Besides the numerous and highly technical methods of ore concentration as practiced in the art of ore dressing, there are a few processes distinctively metallurgical which must be considered.
Not infrequently materials must be dried to avoid freight charges, or in order to supply perfectly dry material for further treatment (as preparatory to electrostatic separation), or preparatory to roasting. Mechanical dryers have been brought to high perfection and can do the work efficiently. Such a revolving tumbling and heating cylinder is seen in Fig. 10. Both the fire box and the flue are at the far end, for in this particular type the gases of combustion travel twice the length of the cylinder before discharging; it has internal partitions for this purpose. The feed hopper is at the far end, and the material discharges from the near end into the rolls.
Calcining in kilns or furnaces may be practiced to get rid of moisture and carbon dioxide preparatory to the shipment of ores, or to put the ores in the best form for reduction. Several types of furnace are used.
Roasting may be performed to get rid of sulphur, to change from sulphide to oxide, to change to sulphate, to rid of arsenic, antimony, or tellurium, with salt to chloridize, or simply to volatilize. The most conservative method is on a flat-hearthed reverberatory; many more recent and improved types are in use. Fig. 11 shows the hand-rabbled reverberatory as used in many countries and for all of the purposes just mentioned. The charge will be pushed slowly the entire length of the furnace, toward the fire box, and finally raked out as we see the man doing in the picture. Sintering. Sintering is becoming more common with iron ores as preparatory to blast-furnace reduction; if carried out in a revolving cylinder it likely will be termed nodulizing. Fig. 12 shows just such a nodulizing cylinder. The ore is fed in through the opening A; B is the cylindrical steel drum revolving on rollers C; the discharge hood is at D; the gas burner is at E; and the gases exhaust through the stack F:
Fig. 10. Pair of On Diyen Feeding into Crashing Rolls Courtesy of Ruggles-Coles Company, New York City.
Fig. 12. Iron-One Nodutising Kiln. A - Feed; B - Rotating Furnace; C - Rollers) D- -Discharge Hood; B - Gas Burner; F- Stack.
This is now the standard preparatory method for sulphide ores before smelting in the lead blast furnace. It leaves the material both desulphurized and chunky. When iron ores are thus treated, the sulphur is lowered and the material is agglomerated as well. The principle of the continuous machine with down draft is seen sketched in Fig. 13. The charge feeds down through the mixer and the moistener upon the continuous-grate system just in front of the ignition burner. When ignited, the glowing cake is carried across and over the suction box as the combustion extends through the mass, so as to complete the work before the cake breaks off, finished, into the waiting car.
Fig. 13. Continuous Roast-Sintering Machine Courtesy of Amtrican Instituts of Mining Engineers.
Fig. 14 shows a pug mill in the background, within which two heavy rollers mix and grind the charge and finally press it through holes in a revolving thick steel disc so as to form the round cakes which are seen piled beside the conveyor extending forward from the pug mill. These particular cakes have been ground up with milk of lime as binder. This process is common for making fine and powdery materials into cakes suitable for blast-furnace reduction. Because of the rapid development of roast-sintering as a preparatory process, briquettes are fast losing in importance.
Fig. 14. Pile of Briquettes with Conveyor Coming from Briquetting Mechine.
The method of chemical solution or fusion is sometimes found necessary in cases where other and cheaper methods are not suitable. The process requires no unusual apparatus. Pure alumina is prepared thus from bauxite; by this process tungsten can be separated from the iron and the lime with which it occurs in nature.