Waelz process

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-Gemeinschaft (German for Waelz association).[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]