Alkaline Leaching Solution Research Articles

Immobilization of Arsenic from Novel Synthesized Scorodite—Analysis on Solubility and Stability

The solubility of scorodite synthesized in a novel atmospheric process was investigated. Stable scorodite (FeAsO4� 2H2O) particles were produced by introducing oxidizing gas into a reaction mixture containing ferrous sulfate and high-concentration arsenic (V) to convert ferrous ion to ferric ion. The obtained scorodite crystals released a very low, almost negligible concentration of arsenic in the pH range of 3 to 6, suggesting its long-term stability. The thermodynamic parameters for the dissolution reaction of scorodite, Ksp andGf, were similar to those reported previously. The results show scorodite released a considerably high concentration of arsenic under specific leaching conditions. In particular, the combination of CaO and NaCl present in the alkaline leaching solution had a significant effect on the mobilization of arsenic. Although this study did not cover all possible natural environments in which scorodite would be stored, it seems practically impossible to develop conditions that would completely prevent the dissolution of arsenic from scorodite. It is crucial to develop scorodite storage methods that will minimize the risk of environmental arsenic contamination based on accurate evaluation of the conditions for the leaching of arsenic. (doi:10.2320/matertrans.M-MRA2008844)

Tungsten recovery from alkaline leach solutions as synthetic scheelite

The recovery of tungsten from alkaline leach solutions has been studied examining the effect of temperature, pH, Ca/WO 3 molar ratio and nature of the precipitated calcium tungstate. The precipitation kinetics of calcium tungstate, upon the addition of aqueous sodium tungstate to calcium solutions, was followed by potentiometric measurements using a calcium ion-selective electrode. Two models, a crystal growth model and a second-order reaction opposed by zero-order reaction, have been used to test the experimental data. Both models show that the apparent activation energy of CaWO 4 precipitation falls in the range 58 to 67 kJ mol − 1 . The kinetic data shows that the maximum recovery of precipitated calcium tungstate occurs at pH ≥ 8.5 with a 10% excess of CaCl 2 at 50 °C over a period of 20 min using sodium tungstate solutions of 100 g L − 1 and 150 g L − 1 WO 3.

Mobility of trace elements from selected Australian fly ashes and its potential impact on aquatic ecosystems

Abstract Batch leaching tests have been performed on fly ashes collected from four Australian power stations fuelled by chemically different coals. Two acidic and two alkaline fly ashes were subjected to long-term (144 h) leaching tests, and the behaviour of As, B, Mo and Se was investigated to obtain data on their potential for mobilisation during fly ash–water interactions. All four elements are mobile under different conditions and over different leaching times. The concentrations of these elements released in leaching solutions with initial pH values of 4, 7 and 10 were used to assess the influence of pH conditions on element mobility from the acidic and alkaline fly ashes. The most mobile of the four elements leached were Mo from alkaline fly ashes and B from acidic fly ashes. Arsenic concentration increased with time in leachate solutions from acidic and alkaline fly ashes; however, in solutions in contact with alkaline fly ashes the As concentration, after reaching a maximum, later decreased with time. Selenium mobility shows a similar pattern to that of As, with similar leaching concentrations. Boron has the highest relative mobility of all four elements. A process possibly responsible for the decrease in concentration of B, As and Se in alkaline leaching solutions is the formation of ettringite. Equilibrium between the solid phase (ash) and the leaching solution was not reached in any of the leaching experiments. The pH of the leaching solution is the key factor affecting the mobility of these trace elements in these fly ashes.

Modified leaching and extraction of uranium from hydrous oxide cake of Egyptian monazite

A modified alkaline dissolution route of monazite was used to separate the hydrous oxide cake containing uranium, thorium, and rare earth elements from the phosphate matrix. It was found that uranium can be selectively leached by a mixture of sodium carbonate, sodium hydroxide, and hydrogen peroxide leaving thorium and rare earth elements as insoluble hydrous oxides. The factors affecting the extraction of uranium from the alkaline solution containing sodium carbonate and hydrogen peroxide were separately studied. Experimental results showed that tricarbonato uranium complexes are selectively extracted by Aliquat-336 diluted in kerosene from alkaline leach solution. Based on these results, uranium was purified from the leach solutions by extraction with Aliquat-336 diluted in kerosene with purity not less than 99%.

Determination of lanthanides (La, Ce, Nd, Sm) and other elements in metallic gallium by instrumental neutron activation analysis

This paper reports the analytical results of lanthanides and other elements in pure elemental gallium. The analyses were performed by instrumental neutron activation analysis (INAA) at the nuclear reactor IEA-R1, IPEN, São Paulo. INAA has the advantage of being a multi-elemental and non-destructive analytical method. After irradiation, the samples were set aside for some days before running high resolution gamma spectrometry with a hyper-pure Ge detector. Gallium was recovered from the Bayer process alkaline leach solution, named by local aluminum industry as ‘weak soda’, (spent liquor) with a concentration of about 150 mg l −1. As a first step, gallium was recovered from the ‘weak soda’ and enriched using a complexing ion-exchanger of the polyamidoxime type specially synthesized in the country for this purpose. After washing the column with water and then with pure sodium hydroxide to remove the interstitial ‘spent liquor’, gallium was eluted from the resin. The eluted gallium solution was made to 4 mol l −1 in NaOH and subject to electrolysis. The metal was then collected from the electrodes and given a final purification step by dilute inorganic acid stripping. The most representative lanthanide elements found and analyzed in the metallic gallium were La, Ce, Nd and Sm. One of the most recent samples exhibited the following results: (μg g −1) La (16.1), Ce (15.5), Nd (11.8) and Sm (3.6). Besides the lanthanides, the following elements were also analyzed: U, Cr, Fe, Co, Zn, Mo, Se, Sb and Ba. The results showed that the metal obtained is highly pure. The purity can be enhanced by stripping the metallic gallium with a dilute mineral acid.

Extraction of zinc from zinc ferrites by fusion with caustic soda

Zinc ferrites are one of the major forms of zinc in some wastes, such as steel mill EAF dusts and the leaching residues of roasted zinc sulfide concentrates. These ferrites can be very difficult to chemically decompose so that the zinc can be recovered. This decomposition is the key to the recovery of zinc in these solid wastes. In this work, the recovery of zinc from synthetic zinc ferrite was investigated. It was found that around 75–80% of the zinc in zinc ferrite can be extracted after being fused directly with NaOH pellets and dissolved in an alkaline leaching solution. The recovery increased to over 90% when the ferrite was hydrolyzed with water or dilute NaOH solution prior to the fusion step.

Removal of zinc cyanide from a leach solution by an anionic ion-exchange resin

The Merrill-Crowe process has been used for the precipitation of silver metal from cyanide solutions. Although the Merrill-Crowe system operates effectively most of the time, if the recycled solution contains a high zinc cyanide concentration, it leads to a decrease in efficiency of the solution as a solvent. In this study an anionic ion-exchange resin, Lewatit MP62, was used for the treatment of excess zinc cyanide from alkaline leach solutions. Ion-exchange tests were conducted in a laboratory-scale glass column using recycled water from the 100th Year Silver Cyanidation Plant in Turkey. It contains 474 mg/l Zn and 720 mg/l CN. The exchange capacity of the resin and the initial pH effect on the loading capacity were determined by breakthrough curves. It was found that zinc cyanide was completely removed from a recycled solution until it reached the breakthrough point.

Production of Zn powder by alkaline treatment of smithsonite Zn–Pb ores

In this work, a production process for Zn powder by alkaline treatment of an oxidized Zn ore, smithsonite (ZnCO 3), was studied. It was found that over 85% of both Zn and Pb, and less than 10% of Al can be leached from the ore when the leaching operation is conducted at over 95°C using 5 M NaOH solution as leaching agent. The dissolution of impurities such as Fe, Ca, etc., was negligible. Leaching of Pb can be improved remarkably with addition of NaCl to the leaching systems. Typical composition of the leach solution is 22–25 g/L Zn, 2.96–3.05 g/L Pb and 0.5–0.7 g/L Al. Zn and Pb contents in the leaching residue were found to be lower than 2.4% and 0.05–0.2%, respectively. Pb present in the leach solution is separated with the addition of sodium sulfide. The Pb-free solution is then used for the electrolysis of metallic Zn using stainless steel electrodes. Zn metal powder with purity higher than 99.5% is obtained. The specific energy for the Zn electrolysis in alkaline leach solution is around 2.4–2.6 kWh/kg Zn, which is lower than the energy consumption of 3.3 kWh/kg Zn required in the conventional process of Zn electrolysis in acidic sulfate electrolytes. The process developed is considered to be cost-effective, simple and easy to be operated and managed, and the flowsheet for the production of Zn from ores is presented. It is proposed that ores be broken into particles smaller than 0.1–0.5 mm. Then, leached with 5 M NaOH solution at a 90–95°C temperature for 1.5 h at a phase ratio [NaOH solution added (ml) to the ore leached (g)] of 6–7. After filtration, the leach solution is subjected to electrolysis for the production of metallic Zn after Pb is separated by sodium sulfide. The NaOH solution is recycled to the next leaching operation after most of the Zn is electrowon. To save energy and keep heat loss minimal, all operations can be conducted in an adiabatic environment, including leaching, filtration and electrolysis. Loss of NaOH was estimated to be less than 50 g for the production of 1 kg of Zn, assuming that most NaOH left in the leaching residues and precipitates of lead sulfide could be recovered by washing with dilute NaOH solution and water. However, loss of NaOH is dependent on the type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in ores would increase the loss of NaOH and deteriorate filtration of leach residues in the alkaline solutions. Therefore, most of the operational cost for this process comprises the consumption of electricity for electrolysis, NaOH, and energy required for heating.

Elimination of Vanadium and Arsenic from VKCs Catalysts

Vanadium and arsenic eliminations from an alkaline leaching solution of spent VKCs catalysts were studied. To this end, several commercial resins or polymers synthesized in the laboratory were evaluated in a solid–liquid batch extraction. Conditions for an effective extraction of vanadium and arsenic were determined. An extraction process, using a glass column, is described for the recovery of the vanadium. The elimination of arsenic by precipitation is proposed.

Extraction of Cesium from an Alkaline Leaching Solution of Spent Catalysts Using an Ion-Exchange Column

The selective extraction of cesium from an alkaline leaching solution of spent catalysts using phenolic resins was studied. The resins were synthesized by alkaline polycondensation of formaldehyde by phenol, resorcinol, catechol, and phloroglucinol. Their ionoselectivities for five alkali metals were evaluated with a solid-liquid extraction, and their ion-exchange capacities were compared. The resin with the best selectivity for cesium was tested with a real solution at different pH values. An on-column extraction is proposed to obtain cesium with high purity.

Uranium leaching from phosphate rock

Uranium in phosphate rock was removed by means of alkaline leach solutions. Ammonium carbonate/bicarbonate solution produced a very stable uranyl carbonate compound which was separated by centrifugation. Radiometric analysis showed that about 40% of uranium was solubilized and it can be recuperated. This process could be used before the manufacture of phosphatic fertilizers and the final products would contain smaller uranium quantities.

Formation of Secondary Minerals in Artificially Weathered Fly Ash

AbstractA leaching experiment was designed to evaluate the chemical, mineralogical, and morphological nature of precipitation products formed during the accelerated laboratory weathering of an alkaline fly ash. The ash, classified as alkaline Modic, was leached with 0.005 mol L−1 H2SO4 in a series of five leaching columns for a period of approximately 90 d. The weathered residues were characterized by x‐ray diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. Three types of precipitation products were identified in the ash. Calcite formed through the dissolution of Ca from the ash and subsequent reaction with CO2 absorbed by the initial alkaline leaching solutions. Iron, dissolved from the ash under acidic conditions, precipitated as amorphous coatings on fly ash particles. Aluminum and Si dissolved from the glass of the ash was translocated and precipitated within alkaline environments as an amorphous aluminosilicate material best described as proto‐imogolite. Results indicated that the products of weathering of fly ash in terrestrial environments are similar to those documented for alkali soils and some soils of volcanic origin.

A Method for Removing Ammonium Ions From a Subterranean Formation After In-Situ Uranium Leaching

Summary Ammonium carbonate and bicarbonate are the preferred carbonate sources in alkaline in-situ leaching of uranium. The ammonium ion exchanges into the clay in the formation and is difficult to remove during restoration operations. A new process is proposed which holds the potential for rapid and effective reduction of ammonia in the formation and groundwater to acceptably low levels. The process employs ph-adjusted, chlorinated water to decompose the ammonia quantitatively. The operation involves flushing the formation with connate water or brine, injecting chlorinated water, and finally flushing with connate water. This process is effective in laboratory tests. Introduction In-situ leaching, or solution mining, is now emerging as a viable technique for recovering uranium from some low-grade ore bodies. As a result, several pilot and commercial in-situ leaching plants are in operation in south Texas - e.g., Mobil Oil Corp., Intercontinental Energy Corp., Wyoming Minerals Corp., Union Carbide Corp., and U.S. Steel Corp.Leaching systems are classified conveniently as either acidic or alkaline. For sandstone ores containing substantial quantities of carbonates, alkaline leaching is preferred. The alkaline leach solutions contain an oxidant (H2O2, NaClO3, or O2) and a mixture of carbonates. Ammonium carbonates have been the most popular source of carbonates. This type of leaching formulation has been used successfully in south Texas.Since the ore bodies generally contain up to 20% or more of cationic exchangeable clays, the NH4+ in the leach solution exchanges with the cations in the clay according to the following equations. (1)(2) The selectivity of clays for NH4+ is high, and the NH4+ ion exchange capacity of the ore may become quite high depending on the overall clay content. A typical NH4+ ion exchange isotherm is presented in Fig. 1, which shows, for example, that for a leach solution containing 10 g/L of NH4HCO3, the equilibrium concentration of NH4+ on the ore is 0.14 meq/g, equivalent to an NH3 concentration of 0.24% of NH3 in the ore body. At the conclusion of the leach operation, the formation is contaminated with this great quantity of NH4+ ion. If not removed, the NH4+ ion will release slowly by exchanging with the incoming cations in the aquifer, resulting in pollution of the groundwater. Government regulations require that water in the leached formation be restored substantially to its original quality. Furthermore, the current Texas Dept. of Water Resources permit procedures require groundwater restoration be completed immediately upon completion of mining of the site. Although baseline levels for NH3 are not set yet, levels in the 10-ppm range have been suggested. Since the ammonia is exchanged into the clays chemically, it cannot be flushed out readily and requires counter ions, such as Na+ or Ca++, for removal by ion exchange. Furthermore, the ion exchange equilibrium limitation makes the restoration of the desired low level difficult and time consuming.To speed up the restoration process, a restoration fluid with high salt content of NaCl or CaCl2 can be used. JPT P. 921^