A process for refining sand for use as frac sand includes the steps of passing the sand through a first fines separation stage to remove fine particles of contaminant from the sand, reducing the water content of the sand (such as to less than 20%), passing the sand into an attrition scrubber unit containing moving blades to delaminate clay and other contaminants from the sand grains, passing the sand from the attrition scrubber unit through a second fines separation stage to separate fine contaminants from the sand, and dewatering the resulting sand product in a further dewatering stage.