Rio Tinto Group, the third-biggest mining company, will gain control of Richards Bay Minerals, or RBM, on South Africa’s east coast in a deal that sees BHP Billiton Ltd. exiting the titanium minerals industry. Rio, operator of the RBM venture, will double its stake to 74 percent after BHP exercised an option to sell 37 percent, the companies said, without disclosing the value of the deal. RBM produces mainly titania slag, which is mostly titanium dioxide, used to make products from paint to toothpaste.