Chemical Behavior Of Fluorine In Rare Earth With Fluorine During Roasting Process

Rare earth concentrate with fluorine is major material in rare earth industry, including single bastnaesite and mixed rare earth concentrate with bastnaesite and monazite.Roasting is the major technology in the treating process of rare earth concentrate with fluorine.Escaping fluorine in roasting course not only seriously polluted the environment but also made the valuable resource wasted.For this reason, the behaviors of fluorine in roasting course of rare earth concentrate with fluorine was studied, including its trending, existing state, escaping condition and reacting process. It provided theory foundation for controlling fluorine and effectually recovery processing, building a new clean manufacturing technology directing at different rare earth mineral.The paper consists of four parts.In the first part, measuring method of fluorine in the gas from rare earth concentrate with fluorine was studied. By comparing and analyzing the current approaches, the method of using Lye absorption, derivative gasification and gas chromatography analysis was convenient and fast. When the absorption of gas flow was controlled in 3Lmin-1 and the concentration of NaOH was 0.001molL-1,the absorption efficiency was up to 96.1%, the relative standard deviation of the derivative gas chromatography analysis was 1.43% and the recoveries of standard were in range of 96.9%-103.3% and RSD was 2.75%.This method had higher precision and accuracy.In the second part, behavior rule of fluorine in roasting process of bastnaesite was investigated. Fluorine in the gas during roasting process of bastnaesite was determined by gas chromatography under the different conditions such as in dry air,60% relative humidity and saturated water vapor. The results showed that the basic condition for fluorine escaping was the presence of water vapor in the roasting atmosphere. With the increasing of roasting temperature, time and air humidity, the content of fluorine in gas was also increasing at a large amount. Under the conditions of saturated water vapor, roasting temperature at 1000℃and time for 120min, escaping ratio of fluorine was up to 98.362%, which can be completely absorbed in 0.001molL·-1 NaOH solution.In the third part, behavior rule of fluorine was studied during the bastnaesite was roasted in the system of CaO-NaCl and CaO-NaCl-CaCl2. During the bastnaesite roasting process, calcium oxide had an obvious function on fixation of fluorine; the fluorine existed in the form of REOF and CaF2 in the roasting products and in the form of HF in gas.Adding NaCl or NaCl-CaCl2 could not affect the bastnaesite decomposition, but could reduce the reaction temperature of CaO and the decomposition product REOF by nearly 50℃and 100℃,at the same time could restrain the fluorine escaping. When the relative humidity was 60% and bastnaesite roasted at 700℃for 120min, the fluorine fixation ratio was 82.42%.In the last part, behavior rule of fluorine was studied during the mixed rare earth concentrate roasting process in the system of CaO-NaCl and CaO-NaCl-CaCl2. During the roasting process of the mixed rare earth concentrate CaO could not only restrain the fluorine escaping but also decompose the monazite. CaO together with CaF2 could decompose REPO4 better than a single CaO, fluorine mainly existed in the form of Ca5F(PO4)3.Adding NaCl or NaCl-CaCl2 could make the fluorine fixation ratio of CaO reach above 92%.By using NaCl-CaCl2 as auxiliary agent, the reaction temperature and the apparent activation energy were lower when monazite was decomposed at 700℃and for 120 min.,the decomposition ratio of the mixed rare earth concentrate was more than 90%.The results of dynamics showed that the chemical reactions apparent activation energy reduced by adding NaCl and NaCl-CaCl2, it advanced the decomposition of monazite, and NaCl-CaCl2 was more obvious.