Heavy inorganics: salt, sulphur, nitrogen and phosphorus

By the end you will be able to

• Explain how the contact, chlor-alkali, Solvay, Haber-Bosch, Ostwald, urea and wet phosphoric acid processes work, step by step
• Estimate raw materials and energy per tonne for each product from stoichiometry and the sources' plant data
• Distinguish energy-exporting processes (sulphuric and nitric acid) from electricity-bought products (chlorine, oxygen, electrolytic hydrogen) and gas-driven ammonia
• Analyse how co-products and wastes such as caustic soda, gypsum and calcium chloride constrain plant economics
• Compare the products on tonnage, energy and uses, and relate their cost drivers to Indian conditions

Contents

1. Part 1 Sulphur and sulphuric acid: the workhorse acid

   Sulphuric acid is made in larger tonnage than any other chemical, yet almost none stays in a final product. Its economics turn on three questions: where the sulphur comes from, how completely the sulphur dioxide is converted, and how much of the plant's heat can be sold as steam and power.

   • Most sulphur is now a by-product of cleaning oil and gas, so supply ignores demand
   • The contact process burns, converts and absorbs, and double absorption removes the last SO2
   • A modern acid plant is also a power station, and much of its margin lies there
   • Acid follows fertiliser, and India built many small plants beside its customers
2. Part 2 Salt: chlor-alkali, hydrochloric acid and soda ash

   Salt is the cheapest major chemical raw material. From it come chlorine, caustic soda and soda ash, with hydrochloric acid as a by-product. Electrolysis splits brine with electricity, so power prices decide its cost. The Solvay process uses limestone and recycled ammonia, so logistics and waste decide its fate.

   • Brine electrolysis makes a fixed bundle: a tonne of chlorine, 1.1 tonnes of caustic soda and some hydrogen
   • Mercury, diaphragm and membrane cells trade electricity, steam and purity against each other
   • Chlorine and caustic sell into different markets, so one of them is always the problem
   • Soda ash: Solvay recycles ammonia but cannot escape calcium chloride, and trona beats it on energy
3. Part 3 Ammonia: fixing nitrogen with natural gas

   Ammonia is the gateway to almost all fixed nitrogen. Making it means building hydrogen from natural gas and steam, cleaning that gas to parts per million, and forcing it to react with nitrogen at high pressure. Energy per tonne has roughly halved since 1950, but gas is still most of the cost.

   • Haber-Bosch is an equilibrium fight won with pressure, a promoted iron catalyst and a recycle loop
   • Most of an ammonia plant is a hydrogen factory: reform, shift, scrub and methanate
   • Energy per tonne roughly halved in fifty years, yet gas is still 70-85% of the cash cost
4. Part 4 From ammonia and rock to fertiliser: nitric acid, urea, ammonium nitrate and phosphates

   Little ammonia is applied to fields as it is. It is turned into nitric acid and ammonium nitrate, into urea with the CO2 from its own plant, or into ammonium phosphates with phosphoric acid made from rock and sulphuric acid. Each step is simple chemistry with a recycle or waste that sets its cost.

   • Nitric acid: ammonia burns on platinum gauze in a millisecond, then absorption takes the time
   • Urea joins ammonia to the CO2 from the same plant, and ammonium nitrate neutralises ammonia with nitric acid
   • Wet-process phosphoric acid trades sulphuric acid for gypsum, about five tonnes per tonne of P2O5
   • DAP, MAP, superphosphates and NPK: the choice is about nutrient per tonne shipped
5. Part 5 Industrial gases, and the heavy inorganics side by side

   Industrial gases are made from air or hydrocarbons and sold by pipeline, as liquid or in cylinders. Their cost is mostly electricity and capital, and plants sit beside their customers. Hydrogen and carbon dioxide tie this industry back to ammonia. The part ends by comparing every product on tonnage, energy and use.

   • Air separation is distillation far below zero, paid for almost entirely in electricity
   • Hydrogen and carbon dioxide are mostly made and used inside the ammonia and refining chain
   • Side by side: what makes each heavy inorganic cheap or dear
6. Wrap-up Key terms, try it in the table, final quiz, sources