Physicochemical Characterization of Moroccan Phosphate and Coke of a Solid Combustible

This paper concerns the study of the phosphate ore and a coke of solid combustible. The determination of their physicochemical characteristics and the study of their structural, textural and morphological aspects before and after thermal treatment are important and necessary for understanding of the reaction mechanisms and of the modelling of the processes of combustion and calcination. The characterization of these materials was accomplished according to various experimental techniques such as: X-ray diffraction; infrared spectroscopy, scanning electron microscopy, porosimetry and the BET method. The results obtained show that the X-ray diffraction pattern of the coke presents only the crystalline phase of the graphite. The diffraction pattern of the raw phosphate as well as the crystallographic parameters at 25°C (a = 9.36 Å, c = 6.90 Å) are consistent with those given in the literature and indeed correspond to those of the crystal lattice of pure fluorapatite. The specific surface of natural phosphate is in the range of 13.36 m2.g and that of the coke is 0.361 m2.g. The total pore volume of the phosphate ore measured by the condensation of nitrogen adsorbed was 0.178 mL and the mean opening diameter of the pores is 0.029 m. The real density of the phosphate (2.96 g.cm) and that of the coke (1.36 g.cm), were determined using an ultra pycnometer. Morphological observation of the grain surface shows fairly homogenous areas and a porous structure in the phosphate ore. The coke grains show on the contrary a difference in the grain size and a non-porous structure.


Introduction
Morocco has important reserves of phosphate ores.According to the chemical and granulometric characteristics of each phosphate type, the mineral extracted is subjected to treatments which are principally washing, drying, calcination and dry enrichment.All these processing operations are energy consuming.
In this context, we have initiated a research project that aims first to contribute to the energy optimization of industrial processes, particularly related to the calcination of phosphate ores, and second, to develop techniques for combustion, using coke as a solid fuel.Many authors have focused on the study of complex phenomena occurring during the combustion of coke (Dubuisson et al., 1995;Salvador et al., 2003;Slaoui & Bounahmidi, 2004).Others analysed the characterisation of the petroleum coke (Heintz, 1996;Suelves et al., 2002;Commandre & Salvador, 2005) and its application (Mé ndez et al., 2006;Murthy et al., 2014).
The present work is part of an extensive research and development program on energy optimization of the calcination process of the Moroccan phosphate ore using coke a solid fuel.We focused as a first step on the characterization of phosphate ore samples and coke samples used in the calcination of Moroccan phosphates.The main objective is to determine the physicochemical and mineralogical characteristics as well as the structural features, textural and morphological aspects of these samples before starting the study of their behavior in terms of combustion and calcination.

Materials and Experimental Techniques
The experimental study is based on two samples: the coke of a solid combustible and bright natural phosphate.These samples were provided by the CERPHOS (Center for studies and research on phosphates of the OCP group).

Sample Preparation
The homogeneity of the material and its particle size must be well controlled.For this, we subjected the samples to preparation operations such as:

 Coning and quartering
Sampling is an operation which splits a large sample into several smaller samples that are virtually identical.This operation can be accomplished using a splitter or manually through a series of divisions-mixtures.

 Granulometric separation
This operation consists of the separation of the desired elements from unwanted particles by sieving into three fractions: 1 < 100 µm, 100 µm < 2 < 400 µm and 3 > 400 µm 2 was kept, because it represents a great part of the sample (nearly 80%).

Materials Characterisation Techniques
The characterization of the samples was achieved using different techniques and experimental processes such as: X-ray diffraction, infrared spectroscopy, scanning electron microscopy; porosimetry and the BET method.

Radiocrystallographic Analyses
The use of X-ray diffraction permits determination of the structure of the samples.It allows, at first, to control the different phases of the studied material, and subsequently the determination of its crystal parameters.
Figures 1 and 2 present the The diffraction pattern performed on powdered samples, obtained by grinding the raw samples (phosphate and cokeof a solid combustible) at 25°C.The diffraction pattern of the raw phosphate sample (Figure 1) shows various lines.Some can be attributed to the carbonate fluorapatite ((CaF)Ca 4 (PO 4 ) 3 ) contained in the ore and others correspond to the carbonate CaCO 3 phase.
The values of the crystallographic parameters of the phosphate ore analysed are: a = 9.36 Å and c = 6.90 Å at 25°C.The X-ray diffraction pattern of the coke (Figure 2) shows a single line corresponding to the crystalline phase of graphite.

Infrared Spectroscopy
The presence of apatite and carbonate phases in bright phosphates are confirmedby infrared spectroscopy technique (Figure 3).The analysis of the infrared spectrum shows that:  between 4000 and 3000 cm -1 , a large peak is observed around 3480 cm -1 corresponds to the water absorption band.
It is a combination of antisymmetric stretching mode, symmetric stretching mode and an overtone of the bendingmode (Bernath, 2002 ;Tennyson et al., 2014),  the 2360 cm -1 peak corresponds to the atmospheric CO 2 ,  the 1644 cm -1 peak corresponds to the third vibration mode of adsorbed water molecule, known as angular deformity,  the 1425 cm -1 doublet isfrom carbonate  3 and the 873 cm -1 peak is from carbonate  2 .(Knubovets et al., 1997;Marshall et al., 1999).

Specific Area Measurement
The specific surface areas of the samples were measured by the BET method which consists of the adsorption of an inert gas at its liquefaction temperature on the solid.The specific surface of the solid is then calculated from conventional formulas proposed by Brunauer, Emmet & Teller (Brunauer et al., 1938).
The study of the adsorption-desorption isotherm is used to determine the distribution of the pore sizes.The isothermal adsorption-desorption of nitrogen on Youssoufia phosphate is reported in Figure 4.
The nitrogen adsorption-desorption isotherm on natural phosphate shows that it is of type II according to BDDT classification (Brunauer et al., 1940).It is characterized by a very gradual increase in the adsorbed amount based on the pressure equilibrium.This type of adsorption is a multimolecular adsorption characteristic, obtained during the adsorption on non-porous surfaces or macroporous adsorbents.
We also note the presence of a hysteresis between the desorption and the adsorption curves.This phenomenon is related primarily to the phase difference between adsorption and desorption of nitrogen in the solid pores.According to the IUPAC classification (Sing, 1985), the observed hysteresis is an H 3 type.It is observed in the case of an adsorbent that forms aggregates and can be attributed to capillary condensation taking place in a non rigid construction, but it is not a defined mesoporosity.Using this method, we have estimated for the phosphate mineral studied a specific surface area of about 13.37 m² .g - .

Analyze of Phosphate Porosity
In any calcination process, it is very useful to know the distribution of pores on the material surface.The best method used to study the pore distribution is the BHJ method (Barrett et al., 1951).This technique is used to determine the ratio (volume/area) of the pore distribution taking into account the change in the thickness of the adsorbed layer and the condensation in the pores of the solid.As a function of pore diameter of the natural phosphate, we report the evolutions of the cumulative volume (Figure 5), the volume distribution of the pore size (Figure 6) and the cumulative surface pores (Figure 7).We can also see the evolutions of the cumulative volume and volume distribution as functions of the equilibrium pressure (Figures 8 and 9).
The analysis of these curves show that:  for a pore size between 30 μm and 320 μm, there is a progressive gas adsorption until the recovery of the monolayer,  for a pore size between 0.1 μm and 31 μm, there is a saturation stage indicating no adsorption despite the increase in the equilibrium pressure.This is interpreted by the fact that the mesopores, in which capillary condensation is occurred, are no longer available for adsorption.
 when the pore size is between 31 μ m and 0.1 μ m, there is a saturation level that indicates a lack of adsorption despite the increased pressure balance.This is interpreted by the fact that the mesopores under which took place capillary condensation is no longer available for adsorption,  for a pore size less than 0.1 μm, the adsorption is produced at so high pressures and the pores are not large enough and do not substantially contribute to the specific surface area.From these results, we can deduce the total pore volume of the natural phosphate measured by the condensation of adsorbed nitrogen: V T = 111 mm 3 .g - and the mean pore opening diameter is: d = 0.029 mm.
The adsorption-desorption isotherm of nitrogen on coke of a solid combustible is illustrated in Figure 10.According to the BDDT classification (Brunauer et al., 1940), it shows that it is a type III and it is different from the adsorption isotherms of type II at low pressure.This change at the beginning of the adsorption isotherm is due to the weaknesses of the adsorbent/adsorbate interactions.Using this method, we found out that the specific surface of the coke of a solid combustible is approximately 0.36 m² .g - .

Scanning Electron Microscopy
This technique allows the observation of the sample morphology before treatment.The morphological observation of phosphate grains before treatment shows a fairly homogeneous zones and a porous structure.The morphology observation of the grain surface of coke of a solid combustible shows that there are different sizes of grains, heterogeneous areas and non-porous structure and anisotropic.

Conclusion
We determine the physicochemical and morphological characteristics of the phosphate and coke of a solid combustible in order to know the properties of these materials.This will help when it comes to analyse its behaviours during the phosphate thermal treatment process as well as during the different reaction mechanisms involved in the process of combustion of coke of a solid combustible and phosphate calcinations.

Figure 1 .
Figure 1.X-ray diffraction pattern of the raw phosphate.

Figure 2 .
Figure 2. X-ray diffraction pattern of the coke of a solid combustible.

Figure 3 .
Figure 3. Infrared spectrum of natural phosphate before treatment.

Figure 7 .
Figure 7. Cumulative surface versus the pore diameter in the natural phosphate.

Figure 8 .
Figure 8. Change in cumulative volume as a function of the relative equilibrium pressure.

Figure 9 .
Figure 9. Volume distribution of the poresize as a function of the relative equilibrium pressure in the natural phosphate.