Methods and compositions for grinding materials are provided. In an embodiment, the present invention provides a method of grinding solid materials. For example, the method can comprise mixing the solid material with the grinding aid, and using a grinding media such as steel balls to grind the solids. The grinding aid composition can comprise one or more glycerin by-products derived from a biodiesel manufacturing process. The grinding aid composition can also comprise one or more glycerin by-products of transesterification reactions involving triglycerides. The grinding aid can be added to the grinding process as a solution.