Aluminum melt purification--alumina ceramic foam filter

The smelting of aluminum alloy is an important part of the aluminum alloy processing process, which is reflected in the alloying, purification, and refinement treatment technology of the smelting process. Taking the flux treatment in the furnace and the foam ceramic filtration outside the stove to remove non-metallic inclusions in the aluminum alloy melt has long been an important means of liquid purification. Foam ceramic is a porous ceramic product with an open porosity of up to 80%~90%, a density of 0.3 g/cm’~0.6 g/om’, a unique three-dimensional network skeleton, and an interconnected pore structure. The earliest foam ceramic filter was successfully developed by FR Mollard and NDavison of Consolidated Aluminum in the United States in 1978 and used in aluminum alloy casting systems. Their research results were published at the American Foundry Annual Conference in April 1980. Since then, foam ceramic filter materials have been developed and developed around the world and are widely used in the metallurgical industry. The production of Ceramic Foam Filter (CFF) generally uses an organic foam plastic with a three-dimensional mesh structure and interconnected pores as a carrier, immerses it in a special ceramic slurry with thixotropy, and adopts a special roller extrusion process to evenly apply the ceramic slurry on the carrier skeleton, and then dry and solidify it and then calcine it at high temperature1.

The application of foam ceramic filter plates in China's aluminum processing industry has been popularized. There are two main modes of application in the purification of deformed aluminum alloy melts: one is on the aluminum alloy semi-continuous casting line, where the melt is refined by spraying flux in the furnace and filtered by foam ceramic plates outside the stove. The ceramic filter plates are replaced every casting. Some aluminum processing plants now add a trough-type online degasser before filtering to improve the purification effect; the second is on the continuous casting and rolling line, where the replacement cycle is determined based on the pressure difference before and after the filter plate. As for the size of the filter plate to be selected, it is determined by the unit time flow rate of aluminum liquid and the total amount of aluminum liquid; the size of the foam ceramic filter plate to be selected is determined by the cleanliness requirements of the final aluminum liquid. The higher the cleanliness requirements of the aluminum liquid, the smaller the selected pore size 16-81. Foam ceramic filtration and purification is the last process of aluminum liquid purification. Before this process, flux powder refining or online rotary degassing treatment was used, so the impurity phase-hydrogen particles that have not been removed in the aluminum liquid are more subtle and dispersed. How to promote the aggregation and growth of impurity phases and hydrogen adsorption in foam ceramic filtration is particularly important. The mechanical interception "filter cake" effect of foam ceramics alone cannot achieve outstanding results. Therefore, the research goal of this topic is to develop a new generation of foam ceramic filter materials that can promote the aggregation and growth of impurity phases and hydrogen adsorption.

1 Characteristic of the new generation of foam ceramic materials

To achieve our determined research goals, a complete set of strict manufacturing process technologies have been studied from material selection, pulping and mixing, carrier pretreatment, infrared drying, sputtering coating, and microwave drying, to the temperature field control of roasting. The new generation of foam ceramic materials developed can effectively improve the cleanliness of aluminum melt. Its characteristics and comparison with commonly used products are shown in Tables 1 and 2.

2 Mechanism of foam ceramic filter and purification of aluminum melt

The mechanism of foam ceramic plate filtration and purification of aluminum melt is generally described as diffusion interception and inertial collision interception or screening mechanism, sedimentation layer, and deep bed filtration mechanism. The National Key Laboratory of Tsinghua University has established a three-dimensional physical model and a two-phase flow model for the foam ceramic filtration mechanism and carried out simulation calculations, which is very helpful for our research on aluminum melt filtration and purification. The filtration and purification process of aluminum melt by foam ceramic plate is very complicated and is a complex process of high-temperature physical chemistry and metallurgical dynamics. As for screening, collision, sedimentation, and interception filtration, they are relatively easy to understand. We have conducted a comprehensive analysis of the many parameters obtained in the research process of foam ceramic plate filtration of aluminum melt and put forward the following insights:

(1) Through screening, collision interception and diffusion interception, friction, sedimentation, and other mechanical filtration, its filtration efficiency is proportional to the mesh size of the foam ceramic plate. The smaller the mesh, the stronger the interception ability of small particle impurities.

(2) Sedimentation layer or filter cake layer effect. With the deposition of impurity particles in the melt and the winding, interconnected, and uneven grid support walls in the filter plate, the capture ability of impurity phase particles is improved.

(3) The rough surface formed by the gaps on the surface of the network support increases the interfacial energy between the aluminum liquid flow and the ceramic body surface, making the flow direction of impurity particles in the aluminum liquid flow more disordered, which is conducive to the capture and deposition of solid particles.

(4) Due to the micro-cracks and pinholes on the surface of the network support, fluoride flux with strong affinity for AbO particles is pre-deposited, which promotes a complete filter cake layer effect and strong chemical adsorption, making the ability to capture and retain impurity phase particles more enhanced.

(5) The temperature field effect in the metal smelting process and heat transfer process will inevitably form a concentration difference in alloy solutes. The metal melt is redistributed, integrated, redistributed, and integrated again. This is also a good alloying process. Some high-melting point metal phases and compounds are re-aggregated and grown. After the fine impurity phases are aggregated and grown, they are conducive to being filtered and captured. This can be explained by the changes in the concentrations of H, A, O, Fe phase, and T phases before and after filtration. In particular, the effect of double-stage filtration or pore gradient thick plate filtration is more obvious. Therefore, under the same conditions, the new generation of foam ceramic plates has a high removal rate of impurity phases in aluminum melt, the static pressure difference before and after filtration changes more with time, and the change in H content in aluminum melt before and after filtration is also obvious. Of course, the smaller the pores of the selected foam ceramic plate, the higher the cleanliness of the aluminum melt.

3 Correctly select the filter plate to improve the cleanliness of the aluminum melt

(1) The selection of foam ceramic filter plates should first be based on the amount of impurities in the aluminum melt, the melt transfer flow rate, and the quality requirements of cast aluminum products.

① If the aluminum melt contains a lot of impurities and the quality requirements of the cast products are average, a foam ceramic filter plate with a larger pore size can be selected, and a glass cloth can be added in front of the filter plate for coarse filtration. In this case, it is important to use glass cloth for coarse filtration when semi-continuously casting aluminum melt. It can intercept the slag flowing out when the furnace eye is opened and the slag is washed down from the flow channel to prevent the filter plate from being blocked.

② When the quality requirements for casting products are high, a new generation of foam ceramic filter plates with small holes or large mesh numbers is selected. For example, Fujian Ruimin Aluminum Co., Ltd. and Southern Aluminum Co., Ltd. produce aluminum foil billets, Henan Mingtai Aluminum Co., Ltd. produces high-quality hot-rolled slabs, Northeast Light Alloy Co., Ltd. produces high-quality military aluminum materials, and Guangdong Zhongshan Hesheng Aluminum Co., Ltd. produces high-quality industrial aluminum profiles.

③ The selection of filter plate specifications is based on the flow rate of aluminum melt passing through per unit of time and the total amount of aluminum melt passing through. If the flow rate and total throughput are large and the filter plate has small holes, a larger filter plate should be selected.

(2) When selecting a filter plate, you should also observe its color, pat its body, and burn its contents.

① If the color of the filter plate is pure and free of impurities, it means that the material is uniform; if it is piercing but not sticky, it means that there is no adhesive sprayed on the surface and it will not cause secondary contamination of the aluminum liquid.

② Hold the filter plate with one hand and pat the filter plate with the other hand in a clapping manner. If ceramsite falls continuously, it is a low-quality filter plate and will cause secondary contamination of the aluminum liquid.

③ Use open flame or electric heating to heat the filter plate to red, and immediately cool it to room temperature without cracking or falling off. It is a good filter plate.

(3) When purchasing a filter plate, you should carefully evaluate the effective specific surface area or porosity of its foam. You can observe the uniformity of the mesh on the surface layer of the filter plate and then cut it open to observe the uniformity of the mesh on the cross-section. If conditions permit, you can use the drainage method to measure and calculate its porosity. If the three-dimensional mesh is uniform and the porosity is relatively large, it is an excellent foam ceramic filter plate.