F. Espiell

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.

Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing by-product

The wastewater produced by the cochineal extract process to obtain the carminic acid colouring pigment (carmin red E120) has high concentrations of phosphates and ammonium. It is known that both ions can be precipitated with magnesium in the form of struvite, MgNH(4)PO(4), or ammonium magnesium phosphate (MAP) compounds. In this study, the use of an alternative MgO-containing by-product is investigated. The optimal pH, reaction time and solid/liquid ratio have been studied. It has been found that the low-grade MgO needed is greater than the stoichiometric value for the full removal of ammonium and phosphate as MAP compounds. Although the low-grade MgO (LG-MgO) reacts slower than pure MgO, it has considerable economic advantages. A batch process has been proposed for the removal of ammonium and phosphates from wastewater obtained in cochineal extracts processing, previously to biological treatment to diminish the COD.

Short-term natural weathering of MSWI bottom ash.

The release of heavy metals from MSWI bottom ash has been the key concern in the management of this material. The leaching distribution values obtained from 100 freshly quenched bottom ash samples, according to the German DIN 38414-S4 procedure test, showed the release of lead, zinc and copper to be the main hazards associated with bottom ash utilisation as a secondary building material. Currently, natural weathering of MSWI bottom ash, for an estimated period of 1-3 months, is the most economic treatment available to ensure the eventual utilisation of this material. The leaching of natural weathered bottom ash in the short-term (up to 9 months) was studied. The most significant changes in the bottom ash were found to occur in the first 90 days. At pH values greater than 12, lead, zinc and copper were the main heavy metals to be released from the MSWI freshly quenched bottom ash samples studied. Natural weathering for a period of about 90 days reduced the leaching of heavy metals, stabilising the bottom ash pH to minimise the solubility of metal hydroxides, and enabled the residue to be used as secondary building material. The profile of the pH neutralisation curve is similar to that described by carbonates, which would suggest that the reaction is controlled by CO(2). The formation of insoluble oxides as well as carbonates control the immobilisation of certain heavy metals, e.g. lead and zinc. The leaching of aluminium increases during this short natural weathering stage due to elemental metal oxidation. Aluminium solubility is controlled by the precipitation of gibbsite or other aluminium-sulphate neoformations. The latter may contribute to the immobilisation of heavy metals.

Short-term natural weathering of MSWI bottom ash as a function of particle size

The chemical and material composition of MSWI bottom ash depends on the particle size; this suggests that the mechanisms and kinetics of natural weathering are also a function of particle size. This paper reports the effects of short-term natural weathering on the leaching of heavy metals (mainly Pb, Cu and Zn) from MSWI bottom ash. Initial concentrations of heavy metals were higher for the smallest particle size fractions, but these levels fell dramatically during the first 50 days of weathering before levelling off. The main differences between size fractions were in the pH and the solubility of calcium and aluminium. For the initial stages of weathering and small size fractions, portlandite solubility seemed to control the pH. In contrast, for fractions bigger than 6 mm, the formation of ettringite was the reaction controlling the pH and the solubility of sulphates, aluminium and calcium.

A proposal for quantifying the recyclability of materials

Abstract It is becoming of empirical importance that recyclability be defined in such a way that engineers, economists, and policy makers can agree upon and use collectively. This paper defines recyclability as the ability of a material to reacquire the properties that it had in its virgin state, where virgin state refers to the material in its purest form before being processed or shaped for a specific use. Anything less than that can be measured as a degree of its recyclability, defined as recycling index ( R ). It is here proposed that R of a material can be estimated by its devaluation (how much the material devalues during its first use), which is reflected by its loss of monetary value. This way, R can be calculated by a mathematical expression. Because of their thermodynamic and kinetic properties, as well as advances in their recycling technologies, most metals are recyclable. They are therefore used to establish a relationship that determines how truly recyclable materials should behave.

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.

Kinetics of the dissolution of pure silver and silver-gold alloys in nitric acid solution

This article describes a kinetic study of the dissolution of silver and silver-gold alloys in nitric acid. For pure Ag, the nitric acid reaction order is two at low concentrations and one for concentrations above 4 M. When attacking alloys, the reaction order is one and drops to zero when the alloy contains 51.1 pct of silver by weight or less and the nitric acid concentration is 6 M or above. The activation energy is 12.1 kcal/mol in both cases. When the molar fraction of silver is 0.70 or more, the rate of silver dissolution from its gold alloys is controlled by the chemical reaction on the solid surface. When the molar fraction of the silver is less than 0.65 and nitric acid activity and temperature are high, the dissolution is controlled by the outward diffusion of silver nitrate through the undissolved gold layer. The dissolution rate is affected by the composition of the alloy. When the molar fraction of silver is 0.76 or more, the gold atoms are found in the lattice isolated or in pairs and silver atoms may dissolve without difficulty. Between 0.55 and 0.76, the reaction rate decreases quickly when the gold content increases because the atoms make up chains into lattice and this makes the dissolution difficult. When the molar fraction is less than 0.54, the alloy does not dissolve since the gold atoms form continuous surfaces that impede the attack.

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.

Kinetics of the reaction of gold cyanidation in the presence of a thallium(I) salt

Abstract The heterogeneous kinetics of gold cyanidation in the presence of a thallium(I) salt, which is known to act as accelerator of the reaction, are described in this paper. The rate of gold dissolution in the conventional gold cyanidation decreases dramatically at pH values higher than 11.5. The presence of thallium increases this maximum value of pH to over 13.5. The kinetic orders found for each of the reagents taking part in the reaction are: 0.92 for cyanide; 0.95 for oxygen and 0.1 for thallium(I). These results, as well as the calculated activation energy (9.2 kJ mol −1 ) and the effect of pH and stirring speed on the reaction rate, have disclosed a possible mechanism by which the electrochemical reaction may take place on the gold surface. According to this mechanism, the gold surface may be divided into two areas, one anodic and another cathodic, where the gold oxidation and the thallium(I) reduction take place, respectively. The cathodic area can also be subdivided into anodic and cathodic subareas, where the oxidation of the thallium deposited on the gold surface and the oxygen reduction take place. In contrast to conventional gold cyanidation, when cyanidation takes place in the presence of a thallium(I) salt, the oxygen reduction yields hydroxide ions as a reduction product, and the rate of gold dissolution is four times faster. The equation of the reaction rate that describes the kinetics of the overall process is also presented.