What was an important technological invention in the Western Han Dynasty?

Wet copper smelting in the Western Han Dynasty is an important technological invention. According to Huainan Wanbi Book, Zeng Qing turned iron into copper. This is a process of replacing copper with iron in copper sulfate solution called bile water, which can be carried out at room temperature with low yield. Coupled with the development of metallurgy in the Western Han Dynasty, it is possible to fake gold. Many people have participated in this activity. What about Hanshu? Zhai Jingji recorded the law of "casting money and abandoning the market" at that time (before 144). This also shows that there were many alchemists at that time.