1. A method of estimating and controlling the concentration of alumina in the bath of a Hall cell, the method including the use of an enhanced Kalman filter-type algorithm that employs two sets of equations, namely, a time update algorithm that contains a dynamic model of the alumina mass balance in the Hall cell and provides estimates of alumina concentration, and a measurement update algorithm that uses a feedback variable from the Hall cell process to modify the alumina estimate provided by the time update algorithm, and one or more tuning parameters, the method comprising using the time update algorithm to estimate the concentration of alumina in the bath at the end of intervals of time by adding to the previous value of said estimate the amount of alumina fed to the cell minus the amount of alumina consumed in the production of aluminum during the interval of time, extrapolating the slope of a voltage-ampere curve of the cell to a voltage value at zero current, feeding back the voltage value at zero current, as the feedback variable, to the measurement update algorithm during measurement update periods, using the same to modify the alumina estimate provided by the time update algorithm to thereby provide the best available estimate of alumina concentration, and employing this estimate to control the amount of alumina fed to the cell.