The freezing point depression, ΔTf is equal to the freezing point of the solvent minus the freezing point of the solution.
It is also proportional to the molal concentration of the solute.
ΔTf=Kfm
Where Kf, the freezing point depression constant, depends only on the solvent. Molality is moles of solute per kg of solvent. So, the concentration of solvent is expressed in terms of mass, and the mass of a substance is not affected by the change in temperature. Hence, molality is independent of temperature.