UPDATE 1-Russian diamond miner Alrosa's second-quarter sales tumble 21 pct q/q

* Alrosa says Q2 net profit 40.5 bln roubles

* Q2 EBITDA down 13 pct q/q to 51.8 bln roubles

* Says Russia to cancel rough diamonds export duty (Adds details, analyst comment, context)

MOSCOW, Aug 29 (Reuters) - Russia’s Alrosa, the world’s largest producer of rough diamonds, said its second-quarter sales fell 21 percent from the previous three months to 9.6 million carats after a recovery in demand seen early this year faded.

The company, which with Anglo American’s unit De Beers produces about half the world’s rough diamonds, reported a second quarter net profit of 40.5 billion roubles ($625 million). That was 61 percent higher than a year ago, helped by the rouble’s weakness against the dollar, but was down 19 percent from a record high profit in the first quarter.

Russia’s government sold a 10.9 percent stake in Alrosa in July, raising $813 million and increasing the company’s free float to 34 percent. It was the biggest Russian privatisation for almost four years.

Alrosa’s second-quarter revenue tumbled 18 percent from the previous three months to 84.3 billion roubles.

“Alrosa maintains a conservative outlook on the diamond market and executes permanent costs control,” CEO Andrey Zharkov said in the results statement.

Earnings before interest, taxes, depreciation and amortisation (EBITDA) dropped 13 percent to 51.8 billion roubles.

“After the sharp recovery observed at the beginning of the year, the seasonal slowdown was broadly expected by the market,” Renaissance Capital said in a recent note on Alrosa. Renaissance said it expected gem-quality diamond sales volumes to decline by 13 percent in the third quarter compared with the second quarter.

Alrosa said its first-half finiancial results were stronger than in the same period a year ago thanks to an increase in diamond sales by 21 percent in carat terms and because of the weaker rouble.

It also said Russia would scrap a 6.5-percent rough diamonds export duty from Sept. 1 in line with World Trade Organization (WTO) rules. ($1 = 64.7994 roubles) (Reporting by Polina Devitt; Editing by Dmitry Solovyov and Susan Fenton)