Research progress on extraction of rare earth from bastnasite by ammonium chloride roasting

China is a large country with rare earth resources, and its rare earth reserves account for about 40% of the world's proven reserves. Most of them are concentrated in Inner Mongolia Bayan Obo Mine, followed by Sichuan Panxi Rare Earth Mine and Shandong Weishan Rare Earth Mine. In addition, Jiangxi, Fujian and other regions have China's unique ionic rare earth ore. At the same time, China is a major producer of rare earth raw materials, and its output is 90% of the world's total rare earth production. About 60% of light rare earth raw materials are produced in Inner Mongolia Baiyun Obo. The fluorocarbon strontium concentrate is used as raw material to produce rare earth products. In China, the main concentrated sulfuric acid intensification roasting method is used to decompose rare earth minerals. After the rare earth concentrate is decomposed, water immersed, and alkali converted, the rare earth chloride is preferentially dissolved to obtain a rare earth chloride. The process produces waste water, waste gas, pollutes the environment, has a long process flow, and consumes a large amount of chemical raw materials. At the same time, the use of this process to extract rare earth in the fluorocarbon strontium concentrate has high requirements for corrosion resistance of the equipment. Therefore, the development of low-cost, low-polluting rare earth extraction new technology for the sustainable development of rare earth metallurgical industry in China, has a very important theoretical and practical significance. Under the support of the former State Planning Commission Rare Earth Office and the National Natural Science Foundation of China, a new process for the extraction and extraction of rare earth from the weathering mud of Panxi Rare Earth Mine by ammonium chloride roasting was proposed. The process utilizes HC1 which is decomposed by NH ₄ Cl at a certain temperature to chlorinate the rare earth in the mineral, and then leaches with hot water to recover the rare earth chloride. The process does not introduce acid or alkali, has less transformation form of rare earth, good chlorination selectivity, high chlorination rate and mild condition, and is a green rare earth extraction process. The rare earth in the black weathered slime of the Panyu manuscript was used in 1998. The intermediate experiment was carried out in 1998 and passed the expert appraisal organized by the Ministry of Education. Further research has found that by changing the process conditions, the selective ammonium chloride roasting decomposition method can also be applied to Panxi rare earth ore and concentrate and tailings, Shandong Weishan medium grade rare earth concentrate and Baiyun Obo medium and high grade mixed rare earth concentrate. The treatment, as well as the desorption (solid) fluorine reaction mechanism, chlorination reaction and chlorination extraction kinetics of these processes. In this paper, the research progress of extracting rare earth from bastnasite by selective ammonium chloride roasting decomposition method is briefly reviewed.

First, the chemical theoretical basis of the ammonium chloride roasting decomposition method

(1) Theoretical basis of thermodynamics, de-solidification and chlorination reaction mechanism

When the bastnasite is heated to about 500 ° C, it decomposes to form a rare earth oxide.

The addition of a defluorination agent (Na ₂ CO ₃ ) or a fluorine-fixing agent (Mg0) can prevent the formation of a solid phase of the rare earth fluoride, thereby facilitating the next step of chlorination of the rare earth to remove the fluorine reaction to:

(3), (4) is an expression of the defluorination reaction of the fluorocarbon lanthanum rare earth ore, and (5) to (8) are reaction expressions of the fluorine fixation process of the Baotou mixed rare earth ore.

After defluorination of the fluorocarbon antimony ore, the ammonium chloride roasting method is used to convert the rare earth into a water-soluble rare earth chloride form, which is advantageous for further separation and purification. Its chlorination reaction can be expressed as:

The main impurities in the fluorocarbon antimony concentrate are compounds containing calcium, magnesium , aluminum , silicon and iron , most of which are present in the form of oxides or converted to oxidation during the desulfurization of the fluorocarbon concentrate. Object. The free energy change ΔG _{T of the} main component of the mineral in the chlorination of ammonium chloride chlorination is shown in Table 1.

Table 1 Free energy change of some oxides reacting with hydrogen chloride

It can be seen from Table 1 that in the optimized chlorination temperature (>600K), in addition to rare earth, calcium, manganese , lead , magnesium and other elements can be chlorinated, the remaining oxides are almost impossible to be chlorinated. Obviously, from the thermodynamic point of view, it is feasible to treat the rare earth ore of the calcined or oxide type after the removal of fluorine from the bastnasite by ammonium chloride roasting, and has good selectivity and can realize rare earth elements. Separation from most non-rare earth elements and radioactive strontium elements.

(II) Kinetics of extraction of rare earth by ammonium chloride roasting

The chlorination kinetics of rare earths in the de-manganese ore from Panxi was studied. The foundation for engineering amplification for the extraction of rare earth from fluorocarbon antimony ore by salt chlorination. The results show that the chlorination of rare earths begins in the spherulitic spheroidal sphere layer, and the chlorination reaction is a dynamic plough controlled by interfacial chemical reaction. The kinetic equation is kt=1-(l-a) ^1/3. The reaction performance activation energy Ea=44.01 kJ·mol ^-l . The kinetic study also shows that the chlorination of rare earths and the oxidation of rare earth chlorides are a pair of competing reactions with the extension of reaction time. At 500 °C, the decomposition of rare earth chlorides is more prominent, resulting in the chlorination of rare earths. The rate decreases as the reaction time increases.

The kinetics of the recovery of rare earth from the decanted mixed rare earth concentrate by the solid-fluorinated ammonium chloride roasting method was studied. Figure 1 shows the chlorination kinetics curve of fluorinated calcine at 350-500 °C. It is a typical multi-phase gas-solid zone reaction kinetics curve. Therefore, the chlorination reaction of fluorinated calcine meets the area proposed by Bagdasarrym. In the reaction rate model, the chlorination reaction process passes through two series reaction steps, and the core of the reaction product grows in one direction. The reaction rate equation follows the Erofeev equation ln[-ln(1-a)]=lnk+nlnt, and the reaction rate constant is expressed as a function of temperature T as k=6.23×10 ^-2 e ^-36381/RT , and the apparent activation energy Ea is 36.381. kJ·mol ^-l , the process limitation link is the internal diffusion control.

The kinetics of the recovery of rare earth from the grade-grade fluorocarbon strontium concentrate in Shandong Weishan by solid-fluorination chlorination roasting method was studied. The kinetic curve is shown in Fig. 2, which is the multi-phase gas-solid region reaction kinetics curve. The chlorination reaction process of the fluorinated calcine meets the regional reaction rate model plough proposed by Bagdasarrym. The chlorination process passes through two series reaction steps, and the nucleus of the reaction product grows in a non-directional direction. The reaction rate equation follows the Erofeev equation ln[-ln(la)]=lnk+nlnt, and the reaction rate constant is expressed as a function of temperature £k=3.972×10 ¹⁰ e ^-167993/RT , and the apparent activation energy Ea is 167.793 kJ·mol ^-l , the process restriction link is mainly the interface chemical reaction control.

(III) Study on the chlorination process and mechanism of rare earth oxides

For bastnasite, the main component elements are lanthanum and cerium, and the sum of the two components is greater than 85%. Therefore, the rare earths in the defluorinated roasting ore are mainly oxides of lanthanum and cerium, but exist in the +4 oxidation state. The main component element is converted into CeO _{2 which} is insoluble in hydrochloric acid during the process of de-solidification of fluorine. Therefore, the in-depth study on the chlorination reaction mechanism of Ce0 ₂ and La ₂ 0 ₃ in the process of ammonium chloride roasting provides a theoretical basis for improving the process conditions.

Fig.1 Kinetic curve of chlorination of 350-500 °C in Baotou mixed rare earth concentrate

Fig. 2 Chlorination kinetics curve of Shandong Weishan carbon lanthanum rare earth concentrate

The chlorination mechanism of Ce0, and La203 and the thermal decomposition oxidation mechanism of CeCl ₃ ·7H ₂ 0 and LaCl ₃ ·7H ₂ 0 were studied. These studies show that CeO ₂ and La ₂ 0 ₃ are chlorinated with NH ₄ Cl in an air atmosphere, and the chlorination rate reaches about 80% at around 300 ° C, while NH ₄ C 1 begins to decompose into NH ₃ and HC 1 at around 328 ° C. Therefore, the chlorination of Ce0 ₂ and La ₂ 0 ₃ is not all chlorination of hydrogen chloride gas generated by thermal decomposition of NH ₄ C1, and NH ₄ Cl is also directly involved in the chlorination of minerals. The reaction first produces the intermediate compounds CeOCl and LaOCl, which are then converted to CeCl ₃ and LaCl ₃ . The chlorination reaction mechanism of ruthenium and osmium is the reaction (5) to (8).

The pyrolysis process of CeCl ₃ ·7H ₂ 0 and LaCl ₃ ·7H ₂ 0 further verified our proposed chlorination mechanism. The TGA curves of LaCl ₃ ·7H ₂ 0 and CeCl ₃ ·7H ₂ 0 are shown in FIG. It can be seen from Fig. 3 that the thermal decomposition of CeCl ₃ ·7H ₂ 0 and LaCl ₃ ·7H ₂ 0 is a multi-step process, in which the first, second, third and fourth steps represent the fourth, fifth, sixth and seventh respectively. Crystal water; but the fifth step is a mixing process, and the weight loss product is a mixture of CeOCl (LaOCl) and Ce0 ₂ (La ₂ 0 ₃ ). In Ce0 ₂ and La ₂₀₃ chlorination process, Ce0 ₂ and La CeCl chloride ₂₀₃ LaCl ₃ and ₃ thermal decomposition (oxidation) occurs simultaneously, and the reaction temperature is too high for too long, will have prompted The resulting CeCl ₃ and LaCl _{3 are} again oxidatively decomposed. Therefore, in order to maximize the chlorination rate of rare earth in chlorinated baking sand after defluorination of bastnasite, the best chlorination temperature should be selected.

And time, on the other hand, also add a sufficient amount of NH ₄ Cl. Excess NH ₄ C1 favors the formation of CeCl ₃ and LaCl ₃ . LaCl ₃ and CeCl ₃ . The decomposition under the air atmosphere makes it more difficult to further increase the recovery rate of the rare earth extracted by the ammonium chloride roasting decomposition method.

Figure 3 TGA curve of CeCl ₃ ·7H ₂ 0 and LaCl ₃ ·7H ₂ 0

Second, the ammonium chloride roasting method proposed the process of rare earth in bastnasite

The rare earth in the weathering mud of Panxi Rare Earth Mine was inspected and studied in the research. It has not only successfully carried out the pilot test, but also applied to the fluorinated sulphuric acid concentrate from Panxi, Shandong Weishan fluorocarbon samarium concentrate and white clouds. Extraction of rare earth from Ebo mixed rare earth concentrate. The following is a brief overview of the research progress in the extraction of rare earth from ammonium chloride roasting.

(1) Study on Separation and Extraction of Rare Earth from Color Weathered Mud

Under the support of the former State Planning Commission Rare Earth Office, Sichuan Provincial Planning Commission and the National Natural Science Foundation, the hydrochloric acid leaching process and rare earth leaching kinetics of black weathered slime were initially studied. Because the hydrochloric acid method has process defects such as high impurity dissolution rate, low recovery rate of the rare earth, serious equipment corrosion and high product cost, it has not been put into industrial application. Later, the process of extracting rare earth from the weathering mud of Panxi Rare Earth Mine by ammonium chloride roasting decomposition method was proposed. Laboratory and pilot studies of the process have shown that the manganese oxide content in the slime has an important effect on the chlorination process. If the manganese oxide content in the slime is more than 5%, the demineralized by S0 ₂ is used before chlorination, and the rare earth is extracted by the NH ₄ Cl roasting process. Under the conditions of calcination temperature of 520 ° C, NH ₄ Cl / slime mass ratio of 0.2 and calcination time of 2.5 h, the recovery of rare earth leaching was 79%, the total recovery of rare earth was 72%, and the recovery of manganese was Up to 64%, the purity of manganese carbonate produced is more than 43.2%, which meets the requirements of industrial products. For rare earth slime containing less than 5% manganese oxide, the black weathered slime can be directly calcined with ammonium chloride at a calcination temperature of 520 ° C, a NH ₄ Cl / slime mass ratio of 0.3, and a calcination time of 2.5 h. Under the conditions, the recovery rate of rare earth is 81%, and the total recovery of rare earth is 75%. The rare earth leaching solution selects potassium permanganate to remove impurities. By controlling pH<3, when the training ωRE/ωWn=100, the loss rate of rare earth is less than 5%, and the condition for preparing crystalline rare earth carbonate is ωRE=1～15g·L- 1. When ωNH ₄ HCO ₃ /ωRE=2.1 and crystallization time is 24h, flaky rare earth carbonate products with good filtration performance can be prepared. The quality of the pilot product is: rare earth chloride ∑REO=45.51%, rare earth chloride ∑REO=98.9%, rare earth carbonate ∑REO=55.20%.

The separation process of RE and Mn extraction methods for black weathered slime chlorination roasting immersion liquid was studied. RE and Mn in black weathered slime chloride immersion liquid were fractionated and extracted by P ₅₀₇ extractant to obtain RECl ₃ solution and MnCl ₂ solution with purity of 99.5%. After precipitation by NH ₄ HC0 ₃ , large-grained rare earth carbonate and industrial grade manganese carbonate were obtained, respectively, and the yields of rare earth and manganese were both greater than 98%. The conditions for preparing crystalline rare earth carbonate from rare earth slime chlorination roasting leachate were studied, and the role of ammonium bicarbonate in the precipitation process was discussed. The optimal preparation conditions of crystalline rare earth carbonate by L ₉ (3 ⁴ ) orthogonal test are as follows: at PH=5, C _RE =1.0g·L ^-l , t=20°C, C _NH4HC03 =15%, NH ₄ When the amount of HC0 ₃ is 2.1 times the mass of RE, the precipitation rate of RE ₂ (C0 ₃ ) ₃ is more than 98%. NH ₄ HC0 ₃ plays a dual role in the preparation of the pH regulator and the RE precipitant. Carbonic acid

The relationship between the optimum pH of rare earth precipitation and the concentration of rare earth and the concentration of NH ₄ HC0 ₃ is: PH = 1.384 - 2 / 3 lg [ RE ³⁺ ] - lg [ HC0-1 ]. In the preparation of crystalline rare earth carbonate, controlling the appropriate supersaturation is the key to the process, and the addition of (NH ₄ ) ₂ S0 ₄ is beneficial to the formation of crystalline products.

In order to improve the concentration of rare earth in the immersion liquid of black weathered slime after chlorination, reduce the relative content of non-rare earth impurities, facilitate the further recovery of rare earth products, conduct column leaching test on chlorinated baking sand, and explore dip The effect of the solvent, leaching temperature and column diameter ratio on the concentration of rare earth in the leachate. The results showed that. Leaching

When the pH of the agent is 7 and the column diameter ratio is h/σ=280mm/50mm, the leaching effect is the best. Column leaching rare earth concentration can be increased from the agitation leaching 4.3g · L ^-l improve soon 44.8g · L ^-l, rare earth leaching rate of 94.43%, the non-rare earth impurities Al, Fe, Ca content with respect to rare earth 6.5% , 3.4%, 6.1%. Adding 5% (NH ₄ ) ₂ S0 ₄ to leaching, the content of non-rare earth impurities in the leachate will be reduced, especially the calcium content will be greatly reduced.

(II) Study on chlorination roasting process of fluorocarbon strontium ore tailings in Panxi

The raw material (REO: 16.78%) with high rare earth grade in Sichuan Suining was used as raw material. After defluorination with Na ₂ C0 ₃ , the process of extracting rare earth by ammonium chloride roasting was studied. The optimized process conditions were determined by L ₁₆ (4 ⁵ ) orthogonal test: when the defluorination agent was added in 25% of the mineral, the defluorination was carried out at a temperature of 650 ° C for 30 min, and the defluorination rate of the raw material reached after the hot water was eluted. More than 95%; defluorinated ore is further recovered by ammonium chloride method, calcined at a chlorination temperature of 325 ° C, m (NH ₄ Cl) / m (mine) = 1:1 for 600 min, the extraction rate of rare earth is 95% the above.

The ammonium chloride roasting method was used to decompose rare earth extracted from Panxi fluorocarbon sulphate concentrate. The process parameters such as defluorination agent, defluorination temperature, chlorinating agent dosage, chlorination temperature and chlorination time were systematically investigated. The impact of the rate. The process optimization conditions were determined: after defluorination of the defluorination agent with 30% of the ore weight in the concentrate, the defluorination was carried out at 500 ° C, and the calcination after defluorination was equivalent to 2 times the weight of the defluorinated calcine. Mixing ammonium chloride, calcining at 480 ° C for 1.5 h, leaching the calcined water with hot water to obtain a chlorinated solution, the rare earth extraction rate is more than 80%, and the content of non-rare earth impurities such as Fe, Al, Si in the leachate is very low, which is beneficial to Further separation and purification. In order to make full use of the fluorocarbon strontium ore resources, the application of ammonium chloride decomposition method in fluorocarbon strontium ore was comprehensively promoted, and the process of recovering rare earth from fluorocarbon tailings by ammonium chloride was studied. The leaching recovery of rare earth leaching in the fluorocarbon tailings of Sichuan is 1/4 of the ore-fixing agent, the amount of ammonium chloride is 1/2 of the ore mass, the calcination temperature is 500 °C, and the calcination time is 1 h. The rate is up to 84%, and the content of iron in the leachate is less than 0.1%, which is beneficial for further impurity removal and recovery of rare earth.

(III) Study on Decomposition of Rare Earth from Shandong Weishan Fluorcarbon Carbide Ore by Ammonium Chloride Roasting Method

The Shandong Weishan Rare Earth Mine is a rare earth ore deposit dominated by bastnasite, and its niobium content is the lowest fluorocarbon antimony concentrate. The rare earth was extracted from the concentrate by ammonium chloride roasting. The test results show that the medium grade concentrate and ammonium chloride are mixed and chlorinated to extract more than 82% of the rare earth. The optimum process conditions determined by the test were as follows: the mass ratio of ammonium chloride to concentrate was 2:1, the calcination temperature was 480 ° C and the calcination time was 90 min. The calcination is leached by hot water, and the leachate is subjected to a full naphthenic acid organic phase process to achieve separation of RE ³⁺ and Ca ²⁺ . The rare earth leaching rate of the process is up to 82.8%, and the purity of the rare earth chloride is 99.2%. The process is

In the second half of 2001, an intermediate test for the treatment of 50kg medium-grade rare earth concentrate was carried out in Shandong Weishan. The continuous chlorination roasting reactor was designed. The recovery rate of rare earth was over 85%. The ammonia produced by the chlorination process was considered in the study. And the recovery and utilization of excess ammonium chloride, the excess ammonium chloride is recovered on the one hand by the cooling trap, and on the other hand, the absorption of the uncollected ammonium chloride while absorbing the ammonia generated by the chlorination with dilute hydrochloric acid . Through the pilot test, it laid a solid foundation for the extraction of rare earth from the industrial production of the bastnasite. Shandong Weishan rare earth concentrate and Sichuan Panxi thin ore are both fluorocarbon antimony ores, and their fluorine content is equivalent. Why Weishan mine can be directly chlorinated, and Panxi mine must undergo pre-defluorination treatment before chlorination The reason for this is to be studied in depth.

(IV) Study on Extraction of Rare Earth from Bayan Obo Mixed Rare Earth Ore by Ammonium Chloride Method

The Baiyun Obo mixed rare earth concentrate mainly exists in the form of bastnasite and monazite. At present, the mine mainly uses rare H ₂ S0 ₄ strong oxidative roasting method to extract rare earth, and its defects are as described above. According to the thermodynamic analysis of Table 1, under the conditions of appropriate dechlorination (fluorination) and chlorination roasting, non-rare earth impurities such as fluorine, iron, silicon and radioactive cesium can not be chlorinated into the solution, so defluorination (fluorine removal) The ammonium chloride roasting method is suitable for the treatment of high-grade mixed rare earth concentrates in Baiyun Obo with high content of antimony. Under the condition of fixed chlorination roasting (the amount of ammonium chloride / defluorination slag = 1.5, the chlorination temperature is 325 ° C and the chlorination time is 60 mm), the amount of defluorination agent, defluorination temperature and defluorination reaction time The impact of the rare earth chlorination process. The test results show that the rare earth chlorination temperature of the defluorination slag is gradually increased with the increase of the amount of defluorination agent when calcined at 550 ° C for 60 min. When the amount of defluorination agent is determined by the quality of defluorination slag When 20% is increased to 30%, the chlorination rate of the rare earth will increase to over 84%, and the amount of defluorination agent will continue to increase, and the rare earth chlorination rate will decrease.

The process of recovery of rare earth from high-grade Baotou mixed rare earth concentrate by solid-fluorinated chlorination roasting method and the mechanism of fluorine fixation were studied. The selective ammonium chloride roasting method generally uses Na ₂ CO ₃ as a defluorinating agent to pre-defluorinate the fluorocarbon cerium concentrate, but the defluorination process requires a large amount of water to elute the NaF in the baking sand, and wash Soluble NaF in the decontamination can pollute the environment. To this end, we have proposed a new process for solid-fluorination chlorination. The optimum process conditions for solid-fluorination chlorination are: adding one-third of the main ore quality to the fluorocarbon strontium concentrate, light burnt magnesium (Mg0) at 600 °C. The bottom is calcined for 80 minutes to fix the fluorine. After the heat, the calcine is mixed with the double mass of ammonium chloride. After roasting at 500 ° C for 80 min, the hot water is leached out of the calcine to obtain a rare earth chloride leaching solution. Under optimized conditions, the rare earth addition rate is above 85%. According to the X-ray diffraction pattern of concentrate powder, fluorinated calcine and chlorinated leaching slag, it is known that in the process of fluorine fixation of mixed rare earth concentrate, the fluorine-fixing agent (Mg0) reacts with minerals to transform F in the mine into difficult. The soluble MgF ₂ , P0 ₄ ^{3- is} converted to poorly soluble Mg ₃ (P0 ₄ ) ₂ , and then part of MgF ₂ and Mg ₃ (P0 ₄ ) _{2 form} poorly soluble Mg ₂ FP0 ₄ at high temperatures. The mechanism of the fluorine fixation reaction of the mixed rare earth concentrate is the reaction (5) to (8). The fluorination agent of the selective rare earth extraction process is cheap and easy to obtain, simple in operation, and saves the water washing and defluorination process and the defluorination process, thereby reducing the production cost and environmental protection. The research on the recovery of rare earth from the Bayan Obo mixed rare earth concentrate by the solid-fluoride ammonium oxide roasting method is only the laboratory test result. Can the fluorine-containing chlorination roasting method be suitable for the process research and fluorine fixation of the Panxi and Shandong Weishan mines? Dynamics and industrial intermediate tests have yet to be further studied.

Third, the conclusion

Through several years of efforts, the research on the extraction of rare earth from bastnasite by ammonium chloride roasting decomposition has made great progress and achieved gratifying results. Firstly, the experimental research on the rare earth extraction process in different types of rare earth minerals was carried out, followed by the research on the application of basic theory, and the industrial experiment on the rare earth extraction process. However, in order to improve the selective ammonium chloride roasting process and put it into industrial production, it is necessary to conduct in-depth research. For example: continue to carry out selective ammonium chloride roasting method to extract the kinetics of de-solidification of rare earths in various types of bastnasite ore, research on physical and chemical problems in the process, industrial application of process and solid-fluorination chlorination Whether the roasting method is suitable for the treatment of Panxi and Shandong Weishan Mine.

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