Achilles Junqueira Bourdot Dutra

The influence of the oxidation state of pyrite and arsenopyrite on the flotation of an auriferous sulphide ore

In this paper, the influence of the oxidation state of pyrite and arsenopyrite on the flotation of an auriferous sulphide ore was investigated by different techniques, including electrode, pulp potentials, DRIFT spectroscopy and microflotation tests. In addition, the gold and sulphide recoveries were also investigate, in a laboratory-scale cell, as a function of pulp potential and reagent concentration. It was verified that the presence of oxidation products on the sulphide mineral surface demands a higher collector concentration in order to achieve a satisfactory sulphide recovery. The electrochemical behaviour of pyrite and arsenopyrite indicated that a nitrogen atmosphere can lead to a low pulp potential, which inhibits the formation of oxidation products, enhancing both the free gold and sulphide recoveries. Although copper sulphate activates even oxidised sulphide surfaces, it does not improve free gold recovery.

Gold electrowinning from thiourea solutions

The use of thiourea as an alternative non-polluting reagent for gold leaching is very well known. However, the processes of gold recovering from thiourea solutions have received little attention, particularly the electrowinning process. Gold electrowinning from thiourea solutions was studied by linear sweep voltammetry and chronopotentiometry. For potentials more cathodic than 0 mV vs. SCE gold reduction is mass transport controlled. The estimated diffusion Coefficient for the complex gold (I)-thiourea, at 25°C was 1.0 × 10−5, cm2s−1. The influence of gold, thiourea and formamidine disulfide concentrations on the cathodic behavior ofgold is described. A passivation due to thiourea adsorption on the electrode surface was observed at −300mV vs. SCE. Experiments shown that gold reduction is favored by low thiourea and formamidine disulfide concentrations. Chronopotentiometric measurements indicated the presence of an adsorption step before the reduction of the complex gold (I) -thiourea, although the reduction of the species in solution is also feasible. The influence of current density on the cathodic current efficiency and gold structure is presented.

Cadmium removal from diluted aqueous solutions by electrowinning in a flow-by cell

Abstract Mineral processing and metal finishing industries produce large amounts of effluents containing a host of heavy metals - cadmium, lead, nickel and chromium - with cadmium as a heavy hazard, since, even in mild concentrations, it may give rise to serious environmental problems. Effluents from electroplating industries and from zinc ore hydrometallurgical processing may contain this metal in the range of 15 to 100 ppm. Cadmium harm to all living systems is aggravated by its strong cumulative effect, because of the long number of years required for its elimination. The electrolytic waste treatment has shown advantages over the classical chemical precipitations by alkalis or carbonates, because it does not produce bulky sludges, which require land for its accumulation, and which may have a retarded pollution effect, due to the redissolution by rain; a great advantage comes from the possibility of the metal recovery in a commercially pure form. In this paper, a laboratory scale electrolytic flow-by cell with a reticulated vitreous carbon (RVC) cathode, developed in our laboratory, is described, which permitted the purification of aqueous solutions containing cadmium ion on the usual order of concentration of metallurgical industries effluents; with this cell, operating in a mass transport regime, it has been possible to decrease cadmium concentration of solutions of 2 liters volume from 200 to 0.1 ppm, with current efficiencies up to 40%, in a matter of 85 minutes.

Gold electrowinning from diluted cyanide liquors: performance evaluation of different reaction systems☆

This work deals with the performance of porous electrodes by using electrolytic cells with similar characteristics to the Zadras one and with mechanical devices to increase the mass transport to the cathode. The flow-by configuration was the experimental arrangements used. Gold electrowinning was carried out with distinct cyanide solutions, namely: pure synthetic solution and synthetic solution with metallic impurities. This study aimed at investigating the effect of some parameters on the direct electrowinning of gold from cyanide solutions and a comparative study of two kinds of three-dimensional electrodes (stainless steel mesh and mild still wool). The current efficiency and the energy consumption were also analysed considering three competing cathodic reactions: gold deposition, oxygen reduction and hydrogen evolution. A mathematical model was also evaluated aiming at determining, experimentally, the mass transport coefficients considering the reaction systems under study.

O processo eletroquímico como alternativa para o tratamento de efluentes cianídricos

Although cyanide is widely employed for leaching precious metals and electroplating certain metals, its high toxicity demands efficient technologies to treat the wastewater generated by these processes. There are a number of available techniques for the treatment of industrial aqueous cyanide-bearing effluents. However, the most employed method is based on its chemical oxidation to the less toxic cyanate which is hydrolyzed to ammonium and bicarbonate ions. The heavy metal complexes, when present in the effluent, will form a sludge composed of hydroxides. Among the most recent techniques for the treatment of effluents containing cyanide, electrochemical oxidation presents some advantages, such as: possible metal recovery from the cyano-complexes, no need for further chemicals additions, compliance to environmental agencies directives. This paper presents a comparison among several processes for treating the aforementioned effluents, with emphasis on the electrochemical process, which is briefly reviewed.

Influência da temperatura de calcinação na resistência à corrosão do eletrodo de Ti/SnO2-Sb para eletrooxidação de fenóis em meio de cloreto

Neste trabalho foi investigada a eficiencia e a resistencia a corrosao do eletrodo de Ti/SnO2 dopado com Sb, preparado pelo metodo Pechini, para oxidacao de fenol em meio de cloreto. Os resultados mostraram que sua resistencia a corrosao depende da temperatura de calcinacao, da morfologia do revestimento e do controle do pH durante a eletrolise. Apos 60 min de eletrolise em solucao 0,34 mol.L-1 de NaCl na presenca de 100 mg.L-1 de fenol a uma densidade de corrente fixa de 10 mA.cm-2, com pH controlado entre 7 e 9, as micrografias de MEV mostraram que apenas o eletrodo calcinado 600oC nao sofreu corrosao. Uma reducao de 90% da absorbância do fenol (269,5 nm) foi obtida apos 60 min de eletrolise a uma densidade de corrente fixa de 10 mA.cm-2 usando o eletrodo calcinado a 600oC.

Eletrorrecuperação de cobre e oxidação de cianeto de efluentes cianídricos diluídos gerados por unidade de galvanoplastia

Despite their high toxicity, cyanide electrolytes are widely used by metal finishing industries due to the quality of metallic deposits obtained. At the Brazilian Mint, the process of copper electroplating onto carbon steel is used to make coins of one and five cents, generating, at the end, two types of wastewaters: one concentrated, after bleeding the contaminated electrolytes; and another diluted, after washing the disks electroplated with copper. In a previous work the possibility of electrolytically recovering copper and oxidizing part of the existing cyanide in the concentrated effluents was evaluated, reaching good results for the final concentration of copper and free cyanide starting with a solution containing 26g/L of copper and 27g/L of total cyanide. The present work aimed at testing in a bench scale an electrolytic cell for the treatment of diluted effluents, starting from a synthetic solution containing 200 mg/L of copper and 130 mg/L of free cyanide. Under flow conditions of 0.37mL/s at 50°C, the final concentrations of 0.7mg/L for copper and 0.08mg/L for free cyanide were obtained for an electric current of 1.5A, and less than 0.5mg/L for copper and 0.08mg/L for free cyanide were obtained for an electric current of 2A, after three hours of electrolysis.

Eletroquímica da pirita e da arsenopirita na presença de amil xantato de potássio

Gold is frequently found in nature associated with sulfides, as pyrite and arsenopyrite. Usually, these sulfides are concentrated by flotation and submitted to posterior hydrometallurgical treatment. The flotability of the sulfide minerals depends on the hydrophilic/hydrophobic balance, which is induced respectively by the oxidation products formed on the minerals surface and by the collector adsorption that can not be completely interrupted by the presence of oxidation products. In the present work, rest potentials measurements for pyrite and arsenopyrite indicated that after the addition of hydrogen peroxide solution 10 % v/v in a 1 mL/min rate, these minerals were oxidized to Fe(OH)3 and FeOOH, respectively. Cyclic voltammetries indicated that ferric xanthate and dixanthogen are the species responsible for the hydrophobicity of pyrite and arsenopyrite respectively. Furthermore, the minerals oxidation leads to the formation of those species at more elevated potentials. Rest potentials measurements indicated that the formation of Fe(OH)3 on the pyrite surface due its oxidation is inhibited when this mineral is connected to arsenopyrite. The rest potential of arsenopyrite in the presence of nitrogen bubbling is more elevated when connected to pyrite, indicating its anodic behavior.

Electrochemical depression of galena aiming at selective sulfide flotation

In this article, the potential at which galena depression occurs is related to the formation of hydrophilic compounds on its surface. Reactions are proposed based on analyses by cyclic voltammetry, thermodynamic data, and X-ray photoelectron spectroscopy (XPS). Electrochemical variables, such as the redox potential, present good perspectives for selectivity in the flotation of galena and sphalerite, because in a given range of potentials, the galena surface becomes hydrophilic, as shown by measurements of the contact angle and floatability tests.

Caracterização de catalisadores automotivos novos e usados visando à reciclagem dos metais

A platina e os metais do grupo da platina (MGP) possuem importantes aplicacoes em catalise. Os catalisadores automotivos geralmente possuem platina, paladio e rodio, que sao uteis na conversao dos gases nocivos produzidos durante a queima do combustivel, em gases menos prejudiciais ao meio ambiente. A demanda pela platina e metais do seu grupo, aliada a sua escassez na crosta terrestre e altos valores de mercado, tornam a sua reciclagem fundamental. Neste contexto, uma vez que nao existe uma formulacao padronizada para a sua fabricacao, a caracterizacao dos catalisadores automotivos e uma etapa primordial para a selecao adequada da rota de reciclagem a ser adotada. Via de regra, os catalisadores monoliticos virgens usados na industria automotiva possuem em torno de 1,89 g/kg de platina, 0,29 g/kg de paladio, alem de 0,10 g/kg de rodio, metais preciosos de altissimo valor agregado. Neste trabalho foram caracterizadas duas amostras de catalisadores automotivos, uma virgem e outra proveniente de um catalisador usado. A microscopia optica, microscopia eletronica de varredura com sistema de microanalise por dispersao de energia, difracao e fluorescencia de raios-X, alem de TG-DTA foram as tecnicas analiticas empregadas neste trabalho. Foi observado que os catalisadores sao constituidos por uma estrutura na forma de colmeia, constituida de cordierita (Mg2Al4Si5O18), recoberta com uma camada de alumina impregnada com os metais do grupo da platina. No catalisador virgem, detectou-se por fluorescencia de raios-X, cerca de 0,99 g/kg de platina, enquanto que no catalisador usado o teor de platina foi de 0,44 g/kg. Na amostra do catalisador exaurido nota-se a deposicao de material carbonoso sobre a camada de alumina, alem da presenca de trincas, que podem levar a perdas dos metais preciosos e queda de atividade do catalisador.