Hearth, multiple

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. 

Multiple-hearth roasting offers ease of operation, abiUty to handle a wide variety of ores or blends, and Httle downtime. On the other hand, these furnaces are no longer being built because of their high capital and labor costs, relatively low sulfur dioxide off-gas, need for added fuel, and marginal opportunity for waste-heat recovery. 

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. 

Roasting . Roasting has been largely abandoned ia modem copper smelters, ia which this function is combiaed with the smelting furnace. In older systems, the multiple-hearth roaster is a brick-lined tower having horizontal brick hearths. The concentrate is iatroduced at the top hearth, where rotating arms with rabble blades turn it over and move it to holes ia the hearth. The concentrate is transferred successively to lower hearths and finally... 

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.)... 

FIG. 23-43 Reactors for solids, (a) Temperature profiles in a rotary cement lain, (h) A multiple hearth reactor, (c) Vertical lain for lime burning, 55 ton/d. (d) Five-stage fluidized bed lime burner, 4 by 14 m, 100 ton/d. (e) A fluidized bed for roasting iron sulfides. (/) Conditions in a vertical moving bed (blast furnace) for reduction of iron oxides, (g) A mechanical salt cake furnace. To convert ton/d to kg/h, multiply by 907. 

Incineration Incineration has been used to reduce the volume of sludge after dewatering. The organic fractions in sludges lend themselves to incineration if the sludge does not have an excessive water content. Multiple-hearth and flmd-bed incinerators have been extensively used for sludge combustion. 

A multiple-hearth incinerator consists of several hearths in a vertical cyhndrical furnace. The dewatered sludge is added to the top hearth and is slowly pushed through the incinerator, dropping by gravity to the next lower layer until it finally reaches the bottom layer. The top layer is used for diying the sludge with the hot gases from the... 

There are four basic types of incinerators used in wastewater treatment plants. They are the multiple hearth incinerator, the fluid bed incinerator, the electric furnace, and the cyclonic furnace. Each system has it s own distinct method of incineration and while one may be more cost efficient, another may have more of an environmental impact. 

The basic configuration and features of the multiple hearth incinerator are illustrated in Figure 23. This incinerator is the most prevalent incinerator technology for the disposal of sewage sludge in the U.S. due to it s low ash discharge. Sludge cake enters the furnace at the top. The interior of the furnace is composed of a series of circular refractory hearths, which are stacked one on top... 

Figure 23. Cross section of multiple hearth furnace.

Solid wastes arc disposed of by two basic methods. The first is by some type of dumping or landfill procedure the second is by incinerating (burning) the waste. This section focuses on incinerators, namely the rotary kiln, liquid injection, fuidized-bed, and multiple-hearth dc ices, which are the four types... 

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. 

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