Evaluation of Marblewood Dust’s ( Marmaroxylon racemosum ) Effect on Ignition Risk

: The paper deals with the selected characteristics, such as moisture, average bulk density, and fraction size, of tropical marblewood dust ( Marmaroxylon racemosum ) that inﬂuence its ignition risk. Research was focused on sieve analysis, granulometric analysis, measurement of moisture level in the dust, and determination of the minimum ignition temperatures of airborne tropical dust and dust layers. Samples were prepared using a Makita 9556CR 1400W grinder and K36 sandpaper for the purpose of selecting the percentages of the various fractions (<63, 63, 71, 100, 200, 315, 500 µ m). The samples were sized on an automatic vibratory sieve machine Retsch AS 200. More than 65% of the particles were determined to be under 100 µ m. The focus was on microfractions of tropical wood dust (particles with a diameter of ≤ 100 µ m) and on the impact assessment of particle size (particle size < 100 µ m) on the minimum ignition temperatures of airborne tropical dust and dust layers. The minimum ignition temperature of airborne marblewood dust decreased with the particle size to the level of 400 ◦ C (particle size 63 µ m).


Introduction
The timber industry is involved in the processing of ecologically safe raw materials with excellent utility properties.Flammability is a controversial feature, where on the one hand we want the product, e.g., wooden beam or door, to be nonflammable [1][2][3], and on the other hand wood as a fuel is required to have the highest possible calorific value.The wood composite materials industry is an important part of the wood processing industry.Its products provide very useful large-area materials with a wide range of uses and significantly contribute to the higher recovery of wood raw material [4,5].Both aspects play a decisive role in the unprecedented dynamism of the sector on a global scale.
The wood materials industry is also strong and promising in Slovakia.It maintains a standard position, retaining on average a 17.5% share in the total number of companies according to [6] and is also one of the dominant producers of waste [7].The character of the wood dust generated during production is influenced by its preparation and the type of wood processed.Significant attention in woodworking practice is given to the issue [8][9][10][11] as well as to the system of their extraction [11].Lesser attention (but not an insignificant amount) is paid to the issue of their existence and the resulting negative impact on the work environment and on the human body.Microclimatic conditions contribute to the stabilization of the generated proportions of wood dust, which forms a dust-air mixture when agitated [12,13] or gets deposited on the surface of technical equipment [14].The number of formed fractions of wood dust particles depends on the type of technological equipment used in the preparation of the wood [15].The type of wood also has a significant effect on the composition of wood dust.Here, several problems come to the fore, which interact with each other and ultimately have a significant impact on both the health of employees and the occurrence of a fire or explosion [16,17].The first problem is the work environment.As part of climate change, the ambient temperature rises regularly, and summer and tropical days increase in the middle of the year.There is a decrease in ambient air humidity and an increase in working temperature [18].All the listed parameters support the existence of dust particles.The second problem is the creation of very fine dust particles, thanks to the use of "eco-friendly" wood processing technologies, i.e., the processing of dry native wood without the addition of chemicals.Emphasis is placed on the quality of the obtained surfaces, for example by grinding and smoothing (leading to the formation of ultra-fine dust particles).Thirdly, also in the context of globalization, tropical woods are being introduced into our market, about which little scientific knowledge is available, especially in terms of their behavior in the event of the creation of undesirable dust-air mixtures during their mechanical processing.Studies [19][20][21] have shown an increase of interest in tropical woody plants for their excellent performance in exteriors and high stability.The preference for their use, especially in interior design, is growing, because the use of domestic wood is focused on industrial processing (such as paper production).
Marblewood (MW) originates from Northeastern South America.This heartwood (1005 kgm −3 ) is yellow to golden brown, with irregular brown, purple, or black streaks [22].Its common uses include flooring, sliced veneer, turned objects, cabinetry, and fine furniture as well as historical artifacts [23,24].There exist works about MW as a resonance material [25].Vivek et al. [26] introduced the possibility of using marblewood dust as a partial replacement for cement and sand in concrete.Marblewood (MW) is one of the most used tropical wood materials in various applications such as timber [27,28] or art objects [23].More studies about marblewood are focused on the structure of marblewood and the quality its surface for its attractive design [29,30].There is a lack of knowledge about the parameters assessing the ignition risk of marblewood and its dust.
The aims of the article are to show the granulometric structure of wood sanding dust from marblewood (Marmaroxylon racemosum), to show that its shape determines its physical properties, and to assess the impact of particle size (particle size < 100 µm) on the minimum ignition temperature of airborne marblewood dusts.

Materials and Methods
MW dust samples were prepared from raw material, marblewood (Marmaroxylon racemosum) with a density of 1000.08 ± 9.98 kgm −3 .The basic samples with dimensions of 120 × 35 × 35 mm 3 and a moisture content of approximately 8-10% were made in a private company that manufactures interior elements based in Žilina (Slovakia) using a wood cutting saw (CNC Panel Saw Machine, Shandong, China) (Figure 1).The dust samples (Figure 1) were sanded using a Makita 9556CR 1400W disc sander (Makita Numazu Corp., Branesti Ilfov, Romania) and K36 sandpaper (Topex, Kinekus, Žilina, Slovakia).A detailed description of the procedure is given in [31].
The moisture value of the tropical wood samples was determined using granulometric analysis according to [32].The bulk density of wood dusts was determined accord- The dust samples (Figure 1) were sanded using a Makita 9556CR 1400W disc sander (Makita Numazu Corp., Branesti Ilfov, Romania) and K36 sandpaper (Topex, Kinekus, Žilina, Slovakia).A detailed description of the procedure is given in [31].
The moisture value of the tropical wood samples was determined using granulometric analysis according to [32].The bulk density of wood dusts was determined according to [33].Sieve analysis was conducted on a Retsch AS 200 sieve shaker vibration machine (Retsch AS 200 control, Retsch GmbH, Haan, Germany) in line with [34].
The microscopic analysis was realized using a Nikon Eclipse Ni (Nikon Corp., Tokyo, Japan) with a Nikon DS-Fi2 camera (Nikon Instruments Inc., Melville, NY, USA) for the determination of the shape and the size of marblewood dust particles.The microscopic analysis (for 500, 315, 200, and 100 µm fractions) was completed at the Institute of Research in Banská Bystrica, Slovakia.The determination of the particle size was limited by the technical specifications of the cameras.The scans of fraction sizes <63, 63, and 71 µm were unreadable.A detailed description is given in [35].
The measures of the minimum ignition temperature of airborne dusts (Figure 2a) were conducted in a test equipment (VVUÚ, a.s., Ostrava, Czech Republic) with the help of the following accessories: automatic weighing machines (Steinberg Systems, Łódź, Poland), HL 100 ZU EINHELL air compressor (Einhell Corp., Landau an der Isar, Germany) and ALMEMO equipment (Ahlborn, Berlin, Germany) (Figure 2a).The procedure was conducted in line with [36] and details are described in [35].The minimum temperature of airborne dusts was tested in 10 experiments with constant volumes of marblewood dust and different air flows.
The minimum ignition temperature of a dust layer (thickness 5 mm) was determined on an electrically heated plate, 185 mm in diameter, according to [36] (Figure 2b).This standard defines the minimum temperature for ignition of a dust layer as the minimum temperature of a hot surface at which ignition can occur of a dust layer with a set thickness located on that hot surface [37].The experiments were performed on a laboratory hot plate apparatus with temperature controller, type CLARE 4.0, with measurement errors of ±0.3 • C (Clasic, Czech Republic) in the experimental laboratory at the Faculty of Safety Engineering in Žilina (Slovakia).Detailed information about the behavior test is available in [38,39].

Results and Discussion
Marblewood (Marmaroxylon racemosum) is ranked among the hardwoods.Everything about working with it revolves around its incredible hardness and density [40].The density is measured based on the mixed particles' size before sieve analysis.
Different types of dry woods, dried to a constant weight (105 °C), have practically the same elementary chemical composition and the same density of wood substance [9].
Disintegration of wood into dust does not change the elemental chemical composition but changes the ratio of surface size to volume of disintegrated particles (example a metal ring indicating the height of the dust layer (5 mm)

Results and Discussion
Marblewood (Marmaroxylon racemosum) is ranked among the hardwoods.Everything about working with it revolves around its incredible hardness and density [40].The density is measured based on the mixed particles' size before sieve analysis.
Different types of dry woods, dried to a constant weight (105 • C), have practically the same elementary chemical composition and the same density of wood substance [9].
Disintegration of wood into dust does not change the elemental chemical composition but changes the ratio of surface size to volume of disintegrated particles (example Figure 3) and changes the density (Table 1).Density and humidity are standard values to be considered when evaluating the examined dust samples.The marblewood dust experiments were performed in agreement with other experiments.Dust is a crushed (dispersed) solid and can be dispersed in air to form a dust-air mixture.The particle size in dust dispersion is determined by sieve analysis (particle size distribution), which determines the percentage of particles of a certain size (Table 2).Dust dispersion has a significant effect on its ignition risk.More-dispersed dust has a considerably large surface area, increased chemical activity, and a lower minimum ignition temperature [42,43], as seen in Table 2.
Marblewood has the highest tested minimum ignition temperature (Table 2) as compared with other tropical wood dusts [35].
It was found that more than 65% of particles were sized below 100 µm.This fact determines the possibility of the formation of airborne dust mixtures.The airborne mixtures of wood dust pose a risk of reaching explosive concentrations.The experimental results show a minimum initiation temperature for the airborne dust mixture of 400 • C. As the particle size increases, the ignition temperature value increases, for 500 µm it is 420 • C. The minimum ignition temperature for airborne dust is the lowest furnace temperature at which ignition occurred in this process, reduced by 20 • C for furnace temperatures above 300 • C [36].This condition for the ignition of marblewood particles was met.
For dust layers, depth is the most important factor affecting the ignition temperature, particle size is not important, and density affected the ignition temperatures only for thin layers [13].
The MIT of a dust layer is the lowest temperature of a surface at which a dust layer resting on the surface can self-ignite.This is usually given for a 5 mm deep layer [44].Pastier (2013) [45] researched a variety of wood dust layer samples.For determining the minimum ignition temperature of dust layers, all samples (Table 3) were sieved through a 0.5 mm sieve.The question of the description of particle size is clearly addressed in the work of [47].Irregularity of particle shapes results from the anatomical structure of the wood mass, as their basis is the vessels in the lamellar structure [48,49].
The goal of all particle size determination techniques is to provide a simple number that indicates the particle size.However, particles are three-dimensional objects for which at least three parameters (length, width, and height) are required in order to provide a complete description.The particle size is given by the longest edge "d" of the rectangular parallelogram described around the particle (Figure 3), where length d is the largest dimension, thickness is the smallest dimension, and width is the intermediate dimension [50].An example of the shape of the marblewood dust particles is shown in Figure 4. Three dimensional scans were performed for fractions of 500, 315, 200, and <100 μm with a magnification of 0.63 zoom (27× magnification of the subject), 1 zoom (44× magnification of the subject), 2 zoom (88× magnification of the subject), and 3 zoom (132× magnification of the subject).Scans determine the shape for selected fractions.
The purpose of microscopic analysis is to show the diversity of the shape of the individual fractions.Figure 4 shows a selected series of pictures for each fraction.The first four are of a monolayer of dust, magnified at 0.63, 1, 2, and 3 zoom.The second series of pictures (foursome) show the same fraction (for example 500 μm) as a 1 mm continuous layer of dust under the microscope, again magnified at 0.63, 1, 2, and 3 zooms.Of course, the smaller the fraction, the greater the problem of preparing the monolayer.
The monolayer of the fraction below 100 μm is no longer pure (Figure 4).The particle An example of the shape of the marblewood dust particles is shown in Figure 4. Three dimensional scans were performed for fractions of 500, 315, 200, and <100 µm with a magnification of 0.63 zoom (27× magnification of the subject), 1 zoom (44× magnification of the subject), 2 zoom (88× magnification of the subject), and 3 zoom (132× magnification of the subject).Scans determine the shape for selected fractions.The purpose of microscopic analysis is to show the diversity of the shape of the individual fractions.Figure 4 shows a selected series of pictures for each fraction.The first four are of a monolayer of dust, magnified at 0.63, 1, 2, and 3 zoom.The second series of pictures (foursome) show the same fraction (for example 500 µm) as a 1 mm continuous layer of dust under the microscope, again magnified at 0.63, 1, 2, and 3 zooms.Of course, the smaller the fraction, the greater the problem of preparing the monolayer.
The monolayer of the fraction below 100 µm is no longer pure (Figure 4).The particle shape varies with the size of the fraction.Fractions of 500, 315, and 200 are µm were fibrous, curled, and articulated.The fraction below 100 µm formed clumps, and the shape was difficult to identify.
The visualization shows the difference in shape and area of the fractions.This fact is also observed on the change of the value of the minimum ignition temperature of airborne dust, also called of the ignition temperature of an explosive mixture according to [44].

Conclusions
Research on marblewood dust is part of the solution of a grant task dealing with the evaluation of fire parameters of domestic and tropical woody plants.The selection of experimental samples was based on the requirements of practice.Tropical woody plants have entered the Slovak market in all forms (decorative elements, furniture, building elements) and have been domesticated.Research into their properties will make it possible to gain a greater overview of the behavior of dust in our environment and will enable the creation of predictive models of its behavior.The fire risk of wood dust increases with the decrease in the size of the dust particles.The result is a decrease in the minimum ignition temperature.
Based on the obtained results of MW dust, it is possible to summarize the following: • MW dust has more than 65% of particles with a size below 100 µm; • at the same time, the most numerous group (39.61%) are particles with a size of 100 µm; • MW dust forms an airborne dust mixture with a minimum initiation temperature of 400 • C at the smallest particle size • an MW dust layer (5 mm) has a minimum initiation temperature of the ignition source of ( • C) <300 • C after 120 s.
Tropical marblewood has comparable parameters to domestic woods.Due to the expansion of the use of this wood in our country, it is possible to assume that it will have a similar behavior in its processing in our wood industry.

Figure 2 .
Figure 2. (a) The test equipment for the measurement of the minimum ignition temperature of airborne dusts; (b) experimental equipment "hot-plate" from [39].

Figure 2 .
Figure 2. (a) The test equipment for the measurement of the minimum ignition temperature of airborne dusts; (b) experimental equipment "hot-plate" from [39].

Figure 4 .
Figure 4. MW dust particles and their shape measured by Nikon Eclipse Ni with Nikon DS-Fi2 camera.Figure 4. MW dust particles and their shape measured by Nikon Eclipse Ni with Nikon DS-Fi2 camera.

Figure 4 .
Figure 4. MW dust particles and their shape measured by Nikon Eclipse Ni with Nikon DS-Fi2 camera.Figure 4. MW dust particles and their shape measured by Nikon Eclipse Ni with Nikon DS-Fi2 camera.

Table 1 .
Comparison of density and moisture of selected wood dusts with experimental sample.

Table 2 .
Experimental result analysis of marblewood dust.

Table 3 .
Results from the testing of the minimum ignition temperature of a 5 mm dust layer.