The thirteen days are all those thirteen companies; this is the detailed answer
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In the 1970s, China introduced 13 large-scale fertilizer production units, leading to the construction of 13 factories. To this day, these thirteen institutions remain the cradle of our coal chemical industry, having trained a large number of talents for the whole country. A large amount of advanced technology has been employed in this field, yielding valuable technical and human capital. They are: Yuntianhua in Yunnan, Huajin Chemical in Liaoning, Dongting Fertilizer Factory, Cangzhou Fertilizer Factory, Liaohe Fertilizer Factory, Qixiashan Fertilizer Factory, Daqing Fertilizer Factory, Anqing Fertilizer Factory, Qilu Second Fertilizer Factory, Hubei Fertilizer Factory, Guangzhou Fertilizer Factory, Sichuan Chemical Plant, and Luzhou Natural Gas Chemical Plant
Many have closed down, but many new ones have also been built over the years.