Method and arrangement for changing the temperature of fluids which form incrustations

A fluid medium is conveyed along a conduit. During its passage through the conduit, the fluid medium undergoes temperature change. The change in temperature of the fluid medium is performed in a step-wise manner by indirect heat-exchange with a fluid flowing countercurrent to the fluid medium, that is, by means of a plurality of heat-exchangers arranged along the conduit. The fluid medium forms incrustations and the rate at which incrustations form is different at different temperatures. This is taken into account by providing for the ratio between the lengths of any two heat-exchangers to approximate the ratio between the rates at which incrustations form at the temperatures of the respective heat-exchangers. Of particular interest is the extraction of bauxite with sodium aluminate lye.