old scape to finish goods

i am utensils dealer. i have old brass and i want to send factory to recycle for make finish goods.
now old bass rate is 250rs par kg and fectory processes is 100rs par kg.
old have 18%gst and new product 12%gst.

how can i send old brass by transport with bill and receive new..
and how much i paid gst and how?
if thare is job work so i will pay only job work tax (100rs kg factory work)?