Patterns in the Synthesis Processes and the Characteristics of Silicate-Spinal Ceramic Pigments When Introducing Mineralizers

The synthesis of ceramic pigments is conventionally carried out at a high temperature (not less than 1,200 °C). Its reduction implies using mineralizing additives, which have a different mechanism of action on the starting components of pigment charges. The effectiveness of the mineralizers is determined by their nature, content, degree of dispersion in the activated reagent. Thus, searching for the most effective mineralizers in the synthesis, in particular, of silicate-spinel ceramic pigments is an important scientific and practical task.<br><br>We have investigated the effect of various mineralizing additives (B2O3, Na2B4O7, Na2O, NaF) on the processes of forming the crystal-phase composition of slag-containing ceramic pigments and the change in their color indicators. A direct dependence has been established between the melting point of the mineralizers and the efficiency of their influence on the formation of spinel phases, which are color carriers in such pigments. The tangible effect of the introduction of sodium fluoride, which has the highest melting point among the examined additives, is achieved as a result of the firing of pigments at a temperature not lower than 1,150 °C. The effect of sodium oxide is effective starting at a temperature of 1,100 °C. The most expedient to apply are the boron-containing compounds. Their introduction makes it possible to lower the firing temperature of slag-containing pigments to 1,050 °C while completely binding the starting components in the spinel solid solutions. The ceramic pigments that are thus synthesized enable the formation of glazed coatings, which, in terms of qualitative indicators, are not inferior to coatings obtained with the addition of high-temperature pigments (a firing temperature of 1,200–1,250 °С). The formation of silicate phases (diopside and wollastonite), which are not color carriers in the examined pigments, undergoes effective mineralized action from the supplements of NaF and B2O3.


Introduction
Ceramic pigments are the most widespread and effective dyes when obtaining glazed ceramic, enameled metallic, and glass articles. The base of the structure of ceramic pigments is the sTable colored crystalline compounds in the form of spinel, silicates, willemites, solid solutions such as corundum, phosphates, molybdates, tungstate, vanadate, etc. The use of pigments is associated with high-temperature processes, so they must exhibit thermal and chemical stability. While in the fine-dispersed state, these dyes cause selective absorption and dispersion of light, due to which a glass layer becomes opaque and acquires the color that complements the absorbed color [1].
Ceramic pigments are mostly obtained by solid-phase synthesis at high temperatures from chemically pure reagents. That is why the main trends in the development of the production of ceramic pigments are to extend the base of raw materials and to reduce the temperature of their synthesis. At present, there is a significant scientific and practical experience of obtaining ceramic pigments using a wide spectrum of anthropogenic raw materials [2][3][4]. The use of anthropogenic raw materials containing the structure-forming and coloring oxides in order to obtain the pigments would reduce the cost of their production, partly to ensure the recycling of industrial waste. In this case, the qualitative indicators of the obtained dyes often are not inferior, and sometimes superior, to the quality of materials synthesized from conventional, technically pure and natural raw components. At the same time, the firing temperature of such pigments remains quite high and is in the interval of 1,200-1,300 °C [5][6][7].
In the intensification of solid-phase reactions of the synthesis of ceramic pigments, an important role belongs to mineralizers. The effectiveness of the mineralizers is determined by their nature, content, degree of dispersion in the activated reagent. Therefore, it is a relevant task to study the comparative influence of different mineralizers on the processes of synthesis of ceramic pigments, in particular silicate-spinel. The spinel and silicate pigments are the most common among the available variety of ceramic dyes.

Literature review and problem statement
Reducing the temperature of the synthesis of ceramic pigments implies the use of various mineralized additives. Mineralizers can, at low temperatures, to form a liquid phase (melt), which significantly affects the crystalline lattice of starting raw materials. This decreases the strength of the crystalline lattice and its transfer to an active state, as well as the increase in the collision surface between the reagents particles, diffusion rate, and the entire process in general [1].
A wide range of compounds is used as mineralizers in pigment technology.
The authors of [8,9] argue that the introduction of boron oxide into the system leads to the emergence of melt at a relatively low temperature and contributes to the transfer of the crystal structure into an active state, without destroying it.
Using B 2 O 3 can also intensify the chromophore properties of ceramic pigments. Moreover, for the effective progress of solid-phase reactions, the amount of the liquid phase in the system should be, if possible, small, because its excess can negatively affect the chromophore properties of ceramic pigments and causes their strong sintering. At the same time, the insufficient content of the liquid phase causes poor wetting of the particulate matter and its subsequent merging, which results in a slower reaction speed in the solid phase [10,11].
The increase in the purity and intensity of green coloration of uvarovite pigments by the equimolecular replacement of free silicon dioxide in their composition with phosphorus (V) oxide was established in paper [12]. The authors noted the introduction of P 2 O 5 into the structure of the mineral uvarovite in the form of tetrahedrons [PO 4 ] 3and a decrease in the firing temperature to 1,100-1,150 °C. At the same time, a considerable effect is achieved with the introduction of a significant amount of P 2 O 5 (exceeding 7 % by weight).
The strong impact on the stability of the crystalline lattice of pigments is exerted by the compounds of fluorine and alkali metals.
The authors of [13,14] argue that the mineralizing effect of fluorine-ions, in particular on silicate pigments, is explained by the fact that they increase the reactivity of the crystalline lattice of the material by weakening the silicon-oxygen frame. This provides for the possibility of diffusion processes in the region of low temperatures, 1,100-1,150 °С.
Effective modifiers of the structure of pigments also include the additives of alkali metal oxides.
In [15], a sol-gel method was used to synthesize a pigment with the structure of olivine (Co 2 SiO 4 ). However, despite the presence of mineralizers (NaCl and KCl), the temperature of pigment synthesis remained high and amounted to 1,200 °C.
It is shown in [16] that LiCl contributes to the transition of the tetragonal form of Nd 2 Si 2 O 7 into monoclinic at a relatively low temperature of pigment thermal treatment (1,100 °C), thereby improving the color indicators. However, it may reduce their temperature resistance.
Thus, the choice of a mineralizer is crucial for the activation of solid-phase reactions during the synthesis of ceramic pigments. At the same time, the lack of detailed studies of the comparative influence of certain mineralized additives on the characteristics of ceramic pigments, specifically spinel and silicate, necessitated our research in this field.

The aim and objectives of the study
The aim of this study is to determine patterns in the processes of mineral formation and changing the color indicators of silicate-spinel ceramic pigments when introducing mineralizing additives. This would make it possible to choose the most effective mineralizing additives, thereby significantly reducing the firing temperature of the pigments of a given type.
To accomplish the aim, the following tasks have been set: -to determine the influence of the nature and content of mineralizing additives on the color indicators of slag-containing silicate-spinel pigments and glass coatings at their introduction; -to establish features in the formation of the crystal-phase composition of the examined pigments in interrelation with the basic technological factors of their manufacture.

Materials and methods to study ceramic pigments with the additives of mineralizers
The examined pigments' main component was the dump open-hearth slag from one of the metallurgical enterprises in the city of Dnipro (Ukraine), which was preliminary magnetically enriched and averaged. In order to obtain brown color, we additionally injected oxides of chromium and nickel; black color -chromium and cobalt oxides in the total amount of up to 23 % by weight.
The formulations of slag-containing pigments that we devised and reported in earlier studies [17,18] provide for high-quality glass coatings.
The additives of mineralizers involved the introduction to the ground pigment charges of orthoboric acid (H 3 BO 3 ), sodium tetraborate, hydrate (Na 2 B 4 O 7 •10H 2 O), sodium carbonate (Na 2 CO 3 ), sodium fluoride (NaF). Their amount was 2.0 and 4.0 mass fractions in recalculation for B 2 O 3 , Na 2 B 4 O 7 , Na 2 O, and NaF, respectively. Pigment charges were prepared by the joint wet grinding of starting raw components. The moisture content of the prepared suspensions was 35 %. Dried to a residual moisture content of 1 %, the pigment charges were fired in an electric furnace in the temperature range 1,050-1,150 °C with aging over 1 hour. The ready pigments were finely ground, adding water to a humidity of 35 %. The dispersion of pigments was characterized by the residue on control sieve No. 0056, which should not exceed 0.4 %. The prepared pigments were dried to the humidity not larger than 0.8 %. To obtain colored glass coatings, the synthesized pigments were injected to the ground transparent fritted glaze, intended for the application onto ceramic tiles, in an amount of 8 % by weight. The glaze coatings were fired at a temperature of 1,100 °C.
The color indicators of the developed pigments and the glass coatings containing them were studied at the colorimetric device KTs-3. For the brown pigments of series 1.2d, we determined the color indicators (color tone (dominating wavelength -λ), color purity -P, lightness -L). For the black pigments of series 8d, we determined only the lightness indicator, which in this case characterizes the degree of blackness of the body. The crystal-phase composition of the pigments was determined by X-ray phase analysis at the diffractometer DRON-3.0 in the Cu-Kα radiation.

1. Results of studying the color indicators of the slag-containing pigments
The objects of our study are the ceramic pigments, synthesized in the system open-hearth slag-Сr 2 O 3 -NiO (dark brown 1.2d) [17], and open-hearth slag-Сr 2 O 3 -CoO (black 8d) [18]. A temperature range of 1,200-1,250 °С is the most rational for firing such pigments; it enables the formation of the assigned mineralogical composition in combination with Table color indicators.
To obtain the slag-containing pigments at a lower temperature, it is necessary to bring their crystalline lattice to a more active state by the additional introduction to their composition of the mineralizers. We studied various additives containing boron oxide, alkaline oxide Na 2 O, and fluoride-ions. Their characteristic is given in Table 1.
The mineralizers were injected into the ground charge of the examined pigments. Synthesis of the obtained compositions was performed in a temperature range of 1,050-1,150 °C.
The dependences of color indicators of pigments on the nature and amount of the introduced mineralizers at different firing temperatures are given in the form of graphic charts in Fig. 1-3. For the black pigments of series 8d, we determined only the indicator of.
The result of our study has established that sodium fluoride has an effective mineralizing effect on the components of the examined pigment charges only at the firing temperature 1,150 °C. This fact was confirmed by a visual assessment of the color of the pigments obtained, as well as the values of their color indicators. The pigment's color 1.2d under an increase in the temperature of its synthesis from 1,100 to 1,150 °C changes from brown to dark brown. The value of the dominant wavelength, in this case, is shifted from the orange (595-612 nm) to the red (620-645 nm) region of the spectrum. The lightness indicator L decreases in total from 13.0-14.9 % to 11.8-12.8 %. The color purity also decreases (from 7-15 to 5-7 %).
For the 8d pigment, the color changes from dark gray to black. The L indicator is reduced from 10.3-11.6 % to 9.7-10.2 %.  Fig. 1. Color tone dependence (nm) of the examined pigments of series 1.2d on the nature and content of the mineralizers fired at temperature: а -1,100 °С; b -1,050 °C The pigments synthesized at a temperature of 1,150 °C with the addition of Na 2 CO 3 , Na 2 B 4 O 7 •10H 2 O, especially H 3 BO 3 , were in the sintered form, which made it difficult to grind them. They were not used to determine the color indicators.
Increasing the firing temperature of the pigments, which contained sodium oxide as a mineralizing additive, to 1,100 °C and increasing the concentration of Na 2 O to 4 mass fractions form the dark brown and black coloration of compositions from series 1.2d and 8d, respectively. Consequently, the chrome-nickel pigment 1.2d demonstrates a shift of λ from the orange (599-610 nm) to a longer wave area of the visible region of the spectrum -red (625 nm) - Fig. 1, a. This is also accompanied by a decrease in the values of L from 13.2-16.1 % (Fig. 2, a) to 12.5 %; and P, from 8-16 % to 7 %, respectively. The synthesized chrome-cobalt pigment 8d is characterized by the enlarged degree of blackness (L falls to 10.0 %) - Fig. 3, a. More substantial changes in the color indicators (color tone - Fig. 1, and lightness -Fig. 2, 3) of the examined pigments occur at the introduction of boron-containing compounds (H 3 BO 3 , Na 2 B 4 O 7 •10H 2 O). This allows us to assert that such compounds are the most effective mineralizing additives in the examined systems. The most effective mineralizing additives in the examined systems are the boron-containing compounds. Their introduction makes it possible to lower the firing temperature of the slag-containing pigments B 2 O 3 or Na 2 B 4 O 7 ) to 2 mass fractions at a synthesis temperature of 1,100 °С. Such pigments, in terms of their qualitative parameters, are not inferior to the pigments fired at 1,200-1,250 °С. The values of color tone for the pigments from series 1.2d in this case are within 625-630 nm (Fig. 1) and correspond to the red region of the spectrum, lightness -12.2-12.7 % (Fig. 2), and the color purity is 7 %. The ceramic pigment from series 8d in turn, after firing in the interval 1,050-1,100 °C, is characterized by a high degree of blackness (L = 9.5-9.7 %) - Fig. 3.
The pigments with boron-containing mineralizing additives were injected into the composition of transparent fritted glaze in the amount of 8 mass fractions followed by the firing of coatings at a maximum temperature of 1,100 °C.
The dependence of color indicators of the obtained glass coatings on the amount of boron-containing additives, introduced to the pigments, is shown in Fig. 4 and Fig. 5.
It was established experimentally that the dynamics of change in the coloration of the ceramic pigments, obtained with the introduction of boron-containing compounds as mineralizers, and the pigment-containing glaze coatings are similar. Namely, the introduction of such pigments to the ground base fritted glaze causes a significant increase in the intensity of the brown and black coloration of a glass layer. Moreover, the most significant intensification of coloring takes place in the case of applying the examined pigments, synthesized using B 2 O 3 , which is introduced by boric acid. And the most signi ficant intensification of coloring takes place when applying the examined pigments, synthesized by using boric acid.
In this case, the glass layer containing the 2d pigment is characterized by the increased brown color. This is confirmed by shifting the λ values from the orange (602-605 nm) to a longer wave region of the spectrum -red (630-637 nm), Fig. 4, b. The black color of glazed coatings, following the introduction of the 8d pigment, synthesized by adding the boron-containing compounds, is also greatly enhanced. This is accompanied by a fall in the lightness indicator values from 5.2-5.6 % (for the glass layer containing the examined pigment without a mineralizer) to 3.8-3.9 and 3.9-4.1 % (when introducing the pigment obtained by using B 2 O 3 and Na 2 B 4 O 7 as a mineralizing additive) - Fig. 5.

2. Results of studying the mineralogical composition of slag-containing pigments
To analyze the mineralizing effect of B 2 O 3 , Na 2 B 4 O 7 , Na 2 O, and NaF, we compared the absolute intensity of the main diffraction maxima that are characteristic of the spinel and the main silicate phase (diopside) - Fig. 6. The results of our X-ray phase analysis of ceramic pigments agree well with the research into their color indicators.
The dynamics of change in the intensity of a diffraction maximum d = 2.50 indicate that in the reactions to form the spinel solid solution the most effective mineralizing action is exerted by the additives of boron-containing compounds, which are the most fusible (Table 1).
Consequently, at a low temperature of 1,050 °C, there is a complete binding of the starting components of pigment charge in the spinel phase, which act as color carriers. In the dark brown 1.2d pigment, this is a solid solution between the ferrite and chromite of nickel and magnesium with their characteristic lines (d = 4.82; 2.93; 2.50; 2.08; 1.60; 1.47 ) - Fig. 7. In the chrome-cobalt 8d pigment, the black coloring is due to the formation of spinel (MgFe 2 O 4 , СоFe 2 O 4 , and СоCr 2 O 4 ), which, due to the similarity of their structure, also produce a solid solution of substitution (Fig. 8). In the presence of B 2 O 3 at a temperature of 1,050 °C, along with the compounds-chromophores, there is a complete formation of silicates in the form of diopside (d = 3.20; 2.98; 2.13; 2.03 ) and wollastonite (d = 2.98; 2.20; 1.73 ), Fig. 7, 8.
A weaker action, compared to the boron-containing compounds, is exerted by NaF and Na 2 O. At the same time, in the reactions to form the silicate phase (diopside), the NaF additive, along with B 2 O 3 , is the most active (Fig. 6).

Discussion of results of studying the influence of mineralizers on the processes of synthesis and the color indicators of slag-containing pigments
In the course of our study, it was established that the color indicators of the examined ceramic pigments, their qualitative and quantitative crystal-phase composition, are determined by the nature and content of mineralizing additives. Moreover, there is a direct dependence between the melting point of mineralizers and their influence on the processes that cause the formation of the predetermined spinel phases in the composition of the examined compositions.
Thus, the tangible effect from the introduction of so dium fluoride, which has the highest melting point (1,040 °C, Table 1), is achieved only as a result of firing the pigment charges at a temperature not lower than 1,150 °C. This defines its inappropriate use as a supplement that intensifies the progress of solid-phase reactions in the specified system, where the main phase is spinel.
Using sodium oxide as a mineralizer, introduced with the help of its carbonic acid salt, is feasible in the amount of 4 mass fractions at the firing temperature of pigments not less than 1,100 °C.
The most effective action on the processes of mineral formation in the examined pigments is exerted by the additives of boron-containing compounds, due to their high fusibility ( Table 1). The result is the intensified synthesis reactions involving the liquid phase. Moreover, the action of boron oxide, introduced by orthoboric acid, is, in this case, is more pronounced compared to sodium tetraborate.
The mechanism of the influence of compounds containing boron oxide is, first, the formation of the liquid phase (melt) at relatively low temperatures. In addition, B 2 O 3 acts on the crystalline lattice of the components of pigment charges, bringing it to an active state without destruction. The reason for this phenomenon lies in the fact that the В 3+ boron ion has a large charge and a small ionic radius (0.02 nm). In this regard, the ion boron has a stronger ability to polarize (deformation) compared to the single-charge ions of alkaline metals, and it significantly influences the reduction in the stability of the crystalline lattice [1]. In addition, it is known that boron oxide positively affects the chromophore properties of the pigments themselves. The role of the main chromophores in the examined dark-brown pigment belongs to the ions of transition metals Ni 2+ , Cr 3+ and Fe 3+ . Their electronic shell is not fully completed and has a high polarization ability. Their coloration varies depending on the polarization ability of ion, specifically oxygen, which is included in the complexes of ions of transition metals. In the presence of В 3+ ions, there is an increase in the ability to polarize the oxygen anion in such complexes, so that the absorption of light in the visible spectrum is increased resulting in the coloration of ceramic pigments.
In general, it follows from the analysis of the acquired experimental data that the introduced mineralizers, in terms of their effect on enhancing the coloration and intensification of spinel-forming reactions of the examined ceramic pigments, can be arranged in the following sequence: The effect of the considered mineralizers on the formation of silicate phases differs from the above series. The effectiveness of NaF and B 2 O 3 additives in this case is almost equivalent.
The presence of fluoride ions, according to literary data [13,14], increases the reactivity of the crystalline lattice of the material due to the weakening of the of the silicon-oxygen frame, which causes the progress of diffusion processes in the region of low temperatures.
Thus, we have established patterns of change in the crystal-phase composition and color indicators of silicate-spinel ceramic pigments, depending on the nature of the mineralizers, which allowed us to recommend the most effective ones. This makes it possible to significantly decrease the firing temperature of such pigments (up to 1,050 °C). At the same time, the application of the established patterns is limited to spinel and silicate pigments. Therefore, it is advisable in the future to consider the comparative effect of different mineralizers on the processes of synthesis of ceramic pigments with the structure of other types.

Conclusions
1. Our study has determined the peculiarities of change in the color indicators of silicate-spinel ceramic pigments, which were obtained on the basis of open-hearth slag, depending on the content and nature of additives-mineralizers. It was established that the tangible effect of the introduction of sodium fluoride, which has the highest melting point among the examined additives, is achieved as a result of firing the pigments at a temperature not lower than 1,150 °C. The effect of sodium oxide is effective starting at a temperature of 1,100 °C. The most expedient is to use additives of boron-containing compounds. Their introduction makes it possible to lower the firing temperature of slag-containing pigments to 1,050 °C or reduce the concentration of the mineralizer (B 2 O 3 ) to 2 mass fractions at the synthesis temperature 1,100 °С. The synthesized pigments ensure the formation of glaze coatings, which, in terms of their qualitative indicators, are not inferior to the coating obtained with the addition of high-temperature pigments (the firing temperature is 1,200-1,250 °C).
2. A direct dependence between the melting point of the mineralizers and their influence on the processes that determine the formation of the predetermined spinel phases in the composition of examined compositions has been established. The introduction of B 2 O 3 as a mineralizer provides for the complete binding of the starting components in the spinel solid solutions as early as at a temperature of 1,050 °C. In the brown pigments from series 1.2d, this is a solid solution between the ferrite and chromite of nickel and magnesium. In the black pigments from series 8d, this is a solid solution between the ferrites of magnesium and cobalt, as well as cobalt chromite. This allows us to argue about the effectiveness of using the boron-containing mineralizers to accelerate the spinel-forming reactions in the synthesis of ceramic pigments.
The formation of silicate phases (diopside and wollastonite), which are not color carriers in the examined pigments, is significantly affected by the mineralizing action exerted by the additives of NaF and B 2 O 3 .