Ceramic Pigments Based on the Dicalcium Silicate M.B. Sedelnikova1, N.V. Liseenko1, V.M. Pogrebenkov1

1Tomsk Polytechnic University, Tomsk, Russia Corresponding-author E-mail: smb@tpu.ru

Abstract The possibility of ceramic pigments synthesis using industrial waste is studied. These raw materials will lower production costs of ceramic pigments. The synthesized pigments can be recommended for obtaining on-glaze ceramic paints and volume coloring of ceramic pastes in the manufacture of construction materials.

Keywords – ceramic pigments, dicalcium silicate, industrial waste, on-glaze paints, crystalline structures

I. INTRODUCTION The present economic conditions make it imperative to

lower the production costs, utilize raw materials fully, and develop waste-free methods of production. The construction industry requires inexpensive ceramic pigments suitable for obtaining protective-decorative coatings and for volume coloring of ceramic pastes. However, stable color can only be obtained as a result of addition of relatively expensive ceramic pigments. Synthesis of stable pigments using inexpensive raw materials, industrial wastes, for instance, which should be utilized, could be a solution to this problem. Examples of such wastes are a by-product of autoclave production of nickel, spent NTK-4 catalyst containing copper compounds as well as GIAP-10 and GIAP-16 catalysts containing zinc and nickel compounds, respectively, and various slags and sludges containing iron oxides and other compounds [1 – 3].

II. EXPERIMENTAL PROCEDURE Nepheline sludge was used for obtaining of ceramic

pigments in this work. The chemical composition of nepheline sludge from the Achinsk Alumina Refinery is as follows (%): 29.12 SiO2, 3.67 Al2O3, 4.55 Fe2O3, 53.20 CaO, 1.45 MgO, 2.16 Na2O, 0.90 K2O, 4.96 calcination loss [4, 5]. The mineral composition of nepheline sludge is basically represented by dicalcium silicate, and calcium hydrosilicates, hydroferrites, etc., are present as minor phases. The presence of calcium and silicon oxides in nepheline sludge makes it possible to obtain ceramic pigments with various structures according to the scheme (Fig.1) and the corresponding reactions:

2CaO·SiO2 + SiO2+2TiO2→2(СaO·TiO2·SiO2) Titanite 2CaO·SiO2 + Al2O3→ 2CaO·Al2O3·SiO2 Gehlenite 2CaO·SiO2 + SiO2 + ZnO→2CaO·ZnO·2SiO2 Hardystonite 2CaO·SiO2 + MgO + SiO2 →2CaO·MgO·2SiO2 Akermanite 2CaO SiO2 + SiO2 → 2(CaO·SiO2) Wollastonite 2CaO SiO2 + 3SiO2 + 2MgO→2(CaO MgO 2SiO2) Diopside 2CaO·SiO2 + Al2O3·2SiO2·2H2O + Al2O3 + SiO2 → 2(CaO·Al2O3·2SiO2) Anorthite

Figure 1. The scheme of ceramic pigments production with dicalcium silicate (nepheline sludge) use

For synthesis of ceramic pigments nepheline sludge was finely ground to the particles size 50 – 60 µm. Mixes were prepared by gel-method [6, 7]. The use of the gel method in our study is based on the capacity of dicalcium silicate to form an amorphous structure under the effect of hydrochloric acid. When HCl was added to dicalcium silicate, gel formation took place, since silicic acid and calcium chloride were formed [8, 9]:

Ca2SiO4 + 4HCl → 2CaCl2+H4SiO4

At this stage the dispersion of the batch and homogenizing of the components at molecular level took place. It was assumed that a structure was capable of assimilating a greater quantity of colorant ions in the form of gel.

The microstructure of the samples was investigated by SEM (Philips SEM 515). Fig. 2 shows micrographs of nepheline sludge samples before the treatment by hydrochloric acid, and after the gel formation. Pigments were obtained using the following method: dicalcium silicate (nepheline sludge) was mixed with other components of the batch and with water-soluble salts of the elements of 3-d subgroup (iron, chromium, nickel, and cobalt) in an amount of 0.1 – 1 mole; then a small quantity of concentrated hydrochloric acid was added into the mixture for the specified reaction to take place. The resulting gel was dried and fired at the temperature of 1100 – 1200 ºC. The resulting material was milled to the particles size 30 – 40 µm.

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Figure 2. Micrographs of a nepheline sludge: a– primary, and б – after the treatment by hydrochloric acid

III. RESULTS AND DISCUSSION

The X-ray phase analysis showed (Table 1) that the pigments had a complex composition.

The crystal structure of sphene samples consists of titanite (d=0.327, 0.283, 0.209 nm) and rutile (d=0.324, 0.249, 0.219 nm) (Fig. 3.). Cobalt titanium oxide and chromium titanium oxide are identified in the sphene pigments structure.

Figure 3. X-ray patterns of sphene sample, С – titanite (СaO·TiO2·SiO2), Р – rutile (TiO2)

Ceramic pigments with diortosilicate structure were obtained at the temperature 1100 oC. Fig. 4 indicated that the height of main characteristic peaks decreased when the content of chromium oxide grew. However the concentration growth cobalt oxide hasnot affected on the main reflex change, excepting the akermanite structure.

Besides the general structure of diortosilicate the spinels identified: NiAl2O4 – in gehlenite pigments, ZnCr2O4,

MgCr2O4 – in hardystonite and akermanite pigments. As the content of chromium oxide in the gehlenite pigments increases to 5-7 wt. %, the oxide was fixed as the autonomous phase Cr2O3 (d = 0.240, 0.208 nm).

TABLE I. THE PHASE CONSTITUTION OF PIGMENTS

Primary structure

Projected structure

Chromo-phore МеnОm (wt. %)

Synthesized structure (ratio crystalline

phase, %)

Dicalcium silicate

(nepheline sludge)

2CaO·SiO2

Titanite СaO·TiO2·SiO2

СоО (5-15 %)

Titanite СаО·TiO2·SiO2

(87-92 %), rutile TiO2 (3-5 %), Co2TiO4 (3-10 %)

Gehlenite 2СаО·Al2O3·SiO2

NiO (7-17 %)

Gehlenite 2СаО·Al2O3·SiO2

(90-96 %), NiAl2O4 (4-10 %)

Hardystonite 2СаО·ZnO·2SiO2

Сr2O3 (13-22 %)

Hardystonite 2СаО·ZnO·2SiO2

(90-93 %), ZnCr2O4 (7-10 %)

Akermanite 2СаО·MgO·2SiO2

Сr2O3

(5-15 %)

Akermanite 2СаО·MgO·2SiO2

(92-97 %), MgCr2O4 (3-8 %)

Wollastonite СаО·SiO2

Сr2O3 (5-10 %)

Wollastonite СаО·SiO2 (94-97 %),

Сr2O3 (3-6 %)

Diopside СаО·MgO·2SiO2

NiO (8-16 %)

Diopside СаО·MgO·2SiO2 (48-

52 %), cristobalite SiО2 (5-10 %),

Akermanite 2СаО·MgO·2SiO2

(35-39 %), NiO (3-8 %)

Anortite СаО·Al2O3·2SiO2

Fe2O3

(8-17 %)

Anortite СаО·Al2O3·2SiO2

(60-63 %), mullite 3Al2O3·2SiO2

(31-33 %), Fe2O3 (4-9 %)

Figure 4. The changes of intensity characteristic peak 1 – gehlenite(d=0.287 nm), 2 – hardystonite(d=0.281 nm), 3 – akermanite

(d=0.287 nm) depending on chromophore content:а– NiО, b – Сr2O3, (Т=1100 ºС)

1 µm

а

b

5 µm

X-ray analysis demonstrated that the wollastonite structure formed more successfully in samples which obtained with gel method usage. For other samples (which obtained without gel-method), the wollastonite was registered too but there were smaller diffraction peaks (Fig. 5).

Figure 5. X-ray patterns of wollastonite samples obtained according to reaction (5) a) – without gel method, b) – with gel method usage,

W – wollastonite

X-ray analysis showed that the crystal structure of samples obtained according to reaction (6) consists of multiple phases: diopside (d=0.323, 0.299, 0.256 nm), cristobalite SiО2 (d=0.407, 0.353, 0.211 nm), akermanite (d=0.351, 0.287 nm) (Fig. 6).

Figure 6. X-ray patterns of diopside samples obtained according to

reaction (6); D – diopside, S – cristobalite, A – akermanite

Chromophore oxides were fixed in the diopside pigments as the autonomous phase at the concentration Cr2O3 5wt. %, NiO 8-10 wt. %, Fe2O3 10 wt. % (Fig. 7). Cobalt oxide was not identified in the pigments up to the content 15 wt. %.

The anorthite was the dominant phase in the pigments obtained according to reaction (7). Mullite (d=0.342, 0.221, 0.169 nm) were identified in them too. The height of anorthite characteristic peak (d=0.32 nm) increased when the temperature grew from 1100 ºC to 1200 ºC.

Figure 7. The changes of intensity characteristic peak of Сr2O3 depending on chromophore content in the pigments: 1 – wollastonite structure,

2 – diopside structure

The chromophore ions influenced on the anorthite structure formation as a mineralizer (Fig. 8). As the content of chromophore oxides in the pigments increased up to the 4-8 wt. % the height of anorthite characteristic peak grew.

The dominant wavelength and purity of the pigment tone were measured with the Cary 100 Scan spectrophotometer. The colour of the pigments is given in Table II.

Figure 8. The changes of X-ray peak of anorthite (d=0.32 nm) intensity

depending on chromophore content: а – CoО, b – Fe2O3

The color of the pigments depends on the structure of their crystal lattice and on the coordination of the chromophore ion.

In the synthesized structures, the chromophore ions can occupy the Ca2+, Mg2+, Zn2+, Al3+ and Ti4+ positions.

It is known that substances containing tetrahedral doubly-charged complex cobalt ions have a blue or green color while octahedral Co2+ complexes impart rose and lilac colors [5]. The cobalt-containing pigments synthesized according to the reactions (1, 2, 3, 4, 5, 7) are blue, dark-blue and blue-green and the pigments synthesized via the reaction (6) are gray-pink. Therefore, a cobalt ion in pigments has in general tetrahedral coordination.

In ceramic pigments, nickel in the form of the complexes [NiO4 ] imparts a blue color; nickel in form [NiO6 ] imparts a color that can vary from brown to yellow-brown and green.

Pigments with nickel oxide are green and yellow-sandy. The color of the synthesized nickel-containing pigments is the evidence of octahedral coordination of Ni2+.

The coordination of Cr3+ ions is octahedral. The oxygen compounds of chromium can impart green, yellow, red, and rose colors. The color of chromium-containing pigments varies from yellow-green to green.

The iron-containing pigments have red-brown, orange and gold-brown color.

It should also be noted that the color of the pigments obtained is a manifestation of the chromophoric properties of the combination of iron oxide and other oxide chromophores, since nepheline sludge also contains 4.55% iron oxide.

Increasing the firing temperature to 1200 ºC has a negative effect on the color properties of the pigments. In addition, some pigments fuse, and it is difficult to grind the resulting agglomeration.

TABLE II. COLOUR CHARACTERISTIC OF PIGMENTS BASED ON DICALCIUM SILICATE, Т=1100 ºC

№ of pig- ment

Colour coordinates Colour

Wave-length,

nm

Purity of tone,

% х у Titanite structure

С6 Ni2+ 0.41 0.36 yellow-sandy 591 37

С15 Fe3+ 0.56 0.32 red-brown 610 70

Wollastonite structure В2 Со2+ 0.31 0.31 dark-blue 482 5

В7 Сr3+ 0.33 0.39 green 546 27

Diopside structure D 8 Сr3+ 0.38 0.40 light-green 557 69

D11 Сo2+ 0.38 0.31 gray-rose 496 13

Sorosilikate structure Gl11 Fe3+ 0.39 0.35 orange 596 24

Gr2 Cо2+ 0.29 0.30 blue 480 10

Gr13 Fe3+ 0.41 0.38 gold-brown 591 33

Anorthite structure А2

Co2+ 0.23 0.31 gray- blue 488 35

А10 Cr3+ 0.36 0.41 yellow-green 565 43

The obtained pigments were applied on the ceramic articles as overglaze paints. The pigments were stable in overglaze painting and almost did not change colour.

IV. CONCLUSION

Thus use of dicalcium silicate (nepheline sludge) for obtaining ceramic pigments with different crystal structures is economically expedient.

The synthesized pigments are resistant to high temperatures and flux and glaze melts. They can be recommended for obtaining on-glaze ceramic paints and volume coloring of ceramic pastes in the manufacture of construction materials.

In addition to the cost reducing of the pigments, problems of utilizing production wastes can be solved and the raw-material base for synthesis of ceramic pigments can be expanded.

The use of the gel method for producing ceramic pigments is justified and promising, since it has a number of advantages. When finely milled material is treated with acolorant salt solution (without the gel-formation stage), the chromophore ions just get adsorbed on the surface of solid particles. The subsequent incorporation of the chromophore ions in the crystal lattice through diffusion to a great extent depends on the size of the initial mineral particles and on the ratio of the ionic radius of the interchangeable ions and their charges. The gel method makes it possible to homogenize the mixture component at the molecular level. The need of joint milling of the components for a long time is eliminated.

The gel method can be recommended for the industrial production of ceramic pigments, as it does not require cardinal modifications of the process scheme and equipment but facilitates a significant improvement of the color properties of pigments and paints.

The pigment compositions developed can be widely used for manufacturing coloured construction materials.

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