1、Application Background
Cement is a powdery hydraulic inorganic gel material that is stirred with water to form a slurry that hardens in air or better in water and binds sand, stone and other materials firmly together. It is an important basic raw material for the construction of national economy. Cement industry is widely used in civil engineering, water conservancy, national defense and other projects.
Cement according to the production of different raw materials, finished products are divided into Portland cement, ordinary cement, slag Portland cement, volcanic ash cement, fly ash cement, and composite Portland cement, they often have different characteristics and use scenarios because of the composition of different.
The quality of cement products is determined by the composition of mineral phases rather than elemental composition. XRD is an effective method for analyzing the phase composition of raw materials, raw materials, clinkers and cement, while providing information that cannot be obtained by other methods. Therefore, the application of portable XRD in cement industry has great advantages and prospects:
1, direct and accurate quantitative and qualitative analysis
2, without making the prototype (boiled method cement stability should try to make bread or specimen), without calibration
3, without the distraction of instrument and human factors
4, automatic on-line inspection
XRD is used in cement production process involving raw materials, raw materials, clinkers and cement products. At the same time in the quality control (such as incoming material testing, cement product identification, etc.) and research and development (cement formula optimization, cement basic discipline research) have great use.
Cement raw materials mainly include limestone (the main material, provide CaO), clay raw materials (provide SiO2, Al2O3 and a small amount of Fe2O3), correction raw materials (supplement some inadequate ingredients), auxiliary raw materials (mineralizer, cosolvent, grinding agent) and so on. Generally speaking, the quality of limestone in the manufacture of cement raw materials accounts for 80%, which is the main manufacturing material of cement. Cement production process is generally divided into raw material preparation, clinker firing, cement grinding and other three processes.
Cement Raw Material Preparation
Calcareous raw materials, clayey raw materials, and a small amount of correction raw materials, after crushing or drying, according to a certain proportion of coordination, grinding, and preparation of suitable composition, uniform quality of raw materials;
Clinker Firing
The raw material was added into the cement kiln and calcined until it was partially melted to obtain cement clinker with calcium silicate as the main component.
Cement Grinding
The clinker is ground to cement with a mixture of gypsum and sometimes mixed materials.
Raw material preparation involves calcareous raw material, clayey raw material, and a small amount of correction raw material (supplemented with SiO2, Al2O3 and Fe2O3). The purity identification of important components in raw materials is very important to ensure the quality of cement. Therefore, XRD can be used to achieve qualitative and quantitative identification.
2、Cement Raw Material Preparation
The most important link in raw material preparation is the guarantee of raw material quality. Therefore, it is very important to identify the chemical composition and purity of key phases of calcareous and clayey materials.
Calcareous Raw Material
Calcareous raw materials are the general term for raw materials that provide calcium carbonate (CaCO3) for cement. Its main component is CaCO3. Divided into natural calcareous raw materials and industrial waste slag two categories. The commonly used natural calcareous raw materials are limestone, marl, chalk, shell, coral, calcium material ginger stone, calcareous pebbles, etc. The industrial waste slag includes calcium carbide slag from chemical plant, sugar filter mud from sugar factory, white mud from paper factory and alkali residue from alkali factory by chlor-alkali method, etc. The main component of CaO after calcination is CaO, and proper technological conditions and the influence of impurities should be paid attention to when using it.
Clayey Material
Clayey raw materials are the general name of raw materials that provide SiO2, Al2O3 and a small amount of Fe2O3 in cement clinker ingredients. Natural clayey raw materials include loess, clay, shale, mudstone, siltstone, river mud and so on. Among them, loess and clay are most used, and the main minerals are one or more of kaolinite, montmorillonite, illite and hydromica. Industrial waste slag such as red mud of aluminum plant, fly ash of power plant, blast furnace slag of ironmaking plant, coal gangue of coal mine, etc., can also be used as clay material of cement production.
3、Clinker Calcined
Clinker firing process involves preheater, calciner, rotary kiln three important equipment. XRD analyzer is very important in various equipment applications.
3.1 Preheating Decomposition Stage - hot raw material
The raw material prepared at this stage needs to be preheated by a repreheater. The appearance of pre-decomposition technology is a technological leap in cement calcination process. It is in the preheater and rotary kiln between the addition of decomposing furnace and the use of kiln tail flue, fuel injection device, so that the heat release process of fuel combustion and raw material carbonate decomposition process, in the decomposing furnace to suspend or fluidized state rapidly, so that the raw material into the kiln decomposition rate increased to more than 90%. The carbonate decomposition task originally carried out in the rotary kiln is moved to the decomposing furnace. Most of the fuel is added from the calcination furnace, and a small part is added from the kiln head, which reduces the heat load of the calcination belt in the kiln, prolongs the life of the lining material, and is conducive to large-scale production. Due to the uniform mixture of fuel and raw material, the fuel combustion heat is transferred to the material in time, so that the combustion, heat transfer and carbonate decomposition process are optimized. Therefore, it has a series of excellent performance and characteristics such as high quality, high efficiency and low consumption.
The application of XRD analyzer in this preheating decomposition stage is as follows:
3.1.1 To reduce consumption of energy
The preheater can display the decarbonization efficiency and energy loss through the degree of decarbonization (CaCO3→CaO+CO2). If a CaO or clinker dust cycle is detected in the preheater using XRD, the presence of albialite, tricalcium silicate (C3S) can be warned of energy loss. The decomposition of CaCO3 is mainly carried out in the decomposing furnace. If CaO and its content are detected in the preheating equipment in the previous process section, the energy loss can be judged. If a certain amount of CaCO3 is detected in the decomposing furnace, it is proved that it is not completely decomposed in the equipment, which can show the degree of decarbonization of CaCO3 in the decomposing furnace.
From the above test results, XRD can give preheating efficiency of preheater, decomposition efficiency of calciner and energy loss warning in the preheating and decomposition stage of raw material, and provide powerful data support in the optimization of this process section.
3.1.2 Improve Clogging Prevention
Ca5(SiO4)2CO3 is a typical blocking mineral in the preheater. Salts such as anhydrous potassium and magnesium alum (Ca-K2Mg2(SO4)3) and potassium salt (KCl) are indicators of fluidity. See Figure 1. If the XRD analyzer detected above material in the preheater, can give congestion early warning in time, avoid congestion by production appeared in the process of production efficiency and safety hidden trouble. At the same time, the phase of blocked minerals in different locations can be given, which points out the direction for improving the blockage prevention and subsequent descaling process Settings.
3.2 High Temperature Calcination Stage -- Clinker
The four main minerals in cement clinker are tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetraalcium ferroaluminate (C4AF). During the clinker firing stage, the XRD analyzer will be used to control the production based on the real physical phase to ensure the purity and quality of clinker.
3.2.1 Check and Control Kiln Operation - Clinker Quality Control
XRD can quickly qualitative and quantitative determination of cement clinker tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetraalcium ferroaluminate (C4AF), provide the percentage content of their components, ensure the quality control of clinker, give the "inspection and control kiln operation" tips. If the cement content proportion or contains not involved in high temperature solid reaction CaO, it is necessary to check or control the kiln operation - flame shape, firing temperature, firing time, etc.
3.2.2 Analysis of Free Calcium Oxide
Free calcium oxide (F-CAO) refers to calcium oxide present in a free state in clinker that has not been absorbed. In the calcination process of cement clinker, most oF CaO can synthesize C2S,C3S,C3A, C4AF and other minerals with acid oxide. However, due to the influence of raw material composition, raw material fineness, raw material uniformity and calcination temperature, There is still a small amount of CaO can not combine with acidic oxides such as SiO2,Al2O3 and Fe2O3 to form minerals and exist as free oxides (F-cao). Free state CaO will directly affect the stability of cement, cement products deformation or cracking, resulting in the destruction of cement slurry. Therefore, it is very important to determine the content of free calcium oxide in clinker to control cement production and ensure the quality requirements of cement.
For external calciner kiln, it has raw material homogenization facilities, cyclone preheating system, higher kiln speed, three air channels or four air channels coal pipe and other technical measures, so that the ability to control F-CAO is far higher than other kiln types, calcining low F-cao clinker is its advantage. At the same time, it must be made clear that the only means of f-CAO content control by the central control operator is the shape of the flame and the calcination temperature.
Therefore, using XRD to analyze and detect fCaO, can achieve the purpose of clinker quality control.
3.2.3 Polycrystalline analysis of C3S and C3A
3.2.3.1 Polycrystalline analysis of tricalcium silicate (C3S)
In cement clinker, the main phase C3S has two monoclinic crystal types (M1 and M3), both of which have different hydraulic properties, such as strength development. Monitoring their hydration properties shows that the strength of M1 modified cement is 10% higher than that of M3 modified cement.
We can now distinguish these homogeneous polycrystals using an XRD analyzer, which allows us to understand and predict the early strength development of cement.
3.2.3.2 C3A polycrystalline analysis
The two crystal types of C3A are cubic crystal and orthogonal crystal. Influence of C3A crystal type on cement properties: setting time, water consumption and early strength change. Cubic type C3A reactivity is orthogonal type low C3A and cubic type C3A hydration reaction more slowly, and the orthogonal type C3A faster, so type cubic C3A have lower early strength, orthogonal type C3A has higher early strength. The ratio of cubic C3A to orthogonal C3A affects the water consumption of cement, and the orthogonal C3A consumes more water. The content of cubic TYPE C3A and orthogonal type C3A can be quantitatively detected by XRD analyzer, and their proportion relationship can be calculated, so as to infer the setting time of cement, water consumption and early strength changes and other parameters, for the actual construction to provide a powerful data basis.
3.2.4 Microphase analysis (quartz, magnesium oxide, sulfate)
The volume stability of cement refers to the uniform function of the volume change in the hardening process of cement. There are many impurities in cement and uneven deformation will occur.
3.2.4.1 Sulphate
A certain amount of sulfate in clinker has a certain effect on the sulfation of C3A. Some research results show that with the increase of sulfate content, namely, the deepening of sulfur acidification, the content of cubic C3A increases while the content of orthogonal C3A decreases. This provides data support for adjusting the crystal type of C3A and its cubic C3A/ orthogonal C3A ratio, which can guide the control and optimization of sulfate in raw ingredients.