Effect of Oxalic Acid on Heat Pretreatment for Serpentine Carbonation

Materials Transactions, Vol. 52, No. 2 (2011) pp. 235 to 238 #2011 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Eﬀect of Oxalic Acid on Heat Pretreatment for Serpentine Carbonation

Myung Gyu Lee1, Kyoung Won Ryu2;*, Young Nam Jang2, Wonbaek Kim2 and Jun-Hwan Bang2

1Resources Recycling Engineering Department, University of Science and Technology of Korea, Gwahang-no 113, Yuseong-gu, Daejeon 305-333, Korea 2Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Gwahang-no 92, Yuseong-gu, Daejeon 305-350, Korea

A novel pretreatment method for the mineral serpentine was proposed to develop an eﬀective carbonation process for CO2 sequestration. Basically, the method involved preheating a mixture of oxalic acid and serpentine and the subsequent aqueous carbonation. The addition of oxalic acid was found to stimulate the decomposition of serpentine during heat treatment. X-ray diﬀraction analysis revealed that serpentine is transformed into magnesium oxalate and magnesium oxide by heat treatments at 200C and 500C, respectively. The addition of oxalic acid was found to enhance the overall carbonation reaction owing to the formation of magnesium oxide during heat treatment. Thermogravimetric analysis showed that approximately 70% of the Mg2þ was transformed into magnesium oxide. Using this method, a carbonation rate as high as 72% could be obtained by aqueous carbonation at 100C under 4 MPa. This novel method can potentially reduce the high energy cost and unavoidable use of expensive chemicals for pH control. [doi:10.2320/matertrans.M2010304]

(Received September 6, 2010; Accepted November 18, 2010; Published January 13, 2011)

Keywords: serpentine, oxalic acid, CO2 sequestration, pretreatment, aqueous carbonation

1. Introduction to the practical application of mineral carbonation for CO2 sequestration. Serpentine has long been the subject of mineral carbon- It was reported that an organic ligand facilitates the ation because it has a high Mg content and it is distributed leaching of metal cations by forming a chelate with cations abundantly worldwide.1,2) Natural minerals like serpentine in acid leaching.12,14,15) In this study, oxalic acid was mixed usually have a highly stable structure and require an with serpentine and heat treated to determine whether it could extremely long reaction time to decompose in a carbonation similarly affect the decomposition process of serpentine and reaction. Various methods have been adopted in attempts to the resultant cation leaching. facilitate the carbonation reaction, including the heat treatment method, internal grinding method, microwave method, 2. Experiments and acid treatment method. Serpentine has typically been heated at 630C in a heat 2.1 Materials treatment method to make its crystal structure unstable by The mineral serpentine used in this study was obtained removing the hydroxyl group. This enhances the leaching of from the Bibong mine in Chungnam, Korea. Its composition Mg2þ and hence the carbonation reaction.1,3–6) The carbon- was determined by Inductively Coupled Plasma (ICP, ation following the heat treatment requires a high temper- 5300DV, PerkinElmer Inc.). The major components were ature of 155 C and high pressure of 12 MPa. Nevertheless, 27.4% MgO and 46.5% SiO2, while the major impurities the highest carbonation rate achieved was no more than were 5.1% Fe2O3,4.4%Al2O3, and 1.2% CaO. The 57%.1) The internal grinding method was designed to remove serpentine sample was crushed in an attrition mill (KMC- the silicate layer on the surface of serpentine particles, which 1B, Korea Machine Engineering Company) for 1 h at is known to be the rate-limiting step. In this method, a 1500 rpm. Particles finer than 43 um were used for the serpentine slurry was stirred with a 25-mm zirconia experiment. medium while introducing air bubbles.7) In the acid treatment method, hydrochloric acid,8–10) 2.2 Methods sulfuric acid,6,9,11) nitric acid,9) and various organic acids12) Because an organic ligand has been shown to be helpful were used as solvents for cation leaching. Among these, in the leaching of metal cations, oxalic acid (Junsei, hydrochloric acid or sulfuric acid appeared to be the most C2H2O42H2O) was mixed with the serpentine before the effective solvent. Most of the cations could be leached out heat treatment. The weight ratio of the serpentine to oxalic in 2 mol/L hydrochloric acid or sulfuric acid by heating at acid in the mixture was 1:2 in accordance with the over 70C for 1 h.9) Nevertheless, this process requires a stoichiometry. The mixture was heated for 2 h at temper- large amount of acid to leach a sufficient amount of Mg2þ. atures between 200 and 500C in air. Moreover, the solution pH should be maintained in the After the heat treatment, aqueous carbonation was carried alkali region, which requires a large quantity of alkali out in a cold-seal type stainless steel autoclave. The internal reagent.7,13) The high energy cost due to the high temper- volume of the autoclave was 1 liter. After sealing the ature and pressure of carbonation and the use of excessive chamber containing water and the sample, CO2 gas was amounts of acid and alkali reagent are the major obstacles introduced. The chamber was then charged into the furnace and agitated at 1000 rpm. The carbonation was conducted *Corresponding author, E-mail: [email protected] under a CO2 partial pressure of 4 MPa, reaction temperature 236 M. G. Lee, K. W. Ryu, Y. N. Jang, W. Kim and J.-H. Bang

Fig. 1 XRD patterns of serpentine and reaction products prepared by heating the mixture of serpentine and oxalic acid at 200C and 500C. of 100C, and reaction time of 1 h without any additives for pH control and activation. Samples were examined using an X-ray diﬀractometer (Phillips, Analytical X-ray B.V. X’pert MPD) at 40 kV and 25 mA. Thermogravimetry was carried out via thermogravimetric-diﬀerential thermal analysis (TG-DTA; DTG-60H, Shimadzu) using Ar gas. The heating rate was 10C/min. The decomposition temperature of CO2 was determined by analyzing the sample after carbonation, and this was then used to calculate the carbonation rate.

3. Results and Discussion

3.1 Pretreatment of serpentine Figure 1 shows the X-ray diffraction patterns of the phases synthesized by heat treatments at 200C and 500C. As can be seen, the major phases in the raw material are lizardite and quartz (SiO2). The quartz was stable during each heat treatment. During the heat treatment at 200C, the lizardite was almost completely transformed into magnesium oxalate (C2MgO4), as shown in Fig. 1. This was probably formed by the chelation between Mg2þ and 2COO. During the heat treatment at 500C, the magnesium oxalate was transformed into magnesium oxide. This is in agreement with the result of Zhang et al. (2006).16) They reported the transformation of magnesium oxalate dihydrate into magnesium oxide at 430590C. The morphology of magnesium oxalate has been reported to be a sphere,17) rod,17) or cube.18) Figure 2 shows SEM micrographs of samples after heat treatment at 200C and 500C. It can be seen that rod-type or cube-type Fig. 2 SEM micrographs of reaction products prepared by heating the mixture of serpentine and oxalic acid at 200C and 500C. magnesium oxalate particles with a size of approximately 2 mm were produced during the heat treatment at 200C (Fig. 2(a)). During the heat treatment at 500C, magnesium oxide from the weight loss. Figure 3 shows the result. An oxide was synthesized from magnesium oxalate via the endothermic peak is seen to occur at approximately 500C, decomposition of carboxylate anion. The magnesium oxide which represents the phase transformation of magnesium is seen to retain the typical morphology of the magnesium oxalate to magnesium oxide, with a weight loss of approx- oxalate (Fig. 2(b)). The higher magnification photograph of imately 34%. The amount of magnesium oxalate in the the synthesized magnesium oxide again confirms that nano- sample heat treated at 200C was approximately 57%, which sized particles of magnesium oxide retain the morphological was determined from the weight of the decomposed carbox- features of magnesium oxalate (Fig. 2(c)). ylate anion. The conversion rate for magnesium oxide Magnesium oxalate is known to transform into magnesium calculated from the Mg2þ content of the serpentine was oxide via the decomposition of carboxylate anion at temper- approximately 70%. atures between 400 and 500C in air.16,19) Therefore, TG- DTA was conducted for the sample heat treated at 200Cto 3.2 Carbonation of pretreated serpentine calculate the transformation rate of serpentine to magnesium The sample heat treated at 500C was mixed with water to Effect of Oxalic Acid on Heat Pretreatment for Serpentine Carbonation 237

Fig. 3 TG-DTA graph of reaction product prepared by heating the mixture of serpentine and oxalic acid at 200C.

Fig. 5 SEM micrograph of reaction products obtained by carbonation of the sample prepared by heating the mixture of serpentine and oxalic acid at 500C.

Fig. 4 X-ray diffraction pattern of reaction products obtained by carbonation of the sample prepared by heating the mixture of serpentine and oxalic acid at 500C. make a slurry for subsequent aqueous carbonation. It is known that an alkali solution (pH 811) is favored for Fig. 6 TG-DTA results for the carbonation product. effective mineral carbonation.7,13) The pH of the slurry was approximately 10.3 and was thus considered suitable for is expected to reduce the high cost from high energy carbonation. consumption and the use of an alkali agent in the conven- The carbonation experiment was conducted at 100C for tional carbonation of serpentine. 1 h under a CO2 partial pressure of 4 MPa. Figure 4 shows the X-ray diffraction pattern of the carbonation product. The 4. Conclusions major phases of the product were magnesium carbonate (MgCO3) and quartz, suggesting that the carbonation A novel pretreatment method for the carbonation of reaction was nearly completed. It should be noted here that serpentine was elaborated. The addition of oxalic acid was the un-indexed reflections (marked as U) correspond to the found to enhance the overall carbonation reaction rate via the unreacted phase in raw serpentine. Most of the MgCO3 rapid formation of magnesium oxide during heat treatment. particles were of the cube type, as shown in an SEM It was found that approximately 70% of the Mg2þ was micrograph (Fig. 5). transformed into magnesium oxide, whereas the remainder The thermal decomposition of MgCO3 was evaluated via was transformed into amorphous serpentine. The carbonation TG-DTA (Fig. 6). It is seen to decompose into CO2 at reaction produced cube-shaped MgCO3. At a relatively lower 450550 C. This suggests that CO2 can be sequestered at temperature of 100 C and pressure of 4 MPa, a carbonation temperatures up to 450 C. The mass of the decomposed CO2 rate as high as 72% could be achieved. This novel method was approximately 22%. The carbonation rate then became can potentially reduce the high energy cost and unavoidable approximately 72%, which was approximately 15% higher use of expensive chemicals for pH control. than O’Conner’s result (2005). It was demonstrated that the pretreatment method sug- Acknowledgment gested in this study could improve the reaction conditions (100C and 4 MPa) compared to the conventional ones This research was supported by the Basic Research Project (155C and 12 MPa). Moreover, the use of a large quantity (GP 2010-018) of the Korea Institute of Geoscience and of alkali agent for pH control in the acid treatment method Mineral Resources (KIGAM), funded by the Ministry of could be avoided. Thus, the method presented in this paper Knowledge Economy, Korea. 238 M. G. Lee, K. W. Ryu, Y. N. Jang, W. Kim and J.-H. Bang

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