Ceramic Foam Filters Research Articles

Sustainable recycling of pure aluminum from waste chips under supergravity-enhanced separation: A cleaning process

Large quantities of chips containing high levels of aluminum are always inevitably produced during machining process of aluminum products, which is a valuable renewable resource. In this paper, an environmental-friendly method for direct and continuous recovery of same-level recycled Al from waste chips under supergravity-induced was proposed. The oxide film covering the surface of the molten Al-chips was easily disrupted under super-gravity, and subsequently almost all of Al melt detached from the oxide film and flowed rapidly through the microporous ceramic foam filter, with a yield ratio of more than 97 %. During this process, all fine broken oxide-film particles and large amounts of primary iron-rich particles in the melt were captured in the complex channels of the filter, resulting in clean 1xxx series recycled Al with free inclusions and impurity iron content of less than 0.28 wt%. In addition, a sustainable process and a continuous centrifugal unit for recycling aluminum chips were designed, the economic and environmental advantages of which demonstrate the feasibility of sustainable regeneration of aluminum chip resources on an industrial scale via supergravity technique.

Open-cell ceramic foam filters for melt filtration: Processing, characterization, improvement and application

Open-cell ceramic foams are widely used as filters for the purification of molten metals due to their high permeability and high efficiency for capturing non-metallic inclusions. With the consideration of manufacture and filtration behaviour, most filters are produced by replica methods. The intrinsic multiscale porous structure requires the ceramic foams to have sufficient strength. As a consequence, different processing approaches, material compositions and characterizations are investigated. Meanwhile, to further enhance the filtration performance, the functionalization of ceramic foam filters is often conducted with surface coating. This review summarizes the processing and characteristics of ceramic foam filters (CFFs), the nature and formation of non-metallic inclusions (NMIs), the filtration mechanism and applications of CFFs. The characterization of mechanical and filtration performance, as well as the corresponding strengthening and functionalization methods are also presented.

Preparation of β-SiAlON/SiC composite ceramic foam filters and their oxidation resistance

In this study, ceramic foam filters with high porosity, strength, and oxidation resistance are produced at low temperatures by preparing porous β-SiAlON/SiC composite ceramic foams using the foam impregnation method with Si, Si, α-Al2O3, AlN, and SiC as the raw materials. The presence of SiC inhibits the growth of β-SiAlON grains, which reduces the amount of β-SiAlON produced. As the proportion of β-SiAlON increases, the porosity of the β-SiAlON/SiC composite ceramic foam decreases, whereas the bulk density and compressive strength increase. Non-isothermal oxidation experiments show that β-SiAlON/SiC composite ceramic foams exhibit high oxidation resistance and their macroscopic morphology does not change significantly after oxidation. As the proportion of β-SiAlON increases, the smooth SiO2 oxide layer on the surface of the ceramic foam gradually changes to acicular and columnar mullite crystals, and the oxidation resistance increases. Therefore, β-SiAlON acts as a high-temperature binder when it is added to SiC. Hence, the prepared β-SiAlON/SiC composite ceramic foams can be used as ceramic filters in the field of molten metal filtration.

Effect of Sodium Tripolyphosphate on The Morphology and Thermal Properties of Anorthite-Based Ceramic Foam Filters for Aluminum Alloy Castings

Effect of Sodium Tripolyphosphate on The Morphology and Thermal Properties of Anorthite-Based Ceramic Foam Filters for Aluminum Alloy Castings

Research on Online Monitoring Technology and Filtration Process of Inclusions in Aluminum Melt.

Online monitoring and real-time feedback on inclusions in molten metal are essential for metal quality control. However, existing methods for detecting aluminum melt inclusions face challenges, including interference, prolonged processing times, and latency. This paper presents the design and development of an online monitoring system for molten metal inclusions. Initially, the system facilitates real-time adjustment of signal acquisition parameters through a multiplexer. Subsequently, it employs a detection algorithm capable of swiftly extracting pulse peaks, with this task integrated into our proprietary host computer software to ensure timely detection and data visualization. Ultimately, we developed a monitoring device integrated with this online monitoring system, enabling the online monitoring of the aluminum alloy filtration process. Our findings indicate that the system can accurately measure the size and concentration of inclusions during the filtration process in real time, offering enhanced detection speed and stability compared to the industrial LiMCA CM (liquid metal cleanliness analyzer continuous monitoring) standard. Furthermore, our evaluation of the filtration process demonstrates that the effectiveness of filtration significantly improves with the increase in inclusion sizes, and the synergistic effect of combining CFF (ceramic foam filter) and MCF (metallics cartridge filter) filtration methods exceeds the performance of the CFF method alone. This system thus provides valuable technical support for optimizing filtration processes and controlling inclusion quality.

Effect of flow velocity on the permeability of ceramic foam filters (CFFs)

The structural uniformity of commercial Al2O3-based Ceramic Foam Filters (CFF) of Grades 30, 50, 65 and 80 was investigated using the permeability of samples from five different locations of the CFF tiles/Blocks, specifically from the four corners and the centre. Statistical analysis at 95 % confidence interval revealed no significant variations in the permeability values among these locations. Additionally, the impact of different flow regimes within the CFF structure, namely laminar, transitional, and turbulent regimes, on the permeability was explored. It was demonstrated that permeability, as a parameter of fluid velocity, exhibited a notable increase with increasing pore diameter. In CFF samples with pore diameters greater than 0.5 mm, the deviation in permeability between turbulent flow and creeping flow was attributed to large eddies occupying a substantial portion of the pore volume. This phenomenon resulted in the narrowing of the flow channels and reduction in the viscous boundary layer. The correlation between filtration efficiency and velocity, as well as CFF pore diameter, is discussed in view of the influence of streamlining and the size of the eddies on the re-entrainment of captured particles.

Towards a new generation of environmentally-friendly ceramic foam filters: contribution of graphene nanoadditives in calcium aluminate-rich coatings

In this contribution, the widely used non-ecological production process of ceramic foam filters was modified into an environmentally friendly procedure, in which no hazardous materials are involved. Firstly, a large number of carbon-bonded ceramic foam filters were prepared by this approach. In the next step, a thin coating consisting of graphene-doped, carbon-bonded calcium aluminate was applied over these filters. Reference samples as well as coated samples were analyzed in detail with a focus on the phase composition, microstructural analysis, and mechanical as well as thermal properties. The results confirmed that the application of the developed coating improved the cold crushing strength of the ceramic foam filters. In addition, by application of graphene nanoadditives, it is also possible to vary the surface microroughness which will subsequently enable improved efficiency of inclusions elimination from metal melts during the filtration process.

Thermal shock resistance of industrial foam filters-a comparative study

For ceramic foams, there is no thermal shock resistance testing standard available at the moment. This work aims to investigate the influence of the different test conditions. Among the descending thermal shock test conditions, heating mode, thermal shock temperature, and quench medium, the latter has the highest impact on the residual mechanical properties. Reference alumina (Al2O3) foams and commercially-available ceramic foam filters (ZrO2, Al2O3–ZrO2, Al2O3–P, and SiC) were subjected to thermal shock with water and air as quenching medium. Due to the low thermal expansion coefficients, SiC and ZrO2 possess the highest calculated thermal shock resistance Rfoam. SiC and ZrO2 foams also had the highest experimentally evaluated residual strength after being quenched in water. An immersion thermal shock procedure with molten metal was tested on Al2O3 reference test pieces. The residual elastic moduli and strengths decrease with increasing temperature which is a function of immersion duration and depends on the usage of an interlayer between the foams and the melt. In order to evaluate the temperature gradient and its development over time, temperature measurements were conducted using a thermocouple inserted and fixed in the midplane of the foams and bulk test pieces.

Study and Modelling of Fluid Flow in Ceramic Foam Filters.

To investigate the fluid flow characteristics of conventional Ceramic Foam Filters (CFFs) of grades 30 and 50, a 2D macro-scale geometry was generated by converting pixel grid images of the filters into vector format images. The flow behaviour through the filter channels was then numerically modelled using the Stocks equation within the Creeping Flow interface of COMSOL Multiphysics®. Through modelling, the average interstitial velocity was estimated and found to be higher than the corresponding value obtained from the Dupuit-Forchheimer equation. The discrepancy obtained suggested that the flow behaviour within the filter channels differed from that based on the simplified assumptions of the equation. The porosity and permeability of the CFFs were evaluated during the post-processing stage using surface integration and user-defined equations. The experimentally determined porosity closely matched the values obtained from the simulation model, demonstrating the reliability of the numerical approach. However, the permeability values from the simulation of CFFs of grades 30 and 50 were higher than those obtained experimentally. This discrepancy can be attributed to the larger channels in the generated geometrical pattern compared to the original CFF structure. The present findings highlight the effectiveness of the proposed methodology in developing a representative macro-scale geometry for CFFs and in simulating fluid flow behaviour.

Lactose/tannin-based calcium aluminate coatings for carbon-bonded alumina foam filters: A novel approach in environment-friendly steel melt filtration

In this paper, a new approach in the development of calcium aluminate (CA) coatings for ceramic foam filters for steel melt filtration is presented. This approach is based on applying an environmentally friendly binder system based on lactose and tannin. The use of such a binder was previously demonstrated for the matrix of carbon-bonded alumina filters. In this research, the influence of the total solid content, the binder content, and the centrifugation speed on the coated ceramic foam filters’ final properties was studied. All of the prepared coating slips were analyzed in terms of their rheology. The most promising slips were applied on carbon-bonded alumina filters, which were prepared using a lactose- and tannin-based binder system. For the selected coating slips, three different centrifugation speeds (600, 1000, and 1400 rpm) were used to optimize the coating layer thickness and the overall amount of the coating material applied to the filter. The coated and thermally treated filters were tested in terms of their cold crushing strength. Furthermore, the microstructure of the filters and mainly the coating layer thickness were studied. It was observed that both the centrifugation speed and the total solid content in the coating slip have a major impact on the filter properties. The most promising results were obtained for a coating containing 72.5 wt% of total solids applied at 1000 rpm centrifugation speed. This batch of coated samples showed a cold crushing strength of more than double the value obtained for the uncoated reference, while having a uniform coating layer with the thickness of ∼177 μm.

Preparing β-SiAlON ceramic foam filters with high oxidation resistance

Herein, to develop ceramic foam filters (CFFs) with high porosity and strength, porous β-SiAlON ceramic foam was prepared using Si, α-Al2O3, and AlN via foam impregnation. Experiments were performed to investigate the effects of the slurry solid content, impregnation times, and sintering temperature on the sample properties. The best green bodies were obtained at a slurry solid content of ∼70 wt%, and the sample subjected to two impregnation times effectively filled the defects. The sintering temperature was found to affect the phase composition, microstructure, sintering behavior, mechanical properties, and oxidation resistance of β-SiAlON ceramic foams. Granular β-SiAlON started to develop at 1550 °C and was almost completely developed at 1600 °C. After sintering at 1650 °C, the porosity remained as high as 85.93% and the compressive strength reached 1.57 MPa. In a nonisothermal oxidation experiment, the maximum oxidation weight gain rate was 14.1% and the sample strength still reached 1.54 MPa at 1600 °C. A high sintering temperature promoted the development of material phases and liquefaction of grains, while the oxidized surface generated an amorphous layer with a high melting point, effectively preventing oxygen diffusion. Therefore, β-SiAlON can be employed to develop CFFs with high strength and excellent oxidation resistance.

Wetting properties and chemical interactions of copper with industrial filter materials

Copper foundries have started using ceramic foam filters to increase copper melt purity and improve cast quality. For the selection of filter materials, the interaction between the copper melt and the filter materials is a significant criterion. This study aims to determine interactions and wetting behavior between commercially available filter materials and pure copper melt using sessile drop technique. SEM/EDX analysis revealed no interactions between Cu and the filter materials, ZrO2, Al2O3–ZrO2, Al2O3–P, and Al2O3–Si. Although Cu and SiC showed no visible reaction, a kind of Cu infiltration into the SiC substrate was observed. The wetting behavior was different for the different substrate materials characterized by the increasing contact angle from Cu/ZrO2, Cu/SiC, Cu/Al2O3–ZrO2, Cu/Al2O3–P to Cu/Al2O3–Si.

The 3D-Printed building and performance of Al2O3 ceramic filters with gradient hole density structures

Ceramic filters with gradient hole density structures will be an attractive candidate for high-temperature alloy melt filtration because it combines good mechanical properties with filtration requirements. This paper focuses on the structural design, mechanical properties and printed forming mechanism of the new pattern Al2O3 filters with gradient hole density structures (Al2O3-GHDS). Al2O3-GHDS filters were designed based on a topologically optimized lattice and then fabricated by stereolithography (SLA) printing technology using our developed Al2O3 paste system. It was found that GHDS filters exhibited the resultant performances combining wide hole sizes and excellent structural stability, and their compressive strength, energy absorption value and bending strength were 4.7 MPa, 77.7 kJ/m3 and 6.7 MPa, respectively, which far exceeded the mechanical properties of the traditional ceramic foam filters. The thermal shock resistance of GHDS filters can reach 2.2 MPa, which was nearly five times the traditional ceramic foam filters. The GHDS ceramic filter finally can exhibit the hole diameter largely ranging from 0.76 mm to 5.35 mm, which covered the hole sizes from 10ppi to 40ppi uniform hole density structure (UHDS). Furthermore, the inner holes of our built GHDS ceramic filters also had the good connectivity without plugging phenomena.

Efficient purification of scrap 1060 aluminum alloys contaminated with Fe and Si by super-gravity separation

1060 aluminum alloy has been used in high-volume commercial applications and correspondingly the mass of its downgraded recycling after contamination is huge, which is primarily due to the gradual accumulation of impurity elements such as Fe and Si. The separation of excess Fe is considered to be a key step towards the same-level recycling of aluminum alloy scrap with extremely economic and environmental benefits. A supergravity-induced technology was employed to purify the contaminating elements from molten aluminum scraps through Al2O3 CFF (Al2O3 ceramic foam filter). During the filtration process, the premature precipitation of Fe-rich phases and Si particles in the molten aluminum scraps were intercepted near the inner wall of Al2O3 CFF. Resultantly, the removal rate of Fe and Si reached 83.98 % and 49.67 %, respectively, at T = 690 ℃ and G= 30 via Al2O3 CFF of P = 80 ppi with H= 40 mm. A continuously supergravity-induced separation on industrial scale was further performed and the removal rate of Fe and Si were 83.5 % and 48.3 %, respectively.

Influence of filter surface roughness on the pressure drop of ceramic foam filters

The pressure drop of a ceramic foam filter is an important characteristic indicating the resistance to fluid flow through the filter. Filtration experiments have shown that filtration efficiency increases with decreasing functional pore size. However, this improvement comes at the cost of a higher pressure drop. Furthermore, trials with increased roughness of filter struts showed an improved filtration behavior. Comparing the influence of these filter properties on the filter efficiency is of high interest in terms of filtration per pressure drop. Therefore, the sensitivity of the pressure drop with respect to surface roughness needs to be known. In the study, the pressure drop of ceramic foam filters was measured for different functional pore sizes, porosities, and surface roughness in a water-based test facility at NTNU in Trondheim, Norway. The flow velocity was varied in the range of 0.6–80 cm/s, allowing the determination of the Darcy and non-Darcy permeability coefficients.

Flame-sprayed alumina molten metal filters for dead-mould casting application

Ceramic foam filters are applied to remove non-metallic inclusions during steel melt filtration. The flame-spray technology can contribute to improving such filters by increasing the deposition capability of inclusions and decreasing the impairment of the filter substrate by the steel melt. This makes them suitable for application to the continuous steel casting process and foundry casting. The ball-on-three-balls test was utilized at elevated temperatures to determine the high-temperature behaviour of thermally sprayed alumina. Additionally, the flame-spray technology was used to produce carbon-free filters composed of flame-sprayed alumina by coating and burning out the Al2O3–C filter substrate during a sintering step. The B3B revealed a ductile behaviour with increasing temperature and a bending strength of 23.28 MPa at 1400 °C. After sintering at 1400 °C, the filters showed a sufficient CCS of 0.16 MPa, which allows the performance of casting tests (immersion and pouring) with molten steel at 1580 °C.

Hydraulic Characterization of Ceramic Foam Filters Used in Aluminum Filtration.

Ceramic Foam Filters (CFF) are frequently used during the filtration of aluminum (Al) melts to produce high-quality products. In the present study, the physical and hydraulic characteristics of alumina (Al2O3)-based CFF from three different suppliers (A, B and C) have been thoroughly investigated. The filters' porosity and pore diameter, i.e., Window and Cell Feret diameters, were measured and the permeability of the different filters calculated based on pressure drop experiments. The comparison of the classification systems of CFF, i.e., Grade and PPI (Pore Per Inch) numbers, using statistical analysis of permeability and Window Feret diameter showed significant variations between the morphological and hydraulic properties of some CFFs of identical Grade and PPI numbers. Moreover, the Fanning friction factor was plotted as a function of interstitial Reynolds numbers (Rei), and laminar, transient, and turbulent flow regimes were identified. The relationship between the Fanning friction factor and the interstitial Reynolds numbers of all the filter samples investigated was processed using regression analysis, and a model equation developed to calculate the pressure drop over the CFF using the Window Feret diameter. The correlation between the experimental pressure drop values and the derived model equation indicates that empirical expressions for calculating the pressure drop over CFFs should be derived based on experimental measurements carried out at the velocity range of the application of the CFF, which is about 10 mm·s-1 for aluminum filtration.

Design, manufacturing and properties of controllable porosity of ceramic filters based on SLA-3D printing technology

The ceramic foam filters are commonly applied in metal casting. However, this kind of filter has a number of drawbacks, such as weak impact resistance, uncontrollable technical parameters, and a long producing cycle. Our work developed a better manufacturing method for ceramic filters based on SLA-3D printing technology in which the structure of filters can be designed according to demands of porosity and hole density. Firstly, it is to propose a ceramic filter model with good mechanical properties under various porosities and hole densities using mechanical simulation and topology optimization. Secondly, SLA-3D printing technology was used to manufacture ceramic filter models with various characteristics. The results demonstrate that the porosity can be achieved with a wide range, and the higher porosity and hole density can approach 90% and 50 PPI, respectively. The preparation error of the internal pillars inside the filters model also can be revised to precisely control the porosity error of filters. Finally, the pore characteristics and compressive performance of the SLA printed ceramic filters were much better than that of the ceramic foam filters.

Purification of primary aluminum liquid through supergravity-induced filtration

Fine particles in primary aluminum jeopardize the quality of aluminum production. Supergravity-induced filtration with an Al2O3 ceramic foam filter (Al2O3 CFF), used to purify primary aluminum, is proposed in this study. During the purification process, Al2O3 particles and Fe-rich phases prematurely precipitated around the Al2O3 particles, and were intercepted either on the surface of the Al2O3 CFF by being wrapped in alumina film or inside the filter. Furthermore, the premature precipitation of the intermetallic near the inner wall of the filter improved the capacity of the Al2O3 CFF to remove metallic impurities; the coarse filter walls acted as nucleation sites. After purification at 700 °C and a gravity coefficient of 80 with an Al2O3 CFF of P = 130 ppi and H = 20 mm, no Al2O3 particles were observed in the filtered aluminum, the removal ratio of Fe (RFe) reached 76%, and the loss percentage of Si (LSi) was 55%. Based on the optimized conditions obtained from the exploratory experiment, a continuous purification was further performed to complete the engineering scale purification of primary aluminum and obtain purified aluminum without Al2O3 particles, with 74% for RFe and 50% for LSi.

Investigation of mechanical and thermo-mechanical strength of ceramic foam filters (CFFs)

During aluminium production, the molten metal will always contain varying amounts of impurities, e.g., non-metallic inclusions, and for high-quality products removing such inclusions is essential. This can be achieved by filtration using ceramic foam filters (CFFs). However, these filters are highly brittle materials subjected to strong mechanical and thermo-mechanical stresses during transport and operation, which occasionally leads to failure of the filter material. In the present study, the compression strength of five different Al2O3-based CFFs was measured at room temperature and elevated temperature (compressed at 730 °C), as well as while submerged in molten aluminium with varying melt compositions (pure aluminium and an aluminium-magnesium alloy). The compression strengths at room temperature were established to be in the range of 1.19–2.09 MPa depending on the filter type tested. In the case of the CFFs compressed at elevated temperature, a reduction in compression strength in the range of 9.2–58.6% was established to exist depending on filter type and heating duration, except in three of the filter/duration-combinations tested. Compression of CFF samples submerged in molten aluminium led to an even further reduction in compression strength in the range of 42.6–69.4% depending on filter type and duration of exposure. With an exposure time of only 5 min, no difference in compression strength was observed between the two aluminium melts.