Development of a 30Kg Aluminium Oil-Fired Crucible Furnace Using Locally Sourced Materials

A crucible furnace is a piece of equipment used in the foundry industry for melting metals for casting operations. This research work focuses on the design of a 30-kilogram capacity aluminium crucible furnace that is fired with diesel fuel. The furnace drum has an overall combustion capacity of 0.1404m 3 . It is fitted with a chimney to allow for the easy escape of combustion gases. The air blower discharges air into the furnace at the rate of 0.3m 3 /s with an air/fuel ratio of 400:1. The aluminium crucible furnace is designed to consume 4 litres of diesel fuel with a rating of 139000kJ/gallon which is required to completely melt 30-kilogram of aluminium over a period of 18 min. The designed operation temperature range of the aluminium crucible furnace is 500°C to 800°C. The cost of the aluminium crucible furnace is one hundred and eighty-two thousand nine hundred naira (N182, 900.00).


Introduction
Technological advancement of a nation depends solely on its capability to harness and convert its useful mineral resources into required finished products (Osarenwinda, 2015). In the processing of mineral resources such as metals, melting and extraction have become the critical industrial practices which are mostly carried out in a furnace. A furnace is equipment used to melt metals for casting or to heat materials to change their shape or properties (heat treatment) (Rajiv Garg, 2006;Furnace, 2015). Furnaces are usually classified according to the purpose for which the material is heated, nature of the transfer of heat to the material, method of firing the furnace and method of handling material through the furnace (Schlesinger et al., 1998). They are refractory lined vessels that contain the material to be melted and provide the energy to melt it (Wikkipedia, 2015a). Based on the method of generating heat, furnaces are broadly classified into two types namely combustion type (using fuels) and electric type (Abed, 2013). The combustion type furnace can be broadly classified as oil fired, coal fired or gas fired (Wikkipedia, 2015b). Modern furnace types include electric arc furnaces (EAF), induction furnaces, cupolas, reverberatory, and crucible furnaces. Furnace choice is dependent on the alloy system and quantities produced. Most aluminum foundries use either an electric resistance or gas heated crucible furnaces or reverberatory furnaces. The oil fired crucible furnace uses the combustion of diesel as a fire source to heat the crucible and melt the solid metal inside it. Some of the advantages of oil fired crucible furnace are low investment costs, easy operation and maintenance ability, capable of melting small batches of various alloys, the melt can be treated directly in the crucible and the alloy can be quickly and easily replaced as necessary (Mastrukov, 1986). As important as oil fired crucible furnaces to the foundry and manufacturing process, its availability seem to be limited in Nigeria and when available most of them are imported and this is costing the country huge sum of foreign exchange.
Previous contributions on furnaces were described by Oyawale and Olawale (2011) in the development of a Mini-Electric Arc Furnace to melt 5kg of steel/cast iron scraps, using locally produced Soderberg electrodes. Performance evaluation on the furnace revealed that it required 60 minutes to heat up the furnace to the range of 1150°C -1400°C. Furthermore, about 95 minutes was required to melt the first charge of 2kg, resulting in a melting rate of 21.05g/minute. Alaneme and Olanrewaju (2010), also developed a diesel fired heat-treatment furnace using locally sourced materials which was targeted at disposing the use of heating elements requiring electric power which is poorly supplied in the country. Evaluation of the furnace was observed to have a fast heating rate of 61.24°C /min to attain a pre-set temperature of 900°C and a fuel consumption rate less than 1.41 litres/hr. According to Ramazan (2007), an electrical furnace with an automatic control to fire ceramic products was also developed. The aim of study was to develop a 30 Kg oil fired crucible furnace using locally sourced materials.

Material Selection
The choice of materials for the development of the furnace was based on the following engineering requirements: a) Weldability: This is the ability of the material to be welded b) Toughness: This is the ability of the material to withstand shock and absorb energy due to impact. c) Fatigue: This is the ability of the material to withstand cyclic stresses. d) Ductility: This is the ability for the material to be drawn into wire. e) Durability: This is the ability to stay strong and in good condition over a long period of time f) Availability: This is the state of the material readily accessible. The mild steel plate used for fabricating most components of the furnace is ductile, thus making it possible for it to be rolled, folded and bent without cracks or fractures.
The under listed materials were specified for the design of the diesel fired crucible furnace. a) Mild steel plate (3mm) b) 2 mm thick (ϕ 45) mild steel pipe c) Gate Valve d) Kaolin sand e) Sodium silicate f) Air blower g) Flexible hose The selected materials for the different components of the furnace are as stated in Table 1. Air pipe resistant to corrosion and heat mild steel 6 furnace insulator/ lining good resistant to heat flow per unit thickness refractory mixture of sodium silicate and kaolin using sawdust and water as binder 7 Fuel tank 2mm thick mild steel bucket The furnace was fuel fired using diesel with slight kaolin in the combustion chamber.

Method of Construction
The 30kg aluminium crucible furnace was designed majorly to melt aluminium and other metals whose melting temperatures falls within its designed operation temperature range of 800°C below. Some of the equipment used in fabricating the various parts of the furnace are as follows: (a) Folding/rolling machine (b) Drilling machine (c) Welding machine (d) Cutting tools (e) Marking/Measuring tools etc.
The major components of the 30kg aluminium crucible furnace are as follows: (i) The Furnace Drum (ii) The furnace cover (iii) The Air Blower (iv) Fire bricks (v) Combustion chamber (vi) Furnace Cover Opening/Closing Mechanism.

Fabrication of the Furnace Drum
The furnace drum was made from a mild steel plate of 3mm thickness by folding the mild steel plate into a cylindrical shape of 800mm diameter with the aid of a rolling/folding machine. A circular mild steel plate of diameter 800mm was cut using a cutting machine and subsequently welded to the bottom of the folded drum with the aid of an arc welding machine.

Fabrication of the Furnace Cover
The cover was made from a 3mm mild steel plate. The mild steel plate was cut with oxyacetylene flame and thereafter welded with an arc welding machine using gauge 12 electrodes. The cover was filled with a refractory mixture comprising of sodium silicate, kaolin, sawdust and water to prevent or reduce the amount of heat loss. In order to firmly secure the insulating materials to the cover, pieces of rods were welded underneath the bottom part of the cover to hold the refractory mixture.

Lining the Furnace Wall with Bricks
The inner surface of the furnace drum was lined round with a double layer of bricks of about 230mm thickness using the refractory mixture comprising of sodium silicate, kaolin, sawdust and water as a binder to fill the spaces in between bricks in order for them to hold firmly together. The base of the furnace was lined with double layer of bricks of about 230mm thickness using the same mixture of kaolin, sodium silicate, sawdust with water as binder.

Fabrication of the Combustion Chamber
This is the space under the crucible pot in the furnace unit meant for burning fuel in order to produce heat for the melting operation. It consists of the burner and its housing, designed to withstand high temperature and pressure developed during the process of combustion. To reduce loss of heat through the furnace walls, the inner and outer walls are separated using refractory materials made of sodium trioxosilicate coupled with kaolin sand. A blower is incorporated to provide sufficient air into the chamber for complete combustion of the fuel.

Fabrication of the Diesel Tank
The diesel tank was made from a mild steel plate of 2mm thickness. A rectangular shaped piece with dimensions of 800mm by 600mm were cut out of the mild steel plate with the aid of a cutting machine and thereafter welded together to form a cylindrical-shaped structure. Ahole of about 80mm diameter was bored at the base of the tank on one side and a pipe of about 12mm diameter was welded to it, to serve as fuel supply line to the furnace

Assembling of Parts and Finished Operation
After all the parts were fabricated, the next step was coupling the furnace drum and cover. Thereafter, the combustion chamber was integrated into the furnace drum. The nozzle was at this point welded to the furnace drum. The air blower which was bought already made was subsequently connected to the nozzle. Lastly the pipe from the diesel fuel tank was linked to the furnace. The entire welded joints were deslagged and thereafter grinded/polished to ensure a smooth finish. Grinding was done with a hand grinding machine. After grinding a first stage painting of the outside body with antirust paint was done and was followed by a second stage and final painting with a black colored paint.

Working Principle of the Crucible Furnace
The furnace is first and foremost preheated before firing it by igniting combustible materials such as coal in the combustible chamber. While the coal is still burning the valves that control the supply of diesel fuel on the diesel fuel supply pipe and air from the air blower through the nozzle respectively are slightly opened to allow in drops of fuel and air under pressure. The air is blown over the fuel to atomize as well as oxidize it for combustion. As the mixture of air and fuel blows over the pre-lit coal it helps to sustain the combustion. As this continues over time, the temperature rises gradually within and around the crucible, thereby melting its content.
The furnace temperature can be read directly from a thermocouple inserted into the furnace through the cover. When the crucible content is fully melted and is ready for pouring, the crucible is lifted out by means of a lifting tong, which is handled by two persons and then poured into the prepared mould cavity. The holes on the sides of the furnace are made to keep a balance between the pressure within and outside the system.

Regulation of the Furnace Temperature
The temperature of the furnace was regulated by simply controlling the air/fuel mixture content. It is done by opening or closing slightly the control valves of either or both of air and diesel fuel, to regulate the air-fuel ratio or the amount of air and diesel fuel entering the combustion chamber. The furnace temperature was determined during melting operations by incorporation of a thermocouple in the furnace designed to enable the temperature of the metal during melting to be monitored.

Design Analysis of the Furnace Drum
The furnace drum was made from a 3mm thick mild steel plate rolled into a cylinder of diameter of 800mm and height 800mm with the overall combustion space of diameter 560mm and height 570mm.
The Detailed dimensions of the furnace drum are as follows: a) Height of the furnace drum before laying bricks (h) = 800mm b) Height of combustible space of the furnace drum after laying of bricks (h 1 ) = 570mm c) Internal diameter of the furnace drum before laying of bricks (d) = 790mm d) Internal diameter of the furnace drum after laying of bricks (d 1 ) = 560mm e) Inlet diameter of the burner nozzle = 102mm f) Outlet diameter of the burner nozzle = 35mm g) Height of the cover = 98mm h) Total height of the drum = height of drum + height of cover = 800 + 98 = 898mm. i) Diameter of the chimney hole (on cover) = 60mm j) Thickness of the metal plate = 3mm Therefore Combustible Volume of furnace after laying of bricks:

Design Analysis of the Air Blower
The air blower is rated as follows; Outlet pressure = 1700Pa, (from blower) Speed = 2850 rpm Power = 0.55 KW Voltage = 220 V Current = 2.5 Amperes Average rate of air flow from blower = 18m 3 /min Average rate of fuel flow = 0.045m 3 /min Ratio of air discharge to fuel discharge = 18/0.45 = 400:1 (2)

Performance Evaluation of the Furnace
The furnace was fired and the time taken to raise the temperature to 660°C was 15 minutes. Thereafter, the 30 kg of aluminium scrap was placed in it.  The total amount of energy consumed in the furnace is calculated by multiplying the number of litres of fuel by the energy content per litres of fuel used.
Energy content of fuel is rated 139000 KJ/gallon (1 gallon = 4. 6 Litres) British Standard Therefore, energy content of the fuel is 13900 30217.

Discussion of Results
From the results obtained above, the efficiency of the furnace was calculated to be 26.5%, which falls within the efficiency range of conventional furnace, showing that most of the heat generated in the furnace was actually used in the melting of the metal.
A crucible furnace of 30kg melting capacity has been designed in line with the set objective of the research work. When compared with the efficiency of 25% obtainable from the conventional crucible furnace, it can be said to be over 80% efficient as well as effective, and can conveniently replace the conventional imported crucible furnace in line with the Federal Government import substitution policy in order to conserve foreign exchange. Moreover, with design temperature range of 500°C to 800°C, aluminium can be conveniently melted using the 30kg melting furnace.

Safety Precaution
There must not be any leakage in fuel along the length of the hose carrying fuel to the combustion chamber. After the melting has been completed, the gate valve that is releasing fuel to the combustion chamber should be tightly locked. All these are meant to avoid fire explosion. Table 5. Summary of BEME.