The alumina concentration in the bath plays a fundamental role on cell operation. Local depletion may lead to an anode effect when using carbon anodes. A mathematical model describing the alumina convection-diffusion process in the bath coupled to the cell magneto-hydrodynamic (MHD) in the presence of small bubbles is presented. Small bubbles may be assumed when slotted anodes are used. The relative importance of the velocity fields generated by the magnetic effects and/or the small bubbles on the alumina concentration in the bath is discussed.