Shares in BHP Billiton ( LON:BLT ), the world's largest mining company, advanced as it hinted it could spin off its non-core aluminium, thermal coal, manganese and nickel assets as part of a plan to simplify its portfolio. "We believe that a portfolio focused on our major iron ore, copper, coal and petroleum assets would retain the benefits of diversification, generate stronger growth in free cash flow and a superior return on investment," the company said in response to market speculation.