Deep purification of copper from zinc sulfate solution using controllable aging FeS nanoparticles through slow-release sulfide precipitation

The purification of impurity ions within zinc hydrometallurgical solutions significantly impacts the subsequent quality of zinc electrodeposition. Traditional methods for copper removal from zinc solutions suffer from issues like limited reactivity, high consumption, and elevated costs. To address these challenges, controllable aging of FeS nanoparticles was designed, employing temperature variations to facilitate comprehensive copper removal from neutral zinc leachates. The aging process imparts a crystalline structure to the FeS nanoparticles, enabling the slow release of sulfide ions. A temperature of 60°C is regarded as the optimal aging temperature, at which the resultant FeS nanoparticles exhibit an optimal sustained-release performance during the aging process. Temperature has been found to be a crucial factor influencing the sustained-release performance of FeS. Once the reaction temperature surpasses 60°C, the utilization efficiency of S ions in the sustained-release agent can reach over 95%, with a maximum of 98.5%. Under optimal conditions featuring a reaction temperature of 60°C, a reaction duration of 30 minutes, and a solution pH maintained at 1, the dosage aged FeS at the S/Cu (II) ratio of 1.2 times, nearly 100% copper removal rate can be achieved. The maximum removal rate for cadmium does not exceed 2.5%, and the residual copper concentration in the zinc leachate is below 0.2 mg/L. The precipitate obtained is high-purity copper sulfide residue, which contains virtually no zinc. Slow-release sulfidation can maximize the utilization efficiency of sulfide ions in the solution, reduce the occurrence of competing reactions, and minimize the intermixing of products. It offers advantages such as high selectivity and deep purification.


Introduction
More than 80% of global zinc production originates from hydrometallurgical procedures, in which zinc concentrates or other zinc-containing raw materials undergo leaching to yield neutral leachate.Throughout this leaching process, compounds of impurity metals, including Cu, Cd, Co, Ni, and other ions, can dissolve into the zinc sulfate solution [1,2].These impurity ions wield considerable influence on ensuing zinc electrowinning processes, culminating in diminished current efficiency, heightened energy consumption, and compromised quality of zinc cathodes [3,4].Consequently, the imperative emerges to purify the solution, with the impurity metal-laden filter cake retrieved post-purification potentially serving as a recyclable source for recovering valuable metals like copper.At present, the elimination of copper and cadmium from zinc leachate involves displacement reactions with zinc powder [5][6][7].Nonetheless, this approach is not without its limitations, as the displacement reaction transpires solely at the surface of zinc powder, resulting in suboptimal efficiency and vulnerability to encapsulation and passivation phenomena.Furthermore, this method necessitates a substantial surplus of zinc powder (exceeding 5 times the stoichiometric requirement), leading to escalated expenditures [8].Furthermore, owing to incomplete reactions, the concentration of valuable elements within the zinc residue remains notably low, rendering the retrieval of these metals from the zinc residue a challenging and costly endeavor.Thus, the imperative of diminishing zinc powder consumption, reducing smelting expenses, and achieving optimal utilization of key constituents (including copper and cobalt) has emerged as a critical quandary within the realm of hydrometallurgical zinc refining processes.
Sulfide precipitation finds wide application in metal extraction owing to its manifold benefits.These encompass the capability for selectively extracting metals, rapid reaction kinetics, achieving higher product quality, and the potential for recycling precipitates through smelting [9].Still, conventional ionic sulfidizing agents exhibit rapid spontaneous hydrolysis and sulfur ion release in aqueous solutions, resulting in low sulfur utilization efficiency, severe product intermixing, and fine particle size that hinders settling [10].The key to addressing these issues is the slow-release rate of S 2-ions, as it can provide a lower sulfur ion saturation in the solution, allowing sufficient time for the binding of other metal ions with sulfur ions [11].Iron sulfide (FeS), characterized by a considerable solubility product (Ksp= 1.59 × 10 -19 ), proves to be an exceptional material for the controlled release of S 2- [12].Synthesized through chemical precipitation methods, amorphous FeS nanoparticles display remarkable reactivity.However, appropriate modifications are necessary to hinder their rapid breakdown in acidic conditions [13].Aging is a traditional method for nanoparticle modification, which affects their structure and reactivity [14,15].Theoretically, different degrees of aging can yield FeS nanoparticles with ideal sustained-release performance.
In this paper, the design of temperature-controlled aging of FeS nanoparticles was explored for achieving deep copper removal and effective resource recovery from neutral zinc leachate in hydrometallurgical zinc refining processes.Aging enabled the development of a crystalline structure in FeS nanoparticles, facilitating the slow release of sulfide ions.The improvement in sulfide ion sustainedrelease performance and subsequent enhancement of removal efficiency were achieved by modifying the influencing conditions.The study also investigated the impact of reaction condition factors on the sustained-release performance and explored suitable process parameters for the sustained-release sulfidizing agent.Finally, the mechanism of FeS as a sustained-release agent for deep copper removal from zinc sulfate solutions was summarized.

Materials
The synthesis of amorphous FeS particles involved the use of FeSO4• 7H2O and Na2S• 9H2O, both obtained from Sinopharm Chemical Reagent Co., Ltd.To create a zinc leaching solution containing impurity ions, ZnSO4• 7H2O, CuSO4• 5H2O, and CdSO4• 8/3H2O were employed.All reagents adhered to analytical grade standards, and deionized water was utilized for solution preparation.

Experimental procedures
2.2.1.Methods for synthesis and aging of FeS nanoparticles.The FeS synthesis was carried out through a chemical precipitation method within a 100 mL volumetric flask.The initial phase involved preparing a FeS suspension through the combination of 0.05 L of 1 mol/L S (-II) solution with an equivalent volume of 1.2 mol/L Fe (II) solution.The synthesis duration was rigorously confined to 10 minutes.In particular, the FeSO4 solution underwent vigorously stirring as the Na2S solution was gradually added, resulting in the formation of a concentrated FeS suspension.The representation of the synthesis procedure is given by Eq. (1).Subsequently, the FeS suspension was moved to a water bath with precise temperature control and subjected to different temperatures (25, 40, 60, and 80℃) for aging periods of 3 hours.The aging process of the nanoparticles was significantly influenced by the varying temperatures.The samples were labelled as FeS-25, FeS-40, FeS-60, and FeS-80, corresponding to the respective aging temperatures.After centrifugation, the solid FeS samples were freeze-dried and used for characterization purposes.On the other hand, the removal experiments utilized the FeS suspension directly, without prior filtration.Na S+FeSO FeS +Na SO → (1) Copper slow-release separation experiment.The preparation of the simulated zinc leaching solution followed a standard procedure, with the Zn (II) concentration fixed at 130 g/L and the Cu (II) concentration held at 0.64 g/L, unless stated otherwise.A solution of sulfuric acid (1%) and sodium hydroxide (1%) was adjusted to obtain the desired pH value.The reaction liquid, amounting to 100 mL, was employed in a 200 mL conical flask, and the reaction was initiated by adding the precalculated FeS suspension.After the reaction, the filtrate underwent filtration with a 0.22 μm filter membrane.The calculation of removal efficiencies was performed using Eq. ( 2).The filter residue underwent three rinses and was subsequently dried at 40°C for 12 hours.The experimental procedures were replicated at least twice, and the results were averaged.
The calculation of ion removal efficiency in the simulated zinc leaching solution was performed using the following equation: where C0 (mg/L), C1 (mg/L) represent the initial and residual concentrations of the elements in the solution, respectively.Additionally, V0 (L), V1 (L) denote the initial and residual volumes, respectively.

Characterization of FeS nanoparticles
The comprehensive exploration of the aging time impact on FeS nanoparticles was undertaken through the implementation of various characterization techniques.As depicted in Fig. 1(a), the extended aging time reveals the conversion of FeS from amorphous to crystalline, evident in the increasing intensity of the four peaks (2θ=17.62°,30.06°, 38.96°, 50.43°) associated with mackinawite.Furthermore, the incremental decrease in the half-maximum width of FeS points to an enlargement in the size parameters of FeS nanoparticles.Notably, a novel greigite phase becomes apparent upon aging of FeS, with distinct peaks at 2θ=29.96°, 36.34°, and 52.35°.Several studies have reported the production of unstable intermediates of greigite under mild oxygen reduction, and the process of greigite formation can be depicted through Eqs. ( 3) and ( 4).
( ) As depicted in Fig. 1(b), the surface the surface morphology of FeS undergoes gradual transformations as the aging time increases.Initially, FeS exhibits a fine sheet-like structure with a notable specific surface area.Yet, as aging progresses, it undergoes a transformation into structures characterized by growth and collapse.The diminishing of surface voids and their transformation into thicker plates over time may pose challenges in the removal of subsequent ions.The information in Table 1 reveals that a continuous increase in the lattice parameter along the a-axis as the aging time extends while the c-axis (interlayer spacing) decreases from 6.36 Å to 5.05 Å, closely resembling the spacing between layers seen in crystalline FeS [16].The ongoing shortening of the Fe-S bond length indicates an incremental stabilization of the structure.The lattice energy, which quantifies the energy generated when a stable crystal breaks down to produce ions, plays a critical role in determining solubility and phase transition behavior.Over an extended aging period, the lattice energy of FeS decreases from -3071 to -3653 kJ/mol, highlighting an increased stability in its crystal structure [17,18].Based on the aforementioned calculations and analyses, an illustration depicting the structural modifications of FeS is presented in Fig. 1(c).The shift from a disordered state to an ordered state within the FeS structure is associated with an augmentation in stability.

Screening of FeS nanoparticles at different aging temperatures
Unlike ionic sulfidizing agents such as Na2S, the release of sulfur by sustained-release sulfidizing agents occurs slowly.This requires the sustained-release agents to possess a certain level of stability to maintain the release of sulfur, while also avoiding excessive stability that hinders the release of sulfur.This delicate balance is worthy of further in-depth research and exploration.Na2S was used for control experiments, while FeS nanoparticles obtained at four different aging temperatures were utilized for copper ion removal experiments at different pH levels to select the optimal sustained-release sulfidizing agent.The reaction conditions included a reagent dosage with a S/Cu (II) molar ratio of 1:1, a reaction temperature of 25°C, and a reaction time of 30 minutes.As shown in the Fig. 2, it can be observed that at pH 1, Na2S exhibits rapid reaction during the copper removal process, reaching equilibrium within 2 minutes with a removal rate of 86.9%.FeS-25, on the other hand, reaches equilibrium after 5 minutes of reaction, achieving a removal rate similar to that of sodium sulfide for copper.This indicates that the addition of FeS has a certain effect in slowing down the reaction rate, demonstrating its sustained-release performance.As the aging temperature increases, the sustained-release performance of FeS continues to improve.FeS-40 and FeS-60 reach equilibrium at 10 minutes and 20 minutes, respectively.However, it is important to note that this comes at the expense of a lower removal rate.FeS-80, on the other hand, has not reached equilibrium within 30 minutes.Under the pH 4 conditions (Fig. 2(b)), all reactions exhibit a slow behavior.The observed behavior is probably a result of the diminished concentration of hydrogen ions, which diminishes the influence of competing reactions.As a result, observable kinetic phenomena can occur between sulfide ions and metal ions.Therefore, taking into consideration the overall findings, FeS with optimal sustained-release performance should possess a certain degree of stability.The FeS product aged at 60°C can be selected for further investigations on sustained-release copper removal experiments.In the subsequent experiments, the removal efficiency can be improved by adjusting the influencing conditions to obtain adjustable sulfide ion sustained-release performance.

Removal behavior 3.3.1 Effect of pH values on Cu (II) removal.
The influence of pH on copper ions removal is shown in Fig. 3 (a).However, it was found that pH had minimal impact on the copper removal under the investigated pH conditions, as the removal efficiency remained relatively stable at around 80% across all conditions.This may be attributed to the fact that the decomposition of aged FeS requires specific acidity, which was not achieved under the pH conditions studied in the paper.Interestingly, a slight increase in the copper removal efficiency was observed at pH 5. The residual concentration of copper ions in the solution is reduced from 0.14g/L at pH=1 to 0.12g/L, which could be attributed to the higher utilization rate of sulfide ions at elevated pH levels.From a thermodynamic perspective, a higher solution pH is more favourable for the removal of copper ions.

Effect of reaction temperatures on Cu (II) removal.
Temperature is an important factor influencing the sustained release efficiency and decomposition activity of FeS, as shown in the Fig. 3 (b).As the reaction temperature increases, the removal efficiency of copper ions is significantly enhanced.As the reaction temperature reaches 80°C, the copper ions removal efficiency increases from 79.5% at 25°C to 98.4%.This can be attributed to the endothermic nature of the FeS decomposition reaction [19], where higher temperatures facilitate the release of S ions from FeS, thereby improving the removal efficiency of copper ions.This also indicated that the maximum utilization efficiency of sulfur ions in the sustained-release agent was 98.4%.Considering that the temperature of zinc leachate in industrial production is maintained between 50-70°C, a reaction temperature of 60°C will be used in the subsequent experiments.At this temperature, the removal efficiency of copper ions exceeds 95%.

Effect of FeS dosage on Cu (II) removal.
As depicted in the Fig. 3 (c), with an increase of FeS/Cu (II) molar ratio from 0.5 to 1.2, the Cu (II) removal efficiency increases from 45.4% to 99.9%.The residual concentration of copper ions decreases from 352 mg/L to 0.15 mg/L.Hence, to achieve the efficient removal and recovery of Cu(II), it is concluded that the optimal molar ratio of FeS to Cu(II) is 1.2:1.

Effect of reaction time on Cu (II) removal.
As shown in the Fig. 3 (d), the copper removal gradually increases with varying time, reaching a plateau around 30 mins, indicating the effective sustainedrelease performance of aged FeS.Moreover, higher temperatures notably enhance the rate of copper removal by FeS.At a reaction temperature of 25°C, the copper removal rate rises gradually with time, reaching equilibrium at 30 mins.However, with a temperature increase to 80°C, the reaction nearly attains equilibrium within just 3 mins.

Effect of copper concentration on Cu (II) removal.
Considering the fluctuation in copper content in zinc leachate in different smelters (0.2-1.2 g/L), the impact of various concentrations of initial copper ions on the removal efficiency was explored.As indicated in the Fig. 3 (e), Under specified conditions with FeS dosage 1.2 times the theoretical amount, 60°C reaction temperature, and 30 minutes reaction time, the removal efficiency of copper ions exceeded 99.9% within the investigated concentration range.The copper content in the solution was consistently below 0.2 mg/L, achieving the objective of deep copper ion removal.This indicates that the sustained release of copper removal by FeS exhibits excellent adaptability, making it suitable for selectively removing copper within a wide concentration range.

Effect of Cadmium concentration on Cu (II) removal.
In addition to copper, zinc leachate also contains a significant amount of other impurity ions, with cadmium ions (0.6-1.2 g/L) being a crucial interfering factor to consider during the copper removal.The impact of interference by different cadmium ion concentrations on the FeS copper removal is illustrated in the Fig. 3 (f).The copper level in the solution was fixed at 0.64 g/L, with FeS dosage set at 1.2 times the theoretical amount, a reaction temperature of 60°C, and 30 minutes.Within the range of cadmium concentrations from 0.3 to 1.5×10-2 mol/L, the copper removal efficiency by FeS remained above 98.5%, while the cadmium removal rate in the solution did not exceed 2.5%.This indicates that FeS, as a sustained-release sulfidizing agent, exhibits excellent selectivity by avoiding interference from impurity ions like Cd during the removal process.It allows for the highly precise separation of copper from the solution and enables the efficient recovery of copper resources.

Characterization of removal products.
The XRD analysis of arsenic removal products from Na2S and FeS at different aging temperatures revealed insights into the phase transformation during the reaction.Fig. 4 illustrated that the products of Na2S and Fe-25 were identified as copper sulfide (CuS), indicating that Na2S and FeS-25 possess high activity and transformation efficiency.Additionally, the intensity of the copper sulfide diffraction peak in FeS-25 was found to be enhanced compared to Na2S, pointing an increase in the crystallinity of the copper removal product in FeS-25.As the aging temperature increased (exceeding 40°C), the enhanced intensity of the diffraction peak at 2θ=29.365° and the presence of a novel diffraction peak at 2θ=49.039° were confirmed as characteristic peaks of CuFeS2.The gradual increase in the corresponding peak intensity signifies a diminishing reactivity of FeS, which is consistent with its enhanced sustained release performance.The strengthening of the FeS structure and the weakening of its decomposition were observed as the aging temperature increased, suggesting the formation of CuFeS2 phase during the sustained release of S ions in FeS.Therefore, it is necessary to select a suitable temperature for aging modification of amorphous FeS, which will determine its slow release performance in zinc leaching solution.
Fig. 4. XRD pattern of the copper removal precipitates of Na2S and FeS with different aging temperatures.The analysis of copper-containing precipitates from Na2S and different types of aged FeS involved SEM and elemental mapping to examine their characteristic morphology and elemental composition, as shown in the Fig. 5.The copper removal product in Na2S exhibited numerous loose and irregular-shaped particles (Fig. 5(a)), which can be attributed to the local oversaturation of S ions in the solution caused by the high concentration of Na2S, resulting in nucleation bursts and the generation of numerous amorphous fine particles.After copper removal using FeS, the point-like particles gradually disappeared, and the copper removal product mainly appeared as block-shaped particles (Fig. 5(b)) with an increased crystallinity, consistent with the XRD analysis.With the increase in aging temperature, the characteristics of the block-shaped particles became more prominent (Fig. 5(c-e)).This structure indicates that S is initially fixed in FeS and slowly released during the reaction process.Elemental analysis results (Fig. 5(f)) revealed the copper grade in the copper removal product of FeS-60 exceeded 60%, indicating a relatively pure copper sulfide, and its purity was close to 85%.The grade of Fe was 5.34%, suggesting that some Fe did not completely decompose, demonstrating the slow release of S ions in FeS.

Proposed removal mechanism
The solubility product of a metal sulfide governs its dissolution characteristics, with precipitates having higher solubility products transforming into those with lower solubility products.Sulfides with larger solubility products transform into those with smaller solubility products.A smaller solubility product for a given metal ion implies a higher probability of preferential precipitation.Therefore, theoretically, the order of precipitation for the four metal ions is as follows: Cu (pK=35.85)> Cd (27.19) > Zn (23.10) > Fe (16.47) [20].In addition to the solubility product principle, inherent characteristics such as metal ion radius and ion charge also have important implications for Cu(II) removal using FeS in highconcentration zinc sulfate solutions.Studies have shown that metal ions with smaller ionic radii are more likely to form strong coordination bonds with sulfur-containing groups [21,22].Copper, having an ionic radius of 0.72 Å, exhibits a smaller size compared to zinc (0.74 Å), cadmium (0.97 Å), and ferrous iron (0.78 Å).The difference in size may result in the generation of new bonds between copper ions and sulfur-containing groups on the surface of aged FeS, enabling selective recovery.Metal ions with higher ion charges have an increased ability to attract electrons and tend to form complexes with sulfur-containing groups [23].The order of ion charges for the four metals is as follows: Cu (2.78) > Zn (2.70) > Fe (2.56) > Cd (2.06).
However, the separation between similar elements is often difficult to achieve in an ideal manner as predicted by theoretical calculations, mainly due to external conditions, particularly the type of sulfidizing agent used [20].To approach the ideal separation state, certain measures need to be taken.As shown in the Fig. 6, sulfur ions gradually bind and become fixed to iron, transitioning from sodium sulfide to amorphous FeS and finally to aged FeS.The strengthening of the Fe-S bond enhances the fixation of sulfur, leading to an improved sustained-release performance of FeS.It has been found that the sustained-release rate can be selectively controlled through heating.The slow release of sulfide ions minimizes the supersaturation of sulfur ions, thereby increasing the utilization efficiency of sulfide ions.This approach brings the separation order of elements closer to the theoretical calculations based on solubility product or other principles, enabling high selectivity and deep purification of copper.Fig. 6.The mechanism and schematic diagram of deep copper removal from zinc leachate using controllable aging FeS.

Conclusions
In this study, controllable aging of FeS nanoparticles was implemented by varying the temperature to achieve deep removal of copper and effective recovery of resources from neutral zinc leachate in hydrometallurgical processes.The aging process resulted in the development of a crystalline structure within the FeS nanoparticles, facilitating the gradual release of sulfide ions.Through the manipulation of influencing conditions, it was possible to attain adjustable sulfide ion sustained-release performance and enhance the copper removal efficiency.Temperature has been found to be a crucial factor influencing the sustained-release performance of FeS.When the temperature exceeds 60°C, the utilization efficiency of S ions in the sustained-release agent can reach over 95%, with a maximum of 98.5%.By optimizing the conditions, the copper removal can approach 100% under the optimal conditions.The residual copper content in the zinc leachate is reduced to 0.2 mg/L, as the removal rate of cadmium does not exceed 2.5%.Slow-release sulfidation can maximize the utilization efficiency of sulfide ions in the solution, reduce the occurrence of competing reactions, and minimize the intermixing of products.It offers advantages such as high selectivity and deep purification.By employing sustained-release sulfidation for copper precipitation, not only can the deep purification and resource recovery of copper from leachate be achieved with high selectivity, but it can also greatly decrease the usage of zinc powder.

Fig. 2 .
Fig. 2. The removal efficiency of Cu (II) by Na2S and FeS from simulated zinc sulfate solution at pH 1 (a) and pH 4 (b).

Fig. 3 .
Fig. 3. Cu (II) removal from simulated zinc sulfate solution by FeS as the function of: (a) pH value, (b) reaction temperature, (c) FeS to Cu (II) mole ratio, (d) reaction time, (e) initial concentration of Cu (II), and (f) initial concentration of Cd (II).

Table 1
The lattice parameters, bond length and lattice energy of FeS with varying aging temperatures