W. Hajjaji

Evaluation of metal-ions containing sludges in the preparation of black inorganic pigments

Inorganic pigments were prepared from industrial wastes: galvanizing sludges resulting from Cr/Ni plating processes (S, G, and T) and a sludge generated from steel wiredraw process that is Fe-rich (F). These industrial wastes were characterized in order to determine the main compositional variations and discover their influence on the colour characteristics of pigments for glazes and ceramics. The toxic character was also investigated and established. Attempting to form the black spinel structure, several combinations of sludges were prepared and then calcined at 1000 °C. XRD and microscopy analysis confirmed the presence of nichromite for compositions with higher Ni amounts, while trevorite was detected in iron-rich formulations. The combination of S and F sludges (SF compositions) generates pigments with higher black colorimetric quality, which is similar to, and sometimes better than, a commercial black pigment. Pigments containing GF and TF sludges develop brown hues on glazes and on porcelain stoneware bodies, being this effect more evident upon industrial firing trials. The addition of minor amounts of cobalt or manganese enhances the black coloration, and might adjust some formulation deviations.

Technological behaviour of some Tunisian clays prepared by dry ceramic processing

Abstract Lower Cretaceous (AJO and AJR) and Oligocene clays (AS) from northern Tunisia were analysed to evaluate their possible use in the production of earthenware tiles by dry processing and fast double-firing. The Cretaceous clays are carbonate-rich (AJO = 20%, AJR = 12%) while the Oligocene ones are carbonate-free. This led to noticeable differences in firing behaviour (shrinkage, sintering rate and loss on ignition) and consequently in functional properties (water absorption, mechanical strength, porosity). The AJO firing shrinkage is very small, which makes this clay suitable for rapid firing. The clays are illite-kaolinite-rich but the AS sample is mostly smectitic (44%) and so is used (10 wt.% maximum) only to adjust the consistency of the powder during pressing. The average agglomerate size ranges from 100 to 350 mm and the distribution is suitable for easy pressing of powders without any special need for further adjustments. Characterization of fired products confirms the high potential of these clays since all properties fall within the ceramic International Standards (ISO). For both technical and economic reasons dry processing is recommended for production of earthenware tiles, in particular for countries in sunnier climates, where solar energy can be exploited for clay drying.

Identification and use of white clayey deposits from the area of Tamra (northern Tunisia) as ceramic raw materials

Abstract White clayey geomaterials were collected from northeast of the Nefza region (northern Tunisia). These deposits belong to the Mio-Pliocene molassic basins (basin of Tamra-Sidi Dris, Boukhchiba) and Oligocene Numidien in the area of El Aouinet. Analysis by X-ray diffraction showed associations of halloysite and kaolinite (Tamra), kaolinite and illite (Aouinet zone) and mixed layers, kaolinite and feldspars (Boukhchiba zone). Semi-industrial processed bricks showed promising characteristics and were visibly free of defects. Ceramic tiles had a bending strength that met the required standards but water absorption was somewhat high. So, to achieve higher quality, optimisation needs to be carried out in the formulation of batches or in the processing conditions. Finally, the incorporation in whitish sanitary-ware glaze formulations generated smooth coating layers that showed the required strong brightness and were free of defects.

Ceramic tiles based on central Tunisian clays (Sidi Khalif formation)

Abstract This study aims to find alternatives to some overexploited deposits in the north of Tunisia. The clays of the Sidi Khalif formation, of Uppermost Jurassic to Lower Cretaceous age, seem suitable to support this strategy, due to their abundance. In terms of mineralogy, they can be divided into two distinct groups, related to their palaeogeographic and eustatic contexts. The clays of the Sidi Khalif formation from Jebel Houareb (JH) and Jebel Meloussi (JM) are richer in clay minerals, while those of Jebel Rhouis (JR) and Jebel Boudinar (JB) zones are dolomite-rich. The compositional differences between the two groups influenced their technological behaviour in production of ceramic tiles. Upon firing, forsterite and gehlenite crystallized in the carbonate-rich JR and JB clays, whereas mullite formed in the JH and JM clays. Concerning their ceramic properties, the tiles made from JR or JB clays showed low bending strength and high water absorption. For JH and JM, the tiles had slightly greater mechanical resistance, but exhibited considerable linear shrinkage. These defects were partially corrected by introducing new formulations based on the same clays and by altering the firing cycles (HM1, HM2, RB1 and RB2). With these modifications, ‘‘Porcelanico’’ and ‘‘Monoporosa’’ technologically conforming tiles were obtained from HM2 and RB2 formulations, respectively.

Production of ceramic bodies from Tunisian Cretaceous clays

Abstract The aim of this research was to assess the potential application of the Late Cretaceous levels as raw material for the production of ceramic tiles. Mineralogical, chemical and grain size distribution studies were carried out on four clays sampled at different sites in the Tunisian Central Atlas. Clays are mainly composed of phyllosilicates, usually illite and kaolinite. Quartz, feldspars and dolomite were also detected. Tiles were prepared by pressing clay bodies at 250 bar and then firing at different temperatures following industrial conditions as closely as possible. Thermal analysis showed the influence of mineralogical composition and grain size distribution on the material behaviour during firing. The physical properties of fired tiles were obtained from specific tests in accordance with international standards (ISO), i.e. the firing shrinkage, water absorption, apparent density and flexural strength. The development of physical properties indicates an optimal firing range between 1050–1150ºC. Results demonstrate the potential of Late Cretaceous materials as raw materials for the production of ceramic tiles.

Effective removal of anionic and cationic dyes by kaolinite and TiO2/kaolinite composites

Abstract The present study investigated the removal of methylene blue (MB) and orange II (OII) dyes from synthetic wastewater by means of adsorption and photocatalysis using natural kaolins. For MB adsorption, the raw kaolinite-rich samples showed the greatest adsorption capacity, with rapid uptake (90% after 20 min). The experimental results were fitted better using the Langmuir isotherm model parameters compared to the Freundlich model, suggesting that the adsorption corresponds to monolayer coverage of MB molecules over the kaolinite surface. For OII, neither the Langmuir nor the Freundlich model gave reliable results, because the adsorption of anionic dye molecules by the clayey particles is not favoured. Mixtures of kaolinite/Degussa TiO2 were also prepared, and their photocatalytic properties under UV-light exposure were investigated. Decolourization of MB solutions was observed, even in a mixture with low TiO2 content. This is related to the combined effect of adsorption and photocatalysis and, unlike the pure clay samples, the efficiency of such mixtures against OII was only slightly weaker (80-94%). For TiO2-impregnated clays, with the kaolinite layers separated by sol-gel TiO2 particles, the MB removal was slow and effective only after >24 h due to the complexity of the bonding of MB molecules. On the other hand, the removal performance against OII solutions was very efficient (nearly 100%) within only 2 h. This excellent performance was attributed to morphological changes in clay particles.

Malayaite Ceramic Pigments Prepared from Industrial Wastes: Formulation and Characterization

In this work chrome-tin red malayaite pigments Ca(Cr,Sn)SiO5 were obtained from industrial wastes: a marble sawing sludge, a galvanizing sludge from the Cr/Ni process and a foundry sand. The pigments were prepared using the common solid state reaction process. The influence of Cr2O3/SnO2 ratio, calcination temperature and introduced sludges compositions, on its structural stability, was investigated by X-ray diffraction coupled with SEM/EDS. Standard formulations, obtained from commercial reagents, were also prepared and characterized for sake of comparison. CIELab colour measurements and UV-Vis spectroscopy were used to evaluate the stability (thermal and chemical) and applicability of the synthesized pigments in colouring distinct ceramic matrixes fired at high temperatures (1050°C-1200°C). The pigment was added (1/20 wt. ratio) to a transparent and shining lead free glaze and to a porcelain stoneware body (1/10 wt. ratio). The incorporation of three industrial wastes, in the chrome-tin red malayaite pigment, was possible without a notorious change in terms of colouring effect allowing decreasing the synthesis temperature.

Novel Ceramic Pigments Based on Industrial Wastes

We report the preparation of ceramic pigments using industrial wa stes as primary sources. An Al-rich sludge (generated in the wastewater treatment unit of an anodising or surface coating industrial plant), a galvanizing sludge (from the Cr/Ni plating process ), a Fe-rich sludge (generated during steel wiredrawing), a foundry sand and a marble sawing sludge are combined to formulate distinct pigment structures via the solid state reaction method. In particular, a black spinel and a novel blue pigment based on the hibonite structure are detailed. A green olivine, and brown and pink sphenes are also reported. The pigments were fully characterized, and then were tested in a standard transparent bright ceramic glaze. Typical working conditions and colour development are fully described.

Former Extractive Activities in Tunisia: Environmental Risk Assessement and Heavy Metals Immobilization

Mining has a very economic importance worldwide and remains an essential part of the economy in different countries (Nansai et al. in Environ Sci Technol 49(4):2022–2031, 2015; Galas and Galas in Resour Policy 49:204–212, 2016).

Production of Lightweight Aggregates from Phosphate Washing Plant Sludge

In this study, we assessed the potentialities of using the phosphate washing by-products as row materials for the production of lightweight aggregates. Sludge samples from different washing plants have been physic-chemically characterized. Slurries were dried, milled and shaped into spherical pellets. These raw aggregates were sintered for 5 min in the laboratory kiln at different temperatures between 1,120 and 1,180 °C. The bloating potential of the lightweight aggregates as well as the effect of the firing temperatures on their properties (expansion, apparent density, water absorption and compressive strength) were assessed. The mineralogical data shows that the collected sludge samples are mainly composed of carbonates, francolite, clinoptilolite, quartz and feldspars; smectite, palygorskite and sepiolite are also present as clayey fractions. Chemically, the samples exhibited significant amount of SiO2, CaO, and P2O5. When sintered, the aggregates expand; this expansion results from the concomitant reaction of gas release and viscous phase formation. The produced lightweight aggregates revealed acceptable technological properties with apparent density values often lower than 0.9 g/cm3. Moreover, good expansion (60 % in volume) and water absorption (close to 10 %) were obtained. These results are encouraging and allow considering the studied sludge as promising material for the production of lightweight aggregates.