Mining giant BHP Billiton (BHP.NYSE, BLT.LSE, BHP.ASX) enjoyed a 16% year-on-year increase in iron ore production in June, but the company is concerned that the good times may not last, BBC reported. Metallurgical coal production was also up 16%. Output has consistently risen over the last three months, and total output for the last 12 months has set a new record of 125 million tons. However, BHP management expressed concern that efforts by Beijing to cool off blistering economic growth – which has stimulated demand for iron ore and coal used in steel needed for construction – will have a negative effect in the second half of the year. China is the company’s single most important customer, consuming a quarter of combined output from BHP and its competitor Rio Tinto (RIO.ASX, RIO.LSE, RTP.NYSE).