Atlantic Richfield Co. said it is considering selling the public 20% of its Arco Chemical Co. unit . . . Thai Airways International signed a contract to buy one McDonnell Douglas DC-10 Series 30ER (extended-range) aircraft and four MD-11 long-range tri-jets with options for four more . . . The Interstate Commerce Commission will vote June 30 on whether to reopen the Santa Fe-Southern Pacific merger case . . . Jefferies Group of Los Angeles said it expects second-quarter earnings will be well below last year's level. The brokerage denied the downturn was attributable to an agreement by its founder Boyd L. Jefferies in March to plead guilty to two felony securities violations . . . America West Airlines of Phoenix said it sold an option to Ansett Airlines of Australia to acquire up to a 20% interest in America West . . . Tonka Corp. said it has discontinued previously announced negotiations with Kidde Inc. to buy that company's Ertl toy division for $89.5 million . . . Federated Department Stores said it completed the purchase of the Blocks unit of Allied Stores for $55 million in cash.