Short answers, pulled from the story.
Nicolas Leblanc, a physician to Louis Philip II, Duke of Orléans, invented and patented the process in 1791. He built the first plant at Saint-Denis that same year, producing 320 tons of soda per year. He was denied the prize of 2400 livres offered by King Louis XVI because of the French Revolution and committed suicide in 1806.
The first step, the Mannheim process, converted sodium chloride to sodium sulfate using sulfuric acid, releasing hydrogen chloride gas as a byproduct. The second step, Leblanc's own contribution, heated the sodium sulfate with coal and crushed limestone in a reverberatory furnace at around 1000 degrees Celsius to produce sodium carbonate. The resulting dark product, called black ash, was then leached with water in a process called lixiviation to extract the soda ash.
The Solvay process, developed by Belgian chemist Ernest Solvay in 1861, produced soda ash more cheaply and with far less pollution, generating only calcium chloride as waste. By 1900-90% of the world's soda was made by the Solvay method. The last Leblanc-based soda ash plant in the West closed in the early 1920s.
For every 8 tons of soda ash produced, the Leblanc process generated 5.5 tons of hydrogen chloride gas, vented into the atmosphere, and 7 tons of calcium sulfide waste known as galligu, which was piled near factories and released hydrogen sulfide as it weathered. British Parliament passed the Alkali Act 1863, the first modern air pollution legislation in the world, specifically in response to these emissions. The act allowed no more than 5% of the hydrogen chloride to escape to the atmosphere.
By the 1870s, British annual soda output had reached 200,000 tons, exceeding the combined output of all other nations. The industry grew rapidly after the repeal of steep salt tariffs in 1824; the first British Leblanc works had opened in 1816 at Walker on the River Tyne. James Muspratt and Charles Tennant built some of the largest soda works in the world in Liverpool and near Glasgow respectively.
Nob End near Bolton is the largest surviving Leblanc process waste site in the United Kingdom, designated as an SSSI and Local Nature Reserve. The calcium sulfide waste, known as galligu, weathered over time into calcium carbonate, creating alkaline soil that supports rare orchid-calcicole flora most unusual in the naturally acidic surrounding region. Only four Leblanc waste sites survived into the new millennium; three hold formal protection as nature reserves.