Reducing mercury emissions in China’s zinc smelting plants

Shuikoushan Nonferrous Metals Group, located in Songbai town, Hunan Province - the industrial powerhouse of central China - is one of the largest state-owned enterprises for metal mining, ore dressing, smelting, processing and associated trades. Employing over 10,000 workers, this over-a-century-old company operates some of the biggest mines, including zinc mines in the province. It produces 150,000 tons of zinc annually.

Globally, the extraction and smelting of zinc is a process that produces emissions of tons of mercury, a silver-white heavy metal that poses a great threat to human health and ecosystems at the local and global levels.

The World Health Organization (WHO) considers mercury to be one of the top ten chemicals of major public health concern. Exposure to it through inhalation or ingestion may cause brain and neurological damages, memory loss, skin rashes, tremors, kidney, heart, vision and respiratory problems, and even death. Meanwhile, mercury emissions deposited in watersheds contaminate aquatic environments.

Mercury emissions can be caused both by natural processes and human activities. It is estimated that anthropogenic sources of mercury emissions account for about 30 per cent of the total amount of mercury entering the atmosphere each year. Globally, artisanal and small-scale gold mining is the largest source of anthropogenic mercury emissions, followed closely by coal combustion. Non-ferrous metal production, including zinc, is another large source, accounting for around 7 per cent of mercury emissions caused by human activities.

China has the second largest reserves of zinc, and has been the world’s largest zinc producer over the last 15 years, producing over four million tons in 2011. The zinc smelting plants are largely congregated in central China, with Hunan Province hosting more than 100 plants which produce 23 per cent of China’s total zinc output.

So far, there is no specific mercury control mechanism or monitoring system in place in China. The majority of workers and managers involved in zinc smelting operations have a very low awareness of the magnitude of mercury emissions in the sector or its effect on human health. The impact on the environment is largely neglected.

This was also the case at the Shuikoushan plant. Mercury emissions resulting from the zinc smelting process normally concentrate in two forms, namely in exhaust gas and wastewater. The plant was emitting over 2,000 kilos of mercury in exhaust gas and another some 450 kilos in wastewater annually. Lacking suitable technologies and unaware of international best practices to deal with the emissions, the plant had been unable to address the issue.

However, now the Shuikoushan plant is turning the tide. It is pioneering some environmentally friendly approaches in its non-ferrous metal production, starting with zinc smelting operations. With the help of the United Nations Industrial Development Organization and the Foreign Economic Cooperation Office (FECO), an institution affiliated to the Ministry of Environmental Protection of China (MEP), as well as research assistance provided by several universities including Tsinghua University, the plant has adopted best available practice technologies and installed some cost-effective equipment to reduce mercury emissions in its zinc smelting operation.

To reduce mercury emissions in exhaust gas, the plant invested in mercury-removal equipment in the smelter, including a desulfursation tower at the exit of the volatilisation kiln; and to lower mercury concentration in wastewater, special filters were installed to sieve some of the mercury before it was carried along with the wastewater to the water treatment station. In addition, an acidic wastewater circulation system was installed to further recycle mercury from the wastewater.

As a result of the technical upgrade, mercury released into the exhaust gas was more than halved - reduced to 845 kilo per year - and the filters contributed to about a 30 per cent of mercury reduction in the wastewater. In total, the plant has reduced mercury emissions in the zinc smelting operation by some 1,200 kilos per year.

Funded by the Global Environment Facility (GEF), the USD 1 million project was implemented to introduce the best available technologies (BATs) and practices in reducing mercury emissions in the zinc smelting operations, thereby promoting an environmentally-sound chemical management approach in China.

Under the same project, another zinc smelting plant, this one in the Sha’anxi Province of central China, also took measures to reduce mercury emissions. Since its waste mercury mainly existed in wastewater, the Shangluo plant adopted the suggestions and guidance of project experts, and gave its wastewater treatment system a comprehensive modernisation upgrade, including adding a settlement system, an acid clay filter system and a wastewater recycling system.

At the end of the project, the plant has demonstrated the capacity to reduce total mercury emissions of over 5,000 kilos per year, with over 4,800 kilos of reduction in wastewater and around 280 kilos in exhaust gas. Meanwhile, with the wastewater recycling system, each year the company now recycles about 80 tons of mud acid, containing about 20 per cent of mercury content. This means the plant can recycle 16 tons of mercury each year from the acid sludge.

Mercury emission is a global problem that knows no national or continental boundaries. To tackle this, a global legally-binding treaty called the Minamata Convention on Mercury was adopted by governments in October 2013. The treaty aims to regulate anthropogenic emissions and releases of mercury and its compounds in order to protect human health and the environment.

“The project has demonstrated the effectiveness of several BATs, which will be considered by the Ministry and evaluated to see how they can serve as the basis for future policies in the sector. As China is the largest producer and consumer of zinc, a detailed insight into this sector has significant value”, said WANG Zuguang, project coordinator at FECO.

China is the world’s biggest producer, user and emitter of mercury. Although it has signed on the Minamata Convention in 2013, it isn’t a ratified member yet, primarily due to big challenges faced in recycling mercury-containing products and controlling emissions from coal burning and smelting.

“Results of the project will contribute to China’s decision on ratifying the Minamata Convention. The technologies and environmentally sound practices have been piloted successfully and have the potential to be scaled up for greater coverage and impacts elsewhere in the country,” added Wang.

UNIDO plays an important role in the United Nations Environment Programme Global Mercury Partnership - the main mechanism and technical advisory group to the Convention for the delivery of immediate actions on mercury.

“Based on its extensive experience in assisting developing countries to comply with multilateral environmental agreements, UNIDO works to reduce global poverty through the promotion of sustainable and inclusive industrial development. To achieve this goal, together with its partners, UNIDO’s Mercury Programme is leading and facilitating the introduction of clean technologies and policy reform to minimize the use and discharge of mercury in these countries,” said UNIDO project manager Riccardo Savigliano.

According to Savigliano, UNIDO’s current mercury portfolio consists of 26 projects in 30 countries across the globe, focusing on Minamata Initial Assessments, National Action Plans and technology transfer for the the artisanal and small-sale gold mining sector, as well as on mercury waste management.

By ZHONG Xingfei
With special thanks to Grace Halla

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