Analysis of the Impact of the Consolidated Material on the Morphology of Briquettes Produced in a Roller Press

This article presents the results of saddle-shaped briquettes produced in a roller press from 15 fine-grained materials subjected to long-term seasoning. Experiments were carried out to determine the capacity of agglomerates, their surface roughness, compressive strength, excessive material, and superficial defects. Although a molding surface with molding cavities with the same shape was used for briquetting, significantly different results were obtained due to the different types of materials used for briquetting. Briquettes made from metallic and inorganic materials were characterized by a strength that increased with their density, which was not observed in carbonaceous materials. The tests are innovative and should be considered as pilot tests.


Introduction
The consolidation of dusty or fine-grained materials offers many benefits (Ref [1][2][3][4][5][6]. A widely used method for the consolidation of loose materials involves pressurized agglomeration ( Ref 7,8). This process involves exerting pressure on a material that causes the grains to move closer and form many types of strong bonds ( Ref 8). The product of the pressurized agglomeration process is compact shapes with relativity low porosity and specific mechanical strength, the sizes of which can be up to several thousand times larger than the input material grains (Ref [7][8][9]. The continuity of the process ensures a smaller energy demand and a longer molding element and roll press lifetime compared with other industrial briquetting machines, e.g., screw or punch ones (Ref [10][11][12][13][14][15][16]. Due to the diverse properties of fine-grained materials, the requirements set forth for blocks are met if the configuration and design properties of the roller press compaction system are correctly selected ( Ref 17). Mutual differentiation of the working surfaces of both rolls prevents unfavorable phenomena from occurring during material consolidation ( Ref 18,19). In this way, an asymmetric compaction unit is established (Fig. 1). This is particularly useful for materials that are difficult to briquette in a roller press, i.e., those that are characterized by high moisture, high compaction degree necessary for consolidation, low bulk density, the presence of hydrophobic grains, those that tend to be suspended in hoppers and dispensers, and those with a high elastic deflection after pressure is removed. The use of an asymmetric compaction unit enables an increase in the moisture range within which the material can be briquetted (Ref 11,20). It also eliminates briquettes from cracking in half along the plane of mutual closure of cavities on both rolls ( Ref 18,20). This results from a more favorable and uniform distribution of pressure exerted on the briquette being molded (Fig. 2) compared with the distribution obtained in a symmetrical system (Ref 20) and, consequently, a better material deformation distribution in the molding cavities ( Fig. 3) ( Ref 21). The essential factor observed over the many years that roller presses have been operated is that the briquetting of materials with various properties, despite using the same molding cavities, does not produce briquettes with the same shapes. Additionally, their volumes are usually smaller or bigger than the rated one, and, depending on the properties of the raw material, different defects appear on the agglomerate surfaces. Briquette materials may also have different storage properties; thus, the briquettes produced from different materials underwent morphological tests after long-term seasoning. The results provide technologically important information about the impact of the material type on the ability to make briquettes and their resulting durability, which limits the assessment of their flowability, compactness, and geometric features. This work analyzes the briquette from the perspective of materials engineering, due to there being an indication of the interactions between structure, properties, manufacturing technology, and the use of resulting briquettes. The analysis is based on the basic physical, chemical, and mechanical properties of the briquetted material.

Materials and Methods
The first stage of the tests was to consolidate the materials to produce briquettes. This was done using a roller press with a 450-mm roll pitch diameter with an installed compaction unit for producing saddle-shaped briquettes (Fig. 1b) with a rated capacity of 6.5 cm 3 . The outline view of the molding surface used to consolidate the material is presented in Fig. 4. The press was equipped with a cycloidal gear motor with a power of 22 kW and a frequency converter that enabled infinitely variable control of the revolutionary speed of the rolls. All materials were consolidated using a gravity feeder with a roller revolutionary speed of 4.25 rpm, which corresponded to the peripheral speed of the rolls equal to 0.1 m/s with an inter-roll gap of 1 mm. After production, the briquettes were seasoned at room temperature for 10±2 years. Fifteen materials were used for tests, which were classified into three groups: materials of organic origin, metallurgical and heavy industry waste, and materials of inorganic origin. Before the consolidation process, the materials were thoroughly mixed and brought to a proper moisture h (provided with the material) enabling them to be consolidated in a roller press. A binder was added to some of them.
The materials of inorganic origin were: • phosphorite (h=3.0%)-marked with the symbol M, • sodium tripolyphosphate-marked with the symbol N, To compare the volume of the tested briquettes with the rated value, we measured the projection of the largest area of the samples and the maximum thickness along the direction normal to the projection. Based on this information and the known volume of the model briquette formed from a completely filled forming cavity, the real volume of briquettes was determined according to the formula: where V rzb -volume capacity of a cuboid designed based on the projection area and the largest thickness of the tested briquette (mm 3 ), V teoret -theoretical briquette volume capacity  due to the die geometry (mm 3 ), and V teoretb -volume of a cuboid determined based on the projection area and the largest thickness for the benchmark briquette model (mm 3 ). The surface area was determined based on an image of the briquettes using LabVIEW environment with Vision Assistant. Also, the area of material allowances and losses was measured against the rated outline of the molding die element that was theoretically determined from a briquette model developed using CAD software. Due to the dimensional and geometric diversity, each briquette was rescaled to make its outline without allowances overlap with the theoretical outline. A measurement performed for four briquettes for each of the tested materials was considered to be representative. To determine the density of the briquettes, their weight was measured using a SA-120CE laboratory scale by Shinko Denshi. To examine the surface structure properties, roughness  where r c -compressive strength (MPa) and HLD-Leeb hardness (HLD). Figure 5 presents images of all briquettes made for tests. The measured thickness range of the briquettes for each material and the range of areas of allowances and losses compared with an ideal rendering of the die shape are presented in Figs. 6 and 7. It varied and was strongly dependent on the properties of the consolidated material. For most materials, the thickness of the briquette was greater than the theoretical value obtained from the die geometry. This was observed in all samples from the group of briquettes made from metallurgical and heavy industry waste, except zinc oxide with a binder and inorganic materials. This discrepancy resulted from the fact that the materials were consolidated in roller presses partly within the elastic strain range and expanded when the pressure was released. This is referred to in the literature as relaxation (Ref 11). This relaxation increased the briquette dimensions compared with the rated dimensions of the molding die. For most briquettes made of carbonaceous materials, the briquette thickness was smaller or similar to the theoretical value. This may result from briquette delamination during consolidation, or from sample shrinkage due to a significant decrease in their moisture during seasoning. The briquettes also differed in their projection areas. Comparing their outline to the outline of the molding pocket shows allowances (the briquetted material protruded beyond The surface structure test results of the examined briquettes differed depending on the type of briquettes material, but they did not differ within the material groups. Four main surface morphology features were observed: cracking, scratches, crumbling, and granularity. The presence of cracking and crumbling ( Fig. 8a and c) indicated a fragile cracking of the surface layer due to tension acting on the material during consolidation in a molding die. Such features were observed on the surface structure of samples of waste blasting sludge with binder and lignite. Scratches formed (Fig. 8b) due to relative motion at the briquetted material-molding interface which caused grooving and micro-cutting on the newly formed surface. This can be seen on the surface image of briquettes with zinc oxide/binder. Such interactions between the consolidated materials and the die also contributed to the rapid destruction of the molding cavity elements (Ref 23). The granularity (Fig. 8d) may indicate small plastic deformation of the material being consolidated and fragile properties of particles being consolidated, which made it difficult for material bridges to form. This feature was observed in the materials, which causes difficulties during molding. The granularity may also be a result of the crumbling of the surface layer of the briquette during formation -the highly granular surface was a fracture surface formed after the material cracked and crumbled off, as visible on the surface of mill scale and binder samples. Among the briquettes made of inorganic materials, some did not have clearly distinguished surface properties (sodium tripolyphosphate, calcium fluoride). Such homogeneous surfaces showed that the structure of the briquettes being molded was not damaged nor was there a distinctive friction interaction between the die part and the surface during consolidation. This was the best outcome due to the properties of the briquette and the lower consumption of the molding surface and clear symptoms of tribological wear ( Ref 24).

Results and Discussion
The granularity share on the surface of the briquettes is related to the R a roughness parameter in all examined briquette groups. Higher R a values were not observed at higher granularity. Other surface morphology features were not directly reflected by R a . Figure 9 presents a summary of the relationships between the compressive strength of the tested briquettes and their density.
The briquettes of carbonic origin and metallurgical and heavy industry waste origin showed the highest specific strength among the tested groups, while the briquettes made of materials of metallic origin showed the lowest. The strength increased with the density for the briquettes made from metallurgical and heavy industry waste and inorganic materials. The opposite trend was observed for briquettes made of Based on the summary of the compressive strength and R a (Fig. 10), it can be stated that the surface roughness of the produced briquettes increased with the strength. This was related to the fact that the particles of briquettes with a higher strength were less susceptible to plastic deformation. In some briquettes, the friction at the interface of the material being consolidated and the molding tool produced numerous grooves and scratches. This initiated surface cracking and increased the  roughness. This was best demonstrated in the briquettes made of waste blasting sludge with binder and manganese blast furnace dust with a binder. The high roughness of the shavings from bearing alloys without binder arose due to the structure of the chips preserved on the briquette surface.

Conclusion
The compaction method employed for all tested materials did not cause briquettes to break in half along the plane of  The R a roughness parameters juxtaposed with the compressive strength of briquettes: A-lignite (Poland), B-coal dust with binder, C -graphite dusts with binder, D-manganese dust with binder, E-mill scale with binder, F-BOF sludge with binder, G-manganese blast furnace dust with binder, H-ferrosilicon with binder, I-waste blasting sludge with binder, J-scale with binder, K-shavings from bearing alloys without binder, L-zinc oxide with binder, M-phosphorite, N-sodium tripolyphosphate, O-calcium fluoride without binder mutual closure of cavities on both rolls. Shrinkage occurred mainly in briquettes containing carbonaceous materials following the briquetting process. In nearly all materials, an increase in the compressive strength of the briquettes also increased the surface roughness R a . It was also found that the strength of the briquettes made of metallic and inorganic materials increases with their density, which was not observed in carbonaceous materials. The microscopic surface examinations made it possible to distinguish four major features of the surface morphology of the briquettes, i.e., cracking, scratches, crumbling, and granularity.

Open Access
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