Research Progress on recovery methods of NdFeB waste
Neodymium iron boron magnet and samarium magnet are two important rare earth permanent magnet materials. With the rapid development of new energy vehicles, wind power generation, electronic equipment, medical equipment, aviation industry, national defense and war, permanent magnet motor and other industries, the amount of rare earth permanent magnet materials is increasing year by year, especially ndfeb magnets, china alone produced nearly 170,000 tons in 2018. NdFeB mainly includes sintering and bonded ndfeb. Because the production of sintered ndfeb is much larger than that of bonded ndfeb, the production process of sintered ndfeb is the main source of materials. In the production process of sintered NdFeB, about 30% of the waste is produced, which contains about 30% of the rare earth elements. In the service process of the permanent magnet material, the material will fail due to oxidation and other reasons, its recycling potential is huge.
With the development of rare earth industry and the progress of society, comprehensive utilization of resources and environmental protection have received extensive attention. In order to improve the reuse rate of rare earth resources, the recovery of rare earth elements in rare earth permanent magnet material waste is imperative. At the same time, the recycling of rare earth is also a requirement for sustainable development. China’s recovery of neodymium iron boron materials lags behind in the research on the recovery of rare earth magnetic materials, basically relying on the traditional wet process. Although the wet process can effectively recover rare earth elements, however, the secondary pollution caused by it should be paid attention to, and this is also against the original intention of reducing the environmental burden caused by rare earth mining. No matter from the perspective of alleviating the environmental problems that may be caused by the development of rare earth mines or from the perspective of strategic reserves of rare earth resources, china should pay attention to the research and development of a new process for recovering rare earth resources from ndfeb materials. Pollution caused by wet recycling process is difficult to meet the requirements of environmental assessment in Europe, America and Japan. Therefore, some foreign researchers are trying to extract rare earth elements from ndfeb materials by using a process that causes relatively less environmental pollution. EU, Japan and other countries have also gradually launched “STROM special project” and “Horizon 2020” plans aimed at developing environmentally friendly and high resource utilization methods for recycling rare earth wastes containing. This paper briefly reviews the research status and development trend of NdFeB magnetic material recovery at home and abroad.
Current situation of domestic research
At present, domestic NdFeB waste recycling mainly includes two methods: wet recovery process and fire process.
Wet process
The wet process for recycling ND-Fe-B waste includes hydrochloric acid total solution method, hydrochloric acid optimal solution method (oxidation roasting-hydrochloric acid solution method), multiple salt precipitation method, natural oxidation pretreatment method, etc.
Hydrochloric acid total solution method
As the name suggests, hydrochloric acid total solution method is to use hydrochloric acid as solvent to dissolve all the rare earth elements and iron elements in the waste of rare earth magnetic materials. The main process of the process is as follows: the waste of NdFeB is broken into a certain particle size powder and dissolved in concentrated hydrochloric acid, the rare earth elements and iron elements are transformed into ionic state, and H2O2 is added to oxidize Fe2 + to Fe3 +, forming rare earth ions and iron and other non-ferrous ions The mixed solution of rare earth ions, and then adjust the pH value of the solution to remove most of Fe3 +. After impurity removal, the solution uses organic extractant to separate rare earth and non rare earth ions. The rare earth ions extracted into organic extractant are back extracted with acid to obtain a relatively pure solution containing rare earth ions. The solution is precipitated by oxalic acid or ammonium bicarbonate, and the precipitate is burned to obtain rare earth oxides. Or no extraction is required The solution containing rare earth ions is directly precipitated with oxalic acid to obtain oxalic acid rare earth precipitation, and the rare earth oxide is obtained after burning.
The biggest advantage of hydrochloric acid method is its high recovery efficiency, which can maximize the recovery of rare earth elements. The equipment is relatively simple, and it can further extract neodymium, which is convenient for industrial production. The disadvantage is that the amount of chemical reagents used is huge, the recovery cost is high, the environmental pollution is great, and the separation of rare earth elements and non rare earth elements such as Fe and CO is not complete.
Hydrochloric acid dominant solution method
Hydrochloric acid combustion method, namely oxidation roasting-hydrochloric acid dissolution method, refers to the control of appropriate roasting conditions, adding hydrochloric acid solution into the roasted rare earth waste to dissolve the rare earth preferentially, the key point of this method lies in the optimal technological conditions for effective control of roasting and optimizing. First, nd-fe-B material is roasted at high temperature to transform FeO into 2o3, and then dissolved with hydrochloric acid, rare Earth can be preferentially leached, and a small amount of iron contained in the leached solution can be removed by adjusting pH value, and relatively pure rare earth compounds can be obtained by extraction or oxalic acid precipitation. In addition to the oxidation reaction, crystal transformation, solid phase reaction, crystal recombination and crystal growth will occur during the roasting of ndfeb powder by this method. The roasting temperature is low, ndfeb is incompletely decomposed, and rare earth and non-rare earth cannot be fully separated when hydrochloric acid is dissolved; If the roasting temperature is too high, the melting and agglomeration phenomenon of material powder will increase, forming dense particles, the specific surface area of the material particles is greatly reduced, and the leaching rate of rare earth is reduced when hydrochloric acid is leached. The method can remove organic matter on the surface, and rare earth and iron can be effectively separated during leaching, but the disadvantage is high energy consumption, large acid consumption, and certain environmental pollution problems.
Multiple salt precipitation method
Complex salt precipitation method is an earlier use of ndfeb recovery method, using rare earth sulfate and sodium sulfate to produce complex salt precipitation and ferrous sulfate exist in the solution of the principle can achieve the separation of rare earth and iron. Through roasting, dissolution leaching, multiple salt precipitation, alkali transfer, acid solution, extraction separation and precipitation roasting processes, valuable elements in neodymium iron boron material can be extracted to obtain neodymium oxide with higher purity, dysprosium oxide and cobalt oxide. Firstly, remove the organic matter and water in ndfeb through roasting, then dissolve the roasting product with sulfuric acid to get the solution containing rare earth sulfate and ferric sulfate/ferrous sulfate, and then add sodium sulfate into the solution, the rare earth sulfate is converted into a complex salt precipitation of rare earth sodium sulfate, and iron sulfate/ferrous sulfate is always dissolved in solution to separate rare earth elements and iron elements. Carry out alkali transfer by precipitation the rare earth sodium sulfate to get rare earth hydroxide precipitation, after washing, we get higher purity rare earth hydroxide, and then dissolve the rare earth hydroxide with hydrochloric acid to get rare earth chloride, or ammonium bicarbonate precipitation, calcination to obtain high purity single rare earth oxide. The advantage of this method is that the rare earth and iron can be separated better, and the purity of the rare earth is higher. The disadvantage is that the process is more cumbersome, the amount of chemical reagents is larger, and the cost is higher.
Natural oxidation pretreatment method
Because the waste of neodymium iron boron contains a large amount of iron, and the natural oxidation of iron occurs when it is exposed to the air after wetting through hydrochloric acid, making the iron element oxidize to trivalent iron in the air, the effect of roasting can reduce the pretreatment cost of NdFeB Material Recovery. Kneading ndfeb material with a certain concentration of dilute hydrochloric acid and placing it under natural conditions for 20 days, the oxidation rate of iron can reach 92.37%, almost reaching the roasting effect, it can meet the requirements of low-cost treatment process for rare earth in NdFeB Material. The method has a good effect on the pretreatment of NdFeB materials, and the cost is low, but the pretreatment period is longer.
Fire craft
The pyrometallurgy process of neodymium iron boron material is an metallurgical process in which the alloy is recovered at high temperature according to the occurrence state of the elements in the material or by means of redox reaction to change the chemical state of the elements in the material. Pyrotechnics has the advantages of short process, being environmentally friendly, high added value of products, etc., but compared with wet technics, pyrotechnics has small processing capacity, low recovery rate, high energy consumption and high cost of recovery, the requirements for raw material quality are high. The domestic fire process for NdFeB Material Recovery includes reduction method, direct reduction-slag gold melting method and direct recovery method.
Reduction method
Chinese patent CN104690277A discloses a method of recovering nd-fe-B oil mud by the reduction diffusion technology, first, the waste of nd-fe-B oil mud is added into water, most organic matters are removed by heating distillation after drying, the dry oil mud is obtained, add excessive amount of hydrochloric acid into the dry oil sludge, heat in the water bath at the temperature of 60-80℃ for 30min and then filter, keep the supernatant, adjust the ph value to 2 with ammonia water, then precipitate by adding oxalic acid, obtain the co-precipitate of rare earth oxalate, after roasting, the mixed oxide of iron and rare earth is obtained, adding an appropriate amount of iron powder and FeB to it, using metallic calcium or CaH2 as reducing agent and CaO as dispersant, carrying out calcium reduction diffusion reaction under the protection of inert gas, the regenerated NdFeB alloy powder is obtained by ultrasonic treatment of the reduction product in magnetic field, which can be directly used for the molding of NdFeB magnet.
Direct reduction-slag gold melting method
Deng yongchun adopted the direct reduction-gold fusion method to recover the iron alloy and rare earth oxide slag from the waste of ndfeb. Mix the waste ndfeb powder and iron ore concentrate powder according to a certain proportion into the reaction tank, then add the reducing agent semi-coke into it and directly reduce it in the reaction tank, according to the characteristics of the difference in redox properties of each element in the material, Fe O and CoO in the material are reduced to Fe and Co, while Al, Mn, RE and other active metals are oxidized into Al 2O3、MnO、RE2O3, metallic Fe and Co are combined to form iron-cobalt alloy. The separation of iron and iron can be directly used in spongy iron-cobalt alloy, slag containing rare earth can be used for smelting other ferroalloys.
Direct reuse
As for the large size, it is not polluted, and NdFeB magnet waste is treated by powder and alloying to prepare new magnets. The recovery rate of NdFeB by mixed powder method reaches 60%, the recovery rate of NdFeB by alloying method reaches 90%, and the magnetic properties of the magnet can reach the available standard. This method can smelting by adding magnet material and high quality alloy powder into the vacuum induction furnace according to the magnet components designed, and then reprepare the magnet.
Current research situation abroad
In recent years, foreign countries have carried out more work in the treatment of ndfeb materials, mainly including room temperature selective oxidation-leaching-precipitation method, hydrogen explosion method, wet process, multilayer carbon nanotube adsorption method, hollow fiber membrane method, ionic liquid recovery method, etc.
Selective oxidation at room temperature-leaching-precipitation
Selective oxidation at room temperature-leaching-precipitation method is to completely immerse the waste ndfeb into the mixed solution of ammonium chloride/sodium chloride and hydrochloric acid with certain concentration first, and after several hours, the rare earth and iron in the material are almost all leached, at this time, the leached iron is the divalent iron, and then by selective electrooxidation, it oxidizes stipulated 2 + to i3 +, adding oxalic acid to the leaching solution, then precipitation 98% of rare earth out, and get rare rare earth oxalic acid, after burning, rare earth oxide with a purity of 99.2% is obtained, and ammonia water is added into the precipitation mother liquor to precipitate iron into three iron hydroxide. The remaining cobalt rich liquid is recovered by electrolysis.
Kataoka used the room temperature selective leaching method to recover the neodymium in the waste of ndfeb, degaussed and ground the material, soaked in 3% nacl solution for 1 week, and continuously pumped into the air to accelerate the corrosion of magnetic powder, the sample after corrosion is leached by 0.1-0.3mol/L hydrochloric acid at room temperature, the leached liquor is precipitated with oxalic acid, rare earth is recovered, insoluble matter is calcined at 800℃ for 5h, iron and boron are recovered, the recovery rate of neodymium can reach 97%, and the purity can reach 97.2%. This method and natural oxidation pretreatment methods studied in China.
Similarly, they all carry out preliminary treatment on ndfeb material and oxidize and pretreat the Fe in the material to replace the roasting process.
Hydrogen explosion method
Hydrogen explosion method firstly conducts hydrogenation of magnet material under certain temperature and hydrogen pressure, and Nd2Fe14B phase and Nd-rich phase in NdFeB magnet can have hydrogen absorption reaction with hydrogen, since Nd2Fe14B compound is a brittle material and the hydrogen absorption reaction is an exothermic reaction, after hydrogen absorption, it causes physical changes such as Nd2Fe14B phase grain boundary fracture, transgranular fracture and crystal expansion, and then, it needs an vacuumizing treatment, in which process, it can make the bulk material of ndfeb break into particles with average particle size of 125μm, and then deal with the powder repeatedly through hydrogen absorption-hydrogen extraction, the particle size can be reduced to less than 10μm. Because after the hydrogen absorption reaction of Nd2Fe14B, the divergence reaction will occur at 600-800℃, that is Nd2Fe14BHx=2NdHx/2+12Fe+Fe2B, after the vacuum, NdHx/2 has the dehydrogenase reaction to generate Nd and H2, at this time, Nd, Fe and Fe2B can react again and combine to Nd2Fe14B, NdFeB powder obtained by hydrogen explosion method has high coercive force, and can be used to prepare bonded NdFeB magnet or hot press dense magnet.
Wet process
The Önal adopts the method of hydrometallurgy. Firstly, break the magnet into the powder under 500 μm, then livize with dilute sulfuric acid at the room temperature for 24 hours. After the powder is completely dissolved, add manganese dioxide into the leaching solution and react at the room temperature for 1 hour, 98% of the iron in the leaching solution is leached and converted into i3 +, and then manganese oxide solids are added into the leaching solution to adjust the ph value of the solution to be 3, so that 99% of the iron can be precipitated, then use oxalic acid to precipitate and recover rare earth, electrolytic method to recover cobalt and manganese, and the recovered manganese can be recycled after oxidation. Padhan et al. finely ground NdFeB Material into 75-106 m powder, then leached it with hydrochloric acid at 90℃ for 2H. The leaching rate of Nd and Dy in the powder reached over 98%, and the leaching rate of Fe reached over 80%, the leaching rate of B reached 93.%, and the infusion solution was sank by adjusting the pH value.
After the majority of Fe is filtered, the rare earth solution is extracted and separated by Cyanex302 saponified with sodium hydroxide. When the equilibrium acidity is at a pH value of 1.2, NaCyanex302 has a high extraction rate for Dy, the extraction rate of Nd is low, and the separation coefficient of Dy and Nd reaches 53, which realizes the efficient separation of Nd and Dy in ndfeb material.
Hollow fiber membrane method
Yadav recover rare earth in NdFeB Material by hollow fiber membrane method, and separate dysprosium by eheipa. When the hollow fiber membrane is used to recover the rare earth, EHEHPA is used as the carrier. The extractant is diluted by heavy paraffin wax to the best fluidity. The magnet material is demagnetized at a temperature of 350℃ for 30min, then broken and ground to get powder, leaching again with nitric acid to get the leaching liquid containing rare earth, then mixing with the extracting agent and then passing through hollow fiber membrane reactor, the extracting agent has a high extraction rate for dysprosium in the extracting solution, while the extraction rate for Pr and Nd is relatively low, thus, dysprosium in the ndfeb material can be effectively recovered, and dysprosium product with a purity of more than 97% can be obtained, with a yield of more than 94%. Pr and Nd can be recovered by other wet metallurgy methods such as precipitation and extraction separation.
Hydrothermal method
The closed reactor is filled with Nd-Fe-B waste, water and sodium chloride. The reaction is held at 250℃ for 18h, in which Nd-rich Nd-phase reacts with water to generate Nd(OH)3, the hydrogen atom combines with Nd2Fe14B to form Nd2Fe14BHx. Due to the formation of Nd(OH)3 and Nd2Fe14BHx, the phase is oxidized to Fe3O4 and Nd(OH)3 by water, then, the separation of rare earth in NdFeB Material is achieved by magnetic separation of Fe3O4 and Nd(OH)3.
Wet-fire combined process
We recover rare earth by wet process, pyrometallurgy and metallographic combination method, dissolve NdFeB Material by nitric acid at room temperature, get mixed metal nitrate, and take advantage of the difference of thermal decomposition properties between rare earth nitrate and iron nitrate, heat the mixed metal nitrate at 200℃, hydrolyze iron nitrate and decompose it into 2O3, and rare earth nitric acid exists in the form of crystal, and after dissolved with water, get rare earth solution rich in rare earth nitric acid, achieve iron, the separation of aluminum and cobalt impurities is used to purify and recover rare earth elements by solvent extraction, ion exchange, ionic liquid and other methods.
Adsorption method
Functional multilayer carbon nanotube adsorption method is to separate neodymium and iron in Nd-Fe waste by utilizing difference of adsorption capacity of Nd and Fe adsorbed by nano-materials, and control the appropriate ph value of Nd-Fe separation coefficient up to 120, the Nd in solution can be extracted with special effects, but the adsorption capacity of this method for Nd is relatively small, which is only suitable for the extraction of Nd in low concentration solution.
Recovery of ionic liquids
Yamada dissolve the ndfeb material with sulfuric acid, precipitate with oxalic acid to get rare earth oxalic acid, and leave most of the iron in the solution. After burning, dissolve the rare earth oxalic acid with nitric acid to get rare earth nitric acid solution, then we will take the solvent of polyvinyl alcohol coated with glutaraldehyde of P507 for adsorption and separate Nd, Dy and Pr by impregnation resin. The purity of dysprosium reaches 99.7%, the yield reaches 96.4%, and the purity of Nd reaches 91.6%, the yield was 45%.
The ionic liquid recovery method is similar to the extraction method of hydrometallurgy, and it is a method of separating metal ions by using different binding abilities of metal cation and organic extractant or ionic liquid. Ionic liquid is composed of organic cation and inorganic or organic anion, at room temperature or near room temperature liquid salts. In recent years, the highly efficient separation and extraction of the ionic liquid of rare earth elements have been widely concerned, for example, the bistrifluoromethylsulfobetaine ionic liquid has a unique extraction effect on scandium, enrichment and extraction of scandium in low concentration scandium solution can be realized. Bifunctional ionic liquid [A336][P507] has good separation effect on light rare earth. The separation coefficient of Ce/La reaches 12.9, and that of Nd Sm reaches 14.6, which is better than the extraction and separation effect of P507. Mixed ionic liquid [N1888][SOPAA] and [P66614][BTMPP] have synergistic extraction effect on heavy rare earth. When extracting Lu, the extraction efficiency of mixed ionic liquid is more than ten times higher than that when using [N1888][SOPAA] or [P66614][BTMPP] alone. Phosphine-organic single-functionalized ionic liquid [c3ump (O)(OEt)2][NTf2] and organophosphate bifunctionalized ionic liquid [bdppie][NTf2] have good extraction effect on Nd 3 +, and can be used for the separation of Pr and Nd. Different from solvent extraction method, ionic liquid has high recovery efficiency, good separation efficiency and high system stability, the characteristic of non-volatilization can be recycled by using ionic liquid to recycle the valuable elements in Nd-Fe-B and samarium-cobalt permanent magnet materials.
Kikuchi uses nitric acid to dissolve the nd-fe-B material to get the mixed solution of rare earth nitric acid and iron nitrate, adjusts the ph value to remove iron, and gets the mixed rare earth nitric acid solution, the mixed ionic liquid system obtained by diluting TBP with the synthetic [A336][no 3](trioctylmethylamine-no 3) ionic liquid, and separate the rare earth nitric acid solution containing 、Pr、 Nd Dy, the extraction capacity of the ionic liquid for the three elements is Nd >Pr> Dy. Adding appropriate amount of NaNO3 to the solution can also increase the extraction capacity of the ionic liquid for rare earth. Hoogerstraete is used to recover the rare earth in ND-Fe-B by using trihexyl (decityl) phosphorus chloride ionic liquid. Firstly, the material is finely ground and then oxidized, most of the iron is left in the slag, and the immersion solution is extracted with trihexyl (decapyl) phosphorus chloride ionic liquid. Since trihexyl (decaveryl) phosphorus chloride ionic liquid has good extraction ability to i3 +, Co2 +, Cu2 +, Zn2 + and Mn2 +, but almost no rare earth elements are extracted, i3 + is extracted into ionic liquid, while rare earth is left in aqueous phase, and oxalic acid is used to precipitate rare earth to achieve the purpose of rare earth recovery. With sanhexyl (Cetyl) phosphorus chloride ionic liquid is similar, trioctyl methylene ammonium chloride ionic liquid can also achieve the separation of rare earth and iron, cobalt, manganese and other elements, for A336 (three octyl methyl amine) the separation coefficient of cobalt and manganese can reach 400 by thiocyanate in ionic liquid, which can efficiently separate the remaining cobalt and manganese elements. The ionic liquid can also be used cooperatively. For example, when extracting the rare earth, the bifunctional ionic liquid similar to A336-C272 and A336-P204, it has a higher extraction rate than using A336, C272 or P204 alone. In the ionic liquid at this time, a336 is a cation, C272 and P507 are anions. Under the same conditions, the extraction abilities of praseopraseohumeral and neodymium are [A336-C272]>[A336-P204]>C272>P204>A336, therefore, these bifunctional ionic liquids can be used to efficiently recover the rare earth in the Nd-fe-B material leaching solution. Compared with 1:1, after two stages of counter-current extraction, the extraction rates of Pr and Nd can reach 99%, with dilute sulfuric acid backextraction, pure solution of prashmanilite and Neodymium sulfate can be obtained, of which [A336-C272]
Ionic liquids have higher separation coefficients for Pr and Nd extraction. Pr and Nd can be separated.
Conclusion
Among the many methods for recycling rare earth magnetic materials, the ionic liquid recovery method is regarded as the most efficient, environmentally friendly and promising method. Compared with the traditional solvent extraction method, ionic liquid has high recovery efficiency, good separation efficiency, high product purity, high stability, non-volatilization, and less acid and alkali consumption in the extraction process, the feature of no environmental pollution can be recycled to realize the efficient separation of rare earth elements from other elements, which is the future development direction of rare earth magnetic materials and other waste materials containing rare earth. Because ionic liquids have excellent selectivity, they can not only achieve efficient separation between rare earth elements and other elements, but also achieve efficient separation between rare earth elements. The development of ionic liquid technology for recovering valuable elements such as rare earth in the waste of rare earth magnetic materials can not only realize efficient and environmentally friendly recycling of waste rare earth magnetic materials, but also can efficiently, green separation provides new technical support and is expected to replace the existing solvent extraction method to separate rare earth elements.
Authors: Ma Ying, Zhao Yongzhi
Source: China Permanent Magnet Manufacturer – www.rizinia.com
