The invention relates to an apparatus for continuous slag treatment of molten silicon for removal of one a more impurity elements from the silicon, the apparatus comprising a vessel (1) intended to contain molten silicon and liquid slag. The vessel (1) has an upwardly extending overflow (4) for slag connected to an outlet opening (3) for slag is the bottom (2) of the vessel, a open-ended pipe (5) arranged about and at a distance from the overflow (4) such than there is an annulus between the pipe (5) and the overflow (4), which pipe (5) extends upwards to a level above the top of the overflow (4) and downwards to the bottom (2) of the vessel, the pipe (5) having at least one opening (6) at the bottom of the vessel (1) a closeable outlet opening (8,9) for treated silicon in the sidewall of the vessel, means (14) for supply of heat energy to the vessel, means for intermittent supply of solid or molten silicon to vessel and means for continuous of substantially continuous supply of solid or liquid slag to the top of the vessel.