The need for new-gen refractories
Today every cement plant is focusing on the efficiency of clinker production and reduction in the specific cost of operation. A permanent cost reduction programme is running in every plant to optimise its competitiveness and profitability. New modern plants are characterised by sophisticated new technologies, i.e., 6-stage preheater/pre-calciner system with tertiary air duct, more efficient burning and cooling system, and a more efficient refractory lining system. However, the required attention is still not given to refractories.
To optimise the energy cost, a significant change of fuels have been developed by using low-grade coal or alternative fuels in kind of waste components. Besides quality and cost improvements, cement manufacturers are also committed to follow environmental regulations.
Considering the cost factors, the refractory materials have a key function by optimising the specific cost of operation. The new generation of refractory materials have been developed to address these existing problems. The main burning zone of cement rotary kiln is traditionally lined with basic refractory materials.
There is a high demand for high-efficient refractory material matching with the need of a new modern cement plant. A new generation of chrome-free refractory concepts have been achieved. In case the traditional basic spinel/chrome bricks are suffering from severe corrosion due excessive thermal attack, instable burning conditions or attack by clinker phases, the material has to be replaced by a more thermal stable refractory material supplied by different manufacturers.
The use of high purity fused raw materials show an outstanding resistance to liquid phase and thermal wear. The thermal flexibility and thermal shock resistance is similar to the well proven magnesia spinel bricks.
High thermal/chemical resistant: In order to counteract the infiltration of liquid phases due to the use of alternative fuels and to prevent any chemical reactions with liquid and volatile compounds, tailormade refractory materials have to be developed. Alkali-resistant high alumina material: In case of high alkali attack in the safety or calcining zone, high alumina refractory material has been developed to counteract the unwanted alkali-spalling reactions as described above. This could be achieved due to the use of silicon carbide (SIC) technology.
Use of alternate fuels
To use alternative fuels to reduce the specific energy costs, the clinker firing unit 'rotary kiln' in Europe has become a useful incinerator for the disposal of hazardous material due to their favourable properties. Eg: tyres, plastics, household waste, animal fat, bone meal, waste oil and solvents. There are a number of cement kilns in Europe [meanwhile] running on that way without any fuel costs, i.e., with negative energy consumption.
On the other hand, the use of alternative and ordinary fuels lead to an introduction of a number of secondary phases and trace elements, which demand a high degree of performance of the installed refractory lining and is on high influence to the environment.
As a result of the introduction of secondary phase, the refractory lining is subjected to a high thermal/chemical wear. There are a number of reactions and interactions experienced between a refractory brick work, kiln gas atmosphere and a kiln feed, which is responsible for the chemical wear rate of refractory materials, i.e., compounds chlorides, sulfates and alkalis. The compounds-SiO2 , Al2O3, Fe2O3, CaO, and MgO-are not affecting the refractory lining, since these compound are a part of the clinker composition itself. In many cases, these compounds are included in the raw material design.
Due to the increasing use of burnable residues as alternative fuels in addition to normal raw material mix, there has been a considerable increase in the chemical load in the atmosphere of cement rotary kiln. The alkalis, sulfates and chlorides are introduced to the kiln atmosphere from two sides either from the kiln inlet with the kiln feed or from the kiln outlet with the burner system.
Degradation of 'refractory lining'
There are three important reaction phenomena observed that is responsible for the premature wear of a refractory lining:
- Reaction with alkali - sulphate - chloride
- High temperature attack on basic refractories
- Attack on high alumina refractories
Al2O3 + K2O + SiO2 K2O Al2O 32 SiO2 + (2,5% vol)
3Al2O32SiO2 + 3K2O + 4 SiO2 3K2OAl2O32SiO2 + (29% vol)
In today's context, running highly-efficient new modern cement plants and following the environmental regulations, the refractory lining has changed to new generation of basic refractory technologies for the cement industry. There is a wide range of basic refractory materials on the market starting with the standard basic chrome contenting brick to chrome ore free high sophisticated basic refractory materials which are going to stay long.
Source: Lothar Kilb, Refratechnik GmbH, Germany