CHAPTER 3

METHODOLOGY

This chapter describes the details of the experimental procedure followed

in this study. Also, the methods in obtaining data as well as brief discussions on

the theoretical framework of the study are presented.

3.1 Determination of Process Temperature and Pressure

The process temperature can be calculated theoretically using equation

2.24. Equilibrium temperature is the temperature in which the formation of

products is equal to formation of reactants. Equilibrium temperatures occurs at a

certain pressure and this happens when ∆ G is zero (0). Enthalpies and entropies

are needed to calculate the equilibrium temperature. Table 3.1 shows the ∆Hf° and

∆S° magnesium, calcium and iron (II) carbonates. The “° “symbol indicates that

the conditions are at STP.

Reaction ∆Hf°, cal/mol-K

∆S°, cal/mol

CaO(s) + CO2 (g) CaCO3 (s) -38560 -32.6MgO(s) + CO2 (g) MgCO3 (s) -27800 -21.45FeO(s) + CO2 (g) FeCO3 (s) -19790 -43.10

Table 3.1. Enthalpy and Entropy of magnesium, calcium and iron (II) Carbonates

From the data given above, equilibrium temperatures can then be

calculated. Consider the formation of calcium carbonate:

CaO(s) + CO2 (g) CaCO3 (s) Eqn. 2.17

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Using equation 2.21, we can derive an empirical equation for the

calculation of Gibb’s free energy.

∆G°=¿∆Hf - T∆S° Eqn. 2.21

Substituting the values in the table,

∆ G=−38560+32.6 T (CaCO3 (s))

∆ G=−27800+41.45T (MgCO3 (s))

∆ G=−19790+43.1 T (FeCO3 (s))

To calculate for the equilibrium temperature, equation 2.24 is used.

T=∆ Hf∆ S

Eqn. 2.24

Substituting the data given in the table, we obtain the following equilibrium

temperatures:

Reaction Temperature ,℃CaO(s) + CO2 (g) CaCO3 (s) 909.67MgO(s) + CO2 (g) MgCO3 (s) 397.54FeO(s) + CO2 (g) FeCO3 (s) 186.01Table 3.2 Equilibrium Temperature of magnesium, calcium and iron (II) Carbonates

Since magnesium silicate is the major component of the Ferronickel slag

from PGMC, the equilibrium temperature of magnesium carbonate is observed.

Since at this temperature, the formation of products is equal to the formation of

reactants, any temperature below the equilibrium temperature can be used.

However partial pressure of CO2 is another consideration. To determine the

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partial pressure of CO2 we used equation 2.26. From our experimental design,

adding pressure is not possible, a standard pressure will be assumed (1 atm).

k=e−∆G°

RT Eqn. 2.26

The temperature that can be used is 350°C, with partial pressure of 0.68

atm and a ∆ G of -1970 J/mol which indicates that the reaction is possible.

3.2 Materials and Equipment

The Ferronickel slag from PGMC in Iligan City was used as a medium in

sequestering carbon dioxide in this study. Chemical composition of the slag prior

to the carbon dioxide sequestration was determined using XRF.

The carbon dioxide used in this experiment was purchased from Sugeco

with at least 95% purity and zero moisture.

The experiment was conducted using a fixed bed reactor having a vertical

electric resistance furnace with an alumina working tube allowed for heating

samples. Thermocouples of either type A or C was equipped for measurement of

temperatures. The furnace could be set at different temperatures, ranging from 0

to 1200°C. A manometer was used to primarily measure the pressure of carbon

dioxide and regulate its flowrate. The flowrate of the carbon dioxide was

determined using bubble test experiment. A ball mill grinder (pulverizer) was

used in reducing the particle size to increase the surface area of the slag. Tyler

mesh screens and a mechanical shaker was used in separating t he different

particle sizes of the ground slag. The different samples were weighed using

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analytical balance. XRD was used to quantify the amounts of carbonates formed

in the slag. Samples were sent to National Institute of Geosciences (NIGS) in

University of the Philippines-Diliman for the XRD Analysis.

3.3 Ferronickel Slag Preparations

The Ferro-Nickel slags are of large or bulk sizes, hard, and dark gray in

color. First, the slag was sun-dried to remove some of the moisture before

grinding to increase the efficiency of obtaining fine particles. Particle size was

reduced using a ball mill to increase the slag surface area. The ground slag was

screened to obtain the desired particle sizes of 1.41mm, 0.177mm, 0.149mm, and

0.074mm using Tyler mesh screens with mesh numbers 12, 80, 100 and 200,

respectively. The Tyler mesh screens were shaken using mechanical shaker to

obtain efficient screening of the slag. After sieving, the pulverized samples were

placed in an oven for two (2) hours at about 200± 5℃ to remove the remaining

moisture. Prepared ferronickel slag samples were placed in resealable plastics and

kept in desiccators.

3.4 Calibration of Instruments

The instruments used in the study were calibrated prior to using. The

muffle furnace’s temperature was checked by placing a standard thermocouple

inside the furnace and a calibrated testometer was used in reading the temperature

outside. The temperatures obtained were compared to the displayed temperature.

The analytical balance was calibrated by first checking the spirit or bubble

of the balance and ensuring that it is on the center. Test weights were used to

check the accuracy and reproducibility of the balance.

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3.5 Experimental Set-up

Figure 3.1: Experimental Design Set-up

Optimal Carbon dioxide gas flowrate and pressure stored in CO2 gas

cylinder was regulated by the manometer. Then the regulated CO2 gas (5 MPa;

2L/minute gas flowrate) was allowed to flow into a braided chemical hose leading

to ¼”-diameter copper tube and finally into the muffle furnace. The braided

chemical hose and the copper tube were fitted to connect together using stainless

steel hose clamp and were tied sealed with thread seal tape to ensure no leaks. The

copper tube encased in a ceramic tube was connected to the exhaust of the muffle

furnace serving as an entry point for the CO2 gas; leaks were sealed using clay.

Another ¼” diameter copper tube was installed in the experimental set-up which

served as exhaust to excess gas. Stainless steel plate was used as the fixed bed for

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the 3g ferronickel slag sample since it is non-reactive to CO2 gas and can resist

temperature up to 1510 °C.

3.6 Bubble Test Method

To determine the optimal gas flowrate of the carbon dioxide, bubble test

experiment was prepared. A burette was filled with water until the 50-mL. The

outlet of the cylinder (braided chemical hose) was placed under the tip of the

burette. The appearance of the first bubble was timed until it reaches the 50-mL

mark of the burette that is filled water. The test was performed in triplicates.

Figure 3.2. Bubble Test Experiment Set-up

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3.7 Carbon Dioxide Sequestration

Three (3) grams of pulverized slag will be obtained having different

particle sizes and was placed in the muffle furnace injected with 2L/minute of

CO2 gas. The calibrated muffle furnace will be heated at 200 ± 10°C, 300 ± 10°C,

and 400 ± 10°C, for 30 minutes, 60 minutes, and 120 minutes reaction time,

respectively with the 3g ferronickel slag sample inside. The carbon dioxide

released from the gas cylinder through a braided chemical hose was regulated by

the manometer and its flowrate was determined using bubble test method. When

the temperature reached 200 ± 10°C, the slag and carbon dioxide was made to

react in the muffle furnace for 30 minutes. After 30 minutes contact time, the

furnace allowed to cooled down to 70°C before the slag was removed from the

furnace. The slags was stored in a desiccator for about ten (10) minutes and

allowed cool. The slag was weighed after cooling. The same procedure was

employed for the samples with different particle sizes, reaction time, and

temperature. During the whole experiment, temperature fluctuating is under

controlled within 10°C difference from the settings.

3.8 XRD Analysis

After the sequestration process, the slag samples were kept in resealable

bags and were sent to National Institute of Geological Sciences (NIGS), UP

Diliman for XRD analysis to verify the presence of carbonates specifically

magnesium carbonate (magnesite) that were formed. Microsoft Excel was used to

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plot and analyze the recorded intensity. The software Mineral Database from

International Center for Diffraction Data (ICDD) was used to identify the peaks.