Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus Slag Lixivium

7/26/2019 Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus Slag Lixivium

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G.-B. LI et al.: Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus, Kem. Ind. 63 (11-12) 405409 (2014) 405

Introduction

Yellow phosphorus slag is a solid waste obtained in the pro-duction of yellow phosphorus in electrical furnace in whichthe main ingredients are SiO2and CaO. It can be used forpreparing silicon and calcium products with high addedvalue. Studied were the kinetic characteristics of calciumsulphate crystallization in nitric acid lixiviumformed in thecourse of treating yellow phosphorus slag with nitric acidsolution. The crystallization kinetics parameters of calciumsulphate were determined.

At present, yellow phosphorus slag is used mainly as siliconfertilizer, cement, calcium phosphorous slag brick, porcelaintiles, microcrystal glass, white carbon black, and so on.18

The calcium in yellow phosphorus slag can be separated inthe form of soluble calcium nitrate into solution. After sep-

arating the calcium and silicon, high quality white carbonblack product with high added value can be obtained byleaching yellow phosphorus slag with nitric acid solution.9In the meantime, we treated the lixiviumcontaining calci-um nitrate with sulphuric acid solution in order to obtaincalcium sulphate of high purity and quality, and convert thecalcium nitrate solution into nitric acid solution which canbe used to leach yellow phosphorus slag so as to implementthe recycle of lixivium. The paper studied the kinetics char-acteristics of calcium sulphate crystallization in nitric acidlixiviumin the course of treating the yellow phosphorus slagwith nitric acid solution.

Experimental

Experimental techniques

The crystallization process of calcium sulphate is composed

of nucleation and crystal growth, and the kinetic character-istics of crystallization are controlled by the nucleation rateand linear growth rate of crystal together. The crystal nucleusformation has the form of primary nucleation and secondarynucleation,10 and the secondary nucleation is applied mostlyin the course of industrial crystallization for it consumes lessenergy, produces crystals easily, and increases the yield ofthe product.

In general, the secondary nucleation is the controlling step inthe crystallization process.11The nucleation rate B0approx-imately equals to the secondary nucleation rate, Bs, and itcan be described by formula

B0= kn GiMTj, (1)

where MTis crystal mass concentration,12knis rate constant,

Gis the crystal growth rate, iandjare empirical kinetic pa-rameters which can be determined by regression analysiswith experimental data.

The crystal growth rate is relevant with nucleation rate B0and crystal number density n0. It can be expressed by for-mula

B0= n0 G, (2)

The research pointed out that in a certain concentration ofcrystal slurry, the mass distribution of the crystal particleswith different sizes was related to the crystal number density.The calculation relationship was as follows:11

* Corresponding author: Prof. Yi Su, e-mail: [email protected]** This manuscript is supported by CNSF Grant #21066003.

Crystallization Kinetics of PrecipitatingCalcium from Yellow PhosphorusSlag Lixivium**

DOI: 10.15255/KUI.2013.029KUI-26/2014

Short communicationReceived September 5, 2013

Accepted December 12, 2013

G.-B. Li,aD. M. Luo,aH.-P. Li,aY. Su,a*and M.-M. Dub

a Institute of Chemical Engineering, Kunming University of Science and Technology,650 500 Kunming, Yunnan, PR China

b Faculty of foreign languages and cultures, Kunming University of Science and Technology,650 500 Kunming, Yunnan, PR China

The crystallization dynamic parameters of calcium sulphate were determined in the course of theprecipitation conversion process of calcium in lixivium. The crystallization dynamic equationsof calcium sulphate were achieved by multiple linear regression. The study results indicate thatCaSO4

2H2O crystal nucleation-growth kinetics equation in nitric acid leaching liquid of yellowphosphorus slag at room temperature is expressed by B0= 2.9041011 G0.83 MT

0.167.

Key words: Crystallization kinetics, yellow phosphorus slag lixivium, calcium sulphate, precipitation
mailto:euyi0311%40163.com%20?subject=http://dx.doi.org/10.15255/KUI.2013.029http://dx.doi.org/10.15255/KUI.2013.029mailto:euyi0311%40163.com%20?subject=


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G.-B. LI et al.: Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus, Kem. Ind. 63 (11-12) 405409 (2014)406

, (3)

where Lis particle size, Nis number concentration of parti-cles in slurry, and nis crystal number size density.

For crystallizer,13if we can ignore the crystal slurry volumechanging with time and the growth of crystal according tothe Llaw and the crystal coalescence and rupture in thecourse of crystallization process, the relationship betweenthe mass distribution of the crystal particles of different sizesand the crystal number density can be described by formula

, (4)

where wiis mass fraction of the particles in the particle sizerange, c is crystal density, k is crystal shape coefcient, Viiscrystal volume.

The study shows that the relationship among the crystal lin-ear growth rate, the crystallization time, the crystal size andthe density of crystal nucleus numbers can be depicted byformula

. (5)

The formula indicates the relationship between the crys-tal number density and the crystal linear growth rate.14By means of plotting with the crystal size as the abscissa andlnnas the ordinate, we can obtain a straight line of which

the slope equals to 1Gtand intercept equals to lnn0. So,the crystal growth rate Gand the crystal number density n0can be calculated on the basis of the test data in the courseof crystallization.

According to eqs. (1) and (2), the test data in crystallizationunder different operating conditions, and by applying multi-ple linear regression, we can obtain the crystallization kinet-ics parameters i,j, knand the crystallization kinetics equationcan be obtained.

Experimental raw material and apparatus

Nitric acid, sulphuric acid, ethanol (all pure for analysis);Precision electric mixer, Jintan Automation Instrument Fac-tory, Jiangsu; Ultrasonic cleaning machine, Kunshan Ultra-sonic Instruments Co. Ltd.; OMEC laser particle size analys-er, Zhuhai OMEC Science and Technology Co. Ltd.

Experimental method

Sulphuric acid solution (20 %) was added into the lixiviumand stirred for 10, 20, 30, 40, 50, and 60 minutes, respec-tively, at stirring speed of 250 min1at room temperature.

After the precipitating reaction was fullled based on re-

tention time, 10 ml of the suspension solution was quicklytransferred into a previously dried and weighed sand corelter for ltering and washing with anhydrous alcohol, andthen dried to constant weight in order to calculate the crys-

tal mass concentration MT. At the same time, the remainingsuspension slurry was ltrated rapidly, washed with alcoholand dried. The dried calcium sulphatewas dispersed andmeasured in anhydrous ethanol with OMEC laser particlesize analyser after the sample was ultrasonic stirred for 2minutes. Then the crystal size distribution was obtained.

Results and discussion

The analysis of the relationship betweencrystal number density and particle size

According to the crystal size analysis data, crystal numberdensity n of each grade range can be calculated by theeq. (4) and the relationship diagram between the crystalnumber density and crystal size L can be made. Fig. 1 isthe relationship diagram between the crystal number den-sity and particle size Lfor 10 minutes residence time. It canbe seen that the crystal number density decreases with the

increase in particle size.

F i g . 1 Relation curve of crystal number size density n andparticle size L

S l i k a 1 Odnos brojnosne gustoe veliine kristalan i veliineesticaL

The crystal nucleus number densityand crystal growth rate

According to eq. (5), the relationship between lnnandLislinear in coordinate diagram of lnnandL, the slope equalsto 1Gtand the intercept equals to lnn

0, so the crystalgrowth rate Gand crystal number density n0can be calcu-lated on the basis ofthe test data in the course of calciumsulphate crystallization.

Fig. 2 shows the relation curve between lnnand L for 10minutes residence time and a linear regression straight line isobtained. It can be seen from Fig. 2 that the 1Gtis 0.1514and lnn

0= 27.09 in which the Gequals to 0.66 mmmin1

and n0

= 5.82

1011

l1

mm1

. With the same method, theGand n0of the residence time of 20, 30, 40, 50, and 60minutes can be obtained and the calculation results are ex-pressed in Table 1.


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F i g . 2 Relation curve between logarithm of crystal number

size density ln n and particle size LS l i k a 2 Odnos logaritma brojnosne gustoe veliine kristala

ln n i veliine esticaL

The effect of residence time on the crystalgrowth rate Gand the nucleation rate B0

According to the crystal theory, the degree of supersatura-tion is an important effect factor of the crystal growth rateand crystal number density for most of the substances in theprocess of crystallization. Fig. 3 shows the relationship be-tween residence time and the crystal growth rate G. Fig. 4is the relationship diagram between the residence time and

the nucleation rate B0

. It can be seen from Table 1 and Fig. 3that the crystal growth rate gradually decreases with extend-ing of residence time. According to Fig. 4, it can be seenthat the calcium sulphate crystal nucleation rate reduceswith the increase in residence time. This is mainly becausethe amount of calcium sulphate crystal increases, the crystalgrows gradually and the supersaturation degree of the solu-tion gradually reduces with the extension of residence time.

Mathematical model of crystallization kinetics

The Gand n0calculated under the condition of different res-idence time, the nucleation rate B0calculated by eq. (2) and

the crystal mass concentration in suspension MTare listed inTable 1. Based on the data of B0, MTand Gin Table 1 andcrystal nucleation-growth kinetics equation described byeq. (1), the indices i= 0.83 andj= 0.167, and coefcient

F i g . 3 Relationship between residence time t and the crystal

growth rate GS l i k a 3 Odnos izmeu trajanja reakcije i brzine kristalnog rasta

F i g . 4 Relationship between residence time t and nucleation

rate B0

S l i k a 4 Odnos izmeu trajanja reakcije ti brzine nukleacije B0

kn= 2.9041011were obtained by the multiple linear re-

gression method. The CaSO4 2H2O crystallization nuclea-

tion-growth kinetics equation in nitric acid leaching liquid of

yellow phosphorus slag was obtained under the condition ofroom temperature:

B0= 2.9041011

G0.83

MT0.167. (6)

T a b l e 1 Results of the experimental data processing

T a b l i c a 1 Rezultati obrade eksperimentalnih podataka

tmin

Gmmmin1

lnn0

n01011l1 mm1

MTgl1

B01011l1 min1

10 0.661 27.090 5.822 27.13 3.450

20 0.338 27.121 6.005 28.13 2.03030 0.225 27.219 6.623 32.99 1.489

40 0.176 27.618 9.871 42.03 1.273

50 0.136 27.436 8.146 30.24 1.109

60 0.112 27.335 6.730 29.96 0.755


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G.-B. LI et al.: Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus, Kem. Ind. 63 (11-12) 405409 (2014)408

Conclusion

(1) The crystal nucleation rate and growth rate of calciumsulphate decreases with the increase in residence time in thecourse of precipitating calcium sulphate from nitrate leach-ing solution of the yellow phosphorus slag with sulphuricacid.

(2) The experimental study shows that the CaSO4 2

H2Ocrystallization kinetics indices i = 0.83, j= 0.167, whichindicates that the effect of suspension density on the nucle-ation rate is not obvious and the effect of crystal growth rateon the nucleation rate is conspicuous.

(3) The CaSO4 2 H2O crystal nucleation-growth kinetics

equation in nitric acid leaching liquid of yellow phosphorusslag at room temperature is expressed as follows:

B0= 2.9041011

G0.83

MT0.167.

List of symbolsPopis simbola

B0 nucleation rate, l1 min1

brzina nukleacije, l1 min1

Bs secondary nucleation rate, l1 min1

brzina sekundarne nukleacije, l1 min1

G crystal growth rate, mmmin1

brzina rasta kristala, mmmin1

i,j kinetic parameters kinetiki parametri

kn rate constant

konstanta brzinekv crystal shape coefcient

koecijent kristalnog oblika

L crystal size, mm veliina kristala, mm

MT crystal mass concentration, gl1 MT= mV

masena koncentracija kristala, gl1

m sample mass, g masa uzorka, g

N crystal number concentration, l1

brojnosna koncentracija kristala, l1

n, n0 crystal number size density, l1 mm1

brojnosna gustoa veliine kristala, l1 mm1

t time, min vrijeme, min

Vi crystal volume, mm3 Vi = Li

3

obujam kristala, mm3 Vi = Li+13 Li

3

wi mass fraction in the crystal size range, % maseni udjel frakcije u rasponu veliine kristala, %

c crystal density, gl1

gustoa kristala, gl1

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SAETAKKinetika kristalizacije kalcija

izluenog iz troske bijelog fosfora

G.-B. Li,aD. M. Luo,aH.-P. Li,aY. Sua*i M.-M. Dub

Odreeni su parametri kinetike kristalizacije kalcijeva sulfata prilikom taloenja kalcija iz luga tro-ske bijelog fosfora. Jednadbe brzine kristalizacije kalcijeva sulfata dobivene su viestrukom line-arnom regresijom. Kinetika kristalizacije CaSO4 2H2O pri sobnoj temperaturi u duino-kiselomlugu troske bijelog fosfora slijedi izraz B0= 2.9041011 G0.83 MT

0.167.

Prispjelo 5. rujna 2013.Prihvaeno 12. prosinca 2013.

a Institute of Chemical Engineering,Kunming University of Science and Technology,650 500 Kunming, Yunnan, Kina.

b Faculty of foreign languages and cultures,Kunming University of Science and Technology,650 500 Kunming, Yunnan, Kina.

Crystallization Kinetics of Precipitating Calcium from Yellow Phosphorus Slag Lixivium

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