Patent Application
20100316560
The present invention refers to a method of inorganic pigments from several industrial wastes. The correct definition of treatment and processing conditions, leads to the formation of colouring materials or pigments, having high thermal and chemical stability and then suitable to be used by distinct industrial products, such as ceramic, glass and plastic.
The coloured pigments to be used in products that are processed at high temperature might be divided in three groups: (i) metallic colloids (e.g. copper); (ii) oxides; and (iii) non-oxides (e.g. cadmium sulphide or selenium).
The group of oxides might further be divided in two sub-groups: (i) simple oxides (e.g. NiO); and (ii) mixed oxides (e.g. spinels).
According to the structure, an inorganic pigment is composed of a crystalline network that hosts the colouring element or chromophore (normally a transition metal cation) and possible modifying components, used to intensify or modify the colour/hue. Frequently, pigments formulations contain fluxing/mineralising additives that improve the reactivity between the components, and consequently contribute to lower the calcination temperature and/or time to produce the pigment.
The production of inorganic pigments normally involves the use of pure oxides or salts (carbonates, chlorides, sulphates and nitrates) of the required metals, that convert to the corresponding oxides during the calcination process.
Concerning the location of the chromophore in the lattice, pigments might be divided into four classes:
In the actual invention, we produce both structural and solid solution metallic oxide inorganic pigments.
The sub-group of mixed-metal-oxide inorganic pigments was classified by the DCMA, Dry Colors Manufacturers Association (EUA), based on their crystalline structure. Table 1 lists all types of such pigments.
Industrial wastes or by-products are normally disposed in proper land fields, with increasing costs for the producer. Moreover, recent EU directives tend to limit or reduce such action, stimulating the search for viable recycling alternatives. In some cases, residues are incinerated in cement kilns. Recycling of such materials attempt to use them as secondary raw materials for distinct alternative products.
Production costs of common pigments have been increasing in recent years, namely because some natural raw materials (e.g. Zn, Ni, Co) became scarce and costly. One of the current trends is the search for alternative and less expensive raw materials. Normally, these raw materials have high-grade chemical purity, but less pure raw materials are often tried since prove to have enough stability and are less expensive. Selected industrial wastes have been investigated for this purpose, and in particular, metal-rich sludges seem to be very promising.
In particular, metal-rich sludges might act as colouring agents, or might be combined with other materials acting in this case as the host for diverse colouring species. A wide range of metals might be interesting, as shown in table 1, if their source transforms in to oxides upon suitable calcination in oxidising atmosphere. As obvious, waste streams should show reasonably chemical constancy or properly pre-treated to assure that criterion.
PT 103269 describes the synthesis of inorganic pigments by solid state reaction, from industrial wastes, in particular sludges generated in the wastewater treatment of galvanizing or surface coating processes, pickling material used in the shipbuilding industry, foundry sands, and fines or sludge from the cutting and polishing of natural stones.
The actual invention describes the synthesis of pigments to be mainly used in ceramics, involving primarily a judicious characterization, selection, and treatment of wastes, attempting to assure their adaptability to form highly stable pigments formulations.
The step of characterization and selection of wastes determines which ones are suitable for this purpose, based on the high concentration of metallic elements that transform, upon calcination in oxidizing atmosphere, into the corresponding oxides. Moreover, when hazardous species are detected in relatively high concentrations, a proper treatment is applied to assure that the manipulation of such material is absolutely safety. This treatment is also important to improve the homogeneity of the material, increasing then the chemical stability of the resulting pigments. This point contradicts what is postulated in the above mentioned PT103269 document, since these preliminary steps of characterization, selection and treatment of wastes are omitted there.
The actual invention describes the synthesis of structural and solid solution pigments. In the last type, the chromophore ions will partially substitute a forming native ion in the lattice, and their admissible concentration is controlled by the solubility limit in the structure. In this way, the hue stability of the pigment is assured. In opposition, in a structural pigment the chromophore specie belongs to native lattice and a judicious balance of all structural components should be assured in the mixture to obtain the desirable stoichiometry. In the present invention, the wastes might introduce any element, independently of its faction in the structure/lattice (chromophore, forming, modifying or mineraliser).
The actual process, involving a correct formulation of well characterized and selected/processed wastes, assures the complete inertization of possible hazardous species in the formed high stable structure, resulting in a product that is absolutely non-hazardous.
The present invention describes the synthesis of coloured mixed-metal-oxide inorganic pigments from several industrial wastes that were primarily selected and treated.
The pigments produced according to the actual process are inorganic compounds that can then be added to vitreous or ceramic matrixes, give them an uniform colour without altering their common physical properties. To be used, those pigments should obey to the following requirements:
The actual invention assumes the use of industrial wastes as raw materials for pigments formulations listed in the DCMA, Dry Colors Manufactures Association, classification (Table 1), after a correct treatment to assure the desirable constancy in the chemical composition.
According the actual invention several wastes might be used, in particular sludges generated in the wastewater treatment of galvanizing or surface coating processes, pickling material used in the shipbuilding industry, foundry sands, and fines or sludge from the cutting and polishing of natural stones.
The waste might substitute one or several primary raw materials normally used to obtain the pigment.
The wastes might be used in the as-received condition or after suffering distinct pretreatments: drying, calcination, milling, etc.
According to the actual invention, the process for the synthesis of the inorganic pigments, consists on the judicious choice and use of metal-rich industrial wastes, alone or combined with distinct components (wastes or primary raw materials). Normally, oxides or salts of the corresponding oxides are used to formulate the pigment. The correct dosage, homogeneous mixing of components, calcination, and washing+milling, are the basic steps to get chemically and thermally stable inorganic pigments.
Thus, the synthesis of pigments involved the following steps:
1. Characterization, Selection and Preparation of Wastes
The characterization of the wastes should be conducted on distinct batches, collected in different periods and/or locations, in order to check their temporal and spatial properties constancy and to design the best manipulation strategy (deposition, mixing, etc) to minimise possible fluctuations in their characteristics.
Accordingly, the characterization should involve:
2. Dosing of Wastes
According to its characteristics, namely the chemical composition, and accounting to any particular pigment formulation, each waste is conveniently weighted to be included in the batch, mixed with several other wastes or commercial raw materials.
Next, we present several examples of pigments formulations listed in table 1 that can be prepared from the mentioned wastes:
Fe-rich wastes might be used in the production of pigments that are based on corundum-hematite (group III DCMA), spinels (group XIII DCMA), and zircon (group XIV DCMA).
The formulations should be composed in a way that the proportions of constituents should give the desirable molar ratio of the pigment structure, in order to maximise the amount of the desirable formed phase.
To assure the desirable quality of the pigment, selected constituents (wastes or primary raw materials) should furnish all the required structural forming elements, in a balanced way that promotes the formation of compounds with the correct stoichiometry. In such way, all used elements will be involved in the development of the final desirable chemical phases, and the chromatic quality of the material is improved.
In solid solution formulations the concentration of the added active chromophore specie should not exceed the solubility limit of the forming element that it will substitute in the lattice. In this case, we must account for that substitution in the formulation, when molar proportions are defined. If the solubility limit is exceeded, the chromophore element will remain uncombined and apart the main structural phase, and chromatic properties are negatively affected. In particular, stability towards processing or using conditions (temperature and oxygen partial pressure) tends to decrease.
As an example, the formula of the sphene indicates that the molar proportion of oxides should respect CaO:SnO2:SiO2=1:1:1, the chromium being the chromophore specie partially substituting Sn ions in the lattice. The solubility limit for Cr corresponds to the ratio Cr/Sn=0.036. Up to this limit pigments showing several hues (light pink, red, etc) can be obtained but when that limit is exceeded crystals of free chromium oxide are visible are responsible for an undesirable green hue.
3. Mixing and Homogenisation of Components
The intimate mixture a homogenisation of components is generally conducted in ball mills and in wet conditions. It is convenient that used components show particle size as fine as possible, since mixing is facilitated. Further reactions upon calcination will be also easier and more complete, since contact area between finer particles is improved.
4. Drying
Once intimately mixed, the batch is dried (at 110° C. for 24 hours), to remove the moisture and then minimise its ignition during further calcination step. The tendency to form and decompose volatile acid compounds upon calcination is then minimised. Accordingly, the corrosion of furnace components is strongly reduced.
5. Calcination
In general, the desirable pigment structure is only obtained at relatively high temperature, and then the calcination of the dried batch is required. The need for a careful control of firing conditions, in terms of temperature and oxygen partial pressure in the atmosphere, deserves the use of proper furnaces equipped with controlling systems of gas exhausting. The thermal cycle used on each pigment is adjusted in order to develop the required phase(s), responsible for the chromatic properties.
In the actual invention, we observed that maximum calcination temperature of waste-containing formulations is lower (50 to 200° C.) than values required to calcine the corresponding waste-free pigments composed of primary conventional raw materials. This advantageous temperature decrease is certainly related with the presence of impurities in the wastes that may act as mineralising or fluxing agents upon firing of the batch.
In order to adjust the calcination temperature of each pigment, several trials should be conducted with small samples for example at intervals of 50° C. The optimal calcination temperature corresponds to the point where the major detected phase (normally by X-ray diffraction) corresponds to the desirable pigment structure and its relative amount is maximum. Below that temperature, secondary undesirable phases are still relevant, and the colour characteristics of the pigment are far from the optimal. Moreover, the presence of unreactive and decomponible compounds might originate defects in the matrix where the pigment was introduced, such as bubbles and bloatings.
In opposition, the use of excessive temperatures (above the optimal point) tends to reduce the brightness of the pigment and their chromatic components (a* and b* in the CIEL*a*b* method, further described), due to excessive formation of glassy phase and partial volatilization of certain chromophore species. The further milling or disintegration step, required to adjust the desirable particle size distribution, is a very hard task and consumes extra time and energy.
The optimal calcination temperature is then function of the pigments composition, being considered as the point that assures maximum chromatic properties.
6. Milling of the Pigment
The pigment, formed upon calcination, needs to be properly milled in order to adjust its particle size distribution, since colour characteristics are strongly affected by the average size of grains. In general, particles should have sizes between 2 and 20 μm; below 2 μm, particles tend to dissolve in the matrix where they were inserted, and colour development is far from the desirable conditions. Coarse particles (above 20 μm) are visible with the naked eye and colour homogeneity is seriously affected. The recommended mills are those that assure a narrow particle size distribution, being rapid jet mills the most common ones. Optimal milling time depends on the equipment characteristics, charge conditions (e.g. solids load), and pigment properties.
7. Washing of the Pigment
The washing step is used to remove soluble salts and is generally complemented by filtration and drying operations. Once present is reasonably amount, soluble salts might diminish the chromatic properties of the pigment and also alter the rheological behaviour of pigment suspensions. The washing in generally performed with water, under permanent stirring or in a ball mill. Then, the pigment is filtered and dried (110° C., 24 hours).
8. Characterization of the Pigment
a) identification of crystalline phases (X-Ray Diffraction), to confirm that main component is the desirable structure of the pigment; it is the main tool to adjust processing conditions, namely calcination conditions.
b) colour measurement (CIEL*a*b* method), to determine the chromatic potential of the pigment; is also a useful tool to check the effect of several processing parameters in the pigment development and than to define optimal conditions. For sake of comparison, we also determine the chromatic characteristics of similar commercial pigments or, in alternative, batches that were composed just by primary conventional raw materials.
c) leaching tests performed according to the DIN 38414-S4 standard, to check the inertization of possible hazardous species. If correctly formulated and processed, any pigment should show leaching levels of hazardous species below the legal limits.
In the actual case, pigments are mainly composed by highly stable structures, both from chemical and thermal aspects, and then they might be used to colour different matrixes, such as ceramic bodies, inks, glazes, enamels, engobes, etc., some of them further processed at high temperature, which also might reinforce the inertness degree.
In those matrixes, a good pigment should maintain their particles almost unreactive and well dispersed in order to assure a homogeneous colouration. Since they are considered technical products, we are talking about a high add-value solution for wastes recycling, assuring at the same time their complete harmfulness for the public health.
The production of inorganic pigments from industrial wastes involved the following steps:
1. Characterization, Selection, and Treatment of Wastes
According to the present invention, the wastes should be characterized and treated in the following way:
2. Dosing of Wastes
According to their characteristics, measured as described in point 1, and to formulate a specific pigment, the wastes were dosing, by accurate weighting. In some cases, conventional primary raw materials were also mixed.
3. Homogeneous Mixing
The mixing of materials was performed in a ball mill, in wet conditions and normally for 1 hour.
4. Drying of the Batch
The drying was conducted at 110° C. for 24 hours in a conventional oven.
5. Calcination of the Batch
The calcination was performed in an electric kiln, at maximum temperatures ranging from 700 to 1650° C., according to the pigment structure to be obtained. A common gas kiln might also be used. Normally, the heating and cooling rate was kept constant (5° C./min.), and the soaking time at maximum temperature was 3 hours.
6. Milling of the Pigment
This operation is very important to reach the desirable grain size distribution of the pigment and was conducted in two steps: (i) pre-milling in dry conditions for 5 min., in an agate mortar; (ii) wet milling for 20 min., in a rapid or jet mill at a typical speed of 1000 rpm.
7. Washing of the Pigment
The pigment was washed in a ball mill, following consecutive cycles of 20 min. and by using the wt % ratio pigment/balls/water=1/15/30. Then the suspension was filtered and dried (110° C. for 24 hours).
8. Characterization of Pigments
The complete characterization of the pigments involves:
a) Identification of main crystalline phases (X-ray diffraction);
b) Colourimetric determinations (CIEL*a*b*);
c) Leaching tests, according to the DIN 38414-S4 standard.
9. Colouring Test of Pigments in Ceramic Bodies and Glazes
Pigments were added to glazes (5 wt %) and/or to ceramic pastes (10 wt %), in order to check their colouring power.
The preparation of samples followed the steps:
a) Dosing of the pigment and of the matrix material;
b) Wet homogeneous mixing in a ball mill for 30 min.;
c) Drying (at 110° C. for 24 hours);
d) Pressing of small cylinders (25 mm diameter);
e) Firing of samples, in an electric kiln and by using heating and cooling rate=5° C./min., and soaking time at maximum temperature of 30 min. This temperature was 1050° C. and 1200° C. for glazes and ceramic bodies, respectively.
The colour of the fired samples was determined by the CIELab method. When necessary, similar commercial pigments were also tested and evaluated, after processing in equal conditions. This helps to check the potential of new wastes-based pigments.
Process for the production of distinct mixed-metal-oxide inorganic pigments from Al-anodizing or surface coating sludge+galvanizing sludge from the Cr/Ni plating process+sludge generated in the steel wiredraw process:
X-ray diffraction of samples, calcined at 1000° C. in an oxidizing atmosphere, reveals the dominance of the following phases:
As listed above, calcination promotes the formation of stable metal-oxide structures, adaptable to be used as secondary raw materials of pigments.
The full characterization and preparation of wastes followed the steps:
As already observed in table 3, only the C-sludge is considered hazardous waste, since leached amount of Ni overpasses the legal limit. As a consequence, this waste deserves a careful manipulation and processes that minimize its dispersion in the atmosphere are obviously preferred.
1.5. Grain Size Distribution
Grain size distribution of sludges C and F reveal the dominance of very fine particles (colloidal size), meaning that they do not need to be pre-milled before mixing. A-sludge was always sieved at 63 μm and just the passant fraction was used in pigment formulation.
2. Dosing of Raw Materials
Two different formulation examples will be given below Comprising wastes in their formulation.
Example of Formulation 1:
Formulation to obtain carmine pigments based on tin and chromium sphene, Ca(Sn, Cr)SiO5, by using the C-sludge as Cr source.
Raw materials of calcium, tin, chromium and silicon are required, and we selected:
In order to get the maximum Cr content in the desired structure (Cr=0.036, sub-stituting Sn) the following ratio should be used: SnO2:Cr2O3:SiO2=1:(1-0.036):(0.036/2):1.
To get 1 mol de CaO we need to add 100.00 g of calcite.
To obtain 0.964 mol SnO2, we should use 145.27 g tin oxide.
To get 0.018 moles Cr2O3 we need to use 37.76 g C-sludge.
Finally, to obtain 1 mol SiO2, we need to add 60.00 g of pure silica sand.
Recalculating the formulation as weight % we get:
29.15% calcite+42.35% SnO2+11.00% C-sludge+17.50 silica sand.
This formulation corresponds to the pigment showing the strongest saturated hue. Different hues might be achieved by changing the mentioned proportions and by keeping Cr ratio below 0.036.
Formulation of a black pigment based on the structure of Ni(Fe, Cr)2O4 spinel, only by the use of wastes.
This is a structural pigment having the formula Ni(Fe, Cr)2O4, and is obtained by fixing the molar ratio NiO:(Fe2O3+Cr2O3)=1:1. It can be formulated mainly by means of combining two of the mentioned wastes:
C-sludge, as source of Ni and Cr; F-sludge to introduce Fe.
Each portion of 100 g of dried C-sludge introduces 14.95 g Cr2O3, (=0.095 mol)+33.17 g NiO (=0.444 mol).
Each 100 g of dried F-sludge will give 64.91 g Fe2O3 (=0.406 mol).
This means that 1 mol NiO is obtained when 225.23 g C-sludge is used, and this amount also introduces 0.215 mol Cr2O3.
We need (1-0.215=0.785) mol Fe2O3, given by 193.35 g of dried F-sludge.
Recalculating in terms of weight % we get: 53.81% C-sludge+46.19% F-sludge.
Application Tests of Pigments in Ceramic Products
The pigments were added to common commercial products, to evaluate the colour development: (i) a transparent and shining lead-free glaze, hereby referred as VTB, to be fired at 1050° C.; (ii) an opaque and shining glaze, referred as VOB, also to be fired at 1050° C.; (iii) a transparent and shining glaze, referred as VTBA, to be fired at 1200° C.; (iv) and a ‘porcellanato’ ceramic body, referred as CB, also to be fired at 1200° C.
All the processing steps were conducted as previously described.
Next, we will detail the formulation and colour characteristics of distinct pigments, obtained by the process hereby described. We give the admissible composition intervals for the components, and the calcination temperature. The colour properties of glazes and ceramic bodies where they were applied are also given.
Pigments Based on the Corundum Structure (Reference 03 DCMA)
These are solid solution pigments, based on the corundum (Al2O3) structure, in which some chromophore trivalent species (Cr3+, Fe3+, and Mn3+) might partially substitute Al3+ ions. Tested formulations involved the use of Fe2O3, Cr2O3, or MnO2 (5 to 20 wt %), while A-sludge was the main component and source of alumina. We might also enlarge the relative proportion of chromophore species up to the solubility limit, and then improve or change the chromatic properties of the resulting pigments. For example, by playing with Cr/Al ratio we might produce green or pink pigments.
Hematite-Based Pigments (Ref. 03)
This is also a structural pigment based on the hematite phase, having the general formula A2O3, in which A represents Cr or Fe trivalent ions.
This type of pigments might be formulated by the exclusive use of wastes that are composed by chromium and iron oxides. As an example, we prepared the 03-Fe-F pigment just from the F-sludge, by calcining the material at temperatures ranging from 850 to 1150° C. By XRD, we proved that obtained material is composed by hematite.
Olivine-Based Pigments (Ref. 05)
This structural pigment is based on the olivine phase (A2SiO4), being A the chromophore specie (Ni2+ or Co2+).
Cassiterite Based Pigments (Ref. 11)
This is a solid solution pigment that is based on the cassiterite structure (SnO2) and in which the chromophore specie partially replaces Se4+ ions. In this example, we test the use of chromium, introduced by the C-sludge. By playing with its relative amount, up to the solubility limit, we can obtain distinct violet hues.
Malayaite-Based Pigments (Ref. 12)
This is a solid solution pigment based on the sphene structure, also named malayaite and having the general formula CaSnSiO5. In this structure, the chromophore specie might partially replace sn4+ ions. In this example we mention the use of chromium, inserted by the C-sludge, in relative amounts up to the solubility limit to get distinct hues changing from the light pink to the dark red (red wine).
Pigments Based on the Spinel Structure (Ref. 13)
This family of structural pigments in based on the spinel structure (AB2O4), where A represents divalent ions (Co2+, Zn2+, Ti2+, Fe2+, Ni2+, Cu2+, Mn2+, Mg2+, and Sn2+) and B represents trivalent species (Al3+, Co3+, Cr3+, Fe3+, and Mn3+). Since those species might be chromophore, distinct colour might by obtained depending on their nature and pro-portions, on their location in the structure and on processing conditions (temperature and oxygen partial pressure of the atmosphere).
The measurement of colour characteristics of applications, according to the CIELab method, assures their potential as inorganic pigments. Moreover, they are thermally and chemically stable, within reasonable processing limits.
According to the actual invention, several formulations have being produced and tested, based on structures such as hematite, olivine, cassiterite, sphene, and spinel. Some examples are given in tables 14-16.
Example of Inertization of Hazardous Species Assured by Processing the Pigment.
To prove the inertization potential of hazardous species assured by the processing of pigments, we selected one composition that is exclusively formulated from industrial wastes (50 wt % C-sludge+50 wt % F-sludge).
Table 17 clearly shows that leaching levels of hazardous species (Ni, Cr, etc) from the 13-Cr, Fe-CF black pigment, calcined at 900° C., are well below the limits that define harmful effects for the public health.
We should also remind that C-sludge, now used to compose half of the pigment formulation, is classed as hazardous waste (see table 3), mainly due to high leaching levels of Ni. This clearly means, that the use of a correct and well controlled process of pigments production also assures the inertization of potential hazardous species, mainly due reactions promoted at high temperatures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103624 | Dec 2006 | PT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2007/055320 | 12/27/2007 | WO | 00 | 6/26/2009 |
