Metallurgical Process For Selective Leaching And Extraction Of Scandium And Sodium From Iron Rich Bauxite Residue

Red mud,namely bauxite residue or red sludge,is a potential valued solid waste produced from the alumina extraction process with substantial reserve of over 2.7 billion tonnes worldwide.Taking into account the rise in iron ore price and s-carcity of rare earth supply worldwide,recovering aluminium,iron,sodium and other valuable elements from red mud is significant in the disposal of problems as-sociated with these solid wastes.The current application status and composition characteristics of red mud are first introduced in the chapter of literature review.Metallurgical processes for aluminium,sodium,iron,titanium,vanadium,scandi-um and other valuable elements recovery from red mud are investigated in detail by subsequent sections.Some mechanisms and performance of solvent extraction involved in the extraction or stripping process are also reviewed in this paper.The result suggests that much work still need to be done for the improvement of leaching and extraction selectivity as well as efficiency,and also for the development of a green recovery process with environmental benignity,low energy requirements and cost.In this doctoral dissertation,a new process regarding extraction of Sc and Na with high efficiency and selectivity was proposed.Red mud typically contains some rare earth elements like Sc and Ga.In the second chapter,we report on the use of Time of Flight-Secondary Ion Mass Spec-trometry（ToF-SIMS）and Electron Probe Micro-Analysis（EPMA）to provide in-sights on affinities of Sc and Ga with the major elements including Fe,Al,Si,Ti and Ca of red mud.Combined the mapping analyses of high mass resolution of ToF-SIMS with individual spot and mapping analyses of EPMA,the substitution of scandium and gallium for these major elements should follow the order of Ti>Fe>A1>Si and Fe>A1>Ti>Ca>Si,respectively.Scandium has an apparent en-richment in the mineral phases of anatase,hematite and goethite.The hetero-valent isomorphism of 4Sc³⁺ → 3Ti⁴⁺ plays a critical role in substitution of Sc³⁺ into the anatase structure and is different from that of Sc³⁺ → Fe³⁺ occurred in hematite or goethite.Part of scandium is also found to be existed in silicon-rich minerals of quartz or zeolite.Gallium is closely coordinated with ferrum and aluminum,which can proxy for these elements in the crystal lattices of hematite,goethite and gibbsite.All the observations are fundamental clues to the extraction technology of scandium and gallium from red mud.Red mud cannot be directly employed as the raw material of iron-making and construction materials for the existence of sodium element.The effects of cooling methods of furnace,air,water and liquid nitrogen on roasted red mud for recover-ing Na+ with water leaching were investigated through the analyses of TG-DTA,QXRD,FTIR,SEM,etc.The faster cooling methods we used,the better leach-ing performance would be obtained.Liquid nitrogen cooling sample therefore dis-played the best leaching result with concentration of 1202 mg·L-1 Na+ at the first leaching stage（～25 wt%total sodium recovery）.Part of cancrinite known as de-silication products transformed into NaCaHSiO₄ and nepheline after the roasting process.In the fast cooling red muds,the new generation of Na₂Ca（CO₃）₂ could dissolve directly into water;the increase of Ca（OH）₂ concentration to～4.8 wt%was beneficial for the dissolution of cancrinite and NaCaHSiO₄ during the leach-ing process;amorphous phase increasing from～4.1 to～13.5 wt%made sodium be more easily leached out from sodium-containing amorphous phase than the same crystalline phase;fluey flakes or plate-shape particles weakened the aggregation behaviour;the increase of specific surface area from 1.898 to 2.177 m2·cm-3 leaded to the contact area increasing between particles and leachant,implying that sodium could be more easily leached out from the fast cooling samples.Selective recovery of scandium and sodium from high alkali Bayer red mud（RM）could be finished via sulfation-roasting-leaching process.Effects of roasting and leaching conditions including roasting time,roasting temperature,concentrated H₂SO₄ addition,leaching temperature,leaching time and liquid to RM solid ratio on the leaching rates of calcium,iron,aluminum,silicon,sodium,titanium,scandium and gallium were first studies and analyzed,suggesting that roasting temperature and roasting time are the two primary constraints on selective recovery of Sc and Na.High leaching temperature also brings a negative effect on the iron leaching rate.Phase transitions and thermal behaviors of sulfated RM indicate that sodium has an inhibitory action on the liberation of SO₂ or SO₃ from metal sulfates,which should follow the decomposition order of TiOSO₄>Ga₂（SO₄）₃>Fe₂（SO₄）₃>NaFe（SO₄）₂>NaAl（SO₄）₂～Al₂（SO₄）₃>Na₃Sc（SO₄）₃>Na₂SO₄>CaSO₄.After water leaching,solid-liquid separation can be carried out extremely smoothly and>95 wt%Na+,～60 wt%Sc in[Sc（H₂O）x（SO₄）n]3-2n（x≤6）with impurities of<1 wt%Fc³⁺,0 wt%Ti⁴⁺,0 wt%Ga³⁺,7 wt%Al³⁺,～29 wt%Ca2+ and<3 wt%Si⁴⁺could be leached into leachant under the optimized roasting and leaching conditions.The alkali-free residue obtained can then be employed as iron-making or building materials.The organophosphorus extractants of di-2-ethylhexyl phosphoric acid（P204,D2EHPA or DEHPA,HDEHP）,2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester（P507,HEHEHP or Ionquest 801,PC-88A,SME 418）and Versatic Acid 10 were used to extract the Sc from the leaching liquor of sulfation-roasting-leaching red mud.P204/sulfonated kerosene（1%v/v）exhibts a novel extraction perfor-mance for Sc under the condition of A:O of 10:1,extraction temperature of 15 ℃.Nearly 97%Sc could be extracted out from the red mud leachate.After the extrac-tion process,the organic phase could be scrubbed with sulphuric acid at pH 0.35～0.5 to remove impurities and then stripped by 2 mol·L-1 NaOH solution.