EQUIPMENT - 3-FOOT-DIAMETER MULTIPLE-HEARTH FURNACE
A 3-foot inside diameter, eight-hearth roaster is available at Hazen for pilot-plant thermal processing studies. Materials most suited for processing in this unit include finely divided ores, and concentrates, samples with adhering characteristics, and samples requiring combustion or pyrolysis over long residence times at controlled temperatures (e.g. carbon activation).
The multiple-hearth furnace is 4.5-feet in diameter by 6.5-feet high, and its perimeter is lined with eight inches of insulating refractory. Hearth floors have an average surface area of about 6.5 square feet. The maximum operational capacity of the furnace is about 8.7 cubic feet when using a two-inch-deep bed on 51.7 square feet of hearth floor. Rabble arms attached to a central vertical shaft agitate and move material from each hearth to the next. Both the vertical shaft and rabble arms are made of cast 309 stainless steel and can be internally cooled with air, allowing operating temperatures up to 1000°C (1850°F).
The multiple hearth is heated to operating temperature using gas burners mounted on each of five different hearths, as shown in the schematic. Each burner has a thermal capacity of 140,000 Btu per hour, providing an overall furnace capability of about 700,000 Btu per hour. Process temperature profiles are adjusted and maintained with the use of the burners and secondary air adjustments. Steam or reformed gas can be injected for gas atmosphere control. Flue gases from the furnace are conveyed through an exhaust duct to a cyclone and baghouse for dust removal. A scrubber is used for final gas cleaning before the gas is exhausted to the atmosphere.
Feed rate and retention time can be varied to accommodate a wide range of operating conditions. Material is introduced using a variable-speed screw located above the top hearth. Feed rates to the furnace range from 25 to 700 pounds per hour, depending on the physical characteristics and processing requirements of the material. Retention times can be controlled to range from 30 minutes to 12 hours. Final product is discharged from the bottom hearth to a canister, to a four-inch water-cooled screw, or to other solids-handling equipment.
The system is equipped with online instrumentation to obtain operating data. Process temperatures, pressures, and gas compositions, including O2, CO2, CO, SO2, NOx, and total hydrocarbons (THC), are monitored and recorded continuously. Gases from individual hearths can be measured. A gas chromatograph and mass spectrometer are available for measuring other gaseous constituents, such as water, hydrogen, nitrogen, and methane. A data-acquisition system is used to monitor and record selected process data. Emission sampling for organic compounds, metals, and other inorganic species can be performed as required, using EPA-approved methods.