A very simple frictional plasticity model for a granular material is presented, including the effects of dilation. The novelty lies in the fact that this is described within the hyperplasticity framework, expressed using the terminology of convex analysis. This allows a consistent mathematical treatment of the dilation constraint. The Fenchel Dual is used to link the force and flow potentials. The resulting model accommodates non-associated flow within a rigorous mathematical framework that ensures compliance with the Laws of Thermodynamics.