In this paper we report on the development of a down-exhaust cyclone separator suitable for use as a primary device for gas-particle separation in circulating fluidized-bed (CFB) boilers. It consists of a cylindrical shell which is joined by an inclined cone, a guide body, and a downward exhaust pipe. The principle of solids separation is similar to that in reverse flow cyclones, except that in this separator the gas leaves the separator axially at the bottom. It has been shown that this separator is capable of achieving a separation efficiency of around 99% in handling high solids loadings such as those in CFB boilers, at a pressure drop of around 400 Pa. A model based on the mechanism of radial solids mixing is proposed, which assumes that the slip between gas and particles in the tangential direction is negligible and that the particles are well-dispersed. This model enables prediction of grade separation efficiency, total separation efficiency, and cut diameter. It may be used in conjunction with a pressure drop model to optimize the design and operating parameters.