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What Is Contact Process?

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  • Written By: Karize Uy
  • Edited By: Lauren Fritsky
  • Last Modified Date: 14 November 2016
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Contact process is a chemical procedure that creates sulfuric acid for industrial purposes such as in paints, detergents, fertilizers, and plastic products. The method was hugely attributed to a British trader named Peregrine Phillips, who patented the process in the year 1831. Since then, contact process has been used worldwide for the production of sulfuric acid.

Before contact process became widespread, the leading method for creating sulfuric acid was the lead chamber process, but it was found out that the former process was more efficient and less expensive. Industries also preferred it to the lead chamber process because it can also be used for the production of other chemical compounds like oleum and sulfur trioxide. The contact process generally has three steps, creating the sulfur dioxide, then converting it to sulfur trioxide, then finally converting it to sulfuric acid.

The first step to creating sulfur dioxide involves two possible methods that use air to provide the oxygen in the sulfur compound. The first method is to burn some molten sulfur that can mix in with the air as it burns, resulting in sulfur dioxide. The second method is to heat materials that have sulfur in them, such as iron pyrites. The evaporating sulfur will then mix in with the oxygenated air and be converted to sulfur dioxide.

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The produced sulfur dioxide may contain some impurities, so it should undergo a purification process, which first involves a dust chamber to eliminate all the particles of dust in the compound. The second step is the washing tower, wherein the sulfur dioxide is water-sprayed to rinse away soluble contaminants. The compound goes through a drying tower to dehydrate it for further purification and goes through a test box to remove the arsenic oxide. Once the sulfur dioxide is thoroughly purified, it can be converted to sulfur trioxide.

To convert the sulfur dioxide to sulfur trioxide, contact process usually uses a compound called vanadium oxide contained in several high-pressured pipes. The pipes are heated as high as 842°F (about 450°C) to make the oxygen from the vanadium oxide rise into the air. Once the sulfur dioxide is put inside the pipes, it reacts with the oxygenated air and becomes sulfur trioxide. Ideally, over 95% of the sulfur dioxide should be converted to sulfur trioxide, and the unconverted sulfur dioxide is removed.

In the final step of the contact process, concentrated sulfuric acid is used to dissolve the sulfur trioxide, producing the oleum. The oleum is then made to react with water and, finally, sulfuric acid is produced in liquid form. The produced sulfuric acid is 30 to 50 percent more than the concentrated sulfuric acid used in the final step of contact process.

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