The Roasting of Sulfides

Other papers on iron ore reduction include those of Warner [26], Edmiston and Grace [27], handler and Komarek [28], Walker and Carpenter [29], and Themelis and Gauvin [30]. A comprehensive text on this subject, covering work published before the mid 1960s, is that of Bogdandy and Engell [8]. 

The roasting of metal sulfides is of considerable technological and commercial interest because the ores of the majority of heavy metals such as copper, nickel, lead, and zinc are in a sulfide form. One important step in the recovery of these metals from their ores involves roasting, that is, their oxidation to metal oxide. 

The tonnages involved are illustrated in Tables 8.1 and 8.2, which indicate the annual production of copper, lead, zinc, and pyrites in the world and in the United States, respectively. The actual course of the roasting operations is illustrated by the following example using lead sulfide  

PbS H- 3O2 -- 2PbO + 2SO2 2PbS + 4O2 = 2PbS04 2PbO + 2SO2 +02 = 2PbS04 

Pyrites, thousand long tons Metals Copper (recoverable content of ores, etc.). 872 — 808 

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The modem process uses a potassium-sulfate-promoted vanadium(V) oxide catalyst on a silica or kie,selguhr support. The SO2 is obtained either by burning pure sulfur or by roasting sulfide minerals (p. 651) notably iron pyrite, or ores of Cu, Ni and Zn during the production of these metals. On a worldwide basis about 65% of the SO2 comes from the burning of sulfur and some 35% by the roasting of sulfide ores but in some countries (e.g, the UK) over 95% conies from the former. 

Reality Check That is 1.78 cubic meters. The roasting of sulfide ores is a major source of the gaseous pollutant S02, which contributes to acid rain (Chapter 14). 

Sulfur dioxide is produced in reactions such as the combustion of sulfur-containing fossil fuels and the roasting of sulfide ores. 

Figure 26-3 Sulfur dioxide, S02(g), that escapes from the roasting of sulfide ores (or the burning of coal in power plants) reacts with water in the atmosphere to form acid rain. This can seriously damage trees as shown in this photo from the southwestern United States.

Sulfur dioxide is produced on a large scale by burning sulfur, Sg. It is also obtained as a by-product of the roasting of sulfide ores (such as FeS2, CuS, ZnS, and PbS). Most of this sulfur dioxide is used to prepare sulfuric acid. Some is used as a bleach for wood pulp and textiles (Figure 22.46) and as a disinfectant and food preservative (for example, in wine and dried fruit). Its use as a food preservative depends on the fact that sulfur dioxide is especially toxic to yeasts, molds, and certain bacteria. Because some people are allergic to sulfur dioxide, foods containing it must be properly labeled. 

Gas-solid reactions play a major role in the technology of most industrialized nations. Gas-solid reactions encompass a very broad field including the extraction of metals from their ores (iron oxide reduction, the roasting of sulfide ores, etc.), the combustion of solid fuels, coal gasification, and the incineration of solid refuse, to cite but a few of the representative examples. Typical equipment used for effecting gas-solid reactions is also rather diverse, as seen from Figs. 1.1-1.3. 

The method can be extended to the reduction of metal oxides by carbon monoxide, or the roasting of sulfides in air or oxygen by, for instance, absorbing the gaseous product in sodium hydroxide solution. 

Perhaps the most commonly used arrangement for the roasting of sulfide ores is the fluidized-bed system. As discussed in Chapter 7, fluidized beds are ideal arrangements for performing reactions where large volumes of solids... 

As discussed in the preceding section dealing with iron oxide reduction, while a great deal of work has been done on the kinetics of the roasting of sulfides, relatively little of this information is being used in the design of... 

THE ROASTING OF SULFIDES 351 Recloimed Oust Screen Returns of Undersize... 

As a final remark on the utility of limestone as an absorbent for sulfur dioxide, the reader is referred to the novel approach of Bartlett and Huang [77] to SO2 absorption in the roasting of sulfide ores. In their suggested process, the sulfur dioxide is trapped at point of generation by lime within the roasting sulfide ore pellet. 

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