Al-rich Sludge Research Articles

Al-rich industrial waste as new alternative of fumed silica for the manufacture of vacuum insulation panels for building energy conservation

Nano-structured fumed silica (FS) is the most used vacuum insulation panel (VIP) core material for building applications. However, the FS VIP core has the majority of the environmental burden associated with conventional VIP manufacture and FS-based VIPs are very expensive. This paper explores aluminium anodizing sludge (ALW), a residue from the aluminium industry, as a new substitute for FS in VIP cores. Al-rich sludge waste is thoroughly characterised, and 100%-based ALW and its hybrid core mixtures with FS are formulated to produce VIPs cores, which are then evaluated. ALW was observed as aggregated clusters of nanosized individual particles in round and smooth-shaped forms typical of hydrated gel-like pure amorphous materials. The microstructure system of the sole ALW VIP core is found to be dissimilar to that of an FS VIP core, with the pore size range of 0.1–0.8 μm for the former and 0.01–0.04 μm for the latter, but the overall volume of large pores being much higher for the FS than ALW cores. The hybridization of ALW and FS resulted in a restructure of the initial ALW core systems. While the thermal conductivity of the ALW VIP core is less proficient than that of the single FS core, the effective thermal conductivity of both ALW50FS50 and ALW25FS75 core compositions at 1 Pa even suppressed that of the sole FS core, presenting 5.8 mW m−1·K−1 against 6.3 mW m−1·K−1 respectively. A life cycle assessment (LCA) study of these hybrid core solutions was also conducted to illustrate the positive environmental benefit of ALW-based VIP compared to FS-based VIP production. The study highlights the potential of using industrial residue ALW in VIP production, contributing to developing more sustainable superinsulation materials.

Synthesis of low-temperature calcium sulfoaluminate-belite cements from industrial wastes and their hydration: Comparative studies between lignite fly ash and bottom ash

The aim of this research was to study the production of calcium sulfoaluminate-belite (CŜAB) cement from industrial waste materials via hydrothermal-calcination process. Lignite fly ash and bottom ash were used as starting materials for comparison. Other waste materials viz., Al-rich sludge and flue gas desulfurization gypsum were also key players in raw mixes for the synthesis of CŜAB cement. For lignite fly ash as a starting material, mixed phases between ye'elimite and larnite were obtained, whereas for lignite bottom ash as starting material, only ye'elimite was obtained The hydration reaction was studied in terms of heat evolution, setting time, compressive strength and hydration product formation with various gypsum contents. The results showed a rapid formation of ettringite as a main hydration product mixed with calcium silicate hydrate, monosulfate and strätlingite phases as minority, with a fast final setting time of 24–26 min and high early compressive strength of 16.0 and 18.0 MPa in 1 day for CŜAB cements made of fly ash and bottom ash, respectively.

Hydration and Physico-mechanical Properties of Blended Calcium Sulfoaluminate-belite Cement Made of Industrial By-products

Portland cement blended Calcium Sulfoaluminate-belite (CŜAB) cements were studied in order to improve its binding properties and workability for specific applications. The binders consisted of calcium sulfoaluminatebelite, Flue Gas Desulfurization-gypsum and Ordinary Portland Cement (OPC). In this research, effects of OPC contents (25, 50, 75 wt%) as a CŜAB replacement on hydration behaviors and physico-mechanical properties of the binders were observed. Used CŜAB cement was synthesized using industrial by-products viz., fly ash, Flue Gas Desulfurization-gypsum, Al-rich sludge as starting materials via hydrothermal-calcination method. The results revealed that the replacement of CŜAB cement with OPC extended the setting times of pastes. The reduction of hydration rate with higher OPC content was due to dilution of fast setting phases such calcium sulfoaluminate and mayenite. Hydration products of calcium sulfoaluminate cement were ettringite responsible for high early strength together with Al(OH)3. From 6 h onwards, hydration of tricalcium silicate phase from the OPC generated calcium silicate hydrate. Stratlingite was also found in low OPC content mix resulting from the reaction between the Al(OH)3 and either alite phase in OPC or belite phase in CŜAB cement. Ettringite could also react with Al(OH)3 to generate monosulfate at later ages. The calcium sulfoaluminate phase was mainly responsible for the early mechanical properties, while OPC played an important role to improve strength at later ages.

Hydrothermal synthesis of calcium sulfoaluminate–belite cement from industrial waste materials

This study investigated the use of industrial waste materials viz., fly ash, Al-rich sludge and flue gas desulfurization gypsum in the production of calcium sulfoaluminate–belite cement using hydrothermal–calcination method. The lime deficit in the waste starting materials was recompensed by the addition of hydrated lime. Various factors viz., type of hydrothermal solvent, time of hydrothermal process and calcinations temperature of the hydrothermal products were studied. Phase developments during various synthesized conditions were studied using X-Ray Diffraction. The cement products were mixed with 20 wt% flue gas desulfurization to allow ettringite formation for solidification. The hydration development was investigated in terms of mineralogical composition, microstructures and compressive strength of the hydration products. The largest numbers of calcium sulfoaluminate–belite cement could be obtained by calcination at temperature of 1050 °C. The synthesized cement developed an acceptable 28-day compressive strength of 30.0 MPa. The main hydration products were ettringite, portlandite, calcium silicate hydrate and calcium aluminosilicate hydrate resulting from the hydration of calcium sulfoaluminate and belite phases.

Production of Sintered α-Alumina by Explosive Compaction from Low Temperature Calcinated Aluminum-Rich Sludge

Up to now the studies performed on alumina nanocrystalline powders using explosive compaction were restricted to the powders produced by specific industrial processes. The research work concerning other sources of alumina, like sludge from industries of aluminum anodizing, only regards to alpha-alumina. This is resulting from a previous heat treatment to high temperatures for which the phase transformation γ → α occurs. In present research is evaluated the possibility to use sludge having, before consolidation, as main phase γ-alumina, that means lower maximum temperature during preliminary calcination. After dried at 110 °C and heat treated at 800 °C during 1 h having as the major phase γ-alumina, the sludge was processed by explosive compaction followed by sintering at 1,200 and 1,650 °C (holding time = 1 h). During shock compaction the γ-alumina was transformed into α-alumina. The role of phase transformation during the explosive compaction followed by sintering is highlighted by the mechanical properties of the bulk material after sintering. The bulk material has hardness values close to α-alumina produced by conventional powder technology. Moreover, it was detected that the sulfur content of 4.5 % (w/w) measured in sludge previously calcinated was further reduced to 0.04 % (w/w), after explosive compaction. This study opens a sustainable possibility to recovery of Al-rich sludge to produce bulk alpha-alumina components/devices.

Impedance Spectroscopy Evolution upon Sintering of Alumina Bodies Containing Al-Rich Anodizing Sludge

Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of alumina-based materials, by using plastic extrusion as shaping technique. Long rods were produced using a vacuum screw extruder, by a careful control of all relevant processing parameters. Then, thick discs were obtained by cutting dried selected rods, to be tested as probes for sintering-dependent electrical properties. The sintering process was followed by common dilatometric/thermal analyses and results were interrelated with the evolution of electrical conductivity, estimated by impedance spectroscopy technique (IS). Results show that sintering-dependent morphological evolution up to 1300°C strongly affects the electrical behaviour of samples, and as a consequence IS seems to be a useful method to follow the firing process.

Shock activation of α-alumina from calcinated Al-rich sludge

Enormous quantities of Al-sludge from aluminum surface treatments are yearly generated all over the world. Despite of being known as important supplier of alumina, the Al-sludge also have some other components that need to be removed. Calcination has revealed to be an inevitable treatment to achieve such objective. It was also found that conventional densification processes are not efficient for this kind of powders and, to overcome that difficulty, a dynamic compaction approach, using detonation generated shock waves have been followed. The calcinated powders compacted by this way have shown up values of density and hardness, after sintering, comparable to the ones of commercial alumina processed by the conventional methods.

Ceramic Properties of Alumina and Alumina-Mullite Pressed Bodies Based on Al-Rich Anodising Sludge

Alumina and alumina-mullite based refractory ceramic materials were produced from formulations based on an industrial Al-rich sludge derived from the wastewater treatment of the aluminium anodising process. Other ceramic raw materials like diatomite was also added, which can be consider as a by-product. Cylindrical samples processed by uniaxial dry pressing were sintered at different temperatures (between 1400-1650°C, 1 hour soaking) to study the ceramic properties evolution. The performance of fired materials was evaluated (firing shrinkage, water absorption, bending strength, thermal expansion coefficient, refractoriness and SEM microstructure) and demonstrated that optimal properties were obtained at 1650°C for alumina and 1450°C for alumina-mullite.

27Al and 29Si NMR and XRD characterisation of clinkers: standard phases and new waste based formulations

27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) and powder X-ray diffraction (XRD) studies of Portland type cement clinkers are reported. Common phases in such cements (C2S, C3S, C3A and C4AF) were prepared with high purity grade chemical reagents to be used as reference, and characterised. Subsequently, clinkers with selected compositions were prepared by combining several industrial wastes and byproducts: Al rich anodising sludge, sludge from the filtration/clarification of potable water, sludge generated in marble sawing processes and foundry sand. The clinkers were found to contain the reference cementitious phases, the two characterisation techniques used (NMR and XRD) being complementary in the information provided. The general similarity of the major phases formed in waste based and reagent based formulations clearly demonstrates the potential of the use of wastes in cement formulations of industrial interest.

Properties of sintered mullite and cordierite pressed bodies manufactured using Al-rich anodising sludge

Mullite and cordierite based refractory ceramic materials were produced from formulations based on an industrial Al-rich sludge derived from the wastewater treatment of the aluminium anodising process. Some common ceramic raw materials, like ball clay, kaolin, diatomite and talc, were also added. Cylindrical samples processed by uniaxial dry pressing were sintered at different temperatures (between 1300 and 1650 °C, 1 h soak at each temperature) to study the ceramic properties. The fired properties of materials was evaluated (firing shrinkage, water absorption, bending strength, thermal expansion coefficient, refractoriness and SEM microstructure) and demonstrated that optimal properties were obtained at 1650 °C for mullite and 1350 °C for cordierite.

Impedance spectroscopy evolution upon sintering of Al-rich anodising sludge-based extruded bodies

Alumina based ceramic materials, containing Al-rich sludge as the major component, were processed by extrusion. The sludge derived from the wastewater treatment of aluminium anodising industrial process. Long rods were produced using a vacuum screw extruder, by a careful control of all relevant processing parameters. Then, thick discs were obtained by cutting dried selected rods, to be tested as probes for sintering-dependent electrical properties. The sintering process was followed by performing common dilatometric/thermal analyses but the evolution of electrical conductivity, estimated by impedance spectroscopy ( S), was also used for this purpose. Results show that sintering-dependent morphological evolution up to 1300oC strongly affects the electrical behaviour of samples, and as a consequence S seems to be a useful technique to follow the firing process.

Electrical Characterization of Mullite Bodies Containing Al-Rich Anodizing Sludge

Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of mullite-based materials, by using unidirectional dry pressing as shaping technique. Mixture with common natural silica and magnesium-containing materials, like ball clay, kaolin and diatomite were prepared in order to achieve interesting final fired refractory and/or electrical insulating materials. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is also studied, by the use of impedance spectroscopy. Finally, different models were used to fit the experimental electrical resistivity values and to estimate the percolation limit of the bi-phase material.

Electrical characterisation of cordierite bodies containing Al-rich anodising sludge

Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of cordierite-based materials, by using unidirectional dry pressing and extrusion as shaping techniques. Mixtures with commercial sand and magnesium-containing materials, like ball clay, and talc were prepared in order to achieve interesting final refractory and/or electrical insulating compositions. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is now studied, by the use of impedance spectroscopy.

High temperature mullite dissolution in ceramic bodies derived from Al-rich sludge

Mullite-based refractory ceramic materials were produced from an industrial Al-rich sludge derived from wastewater treatment of aluminium anodising or surface coating industrial processes, and other low cost raw materials (ball clay, kaolin and diatomite). Cylindrical samples processed by uniaxial dry pressing (32 MPa) were sintered at various temperatures (1250–1650 °C) to study the mullitisation process. The performance of the materials at high temperature (1650 °C) was evaluated through different techniques (XRD, SEM/EDS, optical microscopy, and impedance spectroscopy) to access the microstructural changes occurring under prolonged tests (dwell times up to 100 h). For dwell times <80 h, a preferential dissolution of the smaller mullite grains in the glassy phase and its partial re-precipitation onto the coarser ones, leading to an overall coarsening of the mullite crystals. For dwell times >80 h, coarse α-alumina and Cr-doped alumina developed at the surface of the specimens, being accompanied by the formation of pores in the vicinity of alumina grains. Near alumina grains, additional relevant features include the increase of surface roughness, the appearance of concentration gradients within the glassy phase, which became almost depleted in Al and enriched in alkalines. The continuity of the glassy phase and its enrichment in alkaline species enhanced the electrical conductivity of the material, enabling the use of impedance spectroscopy to access the microstructural changes occurring during prolonged heat treatment.

Predicting processing-sintering-related properties of mullite–alumina ceramic bodies based on Al-rich anodising sludge by impedance spectroscopy

Al-rich sludge produced from industrial anodising and surface treatment processes has been tested in the fabrication of mullite/alumina-based refractory/electrical insulating materials, by dry pressing and extrusion. Different compositions were prepared by using sub-products or common natural silica-containing materials, like ball clay, kaolin and/or diatomite. Ultimate properties are dependent on the shaping method and sintering schedule, since both steps strongly affect the microstructural evolution and crystalline phase formation, which in turn determine the performance of the sintered bodies. The response of sintered bodies to impedance spectroscopy was studied and revealed interesting microstructure–electrical relationships, enabling the use of this technique to predict the processing-sintering-related properties.

Extrusion of Recycled Pastes Based on Al-Rich Anodising Sludge

Al-rich anodising sludge has been tested as main component of alumina, mullite or cordierite based refractory materials and bodies have been processed by using different techniques, such as slip casting, dry pressing and extrusion. The success of the last technique is strongly dependent on the materials composition and proper additives, such as plasticizers and lubricants. Operation conditions might be also adjusted within certain limits and includes the design and size of the die, the flowing speed, and as consequence the applied pressure. In the current work, cordierite hole-tubes of different diameters had been processed by extrusion. The effect of mentioned parameters was evaluated based on the Benbow’s equation and ultimate extrusion quality of samples was estimated.

Production of Al-rich sludge-containing ceramic bodies by different shaping techniques

The present work deals with the processing of refractory ceramic products by using different shaping technologies, namely, unidirectional dry pressing, extrusion, and slip casting. The formulations used include an Al-rich sludge collected in the wastewater treatment unit of an industrial plant dealing with anodizing or surface-coating processes, diatomite and kaolin or ball clay. The effects of a pre-treatment given to the Al-rich sludge and of composition and amount of added dispersant on rheological behavior of suspensions were studied. The green bodies were sintered at different temperatures in the range 1400–1650 °C, and the ultimate material properties (firing shrinkage, water absorption, bending strength, SEM microstructure) were evaluated. The results obtained demonstrated that: (i) the flow behavior of the suspensions is strongly affected by the history of the Al-rich sludge and the composition, as well as by the added amount of deflocculant, (ii) the shaping process has a great influence on the final microstructures and material properties, which derive from different degrees of homogeneity and particle packing achieved in the green state. Soluble salts present in the Al-rich sludge create difficulties in achieving proper dispersion and make slip casting products less reliable compared with those consolidated by other shaping techniques.

Electrical characterization of mullite/alumina bodies based on Al-rich anodizing sludge by impedance spectroscopy

Al-rich sludge produced from an industrial anodising and surface treatment processes had been tested in the fabrication of mullite/alumina-based refractory materials, by using unidirectional dry pressing as shaping technique. Mixtures with common natural silica-containing materials, like ball clay, kaolin and/or diatomite were prepared in order to achieve interesting final fired refractory and/or electrical insulating compositions. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is now studied, by the use of impedance spectroscopy.

Al-rich sludge treatments towards recycling

A treatment method towards recycling of Al-rich sludges to produce high-alumina refractory ceramics is proposed. These sludges are collected in wastewater treatment units of industrial plants dealing with anodising or surface coating processes. This industrial residue usually contains significant amounts of soluble salts such as Na, K, Mg, Ca and Al sulphates that make direct recycling problematic. The removal of these salts from the as-received sludges by washing is ineffective due to its gel-like consistency. The treatment consists of calcining and subsequent washing operations, followed by removal of sulphates by precipitation as BaSO4. The treated sludges could then be well dispersed and processed by slip casting. # 2002 Elsevier Science Ltd. All rights reserved.

Mullite-Alumina Refractory Ceramics Obtained from Mixtures of Natural Common Materials and Recycled Al-Rich Anodizing Sludge

This work describes attempts to reuse Al-rich sludge produced from industrial anodizing and surface treatment processes in the fabrication of mullite-based refractory materials. The complete characterization of the residue is reported, including physical and compositional parameters. Mixtures with common natural silica-containing materials, like ball clay, kaolin, and/or diatomite were prepared in order to achieve interesting final fired compositions. Rheological characterization of the suspensions, their slip casting performance, the thermal behavior of the consolidated bodies, and crystalline phase evolution are also detailed. From the knowledge acquired, pretreatment operations have been suggested to facilitate the recycling of the Al-rich sludge and to improve the characteristics of mullite-based materials.