Process Description

Multiple-hearth furnaces were originally developed in 1889 to roast pyrites for the manufacture of sulfuric acid. Modern multiple-hearth systems have some 120 proven uses, including:

• Burning (or drying) raw sludge, digested sludge, and sewage greases

• Recalcining lime sludge (the burning of CaCO3 into CaO) and waste pond lime from sugar manufacturing

• Pyrolysis: the manufacture and regeneration of activated-granular carbon and the regeneration of diatomaceous earth; pyrolyzing fruit, nut, and lumber waste (peach pits, walnut shells, almond shells, sawdust, and bark) for charcoal briquettes

• Reclaiming oily chips from boring machines to metal briquettes and reclaiming cryolite in aluminum smelting operations

• Other roasting uses like mercury, molybdenum sulfide, carbon, magnesium oxide, uranium yellow cake, and nickel

The multiple-hearth furnace is a simple piece of equipment, consisting primarily of a steel shell lined with refractory on the inside. The refractory can be either castable or brick, depending on the size of the furnace.

The interior is divided by horizontal brick arches into separate compartments called hearths. Alternate hearths have holes at the periphery to allow the feed solids to drop onto the hearth below. The center shaft, driven by a variable-speed motor, rotates the rabble arms situated on each hearth (see Figure 7.50.1). The rabble teeth on these arms are at an angle so that the material is moved inward and then outward on alternate hearths. The shaft and rabble arms are cooled by air introduced at the bottom; this air is recycled as required by the thermal process.

The sludge is fed through the furnace roof by a screw feeder or a belt and flapgate. The rotating rabble arms and rabble teeth push the sludge across the hearth to drop holes where it falls to the next hearth and continues downward until the sterile phosphate-laden ash is discharged at the bottom.

The multiple-hearth system has the following three distinct operating zones:

1. The top hearths where the feed is partially dried

2. The incineration and deodorization zone where temperatures of 1400° to 1800°F (760° to 982°C) are maintained

3. The cooling zone where the hot ash gives up heat to the incoming combustion air.

The warmed air rises to the combustion zone in counter-flow to the sludge flow, and the hot combustion gases

FIG. 7.50.1 Multiple-hearth sludge incineration.

sweep over the cold incoming sludge, evaporating the sludge moisture to about 48% At this moisture content, a phenomenon called thermal jump can occur in the combustion zone. This beneficial energy exchange allows the generation of odor-free exhaust gas at temperatures of 500° to 1100°F (260° to 593°C). Table 7.50.1 gives the typical temperature profile across the sludge furnace.

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