Fig. 8.

The CEZinc-sponsored project considered the effect (on cobalt removal and zinc dust consumption) of different solution constituents and novel activators using both synthetic (CMQ paper) and real CEZinc impure solution (Hydro paper). Small amounts of cadmium and chloride in addition to the activators (Cu/Sb) were found to increase the amount of cobalt removed beyond the level achieved with antimony and copper alone.

Fig. 8: Two stage antimony activation process.

The efficiency of activators for cobalt removal was correlated for the first time to their hydrolysis constants as shown in Figure 9. Activators such as Sb, which have a significantly higher hydrolysis constant (pKOH) than that of Zn adsorb preferentially on the zinc dust surface. Upon adsorption, Sb(III) species is reduced to elemental state forming a favourable substrate for cobalt(II) to adsorb and discharge as shown in Fig. 9.



Fig. 9: Cobalt removal activity correlated with log KOH (left) [44] and (right) schematic of Sb/Co cementation mechanism.

Optimum cobalt removal with minimal zinc-dust dissolution was determined to be at 85°C with the addition of 15 mg/L Cu, 10 mg/L Cd and 2 mg/L Sb, using 3.5 g/L zinc dust. The amount of dissolved zinc dust under these conditions was less than 10% of the initially added zinc dust. Higher temperatures resulted in dramatic increase in Zn dust consumption. Cd plays a critical role acting as co-activator with Sb/Cu (Fig. 10). Combinations of additives were determined to be particularly effective in optimizing cobalt reduction while minimizing zinc dust consumption (Fig. 10). As a result of these findings, CEZinc modified filtration of the first cementation stage (Cd/Cu) to permit a small amount (~10 ppm) of Cd to pass into the 2nd stage hence ensuring optimum results.

Fig. 10. Effect of Cd on Co removal (synthetic solution) (left) [44] and effect of Cd on Co removal efficiency (industrial solution) (right) [45].

B. Manganese Control

Manganese is an impurity in many hydrometallurgical processes that needs to be removed from the process solution prior to any metal recovery [18, 46] In the zinc industry, the manganese present in the concentrate as well as that added as manganese-based oxidant dissolve and build up in the process circuit. In order to regulate the manganese level in solution, bleeding off of electrolyte is often used. However, a chemical separation method is necessary when the manganese concentration is elevated. This is particularly true for the case of processes treating Mn-rich zinc concentrates. In late 90’s CEZinc was faced with the prospect of treating such Mn-rich Zn concentrate and looked into a precipitation process for the removal of the excess Mn from the neutral leach liquor. In this context at the initiative of Lucy Rosato we jointly developed a patented process based on the use of SO2/O2 [17]. This process can be explained with the aid of the following (simplified) reaction:

Mn2+ + SO2 + O2 + 2H2O → MnO2 +SO42- + 4H+

From the above reaction, it can be seen that acid is generated from the oxidation of Mn(II). Therefore a base has to be added to the solution since higher pH improves the manganese oxidation rate [17]. Our research showed the type of neutralizing agent used can impact the efficiency of the process. For example Na2CO3 is not effective as the released CO2 interferes with the SO2/O2 oxidation gas mixture [17]. At the same time there are limitations as to the upper pH that can be used for manganese removal from concentrated zinc sulphate solutions because of the coprecipitation of the latter by hydrolysis. The effectiveness of the SO2/O2 process is complicated by the fact that the solubility of SO2 in aqueous solution is many times



higher than the solubility of O2, so dissolution of the latter may become rate limiting. As a result of the difference in solubility characteristics between the two gases, the concentration ratio [SO2]/[O2] in the solution is higher than that in the gas mixture imposing the risk (if not controlled) in creating (at least locally) reducing conditions. Hence proper design of the gas/liquid transfer mixer (Fig. 11-left) and applied ORP and pH controls are critical for optimum results as determined by Vincent Menard in his MEng study [18,47,48]. Thus as Fig. 11 (right) [48] shows a lower SO2 flowrate and/or SO2/O2 ratio, results in higher redox potential and better total manganese removal.

Fig. 11. Preferred agitator configuration (left) and residual manganese concentration and ORP level as a function of SO2/O2 ratio (right 0

[47,48].

This is, however, achieved at the detriment of slower kinetics. The implication of this behaviour is that from an industrial implementation point of view optimum (fast and maximum) manganese removal should be achieved via a combination of two reactors in series with adjustable SO2/O2 ratio in the two reactors for fast removal of the bulk of manganese in the first reactor followed by removal of the residual manganese level in the second (polishing) reactor. This led us in designing a 2-stage circuit as shown in Fig. 12. Operation of this circuit at 80°C and pH=4 using CEZinc neutral leach solution containing 150 g/L Zn that maintained a certain ORP profile in the 2-reactor cascade and included product recycling yielded well grown manganese oxide precipitate (~10-20 μm in size consisted of a birnessite-like phase, Znx(Mn3+,4+)7O14•yH2O) particles (easily filterable) with no scaling incidence (thanks to the controlled superaturation-via staging- and seeding environment [27]).



Fig. 12: The 2-stage SO2/O2 Mn removal circuit (left) and SEM photos of produced precipitate (right) [47].

V. CONCLUDING REMARKS

In conclusion Lucy Rosato was a force of innovation and guiding beacon of R&D collaborations- a paradigm that more than ever before we need to emulate for building a sustainable mineral/metal/material industry and dynamic research presence in university and government labs. As addressed in a previous opinion article [4], a serious divide has opened between extractive metallurgical industry and teaching of process and materials engineering. On one hand the industry seems to have placed less importance in internal or sponsored in universities R&D. On the other, Universities have moved to the materials science model without ensuring the empowerment of their graduates with strong processing education. In the meantime industry leaders like Chris Twigge-Molecey of Hatch have challenged us with the question “Why can’t we innovate?” [49], arguing of the importance of boosting innovation. Such shift towards smarter more innovative processing and extraction of metals becomes more urgent than ever considering the tremendous global stresses placed by energy, economic, environmental and social factors as elegantly discussed by Bruce Conard (formerly with Inco) in his COM2012 plenary: “The Future of sustainability” [49].

It is only hoped that symposia as the present one honouring Lucy Rosato − a passionate advocate of technical excellence and research collaboration − can mobilize and inspire all parties concerned to work towards sustainable and efficient clean hydrometallurgical processes. Corporate leaders, as remarkably practiced by Lucy Rosato, should engage in collaborative research with universities as the assured way of creating the innovative technical minds that will contribute to a career-satisfying and prosperous future for themselves, our companies and our society at large.

ACKNOWLEDGMENTS

I would like to thank Michael Agnew for inviting me to be part of this symposium honouring the memory and great contributions and influence of Lucy Rosato, distinguished hydrometallurgy leader.

REFERENCES (in bold CEZinc/NTC colleagues)



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12. S. Omelon, Production of High Density Gypsum via an Improved Neutralization Process, MEng Thesis, McGill University, 1998.

13. C. Verbaan, Impurity Uptake and Control during Gypsum Crystallization, MEng Thesis, McGill University, 2000.

14. A. Nelson, G.P. Demopoulos, G. Houlachi and L. Rosato, Testing of New Additives in Conjunction with the Removal of Cobalt from Zinc Electrolyte by Cementation, EPD Congress 1998, B. Mishra, Ed., TMS, 785-796 (1998).

15. A. Nelson, Novel Activators for the Cementation of Cobalt from Zinc Sulphate Electrolytes, MEng Thesis, McGill University, 1998.

16. T. Dreher, A. Nelson, G.P. Demopoulos and D. Filippou, The Kinetics of Cobalt Cementation in an Industrial Electrolyte, Hydrometallurgy, Vol. 60, 105-116 (2001).

17. G.P. Demopoulos, Q. Wang and L. Rosato, A Method for Recovering Manganese from Acidic Sulphate Solutions, U.S. Pat. No. 6,391,270, May 21, 2002.

18. V. Menard, Use of SO2/O2 in Precipitation of MnO2, MEng Thesis, McGill University, 2004. 19. Y. Ling, Production of Alpha Calcium Sulphate Hemihydrate by Reaction of Sulphuric Acid

with Lime, PhD Thesis, McGill University, 2004; Y. Ling and G.P. Demopoulos, Ind. Eng. Chem. Res., 44 (2005), 715-724.

20. Y. Jia and G.P. Demopoulos, Adsorption of Silver onto Carbon from Acidic Media, Ind. Eng. Chem. Res., Vol. 42, 72-79 (2003).

21. G. Houlachi, G. Monteith, and L. Rosato, Process for Removing Selenium and Mercury from Aqueous Solutions, US Patent, No, 6,228,270, May 8, 2001.

22. N. Geoffroy, Se Elimination by Reduction, PhD Thesis, McGill University, 2011.



23. G.P. Demopoulos, J. Zinck and P.D. Kondos, Production of Super Dense Sludges with a Novel Neutralization Process, Waste Proc. and Recycling, R. Rao et al., (Eds.), CIM, 401-412 (1995).

24. T.C. Cheng, G.P. Demopoulos, Y. Shibachi, and H. Masuda, The Precipitation Chemistry and Performance of the Akita Hematite Process, in Hydrometallurgy 2003, C. Young. Ed., TMS, 1657-1674 (2003).

25. Richard Jack De Klerk, Yongfeng Jia, Renaud Daenzer, Mario A Gomez, George P Demopoulos, Continuous Circuit Coprecipitation of Arsenic(V) with Ferric Iron by Lime Neutralization, Hydrometallurgy, Vol. 111-112 (2012) 65–72.

26. G.P. Demopoulos, Preparation and Stability of Scorodite, in Arsenic Metallurgy, R.G. Reddy and V. Ramachandran, Eds., TMS (2005) 25-50.

27. G.P. Demopoulos, Aqueous Precipitation and Crystallization for the Production of Particulate Solids with Desired Properties, Hydrometallurgy, 96, 199–214 (2009).

28. M.A. Gomez, L. Becze, J. N. Cutler and G.P. Demopoulos, Hydrothermal reaction chemistry and characterization of ferric arsenate phases precipitated from Fe2(SO4)3-As2O5-H2SO4 solutions, Hydrometallurgy, 107, 74–90 (2011).

29. L.I. Rosato and M.J. Agnew, Iron disposal options at Canadian Electrolytic Zinc, in Iron Control and Disposal, J.E. Dutrizac and G.B. Harris, Eds. 77–89, CIM (1996).

30. N.L. Piret and A.E. Melin, Impact of environmental issues on iron removal process evolution in electrolyte zinc production in Hydrometallurgy Fundamentals, Process Evolution, and Innovation, J.B. Hiskey and G.W. Warren, Eds. 449–520, SME (1993).

31. G.P. Demopoulos and G. Pouskouleli, Solvent Extraction of Fe(III) from Sulphate Solutions by M2EHPA, Can. Met. Q., 28(1), 13-18 (1989).

32. F. Principe and G.P. Demopoulos, Separation of Iron (III) from Zinc Sulphate - Sulphuric Acid Solutions Using Organophosphorus Extractants OPAP and D2EHPA, Part I: Extraction”, Hydrometallurgy, 74 (2004), 93-102.

33. F. Principe and G.P. Demopoulos, Comparative Study of Iron (III) Separation from Zinc Sulphate - Sulphuric Acid Solutions Using Organophosphorus Extractants, OPAP and D2EHPA: Part II: Stripping, Hydrometallurgy, 79, 97-109 (2005).

34. G.A. Monteith and S. Seyer, Filtration of Jarosite Precipitation Residues, in Solid/Liquid Separation, G.B. Harris and S.J. Omelon, Eds., CIM (1999) 3-23.

35. C. Verbaan, S. Omelon and G.P. Demopoulos, The Effect of Staging and Seeding/Recycling in the Production of High Density and Clean Gypsum from Zinc Process Waste Waters, in Solid-Liquid Separation in Hydrometallurgy, G.B. Harris and S. Omelon, Eds., CIM (1999) 251-264.

36. M.J Collins, Masters, I., Ozberk, E., Krysa, B.D. and Desrochers, G.J., Deportment of Selected Minor Elements at the HBMS Zinc Pressure Leach Plant, in Impurity Control and Disposal in Hydrometallurgical Processes, B. Harris and E. Krause, Eds. CIM (1994) 291-301.

37. M. E. Chalkley, Collins, M.J., Masters, I. and Ozberk, E., 1995, Deportment of Elements in the Sherritt ZPL Process, in Zn & Pb '95, N. Azakami et al., Eds., MMPIJ, Japan, 1995, 612-630.

38. T. Yamada, Kuramochi, S., Sato, Sand Shibachi, Y., The Recent Operation of the Hematite Process, in Zn & Pb Processing, J.E. Dutrizac et al., Eds., CIM (1998) 627-638.

39. T.G. McCristel, Manning, J. and Sanderson, I., Conversion of the Pasminco Hobart Smelter to Paragoethite, in Zinc and Lead Processing, J.E. Dutrizac et al., Eds., CIM (1998) 439-453.

40. S. Omelon and G.P. Demopoulos, Production of High Density Gypsum by Controlled Neutralization of Simulated Zinc Plant Wastewaters, in Waste Proc. Recycl. Mineral. Metall.



Industries (III), S.R. Rao et al., eds., CIM, 435-442 (1998). 41. N. Geoffroy, E. Benguerel, and G. P. Demopoulos, Precipitation of Selenium from Weak

Acidic Solutions Using Sodium Dithionite and Sodium Sulphide, in Zinc and Lead Metallurgy, L. Centomo et al., Eds, CIM (2008) 295-305.

42. N. Geoffroy and G. P. Demopoulos, Reductive Precipitation of Selenium from Selenious Acidic Solutions Using Sodium Dithionite, Ind. Eng. Chem. Res., 48 (2009), 10240–10246.

43. E. Benguerel, S. Seyer, N. Geoffroy, A. Le Regent, Upgrading the Selenium Removal Process for CEZinc’s Acid Plant Effluents, in Lead-Zinc 2010, CIM, 2010, 9p.

44. A. Nelson, G.P. Demopoulos and G. Houlachi, The Effect of Solution Constituents and Activators in the Cementation of Cobalt, Can. Metall. Q., 39 (2), 175-186 (2000).

45. T. Dreher, A. Nelson, G.P. Demopoulos and D. Filippou, The Kinetics of Cobalt Cementation in an Industrial Electrolyte, Hydrometallurgy, Vol. 60, 105-116 (2001).

46. Wensheng Zhang, Chu Yong Cheng, Manganese metallurgy review, Parts I, II & III, Hydrometallurgy, 89 (2007) 137–159, 160–177, 178–188.

47. V. Menard and G.P. Demopoulos, Precipitation of Manganese from an Industrial Zinc Leach Solution Using SO2/O2, EPD Congress 2006, S.M. Howard et al, Eds., TMS,1079-1088 (2006).

48. V. Menard and G.P. Demopoulos, “Mass Transfer and ORP Considerations in Mn Precipitation by SO2/O2 Gas Mixture”, Hydrometallurgy, 89 (2007), 357-368.

49. http://web.cim.org/COM2012/attachments/PDF/Plenary-ppt.pdf

APPENDIX

Table A: McGill hydrometallurgy HQP and non-zinc projects sponsored by Noranda

1. M. Brown, BEng (1984), “Solvent Extraction of Bismuth from CCR spent electrolyte”. (Murray formerly with NTC now works for BBA Associates in Montreal).

2. E. Di Cesare, BEng (1989), “Solvent Extraction of Platinum Metals”. (Enrique works now for Argex Titanium Inc in Montreal).

3. V. Aprahamian, MEng (1989-91), “The Behaviour of Impurities during Solvent Extraction of PGMs”. (Vicken formerly with NTC is now Program Manager; The Royal Canadian Mint, Ottawa, Ont.).

4. E. Benguerel, MSc and PhD (1990-96), “Solvent Extraction of Rhodium from Chloride Solutions”. (Elyse is now Senior Metallurgist, CEZinc, Valleyfield, QC).

5. B. Coté, PhD (1989-94), “Solvent Extraction of Platinum and Palladium”. (Bruno is now, self-employed, Quebec City, QC).

6. S. Ashrafizadeh, PhD (1992-96), “Solvent Extraction and Liquid Membrane Separation of Rhodium”. (Seyed is now Associate Prof., Iran Univ. Sci. Tech., Teheran, Iran).

7. Y. Shang, PDF (1993-95), “Solvent Extraction of Rhodium”. (Yuxing formerly with Falconbridge is now Executive Vice-President, Refining, METALOR Co., Switzerland).

8. D. Droppert, MEng (1994-96), “Ambient Pressure Precipitation of Scorodite from Sulphate Solutions”. (Dave is founder and VP of Solumet MP Inc., Varennes, Que.).

9. T. Dreher, PDF (1997-99), “Solvent extraction of rhodium”. (Trina is with Process Solutions Inc., Melbourne, Australia).

10. G. Moldoveanu, MEng and PhD (1998-05), “Crystallization of Nickel Sulphate with



http://web.cim.org/COM2012/attachments/PDF/Plenary-ppt.pdf

Alcohols”. (Georgiana is now Research Associate, Dept. Chem. Eng. UofT). 11. S. Girgin, PhD (2001-06), “Crystallization of Alpha-Calcium Sulphate Hemihydrate by

Controlling the Aqueous Reaction of Calcium Chloride and Sulphuric Acid”. (Seref is now process engineer with BBA Associates, Montreal).

12. Z. Li, PhD (2002-06), “Measurement and Modelling of Calcium Sulphate Solubilities”. (Zhibao is now Professor, Institute for Process Engineering, Chinese Academy of Sciences, Beijing).

13. A. Al-Othman, M.Eng. (2002-04), “Regeneration of HCl from Spent CaCl2 Solutions”. (Amani is now Assistant Professor, American University of Sharjah, United Arab Emirates).



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