It is added to the new sand to bring the melting point of sand down. Ah for the good old days when recycling glass containers was just washing the damed things and refilling them. In the 70s, we collected bottles of pop music in the park. We would get a whole wagon load in a few minutes, then turn them in for get 5 cents each. Candy bars cost 20 cents each. Happy times.
Up until mid-1990s in the USSR and post-USSR a soda bottle was around 0.2 RUB and the empty bottle could be returned for 0.05 RUB.