Born in New Jersey in 1872, John Van Nostrand Dorr started work as a laboratory assistant in Thomas Edison’s laboratory in Orange, New Jersey, at the age of 16. At the time Edison had become heavily involved in mining and processing low-grade magnetite ores. Thinking that the iron ore deposits in the eastern United States were being exhausted, he saw an opportunity to enter the market for iron ore. The mining and concentration processes of the day were small in scale and expensive, and new high-capacity processes were essential to the economic success of the ventures. Edison established a laboratory to provide data and information about the processes he was planning.
Dorr worked in the laboratory for a little more than 2 years. During his last year there, he worked directly for Edison. He must have absorbed much of Edison’s innovative approach to identifying and solving problems, because Edison’s slogan of “try anything once” was evident in many of Dorr’s achievements. After leaving the laboratory he studied chemistry at Rutgers University and worked at a gold mine in South Dakota one summer. The mine was not profitable because the ore consisted of a clay fraction with no gold and a sands fraction containing gold, and the clay interfered with the extraction of gold. Dorr learned this lesson in the limitations of mineral processing technology well, realizing that if the clay could be separated from the sands and only the sands processed, the operation would be profitable.
After graduation and a brief term as a research assistant at Brooklyn Polytechnic Institute, Dorr returned to South Dakota. At the time, cyanide leaching was being introduced to gold mines in the United States. In 1903, Dorr took part ownership of the
Lundberg, Dorr and Wilson mill that was built near Terry, South Dakota. In 1904, he tested the idea of a sand-slime split using a classifier with reciprocating rakes to replace the classifying cones. It operated by keeping the finer particles in suspension through the stirring induced by the rakes and overflowing them into a launder. The coarser particles fell to the base of the machine through the force of gravity and were continuously removed by being raked into another launder. The Dorr rake classifier gave a coarser and more controllable split than was previously available with cone classifiers. Used for the first time in 1909 in closed grinding circuits at the Real Del Monte mine in Pachuca, Mexico, the Dorr rake classifier was soon used in many circuits.
It is said that when the classifier was built and was first operating, Dorr worked or slept beside it for its first 80 hours of operation. His brother G. H. Dorr wrote that J. V.N. Dorr’s
…ability to work indefinitely on 4 or 5 hours sleep and an instinct for starting a session for the concentrated consideration of some problem at some odd hour of the day or night is one of those maddening characteristics for which his associates were at times inclined to curse his early association with Edison. (Dorr 1941)
Dorr also invented the continuous thickener in 1906, which was the first continuous sedimentation unit in which water could be removed from a dilute slurry and the solids concentrated into a dense slurry. The continuous thickener allowed water to be recycled from the mill tailings pond to points within the mill circuit, greatly reducing the volume of fresh water that had to be used in the mill.
Mineral companies quickly accepted both the rake classifier and the thickener. Wet closed-circuit grinding became part of mineral processing technology and the thickener became essential for recycling process water, which was often very scarce.