Why did the Solvay process replace the Leblanc process in many places?

The Solvay process (or ammonia-soda process) eventually replaced the Leblanc process primarily because it was more economically efficient.12

While the Leblanc process dominated the production of sodium carbonate for much of the 19th century, it faced significant challenges. The process was chemically complex, involving the treatment of salt with sulfuric acid followed by heating with limestone and coal, which produced calcium sulfide as a byproduct—a substance known for its highly unpleasant odor. Additionally, the process initially released hydrogen chloride gas into the atmosphere, causing severe environmental damage to vegetation.34

In contrast, the Solvay process, perfected by Ernest Solvay in the 1860s, allowed for the efficient recovery and reuse of ammonia, which was essential to its economic viability. By producing soda ash more cheaply, the Solvay process gradually supplanted the older method, becoming the standard for industrial alkali production by the early 20th century.5162