Study Of The Technology For The Decomposition Of Baotou Rare Earth Concentrate By Concentrated NaOH Solution

With the rapid development of science and technology, and the sense of environmental protection is deep into people's mind, the technology of alkali decomposing of Baotou rare earth concentrate is becoming favored gradually. In this thesis, the technics for direct decomposition of Baotou rare earth concentrate by concentrated alkali has been studied. The experiment was carried out by roasting of the mixture of Baotou rare earth concentrate and 70% NaOH solution. It was solid-liquid reaction in the start, and changed into solid-solid reaction in the end. The influence of the phase change before and after the reaction on the decomposition efficiency of the rare earth was also investigated.The main contents of the thesis could be divided into three parts:1. Mechanism discussionThermogravimetry and differential thermal analysis of Baotou rare earth concentrate with the addition of NaOH were carried out, and the thermal decomposition mechanism of the complex system was investigated.2. The roasting decomposition and leaching of rare earth concentrate at static conditions The influence of the calcium in Baotou rare earth concentrate on the decomposition efficiency and the recovery ratio of rare earth was investigated in the thesis. The comparison of the experimental results showed that the decomposition efficiencies of rare earth were approximate before and after the removal of calcium, which indicates that the existence of calcium was not the main factor that affecting the decomposition efficiency of rare earth. Therefore, it is possible to carry out the alkali decomposing technology of rare earth concentrate directly without removing calcium by chemical beneficiation.The orthogonal experiments with three factors and four levels were conducted to investigate the influence of the ratio of rare earth concentrate to alkali, the calcination temperature and time on the decomposition efficiency of rare earth. The results showed that the ratio of rare earth concentrate to alkali is the most influential factor in the reaction, and secondly for calcination temperature and time. The optimum process conditions were obtained by experiments:the ratio of rare earth concentrate to alkali was 1:1, the calcination temperature was 400℃, and the calcination time was 90 minutes. The decomposition efficiency could reach 84.80%.In the static roasting experiment, the content of fluorine in the calcined rare earth concentrate reduced with the increase of the calcinations temperature. The effect of fluorine fixation became worse over 400℃, and the air contamination will be caused by fluorine in the form of gas.3. The roasting decomposition and leaching of rare earth concentrate at dynamic conditionsDifferent grade of rare earth concentrate and 70% NaOH solution were mixed at low and medium temperature, and the ratio of rare earth concentrate to alkali were kept at 1:1, after which the dynamic roasting was conducted. The results showed that the decomposition efficiency of 50% rare earth concentrate could reach 93.84%, and the maximum was 98.44%.In the dynamic experiments, bastnasite could be decomposed effectively, and the fluorine existed in the form of F- in water after washing. After acid dissolution of the calcined rare earth concentration, over 98% of the thorium existed in the form of Th4+ in the leaching liquid, which means that the residue after alkali decomposing was not radioactive, and the amount of the residue is small.The alkali decomposing technology could get rid of the pickling procedure in the traditional process. The process is continuous, and there was no production of waste gas and radioactive residue. The chemical elements such as rare earth, thorium, fluorine, and phosphorus could be recycled, which realizes the goal of clean production and resource comprehensive utilization.