Technical Paper


CO Resistance of Monolithic Refractories


Goutam Bhattacharya*, Christoph Wöhrmeyer, Florian Ahouanto, Chris Parr


*Imerys Aluminates, Kolkata, India INTRODUCTION:


Carbon monoxide gas often formed during metallurgical and petrochemical processes, combustion of fuel or during coal gasification penetrates and reacts on refractory pore walls in presence of metallic (free) iron or iron oxides with deposition of carbon and destruction of refractory structure. This reaction, i.e. decomposition of gaseous carbon monoxide to solid carbon and carbon di-oxide is known as Boudouard reaction:


2CO (g) → CO2 (g) + C (s)


The reaction rate is maximum between 400° to 600°C [1]. Metallic iron or iron oxide catalyses the reaction depositing carbon, which develops enormous pressure in the pore or microstructure (?)


structure as they


grow leading to disintegration of refractory [2]. MECHANISM OF REACTION:


Pure CO gas participates in the Boudouard reaction through following steps [3]: 3Fe2


O3


3Fe (s) + 2CO (g) → Fe3 Fe3


(s) + 9CO (g) → 6Fe (s) + 9CO2 C (s) + CO2


(g) C (s) → 3Fe (s) + C (s)


The reaction progresses via continuous formation and decomposition of cementite (Fe3


C). In addition to cementite, iron percarbide (Fe20 C9 ) is


formed by the reaction of cementite and CO and gets decomposed to cementite and carbon.


20Fe3C (s) +14CO → 3Fe20 3Fe20


C9 (s) → 20Fe3 C9 (s) + 7CO2 C (s) + 7C (s)


Hydrogen gas, often present together with CO in several applications including biomass and coal gasification, can accelerate carbon deposition significantly – 2 to 5% H2


(s) (g)


accelerated by alkali metal and Zn vapour [7]. 2K + CO = C + K2


O Zn + CO = C + ZnO


Ammonia and sulphur blocks this carbon deposition reaction. Castables with low gas permeability are more resistant to CO attack. However, alkali vapour can activate iron catalysed CO disintegration [3]. Catalysts play a key role in the reaction. The rate of carbon deposition in pure CO gas is proportional to the amount of iron catalyst present [8]. Bost et al. [5] studied the effect of oxidation state of the catalyst (Fex


Oy (3%) and N2 ) with x=1-3 and y=0-4 and their grain size on the rate of carbon deposition by


Thermo-Gravimetric Analysis (TGA) in a gas mixture with CO (71%), H2 (11%), CO2


loss associated with reduction of Fe2


(15%) that was introduced at 600°C. A weight O3


was first observed, which was


followed by a weight gain due to dominant carbon deposition reaction. No clear correlation was observed between the catalyst grain size and reaction rate, although catalyst with smallest gran size (<35 nm) was found to be more reactive yielding carbon deposit faster than those with the largest. Metallic iron catalyst, unlike other oxide catalysts showed no initial weight loss, gained weight at the fastest rate. However, oxidation states of Fe cannot be easily related with rate of carbon deposition. However, Krause et al. [7] observed that in pure CO atmosphere carbon formation was influenced by the grain size and specific surface area of the catalyst. In this article CO resistance of castable compositions was investigated as a function of their metallic iron and iron oxide content.


STANDARDS OF CO RESISTANCE TESTING:


CO resistance of refractories is generally assessed at 450°C by BS 1902.3.10:1981 and at 500°C by ASTM C288-87 and ISO 12676:2000. Refractory prisms are introduced in the sealed furnace and CO gas is introduced. The prisms are observed visually at regular interval. The experiment is continued nominally 200h or until the disintegration takes place.


According to ASTM C288-87 the prisms are carefully inspected after the experiment for pop outs, cracks and following criteria are used for their classification:


A = no CO- attack


B = no cracks or pop outs with max. 13mm diameter C = cracks and/or pop outs with >13mm diameter D = breakable by hand, brittle with severe cracks


EFFECT OF CASTABLE COMPOSITIONS ON CO RESISTANCE: Experiment 1:


gas in CO test can reduce the reaction time


from 200h to 24h [4]. The rate of carbon deposition in presence of iron or iron oxide catalyst in CO-H2


gas mixture remains significant in the


temperature range 400°-750°C, with the maximum from 500° to 600°C [5]. The main reactions may be summarised as follows [6]:


FexOy Fex


Oy Oy (s) + nH2 (g) → Fex


2CO (g) → CO2 Fex


(s) + pC[Fex Oy-n


(s) + mCO (g) → Fex (g) + C (s)


C] (s) → Fex (s) + n H2 Oy-m Oy-p O (g) (s) + mCO2 (s)+CO (g)


Morphology of the deposited carbon depends on the content of H2 the mix of CO-H2


catalyst in the form of continuous layers, whereas in CO-H2 carbon nanofibers with high content of sp2


carbon forms [5].


FACTORS INFLUENCING THE REACTION: Carbon deposition by CO attack inside refractory lining in blast furnace is


. With only CO, polyaromatic carbon deposits on Fex


gas in Oy


environment Table 1: Iron oxide% of raw materials and composition of castables


Prisms (64 x 56 x 230mm) were prepared by vibration and cured 24h at 20°C, 24h 110°C, 6h at 550°C followed by CO resistance test according to ASTM C288 for 200h at 500°C in DIFK. Although dark spotting was


(g)


To understand the effect of a 50% alumina-containing calcium aluminate cement (CAC 50) on CO resistance of a castable composition, two batches of CAC 50 with a large difference in Fe2


O3 content were chosen together with a pure aggregate (tabular alumina) (Table 1)


REFRACTORIES ENGINEER


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September 2019 Issue


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