Recovery Of Ag Research Articles

Ultralight aerogel based on molecular-modified poly(m-phenylenediamine) crosslinking with polyvinyl alcohol/graphene oxide for flow adsorption.

Poly(m-phenylenediamine) is an excellent adsorbent material. Nevertheless, it is difficult to recover and can even generate secondary pollution due to its powder-like form. Aerogels, monolithic three-dimensional structured materials, that are ultralight and have porous properties are efficient at adsorbing contaminants from water and can solve these problems. Here, we synthesized an aerogel based on molecular-modified poly(m-phenylenediamine) (mPmPD) crosslinking with polyvinyl alcohol (PVA) and graphene oxide (GO) (GO/mPmPD/PVA). Of note is that 3-aminophenylboronic acid was introduced into the polymer structure to induce a crosslinking reaction between boric acid units and PVA to constrain poly(m-phenylenediamine) in the aerogel. The GO/mPmPD/PVA aerogel shows stable mechanical properties in aqueous solution and an effective adsorption capacity for Ag(i); the maximum Ag(i) adsorption capacity is 917.41 mg g−1. The mechanism of Ag(i) adsorption and reduction was clarified in that Ag(i) chelated with imine units, and a redox reaction occured between Ag(i) and the benzenoid amine units. Furthermore, the GO/mPmPD/PVA aerogel also shows excellent adsorption ability toward methyl orange and Congo red dyes. This GO/mPmPD/PVA aerogel shows enormous potential for application to silver recovery and dye removal due to its low cost, effective adsorption capacity and facile separation with aqueous solution.

Recovery of Silver from Dilute Effluents via Electrodeposition and Redox Replacement

In this study, the electrodeposition-redox replacement (EDRR) method was studied for the recovery of minor concentrations of silver from dilute solutions. The parameter optimization was carried out with synthetic solutions similar to silver oxide button battery recycling effluents, consisting of sulfuric acid and concentrated base metal (10 g·L−1 H2SO4, 60 g/L Zn2+) with a minor amount of silver (100 ppm) and a varying amount of Fe3+ ions. Results of these experiments were analyzed both electrochemically and by use of SEM-EDS. The role of dissolved Fe3+ ions was studied by varying the concentration from 0 to 1000 ppm and the results showed that although the presence of Fe ions decreased silver recovery efficiency, final product purity was found to increase slightly. The EDRR process was also found to be more effective for Ag recovery and has less energy consumption when Fe3+ concentrations are relatively low (≤ 100 ppm) when compared with conventional direct current electrowinning. In the final stage, silver was successfully recovered via EDRR, using the optimized conditions, from a real pregnant leaching solution (PLS) obtained from the leaching of silver oxide batteries.

Sustainable industrial technology for recovery of Al nanocrystals, Si micro-particles and Ag from solar cell wafer production waste

Solar cell wafer industry is classified as one of the most complex electronic industries that produces a significant proportion of waste in the form of broken/damaged cells or cells having some defects in their chemical composition that can generally be called Rejected Solar Cell Wafers (RSCWs). Although these wastes contain valuable metals (e.g. Silver (Ag), Aluminum (Al), and Silicon (Si)), unfortunately there is currently no obvious strategy for recovery of these metals and landfilling is considered the primary option for their disposal what can lead to serious soil contamination and human health risks if the waste is not treated properly. This research aims to develop an integrated technology to recover all these metals as high added value nano/micro products with total recovery rate > 98%, thus achieving the sustainability and goals of circular economy. Milling process was used as a pretreatment to break the chemical and mechanical bonds between Ag electrodes, Al electrode, and Si layer of RSCW samples to increase the surface area for reaction. The pretreatment was followed by leaching process using Nitric Acid (HNO3) with concentration > 60% to dissolve Ag and break the chemical bonds between spherical Al microparticles in the Al paste layer. Thus the particles were separated and only a small amount of the Al was dissolved from the surface of the particles since due to the high concentration of HNO3 passivation of Al occurred, forming a thin layer from Aluminum Oxide (Al2O3) that prevented the further dissolution of Al. Under the effect of soundwaves, the separated particles began splitting further and exfoliate into Al nanocrystals. After that, micro-filtration process was used to extract Si microparticles from the leached solution and the extracted particles were purified by etching process using Hydrofluoric acid (HF) to remove Silicon Nitride contamination. Centrifugal process was used to separate Al nanocrystals from the leached solution. Finally, Hydrochloric acid (HCl) was added to the remaining saturated solution to recover AgCl. SEM-EDS, FTIR, ICP, XRD, and TEM were used to analyze and examine the selected wafers and recovered materials. Also, the economic performance, sustainability, and CO2 emissions of the advanced technology were evaluated.

Extraction of Ag and Au from chloride electronic waste leach solutions using ionic liquids

Electronic waste (e-waste) contains valuable materials such as metals. These may be recovered using conventional pyrometallurgical or hydrometallurgical processes. However, these existing metal recovery techniques have a number of limitations such as adverse environmental footprints and high energy costs. This then necessitate an investigation into alternative methods or reagents. In this research, ionic liquids; [Bmim][Tf2N], [Bmim][PF6], and Cyphos 101 were investigated as extractants for Ag and Au from chloride aqua regia leach liquors of printed circuit board samples, with the composition of 162 gt−1 Au and 618 gt−1 Ag. The ionic liquids used were found to be efficient extractants for Au extraction from the leach liquor, with more than 90% of Au extracted in all the ionic liquids tested. Ag extraction was however, found to be lower compared to Au with no extraction recorded for [Bmim][Tf2N] but over 90% extraction was recorded for both Cyphos 101 and [Bmim][PF6]. This showed that ionic liquids are potential alternative extractants for the recovery of Au and Ag from chloride media. Considering both costs and efficiency, Cyphos 101 proved to be the most effective extractant for all metals from chloride solution at room temperature and 1:1 of the IL: sample ratio. The extractions were 95% Ag, 97% Au and 99% Cu under these conditions.

Efficient Recovery of Silver from Crystalline Silicon Solar Cells by Controlling the Viscosity of Electrolyte Solvent in an Electrochemical Process

We present electrowinning of silver (Ag) from crystalline silicon (c-Si) solar cells using a solution of methanesulfonic acid (MSA) as the electrolyte. Ag dissolved effectively in MSA because of its high solubility in MSA; however, the electrochemical recovery of Ag from MSA solutions was found to be inefficient because of the low mobility of Ag ions in MSA, owing to its high viscosity. Therefore, we decreased the viscosity of MSA by adding deionized (DI) water, as a possible method for enhancing the mobility of Ag ions. The concentrations of added DI water were 0, 1.1, 5.0, 9.3, and 20.8 M, respectively. Further, we performed cyclic voltammetry for each solution to calculate the diffusion coefficient using the Randles–Sevcik equation, and analyzed the viscosity of MSA solutions depending on the concentration of added water using a rheometer. The morphologies of the electrochemically recovered Ag particles changed with variations in the amount of the added water, from branch-like structures to dendritic structures with a decreasing size. Moreover, the cathodic current efficiency increased considerably with increasing concentration of the added DI water. Finally, we recovered Ag with >99.9% (3N) purity from c-Si solar cells by electrowinning, as determined by glow discharge mass spectrometry.

Silver recovery as Ag0 nanoparticles from ion-exchange regenerant solution using electrolysis

Many silver (Ag) containing consumer-products (e.g. textiles) release Ag into the environment, posing ecotoxicological risks. Ag recovery mitigates environmental hazards, recycles Ag, and leads to sustainability. In the present work, Ag has been recovered as Ag0 nanoparticles from the spent solution (thiourea (TU) ~0.5 mol/L pH ~1.1–1.2, and Ag ~550 mg/L) obtained from the regeneration of an Ag-loaded resin using a simple undivided electrolytic cell. The reclaimed regenerant solution has been recycled and reused in a closed-loop scheme over multiple cycles. The process parameters, i.e., current (0.05 A) and stirring speed (600 r/min), have been optimized for Ag recovery of ~94% and TU loss of ~2%. The reclaimed regenerant solution has been shown to regenerate Ag-loaded resin samples with >90% regeneration efficiency over 4 cycles of consecutive extraction and regeneration. The recovered Ag0 nanoparticles are monodisperse, consistently spherical in shape, and have a mean diameter of ~6 nm with standard deviation of the Gaussian fit as ~2.66 nm.

A cleaner process for valuable metals recovery from hydrometallurgical zinc residue

Most of hydrometallurgical zinc residues were stored in slag dump and unprocessed worldwide. Heavy metal and acid contained in these residues can be dissolved in rain water and then transfer to soil and groundwater system whereas threatening the health of aquatic systems and ecosystem. In this work, an innovative slag cleaning technique (reducing-matting smelting-thermal decomposition reduction) was proposed to harmlessly co-treat jarosite residue and lead silver residue, which can avoid environmental pollution and at the same time, effectively recover the valuable metal. Thermodynamic analysis and experimental results indicated that reducing atmosphere was a significant smelting parameter for recovery of valuable metal. Sulfur contained in various residues can be fixed in the form of solid sulfide (ferrous matte) without generation and emission of SO2(g). Moreover, the matte product is a good collector for silver collection. In addition, various smelting parameters were investigated in detail to detect and obtain the smelting optimum condition: 12% coke addition, FeO/SiO2 = 1.6, CaO/SiO2 = 0.4, smelting temperature 1300 °C and smelting time 2.5 h. Under these smelting condition, the recovery of Cu, Zn, Pb and Ag can reach 98.82%, 98.84%, 90.35% and 97.17%. The feasibility and reliability of this innovative process were confirmed. The main phases in the slag are CaFe·2SiO2,NaCaAlSi2O7 and silicate. It can be used in cement production so that the secondary pollution of slag can be avoided. Copper matte is comprised of FeS, Cu10(Pb,Fe)S6,Cu5FeS6 and Fe9S11. Ag is inlaid in copper mattes in the form of silver particles.

Production of Nanostructured Silver from Waste Radiographic Films Using a Microwave-Assisted Hydrothermal Method

Radiographic films, widely used in medical and dental diagnosis, are an excellent source of silver (Ag), due to their content of light-sensitive Ag compounds. After their use, the films are commonly incorrectly thrown away in the regular trash, causing potential environmental damage. Thus, sustainable methods for recovering Ag from discarded radiographic films are desirable for economic reasons and environmental preservation. In this study, we performed the Ag recovery from waste radiographic films and carried out its structural, chemical, and morphological characterization. First, a solution of commercial bleach, based on sodium hypochlorite (NaClO), was used to separate Ag from radiographic films. Then, a microwave-assisted hydrothermal method was applied to produce nanometric metallic Ag using sucrose as a green reductant and sodium hydroxide (NaOH). The obtained product was composed of relatively high purity (~ 97%) nanostructured Ag (NS-Ag). Thus, the proposed procedure was efficient, and a promising approach for Ag recovery from radiographic films since the obtained NS-Ag might be used in a wide range of technological applications.

Recovery of Gold and Silver from Secondary Sources of Electronic Waste Processing by Thiourea Leaching

Fine materials, such as flue dust, generated during processing of electronic waste (e-waste) contain valuable metal resources, such as Au and Ag. However, recovery of valuable metals from this flue dust has not been extensively studied. In this study, a leaching process was investigated for effective recovery of Au and Ag from flue dust. In particular, thiourea was used as leaching reagent to replace cyanide, which has environmental problems. The effects of various parameters on the process, such as leaching time (1–10 h), thiourea concentration (20–60 g/L), and pulp density (20–100 g/L) as well as reaction temperature (20–60°C), were systematically examined. One- and two-step leaching methods were proposed to recover Au and Ag, respectively. Au and Ag leaching efficiencies of about 90% and 100% were obtained by one-step thiourea leaching. However, metals such as Fe and Al were also dissolved together. In the two-step process, Fe and Al were dissolved using H2SO4 as a first step, then Au and Ag were recovered using thiourea leaching. Using this two-step process, Au and Ag were dissolved at higher purity than when using the one-step leaching process, achieving leaching efficiencies of about 98% and 100%, respectively. This investigation will lead to a cost-effective method for recovering precious metals from secondary waste.

High-voltage pulse crushing and physical separation of polycrystalline silicon photovoltaic panels

High-voltage pulse crushing technology combined with sieving and dense medium separation was applied to a photovoltaic panel for selective separation and recovery of materials. The panel was first separated into glass and back sheet layers by high-voltage pulse crushing through microexplosions or shock waves transmitted in the Al electrode and Si substrate (primary crushing step). Then the glass and bus-bar electrode could be separated from the encapsulant by high-voltage pulse crushing of the glass layer. The bus-bar electrode of the back sheet layer could also be separated by further high-voltage pulse crushing. After sieving the products obtained from the secondary crushing step of the glass layer, glass was mainly distributed in the size fraction range of 45–850 μm with a small amount of Si powder. However, purification of the glass (removal of Si powder) could be achieved by dense medium separation at a specific gravity of 2.4. Base metals, such as Cu, Sn, and Pb could be recovered in the large size fraction (1.0–8.0 mm). Ag used in the finger and bus-bar electrodes was highly condensed in the sieved product fraction with sizes of less than 20 μm, 2.0–4.0, and 4.0–8.0 mm, and its content exceeded 3000 mg/kg. However, the amount of Ag in these fractions represented only 33.2% of the total amount of Ag in the panel. Thus, to increase the Ag recovery ratio, other separation methods will be needed. We confirmed that dense medium separation at a specific gravity of 3.0 could achieve Ag condensation from the Si and glass, and that this represents a promising option for enhanced Ag recovery from crushed products.

Investigation on the recovery of gold and silver from cyanide tailings using chlorination roasting process

In this work, a chlorination roasting method for the extraction of precious metals (gold and silver) from the cyanide tailings (CT) was investigated. The decomposition mechanism of the CaCl2 used as chloride agent was presented during the chlorination roasting. The Au and Ag recovery as high as 91.6% and 54.7% under the optimum experiment conditions would be achieved. The optimal process conditions were identified to be a CaCl2 content 4%, chlorination temperature 1323 K and chlorination duration of 2 h. The microstructures of CT before and after roasting were examined by using scanning electron microscopy (SEM), and the volatilization process of the chloride of Au and Ag was assessed. The chlorination behaviors of all the phases in the CT in air atmosphere was analyzed using the thermogravimetric analysis (TGA), and the phase transformation of the main phase during roasting were analyzed by using X-ray diffraction (XRD). The chlorination roasting process shows unique advantage for treating the secondary sources of CT, additionally being environmentally benign.

Effect of Impurities in Precious Metal Recovery By Electrodeposition-Redox Replacement Method from Industrial Side-Streams and Process Streams

The recovery of metals from industrial side-streams and process streams is typically performed by solvent extraction, cementation or precipitation, chemical reduction, or electrowinning. However, several industrial process, side- and waste streams do contain also trace amounts of valuable metals such as Ag and Au at lower than ppm scale, making the economics of the recovery challenging. Recently, we have introduced electrodeposition-redox replacement (EDRR) method as a route to extract Ag from Zn process streams [1], where a successful recovery of Ag was possible even from ppb concentrations. In EDRR method, the base metal (such as Zn) is first deposited during the electrodeposition step, followed by a redox replacement step in the absence of applied potential or current. During this latter step, the deposited less noble metal is spontaneously replaced by a more noble metal (such as Ag) still present in the solution. As a result, Ag is enriched on the electrode. This paper concentrates on the effect of impurities on the recovery of valuable metals (Ag and/or Au) from solutions typical for hydrometallurgical industry, such as chloride, sulfate and nitrate based solutions. The measurements were performed in simulated process solutions in which metal and impurity concentrations were close to actual solutions of sulfate based Zn process solution [1], Doré slag leaching [2], or cyanide free gold chloride leaching [3]. The morphology and quality of the recovered metal product was determined by SEM-EDS and the EDRR results showed a high selectivity towards Ag and Au in all investigated hydrometallurgical solutions despite of the impurities present in the solution. Moreover, the impurities investigated were shown to speed up EDRR reactions, regardless of the concentration of the impurity - ppb, ppm or even g/L scale. Acknowledgements Academy of Finland (NoWASTE - Project No: 297962), EU Framework Program for Research and Innovation Horizon 2020 (MSCA-ETN SOCRATES - Grant Agreement No: 721385) and Finnish Steel and Metal Producers (METSEK-project) are acknowledged for financial support. The research utilized “RawMatTERS Finland Infrastructure” (RAMI, funded by Academy of Finland) based at Aalto University.

Bio-electrochemical system for recovery of silver coupled with power generation and wastewater treatment from silver(I) diammine complex

In this study, a bio-electrochemical system was used to investigate the recovery of silver from diammine complex [Ag(NH3)2]+, coupled with power generation and wastewater treatment, measured in terms of chemical oxygen demand (COD) removal. Acetate was oxidized by anodic microorganisms to produce electrons, which were transported to the cathode for the reduction of [Ag(NH3)2]+ complex. After 48 h of operation at different initial [Ag(NH3)2]+ concentrations (1000–3000 mg Ag/L), a Ag removal of over 96%, along with a COD removal of 60–76.80%, and a columbic efficiency of 8.55–14.69%, were achieved. Maximum power density slightly increased by 4.66%, from 3618 to 3795 mW/m3, when 10 mM NaNO3 was added to the catholyte containing 2000 mg Ag/L. The diffusion of [Ag(NH3)2]+ ions through the membrane affected the performance, as the inorganic fouling at the membrane surface increased the internal resistance. This transport was confirmed by the accumulation of silver in anodic biomass (0.853 mg Ag/g biomass) after 58 days. Scanning electron microscope (SEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) analysis demonstrated Ag deposits with a dendritic structure formed at the cathode surface. A deeper understanding on bio-electrochemical technology used for Ag recovery and electricity production from [Ag(NH3)2]+ complex was achieved in this study.

Facile synthesis of dispersed Ag nanoparticles on chitosan-TiO2 composites as recyclable nanocatalysts for 4-nitrophenol reduction

This paper presents a facile, rapid, and controllable procedure for the recovery of trace Ag+ ions and in situ assembly of well dispersed Ag nanoparticles on chitosan-TiO2 composites through bioaffinity adsorption followed by photocatalytic reduction. The prepared Ag nanoparticles are proven to be efficient and recyclable nanocatalysts for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4. Well dispersed quasi-spherical Ag NPs are synthesized in 20 min in the designed inner-irradiated photocatalytic system under a wide range of Ag+ concentrations (50–200 mg l−1), temperatures (10 °C–25 °C) conditions, and UV or visible light irradiation. The synthesized Ag NPs can catalyze the reduction of 4-nitrophenol by NaBH4 at 100% conversion in 120 min and preserve the catalytic activity in five successive cycles. This procedure for trace Ag+ ions recovery and Ag NPs assembly has the potential to be scaled up for the mass production of recyclable Ag nanocatalysts. The present work provides a green and efficient procedure for the conversion of hazardous 4-nitrophenol to industrially important 4-aminophenol and also sheds a light on designing scaled-up procedures for treating high volumes of wastewater with dilute heavy metals to produce recyclable metallic nanocatalysts in aqueous systems.

Enhancement of gold and silver recovery from discarded computer printed circuit boards by Pseudomonas balearica SAE1 using response surface methodology (RSM).

Two-step bioleaching was applied using a cyanogenic bacterium Pseudomonas balearica SAE1 to recover gold (Au) and silver (Ag) from the computer printed circuit boards (CPCBs) via central composite design of a response surface methodology (CCD-RSM). To enhance Au and Ag recovery, factors like pH level, pulp density, temperature and glycine concentration were optimized and their interactions were studied. CCD-RSM optimization resulted in 73.9 and 41.6% dissolution of Au and Ag, respectively, at initial pH 8.6, pulp density 5g/L, temperature 31.2°C, and glycine concentration 6.8g/L, respectively. Two quadratic models were proposed by RSM which can be utilized as an efficient tool to predict Au and Ag recovery through bioleaching. The experimental results are in line with the predicted results, indicating reliability of RSM model in enhancing the Au and Ag recovery from CPCBs. The increased bioleaching yield of Au and Ag from discarded CPCBs has its importance in industrial e-waste recycling and safe disposal.

The Use of Methyl Ethyl Ketone in Nitric Acid Leaching Processes for Enhancement of Ag Recovery from Used X-ray Films

The Use of Methyl Ethyl Ketone in Nitric Acid Leaching Processes for Enhancement of Ag Recovery from Used X-ray Films

Recovery of Ag and Au from synthetic and industrial wastewater by 2-step ferritization and Lt-delafossite process via precipitation

Due to the global rising demand there is an enormous interest to recover noble metals most efficiently from aqueous solutions, in particular industrial wastewater. Herein we demonstrate a new concept to recover Ag and Au from sulfate, chlorine and nitrate containing waters e.g. from catalysis production. Therefore two different treatment procedures, called 2-step ferritization and Lt-delafossite process, are presented which are based on a modified ferrite process. The phase assemblage can be controlled by the adjustment of the reaction parameters. During the 2-step ferritization process, Ag0 and Au0 are precipitated within one separate step. The precipitated phases obtained by the Lt-delafossite process are pure delafossite AgFeO2 and composite particles of Ag0, AgFeO2 and Fe3O4. By using these low energy, fast (<20 min) and easy to handle precipitation processes, Ag and Au can be recovered to ≥99.99% from wastewater. In consequence, these procedures help to avoid additional waste streams.

Evolution of High-Level Magmatic-Hydrothermal Systems: New Insights from Ore Paragenesis of the Veladero High-Sulfidation Epithermal Au-Ag Deposit, El Indio-Pascua Belt, Argentina

AbstractThe world-class Veladero high-sulfidation epithermal Au-Ag deposit is located in the Andean cordillera of Argentina near the northern end of the El Indio-Pascua metallogenic belt. The deposit comprises two nearly coalescing subhorizontal orebodies that are centered on an extensive zone of intense hydrothermal alteration. Intensely altered volcanic rocks are composed of fine-grained groundmass quartz that formed as a result of extreme acid leaching. These quartz grains contain ubiquitous rutile inclusions as well as healed microfractures of vapor-filled inclusions that record magmatic vapor streaming through the Miocene volcanic host succession. Condensation of the magmatic vapor into ambient groundwater generated the highly acidic waters responsible for the alteration. Alunite is present in the fine-grained groundmass quartz and fills vugs in the altered rocks. Stable isotope data indicate that the alunite formed through the disproportionation of SO2 in the condensed magmatic vapor.The fine-grained groundmass quartz is crosscut by later fracture-controlled euhedral quartz that is texturally associated with ore minerals. The euhedral quartz crystals show oscillatory growth zoning and contain rare primary fluid inclusions suggesting that quartz formation occurred at ~200°C from a moderately saline (&lt;5 wt % NaCl equiv) liquid-phase hydrothermal fluid. High-fineness native Au grains are hosted in euhedral quartzlined void spaces and along fractures. In addition to native Au, vugs and fractures in the silicified volcanic rocks host Fe oxide/hydroxide and jarosite that are interpreted to represent the oxidation products of hypogene sulfide minerals that formed during and after the late stages of quartz formation. Results of previous jarosite dating suggest that pervasive oxidation of the orebody commenced during the waning stages of the hydrothermal activity or immediately thereafter. Oxidation of the orebody continued in the supergene environment for at least 3 m.y. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) showed that jarosite, which formed as a result of the oxidation of the orebody, is the principal host for Ag in Veladero ore, explaining the low (ca. 10%) Ag recovery from the oxide ore.The Veladero high-sulfidation epithermal deposit is interpreted to have formed in the shallow part of a magmatic-hydrothermal system. Early alteration related to magmatic vapor discharge was followed by later mineralization from liquid-phase hydrothermal fluids under reduced and slightly acidic to near-neutral conditions. This change from early vapor-dominated to later liquid-dominated magmatic-hydrothermal fluid flow was key in formation of the deposit.

MERCURY POLLUTION ASSESSMENT OF MINING WASTES AND SOILS FROM FORMER SILVER AMALGAMATION AREA IN NORTH-CENTRAL MEXICO

Mining wastes and soils from Cedral, San Luis Potosi, Mexico, were studied to assess mercury (Hg) pollution in areas of historical silver (Ag) amalgamation and in areas where amalgamation mining wastes were recently reprocessed for Hg and Ag recovery. The total, soluble and bioaccessible Hg concentrations from eight mining waste and nine soil cores were determined at varying depths, which fluctuated from the surface up to 1.0 m depth. The total Hg concentrations in the mining wastes samples ranged from 8 to 548 mg/kg. The total Hg concentrations in the soil samples were lower (1 to 116 mg/kg) than those in the mining wastes. Eighty percent of the soil samples exceeded the maximum permissible limit for residential soils according to the Mexican regulations, demonstrating the impacts of the amalgamation process in Cedral. The soluble Hg concentrations were low, from 0.009 to 0.32 mg/kg in the mining wastes, and from 0.003 to 0.02 mg/kg in soils. The latter represents 0.007 % to 0.54 % and < 0.03 % of total Hg, from mining wastes and soils, respectively, indicating low aqueous Hg transport during rainfall events. The bioaccessible Hg concentrations in the mining waste samples (0.1 to 60 mg/kg) and soil samples (0.1 to 17 mg/kg), suggested that humans could be exposed to this toxic element through the accidental ingestion of mining wastes and soil particles. Although the total Hg concentrations in the mining wastes decreased after reprocessing, the soluble and bioaccessible Hg concentrations increased.

Silver Precipitation Using Sodium Dithionite in Cyanide Media

The nature of the reaction and the main parameters affecting Ag precipitation rate in the system Ag+-CN--S2O42- are studied. From the results obtained, a process of chemical precipitation for Ag recovery is proposed, using an environmentally-friendly reducing reagent (sodium dithionite, Na2S2O4), leaving a residual content of Ag into the permitted environmental levels (-1 Ag). The levels of CN- in liquids wastes are of the order of 0.816 mol L-1, and they can be treated by ozonization processes for inerting cyanide. The results obtained indicate that Ag precipitation is achieved in less than 1 hour, with recoveries close to 99%, at room temperature. Silver precipitation is characterized by an induction period followed by a precipitation period, leaving a metal amount in solution less than 1 mg L-1. The main kinetic parameters studied lead to obtain the following apparent reaction orders: with respect to the initial [S2O42-] was n = 1.675, with respect to [CN-] was -1.24, with respect to [OH-] was ≈0 (between 10-4 and 10-2 mol L-1) and with respect to the initial [Ag+] was 0.524. An activation energy of 77 kJ mol-1 was obtained. Under these conditions, Ag precipitation in the system Ag+-CN--S2O42- is controlled by chemical reaction, characterized by high activation energy, and is independent of the hydrodynamic variables of the system.