Knowledge for the Sulphuric Acid Industry
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DKL Engineering, Inc.
May 7, 2009
Pyrometallurgical processes for the smelting and refining or sulphide ores (i.e. copper, lead, zinc, nickel, etc.) produces an off-gas containing dust, fumes and sulphur dioxide which must be treated before discharging the gas to the environment. The most common method of treating the gas is to put the gas through a metallurgical sulphuric acid plant.
The design and operation of a metallurgical sulphuric acid plant varies considerably because the nature of the gas that must be treated is different for each metallurgical operation. Sulphur dioxide concentrations can be as low as 1 to 2% in the case of a molybdenum roaster or over 50% in the gas of some flash smelting process with oxygen enrichment. The contaminants in the gas will depend on the composition of the ore being treated. Some gases may contain only relatively easy to remove dust particles while other gas will contain such components as mercury, chlorides, fluorides, arsenic, selenium, etc. Unlike a sulphur burning acid plant, gas flow and SO2 concentration to a metallurgical sulphuric acid plant may vary considerably.
The design of a metallurgical sulphuric acid plant must take into account all of the above factors in order to produce sulphuric acid of the desired quality.
A feature of all metallurgical sulphuric acid plants is the gas cleaning system which cleans and conditions the gas prior entering the contact section of the acid plant. The first step is to quench and cool the gas in a quench/humidifying tower. This process reduces the temperature, partially cleans and humidifies the gas. Quench and cooling is often followed by a scrubbing stage where a venturi scrubber or similar device is used to remove more of the contaminants. The gas must be further conditioned by cooling and condensing the water from the gas so that the water balance of the acid plant can be satisfied. The final step in a typical gas cleaning system is removal of fine particles and mist by electrostatic precipitation.
The design and operation of the gas cleaning system determines to a large extent the quality of the acid produce and operating problems. Any contaminants in the gas leaving the gas cleaning system will end up in the acid circulating system and eventually the product acid. Fluorides if not removed will attack the ceramic packing and acid brick in the towers as well as the catalyst support which all contain silica. Chlorides and arsenic are catalyst poisons and will deactivate the catalyst reducing overall conversion. Dust that is not removed in the gas cleaning system will collect in the catalyst causing a rise in pressure drop.
If the gas cleaning system is properly designed and operated, the quality of the acid produced from a metallurgical sulphuric acid plant can match the quality obtained from a sulphur burning sulphuric acid plant.
In a metallurgical acid plant the gas entering the contact section of the acid plant is cold and must be heated to the bed 1 inlet temperature. This is done by exchanging heat with gases leaving the beds after they have undergone partial conversion of SO2 to SO3. This is the fundamental difference between the contact sections of a metallurgical and sulphur burning acid plant. In a metallurgical acid plant, the gas strength will determine if there is any excess heat available in the process. At high gas strengths the gas temperature entering the absorber tower(s) will be too high requiring removal of the excess heat in an SO3 Cooler or steam equipment. At low gas strengths there may be insufficient heat to maintain the plant heat balance and the preheater may need to be operated to input additional heat into the system.
The acid system in a metallurgical acid plant must take into consideration that the gas being dried in the drying tower contains larger quantities of water and SO2. The increased amount of water generally means a larger temperature rise across the tower and a higher heatload on the acid cooler. SO2 that passes through the tower will be absorbed into the circulating acid. If left untreated, the SO2 will go into the absorber system and will be stripped out of the acid by the SO2 lean gas entering the final absorber tower. This SO2 will go directly up the stack increasing the plant's emission of SO2. To avoid transferring SO2 in the acid system, the drying and absorber acid systems are always separate circuits operating at different strengths. In a sulphur burning plant the drying and absorber systems can be combined and you will often find a single pump tank and cooler serving both systems. This cannot be done in a metallurgical acid plant since SO2 would be transferred in the common circulating system.
To prevent SO2 transfer, a crossflow stripper is sometimes used to strip SO2 out of the dying acid crossflow stream to the absorber system. The stripping air containing SO2 is sent back to the inlet of the drying tower. Another method is to direct the drying acid crossflow to the inlet of the Interpass Absorber where the SO2 will stripped out of the acid and returned directly into the process. This method works because the acid is hotter which means the SO2 is less soluble and the gas entering the Interpass Absorber has a lower SO2 concentration in the gas than the gas entering the Drying Tower. Equilibrium concentrations of SO2 in the gas and acid are in the direction of SO2 leaving the acid and entering the gas.