Although readers in the US and Canada may associate the name ‘furnace’ with central heating systems, this interpretation will not be discussed in this article. Instead, we will be focusing on furnace technologies and their roles within the ferrous metals industry, for steel production for example.
The earliest recorded furnace applications (circa 2500-1900 BC) were for making ceramic items but in the 21st century, metallurgical furnace technologies outnumber all others. It seems most logical to categorise furnace technologies by the industrial activities that they serve.
The reverberatory furnace
Specifically constructed to prevent direct contact between the charged material and the fuel, but not the combustion gases, this furnace type consists of two adjacent, closed chambers.
Solid fuel like wood, coal or coke is burnt in the first chamber or firebox, allowing the flame to blow across the charge and reverberate back off the arched roof and rear of the second chamber, before escaping via the flue opening.
Heat transfer is maximised by forcing the flame to take the longest possible path through the chamber.
In the steelmaking industry this furnace type was used in the open hearth process that is now obsolete, but the technology is still used in the production of copper, tin and nickel.
Chemical reactions take place between the charge and the combustion gases and also between the charge components.
This furnace is charged through openings in the roof, with slag formed from flux and impurities floating on top of the molten metal to overflow at one end, while the molten matte is tapped at intervals from the deepest part of the ore bath - making the process continuous.
The cupola furnace
First used around 1720 and for many years almost universally in iron foundries, the cupola has now largely been replaced by electric induction melting.
Constructed in vertically orientated, refractory lined steel shells of widely varying dimensions to suit the scale of operation, cupola furnaces resemble large smokestacks and are supported on legs with hinged doors below the hearth - to facilitate cleaning and repairs, cupolas are often fitted with a hood to arrest sparks and fume.
The bottom lining is usually made of sand and clay compressed against the bottom doors and slopes towards the tapping hole, while tuyeres are fitted above the hearth to supply air for combustion. A charging hole is provided in the side about halfway up the height of the cylinder.
Cupola sizes are quoted by the diameter ranging typically from 450mm to 2000mm to provide continuous melting capacity from 1 to 30 tonnes per hour. Cooling jackets and oxygen injection to increase combustion temperature are sometimes fitted.
After starting a wood fire in the hearth, coke is charged on top of the fire and ignited by increasing the air flow until the burning coke bed reaches the desired height. Alternating layers of pig iron and scrap steel, flux and coke are then added-up to the height of the charge doors.
The hot combustion gases percolate up through the charge causing numerous chemical reactions, while molten metal trickles down to collect in the hearth and later tapped into a ladle or receiver.
The cupola’s popularity can be attributed to the simplicity of its construction, high melting rate, ease of start-up and shut-down, flexible campaign life, comparatively high fuel efficiency and low operating cost. These were offset however, by its inherently poor control of harmful emissions and particulate matter.
The blast furnace
This technology initiates the process of extracting steel from iron oxide ore and although it first appeared in the 14th century, has survived well into the 21st century as its size and efficiency still enables the production of hot metal at competitive costs.
Iron ore in the form of raw oxide (Hematite or Magnetite), pellets or sinter containing between 50-70% iron is charged directly into the top of the blast furnace after blending with coke and limestone.
Not unlike cupola furnaces in basic design, the blast furnace is a counter current reactor, where solids descend and gases ascend, however it is far larger reaching heights of about ten stories and melting capacities around 13,000 tonnes per day (tpd).
As with the cupola, fuel and metal are mixed together in the same chamber. The difference implied in the name lies mainly in the air supply, which is preheated in large blast stoves before injection to temperatures reaching 1300°C, and this is sufficient to ignite the coke in the charge.
The hearth has a crucible in which molten pig iron accumulates together with slag before tapping.
As the combustion gases rise through the charged material above, iron oxide ore is reduced and descends after 6-8 hours as impure, high carbon, molten pig iron.
When the blast air reaches the top, 6-8 seconds after entering through the tuyeres, it is hot, dirty and laden with particulates and has gained significant calorific value.
This blast furnace gas is cooled and cleaned to provide fuel gas for the preheating stoves.
Once started, the process will run continuously for 5-6 years, or until the refractory lining needs replacement. Countries having no native coal resource have developed variations, such as the Swedish electric blast furnace.
The basic oxygen furnace
The Basic Oxygen Furnace (BOF) is so named because of the ph of the refractories - calcium oxide and magnesium oxide - that line the vessel to withstand the high temperature of molten metal. Typically with a capacity of 250-400 tonnes, the bath depth is about 1-2 metres and only about 10% of the vertical vessel is filled with molten metal.
The refractory lined vessel is supported on motorised trunnions to facilitate tilting it while charging, sampling and pouring. Around 70% of the world’s steel is produced using BOF technology.
This large-scale process takes place very rapidly converting a 350 tonne charge into steel in about 40 minutes.
The BOF has no heat source, being charged first with up to 30% scrap steel and at least 70% hot metal from the blast furnace. The BOF steelmaking process injects high purity oxygen at a pressure of over 1MPa, either through a top lance into the molten bath, or through bottom tuyeres.
The carbon content of pig iron at about 4-5% reacts exothermically with oxygen raising the temperature to around 1700°C, while silicon and manganese in the iron are also removed.
Fluxes are added during the blow to reduce the sulphur and phosphorous levels and these impurities are absorbed by molten slag that floats on the molten bath.
The electric arc furnace
Heat is generated by an electric arc either between carbon electrodes and the charged metal in direct arc technology, or between the carbon electrodes in indirect arc technology.
The Electric Arc Furnace (EAF) usually consists of a refractory lined, steel shell orientated vertically in AC powered, direct arc furnaces and horizontally in DC powered, indirect arc furnaces. Highly scalable, these furnaces are very popular in steel foundries in the range of 1-10 tonnes per batch.
Laboratory units are made for melting only a few grams, while large units can melt over 100 tonnes per batch.
Negative aspects of EAF include adverse environmental effects caused by noise, dust, off-gases, pollution from electric power generation and disturbances to the power grid that affect other consumers.
Direct arc furnaces usually have a back door for addition of alloying elements, oxygen lancing and slag removal and are tiltable to allow pouring from the front pouring spout.
They are favoured for high melting rates, high pouring temperatures and well controlled melt chemistry. Indirect furnaces are limited to smaller sizes by their single phase DC power and popular for the production of copper based alloys.
Recuperative technology is applied mainly to fuel-fired furnaces and transfers heat from the flue gas flow that would otherwise be wasted, either to the charge materials or the incoming combustion air in order to improve fuel efficiency.
Another important technology to achieve this is oxygen enrichment and this also reduces the production of undesirable nitrogen oxides, by reducing or eliminating nitrogen flow through the furnace. Oxygen technology has been successfully applied on many types of furnaces.
The crucible furnace
These sealable, refractory pot furnaces made of high-grade fire-clay or graphite, were formerly used in the steel industry and directly heated by fire.
The induction furnace
This technology applies high voltage alternating electric current to a hollow section, water-cooled, heavy duty, helically wound primary coil to induce strong eddy currents in the metal charge, thus transferring energy in the form of heat.
Turbulence induced once the metal has melted produces a stirring action that aids mixing of alloying additions and homogenises the temperature throughout the furnace. Varying the primary frequency controls the amount of power available relative to the turbulence produced.
Coreless induction furnace technology has largely replaced the crucible furnace, especially for melting of high melting point alloys including all grades of steel, iron and many non-ferrous alloys. Accurate temperature control and chemical stability make this the ideal technology for re-melting and alloying.
Channel induction furnaces employ an iron cored primary coil and are commonly used for melting low melting point alloys and as holding and superheating units, for higher melting point alloys like cast iron.
Heat treatment furnaces
Many processes used in the manufacture of metal parts or components require heat treatment, usually under a controlled atmosphere.
Furnace Brazing, Sintering, Carburizing, Nitriding, Annealing, and Case Hardening are applied to modify the crystalline structure of metal components, fuse parts made from compressed metal powder, or to join them into composite assemblies.
Typically these operations are performed in electrically heated furnaces with varying degrees of automation, from fully manual temperature control to microprocessor controlled systems that process parts through a preset temperature cycle, while controlling the furnace atmosphere that could consist of hydrogen, ammonia, argon, nitrogen, specific gas blends, or a total vacuum.
Temperature capabilities are limited by the type of electric element employed with nickel/chromium alloy operating up to 1150°C, silicon carbide elements reaching 1350°C, and still higher ranges requiring molybdenum elements that operate only in a pure hydrogen atmosphere.
Continuous furnaces typically transport the parts through preheat, hot and cooling zones on a wire mesh belt but for the higher temperature ranges Walking Beam furnaces are preferred.
Batch operating furnaces of several types are popular too depending on the process parameters and these are usually based on water cooled cold wall construction with an inner heat resistant alloy container or retort, to retain the protective atmosphere and internal electric elements.
Either vertically or horizontally oriented these units maintain external temperatures at or below room temperature.
Similar furnace technology is employed in the electronics industry for manufacturing circuit boards and soldering discreet components onto these. High strength composite materials like carbon fibre and Kevlar are also cured in vacuum furnaces at a controlled temperature during the manufacture of safety helmets, racing and sports car bodies and competitive cycle frames.
The sealed quench is a batch furnace in which the heating and quenching chambers are combined in a single unit. The two chambers are separated by a refractory-lined door which can be opened to allow the hot charge to be transferred from the heating chamber to the cooling chamber.
Laboratory furnaces are available in various sizes that operate up to 3000°C, with hot zones constructed from graphite and rapid heat-up times of around 30 minutes. Inert, high purity helium is the required atmosphere for these extreme temperatures to avoid ionisation that occurs with argon.
Ceramic, frit and glass furnaces
Batch or continuous furnaces for making frit required for enamelling and glazing often use special oil or gas fired burners that use pure oxygen instead of air, to raise the flame temperature, increase productivity and improve fuel economy.
Specialised Fibre Drawing Furnaces operate at 2400°C in the process of drawing glass preforms into optical fibre for the communications industry.
Float Glass Furnaces melt pre-blended raw materials silica sand, calcium oxide, salt cake and cullet at 1500°C and raise its temperature to 2900°C using gas-fired air-fuel or oxy-fuel burners so that the molten glass can be floated in a continuous ribbon onto a bath of molten tin, that ensures a perfectly flat surface.
The ribbon thickness can be controlled between 2mm and 12mm and is cooled gradually, according to precise temperature/time gradient profiles as it is transported through the float furnace and into the annealing chambers called lehrs.
EAF technology has proved to be a low specific energy process for the production of steel long products like structural steel rod, bar and wire because it is possible to use 100% steel scrap feedstock.
Many mini-mills using EAF steelmaking have been established at barely 20% of the cost per tonne of annual installed capacity, compared with integrated steel mills using blast furnace and BOF technology. Where hot metal is available from a blast furnace this, or direct-reduced iron, can be charged into the EAF instead of scrap.