Showing posts with label Inspection (Testing). Show all posts
Showing posts with label Inspection (Testing). Show all posts

January 23, 2010

Alkali Resistance (AR) of Refractory Lining Materials (Bricks and Castables)



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).
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
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.          

November 17, 2009

Refractory Resistance to Carbon Monoxide (CO) Disintegration Attack


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.
The detailed report of this work on the “Resistance of SiC Refractories to the Action of Carbon Monoxide, Alkalies and Slag” [Read].                  
Further study (Related articles)

November 3, 2009

Cold Crushing Strength (CCS) of Refractory Bricks


Cold crushing strength (CCS) of a refractory brick represents its strength. That is it tells us how much load that refractory can bear in cold conditions. The concept of testing CCS of a refractory material has perhaps, come from metallurgy. This is because for any refractory brick it is rather; rare that it would fail simply due to load on it in cold condition and therefore, the determination of cold crushing strength does not appear to be important from that point of view.
But still testing of this property i.e. knowing the strength of the refractory brick is done to check some other properties which are direct result of strength such as ‘abrasion’. The stronger a material is the greater is the resistance to abrasion. Also stronger refractories are expected to have higher resistance to slag attack. The determination of cold crushing strength (CCS), however, is highly important in case of refractory insulating bricks where bricks have to be porous as well as strong.    
It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc. CCS of refractories is determined by placing a suitable Refractory Specimen on a flat surface followed by application of a uniform load to it through a bearing block in a standard mechanical or hydraulic compression testing machine. The load at which cracks appear in the refractory specimen represents the CCS value of the specimen. Load is to be applied uniformly and slowly, depending on the standard testing method followed, with a rate of load varying from 35 – 100 kg/cm2/min. Refractories being anisotropic in nature, the direction of load applied may be stated while reporting the results. The adjacent figure shows an assembly used for conducting the test.

(Fig. Courtsey: Indian Standards Institution, Ref. IS: 1528 of 1974, Part IV)

CCS = Total Load ÷ Total Area
Precaution must be taken that the refractory specimens must have the maximum possible original surfaces, have absolutely parallel and flat faces for applying load, and are free from cracks, voids etc. The size of the refractory test specimen (sample taken for testing) is usually equivalent to a 230 mm standard refractory brick except in case of smaller and other special refractory shapes where the test specimens are of smaller sizes or representative samples of 75 mm cube shape. The value of CCS can be expressed in either lbs. per square inch or kg/cm2.    

June 25, 2009

Slag Corrosion (Slag Attack) Test of Refractories

Slag attack is particularly important. The structural strength of the refractory may be critically reduced by the solvent action of liquid slags. The slag attack on the refractories in contact may be in the two ways:

Corrosion - It is the wear and tear of refractories caused by static chemical attack of slag.

Erosion - It is wear caused my mechanical action i.e. the process of breaking and washing away of refractory materials by molten slag.

The conditions of operation are variable and complex. Hence the standardization of this test is difficult. However, there are various test methods, viz. Crucible test, Solid cube test, Suspended rod test, Model wall test, Cone test, Powder impact test, but none is exact simulative test. It is done in various ways to suit the working conditions. The following are an outline of different methods of Slag Corrosion Test :

=> This method is called ‘Pill test’ is used when the quantity of slag is less as compared to the quantity of refractories. The slag, more often in the form of a pill, is placed on the refractory body or in a cavity made in it and heated. The depth of penetration of the slag inside the refractory, the spread of the molten mass and also the corrosion or bloating is observed. Theses factors form the measures of the attack.

=> This is another method known as immersion method and is used when the quantity of the slag is far in excess of refractory. Here the refractory is subjected to attack of the slag by immersing a small piece of refractory in the molten slag. The depth of penetration of the slag inside the refractory is the measures of the attack. => Another test also known as Impingement method or Powder impact test consists of letting the slag fall on the refractory bricks at high temperature. Many a time a spray of solid powdered slag is directed against the hot refractory brick at an angle of 45O for a certain period and at a certain temperature. The extent of corrosion under gone by the refractory is the measure of the slag attack. Several types of furnaces have been designed for this test.

=> This is Fusion test and consists of making a mixture of different quantities of powdered slag and refractory material and studying the fusion material of the mixture. The interval between softening and flattening of the cone is supposed to indicate the critical range of deformation of refractories in contact with slag.

The extent of penetration of slag is to be carefully studied. The bore diameter, depth in the refractory test specimen, the overall specimen size, fineness, as also the quantity of slag to be tested should be equal in every case for obtaining comparative test results. Overall slag corrosion / erosion will depend on so many factors such as porosity of the refractory brick, the composition, nature of the brick and of the slag, the temperature and duration of the attack, load on the brick at during slag attack, the products of the reaction formed and the rapidity with which they are removed, etc. Thus various refractories are affected variously and therefore it is difficult to simulate the exact conditions encountered in service. Still one can get an approximate an approximate idea by doing the chemical analysis and studying the various phases developed at the slag-refractory interface through Microstructural and XRD (X-ray diffraction) analysis. Out of all the slag corrosion tests described above, most of which give a qualitative and comparative result only, there is one method which has been somehow accepted as standard is the German DIN 1069 based on crucible test.

Related Articles

=> Standard Methods for Testing of Refractories

=> Sampling of the Refractory Specimen for Inspection

=> Iron Making in Mini Blast Furnace (MBF)

=> ‘Black Core’ in Refractory Bricks


June 18, 2009

Permeability of Refractory Bricks and Monolithics

Permeability of any refractory material is defined as the volume of a gas or air which will pass through a cubic centimeter of the material under a pressure of 1 cm of water per second. Permeability is calculated by the following formula:


Permeability = (Vol. of gas/air x thickness) ÷ (Area x Time of flow x Pressure difference)


It is determined by forcing a known volume air or gas through a cube. Time of flow, pressure difference and dimensions of specimens are noted. However, there are ready-made apparatuses and systems available in the market for testing permeability of refractory bricks and monolithics. There is no direct dependence permeability on porosity however, permeability depends upon the existence of closed pores or channel pores and is a measure of these, whereas porosity measures the total pore volume including closed pores.


Permeability for refractory bricks or monolithics is important, wherever molten liquid like metal, slag, glass etc। are in contact with the refractory or when gas under pressure is present. Due to the anisotropic nature of refractories, the result will depend upon on factors such as the direction of flow and presence or absence of the original skin on the test specimen. Low permeability is more important than low porosity from the point of view of slag resistance of the refractory. Uniform permeability is an indication of absence of cracks in the refractory.


Related Articles:


Types of Testing of Refractories


Apparent Porosity and True Porosity of Refractory Samples