Here is what Wikipedia has to say:
IsolationSeveral million tons are produced annually as a by-product of the dairy industry. Whey is made of up 6.5% solids of which 4.8% is lactose that may be purified by crystallisation. Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example in the production of cheese. Lactose makes up about 2-8% of milk by weight. Lactose is purified from whey by adding ethanol. Since it is insoluble in ethanol, lactose precipitates in about 65% yield.