Category Archives: PCN Mining

Sonoma Ore Processing

The following quote describes the processing of magnesite ore at the Rose Fire Brick Company in Oakland, California using production from the Red Mountain Magnesite Mine, the largest Magnesite mine in the US in 1905.  The quote is quite relevant in that it describes the entire ore processing process from the mine through the delivery of the finished product.

“Raw magnesite was shipped by wagons from the Red Mountain mine to the Livermore depot and then loaded into the cars of the Southern Pacific Railroad Company. The cars were transported to the Rose Firebrick plant, where they were pushed up an incline so that the magnesite ore could be dumped into bins. The magnesite was ground into granules in the crusher and raised in automatic elevators to the higher end of the rotary kiln. As the magnesite granules passed through the rotary kiln, they were calcined, whereby carbon dioxide was driven off by high temperatures (2,800 degrees F) to reduce the ore to a dead-burned magnesia, or fine pure crystals of periclase (magnesium oxide). Carbon dioxide was collected and sent to the Pacific carbonic gas plant to be converted into a liquid product. In the pug mill, the periclase was mixed with a little water, a pinch of crushed quartz, and not more than 10 percent iron. This mixture was then sent to the molding room to be power pressed into bricks and then sent to the dryer to drive off any remaining water. After drying, the bricks were fired at a high temperature in the rectangular kiln. The finished magnesite brick was shipped locally by rail and by ship to New York for use in the open hearth furnaces of steel mills.”

It is clear from the photos that plant production at the Sonoma Magnesite plant was not fire bricks.  Rather it was likely to be magnesium oxide crystals that were transported from the plant in canvas bags or metal drums for processing into magnesite bricks by the firm buying the powder shipped in the bags in this photo of a consist from the Sonoma Magnesite Company plant.


But in a much less sophisticated sense, the Sonoma plant and the mines would require a similar process to the Rose Fire Brick company with the exception the powder wasn’t turned into bricks.  Here’s my best guess on how that process worked at the Sonoma Magnesite mine and plant.

This quote in “Mines and mineral resources of the counties of Colusa, Glenn, Lake, Marin …” by Walter Wadsworth Bradley, California State Mining Bureau explains the process at the Sonoma plant.

“The magnesite is being mined at the lower (Cecilia) deposit, where a quarry face has been opened up.  In the calcining plant, a rotary kiln, oil fired, is in operation with a capacity of 30 tons in 24 hours.   A second kiln is stated to be in route for installation.  The crude material is crushed to pass a 2 inch ring, before charging to the kiln: and the calcined material is crushed after cooling in steel bins.  Power is obtained by an oil burning steam plant.  The kiln consumes 3/4 barrel of oil per ton of calcined magnesia obtained which is reduced to 5% CO2.  The finished product is packed in paper lined duck bags for Pacific Coast consumption, and in 400 pound paper lined barrels for the eastern market.  Shipments are now being made via the Panama Canal to New York, an ocean rate of $5 per ton from Tidewater, San Francisco to New York having been obtained.  Seventy men are at work. – W.W.B – July, 1915.”

The process in a modern Magnesite ore crushing plant works like this.


In the 1910s it might have worked like this:

  • The mines in all likelihood used dynamite to break up the magnesite ore into rocks which were transported by gondola to the plant.

  • The ore was sorted, crushed and screened into rocks 2″ in diameter or less.Magnesite_Ore
  • The crushed rock was fed into the rotary kiln for processing.  After being processed and cooled, the calcined ore was crushed into powder.magnasite-powder-500x500
  • Fire bricks were created in San Francisco or New York by the firm receiving the powder.  This is a fire brick produced by the Rose Fire Brick Company in San Francisco, one of the firms that might have been receiving the bags of ore powder.images-2

The following photo is of the Sonoma Magnesite Company Mill located in the area of the mines.


The lean to shed at the left of the photo is likely to be where initial ore crushing into screened rock 2″ or less in diameter occurred.  You can see a hopper protruding from the top of the shed.  It appears the man in the photo may be loading ore from the pole nearer the lean to into the bottom end of the hopper.

The shed at the front of the photo was built when a second rotary kiln was added, presumably to house equipment that had been located in the front gable of the original mill structure.  In modeling the mill, I’m tempted to leave the front lean to off, modeling the mill in its original state.  Doing so would allow me to open the near side of the main building so the rotary kiln and related equipment for viewing.

A commonly used rock crusher in the era was the Blake Crusher.  The following image shows how it works.  The Blake would have been the crusher under the shed to the right of the main building loaded with the hopper.


This is a photo of a Blake Crusher in the era being modeled.


Blake crushers came in a variety of sizes as shown by this photo.  Note the man in the photo.


Blake crushers were initially powered by belt driven by steam engines as indicated in this drawing.


This photo shows a Blake Jaw Crusher encased in a timber structure designed to elevate the crusher and extend the size of the bin accepting incoming ore.  Directly in ffont of the crusher is a stationary steam engine that was presumably used to power the crusher.  Note the bucket system on the left that was used to lift the ore into the hopper.


This is an image of a typical magnesite processing plant.  The rotary kilns are in the middle of the structure.  I believe the large timber structure on the left contains a Blake crusher.  The timber structure on the right would contain post processing equipment.


I suspect the ore came into the Sonoma Magnesite plant from the right via gondola on the spur that turns to the right on the above plant photo.  Once crushed, a conveyer (steam driven) would lift the crushed ore into the input end of the rotary kiln.  The the rotary kiln output of calcined ore would have dropped into the steel bins shown in the rotary kiln photo where they would have cooled.

Once the ore had cooled it would be crushed into powder.  A common machine for crushing ore into powder was the Sturtevant Ring Roll Mill.  They were delivered as single mills or as duplex mills.  A duplex mill is shown in the middle right of the following image.  Cement and Engineering News #31 indicates the capacity of their No.2 Duplex Mill was 16 to 20 tons per hour at 20 mesh (20 mesh openings per inch) and 8 to 14 tons per hour at 80 mesh.  A single No 2 Duplex mill would have been adequate to handle output from the two kilns of 60 ton per day input, 45 ton output in an 8 hour shift.

Sturtevant Crusher

Once the calcined ore had been crushed into powder, it would have been bagged or poured into drums then transported via flat car to the Cazadero area where it would interchange with the narrow gauge.

Sonoma Rotary Kiln

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.

Early English 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.

Norman A1

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.

Rotary Kiln 

In this blow up of the far end, the rear support components are labeled.

Rotary Kiln Stack1

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.

1920s Kiln Gear

Based on an assumption of a six foot man I used a digital caliper for the following takeoffs.

Kiln Takeoff1

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.

Other Equipment

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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