68 



California Division of Mines and Geology 



Bull. 197 



Photo 14. No 

 section located 

 Refractories. 



tividad dolomite processing plant situated at base of hill below quarry (out of picture, top left) . Washing, screening and heavy-media separation 

 upper left. Beneficiated dolomite is stockpiled below (lower left) for final crushing, screening and calcining (center) . Photo courtesy of Kaiser 



The rock is loaded by two 4'/2-cubic-yard capacity, 

 electrically powered shovels into large-capacity dump 

 trucks which haul a short distance to the large pri- 

 mary crusher located at the 700-foot level. Much of the 

 decomposed granitic rock is wasted as the rock is fed 

 over a grizzly to the jaw crusher. The material is 

 crushed to about minus 4 inches then passed over a 

 half-inch mesh screen where the fines, including some 

 granitic material, are wasted. The coarse rock is next 

 conveyed to a stockpile at the washing heavy-media 

 separation sections located about 2.^0 feet lower in 

 elevation. 



At the washing section, the dolomite is scrubbed in 

 a Hardinge mill and passed through an attached trom- 

 mel screen, both of which are supplied with fresh 

 wash-water. Rock larger than 3X inches is sent to a 

 secondary jaw crusher and recycled to the scrubber. 

 The minus fraction is washed over a '/g-inch mesh 

 screen, the fines going to a thickener and then to a 

 waste pile. The plus %-inch dolomite is next conveyed 

 to the heavy-media .separation section for additional 

 beneficiation. At this point, virtually all of the decom- 

 posed granite and other "soft" impurities have been 

 removed from the rock. 



The heavy-media separation (H.M.S.) section was 

 added to the processing plant in 1952 to remove the 



harder, siliceous and granitic impurities so that a high- 

 quality dolomite product could be obtained. This 

 H.M.S. process has been described in detail by Len- 

 hart (1953, p. 89-93), Utley (1952, p. 94-96), and oth- 

 ers. The heavy medium is a suspension of finely 

 divided ferrosilicon and magnetite in water which is 

 kept at a specific gravity of 2.7. Impurities lighter than 

 2.7 are floated off and join the sludge from the thicken- 

 er and are wasted (the lightweight impurities actually 

 appear to be very sound and probably would be useful 

 as aggregate for structural purposes) . The heavier and 

 purer dolomite fragments (maximum specific gravity 

 — 2.85) sink in the heavy medium and are removed 

 from the H.M.S. cone by an air lift. Next, the heavy 

 fraction is washed over a screen and conveyed to the 

 stockpile at the calcining plant below. The ferrosili- 

 con and magnetite are reclaimed from the heavy me- 

 dium by a sequence of steps including magnetizing, 

 thickening, magnetic separation, and demagnetizing. 

 At the calcining plant, the dolomite is crushed and 

 screened to %- by '/-inch and minus '/-inch sizes for 

 calcining in one of three gas-fired rotary kilns. Two of 

 the kilns are used to make caustic calcined dolomite; 

 the third, to make deadburned dolomite. Most of the 

 calcined dolomite is shipped in 25-ton trucks to the 

 company's seawater magnesia plant at Moss Landing. 



