Utilization of formic acid solutions in leaching reaction kinetics of natural magnesite ores

    Utilization of formic acid solutions in leaching reaction kinetics of natural magnesite ores Nadeem Raza, Zafar Iqbal Zafar, Najam Ul-Haq PII: DOI: Reference:

S0304-386X(14)00179-0 doi: 10.1016/j.hydromet.2014.08.008 HYDROM 3942

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

12 November 2013 22 June 2014 16 August 2014

Please cite this article as: Raza, Nadeem, Zafar, Zafar Iqbal, Ul-Haq, Najam, Utilization of formic acid solutions in leaching reaction kinetics of natural magnesite ores, Hydrometallurgy (2014), doi: 10.1016/j.hydromet.2014.08.008

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ACCEPTED MANUSCRIPT Utilization of formic acid solutions in leaching reaction kinetics of natural magnesite ores Nadeem Raza, Zafar Iqbal Zafar, Najam-ul-Haq

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Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, 60800, Pakistan.

ABSTRACT

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In the present research work, dissolution kinetics of natural magnesite is carried out using

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formic acid as a leaching agent. The effect of various reaction parameters such as temperature, acidic solution concentration, particle size and liquid to solid ratio was studied regarding the

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leaching kinetics of natural magnesite. The findings show that the dissolution process is controlled by the chemical reaction (intrinsic) at the liquid-solid interface;

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1  (1  x)1/ 3  59.41  101 e 42078/ RT t .

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The apparent activation energy of the leaching process of magnesite with the formic acid was found to be 42.08 kJmol-1 over the reaction temperature range of 318 to 348 K.

Key words: Magnesite; Formic acid; Dissolution kinetics; Regression analysis, Kinetic model

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Corresponding author. Tel.: 0092300 6340861 Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, 60800, Pakistan. E-mail address: [email protected]

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ACCEPTED MANUSCRIPT 1. INTRODUCTION Magnesium, the 6th most common element does not occur freely in nature because of its

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high reactivity. It is abundant in magnesite, periclase, asbestos, meerschaum, serpentine, talc and

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epsomite (Deangelis et al., 2007). Magnesium is important in many aspects of life and its uses involve synthesis of Grignard reagent, alloy formation, sulfur removal in iron and steel

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production, refractory materials, fertilizers, medicinal products, and fireproofing (Bukovisky,

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1997; Jones et al., 2000).

A wide range of studies have been reported on leaching and dissolution kinetics of rocks

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with variety of leaching agents (Demirkiran, 2008, 2009; Demirkiran and Kunkul, 2007, 2008; Dogan and Yartasi, 2009; Kovacheva et al., 2001; Kuslu and Colak, 2010; Mergene and

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Demirhan, 2009; Sandstrom and Samuelsson, 2010; Zafar, 2008; Zhang and Nichol, 2010). In these research studies it has been described that the leaching kinetics of different metal ores may

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vary with the change in nature and type of rock deposits. Leaching of magnesite rocks can be achieved by acids (inorganic/organic) or bases and their salts (Lacin et al., 2005). The studies on the dissolution process of magnesite in inorganic acids such as HCl, H2SO4 (Abali et al., 2006; Chou et al., 1989) showed that the controlling mechanism for the dissolution of magnesite is chemical process. Inorganic acids have issues of selectivity, froth formation and scaling (Housmanns et al., 1996). Conversely organic acids can act as active leaching reagents because most of the leaching reactions are done in mild acidic conditions (pH 3-5). Moreover organic acids cause low risk of corrosion and can be utilized for the carbonaceous rocks. Another advantage of organic acids as leaching agents is their biodegradability which generally depends on the carbon chain and other attached groups.

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ACCEPTED MANUSCRIPT Raza et al. (2013) performed the magnesite dissolution in ascorbic acid. In this reported study, the shrinking core model was tested by analyzing kinetic data using various statistical and

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graphical methods. The observations revealed that the rate determining stage of the dissolution

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reaction of magnesite ore in ascorbic acid solution is a chemically controlled. From the leaching investigations of naturally occurring magnesite materials in acetic acid, gluconic acid, citric acid,

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and lactic acid solutions, it was found that leaching kinetics is driven by chemically controlled

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mechanism (Bayrak et al., 2010; Lacin et al., 2005; Lacin and Bakan, 2006).

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Magnesite ore deposits are found abundantly in the Khuzdar areas of Balochistan (Pakistan). The Khuzdar area consists of a number of Kraubath type magnesite deposits

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associated with alpine type bela ophiolite dating from the cretaceous period (Bashir et al., 2009).

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These are mostly serpentinized and harzburgite confined mostly in the lowermost segment of bela ophiolite. These deposits have not been considered extensively for leaching kinetic

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investigations up to this time. Moreover no literature has been reported concerning formic acid as a leaching agent for the study of leaching kinetics of indigenous magnesite. Therefore, the current research work has been intended to investigate the leaching reaction kinetics of the indigenous magnesite ore by formic acid. The product formed (magnesium formate) can be used in animal feed formulations and in general chemical applications (Shi et al., 2005). It can also be used for determining pesticides in fruits and vegetables and for the simultaneous determination of tropane alkaloids and glycol alkaloids in grains and seeds.

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ACCEPTED MANUSCRIPT 2. EXPERIMENTAL PROCEDURES 2.1. Sample protocol and analyses

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Samples of magnesite involved in current study were collected from Khuzdar area in the

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province Balochistan (Pakistan). Samples were ground in a ball mill followed by mortar grinder. Different size fractions (500-707, 250-354, 177-210 and 125-177 µm) obtained from screening

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of grounded samples by ASTM sieves, were dried in an electric oven at 100 °C for 24 h. These

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samples were brought to room temperature and stored. Atomic absorption spectrometry, scanning electron microscopy and other conventional methods (Furmann, 1963) were applied for

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the analysis of magnesite rock fractions. Formic acid (Methanoic acid), EDTA (Ethylene diamine tetra acetic acid), EBT (Eriochrome Black T) used during analysis were of reagent

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grade. The preparation of stock solutions and their further dilutions were made by deionized water. Eriochrome Black T and EDTA were used in volumetric quantitative determinations of

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magnesium in leached solutions. In complexometric titrations of magnesium (Gulensoy, 1974), EBT acts as an indicator while EDTA is used as a complexing agent for Mg. 2.2. Detection measurements and analytical procedure Atomic absorption spectrophotometer (Hitachi-1800) was used for the determination of Mg in natural magnesite sample. Scanning electron microscope (JEOL JED-2300) was used to observe the particle morphology of raw magnesite. Leaching studies of magnesite sample were investigated in a reactor made up of glass with 500 mL capacity. A hot plate (IKA C-MAGHS-7) equipped with temperature sensor (ETS-D5) was used to stir, heat and to control temperature (± 0.5 K) of reactor contents. In each experiment a fixed volume of 8% formic acid having L/S ratio of 14:1 mL/g was gradually introduced to the reactor having 5 g of sample. These entities were agitated with stirring rate of 350 rpm at known time and temperature. After completion of 4

ACCEPTED MANUSCRIPT reaction, the hot solution was filtered to remove gangue minerals from magnesium formate. The filtrate solution was analyzed to find the percentage of conversion of magnesite (Gulensoy,

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1974).

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3. RESULTS AND DISCUSSION

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3.1. Sample characterization

To find loss on ignition magnesite rock sample was heated at 950 °C for 24 h in a

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furnace. The carbonates of different elements were converted into their oxides with the liberation

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of CO2. The chemical composition of the calcined magnesite sample (Table 1) was determined by atomic absorption spectrometer. Table 1 indicates that the magnesium is present relatively in

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higher concentration than other elements. The EDX spectrum of the intact ore of magnesite (Fig.

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1) depicts the elemental composition. The elemental composition showing the mass and atomic

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% of different elements in the indigenous magnesite is given in Table 2. From the EDX signature it can be inferred that the material under study is magnesium rich. The profile also exhibits the presence of carbon & oxygen indicating the presence of carbonates. The magnesite in pure form is usually white in appearance but becomes colored due to inclusion of impurities. The SEM image of the raw magnesite ore representing the apparent morphological properties of the magnesite is given in Fig. 2. The material appears to be non-granular with surface roughness which may be due to the evolution of volatiles during weathering and formulation of ore.

3.2. Chemical reactions taking place in glass reactor Chemical reactions occurring in the reaction vessel containing magnesite and formic acid solution is represented as: (a) Formic acid ionization

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ACCEPTED MANUSCRIPT H



C H 2O2

 C H O21

(1)

(b) Diffusion of H+ ions



MgCO3



H 2CO3



Mg 2 

2C H O21



Mg CHO2 2

(3)

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

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(d) Reaction between Mg2+ and formate ions Mg 2 

(2)

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2H 

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(c) Attack of H+ ions on the magnesite particles

The ionization constant of formic acid is pKa = 3.75 at 20 °C and the solubility product constant

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for MgCO3 is 6.8x10-6 at 25 °C (Harned and Embree, 1934; Visscher et al., 2012). Ashraf et al.,

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(2005) described that the magnitudes of the constants (ionization constant & solubility product

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by the kinetics.

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constant) are generally temperature dependent and efficiency of the leaching reaction is restricted

3.3. Effect of time and reaction temperature The reaction temperature was varied from 45 to 75 °C to elucidate the influence of temperature on the rate of dissolution of magnesite while keeping the other experimental conditions constant (177-210 µm particle size, 8% formic acid, liquid/solid ratio of 14:1 mL/g and stirring speed 350 rpm). The results are shown in Fig. 3 which illustrates that increase in reaction temperature causes a rise in the rate of conversion of the magnesite. The elevation of reaction temperature from 55 to 75 °C in 30 min causes an increase in % recovery of magnesite from 49.8 to 84.9 %. This situation indicates that increase in the reaction temperature increases the rate of chemical reaction. Typical rate curves in Fig. 3 represents that the temperature rise reduces the reaction time needed to achieve the maximum conversion. About 95 % dissolution of magnesite is achieved at 75 °C in 40 min of leaching time. A number of experiments were carried 6

ACCEPTED MANUSCRIPT out to determine the effect of formic acid concentration, L/S ratio in the medium and the particle size of magnesite on the dissolution kinetics of magnesite at 65 °C.

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3.4. Influence of acid concentration and L/S ratio In order to find the influence of concentration of formic acid on the leaching of

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magnesite, the concentration of formic acid was varied from 2 to 10 % at temperature 65 °C,

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stirring rate 350 rpm and particle size fraction 177-210 µm. The results are shown in Fig. 4

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which shows that an increase in acid concentration accelerates the rate of leaching of magnesite. However concentrations higher than 8 % do not have an appreciable effect as expressed in Fig. 4.

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This situation may be attributed to the fact that a relatively higher concentration of the leaching agent may attack the gangue minerals present in the ore. Ozmetin et al. (1996) described that an

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increase in the concentration of leaching agent in the reaction vessel may increase the product layer formation and produce a solid film layer surrounding the particles and this may reduce the

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rate of leaching process. The influence of liquid to solid ratio was elucidated by varying the L/S ratio from 6:1 to 16:1 mL/g keeping the other experimental parameters constant. Experimental results have been expressed in Fig. 5. The rate curves in Fig. 5 show that increase in L/S ratio causes an improvement in the dissolution of magnesite. However its effect on leaching process is not as dominant as that of the temperature and formic acid concentration. Higher liquid/solid ratio results in an increase of volume of leaching agent and modifies the ratio between Mg/Formic ions. Moreover the increased volume of reaction mixture may increase filtration time and its handling process.

3.5. Influence of particle size of magnesite Various experiments were performed to probe the particle size impact. The leaching kinetics of magnesite was performed using 4 sample sizes (500-707, 250-354, 177-210 and 1257

ACCEPTED MANUSCRIPT 177 µm) at 65 °C, 8 % formic acid solution, stirring speed 350 rpm, and liquid to solid proportion of 14:1 mL/g. The results are summarized in Fig. 6 which shows that the increase in particle size

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of magnesite samples has an inverse impact on the dissolution process. This situation may be

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related to the fact that with the decrease in particle size of the magnesite, the surface area of the

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particles for reaction may become more available for the leaching process. 4. KINETIC ANALYSIS

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Hydrometallurgical processes usually involve solid-liquid reaction systems. In solid-

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liquid reactions, the leaching kinetics is governed (Levenspiel, 1972; Lacin et al., 2005) by one

b) Ash to product layer,

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a) Diffusion from fluid films,

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of the following mechanisms:

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c) Chemically controlled reaction

The outcome was examined from shrinking core model to evaluate the rate determining step and reaction conditions affecting leaching kinetics of magnesite. On this ground, a general reaction of solid with fluid can be expressed as:

A( fluid )



bB( Solid) 

product

(4)

Only two controlling mechanisms (diffusion from fluid films or chemically controlled reaction) may be considered during the reaction if no ash/product layer is produced. If the conversion fraction of natural magnesite be x at any interval t then the integral rate equations for fluid-solid systems can be denoted as: For the film diffusion controlling mechanism, 8

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

t  k o 1 1  x 



(5)



(6)

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

t  k o 1 1  x 1 / 3

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For the chemically controlled reaction,

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For the ash layer diffusion controlled,

(7)

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  t  k o 1  31  x 2 / 3  2(1  x)  

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Statistical and graphical methods were applied to test the soundness of the experimental data. It was inferred that the experimental results for the dissolution reaction of magnesite follow a

as:

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1  (1  x)1 / 3  kt

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surface chemical reaction. The integral rate for the surface chemical reaction can be expressed

(8)

Using the conversion values for various reaction temperatures, the apparent rate constants k, is determined by plotting 1  (1  x)

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vs. t as shown in Fig. 7. Considering Arrhenius equation,

Eq. 8 can be expressed as:

1  (1  x)1 / 3  k o e  Ea / RT t

(9)

In Eq.9, Ea is the activation energy and R is general gas constant. The values of energy of activation and ko are obtained by plotting ln k vs.

1 as shown in Fig. 8. By putting the values T

of ko and energy of activation, Eq. 9 can be represented as:

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ACCEPTED MANUSCRIPT 1  (1  x)1/ 3  59.41  101 e 42078/ RT t

(10)

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The value of activation energy (42.08 kJ mol-1) depicts that the dissolution process of natural

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magnesite in formic acid solution is a chemically controlled reaction and this value is in accordance with the results described in the published research studies (Ashraf et al., 2005;

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Demir et al., 2003; Lacin et al., 2005).

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In order to find the validation of kinetic model (Eq. 10), experimental conversion values and calculated conversion values for magnesite were plotted and the results are shown in Fig.9.

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From the scatter diagram (Fig. 9) it can be seen that the correlation of the experimentally determined conversion values and calculated conversion values is good and the value of

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correlation coefficient (0.9862) of this agreement also reveals that the workability of the kinetic

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model is good.

5. CONCLUSIONS 

Organic acids can be used as attractive solvents for selective leaching and beneficiation studies for various rocks as these acids have some advantages over the mineral acids.

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The results indicate that formic acid can be employed as a solvent for leaching of magnesite. Formic acid concentration 8 % with liquid solid ratio 14:1 mL/g was found to be promising for the leaching of magnesite.

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The kinetic data analyzed on the basis of different reaction kinetic models illustrate that the dissolution process of natural magnesite in formic acid is controlled by surface chemical reaction. The energy of activation of the leaching reaction is 42.08 kJ mol-1.

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In the dissolution reaction of magnesite in formic acid solution, the reaction product formed (magnesium formate) can be used in various animal feed formulations as well as

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in various more general chemical applications.

ACKNOWLEDGEMENT

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The authors are grateful to the anonymous reviewers for their constructive comments and

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improvement to the manuscript. The authors also like to thank the Institute of Chemical

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Sciences, B.Z.U., Multan for providing the facilities.

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EXPLANATION OF SYMBOLS

activation energy (J mol-1)

t

reaction time (min)

EDX SEM AAS

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T

TE

Ea

reaction temperature (K) energy dispersive X-ray analysis Scanning electron microscope Atomic absorption spectrophotometer

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ACCEPTED MANUSCRIPT References Abali, Y., Yavuz, M., Copur, M., 2006. Determination of the optimum conditions for dissolution

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of magnesite with H2SO4 solutions. Ind. J. Chem. Tech. 13, 391–397.

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Ashraf, M., Zafar, I.Z., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low-

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grade calcareous phosphate rock in succinic acid. Hydrometall. 80, 286–292. Bayrak, B., Lacin, O., Sarac, H., 2010. Kinetic study on the leaching of calcined magnesite in

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gluconic acid solutions. J. Indus. Eng. Chem. 16, 479-484.

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Bashir, E., Naseem, S., Sheikh, S.A., Kaleem, M., 2009. Mineralogy of kraubath-type magnesite deposits of the khuzdar area, balochistan, Pakistan. J. Earth Sci. Application & Res. 30,

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169-180.

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Bukovisky, V., 1997. Yellowing of newspaper after deacidification with methyl magnesium

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carbonate. Int. J. Preserv. Lib. Archiv. Material. 18, 25-38. Chou, L., Garrels, R.M., Wollast, R., 1989. Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals. Chem. Geol. 78, 269-282. Deangelis, M.T., Labotka, T.C., Cole, D.R., Fayek, M., Anovitz, L.M., 2007. Experimental investigation of the breakdown of dolomite in rock cores at 100 MPa, 650-750 °C. Am. Mineral. 92, 510-517. Demir, F., Donmez, B., Colak, S., 2003. Leaching kinetics of magnesite in citric acid solutions. J. Chem. Eng. Jpn. 36, 683– 688. Demirkýran, N., Künkül, A., 2007. Dissolution kinetics of ulexite in perchloric acid solutions. Int. J. Miner. Proc. 83, 76–80.

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ACCEPTED MANUSCRIPT Demirkiran, N., Kunkul, A., 2008. Dissolution kinetics of ulexite in acetic acid solutions. Chem. Eng. Res. Design 86, 1011-1016.

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Demirkiran, N., 2008. A study on dissolution of ulexite in ammonium acetate solutions. Chem.

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Eng. J. 141, 180-186.

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Demirkiran, N., 2009. Dissolution kinetics of ulexite in ammonium nitrate solutions.

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Hydrometall. 95, 198-202.

Dogan, H.T., Yartasi, A., 2009. Kinetic investigation of reaction between ulexite ore and

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phosphoric acid. Hydrometall. 96, 294-299.

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Company, New Jersey, USA.

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Furmann, N.H., 1963. Standard Methods of Chemical Analysis, sixth ed. D. Van Nastrand

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Gulensoy, H., 1974. Kompleksometrik titrasyonlar ve kompleksometrinin temelleri. fatih yayinevi. Istanbul, Turkey.

Habbache, N., Alane, N., Djerad, S., 2009. Leaching of copper oxide with different acid solutions. Chem. Eng. J. 152, 503-508. Harned, H.S., Embree, N.D., 1934. The ionization constant of formic acid from 0 to 60°. J. Am. Chem. Soc. 56, 1042–1044. Housmanns, S., Laufenberg, G., Kunz, B., 1996. Rejection of acetic acid and its improvement by combination with organic acids in dilute solutions using reverse osmosis. Desalination 104, 95-98.

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ACCEPTED MANUSCRIPT Jones, P.T., Blanpain, B., Wollants, P., Ding, R., Hallemans, B., 2000. Degradation mechanisms of magnesia-chromite refractories in vacuum oxygen decarburization ladles during

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production of stainless steel. Iron Making and Steel Making 27, 228-237.

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Kovacheva, P., Arishtirova, K., Vassilev, S., 2001. MgO/NaX zeolite as basic catalyst for

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oxidative methylation of toluene with methane. Appl. Catal. 210, 391-395.

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Kuslu, S., Colak, S., 2010. Leaching kinetics of ulexite in borax pentahydrate solutions saturated with carbon dioxide. J. Indus. Eng. Chem. 16, 673-678.

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Lacin, O., Donmez, B., Demir, F., 2005. Dissolution kinetics of natural magnesite in acetic acid solutions. Int. J. Miner. Proc. 75, 91-99.

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J. Miner. Proc. 80, 27-34.

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Lacin, O., Bakan, F., 2006. Dissolution kinetics of natural magnesite in lactic acid solutions. Int.

York.

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Levenspiel, O., 1972. Chemical Reaction Engineering, second ed. John Wiley & Sons, New

Mergene, A., Demirhan, H.M., 2009. Dissolution kinetics of probertite in boric acid solutions. Int. J. Miner. Proc. 90, 16-20. Ozmetin, C., Kocakerim, M.M., Yapici, S., Yartasi, A., 1996. A semi empirical kinetic model for dissolution of colemanite in aqueous CH3COOH solutions. J. Ind. & Eng. Chem. Res. 35, 2355–2359. Raza, N., Zafar, I.Z., Haq, N., 2013. An analytical model approach for the dissolution kinetics of magnesite ore using ascorbic acid as leaching agent. Int. J. Metals. http://dx.doi.org/10.1155/2013/352496

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ACCEPTED MANUSCRIPT Sandstrom, A., Samuelsson, C., 2010. Dissolution kinetics of tetrahedrite mineral in alkaline sulphide media. Hydrometall. 103, 167-172.

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Shi, D., Yu, X., Roth, L., Morizono, Y., Hathout, N., Allewell, M., Tuchman, M., 2005.

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Expression, purification, crystallization and preliminary X-ray crystallographic studies of a novel acetylcitrulline deacetylase from Xanthomonas campestris. Acta Cryst. F61, 676-

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679.

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Visscher, A.D., Vanderdeelen, J., Nigsberger, E.K., Churagulov, B.R., Ichikuni, M., Tsurumi, M., 2012. IUPAC-NIST solubility data series. 95. Alkaline earth carbonates in aqueous

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systems. Part 1. Introduction, Be and Mg. J. Phys. Chem. Ref. Data 41 (1). Zhang, S., Nichol, J.M., 2010. Kinetics of the dissolution of ilmenite in sulfuric acid solutions

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under reducing conditions. Hydrometall. 103, 196-204.

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ACCEPTED MANUSCRIPT Figure 1. The EDX pattern of natural magnesite. Figure 2. SEM image of natural magnesite.

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Figure 3. Effect of temperature on leaching of magnesite ore (Formic acid concentration, 8%;

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Particle size, 177-210 µm; stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g).

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Figure 4. Effect of Formic acid concentration on leaching ore (Particle size, 177-210 µm; stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g; Temperature, 65 °C).

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Figure 5. Effect of liquid solid ratio on leaching of magnesite ore (Formic acid concentration,

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8%; Particle size, 177-210 µm; stirring speed, 350 rpm; Temperature, 65 °C). Figure 6. Effect of particle size on leaching of magnesite ore (Formic acid concentration, 8%;

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stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g; Temperature, 65 °C).

Figure 6. Plot of lnk and 1/T

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Figure 7. Plot of 1-(1-x)^1/3 and 1/t

of magnesite.

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Figure 9. Agreement between experimental conversion values and calculated conversion values

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 9

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Table 1

MgO

43.47

CaO

0.38

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SiO2

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Fe2O3

1.54 53.5

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Loss on ignition [at 950 oC for 24 h]

Results obtained from analysis of magnesite ore by AAS (Hitachi-1800).

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a

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[Mass.%]

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Component

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Chemical composition of natural magnesite orea.

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Table 2

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Elemental analysis of natural magnesite orea. [keV]

[Mass %]

[Atomic %]

C

0.277

14.591

20.446

O

0.525

57.567

60.501

Mg

1.253

Si

1.739

Ca

3.690

Fe

6.398

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26.086

18.2767

0.7186

0.4314

0.268

0.1127

0.7728

0.2321

Results obtained from EDX (JEOL, JED-2300)

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a

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ACCEPTED MANUSCRIPT Highlights

• Formic acid was used to leach out Mg from magnesite ore. • Effect of some significant parameters was investigated.

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• Leaching of magnesite with formic acid is chemically controlled.

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• Kinetics data were analyzed using graphical and statistical methods.

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• A kinetic model was developed to present the leaching kinetics.

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