Short answers, pulled from the story.
Nicolas Leblac patented his solution in 1791 while serving as physician to Louis Philip II, Duke of Orléans. He constructed the first plant at Saint-Denis that same year which began producing 320 tons of soda annually.
The process involved two distinct chemical stages converting common salt into usable soda ash. First sodium chloride reacted with sulfuric acid to create sodium sulfate and hydrogen chloride gas before crushed limestone mixed with coal reduced the sulfate through heating.
The last Leblanc-based soda ash plant in the West closed in the early 1920s. During World War II Nationalist China temporarily re-established the process when importing complex equipment proved too difficult.
For every eight tons of soda ash the Leblanc process generated five point five tons of hydrogen chloride gas and seven tons of calcium sulfide waste. This solid waste known as galligu had no economic value and was piled in heaps near works where it weathered to release toxic hydrogen sulfide.
Leblanc process waste created a strong case for being the most endangered habitat in the United Kingdom today because the waste weathers down to calcium carbonate producing havens for plants that thrive in lime-rich soils called calcicoles. Only four such sites have survived the new millennium with three protected as local nature reserves.