Depending on the area of application in the kiln almost all refractories (bricks, castables or mortars) are susceptible to alkali attacks. Alkalis mainly, Na2O and K2O are most damaging to refractories, and can reach them either in liquids or in gases. The resistance of Zircon and Zirconia refractories to their attack (at glass-making temperatures) comes mainly from the non-wettability quality of these compositions with respect to alkalis. We have also seen that carbon resists wetting by silicate slags, and graphite is also resistant to wetting by many liquid metals as well as by slags and fluxes. In cement or lime rotary kilns attacks from alkali vapours or alkali salts take the form of an infiltration at the surface of refractory lining, with consequential adverse impacts on the bonding (refractory bricks, castables and mortars). An alkali after reacting with refractory materials forms various ‘low temperature fusing compounds’ which brings down the overall refractoriness and life of the refractory body. Such damage may already occur at temperatures in the range of 800 - 900 degree C.
These Alkali compounds at high temperature react with the Alumino Silicate Refractories to form different Feldspathic phases like Kaliophillite, Kalsilite, and Leucite etc. These phases are accompanied by substantial volume increase which causes the crack in the bricks, called “Alkali Bursting”.
To check Alkali Resistance, first put the specified quantity of anhydrous potassium carbonate (K2CO3) into the hole of the test piece (refractory specimen). See table below -
| Hole diameter (mm) ||Potassium Carbonate (gm)|
Make a lid of firebricks, approximately 80 x 25 mm, or approximately 114 x 114 x 25 mm. Use the lid to cover the hole in the test piece and seal with air setting refractory mortar between lid and the specimen.
Burn (fire) the specimen at a temperature of 1100OC for 5 hours in an electric furnace. Then allow the specimen to cool off. Remove the lid and cut the specimen into two halves for visual inspection.
The addition of Silicon Carbide in Alumino Silicate Refractories to impart alkali resistance is already a known technique and is very common. But the Silicon Carbide is a very expensive material and increases the cost of finished product. It has been found that in comparison to Bauxite based, the Andalusite based Alumino Silicate refractories have greater alkali resistance. Why? Read: A Techno-Commercial Comparison between Andalusite and Bauxite in their applications as Refractory Raw Material
The test described will subject the specimen to an environment which is more hostile than the one normally encountered by refractory lining, but it provides an excellent basis for comparison.
The assessment of the alkali resistance is mainly based on the depth of penetration. If the test reveals a penetration depth of less than 3 mm, without expansion and cracks (alkali bursting), the test result is considered to be satisfactory.
Usually the specification of AR (alkali resistance of refractories) in connection with the material designation on refractory lining drawings if any or depending upon the area of application for the said refractory, specifications that are required in regard to the properties of the various refractory (bricks / castables) concerned to resist chemical attacks of alkali salts in the form of vapour or liquid etc. are provided by the customer to the refractory supplier (vendor).
The supplier / manufacturer of refractories generally furnish conventional information on their materials (Bricks, Castables, and Mortars etc.) such as, compressive and tensile strength, modulus of rupture, chemical analysis, thermal conductivity, density, porosity, refractoriness, resistance to creep and gas permeability etc. In addition with the increasing demand of refractory life etc., also there are some special properties, determined by certain tests that have become standardized now-a-days in the refractory industry. Results obtained from these tests, while not 100% conclusive, do furnish a good indication of the properties of the refractory and its resistance to various exposures within the kiln or furnace, and are the basis for the selection of a refractory particularly suited to any given area of its application.
It can be mentioned here that the term “alkali attack” in case of refractories is not well defined. Only the American standard ASTM C 454-10 describes alkali attack tests on carbon bricks. The definition as well as the related test procedures may need to be more standardized.
Selective infiltration of refractory brick with non-aggressive alkali salts mostly found in cement rotary kilns, 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.
Read: Causes of Refractory Failures in a Cement Rotary Kiln and Remedial Measures for getting longer Refractory Life, Kiln Uptime
Examination of alkali resistance of refractories including chamotte brick, traditional and medium-cement chamotte concrete etc. used most often in power generation boilers operating on wood fuel can be done using the method described in ASTM C 454-83. Similarly, following DIN 51069 standards the AR (alkali resistance of refractories), which is unknown, can be tested following the instructions mentioned hereunder.
Click to find internationally accepted various standard methods for testing of Refractories and Refractory Materials
MAKING TEST SPECIMEN
For Refractory Bricks (Shaped Refractory Lining)
Cut the sample (refractory specimen) from a standard refractory brick. The size must be half of a standard brick, i.e. approximately 114 x 114 x 65 mm, with a hole having a diameter of 35, 40, 45, or 50 mm and a depth of 40 mm. Then dry the sample at 110 degree C.
For Refractory Castables (Monolithic Refractories)
Cast the test specimen (piece) on a vibrating table. The size of the test specimen must be approximately 80 mm in outside diameter, with a height of approximately 65 mm and a hole which is 35 or 40 mm in diameter and 40 mm deep. After casting, the specimen must be dried at 110 degree C and burnt at 1200 degree C for five hours in an electric furnace.