This is the second in a trilogy of short articles on the history, production and usage of zinc sheeting, presented by Rheinzink.
Described as a pyro-metallurgical technique, the imperial smelting process provides for the direct processing of concentrates and secondary raw materials. The main energy source is coke, and in addition to the primary products of zinc and lead, the process also produces slag which can be used in the construction sector. This process accounts for some 15% of the world’s zinc production.
The following processes are available for the production of zinc from these concentrates: zinc electrolysis, the imperial smelting (IS) process and New Jersey zinc distillation (for refining crude zinc). There are also plants for resmelting and liquation, i.e. for thermic separation of zinc scrap.
The individual steps of the IS process are: briquetting – secondary raw materials and “waelz” oxide are hot-briquetted; roasting – concentrates are desulphurised (roasted) via sinter roasting and agglomerated with recycling materials; reduction and condensation – hot briquettes and sinter are reduced with coke in the IS furnace. Zinc is discharged with the blast furnace gas and condensed as crude zinc. Lead is discharged at the lower end of the furnace.
To extract high-grade zinc, this is followed by two-stage distillation according to the New Jersey process, involving removal of the accompanying metals and the production of high-grade zinc of great purity, and zinc distillation according to this process for the production of thermic high-grade zinc involves distilling crude zinc from primary and secondary raw materials in a single or two-stage process.
Here the entire quantity of the crude zinc can be distilled or only part of it, to recover both high-grade zinc and cadmium-free commercial zinc or a cadmium alloy, depending on the requirements. During New Jersey zinc distillation all intermediates undergo further processing so that no waste materials are left over.
Processing zinc scrap involves resmelting and liquation (a metallurgical method for separating metals from an ore or alloy) after the necessary sorting of the scrap material. The individual process steps are: sorting – zinc scrap, aluminium and iron parts are separated from each other; smelting – unmixed scrap is resmelted, clean scrap is melted and liquated, and mixed scrap is melted down in a furnace.
This then undergoes liquation – the constituents of the molten metal are separated according to their different melting points (this process also produces hard zinc, an iron-zinc alloy), and casting – the zinc alloys and zinc are then cast.
The Waelz tube process is used when processing feedstock with a low zinc content – for example, as it occurs during recycling in particular. Here the feedstock is first formed into moist balls (pelletised) and then heated in a rotary furnace (Waelz tube). The zinc contained in the material vaporises, is oxidised and can be recovered as “waelz” oxide after cooling in a filter.
One important source of feedstock with low zinc levels is the filter dust containing zinc produced when recycling galvanised steel parts. Recycling the zinc found in such dust using the above process has represented state-of-the-art technology for many years.
Production of sheets and strips
One major advance in the production of zinc strips in predefined thicknesses was the introduction of the continuous wide-strip casting and rolling process at Rheinzink.
Here an alloy of zinc, copper and titanium is smelted in a coreless induction furnace at a temperature of approximately 760°C. This produces master alloy blocks that are then melted in a core-type induction furnace and mixed with high-grade zinc.
The resulting alloy is next passed to the casting machine in liquid form. Here it is cooled below melting point by a closed water circulation system to produce a solid billet. This process must take place at the same speed as the following steps – rolling and winding.
The billet passes through a cooling section and loop towers to compensate for any minor differences in speed to then arrive at the roll stands. Here the thickness of the material is reduced in steps.
The careful coordination of pressure and cooling is used to influence the metallurgical properties of the material such as ductility, tensile strength and long-time rupture strength.
In the casting and rolling process at Rheinzink zinc strips are produced from liquid metal in the form of a coil using a continuous process. In the casting machine the finished alloy is provided with the input cross-section necessary for the rolling process with simultaneous cooling, and at the end of the rolling train the finished strip is wound into large coils and stored temporarily for cooling.
A stretching, bending and straightening system is used to eliminate stresses in the strip, which is then divided longitudinally and crosswise, depending on further processing.
Part 3 of this trilogy will appear in the next issue of Walls & Roofs in Africa, covering recycling, the environment and sustainability.