Cement is a kind of powdery material. When properly mixed with water, it will turn into slurry. The slurry will gradually harden in air and glue together the granular or fibrous materials such as sand and stone firmly. It is widely used in all aspects of our lives, such as subway construction, bridge construction, and residential building construction. It is an indispensable part of our city.
The production process of silicate cement (also known as Portland cement) is representative in cement production. It usually adopts limestone and clay as main materials. After been crushed, proportioned and ground into appropriate granularity, most of the raw materials will be fed into cement kiln for calcining clinker, and then we usually add an appropriate amount of gypsum (sometimes mixed with other materials or additives) in the cement grinding process, finally obtaining the cement products with a qualified fineness. At cement plant, according to different raw materials preparation methods, cement manufacturing can be divided into the dry process (including semi-dry process) and wet process (including semi-wet process). Next, we will discuss the wet process of cement manufacturing in details.
Wet Process and Dry Process
The wet process of cement manufacturing refers to grinding raw material into slurry after mixing with water and then feeding them into the wet process kiln for drying and calcination and finally forming clinker. The slurry’s water content is usually between 32%-36%. In addition, the raw material slurry can also be dehydrated into raw material blocks and put into the kiln to calcine clinker. This method is called the semi-wet process, which still belongs to the cement wet process production.
Advantages: the wet process of cement production has the characteristics of simple operation, low dust and easy conveying. Because the slurry has fluidity so that its homogeneity is good and the quality of clinker is improved. What’s more, the energy consumption of raw material grinding in the wet process is reduced by nearly 30%.
Disadvantages: the heat consumption of the wet process is too high, usually between 5234-6490 J/kg and the consumption of ball mill vulnerable parts is also large. Compared with other processing methods, the clinker manufactured by the wet process has a low temperature when it comes out of the kiln, so this method is not suitable to produce the clinker with a high silica rate and high aluminum-oxygen rate.
The dry process of cement manufacturing means that after raw materials with different particle sizes are dried, broken and ground into powders of certain fineness, they will be sent into the dry process kiln for calcining, finally forming clinker. Besides, the raw material powder can also be made into raw material balls by adding a proper amount of water and then be directly sent to the Lepol kiln for calcining. This method is called a semi-dry process, which belongs to the cement dry process production.
Advantages: as the dry process is to directly feed raw material powder into the rotary kiln for calcination, and the moisture content of raw materials is about 1% – 2%, it saves the heat consumption needed for the moisture evaporation. Therefore, this method has the advantages of energy-saving, high production efficiency and stable output, which can meet the production needs of large cement plants. At the same time, there is less sewage discharged in the dry process cement production. It is conducive to environmental protection. Nowadays, we call the production line with preheater and precalciner as the new dry process cement production line, which is the development direction of dry process cement manufacturing in the future.
Disadvantages: the disadvantages of dry process are the inhomogeneous composition of raw materials, high power consumption and the high dust content in the workshop.
Procedures of the Wet Process
The procedures of the wet process are basically the same with the dry process, which can be divided into three stages: raw materials preparation, clinker calcination, and the cement grinding. All of these stages are covered in the article What You Need to Know about Portland Cement Manufacturing Process we mentioned before.
Similar to the dry process, materials also need to undergo quarrying, primary crushing, secondary crushing, proportioning and grinding in the raw materials preparation stage of the wet process. The biggest difference between the two methods is that in the wet process, water is usually required as a process media added in the raw mix to form slurry. After mixing and blending, the slurry will be stored in the slurry tank waiting for further processing. While in the dry method cement production line, the raw mix doesn’t need water.
In the calcination stage, the cement kiln used by the wet process is longer in comparison to the dry process, and there is no preheater and precalciner in front of the kiln. The temperature in cement kiln can reach 1400-1500℃, slurry in it is heated and dried and finally forming the clinker compounds, namely Di-calcium Silicate, Tricalcium Silicate, Tri-calcium Aluminate and Tetra Calcium Alumino-Ferrite. Clinker is a kind of particle with a variety of size and dark green color. After cooled down in the grate cooler, they will be sent into the grinding mill for the last processing.
In the last stage, clinker will be ground into qualified fineness in grinding mills. During this process, we usually add some gypsum and other materials into clinker to give the final cement product different properties and usages. For example, we add gypsum to obtain the ordinary Portland cement and add gypsum and fly ash to obtain the Pozzolana Portland Cement.