Xiangchun Liu,Li Feng*,Xinhua Wang,Ying Zhang,Haiyan Tang

Key Laboratory of Coal Processing and Ef fi cient Utilization of Ministry of Education,School of Chemical Engineering and Technology,China University of Mining&Technology,Jiangsu 221116,China

Keywords:Ion-exchange FT-IR Equilibrium adsorption water content Surface

A B S T R A C T Mg ion-exchanged samples were prepared with acid-washed Shengli lignite.The chemical composition of the ash of the raw sample was determined by X-ray fluorescence.The equilibrium adsorption water contents of samples were determined in a range of relative humidity.The ion-exchange process was characterized by FT-IR,ash content,and p H value.A possible mechanism is proposed for equilibrium adsorption water of ion-exchanged samples at different humidities.The extent of ion-exchange reaction between Mg2+and lignite is controlled by the concentration of Mg2+in MgSO4 solution.The effect of Mg2+on equilibrium adsorption water content varies with relative humidity and content of Mg2+.The factor that controls equilibrium adsorption water content at low relative humidity is water interactions with sorption sites,which are Mg2+-carboxyl group complex.At middle relative humidity capillary force between Mg2+-water clusters Mg+(H2O)n and capillary is more important.At high relative humidity,free water-free water interactions are more significant.

1.Introduction

Lignite contains high moisture content.Water in lignite is categorized into three types depending on the strength of lignite-water interactions,which are non-freezing water because of strong interactions with functional groups in the lignite,bound water held in the capillaries,and free water without specific interaction with lignite[1,2].It is known that high water content of lignite has important effect on its utilization.A number of factors have important effects on the hydrophilicity of lignite.Brie fly,oxygen-containing functional groups and highly porous nature of lignite play an important role in the high water content.The importance of cation in lignite to the high water content has been confirmed[3-7].

Mg2+is one of the main alkaline earth metal ions in lignite.It presents primarily as a cation bonded to oxygen anions in carboxyl groups in lignite[3].The effect of Mg2+on w ater content in lignite can be explained by hydration in aqueous solution.Hydration of Mg2+has been studied by Watanabe et al.[8].Total and sequential binding energies for Mg2+-water clusters Mg+(H2O)ncontaining 1-6 water molecules are evaluated.Delphine and Philippe[9]have proposed that high molecular forces between water molecules induce the formation of water clusters.Water adsorption is involved at primary sorption sites at the surface of lignite,and then the water molecules play the role of secondary sites to form water clusters due to interactions between water molecules.Allardice et al.[10]have reported that the factors that control water adsorption at 92%relative humidity or less differ from those that control extra water adsorbed at higher relative humidity.Lignite differs significantly in Mg2+content so it is meaningful to investigate the effect of Mg2+content on equilibrium adsorption water content(EDWC).

The focus of this study is to access the effects of Mg2+content of ion-exchanged Shengli Lignite on its EDWC.The EDWCs of raw,acid w ashed and ion-exchanged samples are determined in a range of relative humidity.Fourier transform infrared(FT-IR)analysis,ash contents and p H values are used to characterize the samples.A possible mechanism for EDWC of ion-exchanged samples at different humidities is proposed.

2.Experimental

2.1.Materials

The lignite sample in this study was taken from Shengli,Inner Mongolia,China.The lignite was ground to pass a 177 μm sieve.The raw lignite(SLR)was analyzed for ultimate and proximate analyses according to the Chinese Standard Method[11].The results are collected in Table 1.

2.2.Fourier transform infrared and X-ray fluorescence analyses

The FT-IR spectra(4000-500 cm-1)of KBr pellet were recorded on a Nicolet iS5 FT-IR spectrometer(American).Each KBr pellet wasprepared with a lignite sample to KBr ratio of about 0.5%.FT-IR spectra of all samples were recorded in the transmission model with a resolution of 4 cm-1.Beam diameter at the sample location was 1 cm.

Table 1Proximate and ultimate analyses of the lignite sample

Bruker S8 Tiger X-ray fluorescence(XRF)spectrometry was used to quantify elements in the ash of raw lignite.3.00 g of the raw lignite was finely ground with grinding aid tablets in a ratio 5:1.Then the mixture was prepared with the use of a hand press at constant pressure within 20 s to form a pellet.The concentrations of major elements obtained by XRF analysis are listed in Table 2.The ash consists largely of Si and Al.The total content of Na2O,MgO,CaO,and K2O is 10.15%.The content of Fe is also high in the lignite.Since alkali and alkaline earth metals present primarily as cations bonded to oxygen anions in carboxyl groups,which has important effect on EDWC[3].Mg2+was chosen as the research object,because of its apparent effect on EDWC.

Table 2Concentrations of major elements of the ash of raw lignite determined by XRF

2.3.Acid washing and ion-exchange process

For acid washing 15 g raw lignite was stirred with 150 ml of distilled water,60 ml of 37%HCl solution,30 ml of 66%HNO3solution and 30 ml of pure HF at room temperature for 24 h.Then,it was filtered and washed with distilled water until no Cl-was detected.

The acid-w ashed lignite(10.00 g)was prepared by mixing with 1000 ml MgSO4solution of different concentrations(0,0.01,0.05,0.10,0.20 and 0.50 mol·L-1),designated as SLA,SL0.01,SL0.05,SL0.10,SL0.20 and SL0.50.Shengli raw lignite itself is called SLR.Each mixture was stirred for 24 h,and then the sample was filtered.The residue was dried at 378 Kfor 6 h.The treated products were stored for further analysis.

2.4.EDWC determination

EDWC was determined by the desiccator method.Brie fly,vacuum dried samples(1.0000 g)were separately placed in weighing bottles(m1is the mass of weighing bottle,lid and lignite).Then the open weighing bottles were positioned in a desiccator with saturated salt solution in its bottom at 293 K,which provides specific humidity as shown in Table 3[12].After several hours the weighing bottles were taken out,closed with lids and weighed(mi).Experiments were repeated until reaching the equilibrium.The adsorption water content,on a dry basis,was calculated using the following equation:adsorption water content=[(mi-m1)/1.0000]×100%.

Table 3Relative humidity at 293 K of saturated salt-water mixture

3.Results and Discussion

3.1.The effect of Mg2+content on equilibrium adsorption water content

Fig.1 shows adsorption water contents of the raw,acid washed and Mg ion-exchanged samples at different humidities varied with time.The adsorption water content increases steeply for all samples during the first 20 h,and then changes a litter,which suggests that equilibrium is attained.The equilibrium time increases with the humidity,which is in agreement with the observation of Yi et al.[13].Compared with raw lignite,the individual equilibrium adsorption water values of acid w ashed samples are not different significantly at these humidities.One possible explanation for these trends is attributed to the changes in cation content and structure during acid w ashing process.

Yi et al.[5]have compared the relative effectiveness of different cations in increasing the equilibrium moisture contents at four different relative vapor pressures.They report that the effectiveness of cations in changing equilibrium moisture content is dependent on relative vapor pressures.Our attention is paid on the effect of the amount of loaded Mg2+on the equilibrium adsorption water content.It is noted that in all cases the EDWCs of ion-exchanged samples increase with the concentration of MgSO4solution,which undergoes exchange reaction with lignite,and the extent of increase varies with humidity.Furthermore,significant effect of ion-exchange on EDWC at humidity of 97%can be observed.It is of interest to see that the EDWC of SLR was the highest at 43%relative humidity among all samples,which can be attributed to the synergistic effect of all kinds of cations on EDWC.In addition,EDWC of SLR was higher than that of SL0.01 at 43%relative humidity.Since the amount of loaded Mg2+of SL0.01 is low,its total capillary force is weaker than that of SLR(Section 3.2.3 shows that the middle humidity the factor that controls water adsorption is capillary force).

3.2.Possible mechanism for EDWC at different humidities

3.2.1.Ion-exchange between Mg2+and carboxyl groups

Fig.2 show s the FT-IR spectra of the raw,acid washed and Mg ionexchanged samples.The bands between inorganic and organic matters not only influence the intensity of some specific bands,their positions may also be changed[14,15].The 1700 cm-1band of acid washed samples,typical for C═O stretching band of carboxyl groups[16],is considerably stronger than that of the raw lignite due to the increasing concentration of carboxyl groups in the acid washing procedure[Fig.2(a)].Since the capacity of ion-exchanged samples is controlled by the concentration of carboxyl groups[17],which are not associated with inherent mental cations,increasing the concentration of carboxyl groups will increase the capacity of metal exchange to lignite.

Ion-exchange is also influenced by p H value.The p H of SLA solution was 3.40(see Fig.3).With MgSO4added into it,ion-exchange reaction is primary between Mg2+and carboxyl groups.Compared with sample SLA,the intensity of the peak at 1700 cm-1of ion-exchanged samples decreases drastically and instead the intensity of the band at 1600 cm-1increases,which is attributed to carboxyl groups that may be in their metal-bonded carboxylate form[Fig.2(b)].These observations suggest that Mg2+of the MgSO4solution is successfully loaded on the site of H+of carboxyl groups,that is,on lignite during the ionexchange process.Evangelou et al.[18]have reported that the reaction between metal ion and carboxyl groups of humic acid,forming a mental-ion complex,results in the disappearance of the peak at 1700 cm-1and the appearance of new bands at low er w ave number.

3.2.2.The amount of Mg2+loaded on lignite during the ion-exchange process

Fig.1.Water adsorption on raw,acid washed and Mg ion-exchanged samples at different humidities.

Table 4 shows the ash contents of the raw,acid washed and Mg ionexchanged samples.A sample of 0.50 g was weighed into a glass beaker and 100 ml of distilled water was added.The solution was steadily stirred for 10 min and the pH value was read by pH meter,as is shown in Fig.3.Ash content and pH value were used to determine the amount of Mg2+loaded on lignite during the ion-exchange process.The acid washed sample marginally reduces the ash content compared with raw lignite.This is consistent with the removal of minerals during the acid washing process.Most carbonates,pyrite and quartz are dissolved and removed from lignite by treating with HCl,HNO3and HF[19].Notethat the ash contents of ion-exchanged samples increase with Mg2+concentration in the solution.It can beinferred that the amount of loaded Mg2+increases.Eligwe and Okolue[20]have demonstrated that the amount of ionsadsorbed increases with the concentration of metal ions in the solution.

Fig.3.p H values of raw,acid washed and Mg ion-exchanged samples.

Table 4The ash content of raw,acid w ashed and Mg ion-exchanged samples

With H+loaded on the site of carboxyl groups of SLA,the p Hvalue of SLA was low er than that of SLR(Fig.3).Furthermore,Mg2+was loaded on the site of H+of carboxyl groups and H+was leached from the lignite.Therefore,the p H value increased gradually from the initial 3.67 to 4.99 with increasing amount of loaded Mg2+.

3.2.3.Analysis on the mechanism for EDWC of ion-exchanged samples

Fig.4.EDWCs at different relative humidities.

Fig.4 show s that the equilibrium adsorption water contents increase with the relative humidity.Allardice et al.[10]have found that the factors that control water adsorption at 92%relative humidity or less differ from those controlling the extra water adsorbed at higher relative humidity.One possible explanation is that at lower relative humidity the effect on equilibrium moisture content of water interaction with cations and functional groups in lignite is more important than at higher relative humidity,where water water interactions are more significant[5].It is generally accepted that water in lignite is classified into three types:free water,bound water and non-freezing water.These investigations and the change in the shape of curves imply that the mechanism of water adsorption on ion-exchanged samples may vary with relative humidity.Firstly,water adsorbs primarily at the major sorption sites on the surface of lignite,which seem to be carboxyl groups for SLA[21],while the preferential sites of adsorption for ion-exchanged samples are represented by Mg2+-carboxyl group complexes formed by Mg2+with carboxyl group interaction.With the hydration of Mg2+,the lignite adsorption capacity increases.At low humidity,water adsorbs on the primary sites in the form of monolayer,the content of which is strongly influenced by the Mg2+-carboxyl group complex,so the EDWCs of ionexchanged samples increase with the amount of loaded Mg2+.Secondly,due to high molecular forces between a water molecule and that already adsorbed on primary sites,secondary site adsorption occurs.Adsorbed water forms Mg+-water clusters Mg+(H2O)n.Middle humidity represents multilayer adsorption[22],Mg2+-water clusters are bound in capillaries,and the factor that controls water adsorption is capillary force attributed to Mg2+-water clusters condensed in the capillary with diameters of less than several microns.In other words,the key factor is interactions between water clusters and pores.At high humidity,water is considered to be in the form of free water without specific interactions with lignite,the content of which is strongly influenced by free water-free water interactions.

In summary,the results indicate that the amount of Mg2+loaded on the lignite increases with the concentration of Mg2+in the solution.With the hydration of Mg2+,the equilibrium adsorption water contents of ion-exchanged samples increase with the amount of loaded Mg2+.At low relative humidity water interactions with sorption sites,which is the Mg2+-carboxyl group complex,appear to be important in controlling water adsorption content.At middle relative humidity,interactions between Mg2+-water clusters Mg+(H2O)nand capillary are more important,while at high humidity the free water-free water interactions appear to be the key factor.