The Waelz process is a method of recovering

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

and other relatively low boiling point metals from metallurgical waste (typically

electric arc furnace An electric arc furnace (EAF) is a Industrial furnace, furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundry, foundries for producin ...

flue dust) and other recycled materials using a

rotary kiln A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: * Cement * Lime * Refractories * Metakaolin * Titanium dioxide * ...

(''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

for the recovery of Zinc by volatization dates to at least 1888. 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). The process consists of treating zinc containing material, in which zinc can be in the form

zinc oxide Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder which is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, Zinc metabolism, food supplements, rubbe ...

, zinc silicate, zinc ferrite,

zinc sulphide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...

together with a carbon containing reductant/fuel, within a rotary kiln at 1000 °C to 1500 °C. The kiln feed material comprising zinc 'waste', fluxes and reductant (coke) is typically pelletized before addition to the kiln. The chemical process involves the reduction of zinc compounds to elemental zinc (boiling point 907 °C) which volatilises, 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. Kiln size is typically 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). The process also captures other low boiling metals in the ''waelz oxide'' including lead, cadmium and silver. Halogen compounds are also present in the product oxide. Increased use of

galvanised Galvanization ( also spelled galvanisation) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are coated by submerging them in a bath o ...

steel has resulted in increased levels of zinc in steel scrap which in turn leads to higher levels of zinc in

flue dusts. As of 2000, the waelz process is considered to be a "

best available technology The best available technology or best available techniques (BAT) is the technology approved by legislators or regulators for meeting output Technical standard, standards for a particular process, such as pollution abatement. Similar terms are ''be ...

" for flue dust zinc recovery and the process is used at industrial scale worldwide. As of 2014, the Waelz process is the preferred or most widely used process for zinc recovery of zinc from

dust (90%). Alternative production and experimental scale zinc recovery processes include the rotary hearth treatment of pelletised zinc containing dust ( Kimitsu works, Nippon Steel); the SDHL (Saage, Dittrich, Hasche, Langbein) process, an efficiency modification of the Waelz process; the "DK process" a modified blast furnace process producing pig iron and zinc (oxide) dust from blast furnace dusts, sludges and other wastes; and the PRIMUS process (multi-stage zinc volatilisation furnace).

History and description

References

Sources

External links