Ž.
Int.J.Miner.Process.612001273–288
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Sorption of atmospheric carbon dioxide and structural changes of Ca and Mg silicate minerals
during grinding
I.Diopside
Elena V.Kalinkina a,b,Alexander M.Kalinkin a,b,Willis Forsling a,),
Victor N.Makarov b
a DiÕision of Inorganic Chemistry,Lulea UniÕersity of Technology,S-97187Lulea,Sweden
˚˚
b Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials,Kola Science Centre,
Apatity,Murmansk Region,184200,Russia
Received17February2000;received in revised form4May2000;accepted7September2000 Abstract
Considerable sorption of atmospheric carbon dioxide by the ground mineral,alongside hydration due to atmospheric moisture,occurs in the course of prolonged dry grinding of natural and synthetic diopside in laboratory conditions.Grinding of natural diopside for36h results in
about10wt.%of CaCO in the ground sample.A unique double peak in the1430–1515cm y1
3
region in the FT-IR spectrum,attributable to the CO2y group,shows that carbon dioxide is
3
戴玉庆present in the ground diopside,in the same form as in synthetic and natural silicate glasses after
dissolution of CO at high temperatures and pressures relevant to the magma state.This 2
conclusion is supported by the13C CP r1H-decoupling MAS-NMR spectrum of ground diopside, whic
h has a strong resonance signal at167.4ppm.Carbonate groups are present not only on the surface,but also in the bulk of mineral grains.The appearance of the new peak at approximately y108ppm in29Si MAS-NMR spectrum of the ground diopside after36h of grinding shows that
the Q structure of crystalline diopside is partially transformed into the Q structure.Together 24
with XRD data,this result indicates the formation of quartz,which may occur through a re-polymerisation of an amorphous phase.q2001Elsevier Science B.V.All rights reserved. Keywords:diopside;dry grinding;atmospheric carbon dioxide;sorption;structural changes
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)Corresponding author.Fax:q46-920-91199. Ž.
E-mail address:wifo@km.luth.se W.Forsling.
0301-7516r01r$-see front matter q2001Elsevier Science B.V.All rights reserved.
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PII:S0301-75160000035-1
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E.V.Kalinkina et al.r Int.J.Miner.Process.612001273–288
1.Introduction
Comminution of minerals,including crushing and grinding,is an important operation
in mineral processing,building materials production and in sample preparation for precise laboratory experiments.In mineral processing,the change of mineral surface properties during grinding influences the effectiveness of flotation,filtration,magnetic, and electrostatic separation.On the basis of changes of mineral properties caused by mechanical treatment,it is possible to speed up many technological processes:decompo-sition of ores,leaching of components from mineral raw materials,obtaining of building materials with higher quality,improving of agrochemical properties of fertilisers ŽHeinicke,1984;Avvakumov,1986;Molchanov and Yusupov,1981;Schrader and
.
Hoffman,1973;Khodakov,1972.Such processes are referred to as mechanical activation.
Ž.
The fine grinding of minerals to grain size of a few micrometers and less has been
Ž
studied systematically only in relatively recent time Heinicke,1984;Boldyrev,1983;
.
Avvakumov,1986;Juhasz and Opoczky,1990.The fine grinding results in high reactivity of minerals due to the breakage of valence bonds and the crystal structure distortions that leads to partial or complete amorphization of the ground substance.
Ž. Investigation of structural changes of kaolinite Schrader et al.,1970,lepidolite Ž.Ž.
kta
Lapukhova et al.,1978,montmorillonite Cicel and Kranz,1981,and spodumen Ž.
Berger et al.,1981during the fine grinding shows that at first the valence bonds of the atoms,which are in the octahedral positions,are broken;tetrahedral layers are more stable to the mechanical treatment.
Fine grinding can result not only in structural,but also in chemical changes on the
Ž
surface of mineral grains,as a result of mechanochemical reactions Heinicke,1984;
.
Molchanov and Yusupov,1981;Boldyrev,1983;Tkacova,1989.It should be noted, that chemical reactions occur during the coarse grinding as well,albeit to a smaller extent.Reaction of gas with the mineral surface during grinding is one of the most important mechanochemical processes from technological and geochemical points of Ž.
view Heinicke,1984.It was found that crushing of quartz,nepheline syenites,granites,
Ž
basalts and some other rocks Ikorsky and Evetskaya,1975;Barker and Torkelson, .Ž.
1975may promote small surface adsorption-0.02wt.%of CO,H O and some
22
other gases.
As the research object we have chosen diopside CaMgSi O,a member of pyroxene
26
秸秆成型机
group of minerals.Pyroxenes are among the most wide-spread rock-forming minerals Ž. Deer et al.,1992and they are often involved as gangue minerals in mineral processing
Žoperations.Investigations of surface changes of diopside in a previous work Peck et al.,
.
1988;Eggleston et al.,1989show that short-term grinding results in the production of a disordered surface phase and its hydration due to atmospheric moisture,which influ-ences the kinetics of mineral dissolution.However,we have not found any literature data on extensive sorption of atmospheric carbon dioxide by diopside during grinding.In
this paper,we present new data on the atmospheric CO sorption ability of synthetic and
2
Ž.
natural diopside,and sample transformations,during extended up to36h dry grinding in a laboratory mechanical mill.
()
E.V.Kalinkina et al.r Int.J.Miner.Process.612001273–288275
2.Experimental methods and materials
The synthetic diopside was prepared as follows.Stoichiometric amounts of reagent
grade silica,CaCO,and MgCO were thoroughly mixed and heated at14508C.Then 33
Ž
the melt was slowly cooled.In order to obtain a pure crystalline phase without glassy .
material,heating at11008C and slow cooling were repeated several times.
Natural diopside was from the Pechenga deposit,Kola peninsula,Russia.Optical and X-ray examination revealed the synthetic and natural samples to be almost entirely
Ž.
diopside,although in the latter,small amounts-2%of antigorite and sulphides were detected.About50g of synthetic and natural diopsides were crushed and ground for15
Ž. min in the agate laboratory mechanical pestle-and-mortar mill Retsch,type RMO and then sieved to yield a size fraction less than106m m.These samples were referred to as ‘initial’.Then12g of initial,both synthetic and natural samples,were ground for up to 36h.During grinding,1.5g of the sample was removed from the mill every6h for
Ž.
diffuse reflectance Fourier transform infrared DRIFT spectroscopy,powder X-ray diffraction,1H-13C CP MAS-NMR and29Si MAS-NMR spectroscopy,BET surface area
and CO-content measurements.In addition,4g of initial synthetic diopside was ground 2
for36h.The ground synthetic and natural samples are identified as T synt N g and
Ž.
T pech N g,respectively,where T denotes time of grinding h and N the sample load in Ž.
the mill g.Initial samples are denoted as0synt and0pech.
A Perkin-Elmer Fourier transform infrared spectrometer2000,equipped with a
Ž.Ž. triglycine sulfate TGS detector,was used to obtain diffuse reflectance FT-IR DRIFT spectra.Normally,50scans were accumulated at a resolution4cm y1.For each
Ž. measurement,about5mg of sample was mixed with200mg of oven-dried at1108C spectroscopic grade KBr.Pure KBr was used as a reference.
Ž.
X-ray diffraction XRD measurements were performed using a powder difractometer Ž.
Siemens,Model D5000operating in the reflection mode with Cu-K a radiation at scanning rate of0.01
8s y1.
Ž.
Solid state nuclear magnetic resonance NMR spectra were recorded on a Chemag-netics Infinity CMX-360spectrometer operating at the magnetic field of8.46T.29Si
Ž.
magic-angle-spinning MAS spectra were obtained at the resonance frequency of71.5 MHz using a single pulse experiment without proton decoupling.The spinning fre-quency was6000Hz and stabilized to"3Hz using an in-built stabilization device.The silicon908pulse duration was4.5m s.For all samples,1024transients,with a relaxation
Ž.
delay of5s,were accumulated.Powder samples ca.700mg were packed into zirconium dioxide standard double bearing7.5-mm rotors.All29Si chemical shift data
Ž.13
were externally referenced to tetramethylsilane TMS.Natural abundance C CP r MAS spectra were acquired at the resonance frequency of90.5MHz using cross polarization from protons with proton decoupling.The proton90-pulse duration was5.0m s and the 1H–13C contact time was2ms.The proton rf field during decoupling corresponded to the proton nutation frequency of30kHz.20480and1024transients were collected for ground and initial synthetic diopside,respectively.The spinning frequency was5070"3
Ž.
Hz.Chemical shifts are quoted with reference to TMS externally referenced.All solid
Ž.
state MAS-NMR spectra were recorded at room temperature ca.300K.
()E.V.Kalinkina et al.r Int.J.Miner.Process.612001273–288
276Specific surface areas were determined by the N r BET method using a Micrometrics 2ASAP 2010surface area analyser.
CO -content in samples was measured by a Bruel and Kjær A r S type 1301gas ¨2analyser,which photoacoustically measures the absorption of infrared light by carbon Ž.Ž.dioxide Liu et al.,1999.Briefly,an exact amount 0.2–0.25g of the sample was added to the reactor with 50ml of distilled deionised water and the suspension thus obtained was purged by pure N up to a background CO concentration of less than 422ppm.Then the nitrogen outlet was closed and an excess amount of 0.5M HCl was injected into the reactor under continuous magnetic stirring in order to release the carbon dioxide from the sample.The concentration of CO produced by the sample was 2measured by the gas analyser connected to the reactor.As a rule,two parallel measurements were carried out for each sample and an average value was calculated.The reproducibility of parallel measurements of CO -content was within two relative 2percent.
In dissolution tests distilled deionised water and reagent grade hydrochloric acid were used.Concentrations of Ca,Mg,and Si in solutions were measured with a Perkin-Elmer Atomic Absorption Spectrometer 3100.
3.Results
3.1.FT-IR measurements
FT-IR spectra of initial and ground natural diopside are shown in Fig.1.In the FT-IR Ž.y 1spectra of the initial natural diopside Fig.1a bands in the region ca.700cm can be
风雨张居正
比较优势Ž.Ž.Fig.1.FT-IR spectra of natural diopside:a initial;b,c,and d after 6,12,and 36h of grinding,respectively.Ž.y 1Concentrations of CaCO mg r g of sample near the bands in the 1430–1515cm region are shown.3
()
E.V.Kalinkina et al.r Int.J.Miner.Process.612001273–288277 attributed to the bending vibrations,and bands in the region850–1100cm y1correspond
to the stretching vibrations of the silicate structure.The latter is characteristic A diopside Ž.y1 type B bands Rutstein and White,1971;Omori,1971.The sharp peak at3670cm Ž.Ž.
OH stretching vibrations in the spectra of the initial natural diopside Fig.1a is likely
Ž
caused by the presence of antigorite,which has a similar peak in this region Marel van
.
der and Beutelspacher,1976.As can be clearly seen,during the process of grinding, dramatic changes in the samples occur.In the850–1100cm y1region,the intensities of the characteristic A diopside B type bands gradually decrease.A very broad peak in the 3300–3500cm y1region and a double peak at1434–1511cm y1appear.
After36h of grinding the two bands at ca.700cm y1practically disappear and the diopside type bands in the850–1100cm y1region are notably smoothed,although the
Ž.
main peak corresponding to Si–O stretching vibrations remains Fig.1d.Changes of the diopside spectra visible in the700–1100cm y1region can be explain by silicate structure disordering and amorphization during prolonged mechanical treatment.It should be noted that grinding generally leads to a decrease of the particle size as well. For transmission spectra measured with the use of KBr discs the decrease of particle size
Žcauses the increase of intensities of the strong bands relative to the weaker ones e.g.,
.
Tuddenham and Lyon,1960;Farmer and Russell,1966.For the DRIFT spectra the
Ž.
effect of the particle size decrease is similar Fuller and Griffiths,1978and should lead to the sharpening and narrowing of the strong Si–O stretching bands in the850–1100 y1Ž. cm region,which is not the case for the natural ground diopside Fig.1b,c,and d. Thus,from Fig.1we can conclude that the spectra changes in700–1100cm y1region are due to structural disorder and amorphization of diopside rather than due to the particle size effect.
29Ž.
From Si MAS-NMR data it was concluded previously Peck et al.,1988that a disordered surface phase similar to bulk diopside glass was formed on the mineral grains Ž.
during short-term several minutes grinding.At the same time,grinding resulted in a
Žnotable hydration of the newly formed surface phase,due to atmospheric moisture Peck .y1Ž.
et al.,1988.Appearance of bands in the3300–3500cm OH-stretching and1640 y1Ž.
cm OH-bending regions in Fig.1shows that,during grinding of our samples, perceivable hydration also takes place.Although the water content was not analysed directly,it is obvious from the spectra that the longer the grinding time the more the degree of hydration.A similar behavior was observed for the dry grinding of micas Ž.
Klyarovskiy et al.,1965using IR spectroscopy.
A double peak in the1430–1515cm y1region is close to the absorption bands of the
CO2y group in carbonate minerals.The spectra of carbonate minerals such as calcite 3
Ž.
CaCO,magnesite MgCO,and dolomite CaMg CO are characterised by a single 3332
peak in the same region,which corresponds to n degenerate antisymmetric stretching
3
2yŽ.
of an undistorted CO group White,1974;Nakamoto,1997.The distortion of this 3
group in silicate glasses eliminates the degeneracy and results in two unique infrared
y1Ž.
absorption bands at1435and1515cm Fine and Stolper,1985b.We have found that the double peak at1430–1515cm y1in the spectra of the ground diopside samples is very similar in shape and position to the one in the spectra of synthetic diopside glasses, calcium aluminosilicate glasses,and in Ca q Mg-bearing natural basaltic glasses con-
Ž.
taining dissolved carbon dioxide Fine and Stolper,1985b.It is remarkable that the
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