The impact of desilication product on bauxite residue flocculation

The pre-desilication step within the Bayer Process seeks to transform reactive silica in bauxite into desilication product (DSP) prior to digestion, thereby reducing post-digestion precipitation and scaling. The precipitated DSP is removed with the other residue phases in the primary settling stage. While the proportion of DSP within the residue can be significant, there are surprisingly few open-literature studies that consider the impact of DSP on residue flocculation and settling. Such studies typically involve bauxites of a fixed composition, which limits the scope to vary residue properties while investigating flocculation mechanisms. In this study, DSP has been formed from the reaction of standard kaolin in synthetic Bayer liquor in the presence of iron oxides. Variation of the reaction conditions (temperature, duration, kaolin to iron oxide ratio) has produced a range of synthetic residue slurries that have been characterised in terms of their physical and flocculation properties. Effective comparison of the latter could only be achieved after detailed optimisation of test conditions, which included (i) slurry stability over time, (ii) solids concentration, (iii) temperature and concentration of the dosed flocculant, and (iv) flocculant make-up/shelf life. The detrimental impact of DSP is clearly reflected in lower settling rates, higher supernatant solids and poor consolidation over a range of solid densities and DSP contents of operational interest. Flocculant dosage response curves are also less steep and shifted towards higher demand. A number of flocculants are contrasted in terms of their suitability for high DSP residues, with the practical implications of their application discussed

Leaching nickel and nickel sulfides in ammonia/ammonium carbonate solutions

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)

Effect of thiosulfate, sulfide, copper(II), cobalt(II)/(III) and iron oxides on the ammoniacal carbonate leaching of nickel and ferronickel in the Caron process

Previous studies have related the low recovery of nickel and cobalt in the Caron roast-leach process to the formation of less reactive sulfides during roasting and/or the passivation or surface blockage of ferronickel by oxides and/or sulfides. This study examines the different types of reactions and background reagents which may affect the dissolution of nickel and ferronickel alloy in oxygenated NH3/NH4+/HCO 3- solutions at 45 °C based on equilibrium constants and measured leach results at a low solid/liquid ratio of 1 g/dm3. Some of the additives tested in the present study represent interim leach products. Initial leaching rates of nickel during oxygenation of presoaked Fe-Ni alloys decreases with increasing iron mole fraction. The Fe-Ni(45%) alloy continues to react and dissolve about 90% Ni over the first 15-40 min, depending upon the additives. In contrast, iron leaching reaches a broad maximum of ~ 10% over 20-35 min, or a sharp maximum of 6% after 5 min in the absence or presence of additives, respectively. This is followed by a decrease in iron extraction to ≤ 2% after 45-60 min due to the precipitation of red/black oxides and sulfides. Direct involvement of S2O32- and redox mediation by Cu(II) or Co(III) is evident from the enhanced initial rates of nickel leaching from Fe-Ni(45%) alloy in the order: O2/HS - O2 < O2/S2O32- < O2/Co(III)/S2O32- < O2/Cu(II)/S2O32-. While the added S2O32- has a detrimental effect on iron leaching, HS- retards the leaching of both iron and nickel from Fe-Ni(45%) alloy to < 1%. The final leaching of 95% Ni from Fe-Ni(45%) alloy after 3 h is unaffected by Fe2O3 and Fe3O 4, but FeOOH causes about 5% decrease in nickel leaching. Thermodynamics predict the passivation of nickel and ferronickel by M(OH) 3-MOOH as well as the formation of MFe2O4

Sodalite solids formation at the surface of iron oxide and its impact on flocculation

Sodalite represents the main desilication product (DSP) phase formed from reactive silica during alkaline digestion of bauxite in the Bayer Process. Previous studies into DSP effects on bauxite residue flocculation have focused on flocculant selection or digestion optimisation, not answering a fundamental question: does DSP coat the residue and thereby change surface properties? This study sought to answer that question by contrasting the physicochemical properties of hematite slurries (as a model phase for residue) containing DSP where it was either made in-situ or added as a physical mixture. On the basis of differences found in dewatering behaviour, zeta potential, desilication rates and microscopy of the solids, it is proposed that DSP nuclei initially associate with the hematite surface and subsequently affect flocculant adsorption chemistry, resulting in different extents of flocculant adsorption and smaller aggregates. The practical implications for flocculation processes are discussed

Comparative leaching of spent zinc-manganese-carbon batteries using sulfur dioxide in ammoniacal and sulfuric acid solutions

The non-magnetic fraction of spent zinc-manganese-carbon batteries containing 20.8% Zn, 22.7% Mn, 2.65% Fe, and < 0.1% Hg, Ni, Co, Cu and Pb was leached in H2SO4, H2SO4/SO 2, NH3 and NH3/SO2 at 30-60 °C. In acid media the complete dissolution of zinc is unaffected by SO2. However, the reductive role of SO2 increases the leaching of manganese in H2SO4 from 25% to 100% and iron from 1% to 25%. Literature results of leaching with other reductants are compared. The XRD analysis of leach residues from ammonia solutions shows the conversion of Zn5(OH)8Cl2•H2O in the feed to ZnO with low dissolution of zinc, manganese and iron. Low leaching of iron in NH3/SO2 is due to the formation of Fe(NH4) 2(SO3)2 and Fe(NH4) 2(SO4)2 identified by XRD analysis of leach residues. However, the formation of Zn(NH3)42+ facilitates the selective leaching of zinc in buffered SO2/NH 3 solutions that contains NH4+. Complete dissolution of copper also occurs in both H2SO4/SO 2 and NH3/SO2. The dissolution of mercury by H2SO4 is retarded in the presence of SO2, and enhanced by NH3/SO2. Lead remains insoluble in all media, whilst the partial dissolution of nickel and cobalt is retarded by NH 3/SO2

Sodalite solids formation at the surface of iron oxide and its impact on flocculation

© The Minerals, Metals & Materials Society 2016. Sodalite represents the main desilication product (DSP) phase formed from reactive silica during alkaline digestion of bauxite in the Bayer Process. Previous studies into DSP effects on bauxite residue flocculation have focused on flocculant selection or digestion optimisation, not answering a fundamental question: does DSP coat the residue and thereby change surface properties? This study sought to answer that question by contrasting the physicochemical properties of hematite slurries (as a model phase for residue) containing DSP where it was either made in-situ or added as a physical mixture. On the basis of differences found in dewatering behaviour, zeta potential, desilication rates and microscopy of the solids, it is proposed that DSP nuclei initially associate with the hematite surface and subsequently affect flocculant adsorption chemistry, resulting in different extents of flocculant adsorption and smaller aggregates. The practical implications for flocculation processes are discussed