SHANGHAI, May 21 (SMM) – Spot copper premiums were quoted between positive RMB 200-280/mt in Shanghai in the morning business. Traded prices for standard-quality copper were between RMB 56,550-56,670/mt, and RMB 56,600-56,750/mt for high-quality copper. SHFE copper prices rebounded by more than 1.5%, and the SHFE/LME copper price ratio improved significantly, cutting losses in imported copper. Spot copper supply increased slightly in consequence. But cargo-holders were optimistic about future copper premiums and thus insisted on higher premium quotes. Downstream producers stood on the sidelines during the first trading day of the week following purchases at the lows the previous week and due to doubts about copper price rebounds. This led to limited market transaction volumes.