What is oxygen gas used for? Industrial medical and process uses with the purity each one needs

Oxygen is bought for one property: it makes oxidation faster. Air is 20.95 % oxygen by volume and the other 79 % is dead weight that must be heated, pumped and vented. Replace it with oxygen at 99.5 % and a furnace runs hotter, a cutting jet burns steel instead of merely heating it, a patient receives five times the partial pressure of oxygen per breath, and an aeration tank dissolves oxygen about five times as fast, so it can treat more organic load in the same volume. Every use below is a version of that one fact, and the only real variable is how pure the oxygen must be.

IS 309:2023, the Indian specification for compressed and liquid oxygen, replaced the old single figure with three grades. The grade designation follows the industry "N" convention: the first digit is the number of nines, the decimal is the next digit, so N2.5 means 99.5 % and N3.5 means 99.95 %.

Source: IS 309:2023 Table 1 and clause 4. Note 1 to the table adds that oxygen made by air separation shall not normally contain CO, CO₂ or hydrocarbons, so for air-separation product these are tested only by agreement between buyer and seller. KIGL industrial oxygen is supplied to Grade 2 (N2.5), minimum 99.5 %.

Medical oxygen sits outside this table altogether. IS 309 clause 1 excludes "oxygen for medical or inhalation purposes"; the governing document is the Indian Pharmacopoeia monograph, which asks for not less than 99.0 % v/v oxygen with carbon dioxide at 300 ppm maximum and carbon monoxide at 5 ppm maximum, filled under a drug licence. The difference is explained on industrial vs medical oxygen.

The basic oxygen furnace (BOF) is a pear-shaped vessel into which a water-cooled lance blows near-pure oxygen at supersonic velocity onto molten pig iron. Carbon in the iron burns to CO and CO₂, silicon, manganese and phosphorus go into the slag, and the heat of those reactions alone raises the melt from about 1,350 °C to tapping temperature with no external fuel. Electric arc furnaces use oxygen differently, through lances and oxy-fuel burners that speed the melt-down of scrap and refine the bath, but the gas is the same.

The volumes are large enough that integrated steelworks typically run their own cryogenic air separation units and pipe oxygen to the converter shop. Merchant oxygen in cylinders serves the foundries, re-rollers and fabricators around the works rather than the converter itself. Oxygen is also injected into blast-furnace blast and into cupolas: enriching the blast from 21 % to 23 or 24 % oxygen raises the flame temperature in the raceway and lets the furnace burn more injected coal per tonne of hot metal.

Oxy-fuel cutting is the use most buyers meet first, and it is widely misunderstood. The flame does not cut the steel. A preheat flame of acetylene and oxygen brings a spot on the plate to its ignition temperature, around 870 to 900 °C for mild steel, and then a separate jet of pure oxygen burns the iron to iron oxide. The reaction Fe + ½O₂ → FeO is strongly exothermic and supplies most of the heat that keeps the cut moving; the preheat flame only sustains the ignition temperature at the top of the kerf. This is why cutting oxygen purity matters so much more than preheat gas purity: nitrogen or argon in the jet dilutes the reaction, slows the cut and leaves a rougher, more adherent dross. The standard the trade works to is 99.5 % minimum, IS 309 Grade 2, and the full mechanism is on the oxy-acetylene welding and cutting page.

For the preheat and for welding, oxygen is paired with acetylene because the oxy-acetylene flame, at about 3,100 °C, is the hottest of the industrial fuel gases. The stoichiometric reaction 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O needs 2.5 volumes of oxygen per volume of acetylene, but a neutral welding flame is set at roughly 1:1 at the torch and draws the rest from the surrounding air in the outer envelope. A plate-cutting torch at 25 mm thickness is a modest consumer; a profile-cutting bay running several torches on a shift is the kind of load for which fabricators move from cylinders to a manifold or to an oxygen pipeline, and the cleanliness rules for that pipework are on oxygen pipeline cleanliness.

Related uses in the same workshop: oxy-fuel brazing and heating for bending, flame gouging, scarfing of billets in steel mills, and flame hardening of gears and ways.

Oxygen therapy raises the partial pressure of oxygen in the lungs. Air at sea level delivers about 21 kPa of oxygen partial pressure; breathing pure oxygen through a mask delivers up to five times that. Hospitals take it in three forms: cylinders of medical oxygen at 99.0 % or better to the Pharmacopoeia monograph, bulk liquid oxygen vaporised into a medical gas pipeline system, and on-site pressure-swing adsorption plants.

PSA plants produce 90 to 96 % oxygen with the balance argon and nitrogen, and the Pharmacopoeia covers this under a separate "Oxygen 93 per cent" monograph. The distinction matters when reading a tender: a hospital specifying "IP oxygen" for cylinders is asking for the 99.0 % product, while one installing a PSA oxygen plant is working to the 93 % monograph. How the adsorption process reaches that purity, and why it does not go higher without a second stage, is on how PSA oxygen generation works.

The cylinder colour difference is a legal one. Industrial oxygen cylinders are Graphite Black RAL 9011 all over under IS 4379:2021; medical cylinders are black with a white RAL 9003 shoulder under IS 3933:2021. A black cylinder with a white shoulder in a fabrication shop, or a plain black one on a ward, is a compliance failure.

Ozone is the strongest disinfectant in routine use for drinking water, bottled water and swimming pools, and it is generated by passing oxygen through a silent electrical discharge. Feeding the generator with oxygen rather than dried air roughly doubles the ozone concentration achievable per unit of power and removes the nitrogen oxides that air-fed generators produce. Oxygen is also dosed directly into aeration tanks at sewage works and at pulp, dairy, brewery and chemical plants whose effluent carries a high biochemical oxygen demand. The driving force for dissolving oxygen is its partial pressure, so pure oxygen at the same pressure transfers about five times as fast as air into the same water, which lets an overloaded tank meet its discharge consent without being rebuilt. Fish farms and hatcheries use the same principle to hold higher stocking densities.

Oxy-fuel combustion, in which natural gas or oil burns with oxygen instead of air, is standard in glass melting furnaces, in copper and lead smelting and in cement kilns. Taking the nitrogen out of the flame raises its temperature, cuts the volume of flue gas that must be heated and exhausted, and largely eliminates thermal NOx. In glassworks the gain is cleaner melt and lower fuel per tonne; in secondary aluminium and copper it is faster melting in rotary furnaces. The chemical industry uses oxygen as a feedstock: ethylene oxide, vinyl acetate, titanium dioxide by the chloride route, and partial oxidation of hydrocarbons to synthesis gas all run on oxygen from dedicated air separation units.