Rotary calciners are indirectly-heated, continuous-process heat transfer devices for medium to high-temperature processing of bulk solids materials. The rotary calciner consists principally of an alloy rotary shell, enclosed in and heated on its exterior in a stationary furnace. The process material moves through the interior of the rotary shell, where it is heated through a combined radiative and convective/conductive mode of heat transfer through the rotary shell wall. Rotary calciners are ideal for specialized processing due to the indirect heating mechanism. As the heat source is physically separated from the process environment, specific process atmospheres can be maintained. Processes requiring inert, reducing, oxidizing, or dehumidified atmospheres, or those with a solids/gas phase reaction can be accommodated. Depending on the process requirements, rotary calciners can operate under positive or negative pressure, and a variety of seal arrangements are available.