Metallurgical sulfuric acid plants

In drying towers of sulfur-burning plants, mesh pads or inertial impaction-type mist eliminators are usually adequate. High efficiency mist eliminators are usually used in drying towers of spent acid or metallurgical plants. 

Fig. 25.10. Modern metallurgical sulfuric acid plant with view of preheating furnace in foreground. (Courtesy of Kennecott, Monsanto Enviro-Chem, and Manly Prim Photography.)...

Bhambri, N., Fell, R. C., Fries, R. M., and Ritschel, P. M., Metallurgical Sulfuric Acid Plants for the New Millennium, Sulphur 98 Conference, British Sulphur Corporation, London. 

Smelter acid, on the other hand, must be made from byproduct S02(g) at the smelter and transported to its point of use. An example of this is production of acid at the Cu-Ni smelters in Sudbury, Canada and rail transport of the product acid to fertilizer plants in Florida. A new metallurgical sulfuric acid plant (3760 tonnes of acid per day) is costing 59 million U.S. dollars (Sulfuric 2005). 

The use of cyclic absorption processes for concentrating sulfur dioxide from smelter gases is still very limited. If and when sulfur dioxide reduction is practiced, concentration processes must be used more extensively unless metallurgical processes are used that deliver richer off-gases, probably with sulfur dioxide concentrations not lower than 20-25%. The Asarco DMA absorption process (9, 10) is coming into renewed use, partly to produce liquid sulfur dioxide and partly to provide enriched feed to sulfuric acid plants. 

These studies, associated with pilot-plant trials, indicated that Sirosmelt technology, because of its flexibility, versatility and simplicity, was the most suitable process to replace the conventional sinter machine-blast furnace. The process has been optimized since the start-up of the furnace based on increased industrial experience and pilot plant trials at the Metaleurop Research Centre. Improved automation and expert systems are necessary to stabilize the process because of the short residence times involved. The bath smelting process operates with a large range of primary and secondary feed materials, and carries out different metallurgical operations in the same reactor.The exhaust gases produced are processed in a sulfuric acid plant, and final SO2 conversion efficiencies of more than 99.5 % are achieved. All the emissions have been drastically reduced CO2 emissions have decreased by 60 %. 

Sulfuric acid plants based on sulfur are auto-thermal in nature. However, if the plant is running on metallurgical ores like iron pyrites, copper and zinc sulfides etc., it occasionally needs additional fuel for maintaining the process conditions because the sulfur dioxide percentage in the gases from the burner is sometimes insufficient for auto-thermal operations. 

Acid mist captured on the candle filters drains by gravity through collection pipes to the top of the packed bed or to a collection tank external to the absorption tower. These candle drips are sometimes treated to remove nitrate compounds that form when nitrogen oxides (mainly NO(g) and N02(g)) in the acid plant feed gas react with sulfuric acid to form nitrosyl sulfuric acid (HN0S04(f) Daum, 2000 Lyne et al., 2002). High nitrogen oxide concentration feed gas is commonly found in metallurgical acid plants. 

Steady operation and control of catalyst bed temperatures result in lower acid plant SO2 emissions. This is relatively easy for sulfur burning acid plants, but much more challenging for metallurgical acid plants, especially those treating Peirce-Smith converter gases that are intermittent and highly variable in terms of flow and composition. 

The investment costs in this chapter focus on sulfur burning and metallurgical type acid plants. Costs for spent acid regeneration acid plants are expected to be slightly higher than sulfur burning acid plants due to their increased furnace complexity and additional gas cleaning equipment. 

This is not the case for metallurgical sulfuric acid plants which often treat feed gas with intermittent flow rates and fluctuating SO2 concentrations depending on the metallurgical processes that produce the feed gas (Chapter 4). Actual acid production rates are perhaps 60-80% of the design acid production rate. Thus, it is more accurate to compare the investment costs for metallurgical acid plants using their feed gas flow and acid production rate. 

Figure 31.3 shows that a metallurgical sulfuric acid plant producing 1500 toime/ day H2SO4 could have a feed gas flow of 100,000 Nm /h with 14 volume% SO2 in its feed gas or a feed gas flow of 240,000 Nm /h with 6 volume% SO2 in its feed gas. The higher gas flow rate acid plant is expected to cost nearly twice as much as the lower gas flow rate acid plant. 

Electricity costs make up the majority of the production costs for metallurgical acid plants. Table 31.3 shows a production cost estimate for a metallurgical sulfuric acid plant including weak acid blowdown treatment. The example shown does not include heat recovery equipment to produce or superheat steam. Many metallurgical acid plants do not have steam equipment which simplifies their operation and lowers their investment cost. The unit costs are the same as those used in Table 31.2. ... 

This chapter has provided study estimate level investment and production cost estimates for sulfur burning and metallurgical type sulfuric acid plants. Spent acid regeneration type acid plants are expected to have slightly higher investment costs than sulfur burning type acid plants. 

Metallurgical acid plants have a gas cleaning step ahead of the acidmaking plant and are usually compared on both feed gas flow and acid production rate. Many metallurgical acid plants do not have heat recovery equipment to produce steam. Their investment costs are typically less than sulfur burning acid plants. Their electricity consumption makes up the majority of their production costs. 

Fig. 14.9. Metallurgical gas sulfuric acid plant. (Courtesy Texasgulf Inc.)...

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