where mF and mP are the solids rates of the feed and product streams respectively, and GF,i, GP,i, and GT,i are the grade of species i in the feed (F), product (P) and tailings (T) streams respectively. These equations assume no change in the properties of the particles, for example, due to breakage or reactions, as they pass through the system, and that the system is operating at steady state. Equation 1 also appears in other fields in various guises, sometimes referred to as the mixture rule or the lever rule. In this paper we attach the subscript i to the yield value to indicate that the yield was calculated using the assay values of species i. For a steady state system with no sampling or measurement errors, the calculated yield should of course be the same regardless of which species' assays were used.