Sulphur is the tenth most abundant element in the universe. A brittle, non-metallic element, one of its oxides finds a myriad of uses in modern day industry.

Sulphur dioxide (SO2) is produced by burning sulphur or sulphur-containing compounds and can also be produced by burning coal and producing metals from their sulphides, such as copper sulphide.

Natural sources of SO2 are volcanoes and algae-producing di-methyl sulphide, which is converted to SO2 in the ambient atmosphere.

Sulphur dioxide can be liquefied under moderate pressures at room temperatures; the liquid freezes at -73°C and boils at -10°C under atmospheric pressure.

Industrial production
Sulphur dioxide is a heavy, colourless, poisonous gas with a pungent, irritating odour familiar as the smell of a just-struck match.

Occurring in nature in volcanic gases and in solution in the waters of some warm springs, sulphur dioxide is usually prepared industrially by the burning of sulphur or such compounds in air or oxygen.

Large quantities of sulphur dioxide are formed in the combustion of sulphur-containing fuels.

The principal use of SO2 has historically been the production of sodium hydrosulphite, which is consumed primarily as a bleaching agent by the textile and the pulp & paper industries.

A growing use in the pulp & paper industry is to stabilise mechanical pulp after bleaching with hydrogen peroxide. In this application, sulphur dioxide acts to maintain the pulp brightness by destroying excess hydrogen peroxide.

Of the various sulphur dioxide market segments, the pulp & paper industry has the greatest potential for future growth because of the increasing quantities of paper being recycled – a process requiring bleaching.

Sulphur dioxide is important in the agricultural and food processing markets, where it acts as a preservative, fumigant, bleach and/or steeping agent, depending on the application.

It is used as an antimicrobial in the manufacture of high-fructose corn syrup, for example.

Sulphur dioxide is sometimes used as a preservative for dried fruits, especially in apricots due to its antimicrobial properties, and is called E220 when used in this way.

As a preservative, it maintains the appearance of the fruit and delays rotting. However, its presence is also known to give the fruit a distinctive chemical taste.

Alcohol industry
Sulphur dioxide is an important compound in winemaking, and is designated as parts per million in wine.

It is present even in so-called unsulphurated wine at concentrations of up to 10 milligrams per litre and serves as both an antibiotic and antioxidant – protecting wine from spoilage by bacteria and oxidation.

Sulphur dioxide is a very important element in winery sanitation, with wineries and equipment having to be kept clean, and because bleach cannot be used in a winery.

A mixture of SO2, water and citric acid is commonly used to clean and sanitise equipment in a safe and effective manner.

Growth and future prospects
Market growth has been limited by the trend of large consumers to install sulphur burning equipment, and generate their own SO2 on-site as required.

This has been the case particularly in corn processing and the pulp & paper industry, where one driver is the elimination of stored, liquid SO2 – a safety issue, particularly in populated areas.

The SO2 usage has been stable and should remain so even with the widely fluctuating sulphur prices last year, due to increased fertilizer activity.

Industry capacity is more than adequate to meet future needs.

Most application areas are mature. Where there is growth (paper and foods), industry gains are periodically set back when large consumers begin generating their own requirements by burning sulphur.

Demand growth for the forecast period is in the low single digit margin.