Melting Sulfur in the Production of Sulfuric Acid
Melting Sulfur
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| Sulfur Ore |
Sulfur Melting is the first step of the production of sulfuric acid.
Sulfur
Actually, most of the elemental sulfur used for the production of sulfuric acid is a by-product of natural gas and petroleum refining. These by-products contain mainly 99.9% sulfur. The main impurity is carbon from natural gas or petroleum. Sulfur melting point is 115-120 0C, depending on the crystal structure. It is melted with pressurized steam pipes easily.
Viscosity
The main features of the sulfur viscosity could be explained as follows:
(i) minimum viscosity is at 160 0C
(ii) 10,000-fold viscosity increase just above 160 0C.
Sulfur burners, the second step of the production of sulfuric acid, are fed with 140 0C molten sulfur, near the viscosity minimum, under the steep viscosity increase. Sulfur temperature is maintained by circulating 150 0C steam through steam pipes around molten sulfur storage tanks and all molten sulfur delivered pipes are also jacketed with steam to ensure the molten state of sulfur. Insulated below ground or above ground molten sulfur storage tanks are used.
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Molten sulfur viscosity as a function of temperature
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The huge increase in sulfur viscosity just above 160 0C is because of a transformation of sulfur from S8 ring molecules to long interlocking sulfur chain molecules.
Sulfur Melters
Sulfur melters can be either above ground tanks or below ground pits having steam coils for melting the sulfur. The tank or pit is fitted with an agitator which will increase the rate of sulfur melting. The raw of sulfur is generally delivered from a hopper to the melting tank or by a conveyor system. Lime should be added during melting to neutralize any acidity in the sulfur to prevent corrosion of equipment.
Each percent of moisture in the sulfur will increase the heat load by typically 20%. Moisture may cause foaming in the melting tanks, and that may decrease the capacity of the tank. Moisture is also undesirable because it will accelerate corrosion in the presence of small amount of SO3 gas to form traces of corrosive H2SO4 acid.
About 70 BTU are wanted to convert one pound of solid sulfur at 60oF to liquid sulfur at a temperature of 280oF.
Above ground melting Tanks
The above-ground melting tank is mainly made of carbon steel lined with acid resistant brick. Tanks should be equipped with an overflow to discharge molten sulfur formed to other underground tanks as it discharges through the melting tanks by gravity. A drain should be installed at the low point of the tank.
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| Above Ground Sulfur Melting Tank |
Melting underground Pits
A melting underground pit is generally rectangular and is made of reinforced concrete lined with acid resistant brick. The concrete resists the corrosive affinity that can be found because of the possibility of presence traces of sulfuric acid with molten sulfur. The wall of the underground pit should be brick lined at the liquid-air interface.
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| Melting Underground Pit |
Heating/Melting Coils
Heating coils are usually fabricated of 1 1/2” or 2” schedule 80 steel pipe. They should be located below the minimum sulfur level in order to reduce corrosion and ensure the sulfur in its molten shape. Coils may be mad in the shape of trombone, helical and other shapes. Finned tubes are generally used as they increase the surface area of heat transfer.
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