search.noResults

search.searching

dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
Figure 3: The expansion versus temperature curve is used to determine the thermal expansion of the various mullites.


important to maintaining a consistently screened product. The rest of the Premium Grade


Virginia Mullite™ goes to a special ball mill for grinding. This ball mill is lined with granite and uses alumina milling media. Using steel balls or liners would negate the effort put into making a lower iron product. The material exiting the mill passes through an air classifier to send any large particles back to the mill for further grinding.


Testing Testing throughout the process is vital to make a consistent product. Testing begins at the mill side of the float to determine the kyanite percent of the raw ore. Dozens of tests are done in the flotation building to check on the level of beneficiation of the kyanite from the other minerals. At the dryer, particle size testing is done to check the coarseness of the product. A field XRF unit is also used to monitor the iron removal process. Helium pycnometers are used at the kilns as a field test to check to see if the kyanite was completely converted to mullite during the calcination process. All material is sent to the main


Quality Control Lab for final testing. Each bag of product is tested for particle size distribution, chemistry, and mineralogy. For stuccos, a Ro-Tap is used to screen size the product. For finer meshes, an air sieve is used. Laser light scatter analysis can also be done on the flour products using a Microtrac particle size analyzer. Chemistry is checked on an x-ray fluorescence analyzer. An


24 ❘ February 2020 ®


Figure 4: The change in length versus time curve can be used to determine creep resistance.


x-ray diffraction unit is used to check the mullite for any unconverted kyanite and determine the presence of other minerals or compounds of interest, such as cristobalite content.


Why It Matters Producing mullite by beneficiating kyanite from the ore body and then calcining is a time consuming and meticulous multi-step process. However, the effort is worth the time and resources spent as this method of mullite production creates a final raw material with some unique and useful properties. There are several differences between mullite made via calcination of kyanite versus mullite made from calcining clay minerals. For one, each individual mullite crystal produced comes from a kyanite crystal instead of an agglomeration of crystals. This allows the mullite particles to have a high aspect ratio. When agglomerated to form an aggregate, the natural aspect ratio of mullite is neutralized by the shape of the aggregate. Kyanite is a blade shaped mineral. Mullite made from kyanite maintains this aspect ratio throughout calcination. The high aspect ratio of the particles has been shown to increase hot MOR strength, reduce mold splits (cracking), and has led to the reduction of the number of backup coats needed on the mold.


Another difference is the homogeneity of the mullites. Impurities in mullites produced from clay minerals are often well dispersed throughout the entirety of the aggregate. While


homogeneity is typically a good thing, this can be a detriment concerning creep resistance. If the impurities are dispersed throughout the material, then the aggregate will creep. Mullite from kyanite, on the other hand, is a single blade particle. The impurities in the material are stuck to the sides of the kyanite/mullite crystal surface and are thus localized. This means that a small area of the grain may creep, but the rest of the crystal, where the impurities are not located, will not. Stacking several of these blades together will spread that creep around and lead greater creep resistance than a typical blended aggregate mullite material. While mullites generally exhibit excellent creep resistance, the iron content


limits the maximum usage


temperature before the on-set of creep. Iron oxides act as a flux when combined with silica which drastically lowers the melting and softening point. This forms glass, which is fluid, and allows the shell to creep. It is shell creep (bulge) that can lead to poor dimensional control so castings may be out of tolerance to customer specifications without additional rework. Small changes in iron content can have very large effects on the usage temperature and the stability of the shell. This concept is what led to the production of the Premium Grade Virginia Mullite™. This material has an iron content of less than 0.2% vs the standard Virginia Mullite™ that contains a maximum of 0.75%.


Dilatometer testes were done in order to examine the effect of iron on


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104