M. A. Fernández

Characterization of the bottom ash in municipal solid waste incinerator

Abstract The particles with diameter >1 mm present in the bottom ash of Municipal solid waste incinerator (MSWI) were characterized by identifying the main constituent materials. This characterization may be used to evaluate the potential applications of bottom ash and its environmental hazards, and to evaluate the possibilities of recycling its main components. The effectiveness of the voluntary recycling programs of bottom ash can also be assessed. The main components of the bottom ash are glass, magnetic metals, minerals, synthetic ceramics, paramagnetic metals and unburned organic matter. The 4–25 mm size fraction accounts for approximately 50% of the bottom ash weight and comprises mainly glass (>50% of this fraction), synthetic ceramics (>26%) and minerals (>8%), and thus appears to be suitable for reuse as secondary building materials or for glass recycling. Magnetic metals accumulate in the 1–6 mm particle size fraction (6% of this fraction). Heavy metals accumulate in the fraction under 1 mm, unlikely the acid-soluble fraction, which diminishes as particle size diminishes.

Kinetic study of carbonation of MgO slurries

Abstract Carbonation of MgO slurries at atmospheric pressure using natural magnesite from Navarra, Spain, was studied. Two processes were observed: the formation of a magnesium bicarbonate solution and the precipitation of magnesium carbonate. Under different conditions, each process was favoured so as to find the optimum conditions to produce the Mg(HCO 3 ) 2 solution that acts as raw material in the production of basic magnesium carbonate. In the absence of precipitation, the kinetic data were examined according to the plot of rate equation 1−(1− X MgO) 1/3 against time. The activation energy of 29.1 kJ/mol suggested a chemical reaction controlling step. A mechanism for carbonation is proposed. The effect of specific surface area on the MgO conversion, as a function of time and temperature of calcination, was also investigated.

Selective leaching of arsenic and antimony contained in the anode slimes from copper refining

Abstract Selenium, tellurium, silver and gold are recovered from the anode slimes of copper electrorefining. Silver-copper selenide-telluride phases must be previously oxidized to obtain selectively leachable compounds. Arsenic and antimony, present in anode slimes as AsSb oxidized compounds, may be selectively and almost completely dissolved in 0.4 M KOH at 80δC. After this extraction, alkaline roasting of anode slimes in the presence of K 2 CO 3 at 600°C solubilizes Se (99%) and less than 2% As and 0.1% of Sb. After Se leaching, Cu and Te can be dissolved in 1.2 M HCl at 25°C as well as in CuSO 4 H 2 SO 4 solution. The residue contains BaSO 4 , PbSO 4 , SiO 2 , Au, Ag and small amounts of Se, Cu and Te and can be smelted in order to obtain a bullion for Ag and Au recovery by the conventional electrorefining process.

Gold cyanidation using hydrogen peroxide

Gold cyanidation, using hydrogen peroxide as an oxidising agent, has been studied by performing kinetic experiments, open circuit potential measurements and voltammetry. Small amounts of hydrogen peroxide make the cyanidation rate lower than that in conventional cyanidation, but higher amounts can make the cyanidation rate double. At low temperatures, the process is controlled by a chemical reaction, whereas at higher temperatures, it is controlled by diffusion with higher rates with respect to conventional gold cyanidation at atmospheric pressure. As the presence of hydrogen peroxide displaces the mixed potential of cyanidation to more anodic potentials, where oxygen activity is nil, cyanidation can be performed under conditions of low oxygen pressure. In these conditions, cyanidation is established at transpassive potential regions of gold anodic dissolution and the cyanidation rate is determined by the cyanide concentration. The cyanidation rate can be tripled by adding small amounts of thallium (I) ions to cyanidation with hydrogen peroxide.

Gold cyanidation with potassium persulfate in the presence of a thallium (I) salt

Abstract The use of potassium persulfate as oxidant in gold cyanidation is proposed at pH values between 10 and 11. The addition of thallium (I) ions enables high cyanidation rates to be maintained at pH values higher than 11, as it acts as a catalyst of the process. When gold cyanidation is performed in the simultaneous presence of 10 mM persulfate and 0.5 mM thallium (I) the dissolution rate is eight times higher than that obtained by conventional cyanidation, and the working pH can achieve values near 13. The kinetics of the process have been studied, and the results compared with those obtained by conventional cyanidation. The process is shown to be diffusion-controlled with an activation energy of 19.2 kJ mol−1. Finally, an equation of the reaction rate that describes the kinetics of the overall process for gold cyanidation with potassium persulfate and thallium (I) is presented, and the [CN−]/[S2O82−] ratio at which the dissolution rate reaches its limiting value is shown to be 2.5.

Adsorption kinetics of dicyanoaurate and dicyanoargentate ions in activated carbon

A kinetic model describing the adsorption of dicyanoaurate and dicyanoargentate ions in activated carbon has been derived assuming that the dicyanoions adsorbed onto the external surface are in equilibrium with those in solution. This equilibrium may be described by a modified Langmuir-Hinselwood equation as is usual in heterogeneous catalysis systems. Once adsorbed onto the outer surface, the dicyanoions can diffuse toward the inner surface of the particle through the pores of the carbon, or they can be desorbed back to the liquid phase. The two exit stages are fast enough to maintain the outer surface in a pseudostationary state in normal experimental conditions. Finally, the kinetic parameters for the process are determined. The fitting of the model with experimental data is satisfactory and the range of applications is wide.

Characterization of fire-refined copper recycled from scrap

Certain properties of fire-refined copper recycled from scrap have been characterized. A method is presented to calculate the half-softening temperature and the annealing temperature that allows 30% elongation to failure, hereinafter referred to as ε30% temperature, on the basis of hardness measurements. The relation between ultimate strain and ultimate elongation has been studied and is described by a mathematicale xpression that seems to be independent of copper composition and annealing temperature. The microstructure of annealed samples reveals that recrystallization begins at half-softening temperature, and is ending at ε30% temperature, although grain growth is not observed. An optimal range of oxygen content has been found that gives the minimum ε30% temperature for each studied composition, and a mathematical expression with which to calculate those minimum temperatures is developed. The influence of cold-working degree on ε30% temperature is also described; these temperatures reach a constant minimum value for each composition at high deformation degrees of cold-working.