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The oxidizing gases flow upward, i.e., counter-current to the descending charge. In a well-insulated roaster, external heating is unnecessary except when the charge is highly moist. The hearth at the top of the roaster dries and heats the charge. Ignition and oxidation of the charge occur lower down. [1]
The reactions are slow and can be sustained only by the addition of fuel. Multiple hearth roasters are unpressurized and operate at about 690 °C (1,270 °F). Operating time depends upon the composition of concentrate and the amount of the sulfur removal required. Multiple hearth roasters have the capability of producing a high-purity calcine. [1]
Initial copper production used two multiple-hearth roasters, a single coal-fired reverberatory furnace and two Peirce-Smith converters to produce a design 1,500 tons of blister copper per month (18,000 tons per year). [20] The copper smelter produced 15,000 tons of copper during 1953. [21]
The facility has roasting capacity of 35 million pounds of molybdenum per year. Four multiple-hearth furnaces are used for the conversion (roasting) of molybdenum disulfide concentrates into technical grade molybdenum oxide (tech oxide), which is sold in powder form or briquettes or converted into pure molybdenum oxide or ferromolybdenum.
Early sulfide roasting was practiced in this manner in "open hearth" roasters, which were manually stirred (a practice called "rabbling") using rake-like tools to expose unroasted ore to oxygen as the reaction proceeded. This process released large amounts of acidic, metallic, and other toxic compounds.
Thermal drying is usually used for fine particles and to remove low water content in the particles. Some common processes include rotary dryers, fluidized beds, spray driers, hearth dryers and rotary tray dryers. This process is usually expensive to operate due to the fuel requirement of the dryers.