Arch Coal Inc. ACI +11.35% shares surged as much as 16% to $8.48 at the open Friday after the coal miner turned in surprisingly strong third-quarter results. Arch, citing strict cost and spending controls, posted adjusted earnings of 20 cents a share on $1.09 billion in revenue. Analysts surveyed by FactSet had predicted the company would post a loss of 15 cents a share on $1.04 billion in sales. Arch Coal also said it sees coal demand picking up in 2013 as natural gas prices climb. Abundant supplies of cheap natural gas have cut deeply into coal demand over the past year, especially for power generation. Including today's gains, Arch Coal shares are still down 49% from where they traded a year ago.