SAFETY-KLEEN <SK> TO BUY MCKESSON <MCK> UNIT Safety-Kleen Corp said it agreed in principle to acquire McKesson Envirosystems Co, a subsidiary of McKesson Corp. It said McKesson Envirosystems' current annual gross revenues are about 14 mln dlrs. The company collects flammable solvents from its industrial customers for a fee, after which it analyzes and processes the solvents before they are burned.