M.C. Ruiz

Drop size distribution in a batch mixer under breakage conditions

Abstract The equilibrium size distribution, F 0 ( v ), of organic drops produced in a batch mixing vessel has been determined experimentally for very low dispersed phase fractions (0.006) in order to reduce the coalescence between drops to negligible values. The organic phase used was a 1:1 mixture of a salicylaldoxime (LIX 860N-IC) and a ketoxime (LIX 64-IC) in an aliphatic diluent (Escaid 103). The aqueous phase was a 0.25-M sodium sulfate solution. The results indicated that the system reaches an equilibrium drop size distribution in less than 30 min of stirring time. An increase in the stirring speed increased the tendency of the organic drops to break, shifting the drop size distribution toward the smaller drop sizes. An increase in temperature from 22 to 32 °C decreased the size of the organic drops, while a change in the concentration of the extractant in the organic phase from 7% to 20% by weight had little effect. Both effects can be attributed to changes in the physical properties of the system. A decrease in pH of the aqueous phase from 5.7 to 2.0 increased progressively the tendency of the organic drops to undergo breakage giving finer drop size distributions due to changes in the surface charge of the organic drops produced by the pH change. In all cases, the experimental drop size distributions could be accurately represented by a lognormal distribution.

Separation of liquid−liquid dispersions in a deep-layer gravity settler: Part I. Experimental study of the separation process

Abstract The separation of liquid-liquid dispersions is a crucial operation in the solvent extraction of metals because very often the throughputs of solvent extraction plants are limited by phase separation characteristics rather than mass transfer rates in the mixer. The separation of phases in a laboratory scale mixer-settler has been investigated in a system consisting of 10% Acorga M5640 in Escaid 103–0.25 M Na 2 SO 4 solution. The thickness of the dispersion band, the local dispersed phase hold-up, and the droplet size distribution at different positions in the dispersion band were determined for various operational conditions. Based on the flow pattern of the dispersion, two well defined zones were identified in the band: a narrow distribution zone just above the passive interface characterized by a horizontal velocity of the drops, and above it a dense zone where the drops have essentially vertical movement. The drop size measurements indicated little variation in size in the horizontal direction but a dramatic increase in the vertical direction of the dispersion band. The thickness of the band was found to be a function of the specific flow rate of the dispersed phase, and independent of the phase ratio.

Analysis of breakage functions for liquid–liquid dispersions

Abstract In the solvent extraction process in the mining industry, the drop size distribution produced in the mixing vessel is crucial for the global performance of the extraction process, because it affects the rate of copper extraction in the mixing step and the phase separation in the settler as well. In this work, experimental drop size distributions were obtained in a batch-mixing vessel using a low dispersed phase fraction in a hydryoximes–sodium sulfate system. The data were analyzed using a population balance model neglecting the coalescence terms to obtain appropriate analytical forms for the functions that describe the drop breakage process. From the comparison of the experimental and simulated transient drop size distributions, it was determined that the specific breakage frequency function used has little effect on the evolution of the drop size distribution. On the other hand, the daughter drop density function has a large influence on the shape of the resulting drop size distribution. The most appropriate daughter drop density function is a U-shaped distribution with minimum probability of producing two equally sized daughter drops from the breakage of a mother drop.

Copper removal from molybdenite concentrate by sodium dichromate leaching

Abstract Molybdenite flotation concentrate was leached with sodium dichromate–sulfuric acid solutions in order to remove the copper sulfide impurities. The experimental results indicated that the temperature has a large influence on the copper dissolution. The concentration of dichromate has also a pronounced effect for concentrations under 0.10 M, while sulfuric acid concentration over the stoichiometric value has a deleterious effect on the copper dissolution. The purification of molybdenite concentrates by this method is feasible, and the best results were obtained by leaching the concentrate with the stoichiometric concentration of H2SO4 and 0.12 M Na2Cr2O7 solution for 90 min at 100°C. Under these conditions, about 95% of the copper was dissolved with little dissolution of molybdenum. The kinetic study showed that the dissolution of copper from the concentrate is well represented by a shrinking core model controlled by diffusion through a porous layer, 1/3 ln(1−α)−[1−(1−α)−1/3]=kt. The apparent activation energy obtained was 40 kJ/mol for the temperature range 50 to 100°C.

Separation of liquid-liquid dispersions in a deep-layer gravity settler: Part II. Mathematical modeling of the settler

Abstract A mathematical model for the steady-state operation of a deep-layer gravity settler has been developed by using a population balance approach. This model takes into account the size distribution of drops within the dispersion band and uses rate expressions for the description of drop-drop and drop-interface coalescence phenomena. The model was validated with experimental data obtained in a laboratory mixer-settler unit, for a system consisting of 10% Acorga M5640 in Escaid 103–0.25 M Na2SO4 aqueous solution. The model successfully predicts the thickness of the dispersion band and the growth of the dispersed phase drops occurring by drop-drop coalescence within the dispersion band.

Removal of Arsenic from Enargite Rich Copper Concentrates

Arsenic is a troublesome impurity in copper concentrates where it is usually present as enargite (Cu3AsS4). When the enargite content is high in the concentrates, they cannot be treated by direct smelting processes because of the high risk of arsenic emissions to the atmosphere. In this work, some experimental results are presented on a novel alternative process to remove selectively the arsenic from an enargite rich copper concentrate. The process includes an alkaline (Na2S-NaOH) baking step followed by leaching with water. The experimental results showed that temperature and Na2S concentration are important variables in the baking step for efficient arsenic removal. In experiments using a chalcopyritic copper concentrate containing 2.2% arsenic as enargite, 94% of the arsenic was removed when the concentrate was baked at 85 °C for 10 min by using 2.5 times the stoichiometric Na2S requirement for arsenic dissolution. Copper dissolution was negligible under these conditions.

Galvanic Effect of Pyrite on Chalcopyrite Leaching in Sulfate-Chloride Media

Chalcopyrite concentrate and mixtures of pyrite/chalcopyrite were leached in H2SO4-NaCl-O2 solutions to assess the effect of pyrite on the chalcopyrite dissolution rate in this media. The results showed that the addition of pyrite increases the dissolution rate of chalcopyrite at short leaching times but the catalytic effect of pyrite decreases as the reaction progresses. Based on the experimental evidence, it was concluded that the presence of pyrite in the leaching of chalcopyrite in H2SO4-NaCl-O2 solutions enhances the dissolution of copper through a galvanic interaction between pyrite and chalcopyrite. Additions of ferrous sulfate also increase chalcopyrite leaching rate effectively.

Kinetics of the Cupric Catalyzed Oxidation of FeII by Oxygen at High Temperature and High Pressure

ABSTRACT The oxidation of FeII to FeIII by oxygen in sulfate media plays an important role in the pressure leaching of copper sulfide concentrates because ferric ion is a more effective oxidant of copper sulfide minerals than oxygen. In this work, the cupric catalyzed oxidation of ferrous sulfate by oxygen at high temperature and high pressure has been investigated. The effects of the variables temperature (393 to 493 K)), sulfuric acid concentration (0.15 to 1.2 M), copper sulfate (0 to1.6×10–2 M) and partial pressure of oxygen (343 to 1379 kPa) were studied. The presence of cupric ions in the solution greatly accelerated the rate of ferrous oxidation in the range of experimental conditions studied but the rate was not a function of the cupric concentration over 4×10–3 M. The following empirical equation was found to accurately describe the rate of the cupric catalyzed oxidation of FeII by oxygen: The experimental activation energy for the cupric catalyzed ferrous oxidation was 36.5 kJ/mol while in the absence of cupric ions in the solution, the oxidation of FeII by oxygen proceeds through a second order model with respect to FeII concentration with an activation energy of 78.0 kJ/mol.

Copper Removal from Molybdenite by Sulfidation‐Leaching Process

Molybdenite (MoS2) concentrates are produced by differential flotation from bulk copper-molybdenum concentrates. However, complete separation of molybdenum by flotation is very difficult, thus, molybdenum concentrates produced by this route requires a further step of chemical purification in order to reduce the copper content to a suitable level for marketing. Refractory chalcopyrite (CuFeS2) is the major copper mineral impurity in these concentrates; thus the elimination of copper by leaching requires highly aggressive solutions. This paper is concerned with a novel process of chemical purification of molybdenite concentrates containing chalcopyrite as the main copper impurity. The process involves a sulfidation of the molybdenum concentrate with 82(g) at 380 °C followed by a leaching stage. H2SO4-NaCl-O2 leaching of a sulfidized concentrate containing 3.4% Cu for 90 min at 100°C produced a molybdenite concentrate with less than 0.2% Cu, which is appropriate for marketing. Molybdenite dissolution was negligible in these conditions.

Arsenic and Antimony Removal from Copper Concentrates by Digestion with Nahs‐Naoh

Copper concentrates with high arsenic contents must be pretreated before conventional smelting to prevent environmental pollution with arsenic compounds. In this work, experimental results concerning the selective removal of arsenic and antimony from copper concentrates are presented. The process consists of an alkaline digestion using concentrated NaHS-NaOH solutions to transform the arsenic and antimony sulfides into soluble compounds. A water leaching follows the digestion to dissolve the arsenic and antimony, leaving clean copper sulfide solid residues. The laboratory scale tests were carried out using a copper-arsenic concentrate with 15.05% As and 1.42% Sb. The results showed that the most important digestion variables were temperature and concentrations of NaHS and NaOH. Over 97% of arsenic and 92% of antimony could be removed in 10 min of digestion using 8.9 M NaOH and 100% excess of NaHS at 80 °C. The subsequent water leaching was performed at 80°C for 20 min.