Thermochemical decomposition of phosphogypsum with Fe-P slag via a solid-state reaction

Phosphogypsum is a solid waste sourced from the wet-process phosphoric acid industry,which causes severe environmental damages.Its utilization was limited by its high decomposition temperature and high energy consumption.Herein,an Fe-P slag,which is a solid waste that mainly comprises iron phosphide(FeP)and diiron phosphide(Fe_(2)P),can dramatically decrease the decomposition temperature of phosphogypsum.It was found that the Fe-P slag and CaSO_(4) can react as shown in the following reaction equation:2Fe_(1.5)P+3CaSO_(4)+6CO_(2)→Ca_(3)(PO_(4))_(2)+Fe_(3)O_(4)+3SO_(2)+6CO.Its reaction mechanism was further determined using the thermodynamic method.It was found that CaS was the key intermediate for this reaction.The CaSO_(4) conversion for this method can reach approximately 97%under the optimized roasting conditions:the molar ratio between Fe_(1.5) P and CaSO_(4) of 2:3,roasting temperature of 900℃,a roasting time of 8 h.

A comprehensive study on the oxidation behavior and failure mechanism of(γ’+β)two-phase Ni-34Al-0.1Dy coating treated by laser shock processing

Laser shock processing(LSP),as a novel surface modification technology,exhibits tremendous potential to improve the oxidation resistance of alloys.In this paper,(γ’+β)two-phase Ni-34Al-0.1Dy coatings were prepared by electron beam physical vapor deposition(EB-PVD)and then treated by LSP with different pulse energy(4 and 5 J).Their oxidation behavior(including isothermal oxidation and cyclic oxidation)at 1150°C was compared.In the isothermal oxidation case,the LSP-treated samples exhibited good ox-idation resistance due to high dislocation density and refined grains caused by the LSP.For the cyclic oxidation,however,the LSP-treated samples were subjected to undesirable thermally grown oxide(TGO)spallation.In the early oxidation stage(within 30 min),the residual compressive stress introduced by the LSP rapidly released,resulting in a severe plastic deformation at the coating/TGO interface.As a result,the mismatch between coating and TGO gave rise to the TGO spallation.With the increase in the oxida-tion time,work hardening,which continuously limited the synchronous deformation of the TGO/coating interface,became the dominant mechanism controlling the TGO spallation of 5 J-treated coatings.The continuous TGO spallation affected the Al supply and thus deteriorated the cyclic oxidation behavior.

Mechanism of adding rhenium to improve hot corrosion resistance of nickel-based single-crystal superalloys

Hot corrosion behavior in sulfate salt at 950℃ of Rene N5 single-crystal superalloys with 3 wt%rhenium(NSR)was investigated compared with that of nickel-based single-crystal superalloys without rhenium(NS).After 30-h corrosion,the surface of the NS superalloy is seriously corroded.Many holes and exfoliation appear on the surface.The NSR superalloys exhibit better hot corrosion resistance than the NS superalloys.After 30-h corrosion,a continuous and compact Al_(2)O_(3) film is observed on its surface.The Al_(2)O_(3) film with dense structure formed on the surface provides protection for the matrix.The characterization results show that A1 is aggregated in theγ’phase,while Re is aggregated in the y phase during the formation of oxide scale.Considering that Re can inhibit the diffusion of A1 in the nickel matrix,it is inferred that Re can inhibit the outward diffusion of A1 and prevent the decrease of Al concentration in theγ’phase.High concentration of Al hinders the decomposition of Al_(2)O_(3) due to the reaction of acid and basic dissolution.Al_(2)O_(3) keeps its structure intact and provides protection for the matrix.