Maximising performance: Steps for selecting refractory

Refractory selection is the most important step for the maximisation of its performance. The major deciding factor is the working or operating environment where the refractory would be used.  The working environment, in general, is defined by the following parameters Temperature, Chemical Condition, Thermal Shock, Mechanical Stress and Abrasion. All these parameters are not relevant for every application but identification of the critical parameters, for a given working environment, is vital for refractory life maximization at optimal cost.   
 
In cement industry, the operating condition for various parts of the kiln system constitutes the concentration of raw meal and the new phases formed there from, at different stage of the processing (see Figure 1).  For a theoretical raw meal, ie, a mix of limestone, ouartz, clay and some lateritic material, the operating condition is not very severe, except for the burning zone where the temperature is as high as 1450°C and the liquid content of the feed material would be in the range of 25-27 per cent.
  
By the time the raw mix attains 900°C, the lime stone of raw meal is decomposed into CaO, clay is dehydrated and quartz undergoes polymorphic transformation. Simultaneously formation of some of the cement constituents, eg, C2S and C3A, commences. None of these reactions has any adverse impact on the refractory.  As temperature rises to 1450° C, the liquid phase forms and on cooling phases like C3S and C4AF precipitates out from the melt.  As the clinker cools down, the reactivity of the mass reduces, ie, refractory is not chemically affected.  The clinker, on cooling, however, becomes abrasive and has a tendency to erode the refractory. The situation worsens in the modern pre-heater, pre-calciner kilns, where the clinker is dustier.  The cooler refractories, including the cooler take off duct, bull nose and tertiary air ducts are impinged upon by dust laden gas. Except for burning zone, thus, the operating condition of cement kiln, is apparently of moderate severity. The temperature in non burning zone part of the kiln system is not high enough for reaction between the aluminous refractory and lime bearing raw meal /clinker.  Chemically non-compatible, ie, alumino-silicate, refractories, hence, should suffice for majority part of the kiln system.

Volatile Cycle
The reality is raw meal and fuel brings in potassium, sodium, sulphur, chloride in cement rotary kiln system.  These constituents combine to form a varied range of alkali compounds.  The nature of compounds formed, ie, the ones available in the kiln environment is determined the alkali sulphur ratio (ASR), which is expressed by the following relationship.  ASR is calculated based on the constituents in the kiln gaseous environment,
        {SO3/80}
Q(ASR) =
    {(Na2O/62) + (K2O/94)-(CI/71)}
 
When,
Q = 1 KCl and K2SO4 in the environment
Q > 1, KCl and K2SO4 as well as free SO3
Q < 1, KCl and K2SO4 as well as free K2O
 
The situation worsens when alternate fuel is used, since they have significantly higher level of alkali and chlorine in them. Table I below presents the melting point of the compounds which form in the kiln environment.   
 
Volatile Components    Melting Point, 0C
KCl    770
NaCl    801
K2SO4    1069
Na2SO4    885
K2O    400
Na2O    1100
 
 
Since the melting points of newly formed alkali compounds are significantly lower than the maximum operating temperature of the kiln, part of these compounds partially vapourise in the kiln and travel along with the flue gas towards the kiln inlet areas, whereas the rest escape the kiln system by combining with the clinker. The alkali bearing compounds in the flue gas get deposited on the incoming raw meal at the corresponding freezing point of these alkali compounds. The alkali enriched raw materials travel back in the kiln and the aforementioned process gets repeated. Since the chloride compounds have lower melting point than the sulphates, they have the ability to travel further back in the kiln system, compared to the sulphur bearing compounds. Owing to the cycling process, the kiln environment becomes richer in alkali.
 
Build Up Formation 
In addition to the deposition of these alkali compounds on the incoming raw meals, they also get deposited on the refractory surface.  Incoming raw meal sticks to the alkali coated refractory surface, where the temperatures are in the range of the freezing points of the alkali compounds. This process continues and the build up occurs in the kiln system, where the temperature is in the vicinity of melting points of the compounds. The kiln inlet, smoke chamber and lower riser ducts are the most vulnerable areas for build up formation since temperature in these regions fall in the range of melting point of the alkali compounds.  It is, hence, evident that the build-up problem of the kiln inlet region originates from the cement manufacturing process, not refractory. The build up hinders the flow of raw meal and many a time the production needs to be stopped for their dislodgement, which is either by thermal or mechanical shock. Both these methods of build up dislodgement put stress on the refractories and may cause premature failure.
 
Solution of Build Up through Refractory
There exists solution for the build - up problem through refractory.  The build up can be minimized, if not eliminated, by using SiC based castable, e. g. Ace Calderys' ACCMON CRC and its extension like ACCMON AF 1 and ACCMON AF 2, wherever such problem exists.  The basic mechanism for build up resistance is by virtue of glassy phase formation on the refractory surface, via following reactions: 
 
SiC (s) + 3/2O2 (g) = SiO2 (s) + CO (g) (I)
 
SiO2 (s) + Na2CO3 (l) = Na2SiO3 (Glass) + CO2 (g), and
SiO2 (s) + Na2SO4 (l) + 2C (s) + 3/2O2 (g) = Na2SiO3 (Glass) + SO2 (g) + 2CO2 (g)
 
Since glassy surface is 'slimy', the adherence of build up on the refractory surface is weak and hence, falls off under its own weight or by light mechanical shock.  The elimination of build up in ACCMON CRC is by an unconventional process, i. e. by allowing the refractory to react with the environmental constituents.  Since the build up adherence is a physical phenomenon, we recommend castable as it offers lesser foothold, compared to bricks, owing to lesser number of joints
 
Alkali Bursting
No refractory is impervious and hence, alkali compounds present in the kiln environment not only travel to the cooler part of the kiln but also travel to the cold face of the refractory. While travelling, the alkali compounds can interact with the refractories. The resultant of the alkali - refractory interaction are that the Chemical interaction with refractory, ie, formation of new compounds, and /or physical interaction, ie, alkali impregnation, which leads to densification of brick texture resulting in structural inflexibility.  This leads to eventual refractory failure by thermal Spalling.

The alkali bursting occurs due to formation of following feldspathic compounds by reaction between alkali and alumino-silicate refractories.  It is evident that formation of all these feldspathic compounds is expansive in nature, which induces stress in the refractory, leading to their ing.
 
Refractories for Kiln Down Stream
In the down stream, the clinker becomes more abrasive since it becomes cooler and volatile components may still be in the atmosphere.  temperature fluctuation, ie, thermal shock, always remains an Issue in the tip casting area.  In short, refractories in majority areas of down stream should be resistant to abrasion and thermal shock. For areas, with relatively higher temperature, ie, the locations where operating temperature is higher than the melting point of the alkali compounds, alkali resistance in refractories is desirable.  For the areas, which require only abrasion resistance, eg, tertiary air duct, ACCMON CAR is recommend.  The abrasion resistances for some of Ace Calderys Ltd products are illustrated in the Figure. 
Prior to the development of ACCMON CAR, ACCMON 90 has been the most abrasion resistance refractory.  The data from the Figure reveal that abrasion resistance of ACCMON CAR is over 3 times better than that of ACCMON 90.  Field trial results indicate significant improvement in performance while using ACCMON CAR in high abrasion areas of cement kiln.  ACCMON CAR has become synonymous with abrasion resistant refractory in the Indian market.
 
Exploitation of ACCMON CRC Versions for Other Applications
Since ACCMON CRC is based on SiC, its thermal conductivity is high and as a fall out of the same, it has excellent thermal spalling resistance.  SiC also is known for its high abrasion resistance.  Aforementioned SiC based castables, hence, is recommended for tip casting, cooler bull nose and cooler bench, since refractories in these locations are exposed to high abrasion, thermal shock and occasionally to alkali attack.
 
Conclusion
Generic alumino-silicate refractories suffice in majority of locations, except for upper as well as lower transition zones and burning zone.  Such refractories are definite choice for upper cyclones since they are not exposed to high abrasion or alkali attack.  Lower cyclones and kiln inlet areas, however, would require special refractories, eg, ACCMON CRC or its derivatives, when alkali concentration in the atmosphere is high.  Down stream of burning zone mainly requires abrasion as well as thermal shock resistant refractories, and sometimes alkali resistance also is desirable.  For areas with only abrasion, eg, in tertiary air duct, ACCMON CAR is recommended. For rest of the locations, eg, cooler bull nose, cooler bench and cooler take off duct, either ACCMON CAR or ACCMON CRC is recommended depending on the exact operating condition.

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