James L. Hendrix

A mathematical model for heap leaching of one or more solid reactants from porous ore pellets

A mathematical model is derived in dimensionless form for heap leaching of one or more solid reactants from nonreactive, porous, spherical ore particles. The model is for the interpretation of column and heap leaching data and for use in the design and scaleup of heap leaching pro-cesses. Data from experimental column leaching tests are presented which validate the model. The scope of the present study is limited to the case where the rate-controlling reagent is a component of the lixiviant solution only and not of the gas phase. The effects of particle-scale kinetic factors, heap scale and operating variables, competition between multiple solid reactants, and particle size distribution are examined using the concept of the heap effectiveness factor. It is shown that heaps operate either «homogeneously,» with distribution of reagent throughout the heap at all times, or in a «zone-wise» manner, depending mostly on a single parameter. The observed value of this parameter deviates from the predicted value in inverse proportion to the degree of contact effectiveness between the lixiviant solution and the ore particles. A rough correlation between the contact effectiveness and Reynolds number is generated from the simulation of column test results.

Affinity of activated carbon towards some gold(I) complexes

Abstract The adsorption of dicyanoaurate(I), dithioureaaurum(I), dithiocyanatoaurate(I), and dithiosulfatoaurate(I) complexes onto activated carbon has been investigated. A flow-through miniature column connected on-line to a flow injection analysis system with a flame atomic absorption spectrometry detector provided information on the relative rates and extent of adsorption of gold(I) complexes. The corresponding adsorption isotherms have also been determined. Scanning electron micrographs of the barren and loaded carbon surfaces (from K[Au(SCN)2]) are presented. These show that Au(SCN)2−1 is reduced to metallic gold on the carbon surface. It is established that the effectiveness of activated carbon in removing complexed gold from aqueous solutions decreases in the following ligand order: SCN− > SC(NH2)2 > CN− ⪢ S2O32−.

A General Model for Leaching of One or More Solid Reactants from Porous Ore Particles

A general, unsteady-state model for leaching of one or more solid reactant species from a nonreactive, porous, spherical pellet is derived in dimensionless form for the interpretation of leaching data and for use in the design of coarse-ore leaching processes. Numerical solutions of the model equations are obtained with implicit finite difference approximations. The effects of diffusion rate, chemical reaction rate, and competition between multiple solid reactants are examined using the concept of the effectiveness factor. The importance of deposits of solid reactants on the pellet surface is ascertained. It is shown that the model is capable of simulating both “reaction-zone” and “homogeneous” kinetics, depending on the choice of parameters. The model is also capable of simulating leaching from a distribution of particle sizes, and techniques for the modification of model parameters for this purpose are discussed.

Comparative catalytic activity of selected metal oxides and sulfides for the photo-oxidation of cyanide

Abstract The photocatalytic oxidation of cyanide by TiO 2 (anatase and rutile), ZnS, ZnO, CdS, V 2 O 5 , SiO 2 and Fe 2 O 3 was investigated and compared with the photo-oxidation in the absence of catalyst. Only TiO 2 and ZnO are satisfactory catalysts. Cyanide is first oxidized to cyanate and then via nitrite to nitrate. In most systems cyanide is also volatilized as HCN. The amount of HCN volatilized is a function of the pH of the solution and the surface area of the catalyst and generally increases with decreasing pH and increasing surface area.

Photocatalytic oxidation of cyanide to nitrate at TiO2 particles

Abstract The photocatalytic oxidation of cyanide at titanium(IV) oxide powder induced by UV irradiation was investigated. It is verified that cyanide is first oxidized to cyanate. However, contrary to earlier studies, it is established that cyanate is subsequently further photocatalytically oxidized via nitrite all the way to nitrate. Experimental conditions were varied and the effects of various sparging gases and pH, as well as the concentrations of both cyanide and photocatalyst, were investigated. As nitrate is a significantly less hazardous contaminant than cyanide, complete photocatalytic oxidation of cyanide to nitrate may be a viable method for decontamination of cyanide wastes.

Rapid Distillationless "Free Cyanide" Determination by a Flow Injection Ligand Exchange Method

In the first part of this research, extensive species-dependent cyanide recoveries studies were performed using the approved standard methods available for determination of free cyanide. The data obtained show that serious problems are associated with both the CATC (cyanide amenable to chlorination) and WAD (weak and dissociable cyanide) methods. In the second part, a novel flow injection gas-diffusion method for the determination of free cyanide was developed. Complete cyanide recoveries even in the presence of a large excess of the free CN- ion were found for the following species : [Zn(CN) 4 ] 2- , [Cd(CN) 4 ) 2- , [Nl(CN) 4 ) 2- , [CU(CN) 4 ] 3- , [Ag(CN) 2 ] - , [Hg(CN) 4 ] 2- , and Hg(CN) 2 . No recoveries of CN- were obtained from the species that are considered as non free cyanide producing ones, such as [F e (CN) 6 ) 3-/4- , [Co(CN) 6 ) 3- , and [Au(CN) 2 ] - . The method developed is rapid, selective, reproducible, and easy to automate.

Photocatalytic oxidation of aqueous ammonia (ammonium ion) to nitrite or nitrate at TiO2 particles

SummaryThe photocatalytic oxidation of ammonia (ammonium ion) in the presence of air or pure oxygen at titanium(IV) oxide powder surfaces and induced by UV light was investigated. Experimental conditions were varied and the effect ofpH, sparging gas, as well as the concentration of both ammonia (ammonium ion) and photocatalyst were investigated. It has been established that the principal product of photocatalytic oxidation depends onpH, and is either nitrite or nitrate. A gas-diffusion flow injection analysis system with conductivity detection was used for on-line monitoring of ammonia concentration. In addition, ion-chromatography with UV detection was utilized for offline monitoring of nitrite and nitrate concentrations. The use of a photocatalyst such as TiO2 may prove to be a viable method for conversion of ammonia (ammonium ion) to either nitrite or nitrate.ZusammenfassungEs wurde die photokatalytische Oxidation von Ammoniak (Ammoniumion) mittels Luft oder reinem Sauerstoff an Titan(IV)oxid-Pulveroberflächen unter Induktion von UV-Licht untersucht. Die experimentellen Bedingungen wurden variiert, wobei der Effekt despH-Wertes, des Spülgases und sowohl die Ammoniak- als auch die Photokatalysator-Konzentration berücksichtigt wurde. Dabei ergab sich, daß das Hauptprodukt der photokatalytischen Oxidation vompH abhängt und entweder Nitrit oder Nitrat ist. Ein Gasdiffusions-Fließinjektions-Analysensystem wurde zur on-line-Kontrolle der Ammoniak-Konzentration verwendet. Off-line-Ionenchromatographie mit UV-Detektion wurde zur Konzentrationsbestimmung für Nitrit und Nitrat herangezogen. Die Verwendung von Photokatalysatoren wie Titanoxid könnte sich als gangbarer Weg erweisen, um Ammoniak (Ammoniumion) entweder zu Nitrit oder zu Nitrat umzusetzen.

Characterization of Carlin-Type Gold Ore by Photoacoustic, Raman, and EPR Spectroscopy

Low-grade carbonaceous gold ore (0.01 to 1.8 oz/ton of gold) from the Carlin mine in Elko county, Nevada, has been characterized by a variety of analytical and spectroscopic techniques with emphasis on vibrational spectroscopy. A particular concern was determining the nature of the carbon-containing material in the ore. It was found that in addition to the anticipated carbonate, finely disseminated elemental carbon is present in amounts ranging from 0.062 to 0.43 wt. %. Contrary to previous reports, no detectable amounts of humic acid materials or organic hydrocarbons are present in any of the samples investigated.

Photocatalytic detoxification of cyanide and metal cyano-species from precious-metal mill effluents

Photochemically-induced detoxification of various cyanide species from a selected precious-metal mills tailings was investigated in detail. The studies were conducted in the presence or absence of a titanium (IV) oxide semiconductor photocatalyst utilizing simulated sunlight as the irradiation source. It was established that the cyanide ion (both free and complexed) was photocatalytically oxidized to nitrate via cyanate and nitrite. In addition iron, copper, zinc, mercury and arsenic were stabilized as precipitates. The results obtained may have ramifications for the possible use of solar energy for the efficient treatment of large quantities of precious-metal mill-tailing wastes that contain various cyano-species.

Indirect determination of chloride by gas-diffusion flow injection with amperometric detection

A rapid, indirect gas-diffusion flow injection (FI) method with amperometric detection has been developed for the selective and sensitive determination of Cl–. The method is based on permanganate oxidation of Cl– to chlorine. The chlorine diffuses through the micro-porous membrane and is quantified amperometrically at a platinum working electrode. Calibration graphs were linear up to the maximum concentration of Cl– investigated (10 mmol dm–3). The precision of the technique was better than a relative standard deviation of 1% at 2 mmol dm–3 levels and better than 2% at 10 µmol dm–3, with a throughput of 30 samples h–1. At elevated temperatures (50 °C) and higher acidities (5 mol dm–3 H2SO4), the detection limit was 0.1 µmol dm–3(0.7 ng of Cl–). The effects of temperature, sample acidity, working potential and interferents on the FI signals were studied. The method was successfully applied to the determination of Cl– in natural and tap waters.