The dominant piece of stationary equipment in the PC&N Mining District is the rotary kiln used to process the magnesite ore. The first rotary kiln patents were in the 1870s in England, followed in the same decade by some US patents. Rotary kilns are most commonly used in the manufacture of concrete but played a significant role in magnesite processing.
This quote from Wikipedia describes how they operate.
“The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the ‘burner pipe’, producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.
In this is a diagram of a rotary kiln, it is mounted slanted with the feed being on the high end. attaching an engine to the drive gear allows the kiln to be continuously rotated. The ore moves down the kiln toward the flame. The kiln is lined with brick allowing it to sustain a very high temperature without melting the steel sleeve on the outside. When the ore hits the flame it is oxidized. The processed product is deposited at the end of the tube.
In 1885, an English engineer, F. Ransome, patented a slightly tilted horizontal kiln which could be rotated so that material moved gradually from one end to the other. Because this new type of kiln had much greater capacity and burned more thoroughly and uniformly, it rapidly displaced the older type.
Thomas A. Edison was a pioneer in the further development of the rotary kiln. In 1902, in his Edison Portland Cement Works in New Village, NJ, he introduced the first long kilns used in the industry-150 feet long in contrast to the customary 60 to 80 feet. Today, some kilns are more than 500 feet long. Parallel improvements in crushing and grinding equipment also influenced the rapid increase in production.
This quote is from the 1920 “Transactions of the American Institute of Chemical Engineers.”
This is an image of an early English rotary kiln said to be the first successful rotary kiln.
Image of another English kiln very similar in design to the Sonoma Magnesite Company kiln. Note that the end hood has been pulled back along the rail allowing the interior of the kiln to be serviced.
This diagram shows common characteristics of early kilns. Click on the image for a larger higher resolution version.
The following quote is from an article by Dylan Moore in 1911 along with the above image.
“The shell was built up, in a manner that continued to be standard for several decades, from semi-circularly rolled sheets of mild steel plate, 20 mm thick. These were butt-jointed together with riveted 20 mm thick steel plate straps. The tyres were held away from the shell by chairs with a small “breathing space” to allow for expansion. Because of this gap, the tyres would gradually precess around the kiln, ensuring uniform wear. The turning gear was attached to the kiln via tangentially-mounted spring plates, thus allowing for expansion. It soon afterwards became practice to add a further 20 mm thickness of “wrapper plate” to the tyre and turning gear areas to improve rigidity at these points where the flexural stresses are greatest. The drive train was open, allowing liberal application of lubricants, but also exposing the gear to air thick with dust and grit. Closed gear boxes came much later.”
There is a tremendous amount of useful information on rotary kilns at Dylan’s site including a very useful page on rotary kiln design considerations.
Another great source focuses on the cement industry in 1917, “The Portland Cement Industry” by William Alden Brown.
This image is of the Sonoma Magnesite Company’s rotary kiln. we are looking at the lower end discharge chute end of the kiln. The drive gear is likely to be the large ring with holes. You can see the inlet chute above the far end of the kiln. Note that the near the end tube is made up of fire brick. As in the above drawing, it appears as though the exhaust is at the opposite end from the burner. That means we are looking at both the inlet chute and the chimney above the far end of the tube. Click photo for a larger image.
In this blow up of the far end, the rear support components are labeled.
This is a very similar kiln located at Porterville, CA. Photos of both kilns date to the late teens or early 20s. Lines entering the end of both kilns are likely to be oil lines to the burners.
This shot of 1920s rotary kiln shows the gears and rivets.
Based on an assumption of a six foot man I used a digital caliper for the following takeoffs.
The rails that support the near end would presumably allow the end to be slid off the tube for cleaning and maintenance. These rails appear on both photos.
Because the raw material enters down a diagonal tube on the far end (see drawing) some form of apparatus would be needed to lift the material to the inlet chute. The piece of the equipment on the photo to the right of the near end of the Sonoma mine photo is likely to be the steam engine that powers turning the cylinder although it is not obvious how the connection is made. There would also be a boiler in this area. The presence of a boiler would explain why there are chimneys coming out of both ends of the photo of the mill structure that follows. Click photo for a larger image and labeling of sub-structures.
In this photo I am assuming that the far side of the building is the output side and the photo of the rotary kiln is taken from this far end. The two piles outside the structure are likely to be ore to be processed near the building and tailings at the far right side of the photo. The chimney on the far end would be the chimney for the steam boiler in the power house. The chimney on the near end would be the exhaust from the kiln. Note that in this photo it appears as though electrical lines are coming down from above to the support scaffolding on the top of the small shed off the main shed. This might indicate that the rotation of the kiln was powered by an electrical motor rather than steam which would explain why there is no obvious connection between the steam engine and the large circular gear used in rotating the kiln. Most kiln rotations were electrically powered. Inside the lean to shed is the lower end of the conveyor which the man appears to be loading.
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