Multiple-hearth furnace

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

A more simplified description is a unit that combusts materials in the presence of oxygen at temperatures normally ranging from 800 to 1650°C. A typical configuration of an incinerator is shown in Figure 9. Typical types of incineration units that are discussed herein are catalytic oxidation, fluidized beds, hquid injection, multiple hearth furnaces, and rotary kiln. Thermal desorption is also discussed. However, an overview of the main factors affecting incinerator performance is presented first, below. 

PAG sludge can be regenerated by wet air oxidation (WAO) or by a multiple-hearth furnace. Capacity losses might be high in WAO, particulady with low molecular weight organics. Weight loss in a furnace may exceed 20%. 

Incineration can be accompHshed ia multiple-hearth furnaces, ia which the sludge passes vertically through a series of hearths. In a fluidized-bed sludge, particles are fed iato a bed of sand fluidized by upwardly moving air. 

Charcoal is produced commercially from primary wood-processing residues and low quaUty roundwood in either kilns or continuous furnaces. A kiln is used if the raw material is in the form of roundwood, sawmill slabs, or edgings. In the United States, most kilns are constmcted of poured concrete with a capacity of 40 to 100 cords of wood and operating on a 7- to 12-d cycle. Sawdust, shavings, or milled wood and bark are converted to charcoal in a continuous multiple-hearth furnace commonly referred to as a Herreshoff furnace. The capacity is usually at least 1 ton of charcoal per hour. The yield is - 25% by weight on a dry basis. 

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. 

In addition to the multiple-hearth furnace, the reactivation system is comprised of additional equipment to transport, store, dewater, and qiieucn the carbon. 

FIG. 16-50 Multiple hearth furnace for carhon reactivation. (Reprinted with permission of EPA. Reference EPA, Process Design Manual for Carhon Adsorption, U.S. Envir Protect. Agency., Cincinnati, 1.973.)... 

Figure 23. Cross section of multiple hearth furnace.

Typical multiple hearth furnaces of the Hereshoff patent. 

Continuous production ol charcoal is typically performed in multiple hearth furnaces, as illustrated in the Herreshoff patent shown in Figure 2. Raw material is carried by a screw conveyor to the uppermost of a series of hearths, /kir is supplied counter-currently and burns some of the wood to supply process heat. As the layers of wood carbonize, they are transported to the lower (hotter) hearths by rakes. The hot charcoal product is discharged onto a conveyor belt and cooled with a water spray. 

The roasting process mostly applied to sulfidic sources is carried out in appliances of different types with different design features. Without describing their features and constructional details, it may simply be mentioned here that some of the widely acclaimed roasting units are traveling grate furnaces, multiple hearth furnace, rotary kilns, and fluid-ized-bed roasters. 

Solid effluents arising from metallurgical operations occur principally in two forms fine particulate solids or dusts, and solid wastes. As an example, blast furnace gas may contain up to 170 kg of dust per ton of pig iron produced. Suitable methods must be devised for processing the solid effluents for two reasons (i) to prevent pollution of the environment and (ii) to recover their valuable content, if any. As far as the latter is concerned, reference may be drawn, as an example, to the recovery of rhenium from the exit gas from molybdenite roasting in a multiple-hearth furnace. 

In a suspension roaster, the concentrates are blown into a combustion chamber. The roaster consists of a refractory-lined cylindrical shell, with a large combustion space at the top and two to four hearths in the lower portion. Additional grinding, beyond that required for a multiple-hearth furnace, is normally required to ensure that heat transfer to the material is sufficiently rapid for the desulfurization and oxidation reaction to occur in the furnace chamber. Suspension roasters are also unpressurized and operate at about 980°C (1800°F). 

Multiple hearth furnaces, in thermal waste treatment, 25 833... 

Figure 16 Process flowsheet of a GAC system with regeneration. In this complete GAC adsorption and regeneration system, four GAC columns can be operated in parallel or in series. Spent carbon is transferred to a multiple-hearth furnace for thermal regeneration. Regenerated carbon is mixed with virgin makeup and pumped back to the GAC columns. The GAC columns are backwashed periodically. (From Ref. 27.)...

Figure 11 shows a typical flow diagram for a thermal regeneration system [11]. Thermal regeneration is conventionally carried out in a multiple hearth furnace or a rotary kiln at... 

The capacity of regeneration furnaces is selected so that they operate 80-90% of the time. In multiple-hearth furnaces the loading is 70-80 lb/(sqft)(day). In countercurrent direct fired rotary kilns, a 6% volumetric loading is used with 45 min at activation temperature. 

Rotary, combustion gas heated Rotary, hot air heated Rotary, steam tube heated Cabinet dryers Spray dryers Multiple hearth furnace... 

The Soot Ash Removal Unit (SARU) is shown in Figure 5.23. In this step the carbon slurry is flashed to atmospheric pressure in the slurry tank. The slurry is then filtered to produce 1) a filter cake (with about 80% residual moisture) and 2) a clear water filtrate. The filter cake then goes through a controlled oxidation process in a multiple hearth furnace. The furnace allows the combustion of the carbon to take place under conditions where vanadium oxides neither melt nor corrode the furnace. The product is a vanadium concentrate that contains about 75% V2Os and can be sold to metals reclaiming companies. The process is autothermal because the heat of combustion of the carbon is sufficient to evaporate the moisture in the filter cake180. 

Oxidation of sulfide ores (MeS + 1.502 =>MeO + S02, where Me is Fe, Mo, Pb, Cu, or Ni). Iron sulfide (pyrite) is burned with air for recovery of sulfur and to make the iron oxide from which the metal is more easily recovered. Sulfides of other metals also are roasted. A multiple-hearth furnace, as shown in Fig. 19-24, is used. In some designs, the plates rotate in others, the scraper arms rotate or oscillate and discharge the material to lower plates. Material charged at the top drops to successively lower plates while reactant and combustion... 

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