The Waelz process is a method of recovering

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc 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 periodi ...

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 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 * Al ...

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 Metallgesellschaft AG was formerly one of Germany's largest industrial conglomerates based in Frankfurt. It had over 20,000 employees and revenues in excess of 10 billion US dollars. It had over 250 subsidiaries specializing in mining, specialty c ...

(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 formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...

, 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 or galvanizing ( also spelled galvanisation or galvanising) 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 submerged ...

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

electric arc furnace An electric arc furnace (EAF) is a 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 foundries for producing cast iron products) up to ...

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 standards for a particular process, such as pollution abatement. Similar terms are ''best practicable means ...

" 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