Study On Leaching Technic Of Valuable Metals From Superalloy Scraps

Superalloys are primarily used for aerospace,petrochemical,nuclear industry,shipbuilding because of their excellent corrosion resistivity and mechanical strength.The extensive use of superalloys has accumulated more and more scraps.However,the recovery technology of superalloy scraps in China is immature,a large number of superalloy scraps are not fully utilized,resulting in huge resource waste and environmental pollution.Therefore,it is urgent to develop energy saving,environment-friendly and efficient recovery technology of superalloy scraps to alleviate the crisis of metal mineral resources,energy and environment.Currently,superalloy scraps are mainly recovered by hydrometallurgy processes.However,the excellent corrosion resistance of the superalloy makes it easy to form passivation film during the chemical or electrochemical dissolution process,which hinders the dissolution of superalloy and reduces the dissolution rate of superalloy.In addition,the high hardness and high mechanical strength of the superalloy make it difficult to crush the superalloy.Hence,the contact area with the leaching solution is small,and the dissolution rate is low,which seriously affects the recovery efficiency of the superalloy.Therefore,how to efficiently and quickly dissolve superalloys has become one of the key processes for recycling superalloy scraps in the hydrometallurgy process.Based on the above problems,combining with the current research progress,different sizes of superalloy scraps dissolution processes are developed.The main research conclusions are summarized as follows:（1）Nickel-based single crystal superalloy（DD5）grinding waste was chosen as the research object,the atmospheric ternary oxide dissolution system was used to achieve rapid dissolution and leaching of the superalloy,and the stirring speed,leaching time,the dosage of HCl,HNO₃ and H₂O₂ were optimized.The results showed that when V（HNO₃）/V（H₂O）was 0.24,V（H₂O₂）/V（H₂O）was 0.24,V（HCl）/V（H₂O）was 2,stirring speed was 200 rpm/min,and leaching for 3 min.Under the optimal conditions,the leaching rates of Ni,Co,Cr and Re were above 95%,and the maximum leaching rates of W were 53%.The further study indicated that the leaching of Ni,Co,Cr,Re,W conformed to the chemical control model,and the total leaching rate was controlled by the chemical reaction rate,Ni,Co,Cr and Re were mainly in the form of M_x Cl_y.In addition,the results demonstrated that the leaching residue was mainly composed of WO₃,TaO and Ta₂O₅.（2）Based on the research results in the first part,an efficient embrittlement method for a nickel-based single crystal superalloy had been achieved by encapsulating the superalloy with Zn in vaccum quartz tubes and heat treating at the temperature of 700 ℃,900 ℃,1100 ℃ and 1200 ℃ for 8 h.SEM,XRD and EPMA were used to observe the evolution of microstructures,identify phases in alloys and detect elements distribution.The following results were obtained.Firstly,the occurrence of embrittlement was due to the dissolution and reaction of superalloy.Secondly,the process of reaction was a mutual diffusion result.Thirdly,the formation of Ni₅Zn₂₁ intermetallic compound in superalloy was the main cause of failure and embrittlement of superalloy.Fourthly,study also found W,Re and Ta would gather together to form WRe_x Ta_y after superalloy was treated with Zn.Besides,results also indicated that Ni,Al and Co,Ni,Co and Cr would form NiAl_x Co_y and NiCoCr at low temperature,respectively.However,as temperature was high enough,NiAl_x Co_y could disappear,while NiCoCr would change to CoCr compounds.（3）Based on the research results in the second part,the microstructure change,phase identification and elements distribution of the superalloy after reacted with Zn at 1100 ℃ for 2 h,4 h,8 h and 10 h were investigated and analyzed in detail through the method of XRD,SEM and EPMA.The following results were obtained.First of all,the Zn and Ni in the superalloy reacted to form Ni₅Zn₂₁ intermetallic compounds,at the same time,some Zn（Ni）solid solutions were formed.With the extension of reaction time,the Zn（Ni）solid solutions converted into more Ni₅Zn₂₁ intermetallic compounds.Secondly,the Cr₂₃C₆ carbide with low C and rich in Cr was formed.Finally,the Cr₂₃C₆ compound was decomposed,part of Cr was dissolved in Zn,and part of the free state Cr reacted with Ni to form Cr₇Ni₃ intermetallic alloy.The study also found that W,Re and Ta would gather together to form WRe_x Ta_y.Besides,results also documented that Ni,Al and Co,Ni,Co and Cr would form NiAl_x Co_y and NiCoCr in a short reaction time,respectively.But as reaction time was long enough,NiAl_x Co_y significantly decreased and gradually disappeared,while NiCoCr gradually changed to CoCr compounds.