In addition to the two methods of sulfuric acid roasting decomposition and sodium hydroxide decomposition, high temperature chlorination, molten salt extraction and acid-base combination methods have also been studied. These methods based on the properties of minerals, there are unique, but the process of the three kinds of decomposition methods are not a complete recovery of valuable minerals, fluorine, phosphorus, thorium, barium step of setting, these elements often remain in the exhaust gas, Wastewater and waste residue pollute the environment. Although some of the research schemes can recycle two kinds of elements in the process, the equipment involved is complicated, the chemical raw materials are expensive to consume, the process is long and difficult to operate, and it is difficult to produce. achieve. In recent years, based on the purpose of environmental protection, some progress has been made in the process of decomposing mixed rare earth concentrates, among which there are several reasons. 1. NaCO 3 roasting method At high temperatures, sodium carbonate can decompose rare earth fluorocarbonate and phosphate in the mixed rare earth concentrate into rare earth oxides. During the decomposition process, other components in the mineral will also participate in the reaction, complicating the composition of the calcined product. The characteristics of sodium carbonate roasting are: 1. During the roasting process, the rare earth mineral is decomposed into a rare earth complex salt soluble in rare earth oxide, and the tritium is trivalently oxidized to tetravalent; 2. The calcined product contains Na 3 PO 4 Non-rare earth magazines such as BaCO 3 , Na 2 SO 4 , CaCO 3 , NaF. In order to prevent these impurities from forming rare-acid rare earth sulfate double salt and rare earth phosphate with rare earth during sulfuric acid leaching, the rare earth loss is caused. Before the sulfuric acid leaching, the calcined product is pretreated by washing with water and pickling; 3. The rare earth sulfate solution can be connected. Solvent extraction and extraction of ruthenium and recovery of ruthenium; 4, roasting waste gas and leaching waste slag and wastewater are less polluting to the ring. The sodium carbonate roasting method still has problems such as agglomeration of the calcined product in the rotary kiln during the roasting process and has not been used for industrial production. Shown in Figure 1 is the industrial test procedure. Figure 1 Process flow of sodium carbonate decomposition mixed rare earth concentrate (a) calcination reaction When the mixed rare earth mineral is calcined with Na 2 CO 3 at 600 to 700 ° C, the following chemical reaction occurs: 2REFCO 3 +Na 2 CO 3 =RE 2 O 3 +2NaF+3CO 2 (1) 2CeFCO 3 +Na 2 CO 3 +(1/2)O 2 =2CeO 2 +2NaF+3CO 2 (2) 2REPO 4 +3Na 2 CO 3 =RE 2 O 3 +2Na 3 PO 4 +3CO 2 (3) Th 3 (PO 4 ) 4 +6NaCO 3 =3ThO 2 +4Na 3 PO 4 +6CO 2 (4) At 750-780 ° C, the fluorite (CaF 2 ) in the concentrate is further reacted with Na 3 PO 4 and NaF formed by the above reaction to form NaREF 4 , NanREPO 4 Fn and Na 3 RE (PO 4 ) which are soluble in acid. ) 2 : CaF 2 +Na 3 PO 4 =2NaF+NaCaPO 4 (5) REPO 4 +nNaF=NanREPO 4 Fn (6) REPO 4 +Na 3 PO 4 =Na 3 RE(PO 4 ) 2 (7) X-ray diffraction method was used to test the calcined products at different temperatures. It was found that at 850 ° C, when the amount of Na 2 CO 3 added was insufficient (15%), there was still REPO 4 in the calcined product, and Ca 8 RE (PO) appeared. 4 ) 5 O 2 . The remaining REPO 4 forms a solid solution with the CaO 3 decomposition product CaO in the mineral, and a chemical reaction occurs along with the formation of the solid solution, namely: REPO 4 +2Ca 3 (PO 4 ) 2 +2CaO=Ca8RE(PO 4 ) 5 O 2 (8) 2REPO 4 +3CaO=RE 2 O 3 +Ca 3 (PO 4 ) 2 (9) In addition, some of the fluorite, barite , and apatite participate in the reaction during the roasting process: CaF 2 +Na 2 CO 3 =CaCO 3 +2NaF (10) BaSO 4 +Na 2 CO 3 =BaCO 3 +Na 2 SO 4 (11) Ca 5 (PO 4 ) 3 F+5Na 2 CO 3 =5CaCO 3 +3Na 3 PO 4 +NaF (12) 2Na 3 PO 4 +3BaCO 3 =Ba 3 (PO 4 ) 2 +3Na 2 CO 3 (13) 2Na 3 PO 4 +3CaCO 3 =Ca 3 (PO 4 ) 2 +3Na 2 CO 3 (14) The decomposition rate of concentrate in the roasting process is greatly affected by the addition amount of sodium carbonate and the calcination temperature. Before 700 °C, the decomposition rate increases with the addition of sodium carbonate, but when the calcination temperature is greater than 700 ° C, sodium carbonate After the addition amount exceeds 20%, the reaction process is further complicated due to the enhanced action of Na 2 CO 3 and SiO 2 in the mineral, and the formation of the poorly soluble acid compound NaRE 4 (SiO 4 ) 3 F is promoted, resulting in decomposition. The rate has declined. Too high a temperature will cause the decomposition of soluble Na 3 RE(PO 4 ) 2 and the formation of the acid-insoluble compound NaRE4(SiO 4 ) 3 F, which will also result in a decrease in the decomposition rate. (2) Sulfuric acid leaching and rare earth extraction The calcined product contains non-rare earth impurities such as Na 3 PO 4 , BaCO 3 , Na 2 SO 4 , CaCO 3 , NaF, etc. in addition to REO. Na 3 PO 4 during sulfuric acid leaching with sulfuric acid to generate H 3 PO 4 and Na 2 SO 4, and H 3 PO 4 and Na 2 SO 4 and the turn to form insoluble rare earth rare earth double sulfate and a rare earth phosphate, resulting in Loss of rare earth. When BaCO 3 and CaCO 3 are leached by sulfuric acid, BaSO 4 and CaSO 4 poorly soluble compounds are formed and precipitated in the leaching slag. However, the crystal grains formed by CaSO 4 during the leaching process are small and the precipitation rate is slow, and it is difficult to completely exit the filtration. Therefore, the calcined product is subjected to water washing and pickling to remove these impurities in advance before the sulfuric acid is leached. The leaching solution has a sulfuric acid concentration of about 1.5 mol/L and a cerium oxidation rate of more than 90%. The approximate composition is shown in Table 1. Table 1 Chemical composition of sulfuric acid leaching solution chemical composition REO ThO 2 F Fe CaO content/ g·L -1 50~60 0.2 to 0.3 3~7 2 to 15 About 4 Based on the difference in chemical properties between tetravalent cerium and trivalent rare earth elements, this solution can be first separated by bismuth sulfate precipitation or solvent extraction, but the method of precipitation of sulphate double salt has long process flow, consumes more chemical raw materials, and costs of production. Compared with the sulfate double salt precipitation method, the solvent extraction method overcomes these shortcomings, and also has the advantages of high purity of the ruthenium product and high recovery rate of the rare earth. The disadvantage is that the interference of the F on the extraction process is large, which affects the normal production. Second, ammonium chloride decomposition method The rare earth extraction process of ammonium chloride roasting and decomposing mixed rare earth concentrate is a method for chlorinating rare earth minerals into rare earth chlorides by decomposing HCl with NH 4 Cl under certain temperature conditions. In this process, in order to overcome the problem that a large amount of water is used in the sodium carbonate roasting process to wash off the NaF in the calcined product, two calcination methods are employed. For the first time, calcined magnesium (MgO) and Baotou mixed rare earth concentrate (52.1% based on rare earth oxides) were mixed and calcined to decompose the monazite bastnasite in the mixed rare earth concentrate into rare earth. The oxide and magnesium fluoride are calcined for the second time, and the rare earth oxide formed in the first calcination is chlorinated to a rare earth chloride with ammonium chloride. The rare earth extraction process can be carried out by extracting the rare earth with the calcined product without directly introducing water and alkali, and the rare earth conversion form is less. The recovery rate of the rare earth in the small experiment is more than 85%, which is a rare earth extraction process worthy of further study. The first calcination reaction is as follows: 2REFCO 3 +MgO=MgF 2 +RE 2 O 3 +2CO 2 (15) 4CeFCO 3 +2MgO+O 2 =2MgF 2 +4CeO 2 +4CO 2 (16) 2REPO 4 +3MgO=RE 2 O 3 +Mg 3 (PO 4 ) 2 (17) MgF 2 +Mg 3 (PO 4 ) 2 =2Mg 2 FPO 4 (18) It was found in the experiment that the rare earth recovery rate was the highest when the mass ratio of rare earth concentrate to magnesium oxide was 3:1. Continued to increase MgO, the rare earth recovery mainly declines, because when MgO is excessive, MgO will be chlorinated during the chlorination process and affect the chlorination of the rare earth; the optimum temperature for calcination is 600 °C. The reaction temperature is low, which is not conducive to the calcination reaction; the temperature is further increased, and the temperature has little effect on the calcination reaction. The optimum calcination time of the mixed rare earth concentrate is 80 min. The reaction of the second baking is as follows: NH 4 Cl=NH 3 +HCl (328 ° C) (19) RE 2 O 3 +6HCl=2RECl 3 +3H 2 O (20) 2CeO 2 +8HCl=2CeCl 3 +Cl 2 +4H 2 O (21) RE 2 O 3 +3Cl 2 =2RECl 3 +(3/2)O 2 (22) The experimental results show that when the mass ratio of rare earth concentrate to ammonium chloride is 1:2, the recovery rate of rare earth can reach more than 85%, and then the amount of ammonium chloride is increased, which is not beneficial to the recovery of rare earth. In the range of 350-500 °C, the recovery of rare earth increases gradually with the increase of reaction temperature. When the temperature is 500 °C, the recovery rate of rare earth is the highest. Further increasing the reaction temperature, the recovery rate of the rare earth is decreased, which may be caused by the reoxidation of the rare earth chloride. Third, CaO decomposition method The decomposition of mixed rare earth concentrate by CaO-NaCl is a method of increasing the decomposition of REPO 4 and REFCO 3 by CaO by means of solvent NaCl. At a calcination temperature of 600 to 900 ° C, REPO 4 and REFCO 3 are decomposed into REO and Ca5F(PO 4 ) 3 , and while decomposed, Ce 2 O 3 is oxidized to CeO 2 by oxygen in the air. The main component of the exhaust gas generated during the roasting process is CO 2 , which is environmentally friendly. The calcined product was removed with dilute acid post Ca 5 F (PO 4) 3 and NaCl, leaching with sulfuric acid REO, CeO 2, ThO 2, ThO 2 leaching residue is less than 0.001% g / L, a low radioactive residue, can as a general Waste residue treatment. The leachate can be extracted by solvent extraction to extract lanthanum, cerium and non-cerium rare earth elements. The process is a clean production process that is environmentally friendly. It is still in the research stage and needs to be systematically studied before it can be applied to production. The study of CaO decomposition of rare earth minerals has focused on the study of monazite. In 1980, Yasuo.Hikichi et al. studied the chemical effects of SiO 2 , Al 2 O 3 , CaO and rare earth phosphates. It was found in the experiment that the reaction of rare earth phosphate with SiO 2 is above 1700 ° C; and the reaction with CaO can be carried out at 700 ° C. Since the chemical reaction between CaO and rare earth phosphate is a solid phase reaction, the reaction rate is limited by the diffusion rate, and the decomposition rate is very low. Only in the presence of the liquid phase, the reaction proceeds relatively completely, and the decomposition rate can reach 78%. The composition of the mixed rare earth concentrate is more complicated than monazite, and the chemical reaction during the roasting process is not exactly the same as the monazite. In recent years, some research results have been obtained in the research on the decomposition of mixed rare earth concentrate by calcination of calcium oxide. (1) Decomposition reaction in the roasting process The TG-DTA thermal analysis technique was used to study the calcination process of the mixed rare earth concentrate with CaO and NaCl at 100-1000 °C, and the results shown in Fig. 2 were obtained. The results show that the process of CaO-NaCl roasting and decomposing mixed rare earth concentrate is divided into two stages: the first stage is between 417 and 530 °C, and the main decomposition reaction is the decomposition of REFCO 3 and the oxidation of Ce 2 O 3 : REFCO 3 =REOF+CO 2 (23) 3REFCO 3 +H 2 O=RE 2 O 3 +REOF+2HF+3CO 2 (24) Ce 2 O 3 +(1/2)O 2 =2CeO 2 (25) The second stage is between 600 and 800 °C, mainly CaO decomposes REPO 4 and REOF. The decomposition products are Ca 3 (PO 4 ) 2 , RE 2 O 3 , CaF 2 , and CaF 2 also participates in CaO decomposition REPO. The reaction of 4 and promoted the reaction of CaCO 3 to decompose monazite: 3CaO+2REPO 4 =RE 2 O 3 +Ca 3 (PO 4 ) 2 (26) CaO+2REOF=RE 2 O 3 +CaF 2 (27) 9CaO+CaF 2 +6REPO 4 =3RE 2 O 3 +2Ca 5 F(PO 4 ) 3 (28) At this stage, the added NaCl provides a liquid phase in the reaction system, which enhances the mass transfer process between the solid phase reactants and significantly improves the decomposition of the mixed rare earth concentrate. When the amount of CaF 2 in the reaction system is insufficient, NaCl may also participate in the decomposition reaction: 15CaO+3NaCl+10REPO 4 =3CaCl(PO 4 ) 3 +Na 3 PO 4 +5RE 2 O 3 (29) However, when there is sufficient CaF 2 in the reaction system, since Ca 5 F(PO 4 ) 3 and Ca 5 Cl(PO 4 ) 3 belong to the hexagonal system, the lattice constant is close, and the radius of the F atom (0.136 nm) is smaller than The atomic radius of Cl (O.181 nm), so CaF 2 is more likely to participate in the reaction of CaO to decompose REPO 4 than NaCl. Therefore, when both REFCO 3 and REPO 4 are present in the reaction system, the reaction formula (28) has a greater tendency to react than the reaction formula (29). (B) the effect of CaO addition on the decomposition rate The addition amount of NaCl was set to 10%, and the calcination temperature was changed to 700 ° C, 780 ° C, and 860 ° C, and an isotherm curve was obtained in which the decomposition rate (Y) varied with the amount of CaO added (C) (see FIG. 3). It can be seen from the curve in Fig. 3 that the process of the decomposition rate (Y) changing with the increase of the CaO addition amount is divided into three stages: in the first stage, the decomposition rate increases rapidly with the increase of CaO, and as the temperature increases, this The stage becomes longer; in the second stage, as the amount of CaO increases, the decomposition rate tends to be flat; in the third stage, CaO continues to increase, and the decomposition rate has a tendency to be low. Therefore, the conditions of the temperature should be noted when determining the amount of CaO added. If a high decomposition rate is obtained, a condition of 900 ° C and a CaO addition amount of 30% can be selected. (III) Effect of NaCl addition on decomposition rate Figure 4 is a graph showing the relationship between the amount of NaCl added and the decomposition rate. The curve in the graph changes in a linear shape as the amount of NaCl added increases. When the calcination temperature is 800 °C, the decomposition rate of NaCl increases and the change is large. When the temperature is 10% to 20%, the decomposition rate increases, the rate of change decreases, and the decomposition rate reaches the maximum at about 20%. When the amount of NaCl added exceeds 20%, the decomposition rate shows a downward trend. The main role of NaCl in the calcination process is to provide a liquid phase for the reactants, which promotes the progress of the reaction and improves the decomposition rate of the rare earth concentrate. When the amount of NaCl added is increased from 10% to 20%, the decomposition rate does not change much. Moreover, the addition of NaCl has many disadvantages, such as the calcination product is easy to agglomerate and is easy to be sintered, which brings a lot of inconvenience to the leaching process, and increases the amount of water washing before the leaching of sulfuric acid, because if the washing is not complete, the NaCl will be converted into Na 2 SO when leaching. 4 The rare earth double salt formed by the rare earth remains in the slag, which seriously affects the recovery rate of the rare earth. Therefore, from all aspects, the amount of NaCl added is preferably 10%. Figure 2 Adding 15% of mixed rare earth concentrate TG-DTA test results of CaO+10% NaCl Figure 3 Relationship between CaO addition amount and decomposition rate Figure 4 Relationship between the amount of NaCl added and the decomposition rate Figure 5 Effect of calcination temperature on decomposition rate (4) Effect of calcination temperature on decomposition rate From the temperature-decomposition rate curve in Fig. 5, the decomposition rate increases with the increase of temperature, because the decomposition reaction is an endothermic reaction, and the temperature rises, which is favorable for the reaction. At the same time, the temperature rises, and the low-melting melt composed of NaCl or NaCl and CaF 2 starts to melt, and a liquid phase appears, which is beneficial to the mass transfer between the reactants and completes the decomposition of the concentrate. However, it should be noted that if the temperature is too high, the molten salt will be volatilized, and the effect of the low melting point body will be reduced, which is not conducive to the decomposition reaction. Further, if the temperature is too high, the CeO 2 in the calcined product is not easily leached by the acid solution, and the rare earth recovery rate is lowered. Therefore, the choice of temperature is very important. For example, the decomposition rate of the mixed rare earth concentrate at the ratio of concentrate:CaO:NaCl=1:0.35:0.1, respectively at 670 ° C, 780 ° C, 900 ° C for 1 h is 71.12%, 89.27%, 91.08 %. This indicates that the increase of temperature before 780 °C has a significant effect on increasing the decomposition rate, while the effect between 780 °C and 900 °C is not significant, so the temperature control is reasonable at 780-800 °C. (5) Method for recovering rare earth from calcined product After the calcined product is washed by dilute acid to remove Ca 5 F(PO 4 ) 3 and NaCl, the REO, CeO 2 and ThO 2 are leached with sulfuric acid according to the experimental procedure shown in FIG. 6 , and the rare earth recovery rate can reach 92% or more. After two leaching, the main component in the leaching residue of the two water washings is CaSO 4 , and the ThO 2 is less than 0.001 g/l. Figure 6 Experimental process for recovering rare earth from sulfuric acid leaching from calcined products Floor Heat Pump,Multi Function Heat Pump,Air Source Multi Function Heat Pump,Air Source Multi Function Heat Pump Guangdong Shunde O.S.B. Environmental Technology CO.,LTD. , https://www.heatpump-osb.com