Examination of Alkali Resistance of Refractory Bricks Containing Fired Alumina
The Refractory Division of STEULER-KCH GmbH supported a diploma thesis dealing with the alkali resistance of different types of fired alumina bricks. Three different basic types were examined based on fireclay, andalusite and bauxite. To each type additionally different amounts and types of phosphate was added to see the possible influence on alkali resistance. The phosphate contents were zero, 2 x 1.5 % (in a liquid and a solid form) and
Mixtures of 1:1 (Mole) NaCl / KCl, Na2CO3 / K2CO3 and Na2SO4 / K2SO4 were used as alkali media executing crucible tests according to DIN CEN/TS 15418. The samples were hold for
5 h at 1100 °C and cross cut dry afterwards for visual examination.
Neither chlorides nor sulphates were aggressive to any type of the bricks. Andalusite bricks were not attacked at all. The bauxite brick without any phosphate addition showed the highest attack and intensive bursting by carbonates. The fireclay brick without phosphate and the bauxite brick with the highest phosphate content both showed a slight but remarkable attack, also only with carbonates. All other samples only showed infiltration by alkali melts, in most cases infiltration decreased with increasing phosphate content.
The degree of infiltration may be related to the formation of alkali phosphates. Samples with alkali bursting showed the formation of feldspatoides with the combined volume expansion, as expected.
As a conclusion alkali attack as bursting only appears when tested with carbonates. There may be others kinds of alkali attacks which show different effects, such as loss of refractoriness and loss of thermal shock resistance. Phosphate addition can reduce infiltration as well as bursting. Andalusite Brick show no alkali sensivity at all in this test series.
It should be mentioned now that the term “alkali attack” is not well defined. Only the American standard ASTM C 454-10 describes alkali attack tests on carbon bricks. A suggestion to the official institutions should be released that the definition as well as the related test procedures should be standardized.
Additional work showed that selective infiltration of refractory brick with non-aggressive alkali salts decreases infiltration and corrosion in service, for example of metal melts. This selected infiltration reduces porosity, increases bulk density and cold crushing strength and has no negative effect on thermal shock resistance.
There are some salt impregnated refractories available on the market, mostly basic bricks, but also phosphate impregnated Suprema SA 601 PP produced by STEULER-KCH.
Dipl.-Ing. Werner Schönwelski (R&D and Application Technology)
Dipl.-Ing. Bernd Schladt (Quality Department)
Dipl-Ing. Jens Sperber (Head of R&D and Application Technology)
dgfs | 5TH Conference Refractory und Chimney Engineering
Meet us on tuesday, 16.06.2015 | 11.00 h | Düsseldorf Exhibition Grounds, CCD. East
Heavy alkali attack „Bursting“ of carbonates on fired Bauxite brick without phosphate addition
Small but remarkable alkali attack of carbonates on fireclay brick with no phosphate addition
Alkali bursting detected on a mullite
brick from an alumina calcination kiln
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