A tungsten lightbulb is seen in this illustration photo August 12, 2017. REUTERS/Thomas White/Illustration

A tungsten lightbulb is seen in this illustration photo August 12, 2017. REUTERS/Thomas White/Illustration

Tungsten lightbulbs are seen in this illustration photo August 12, 2017. REUTERS/Thomas White/Illustration

A man walks past a company logo outside a closed exchange centre of the Fanya Metal Exchange in downtown Kunming, Yunnan province, China, September 22, 2015. Hundreds of investors gathered outside the headquarters of China's securities watchdog on Monday to protest against a metals exchange in a southwest province they accuse of defrauding them of billions of yuan. The Fanya Metal Exchange, located in the city of Kunming in Yunnan province and authorised by the local government, trades 14 minor and rare metals, including indium, bismuth, tungsten, antimony and cobalt. REUTERS/Stringer

An artisanal gold miner carries a sack of sediment from tunnels in an abandoned industrial shaft at Makala Mine, near the town of Mogbwalu, in the Ituri district of the Democratic Republic of Congo April 28, 2012. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken April 28, 2012. To match Feature GOLD-CONFLICT/ REUTERS/Jonny Hogg (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENVIRONMENT BUSINESS EMPLOYMENT CONFLICT SOCIETY COMMODITIES)

Artisanal gold miners carry sacks of sediment from tunnels in an abandoned industrial shaft at Makala Mine, near the town of Mogbwalu, in the Ituri district of the Democratic Republic of Congo April 28, 2012. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken April 28, 2012. To match Feature GOLD-CONFLICT/ REUTERS/Jonny Hogg (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENVIRONMENT CONFLICT SOCIETY BUSINESS COMMODITIES)

An artisanal gold miner poses with the tools of his trade near the town of Mongbwalu, in the Ituri district of the Democratic Republic of Congo, April 28, 2012. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken April 28, 2012. To match Feature GOLD-CONFLICT/ REUTERS/Jonny Hogg (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENVIRONMENT CONFLICT SOCIETY BUSINESS COMMODITIES)

Artisanal gold miners pan sediment for gold near the town of Mogbwalu, in the Ituri district of the Democratic Republic of Congo, April 28, 2012. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken April 28, 2012. To match Feature GOLD-CONFLICT/ REUTERS/Jonny Hogg (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENERGY ENVIRONMENT BUSINESS EMPLOYMENT CONFLICT SOCIETY)

Artisanal gold miners pass up pans of sediment from an open-cast artisanal mine near the town of Mongbwalu, in the Ituri district of the Democratic Republic of Congo, April 28, 2012. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken April 28, 2012. To match Feature GOLD-CONFLICT/ REUTERS/Jonny Hogg (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENERGY ENVIRONMENT BUSINESS EMPLOYMENT CONFLICT SOCIETY)

Boys pan for gold on a riverside at Iga Barriere, 25 km (15 miles) from Bunia, in the resource-rich Ituri region of eastern Congo February 16, 2009. Ituri is one of many areas of the country to have experienced bitter ethnic conflict between rival tribes in recent years. Massacres have left tens of thousands dead. It is this fighting that led U.S. authorities to take the unprecedented step of naming Congo in section 1502 of the Dodd-Frank financial regulation act, which says U.S.-listed companies that source gold, tungsten, tantalum and tin from Congo or its neighbours must assure the U.S. stock exchange regulator that their business is not helping fund conflict. Picture taken February 16, 2009. To match Feature GOLD-CONFLICT/ REUTERS/Finbarr O'Reilly/Files (DEMOCRATIC REPUBLIC OF CONGO - Tags: ENERGY ENVIRONMENT BUSINESS EMPLOYMENT CONFLICT SOCIETY)

Workers install a drill bit before drilling for gold in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken April 24, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

A worker watches the area in which gold prospecting is taking place in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken April 24, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

A worker marks samples of gold deposits after drilling in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken April 24, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

Workers collect samples of gold deposits after drilling in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken 24 April, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

A geologist examines samples of gold deposits at a warehouse in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken 24 April, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

Workers carry a box with samples of the gold deposits in a warehouse in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken 24 April, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

A worker prepares samples of gold deposits for study at a warehouse in the southern Portuguese town of Boa Fe April 24, 2012. Faced with the deepest recession in three decades, record- high unemployment, a stubbornly high debt and a 78-billion-euro bailout from the European Union and IMF, Portugal is looking to turn natural riches into much needed income. The centre-right government is trying to draw mining giants to the country to extract everything from iron ore to gold, silver and tungsten in the hope of cashing in on the 4 percent of revenues it will gain from each operation. Photo taken 24 April, 2012.
REUTERS/Jose Manuel Ribeiro (PORTUGAL - Tags: BUSINESS COMMODITIES)

An employee operates a grinding machine at an ore-dressing mill at Taiga settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS EMPLOYMENT)

People walk along the street of ore-dressing mill settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. 1200 employees work at a mill. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS EMPLOYMENT)

An employee controls the quality of tungsten concentrate at an ore-dressing mill at a Taiga settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS EMPLOYMENT)

An employee works on a grinding machine at a ore-dressing mill at a Taiga settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS EMPLOYMENT)

An employee walks in an ore-dressing mill at a Taiga settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS EMPLOYMENT)

An employee works on a flotator at ore-dressing mill at a Taiga settlement Vostok, 600 km (373 miles) north of Vladivostok, November 18, 2010. The mill belongs to AIR mining company and produces 5 percent of world tungsten concentrate, local officials said. REUTERS/Yuri Maltsev (RUSSIA - Tags: BUSINESS)

Gold miners form a human chain while digging an open pit at the Chudja mine in the Kilomoto concession near the village of Kobu, 100 km (62 miles) from Bunia in north-eastern Congo, in this February 23, 2009 file photo. Home to the world's biggest reserves of cobalt -- used in batteries, ceramics and dyes -- Congo has gold, silver and diamond mines, and holds some of the world's largest stores of copper, tin and metals such as tungsten, a component of many mobile phones. To match feature CONGO-DEMOCRATIC/ REUTERS/Finbarr O'Reilly/Files (DEMOCRATIC REPUBLIC OF CONGO - Tags: BUSINESS ENVIRONMENT)

A worker inspects a production unit of a tungsten mine that is currently out of service near the settlement of Svetlogore in the far-eastern Primorsk region April 9, 2007. The Primorsk region produces the bulk of Russia's tungsten concentrate - the metal that is used in machine tools and light bulbs. Picture taken April 9, 2007. REUTERS/Yuri Maltsev (RUSSIA)

Workers inspect a production unit of a tungsten mine that is currently out of service near the settlement of Svetlogorsk in the far-eastern Primorsk region April 9, 2007. The Primorsk region produces the bulk of Russia's tungsten concentrate - the metal that is used in machine tools and light bulbs. Picture taken April 9, 2007. REUTERS/Yuri Maltsev (RUSSIA)