Complex sentence: Coins were returned from commercial banks to the Fed, were being processed and cleaned, and were then sent to banks needing them, representing the bulk of what the Fed shipped, with only perhaps a fifth being new coins.
Split into simpler sentences: Coins were returned from commercial banks to the Fed, were processed and cleaned, and were then sent to banks needing them. These recycled pieces represented the bulk of what the Fed shipped, with only perhaps a fifth being new coins.