Bessemer Converter Process Explained: Mass Steel Production

In 1876, Sydney Gilchrist Thomas found a solution to the phosphorus problem in Bessemer steel. Thomas discovered that by adding a basic flux such as lime to the Bessemer process, the lime draws phosphorus from the pig iron into the slag, allowing the unwanted element to be removed. However, the basic slag produced would have worn down the acidic refractory lining of the converter. To solve this, Thomas replaced the acidic lining with a basic lining. Together with Percy Gilchrist, they developed a basic-lined converter in which calcium dolomite was the refractory material. This innovation allowed the use of iron ores rich in phosphorus, which had previously been unusable for steel production.

The Bessemer process faced a critical limitation: high-phosphorus iron ores produced brittle steel unsuitable for critical applications. Most countries lacked access to low-phosphorus Swedish ores. Sidney Thomas and his cousin Gilchrist solved this in 1877 by lining converters with dolomite instead of silica-based refractories. When lime was added to molten iron, it reacted preferentially with the dolomite lining rather than phosphorus, allowing phosphorus to migrate into slag. This breakthrough enabled steel production from previously unusable iron ores worldwide. Combined with the Martin-Siemens open-hearth process, which offered superior quality control and enabled recycling of scrap metal, this solved the phosphorus problem and enabled global steel industry expansion.

The Thomas process solved the critical phosphorus problem in steel production by using basic refractory materials (dolomite or limestone) instead of acidic refractories. This allowed phosphorus to be chemically removed from molten steel, converting it into a form that could be sold as fertilizer. The process was particularly important for regions with high-phosphorus iron ore, such as Germany. It enabled the production of usable steel from previously unusable ore deposits and represented a major breakthrough in steelmaking technology.

The Thomas Converter, also known as the Thomas-Gilchrist process, is a modification of the Bessemer process developed to solve the phosphorus problem in iron. To achieve this, the converter uses a basic lining of lime or dolomite, which allows phosphorus to be eliminated from the metal during the process. This represents an improvement over the Bessemer process, enabling the use of iron with high phosphorus content. Key characteristics include being a variant of the Bessemer process, the ability to work with high-phosphorus iron ore, and the use of special basic lining. Applications were primarily in countries where iron ore contained high phosphorus levels for industrial steel production. Advantages include the ability to use previously unsuitable iron ore and increased steel production in various countries. Disadvantages include complicated process control, lower steel quality compared to modern processes, and current obsolescence.

Problem three was solved by Sidney Thomas and Percy Gilchrist who discovered that the composition of the converter lining was causing the problem. The original acidic lining worked well with high-quality phosphorus-free iron ore. Thomas-Gilchrist invented a basic lining using dolomite and magnesium bricks, which solved the problem of using phosphoric iron ore. Both the acid-lined and basic-lined furnaces removed impurities in about 18 minutes, with the basic lining furnace also removing phosphorus. Mushet's invention was critical for controlling the process timing to produce steel of specific carbon content.

The Second Industrial Revolution, occurring approximately from 1870 to the start of World War I, brought new technological accelerations. The production of steel—purified iron that was stronger, lighter, and more flexible than cast iron—was transformed by the Bessemer process. This invention involved forcing compressed air through molten iron in a pear-shaped converter lined with refractory bricks. The result was a controlled volcanic eruption—rain of sparks, deafening roar, and blinding white flame—as oxygen burned away impurities in minutes instead of days. Steel became the new skeleton of the world, enabling the construction of skyscrapers with steel frames that could reach unprecedented heights, railways with steel tracks lasting ten times longer than iron, and ships with steel hulls that were larger and faster. The world was being rebuilt, taller and stronger, on a foundation of steel.

The Second Industrial Revolution (Industry 2.0) emerged from the interplay between iron/steel production, railroads, and coal, where innovations like the hot blast furnace (James Beaumont Neilson, 1828), Bessemer steel process (Sir Henry Bessemer), Thomas-Gilchrist basic oxygen process (1878), and Siemens-Martin open-hearth furnace collectively enabled mass steel production, which then transformed transportation through durable steel rails (Robert Mushit, 1857) that outlasted iron rails by tenfold, creating a self-reinforcing cycle that drove industrialization forward.

By 1900, Bessemer and Thomas converters had reached peak use while the Siemens-Martin open hearth process dominated 90% of the world's steel market. Since the 1870s, steel production rose exponentially while prices fell to fractions of earlier costs. For the first time in history, large objects could be cast from steel—multiple tons at a time, quickly and cheaply—with precise alloy ratios. This enabled unprecedented possibilities: I-beams for skyscrapers, superheated steam engines small enough to pull trains, steel-armored battleships, and hardened steel gun barrels.

The steel industry became a fundamental engine of the Second Industrial Revolution, with the Martin-Siemens process (1865) enabling mass steel production. Steel began shaping urban landscapes of new metropolises and became the material for monuments to modernity: the Brooklyn Bridge (1883), the Eiffel Tower (1889), and ocean-crossing ships. This represented the material foundation of industrial civilization. The revolution also transformed transportation, with the movement of goods, people, and capital creating the first true globalization of the world economy. The term 'globalization' simply means stronger integration of the international economy, with increased movements of goods, services, and people across national boundaries.

The Second Industrial Revolution (late 19th century) transformed America through mass production technologies. Key innovations included the Bessemer process for cheaper steel production, enabling railroad expansion to 40,000 miles of track—more than all of Europe combined. This created a self-sustaining cycle where steel built railroads, which generated profits for more steel production. The revolution shifted manufacturing from small-scale craftsmanship to industrial-scale operations, laying foundations for modern urban economies and national market integration.

The Siemens-Martin open hearth furnace (1860s) used regenerative heating to achieve high temperatures, allowing flexible charging of pig iron, scrap, or combinations. Robert Dorn's basic oxygen process (mid-20th century) used pure oxygen instead of air, combining Bessemer speed with open hearth quality control. Paul Héroult's electric arc furnace (1907) enabled melting scrap steel directly with graphite electrodes, now accounting for over 30% of global production. These technologies progressively reduced costs and expanded applications from weapons to infrastructure.

Steel production employs two main techniques: Basic Oxygen Furnace (BOF) and Electric Arc Furnace. BOF loads scrap metal and molten pig iron into refractory converters, injecting oxygen, argon, and nitrogen under pressure through lances for approximately 45 minutes to produce liquid steel. Electric Arc Furnace uses three graphite electrodes applying 140 megavolts to melt scrap metal, iron briquettes, or pig iron. Both methods produce liquid steel poured into ladles where ferroalloys adjust chemical composition. Secondary metallurgy processes include ladle metallurgy, chemical reheating, and dual degassing to reduce hydrogen and carbon content. Liquid steel is then cast into slabs using continuous casting machines, passing through rollers with cooling systems until solidified, then cut to required dimensions.

EAF is the other main method of making steel, predominantly using cold steel scrap, making it one of the world's largest recycling processes. EAFs make up to 150 tons of steel in a single melt in less than an hour. The furnace is filled with recycled steel scrap, the roof swings into place, and three graphite electrodes are lowered. A powerful electric current creates an arc that melts the scrap. Lime and fluorspar are added, and carbon and oxygen are blown into the melt. Impurities combine to form slag. Once correct temperature and composition are reached, the steel is tapped off. Final adjustments to customer specifications are made by adding alloying elements. These furnaces provide precise control over composition and are used for producing alloy steels and stainless steels. Mini mills, small plants closely linked to casting and rolling facilities, often operate EAFs and produce specialist products near demand centers.

An electric arc furnace (EAF) is a primary steelmaking technology that melts scrap metal and direct reduced iron using electrical arcs generated by graphite electrodes, with the process involving charging phases where materials are added, melting phases where arcs heat the charge to temperatures up to 3000°C, and refining phases where oxygen injection removes impurities like silicon, sulfur, and phosphorus while forming a protective slag blanket; the furnace can operate with 100% scrap or 100% direct reduced iron, and evolved gases are recycled for preheating subsequent charges.

Steelmaking refines pig iron by reducing carbon, manganese, silicon, and other elements to achieve desired properties. Four furnace types exist: open hearth (replaced by more efficient methods), electric arc furnaces (using three graphite electrodes generating 1,925°C temperatures), vacuum furnaces (removing air to eliminate gas impurities and prevent oxidation), and basic oxygen furnaces (most common, using oxygen lances to oxidize impurities). Electric arc furnaces melt pig iron, scrap, carbon, and limestone within two hours. Basic oxygen furnaces charge 200 tons of pig iron and 90 tons of scrap, then blow pure oxygen through a water-cooled lance to oxidize impurities. The resulting molten steel is poured into ladles for further processing.