Fluid bed roaster

For environmental and economic reasons, the eady practice of roasting zinc sulfide and discharging the sulfur dioxide to the atmosphere gave way to plants where the sulfur dioxide is converted to sulfuric acid. Desulfurization takes place while the ore particles are suspended in hot gases. Called flash-and fluid-bed roasters, these processes are described below. Some plants use combinations of roasters and sintering for desulfurization. 

St. Joe Minerals Corporation uses a fluid-bed roaster to finish the roasting at 950°C of material that has been deleaded in a modified multiple-hearth furnace operated with insufficient oxidation (34). First, sulfur is reduced from 31 to 22% and lead from 0.5 to 0.013%. Somewhat aggregated, the product is hammer-milled before final roasting. Half of the calcined product is bed overflow and special hot cyclones before the boiler remove the other half total sulfur is ca 1.5%. Boiler and precipitator dusts are higher in sulfur, lead, etc, and are separated. 

The ziac concentrate is first roasted ia a fluid-bed roaster to convert the ziac sulfide to the oxide and a small amount of sulfate. Normally, roasting is carried out with an excess of oxygen below 1000°C so that comparatively Htfle cadmium is eliminated from the calciaed material ia this operation (3). Siace the advent of the Imperial Smelting Ziac Furnace, the preliminary roasting processes for ziac and ziac-lead concentrates result ia cadmium recovery as precipitates from solution or as cadmium—lead fume, respectively, as shown ia Figure 1. 

See also Fluidized-bed entries Fluid-bed direct oxidation process, 10 656 Fluid-bed dryers, 9 122-123, 130-131 two-stage, 9 125 Fluid-bed roasters, 16 141 Fluid catalytic cracking (FCC), 11 678-699, 700-734 18 651, 653 20 777 24 257, 271. See also FCC entries Fluidized-bed catalytic cracking (FCC) clean fuels production and, 11 686-689 defined, 11 700... 

Murthy, K.V., Srinivasa Rao, RN. and Ramesh, T., Continuous vibro fluid bed roaster for breakfast cereals, Indian Food Industry, 14 (1995) 35-38. 

Sulfur dioxide is manufactured mostly by combustion of sulfur or its iron sulfide mineral, pyrite, FeS2, in air. The flame temperatures for such combustion of sulfur in the air are usually in the range 1,200 to 1,600°C. Many types of sulfur burners are available and are used to produce sulfur dioxide. They include rotary-kiln, spray, spinning-cup and air-atomizing sulfur burners. Selection and design of burners depend on quality of sulfur to be burned, and rate and concentration of sulfur dioxide to be generated. Pyrites or other metal sulfides may be burned in air in fluid-bed roasters to form sulfur dioxide. 

Over the period 1955-1961, the electrolytic plant processed only calcined zinc oxide having a composition of 60% Zn, 3-4% Pb and 0.03% Cd. The zinc oxide was produced by calcination of the fumes from the Waelz process at temperatures from 1420 to 1470 K. Zinc carbonate ores, Zn-Fe residues and other zinc-bearing materials were used as raw materials in the Waelz process. After enhancement of the electrolytic zinc plant in 1962, the importance of roasted sulphide concentrates was continuously growing in the feed. Currently, only the calcine from fluid-bed roasters is processed. Table I shows the analysis of the raw materials processed over the period 1998-1999. 

Beside the highest possible cobalt and copper recoveries, the auto-thermal operation of those fluid bed roasters, were of prime importance in order to avoid the extra operating cost of auxiliary fuel consumption. In order to limit any fuel (coal) addition, the feed slurry was maintained at 75 wt% solids to minimize the energy loss to water vaporization. This resulted in a slurry that presented a significant challenge in the roaster feed system design due to its high viscosity and solid content. 

The aim of the fluid bed roaster is generally to maximize the cobalt and copper recovery, while minimizing the leached iron into the atmospheric acid extraction following the calcine cooler. 

Figure 5 Sketch of fluid bed roaster and auxiliary equipment...

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