Kurilenko Nikolay

Mining and geology institute
Speciality: Mineral Processing

Theme of master's work:

Research of parameters of coal sediments which define results of its dewatering

Scientific adviser: Nazimko E.

Introduction

Today’s most urgent problem in coal preparation practice is improving of coal concentrate quality and efficiency of fine coal classes’ treatment. End products of coal preparation should satisfy some definite demands not only in terms of quality characteristics but also in terms of residual moisture content in them. The most labor-consuming and energy demanding of them is a process of moist elimination from fine-dispersed sludge. The content of grains and fine particles in raw materials grows constantly. In particular, in run-of-mine coal the content of particles of the size from 0 to1 mm grew from 20 to 28 % with simultaneous growth of ash content. One of the most widely spread processes of slime dehydration from flotation concentrates is a process named filtering.


The Problem Actuality

Total costs of dehydrating process can be lessened by conversion from thermal dehydration methods to mechanical ones. Reducing of flotation concentrate moisture content can be reached by means of improvement of the existing methods and technologies of dehydration as well as by creating of new types of dehydrating equipment. That is why researches, which aimed to discover a new ways to effect sludge and to hasten moisture eliminating process, are so topical nowadays.


The Goal of the Research Work

The major goal of the graduation work is to research mechanical properties of fine dispersed sludge of coal preparation which are important for dehydrating process.


The main part

The typical approach for researching of different complex processes is to use the so called ideal environment models. In this case we can examine an ideal environment of sludge formed by spherical particles. On condition of different laying of particles against each other different types of sludge with a variety of porous environment properties appears. In theory there are nine types of particle laying. According to this porosity of pore space varies in a quite wide range - from the high value of n=0.78 to the low value of n=0.26. In the first case each spherical particle contacts three neighboring particles, in the second case the contact is provided by twelve particles and the sludge has denser packaging and capillary channels of a smaller diameter correspondingly.

The main characteristic that defines sludge porosity level is a disposition of spherical particles against each other. That is a way of their laying. Different way of particles’ packaging in sludge may cause difference between capillary diameters up to 2.64 times.

On real condition the process of sludge formation has a random character for particles’ forms can vary dramatically so as a way of their laying. That is why a number of contact points, form and size of capillaries formed have a random character as well. So we can assume that porous environment of different types of real sludge has a complex structure.

The scheme of changing of a sludge structure caused by a slap of shear deformation of sludge with its hexagonal (densest) packaging is shown on the picture 1.

Picture 1 – Process circuit (animation: size 33 KB, 17 frames, 5 cycles, editor Adobe EmageReady)


The solid lines depict hexagonal packaging of spherical particles in the sludge. When a slap of shear deformation of sludge takes place the particles 1 and 2 relocate to the position 1' and 2', which are shown with the dashed line. The size of the capillary between the particles increase from 3 to 3' (shown with the dashed line as well). The height of the sludge is increasing at extent of ?Н too at the expense of the capillary growth. With the slap of shearing forces and densest packaging of the sludge its decompaction is taking place.

Filtrating feeding effected by pressure differential is represented by suspensions. Increase of a solid phase concentration in a suspension causes formation of complex bulk structures from packaged particles (especially when there are fine argillaceous materials in a suspension). When aggregates have enough stability and save their form while sludge on filtrating cloth is forming, moisture is removed through inter-aggregate channels. There are two types of pores: inter-aggregate pores and those pores within aggregates. A size of inter-aggregate channels cross-section depends on aggregate fineness and its granulometric composition. A size of pores within aggregates is defined of a size of particles they consist of and porosity of those aggregates.

Prolongation of filtrating process may lead to mashing and destruction of aggregates because of moisture elimination.

If free moisture elimination takes place any internal pressure that effect water-saturated bounded sludge, the elimination alters its moisture content and density, thus effecting a total resistance to a shear. Characteristics of the resistance depend on the pressure applied and conditions in contact points of particles.

Complex processes interaction researches in coal dehydration with different methods of effecting them give an opportunity to find out ways to increase industrial slime usage efficiency.

Several main modes of deformation may take place in dehydrating process. First of them is a mode of compressive squeezing, when normal deformations of compression only develop (increase) in sludge. As a result liquid phase is squeezed from pores and catches in filtrating mode. The second mode of a simple shear, when three cubic strain types may emerge: squeezing in case of a simple shear in non-consolidated sludge, decompaction in case of over-consolidated sludge and a simple shear without volume changing in the borderline case. The surd mode is a complex one, when a simple shear with compression may take place. The modes enumerated are not only typical for press-filters or vacuum filter but also for sedimentation centrifuges.

Any mode or a sub-mode with a shear influences filtrating process in a favorable way as they destruct one-side open pores and actively restructure sludge. That lead to increasing permeability and fastening of a liquid phase elimination. Considering soil mechanics, shearing tests are given after previous ramming (consolidation) of a sample. Besides the very mode of sludge consolidation is practically useful for such modes take place everywhere in many mechanisms, which are used in preparation and dehydration techniques. In connection with the fact there is a strong necessity to test preparation sludge in these modes.

Sludge compression tests (which need to be dehydrated in preparation manufacturing schemes) haven’t been given before, but these test are widely used in soil mechanics. These tests give an opportunity to estimate a range of sludge compression, a range of their porosity dependency and water permeability on pressure and other important characteristics, those let us adjust dehydrating process properly.


The Sample Normal Compression Test

At the first stage of compression tests dependency of sludge filtration speed on normal deformation is defined. For this purpose my scientific adviser Doctor of Engineering, professor of “Mineral processing” department Nazimenko designed a facility to test sludge water permeability, which was named «Испытательная ячейка» (“The probationary cell”) (picture 2).

1 - the base; 2- the lid; 3 – the built-up frame; 4 – the sludge sample; 5 – the piston; 6 - the hold-down bolt; 7 – the water inlet manifold; 8 – the filtrate discharge manifold.

Picture 2 - “The probationary cell”; a) exterior view b) cutaway view of the facility


The facility use principle of material strength test in response to biaxial compression Thus the sample is effected with normal deformations in two planes perpendicularly to each other. Endwise abscissa axis the hold-down bolt of the piston 1 is responsible for load of compression intensity, endwise ordinate axis the hold-down bolt of the piston 4 is responsible for it. Pressure value which is performed by the pistons on the sample facet is estimated through tightening torque of the corresponding hold-down bolt.

On the basis of compression tests results liquid filtration speed is estimated with the help of the sludge sample and the pressure applied. We can also define the sample deformation rate. The estimated data are used in construction of diagrams of filtration speed dependency on the pressure applied for compression curves.

a)

b)

Picture 3 – a) the diagram of filtration speed dependency on the pressure applied b) the compression curve.


Having analyzed the diagrams we can consider the maximum pressure and normal deformation, due to which liquid filtration through sludge ceases. On the ground of the received pressure values we can estimate value of the maximum or critical deformation, define the rate of consolidation or over-consolidation of sludge.


The Researches of Dependency of Fine Material Water Permeability
on Application Speed of Shear Deformations

Characteristics of shear resistance depend on the pressure applied and the conditions in particles contact points. The value of boundary shear resistance while direct in-plane shear is estimated with the help of sludge tests performed with the same facility, which was used in sludge normal compression tests but with some certain changes.

In the device shear circumferential loading is applied till one of the part shears against the other. Simultaneously with shear loading supplication sludge shift caused by decompaction is estimated with the help of clock type sensors.

Shearing tests are performed after preliminary compaction (consolidation) of a sample.

Compression tests let us estimate a range of sludge compression, dependency of their porosity and water permeability on pressure and other important characteristics, those let us estimate and adjust dehydrating process properly. The result of compression tests is a normal compression curve.

On this curve for the sludge given we can define the value of structural robustness of the sludge in compressive squeezing. A compressive curve shows dependency of relative deformation of samples or their porosity on the value of the loading applied on condition of compressive squeezing.


Summery

With the help of the received diagrams we can estimate the value of consolidating parameters: comparative shrinkage and corresponding instant time, consolidating parameters with different values of loading.

The results received at the end of the scientific efforts will give an opportunity to substantiate instant time of shrinkage applied for intensification of dehydration process, and this also will help to choose efficient modes of dehydration in shearing deformation areas.

The further investigation can aim experimental definition of a range of mechanical properties of coal preparation sludge in filtrating process.


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