Thiourea Leaching Research Articles

Preliminary Study on Gold Recovery from High Grade E-Waste by Thiourea Leaching and Electrowinning

The present paper is focused on the extraction of gold from high-grade e-waste, i.e., spent electronic connectors and plates, by leaching and electrowinning. These connectors are usually made up of an alloy covered by a layer of gold; sometimes, in some of them, a plastic part is also present. The applied leaching system consisted of an acid solution of diluted sulfuric acid (0.2 mol/L) with thiourea (20 g/L) as a reagent and ferric sulfate (21.8 g/L) as an oxidant. This system was applied on three different high-grade e-waste, namely: (1) Connectors with the partial gold-plated surface (Au concentration—1139 mg/kg); (2) different types of connectors with some of which with completely gold-plated surface (Au concentration—590 mg/kg); and (3) connectors and plates with the completely gold-plated surface (Au concentration—7900 mg/kg). Gold dissolution yields of 52, 94, and 49% were achieved from the first, second, and third samples, respectively. About 95% of Au recovery was achieved after 1.5 h of electrowinning at a current efficiency of only 4.06% and current consumption of 3.02 kWh/kg of Au from the leach solution of the third sample.

Recovery of gold from sulfide refractory gold ore: Oxidation roasting pretreatment and gold extraction

Recovering gold from sulfide refractory gold ore has been an important guarantee for the sustainable development of gold industry, while there are many challenges in the treatment of sulfide refractory gold ore, such as environmental pollution and low gold recovery. In this study, A novel method for gold recovery from refractory gold ore was proposed with the purpose of improving recovery efficiency of gold and reducing environmental pollution. The method consisted of two stages roasting for arsenic and sulfur removal, gold recovery by thiourea leaching, followed by an enhanced chlorination roasting for gold recovery in the presence of pyrite. In the two-stage roasting process, the phase transition of FeAsS and FeS2 during the roasting process was analyzed. In the first stage roasting process, the removal efficiency of arsenic reached 96.98% with only 43.07% sulfur was removed. We achieved removal efficiency of 97.19% for sulfur and 84.11% of gold was exposed during the second stage roasting process. The thiourea leaching combined with pyrite enhanced chlorination roasting was first proposed to effectively extract gold from the second stage roasting slag, and the pyrite shown a great role in promoting gold extraction during chlorination roasting. In this process, gold could be effectively recovered from the sulfide refractory gold ore with a recovery efficiency of 98.06%.

Leaching of gold from flotation waste by thiourea

Throughout human history, gold has been the most sought-after and well-known precious metal in the world. It is synonymous with wealth in the financial market and jewellery. However, its use has a wide range of applicability in biomedicine, electronic engineering and other industries. Primary gold resources are limited and have been significantly depleted in recent decades. To cover the current demand, it is necessary to process secondary sources containing gold such as heaps, tailings, lean ores or electrical waste. New and especially more efficient technological procedures are needed for the processing of these secondary sources. From an environmental point of view, a hydrometallurgical process is considered to be more appropriate than a pyrometallurgical process, in which very toxic fumes enter the air. This work investigates the leaching of gold from flotation wastes using thiourea. It is flotation waste that is the target group of materials that are a source of precious metals. An interesting gold content is present in our waste sample (Hodruša Hámre tailings pond), but its evaluation depends on the setting of suitable technological conditions. In this work, thiourea is used as an alternative leaching agent to toxic cyanide, which is banned in Slovakia and many countries around the world due to its high environmental risk. The results of the study are described with respect to the gold recovery under optimized thiourea leaching conditions.

Gold Leaching Using Thiourea from Uranium Tailing Material, Gabal El-Missikat, Central Eastern Desert, Egypt

The objective of this study was to investigate thiourea leaching of gold present in the uranium tailings. Sulfuric acid treatment for raw material is important in order to dissolve uranium as well as to remove easily leachable metals. The acidic thiourea solution was proposed as a non-toxic possible alternative process route to cyanide in gold dissolution. The acidic system of thiourea-sodium sulfite was studied in the gold dissolution process. The presence of sodium sulfite can significantly reduce thiourea decomposition, accelerate the gold dissolution process, decrease the leaching time, and decrease the activation energy of gold dissolution. The activation energy for gold dissolution was evaluated. The activation energy solution with sodium sulfite is 7.62 kJ mol−1 which is much lower than 9.32 kJ mol−1 in the absence of sodium sulfite. Low activation energy confirms the surface reaction dissolution mechanism. This method demonstrates the possibility of extracting gold directly from low-grade ores without any physical or chemical pre-concentration method with a more environmentally friendly process.

Analysis and Prediction of the Thiourea Gold Leaching Process Using Grey Relational Analysis and Artificial Neural Networks

The thiourea (TU) leaching of gold from refractory ores can be considered an alternative to cyanidation. However, the high reagent consumption causes an increase in cost, which seriously limits its use. In order to effectively reduce the TU consumption, it is necessary to analyze the influencing parameters of gold recovery and TU consumption and apply them to the prediction of the TU leaching process. This paper investigated six potential influencing parameters and used grey relational analysis (GRA) to analyze the relational degree between each parameter and gold recovery and TU consumption. Then, the artificial neural network (ANN) model was established to simultaneously predict the gold recovery and TU consumption in the TU gold leaching process. The results of the GRA indicated that the leaching time, initial pH, temperature, TU dosage, stirring speed, and ferric iron concentration were all well related to the gold recovery and TU consumption. Therefore, the incorporation of these parameters can significantly improve the ANN model validation. The predictive results noted that the prediction accuracy of gold recovery varied from 94.46% to 98.06%, and the TU consumption varied from 95.15% to 99.20%. Thus, the predicted values corresponded closely to the experimental results, which suggested that the ANN model can accurately reflect the relationship between the operational conditions and the gold recovery and TU consumption. This prediction method can be used as an auxiliary decision-making tool in the TU gold leaching process, and it has broad engineering application prospects in engineering.

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.

An alternative to cyanide leaching of waste activated carbon ash for gold and silver recovery via synergistic dual-lixiviant treatment

This study reports the development of a hydrometallurgical treatment for activated carbon ash (ACA); a waste product of the carbon-in-pulp (CIP) process used in the gold mining industry, rich in adsorbed precious metals. After an initial screening of known leaching chemistries, the research focusses on dual-lixiviant (thiourea and thiocyanate) and thiourea systems, both of which have lower environmental impact than traditional cyanide leaches. Comparing ferric sulfate and hydrogen peroxide as oxidants showed that a thiourea leach is more suited to ferric sulfate whereas the dual-lixiviant leach achieved greater extraction with hydrogen peroxide. This was believed to be due to a more favourable effective [thiourea]:[thiocyanate] molar ratio in solution. The latter demonstrated faster kinetics and improved efficiency for dissolution of gold and silver. However, both leaches had issues with silver extraction due to formation of passivating layers on the surface of the silver nanoparticles. Kinetic modelling showed both systems tended towards a mixed-controlled process. Gold extraction of 89%, with leaching reaching equilibrium within 100min, using the dual-lixiviant process demonstrated that there is an alternative to cyanide leaches employed in gold recovery.

Base metal citrate pretreatment of complex ores to improve gold and silver leaching with thiourea

A pretreatment to remove and recover base metals present in complex sulfide refractory ores is proposed as a method to improve the extraction of precious metals with thiourea. Sodium citrate solutions at pH 8 and 5 were employed to extract lead and iron, respectively, from two concentrates: one having silver in a primarily galena matrix and the other, with gold associated with pyrite. The pretreatment solutions, containing the base metals, were subjected to electrolysis and pH adjustment, before being recycled back to the pretreatment stage. The results showed that citrate pretreatment successfully extracted lead and some iron, liberating the precious metals for thiourea leaching. The citrate pretreatment solution could be reutilized, after base metal removal, decreasing the reagent and water requirements. In all cases, the citrate pretreatment improved precious metal extraction.

Hydrometallurgical metal recovery from waste printed circuit boards pretreated by microwave pyrolysis

Waste printed circuit boards (PCBs) were pretreated by either microwave or conventional pyrolysis prior to acid leaching processes. Microwave pyrolysis can provide higher weight loss of waste PCBs and better metal recycling than conventional pyrolysis. After microwave and conventional pyrolysis of waste PCBs, the overall copper recovery rates of two-stage acid leaching can be up to approximately 96 and 75%, respectively. The gold recovery rates of thiourea leaching from microwave- and conventional-pyrolyzed PCBs were approximately 80 and 69%, respectively. Therefore, size-reduction processes for the pretreatment of waste PCBs would not be necessary, as long as microwave pyrolysis is applied prior to hydrometallurgical processes. The gold recovery rates of this study should be credible, since the sum of recovery and non-recovery rates was close to 100%. The initial acid leaching of copper may be governed by the copper content in the PCBs as well as the concentration of oxidizing agent in the leaching solution. Besides, the reaction rate of thiourea leaching of microwave-pyrolyzed PCBs may be largely influenced by the thickness of product layer. Both acid and thiourea leaching experimental results were better fitted by the pseudo-second-order reaction model. However, the maximum recovery rate determined by using the pseudo-first-order reaction model should be more reasonable.

Leaching of Gold from Fine-grained Flotation Tailings

This study focuses on leaching of gold from fine-grained waste materials from flotation. The input raw material is the waste product after flotation containing about 1–2.5 g/t of Au. This paper makes part of wider research to identify all available methods and select the most appropriate technology to obtain residual gold from fine-grained waste materials. An alternative method to conventional cyanide methods, which have negative environmental impacts [1], has been tested under laboratory conditions, namely thiourea leaching. We observed the effect of the grain size of input raw materials with concentration of thiourea and Fe3+ ions in dependence on temperature and pH during research. The pH values were maintained low during all experiments to avoid dissociation of the leaching solution. In optimal condition of leaching we achieved 85% success rate of gold recovery. The data may be useful to improve the processes of recovery of useful constituents from unused wastes with the content of gold. Keywords: Flotation; Gold; Leaching; Thiourea. DOI 10.35180/gse-2020-0036

Technology of Gold Extraction from Mature Dumps by Thiourea Leaching

Tajikistan has a large volume of gold-containing dumps, which on conversion to gold equals ~18 t. These dumps are products of amalgamation and cyanidation processes, resulting from treating the ores from Tarorskoe, one of the largest deposits in Tajikistan, and from a few other nearby deposits: Dzhilau, Khirskhon, and Olimpiiskoe. The mineralogical composition of the dumps is represented primarily by quartz, feldspar, and clay minerals. Their average content of gold is rather high, 2.4 g/t. Therefore, it is expedient to treat the dumps on the premises of JV Zerafshan plant (in the city of Pendzhikent, Tajikistan), which processes ores of the Tarorskoe deposit and has an unloaded technological cycle. The treatment of these dumps does not require an ore pretreatment stage, since, for the actual grain-size class (–0.074 mm, 58.11%), the content of cyanided gold is 89.7%. This work presents the results of studying the extraction of gold from the dumps by thiourea leaching. The purpose of this work is to find ways to decrease the consumption of expensive reagent thiourea. It is found that the sample of the dumps contains sorption-active minerals, which lead to the loss of gold with tails. Therefore, it is suggested that raw material should undergo a preliminary acid treatment to reduce the consumption of thiourea and sodium sulfite should be added to the pulp during carbon in thiourea leach. A high degree of extraction (~89%) is achieved when thiourea is loaded in the amount of 2 kg/t, ferric sulfate 7 kg/t, sodium sulfite 12 kg/t, and the initial concentration of sulfuric acid of 0.5% (at the stage of acid treatment). In this case, the consumption of thiourea is 0.8 kg/t. A process flow diagram for the dump treatment is proposed, which includes the following operations: preliminary acid treatment, carbon in thiourea leach, desorption, reactivation, electrolysis, and melting.

Recovery of gold from mature dumps by thiourea leaching

Tajikistan has a large amount of gold-bearing dumps making up - 18 tons in gold equivalent. These dumps are tails of amalgamation and cyanidation processes obtained in the processing of ores from the largest deposit in Tajikistan - Tarorskoe and a number of adjacent deposits - Jilau, Khirskhon, Olimpiyskoe. In terms of their mineralogical composition, dumps consist primarily of quartz, feldspar, and clay minerals. Average gold content in them is quite high - 2.4 g/t. Therefore, it is feasible to involve them in processing based on the existing Joint Venture «Zarafshon» plant (Penjikent, Tajikistan) processing ores from the Tarorskoe deposit and having underutilized process capacity. These dumps require no ore preparation for processing as the cyanided gold content is 89.7 % at the existing grain size class (-0.074 mm, 58.11 %). This article presents the results of studies on gold extraction from dumps by thiourea leaching. The paper is aimed at searching for ways to reduce the consumption of an expensive component - thiourea. It was found that dump samples contain sorption-active minerals that lead to the loss of gold with tails. In this regard, it is proposed to conduct gold sorption leaching. It was established that thiourea consumption can be reduced by acid pretreatment of raw materials with Na 2 SO 4 added to pulp during the thiourea leaching process. A high degree of extraction (-89 %) is achieved when loading 2 kg/t of thiourea, 7 kg/t of ferrous sulfate (III), 12 kg/t Na 2 SO 4 and the initial concentration of sulfuric acid of 0.5 % (at the stage of acid pretreatment). Thus, the thiourea consumption is 0.8 kg/t. A dump processing flow chart is presented that includes the following operations: acid pretreatment, thiourea sorption leaching, desorption, reactivation, electrolysis, and melting.

Stepwise extraction of gold and silver from refractory gold concentrate calcine by thiourea

Cyanide leaching, as a typical traditional method, hardly achieves the cleaner production of gold due to high consumption of auxiliary materials, inefficient production and heavy environmental pollution. In this work, stepwise thiourea leaching was used to extract gold and silver from refractory gold concentrate calcine. In the first stage, gold and silver extractions of 84.42% and 44.15% are obtained by thiourea leaching without any additive, respectively, which are further enhanced to 88.71% and 52.65% by adding the additives of lignin sulfonate sodium, urea and iron ion. The spent thiourea can be recycled to leach next batch of calcine after the treatment of replacing gold by iron powder. In the second stage with mechanical activation, higher gold and silver extractions are achieved 96.51% and 70.43%, respectively. Furthermore, a pilot-scale test is carried out to prove the reliability of this technology, which is hopefully adopted in industry to extract gold and silver efficiently and cleanly

Recovery of Gold from Pregnant Thiourea Leaching Solution by Synergistic Extraction

Aim of this research is to investigate the ability of synergist system of D2EHPA-isodecanol to recover gold from thiourea leaching solution. Synergistic extraction was introduced to replace single extraction due to enhance the extraction efficiency of heavy metal such as gold, silver copper and etc. In order to improve extraction efficiency, di-(2ethylhexyl) phosphoric acid (D2EHPA) was mixed with isodecanol to provide synergistic effect together with leaching solution which thiourea. From the synergist extraction system. The optimum pH is at 2.5 which in acidic range producing the highest yield of %E of 94.6%. It was found that synergistic D2EHPA-isodecanol extraction is successful as it yield %E of 99.4% if the ratio aqueous: organic is set to 1:2.

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.

Use of Thiourea Leaching During Gold-Containing Dump Treatment

In Tadzhikistan the volume of gold-containing waste converted to gold is 18 tons. This waste is products of amalgamation and cyaniding processes., and with respect to end component content they may be classified as a very rich type of raw material since they contain 1.7–8.5 g/ton of gold, whereas the average gold content in ores extracted for processing in one of the richest deposits of the republic, i.e., Tarror, comprises 2.4 g/ton. In addition, treatment of these dumps does not require crushing and refining since they have adequate coarseness of a finished class. In view of this the research object is goldcontaining waste of Tarror ore and a number of other adjacent deposits that have been accumulated over a long time. Results are provided for the raw material substance composition of the test raw material and the possibilities of using thiourea leaching for processing them. The main process parameters are optimized and the degree of gold extraction at the level of 80% is demonstrated. A fundamental production scheme is presented for processing dumps, including the following operations: preliminary acid treatment; sorption thiourea leaching; desorption; reactivation; electrolysis; melting.

DETERMINACIÓN DE TIOUREA PARA EL PROCESO DE LIXIVIACIÓN DE METALES PRECIOSOS POR TITULACIÓN CON YODATO

The thiourea leaching process for precious metals recovery has been studied as an alternative process to cyanidation. The accurate and simple determination of thiourea is important to guarantee the efficiency of the process and to evaluate the thiourea consumption during the gold and silver leaching from minerals; however, there is not, until now, any trustable method to determine the thiourea concentration for the high levels used in the extractive metallurgy industry. The present work, performed at CINVESTAV Saltillo, studies thermodynamically and experimentally the thiourea determination by iodate titration in the search of a precise technique appropriate for the high thiourea concentrations used in the leaching of precious metals. The thiourea titration by iodate in acid media permits the precise determination of high thiourea concentrations (up to 10,000 ppm). The acidification of thiourea (CS(NH2)2) produces protonated thiourea (HCS(NH2)2+) which reacts with iodate to form the protonated formamidine disulfide ([CS(NH2)2]22+).  The presence of oxidant ions like cupric or ferric ions can oxidize the thiourea into formamidine disulfide (FDS2+), decreasing the leaching strength of the solution. When zinc dust is added to the solution before the titration, it reduces FDS2+ by regenerating the thiourea.

Recovery of Precious Metals from Discarded Mobile Phones by Thiourea Leaching

The objective of this work was to recover gold and silver from the printed circuit board (PCB) of discarded mobile phones by thiourea leaching. Effects of thiourea concentration, leaching temperature, leaching time, and ferric ion (Fe3+) concentration on the recovery of gold and silver were investigated. The PCB was pretreated physically to reduce the thiourea consumption and enhance the leaching process. It was found that the percentage of gold leaching was higher than that of silver at different conditions studied. The highest percentages of gold (96%) and silver (90%) leachings were achieved with 20 g/L of thiourea and 8 vol% of Fe3+ at 4 h of leaching time and 25°C of leaching temperature.

Extraction and Stripping Behaviour Study on Gold Recovery from Printed Circuit Board

Single extraction was used to investigate the suitable condition parameters of temperature and agitation speed in extracting gold, Au (III) from printed circuit board (PCB). Extractant di-(2-ethylhexyl) phosphoric acid (D2EHPA) was mixed with thiourea leaching solution. From the extraction system, the optimised conditions are the temperature of 45°C and the agitation speed of 300rpm and obtained 88% of percentage of extraction (%E). In order to enhance the recovery of Au(III), three samples underwent stripping process, the results from three samples which are sample 1,2 and 3 showed the percentage of stripping efficiency (%S) were 50.37%, 29.58% and 25.64% respectively. The results of FTIR showed the presence of C-H and C-O bond in waste cooking oil and after addition of D2EHPA into the diluent the P-O-H, P-O-C and O-H vibration band were appeared.

Recovery and separation of silver and mercury from hazardous zinc refinery residues produced by zinc oxygen pressure leaching

Hazardous zinc refinery residues that contain Ag and Hg are a typical complex material obtained from zinc oxygen pressure leaching and currently there are few economically viable methods for disposal or reuse. The research presented here offers an effective approach for the comprehensive recovery of Zn, Fe, Cu, Ag and Hg from this highly toxic waste. During the initial hot-acid leaching stage, leaching efficiencies of 96.3% Zn, 96.0% Fe and 97.5% Cu were obtained, whereas the leaching of Ag and Hg were <0.2%. This resulted in the simultaneous separation of Zn, Fe and Cu and the enrichment of Ag and Hg ca. 300%. Subsequently, the leaching residues obtained from hot-acid leaching was further leached using acidic thiourea solution (pH = 1). The results of this second leaching step showed the almost 93% of Ag and 98% of Hg could be extracted using 20 g/L thiourea and 4 g/L Fe3+, with a L/S ratio of 8 and a temperature of 40 °C for 2 h. The residue after thiourea leaching could be used as raw material in lead smelting. In the next step based on zinc powder cementation, the recovery of Ag and Hg both reached 99.0% and the cementation residue comprised of 53.1 wt% Ag and 11.7 wt% Hg. The main phases present included Ag, Hg3Ag2 and Ag2S and these can be further treated by vacuum distillation in order to separate Hg from Ag. These findings demonstrate that high recoveries of Zn, Fe, Cu, Ag and Hg from the toxic waste could be achieved with stepwise leaching followed by zinc powder cementation. This treatment protocol for toxic zinc refinery residues not only avoids the potential harm to the environment but also significantly improves the economics of the process.