The word ´refractory´ is derived from a Latin word that means ´stubborn´. Ideally, a refractory material needs to be stubborn to any change either in shape or size or its chemistry, in the face of high temperature and attacks by corrosive/abrasive solids, liquids or gases during the life-cycle of the product being produced.
To appreciate the importance of refractories, one needs to know that modern life cannot exist without refractories. There can be no metals, ferrous and non-ferrous, cements, fuel oils, petrochemicals, plastics, glass, fertilisers, ceramics and umpteen other items of daily use without refractories. The capital & replacement costs of refractories are very small as a percentage of total cost; but failure of refractories brings the entire plant to a standstill.
The earliest refractories were the cooking pots of clay used by the ancient civilisations of Egypt, Mesopotamia and the Indus Valley. As man discovered ores, he searched for better refractories to melt these to produce metals.
Initially through the industrial revolution in the 1700´s till the mid-twentieth century, refractories were predominantly pre- shaped and fired. Since shaped refractories required a laborious production cycle, these had to be ordered well in time and stocked; leading to higher inventory and longer delivery time. Today, the major industries consuming refractories are as shown in Fig. 1. The cement industry is the second major consumer, although a distant second from the steel industry.
Towards the 1960s, production of refractory cements of high purity gave birth to the first castables. These castables initially had properties inferior to shaped refractories; but still the convenience of using castables, these being much simpler to manufacture, and consequently lower lead time, led to a steady increase in sales. Towards the 1980s, nano particle refractory powders became available and low cement castables started becoming produced. Today refractory castables are plus 80 per cent of sales of all refractories, as shown in Fig.2 , and are technically superior to shaped refractories:
Chemistry Of Refractory Monoliths:
OPC technologists are familiar with the main phases of OPC clinker, namely, C2S, C3S, C3A, C4AF. The first 3 are hydraulic, contributing to various properties of concrete like early strength, etc. In case of refractory Alumina Cements too, various phases contribute to different properties of refractory concrete. (Legend C:CaO, S:SiO2, A: Al2O3, F: Fe2O3, H:H2O)
Alumina Cements are basically a binary product of the A12O3-CaO system. Tertiary products like Ghelinite, C2AS are present as unwanted phases due to presence of SiO2 in the lime or alumina raw material source.
These cements are the heart of conventional & low cement castables. They are manufactured by inter-grinding raw materials containing A12O3 & CaO, palletisation & sintering in a rotary kiln at temperatures ranging from 1300° to 1450°. There are essentially two types of cements manufactured, a low purity one & a high purity cement.
Conventional & Low Cement Castables
Before we study in some depth the products of hydration of high purity alumina cements, let us consider the essential features of conventional and low cement castables.
Conventional castables are very much like civil engineering concrete, essentially consisting of aggregates and cement. Low cement castables are similar to M-80 & M-120 grades concrete containing super plasticisers and nano particles.
Any one sized particle, loosely filled, will have as much as 40 per cent of porosity. Concrete or conventional castables are made up of different sized aggregates, the smaller sizes filling the inter-particle porosity, thus compacting the concrete. The finest particles 20-45 microns in fineness are obtained by grinding.
Even the best graded concretes could achieve up to 20 per cent porosity, thus limiting the strength of the cast concretes to 500 kg/cm2. This was due to the fact that the porosity caused by the finest particles could not be filled with micro-fine powders. It was only after the 1980s that nano particle (sub-micron) powders became available. Use of these led to further compaction in concretes bringing down the cast concretes´ porosity to 12-14 per cent. Use of these nano particles reduced the flow of the concretes and necessitated the use of super plasticises to restore flow. Thus, dawned the era of superior concretes. This is better illustrated by Tables 2 & 3 and Figure 4.
Typical strength profiles of these 2 types of castables are better illustrated by Fig. 3;
Hydration of Alumina Cements and their influence on castable properties:
When alumina cements are produced, different phases are formed during pyro-processing. This is particularly dependent on impurities present in the raw materials and the firing temperature.
A careful look at above table reveals that the phases C2AS & C4AF are formed due to impurities like SiO2 & Fe2O3. They are to be avoided and being non-hydraulic do not contribute to strength of cement & concrete.
Presence of C12A7 makes the cement too fast setting. Conversely, presence of excessive amounts of CA2 makes the cement too slow setting.
The most useful phase is CA being of highest density and CCS, having a moderate setting time, and a high melting point.
On adding water to a castable, the main useful phase reacts with water and forms different hydrates depending on casting and curing conditions as given in Table 9 below. It must be understood that some hydrates are stable compounds, others being meta-stable, and the reactions are temperature dependent.
We shall discuss the significance of these properties a little later in the article. This paper is written for cement technologists and it would be pertinent to compare the setting time and heat of hydration of alumina cements & OPC.
It needs to be noted that alumina cements set slower than OPCs.
Basic difference between concretes made from OPC and refractory concretes is that the latter has to be heated up during service or before service. The hydrates, as given in table 10 above break up releasing water. Eg. CAH10 loses 53.5 per cent water while C3AH6 loses 28.6 per cent.
- In conventional castables, percentage of alumina cement varies between 15 to 25 per cent
- Since percentage of cement is high, heat of hydration is tremendous (Table 4)
- Setting time of alumina cements is slow (Table 5). To prevent premature dehydration, before strength can develop, temperature of curing is controlled to < 20° by using ice cold water for mixing.
- Since cement content is high, large quantities of CAH10 is formed. As CAH10 on heating up has 53.5 per cent water, large quantities of water will have to be expelled during heat-up of the castable.
- Heating rate needs to be controlled, to avoid explosive spalling.
- This large evaporation of water during heat-up explains the fall in strength of conventional castables at intermediate temperatures (Fig. 4).
Low Cement Castables
- In low & ultra-low cement castables, percentage of alumina cement varies between 1.5 to 6 per cent.
- Since percentage of cement is low, heat of hydration is much less.
- Since quantity of heat generated is less, increase in temperature is negligible.
- There is thus no need to limit temperature < 20°. In fact, high curing temperatures are necessary to give economic setting time.
- As temperature of curing > 35¦, predominantly the hydrate formed during curing will be C3AH6
- Quantity of water expelled during heat-up will be only 28.6 per cent (Table 10). Heating rate still needs to be controlled, to avoid explosive spalling, due to very compact, closed structure. Here, even a small amount of steam can lead to explosive spalling.
- Heat-up of the low cement castable is aided by addition of organic fibres that char/melt below 100°, creating capillaries for safe escape of steam.
To summarise, careful gradation, low cement content, use of nano-particles, plasticisers, dispersants, set adjusters and additives that assist in safe burnout of water during heat-up, has produced a superman amongst refractory castables. This technology leap helped all industries that use refractories to replace cumbersome shaped products with easy to use castables. These had superior properties than the shaped refractories; but they suffered from poor thermal spalling resistance. Thus, cyclic industrial furnaces which are periodically heated and cooled avoided low cement castables.
Also low cement castables, had a hot MOR, only moderately higher than that of conventional castables (Table 3) The castables´ only drawback is that it is supplied in two components. The CCS after drying is a low, just 250 kg/cm2; but this is only of academic interest, since the castable is used at high temperature where strength pick-up is excellent. Its Hot MOR is equal to that of shaped refractories; it has excellent spalling resistance. In fact this is possible, since CaO is eliminated from the matrix and the castable reduces to a two-component one: Al2O3-SiO2. Fig. 4 & 5 below illustrate this succinctly.
Some Key applications of Castables in Cement Plants
Burner Lance is a critical area with specific needs. Strong thermal loads, abrasions, infiltrations and chemical attacks must be considered. Coupled with this a constricted area requires a self flow castable. LCC 80 is recommended.
Cyclone systems and connecting ducts, are important areas. The castable chosen must be able to withstand build-ups & alkali & chlorine attacks. In stage 1 &2, LCC 45, 50 are recommended while in the more abrasive stages 3, 4, 5, LCC containing 10 to 30 per cent SiC are recommended.
Calciner: This is the first step of the clinker making process. The castable must have the ability to withstand chemical attacks from hot gasses of the process. In medium severe areas, LCC 50 or LCC containing 10 per cent SiC are recommended, while in severe areas or LCC containing 30 to 60 per cent SiC are recommended.
Riser Duct: This also is a problematic area where build-up and alkali and chemical attack is a threat. Here 50 per cent LCC are used in square sections.
Nose Ring: In this critical area high mechanical strength, high thermal shock and chemical resistance are needed. Here Andalusite based LCC gives a good performance.
Kiln Hood: Kiln hood castables need to have a low thermal conductivity, good abrasion resistance, and resistance to chemical attack. Mullite based LCC 60 per cent are recommended in less severe areas and Sic 30 to 60 per cent containing LCC for the most severe areas.