The Essential Chemical Industry - online

Sulfur is one of the most important elements.  It is used to make sulfuric acid, which is the most abundantly made industrial chemical. The element itself is an essential plant nutrient.

Uses of sulfur

About 90% of sulfur produced or extracted is used to make sulfur dioxide, which is then converted to sulfuric acid.  The majority of the acid is used in the production of phosphate fertilizers, a crucial component in the provision of food in the World.

Another important use of elemental sulfur is in fertilizers as a soil nutrient in its own right, particularly where there is a deficiency of sulfur in the soil.

Sulfur is beginning to be used as a component of infrastructre band structural materials. For example, adding sulfur pellets to asphalt makes road surfaces more durable and resistant to damage by cracking when the conditions are very cold. Sulfur concrete is produced by adding sulfur as concrete is being made.  It has a smoother surface, making it easy to paint. It is strong and is able to withstand acidic and salt waters, making it a possible material for sea barriers and dams.

Annual production of sulfur

Data from:
U.S. Geological Survey, Mineral Commodity Summaries, 2016

Manufacture of sulfur

The vast majority of sulfur is recovered from natural gas and oil, which contain hydrogen sulfide and a wide variety of organic compounds that contain sulfur (such as CH₃SH).  They have to be removed before the natural gas and oil can be used as a feedstock for the chemical industry as they poison the catalysts that are used in the various processes to make chemicals.  Equally, they have to be removed if natural gas or oil are to be used as fuels.  Otherwise large amounts of sulfur dioxide, a very dangerous pollutant, will be released into the atmosphere when the fuels are burnt.  The organic sulfur compounds are converted to hydrogen sulfide by reduction with hydrogen. For example:

Hydrogen sulfide gas is separated from other gases present by dissolving it in an aqueous solution of an organic base, such as, for example, 2-aminoethanol (monoethanolamine). Although this is a complex process, the following equation shows the overall reaction:

Figure 1   Much natural gas and oil contain large quantities of hydrogen sulfide and organic sulfur compounds which must be removed to produce gas and oil which can be used as fuels.  They are converted into elemental sulfur which is heated so that it becomes molten and then piped into large moulds where it solidifies.  It is stored in the open until it is used to make sulfuric acid.  This photo shows sulfur recovered in the immense Lacq field in southwestern France where the gas is very 'sour', containing 16% hydrogen sulfide. By kind permission of Arkema.

Heating the solution of the salt liberates pure hydrogen sulfide and regenerates the amine.  Hydrogen sulfide is then burned with a limited amount of air to give sulfur dioxide, sulfur and water vapour, and unreacted hydrogen sulfide:

The sulfur is condensed and separated.  The remaining gases are passed over a catalyst bed of alumina to convert more hydrogen sulfide to sulfur:

These two stages are repeated in three separate cycles to achieve over 95% recovery of sulfur.

Date last amended: 11th October 2016

Sulfuric acid is one of the most important compounds made by the chemical industry.  It is used to make, literally, hundreds of compounds needed by almost every industry.

Uses of sulfuric acid

By far the largest amount of sulfuric acid is used to make phosphoric acid, used, in turn, to make the phosphate fertilizers, calcium dihydrogenphosphate and the ammonium phosphates. It is also used to make ammonium sulfate, which is a particularly important fertilizer in sulfur-deficient.

Figure 1  Uses of sulfuric acid.

It is widely used in metal processing for example in the manufacture of copper and the manufacture of zinc and in cleaning the surface of steel sheet, known as 'pickling', prior to it being covered in a thin layer of tin, used to make cans for food.

It is also used to make caprolactam, which is converted into polyamide 6 and in the manufacture of titanium dioxide, used, for example, as a pigment.

Amongst its many other uses is in the manufacture of hydrofluoric acid and phenol with propanone all of which are used in many industries.

Annual production of sulfuric acid

Data estimated from:
Merchant Research & Consulting Ltd.

These figures relate to 2011 - 2012.  It was expected that by 2012, the World production would be over 250 million tonnes (mcgroup.com) and 260 million tonnes by 2018 (marketsandmarkets.com) with the upward trend forecasted to at least 2023 (transparencymarketresearch.com).

Manufacture of sulfuric acid

The process for producing sulfuric acid has four stages:
a) extraction of sulfur
b) conversion of sulfur to sulfur dioxide
c) conversion of sulfur dioxide to sulfur trioxide
d) conversion of sulfur trioxide to sulfuric acid

(a) Extraction of sulfur

Easily the most important source of sulfur is its recovery from natural gas and oil.  These contain sulfur compounds, both organic and hydrogen sulfide both of which must be removed before they are used as fuels or chemical feedstock.

Another important source of sulfur is as sulfur dioxide from metal refining. Many metal ores occur as sulfides and are roasted to form an oxide and sulfur dioxide, for example, in the manufacture of lead:

Other metals manufactured from their sulfide ores include copper, nickel and zinc.

Worldwide about 35% of the sulfur is obtained as sulfur dioxide from sulfide ore roasting and this is increasing, as plants which traditionally passed the sulfur dioxide to atmosphere are recovering it as sulfuric acid. In particular, China makes most of its sulfuric acid from pyrites, an iron sulfide ore.

Sulfuric acid is also obtained from ammonium sulfate, a by-product in the manufacture of poly(methyl 2-methylpropenoate) and also recovered from 'spent' (i.e. used) sulfuric acid.

(b) Conversion of sulfur to sulfur dioxide

If sulfur is the feedstock, it must first be converted to sulfur dioxide. Molten sulfur is sprayed into a furnace and burnt in a blast of dry air at about 1300 K.  The sulfur burns with a characteristic blue flame:

As excess air is used the emerging gas contains about 10-12% sulfur dioxide and 10% oxygen, by volume.  The gases are very hot and so are passed through heat exchangers (waste heat boilers).

The gases are cooled to about 700 K and the water in the surrounding boiler pipes is converted into steam. In manufacturing one tonne of sulfuric acid, one tonne of high pressure steam is also produced.

(c) Conversion of sulfur dioxide to sulfur trioxide (The Contact Process)

A typical plant contains one cylindrical vessel which acts as a fixed bed reactor with four separate beds of catalyst, known as a converter, heated to 700 K, through which the sulfur dioxide and air pass:

The catalyst, vanadium(V) oxide on silica, is generally in the form of small pellets, to which caesium sulfate has been added as a promoter (Figure 2). The function of the promoter is to lower the melting point of vanadium(V) oxide so that it is molten at 700 K.

Figure 3  A flow diagram of the Contact Process. As shown above, it is an exothermic reaction so, for a satisfactory yield of sulfur trioxide (above 99.5% conversion is needed), as low a temperature as economically possible is used.  Thus, heat is removed from the gas leaving each bed using heat exchangers.  The sulfur trioxide produced is removed between the third and fourth beds and flows to the next stage, the conversion of sulfur trioxide to sulfuric acid. However, a small amount of the sulfur dioxide is not converted and is passed through the fourth bed of catalyst and the resulting gases, mainly sulfur trioxide, flow to the next stage. Based on diagram given by Mike Sellars.

(d) Conversion of sulfur trioxide to sulfuric acid

The sulfur trioxide formed from the third bed (and the small amount from the fourth bed) are now converted to
sulfuric acid.
Sulfur trioxide reacts with water and the reaction can be expressed as:

However, water itself cannot be used for absorption as there is a large temperature rise, and a sulfuric acid mist is formed, which is difficult to handle.  Instead, sulfuric acid of about 98% concentration is used.  This is kept at this concentration by addition of water and removal of acid at that concentration.
To keep the temperature at about 400 K, the heat is removed by heat exchangers, Figure 4.

Figure 4  A line diagram illustrating a heat exchanger used in the manufacture of sulfur trioxide.

The gases not absorbed contain about 95% nitrogen, 5% oxygen, and traces of sulfur dioxide.  The gas stream is filtered to remove any traces of sulfuric acid mist and is returned to the atmosphere using a high stack.

Date last amended: 9th October 2016

Titanium dioxide has remarkable optical properties, with a very high refractive index close to that of diamond.  It is also a very stable compound and can be heated to over 2000 K before melting.  It is these properties that underlie its most important uses.

Uses of titanium dioxide

Titanium dioxide is the most used white pigment and provides whiteness and opacity for paints and coatings (including glazes), plastics and paper.

It has many speciality uses. It is resistant to UV radiation and thus does not discolour over a long period of time and ultra pure and fine crystal titanium dioxide grades are being increasingly used for sun screens.
It is also being used in novel nanotechnology applications.

Figure 1  Uses of titanium dioxide

Data for 2013, Merchant Research and Consulting, 2015

Much smaller amounts of titanium dioxide are used as a semi-conductor and to catalyse the photodecomposition of water into hydrogen and oxygen.

It also has a high dielectric constant and a high resistance and thus it is used to make capacitors.

The strong bonding between titanium and oxygen gives great thermal stability, melting at 2100 K.  It is thus used as a ceramic material.  It is also used to increase the acid resistance of vitreous enamels.

Annual production of titanium dioxide

1.  Data for 2013, Merchant Research and Consulting, 2015
2.  2018 Elements of the Business of Chemistry, American Chemistry Council.

Manufacture of titanium dioxide

There are two main processes, the Sulfate Process and the Chloride Process (Table 1) which use the two principal ores, ilmenite and rutile, respectively.  Ilmenite contains 45-60% TiO₂ and rutile contains up to 99% TiO₂.  The ores are mined worldwide but most production is in Australia and South Africa.
Each large producer of titanium dioxide balances its production between the two processes.  Each produces the oxide in the rutile crystal form but the Sulfate Process can also produce another form of the oxide, anatase, which is softer and which is used for a small number of specialist applications.

The Sulfate Process is run as a batch process; the Chloride Process is run as a continuous process.

It is estimated that about 65% of the world's production is based on the Chloride Process.

Table 1  Comparison of the two processes for the manufacture of titanium dioxide.

The Sulfate Process

The chemistry of the process involves three main stages:
a) dissolving the ore
b) formation of hydrated titanium dioxide
c) formation of anhydrous titanium dioxide

(a) Dissolving the ore

The ore is usually ilmenite, FeTiO₃.  It is ground finely and dissolved in sulfuric acid to form a mixture of sulfates:

Before the titanium dioxide is extracted, the iron ions must be removed from the solution so that the colour of the final product is not spoiled.  The solution is therefore reacted with recycled iron sources to convert any iron(lll) ions that may be present to iron(ll) ions:

The solution is allowed to stand so that the unreacted solid settles, and the clear liquid is poured off before being concentrated by evaporation.  Cooling then allows light green crystals of iron(ll) sulfate to form and to be filtered off.  These are sold separately.  The remaining solution contains titanyl sulfate, TiOSO₄.

(b) Formation of hydrated titanium dioxide

The next stage involves the hydrolysis of the titanyl sulfate in solution to give insoluble, hydrated titanium dioxide:

This is a critical stage and the conditions must be controlled to ensure that the precipitate is suitable for filtering and roasting.

(c) Formation of anhydrous titanium dioxide

The final stage of the process is the heating of the solid in a furnace, known as a calciner.  This is a rotating cylinder which is typically heated by gas flames.  As the cylinder turns, the titanium dioxide passes along it and its temperature rises from 313 K, as it enters, to over 1000 K as it leaves:

Heating evaporates the water and decomposes any remaining sulfuric acid in the solid. After cooling, the product is 'milled' to form crystals of the size needed.  Crystals may also be coated with another substance, such as aluminium oxide or silica, to make the titanium dioxide mix more easily with liquids or to make the water-based paints made from it last longer.

This coating is achieved by dispersing the dry product from the calciner in water containing the dissolved coating chemicals which precipitate from solution onto the TiO₂ crystals.  The coating is usually between 3 and 8% by weight in the final dried pigment.  This coating is achieved by changing the temperature and pH of the solution.  Each TiO₂ crystal needs to be coated uniformly to maximize the effectiveness of the coating.  The coated TiO₂ crystals are filtered from the water and dried before being packed for dispatch to the final customer.

The Chloride Process

There are two main stages:
a) the conversion of rutile to titanium(IV) chloride
b) the oxidation of titanium(IV) chloride

(a) The conversion of rutile to titanium(IV) chloride

The rutile is fed into a heated bed together with a source of carbon, usually coke.  Chlorine is fed into the bed and the reaction takes place to form titanium(IV) chloride in the vapour form which is removed from the bed.  Iron and other metals in the ore are chlorinated and also leave the bed in the vapour state.  The oxygen in the ores is combined with the carbon to form carbon monoxide and dioxide.  The vapour stream is cooled and the metal chlorides other than titanium(IV) chloride are condensed and solidified.  The titanium(IV) chloride vapour, which contains almost pure titanium(IV) chloride and has a lower boiling point, is then condensed and stored as liquid.  It is then reboiled and distilled to give a purer product to feed to the next stage.

(b) The oxidation of titanium(IV) chloride

Liquid titanium(IV) chloride is vaporised and burnt in oxygen, together with a hydrocarbon fuel source (for example, methane) to a high temperature to initiate the reaction and keep the temperature high enough for the reaction to proceed:

The titanium dioxide is formed (by adding seed crystals) as a fine solid in the gas stream and is filtered out of the waste gases using cyclones or filters.  Once again control of crystal growth is important to give particles of the correct size for pigments.  This is done by adding nucleating agents to the gas stream (e.g. water or aluminium chloride) and by cooling the products.  The chlorine in titanium(IV) chloride is released and recycled to the chlorination stage of the process above.

The product contains small amounts of absorbed chlorine gas which are removed.  The product is washed and dried before milling and surface treatment in an identical manner to that used in the Sulfate Process described.

Date last amended: 27th November 2018

Sodium hydroxide (caustic soda) is a co-product from the manufacture of chlorine using a solution of the readily available raw material, rock salt (sodium chloride).  These factors contribute to it being the cheapest and most widely used strong alkali.

Uses of sodium hydroxide

Figure 1  Uses of sodium hydroxide.

While Figure 1 shows the worldwide uses of sodium hydroxide, the proportion varies country by country.  For example the manufacture of aluminium only accounts for 3% in the US whereas 35% is used to make organic chemicals, a reflection of the industrial make up of the US.

Sodium hydroxide is used in a wide range of industries as can be seen in Figure 1.  Much is used to scrub gases to remove acids before emitting them to the open environment.  For example gases emitted from burning fossil fuels which contain considerable amounts of sulfur dioxide.  There are many ways of trapping the sulfur dioxide, one which is used widely involves scrubbing the gases with a solution of sodium and calcium hydroxides.  This technique is also used in refining processes such as the purification of bauxite (Figures 2).

Figure 2  Sodium hydroxide is used in the purification of the ore, bauxite, prior to it being used to make aluminium.  This picture shows purified bauxite being unloaded from a ship in Iceland, on its way to an aluminium extraction plant. By kind permission of Alcoa.

Another major use of sodium hydroxide is in the manufacture of paper from wood. In the most used process, the Kraft process, wood is treated with a solution containing a mixture of sodium sulfide and sodium hydroxide.  Most of the unwanted material in the wood, such as the lignins, dissolve in the liquor, leaving relatively pure cellulose which is filtered off.  It is this cellulose which after further purification forms the basis of paper.

Other uses include the production of surfactants, soaps and bleaches, the latter being usually produced by passing chlorine gas into a solution of sodium hydroxide, which generates a solution containing sodium chlorate(I) (sodium hypochlorite):

Sodium hydroxide is used in making organic chemicals, one of the most important being epoxypropane (propylene oxide), used in turn to make polyurethanes.  Propene is treated with chlorine in water to make a mixture of 1-chloropropan-2-ol and 2-chloropropan-1-ol.  Epoxypropane forms on addition of a solution of either sodium hydroxide or calcium hydroxide.

Annual production of sodium hydroxide

1.  Estimated from data for chlorine
2.  2018 Elements of the Business of Chemistry, American Chemical Council.

Manufacture of sodium hydroxide

Sodium hydroxide and chlorine are manufactured together by the electrolysis of sodium chloride.

Large deposits of sodium chloride (rock salt) are found in many parts of the world.  These deposits are almost pure sodium chloride and are often several hundred metres deep (some are up to 3000 metres deep) with seams of 30 m to 500 m thick (Figure 3).  They were evaporated from trapped seas in the Triassic period, 200 million years ago.  For example, in Europe, the seas produced deposits which stretch, although not continuously, from Cheshire, Lancashire, Staffordshire and Cleveland in the UK to Poland.  They are also found throughout the US, particularly in Louisiana and Texas.  It is thought that the global demand for salt is about 350 million tonnes annually¹.

1.  Sojitz Corporation, 2014

Figure 3  Although much rock salt is pumped to the surface as brine, some
is mined, as is being done in this large underground deposit in Cheshire.
By kind permission of the Salt Association.

A small amount is mined as rock salt, the majority is solution-mined by the controlled pumping of water at high pressure into the salt seam.  A proportion of the solution-mined brine produced in this manner is evaporated to produce dry salt.

Solar salt, produced by the evaporation of sea water using solar heating, is also a source of sodium chloride.

Figure 4  Salt (sodium chloride) is being transferred to a large silo for
storage, prior to being converted to chlorine and sodium hydroxide.
By kind permission of AkzoNobel.

Saturated brine, prior to the electrolysis, is purified to precipitate calcium, magnesium and other detrimental cations by addition of sodium carbonate, sodium hydroxide and other reagents.  The suspended solids are removed from the brine by settling and filtration.

There are three electrolytic processes employed today. The concentration of caustic soda produced from each of the processes varies:

• Membrane Cells: Caustic soda is produced as a pure ca 30% (w/w) solution which is normally concentrated by evaporation to a 50% (w/w) solution using steam under pressure.
• Mercury Cells: Caustic soda is produced as a pure 50% (w/w) solution, which is the concentration most commonly sold on the world market.  Some is concentrated by evaporation to 75% and then heated at 750-850 K to obtain solid sodium hydroxide.
• Diaphragm Cells: The caustic soda is produced as an impure solution called 'diaphragm cell liquor' (DCL) with typical concentrations 10-12% (w/w) sodium hydroxide and 15% (w/w) sodium chloride.  In order to produce the 50% (w/w) strength that is usually required, the DCL has to be concentrated using evaporation units that are much larger and more complex than those used on membrane cell plants.  Large quantities of salt are precipitated during this process, which is normally reused to produce a saturated brine feed to the cells.  An additional aspect of the sodium hydroxide produced in the diaphragm cell is that the product has a small amount (1%) of salt present as a contaminant, which may make the material unsuitable for some purposes.

Date last amended: 27th November 2018