David G. Dixon

Thermophilic heap leaching of a chalcopyrite concentrate

Abstract Thermophiles have been shown to be the only micro-organisms to leach chalcopyrite successfully. Heap leaching may be a feasible alternative to conventional bio-reactors, providing a high temperature environment can be maintained within the heap without external heating. In the present study thermophilic heap leaching of a chalcopyrite concentrate coated onto inert support rocks (the GEOCOAT™ process) was studied in sets of small heated columns. The temperature was gradually increased to 70 °C, while successively introducing various mesophile and thermophile cultures. Individual columns were dismantled after progressively longer leach periods and the residual concentrates analysed. Copper extractions in excess of 90% were achieved within 100 days. On the basis of head and residue analyses the rate of reaction heat generated was calculated. A comprehensive heap heat conservation model was used to determine whether the experimental temperatures can be achieved and maintained in a full scale heap. Results indicate that operating hot heaps successfully is possible within a certain range of process parameters.

Analysis of heat conservation during copper sulphide heap leaching

Abstract An analysis of heat conservation during copper sulphide heap leaching has been carried out with the aid of a computer model. As a result, a new mode of heap behaviour called “evaporative autocatalysis” is postulated. In this mode, air is blown upward through the heap at a rate sufficient to drive the net advection of heat upward through the heap, resulting in much higher and more uniform internal heap temperatures than can be achieved in the absence of forced aeration. The effects of flow rate, heat generation rate, solution and air heating, and application of an evaporation shield to the heap surface are also investigated.

The Active-Passive Behavior of Chalcopyrite Comparative Study Between Electrochemical and Leaching Responses

The oxidative behavior of chalcopyrite in H 2 O-H2S04-Fe2(S04)3-FeS04 media was studied using both electrochemical techniques and leaching experiments. The results of the two methods demonstrate that chalcopyrite oxidation displays the classical active-passive behavior observed in passivating metals; values predicted electrochemically for the passivation potential E PP are in excellent agreement with leaching experiments. This result substantially improves the knowledge of the anodic behavior of chalcopyrite, which has been reported so far mostly as pseudopassive when massive chalcopyrite electrodes are used. Imposing a continuous series of potentiostatic pulses (increasing by 10 mV/h), three-dimensional current density-time-potential surfaces were generated in order to establish the effects of acidity and temperature on the passivation potential, the passive current, and the critical current of chalcopyrite leaching. The concentration of sulfuric acid was systematically varied from 2 to 100 g/L and temperature from 25 to 80°C. E PP increases with increasing temperature from 440 mV at 25°C up to 515 mV at 80°C; it is practically insensitive to acidity at low (25-40°C) and high (60-80°C) temperatures. At 50°C an acid-dependent transition of E pp was observed from 440 to 515 mV. The passive currents were at most one order of magnitude lower than the maximum critical current.

A mathematical model for heap leaching of one or more solid reactants from porous ore pellets

A mathematical model is derived in dimensionless form for heap leaching of one or more solid reactants from nonreactive, porous, spherical ore particles. The model is for the interpretation of column and heap leaching data and for use in the design and scaleup of heap leaching pro-cesses. Data from experimental column leaching tests are presented which validate the model. The scope of the present study is limited to the case where the rate-controlling reagent is a component of the lixiviant solution only and not of the gas phase. The effects of particle-scale kinetic factors, heap scale and operating variables, competition between multiple solid reactants, and particle size distribution are examined using the concept of the heap effectiveness factor. It is shown that heaps operate either «homogeneously,» with distribution of reagent throughout the heap at all times, or in a «zone-wise» manner, depending mostly on a single parameter. The observed value of this parameter deviates from the predicted value in inverse proportion to the degree of contact effectiveness between the lixiviant solution and the ore particles. A rough correlation between the contact effectiveness and Reynolds number is generated from the simulation of column test results.

Principles, Mechanisms And Dynamics Of Chalcocite Heap Bioleaching

Solution is irrigated (continuously or intermittently) over the top surface of the heap and left to seep through the ore bed where it can react with the target minerals. Dissolved metals are then transported with the flowing solution to the bottom of the heap from where they are drained via the drainage system into a collection pond as the pregnant leach solution (PLS). The target metal is removed from the PLS through a suitable technology (by solvent extraction, cementation or adsorption), and the barren solution (usually referred to as the raffinate) returned to the top.

The influence of applied potentials and temperature on the electrochemical response of chalcopyrite during bacterial leaching

Abstract Electrochemical techniques were conducted to clarify the role of solution potential and temperature under a variety of experimental conditions similar to those found during the mesophilic and thermophilic biooxidation of chalcopyrite (CuFeS2). Despite a large number of publications dealing with the bacterial leaching of CuFeS2, three central aspects remain unclear: How to dissolve preferentially copper from CuFeS2, the effect of temperature on the extent of CuFeS2 passivation, and the behavior of ferric ions on a polarized CuFeS2 surface. Anodic characteristics showed that CuFeS2 passivation was more severe in the potential range 0.45–0.65 V saturated calomel electrode at 25 °C. However, there was no evidence of CuFeS2 passivation at higher temperatures (45 and 65 °C). Cu was preferentially dissolved from CuFeS2 at lower potentials and high temperatures at a ratio copper to iron of about 3:2. Cathodic characteristics showed that the ferric ions inhibited the leaching process when the CuFeS2 surface was polarized at high potentials and low temperatures.

Modeling the cyanide heap leaching of cupriferous gold ores: Part 1: Introduction and interpretation of laboratory column leaching data

Abstract A kinetic and thermodynamic model of cyanide heap leaching of copper-bearing gold ores was developed in order to facilitate the evaluation and treatment of these complex ores [F. Coderre, Modeling the cyanide heap leaching of cupriferous gold ores. M.A.Sc. Thesis, U. British Columbia, 1998.]. In this first paper on the work, the behaviour of copper minerals in cyanide solutions is reviewed, and data from a series of laboratory column leaching tests performed under different influent cyanide concentrations are presented and interpreted in support of model development. The cyanide leaching is shown to occur in four distinct stages: (1) reductive dissolution of Cu(II) salts, during which copper and gold precipitate as cyanide salts, and no gold appears in the column effluent; (2) redissolution of the copper and gold cyanide precipitates; (3) leaching of copper sulphide minerals and elemental gold; and (4) free cyanide breakthrough in the effluent, which is accompanied by the appearance of dissolved iron from the redissolution of metal hexacyanoferrates. During the leaching step, it appears as if elemental gold dissolves in the absence of free cyanide, but that the copper sulphides do not.

The Active-to-passive Transition of Chalcopyrite

The active-to-passive transition of chalcopyrite is studied using electrochemical techniques and leaching tests to establish the conditions under which chalcopyrite electrodes display active-passive behavior. Results from potentiostatic anodic polarizations are predictive and agree very well with leaching results while potentiodynamic techniques give only erratic values. The critical current and oxidation rate in the active potential range increases with the concentration of acid, but the passivation potential decreases. The corrosion rate of chalcopyrite was studied at various temperatures and concentrations of acid in electrolytes containing only ferric (no ferrous) using electrochemical impedance spectroscopy; the higher the concentration of acid and the lower the temperature, the lower the corrosion rate.

On the rate-limiting steps of pyritic refractory gold ore heap leaching: results from small and large column tests

A pyritic refractory gold ore was oxidized in short and tall isothermal columns at different temperatures (22, 45 and 65 °C) with consortia of mesophiles, moderate and extreme thermophiles under a variety of experimental conditions. These experiments were performed as part of a heap leach model validation program. Severe air channeling and bed compaction occurred in columns as short as 2 m packed with non-agglomerated ore particles. Similar problems were encountered in columns loaded with 9 mm agglomerates prepared with smaller particles (<3.3 mm). Although aeration was key to maintaining high potentials, neither the irrigation flow rate, nor the inoculation procedure, nor CO 2 enrichment had any significant effect on the oxidation kinetics at ambient temperature. Elemental sulfur oxidation was more rapid at elevated temperatures. Reasons why sulfide oxidation kinetics in the 65 °C column were comparable to the 45 °C test, and thus slower than predicted by the intrinsic Arrhenius activation energy of pyrite, are discussed.

The multiple convolution integral: A new method for modeling multistage continuous leaching reactors

Abstract A new method is presented for modeling multiparticle leaching kinetics in multistage continuous reactors at steady state. The method involves recasting the kinetics of any given leaching reaction into a linear function of time, and then solving a multiple convolution integral over residence time for the mass-weighted probability density of the resulting time-linear variable after passing through N mixed-flow reactors in series. The fraction of leachable solids unreacted is then found by integrating this probability density function over all values of the time-linear variable, which is a known function of particle size. The entire model may be solved numerically in a single step using Gaussian quadrature. This new method represents an alternative to both the segregated flow model and the population balance model, combining the simplicity and ease of application of the former with the rigorousness of the latter.