Sulfur, in elemental form and in minerals, ranks as the third most common raw industrial material, after oxygen and silicon. It is one of the more important chemical elements because of its major derivative, sulfuric acid. However, its dioxide form, SO2, is considered a major pollutant from today’s industry.
Though elemental sulfur remains a primary source for sulfur raw materials, secondary sources can include pyrite mineral, sulfidic nonferrous metals, coal, natural gas and oil. Sulfur is recovered from secondary sources mainly by capturing off-gases from smelters and roasters, as well as from fossil-fired power plants, by transforming the SO2 content into sulfuric acids. Another source of sulfuric acid as a byproduct is petroleum refining and natural gas processing.
The majority of SO2 is released by fossil fuel-fired power plants and can lead to the formation of acid rain. As early as the 1970s, flue gas desulfurization (FGD) using limestone was introduced to capture SO2 before exiting power plant stacks. The removal rate today is up to 95 percent. In addition to SO2, the same plants emit certain amounts of NOX, which is responsible for smog formation, as well as mercury and particulate matter (PM) during combustion.
Sulphuric Acid is the highest volume inorganic chemical manufactured world-wide.Approximately 60% of world production is consumed in producing phosphate Fertilizers,the balance being used in literally thousands of other applications including Lead AcidBatteries, the production of other Acids, Pharmaceuticals, Plastics, Dyes & Pigments (e.g.Titanium Dioxide), Electroplating, Explosives, Metals Processing and Petroleum Refining.Sulphuric Acid is a clear or slightly cloudy, heavy, oily, toxic liquid. Other names usedinclude Oil of Vitriol, ROV (Rectified oil of Vitriol), BOV (Brown Oil of Vitriol) andBatteryAcid. At concentrations over 98% it has a pungent smell due to the presence of free Sulphur Trioxide, but below 98% it has little odour. Sulphuric Acid is commercially available in concentrations from 33%, used in storage batteries, up to 100%.Oleum, or Fuming Sulphuric Acid, is a solution of excess Sulphur Trioxide in anhydrous (100%) Sulphuric Acid. Since Oleum can be diluted with water to produce 100% Sulphuric Acid, the equivalent acid concentration is often used. For example, 66% Oleum is also known as 114.6% Sulphuric Acid
Over the last decades the ‘Contact’ process has been used to produce Sulphuric Acid, replacing the traditional ‘Lead Chamber’ process dating back to the 18th Century.In the Contact process,Sulphuris burnt in the presence of air to produce Sulphur Dioxide(SO2), which is then oxidized to Sulphur Trioxide (SO3) at high temperature in the presence of a Vanadium catalyst. The SO3 is absorbed into 98% to 99% Sulphuric Acid, increasing its concentration. Water is added to maintain the acid concentration, and the excess produced is drawn-off as product acid. Sulphurand Sulphur Dioxide are by-products of other industrial processes such as Metals Smelting, Oil & Natural Gas Desulphurization, and Flue Gas Desulphurization from Power Generation. Increasing legislation and awareness of environmental effects have led to the local construction of Sulphuric Acid plants to utilize these sources and hence reduce SO2 emissions. As energy prices have risen in recent decades, the valuable energy generated during the Sulphuric Acid production process has become a valuable commodity. Major changes have been made in process and plant design to maximize energy recovery and then use this heat to generate high-pressure steam and/or electricity. This secondary function complicates the plant and it’s operation, but sharply reduces the cost of the acid produced. A modern Sulphuric Acid plant has extensive boiler and steam systems in addition to the acid streams. Process economics and legislation reducing allowable SO2 emissions have resulted in more complex plants with higher conversion efficiencies. The older ‘single absorption’ process has been largely replaced by the ‘double absorption’ process (also called ‘double catalyst’) which increases yield of acid and reduces emissions. A modernSulphurburning plant may achieve conversion efficiencies of 99.7% or better, and plants using smelter gases 99.5%. A typical flowchart for aSulphurburning double absorption Sulphuric Acid plant is shown in Figure 1. MoltenSulphuris pumped (using a heated pump and lines) to burners in a furnace, where it meets dried air from the Turbo-blower (fan). Where roastedSulphurores are used rather than elementalSulphur, fluidized beds or rotary roasters are used in the furnace. Spent Sulphuric Acid from other production processes may also be decomposed to provide SO2 gas. The gas produced (containing around 11% or so of SO2) is passed through a heat recovery boiler then filtered before being passed to the converter. Vanadium catalyst beds in the converter encourage the reaction of SO2 with Oxygen to produce SO3 (Sulphur Trioxide). The reaction is exothermic, and the gas passes through several heat exchangers as it makes its way down the converter, controlling the temperatures at each catalyst bed to obtain optimum speed and efficiency of the conversion. The converter operates at temperatures over 400°C (750°F), and is usually brick-lined. In the double absorption process, gas from part-way down the converter is cooled, then passes through an interpass absorber before being reheated and sent back to the final stage of the converter. After the final stage, the gas passes through another heat recovery exchanger then to a final absorber. In the absorbers the SO3 laden gas is passed through 98% to 99% Sulphuric Acid, increasing the concentration of the acid. The acid circulates back to a tank, where it is diluted with water (or dilute acid) then the excess drawn off as product. The process is again exothermic, and the acid is cooled as it circulates. Absorbers are usually lined with acid-resistant brick. In some plants the gas passes through an Oleum tower before going to the absorber, where 98% to 99% acid is added to the circulating acid instead of water. Oleum is then produced and drawn off as product. Product acid may be pumped to stock tanks, and then to road or rail tankers.