J.V.N. Dorr’s first development was the Dorr classifier. John Dorr was one of the great inventors of methods to process solid particles and he was quick to take these methods from ideas to operating practice.Wet ball mills were being tested on the gold ores in South Africa and copper ores in Arizona and the Dorr classifier made continuous closed circuit grinding a reality. The ball mill – rake classifier circuit (spiral classifiers were also used) were used for 50 years until the development of better pumps enabled hydrocyclones to replace mechanical classifiers.John Dorr invented other machines which were widely used for processing minerals, for example the Dorr continuous thickener and the Dorr agitator, but it was the classifier which transformed the technology.In 1910 the Dorr Cyanide Machinery Co was formed to become in 1916 the Dorr Co, Engineers. The technological developments in ore dressing and wet metallurgy during the 1920s greatly increased the demands for Dorr machinery and engineering services in this country and abroad and territorial growth began. The Dorr Co built offices in many cities. Originally its business was related directly to the cyanidation of gold and silver ores in the US and in Mexico. The iron, lead, copper, phosphate, and sand fields were also entered in a small way. Ultimately not only all of wet metallurgy, but also most of heavy chemical and industrial processing, sugar manufacture, and sewage, water, and trade-waste treatment became Dorr customers.John Dorr, in association with Francis Bosqui, published two editions of “Cyanidation and Concentration of Gold and Silver Ores”. This was a seminal book for mineral processors.Dorr-Oliver ultimately became part of FLSmidth.