Classically , there are two pyrometallurgical methods and one hydrometallurgical method for the treatment of metal chlorides available. These processes decompose the metal chloride to produce the oxide and the acid. The pyrometallurgical route is available for all elements listed in the first post. The hydrometallurgical process route is proven for iron in the field of steel mill pickling lines . For other elements and areas, the hydrometallurgical route is a promising technology. The required processes are challenging. There are blanket statements about the possible practical applications and each case has to be analyzed separately.
As pyrometallurgical processes, the spray roaster and the fluidized-bed process are available. Both methods operate at temperatures > 400 ° C and the chemical reactions take place in the gas phase.
The spray roasting chloride solution is injected through lances and dispersed at the top of the hot reactor. In the lower region of the reactor, tangentially arranged gas burners supply the required energy. The chemical decomposition reaction of the chloride to oxide and hydrogen chloride take place during the free fall of the drops. The resulting oxide is drawn off downwards and the hot HCl / vapor mixture leaves the system at the top of the reactor. In case of recovering used pickling liquor from steel mills fine red Fe2O3 powder is produced. The resulting HCl gas is washed out of the gas stream by a scrubber system and hydrochloric acid is produced.
In the fluidized bed process, the chloride solution is fed without spraying directly to a hot fluidized bed. The energy is supplied by combustion of natural gas directly in the fluidized bed. The chemical reaction takes place on the surface of the particles present in the bed. An onion structure is formed at the particle. Oxide spheres are removed hot at the bottom of the fluidized bed. The oxide has a statistical grain size distribution and up to 1.5 to 2 mm in diameter. The HCl / vapor mixture is discharged at the top of the reactor and the absorption is analogous to the spray roasting process.
The so-called PORI process is available as hydrometallurgical process. In this method, the chloride is decomposed at < 250 ° C in the liquid phase to solid oxide and the gaseous HCl. The energy is supplied indirectly by heat exchangers. The resulting HCl could reach, in contrast to the previously described methods, concentrations above the azeotropic point. Another advantage of this method is that due to lower process temperature, residual energy can be used and, in general, the energy consumption is lower. The challenge is in the process control and material selection. For example, a solid / liquid separation at high temperatures in a corrosive medium has to be executed.
To be continued …