Pradeep Kumar Chouhan, General Manager – Quality Control & Environment, Udaipur Cement Works Limited, gives a detailed account of different types of refractories used in a cement plant, the current improvements in automation and the eco-friendly innovations required.

What are the key materials used in building a refractory lining to the kiln in your organisation?
For kiln lining, UCWL uses alumina magnesium bricks for burning zones i.e., 7 to 27 metres. The kiln length at UCWL is 66 metres. (0 metre is considered at kiln outlet and 66 metre is at kiln inlet) 40 per cent to 70 per cent alumina bricks for the transition and pre-transition zone.

What are the key properties of a refractory that support the cement making process?
For bricks, chemical properties are mainly a percentage of Al2O3, Fe2O3 and silica is more important, the percentage of Al2O3 normally varies with application area temperature. Fe2O3 percentage is kept as minimum as possible to avoid self-damages. Silica percentage is monitored to check whether the refractory is having any other foreign material in it.
Bulk Density: The bulk density (BD) is the amount of refractory material within a volume (kg/m3). An increase in bulk density of a given refractory increases its volume stability, heat capacity and resistance to slag penetration.
Cold Crushing Strength: The Cold Crushing Strength (CCS) represents the ability of a product to resist failure under compressive load at room temperature. It has an indirect relevance to refractory performance, and is used as one of the indicators of abrasion resistance. The higher the CCS of a material is the greater should be the resistance to abrasion. Refractories with high CCS are also expected to have higher resistance to slag attack. The determination of cold crushing strength (CCS) is also highly important in case of refractory insulating bricks where bricks must be porous as well as strong.
Apparent Porosity: The apparent porosity or open porosity (oPo) is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the refractory. This property is important when the refractory is in contact with molten charge and slag. A low apparent porosity prevents molten material from penetrating into the refractory and therefore enhance it resistance to corrosion
Permanent Linear Change: Permanent Linear Change (PLC) is a crucial parameter for the design of refractory lining.it is a factor used to judge the suitability of refractories in ranges of temperature limits. Refractory materials can undergo mineral formation, phase transformation or shrinkage when heated. These processes may result in either volume expansion or reduction. Upon cooling to room temperature, the material will possibly be larger or smaller than the original dimensions. PLC is the property of shaped refractory to retain their original size after undergoing through a given temperature-time treatment and subsequent cooling down to room temperature.

Explain the types of refractories that you have in your manufacturing unit. What are their respective purposes?
Udaipur Cement Works limited (UCWL) has the following type of refractory for insulation in the kiln, pre-heater, calciner and the cooler area.

Tell us more about the porosity and permeability of the refractory.
The apparent porosity or open porosity (oPo) is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the refractory. This property is important when the refractory is in contact with molten charge.
A low apparent porosity prevents molten material from penetrating into the refractory, it makes a material-to-material bond and develops a good and stable coating on refractory / bricks, which enhances its life and its resistance to corrosion.
The permeability of refractories is a governing factor in the deterioration of linings by liquids and gases. The permeability of any refractory material is defined as the volume of the gas or air, which passes through a cubic centimetre of material under a pressure of 10 mmWG per seconds.

What is the maximum temperature that a refractory can withhold? How does its strength differ from ambient temperature to high temperature?
The temperature range in which the softening of refractory products occurs is not identical with the melting range of the pure raw material; Refractoriness Under Load (RUL) is a measure of the deformation behaviour of refractory subjected to a constant load and increasing temperature. The RUL has importance to check the suitability of refractory products for high-temperature applications; it gives an indication of the temperature at which the bricks will collapse.
For cement plant application, refractories can hold a temperature of 1450°C.
Refractory materials that keep their chemical and physical strength at temperatures above 500°C are of high importance for metallurgical and other industrial processes. They consist of alumina, magnesia, silica, lime, and other metal oxides. As these materials are dedicated for high temperature, the production is carried out at temperatures of more than 1000°C, making the process highly energy consuming.

Tell us about the installation and operating process of refractories in the kiln.
Installation of a refractory in a kiln requires specific skill. Brick installation is normally without any bonding material. Most of the cement plants use civil masons for refractory bricks installation, whereas brick installation in kilns is quite different from the normal civil masonry.
Installation of bricks is normally a combination of two types of bricks. Based on kiln diameter per ring numbers of bricks are worked out and accordingly it is laid line by line. Nowadays brick lining machines are available, which work very fast and less moan power is required. Lining with machines also eliminated the requirement of heavy jacking and rotation of the kiln.
For castable laying, the gunning castable has become more popular instead of the old conventional method by putting shuttering etc. Castable are also available as chemical bonded castable, which reduces the requirement of ICE cooling while mixing.

What are the standards set for refractories in a cement kiln?
For cement kilns, normally following standards are used but it is observed that use of different and various kinds of refractory as per the suitability of raw meal, temperature profile and plant design.

What is the role of technology and automation in refractories for cement kilns?
Automation and technology have helped the cement kiln refractory a lot. The shell scanner monitors real time shell radiation and gives a full picture of kiln coating and bricks health, throughout the
kiln length.
Shell scanner helps the kiln operation for the condition of hot zones, coating level, over coating zones, development of ring formation, if any. Timely corrective action can be taken which enhances the life of refractory and avoids heating of mechanical parts. For the application part, the de-braking machine, brick laying machine and castable gunning techniques are good examples of use of technology in refractory work. Mechanised working saves shutdown time and manpower, ultimately increasing the productivity of the cement kilns.

What tests are employed to check the refractory for defects and at what intervals are these tests done?
With advanced technology, most of the refractories are supplied to cement plants with good quality and assured guarantees of refractory life. However, CCS, apparent porosity, RUL and thermal conductivity is normally tested at the plant. Suppliers also provide us with test certificates for the same.

What are the major challenges your organisation comes across with the refractory kiln?
Most often, there is no premature failure of refractory at UCWL. The UCWL limestone contains very high grain size of calcite and quartz, it affects the development of stable coating and ultimately due to abrasive nature it affects the bricks’ life. However, at the refractory application part is found that skilled manpower with good workmanship is not available. Since the growth of cement industry is very fast and maximum industries are increasing their capacity, the availability of skilled manpower for application work will be a big challenge in near future.

What innovations in the refractory sector do you expect to see in the near future that will help better it?
Refractory suppliers are doing much research and continuously improving their products and are also providing tailor-made products. Precast modular lining at kiln inlet and precast tip casting are popular in the current times due to its long life and good thermal insulation properties.
Since the cement industry is consuming maximum hazardous waste in cement kilns, as hazardous material contains toxic element like chloride, metal parts like Zn, Mn, Pb etc. Refractory bricks of kiln are likely to be affected more, not in the kiln but also in lower cyclone and pre-calciner, so there will be a requirement to address the problems causes to refractory due to the hazardous materials. Refractory manufacturers should work upon developing eco-friendly refractories.

–Kanika Mathur