The current invention discloses a novel process and system to improve the quality of the slag product from a gasification process, thereby producing low-carbon marketable aggregate product. The inventive processes and systems employ a hindered-bed settler in conjunction with optional disengager and de-watering devices. A slag slurry stream from a gasification process is de-watered and the solids content is increased from less than 5% to greater than 30% via the de-waterer before being conveyed to a hindered-bed settler, wherein the carbon content is reduced from as much as 70% to less than 5%. Particles with a high carbon content are conveyed to a gravity settler, whereby they are concentrated and then recycled to the gasification reactor.