Thiourea System Research Articles

A highly efficient clean hydrometallurgy process for gold leaching in a Fenton oxidation assisted thiourea system

Clean hydrometallurgy processes, formed by “oxidant-ligand” systems, present promising strategies for gold leaching to mitigate the environmental impact of cyanidation. In this work, a Fenton oxidation-assisted thiourea system was proposed, employing reactive oxygen species (ROS, generated from Fenton reaction) as oxidant and thiourea as the ligand, respectively. By precise parameters control, 100% leaching of gold was rapidly achieved within 30 min from a roasted gold concentrate, highlighting extremely fast kinetics. Furthermore, this system was optimized by incorporating nitrilotriacetic acid (NTA) as an additive, which significantly reduced the thiourea consumption and produced the same gold leaching (100%, 60 min). Experimental results demonstrate that NTA lowers the solution potential (ORP) and eliminates the side reactions between iron ions and thiourea, thereby reducing thiourea decomposition. Surprisingly, the combination of Fenton oxidation and thiourea showed a high selectivity of the generation of ROS, with superoxide radical anion (•O2−) dominating, singlet oxygen (1O2) co-existing, but hydroxyl radical (•OH) absenting. Importantly, the moderate •O2− primarily contributes to gold leaching, which is generated through a chain reaction pathway between iron complex and H2O2 (Fe3+-Ligand+H2O2 → Ligand-Fe3+-OOH → Fe2+-Ligand+•O2−). Besides, the generation process of •O2− can be regulated by adding NTA. This work presented a prospective way in establishing the clean hydrometallurgy processes for efficient gold leaching.

Facile and novel method for the recovery of gold from a calcined concentrate via copper–ammonia -thiourea system and activated carbon

Cyanidation, as the primary method for gold extraction, poses significant risks to the environment and animals. Therefore, an eco-friendly, sustainable, and effective leaching approach has been devised for extracting gold from a gold concentrate after calcination, utilizing copper(II)-ammonia and thiourea. This research focuses primarily on optimizing leaching conditions, gold recovery from leach solution, and cyclic utilization of the leaching solution. Furthermore, we investigated the potential mechanisms behind the leaching and recovery processes using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and thermodynamic analysis. The leaching process exhibits a spontaneous reaction, and the mechanism for alkaline thiourea leaching of gold is as follows: (a) reduction of Cu(NH3)42+ to Cu(TU)4+, (b) oxidation of Au(0) to Au(TU)2+ and (c) oxidation of Cu(TU)4+ back to Cu(NH3)42+, in the presence of oxygen, regenerating the catalyst. Under optimal leaching conditions, the proposed method achieves a gold extraction efficiency of approximately 86.6%, approaching that of cyanidation. The barren leach solution can be reused for subsequent leaching experiments, with consistent gold extraction efficiency and thiourea consumption. Therefore, this method presents a promising alternative to gold leaching via cyanidation.

Uticaj tiouree na otpornost na koroziju mekog čelika uronjenog u simulirani rastvor betonskih pora

The corrosion resistance of mild steel in simulated concrete pore solution (SCPS) in the absence and presence of thiourea has been investigated by polarization study and AC impedance spectra. Polarisation study reveals that thiourea system functions as anodic inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. When mild steel is used as rebar, thiourea may be mixed with concrete. Thus the mild steel will be protected from corrosion. The protective film consists of iron-thiourea complex formed on metal surface. In the presence of thiourea the linear polarisation resistance increases, corrosion current decreases, charge transfer resistance (Rt) increases, impedance increases, phase angle increases and double layer capacitance (Cdl) value decreases. This formulation may find application in concrete technology.

Kinetics on the Oxidation of Aminoiminomethanesulfonic Acid by Hypochlorous Acid: A Novel Product in the Chlorination of Aminoiminomethanesulfonic Acid

AbstractThe oxidation of aminoiminomethanesulfonic acid (AIMSOA) by HOCl is too fast to be traced in both acidic and neutral solutions, while extreme alkalinity will hasten the decomposition of AIMSOA. Nonetheless, in weakly alkaline solutions the oxidation kinetics were monitored successfully by stopped flow at 25 °C. The results show that the rate law is first order with respect to both [HOCl]tot and [AIMSOA] but shows an inverse dependence on [OH−]. The calculated kHOCl values of (9.2±0.2)×106 M−1 s−1 and a statistically insignificant kOCl‐ value indicate that the conjugate acid HOCl is regarded as the primary oxidant. The reaction is supposed to be a Cl+ transfer mechanism followed by the formation of a novel unreported oxidation product, AIMSOA‐chloramine. Various technologies were employed to investigate the structure and decomposition of AIMSOA‐chloramine. The above evidence showed that the product, a kind of chloramine rather than sulfate, was characterized by a distinct structure consisting of N−Cl bond. The chloramine could also slowly decompose to sulfate and chloride due to the cleavage of both C−S and N−Cl bonds. Based on the results, the mechanism involving the production of chloride and sulfate from the oxidation of AIMSOA in Cl‐containing oxides ‐ thiourea system should be excluded. In addition, the results also provide additional insights into the bromination for the HOBr‐AIMSOA reaction, in which priority may be given to the formation of bromamine.

Recovery of gold from refractory gold ores: Effect of pyrite on the stability of the thiourea leaching system

The extraction of gold from refractory gold ores (RGOs) without side reactions is an extremely promising endeavor. However, most RGOs contain large amounts of sulfide, such as pyrite. Thus, investigation of the influence of sulfide on the gold leaching process is important to maximize the utilization of RGOs. In this work, the effects of pyrite on the stability of the thiourea system were systematically investigated under different conditions. Results showed that the decomposition rate of thiourea was accelerated sharply in the presence of pyrite. The effect of pyrite on gold recovery in thiourea leaching systems was then confirmed via a series of experiments. The decomposition efficiency of thiourea decreased by 40% and the recovery efficiency of gold increased by 56% after the removal of sulfide by roasting. Under optimal conditions, the efficiency of the gold recovery system increased to 83.69% and only 57.92% of thiourea decomposition was observed. The high consumption of thiourea by the leaching system may be attributed to not only adsorption by mineral particles but also catalytic decomposition by some impurities in the ores, such as pyrite and soluble ferric oxide.

Potentiometric Study of the Copper(II) Ions Complexation Process with Thiourea in Acidic Medium

Potentiometric titration method was used to study the copper(II) ions complexation process with thiourea in medium containing 1 mol/l of hydrochloric acid at a temperature of 298 K. It was found out that mononuclear (CuTMn) and binuclear (Cu2TMn) complexes were observed in the copper(II) --- thiourea system depending on the copper ions concentration. It was determined that in acid medium with copper(II) ions concentration less than 5 · 10--4 mol/l and mononuclear complexes of the composition are generated in the system, which constant is equal to lg β3 = 11.9. With an increase in the copper(II) ions concentration (CCu+2 > 5 · 10--4 mol/l), binuclear thiourea complex dominates in the system. Stability constant logarithm of the Cu2TM6 composition binuclear complex is equal to 27.5 and was calculated using the modified Leden's method. Based on the constants, complexes existence regions were determined depending on the ratio CTM/CCu+2. With the CTM/CCu+2 ≥ 5 relations, existence of a dominant complex significantly depends on the copper(II) ions concentration. It turned out that fractions of all complex particles were growing with an increase in the copper(II) ions concentration in the system. It was found that stability of thiourea complexes in the CuLi2+, AgLi+, AuLi+ rows was increasing

Bio-Oxidation of a Double Refractory Gold Ore and Investigation of Preg-Robbing of Gold from Thiourea Solution

Carbonaceous sulfidic gold ores are commonly double refractory and thus require pretreatment before gold extraction. In this paper, the capacity of pre-bio-oxidation can simultaneously decompose sulfides or deactivate carbonaceous matters (CM) from a double refractory gold ore (DRGO) using pure cultures of A. ferrooxidans or L. ferrooxidans, and a mixed culture containing A. ferrooxidans and L. ferrooxidans was investigated. The results showed that direct thiourea leaching of the as-received DRGO yielded only 28.7% gold extraction, which was due to the encapsulation of sulfides on gold and the gold adsorption of CM. After bio-oxidation, thiourea leaching of the DRGO resulted in gold extraction of over 75–80%. Moreover, bio-oxidation can effectively reduce the adsorption of carbon to gold. XRD, SEM-EDS and FTIR analysis showed that many oxygen-containing groups were introduced on the surface of DRGO during bio-oxidation, while the C=C bond was cleaved and the O–C–O and C–N bonds were degraded, causing a decrease in active sites for gold adsorption. Moreover, passivation materials such as jarosite were formed on the surface of DRGO, which might reduce the affinity of CM for gold in solutions. In addition, the cleavage of the S–S band indicated that sulfides were oxidized by bacteria. This work allows us to explain the applicability of pre-bio-oxidation for degrading both sulfides and CM and increasing gold recovery from DRGO in the thiourea system.

Effect of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans on preg-robbing of gold by graphite from thiourea leaching solution

Carbonaceous matter (CM) is one of the main reasons for the decrease of gold recovery in refractory gold ores. Bio-pretreatment of carbonaceous gold ores makes the reduction the preg-robbing of gold, and has attracted significant attention recently due to its economic feasibility and environmental friendliness. In this study, the graphite was used as surrogate material to investigate the effect of Acidithiobacillus ferrooxidans (A. ferrooxidans) and Leptospirillum ferrooxidans (L. ferrooxidans) on the gold adsorption by graphite in thiourea system. The results showed that graphite had a significant inhibitory effect on the growth of A. ferrooxidans and L. ferrooxidans. After bio-oxidation, the lamellar structure of graphite surface was blurred, and there was the formation of jarosite. In addition, some oxygen-containing groups such as O–H, SO and Fe–O appeared on the surface, but the CC bond cleaved. The adsorption capacity of graphite to gold decreased from 35.8% to 16.6% (A. ferrooxidans) and 18.4% (L. ferrooxidans) respectively. It can be concluded that the bio-oxidation by A. ferrooxidans and L. ferrooxidans can passivate and erode graphite, and contribute to decrease preg-robbing of gold from thiourea leaching solution.

Transesterification by air/moisture-tolerant bifunctional organocatalyst to produce ‘nonstrained’ γ-butyrolactone-based aliphatic copolyesters: Turning a bane into a boon

It has been a challenge in effective ring-opening copolymerization (ROC) of “nonstrained” γ-butyrolactone (BL) with high-strained cyclic esters or lactones, due to their large difference in reactivity. Transesterification (TEster) would be a promising alternative strategy to ROC for the synthesis of BL-based copolyesters if advanced catalysts could be rationally designed by enabling air/moisture tolerance and preference toward BL ring-opening over chain breakage. Here we report the development of air/moisture-tolerant bifunctional oganocatalyst based on 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)/thiourea(urea) system for efficient TEster reaction of poly(δ-valerolactone) (PVL)/BL or poly(ε-caprolactone) (PCL)/BL, successfully affording BL-based copolyesters, especially poly(BL-co-VL)s, with relatively high BL incorporation (up to 38.3%) and high molecular weight (Mn up to 40.6 kg/mol). Mechanistic study reveals that the enhancement of BL incorporation in TBD/TU(U)-mediated TEster reaction is presumably originated from the preferential activation of BL monomer over the polymer chain, rather than the enhanced bifunctional activation. Thanks to more homosequences present in the copolyesters produced by TEster, the resulting poly(BL-co-VL)s exhibit higher melting temperature (Tm) over those obtained by ROC at similar level of BL incorporation.

Solvent extraction separation and spectrophotometric determination of ruthenium(III) with p-methylphenyl thiourea: sequential separation of ruthenium, osmium and iron

ABSTRACT A selective and sensitive solvent extraction and spectrophotometric study of the ruthenium(III)–p-methylphenyl thiourea (PMPT) system is presented. The optimum conditions were determined by a critical study of acid concentration, reagent concentration, equilibration period, heating time and effect of solvent on the equilibrium. Ruthenium(III) forms 1:1 complex with PMPT in 20% ethanol and extracted into chloroform. Conformity to Beer’s law at 600 nm was observed up to 40 µg mL–1 of ruthenium. Molar absorptivity and Sandell’s sensitivity were found to be 2.31 × 103 L mol−1cm−1 and 0.044 μg cm−2, respectively. The detection limits were 0.11 μg mL−1 of ruthenium. The method is free from interferences from large number of cations and anions. Proposed method was successfully applied to the separation and determination of ruthenium from synthetic alloys, catalyst and water samples. Sequential separation and determination method for ruthenium(III), osmium(VIII) and iron(II) has been developed.

Low-temperature vulcanizable chloroprene rubber–bromobutyl rubber blend for encapsulation of undersea sensors: Nonconventional cure systems and reaction kinetics

Blends of chloroprene rubber (CR) and bromobutyl rubber (BIIR) are used in making the undersea sensors watertight by a process of encapsulation. The encapsulation process is conventionally done at high temperature approximately 150°C and above using high-temperature vulcanization (HTV). However, the new class of acoustic sensors like polyvinilidenefluride (PVDF) and thin film PZT are highly temperature sensitive and fragile in nature and hence they require low-temperature vulcanization (LTV) process to avoid damages and protect their full functionalities. However, conventional cure systems are not adoptable in LTV process and hence there is a need for the search of alternate cure systems. Not much work has been reported in this area. This article reports a nonconventional cure system vulcanizable with LTV and the associated reaction kinetics for a commonly used CR–BIIR blend for encapsulation of undersea sensors. Formulations have been attempted with cure systems based on red lead (Pb3O4) and zinc oxide (ZnO) for CR–BIIR blend in 80:20 weight ratio, instead of zinc oxide, magnesium oxide, and ethylene thiourea system, which are conventionally used in HTV. The cure parameters at low temperature between 70°C and 120°C and the activation energy for cure reactions ( E a) were estimated using MDR 2000 rheometer. Essential prerequisites like water resistance, electrical resistivity, and physicomechanical properties for sensor application are qualitatively analyzed for the blend cured at 90°C. The results reveal that the proposed nonconventional cure systems are able to bring down the cure temperature of CR–BIIR blend to 90°C from 150°C enabling the suitability of the materials for undersea sensor encapsulation.

Non-toxic growth of CuxS thin films in alkaline medium by ammonia free chemical bath deposition

CuxS thin films were deposited on commercial glass substrates at different temperatures by an ammonia-free chemical bath deposition process. The deposition process is based in a copper chloride- sodium citrate- thiourea system. The pH value in the reactive solution was adjusted at 10 by addition of KOH and buffer solution. In order to investigate the effect of the deposition temperature and time, four reaction baths were prepared and set at temperatures 25, 40, 55 and 70 °C. The deposited films were studied by X-ray diffraction, Raman spectroscopy, 3D optical microscopy, energy dispersive X-ray spectroscopy, electron probe X-ray microanalysis, UV–vis spectroscopy and two-points probe electrical measurements. The chemically deposited CuxS films showed good adherence to the substrate, homogeneity, metallic luster and light reddish-brown appearance. The thickness of the films varied in the range of 40–80 nm, depending on the deposition conditions. The X-ray diffraction revealed that the films present amorphous nature, excepting those deposited at the highest temperature, which had CuS covellite crystalline structure. The chemical composition of the films exhibited the presence of the covellite phase with excess of sulphur atoms in all cases. The electrical analysis showed a strong decrease in the resistivity with increasing the deposition temperature with values in the range from 106 to 10−2 Ωcm.

Acid decomposition and thiourea leaching of silver from hazardous jarosite residues: Effect of some cations on the stability of the thiourea system

The recovery of silver from hazardous jarosite residues was studied employing thiourea as leaching agent at acid pH and 90°C. The stability of the thiourea in synthetic solutions was evaluated in the presence of some cations that can be present in this leaching system: cupric and ferric ions as oxidant species, and zinc, lead and iron as divalent ions. Two silver leaching methods were studied: the simultaneous jarosite decomposition-silver leaching, and the jarosite decomposition followed by the silver leaching.The study with synthetic solutions demonstrated that cupric and ferric ions have a negative effect on thiourea stability due to their oxidant properties. The effect of cupric ions is more significant than the effect of ferric ions; other studied cations (Fe2+, Zn2+, Pb2+) had no effect on the stability of thiourea.When the decomposition of jarosite and the silver leaching are carried out simultaneously, 70% of the silver can be recovered. When the acid decomposition was performed at pH 0.5 followed by the leaching step at pH 1, total silver recovery increased up to 90%. The zinc is completely dissolved with any of these processes while the lead is practically insoluble with these systems producing a lead-rich residue.

Dissolution of gold with cyanide replacing reagents

Cyanide is used in the leaching phase of hydrometallurgical recovery of gold. The toxicity and environmental risks have created a need for safer alternatives. There is a vast amount of information about gold extraction and a selection has been collected and formalized to be used in a decision support tool. The tool Auric Advisor uses case-based reasoning (CBR) to provide process alternatives for the user queries. The aim of this paper is to study leaching of gold with cyanide replacing alternatives such as ammonium thiosulfate, chloride-hypochlorite, thiourea and thiocyanate to provide new cases for the tool. The behaviour of gold was examined with electrochemical tests and weight loss measurements a using quartz crystal microbalance (QCM) were used to determine the dissolution rate. Based on literature, the dissolution rate determined for cyanide solution was 2.5 mgcm -2 h -1 . Based on polarisation experiments and potential measurements, the ammonium thiosulfate system with cupric ion as oxidant was estimated to dissolve gold slower than cyanide. In the QCM tests the chloride-hypochlorite alternative showed highest dissolution rate of 8.6 mgcm -2 h -1 at the redox potential of 900 mV vs. SHE. The thiocyanate system with ferric ion dissolved gold at a rate of 3.4 mgcm -2 h -1 at the redox potential of 620 mV vs. SHE. Thiourea system with ferric ion showed the dissolution rate of 1.65 mg cm -2 h -1 at the redox potential of 450 mV vs. SHE. The results of the rapid tests were comparable with literature and were included in the case base of the Auric Advisor decision-support tool.

Contribution of Methyl Substituent on the Conductivity Properties and Behaviour of CMC-Alkoxy Thiourea Polymer Electrolyte

An essentially linear conjugated thiourea system provides wide range of electronic properties as they consist of rigid π-systems on their structures. This study reported the synthetic, characterization and theoretical evaluation of molecular wire candidate bearing alkoxy thiourea derivative featuring methylbenzene head group. It was applied as dopant in Carboxymethyl-Cellulose (CMC) solution in order to form a conductive biodegradable thin film. According to the conductivity result, the developed system can be developed as electrical conductor as proven via both experimental and theoretical studies. Therefore, this type of A-ArC(O)NHC(S)NHAr-D molecular framework has opened wide possibilities to be applied in many micro-electronic application devices.

Origin of the synergistic effect between polysaccharide and thiourea towards adsorption and corrosion inhibition for mild steel in sulphuric acid

The origin of the synergistic effect between polysaccharides and thiourea towards adsorption on mild steel in 0.5 M H2SO4 and their effect on corrosion inhibition has been studied. Two different polysaccharides, gum arabic and agar agar are used for this purpose. Results obtained from potentiodynamic polarization and electrochemical impedance spectroscopic studies reveal that thiourea imparts a profound synergistic effect towards the corrosion inhibition of mild steel in H2SO4 by the polysaccharides. The polysaccharide–thiourea system acts essentially as a mixed type corrosion inhibitor and the phenomenon of adsorption mechanism on metal surface is proposed as physisorption. Detailed FTIR studies of the surface adsorbed layers of inhibitors have been done to elucidate the origin of the synergistic effect towards the coadsorption and subsequent corrosion inhibition of the metal.

Nonlinear effects in proline–thiourea host–guest complex catalyzed aldol reactions in nonpolar solvents

An aldol reaction catalyzed by a proline–thiourea host–guest complex in a nonpolar solvent shows excellent nonlinear effects. This proline–thiourea system has the ability to form a hydrogen-bonding network. The enantiomeric excess of proline in a solution can be significantly enhanced by its incorporation with a urea molecule into its solid racemate. This suggests a general and facile route to homochirality, which may be involved in the origin of chirality on earth.

Ethyl 4-(3-benzoylthioureido)benzoate

The title compound, C17H16N2O3S, crystallizes in the thio­amide form with an intra­molecular N—H⋯O hydrogen bond across the thio­urea system. Mol­ecules are connected in chains parallel to [10] by hydrogen bonds from the second thio­urea N—H group to the benzoate C=O function.

Temporal and Spatial Organization of Chemical and Hydrodynamic Processes. The System Pb2+−Chlorite−Thiourea

Precise spatio-temporal organization of chemical, hydrodynamic, and mechanical processes is typical for biological systems where particular chemical reactions have to accrue in precisely assignment place and time. It is rarely studied and observed in chemical systems. We report unusual precipitation pattern formation of PbSO(4) in chemical media (Pb(2+)-Chlorite-Thiourea System). We have found that there is a region in a plane of initial concentrations of chlorite ions and thiourea where precipitation of lead sulfate appears in a form of ring if a pellet of lead nitrate is placed into the system. The whole process may be divided into three stages: movement of first circular front of lead containing solution, formation of a ringlike pattern of lead sulfate, and finally, propagation of this pattern resulting in a formation of ring with final inside diameter. Our experiments indicate that the following values are reproducible and quantify the PbSO(4) ring evolution: induction time, radius of the ring birth, speed of ring propagation toward the center, and final inside radius of the ring. Numerical solution of kinetic equations allowed us to give a qualitative explanation for the phenomenon observed. Formation and evolution of the PbSO(4) rings are caused by interplay of concentration gradients in the system and chemical reactions that occur in excitable chlorite-thiourea system. Chemical reactions and hydrodynamic processes form a complex causal network that made morphogenesis of this unusual pattern possible.

Electrodeposition of a Silver-Gold Alloy (DORÉ) from Thiourea Solutions in the Presence of Other Metallic Ion Impurities

With the increasing complexity of gold and silver containing minerals, the thiourea system has been considered a possible alternative for precious metal leaching. In the present work, direct electrodeposition is proposed as a viable method to recover gold and silver values from thiourea leaching solutions. With this purpose in mind, thermodynamic and electrochemical analyses of this complex system were undertaken. The thermodynamic reduction potentials of copper and lead ions were shown to be very close to those of gold and silver, favoring the formation of codeposits of these metals. In order to complement the thermodynamic analysis, kinetic studies were performed using potentiostatic electrodeposition with anodic stripping. A vitreous carbon electrode was shown to be sufficiently inert over the entire potential range studied to allow its use for the investigation of the metal deposits formed. Regarding these deposits, it was demonstrated that the presence of a small amount of gold (10 ppm), in solutions containing silver, favorably affects the energy and kinetics of the gold-silver alloy (doré) deposition process. In this same manner, it was also shown that the presence of lead(II) (10 ppm), zinc(II) (10 ppm) or iron(II) (100 ppm) favors the alloy deposition by shifting the initial reduction potential to less negative values while maintaining the same deposition rate. On the other hand, the addition of 200 ppm of copper (II) provokes a slight decrease in the reduction rate, although the potential at which the deposition begins is practically the same as that observed for solutions of only gold and silver.