Dissolution Of Zn Research Articles

Characterization of zinc solubilization potential of arsenic tolerant Burkholderia spp. isolated from rice rhizospheric soil.

In this study, experiments were conducted to isolate, characterize, and evaluate rice rhizosphere bacteria for their arsenic (As) tolerance ability and zinc (Zn) solubilization potential in culture media and soil. Among 20 bacterial isolates recovered, six were found to solubilize inorganic Zn salt(s) efficiently under in vitro culture conditions. 16S rRNA gene sequence-based phylogenetic analysis indicated the affiliation of efficient Zn solubilizing bacteria (ZSB) to Burkholderia vietnamiensis and Burkholderia seminalis. Zinc solubilizing efficiency (ZSE) of the bacteria varied with the concentrations and types of Zn salts used in the experiments. Increasing trend in ZSE of the bacteria was noticed when the percentage of ZnO increased from 0.1 to 0.5 but the same decreased at 1.0%. Increased Zn solubilization was noticed when bacteria were incubated with lower concentration of Zn3(PO4)2 and ZnCO3. In general, Zn solubilization increased with increasing incubation time in lower volume medium, while some isolates failed to solubilize one or more tested Zn salts. However, enriched concentrated cells of the ZSB in glucose amended medium with 0.5% ZnO showed an increasing trend of Zn solubilization with time and were able to solubilize more than 300mg/L Zn. This increased rate of Zn release by the ZSB was attributed to marked decline in pH that might be due to the enhanced gluconic acid production from glucose. As evident from the decreased ZSE of the bacteria in the presence of As(V) in particular, it seems arsenic imparts a negative effect on Zn solubilization. The ZSB were also able to increase the rate of Zn release in soil. A microcosm-based soil incubation study amending the enriched bacteria and 0.5% ZnO in soil showed an elevated level of both water-soluble and available Zn compared to un-inoculated control. During Zn solubilization in microcosms, viable cells in terms of colony-forming unit (CFU) declined by the same order of magnitude both in the presence and absence of ZnO that might be due to the nutrients limiting condition aroused during the incubation period rather than Zn toxicity. The bacteria in this study also exhibited plant growth promoting traits, such as growth in nitrogen-free medium, production of indole acetic acid (IAA), and solubilization of potassium and phosphate. Our findings suggested that Burkholderia spp. could be the potential candidates for enhancing Zn dissolution in the soil that might reduce the rate of inorganic Zn fertilization in agricultural soil.

Combined thermochemical treatment of steel construction parts to increase their corrosion resistance

Electrochemical corrosion of construction parts made of structural steels appears due to different electrochemical potentials (ECP) of contacting materials. The values of ECP of carbon steels may be more or less compensated by forming of alloyed layers or coatings by methods of thermochemical treatment (TCT). The following combinations of TCT were examined for surface modification of steels: (a) application of zinc coatings by a method of cold zinc-plating with subsequent classical gas nitriding in ammonia; (b) joint diffusion saturation by zinc and nitrogen; (c) joint diffusion saturation with chromium and nickel. Metallographic and microhardness methods were used for the study of modified layers and coatings. Strengthened layers with a thickness up to 140…150 microns were formed in low-carbon steels after nitriding of zinc coatings at 540°C for 24 hours, and after combined TCT with multicomponent saturation by Cr+Ni at 1000°C for 7 hours. Both methods of TCT increase the corrosion resistance of low-carbon steels and low-alloyed steels. Corrosion products examination in 5% NaCl salt fog shows that the dissolution of nitrided Zn coating and formation of corrosion products slow down with the increase of zinc concentration in the coating.

Effect of Al on Corrosion Behavior of Imitation-Gold Cu-Zn-Ni-Sn Alloys in 3.5 wt.% NaCl solution

The corrosion behavior of imitation-gold Cu-Zn-Ni-Sn alloys with and without Al was investigated by means of color difference testing, static corrosion measurements, scanning electron microscopy analysis, electrochemical impedance measurements, and x-ray photoelectron spectroscopy analysis. The results showed that aluminum inhibited dissolution of Zn and Sn by the formation of an oxide film on the surface of the Cu-Zn-Ni-Sn alloy during corrosion, effectively improving its corrosion resistance. A transition of the corrosion product from Cu2O and Cu(OH)2 to CuO occurred on the surface of Cu-Zn-Ni-Sn-Al alloy, and the corrosion rate of the alloy decreased with increasing corrosion time due to the formation of the oxidation film and the hydrophobic effect of Zn5(CO3)2(OH)6 in the product layer.

Brass dezincification with a bipolar electrochemistry technique

A bipolar electrochemistry approach was applied to characterise the dezincification of brass. The dissolution of both Cu and Zn resulted in the formation of Cu2O, CuO, ZnO and CuClx, with corrosion product formation characterised as a function of the potential gradient. At low potentials, protective corrosion products formed, with compositional changes observed via dezincification up to a critical applied potential threshold. Above the critical potential, no corrosion products could form, resulting in higher corrosion rates. The influence of corrosion product formation after bipolar electrochemistry assessment is compared to results from standard three-electrode potentiodynamic polarisation tests.

Recovery of zinc and lead from Yahyali non-sulphide flotation tailing by sequential acidic and sodium hydroxide leaching in the presence of potassium sodium tartrate

The recovery of zinc and lead from Yahyali non-sulphide flotation tailing using sulfuric acid followed by sodium hydroxide leaching in the presence of potassium sodium tartrate was experimentally investigated. In the acidic leaching stage, the effects of pH, solid-to-liquid ratio and temperature on the dissolution of zinc from the tailing were explored. 82.3% Zn dissolution was achieved at a pH of 2, a temperature of 40 °C, a solid-to-liquid ratio of 20% and a leaching time of 2 h, whereas the iron and lead dissolutions were determined to be less than 0.5%. The sulfuric acid consumption was found to be 110.6 kg/t (dry tailing). The leaching temperature had no beneficial effect on the dissolution of zinc from the tailing. The acidic leach solution was subjected to an electrowinning test. The cathode product consisted of 99.8% Zn and 0.15% Fe. In the alkaline leaching stage, the Pb dissolution increased slightly in the presence of potassium sodium tartrate. More than 60% of Pb was taken into the leach solution when the leaching temperature increased from 40 to 80 °C. The final leach residue was analyzed by XRD and XRF. The XRD results indicated that the major peaks originated from the goethite and quartz while minor peaks stem from smithsonite and cerussite. The XRF analysis demonstrated that the residue contained 70.3% iron oxide. Based on the sequential leaching experiments, the zinc and lead were excellently depleted from the flotation tailing, leaving a considerable amount of iron in the final residue.

Development of Alkaline Zn-Based Batteries for Grid Storage

For energy storage to become ubiquitous in the electric grid, safe, reliable low-cost electrochemical storage technologies manufactured at high volumes with low capital expenditures are needed. Alkaline batteries based on high capacity multi-electron conversion electrodes from low cost, abundant and safe materials, such as a Zn/MnO2 are a promising technology. These batteries have a theoretical specific energy rivaling that of Li-ion systems (Zn @820 mAh/g and MnO2 @617 mAh/g, with ~400 Wh/L) and costs reducible to <$50/kWh, when produced at scale (S. Banerjee et al.). While recent advances by Yadav et al. have demonstrated highly reversible Bi- and Cu-stabilized MnO2 electrodes that can achieve the full 2e- capacity of MnO2 in alkaline electrolyte, the ability to pair this electrode with Zn over 5000+ cycles, which equates to ~10-15 years of battery life, remains a difficult challenge. Zn anodes suffer from irreversible shape change, the redistribution of active material, and passivation over repeated charge and discharge, limiting their achievable capacity and lifetime. Pre-saturating the electrolyte with zincate [Zn(OH)4 2-], which minimizes dissolution of Zn from the anode and reduces the rate of hydrogen evolution, has recently been shown to enhance cycle life for ~10-35% depth-of-discharge (DOD) Zn anodes by ~100-200% (M. Lim et al.); however, Zn(OH)4 2- saturated electrolyte is incompatible with high DOD MnO2, and exacerbates the formation of electrochemically inactive phases, such as ZnMn2O4 at the cathode. Hence, using zincate-blocking separators, able to entrap the zincate within the anode and effectively isolate the MnO2 cathode from Zn(OH)4 2- crossover, while maintaining hydroxide/cation conductivity, is one approach to improve the reversible cycling of a Zn/MnO2 cell at high DOD of both MnO2 and Zn. More recently we have developed a series of permselective polymeric separators (Kolesnichenko et al.) and screened them using our newly-developed anodic stripping voltammetry crossover assay (Duay et al.) to identify those with Zn(OH)4 2- blocking ability. A primary discharge of MnO2 was used to demonstrate that a sustained 2nd e- discharge plateau, indicative of the absence of zinc species at the cathode, was observed only for Zn/MnO2 batteries that utilized our selective polymeric separators. Finally, application of these polymeric separators in rechargeable Zn/MnO2 batteries increased cycle life with higher coulombic efficiencies. Various aspects involved in improving the cycle life of Zn anodes at increased DOD, and of the application of polymeric separators in isolating the MnO2 cathode from soluble zincate and their ability to enable higher DOD cycling in Zn, will be discussed. This work was supported by the U.S. Department of Energy, Office of Electricity, and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

(Invited) Zinc-Deposited Aluminum Electrode Prepared Using Spontaneous Deposition Method for Use in Aqueous Hybrid Capacitors

Aqueous electrolytes have advantages compared to nonaqueous electrolytes, such as high ionic conductivity, nonflammability, low environmental load, low cost, and so on. However, the maximum operating voltages of electric double-layer capacitors (EDLCs) with aqueous electrolytes are much lower than those with nonaqueous electrolytes, which can operate at voltages over 2.5 V, because the former are limited to 1.23 V, the theoretical decomposition voltage of water, leading to low energy and power densities. So, a key to improve energy and power densities is to increase the maximum operating voltage. Hybrid capacitors (HCs), which consists of activated carbon (AC) and redox electrodes, can greatly improve the energy and power densities of EDLCs because of widened potential windows. As for aqueous HCs, metal hydroxides/oxides as a redox positive electrode have realized the maximum operating voltages of 1.4–2.0 V because they have higher potential limits and higher oxygen overpotentials than AC,1-3 leading to higher energy and power densities. In the same manner, we reported that an HC with a Zn negative electrode whose hydrogen overpotential was larger than AC also increased the maximum operating voltage due to the extension of the negative potential limits, leading to low energy and power densities.4 A copper sheet was used as a substrate for zinc electrodeposition in the previous study.4 Aluminum is a light metal and cheap. Since the standard potential of Al is more negative than that of Zn, Zn can be spontaneously deposited on an Al sheet only by immersing it in a zincate solution. In this study, we prepared Zn-deposited Al sheet negative electrodes and evaluated their electrochemical properties and capacitor performance of a HC with the negative electrode.A polished Al sheet was immersed in a zincate solution to deposit Zn metal spontaneously. After the deposition, the mirror image of the Al sheet was cloudy due to zinc deposits. EDX and XPS analyses showed that Zn deposits covered almost the whole Al surface, indicating that spontaneous deposition of Zn occurred as follows.2Al + 3Zn(OH)4 2- → 3Zn + 4OH- + 2Al(OH)4 - Cyclic voltammograms of a Zn/Al electrode were recorded in 7 M KOH aqueous solution containing 0.6 M ZnO. A reduction peak due to Zn deposition and an oxidation peak due to Zn dissolution were observed in cyclic voltammograms, which was similar to a cyclic voltammogram of a Cu electrode in the same electrolyte.An aqueous HC cell with a Zn/Al negative electrode and AC positive electrode was assembled, and its capacitor performance will be reported. References V. Khomenko, E. Raymundo-Piñero, F. Béguin, J. Power Sources, 153, 183 (2006).T. Cottineau, M. Toupin, T. Brousse, D. Bélanger, Appl. Phys. A: Mater. Sci. Process., 82, 599 (2006).H. Inoue, Y. Namba, E. Higuchi, J. Power Sources, 195, 6239 (2010).H. Inoue, T. Morimoto, S. Nohara, Electrochem. Solid-State Lett., 10, A261 (2007).

Enhanced Cycling Performance of Rechargeable Zinc-Air Flow Batteries Using Potassium Persulfate as Electrolyte Additive.

Zinc–air batteries (ZABs) offer high specific energy and low-cost production. However, rechargeable ZABs suffer from a limited cycle life. This paper reports that potassium persulfate (KPS) additive in an alkaline electrolyte can effectively enhance the performance and electrochemical characteristics of rechargeable zinc–air flow batteries (ZAFBs). Introducing redox additives into electrolytes is an effective approach to promote battery performance. With the addition of 450 ppm KPS, remarkable improvement in anodic currents corresponding to zinc (Zn) dissolution and limited passivation of the Zn surface is observed, thus indicating its strong effect on the redox reaction of Zn. Besides, the addition of 450 ppm KPS reduces the corrosion rate of Zn, enhances surface reactions and decreases the solution resistance. However, excess KPS (900 and 1350 ppm) has a negative effect on rechargeable ZAFBs, which leads to a shorter cycle life and poor cyclability. The rechargeable ZAFB, using 450 ppm KPS, exhibits a highly stable charge/discharge voltage for 800 cycles. Overall, KPS demonstrates great promise for the enhancement of the charge/discharge performance of rechargeable ZABs.

Tuning of CeO2-MnO2 nano composite in Al-Zn alloy matrix for effective activation

Efficient Al + 5 wt% Zn sacrificial anode was prepared and its activity was further enhanced by tuning CeO2-MnO2 nano composite in the alloy matrix. The nano composite was prepared by thermal decomposition method and was characterised by XRD and HRTEM. Al + 5 wt% Zn were fabricated by the incorporation of the composite in the alloy matrix. The composite acted as lattice expander in the Al-Zn alloy and increased dissolution of Zn. Very high galvanic efficiency of 92% was achieved by tuning the amount of the composite in the Al-Zn alloy anode. Its uniform corrosion behaviour was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy, cyclic polarisation and post consumption microscopic evaluation. A significant lowering in impedance value was achieved by increasing the amount of nano composite in the alloy due to the increased penetration of the oxide film by chloride ions. The composite and Zn in the alloy synergistically activated the Al alloy which facilitated uniform corrosion suppressing pitting corrosion.

Galvanic oxidation of bimetallic Zn-Fe nanoparticles for oxygen scavenging

Bimetallic nanoparticles (NP) have demonstrated outstanding multifunctional characteristics, which depend on their size, distribution and composition. In this study, we show the possibility of tailoring the oxidation behavior of Zn-Fe bimetallic nanoparticles produced by magnetron sputtering and gas agglomeration system. Zn and Fe metals were coupled to promote faster oxidation of Zn, stimulating a galvanic mechanism due to the dissimilar corrosion potential in the nanoparticles. The results revealed Zn dissolution occurring at high humidity environments for bimetallic Zn-Fe nanoparticles where no intermix exists between Zn and Fe; however, such dissolution is excluded for ZnFe alloys. The effect of the galvanic couple on the Zn dissolution was confirmed by molecular dynamic simulations. This bimetallic system can be exploited as moisture-activated oxygen scavenger materials due to the acceleration in the oxidation mechanism.

Microstructural engineering of ZnCr coating by incorporation of graphene oxide and its effect on the corrosion behaviour of the coating

In this report, the effect of different amounts of graphene oxide on the corrosion properties and microstructural engineering of ZnCr composite coatings were investigated and correlated. Electrodeposition of ZnCr-graphene oxide (ZnCr-GO) composite coatings was conducted over mild steel substrate using electrolyte baths with different concentrations of dispersed graphene oxide. Incorporation of GO produced relatively more compact, finer and fibrous morphology in the case of ZnCr-GO coatings whereas the ZnCr coating exhibited rougher nodular morphology. Microstructure of the ZnCr coating contained randomly distributed highly Zn and Cr-rich phases whereas, the incorporation of graphene oxide led to evolution of a layered microstructure containing Zn and Cr rich phase layers stacked over each other perpendicular to the coating growth direction. Corrosion properties measured using the potentiodynamic polarization and impedance spectroscopy methods revealed that both for 1 h and 48 h exposure times, composite coatings were more corrosion resistance than the ZnCr coatings and the corrosion resistance substantially increased with GO content increase in the coatings. Corrosion resistance increase was attributed to the formation of compact morphology, evolution of layered ZnCr microstructure which can provide two stage protection involving sacrificial dissolution of Zn and formation of protective Cr-oxide layer in addition to the impermeability of the GO to the passage of the corrosive media.

Single versus multi-metal sulfide systems: The role of cysteine and complex environmental conditions

The presence of dissolved organic matter (DOM) can impact metal sulfide (MeS) precipitation and mobility. Thiol containing ligands such as cysteine have been shown to be effective capping agents in single metal MeS studies, allowing NPs to persist in oxic environments. In this study, both single (Cd or Zn) and multi-MeS (Cu, Pb, Cd, Zn, and As) nanoparticle (NP) formation was characterized to understand the impact of the thiol cysteine (CYS) on early stage (3 h) MeS NP behavior. Short duration single metal batch experiments, in the absence and presence of CYS, confirmed that MeS species readily formed solids with limited dissolved fraction; however, multi-metal systems exhibited divergent behavior reflecting a wider range of NP sizes and an increased dissolved concentration. Multi-metal batch experiments revealed that metals were generally sequestered into MeS solids in accordance with MeS solubility products (i.e., from least to most soluble: Cu > Pb ~ Cd > Zn). CYS concentrations in excess of sulfide (10:1 CYS:S ratio) stabilized MeS within the Small NP size fraction (3.2 nm < d < 43 nm) and limited Pb, Cd, and Zn dissolution compared to molar ratios of 1:1. In the combined presence of CYS and Ca2+, multi-MeS particle aggregation increased substantially compared to monovalent systems. Dissolution increased for Pb and Zn as a function of matrix ionic strength whereas dissolved Cu trends changed as a function of cation valence state (e.g., Na+ vs. Ca2+). Most noteworthy, single-metal Zn and Cd batch experiments demonstrated that single-metal studies can overestimate MeS NP resistance to oxidative dissolution compared to multi-metal counterparts. Thus, caution should be taken when broadly applying mechanisms and rates elucidated from single-metal systems.

Changes of thermal properties and microstructure of nano‐ZnO/polylactic acid composite films during Zn migration

In this study, polylactic acid (PLA) films with different nano‐ZnO concentration (0, 1, 3, 5, 7, 9, and 15%wt) were obtained by casting method. Overall migration values and specific migration values were studied in different food simulants (10% [v/v] ethanol and isooctane). The results demonstrated that overall migration of the composite films and specific migration of Zn2+ was higher in 10% (v/v) ethanol solution than in isooctane solution. The release of Zn2+ was affected by the percentage of nano‐ZnO in the composite films and food simulants. The migration of Zn in this study was mainly caused by polymer softening and degradation leading to the dissolution of Zn. Scanning electron microscope analysis indicated that with the migration of Zn2+, the structure of the composite films changed greatly. Differential scanning calorimeter analysis indicated that the thermal stability was decreased and the glass transition temperature and crystallization temperature were reduced.

Mechanically mixed ZnO‐Al2O3 catalysts in the synthesis of propylene carbonate via alcoholysis of urea

AbstractAlcoholysis of urea with 1,2‐propylene glycol (PG) is a potential industrial process for the synthesis of propylene carbonate (PC) with many advantages. However, the preparation of industrial catalysts such as Zn‐Al oxide by the co‐precipitation method will inevitably produce a large amount of salty water and building desalting equipment will increase the investment cost. The present work attempts to use a mechanically mixed catalyst consisting of commercial ZnO and Al2O3 to solve these problems. This type of catalyst was capable of catalyzing the reaction more than 5 times. The PC yield was gradually increased with each use. The best PC yield of 96.7% is comparable to the co‐precipitated Zn‐Al oxide catalysts and was achieved during the third use. In the reaction, ZnO and Al2O3 were dissolved into the reactants to form complexes, which homogenously catalyzed the reaction. It was found that the dissolved Zn and Al complex significantly influenced the PC yield and re‐precipitated in the late stage of the reaction. Furthermore, the partial dissolution of Al assisted the dissolution and precipitation of Zn, which improved the PC yield. After several dissolution‐precipitation cycles, ZnO‐Al2O3 was homogeneously mixed at the atomic scale. Interestingly, there was an adequate linear relationship between the amount of dissolved Zn and Al in each reaction, the linear correlation coefficient improved after each reaction, and the slope of the line (the ratio of dissolved Zn/Al) was 7.26 in the third use.

Influence of Current Density on the Mechanism of Electrodeposition and Dissolution of Zn–Fe–Co Alloys

The electrodeposition of zinc–iron–cobalt alloys from sulfate bath was carried out galvanostatically on steel. A complementary approach based on of a combination of different electrochemical techniques was used to clarify the deposition mechanism. The electrodeposition of the alloys was of an anomalous type. The corrosion behavior of the coatings was evaluated from polarization measurements in hydrochloric acid. The deposition at different current densities exhibited varied influence on the corrosion potential of alloys obtained from sulfate baths. It was shown that increased corrosion protection of steel can be achieved by electrochemical deposition of ternary alloys with higher amounts of the Co and Fe. The composition of the ternary alloy can be optimized by proper choice of the current density.

Effects of the Combined Addition of Zn and Mg on Corrosion Behaviors of Electropainted AlSi-Based Metallic Coatings Used for Hot-Stamping Steel Sheets.

The effects of the combined addition of Zn and Mg on the corrosion resistance of AlSi-based coating for automotive steel sheets were investigated using a variety of analytical and electrochemical techniques. The preferential dissolution of Mg and Zn from MgZn2/Mg2Si phases occurred on the AlSi-based coating that had been alloyed with a smaller portion of Zn and Mg, which contributed to the rapid surface coverage by corrosion products with a protective nature, reducing the corrosion current density. On the other hand, localized corrosion attacks caused by the selective dissolution of Mg were also observed in the AlSi-based coating with a smaller portion of Zn and Mg. Such alloying can also worsen its corrosion resistance when coated additionally with electrodeposited paint. The mechanistic reasons for these conflicting results are also discussed.

Geochemistry of Engineered Nanoparticles (CdSe/ZnS Quantum Dots) in Surface Waters

The difficulties when studying the behavior of engineered nanoparticles (ENPs), and the subsequent metal speciation in aquatic ecosystems, at concentrations relevant of environment (i.e. ppt level) are often related to the occurrence of ENPs constitutive elements at high concentrations as a background in aquatic media (i.e.). In this study, the physico-chemical behavior of CdSe/ZnS quantum dots (QDs) when spread at very low concentration in surface waters was investigated. The above-mentioned issues were overcome with the use of isotopically labeled QDs, separated by centrifugal ultrafiltration (CU) and measured by HR-ICP-MS, combined with the detection of free and labile metal ions by Scanned Stripping ChronoPotentiometry (SSCP). They firmly provided a thorough comprehension regarding the transformation of QDs in surface waters. The physicochemical conditions of the medium including the presence of an analogue of natural organic matter and a mineral (i.e. fulvic acid, goethite), the manufactured coating of the QDs (here thioglycolic acid), and the occurrence of added Zn in the medium, were considered in the study. The overall results show that, in the absence of mineral/organic matter, the TGA ligands in solution detached from the QDs surface after dissolution control the metal ions speciation, especially for Cd. Conversely in a more representative aquatic ecosystem conditions (i.e. with Zn in the background media together with fulvic acid and goethite), almost no Zn nor Cd dissolution from the QDs is detected. SSCP measurements reveal that the Zn complexes formed with the organic/mineral material in the system are inert, whereas the speciation model calculations indicated that Cd2+ is bound to TGA ligands in solution and organic/inorganic matter – therefore suggesting that, under the studied conditions, aquatic organisms will be exposed to a very low concentration of free and labile metal ions issued from the QDs.

Efficiency of Chemical and Biological Leaching of Copper Slag for the Recovery of Metals and Valorisation of the Leach Residue as Raw Material in Cement Production

Copper slags produced in vast quantities in smelting operations could be considered as secondary material sources instead of stockpiling them in landfills. This study investigates the recovery of valuable metals from copper slag and the valorisation of the leach residue as construction material in line with the principles of a circular economy. By taking into account that the environmental characterization of the as-received copper slag did not allow its disposal in landfills without prior treatment, chemical and biological leaching were tested for the recovery of metals. Pre-treatment with acids, namely HNO3 and H2SO4, resulted in the extraction of several target metals and the production of an almost inert waste. Despite the clearly better oxidative conditions prevailing in the bioleaching reactors, chemical leaching resulted in the higher dissolution of Cu (71% vs. 51%), Co (70% vs. 36%), and Zn (65% vs. 44%). The acid consumption was much lower during the bioleaching experiments compared to the chemical leaching. The bioleach residue was suitable for its use as supplementary cementitious material, showing a better performance than the reference sample without causing any detrimental effects to the calcium aluminate cement (CAC) quality. The complete valorisation of copper slags is expected to improve the economics of the process, by avoiding landfill costs and producing saleable products with high added value.

Synergistic enhancement of sinterability and corrosion resistance of ZnCr2O4 spinel by TiO2 addition for carbon-free aluminum electrolysis

ZnCr2O4 is a corrosive-resistant spinel with potential as a sidewall material candidate for advanced carbon-free aluminum electrolysis. However, synergistic enhancement of sinterability and corrosion resistance of ZnCr2O4 is still a challenge. To address this issue, ZnCr2O4 bulks were sintered by using TiO2 as additive, and their corrosion performance was evaluated in the electrolysis of NaF19.69-KF25.84-(AlF3)54.47 bath at 800 °C by both static and dynamic corrosion experiments. The results showed that the doped Ti4+ was located in the octahedral space of ZnCr2O4, and the formed cation vacancies at B-site of spinel promoted the sinterability. The spinel with 10 wt% TiO2 showed the highest density which was composed of the main phase Zn(Cr, Ti)2O4 and the second phase Cr2Ti5O13. The compact spinels can resist the penetration of electrolyte and further reduce the dissolution of Zn(Cr, Ti)2O4. Consequently, the corrosion rate significantly decreased from 0.028 cm·a−1 for ZnCr2O4 to 0.016 cm·a−1 for 10 wt% TiO2 added spinel. Based on the experimental analyses and ab initio thermodynamics calculations, the corrosion mechanism of the obtained spinels was revealed. The as-introduced strategy for performance synergy improvement of ZnCr2O4 spinel is expected to allow successful application of new materials in the promising electrolytic cell and other severe corrosion environments.

Reexamination of Nucleation and Growth Behavior of Zn during Zincating on Al: Role of Nitric Acid Etching

Deposition of various coatings on Al surface is required for variety of functional applications such as for better corrosion resistance, improvement in conductivity, wear resistance etc. These coatings are normally applied on Al surface followed by well-known zincating process (galvanic displacement) in order to obtain subsequent adherent overlayer coatings. Adhesion of these overlayers depends on uniformity of Zn films on Al surface and researchers normally follow either single zincating or double zincating process to eliminate oxide layer interference. However, it has been established over the years that double zincating is a better technique in comparison to single zincating to produce a uniform Zn-films on Al surface. It is to mention here that Al substrate undergoes nitric acid dip followed by single zincating prior to double zincating and is a very important step. A quiet good amount of research has already been devoted to obtain various adherent coatings on Al and Al-alloys and to analyze the nucleation mechanism during single and double zincating processes. However, the underline mechanism of nitric acid etching and its role on subsequent Zn-nucleation is yet to be fully understood. In view of this, in the present study, a systematic effort has been made to elucidate the role of nitric acid etching followed by single zincating on Zn nucleation and growth mechanism during subsequent second zincating. For this, we have studied the time-dependent zinc nucleation on 99.95% pure Al during single as well as double zincating. These Zn-nucleation and growth behavior on Al surface was monitored using AFM, FESEM and FIB-cut cross section image investigations in detail. Difference in nucleation and growth pattern of Zn on Al during two zincating processes was analyzed. Apart from these, the porous surface obtained after nitric acid dissolution was investigated in depth under AFM and FESEM along with EDS. Non-gravimetric method such as Rutherford Backscattering Spectrometry (RBS) was used to determine the deposition rate of Zn during competitive galvanic deposition of Zn and alkaline dissolution of Al while zincating. The Zn grain size obtained from AFM image was verified by GIXRD analysis. Open circuit potential (OCP) was monitored during single as well as double galvanic displacement processes to understand Zn nucleation behavior and correlated with the morphological data. The observed results will be discussed in terms of electrochemical processes involved, change in chemical nature of the Al surface and porous layer formation due to nitric acid etching and thermodynamic supersaturation on Zn nucleation. Figure 1