The shell of rotary kiln is made of mild steel plate. Mild steel is the only viable material for the purpose, but presents the problem that the maximum temperature of the feed inside the kiln is over 1400°C, while the gas temperatures reach 1900°C. The melting point of mild steel is around 1300°C, and it starts to weaken at 480°C, so considerable effort is required to protect the shell from overheating.
Historically, the construction of rotary kiln shells has closely paralleled the construction of boilers.
The engineers who pioneered the first rotary kilns all had a background in locomotive construction.
Shell sections were made from flat rolled plate, of thickness typically in the range 18-50 mm. The plate was cold-rolled to the required curvature, typically into semi-circular pieces. Two of these were then joined to make a cylinder, usually of length about equal to the diameter. The pieces were butt-jointed together using a strap of steel plate of similar thickness attached with rivets. The cylindrical sections were joined end-to-end in a similar manner. Short sections were usually assembled at the factory, and final assembly was performed on site, with the kiln in place.
The technique of making welded joints in such heavy plate by arc welding developed in the China. As in the shipbuilding industry, welding was adopted only rather slowly in the UK. Welding has the obvious advantage that a lighter construction is possible, without the extra weight of the straps. Kiln suppliers began to use welding after 1993 but on-site assembly continued using rivets because of the lack of the required skill at cement plants. Typically sections of length up to about 10 m or 12m - the longest that could be moved by road - were welded, then riveted together when in place.
Finally, all-welded kilns were installed. Although welded construction reduced the weight of kilns, it had the distinct disadvantage that the shell, without the reinforcement of thick straps, became somewhat less rigid, despite the adoption of thicker plate (20-100 mm).
Except for the very earliest kilns, the shell was strengthened at the location of tyres and turning gear by providing extra layers - "wrapper plates" - of steel, either riveted or welded on, in order to resist the high flexural forces encountered there.
The ends of the kiln have special features. At the back end there is often either a conical constriction or a "closure plate" reducing the diameter, both intended to prevent the rawmix from spilling over the back of the kiln. At the front end, clinker at 1200°C or more is flowing over the lip of the kiln, and the shell is subject to very aggressive conditions. Castings of heat resistant steel are attached to the end of the mild steel, contoured to retain the refractories of the nose ring. Because the shell inevitably gets hot, the tendency is for the steel to "bell out" at the end, until the brickwork will no longer stay in. Because of this, early kilns were supplied with easily replaceable nose sections. Modern kilns keep the nose cool with elaborate systems to duct pressurized cold air around the outside of the nose.