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Javier Calbo, M. Ángeles Tena, Guillermo Monrós, M. Llusar, Roberto Galindo and J.A. BadenesInorganic Chemistry Area, Inorganic and Organic Chemistry Dept., Jaume I University, Castellón, Spain

Black ceramic pigments used in the ceramic industry are based on two crys-talline structures: hematite and spinel. Hematite solid solution (Fe,Cr)2O3 is used in bulkcoloration of polished, unglazed, porcelainized stoneware because of its instabilitywhen glazed to decorate traditional wall and floor tile. For example, this pigment can-not be used in modern zinc-containing enamels, because it decomposes to franklinite(Zn(Fe,Cr)2O4; DCMA1 No. 13-37-7, which is used as brown pigment) and skolaite (whichinvolves brown-green coloration).2

Spinel-system black pigments are widely used to decorate glazed wall and floor tile,because they remain stable when mixed with the glaze and when fired at high tempera-tures (>1000°C). Spinel black pigments are usually synthesized from cobalt ferrochromite(DCMA No. 13-40-9) and nickel ferrochromite (DCMA No. 13-50-9) and are doped with anelement, such as manganese.Therefore, most industrial pigments involve chromium, iron,nickel and manganese in their composition.

The use of several additives known as mineralizers are important in the synthesis ofpigments. Mineralizers enhance the reactivity of the solid-state reaction3 and the crys-talline structure formation. These effects can be achieved in various ways:4

• Classical mineralization (molybdates, borates, alkaline or alkaline-earth halides,nitrates, etc.) by forming flux phases in the reacting media that increase the rate ofionic interdiffusion processes;

• Gas-phase mineralization by in-situ generation of volatile species that facilitate somereactions (e.g., fluoride addition generates volatile SiF4(g), which allows the transport ofSiO2 toward ZrO2 reaction sites, as suggested by Eppler);5

• Surface reaction site activation due to flux decomposition (e.g., formation of nitro-genated vapors between 300 and 500°C as result of nitrate decomposition); and

• Redox-stabilizing mineralization, where the oxidation state of some reactive speciescan be controlled or established by adding the appropriate compound (e.g., stabiliza-tion of Fe2+ by iodide, polycarboxylate, thiosulfate, sulfate, tin(II) and other chelate addition).

Flux Agent Effect on Nickel FerriteBlack Pigment

Three mineralizers were added to Ni(Fe,Cr)2O4 spinel systemblack pigments to improve color strength, decrease firing temperature and minimize hazardous cation composition.

On the other hand, the use of mineralizers can cause negative environmental effects. Theemission of pollutants—such as NOx, SO2, HCl, HF, borates and metal aerosols—into the atmos-phere and the leaching of pollutants—such as heavy metals, nitrates and sulfates—into thewashing waters damage the environment. To minimize these negative effects, environmentaland workplace health limits are becoming more restrictive. Legislation about workplace healthhas caused industry to pay more attention to environmental impact. Therefore, black pigmentshave been environmentally and toxically optimized in a previous work,6 while maintainingsimilar color and stability characteristics of other commercial pigments.

In this work, mineralization of black ceramic pigments has been studied to improve theircharacteristics compared with other commercial pigments. Spinel black pigments with com-positions (Fe0.8Mg0.2)(Fe0.2Ni0.8Cr)O4 and (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 have been environmen-tally optimized in a previous work. The objective of this work is to use these compositions andto improve the microstructure and color strength of black spinel pigments with mineralizers,while decreasing the synthesis temperature and soaking time.

The ExperimentSpinel black pigments (Fe0.8Mg0.2)(Fe0.2Ni0.8Cr)O4 and (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 environmen-tally optimized in a previous work were prepared using the traditional ceramic procedure.Fe2O3 (98%, J.T. Baker Chemicals, Phillipsburg, N.J.), NiO (99%, Prolabo), MgO (97%, Merck,Darmstadt, Germany), Al(OH)3 (99.5%, Merck), Cr2O3 (99%, Merck) and ZnO (98%, Merck) wereused as precursors.

Several mineralizers with various characteristics and melting points were used: KNO3 (99%,Panreac, Montplet and Esteban S.A., Barcelona, Spain); 2NaCl·NaF eutectic mixture (99 and 98%,respectively, Panreac); and Na2SiF6 (industrial, 98%). KNO3 was considered a low-melting-pointmineralizer (decomposes at 300°C to NO2(g) + K2O to form a local oxidizing atmosphere and fluidphases). 2NaCl·NaF melts at an intermediate temperature (674°C) to form fluid phases, andNa2SiF6 is considered a relatively high-temperature flux (~1000°C), which decomposes to SiF4(g)+ NaF to form a local reducing atmosphere and fluid phases.

To prepare the samples, the appropriate amount of each precursor to obtain 20 g of pigmentand 20 mol% of corresponding mineralizer were ball-milled in acetone media for 20 min at800 rpm. Residual acetone then was removed by evaporation, and the dried powders werehomogenized in an agate mortar. These powders were fired in an electric furnace (Nannetti) at1000°C at a heating rate of 5°C/min and were soaked for 6 h at the maximum temperature. Thefired powders then were again homogenized in an agate mortar.

X-ray diffractometry (XRD; Model D-500, Siemens, Karlsruhe, Germany) was used to analyze thedevelopment of crystalline phases with firing temperature and composition. Patterns of the cal-cined samples were obtained using conventional powder techniques with Ni-filtered CuKαradiation (20–70° 2θ, with a goniometer speed of 0.05° 2θ, and 1 s of counting time per step).

Fired powders were washed according to the industrial procedure (exhaustive ball milling for20 min at a pigment/water/ball ratio of 1/30/15). The washing process eliminated undesirablesoluble salts that would have negative effect during the enamel-firing process. Chromium(VI)concentration in washing waters was measured by adding HNO3 to oxidize all the chromiumto chromium(VI) and measuring the absorption at 350 nm with a spectrophotometer (ModelLambda 2000, Perkin–Elmer, Norwalk, Conn.).

Nonmineralized and mineralized samples were 5% enameled with a transparent single-firingglaze into ceramic bisques to test their efficiency as black ceramic pigments. The samples thenwere fired using a fast-firing scheme (52 min of duration from cool to cool at a maximum tem-perature of 1085°C).

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L*a*b* color parameters of enameled samples were measured using a spectrophotometer(Model Lambda 2000, Perkin–Elmer) with standard lighting C, according to the L*a*b* colori-metric method recommended by Commission Internationale de l’Eclairage (CIE).7 In thismethod, L* is the lightness axis (black (0) → white (100)), b* is the blue (–) → yellow (+) axisand a* is the green (–) → red (+) axis.

Microstructure and chemical homogeneity of some representative fired samples were studiedusing scanning electron microscopy (SEM; Model Leo-440i, Leica, Buffalo, N.Y.) with energy-dis-persive X-ray spectroscopy (EDAX) by Oxford University.

The toxicity levels of the powders were analyzed using a leaching method and then usingthe photobacteria phosphoreum toxicity test.8 First, the powders were leached in acetic acidmedia (pH <4.5) for 24 h. Second, the samples were filtered, which separated the solids fromthe leaching waters. Third, the toxicities of the leaching waters were analyzed using the EC50(effective concentration 50%) values, which were obtained using the bioluminescence testwith photobacteria phosphoreum.

Photobacteria emit light during respiration process, and EC50 is defined as the toxic concen-tration necessary to decrease light emission by 50% after 15 min of exposure at 15°C. Samplesanalyzed with EC50 < 3000 ppm are considered hazardous.

XRD Characterization(Fe0.8Mg0.2)(Fe0.2Ni0.8Cr)O4 and (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 samples were synthesized with thethree mineralizers (Table 1). Two samples without mineralizer (A0 and B0) obtained in a previ-ous work6 were used to compare with the prepared samples.

XRD patterns of fired samples (Table 2) show that trevorite (NiFe2O4) is the primary phase inall samples and (Cr2–xFex)O3 is the secondary phase. Mineralized samples present higher spineldiffraction peaks than nonmineralized samples (B0/1000°C), and peaks assigned to (Cr2–xFex)O3show lower intensity (Fig. 1). This result indicates better reactivity achieved with the use of thestudied mineralizers. Also, the spinel peaks are stronger in A2 and B2 samples than in theother mineralized samples (Table 1).

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Figure 1 XRD evolution of samples (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 depending on the mineralizer used(crystalline phases: T is NiFeO4 and CF is (Cr2–xFex)O3.

2θ (deg)

Furthermore, the corresponding peaks of (Cr2–xFex)O3 are lowerfor A2 and B2 samples than for A1/B1 or A3/B3 samples becauseof the high degree of reaction reached in samples mineralizedwith 2NaCl·NaF. XRD results show that the mineralizer systemimproves the reactivity with respect to nonmineralized samplesand that the eutectic mixture 2NaCl·NaF presents the best reactivi-ty of all samples. On the other hand, when a mineralizer system isused, samples with zinc (compositions B) present strongertrevorite peaks than do samples without zinc (compositions A),which indicates better development of the spinel achieved in sam-ples with compositions B.

Color MeasurementThe samples were 5% glazed, and the CIE–L*a*b* colorimetricparameters were measured (Table 2).

Enameled samples A1 and B1 (KNO3 mineralized) show visualbrown coloration, as expected from the high a* value measured.Samples A3 and B3 are blacker than A1 and B1; a* and b* arealmost 0, although L* is sensibly higher. A2 and, in particular, B2present the best black coloration of the studied samples; a* and b*are almost 0 (expected values for a black pigment) and have thelowest L* (more intense) value of the mineralized samples.

Colorimetric measurements show that samples mineralized withthe alkaline mixture 2NaCl·NaF present the best black color pig-ment for magnesium-doped samples (A) and magnesium- andzinc-doped samples (B). Because of the different compositions of Aand B, magnesium- and zinc-doped samples (B) always presentblacker coloration than magnesium-doped samples (A), when thesame mineralizer is used. This difference in the black color is thesame in A and B compositions when no mineralizer is used.

When the mineralized samples are compared with the commer-cial pigment (L*a*b* = 33.0/1.0/0.0), sample B2 presents a similarcolor strength that is more intense (lower L*) but with a light-redcomponent (a* of 1.7). However, this pigment has been synthe-sized at lower temperature (1000°C for 6 h) than the commercialpigment, and its composition has been environmentally opti-mized.6

Microstructure CharacterizationMicrostructure and chemical homogeneity of samples were studied using SEM/EDX (Fig. 2).

The (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 nonmineralized sample (B0) shows two types of particles(Fig. 2(a)). The larger particles (~1 mm) have an octahedral shape that corresponds to spinelformation. Dispersed smaller particles (200–500 nm) have an irregular round shape. EDX andmapping analysis not shown in this article indicate a homogeneous distribution of elementsin the sample.

Sample B1 also shows two types of particles (Fig. 2(b)): octahedral particles (1–2 µm) andaggregates of smaller round-shaped particles (200–400 nm). EDX analysis (Fig. 3(a)) shows thepresence of potassium in the sample. The sample has a clear heterogeneity in element distrib-ution, with potassium and chromium aggregates (Fig. 3(b)). The use of KNO3 as a mineralizer

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Figure 2 SEM micrographs of samples (a) B0, (b)B1, (c) B3 and (d) B2.

(a)

(b)

(c)

(d)

involves the formation of K2CrO4, which avoids the formation of the stoichiometrically desiredcomposition. The presence of soluble chromium(VI) compounds is environmentally undesir-able because of their high toxicity level. Chromium(VI) is an extremely hazardous material: itscompounds are carcinogenic and are classified as A1 by ACGIH.9

Sample B3 has octahedral-shaped particles of spinel (~1–2 µm) and smaller irregular-shapedparticles (200–500 nm) (Fig. 2(c)). The elements are homogeneously distributed and no segre-gation can be detected in this sample.

Sample B2 has octahedral-shaped crystals associated with the spinel phase that are larger(1–4 µm) than in the other samples (Fig. 2 (d)). This larger particle size is in agreement with thehigh reaction level of spinel formation achieved in this composition. EDX and mapping analy-sis (Fig. 4) show a homogenous distribution of the elements, and no segregation is observedin samples B1 and B3.

Sample B2 stands out for its larger crystal size and better crystal formation. This is importantin the glazing process, because pigment particles dispersed in the glaze are attacked by theglaze, which partially dissolves them. If the particle size is too small, the particles are complete-ly dissolved by the glaze, and color strength is modified. SEM/EDX results, which are supportedby the L*a*b* measurements and XRD results, show that sample B2 presents the best col-oration because of the high level of development of the spinel and because of the crystal sizeachieved in this sample.

Powder Toxicity LevelToxicity levels of powders were analyzed using a leaching method and the photobacteriaphosphoreum toxicity test. Pigment powders were washed to eliminate undesirable sub-stances that could be adsorbed on the particle surface and to eliminate soluble impurities,such as K2CrO4 formed in samples mineralized with KNO3. Chromates were analyzed in wash-ing waters to determine the amount of chromium leached (Table 2). Samples A1 and B1leached more chromium than the other samples (51.69 and 46.23 mmol/(mol of pigment),respectively) from the K2CrO4 dissolution, as detected using EDX.

The K2CrO4 formation eliminated chromium from the system, and the formulated stoichiome-try was modified. Samples mineralized with 2NaCl·NaF (A2 and B2) leached less than one-tenth part than did samples A1 and B1. A3 and B3 leached an intermediate amount of chromi-um. Considering the chromium leached, the alkaline mixture 2NaCl·NaF used was the mostenvironmentally appropriate mineralizer system. However, nonmineralized samples leachednegligible amounts of chromium; therefore, the use of mineralizers must be optimized from anenvironmental consideration.

The toxicity level of the pigment powders was tested. EC50 of washed and nonwashed pow-ders were measured using the photobacteria phosphoreum bioluminescence test of samplessubjected to the leaching process (Table 3). These results showed that all washed mineralizedsamples had EC50 values >6000 ppm (only samples with EC50 < 3000 ppm are considered haz-ardous) and nonwashed mineralized samples had EC50 values <3000 ppm, except B2 and B3.The washing process decreased notably the toxicity of the pigments. Nonwashed powdersmineralized with KNO3 (samples A1 and B1), presented the lowest EC50 values, as was expectedfrom the large amount of chromium leached during the washing process.

Results of the nonwashed samples showed that magnesium- and zinc-doped samples mineral-ized with alkaline mixture and that Na2SiF6 had EC50 values higher than the limit (3000 ppm),which was contrary to the only-magnesium-doped samples.This was due to the better develop-ment level of the spinel achieved in zinc-doped samples, which was in agreement with XRD

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results.The washing process allowed a decrease in the toxicity level of powders and samples, asshown by EC50 values up to 6000 ppm, which was up to twice that of the limit level of 3000 ppm.

Mineralizers Improve ReactivityMineralizers improve the reactivity of the system and increase spinel development. In(Fe0.8Mg0.2)(Fe0.2Ni0.8Cr)O4 and (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 systems, samples mineralized with2NaCl·NaF (A2 and B2) show higher-level development than the other samples using differentmineralizers and nonmineralized samples.

The use of KNO3 as mineralizer in this system involves the formation of K2CrO4, which elimi-nates chromium from the composition and changes the desired stoichiometry. Formation ofK2CrO4 also is environmentally undesirable, because it is dissolved during the washing process,which introduces hazardous chromium(VI).

Nonwashed samples show higher toxicity levels than washed samples that are not consideredhazardous, according to the photobacteria phosphoreum bioluminescence test.The washingprocess decreases notably the toxicity of pigments. Samples mineralized with KNO3 and notwashed present high toxicity levels. However, when these samples are washed, they are not con-sidered hazardous.

Samples with (Fe0.8Mg0.1Zn0.1)(Fe0.2Ni0.8Cr)O4 composition present a higher level of develop-ment of the spinel phase and are less environmentally hazardous than (Fe0.8Mg0.2)-(Fe0.2Ni0.8Cr)O4. �

Acknowledgement

The authors gratefully acknowledge the financial support of Spanish CICYT (MAT 2005 program) andFundación Caja de Castellón Bancaixa.

References1Dry Colors Manufacturer’s Association (DCMA), Classification and Chemical Description of the Mixed MetalOxide Inorganic Colored Pigments, 2nd Ed. Mixed Metal Oxide and Ceramic Colors Subcommittee, EcologyCommittee, Washington, D.C., 1982.

2D. Eppler and R. Eppler,“The Relative Stability of Ceramic Pigments,” Ceram. Eng. Sci. Proc., 18 [2] (1997).

3M.A. Tena, G. Monrós, J. Carda, P. Escribano and J. Alarcon, Bol. Soc. Esp. Ceram. Vidr., 29 [3] 177 (1990).

4M. Llusar, J.A. Badenes, J. Calbo, M.A. Tena and G. Monrós,“Environmental and Colour Optimisation ofMineraliser Addition in Synthesis of Iron Zircon Ceramic Pigment,” Br. Ceram. Trans., 99 [1] 14–22 (2000).

5R.A. Eppler,“Mechanism of Formation of Zircon Stains,” J. Am. Ceram. Soc., 53 [8] 457 (1970).

6J. Calbo, S. Sorlí, M. Llusar, M.A. Tena and G. Monrós,“Minimization of Toxicity Level on Nickel Ferrite BlackPigment,” Br. Ceram. Trans., 103 [1] 3–9 (2003).

7CIE, Recommendations on Uniform Colour Spaces, Colour Difference Equations, Psychometrics Colour Terms;Suplement.

8ECDJ 84/449/CEE of April 25, 1984; DOCE No. L251.

9American Conference of Governmental Industry Hygienists (ACGIH), Threshold Limit Values for ChemicalSubstances in the Work Environment, Cincinnati, Ohio, 1988.

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