Secondary aluminum salt slag recovery
During the re-melting process, the Secondary (recycled) Aluminum
industry produces dross which typically contains metallic Aluminum
(about10%wt), Aluminum Oxide (about 50%wt) and a mixture of Potassium Chloride
and Sodium Chloride (about 40%wt). Metallic Aluminum is recovered by mechanical
means. For environmental reasons, the remaining part of the dross cannot be
disposed easily.
In GEA-Messo PT systems, the salts are separated
from the insoluble Aluminum Oxide (this is eventually filtered out) in a hot
leach step. Ammonia generated in the recovery process can be neutralized with
Sulfuric acid, and fertilizer-grade Ammonium Sulphate can be produced. The
Potassium Chloride and Sodium Chloride contained in the leachate can be
crystallized separately or as a mixed salt solid. The salts can be recycled to
the Aluminum Recovery section or sold for industrial uses. Evaporative
Crystallization units utilizing Thermal Vapor Recompression or multiple effect
evaporation are used for this task. These recovery systems can be designed to
operate as Zero Liquid Discharge plants.
Zinc oxide wastewater treatment plants
When leaching Waelz oxide (zinc) from recycling of steel plant dust the
resulting solutions are containing sodium and potassium chloride, as well as
sulphate compounds. Heavy metals, like Zn, are precipitated with soda ash and
separated by filtration.
With GEA Messo PT evaporation and
crystallization experiences a processing concept has been established in the
respective recycling market, which is operated under zero liquid discharge
conditions. Heating is realized by MVR or Thermocompression. The
crystallization of Glaserite, which is one of the compounds crystallizing as
well as the material resistance, had been the challenging development targets.
The obtained condensate is internally recycled.