Showing posts with label Testing. Show all posts
Showing posts with label Testing. Show all posts

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)

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

May 25, 2009

Pore Size Distribution in Refractories

It is highly essential to measure pore size and its distribution to get the desired optimum properties in any refractory brick. Actually ‘pore size’ and ‘pore size distribution’ are two different things. First, let us discuss the difference between these two terms. The ‘pore size’ is a measure of the diameter of the largest pore whereas; the ‘pore size distribution’ is a measure of the range of pore sizes. The range of pore sizes can be normally distributed, and the spread can be quite narrow (e.g. the ratio of largest to smallest may be less than 2). On the other hand, pore size distribution can be very heterogeneous. In the case of large spreads and heterogeneity, which is not desired, the pore size will be far less predictive.

It is measured by analytical instrument known as Porosimeter based on mercury intrusion and capillary flow methods. The volume of mercury penetrating the pores measured directly as function of applied pressure. Pore Size information is best described graphically, rather than with a single number. An alternative method of describing the distribution curve is to use a collection of values that better describe the distribution curve in more detail, i.e., upper limit, lower limit, mean, standard distribution, etc. This P-V information serves as unique characterization of pore structure. This data together with Microstructural analysis and few other test reports of the refractory sample can be of great help in optimizing the properties as well as for any failure analysis.

Related Articles:

Types of Testing of Refractories [Read]

Apparent Porosity and True Porosity of Refractory Samples [Read]

Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks [Read]

May 7, 2009

Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks

Apparent porosity (AP) is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. AP is one of the most important physical properties for any type of refractory brick that will be certainly mentioned in its specifications. Hence determination of Apparent Porosity is almost a compulsory part of Inspection / Testing of a Refractory Lot. This property becomes more stringent when the refractory brick is of any complicated shape like, checker bricks or nozzle bricks having tongue - groove etc. In such cases often, it remains a cause of worries for the laboratory person conducting inspection.

The results of AP can be manipulated to show less AP% than what the brick has actually and thus, an inspector may be deceived by a laboratory person. But such actions or manipulation of results are undeniably wrong and liable to be penalized or even the whole Refractory Lot may get rejected, if caught red-handed.

Nevertheless, this article is not to discuss the merits-demerits or right-wrong of the action. That is for you to decide. Here I assume that you have made a conscious decision to learn the trick as how to manipulate the results of Apparent Porosity in order to show a better result of a porous brick. Having so decided, below is a guide (trick) on how you can do it:

=>> Apparent Porosity (%) = {(Soaked Wt - Dry Wt) ÷ (Soaked Wt - Suspended Wt)} x 100

=>> To show less AP% we need to increase Dry wt as much as possible.

=>> Make 30% conc. salt solution (i.e. 30gm salt in 100gm water).

=>> Example: To bring down 75 mm std. Brick having actually AP-25% to AP-18%, soak the brick in approximately 30ml quantity of above salt solution so that its Dry Wt. is increased by approx 9 gm. After applying solution from different sides and its complete soaking put the brick in the drier & properly clean itssurfaces after drying.

Better try to know the actual AP before applying/manipulating.

Types of Testing of Refractories [Read]