Gas furnaces for melting aluminum: selection criteria

Homemade furnace for melting aluminum at home, made by a craftsman with his own hands: detailed photo instructions for making a crucible furnace.

As you know, aluminum is often used by DIYers to make various homemade parts. The average melting point of aluminum is 650–660 ℃, so this metal can be melted at home, cast pigs and blanks for parts.

The manufacturing process of a crucible furnace for melting aluminum is presented in detail in the photo.

Basic types of reverberatory furnaces

The basic types of gas reverberatory furnaces for melting aluminum are [1]:

Dry hearth reverberatory furnaces, in which the metal is heated on an inlet inclined hearth before being melted (Figure 1).
Wet bath reverberatory furnaces , in which metal is loaded directly into the aluminum melt, usually without preheating (Figure 2).
Shaft smelting furnaces (stack (shaft) furnaces) are characterized by increased energy efficiency, as they use the heat of exhaust gases to heat the loaded aluminum charge (Figure 3). Shaft furnaces are a modified version of reverberatory furnaces

Figure 1 – Gas reverberatory melting furnace with dry hearth [2]

Figure 2 – Bath-type gas reverberatory melting furnace [3]

Figure 3 – Gas shaft melting furnace [4]

Another type of gas furnace is crucible furnace, which is used for small casting volumes, as well as for special alloys. These furnaces require separate consideration.

A simple coal forge A forge for heating and melting metals in a couple of hours.

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#1 romka10.06

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Group: Users
Posts: 270
Registration: 27 May 11

While cleaning up the yard after renovating the house, I collected two dozen red bricks, and I had a desire to try to make a forge, especially since I have long dreamed of mastering aluminum casting, and now I need to put the old brick somewhere. We have earth - just dig and the clay will go, well, I dug, dug out a lump of clay, kneaded it in a basin with some water, stacked two bricks in length, two more bricks in parallel at a distance of 100 mm, it turned out like a corridor, put a thick layer of clay on top and pressed across the corridor there are 6 thick nails with a gap of 5 mm, and the grate is ready! There were four more bricks in the well for this entire structure, it turned out to be a firebox, one end of the corridor (now it has become a ashpit and part-time ash pit) was covered with half a brick, the other with a cooler from a burnt computer power supply. I took an old battery from UPS, connected the cooler, threw in coal that is sold for barbecue, a piece of newspaper under the bottom and set it on fire. It will hum and flare up! I built up the well with four more bricks, and covered one on top like a lid, everything inside heated up to a light yellow glow. I threw an aluminum spoon there, it didn’t last even 30 seconds, it quickly shrunk and flowed away! I haven’t felt such delight for a long time. In an hour, such a cool thing came out of all the trash. Unfortunately, I couldn’t take a photo; my hands were always covered in clay or soot. This inspired me to build a more solid structure, yesterday I went to the nearest building materials store, bought 10 pieces of stove bricks, 10 fireclay bricks, fireproof clay and a cast iron grate, for only 600 rubles. I put a new forge on a piece of flat slate, took an old fan, asked a friend for a bucket of coal (wood is expensive and melts quickly, right before our eyes) and lit the whole thing, well, of course it’s burning, where should it go. I spent two hours on everything about everything, including going to the store. Here is the fan, it’s too small, I plugged the gap on the side with a piece of paper to seal it

I took a piece of an old car shock absorber, the one with the bottom, stuffed it with aluminum wire, aluminum radiators from a computer power supply (where I got the fan from) and shoved it inside, covering the sides with the remains of a brick.

When the whole thing melted and shone with a bright yellow light (it warmed up for about 10 minutes), I dug a hole in the wet clay with my finger. Of course, I understand that this is not the right thing to do, you need to cook aluminum with fluxes, and the crucible should be sterile inside and coated with liquid glass and chalk. , and the form should be dried and heated to at least 200 degrees. Just in case, I put on an old jacket, thick mittens, glasses (liquid metal + wet clay = flying metal splashes, I did this on lead as a child), and poured it at arm’s length the contents of the improvised crucible into the hole.

To my surprise, the pouring took place without any ejection of metal, it boiled a little and everything froze, so I waited a minute and let’s pick it out with a nail - Figurines, everything froze on the outside, and when I tried to lift the casting, it slid off the nail in a nasty wrinkled lump. Well, I cooled the matter and rather, use an angle grinder - a cut to see where they are, these difficult-to-remove casting pores with which lovers of aluminum casting unsuccessfully struggle. I didn’t see anything particularly scary with the naked eye; the metal was quite suitable for my needs! https://rutube.ru/tracks/5729089.html

Now I will try to cast something worthwhile, maybe a cylinder for a steam engine. Once done I will post pictures.

So, I recommend to all DIYers living in the private sector or at the dacha to build a forge, there are no costs, but the benefits can be considerable, how often do you need to cast a body, or forge a knife - harden, anneal the hardened steel. And how much pleasure there is in the process itself, there’s nothing to say here!

A forge is equipment that allows you to heat metal to a temperature of 1200 degrees. Its dimensions vary widely. Some are large and stationary, others are small and portable.

Read also: DIY wire feed mechanism for a semi-automatic machine

You can make a forge with your own hands of any design, which will work on both solid and gaseous fuel.

Additional functions of gas furnaces

Each of these three basic oven types may have additional options, as well as elements common to other oven types, such as:

all three types of furnaces can have external wells for loading crushed charge, for example, shavings, into the melt;
inclined hearth furnaces and shaft furnaces may have additional loading windows for loading the charge directly into the melt;
bath furnaces and shaft furnaces can have a wide threshold or a short inclined hearth at the entrance to the furnace for preheating ingots and massive, for example, packaged scrap, as well as for the convenience of removing slag from the surface of the melt;
All types of furnaces can have a melt mixing system, both in the loading well and in the aluminum melt bath.

Useful tips

The melt must be hot so that the mold is filled evenly. It is poured when it acquires the consistency of mercury.
Castings with complex configurations are made quickly; they try to fill the molds right away so that the metal does not have time to set and no partitions or voids are formed.
Metal can be poured into plaster over wax or foam; high temperatures cause the wax and foam to burn out. The surface of the casting will be smooth.
To cool the casting, do not immerse it in water; the casting will crack.
When melting pure aluminum, the technology of using protective fluxes is used; they protect the metal from oxidation.

It is necessary to observe fire safety, the casting process involves the use of an open flame

It is important to use personal protective equipment: gloves, goggles

Dry hearth reverberatory melting furnaces

In dry hearth furnaces, the loaded aluminum charge is placed on an inclined hearth, which is located above the level of the molten metal (Figure 4). Hot exhaust gases pass through the charge and quickly heat it up. The charge melts and gradually flows from the inclined hearth into a chamber for accumulating molten metal, which is therefore often called a storage tank.

Figure 4 – Reflective melting furnace with a dry hearth. Heated ingots are visible at the entrance to the furnace [5]

Dry hearth ovens are usually used when:

Most of the loaded aluminum charge is melting ingots, including large ones, that is, the sheet elements have a minimal surface.
The aluminum charge contains, for example, steel elements, as well as residues from casting molds, such as sand castings. The aluminum part of the charge is melted on an inclined hearth and the aluminum flows down the inclined hearth, and foreign materials remain on the hearth and are removed from the furnace using scrapers.
The aluminum charge may contain water or be wet.

Advantages of dry hearth furnaces:

Safety. The entire charge is melted on an inclined hearth, and not under the surface of the melt. Such heating prevents water from entering the melt and the threat of explosion.
Preventing melt contamination. Allows you to remove foreign, non-aluminum materials from the charge that remain on the inclined hearth.
Ability to produce large volumes of liquid aluminum almost continuously.

Disadvantages of dry hearth furnaces:

Typically these stoves have two chimneys, one above the inclined hearth and the other above the boiler. This requires a burner control system for optimal operation [1].
High loss of aluminum (7-12%) when melting in a furnace atmosphere at a temperature of 700-760 degrees Celsius. Waste is the loss of aluminum metal contained in the loaded charge as a result of its oxidation and transformation into slag.
Low energy efficiency. The efficiency of furnaces of this type increases with increasing ratio of the volume of the loaded charge to its outer surface. Maximum efficiency is achieved when melting large aluminum ingots, sometimes weighing up to 500 kg [1].

General operating principle

With your own hands, using scrap materials, you can create an effective system that will allow you to carry out forging work. There are a huge number of types of designs that you create with your own hands. A professional forge has the following elements:

the table is made of fire-resistant material;
there is a firebox with a grate;
air chamber to improve fuel combustion efficiency;
air drainage with supply air duct;
valve for regulating air supply;
tent;
window for feeding workpieces;
umbrella;
chimney;
crucible;
hardening bath;
gas-air chamber.

An example of a furnace design for aluminum smelting

The type of forging equipment in question can be called a classic forge, which is used in blacksmithing. It is quite difficult to create it with your own hands, but you can create a simpler design for melting aluminum.

Bath type reverberatory melting furnaces

In these furnaces, the combustion products are in direct contact with the surface of the melt and heat transfer occurs through a combination of convection and radiation. These furnaces have a lower roof to maximize the use of radiant heat, the most efficient way to transfer heat into the aluminum. The heat source (burners) can be located at a distance of 45-60 mm from the aluminum melt mirror. The closer the heat source is to the surface of the melt, the faster the heat is transferred to it [1].

Figure 5 – Bath-type reverberatory melting furnace [6]

Bath-type reverberatory melting furnaces are used when:

The aluminum charge consists mainly of one alloy or related alloys, and changes in alloys occur rarely.
The charge can have a large surface, and its components can have different sizes, including small thicknesses, for example, scrap aluminum profiles, as well as packaged aluminum scrap.
Large volumes of molten aluminum are required.

Advantages of bath-type reverberatory furnaces:

The installation cost of such furnaces is the lowest compared to dry hearth and shaft furnaces.
All metal melts under the surface of the aluminum melt, which ensures a relatively low loss of aluminum metal of 2-5%.
Allows various additions depending on the conditions of specific foundries: a small under or threshold for preheating ingots or packaged scrap, a loading well for crushed scrap (Figure 5), mixing the melt, a window for easy degassing, etc.

Disadvantages of bathtub type stoves:

They have an increased ratio of the furnace capacity for molten metal to the melting speed of the furnace, which requires additional production space. A typical ratio is 8 to 1.
If melting ingots are used, they require preheating on the threshold or hearth before being loaded into the melt.
A lower oven roof requires extra attention when cleaning it.

Brick construction

It is better to make it yourself using scrap materials. A mini forge of 6 bricks is suitable for obtaining various parts that are used to decorate metal objects, buckets or other parts. The creation process can be divided into the following stages:

We create the body from fireclay bricks. Such a brick can withstand exposure to high temperatures for a long time. When choosing a brick, you should pay attention to the fact that it should not have damage or defects. This is because cracks will lead to heat loss.
We create the grid grate shelves from scraps of steel pipes. Steel pipes should be of small diameter. Fusible material options cannot be used.
The grate is made from steel strips, the thickness of which is 4-6 millimeters.
We twist the grate at an angle with a screw.
The fuel can be coal or coke.
Ignition is usually carried out using a blowtorch. You can also use a gas burner.
A window under the nozzle must be created when using a blowtorch. This is because the forge emits a lot of heat, which can cause the lamp tank to explode.

This type of design, which you can quickly create with your own hands, should be used exclusively outdoors, since there is no umbrella with a chimney. During smelting, a large amount of combustion products is released.

When considering the question of how to make a forge for melting aluminum, we note the possibility of creating portable versions. An example is a portable version of the track, which with additional elements are installed on a common base. The lining is represented by fireclay marl with fireclay sand. Another design element can be called an air supply system to the track, for which you can use, for example, a manual siren. It can run on coke or charcoal.

Shaft melting furnaces

Shaft smelting furnaces have higher energy efficiency due to better sealing of the furnace from the penetration of atmospheric air and the use of exhaust gases to preheat the aluminum charge.

The aluminum charge passes down the charging shaft and gradually reaches the melting zone, where it is melted by the burners and flows down into the storage tank. Hot exhaust gases pass from the melting zone through the shaft and heat the loaded charge.

Figure 6 – Shaft melting furnace [7]

Advantages of shaft furnaces

Thanks to preheating of the charge with combustion waste products, energy efficiency reaches 40-50%.
The finished aluminum melt has a reduced content of hydrogen and non-metallic inclusions. This is achieved by separating the melting zone and the preheating zone. Moisture and other pollutants evaporate or burn while still in the mine, and a dry charge with a minimum of polluting materials enters the melting zone.
The absence of excessive contact of the melt with atmospheric oxygen, which ensures a reduced level of waste of metallic aluminum (0.8-1.5%).

Disadvantages of shaft furnaces

In order for shaft furnaces to achieve low levels of waste and high energy efficiency, they must be operated strictly in accordance with the rules established for them, including the use of a sufficiently dense aluminum charge. With a lightweight and thin-walled charge, shaft furnaces may have increased aluminum waste and reduced energy efficiency.
Shaft furnaces generally have a fairly large height (about 6 m), which is not suitable for all industrial premises.
The refractory lining at the bottom of the charging well of a shaft furnace is subject to increased mechanical shock loads, which leads to the need for more frequent maintenance.
Shaft furnaces are more complex to operate than reverberatory furnaces and therefore require additional qualified personnel.

How the forge works

In order for a user to make a forge on his own, he first needs to understand the principle of its operation, so that he can subsequently work with what he has without losing quality.
There is a chemical reaction for the combustion of carbon: 2C + O2 = 2CO2 + 188.1 kcal. It is this that underlies the operation of any device. The reaction immediately shows that carbon is a fairly strong reducing agent, that is, it greedily combines with oxygen. This reducing ability of carbon is partly used in the forge to prevent oxidation of the workpiece, that is, to prevent the metal in the device from burning

Another very important property of carbon is its high calorific value. This property is that if you blow enough air into the fuel mass, which would be enough for carbon, you can burn it quite quickly, which will lead to more heat being released

This will allow it to develop a higher temperature.

To prevent oxidation of the metal, the blast in the forge should be adjusted so that the fuel is slightly lacking in oxygen. But it should be remembered that if the workpiece is overexposed, carburization will occur. This means that the metal will become overdried, that is, brittle.