Waelz process

The Waelz process is a method of recovering zinc and other relatively low boiling point metals from metallurgical waste (typically EAF flue dust) and other recycled materials using a rotary kiln (waelz kiln).

The zinc enriched product is referred to as waelz oxide, and the reduced zinc by product as waelz slag.

History and description

The concept of using a rotary kiln for the recovery of Zinc by volatization dates to at least 1888.[1] A process was patented by Edward Dedolph in 1910. Subsequently the Dedpolph patent was taken up and developed by Metallgesellschaft (Frankfurt) with Chemische Fabrik Griesheim-Elektron but without leading to a production scale ready process. In 1923 the Krupp Grusonwerk independently developed a process (1923), named the Waelz process (from the German Waelzen, a reference to the motion of the materials in the kiln); the two German firms later collaborated and improved the process marketing under the name Waelz-Gemeinschaf.[2]

The process consists of treating zinc containing material, in which zinc can be in the form zinc oxide, zinc silicate, zinc ferrite, zinc sulphide together with a carbon containing reductant/fuel, within a rotary kiln at 1000 °C to 1500 °C.[1] The kiln feed material comprising zinc 'waste', fluxes and reductant (coke) is typically pelletized before addition to the kiln.[3] The chemical process involves the reduction of zinc compounds to elemental zinc (boiling point 907 °C) which volatalises, which oxidises in the vapour phase to zinc oxide. The zinc oxide is collected from the kiln outlet exhaust by filters/electrostatic precipitators/settling chambers etc.[4][3]

Kiln size is typically 50 by 3.6 metres (164 by 12 ft) long / internal diameter, with a rotation speed of around 1 rpm. The recovered dust (Waelz oxide) is enriched in zinc oxide and is a feed product for zinc smelters, the zinc reduced by-product is known as Waelz slag. Sub-optimal features of the process are high energy consumption, and lack of iron recovery (and iron rich slag).[3] The process also captures other low boiling metals in the waelz oxide including lead, cadmium and silver.[5] Halogen compounds are also present in the product oxide.[6]

Increased use of galvanised steel has resulted in increased levels of zinc in steel scrap which in turn leads to higher levels of zinc in electric arc furnace flue dusts - as of 2000 the waelz process is considered to be a "best available technology" for flue dust zinc recovery, and the process is used at industrial scale worldwide.[7]

As of 2014 the Waelz process is the preferred or most widely used process for zinc recovery of zinc from electric arc furnace dust (90%).[8]

Alternative production and experimental scale zinc recovery processes include the rotary hearth treatment of pelletised zinc containing dust (Kimitsu works, Nippon Steel);[9][10] the SDHL (Saage, Dittrich, Hasche, Langbein) process, an efficiency modification of the Waelz process;[3] the "DK process" a modified blast furnace process producing pig iron and zinc (oxide) dust from blast furnace dusts, sludges and other wastes;[11] and the PRIMUS process (multi-stage zinc volatilisation furnace).[12][13]

References

  1. 1 2 Clay & Schoonraad 1976, p. 11.
  2. ↑ Harris 1936.
  3. 1 2 3 4 Stewart, Daley & Stephens 2000, Recovery of Zinc Oxide from secondary raw materials : New developments of the Waelz Process
  4. ↑ Clay & Schoonraad 1976, pp. 11, 13.
  5. ↑ Antrekowitsch et al. 2014, p. 118.
  6. ↑ Antrekowitsch et al. 2014, p. 119.
  7. ↑ Stewart, Daley & Stephens 2000, Recovery of Zinc Oxide from secondary raw materials : New developments of the Waelz Process.
  8. ↑ Antrekowitsch et al. 2014, pp. 117-118, 119.
  9. ↑ Oda, Hiroshi; Ibaraki, Tetsuharu; Takahashi, Masaharu (July 2002), "Dust Recycling Technology by the Rotary Hearth Furnace" (PDF), Nippon Steel Technical Report (86)
  10. ↑ Oda, Hiroshi; Ibaraki, Tetsuharu; Abe, Youichi (July 2006), "Dust Recycling System by the Rotary Hearth Furnace" (PDF), Nippon Steel Technical Report (94)
  11. ↑ Hillmann, Carsten; Sassen, Karl-Josef (2006), "Solutions for dusts and sludges from the BOF process" (PDF), Stahl und Eisin 126 (11)
  12. ↑ J. L., Roth; R., Frieden; Hansmann, T.; Monai, J.; Solvi, M. (Nov 2001). "PRIMUS, a new process for recycling by-products and producing virgin iron". Revue de Métallurgie 98 (11): 987–996. doi:10.1051/metal:2001140.
  13. ↑ "The PRIMUS process by Paul Wurth: cutting-edge technology for recycling iron and steel by-products by direct reduction", www.innovation.public.lu, 3 Aug 2003

Sources

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