Innovation–future driven I-06
NEW SILICON CARBIDE CONTAINING REFRACTORY CASTABLE
FOR RESOLVING PROBLEMS IN CEMENT KILN
Chanvit Chawengkul1 Thanapong Prasertphol2 Punyoot Huantanom3
(1)Senior Researcher, The Siam Refractory Industry Co. Ltd., Thailand
(2)Researcher, The Siam Refractory Industry Co. Ltd., Thailand
(3)Researcher, The Siam Refractory Industry Co. Ltd., Thailand
ABSTRACT
A major and frequentproblem occurring in the outlet zone and burner pipe of cement rotary kilns are thermo-chemical and mechanical wear which shortens the life of refractory monolithic liningsto less than 1 shutdown (4-6 months). The loads generally act on the refractory lining come from abrasion of clinker ball, abrasion and chemical reaction of clinker dust, and thermal cycling of secondary air from cooler.
To find a solution to such problem, a new and highly thermal spalling resistance property silicon carbide containing refractory castablehas been developed. It also helps resist chemicals attack of alkali and clinker during the operation, thereby formation of protective film on surface of lining.The important features of silicon carbide containing refractory castableenabling it to overcome the problems at the outlet zone and burner pipe of cement rotary kilns are its high level of hardness, outstanding mechanical strength at elevated temperature, excellent thermal shock resistance etc.
The development of silicon carbide containing refractory castablewas started from applying QFD and DOE for finding the optimum content of silicon carbide in order to help solving problem of refractory in these areas. Silicon Carbide is the main material to generate thermal spalling resistance property as well as its abrasion resistance.Special additives were studied and experimented to preserve the good properties of silicon carbide in high oxidation atmosphere. Thermal spalling resistance of silicon carbide containing refractory castable is relatively higher than other types of refractory castable. The abrasion resistance of this castable has been improved which also related to amount of silicon carbide in the recipe.
The performance of newly developed silicon carbide containing refractory castable after several trials at SCG cement kilns has showed an obviously improvement of service life at outlet zone and burner pipe up to 1 shutdown.
Keywords: Thermal spalling, Silicon Carbide containing refractory castable, Monolithic
INTRODUCTION
Regarding high competition of cement market, cement manufacturers must cut down their operating cost and prolong kiln operation for high productivity. Kiln operation with high running factor is targeted and needed for all cement manufacturers. Frequent break down time is not preferred. Generally, mechanical and electrical problems are main factors of break down time. Another factor was shut down of relining refractory bricks which also affected cement production loss. This major relining refractory should bedone atabout once a year. Several minor shut down times of refractory works basically mustbe avoided.So that, these minor works could affect the running factor of the kiln and need to be resolved.
Surveying customers’ needs
According to continuous development of refractory materials with advanced technology and TQM concept of Siam Refractory people, High alumina brick and Magnesia Spinel brick using in cement kiln should last up to one shutdown plan at least. However, some part of refractory lining at critical area such as outlet and main burner pipe faced short service life and they needed to reline prior the targetedplanned shutdown.The records showed that the relining time of this minor shut down was about 5 days in average of production loss. Most refractory materialsusing at outlet and main burner arerefractory castable(monolithic) which requires careful setting, curing and preheatingprocess.So that critical path of minor relining time is the castable works at the outlet of cement kiln. In order to improve service life of the castable at outlet and burner, Siam Refractory set up a P-team (Product team) which included teammates from Product development, Quality assurance, Production and Marketing department observing problem and finding solution. By this, observation of P-team at customer’s sites was set upin order to see the fact beginning from installation, heating, operation, demolition to disposal. Details of each step were noted and distinguished customers’ needs. The interviews with related persons in the route were done such as masons, contractors, operators, supervisors, engineers and managers. Questionnaires were also prepared and send to customers for checking and prioritizing their needs. Reverse engineering for failed samples weredeeply investigated as well as bench marking of other products was tested and compared. In doing so, the closed relationship and good communication with customers were simultaneously developed.
The observation of P-team found that the major and frequent problems occurring in the outlet zone and burner pipe of cement rotary kilns were thermo-chemical and mechanical wears. The loads generally acted on the refractory lining coming from abrasion of clinker ball, abrasion and chemical reaction of clinker dust, and strong thermal cycling of secondary air from cooler as showed in figure 1. However, improper installation and installation techniques also affected short service life of them. These fact finding factors need to be resolved by new development of refractory castable.
Quality function Deployment (QFD)
The needs and fact finding were reviewed, grouped and prepared in matrix table. Quality characteristics of product were related, weighed and considered. Deployment of customers’ needs and quality characteristics was developed following QFD stages and QFD tables were prepared as shown.
Table 1: The QFD table
Development stage
From above table 1, quality characteristics of products were related with quality of raw materials and processes.Development plan was set up. Materials and equipments were also prepared for the development.
Planning stage
Most refractory castable are Alumina materials which are widely used by all refractory manufacturers such as Calcined Kaolin, Calcined Bauxite, Tabular alumina, etc. In order to develop innovative product, selection of new raw materials was a key of development because the new raw materials should have superior properties than ordinary Alumina materials. Properties of various kinds of raw materials were studied comparing with the ordinary Alumina materials. Silicon carbide material was found that its properties could cope with the needs.
From the table 2, Silicon carbide material is inert, high hardness and low thermal expansion material which could generate excellent properties of castable such as good abrasion resistance, good chemical resistance, excellent thermal shock resistance and high refractoriness. This could be used in the critical conditions of outlet and burner pipe. However, its resistance to high oxidation atmosphere at high temperature is poor and its structure could be transformed from Silicon carbide to fluffy Silicon Dioxide which could be quickly abraded. However, P-team though that this important property could not be compromised. In vice versa, if oxidation resistant property of Silicon carbide could be developed, it will be an excellent material which suit for using at outlet and burner pipe of cement kiln. Hence, the development has to be done in experiments.
Table 2: Selection of alternative material technology
Customers’ needs / Characteristics of products / Ordinary alumina materials / Silicon Carbide materialResist to abrasion of clinker / High abrasion resistance / **** / *****
Resist to abrasion of dust / High abrasion resistance / *** / *****
Resist to clinker attack / High chemical resistance / *** / ****
Resist to dust attack / High chemical resistance / *** / ****
Resist to alkali attack / High chemical resistance / ** / *****
Resist to thermal cycling / High thermal shock resistance / *** / *****
Resist high temperature above 1500 Degree Celsius / Refractoriness > 1500 C / ***** / *****
Resist high temperature with high oxidation atmosphere / High oxidation resistance / ***** / *
Note: * poor**fair***good****very good*****excellent
Experiments stage
Designs of Experiments (DOE) were applied to find out the optimum using Silicon carbide materials. Typesand amount of additives for oxidation resistance (additive A, B and C) were also selective and experimented. The design was showed in table 3.
Table 3: The planned experiments
Factor / Description / Level 1 / Level 2 / Level 3 / Level 4A / % Silicon carbide / 0 / 30 / 60 / 80
B / Additive A / 0 / 2 / - / -
C / Additive B / 0 / 2 / - / -
D / Additive C / 3 / 10 / - / -
From above design, 32 experiments were done and tested in Laboratory and results were analyzed by statistical program.Analysis results were showed in figure 2, 3, 4 and 5 as followed
From test results, amount of Silicon Carbide was directly related to castable properties such as abrasion resistance, Thermal shock resistance and oxidation resistance. Increasing of Silicon carbide (SiC) affected higher abrasion resistance, better thermal shock resistance but oxidation resistance was poorer. Increasing values of oxidation mean higher oxidation of SiC because Oxygen (O2) substituted to molecular structure of Carbon (C) and Silicon Dioxide (SiO2) was occurred with higher molecular weight than Silicon Carbide. Despite of prevented Silicon Carbide from the oxidation reaction, additive C must be added about 10% by weight as showed in figure 4. Both additive A and B could slightly reduce oxidation rate. The experiments also showed that they had no interaction of the factors.
Checking stage
The test results were verified and reviewed in QFD table again. Negative effects of using higher percentage of Silicon carbide were also considered and discussed among P-team such as cost, price per performance, and thermal conductivity values. Higher amount of Silicon Carbide could generate higher heat loss due to its high thermal conductivity characteristic. If using 80% Silicon Carbide product is applied, kiln shell and anchor of cement kiln will be destroyed by heat load. So that medium amount of Silicon Carbide product (60% SiC) wasconsidered.
Action stage
From checked and discussed results, reproduced samples were done and tested again. Test results showed that new refractory castable with 60% Silicon carbide and 10% additive C gave optimum test values which properly applied to outlet and burner of cement kiln. Samples of new anti-oxidant Silicon Carbide castable were tested and fired at temperature 1500 Degree Celsius for 5 hours in oxidation atmosphere comparing with normal Silicon Carbide castable. The fired samples showed that forming of protective film could be perfectly covered Silicon carbide material and oxidation reaction could be prevented as showed in figure 6. Hence, all excellent properties of Silicon Carbide could be preserved by new mixes design. New Silicon Carbide containing castable was trialed and launched to market.
Figure 6: Left; Oxidized general Silicon Carbide castable and Right; Anti-Oxidant Silicon Carbide castable
Customer trials
Marketing materials of new refractory castable were prepared and presented to a customer in order to make trial lot in the real application. A cement plant in SCG group accepted the concept of development and works trial of new Silicon Carbide containing castable was produced. New castable was installed at area of outlet and burner tip by 60% Silicon carbide containing castable namely “TufDura S600”. It could show outstanding performance after running 1 shut down as showed in figure 7 and 8. This was much improvement and could satisfy the customer.
Commercial stage
After finishedthe first trial, this new castable namely “TufDura S600” was launched to world-wide market. Performance of these castable could satisfy customers due to their tough and durable properties. Siam Refractory sold this type of our products to more than 10 countries such as Malaysia, The Philippines, Vietnam, India, Pakistan, Nigeria, Indonesia and Australia, etc.
Conclusion
Changing raw material based from Alumina materials to Silicon carbide material could produce higher performance refractory castable which has excellent thermal shock, chemical and abrasion resistance. Added additive C with certain content could protect Silicon Carbide from oxidation reaction and preserve excellent characteristics of the same. This type of new refractory castable could satisfy, resolve problem and fulfill customers’ needs. The most important step of development is how to find out what customers need and how developed products or services can satisfy or delight them.These need good vehicles, tools and techniques of TQM such as QFD, DOE, and other statistical models could help to be easier and faster development. This also was pushed by policy deployment of top management as well as daily management of teammates. An example of this development of newsilicon carbide containing refractory castable could satisfy the customer trough TQM steps.
References
The Technical Association of Refractories, Japan (1998), “Refractories Handbook”,The Technical Association of Refractories, Japan
Subrata Banerjee (1998), “Monolithic Refractories”, World Scientific Publishing Co., Pte.Ltd., Singapore
J.P. Saxena (2003), “Refractory Engineering and Kiln Maintenance in Cement Plants”, Tech Books International, India
Gerald Routschka (2004), “Refractories materials”, Vulkan-Verlag Essen, Germany
Shigeru Mizuno, Yoji Akao (1994), “ QFD The customer-driven approach to quality planning and deployment”, Asian Productivity Organization, Japan