After Du Bing graduated from the University of Mining and Technology, he entered a county-owned state-owned enterprise. This enterprise has two coal mines, Yong'an and Yongfa, a carton factory and a real estate company. Du Bing worked in the technical department of Yong'an Coal Mine.This coal mine is a long time ago, how old has it been? The factory building of the coal washing factory was built in the second five-year plan. The slogans of the Great Times are vaguely seen on the wall, and there are mottled red stars in front of the office building.There is a sign next to the carton factory and Yong'an Coal Mine, each with a small door connected to the middle. When Du Bing first entered the mine, he was puzzled when he saw this small door. Although the two factories belong to one, their main businesses are different. Why do you have to open such a small door?The coal mine is located in a remote town. To the east is a village called Ning'an Village. The village is not big, with less than fifty families. Most of the young people in the village work in the mine.