PVC Production by Calcium Carbide Process - A Challenge to Mercury Emission Cuts   

By Lu Yuanli  

Mercury is closely watched as a scarce resource and given its environmental impact. As several heavy-metal or metalloid pollution accidents have occurred one after another in China since 2009, the prevention and treatment of mercury pollution and other heavy-metals pollution has been put on government agenda. The Ministry of Environmental Protection, the National Development and Reform Commission and the Ministry of Information Industry, and other five ministries in November 2009 jointly issued a work guidance to strengthen the prevention and treatment of heavy-metal pollution. The environmental ministry also has started the preparation of the national prevention and treatment plan for heavy metal pollution, which includes specific plans for mercury. At present, a nationwide survey on mercury pollution is under way.
   China's Polyvinyl Chloride (PVC) producers mostly adopt calcium carbide process for making PVC, in which mercury chloride catalyst is used for the synthesis of vinyl chloride monomer. In doing so, about 6 960 tons of mercury chloride catalyst, or 566 tons of mercury, is needed every year, posing a big challenge to mercury pollution treatment.
   It has become a global trend to limit the emission of mercury. The international community is gradually taking concrete measures to limit mercury emissions. The United States reportedly plans to reduce mercury emissions into the atmosphere by 69% by 2010; the Europe Union will ban mercury exports from July 2011 and European nations will gradually stop using mercury thermometer to prompt the whole world to stop random dumping of this toxic substance.
   China's mercury resource used to be optimistic, but the development of the PVC and battery industries over the past 20 years has consumed almost all the resources. Now, China relies on imports for 50% of its mercury needs as domestic mercury production is far short of local demand. And a future international mercury treaty is likely to order the closure of all mercury mines globally, disrupting supplies.
   In the face of a lack of mercury resources and a serious environment situation, China's calcium carbide based PVC industry should lower mercury consumption and reduce mercury pollution by technology upgrades. A low mercury catalyst technology has been successfully developed and widely used by more than 20 players including Xinjiang Tianye Group, Hebei Shenghua Chemical Industry Co., Ltd., Qingdao Haijing Chemical Group Co., Ltd. About 583 tons of low-mercury catalysts were used in China in 2009. The new technology could reduce mercury chloride content in the catalyst to around 6%. And if the technology is used in the whole industry, it could reduce mercury use by half.
   Meanwhile, China's PVC industry is speeding up the R & D of the clean production technologies. So far, the oxygen-cut carbonization process to recover mercury chloride and the high-efficiency mercury-recycling technology has been checked by relevant authorities and are being promoted. The PVC companies also cooperate with research institutes and colleagues on the development of some key technologies regarding the prevention and treatment of mercury pollution in the calcium carbide process PVC industry. Among them, the development of the new mercury-capture by molecular sieve catalyst technology and the vinyl chloride synthetic gas for high recycling mercury technology has made major breakthrough; the waste mercury catalyst recycling technology, the vinyl chloride fluidized bed reactor technology, and the mercury-free catalyst technology are being developed.
    To advance the mercury emission cut effort, China Petroleum and Chemical Industry Association and China Chlor-Alkali Industry Association jointly drafted a guidance in 2009 to strengthen mercury control in the PVC industry and a mercury pollution prevention and a treatment plan for the calcium carbide based PVC industry, which outlined some tasks and goals for the PVC industry during the 12th Five-Year Program period (2011-2015). With the theme to reduce mercury and abandon mercury use, China aims to fundamentally eliminate mercury consumption and pollution, realizing a mercury-free industry and strengthening the competitiveness of the domestic calcium carbide based PVC industry.