Why is granulated slag retreated after tin smelting?

In the context of tin processing, granulated slag is retreated because it may still contain residual amounts of tin that were not successfully recovered during the initial smelting process.1

During smelting, a pool of molten tin is produced with a layer of slag floating on top, which contains most of the unwanted impurities. Once the impure tin is tapped off, the remaining slag is solidified into granules by being poured into water tanks. Retreating this granulated slag allows for the recovery of the remaining tin, ensuring a more efficient extraction process.1