The Federal Deposit Insurance Corp. said yesterday that it plans to sell $1.8 billion in bad loans that it absorbed from the 1984 bailout of Chicago's Continental Illinois National Bank & Trust Co. in what would be the largest auction of its kind.

"We've never attempted anything of this size or magnitude before," said Alan Whitney, a spokesman for the FDIC. He said the $1.8 billion portfolio is left from about $3.5 billion in bad loans absorbed by the agency when Continental nearly failed four years ago.

Whitney said the agency plans to accept sealed bids for each of six categories of loans contained in the portfolio: energy, international, commercial, maritime, real estate and personal.

"These are what you would characterize generally as problem credits," said Whitney. "There's still value there. The prospective bidders are going to have to do their own analysis of how much value is there and how much of it is collectable and make their bids accordingly."

He added that the agency might accept one bid for all of the loans if such an an offer exceeds the combined high bids in each of the six areas. The FDIC hasn't set a minimum price for the loans or a date for the bidding, he said.

"We've increasingly in the last couple of years attempted to package loans and sell them on a bulk basis," Whitney said. "What's unique here is the size of this undertaking.

"We've talked to some prospective bidders, and we think there's enough interest out there to go through with this," he added.

Although owned by the FDIC, the bad loans still are being administered by a special 100-person department at Continental under a service contract with the FDIC.

In October, the FDIC's suburban Chicago office sold $7.2 million in loans from two failed Illinois banks in its first open auction of troubled credits. The loans were from the State Bank of Cuba, which was closed Jan. 9, 1987, and the Lewistown Bank, which closed Feb. 27, 1987.

Whitney said the FDIC's net worth stands at $18.3 billion, even though it helped 19 troubled banks last year and 184 banks failed.