We first examine the three main components required to construct a realistic and tractable model of the working of a spiral, namely (1) geometry of the spiral and its trough, (2) principal forces acting on a particle which are gravity, centrifugal, hydrodynamic drag and lift and friction forces, and (3) flow of fluid down the curvilinear tightly turning path of the spiral, Next we combine these elements seamlessly by assuming that the particles eventually attain dynamic equilibrium in the forward longitudinal direction and static equilibrium in the transverse direction. The resulting force function provides a spectrum of the particles＇ radial location on the trough according to their size and density. Simulation results are highly encouraging. The model faithfully reflects the present empirical spiral design philosophy for treating different feeds such as coal or heavy density minerals. It seemingly has potential for computer-aided spiral design and in control and optimization of spiral circuits.