| The scale of Mianning rare earth ore deposit is in the first place in Sichuan.80% of the rare earth oxides in the ore concentrate in bastnaesite. The currently widely used process is oxidizing roasting-preferred leaching of non-cerium rare earth by dilute hydrochloric acid-alkali decomposition. In production it’s difficult to deal with wastewater containing fluorine which leading serious environmental pollution. In order to solve these problems and develop clean metallurgy process that can efficiently recycle valuable resources like rare earth and fluorine, this paper puts forward the new decomposition craft of bastnaesite concentrate by Ca(OH)2-NaOH. The thesis is divided into the following four parts:The first part:study on the thermal decomposition process and mechanism has been done by the method of TG-DTA and XRD. The results shows that bastnaesite in the ore decompose after affected by the heat. Rare earth oxides are formed from the reaction between the thermal decomposition products and Ca(OH)2、NaOH. At the same time, the fluorine in the ore is fixed in the roasted products in the form of calcium fluoride. Sodium fluoride is also generated in the process. The addition of low melting point NaOH can reduce mineral decomposition temperature and make the product particles gather, forming some large agglomerate.The second part:in the way of quadratic regression orthogonal experiment the influence factors of bastnaesite roasting process are studied, through which the optimal decomposition process conditions are obtained as follows:the quantity of Ca(OH)2 is 15%(mass percent), the quantity of NaOH is 8%, the calcination temperature is 550℃ and the time is 75min. The decomposition rate of bastnaesite concentrate can be 98.13% under this condition.The third part:the phase distribution of bastnaesite concentrate and roasted ore are obtained through MLA et al. The results show that as the main target minerals the mineral association of rare earth ores and fluorite weaken after roast process while mineral liberations increase, which is in favour of the flotation separation between rare earth oxides and fluorite.The fourth part:based on reaction kinetics model and TG-DTA curves the research of roasting reaction kinetics is done. The study results show that reaction is controlled by diffusion and chemical reaction. When the temperature is in the range of 400～450℃, the reaction rate is mainly controlled by chemical reaction mode. When the temperature is in the range of 450～500℃, the reaction rate is controlled by diffusion mode and chemical reaction mode. When the temperature is up to 500℃, reaction rate slows along with the rise of temperature and the influence of chemical reaction speed on the reaction rate is gradually weak. The apparent activation energy of the reaction decreases after NaOH is added into mineral, which would promote reaction.Compared with the technology of oxidizing roasting-preferred leaching, the process engineering proposed in this paper would not produce fluoride waste gas and the fluorine can be effectively recycled. The technology has lower cost of production when compared with NaOH or Na2CO3 roast method and explores a new way for cleaner production of Sichuan bastnaesite. These have important significance in clean and efficient utilization of rare earth resources. |
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