In this guide, we will discuss the basics of coal clinker, how it is formed, and how it can be managed for efficient and safe combustion. We’ll also provide tips on recognizing common issues with coal clinker, as well as solutions to ensure optimal performance in a boiler.
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Coal clinker is an aggregate product formed through the burning of coal. It is composed primarily of carbonates, sesquioxides, alumino-silicates, and sometimes quartz or glassy material that has been fused together by heat during the combustion process.
This aggregate material is a major component of coal ash, which is the solid residue remaining after burning coal. The clinker primarily consists of inorganic compounds and minerals that are fused together due to high temperatures during combustion.
Coal ash, also known as coal combustion residuals (CCR), is the by-product of burning coal for the generation of electricity. It includes fly ash, bottom ash, and flue gas desulphurization sludge that are collected from the emission control devices of a coal-fired boiler.
Fly ash is composed primarily of silica, alumina, and iron oxide, while bottom ash is composed of heavier elements such as metals and other trace elements.
Coal ash can be used in several industrial applications, such as cement production, road construction, and fill material. It can also be recycled for use in the production of bricks or as an additive to concrete.
The presence of toxic substances in coal ash has raised some environmental concerns, however, as these substances can leach into the environment if not stored properly. As such, proper disposal of coal ash is crucial to maintain a safe and healthy environment.
As discussed, coal ash is an important component in the production of clinker, where clinker is obtained by heating coal ash to a very high temperature. This causes the coal ash to fuse together into an aggregate.
Clinker is then mixed with gypsum and other substances to create cement, a popular building material. The properties of clinker depend on its composition and how it was processed during production. Thus, understanding the process of creating coal clinker is essential for creating high-quality cement.
The process of creating coal clinker is complex and requires careful control to ensure the quality of the end product. The most important step in this process is heating the coal ash, as this determines the chemistry of the resulting clinker.
The temperature, duration, and other parameters must be carefully adjusted to achieve desired properties such as strength and durability. These parameters must be closely monitored to ensure that the clinker produced meets the required specifications.
By understanding coal clinker, manufacturers can create high-quality cement and other materials that meet the needs of their customers. Moreover, proper disposal of coal ash is essential for protecting the environment from harmful pollutants present in this material.
Coal clinker plays an important role in efficient and safe coal combustion. It helps to ensure that the coal is burned evenly and at a consistent temperature, resulting in clean, efficient combustion. The clinker can also trap certain pollutants, such as sulfur dioxide and nitrogen oxide, preventing them from entering the atmosphere. It’s also an important source of minerals for use in making cement, glass, and other products.
It’s also important to manage coal clinker correctly, as it will help ensure optimal performance in a boiler. One way to do this is by regularly measuring the ash content of the fuel.
The ash should be removed from the boiler and disposed of properly before it builds up and interferes with the efficiency of the boiler. Additionally, the clinker should be closely monitored for signs of degradation, such as cracks and chips.
These can lead to inefficiencies in the combustion process and should be resolved before they cause serious damage.
In addition to being used as a fuel source, coal clinker also has many other uses. It is often used in the production of cement and glass, and it’s also an important component of refractory bricks and mortar.
It can also be used as a soil amendment or fertilizer, which helps to improve its water-holding capacity and nutrient content.
In addition, coal clinker can be used as a catalyst in certain chemical reactions, or it can be converted into activated carbon for use in industrial processes such as water filtration or wastewater treatment.
Manufacturers use coal clinker as a material for cement production, as it is made up of calcium-rich aluminosilicates. The clinker is heated in large kilns until it reaches a temperature of around 2,500°F, which helps to form the nodules. The nodules are then ground into a fine powder, which can be blended with other materials to create various types of cement.
Coal clinker is an ideal material for manufacturing cement due to its low cost and abundance. It also has a high calcium content, which makes it a great binding agent for concrete and other construction materials.
Furthermore, its high temperature tolerance makes it ideal for use in a variety of industrial applications such as metallurgy and glass-making. Additionally, the nodules produced from coal clinker are small enough to be safely handled without any risk of injury.
If you’re looking for coal clinker or other raw materials, contact PermuTrade today. Our knowledgeable staff can help you find the perfect material for your needs. We also offer a wide selection of minerals and raw materials to meet any requirement.
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Clinker is the primary material of cement, and it is called as semi-finished product. Clinker is a granulated material made of raw meal powder obtained by grinding limestone and clay together by sintering the raw meal in a rotary furnace at °C-°C degrees.
Produced clinker’s mineralogical structure effects clinker’s grinding energy. And it determines the behavior of cement by changing the parameters such as strength and setting time.
Our post deals with clinker mineral structure’s grindability effects.
Figure 1: Gray Cement Clinker
Cement production is an energy-intense process and approximately one third of the energy required to produce one ton of cement is consumed during grinding of clinker and additive materials.
Cement industry is one of the biggest industries using almost 3.5% of the world’s energy. In cement industries, 40% of the total energy consumed during the production process is used for grinding.
Grindability in cement production is important due to two factors. First, cement’s properties depend on the fineness and grain size distribution of the cement other than chemical and mineralogical compound of it. Second; 1/3 of energy expense in cement cost is spent for grinding. 80% more energy is spent in grinding hard clinker compared to soft clinker.
Chemical Compound and Grindability
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Chemical and mineralogical compound play an important role in the grindability of clinker. Grindability is observed to decrease with the increase of Silicate module, Aluminum oxide and free Calcium oxide determined through the chemical analysis of clinkers.
Microstructure also has an effect on grindability alongside chemical and mineralogical elements. Heating and cooling speeds and furnace type are also effective in the formation of microstructure. Fine crystalline structure, especially small calcium silicate crystals, improve grinding. Large crystals not only make breaking down difficult, but they also increase the number of breaking areas.
Raw meal powder enters into the furnace system, it transitions into the melt phase, and clinker is obtained once furnace and cooling reactions are completed.
The first melt phase is formed at °C-°C. Clinker melt’s rate increases as the temperature rises and it reaches up to 20-30% in weight at °C depending on the chemical compound.
These temperatures enable alite formation which is the main content of Portland cement. As soon as sintering starts, high amounts of free Calcium oxide along with belite come out. With the melt phase, Calcium oxide and belite enter into the solid solution.
Reactions occur in the rotary furnace at °C-°C. Clinker should be cooled off rapidly once the reactions are completed. This process preserves the presence of alite which is the main phase of clinker. Belite and tricalcium aluminate phase emerges as a result of clinker melt entering into reaction with a certain proportion of alite due to slow cooling process. In addition, alite is not stable under °C and it tends to decompose into belite and free Calcium oxide. Therefore, clinker requires a cooling process fast enough not to allow these reactions.
High cooling speed has the following effects: Grinding improves due to stress cracks in clinker. No alite dissolution occurs and this phase has a high amount. The fine crystalline aluminate and ferrite phases formed by shock cooling of clinker slow down cement hardening.
Clinker phases emerge as a result of reactions which occur during sintering of raw meal powder in the rotary furnace. These phases affect the behavior of clinker, thus the behavior of cement. Main phases of typical Portland cement are as follows: Alite (Tricalcium Silicate-C3S), Belite (Dicalcium Silicate-C2S), C3A (Tricalcium aluminate), C4AF (Tetracalcium Alumina Ferrite) Grinding becomes difficult when Alite/Belite rate and (Alite+Belite)/(C3A+C4AF) decrease in clinker phases.
Figure 2: Clinker View Under Microscope
Clinker view under microscope: Crystals with brown corners are alite crystals. Blue circular crystals are belite crystals. Frays seen around the circles are called fingertype and they indicate that alite crystals transformed into belites by decaying as a result of slow cooling.In addition, the sizes of crystals are also assessed. Crystal size is effective in grinding energy.
Alite may range between 40-70% of the clinker mass. It reacts with water, and it is the phase which controls cement strength and hydration temperature. If the cooling speed is not too slow under °C, it can preserve its stability down to normal temperatures. At very slow cooling speeds, however, some parts of alite dissolve and belite forms. This is the component with high hydration capacity as colorless crystals provide the first high strength in clinker. The quality of cement is measured by its alite concentration. Depending on cooling degrees, clinker’s alite rate reaches 59.8% in slow cooling, 65.2% in fast cooling, and 70% in express cooling.
Belite may range between 15-45% of the clinker mass. It has a circular crystalline form. Its crystal size is between 5-40 μm. Belite is less reactive compared to alite and contributes to strength at later ages. It is a hydraulic binder which hardens slowly.
It may range between 1-15% of the clinker mass. It is in cubic or orthorhombic form. Its crystal size varies between 1-60 μm. Its reaction with water is very fast and amorphous in structure. Hydration temperature is very high. It hardens immediately with water.
It may range between 0-18% of the clinker mass. Its crystalline structure is dendritic and prismatic. It presents a thin and long view in the form of a sword in the crystalline section.
Clinker silicate crystals’ grain size may be put forth by calculating equivalent diameters. Equivalent diameter is determined by calculating the arithmetic mean of the longest and shortest lengths which pass through the center of gravity of the cross-sectional area in a crystal cross-section. Alite crystals with equivalent diameters of 15-20 μm have positive impacts on the ease of grinding of clinker as well as the early strength of the cement.
Belite crystals with equivalent diameters of 25-60 μm may be a result of insufficient grinding of quartz grains in the raw mixture or of heating-cooling regime.
In summary, examining grain size distributions of silicate crystals through polarizing optic microscope and image processing software would guide the manufacturer for the raw material choice and optimization of heating-cooling regime.
In Rapidly Cooled Clinkers: There are cracks on the alite crystals in hexagonal crystal structure which are formed due to thermal pressing during express cooling process. These cracks increase hydraulic activity and grindability capacity. Crystal ends are sharp in a clinker which was properly cooled.
The size and shapes of pores (porosity) provide information about sintering conditions in microscopic examinations of the clinker. High porosity and large, long, and interconnected pores may indicate that the clinker was not adequately sintered. Small and round pores, on the other hand, are a sign of a good sintering.
Alite Phase Amount and Grindability: Job index value (kwh/ton;) decreases with the increase of alite amount in the clinker. This shows us that the energy amount required for grinding decreased.
Figure 3: The Relationship Between the Amount % of Alite Phase (C3S) in Clinker and Grindability of Clinker
Belite Phase Amount and Grindability: When belite amount is below 20% in the clinker, the energy demand required for grindability decreases with the increase in belite %. However, in case there is a certain amount of belite (around 22%), the grindability energy demand increases once again.
Figure 4: The Relationship Between the Amount % of Belite Phase (C2S) in Clinker and Grindability of Clinker
Intermediate Phase (C3A and C4AF) Amount and Grindability: With an increase in the intermediate phase of more than 6 – 6.5 %, energy requirement (kwh/ton) for grindability is observed to decrease.
Figure 5: The Relationship Between the Amount % of Intermediate Phase (C3A+ C4AF) in Clinker and Grindability of Clinker
Porosity and Grindability: In contrast to literature, the required energy demand for grindability was observed to increase in parallel with the increase in porosity amount in the clinker. The reason for this is seen when porosity’s fineness module is examined.
Figure 6: The Relationship Between the Amount % of Porosity in Clinker and Grindability of Clinker
Table-1: Comparison of Silicate Phases and Porosity’s Fineness Modules in Clinker Samples with Job Indexes
Grindability becomes easier with high alite content in the clinker, and it becomes more difficult with high belite content. High amount of liquid phase makes grindability of clinker easier.
Cement behavior may be altered by conducting studies on clinker chemistry and mineralogical phases during clinker production process. Therefore, clinker grinding energy varies as well depending on the phase structure. Energy recovery may be obtained in grinding by bringing the phase structures to the desired levels.
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