Gamini Senanayake

Monazite, a rare earth phosphate mineral, is the second most important primary source of rare earth elements (REEs). Current technologies for processing monazite ore using sulfuric acid are primarily focused on REEs recovery. However, these technologies result in the loss of phosphorus in waste streams. Therefore, there is a need for efficient processing methods that could recover both REEs and phosphorus from monazite ore. This study presents a method for recovering both REEs and phosphorus as potential feed material for iron phosphate battery precursor from monazite ore by sulfuric acid baking with the addition of sulfate salts. The leaching efficiency of REEs and phosphorus varied depending on the additive used, with the highest efficiencies observed for the ferric sulfate system. As the temperature increased, the leaching efficiency of REEs and phosphorus decreased when baking with no additive. However, the addition of ferric sulfate salt to the baking reactants improved leaching efficiency of REEs and favourably enriched P in the residue for subsequent processing. The XRD confirmed the successful constraining of P and Fe in the residue while more than 95% REEs were selectively leached. The results suggest that this method could be a promising alternative to conventional methods for processing monazite ore. An integrated flowsheet was proposed to produce a marketable rare earth oxide product of over 99% purity.

Mineral carbonation offers permanent and safe disposal of anthropogenic CO2. Well distributed and abundant resources of serpentine minerals and natural weathering of these mineral to stable and environmentally benign carbonates favour the exploitation of these minerals as the most suitable raw material for mineral carbonation. However, slow dissolution kinetics are impeding the large scale implementation of mineral carbonation. Heat treatment of serpentine minerals results in enhanced reactivity for subsequent carbonation processes at the expense of an additional energy penalty4. Heat treatment of these minerals results in the removal of structurally bound hydroxyl groups which leads to partial amorphisation of the structure and enhanced reactivity. Therefore, understanding the role of the mineralogical changes during dehydroxylation and determination of activation energy (Ea) is crucial for providing an energy efficient solution for commercialisation of mineral carbonation...

Thiosulphate has received much attention as an alternative non-cyanide lixiviant for gold recovery over the last three decades. In particular, a number of studies have shown that an ammoniacal copper(II)/thiosulphate system offers fast leaching kinetics, but there are difficulties in controlling the complex solution chemistry and there are concerns over the use of ammonia. Recently, thiosulphate leaching of gold in the absence of ammonia has shown to be one of the most promising alternatives to cyanide, as evident from the thiosulphate gold processing plant recently commissioned at Barrick Goldstrike for treating pressure-oxidized double refractory ore. However, the published information on non-ammoniacal thiosulphate systems is limited. In this work, the dissolution of gold in non-ammoniacal thiosulphate solutions has been studied using a rotating electrochemical quartz crystal microbalance (REQCM), rotating gold disk, gold powder, and selected sulphidic gold ores. The electrochemical studies found that gold oxidation is enhanced by increases in temperature, thiosulphate concentration, and the addition of low levels of copper. Oxygen reduction was found to occur much more readily on sulphide mineral surfaces than on the gold surface, offering an opportunity for galvanic interaction. The subsequent leaching tests using REQCM showed that the gold leach rate in the oxygen-thiosulphate system without any additives is in the order of 10-7 mol m-2 s-1 , two orders of magnitude lower than a typical cyanidation rate. However, the use of elevated temperature, high oxygen concentration, and copper addition, in conjunction with the galvanic effect of sulphide minerals, dramatically improved the gold leach rate to the same order of magnitude as a typical cyanidation rate. This was supported by the results obtained from prolonged leaching tests using gold powder and sulphidic gold ores. This study hence shows that the oxygen-thiosulphate system could be a promising alternative to cyanidation for treating some sulphidic gold ores.

A preliminary investigation has been conducted to determine the conditions for the precipitation of phosphate from the liquor produced after a hydrochloric acid pre-leach and rare-earth recovery from a rare earth containing fluorapatite concentrate from the Nolans rare earth deposit in the Northern Territory of Australia. The results show that at 50oC the phosphate can be precipitated in two stages using limestone to produce a Stage 1 precipitate containing most of the impurities including iron (94%), aluminium (92%), fluoride (98%), uranium (84%) and thorium (94%), followed by a relatively pure Stage 2 dicalcium phosphate precipitate (DCP) containing only 0.05% Fe, 0.05% Al, 0.1% F, 0.01% U, and <0.01% Th. The initial Stage 1 precipitate contained about 25% of the phosphate while the remaining 75% reported to the Stage 2 phosphate precipitate. Depending on the required radioactivity specifications, the amount of phosphate in the Stage 1 precipitate could be reduced. Reagent requirements were 11.4 kg limestone (as pure CaCO3) per cubic metre of solution for the Stage 1 precipitate and 39.7 kg limestone (as pure CaCO3) per cubic metre for Stage 2. The analysis of radionuclide balance indicated that radionuclides deported mainly to the final filtrate (86.4%) compared to Stage 1 precipitate (5.4%) and Stage 2 precipitate (8.2%). Settling and pressure filtration tests indicated the precipitates were both readily settled and filtered. The final moisture contents of the Stage 1 and Stage 2 precipitates were 62.5% and 49.3%, respectively.

The extraction of rare earth metals from fluoroapatite rich phosphate concentrates often involves a weak acid leach of fluoroapatite and a sulphuric acid bake of leach residue followed by water leach and precipitation to obtain an intermediate product. The main aim of this study was to determine and rationalize the solubility of rare earth metal ions in synthetic solutions representing various process liquors at three temperatures 40, 60 and 80oC containing different acid and metal ion concentrations with respect to sulphuric acid, phosphoric acid and sulphate salts of sodium, magnesium, aluminium, potassium, calcium and iron(III). Solid mixture of rare earth metal carbonates was used as the source of rare earth metal ions. The solubility tests and characterization of solids using XRD were conducted at Murdoch University laboratories. The composition of initial carbonate solids and the solids and solutions formed after saturation were analysed for rare earth and other elements at TSW Analytical Laboratories in Perth using ICP-MS and ICP-OES. The precipitated solid in sodium rich acidic sulphate solutions is the double salt NaRE(SO4)2 based on the solid assays. However, in acidic solutions free of sodium or of low concentrations of sodium the precipitated solid appears to be RE2(SO4)3.

A number of sulfide minerals of copper, nickel and iron can only be effectively leached by methods that require elevated temperatures in acidic solutions. While suitable for concentrates, these processes are not applicable to tank or heap leaching of low grade ores. It is now generally accepted that many of these minerals are subject to so-called passivation under oxidising conditions. However, the mechanisms involved in this passivation are not well understood and this study will review some of these mechanisms and show that the passivation of several of these important minerals appears to follow similar trends. Electrochemical measurements using millerite, covellite and chalcopyrite electrodes were used to study the similarities of leaching and passivation processes taking place in oxidative acidic media. Thus, potentiostatic current-time transient experiments have demonstrated the resemblance with selective dissolution of metals from alloy systems. A model will be proposed which involves depletion of metal from outer layers of the minerals and the limiting step being diffusion of the metal ions through sulfur sub-lattice in these minerals. Potential step polarization experiments have enabled estimates to be made of the selfdiffusion coefficients of copper, nickel and iron which were in the range of published values extrapolated from higher temperatures. The well known parabolic leaching behaviour of these minerals is supported by solid state diffusion as the rate limiting step.

In the Caron process, ferronickel produced by reductive roasting of laterite ores is leached in ammonia/ammonium carbonate solution. Thiosulfate ions in the leach liquor, originated from sulfur containing fuels, interfere with the leaching process. A comparative fundamental study of the reaction equilibria, Eh-pH diagrams, and leaching kinetics of Ni, Ni3S 2, NiS, and NiO has been conducted in the presence of different oxidants/anions. Typical Caron conditions were employed in order to examine whether the formation of nickel sulfides/oxide is responsible for incomplete leaching. The relative rates of nickel extraction in the stirred reactors after two hours, decrease in the following order: Ni > Ni3S2 > NiS > NiO. Although Cu(II) and thiosulfate show a synergistic effect to improve the dissolution of metallic nickel, the presence of thiosulfate depresses the dissolution of both nickel sulfides. In the oxygenated Cu(II) leaching system, thiosulfate and hydrosulfide have a beneficial effect on leaching metallic nickel and sulfite has a beneficial effect on both nickel sulfides. Equilibria and relative rates are used to shed more light on possible surface reaction mechanism(s).

The beneficial effect of silver in gold leaching by oxygen in alkaline cyanide and thiosulfate media has been reported by previous researchers. This paper describes a preliminary study of a similar effect in thiosulfate leaching of gold by ammoniacal copper(II) at ambient temperature. Silver dissolves faster than gold in ammoniacal copper(II) thiosulfate solutions. The results based on gold colloids show that the addition of silver colloids or silver(I) nitrate has a beneficial effect on gold dissolution. The chemical controlled dissolution of gold by copper(II) is slower than the diffusion controlled dissolution of silver. The rate of dissolution of gold powder is also enhanced in the presence of silver(I) nitrate. The dissolution curves of gold colloids were analysed on the basis of a shrinking sphere kinetic model. Species distribution diagrams, electrode potential calculations, and gold and silver dissolution kinetics were used to shed more light on the surface reaction mechanism.

Despite the research interest on non-cyanide gold lixiviants over the past three decades all the non-cyanide gold processes are still at the developmental stages with limited understanding on solution and pulp chemistry in some cases. This is partly associated with: 1. the difficulties in measuring reliable equilibrium data for various gold(I/III) complexes with non-cyanide ligands; 2. the lack of knowledge on mixed-ligand complexes; and 3. different kinetic stabilities of gold(I) complexes with respect to disproportionation. This paper presents reliable equilibrium data for complex formation, dissolution, precipitation, hydrolysis and disproportionation reactions of gold(I/III) compounds with a range of non-cyanide ligands: S 2-, HS -, S 2O 3 2-, SO 3 2-, SCN -, SC(NH 2) 2, OH -, NH 3, Cl -. Solubility of gold(III) compounds with non-cyanide ligands follows the order: Cl - > NH 3 > OH -. In the case of gold metal the solubility order at a given temperature is MgS > Fe(III) or Cu(II) / Cl - > Cu(II)/NH 3 > NaOH, but the solubility increases with increasing temperature. Rate of gold oxidation with different non-cyanide lixiviants depends on the oxidant, ligand and temperature and follows the general order of oxidants: Fe(III) or Cu(II) > X 2 or OX - > O 2 (X = halogen). In the case of metal ion oxidants oxygen is required to reoxidise Cu(I) or Fe(II). Some of the reported data on chloride and thiosulfate leaching of gold are presented to highlight the effect of additives and the copper, silver and sulfur contents of the starting material on the success of non-cyanide leaching processes.