How calcium nitrate is manufactured: limestone and nitric acid, the nitrophosphate (Odda) route, and how the commercial forms are made
Calcium nitrate is made by neutralising nitric acid with limestone or lime: CaCO3 + 2 HNO3 → Ca(NO3)2 + CO2 + H2O.[18] It is also produced as a by-product of the nitrophosphate (Odda) process, where phosphate rock is digested in about 60–62% nitric acid and calcium nitrate tetrahydrate is crystallised out by cooling.[1][14] The solution is then filtered, usually neutralised with a little ammonia, evaporated and formed into prills, granules, flakes or powder; commercial prilled and granulated material is typically 77–80% Ca(NO3)2, 6–8% ammonium nitrate and 14–17% crystal water.[2] The product was first made at industrial scale in Norway from 1905 using the Birkeland–Eyde electric-arc process.[11][15]
Key facts at a glance
| Item | Value / answer |
|---|---|
| Main reaction (limestone route) | CaCO3 + 2 HNO3 → Ca(NO3)2 + CO2 + H2O[18] |
| Other routes listed | Odda (nitrophosphate) by-product, and ammonium nitrate reacted with calcium hydroxide in water[18] |
| Nitrophosphate digestion acid | Rock phosphate dissolved in dilute nitric acid, about 60–62%, in a series of digesters (GNFC Odda plant, India)[14] |
| Odda by-product form | Calcium nitrate tetrahydrate, Ca(NO3)2·4H2O, crystallised by cooling and filtered off[1][14] |
| Why the Odda route is used | Avoids sulphuric acid and the gypsum by-product of sulphuric-acid routes[1][19] |
| Tetrahydrate composition and melting point | About 68–69 wt% Ca(NO3)2 and about 30 wt% water; melts at about 42 °C[2] |
| Prilled / granulated commercial composition | 77–80 wt% Ca(NO3)2, 6–8 wt% ammonium nitrate, 14–17 wt% crystal water[2] |
| Why ammonium nitrate is added | Pure melt solidifies at only 43–50 °C with undercooling, which makes prilling and granulation difficult[3] |
| Granulation feed (patent example) | Concentrated liquid at 85–90% m/m, sprayed at 130–140 °C into a fluidised bed[4] |
| Granule analysis in that example | 18.9% Ca, 1.45% ammoniacal N, 14.7% nitrate N[4] |
| Anhydrous powder route (patent) | Turbo-dryer at 200–300 °C giving 92–99.9 wt% Ca(NO3)2; anhydrous melting point above 500 °C[2] |
| Critical relative humidity at 30 °C | Calcium nitrate 46.7%; ammonium nitrate 59.4%[22] |
| Birkeland–Eyde energy use | About 2.4–3.1 MJ per mol N, versus about 0.5–0.6 MJ per mol N for Haber–Bosch[9] |
| Nitric acid plant N2O (European average, 1990s data) | About 6 kg N2O per tonne HNO3, about 2 t CO2-eq per tonne[1] |
What is the basic chemistry of making calcium nitrate?
Calcium nitrate is the calcium salt of nitric acid. The simplest industrial route is acid–base neutralisation: limestone (calcium carbonate) or lime is reacted with nitric acid, giving calcium nitrate, carbon dioxide and water.[18] A third route in the literature reacts ammonium nitrate with calcium hydroxide in aqueous solution.[18] Because the salt is extremely soluble (121.2 g/100 mL of water, about 1,200 g/L),[12] the product stays dissolved and the work of manufacture is mostly removing water and forming a handleable solid.
Two simple mass-balance figures follow from the formula (my own arithmetic from molar masses, not a source value): making 1 tonne of anhydrous Ca(NO3)2 needs about 0.77 t of 100% HNO3 and about 0.61 t of CaCO3, and releases about 0.27 t of CO2. Nitric acid is therefore the dominant raw-material cost and the main environmental load, which is why the nitric-acid chain is covered below.
How is calcium nitrate made from limestone and nitric acid, step by step?
Patent literature describes the commercial sequence for the limestone route. A calcium nitrate solution is produced by neutralising about 60% nitric acid with limestone, then neutralising with ammonia so that roughly one ammonium nitrate is formed for every five calcium nitrate (a molar ratio of about 5:1).[4] A published example filters the solution to remove insoluble matter before ammoniation and concentrates it by evaporation.[4][7] Older patents show that neutralisation of nitric acid with lime-based materials has been used for this purpose for about a century.[7]
- Neutralise. Limestone (or lime) is added to about 60% nitric acid; CO2 is released.[4][18]
- Clarify. The hot solution is filtered to remove insoluble matter from the stone.[4]
- Optional ammoniation. A small amount of ammonia converts part of the nitrate to ammonium nitrate, giving about a 5:1 Ca:NH4 molar ratio.[4]
- Concentrate. Water is evaporated; granulation feeds in the patent example are 85–90% m/m.[4]
- Form the solid. Prilling, fluidised-bed or pan granulation, flaking, or drying to powder (see forms below).[2][4][8]
- Cool, screen and pack. Granules are cooled below 40 °C in the patent example before bagging; dry air is used because the product is hygroscopic.[4][22]
What is the Odda (nitrophosphate) process and why does it make calcium nitrate?
In the nitrophosphate route, phosphate rock is dissolved in nitric acid instead of sulphuric acid. In the GNFC plant described to IFA, rock is digested in about 60–62% nitric acid, sand is removed, and the solution of phosphoric acid, calcium nitrate and excess nitric acid is cooled in batch crystallisers so that calcium nitrate crystallises as the tetrahydrate; the crystals are filtered and washed to remove adherent phosphoric and nitric acid.[14] The EU BAT reference document describes the same logic: cooling separates most calcium from the digest, and the route avoids the gypsum formed in sulphuric-acid processes.[1] The process is credited to Erling Johnson at Odda, Norway; sources differ on whether the date is the late 1920s or 1930s.[14][19]
What happens to the calcium nitrate is a commercial choice. The BAT document says it is either converted to ammonium nitrate and calcium carbonate for CAN fertiliser or, after purification, marketed as calcium nitrate fertiliser.[1] Five nitrophosphate plants were operating in Europe when that document was written, including one at Porsgrunn.[1] Norwegian sources describe Herøya near Porsgrunn as one of the world’s most significant calcium nitrate production sites.[15] The BAT document also notes that calcium nitrate can alternatively be produced by neutralising nitric acid with lime.[1]
What was the Birkeland–Eyde process and Norgessalpeter?
Before ammonia synthesis, nitrogen was fixed with electricity. Kristian Birkeland and Sam Eyde tested an electric-arc process in 1903 at what is now the University of Oslo; air passed through an arc formed nitrogen oxides, which were converted to nitric acid.[9][20] Hydro dates its company history from 1905,[16] and calcium nitrate was sold from 1905 under the name Norgessalpeter (Norwegian saltpetre), containing roughly 13% nitrogen, the first artificial nitrogen fertiliser made from air at industrial scale.[15] The Rjukan–Notodden industrial complex, built to make fertiliser from atmospheric nitrogen, was inscribed as a UNESCO World Heritage Site in 2015.[11]
The method was abandoned because it was energy-hungry: published estimates are about 2.4–3.1 MJ per mol of nitrogen fixed at 1–2 mol% NO in the gas, against about 0.5–0.6 MJ per mol N for Haber–Bosch ammonia.[9] Norsk Hydro moved to ammonia-based production around 1928–1929 and introduced kalksalpeter, a calcium nitrate–ammonium nitrate double salt that was less hygroscopic than pure calcium nitrate.[15]
What forms does calcium nitrate come in, and how does each differ?
| Form | Typical composition | How it is made | Notes |
|---|---|---|---|
| Solution | 45–50% Ca(NO3)2 | Neutralisation, filtration, partial evaporation | High handling cost because it carries much water[3] |
| Tetrahydrate crystals | About 68–69% Ca(NO3)2, about 30% water | Cooling crystallisation below saturation (Odda by-product, or from solution) | Melts at about 42 °C; caking and dust are known drawbacks[2][3] |
| Prills / granules (CAN-type double-salt composition) | 77–80% Ca(NO3)2, 6–8% AN, 14–17% water | Melt of Ca nitrate, AN and crystal water prilled or granulated[2][3] | AN was added to the melt to improve solidification and avoid undercooling[3] |
| Anhydrous powder | 92–99.9% Ca(NO3)2 (patent) | Turbo-drying at 200–300 °C | Anhydrous salt absorbs moisture back to hydrate[2][18] |
| Water-soluble fertiliser powder / fine granule | About 15.5% N and 19% Ca is the common horticultural analysis | Depends on producer | Check the TDS for the exact ammoniacal vs nitrate N split |
The “15.5-0-0 + 19% Ca” horticultural product corresponds closely to the double salt 5Ca(NO3)2·NH4NO3·10H2O. Calculated from that formula (my arithmetic, not a source figure), the pure double salt is about 18.5% Ca and 15.5% N, of which about 1.3 points is ammoniacal. Double salts of calcium nitrate and ammonium nitrate have been patented and studied for roughly a century,[5][6] and the decahydrate has been characterised as a defined crystalline phase.[5] Pure tetrahydrate, by the same arithmetic, is only about 17% Ca and 12% N, which is one reason commercial fertiliser products are not simply the tetrahydrate.
Why do prills and granules contain ammonium nitrate, and why does powder differ?
Pure calcium nitrate melt has a low solidification temperature (43–50 °C) and undercools, so it is very hard to form clean particles by conventional prilling or granulation; manufacturers added ammonium nitrate, at the cost of some ammonia evolution in applications where that matters.[3] In the granulation example, a hot 85–90% liquid is sprayed onto seed particles in a fluidised bed supplied with conditioned air of relative humidity below 30% at 40 °C, then cooled and screened.[4] Equipment patents exist for nitrate granulation plants.[8]
Powder differs for practical reasons. It is a finer, dustier solid with a larger surface area, so it dissolves quickly in make-up tanks but also takes up moisture faster (this is physical reasoning, not a measured comparison). Calcium nitrate starts absorbing water from air above about 46.7% relative humidity at 30 °C, lower than ammonium nitrate at 59.4%,[22] so powder needs sealed packaging and dry storage. Granules flow better and generate less dust, which suits bulk spreading; powder suits dissolving for fertigation, hydroponics or process baths. See powder vs granular calcium nitrate.
What quality parameters define a calcium nitrate grade?
| Parameter | Why it matters | What to ask for |
|---|---|---|
| Calcium (Ca or CaO) | Dose by calcium, not by bag weight | Guaranteed % Ca on the COA; hydrate form |
| Nitrogen: nitrate vs ammoniacal | Double-salt products carry some ammoniacal N; this affects use in ammonium-sensitive systems | Total N and the nitrate/ammoniacal split |
| Water-insoluble matter | Limestone-derived grit blocks drip emitters and fouls dip tanks and filters | % insolubles per batch; clear-solution test |
| Moisture | Controls caking and effective Ca/N per kg; for comparison a draft CAN fertiliser standard caps moisture at 1% m/m[13] | Moisture or loss-on-drying; packaging type |
| pH of a stated solution | Residual acid or alkalinity shifts nutrient solution pH | pH at a stated concentration (no standard value is cited here) |
| Heavy metals | Regulated for fertilisers; draft CAN standards include a heavy-metal limits table[13] | Metals analysis for the destination market |
| Particle size | Draft CAN standard requires at least 90% between 1 and 4 mm for pellets[13]; powder is specified differently | Sieve analysis if relevant |
Note that the CAN standard cited is for calcium ammonium nitrate (about 26% N minimum, with carbonate), a different product from calcium nitrate fertiliser;[13] it is quoted only to show how fertiliser standards frame moisture, metals and sizing. See calcium nitrate vs CAN.
Where does the nitric acid come from, and what are the emissions?
Nitric acid is made from ammonia by the Ostwald process: ammonia is burned over a platinum–rhodium gauze at about 900 °C and pressures up to about 8 atm, and the nitrogen oxides are absorbed in water.[21] US EPA describes nitric acid production as catalytic oxidation of ammonia and requires N2O reporting from each nitric acid train.[10] So calcium nitrate is, in effect, a downstream product of ammonia, which in turn comes from Haber–Bosch synthesis.[9]
- N2O. The ammonia burner produces nitrous oxide as a by-product. The EU BAT document gave an average of about 6 kg N2O per tonne HNO3 (about 2 t CO2-eq) for European plants of the time;[1] an industry initiative cites 6–9 kg per tonne, a global warming potential of 273 for N2O and about 95% abatement with secondary or tertiary catalysts.[17] Using the BAT average, an uncontrolled plant would be associated with roughly 4.6 kg N2O per tonne of anhydrous calcium nitrate (0.77 t HNO3 × 6 kg; illustrative arithmetic only). Modern abated plants are far lower.
- NOx and dust. The BAT document gives NOx levels of 5–75 ppmv for new nitric acid plants, and for calcium nitrate pan granulation reports aerosol emissions normally below 4 mg nitrate-N per Nm3 of off-gas, with scrubbing and return of condensate to the process.[1]
- CO2. The limestone route releases the carbonate carbon as CO2; the nitrophosphate route instead trades a gypsum by-product for calcium nitrate that must be sold or converted.[1][18]
How do the grades map to fertiliser, industrial and technical uses?
| Use family | Form usually specified | What the buyer must pin down |
|---|---|---|
| Fertiliser (soil, fertigation, hydroponics, foliar) | Water-soluble powder or granule, about 15.5% N and 19% Ca | Ca and N, insolubles, ammoniacal N share, local fertiliser regulation |
| Industrial (concrete admixtures, wastewater, corrosion control) | Powder to dissolve, or solution | Chloride, insolubles and metals relevant to your process; ask for a trial sample |
| Technical (latex coagulant, heat storage, molten salt) | Often anhydrous or low-water material | Water content and purity limits in writing; these uses have tighter needs than fertiliser |
There is no single universal grade standard for these three families in the sources reviewed here; the specification is normally agreed between buyer and supplier. See uses of calcium nitrate for use-by-use detail.
When calcium nitrate is NOT the right choice
- Do not treat the patent figures here as a plant recipe. They are published examples from specific inventions and vary by producer and product.[2][3][4]
- Do not assume one “calcium nitrate” fits every use. Tetrahydrate, CAN-type double-salt granules and anhydrous powder differ in Ca, N, water and ammoniacal nitrogen, so a technical user and a fertiliser user need different specifications.[2]
- Do not store or ship it as if it were non-hygroscopic. Calcium nitrate takes up water above about 46.7% relative humidity at 30 °C, and the tetrahydrate melts near 42 °C.[2][22]
- Do not infer a supplier’s process from this page. Which route a plant uses (limestone, nitrophosphate by-product or other) changes the impurity profile; ask for the TDS and batch COA rather than assuming.[1]
What HRSU supplies — and what it does not
HRSU Indore manufactures and exports fertilizer-grade calcium nitrate powder: 18.5–19% calcium, 15.5% minimum nitrogen, 100% water-soluble with 0% insoluble residue, packed in 25 kg and 50 kg bags with a batch Certificate of Analysis, for bulk and export supply. A Technical Data Sheet and Safety Data Sheet are available on request. This page describes industry-wide manufacturing routes from public sources; it does not describe HRSU’s own plant or process. HRSU does not claim food, pharmaceutical, glove, construction-chemical or other specialised certification for this grade. If you need a specific analysis (for example the ammoniacal-nitrogen share, moisture or metals), ask for the TDS and a sample before ordering.
Request the TDS and a batch COA for HRSU calcium nitrate
100% water-soluble · 0% insoluble residue · 25 kg & 50 kg bags · bulk/export available.
Buy online WhatsApp usFrequently asked questions
How is calcium nitrate made?
What is the Odda process?
The Odda or nitrophosphate process digests phosphate rock in about 60–62% nitric acid instead of sulphuric acid. Cooling crystallises calcium nitrate tetrahydrate, which is filtered off; this avoids gypsum formation. The calcium nitrate is then sold as fertiliser or converted to ammonium nitrate and calcium carbonate.[1][14]
Who invented the first industrial calcium nitrate process?
Why does commercial calcium nitrate contain ammonium nitrate?
What is 5Ca(NO3)2·NH4NO3·10H2O?
What is the difference between tetrahydrate and anhydrous calcium nitrate?
What are the environmental impacts of making calcium nitrate?
Does HRSU make its calcium nitrate by the limestone route?
HRSU supplies fertilizer-grade calcium nitrate powder (18.5–19% Ca, 15.5% min N, 100% soluble, 0% insolubles) with batch COA. This page does not describe HRSU’s process; request the TDS and COA for the specification.
Related guides
- Powder vs granular calcium nitrate
- Calcium nitrate vs calcium ammonium nitrate (CAN)
- Uses of calcium nitrate: the complete guide
- Calcium nitrate safety, handling and storage
References
- Reference Document on Best Available Techniques for the Manufacture of Large Volume Inorganic Chemicals: Ammonia, Acids and Fertilisers (LVIC-AAF BREF, 2007) — European Commission, Joint Research Centre (IPTS / EIPPCB). EU regulator BAT document
- US 9,908,787 B2 — Method for producing anhydrous calcium nitrate powder — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 6,610,267 B1 — Method for manufacturing a calcium nitrate melt and product thereof — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 7,029,504 B2 — Method for producing calcium nitrate granules — U.S. Patent and Trademark Office, via Google Patents. Patent
- US 1,776,694 A — Manufacture of double salts of calcium nitrate — U.S. Patent Office, via Google Patents. Patent (historical)
- US 3,173,756 A — Double nitrate salts and methods for their preparation — U.S. Patent Office, via Google Patents. Patent (historical)
- US 1,893,946 A — Production of calcium nitrate — U.S. Patent Office, via Google Patents. Patent (historical)
- US 7,727,485 B2 — Apparatus for producing nitrate granules — U.S. Patent and Trademark Office, via Google Patents. Patent
- From the Birkeland-Eyde process towards energy-efficient plasma-based NOx synthesis: a techno-economic analysis (Rouwenhorst et al.) — Energy & Environmental Science, Royal Society of Chemistry, 2021 (open access via PubMed Central). Peer-reviewed
- Greenhouse Gas Reporting Program, Subpart V: Nitric Acid Production information sheet — U.S. Environmental Protection Agency (archived snapshot). Government regulator
- Rjukan-Notodden Industrial Heritage Site (World Heritage List, ref. 1486) — UNESCO World Heritage Centre. Intergovernmental body
- Calcium nitrate (PubChem compound summary) — U.S. National Library of Medicine / NCBI. Government database
- DUS 758:2017 Draft Uganda Standard — Calcium ammonium nitrate fertilizer: Specification (WTO TBT notification) — Uganda National Bureau of Standards, via WTO. National standard (draft)
- GNFC experience with nitrophosphate Odda process (Patwa and Shamsi), IFA Technical Conference, Marrakech, 1998 — International Fertilizer Industry Association. Industry association paper
- Kalsiumnitrat (Calcium nitrate) — Store norske leksikon (Norwegian encyclopedia). Encyclopedia (Norwegian)
- 1905: A new working day, 2 December 1905 — Hydro (Norsk Hydro company history). Company history
- Nitrous oxide emissions from nitric acid production — Nitric Acid Climate Action Group. Industry/NGO initiative
- Calcium nitrate — Wikipedia. Encyclopedia
- Nitrophosphate process — Wikipedia. Encyclopedia
- Birkeland-Eyde process — Wikipedia. Encyclopedia
- Ostwald process — Wikipedia. Encyclopedia
- Critical relative humidity (table of pure salts at 30 C) — Wikipedia. Encyclopedia