Leaching Step Research Articles

Process development for recovery of copper and precious metals from waste printed circuit boards with emphasize on palladium and gold leaching and precipitation

A novel hydrometallurgical process was proposed for selective recovery of Cu, Ag, Au and Pd from waste printed circuit boards (PCBs). More than 99% of copper content was dissolved by using two consecutive sulfuric acid leaching steps in the presence of H2O2 as oxidizing agents. The solid residue of 2nd leaching step was treated by acidic thiourea in the presence of ferric iron as oxidizing agent and 85.76% Au and 71.36% Ag dissolution was achieved. The precipitation of Au and Ag from acidic thiourea leachate was investigated by using different amounts of sodium borohydride (SBH) as a reducing agent. The leaching of Pd and remained gold from the solid reside of 3rd leaching step was performed in NaClO-HCl-H2O2 leaching system and the effect of different parameters was investigated. The leaching of Pd and specially Au increased by increasing the NaClO concentration up to 10V% and any further increasing the NaClO concentration has a negligible effect. The leaching of Pd and Au increased by increasing the HCl concentration from 2.5 to 5M. The leaching of Pd and Au were endothermic and raising the temperature had a positive effect on leaching efficiency. The kinetics of Pd leaching was quite fast and after 30min complete leaching of Pd was achieved, while the leaching of Au need a longer contact time. The best conditions for leaching of Pd and Au in NaClO-HCl-H2O2 leaching system were determined to be 5M HCl, 1V% H2O2, 10V% NaClO at 336K for 3h with a solid/liquid ratio of 1/10. 100% of Pd and Au of what was in the chloride leachate were precipitated by using 2g/L SBH. Finally, a process flow sheet for the recovery of Cu, Ag, Au and Pd from PCB was proposed.

The Effect of Si on the Formation of the ${\rm La}({\rm Fe},{\rm Si})_{13}$ Phase Synthesized by the Reduction-Diffusion (R/D) Process

In this contribution, the influence of Si content on the formation of the of LaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(13-x)</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(x)</sub> (1:13 phase) synthesized by the reduction-diffusion (R/D) process is addressed. The raw materials used were Fe and Si, in powder form, La oxide (La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), and Ca granules as reduction agent. The amount of Si per unit formula, represented by x, was varied from 1.3 to 1.7, for three different compositions. After a leaching step, subsequent to the actual R/D process and necessary to extract the reaction byproducts, such as CaO and nonreacted Ca, an alloy in powder form is obtained, with an average particle size of about 10 μm. In order to know the relative phase-amount evolution with the increasing Si content for these three different alloys, X-ray diffraction (XRD), supported by Rietveld refinement, was performed. Magnetic measurements carried out on a vibrating sample magnetometer (VSM) were used as an alternative means to infer the amount of formed 1:13 phase. The agreement found for the two methods (XRD and VSM) was good and showed that, for predetermined conditions, the amount of 1:13 phase formed, is directly related to the Si content added in powder form, as raw material. Moreover the transition-temperature (TC) of the target 1:13 phase, at which the magnetocaloric effect is highest, was accessed by differential scanning calorimetry (DSC) in order to check homogeneity and stoichiometry deviations from the predicted composition. The different values measured for distinct preparation conditions, point out to different Si contents incorporated to the targeted 1:13 phase, what directly affects phase formation and stability. The well-defined DSC-peaks, however, point out to good phase homogeneity.

Counter-current acid leaching process for the removal of Cu, Pb, Sb and Zn from shooting range soil

This research explores the performance of a counter-current leaching process (CCLP) for Cu, Pb, Sb and Zn extraction in a polluted shooting range soil. The initial metal concentrations in the soil were 1790 mg Cu/kg, 48,300 mg Pb/kg, 840 mg Sb/kg and 368 mg Zn/kg. The leaching process consisted of five one-hour acid leaching steps, which used 1 M H 2SO 4+4 M NaCl (20°C, soil suspension = 100 g/L) followed by two water rinsing steps. Ten counter-current remediation cycles were completed and the average metal removal yields were 98.3±0.3% of Cu, 99.5±0.1% of Pb, 75.5±5.1% of Sb and 29.1±27.2% of Zn. The quality of metal leaching did not deteriorate throughout the 10 remediation cycles completed for this study. The CCLP reduced acid and salt use by approximately 68% and reduced water consumption by approximately 60%, exceeding reductions achieved by a standard acid leaching process.

The Bayer digestion behaviour of transition aluminas formed from roasted gibbsite

Bauxite with reactive silica content greater than ~8% cannot be economically processed by the Bayer process. Roast-leach technology (sometimes called thermo-chemical processing) is potentially one of the options for removal of silica from bauxite using a thermal pre-treatment and leach step. However, the thermal pre-treatment transforms aluminium oxides in both kaolinite and valuable phases (gibbsite and boehmite) to transition aluminas which are difficult to extract using normal Bayer digestion. To better understand and solve this problem, we simplified the roast-leach process by roasting pure gibbsite and digesting the resultant transition aluminas. We investigated the relationship between roast temperature, dehydration extent, transition aluminas and Bayer extraction behaviour. We found that transition aluminas with thermodynamically relatively stable structures and better crystallinity were the cause of extraction difficulty. Extraction can be enhanced using higher caustic concentration and digestion temperature although these measures are not preferred in reality. The practical and effective method is perhaps the counter current double digestion which can extract all transition aluminas and produce pregnant liquor with desired aluminate concentration.

Recovering lead from cathode ray tube funnel glass by mechano-chemical extraction in alkaline solution

This study evaluates the efficiency of lead (Pb) extraction from cathode ray tube (CRT) funnel glass in strongly alkaline solution using mechanical activation in a ball mill as the chemical breakage and defects formed in the inner structures will contribute to the easy dissolution of the activated Pb glass. The combination of mechanical activation and a chemical leaching process in a single operation (mechano-chemical leaching) is more effective than the mechanical activation and subsequent chemical leaching. More than 97% of Pb in the CRT funnel glass can be extracted with a stirring ball mill leaching process in 5 M sodium hydroxide at 70°C. The diameter of the stainless steel balls as the activation medium is 5 mm; the mass ratio of ball to raw materials is 25:1. Pb powder with a purity of 97% can be obtained by electrowinning from the leaching solution. The Pb-depleted solution can be recycled into the leaching step. After Pb is removed, the solid leaching residues can be used for preparation of foam glass. Thus, a novel hydrometallurgical process for recovering Pb from CRT funnel glass in alkaline solution is proposed.

Two-step oxidation of a refractory gold-bearing sulfidic concentrate and the effect of organic nutrients on its biooxidation

Methods for improving the treatment efficiency of a refractory gold-bearing sulfidic concentrate are proposed. These methods consist of the oxidation of the concentrate during a two-step process, which includes a high temperature ferric leaching step and a subsequent biooxidation step, and the use of organic nutrients during the biooxidation step. The concentrate contained 34.7% pyrite and 7.9% arsenopyrite. The biooxidation of the concentrate (for a one-step process) was conducted at 45°C in two bioreactors that were connected in series under continuous conditions. The pyrite and arsenopyrite oxidation levels after 240h were 60.2% and 92.0%, and the gold recovery level by carbon-in-pulp cyanidation was 65.7%. The two-step process included the leaching of the concentrate by a biologically generated Fe3+-containing solution and the subsequent biooxidation of the leach residue. In this case, the pyrite and arsenopyrite oxidation levels after 240h of biooxidation were 65.7% and 94.1%, and the gold recovery level was 71.7%.The effect of an organic nutrient (yeast extract) on biooxidation during the two-step process was studied. The pyrite and arsenopyrite oxidation levels after 240h of biooxidation under mixotrophic conditions were 73.5% and 95.1%, and the gold recovery level was 77.9%. The effect of the organic nutrient on the microbial population was determined. Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus were the predominant microorganisms studied under both autotrophic and mixotrophic conditions. Archaeon Acidiplasma sp. MBA-1 was a minor component of the microbial community under autotrophic conditions but was one of the predominant microorganisms studied under mixotrophic conditions. These results suggest that the organic nutrient changed the composition and increased the activity of the microbial population.Thus, a two-step process with organic nutrients added during biooxidation may be considered as an effective strategy for treating refractory pyrite–arsenopyrite concentrates.

Leaching Characteristics of Organic and Inorganic Matter from Biomass by Water: Differences between Batch and Semi-continuous Operations

Over 30% and ∼2% (on a carbon basis) organic matter can be leached from mallee leaf and wood by water, respectively, producing acidic leachates containing organic acids. As a result, there are significant differences in the leaching characteristics of both organic and inorganic species in biomass between batch and semi-continuous leaching operations. Under conventional batch leaching, the acidic leachate continuously contacts the biomass for a prolonged period, resulting in the leaching of at least some water-insoluble inorganic species (e.g., organically bound) from biomass. Therefore, the batch leaching method clearly overestimates the amount of water-soluble inorganic species in biomass and exhibits two-step leaching kinetics, i.e. a rapid leaching step for an initial short period followed by a slow leaching step for a relatively long period. This study further develops a semi-continuous leaching method to address this issue via minimizing the contact between the leachate and the biomass sample. The se...

Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36Cl applied to coexisting limestone

This paper highlights potential complications that may arise while using in situ produced 10Be to date exposure or burial events using diagenetic silica (chert). The initiation and evolution of large gravitational collapses in sedimentary rocks were constrained using cosmic ray exposure dating. Because these collapses occurred in a stratigraphic level composed of chert (diagenetic silica) concretions interbedded in limestone layers, their development was studied by performing in situ-produced 36Cl and 10Be concentration measurements in both the limestone and coexisting diagenetic silica (chert), respectively. Following the routinely used decontamination and preparation protocols for 10Be produced in diagenetic silica, large discrepancies were observed with exposure ages determined by 36Cl within carbonate for samples originating from the same scarp. While 36Cl exposure ages were clustered as expected for a unique single gravitational event, 10Be exposure ages were scattered along the same studied scarps. To determine the origin of such a bias, petrological investigations were carried out for chert (diagenetic silica). Thin sections highlighted a complex mineralogical texture characterized by remnant silicified ooids showing calcitic cores, calcite inclusions and a dominant amorphous hydrated silica (grain > 20 μm). To decipher and characterize the potential origins of the excess measured 10Be within diagenetic silica, all samples were first reprocessed following the routine decontamination protocol (HCL–H2SiF6 leachings and three partial HF dissolutions) but starting from three different grain size fractions (GS1: 1000–500, GS2: 500–250 and GS3: 250–50 μm). The resulting concentrations clearly showed a decreasing 10Be content as a function of the grain size, but still yielded 10Be exposure ages significantly higher than 36Cl counterparts. Because potential adsorption of 10Be at the surface of amorphous silica grains was suspected, partial dissolution steps following by a leaching step in hydroxylamine were investigated. Finally, it seems that an additional leaching in KOH allowed removal of the amorphous silica phase and the measured 10Be concentrations yielded 10Be exposure ages agreeing within uncertainties with the 36Cl ones. This work suggests that measuring in situ produced 10Be within chert (amongst other types of diagenetic silica, e.g. flint, hornstone, jasper, etc.) containing amorphous silica requires caution.

Bioleaching and electrochemical properties of chalcopyrite by pure and mixed culture of Leptospirillum ferriphilum and Acidthiobacillus thiooxidans

The bioleaching of chalcopyrite was investigated using a pure and mixed culture consisting of iron-oxidizing Leptospirillum ferriphilum (L. ferriphilum) and sulfur-oxidizing Acidthiobacillus thiooxidans (A. thiooxidans). The electrochemical tests were conducted to investigate the bioleaching behavior of chalcopyrite by various bacteria. Bioleaching efficiency of chalcopyrite in mixed culture is higher than that in the pure culture of L. ferriphilum alone. The iron-oxidizing L. ferriphilum plays a dominant role during bioleaching of chalcopyrite in the mixed culture of L. ferriphilum and A. thiooxidans. During bioleaching, certain values of redox potential are beneficial to the decomposition of chalcopyrite. Jarosite and sulfur are observed as products of bioleaching. The addition of A. thiooxidans during leaching by L. ferriphilum can change the electrochemical control steps of leaching. The corrosion current density is substantially promoted in the culture involving bacteria, especially in the mixed culture.

Invertebrate shredder as a factor controlling the fixation potential for metals/metalloids in organic matter during decay

Wetlands are well known for water treatment and as a sink for elements (e.g. metals/metalloids). The elements are mainly fixed by sediments, microbial biofilms and plants, whereas the organic part of the sediment has a potential important role. These organic sediments underlie the process of organic matter decomposition, which proceeds in the three distinct steps of leaching, microbial conditioning and fragmentation by aquatic fauna. The effect of organic matter fragmentation on elemental fixation was shown previously only under neutral pH conditions. Hence, a batch experiment was conducted to assess the effect of invertebrate shredder on elemental accumulation during litter decay under low acidic conditions. Here it is shown that this fragmentation of the organic material by aquatic fauna enhances the elemental fixation potential of the organic sediments significantly, also under low acidic conditions. During the process of fragmentation the surface area of the organic matter increases by increasing amount of smaller particles. This in turn results in higher adsorption capacity of a larger amount of growing biofilm enhancing the surface even more (heterotrophic microorganisms). The growing biofilm (periphyton) with its larger surface area and hence more functional groups result in higher elemental fixation. Therefore, invertebrate shredders as ecosystem engineers enhance the capability for elemental fixation by fragmentation of organic matter, which is an important process in wetlands ecosystems usable in a broad pH range.

Production of copper cathode from oxidized copper ores by acidic leaching and two-step precipitation followed by electrowinning

A step by step hydrometallurgical process for the production of copper cathode was developed after a two-step precipitation from leaching solution of copper oxidized ore, followed by copper concentrate leaching and electrowinning. The copper oxidized ore was primarily comminuted to a size below 100μ, followed by acidic leaching at 25°C for 40min in H2SO4 solution, in which recovery of copper and iron were 95.95% and 12.63%, respectively. To remove iron impurity, at the first step of precipitation, NaOH was added to increase pH from about 1.5 to the optimum pH of 3.8 at 60°C for 60min; thus iron precipitation with recovery of over 80% was achieved. Copper precipitate as concentrate was obtained in the same method from iron-removed solution. The optimum condition of copper precipitation was found to be pH of 5.5, 25°C and 45min with 98.69% recovery. One of the advantages of this process was production of Na2SO4 with 99.1% purity after vaporization of the remaining solution from two-step precipitation. The obtained copper concentrate was leached at approximately the same condition of the first leaching step, and then the provided pregnant solution proceeded to an electrowinning cell with lead alloy anode contained antimony and steel sheet cathode under the following condition: temperature of 50°C, reaction voltage of 2V and current density of 300Am−2. Finally, a scale-up experiment was carried out and the copper cathode with 99.99% purity produced.

Dynamics of particle-size distributions in continuous leaching reactors and autoclaves

Leaching plays a central role in hydrometallurgical processing. However, frequently it is the leaching step that is the least efficient part of the hydrometallurgical process. Part of any drive to improve the efficiency of the operation therefore requires an understanding of the behaviour of leaching reactors. Surprisingly little work has been done on studying the effect of the particle-size distribution on the performance of leaching reactors. The objective of this work is to examine the impact of the mean and standard deviation of several different functional forms of the particle-size distribution on the performance of the reactor. Previous work showed that the Leaching Number, defined as (linear rate of shrinkage)×(residence time)/(mean particle size), is a key parameter in the performance of leaching reactors. The results of these calculations demonstrate the following: (i) the number distribution of the particles leaving the reactor reflects the interaction between particle-size distribution in the feed and the residence-time distribution, which is correctly described by the population balance; (ii) the mean size of the particles leaving the reactor can either increase or decrease, depending on the covariance of the particle-size distribution of the feed; (iii) changing the covariance over a wide range, from 0.1 to 1.8, increases the conversion by about 10%; and (iv) the Leaching Number is the primary variable in determining the performance of leaching reactors.

A novel sequential process of bioleaching and chemical leaching for dissolving Ni, V, and Mo from spent petroleum refinery catalyst

Two step leaching experiments were carried out to extract nickel, vanadium, and molybdenum present in spent refinery catalyst. A bioleaching process was applied in the first step. Pulp density, initial Fe(II) concentration, initial pH, particle size, and temperature were varied to optimize the bioleaching process. Ni, V, and Mo were leached out with maximum recoveries of 97%, 92% and 53%, respectively, at an optimized bioleaching condition of initial ferrous ion of 2g/L, initial pH of 2, pulp density of 10% (w/v), particle size of (−106+45) μm, and a temperature of 35°C. As the Mo leaching rate was very low, a second leaching step for the bioleached residue was applied with different concentrations of (NH4)2CO3, Na2CO3, or H2SO4. The second step leaching was optimum at a concentration of 30g/L (NH4)2CO3 with respect to Mo extraction. The rate of Mo dissolution with respect to concentration of lixiviant in the second leaching step was evaluated. The percentages of Ni, V, and Mo leached were 97%, 97% and 99%, respectively, by combining the first step under optimized conditions and the second step with 30g/L (NH4)2CO3.

The Effect of Shredding and Particle Size in Physical and Chemical Processing of Printed Circuit Boards Waste

Circuit boards present in most electric and electronic devices are very important components, which should be removed during sorting and dismantling operations in order to allow further adequate treatment for recovering valuable metals such as copper, nickel, zinc, lead, tin and rare elements. This recovery can be made by physical and chemical processes being size reduction by shredding the first step. In this paper, the effect of particle size in physical and chemical processing of printed circuit boards is presented and discussed. Shredding using cutting-based equipment allowed the comminution of boards and the liberation of particles composed by different materials (mainly metals and resin). Particle sizes less than 1 mm seems to be appropriate to attain high liberation of materials, which is crucial for the physical separation using gravity or electrostatic processes. Concerning chemical treatment, hydrometallurgical processing involves a leaching operation which can be also influenced by particle size of shredded boards. Samples with different granulometries were leached with 1 M HNO3 solutions, being leaching yields evaluated. It was concluded that particle size can be an important factor for the solubilization of some metals, but the effect is not similar for all elements. When average diameters change from 2.0 to 0.20 mm, nickel, aluminium and tin reactivity were not significantly affected, being this effect important for copper. Zinc behavior was very dependent from extreme particle sizes but was less affected in intermediate granulometries. Lead leaching showed also a peculiar behavior, exhibiting high and almost constant yields (80-90%) for particle size of solids up to 1.2 mm, and decreasing suddenly for higher granulometries. The effect of time on chemical reactivity for samples with different granulometries demonstrated that particle size affects reaction rates but eventually similar efficiencies can be obtained for long time periods. Therefore the relationship between results from shredding operation and chemical leaching step needs to be optimized, considering the balance between factors like consumption of energy during grinding operation and residence time in leaching.

Effects of process parameters on tungsten boride production from WO3 by self propagating high temperature synthesis

► We produced tungsten boride compounds by SHS method. ► Mg containing byproducts were leached out by using a hot aqueous HCl media. ► The ratio of W 2 B 5 /WB was found to be ∼2.0 containing minor phases of W 2 B and W. In the present study, the production parameters of tungsten boride compounds by self-propagating high-temperature synthesis (SHS) method and following leaching process were investigated. In the SHS stage, the products consisting of tungsten borides, magnesium oxide, magnesium borate, and also minor compounds were obtained by using different initial molar ratios of WO 3 , Mg and B 2 O 3 as starting materials. In the leaching step, Mg containing byproducts, i.e. MgO and Mg 3 B 2 O 6 , existed in the selected SHS product synthesized at 1:8:2.5 initial molar ratio of WO 3 :Mg:B 2 O 3 were leached out by using aqueous HCl solution to obtain clean tungsten boride compounds at different experimental parameters which are time, acid concentration and temperature. The acid leaching experiments of the SHS product showed that optimum leaching conditions could be achieved by using 5.8 M HCl at 1/10 S/L ratio and the temperature of 80 °C for 60 min.

Counter-current acid leaching process for copper azole treated wood waste

This study explores the performance of a counter-current leaching process (CCLP) for copper extraction from copper azole treated wood waste for recycling of wood and copper. The leaching process uses three acid leaching steps with 0.1 M H2SO4 at 75°C and 15% slurry density followed by three rinses with water. Copper is recovered from the leachate using electrodeposition at 5 amperes (A) for 75 min. Ten counter-current remediation cycles were completed achieving≥94% copper extraction from the wood during the 10 cycles; 80–90% of the copper was recovered from the extract solution by electrodeposition. The counter-current leaching process reduced acid consumption by 86% and effluent discharge volume was 12 times lower compared with the same process without use of counter-current leaching. However, the reuse of leachates from one leaching step to another released dissolved organic carbon and caused its build-up in the early cycles.

Carbon catalyst codoped with boron and nitrogen for oxygen reduction reaction in acid media

A carbon matrix codoped with boron and nitrogen atoms was synthesized for use as an oxygen reduction reaction (ORR) catalyst in acid media. The sol–gel route developed to produce CO2 adsorbents was modified for the synthesis of the present catalysts using boric acid, urea, activated carbon, and iron acetate precursors. The effects of doped boron and nitrogen atoms as well as iron species on ORR activity and selectivity were investigated by analyzing X-ray photoelectron spectra, field emission-transmission electron microscope images, X-ray diffraction patterns, Brunauer–Emmett–Teller surface areas, cyclic voltammograms, rotating disk electrode voltammograms, and rotating ring-disk electrode voltammograms. Both boron and nitrogen atoms were simultaneously doped into a carbon matrix by two heat-treatment steps, the first under N2 gas at 1173K and the second under NH3 gas at 1223K with the remaining boric acid-derived oxygen species that limited ORR activity. The remaining oxygen atoms were primarily bonded to the boron atoms and removed by adding iron acetate in the precursor dispersion, resulting in increased ORR activity. The iron species also played a role in increasing edge sites in the carbon matrix, whereas boron doping increased the amount of pyridinic nitrogen. The ORR activity was maximized by the simultaneous doping of boron and nitrogen atoms into the carbon matrix in the presence of iron species. The optimum mass fraction of iron for both ORR activity and selectivity was 0.014, and the activity and selectivity levels were retained after the acid leaching step and reheat treatment under Ar gas. Based on the experimental results, the edge sites in the carbon matrix containing pyridinic-nitrogen atoms near the boron atoms and/or the boron atoms were suggested to be responsible for the ORR activity.

Determinación experimental de los parámetros óptimos de operación en el proceso de electrolixiviación y electrodeposición secuencial de oro en soluciones ácidas de tiourea a partir de sulfuros concentrados

Se investigó a nivel de laboratorio las variables de operación (concentración de reactivos, pH y potencial de oxidación) del proceso de lixiviación de piritas auríferas procedentes de río Tablachaca (Ancash Perú) con Tiourea (TU) seguido de electro-deposición de oro. El estudio experimental, se desarrolló en tres etapas: Oxidación de Tiourea, Lixiviación de oro y Electro-deposición de oro. Como una alternativa a los agentes químicos, la oxidación de TU se realizó vía electrólisis para producir Disulfuro Formamidina (FDS), el cual, sirve como oxidante en la disolución de oro. El grado de conversión óptima de tiourea a disulfuro formamidina se obtiene en 2,5 h aplicando una FEM de 4,5 V con un pH igual 1,5. Los compartimientos anódico y catódico del reactor electroquímico fueron separados por una membrana aniónica con la finalidad de eliminar reacciones indeseables en el cátodo. Previo a la etapa de lixiviación, el mineral fue lavado con ácido sulfúrico para eliminar los agentes consumidores de ácido y mantener de este modo un pH constante durante el tiempo que dure la lixiviación. Durante la lixiviación de oro, se estudió las velocidades de disolución en función de las concentraciones de TU, FDS y pH. Finalmente, se efectuó una prueba de lixiviación en tiempos prolongados usando los parámetros óptimos de operación obtenidos en las pruebas anteriores. La prueba de electro-deposición se llevó a cabo usando la técnica de electrodiálisis en una celda de tres compartimentos El bajo consumo de Tiourea (1,76 kg/TM mineral) demuestra que es una alternativa viable desde el punto de vista económico.

Recovery of rare earths and base metals from spent nickel-metal hydride batteries by sequential sulphuric acid leaching and selective precipitations

This work describes a hydrometallurgical process to recover rare earth and basemetals from spent NiMH batteries. Laboratory tests were conducted by using NiMH powders obtained in an industrial scale grinding process after a preliminary sieving (<500 μm).The most important metals present in the initial powder were nickel (29% w/w), manganese (13% w/w), zinc (8% w/w) and rare earths (lanthanum and cerium, (5% w/w); other elements were iron (1% w/w), potassium and cobalt (2.5% w/w).The results obtained showed that two sequential leaching steps were needed to dissolve almost completely lanthanum and cerium present in the samples. In the leaching experiments three parameters were studied and optimized according to a factorial experiment: sulfuric acid concentration, acid citric concentration and temperature. The average dissolution yields after the second leaching step were 100% for manganese, cobalt, zinc and rare earths and 99% for Ni. A total recovery of RE 99% was obtained after selective precipitation.Rare earths were recovered by selective precipitation with sodium hydroxide at pH less than 2 after leaching: in these conditions a precipitate composed of lanthanum and cerium sulphates are produced.A suitable flowsheet to recover RE from NiMH spent batteries with recoveries larger than 99% has been proposed.

Use of rice husk for emission neutral energy generation and synthesis of solar-grade silicon feedstock

The potential use of rice husk as a source of energy and also for producing high-purity silicon has been investigated by a combination of pilot trials, theoretical calculations and laboratory experiments. Rice husk was combusted at 850°C. Mass balance together with the pilot results indicates an electricity generation potential of 1 MWh per tonne rice husk. The process also generates 180 kg rice husk ash (RHA) that is used for production of high-purity silicon through a hybrid pyrometallurgical–hydrometallurgical process. The process consists of three major steps: purification of RHA by leaching and roasting, reduction by magnesium and recovery of silicon through two steps of leaching. The silicon generation process steps were optimized, showing that about 85% of silicon in the RHA can be recovered. The silicon product has a total impurity of <0.7%, hence it may be used as a suitable feedstock for production of solar-grade silicon.