The surprise surge in coal prices will provide an unexpected profit boost for the world&rsquo;s biggest mining companies as they navigate a commodity downturn that sapped earnings.<br /> <br /> Gains for coking coal, used with iron ore to make steel, accelerated in recent weeks and benchmark prices from top producer Australia jumped 28 percent this week, the most in data going back to 2013. At current prices, that would add almost $5 billion to BHP Billiton Ltd.&rsquo;s underlying earnings before interest, tax, depreciation and amortization in the year ending June 30, according to Liberum Capital Ltd.