Cement is a kind of powdery hydraulic inorganic cementitious material, which is the most basic and widely used building material in modern society. Cement can be hardened in the air or in water, and can firmly bond sand and stone together. It is widely used in civil engineering, water conservancy, national defense, and other projects.
The most commonly seen and used cement is portland cement, or ordinary portland cement (OPC). This type of cement was invented in the early 19th century by Joseph Aspdin. It is named ‘portland cement’ because its property is similar to the stone quarried on the Ise of Portland, England.
The making process of portland cement in the modern industry can be divided into the wet process, dry process, and semi-dry process. Nowadays, the dry process is the most popular cement making process which is widely adopted by cement plants all over the world for its great advantages in energy saving and environmental protection.
Cement Making Process
The cement making dry process includes six phases:
- Raw material extraction/ Quarry
- Proportioning, Blending, and Grinding
- Pre-heater Phase
- Kiln Phase
- Cooling and Final Grinding
- Packing & Shipping
Phase I: Raw Material Extraction
The raw materials needed for cement production mainly include limestone (main material, providing CaO), clay materials (providing SiO2, Al2O3 and a small amount of Fe2O3), correction raw materials (to supplement some insufficient ingredients), and auxiliary raw materials (such as mineralizer, cosolvent, grinding aid), etc. Generally, the limestone accounts for 80% of cement raw materials, which is the main cement manufacturing material.
Cement plants are usually built near the quarry of limestone so the quarried limestone can be transported to the cement plant directly by belt conveyor or other conveying systems. If the distance between the quarry and the cement plant is too long to use conveyors, the limestones will have to be transported by trucks, which means higher transportation fees and inconvenience. The short distance between the cement plant and the quarry saves transportation fees and fuel costs and makes the cement making process economical.
Apart from limestone, there are also other raw materials used in the cement making process, such as clay, fly ash, iron ore, and coal. The need for these raw materials in cement production is relatively small so it is OK to just buy them from a supplier.
Before raw materials being transported to the cement plant, they are crushed into smaller size pieces by the crusher at the quarry. Compared to large pieces, raw materials in smaller pieces are easier to be loaded and transported, and more convenient for subsequent processing.
Phase II: Raw Material Proportioning, Blending, and Grinding
The samples of limestones from the quarry are first sent to the laboratory of the cement plant, where they are tested and analyzed for proportioning. The proper proportioning of limestone and other raw materials is a necessary job before the beginning of grinding. The proportioning of cement raw materials is not all the same but should be determined according to the actual situation. The proportion of raw materials of different specifications of cement is also different. Generally, the proportion of components in cement raw materials is 67-75% limestone, 10-15% clay, 0.5-1.5% iron ore and 8.5-11% coal.
With the help of vertical roller mills or other types of crushers, the raw materials are blended and further ground into smaller pieces of raw meal in the cement plant. In a roller mill, the rotating roller rotates continuously on the grinding table and crushes the material to fine powders, and finishes the job. The raw mix is stored in a pre-homogenization pile after it was ground to fine powders.
Phase III: Raw Meal Preheating
After being crushed by the raw mill, the raw meal is ready to enter the preheater. The cyclone preheater is one of the core equipment for dry process cement production, which consists of a series of vertical cyclones locate on several stages. Inside the cyclones, raw meal meats with the exhaust gas emitting from the rotary kiln and performs suspension heat exchange with it. The suspension pre-heating process helps cement plants save energy and reduce environmental pollution.
Phase IV: Calcination
The calcination is the core phase of the cement making dry process. The calcination of the preheated raw meal takes place in the rotary kiln of the cement plant. The rotary kiln is a huge rotating furnace in which the raw meal is heated up to 1450 ⁰C and turned to clinker.
The high temperature in the rotary kiln initiates a series of chemical reactions between calcium and silicon dioxide compounds, which eventually turn the raw meal into cement clinker.
The heat source of the rotary kiln is the flame from the burner located on the kiln’s front end. Using natural gas or coal as fuels, the burner shoots high-temperature flames to calcine the raw meal. After calcination, the cement clinker will enter a cooler for cooling.
Phase V: Clinker Cooling & Final Grinding
The hot clinker discharged from the rotary kiln is cooled in the grate cooler by forced air, which is extracted from the outer atmosphere by grate cooling fans. The temperature of the clinker after being cooled drops from 1350 – 1450⁰C to around 120⁰C. The cooled clinker is then transported to clinker silos or hoppers directly by conveyors for storage and later the cement grinding process. The hot air in the cooler is recirculated back to the rotary kiln for reuse, which further saves the energy consumption of the cement plant.
The cooled clinker is then fed to cement mills for final grinding. Many factories nowadays prefer to use ball mills for cement grinding, since this kind of mill has small product particle size distribution and is easy to operate.
The cement ball mill is a horizontal cylinder filled with steel balls or other grinding media. Inside the cylinder, the steel balls are rotated and tumbled and crush the clinker into very fine powders, which are product cement powders. During the grinding process, a small percentage of gypsum is added to the mix to control the setting time of the cement.
Phase VI: Storage & Packaging
The product cement discharged from the cement grinding mills is conveyed to the cement storage silos. Further, it is packed into bags or shipped to customers in bulk quantities by trucks, rail freight wagons, or ships. The most popular cement packing machines are roto-packers which can pack cement fast and in large quantities.