Metallurgical refining of crude silicon for low-cost, mass production of solar silicon has been studied extensively over the past two decades. One of the methods attempted to target the deleterious impurities of Si is slag refining, where liquid Si or its alloys are treated with a flux. This article presents an overview of the impurity response to various slag treatment conditions for crude Si metal from both fundamental and practical perspectives, aiming to provide a basis for the design and application of more effective fluxes and alloys. It was found that removal of B and P through slag refining alone is not practical on an industrial scale as it would require a large volume of slag, however the removal percentage can be significantly increased if the silicon has been alloyed with a suitable metal.