Cassiterite, $\mathrm{SnO}{}_2$, contains wolframite, $\mathrm{FeWO}{}_4$, as impurity. The impurity is removed by

Wolframite is magnetic whereas cassiterite is non-magnetic, so the crushed ore passed over a magnetic roller separates the two. Froth flotation is for sulphides, leaching is for ores like bauxite, and calcination only removes volatile matter.