Development of a Robust Quality Control Procedure to Measure Particle Size Distributions in Fly Ash

Transcription

1 1 Development of a Robust Quality Control Procedure to Measure Particle Size Distributions in Fly Ash J H Potgieter 1, S S Potgieter 2, R A Kruger 3 and J Krüger 4 1. Department of Chemical & Metallurgical Engineering, Technikon Pretoria, Private Bag X680, Pretoria, 0001, South Africa 2. Department of Chemistry & Physics, Technikon Pretoria, Private Bag X680, Pretoria, 0001, South Africa 3. Ash Resources, P O Box 3017, Randburg, 2125, South Africa William Drive, Silverton, 0184, South Africa ABSTRACT This paper describes an alternative quality control approach to laser diffractometry particle size measurement. The investigation was prompted by the need to find a robust way to discriminate between various particle size fractions obtained in a fly ash classifier plant by unskilled operators. Different fly ash particle sizes were placed in glass jars of varying volumes and masses and rolled down a fixed incline unto a horizontal plane. The effect of different incline angles, jar sizes and masses of each fly ash particle size fraction on the distance traveled on the horizontal plane were investigated. It was found that a unique relationship exists between the distance traveled by a specific mass of material at a constant incline angle and the size fraction of the fly ash in a given flask/ jar. It was thus proven that this approach could successfully render a robust plant quality control procedure. INTRODUCTION The rate of a reaction in any solid is governed by its available surface area. This is of particular importance in powdery materials, and therefore the measurement of particle size and the subsequent surface area due to it, forms a routine measurement in most industries producing and utilising such materials. It is especially important for mineral admixtures like fly ash, silica fume and slag that are used in the building and construction industry that their particle sizes be controlled closely, because it can have a major influence on the physical and mechanical behaviour of the cement mortars and concrete produced from it.

2 2 Fly ash is a byproduct of coal combustion in the power generating industry. South Africa currently produces approximately 30 million tons of fly ash annually, of which only about 1.2 million tons are consumed again. The majority of the utilised fraction goes into cement and concrete manufacturing, while the very fine fractions are utilised as fillers for paint and polymer production. The separation of the fine fraction of fly ash, which is called cenospheres, from the bulk unclassified portion obtained from the electrostatic precipitators of the power station, is accomplished through the use of a series of cyclones and results in a number of fractions with a narrow size range. The preferred method of particle size analysis nowadays is through measurements on dedicated instruments using laser diffraction methodology [15], as well as sedimentation technology. While these methods yield very reliable results, they are expensive and require welltrained technicians to ensure smooth and errorfree operation. In an unsophisticated economy of a developing country where one often has to rely on semiskilled labourers to perform analyses, it is preferable to have a more robust and cheaper way of measurement. If one does use sophisticated equipment, it is advisable to have a backup procedure to validate results or to use in cases of breakdown of automated equipment. Sieving can be used for this purpose, but is not suitable with very small particle size materials and becomes unreliable at particle sizes < 45 µm. The major aim of this investigation was therefore to develop a robust method of particle size measurement for use by semiskilled operators to control particle size separations by classifiers/ cyclones in a fly ash plant. A further aim was to have a backup analysis procedure available in cases of malfunctioning of a laser diffractometer measuring apparatus. This paper will describe the approach adopted to achieve these aims, the alternative equipment developed and the results obtained. EXPERIMENTAL PROCEDURE Materials: Cenospheres with various particle size distributions were supplied by Ash Resources (Pty) Ltd. after separation in a fly ash plant by a series of cyclones. The materials investigated are noted in Table 1.

3 3 Table 1. Materials used in the investigation Sample Description Explanation /45 Cenospheres with an average particle size of 5 µm Cenospheres with an average particle size of 15 µm Classified fly ash with 85 % < 45 µm particles Tailings from the ash dump Ordinary Portland Cement Equipment: A laser particle size analyser, Malvern Masterizer 2000, was used to measure the particle size distributions of each material. A 1% Calgon solution was employed as a dispersing agent during the measurements. The chemical analyses of the different fly ashes and were done on a Bruker SRS 3300 XRF spectrometer after fusion of the different materials with lithiumtetraborate flux [6]. Proposed alternative method: A rolling table with a partly adjustable incline piece was constructed from hardboard and mounted on a steel frame. It was varnished with ordinary wood varnish to present a smooth surface along which movement could take place. Glass jars of various sizes were filled with different masses of the various materials and rolled from a fixed position on the inclined part down the rolling table. The positions where the jars came to a stand still on the flat part of the table were recorded and noted. Each measurement was performed three times and the values obtained were averaged. It was found that the values measured rarely differed by more than 5 mm in each particular case. The constructed rolling table is shown in Fig. 1. The influence of a number of variables on the distance that the jars rolled down from the inclined part onto the horizontal level of the rolling table was investigated. These include: (1) The effect of the jar size (2) The effect of the sample mass in each jar size (3) The type of material (4) The effect of various particle sizes of the fly ash (5) The effect of the angle of inclination.

4 4 Fig. 1 Rolling table used in the investigation. RESULTS AND DISCUSSION The chemical composition of the various different fly ash fractions was remarkably constant, and the mean composition thereof and of are summarized in Table 2. Table 2. Main element composition (% m/m) of the different fly ash fractions and used in the investigation Element as oxide Fly ash SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO TiO The particle size distributions of the various materials as measured by the Malvern instrument, are shown in Figs. 26. The most important characteristics, such as the density of each material, the mean particle diameter (D 50 ), and maximum particle size of 90% of the particles (D 90 ), which can be deduced from the graphs, are presented in Table 3.

5 5 Table 3. Important physical parameters of the various fly ash fractions and used in the investigation Sample D 50 (µm) D 90 (µm) Relative density (g/cm 3 ) / Fig. 2 Particle size distribution of cenospheres used. Fig. 3 Particle size distribution of cenospheres used.

6 6 Fig. 4 Particle size distribution of /45 cenospheres used. Fig. 5 Particle size distribution of cenospheres used. Fig. 6 Particle size distribution of used.

7 7 From the curves shown in Figs. 26 and the data summarized in Table 3, the following conclusions can be drawn: (1) The relative density of the Plasfill products increase with increasing mean particle size. (2) The cement has a higher relative density than any of the fly ash fractions. (3) The mean particle size D 50 of the cement is only smaller than that of the coarsest fly ash fraction,. One of the first important observations made in this investigation, was that the compaction of the material inside the jar had an effect on the distance that it rolled along the horizontal part of the table. The difference in distances that the jar rolled when it was shaken after each roll, only at the start of the rolling tests or only after every 5 th roll down the incline, can be seen very clearly in the data summarized in Tables 4 & 5. Table 4. The effect of shaking on the rolling distance of a jar down a 15 incline onto a horizontal table. Material Volume of jar (ml) Mass of material (g) Distance traveled (mm) when shaking At the start After each roll / /

8 8 Table 5. The effect of the frequency of shaking on the rolling distance of a jar down a 15 incline onto a horizontal table. Material Volume of jar (ml) Mass of material (g) Distance traveled (mm) when shaking After each roll After every 5 th roll / / / The results in table 4 and 5 indicate that frequency of shaking the material inside the jar has a very definite effect on the distance that the jar travels along the horizontal plane after rolling down an incline. This can be ascribed to compaction and/or agglomeration of the powdery material inside the jar due to the rolling motion. It is obvious that when the jar is shaken at the start of each test, that a larger distance is traveled than when it is only shaken at the beginning of each series of recordings or only after the 5 times it has rolled down the incline. All subsequent tests were consequently carried out by shaking the jar and its constituents every time before it was allowed to roll down the incline. The effect of the sample mass and type of material on the distance that the various jars rolled down a fixed incline of 14 is given in Table 6.

9 9 Table 6. Effect of jar size and sample mass on the distance traveled along the horizontal table from a 14 incline. Material Volume of jar (ml) Distance traveled (mm) with masses of 10g 20g 40g 50g = jar rolled past the end of the table. From the results summarized in Table 6, the following conclusions can be drawn: (1) Regardless of the jar size, the distance traveled along the horizontal plane decreases as the mass of the particular material increases. There is an exception in this regard, as can be observed with the extremely fine cenospheres Pozzfill 5. For the smallest jar size, the jar rolled past the end of the table with increased sample mass. The only explanation for this behaviour is that the material is so fine that it does not undergo significant compaction during the rolling motion and the greater masses gave the jars increased kinetic energy that causes it to keep on rolling past the end point of the table. (2) As the particle sizes of the materials increase, there is a decrease in the distance that the jar travels before coming to rest on the horizontal plane of the table. (3) There is a large enough distance between the distances traveled by the various fly ash fractions in the different jars to characterise them each uniquely.

10 10 (4) The caused the different sizes of jars to come to a stop at a shorter distance than similar masses of the various fly ash fractions. The effect of different inclination angles on the distance traveled by the various jars with their various materials is given in Table 7. Table 7. The effect of inclination angle on distance traveled by jars on a horizontal rolling table. Material Volume of jar (ml) Mass of material Distance traveled (mm) from incline of / g / g / g 40g 40g 40g 40g From the values given in Table 7, it can be seen that the distance traveled by any particular jar containing a fixed mass of material increases with the angle of inclination. Figures 712 show the relationships between the distances traveled by the jars along the horizontal plane of the rolling angle from a fixed angle of inclination of 15 º and the ratios (mass of material: mass of jar) and (mass of material: volume of jar) for 4 different materials used in the 3 different jar sizes.

11 mass material/mass bottle Distance (m) Distance traveled (m) /45 Type of material Fig. 7 Relationship between distance traveled by a 156 ml jar and the ratio of mass of material to mass of the jar from a 15 º incline Distance traveled (m) mass materail/ volume bottle Distance (m) /45 Type of material Fig. 8 Relationship between distance traveled by a 156 ml jar and the ratio of mass of material to volume of the jar from a 15 º incline. 2.5 mass material/mass bottle Distance (m) 2 Distance traveled (m) /45 Type of material Fig. 9 Relationship between distance traveled by a 290 ml jar and the ratio of mass of material to mass of the jar from a 15 º incline.

12 mass material/ volume bottle Distance (m) Distance travelled (m) /45 Type of material Fig. 10 Relationship between distance traveled by a 290 ml jar and the ratio of mass of material to volume of the jar from a 15 º incline Distance traveled (m) mass material/mass bottle Distance (m) /45 Type of material Fig. 11 Relationship between distance traveled by a 420 ml jar and the ratio of mass of material to mass of the jar from a 15 º incline Distance traveled (m) mass material/ volume bottle Distance (m) /45 Type of material Fig. 12 Relationship between distance traveled by a 420 ml jar and the ratio of mass of material to volume of the jar from a 15 º incline.

13 13 It is clear from the figures that for each particular jar size, the forms of the curves of the ratios (i) mass of material : mass of the jar and (ii) mass of material : volume of the jar are the same. However, they are completely different for each of the jar sizes used in the investigation. It can furthermore be observed that there is no single simple linear relationship between either of the ratios and the different materials employed in anyone of the jar sizes used in the investigation. Resorting to any one of these 2 ratios as a way to discriminate between various particle size fractions of the fly ash is therefore not a very sensitive and reliable and thus not recommended as a quality control measure. CONCLUSIONS From the investigation it can be concluded that there is a unique relationship between the distance traveled along the horizontal plane of a rolling table by a jar of a particular volume with a fixed mass of material and the specific particle size and distribution of the material. This was proven conclusively for a variety of fly ashes with different particle size fractions, and it was also shown to differ from the behaviour displayed by. This approach can therefore be utilised by unskilled operators as a robust quality control test/tool on a fly ash plant using cyclones/classifiers to separate the different size fractions of cenospheres from unclassified fly ash. REFERENCES 1. P MORETON, Measuring particle size and zeta potential, Lab Africa, pp (1998) 2. P K KOLAY and D N SINGH, Physical, chemical, mineralogical and thermal properties of cenospheres from an ash lagoon, Cem & Concr. Res., 31, pp (2001) 3. A RAWLE, Repeatable particle size analysis, World Cement, pp (June 1999) 4. M BUMILLER and A MALCOLMSON, Particle size analysis: a review, World Cement, pp (June 1998). 5. M CYR and A TAGMIT HAMAN, Particle size distribution of fine powders by laser diffraction spectrometry. Case of cementitions materials, Materials & Structures, 34, pp. 342

14 (2001). 6. K. NORRISH and HUTTON, Standard XRF procedure for sample fusion & analysis, Geochim. & Cosmochim. Acta, (1969)