A Novel Approach to the Screw Feeder Design to Improve the Reliability of Briquetting Process in the Roller Press

compaction


Introduction
Maintaining the reliable and efficient functioning of mining, metallurgical and chemical enterprises is one of the important challenges in the modern economy [23,3,30,31]. Roller presses with profiled rollers are widely used in the briquetting processes of fine-grained raw materials in various industries [4,9,35,7,8]. Under certain technological conditions, the roller presses can be equipped with mechanical devices for feeding material into the gap between the rolls. Although some methods of briquetting process optimization, design changes, advanced simulation and visualization [2] techniques have been proposed, the production of high-quality briquettes still remains a challenge.
The use of a feeder pre-compaction device is necessary for the production of briquettes from materials with low bulk density (bulk≤ 0.6 g/cm 3 ), requiring forced material feeding into the gap between the rollers. Such materials are prone to fluidization, sagging and bridging during gravitational feeding to the deformation area [10]. Typically, to obtain high-quality briquettes the materials should have a compaction factor Ky equal to or greater than 3.0 (units). The compaction factor Ky of a material is the ratio of the density of the formed briquette br to the bulk density bulk of the original material, which is investigated in work [14] for different raw materials. The most used devices in the practice of roller briquetting process for forced feed and pre-compaction of fine fraction materials are the pre-compaction screw feeders. They can be equipped, subject to technological conditions, with different types of screws -cylindrical, conical, and complex, including as well as screws with a constant and variable pitch. 3 -support-bearing assembly of the screw feeder; 4 -screw; 5 -loading section of the screw feeder; 6 -roller press feeder; 7rollers of roller press; 8 -consolidated material; (c) general view of the screw. There are quite a few publications on the study of the relationship between the design and technological parameters of screw feeders of roller presses. Probably, this is caused by the complexity of investigating the deformation area of a roller press equipped with a screw feeder and competition between manufacturers of roller presses. Quite often the selection of rational parameters of screw feeders of roller presses is based on the experimental results [6,5,27,34,18,21].
Dec and Komarek [6] introduced experimental tests of a screw feeder system in the roller press using some specific experimental devices. This experimental study provided information that helped understand the feeding and precompaction process that can be useful when developing new mathematical models of the pressing process, especially for the mathematical model verification.
The research presented by Dai and Grace in [5] was conducted to find the mechanisms of blockage in screw feeding and to determine the effects of particle mean size (0.5-15.0 mm), size distribution, shape, moisture content (10-60%), density and compressibility on biomass particle feeding at room temperature. The results of these experimental studies showed that large particles, wide size distributions, large bulk densities and high moisture contents generally led to larger torque requirements for screw feeding. It was also detected that the "choke section" and seal plug are important for the determination of the required torque.
In [27], the capacity of a screw feeder and the capacity of a high-pressure roller mill equipped with it with quartz materials were analyzed. It was shown the following. 1) The feeder capacity is proportional to the screw speed and does not depend on the counter-pressure created by varying the outlet resistance.
2) The mill capacity at different roller velocities is proportional to the screw speed.
3) The screw speed has to exceed the lower limit to ensure proper mill operation and this limit depends on the feed fineness; the capacity of screw feeders can be doubled at the same roller velocity. 4) The additional power draft can be kept at less than 10% of the total power if the screw speed is adjusted properly.
The results of these studies may be useful for the development of experimental methods for evaluating the efficiency of calculation models of screw feeders of roll presses.
In [34], the influence of vacuum on the process of roller compaction of powders with the extremely poor flow is examined. This influence for materials under consideration is qualitatively correlated with the sensitivity of their bulk densities to the pressure. The impact of bulk density, roller gap, and deaeration (vacuum level) on the roller compaction process are investigated. Detailed information about the ability of powder processing helps to determine how the use of the vacuum line may improve the roller compaction process by screw feeders.
The study described in [18] analyzed statistically a pharmaceutical compaction process using an industrial-scale roller compactor. The roll speed was kept constant and the gap between rollers was uncontrolled. Microcrystalline cellulose was used as a model material. Some significant process parameters were determined, such as the screw speed to roll speed ratio and the roll pressure. The relationships between the process parameters and the resultant ribbon/granule/tablet characteristics were established.
In the paper [36], the discrete element method is used to simulate and analyze powder transportation. A 3D model of the screw conveyor is created and imported into the EDEM simulation software. The key parameters of a screw conveyor are the internal diameter of the screw, pitch and speed as design variables. The purpose of the research was to find the optimal operation modes of the screw conveyor. A specific practical problem for one material was solved. The results of this study allow making a solution to similar problems for determining the rational parameters of roller presses equipped with screw feeders.
In [24], it was examined how operating conditions influence the performance of a screw conveyor by applying the Discrete Element Method (DEM). A single-pitch screw conveyor with periodic boundary conditions was simulated.
The DEM modelling gives predictions of screw conveyor performance in terms of variations of such parameters as particle speeds, mass flow rate, energy dissipation and power consumption, due to changes in the operating conditions.
In [32], a numerical analysis was carried out using the three-dimensional discrete element method (DEM) to study the performance of screw conveyors. The modelling of horizontal and vertical types of screw conveyors was studied.
The results are compared with previous work and empirical equations.
The study [25] concerns screw conveyors with fully enclosed tubular casings. A theoretical approach to predict the performance of screw conveyors of any given geometry is presented. The influence of flow properties of loose material on conveyor performance was studied.
The above-mentioned studies indicate that the DEM is a promising tool for researching screw mechanisms requiring calibration of bulk material model [11] after which it can be conducted the optimization of a screw conveyor's exploitation parameters [12] by simulation of the real conditions [13].
Nevertheless, methods, in which the grained material is

Method of model-based design the parameters of the roller press screw feeder
Before proceeding with the design, analysis and selection of the parameters of the roller press screw feeder, the conditions of its use should be defined based on the physical and mechanical properties of the material to be briquetted [15][16][17][18][19][20][21][22].
In [22], a graph analytical method for determining the type of device for feeding fine-grained charge material into a roller press is proposed by Kosturkiewicz. The essence of this approach is as follows: the compaction characteristics of the fine-grained material are determined experimentally, and the dependency curve of compaction factor Ky on compression pressure p is plotted. Next, the preferred method of feeding the fine-grained charge material into the deformation area based on the following conditions is to be chosen: gravitational feed: screw feeder with a cylindrical screw: screw feeder with a conical screw: where Kybr.max -maximum possible compaction coefficient of the fine-grained material to be briquetted.
In practice, it often happens that when briquetting materials in a roller press with pre-compaction by screw feeders equipped with cylindrical screws, the value of the compaction factor becomes more than 2.6. At the same time, there are some difficulties in using conical screws, namely, high manufacture cost, and frequent material jamming in the screw channel when changing the physical and mechanical properties of the material handled. Therefore, unless otherwise specified, the use of pre-compaction screw feeders with cylindrical screws is completely justified for most low bulk density materials (bulk ≤ 0.6 g/cm 3 ). Therefore, in this work, the parameters of pre-compaction feeders with a cylindrical screw are investigated. In addition, a serious drawback of the models proposed by Kosturkiewicz in works [15][16][17][18][19][20][21][22] is the omission of the effect of feeding the material to the screw feeder and the impact of the geometry and parameters of the roller press operation on the operation of the screw feeder. Analysis shows that conditions (1-3) are empirical, and there is no clear justification for them.
Therefore, other principles for choosing the type of material feeding to the deformation centre should be found proposed.
It is known from the practice of pressing fine-grained materials that it is very difficult to achieve compact body density, as a rule, the maximum density achieved is 5…15% less than the density of a compact body made of this material.
The choice of the appropriate scheme of material feeding into the gap between the rollers depends on the possibility of achieving the maximum value of the compaction coefficient that is determined by the expression: where pikn -picnometric (true) density of fine-grained material particles, g/cm 3 ; bulk -bulk density of fine-grained material, g/cm 3 .
The feeding scheme of the material in the rollers is selected depending on what degree of compaction can be achieved with a particular feeding scheme in a particular case and after comparing it with the maximum value, these conditions can be presented as follows: gravitational feed: pre-compaction screw feeder: where Kyroll.max -the maximum possible coefficient of charge compaction in the deformation area [1]: where D0 -is the reduced diameter of press rolls, mm; Hbr -is the thickness of the briquette on the rollers centres line, mm.
Parameter 0 is the maximum pressing angle that can be determined by the specific parameters of the rollers seizing the charge material pick-up: for rollers with toothed-grooved forming elements [1]: for rollers with symmetrical forming elements (lenticular, pillow-shaped) [1]: where roll -the gap between the rollers, mm;  -slope angle of the sliding lines, degree; c -extreme feed angle for a row of grooved forming elements, degree: where f2 -internal friction coefficient, where ch -extreme feed angle for a row of grooved forming elements, degree; f1 -external friction coefficient;  -lateral pressure coefficient: The values of the coefficients of external f1 and internal f2 friction are determined according to the experimental procedure.
The authors have developed their method for analyzing the process of pressing fine fractional materials and calculating the parameters of the briquetting process and a further selection of design parameters of press equipment. Namely, they use a functional relationship between pressing pressure p and compaction coefficient Ky that was established experimentally and described analytically [14]. According to this method, the power function is used for describing this relation, thus, this relationship can be written as follows: where a and b -coefficients in the equation of approximation of the experimentally established compression curve of fine fraction charge, indicating the degree of its compression resistance; Ky -compaction factor, can be determined as: where br -briquette density, g/cm 3 .
However, equation (13) cannot be used to determine the pressing parameters in the low-pressure range (up to 5 MPa), which is typical for material output from the screw feeder.
Therefore, an exponential function is used in this work to describe the experimentally established compression curve for describing the behaviour of the material in the screw feeder, as follows: where a, b and c -coefficients in the approximation equation of the compaction curve of fine-grained material.
To make the briquetting process in the roller press with a screw feeder more stable, the condition of a constant flow rate of the briquetted charge material must be strictly met, which means, both the roller press and screw feeder productivity should match each other: where Qpr -roller press productivity, t/h; Qscr -screw feeder where Qscr.req -required productivity of the screw feeder, t/h and kQ -productivity coefficient that can be determined as: where 8760 -number of hours per year; Te -number of press working hours per year.
As a rule, the value of kQ is within the range of 1.1-1.5, therefore, taking into account the practice of roller presses operation in real production conditions, kQ =1.1 is accepted.
The required productivity of the screw feeder can be calculated as: where So, based on stated in [33] and the fact that the movement of the material along the screw feeder belt and the body of the press is determined by the conditions of external friction, it is assumed that: the pitch of screw helix line Sscr is: where e -the angle of inclination of the screw helical line on the effective diameter, degree and Dscr.e -effective screw diameter, mm can be calculated as: and correspondingly.
The values of D, d and 1 angles are defined as follows: The pressure in the charge after leaving the screw in the area of the material seizure by the press rollers is the precompaction pressure of the material and is determined according to the following expression [29,28]: lscr -length of the working part of the screw in (28) (it is based on analytical studies in [32,25]), mm; hscr is the height of screw coil, mm; and bscr is screw width, mm that can be determined as: p0 -initial pressure in the material at the screw feeder inlet, MPa: where hbulk is the height of the material stack above the screw feeder inlet, mm and  is the angle between the direction of movement of the material and the surface of the helical line of the screw, degree, can be determined as: Compaction coefficient of the material in the roller press using the screw feeder: where Kypc -is the pre-compaction coefficient of the material after leaving the screw feeder, at the entrance to the area of the material seizure by rollers; Kyroll -coefficient of material compaction by the press rollers, which is determined by the geometric parameters of the deformation area in the roller press [34,18]: where D0 -press rollers reduced diameter, mm; Hbr -the briquette thickness on the rolls centres line, mm; prpressing angle, deg.
For working conditions of the press with the screw feeder, it is accepted: Determining the value of the material compaction by rolls according to expressions (33) using the values of geometric parameters of the rolls forming the elements of the press and expressions (15,23,(26)(27)(28)(29)(30) it becomes possible to determine the value of the previous seal Kypc: The density of briquettes taking into account the elastic aftereffect: where V -elastic aftereffect, %.
The following condition is proposed to evaluate the efficiency of the adopted screw design: (42) where uscr.drive -gear ratio of the screw feeder drive.
Power consumed by the screw feeder, kW is calculated as: According to equations (1-43), the algorithm is developed (see Figure 2) of design, power, and technological parameters of the screw feeder calculation and the analysis.   The calculations performed based on the following parameters of the press rolls: • forming elements design: saddle; • forming element dimensions: 40.0х38.5х17.5 mm; • press rollers diameter: 648mm; • press rollers reduced diameter: 630.5 mm; • width of the working roller surface: 200.0 mm; • briquette thickness (excluding elastic aftereffect): 17.5 mm; • gap between the rolls: 0.5 mm.

Results of Calculation on the Model
Using expressions (1-43) for the received parameters of rolls and screw calculations were performed and several data sets were obtained. They allow analyzing dependence between working parameters of the screw feeder accepted in work at briquetting of the materials specified in Table 1.
The main parameter that determines the efficiency of the screw feeder is the pre-compaction pressure and the degree of its effect on the total degree of material compaction in the roller press. The pre-compaction ratio is significantly affected by the height of the material column at the screw entrance. The pre-compaction pressure at the material outlet from the screw feeder and its seizure by the rolls and the compaction pressure in the deformation area of the roller press determine the total value of the material compaction, expressed by the compaction coefficient. Such values of compaction ratio increasing are essential and, as practice shows, allow to receive high-quality briquettes. At that time, in the absence of a screw feeder, the briquettes from the studied peat and lignin on a roller press could not be obtained.
Computational and analytical studies revealed that when the press and roller capacities are matched and there is no overpressure, screw pitch has little effect on the precompaction pressure. It is largely due to the physical and mechanical properties of the material and the size of its column at the screw inlet.
However, a change of the screw pitch affects the productivity of the screw feeder and the value of friction force of the charge against the walls of the feeder housing, and, consequently, the parameters of the power drive.    • pre-compaction pressurepitch of the screw turnstorque on the screw shaft; • size of the material stack above the screw feeder inlet -pitch of the screw turns -pre-compaction pressure; • size of the material stack above the screw feeder inlet -pitch of the screw turnsmoment on the screw shaft/power of the screw feeder's drive, etc.
The presented instructions for using the graphical and analytical method allow a reasonable choice of design and technological parameters of screw feeders of roller presses.

Industrial validation
Some tests using different materials were carried out on the roller press design similar to the one shown in Figure 1.
The main design parameters of the feeder and the press, except the variable length of the screw, are corresponding to those mentioned earlier. Figure 9 shows The data obtained in the process of work on the design, production, and exploitation of roller presses with screw feeders are presented in Table 2. It was difficult to bring these data into a well-structured form because they are obtained from the various tests of screw feeders supplied to the customers and not from planned lab experiments.
The feature of the tests on real industrial equipment was that they were carried out in two modes of operation of the roller press with a screw feeder (see Table 2). Table 2. Experimental and industrial pressing on roller presses using a screw feeder of an industrial roller press.  In the first mode, without preliminary support of the material -the press operates in normal mode. It prevents the material from being displaced from the deformation zone and creates a pre-compaction pressure sufficient to achieve the required degree of material compaction. In this mode (Table 2), the experimental and calculated values of strength parameters and density of briquettes had a slight discrepancy. This confirms the theoretical positions stated above.
In the second mode, with preliminary support of the material -when the feeder was put into operation before the press rolls. This was done to create the so-called "initial plug", by analogy with extrusion moulding; the delay was 2-3 seconds. During this time, a small portion of the material was pressed into the space between the rolls. During the start-up of the rolls, a short-term overload of the drive was observed, but this did not lead to its emergency shutdown. Then the press switched to the nominal operating mode. This method allows for increasing the degree of compaction of the material, but at the same time, there was a significant increase in energy consumption parameters.

Discussion
As a result, comparing the data given in Table 2, we found that the data of theoretical calculations correlate quite well with the data obtained during the experiment, the discrepancy is, on average, up to 15%. Such good correlation allows using the developed models in strength capacity calculations and drives power estimation at the design stage of screw feeders depending on roller press productivity and the expected properties variation of the different fractions of briquetted materials.
It was determined that the most rational, in terms of technological and energy indicators of the briquetting process, is 3.5-4.5 turns of the screw. It has been proven that the height of the column of material at the entrance to the screw cavity has a significant impact on the performance characteristics of the screw feeder. The use of a screw feeder for the studied materials allows for increasing the degree of their compaction by 17-27%.
In the future, the results of the above theoretical studies could be expanded on a wider range of materials with different mechanical properties (granulometric composition, moisture content, bulk density, etc.). This will make it possible to improve the proposed method and more accurately determine its applicability limits.

Conclusions
A computational and analytical method has been developed to determine and evaluate the influence of the design parameters of the screw feeder and the physical and mechanical characteristics of the briquetted material on the compaction parameters and power characteristics of the screw feeder of the roller press. An algorithm for the implementation of this method has been also developed.
Taking into account the physical and mechanical properties of fine-grained materials and geometric parameters of deformation, the conditions for determining the need for a screw feeder in the roller press design are formulated. Under these conditions, the screw feeder or any other forced material feed and pre-compacting device can be used in the roller press, if the required (or maximum) degree of material compaction cannot be achieved by gravitational feed into the gap between rolls. Graphical and analytical relationships between design, power and other technological parameters of the screw feeder of the roller press that can be used for finding optimum design solutions are established.
The analytical studies and industrial measurements of the screw feeder parameters on the roller press were carried out.
The obtained calculation and experimental data justify the theoretical provisions assumed and the method developed.
The results of this study allow us to prevent overloading of both screw feeder and roller press providing their reliable operation within the whole technological complex with the high quality of output product (briquettes).