A Mechanical Auger Pulverizer for Crushing Soil Samples

Transcription

1 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 18 ISSN The Journal of the Association of Professional Engineers of Trinidad and Tobago Vol.40, No.2, October/November 2011, pp A Mechanical Auger Pulverizer for Crushing Soil Samples Edwin I. Ekwue a Ψ and Sasha Seepersad-Singh b Department of Mechanical & Manufacturing Engineering, The University of the West Indies, St Augustine, Trinidad and Tobago, West Indies a Edwin.Ekwue@sta.uwi.edu b Sasha_seeps@hotmail.com Ψ Corresponding Author (Received 25 May 2011; Revised 29 August 2011; Accepted 22 September 2011) Abstract: The design, construction and testing of a soil auger pulverizer capable of crushing different soils into consistent finer grain samples with varying uniformity and gradation for the purpose of soil testing is described. The design was generated after a thorough research was done to examine the existing models. Most present commercial pulverizers are very expensive due to their complicated operative processes. The major components of this machine include a cylindrical drum (housing), auger and shaft supported at its ends by two pillow block bearings, a hopper, a perforated exit plate, a grating plate and the main frame. It is powered by a 2 Hp motor. This equipment was tested using three Trinidad soils: a sandy loam, a clay loam and clay at three water contents (5%, 7% and 9%), two varying speeds of 120 rpm and 180 rpm and 2 mm, 4 mm and 6 mm spacing between the two steel grating plates. The aggregate size distributions of the pulverized soils were obtained and the analysis of variance of the results was carried out in order to test the effect of the experimental factors on the varying grain sizes, soil uniformity and gradation produced by the pulverizer. The greatest pulverization of the soil was produced when the equipment operated at a plate spacing of 2 mm and the lowest water content of 5%. Best results in terms of crushed non-uniform and well-graded soils were obtained using a plate spacing of 6 mm and a water content of 9%. Keywords: Pulverizer, Soil, Particle, Size, Distribution, Auger 1. Introduction Pulverization is a reduction process whereby a large solid substance is pounded, ground and/or crushed into a range of finer particles. Soil pulverization is the fragmentation of the clumpiness and clods of soil, turning it into finer micro aggregates and primary particles. This process results in changing the structure of the soils, whereas their parent compositions are retained. Soils are mainly pulverized for testing purposes whereby dried and sieved soil samples are used. In construction, soil engineers upon the development of a new building or road or any infrastructure investigate the proposed site and analyse its conditions by measures of testing, so that recommendations for drainage, grading, septic systems and foundation designs could be made. This reduces the risks of potential hazards in long- and short-term periods. This normally involves collecting soils from a depth of 1 metre below the earth s surface and pulverizing it into a finer state so that the assessment could be carried out. In agriculture, in order to ensure sustainable productivity of their soils, farmers conduct soil testing to investigate whether any diagnosis could be made to improve the soil quality and fertility. In the case of plants grown in greenhouses, fresh pulverized soil is always adopted in order to simulate growth and create an enabling environment for the roots to have sufficient space to grow. The pulverized soil is uncontaminated from all debris including sticks, stones and other refuse. The normal manual methods of size reduction include the mortar and pestle, the molcajete, the millstone, the stone and Muller and the Dhal Mill. These manual methods are usually tedious and apply to small soil samples. The existing mechanical soil pulverizers utilise a wide range of measures to crush soils. One distinct disadvantage of present commercial pulverizers is that they are very expensive due to complicated operative processes. Most of them were developed for the secondary and fine crushing of hard and medium hard mineral stones such as iron ore, copper and quartz, rather than ordinary soils that could be used for testing. The present design seeks to provide cheap, easy-to-operate equipment capable of crushing soil samples to varying levels of sizes, uniformity and gradation by varying the soil properties or the operating parameters of the pulverizer. 2. Existing Commercial Pulverizers The design and operation of existing pulverizers are fully described by Wills (1979) and illustrated by Goodquarry

2 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 19 website (undated.). These pulverizers include: 2.1 Jaw Crushers This primary crusher is known for its distinctive feature: a set of vertical jaws, one being fixed and the other sitting and closing (moving back and forth) relative to it by the cam or pitman mechanism (Wills, 1979). The jaws are set at an acute angle of approximately 26 o apart; forming a tapered chute where the material then fed into the jaw is nipped progressively smaller and smaller as it travels downwards, before being eventually released from the discharge aperture. 2.2 Gyratory Crusher The features of such a crusher are reduced to three elements. A long spindle, posing the ability to turn axially, freely carried a hard conical steel grinding element seated in an electric sleeve. The spindle suspended from a spider sweeps out a conical path within the fixed crushing chamber or shell due to the gyratory action of the eccentric as it rotates. More features can be found in the Goodquarry website (undated). 2.3 Cone Crushers Defined as a modified gyratory crusher, cone crushers display similar operations with less steepness in the crushing chamber and a parallel section between the crushing zones. Due to their ability to operate at higher speeds, they facilitate particles to flow freely through the crusher and the wide travel of the heads creates a large opening between it and the bowl in the fully open position. media. 2.7 Ball Mills These are manufactured with a cylindrical drum and the main mode of grinding occurs due to the impact of the material and the steel balls whose length to diameter ratio is 1.5 to 1.0 or less. These forged high carbon alloyed steel balls are designed as small as possible to ensure that the charge is graded in such a manner, that the largest balls are just heavy enough to grind the largest and hardest particles fed through the chamber. 3. Description of the Constructed Auger Pulverizer The diagram of the constructed auger pulverizer is shown in Figure 1. The major mode of operation is that of a spiral auger (see Figure 2) within the cylindrical housing in form of a drum (i.e., 782 mm long and 159 mm diameter) which formed the pulverizing chamber. As the soil enters the pulverizing chamber through the hopper (i.e., mm by 254 mm on the outer surface), the rotational effect of the auger transports the soil along the drum as the soil particles simultaneously break up along the walls of the drum. 2.4 Crushing Rolls Also called roll crushers, they comprise of either a toothed single roll or toothed (for coarse crushing or smooth (for fine crushing) double rolls. 2.5 Impact Crushers/Hammer Mills In crushing the material, the breaking force is mainly derived from impact as opposed to pressure due to the sharp blow contributed at high speeds to free falling rock. The moving parts generally termed beaters create internal stresses within the particles, causing shattering to occur. 2.6 Tumbling Rolls and Rod Mills A grinding media consisting of either steel rods, steel or ceramic balls produces the fracture of the particles within the crushing chamber. These loose bodies in comparison to the particles to be pulverized are generally larger, but in the analysis of its volume to the mill itself, it occupies less than half the existing volume. Rod mills are classed as older tumbling mills whereby long steel rods of a comparable length to that of the drum are the crushing Keys: 1) Driver Pulley; 2) Link Belt; 3) Driven Pulley; 4) Cylindrical Drum with Inner Auger; 5) Entry Hopper; 6) Shaft; 7) Fixed Perforated Exit Plate; 8) Rotational Grating Plate; 9) Pillow Block Bearing; 10) Collection Pan; 11) Table, and 12) Motor Figure 1. The constructed auger pulverizer Figure 2. The spiral auger

3 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 20 This pulverization process weakens the particles as the auger blade contributes a shearing effect as well. As the auger transports the crushed soil to the end of the drum, it then exits through the perforated steel plate, 12.7 mm thick and mm diameter (see Figure 3) where the final pulverization is done as the exiting soil grates between the toothed, adjustable grating plate (see Figure 4). The pulverized soil exits the chute into the collection bin supported upon the design of a table, which ensures stability during the pulverization process. The pulverizer was designed to pulverize soils with shear strengths up to 200 kpa, which is above the maximum strength expected of dry clay soils in Trinidad (Ekwue and Stone, 1995). All Dimensions are in mm. Figure 3. The fixed perforated steel exit plate All Dimensions are in mm. Figure 4. The rotating adjustable steel grating plate 4. Testing of the Constructed Auger Pulverizer 4.1 Purpose of the tests Tests were conducted to ensure that the constructed mechanical auger pulverizer is capable of crushing different soils into consistent finer grain samples for the purpose of soil testing, hence every test was performed twice on a particular soil sample. Besides, they investigate the effect of soil type, soil water content, pulverization speed and the spacing between the two steel grating plates (see Figures 3 and 4) on the sizes, gradation and uniformity of the crushed soil samples. The soil aggregate size distributions of the crushed samples were determined in order to obtain the amount, uniformity and shape of the aggregate fractions. 4.2 Procedure for soil testing Three agriculturally important soil series in Trinidad were used for the testing of the equipment (see Table 1). The soils are Piarco sandy loam, Maracas clay loam and Talparo clay. These soils were used since they are very common in Trinidad. The soils also required different efforts to pulverize them with the sandy loam being the easiest to pulverize followed by the clay loam, with the clay requiring the greatest pulverization effort particularly at dry conditions. Cooper and Georges (1982) emphasised that Trinidad clay soils are very difficult to pulverize, particularly at their dry states as a result of their high shear strengths. The particle-size analysis of the soils was carried out using the hydrometer method (Lambe, 1951) while the organic matter contents were determined using the Walkley and Black method (1934). Prior to the testing, three sets of samples of each soil were put to air-dry. The air-dry water content for clay was 5% while the sandy loam and the clay loam air-dried at 2.5% and 4% respectively. These latter soils were added water to make up to 5% water contents, in order to obtain common starting water content for testing the samples. The three soils were then added water to make up to 7% and 9% contents. The soils were thorough mixed with water and left overnight in a humidification chamber to ensure equal distribution of water throughout the samples. Although soils are normally pulverized at air-dry conditions, it was decided to test the efficiency of the pulverizer in operating at different water contents and in case soils are not properly air-dried or are pulverized at higher water contents. Moreover, testing the three soils at the same three water contents ensured that reliable statistical analysis could be carried out. Before the start of all operations of the pulverizer, all the nuts and bolts were properly fastened onto the machine. The motor and voltage regulator were then turned on and a voltage of 9.8 V was set in order to attain the maximum operating speed of 180 rpm. 1,000 grams of soil were pulverized in each test. The machine was allowed to operate at this maximum speed in order to ensure its entity and that all included components functioned as required with minimal vibrations and noise whist ensuring that the auger and the drum did not come into contact during the operation. After this initial run, an additional operating speed of 120 rpm was achieved using a 6.8V setting of the motor. The total time required to pulverize the soils were initially obtained as 8 min and 10 min for the 120 rpm and 180 rpm speeds, respectively.

4 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 21 Table 1. Classification, organic matter, and the particle size distribution (%) of the soils Organic Particle size Distribution (%) Matter Sand Silt Clay Soil Content ( ) ( ) (<0.002) Series Classification* (%) mm mm mm Piarco Aquoxic Tropudults ** Maracas Orthoxic Tropudults Talparo Aquentic Chromuderts * Classification according to the Soil Taxonomy System (Soil Survey Staff, 1999). ** All values are means of three replicates The actual tests were a factorial experiment involving the three soils at three water contents (5%, 7% and 9%), operating pulverization speeds of 120 rpm and 180 rpm and three different place spacing (between the perforated steel plate and adjustable grating plate) of 2 mm, 4 mm and 6 mm. Each test was replicated twice to give a total number of 108 tests. The pulverized samples were then transferred into the top of a stack of sieves arranged in decreasing opening sizes of sieves (0.075 to 4.75 mm diameter) and put on a mechanical shaker, where they were sieved for 5 min each. It was found that after 5 min, there was no more considerable breakdown of the soils in the sieves. The largest sieve opening (4.75 mm) was placed on top and the receiving pan on the bottom. When the oscillation of the mechanical shaker was completed, the sieve stack was removed and carefully disassembled. The mass of soil retained in each sieve was determined by weighing and the percentage of soil that passed each sieve was determined. The full mechanical sieving process and the analysis of results were fully described by Lambe (1951) and Eccles and Ekwue (2008). 5. Results and Discussion During the testing, it was observed that the constructed auger pulverizer produced the desired motion in a manner that was quiet and with little or no vibration. The design also facilitated flexible use by allowing some adjustments in its operating parameters. The percentages of soil that passed each sieve size were determined for each pulverized soil sample and these were used to plot the aggregate size distribution curves for each soil sample (see Figure 5). Three basic soil parameters (Das, 2002) were obtained from the distribution curves and used to classify the pulverized soil samples. These parameters are effective size, uniformity coefficient and coefficient of gradation. The effective size of a soil is defined as the diameter in the particle size distribution curve corresponding to 10% finer and is denoted as D 10 (Das, 2002). Table 2 details the values of these basic soil parameters of the soils for the four experimental factors. Since all the D 10 values are less than 0.25 mm, the pulverizer can be said to have crushed all the soil samples to their fine states. The uniformity coefficient is a measure of the particle size range. It is the ratio of the diameter corresponding to 60% finer (D 60 ) to the effective size (Das, 2002). Piarco sandy loam Maracas clay loam Talparo clay Figure 5. Aggregate-size distribution for the three soils for the 5% water content, three plate spacings and the 180rpm pulverization speed. (Only representative data were plotted)

5 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 22 All the crushed soil samples can be classified as nonuniform since they all have uniformity coefficients greater than 5. The coefficient of gradation is the measure of the shape of the aggregate size curve (Das, 2002) and is defined as shown in Table 2. Since most of the samples had the coefficient of gradation outside the range of 1 to 3, they are all classified as poorly- or gap-graded. This means that the smaller particles will not pack between the larger ones. There is no fairly evenly distribution of the proportions of all the different aggregate sizes. However, for most values for the 6 mm plate spacing (see Table 2), the coefficient of gradation was within the required range of 1-3 required to make soils to be well graded. Table 2. Values of some particle size parameters of the soils obtained by the constructed soil pulverizer operating at different speeds and plate spacing for the three soils at three water contents Soil type and Plate Pulverization speed moisture content spacing 120 rpm 180 rpm (mm) a D 10 D 30 D 60 Cu b Cz c D 10 D 30 D 60 Cu Cz Piarco sandy loam: % water content % water content % water content Maracas clay loam: % water content % water content % water content Talparo clay: % water content % water content % water content Remarks: a - Diameter (mm) of the particle to which 10% is finer is defined as effective size; b - Uniformity coefficient, UC = D 60 /D 10 ; c - Coefficient of gradation, Cz = D 2 30 /(D 60 x D 10 )

6 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 23 Table 3 shows the mean values of the basic soil parameters for the four experimental factors. Results showed that mean effective size, D 10 as well as D 30 and D 60 increased with increasing clay content in the soils as well as increasing water contents and plate spacing. This shows that as the clay content of the soils as well as the water content and the plate spacing increased, there was less crushing of the samples by the pulverizer. Clay soils are known to have greater shear strengths than sandy soils (Ekwue and Harrilal, 2010). Values for the mean pulverization speeds of 120 rpm and 180 rpm were more or less the same. The mean values for the various water contents were more or less the same showing that the pulverizer was consistent in crushing soils, within the limits of the water contents tested. As soils become plastic and difficult to pulverize with greater water content, the tendency in the results is that less pulverization took place as water content increased. All the values of soil parameters (see Table 3) increased with the spacing between the two steel grating plates showing that as the spacing increased, there was less crushing by the pulverizer. It could be explained because the greater the distance, the soil would have a better chance of exiting the plate without being crushed between the plates. It must be observed, however, that if the soil testing required a soil that is less uniform and well-graded, then the plate spacing of 6 mm will provide better results than 4 mm and 2 mm spacing. Mean values for the two pulverization speeds were more or less the same showing that within the range of 120 rpm and 180 rpm, speed of pulverization was not important. Values of the mean uniformity coefficient confirmed that all the samples were non-uniform since they were all greater than 5. There was therefore a wide distribution of grain sizes present (Bowles, 1984). This shows the effectiveness and the efficiency of the varied distance between the grating plates. Mean uniformity coefficient values increased with increasing plate spacing and pulverization speeds. Mean values of coefficient of gradation showed that only the Talparo clay soil, water contents of 5% and 9% as well as the 6 cm plate spacing and the two pulverization speeds had the values within the 1 to 3 range meaning well graded samples. At these situations, grains of each possible size between the upper and low gradation limits existed (Bowles, 1984). Analysis of variance performed for all the soil parameters (see Table 4) shows that the effects of most experimental factors were significant at 1% level as depicted by the significant F values. The effect of plate spacing was the greatest, followed by that of soil type and water content. The effect of speed of pulverization on the soil parameters was not significant. This clearly demonstrates that the spacing between the grating plates controls the level of pulverization expected as well as the level of uniformity and gradation obtained from the crushing process. Table 3. Mean a values of soil effective size, D 10 ; D 30 ; D 60, uniformity and coefficient of gradation for the four experimental factors Factor level Mean Mean Mean Mean Mean effective D 30 D 60 uniformity coefficient of size, D 10 coefficient gradation (mm) (mm) (mm) Soil type Piarco sandy loam Maracas clay loam Talparo clay Water content 5% % % Plate spacing 2cm cm cm Pulverisation speed 120 rpm rpm Remarks: a Mean values for each factor were obtained by averaging the measured values over the levels of the other three experimental factors. Number of experimental points is 108 representing a factorial experiment with 3 soil types, 3 water contents, three plate spacings and two pulverization speeds.

7 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples 24 Table 4. F values in the analysis of variance for soil D 10 ; D 30 ; D 60, uniformity and coefficient of gradation Factor level Degrees Mean Mean Mean Mean Mean of effective D 30 D 60 uniformity coefficient of freedom size, D 10 coefficient gradation (mm) (mm) (mm) (%) Soil type Water content * 3.0* * Plate spacing Pulverization speed 1 1.4* a 0.0* 0.0* 0.8* 0.1* Soil type x water content 4 2.8* 3.3* 1.3* 1.4* 3.5 Soil type x spacing * 1.8* 11.5 Soil type x speed 2 1.9* 3.0* * Water content x spacing 4 3.1* Water content x speed * 1.9* 8.8 Plate spacing x speed 2 2.0* 3.4* Remarks: a - Values of F followed by asterisks were not significant at the 1% level. The interactions of the factors were also important meaning that the effect of some of the factors on soil particle parameters may depend on the other ones used in the same combination. The most important interaction was that between water content and plate spacing, followed by those between soil type and place spacing, water content and pulverization speed and plate spacing and pulverization speed. The interaction of water content and plate spacing is illustrated in Figure 6. It is showed that the best soil pulverization was obtained when a plate spacing of 2 mm was used at the lowest water content of 5%. This combination also gave the greatest uniformity and the lowest gradation. Best results in terms of crushed non-uniform and well-graded soils were obtained using a plate spacing of 6 mm at a water content of 9%. The best combinations of factors for pulverization in this equipment will therefore depend on what the user prefers: greater pulverization or greater non-uniformity and gradation of the pulverized soil. Figure 6. Interaction between water content and plate spacing for the different aggregate size parameters

8 E.I. Ekwue and S. Seepersad-Singh: A Mechanical Auger Pulverizer for Crushing Soil Samples Conclusion The major objective of this research was to design, construct and test a soil pulverizer capable of crushing different classification of soils into a consistent finer grain sample sizes for the propose of engineering or agricultural testing. Based on the detailed experimental testing, the constructed soil auger pulverizer was found to be very efficient, user friendly and easy to operate. It was also well suited for laboratory work. It was found to be able to produce soil particles of a given size distribution, uniformity and particle shape; and based on what properties are desired in soil tests, these could be obtained by mainly adjusting the spacing between the two grating plates in the pulverizer and working a different soil water contents. Generally, for a well-graded and non-uniform soil, best results were obtained at a grating plate distance of 6 mm and water content of 9%. A plate distance of 2 mm is the best for pulverization at the moisture content of 2%. Sandy soils will be grinded to smaller diameters than clays, but the level of pulverization in different soils was found to be sufficient since the pulverizer was designed using the shear strength of the strongest clay soil. The pulverizer was also found to be able to operate at different water contents of the soil. The machine was found to operate effectively and efficiently and conformed to all of the design specifications. Cost minimisation was also an important factor of the design. As such, materials were chosen which would reduce the cost of the design. However, these materials were also chosen so that the long life of the design was ensured. Acknowledgements Thanks to the technicians in the Mechanical Engineering Laboratories and the Faculty of Engineering Workshop especially Mr. Oswald Lawrence, Mr. Amrit Bandoo and Mr. Steve Ramouter for their help in completing this research. References: Bowles, J.E. (1984), Physical and Geotechnical Properties of Soils, 2nd Edition, McGraw-Hill Book Company, p Das, B.M. (2002), Principles of Geotechnical Engineering, 5th. Edition, Pacific Grove, California: Brooks/Cole. Cooper, B.R. and Georges, J.E.W. (1982), Importance of sugar cane in the management of clay soils in Trinidad, Tropical Agriculture, Vol.59, pp Eccles, C. and Ekwue, E.I. (2008), A mechanical shaker for sieving dry soil samples, West Indian Journal of Engineering, Vol.30, No.2, pp Ekwue, E.I. and Harrilal, A. (2010), Effect of soil type, slope, compaction effort and their interactions on infiltration, runoff and raindrop erosion of some Trinidadian soils, Biosystems Engineering, Vol.105, pp Ekwue, E.I. and Stone, R.J. (1995), Organic matter effect on the strength properties of compacted agricultural soils, Transactions of the American Society of Agricultural Engineers, Vol.33, pp Lambe T.W. (1951), Soil Testing for Engineers, John Wiley, New York Goodquarry (undated), Technology: Extraction and Crushing, available at: [Assessed May 7, 2011] Soil Survey Staff (1999), Soil taxonomy: A basic system for making and interpreting soil surveys, Agriculture Handbook, 2nd Edition, USDA, Washington-DC. US Govt. Printing Office, 436: pp Walkley, A. and Black, I.A. (1934), An examination of the effect of Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method, Soil Science, Vol.37, pp Wills, B.A. (1979), Mineral Processing Technology, An Introduction to the Physical Aspects of Ore Treatment and Mineral Recovery in SI Metric Units, Pergamon Press, London Authors Biographical Notes: Edwin I. Ekwue is presently the Head of the Department of Mechanical and Manufacturing Engineering and Professor in charge of the Biosystems Engineering program at The University of the West Indies, St Augustine, Trinidad and Tobago. He is a member of the Editorial Board of the West Indian Journal of Engineering. His specialty is in Water Resources, Hydrology, Soil and Water Conservation and Irrigation. His subsidiary areas of specialisation are Structures and Environment, Solid and Soil Mechanics, where he has teaching capabilities. Professor Ekwue has published widely. He had served as the Deputy Dean (Undergraduate Student Affairs), the Deputy Dean (Post-graduate Affairs and Outreach), the Chairman of Continuing Education Committee, and the Manager of the Engineering Institute in the Faculty of Engineering at UWI. Sasha Seepersad-Singh graduated with a BSc. (Eng) in Mechanical Engineering from The University of the West Indies in Her final year project was highly rated and she made the grade of A. She is presently seeking employment. Her areas of interest are in mechanical engineering design and manufacturing.