Refractories face the challenge of AFR
Refractories face the challenge of AFR

Refractories face the challenge of AFR

Despite a lot of talk about the usage of alternate fuels and raw materials (AFR) in a cement industry, AFRs are not easy to use as it comes with a set of problems. The cover story discusses these problems in detail.

I n the present day, when energy cost is very high, the cement industry is compelled to use alternate fuels, which are nothing but chemical waste from the industry, RDF (refuse-derived fuel), animal meal, fly ash and many more. The cement industry will go on trying different options.

Initially, the use of these kind of alternate fuels did not pose much of a problem, but once the volatile compounds (chlorine, sulphur and alkali) starts getting accumulated in the system, severe operational problems gets created at a later stage. These compounds form coatings on the kiln lining causing premature wear of refractory lining. One measure taken is to provide a bypass for chlorine.

Some areas that are exposed to chemical attack are cooler bull nose, clinker bed, inlet chamber, riser duct and kiln burner lining. These areas are lined with refractory castables or monoliths. Leading refractory manufacturers are offering a variety of products to suit the requirement. The new development of materials show very good mechanical strength at room temperature, no pre-drying of lining is required, the kiln can be put to use in a very short time.

Application for bullnose
Use of alternate fuels, which are more corrosive than fossil fuels, is one of the cause of failures. The chemical composition of clinker, moduli values of raw meal and volatile alkali salts are affecting the refractory lining. The chemically bonded refractory monoliths with some addition of zirconium oxide and Sic are finding it more user friendly. These are no cement materials with enhanced properties by making use of some chemical additives. Despite higher costs, these materials prove to be better ones in the long run.

Clinker bed
It is another area of application where the life of a refractory can be extended. In this area, the expectation from refractory material is high resistance to high thermal stress and abrasion resistance. The location of application is such that it is difficult to dry and heat. High grade low cement castable still finds its way here. Use of Sic-based castables are becoming popular for these kind of applications.

Rotary kiln burner
The basic requirement here is high strength material and proper pre drying. The lining is exposed to a different kind of wear mechanism, high dust loading, and deposit of alkali salts are more corrosive for castable lining. More and more use of cementless castables are finding its application here.

Inlet chamber-riser duct
This is a typical area where gunning technique is used with low cement or no cement castables. The problems here are increased many fold due to use of alternate fuels. which leads to increase in sulphur and chlorine content. The excessive alkali [if present] will only add to aggravate the chocking. All this has to be taken into account while selecting the refractory. Up to a maximum of 30 per cent of Sic has been proved successful in gunning materials.

Use of alternate fuels is the need of hour, considering both the environment and availability of fossil fuels. Therefore for any kind of problems posed by use of alternate fuels is to be taken a challenge. Any new development in the area of refractories is to be viewed from that perspective. The refractory producers will have to come out with not only cost effective solutions, but which are user and environment friendly, better in terms of life.

RHI AG is an Austria-based leading manufacturer of refractories to the cement industry across the globe. It is a solution provider, not just a product supplier. The products designed by the company are able to offer solutions to the above-mentioned problems. It is interesting to know about the new technologies developed by RHI AG.

SOL Mixes (Castables)
In the case of sol bonded mixes (castables), the binder is delivered separately from the mix. These cannot be called as castables because water is not used for hydration reaction. The liquid binder that is used contains nano particle of silica in an aqueous solution. This binder is added during the mixing procedure instead of water. The mixing procedure is similar to that for cement-bonded castables, except the binder is added instead of water and the mixing time is slightly shorter. The setting mechanism is based on a condensation reaction, during which water is released and Si-O-Si bonds are created. In contrast to the cement bonding, no hydrate phases are formed, which makes the residual water easy to remove. Accordingly no pre-drying or heating up procedure is required, just following the typical 36-hours heating up schedule applied for rotary kiln bricks is sufficient.

Advantages of SOL mixes:

  • No pre-drying requirement.
  • Fast and easy heating-up procedure.
  • Reduced ambient temperature impact on setting properties.
  • Reduced sensitivity to incorrect dosing of the mixing fluid.
  • Easily adjusted to the desired setting time.
  • Higher refractoriness compared to equivalent cement-bonded products.
  • Improved chemical resistance (e.g., alkali as well as sulphur) compared to equivalent cement-bonded products.
  • Longer shelf life than low cement castables (LCC).

The sol-bonding technology is available in a wide range of mixes and even gunning mixes (dry gunning), and recently also a selected range of shotcreting mixes (wet gunning) were also developed. No special equipment is required for either casting or gunning and conventional mixers and gunning machines can be used. The technology has rapidly spread worldwide and the references gained so far are impressive.

SOL castables is a new group of alumina-based no-cement castables, which combine the high performance of no-cement castables, but the application technique is same as that of conventional castables.

Besides the mentioned advantages in physical and chemical properties, we can rely on very good workability under difficult circumstances (e.g. winter repair period). The shelf life of the products is very high, and except for the binder liquid, not sensitive to storage conditions.

In particular, the very fast and easy heat-up procedure of sol-bonded castables compared to conventional low cement mixes provides a big advantage for applications where complex heat-ups cannot be maintained (intermediate repairs, high volume installations, fast repairs...) The products are designed for applications where the exposure of refractory material is very high and service conditions are challenging.

Low cement castables with SiC
How it works? At temperatures of 700¦C to 800¦C and in an oxidising atmosphere, SiC starts being oxidised and converted into a highly viscous silicate glass. This glass coating seals the pores of the refractory material very effectively. SiC is difficult to moisten and closes the pores effectively due to the volume increase during oxidation. This results in an outstanding resistance to alkaline attack. Because of the formation of a protective coating, the alkaline salts can attack only the surface and a penetration of alkalis can be prevented. The formation of a deposit can thus be avoided completely, with the bond to the surface being so weak that these build-ups are pulled off by their own weight or can be removed very easily. Low cement castables with SiC show outstanding resistance to alkaline attack and coating repellent behaviour.

Advantages of SiC

  • Very hard - high abrasion resistance
  • Improves alkali resistance
  • High strength, even at higher temperatures
  • Advantages of LCCs - Low Cement Castables
  • Low cement content, therefore low amount of mixing water
  • Very dense, low porosity
  • Very wear-resistant, high abrasion resistance
  • Higher resistance to infiltrations, e.g. to alkalis

Source: Innovative Refractory Mixes Technology by Uwe Schneider, Solnhfer Portlandzementwerke, Germany and Roland Krischanitz and Peter Fritsch, RHI AG, Austria.

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