Potassium aluminum sulfate or alum crystals can be very easily grown by mixing the salt in hot water, stirring it to aid solution, and letting it cool down to room temperature. The water will evaporate, leaving behind alum crystals.
Each liquid has a certain solubility for a salt at a given temperature. As the temperature increase, so does the solubility. At higher temperatures relatively higher concentrations of substance can be found in the hot solution. When the temperature decreases, this supersaturated salt quantity will move out of the solution, leaving behind crystals, by the process of crystallization. If excess liquid is there, then the process of decanting is used.
In this case, the solvent (water here) will evaporate when the salt solution is cooling thus leaving behind salt crystals.
So the physical separation technique causing solid-liquid separation is evaporation.