**3. XRD analysis and QXRD (semi-quantitative XRD)**

XRD analysis is useful in the determination of crystalline phases that exists in a powdered sample [30]. Each phase has a specific, identifiable x-ray diffraction pattern, which is used to determine different phases present in the sample. X-rays are generated from the emission of high energy electrons from hot tungsten elements, which are bombarded on a copper metal target. This bombardment causes an electron emission from target atoms, thus generating an electron vacancy which is filled by an electron from higher energy orbitals and this transition generates x-rays. Filtration of these x-rays is performed to get monochromatic radiation which is bombarded on the sample being analysed. Bragg's equation is the main law used in XRD diffraction pattern analysis [31].

$$\mathbf{n}\lambda = \mathbf{2}\text{d}\text{sin}\Theta\tag{1}$$

λ = wavelength of x-rays, *n* = integer, *d* = plane spacing, Ɵ = Bragg's diffraction angle.

To derive Bragg's law, consider two x-rays (A and D) impinging on the atom B and E of a crystal and the angle of incident and angle of reflectance are equal as shown in **Figure 3**. Incident waves A and D are in phase with each other although wave D has to travel an extra distance of GE + EH to remain in the same phase as wave A. This extra

**Figure 3.** *Schematic of X-ray diffraction (left), Bragg–Brentano geometry (right).*

### *Testing and Validating Instruments for Feedstocks of Mineral Carbonation DOI: http://dx.doi.org/10.5772/intechopen.101175*

distance must have been an integral (*n*) multiple of wavelength (λ). The length GE and EH are equal and GE equals to *d sinn*Ɵ. Bragg–Brentano design is the most commonly used instrument geometry for high-resolution powder diffraction. The incident beam through a number of slits diverges towards the sample, the diffracted signal from the sample again converges through a number of slits towards the detector. A Ɵ/2Ɵ rotation is employed to keep incident and diffracted wave paths in symmetry. During sample scanning, the sample rotates by Ɵ while the detector is rotated by 2Ɵ with each step [32].

In practice, finely ground feed materials and carbonated samples (up to 100 μm size particles) were put in the instrument holder for analysis. Samples were finely grounded in order to avoid intensity fluctuations and preferred orientation. XRD analyses were performed using Philips X'Pert Pro multipurpose diffractometer with Cu radiation and 2θ from 5 to 90° or 11 to 31° depending upon the sample being analysed. Collection time used was 1 s with a step size of 0.02°. The patterns from XRD were matched with the International Centre for Diffraction Data ® (ICCD) using X'Pert Highscore® in order to identify crystalline phases. A typical x-ray diffraction pattern for feed dunite is shown in **Figure 4**. Phases identified are lizardite, olivine, brucite and magnetite.

If the reference intensity ratio (RIR) of an analytical phase i (such as silicon) is known, then its concentration can be calculated by doping the original sample with the analytical phase. This can be done by the addition of a known amount of standard (silicon) of which the RIR is known. For semiquantitative method details please see below.

After obtaining the diffraction pattern of the doped sample, the concentration Ci in the original sample is calculated as follows:

$$\mathbf{C}\_{\mathbf{i}} = \mathbf{A}\_{\mathbf{x}} \times \left(\frac{\mathbf{I}\_{\mathbf{i}}}{I\_{\mathbf{x}}}\right) \times \left(\frac{\mathbf{RIR}\_{X}}{\mathbf{RIR}\_{\mathbf{i}}}\right) \tag{2}$$

Ci = concentration of given phase i in the original sample Axe = known amount of standard (silicon) added to the original sample

**Figure 4.** *Typical x-ray diffraction pattern for dunite. L, Lizardite; O, olivine; B, Brucite; M, magnetite.*


### **Table 2.**

*Semiquantitative XRD analysis.*

Ii, Ix = intensities (peak areas) of phases i and x in the doped sample RIRi, RIRx = reference intensity ratio values of i and x respectively The procedure is described below in detail (for calculation detail see **Table 2**)


### **3.1 ICP-OES (inductively coupled plasma: Optical emission spectrometry)**

The elemental composition of solid and liquid samples can be determined using ICP-OES. ICP-OES consists of two major components; the torch and optical spectrometer. The torch comprises quartz tubes [33]. To produce plasma, argon gas is normally used, which passes through the tubes around the induction coil. The argon gas is "ignited" by the Telsa unit and the ionisation process (plasma formation) is initiated. The ionisation of argon gas occurs at this stage. A plasma having approximately 7000 K temperature is generated because of collisions between neutral argon atoms and charged particles [34]. Using a peristaltic pump, an aqueous sample is continuously supplied to the nebuliser where it changes to mist and moves to the plasma envelope. The introduced sample interacts with electrons and ions in the plasma and is converted into charged ions. This causes the decomposition of different molecules into respective atoms that lose electrons to induce the emission of radiation of distinctive wavelengths of elements present inside the sample. The optical spectrometer separates these wavelengths into component wavelengths. Intensities are

*Testing and Validating Instruments for Feedstocks of Mineral Carbonation DOI: http://dx.doi.org/10.5772/intechopen.101175*

**Figure 5.**

*Photo of the ICP-OES set-up and microwave digestion system. A, autosampler with standards and sample tube holders; B, plasma chamber; C, gases exhaust; D, computer for analysis output; E, argon gas cylinders; F, chiller; G, microwave digestion system.*

compared with the intensities of standard solutions of known element compositions and elements concentrations are computed based on the calibration curves. ICP-OES set-up and microwave digestion system is shown in **Figure 5**.

Solid samples used in the present investigation were first digested in acidic solution. Dunite sample (0.1 g) was digested in a microwave oven (**Figure 5**) using a mixture of 4.5 mL HNO3 (65%), 4.5 mL HCl (37%) and 3 mL HBF4 (tetrafluoroboric acid, 50%). Thulium (50 μL) was added as a tracking element. The volume of this mixture was increased to 20 mL by the addition of 2% nitric acid prior to its digestion in the microwave. Digestion was not required for supernatant solution samples and they are diluted using 2% nitric acid to the required level (50%/100% dilution) prior to their analysis by ICP-OES (Varian, Australia). The typical curve for ICP-OES is shown in **Figure 6**. Mg concentration drop with the passage of time due to magnesite precipitation. Si concentration increases during the first hour but then it stays constant, which is due to simultaneous silicon leaching from dunite and its precipitation in the form of silica.

### **Figure 6.**

*Typical curve for ICP-OES. The graph represents Mg and Si concentrations variation with time for supernatant solution of sub 75 μm heat-activated dunite carbonated sample. Carbonation reaction was performed with 15% solids slurry at 185°C, 130 bar pressure and using 0.64 M NaHCO3.*

## *3.1.1 Scanning Electron microscope (SEM)/energy dispersive scattering (EDS)*

Morphology, surface topography and elemental compositions of feed materials and carbonated products were determined using SEM (Zeiss Sigma VP FESEM) and EDS (Bruker). SEM scans a fine electron beam over the material being analysed and uses different detectors to reconstruct the image from signals produced from the sample [35]. SEM consists of different parts, e.g., microscope column which also includes electron gun and electron beam travels in this column; the computer that drives the microscope; ancillary equipment which analyses the composition. SEM can magnify objects from 10 times to 300,000 times. Scanning from an electron microscope can be compared with a person having a torch and looking for objects on the wall. As a person builds an image in his/her memory, SEM works in the same way and uses a fine electron beam instead of the torch to build an image.

EDS is a technique that provides information about the chemical composition of the sample. For EDS, an electron beam is focussed on the sample during SEM analysis and these electrons interact with the atoms. X-rays are produced from these interactions and an energy dispersive detector detects these x-rays and displays a signal in the form of spectrum, histogram or intensity versus x-ray energy. This makes it possible to identify elements present in the sample.

Sample preparation is important for SEM. Samples are gold (imaging) or carbon (EDS) coated prior to their analysis. Gold coating provides a thin layer to the samples and samples were coated four times at a 90° angle and fifth time from the top. A typical SEM micrograph and EDS spectrum of dunite feed sample are shown in **Figure 7**. SEM shows an image of the dunite feed and the EDS spectrum indicates intensities of the elements present in the sample. Polished resin blocks were used to study the silica-rich layers. Polished resin blocks were prepared using feed material, carbonated products and resin. Photo of polished resin blocks and sample holders is shown in **Figure 8**. The polished resin block samples preparation procedure is given in appendix 3D.

### *3.1.2 Transmission Electron microscope (TEM)*

TEM is useful to study the structure, properties and compositions of different mineral powders, especially in the submicron range. Mineral particles should have

**Figure 7.**

*SEM micrograph and EDS spectrum of dunite feed. (a) Dunite feed SEM micrograph, 10 μm is a resolution of the SEM (b) EDS spectrum of dunite feed, intensities of different elements are shown, Mg, magnesium; Si, silicon; O, oxygen; Fe, iron.*

**Figure 8.** *Photo of the sample holders (left) and polished resin blocks (right).*

been in 50–100 nanometres size to be properly analysed by TEM. Electrons transmission through the mineral particles enables detailed analysis of the particle features especially its crystal structure, orientation and chemical composition. In the present study, TEM was used to identify shell (silica-rich layers) and core part of the reacted mineral particles and study the corresponding elemental compositions and structure.

To prepare samples for TEM (JEOL 2100 TEM) analysis, 10 mg of sample powder was added to the pestle and mortar. Ethanol (4 ml) was mixed with the sample powder and contents were ground for 3 min. Ground sample was moved into a 5 ml plastic vile and sonicated for 20 min. Using pipette 1–2 drops were dropped on the TEM grid (200 mesh Cu, ProSciTech) and air-dried overnight prior to TEM analysis.
