Pyrite Roasting, an Alternative to Sulphur Burning

The roasting of sulphide ores and concentrates is often the first step in the production of metals or chemicals. In many processes, the production of sulphuric acid is viewed as a by-product, while in some plants production is an important economic factor. Regardless of the purpose, a pyrite roasting plant consists of mainly three plant sections: roasting, gas cleaning and sulphuric acid. With the addition of air, the pyrite concentrates are transformed into solid oxides and gaseous sulphur dioxide at temperatures of 600 1000° C. After cleaning and cooling, the sulphur dioxide in the roasting gas is further processed to sulphuric acid. Two types of reactors are used depending on the application: stationary or circulating fluid bed . For over 60 years, Outotec has progressively been developing the principle of fluidised bed technology in several different reactor types for a multitude of process applications. The versatility of the fluidised bed reactor system has manifested itself in the treatment of minerals, including solid fuels, and for metallurgical processes both in the ferrous and non-ferrous fields. Process applications have included roasting, calcining, combustion and charring of coals, as well as off-gas treatment. This paper provides a summary of the pyrite roasting technology currently used along with a simple cost comparison of pyrite roasting and sulphur burning processes. Introduction Pyrite roasting and sulphur burning plants are built for the production of sulphuric acid. Whereas the burning of elemental sulphur is the main source for sulphuric acid, the roasting process is an interesting alternative once pyrite concentrate is available. In addition to these two applications, Figure 1 shows a wider variety of sources for the production of sulphuric acid, as there are also sulphidic ores, tail/flue gases, iron sulphate to name only a few. In some coutnries, pyrites and iron sulfide ores still constitute today, an important raw material basis for sulfur dioxide production, especially as the primary stage for sulfuric acid manufacture. The most important iron sulfide minerals are pyrite FeS2 as well as pyrrhotite Fe7S8. They occur in varying purity as sedimentary rocks in all formations, and as minor constituents in coal deposits. , Apart from gangue materials which are present in varying proportions in pyritic ores, other constituents includee sulfides of nonferrous metals, especially copper, zinc, and lead and to a lesser extent cobalt, nickel, and gold. In some cases, pyrites may also contain significant amount of arsenic as well as fluorides and chlorides. The production of pyrites, either as crude pyrites or as beneficiated pyrites, The Southern African Institute of Mining and Metallurgy Sulphur and Sulphuric Acid Conference 2009 M Runkel and P Sturm ________________________________________________________________ Page 102 is highly dependent on the fertilizer market, sulphur prices and the amount of produced sulfuric acid. Flotation pyrites are, however, obtained in marketable quantities as a byproduct of nonferrous metal sulfide beneficiation plants. Figure 1: Source and Demand for sulphuric acid plants The roasting processes as shown in Figure 1 are related to the fluid bed reactor. The largest single line stationary fluid bed roasting plant in operation today has a capacity of 1,130 t/d of pyrite/pyrrhotite concentrate. Further development has now been done to increase this maximum capacity for a single roasting line up to 1,800 t/d of concentrate. From an economic and technical standpoint, this throughput can only be achieved for one line by applying circulating fluid bed technology. In similar applications, the throughput e.g. in whole ore gold roasting plants can reach up to 3,800 t/d. The Southern African Institute of Mining and Metallurgy Sulphur and Sulphuric Acid Conference 2009 M Runkel and P Sturm ________________________________________________________________