Alumina foam ceramic filter

This article relates to an alumina ceramic foam filter suitable for aluminum and aluminum alloys. It is made of ceramic powder composed of alumina, silica, talc, and kaolin, and then added with carboxymethyl cellulose, polyvinyl alcohol, silica sol, and Water used to prepare a slurry and soft polyurethane foam is used as a carrier. The slurry is impregnated, extruded to form a blank, and then dried and sintered to obtain an alumina foam ceramic filter.

The foam ceramic filter has good high-temperature stability in the aluminum alloy melt, does not pollute the alloy, and has good filtration and adsorption capabilities for oxidation and flux inclusions in the metal melt. In the filtration and purification test, all performances were good.

Background:

In the smelting and pouring process of various metal liquid alloys, the casting rejection rate due to casting defects such as non-metallic inclusions and shrinkage holes is generally as high as 50~60% of the total scraps. Inclusion defects not only seriously reduce the mechanical properties of castings and casting performance but also harm the cutting processing and appearance of castings. Purifying liquid casting alloys and reducing or eliminating various non-metallic inclusions and exhaust gases are undoubtedly important technical measures to obtain high-quality castings.

Filtration technology has been used in casting production for decades, but initially, it was just a simple filter such as wire mesh, perforated steel plate, porous mud core, etc. inserted into the pouring system to filter out large inclusions.

Since the 1960s, two-dimensional structural internal filters such as aluminum silicate fiber, molybdenum silk, and boron nitride fiber have appeared in Russia, the United States, the United Kingdom, and middle-income countries, and have been used in production, and have achieved a certain effect. However, all these filters can only filter out large inclusions and very few small inclusions in the molten metal through mechanical screening. Since the aluminum silicate fiber filter has low refractoriness and strength, it can only be used for filtration of non-ferrous alloys, cast iron, and small steel castings, and it is difficult to withstand the impact of high-temperature metal fluids for a long time;

Although the molybdenum silk filter developed in the UK and the boron nitride fiber filter developed in the United States can be used to filter high-temperature alloys such as cast steel, their applications are limited due to their high price.

There are also straight-hole core ceramic foam filters and refractory particle filters used for the filtration of cast alloys, but their porosity is small, the former’s filtration efficiency is still low and the filtration effect is unstable. It has no bonding effect, making it easy to leak particles and inconvenient to use.

Although the sintered porous ceramic filter first successfully developed in the United States in the early 1970s solved the problems of easy leakage and inconvenient use of refractory particle filters, it is different from the honeycomb straight-hole ceramic filter first successfully developed in the United States in the early 1980s. Like the device, the porosity ratio is still small, generally less than 50%, making the flow rate of molten metal low.

Ceramic foam filters are the third generation of filtration products after refractory fiber filters and honeycomb straight-hole ceramic filters. The earliest ceramic foam filter was a ceramic foam filter for aluminum alloy successfully developed in 1978 by the American Consolidated Aluminum Company. The product name is Selee/Al.

In 1984, Selee/Fe, a foam ceramic filter for filtering ferrous metals, was developed.

Foam ceramic filters use polyurethane foam as a carrier, which is immersed in a slurry made of ceramic powder, binders, sintering aids, suspending agents, etc., and then the excess slurry is squeezed out to make the ceramic slurry evenly coated The carrier skeleton becomes a green body, and then the green body is dried and roasted at high temperature. Foam ceramic filters have a pore structure similar to foam plastics, with a porosity of 85-90%.

Due to the special filtration and purification mechanism of the unique three-dimensional mesh structure (mechanical interception; exhaust rectification scum; deep adsorption of fine slag inclusions), the purification effect of the foam ceramic filter is that of refractory fiber pure filter and honeycomb straight hole ceramic The filter disc is incomparable. It is characterized by a uniform rigid skeleton structure, small mass dispersion of filtered molten metal, and good filtration precision. It can not only filter out most of the tiny solid suspended inclusions as small as several microns in the molten metal, but It can also filter out liquid slag and gas.

Using a foam ceramic filter to filter molten metal liquid can also make the flow rate of the metal liquid slow and orderly, greatly improve the microstructure and mechanical properties of castings, improve the quality and density of castings, and reduce tool wear during casting cutting. Practice has proven that the filtration efficiency of foam ceramic filters is as high as 75-97%. Therefore, foam ceramic filters have been widely used in the melt purification of aluminum, copper, and steel, and are favored by more and more metallurgical and foundry companies.

In the past ten years, the world has developed a variety of foam ceramic filters of different materials to filter molten metal based on the characteristics of different alloys. For example, AdTech Metallurgical Materials Co., Ltd's Ceramic Foam Filter-PAL.

During the process of smelting and forming castings, aluminum alloys are prone to absorbing gases and mixing in non-metallic impurities, thereby reducing the performance and processing performance of the castings. The use of foam ceramic filters is the most effective way to remove heterogeneous impurities in aluminum melts. Studies have shown that using foam ceramic filters to filter aluminum alloy melts can also reduce the hydrogen content in aluminum melts while removing slag content.

Since the temperature of molten metals such as aluminum, aluminum alloys, and copper is relatively low, the use temperature of alumina ceramic foam filters is also low. On the premise of meeting the use requirements, a low-temperature sintering process is used to produce alumina ceramic foam filters. The machine is the most economical and reasonable. As the quality requirements for aluminum products become higher and higher, the requirements for alumina foam ceramic filters are also getting higher and higher.

Preparation method of foam ceramic filter:

Raw material:

According to the weight ratio of 68.5:23:6.5:2, thoroughly mix alumina, silica, talc, and kaolin to make ceramic powder.

Mix ceramic powder, carboxymethyl cellulose, polyvinyl alcohol, silica sol, and water according to the weight ratio of 65:0.5:1:12:25, add it to the mixer and stir for 2~3 hours until the mixture is evenly mixed to make a ceramic slurry.

Use 10PPI soft polyurethane foam, process it into a size of 50mm x 50mm x 22mm, soak it in a 60C alkali solution with a pH value of 7.5 for 20 minutes to remove the film in the foam holes, and then rinse with water to remove the alkali solution, and then naturally dried to remove moisture and used as a carrier for ceramic slurry.

The roller pressing dipping process is used to add ceramic slurry to the foam plastic carrier, and then the carrier is input into the dipping machine for extrusion and dipping, and the excess slurry is squeezed out to make a foam ceramic filter blank.

Put the foam ceramic filter blank into a drying box, control the temperature within the range of 100~105°C, and dry it for 0.5~1h to harden.

Put the dried billet into the sintering furnace and slowly heat it from room temperature to 500°C at a heating rate of 90°C/h.

C, the plastic is discharged at low temperatures. After the foam plastic is burned and volatilized, it is quickly heated to 1000 °C at a heating rate of 150°C/h.

Heat it to 1400°C at a heating rate of 120°C/h, keep it at this temperature for 1 hour, and then naturally cool it to room temperature and take it out of the furnace to obtain an alumina foam ceramic filter.