A method for recycling spent carbon cathode of aluminum electrolysis includes the following steps: (1) crushing and sieving spent carbon cathode, to obtain carbon particles; (2) mixing the carbon particles with a sulfuric acid solution, to obtain a slurry A, and then performing pressure leaching, to obtain a slurry B; (3) evaporating and concentrating the slurry B until a mass percentage of water is lower than 8%, to obtain a slurry C; (4) adding concentrated sulfuric acid to the slurry C to obtain a slurry D, then roasting the slurry D at 150-300° C. for 0.5-10 h, and then roasting at 300-600° C. for 0.5-8 h, to obtain the roasted carbon; and calcining the roasted carbon at a high temperature, to obtain the purified carbon, or mixing the roasted carbon with a leaching agent, and performing leaching, filtering, and washing, to obtain the purified carbon.

A method of treating a waste liquid includes: an aluminum dissolution step of dissolving aluminum in an acidic waste liquid and performing separation into a first treated water and a reduced heavy metal precipitate; a gypsum recovery step of adding a calcium compound to the first treated water at a liquid property of a pH of 4 or less, and performing separation into a second treated water and gypsum; an aluminum and fluorine removal step of adding an alkali to the second treated water and performing separation into a third treated water and a precipitate containing aluminum and fluorine; and a neutralization step of adding an alkali to the third treated water and performing separation into an alkali neutralization treated water and a neutralized precipitate of a heavy metal hydroxide.

New methods of producing aluminum fluoride from cryolite are disclosed. A method may include a step of reacting cryolite bath materials with aluminum sulfate, thereby producing a reactant product, the reactant product comprising aluminum fluoride. The method may further include a step of removing impurities from the reactant product, thereby creating a purified product comprising the aluminum fluoride. The removed impurities may comprise at least one of sodium (Na), magnesium (Mg), and calcium (Ca). In one embodiment, due to the removing step, the purified product contains not greater than 0.2 wt. % of calcium.

New methods of producing aluminum fluoride from cryolite are disclosed. A method may include a step of reacting cryolite bath materials with aluminum sulfate, thereby producing a reactant product, the reactant product comprising aluminum fluoride. The method may further include a step of removing impurities from the reactant product, thereby creating a purified product comprising the aluminum fluoride. The removed impurities may comprise at least one of sodium (Na), magnesium (Mg), and calcium (Ca). In one embodiment, due to the removing step, the purified product contains not greater than 0.2 wt. % of calcium.

A method of treating a waste liquid: an aluminum dissolution step of dissolving aluminum in an acidic waste liquid and performing separation into a first treated water and a reduced heavy metal precipitate; a gypsum recovery step of adding a calcium compound to the first treated water at a pH of 4 or less, and performing separation into a second treated water and gypsum; a heavy metal coprecipitation step of adding a ferric compound to the second treated water and performing separation into a third treated water and a heavy metal coprecipitate; an aluminum and fluorine removal step of adding an alkali to the third treated water and performing separation into a fourth treated water and a precipitate containing aluminum and fluorine; and a neutralization step of adding an alkali to the fourth treated water and performing separation into an alkali neutralization treated water and a neutralized heavy metal hydroxide.

A method for recycling spent carbon cathode of aluminum electrolysis includes the following steps: (1) crushing and sieving spent carbon cathode, to obtain carbon particles; (2) mixing the carbon particles with a sulfuric acid solution, to obtain a slurry A, and then performing pressure leaching, to obtain a slurry B; (3) evaporating and concentrating the slurry B until a mass percentage of water is lower than 8%, to obtain a slurry C; (4) adding concentrated sulfuric acid to the slurry C to obtain a slurry D, then roasting the slurry D at 150-300° C. for 0.5-10 h, and then roasting at 300-600° C. for 0.5-8 h, to obtain the roasted carbon; and calcining the roasted carbon at a high temperature, to obtain the purified carbon, or mixing the roasted carbon with a leaching agent, and performing leaching, filtering, and washing, to obtain the purified carbon.

Apparatus for converting Spent Pot Lining (SPL) into inert slag, aluminum fluoride and energy includes a plasma arc furnace such that the destruction of SPL occurs therein. The furnace generates an electric arc within the waste, which arc travels from an anode to a cathode and destroys the waste due to the arc's extreme temperature, thereby converting a mineral fraction of SPL into vitrified inert slag lying within a crucible of the furnace. The furnace gasifies the carbon content of the SPL and produces a well-balanced syngas. The gasification takes place due to the controlled intake of air and steam into the furnace. The gasification reaction liberates significant amount of energy. Steam captures this excess energy, to provide part of the oxygen requirement for gasification and to contribute to raise the syngas H2 content. Steam also contributes to converting some SPL fluorides (NaF and Al2F3) into hydrogen fluoride. The plasma SPL processing system is compact (occupying less area than some competitive methods of SPL treatment), can be installed in close proximity to the aluminium plant (minimizing transportation of SPL and AlF3), and requires only electricity as its energy source and thus no fossil fuels.

Apparatus for converting Spent Pot Lining (SPL) into inert slag, aluminum fluoride and energy includes a plasma arc furnace such that the destruction of SPL occurs therein. The furnace generates an electric arc within the waste, which arc travels from an anode to a cathode and destroys the waste due to the arc's extreme temperature, thereby converting a mineral fraction of SPL into vitrified inert slag lying within a crucible of the furnace. The furnace gasifies the carbon content of the SPL and produces a well-balanced syngas. The gasification takes place due to the controlled intake of air and steam into the furnace. The gasification reaction liberates significant amount of energy. Steam captures this excess energy, to provide part of the oxygen requirement for gasification and to contribute to raise the syngas H2 content. Steam also contributes to converting some SPL fluorides (NaF and Al2F3) into hydrogen fluoride. The plasma SPL processing system is compact (occupying less area than some competitive methods of SPL treatment), can be installed in close proximity to the aluminium plant (minimizing transportation of SPL and AlF3), and requires only electricity as its energy source and thus no fossil fuels.

A corrosion-resistant member including: a metal base material (10); and a corrosion-resistant coating (30) formed on the surface of the base material (10). The corrosion-resistant coating (30) is a stack of a magnesium fluoride layer (31) and an aluminum fluoride layer (32) in order from the base material (10) side. The aluminum fluoride layer (32) is a stack of a first crystalline layer (32A) containing crystalline aluminum fluoride, an amorphous layer (32B) containing amorphous aluminum fluoride, and a second crystalline layer (32C) containing crystalline aluminum fluoride in order from the magnesium fluoride layer (31) side. The first crystalline layer (32A) and the second crystalline layer (32C) are layers in which diffraction spots are observed in electron beam diffraction images obtained by electron beam irradiation and the amorphous layer (32B) is a layer in which a halo pattern is observed in an electron beam diffraction image obtained by electron beam irradiation.

A corrosion-resistant member including: a metal base material (10); and a corrosion-resistant coating (30) formed on the surface of the base material (10). The corrosion-resistant coating (30) is a stack of a magnesium fluoride layer (31) and an aluminum fluoride layer (32) in order from the base material (10) side. The aluminum fluoride layer (32) is a stack of a first crystalline layer (32A) containing crystalline aluminum fluoride, an amorphous layer (32B) containing amorphous aluminum fluoride, and a second crystalline layer (32C) containing crystalline aluminum fluoride in order from the magnesium fluoride layer (31) side. The first crystalline layer (32A) and the second crystalline layer (32C) are layers in which diffraction spots are observed in electron beam diffraction images obtained by electron beam irradiation and the amorphous layer (32B) is a layer in which a halo pattern is observed in an electron beam diffraction image obtained by electron beam irradiation.