氯化镁焙烧分解磷酸镧的反应机制研究

Decomposition Mechanism of Lanthanum Phosphate Roasted with Magnesium Chloride

针对处理独居石的工艺条件苛刻、废渣废液处理难度大,以磷酸镧为代表,研究了采用氯化镁焙烧分解独居石的机制。根据已有的工作基础,对焙烧温度、焙烧时间、矿盐比等工艺参数对独居石焙烧分解的影响进行研究,利用X射线衍射(XRD)、热重分析(DSC-TG)、扫描电镜(SEM)和能谱(EDS)分析等表征手段对焙烧产物进行分析,研究磷酸镧氯化焙烧反应机制,推断分解化学反应。实验结果表明:氯化镁在300℃分解生成碱式氯化镁和氯化氢,碱式氯化镁和氯化氢在400℃与磷酸镧反应生成氯化镧,氯化镧又分解生成氯氧化镧;在480℃氯化镁分解产物氧化镁和氯化氢与磷酸镧反应生成氯氧化镧和磷酸镁;在550℃以后,氯氧化镧和磷酸镁又重新反应生成磷酸镧、氧化镁和氯化氢;温度超过600℃氯化镁不能分解磷酸镧。氯化镁分解产物氯化氢和水在磷酸镧的分解过程中具有催化分解作用。

With the gradual awareness of environmental protection,how to smelt rare earth in green,high efficiency,low consumption and moderate conditions was the direction of people’s efforts. As one of the main rare earth minerals,monazite was smelted by concentrated sulfuric acid roasting and concentrated lye roasting,both of which existed the problems of waste gas,waste liquid,waste residue and serious environmental pollution. Monazite composed of phosphate of a variety of rare earth elements was also known as phosphorus lanthanum cerium stone,of which lanthanum accounted for more than 30%. In view of the harsh processing conditions of monazite and the difficulty in the treatment of waste residue and liquid,this paper proposed a clean roasting technology for monazite with MgCl2,represented by lanthanum LaPO4. Based on existing work,artificial synthesis of LaPO4 as the research object,the effects of roasting temperature,roasting time and salt mineral ratio on rare earth extraction ration were researched,by means of X-ray diffraction(XRD),thermogravimetric analysis(TGA),scanning electron microscopy(SEM)and energy dispersive analysis(EDS). The reaction mechanism of LaPO4 roasting with MgCl2 was studied and its chemical reaction was also inferred. According to the thermogravimetric analysis of LaPO4 and MgCl2,the weight loss of the mixed mineral was between the amount of H_(2)O and HCl produced,MgCl2 was decomposed while LaPO4 was not at 290 ℃. The weight loss of mixed ore was obviously lower than the amount of HCl produced in the range of 450-550 ℃,and the weight loss of the mixed ore was close to the theoretical release of HCl above 600 ℃ MgCl2,indicated that LaPO4 was decomposed with MgCl2 at 450 ℃,while LaPO4 was not decomposed above 600 ℃. From the experiments on the roasting temperature,at 300 ℃ MgCl2 was decomposed to Mg(OH)Cl and HCl,which reacted with LaPO4 to form LaOCl at 400 ℃. MgO and HCl reacted with LaPO4 to LaOCl and Mg3(PO4)2 at 480 ℃. Beyond 550 ℃,LaOCl and Mg3(PO4)2 re-reacted to LaPO4,MgO and HCl. Over 600 ℃ LaPO4 could not decompose with MgCl2. In thermodynamic calculation,the Gibbs free energy of the reaction was 0 at495 ℃. That was to say,when the temperature was lower than 495 ℃,LaPO4 was decomposed into LaOCl,which was forward reaction,while LaOCl regenerated LaPO4 at above 495 ℃,in the opposite direction. From the experiments on the roasting time,with the increase of roasting time,LaPO4 was decomposed into LaOCl at 400 ℃,which was the forward reaction. However,at the early stage of roasting the reaction was forward,and when the time was extended to 120 min,the reaction was carried out in the reverse direction,LaOCl and HCl formed LaPO4 again at 550 ℃. The roasting time would not exceed 60 min. The ratio of MgCl2 had a certain effect on the decomposition rate of monazite. Increasing the ratio of magnesium chloride was beneficial to the decomposition rate of LaPO4 at480 ℃,to the decomposition rate of LaPO4. Excessive MgCl2 was decomposed to MgO,which had no obvious effect on the decomposition rate of LaPO4,and the decomposition rate of LaPO4 could not be improved by increasing the amount of MgCl2 above 550 ℃. The supplemental ratio of MgCl2 was less than 2.6 times of LaPO4. The decomposition products of LaPO4 and MgCl2 was mainly LaOCl,MgO and LaPO4 at 480 ℃,in which MgO existed as needle-like crystal,LaOCl as columnar accumulation. At the same time,the part of non-decomposed LaPO4 existed in the roasted ore. The results could be well verified by EDS and XRD of the roasted ore. It could be concluded from the experiment that temperature had a great effect on the decomposition of LaPO4,the optimum conditions for the decomposition of LaPO4 by MgCl2 were as follows:the mass ratio of ore to salt was 1.0∶1.3-2.6,the roasting temperature was 480 ℃,the roasting time was 60 min. When the temperature exceeded 600 ℃,MgCl2 could not decompose LaPO4. Reaction mechanism of decomposition of LaPO4 by MgCl2 was that Mg(OH)Cl and HCl(the decomposition products of MgCl2)reacted with LaPO4 to form LaCl_(3) and Mg3(PO4)2 between 400 and 480 ℃,and then LaCl_(3) was decomposed to LaOCl. MgO and HCl were the decomposition products of MgCl2 reacted with LaPO4 to form LaOCl and Mg3(PO4)2 between 480 and 550 ℃. Beyond 550 ℃,LaOCl and Mg3(PO4)2 re-reacted to LaPO4. As the decomposition products of MgCl2,HCl(g)and H_(2)O(g)in the reaction system as both products and reactants,cycling,in the decomposition process of lanthanum phosphate played a catalytic role in the decomposition of LaPO4.