UPDATE 1-China Sept iron ore imports surge to 20-mth high on firm demand

* Sept iron ore imports 99.36 mln T vs 94.85 mln Aug - Rtrs calcs

* Shipments for first nine months down 2.4% on year ago (Adds details, background and analyst quotes)

BEIJING, Oct 14 (Reuters) - China’s iron ore imports rose for a third straight month in September to a 20-month high, according to customs data released on Monday, fuelled by firm demand at steel mills and stable shipments from big miners.

The world’s top iron ore consumer brought in 99.36 million tonnes of the mineral last month, the biggest purchase since January 2018, Reuters calculations showed. That was up 4.8% from 94.85 million tonnes in August and compared with 93.47 million tonnes a year earlier.

For the first nine months of the year, arrivals of the key steelmaking ingredient totalled 784 million tonnes, down 2.4% from 803.34 million tonnes in the same period a year ago.

Demand for iron ore at China’s steel mills has been firm despite some curbs last month when China implemented special measures to cut factory pollution ahead of celebrations for the 70th anniversary of the founding of the People’s Republic on Oct. 1.

Utilisation rates at steel mills across China were around 68% in the first three weeks of September, but fell to 57.6% in the final week, the lowest on record since August 2012, data compiled by consultancy Mysteel showed.

“The curbs ahead of National Day were strict but were a one-off,” said Zhao Yu, analyst with Huatai Futures, speaking before data was released, noting that steel output has been running at high levels this year.

“Shipments from Brazil and Australia that were due to discharged in China last month were stable,” she added.

Stocks of iron ore at China’s major ports stood at 125.55 million tonnes as of end-September, little changed from end-August, data compiled by consultancy SteelHome showed.

China’s Ministry of Ecology and Environment said in September it would set stricter targets for polluting cities for this year’s autumn-winter heating season.

But analysts don’t expect a major impact on steel production.

“Steel mills have adapted to the pace and measures of output curbs over the past few years, so actual influence on production could be relatively small,” Zhao said. (Reporting by Min Zhang and Shivani Singh; editing by Richard Pullin)