Review of metal sulphide precipitation

Hydrometallurgy 104 (2010) 222–234

Contents lists available at ScienceDirect

Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Review of metal sulphide precipitation Alison Emslie Lewis ⁎ Crystallization and Precipitation Unit, Chemical Engineering Department, University of Cape Town, Rondebosch, Cape Town, South Africa, 7701

a r t i c l e

i n f o

Article history: Received 14 December 2009 Received in revised form 13 June 2010 Accepted 16 June 2010 Available online 1 July 2010 Keywords: Metal sulphide precipitation Solubility Mechanisms Bisulphide Nano crystals Metal (Zn, Cu, Fe, Pb, Rh, Mn, Co, Ni, Mb, V, Cd, Hg, Bi, Ag) sulphides Acid mine drainage

a b s t r a c t Although there have been numerous studies on metal sulphide precipitation, the research field as a whole is not well integrated. This paper reviews the disparate areas of study into metal sulphide precipitation in an attempt to summarise the current work, as well as to suggest potential for future consolidation in the field. The review encompasses (1) fundamental studies into metal sulphide precipitation, which usually focus on mechanisms and are carried out at very low (micromolar) concentrations; (2) applied studies focussing on metal removal and reaction kinetics (mostly via the aqueous phase); (3) studies that focus on the solid phase and address the crystallization kinetics of the formed particles; (4) studies into precipitation of metal sulphide nanocrystals and lastly, (5) applications of metal sulphide precipitation to effluent treatment processes such as Acid Mine Drainage (AMD) treatment as well as industrial hydrometallurgical processes. The review found that, besides lack of integration between the fundamental and applied areas of study, the applied studies have often used flawed methods to evaluate the efficiency of the metal sulphide precipitation process. Applying classical crystallization theory has also not been entirely successful because of the sparingly soluble nature of the systems. The studies that investigate nanocrystal formation tend to focus mostly on recipes and characterization of the formed particles. The industrial and effluent treatment studies form another area of research that stands relatively isolated from its more scientific counterparts. One of the key insights obtained from the summary of these disparate areas of work is that the level of scientific understanding in each of the fields is vastly different. The fundamental studies into mechanistic aspects of metal sulphide precipitation are far advanced of the other areas. However, they are restricted to very low concentrations, which are of limited value in most process-based hydrometallurgical applications. Most of the applied studies are still at a relatively empirical level, with the findings being highly systemdependent. Truly generic findings are still to be realised in these areas. Developing robust measurement techniques to be able to collect consistent data and thus model the simultaneous nucleation, growth, aggregation and attrition of the nano to micro scale particles is one of the challenges in the field. Understanding and characterising the complex aqueous chemistry, taking into account multiple sulphide and metal species interactions, is another. It is proposed that, by integrating the different priorities of the various study areas (chemistry, reaction mechanisms, crystallization mechanisms, particle characterization and industrial applications) the research field as a whole would benefit. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Metal sulphide precipitation is an important process in hydrometallurgical treatment of ores and effluents. Although hydroxide precipitation is widely used in industry for metal removal, there are some advantages to sulphide precipitation, including the lower solubility of metal sulphide precipitates, potential for selective metal removal, fast reaction rates, better settling properties and potential for re-use of sulphide precipitates by smelting. However, sulphide precipitation is not used as widely as it could be because the dosing of sulphide is seen to be difficult to control (due to the very low solubility of the metal sulphides and thus ⁎ Tel.: +27 21 650 4091. E-mail address: [email protected]. 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.06.010

the sensitivity of the process to the dose) and because of concerns about the toxicity and corrosiveness of excess sulphide (Veeken et al., 2003a). Sulphide precipitation can be effected using either solid (FeS, CaS), aqueous (Na2S, NaHS, NH4S) or gaseous sulphide sources (H2S). There is also the possibility of using the degeneration reaction of sodium thiosulphate (Na2S2O3) as a source of sulphide for metal precipitation. In the preparation of nanocrystals, a number of other sulphide sources, including thiourea, thioacetamide, polyphenylene sulphide and carbon disulphide are used. The thermodynamic equilibria involved in metal sulphide precipitation can be expressed as:

Kp1




þ

H2 S ⇄ HS þ H

Kp1 ¼

½HS
½Hþ  ½H2 S

pK1 ¼ 6:99

ð1Þ

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234


Kp2

2


HS ⇄ S

þH

þ

2


Kp2 ¼

½S ½H  ½HS


2þ

þ S ⇄MSðsÞ

2þ

þ HS ⇄MSðsÞ + H

M

M

þ

pK2 ¼ 17:4

2





ð2Þ ð3Þ

þ

ð4Þ

The concentration of sulphur species is a strong function of pH, as shown in Fig. 1. The pK2 value in Eq. (2) is currently the most reliable value, as measured by Migdisov et al. (2002). The solubilities of various metal sulphides as a function of pH are illustrated in Fig. 2. These were calculated using Stream Analyser (OLI Systems Inc., 2009), which uses the revised Helgeson–Kirkham– Flowers (HKF) model for the calculation of standard thermodynamic properties of aqueous species and the frameworks of Bromley, Zemaitis, Pitzer, Debye-Huckel, and others for the excess terms. For comparison purposes, the equivalent diagram for metal hydroxides is given in Fig. 3. The apparently strange shape of the lead hydroxide solubility is due to the formation of multiple soluble lead species over the investigated pH range (PbOH+ 1, PbSO4, PbO, Pb+ 2 and HPbO−1 2 ). Metal sulphides are also of interest as nanocrystals due to their special applications as, amongst others, semi conductors, solar radiation absorbers, catalysts, polymer surface coatings, cathodic materials and nanometer scale switches (Mathew et al., 2008); solar cell components, electro luminescent devices and multilayer dielectric filters (Pawaskar et al., 2002). In this paper, various aspects of metal sulphide precipitation are reviewed, with particular focus on the following areas: 1 Fundamental studies in metal sulphide precipitation, 2 Metal sulphide precipitation studies focussing on metal removal and reaction kinetics; 3 Metal sulphide precipitation studies focussing on particles and crystallization kinetics 4 Precipitation of metal sulphide nanocrystals, 5 Applications of metal sulphide precipitation to environmental and industrial systems a Effluent treatment processes such as Acid Mine Drainage (AMD) treatment b Industrial processes 2. Fundamental studies in metal sulphide precipitation Although the physics of nucleation and crystal growth are apparently well known, the chemical or molecular processes involved

Fig. 1. pH dependence of sulphide speciation.

223

in the transformation of simple dissolved species to solid products are not well understood (Luther et al., 1999). As a result, many of the fundamental studies in metal sulphide precipitation focus on identifying the precise forms of reduced sulphur species in aqueous environments. Metal sulphide species can be present as metal-sulphide complexes (truly dissolved species for which a chemical potential can be defined); nanoclusters (in the range of 2–10 nm; too small to have bulk-like electronic wave functions but exhibiting bulk-like crystal structure); or colloids (small particles in the range 1 nm–1 μm that are formed in supersaturated solutions (Sukola et al., 2005). Many of the studies focus on the formation of metal bi-sulphide complexes, partly because they are often intermediates in metal sulphide precipitation, but also because they can account for puzzlingly high concentrations of metals in the environment. 2.1. Zinc Zinc is an often chosen model metal for metal sulphide precipitation studies, partly because of its relevance in sediments and natural environments, but also because it only has one redox state (II), which simplifies the chemistry. In a study that investigated the formation of ZnS from aqueous solution (at 1–20 μM concentrations of reagents), Luther et al. (1999) proposed a stepwise process for the formation of stable ZnS phases. This process starts with Zn (II) in aqueous solution as the hexaaquo Zn(H2O)2+ 6 ion. The Zn then forms soluble complexes that aggregate into soluble rings and clusters. Both neutral sixmembered rings (Zn3S3) and an anionic tetrameric complex (Zn4S4− 6 ) were found, explaining the observation of neutral and anionic metal complexes found in sulphidic waters at micromolar Zn(II) levels. The neutral Zn3S3 clusters are ideal for condensation reactions with other neutral or anionic clusters and it is these that eventually form the stable ZnS phase. These results showed that molecular clusters play a role as intermediates in ZnS formation and underline the applicability of Ostwald's rule or the rule of stages. This rule states that the precipitate with the highest solubility will form first in a consecutive precipitation reaction. A corollary to this rule is that the final solution moiety should have a form close to the moiety in the first solid product—a hypothesis neatly addressed in this work. 2.2. Copper Copper is one of the difficult metals to study in this context because of its ability to reduce in sulphidic solutions. Consequently, it forms a range of bisulphide, sulphide and polysulphide complexes which may be mixed ligand type as well as mononuclear and multinuclear complexes (Luther et al., 1996). Pattrick et al. (1997) detailed the complex nature of the Cu–S system, giving a table of stable, metastable and intermediate compositions that occur between the Cu2S mineral (chalcocite) and CuS (covellite). In generating the precipitates by the reaction of Cu(NO)3 with Na2S or H2S, no difference was found in the products when the different sulphide sources were used. The colour of the initial precipitates depended on the reagent concentrations: brown for low concentrations (0.5–5 mM); a blue/black floc at intermediate (≥ 10 mM) concentrations and dark blue with a greenish tint at the highest concentrations (50 mM). All the precipitates transformed to a dark blue/green precipitate with ageing. Interestingly, the structural environment for Cu in the precipitates was independent of the reagent concentrations. A “primitive” structure of two S shells around Cu was found in all the initial precipitates—regardless of colour. In all cases, the aged precipitates developed the “evolved” structure, very similar to CuS (covellite). Although this implies that colour cannot be used to deduce information about structure, the

224

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

However, in a later paper from the same group, Ciglenecki et al. (2005) proposed that in fact such conditions generate nanoparticles (radius 1–1000 nm) as follows: 1 2þ
0 þ þ Cu þ HS ⇄ Cu2 S×S ðbrown solÞ þ H ⇄CuSðgreen solÞ þ H 2 ð6Þ

Ciglenecki and co-workers challenged the inconsistent results of previous voltammetry-based copper bisulphide complexing studies and maintained that the diagnostic peaks have been previously misconstrued. They proposed that these peaks indicate particles, as opposed to aqueous species and show that CuS nanoparticles are electrochemically active. 2.3. Iron 2.3.1. Stoichiometry Wei and Osseo-Asare (1995) carried out a stopped flow spectrophotometric study of iron monosulphide formation from ferrous chloride (0.1 M) and sodium sulphide (0.1 M). The stopped flow technique is useful for tracking intermediates because of the rapidity with which the measurements can be taken. They found that an intermediate product was formed within the first few seconds, which then decomposed over the next few minutes. The intermediate was identified as FeHS+, which is corroborated by the work of Luther (1991), who found evidence for the complex using polarographic methods. The kinetics of the Fe(HS)+ formation were found to be first order with respect to both Fe and HS−.

Fig. 2. pH dependence of metal sulphide solubilities.

authors do observe that the colour change from blue/black to dark blue/green appears to correlate with the change from the primitive to the evolved structure. Furthermore, the authors go on to stress that colour changes do not represent a change in the formal oxidation state of Cu, as all of their precipitates only contained Cu(I) but also exhibited the full range of colours. Instead, the colours relate to changes in electronic structure or grain size. Shea and Helz (1988), when measuring the solubility of covellite (CuS) in bisulphide solutions, proposed that the following complexes 3− 3− 3− are formed: CuS(HS)2− and 2 , CuS(HS)3 , CuS(S5)2 , Cu(S4)(S5) 2− CuS(S5) , with the following equilibria:



n

CuSðsÞ þ nHS ⇄CuSðHSÞn where n ¼ 2 and 3

2.3.2. Formation conditions In a later study, Wei and Osseo-Asare (1996) synthesized microsized pyrite (FeS2) particles by reaction of ferric with sulphide in aqueous solutions. Eh and pH strongly affected the pyrite formation, with pyrite forming between pH 3.6 to 5.7 and Eh −0.05 to 0.01 V. The intermediate products discussed in Wei and Osseo-Asare (1995) were found to be essential for pyrite formation. The fact that pyrite could not be formed below pH 3.6 was attributed to undersaturation of FeS or the absence of Fe(HS)+. Another explanation might be found in Karbanee et al. (2008), who attempted to establish which sulphide species (H2S, HS− or S2−)

ð5Þ

According to their interpretation, the dominant copper species 2− above 0.1 M HS− is CuS(HS)3− species does not 3 . The CuS(HS)2 − become significant until the HS concentration limit falls below 0.2 M. Van Hille et al. (2005) used these findings as one possible explanation for the presence of dissolved copper in the effluent stream of their copper sulphide precipitation reactor. The copper in solution was not removed to the expected levels, despite excess sulphide being available, despite instantaneous reaction kinetics for the initial precipitation reaction and despite low CuS solubility over the entire pH range. Luther et al. (2002) used a combination of experimental techniques to show that Cu(II) is reduced by sulphide in solution to Cu(I) before precipitation. Working at micromolar levels, they inferred that six-membered Cu3S3 rings formed in solution and that these are the building blocks for the aqueous CuS clusters, which are themselves the precursors to CuS precipitation. In other words, the major structural elements of the CuS precipitate are preformed in solution and determine the development of the solid precipitate.

Fig. 3. pH dependence of metal hydroxide solubilities.

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

was responsible for metal sulphide precipitation. They found that the H2S(aq) sulphide species could not precipitate Ni2+ at ambient conditions, so perhaps this might be a factor in the pyrite case as well, as H2S is the dominant species at pH 3.5. Wei and Osseo-Asare (1996) proposed that the inability to form pyrite at about pH 5.7 was due to the Eh conditions which favoured FeOOH. Analysis of the microsized pyrite was presented in Wei and OsseoAsare (1997) and found to be 99.46% FeS2, with an average particle size of 1.5 μm. 2.4. Lead 2.4.1. Surface chemical studies Stén and Forsling (2000) examined the surface chemical properties of precipitated lead sulphide. They found that precipitation conditions (speed of titration) affected the results. When the lead nitrate (1 g/ 30 mL, 160 mM) was very slowly titrated with 0.1 M sodium sulphide, they found that both lead and sulphide adsorbed onto the precipitate surface. There was also evidence of lead nitrate occlusion in the final lead sulphide precipitate, probably due to the rapid precipitation reaction being too fast for the desorption of nitrate from the surface. 2.5. Rhodium Mc George et al. (2009) investigated the mechanism and kinetics of rhodium (III) co-precipitation with copper sulphide and proposed a new cationic substitution path. It is proposed that Rh3+ and Cu2+ compete for the available sulphide during the initial ionic precipitation, during which time the bulk of the Rh precipitation occurs. Since Rh2S3 has a much lower solubility than CuS, it continues to precipitate via a cationic substitution, thus replacing the Cu in the CuS precipitate, resulting in the enrichment of Rh towards the edge of the CuS particles. 2.6. Mixed metals Luther et al. (1996) used voltammetric techniques to determine the stoichiometry of metal sulphide complexes for Mn, Fe, Co, Ni, Cu and Zn. They also determined the stability constants of the complexes. For the investigation, the sulphide solutions were held at 1–10 μM and the metal was added by titration. 2.6.1. Manganese, iron, cobalt and nickel They found that Mn, Fe, Co and Ni behave similarly and form M(SH)+, M2(SH)3+ and M3(SH)5+ complexes. Titrations showed that the complexes are HS− complexes at pH ≥ 7. Between pH 5–7, there is evidence for complex dissociation and H2S formation. Below pH 5, there is evidence for H3S+. 2.6.2. Copper and zinc On the other hand, Cu and Zn formed strong complexes that did not dissociate under diffusion control conditions (no stirring). Also, the titrations showed that they were sulphide complexes and that the complexes remained in solution without precipitation occurring. The proposed stoichiometry of the complexes is MS and M2S2− 3 . The Zn complex dissociates below pH 6.7, releasing H2S, whereas the Cu complex does not fully dissociate below pH 2 because of reduction of the Cu (II). These findings contrast with those of Shea and Helz (1988) who proposed a complex of the form CuS(HS)nn-. In an investigation into the interactions of trace metals with sulphides in anoxic sediments, Morse and Luther (1999) used the water exchange reaction (Eq. (7)) as a guide to the relative reactivities of the various metals. 2þ



 2þ

MðH2 OÞ6 þ H2 O ⇄MðH2 OÞ5 ðH2 O Þ 

þ H2 O

Where H2 O ¼ radiolabelled water

ð7Þ

225

These relative reactivities were then used to compare the rate of metal sulphide precipitation with that of FeS2 (pyrite) precipitation. They found that, in accordance with the kinetics of the water exchange reaction, PbS, CdS and ZnS were faster than FeS precipitation; CoS and NiS were slower. However Cu, Mn and Hg did not follow the pattern. The explanations were as follows: Cu exists in a variety of oxidation states and forms strong complexes; Mn occurs mainly as a carbonate mineral and Hg has a strong tendency to form chloride complexes. In a more recent paper, Luther and Rickard (2005) focussed on clusters, which are important because they are seen to be the precursors to the formation of metal sulphide. They extended the classical definition of a cluster and describe it as “a quantum-sized particle or complex which contains a discrete number of atoms in a molecule or ion that is small enough to behave as a dissolved species”. Using molecular modelling as well as electrochemical titration techniques and operating at micromolar concentrations, they showed that sulphide cluster complexes exist for Fe, Cu, Zn, Ag and Pb. The size of the clusters could be controlled at concentrations below 2 μM in each reactant but significant aggregation occurred above 10 μM. They also confirmed the findings of their earlier paper (Luther et al., 1999), that the form of the basic cluster complex is very similar to the basic structural element of the condensed phase. Conversely, Sukola et al. (2005) found that multiple lines of evidence did not support the presence of soluble molecular metal sulphide clusters. Instead they proposed that the metal sulphide species were more likely a mixture of truly dissolved metal sulphide complexes and dynamic metal sulphide colloids. 3. Metal sulphide precipitation studies focussing on metal removal and reaction kinetics Although there are an abundance of studies in the literature that evaluate the effectiveness of metal sulphide precipitation, almost all of them do so via measurement of the remaining concentration of metal in solution after precipitation. Since the solutions are usually filtered in order to perform the analysis, any particles that are smaller than the filter size will pass through and form part of the aqueous fraction. This will cause the metal removal efficiency to be unexpectedly low. In addition, most of these studies do not acknowledge that the extremely small particle sizes produced by metal sulphide precipitation processes do not lend themselves to solid liquid separation. Thus, no matter how low the residual metal concentration, removal can only be said to have been achieved if the formed particles can be effectively separated. This issue was addressed by Lewis and van Hille (2006) in a paper comparing nickel, cobalt and copper sulphide precipitation in various reactor configurations. They found that the rate of fines formation (particles passing through a 0.45 μm filter) was related to the supersaturation of the system (and thus indirectly to the solubility of the metal sulphide). For the nickel and cobalt sulphide cases, fines formation occurred after 10 min in a fluidised bed reactor, whereas, for the copper sulphide case, fines were formed immediately. Using a gaseous source of sulphide (H2S) increased the particle size of the precipitate, which was then retained on a 0.45 μm filter. Batch tests addressed the issue of excess sulphide causing “redissolution” of the formed precipitates. A reaction of the form given in Eq. (8) based on the work of Shea and Helz (1988) was suggested: −

−

MSðsÞ + HS ðaqÞ→MSðHSÞ ðaqÞ

ð8Þ

This would account for the fact that the pH of the solution did not change and that the free sulphide in solution continued to decrease over

226

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

time. However, based on the work of Shea and Helz (1988), it would appear that this reaction does not occur in the form given in Eq. (8), and that the more likely reaction form for these conditions is that given in Eq. (5), with n = 2 or 3. Shea and Helz (1988) verified a similar reaction for copper sulphide in solutions containing various concentrations of bisulphide. These aqueous complexes were found to be stable in the presence of free HS− (N1 μM), a condition which was satisfied by the work of Lewis and van Hille (2006). An implication of this work for industrial practice is that sulphide addition must be carefully controlled to avoid areas of high local sulphide concentration. 3.1. Zinc Peters et al. (1984) investigated residual metal concentrations for the precipitation of zinc sulphide in the presence of EDTA. They found that EDTA interfered with the precipitation reaction, but that other chelating agents did not. The zinc sulphide precipitation reaction was found to be extremely fast, with very low residual zinc concentrations, corresponding to zinc removals of 99.7%. However, it was acknowledged that the very small particle sizes (of the order of 5–7 μm) would render sedimentation and filtration extremely difficult. They observed that the residual metal concentration was always higher than that predicted by the literature values for zinc sulphide solubility. With the aid of microscopic observations, they concluded that the zinc sulphide probably precipitates in an amorphous form (with a higher solubility) that transforms to the more stable crystalline form over time. Grootscholten et al. (2008) developed a model for ZnS precipitation in a stirred tank reactor. They found that only 19% of the influent sulphide was available for sulphide precipitation and included an efficiency factor to take this into account. They were also able to estimate the reaction constant for ZnS precipitation using measured and estimated process variables. Esposito et al. (2006) compared chemical and biogenic sulphide sources for ZnS precipitation. For a 3 g/L (≈ 45 mM) zinc influent, a 10−12 mM S2− concentration was found to be optimal, resulting in a residual zinc concentration of 0.07 mg/L, for both chemical and biogenic sulphide. The mean particle sizes for both sources of sulphide were similar (7.5 μm for biogenic and 10.2 μm for chemical sulphide). Biogenic sulphide caused a slight decrease in efficiency of the process. This was attributed to the presence of phosphate, micronutrients, acetate and EDTA present in the sulphate-reducing reactor effluent.

attention paid to the particle size distributions or separation properties of the precipitates. In a review article, Zhang and Cheng (2007) reviewed various methods of purifying manganese solutions for electrowinning. One of the methods mentioned is the use of H2S gas to remove impurities (Ni, Co, Zn, Pb, As, Sb, Fe, Cu, Cd) from MnSO4 solutions, and also sulphide precipitation for Zn removal using Na2S. 3.4. Iron Harmandas and Koutsoukos (1996) studied the kinetics of iron sulphide precipitation in low ionic strength (0.01 mM) solutions using the constant supersaturation method. The mineral mackinawite (an iron monosulphide) was formed at neutral pH and amorphous microcrystalline FeS at more acidic (pH = 5.5) conditions. They found that the rate of precipitation was proportional to the relative supersaturation as described by Eq. (9). m

Rp ¼ kσ s Where Rp k σs IAP Ksp m

the rate of precipitation (mol/L/s) the reaction rate constant (mol/L/s) the relative solution supersaturation = (IAP/Ksp)1/2 − 1 ionic activity product solubility product the order of the reaction

They found the order of the precipitation reaction (m) to be 2, and proposed that this is typical of the values found for spiral growth. However, this is only true if growth accounts for all of the mass deposition, and if nucleation and aggregation are negligible. But this does not correlate with the discussion in the text nor the evidence of the Scanning Electron Micrograph images, which show clear evidence of aggregation. So it appears that this analysis technique, although it offers some promise, has been applied too simplistically in this case. Rickard (1995) reported on the competing reaction mechanisms for the precipitation reaction between Fe (II) and dissolved sulphide. The two reactions are given in Eq. (10):

2 +

3.2. Nickel Okuwaki et al. (1984) defined the formation conditions for Ni3S2 from sulphate solutions using H2S gas in the presence of reduced iron powder. The reduced iron powder acts as a reductant for the nickel ion. The formation of Ni3S2 is competitive with NiS, but Ni3S2 is more reactive in subsequent leaching steps. The conditions were: 363 K; PH2S 31 kPa; Ni N 4.0 ppm; 3[Fe°]/[Ni2+] 1.25 to 1.5; H2S flow rate 70 to 100 cm3/min and 45–60 min retention time. The reaction on the surface of the iron was rate limiting in the early stages, but Ni3S2 was formed within 10 min.

ð9Þ

Fe 2 Fe

+

þ

+ H2 S→FeSðsÞ + 2H − + 2HS →FeðHSÞ2 ðsÞ

ð10Þ

Both rates are direct functions of the dissolved Fe activity. In neutral to alkaline waters, the bisulphide pathway resulting in the formation of Fe(HS)2 dominates, with the H2S pathway dominating in acidic environments. The condensation of Fe(HS)2 occurs as follows, with the release of dissolved sulphide back into solution. FeðHSÞ2 ðsÞ→FeSðsÞ + H2 S

ð11Þ

3.3. Manganese 3.5. Cadmium Jandová et al. (2005) focussed on the selective precipitation of copper, cobalt and nickel from concentrated manganese sulphate solutions. They found that, as predicted by the solubility, it is possible to precipitate copper sulphide in the acidic pH range (pH ≈ 1), cobalt and nickel sulphides in the weakly acidic pH range (pH ≈ 3), and manganese sulphide in the alkaline pH range. Although more than 98% removal of Cu, Co, Ni and Zn was measured, there was no

Koumanakos et al. (1990) investigated the mechanism of cadmium sulphide precipitation and found that the rate of precipitation of α-CdS was proportional to the supersaturation with a fourth order dependence on the supersaturation. The high order was ascribed to the fact that initial rates (when both nucleation and growth are operative) were used. The precipitation rate was measured using conductivity i.e. the

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

rate of depletion of ions from solution. An induction period preceded the precipitation and the rate of precipitation was affected by the presence of other anions, as would be expected for the proposed mechanism of surface diffusion-controlled growth. Ennaassia et al. (2002) investigated the removal of cadmium (0.5 mM) from phosphoric acid solutions by precipitation of CdS with Na2S (0.1 M or 1 M). They measured the efficiency of the process in terms of the residual cadmium concentration, and found that the removal efficiency decreased with increase in acidity level (either H3PO4 or H2SO4) and an increase in temperature. An increase in the molar ratio of sulphide to cadmium improved the precipitation efficiency. In order to purify the phosphoric acid to international regulatory levels (b20 mg Cd/kg P2O5), a large excess of sulphide was required (up to 20:1 molar ratio of Na2S to Cd). A large discrepancy was found between the theoretical curves (calculated using equilibrium solubilities) and the experimental data, and this was ascribed to the formation of colloidal particles of CdS. With the formation of colloidal particles, the separation of the solid from the aqueous phase would not be complete and thus the analysis of the aqueous phase would include the contribution of the colloidal CdS particles, thereby “underestimating” the removal compared to the equilibrium calculation. If the findings of Shea and Helz (1988) and Lewis and van Hille (2006) are applied to this work, then it is possible that the discrepancy could be explained by the formation of bisulphide complexes, as given in Eq. (8). 3.6. Molybdenum and vanadium Zeng and Cheng (2009), in a review of the recovery of molybdenum and vanadium from spent catalysts, briefly covered sulphide precipitation. H2S gas is usually injected into the acid solution to precipitate MoS3. Then the pH is adjusted using calcium carbonate and the vanadium is recovered. High levels of recovery are achieved. Vanadium can also be precipitated out as a hydrated oxide. 3.7. Mixed metals In one of the early studies, Bhattacharyya et al. (1979) used Na2S as the sulphide form to separate the metal cations Cd, Zn, Cu and Pb and oxyanions of Ar and Se precipitate from complex wastewaters at concentrations of between 0.02–0.3 mM. Metal removal was measured by analysis of the solution after it was filtered through a 0.45 μm filter. No particle analysis was carried out. Since the pH was adjusted to 8.5, much of the metal removal was due to hydroxide formation. Lead precipitated out as PbSO4. Bhattacharyya et al. (1980) investigated the feasibility of a combined sulphide/hydroxide precipitation process. They found that, in this process, the consumption of Na2S was only 60% of the theoretical requirement (but the reason for this was not explored). Effective separation of metals and arsenic was achieved. H2S gas generation was not found to be a problem due to the high reactivity of the sulphide and metals. 3.7.1. PSD + contaminants Peters and Ku (1985) addressed the issue of the particle size distributions (PSDs) of the formed precipitates. They also addressed the effect of contaminants on the removal of heavy metals. The experimental system used a sulphide dose 1.05× the stoichiometric requirement, pH between 5–8 and metal (zinc, cadmium or nickel) concentrations of 100 mg/L (0.5–1.5 mM) in the presence of various complexing agents (citrate, phosphate and EDTA). They found that, for ZnS precipitated at pH 8, the complexing agents reduced the nucleation rate, as shown by fewer particles at smaller sizes, but promoted the aggregation rate, as shown by the larger number of particles at larger sizes. For CdS precipitation at pH 5.3–5.7, the

227

complexing agents (ammonia, EDTA and 18-Crown-6 ether) all inhibited CdS precipitation, with the largest effect experienced with EDTA. All the complexing agents, except phosphate, reduced the number of small particles as well as the number of larger particles i.e. the total number of particles was reduced, but this analysis was not pursued in the paper. For NiS precipitation at pH 7.6, the complexing agents: phosphate, EDTA and crown ether all gave PSDs similar to the ZnS system, with reduced nucleation and increased aggregation rates. In another study, Veeken and Rulkens (2003), used synthetic wastewaters containing Cd, Cu, Ni, Pb or Zn (1 mM–3 mM). In continuous experiments, Pb and Zn were removed to levels b0.05 ppm at pH 6 by controlling the sulphide concentration at b0.02 ppm. However, the authors observed that the removal of Cu and Ni was lower than expected, possibly due to the fact that metal sulphide particles smaller than 0.45 μm (the filter size) were produced. Considering the experimental conditions and the fact that metal removal was measured by analysis of the filtered solution, this is a likely explanation. A full scale design that used a membrane for the precipitation of Zn was also tested, and was found to be a successful mechanism for generating large particles, at least for the ZnS system. Tokuda et al. (2008) reported on selective precipitation of Cu, Zn, Ni and Sn with H2S at concentration levels of about 100 mg/L (0.5– 1.7 mM). Their objective was to optimise selective precipitation by careful control of the pH. The optimal pH values were 1.5 for CuS and SnS, 4.5 for ZnS and 6.5–7.0 for NiS. The selectivity of metal precipitation was N95% in the Cu–Zn–Ni system and N91% in the Sn–Zn system. The reaction kinetics for the single metal systems were almost the same as those for the multi-metal systems, indicating that the precipitation of one metal is not affected by the others. 4. Metal sulphide precipitation studies focussing on particles and crystallization kinetics The studies that focus on crystallization kinetics are potentially more useful, since they take into account the fact that the particle size distribution of the particles formed by the precipitation reaction will be critical in determining the success (or not) of the subsequent solid–liquid separation. However, as can be seen in the published work, measuring the crystallization kinetics of sparingly soluble salts with rapid reaction rates can be challenging. 4.1. Zinc Peters et al. (1984) carried out a study on zinc sulphide precipitation using a Mixed Suspension Mixed Product Removal (MSMPR) reactor in an attempt to characterise the precipitation kinetics (nucleation and growth) of the metal sulphide precipitation. In order to apply the MSMPR analysis, the population balance of the crystallizer at steady state must be reduced to the form that only considers the processes of nucleation and crystal growth:

L 0 n ¼ n exp Gτ

ð12Þ

Where n n0 G L τ

population density distribution function (number/m3) the population density of nuclei (number/m3) the linear growth rate (ms−1) the crystal size (m) the residence time (s)

The growth and nucleation rates can then be determined directly from the gradient (−1/Gτ) and the intercept (ln n0) of the straight

228

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

line in a logarithmic-linear plot of Eq. (12). However, this analysis is predicated on the assumption that nucleation and growth are the only active processes, and that aggregation and attrition are negligible—an assumption that is mostly not true for precipitation of sparingly soluble salts. Although Peters et al. (1984) used the MSMPR analysis and derived nucleation rates (1.47 × 104–4.61 × 104 number/ml/min) and growth rates (0.136–0.390 μm/min) for the zinc sulphide system, they did acknowledge that the expected straight line is in fact concave, with the degree of concavity increasing as the residence time was increased. This indicates that nucleation and growth are not the only precipitation mechanisms, and that some other factors are at work to influence the measured particle size distributions. Concavity in the log-linear plot usually indicates aggregation, as the number of large particles increases at the expense of the number of mid-size particles, although the authors did not believe that aggregation was a plausible explanation for the concavity in the plot. They ascribed the concavity to their lack of ability to measure the very small particles in the distribution. Much later, Al-Tarazi et al. (2004a) also used the MSMPR method to measure the crystallization kinetics of ZnS precipitation. Although they derived expressions for the nucleation, growth and aggregation rates, as given in Eqs. (13), (14) and (15), they acknowledged that the MSMPR method is not optimal for examining fast precipitation reactions since the assumption of perfect mixing does not hold. 

7


0:3

GL ¼ 2:98 10 ðS
1Þ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ½Zn2þ ½S2
 where S ¼ Ksp 9

Bo = 9:31*10 ðS
1Þ 

βo ¼ 2:27 10

0:77


44 0:3
1:47 Bo Gv

where Gv =

π 2 d G 2 s L

ð13Þ

ð14Þ

ð15Þ

Where GL Gv S Ksp Bo β ds

the linear average crystal growth rate (m/s) volumetric average crystal growth rate (m3/s) relative supersaturation (−) solubility constant (mol2/m6) primary nucleation rate (number/m3/s) aggregation kernel in volume co-ordinates (m3/number/s) Sauter average particle size (m)

Veeken and co-authors carried out a number of studies investigating ZnS precipitation. The general focus was on selective removal and precipitation of Zn as ZnS. Veeken et al. (2003a) showed that controlling the stoichiometric addition of sulphide to a metal metal-containing was critical to avoid excess metals or excess sulphide remaining in solution. The control could be achieved through controlling the sulphide concentration in the reactor by applying a combination of a sulphide ion-selective electrode and a pH electrode. This resulted in low effluent concentrations of both metals and sulphide. In the next study, Veeken et al. (2003b) investigated the effect of the sulphide concentration on the particle size of the zinc sulphide precipitates. They found that, at the optimum reagent concentrations, both zinc and sulphide were removed to at least 0.03 ppm (from a starting concentration of 800–5800 mg/L: 12 mM–89 mM), and the ZnS precipitate consisted of particles with a mean geometric diameter of about 10 μm. Although they concluded that aggregation played an important role in ZnS particle formation, the aggregation

rate was not calculated, this conclusion being drawn from the fact that the classical nucleation/growth model did not fit the data very well. Thus, although growth and nucleation rates were calculated from the data, their validity is probably uncertain due to the flawed analysis. Al-Tarazi et al. (2005) used a bubble column reactor and H2S as the sulphide source to study the precipitation of ZnS in semi-batch mode from 1–500 mM (65–32,000 mg/L) ZnSO4 solutions. They found that increasing either the initial zinc or H2S concentration decreased the average particle size of the precipitate, due to increased local supersaturation and thus increased nucleation. The maximum particle size obtained for the zinc sulphides was about 0.5 μm. Al-Tarazi et al. (2004b) also developed a mathematical model to describe simultaneous precipitation of copper and zinc sulphides, using concentrations similar to that of an industrial wastewater (± 400 mg/L, 6 mM). The model assumed no aggregation or attrition (contradicting the results measured by the same group (Al-Tarazi et al., 2004a)) and was used to predict the selectivity of the precipitation and the size distribution of the metal sulphide crystals. The precipitation rate was fully controlled by the H2S mass transfer rate and selective precipitation was possible where there was a large difference in solubilities between the two metal sulphides. 4.2. Copper Al-Tarazi et al. (2005) also carried out precipitation in the H2S bubble reactor with 1–100 mM Cu solutions. The average size of the produced particles scarcely varied with the concentration of copper ions. Based on the difference in solubility products of ZnS and CuS (CuS being much less soluble), much smaller particles were expected for the copper. However, the copper sulphide particles were found to be surface active and clustered at the gas–liquid interface, forming aggregates of the order 10–15 μm. Sampaio et al. (2009) carried out selective precipitation of copper from a mixed copper–zinc sulphate solution at 600 ppm (±9 mM) in a continuously stirred tank reactor. Recovery was measured via analysis of the solution, which was filtered through a 0.45 μm filter. Anything that passed through the filter was considered to be soluble. Copper recovery and purity was found to be almost 100%, with the copper being precipitated as highly crystalline covellite (CuS). The mode of the PSD of the precipitated CuS particles was ≈ 36 μm. If allowed to settle, the PSD increased to 180 μm. If the same sample was vigorously stirred, then the PSD decreased to below 3 μm. Lower supersaturation levels were correlated with the larger particles. Surprisingly, X-ray Diffraction (XRD) analysis identified the precipitates as highly crystalline structures, with copper precipitating as covellite (CuS) and zinc as sphalerite (ZnS). 4.3. Manganese and cobalt Bryson and Bijsterveld (1991) examined the selective precipitation of cobalt from concentrated manganese sulphate solutions (Mn = 30 g/L (0.55 mM) and Co = 15 g/L (0.3 mM)) and found that first order kinetics could be used to model the precipitation of manganese sulphide. The cobalt sulphide precipitation reaction exhibited an induction period, followed by rapid precipitation and finally a slow approach to equilibrium. Although one of the aims of the work was to apply classical crystallization theory to the precipitation process, the observed particle sizes were extremely small, amorphous and tended to form aggregates. This crystallization theory approach was therefore not pursued. Chiang et al. (2007) investigated how various mixing configurations could be used to control particle size in MnS precipitation. They used micromixing devices to control the micromixing time and showed that these improved the particle characteristics (narrower

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

particle size distributions) of the precipitates compared to direct addition into a stirred tank. 4.4. Zinc and cobalt In a very similar study to that of Bryson and Bijsterveld (1991), Mishra and Das (1992) precipitated zinc and cobalt sulphides from an ammonium sulphate solution by controlled addition of sodium sulphide. Initial metal concentrations varied from 50–80 mg/L (0.7– 1.3 mM). They found the kinetics of the zinc sulphide precipitation to be first order. In accordance with the findings of Bryson and Bijsterveld, they also found that cobalt sulphide precipitation exhibited three kinetic regions with an induction period in the first region. 4.5. Nickel and cobalt In contrast, Lewis and Swartbooi (2006) found that, for a mixed nickel (2000 ppm, 34 mM) and cobalt (1000 ppm, 17 mM) system , there was no induction time for cobalt sulphide precipitation, that both the nickel and cobalt sulphide precipitation reactions were first order and that removals of 90% for both metals were achieved. The lack of an induction time for the Co was probably due to the presence of the NiS precipitates, which acted as heterogeneous nuclei. When excess sulphide was used, the removals decreased, and this was ascribed to the formation of aqueous polysulphide species, which consumed the aqueous sulphide and made it unavailable for precipitation. This is dealt with in more detail in Luther et al. (1996), although the concentrations used by Luther III and other workers focussing on fundamental aspects of metal sulphide precipitation are far lower (micromolar) than the ones used in industrially-based studies (millimolar to molar). 5. Precipitation of metal sulphide nanocrystals Controlled synthesis of metal sulphide nanocrystals has attracted a great deal of attention in both fundamental and technical applications in recent years (Geng et al., 2007). The attraction of this research field is the numerous special optical, structural and electronic properties of the nanocrystals compared to the same crystals at larger sizes. The main reasons for these special properties are the extremely large surface to volume ratio of the nanocrystals, as well as the quantum confinement effect. For all of these special applications, highly crystalline particles with an almost monodisperse particle size distribution are required. However, these as-prepared nanocrystals also have a tendency to continue to grow by coarsening or epitaxial attachment because of the lack of strong repulsive forces between the particles. This results in poor stability of the nanocrystals and thus affects their special properties. Consequently, there is a still a major research effort in exploring robust methods that are able to systematically modify both the shape and size of formed nanocrystals. 5.1. General synthesis Shen et al. (2003) proposed a general polyol route for synthesising a range of binary metal sulphide nanocrystals (NiS, CunS, PbS, Ag2S, In2S3, Fe3S4). The metal sulphides were obtained by refluxing the different metal salts with thiourea ((NH2)2CS) in glycol at appropriate conditions. Powder XRD, energy dispersive X-ray analyses and transmission electron microscopy were used to investigate the crystallinity, size and morphology. Hexagonal phase NiS nanocrystals of about 35 nm in diameter, pseudo-cubic phase Cu1.8S of about 65 nm and cubic phase PbS with average size between 15–120 nm were formed. Nano crystallites with flaky morphology were obtained in the case of In2S3 precipitation. No further information was given about the nanocrystals of silver and iron sulphides.

229

5.1.1. Zinc Pawaskar et al. (2002) used a liquid–liquid interface reaction technique to grow nanoparticle films of ZnS and reported its structural and optical properties. A solution of zinc sulphate (0.1 mM) was contacted with a solution of H2S in carbon tetrachloride. When the tetrachloride evaporated, H2S reacted with the zinc sulphate to form a film of zinc sulphide. The stoichiometry was determined to be ZnS, with the phase made up of a mixture of hexagonal and metastable cubic forms. The XRD characterisation did not reveal any structural features, indicating that the particles are either non-crystalline or that the particle size was very small. Jovanovic et al. (2007) synthesised zinc sulphide nanocrystals in micro emulsions and were able to prepare two different morphologies of ZnS nanocrystals: either cubes or nanowires. The cubes had an average size of about 25 nm, whilst the wires were about 25 Å diameter and lengths ranging from several hundred nanometers up to a few microns. XRD analysis showed cubic phase ZnS. 5.1.2. Copper In a relatively early paper, Haram et al. (1998) synthesised copper sulphide (CuS) nanocrystals by reacting a copper–ammonia complex with thiourea in aqueous micellar solutions of cationic, non-ionic and anionic surfactants. A blue shift in the absorption spectra revealed the formation of size quantized nanoparticles. The XRD analysis confirmed that the particles were the hexagonal form of cupric sulphide (CuS). This paper makes the important comment that the great challenge in the field is not the synthesis but the stabilisation of particles generated at the desired size. Mathew et al. (2008) synthesised copper sulphide nanocrystals from aqueous copper sulphate (CuSO4) and sodium thiosulphate (Na2S2O3) using a UV source to decompose the thiosulphate. They found that the particles were crystalline and that they displayed the characteristic XRD peaks corresponding to the hexagonal structure CuxS. They also found that the quantity of particles formed was a function of the duration of the irradiation. Roy and Srivastava (2007) synthesised CuS nanorods using a wet chemical method at 105 °C using CuCl2·2H2O and CS2. They suggested that the morphology is affected by the reaction temperature since growth is favoured at lower temperatures, but nucleation begins to predominate at higher temperatures. 5.1.3. Iron Wei and Osseo-Asare (1996) were able to synthesise pyrite (FeS2) nanocrystals by reaction of ferric ions with sulphide ions in aqueous solution and by lowering the concentrations of Fe3+ and HS− in the early stages of the reaction. They found that the particle size could be reduced to ± 50 nm when the concentration of FeCl3 was b6.7 × 10−3 M. However, they also found, after about 5 days, the particle size became 1.5 μm for any initial concentration of reactants. 5.1.4. Cadmium Ge et al. (2002) prepared cadmium sulphide nanorods using precipitation of CdSO4 with thioacetamide (CH3CSNH2). The technique involved γ-sradiation at room temperature using urea as the template. XRD confirmed that the product was hexagonal CdS with a rod-like morphology. The mean diameter of the rods was 40 nm, and the length was up to 100 nm. The urea was found to play an important role in controlling the morphology of the crystals, since the urea crystals contained the embedded reagent solution. The role of the γradiation was to provoke a series of reactions in the embedded solution in the urea matrix and thus precipitate out the CdS nanorods. Silver nano-ribbons were also prepared by the same workers, using the same conditions but different reagents. No further information was given about the nano-ribbons. Hassan and Ali (2008) prepared nano structured cadmium and zinc sulphides by precipitation of metal chloride solutions with

230

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

sodium sulphide in the presence of polyethyleneimine (PEI) hyperbranched polymer. The role of the PEI was to act as a stabiliser and dispersant. The sizes of the particles were about 4–17 nm for the cadmium sulphide and 7–53 nm for the zinc sulphide particles, according to the proportion of PEI used. Kanade et al. (2006) synthesised nanocrystals of CdS using polyphenylene sulphide (PPS) and cadmium iodide through heating at the melting temperature of PPS (285 °C). Two different molar ratios of PPs to CdI2 were used: 1:1 and 10:1. For the 1:1 molar ratio, XRD analyses showed mainly hexagonal phases of CdS, with some cubic phases evident. The average particle size, as found by Schrerrer's formula, was 15 nm. For the 10:1 molar ratio case, both cubic and hexagonal phases were found, as well as excess PPS. The average particle size was 12 nm. Ni et al. (2001) irradiated an ethanol solution of cadmium chloride and carbon disulphide with 60Co γ-rays to produce cadmium sulphide nanocrystals. The powders consisted of uniform size spherical particles, in which the small particles had aggregated into secondary particles. The powders were identified as cubic phase CdS. 5.1.5. Transition metals—cadmium, zinc, nickel, cobalt and copper Deng et al. (2006) synthesised CunS (n = 1, 2) microtubes and transition metal sulphide (CdS, ZnS, Nis, CoS, CuS, Cu2S) microspheres through a hydrothermal reaction. Metal nitrate and sodium thiosulphate (Na2S2O3) were mixed together and then heated to instigate the decomposition of the thiosulphate and thus the metal sulphide precipitation reaction. The recipes were adjusted slightly to form either microspheres or microtubes. The powder XRD patterns corresponded to hexagonal phase CdS, cubic phase β-ZnS, triclinic phase NiS2, triclinic phase CoS, hexagonal phase CuS and cubic phase Cu2S. The Cd, Zn, Ni and Co products formed spherical morphologies with diameters about 1-2 μm and both the CuS and the Cu2S formed hollow microspheres with diameters ranging from 0.5-1 μm for the CuS and about 2 μm for the Cu2S. 5.1.6. Copper, mercury, zinc, bismuth, lead Many researchers, including Zhu and Lie (2009) have focussed on the bismuth sulphide (Bi 2 S 3 ) system, mostly because of its application in the semiconductor field. Much of the research has focused on how the synthesis method affects the morphology. Zhu and Lie (2009) found that the sulphur source had a significant effect on the morphology. They proposed that, when the sulphur source, such as urea, formed a complex with the Bi3+, this prevented the explosive production of bismuth sulphide and favoured oriented growth. Liao et al. (2001) prepared nanocrystals of Cu, Hg, Zn, Bi and Pb using a formaldehyde solution of metal salt and thioacetamide subjected to microwave irradiation. XRD patterns showed that the particles were pure orthorhombic cubic phase Bi2S3, cubic phase HgS, hexagonal phase CuS, cubic phase ZnS and cubic phase PbS, respectively. 5.1.7. Cadmium, silver and mercury Xu et al. (2004) used a disproportionation reaction to synthesise metal sulphides of CdS, Ag2S and HgS at room temperature. CdCl2, AgNO3 or Hg(NO3)2 dissolved in ethanol were used as the sources of the metals with carbon disulphide as the sulphide source. The reaction was carried out in the presence of polyvinylpyrrolidone (PVP) at pH 9 using ammonia. Hexagonal CdS appearing as flakes, monoclinic Ag2S as elliptical and Y shaped flakes and cubic HgS as star shaped crystals were identified. 5.1.8. Silver Choi et al. (2009) found that silver nanoparticles were highly reactive with sulphide to form new AgxSy complexes or precipitates which did not oxidise after prolonged aeration. This has potential

implications for reduction of nanosilver toxicity in wastewaters via sulphide complexation. 5.1.9. Bismuth Li et al. (2002) prepared Bi2S3 nano fibres using BiCl3 and thiourea as the starting materials. The fibres had diameters less than 10 nm and lengths ranging from 400 to 1000 nm. Potassium oleate played an important role in controlling the morphology. 6. Applications of metal sulphide precipitation to environmental and industrial systems 6.1. Effluent treatment processes such as Acid Mine Drainage (AMD) treatment Mine waters and industrial effluents that contain high sulphate and metal concentrations can be treated using a combination of bacterial sulphate reduction to generate sulphide, followed by removal of the metals as metal sulphide precipitates. Many studies have been carried out in this area of application—this review will focus on the metal sulphide precipitation aspects of the investigations. Since most of the Acid Mine Drainage treatment processes produce amorphous metal sulphide precipitates, it is important to understand the solubility differences between these and the solubilities of the crystalline metal sulphides that are most often reported in the literature. The environmental implications of producing a more soluble metal sulphide are often not taken into account. However, authors such as Gammons and Frandzen (2001) have started to address this question in a paper that points out the differences between the theoretical and measured solubilities of metal sulphides removed in a treatment wetland. 6.1.1. Mixed metals Jong and Parry (2003) treated mildly acidic metal (Cu, Zn, Ni, Fe, Al and Mg), arsenic and sulphate waters using a bench scale, Upflow Anaerobic Packed Bed (UAPB) reactor filled with silica sand. The initial metal concentrations were approximately 10 mg/L (0.1–0.8 mM). They found that the reactor removed more than 97.5% of the initial concentrations of Cu, Zn and Ni, while only N77.5% of the As and N82% of the Fe were removed. Mg and Al levels were unchanged. They found that black precipitates (metal sulphides) were formed in all of the experiments that were supplied with lactate (as a carbon source for the sulphate reducing bacteria). The precipitates appeared inbetween the sand particles and adhered onto the surface of the sand particles themselves. This is contrary to the findings of van Hille et al. (2005), who used a Fluidised Bed Reactor (FBR) filled with silica sand and aqueous sodium sulphide to precipitate copper sulphide from a copper sulphate solution (50–180 mg/L, 0.8–2.8 mM). They found that the copper sulphide precipitated as very fine particles, did not adhere to the silica sand and exited the system with the aqueous effluent. Although the reactor systems were different, perhaps it was also the presence of the sulphate reducing bacteria in the system of Jong and Parry (2003) that contributed to the high percentage removal of the metal sulphides, especially the copper sulphides, by acting as adsorption sites? Alvarez et al. (2007) point out that precipitating the metal sulphides with the biomass means that the precipitated metals are lost with the wasted biomass, although they can be recovered from the bio-sludge. They used a two stage process which separated out the metal precipitation and biological sulphate reduction steps. They then evaluated the efficiency of metal removal using synthetic mine effluent containing zinc (1.34 mM), copper (0.018 mM) and lead (2.3 μM) and treating it using biologically generated H2S. The sulphide-rich supernatant from the biofilm reactor was collected and combined with the same volume of metal leachate. The precipitation reactor had a volume of 150 ml and was designed such

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

that the metals leachate and H2S were well mixed. The authors found that copper was 100% removed, zinc N94% and lead N92% as measured by decrease in solution concentrations of these metals. They highlighted the advantage of the increase in wastewater pH due to the biological process. However, the disadvantages of the two stage method are that additional pumping facilities are required for gas recycling; the H2S depends on the quantity of sulphate available; the final effluent may require treatment for sulphide removal and there are concerns about odour and corrosion problems and poor settling of precipitates (Hao, 2000). Foucher et al. (2001) treated synthetic acid mine drainage containing a range of metals at different concentrations (Fe: 1540 mg/L (27.6 mM); Zn: 320 mg/L (4.89 mM); Cu: 160 mg/L (2.52 mM); Al: 210 mg/L (7.8 mM); Mn: 5.5 mg/L (0.1 mM); Co: 0.06 mg/L (0.001 mM); Ni: 0.4 mg/L (0.007 mM); Pb: 0.5 mg/L (0.002 mM)) using H2S gas generated by bacterial sulphate reduction. They selectively recovered Cu at pH 2.8 and Zn at pH 3.5. Other impurities such as Ni and Fe were removed at pH 6. Cao et al. (2009) used biogenic H2S to recover metals as sulphides from bioleaching solutions. They used synthetic solutions (Mg: 20 g/L (823 mM); Fe: 5 g/L (90 mM); Ni: 2 g/L (34 mM) and Cu: 0.5 g/L (7.8 mM)) and found that Fe, Ni and Cu were N99% removed, whereas Mg remained in solution. Increasing the temperature in the precipitation reactor increased the removal efficiency, with the highest removal occurring at 60 °C. Although it is stated that the pH in the reactor had a significant effect, since the rate of H2S dissolution is faster at high pH levels, the removal levels were very high at all pH values. Bhagat et al. (2004) also used biogenic sulphide gas (generated by bacterial reduction of sulphate) to precipitate metals as sulphides from wastewater (Fe: 475–1900 mg/L (8.5–34 mM); Au: 5–20 mg/L (0.03–0.1 mM) and Cu: 540–2160 mg/L (8.5–34 mM)). They found that iron was 30% removed, whereas gold (removed as Au2S) and copper were 100% removed. 6.1.2. Zinc Azabou et al. (2007) evaluated the effectiveness of zinc removal in an environment of sulphate reducing bacteria. They found that zinc (measured by the decrease in zinc in solution) was removed effectively to less than 5% from initial concentrations of 150 mg/L (≈2 mM). It was suggested that the precipitation was aided by adsorption onto the biomass although this was not quantified further. Other authors have also suggested that the bacteria facilitate solids removal by binding the metal in their cell walls (Jalali and Baldwin, 2000) or by producing extracellular polymeric substances that take up soluble metals (Beech and Cheung, 1995). Radhika et al. (2006) showed that zinc could be successfully removed from the sulphate solution at concentrations from 10–20 000 mg/L (0.15–305 mM). The precipitates themselves conformed to wurtzite structures. About 16% of the zinc was removed by biosorption. 6.1.3. Zinc and iron Kaksonen et al. (2003) used a zinc (170–230 mg/L) (2.6–3.5 mM) and iron (58 mg/L) (1 mM) sulphate wastewater in a fluidized bed and used sulphate reducing bacteria to produce the sulphide for metal sulphide precipitation. They found that 14–23% of the metal sulphides were washed out of the reactors—a washout level that was considered to be low. The authors propose that the metal sulphide precipitates contributed to the retention of the microbes in the bed, and not the other way around. In any case, both microbes and precipitates were successfully retained in the reactor bed. Remoundaki et al. (2008) characterized the precipitates from a sulphate reducing reactor treating a mixed zinc (50–400 mg/L (0.76– 6 mM)) and iron (100 mg/L (1.8 mM)) effluent, and found them to

231

consist of amorphous solid phases of iron and zinc sulphides. They found that the solids were precipitated as either amorphous phases deposited on the bed material and on crystal surfaces or as rod-shaped solids characterized by a rough hazy surface, indicating the encapsulation of bacterial cells by amorphous metal sulphides. 6.1.4. Zinc and cadmium Gonçalves et al. (2007) used sulphate reduction to generate sulphide that was then used to remove metals from synthetic wastewater containing 50–70 mg/L Zn (1 mM) and 2 mg/L cadmium (17 μM). Metals were removed to below discharge limits (b5 mg/L for Zn and b0.2 mg/L for Cd), but the quantity of sulphide used was below the stoichiometric requirement. This suggested that sulphide precipitation was not the only form responsible for metal removal. 6.1.5. Copper and iron García et al. (2001) investigated the use of sulphate reduction for bioremediation of an industrial acid mine water. Iron at 30 mg/L (0.5 mM) and different concentrations of copper (25–200 mg/L (0.39–3 mM)) were used for the bio-precipitation tests. Total removal of the copper from solution was achieved, whilst iron was 97% removed. The precipitates were analysed to be FeS2 (pyrite) and Cu7S4. 6.1.6. Lead Velasco et al. (2008) used biological hydrogen sulphide production to precipitate lead nitrate (20–200 mg/L) (0.1–1 mM) from synthetic wastewater as lead sulphide. The process efficiency was evaluated using the residual lead concentration, which remained below 0.2 mg/L, probably due to constructive interaction between the microbes and the metal sulphide precipitates. 6.1.7. Cadmium Shpiner et al. (2009) used Na2S to treat oil well produced water (with Cr and Cd at micromolar concentrations) and found that nearly 100% chromium and cadmium were removed. However, the nickel removal was only 60%. It became apparent that this low value was partly due to the size of the filter, since the removal was measured using the filtrate generated through a 0.45 μm filter. They did find that all the removals were lower when a higher sulphide dose was used since the higher sulphide concentration resulted in much smaller particles (20% by mass between the 0.22 μm and 0.45 μm sizes). They also found that high ionic strength solutions (contrary to what was expected) resulted in increased metal removals. Theoretically, the solubility should increase at high ionic strengths, but these authors proposed that the compression of the diffuse part of the double layer around the particles led to a reduction in the double-layer repulsion, and allowed particles to approach close enough for the van der Waals forces to dominate. Sahinkaya et al. (2009) focussed on selective recovery of metals (at 50–100 mg/L) from acid mine drainage using biogenic sulphide. Cu was successfully precipitated at pH b2 using sulphide transported from the biological reactor using N2 gas. Zn did not precipitate during Cu removal but was 84–98% removed in a separate reactor that used biologically generated alkalinity to increase the pH to above 7. The mode of the particle size distributions were 17 and 46 μm for the zinc and copper sulphide precipitates respectively. Jiménez-Rodríguez et al. (2009) looked at the effect of pH on combined metal removal from acid mine drainage using biogenic sulphide. They found that a pH of 5.5 maximised the metal removal with Fe, Cu, Zn and Al being removed to 91.3, 96.1, 79.0, and 99.0% respectively, the Al precipitating as a hydroxide under these conditions. The removal figures were based on measurements of metals in solution using Atomic Absorption Spectroscopy. No filtration procedure was mentioned.

232

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

6.2. Industrial processes Although most descriptions of industrial processes are not covered extensively in the scientific literature, but rather in the patent domain, some early papers do describe and evaluate industrial applications of metal sulphide precipitation. Simons (1964) presented the many advantages of H2S as a hydrometallurgical reagent in a comprehensive paper that describes both basic and applied aspects. The paper covers two general applications: (1) the bulk precipitation of metal sulphides from weak aqueous solutions for concentration purposes and (2) selective precipitation of a specific metal in the presence of others. The general message of the paper is that the objections to H2S as a reagent are not significant and that H2S has unique advantages in metal sulphide precipitation processes. Aspects of the NiS precipitation process at Moa Bay in Cuba, such as the raised temperature necessary for rapid nickel sulphide precipitation and the necessity for a recycle stream, are discussed. The nickel solution purification process at Freeport Nickel Company refinery in Louisiana is also described in some detail. The potential for selective metal recovery with careful control of H2S injection is considered. The paper by Roy (1961) provides a comprehensive description of the Moa Bay process, including the effect of seeding, temperature, initial pH, agitation, H2S pressure on the nickel sulphide precipitation rates. Jha et al. (1981) described the selective precipitation of nickel and cobalt from leach liquors in the AMAX Acid Leach Process. They highlighted how almost complete precipitation is obtained within an hour, so long as adequate agitation and a high concentration of recycled solids are available. In another early paper, Kim (1981) described various sulphide precipitation methods: soluble sulphide precipitation (SSP) using reagents such as Na2S and NaHS and insoluble sulphide precipitation (ISP) using reagents such as FeS and CaS. He also described a process that precipitates metal sulphides and hydroxides and separates the toxic from the non-toxic solids. The paper gives some examples of applications of the process for treating wastewaters, copper wire plant wastes, metal finishing wastes, electroplating, pickling and aluminium etching process wastes. For industrial metal sulphide precipitation processes, Karbanee et al. (2008) proposed a 1.66:1 molar ratio of NaOH to H2S (g) in order to avoid sulphide wastage. They found that, above these levels, sulphide wastage occurs through either accumulation in the reaction solution or being lost as unabsorbed H2S. In a wastewater treatment application, Banfalvi (2006) proposed the use of bentonite (aluminium silicate clay) in combination with metal sulphide precipitation to adsorb the precipitates. He used crude bentonite mixed with sodium sulphide and inserted the mixture into tea bags which were then immersed in heavy-metal containing water. The concept was that the sodium sulphide would remain associated with the bentonite, so that the metal sulphide would precipitate on the surface of bentonite and thus remain inside the bag. The remaining sulphide in solution was removed using carbogen gas (5% CO2 + 95% O2) to convert the sulphide to sulphur. The Murrin Murrin nickel laterite process uses sulphide precipitation to separate Ni, Co, Cu and Zn from Mn, Mg and Ca impurities. The mixed Ni, Co, Cu and Zn precipitate is re-leached and further treated to recover the metals (Moskalyk and Alfantazi, 2002). Adams et al. (2008) proposed a process that uses biogenic generation of hydrogen sulphide by reduction of elemental sulphur. The biogenic sulphide is then used in an anaerobic agitated contactor to selectively precipitate the metal to be recovered as sulphide. There is the possibility of producing separate sulphide concentrates of Cu, Zn, Ni-Co and Mn. This particular advantage of this technology is that it can be used for profitable operation in cases where the solutions are too low grade for solvent extraction-electrowinning (SX-EW). In one example, N99.5% copper was recovered from a solution containing

220–360 mg/L (3.5–5.5 mM) copper. The copper concentrate was thickened and dewatered using conventional equipment, with the filtered concentrate (40–45% copper) smelted and recovered as metal. 7. Conclusions Although there have been numerous studies on metal sulphide precipitation, those that focus on the fundamental aspects have not been well incorporated into the applied studies that focus on metal removal and process efficiency. The applied studies, in turn, have often used flawed methods to evaluate the efficiency of the metal sulphide precipitation process, which has led to inconsistent results across studies. Attempting to apply classical crystallization theory to these processes has also not been entirely successful, as the sparingly soluble nature of the precipitates and the extremely fast reaction rates mean that multiple crystallization mechanisms are active. The studies that investigate nanocrystal formation represent yet another disparate branch of the field, with these studies mostly focussing on recipes and characterization of the formed particles. The industrial and effluent treatment studies are another area of research that could be profitably linked up with fundamental studies into mechanisms, chemistry and particle formation processes. One of the key insights obtained from the summary of these disparate areas of work is that the level of scientific understanding in each of the fields is vastly different. The fundamental studies into mechanistic aspects of metal sulphide precipitation are far advanced of the other areas. However, they are restricted to very low concentrations, which are of limited value in most process-based hydrometallurgical applications. Most of the applied studies are still at a relatively empirical level, with the findings being highly systemdependent. Truly generic findings are still to be realised in these areas. Some suggestions as to how the applied studies could be improved would be: Firstly, to use appropriate and consistent methods to evaluate the effectiveness of metal sulphide removal. The measurement techniques that are now available can track metal sulphide particles across the full range of sizes. Secondly, if appropriate measurement techniques are applied (that take into account the non ideality of mixing), then there is no reason why population balance modelling cannot be used to comprehensively model the simultaneous nucleation, growth, aggregation and attrition mechanisms involved in metal sulphide precipitation processes. Thirdly, using the current understanding and characterisation of the complex aqueous chemistry and taking into account multiple sulphide and metal species interactions could be another way that the applied studies could be improved. Conversely, the fundamental studies could be of enormous use to the applied practitioners if their findings could be transferred to high concentration, multiple component systems. In summary, linking these disparate focus areas together would represent a significant step forward. This would benefit the individual focus areas, as well as the field as a whole. References Adams, M., Lawrence, R., Bratty, M., 2008. Biogenic sulphide for cyanide recycle and copper recovery in gold-copper ore processing. Minerals Engineering 21 (6), 509–517. Al-Tarazi, M., Heesink, A.B.M., Azzam, M., Abu Yahya, S., Versteeg, G.F., 2004a. Crystallization kinetics of ZnS precipitation; an experimental study using the mixed-suspension-mixed-product-removal (MSMPR) method. Crystal Research and Technology 39 (8), 675–685. Al-Tarazi, M., Heesink, A.B.M., Versteeg, G.F., 2004b. Precipitation of metal sulphides using gaseous hydrogen sulphide: Mathematical modelling. Chemical Engineering Science 59 (3), 567–579.

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234 Al-Tarazi, M., Heesink, A.B.M., ersteeg, G.F., Azzam, M.O.J., Azzam, K., 2005. Precipitation of CuS and ZnS in a bubble column reactor. AIChE Journal 51 (1), 235–246. Alvarez, M.T., Crespo, C., Mattiasson, B., 2007. Precipitation of Zn(II), Cu(II) and Pb(II) at bench-scale using biogenic hydrogen sulfide from the utilization of volatile fatty acids. Chemosphere 66 (9), 1677–1683. Azabou, S., Mechichi, T., Sayadi, S., 2007. Zinc precipitation by heavy-metal tolerant sulfate-reducing bacteria enriched on phosphogypsum as a sulfate source. Minerals Engineering 20 (2), 173–178. Banfalvi, G., 2006. Removal of insoluble heavy metal sulfides from water. Chemosphere 63 (7), 1231–1234. Beech, I.B., Cheung, C.W.S., 1995. Interactions of exopolymers produced by sulphatereducing bacteria with metal ions. International Biodeterioration and Biodegradation 35 (1–3), 59–72. Bhagat, M., Burgess, J.E., Antunes, A.P.M., Whiteley, C.G., Duncan, J.R., 2004. Precipitation of mixed metal residues from wastewater utilising biogenic sulphide. Minerals Engineering 17 (7–8), 925–932. Bhattacharyya, D., Jumawan, A.B., Grieves, R.B., 1979. Separation of toxic heavy metals by sulfide precipitation. Separation Science and Technology 14 (5), 441–452. Bhattacharyya, D., Jumawan, A.B., Sun, G., Sund-Hagelberg, C., Schwitzgebel, K., 1980. Precipitation of heavy metals with sodium sulfide: Bench-scale and full-scale results. AIChE Symposium Series 209 (77), 31–38. Bryson, A.W., Bijsterveld, C.H., 1991. Kinetics of the precipitation of manganese and cobalt sulphides in the purification of a manganese sulphate electrolyte. Hydrometallurgy 27 (1), 75–84. Cao, J., Zhang, G., Mao, Z., Fang, Z., Yang, C., 2009. Precipitation of valuable metals from bioleaching solution by biogenic sulfides. Minerals Engineering 22 (3), 289–295. Chiang, Y.L., Nathoo, J., Lewis, A.E., 2007. Investigating the control of manganese sulphide precipitation. The Journal of The South African Institute of Mining and Metallurgy 107, 1–6. Choi, O., et al., 2009. Role of sulfide and ligand strength in controlling nanosilver toxicity. Water Research 43 (7), 1879–1886. Ciglenecki, I., Krznaric, D., Helz, G.R., 2005. Voltammetry of copper sulfide particles and nanoparticles: investigation of the cluster hypothesis. Environmental Science & Technology 39 (19), 7492–7498. Deng, H., Chen, C., Peng, Q., Li, Y., 2006. Formation of transition-metal sulfide microspheres or microtubes. Materials Chemistry and Physics 100 (2–3), 224–229. Ennaassia, E.l., Kacemi, K., Kossir, A., Cote, G., 2002. Study of the removal of Cd(II) from phosphoric acid solutions by precipitation of CdS with Na2S. Hydrometallurgy 64 (2), 101–109. Esposito, G., Veeken, A., Weijma, J., Lens, P.N.L., 2006. Use of biogenic sulfide for ZnS precipitation. Separation and Purification Technology 51 (1), 31–39. Foucher, S., Battaglia-Brunet, F., Ignatiadis, I., Morin, D., 2001. Treatment by sulfatereducing bacteria of Chessy acid-mine drainage and metals recovery. Chemical Engineering Science 56 (4), 1639–1645. Gammons, C.H., Frandzen, A.K., 2001. Fate and transport of metals in H2S-rich waters at a treatment wetland. Geochemical Transactions 2, 1–15. García, C., Moreno, D.A., Ballester, A., Blázquez, M.L., González, F., 2001. Bioremediation of an industrial acid mine water by metal-tolerant sulphate-reducing bacteria. Minerals Engineering 14 (9), 997–1008. Ge, X., Ni, Y., Zhang, Z., 2002. A novel route to prepare cadmium sulfide nano-rods. Radiation Physics and Chemistry 64 (3), 223–227. Geng, B., Liu, X., Ma, J., Du, Q., 2007. A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal sulfides. Materials Science and Engineering B 145 (1–3), 17–22. Gonçalves, M.M.M., da Costa, A.C.A., Leite, S.G.F., Sant'Anna Jr., G.L., 2007. Heavy metal removal from synthetic wastewaters in an anaerobic bioreactor using stillage from ethanol distilleries as a carbon source. Chemosphere 69 (11), 1815–1820. Grootscholten, T., Keesman, K., Lens, P., 2008. Modelling and on-line estimation of zinc sulphide precipitation in a continuously stirred tank reactor. Separation and Purification Technology 63 (3), 654–660. Hao, O.J., 2000. Metal Effects on Sulphur Cycle Bacteria Nd Metal Removal by SulphateReducing Bacteria. In: Lens, P.N.L., Pol, L.H. (Eds.), Environmental Technologies to Treat Sulfur Pollution: Principles and Engineering IWA Publishing, pp. 393–414. London, UK. Haram, S.K., Mahadeshwar, A.R., Dixit, S.G., 1998. Synthesis and characterization of copper sulphide nanoparticles in aqueous surfactant solutions. Adsorption Science and Technology 16, 667–677. Harmandas, N.G., Koutsoukos, P.G., 1996. The formation of iron(II) sulfides in aqueous solutions. Journal of Crystal Growth 167 (3–4), 719–724. Hassan, M.L., Ali, A.F., 2008. Synthesis of nanostructured cadmium and zinc sulfides in aqueous solutions of hyperbranched polyethyleneimine. Journal of Crystal Growth 310 (24), 5252–5258. Jalali, K., Baldwin, S.A., 2000. The role of sulphate reducing bacteria in copper removal from aqueous sulphate solutions. Water Research 34 (3), 797–806. Jandová, J., Lisá, K., Vu, H., Vranka, F., 2005. Separation of copper and cobalt–nickel sulphide concentrates during processing of manganese deep ocean nodules. Hydrometallurgy 77 (1–2), 75–79. Jha, M.C., Meyer, G.A., Wicker, G.R., 1981. An improved process for precipitating nickel sulfide from acidic laterite leach liquors. Journal of Metals 33 (11), 48–53. Jiménez-Rodríguez, A.M., et al., 2009. Heavy metals removal from acid mine drainage water using biogenic hydrogen sulphide and effluent from anaerobic treatment: effect of pH. Journal of Hazardous Materials 165 (1–3), 759–765. Jong, T., Parry, D.L., 2003. Removal of sulfate and heavy metals by sulfate reducing bacteria in short-term bench scale upflow anaerobic packed bed reactor runs. Water Research 37 (14), 3379–3389.

233

Jovanovic, D.J., Lj Validzic, I., Jankovic, I.A., Bibic, N., Nedeljkovic, J.M., 2007. Synthesis and characterization of shaped ZnS nanocrystals in water in oil microemulsions. Materials Letters 61 (22), 4396–4399. Kaksonen, A.H., Riekkola-Vanhanen, M.L., Puhakka, J.A., 2003. Optimization of metal sulphide precipitation in fluidized-bed treatment of acidic wastewater. Water Research 37 (2), 255–266. Kanade, K.G., Baeg, J.-O., Mulik, U.P., Amalnerkar, D.P., Kale, B.B., 2006. Nano-CdS by polymer-inorganic solid-state reaction: Visible light pristine photocatalyst for hydrogen generation. Materials Research Bulletin 41 (12), 2219–2225. Karbanee, N., Van Hille, R.P., Lewis, A.E., 2008. Controlled nickel sulfide precipitation using gaseous hydrogen sulfide. Industrial and Engineering Chemistry Research 47 (5), 1596–1602. Kim, B.M., 1981. Treatment of metal containing wastewater with calcium sulfide. AIChE Symposium Series 77 (209), 39–48. Koumanakos, E., Dalas, E., Koutsoukos, P.G., 1990. The precipitation of cadmium sulphide in aqueous solutions. Journal of the Chemical Society, Faraday Transactions 86 (6), 973–977. Lewis, A., Swartbooi, A., 2006. Factors affecting metal removal in mixed sulfide precipitation. Chemical Engineering and Technology 29 (2), 277–280. Lewis, A., van Hille, R., 2006. An exploration into the sulphide precipitation method and its effect on metal sulphide removal. Hydrometallurgy 81 (3–4), 197–204. Li, Q., Shao, M., Wu, J., Yu, G., Qian, Y., 2002. Synthesis of nano-fibrillar bismuth sulfide by a surfactant-assisted approach. Inorganic Chemistry Communications 5 (11), 933–936. Liao, X.H., Zhu, J.J., Chen, H.Y., 2001. Microwave synthesis of nanocrystalline metal sulfides in formaldehyde solution. Materials Science and Engineering B 85 (1), 85–89. Luther, G.W., 1991. Pyrite synthesis via polysulfide compounds. Geochimica et Cosmochimica Acta 55 (10), 2839–2849. Luther III, G.W., Rickard, D., 2005. Metal sulfide cluster complexes and their biogeochemical importance in the environment. Journal of Nanoparticle Research 7 (7), 389–407. Luther III, G.W., Rickard, D.T., Theberge, S., Olroyd, A., 1996. Determination of metal (Bi) sulfide stability constants of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ by voltammetric methods. Environmental Science & Technology 30 (2), 671–679. Luther III, G.W., Theberge, S.M., Rickard, D.T., 1999. Evidence for aqueous clusters as intermediates during zinc sulfide formation. Geochimica et Cosmochimica Acta 63 (19–20), 3159–3169. Luther III, G.W., et al., 2002. Aqueous copper sulfide clusters as intermediates during copper sulfide formation. Environmental Science & Technology 36 (3), 394–402. Mathew, S.K., Rajesh, N.P., Ichimura, M., Udayalakshmi, 2008. Preparation and characterization of copper sulphide particles by photochemical method. Materials Letters 62, 591–593. Mc George, B., Gaylard, P.G., Lewis, A.E., 2009. Mechanism of rhodium(III) coprecipitation with copper sulfide (at low Rh concentrations) incorporating a new cationic substitution reaction path. Hydrometallurgy 96 (3), 235–245. Migdisov, A.A., Williams-Jones, A.E., Lakshtanov, L.Z., Alekhin, Y.V., 2002. Estimates of the second dissociation constant of H2S from the surface sulfidation of crystalline sulfur. Geochimica et Cosmochimica Acta 66 (10), 1713–1725. Mishra, P.K., Das, R.P., 1992. Kinetics of zinc and cobalt sulphide precipitation and its application in hydrometallurgical separation. Hydrometallurgy 28 (3), 373–379. Morse, J.W., Luther, G.W., 1999. Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63 (19– 20), 3373–3378. Moskalyk, R.R., Alfantazi, A.M., 2002. Nickel laterite processing and electrowinning practice. Minerals Engineering 15 (8), 593–605. Ni, Y., Ge, X., Liu, H., Xu, X., Zhang, Z., 2001. [Gamma]-Irradiation preparation of CdS nano-particles and their formation mechanism in non-water system. Radiation Physics and Chemistry 61 (1), 61–64. Okuwaki, A., Kanome, O., Okabe, T., 1984. The precipitation of Ni sub 3 S sub 2 from sulfate solutions. Metallurgical and Materials Transactions B 15B (4), 609–615. OLI Systems Inc, 2009. Stream Analyzer. OLI Systems Inc, Morris Plains, New Jersey. Pattrick, R.A.D., et al., 1997. The structure of amorphous copper sulfide precipitates: an X-ray absorption study. Geochimica et Cosmochimica Acta 61 (10), 2023–2036. Pawaskar, N.R., Sathaye, S.D., Bhadbhade, M.M., Patil, K.R., 2002. Applicability of liquid– liquid interface reaction technique for the preparation of zinc sulfide nano particulate thin films. Materials Research Bulletin 37 (9), 1539–1546. Peters, R.W., Ku, Y., 1985. Batch precipitation studies for heavy metal removal by sulphide precipitation. AIChE Symposium Series 243 (81), 9–27. Peters, R.W., Ku, Y., Chang, T.K., 1984. Heavy metal crystallization kinetics in an MSMPR crystallizer emloying sulphide precipitation. AIChE Symposium Series-Advances in Crystallization from Solutions 240 (80), 55–75. Radhika, V., Subramanian, S., Natarajan, K.A., 2006. Bioremediation of zinc using Desulfotomaculum nigrificans: Bioprecipitation and characterization studies. Water Research 40 (19), 3628–3636. Remoundaki, E., et al., 2008. Characterization, morphology and composition of biofilm and precipitates from a sulphate-reducing fixed-bed reactor. Journal of Hazardous Materials 153 (1–2), 514–524. Rickard, D., 1995. Kinetics of FeS precipitation: Part 1. Competing reaction mechanisms. Geochimica et Cosmochimica Acta 59 (21), 4367–4379. Roy, K.T., 1961. Preparing nickel and cobalt concentrates. Journal of Industrial and Engineering Chemistry 53 (7), 559–566. Roy, P., Srivastava, S.K., 2007. Low-temperature synthesis of CuS nanorods by simple wet chemical method. Materials Letters 61 (8–9), 1693–1697. Sahinkaya, E., Gungor, M., Bayrakdar, A., Yucesoy, Z., Uyanik, S., 2009s. Separate recovery of copper and zinc from acid mine drainage using biogenic sulfide. Journal of Hazardous Materials 171 (1–3), 901–906.

234

A.E. Lewis / Hydrometallurgy 104 (2010) 222–234

Sampaio, R.M.M., Timmers, R.A., Xu, Y., Keesman, K.J., Lens, P.N.L., 2009. Selective precipitation of Cu from Zn in a pS controlled continuously stirred tank reactor. Journal of Hazardous Materials 165 (1–3), 256–265. Shea, D., Helz, G.R., 1988. The solubility of copper in sulfidic waters: sulfide and polysulfide complexes in equilibrium with covellite. Geochimica et Cosmochimica Acta 52 (7), 1815–1825. Shen, G., et al., 2003. General synthesis of metal sulfides nanocrystallines via a simple polyol route. Journal of Solid State Chemistry 173 (1), 232–235. Shpiner, R., Vathi, S., Stuckey, D.C., 2009. Treatment of oil well “produced water” by waste stabilization ponds: removal of heavy metals. Water Research 43 (17), 4258–4268. Simons, C.S., 1964. Hydrogen Sulfide as a Hydrometallurgical Reagent. In: Wadsworth, M.E., Davids, F.T. (Eds.), Unit Processes in Hydrometallurgy. Gordon and Breach, New York, pp. 592–615. Stén, P., Forsling, W., 2000. Precipitation of lead sulfide for surface chemical studies. Colloids and Surfaces A: Physicochemical and Engineering Aspects 172 (1–3), 17–31. Sukola, K., Wang, F., Tessier, A., 2005. Metal-sulfide species in oxic waters. Analytica Chimica Acta 528 (2), 183–195. Tokuda, H., et al., 2008. Study on reaction kinetics and selective precipitation of Cu, Zn, Ni and Sn with H2S in single-metal and multi-metal systems. Chemosphere 73 (9), 1448–1452. van Hille, R.P., Peterson, K.A., Lewis, A.E., 2005. Copper sulphide precipitation in a fluidised bed reactor. Chemical Engineering Science 60 (10), 2571–2578. Veeken, A.H.M., Rulkens, W.H., 2003. Innovative developments in the selective removal and reuse of heavy metals from wastewaters. Water Science and Technology 47 (10), 9–16. Veeken, A., de Vries, S., van der Mark, A., Rulkens, W., 2003a. Selective Precipitation of Heavy Metals as Controlled by a Sulfide-Selective Electrode. Separation Science & Technology. Taylor & Francis Ltd., p. 1.

Veeken, A.H.M., Akoto, L., Hulshoff Pol, L.W., Weijma, J., 2003b. Control of the sulfide (S2-) concentration for optimal zinc removal by sulfide precipitation in a continuously stirred tank reactor. Water Research 37 (15), 3709–3717. Velasco, A., Ramírez, M., Volke-Sepúlveda, T., González-Sánchez, A., Revah, S., 2008. Evaluation of feed COD/sulfate ratio as a control criterion for the biological hydrogen sulfide production and lead precipitation. Journal of Hazardous Materials 151 (2–3), 407–413. Wei, D., Osseo-Asare, K., 1995. Formation of iron monosulfide: a spectrophotometric study of the reaction between ferrous and sulfide ions in aqueous solutions. Journal of Colloid and Interface Science 174 (2), 273–282. Wei, D., Osseo-Asare, K., 1996. Particulate pyrite formation by the Fe3+/HS-reaction in aqueous solutions: effects of solution composition. Colloids and Surfaces A: Physicochemical and Engineering Aspects 118 (1–2), 51–61. Wei, D., Osseo-Asare, K., 1997. Aqueous synthesis of finely divided pyrite particles. Colloids and Surfaces A: Physicochemical and Engineering Aspects 121 (1), 27–36. Xu, C., Zhang, Z., Ye, Q., 2004. A novel facile method to metal sulfide (metal = Cd, Ag, Hg) nano-crystallite. Materials Letters 58 (11), 1671–1676. Zeng, L., Cheng, C.Y., 2009. A literature review of the recovery of molybdenum and vanadium from spent hydrodesulphurisation catalysts: Part I: Metallurgical processes. Hydrometallurgy 98 (1–2), 1–9. Zhang, W., Cheng, C.Y., 2007. Manganese metallurgy review. Part II: Manganese separation and recovery from solution. Hydrometallurgy 89 (3–4), 160–177. Zhu, G., Lie, P., 2009. Low-temperature urea-assisted hydrothermal synthesis of B2S3 nanostructures with different morphologies. Crystal Research and Technology 44 (7), 713–720.