9/20/2012 8:21 AM ET (RTTNews) - Spain conducted a successful bond auction on Thursday, with borrowing costs falling amid hopes the country will request aid paving the way for the European Central Bank to purchase sovereign debt.In the first bond auction held by Spain after the European Central Bank announced its new bond purchase plan earlier this month, the treasury raised EUR 4.8 billion from the sale of its benchmark 10-year bond and a new 3-...