Chapter 2
Inorganic Chemicals

2.1 Sulfuric acid

Sulfuric acid (H2SO4) is a large volume chemical made from sulfur dioxide, which in turn can be made from elemental sulfur. Because sulfuric acid is of prime importance to the world's fertilizer and manufacturing industries, consumption of sulfuric acid has been regarded as one of the indexes of a nation's industrial development [1]. Sulfuric acid is the largest volume chemical produced in the world. Sulfur is oxidized to sulfur dioxide. Sulfur dioxide is then further oxidized to sulfur trioxide. Temperatures of 400 °C to 450 °C are typical and a vanadium catalyst such as vanadium pentoxide (V2O5) is commonly used [2, 3]. At a lower temperature, sulfur trioxide combines with water to form sulfuric acid. The following reactions give a synopsis of the chemistry.

There are several sources of sulfur. Elemental sulfur is naturally occurring and can be mined by a process invented in the late 19th century by Herman Frasch. The Frasch process takes advantage of the relatively low melting point of sulfur at 115 °C. Superheated water at 168 °C is pumped through pipes inserted into a well and molten sulfur is pumped from the well [4].

Another source is pyrite. Pyrite is iron sulfide (FeS2). Pyrite is also known as fool's gold because of its visual resemblance to the precious metal. With water and oxygen, pyrite can be converted to sulfuric acid. China is a leading miner of pyrite and extraction of sulfuric acid from pyrite is an important process in China. The pyrite is roasted to form sulfur dioxide which is then purified and converted to sulfuric acid [5].

A major source of sulfur is refinery and natural gas streams. This is done by the Claus process which was discovered more than 100 years ago and has been used by the natural gas and refinery industries for 50 years. In the Claus process, hydrogen sulfide from the gas stream is converted to elemental sulfur. Air is introduced into a furnace to oxidize about one-third of the hydrogen sulfide to sulfur dioxide. In the next stage, the reaction furnace, unconverted hydrogen sulfide reacts with the sulfur dioxide to form elemental sulfur. The Claus process generally produces an overall recovery of sulfur of 95–97%, but several modifications have been invented and sulfur recoveries of 99.9% are now achievable [6]. The chemistry is represented by the following reactions; the equilibrium to form elemental sulfur is favored at lower temperatures.

The leading U.S. producers are the refining companies such as Valero Energy Corp., Exxon Mobil Corp., Conoco Phillips Co., Chevron Oil Co., and Shell Oil Co. In 2011, elemental sulfur and byproduct sulfuric acid were produced in the United States at 109 operations in 29 states and St. Croix with total shipments being valued at $1.6 billion [7]. The production took place at petroleum refineries, natural-gas processing plants, and coking plants. About 90% of consumed sulfur is in the form of sulfuric acid. Production of fertilizers is the major use of sulfuric acid, followed by petroleum refining and metal mining. U.S. production data for 2009 is estimated at 9.8 million metric tons of sulfur. If this number is converted to a sulfuric acid basis, it becomes 30 million metric tons.

The population of the U.S. is around 300 million people, so this is equivalent to each and every person in the United States making 0.1 metric tons each year. A metric ton is 1,000 kg or 2,240 pounds (not to be confused with a U.S. ton which is 2,000 pounds) so this is equivalent to each person making 224 pounds of sulfuric acid every year. The actual amount used is about 20% higher because of imports.

Reagent sulfuric acid is 95–98% by weight sulfuric acid with the remainder being water. It is a clear, colorless, oily, corrosive liquid. Fuming sulfuric acid, also known as oleum, is sulfuric acid with dissolved sulfur trioxide. Oleum is sold at a variety of percentages that indicate the amount of sulfur trioxide. For example, 24% oleum is 24 g sulfur trioxide per 100 g total. Recognize that this is also true for other mass units. So 24% oleum is also 24 pounds sulfur trioxide per 100 pounds total or 24 tons sulfur trioxide per 100 tons total. This is also sometimes expressed as 105.4% H2SO4, because 100 g of 24% oleum can make 105.4 g H2SO4 when the oleum reacts with water. This is a theoretical exercise and generally should not actually be done because oleum reacts violently with water. It is worthwhile to review the arithmetic because such exercises commonly need to be done in industry. The calculation can be done by converting the grams of sulfur trioxide per 100g total to moles of sulfur trioxide, multiplying by 18 g per mole of water, and adding that number to 100. The calculation is demonstrated for 34% oleum, which can be called 107.65% H2SO4.

Sulfuric acid has many uses but one of the largest uses is for fertilizer manufacture. Primary nutrients for plants are nitrogen, phosphorus and potassium. The major use of sulfuric acid in fertilizer manufacture is in the production of phosphoric acid, which in turn is used to produce phosphorus-containing fertilizers.

2.2 Phosphoric acid

A major use of sulfuric acid is in the manufacture of phosphoric acid (H3PO4), also known as orthophosphoric acid. The principle use of phosphoric acid is to make fertilizers. In the United States, phosphate rock ore is mined by six companies at 12 mines and upgraded to 28 million metric tons [8]. Most of the mining is done in Florida and North Carolina. China is the leading producer of phosphate rock with 72 million metric tons of the 191 million metric tons manufactured worldwide.

Phosphorus is an essential element for plant and animal nutrition and phosphate rock is the only significant resource of phosphorus [9]. Phosphate rock is converted into phosphoric acid, which in turn is converted into phosphorus fertilizers. Common examples of phosphorous-containing fertilizers are triple superphosphate, monoammonium phosphate, diammonium phosphate, and ammonium polyphosphate. The latter three examples also provide nitrogen, another primary nutrient. Triple superphosphate is calcium dihydrogen phosphate (CaH2PO4). Two processes, the dry process and the wet process, are used to convert phosphate rock to phosphoric acid. In the manufacture of phosphoric acid by the dry process, phosphate rock is reduced in an electric furnace, and the resulting yellow phosphorus is burnt into the oxide, P4O10, commonly referred to by its empirical formula, P2O5, and therefore called phosphorous pentoxide. The phosphorous pentoxide is hydrated to obtain phosphoric acid. This process has high energy costs. The wet process is the more commonly used. In the wet process, phosphate rock is decomposed with sulfuric acid, and the produced calcium sulfate is separated to obtain dilute phosphoric acid, which is then concentrated into high concentration acid [10]. Direct acidification with sulfuric acid is problematic because the outside surface of the rock reacts with the sulfuric acid resulting in a gypsum layer on the outside of the rock. Gypsum is calcium sulfate dihydrate and is used for plaster and drywall. This gypsum layer limits the reaction on the inside of the rock. To address this problem, the phosphate rock is first acidified with phosphoric acid to make calcium dihydrogen phosphate. Sulfuric acid is then added to make phosphoric acid [11].

This process requires that 12 of the 18 moles, or 67%, of phosphoric acid produced be recycled to treat more phosphate rock. There are other metals in the phosphate rock and upon acidification they become salts. The insoluble salts remain with the calcium sulfate precipitate but the soluble salts such as magnesium sulfate or iron sulfate remain in solution with the phosphoric acid. Generally, high purity is not required for fertilizer grade phosphoric acid. The crude phosphoric acid reacts, for example, with ammonia to make ammonium dihydrogen phosphate.

If higher purity is required, such as for food applications, there are various processes to improve the purity of wet process phosphoric acid. Generally they involve a solvent which solubilizes the phosphoric acid, but not the impurities [12]. Alternatively, dry process phosphoric acid can be used. However, purification of wet process phosphoric acid is the dominant route because of the higher costs associated with the thermal process.

Phosphoric acid is a low melting solid and it is usually sold as an aqueous solution, such as 85% phosphoric acid, which freezes at about 21 °C. More dilute solutions are also sold and they have lower freezing points. For example, 75% phosphoric acid has a freezing point of −18 °C and can be used when heated storage is not available. Sometimes phosphoric acid is classified or sold based upon the amount of phosphorous pentoxide present. A solution of 85% phosphoric acid is 61.5% P2O5. This can be calculated by taking into account that it takes two moles of phosphoric acid to make one mole of phosphorus pentoxide and using the molecular weights.

Although fertilizer production is the major use and represents almost 90% of U.S. consumption [13], there are many other applications for phosphoric acid. For example, it can be used to remove scale from boilers, to treat metals for improved corrosion resistance, or as a dilute solution in foods as an acidulant and flavoring agent.

2.3 Lime

Lime is calcium oxide (CaO). At the end of 2011, there were 72 lime plants operating in the United States [14]. They manufactured 19 million metric tons. This compares with a...