WO2014053189A1

WO2014053189A1 - Lost wax process and calcination furnace therefor

Info

Publication number: WO2014053189A1
Application number: PCT/EP2012/069753
Authority: WO
Inventors: Ricardo MORALES
Original assignee: L'air Liquide Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude; Air Liquide Brasil Ltda
Priority date: 2012-10-05
Filing date: 2012-10-05
Publication date: 2014-04-10

Abstract

Lost-wax metal casting process and calcination furnace therefor, whereby black smoke is reduced or prevented during calcination of dewaxed ceramic casting shells (20) in the calcination furnace, which is heated by the combustion of fuel with oxidizer therein, by injecting additional oxygen-rich oxidizing fluid in the calcination chamber (10) or in the flue gas stack (80) of the calcination furnace during at least part of the calcination step.

Description

LOST WAX PROCESS AND CALCINATION FURNACE THEREFOR

The present invention relates to the lost-wax process for precision casting of metal pieces.

The lost-wax process is used in artistic and industrial casting of metal pieces, both for the casting of a single piece and for the casting of larger numbers of a part. The process is used for a wide range of metals such as steel, aluminium, bronze and numerous alloys. The process is particularly suited for the casting of complex metal parts. Metal parts produced by the lost-wax process typically weigh up to 50 kg.

The lost wax process typically comprises the following steps (illustrated hereafter by an example for the casting of several copies of a more or less intricate part, such as an engine or a machine assembly part):

1) Preparation of the die

Metal dies are produced in a shape corresponding to the pieces to be cast. However, the dimensions of the dies are corrected to take into account the contraction and/or expansion of the materials used in the lost-wax process.

2) Making of wax models

"Wax models" or "wax patterns" of pieces to be cast are made by filling the assembled dies or moulds with "wax" and then removing the wax models from the dies, whereby, in the context of the lost-wax process, the term "wax" refers to a mixture of solid materials, such as carnouba wax, paraffin, tar and thermoplastic resins, which melt when heated.

3) Spruing

The wax models are sprued with a tree-like wax structure known as the "wax pattern tree" that will provide for flow paths for the molten metal into the mold or shell and escape paths for air from the mold or shell. The wax pattern tree generally comprises a sprue cup, which usually widens towards the open end of the pattern tree. One or more wax models may be mounted on any one wax pattern tree.

4} Coating

Before coating the wax pattern tree, it is cleaned to eliminate grease and other mould residues.

The cleaned wax pattern tree is coated with ceramic shell material, for example by dipping the tree into a silica slurry, allowing it to drain, then covering it with dry refractory material of controlled grain size and allowing the dipped and covered wax pattern tree to dry and harden, whereby the temperature and the humidity during drying and hardening are controlled to guarantee dimensional reproducibility of the parts. This process is repeated until the required thickness and strength of the ceramic shell is achieved.

For an optimum surface finish of the cast metal pieces, a smaller granulometry dry refractory material will be used for the first layer or layers applied onto the tree which, during casting, come into contact with the molten metal.

5) Dewaxing

After the ceramic shell has been completed and dried, it is put into an autoclave with the sprue cup at the bottom opening downwardly.

In the autoclave, which may for example be a steam autoclave, the wax is melted and allowed to drain from the interior of the shell via the sprue cup.

The molten wax is generally recovered and recycled in a subsequent lost-wax process.

The temperature (typically from 100°C to 120°C) and pressure in the autoclave are controlled to avoid any deformation of the shell.

6) Calcination

The dewaxed ceramic shell is then taken to a calcination furnace to be calcined and thereby to acquire the necessary mechanical resistance to support the liquid metal during the casting step. Calcination of the ceramic shell takes place at a high temperature, referred to as "calcination temperature", which depends on the ceramic material and is typically from 1000°C to 1100°C.

At these elevated temperatures, any wax residues which remained within the shell after the dewaxing step decompose and escape from the shell.

7) Casting

Molten metal is poured into the calcined ceramic shell, typically via a sprue cup, to fill the voids therein and allowed to solidify.

Prior to the pouring of the molten metal, the shell is usually preheated to prevent or substantially reduce the thermal shock to the shell.

8) Release and finishing

The ceramic shell is broken to reveal the cast metal parts and the metal sprues are cut off.

The metal gates and risers produced by the sprues are removed from the metal parts, which are usually further subjected to finishing steps, such as grinding and sandblasting, and/or structural and mechanical tests. In known processes, black smoke is observed during the calcination step due to the decomposition of wax residues. Said black smoke is evacuated from the calcination furnace through its flue gas stack together with the combustion flue gas generated by the combustion process used for heating the calcination furnace.

There are increasingly stringent environmental regulations for the release of flue gas into the environment.

It is, in particular, frequently prohibited to release black smoke into the atmosphere, so that additional flue gas treatment equipment and processes are required to remove said black smoke from the flue gases of the calcination furnace before they can be released into the atmosphere via the flue gas stack.

These additional equipments and processes for removing black smoke from the flue gases substantially increase the costs of the lost-wax process and the complexity of the installation.

The present invention aims to at least partially overcome the above disadvantages by providing a simple and reliable method for cost effectively eliminating or significantly reducing the presence of any black smoke in the flue gases.

In accordance with the invention, the presence of black smoke in the flue gases of the calcination furnace is effectively reduced or even eliminated, so that additional equipment and processes removing black smoke from the flue gases are no longer necessary.

In accordance with the invention, this is achieved by injecting supplementary or additional oxygen in a controlled manner into the calcination furnace during at least part of the calcination step.

The present invention thus relates to a lost-wax metal casting process comprising a step of calcining a dewaxed ceramic casting shell in a calcination chamber of a furnace. During said calcination step, the calcination chamber is heated to at least the calcination temperature of the ceramic casting shell by one or more burners. Said burners combust a fuel with an oxidizer, thereby generating heat and flue gas inside the calcination chamber. The flue gas is evacuated from the calcination chamber via a flue gas stack. According to the present invention, an additional oxidizing fluid having an oxygen content of between 22%vol and 100%vol is injected into the calcination chamber or near the calcination chamber in the flue gas stack in addition to the oxidizer for combusting the fuel and this during at least part of the calcination.

In the present context, the "calcination temperature" refers to the minimum temperature at which the calcination reactions take place in the shell, thereby giving the shell the required mechanical resistance. The exact calcination temperature depends in particular on the composition of the uncalcined dewaxed shell. The calcination temperature of the ceramic shell is typically at least 1000°C or more.

The additional oxidizing fluid advantageously has an oxygen content of between 30%vol and 100%vol, preferably between 50%vol and 100%vol, and more preferably between 80%vol and 100%vol.

It will be appreciated that the overall lost-wax process also comprises the other process steps of preparation of the dies, the making of wax models; etc. as described in points 1 to 5, 7 and 8 above (step 6 being the calcination step).

By thus injecting oxidizing fluid into the calcination chamber during at least part of the calcination step, in addition to the oxidizer for fuel combustion, not only is the fuel used for heating the calcination furnace completely or substantially completely combustion, but the partial decomposition products of any residual wax which had remained in the dewaxed shell are also completely or substantially completely combusted within the calcination chamber or in the flue gas stack, so that no black smoke is present in the flue gases and no specific treatment of the flue gases to remove black smoke therefrom is required. Indeed, said partial decomposition products of residual wax are the main or even the only source of black smoke in the calcination chamber. In the present context, the expressions "wax partial decomposition products" and "partial wax decomposition products" are used to identify wax decomposition products which may further be oxidized to C0₂ and H₂0, for example by means of combustion with oxygen.

The additional oxidizing fluid is preferably injected into the calcination chamber itself, rather than into the flue gas stack, so that the heat generated by the combustion of the residual wax partial decomposition products with the additional oxidizing fluid is released within the calcination chamber and thus contributes to the calcination process.

The additional oxidizing fluid is typically injected in gaseous form, but may be stored and/or transported to the calcination chamber in liquid form.

The additional oxidizing fluid may have the same composition as the oxidizer for fuel combustion or may have a different composition. Preferably, the additional oxidizing fluid has a higher oxygen content than the oxidizer for fuel combustion. This latter embodiment is particularly advantageous when the oxidizer for fuel combustion is air.

The additional oxidizing fluid may be injected by means of a separate lance for the oxidizing fluid. Alternatively, or in combination therewith, the additional oxidizing fluid may also be injected into the calcination chamber by at least one of the one or more burners of the calcination chamber.

In the latter case, the additional oxidizing fluid may be injected into the calcination chamber by mixing the oxidizer for combusting the fuel with the oxidizing fluid in or upstream of the at least one burner.

The calcination step may be a compound step consisting of several successive sub-steps. The calcination step of the process according to the invention may in particular comprise three successive sub-steps:

a first sub-step, which follows the introduction of the dewaxed ceramic casting shell into the calcination chamber, during which the temperature in the calcination chamber decreases (with respect to the initial temperature of the calcination chamber), a second sub-step in which the temperature in the calcination chamber increases until it reaches the calcination temperature of the ceramic casting shell, and

a third sub-step in which the temperature in the calcination chamber is maintained at or above the calcination temperature of the ceramic casting shell.

The calcination step of the process according to the invention may, for example, be as follows: a first sub-step during which the temperature in the calcination chamber decreases and may even reach a lower limit of 760°C or less,

a second sub-step in which the temperature in the calcination chamber increases from said lower limit to 1000°C, and

a third sub-step during which the temperature in the calcination chamber is maintained at 1000°C or more.

According to a first embodiment of the invention, the additional oxidizing fluid is injected into the calcination chamber from the first sub step until the last one.

However, the operator of the furnace may wish to limit the consumption of the additional oxidizing fluid by injecting the additional oxidizing fluid during only part of the calcination step.

The operator may base his selection of the part of the calcination step during which additional oxidizing fluid is to be injected on experience gained in previous calcination steps. It may, for example, be opportune to inject the additional oxidizing fluid only during the first sub-step or only during the first and second sub-steps of the calcination step.

The operator may also inject the additional oxidizing fluid starting from the point at which a certain temperature is reached in the calcination chamber and/or for a particular duration. The operator may also vary the flow rate at which the additional oxidizing fluid is injected during the calcination step or parts thereof. This experience-based approach is more particularly possible in the case of repeated lost-wax casting processes for the production of the same metal part or parts with substantially identical casting shells, but will generally not be possible in the case of varying shapes, sizes and/or numbers of the metal part or parts to be cast or in case of casting shells of varying ceramic materials.

It is therefore preferred to equip the calcination chamber and/or the flue gas stack with detectors or sensors capable of determining when the injection of additional oxidizing fluid is required to prevent or substantially prevent the formation of black smoke.

According to an embodiment of the process of the invention an optical detection method is used.

For example, the opacity of the atmosphere in the calcination chamber and/or of the flue gas entering the flue gas stack may be detected and additional oxidizing fluid is injected into when an increase in opacity is detected or when the detected opacity reaches or exceeds a predetermined level of opacity. The opacity of the furnace atmosphere or of the flue gas can for example be determined by directing a laser beam through the atmosphere or through the flue gas and by measuring the degree to which the light of the laser beam is absorbed by the atmosphere or flue gas.

Alternative optical detection methods involve the use of an infrared detector or sensor or of an ultraviolet detector or sensor.

For example, an appropriately positioned infrared detector or sensor may be used to detect temperature changes or variations in the calcination chamber (typically in the vicinity of the dewaxed shell or shells) or in the flue gas stack due to the presence of residual wax partial decomposition products in the furnace atmosphere or in the flue gas. An appropriately positioned ultraviolet detector or sensor can be used to detect local combustion in the calcination chamber or in the flue gas stack due to the presence of residual wax partial decomposition products in the furnace atmosphere or in the flue gas. In industrial furnaces in which a large number of dewaxed shells may be simultaneously calcined, it is frequently more practical to apply the infrared and/or ultraviolet detection methods to the flue gas in the flue gas stack.

For example, the flue gas stack may comprise an air gap in the vicinity of the calcination chamber. In that case, an ultraviolet or infrared detector may be used to detect local combustion, respectively a temperature increase in the flue gas stack downstream of said air gap due to the presence of residual wax partial decomposition products in the flue gas and additional oxidizing gas may be injected into the calcination chamber when the sensor detects such local combustion or temperature rise.

A further option is to use sensors for detecting the concentration of combustible substances, such as for example CO, in the flue gas in the flue gas stack, which are indicative of the presence of partial decomposition products in the flue gas, and to inject additional oxidizing gas into the calcination chamber when the detected concentration of combustible substances in the flue gas reaches or exceeds a predetermined level.

Preferably, the oxidizing fluid is an oxygen-rich oxidizing fluid, i.e. it has an oxygen concentration higher than the oxygen concentration of air. In that case, the oxidizing fluid advantageously has an oxygen content between 22%vol and 100%vol, preferably between 75%vol and 100%vol, more preferably between 80%vol and 100%vol and most preferably between 90%vol and 100%vol. The higher the oxygen concentration in the oxidizing gas, the higher the efficiency of the substantial or complete prevention of black smoke in the flue gas leaving the stack and the smaller the variations in the flue gas volume arising from the injection of the additional oxidizing gas.

An additional advantage of using an oxygen-rich oxidizing fluid is that thereby it is possible to substantially or completely prevent the occurrence of black smoke in the flue gas without a substantial increase in the flue gas flow leaving the calcination furnace, which could lead to an increased loss of thermal energy from the calcination chamber via the flue gases, pressure variations in the calcination chamber and/or problems in the downstream flue gas system. Indeed, air consists for 79%vol of inert ballast gas and combustion with air therefore generates more flue gas than combustion with an oxygen-rich oxidizer. In this regard, the higher oxygen content ranges are preferred for the oxidizing fluid.

The present invention also relates to a furnace adapted for the implementation of the calcination of a dewaxed ceramic casting shell in accordance with the process of the present invention.

The present invention thus relates to a furnace for the calcination of dewaxed ceramic casting shells. The furnace comprises:

• a calcination chamber equipped with one or more burners for heating the calcination chamber by combusting a fuel with an oxidizer,

• a flue gas stack for evacuating flue gas from the calcination chamber. According to the invention, the calcination chamber is further equipped with at least one injector for injecting an oxidizing fluid into the calcination chamber or near the calcination chamber into the flue gas stack, whereby said injector is connected to a source of an oxidizing fluid having an oxygen content of between 22%vol and 100%vol.

The injector is preferably positioned for the injection of oxidizing gas into the calcination chamber.

Said injector may, for example, be an injection lance or may be integrated in at least one of the burners. In the latter case, the oxidizing fluid injector may correspond to an oxidizer injector of the burner or one of the burners.

The furnace of the invention preferably also comprises a detector or sensor for the detection of the presence or level partial decomposition products of wax in the calcination chamber atmosphere or in the flue gas stack.

The detector or sensor is advantageously an optical sensor. The detector or sensor may be an opacity sensor, in particular an opacity sensor comprising a laser device. The detector or sensor may also be an ultraviolet sensor or detector or an infrared sensor or detector.

The detector or sensor is advantageously mounted in the flue gas stack.

If the flue gas stack comprises an air gap in the vicinity of the calcination chamber, an ultraviolet or an infrared sensor or detector is usefully mounted in the flue gas stack downstream of the air gap.

The furnace preferably comprises a central processing unit for the regulation of the injection of oxidizing gas into the calcination chamber or, near the calcination chamber, into the flue gas stack by means of the injector, whereby said central processing unit is connected to the detector or sensor for the detection of partial wax decomposition products.

The present invention is further illustrated in the following example with reference to figures 1 and 2 which are schematic perspective representations of furnaces for the calcination of dewaxed casting shells in accordance with the present invention.

The furnace comprises a calcination chamber into which a number of dewaxed casting shells 20 for the casting of multiple metal parts have been placed on top of a grid 11 with downward facing sprue cups 21.

A burner 30 is mounted in side wall 12 of the calcination chamber. Said burner 30 comprises a fuel supply line 31 and a combustion-oxidizer supply line 32 so that, in operation, burner 30 generates a flame 40 inside calcination chamber 10 so as to heat said calcination chamber 10 to temperatures of at least 1000°C in order to calcine the dewaxed casting shells 20. Although only a single burner is shown, the calcination chamber 10 may comprise several burners, each with a fuel and oxidizer supply line.

The operation of the burner 30 and the supply of fuel and oxidizer to the burner 30 is controlled by a central processing unit 50.

A temperature detector 60, such as a thermocouple, measures the temperature in the calcination chamber and the measured temperature is supplied to the central processing unit 50 for the optimization of the bumer operation so as to achieve the desired temperature and temperature evolution inside the calcination chamber.

The flue gas 70 generated by the combustion of fuel and oxidizer in the calcination chamber is evacuated via flue gas stack 80 for subsequent release into the atmosphere.

In the embodiment shown in figure 1 , stack 80 comprises an air gap 81 in the vicinity of the calcination chamber. Ambiant air is sucked into stack 80 by the flue gas 70 flowing thereto.

When residual wax which remained in the dewaxed shells 20 after the dewaxing steps decomposes in the calcination chamber 10, the decomposition products are entrained in the form of black smoke by the flue gas into stack 80. Due to the high temperature of the flue gas 70 as it leaves the calcination chamber 10, some partial wax decomposition products, if present in the flue gas 70, combust when they come into contact with air at the air gap 81. Stack 80 is equipped with an ultraviolet or infrared sensor 82 which detects the combustion of partial wax decomposition products inside the stack 80 and sends the corresponding signal to the central processing unit 50. Preferably, sensor 82 detects the intensity of said combustion of partial wax decomposition products inside the stack 80.

An injection lance 100 is further mounted in wall 12 of the calcination chamber 10. Said lance is connected via pipeline 101 to a source 110 of liquid oxygen.

When sensor 82 detects combustion of partial wax decomposition products downstream of air gap 81, central processing unit 50 opens valve 102 so as to inject a controlled amount of oxygen into the calcination chamber 10, whereby said oxygen causes the complete combustion of the partial wax decomposition products, thereby generating additional heat in the calcination chamber 10 and preventing the presence of partial wax decomposition products in the flue gas leaving stack 80. When sensor 82 no longer detects combustion of partial wax decomposition products downstream of air gap 81, the central processing unit closes valve 102, thereby interrupting the injection of oxygen into the calcination chamber 10 through lance 100. In the embodiment shown in figure 2, flue gas stack 80 does not comprise an air gap. Instead, the presence of partial wax decomposition products in flue gas 70 are detected by a light absorption detection device 83 which measures the opacity of the flue gas flowing through the stack 80.

In said embodiment, supply line 101 connects the oxygen tank 1 10 to the combustion oxidizer supply line of the burner 30.

When the detected degree of light absorption by the flue gas exceeds a predetermined lower limit, the central processing opens valve 102 for the controlled oxygen enrichment of the combustion oxidizer, which is usually air and the partial wax decomposition products burn inside the calcination chamber 10 with the excess oxygen thus injected into the calcination chamber 10 by burner 30. As a consequence thereof, the light absorption in flue gas stack 80 again decreases, after which the central processing unit closes valve 102 thereby halting the oxygen enrichment of the combustion oxidizer.

The present invention thus provides an easy and cost-effective method for eliminating or substantially preventing the presence of black smoke in the flue gas leaving the calcination chamber 10 via stack 80, so that said flue gas generally remains within the limits of environmental regulations regarding the release of flue gas into the atmosphere.

Claims

1) Lost-wax metal casting process comprising a step of calcining a dewaxed ceramic casting shell (20) in a calcination chamber (10) of a furnace, whereby said calcination chamber (10) is heated to at least the calcination temperature of the ceramic casting shell by one or more burners (30) combusting a fuel with an oxygen-containing oxidizer thereby generating heat and flue gas (70) inside the calcination chamber (10), whereby the flue gas (70) is evacuated from the calcination chamber (10) via a flue gas stack (80), and whereby, during at least part of the calcination process, an oxidizing fluid having an oxygen content between 22%vol and 100%vol is injected into the calcination chamber (10) or near the calcination chamber (10) in the flue gas stack (80) in addition to the oxidizer for combusting the fuel.

2) Process according to claim 1 , whereby the oxidizing fluid is injected into the calcination chamber (10).

3) Process according to claims 1 or 2, whereby the oxidizing fluid is injected by means of a lance (100) for the oxidizing fluid. 4) Process according to claim 2, whereby the oxidizing fluid is injected into the calcination chamber (10) by at least one of the one or more burners (30).

5) Process according to claim 4, whereby the oxidizing fluid is injected into the calcination chamber (10) by enriching the oxidizer for combusting the fuel with the oxidizing fluid.

6) Process according to any one of the preceding claims, whereby a detector or sensor (82, 83) detects the presence or level of partial wax decomposition products in the calcination chamber atmosphere or in the flue gas stack (80) and oxidizing fluid is injected when the detector or sensor (82, 83) detects the presence or a minimum level of partial wax decomposition products. 7) Process according to claim 6, whereby oxidizing fluid is injected when the detector or sensor (82, 83) detects a level of partial wax decomposition products equal to or higher than a predetermined limit. 8) Process according to any one of claims 1 to 4, whereby the CO content of the calcination chamber atmosphere and/or of the flue gas in the flue gas stack (80) is detected and whereby the oxidizing fluid is injected when the detected CO content exceeds a predetermined limit.

9) Process according to any one of the preceding claims, whereby the oxidizing fluid has an oxygen content of at least 30%vol, preferably of at least 80%vol, more preferably of at least

100%vol.

10) Furnace for the calcination of dewaxed ceramic casting shells (20), the furnace comprising a calcination chamber (10) equipped with one or more burners (30) for heating the calcination chamber (10) by combusting a fuel with an oxidizer, a flue gas stack (80) for evacuating flue gas (70) from the calcination chamber (10), whereby the calcination chamber (10) is further equipped with at least one injector for injecting an oxidizing fluid into the calcination chamber (10) or in the flue gas stack (80) near the calcination chamber (10) and whereby said injector is connected to a source of an oxidizing fluid having an oxygen content of at least 22%vol.

1 1) Furnace according to claim 10, whereby the injector is adapted for injecting an oxidizing fluid into the calcination chamber (10).

12) Furnace according to claim 10 or 11 , whereby the injector is an injection lance (100).

13) Furnace according to claim 10 or 11 , whereby the injector is integrated in at least one of the burners (30).

14) Furnace according to any one of claims 10 to 13, further comprising a detector or sensor (82, 83) for detecting the presence or level of partial wax decomposition products in the combustion chamber atmosphere or in the flue gas stack (80). 15) Furnace according to claim 14, further comprising a central processing unit connected to the detector or sensor (82, 83) and adapted for controlling the flow of oxidizing fluid through the at least one injector.