Thermal decomposition and oxidation of bastnaesite concentrate in inert and oxidative atmosphere

To clearly elucidate the oxidative roasting behaviors of the bastnaesite, the thermal decomposition and oxidation of the bastnaesite concentrate in inert and oxidative atmosphere have been investigated in detail. Experimental data indicated that the initial decomposition temperature of the concentrate under N2 atmosphere is 150 ℃ higher than that under O2 atmosphere,most likely because the oxidation of the cerium induces the decomposition of the concentrate. For the roasted samples under N2 atmosphere at500 ℃ and above,the oxidation efficiency of the cerium is 19.8%-26.8% because of the fact that rareearth fluorocarbonate is first decomposed to form rare-earth oxyfluoride and CO2, and the cerium oxyfluoride is then partially oxidized by the CO2 gas. The rest cerium in these samples can be further oxidized in air at room temperature, with the oxidation efficiency of the cerium gradually increasing to above 80% in 7 d. This can be attributed to the obvious changes in the inner morphology of the roasted samples under N2 atmosphere at high temperatures, which largely induce the diffusion of the air and improves the oxidation activity of CeOF, and further induces the oxidation of CeOF by the air. XRD and XPS techniques were used to further verify the significant differences in the thermal decomposition behaviors of the bastnaesite concentrate under N2 and O2 atmosphere. Moreover, no oxidation of Pr^（3＋） to Pr^（4＋） in the roasted samples under both N2 and O2 atmosphere is observed. This gives an overall understanding of the oxidative roasting of the bastnaesite concentrate without additives.

Synthesis of lanthanum oxide nanosheets by a green carbonation process

Two-dimensional metal oxide nanostructures,such as nanoplates,nanowalls,and nanosheets,have received further attentions in recent years,due to their outstanding properties resulted from the small thickness and quantum size effects.In this work,lanthanum oxide nanosheets with the thickness of 56 nm had been successfully prepared by an originally simple method of carbon dioxide carbonation;the preparation process was green without using surfactants and detergent.The X-ray diffraction,transmission electron microscopy,and scanning electron microscopy were used to characterize the products.Thermogravimetric-differential thermal analysis and FT-IR were introduced to prove that the lanthanum oxide precursor with lamellar morphology was La2(CO3)3 3H2O.The carbon dioxide carbonation method would provide significant benefits containing mild reaction conditions,high efficiency,low cost,and easy to realize large-scale production.

Recovery of rare earths and aluminum from FCC waste slag by acid leaching and selective precipitation

This paper investigated the recovery of rare earth elements(REEs) and aluminum(AI) from the waste slag discharged by FCC catalyst factory(FCC waste slag) via acid leaching and selective precipitation.Analysis methods such as ICP-AES, XRF and XRD were applied to obtain experiment data. The maximum leaching efficiency of REEs and Al was achieved at pH value of 1 and with liquid to solid ratio of 4:1,Under such conditions, 91.01%, 92.24% and 94.77% of La, Ce and Al were extracted at 20 ℃ for 2 h from the FCC waste slag, respectively. The SiOcontent in the leaching residue was 88.3%, which can be used as an available silicon resource. The REEs can be precipitated in the form of REEs and sodium double sulfate(NaRE(SO)·xHO) by adding NaSOto the leaching solution, while Al remained in the solution. Afterwards, the pH value of the filtrate was adjusted to 4.5, and Al was precipitated as AI(OH). Finally,NaRE(SO)·xHO and Al(OH)were converted into RECland Al(SO4)solution,respectively, which were recycled to manufacture zeolite. This process recovered REEs and Al from the FCC waste slag and reduced the emissions of waste slag simultaneously, which has an important economic and environment significance.

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.