The steel industry

The use of mass spectrometers can bring the following benefits to steel production: optimisation of the steelmaking process, maximisation of production output, minimisation of the use of process gases such as oxygen and argon, operational safety, and reduction of exhaust emissions. In the steel industry, the use of process mass spectrometers for gas analysis encompasses: coke ovens, blast furnaces, direct reduction furnaces, oxygen top-blown converters, electric arc furnaces, argon-oxygen decarburisation converters, vacuum oxygen decarburisation furnaces and cyclic vacuum decarburisation furnaces. Furthermore, process mass spectrometry can also be used to characterise the combustion properties of fuels employed in the steel industry.

Mass spectrometers for gas analysis in steelmaking as a means of process control.

Applications in steelworks include:

Blast furnace production process

Primary steel production process

Secondary steel production process

Electric arc furnace steelmaking process

Combustible gas treatment process

Analysis of high-purity hydrogen in steelworks reduction processes

Blast furnace gas analysis:

The fundamental process in most steelmaking involves the reduction of iron ore (iron oxides) to iron using coke (carbon) in a blast furnace, as shown by the following equation:

FeO + C → Fe + CO

Monitoring of CO and CO₂ is used to control coke consumption; monitoring of H₂ enables early warning, which aids in blast furnace maintenance; and monitoring of N₂ helps to increase iron yield. Coke is also used as fuel in the blast furnace process.

Gas Analysis in Refining Processes:

Applied in processes such as Vacuum Oxidation Decarburisation (VOD), argon oxidation decarburisation (AOD), and double-tube circulating vacuum degassing (RH). By analysing the composition of tail gas and combining this with continuous measurement of CO and CO₂ levels during the decarburisation process, precise control of residual carbon content in molten steel is achieved, thereby effectively improving steel output and quality whilst reducing consumption. Coke produced in coking plants is used in blast furnaces to reduce iron ore (iron oxides) into iron.

In oxygen-top-blown converters, iron is converted into steel by removing carbon and other impurities using oxygen. Further refining takes place during the AOD, VOD or RH processes. Continuous casting transforms the steel into steel ingots, which are then processed into hot-rolled coils and heavy plates. Hot rolling mills melt steel to produce various products such as steel sheets, steel strips, steel bars and steel coils.

Another method is direct reduction, which produces iron from iron ore and natural gas. The product is used in electric arc furnaces for steelmaking. In the electric arc furnace process, the main raw material is scrap steel.

Key analytes: CO, CO₂, H₂, N2, O₂, Ar, methane, ethane, propane, etc.

Features of the ProC-TOF online mass spectrometer:

Capable of multi-component analysis; most installed systems routinely measure 4–8 different components simultaneously. Multi-flow path analysis is also possible, with switching between flow paths achieved simply by using a multi-port injection valve.

Maintenance requirements for the mass spectrometer are minimal, with normal uptime reaching >95 per cent.

Fast analysis speed

Multi-component measurement – all components to be analysed can be measured on a single instrument

Multi-flow path measurement – enables the measurement of multiple sample streams on the same instrument

High analytical performance: extremely high sensitivity, stability and a wide linear range

The economic value and benefits derived from gas component analysis and plant data control far exceed the initial investment

BOF (Basic Oxygen Converter) steelmaking

The fundamental process of steelmaking involves desulphurisation and reducing the carbon content to the required level.

The oxygen converter steelmaking process, also known as BOS, BOF and LD, typically reduces the carbon content from 4 per cent to 0.05 per cent. A vertical cold oxygen lance is inserted into the furnace from the top at supersonic speed; the oxygen reacts with the carbon to form CO and CO₂, a process that lasts approximately 30 minutes. Gas analysis is used to control the operation of the oxygen lance to improve efficiency and determine the end point of the process. The benefits are clear: process control is improved, and re-melting—which would otherwise result in metal oxidation—is avoided.

Representation and typical data for PMS on a BOF unit

Analytical speed is particularly important during BOF monitoring.

AOD (Argon-Oxygen Decarburisation)

Argon-oxygen decarburisation is a process used to produce stainless steel, i.e. steel containing approximately 17 per cent chromium and less than 0.02 per cent carbon. In addition to stainless steel, the AOD process is also used to produce most forged and cast grades of ferroalloys. Successful commercial production of silicon, tool, nickel-based, cobalt-based, military-specification and other special-purpose alloys has also been achieved using the AOD process.

A typical raw material charge in an AOD vessel, or ‘charge’, contains 1.5 per cent carbon and 18.5 per cent chromium. The aim of this process is to reduce the carbon content through oxygen decarburisation without incurring a loss of chromium. If decarburisation were achieved solely by oxygen blowing, the chromium content would decrease to approximately 3 per cent as the carbon level is reduced from 1.5 per cent to 0.02 per cent. Chromium oxidation depends on the partial pressure of carbon monoxide and temperature; therefore, process conditions are maintained to control the CO partial pressure and the exothermic reaction.

The key to the AOD process is the dilution of the oxygen used for blowing with argon to reduce the partial pressure of carbon monoxide released. As the oxygen is gradually diluted with argon, this controls the temperature rise within the vessel and ensures that virtually all the chromium present in the initial charge is retained, whilst the carbon is removed to the required level. Rapid and accurate gas analysis is crucial for monitoring the gas flow during each blowing cycle.

Many of the benefits of converter gas applications also apply to the AOD process. Prior to the use of process mass spectrometry, it was difficult to control this process effectively. Operators would judge the end point visually—by observing the colour of the flame—which is not a very objective method. This frequently resulted in re-melting. Another approach was to always use a longer blowing time to ensure the end point was reached, but this was inefficient and resulted in poorer product quality due to chromium oxidation. However, another method involves interrupting the process to take multiple samples—which reduces productivity.

When using the ProC-TOF process mass spectrometer to control the AOD process, the return on investment for the instrument has been calculated at 3–6 months.

Basic Oxygen Process (BOP)

Process mass spectrometry can be used to improve the Basic Oxygen Process (BOP) and successfully produce high-quality stainless steel. The BOP is relied upon to calculate and control the residual carbon content in liquid metal batches. The BOP utilises oxygen to burn off excess carbon, converting it into carbon monoxide and carbon dioxide. The percentage of carbon removed is calculated using measured concentrations of carbon monoxide and carbon dioxide, along with gas flow rates. As the gas flows extremely rapidly, calculations must be performed as quickly as possible; mass spectrometers are well-suited to this task.

Gas Blending Station

Steelworks produce more than just steel; the process also generates usable fuel gas. During steel processing, waste gas streams containing varying levels of combustible gases can be combusted to produce usable fuel. However, to ensure the quality of the fuel gas, it is important to understand more than just the BTU value. The precise composition of the combustible components is crucial for process control. As these streams are recycled process streams, their composition varies considerably. Various waste streams and tail gas are blended at the gas blending station to ensure high-quality fuel. Given the very high gas throughput at the gas blending station, it is essential to perform a complete compositional analysis within a matter of seconds.

DRI (Direct Reduction)

Another method of steelmaking is direct reduction, a process that does not require coke; instead, iron ore is reduced using CO and H₂.

3Fe₂O₃+9CO-=6Fe+9CO₂

3Fe₂O₃ + 9H₂ – = 6Fe + 9H₂O

Reducing gases such as CO and H₂ are produced from natural gas (CH₄) and as by-products of the direct reduction process.

CH₄ + CO₂ → 2CO + 2H₂

CH₄ + H₂O → CO + 3H₂

These gas reactions take place within the reactor.

Natural gas is also used as a fuel in the process. Some direct reduction steelworks use high-hydrocarbon fuels, or even oil or coal, in place of natural gas; the underlying principles remain the same. The scarcity of coke for blast furnaces, combined with the low cost of natural gas and other fuels, has led many ironworks to adopt the direct reduction process.

As can be seen from the key reactions in the direct reduction ironmaking process described above, this process requires precise online monitoring of the gas streams H₂, CH₄, CO and CO₂.

The ProC-TOF mass spectrometer provides rapid and accurate gas analysis, whilst replacing other analytical methods such as infrared spectroscopy and gas chromatography. Controlling the DRI process actually requires monitoring the gas composition in at least six sample streams. The standard gas analysis samples are listed below.

EAF electric arc furnace steelmaking

The electric arc furnace is filled with various mixtures of feedstock, which are heated at a power of 100 megawatts to produce the final product. During the steelmaking process, oxygen is injected to raise the temperature, and substances such as limestone are used to purify the steel and reduce its carbon content. The ProC-TOF mass spectrometer monitors the entire process and is capable of:

Improve oxygen utilisation efficiency

Reduce electricity consumption

Ensure safe operation

Electric arc furnaces utilise electrical and chemical energy to convert scrap into steel. The amount of oxygen blown in via the oxygen lance is critical for controlling the combustion of CO into CO₂. This is because unoxidised CO burns at the exhaust outlet, resulting in a loss of thermal energy. The oxidation process in an electric arc furnace takes 1–2 hours, and the gases emitted from the furnace contain H₂, CH₄, CO, N₂, O₂, Ar, CO₂ and H₂O. The primary purpose of gas analysis is to provide data for controlling the end point of oxidation, thereby minimising CO losses—hence the need for precise monitoring of CO₂ and CO. Monitoring the formation of H₂ caused by cooling water entering the furnace is also crucial, as this can result in incalculable energy losses.

Reheating furnace

Reheating furnaces are used to melt steel in order to produce various steel products—such as plates, bars, rods, coils, sheets, tubes, pipes and other shapes—in various rolling mills. The fuel gas for reheating furnaces is typically a mixture of different gases, such as coke oven gas, blast furnace gas and converter gas. Rapid and comprehensive gas analysis is required to optimise the blending of these gases, ensuring that the mixed gas supplied via the gas mixing station maintains a constant calorific value. Even when the gases are not blended, there will still be considerable variation in their composition and, consequently, in their combustion characteristics, such as calorific value, density, specific gravity, the Wobbe index and stoichiometric air requirement.

To maintain consistent burner performance and maximise efficiency, these combustion characteristics need to be accurately monitored. This is where mass spectrometry offers a significant advantage over other monitoring methods: the analysis is comprehensive—everything is measured—whilst being faster.

Pure Oxygen Top-Blown Converter Steelmaking

When using the pure oxygen top-blown converter steelmaking process, a mass spectrometer can easily and accurately measure the concentrations of CO, CO₂, N₂ and O₂. After the raw steel is discharged from the blast furnace, its carbon content is reduced from 4 per cent to 0.2 per cent. Pure oxygen is injected directly into the molten steel at 3,000 degrees Celsius, oxidising the carbon in the melt to produce gaseous CO and CO₂. The decarburisation reaction lasts for approximately 20 minutes. During this process, a mass spectrometer can precisely measure the concentrations of O₂, CO and CO₂. In this decarburisation step, fluxing agents are often added to remove other impurities from the molten steel along with the trapped nitrogen.

Gas impurity removal process

Many large steelworks require substantial quantities of gases during production, including N₂, O₂, argon and other gases separated from air. Air is passed through molecular sieves and water to remove carbon dioxide and nitrogen oxides. Subsequently, a mixture of dry nitrogen, argon and O₂ enters the distillation unit where it is ultimately separated. The ProC-TOF mass spectrometer plays a vital role throughout the gas mixture separation and purification process—or even the entire process—meaning that the mass spectrometer is not only used to monitor the steelmaking process but also to continuously monitor pollutant emissions. For example, the mass spectrometer can detect the presence of nitrogen during the separation of pure oxygen.

Flammable Gas Treatment Process

The flammable gas treatment process is quite complex; in theory, a gas analyser can be used to obtain information on the flammable gases involved in this process and to predict their combustion efficiency. The ideal outcome is for all numerical data to be recorded continuously and comprehensively, such as the continuous Wobbe index, gas density, calorific value, complete gas composition and gas volume. A mass spectrometer is fully capable of undertaking this task, as it can rapidly and continuously provide data on gases such as CO, CO₂, H₂, CH₄, N₂, O₂, C₂H₆, C₃H₈, C₄H₁₀, C₆H₆ and other gases. This comprehensive gas composition data, incorporating the Wobbe index, enables better control and predictive analysis of gas combustion efficiency and gas consumption.

Advantages of mass spectrometry over other analytical methods

Gas chromatography

Gas chromatography is a reliable and precise method for online gas analysis; its main drawback is its slow analysis speed—typically requiring 5–30 minutes per run—which is clearly too slow for process control in the steel industry. Another drawback is its inability to distinguish between certain gases, such as argon and oxygen.

Infrared analysis

This technique is also slower than process mass spectrometry and can only analyse gas components containing two or more atoms (i.e. molecules with a dipole moment), such as CO and CO₂. Consequently, this analytical method also has its limitations.

Thermal Conductivity Analysis

The thermal conductivity method is used to analyse hydrogen, but its accuracy and linearity are inferior to those of process mass spectrometry.

Laser Methods

Laser methods have recently emerged for gas analysis. They utilise embedded installation and incorporate built-in calibration based on the attenuation of light by particles; consequently, they require no sample pretreatment and are more cost-effective than process mass spectrometry. However, laser analysis typically only measures one or two components, and the range of measurable components is also very limited; it is therefore a relatively expensive technology.

Other

A number of other single-point analysers are also used in the steel industry, such as oxygen analysers and lead acetate test strips for hydrogen sulphide, which provide high-sensitivity and reliable analysis of specific samples and are used only under specific requirements.

Compared with other instruments, the main advantages of the ProC-TOF process mass spectrometer are:

It provides rapid and multi-component analysis, with a single instrument capable of measuring all components to be analysed in the process. Process mass spectrometry not only analyses gas components in the process but also provides further information for process control. For example, by using a tracer gas—introducing helium at a known flow rate and then measuring its concentration—the flow rate of the entire gas stream can be calculated. In the decarburisation process, for instance, the amount of decarburisation can be precisely calculated. Rapid online gas analysis using mass spectrometry is widely applied in the steel industry to achieve better process control, reduce costs, ensure products meet specifications, determine end points and improve processes.