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Showing posts with label Alkali Resistance (AR). Show all posts
Showing posts with label Alkali Resistance (AR). Show all posts

How to check Alkali Resistance (AR) of Refractory Lining - Bricks and Castables

- 1 comment

23-Jan-2010

BACKGROUND
Refractory lining materials such as bricks and castables etc. are susceptible to alkali attacks. As we already know that alkalies (Na2O and K2O) are very 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 alkalies. 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 (bricks, castables or mortars).
Such damage may already occur at temperatures in the range of 800 - 900OC. Refractory lining, the alkali resistance (AR) of which is unknown, can be tested according to the following instructions (based on DIN 51069 standards).
Preapring Test Specimen  

Refractory Bricks (Lining)
Cut the sample 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 110OC.
Refractory Castable
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 110OC and burnt at 1200OC for five hours in an electric furnace.
Testing of Refractories
Put the specified quantity of anhydrous potassium carbonate (K2CO3) into the hole of the test piece or specimen. See table below -
Hole diameter (mm)
Potassium Carbonate (gm)
35
40
45
50
32
38
44
50

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.
Assessment of Alkali Resistance
The test described will subject the specimen to an environment which is more hostile than the one normally encountered by refractory lining materials, 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 Alkali Resistance (AR) in connection with the material designation on refractory lining drawings if any, or specifications that are required in regard to the properties of the various refractory materials 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). Manufacturers 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, there are some special properties, determined by certain tests that have become standardized 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, and are the basis for the selection of a refractory particularly suited to any given area of the kiln.          

Related Article:

Refractory Resistance to Carbon Monoxide (CO) Disintegration Attack

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17-Nov-2009

Chemical attacks on refractories are mainly caused due to slags, gases like carbon monoxide (CO), and glasses etc. The test of determination of resistance of refractories to Carbon Monoxide (CO) disintegration is very important for fire clay bricks used in blast furnace stacks and other furnaces where CO is encountered, as in carbide manufacture and in carbonization of coal.
Depending on their composition, many refractories may begin to deposit carbon when exposed to a Carbon Monoxide (CO) atmosphere over a certain range of temperature and period. The dissociation reaction takes place as follows (Bell’s Reaction):
2CO = CO2 + C (soot)
Any form of iron present in the refractory acts as a nucleation site for deposition of Carbon. This is one of the most common and possible factors including disintegration of blast-furnace linings where disintegration is caused by deposition of soot carbon as a result of Bell’s Reaction.

Test of Resistance to CO (Carbon Monoxide) Disintegration [in Brief]
The mechanism of carbon deposition on refractory pores is technically known as VLS (vapour - liquid - solid) mechanism. The various test methods for verification of Resistance of Refractories to Carbon Monoxide (CO) Disintegration are BS 1902-3.10, ISO 12676, ASTM C288-87 (2009) etc. These test methods are used to determine the relative resistance of different type of refractories to disintegration caused by exposure to CO (Carbon Monoxide) atmosphere. The results obtained by these methods can be used to select refractories that are resistant to CO disintegration (attack). There are both qualitative and quantitative methods of testing although the standard method is for qualitative tests only. It comprises selection of two refractory test specimens. One of the test specimens is cut fro the center of a refractory and the second specimen is cut from the exterior of another refractory shape. The specimens so cut are of cylindrical shapes of 50 mm length and not less than 30 mm diameter. The refractory specimens may also be cut to rectangular or prismatic shapes. The two refractory specimens are placed in a wire-wound furnace of a suitable size which is purged with purified nitrogen. The furnace is heated to 450OC and purified CO is then allowed to pass through the furnace at the rate of 2 liters per hour. The test is continued for 100 hours or until the test specimens (refractories) disintegrate if it occurs earlier. The test specimens therefore, should be examined at regular intervals of time for discoloration, carbon decomposition and disintegration that may take place during the course of test. The entire test is to be carried out over a range of temperature under a constant supply of carbon monoxide. The time after which carbon deposition and disintegration takes place is taken as a measure resistance of the refractory to CO (Carbon Monoxide) attack. Purification of CO (Carbon Monoxide) and nitrogen is carried out to remove carbon dioxide, oxygen and water vapour.

Effect of CO (Carbon Monoxide) Attack on SiC and SiN Refractories
Here it would not be irrelevant to discuss about one report of former Ukrainian Scientific Research Institute of Refractories according to which, SiC (Silicon Carbide) is destructed most rapidly at 1200OC while Silicon Nitride (SiN) virtually do not change on heating up to 1400OC in presence of CO. At 1200OC Carbon Monoxide (CO) and alkalies significantly influence the property variation of the Silicon carbide refractories (SiC) containing a SiN-based binder only during first 2 hours of holding, which was confirmed by abrupt decrease of the open porosity and the apparent porosity during this period. Further increase in the holding period up to 16 hours does not above a significant change of the properties of the products owing to the protective glassy coating formed on the refractory surface as a result of partial oxidation of SiC. According to thermodynamic data given in the above report, when Silicon Carbide (SiC) is heated in CO the most probable reactions include -
SiC + CO = SiO + 2C,
SiC + 2CO = SiO2 + 3C,
SiO + C = Si + CO.