Particle Size and Shape Analysis in CEMENT INDUSTRY.

Transcription

1 Particle Size and Shape Analysis in CEMENT INDUSTRY

2 Generalities Cement is made from a mixture of limestone (CaCO 3 ) and clay (silica and alumina) and minority constituents such as iron oxide in a rotary kiln heated to145 C. The resulting product is known as clinker. The cement is obtained by grinding the clinker with gypsum (CaSO 4 ). The steps involved in the manufacture of cement are shown in figure 1. RAW MATERIALS 8% limestone (CaCO 3 ) 2% clay (SiO 2 -Al 2 O 3 ) Additives : bauxite (Al 2 O 3 -Fe 2 O 3 ), iron oxide (Fe 2 O 3 ) Grinding <2 µm RAW MIX Lime (CaO) 65 to 7% Chemical composition (by weight) Silica Alumina Ferric oxide (SiO 2 ) (Al 2 O 3 ) (Fe 2 O 3 ) 18 to 24% 4 to 8% 1 to 6% Heating to 145 C 4 main crystalline phases Symbolic notation Name Chemical formula %average by weight CLINCKER C3S C2S Tricalcium silicate or alite Bicalcique silicate or bélite 3 CaO, SiO 2 2 CaO, SiO Grinding <1 µm with gypsum C3A C4AF Tricalcique aluminate Tetracalcium alumino-ferrite 3 CaO, Al 2 O 3 4CaO, Al 2 O 3, Fe 2 O CEMENT Clinker + other possible constituents : fly ash, silica fumes, calcium carbonates Figure 1: Cement manufacturing steps [1] Why analyze the particle size and shape? The particle size of the cement is controlled at the grinding stage, a very energy-consuming step. Properly managing the grinding time is consequently an important economic consideration and may be carried out at different steps in the cement manufacturing process (figure 2). The steps in manufacturing cement and particle size analysis The physical and chemical properties and, in particular, the hydration kinetics of the cement depend on the size of the particles and their specific surface. Too high kinetics can lead to a too intense exothermic reaction that can lead to an instability of the cement, which, during the cooling phase, will lead to the formation of fissures or a degradation in the mechanical properties. PSA PSA PSA RAW MATERIALS Grinding <2 µm RAW MIX Heating to 145 C CLINCKER Grinding <1 µm with gypsum CEMENT Figure 2: The need to characterise particle size in cement manufacturing steps PSA : Particle Size Analysis [1] Brochure «Collection Technique» CIMBETON B51

3 Specific surface area determination The specific surface area is the value of the surface area of the particles per unit of mass. It is measured in the cement industry by the Blaine Test, also known as the air permeability test, and which is based on the Kozeny-Carman relation. The handling enabling the measurement of the Blaine value is long but it can be deduced from the measurement of the particle size according to the following relation: d S = 6 d ρ S k 3 nidi i= 1 = d[ 3,2] = k 2 nidi i= 1 ρ s is the density of the material d is the mean diameter of the particle size distribution (Sauter) n i is the number of particles for the i st size class d i is the mean diameter of the i st size class k is the number of the size classes The correlation between the surface area measured by the Blaine method and by laser diffraction is very good for the majority of cements, the specific surface of which is between 2 and 6 m 2 /g [2] (figure 3). 7 6 S,Laser (cm 2 /g) S,Koz (cm 2 /g) Figure 3: Specific surfaces measured by air permeability and laser diffraction Particle size distribution with Rosin Rammler law Rosin Rammler law is a useful model to represent ciment particle size distribution. Mathematical equation can be written as follow : Di R(D i) = 1 exp D A i is the diameter for the i st class (µm) A and B are parameters calculated using the least squares minimisation method. B is a measure of the particle size dispersion (size range) of the powder The peak of the frequency curve for n)1 is at (1/e)=36.8% and denoting the mode by x m in the relation gives b=1/x m The sieve apparture for R=36.8% is used to characterize the degree of comminution (grinding) of the material B [2] Septième édition des Journées scientifiques du Regroupement francophone pour la recherche et la formation sur le béton (RF)2B Toulouse 19-2 Juin 26

4 Morphological analysis The grinding of the cement leads to the formation of particles of specific morphology and which have, in particular, sharp edges (figure 4). Figure 4: Modelled views of two particles of a certified cement of equivalent diameter equal to a) 36 µm and b) 15 µm [3]. The graph in figure 5.a shows that the particles of cement have a shape that differs from sphericity. This conclusion may also be made for particles whose image is a 2D projection (figure 5.b) Cement Spherical particles y = x.5523 Perimeter (µm) y = x Surface area (µm 2 ) a) b) Figure 5: a) Relation between Volume and Surface [3] and b) Surface and Perimeter for spherical particles and cement This difference is especially more marked for particles of small size (figure 6). In the case of non-homogeneous materials, which include cement, the fine particles resulting from the fragmentation of the larger particles are not subjected to the grinding process. Consequently, only the largest particles are ground, which leads to them becoming more spherical. Figure 6: Relation between Surface and Size of cement particles [3] E.J. Garboczi and J.W. Bullard, Cement and Concrete Research, 34 (1), (24)

5 Separation efficiency curve or Tromp curve The grinding of the cement is generally followed by the sorting of particles by means of a cyclone. The particle size distribution of a cement may be described as in figure 7, split up into a population of fine and coarse particles. Particle size analysis then makes it possible to evaluate the efficiency of the sorting by means of the separation efficiency curve T(x) (figure 8). Figure 7: Differential distributions weighted for the filler material, q r,s (x), the fine fraction, v r,f q r,f (x), and the coarse fraction, v r,c q r,c (x) [4] Figure 8: Ideal separation efficiency curve T(x) [4] How to analyse? Particle size range Particle size distribution comprised between 1 et 1 µm. The D 5 is generally situated between 5 and 5 µm. Dry mode Vibration frequency : 4-6 Hz Cyclical ratio amplitude : 4-6 % Air pressure : 25-5 bars Liquide Mode Carried liquid : Isopropylique alcohol, ethanol, 1-butanol, 2-butanol Dispersant : none Ultrasounds : 6s during dispersion Mathematical model : Mie with n=1,68,1i [5] Notes In the case of a measurement by liquid process in an alcohol, the user has to envisage the use of suitable tubes and a pump unit adapted to these tubes. The use of an alcohol re-circulator may be envisaged for the regeneration of the solvent used and to limit its consumption. [4] Norme ISO :21, Representation of results of particle size analysis - Part 4 [5] Cement, Concrete, and Aggregates, Dec. 24, Vol. 26, No. 2 Measurement of Particle Size Distribution in Portland Cement Powder Analysis of ASTM Round Robin Studies

6 Particle size analysis in liquid mode Carried liquid : Ethanol Dispersing agent : none Ultrasounds : 6s during dispersion Special diameters of the particle size distribution shown in figure 9: - D 1 = 2.97 µm - D 5 = µm - D 9 = µm Volume (%) Butanol 2-Propanol WATER Particle size (µm) Volume (%) Particle size (µm) Figure 9: Particle size distribution obtained in liquid mode

7 Particle size analysis in dry mode Vibration frequency = 5 Hz Cyclical ratio amplitude = 5% Air pressure : 5 mbars Special diameters of the particle size distribution shown in figure 1 : - D 1 = 1.85 µm - D 5 = 16.2 µm - D 9 = 49.4 µm Volume (%) Particle size (µm) Figure 1: Particle size distribution obtained in dry mode

8 Shape parameters adapted for morphological analysis An example of characteristic image is shown in figure 11. This image was obtained by optical microscopy with x4 magnification. 2 µm Figure 11: Characteristic image of a sample of cement obtained by optical microscopy The morphological parameters that may be used for this application are: Circularity Ratio Proportional to the ratio of the area over the perimeter squared. In the case presented CR=.78. This parameter highlights the effect of the perimeter in relation to the surface area. A CR = 4 Π 2 P Sphericity Ratio of the radiuses of the inscribed and circumscribed circles. R Sphericity = R inscribed circonscribed R circonscribed In the case presented, Sphericity =.54. This parameter enables the deviation from circularity to be quantified. R inscribed