Name
Premium Grade Virginia Mullite™
Virginia Mullite™ M 1 M 2 M 3 M 4
Al2O3 56.08 55.25
66.74 61.92 52.38 48.10
Table 2: Chemical Analysis of the materials tested.
the 100,000 – 120,000 tons of annual demand for the company’s purified kyanite and mullite products. The first step in the comminution
process is a large jaw crusher, where rocks weighing several tons each are reduced in size to ones weighing only a few dozen pounds. After
this, the
material passes through several further stages of crushing (mainly gyratory) and screening to progressively reduce the size of the ore to roughly one inch by down in size.
Wet Processing
After dry comminution, the ore stream is fed into what is called the Float. It is called “the Float” as several of the steps in the wet processing of the material involve froth floatation, but many other size reduction, size sorting, and beneficiation technologies are involved in the removal of impurities in this part of the process. This wet processing, as the name suggests, is very water intensive, using 7,000 – 8,000 gallons per minute, and runs 24 hours a day, usually for five days a week. There are well over twenty steps in this part of the beneficiation circuit (rod mills, ball mills, hydro- sizers, hydro-cyclones, screens, float cells, spirals, extractors, etc.), and at the end of the elaborate set of mineral purification steps the all the various forms of clay & dirt have been removed, as have almost all of the pyrite and mica. Alkali and alkaline earth oxides have been reduced to less than 0.05% and the titania to around 1.5%. The quartz content has also been reduced to 4-6%. The iron oxides are the only impurity that have been left untouched in this part of the process, and they make up
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anywhere from 5%-10% of the “float concentrate” at this point in the process. Their removal takes place at the next part of the beneficiation process.
Dry Processing The removal of the various naturally occurring forms of iron occurs in the dry processing phase of the company’s beneficiation circuit. The damp “float concentrate” is dewatered in several steps, and then is fed into a fluid bed dryer, being mixed and blended along the way. The homogenized material falls into the top chamber of heated fluidized bed, similar to those used in stucco application at some foundries, where the material is fully dried. It then makes its way into another chamber of this vessel where it is heated to over 1000°F (540°C) before undergoing a special process to convert all of the various iron oxides into a magnetic form. After which, the kyanite/iron mixture is cooled in a rotary cooler. Once it has cooled down to less than about 250°F (120°C), the material is run through several banks of magnets: some low intensity permanent magnets, some high intensity variable magnets, as well as some rare earth magnets. These banks of magnets bring the total iron content down into the 0.4%-0.6% range. This part of the process runs for extended campaigns 24 hours a day, seven days a week in order to prolong the lifetime of the refractories. Recently, new processes have been
developed in order to further reduce the iron content of a portion of the company’s kyanite production. This has allowed for the production of an even lower iron oxide product: Premium Grade Virginia Kyanite™. The iron oxide content in this
material is less than 0.2%. It is these new separation techniques that create the lower iron oxide containing kyanite which is the precursor for Premium Grade Virginia Mullite™.
Calcination The Premium Grade Virginia Kyanite™ is taken by truck to one of our two calcination plants.
The kyanite is
put through a rotary kiln where it is exposed to temperatures in excess of 2700°F (1480°C). There are three kilns in operation. The heating of the kyanite causes the crystal structure to rearrange and form mullite, as previously discussed. This reaction was shown in Equation 1. The silica produced as a byproduct of the reaction is very fine and amorphous. This amorphous particle is stuck to the side of new mullite crystal blades. When kyanite is converted to mullite it expands seventeen volume percent. The mullite also maintains the very high aspect ratio (acicular or needle shape) of kyanite through the conversion.
Grinding and Sizing After calcination the Premium Grade Virginia Mullite™ will take one of two routes: screening for stuccos or milling for flours. To make the stuccos, some of the mullite is sent over vibratory sieves. This is a multi-deck sieve stack that creates both the 20x50 and the 50x100 stucco products. The elongated needle shape of the mullite makes this a difficult process to control because the blades can stand on end and pass through the screen. Controlling the feed rate and vibration frequency are very
Continued February 2020 ❘ 23
SiO2 43.8
43.08 28.40 33.51 43.98 49.79
Fe2O3 0.18 0.44
1.23 1.27 0.94 0.77
Alkalis/Alkaline Earth 0.05 0.06
0.16 0.13 0.17 0.46
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