Archeology of Consciousness Studies

Chu Yong Cheng and Frank Lawson, 1991. The kinetics of leaching covellite in acidic oxygenated sulphate-chloride solutions. Hydrometallurgy, 27: 269-284. Synthetic covellite, CuS, was leached in oxygenated acidic sulphate-chloride solutions in a series of experiments with the parameters temperature, oxygen partial pressure, particle size, stirring speed and acidity being changed. The shrinking core model can be used to describe the leaching which is controlled by the surface chemical processes in the initial leaching stage and by both chemical and pore diffusion processes subsequently. The apparent activation energy measured in the initial leaching stage is 77 k J/tool over the temperature range 75-95 °C. About 85% of the copper dissolved at 90°C in 0.5 M H2804/0.5 MNaCI solution with pure oxygen bubbled through in 6 h. Some 10% of sulphide sulphur was oxidised through to sulphate; the remainder was converted to xylene soluble elemental sulphur. When no chloride ions were present in the solution, or the chloride concentration was very low, covellite was essentially not dissolved; most probably because of the formation, on the reaction surface, of a tight passivating sulphur layer with a cryptocrystalline or amorphous structure. The effect of the chloride ion is to promote the formation of large sulphur crystals to allow the reactants to penetrate the sulphur layer.

The ability of algae and bacteria to accumulate heavy metals from the surrounding environment is a widely recognized phenomenon that has a number of important implications. This work reports on the development of a quantitative model that addresses the basic mechanisms inherent in many uptake processes. The model postulates two mechanisms: an initial rapid metal ion uptake due to attachment onto the cell wall followed by a relatively slow uptake due to membrane transport of the metal into the cell. The mathematical model has been tested using the alga Chlorella vulgaris in the presence of cadmium and zinc in solution under various experi menta I conditions.

The mathematical model previously developed in the Department of Chemical Engineering, Monash University for the metal uptake by living algal cells has been revised to take into account the actual surface area through which transfer occurs, Cadmium uptake data obtained using the three algal species Chlorella vulgaris, Chlorellu pyrenoidosa and Chlatnydomonas reinhardfii were able to be fitted satisfactorily by the revised model.

In the industries involving alkaline solutions in dierent process streams, the nature and stability of oxide ®lms formed on the metallic surfaces determine the rates of erosion±corrosion of the equipment. In the present study the characteristics of the oxide ®lms formed on AISI 1020 steel in a 2.75 M sodium hydroxide solution at temperatures up to 175°C, have been investigated by employing electrochemical techniques of cyclic voltammetry and chronoamperometry. The experiments were carried out in an autoclave system based upon a Ôrotating cylinder electrodeÕ geometry to determine the eects of turbulence on the stability of the ®lms. The results suggest that little protection is aorded in the active region (at about À0.8 V SHE). In the passive region at low potentials (À0.6 V to À0.4 V SHE), it appears the ®lms are compact and more stable, and therefore provide good protection. At higher potentials (> À0.4 V SHE) in the passive region, the results suggest that ®lm formation and dissolution occur simultaneously and the increase in temperature and turbulence causes a breakdown of the passive ®lm resulting in a situation similar to nonprotective magnetite growth.

Empirical correlation equations for the cementation of cadmium on zinc, of copper on zinc and of copper on cadmium have been developed. For each of the three different systems when the cementation reaction is carried out under an inert atmosphere using a rotating disc geometry under laminar flow conditions, the correlation equation in the initial stage follows the Levich expression written in dimensionless form: Sh = 0.62 (Re) °'5 (Sc) °'333 For the enhanced rate period, the data were correlated by Sh = A (Re) °'74 (Sc) °'29 The constant A depends on the actual deposit morphology and has the values of 0.37, 0.15 and 0.11 for the cadmium/zinc, copper/zinc and copper/cadmium precipitation systems when the deposition is from dilute aqueous solutions.

The kinetics and mechanism of pentlandite leaching in aerated acidic chloridersulfate solutions were studied using electrochemical and chemical techniques. The electrochemical experiments showed that pentlandite experiences passivation at low overpotentials. These results were consistent with the leaching studies which showed that pentlandite leaching proceeds at a low rate. However, the addition of chloride ions was found to increase the leaching rate, and it is believed that the role of chloride is to allow the formation of a more porous sulfur product. Both temperature and oxygen partial pressure had a substantial effect on the reaction rate, with optimum leaching kinetics being obtained at 858C and an oxygen pressure of 1 atm. Under such conditions, 96% nickel extraction could be achieved during leaching in the presence of chloride ions.

Lead smelter slags from two different sources have been leached using sulfurous acid/oxygen as the leachant so as to dissolve zinc and copper values but not the contained iron. The differences in the leaching behaviour are explained in terms of the different slag compositions. The mixed control shrinking core model adequately described the relationship between fractional extraction and time.

The solubility of oxygen in a number of aqueous electrolyte solutions was determined using a colorimetric method based on the oxidation of indigo carmine. The effects of changes in electrolyte concentration, temperature and partial pressure of oxygen were investigated. The ratio of the solubility of oxygen in the electrolyte solutions to that in pure water decreases as the concentration of the electrolyte increases but is essentially independent of both temperature (over the range 298-348 K) and the oxygen partial pressure (over the range 0.1-1.0 atm). Empirical relationships for this solubility ratio as a function of the electrolyte concentration and temperature were developed. The predictions made using these relationships are found to be superior to the alternative methods suggested in the literature, particularly for those conditions frequently found in hydrometallurgical systems, namely at high electrolyte concentrations. The significance of changes in the solubility of oxygen in aqueous electrolyte solutions is briefly discussed with particular reference to the hydrometallurgical implications.

The heterogeneous reduction of As (V) with SO2 gas has been studied; the results indicate that the homogeneous reactions between undissociated As(V) species and undissociated sulphurous acid and bisulphite ions are rate limiting as the solution of S02 is sufficiently rapid to maintain a constant concentration of SO2 in solution provided the gas flow rate is reasonably high. The reactions are: H3As04 + SO2 (aq) + H20-~ H3As03 + H + + HSO4-H3AsO4 + HS03-* H3AsO~ + HSO4-A general rate equation is given by d[As(V) ]-hi [H3As04] [SO2 (aq) ] +k2 [H3As04] [HS03-] dt where k, and k2, the rate constants, are functions of temperature. The reduction rate is a direct function of the partial pressure of S02 but is independent of 02 partial pressure in the gas phase. As the solubility of SO2 decreases with increased temperature, the overall rate of reduction of As (V) to As (III) is almost constant in the temperature range 313-343 K.

This paper describes the application of mixed chloride-sulfate leaching at atmospheric Ž. pressure for the treatment of a CuFeS chalcopyrite concentrate. It was found that excellent 2 leaching kinetics exist for solutions containing chloride, while for solutions without chloride, the leaching was very slow. It is believed that the role of chloride is to promote the formation of a more porous sulfur product, thus permitting the dissolution reaction to proceed at a reasonable rate. It was established that temperature and oxygen partial pressure had a substantial effect on the reaction rate, with very good leaching kinetics being obtained at 958C. At an oxygen pressure of 1 atm, 86% of the copper was extracted in 6 h, and 97% extracted in 9 h. Decreasing the particle size was also found to increase the reaction rate.

Leaching studies of nickel(II) oxide and hydroxide, and high valence nickel hydroxide in sulphuric acid and in ammonium carbonate solutions were carried out. The dissolution rate of nickel(II) oxide in both solvents was influenced greatly by the heat treatment given to the oxide prior to leaching and, under the conditions studied, was found to be limited by the rate of the surface chemical reactions. The rates of leaching of nickel(II) and nickel(III) hydroxides on the other hand were very fast. The nickel(III) hydroxide is unstable and decomposes to liberate oxygen in acid solutions and oxidises ammonia to nitrogen and nitrite in ammoniacal solutions.

The dissolution behaviour of cobaltous oxide and cobalto-cobaltic oxide in ammonia and in acid solutions has been investigated. The dissolution reaction for cobaltous oxide was found to be chemically controlled with apparent activation energies of 54.0 kJ tool-1 and 56.5 kJ tool-1 respectively in ammonia and in acid solutions. The dissolution rate of cobalto-cobaltic oxide in ammonia-ammonium carbonate is very slow at low temperatures. At higher temperatures, the amount of cobalt remaining in solution is low, less than 2 × 10-s tool din-3 at 150 ° C, due to the precipitation of cobalt carbonate and cobalto-cobaltic oxide.

The electrochemical oxidation of a CuFeS chalcopyrite concentrate in a mixed chloride-2 sulfate electrolyte was studied using carbon paste electrodes. This technique was found to give very reproducible results, and has the advantage that actual concentrate particles can be used in place of large crystalline mineral samples. It will be demonstrated that during the initial stages of leaching, the oxidation of chalcopyrite at low overpotentials is hindered by the formation of a metal-deficient sulfide surface. When chloride ions are present, it will be shown by both transientand steady-state techniques that the oxidation of the mineral is significantly enhanced. These results have also been found to be in very good agreement with those obtained by direct leaching of the same concentrate.

Chu Yong Cheng and Frank Lawson, 1991. The kinetics of leaching chalcocite in acidic oxygenated sulphate-chloride solutions. Hydrometallurgy, 27: 249-268. Narrowly sized samples of synthetic chalcocite, CH2S , were leached in acidic oxygenated sulphatechloride solutions. The leaching process occurs in two stages. In the first rapid leaching stage, cuprous ions diffuse through the solid to the particle surface and are oxidised to divalent state. This results in the formation of a series of copper-deficient chalcocite intermediates, the presence of which was confirmed by X-ray diffraction analyses. The rate-limiting step is the diffusion of oxygen through the liquid boundary layer around the particle to the reaction surface. The activation energy was found to be 33.5 kJ/mole. The first stage of leaching is completed when the surface has been converted to covellite (CuS). In the much slower second stage, the dissolution can be described in terms of the traditional shrinking core model in which a shrinking core of unreacted covellite is surrounded by a thickening shell of elemental sulphur. The rate of copper leaching from the covellite is initially limited by surface chemical reactions and in the late second stage by both the surface chemical reaction and pore diffusion processes. The apparent activation energy in this second leaching stage is 69.0 k J/mole, characteristic of a reaction under chemical reaction or mixed control with the chemical rate limitation being dominant. It is believed that the role of the chloride ion is to disrupt the passivating sulphur layer on the particle surface; probably by promoting the formation of crystalline sulphur rather than a cryptocrystalline or amorphous product. The complexing effect of chloride with copper ions is believed to be of secondary importance. Over 97% of the copper in the Cu2S dissolved in a 0.5 M H2804/0.5 M NaCI with pure oxygen bubbling at 85 °C in 3 h. About 1% of the sulphide sulphur was oxidised to soluble sulphate while the remainder was converted to solid elemental sulphur extractable with xylene.

The leaching of metallic tin in solutions of sodium hydroxide was studied using a rotating disc geometry. The rate-limiting step was found to be the diffusion of dissolved oxygen through the liquid boundary layer, the overall reaction being: 2Sn + 2OH-+ O~ = 2HSnO2at pH values of about 14. The oxygen concentration on the metal surface was found to be independent of disc rotation speed but increased with decreasing temperature. Above 290 K the activation energy was found to be 12.6 kJ moli (3 kcal mol-1). All results confirm that at these temperatures, the reaction under laminar flow conditions was mass transfer controlled. When the sodium hydroxide concentration was increased, the observed rate of dissolution of tin decreased. This was shown to be due to the decrease in the solubility and diffusivity of oxygen in these solutions of higher sodium hydroxide concentration.

The cementation of uncomplexed silver ions on to zinc and on to ferrous alloys. Hydrometallurgy, 14: 1-21. The kinetics of the cementation of uncomplexed silver ions from low concentration solutions on to zinc and on to ferrous materials have been studied using a rotating disc reactor system. The silver/zinc reaction is mass transfer controlled whereas the deposition on to ferrous precipitants is under mixed control. The effects of changes in temperature, disc rotation speed and solution pH were investigated and the observed behaviour explained in terms of changes in the deposit morphology.