Influence of phase and microstructure on the rate of hydrochloric acid leaching in pretreated Panzhihua ilmenite

The present study investigated the influence of high temperature oxidation and reduction pretreatments on the leaching rate ofPanzhihua ilmenite. The as-pretreated ilmenite was leached with 20% HCI at 105 ℃, The leaching process was controlled by the phases and microstructures that evolved during the pretreatment processes. The leaching kinetics of pure hematite, ilmenite and pseudobrookite were characterized to clarify the phase effect on the iron-leaching rate; the rate of iron leaching occurs in the following order in the HCI solution： hematite （ferric iron） 〉 ilmenite （ferrous iron） 〉〉 pseudobrookite （ferric iron）. Therefore, the often-cited notion that ferrous iron dissolves faster in HCl solutions than ferric iron when explaining the pretreatment effects is inaccurate. Moreover, the oxidation pretreatment （at 600-1000 ℃ for 4 h） cannot destroy the dense structure of the Panzhihua ilmenite. Therefore, the influence exerted by the oxidation on the leaching process is primarily determined by the phase change; oxidation at 600 and 700℃ slightly increased the rate of iron leaching because the ilmenite was transformed into hematite, while the oxidation at 900-1000℃ significantly reduced the rate of iron leaching because a pseudobrookite phase formed. The reduction effect was subsequently investigated; the as-oxidized ilmenite was reduced under H2 at 750 ℃ for 30 min. The reduction significantly accelerated the rate of subsequent iron leaching such that nearly all of the iron had dissolved after leaching for 2 h in 20% HCl at 105 ℃. This enhanced iron-leaching rate is mainly attributed to the cracks and holes that formed during the reduction process.

Aqueous stability of rare earth and thorium elements during hydrochloric acid leaching of roasted bastnaesite

In order to study the aqueous stability of rare earth and thorium elements and the reaction mechanism of hydrochloric acid leaching of roasted bastnaesite,Eh-pH diagrams for La-,Nd-,Ce-,Th-（Cl）-（F）-H2 O systems at 20 ℃ were depicted using HSC Chemistry 6.0 software. Eh-pH diagrams of La-, Ce-,Nd-and Th-Cl-H2 O systems show that trivalent rare earth would be leached into solution by adjusting the acidity of the leaching solution, while tetravalent cerium and thorium would be remained in the leaching residue. And in the case that the pH value of the leaching solution is lower than 2, tetravalent cerium would be partially reduced by chlorine ions（Cl^-）, which is well agreed with the industrial production. It can be inferred from Eh-pH diagrams for the systems La-, Nd-,Ce-, Th-F-H2 O that the leached trivalent rare earth ions（RE^（3＋）） and tetravalent thorium ions（Th^（4＋）） are preferentially combined with fluorine ions（F） to form sediment during non-reductive acid leaching of roasted bastnaesite. However,when controlling the pH value of the leaching solution below 0, fluorine and tetravalent cerium in the roasted bastnaesite would be leached out in the form of soluble [CeF3]~＋ complex. That means the precipitation of REF3 and ThF4·2.5 H2 O can be avoided in the leaching step. According to Eh-pH diagrams for the system Ce-F-Cl-H2 O, [CeF3]~＋ in the leaching solution would be reduced to CeF3 precipitate in the presence of Cl^-, that is to say, tetravalent cerium and fluorine would be firstly leached out to form[CeF3]~＋,which would then be reduced to CeF3 precipitate by Cl^-in the hydrochloric acid leaching process of roasted bastnaesite.

Hydrometallurgical application for treating a Nigerian chalcopyrite ore in chloride medium:Part I.Dissolution kinetics assessment

The dissolution kinetics of a Nigerian chalcopyrite ore in hydrochloric acid was studied in this article. Acid concentration, reaction temperature, and ore particle size were chosen as experimental parameters. The chemical and morphological studies of the ore before and after leaching at optimal conditions were carried out by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. It is revealed that increasing the acid concentration and system temperature and decreasing the ore particle size greatly enhances the dissolution rate. The dissolution kinetics was found to follow the shrinking core model for the diffusion control mechanism where the activation energy （Ea） of 32.92 kJ.mo1-1 was obtained for the process and supported by morphological changes at a higher dissolution of 91.33%.