WO2019013728A1

WO2019013728A1 - Chamotte refractory bricks (alumina-silica bricks) from waste sand of investment casting (lost wax casting, precision casting) mold

Info

Publication number: WO2019013728A1
Application number: PCT/TR2018/050036
Authority: WO
Inventors: Tuba Bahtli
Original assignee: Tuba Bahtli
Priority date: 2017-07-14
Filing date: 2018-02-01
Publication date: 2019-01-17
Also published as: TR201710403A2

Abstract

The invention is related to the recycling of waste sand obtained after the use as low wax casting molds and the use as raw material in the production of chamotte refractory bricks. All the raw material contents required for chamotte refractory bricks are contained at certain ratios in low wax casting mold. Also ZrO2, which is present in the waste, will also increase the mechanical properties, thermal shock and corrosion resistance of Chamotte refractories. Recycle; is made by adding the waste sand, after the use of the low wax casting molds, with different sizes and different ratios in Chamotte refractory bricks in order to obtain optimum chemical content, porosity and density for the chamotte refractory brick.

Description

CHAMOTTE REFRACTORY BRICKS (ALUMINA-SILICA BRICKS) FROM WASTE SAND OF INVESTMENT CASTING (LOST WAX CASTING, PRECISION

CASTING) MOLD

TECHNICAL FIELD

The invention is to regain a waste material whose amount is increasing day by day to a different production sector by providing the use of casting sand, which is known as lost wax casting mold in the industry and which is used for casting of cobalt, 4140 steel and stainless steel as precision casting and which has a certain end use life and which is solid waste and does not have mold feature, in order to produce alumina based chamotte refractory brick or low alumina bricks as known in refractory industry. Thus, achieving an economic gain and contribution to nature are the main objectives. It is also to obtain bulk density,% visible porosity, cold compressive strength, thermal properties and corrosion resistance properties which are the same or improved with the existing products produced in the industry by using this waste in the production of refractory materials. PREVIOUS TECHNIQUE

Refractory materials are materials that can withstand high temperatures and maintain their physical and chemical properties at high temperatures and under atmospheric conditions. Properties expected from refractory materials are to be durable at high temperatures without deformation for the purpose of their use, protect shape and rigidity in furnace atmosphere, carry the weigth they are loaded at high temperatures, resistant to thermal shocks and not to crumble, not crack and spill, resistant to the chemical effects of the environment within the meaning of high corrosion resistance, have very small even not the dimensional changes at high temperature and temperature changes, isolate or transfere the heat according to the place of their use.

Casting; is a process which depends on filling the die, in which have the cavity with the shape of the part desired to be produced, with the liquid metal. The dimensions of the mold cavity are made slightly larger than the part to be produced. Thus, size shrinkages after cooling are balanced. Metal casting is very important for many modern machine parts and components. For example, there is casting more than 50% of weight of a tractor. On a car engine this exceeds 90%. On avarage, one- tenth of the iron and steel has been produced as casting in the last 20 years. 75% of all metal castings are gray (Camel Graphite) cast iron. Steel, malleable iron, copper alloys, Aluminum, Zinc (Zn) alloys, Magnesium-Nickel (Mg-Ni) based alloys, Lead (Pb) and Titanium castings follow it. According to available historical informations, copper alloys have been casting for at least 6000 years and gray iron for 4000 years. Aluminum has been casting since1900, nickel-based alloys since 1940 and titanium since 1950.

Sand Casting is one of the oldest and most used methods. The lower cost, casting of very different sized pieces are the mean reasons of its preference. Silica sand particles are connected to each other by adding some water and clay. The material prepared in this way has certain strength, toughness and permeability. The wood or metal model of the casting part is prepared. The model needs to be made somewhat larger due to the shrinkage during casting. The model is placed in a container and covered with sand to provide the shape of the model. When the model is removed, the mold is hollowed out. The liquid metal solidifies by filling the void when it is poured. The sand mold is broken and the part is removed and cleaned. This product is called casting. The broken mold sands are used again in mold making. In this process, every part to be produced has a mold making requirement.

Pressure Casting is the casting of liquid metal by filling metal mold under pressure. Pressure die casting is very similar to metal die casting. The most important difference between them is that liquid metal is filled under pressure in the pressure casting. The fact that the mold filling speed of the metal in the pressure casting is too high makes it possible to cast very complex parts.

Centrifugal Casting is used for casting of cylindrical parts. In principle, the liquid metal is sent into the mold under centrifugal force. The mold axis can be vertical, horizontal or inclined. The molten metal poured into the mold, which is in vertical position and rotate around its axis, is blown outwards means the inner surface of the mold by the effect of the rotation. Thus, the molten metal takes the form of the outer mold. In Ceramic Mold Casting, mortar consisting of refractory granules and a ceramic binder is poured on models made of wood, plaster or metal. Generally, the model is removed after this mixture containing a gel-forming agent is expected to be gelled. Then the volatile substances in the mixture are burned with a blower and the mold is cooked. As a result a mold with high refractoriness, where all metals including steels could be casted in, is obtained. It is preferred when it is too large to be produced by other precision casting processes or when the number of parts is very small. The only difference from the precision casting is that the mold material can be reused as in the sand molding process.

Invesment Casting (Precision Casting, Low wax casting) is the processes of coating with the ceramic around the meltable waxy pieces at the appropriate temperature, then firing the ceramics and transferring molten metal into the ceramic mold cavity vocated by the wax melting. The other name is "Low wax casting". First, the model made out of wax or plastic is covered by refractory mud that hardens at room temperature. When heated, the wax melts away. In this method, as many models as the number of parts to be produced are required, wax and plastic-like models are used. Models are produced with a metal mold injection of wax or plastic and many of models are arranged in bunches connected to a common path. Even the most complex parts model can be produced very quickly by using those methods. Mixed shaped parts, which are generally difficult to manufacture in machines, are produced by this method. After casting it can be used almost without any further processing. According to other casting methods, it gives opportunity to obtain high dimensional precision and smoother surface. Casting with this method has started to be widely used in recent years.

Waste recycling practices remain on the second plenary due to prioritize production, product quality and cost in manufacturing industry. However, it is very important to reduce the consumption of materials and to use of natural resources efficiently by recycling the evaluable wastes. Converting this waste sand, which is produced in high quantities after casting, into beneficial products which are economic value by using environmentally friendly methods instead of disposal. Casting sand is often disposed of at regular storage facilities without being reevaluated. The increased storage cost has become a major problem for the casting industry. For this reason, recycling instead of disposal; will provide significant savings in both the production costs and the waste disposal costs by reducing the amount of waste. It is important to prevent the depletion of natural resources, convert them to an input for the economy by removing them from being a threat to the environment and human health, to reintegrate the economy by being converted into value-added products.

Approximately 450.000 tons of waste is produced by the production made in our country. Approximately 65% of this amount is composed of sand, 10% is slag, 15% is dust-mud and 10% are refractory, oil, stone, paint, barrel. The contents of waste sand of investment casting were analyzed as the following table.

Table 1. XRF analysis of waste sand of investment casting (low wax casting) Sudden temperature change during casting cracks the sand particles, refractoriness and gas permeability of the sand decrease, it is known as technical knowledge that sand loses its binding ability by burning of clay that gives the plasticity. Because of the acceptance that there is no study on the usability of recycled waste sand obtained from low wax casting molds (invesment molding) as a refractory material.

BRIEF SUMMARY OF THE INVENTION

The invention is related to the recycling of waste sand obtained after the use as low wax casting molds and the use as raw material for the production of chamotte refractory bricks. All the raw material contents required for chamotte refractory bricks are contained at certain ratios in low wax casting mold. Also ZrO2, which is present in the waste, will also increase the mechanical properties, thermal shock and corrosion resistance of Chamotte refractories. Recycle; is made by adding the waste sand, after the use of the low wax casting molds, with different sizes and different ratios in Chamotte refractory bricks in order to obtain optimum chemical content, porosity and density for the chamotte refractory brick. DETAILED DESCRIPTION OF THE INVENTION

The invention is important for both the economy and the environment to be able to be recycled and regained of wastes used as input in different sectors. The importance of the invention emerges when consider the casting sand material is converted into waste and no use as casting sand, disposal costs, storage costs, and non-compensatory losses to the environment.

It is seen that there is no scientific and technological study which investigates the usability of casting sand material converted into waste in refractory sector. The main reason for this is the technical knowledge known as "Sudden temperature change during casting cracks the sand particles, refractoriness of the sand and gas permeability are reduced, the clay gives plasticity is fired and loses its binding property ".

Both the waste material will be recovered and economic gain will be obtained by this invention. It is thought that this is a study for the transfer of original scientific studies which can be an international informative contribution to industrial application.

Zircon is found as well as silica and alumina in the chemical analysis and phase analysis of waste casting sand. The components as percentage are given in Table 1 .

Zircon is decomposed at high temperatures to form Silicon Dioxide (S1O2) and Zirconium Dioxide (ZrO2), and Zirconium Dioxide (ZrO2) provides increase in the toughness. Refractory bricks with higher mechanical, thermal shock and corrosion resistance and thus longer service life will be produced by the zircon in this waste. Therefore, zircon-which is an important input in refractory sector- will be added to the structure and extra zircon raw material will not be used. For the chamotte refractory brick, a proportion of alumina and silica will be supplied from this waste, so the amount of raw material will be reduced and production cost will be reduced.

The alumina silicate (S1O2-AI2O3) refractories have three phases: the silica phase, the mullite (3AI2O3.2SiO2) and the glassy phase. The less the glassy phase, the refractory with higher strength is obtained. Bricks containing 85% or more of alumina are preferred due to their high temperature resistance. The corundum refractors have high refractoriness.

Shaped or monolithic refractory materials made from high alumina raw materials, forms formed by casting electro-fused materials, are commonly known as fused cast refractories. The raw materials used in the production of high alumina refractories are:

• Diaspore (AI2O3.H2O)

· Bauxite (AI2O3.H2O + AI2O3.3H2O)

• Diaspore clay (refractory clay, scattered in diasporon nodules and is called as nodule shaft)

• Bauxitic Kaolin (Bauxitin nodules scattered.)

• Kyanite (AI2O3, S1O2)

· Andaluzite (AI2O3, S1O2)

• Silimanite (AI2O3, S1O2)

• Refined Calcined Alumina (AI2O3)

• Granulated Fused and Sinter Alumina (AI2O3)

• Mullite (3AI2O3.2S1O2)

If the alumino-silicate group contains more than 45% of Aluminum Oxide (AI2O3) in refractory materials, it is named as high alumina refractory material. Thesecan be divided into two as 45-56% Aluminum Oxide (AI2O3) and more than 56% Aluminum Oxide (AI2O3). According to the desired amount of alumina and their properties many alumina refractories are obtained by the mixture of plastic or flint clay and bauxite, diaspore clay or a mixture of both. Taking into consideration the high firing shrinkage of bauxite and diaspor clay, they should be subjected to precalcine treatment before refractory material production. In some cases, fused or calcined alumina is added to improve certain properties. The mineral of the class known as mullite refractory is mullite (contains 71 .8% Aluminum Oxide (AI2O3) and 28.2% Silicon Dioxide (S1O2)). Refractory class containing 99% Aluminum Oxide (AI2O3) is called corundum. The mineral melts at 3720 °F is corundum (crystal Aluminum Oxide (AI2O3)). High alumina bricks are very resistant to smoke and gases, the attack of different slags. It is more durable than the chamotte bricks at high temperatures. It is very resistant to thermal shocks. Mullite refractories containing mullite mineral (3AI2O3.2S1O2) are produced from natural minerals and synthetic mullite. Mullite bricks attracts attention because of their high carrying capacity, volume stability, resistance to fluids in high temperatures. The high alumina products used for monolithic coatings are supplied with a wide composition and physical properties. These include ramming mortars, castable mortars, hot patching mortars and plastic refractories.

The raw material of the chamotte refractory materials is hydro aluminum silicate, which contains other minerals in a small amount. The general formula of those alumina-silicates is AI2O3.2S1O2 that contain 39.5% Aluminum Oxide (AI2O3), 46.5% Silicone Dioxide (S1O2) and 14% Water (H2O). The most common member of this group is KAOLINITE. The bound water disappears at high temperatures; the remaining raw material theoretically contains 45.9% Aluminum Oxide (AI2O3) and 54.1 % Silicon Dioxide (S1O2). However, even the purest one has very little impurities such as iron, calcium, magnesium, titanium, sodium, potassium, lithium and free silica. Some of these clays are firedand made into a material called chamotte. CHAMOTTE REFRACTORIES are produced by binding CHAMOTTE materials with plastic clays. The bricks in this class contain approximately 18-44% Aluminum Oxide (AI2O3). High resistant fire-clay bricks are usually made by mixing of a few clays. Flint clays and high grade kaolins have high refractoriness. Most of these bricks are resistant to shrinkage and breakage against rapid temperature changes. Some of them are fired at higher temperatures. These bricks have volume stability. Their resistance to fluids has been increased and they are inert to decomposition due to carbon deposition in carbon monoxide gas atmosphere.

Zircon is a chemical compound having the chemical formula ZrO2.SiO2 and theoretically containing (by weight) 67.23% Zirconium Dioxide (ZrO2) and 32.77% Silicon Dioxide (S1O2). It has tetrahedral crystal structure and a = 6.60A °, c = 5.88A ⁰ lattice parameters. Its average density is 4.6 g/cm³. Zircon or zirconium silicate mineral represented as ZrSiO4 or ZrO2.SiO2 have been used as opacifiers in the refractory and traditional ceramics industry for many years due to their purity and high quality, their chemical and physical properties, and the technological developments resulting from the researches. It is the main source of Zirconium Dioxide (ZrO2) used in the production of zirconate products. Zircon bricks are used in glass furnaces and inner coatings of crucibles. Zircon compositions are used in tandish nozzles because of their excellent spalling and corrosion resistance, and non-wetting properties with molten steel. Zircon have properties such as good thermal shock resistance, high erosion resistance, dimensional stability and high resistance to molten metal penetration. For this reason, it's preferred over other refractories in metallurgical industry and various ceramic industries.

Zirconia is a refractory material with high temperature resistance and excellent insulation properties. Especially it has high corrosion resistance against basic slags in continuous casting. Sintered zirconia has high stability against temperature and chemical effects. Crucibles and firing boats produced from sintered dense Zirconium Dioxide (ZrO2) can be used up to 2500 °C. Depending on the type of initiator component, the melting temperature of stabilized Zirconium Dioxide (ZrO2) is lower than pure Zirconium Dioxide (ZrO2).

A similar martensitic transformation in the metals takes place in zirconia. When monoclinic zirconia crystals are heated to the transformation temperature, tetragonal phase domains begin to form in the monoclinic matrix. A significant amount of stress energy is generated during the conversion. Because domain boundaries are tied together and there is a significant volume difference between those two phases. The transformation of the monoclinic zirconia proceeds with small movements (small distance between atoms) in the plane of the oxygen atom (100). The tetragonal phase is unstable above the critical crystal size of about 300 A°. If monoclinic and tetragonal phases are present together in critical crystal size of 300 A°, their free energies must be equal.

The transformation of zirconia from tetragonal to monoclinic and the increase in strength and toughness of ceramic materials is a well-known phenomenon. Although zirconia toughness has been applied to many ceramics, the most studied system is zirconia toughened alumina. The addition of zirconia as the second component to the sintered alumina causes the final grain size of the alumina to be small. In addition, the second phase in grain boundaries prevents simultaneous grain growth during plastic deformation at high temperatures.

Invesment Casting (Precision Casting, Low wax casting) is the processes of coating with the ceramic around the meltable waxy pieces at the appropriate temperature, then firing the ceramics and transferring molten metal into the ceramic mold cavity vocated by the wax melting. The other name is "Low wax casting". First, the model made from wax or plastic is covered by refractory mud that hardens at room temperature. When heated, the wax melts away. In this method, as many models as the number of parts to be produced are required, wax and plastic-like models are used. Models are produced with a metal mold injection of wax or plastic and many of models are arranged in bunches connected to a common path. Even the most complex parts model can be produced very quickly by using those methods. Mixed shaped parts, which are generally difficult to manufacture in machines, are produced by this method. After casting it can be used almost without any further processing. According to other casting methods, it gives opportunity to obtain high dimensional precision and smoother surface. Casting with this method has started to be widely used in recent years.

Waste sand formed after Invesment Casting (Precision Casting, Low Wax Casting) is processed by the use in certain dimensions in order to obtain optimum density in chamotte refractory and be grinded and/or sieved if necessary to obtain optimum size determination. Considering the chemical content of the chamotte bricks, they are mixed at different ratios with other raw materials used for the bricks during the production of the chamotte refractory bricks. The amount of Zirconium Dioxide (ZrO2) in the waste sand will also be taken into consideration when the brick prescription is adjusted. Silicon Dioxide (S1O2) and Aluminum Oxide (AI2O3), which are the two basic oxides for chamotte bricks, will come from waste sand, and utilization ratio of chamotte raw material will also decrease. The proportion of the weight of waste sand, obtined after invesment (precision casting, low wax casting) casting, to the amount of raw materials to be used in the refractory brick will be maximum 50%.

Claims

1. Chamotte refractory brick, characterized by; the presence of waste sand formed after precision casting (low wax molding).