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.

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