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Processes of granular charges pre-compaction.

INTRODUCTION

In dispersed materials processing technologies in many industries for a given quality of the molded product, agglomeration processes carried out by extrusion, briquetting, pelletizing, etc are usually used. The selecting molding method depends on the physical and chemical properties of powdered materials, their dispersibility, the requirements for physical and mechanical parameters of the final product and other factors (Iljina, T.N., 2009). The increasing amount of powder and dust waste in the production of building materials and products provides the necessity of their utilization with the help of compacting method (Glagolev, S.N., 2010).

However, some anthropogenic materials have a low bulk density (less than 200 kg/[m.sup.3]), and high dispersibility. To obtain a granulated product from such charges a stepwise process of forming a compulsory stage of pre-compaction and formation centers of granules (Iljina, T.N., 2010) is required. These materials include powdered waste products of circulite, vermiculite, lime, etc. The study of the characteristics of the forming process of anthropogenic charges, as well as obtaining analytical dependence of filtration processes of gas and liquid phases and recommendations on the choice of the method and apparatus for pre-compression of the dispersed phase of the system is the purpose of the given work.

Main part:

Various processing methods for removing gaseous phase while forming powder mixtures: vibration compaction, vacuumizing, mechanical action with the help of centrifugal forces, rolling, screw pre-compaction, etc are well known. Each of the above processing methods has its advantages and disadvantages (Rieschel, H., 1971; Ravich, B.M., 1975; Gridchin, A.M., 2006; Min'ko, E.A., 2009; Mulevanov, S.V., 2009; Klassen, P.V., 1991).

In our point of view, the most effective way to remove the gaseous phase (deaeration) during compaction of powder materials is a combination of vibration and mechanical impact. It can provide not only a rational way of compacted mixture pretreatment for granules formation with minimum energy consumption, but will make it possible to combine two processes: removal of the gaseous phase and getting micro-granulars, which are centers of granules.

The vibrating roller method of pre-compression of the charge can be the technical solution of this process (Fig. 1).

The physical and mechanical parameters of the charge produce a great effect on the process of its precompression: material flowability, depending on granulometric composition, moisture content of the material and shape of its particles, coefficients of internal and external friction, the rate of removal of the gaseous phase, etc. (Iljina, T.N., 2009).

The main factors determining the flowability of powder materials are friction and coalescence of particles, complicating their movement.

In case of vibration effect on a layer of the charge moving downwards and its mechanical packing in rolls, the gas phase (air) filtration through the material layer between the cylindrical surfaces of the rolls and the side walls of the boot device is observed. The air rises through the material layer, worsening the flowability of the material and the conditions of its feeding to the deformation zone of the rolls.

The filtration rate of the gaseous phase through the section of the compacting zone thickness [H.sub.0] and width B can be determined by the formula:

[[??].sub.g.p] = [Q.sub.g.p] x [rho] [P.sub.m.sp]/H0 x B ([[rho].sub.m.sp] - [[rho].sub.0]), (1)

where [Q.sub.g.p]--consumption of gaseous phase, [m.sup.3]/c;

[[rho].sub.m.sp]--density of moulded microspheres, kg/[m.sup.3];

[[rho].sub.0]--initial density (bulk weight) of material, k/[m.sup.3].

The stage of the granules final forming with the application of dynamic impact follows after the stage of charge precompression and its microgranulation. In this connection it is useful to know the range of variation values of the densification rate of the material in the study of the granulation process materials having different physical and mechanical characteristics : materials with a high bulk density (waste iron ore products

[[rho].sub.0] > 4 x [10.sup.3]kg/[m.sup.3]), carbonate and clay materials ([rho] = (2 ... 4) - [10.sup.3]kg/[m.sup.3]); as well as anthropogenic materials with a bulk density [[rho].sup.0] = 800-2000 kg/[m.sup.3] and materials with a low initial bulk density, [[rho].sub.0] < 800kg/[m.sup.3] (inflated vermiculite, pearlite, collected dust of rotary kiln of cement, lime and other industries).

The minimum speed of pre-compression of the charge is determined by the medium that prevents from the free passage of powder materials through the roll gap, which determines the diameter of the particles or microgranular [d.sub.m.sp] or height microspheres [h.sub.m.sp] (due to their irregular shape). In the given case the free-flowing of the compacted material q, which determines the free passage of material through the roll gap should be considered (Fig. 1). In this case the free-flowing compacted material q, which determines the free passage of material through the roll gap [h.sub.G] should be considered (Fig. 1).

The maximum rate of microgranulation [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is limited by the filtration rate of the gaseous phase which moves towards the layer of powder material and complicates its delivering to the compacted zone between the rolls.

The critical filtration rate of the gaseous phase through the layer of powder material limits the data [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and is determined by the data of the Reynold's number:

[Re.sub.cr] = [Ar.sup.0,68]/600 [[H.sub.C]/[h.sub.m.sp] + ([R.sub.1] + [R.sub.2])(1 - cos [[alpha].sub.comp.])], (3)

where [H.sub.C]--layer thickness of sealed charge in compaction zone, m;

[R.sub.1], [R.sub.2]--radii of forming rolls, m;

[h.sub.m.sp] - microsphere size, m (see Figure 1).

Assuming that the volumetric flow rate of the gas phase through the upper section of the seal area with the thickness [H.sub.0] = [h.sub.m.sp] - ([R.sub.1] - [R.sub.2])(1 - cos[[alpha].sub.comp]) is [Q.sub.g.p] = B[H.sub.0] [[??].sub.g.p], we define the critical filtration rate of the gaseous phase, as follows:

[[??].sub.g.p] = [[??].sub.max] ([[rho].sub.m.sp]/[[rho].sub.0] - 1) [h.sub.m.sp]/[h.sub.m.sp] + ([R.sub.1] + [R.sub.2]) (1 - cos[[alpha].sub.comp]), (4)

In terms of [Re.sub.cr] = [[??].sub.cr] x [d.sub.av.p]/v , where [d.sub.av.p] is the average particle size of the mouldable mixture, m; v is the kinematic viscosity of the gas phase, [m.sup.2]/s, and also in terms of expressions (3) and (4) we obtain a maximum speed of pre-compression of the charge:

[[??].sub.max] = [Ar.sup.0,68] x [H.sub.0] x v x [[rho].sub.0]/600 x [h.sub.m.sp] ([[rho].sub.m.sp] - [[rho].sub.0]) [d.sub.av.p] (5)

Thus, during development of structural and technological parameters of roller-type device for implementing the process of microgranulation powdered particles it is necessary to meet the condition [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[pi] [[??].sub.1] [n.sub.R] [less than or equal to] [Ar.sup.0,68] x [H.sub.0] x v x [[rho].sub.0]/600 x [h.sub.m.sp] ([[rho].sub.m.sp] - [[rho].sub.0]) [d.sub.av.p] (6)

where rotating frequency of mouldable roller is:

[n.sub.R] [less than or equal to] [Ar.sup.0,68] x [H.sub.0] x v x [[rho].sub.0]/1884 x [[??].sub.1] [h.sub.m.sp] ([[rho].sub.m.sp] - [[rho].sub.0]) [d.sub.av.p]

If the process of pre-compression of molding powder mixture is not followed by significant power consumption, the deformation process of sealed moisture-laden charge is followed by the structural and mechanical changes of molded samples (reorientation and packaging of polydisperse particles, their adhesive interactions, etc.) and migration of the liquid phase from zones of maximum stress.

The liquid phase, which fills the pores between the particles, due to its incompressibility, provides elastic resistance to the external stresses together with the solid phase of structural system, and while exceeding material yield point, moves to lower pressure zone.

While studying the liquid phase migration in the powder charge and the use of Darcy's law (Zhuzhikov, V.A., 1971), the analytical expression for the time required for liquid filtration through a granular layer the molding mixture was received:

[[tau].sub.f] = 0,5[[eta].sub.f] x [r.sub.0] x [X.sub.0] [V.sup.2.sub.f]/[S.sup.2] x [DELTA] [bar.P], (8)

where [V.sub.f]--the volume of filterable fluid, [m.sup.3];

[[tau].sub.[??]]--filtration time, s;

S--seepage face, [m.sup.2];

[DELTA][bar.P]--pressure difference, N/[m.sup.2];

[[eta].sub.f]--viscosity of filterable fluid, Pa x sec;

[R.sub.g,m]--resistance of granular medium, [m.sup.-1]([R.sub.g.m] = [r.sub.0] X [X.sub.0] [V.sub.f]/S), where [r.sub.0], [X.sub.0]--resistivity and layer thickness of granular medium is given, accordingly, [m.sup.-1]

The resulting analytical expression is essential for the practice of molding moisture saturation of powder mixtures and for obtaining qualitative granules from them.

Regardless of the force action method (compaction, centrifugal molding, vibro influence, etc.) during granules forming and aggregate design development for granulating materials it is necessary to meet the condition of gaseous and liquid phases filtering, defined by formulas (7) and (8) respectively.

The study of filtering criteria of gaseous and liquid phases during compaction of powder materials and obtained analytical expressions for the timing of air filtration and migration of the liquid phase in molding charge at the stage of pre-compaction were used in the design and creation of high-technology equipment for the stage--by--stage process of granulation - vibration -centrifugal granulator (Iljina, T.N., 2010) (Fig. 2). The study (Sevostyanov, V.S., 2011) shows that it is necessary to meet the condition for the effective process of microgranulation of water-saturated mixtures and filtration of the liquid phase:

[n.sub.m.s] [less than or equal to] 2 x [square root of F (1 - [[mu].sup.2])] x [S.sup.2] [DELTA][bar.P]/0,5 x [pi] x [[eta].sub.f] [r.sub.0] [X.sub.0] [V.sup.2.sub.f] x [square root of [[pi] x [E.sub.m.s] [R.sub.m.s]], (9)

where [mu]--Poisson's ratio, for elastic material such as rubber [mu] = 0,4 ... 0,5; [E.sub.m.s]--modulus of elasticity of roller molding surface, for rubber. [E.sub.m.s] = (10 ... 100) x [10.sup.5] H/[M.sup.2]; F--force of compression, N/m.

To provide for specified productivity of the granulator it is also necessary to take into account its capacity in preparation of high-quality products, which should also take into account the values obtained [n.sub.m.s]

The process of granulation (formation of compacted nuclei) strongly effects structure formation granules during their stage--by--stage formation, which stabilizes the granule quality characteristics (their dimensions and shape, density distribution by volume, etc.). In this connection, while considering conditions of microgranulation material in precompaction we received the following expression:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (10)

[K.sub.i.c]--coefficient, which takes into account intercontact particles interaction in precompaction;

[K.sub.[phi]]--coefficient, which takes into account internal friction of particles.

Values [K.sub.i.c] H [K.sub.[phi]] are related by [K.sub.i.c] = 2[K.sub.ad] cos [phi]; [K.sub.[phi]] = sin[phi], where [K.sub.ad]--coefficient of adhesion of particles of moulding mixture; [phi]--angle of internal friction of particles in mixture.

The coefficient of internal friction f is bound by angle of internal friction [phi] with dependence f = tg[phi] or [phi] = arctgf.

Using the values [K.sub.i.c] = 2[[KAPPA].sub.ad ] cos [phi] and [K.sub.[phi]] = sin [phi], we finally obtain

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (11)

where [zeta] = 2[[kappa].sub.p]/1 + [[kappa].sub.[phi]]/1 + [k.sub.[phi]] = 2 [sin.sub.[phi]]/1 + sin [phi].

The received analytical expressions that characterize the general conditions of the process of particles microgranulation by using the method of volume compression, allow us to preset not only a high-speed parameter of the dynamic impact on the condensed particles for this technical solutions (sealing rollers), but also to determine the effect of other physical and mechanical characteristics and process parameters on microgranulation process. This, in turn, enables us to use the developed various physics and chemical techniques to improve the process of granule formation.

The analytical expressions (7, 9, 11) were used to calculate the speed of sealing rollers of developed experimental and industrial vibration and centrifugal granulator (Fig. 2). For materials with low bulk density, for example, mixtures having pearlite [[rho].sub.l.b] = 150 kg/[m.sup.3], pg =1000 kg/m3 for the parameters of sealing devices VCG l = 0,041 m, [delta]l = 0,002 m, (l, [DELTA]l--the length and width of the gap between the rolls, respectively) [D.sub.R] = 0.25 m, rotation frequency of the roll should not exceed the value [n.sub.e] < 2,7[ c.sup.-1].

The rotation frequency of forming rollers (7) ensuring the conditions of gaseous phase removal and charge compaction is nR < 0,04 sec -1, i.e. the gaseous phase removal occurs at low values of the rotational speed of the rollers.

Based on experimentally obtained data for pearlite mixtures we accept: [K.sub.ad] = 0,93 x [10.sup.6] Pa, [[sigma].sub.r] = 2,0 x [10.sup.6] Pa, [r.sub.0] = 0,05 x [10.sup.-3] m, [r.sub.0m.sp] = 2 x [10.sup.-3] m. At the same time rotating frequency of sealing rollers, calculated according to equation 11, is [n.sub.m,s]. [less than or equal to] 2,8 [sec.sup.-1].

For conditions [E.sub.m.s] = 1 x [10.sup.6] Pa, [mu] = 0,5 (for rubber), l = 0,041 m, F = 0,22 x [10.sup.6] N/m (for pearlite containing mixtures) calculated by the equation (9) the frequency of rotation of the forming rolls is [n.sub.m.s] [less than or equal to] 2,0 [sec.sup.-1].

Thus, for conditions of the gaseous phase removing, liquid filtration, charge compaction with microgranules forming for mixtures having pearlite with (Q = 50-100 kg/h) capability, roll forming speed should be [n.sub.m.s] = 1,1 - 2,0 [sec.sup.-1].

The conducted complex study of dispersed materials having different physical and chemical properties allowed developing the classification of dispersed materials with recommendations for organizing processes of their agglomeration (Iljina, T.N., 2013).

Findings:

The analytical expressions that describe the processes of removing the gas phase, liquid phase movement in molding charge, have been applied for the development of roller device for pre-compression of the charge vibration and centrifugal granulator. This device allows implementing pre-compression molding stage of the charge with a bulk density of less than 200 kg/[m.sup.3] with microspheres formation, and their growth and compaction is carried out in molding unit VCG, which consists of three cylinders.

Conclusions:

The analytical expressions for the calculation of the speed of forming rolls, providing the necessary conditions for gaseous phase removal, liquid phase filtration, production microspheres during three-phase system compaction were obtained.

The patent-protected experimental industrial vibration and centrifugal granulator for granular products of anthropogenic materials having a low bulk density, like dust wastes of pearlite, cement, limestone and other production processes, was designed and manufactured. The design and technological parameters of the granulator were determined.

Appreciation:

The study was performed with the support of the President of the Russian Federation Council for Grants (Project code NSH-588.2012.8), and also the Ministry of Education and Science of the Russian Federation within the framework of the Program of Strategic Development of Belgorod State Technological University named after V. G. Shukhov 2012-2016 (2011/ PR -146). The authors thank their colleagues who are not the authors of the paper, but who rendered assistance in conducting the above study.

ARTICLE INFO

Article history:

Received 25 January 2014

Received in revised form 12 March 2014

Accepted 14 April 2014

Available online 5 May 2014

REFERENCES

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Min'ko, E.A., E.A. Laz'ko, Doroganov, 2009. Effect of finely disperse cullet on glass batch briquetting: Glass and Ceramics, Springer US, 1: 305-309.

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Patent RF N[degrees] 2412753/ 27.02.2011. Iljina, T.N., Sevostyanov, M.V., Shkarpetkin, E.A., Uralsky, V.I. 2011. Vibration- centrifugal granulator. Bulletin N[degrees] 6.

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Rieschel, H., 1971. Uber den Verdichtungsvorgang beim Briketieren Aufbereitungs. Technik, 11: 691-698.

Sevostyanov, V.S., T.N. Iljina, M.V. Sevostyanov, E.A. Shkarpetkin, 2011. Research of conditions of microgranulation process in disperse systems: Vestnik BSTU, 1: 81-86.

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Vladimir Semenovich Sevostyanov, Tatiana Nikolayevna Iljina, Maksim Vladimirovich Sevostyanov, Dmitrij Aleksandrovich Emelyanov

Belgorod State Technological University 308012, Belgorod, Kostyukova, 46, Russia

Corresponding Author: Vladimir Semenovich Sevostyanov, Belgorod State Technological University, Russia, 308012, Belgorod, Kostyukova, 46
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Author:Sevostyanov, Vladimir Semenovich; Iljina, Tatiana Nikolayevna; Sevostyanov, Maksim Vladimirovich; Em
Publication:Advances in Natural and Applied Sciences
Date:Apr 1, 2014
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