Preparation of pollucite and analcime zeolites as a method to valorizealuminum saline slags

Alejandro Jiménez1, Alexander Misol1, Álvaro Morato1, Miguel Ángel, Vicente Rives1, Raquel Trujillano1, Antonio Gil2, Sophia Korili2

1GIR- QUESCAT, Departamento de Química Inorgánica, Universidad de Salamanca, 37008 Salamanca2INAMAT^2-Departamento de Ciencias, Edificios de los Acebos, Universidad Pública de Navarra, 31006 Pamplona

Introduction

Experimental

Results

Acknowledgement

Conclusions

c

a

Aluminum properties, such as corrosion resistance, low melting point (660 °C) or low density (2.70 g/cm3) make it an ideal material for many applications. This element can be recycled and reused without losing its properties. Recycling process requires less energy than primary aluminum production (combination of the Bayer and Hall–Héroult processes but other wastes are also generated, the most important is the so-called Salt Cake or Saline Slag. It is produced when the flux salts (mainly NaCl and KCl) are used for melting aluminum. Salt cake is considered a hazardous waste in the European Union.

Analcime and Pollucite belong to zeolites from the analcime family and their structure are similar. The diameter of channels in pollucite is 2.80 Å while the diameter of Cs is 3.34 Å and Cs+ is immobilized inside the pollucite structure. For this reason, pollucite is one of the most interesting materials for storing 137Cs for a long period of time in a safe way. Analcime could be used as ion exchanger.

The objective of this work is to use the salt cake in the synthesis of applicable zeolites. Aluminum from the non-metallic fraction of salt cake should be recovered under reflux conditions. The resulting liquor will be used to synthesized zeolitic materials under hydrothermal conditions.

MINECO and Fondos ERDF Funds (MAT2016-78863-C2-R).

- Analcime and pollucite zeolities can be synthesized from aluminum saline slag. First, it is necessary to recover aluminum from the slag under reflux conditions with different alkaline hydroxide.

- Extraction performance is improved when increasing the alkaline hydroxide concentration.

- The best extraction is obtained at high concentration of CsOH, keeping time reflux in 2 h and ratio small fraction/dissolution volume constant.

- The liquor from the extraction is used as a source of Al in the preparation of zeolites.

- Hydrothermal Synthesis is carried out at 200ºC for 24 hours.- Crystallinity and water content is higher for analcime than pollucite.- Si/Al ratio is high in both cases, so substitution of Si4+ by Al3+ is small.

Therefore, to use analcime as ion exchanger or pollucite for cesium storage, this molar ratio should be smaller, to increase the performance of these materials in these applications.

Analcime

NaAlSi2O6·nH2O

Pollucite

CsAlSi2O6·nH2OSaline slag

Reflux treatment

Extractionliquor

Hydrothermaltreatment

Grinding.Sieving 1mm.Washing.Sieving 0.4 mm.

Fraction lessthan 0.4 mm

Silicasolution

Component Raw salt cake Intermediatefraction

Small fraction

Al2O3 21.30 70.00 76.80

Na2O 18.90 Not detected Not detected

MgO 1.30 4.50 10.30

SiO2 2.20 15.00 5.30

SO3 0.24 0.29 0.50

Cl 33.90 0.90 0.21

K2O 18.90 1.10 0.45

CaO 0.72 4.10 2.12

TiO2 0.19 0.70 0.76

Fe2O3 0.70 1.40 2.00

CuO 0.34 1.10 0.78

ZnO 0.15 0.60 0.28

Elements with oxide content ≤ 0.1 % are not given

Chemical compositions of the raw salt cake and the fractions with particle size smaller tan 1 mm

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

intermediate fraction small fraction

Inte

nsity

(cps

)

2q (deg)

100

Al2O3Al(OH)3Al metalMgAl2O4

1 2 3 40

5

10

15

20

25

30

35

40

45

50

CsOHNaOH

% A

lum

inum

reco

vere

d (%

)

MOH Concentration (M)

X-ray patterns of Intermediate and small fractions

Percentage of recovered aluminum for different alkaline hydroxide

concentrations

Element

Sample Al (mg/L) Si (mg/L) Na (mg/L) K (mg/L) Cs (mg/L)

CsOH-1M-2h 5934 114 545 355 64081

NaOH-1M-2h 7159 129 10230 448 -

Chemical compisitions of extractions liquors used in preparation of zeolitic materials

10 15 20 25 30 35 40 45 50

(721)(461)(532)(440)

(521)

(431)

(332)

(400)

(321)(220)

Inte

nsity

(cps

)

2q (deg)

POL ANA

2000

(211)

(422)

4000 3500 3000 2500 2000 1500 1000 500

POL ANA

Tran

smitt

ance

(%)

Wavenumber (cm-1)

3436

1629

1042

763

727

625

444

36161641

1009

740

683

616

443

10

0 100 200 300 400 500 600 700 800 90091

92

93

94

95

96

97

98

99

100

ANA POL

Mas

s (%

)

Temperature (ºC)

V

II III

VI

X-ray patterns of both solid prepared FT – IR spectra of ANA and POL

SEM micrograph of ANA SEM micrograph of POL

TEM micrograph of ANA SEM micrograph of POL

TG curves of ANA and POL

0 100 200 300 400 500 600 700 800-0.2

0.0

0.2

0.4

0.6

ANA POL

Hea

t Flo

w (W

/g)

Temperature (°C)

308

200

DSC curves of ANA and POL

Element

Sample Al Si Na Cs

ANA 10.87 42.36 12.22 -

POL 8.34 32.67 0.95 38.99

Chemical composition of the final solids (wt%)