Ferric Chloride Media Research Articles

Kinetic limitations of gold leaching in ferric chloride media Part I: Batch reactor studies

The global gold industry is under increasing pressure to search for alternative technologies to replace cyanidation due to the inefficiency of cyanide in treating low grade refractory gold ores and increased public scrutiny on the use of cyanide in gold mining. Chloride has been identified as a promising candidate to replace cyanide. Despite extensive efforts to investigate gold leaching in chloride media, gold leaching in ferric chloride media at moderate temperatures relevant to heap leaching has not been well studied. Sponsored by Barrick Gold Corporation, we investigated the thermodynamics and kinetics of gold dissolution in ferric chloride media at moderate temperatures using batch leaching and electrochemical techniques. The Part I of this study reports the effects of four key process variables, initial ferric concentration, total chloride concentration, redox potential of the leaching solution, and the type of chloride salts, on the rate and extent of gold dissolution in ferric chloride media. A series of batch reactor leaching tests was carried out using pure gold as the model mineral. The experimental results showed that the leaching process was either under thermodynamic or kinetic control depending on the leaching conditions. The gold extraction increased with the initial ferric concentration up to 0.3 M, after which a negative effect was observed. This was thought to be caused by the transition from cathodic reduction of the oxidant controlling the kinetics to anodic oxidation of gold controlling the kinetics. Increasing total chloride concentration and the redox potential of the leaching solution also increased the gold extraction. Gold extraction was higher in the presence of the monovalent salts than in the presence of the divalent salts.

The process chemistry and mineralogy of brannerite leaching

Brannerite (UTi2O6) is the most important uranium mineral after uraninite and coffinite, and the most common refractory uranium mineral. As the more easily leachable uranium ores are becoming exhausted, it is necessary to process the complex and refractory ores in order to meet the growing demand for uranium as an energy source. This typically requires either more intense leaching conditions or a better-designed process based on sound understanding of feed mineralogy and reaction chemistry. The present study was carried out to provide information that will enable the development of a more effective processing strategy for the extraction of uranium from ores containing brannerite. The leaching behaviour of brannerite in sulphate media under moderate temperature conditions was investigated and compared with its relative leachability in alternative acid and alkaline systems. The feed and the leached residues were characterized by XRD and SEM-EDX techniques. Brannerite dissolutions of up to 95% after 5 hours of leaching in ferric sulphate media, up to 89% in ferric chloride media under similar conditions, and up to 82% in 24 hours in sodium carbonate media were obtained. Since alkaline leaching was considered promising for acid-consuming ores, leaching was repeated with a high-carbonate brannerite-bearing ore, with comparable extractions. Mineralogical characterization showed that altered and amorphous regions are a regular feature of brannerite, and that pitting is typically observed on the surface of the leached grains. The leaching results, coupled with mineralogical data, showed that the uranium and titanium in brannerite generally dissolve congruently, with faster dissolution of the altered and amorphous regions in the brannerite grains than of the crystalline regions. We conclude that the extent of brannerite alteration is a key factor in process selection, along with the grade, liberation size, and gangue mineralogy.

Preparation of lead sulfate powder directly from galena concentrates

A new approach to prepare PbSO 4 powder is studied. Using the methods of the leaching of galena concentrates in the ferric chloride media, selective purification and chemical sedimentation, PbS concentrate can be converted into PbSO 4. The conversion recovery is 97.39%, the purity of PbSO 4 powder is above 99% and its average crystallite size is about 42 nm. In this process, emission of lead vapour and SO 2 cannot occur. The experimental results demonstrate the feasibility of realizing a green route to prepare the lead sulfate powder.

The precipitation of hematite from ferric chloride media at atmospheric pressure

The precipitation of hematite from ferric chloride media at temperatures 66 pct Fe and <1 pct Cl, and its composition does not change appreciably on repeated recycling. The amount of product formed increases significantly with increasing FeCl3 concentrations to ∼0.2 M FeCl3, but nearly constant product yields are obtained thereafter; the precipitates consist only of hematite, provided that an adequate amount of seed is present. The contamination with Zn, Ca, and Na is <0.1 pct, even for high concentrations of dissolved ZnCl2, CaCl2, or NaCl. The extent of the precipitation reaction depends principally on the temperature and the free-acid concentration; accordingly, the controlled addition of a base allows the nearly complete elimination of the iron from metallurgical processing streams or effluents, as readily filterable Fe2O3.

The precipitation of hematite from ferric chloride media

The precipitation of hematite from ferric chloride media was systematically investigated in a series of autoclave experiments. The minimum temperature to form hematite in the absence of hematite seed is ∼ 125°C; akaganeite, β-FeO · OH, is precipitated at lower temperatures for the < 6 h reaction times used. In the presence of hematite seed, Fe 2O 3 is precipitated at temperatures as low as 100°C. The amount of iron precipitated and the concentration of co-generated HCl increase systematically as the initial concentration of ferric chloride increases from 0.025 to 0.3 M FeCl 3. Further increases in the FeCl 3 concentration have a negligible effect on the quantity of Fe 2O 3 precipitated or on the concentration of HCl generated. Increasing initial HClconcentrations, however, sharply reduce the amount of hematite precipitated. In contrast, pre-neutralization of the solution significantly increases the extent of hematite precipitation. The presence of soluble species such as FeCl 2, NaCl, CaCl 2, ZnCl 2 and PbCl 2 sometimes decreases the extent of iron precipitation but has only a minor effect on the purity of the hematite product; the precipitates typically contain > 65% Fe, < 0.5% Cl and less than 0.1% of Na, Ca, Zn or Pb. The precipitated hematite is very fine, but its filterability is greatly improved by the presence of hematite seed.

The effect of the iron content of sphalerite on its rate of dissolution in ferric sulphate and ferric chloride media

Fifteen sphalerite samples having solid solution iron contents ranging from 0.04 to 14.7% Fe were leached at 20–90%C in both 0.3 M Fe(SO 4 1.5−0.3 M H 2SO 4 and 0.3 M FeCl 3-0.3 MHCl media. Linear kinetics were observed for all the sphalerites in both leaching systems. The sphalerite leaching rate increased in a linear manner with increasing iron content. At 80°C in the ferric sulphate system, the rate, as defined by k=(1 −1 (1 − α) 1 3 )/t , increased from 0.38 gg −1 m −2 h −1 to 4.33 g g −1 m −2 h −1 as the iron content of the sphalerite increased from 0.04 to 12.5% Fe; the corresponding values in the ferric chloride system went from 0.26 g g −1 m −2 h −1 to 9.73 g g −1 m −2 h −1. Activation energies in the 39–81 kJ/mol range were observed in both systems. The activation energy decreased as the iron content of the sphalerite increased. The kinetics of sphalerite dissolution are similar in both the Fe(SO 4) 1.5H 2SO 4 and FeCl 3HCl system, and the sphalerite leaching reaction seems to be controlled by charge transfer at the sphalerite surface.

The effect of the elemental sulfur reaction product on the leaching of galena in ferric chloride media

The dissolution of galena in 0.3 M FeCl3-0.3 M HC1 solutions containing 0 to 6 M LiCl was studied at 80 °C, and parabolic kinetics were observed at all LiCl concentrations. The leaching rate increases gradually with increasing LiCl concentrations to ≈4 M LiCl; the presence of >4 M LiCl results in a rapid increase in the leaching rate. The solubility of PbCl2 in 0.3 M FeCl3-0.3 M HC1 solutions containing 0 to 6.5 M LiCl was measured over the temperature range of 50 °C to 90 °C. The solubility increases systematically with increasing temperature and LiCl concentration. The parabolic kinetics, coupled with the correlation between the leaching rate and the solubility of PbCl2, suggest that the dissolution of galena is controlled by the outward diffusion of the PbCl2 reaction product through the constantly thickening layer of elemental sulfur formed during leaching. This conclusion is also supported by various morphological studies which consistently indicated a thin layer of PbCl2 between the corroding galena and the porous elemental sulfur reaction product.

The dissolution of galena in ferric chloride media

The dissolution of galena (PbS) in ferric chloride-hydrochloric acid media has been investigated over the temperature range 28 to 95 °C and for alkali chloride concentrations from 0 to 4.0 M. Rapid parabolic kinetics were observed under all conditions, together with predominantly (>95 pet) elemental sulfur formation. The leaching rate decreased slightly with increasing FeCl3 concentrations in the range 0.1 to 2.0 M, and was essentially independent of the concentration of the FeCl2 reaction product. The rate was relatively insensitive to HCl concentrations <3.0 M, but increased systematically with increasing concentrations of alkali or alkaline earth chlorides. Most significantly, the leaching rate decreased sharply and linearly with increasing initial concentrations of PbCl2 in the ferric chloride leaching media containing either 0.0 or 3.0 M NaCl. Although the apparent activation energy was in the range 40 to 45 kJ/mol (∼10 kcal/mol), this value was reduced to 16 kJ/mol (3.5 kcal/mol) when the influence of the solubility of lead chloride on the reaction rate was taken into consideration. The experimental results are consistent with rate control by the outward diffusion of the PbCl2 reaction product through the solution trapped in pores in the constantly thickening elemental sulfur layer formed on the surface of the galena.

The dissolution of chalcopyrite in ferric sulfate and ferric chloride media

The literature on the ferric ion leaching of chalcopyrite has been surveyed to identify those leaching parameters which are well established and to outline areas requiring additional study. New experimental work was undertaken to resolve points still in dispute. It seems well established that chalcopyrite dissolution in either ferric chloride or ferric sulfate media is independent of stirring speeds above those necessary to suspend the particles and of acid concentrations above those required to keep iron in solution. The rates are faster in the chloride system and the activation energy in that medium is about 42 kJ/mol; the activation energy is about 75 kJ/mol in ferric sulfate solutions. It has been confirmed that the rate is directly proportional to the surface area of the chalcopyrite in both chloride and sulfate media. Sulfate concentrations, especially FeSO4 concentrations, decrease the leaching rate substantially; furthermore, CuSO4 does not promote leaching in the sulfate system. Chloride additions to sulfate solutions accelerate slightly the dissolution rates at elevated temperatures. It has been confirmed that leaching in the ferric sulfate system is nearly independent of the concentration of Fe3+, ka[Fe3+]0.12. In ferric chloride solutions, the ferric concentration dependence is greater and appears to be independent of temperature over the interval 45 to 100 °C.